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The Origin of Species

by Charles Darwin

Full Title:

On the Origin of Species by Means of Natural Selection, or the Preservation
of Favoured Races in the Struggle for Life.

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Table of Contents

* Preface
* Introduction
* Chapter 1 - Variation Under Domestication
* Chapter 2 - Variation Under Nature
* Chapter 3 - Struggle for Existence
* Chapter 4 - Natural Selection
* Chapter 5 - Laws of Variation
* Chapter 6 - Difficulties on Theory
* Chapter 7 - Instinct
* Chapter 8 - Hybridism
* Chapter 9 - On the Imperfection of the Geological Record
* Chapter 10 - On The Geological Succession of Organic Beings
* Chapter 11 - Geographical Distribution
* Chapter 12 - Geographical Distribution continued
* Chapter 13 - Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs
* Chapter 14 - Recapitulation and Conclusion
* Glossary

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Preface

FIRST EDITION OF THIS WORK AN HISTORICAL SKETCH OF THE PROGRESS OF OPINION
ON THE ORIGIN OF SPECIES PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION
OF THIS WORK

I WILL here give a brief sketch of the progress of opinion on the Origin of
Species. Until recently the great majority of naturalists believed that
species were immutable productions, and had been separately created. This
view has been ably maintained by many authors. Some few naturalists, on the
other hand, have believed that species undergo modification, and that the
existing forms of life are the descendants by true generation of
pre-existing forms. Passing over allusions to the subject in the classical
writers,* the first author who in modern times has treated it in a
scientific spirit was Buffon. But as his opinions fluctuated greatly at
different periods, and as he does not enter on the causes * Aristotle, in
his 'Physicae Auscultationes' (lib. 2, cap. 8, s. 2), after remarking that
rain does not fall in order to make the corn grow, any more than it falls to
spoil the farmer's corn when threshed out of doors, applies the same
argument to organization: and adds (as translated by Mr Clair Grece, who
first pointed out the passage to me), 'So what hinders the different parts
[of the body] from having this merely accidental relation in nature? as the
teeth, for example, grow by necessity, the front ones sharp, adapted for
dividing, and the grinders flat, and serviceable for masticating the food;
since they were not made for the sake of this, but it was the result of
accident. And in like manner as to the other parts in which there appears to
exist an adaptation to an end. Wheresoever, therefore, all things together
(that is all the parts of one whole) happened like as if they were made for
the sake of something, these were preserved, having been appropriately
constituted by an internal spontaneity, and whatsoever things were not thus
constituted, perished, and still perish. or means of the transformation of
species, I need not here enter on details.

Lamarck was the first man whose conclusions on the subject excited much
attention. This justly-celebrated naturalist first published his views in
1801; he much enlarged them in 1809 in his "Philosophie Zoologique,' and
subsequently, in 1815, in the Introduction to his "Hist. Nat. des Animaux
sans Vertébres.' In these works he upholds the doctrine that species,
including man, are descended from other species. He first did the eminent
service of arousing attention to the probability of all change in the
organic, as well as in the inorganic world, being the result of law, and not
of miraculous interposition. Lamarck seems to have been chiefly led to his
conclusion on the gradual change of species, by the difficulty of
distinguishing species and varieties, by the almost perfect gradation of
forms in certain groups, and by the analogy of domestic productions. With
respect to the means of modification, he attributed something to the direct
action of the physical conditions of life, something to the crossing of
already existing forms, and much to use and disuse, that is, to the effects
of habit. To this latter agency he seemed to attribute all the beautiful
adaptations in nature; --- such as the long neck of the giraffe for
browsing on the branches of trees. But he likewise believed in a law of
progressive development; and as all the forms of life thus tend to progress,
in order to account for the existence at the present day of simple
productions, he maintains that such forms are now spontaneously generated.*

We here see the principle of natural selection shadowed forth, but how
little Aristotle fully comprehended the principle, is shown by his remarks
on the formation of the teeth. *I have taken the date of the first
publication of Lamarck from Isid. Geoffroy Saint-Hilaire's ('Hist. Nat.
Générale,' tom. ii. p. 405, 1859) excellent history of opinion on this
subject. In this work a full account is given of Buffon's conclusions on the
same subject. It is curious how largely my grandfather, Dr Erasmus Darwin,
anticipated the views and erroneous grounds of opinion of Lamarck in his
'Zoonomia' (vol. i. pp. 500-510), published in 1794. According to Isid.
Geoffroy there is no doubt that Goethe was an extreme partisan of similar
views, as shown in the Introduction to a work written in 1794 and 1795, but
not published till long afterwards: he has pointedly remarked ('Goethe als
Naturforscher,' von Dr Karl Medinge s. 34) that the future question for
naturalists will be how, for instance, cattle got their horns, and not for
what they are used. It is rather a singular instance of the manner in which
similar views arise at about the same time, that Goethe in Germany, Dr
Darwin in England, and Geoffroy Saint-Hilaire (as we shall immediately see)
in France; came to the same conclusion on the origin of species, in the
years 1794-5.

Geoffroy Saint-Hilaire, as is stated in his 'Life,' written by his son,
suspected, as early as 1795, that what we call species are various
degenerations of the same type. It was not until 1828 that he published his
conviction that the same forms have not been perpetuated since the origin of
all things. Geoffroy seems to have relied chiefly on the conditions of life,
or the 'monde ambiant' as the cause of change. He was cautious in drawing
conclusions, and did not believe that existing species are now undergoing
modification; and, as his son adds, "C'est donc un problème à réserver
entièrement à l'avenir, supposé meme que l'avenir doive avoir prise sur
lui.'

In 1813, Dr W. C. Wells read before the Royal Society 'An Account of a White
female, part of whose skin resembled that of a Negro'; but his paper was not
published until his famous 'Two Essays upon Dew and Single Vision' appeared
in 1818. In this paper he distinctly recognises the principle of natural
selection, and this is the first recognition which has been indicated; but
he applies it only to the races of man, and to certain characters alone.
After remarking that negroes and mulattoes enjoy an immunity from certain
tropical diseases, he observes, firstly, that all animals tend to vary in
some degree, and, secondly, that agriculturists improve their domesticated
animals by selection; and then, he adds, but what is done in this latter
case 'by art, seems to be done with equal efficacy, though more slowly, by
nature, in the formation of varieties of mankind, fitted for the country
which they inhabit. Of the accidental varieties of man, which would occur
among the first few and scattered inhabitants of the middle regions of
Africa, some one would be better fitted than the others to bear the diseases
of the country. This race would consequently multiply, while the others
would decrease; not only from their inability to sustain the attacks of
disease, but from their incapacity of contending with their more vigorous
neighbours. The colour of this vigorous race I take for granted, from what
has been already said, would be dark. But the same disposition to form
varieties still existing, a darker and a darker race would in the course of
time occur: and as the darkest would be the best fitted for the climate,
this would at length become the most prevalent; if not the only race, in the
particular country in which it had originated.' He then extends these same
views to the white inhabitants of colder climates. I am indebted to Mr
Rowley, of the United States, for having called my attention, through Mr
Brace, to the above passage in Dr Wells' work.

The Hon. and Rev. W. Herbert, afterwards Dean of Manchester, in the fourth
volume of the 'Horticultural Transactions,' 1822, and in his work on the
'Amaryllidaceae' (1837, pp. 19, 339), declares that 'horticultural
experiments have established, beyond the possibility of refutation, that
botanical species are only a higher and more permanent class of varieties.'
He extends the same view to animals. The Dean believes that single species
of each genus were created in an originally highly plastic condition, and
that these have produced, chiefly by intercrossing, but likewise by
variation, all our existing species.

In 1826 Professor Grant, in the concluding paragraph in his well-known paper
('Edinburgh philosophical journal,' vol. xiv. p. 283) on the Spongilla,
clearly declares his belief that species are descended from other species,
and that they become improved in the course of modification. This same view
was given in his 55th Lecture, published in the 'Lancet' in 1834.

In 1831 Mr Patrick Matthew published his work on 'Naval Timber and
Arboriculture,' in which he gives precisely the same view on the origin of
species as that (presently to be alluded to) propounded by Mr Wallace and
myself in the 'Linnean journal,' and as that enlarged in the present volume.
Unfortunately the view was given by Mr Matthew very briefly in scattered
passages in an Appendix to a work on a different subject, so that it
remained unnoticed until Mr Matthew himself drew attention to it in the
'Gardener's Chronicle,' on April 7th, 1860. The differences of Mr Matthew's
view from mine are not of much importance; he seems to consider that the
world was nearly depopulated at successive periods, and then re-stocked; and
he gives as an alternative, that new forms may be generated ' without the
presence of any mould or germ of former aggregates.' I am not sure that I
understand some passages; but it seems that he attributes much influence to
the direct action of the conditions of life. He clearly saw, however, the
full force of the principle of natural selection.

The celebrated geologist and naturalist, Von Buch, in his excellent
'Description physique des Isles Canaries' (1836, p. 147), clearly expresses
his belief that varieties slowly become changed into permanent species,
which are no longer capable of intercrossing.

Rafinesque, in his 'New Flora of North America,' published in 1836, wrote
(p. 6) as follows:- 'All species might have been varieties once, and many
varieties are gradually becoming species by assuming constant and peculiar
characters'; but farther on (p. 18) he adds, 'except the original types or
ancestors of the genus.'

In 1843-44 Professor Haldeman ('Boston journal of Nat. Hist. U. States, vol.
iv. p. 468) has ably given the arguments for and against the hypothesis of
the development and modification of species: he seems to lean towards the
side of change.

The 'Vestiges of Creation' appeared in 1844. In the tenth and much improved
edition (1853) the anonymous author says (p. 155):- 'The proposition
determined on after much consideration is, that the several series of
animated beings, from the simplest and oldest up to the highest and most
recent, are, under the providence of God, the results, first, of an impulse
which has been imparted to the forms of life, advancing them, in definite
times, by generation, through grades of organisation terminating in the
highest dicotyledons- and vertebrata, these grades being few in number, and
generally marked by intervals of organic character, which we find to be a
practical difficulty in ascertaining affinities; second, of another impulse
connected with the vital forces, tending, in the course of generations, to
modify organic structures in accordance with external circumstances, as
food, the nature of the habitat, and the meteoric agencies, these being the
''adaptations'' of the natural theologian.' The author apparently believes
that organisation progresses by sudden leaps, but that the effects produced
by the conditions of life are gradual. He argues with much force on general
grounds that species are not immutable productions. But I cannot see how the
two supposed 'impulses' account in a scientific sense for the numerous and
beautiful co-adaptations which we see throughout nature; I cannot see that
we thus gain any insight how, for instance, a woodpecker has become adapted
to its peculiar habits of Life. The work, from its powerful and brilliant
style, though displaying in the earlier editions little accurate knowledge
and a great want of scientific caution, immediately had a very wide
circulation. In my opinion it has done excellent service in this country in
calling attention to the subject, in removing prejudice, and in thus
preparing the ground for the reception of analogous views.

In 1846 the veteran geologist N. J. d'Omalius d'Halloy published in an
excellent though short paper ("Bulletins de l'Acad. Roy Bruxelles,' tom.
xiii. p. 581) his opinion that it is more probable that new species have
been produced by descent with modification than that they have been
separately created: the author first promulgated this opinion in 1831.

Professor Owen, in 1849 ('Nature of Limbs,' p. 86), wrote as follows:- "The
archetypal idea was manifested in the flesh under diverse such
modifications, upon this planet, long prior to the existence of those animal
species that actually exemplify it. To what natural laws or secondary causes
the orderly succession and progression of such organic phenomena may have
been committed, we, as yet, are ignorant.' In his Address to the British
Association, in 1858, he speaks (p. li.) of "the axiom of the continuous
operation of creative power, or of the ordained becoming of living things.'
Farther on (p. xc.), after referring to geographical distribution, he adds,
'These phenomena shake our confidence in the conclusion that the Apteryx of
New Zealand and the Red Grouse of England were distinct creations in and for
those islands respectively. Always, also, it may be well to bear in mind
that by the word ''creation'' the zoologist means '"a process he knows not
what.'' He amplifies this idea by adding that when such cases as that of the
Red Grouse are enumerated by the zoologists as evidence of distinct creation
of the bird in and for such islands, he chiefly expresses that he knows not
how the Red Grouse came to be there, and there exclusively; signifying also,
by this mode of expressing such ignorance, his belief that both the bird and
the islands owed their origin to a great first Creative Cause.' If we
interpret these sentences given in the same Address, one by the other, it
appears that this eminent philosopher felt in 1858 his confidence shaken
that the Apteryx and the Red Grouse first appeared in their respective
homes, 'he knew not how,' or by some process 'he knew not what.'

This Address was delivered after the papers by Mr Wallace and myself on the
Origin of Species, presently to be referred to, had been read before the
Linnean Society. When the first edition of this work was published, I was so
completely deceived, as were many others, by such expressions as 'the
continuous operation of creative power,' that I included Professor Owen with
other palaeontologists as being firmly convinced of the immutability of
species; but it appears ('Anat. of Vertebrates,' vol. iii. p. 796) that this
was on my part a preposterous error. In the last edition of this work I
inferred, and the inference still seems to me perfectly just, from a passage
beginning with the words 'no doubt the type-form,' &c. (Ibid. vol. i. p.
xxxv.), that Professor Owen admitted that natural selection may have done
something in the formation of a new species; but this it appears (Ibid. vol.
nl. p. 798) is inaccurate and without evidence. I also gave some extracts
from a correspondence between Professor Owen and the Editor of the 'London
Review,' from which it appeared manifest to the Editor as well as to myself,
that Professor Owen claimed to have promulgated the theory of natural
selection before I had done so; and I expressed my surprise and satisfaction
at this announcement; but as far as it is possible to understand certain
recently published passages (Ibid. vol. iii. p. 798) I have either partially
or wholly again fallen into error. It is consolatory to me that others find
Professor Owen's controversial writings as difficult to understand and to
reconcile with each other, as I do. As far as the mere enunciation of the
principle of natural selection is concerned, it is quite immaterial whether
or not Professor Owen preceded me, for both of us, as shown in this
historical sketch, were long ago preceded by Dr Wells and Mr Matthews.

M. Isidore Geoffroy Saint-Hilaire, in his lectures delivered in 1850 (of
which a Résumé appeared in the 'Revue et Nag. de Zoolog.,' Jan. 1851),
briefly gives his reason for believing that specific characters "sont fixés,
pour chaque espèce, tant qu'elle se perpétue au milieu des mèmes
circonstances: ils se modifient, si les circonstances ambiantes viennent à
changer.' 'En résumé, l'observation des animaux sauvages démontre déjà la
variabilité limité des espèces. Les expériences sur les animaux sauvages
devenus domestiques, et sur les animaux domestiques redevenus sauvages, la
démontrent plus clairement encore. Ces memes expériences prouvent, de plus,
que les différences produites peuvent etre de valeur générique.' In his
'Hist. Nat. Généralé (tom. ii. p. 430, 1859) he amplifies analogous
conclusions.

From a circular lately issued it appears that Dr Freke, in 1851 ("Dublin
Medical Press,' p. 322), propounded the doctrine that all organic beings
have descended from one primordial form. His grounds of belief and treatment
of the subject are wholly different from mine; but as Dr Freke has now
(1861) published his Essay on the 'Origin of Species by means of Organic
Affinity,' the difficult attempt to give any idea of his views would be
superfluous on my part.

Mr Herbert Spencer, in an Essay (originally published in the 'Leader,'
March, 1852, and republished in his 'Essays,' in 1858), has contrasted the
theories of the Creation and the Development of organic beings with
remarkable skill and force. He argues from the analogy of domestic
productions, from the changes which the embryos of many species undergo,
from the difficulty of distinguishing species and varieties, and from the
principle of general gradation, that species have been modified; and he
attributes the modification to the change of circumstances. The author
(1855) has also treated psychology on the principle of the necessary
acquirement of each mental power and capacity by gradation.

In 1852 M. Naudin, a distinguished botanist, expressly stated, in an
admirable paper on the Origin of Species ('Revue Horticole, p. 102; since
partly republished in the 'Nouvelles Archives du Muséum,' tom. i. p. 171),
his belief that species are formed in an analogous manner as varieties are
under cultivation; and the latter process he attributes to man's power of
selection. But he does not show how selection acts under nature. He
believes, like Dean Herbert, that species, when nascent, were more plastic
than at present. He lays weight on what he calls the principle of finality,
'puissance mystérieuse, indéterminée; fatalité pour les uns; pour les autres
volonté providentielle, dont l'action incessante sur les ètres vivants
détermine, à toutes les époques de l'existence du monde, la forme, le
volume, et la durée de chacun d'eux, en raison de sa destinée dans l'ordre
de choses dont il fait partie. C'est cette puissance qui harmonise chaque
membre à l'ensemble, en l'appropriant à la fonction qu'il doit remplir dans
l'organisme général de la nature, fonction qui est pour lui sa raison
d'ètre.'*

* From references in Bronn's 'Untersuchungen über die
Entwickenlungs-Gesetze,' it appears that the celebrated botanist and
palaeontologist Unger published, in 1852, his belief that species undergo
development and modification. Dalton, likewise, in Pander and Dalton's work
on Fossil Sloths, expressed, in 1821 a similar belief. Similar views have,
as is well known, been maintained by Oken in his mystical
'Natur-philosophie.' From other references in Godron's work 'Sur l'Espéce,'
it seems that Bory St Vincent, Burdach, Poiret, and Fries, have all admitted
that new species are continually being produced.

In 1853 a celebrated geologist, Count Keyserling ("Bulletin de la Soc.
Gèolog.,' 2nd Ser., tom. x. p. 357), suggested that as new diseases,
supposed to have been caused by some miasma, have arisen and spread over the
world, so at certain periods the germs of existing species may have been
chemically affected by circumambient molecules of a particular nature, and
thus have given rise to new forms.

In this same year, 1853, Dr Schaaffhausen published an excellent pamphlet
('Verhand. des Naturhist. Vereins der preuss. Rheinlands,' &c.), in which he
maintains the development of organic forms on the earth. He infers that many
species have kept true for long periods, whereas a few have become modified.
The distinction of species he explains by the destruction of intermediate
graduated forms. 'Thus living plants and animals are not separated from the
extinct by new creations, but are to be regarded as their descendants
through continued reproduction.'

I may add, that of the thirty-four authors named in this Historical Sketch,
who believe in the modification of species, or at least disbelieve in
separate acts of creation, twenty-seven have written on special branches of
natural history or geology.

A well-known French botanist, M. Lecoq, writes in 1854 ('Etudes sur
Géograph. Bot.,' tom. i. p. 250), 'On voit que nos recherches sur la fixité
ou la variation de l'espèce, nous conduisent directement aux idées émises,
par deux hommes justement célèbres, Geoffroy Saint-Hilaire et Goethe.' Some
other passages scattered through M. Lecoq's large work, make it a little
doubtful how far he extends his views on the modification of species.

The 'Philosophy of Creation' has been treated in a masterly manner by the
Rev. Baden Powell, in his "Essays on the Unity of Worlds,' 1855. Nothing can
be more striking than the manner in which he shows that the introduction of
new species is "a regular, not a casual phenomenon,' or, as Sir John
Herschel expresses it, 'a natural in contradistinction to a miraculous,
process.'

The third volume of the "Journal of the Linnean Society' contains papers,
read July 1st, 1858, by Mr Wallace and myself, in which, as stated in the
introductory remarks to this volume, the theory of Natural Selection is
promulgated by Mr Wallace with admirable force and clearness.

Von Baer, towards whom all zoologists feel so profound a respect, expressed
about the year 1859 (see Prof. Rudolph Wagner, a
"Zoologisch-Anthropologische Untersuchungen,' 1861, s. 51) his conviction,
chiefly grounded on the laws of geographical distribution, that forms now
perfectly distinct have descended from a single parent-form.

In June, 1859, Professor Huxley gave a lecture before the Royal Institution
on the 'Persistent Types of Animal Life.' Referring to such cases, he
remarks, "It is difficult to comprehend the meaning of such facts as these,
if we suppose that each species of animal and plant, or each great type of
organisation, was formed and placed upon the surface of the globe at long
intervals by a distinct act of creative power; and it is well to recollect
that such an assumption is as unsupported by tradition or revelation as it
is opposed to the general analogy of nature. If, on the other hand, we view
'Persistent Types' in relation to that hypothesis which supposes the species
living at any time to be the result of the gradual modification of
pre-existing species a hypothesis which, though unproven, and sadly damaged
by some of its supporters, is yet the only one to which physiology lends any
countenance; their existence would seem to show that the amount of
modification which living beings have undergone during geological time is
but very small in relation to the whole series of changes which they have
suffered.'

In December, 1859, Dr Hooker published his 'Introduction to the Australian
Flora.' In the first part of this great work he admits the truth of the
descent and modification of species, and supports this doctrine by many
original observations.

The first edition of this work was published on November 24th, 1859, and the
second edition on January 7th, 1860.

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Introduction

WHEN on board H.M.S. Beagle, as naturalist, I was much struck with certain
facts in the distribution of the inhabitants of South America, and in the
geological relations of the present to the past inhabitants of that
continent. These facts seemed to me to throw some light on the origin of
species --- that mystery of mysteries, as it has been called by one of
our greatest philosophers. On my return home, it occurred to me, in 1837,
that something might perhaps be made out on this question by patiently
accumulating and reflecting on all sorts of facts which could possibly have
any bearing on it. After five years' work I allowed myself to speculate on
the subject, and drew up some short notes; these I enlarged in 1844 into a
sketch of the conclusions, which then seemed to me probable: from that
period to the present day I have steadily pursued the same object. I hope
that I may be excused for entering on these personal details, as I give them
to show that I have not been hasty in coming to a decision.

My work is now nearly finished; but as it will take me two or three more
years to complete it, and as my health is far from strong, I have been urged
to publish this Abstract. I have more especially been induced to do this, as
Mr Wallace, who is now studying the natural history of the Malay
archipelago, has arrived at almost exactly the same general conclusions that
I have on the origin of species. Last year he sent to me a memoir on this
subject, with a request that I would forward it to Sir Charles Lyell, who
sent it to the Linnean Society, and it is published in the third volume of
the journal of that Society. Sir C. Lyell and Dr Hooker, who both knew of my
work --- the latter having read my sketch of 1844 --- honoured me by
thinking it advisable to publish, with Mr Wallace's excellent memoir, some
brief extracts from my manuscripts.

This Abstract, which I now publish, must necessarily be imperfect. I cannot
here give references and authorities for my several statements; and I must
trust to the reader reposing some confidence in my accuracy. No doubt errors
will have crept in, though I hope I have always been cautious in trusting to
good authorities alone. I can here give only the general conclusions at
which I have arrived, with a few facts in illustration, but which, I hope,
in most cases will suffice. No one can feel more sensible than I do of the
necessity of hereafter publishing in detail all the facts, with references,
on which my conclusions have been grounded; and I hope in a future work to
do this. For I am well aware that scarcely a single point is discussed in
this volume on which facts cannot be adduced, often apparently leading to
conclusions directly opposite to those at which I have arrived. A fair
result can be obtained only by fully stating and balancing the facts and
arguments on both sides of each question; and this cannot possibly be here
done.

I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very many
naturalists, some of them personally unknown to me. I cannot, however, let
this opportunity pass without expressing my deep obligations to Dr Hooker,
who for the last fifteen years has aided me in every possible way by his
large stores of knowledge and his excellent judgement.

In considering the Origin of Species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on their
embryological relations, their geographical distribution, geological
succession, and other such facts, might come to the conclusion that each
species had not been independently created, but had descended, like
varieties, from other species. Nevertheless, such a conclusion, even if well
founded, would be unsatisfactory, until it could be shown how the
innumerable species inhabiting this world have been modified so as to
acquire that perfection of structure and co-adaptation which most justly
excites our admiration. Naturalists continually refer to external
conditions, such as climate, food, &c., as the only possible cause of
variation. In one very limited sense, as we shall hereafter see, this may be
true; but it is preposterous to attribute to mere external conditions, the
structure, for instance, of the woodpecker, with its feet, tail, beak, and
tongue, so admirably adapted to catch insects under the bark of trees. In
the case of the misseltoe, which draws its nourishment from certain trees,
which has seeds that must be transported by certain birds, and which has
flowers with separate sexes absolutely requiring the agency of certain
insects to bring pollen from one flower to the other, it is equally
preposterous to account for the structure of this parasite, with its
relations to several distinct organic beings, by the effects of external
conditions, or of habit, or of the volition of the plant itself.

The author of the 'Vestiges of Creation' would, I presume, say that, after a
certain unknown number of generations, some bird had given birth to a
woodpecker, and some plant to the misseltoe, and that these had been
produced perfect as we now see them; but this assumption seems to me to be
no explanation, for it leaves the case of the coadaptations of organic
beings to each other and to their physical conditions of life, untouched and
unexplained.

It is, therefore, of the highest importance to gain a clear insight into the
means of modification and coadaptation. At the commencement of my
observations it seemed to me probable that a careful study of domesticated
animals and of cultivated plants would offer the best chance of making out
this obscure problem. Nor have I been disappointed; in this and in all other
perplexing cases I have invariably found that our knowledge, imperfect
though it be, of variation under domestication, afforded the best and safest
clue. I may venture to express my conviction of the high value of such
studies, although they have been very commonly neglected by naturalists.

From these considerations, I shall devote the first chapter of this Abstract
to Variation under Domestication. We shall thus see that a large amount of
hereditary modification is at least possible, and, what is equally or more
important, we shall see how great is the power of man in accumulating by his
Selection successive slight variations. I will then pass on to the
variability of species in a state of nature; but I shall, unfortunately, be
compelled to treat this subject far too briefly, as it can be treated
properly only by giving long catalogues of facts. We shall, however, be
enabled to discuss what circumstances are most favourable to variation. In
the next chapter the Struggle for Existence amongst all organic beings
throughout the world, which inevitably follows from their high geometrical
powers of increase, will be treated of. This is the doctrine of Malthus,
applied to the whole animal and vegetable kingdoms. As many more individuals
of each species are born than can possibly survive; and as, consequently,
there is a frequently recurring struggle for existence, it follows that any
being, if it vary however slightly in any manner profitable to itself, under
the complex and sometimes varying conditions of life, will have a better
chance of surviving, and thus be naturally selected. From the strong
principle of inheritance, any selected variety will tend to propagate its
new and modified form.

This fundamental subject of Natural Selection will be treated at some length
in the fourth chapter; and we shall then see how Natural Selection almost
inevitably causes much Extinction of the less improved forms of life and
induces what I have called Divergence of Character. In the next chapter I
shall discuss the complex and little known laws of variation and of
correlation of growth. In the four succeeding chapters, the most apparent
and gravest difficulties on the theory will be given: namely, first, the
difficulties of transitions, or understanding how a simple being or a simple
organ can be changed and perfected into a highly developed being or
elaborately constructed organ; secondly the subject of Instinct, or the
mental powers of animals, thirdly, Hybridism, or the infertility of species
and the fertility of varieties when intercrossed; and fourthly, the
imperfection of the Geological Record. In the next chapter I shall consider
the geological succession of organic beings throughout time; in the eleventh
and twelfth, their geographical distribution throughout space; in the
thirteenth, their classification or mutual affinities, both when mature and
in an embryonic condition. In the last chapter I shall give a brief
recapitulation of the whole work, and a few concluding remarks.)

No one ought to feel surprise at much remaining as yet unexplained in regard
to the origin of species and varieties, if he makes due allowance for our
profound ignorance in regard to the mutual relations of all the beings which
live around us. Who can explain why one species ranges widely and is very
numerous, and why another allied species has a narrow range and is rare? Yet
these relations are of the highest importance, for they determine the
present welfare, and, as I believe, the future success and modification of
every inhabitant of this world. Still less do we know of the mutual
relations of the innumerable inhabitants of the world during the many past
geological epochs in its history. Although much remains obscure, and will
long remain obscure, I can entertain no doubt, after the most deliberate
study and dispassionate judgement of which I am capable, that the view which
most naturalists entertain, and which I formerly entertained --- namely,
that each species has been independently created --- is erroneous. I am
fully convinced that species are not immutable; but that those belonging to
what are called the same genera are lineal descendants of some other and
generally extinct species, in the same manner as the acknowledged varieties
of any one species are the descendants of that species. Furthermore, I am
convinced that Natural Selection has been the main but not exclusive means
of modification.

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Chapter 1 - Variation Under Domestication

[* ] Causes of Variability [* ] Effects of Habit [* ] Correlation of Growth
[* ] Inheritance [* ] Character of Domestic Varieties [* ] Difficulty of
distinguishing between Varieties and Species [* ] Origin of Domestic
Varieties from one or more Species [* ] Domestic pigeons, their Differences
and Origin [* ] Principle of Selection anciently followed, its Effects [* ]
Methodical and Unconscious Selection [* ] Unknown Origin of our Domestic
Productions [* ] Circumstances favourable to Man's power of Selection
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WHEN we look to the individuals of the same variety or sub-variety of our
older cultivated plants and animals, one of the first points which strikes
us, is, that they generally differ much more from each other, than do the
individuals of any one species or variety in a state of nature. When we
reflect on the vast diversity of the plants and animals which have been
cultivated, and which have varied during all ages under the most different
climates and treatment, I think we are driven to conclude that this greater
variability is simply due to our domestic productions having been raised
under conditions of life not so uniform as, and somewhat different from,
those to which the parent-species have been exposed under nature. There is,
also, I think, some probability in the view propounded by Andrew Knight,
that this variability may be partly connected with excess of food. It seems
pretty clear that organic beings must be exposed during several generations
to the new conditions of life to cause any appreciable amount of variation;
and that when the organisation has once begun to vary, it generally
continues to vary for many generations. No case is on record of a variable
being ceasing to be variable under cultivation. Our oldest cultivated
plants, such as wheat, still often yield new varieties: our oldest
domesticated animals are still capable of rapid improvement or modification.

It has been disputed at what period of time the causes of variability,
whatever they may be, generally act; whether during the early or late period
of development of the embryo, or at the instant of conception. Geoffroy St
Hilaire's experiments show that unnatural treatment of the embryo causes
monstrosities; and monstrosities cannot be separated by any clear line of
distinction from mere variations. But I am strongly inclined to suspect that
the most frequent cause of variability may be attributed to the male and
female reproductive elements having been affected prior to the act of
conception. Several reasons make me believe in this; but the chief one is
the remarkable effect which confinement or cultivation has on the functions
of the reproductive system; this system appearing to be far more susceptible
than any other part of the organization, to the action of any change in the
conditions of life. Nothing is more easy than to tame an animal, and few
things more difficult than to get it to breed freely under confinement, even
in the many cases when the male and female unite. How many animals there are
which will not breed, though living long under not very close confinement in
their native country! This is generally attributed to vitiated instincts;
but how many cultivated plants display the utmost vigour, and yet rarely or
never seed! In some few such cases it has been found out that very trifling
changes, such as a little more or less water at some particular period of
growth, will determine whether or not the plant sets a seed. I cannot here
enter on the copious details which I have collected on this curious subject;
but to show how singular the laws are which determine the reproduction of
animals under confinement, I may just mention that carnivorous animals, even
from the tropics, breed in this country pretty freely under confinement,
with the exception of the plantigrades or bear family; whereas, carnivorous
birds, with the rarest exceptions, hardly ever lay fertile eggs. Many exotic
plants have pollen utterly worthless, in the same exact condition as in the
most sterile hybrids. When, on the one hand, we see domesticated animals and
plants, though often weak and sickly, yet breeding quite freely under
confinement; and when, on the other hand, we see individuals, though taken
young from a state of nature, perfectly tamed, long-lived, and healthy (of
which I could give numerous instances), yet having their reproductive system
so seriously affected by unperceived causes as to fail in acting, we need
not be surprised at this system, when it does act under confinement, acting
not quite regularly, and producing offspring not perfectly like their
parents or variable.

Sterility has been said to be the bane of horticulture; but on this view we
owe variability to the same cause which produces sterility; and variability
is the source of all the choicest productions of the garden. I may add, that
as some organisms will breed most freely under the most unnatural conditions
(for instance, the rabbit and ferret kept in hutches), showing that their
reproductive system has not been thus affected; so will some animals and
plants withstand domestication or cultivation, and vary very slightly
perhaps hardly more than in a state of nature.

A long list could easily be given of 'sporting plants;' by this term
gardeners mean a single bud or offset, which suddenly assumes a new and
sometimes very different character from that of the rest of the plant. Such
buds can be propagated by grafting, &c., and sometimes by seed. These
'sports' are extremely rare under nature, but far from rare under
cultivation; and in this case we see that the treatment of the parent has
affected a bud or offset, and not the ovules or pollen. But it is the
opinion of most physiologists that there is no essential difference between
a bud and an ovule in their earliest stages of formation; so that, in
fact,'sports' support my view, that variability may be largely attributed to
the ovules or pollen, or to both, having been affected by the treatment of
the parent prior to the act of conception. These cases anyhow show that
variation is not necessarily connected, as some authors have supposed, with
the act of generation.

Seedlings from the same fruit, and the young of the same litter, sometimes
differ considerably from each other, though both the young and the parents,
as Muller has remarked, have apparently been exposed to exactly the same
conditions of life; and this shows how unimportant the direct effects of the
conditions of life are in comparison with the laws of reproduction, and of
growth, and of inheritance; for had the action of the conditions been
direct, if any of the young had varied, all would probably have varied in
the same manner. To judge how much, in the case of any variation, we should
attribute to the direct action of heat, moisture, light, food, &c., is most
difficult: my impression is, that with animals such agencies have produced
very little direct effect, though apparently more in the case of plants.
Under this point of view, Mr Buckman's recent experiments on plants seem
extremely valuable. When all or nearly all the individuals exposed to
certain conditions are affected in the same way, the change at first appears
to be directly due to such conditions; but in some cases it can be shown
that quite opposite conditions produce similar changes of structure.
Nevertheless some slight amount of change may, I think, be attributed to the
direct action of the conditions of life as, in some cases, increased size
from amount of food, colour from particular kinds of food and from light,
and perhaps the thickness of fur from climate.

Habit also has a deciding influence, as in the period of flowering with
plants when transported from one climate to another. In animals it has a
more marked effect; for instance, I find in the domestic duck that the bones
of the wing weigh less and the bones of the leg more, in proportion to the
whole skeleton, than do the same bones in the wild-duck; and I presume that
this change may be safely attributed to the domestic duck flying much less,
and walking more, than its wild parent. The great and inherited development
of the udders in cows and goats in countries where they are habitually
milked, in comparison with the state of these organs in other countries, is
another instance of the effect of use. Not a single domestic animal can be
named which has not in some country drooping ears; and the view suggested by
some authors, that the drooping is due to the disuse of the muscles of the
ear, from the animals not being much alarmed by danger, seems probable.

There are many laws regulating variation, some few of which can be dimly
seen, and will be hereafter briefly mentioned. I will here only allude to
what may be called correlation of growth. Any change in the embryo or larva
will almost certainly entail changes in the mature animal. In monstrosities,
the correlations between quite distinct parts are very curious; and many
instances are given in Isidore Geoffroy St Hilaire's great work on this
subject. Breeders believe that long limbs are almost always accompanied by
an elongated head. Some instances of correlation are quite whimsical; thus
cats with blue eyes are invariably deaf; colour and constitutional
peculiarities go together, of which many remarkable cases could be given
amongst animals and plants. From the facts collected by Heusinger, it
appears that white sheep and pigs are differently affected from coloured
individuals by certain vegetable poisons. Hairless dogs have imperfect
teeth; long-haired and coarse-haired animals are apt to have, as is
asserted, long or many horns; pigeons with feathered feet have skin between
their outer toes; pigeons with short beaks have small feet, and those with
long beaks large feet. Hence, if man goes on selecting, and thus augmenting,
any peculiarity, he will almost certainly unconsciously modify other parts
of the structure, owing to the mysterious laws of the correlation of growth.

The result of the various, quite unknown, or dimly seen laws of variation is
infinitely complex and diversified. It is well worth while carefully to
study the several treatises published on some of our old cultivated plants,
as on the hyacinth, potato, even the dahlia, &c.; and it is really
surprising to note the endless points in structure and constitution in which
the varieties and sub varieties differ slightly from each other. The whole
organization seems to have become plastic, and tends to depart in some small
degree from that of the parental type.

Any variation which is not inherited is unimportant for us. But the number
and diversity of inheritable deviations of structure, both those of slight
and those of considerable physiological importance, is endless. Dr Prosper
Lucas's treatise, in two large volumes, is the fullest and the best on this
subject. No breeder doubts how strong is the tendency to inheritance: like
produces like is his fundamental belief: doubts have been thrown on this
principle by theoretical writers alone. When a deviation appears not
unfrequently, and we see it in the father and child, we cannot tell whether
it may not be due to the same original cause acting on both; but when
amongst individuals, apparently exposed to the same conditions, any very
rare deviation, due to some extraordinary combination of circumstances,
appears in the parent say, once amongst several million individuals and it
reappears in the child, the mere doctrine of chances almost compels us to
attribute its reappearance to inheritance. Every one must have heard of
cases of albinism, prickly skin, hairy bodies, &c. appearing in several
members of the same family. If strange and rare deviations of structure are
truly inherited, less strange and commoner deviations may be freely admitted
to be inheritable. Perhaps the correct way of viewing the whole subject,
would be, to look at the inheritance of every character whatever as the
rule, and non-inheritance as the anomaly.

The laws governing inheritance are quite unknown; no one can say why the
same peculiarity in different individuals of the same species, and in
individuals of different species, is sometimes inherited and sometimes not
so; why the child often reverts in certain characters to its grandfather or
grandmother or other much more remote ancestor; why a peculiarity is often
transmitted from one sex to both sexes or to one sex alone, more commonly
but not exclusively to the like sex. It is a fact of some little importance
to us, that peculiarities appearing in the males of our domestic breeds are
often transmitted either exclusively, or in a much greater degree, to males
alone. A much more important rule, which I think may be trusted, is that, at
whatever period of life a peculiarity first appears, it tends to appear in
the offspring at a corresponding age, though sometimes earlier. In many
cases this could not be otherwise: thus the inherited peculiarities in the
horns of cattle could appear only in the offspring when nearly mature;
peculiarities in the silkworm are known to appear at the corresponding
caterpillar or cocoon stage. But hereditary diseases and some other facts
make me believe that the rule has a wider extension, and that when there is
no apparent reason why a peculiarity should appear at any particular age,
yet that it does tend to appear in the offspring at the same period at which
it first appeared in the parent. I believe this rule to be of the highest
importance in explaining the laws of embryology. These remarks are of course
confined to the first appearance of the peculiarity, and not to its primary
cause, which may have acted on the ovules or male element; in nearly the
same manner as in the crossed offspring from a short-horned cow by a
long-horned bull, the greater length of horn, though appearing late in life,
is clearly due to the male element.

Having alluded to the subject of reversion, I may here refer to a statement
often made by naturalists namely, that our domestic varieties, when run
wild, gradually but certainly revert in character to their aboriginal
stocks. Hence it has been argued that no deductions can be drawn from
domestic races to species in a state of nature. I have in vain endeavoured
to discover on what decisive facts the above statement has so often and so
boldly been made. There would be great difficulty in proving its truth: we
may safely conclude that very many of the most strongly-marked domestic
varieties could not possibly live in a wild state. In many cases we do not
know what the aboriginal stock was, and so could not tell whether or not
nearly perfect reversion had ensued. It would be quite necessary, in order
to prevent the effects of intercrossing, that only a single variety should
be turned loose in its new home. Nevertheless, as our varieties certainly do
occasionally revert in some of their characters to ancestral forms, it seems
to me not improbable, that if we could succeed in naturalising, or were to
cultivate, during many generations, the several races, for instance, of the
cabbage, in very poor soil (in which case, however, some effect would have
to be attributed to the direct action of the poor soil), that they would to
a large extent, or even wholly, revert to the wild aboriginal stock. Whether
or not the experiment would succeed, is not of great importance for our line
of argument; for by the experiment itself the conditions of life are
changed. If it could be shown that our domestic varieties manifested a
strong tendency to reversion, that is, to lose their acquired characters,
whilst kept under unchanged conditions, and whilst kept in a considerable
body, so that free intercrossing might check, by blending together, any
slight deviations of structure, in such case, I grant that we could deduce
nothing from domestic varieties in regard to species. But there is not a
shadow of evidence in favour of this view: to assert that we could not breed
our cart and race-horses, long and short-horned cattle and poultry of
various breeds, and esculent vegetables, for an almost infinite number of
generations, would be opposed to all experience. I may add, that when under
nature the conditions of life do change, variations and reversions of
character probably do occur; but natural selection, as will hereafter be
explained, will determine how far the new characters thus arising shall be
preserved.

When we look to the hereditary varieties or races of our domestic animals
and plants, and compare them with species closely allied together, we
generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races of the same
species, also, often have a somewhat monstrous character; by which I mean,
that, although differing from each other, and from the other species of the
same genus, in several trifling respects, they often differ in an extreme
degree in some one part, both when compared one with another, and more
especially when compared with all the species in nature to which they are
nearest allied. With these exceptions (and with that of the perfect
fertility of varieties when crossed, a subject hereafter to be discussed),
domestic races of the same species differ from each other in the same manner
as, only in most cases in a lesser degree than, do closely-allied species of
the same genus in a state of nature. I think this must be admitted, when we
find that there are hardly any domestic races, either amongst animals or
plants, which have not been ranked by some competent judges as mere
varieties, and by other competent judges as the descendants of aboriginally
distinct species. If any marked distinction existed between domestic races
and species, this source of doubt could not so perpetually recur. It has
often been stated that domestic races do not differ from each other in
characters of generic value. I think it could be shown that this statement
is hardly correct; but naturalists differ most widely in determining what
characters are of generic value; all such valuations being at present
empirical. Moreover, on the view of the origin of genera which I shall
presently give, we have no right to expect often to meet with generic
differences in our domesticated productions.

When we attempt to estimate the amount of structural difference between the
domestic races of the same species, we are soon involved in doubt, from not
knowing whether they have descended from one or several parent-species. This
point, if could be cleared up, would be interesting; if, for instance, it
could be shown that the greyhound, bloodhound, terrier, spaniel, and
bull-dog, which we all know propagate their kind so truly, were the
offspring of any single species, then such facts would have great weight in
making us doubt about the immutability of the many very closely allied and
natural species for instance, of the many foxes inhabiting different
quarters of the world. I do not believe, as we shall presently see, that all
our dogs have descended from any one wild species; but, in the case of some
other domestic races, there is presumptive, or even strong, evidence in
favour of this view.

It has often been assumed that man has chosen for domestication animals and
plants having an extraordinary inherent tendency to vary, and likewise to
withstand diverse climates. I do not dispute that these capacities have
added largely to the value of most of our domesticated productions; but how
could a savage possibly know, when he first tamed an animal, whether it
would vary in succeeding generations, and whether it would endure other
climates? Has the little variability of the ass or guinea-fowl, or the small
power of endurance of warmth by the reindeer, or of cold by the common
camel, prevented their domestication? I cannot doubt that if other animals
and plants, equal in number to our domesticated productions, and belonging
to equally diverse classes and countries, were taken from a state of nature,
and could be made to breed for an equal number of generations under
domestication, they would vary on an average as largely as the parent
species of our existing domesticated productions have varied.

In the case of most of our anciently domesticated animals and plants, I do
not think it is possible to come to any definite conclusion, whether they
have descended from one or several species. The argument mainly relied on by
those who believe in the multiple origin of our domestic animals is, that we
find in the most ancient records, more especially on the monuments of Egypt,
much diversity in the breeds; and that some of the breeds closely resemble,
perhaps are identical with, those still existing. Even if this latter fact
were found more strictly and generally true than seems to me to be the case,
what does it show, but that some of our breeds originated there, four or
five thousand years ago? But Mr Horner's researches have rendered it in some
degree probable that man sufficiently civilized to have manufactured pottery
existed in the valley of the Nile thirteen or fourteen thousand years ago;
and who will pretend to say how long before these ancient periods, savages,
like those of Tierra del Fuego or Australia, who possess a semi-domestic
dog, may not have existed in Egypt?

The whole subject must, I think, remain vague; nevertheless, I may, without
here entering on any details, state that, from geographical and other
considerations, I think it highly probable that our domestic dogs have
descended from several wild species. In regard to sheep and goats I can form
no opinion. I should think, from facts communicated to me by Mr Blyth, on
the habits, voice, and constitution, &c., of the humped Indian cattle, that
these had descended from a different aboriginal stock from our European
cattle; and several competent judges believe that these latter have had more
than one wild parent. With respect to horses, from reasons which I cannot
give here, I am doubtfully inclined to believe, in opposition to several
authors, that all the races have descended from one wild stock. Mr Blyth,
whose opinion, from his large and varied stores of knowledge, I should value
more than that of almost any one, thinks that all the breeds of poultry have
proceeded from the common wild Indian fowl (Gallus bankiva). In regard to
ducks and rabbits, the breeds of which differ considerably from each other
in structure, I do not doubt that they all have descended from the common
wild duck and rabbit.

The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some authors.
They believe that every race which breeds true, let the distinctive
characters be ever so slight, has had its wild prototype. At this rate there
must have existed at least a score of species of wild cattle, as many sheep,
and several goats in Europe alone, and several even within Great Britain.
One author believes that there formerly existed in Great Britain eleven wild
species of sheep peculiar to it! When we bear in mind that Britain has now
hardly one peculiar mammal, and France but few distinct from those of
Germany and conversely, and so with Hungary, Spain, &c., but that each of
these kingdoms possesses several peculiar breeds of cattle, sheep, &c., we
must admit that many domestic breeds have originated in Europe; for whence
could they have been derived, as these several countries do not possess a
number of peculiar species as distinct parent-stocks? So it is in India.
Even in the case of the domestic dogs of the whole world, which I fully
admit have probably descended from several wild species, I cannot doubt that
there has been an immense amount of inherited variation. Who can believe
that animals closely resembling the Italian greyhound, the bloodhound, the
bull-dog, or Blenheim spaniel, &c. so unlike all wild Canidae ever existed
freely in a state of nature? It has often been loosely said that all our
races of dogs have been produced by the crossing of a few aboriginal
species; but by crossing we can get only forms in some degree intermediate
between their parents; and if we account for our several domestic races by
this process, we must admit the former existence of the most extreme forms,
as the Italian greyhound, bloodhound, bull-dog, &c., in the wild state.
Moreover, the possibility of making distinct races by crossing has been
greatly exaggerated. There can be no doubt that a race may be modified by
occasional crosses, if aided by the careful selection of those individual
mongrels, which present any desired character; but that a race could be
obtained nearly intermediate between two extremely different races or
species, I can hardly believe. Sir J. Sebright expressly experimentised for
this object, and failed. The offspring from the first cross between two pure
breeds is tolerably and sometimes (as I have found with pigeons) extremely
uniform, and everything seems simple enough; but when these mongrels are
crossed one with another for several generations, hardly two of them will be
alike, and then the extreme difficulty, or rather utter hopelessness, of the
task becomes apparent. Certainly, a breed intermediate between two very
distinct breeds could not be got without extreme care and long-continued
selection; nor can I find a single case on record of a permanent race having
been thus formed.

On the Breeds of the Domestic pigeon.

Believing that it is always best to study some special group, I have, after
deliberation, taken up domestic pigeons. I have kept every breed which I
could purchase or obtain, and have been most kindly favoured with skins from
several quarters of the world, more especially by the Hon. W. Elliot from
India, and by the Hon. C. Murray from Persia. Many treatises in different
languages have been published on pigeons, and some of them are very
important, as being of considerably antiquity. I have associated with
several eminent fanciers, and have been permitted to join two of the London
Pigeon Clubs. The diversity of the breeds is something astonishing. Compare
the English carrier and the short-faced tumbler, and see the wonderful
difference in their beaks, entailing corresponding differences in their
skulls. The carrier, more especially the male bird, is also remarkable from
the wonderful development of the carunculated skin about the head, and this
is accompanied by greatly elongated eyelids, very large external orifices to
the nostrils, and a wide gape of mouth. The short-faced tumbler has a beak
in outline almost like that of a finch; and the common tumbler has the
singular and strictly inherited habit of flying at a great height in a
compact flock, and tumbling in the air head over heels. The runt is a bird
of great size, with long, massive beak and large feet; some of the
sub-breeds of runts have very long necks, others very long wings and tails,
others singularly short tails. The barb is allied to the carrier, but,
instead of a very long beak, has a very short and very broad one. The pouter
has a much elongated body, wings, and legs; and its enormously developed
crop, which it glories in inflating, may well excite astonishment and even
laughter. The turbit has a very short and conical beak, with a line of
reversed feathers down the breast; and it has the habit of continually
expanding slightly the upper part of the oesophagus. The Jacobin has the
feathers so much reversed along the back of the neck that they form a hood,
and it has, proportionally to its size, much elongated wing and tail
feathers. The trumpeter and laugher, as their names express, utter a very
different coo from the other breeds. The fantail has thirty or even forty
tail-feathers, instead of twelve or fourteen, the normal number in all
members of the great pigeon family; and these feathers are kept expanded,
and are carried so erect that in good birds the head and tail touch; the
oil-gland is quite aborted. Several other less distinct breeds might have
been specified.

In the skeletons of the several breeds, the development of the bones of the
face in length and breadth and curvature differs enormously. The shape, as
well as the breadth and length of the ramus of the lower jaw, varies in a
highly remarkable manner. The number of the caudal and sacral vertebrae
vary; as does the number of the ribs, together with their relative breadth
and the presence of processes. The size and shape of the apertures in the
sternum are highly variable; so is the degree of divergence and relative
size of the two arms of the furcula. The proportional width of the gape of
mouth, the proportional length of the eyelids, of the orifice of the
nostrils, of the tongue (not always in strict correlation with the length of
beak), the size of the crop and of the upper part of the oesophagus; the
development and abortion of the oil-gland; the number of the primary wing
and caudal feathers; the relative length of wing and tail to each other and
to the body; the relative length of leg and of the feet; the number of
scutellae on the toes, the development of skin between the toes, are all
points of structure which are variable. The period at which the perfect
plumage is acquired varies, as does the state of the down with which the
nestling birds are clothed when hatched. The shape and size of the eggs
vary. The manner of flight differs remarkably; as does in some breeds the
voice and disposition. Lastly, in certain breeds, the males and females have
come to differ to a slight degree from each other.

Altogether at least a score of pigeons might be chosen, which if shown to an
ornithologist, and he were told that they were wild birds, would certainly,
I think, be ranked by him as well-defined species. Moreover, I do not
believe that any ornithologist would place the English carrier, the
short-faced tumbler, the runt, the barb, pouter, and fantail in the same
genus; more especially as in each of these breeds several truly-inherited
sub-breeds, or species as he might have called them, could be shown him.

Great as the differences are between the breeds of pigeons, I am fully
convinced that the common opinion of naturalists is correct, namely, that
all have descended from the rock-pigeon (Columba livia), including under
this term several geographical races or sub-species, which differ from each
other in the most trifling respects. As several of the reasons which have
led me to this belief are in some degree applicable in other cases, I will
here briefly give them. If the several breeds are not varieties, and have
not proceeded from the rock-pigeon, they must have descended from at least
seven or eight aboriginal stocks; for it is impossible to make the present
domestic breeds by the crossing of any lesser number: how, for instance,
could a pouter be produced by crossing two breeds unless one of the
parent-stocks possessed the characteristic enormous crop? The supposed
aboriginal stocks must all have been rock-pigeons, that is, not breeding or
willingly perching on trees. But besides C. livia, with its geographical
sub-species, only two or three other species of rock-pigeons are known; and
these have not any of the characters of the domestic breeds. Hence the
supposed aboriginal stocks must either still exist in the countries where
they were originally domesticated, and yet be unknown to ornithologists; and
this, considering their size, habits, and remarkable characters, seems very
improbable; or they must have become extinct in the wild state. But birds
breeding on precipices, and good fliers, are unlikely to be exterminated;
and the common rock-pigeon, which has the same habits with the domestic
breeds, has not been exterminated even on several of the smaller British
islets, or on the shores of the Mediterranean. Hence the supposed
extermination of so many species having similar habits with the rock-pigeon
seems to me a very rash assumption. Moreover, the several above-named
domesticated breeds have been transported to all parts of the world, and,
therefore, some of them must have been carried back again into their native
country; but not one has ever become wild or feral, though the
dovecot-pigeon, which is the rock-pigeon in a very slightly altered state,
has become feral in several places. Again, all recent experience shows that
it is most difficult to get any wild animal to breed freely under
domestication; yet on the hypothesis of the multiple origin of our pigeons,
it must be assumed that at least seven or eight species were so thoroughly
domesticated in ancient times by half-civilized man, as to be quite prolific
under confinement.

An argument, as it seems to me, of great weight, and applicable in several
other cases, is, that the above-specified breeds, though agreeing generally
in constitution, habits, voice, colouring, and in most parts of their
structure, with the wild rock-pigeon, yet are certainly highly abnormal in
other parts of their structure: we may look in vain throughout the whole
great family of Columbidae for a beak like that of the English carrier, or
that of the short-faced tumbler, or barb; for reversed feathers like those
of the jacobin; for a crop like that of the pouter; for tail-feathers like
those of the fantail. Hence it must be assumed not only that half-civilized
man succeeded in thoroughly domesticating several species, but that he
intentionally or by chance picked out extraordinarily abnormal species; and
further, that these very species have since all become extinct or unknown.
So many strange contingencies seem to me improbable in the highest degree.

Some facts in regard to the colouring of pigeons well deserve consideration.
The rock-pigeon is of a slaty-blue, and has a white rump (the Indian
sub-species, C. intermedia of Strickland, having it bluish); the tail has a
terminal dark bar, with the bases of the outer feathers externally edged
with white; the wings have two black bars: some semi-domestic breeds and
some apparently truly wild breeds have, besides the two black bars, the
wings chequered with black. These several marks do not occur together in any
other species of the whole family. Now, in every one of the domestic breeds,
taking thoroughly well-bred birds, all the above marks, even to the white
edging of the outer tail-feathers, sometimes concur perfectly developed.
Moreover, when two birds belonging to two distinct breeds are crossed,
neither of which is blue or has any of the above-specified marks, the
mongrel offspring are very apt suddenly to acquire these characters; for
instance, I crossed some uniformly white fantails with some uniformly black
barbs, and they produced mottled brown and black birds; these I again
crossed together, and one grandchild of the pure white fantail and pure
black barb was of as beautiful a blue colour, with the white rump, double
black wing-bar, and barred and white-edged tail-feathers, as any wild
rock-pigeon! We can understand these facts, on the well-known principle of
reversion to ancestral characters, if all the domestic breeds have descended
from the rock-pigeon. But if we deny this, we must make one of the two
following highly improbable suppositions. Either, firstly, that all the
several imagined aboriginal stocks were coloured and marked like the
rock-pigeon, although no other existing species is thus coloured and marked,
so that in each separate breed there might be a tendency to revert to the
very same colours and markings. Or, secondly, that each breed, even the
purest, has within a dozen or, at most, within a score of generations, been
crossed by the rock-pigeon: I say within a dozen or twenty generations, for
we know of no fact countenancing the belief that the child ever reverts to
some one ancestor, removed by a greater number of generations. In a breed
which has been crossed only once with some distinct breed, the tendency to
reversion to any character derived from such cross will naturally become
less and less, as in each succeeding generation there will be less of the
foreign blood; but when there has been no cross with a distinct breed, and
there is a tendency in both parents to revert to a character, which has been
lost during some former generation, this tendency, for all that we can see
to the contrary, may be transmitted undiminished for an indefinite number of
generations. These two distinct cases are often confounded in treatises on
inheritance.

Lastly, the hybrids or mongrels from between all the domestic breeds of
pigeons are perfectly fertile. I can state this from my own observations,
purposely made on the most distinct breeds. Now, it is difficult, perhaps
impossible, to bring forward one case of the hybrid offspring of two animals
clearly distinct being themselves perfectly fertile. Some authors believe
that long-continued domestication eliminates this strong tendency to
sterility: from the history of the dog I think there is some probability in
this hypothesis, if applied to species closely related together, though it
is unsupported by a single experiment. But to extend the hypothesis so far
as to suppose that species, aboriginally as distinct as carriers, tumblers,
pouters, and fantails now are, should yield offspring perfectly fertile,
inter se, seems to me rash in the extreme.

From these several reasons, namely, the improbability of man having formerly
got seven or eight supposed species of pigeons to breed freely under
domestication; these supposed species being quite unknown in a wild state,
and their becoming nowhere feral; these species having very abnormal
characters in certain respects, as compared with all other Columbidae,
though so like in most other respects to the rock-pigeon; the blue colour
and various marks occasionally appearing in all the breeds, both when kept
pure and when crossed; the mongrel offspring being perfectly fertile; from
these several reasons, taken together, I can feel no doubt that all our
domestic breeds have descended from the Columba livia with its geographical
sub-species.

In favour of this view, I may add, firstly, that C. livia, or the
rock-pigeon, has been found capable of domestication in Europe and in India;
and that it agrees in habits and in a great number of points of structure
with all the domestic breeds. Secondly, although an English carrier or
short-faced tumbler differs immensely in certain characters from the
rock-pigeon, yet by comparing the several sub-breeds of these breeds, more
especially those brought from distant countries, we can make an almost
perfect series between the extremes of structure. Thirdly, those characters
which are mainly distinctive of each breed, for instance the wattle and
length of beak of the carrier, the shortness of that of the tumbler, and the
number of tail-feathers in the fantail, are in each breed eminently
variable; and the explanation of this fact will be obvious when we come to
treat of selection. Fourthly, pigeons have been watched, and tended with the
utmost care, and loved by many people. They have been domesticated for
thousands of years in several quarters of the world; the earliest known
record of pigeons is in the fifth Aegyptian dynasty, about 3000 B.C., as was
pointed out to me by Professor Lepsius; but Mr Birch informs me that pigeons
are given in a bill of fare in the previous dynasty. In the time of the
Romans, as we hear from Pliny, immense prices were given for pigeons; 'nay,
they are come to this pass, that they can reckon up their pedigree and
race.' Pigeons were much valued by Akber Khan in India, about the year 1600;
never less than 20,000 pigeons were taken with the court. 'The monarchs of
Iran and Turan sent him some very rare birds;' and, continues the courtly
historian, 'His Majesty by crossing the breeds, which method was never
practised before, has improved them astonishingly.' About this same period
the Dutch were as eager about pigeons as were the old Romans. The paramount
importance of these considerations in explaining the immense amount of
variation which pigeons have undergone, will be obvious when we treat of
Selection. We shall then, also, see how it is that the breeds so often have
a somewhat monstrous character. It is also a most favourable circumstance
for the production of distinct breeds, that male and female pigeons can be
easily mated for life; and thus different breeds can be kept together in the
same aviary.

I have discussed the probable origin of domestic pigeons at some, yet quite
insufficient, length; because when I first kept pigeons and watched the
several kinds, knowing well how true they bred, I felt fully as much
difficulty in believing that they could ever have descended from a common
parent, as any naturalist could in coming to a similar conclusion in regard
to the many species of finches, or other large groups of birds, in nature.
One circumstance has struck me much; namely, that all the breeders of the
various domestic animals and the cultivators of plants, with whom I have
ever conversed, or whose treatises I have read, are firmly convinced that
the several breeds to which each has attended, are descended from so many
aboriginally distinct species. Ask, as I have asked, a celebrated raiser of
Hereford cattle, whether his cattle might not have descended from long
horns, and he will laugh you to scorn. I have never met a pigeon, or
poultry, or duck, or rabbit fancier, who was not fully convinced that each
main breed was descended from a distinct species. Van Mons, in his treatise
on pears and apples, shows how utterly he disbelieves that the several
sorts, for instance a Ribston-pippin or Codlin-apple, could ever have
proceeded from the seeds of the same tree. Innumerable other examples could
be given. The explanation, I think, is simple: from long-continued study
they are strongly impressed with the differences between the several races;
and though they well know that each race varies slightly, for they win their
prizes by selecting such slight differences, yet they ignore all general
arguments, and refuse to sum up in their minds slight differences
accumulated during many successive generations. May not those naturalists
who, knowing far less of the laws of inheritance than does the breeder, and
knowing no more than he does of the intermediate links in the long lines of
descent, yet admit that many of our domestic races have descended from the
same parents may they not learn a lesson of caution, when they deride the
idea of species in a state of nature being lineal descendants of other
species?

Selection

Let us now briefly consider the steps by which domestic races have been
produced, either from one or from several allied species. Some little effect
may, perhaps, be attributed to the direct action of the external conditions
of life, and some little to habit; but he would be a bold man who would
account by such agencies for the differences of a dray and race horse, a
greyhound and bloodhound, a carrier and tumbler pigeon. One of the most
remarkable features in our domesticated races is that we see in them
adaptation, not indeed to the animal's or plant's own good, but to man's use
or fancy. Some variations useful to him have probably arisen suddenly, or by
one step; many botanists, for instance, believe that the fuller's teazle,
with its hooks, which cannot be rivalled by any mechanical contrivance, is
only a variety of the wild Dipsacus; and this amount of change may have
suddenly arisen in a seedling. So it has probably been with the turnspit
dog; and this is known to have been the case with the ancon sheep. But when
we compare the dray-horse and race-horse, the dromedary and camel, the
various breeds of sheep fitted either for cultivated land or mountain
pasture, with the wool of one breed good for one purpose, and that of
another breed for another purpose; when we compare the many breeds of dogs,
each good for man in very different ways; when we compare the gamecock, so
pertinacious in battle, with other breeds so little quarrelsome, with
'everlasting layers' which never desire to sit, and with the bantam so small
and elegant; when we compare the host of agricultural, culinary, orchard,
and flower-garden races of plants, most useful to man at different seasons
and for different purposes, or so beautiful in his eyes, we must, I think,
look further than to mere variability. We cannot suppose that all the breeds
were suddenly produced as perfect and as useful as we now see them; indeed,
in several cases, we know that this has not been their history. The key is
man's power of accumulative selection: nature gives successive variations;
man adds them up in certain directions useful to him. In this sense he may
be said to make for himself useful breeds.

The great power of this principle of selection is not hypothetical. It is
certain that several of our eminent breeders have, even within a single
lifetime, modified to a large extent some breeds of cattle and sheep. In
order fully to realise what they have done, it is almost necessary to read
several of the many treatises devoted to this subject, and to inspect the
animals. Breeders habitually speak of an animal's organisation as something
quite plastic, which they can model almost as they please. If I had space I
could quote numerous passages to this effect from highly competent
authorities. Youatt, who was probably better acquainted with the works of
agriculturalists than almost any other individual, and who was himself a
very good judge of an animal, speaks of the principle of selection as 'that
which enables the agriculturist, not only to modify the character of his
flock, but to change it altogether. It is the magician's wand, by means of
which he may summon into life whatever form and mould he pleases.' Lord
Somerville, speaking of what breeders have done for sheep, says: 'It would
seem as if they had chalked out upon a wall a form perfect in itself, and
then had given it existence.' That most skilful breeder, Sir John Sebright,
used to say, with respect to pigeons, that 'he would produce any given
feather in three years, but it would take him six years to obtain head and
beak.' In Saxony the importance of the principle of selection in regard to
merino sheep is so fully recognised, that men follow it as a trade: the
sheep are placed on a table and are studied, like a picture by a
connoisseur; this is done three times at intervals of months, and the sheep
are each time marked and classed, so that the very best may ultimately be
selected for breeding.

What English breeders have actually effected is proved by the enormous
prices given for animals with a good pedigree; and these have now been
exported to almost every quarter of the world. The improvement is by no
means generally due to crossing different breeds; all the best breeders are
strongly opposed to this practice, except sometimes amongst closely allied
sub-breeds. And when a cross has been made, the closest selection is far
more indispensable even than in ordinary cases. If selection consisted
merely in separating some very distinct variety, and breeding from it, the
principle would be so obvious as hardly to be worth notice; but its
importance consists in the great effect produced by the accumulation in one
direction, during successive generations, of differences absolutely
inappreciable by an uneducated eye differences which I for one have vainly
attempted to appreciate. Not one man in a thousand has accuracy of eye and
judgement sufficient to become an eminent breeder. If gifted with these
qualities, and he studies his subject for years, and devotes his lifetime to
it with indomitable perseverance, he will succeed, and may make great
improvements; if he wants any of these qualities, he will assuredly fail.
Few would readily believe in the natural capacity and years of practice
requisite to become even a skilful pigeon-fancier.

The same principles are followed by horticulturists; but the variations are
here often more abrupt. No one supposes that our choicest productions have
been produced by a single variation from the aboriginal stock. We have
proofs that this is not so in some cases, in which exact records have been
kept; thus, to give a very trifling instance, the steadily-increasing size
of the common gooseberry may be quoted. We see an astonishing improvement in
many florists' flowers, when the flowers of the present day are compared
with drawings made only twenty or thirty years ago. When a race of plants is
once pretty well established, the seed-raisers do not pick out the best
plants, but merely go over their seed-beds, and pull up the 'rogues,' as
they call the plants that deviate from the proper standard. With animals
this kind of selection is, in fact, also followed; for hardly any one is so
careless as to allow his worst animals to breed.

In regard to plants, there is another means of observing the accumulated
effects of selection namely, by comparing the diversity of flowers in the
different varieties of the same species in the flower-garden; the diversity
of leaves, pods, or tubers, or whatever part is valued, in the
kitchen-garden, in comparison with the flowers of the same varieties; and
the diversity of fruit of the same species in the orchard, in comparison
with the leaves and flowers of the same set of varieties. See how different
the leaves of the cabbage are, and how extremely alike the flowers; how
unlike the flowers of the heartsease are, and how alike the leaves; how much
the fruit of the different kinds of gooseberries differ in size, colour,
shape, and hairiness, and yet the flowers present very slight differences.
It is not that the varieties which differ largely in some one point do not
differ at all in other points; this is hardly ever, perhaps never, the case.
The laws of correlation of growth, the importance of which should never be
overlooked, will ensure some differences; but, as a general rule, I cannot
doubt that the continued selection of slight variations, either in the
leaves, the flowers, or the fruit, will produce races differing from each
other chiefly in these characters.

It may be objected that the principle of selection has been reduced to
methodical practice for scarcely more than three-quarters of a century; it
has certainly been more attended to of late years, and many treatises have
been published on the subject; and the result, I may add, has been, in a
corresponding degree, rapid and important. But it is very far from true that
the principle is a modern discovery. I could give several references to the
full acknowledgement of the importance of the principle in works of high
antiquity. In rude and barbarous periods of English history choice animals
were often imported, and laws were passed to prevent their exportation: the
destruction of horses under a certain size was ordered, and this may be
compared to the 'roguing' of plants by nurserymen. The principle of
selection I find distinctly given in an ancient Chinese encyclopaedia.
Explicit rules are laid down by some of the Roman classical writers. From
passages in Genesis, it is clear that the colour of domestic animals was at
that early period attended to. Savages now sometimes cross their dogs with
wild canine animals, to improve the breed, and they formerly did so, as is
attested by passages in Pliny. The savages in South Africa match their
draught cattle by colour, as do some of the Esquimaux their teams of dogs.
Livingstone shows how much good domestic breeds are valued by the negroes of
the interior of Africa who have not associated with Europeans. Some of these
facts do not show actual selection, but they show that the breeding of
domestic animals was carefully attended to in ancient times, and is now
attended to by the lowest savages. It would, indeed, have been a strange
fact, had attention not been paid to breeding, for the inheritance of good
and bad qualities is so obvious.

At the present time, eminent breeders try by methodical selection, with a
distinct object in view, to make a new strain or sub-breed, superior to
anything existing in the country. But, for our purpose, a kind of Selection,
which may be called Unconscious, and which results from every one trying to
possess and breed from the best individual animals, is more important. Thus,
a man who intends keeping pointers naturally tries to get as good dogs as he
can, and afterwards breeds from his own best dogs, but he has no wish or
expectation of permanently altering the breed. Nevertheless I cannot doubt
that this process, continued during centuries, would improve and modify any
breed, in the same way as Bakewell, Collins, &c., by this very same process,
only carried on more methodically, did greatly modify, even during their own
lifetimes, the forms and qualities of their cattle. Slow and insensible
changes of this kind could never be recognised unless actual measurements or
careful drawings of the breeds in question had been made long ago, which
might serve for comparison. In some cases, however, unchanged or but little
changed individuals of the same breed may be found in less civilised
districts, where the breed has been less improved. There is reason to
believe that King Charles's spaniel has been unconsciously modified to a
large extent since the time of that monarch. Some highly competent
authorities are convinced that the setter is directly derived from the
spaniel, and has probably been slowly altered from it. It is known that the
English pointer has been greatly changed within the last century, and in
this case the change has, it is believed, been chiefly effected by crosses
with the fox-hound; but what concerns us is, that the change has been
effected unconsciously and gradually, and yet so effectually, that, though
the old Spanish pointer certainly came from Spain, Mr Barrow has not seen,
as I am informed by him, any native dog in Spain like our pointer.

By a similar process of selection, and by careful training, the whole body
of English racehorses have come to surpass in fleetness and size the parent
Arab stock, so that the latter, by the regulations for the Goodwood Races,
are favoured in the weights they carry. Lord Spencer and others have shown
how the cattle of England have increased in weight and in early maturity,
compared with the stock formerly kept in this country. By comparing the
accounts given in old pigeon treatises of carriers and tumblers with these
breeds as now existing in Britain, India, and Persia, we can, I think,
clearly trace the stages through which they have insensibly passed, and come
to differ so greatly from the rock-pigeon.

Youatt gives an excellent illustration of the effects of a course of
selection, which may be considered as unconsciously followed, in so far that
the breeders could never have expected or even have wished to have produced
the result which ensued namely, the production of two distinct strains. The
two flocks of Leicester sheep kept by Mr Buckley and Mr Burgess, as Mr
Youatt remarks, 'have been purely bred from the original stock of Mr
Bakewell for upwards of fifty years. There is not a suspicion existing in
the mind of any one at all acquainted with the subject that the owner of
either of them has deviated in any one instance from the pure blood of Mr
Bakewell's flock, and yet the difference between the sheep possessed by
these two gentlemen is so great that they have the appearance of being quite
different varieties.'

If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one animal
particularly useful to them, for any special purpose, would be carefully
preserved during famines and other accidents, to which savages are so
liable, and such choice animals would thus generally leave more offspring
than the inferior ones; so that in this case there would be a kind of
unconscious selection going on. We see the value set on animals even by the
barbarians of Tierra del Fuego, by their killing and devouring their old
women, in times of dearth, as of less value than their dogs.

In plants the same gradual process of improvement, through the occasional
preservation of the best individuals, whether or not sufficiently distinct
to be ranked at their first appearance as distinct varieties, and whether or
not two or more species or races have become blended together by crossing,
may plainly be recognised in the increased size and beauty which we now see
in the varieties of the heartsease, rose, pelargonium, dahlia, and other
plants, when compared with the older varieties or with their parent-stocks.
No one would ever expect to get a first-rate heartsease or dahlia from the
seed of a wild plant. No one would expect to raise a first-rate melting pear
from the seed of a wild pear, though he might succeed from a poor seedling
growing wild, if it had come from a garden-stock. The pear, though
cultivated in classical times, appears, from Pliny's description, to have
been a fruit of very inferior quality. I have seen great surprise expressed
in horticultural works at the wonderful skill of gardeners, in having
produced such splendid results from such poor materials; but the art, I
cannot doubt, has been simple, and, as far as the final result is concerned,
has been followed almost unconsciously. It has consisted in always
cultivating the best known variety, sowing its seeds, and, when a slightly
better variety has chanced to appear, selecting it, and so onwards. But the
gardeners of the classical period, who cultivated the best pear they could
procure, never thought what splendid fruit we should eat; though we owe our
excellent fruit, in some small degree, to their having naturally chosen and
preserved the best varieties they could anywhere find.

A large amount of change in our cultivated plants, thus slowly and
unconsciously accumulated, explains, as I believe, the well-known fact, that
in a vast number of cases we cannot recognise, and therefore do not know,
the wild parent-stocks of the plants which have been longest cultivated in
our flower and kitchen gardens. If it has taken centuries or thousands of
years to improve or modify most of our plants up to their present standard
of usefulness to man, we can understand how it is that neither Australia,
the Cape of Good Hope, nor any other region inhabited by quite uncivilised
man, has afforded us a single plant worth culture. It is not that these
countries, so rich in species, do not by a strange chance possess the
aboriginal stocks of any useful plants, but that the native plants have not
been improved by continued selection up to a standard of perfection
comparable with that given to the plants in countries anciently civilised.

In regard to the domestic animals kept by uncivilised man, it should not be
overlooked that they almost always have to struggle for their own food, at
least during certain seasons. And in two countries very differently
circumstanced, individuals of the same species, having slightly different
constitutions or structure, would often succeed better in the one country
than in the other, and thus by a process of 'natural selection,' as will
hereafter be more fully explained, two sub-breeds might be formed. This,
perhaps, partly explains what has been remarked by some authors, namely,
that the varieties kept by savages have more of the character of species
than the varieties kept in civilised countries.

On the view here given of the all-important part which selection by man has
played, it becomes at once obvious, how it is that our domestic races show
adaptation in their structure or in their habits to man's wants or fancies.
We can, I think, further understand the frequently abnormal character of our
domestic races, and likewise their differences being so great in external
characters and relatively so slight in internal parts or organs. Man can
hardly select, or only with much difficulty, any deviation of structure
excepting such as is externally visible; and indeed he rarely cares for what
is internal. He can never act by selection, excepting on variations which
are first given to him in some slight degree by nature. No man would ever
try to make a fantail, till he saw a pigeon with a tail developed in some
slight degree in an unusual manner, or a pouter till he saw a pigeon with a
crop of somewhat unusual size; and the more abnormal or unusual any
character was when it first appeared, the more likely it would be to catch
his attention. But to use such an expression as trying to make a fantail,
is, I have no doubt, in most cases, utterly incorrect. The man who first
selected a pigeon with a slightly larger tail, never dreamed what the
descendants of that pigeon would become through long-continued, partly
unconscious and partly methodical selection. Perhaps the parent bird of all
fantails had only fourteen tail-feathers somewhat expanded, like the present
Java fantail, or like individuals of other and distinct breeds, in which as
many as seventeen tail-feathers have been counted. Perhaps the first
pouter-pigeon did not inflate its crop much more than the turbit now does
the upper part of its oesophagus, a habit which is disregarded by all
fanciers, as it is not one of the points of the breed.

Nor let it be thought that some great deviation of structure would be
necessary to catch the fancier's eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however slight,
in one's own possession. Nor must the value which would formerly be set on
any slight differences in the individuals of the same species, be judged of
by the value which would now be set on them, after several breeds have once
fairly been established. Many slight differences might, and indeed do now,
arise amongst pigeons, which are rejected as faults or deviations from the
standard of perfection of each breed. The common goose has not given rise to
any marked varieties; hence the Thoulouse and the common breed, which differ
only in colour, that most fleeting of characters, have lately been exhibited
as distinct at our poultry-shows.

I think these views further explain what has sometimes been noticed namely
that we know nothing about the origin or history of any of our domestic
breeds. But, in fact, a breed, like a dialect of a language, can hardly be
said to have had a definite origin. A man preserves and breeds from an
individual with some slight deviation of structure, or takes more care than
usual in matching his best animals and thus improves them, and the improved
individuals slowly spread in the immediate neighbourhood. But as yet they
will hardly have a distinct name, and from being only slightly valued, their
history will be disregarded. When further improved by the same slow and
gradual process, they will spread more widely, and will get recognised as
something distinct and valuable, and will then probably first receive a
provincial name. In semi-civilised countries, with little free
communication, the spreading and knowledge of any new sub-breed will be a
slow process. As soon as the points of value of the new sub-breed are once
fully acknowledged, the principle, as I have called it, of unconscious
selection will always tend, perhaps more at one period than at another, as
the breed rises or falls in fashion, perhaps more in one district than in
another, according to the state of civilisation of the inhabitants slowly to
add to the characteristic features of the breed, whatever they may be. But
the chance will be infinitely small of any record having been preserved of
such slow, varying, and insensible changes.

I must now say a few words on the circumstances, favourable, or the reverse,
to man's power of selection. A high degree of variability is obviously
favourable, as freely giving the materials for selection to work on; not
that mere individual differences are not amply sufficient, with extreme
care, to allow of the accumulation of a large amount of modification in
almost any desired direction. But as variations manifestly useful or
pleasing to man appear only occasionally, the chance of their appearance
will be much increased by a large number of individuals being kept; and
hence this comes to be of the highest importance to success. On this
principle Marshall has remarked, with respect to the sheep of parts of
Yorkshire, that 'as they generally belong to poor people, and are mostly in
small lots, they never can be improved.' On the other hand, nurserymen, from
raising large stocks of the same plants, are generally far more successful
than amateurs in getting new and valuable varieties. The keeping of a large
number of individuals of a species in any country requires that the species
should be placed under favourable conditions of life, so as to breed freely
in that country. When the individuals of any species are scanty, all the
individuals, whatever their quality may be, will generally be allowed to
breed, and this will effectually prevent selection. But probably the most
important point of all, is, that the animal or plant should be so highly
useful to man, or so much valued by him, that the closest attention should
be paid to even the slightest deviation in the qualities or structure of
each individual. Unless such attention be paid nothing can be effected. I
have seen it gravely remarked, that it was most fortunate that the
strawberry began to vary just when gardeners began to attend closely to this
plant. No doubt the strawberry had always varied since it was cultivated,
but the slight varieties had been neglected. As soon, however, as gardeners
picked out individual plants with slightly larger, earlier, or better fruit,
and raised seedlings from them, and again picked out the best seedlings and
bred from them, then, there appeared (aided by some crossing with distinct
species) those many admirable varieties of the strawberry which have been
raised during the last thirty or forty years.

In the case of animals with separate sexes, facility in preventing crosses
is an important element of success in the formation of new races, at least,
in a country which is already stocked with other races. In this respect
enclosure of the land plays a part. Wandering savages or the inhabitants of
open plains rarely possess more than one breed of the same species. Pigeons
can be mated for life, and this is a great convenience to the fancier, for
thus many races may be kept true, though mingled in the same aviary; and
this circumstance must have largely favoured the improvement and formation
of new breeds. Pigeons, I may add, can be propagated in great numbers and at
a very quick rate, and inferior birds may be freely rejected, as when killed
they serve for food. On the other hand, cats, from their nocturnal rambling
habits, cannot be matched, and, although so much valued by women and
children, we hardly ever see a distinct breed kept up; such breeds as we do
sometimes see are almost always imported from some other country, often from
islands. Although I do not doubt that some domestic animals vary less than
others, yet the rarity or absence of distinct breeds of the cat, the donkey,
peacock, goose, &c., may be attributed in main part to selection not having
been brought into play: in cats, from the difficulty in pairing them; in
donkeys, from only a few being kept by poor people, and little attention
paid to their breeding; in peacocks, from not being very easily reared and a
large stock not kept; in geese, from being valuable only for two purposes,
food and feathers, and more especially from no pleasure having been felt in
the display of distinct breeds.

To sum up on the origin of our Domestic Races of animals and plants. I
believe that the conditions of life, from their action on the reproductive
system, are so far of the highest importance as causing variability. I do
not believe that variability is an inherent and necessary contingency, under
all circumstances, with all organic beings, as some authors have thought.
The effects of variability are modified by various degrees of inheritance
and of reversion. Variability is governed by many unknown laws, more
especially by that of correlation of growth. Something may be attributed to
the direct action of the conditions of life. Something must be attributed to
use and disuse. The final result is thus rendered infinitely complex. In
some cases, I do not doubt that the intercrossing of species, aboriginally
distinct, has played an important part in the origin of our domestic
productions. When in any country several domestic breeds have once been
established, their occasional intercrossing, with the aid of selection, has,
no doubt, largely aided in the formation of new sub-breeds; but the
importance of the crossing of varieties has, I believe, been greatly
exaggerated, both in regard to animals and to those plants which are
propagated by seed. In plants which are temporarily propagated by cuttings,
buds, &c., the importance of the crossing both of distinct species and of
varieties is immense; for the cultivator here quite disregards the extreme
variability both of hybrids and mongrels, and the frequent sterility of
hybrids; but the cases of plants not propagated by seed are of little
importance to us, for their endurance is only temporary. Over all these
causes of Change I am convinced that the accumulative action of Selection,
whether applied methodically and more quickly, or unconsciously and more
slowly, but more efficiently, is by far the predominant power.

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Chapter 2 - Variation Under Nature

[* ] Variability [* ] Individual differences [* ] Doubtful species [* ] Wide
ranging, much diffused, and common species vary most [* ] Species of the
larger genera in any country vary more than the species of the smaller
genera [* ] Many of the species of the larger genera resemble varieties in
being very closely, but unequally, related to each other, and in having
restricted ranges
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BEFORE applying the principles arrived at in the last chapter to organic
beings in a state of nature, we must briefly discuss whether these latter
are subject to any variation. To treat this subject at all properly, a long
catalogue of dry facts should be given; but these I shall reserve for my
future work. Nor shall I here discuss the various definitions which have
been given of the term species. No one definition has as yet satisfied all
naturalists; yet every naturalist knows vaguely what he means when he speaks
of a species. Generally the term includes the unknown element of a distinct
act of creation. The term 'variety' is almost equally difficult to define;
but here community of descent is almost universally implied, though it can
rarely be proved. We have also what are called monstrosities; but they
graduate into varieties. By a monstrosity I presume is meant some
considerable deviation of structure in one part, either injurious to or not
useful to the species, and not generally propagated. Some authors use the
term 'variation' in a technical sense, as implying a modification directly
due to the physical conditions of life; and 'variations' in this sense are
supposed not to be inherited: but who can say that the dwarfed condition of
shells in the brackish waters of the Baltic, or dwarfed plants on Alpine
summits, or the thicker fur of an animal from far northwards, would not in
some cases be inherited for at least some few generations? and in this case
I presume that the form would be called a variety.

Again, we have many slight differences which may be called individual
differences, such as are known frequently to appear in the offspring from
the same parents, or which may be presumed to have thus arisen, from being
frequently observed in the individuals of the same species inhabiting the
same confined locality. No one supposes that all the individuals of the same
species are cast in the very same mould. These individual differences are
highly important for us, as they afford materials for natural selection to
accumulate, in the same manner as man can accumulate in any given direction
individual differences in his domesticated productions. These individual
differences generally affect what naturalists consider unimportant parts;
but I could show by a long catalogue of facts, that parts which must be
called important, whether viewed under a physiological or classificatory
point of view, sometimes vary in the individuals of the same species. I am
convinced that the most experienced naturalist would be surprised at the
number of the cases of variability, even in important parts of structure,
which he could collect on good authority, as I have collected, during a
course of years. It should be remembered that systematists are far from
pleased at finding variability in important characters, and that there are
not many men who will laboriously examine internal and important organs, and
compare them in many specimens of the same species. I should never have
expected that the branching of the main nerves close to the great central
ganglion of an insect would have been variable in the same species; I should
have expected that changes of this nature could have been effected only by
slow degrees: yet quite recently Mr Lubbock has shown a degree of
variability in these main nerves in Coccus, which may almost be compared to
the irregular branching of the stem of a tree. This philosophical
naturalist, I may add, has also quite recently shown that the muscles in the
larvae of certain insects are very far from uniform. Authors sometimes argue
in a circle when they state that important organs never vary; for these same
authors practically rank that character as important (as some few
naturalists have honestly confessed) which does not vary; and, under this
point of view, no instance of any important part varying will ever be found:
but under any other point of view many instances assuredly can be given.

There is one point connected with individual differences, which seems to me
extremely perplexing: I refer to those genera which have sometimes been
called 'protean' or 'polymorphic,' in which the species present an
inordinate amount of variation; and hardly two naturalists can agree which
forms to rank as species and which as varieties. We may instance Rubus,
Rosa, and Hieracium amongst plants, several genera of insects, and several
genera of Brachiopod shells. In most polymorphic genera some of the species
have fixed and definite characters. Genera which are polymorphic in one
country seem to be, with some few exceptions, polymorphic in other
countries, and likewise, judging from Brachiopod shells, at former periods
of time. These facts seem to be very perplexing, for they seem to show that
this kind of variability is independent of the conditions of life. I am
inclined to suspect that we see in these polymorphic genera variations in
points of structure which are of no service or disservice to the species,
and which consequently have not been seized on and rendered definite by
natural selection, as hereafter will be explained.

Those forms which possess in some considerable degree the character of
species, but which are so closely similar to some other forms, or are so
closely linked to them by intermediate gradations, that naturalists do not
like to rank them as distinct species, are in several respects the most
important for us. We have every reason to believe that many of these
doubtful and closely-allied forms have permanently retained their characters
in their own country for a long time; for as long, as far as we know, as
have good and true species. practically, when a naturalist can unite two
forms together by others having intermediate characters, he treats the one
as a variety of the other, ranking the most common, but sometimes the one
first described, as the species, and the other as the variety. But cases of
great difficulty, which I will not here enumerate, sometimes occur in
deciding whether or not to rank one form as a variety of another, even when
they are closely connected by intermediate links; nor will the
commonly-assumed hybrid nature of the intermediate links always remove the
difficulty. In very many cases, however, one form is ranked as a variety of
another, not because the intermediate links have actually been found, but
because analogy leads the observer to suppose either that they do now
somewhere exist, or may formerly have existed; and here a wide door for the
entry of doubt and conjecture is opened.

Hence, in determining whether a form should be ranked as a species or a
variety, the opinion of naturalists having sound judgement and wide
experience seems the only guide to follow. We must, however, in many cases,
decide by a majority of naturalists, for few well-marked and well-known
varieties can be named which have not been ranked as species by at least
some competent judges.

That varieties of this doubtful nature are far from uncommon cannot be
disputed. Compare the several floras of Great Britain, of France or of the
United States, drawn up by different botanists, and see what a surprising
number of forms have been ranked by one botanist as good species, and by
another as mere varieties. Mr H. C. Watson, to whom I lie under deep
obligation for assistance of all kinds, has marked for me 182 British
plants, which are generally considered as varieties, but which have all been
ranked by botanists as species; and in making this list he has omitted many
trifling varieties, but which nevertheless have been ranked by some
botanists as species, and he has entirely omitted several highly polymorphic
genera. Under genera, including the most polymorphic forms, Mr Babington
gives 251 species, whereas Mr Bentham gives only 112, a difference of 139
doubtful forms! Amongst animals which unite for each birth, and which are
highly locomotive, doubtful forms, ranked by one zoologist as a species and
by another as a variety, can rarely be found within the same country, but
are common in separated areas. How many of those birds and insects in North
America and Europe, which differ very slightly from each other, have been
ranked by one eminent naturalist as undoubted species, and by another as
varieties, or, as they are often called, as geographical races! Many years
ago, when comparing, and seeing others compare, the birds from the separate
islands of the Galapagos Archipelago, both one with another, and with those
from the American mainland, I was much struck how entirely vague and
arbitrary is the distinction between species and varieties. On the islets of
the little Madeira group there are many insects which are characterized as
varieties in Mr Wollaston's admirable work, but which it cannot be doubted
would be ranked as distinct species by many entomologists. Even Ireland has
a few animals, now generally regarded as varieties, but which have been
ranked as species by some zoologists. Several most experienced
ornithologists consider our British red grouse as only a strongly-marked
race of a Norwegian species, whereas the greater number rank it as an
undoubted species peculiar to Great Britain. A wide distance between the
homes of two doubtful forms leads many naturalists to rank both as distinct
species; but what distance, it has been well asked, will suffice? if that
between America and Europe is ample, will that between the Continent and the
Azores, or Madeira, or the Canaries, or Ireland, be sufficient? It must be
admitted that many forms, considered by highly-competent judges as
varieties, have so perfectly the character of species that they are ranked
by other highly-competent judges as good and true species. But to discuss
whether they are rightly called species or varieties, before any definition
of these terms has been generally accepted, is vainly to beat the air.

Many of the cases of strongly-marked varieties or doubtful species well
deserve consideration; for several interesting lines of argument, from
geographical distribution, analogical variation, hybridism, &c., have been
brought to bear on the attempt to determine their rank. I will here give
only a single instance, the well-known one of the primrose and cowslip, or
Primula veris and elatior. These plants differ considerably in appearance;
they have a different flavour and emit a different odour; they flower at
slightly different periods; they grow in somewhat different stations; they
ascend mountains to different heights; they have different geographical
ranges; and lastly, according to very numerous experiments made during
several years by that most careful observer Gärtner, they can be crossed
only with much difficulty. We could hardly wish for better evidence of the
two forms being specifically distinct. On the other hand, they are united by
many intermediate links, and it is very doubtful whether these links are
hybrids; and there is, as it seems to me, an overwhelming amount of
experimental evidence, showing that they descend from common parents, and
consequently must be ranked as varieties.

Close investigation, in most cases, will bring naturalists to an agreement
how to rank doubtful forms. Yet it must be confessed, that it is in the
best-known countries that we find the greatest number of forms of doubtful
value. I have been struck with the fact, that if any animal or plant in a
state of nature be highly useful to man, or from any cause closely attract
his attention, varieties of it will almost universally be found recorded.
These varieties, moreover, will be often ranked by some authors as species.
Look at the common oak, how closely it has been studied; yet a German author
makes more than a dozen species out of forms, which are very generally
considered as varieties; and in this country the highest botanical
authorities and practical men can be quoted to show that the sessile and
pedunculated oaks are either good and distinct species or mere varieties.

When a young naturalist commences the study of a group of organisms quite
unknown to him, he is at first much perplexed to determine what differences
to consider as specific, and what as varieties; for he knows nothing of the
amount and kind of variation to which the group is subject; and this shows,
at least, how very generally there is some variation. But if he confine his
attention to one class within one country, he will soon make up his mind how
to rank most of the doubtful forms. His general tendency will be to make
many species, for he will become impressed, just like the pigeon or
poultry-fancier before alluded to, with the amount of difference in the
forms which he is continually studying; and he has little general knowledge
of analogical variation in other groups and in other countries, by which to
correct his first impressions. As he extends the range of his observations,
he will meet with more cases of difficulty; for he will encounter a greater
number of closely-allied forms. But if his observations be widely extended,
he will in the end generally be enabled to make up his own mind which to
call varieties and which species; but he will succeed in this at the expense
of admitting much variation, and the truth of this admission will often be
disputed by other naturalists. When, moreover, he comes to study allied
forms brought from countries not now continuous, in which case he can hardly
hope to find the intermediate links between his doubtful forms, he will have
to trust almost entirely to analogy, and his difficulties will rise to a
climax.

Certainly no clear line of demarcation has as yet been drawn between species
and sub-species that is, the forms which in the opinion of some naturalists
come very near to, but do not quite arrive at the rank of species; or,
again, between sub-species and well-marked varieties, or between lesser
varieties and individual differences. These differences blend into each
other in an insensible series; and a series impresses the mind with the idea
of an actual passage.

Hence I look at individual differences, though of small interest to the
systematist, as of high importance for us, as being the first step towards
such slight varieties as are barely thought worth recording in works on
natural history. And I look at varieties which are in any degree more
distinct and permanent, as steps leading to more strongly marked and more
permanent varieties; and at these latter, as leading to sub-species, and to
species. The passage from one stage of difference to another and higher
stage may be, in some cases, due merely to the long-continued action of
different physical conditions in two different regions; but I have not much
faith in this view; and I attribute the passage of a variety, from a state
in which it differs very slightly from its parent to one in which it differs
more, to the action of natural selection in accumulating (as will hereafter
be more fully explained) differences of structure in certain definite
directions. Hence I believe a well-marked variety may be justly called an
incipient species; but whether this belief be justifiable must be judged of
by the general weight of the several facts and views given throughout this
work.

It need not be supposed that all varieties or incipient species necessarily
attain the rank of species. They may whilst in this incipient state become
extinct, or they may endure as varieties for very long periods, as has been
shown to be the case by Mr Wollaston with the varieties of certain fossil
land-shells in Madeira. If a variety were to flourish so as to exceed in
numbers the parent species, it would then rank as the species, and the
species as the variety; or it might come to supplant and exterminate the
parent species; or both might co-exist, and both rank as independent
species. But we shall hereafter have to return to this subject.

From these remarks it will be seen that I look at the term species, as one
arbitrarily given for the sake of convenience to a set of individuals
closely resembling each other, and that it does not essentially differ from
the term variety, which is given to less distinct and more fluctuating
forms. The term variety, again, in comparison with mere individual
differences, is also applied arbitrarily, and for mere convenience sake.

Guided by theoretical considerations, I thought that some interesting
results might be obtained in regard to the nature and relations of the
species which vary most, by tabulating all the varieties in several
well-worked floras. At first this seemed a simple task; but Mr H. C. Watson,
to whom I am much indebted for valuable advice and assistance on this
subject, soon convinced me that there were many difficulties, as did
subsequently Dr Hooker, even in stronger terms. I shall reserve for my
future work the discussion of these difficulties, and the tables themselves
of the proportional numbers of the varying species. Dr Hooker permits me to
add, that after having carefully read my manuscript, and examined the
tables, he thinks that the following statements are fairly well established.
The whole subject, however, treated as it necessarily here is with much
brevity, is rather perplexing, and allusions cannot be avoided to the
'struggle for existence,' 'divergence of character,' and other questions,
hereafter to be discussed.

Alph. De Candolle and others have shown that plants which have very wide
ranges generally present varieties; and this might have been expected, as
they become exposed to diverse physical conditions, and as they come into
competition (which, as we shall hereafter see, is a far more important
circumstance) with different sets of organic beings. But my tables further
show that, in any limited country, the species which are most common, that
is abound most in individuals, and the species which are most widely
diffused within their own country (and this is a different consideration
from wide range, and to a certain extent from commonness), often give rise
to varieties sufficiently well-marked to have been recorded in botanical
works. Hence it is the most flourishing, or, as they may be called, the
dominant species, those which range widely over the world, are the most
diffused in their own country, and are the most numerous in individuals,
which oftenest produce well-marked varieties, or, as I consider them,
incipient species. And this, perhaps, might have been anticipated; for, as
varieties, in order to become in any degree permanent, necessarily have to
struggle with the other inhabitants of the country, the species which are
already dominant will be the most likely to yield offspring which, though in
some slight degree modified, will still inherit those advantages that
enabled their parents to become dominant over their compatriots.

If the plants inhabiting a country and described in any Flora be divided
into two equal masses, all those in the larger genera being placed on one
side, and all those in the smaller genera on the other side, a somewhat
larger number of the very common and much diffused or dominant species will
be found on the side of the larger genera. This, again, might have been
anticipated; for the mere fact of many species of the same genus inhabiting
any country, shows that there is something in the organic or inorganic
conditions of that country favourable to the genus; and, consequently, we
might have expected to have found in the larger genera, or those including
many species, a large proportional number of dominant species. But so many
causes tend to obscure this result, that I am surprised that my tables show
even a small majority on the side of the larger genera. I will here allude
to only two causes of obscurity. Fresh-water and salt-loving plants have
generally very wide ranges and are much diffused, but this seems to be
connected with the nature of the stations inhabited by them, and has little
or no relation to the size of the genera to which the species belong. Again,
plants low in the scale of organisation are generally much more widely
diffused than plants higher in the scale; and here again there is no close
relation to the size of the genera. The cause of lowly-organised plants
ranging widely will be discussed in our chapter on geographical
distribution.

From looking at species as only strongly-marked and well-defined varieties,
I was led to anticipate that the species of the larger genera in each
country would oftener present varieties, than the species of the smaller
genera; for wherever many closely related species (i.e. species of the same
genus) have been formed, many varieties or incipient species ought, as a
general rule, to be now forming. Where many large trees grow, we expect to
find saplings. Where many species of a genus have been formed through
variation, circumstances have been favourable for variation; and hence we
might expect that the circumstances would generally be still favourable to
variation. On the other hand, if we look at each species as a special act of
creation, there is no apparent reason why more varieties should occur in a
group having many species, than in one having few.

To test the truth of this anticipation I have arranged the plants of twelve
countries, and the coleopterous insects of two districts, into two nearly
equal masses, the species of the larger genera on one side, and those of the
smaller genera on the other side, and it has invariably proved to be the
case that a larger proportion of the species on the side of the larger
genera present varieties, than on the side of the smaller genera. Moreover,
the species of the large genera which present any varieties, invariably
present a larger average number of varieties than do the species of the
small genera. Both these results follow when another division is made, and
when all the smallest genera, with from only one to four species, are
absolutely excluded from the tables. These facts are of plain signification
on the view that species are only strongly marked and permanent varieties;
for whenever many species of the same genus have been formed, or where, if
we may use the expression, the manufactory of species has been active, we
ought generally to find the manufactory still in action, more especially as
we have every reason to believe the process of manufacturing new species to
be a slow one. And this certainly is the case, if varieties be looked at as
incipient species; for my tables clearly show as a general rule that,
wherever many species of a genus have been formed, the species of that genus
present a number of varieties, that is of incipient species, beyond the
average. It is not that all large genera are now varying much, and are thus
increasing in the number of their species, or that no small genera are now
varying and increasing; for if this had been so, it would have been fatal to
my theory; inasmuch as geology plainly tells us that small genera have in
the lapse of time often increased greatly in size; and that large genera
have often come to their maxima, declined, and disappeared. All that we want
to show is, that where many species of a genus have been formed, on an
average many are still forming; and this holds good.

There are other relations between the species of large genera and their
recorded varieties which deserve notice. We have seen that there is no
infallible criterion by which to distinguish species and well-marked
varieties; and in those cases in which intermediate links have not been
found between doubtful forms, naturalists are compelled to come to a
determination by the amount of difference between them, judging by analogy
whether or not the amount suffices to raise one or both to the rank of
species. Hence the amount of difference is one very important criterion in
settling whether two forms should be ranked as species or varieties. Now
Fries has remarked in regard to plants, and Westwood in regard to insects,
that in large genera the amount of difference between the species is often
exceedingly small. I have endeavoured to test this numerically by averages,
and, as far as my imperfect results go, they always confirm the view. I have
also consulted some sagacious and most experienced observers, and, after
deliberation, they concur in this view. In this respect, therefore, the
species of the larger genera resemble varieties, more than do the species of
the smaller genera. Or the case may be put in another way, and it may be
said, that in the larger genera, in which a number of varieties or incipient
species greater than the average are now manufacturing, many of the species
already manufactured still to a certain extent resemble varieties, for they
differ from each other by a less than usual amount of difference.

Moreover, the species of the large genera are related to each other, in the
same manner as the varieties of any one species are related to each other.
No naturalist pretends that all the species of a genus are equally distinct
from each other; they may generally be divided into sub-genera, or sections,
or lesser groups. As Fries has well remarked, little groups of species are
generally clustered like satellites around certain other species. And what
are varieties but groups of forms, unequally related to each other, and
clustered round certain forms that is, round their parent-species?
Undoubtedly there is one most important point of difference between
varieties and species; namely, that the amount of difference between
varieties, when compared with each other or with their parent-species, is
much less than that between the species of the same genus. But when we come
to discuss the principle, as I call it, of Divergence of Character, we shall
see how this may be explained, and how the lesser differences between
varieties will tend to increase into the greater differences between
species.

There is one other point which seems to me worth notice. Varieties generally
have much restricted ranges: this statement is indeed scarcely more than a
truism, for if a variety were found to have a wider range than that of its
supposed parent-species, their denominations ought to be reversed. But there
is also reason to believe, that those species which are very closely allied
to other species, and in so far resemble varieties, often have much
restricted ranges. For instance, Mr H. C. Watson has marked for me in the
well-sifted London Catalogue of plants (4th edition) 63 plants which are
therein ranked as species, but which he considers as so closely allied to
other species as to be of doubtful value: these 63 reputed species range on
an average over 6.9 of the provinces into which Mr Watson has divided Great
Britain. Now, in this same catalogue, 53 acknowledged varieties are
recorded, and these range over 7.7 provinces; whereas, the species to which
these varieties belong range over 14.3 provinces. So that the acknowledged
varieties have very nearly the same restricted average range, as have those
very closely allied forms, marked for me by Mr Watson as doubtful species,
but which are almost universally ranked by British botanists as good and
true species.

Finally, then, varieties have the same general characters as species, for
they cannot be distinguished from species, except, firstly, by the discovery
of intermediate linking forms, and the occurrence of such links cannot
affect the actual characters of the forms which they connect; and except,
secondly, by a certain amount of difference, for two forms, if differing
very little, are generally ranked as varieties, notwithstanding that
intermediate linking forms have not been discovered; but the amount of
difference considered necessary to give to two forms the rank of species is
quite indefinite. In genera having more than the average number of species
in any country, the species of these genera have more than the average
number of varieties. In large genera the species are apt to be closely, but
unequally, allied together, forming little clusters round certain species.
Species very closely allied to other species apparently have restricted
ranges. In all these several respects the species of large genera present a
strong analogy with varieties. And we can clearly understand these
analogies, if species have once existed as varieties, and have thus
originated: whereas, these analogies are utterly inexplicable if each
species has been independently created.

We have, also, seen that it is the most flourishing and dominant species of
the larger genera which on an average vary most; and varieties, as we shall
hereafter see, tend to become converted into new and distinct species. The
larger genera thus tend to become larger; and throughout nature the forms of
life which are now dominant tend to become still more dominant by leaving
many modified and dominant descendants. But by steps hereafter to be
explained, the larger genera also tend to break up into smaller genera. And
thus, the forms of life throughout the universe become divided into groups
subordinate to groups.

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Chapter 3 - Struggle for Existence

[* ] Bears on natural selection [* ] The term used in a wide sense [* ]
Geometrical powers of increase [* ] Rapid increase of naturalised animals
and plants [* ] Nature of the checks to increase [* ] Competition universal
[* ] Effects of climate [* ] Protection from the number of individuals [* ]
Complex relations of all animals and plants throughout nature [* ] Struggle
for life most severe between individuals and varieties of the same species;
often severe between species of the same genus [* ] The relation of organism
to organism the most important of all relations
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BEF0RE entering on the subject of this chapter, I must make a few
preliminary remarks, to show how the struggle for existence bears on Natural
Selection. It has been seen in the last chapter that amongst organic beings
in a state of nature there is some individual variability; indeed I am not
aware that this has ever been disputed. It is immaterial for us whether a
multitude of doubtful forms be called species or sub-species or varieties;
what rank, for instance, the two or three hundred doubtful forms of British
plants are entitled to hold, if the existence of any well-marked varieties
be admitted. But the mere existence of individual variability and of some
few well-marked varieties, though necessary as the foundation for the work,
helps us but little in understanding how species arise in nature. How have
all those exquisite adaptations of one part of the organisation to another
part, and to the conditions of life, and of one distinct organic being to
another being, been perfected? We see these beautiful co-adaptations most
plainly in the woodpecker and missletoe; and only a little less plainly in
the humblest parasite which clings to the hairs of a quadruped or feathers
of a bird; in the structure of the beetle which dives through the water; in
the plumed seed which is wafted by the gentlest breeze; in short, we see
beautiful adaptations everywhere and in every part of the organic world.

Again, it may be asked, how is it that varieties, which I have called
incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more than
do the varieties of the same species? How do those groups of species, which
constitute what are called distinct genera, and which differ from each other
more than do the species of the same genus, arise? All these results, as we
shall more fully see in the next chapter, follow inevitably from the
struggle for life. Owing to this struggle for life, any variation, however
slight and from whatever cause proceeding, if it be in any degree profitable
to an individual of any species, in its infinitely complex relations to
other organic beings and to external nature, will tend to the preservation
of that individual, and will generally be inherited by its offspring. The
offspring, also, will thus have a better chance of surviving, for, of the
many individuals of any species which are periodically born, but a small
number can survive. I have called this principle, by which each slight
variation, if useful, is preserved, by the term of Natural Selection, in
order to mark its relation to man's power of selection. We have seen that
man by selection can certainly produce great results, and can adapt organic
beings to his own uses, through the accumulation of slight but useful
variations, given to him by the hand of Nature. But Natural Selection, as we
shall hereafter see, is a power incessantly ready for action, and is as
immeasurably superior to man's feeble efforts, as the works of Nature are to
those of Art.

We will now discuss in a little more detail the struggle for existence. In
my future work this subject shall be treated, as it well deserves, at much
greater length. The elder De Candolle and Lyell have largely and
philosophically shown that all organic beings are exposed to severe
competition. In regard to plants, no one has treated this subject with more
spirit and ability than W. Herbert, Dean of Manchester, evidently the result
of his great horticultural knowledge. Nothing is easier than to admit in
words the truth of the universal struggle for life, or more difficult at
least I have found it so than constantly to bear this conclusion in mind.
Yet unless it be thoroughly engrained in the mind, I am convinced that the
whole economy of nature, with every fact on distribution, rarity, abundance,
extinction, and variation, will be dimly seen or quite misunderstood. We
behold the face of nature bright with gladness, we often see superabundance
of food; we do not see, or we forget, that the birds which are idly singing
round us mostly live on insects or seeds, and are thus constantly destroying
life; or we forget how largely these songsters, or their eggs, or their
nestlings are destroyed by birds and beasts of prey; we do not always bear
in mind, that though food may be now superabundant, it is not so at all
seasons of each recurring year.

I should premise that I use the term Struggle for Existence in a large and
metaphorical sense, including dependence of one being on another, and
including (which is more important) not only the life of the individual, but
success in leaving progeny. Two canine animals in a time of dearth, may be
truly said to struggle with each other which shall get food and live. But a
plant on the edge of a desert is said to struggle for life against the
drought, though more properly it should be said to be dependent on the
moisture. A plant which annually produces a thousand seeds, of which on an
average only one comes to maturity, may be more truly said to struggle with
the plants of the same and other kinds which already clothe the ground. The
missletoe is dependent on the apple and a few other trees, but can only in a
far-fetched sense be said to struggle with these trees, for if too many of
these parasites grow on the same tree, it will languish and die. But several
seedling missletoes, growing close together on the same branch, may more
truly be said to struggle with each other. As the missletoe is disseminated
by birds, its existence depends on birds; and it may metaphorically be said
to struggle with other fruit-bearing plants, in order to tempt birds to
devour and thus disseminate its seeds rather than those of other plants. In
these several senses, which pass into each other, I use for convenience sake
the general term of struggle for existence.

A struggle for existence inevitably follows from the high rate at which all
organic beings tend to increase. Every being, which during its natural
lifetime produces several eggs or seeds, must suffer destruction during some
period of its life, and during some season or occasional year, otherwise, on
the principle of geometrical increase, its numbers would quickly become so
inordinately great that no country could support the product. Hence, as more
individuals are produced than can possibly survive, there must in every case
be a struggle for existence, either one individual with another of the same
species, or with the individuals of distinct species, or with the physical
conditions of life. It is the doctrine of Malthus applied with manifold
force to the whole animal and vegetable kingdoms; for in this case there can
be no artificial increase of food, and no prudential restraint from
marriage. Although some species may be now increasing, more or less rapidly,
in numbers, all cannot do so, for the world would not hold them.

There is no exception to the rule that every organic being naturally
increases at so high a rate, that if not destroyed, the earth would soon be
covered by the progeny of a single pair. Even slow-breeding man has doubled
in twenty-five years, and at this rate, in a few thousand years, there would
literally not be standing room for his progeny. Linnaeus has calculated that
if an annual plant produced only two seeds and there is no plant so
unproductive as this and their seedlings next year produced two, and so on,
then in twenty years there would be a million plants. The elephant is
reckoned to be the slowest breeder of all known animals, and I have taken
some pains to estimate its probable minimum rate of natural increase: it
will be under the mark to assume that it breeds when thirty years old, and
goes on breeding till ninety years old, bringing forth three pairs of young
in this interval; if this be so, at the end of the fifth century there would
be alive fifteen million elephants, descended from the first pair.

But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonishingly rapid
increase of various animals in a state of nature, when circumstances have
been favourable to them during two or three following seasons. Still more
striking is the evidence from our domestic animals of many kinds which have
run wild in several parts of the world: if the statements of the rate of
increase of slow-breeding cattle and horses in South America, and latterly
in Australia, had not been well authenticated, they would have been quite
incredible. So it is with plants: cases could be given of introduced plants
which have become common throughout whole islands in a period of less than
ten years, Several of the plants now most numerous over the wide plains of
La Plata, clothing square leagues of surface almost to the exclusion of all
other plants, have been introduced from Europe; and there are plants which
now range in India, as I hear from Dr Falconer, from Cape Comorin to the
Himalaya, which have been imported from America since its discovery. In such
cases, and endless instances could be given, no one supposes that the
fertility of these animals or plants has been suddenly and temporarily
increased in any sensible degree. The obvious explanation is that the
conditions of life have been very favourable, and that there has
consequently been less destruction of the old and young, and that nearly all
the young have been enabled to breed. In such cases the geometrical ratio of
increase, the result of which never fails to be surprising, simply explains
the extraordinarily rapid increase and wide diffusion of naturalised
productions in their new homes.

In a state of nature almost every plant produces seed, and amongst animals
there are very few which do not annually pair. Hence we may confidently
assert, that all plants and animals are tending to increase at a geometrical
ratio, that all would most rapidly stock every station in which they could
any how exist, and that the geometrical tendency to increase must be checked
by destruction at some period of life. Our familiarity with the larger
domestic animals tends, I think, to mislead us: we see no great destruction
falling on them, and we forget that thousands are annually slaughtered for
food, and that in a state of nature an equal number would have somehow to be
disposed of.

The only difference between organisms which annually produce eggs or seeds
by the thousand, and those which produce extremely few, is, that the
slow-breeders would require a few more years to people, under favourable
conditions, a whole district, let it be ever so large. The condor lays a
couple of eggs and the ostrich a score, and yet in the same country the
condor may be the more numerous of the two: the Fulmar petrel lays but one
egg, yet it is believed to be the most numerous bird in the world. One fly
deposits hundreds of eggs, and another, like the hippobosca, a single one;
but this difference does not determine how many individuals of the two
species can be supported in a district. A large number of eggs is of some
importance to those species, which depend on a rapidly fluctuating amount of
food, for it allows them rapidly to increase in number. But the real
importance of a large number of eggs or seeds is to make up for much
destruction at some period of life; and this period in the great majority of
cases is an early one. If an animal can in any way protect its own eggs or
young, a small number may be produced, and yet the average stock be fully
kept up; but if many eggs or young are destroyed, many must be produced, or
the species will become extinct. It would suffice to keep up the full number
of a tree, which lived on an average for a thousand years, if a single seed
were produced once in a thousand years, supposing that this seed were never
destroyed, and could be ensured to germinate in a fitting place. So that in
all cases, the average number of any animal or plant depends only indirectly
on the number of its eggs or seeds.

In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind never to forget that every single organic
being around us may be said to be striving to the utmost to increase in
numbers; that each lives by a struggle at some period of its life; that
heavy destruction inevitably falls either on the young or old, during each
generation or at recurrent intervals. Lighten any check, mitigate the
destruction ever so little, and the number of the species will almost
instantaneously increase to any amount. The face of Nature may be compared
to a yielding surface, with ten thousand sharp wedges packed close together
and driven inwards by incessant blows, sometimes one wedge being struck, and
then another with greater force.

What checks the natural tendency of each species to increase in number is
most obscure. Look at the most vigorous species; by as much as it swarms in
numbers, by so much will its tendency to increase be still further
increased. We know not exactly what the checks are in even one single
instance. Nor will this surprise any one who reflects how ignorant we are on
this head, even in regard to mankind, so incomparably better known than any
other animal. This subject has been ably treated by several authors, and I
shall, in my future work, discuss some of the checks at considerable length,
more especially in regard to the feral animals of South America. Here I will
make only a few remarks, just to recall to the reader's mind some of the
chief points. Eggs or very young animals seem generally to suffer most, but
this is not invariably the case. With plants there is a vast destruction of
seeds, but, from some observations which I have made, I believe that it is
the seedlings which suffer most from germinating in ground already thickly
stocked with other plants. Seedlings, also, are destroyed in vast numbers by
various enemies; for instance, on a piece of ground three feet long and two
wide, dug and cleared, and where there could be no choking from other
plants, I marked all the seedlings of our native weeds as they came up, and
out of the 357 no less than 295 were destroyed, chiefly by slugs and
insects. If turf which has long been mown, and the case would be the same
with turf closely browsed by quadrupeds, be let to grow, the more vigorous
plants gradually kill the less vigorous, though fully grown, plants: thus
out of twenty species growing on a little plot of turf (three feet by four)
nine species perished from the other species being allowed to grow up
freely.

The amount of food for each species of course gives the extreme limit to
which each can increase; but very frequently it is not the obtaining food,
but the serving as prey to other animals, which determines the average
numbers of a species. Thus, there seems to be little doubt that the stock of
partridges, grouse, and hares on any large estate depends chiefly on the
destruction of vermin. If not one head of game were shot during the next
twenty years in England, and, at the same time, if no vermin were destroyed,
there would, in all probability, be less game than at present, although
hundreds of thousands of game animals are now annually killed. On the other
hand, in some cases, as with the elephant and rhinoceros, none are destroyed
by beasts of prey: even the tiger in India most rarely dares to attack a
young elephant protected by its dam.

Climate plays an important part in determining the average numbers of a
species, and periodical seasons of extreme cold or drought, I believe to be
the most effective of all checks. I estimated that the winter of 1854-55
destroyed four-fifths of the birds in my own grounds; and this is a
tremendous destruction, when we remember that ten per cent. is an
extraordinarily severe mortality from epidemics with man. The action of
climate seems at first sight to be quite independent of the struggle for
existence; but in so far as climate chiefly acts in reducing food, it brings
on the most severe struggle between the individuals, whether of the same or
of distinct species, which subsist on the same kind of food. Even when
climate, for instance extreme cold, acts directly, it will be the least
vigorous, or those which have got least food through the advancing winter,
which will suffer most. When we travel from south to north, or from a damp
region to a dry, we invariably see some species gradually getting rarer and
rarer, and finally disappearing; and the change of climate being
conspicuous, we are tempted to attribute the whole effect to its direct
action. But this is a very false view: we forget that each species, even
where it most abounds, is constantly suffering enormous destruction at some
period of its life, from enemies or from competitors for the same place and
food; and if these enemies or competitors be in the least degree favoured by
any slight change of climate, they will increase in numbers, and, as each
area is already fully stocked with inhabitants, the other species will
decrease. When we travel southward and see a species decreasing in numbers,
we may feel sure that the cause lies quite as much in other species being
favoured, as in this one being hurt. So it is when we travel northward, but
in a somewhat lesser degree, for the number of species of all kinds, and
therefore of competitors, decreases northwards; hence in going northward, or
in ascending a mountain, we far oftener meet with stunted forms, due to the
directly injurious action of climate, than we do in proceeding southwards or
in descending a mountain. When we reach the Arctic regions, or snow-capped
summits, or absolute deserts, the struggle for life is almost exclusively
with the elements.

That climate acts in main part indirectly by favouring other species, we may
clearly see in the prodigious number of plants in our gardens which can
perfectly well endure our climate, but which never become naturalised, for
they cannot compete with our native plants, nor resist destruction by our
native animals.

When a species, owing to highly favourable circumstances, increases
inordinately in numbers in a small tract, epidemics at least, this seems
generally to occur with our game animals often ensue: and here we have a
limiting check independent of the struggle for life. But even some of these
so-called epidemics appear to be due to parasitic worms, which have from
some cause, possibly in part through facility of diffusion amongst the
crowded animals, been disproportionably favoured: and here comes in a sort
of struggle between the parasite and its prey.

On the other hand, in many cases, a large stock of individuals of the same
species, relatively to the numbers of its enemies, is absolutely necessary
for its preservation. Thus we can easily raise plenty of corn and rape-seed,
&c., in our fields, because the seeds are in great excess compared with the
number of birds which feed on them; nor can the birds, though having a
superabundance of food at this one season, increase in number proportionally
to the supply of seed, as their numbers are checked during winter: but any
one who has tried, knows how troublesome it is to get seed from a few wheat
or other such plants in a garden; I have in this case lost every single
seed. This view of the necessity of a large stock of the same species for
its preservation, explains, I believe, some singular facts in nature, such
as that of very rare plants being sometimes extremely abundant in the few
spots where they do occur; and that of some social plants being social, that
is, abounding in individuals, even on the extreme confines of their range.
For in such cases, we may believe, that a plant could exist only where the
conditions of its life were so favourable that many could exist together,
and thus save each other from utter destruction. I should add that the good
effects of frequent intercrossing, and the ill effects of close
interbreeding, probably come into play in some of these cases; but on this
intricate subject I will not here enlarge.

Many cases are on record showing how complex and unexpected are the checks
and relations between organic beings, which have to struggle together in the
same country. I will give only a single instance, which, though a simple
one, has interested me. In Staffordshire, on the estate of a relation where
I had ample means of investigation, there was a large and extremely barren
heath, which had never been touched by the hand of man; but several hundred
acres of exactly the same nature had been enclosed twenty-five years
previously and planted with Scotch fir. The change in the native vegetation
of the planted part of the heath was most remarkable, more than is generally
seen in passing from one quite different soil to another: not only the
proportional numbers of the heath-plants were wholly changed, but twelve
species of plants (not counting grasses and carices) flourished in the
plantations, which could not be found on the heath. The effect on the
insects must have been still greater, for six insectivorous birds were very
common in the plantations, which were not to be seen on the heath; and the
heath was frequented by two or three distinct insectivorous birds. Here we
see how potent has been the effect of the introduction of a single tree,
nothing whatever else having been done, with the exception that the land had
been enclosed, so that cattle could not enter. But how important an element
enclosure is, I plainly saw near Farnham, in Surrey. Here there are
extensive heaths, with a few clumps of old Scotch firs on the distant
hill-tops: within the last ten years large spaces have been enclosed, and
self-sown firs are now springing up in multitudes, so close together that
all cannot live. When I ascertained that these young trees had not been sown
or planted, I was so much surprised at their numbers that I went to several
points of view, whence I could examine hundreds of acres of the unenclosed
heath, and literally I could not see a single Scotch fir, except the old
planted clumps. But on looking closely between the stems of the heath, I
found a multitude of seedlings and little trees, which had been perpetually
browsed down by the cattle. In one square yard, at a point some hundreds
yards distant from one of the old clumps, I counted thirty-two little trees;
and one of them, judging from the rings of growth, had during twenty-six
years tried to raise its head above the stems of the heath, and had failed.
No wonder that, as soon as the land was enclosed, it became thickly clothed
with vigorously growing young firs. Yet the heath was so extremely barren
and so extensive that no one would ever have imagined that cattle would have
so closely and effectually searched it for food.

Here we see that cattle absolutely determine the existence of the Scotch
fir; but in several parts of the world insects determine the existence of
cattle. Perhaps Paraguay offers the most curious instance of this; for here
neither cattle nor horses nor dogs have ever run wild, though they swarm
southward and northward in a feral state; and Azara and Rengger have shown
that this is caused by the greater number in Paraguay of a certain fly,
which lays its eggs in the navels of these animals when first born. The
increase of these flies, numerous as they are, must be habitually checked by
some means, probably by birds. Hence, if certain insectivorous birds (whose
numbers are probably regulated by hawks or beasts of prey) were to increase
in Paraguay, the flies would decrease then cattle and horses would become
feral, and this would certainly greatly alter (as indeed I have observed in
parts of South America) the vegetation: this again would largely affect the
insects; and this, as we just have seen in Staffordshire, the insectivorous
birds, and so onwards in ever-increasing circles of complexity. We began
this series by insectivorous birds, and we have ended with them. Not that in
nature the relations can ever be as simple as this. Battle within battle
must ever be recurring with varying success; and yet in the long-run the
forces are so nicely balanced, that the face of nature remains uniform for
long periods of time, though assuredly the merest trifle would often give
the victory to one organic being over another. Nevertheless so profound is
our ignorance, and so high our presumption, that we marvel when we hear of
the extinction of an organic being; and as we do not see the cause, we
invoke cataclysms to desolate the world, or invent laws on the duration of
the forms of life!

I am tempted to give one more instance showing how plants and animals, most
remote in the scale of nature, are bound together by a web of complex
relations. I shall hereafter have occasion to show that the exotic Lobelia
fulgens, in this part of England, is never visited by insects, and
consequently, from its peculiar structure, never can set a seed. Many of our
orchidaceous plants absolutely require the visits of moths to remove their
pollen-masses and thus to fertilise them. I have, also, reason to believe
that humble-bees are indispensable to the fertilisation of the heartsease
(Viola tricolor), for other bees do not visit this flower. From experiments
which I have tried, I have found that the visits of bees, if not
indispensable, are at least highly beneficial to the fertilisation of our
clovers; but humble-bees alone visit the common red clover (Trifolium
pratense), as other bees cannot reach the nectar. Hence I have very little
doubt, that if the whole genus of humble-bees became extinct or very rare in
England, the heartsease and red clover would become very rare, or wholly
disappear. The number of humble-bees in any district depends in a great
degree on the number of field-mice, which destroy their combs and nests; and
Mr H. Newman, who has long attended to the habits of humble-bees, believes
that 'more than two thirds of them are thus destroyed all over England.' Now
the number of mice is largely dependent, as every one knows, on the number
of cats; and Mr Newman says, 'Near villages and small towns I have found the
nests of humble-bees more numerous than elsewhere, which I attribute to the
number of cats that destroy the mice.' Hence it is quite credible that the
presence of a feline animal in large numbers in a district might determine,
through the intervention first of mice and then of bees, the frequency of
certain flowers in that district!

In the case of every species, many different checks, acting at different
periods of life, and during different seasons or years, probably come into
play; some one check or some few being generally the most potent, but all
concurring in determining the average number or even the existence of the
species. In some cases it can be shown that widely-different checks act on
the same species in different districts. When we look at the plants and
bushes clothing an entangled bank, we are tempted to attribute their
proportional numbers and kinds to what we call chance. But how false a view
is this! Every one has heard that when an American forest is cut down, a
very different vegetation springs up; but it has been observed that the
trees now growing on the ancient Indian mounds, in the Southern United
States, display the same beautiful diversity and proportion of kinds as in
the surrounding virgin forests. What a struggle between the several kinds of
trees must here have gone on during long centuries, each annually scattering
its seeds by the thousand; what war between insect and insect between
insects, snails, and other animals with birds and beasts of prey all
striving to increase, and all feeding on each other or on the trees or their
seeds and seedlings, or on the other plants which first clothed the ground
and thus checked the growth of the trees! Throw up a handful of feathers,
and all must fall to the ground according to definite laws; but how simple
is this problem compared to the action and reaction of the innumerable
plants and animals which have determined, in the course of centuries, the
proportional numbers and kinds of trees now growing on the old Indian ruins!

The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This is
often the case with those which may strictly be said to struggle with each
other for existence, as in the case of locusts and grass-feeding quadrupeds.
But the struggle almost invariably will be most severe between the
individuals of the same species, for they frequent the same districts,
require the same food, and are exposed to the same dangers. In the case of
varieties of the same species, the struggle will generally be almost equally
severe, and we sometimes see the contest soon decided: for instance, if
several varieties of wheat be sown together, and the mixed seed be resown,
some of the varieties which best suit the soil or climate, or are naturally
the most fertile, will beat the others and so yield more seed, and will
consequently in a few years quite supplant the other varieties. To keep up a
mixed stock of even such extremely close varieties as the variously coloured
sweet-peas, they must be each year harvested separately, and the seed then
mixed in due proportion, otherwise the weaker kinds will steadily decrease
in numbers and disappear. So again with the varieties of sheep: it has been
asserted that certain mountain-varieties will starve out other
mountain-varieties, so that they cannot be kept together. The same result
has followed from keeping together different varieties of the medicinal
leech. It may even be doubted whether the varieties of any one of our
domestic plants or animals have so exactly the same strength, habits, and
constitution, that the original proportions of a mixed stock could be kept
up for half a dozen generations, if they were allowed to struggle together,
like beings in a state of nature, and if the seed or young were not annually
sorted.

As species of the same genus have usually, though by no means invariably,
some similarity in habits and constitution, and always in structure, the
struggle will generally be more severe between species of the same genus,
when they come into competition with each other, than between species of
distinct genera. We see this in the recent extension over parts of the
United States of one species of swallow having caused the decrease of
another species. The recent increase of the missel-thrush in parts of
Scotland has caused the decrease of the song-thrush. How frequently we hear
of one species of rat taking the place of another species under the most
different climates! In Russia the small Asiatic cockroach has everywhere
driven before it its great congener. One species of charlock will supplant
another, and so in other cases. We can dimly see why the competition should
be most severe between allied forms, which fill nearly the same place in the
economy of nature; but probably in no one case could we precisely say why
one species has been victorious over another in the great battle of life.

A corollary of the highest importance may be deduced from the foregoing
remarks, namely, that the structure of every organic being is related, in
the most essential yet often hidden manner, to that of all other organic
beings, with which it comes into competition for food or residence, or from
which it has to escape, or on which it preys. This is obvious in the
structure of the teeth and talons of the tiger; and in that of the legs and
claws of the parasite which clings to the hair on the tiger's body. But in
the beautifully plumed seed of the dandelion, and in the flattened and
fringed legs of the water-beetle, the relation seems at first confined to
the elements of air and water. Yet the advantage of plumed seeds no doubt
stands in the closest relation to the land being already thickly clothed by
other plants; so that the seeds may be widely distributed and fall on
unoccupied ground. In the water-beetle, the structure of its legs, so well
adapted for diving, allows it to compete with other aquatic insects, to hunt
for its own prey, and to escape serving as prey to other animals.

The store of nutriment laid up within the seeds of many plants seems at
first sight to have no sort of relation to other plants. But from the strong
growth of young plants produced from such seeds (as peas and beans), when
sown in the midst of long grass, I suspect that the chief use of the
nutriment in the seed is to favour the growth of the young seedling, whilst
struggling with other plants growing vigorously all around.

Look at a plant in the midst of its range, why does it not double or
quadruple its numbers? We know that it can perfectly well withstand a little
more heat or cold, dampness or dryness, for elsewhere it ranges into
slightly hotter or colder, damper or drier districts. In this case we can
clearly see that if we wished in imagination to give the plant the power of
increasing in number, we should have to give it some advantage over its
competitors, or over the animals which preyed on it. On the confines of its
geographical range, a change of constitution with respect to climate would
clearly be an advantage to our plant; but we have reason to believe that
only a few plants or animals range so far, that they are destroyed by the
rigour of the climate alone. Not until we reach the extreme confines of
life, in the arctic regions or on the borders of an utter desert, will
competition cease. The land may be extremely cold or dry, yet there will be
competition between some few species, or between the individuals of the same
species, for the warmest or dampest spots.

Hence, also, we can see that when a plant or animal is placed in a new
country amongst new competitors, though the climate may be exactly the same
as in its former home, yet the conditions of its life will generally be
changed in an essential manner. If we wished to increase its average numbers
in its new home, we should have to modify it in a different way to what we
should have done in its native country; for we should have to give it some
advantage over a different set of competitors or enemies.

It is good thus to try in our imagination to give any form some advantage
over another. Probably in no single instance should we know what to do, so
as to succeed. It will convince us of our ignorance on the mutual relations
of all organic beings; a conviction as necessary, as it seems to be
difficult to acquire. All that we can do, is to keep steadily in mind that
each organic being is striving to increase at a geometrical ratio; that each
at some period of its life, during some season of the year, during each
generation or at intervals, has to struggle for life, and to suffer great
destruction. When we reflect on this struggle, we may console ourselves with
the full belief, that the war of nature is not incessant, that no fear is
felt, that death is generally prompt, and that the vigorous, the healthy,
and the happy survive and multiply.

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Chapter 4 - Natural Selection

[* ] Natural Selection [* ] its power compared with man's selection [* ] its
power on characters of trifling importance [* ] its power at all ages and on
both sexes [* ] Sexual Selection [* ] On the generality of intercrosses
between individuals of the same species [* ] Circumstances favourable and
unfavourable to Natural Selection, namely, intercrossing, isolation, number
of individuals [* ] Slow action [* ] Extinction caused by Natural Selection
[* ] Divergence of Character, related to the diversity of inhabitants of any
small area, and to naturalisation [* ] Action of Natural Selection, through
Divergence of Character and Extinction, on the descendants from a common
parent [* ] Explains the Grouping of all organic beings
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How will the struggle for existence, discussed too briefly in the last
chapter, act in regard to variation? Can the principle of selection, which
we have seen is so potent in the hands of man, apply in nature? I think we
shall see that it can act most effectually. Let it be borne in mind in what
an endless number of strange peculiarities our domestic productions, and, in
a lesser degree, those under nature, vary; and how strong the hereditary
tendency is. Under domestication, it may be truly said that the, whole
organisation becomes in some degree plastic. Let it be borne in mind how
infinitely complex and close-fitting are the mutual relations of all organic
beings to each other and to their physical conditions of life. Can it, then,
be thought improbable, seeing that variations useful to man have undoubtedly
occurred, that other variations useful in some way to each being in the
great and complex battle of life, should sometimes occur in the course of
thousands of generations? If such do occur, can we doubt (remembering that
many more individuals are born than can possibly survive) that individuals
having any advantage, however slight, over others, would have the best
chance of surviving and of procreating their kind? On the other hand, we may
feel sure that any variation in the least degree injurious would be rigidly
destroyed. This preservation of favourable variations and the rejection of
injurious variations, I call Natural Selection. Variations neither useful
nor injurious would not be affected by natural selection, and would be left
a fluctuating element, as perhaps we see in the species called polymorphic.

We shall best understand the probable course of natural selection by taking
the case of a country undergoing some physical change, for instance, of
climate. The proportional numbers of its inhabitants would almost
immediately undergo a change, and some species might become extinct. We may
conclude, from what we have seen of the intimate and complex manner in which
the inhabitants of each country are bound together, that any change in the
numerical proportions of some of the inhabitants, independently of the
change of climate itself, would most seriously affect many of the others. If
the country were open on its borders, new forms would certainly immigrate,
and this also would seriously disturb the relations of some of the former
inhabitants. Let it be remembered how powerful the influence of a single
introduced tree or mammal has been shown to be. But in the case of an
island, or of a country partly surrounded by barriers, into which new and
better adapted forms could not freely enter, we should then have places in
the economy of nature which would assuredly be better filled up, if some of
the original inhabitants were in some manner modified; for, had the area
been open to immigration, these same places would have been seized on by
intruders. In such case, every slight modification, which in the course of
ages chanced to arise, and which in any way favoured the individuals of any
of the species, by better adapting them to their altered conditions, would
tend to be preserved; and natural selection would thus have free scope for
the work of improvement.

We have reason to believe, as stated in the first chapter, that a change in
the conditions of life, by specially acting on the reproductive system,
causes or increases variability; and in the foregoing case the conditions of
life are supposed to have undergone a change, and this would manifestly be
favourable to natural selection, by giving a better chance of profitable
variations occurring; and unless profitable variations do occur, natural
selection can do nothing. Not that, as I believe, any extreme amount of
variability is necessary; as man can certainly produce great results by
adding up in any given direction mere individual differences, so could
Nature, but far more easily, from having incomparably longer time at her
disposal. Nor do I believe that any great physical change, as of climate, or
any unusual degree of isolation to check immigration, is actually necessary
to produce new and unoccupied places for natural selection to fill up by
modifying and improving some of the varying inhabitants. For as all the
inhabitants of each country are struggling together with nicely balanced
forces, extremely slight modifications in the structure or habits of one
inhabitant would often give it an advantage over others; and still further
modifications of the same kind would often still further increase the
advantage. No country can be named in which all the native inhabitants are
now so perfectly adapted to each other and to the physical conditions under
which they live, that none of them could anyhow be improved; for in all
countries, the natives have been so far conquered by naturalised
productions, that they have allowed foreigners to take firm possession of
the land. And as foreigners have thus everywhere beaten some of the natives,
we may safely conclude that the natives might have been modified with
advantage, so as to have better resisted such intruders.

As man can produce and certainly has produced a great result by his
methodical and unconscious means of selection, what may not nature effect?
Man can act only on external and visible characters: nature cares nothing
for appearances, except in so far as they may be useful to any being. She
can act on every internal organ, on every shade of constitutional
difference, on the whole machinery of life. Man selects only for his own
good; Nature only for that of the being which she tends. Every selected
character is fully exercised by her; and the being is placed under
well-suited conditions of life. Man keeps the natives of many climates in
the same country; he seldom exercises each selected character in some
peculiar and fitting manner; he feeds a long and a short beaked pigeon on
the same food; he does not exercise a long-backed or long-legged quadruped
in any peculiar manner; he exposes sheep with long and short wool to the
same climate. He does not allow the most vigorous males to struggle for the
females. He does not rigidly destroy all inferior animals, but protects
during each varying season, as far as lies in his power, all his
productions. He often begins his selection by some half-monstrous form; or
at least by some modification prominent enough to catch his eye, or to be
plainly useful to him. Under nature, the slightest difference of structure
or constitution may well turn the nicely-balanced scale in the struggle for
life, and so be preserved. How fleeting are the wishes and efforts of man!
how short his time! and consequently how poor will his products be, compared
with those accumulated by nature during whole geological periods. Can we
wonder, then, that nature's productions should be far 'truer' in character
than man's productions; that they should be infinitely better adapted to the
most complex conditions of life, and should plainly bear the stamp of far
higher workmanship?

It may be said that natural selection is daily and hourly scrutinising,
throughout the world, every variation, even the slightest; rejecting that
which is bad, preserving and adding up all that is good; silently and
insensibly working, whenever and wherever opportunity offers, at the
improvement of each organic being in relation to its organic and inorganic
conditions of life. We see nothing of these slow changes in progress, until
the hand of time has marked the long lapses of ages, and then so imperfect
is our view into long past geological ages, that we only see that the forms
of life are now different from what they formerly were.

Although natural selection can act only through and for the good of each
being, yet characters and structures, which we are apt to consider as of
very trifling importance, may thus be acted on. When we see leaf-eating
insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in
winter, the red-grouse the colour of heather, and the black-grouse that of
peaty earth, we must believe that these tints are of service to these birds
and insects in preserving them from danger. Grouse, if not destroyed at some
period of their lives, would increase in countless numbers; they are known
to suffer largely from birds of prey; and hawks are guided by eyesight to
their prey, so much so, that on parts of the Continent persons are warned
not to keep white pigeons, as being the most liable to destruction. Hence I
can see no reason to doubt that natural selection might be most effective in
giving the proper colour to each kind of grouse, and in keeping that colour,
when once acquired, true and constant. Nor ought we to think that the
occasional destruction of an animal of any particular colour would produce
little effect: we should remember how essential it is in a flock of white
sheep to destroy every lamb with the faintest trace of black. In plants the
down on the fruit and the colour of the flesh are considered by botanists as
characters of the most trifling importance: yet we hear from an excellent
horticulturist, Downing, that in the United States smooth-skinned fruits
suffer far more from a beetle, a curculio, than those with down; that purple
plums suffer far more from a certain disease than yellow plums; whereas
another disease attacks yellow-fleshed peaches far more than those with
other coloured flesh. If, with all the aids of art, these slight differences
make a great difference in cultivating the several varieties, assuredly, in
a state of nature, where the trees would have to struggle with other trees
and with a host of enemies, such differences would effectually settle which
variety, whether a smooth or downy, a yellow or purple fleshed fruit, should
succeed.

In looking at many small points of difference between species, which, as far
as our ignorance permits us to judge, seem to be quite unimportant, we must
not forget that climate, food, &c., probably produce some slight and direct
effect. It is, however, far more necessary to bear in mind that there are
many unknown laws of correlation of growth, which, when one part of the
organisation is modified through variation, and the modifications are
accumulated by natural selection for the good of the being, will cause other
modifications, often of the most unexpected nature.

As we see that those variations which under domestication appear at any
particular period of life, tend to reappear in the offspring at the same
period; for instance, in the seeds of the many varieties of our culinary and
agricultural plants; in the caterpillar and cocoon stages of the varieties
of the silkworm; in the eggs of poultry, and in the colour of the down of
their chickens; in the horns of our sheep and cattle when nearly adult; so
in a state of nature, natural selection will be enabled to act on and modify
organic beings at any age, by the accumulation of profitable variations at
that age, and by their inheritance at a corresponding age. If it profit a
plant to have its seeds more and more widely disseminated by the wind, I can
see no greater difficulty in this being effected through natural selection,
than in the cotton-planter increasing and improving by selection the down in
the pods on his cotton-trees. Natural selection may modify and adapt the
larva of an insect to a score of contingencies, wholly different from those
which concern the mature insect. These modifications will no doubt affect,
through the laws of correlation, the structure of the adult; and probably in
the case of those insects which live only for a few hours, and which never
feed, a large part of their structure is merely the correlated result of
successive changes in the structure of their larvae. So, conversely,
modifications in the adult will probably often affect the structure of the
larva; but in all cases natural selection will ensure that modifications
consequent on other modifications at a different period of life, shall not
be in the least degree injurious: for if they became so, they would cause
the extinction of the species.

Natural selection will modify the structure of the young in relation to the
parent, and of the parent in relation to the young. In social animals it
will adapt the structure of each individual for the benefit of the
community; if each in consequence profits by the selected change. What
natural selection cannot do, is to modify the structure of one species,
without giving it any advantage, for the good of another species; and though
statements to this effect may be found in works of natural history, I cannot
find one case which will bear investigation. A structure used only once in
an animal's whole life, if of high importance to it, might be modified to
any extent by natural selection; for instance, the great jaws possessed by
certain insects, and used exclusively for opening the cocoon or the hard tip
to the beak of nestling birds, used for breaking the egg. It has been
asserted, that of the best short-beaked tumbler-pigeons more perish in the
egg than are able to get out of it; so that fanciers assist in the act of
hatching. Now, if nature had to make the beak of a full-grown pigeon very
short for the bird's own advantage, the process of modification would be
very slow, and there would be simultaneously the most rigorous selection of
the young birds within the egg, which had the most powerful and hardest
beaks, for all with weak beaks would inevitably perish: or, more delicate
and more easily broken shells might be selected, the thickness of the shell
being known to vary like every other structure.

Sexual Selection

Inasmuch as peculiarities often appear under domestication in one sex and
become hereditarily attached to that sex, the same fact probably occurs
under nature, and if so, natural selection will be able to modify one sex in
its functional relations to the other sex, or in relation to wholly
different habits of life in the two sexes, as is sometimes the case with
insects. And this leads me to say a few words on what I call Sexual
Selection. This depends, not on a struggle for existence, but on a struggle
between the males for possession of the females; the result is not death to
the unsuccessful competitor, but few or no offspring. Sexual selection is,
therefore, less rigorous than natural selection. Generally, the most
vigorous males, those which are best fitted for their places in nature, will
leave most progeny. But in many cases, victory will depend not on general
vigour, but on having special weapons, confined to the male sex. A hornless
stag or spurless cock would have a poor chance of leaving offspring. Sexual
selection by always allowing the victor to breed might surely give
indomitable courage, length to the spur, and strength to the wing to strike
in the spurred leg, as well as the brutal cock-fighter, who knows well that
he can improve his breed by careful selection of the best cocks. How low in
the scale of nature this law of battle descends, I know not; male alligators
have been described as fighting, bellowing, and whirling round, like Indians
in a war-dance, for the possession of the females; male salmons have been
seen fighting all day long; male stag-beetles often bear wounds from the
huge mandibles of other males. The war is, perhaps, severest between the
males of polygamous animals, and these seem oftenest provided with special
weapons. The males of carnivorous animals are already well armed; though to
them and to others, special means of defence may be given through means of
sexual selection, as the mane to the lion, the shoulder-pad to the boar, and
the hooked jaw to the male salmon; for the shield may be as important for
victory, as the sword or spear.

Amongst birds, the contest is often of a more peaceful character. All those
who have attended to the subject, believe that there is the severest rivalry
between the males of many species to attract by singing the females. The
rock-thrush of Guiana, birds of paradise, and some others, congregate; and
successive males display their gorgeous plumage and perform strange antics
before the females, which standing by as spectators, at last choose the most
attractive partner. Those who have closely attended to birds in confinement
well know that they often take individual preferences and dislikes: thus Sir
R. Heron has described how one pied peacock was eminently attractive to all
his hen birds. It may appear childish to attribute any effect to such
apparently weak means: I cannot here enter on the details necessary to
support this view; but if man can in a short time give elegant carriage and
beauty to his bantams, according to his standard of beauty, I can see no
good reason to doubt that female birds, by selecting, during thousands of
generations, the most melodious or beautiful males, according to their
standard of beauty, might produce a marked effect. I strongly suspect that
some well-known laws with respect to the plumage of male and female birds,
in comparison with the plumage of the young, can be explained on the view of
plumage having been chiefly modified by sexual selection, acting when the
birds have come to the breeding age or during the breeding season; the
modifications thus produced being inherited at corresponding ages or
seasons, either by the males alone, or by the males and females; but I have
not space here to enter on this subject.

Thus it is, as I believe, that when the males and females of any animal have
the same general habits of life, but differ in structure, colour, or
ornament, such differences have been mainly caused by sexual selection; that
is, individual males have had, in successive generations, some slight
advantage over other males, in their weapons, means of defence, or charms;
and have transmitted these advantages to their male offspring. Yet, I would
not wish to attribute all such sexual differences to this agency: for we see
peculiarities arising and becoming attached to the male sex in our domestic
animals (as the wattle in male carriers, horn-like protuberances in the
cocks of certain fowls, &c.), which we cannot believe to be either useful to
the males in battle, or attractive to the females. We see analogous cases
under nature, for instance, the tuft of hair on the breast of the
turkey-cock, which can hardly be either useful or ornamental to this bird;
indeed, had the tuft appeared under domestication, it would have been called
a monstrosity.

Illustrations of the action of Natural Selection

In order to make it clear how, as I believe, natural selection acts, I must
beg permission to give one or two imaginary illustrations. Let us take the
case of a wolf, which preys on various animals, securing some by craft, some
by strength, and some by fleetness; and let us suppose that the fleetest
prey, a deer for instance, had from any change in the country increased in
numbers, or that other prey had decreased in numbers, during that season of
the year when the wolf is hardest pressed for food. I can under such
circumstances see no reason to doubt that the swiftest and slimmest wolves
would have the best chance of surviving, and so be preserved or selected,
provided always that they retained strength to master their prey at this or
at some other period of the year, when they might be compelled to prey on
other animals. I can see no more reason to doubt this, than that man can
improve the fleetness of his greyhounds by careful and methodical selection,
or by that unconscious selection which results from each man trying to keep
the best dogs without any thought of modifying the breed.

Even without any change in the proportional numbers of the animals on which
our wolf preyed, a cub might be born with an innate tendency to pursue
certain kinds of prey. Nor can this be thought very improbable; for we often
observe great differences in the natural tendencies of our domestic animals;
one cat, for instance, taking to catch rats, another mice; one cat,
according to Mr. St. John, bringing home winged game, another hares or
rabbits, and another hunting on marshy ground and almost nightly catching
woodcocks or snipes. The tendency to catch rats rather than mice is known to
be inherited. Now, if any slight innate change of habit or of structure
benefited an individual wolf, it would have the best chance of surviving and
of leaving offspring. Some of its young would probably inherit the same
habits or structure, and by the repetition of this process, a new variety
might be formed which would either supplant or coexist with the parent-form
of wolf. Or, again, the wolves inhabiting a mountainous district, and those
frequenting the lowlands, would naturally be forced to hunt different prey;
and from the continued preservation of the individuals best fitted for the
two sites, two varieties might slowly be formed. These varieties would cross
and blend where they met; but to this subject of intercrossing we shall soon
have to return. I may add, that, according to Mr. Pierce, there are two
varieties of the wolf inhabiting the Catskill Mountains in the United
States, one with a light greyhound-like form, which pursues deer, and the
other more bulky, with shorter legs, which more frequently attacks the
shepherd's flocks.

Let us now take a more complex case. Certain plants excrete a sweet juice,
apparently for the sake of eliminating something injurious from their sap:
this is effected by glands at the base of the stipules in some Leguminosae,
and at the back of the leaf of the common laurel. This juice, though small
in quantity, is greedily sought by insects. Let us now suppose a little
sweet juice or nectar to be excreted by the inner bases of the petals of a
flower. In this case insects in seeking the nectar would get dusted with
pollen, and would certainly often transport the pollen from one flower to
the stigma of another flower. The flowers of two distinct individuals of the
same species would thus get crossed; and the act of crossing, we have good
reason to believe (as will hereafter be more fully alluded to), would
produce very vigorous seedlings, which consequently would have the best
chance of flourishing and surviving. Some of these seedlings would probably
inherit the nectar-excreting power. Those in individual flowers which had
the largest glands or nectaries, and which excreted most nectar, would be
oftenest visited by insects, and would be oftenest crossed; and so in the
long-run would gain the upper hand. Those flowers, also, which had their
stamens and pistils placed, in relation to the size and habits of the
particular insects which visited them, so as to favour in any degree the
transportal of their pollen from flower to flower, would likewise be
favoured or selected. We might have taken the case of insects visiting
flowers for the sake of collecting pollen instead of nectar; and as pollen
is formed for the sole object of fertilisation, its destruction appears a
simple loss to the plant; yet if a little pollen were carried, at first
occasionally and then habitually, by the pollen-devouring insects from
flower to flower, and a cross thus effected, although nine-tenths of the
pollen were destroyed, it might still be a great gain to the plant; and
those individuals which produced more and more pollen, and had larger and
larger anthers, would be selected.

When our plant, by this process of the continued preservation or natural
selection of more and more attractive flowers, had been rendered highly
attractive to insects, they would, unintentionally on their part, regularly
carry pollen from flower to flower; and that they can most effectually do
this, I could easily show by many striking instances. I will give only one
not as a very striking case, but as likewise illustrating one step in the
separation of the sexes of plants, presently to be alluded to. Some
holly-trees bear only male flowers, which have four stamens producing rather
a small quantity of pollen, and a rudimentary pistil; other holly-trees bear
only female flowers; these have a full-sized pistil, and four stamens with
shrivelled anthers, in which not a grain of pollen can be detected. Having
found a female tree exactly sixty yards from a male tree, I put the stigmas
of twenty flowers, taken from different branches, under the microscope, and
on all, without exception, there were pollen-grains, and on some a profusion
of pollen. As the wind had set for several days from the female to the male
tree, the pollen could not thus have been carried. The weather had been cold
and boisterous, and therefore not favourable to bees, nevertheless every
female flower which I examined had been effectually fertilised by the bees,
accidentally dusted with pollen, having flown from tree to tree in search of
nectar. But to return to our imaginary case: as soon as the plant had been
rendered so highly attractive to insects that pollen was regularly carried
from flower to flower, another process might commence. No naturalist doubts
the advantage of what has been called the 'physiological division of
labour;' hence we may believe that it would be advantageous to a plant to
produce stamens alone in one flower or on one whole plant, and pistils alone
in another flower or on another plant. In plants under culture and placed
under new conditions of life, sometimes the male organs and sometimes the
female organs become more or less impotent; now if we suppose this to occur
in ever so slight a degree under nature, then as pollen is already carried
regularly from flower to flower, and as a more complete separation of the
sexes of our plant would be advantageous on the principle of the division of
labour, individuals with this tendency more and more increased, would be
continually favoured or selected, until at last a complete separation of the
sexes would be effected.

Let us now turn to the nectar-feeding insects in our imaginary case: we may
suppose the plant of which we have been slowly increasing the nectar by
continued selection, to be a common plant; and that certain insects depended
in main part on its nectar for food. I could give many facts, showing how
anxious bees are to save time; for instance, their habit of cutting holes
and sucking the nectar at the bases of certain flowers, which they can, with
a very little more trouble, enter by the mouth. Bearing such facts in mind,
I can see no reason to doubt that an accidental deviation in the size and
form of the body, or in the curvature and length of the proboscis, &c., far
too slight to be appreciated by us, might profit a bee or other insect, so
that an individual so characterised would be able to obtain its food more
quickly, and so have a better chance of living and leaving descendants. Its
descendants would probably inherit a tendency to a similar slight deviation
of structure. The tubes of the corollas of the common red and incarnate
clovers (Trifolium pratense and incarnatum) do not on a hasty glance appear
to differ in length; yet the hive-bee can easily suck the nectar out of the
incarnate clover, but not out of the common red clover, which is visited by
humble-bees alone; so that whole fields of the red clover offer in vain an
abundant supply of precious nectar to the hive-bee. Thus it might be a great
advantage to the hive-bee to have a slightly longer or differently
constructed proboscis. On the other hand, I have found by experiment that
the fertility of clover greatly depends on bees visiting and moving parts of
the corolla, so as to push the pollen on to the stigmatic surface. Hence,
again, if humble-bees were to become rare in any country, it might be a
great advantage to the red clover to have a shorter or more deeply divided
tube to its corolla, so that the hive-bee could visit its flowers. Thus I
can understand how a flower and a bee might slowly become, either
simultaneously or one after the other, modified and adapted in the most
perfect manner to each other, by the continued preservation of individuals
presenting mutual and slightly favourable deviations of structure.

I am well aware that this doctrine of natural selection, exemplified in the
above imaginary instances, is open to the same objections which were at
first urged against Sir Charles Lyell's noble views on 'the modern changes
of the earth, as illustrative of geology;' but we now very seldom hear the
action, for instance, of the coast-waves, called a trifling and
insignificant cause, when applied to the excavation of gigantic valleys or
to the formation of the longest lines of inland cliffs. Natural selection
can act only by the preservation and accumulation of infinitesimally small
inherited modifications, each profitable to the preserved being; and as
modern geology has almost banished such views as the excavation of a great
valley by a single diluvial wave, so will natural selection, if it be a true
principle, banish the belief of the continued creation of new organic
beings, or of any great and sudden modification in their structure.

On the Intercrossing of Individuals

I must here introduce a short digression. In the case of animals and plants
with separated sexes, it is of course obvious that two individuals must
always unite for each birth; but in the case of hermaphrodites this is far
from obvious. Nevertheless I am strongly inclined to believe that with all
hermaphrodites two individuals, either occasionally or habitually, concur
for the reproduction of their kind. This view, I may add, was first
suggested by Andrew Knight. We shall presently see its importance; but I
must here treat the subject with extreme brevity, though I have the
materials prepared for an ample discussion. All vertebrate animals, all
insects, and some other large groups of animals, pair for each birth. Modern
research has much diminished the number of supposed hermaphrodites, and of
real hermaphrodites a large number pair; that is, two individuals regularly
unite for reproduction, which is all that concerns us. But still there are
many hermaphrodite animals which certainly do not habitually pair, and a
vast majority of plants are hermaphrodites. What reason, it may be asked, is
there for supposing in these cases that two individuals ever concur in
reproduction? As it is impossible here to enter on details, I must trust to
some general considerations alone.

In the first place, I have collected so large a body of facts, showing, in
accordance with the almost universal belief of breeders, that with animals
and plants a cross between different varieties, or between individuals of
the same variety but of another strain, gives vigour and fertility to the
offspring; and on the other hand, that close interbreeding diminishes vigour
and fertility; that these facts alone incline me to believe that it is a
general law of nature (utterly ignorant though we be of the meaning of the
law) that no organic being self-fertilises itself for an eternity of
generations; but that a cross with another individual is occasionally
perhaps at very long intervals -- indispensable.

On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any other
view are inexplicable. Every hybridizer knows how unfavourable exposure to
wet is to the fertilisation of a flower, yet what a multitude of flowers
have their anthers and stigmas fully exposed to the weather! but if an
occasional cross be indispensable, the fullest freedom for the entrance of
pollen from another individual will explain this state of exposure, more
especially as the plant's own anthers and pistil generally stand so close
together that self-fertilisation seems almost inevitable. Many flowers, on
the other hand, have their organs of fructification closely enclosed, as in
the great papilionaceous or pea-family; but in several, perhaps in all, such
flowers, there is a very curious adaptation between the structure of the
flower and the manner in which bees suck the nectar; for, in doing this,
they either push the flower's own pollen on the stigma, or bring pollen from
another flower. So necessary are the visits of bees to papilionaceous
flowers, that I have found, by experiments published elsewhere, that their
fertility is greatly diminished if these visits be prevented. Now, it is
scarcely possible that bees should fly from flower to flower, and not carry
pollen from one to the other, to the great good, as I believe, of the plant.
Bees will act like a camel-hair pencil, and it is quite sufficient just to
touch the anthers of one flower and then the stigma of another with the same
brush to ensure fertilisation; but it must not be supposed that bees would
thus produce a multitude of hybrids between distinct species; for if you
bring on the same brush a plant's own pollen and pollen from another
species, the former will have such a prepotent effect, that it will
invariably and completely destroy, as has been shown by Gärtner, any
influence from the foreign pollen.

When the stamens of a flower suddenly spring towards the pistil, or slowly
move one after the other towards it, the contrivance seems adapted solely to
ensure self-fertilisation; and no doubt it is useful for this end: but, the
agency of insects is often required to cause the stamens to spring forward,
as Kölreuter has shown to be the case with the barberry; and curiously in
this very genus, which seems to have a special contrivance for
self-fertilisation, it is well known that if very closely-allied forms or
varieties are planted near each other, it is hardly possible to raise pure
seedlings, so largely do they naturally cross. In many other cases, far from
there being any aids for self-fertilisation, there are special contrivances,
as I could show from the writings of C. C. Sprengel and from my own
observations, which effectually prevent the stigma receiving pollen from its
own flower: for instance, in Lobelia fulgens, there is a really beautiful
and elaborate contrivance by which every one of the infinitely numerous
pollen-granules are swept out of the conjoined anthers of each flower,
before the stigma of that individual flower is ready to receive them; and as
this flower is never visited, at least in my garden, by insects, it never
sets a seed, though by placing pollen from one flower on the stigma of
another, I raised plenty of seedlings; and whilst another species of Lobelia
growing close by, which is visited by bees, seeds freely. In very many other
cases, though there be no special mechanical contrivance to prevent the
stigma of a flower receiving its own pollen, yet, as C. C. Sprengel has
shown, and as I can confirm, either the anthers burst before the stigma is
ready for fertilisation, or the stigma is ready before the pollen of that
flower is ready, so that these plants have in fact separated sexes, and must
habitually be crossed. How strange are these facts! How strange that the
pollen and stigmatic surface of the same flower, though placed so close
together, as if for the very purpose of self-fertilisation, should in so
many cases be mutually useless to each other! How simply are these facts
explained on the view of an occasional cross with a distinct individual
being advantageous or indispensable!

If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority, as I have
found, of the seedlings thus raised will turn out mongrels: for instance, I
raised 233 seedling cabbages from some plants of different varieties growing
near each other, and of these only 78 were true to their kind, and some even
of these were not perfectly true. Yet the pistil of each cabbage-flower is
surrounded not only by its own six stamens, but by those of the many other
flowers on the same plant. How, then, comes it that such a vast number of
the seedlings are mongrelised? I suspect that it must arise from the pollen
of a distinct variety having a prepotent effect over a flower's own pollen;
and that this is part of the general law of good being derived from the
intercrossing of distinct individuals of the same species. When distinct
species are crossed the case is directly the reverse, for a plant's own
pollen is always prepotent over foreign pollen; but to this subject we shall
return in a future chapter.

In the case of a gigantic tree covered with innumerable flowers, it may be
objected that pollen could seldom be carried from tree to tree, and at most
only from flower to flower on the same tree, and that flowers on the same
tree can be considered as distinct individuals only in a limited sense. I
believe this objection to be valid, but that nature has largely provided
against it by giving to trees a strong tendency to bear flowers with
separated sexes. When the sexes are separated, although the male and female
flowers may be produced on the same tree, we can see that pollen must be
regularly carried from flower to flower; and this will give a better chance
of pollen being occasionally carried from tree to tree. That trees belonging
to all Orders have their sexes more often separated than other plants, I
find to be the case in this country; and at my request Dr Hooker tabulated
the trees of New Zealand, and Dr Asa Gray those of the United States, and
the result was as I anticipated. On the other hand, Dr Hooker has recently
informed me that he finds that the rule does not hold in Australia; and I
have made these few remarks on the sexes of trees simply to call attention
to the subject.

Turning for a very brief space to animals: on the land there are some
hermaphrodites, as land-mollusca and earth-worms; but these all pair. As yet
I have not found a single case of a terrestrial animal which fertilises
itself. We can understand this remarkable fact, which offers so strong a
contrast with terrestrial plants, on the view of an occasional cross being
indispensable, by considering the medium in which terrestrial animals live,
and the nature of the fertilising element; for we know of no means,
analogous to the action of insects and of the wind in the case of plants, by
which an occasional cross could be effected with terrestrial animals without
the concurrence of two individuals. Of aquatic animals, there are many
self-fertilising hermaphrodites; but here currents in the water offer an
obvious means for an occasional cross. And, as in the case of flowers, I
have as yet failed, after consultation with one of the highest authorities,
namely, Professor Huxley, to discover a single case of an hermaphrodite
animal with the organs of reproduction so perfectly enclosed within the
body, that access from without and the occasional influence of a distinct
individual can be shown to be physically impossible. Cirripedes long
appeared to me to present a case of very great difficulty under this point
of view; but I have been enabled, by a fortunate chance, elsewhere to prove
that two individuals, though both are self-fertilising hermaphrodites, do
sometimes cross.

It must have struck most naturalists as a strange anomaly that, in the case
of both animals and plants, species of the same family and even of the same
genus, though agreeing closely with each other in almost their whole
organisation, yet are not rarely, some of them hermaphrodites, and some of
them unisexual. But if, in fact, all hermaphrodites do occasionally
intercross with other individuals, the difference between hermaphrodites and
unisexual species, as far as function is concerned, becomes very small.

From these several considerations and from the many special facts which I
have collected, but which I am not here able to give, I am strongly inclined
to suspect that, both in the vegetable and animal kingdoms, an occasional
intercross with a distinct individual is a law of nature. I am well aware
that there are, on this view, many cases of difficulty, some of which I am
trying to investigate. Finally then, we may conclude that in many organic
beings, a cross between two individuals is an obvious necessity for each
birth; in many others it occurs perhaps only at long intervals; but in none,
as I suspect, can self-fertilisation go on for perpetuity.

Circumstances favourable to Natural Selection

This is an extremely intricate subject. A large amount of inheritable and
diversified variability is favourable, but I believe mere individual
differences suffice for the work. A large number of individuals, by giving a
better chance for the appearance within any given period of profitable
variations, will compensate for a lesser amount of variability in each
individual, and is, I believe, an extremely important element of success.
Though nature grants vast periods of time for the work of natural selection,
she does not grant an indefinite period; for as all organic beings are
striving, it may be said, to seize on each place in the economy of nature,
if any one species does not become modified and improved in a corresponding
degree with its competitors, it will soon be exterminated.

In man's methodical selection, a breeder selects for some definite object,
and free intercrossing will wholly stop his work. But when many men, without
intending to alter the breed, have a nearly common standard of perfection,
and all try to get and breed from the best animals, much improvement and
modification surely but slowly follow from this unconscious process of
selection, notwithstanding a large amount of crossing with inferior animals.
Thus it will be in nature; for within a confined area, with some place in
its polity not so perfectly occupied as might be, natural selection will
always tend to preserve all the individuals varying in the right direction,
though in different degrees, so as better to fill up the unoccupied place.
But if the area be large, its several districts will almost certainly
present different conditions of life; and then if natural selection be
modifying and improving a species in the several districts, there will be
intercrossing with the other individuals of the same species on the confines
of each. And in this case the effects of intercrossing can hardly be
counterbalanced by natural selection always tending to modify all the
individuals in each district in exactly the same manner to the conditions of
each; for in a continuous area, the conditions will generally graduate away
insensibly from one district to another. The intercrossing will most affect
those animals which unite for each birth, which wander much, and which do
not breed at a very quick rate. Hence in animals of this nature, for
instance in birds, varieties will generally be confined to separated
countries; and this I believe to be the case. In hermaphrodite organisms
which cross only occasionally, and likewise in animals which unite for each
birth, but which wander little and which can increase at a very rapid rate,
a new and improved variety might be quickly formed on any one spot, and
might there maintain itself in a body, so that whatever intercrossing took
place would be chiefly between the individuals of the same new variety. A
local variety when once thus formed might subsequently slowly spread to
other districts. On the above principle, nurserymen always prefer getting
seed from a large body of plants of the same variety, as the chance of
intercrossing with other varieties is thus lessened.

Even in the case of slow-breeding animals, which unite for each birth, we
must not overrate the effects of intercrosses in retarding natural
selection; for I can bring a considerable catalogue of facts, showing that
within the same area, varieties of the same animal can long remain distinct,
from haunting different stations, from breeding at slightly different
seasons, or from varieties of the same kind preferring to pair together.

Intercrossing plays a very important part in nature in keeping the
individuals of the same species, or of the same variety, true and uniform in
character. It will obviously thus act far more efficiently with those
animals which unite for each birth; but I have already attempted to show
that we have reason to believe that occasional intercrosses take place with
all animals and with all plants. Even if these take place only at long
intervals, I am convinced that the young thus produced will gain so much in
vigour and fertility over the offspring from long-continued
self-fertilisation, that they will have a better chance of surviving and
propagating their kind; and thus, in the long run, the influence of
intercrosses, even at rare intervals, will be great. If there exist organic
beings which never intercross, uniformity of character can be retained
amongst them, as long as their conditions of life remain the same, only
through the principle of inheritance, and through natural selection
destroying any which depart from the proper type; but if their conditions of
life change and they undergo modification, uniformity of character can be
given to their modified offspring, solely by natural selection preserving
the same favourable variations.

Isolation, also, is an important element in the process of natural
selection. In a confined or isolated area, if not very large, the organic
and inorganic conditions of life will generally be in a great degree
uniform; so that natural selection will tend to modify all the individuals
of a varying species throughout the area in the same manner in relation to
the same conditions. Intercrosses, also, with the individuals of the same
species, which otherwise would have inhabited the surrounding and
differently circumstanced districts, will be prevented. But isolation
probably acts more efficiently in checking the immigration of better adapted
organisms, after any physical change, such as of climate or elevation of the
land, &c.; and thus new places in the natural economy of the country are
left open for the old inhabitants to struggle for, and become adapted to,
through modifications in their structure and constitution. Lastly,
isolation, by checking immigration and consequently competition, will give
time for any new variety to be slowly improved; and this may sometimes be of
importance in the production of new species. If, however, an isolated area
be very small, either from being surrounded by barriers, or from having very
peculiar physical conditions, the total number of the individuals supported
on it will necessarily be very small; and fewness of individuals will
greatly retard the production of new species through natural selection, by
decreasing the chance of the appearance of favourable variations.

If we turn to nature to test the truth of these remarks, and look at any
small isolated area, such as an oceanic island, although the total number of
the species inhabiting it, will be found to be small, as we shall see in our
chapter on geographical distribution; yet of these species a very large
proportion are endemic, that is, have been produced there, and nowhere else.
Hence an oceanic island at first sight seems to have been highly favourable
for the production of new species. But we may thus greatly deceive
ourselves, for to ascertain whether a small isolated area, or a large open
area like a continent, has been most favourable for the production of new
organic forms, we ought to make the comparison within equal times; and this
we are incapable of doing.

Although I do not doubt that isolation is of considerable importance in the
production of new species, on the whole I am inclined to believe that
largeness of area is of more importance, more especially in the production
of species, which will prove capable of enduring for a long period, and of
spreading widely. Throughout a great and open area, not only will there be a
better chance of favourable variations arising from the large number of
individuals of the same species there supported, but the conditions of life
are infinitely complex from the large number of already existing species;
and if some of these many species become modified and improved, others will
have to be improved in a corresponding degree or they will be exterminated.
Each new form, also, as soon as it has been much improved, will be able to
spread over the open and continuous area, and will thus come into
competition with many others. Hence more new places will be formed, and the
competition to fill them will be more severe, on a large than on a small and
isolated area. Moreover, great areas, though now continuous, owing to
oscillations of level, will often have recently existed in a broken
condition, so that the good effects of isolation will generally, to a
certain extent, have concurred. Finally, I conclude that, although small
isolated areas probably have been in some respects highly favourable for the
production of new species, yet that the course of modification will
generally have been more rapid on large areas; and what is more important,
that the new forms produced on large areas, which already have been
victorious over many competitors, will be those that will spread most
widely, will give rise to most new varieties and species, and will thus play
an important part in the changing history of the organic world.

We can, perhaps, on these views, understand some facts which will be again
alluded to in our chapter on geographical distribution; for instance, that
the productions of the smaller continent of Australia have formerly yielded,
and apparently are now yielding, before those of the larger Europaeo-Asiatic
area. Thus, also, it is that continental productions have everywhere become
so largely naturalised on islands. On a small island, the race for life will
have been less severe, and there will have been less modification and less
extermination. Hence, perhaps, it comes that the flora of Madeira, according
to Oswald Heer, resembles the extinct tertiary flora of Europe. All
fresh-water basins, taken together, make a small area compared with that of
the sea or of the land; and, consequently, the competition between
fresh-water productions will have been less severe than elsewhere; new forms
will have been more slowly formed, and old forms more slowly exterminated.
And it is in fresh water that we find seven genera of Ganoid fishes,
remnants of a once preponderant order: and in fresh water we find some of
the most anomalous forms now known in the world, as the Ornithorhynchus and
Lepidosiren, which, like fossils, connect to a certain extent orders now
widely separated in the natural scale. These anomalous forms may almost be
called living fossils; they have endured to the present day, from having
inhabited a confined area, and from having thus been exposed to less severe
competition.

To sum up the circumstances favourable and unfavourable to natural
selection, as far as the extreme intricacy of the subject permits. I
conclude, looking to the future, that for terrestrial productions a large
continental area, which will probably undergo many oscillations of level,
and which consequently will exist for long periods in a broken condition,
will be the most favourable for the production of many new forms of life,
likely to endure long and to spread widely. For the area will first have
existed as a continent, and the inhabitants, at this period numerous in
individuals and kinds, will have been subjected to very severe competition.
When converted by subsidence into large separate islands, there will still
exist many individuals of the same species on each island: intercrossing on
the confines of the range of each species will thus be checked: after
physical changes of any kind, immigration will be prevented, so that new
places in the polity of each island will have to be filled up by
modifications of the old inhabitants; and time will be allowed for the
varieties in each to become well modified and perfected. When, by renewed
elevation, the islands shall be re-converted into a continental area, there
will again be severe competition: the most favoured or improved varieties
will be enabled to spread: there will be much extinction of the less
improved forms, and the relative proportional numbers of the various
inhabitants of the renewed continent will again be changed; and again there
will be a fair field for natural selection to improve still further the
inhabitants, and thus produce new species.

That natural selection will always act with extreme slowness, I fully admit.
Its action depends on there being places in the polity of nature, which can
be better occupied by some of the inhabitants of the country undergoing
modification of some kind. The existence of such places will often depend on
physical changes, which are generally very slow, and on the immigration of
better adapted forms having been checked. But the action of natural
selection will probably still oftener depend on some of the inhabitants
becoming slowly modified; the mutual relations of many of the other
inhabitants being thus disturbed. Nothing can be effected, unless favourable
variations occur, and variation itself is apparently always a very slow
process. The process will often be greatly retarded by free intercrossing.
Many will exclaim that these several causes are amply sufficient wholly to
stop the action of natural selection. I do not believe so. On the other
hand, I do believe that natural selection will always act very slowly, often
only at long intervals of time, and generally on only a very few of the
inhabitants of the same region at the same time. I further believe, that
this very slow, intermittent action of natural selection accords perfectly
well with what geology tells us of the rate and manner at which the
inhabitants of this world have changed.

Slow though the process of selection may be, if feeble man can do much by
his powers of artificial selection, I can see no limit to the amount of
change, to the beauty and infinite complexity of the coadaptations between
all organic beings, one with another and with their physical conditions of
life, which may be effected in the long course of time by nature's power of
selection.

Extinction

This subject will be more fully discussed in our chapter on Geology; but it
must be here alluded to from being intimately connected with natural
selection. Natural selection acts solely through the preservation of
variations in some way advantageous, which consequently endure. But as from
the high geometrical powers of increase of all organic beings, each area is
already fully stocked with inhabitants, it follows that as each selected and
favoured form increases in number, so will the less favoured forms decrease
and become rare. Rarity, as geology tells us, is the precursor to
extinction. We can, also, see that any form represented by few individuals
will, during fluctuations in the seasons or in the number of its enemies,
run a good chance of utter extinction. But we may go further than this; for
as new forms are continually and slowly being produced, unless we believe
that the number of specific forms goes on perpetually and almost
indefinitely increasing, numbers inevitably must become extinct. That the
number of specific forms has not indefinitely increased, geology shows us
plainly; and indeed we can see reason why they should not have thus
increased, for the number of places in the polity of nature is not
indefinitely great, not that we have any means of knowing that any one
region has as yet got its maximum of species. probably no region is as yet
fully stocked, for at the Cape of Good Hope, where more species of plants
are crowded together than in any other quarter of the world, some foreign
plants have become naturalised, without causing, as far as we know, the
extinction of any natives.

Furthermore, the species which are most numerous in individuals will have
the best chance of producing within any given period favourable variations.
We have evidence of this, in the facts given in the second chapter, showing
that it is the common species which afford the greatest number of recorded
varieties, or incipient species. Hence, rare species will be less quickly
modified or improved within any given period, and they will consequently be
beaten in the race for life by the modified descendants of the commoner
species.

From these several considerations I think it inevitably follows, that as new
species in the course of time are formed through natural selection, others
will become rarer and rarer, and finally extinct. The forms which stand in
closest competition with those undergoing modification and improvement, will
naturally suffer most. And we have seen in the chapter on the Struggle for
Existence that it is the most closely-allied forms, varieties of the same
species, and species of the same genus or of related genera, which, from
having nearly the same structure, constitution, and habits, generally come
into the severest competition with each other. Consequently, each new
variety or species, during the progress of its formation, will generally
press hardest on its nearest kindred, and tend to exterminate them. We see
the same process of extermination amongst our domesticated productions,
through the selection of improved forms by man. Many curious instances could
be given showing how quickly new breeds of cattle, sheep, and other animals,
and varieties of flowers, take the place of older and inferior kinds. In
Yorkshire, it is historically known that the ancient black cattle were
displaced by the long-horns, and that these 'were swept away by the
short-horns' (I quote the words of an agricultural writer) 'as if by some
murderous pestilence.'

Divergence of Character

The principle, which I have designated by this term, is of high importance
on my theory, and explains, as I believe, several important facts. In the
first place, varieties, even strongly-marked ones, though having somewhat of
the character of species as is shown by the hopeless doubts in many cases
how to rank them yet certainly differ from each other far less than do good
and distinct species. Nevertheless, according to my view, varieties are
species in the process of formation, or are, as I have called them,
incipient species. How, then, does the lesser difference between varieties
become augmented into the greater difference between species? That this does
habitually happen, we must infer from most of the innumerable species
throughout nature presenting well-marked differences; whereas varieties, the
supposed prototypes and parents of future well-marked species, present
slight and ill-defined differences. Mere chance, as we may call it, might
cause one variety to differ in some character from its parents, and the
offspring of this variety again to differ from its parent in the very same
character and in a greater degree; but this alone would never account for so
habitual and large an amount of difference as that between varieties of the
same species and species of the same genus.

As has always been my practice, let us seek light on this head from our
domestic productions. We shall here find something analogous. A fancier is
struck by a pigeon having a slightly shorter beak; another fancier is struck
by a pigeon having a rather longer beak; and on the acknowledged principle
that 'fanciers do not and will not admire a medium standard, but like
extremes,' they both go on (as has actually occurred with tumbler-pigeons)
choosing and breeding from birds with longer and longer beaks, or with
shorter and shorter beaks. Again, we may suppose that at an early period one
man preferred swifter horses; another stronger and more bulky horses. The
early differences would be very slight; in the course of time, from the
continued selection of swifter horses by some breeders, and of stronger ones
by others, the differences would become greater, and would be noted as
forming two sub-breeds; finally, after the lapse of centuries, the
sub-breeds would become converted into two well-established and distinct
breeds. As the differences slowly become greater, the inferior animals with
intermediate characters, being neither very swift nor very strong, will have
been neglected, and will have tended to disappear. Here, then, we see in
man's productions the action of what may be called the principle of
divergence, causing differences, at first barely appreciable, steadily to
increase, and the breeds to diverge in character both from each other and
from their common parent.

But how, it may be asked, can any analogous principle apply in nature? I
believe it can and does apply most efficiently, from the simple circumstance
that the more diversified the descendants from any one species become in
structure, constitution, and habits, by so much will they be better enabled
to seize on many and widely diversified places in the polity of nature, and
so be enabled to increase in numbers.

We can clearly see this in the case of animals with simple habits. Take the
case of a carnivorous quadruped, of which the number that can be supported
in any country has long ago arrived at its full average. If its natural
powers of increase be allowed to act, it can succeed in increasing (the
country not undergoing any change in its conditions) only by its varying
descendants seizing on places at present occupied by other animals: some of
them, for instance, being enabled to feed on new kinds of prey, either dead
or alive; some inhabiting new stations, climbing trees, frequenting water,
and some perhaps becoming less carnivorous. The more diversified in habits
and structure the descendants of our carnivorous animal became, the more
places they would be enabled to occupy. What applies to one animal will
apply throughout all time to all animals that is, if they vary for otherwise
natural selection can do nothing. So it will be with plants. It has been
experimentally proved, that if a plot of ground be sown with several
distinct genera of grasses, a greater number of plants and a greater weight
of dry herbage can thus be raised. The same has been found to hold good when
first one variety and then several mixed varieties of wheat have been sown
on equal spaces of ground. Hence, if any one species of grass were to go on
varying, and those varieties were continually selected which differed from
each other in at all the same manner as distinct species and genera of
grasses differ from each other, a greater number of individual plants of
this species of grass, including its modified descendants, would succeed in
living on the same piece of ground. And we well know that each species and
each variety of grass is annually sowing almost countless seeds; and thus,
as it may be said, is striving its utmost to increase its numbers.
Consequently, I cannot doubt that in the course of many thousands of
generations, the most distinct varieties of any one species of grass would
always have the best chance of succeeding and of increasing in numbers, and
thus of supplanting the less distinct varieties; and varieties, when
rendered very distinct from each other, take the rank of species.

The truth of the principle, that the greatest amount of life can be
supported by great diversification of structure, is seen under many natural
circumstances. In an extremely small area, especially if freely open to
immigration, and where the contest between individual and individual must be
severe, we always find great diversity in its inhabitants. For instance, I
found that a piece of turf, three feet by four in size, which had been
exposed for many years to exactly the same conditions, supported twenty
species of plants, and these belonged to eighteen genera and to eight
orders, which shows how much these plants differed from each other. So it is
with the plants and insects on small and uniform islets; and so in small
ponds of fresh water. Farmers find that they can raise most food by a
rotation of plants belonging to the most different orders: nature follows
what may be called a simultaneous rotation. Most of the animals and plants
which live close round any small piece of ground, could live on it
(supposing it not to be in any way peculiar in its nature), and may be said
to be striving to the utmost to live there; but, it is seen, that where they
come into the closest competition with each other, the advantages of
diversification of structure, with the accompanying differences of habit and
constitution, determine that the inhabitants, which thus jostle each other
most closely, shall, as a general rule, belong to what we call different
genera and orders.

The same principle is seen in the naturalisation of plants through man's
agency in foreign lands. It might have been expected that the plants which
have succeeded in becoming naturalised in any land would generally have been
closely allied to the indigenes; for these are commonly looked at as
specially created and adapted for their own country. It might, also, perhaps
have been expected that naturalised plants would have belonged to a few
groups more especially adapted to certain stations in their new homes. But
the case is very different; and Alph. De Candolle has well remarked in his
great and admirable work, that floras gain by naturalisation, proportionally
with the number of the native genera and species, far more in new genera
than in new species. To give a single instance: in the last edition of Dr
Asa Gray's 'Manual of the Flora of the Northern United States,' 260
naturalised plants are enumerated, and these belong to 162 genera. We thus
see that these naturalised plants are of a highly diversified nature. They
differ, moreover, to a large extent from the indigenes, for out of the 162
genera, no less than 100 genera are not there indigenous, and thus a large
proportional addition is made to the genera of these States.

By considering the nature of the plants or animals which have struggled
successfully with the indigenes of any country, and have there become
naturalised, we can gain some crude idea in what manner some of the natives
would have had to be modified, in order to have gained an advantage over the
other natives; and we may, I think, at least safely infer that
diversification of structure, amounting to new generic differences, would
have been profitable to them.

The advantage of diversification in the inhabitants of the same region is,
in fact, the same as that of the physiological division of labour in the
organs of the same individual body a subject so well elucidated by Milne
Edwards. No physiologist doubts that a stomach by being adapted to digest
vegetable matter alone, or flesh alone, draws most nutriment from these
substances. So in the general economy of any land, the more widely and
perfectly the animals and plants are diversified for different habits of
life, so will a greater number of individuals be capable of there supporting
themselves. A set of animals, with their organisation but little
diversified, could hardly compete with a set more perfectly diversified in
structure. It may be doubted, for instance, whether the Australian
marsupials, which are divided into groups differing but little from each
other, and feebly representing, as Mr Waterhouse and others have remarked,
our carnivorous, ruminant, and rodent mammals, could successfully compete
with these well-pronounced orders. In the Australian mammals, we see the
process of diversification in an early and incomplete stage of development.

After the foregoing discussion, which ought to have been much amplified, we
may, I think, assume that the modified descendants of any one species will
succeed by so much the better as they become more diversified in structure,
and are thus enabled to encroach on places occupied by other beings. Now let
us see how this principle of great benefit being derived from divergence of
character, combined with the principles of natural selection and of
extinction, will tend to act.

The accompanying diagram will aid us in understanding this rather perplexing
subject. Let A to L represent the species of a genus large in its own
country; these species are supposed to resemble each other in unequal
degrees, as is so generally the case in nature, and as is represented in the
diagram by the letters standing at unequal distances. I have said a large
genus, because we have seen in the second chapter, that on an average more
of the species of large genera vary than of small genera; and the varying
species of the large genera present a greater number of varieties. We have,
also, seen that the species, which are the commonest and the most
widely-diffused, vary more than rare species with restricted ranges. Let (A)
be a common, widely-diffused, and varying species, belonging to a genus
large in its own country. The little fan of diverging dotted lines of
unequal lengths proceeding from (A), may represent its varying offspring.
The variations are supposed to be extremely slight, but of the most
diversified nature; they are not supposed all to appear simultaneously, but
often after long intervals of time; nor are they all supposed to endure for
equal periods. Only those variations which are in some way profitable will
be preserved or naturally selected. And here the importance of the principle
of benefit being derived from divergence of character comes in; for this
will generally lead to the most different or divergent variations
(represented by the outer dotted lines) being preserved and accumulated by
natural selection. When a dotted line reaches one of the horizontal lines,
and is there marked by a small numbered letter, a sufficient amount of
variation is supposed to have been accumulated to have formed a fairly
well-marked variety, such as would be thought worthy of record in a
systematic work.

The intervals between the horizontal lines in the diagram, may represent
each a thousand generations; but it would have been better if each had
represented ten thousand generations. After a thousand generations, species
(A) is supposed to have produced two fairly well-marked varieties, namely a1
and m1. These two varieties will generally continue to be exposed to the
same conditions which made their parents variable, and the tendency to
variability is in itself hereditary, consequently they will tend to vary,
and generally to vary in nearly the same manner as their parents varied.
Moreover, these two varieties, being only slightly modified forms, will tend
to inherit those advantages which made their common parent (A) more numerous
than most of the other inhabitants of the same country; they will likewise
partake of those more general advantages which made the genus to which the
parent-species belonged, a large genus in its own country. And these
circumstances we know to be favourable to the production of new varieties.

If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand generations.
And after this interval, variety a1 is supposed in the diagram to have
produced variety a2, which will, owing to the principle of divergence,
differ more from (A) than did variety a1. Variety m1 is supposed to have
produced two varieties, namely m 2 and s2, differing from each other, and
more considerably from their common parent (A). We may continue the process
by similar steps for any length of time; some of the varieties, after each
thousand generations, producing only a single variety, but in a more and
more modified condition, some producing two or three varieties, and some
failing to produce any. Thus the varieties or modified descendants,
proceeding from the common parent (A), will generally go on increasing in
number and diverging in character. In the diagram the process is represented
up to the ten-thousandth generation, and under a condensed and simplified
form up to the fourteen-thousandth generation.

But I must here remark that I do not suppose that the process ever goes on
so regularly as is represented in the diagram, though in itself made
somewhat irregular. I am far from thinking that the most divergent varieties
will invariably prevail and multiply: a medium form may often long endure,
and may or may not produce more than one modified descendant; for natural
selection will always act according to the nature of the places which are
either unoccupied or not perfectly occupied by other beings; and this will
depend on infinitely complex relations. But as a general rule, the more
diversified in structure the descendants from any one species can be
rendered, the more places they will be enabled to seize on, and the more
their modified progeny will be increased. In our diagram the line of
succession is broken at regular intervals by small numbered letters marking
the successive forms which have become sufficiently distinct to be recorded
as varieties. But these breaks are imaginary, and might have been inserted
anywhere, after intervals long enough to have allowed the accumulation of a
considerable amount of divergent variation.

As all the modified descendants from a common and widely-diffused species,
belonging to a large genus, will tend to partake of the same advantages
which made their parent successful in life, they will generally go on
multiplying in number as well as diverging in character: this is represented
in the diagram by the several divergent branches proceeding from (A). The
modified offspring from the later and more highly improved branches in the
lines of descent, will, it is probable, often take the place of, and so
destroy, the earlier and less improved branches: this is represented in the
diagram by some of the lower branches not reaching to the upper horizontal
lines. In some cases I do not doubt that the process of modification will be
confined to a single line of descent, and the number of the descendants will
not be increased; although the amount of divergent modification may have
been increased in the successive generations. This case would be represented
in the diagram, if all the lines proceeding from (A) were removed, excepting
that from a1 to a10 In the same way, for instance, the English race-horse
and English pointer have apparently both gone on slowly diverging in
character from their original stocks, without either having given off any
fresh branches or races.

After ten thousand generations, species (A) is supposed to have produced
three forms, a10, f10, and m10, which, from having diverged in character
during the successive generations, will have come to differ largely, but
perhaps unequally, from each other and from their common parent. If we
suppose the amount of change between each horizontal line in our diagram to
be excessively small, these three forms may still be only well-marked
varieties; or they may have arrived at the doubtful category of sub-species;
but we have only to suppose the steps in the process of modification to be
more numerous or greater in amount, to convert these three forms into
well-defined species: thus the diagram illustrates the steps by which the
small differences distinguishing varieties are increased into the larger
differences distinguishing species. By continuing the same process for a
greater number of generations (as shown in the diagram in a condensed and
simplified manner), we get eight species, marked by the letters between a14
and m14, all descended from (A). Thus, as I believe, species are multiplied
and genera are formed.

In a large genus it is probable that more than one species would vary. In
the diagram I have assumed that a second species (I) has produced, by
analogous steps, after ten thousand generations, either two well-marked
varieties (w10 and z10) or two species, according to the amount of change
supposed to be represented between the horizontal lines. After fourteen
thousand generations, six new species, marked by the letters n14 to z14, are
supposed to have been produced. In each genus, the species, which are
already extremely different in character, will generally tend to produce the
greatest number of modified descendants; for these will have the best chance
of filling new and widely different places in the polity of nature: hence in
the diagram I have chosen the extreme species (A), and the nearly extreme
species (I), as those which have largely varied, and have given rise to new
varieties and species. The other nine species (marked by capital letters) of
our original genus, may for a long period continue transmitting unaltered
descendants; and this is shown in the diagram by the dotted lines not
prolonged far upwards from want of space.

But during the process of modification, represented in the diagram, another
of our principles, namely that of extinction, will have played an important
part. As in each fully stocked country natural selection necessarily acts by
the selected form having some advantage in the struggle for life over other
forms, there will be a constant tendency in the improved descendants of any
one species to supplant and exterminate in each stage of descent their
predecessors and their original parent. For it should be remembered that the
competition will generally be most severe between those forms which are most
nearly related to each other in habits, constitution, and structure. Hence
all the intermediate forms between the earlier and later states, that is
between the less and more improved state of a species, as well as the
original parent-species itself, will generally tend to become extinct. So it
probably will be with many whole collateral lines of descent, which will be
conquered by later and improved lines of descent. If, however, the modified
offspring of a species get into some distinct country, or become quickly
adapted to some quite new station, in which child and parent do not come
into competition, both may continue to exist.

If then our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have become
extinct, having been replaced by eight new species (a14 to m14); and (I)
will have been replaced by six (n14 to z14) new species.

But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally the
case in nature; species (A) being more nearly related to B, C, and D, than
to the other species; and species (I) more to G, H, K, L, than to the
others. These two species (A) and (I), were also supposed to be very common
and widely diffused species, so that they must originally have had some
advantage over most of the other species of the genus. Their modified
descendants, fourteen in number at the fourteen-thousandth generation, will
probably have inherited some of the same advantages: they have also been
modified and improved in a diversified manner at each stage of descent, so
as to have become adapted to many related places in the natural economy of
their country. It seems, therefore, to me extremely probable that they will
have taken the places of, and thus exterminated, not only their parents (A)
and (I), but likewise some of the original species which were most nearly
related to their parents. Hence very few of the original species will have
transmitted offspring to the fourteen-thousandth generation. We may suppose
that only one (F), of the two species which were least closely related to
the other nine original species, has transmitted descendants to this late
stage of descent.

The new species in our diagram descended from the original eleven species,
will now be fifteen in number. Owing to the divergent tendency of natural
selection, the extreme amount of difference in character between species a14
and z14 will be much greater than that between the most different of the
original eleven species. The new species, moreover, will be allied to each
other in a widely different manner. Of the eight descendants from (A) the
three marked a14, q14, p14, will be nearly related from having recently
branched off from a14; b14 and f14, from having diverged at an earlier
period from a5, will be in some degree distinct from the three first-named
species; and lastly, o14, e14, and m14, will be nearly related one to the
other, but from having diverged at the first commencement of the process of
modification, will be widely different from the other five species, and may
constitute a sub-genus or even a distinct genus. The six descendants from
(I) will form two sub-genera or even genera. But as the original species (I)
differed largely from (A), standing nearly at the extreme points of the
original genus, the six descendants from (I) will, owing to inheritance,
differ considerably from the eight descendants from (A); the two groups,
moreover, are supposed to have gone on diverging in different directions.
The intermediate species, also (and this is a very important consideration),
which connected the original species (A) and (I), have all become, excepting
(F), extinct, and have left no descendants. Hence the six new species
descended from (I), and the eight descended from (A), will have to be ranked
as very distinct genera, or even as distinct sub-families.

Thus it is, as I believe, that two or more genera are produced by descent,
with modification, from two or more species of the same genus. And the two
or more parent-species are supposed to have descended from some one species
of an earlier genus. In our diagram, this is indicated by the broken lines,
beneath the capital letters, converging in sub-branches downwards towards a
single point; this point representing a single species, the supposed single
parent of our several new sub-genera and genera.

It is worth while to reflect for a moment on the character of the new
species F14, which is supposed not to have diverged much in character, but
to have retained the form of (F), either unaltered or altered only in a
slight degree. In this case, its affinities to the other fourteen new
species will be of a curious and circuitous nature. Having descended from a
form which stood between the two parent-species (A) and (I), now supposed to
be extinct and unknown, it will be in some degree intermediate in character
between the two groups descended from these species. But as these two groups
have gone on diverging in character from the type of their parents, the new
species (F14) will not be directly intermediate between them, but rather
between types of the two groups; and every naturalist will be able to bring
some such case before his mind.

In the diagram, each horizontal line has hitherto been supposed to represent
a thousand generations, but each may represent a million or hundred million
generations, and likewise a section of the successive strata of the earth's
crust including extinct remains. We shall, when we come to our chapter on
Geology, have to refer again to this subject, and I think we shall then see
that the diagram throws light on the affinities of extinct beings, which,
though generally belonging to the same orders, or families, or genera, with
those now living, yet are often, in some degree, intermediate in character
between existing groups; and we can understand this fact, for the extinct
species lived at very ancient epochs when the branching lines of descent had
diverged less.

I see no reason to limit the process of modification, as now explained, to
the formation of genera alone. If, in our diagram, we suppose the amount of
change represented by each successive group of diverging dotted lines to be
very great, the forms marked a214 to p14, those marked b14 and f14, and
those marked o14 to m14, will form three very distinct genera. We shall also
have two very distinct genera descended from (I) and as these latter two
genera, both from continued divergence of character and from inheritance
from a different parent, will differ widely from the three genera descended
from (A), the two little groups of genera will form two distinct families,
or even orders, according to the amount of divergent modification supposed
to be represented in the diagram. And the two new families, or orders, will
have descended from two species of the original genus; and these two species
are supposed to have descended from one species of a still more ancient and
unknown genus.

We have seen that in each country it is the species of the larger genera
which oftenest present varieties or incipient species. This, indeed, might
have been expected; for as natural selection acts through one form having
some advantage over other forms in the struggle for existence, it will
chiefly act on those which already have some advantage; and the largeness of
any group shows that its species have inherited from a common ancestor some
advantage in common. Hence, the struggle for the production of new and
modified descendants, will mainly lie between the larger groups, which are
all trying to increase in number. One large group will slowly conquer
another large group, reduce its numbers, and thus lessen its chance of
further variation and improvement. Within the same large group, the later
and more highly perfected sub-groups, from branching out and seizing on many
new places in the polity of Nature, will constantly tend to supplant and
destroy the earlier and less improved sub-groups. Small and broken groups
and sub-groups will finally tend to disappear. Looking to the future, we can
predict that the groups of organic beings which are now large and
triumphant, and which are least broken up, that is, which as yet have
suffered least extinction, will for a long period continue to increase. But
which groups will ultimately prevail, no man can predict; for we well know
that many groups, formerly most extensively developed, have now become
extinct. Looking still more remotely to the future, we may predict that,
owing to the continued and steady increase of the larger groups, a multitude
of smaller groups will become utterly extinct, and leave no modified
descendants; and consequently that of the species living at any one period,
extremely few will transmit descendants to a remote futurity. I shall have
to return to this subject in the chapter on Classification, but I may add
that on this view of extremely few of the more ancient species having
transmitted descendants, and on the view of all the descendants of the same
species making a class, we can understand how it is that there exist but
very few classes in each main division of the animal and vegetable kingdoms.
Although extremely few of the most ancient species may now have living and
modified descendants, yet at the most remote geological period, the earth
may have been as well peopled with many species of many genera, families,
orders, and classes, as at the present day.

Summary of Chapter

If during the long course of ages and under varying conditions of life,
organic beings vary at all in the several parts of their organisation, and I
think this cannot be disputed; if there be, owing to the high geometrical
powers of increase of each species, at some age, season, or year, a severe
struggle for life, and this certainly cannot be disputed; then, considering
the infinite complexity of the relations of all organic beings to each other
and to their conditions of existence, causing an infinite diversity in
structure, constitution, and habits, to be advantageous to them, I think it
would be a most extraordinary fact if no variation ever had occurred useful
to each being's own welfare, in the same way as so many variations have
occurred useful to man. But if variations useful to any organic being do
occur, assuredly individuals thus characterised will have the best chance of
being preserved in the struggle for life; and from the strong principle of
inheritance they will tend to produce offspring similarly characterised.
This principle of preservation, I have called, for the sake of brevity,
Natural Selection. Natural selection, on the principle of qualities being
inherited at corresponding ages, can modify the egg, seed, or young, as
easily as the adult. Amongst many animals, sexual selection will give its
aid to ordinary selection, by assuring to the most vigorous and best adapted
males the greatest number of offspring. Sexual selection will also give
characters useful to the males alone, in their struggles with other males.

Whether natural selection has really thus acted in nature, in modifying and
adapting the various forms of life to their several conditions and stations,
must be judged of by the general tenour and balance of evidence given in the
following chapters. But we already see how it entails extinction; and how
largely extinction has acted in the world's history, geology plainly
declares. Natural selection, also, leads to divergence of character; for
more living beings can be supported on the same area the more they diverge
in structure, habits, and constitution, of which we see proof by looking at
the inhabitants of any small spot or at naturalised productions. Therefore
during the modification of the descendants of any one species, and during
the incessant struggle of all species to increase in numbers, the more
diversified these descendants become, the better will be their chance of
succeeding in the battle of life. Thus the small differences distinguishing
varieties of the same species, will steadily tend to increase till they come
to equal the greater differences between species of the same genus, or even
of distinct genera.

We have seen that it is the common, the widely-diffused, and widely-ranging
species, belonging to the larger genera, which vary most; and these will
tend to transmit to their modified offspring that superiority which now
makes them dominant in their own countries. Natural selection, as has just
been remarked, leads to divergence of character and to much extinction of
the less improved and intermediate forms of life. On these principles, I
believe, the nature of the affinities of all organic beings may be
explained. It is a truly wonderful fact the wonder of which we are apt to
overlook from familiarity that all animals and all plants throughout all
time and space should be related to each other in group subordinate to
group, in the manner which we everywhere behold namely, varieties of the
same species most closely related together, species of the same genus less
closely and unequally related together, forming sections and sub-genera,
species of distinct genera much less closely related, and genera related in
different degrees, forming sub-families, families, orders, sub-classes, and
classes. The several subordinate groups in any class cannot be ranked in a
single file, but seem rather to be clustered round points, and these round
other points, and so on in almost endless cycles. On the view that each
species has been independently created, I can see no explanation of this
great fact in the classification of all organic beings; but, to the best of
my judgment, it is explained through inheritance and the complex action of
natural selection, entailing extinction and divergence of character, as we
have seen illustrated in the diagram.

The affinities of all the beings of the same class have sometimes been
represented by a great tree. I believe this simile largely speaks the truth.
The green and budding twigs may represent existing species; and those
produced during each former year may represent the long succession of
extinct species. At each period of growth all the growing twigs have tried
to branch out on all sides, and to overtop and kill the surrounding twigs
and branches, in the same manner as species and groups of species have tried
to overmaster other species in the great battle for life. The limbs divided
into great branches, and these into lesser and lesser branches, were
themselves once, when the tree was small, budding twigs; and this connexion
of the former and present buds by ramifying branches may well represent the
classification of all extinct and living species in groups subordinate to
groups. Of the many twigs which flourished when the tree was a mere bush,
only two or three, now grown into great branches, yet survive and bear all
the other branches; so with the species which lived during long-past
geological periods, very few now have living and modified descendants. From
the first growth of the tree, many a limb and branch has decayed and dropped
off; and these lost branches of various sizes may represent those whole
orders, families, and genera which have now no living representatives, and
which are known to us only from having been found in a fossil state. As we
here and there see a thin straggling branch springing from a fork low down
in a tree, and which by some chance has been favoured and is still alive on
its summit, so we occasionally see an animal like the Ornithorhynchus or
Lepidosiren, which in some small degree connects by its affinities two large
branches of life, and which has apparently been saved from fatal competition
by having inhabited a protected station. As buds give rise by growth to
fresh buds, and these, if vigorous, branch out and overtop on all sides many
a feebler branch, so by generation I believe it has been with the great Tree
of Life, which fills with its dead and broken branches the crust of the
earth, and covers the surface with its ever branching and beautiful
ramifications.

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Chapter 5 - Laws of Variation

[* ] Effects of external conditions [* ] Use and disuse, combined with
natural selection; organs of flight and of vision [* ] Acclimatisation [* ]
Correlation of growth [* ] Compensation and economy of growth [* ] False
correlations [* ] Multiple, rudimentary, and lowly organised structures
variable [* ] Parts developed in an unusual manner are highly variable:
specific character more variable than generic: secondary sexual characters
variable [* ] Species of the same genus vary in an analogous manner [* ]
Reversions to long-lost characters [* ] Summary
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I HAVE hitherto sometimes spoken as if the variations so common and
multiform in organic beings under domestication, and in a lesser degree in
those in a state of nature had been due to chance. This, of course, is a
wholly incorrect expression, but it serves to acknowledge plainly our
ignorance of the cause of each particular variation. Some authors believe it
to be as much the function of the reproductive system to produce individual
differences, or very slight deviations of structure, as to make the child
like its parents. But the much greater variability, as well as the greater
frequency of monstrosities, under domestication or cultivation, than under
nature, leads me to believe that deviations of structure are in some way due
to the nature of the conditions of life, to which the parents and their more
remote ancestors have been exposed during several generations. I have
remarked in the first chapter but a long catalogue of facts which cannot be
here given would be necessary to show the truth of the remark that the
reproductive system is eminently susceptible to changes in the conditions of
life; and to this system being functionally disturbed in the parents, I
chiefly attribute the varying or plastic condition of the offspring. The
male and female sexual elements seem to be affected before that union takes
place which is to form a new being. In the case of 'sporting' plants, the
bud, which in its earliest condition does not apparently differ essentially
from an ovule, is alone affected. But why, because the reproductive system
is disturbed, this or that part should vary more or less, we are profoundly
ignorant. Nevertheless, we can here and there dimly catch a faint ray of
light, and we may feel sure that there must be some cause for each deviation
of structure, however slight.

How much direct effect difference of climate, food, &c., produces on any
being is extremely doubtful. My impression is, that the effect is extremely
small in the case of animals, but perhaps rather more in that of plants. We
may, at least, safely conclude that such influences cannot have produced the
many striking and complex co-adaptations of structure between one organic
being and another, which we see everywhere throughout nature. Some little
influence may be attributed to climate, food, &c.: thus, E. Forbes speaks
confidently that shells at their southern limit, and when living in shallow
water, are more brightly coloured than those of the same species further
north or from greater depths. Gould believes that birds of the same species
are more brightly coloured under a clear atmosphere, than when living on
islands or near the coast. So with insects, Wollaston is convinced that
residence near the sea affects their colours. Moquin-Tandon gives a list of
plants which when growing near the sea-shore have their leaves in some
degree fleshy, though not elsewhere fleshy. Several other such cases could
be given.

The fact of varieties of one species, when they range into the zone of
habitation of other species, often acquiring in a very slight degree some of
the characters of such species, accords with our view that species of all
kinds are only well-marked and permanent varieties. Thus the species of
shells which are confined to tropical and shallow seas are generally
brighter-coloured than those confined to cold and deeper seas. The birds
which are confined to continents are, according to Mr Gould,
brighter-coloured than those of islands. The insect-species confined to
sea-coasts, as every collector knows, are often brassy or lurid. Plants
which live exclusively on the sea-side are very apt to have fleshy leaves.
He who believes in the creation of each species, will have to say that this
shell, for instance, was created with bright colours for a warm sea; but
that this other shell became bright-coloured by variation when it ranged
into warmer or shallower waters.

When a variation is of the slightest use to a being, we cannot tell how much
of it to attribute to the accumulative action of natural selection, and how
much to the conditions of life. Thus, it is well known to furriers that
animals of the same species have thicker and better fur the more severe the
climate is under which they have lived; but who can tell how much of this
difference may be due to the warmest-clad individuals having been favoured
and preserved during many generations, and how much to the direct action of
the severe climate? for it would appear that climate has some direct action
on the hair of our domestic quadrupeds.

Instances could be given of the same variety being produced under conditions
of life as different as can well be conceived; and, on the other hand, of
different varieties being produced from the same species under the same
conditions. Such facts show how indirectly the conditions of life must act.
Again, innumerable instances are known to every naturalist of species
keeping true, or not varying at all, although living under the most opposite
climates. Such considerations as these incline me to lay very little weight
on the direct action of the conditions of life. Indirectly, as already
remarked, they seem to play an important part in affecting the reproductive
system, and in thus inducing variability; and natural selection will then
accumulate all profitable variations, however slight, until they become
plainly developed and appreciable by us.

Effects of Use and Disuse

From the facts alluded to in the first chapter, I think there can be little
doubt that use in our domestic animals strengthens and enlarges certain
parts, and disuse diminishes them; and that such modifications are
inherited. Under free nature, we can have no standard of comparison, by
which to judge of the effects of long-continued use or disuse, for we know
not the parent-forms; but many animals have structures which can be
explained by the effects of disuse. As Professor Owen has remarked, there is
no greater anomaly in nature than a bird that cannot fly; yet there are
several in this state. The logger-headed duck of South America can only flap
along the surface of the water, and has its wings in nearly the same
condition as the domestic Aylesbury duck. As the larger ground-feeding birds
seldom take flight except to escape danger, I believe that the nearly
wingless condition of several birds, which now inhabit or have lately
inhabited several oceanic islands, tenanted by no beast of prey, has been
caused by disuse. The ostrich indeed inhabits continents and is exposed to
danger from which it cannot escape by flight, but by kicking it can defend
itself from enemies, as well as any of the smaller quadrupeds. We may
imagine that the early progenitor of the ostrich had habits like those of a
bustard, and that as natural selection increased in successive generations
the size and weight of its body, its legs were used more, and its wings
less, until they became incapable of flight.

Kirby has remarked (and I have observed the same fact) that the anterior
tarsi, or feet, of many male dung-feeding beetles are very often broken off;
he examined seventeen specimens in his own collection, and not one had even
a relic left. In the Onites apelles the tarsi are so habitually lost, that
the insect has been described as not having them. In some other genera they
are present, but in a rudimentary condition. In the Ateuchus or sacred
beetle of the Egyptians, they are totally deficient. There is not sufficient
evidence to induce us to believe that mutilations are ever inherited; and I
should prefer explaining the entire absence of the anterior tarsi in
Ateuchus, and their rudimentary condition in some other genera, by the
long-continued effects of disuse in their progenitors; for as the tarsi are
almost always lost in many dung-feeding beetles, they must be lost early in
life, and therefore cannot be much used by these insects.

In some cases we might easily put down to disuse modifications of structure
which are wholly, or mainly, due to natural selection. Mr. Wollaston has
discovered the remarkable fact that 200 beetles, out of the 550 species
inhabiting Madeira, are so far deficient in wings that they cannot fly; and
that of the twenty-nine endemic genera, no less than twenty-three genera
have all their species in this condition! Several facts, namely, that
beetles in many parts of the world are very frequently blown to sea and
perish; that the beetles in Madeira, as observed by Mr Wollaston, lie much
concealed, until the wind lulls and the sun shines; that the proportion of
wingless beetles is larger on the exposed Dezertas than in Madeira itself;
and especially the extraordinary fact, so strongly insisted on by Mr.
Wollaston, of the almost entire absence of certain large groups of beetles,
elsewhere excessively numerous, and which groups have habits of life almost
necessitating frequent flight; these several considerations have made me
believe that the wingless condition of so many Madeira beetles is mainly due
to the action of natural selection, but combined probably with disuse. For
during thousands of successive generations each individual beetle which flew
least, either from its wings having been ever so little less perfectly
developed or from indolent habit, will have had the best chance of surviving
from not being blown out to sea; and, on the other hand, those beetles which
most readily took to flight will oftenest have been blown to sea and thus
have been destroyed.

The insects in Madeira which are not ground-feeders, and which, as the
flower-feeding coleoptera and lepidoptera, must habitually use their wings
to gain their subsistence, have, as Mr. Wollaston suspects, their wings not
at all reduced, but even enlarged. This is quite compatible with the action
of natural selection. For when a new insect first arrived on the island, the
tendency of natural selection to enlarge or to reduce the wings, would
depend on whether a greater number of individuals were saved by successfully
battling with the winds, or by giving up the attempt and rarely or never
flying. As with mariners ship-wrecked near a coast, it would have been
better for the good swimmers if they had been able to swim still further,
whereas it would have been better for the bad swimmers if they had not been
able to swim at all and had stuck to the wreck.

The eyes of moles and of some burrowing rodents are rudimentary in size, and
in some cases are quite covered up by skin and fur. This state of the eyes
is probably due to gradual reduction from disuse, but aided perhaps by
natural selection. In South America, a burrowing rodent, the tuco-tuco, or
Ctenomys, is even more subterranean in its habits than the mole; and I was
assured by a Spaniard, who had often caught them, that they were frequently
blind; one which I kept alive was certainly in this condition, the cause, as
appeared on dissection, having been inflammation of the nictitating
membrane. As frequent inflammation of the eyes must be injurious to any
animal, and as eyes are certainly not indispensable to animals with
subterranean habits, a reduction in their size with the adhesion of the
eyelids and growth of fur over them, might in such case be an advantage; and
if so, natural selection would constantly aid the effects of disuse.

It is well known that several animals, belonging to the most different
classes, which inhabit the caves of Styria and of Kentucky, are blind. In
some of the crabs the foot-stalk for the eye remains, though the eye is
gone; the stand for the telescope is there, though the telescope with its
glasses has been lost. As it is difficult to imagine that eyes, though
useless, could be in any way injurious to animals living in darkness, I
attribute their loss wholly to disuse. In one of the blind animals, namely,
the cave-rat, the eyes are of immense size; and Professor Silliman thought
that it regained, after living some days in the light, some slight power of
vision. In the same manner as in Madeira the wings of some of the insects
have been enlarged, and the wings of others have been reduced by natural
selection aided by use and disuse, so in the case of the cave-rat natural
selection seems to have struggled with the loss of light and to have
increased the size of the eyes; whereas with all the other inhabitants of
the caves, disuse by itself seems to have done its work.

It is difficult to imagine conditions of life more similar than deep
limestone caverns under a nearly similar climate; so that on the common view
of the blind animals having been separately created for the American and
European caverns, close similarity in their organisation and affinities
might have been expected; but, as Schiödte and others have remarked, this is
not the case, and the cave-insects of the two continents are not more
closely allied than might have been anticipated from the general resemblance
of the other inhabitants of North America and Europe. On my view we must
suppose that American animals, having ordinary powers of vision, slowly
migrated by successive generations from the outer world into the deeper and
deeper recesses of the Kentucky caves, as did European animals into the
caves of Europe. We have some evidence of this gradation of habit; for, as
Schiödte remarks, 'animals not far remote from ordinary forms, prepare the
transition from light to darkness. Next follow those that are constructed
for twilight; and, last of all, those destined for total darkness.' By the
time that an animal had reached, after numberless generations, the deepest
recesses, disuse will on this view have more or less perfectly obliterated
its eyes, and natural selection will often have effected other changes, such
as an increase in the length of the antennae or palpi, as a compensation for
blindness. Notwithstanding such modifications, we might expect still to see
in the cave-animals of America, affinities to the other inhabitants of that
continent, and in those of Europe, to the inhabitants of the European
continent. And this is the case with some of the American cave-animals, as I
hear from Professor Dana; and some of the European cave-insects are very
closely allied to those of the surrounding country. It would be most
difficult to give any rational explanation of the affinities of the blind
cave-animals to the other inhabitants of the two continents on the ordinary
view of their independent creation. That several of the inhabitants of the
caves of the Old and New Worlds should be closely related, we might expect
from the well-known relationship of most of their other productions. Far
from feeling any surprise that some of the cave-animals should be very
anomalous, as Agassiz has remarked in regard to the blind fish, the
Amblyopsis, and as is the case with the blind Proteus with reference to the
reptiles of Europe, I am only surprised that more wrecks of ancient life
have not been preserved, owing to the less severe competition to which the
inhabitants of these dark abodes will probably have been exposed.

Acclimatisation

Habit is hereditary with plants, as in the period of flowering, in the
amount of rain requisite for seeds to germinate, in the time of sleep, &c.,
and this leads me to say a few words on acclimatisation. As it is extremely
common for species of the same genus to inhabit very hot and very cold
countries, and as I believe that all the species of the same genus have
descended from a single parent, if this view be correct, acclimatisation
must be readily effected during long-continued descent. It is notorious that
each species is adapted to the climate of its own home: species from an
arctic or even from a temperate region cannot endure a tropical climate, or
conversely. So again, many succulent plants cannot endure a damp climate.
But the degree of adaptation of species to the climates under which they
live is often overrated. We may infer this from our frequent inability to
predict whether or not an imported plant will endure our climate, and from
the number of plants and animals brought from warmer countries which here
enjoy good health. We have reason to believe that species in a state of
nature are limited in their ranges by the competition of other organic
beings quite as much as, or more than, by adaptation to particular climates.
But whether or not the adaptation be generally very close, we have evidence,
in the case of some few plants, of their becoming, to a certain extent,
naturally habituated to different temperatures, or becoming acclimatised:
thus the pines and rhododendrons, raised from seed collected by Dr Hooker
from trees growing at different heights on the Himalaya were found in this
country to possess different constitutional powers of resisting cold. Mr
Thwaites informs me that he has observed similar facts in Ceylon, and
analogous observations have been made by Mr H. C. Watson on European species
of plants brought from the Azores to England. In regard to animals, several
authentic cases could be given of species within historical times having
largely extended their range from warmer to cooler latitudes, and
conversely; but we do not positively know that these animals were strictly
adapted to their native climate, but in all ordinary cases we assume such to
be the case; nor do we know that they have subsequently become acclimatised
to their new homes.

As I believe that our domestic animals were originally chosen by uncivilised
man because they were useful and bred readily under confinement, and not
because they were subsequently found capable of far-extended transportation,
I think the common and extraordinary capacity in our domestic animals of not
only withstanding the most different climates but of being perfectly fertile
(a far severer test) under them, may be used as an argument that a large
proportion of other animals, now in a state of nature, could easily be
brought to bear widely different climates. We must not, however, push the
foregoing argument too far, on account of the probable origin of some of our
domestic animals from several wild stocks: the blood, for instance, of a
tropical and arctic wolf or wild dog may perhaps be mingled in our domestic
breeds. The rat and mouse cannot be considered as domestic animals, but they
have been transported by man to many parts of the world, and now have a far
wider range than any other rodent, living free under the cold climate of
Faroe in the north and of the Falklands in the south, and on many islands in
the torrid zones. Hence I am inclined to look at adaptation to any special
climate as a quality readily grafted on an innate wide flexibility of
constitution, which is common to most animals. On this view, the capacity of
enduring the most different climates by man himself and by his domestic
animals, and such facts as that former species of the elephant and
rhinoceros were capable of enduring a glacial climate, whereas the living
species are now all tropical or sub-tropical in their habits, ought not to
be looked at as anomalies, but merely as examples of a very common
flexibility of constitution, brought, under peculiar circumstances, into
play.

How much of the acclimatisation of species to any peculiar climate is due to
mere habit, and how much to the natural selection of varieties having
different innate constitutions, and how much to means combined, is a very
obscure question. That habit or custom has some influence I must believe,
both from analogy, and from the incessant advice given in agricultural
works, even in the ancient Encyclopaedias of China, to be very cautious in
transposing animals from one district to another; for it is not likely that
man should have succeeded in selecting so many breeds and sub-breeds with
constitutions specially fitted for their own districts: the result must, I
think, be due to habit. On the other hand, I can see no reason to doubt that
natural selection will continually tend to preserve those individuals which
are born with constitutions best adapted to their native countries. In
treatises on many kinds of cultivated plants, certain varieties are said to
withstand certain climates better than others: this is very strikingly shown
in works on fruit trees published in the United States, in which certain
varieties are habitually recommended for the northern, and others for the
southern States; and as most of these varieties are of recent origin, they
cannot owe their constitutional differences to habit. The case of the
Jerusalem artichoke, which is never propagated by seed, and of which
consequently new varieties have not been produced, has even been advanced
for it is now as tender as ever it was -- as proving that acclimatisation
cannot be effected! The case, also, of the kidney-bean has been often cited
for a similar purpose, and with much greater weight; but until some one will
sow, during a score of generations, his kidney-beans so early that a very
large proportion are destroyed by frost, and then collect seed from the few
survivors, with care to prevent accidental crosses, and then again get seed
from these seedlings, with the same precautions, the experiment cannot be
said to have been even tried. Nor let it be supposed that no differences in
the constitution of seedling kidney-beans ever appear, for an account has
been published how much more hardy some seedlings appeared to be than
others.

On the whole, I think we may conclude that habit, use, and disuse, have, in
some cases, played a considerable part in the modification of the
constitution, and of the structure of various organs; but that the effects
of use and disuse have often been largely combined with, and sometimes
overmastered by, the natural selection of innate differences.

Correlation of Growth

I mean by this expression that the whole organisation is so tied together
during its growth and development, that when slight variations in any one
part occur, and are accumulated through natural selection, other parts
become modified. This is a very important subject, most imperfectly
understood. The most obvious case is, that modifications accumulated solely
for the good of the young or larva, will, it may safely be concluded, affect
the structure of the adult; in the same manner as any malconformation
affecting the early embryo, seriously affects the whole organisation of the
adult. The several parts of the body which are homologous, and which, at an
early embryonic period, are alike, seem liable to vary in an allied manner:
we see this in the right and left sides of the body varying in the same
manner; in the front and hind legs, and even in the jaws and limbs, varying
together, for the lower jaw is believed to be homologous with the limbs.
These tendencies, I do not doubt, may be mastered more or less completely by
natural selection: thus a family of stags once existed with an antler only
on one side; and if this had been of any great use to the breed it might
probably have been rendered permanent by natural selection.

Homologous parts, as has been remarked by some authors, tend to cohere; this
is often seen in monstrous plants; and nothing is more common than the union
of homologous parts in normal structures, as the union of the petals of the
corolla into a tube. Hard parts seem to affect the form of adjoining soft
parts; it is believed by some authors that the diversity in the shape of the
pelvis in birds causes the remarkable diversity in the shape of their
kidneys. Others believe that the shape of the pelvis in the human mother
influences by pressure the shape of the head of the child. In snakes,
according to Schlegel, the shape of the body and the manner of swallowing
determine the position of several of the most important viscera.

The nature of the bond of correlation is very frequently quite obscure. M.
Is. Geoffroy St Hilaire has forcibly remarked, that certain malconformations
very frequently, and that others rarely coexist, without our being able to
assign any reason. What can be more singular than the relation between blue
eyes and deafness in cats, and the tortoise-shell colour with the female
sex; the feathered feet and skin between the outer toes in pigeons, and the
presence of more or less down on the young birds when first hatched, with
the future colour of their plumage; or, again, the relation between the hair
and teeth in the naked Turkish dog, though here probably homology comes into
play? With respect to this latter case of correlation, I think it can hardly
be accidental, that if we pick out the two orders of mammalia which are most
abnormal in their dermal coverings, viz. Cetacea (whales) and Edentata
(armadilloes, scaly ant-eaters, &c.), that these are likewise the most
abnormal in their teeth.

I know of no case better adapted to show the importance of the laws of
correlation in modifying important structures, independently of utility and,
therefore, of natural selection, than that of the difference between the
outer and inner flowers in some Compositous and Umbelliferous plants. Every
one knows the difference in the ray and central florets of, for instance,
the daisy, and this difference is often accompanied with the abortion of
parts of the flower. But, in some Compositous plants, the seeds also differ
in shape and sculpture; and even the ovary itself, with its accessory parts,
differs, as has been described by Cassini. These differences have been
attributed by some authors to pressure, and the shape of the seeds in the
ray-florets in some Compositae countenances this idea; but, in the case of
the corolla of the Umbelliferae, it is by no means, as Dr Hooker informs me,
in species with the densest heads that the inner and outer flowers most
frequently differ. It might have been thought that the development of the
ray-petals by drawing nourishment from certain other parts of the flower had
caused their abortion; but in some Compositae there is a difference in the
seeds of the outer and inner florets without any difference in the corolla.
Possibly, these several differences may be connected with some difference in
the flow of nutriment towards the central and external flowers: we know, at
least, that in irregular flowers, those nearest to the axis are oftenest
subject to peloria, and become regular. I may add, as an instance of this,
and of a striking case of correlation, that I have recently observed in some
garden pelargoniums, that the central flower of the truss often loses the
patches of darker colour in the two upper petals; and that when this occurs,
the adherent nectary is quite aborted; when the colour is absent from only
one of the two upper petals, the nectary is only much shortened.

With respect to the difference in the corolla of the central and exterior
flowers of a head or umbel, I do not feel at all sure that C. C. Sprengel's
idea that the ray-florets serve to attract insects, whose agency is highly
advantageous in the fertilisation of plants of these two orders, is so
far-fetched, as it may at first appear: and if it be advantageous, natural
selection may have come into play. But in regard to the differences both in
the internal and external structure of the seeds, which are not always
correlated with any differences in the flowers, it seems impossible that
they can be in any way advantageous to the plant: yet in the Umbelliferae
these differences are of such apparent importance the seeds being in some
cases, according to Tausch, orthospermous in the exterior flowers and
coelospermous in the central flowers, that the elder De Candolle founded his
main divisions of the order on analogous differences. Hence we see that
modifications of structure, viewed by systematists as of high value, may be
wholly due to unknown laws of correlated growth, and without being, as far
as we can see, of the slightest service to the species.

We may often falsely attribute to correlation of growth, structures which
are common to whole groups of species, and which in truth are simply due to
inheritance; for an ancient progenitor may have acquired through natural
selection some one modification in structure, and, after thousands of
generations, some other and independent modification; and these two
modifications, having been transmitted to a whole group of descendants with
diverse habits, would naturally be thought to be correlated in some
necessary manner. So, again, I do not doubt that some apparent correlations,
occurring throughout whole orders, are entirely due to the manner alone in
which natural selection can act. For instance, Alph. De Candolle has
remarked that winged seeds are never found in fruits which do not open: I
should explain the rule by the fact that seeds could not gradually become
winged through natural selection, except in fruits which opened; so that the
individual plants producing seeds which were a little better fitted to be
wafted further, might get an advantage over those producing seed less fitted
for dispersal; and this process could not possibly go on in fruit which did
not open.

The elder Geoffroy and Goethe propounded, at about the same period, their
law of compensation or balancement of growth; or, as Goethe expressed it,
'in order to spend on one side, nature is forced to economise on the other
side.' I think this holds true to a certain extent with our domestic
productions: if nourishment flows to one part or organ in excess, it rarely
flows, at least in excess, to another part; thus it is difficult to get a
cow to give much milk and to fatten readily. The same varieties of the
cabbage do not yield abundant and nutritious foliage and a copious supply of
oil-bearing seeds. When the seeds in our fruits become atrophied, the fruit
itself gains largely in size and quality. In our poultry, a large tuft of
feathers on the head is generally accompanied by a diminished comb, and a
large beard by diminished wattles. With species in a state of nature it can
hardly be maintained that the law is of universal application; but many good
observers, more especially botanists, believe in its truth. I will not,
however, here give any instances, for I see hardly any way of distinguishing
between the effects, on the one hand, of a part being largely developed
through natural selection and another and adjoining part being reduced by
this same process or by disuse, and, on the other hand, the actual
withdrawal of nutriment from one part owing to the excess of growth in
another and adjoining part.

I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more general
principle, namely, that natural selection is continually trying to economise
in every part of the organisation. If under changed conditions of life a
structure before useful becomes less useful, any diminution, however slight,
in its development, will be seized on by natural selection, for it will
profit the individual not to have its nutriment wasted in building up an
useless structure. I can thus only understand a fact with which I was much
struck when examining cirripedes, and of which many other instances could be
given: namely, that when a cirripede is parasitic within another and is thus
protected, it loses more or less completely its own shell or carapace. This
is the case with the male Ibla, and in a truly extraordinary manner with the
Proteolepas: for the carapace in all other cirripedes consists of the three
highly-important anterior segments of the head enormously developed, and
furnished with great nerves and muscles; but in the parasitic and protected
Proteolepas, the whole anterior part of the head is reduced to the merest
rudiment attached to the bases of the prehensile antennae. Now the saving of
a large and complex structure, when rendered superfluous by the parasitic
habits of the Proteolepas, though effected by slow steps, would be a decided
advantage to each successive individual of the species; for in the struggle
for life to which every animal is exposed, each individual Proteolepas would
have a better chance of supporting itself, by less nutriment being wasted in
developing a structure now become useless.

Thus, as I believe, natural selection will always succeed in the long run in
reducing and saving every part of the organisation, as soon as it is
rendered superfluous, without by any means causing some other part to be
largely developed in a corresponding degree. And, conversely, that natural
selection may perfectly well succeed in largely developing any organ,
without requiring as a necessary compensation the reduction of some
adjoining part.

It seems to be a rule, as remarked by Is. Geoffroy St Hilaire, both in
varieties and in species, that when any part or organ is repeated many times
in the structure of the same individual (as the vertebrae in snakes, and the
stamens in polyandrous flowers) the number is variable; whereas the number
of the same part or organ, when it occurs in lesser numbers, is constant.
The same author and some botanists have further remarked that multiple parts
are also very liable to variation in structure. Inasmuch as this 'vegetative
repetition,' to use Prof. Owen's expression, seems to be a sign of low
organisation; the foregoing remark seems connected with the very general
opinion of naturalists, that beings low in the scale of nature are more
variable than those which are higher. I presume that lowness in this case
means that the several parts of the organisation have been but little
specialised for particular functions; and as long as the same part has to
perform diversified work, we can perhaps see why it should remain variable,
that is, why natural selection should have preserved or rejected each little
deviation of form less carefully than when the part has to serve for one
special purpose alone. In the same way that a knife which has to cut all
sorts of things may be of almost any shape; whilst a tool for some
particular object had better be of some particular shape. Natural selection,
it should never be forgotten, can act on each part of each being, solely
through and for its advantage.

Rudimentary parts, it has been stated by some authors, and I believe with
truth, are apt to be highly variable. We shall have to recur to the general
subject of rudimentary and aborted organs; and I will here only add that
their variability seems to be owing to their uselessness, and therefore to
natural selection having no power to check deviations in their structure.
Thus rudimentary parts are left to the free play of the various laws of
growth, to the effects of long-continued disuse, and to the tendency to
reversion.

A part developed in any species in an extraordinary degree or manner, in
comparison with the same part in allied species, tends to be highly
variable.

Several years ago I was much struck with a remark, nearly to the above
effect, published by Mr Waterhouse. I infer also from an observation made by
Professor Owen, with respect to the length of the arms of the ourang-outang,
that he has come to a nearly similar conclusion. It is hopeless to attempt
to convince any one of the truth of this proposition without giving the long
array of facts which I have collected, and which cannot possibly be here
introduced. I can only state my conviction that it is a rule of high
generality. I am aware of several causes of error, but I hope that I have
made due allowance for them. It should be understood that the rule by no
means applies to any part, however unusually developed, unless it be
unusually developed in comparison with the same part in closely allied
species. Thus, the bat's wing is a most abnormal structure in the class
mammalia; but the rule would not here apply, because there is a whole group
of bats having wings; it would apply only if some one species of bat had its
wings developed in some remarkable manner in comparison with the other
species of the same genus. The rule applies very strongly in the case of
secondary sexual characters, when displayed in any unusual manner. The term,
secondary sexual characters, used by Hunter, applies to characters which are
attached to one sex, but are not directly connected with the act of
reproduction. The rule applies to males and females; but as females more
rarely offer remarkable secondary sexual characters, it applies more rarely
to them. The rule being so plainly applicable in the case of secondary
sexual characters, may be due to the great variability of these characters,
whether or not displayed in any unusual manner of which fact I think there
can be little doubt. But that our rule is not confined to secondary sexual
characters is clearly shown in the case of hermaphrodite cirripedes; and I
may here add, that I particularly attended to Mr. Waterhouse's remark,
whilst investigating this Order, and I am fully convinced that the rule
almost invariably holds good with cirripedes. I shall, in my future work,
give a list of the more remarkable cases; I will here only briefly give one,
as it illustrates the rule in its largest application. The opercular valves
of sessile cirripedes (rock barnacles) are, in every sense of the word, very
important structures, and they differ extremely little even in different
genera; but in the several species of one genus, Pyrgoma, these valves
present a marvellous amount of diversification: the homologous valves in the
different species being sometimes wholly unlike in shape; and the amount of
variation in the individuals of several of the species is so great, that it
is no exaggeration to state that the varieties differ more from each other
in the characters of these important valves than do other species of
distinct genera.

As birds within the same country vary in a remarkably small degree, I have
particularly attended to them, and the rule seems to me certainly to hold
good in this class. I cannot make out that it applies to plants, and this
would seriously have shaken my belief in its truth, had not the great
variability in plants made it particularly difficult to compare their
relative degrees of variability.

When we see any part or organ developed in a remarkable degree or manner in
any species, the fair presumption is that it is of high importance to that
species; nevertheless the part in this case is eminently liable to
variation. Why should this be so? On the view that each species has been
independently created, with all its parts as we now see them, I can see no
explanation. But on the view that groups of species have descended from
other species, and have been modified through natural selection, I think we
can obtain some light. In our domestic animals, if any part, or the whole
animal, be neglected and no selection be applied, that part (for instance,
the comb in the Dorking fowl) or the whole breed will cease to have a nearly
uniform character. The breed will then be said to have degenerated. In
rudimentary organs, and in those which have been but little specialized for
any particular purpose, and perhaps in polymorphic groups, we see a nearly
parallel natural case; for in such cases natural selection either has not or
cannot come into full play, and thus the organisation is left in a
fluctuating condition. But what here more especially concerns us is, that in
our domestic animals those points, which at the present time are undergoing
rapid change by continued selection, are also eminently liable to variation.
Look at the breeds of the pigeon; see what a prodigious amount of difference
there is in the beak of the different tumblers, in the beak and wattle of
the different carriers, in the carriage and tail of our fantails, &c., these
being the points now mainly attended to by English fanciers. Even in the
sub-breeds, as in the short-faced tumbler, it is notoriously difficult to
breed them nearly to perfection, and frequently individuals are born which
depart widely from the standard. There may be truly said to be a constant
struggle going on between, on the one hand, the tendency to reversion to a
less modified state, as well as an innate tendency to further variability of
all kinds, and, on the other hand, the power of steady selection to keep the
breed true. In the long run selection gains the day, and we do not expect to
fail so far as to breed a bird as coarse as a common tumbler from a good
short-faced strain. But as long as selection is rapidly going on, there may
always be expected to be much variability in the structure undergoing
modification. It further deserves notice that these variable characters,
produced by man's selection, sometimes become attached, from causes quite
unknown to us, more to one sex than to the other, generally to the male sex,
as with the wattle of carriers and the enlarged crop of pouters.

Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other species of
the same genus, we may conclude that this part has undergone an
extraordinary amount of modification, since the period when the species
branched off from the common progenitor of the genus. This period will
seldom be remote in any extreme degree, as species very rarely endure for
more than one geological period. An extraordinary amount of modification
implies an unusually large and long-continued amount of variability, which
has continually been accumulated by natural selection for the benefit of the
species. But as the variability of the extraordinarily-developed part or
organ has been so great and long-continued within a period not excessively
remote, we might, as a general rule, expect still to find more variability
in such parts than in other parts of the organisation, which have remained
for a much longer period nearly constant. And this, I am convinced, is the
case. That the struggle between natural selection on the one hand, and the
tendency to reversion and variability on the other hand, will in the course
of time cease; and that the most abnormally developed organs may be made
constant, I can see no reason to doubt. Hence when an organ, however
abnormal it may be, has been transmitted in approximately the same condition
to many modified descendants, as in the case of the wing of the bat, it must
have existed, according to my theory, for an immense period in nearly the
same state; and thus it comes to be no more variable than any other
structure. It is only in those cases in which the modification has been
comparatively recent and extraordinarily great that we ought to find the
generative variability, as it may be called, still present in a high degree.
For in this case the variability will seldom as yet have been fixed by the
continued selection of the individuals varying in the required manner and
degree, and by the continued rejection of those tending to revert to a
former and less modified condition.

The principle included in these remarks may be extended. It is notorious
that specific characters are more variable than generic. To explain by a
simple example what is meant. If some species in a large genus of plants had
blue flowers and some had red, the colour would be only a specific
character, and no one would be surprised at one of the blue species varying
into red, or conversely; but if all the species had blue flowers, the colour
would become a generic character, and its variation would be a more unusual
circumstance. I have chosen this example because an explanation is not in
this case applicable, which most naturalists would advance, namely, that
specific characters are more variable than generic, because they are taken
from parts of less physiological importance than those commonly used for
classing genera. I believe this explanation is partly, yet only indirectly,
true; I shall, however, have to return to this subject in our chapter on
Classification. It would be almost superfluous to adduce evidence in support
of the above statement, that specific characters are more variable than
generic; but I have repeatedly noticed in works on natural history, that
when an author has remarked with surprise that some important organ or part,
which is generally very constant throughout large groups of species, has
differed considerably in closely-allied species, that it has, also, been
variable in the individuals of some of the species. And this fact shows that
a character, which is generally of generic value, when it sinks in value and
becomes only of specific value, often becomes variable, though its
physiological importance may remain the same. Something of the same kind
applies to monstrosities: at least Is. Geoffroy St. Hilaire seems to
entertain no doubt, that the more an organ normally differs in the different
species of the same group, the more subject it is to individual anomalies.

On the ordinary view of each species having been independently created, why
should that part of the structure, which differs from the same part in other
independently-created species of the same genus, be more variable than those
parts which are closely alike in the several species? I do not see that any
explanation can be given. But on the view of species being only strongly
marked and fixed varieties, we might surely expect to find them still often
continuing to vary in those parts of their structure which have varied
within a moderately recent period, and which have thus come to differ. Or to
state the case in another manner: the points in which all the species of a
genus resemble each other, and in which they differ from the species of some
other genus, are called generic characters; and these characters in common I
attribute to inheritance from a common progenitor, for it can rarely have
happened that natural selection will have modified several species, fitted
to more or less widely-different habits, in exactly the same manner: and as
these so-called generic characters have been inherited from a remote period,
since that period when the species first branched off from their common
progenitor, and subsequently have not varied or come to differ in any
degree, or only in a slight degree, it is not probable that they should vary
at the present day. On the other hand, the points in which species differ
from other species of the same genus, are called specific characters; and as
these specific characters have varied and come to differ within the period
of the branching off of the species from a common progenitor, it is probable
that they should still often be in some degree variable, at least more
variable than those parts of the organisation which have for a very long
period remained constant.

In connexion with the present subject, I will make only two other remarks. I
think it will be admitted, without my entering on details, that secondary
sexual characters are very variable; I think it also will be admitted that
species of the same group differ from each other more widely in their
secondary sexual characters, than in other parts of their organisation;
compare, for instance, the amount of difference between the males of
gallinaceous birds, in which secondary sexual characters are strongly
displayed, with the amount of difference between their females; and the
truth of this proposition will be granted. The cause of the original
variability of secondary sexual characters is not manifest; but we can see
why these characters should not have been rendered as constant and uniform
as other parts of the organisation; for secondary sexual characters have
been accumulated by sexual selection, which is less rigid in its action than
ordinary selection, as it does not entail death, but only gives fewer
offspring to the less favoured males. Whatever the cause may be of the
variability of secondary sexual characters, as they are highly variable,
sexual selection will have had a wide scope for action, and may thus readily
have succeeded in giving to the species of the same group a greater amount
of difference in their sexual characters, than in other parts of their
structure.

It is a remarkable fact, that the secondary sexual differences between the
two sexes of the same species are generally displayed in the very same parts
of the organisation in which the different species of the same genus differ
from each other. Of this fact I will give in illustration two instances, the
first which happen to stand on my list; and as the differences in these
cases are of a very unusual nature, the relation can hardly be accidental.
The same number of joints in the tarsi is a character generally common to
very large groups of beetles, but in the Engidae, as Westwood has remarked,
the number varies greatly; and the number likewise differs in the two sexes
of the same species: again in fossorial hymenoptera, the manner of neuration
of the wings is a character of the highest importance, because common to
large groups; but in certain genera the neuration differs in the different
species, and likewise in the two sexes of the same species. This relation
has a clear meaning on my view of the subject: I look at all the species of
the same genus as having as certainly descended from the same progenitor, as
have the two sexes of any one of the species. Consequently, whatever part of
the structure of the common progenitor, or of its early descendants, became
variable; variations of this part would it is highly probable, be taken
advantage of by natural and sexual selection, in order to fit the several
species to their several places in the economy of nature, and likewise to
fit the two sexes of the same species to each other, or to fit the males and
females to different habits of life, or the males to struggle with other
males for the possession of the females.

Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of generic
characters, or those which the species possess in common; that the frequent
extreme variability of any part which is developed in a species in an
extraordinary manner in comparison with the same part in its congeners; and
the not great degree of variability in a part, however extraordinarily it
may be developed, if it be common to a whole group of species; that the
great variability of secondary sexual characters, and the great amount of
difference in these same characters between closely allied species; that
secondary sexual and ordinary specific differences are generally displayed
in the same parts of the organisation, are all principles closely connected
together. All being mainly due to the species of the same group having
descended from a common progenitor, from whom they have inherited much in
common, to parts which have recently and largely varied being more likely
still to go on varying than parts which have long been inherited and have
not varied, to natural selection having more or less completely, according
to the lapse of time, overmastered the tendency to reversion and to further
variability, to sexual selection being less rigid than ordinary selection,
and to variations in the same parts having been accumulated by natural and
sexual selection, and thus adapted for secondary sexual, and for ordinary
specific purposes.

Distinct species present analogous variations; and a variety of one species
often assumes some of the characters of an allied species, or reverts to
some of the characters of an early progenitor.

These propositions will be most readily understood by looking to our
domestic races. The most distinct breeds of pigeons, in countries most
widely apart, present sub-varieties with reversed feathers on the head and
feathers on the feet, characters not possessed by the aboriginal
rock-pigeon; these then are analogous variations in two or more distinct
races. The frequent presence of fourteen or even sixteen tail-feathers in
the pouter, may be considered as a variation representing the normal
structure of another race, the fantail. I presume that no one will doubt
that all such analogous variations are due to the several races of the
pigeon having inherited from a common parent the same constitution and
tendency to variation, when acted on by similar unknown influences. In the
vegetable kingdom we have a case of analogous variation, in the enlarged
stems, or roots as commonly called, of the Swedish turnip and Ruta baga,
plants which several botanists rank as varieties produced by cultivation
from a common parent: if this be not so, the case will then be one of
analogous variation in two so-called distinct species; and to these a third
may be added, namely, the common turnip. According to the ordinary view of
each species having been independently created, we should have to attribute
this similarity in the enlarged stems of these three plants, not to the vera
causa of community of descent, and a consequent tendency to vary in a like
manner, but to three separate yet closely related acts of creation.

With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars on the
wings, a white rump, a bar at the end of the tail, with the outer feathers
externally edged near their bases with white. As all these marks are
characteristic of the parent rock-pigeon, I presume that no one will doubt
that this is a case of reversion, and not of a new yet analogous variation
appearing in the several breeds. We may I think confidently come to this
conclusion, because, as we have seen, these coloured marks are eminently
liable to appear in the crossed offspring of two distinct and differently
coloured breeds; and in this case there is nothing in the external
conditions of life to cause the reappearance of the slaty-blue, with the
several marks, beyond the influence of the mere act of crossing on the laws
of inheritance.

No doubt it is a very surprising fact that characters should reappear after
having been lost for many, perhaps for hundreds of generations. But when a
breed has been crossed only once by some other breed, the offspring
occasionally show a tendency to revert in character to the foreign breed for
many generations some say, for a dozen or even a score of generations. After
twelve generations, the proportion of blood, to use a common expression, of
any one ancestor, is only 1 in 2048; and yet, as we see, it is generally
believed that a tendency to reversion is retained by this very small
proportion of foreign blood. In a breed which has not been crossed, but in
which both parents have lost some character which their progenitor
possessed, the tendency, whether strong or weak, to reproduce the lost
character might be, as was formerly remarked, for all that we can see to the
contrary, transmitted for almost any number of generations. When a character
which has been lost in a breed, reappears after a great number of
generations, the most probable hypothesis is, not that the offspring
suddenly takes after an ancestor some hundred generations distant, but that
in each successive generation there has been a tendency to reproduce the
character in question, which at last, under unknown favourable conditions,
gains an ascendancy. For instance, it is probable that in each generation of
the barb-pigeon, which produces most rarely a blue and black-barred bird,
there has been a tendency in each generation in the plumage to assume this
colour. This view is hypothetical, but could be supported by some facts; and
I can see no more abstract improbability in a tendency to produce any
character being inherited for an endless number of generations, than in
quite useless or rudimentary organs being, as we all know them to be, thus
inherited. Indeed, we may sometimes observe a mere tendency to produce a
rudiment inherited: for instance, in the common snapdragon (Antirrhinum) a
rudiment of a fifth stamen so often appears, that this plant must have an
inherited tendency to produce it.

As all the species of the same genus are supposed, on my theory, to have
descended from a common parent, it might be expected that they would
occasionally vary in an analogous manner; so that a variety of one species
would resemble in some of its characters another species; this other species
being on my view only a well-marked and permanent variety. But characters
thus gained would probably be of an unimportant nature, for the presence of
all important characters will be governed by natural selection, in
accordance with the diverse habits of the species, and will not be left to
the mutual action of the conditions of life and of a similar inherited
constitution. It might further be expected that the species of the same
genus would occasionally exhibit reversions to lost ancestral characters.
As, however, we never know the exact character of the common ancestor of a
group, we could not distinguish these two cases: if, for instance, we did
not know that the rock-pigeon was not feather-footed or turn-crowned, we
could not have told, whether these characters in our domestic breeds were
reversions or only analogous variations; but we might have inferred that the
blueness was a case of reversion, from the number of the markings, which are
correlated with the blue tint, and which it does not appear probable would
all appear together from simple variation. More especially we might have
inferred this, from the blue colour and marks so often appearing when
distinct breeds of diverse colours are crossed. Hence, though under nature
it must generally be left doubtful, what cases are reversions to an
anciently existing character, and what are new but analogous variations, yet
we ought, on my theory, sometimes to find the varying offspring of a species
assuming characters (either from reversion or from analogous variation)
which already occur in some members of the same group. And this undoubtedly
is the case in nature.

A considerable part of the difficulty in recognising a variable species in
our systematic works, is due to its varieties mocking, as it were, come of
the other species of the same genus. A considerable catalogue, also, could
be given of forms intermediate between two other forms, which themselves
must be doubtfully ranked as either varieties or species, that the one in
varying has assumed some of the characters of the other, so as to produce
the intermediate form. But the best evidence is afforded by parts or organs
of an important and uniform nature occasionally varying so as to acquire, in
some degree, the character of the same part or organ in an allied species. I
have collected a long list of such cases; but here, as before, I lie under a
great disadvantage in not being able to give them. I can only repeat that
such cases certainly do occur, and seem to me very remarkable.

I will, however, give one curious and complex case, not indeed as affecting
any important character, but from occurring in several species of the same
genus, partly under domestication and partly under nature. It is a case
apparently of reversion. The ass not rarely has very distinct transverse
bars on its legs, like those of a zebra: it has been asserted that these are
plainest in the foal, and from inquiries which I have made, I believe this
to be true. It has also been asserted that the stripe on each shoulder is
sometimes double. The shoulder-stripe is certainly very variable in length
and outline. A white ass, but not an albino, has been described without
either spinal or shoulder-stripe; and these stripes are sometimes very
obscure, or actually quite lost, in dark-coloured asses. The koulan of
Pallas is said to have been seen with a double shoulder-stripe; but traces
of it, as stated by Mr Blyth and others, occasionally appear: and I have
been informed by Colonel Poole that foals of this species are generally
striped on the legs, and faintly on the shoulder. The quagga, though so
plainly barred like a zebra over the body, is without bars on the legs; but
Dr Gray has figured one specimen with very distinct zebra-like bars on the
hocks.

With respect to the horse, I have collected cases in England of the spinal
stripe in horses of the most distinct breeds, and of all colours; transverse
bars on the legs are not rare in duns, mouse-duns, and in one instance in a
chestnut: a faint shoulder-stripe may sometimes be seen in duns, and I have
seen a trace in a bay horse. My son made a careful examination and sketch
for me of a dun Belgian cart-horse with a double stripe on each shoulder and
with leg-stripes; and a man, whom I can implicitly trust, has examined for
me a small dun Welch pony with three short parallel stripes on each
shoulder.

In the north-west part of India the Kattywar breed of horses is so generally
striped, that, as I hear from Colonel Poole, who examined the breed for the
Indian Government, a horse without stripes is not considered as purely-bred.
The spine is always striped; the legs are generally barred; and the
shoulder-stripe, which is sometimes double and sometimes treble, is common;
the side of the face, moreover, is sometimes striped. The stripes are
plainest in the foal; and sometimes quite disappear in old horses. Colonel
Poole has seen both gray and bay Kattywar horses striped when first foaled.
I have, also, reason to suspect, from information given me by Mr. W. W.
Edwards, that with the English race-horse the spinal stripe is much commoner
in the foal than in the full-grown animal. Without here entering on further
details, I may state that I have collected cases of leg and shoulder stripes
in horses of very different breeds, in various countries from Britain to
Eastern China; and from Norway in the north to the Malay Archipelago in the
south. In all parts of the world these stripes occur far oftenest in duns
and mouse-duns; by the term dun a large range of colour is included, from
one between brown and black to a close approach to cream-colour.

I am aware that Colonel Hamilton Smith, who has written on this subject,
believes that the several breeds of the horse have descended from several
aboriginal species one of which, the dun, was striped; and that the
above-described appearances are all due to ancient crosses with the dun
stock. But I am not at all satisfied with this theory, and should be loth to
apply it to breeds so distinct as the heavy Belgian cart-horse, Welch
ponies, cobs, the lanky Kattywar race, &c., inhabiting the most distant
parts of the world.

Now let us turn to the effects of crossing the several species of the
horse-genus. Rollin asserts, that the common mule from the ass and horse is
particularly apt to have bars on its legs. I once saw a mule with its legs
so much striped that any one at first would have thought that it must have
been the product of a zebra; and Mr. W. C. Martin, in his excellent treatise
on the horse, has given a figure of a similar mule. In four coloured
drawings, which I have seen, of hybrids between the ass and zebra, the legs
were much more plainly barred than the rest of the body; and in one of them
there was a double shoulder-stripe. In Lord Moreton's famous hybrid from a
chestnut mare and male quagga, the hybrid, and even the pure offspring
subsequently produced from the mare by a black Arabian sire, were much more
plainly barred across the legs than is even the pure quagga. Lastly, and
this is another most remarkable case, a hybrid has been figured by Dr Gray
(and he informs me that he knows of a second case) from the ass and the
hemionus; and this hybrid, though the ass seldom has stripes on its legs and
the hemionus has none and has not even a shoulder-stripe, nevertheless had
all four legs barred, and had three short shoulder-stripes, like those on
the dun Welch pony, and even had some zebra-like stripes on the sides of its
face. With respect to this last fact, I was so convinced that not even a
stripe of colour appears from what would commonly be called an accident,
that I was led solely from the occurrence of the face-stripes on this hybrid
from the ass and hemionus, to ask Colonel Poole whether such face-stripes
ever occur in the eminently striped Kattywar breed of horses, and was, as we
have seen, answered in the affirmative.

What now are we to say to these several facts? We see several very distinct
species of the horse-genus becoming, by simple variation, striped on the
legs like a zebra, or striped on the shoulders like an ass. In the horse we
see this tendency strong whenever a dun tint appears a tint which approaches
to that of the general colouring of the other species of the genus. The
appearance of the stripes is not accompanied by any change of form or by any
other new character. We see this tendency to become striped most strongly
displayed in hybrids from between several of the most distinct species. Now
observe the case of the several breeds of pigeons: they are descended from a
pigeon (including two or three sub-species or geographical races) of a
bluish colour, with certain bars and other marks; and when any breed assumes
by simple variation a bluish tint, these bars and other marks invariably
reappear; but without any other change of form or character. When the oldest
and truest breeds of various colours are crossed, we see a strong tendency
for the blue tint and bars and marks to reappear in the mongrels. I have
stated that the most probable hypothesis to account for the reappearance of
very ancient characters, is that there is a tendency in the young of each
successive generation to produce the long-lost character, and that this
tendency, from unknown causes, sometimes prevails. And we have just seen
that in several species of the horse-genus the stripes are either plainer or
appear more commonly in the young than in the old. Call the breeds of
pigeons, some of which have bred true for centuries, species; and how
exactly parallel is the case with that of the species of the horse-genus!
For myself, I venture confidently to look back thousands on thousands of
generations, and I see an animal striped like a zebra, but perhaps otherwise
very differently constructed, the common parent of our domestic horse,
whether or not it be descended from one or more wild stocks, of the ass, the
hemionus, quagga, and zebra.

He who believes that each equine species was independently created, will, I
presume, assert that each species has been created with a tendency to vary,
both under nature and under domestication, in this particular manner, so as
often to become striped like other species of the genus; and that each has
been created with a strong tendency, when crossed with species inhabiting
distant quarters of the world, to produce hybrids resembling in their
stripes, not their own parents, but other species of the genus. To admit
this view is, as it seems to me, to reject a real for an unreal, or at least
for an unknown, cause. It makes the works of God a mere mockery and
deception; I would almost as soon believe with the old and ignorant
cosmogonists, that fossil shells had never lived, but had been created in
stone so as to mock the shells now living on the sea-shore.

Summary

Our ignorance of the laws of variation is profound. Not in one case out of a
hundred can we pretend to assign any reason why this or that part differs,
more or less, from the same part in the parents. But whenever we have the
means of instituting a comparison, the same laws appear to have acted in
producing the lesser differences between varieties of the same species, and
the greater differences between species of the same genus. The external
conditions of life, as climate and food, &c., seem to have induced some
slight modifications. Habit in producing constitutional differences, and use
in strengthening, and disuse in weakening and diminishing organs, seem to
have been more potent in their effects. Homologous parts tend to vary in the
same way, and homologous parts tend to cohere. Modifications in hard parts
and in external parts sometimes affect softer and internal parts. When one
part is largely developed, perhaps it tends to draw nourishment from the
adjoining parts; and every part of the structure which can be saved without
detriment to the individual, will be saved. Changes of structure at an early
age will generally affect parts subsequently developed; and there are very
many other correlations of growth, the nature of which we are utterly unable
to understand. Multiple parts are variable in number and in structure,
perhaps arising from such parts not having been closely specialized to any
particular function, so that their modifications have not been closely
checked by natural selection. It is probably from this same cause that
organic beings low in the scale of nature are more variable than those which
have their whole organisation more specialized, and are higher in the scale.
Rudimentary organs, from being useless, will be disregarded by natural
selection, and hence probably are variable. Specific characters that is, the
characters which have come to differ since the several species of the same
genus branched off from a common parent are more variable than generic
characters, or those which have long been inherited, and have not differed
within this same period. In these remarks we have referred to special parts
or organs being still variable, because they have recently varied and thus
come to differ; but we have also seen in the second Chapter that the same
principle applies to the whole individual; for in a district where many
species of any genus are found that is, where there has been much former
variation and differentiation, or where the manufactory of new specific
forms has been actively at work there, on an average, we now find most
varieties or incipient species. Secondary sexual characters are highly
variable, and such characters differ much in the species of the same group.
Variability in the same parts of the organisation has generally been taken
advantage of in giving secondary sexual differences to the sexes of the same
species, and specific differences to the several species of the same genus.
Any part or organ developed to an extraordinary size or in an extraordinary
manner, in comparison with the same part or organ in the allied species,
must have gone through an extraordinary amount of modification since the
genus arose; and thus we can understand why it should often still be
variable in a much higher degree than other parts; for variation is a
long-continued and slow process, and natural selection will in such cases
not as yet have had time to overcome the tendency to further variability and
to reversion to a less modified state. But when a species with any
extraordinarily-developed organ has become the parent of many modified
descendants which on my view must be a very slow process, requiring a long
lapse of time in this case, natural selection may readily have succeeded in
giving a fixed character to the organ, in however extraordinary a manner it
may be developed. Species inheriting nearly the same constitution from a
common parent and exposed to similar influences will naturally tend to
present analogous variations, and these same species may occasionally revert
to some of the characters of their ancient progenitors. Although new and
important modifications may not arise from reversion and analogous
variation, such modifications will add to the beautiful and harmonious
diversity of nature.

Whatever the cause may be of each slight difference in the offspring from
their parents and a cause for each must exist it is the steady accumulation,
through natural selection, of such differences, when beneficial to the
individual, that gives rise to all the more important modifications of
structure, by which the innumerable beings on the face of this earth are
enabled to struggle with each other, and the best adapted to survive.

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Chapter 6 - Difficulties on Theory

[* ] Difficulties on the theory of descent with modification [* ]
Transitions [* ] Absence or rarity of transitional varieties [* ]
Transitions in habits of life [* ] Diversified habits in the same species
[* ] Species with habits widely different from those of their allies [* ]
Organs of extreme perfection [* ] Means of transition [* ] Cases of
difficulty [* ] Natura non facit saltum [* ] Organs of small importance [* ]
Organs not in all cases absolutely perfect [* ] The law of Unity of Type and
of the Conditions of Existence embraced by the theory of Natural Selection
----------------------------------------------------------------------------
LONG before having arrived at this part of my work, a crowd of difficulties
will have occurred to the reader. Some of them are so grave that to this day
I can never reflect on them without being staggered; but, to the best of my
judgment, the greater number are only apparent, and those that are real are
not, I think, fatal to my theory.

These difficulties and objections may be classed under the following
heads:-Firstly, why, if species have descended from other species by
insensibly fine gradations, do we not everywhere see innumerable
transitional forms? Why is not all nature in confusion instead of the
species being, as we see them, well defined?

Secondly, is it possible that an animal having, for instance, the structure
and habits of a bat, could have been formed by the modification of some
animal with wholly different habits? Can we believe that natural selection
could produce, on the one hand, organs of trifling importance, such as the
tail of a giraffe, which serves as a fly-flapper, and, on the other hand,
organs of such wonderful structure, as the eye, of which we hardly as yet
fully understand the inimitable perfection?

Thirdly, can instincts be acquired and modified through natural selection?
What shall we say to so marvellous an instinct as that which leads the bee
to make cells, which have practically anticipated the discoveries of
profound mathematicians?

Fourthly, how can we account for species, when crossed, being sterile and
producing sterile offspring, whereas, when varieties are crossed, their
fertility is unimpaired?

The two first heads shall be here discussed Instinct and Hybridism in
separate chapters.

On the absence or rarity of transitional varieties. As natural selection
acts solely by the preservation of profitable modifications, each new form
will tend in a fully-stocked country to take the place of, and finally to
exterminate, its own less improved parent or other less-favoured forms with
which it comes into competition. Thus extinction and natural selection will,
as we have seen, go hand in hand. Hence, if we look at each species as
descended from some other unknown form, both the parent and all the
transitional varieties will generally have been exterminated by the very
process of formation and perfection of the new form.

But, as by this theory innumerable transitional forms must have existed, why
do we not find them embedded in countless numbers in the crust of the earth?
It will be much more convenient to discuss this question in the chapter on
the Imperfection of the geological record; and I will here only state that I
believe the answer mainly lies in the record being incomparably less perfect
than is generally supposed; the imperfection of the record being chiefly due
to organic beings not inhabiting profound depths of the sea, and to their
remains being embedded and preserved to a future age only in masses of
sediment sufficiently thick and extensive to withstand an enormous amount of
future degradation; and such fossiliferous masses can be accumulated only
where much sediment is deposited on the shallow bed of the sea, whilst it
slowly subsides. These contingencies will concur only rarely, and after
enormously long intervals. Whilst the bed of the sea is stationary or is
rising, or when very little sediment is being deposited, there will be
blanks in our geological history. The crust of the earth is a vast museum;
but the natural collections have been made only at intervals of time
immensely remote.

But it may be urged that when several closely-allied species inhabit the
same territory we surely ought to find at the present time many transitional
forms. Let us take a simple case: in travelling from north to south over a
continent, we generally meet at successive intervals with closely allied or
representative species, evidently filling nearly the same place in the
natural economy of the land. These representative species often meet and
interlock; and as the one becomes rarer and rarer, the other becomes more
and more frequent, till the one replaces the other. But if we compare these
species where they intermingle, they are generally as absolutely distinct
from each other in every detail of structure as are specimens taken from the
metropolis inhabited by each. By my theory these allied species have
descended from a common parent; and during the process of modification, each
has become adapted to the conditions of life of its own region, and has
supplanted and exterminated its original parent and all the transitional
varieties between its past and present states. Hence we ought not to expect
at the present time to meet with numerous transitional varieties in each
region, though they must have existed there, and may be embedded there in a
fossil condition. But in the intermediate region, having intermediate
conditions of life, why do we not now find closely-linking intermediate
varieties? This difficulty for a long time quite confounded me. But I think
it can be in large part explained.

In the first place we should be extremely cautious in inferring, because an
area is now continuous, that it has been continuous during a long period.
Geology would lead us to believe that almost every continent has been broken
up into islands even during the later tertiary periods; and in such islands
distinct species might have been separately formed without the possibility
of intermediate varieties existing in the intermediate zones. By changes in
the form of the land and of climate, marine areas now continuous must often
have existed within recent times in a far less continuous and uniform
condition than at present. But I will pass over this way of escaping from
the difficulty; for I believe that many perfectly defined species have been
formed on strictly continuous areas; though I do not doubt that the formerly
broken condition of areas now continuous has played an important part in the
formation of new species, more especially with freely-crossing and wandering
animals.

In looking at species as they are now distributed over a wide area, we
generally find them tolerably numerous over a large territory, then becoming
somewhat abruptly rarer and rarer on the confines, and finally disappearing.
Hence the neutral territory between two representative species is generally
narrow in comparison with the territory proper to each. We see the same fact
in ascending mountains, and sometimes it is quite remarkable how abruptly,
as Alph. De Candolle has observed, a common alpine species disappears. The
same fact has been noticed by Forbes in sounding the depths of the sea with
the dredge. To those who look at climate and the physical conditions of life
as the all-important elements of distribution, these facts ought to cause
surprise, as climate and height or depth graduate away insensibly. But when
we bear in mind that almost every species, even in its metropolis, would
increase immensely in numbers, were it not for other competing species; that
nearly all either prey on or serve as prey for others; in short, that each
organic being is either directly or indirectly related in the most important
manner to other organic beings, we must see that the range of the
inhabitants of any country by no means exclusively depends on insensibly
changing physical conditions, but in large part on the presence of other
species, on which it depends, or by which it is destroyed, or with which it
comes into competition; and as these species are already defined objects
(however they may have become so), not blending one into another by
insensible gradations, the range of any one species, depending as it does on
the range of others, will tend to be sharply defined. Moreover, each species
on the confines of its range, where it exists in lessened numbers, will,
during fluctuations in the number of its enemies or of its prey, or in the
seasons, be extremely liable to utter extermination; and thus its
geographical range will come to be still more sharply defined.

If I am right in believing that allied or representative species, when
inhabiting a continuous area, are generally so distributed that each has a
wide range, with a comparatively narrow neutral territory between them, in
which they become rather suddenly rarer and rarer; then, as varieties do not
essentially differ from species, the same rule will probably apply to both;
and if we in imagination adapt a varying species to a very large area, we
shall have to adapt two varieties to two large areas, and a third variety to
a narrow intermediate zone. The intermediate variety, consequently, will
exist in lesser numbers from inhabiting a narrow and lesser area; and
practically, as far as I can make out, this rule holds good with varieties
in a state of nature. I have met with striking instances of the rule in the
case of varieties intermediate between well-marked varieties in the genus
Balanus. And it would appear from information given me by Mr Watson, Dr Asa
Gray, and Mr Wollaston, that generally when varieties intermediate between
two other forms occur, they are much rarer numerically than the forms which
they connect. Now, if we may trust these facts and inferences, and therefore
conclude that varieties linking two other varieties together have generally
existed in lesser numbers than the forms which they connect, then, I think,
we can understand why intermediate varieties should not endure for very long
periods; why as a general rule they should be exterminated and disappear,
sooner than the forms which they originally linked together.

For any form existing in lesser numbers would, as already remarked, run a
greater chance of being exterminated than one existing in large numbers; and
in this particular case the intermediate form would be eminently liable to
the inroads of closely allied forms existing on both sides of it. But a far
more important consideration, as I believe, is that, during the process of
further modification, by which two varieties are supposed on my theory to be
converted and perfected into two distinct species, the two which exist in
larger numbers from inhabiting larger areas, will have a great advantage
over the intermediate variety, which exists in smaller numbers in a narrow
and intermediate zone. For forms existing in larger numbers will always have
a better chance, within any given period, of presenting further favourable
variations for natural selection to seize on, than will the rarer forms
which exist in lesser numbers. Hence, the more common forms, in the race for
life, will tend to beat and supplant the less common forms, for these will
be more slowly modified and improved. It is the same principle which, as I
believe, accounts for the common species in each country, as shown in the
second chapter, presenting on an average a greater number of well-marked
varieties than do the rarer species. I may illustrate what I mean by
supposing three varieties of sheep to be kept, one adapted to an extensive
mountainous region; a second to a comparatively narrow, hilly tract; and a
third to wide plains at the base; and that the inhabitants are all trying
with equal steadiness and skill to improve their stocks by selection; the
chances in this case will be strongly in favour of the great holders on the
mountains or on the plains improving their breeds more quickly than the
small holders on the intermediate narrow, hilly tract; and consequently the
improved mountain or plain breed will soon take the place of the less
improved hill breed; and thus the two breeds, which originally existed in
greater numbers, will come into close contact with each other, without the
interposition of the supplanted, intermediate hill-variety.

To sum up, I believe that species come to be tolerably well-defined objects,
and do not at any one period present an inextricable chaos of varying and
intermediate links: firstly, because new varieties are very slowly formed,
for variation is a very slow process, and natural selection can do nothing
until favourable variations chance to occur, and until a place in the
natural polity of the country can be better filled by some modification of
some one or more of its inhabitants. And such new places will depend on slow
changes of climate, or on the occasional immigration of new inhabitants,
and, probably, in a still more important degree, on some of the old
inhabitants becoming slowly modified, with the new forms thus produced and
the old ones acting and reacting on each other. So that, in any one region
and at any one time, we ought only to see a few species presenting slight
modifications of structure in some degree permanent; and this assuredly we
do see.

Secondly, areas now continuous must often have existed within the recent
period in isolated portions, in which many forms, more especially amongst
the classes which unite for each birth and wander much, may have separately
been rendered sufficiently distinct to rank as representative species. In
this case, intermediate varieties between the several representative species
and their common parent, must formerly have existed in each broken portion
of the land, but these links will have been supplanted and exterminated
during the process of natural selection, so that they will no longer exist
in a living state.

Thirdly, when two or more varieties have been formed in different portions
of a strictly continuous area, intermediate varieties will, it is probable,
at first have been formed in the intermediate zones, but they will generally
have had a short duration. For these intermediate varieties will, from
reasons already assigned (namely from what we know of the actual
distribution of closely allied or representative species, and likewise of
acknowledged varieties), exist in the intermediate zones in lesser numbers
than the varieties which they tend to connect. From this cause alone the
intermediate varieties will be liable to accidental extermination; and
during the process of further modification through natural selection, they
will almost certainly be beaten and supplanted by the forms which they
connect; for these from existing in greater numbers will, in the aggregate,
present more variation, and thus be further improved through natural
selection and gain further advantages.

Lastly, looking not to any one time, but to all time, if my theory be true,
numberless intermediate varieties, linking most closely all the species of
the same group together, must assuredly have existed; but the very process
of natural selection constantly tends, as has been so often remarked, to
exterminate the parent forms and the intermediate links. Consequently
evidence of their former existence could be found only amongst fossil
remains, which are preserved, as we shall in a future chapter attempt to
show, in an extremely imperfect and intermittent record.

On the origin and transitions of organic beings with peculiar habits and
structure. It has been asked by the opponents of such views as I hold, how,
for instance, a land carnivorous animal could have been converted into one
with aquatic habits; for how could the animal in its transitional state have
subsisted? It would be easy to show that within the same group carnivorous
animals exist having every intermediate grade between truly aquatic and
strictly terrestrial habits; and as each exists by a struggle for life, it
is clear that each is well adapted in its habits to its place in nature.
Look at the Mustela vison of North America, which has webbed feet and which
resembles an otter in its fur, short legs, and form of tail; during summer
this animal dives for and preys on fish, but during the long winter it
leaves the frozen waters, and preys like other polecats on mice and land
animals. If a different case had been taken, and it had been asked how an
insectivorous quadruped could possibly have been converted into a flying
bat, the question would have been far more difficult, and I could have given
no answer. Yet I think such difficulties have very little weight.

Here, as on other occasions, I lie under a heavy disadvantage, for out of
the many striking cases which I have collected, I can give only one or two
instances of transitional habits and structures in closely allied species of
the same genus; and of diversified habits, either constant or occasional, in
the same species. And it seems to me that nothing less than a long list of
such cases is sufficient to lessen the difficulty in any particular case
like that of the bat.

Look at the family of squirrels; here we have the finest gradation from
animals with their tails only slightly flattened, and from others, as Sir J.
Richardson has remarked, with the posterior part of their bodies rather wide
and with the skin on their flanks rather full, to the so-called flying
squirrels; and flying squirrels have their limbs and even the base of the
tail united by a broad expanse of skin, which serves as a parachute and
allows them to glide through the air to an astonishing distance from tree to
tree. We cannot doubt that each structure is of use to each kind of squirrel
in its own country, by enabling it to escape birds or beasts of prey, or to
collect food more quickly, or, as there is reason to believe, by lessening
the danger from occasional falls. But it does not follow from this fact that
the structure of each squirrel is the best that it is possible to conceive
under all natural conditions. Let the climate and vegetation change, let
other competing rodents or new beasts of prey immigrate, or old ones become
modified, and all analogy would lead us to believe that some at least of the
squirrels would decrease in numbers or become exterminated, unless they also
became modified and improved in structure in a corresponding manner.
Therefore, I can see no difficulty, more especially under changing
conditions of life, in the continued preservation of individuals with fuller
and fuller flank-membranes, each modification being useful, each being
propagated, until by the accumulated effects of this process of natural
selection, a perfect so-called flying squirrel was produced.

Now look at the Galeopithecus or flying lemur, which formerly was falsely
ranked amongst bats. It has an extremely wide flank-membrane, stretching
from the corners of the jaw to the tail, and including the limbs and the
elongated fingers: the flank membrane is, also, furnished with an extensor
muscle. Although no graduated links of structure, fitted for gliding through
the air, now connect the Galeopithecus with the other Lemuridae, yet I can
see no difficulty in supposing that such links formerly existed, and that
each had been formed by the same steps as in the case of the less perfectly
gliding squirrels; and that each grade of structure had been useful to its
possessor. Nor can I see any insuperable difficulty in further believing it
possible that the membrane-connected fingers and fore-arm of the
Galeopithecus might be greatly lengthened by natural selection; and this, as
far as the organs of flight are concerned, would convert it into a bat. In
bats which have the wing-membrane extended from the top of the shoulder to
the tail, including the hind-legs, we perhaps see traces of an apparatus
originally constructed for gliding through the air rather than for flight.

If about a dozen genera of birds had become extinct or were unknown, who
would have ventured to have surmised that birds might have existed which
used their wings solely as flappers, like the logger-headed duck
(Micropterus of Eyton); as fins in the water and front legs on the land,
like the penguin; as sails, like the ostrich; and functionally for no
purpose, like the Apteryx. Yet the structure of each of these birds is good
for it, under the conditions of life to which it is exposed, for each has to
live by a struggle; but it is not necessarily the best possible under all
possible conditions. It must not be inferred from these remarks that any of
the grades of wing-structure here alluded to, which perhaps may all have
resulted from disuse, indicate the natural steps by which birds have
acquired their perfect power of flight; but they serve, at least, to show
what diversified means of transition are possible.

Seeing that a few members of such water-breathing classes as the Crustacea
and Mollusca are adapted to live on the land, and seeing that we have flying
birds and mammals, flying insects of the most diversified types, and
formerly had flying reptiles, it is conceivable that flying-fish, which now
glide far through the air, slightly rising and turning by the aid of their
fluttering fins, might have been modified into perfectly winged animals. If
early transitional state they had been inhabitants of the open ocean, and
had used their incipient organs of flight exclusively, as far as we know, to
escape being devoured by other fish?

When we see any structure highly perfected for any particular habit, as the
wings of a bird for flight, we should bear in mind that animals displaying
early transitional grades of the structure will seldom continue to exist to
the present day, for they will have been supplanted by the very process of
perfection through natural selection. Furthermore, we may conclude that
transitional grades between structures fitted for very different habits of
life will rarely have been developed at an early period in great numbers and
under many subordinate forms. Thus, to return to our imaginary illustration
of the flying-fish, it does not seem probable that fishes capable of true
flight would have been developed under many subordinate forms, for taking
prey of many kinds in many ways, on the land and in the water, until their
organs of flight had come to a high stage of perfection, so as to have given
them a decided advantage over other animals in the battle for life. Hence
the chance of discovering species with transitional grades of structure in a
fossil condition will always be less, from their having existed in lesser
numbers, than in the case of species with fully developed structures.

I will now give two or three instances of diversified and of changed habits
in the individuals of the same species. When either case occurs, it would be
easy for natural selection to fit the animal, by some modification of its
structure, for its changed habits, or exclusively for one of its several
different habits. But it is difficult to tell, and immaterial for us,
whether habits generally change first and structure afterwards; or whether
slight modifications of structure lead to changed habits; both probably
often change almost simultaneously. Of cases of changed habits it will
suffice merely to allude to that of the many British insects which now feed
on exotic plants, or exclusively on artificial substances. Of diversified
habits innumerable instances could be given: I have often watched a tyrant
flycatcher (Saurophagus sulphuratus) in South America, hovering over one
spot and then proceeding to another, like a kestrel, and at other times
standing stationary on the margin of water, and then dashing like a
kingfisher at a fish. In our own country the larger titmouse (Parus major)
may be seen climbing branches, almost like a creeper; it often, like a
shrike, kills small birds by blows on the head; and I have many times seen
and heard it hammering the seeds of the yew on a branch, and thus breaking
them like a nuthatch. In North America the black bear was seen by Hearne
swimming for hours with widely open mouth, thus catching, like a whale,
insects in the water. Even in so extreme a case as this, if the supply of
insects were constant, and if better adapted competitors did not already
exist in the country, I can see no difficulty in a race of bears being
rendered, by natural selection, more and more aquatic in their structure and
habits, with larger and larger mouths, till a creature was produced as
monstrous as a whale.

As we sometimes see individuals of a species following habits widely
different from those both of their own species and of the other species of
the same genus, we might expect, on my theory, that such individuals would
occasionally have given rise to new species, having anomalous habits, and
with their structure either slightly or considerably modified from that of
their proper type. And such instances do occur in nature. Can a more
striking instance of adaptation be given than that of a woodpecker for
climbing trees and for seizing insects in the chinks of the bark? Yet in
North America there are woodpeckers which feed largely on fruit, and others
with elongated wings which chase insects on the wing; and on the plains of
La Plata, where not a tree grows, there is a woodpecker, which in every
essential part of its organisation, even in its colouring, in the harsh tone
of its voice, and undulatory flight, told me plainly of its close
blood-relationship to our common species; yet it is a woodpecker which never
climbs a tree!

Petrels are the most aërial and oceanic of birds, yet in the quiet Sounds of
Tierra del Fuego, the Puffinuria berardi, in its general habits, in its
astonishing power of diving, its manner of swimming, and of flying when
unwillingly it takes flight, would be mistaken by any one for an auk or
grebe; nevertheless, it is essentially a petrel, but with many parts of its
organisation profoundly modified. On the other hand, the acutest observer by
examining the dead body of the water-ouzel would never have suspected its
sub-aquatic habits; yet this anomalous member of the strictly terrestrial
thrush family wholly subsists by diving, grasping the stones with its feet
and using its wings under water.

He who believes that each being has been created as we now see it, must
occasionally have felt surprise when he has met with an animal having habits
and structure not at all in agreement. What can be plainer than that the
webbed feet of ducks and geese are formed for swimming; yet there are upland
geese with webbed feet which rarely or never go near the water; and no one
except Audubon has seen the frigate-bird, which has all its four toes
webbed, alight on the surface of the sea. On the other hand, grebes and
coots are eminently aquatic, although their toes are only bordered by
membrane. What seems plainer than that the long toes of grallatores are
formed for walking over swamps and floating plants, yet the water-hen is
nearly as aquatic as the coot; and the landrail nearly as terrestrial as the
quail or partridge. In such cases, and many others could be given, habits
have changed without a corresponding change of structure. The webbed feet of
the upland goose may be said to have become rudimentary in function, though
not in structure. In the frigate-bird, the deeply-scooped membrane between
the toes shows that structure has begun to change.

He who believes in separate and innumerable acts of creation will say, that
in these cases it has pleased the Creator to cause a being of one type to
take the place of one of another type; but this seems to me only restating
the fact in dignified language. He who believes in the struggle for
existence and in the principle of natural selection, will acknowledge that
every organic being is constantly endeavouring to increase in numbers; and
that if any one being vary ever so little, either in habits or structure,
and thus gain an advantage over some other inhabitant of the country, it
will seize on the place of that inhabitant, however different it may be from
its own place. Hence it will cause him no surprise that there should be
geese and frigate-birds with webbed feet, either living on the dry land or
most rarely alighting on the water; that there should be long-toed
corncrakes living in meadows instead of in swamps; that there should be
woodpeckers where not a tree grows; that there should be diving thrushes,
and petrels with the habits of auks.

Organs of extreme perfection and complication. To suppose that the eye, with
all its inimitable contrivances for adjusting the focus to different
distances, for admitting different amounts of light, and for the correction
of spherical and chromatic aberration, could have been formed by natural
selection, seems, I freely confess, absurd in the highest possible degree.
Yet reason tells me, that if numerous gradations from a perfect and complex
eye to one very imperfect and simple, each grade being useful to its
possessor, can be shown to exist; if further, the eye does vary ever so
slightly, and the variations be inherited, which is certainly the case; and
if any variation or modification in the organ be ever useful to an animal
under changing conditions of life, then the difficulty of believing that a
perfect and complex eye could be formed by natural selection, though
insuperable by our imagination, can hardly be considered real. How a nerve
comes to be sensitive to light, hardly concerns us more than how life itself
first originated; but I may remark that several facts make me suspect that
any sensitive nerve may be rendered sensitive to light, and likewise to
those coarser vibrations of the air which produce sound.

In looking for the gradations by which an organ in any species has been
perfected, we ought to look exclusively to its lineal ancestors; but this is
scarcely ever possible, and we are forced in each case to look to species of
the same group, that is to the collateral descendants from the same original
parent-form, in order to see what gradations are possible, and for the
chance of some gradations having been transmitted from the earlier stages of
descent, in an unaltered or little altered condition. Amongst existing
Vertebrata, we find but a small amount of gradation in the structure of the
eye, and from fossil species we can learn nothing on this head. In this
great class we should probably have to descend far beneath the lowest known
fossiliferous stratum to discover the earlier stages, by which the eye has
been perfected.

In the Articulata we can commence a series with an optic nerve merely coated
with pigment, and without any other mechanism; and from this low stage,
numerous gradations of structure, branching off in two fundamentally
different lines, can be shown to exist, until we reach a moderately high
stage of perfection. In certain crustaceans, for instance, there is a double
cornea, the inner one divided into facets, within each of which there is a
lens shaped swelling. In other crustaceans the transparent cones which are
coated by pigment, and which properly act only by excluding lateral pencils
of light, are convex at their upper ends and must act by convergence; and at
their lower ends there seems to be an imperfect vitreous substance. With
these facts, here far too briefly and imperfectly given, which show that
there is much graduated diversity in the eyes of living crustaceans, and
bearing in mind how small the number of living animals is in proportion to
those which have become extinct, I can see no very great difficulty (not
more than in the case of many other structures) in believing that natural
selection has converted the simple apparatus of an optic nerve merely coated
with pigment and invested by transparent membrane, into an optical
instrument as perfect as is possessed by any member of the great Articulate
class.

He who will go thus far, if he find on finishing this treatise that large
bodies of facts, otherwise inexplicable, can be explained by the theory of
descent, ought not to hesitate to go further, and to admit that a structure
even as perfect as the eye of an eagle might be formed by natural selection,
although in this case he does not know any of the transitional grades. His
reason ought to conquer his imagination; though I have felt the difficulty
far too keenly to be surprised at any degree of hesitation in extending the
principle of natural selection to such startling lengths.

It is scarcely possible to avoid comparing the eye to a telescope. We know
that this instrument has been perfected by the long-continued efforts of the
highest human intellects; and we naturally infer that the eye has been
formed by a somewhat analogous process. But may not this inference be
presumptuous? Have we any right to assume that the Creator works by
intellectual powers like those of man? If we must compare the eye to an
optical instrument, we ought in imagination to take a thick layer of
transparent tissue, with a nerve sensitive to light beneath, and then
suppose every part of this layer to be continually changing slowly in
density, so as to separate into layers of different densities and
thicknesses, placed at different distances from each other, and with the
surfaces of each layer slowly changing in form. Further we must suppose that
there is a power always intently watching each slight accidental alteration
in the transparent layers; and carefully selecting each alteration which,
under varied circumstances, may in any way, or in any degree, tend to
produce a distincter image. We must suppose each new state of the instrument
to be multiplied by the million; and each to be preserved till a better be
produced, and then the old ones to be destroyed. In living bodies, variation
will cause the slight alterations, generation will multiply them almost
infinitely, and natural selection will pick out with unerring skill each
improvement. Let this process go on for millions on millions of years; and
during each year on millions of individuals of many kinds; and may we not
believe that a living optical instrument might thus be formed as superior to
one of glass, as the works of the Creator are to those of man?

If it could be demonstrated that any complex organ existed, which could not
possibly have been formed by numerous, successive, slight modifications, my
theory would absolutely break down. But I can find out no such case. No
doubt many organs exist of which we do not know the transitional grades,
more especially if we look to much-isolated species, round which, according
to my theory, there has been much extinction. Or again, if we look to an
organ common to all the members of a large class, for in this latter case
the organ must have been first formed at an extremely remote period, since
which all the many members of the class have been developed; and in order to
discover the early transitional grades through which the organ has passed,
we should have to look to very ancient ancestral forms, long since become
extinct.

We should be extremely cautious in concluding that an organ could not have
been formed by transitional gradations of some kind. Numerous cases could be
given amongst the lower animals of the same organ performing at the same
time wholly distinct functions; thus the alimentary canal respires, digests,
and excretes in the larva of the dragon-fly and in the fish Cobites. In the
Hydra, the animal may be turned inside out, and the exterior surface will
then digest and the stomach respire. In such cases natural selection might
easily specialise, if any advantage were thus gained, a part or organ, which
had performed two functions, for one function alone, and thus wholly change
its nature by insensible steps. Two distinct organs sometimes perform
simultaneously the same function in the same individual; to give one
instance, there are fish with gills or branchiae that breathe the air
dissolved in the water, at the same time that they breathe free air in their
swimbladders, this latter organ having a ductus pneumaticus for its supply,
and being divided by highly vascular partitions. In these cases, one of the
two organs might with ease be modified and perfected so as to perform all
the work by itself, being aided during the process of modification by the
other organ; and then this other organ might be modified for some other and
quite distinct purpose, or be quite obliterated.

The illustration of the swimbladder in fishes is a good one, because it
shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into one for
a wholly different purpose, namely respiration. The swimbladder has, also,
been worked in as an accessory to the auditory organs of certain fish, or,
for I do not know which view is now generally held, a part of the auditory
apparatus has been worked in as a complement to the swimbladder. All
physiologists admit that the swimbladder is homologous, or 'ideally
similar,' in position and structure with the lungs of the higher vertebrate
animals: hence there seems to me to be no great difficulty in believing that
natural selection has actually converted a swimbladder into a lung, or organ
used exclusively for respiration.

I can, indeed, hardly doubt that all vertebrate animals having true lungs
have descended by ordinary generation from an ancient prototype, of which we
know nothing, furnished with a floating apparatus or swimbladder. We can
thus, as I infer from Professor Owen's interesting description of these
parts, understand the strange fact that every particle of food and drink
which we swallow has to pass over the orifice of the trachea, with some risk
of falling into the lungs, notwithstanding the beautiful contrivance by
which the glottis is closed. In the higher Vertebrata the branchiae have
wholly disappeared the slits on the sides of the neck and the loop-like
course of the arteries still marking in the embryo their former position.
But it is conceivable that the now utterly lost branchiae might have been
gradually worked in by natural selection for some quite distinct purpose: in
the same manner as, on the view entertained by some naturalists that the
branchiae and dorsal scales of Annelids are homologous with the wings and
wing-covers of insects, it is probable that organs which at a very ancient
period served for respiration have been actually converted into organs of
flight.

In considering transitions of organs, it is so important to bear in mind the
probability of conversion from one function to another, that I will give one
more instance. Pedunculated cirripedes have two minute folds of skin, called
by me the ovigerous frena, which serve, through the means of a sticky
secretion, to retain the eggs until they are hatched within the sack. These
cirripedes have no branchiae, the whole surface of the body and sack,
including the small frena, serving for respiration. The Balanidae or sessile
cirripedes, on the other hand, have no ovigerous frena, the eggs lying loose
at the bottom of the sack, in the well-enclosed shell; but they have large
folded branchiae. Now I think no one will dispute that the ovigerous frena
in the one family are strictly homologous with the branchiae of the other
family; indeed, they graduate into each other. Therefore I do not doubt that
little folds of skin, which originally served as ovigerous frena, but which,
likewise, very slightly aided the act of respiration, have been gradually
converted by natural selection into branchiae, simply through an increase in
their size and the obliteration of their adhesive glands. If all
pedunculated cirripedes had become extinct, and they have already suffered
far more extinction than have sessile cirripedes, who would ever have
imagined that the branchiae in this latter family had originally existed as
organs for preventing the ova from being washed out of the sack?

Although we must be extremely cautious in concluding that any organ could
not possibly have been produced by successive transitional gradations, yet,
undoubtedly, grave cases of difficulty occur, some of which will be
discussed in my future work.

One of the gravest is that of neuter insects, which are often very
differently constructed from either the males or fertile females; but this
case will be treated of in the next chapter. The electric organs of fishes
offer another case of special difficulty; it is impossible to conceive by
what steps these wondrous organs have been produced; but, as Owen and others
have remarked, their intimate structure closely resembles that of common
muscle; and as it has lately been shown that Rays have an organ closely
analogous to the electric apparatus, and yet do not, as Matteuchi asserts,
discharge any electricity, we must own that we are far too ignorant to argue
that no transition of any kind is possible.

The electric organs offer another and even more serious difficulty; for they
occur in only about a dozen fishes, of which several are widely remote in
their affinities. Generally when the same organ appears in several members
of the same class, especially if in members having very different habits of
life, we may attribute its presence to inheritance from a common ancestor;
and its absence in some of the members to its loss through disuse or natural
selection. But if the electric organs had been inherited from one ancient
progenitor thus provided, we might have expected that all electric fishes
would have been specially related to each other. Nor does geology at all
lead to the belief that formerly most fishes had electric organs, which most
of their modified descendants have lost. The presence of luminous organs in
a few insects, belonging to different families and orders, offers a parallel
case of difficulty. Other cases could be given; for instance in plants, the
very curious contrivance of a mass of pollen-grains, borne on a foot-stalk
with a sticky gland at the end, is the same in Orchis and Asclepias, genera
almost as remote as possible amongst flowering plants. In all these cases of
two very distinct species furnished with apparently the same anomalous
organ, it should be observed that, although the general appearance and
function of the organ may be the same, yet some fundamental difference can
generally be detected. I am inclined to believe that in nearly the same way
as two men have sometimes independently hit on the very same invention, so
natural selection, working for the good of each being and taking advantage
of analogous variations, has sometimes modified in very nearly the same
manner two parts in two organic beings, which owe but little of their
structure in common to inheritance from the same ancestor.

Although in many cases it is most difficult to conjecture by what
transitions an organ could have arrived at its present state; yet,
considering that the proportion of living and known forms to the extinct and
unknown is very small, I have been astonished how rarely an organ can be
named, towards which no transitional grade is known to lead. The truth of
this remark is indeed shown by that old canon in natural history of 'Natura
non facit saltum.' We meet with this admission in the writings of almost
every experienced naturalist; or, as Milne Edwards has well expressed it,
nature is prodigal in variety, but niggard in innovation. Why, on the theory
of Creation, should this be so? Why should all the parts and organs of many
independent beings, each supposed to have been separately created for its
proper place in nature, be so invariably linked together by graduated steps?
Why should not Nature have taken a leap from structure to structure? On the
theory of natural selection, we can clearly understand why she should not;
for natural selection can act only by taking advantage of slight successive
variations; she can never take a leap, but must advance by the shortest and
slowest steps.

Organs of little apparent importance. As natural selection acts by life and
death, by the preservation of individuals with any favourable variation, and
by the destruction of those with any unfavourable deviation of structure, I
have sometimes felt much difficulty in understanding the origin of simple
parts, of which the importance does not seem sufficient to cause the
preservation of successively varying individuals. I have sometimes felt as
much difficulty, though of a very different kind, on this head, as in the
case of an organ as perfect and complex as the eye.

In the first place, we are much too ignorant in regard to the whole economy
of any one organic being, to say what slight modifications would be of
importance or not. In a former chapter I have given instances of most
trifling characters, such as the down on fruit and the colour of the flesh,
which, from determining the attacks of insects or from being correlated with
constitutional differences, might assuredly be acted on by natural
selection. The tail of the giraffe looks like an artificially constructed
fly-flapper; and it seems at first incredible that this could have been
adapted for its present purpose by successive slight modifications, each
better and better, for so trifling an object as driving away flies; yet we
should pause before being too positive even in this case, for we know that
the distribution and existence of cattle and other animals in South America
absolutely depends on their power of resisting the attacks of insects: so
that individuals which could by any means defend themselves from these small
enemies, would be able to range into new pastures and thus gain a great
advantage. It is not that the larger quadrupeds are actually destroyed
(except in some rare cases) by the flies, but they are incessantly harassed
and their strength reduced, so that they are more subject to disease, or not
so well enabled in a coming dearth to search for food, or to escape from
beasts of prey.

Organs now of trifling importance have probably in some cases been of high
importance to an early progenitor, and, after having been slowly perfected
at a former period, have been transmitted in nearly the same state, although
now become of very slight use; and any actually injurious deviations in
their structure will always have been checked by natural selection. Seeing
how important an organ of locomotion the tail is in most aquatic animals,
its general presence and use for many purposes in so many land animals,
which in their lungs or modified swim-bladders betray their aquatic origin,
may perhaps be thus accounted for. A well-developed tail having been formed
in an aquatic animal, it might subsequently come to be worked in for all
sorts of purposes, as a fly-flapper, an organ of prehension, or as an aid in
turning, as with the dog, though the aid must be slight, for the hare, with
hardly any tail, can double quickly enough.

In the second place, we may sometimes attribute importance to characters
which are really of very little importance, and which have originated from
quite secondary causes, independently of natural selection. We should
remember that climate, food, &c., probably have some little direct influence
on the organisation; that characters reappear from the law of reversion;,
that correlation of growth will have had a most important influence in
modifying various structures; and finally, that sexual selection will often
have largely modified the external characters of animals having a will, to
give one male an advantage in fighting with another or in charming the
females. Moreover when a modification of structure has primarily arisen from
the above or other unknown causes, it may at first have been of no advantage
to the species, but may subsequently have been taken advantage of by the
descendants of the species under new conditions of life and with newly
acquired habits.

To give a few instances to illustrate these latter remarks. If green
woodpeckers alone had existed, and we did not know that there were many
black and pied kinds, I dare say that we should have thought that the green
colour was a beautiful adaptation to hide this tree-frequenting bird from
its enemies; and consequently that it was a character of importance and
might have been acquired through natural selection; as it is, I have no
doubt that the colour is due to some quite distinct cause, probably to
sexual selection. A trailing bamboo in the Malay Archipelago climbs the
loftiest trees by the aid of exquisitely constructed hooks clustered around
the ends of the branches, and this contrivance, no doubt, is of the highest
service to the plant; but as we see nearly similar hooks on many trees which
are not climbers the hooks on the bamboo may have arisen from unknown laws
of growth, and have been subsequently taken advantage of by the plant
undergoing further modification and becoming a climber. The naked skin on
the head of a vulture is generally looked at as a direct adaptation for
wallowing in putridity; and so it may be, or it may possibly be due to the
direct action of putrid matter; but we should be very cautious in drawing
any such inference, when we see that the skin on the head of the
clean-feeding male turkey is likewise naked. The sutures in the skulls of
young mammals have been advanced as a beautiful adaptation for aiding
parturition, and no doubt they facilitate, or may be indispensable for this
act; but as sutures occur in the skulls of young birds and reptiles, which
have only to escape from a broken egg, we may infer that this structure has
arisen from the laws of growth, and has been taken advantage of in the
parturition of the higher animals.

We are profoundly ignorant of the causes producing slight and unimportant
variations; and we are immediately made conscious of this by reflecting on
the differences in the breeds of our domesticated animals in different
countries, more especially in the less civilized countries where there has
been but little artificial selection. Careful observers are convinced that a
damp climate affects the growth of the hair, and that with the hair the
horns are correlated. Mountain breeds always differ from lowland breeds; and
a mountainous country would probably affect the hind limbs from exercising
them more, and possibly even the form of the pelvis; and then by the law of
homologous variation, the front limbs and even the head would probably be
affected. The shape, also, of the pelvis might affect by pressure the shape
of the head of the young in the womb. The laborious breathing necessary in
high regions would, we have some reason to believe, increase the size of the
chest; and again correlation would come into play. Animals kept by savages
in different countries often have to struggle for their own subsistence, and
would be exposed to a certain extent to natural selection, and individuals
with slightly different constitutions would succeed best under different
climates; and there is reason to believe that constitution and colour are
correlated. A good observer, also, states that in cattle susceptibility to
the attacks of flies is correlated with colour, as is the liability to be
poisoned by certain plants; so that colour would be thus subjected to the
action of natural selection. But we are far too ignorant to speculate on the
relative importance of the several known and unknown laws of variation; and
I have here alluded to them only to show that, if we are unable to account
for the characteristic differences of our domestic breeds, which
nevertheless we generally admit to have arisen through ordinary generation,
we ought not to lay too much stress on our ignorance of the precise cause of
the slight analogous differences between species. I might have adduced for
this same purpose the differences between the races of man, which are so
strongly marked; I may add that some little light can apparently be thrown
on the origin of these differences, chiefly through sexual selection of a
particular kind, but without here entering on copious details my reasoning
would appear frivolous.

The foregoing remarks lead me to say a few words on the protest lately made
by some naturalists, against the utilitarian doctrine that every detail of
structure has been produced for the good of its possessor. They believe that
very many structures have been created for beauty in the eyes of man, or for
mere variety. This doctrine, if true, would be absolutely fatal to my
theory. Yet I fully admit that many structures are of no direct use to their
possessors. Physical conditions probably have had some little effect on
structure, quite independently of any good thus gained. Correlation of
growth has no doubt played a most important part, and a useful modification
of one part will often have entailed on other parts diversified changes of
no direct use. So again characters which formerly were useful, or which
formerly had arisen from correlation of growth, or from other unknown cause,
may reappear from the law of reversion, though now of no direct use. The
effects of sexual selection, when displayed in beauty to charm the females,
can be called useful only in rather a forced sense. But by far the most
important consideration is that the chief part of the organisation of every
being is simply due to inheritance; and consequently, though each being
assuredly is well fitted for its place in nature, many structures now have
no direct relation to the habits of life of each species. Thus, we can
hardly believe that the webbed feet of the upland goose or of the
frigate-bird are of special use to these birds; we cannot believe that the
same bones in the arm of the monkey, in the fore leg of the horse, in the
wing of the bat, and in the flipper of the seal, are of special use to these
animals. We may safely attribute these structures to inheritance. But to the
progenitor of the upland goose and of the frigate-bird, webbed feet no doubt
were as useful as they now are to the most aquatic of existing birds. So we
may believe that the progenitor of the seal had not a flipper, but a foot
with five toes fitted for walking or grasping; and we may further venture to
believe that the several bones in the limbs of the monkey, horse, and bat,
which have been inherited from a common progenitor, were formerly of more
special use to that progenitor, or its progenitors, than they now are to
these animals having such widely diversified habits. Therefore we may infer
that these several bones might have been acquired through natural selection,
subjected formerly, as now, to the several laws of inheritance, reversion,
correlation of growth, &c. Hence every detail of structure in every living
creature (making some little allowance for the direct action of physical
conditions) may be viewed, either as having been of special use to some
ancestral form, or as being now of special use to the descendants of this
form either directly, or indirectly through the complex laws of growth.

Natural selection cannot possibly produce any modification in any one
species exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by, the
structure of another. But natural selection can and does often produce
structures for the direct injury of other species, as we see in the fang of
the adder, and in the ovipositor of the ichneumon, by which its eggs are
deposited in the living bodies of other insects. If it could be proved that
any part of the structure of any one species had been formed for the
exclusive good of another species, it would annihilate my theory, for such
could not have been produced through natural selection. Although many
statements may be found in works on natural history to this effect, I cannot
find even one which seems to me of any weight. It is admitted that the
rattlesnake has a poison-fang for its own defence and for the destruction of
its prey; but some authors suppose that at the same time this snake is
furnished with a rattle for its own injury, namely, to warn its prey to
escape. I would almost as soon believe that the cat curls the end of its
tail when preparing to spring, in order to warn the doomed mouse. But I have
not space here to enter on this and other such cases.

Natural selection will never produce in a being anything injurious to
itself, for natural selection acts solely by and for the good of each. No
organ will be formed, as Paley has remarked, for the purpose of causing pain
or for doing an injury to its possessor. If a fair balance be struck between
the good and evil caused by each part, each will be found on the whole
advantageous. After the lapse of time, under changing conditions of life, if
any part comes to be injurious, it will be modified; or if it be not so, the
being will become extinct, as myriads have become extinct.

Natural selection tends only to make each organic being as perfect as, or
slightly more perfect than, the other inhabitants of the same country with
which it has to struggle for existence. And we see that this is the degree
of perfection attained under nature. The endemic productions of New Zealand,
for instance, are perfect one compared with another; but they are now
rapidly yielding before the advancing legions of plants and animals
introduced from Europe. Natural selection will not produce absolute
perfection, nor do we always meet, as far as we can judge, with this high
standard under nature. The correction for the aberration of light is said,
on high authority, not to be perfect even in that most perfect organ, the
eye. If our reason leads us to admire with enthusiasm a multitude of
inimitable contrivances in nature, this same reason tells us, though we may
easily err on both sides, that some other contrivances are less perfect. Can
we consider the sting of the wasp or of the bee as perfect, which, when used
against many attacking animals, cannot be withdrawn, owing to the backward
serratures, and so inevitably causes the death of the insect by tearing out
its viscera?

If we look at the sting of the bee, as having originally existed in a remote
progenitor as a boring and serrated instrument, like that in so many members
of the same great order, and which has been modified but not perfected for
its present purpose, with the poison originally adapted to cause galls
subsequently intensified, we can perhaps understand how it is that the use
of the sting should so often cause the insect's own death: for if on the
whole the power of stinging be useful to the community, it will fulfil all
the requirements of natural selection, though it may cause the death of some
few members. If we admire the truly wonderful power of scent by which the
males of many insects find their females, can we admire the production for
this single purpose of thousands of drones, which are utterly useless to the
community for any other end, and which are ultimately slaughtered by their
industrious and sterile sisters? It may be difficult, but we ought to admire
the savage instinctive hatred of the queen-bee, which urges her instantly to
destroy the young queens her daughters as soon as born, or to perish herself
in the combat; for undoubtedly this is for the good of the community; and
maternal love or maternal hatred, though the latter fortunately is most
rare, is all the same to the inexorable principle of natural selection. If
we admire the several ingenious contrivances, by which the flowers of the
orchis and of many other plants are fertilised through insect agency, can we
consider as equally perfect the elaboration by our fir-trees of dense clouds
of pollen, in order that a few granules may be wafted by a chance breeze on
to the ovules?

Summary of Chapter. We have in this chapter discussed some of the
difficulties and objections which may be urged against my theory. Many of
them are very grave; but I think that in the discussion light has been
thrown on several facts, which on the theory of independent acts of creation
are utterly obscure. We have seen that species at any one period are not
indefinitely variable, and are not linked together by a multitude of
intermediate gradations, partly because the process of natural selection
will always be very slow, and will act, at any one time, only on a very few
forms; and partly because the very process of natural selection almost
implies the continual supplanting and extinction of preceding and
intermediate gradations. Closely allied species, now living on a continuous
area, must often have been formed when the area was not continuous, and when
the conditions of life did not insensibly graduate away from one part to
another. When two varieties are formed in two districts of a continuous
area, an intermediate variety will often be formed, fitted for an
intermediate zone; but from reasons assigned, the intermediate variety will
usually exist in lesser numbers than the two forms which it connects;
consequently the two latter, during the course of further modification, from
existing in greater numbers, will have a great advantage over the less
numerous intermediate variety, and will thus generally succeed in
supplanting and exterminating it.

We have seen in this chapter how cautious we should be in concluding that
the most different habits of life could not graduate into each other; that a
bat, for instance, could not have been formed by natural selection from an
animal which at first could only glide through the air.

We have seen that a species may under new conditions of life change its
habits, or have diversified habits, with some habits very unlike those of
its nearest congeners. Hence we can understand bearing in mind that each
organic being is trying to live wherever it can live, how it has arisen that
there are upland geese with webbed feet, ground woodpeckers, diving
thrushes, and petrels with the habits of auks.

Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is more than enough to stagger any one; yet in
the case of any organ, if we know of a long series of gradations in
complexity, each good for its possessor, then, under changing conditions of
life, there is no logical impossibility in the acquirement of any
conceivable degree of perfection through natural selection. In the cases in
which we know of no intermediate or transitional states, we should be very
cautious in concluding that none could have existed, for the homologies of
many organs and their intermediate states show that wonderful metamorphoses
in function are at least possible. For instance, a swim-bladder has
apparently been converted into an air-breathing lung. The same organ having
performed simultaneously very different functions, and then having been
specialised for one function; and two very distinct organs having performed
at the same time the same function, the one having been perfected whilst
aided by the other, must often have largely facilitated transitions.

We are far too ignorant, in almost every case, to be enabled to assert that
any part or organ is so unimportant for the welfare of a species, that
modifications in its structure could not have been slowly accumulated by
means of natural selection. But we may confidently believe that many
modifications, wholly due to the laws of growth, and at first in no way
advantageous to a species, have been subsequently taken advantage of by the
still further modified descendants of this species. We may, also, believe
that a part formerly of high importance has often been retained (as the tail
of an aquatic animal by its terrestrial descendants), though it has become
of such small importance that it could not, in its present state, have been
acquired by natural selection, a power which acts solely by the preservation
of profitable variations in the struggle for life.

Natural selection will produce nothing in one species for the exclusive good
or injury of another; though it may well produce parts, organs, and
excretions highly useful or even indispensable, or highly injurious to
another species, but in all cases at the same time useful to the owner.
Natural selection in each well-stocked country, must act chiefly through the
competition of the inhabitants one with another, and consequently will
produce perfection, or strength in the battle for life, only according to
the standard of that country. Hence the inhabitants of one country,
generally the smaller one, will often yield, as we see they do yield, to the
inhabitants of another and generally larger country. For in the larger
country there will have existed more individuals, and more diversified
forms, and the competition will have been severer, and thus the standard of
perfection will have been rendered higher. Natural selection will not
necessarily produce absolute perfection; nor, as far as we can judge by our
limited faculties, can absolute perfection be everywhere found.

On the theory of natural selection we can clearly understand the full
meaning of that old canon in natural history, 'Natura non facit saltum.'
This canon, if we look only to the present inhabitants of the world, is not
strictly correct, but if we include all those of past times, it must by my
theory be strictly true.

It is generally acknowledged that all organic beings have been formed on two
great laws Unity of Type, and the Conditions of Existence. By unity of type
is meant that fundamental agreement in structure, which we see in organic
beings of the same class, and which is quite independent of their habits of
life. On my theory, unity of type is explained by unity of descent. The
expression of conditions of existence, so often insisted on by the
illustrious Cuvier, is fully embraced by the principle of natural selection.
For natural selection acts by either now adapting the varying parts of each
being to its organic and inorganic conditions of life; or by having adapted
them during long-past periods of time: the adaptations being aided in some
cases by use and disuse, being slightly affected by the direct action of the
external conditions of life, and being in all cases subjected to the several
laws of growth. Hence, in fact, the law of the Conditions of Existence is
the higher law; as it includes, through the inheritance of former
adaptations, that of Unity of Type.

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Chapter 7 - Instinct

[* ] Instincts comparable with habits, but different in their origin [* ]
Instincts graduated [* ] Aphides and ants [* ] Instincts variable [* ]
Domestic instincts, their origin [* ] Natural instincts of the cuckoo,
ostrich, and parasitic bees [* ] Slave-making ants [* ] Hive-bee, its
cell-making instinct [* ] Difficulties on the theory of the Natural
Selection of instincts [* ] Neuter or sterile insects [* ] Summary
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THE subject of instinct might have been worked into the previous chapters;
but I have thought that it would be more convenient to treat the subject
separately, especially as so wonderful an instinct as that of the hive-bee
making its cells will probably have occurred to many readers, as a
difficulty sufficient to overthrow my whole theory. I must premise, that I
have nothing to do with the origin of the primary mental powers, any more
than I have with that of life itself. We are concerned only with the
diversities of instinct and of the other mental qualities of animals within
the same class.

I will not attempt any definition of instinct. It would be easy to show that
several distinct mental actions are commonly embraced by this term; but
every one understands what is meant, when it is said that instinct impels
the cuckoo to migrate and to lay her eggs in other birds' nests. An action,
which we ourselves should require experience to enable us to perform, when
performed by an animal, more especially by a very young one, without any
experience, and when performed by many individuals in the same way, without
their knowing for what purpose it is performed, is usually said to be
instinctive. But I could show that none of these characters of instinct are
universal. A little dose, as Pierre Huber expresses it, of judgment or
reason, often comes into play, even in animals very low in the scale of
nature.

Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, a remarkably accurate
notion of the frame of mind under which an instinctive action is performed,
but not of its origin. How unconsciously many habitual actions are
performed, indeed not rarely in direct opposition to our conscious will! yet
they may be modified by the will or reason. Habits easily become associated
with other habits, and with certain periods of time and states of the body.
When once acquired, they often remain constant throughout life. Several
other points of resemblance between instincts and habits could be pointed
out. As in repeating a well-known song, so in instincts, one action follows
another by a sort of rhythm; if a person be interrupted in a song, or in
repeating anything by rote, he is generally forced to go back to recover the
habitual train of thought: so P. Huber found it was with a caterpillar,
which makes a very complicated hammock; for if he took a caterpillar which
had completed its hammock up to, say, the sixth stage of construction, and
put it into a hammock completed up only to the third stage, the caterpillar
simply re-performed the fourth, fifth, and sixth stages of construction. If,
however, a caterpillar were taken out of a hammock made up, for instance, to
the third stage, and were put into one finished up to the sixth stage, so
that much of its work was already done for it, far from feeling the benefit
of this, it was much embarrassed, and, in order to complete its hammock,
seemed forced to start from the third stage, where it had left off, and thus
tried to complete the already finished work.

If we suppose any habitual action to become inherited and I think it can be
shown that this does sometimes happen then the resemblance between what
originally was a habit and an instinct becomes so close as not to be
distinguished. If Mozart, instead of playing the pianoforte at three years
old with wonderfully little practice, had played a tune with no practice at
all, be might truly be said to have done so instinctively. But it would be
the most serious error to suppose that the greater number of instincts have
been acquired by habit in one generation, and then transmitted by
inheritance to succeeding generations. It can be clearly shown that the most
wonderful instincts with which we are acquainted, namely, those of the
hive-bee and of many ants, could not possibly have been thus acquired.

It will be universally admitted that instincts are as important as corporeal
structure for the welfare of each species, under its present conditions of
life. Under changed conditions of life, it is at least possible that slight
modifications of instinct might be profitable to a species; and if it can be
shown that instincts do vary ever so little, then I can see no difficulty in
natural selection preserving and continually accumulating variations of
instinct to any extent that may be profitable. It is thus, as I believe,
that all the most complex and wonderful instincts have originated. As
modifications of corporeal structure arise from, and are increased by, use
or habit, and are diminished or lost by disuse, so I do not doubt it has
been with instincts. But I believe that the effects of habit are of quite
subordinate importance to the effects of the natural selection of what may
be called accidental variations of instincts; that is of variations produced
by the same unknown causes which produce slight deviations of bodily
structure.

No complex instinct can possibly be produced through natural selection,
except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which
each complex instinct has been acquired for these could be found only in the
lineal ancestors of each species but we ought to find in the collateral
lines of descent some evidence of such gradations; or we ought at least to
be able to show that gradations of some kind are possible; and this we
certainly can do. I have been surprised to find, making allowance for the
instincts of animals having been but little observed except in Europe and
North America, and for no instinct being known amongst extinct species, how
very generally gradations, leading to the most complex instincts, can be
discovered. The canon of 'Natura non facit saltum' applies with almost equal
force to instincts as to bodily organs. Changes of instinct may sometimes be
facilitated by the same species having different instincts at different
periods of life, or at different seasons of the year, or when placed under
different circumstances, &c.; in which case either one or the other instinct
might be preserved by natural selection. And such instances of diversity of
instinct in the same species can be shown to occur in nature.

Again as in the case of corporeal structure, and conformably with my theory,
the instinct of each species is good for itself, but has never, as far as we
can judge, been produced for the exclusive good of others. One of the
strongest instances of an animal apparently performing an action for the
sole good of another, with which I am acquainted, is that of aphides
voluntarily yielding their sweet excretion to ants: that they do so
voluntarily, the following facts show. I removed all the ants from a group
of about a dozen aphides on a dock-plant, and prevented their attendance
during several hours. After this interval, I felt sure that the aphides
would want to excrete. I watched them for some time through a lens, but not
one excreted; I then tickled and stroked them with a hair in the same
manner, as well as I could, as the ants do with their antennae; but not one
excreted. Afterwards I allowed an ant to visit them, and it immediately
seemed, by its eager way of running about, to be well aware what a rich
flock it had discovered; it then began to play with its antennae on the
abdomen first of one aphis and then of another; and each aphis, as soon as
it felt the antennae, immediately lifted up its abdomen and excreted a
limpid drop of sweet juice, which was eagerly devoured by the ant. Even the
quite young aphides behaved in this manner, showing that the action was
instinctive, and not the result of experience. But as the excretion is
extremely viscid, it is probably a convenience to the aphides to have it
removed; and therefore probably the aphides do not instinctively excrete for
the sole good of the ants. Although I do not believe that any animal in the
world performs an action for the exclusive good of another of a distinct
species, yet each species tries to take advantage of the instincts of
others, as each takes advantage of the weaker bodily structure of others. So
again, in some few cases, certain instincts cannot be considered as
absolutely perfect; but as details on this and other such points are not
indispensable, they may be here passed over.

As some degree of variation in instincts under a state of nature, and the
inheritance of such variations, are indispensable for the action of natural
selection, as many instances as possible ought to have been here given; but
want of space prevents me. I can only assert, that instincts certainly do
vary for instance, the migratory instinct, both in extent and direction, and
in its total loss. So it is with the nests of birds, which vary partly in
dependence on the situations chosen, and on the nature and temperature of
the country inhabited, but often from causes wholly unknown to us: Audubon
has given several remarkable cases of differences in nests of the same
species in the northern and southern United States. Fear of any particular
enemy is certainly an instinctive quality, as may be seen in nestling birds,
though it is strengthened by experience, and by the sight of fear of the
same enemy in other animals. But fear of man is slowly acquired, as I have
elsewhere shown, by various animals inhabiting desert islands; and we may
see an instance of this, even in England, in the greater wildness of all our
large birds than of our small birds; for the large birds have been most
persecuted by man. We may safely attribute the greater wildness of our large
birds to this cause; for in uninhabited islands large birds are not more
fearful than small; and the magpie, so wary in England, is tame in Norway,
as is the hooded crow in Egypt.

That the general disposition of individuals of the same species, born in a
state of nature, is extremely diversified, can be shown by a multitude of
facts. Several cases also, could be given, of occasional and strange habits
in certain species, which might, if advantageous to the species, give rise,
through natural selection, to quite new instincts. But I am well aware that
these general statements, without facts given in detail, can produce but a
feeble effect on the reader's mind. I can only repeat my assurance, that I
do not speak without good evidence.

The possibility, or even probability, of inherited variations of instinct in
a state of nature will be strengthened by briefly considering a few cases
under domestication. We shall thus also be enabled to see the respective
parts which habit and the selection of so-called accidental variations have
played in modifying the mental qualities of our domestic animals. A number
of curious and authentic instances could be given of the inheritance of all
shades of disposition and tastes, and likewise of the oddest tricks,
associated with certain frames of mind or periods of time. But let us look
to the familiar case of the several breeds of dogs: it cannot be doubted
that young pointers (I have myself seen a striking instance) will sometimes
point and even back other dogs the very first time that they are taken out;
retrieving is certainly in some degree inherited by retrievers; and a
tendency to run round, instead of at, a flock of sheep, by shepherd-dogs. I
cannot see that these actions, performed without experience by the young,
and in nearly the same manner by each individual, performed with eager
delight by each breed, and without the end being known, for the young
pointer can no more know that he points to aid his master, than the white
butterfly knows why she lays her eggs on the leaf of the cabbage, I cannot
see that these actions differ essentially from true instincts. If we were to
see one kind of wolf, when young and without any training, as soon as it
scented its prey, stand motionless like a statue, and then slowly crawl
forward with a peculiar gait; and another kind of wolf rushing round,
instead of at, a herd of deer, and driving them to a distant point, we
should assuredly call these actions instinctive. Domestic instincts, as they
may be called, are certify far less fixed or invariable than natural
instincts; but they have been acted on by far less rigorous selection, and
have been transmitted for an incomparably shorter period, under less fixed
conditions of life.

How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown when
different breeds of dogs are crossed. Thus it is known that a cross with a
bull-dog has affected for many generations the courage and obstinacy of
greyhounds; and a cross with a greyhound has given to a whole family of
shepherd-dogs a tendency to hunt hares. These domestic instincts, when thus
tested by crossing, resemble natural instincts, which in a like manner
become curiously blended together, and for a long period exhibit traces of
the instincts of either parent: for example, Le Roy describes a dog, whose
great-grandfather was a wolf, and this dog showed a trace of its wild
parentage only in one way, by not coming in a straight line to his master
when called.

Domestic instincts are sometimes spoken of as actions which have become
inherited solely from long-continued and compulsory habit, but this, I
think, is not true. No one would ever have thought of teaching, or probably
could have taught, the tumbler-pigeon to tumble, an action which, as I have
witnessed, is performed by young birds, that have never seen a pigeon
tumble. We may believe that some one pigeon showed a slight tendency to this
strange habit, and that the long-continued selection of the best individuals
in successive generations made tumblers what they now are; and near Glasgow
there are house-tumblers, as I hear from Mr Brent, which cannot fly eighteen
inches high without going head over heels. It may be doubted whether any one
would have thought of training a dog to point, had not some one dog
naturally shown a tendency in this line; and this is known occasionally to
happen, as I once saw in a pure terrier. When the first tendency was once
displayed, methodical selection and the inherited effects of compulsory
training in each successive generation would soon complete the work; and
unconscious selection is still at work, as each man tries to procure,
without intending to improve the breed, dogs which will stand and hunt best.
On the other hand, habit alone in some cases has sufficed; no animal is more
difficult to tame than the young of the wild rabbit; scarcely any animal is
tamer than the young of the tame rabbit; but I do not suppose that domestic
rabbits have ever been selected for tameness; and I presume that we must
attribute the whole of the inherited change from extreme wildness to extreme
tameness, simply to habit and long-continued close confinement.

Natural instincts are lost under domestication: a remarkable instance of
this is seen in those breeds of fowls which very rarely or never become
'broody,' that is, never wish to sit on their eggs. Familiarity alone
prevents our seeing how universally and largely the minds of our domestic
animals have been modified by domestication. It is scarcely possible to
doubt that the love of man has become instinctive in the dog. All wolves,
foxes, jackals, and species of the cat genus, when kept tame, are most eager
to attack poultry, sheep, and pigs; and this tendency has been found
incurable in dogs which have been brought home as puppies from countries,
such as Tierra del Fuego and Australia, where the savages do not keep these
domestic animals. How rarely, on the other hand, do our civilised dogs, even
when quite young, require to be taught not to attack poultry, sheep, and
pigs! No doubt they occasionally do make an attack, and are then beaten; and
if not cured, they are destroyed; so that habit, with some degree of
selection, has probably concurred in civilising by inheritance our dogs. On
the other hand, young chickens have lost, wholly by habit, that fear of the
dog and cat which no doubt was originally instinctive in them, in the same
way as it is so plainly instinctive in young pheasants, though reared under
a hen. It is not that chickens have lost all fear, but fear only of dogs and
cats, for if the hen gives the danger-chuckle, they will run (more
especially young turkeys) from under her, and conceal themselves in the
surrounding grass or thickets; and this is evidently done for the
instinctive purpose of allowing, as we see in wild ground-birds, their
mother to fly away. But this instinct retained by our chickens has become
useless under domestication, for the mother-hen has almost lost by disuse
the power of flight.

Hence, we may conclude, that domestic instincts have been acquired and
natural instincts have been lost partly by habit, and partly by man
selecting and accumulating during successive generations, peculiar mental
habits and actions, which at first appeared from what we must in our
ignorance call an accident. In some cases compulsory habit alone has
sufficed to produce such inherited mental changes; in other cases compulsory
habit has done nothing, and all has been the result of selection, pursued
both methodically and unconsciously; but in most cases, probably, habit and
selection have acted together.

We shall, perhaps, best understand how instincts in a state of nature have
become modified by selection, by considering a few cases. I will select only
three, out of the several which I shall have to discuss in my future work,
namely, the instinct which leads the cuckoo to lay her eggs in other birds'
nests; the slave-making instinct of certain ants; and the comb-making power
of the hive-bee: these two latter instincts have generally, and most justly,
been ranked by naturalists as the most wonderful of all known instincts.

It is now commonly admitted that the more immediate and final cause of the
cuckoo's instinct is, that she lays her eggs, not daily, but at intervals of
two or three days; so that, if she were to make her own nest and sit on her
own eggs, those first laid would have to be left for some time unincubated,
or there would be eggs and young birds of different ages in the same nest.
If this were the case, the process of laying and hatching might be
inconveniently long, more especially as she has to migrate at a very early
period; and the first hatched young would probably have to be fed by the
male alone. But the American cuckoo is in this predicament; for she makes
her own nest and has eggs and young successively hatched, all at the same
time. It has been asserted that the American cuckoo occasionally lays her
eggs in other birds' nests; but I hear on the high authority of Dr. Brewer,
that this is a mistake. Nevertheless, I could give several instances of
various birds which have been known occasionally to lay their eggs in other
birds' nests. Now let us suppose that the ancient progenitor of our European
cuckoo had the habits of the American cuckoo; but that occasionally she laid
an egg in another bird's nest. If the old bird profited by this occasional
habit, or if the young were made more vigorous by advantage having been
taken of the mistaken maternal instinct of another bird, than by their own
mother's care, encumbered as she can hardly fail to be by having eggs and
young of different ages at the same time; then the old birds or the fostered
young would gain an advantage. And analogy would lead me to believe, that
the young thus reared would be apt to follow by inheritance the occasional
and aberrant habit of their mother, and in their turn would be apt to lay
their eggs in other birds' nests, and thus be successful in rearing their
young. By a continued process of this nature, I believe that the strange
instinct of our cuckoo could be, and has been, generated. I may add that,
according to Dr. Gray and to some other observers, the European cuckoo has
not utterly lost all maternal love and care for her own offspring.

The occasional habit of birds laying their eggs in other birds' nests,
either of the same or of a distinct species, is not very uncommon with the
Gallinaceae; and this perhaps explains the origin of a singular instinct in
the allied group of ostriches. For several hen ostriches, at least in the
case of the American species, unite and lay first a few eggs in one nest and
then in another; and these are hatched by the males. This instinct may
probably be accounted for by the fact of the hens laying a large number of
eggs; but, as in the case of the cuckoo, at intervals of two or three days.
This instinct, however, of the American ostrich has not as yet been
perfected; for a surprising number of eggs lie strewed over the plains, so
that in one day's hunting I picked up no less than twenty lost and wasted
eggs.

Many bees are parasitic, and always lay their eggs in the nests of bees of
other kinds. This case is more remarkable than that of the cuckoo; for these
bees have not only their instincts but their structure modified in
accordance with their parasitic habits; for they do not possess the
pollen-collecting apparatus which would be necessary if they had to store
food for their own young. Some species, likewise, of Sphegidae (wasp-like
insects) are parasitic on other species; and M. Fabre has lately shown good
reason for believing that although the Tachytes nigra generally makes its
own burrow and stores it with paralysed prey for its own larvae to feed on,
yet that when this insect finds a burrow already made and stored by another
sphex, it takes advantage of the prize, and becomes for the occasion
parasitic. In this case, as with the supposed case of the cuckoo, I can see
no difficulty in natural selection making an occasional habit permanent, if
of advantage to the species, and if the insect whose nest and stored food
are thus feloniously appropriated, be not thus exterminated.

Slave-making instinct. This remarkable instinct was first discovered in the
Formica (Polyerges) rufescens by Pierre Huber, a better observer even than
his celebrated father. This ant is absolutely dependent on its slaves;
without their aid, the species would certainly become extinct in a single
year. The males and fertile females do no work. The workers or sterile
females, though most energetic and courageous in capturing slaves, do no
other work. They are incapable of making their own nests, or of feeding
their own larvae. When the old nest is found inconvenient, and they have to
migrate, it is the slaves which determine the migration, and actually carry
their masters in their jaws. So utterly helpless are the masters, that when
Huber shut up thirty of them without a slave, but with plenty of the food
which they like best, and with their larvae and pupae to stimulate them to
work, they did nothing; they could not even feed themselves, and many
perished of hunger. Huber then introduced a single slave (F. fusca), and she
instantly set to work, fed and saved the survivors; made some cells and
tended the larvae, and put all to rights. What can be more extraordinary
than these well-ascertained facts? If we had not known of any other
slave-making ant, it would have been hopeless to have speculated how so
wonderful an instinct could have been perfected.

Formica sanguinea was likewise first discovered by P. Huber to be a
slave-making ant. This species is found in the southern parts of England,
and its habits have been attended to by Mr. F. Smith, of the British Museum,
to whom I am much indebted for information on this and other subjects.
Although fully trusting to the statements of Huber and Mr. Smith, I tried to
approach the subject in a sceptical frame of mind, as any one may well be
excused for doubting the truth of so extraordinary and odious an instinct as
that of making slaves. Hence I will give the observations which I have made
myself made, in some little detail. I opened fourteen nests of F. sanguinea,
and found a few slaves in all. Males and fertile females of the
slave-species are found only in their own proper communities, and have never
been observed in the nests of F. sanguinea. The slaves are black and not
above half the size of their red masters, so that the contrast in their
appearance is very great. When the nest is slightly disturbed, the slaves
occasionally come out, and like their masters are much agitated and defend
their nest: when the nest is much disturbed and the larvae and pupae are
exposed, the slaves work energetically with their masters in carrying them
away to a place of safety. Hence, it is clear, that the slaves feel quite at
home. During the months of June and July, on three successive years, I have
watched for many hours several nests in Surrey and Sussex, and never saw a
slave either leave or enter a nest. As, during these months, the slaves are
very few in number, I thought that they might behave differently when more
numerous; but Mr. Smith informs me that he has watched the nests at various
hours during May, June and August, both in Surrey and Hampshire, and has
never seen the slaves, though present in large numbers in August, either
leave or enter the nest. Hence he considers them as strictly household
slaves. The masters, on the other hand, may be constantly seen bringing in
materials for the nest, and food of all kinds. During the present year,
however, in the month of July, I came across a community with an unusually
large stock of slaves, and I observed a few slaves mingled with their
masters leaving the nest, and marching along the same road to a tall
Scotch-fir-tree, twenty-five yards distant, which they ascended together,
probably in search of aphides or cocci. According to Huber, who had ample
opportunities for observation, in Switzerland the slaves habitually work
with their masters in making the nest, and they alone open and close the
doors in the morning and evening; and, as Huber expressly states, their
principal office is to search for aphides. This difference in the usual
habits of the masters and slaves in the two countries, probably depends
merely on the slaves being captured in greater numbers in Switzerland than
in England.

One day I fortunately chanced to witness a migration from one nest to
another, and it was a most interesting spectacle to behold the masters
carefully carrying, as Huber has described, their slaves in their jaws.
Another day my attention was struck by about a score of the slave-makers
haunting the same spot, and evidently not in search of food; they approached
and were vigorously repulsed by an independent community of the slave
species (F. fusca); sometimes as many as three of these ants clinging to the
legs of the slave-making F. sanguinea. The latter ruthlessly killed their
small opponents, and carried their dead bodies as food to their nest,
twenty-nine yards distant; but they were prevented from getting any pupae to
rear as slaves. I then dug up a small parcel of the pupae of F. fusca from
another nest, and put them down on a bare spot near the place of combat;
they were eagerly seized, and carried off by the tyrants, who perhaps
fancied that, after all, they had been victorious in their late combat.

At the same time I laid on the same place a small parcel of the pupae of
another species, F. flava, with a few of these little yellow ants still
clinging to the fragments of the nest. This species is sometimes, though
rarely, made into slaves, as has been described by Mr Smith. Although so
small a species, it is very courageous, and I have seen it ferociously
attack other ants. In one instance I found to my surprise an independent
community of F. flava under a stone beneath a nest of the slave-making F.
sanguinea; and when I had accidentally disturbed both nests, the little ants
attacked their big neighbours with surprising courage. Now I was curious to
ascertain whether F. sanguinea could distinguish the pupae of F. fusca,
which they habitually make into slaves, from those of the little and furious
F. flava, which they rarely capture, and it was evident that they did at
once distinguish them: for we have seen that they eagerly and instantly
seized the pupae of F. fusca, whereas they were much terrified when they
came across the pupae, or even the earth from the nest of F. flava, and
quickly ran away; but in about a quarter of an hour, shortly after all the
little yellow ants had crawled away, they took heart and carried off the
pupae.

One evening I visited another community of F. sanguinea, and found a number
of these ants entering their nest, carrying the dead bodies of F. fusca
(showing that it was not a migration) and numerous pupae. I traced the
returning file burthened with booty, for about forty yards, to a very thick
clump of heath. whence I saw the last individual of F. sanguinea emerge,
carrying a pupa; but I was not able to find the desolated nest in the thick
heath. The nest, however, must have been close at hand, for two or three
individuals of F. fusca were rushing about in the greatest agitation, and
one was perched motionless with its own pupa in its mouth on the top of a
spray of heath over its ravaged home.

Such are the facts, though they did not need confirmation by me, in regard
to the wonderful instinct of making slaves. Let it be observed what a
contrast the instinctive habits of F. sanguinea present with those of the F.
rufescens. The latter does not build its own nest, does not determine its
own migrations, does not collect food for itself or its young, and cannot
even feed itself: it is absolutely dependent on its numerous slaves. Formica
sanguinea, on the other hand, possesses much fewer slaves, and in the early
part of the summer extremely few. The masters determine when and where a new
nest shall be formed, and when they migrate, the masters carry the slaves.
Both in Switzerland and England the slaves seem to have the exclusive care
of the larvae, and the masters alone go on slave-making expeditions. In
Switzerland the slaves and masters work together, making and bringing
materials for the nest: both, but chiefly the slaves, tend, and milk as it
may be called, their aphides; and thus both collect food for the community.
In England the masters alone usually leave the nest to collect building
materials and food for themselves, their slaves and larvae. So that the
masters in this country receive much less service from their slaves than
they do in Switzerland.

By what steps the instinct of F. sanguinea originated I will not pretend to
conjecture. But as ants, which are not slave-makers, will, as I have seen,
carry off pupae of other species, if scattered near their nests, it is
possible that pupae originally stored as food might become developed; and
the ants thus unintentionally reared would then follow their proper
instincts, and do what work they could. If their presence proved useful to
the species which had seized them if it were more advantageous to this
species to capture workers than to procreate them the habit of collecting
pupae originally for food might by natural selection be strengthened and
rendered permanent for the very different purpose of raising slaves. When
the instinct was once acquired, if carried out to a much less extent even
than in our British F. sanguinea, which, as we have seen, is less aided by
its slaves than the same species in Switzerland, I can see no difficulty in
natural selection increasing and modifying the instinct always supposing
each modification to be of use to the species until an ant was formed as
abjectly dependent on its slaves as is the Formica rufescens.

Cell-making instinct of the Hive-Bee. I will not here enter on minute
details on this subject, but will merely give an outline of the conclusions
at which I have arrived. He must be a dull man who can examine the exquisite
structure of a comb, so beautifully adapted to its end, without enthusiastic
admiration. We hear from mathematicians that bees have practically solved a
recondite problem, and have made their cells of the proper shape to hold the
greatest possible amount of honey, with the least possible consumption of
previous wax in their construction. It has been remarked that a skilful
workman, with fitting tools and measures, would find it very difficult to
make cells of wax of the true form, though this is perfectly effected by a
crowd of bees working in a dark hive. Grant whatever instincts you please,
and it seems at first quite inconceivable how they can make all the
necessary angles and planes, or even perceive when they are correctly made.
But the difficulty is not nearly so great as it at first appears: all this
beautiful work can be shown, I think, to follow from a few very simple
instincts.

I was led to investigate this subject by Mr. Waterhouse, who has shown that
the form of the cell stands in close relation to the presence of adjoining
cells; and the following view may, perhaps, be considered only as a
modification of this theory. Let us look to the great principle of
gradation, and see whether Nature does not reveal to us her method of work.
At one end of a short series we have humble-bees, which use their old
cocoons to hold honey, sometimes adding to them short tubes of wax, and
likewise making separate and very irregular rounded cells of wax. At the
other end of the series we have the cells of the hive-bee, placed in a
double layer: each cell, as is well know, is an hexagonal prism, with the
basal edges of its six sides bevelled so as to join on to a pyramid, formed
of three rhombs. These rhombs have certain angles, and the three which form
the pyramidal base of a single cell on one side of the comb, enter into the
composition of the bases of three adjoining cells on the opposite side. In
the series between the extreme perfection of the cells of the hive-bee and
the simplicity of those of the humble-bee, we have the cells of the Mexican
Melipona domestica, carefully described and figured by Pierre Huber. The
Melipona itself is intermediate in structure between the hive and humble
bee, but more nearly related to the latter: it forms a nearly regular waxen
comb of cylindrical cells, in which the young are hatched, and, in addition,
some large cells of wax for holding honey. These latter cells are nearly
spherical and of nearly equal sizes, and are aggregated into an irregular
mass. But the important point to notice, is that these cells are always made
at that degree of nearness to each other, that they would have intersected
or broken into each other, if the spheres had been completed; but this is
never permitted, the bees building perfectly flat walls of wax between the
spheres which thus tend to intersect. Hence each cell consists of an outer
spherical portion and of two, three, or more perfectly flat surfaces,
according as the cell adjoins two, three or more other cells. When one cell
comes into contact with three other cells, which, from the spheres being
nearly of the same size, is very frequently and necessarily the case, the
three flat surfaces are united into a pyramid; and this pyramid, as Huber
has remarked, is manifestly a gross imitation of the three-sided pyramidal
basis of the cell of the hive-bee. As in the cells of the hive-bee, so here,
the three plane surfaces in any one cell necessarily enter into the
construction of three adjoining cells. It is obvious that the Melipona saves
wax by this manner of building; for the flat walls between the adjoining
cells are not double, but are of the same thickness as the outer spherical
portions, and yet each flat portion forms a part of two cells.

Reflecting on this case, it occurred to me that if the Melipona had made its
spheres at some given distance from each other, and had made them of equal
sizes and had arranged them symmetrically in a double layer, the resulting
structure would probably have been as perfect as the comb of the hive-bee.
Accordingly I wrote to Professor Miller, of Cambridge, and this geometer has
kindly read over the following statement, drawn up from his information, and
tells me that it is strictly correct:-

If a number of equal spheres be described with their centres placed in two
parallel layers; with the centre of each sphere at the distance of radius X
/sqrt[2] or radius X 1.41421 (or at some lesser distance), from the centres
of the six surrounding spheres in the same layer; and at the same distance
from the centres of the adjoining spheres in the other and parallel layer;
then, if planes of intersection between the several spheres in both layers
be formed, there will result a double layer of hexagonal prisms united
together by pyramidal bases formed of three rhombs; and the rhombs and the
sides of the hexagonal prisms will have every angle identically the same
with the best measurements which have been made of the cells of the
hive-bee.

Hence we may safely conclude that if we could slightly modify the instincts
already possessed by the Melipona, and in themselves not very wonderful,
this bee would make a structure as wonderfully perfect as that of the
hive-bee. We must suppose the Melipona to make her cells truly spherical,
and of equal sizes; and this would not be very surprising, seeing that she
already does so to a certain extent, and seeing what perfectly cylindrical
burrows in wood many insects can make, apparently by turning round on a
fixed point. We must suppose the Melipona to arrange her cells in level
layers, as she already does her cylindrical cells; and we must further
suppose, and this is the greatest difficulty, that she can somehow judge
accurately at what distance to stand from her fellow-labourers when several
are making their spheres; but she is already so far enabled to judge of
distance, that she always describes her spheres so as to intersect largely;
and then she unites the points of intersection by perfectly flat surfaces.
We have further to suppose, but this is no difficulty, that after hexagonal
prisms have been formed by the intersection of adjoining spheres in the same
layer, she can prolong the hexagon to any length requisite to hold the stock
of honey; in the same way as the rude humble-bee adds cylinders of wax to
the circular mouths of her old cocoons. By such modifications of instincts
in themselves not very wonderful, hardly more wonderful than those which
guide a bird to make its nest, I believe that the hive-bee has acquired,
through natural selection, her inimitable architectural powers.

But this theory can be tested by experiment. Following the example of Mr
Tegetmeier, I separated two combs, and put between them a long, thick,
square strip of wax: the bees instantly began to excavate minute circular
pits in it; and as they deepened these little pits, they made them wider and
wider until they were converted into shallow basins, appearing to the eye
perfectly true or parts of a sphere, and of about the diameter of a cell. It
was most interesting to me to observe that wherever several bees had begun
to excavate these basins near together, they had begun their work at such a
distance from each other, that by the time the basins had acquired the above
stated width (i.e. about the width of an ordinary cell), and were in depth
about one sixth of the diameter of the sphere of which they formed a part,
the rims of the basins intersected or broke into each other. As soon as this
occurred, the bees ceased to excavate, and began to build up flat walls of
wax on the lines of intersection between the basins, so that each hexagonal
prism was built upon the festooned edge of a smooth basin, instead of on the
straight edges of a three-sided pyramid as in the case of ordinary cells.

I then put into the hive, instead of a thick, square piece of wax, a thin
and narrow, knife-edged ridge, coloured with vermilion. The bees instantly
began on both sides to excavate little basins near to each other, in the
same way as before; but the ridge of wax was so thin, that the bottoms of
the basins, if they had been excavated to the same depth as in the former
experiment, would have broken into each other from the opposite sides. The
bees, however, did not suffer this to happen, and they stopped their
excavations in due time; so that the basins, as soon as they had been a
little deepened, came to have flat bottoms; and these flat bottoms, formed
by thin little plates of the vermilion wax having been left ungnawed, were
situated, as far as the eye could judge, exactly along the planes of
imaginary intersection between the basins on the opposite sides of the ridge
of wax. In parts, only little bits, in other parts, large portions of a
rhombic plate had been left between the opposed basins, but the work, from
the unnatural state of things, had not been neatly performed. The bees must
have worked at very nearly the same rate on the opposite side of the ridge
of vermilion wax, as they circularly gnawed away and deepened the basins on
both sides, in order to have succeeded in thus leaving flat plates between
the basins, by stopping work along the intermediate planes or planes of
intersection.

Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip of wax,
perceiving when they have gnawed the wax away to the proper thinness, and
then stopping their work. In ordinary combs it has appeared to me that the
bees do not always succeed in working at exactly the same rate from the
opposite sides; for I have noticed half-completed rhombs at the base of a
just-commenced cell, which were slightly concave on one side, where I
suppose that the bees had excavated too quickly, and convex on the opposed
side, where the bees had worked less quickly. In one well-marked instance, I
put the comb back into the hive and allowed the bees to go on working for a
short time and again examined the cell, and I found that the rhombic plate
had been completed, and had become perfectly flat: it was absolutely
impossible, from the extreme thinness of the little rhombic plate, that they
could have affected this by gnawing away the convex side; and I suspect that
the bees in such cases stand in the opposed cells and push and bend the
ductile and warm wax (which as I have tried is easily done) into its proper
intermediate plane, and thus flatten it.

From the experiment of the ridge of vermilion wax, we can clearly see that
if the bees were to build for themselves a thin wall of wax, they could make
their cells of the proper shape, by standing at the proper distance from
each other, by excavating at the same rate, and by endeavouring to make
equal spherical hollows, but never allowing the spheres to break into each
other. Now bees, as may be clearly seen by examining the edge of a growing
comb, do make a rough, circumferential wall or rim all round the comb; and
they gnaw into this from the opposite sides, always working circularly as
they deepen each cell. They do not make the whole three-sided pyramidal base
of any one cell at the same time, but only the one rhombic plate which
stands on the extreme growing margin, or the two plates, as the case may be;
and they never complete the upper edges of the rhombic plates, until the
hexagonal walls are commenced. Some of these statements differ from those
made by the justly celebrated elder Huber, but I am convinced of their
accuracy; and if I had space, I could show that they are conformable with my
theory.

Huber's statement that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly correct;
the first commencement having always been a little hood of wax; but I will
not here enter on these details. We see how important a part excavation
plays in the construction of the cells; but it would be a great error to
suppose that the bees cannot build up a rough wall of wax in the proper
position that is, along the plane of intersection between two adjoining
spheres. I have several specimens showing clearly that they can do this.
Even in the rude circumferential rim or wall of wax round a growing comb,
flexures may sometimes be observed, corresponding in position to the planes
of the rhombic basal plates of future cells. But the rough wall of wax has
in every case to be finished off, by being largely gnawed away on both
sides. The manner in which the bees build is curious; they always make the
first rough wall from ten to twenty times thicker than the excessively thin
finished wall of the cell, which will ultimately be left. We shall
understand how they work, by supposing masons first to pile up a broad ridge
of cement, and then to begin cutting it away equally on both sides near the
ground, till a smooth, very thin wall is left in the middle; the masons
always piling up the cut-away cement, and adding fresh cement, on the summit
of the ridge. We shall thus have a thin wall steadily growing upward; but
always crowned by a gigantic coping. From all the cells, both those just
commenced and those completed, being thus crowned by a strong coping of wax,
the bees can cluster and crawl over the comb without injuring the delicate
hexagonal walls, which are only about one four-hundredth of an inch in
thickness; the plates of the pyramidal basis being about twice as thick. By
this singular manner of building, strength is continually given to the comb,
with the utmost ultimate economy of wax.

It seems at first to add to the difficulty of understanding how the cells
are made, that a multitude of bees all work together; one bee after working
a short time at one cell going to another, so that, as Huber has stated, a
score of individuals work even at the commencement of the first cell. I was
able practically to show this fact, by covering the edges of the hexagonal
walls of a single cell, or the extreme margin of the circumferential rim of
a growing comb, with an extremely thin layer of melted vermilion wax; and I
invariably found that the colour was most delicately diffused by the bees as
delicately as a painter could have done with his brush by atoms of the
coloured wax having been taken from the spot on which it had been placed,
and worked into the growing edges of the cells all round. The work of
construction seems to be a sort of balance struck between many bees, all
instinctively standing at the same relative distance from each other, all
trying to sweep equal spheres, and then building up, or leaving ungnawed,
the planes of intersection between these spheres. It was really curious to
note in cases of difficulty, as when two pieces of comb met at an angle, how
often the bees would entirely pull down and rebuild in different ways the
same cell, sometimes recurring to a shape which they had at first rejected.

When bees have a place on which they can stand in their proper positions for
working, for instance, on a slip of wood, placed directly under the middle
of a comb growing downwards so that the comb has to be built over one face
of the slip in this case the bees can lay the foundations of one wall of a
new hexagon, in its strictly proper place, projecting beyond the other
completed cells. It suffices that the bees should be enabled to stand at
their proper relative distances from each other and from the walls of the
last completed cells, and then, by striking imaginary spheres, they can
build up a wall intermediate between two adjoining spheres; but, as far as I
have seen, they never gnaw away and finish off the angles of a cell till a
large part both of that cell and of the adjoining cells has been built. This
capacity in bees of laying down under certain circumstances a rough wall in
its proper place between two just-commenced cells, is important, as it bears
on a fact, which seems at first quite subversive of the foregoing theory;
namely, that the cells on the extreme margin of wasp-combs are sometimes
strictly hexagonal; but I have not space here to enter on this subject. Nor
does there seem to me any great difficulty in a single insect (as in the
case of a queen-wasp) making hexagonal cells, if she work alternately on the
inside and outside of two or three cells commenced at the same time, always
standing at the proper relative distance from the parts of the cells just
begun, sweeping spheres or cylinders, and building up intermediate planes.
It is even conceivable that an insect might, by fixing on a point at which
to commence a cell, and then moving outside, first to one point, and then to
five other points, at the proper relative distances from the central point
and from each other, strike the planes of intersection, and so make an
isolated hexagon: but I am not aware that any such case has been observed;
nor would any good be derived from a single hexagon being built, as in its
construction more materials would be required than for a cylinder.

As natural selection acts only by the accumulation of slight modifications
of structure or instinct, each profitable to the individual under its
conditions of life, it may reasonably be asked, how a long and graduated
succession of modified architectural instincts, all tending towards the
present perfect plan of construction, could have profited the progenitors of
the hive-bee? I think the answer is not difficult: it is known that bees are
often hard pressed to get sufficient nectar; and I am informed by Mr.
Tegetmeier that it has been experimentally found that no less than from
twelve to fifteen pounds of dry sugar are consumed by a hive of bees for the
secretion of each pound of wax; so that a prodigious quantity of fluid
nectar must be collected and consumed by the bees in a hive for the
secretion of the wax necessary for the construction of their combs.
Moreover, many bees have to remain idle for many days during the process of
secretion. A large store of honey is indispensable to support a large stock
of bees during the winter; and the security of the hive is known mainly to
depend on a large number of bees being supported. Hence the saving of wax by
largely saving honey must be a most important element of success in any
family of bees. Of course the success of any species of bee may be dependent
on the number of its parasites or other enemies, or on quite distinct
causes, and so be altogether independent of the quantity of honey which the
bees could collect. But let us suppose that this latter circumstance
determined, as it probably often does determine, the numbers of a humble-bee
which could exist in a country; and let us further suppose that the
community lived throughout the winter, and consequently required a store of
honey: there can in this case be no doubt that it would be an advantage to
our humble-bee, if a slight modification of her instinct led her to make her
waxen cells near together, so as to intersect a little; for a wall in common
even to two adjoining cells, would save some little wax. Hence it would
continually be more and more advantageous to our humble-bee, if she were to
make her cells more and more regular, nearer together, and aggregated into a
mass, like the cells of the Melipona; for in this case a large part of the
bounding surface of each cell would serve to bound other cells, and much wax
would be saved. Again, from the same cause, it would be advantageous to the
Melipona, if she were to make her cells closer together, and more regular in
every way than at present; for then, as we have seen, the spherical surfaces
would wholly disappear, and would all be replaced by plane surfaces; and the
Melipona would make a comb as perfect as that of the hive-bee. Beyond this
stage of perfection in architecture, natural selection could not lead; for
the comb of the hive-bee, as far as we can see, is absolutely perfect in
economising wax.

Thus, as I believe, the most wonderful of all known instincts, that of the
hive-bee, can be explained by natural selection having taken advantage of
numerous, successive, slight modifications of simpler instincts; natural
selection having by slow degrees, more and more perfectly, led the bees to
sweep equal spheres at a given distance from each other in a double layer,
and to build up and excavate the wax along the planes of intersection. The
bees, of course, no more knowing that they swept their spheres at one
particular distance from each other, than they know what are the several
angles of the hexagonal prisms and of the basal rhombic plates. The motive
power of the process of natural selection having been economy of wax; that
individual swarm which wasted least honey in the secretion of wax, having
succeeded best, and having transmitted by inheritance its newly acquired
economical instinct to new swarms, which in their turn will have had the
best chance of succeeding in the struggle for existence.

No doubt many instincts of very difficult explanation could be opposed to
the theory of natural selection, cases, in which we cannot see how an
instinct could possibly have originated; cases, in which no intermediate
gradations are known to exist; cases of instinct of apparently such trifling
importance, that they could hardly have been acted on by natural selection;
cases of instincts almost identically the same in animals so remote in the
scale of nature, that we cannot account for their similarity by inheritance
from a common parent, and must therefore believe that they have been
acquired by independent acts of natural selection. I will not here enter on
these several cases, but will confine myself to one special difficulty,
which at first appeared to me insuperable, and actually fatal to my whole
theory. I allude to the neuters or sterile females in insect-communities:
for these neuters often differ widely in instinct and in structure from both
the males and fertile females, and yet, from being sterile, they cannot
propagate their kind.

The subject well deserves to be discussed at great length, but I will here
take only a single case, that of working or sterile ants. How the workers
have been rendered sterile is a difficulty; but not much greater than that
of any other striking modification of structure; for it can be shown that
some insects and other articulate animals in a state of nature occasionally
become sterile; and if such insects had been social, and it had been
profitable to the community that a number should have been annually born
capable of work, but incapable of procreation, I can see no very great
difficulty in this being effected by natural selection. But I must pass over
this preliminary difficulty. The great difficulty lies in the working ants
differing widely from both the males and the fertile females in structure,
as in the shape of the thorax and in being destitute of wings and sometimes
of eyes, and in instinct. As far as instinct alone is concerned, the
prodigious difference in this respect between the workers and the perfect
females, would have been far better exemplified by the hive-bee. If a
working ant or other neuter insect had been an animal in the ordinary state,
I should have unhesitatingly assumed that all its characters had been slowly
acquired through natural selection; namely, by an individual having been
born with some slight profitable modification of structure, this being
inherited by its offspring, which again varied and were again selected, and
so onwards. But with the working ant we have an insect differing greatly
from its parents, yet absolutely sterile; so that it could never have
transmitted successively acquired modifications of structure or instinct to
its progeny. It may well be asked how is it possible to reconcile this case
with the theory of natural selection?

First, let it be remembered that we have innumerable instances, both in our
domestic productions and in those in a state of nature, of all sorts of
differences of structure which have become correlated to certain ages, and
to either sex. We have differences correlated not only to one sex, but to
that short period alone when the reproductive system is active, as in the
nuptial plumage of many birds, and in the hooked jaws of the male salmon. We
have even slight differences in the horns of different breeds of cattle in
relation to an artificially imperfect state of the male sex; for oxen of
certain breeds have longer horns than in other breeds, in comparison with
the horns of the bulls or cows of these same breeds. Hence I can see no real
difficulty in any character having become correlated with the sterile
condition of certain members of insect-communities: the difficulty lies in
understanding how such correlated modifications of structure could have been
slowly accumulated by natural selection.

This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be applied to
the family, as well as to the individual, and may thus gain the desired end.
Thus, a well-flavoured vegetable is cooked, and the individual is destroyed;
but the horticulturist sows seeds of the same stock, and confidently expects
to get nearly the same variety; breeders of cattle wish the flesh and fat to
be well marbled together; the animal has been slaughtered, but the breeder
goes with confidence to the same family. I have such faith in the powers of
selection, that I do not doubt that a breed of cattle, always yielding oxen
with extraordinarily long horns, could be slowly formed by carefully
watching which individual bulls and cows, when matched, produced oxen with
the longest horns; and yet no one ox could ever have propagated its kind.
Thus I believe it has been with social insects: a slight modification of
structure, or instinct, correlated with the sterile condition of certain
members of the community, has been advantageous to the community:
consequently the fertile males and females of the same community flourished,
and transmitted to their fertile offspring a tendency to produce sterile
members having the same modification. And I believe that this process has
been repeated, until that prodigious amount of difference between the
fertile and sterile females of the same species has been produced, which we
see in many social insects.

But we have not as yet touched on the climax of the difficulty; namely, the
fact that the neuters of several ants differ, not only from the fertile
females and males, but from each other, sometimes to an almost incredible
degree, and are thus divided into two or even three castes. The castes,
moreover, do not generally graduate into each other, but are perfectly well
defined; being as distinct from each other, as are any two species of the
same genus, or rather as any two genera of the same family. Thus in Eciton,
there are working and soldier neuters, with jaws and instincts
extraordinarily different: in Cryptocerus, the workers of one caste alone
carry a wonderful sort of shield on their heads, the use of which is quite
unknown: in the Mexican Myrmecocystus, the workers of one caste never leave
the nest; they are fed by the workers of another caste, and they have an
enormously developed abdomen which secretes a sort of honey, supplying the
place of that excreted by the aphides, or the domestic cattle as they may be
called, which our European ants guard or imprison.

It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful and
well-established facts at once annihilate my theory. In the simpler case of
neuter insects all of one caste or of the same kind, which have been
rendered by natural selection, as I believe to be quite possible, different
from the fertile males and females, in this case, we may safely conclude
from the analogy of ordinary variations, that each successive, slight,
profitable modification did not probably at first appear in all the
individual neuters in the same nest, but in a few alone; and that by the
long-continued selection of the fertile parents which produced most neuters
with the profitable modification, all the neuters ultimately came to have
the desired character. On this view we ought occasionally to find
neuter-insects of the same species, in the same nest, presenting gradations
of structure; and this we do find, even often, considering how few
neuter-insects out of Europe have been carefully examined. Mr F. Smith has
shown how surprisingly the neuters of several British ants differ from each
other in size and sometimes in colour; and that the extreme forms can
sometimes be perfectly linked together by individuals taken out of the same
nest: I have myself compared perfect gradations of this kind. It often
happens that the larger or the smaller sized workers are the most numerous;
or that both large and small are numerous, with those of an intermediate
size scanty in numbers. Formica flava has larger and smaller workers, with
some of intermediate size; and, in this species, as Mr F. Smith has
observed, the larger workers have simple eyes (ocelli), which though small
can be plainly distinguished, whereas the smaller workers have their ocelli
rudimentary. Having carefully dissected several specimens of these workers,
I can affirm that the eyes are far more rudimentary in the smaller workers
than can be accounted for merely by their proportionally lesser size; and I
fully believe, though I dare not assert so positively, that the workers of
intermediate size have their ocelli in an exactly intermediate condition. So
that we here have two bodies of sterile workers in the same nest, differing
not only in size, but in their organs of vision, yet connected by some few
members in an intermediate condition. I may digress by adding, that if the
smaller workers had been the most useful to the community, and those males
and females had been continually selected, which produced more and more of
the smaller workers, until all the workers had come to be in this condition;
we should then have had a species of ant with neuters very nearly in the
same condition with those of Myrmica. For the workers of Myrmica have not
even rudiments of ocelli, though the male and female ants of this genus have
well-developed ocelli.

I may give one other case: so confidently did I expect to find gradations in
important points of structure between the different castes of neuters in the
same species, that I gladly availed myself of Mr F. Smith's offer of
numerous specimens from the same nest of the driver ant (Anomma) of West
Africa. The reader will perhaps best appreciate the amount of difference in
these workers, by my giving not the actual measurements, but a strictly
accurate illustration: the difference was the same as if we were to see a
set of workmen building a house of whom many were five feet four inches
high, and many sixteen feet high; but we must suppose that the larger
workmen had heads four instead of three times as big as those of the smaller
men, and jaws nearly five times as big. The jaws, moreover, of the working
ants of the several sizes differed wonderfully in shape, and in the form and
number of the teeth. But the important fact for us is, that though the
workers can be grouped into castes of different sizes, yet they graduate
insensibly into each other, as does the widely-different structure of their
jaws. I speak confidently on this latter point, as Mr Lubbock made drawings
for me with the camera lucida of the jaws which I had dissected from the
workers of the several sizes.

With these facts before me, I believe that natural selection, by acting on
the fertile parents, could form a species which should regularly produce
neuters, either all of large size with one form of jaw, or all of small size
with jaws having a widely different structure; or lastly, and this is our
climax of difficulty, one set of workers of one size and structure, and
simultaneously another set of workers of a different size and structure; a
graduated series having been first formed, as in the case of the driver ant,
and then the extreme forms, from being the most useful to the community,
having been produced in greater and greater numbers through the natural
selection of the parents which generated them; until none with an
intermediate structure were produced.

Thus, as I believe, the wonderful fact of two distinctly defined castes of
sterile workers existing in the same nest, both widely different from each
other and from their parents, has originated. We can see how useful their
production may have been to a social community of insects, on the same
principle that the division of labour is useful to civilised man. As ants
work by inherited instincts and by inherited tools or weapons, and not by
acquired knowledge and manufactured instruments, a perfect division of
labour could be effected with them only by the workers being sterile; for
had they been fertile, they would have intercrossed, and their instincts and
structure would have become blended. And nature has, as I believe, effected
this admirable division of labour in the communities of ants, by the means
of natural selection. But I am bound to confess, that, with all my faith in
this principle, I should never have anticipated that natural selection could
have been efficient in so high a degree, had not the case of these neuter
insects convinced me of the fact. I have, therefore, discussed this case, at
some little but wholly insufficient length, in order to show the power of
natural selection, and likewise because this is by far the most serious
special difficulty, which my theory has encountered. The case, also, is very
interesting, as it proves that with animals, as with plants, any amount of
modification in structure can be effected by the accumulation of numerous,
slight, and as we must call them accidental, variations, which are in any
manner profitable, without exercise or habit having come into play. For no
amount of exercise, or habit, or volition, in the utterly sterile members of
a community could possibly have affected the structure or instincts of the
fertile members, which alone leave descendants. I am surprised that no one
has advanced this demonstrative case of neuter insects, against the
well-known doctrine of Lamarck.

Summary. I have endeavoured briefly in this chapter to show that the mental
qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that instincts vary
slightly in a state of nature. No one will dispute that instincts are of the
highest importance to each animal. Therefore I can see no difficulty, under
changing conditions of life, in natural selection accumulating slight
modifications of instinct to any extent, in any useful direction. In some
cases habit or use and disuse have probably come into play. I do not pretend
that the facts given in this chapter strengthen in any great degree my
theory; but none of the cases of difficulty, to the best of my judgment,
annihilate it. On the other hand, the fact that instincts are not always
absolutely perfect and are liable to mistakes; that no instinct has been
produced for the exclusive good of other animals, but that each animal takes
advantage of the instincts of others; that the canon in natural history, of
'natura non facit saltum' is applicable to instincts as well as to corporeal
structure, and is plainly explicable on the foregoing views, but is
otherwise inexplicable, all tend to corroborate the theory of natural
selection.

This theory is, also, strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but certainly distinct,
species, when inhabiting distant parts of the world and living under
considerably different conditions of life, yet often retaining nearly the
same instincts. For instance, we can understand on the principle of
inheritance, how it is that the thrush of South America lines its nest with
mud, in the same peculiar manner as does our British thrush: how it is that
the male wrens (Troglodytes) of North America, build 'cock-nests,' to roost
in, like the males of our distinct Kitty-wrens, a habit wholly unlike that
of any other known bird. Finally, it may not be a logical deduction, but to
my imagination it is far more satisfactory to look at such instincts as the
young cuckoo ejecting its foster-brothers, ants making slaves, -- the larvae
of ichneumonidae feeding within the live bodies of caterpillars, not as
specially endowed or created instincts, but as small consequences of one
general law, leading to the advancement of all organic beings, namely,
multiply, vary, let the strongest live and the weakest die.

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Chapter 8 - Hybridism

[* ] Distinction between the sterility of first crosses and of hybrids [* ]
Sterility various in degree, not universal, affected by close interbreeding,
removed by domestication [* ] Laws governing the sterility of hybrids [* ]
Sterility not a special endowment, but incidental on other differences [* ]
Causes of the sterility of first crosses and of hybrids [* ] Parallelism
between the effects of changed conditions of life and crossing [* ]
Fertility of varieties when crossed and of their mongrel offspring not
universal [* ] Hybrids and mongrels compared independently of their
fertility [* ] Summary
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THE view generally entertained by naturalists is that species, when
intercrossed, have been specially endowed with the quality of sterility, in
order to prevent the confusion of all organic forms. This view certainly
seems at first probable, for species within the same country could hardly
have kept distinct had they been capable of crossing freely. The importance
of the fact that hybrids are very generally sterile, has, I think, been much
underrated by some late writers. On the theory of natural selection the case
is especially important, inasmuch as the sterility of hybrids could not
possibly be of any advantage to them, and therefore could not have been
acquired by the continued preservation of successive profitable degrees of
sterility. I hope, however, to be able to show that sterility is not a
specially acquired or endowed quality, but is incidental on other acquired
differences.

In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded together; namely,
the sterility of two species when first crossed, and the sterility of the
hybrids produced from them.

Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no offspring.
Hybrids, on the other hand, have their reproductive organs functionally
impotent, as may be clearly seen in the state of the male element in both
plants and animals; though the organs themselves are perfect in structure,
as far as the microscope reveals. In the first case the two sexual elements
which go to form the embryo are perfect; in the second case they are either
not at all developed, or are imperfectly developed. This distinction is
important, when the cause of the sterility, which is common to the two
cases, has to be considered. The distinction has probably been slurred over,
owing to the sterility in both cases being looked on as a special endowment,
beyond the province of our reasoning powers.

The fertility of varieties, that is of the forms known or believed to have
descended from common parents, when intercrossed, and likewise the fertility
of their mongrel offspring, is, on my theory, of equal importance with the
sterility of species; for it seems to make a broad and clear distinction
between varieties and species.

First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of those
two conscientious and admirable observers, Kölreuter and Gärtner, who almost
devoted their lives to this subject, without being deeply impressed with the
high generality of some degree of sterility. Kölreuter makes the rule
universal; but then he cuts the knot, for in ten cases in which he found two
forms, considered by most authors as distinct species, quite fertile
together, he unhesitatingly ranks them as varieties. Gärtner, also, makes
the rule equally universal; and he disputes the entire fertility of
Kölreuter's ten cases. But in these and in many other cases, Gärtner is
obliged carefully to count the seeds, in order to show that there is any
degree of sterility. He always compares the maximum number of seeds produced
by two species when crossed and by their hybrid offspring, with the average
number produced by both pure parent-species in a state of nature. But a
serious cause of error seems to me to be here introduced: a plant to be
hybridised must be castrated, and, what is often more important, must be
secluded in order to prevent pollen being brought to it by insects from
other plants. Nearly all the plants experimentised on by Gärtner were
potted, and apparently were kept in a chamber in his house. That these
processes are often injurious to the fertility of a plant cannot be doubted;
for Gärtner gives in his table about a score of cases of plants which he
castrated, and artificially fertilised with their own pollen, and (excluding
all cases such as the Leguminosae, in which there is an acknowledged
difficulty in the manipulation) half of these twenty plants had their
fertility in some degree impaired. Moreover, as Gärtner during several years
repeatedly crossed the primrose and cowslip, which we have such good reason
to believe to be varieties, and only once or twice succeeded in getting
fertile seed; as he found the common red and blue pimpernels (Anagallis
arvensis and coerulea), which the best botanists rank as varieties,
absolutely sterile together; and as he came to the same conclusion in
several other analogous cases; it seems to me that we may well be permitted
to doubt whether many other species are really so sterile, when
intercrossed, as Gärtner believes.

It is certain, on the one hand, that the sterility of various species when
crossed is so different in degree and graduates away so insensibly, and, on
the other hand, that the fertility of pure species is so easily affected by
various circumstances, that for all practical purposes it is most difficult
to say where perfect fertility ends and sterility begins. I think no better
evidence of this can be required than that the two most experienced
observers who have ever lived, namely, Kölreuter and Gärtner, should have
arrived at diametrically opposite conclusions in regard to the very same
species. It is also most instructive to compare but I have not space here to
enter on details the evidence advanced by our best botanists on the question
whether certain doubtful forms should be ranked as species or varieties,
with the evidence from fertility adduced by different hybridisers, or by the
same author, from experiments made during different years. It can thus be
shown that neither sterility nor fertility affords any clear distinction
between species and varieties; but that the evidence from this source
graduates away, and is doubtful in the same degree as is the evidence
derived from other constitutional and structural differences.

In regard to the sterility of hybrids in successive generations; though
Gärtner was enabled to rear some hybrids, carefully guarding them from a
cross with either pure parent, for six or seven, and in one case for ten
generations, yet he asserts positively that their fertility never increased,
but generally greatly decreased. I do not doubt that this is usually the
case, and that the fertility often suddenly decreases in the first few
generations. Nevertheless I believe that in all these experiments the
fertility has been diminished by an independent cause, namely, from close
interbreeding. I have collected so large a body of facts, showing that close
interbreeding lessens fertility, and, on the other hand, that an occasional
cross with a distinct individual or variety increases fertility, that I
cannot doubt the correctness of this almost universal belief amongst
breeders. Hybrids are seldom raised by experimentalists in great numbers;
and as the parent-species, or other allied hybrids, generally grow in the
same garden, the visits of insects must be carefully prevented during the
flowering season: hence hybrids will generally be fertilised during each
generation by their own individual pollen; and I am convinced that this
would be injurious to their fertility, already lessened by their hybrid
origin. I am strengthened in this conviction by a remarkable statement
repeatedly made by Gärtner, namely, that if even the less fertile hybrids be
artificially fertilised with hybrid pollen of the same kind, their
fertility, notwithstanding the frequent ill effects of manipulation,
sometimes decidedly increases, and goes on increasing. Now, in artificial
fertilisation pollen is as often taken by chance (as I know from my own
experience) from the anthers of another flower, as from the anthers of the
flower itself which is to be fertilised; so that a cross between two
flowers, though probably on the same plant, would be thus effected.
Moreover, whenever complicated experiments are in progress, so careful an
observer as Gärtner would have castrated his hybrids, and this would have
insured in each generation a cross with the pollen from a distinct flower,
either from the same plant or from another plant of the same hybrid nature.
And thus, the strange fact of the increase of fertility in the successive
generations of artificially fertilised hybrids may, I believe, be accounted
for by close interbreeding having been avoided.

Now let us turn to the results arrived at by the third most experienced
hybridiser, namely, the Hon. and Rev. W. Herbert. He is as emphatic in his
conclusion that some hybrids are perfectly fertile as fertile as the pure
parent-species as are Kölreuter and Gärtner that some degree of sterility
between distinct species is a universal law of nature. He experimentised on
some of the very same species as did Gärtner. The difference in their
results may, I think, be in part accounted for by Herbert's great
horticultural skill, and by his having hothouses at his command. Of his many
important statements I will here give only a single one as an example,
namely, that 'every ovule in a pod of Crinum capense fertilised by C.
revolutum produced a plant, which (he says) I never saw to occur in a case
of its natural fecundation.' So that we here have perfect, or even more than
commonly perfect, fertility in a first cross between two distinct species.

This case of the Crinum leads me to refer to a most singular fact, namely,
that there are individual plants, as with certain species of Lobelia, and
with all the species of the genus Hippeastrum, which can be far more easily
fertilised by the pollen of another and distinct species, than by their own
pollen. For these plants have been found to yield seed to the pollen of a
distinct species, though quite sterile with their own pollen,
notwithstanding that their own pollen was found to be perfectly good, for it
fertilised distinct species. So that certain individual plants and all the
individuals of certain species can actually be hybridised much more readily
than they can be self-fertilised! For instance, a bulb of Hippeastrum
aulicum produced four flowers; three were fertilised by Herbert with their
own pollen, and the fourth was subsequently fertilised by the pollen of a
compound hybrid descended from three other and distinct species: the result
was that 'the ovaries of the three first flowers soon ceased to grow, and
after a few days perished entirely, whereas the pod impregnated by the
pollen of the hybrid made vigorous growth and rapid progress to maturity,
and bore good seed, which vegetated freely.' In a letter to me, in 1839, Mr
Herbert told me that he had then tried the experiment during five years, and
he continued to try it during several subsequent years, and always with the
same result. This result has, also, been confirmed by other observers in the
case of Hippeastrum with its sub-genera, and in the case of some other
genera, as Lobelia, Passiflora and Verbascum. Although the plants in these
experiments appeared perfectly healthy, and although both the ovules and
pollen of the same flower were perfectly good with respect to other species,
yet as they were functionally imperfect in their mutual self-action, we must
infer that the plants were in an unnatural state. Nevertheless these facts
show on what slight and mysterious causes the lesser or greater fertility of
species when crossed, in comparison with the same species when
self-fertilised, sometimes depends.

The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, &c., have been crossed, yet many of these hybrids
seed freely. For instance, Herbert asserts that a hybrid from Calceolaria
integrifolia and plantaginea, species most widely dissimilar in general
habit, 'reproduced itself as perfectly as if it had been a natural species
from the mountains of Chile.' I have taken some pains to ascertain the
degree of fertility of some of the complex crosses of Rhododendrons, and I
am assured that many of them are perfectly fertile. Mr C. Noble, for
instance, informs me that he raises stocks for grafting from a hybrid
between Rhod. Ponticum and Catawbiense, and that this hybrid 'seeds as
freely as it is possible to imagine.' Had hybrids, when fairly treated, gone
on decreasing in fertility in each successive generation, as Gärtner
believes to be the case, the fact would have been notorious to nurserymen.
Horticulturists raise large beds of the same hybrids, and such alone are
fairly treated, for by insect agency the several individuals of the same
hybrid variety are allowed to freely cross with each other, and the
injurious influence of close interbreeding is thus prevented. Any one may
readily convince himself of the efficiency of insect-agency by examining the
flowers of the more sterile kinds of hybrid rhododendrons, which produce no
pollen, for he will find on their stigmas plenty of pollen brought from
other flowers.

In regard to animals, much fewer experiments have been carefully tried than
with plants. If our systematic arrangements can be trusted, that is if the
genera of animals are as distinct from each other, as are the genera of
plants, then we may infer that animals more widely separated in the scale of
nature can be more easily crossed than in the case of plants; but the
hybrids themselves are, I think, more sterile. I doubt whether any case of a
perfectly fertile hybrid animal can be considered as thoroughly well
authenticated. It should, however, be borne in mind that, owing to few
animals breeding freely under confinement, few experiments have been fairly
tried: for instance, the canary-bird has been crossed with nine other
finches, but as not one of these nine species breeds freely in confinement,
we have no right to expect that the first crosses between them and the
canary, or that their hybrids, should be perfectly fertile. Again, with
respect to the fertility in successive generations of the more fertile
hybrid animals, I hardly know of an instance in which two families of the
same hybrid have been raised at the same time from different parents, so as
to avoid the ill effects of close interbreeding. On the contrary, brothers
and sisters have usually been crossed in each successive generation, in
opposition to the constantly repeated admonition of every breeder. And in
this case, it is not at all surprising that the inherent sterility in the
hybrids should have gone on increasing. If we were to act thus, and pair
brothers and sisters in the case of any pure animal, which from any cause
had the least tendency to sterility, the breed would assuredly be lost in a
very few generations.

Although I do not know of any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have some reason to believe that the
hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus
with p. torquatus and with p. versicolor are perfectly fertile. The hybrids
from the common and Chinese geese (A. cygnoides), species which are so
different that they are generally ranked in distinct genera, have often bred
in this country with either pure parent, and in one single instance they
have bred inter se. This was effected by Mr Eyton, who raised two hybrids
from the same parents but from different hatches; and from these two birds
he raised no less than eight hybrids (grandchildren of the pure geese) from
one nest. In India, however, these cross-bred geese must be far more
fertile; for I am assured by two eminently capable judges, namely Mr Blyth
and Capt. Hutton, that whole flocks of these crossed geese are kept in
various parts of the country; and as they are kept for profit, where neither
pure parent-species exists, they must certainly be highly fertile.

A doctrine which originated with Pallas, has been largely accepted by modern
naturalists; namely, that most of our domestic animals have descended from
two or more aboriginal species, since commingled by intercrossing. On this
view, the aboriginal species must either at first have produced quite
fertile hybrids, or the hybrids must have become in subsequent generations
quite fertile under domestication. This latter alternative seems to me the
most probable, and I am inclined to believe in its truth, although its rests
on no direct evidence. I believe, for instance, that our dogs have descended
from several wild stocks; yet, with perhaps the exception of certain
indigenous domestic dogs of South America, all are quite fertile together;
and analogy makes me greatly doubt, whether the several aboriginal species
would at first have freely bred together and have produced quite fertile
hybrids. So again there is reason to believe that our European and the
humped Indian cattle are quite fertile together; but from facts communicated
to me by Mr Blyth, I think they must be considered as distinct species. On
this view of the origin of many of our domestic animals, we must either give
up the belief of the almost universal sterility of distinct species of
animals when crossed; or we must look at sterility, not as an indelible
characteristic, but as one capable of being removed by domestication.

Finally, looking to all the ascertained facts on the intercrossing of plants
and animals, it may be concluded that some degree of sterility, both in
first crosses and in hybrids, is an extremely general result; but that it
cannot, under our present state of knowledge, be considered as absolutely
universal.

Laws governing the Sterility of first Crosses and of Hybrids. We will now
consider a little more in detail the circumstances and rules governing the
sterility of first crosses and of hybrids. Our chief object will be to see
whether or not the rules indicate that species have specially been endowed
with this quality, in order to prevent their crossing and blending together
in utter confusion. The following rules and conclusions are chiefly drawn up
from Gärtner's admirable work on the hybridisation of plants. I have taken
much pains to ascertain how far the rules apply to animals, and considering
how scanty our knowledge is in regard to hybrid animals, I have been
surprised to find how generally the same rules apply to both kingdoms.

It has been already remarked, that the degree of fertility, both of first
crosses and of hybrids, graduates from zero to perfect fertility. It is
surprising in how many curious ways this gradation can be shown to exist;
but only the barest outline of the facts can here be given. When pollen from
a plant of one family is placed on the stigma of a plant of a distinct
family, it exerts no more influence than so much inorganic dust. From this
absolute zero of fertility, the pollen of different species of the same
genus applied to the stigma of some one species, yields a perfect gradation
in the number of seeds produced, up to nearly complete or even quite
complete fertility; and, as we have seen, in certain abnormal cases, even to
an excess of fertility, beyond that which the plant's own pollen will
produce. So in hybrids themselves, there are some which never have produced,
and probably never would produce, even with the pollen of either pure
parent, a single fertile seed: but in some of these cases a first trace of
fertility may be detected, by the pollen of one of the pure parent-species
causing the flower of the hybrid to wither earlier than it otherwise would
have done; and the early withering of the flower is well known to be a sign
of incipient fertilisation. From this extreme degree of sterility we have
self-fertilised hybrids producing a greater and greater number of seeds up
to perfect fertility.

Hybrids from two species which are very difficult to cross, and which rarely
produce any offspring, are generally very sterile; but the parallelism
between the difficulty of making a first cross, and the sterility of the
hybrids thus produced two classes of facts which are generally confounded
together is by no means strict. There are many cases, in which two pure
species can be united with unusual facility, and produce numerous
hybrid-offspring, yet these hybrids are remarkably sterile. On the other
hand, there are species which can be crossed very rarely, or with extreme
difficulty, but the hybrids, when at last produced, are very fertile. Even
within the limits of the same genus, for instance in Dianthus, these two
opposite cases occur.

The fertility, both of first crosses and of hybrids, is more easily affected
by unfavourable conditions, than is the fertility of pure species. But the
degree of fertility is likewise innately variable; for it is not always the
same when the same two species are crossed under the same circumstances, but
depends in part upon the constitution of the individuals which happen to
have been chosen for the experiment. So it is with hybrids, for their degree
of fertility is often found to differ greatly in the several individuals
raised from seed out of the same capsule and exposed to exactly the same
conditions.

By the term systematic affinity is meant, the resemblance between species in
structure and in constitution, more especially in the structure of parts
which are of high physiological importance and which differ little in the
allied species. Now the fertility of first crosses between species, and of
the hybrids produced from them, is largely governed by their systematic
affinity. This is clearly shown by hybrids never having been raised between
species ranked by systematists in distinct families; and on the other hand,
by very closely allied species generally uniting with facility. But the
correspondence between systematic affinity and the facility of crossing is
by no means strict. A multitude of cases could be given of very closely
allied species which will not unite, or only with extreme difficulty; and on
the other hand of very distinct species which unite with the utmost
facility. In the same family there may be a genus, as Dianthus, in which
very many species can most readily be crossed; and another genus, as Silene,
in which the most persevering efforts have failed to produce between
extremely close species a single hybrid. Even within the limits of the same
genus, we meet with this same difference; for instance, the many species of
Nicotiana have been more largely crossed than the species of almost any
other genus; but Gärtner found that N. acuminata, which is not a
particularly distinct species, obstinately failed to fertilise, or to be
fertilised by, no less than eight other species of Nicotiana. Very many
analogous facts could be given.

No one has been able to point out what kind, or what amount, of difference
in any recognisable character is sufficient to prevent two species crossing.
It can be shown that plants most widely different in habit and general
appearance, and having strongly marked differences in every part of the
flower, even in the pollen, in the fruit, and in the cotyledons, can be
crossed. Annual and perennial plants, deciduous and evergreen trees, plants
inhabiting different stations and fitted for extremely different climates,
can often be crossed with ease.

By a reciprocal cross between two species, I mean the case, for instance, of
a stallion-horse being first crossed with a female-ass, and then a male-ass
with a mare: these two species may then be said to have been reciprocally
crossed. There is often the widest possible difference in the facility of
making reciprocal crosses. Such cases are highly important, for they prove
that the capacity in any two species to cross is often completely
independent of their systematic affinity, or of any recognisable difference
in their whole organisation. On the other hand, these cases clearly show
that the capacity for crossing is connected with constitutional differences
imperceptible by us, and confined to the reproductive system. This
difference in the result of reciprocal crosses between the same two species
was long ago observed by Kölreuter. To give an instance: Mirabilis jalappa
can easily be fertilised by the pollen of M. longiflora, and the hybrids
thus produced are sufficiently fertile; but Kölreuter tried more than two
hundred times, during eight following years, to fertilise reciprocally M.
longiflora with the pollen of M. jalappa, and utterly failed. Several other
equally striking cases could be given. Thuret has observed the same fact
with certain sea-weeds or Fuci. Gärtner, moreover, found that this
difference of facility in making reciprocal crosses is extremely common in a
lesser degree. He has observed it even between forms so closely related (as
Matthiola annua and glabra) that many botanists rank them only as varieties.
It is also a remarkable fact, that hybrids raised from reciprocal crosses,
though of course compounded of the very same two species, the one species
having first been used as the father and then as the mother, generally
differ in fertility in a small, and occasionally in a high degree.

Several other singular rules could be given from Gärtner: for instance, some
species have a remarkable power of crossing with other species; other
species of the same genus have a remarkable power of impressing their
likeness on their hybrid offspring; but these two powers do not at all
necessarily go together. There are certain hybrids which instead of having,
as is usual, an intermediate character between their two parents, always
closely resemble one of them; and such hybrids, though externally so like
one of their pure parent-species, are with rare exceptions extremely
sterile. So again amongst hybrids which are usually intermediate in
structure between their parents, exceptional and abnormal individuals
sometimes are born, which closely resemble one of their pure parents; and
these hybrids are almost always utterly sterile, even when the other hybrids
raised from seed from the same capsule have a considerable degree of
fertility. These facts show how completely fertility in the hybrid is
independent of its external resemblance to either pure parent.

Considering the several rules now given, which govern the fertility of first
crosses and of hybrids, we see that when forms, which must be considered as
good and distinct species, are united, their fertility graduates from zero
to perfect fertility, or even to fertility under certain conditions in
excess. That their fertility, besides being eminently susceptible to
favourable and unfavourable conditions, is innately variable. That it is by
no means always the same in degree in the first cross and in the hybrids
produced from this cross. That the fertility of hybrids is not related to
the degree in which they resemble in external appearance either parent. And
lastly, that the facility of making a first cross between any two species is
not always governed by their systematic affinity or degree of resemblance to
each other. This latter statement is clearly proved by reciprocal crosses
between the same two species, for according as the one species or the other
is used as the father or the mother, there is generally some difference, and
occasionally the widest possible difference, in the facility of effecting an
union. The hybrids, moreover, produced from reciprocal crosses often differ
in fertility.

Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely different
in degree, when various species are crossed, all of which we must suppose it
would be equally important to keep from blending together? Why should the
degree of sterility be innately variable in the individuals of the same
species? Why should some species cross with facility, and yet produce very
sterile hybrids; and other species cross with extreme difficulty, and yet
produce fairly fertile hybrids? Why should there often be so great a
difference in the result of a reciprocal cross between the same two species?
Why, it may even be asked, has the production of hybrids been permitted? To
grant to species the special power of producing hybrids, and then to stop
their further propagation by different degrees of sterility, not strictly
related to the facility of the first union between their parents, seems to
be a strange arrangement.

The foregoing rules and facts, on the other hand, appear to me clearly to
indicate that the sterility both of first crosses and of hybrids is simply
incidental or dependent on unknown differences, chiefly in the reproductive
systems, of the species which are crossed. The differences being of so
peculiar and limited a nature, that, in reciprocal crosses between two
species the male sexual element of the one will often freely act on the
female sexual element of the other, but not in a reversed direction. It will
be advisable to explain a little more fully by an example what I mean by
sterility being incidental on other differences, and not a specially endowed
quality. As the capacity of one plant to be grafted or budded on another is
so entirely unimportant for its welfare in a state of nature, I presume that
no one will suppose that this capacity is a specially endowed quality, but
will admit that it is incidental on differences in the laws of growth of the
two plants. We can sometimes see the reason why one tree will not take on
another, from differences in their rate of growth, in the hardness of their
wood, in the period of the flow or nature of their sap, &c.; but in a
multitude of cases we can assign no reason whatever. Great diversity in the
size of two plants, one being woody and the other herbaceous, one being
evergreen and the other deciduous, and adaptation to widely different
climates, does not always prevent the two grafting together. As in
hybridisation, so with grafting, the capacity is limited by systematic
affinity, for no one has been able to graft trees together belonging to
quite distinct families; and, on the other hand, closely allied species, and
varieties of the same species, can usually, but not invariably, be grafted
with ease. But this capacity, as in hybridisation, is by no means absolutely
governed by systematic affinity. Although many distinct genera within the
same family have been grafted together, in other cases species of the same
genus will not take on each other. The pear can be grafted far more readily
on the quince, which is ranked as a distinct genus, than on the apple, which
is a member of the same genus. Even different varieties of the pear take
with different degrees of facility on the quince; so do different varieties
of the apricot and peach on certain varieties of the plum.

As Gärtner found that there was sometimes an innate difference in different
individuals of the same two species in crossing; so Sagaret believes this to
be the case with different individuals of the same two species in being
grafted together. As in reciprocal crosses, the facility of effecting an
union is often very far from equal, so it sometimes is in grafting; the
common gooseberry, for instance, cannot be grafted on the currant, whereas
the currant will take, though with difficulty, on the gooseberry.

We have seen that the sterility of hybrids, which have their reproductive
organs in an imperfect condition, is a very different case from the
difficulty of uniting two pure species, which have their reproductive organs
perfect; yet these two distinct cases run to a certain extent parallel.
Something analogous occurs in grafting; for Thouin found that three species
of Robinia, which seeded freely on their own roots, and which could be
grafted with no great difficulty on another species, when thus grafted were
rendered barren. On the other hand, certain species of Sorbus, when grafted
on other species, yielded twice as much fruit as when on their own roots. We
are reminded by this latter fact of the extraordinary case of Hippeastrum,
Lobelia, &c., which seeded much more freely when fertilised with the pollen
of distinct species, than when self-fertilised with their own pollen.

We thus see, that although there is a clear and fundamental difference
between the mere adhesion of grafted stocks, and the union of the male and
female elements in the act of reproduction, yet that there is a rude degree
of parallelism in the results of grafting and of crossing distinct species.
And as we must look at the curious and complex laws governing the facility
with which trees can be grafted on each other as incidental on unknown
differences in their vegetative systems, so I believe that the still more
complex laws governing the facility of first crosses, are incidental on
unknown differences, chiefly in their reproductive systems. These
differences, in both cases, follow to a certain extent, as might have been
expected, systematic affinity, by which every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The facts
by no means seem to me to indicate that the greater or lesser difficulty of
either grafting or crossing together various species has been a special
endowment; although in the case of crossing, the difficulty is as important
for the endurance and stability of specific forms, as in the case of
grafting it is unimportant for their welfare.

Causes of the Sterility of first Crosses and of Hybrids. We may now look a
little closer at the probable causes of the sterility of first crosses and
of hybrids. These two cases are fundamentally different, for, as just
remarked, in the union of two pure species the male and female sexual
elements are perfect, whereas in hybrids they are imperfect. Even in first
crosses, the greater or lesser difficulty in effecting a union apparently
depends on several distinct causes. There must sometimes be a physical
impossibility in the male element reaching the ovule, as would be the case
with a plant having a pistil too long for the pollen-tubes to reach the
ovarium. It has also been observed that when pollen of one species is placed
on the stigma of a distantly allied species, though the pollen-tubes
protrude, they do not penetrate the stigmatic surface. Again, the male
element may reach the female element, but be incapable of causing an embryo
to be developed, as seems to have been the case with some of Thuret's
experiments on Fuci. No explanation can be given of these facts, any more
than why certain trees cannot be grafted on others. Lastly, an embryo may be
developed, and then perish at an early period. This latter alternative has
not been sufficiently attended to; but I believe, from observations
communicated to me by Mr. Hewitt, who has had great experience in
hybridising gallinaceous birds, that the early death of the embryo is a very
frequent cause of sterility in first crosses. I was at first very unwilling
to believe in this view; as hybrids, when once born, are generally healthy
and long-lived, as we see in the case of the common mule. Hybrids, however,
are differently circumstanced before and after birth: when born and living
in a country where their two parents can live, they are generally placed
under suitable conditions of life. But a hybrid partakes of only half of the
nature and constitution of its mother, and therefore before birth, as long
as it is nourished within its mother's womb or within the egg or seed
produced by the mother, it may be exposed to conditions in some degree
unsuitable, and consequently be liable to perish at an early period; more
especially as all very young beings seem eminently sensitive to injurious or
unnatural conditions of life.

In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is very different. I have more than once
alluded to a large body of facts, which I have collected, showing that when
animals and plants are removed from their natural conditions, they are
extremely liable to have their reproductive systems seriously affected.
This, in fact, is the great bar to the domestication of animals. Between the
sterility thus superinduced and that of hybrids, there are many points of
similarity. In both cases the sterility is independent of general health,
and is often accompanied by excess of size or great luxuriance. In both
cases, the sterility occurs in various degrees; in both, the male element is
the most liable to be affected; but sometimes the female more than the male.
In both, the tendency goes to a certain extent with systematic affinity, or
whole groups of animals and plants are rendered impotent by the same
unnatural conditions; and whole groups of species tend to produce sterile
hybrids. On the other hand, one species in a group will sometimes resist
great changes of conditions with unimpaired fertility; and certain species
in a group will produce unusually fertile hybrids. No one can tell, till he
tries, whether any particular animal will breed under confinement or any
plant seed freely under culture; nor can he tell, till he tries, whether any
two species of a genus will produce more or less sterile hybrids. Lastly,
when organic beings are placed during several generations under conditions
not natural to them, they are extremely liable to vary, which is due, as I
believe, to their reproductive systems having been specially affected,
though in a lesser degree than when sterility ensues. So it is with hybrids,
for hybrids in successive generations are eminently liable to vary, as every
experimentalist has observed.

Thus we see that when organic beings are placed under new and unnatural
conditions, and when hybrids are produced by the unnatural crossing of two
species, the reproductive system, independently of the general state of
health, is affected by sterility in a very similar manner. In the one case,
the conditions of life have been disturbed, though often in so slight a
degree as to be inappreciable by us; in the other case, or that of
hybrids,the external conditions have remained the same, but the organisation
has been disturbed by two different structures and constitutions having been
blended into one. For it is scarcely possible that two organisations should
be compounded into one, without some disturbance occurring in the
development, or periodical action, or mutual relation of the different parts
and organs one to another, or to the conditions of life. When hybrids are
able to breed inter se, they transmit to their offspring from generation to
generation the same compounded organisation, and hence we need not be
surprised that their sterility, though in some degree variable, rarely
diminishes.

It must, however, be confessed that we cannot understand, excepting on vague
hypotheses, several facts with respect to the sterility of hybrids; for
instance, the unequal fertility of hybrids produced from reciprocal crosses;
or the increased sterility in those hybrids which occasionally and
exceptionally resemble closely either pure parent. Nor do I pretend that the
foregoing remarks go to the root of the matter: no explanation is offered
why an organism, when placed under unnatural conditions, is rendered
sterile. All that I have attempted to show, is that in two cases, in some
respects allied, sterility is the common result, in the one case from the
conditions of life having been disturbed, in the other case from the
organisation having been disturbed by two organisations having been
compounded into one.

It may seem fanciful, but I suspect that a similar parallelism extends to an
allied yet very different class of facts. It is an old and almost universal
belief, founded, I think, on a considerable body of evidence, that slight
changes in the conditions of life are beneficial to all living things. We
see this acted on by farmers and gardeners in their frequent exchanges of
seed, tubers, &c., from one soil or climate to another, and back again.
During the convalescence of animals, we plainly see that great benefit is
derived from almost any change in the habits of life. Again, both with
plants and animals, there is abundant evidence, that a cross between very
distinct individuals of the same species, that is between members of
different strains or sub-breeds, gives vigour and fertility to the
offspring. I believe, indeed, from the facts alluded to in our fourth
chapter, that a certain amount of crossing is indispensable even with
hermaphrodites; and that close interbreeding continued during several
generations between the nearest relations, especially if these be kept under
the same conditions of life, always induces weakness and sterility in the
progeny.

Hence it seems that, on the one hand, slight changes in the conditions of
life benefit all organic beings, and on the other hand, that slight crosses,
that is crosses between the males and females of the same species which have
varied and become slightly different, give vigour and fertility to the
offspring. But we have seen that greater changes, or changes of a particular
nature, often render organic beings in some degree sterile; and that greater
crosses, that is crosses between males and females which have become widely
or specifically different, produce hybrids which are generally sterile in
some degree. I cannot persuade myself that this parallelism is an accident
or an illusion. Both series of facts seem to be connected together by some
common but unknown bond, which is essentially related to the principle of
life.

Fertility of Varieties when crossed, and of their Mongrel off-spring. It may
be urged, as a most forcible argument, that there must be some essential
distinction between species and varieties, and that there must be some error
in all the foregoing remarks, inasmuch as varieties, however much they may
differ from each other in external appearance, cross with perfect facility,
and yield perfectly fertile offspring. I fully admit that this is almost
invariably the case. But if we look to varieties produced under nature, we
are immediately involved in hopeless difficulties; for if two hitherto
reputed varieties be found in any degree sterile together, they are at once
ranked by most naturalists as species. For instance, the blue and red
pimpernel, the primrose and cowslip, which are considered by many of our
best botanists as varieties, are said by Gärtner not to be quite fertile
when crossed, and he consequently ranks them as undoubted species. If we
thus argue in a circle, the fertility of all varieties produced under nature
will assuredly have to be granted.

If we turn to varieties, produced, or supposed to have been produced, under
domestication, we are still involved in doubt. For when it is stated, for
instance, that the German Spitz dog unites more easily than other dogs with
foxes, or that certain South American indigenous domestic dogs do not
readily cross with European dogs, the explanation which will occur to
everyone, and probably the true one, is that these dogs have descended from
several aboriginally distinct species. Nevertheless the perfect fertility of
so many domestic varieties, differing widely from each other in appearance,
for instance of the pigeon or of the cabbage, is a remarkable fact; more
especially when we reflect how many species there are, which, though
resembling each other most closely, are utterly sterile when intercrossed.
Several considerations, however, render the fertility of domestic varieties
less remarkable than at first appears. It can, in the first place, be
clearly shown that mere external dissimilarity between two species does not
determine their greater or lesser degree of sterility when crossed; and we
may apply the same rule to domestic varieties. In the second place, some
eminent naturalists believe that a long course of domestication tends to
eliminate sterility in the successive generations of hybrids, which were at
first only slightly sterile; and if this be so, we surely ought not to
expect to find sterility both appearing and disappearing under nearly the
same conditions of life. Lastly, and this seems to me by far the most
important consideration, new races of animals and plants are produced under
domestication by man's methodical and unconscious power of selection, for
his own use and pleasure: he neither wishes to select, nor could select,
slight differences in the reproductive system, or other constitutional
difference correlated with the reproductive system. He supplies his several
varieties with the same food; treats them in nearly the same manner, and
does not wish to alter their general habits of life. Nature acts uniformly
and slowly during vast periods of time on the whole organization, in any way
which may be for each creature's own good; and thus she may, either
directly, or more probably indirectly, through correlation, modify the
reproductive system in the several descendants from any one species. Seeing
this difference in the process of selection, as carried on by man and
nature, we need not be surprised at some difference in the result.

I have as yet spoken as if the varieties of the same species were invariably
fertile when intercrossed. But it seems to me impossible to resist the
evidence of the existence of a certain amount of sterility in the few
following cases, which I will briefly abstract. The evidence is at least as
good as that from which we believe in the sterility of a multitude of
species. The evidence is, also, derived from hostile witnesses, who in all
other cases consider fertility and sterility as safe criterions of specific
distinction. Gärtner kept during several years a dwarf kind of maize with
yellow seeds, and a tall variety with red seeds, growing near each other in
his garden; and although these plants have separated sexes, they never
naturally crossed. He then fertilized thirteen flowers of the one with the
pollen of the other; but only a single head produced any seed, and this one
head produced only five grains. Manipulation in this case could not have
been injurious, as the plants have separated sexes. No one, I believe, has
suspected that these varieties of maize are distinct species; and it is
important to notice that the hybrid plants thus raised were themselves
perfectly fertile; so that even Gärtner did not venture to consider the two
varieties as specifically distinct.

Girou de Buzareingues crossed three varieties of gourd, which like the maize
has separated sexes, and he asserts that their mutual fertilization is by so
much the less easy as their differences are greater. How far these
experiments may be trusted, I know not; but the forms experimentised on, are
ranked by Sagaret, who mainly founds his classification by the test of
infertility, as varieties.

The following case is far more remarkable, and seems at first quite
incredible; but it is the result of an astonishing number of experiments
made during many years on nine species of Verbascum, by so good an observer
and so hostile a witness, as Gärtner: namely, that yellow and white
varieties of the same species of Verbascum when intercrossed produce less
seed, than do either coloured varieties when fertilized with pollen from
their own coloured flowers. Moreover, he asserts that when yellow and white
varieties of one species are crossed with yellow and white varieties of a
distinct species, more seed is produced by the crosses between the same
coloured flowers, than between those which are differently coloured. Yet
these varieties of Verbascum present no other difference besides the mere
colour of the flower; and one variety can sometimes be raised from the seed
of the other.

From observations which I have made on certain varieties of hollyhock, I am
inclined to suspect that they present analogous facts.

Kölreuter, whose accuracy has been confirmed by every subsequent observer,
has proved the remarkable fact, that one variety of the common tobacco is
more fertile, when crossed with a widely distinct species, than are the
other varieties. He experimentised on five forms, which are commonly reputed
to be varieties, and which he tested by the severest trial, namely, by
reciprocal crosses, and he found their mongrel offspring perfectly fertile.
But one of these five varieties, when used either as father or mother, and
crossed with the Nicotiana glutinosa, always yielded hybrids not so sterile
as those which were produced from the four other varieties when crossed with
N. glutinosa. Hence the reproductive system of this one variety must have
been in some manner and in some degree modified.

From these facts; from the great difficulty of ascertaining the infertility
of varieties in a state of nature, for a supposed variety if infertile in
any degree would generally be ranked as species; from man selecting only
external characters in the production of the most distinct domestic
varieties, and from not wishing or being able to produce recondite and
functional differences in the reproductive system; from these several
considerations and facts, I do not think that the very general fertility of
varieties can be proved to be of universal occurrence, or to form a
fundamental distinction between varieties and species. The general fertility
of varieties does not seem to me sufficient to overthrow the view which I
have taken with respect to the very general, but not invariable, sterility
of first crosses and of hybrids, namely, that it is not a special endowment,
but is incidental on slowly acquired modifications, more especially in the
reproductive systems of the forms which are crossed.

Hybrids and Mongrels compared, independently of their fertility.
Independently of the question of fertility, the offspring of species when
crossed and of varieties when crossed may be compared in several other
respects. Gärtner, whose strong wish was to draw a marked line of
distinction between species and varieties, could find very few and, as it
seems to me, quite unimportant differences between the so-called hybrid
offspring of species, and the so-called mongrel offspring of varieties. And,
on the other hand, they agree most closely in very many important respects.

I shall here discuss this subject with extreme brevity. The most important
distinction is, that in the first generation mongrels are more variable than
hybrids; but Gärtner admits that hybrids from species which have long been
cultivated are often variable in the first generation; and I have myself
seen striking instances of this fact. Gärtner further admits that hybrids
between very closely allied species are more variable than those from very
distinct species; and this shows that the difference in the degree of
variability graduates away. When mongrels and the more fertile hybrids are
propagated for several generations an extreme amount of variability in their
offspring is notorious; but some few cases both of hybrids and mongrels long
retaining uniformity of character could be given. The variability, however,
in the successive generations of mongrels is, perhaps, greater than in
hybrids.

This greater variability of mongrels than of hybrids does not seem to me at
all surprising. For the parents of mongrels are varieties, and mostly
domestic varieties (very few experiments having been tried on natural
varieties), and this implies in most cases that there has been recent
variability; and therefore we might expect that such variability would often
continue and be super-added to that arising from the mere act of crossing.
The slight degree of variability in hybrids from the first cross or in the
first generation, in contrast with their extreme variability in the
succeeding generations, is a curious fact and deserves attention. For it
bears on and corroborates the view which I have taken on the cause of
ordinary variability; namely, that it is due to the reproductive system
being eminently sensitive to any change in the conditions of life, being
thus often rendered either impotent or at least incapable of its proper
function of producing offspring identical with the parent-form. Now hybrids
in the first generation are descended from species (excluding those long
cultivated) which have not had their reproductive systems in any way
affected, and they are not variable; but hybrids themselves have their
reproductive systems seriously affected, and their descendants are highly
variable.

But to return to our comparison of mongrels and hybrids: Gärtner states that
mongrels are more liable than hybrids to revert to either parent-form; but
this, if it be true, is certainly only a difference in degree. Gärtner
further insists that when any two species, although most closely allied to
each other, are crossed with a third species, the hybrids are widely
different from each other; whereas if two very distinct varieties of one
species are crossed with another species, the hybrids do not differ much.
But this conclusion, as far as I can make out, is founded on a single
experiment; and seems directly opposed to the results of several experiments
made by Kölreuter.

These alone are the unimportant differences, which Gärtner is able to point
out, between hybrid and mongrel plants. On the other hand, the resemblance
in mongrels and in hybrids to their respective parents, more especially in
hybrids produced from nearly related species, follows according to Gärtner
the same laws. When two species are crossed, one has sometimes a prepotent
power of impressing its likeness on the hybrid; and so I believe it to be
with varieties of plants. With animals one variety certainly often has this
prepotent power over another variety. Hybrid plants produced from a
reciprocal cross, generally resemble each other closely; and so it is with
mongrels from a reciprocal cross. Both hybrids and mongrels can be reduced
to either pure parent-form, by repeated crosses in successive generations
with either parent.

These several remarks are apparently applicable to animals; but the subject
is here excessively complicated, partly owing to the existence of secondary
sexual characters; but more especially owing to prepotency in transmitting
likeness running more strongly in one sex than in the other, both when one
species is crossed with another, and when one variety is crossed with
another variety. For instance, I think those authors are right, who maintain
that the ass has a prepotent power over the horse, so that both the mule and
the hinny more resemble the ass than the horse; but that the prepotency runs
more strongly in the male-ass than in the female, so that the mule, which is
the offspring of the male-ass and mare, is more like an ass, than is the
hinny, which is the offspring of the female-ass and stallion.

Much stress has been laid by some authors on the supposed fact, that mongrel
animals alone are born closely like one of their parents; but it can be
shown that this does sometimes occur with hybrids; yet I grant much less
frequently with hybrids than with mongrels. Looking to the cases which I
have collected of cross-bred animals closely resembling one parent, the
resemblances seem chiefly confined to characters almost monstrous in their
nature, and which have suddenly appeared such as albinism, melanism,
deficiency of tail or horns, or additional fingers and toes; and do not
relate to characters which have been slowly acquired by selection.
Consequently, sudden reversions to the perfect character of either parent
would be more likely to occur with mongrels, which are descended from
varieties often suddenly produced and semi-monstrous in character, than with
hybrids, which are descended from species slowly and naturally produced. On
the whole I entirely agree with Dr Prosper Lucas, who, after arranging an
enormous body of facts with respect to animals, comes to the conclusion,
that the laws of resemblance of the child to its parents are the same,
whether the two parents differ much or little from each other, namely in the
union of individuals of the same variety, or of different varieties, or of
distinct species.

Laying aside the question of fertility and sterility, in all other respects
there seems to be a general and close similarity in the offspring of crossed
species, and of crossed varieties. If we look at species as having been
specially created, and at varieties as having been produced by secondary
laws, this similarity would be an astonishing fact. But it harmonizes
perfectly with the view that there is no essential distinction between
species and varieties.

Summary of Chapter. First crosses between forms sufficiently distinct to be
ranked as species, and their hybrids, are very generally, but not
universally, sterile. The sterility is of all degrees, and is often so
slight that the two most careful experimentalists who have ever lived, have
come to diametrically opposite conclusions in ranking forms by this test.
The sterility is innately variable in individuals of the same species, and
is eminently susceptible of favourable and unfavourable conditions. The
degree of sterility does not strictly follow systematic affinity, but is
governed by several curious and complex laws. It is generally different, and
sometimes widely different, in reciprocal crosses between the same two
species. It is not always equal in degree in a first cross and in the hybrid
produced from this cross.

In the same manner as in grafting trees, the capacity of one species or
variety to take on another, is incidental on generally unknown differences
in their vegetative systems, so in crossing, the greater or less facility of
one species to unite with another, is incidental on unknown differences in
their reproductive systems. There is no more reason to think that species
have been specially endowed with various degrees of sterility to prevent
them crossing and blending in nature, than to think that trees have been
specially endowed with various and somewhat analogous degrees of difficulty
in being grafted together in order to prevent them becoming inarched in our
forests.

The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several circumstances; in
some cases largely on the early death of the embryo. The sterility of
hybrids, which have their reproductive systems imperfect, and which have had
this system and their whole organisation disturbed by being compounded of
two distinct species, seems closely allied to that sterility which so
frequently affects pure species, when their natural conditions of life have
been disturbed. This view is supported by a parallelism of another kind;
namely, that the crossing of forms only slightly different is favourable to
the vigour and fertility of their offspring; and that slight changes in the
conditions of life are apparently favourable to the vigour and fertility of
all organic beings. It is not surprising that the degree of difficulty in
uniting two species, and the degree of sterility of their hybrid-offspring
should generally correspond, though due to distinct causes; for both depend
on the amount of difference of some kind between the species which are
crossed. Nor is it surprising that the facility of effecting a first cross,
the fertility of the hybrids produced, and the capacity of being grafted
together though this latter capacity evidently depends on widely different
circumstances should all run, to a certain extent, parallel with the
systematic affinity of the forms which are subjected to experiment; for
systematic affinity attempts to express all kinds of resemblance between all
species.

First crosses between forms known to be varieties, or sufficiently alike to
be considered as varieties, and their mongrel offspring, are very generally,
but not quite universally, fertile. Nor is this nearly general and perfect
fertility surprising, when we remember how liable we are to argue in a
circle with respect to varieties in a state of nature; and when we remember
that the greater number of varieties have been produced under domestication
by the selection of mere external differences, and not of differences in the
reproductive system. In all other respects, excluding fertility, there is a
close general resemblance between hybrids and mongrels. Finally, then, the
facts briefly given in this chapter do not seem to me opposed to, but even
rather to support the view, that there is no fundamental distinction between
species and varieties.

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Chapter 9 - On the Imperfection of the Geological Record

[* ] On the absence of intermediate varieties at the present day [* ] On the
nature of extinct intermediate varieties; on their number [* ] On the vast
lapse of time, as inferred from the rate of deposition and of denudation
[* ] On the poorness of our palaeontological collections [* ] On the
intermittence of geological formations [* ] On the absence of intermediate
varieties in any one formation [* ] On their sudden appearance in the lowest
known fossiliferous strata
----------------------------------------------------------------------------
IN the sixth chapter I enumerated the chief objections which might be justly
urged against the views maintained in this volume. Most of them have now
been discussed. One, namely the distinctness of specific forms, and their
not being blended together by innumerable transitional links, is a very
obvious difficulty. I assigned reasons why such links do not commonly occur
at the present day, under the circumstances apparently most favourable for
their presence, namely on an extensive and continuous area with graduated
physical conditions. I endeavoured to show, that the life of each species
depends in a more important manner on the presence of other already defined
organic forms, than on climate; and, therefore, that the really governing
conditions of life do not graduate away quite insensibly like heat or
moisture. I endeavoured, also, to show that intermediate varieties, from
existing in lesser numbers than the forms which they connect, will generally
be beaten out and exterminated during the course of further modification and
improvement. The main cause, however, of innumerable intermediate links not
now occurring everywhere throughout nature depends on the very process of
natural selection, through which new varieties continually take the places
of and exterminate their parent-forms. But just in proportion as this
process of extermination has acted on an enormous scale, so must the number
of intermediate varieties, which have formerly existed on the earth, be
truly enormous. Why then is not every geological formation and every stratum
full of such intermediate links? Geology assuredly does not reveal any such
finely graduated organic chain; and this, perhaps, is the most obvious and
gravest objection which can be urged against my theory. The explanation
lies, as I believe, in the extreme imperfection of the geological record.

In the first place it should always be borne in mind what sort of
intermediate forms must, on my theory, have formerly existed. I have found
it difficult, when looking at any two species, to avoid picturing to myself,
forms directly intermediate between them. But this is a wholly false view;
we should always look for forms intermediate between each species and a
common but unknown progenitor; and the progenitor will generally have
differed in some respects from all its modified descendants. To give a
simple illustration: the fantail and pouter pigeons have both descended from
the rock-pigeon; if we possessed all the intermediate varieties which have
ever existed, we should have an extremely close series between both and the
rock-pigeon; but we should have no varieties directly intermediate between
the fantail and pouter; none, for instance, combining a tail somewhat
expanded with a crop somewhat enlarged, the characteristic features of these
two breeds. These two breeds, moreover, have become so much modified, that
if we had no historical or indirect evidence regarding their origin, it
would not have been possible to have determined from a mere comparison of
their structure with that of the rock-pigeon, whether they had descended
from this species or from some other allied species, such as C. oenas.

So with natural species, if we look to forms very distinct, for instance to
the horse and tapir, we have no reason to suppose that links ever existed
directly intermediate between them, but between each and an unknown common
parent. The common parent will have had in its whole organisation much
general resemblance to the tapir and to the horse; but in some points of
structure may have differed considerably from both, even perhaps more than
they differ from each other. Hence in all such cases, we should be unable to
recognise the parent-form of any two or more species, even if we closely
compared the structure of the parent with that of its modified descendants,
unless at the same time we had a nearly perfect chain of the intermediate
links.

It is just possible by my theory, that one of two living forms might have
descended from the other; for instance, a horse from a tapir; and in this
case direct intermediate links will have existed between them. But such a
case would imply that one form had remained for a very long period
unaltered, whilst its descendants had undergone a vast amount of change; and
the principle of competition between organism and organism, between child
and parent, will render this a very rare event; for in all cases the new and
improved forms of life will tend to supplant the old and unimproved.

By the theory of natural selection all living species have been connected
with the parent-species of each genus, by differences not greater than we
see between the varieties of the same species at the present day; and these
parent-species, now generally extinct, have in their turn been similarly
connected with more ancient species; and so on backwards, always converging
to the common ancestor of each great class. So that the number of
intermediate and transitional links, between all living and extinct species,
must have been inconceivably great. But assuredly, if this theory be true,
such have lived upon this earth.

On the lapse of Time. Independently of our not finding fossil remains of
such infinitely numerous connecting links, it may be objected, that time
will not have sufficed for so great an amount of organic change, all changes
having been effected very slowly through natural selection. It is hardly
possible for me even to recall to the reader, who may not be a practical
geologist, the facts leading the mind feebly to comprehend the lapse of
time. He who can read Sir Charles Lyell's grand work on the Principles of
Geology, which the future historian will recognise as having produced a
revolution in natural science, yet does not admit how incomprehensibly vast
have been the past periods of time, may at once close this volume. Not that
it suffices to study the Principles of Geology, or to read special treatises
by different observers on separate formations, and to mark how each author
attempts to give an inadequate idea of the duration of each formation or
even each stratum. A man must for years examine for himself great piles of
superimposed strata, and watch the sea at work grinding down old rocks and
making fresh sediment, before he can hope to comprehend anything of the
lapse of time, the monuments of which we see around us.

It is good to wander along lines of sea-coast, when formed of moderately
hard rocks, and mark the process of degradation. The tides in most cases
reach the cliffs only for a short time twice a day, and the waves eat into
them only when they are charged with sand or pebbles; for there is reason to
believe that pure water can effect little or nothing in wearing away rock.
At last the base of the cliff is undermined, huge fragments fall down, and
these remaining fixed, have to be worn away, atom by atom, until reduced in
size they can be rolled about by the waves, and then are more quickly ground
into pebbles, sand, or mud. But how often do we see along the bases of
retreating cliffs rounded boulders, all thickly clothed by marine
productions, showing how little they are abraded and how seldom they are
rolled about! Moreover, if we follow for a few miles any line of rocky
cliff, which is undergoing degradation, we find that it is only here and
there, along a short length or round a promontory, that the cliffs are at
the present time suffering. The appearance of the surface and the vegetation
show that elsewhere years have elapsed since the waters washed their base.

He who most closely studies the action of the sea on our shores, will, I
believe, be most deeply impressed with the slowness with which rocky coasts
are worn away. The observations on this head by Hugh Miller, and by that
excellent observer Mr. Smith of Jordan Hill, are most impressive. With the
mind thus impressed, let any one examine beds of conglomerate many thousand
feet in thickness, which, though probably formed at a quicker rate than many
other deposits, yet, from being formed of worn and rounded pebbles, each of
which bears the stamp of time, are good to show how slowly the mass has been
accumulated. Let him remember Lyell's profound remark, that the thickness
and extent of sedimentary formations are the result and measure of the
degradation which the earth's crust has elsewhere suffered. And what an
amount of degradation is implied by the sedimentary deposits of many
countries! Professor Ramsay has given me the maximum thickness, in most
cases from actual measurement, in a few cases from estimate, of each
formation in different parts of Great Britain; and this is the result:-

Feet

Palaeozoic strata (not including igneous beds) 57,154

Secondary strata 13,190

Tertiary strata 2,240

making altogether 72,584 feet; that is, very nearly thirteen and
three-quarters British miles. Some of these formations, which are
represented in England by thin beds, are thousands of feet in thickness on
the Continent. Moreover, between each successive formation, we have, in the
opinion of most geologists, enormously long blank periods. So that the lofty
pile of sedimentary rocks in Britain, gives but an inadequate idea of the
time which has elapsed during their accumulation; yet what time this must
have consumed! Good observers have estimated that sediment is deposited by
the great Mississippi river at the rate of only 600 feet in a hundred
thousand years. This estimate may be quite erroneous; yet, considering over
what wide spaces very fine sediment is transported by the currents of the
sea, the process of accumulation in any one area must be extremely slow.

But the amount of denudation which the strata have in many places suffered,
independently of the rate of accumulation of the degraded matter, probably
offers the best evidence of the lapse of time. I remember having been much
struck with the evidence of denudation, when viewing volcanic islands, which
have been worn by the waves and pared all round into perpendicular cliffs of
one or two thousand feet in height; for the gentle slope of the
lava-streams, due to their formerly liquid state, showed at a glance how far
the hard, rocky beds had once extended into the open ocean. The same story
is still more plainly told by faults, those great cracks along which the
strata have been upheaved on one side, or thrown down on the other, to the
height or depth of thousands of feet; for since the crust cracked, the
surface of the land has been so completely planed down by the action of the
sea, that no trace of these vast dislocations is externally visible.

The Craven fault, for instance, extends for upwards of 30 miles, and along
this line the vertical displacement of the strata has varied from 600 to
3000 feet. Prof. Ramsay has published an account of a downthrow in Anglesea
of 2300 feet; and he informs me that he fully believes there is one in
Merionethshire of 12,000 feet; yet in these cases there is nothing on the
surface to show such prodigious movements; the pile of rocks on the one or
other side having been smoothly swept away. The consideration of these facts
impresses my mind almost in the same manner as does the vain endeavour to
grapple with the idea of eternity.

I am tempted to give one other case, the well-known one of the denudation of
the Weald. Though it must be admitted that the denudation of the Weald has
been a mere trifle, in comparison with that which has removed masses of our
Palaeozoic strata, in parts ten thousand feet in thickness, as shown in
Prof. Ramsay's masterly memoir on this subject. Yet it is an admirable
lesson to stand on the North Downs and to look at the distant South Downs;
for, remembering that at no great distance to the west the northern and
southern escarpments meet and close, one can safely picture to oneself the
great dome of rocks which must have covered up the Weald within so limited a
period as since the latter part of the Chalk formation. The distance from
the northern to the southern Downs is about 22 miles, and the thickness of
the several formations is on an average about 1100 feet, as I am informed by
Prof. Ramsay. But if, as some geologists suppose, a range of older rocks
underlies the Weald, on the flanks of which the overlying sedimentary
deposits might have accumulated in thinner masses than elsewhere, the above
estimate would be erroneous; but this source of doubt probably would not
greatly affect the estimate as applied to the western extremity of the
district. If, then, we knew the rate at which the sea commonly wears away a
line of cliff of any given height, we could measure the time requisite to
have denuded the Weald. This, of course, cannot be done; but we may, in
order to form some crude notion on the subject, assume that the sea would
eat into cliffs 500 feet in height at the rate of one inch in a century.
This will at first appear much too small an allowance; but it is the same as
if we were to assume a cliff one yard in height to be eaten back along a
whole line of coast at the rate of one yard in nearly every twenty-two
years. I doubt whether any rock, even as soft as chalk, would yield at this
rate excepting on the most exposed coasts; though no doubt the degradation
of a lofty cliff would be more rapid from the breakage of the fallen
fragments. On the other hand, I do not believe that any line of coast, ten
or twenty miles in length, ever suffers degradation at the same time along
its whole indented length; and we must remember that almost all strata
contain harder layers or nodules, which from long resisting attrition form a
breakwater at the base. Hence, under ordinary circumstances, I conclude that
for a cliff 500 feet in height, a denudation of one inch per century for the
whole length would be an ample allowance. At this rate, on the above data,
the denudation of the Weald must have required 306,662,400 years; or say
three hundred million years.

The action of fresh water on the gently inclined Wealden district, when
upraised, could hardly have been great, but it would somewhat reduce the
above estimate. On the other hand, during oscillations of level, which we
know this area has undergone, the surface may have existed for millions of
years as land, and thus have escaped the action of the sea: when deeply
submerged for perhaps equally long periods, it would, likewise, have escaped
the action of the coast-waves. So that in all probability a far longer
period than 300 million years has elapsed since the latter part of the
Secondary period.

I have made these few remarks because it is highly important for us to gain
some notion, however imperfect, of the lapse of years. During each of these
years, over the whole world, the land and the water has been peopled by
hosts of living forms. What an infinite number of generations, which the
mind cannot grasp, must have succeeded each other in the long roll of years!
Now turn to our richest geological museums, and what a paltry display we
behold!

On the poorness of our Palaeontological collections. That our
Palaeontological collections are very imperfect, is admitted by every one.
The remark of that admirable Palaeontologist, the late Edward Forbes, should
not be forgotten, namely, that numbers of our fossil species are known and
named from single and often broken specimens, or from a few specimens
collected on some one spot. Only a small portion of the surface of the earth
has been geologically explored, and no part with sufficient care, as the
important discoveries made every year in Europe prove. No organism wholly
soft can be preserved. Shells and bones will decay and disappear when left
on the bottom of the sea, where sediment is not accumulating. I believe we
are continually taking a most erroneous view, when we tacitly admit to
ourselves that sediment is being deposited over nearly the whole bed of the
sea, at a rate sufficiently quick to embed and preserve fossil remains.
Throughout an enormously large proportion of the ocean, the bright blue tint
of the water bespeaks its purity. The many cases on record of a formation
conformably covered, after an enormous interval of time, by another and
later formation, without the underlying bed having suffered in the interval
any wear and tear, seem explicable only on the view of the bottom of the sea
not rarely lying for ages in an unaltered condition. The remains which do
become embedded, if in sand or gravel, will when the beds are upraised
generally be dissolved by the percolation of rain-water. I suspect that but
few of the very many animals which live on the beach between high and low
watermark are preserved. For instance, the several species of the
Chthamalinae (a sub-family of sessile cirripedes) coat the rocks all over
the world in infinite numbers: they are all strictly littoral, with the
exception of a single Mediterranean species, which inhabits deep water and
has been found fossil in Sicily, whereas not one other species has hitherto
been found in any tertiary formation: yet it is now known that the genus
Chthamalus existed during the chalk period. The molluscan genus Chiton
offers a partially analogous case.

With respect to the terrestrial productions which lived during the Secondary
and Palaeozoic periods, it is superfluous to state that our evidence from
fossil remains is fragmentary in an extreme degree. For instance, not a land
shell is known belonging to either of these vast periods, with one exception
discovered by Sir C. Lyell in the carboniferous strata of North America. I n
regard to mammiferous remains, a single glance at the historical table
published in the Supplement to Lyell's Manual, will bring home the truth,
how accidental and rare is their preservation, far better than pages of
detail. Nor is their rarity surprising, when we remember how large a
proportion of the bones of tertiary mammals have been discovered either in
caves or in lacustrine deposits; and that not a cave or true lacustrine bed
is known belonging to the age of our secondary or palaeozoic formations.

But the imperfection in the geological record mainly results from another
and more important cause than any of the foregoing; namely, from the several
formations being separated from each other by wide intervals of time. When
we see the formations tabulated in written works, or when we follow them in
nature, it is difficult to avoid believing that they are closely
consecutive. But we know, for instance, from Sir R. Murchison's great work
on Russia, what wide gaps there are in that country between the superimposed
formations; so it is in North America, and in many other parts of the world.
The most skilful geologist, if his attention had been exclusively confined
to these large territories, would never have suspected that during the
periods which were blank and barren in his own country, great piles of
sediment, charged with new and peculiar forms of life, had elsewhere been
accumulated. And if in each separate territory, hardly any idea can be
formed of the length of time which has elapsed between the consecutive
formations, we may infer that this could nowhere be ascertained. The
frequent and great changes in the mineralogical composition of consecutive
formations, generally implying great changes in the geography of the
surrounding lands, whence the sediment has been derived, accords with the
belief of vast intervals of time having elapsed between each formation.

But we can, I think, see why the geological formations of each region are
almost invariably intermittent; that is, have not followed each other in
close sequence. Scarcely any fact struck me more when examining many hundred
miles of the South American coasts, which have been upraised several hundred
feet within the recent period, than the absence of any recent deposits
sufficiently extensive to last for even a short geological period. Along the
whole west coast, which is inhabited by a peculiar marine fauna, tertiary
beds are so scantily developed, that no record of several successive and
peculiar marine faunas will probably be preserved to a distant age. A little
reflection will explain why along the rising coast of the western side of
South America, no extensive formations with recent or tertiary remains can
anywhere be found, though the supply of sediment must for ages have been
great, from the enormous degradation of the coast-rocks and from muddy
streams entering the sea. The explanation, no doubt, is, that the littoral
and sub-littoral deposits are continually worn away, as soon as they are
brought up by the slow and gradual rising of the land within the grinding
action of the coast-waves.

We may, I think, safely conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during subsequent
oscillations of level. Such thick and extensive accumulations of sediment
may be formed in two ways; either, in profound depths of the sea, in which
case, judging from the researches of E. Forbes, we may conclude that the
bottom will be inhabited by extremely few animals, and the mass when
upraised will give a most imperfect record of the forms of life which then
existed; or, sediment may be accumulated to any thickness and extent over a
shallow bottom, if it continue slowly to subside. In this latter case, as
long as the rate of subsidence and supply of sediment nearly balance each
other, the sea will remain shallow and favourable for life, and thus a
fossiliferous formation thick enough, when upraised, to resist any amount of
degradation, may be formed.

I am convinced that all our ancient formations, which are rich in fossils,
have thus been formed during subsidence. Since publishing my views on this
subject in 1845, I have watched the progress of Geology, and have been
surprised to note how author after author, in treating of this or that great
formation, has come to the conclusion that it was accumulated during
subsidence. I may add, that the only ancient tertiary formation on the west
coast of South America, which has been bulky enough to resist such
degradation as it has as yet suffered, but which will hardly last to a
distant geological age, was certainly deposited during a downward
oscillation of level, and thus gained considerable thickness.

All geological facts tell us plainly that each area has undergone numerous
slow oscillations of level, and apparently these oscillations have affected
wide spaces. Consequently formations rich in fossils and sufficiently thick
and extensive to resist subsequent degradation, may have been formed over
wide spaces during periods of subsidence, but only where the supply of
sediment was sufficient to keep the sea shallow and to embed and preserve
the remains before they had time to decay. On the other hand, as long as the
bed of the sea remained stationary, thick deposits could not have been
accumulated in the shallow parts, which are the most favourable to life.
Still less could this have happened during the alternate periods of
elevation; or, to speak more accurately, the beds which were then
accumulated will have been destroyed by being upraised and brought within
the limits of the coast-action.

Thus the geological record will almost necessarily be rendered intermittent.
I feel much confidence in the truth of these views, for they are in strict
accordance with the general principles inculcated by Sir C. Lyell; and E.
Forbes independently arrived at a similar conclusion.

One remark is here worth a passing notice. During periods of elevation the
area of the land and of the adjoining shoal parts of the sea will be
increased, and new stations will often be formed; all circumstances most
favourable, as previously explained, for the formation of new varieties and
species; but during such periods there will generally be a blank in the
geological record. On the other hand, during subsidence, the inhabited area
and number of inhabitants will decrease (excepting the productions on the
shores of a continent when first broken up into an archipelago), and
consequently during subsidence, though there will be much extinction, fewer
new varieties or species will be formed; and it is during these very periods
of subsidence, that our great deposits rich in fossils have been
accumulated. Nature may almost be said to have guarded against the frequent
discovery of her transitional or linking forms.

From the foregoing considerations it cannot be doubted that the geological
record, viewed as a whole, is extremely imperfect; but if we confine our
attention to any one formation, it becomes more difficult to understand, why
we do not therein find closely graduated varieties between the allied
species which lived at its commencement and at its close. Some cases are on
record of the same species presenting distinct varieties in the upper and
lower parts of the same formation, but, as they are rare, they may be here
passed over. Although each formation has indisputably required a vast number
of years for its deposition, I can see several reasons why each should not
include a graduated series of links between the species which then lived;
but I can by no means pretend to assign due proportional weight to the
following considerations.

Although each formation may mark a very long lapse of years, each perhaps is
short compared with the period requisite to change one species into another.
I am aware that two palaeontologists, whose opinions are worthy of much
deference, namely Bronn and Woodward, have concluded that the average
duration of each formation is twice or thrice as long as the average
duration of specific forms. But insuperable difficulties, as it seems to me,
prevent us coming to any just conclusion on this head. When we see a species
first appearing in the middle of any formation, it would be rash in the
extreme to infer that it had not elsewhere previously existed. So again when
we find a species disappearing before the uppermost layers have been
deposited, it would be equally rash to suppose that it then became wholly
extinct. We forget how small the area of Europe is compared with the rest of
the world; nor have the several stages of the same formation throughout
Europe been correlated with perfect accuracy.

With marine animals of all kinds, we may safely infer a large amount of
migration during climatal and other changes; and when we see a species first
appearing in any formation, the probability is that it only then first
immigrated into that area. It is well known, for instance, that several
species appeared somewhat earlier in the palaeozoic beds of North America
than in those of Europe; time having apparently been required for their
migration from the American to the European seas. In examining the latest
deposits of various quarters of the world, it has everywhere been noted,
that some few still existing species are common in the deposit, but have
become extinct in the immediately surrounding sea; or, conversely, that some
are now abundant in the neighbouring sea, but are rare or absent in this
particular deposit. It is an excellent lesson to reflect on the ascertained
amount of migration of the inhabitants of Europe during the Glacial period,
which forms only a part of one whole geological period; and likewise to
reflect on the great changes of level, on the inordinately great change of
climate, on the prodigious lapse of time, all included within this same
glacial period. Yet it may be doubted whether in any quarter of the world,
sedimentary deposits, including fossil remains, have gone on accumulating
within the same area during the whole of this period. It is not, for
instance, probable that sediment was deposited during the whole of the
glacial period near the mouth of the Mississippi, within that limit of depth
at which marine animals can flourish; for we know what vast geographical
changes occurred in other parts of America during this space of time. When
such beds as were deposited in shallow water near the mouth of the
Mississippi during some part of the glacial period shall have been upraised,
organic remains will probably first appear and disappear at different
levels, owing to the migration of species and to geographical changes. And
in the distant future, a geologist examining these beds, might be tempted to
conclude that the average duration of life of the embedded fossils had been
less than that of the glacial period, instead of having been really far
greater, that is extending from before the glacial epoch to the present day.

In order to get a perfect gradation between two forms in the upper and lower
parts of the same formation, the deposit must have gone on accumulating for
a very long period, in order to have given sufficient time for the slow
process of variation; hence the deposit will generally have to be a very
thick one; and the species undergoing modification will have had to live on
the same area throughout this whole time. But we have seen that a thick
fossiliferous formation can only be accumulated during a period of
subsidence; and to keep the depth approximately the same, which is necessary
in order to enable the same species to live on the same space, the supply of
sediment must nearly have counterbalanced the amount of subsidence. But this
same movement of subsidence will often tend to sink the area whence the
sediment is derived, and thus diminish the supply whilst the downward
movement continues. In fact, this nearly exact balancing between the supply
of sediment and the amount of subsidence is probably a rare contingency; for
it has been observed by more than one palaeontologist, that very thick
deposits are usually barren of organic remains, except near their upper or
lower limits.

It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation composed of
beds of different mineralogical composition, we may reasonably suspect that
the process of deposition has been much interrupted, as a change in the
currents of the sea and a supply of sediment of a different nature will
generally have been due to geographical changes requiring much time. Nor
will the closest inspection of a formation give any idea of the time which
its deposition has consumed. Many instances could be given of beds only a
few feet in thickness, representing formations, elsewhere thousands of feet
in thickness, and which must have required an enormous period for their
accumulation; yet no one ignorant of this fact would have suspected the vast
lapse of time represented by the thinner formation. Many cases could be
given of the lower beds of a formation having been upraised, denuded,
submerged, and then re-covered by the upper beds of the same formation,
facts, showing what wide, yet easily overlooked, intervals have occurred in
its accumulation. In other cases we have the plainest evidence in great
fossilised trees, still standing upright as they grew, of many long
intervals of time and changes of level during the process of deposition,
which would never even have been suspected, had not the trees chanced to
have been preserved: thus, Messrs Lyell and Dawson found carboniferous beds
1400 feet thick in Nova Scotia, with ancient root-bearing strata, one above
the other, at no less than sixty-eight different levels. Hence, when the
same species occur at the bottom, middle, and top of a formation, the
probability is that they have not lived on the same spot during the whole
period of deposition, but have disappeared and reappeared, perhaps many
times, during the same geological period. So that if such species were to
undergo a considerable amount of modification during any one geological
period, a section would not probably include all the fine intermediate
gradations which must on my theory have existed between them, but abrupt,
though perhaps very slight, changes of form.

It is all-important to remember that naturalists have no golden rule by
which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species,
unless they are enabled to connect them together by close intermediate
gradations. And this from the reasons just assigned we can seldom hope to
effect in any one geological section. Supposing B and C to be two species,
and a third, A, to be found in an underlying bed; even if A were strictly
intermediate between B and C, it would simply be ranked as a third and
distinct species, unless at the same time it could be most closely connected
with either one or both forms by intermediate varieties. Nor should it be
forgotten, as before explained, that A might be the actual progenitor of B
and C, and yet might not at all necessarily be strictly intermediate between
them in all points of structure. So that we might obtain the parent-species
and its several modified descendants from the lower and upper beds of a
formation, and unless we obtained numerous transitional gradations, we
should not recognise their relationship, and should consequently be
compelled to rank them all as distinct species.

It is notorious on what excessively slight differences many palaeontologists
have founded their species; and they do this the more readily if the
specimens come from different sub-stages of the same formation. Some
experienced conchologists are now sinking many of the very fine species of
D'Orbigny and others into the rank of varieties; and on this view we do find
the kind of evidence of change which on my theory we ought to find.
Moreover, if we look to rather wider intervals, namely, to distinct but
consecutive stages of the same great formation, we find that the embedded
fossils, though almost universally ranked as specifically different, yet are
far more closely allied to each other than are the species found in more
widely separated formations; but to this subject I shall have to return in
the following chapter.

One other consideration is worth notice: with animals and plants that can
propagate rapidly and are not highly locomotive, there is reason to suspect,
as we have formerly seen, that their varieties are generally at first local;
and that such local varieties do not spread widely and supplant their
parent-forms until they have been modified and perfected in some
considerable degree. According to this view, the chance of discovering in a
formation in any one country all the early stages of transition between any
two forms, is small, for the successive changes are supposed to have been
local or confined to some one spot. Most marine animals have a wide range;
and we have seen that with plants it is those which have the widest range,
that oftenest present varieties; so that with shells and other marine
animals, it is probably those which have had the widest range, far exceeding
the limits of the known geological formations of Europe, which have oftenest
given rise, first to local varieties and ultimately to new species; and this
again would greatly lessen the chance of our being able to trace the stages
of transition in any one geological formation.

It should not be forgotten, that at the present day, with perfect specimens
for examination, two forms can seldom be connected by intermediate varieties
and thus proved to be the same species, until many specimens have been
collected from many places; and in the case of fossil species this could
rarely be effected by palaeontologists. We shall, perhaps, best perceive the
improbability of our being enabled to connect species by numerous, fine,
intermediate, fossil links, by asking ourselves whether, for instance,
geologists at some future period will be able to prove, that our different
breeds of cattle, sheep, horses, and dogs have descended from a single stock
or from several aboriginal stocks; or, again, whether certain sea-shells
inhabiting the shores of North America, which are ranked by some
conchologists as distinct species from their European representatives, and
by other conchologists as only varieties, are really varieties or are, as it
is called, specifically distinct. This could be effected only by the future
geologist discovering in a fossil state numerous intermediate gradations;
and such success seems to me improbable in the highest degree.

Geological research, though it has added numerous species to existing and
extinct genera, and has made the intervals between some few groups less wide
than they otherwise would have been, yet has done scarcely anything in
breaking down the distinction between species, by connecting them together
by numerous, fine, intermediate varieties; and this not having been
effected, is probably the gravest and most obvious of all the many
objections which may be urged against my views. Hence it will be worth while
to sum up the foregoing remarks, under an imaginary illustration. The Malay
Archipelago is of about the size of Europe from the North Cape to the
Mediterranean, and from Britain to Russia; and therefore equals all the
geological formations which have been examined with any accuracy, excepting
those of the United States of America. I fully agree with Mr Godwin-Austen,
that the present condition of the Malay Archipelago, with its numerous large
islands separated by wide and shallow seas, probably represents the former
state of Europe, when most of our formations were accumulating. The Malay
Archipelago is one of the richest regions of the whole world in organic
beings; yet if all the species were to be collected which have ever lived
there, how imperfectly would they represent the natural history of the
world!

But we have every reason to believe that the terrestrial productions of the
archipelago would be preserved in an excessively imperfect manner in the
formations which we suppose to be there accumulating. I suspect that not
many of the strictly littoral animals, or of those which lived on naked
submarine rocks, would be embedded; and those embedded in gravel or sand,
would not endure to a distant epoch. Wherever sediment did not accumulate on
the bed of the sea, or where it did not accumulate at a sufficient rate to
protect organic bodies from decay, no remains could be preserved.

In our archipelago, I believe that fossiliferous formations could be formed
of sufficient thickness to last to an age, as distant in futurity as the
secondary formations lie in the past, only during periods of subsidence.
These periods of subsidence would be separated from each other by enormous
intervals, during which the area would be either stationary or rising;
whilst rising, each fossiliferous formation would be destroyed, almost as
soon as accumulated, by the incessant coast-action, as we now see on the
shores of South America. During the periods of subsidence there would
probably be much extinction of life; during the periods of elevation, there
would be much variation, but the geological record would then be least
perfect.

It may be doubted whether the duration of any one great period of subsidence
over the whole or part of the archipelago, together with a contemporaneous
accumulation of sediment, would exceed the average duration of the same
specific forms; and these contingencies are indispensable for the
preservation of all the transitional gradations between any two or more
species. If such gradations were not fully preserved, transitional varieties
would merely appear as so many distinct species. It is, also, probable that
each great period of subsidence would be interrupted by oscillations of
level, and that slight climatal changes would intervene during such lengthy
periods; and in these cases the inhabitants of the archipelago would have to
migrate, and no closely consecutive record of their modifications could be
preserved in any one formation.

Very many of the marine inhabitants of the archipelago now range thousands
of miles beyond its confines; and analogy leads me to believe that it would
be chiefly these far-ranging species which would oftenest produce new
varieties; and the varieties would at first generally be local or confined
to one place, but if possessed of any decided advantage, or when further
modified and improved, they would slowly spread and supplant their
parent-forms. When such varieties returned to their ancient homes, as they
would differ from their former state, in a nearly uniform, though perhaps
extremely slight degree, they would, according to the principles followed by
many palaeontologists, be ranked as new and distinct species.

If then, there be some degree of truth in these remarks, we have no right to
expect to find in our geological formations, an infinite number of those
fine transitional forms, which on my theory assuredly have connected all the
past and present species of the same group into one long and branching chain
of life. We ought only to look for a few links, some more closely, some more
distantly related to each other; and these links, let them be ever so close,
if found in different stages of the same formation, would, by most
palaeontologists, be ranked as distinct species. But I do not pretend that I
should ever have suspected how poor a record of the mutations of life, the
best preserved geological section presented, had not the difficulty of our
not discovering innumerable transitional links between the species which
appeared at the commencement and close of each formation, pressed so hardly
on my theory.

On the sudden appearance of whole groups of Allied Species. The abrupt
manner in which whole groups of species suddenly appear in certain
formations, has been urged by several palaeontologists, for instance, by
Agassiz, Pictet, and by none more forcibly than by Professor Sedgwick, as a
fatal objection to the belief in the transmutation of species. If numerous
species, belonging to the same genera or families, have really started into
life all at once, the fact would be fatal to the theory of descent with slow
modification through natural selection. For the development of a group of
forms, all of which have descended from some one progenitor, must have been
an extremely slow process; and the progenitors must have lived long ages
before their modified descendants. But we continually over-rate the
perfection of the geological record, and falsely infer, because certain
genera or families have not been found beneath a certain stage, that they
did not exist before that stage. We continually forget how large the world
is, compared with the area over which our geological formations have been
carefully examined; we forget that groups of species may elsewhere have long
existed and have slowly multiplied before they invaded the ancient
archipelagoes of Europe and of the United States. We do not make due
allowance for the enormous intervals of time, which have probably elapsed
between our consecutive formations, longer perhaps in some cases than the
time required for the accumulation of each formation. These intervals will
have given time for the multiplication of species from some one or some few
parent-forms; and in the succeeding formation such species will appear as if
suddenly created.

I may here recall a remark formerly made, namely that it might require a
long succession of ages to adapt an organism to some new and peculiar line
of life, for instance to fly through the air; but that when this had been
effected, and a few species had thus acquired a great advantage over other
organisms, a comparatively short time would be necessary to produce many
divergent forms, which would be able to spread rapidly and widely throughout
the world.

I will now give a few examples to illustrate these remarks; and to show how
liable we are to error in supposing that whole groups of species have
suddenly been produced. I may recall the well-known fact that in geological
treatises, published not many years ago, the great class of mammals was
always spoken of as having abruptly come in at the commencement of the
tertiary series. And now one of the richest known accumulations of fossil
mammals belongs to the middle of the secondary series; and one true mammal
has been discovered in the new red sandstone at nearly the commencement of
this great series. Cuvier used to urge that no monkey occurred in any
tertiary stratum; but now extinct species have been discovered in India,
South America, and in Europe even as far back as the eocene stage. The most
striking case, however, is that of the Whale family; as these animals have
huge bones, are marine, and range over the world, the fact of not a single
bone of a whale having been discovered in any secondary formation, seemed
fully to justify the belief that this great and distinct order had been
suddenly produced in the interval between the latest secondary and earliest
tertiary formation. But now we may read in the Supplement to Lyell's
'Manual,' published in 1858, clear evidence of the existence of whales in
the upper greensand, some time before the close of the secondary period.

I may give another instance, which from having passed under my own eyes has
much struck me. In a memoir on Fossil Sessile Cirripedes, I have stated
that, from the number of existing and extinct tertiary species; from the
extraordinary abundance of the individuals of many species all over the
world, from the Arctic regions to the equator, inhabiting various zones of
depths from the upper tidal limits to 50 fathoms; from the perfect manner in
which specimens are preserved in the oldest tertiary beds; from the ease
with which even a fragment of a valve can be recognised; from all these
circumstances, I inferred that had sessile cirripedes existed during the
secondary periods, they would certainly have been preserved and discovered;
and as not one species had been discovered in beds of this age, I concluded
that this great group had been suddenly developed at the commencement of the
tertiary series. This was a sore trouble to me, adding as I thought one more
instance of the abrupt appearance of a great group of species. But my work
had hardly been published, when a skilful palaeontologist, M. Bosquet, sent
me a drawing of a perfect specimen of an unmistakeable sessile cirripede,
which he had himself extracted from the chalk of Belgium. And, as if to make
the case as striking as possible, this sessile cirripede was a Chthamalus, a
very common, large, and ubiquitous genus, of which not one specimen has as
yet been found even in any tertiary stratum. Hence we now positively know
that sessile cirripedes existed during the secondary period; and these
cirripedes might have been the progenitors of our many tertiary and existing
species.

The case most frequently insisted on by palaeontologists of the apparently
sudden appearance of a whole group of species, is that of the teleostean
fishes, low down in the Chalk period. This group includes the large majority
of existing species. Lately, Professor Pictet has carried their existence
one sub-stage further back; and some palaeontologists believe that certain
much older fishes, of which the affinities are as yet imperfectly known, are
really teleostean. Assuming, however, that the whole of them did appear, as
Agassiz believes, at the commencement of the chalk formation, the fact would
certainly be highly remarkable; but I cannot see that it would be an
insuperable difficulty on my theory, unless it could likewise be shown that
the species of this group appeared suddenly and simultaneously throughout
the world at this same period. It is almost superfluous to remark that
hardly any fossil-fish are known from south of the equator; and by running
through Pictet's palaeontology it will be seen that very few species are
known from several formations in Europe. Some few families of fish now have
a confined range; the teleostean fish might formerly have had a similarly
confined range, and after having been largely developed in some one sea,
might have spread widely. Nor have we any right to suppose that the seas of
the world have always been so freely open from south to north as they are at
present. Even at this day, if the Malay Archipelago were converted into
land, the tropical parts of the Indian Ocean would form a large and
perfectly enclosed basin, in which any great group of marine animals might
be multiplied; and here they would remain confined, until some of the
species became adapted to a cooler climate, and were enabled to double the
southern capes of Africa or Australia, and thus reach other and distant
seas.

From these and similar considerations, but chiefly from our ignorance of the
geology of other countries beyond the confines of Europe and the United
States; and from the revolution in our palaeontological ideas on many
points, which the discoveries of even the last dozen years have effected, it
seems to me to be about as rash in us to dogmatize on the succession of
organic beings throughout the world, as it would be for a naturalist to land
for five minutes on some one barren point in Australia, and then to discuss
the number and range of its productions.

On the sudden appearance of groups of Allied Species in the lowest known
fossiliferous strata. There is another and allied difficulty, which is much
graver. I allude to the manner in which numbers of species of the same
group, suddenly appear in the lowest known fossiliferous rocks. Most of the
arguments which have convinced me that all the existing species of the same
group have descended from one progenitor, apply with nearly equal force to
the earliest known species. For instance, I cannot doubt that all the
Silurian trilobites have descended from some one crustacean, which must have
lived long before the Silurian age, and which probably differed greatly from
any known animal. Some of the most ancient Silurian animals, as the
Nautilus, Lingula, &c., do not differ much from living species; and it
cannot on my theory be supposed, that these old species were the progenitors
of all the species of the orders to which they belong, for they do not
present characters in any degree intermediate between them. If, moreover,
they had been the progenitors of these orders, they would almost certainly
have been long ago supplanted and exterminated by their numerous and
improved descendants.

Consequently, if my theory be true, it is indisputable that before the
lowest Silurian stratum was deposited, long periods elapsed, as long as, or
probably far longer than, the whole interval from the Silurian age to the
present day; and that during these vast, yet quite unknown, periods of time,
the world swarmed with living creatures.

To the question why we do not find records of these vast primordial periods,
I can give no satisfactory answer. Several of the most eminent geologists,
with Sir R. Murchison at their head, are convinced that we see in the
organic remains of the lowest Silurian stratum the dawn of life on this
planet. Other highly competent judges, as Lyell and the late E. Forbes,
dispute this conclusion. We should not forget that only a small portion of
the world is known with accuracy. M. Barrande has lately added another and
lower stage to the Silurian system, abounding with new and peculiar species.
Traces of life have been detected in the Longmynd beds beneath Barrande's
so-called primordial zone. The presence of phosphatic nodules and bituminous
matter in some of the lowest azoic rocks, probably indicates the former
existence of life at these periods. But the difficulty of understanding the
absence of vast piles of fossiliferous strata, which on my theory no doubt
were somewhere accumulated before the Silurian epoch, is very great. If
these most ancient beds had been wholly worn away by denudation, or
obliterated by metamorphic action, we ought to find only small remnants of
the formations next succeeding them in age, and these ought to be very
generally in a metamorphosed condition. But the descriptions which we now
possess of the Silurian deposits over immense territories in Russia and in
North America, do not support the view, that the older a formation is, the
more it has suffered the extremity of denudation and metamorphism.

The case at present must remain inexplicable; and may be truly urged as a
valid argument against the views here entertained. To show that it may
hereafter receive some explanation, I will give the following hypothesis.
From the nature of the organic remains, which do not appear to have
inhabited profound depths, in the several formations of Europe and of the
United States; and from the amount of sediment, miles in thickness, of which
the formations are composed, we may infer that from first to last large
islands or tracts of land, whence the sediment was derived, occurred in the
neighbourhood of the existing continents of Europe and North America. But we
do not know what was the state of things in the intervals between the
successive formations; whether Europe and the United States during these
intervals existed as dry land, or as a submarine surface near land, on which
sediment was not deposited, or again as the bed of an open and unfathomable
sea.

Looking to the existing oceans, which are thrice as extensive as the land,
we see them studded with many islands; but not one oceanic island is as yet
known to afford even a remnant of any palaeozoic or secondary formation.
Hence we may perhaps infer, that during the palaeozoic and secondary
periods, neither continents nor continental islands existed where our oceans
now extend; for had they existed there, palaeozoic and secondary formations
would in all probability have been accumulated from sediment derived from
their wear and tear; and would have been at least partially upheaved by the
oscillations of level, which we may fairly conclude must have intervened
during these enormously long periods. If then we may infer anything from
these facts, we may infer that where our oceans now extend, oceans have
extended from the remotest period of which we have any record; and on the
other hand, that where continents now exist, large tracts of land have
existed, subjected no doubt to great oscillations of level, since the
earliest silurian period. The coloured map appended to my volume on Coral
Reefs, led me to conclude that the great oceans are still mainly areas of
subsidence, the great archipelagoes still areas of oscillations of level,
and the continents areas of elevation. But have we any right to assume that
things have thus remained from eternity? Our continents seem to have been
formed by a preponderance, during many oscillations of level, of the force
of elevation; but may not the areas of preponderant movement have changed in
the lapse of ages? At a period immeasurably antecedent to the silurian
epoch, continents may have existed where oceans are now spread out; and
clear and open oceans may have existed where our continents now stand. Nor
should we be justified in assuming that if, for instance, the bed of the
Pacific Ocean were now converted into a continent, we should there find
formations older than the silurian strata, supposing such to have been
formerly deposited; for it might well happen that strata which had subsided
some miles nearer to the centre of the earth, and which had been pressed on
by an enormous weight of superincumbent water, might have undergone far more
metamorphic action than strata which have always remained nearer to the
surface. The immense areas in some parts of the world, for instance in South
America, of bare metamorphic rocks, which must have been heated under great
pressure, have always seemed to me to require some special explanation; and
we may perhaps believe that we see in these large areas, the many formations
long anterior to the silurian epoch in a completely metamorphosed condition.

The several difficulties here discussed, namely our not finding in the
successive formations infinitely numerous transitional links between the
many species which now exist or have existed; the sudden manner in which
whole groups of species appear in our European formations; the almost entire
absence, as at present known, of fossiliferous formations beneath the
Silurian strata, are all undoubtedly of the gravest nature. We see this in
the plainest manner by the fact that all the most eminent palaeontologists,
namely Cuvier, Owen, Agassiz, Barrande, Falconer, E. Forbes, &c., and all
our greatest geologists, as Lyell, Murchison, Sedgwick, &c., have
unanimously, often vehemently, maintained the immutability of species. But I
have reason to believe that one great authority, Sir Charles Lyell, from
further reflexion entertains grave doubts on this subject. I feel how rash
it is to differ from these great authorities, to whom, with others, we owe
all our knowledge. Those who think the natural geological record in any
degree perfect, and who do not attach much weight to the facts and arguments
of other kinds even in this volume, will undoubtedly at once reject my
theory. For my part, following out Lyell's metaphor, I look at the natural
geological record, as a history of the world imperfectly kept, and written
in a changing dialect; of this history we possess the last volume alone,
relating only to two or three countries. Of this volume, only here and there
a short chapter has been preserved; and of each page, only here and there a
few lines. Each word of the slowly-changing language, in which the history
is supposed to be written, being more or less different in the interrupted
succession of chapters, may represent the apparently abruptly changed forms
of life, entombed in our consecutive, but widely separated formations. On
this view, the difficulties above discussed are greatly diminished, or even
disappear.

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Chapter 10 - On The Geological Succession of Organic Beings

[* ] On the slow and successive appearance of new species [* ] On their
different rates of change [* ] Species once lost do not reappear [* ] Groups
of species follow the same general rules in their appearance and
disappearance as do single species [* ] On Extinction [* ] On simultaneous
changes in the forms of life throughout the world [* ] On the affinities of
extinct species to each other and to living species [* ] On the state of
development of ancient forms [* ] On the succession of the same types within
the same areas [* ] Summary of preceding and present chapters
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Let us now see whether the several facts and rules relating to the
geological succession of organic beings, better accord with the common view
of the immutability of species, or with that of their slow and gradual
modification, through descent and natural selection.

New species have appeared very slowly, one after another, both on the land
and in the waters. Lyell has shown that it is hardly possible to resist the
evidence on this head in the case of the several tertiary stages; and every
year tends to fill up the blanks between them, and to make the percentage
system of lost and new forms more gradual. In some of the most recent beds,
though undoubtedly of high antiquity if measured by years, only one or two
species are lost forms, and only one or two are new forms, having here
appeared for the first time, either locally, or, as far as we know, on the
face of the earth. If we may trust the observations of Philippi in Sicily,
the successive changes in the marine inhabitants of that island have been
many and most gradual. The secondary formations are more broken; but, as
Bronn has remarked, neither the appearance nor disappearance of their many
now extinct species has been simultaneous in each separate formation.

Species of different genera and classes have not changed at the same rate,
or in the same degree. In the oldest tertiary beds a few living shells may
still be found in the midst of a multitude of extinct forms. Falconer has
given a striking instance of a similar fact, in an existing crocodile
associated with many strange and lost mammals and reptiles in the
sub-Himalayan deposits. The Silurian Lingula differs but little from the
living species of this genus; whereas most of the other Silurian Molluscs
and all the Crustaceans have changed greatly. The productions of the land
seem to change at a quicker rate than those of the sea, of which a striking
instance has lately been observed in Switzerland. There is some reason to
believe that organisms, considered high in the scale of nature, change more
quickly than those that are low: though there are exceptions to this rule.
The amount of organic change, as Pictet has remarked, does not strictly
correspond with the succession of our geological formations; so that between
each two consecutive formations, the forms of life have seldom changed in
exactly the same degree. Yet if we compare any but the most closely related
formations, all the species will be found to have undergone some change.
When a species has once disappeared from the face of the earth, we have
reason to believe that the same identical form never reappears. The
strongest apparent exception to this latter rule, is that of the so-called
`colonies' of M. Barrande, which intrude for a period in the midst of an
older formation, and then allow the pre-existing fauna to reappear; but
Lyell's explanation, namely, that it is a case of temporary migration from a
distinct geographical province, seems to me satisfactory.

These several facts accord well with my theory. I believe in no fixed law of
development, causing all the inhabitants of a country to change abruptly, or
simultaneously, or to an equal degree. The process of modification must be
extremely slow. The variability of each species is quite independent of that
of all others. Whether such variability be taken advantage of by natural
selection, and whether the variations be accumulated to a greater or lesser
amount, thus causing a greater or lesser amount of modification in the
varying species, depends on many complex contingencies, on the variability
being of a beneficial nature, on the power of intercrossing, on the rate of
breeding, on the slowly changing physical conditions of the country, and
more especially on the nature of the other inhabitants with which the
varying species comes into competition. Hence it is by no means surprising
that one species should retain the same identical form much longer than
others; or, if changing, that it should change less. We see the same fact in
geographical distribution; for instance, in the land-shells and coleopterous
insects of Madeira having come to differ considerably from their nearest
allies on the continent of Europe, whereas the marine shells and birds have
remained unaltered. We can perhaps understand the apparently quicker rate of
change in terrestrial and in more highly organised productions compared with
marine and lower productions, by the more complex relations of the higher
beings to their organic and inorganic conditions of life, as explained in a
former chapter. When many of the inhabitants of a country have become
modified and improved, we can understand, on the principle of competition,
and on that of the many all-important relations of organism to organism,
that any form which does not become in some degree modified and improved,
will be liable to be exterminated. Hence we can see why all the species in
the same region do at last, if we look to wide enough intervals of time,
become modified; for those which do not change will become extinct.

In members of the same class the average amount of change, during long and
equal periods of time, may, perhaps, be nearly the same; but as the
accumulation of long-enduring fossiliferous formations depends on great
masses of sediment having been deposited on areas whilst subsiding, our
formations have been almost necessarily accumulated at wide and irregularly
intermittent intervals; consequently the amount of organic change exhibited
by the fossils embedded in consecutive formations is not equal. Each
formation, on this view, does not mark a new and complete act of creation,
but only an occasional scene, taken almost at hazard, in a slowly changing
drama.

We can clearly understand why a species when once lost should never
reappear, even if the very same conditions of life, organic and inorganic,
should recur. For though the offspring of one species might be adapted (and
no doubt this has occurred in innumerable instances) to fill the exact place
of another species in the economy of nature, and thus supplant it; yet the
two forms the old and the new would not be identically the same; for both
would almost certainly inherit different characters from their distinct
progenitors. For instance, it is just possible, if our fantail-pigeons were
all destroyed, that fanciers, by striving during long ages for the same
object, might make a new breed hardly distinguishable from our present
fantail; but if the parent rock-pigeon were also destroyed, and in nature we
have every reason to believe that the parent-form will generally be
supplanted and exterminated by its improved offspring, it is quite
incredible that a fantail, identical with the existing breed, could be
raised from any other species of pigeon, or even from the other
well-established races of the domestic pigeon, for the newly-formed fantail
would be almost sure to inherit from its new progenitor some slight
characteristic differences.

Groups of species, that is, genera and families, follow the same general
rules in their appearance and disappearance as do single species, changing
more or less quickly, and in a greater or lesser degree. A group does not
reappear after it has once disappeared; or its existence, as long as it
lasts, is continuous. I am aware that there are some apparent exceptions to
this rule, but the exceptions are surprisingly few, so few, that E. Forbes,
Pictet, and Woodward (though all strongly opposed to such views as I
maintain) admit its truth; and the rule strictly accords with my theory. For
as all the species of the same group have descended from some one species,
it is clear that as long as any species of the group have appeared in the
long succession of ages, so long must its members have continuously existed,
in order to have generated either new and modified or the same old and
unmodified forms. Species of the genus Lingula, for instance, must have
continuously existed by an unbroken succession of generations, from the
lowest Silurian stratum to the present day.

We have seen in the last chapter that the species of a group sometimes
falsely appear to have come in abruptly; and I have attempted to give an
explanation of this fact, which if true would have been fatal to my views.
But such cases are certainly exceptional; the general rule being a gradual
increase in number, till the group reaches its maximum, and then, sooner or
later, it gradually decreases. If the number of the species of a genus, or
the number of the genera of a family, be represented by a vertical line of
varying thickness, crossing the successive geological formations in which
the species are found, the line will sometimes falsely appear to begin at
its lower end, not in a sharp point, but abruptly; it then gradually
thickens upwards, sometimes keeping for a space of equal thickness, and
ultimately thins out in the upper beds, marking the decrease and final
extinction of the species. This gradual increase in number of the species of
a group is strictly conformable with my theory; as the species of the same
genus, and the genera of the same family, can increase only slowly and
progressively; for the process of modification and the production of a
number of allied forms must be slow and gradual, one species giving rise
first to two or three varieties, these being slowly converted into species,
which in their turn produce by equally slow steps other species, and so on,
like the branching of a great tree from a single stem, till the group
becomes large.

On Extinction

We have as yet spoken only incidentally of the disappearance of species and
of groups of species. On the theory of natural selection the extinction of
old forms and the production of new and improved forms are intimately
connected together. The old notion of all the inhabitants of the earth
having been swept away at successive periods by catastrophes, is very
generally given up, even by those geologists, as Elie de Beaumont,
Murchison, Barrande, &c., whose general views would naturally lead them to
this conclusion. On the contrary, we have every reason to believe, from the
study of the tertiary formations, that species and groups of species
gradually disappear, one after another, first from one spot, then from
another, and finally from the world. Both single species and whole groups of
species last for very unequal periods; some groups, as we have seen, having
endured from the earliest known dawn of life to the present day; some having
disappeared before the close of the palaeozoic period. No fixed law seems to
determine the length of time during which any single species or any single
genus endures. There is reason to believe that the complete extinction of
the species of a group is generally a slower process than their production:
if the appearance and disappearance of a group of species be represented, as
before, by a vertical line of varying thickness, the line is found to taper
more gradually at its upper end, which marks the progress of extermination,
than at its lower end, which marks the first appearance and increase in
numbers of the species. In some cases, however, the extermination of whole
groups of beings, as of ammonites towards the close of the secondary period,
has been wonderfully sudden.

The whole subject of the extinction of species has been involved in the most
gratuitous mystery. Some authors have even supposed that as the individual
has a definite length of life, so have species a definite duration. No one I
think can have marvelled more at the extinction of species, than I have
done. When I found in La Plata the tooth of a horse embedded with the
remains of Mastodon, Megatherium, Toxodon, and other extinct monsters, which
all co-existed with still living shells at a very late geological period, I
was filled with astonishment; for seeing that the horse, since its
introduction by the Spaniards into South America, has run wild over the
whole country and has increased in numbers at an unparalleled rate, I asked
myself what could so recently have exterminated the former horse under
conditions of life apparently so favourable. But how utterly groundless was
my astonishment! Professor Owen soon perceived that the tooth, though so
like that of the existing horse, belonged to an extinct species. Had this
horse been still living, but in some degree rare, no naturalist would have
felt the least surprise at its rarity; for rarity is the attribute of a vast
number of species of all classes, in all countries. If we ask ourselves why
this or that species is rare, we answer that something is unfavourable in
its conditions of life; but what that something is, we can hardly ever tell.
On the supposition of the fossil horse still existing as a rare species, we
might have felt certain from the analogy of all other mammals, even of the
slow-breeding elephant, and from the history of the naturalisation of the
domestic horse in South America, that under more favourable conditions it
would in a very few years have stocked the whole continent. But we could not
have told what the unfavourable conditions were which checked its increase,
whether some one or several contingencies, and at what period of the horse's
life, and in what degree, they severally acted. If the conditions had gone
on, however slowly, becoming less and less favourable, we assuredly should
not have perceived the fact, yet the fossil horse would certainly have
become rarer and rarer, and finally extinct; its place being seized on by
some more successful competitor.

It is most difficult always to remember that the increase of every living
being is constantly being checked by unperceived injurious agencies; and
that these same unperceived agencies are amply sufficient to cause rarity,
and finally extinction. We see in many cases in the more recent tertiary
formations, that rarity precedes extinction; and we know that this has been
the progress of events with those animals which have been exterminated,
either locally or wholly, through man's agency. I may repeat what I
published in 1845, namely, that to admit that species generally become rare
before they become extinct to feel no surprise at the rarity of a species,
and yet to marvel greatly when it ceases to exist, is much the same as to
admit that sickness in the individual is the forerunner of death to feel no
surprise at sickness, but when the sick man dies, to wonder and to suspect
that he died by some unknown deed of violence.

The theory of natural selection is grounded on the belief that each new
variety, and ultimately each new species, is produced and maintained by
having some advantage over those with which it comes into competition; and
the consequent extinction of less-favoured forms almost inevitably follows.
It is the same with our domestic productions: when a new and slightly
improved variety has been raised, it at first supplants the less improved
varieties in the same neighbourhood; when much improved it is transported
far and near, like our short-horn cattle, and takes the place of other
breeds in other countries. Thus the appearance of new forms and the
disappearance of old forms, both natural and artificial, are bound together.
In certain flourishing groups, the number of new specific forms which have
been produced within a given time is probably greater than that of the old
forms which have been exterminated; but we know that the number of species
has not gone on indefinitely increasing, at least during the later
geological periods, so that looking to later times we may believe that the
production of new forms has caused the extinction of about the same number
of old forms.

The competition will generally be most severe, as formerly explained and
illustrated by examples, between the forms which are most like each other in
all respects. Hence the improved and modified descendants of a species will
generally cause the extermination of the parent-species; and if many new
forms have been developed from any one species, the nearest allies of that
species, i.e. the species of the same genus, will be the most liable to
extermination. Thus, as I believe, a number of new species descended from
one species, that is a new genus, comes to supplant an old genus, belonging
to the same family. But it must often have happened that a new species
belonging to some one group will have seized on the place occupied by a
species belonging to a distinct group, and thus caused its extermination;
and if many allied forms be developed from the successful intruder, many
will have to yield their places; and it will generally be allied forms,
which will suffer from some inherited inferiority in common. But whether it
be species belonging to the same or to a distinct class, which yield their
places to other species which have been modified and improved, a few of the
sufferers may often long be preserved, from being fitted to some peculiar
line of life, or from inhabiting some distant and isolated station, where
they have escaped severe competition. For instance, a single species of
Trigonia, a great genus of shells in the secondary formations, survives in
the Australian seas; and a few members of the great and almost extinct group
of Ganoid fishes still inhabit our fresh waters. Therefore the utter
extinction of a group is generally, as we have seen, a slower process than
its production.

With respect to the apparently sudden extermination of whole families or
orders, as of Trilobites at the close of the palaeozoic period and of
Ammonites at the close of the secondary period, we must remember what has
been already said on the probable wide intervals of time between our
consecutive formations; and in these intervals there may have been much slow
extermination. Moreover, when by sudden immigration or by unusually rapid
development, many species of a new group have taken possession of a new
area, they will have exterminated in a correspondingly rapid manner many of
the old inhabitants; and the forms which thus yield their places will
commonly be allied, for they will partake of some inferiority in common.

Thus, as it seems to me, the manner in which single species and whole groups
of species become extinct, accords well with the theory of natural
selection. We need not marvel at extinction; if we must marvel, let it be at
our presumption in imagining for a moment that we understand the many
complex contingencies, on which the existence of each species depends. If we
forget for an instant, that each species tends to increase inordinately, and
that some check is always in action, yet seldom perceived by us, the whole
economy of nature will be utterly obscured. Whenever we can precisely say
why this species is more abundant in individuals than that; why this species
and not another can be naturalised in a given country; then, and not till
then, we may justly feel surprise why we cannot account for the extinction
of this particular species or group of species.

On the Forms of Life changing almost simultaneously throughout the World

Scarcely any palaeontological discovery is more striking than the fact, that
the forms of life change almost simultaneously throughout the world. Thus
our European Chalk formation can be recognised in many distant parts of the
world, under the most different climates, where not a fragment of the
mineral chalk itself can be found; namely, in North America, in equatorial
South America, in Tierra del Fuego, at the Cape of Good Hope, and in the
peninsula of India. For at these distant points, the organic remains in
certain beds present an unmistakeable degree of resemblance to those of the
Chalk. It is not that the same species are met with; for in some cases not
one species is identically the same, but they belong to the same families,
genera, and sections of genera, and sometimes are similarly characterised in
such trifling points as mere superficial sculpture. Moreover other forms,
which are not found in the Chalk of Europe, but which occur in the
formations either above or below, are similarly absent at these distant
points of the world. In the several successive palaeozoic formations of
Russia, Western Europe and North America, a similar parallelism in the forms
of life has been observed by several authors: so it is, according to Lyell,
with the several European and North American tertiary deposits. Even if the
few fossil species which are common to the Old and New Worlds be kept wholly
out of view, the general parallelism in the successive forms of life, in the
stages of the widely separated palaeozoic and tertiary periods, would still
be manifest, and the several formations could be easily correlated.

These observations, however, relate to the marine inhabitants of distant
parts of the world: we have not sufficient data to judge whether the
productions of the land and of fresh water change at distant points in the
same parallel manner. We may doubt whether they have thus changed: if the
Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to Europe
from La Plata, without any information in regard to their geological
position, no one would have suspected that they had coexisted with still
living sea-shells; but as these anomalous monsters coexisted with the
Mastodon and Horse, it might at least have been inferred that they had lived
during one of the latter tertiary stages.

When the marine forms of life are spoken of as having changed simultaneously
throughout the world, it must not be supposed that this expression relates
to the same thousandth or hundred-thousandth year, or even that it has a
very strict geological sense; for if all the marine animals which live at
the present day in Europe, and all those that lived in Europe during the
pleistocene period (an enormously remote period as measured by years,
including the whole glacial epoch), were to be compared with those now
living in South America or in Australia, the most skilful naturalist would
hardly be able to say whether the existing or the pleistocene inhabitants of
Europe resembled most closely those of the southern hemisphere. So, again,
several highly competent observers believe that the existing productions of
the United States are more closely related to those which lived in Europe
during certain later tertiary stages, than to those which now live here; and
if this be so, it is evident that fossiliferous beds deposited at the
present day on the shores of North America would hereafter be liable to be
classed with somewhat older European beds. Nevertheless, looking to a
remotely future epoch, there can, I think, be little doubt that all the more
modern marine formations, namely, the upper pliocene, the pleistocene and
strictly modern beds, of Europe, North and South America, and Australia,
from containing fossil remains in some degree allied, and from not including
those forms which are only found in the older underlying deposits, would be
correctly ranked as simultaneous in a geological sense.

The fact of the forms of life changing simultaneously, in the above large
sense, at distant parts of the world, has greatly struck those admirable
observers, MM. de Verneuil and d'Archiac. After referring to the parallelism
of the palaeozoic forms of life in various parts of Europe, they add, `If
struck by this strange sequence, we turn our attention to North America, and
there discover a series of analogous phenomena, it will appear certain that
all these modifications of species, their extinction, and the introduction
of new ones, cannot be owing to mere changes in marine currents or other
causes more or less local and temporary, but depend on general laws which
govern the whole animal kingdom.' M. Barrande has made forcible remarks to
precisely the same effect. It is, indeed, quite futile to look to changes of
currents, climate, or other physical conditions, as the cause of these great
mutations in the forms of life throughout the world, under the most
different climates. We must, as Barrande has remarked, look to some special
law. We shall see this more clearly when we treat of the present
distribution of organic beings, and find how slight is the relation between
the physical conditions of various countries, and the nature of their
inhabitants.

This great fact of the parallel succession of the forms of life throughout
the world, is explicable on the theory of natural selection. New species are
formed by new varieties arising, which have some advantage over older forms;
and those forms, which are already dominant, or have some advantage over the
other forms in their own country, would naturally oftenest give rise to new
varieties or incipient species; for these latter must be victorious in a
still higher degree in order to be preserved and to survive. We have
distinct evidence on this head, in the plants which are dominant, that is,
which are commonest in their own homes, and are most widely diffused, having
produced the greatest number of new varieties. It is also natural that the
dominant, varying, and far-spreading species, which already have invaded to
a certain extent the territories of other species, should be those which
would have the best chance of spreading still further, and of giving rise in
new countries to new varieties and species. The process of diffusion may
often be very slow, being dependent on climatal and geographical changes, or
on strange accidents, but in the long run the dominant forms will generally
succeed in spreading. The diffusion would, it is probable, be slower with
the terrestrial inhabitants of distinct continents than with the marine
inhabitants of the continuous sea. We might therefore expect to find, as we
apparently do find, a less strict degree of parallel succession in the
productions of the land than of the sea.

Dominant species spreading from any region might encounter still more
dominant species, and then their triumphant course, or even their existence,
would cease. We know not at all precisely what are all the conditions most
favourable for the multiplication of new and dominant species; but we can, I
think, clearly see that a number of individuals, from giving a better chance
of the appearance of favourable variations, and that severe competition with
many already existing forms, would be highly favourable, as would be the
power of spreading into new territories. A certain amount of isolation,
recurring at long intervals of time, would probably be also favourable, as
before explained. One quarter of the world may have been most favourable for
the production of new and dominant species on the land, and another for
those in the waters of the sea. If two great regions had been for a long
period favourably circumstanced in an equal degree, whenever their
inhabitants met, the battle would be prolonged and severe; and some from one
birthplace and some from the other might be victorious. But in the course of
time, the forms dominant in the highest degree, wherever produced, would
tend everywhere to prevail. As they prevailed, they would cause the
extinction of other and inferior forms; and as these inferior forms would be
allied in groups by inheritance, whole groups would tend slowly to
disappear; though here and there a single member might long be enabled to
survive.

Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the world,
accords well with the principle of new species having been formed by
dominant species spreading widely and varying; the new species thus produced
being themselves dominant owing to inheritance, and to having already had
some advantage over their parents or over other species; these again
spreading, varying, and producing new species. The forms which are beaten
and which yield their places to the new and victorious forms, will generally
be allied in groups, from inheriting some inferiority in common; and
therefore as new and improved groups spread throughout the world, old groups
will disappear from the world; and the succession of forms in both ways will
everywhere tend to correspond.

There is one other remark connected with this subject worth making. I have
given my reasons for believing that all our greater fossiliferous formations
were deposited during periods of subsidence; and that blank intervals of
vast duration occurred during the periods when the bed of the sea was either
stationary or rising, and likewise when sediment was not thrown down quickly
enough to embed and preserve organic remains. During these long and blank
intervals I suppose that the inhabitants of each region underwent a
considerable amount of modification and extinction, and that there was much
migration from other parts of the world. As we have reason to believe that
large areas are affected by the same movement, it is probable that strictly
contemporaneous formations have often been accumulated over very wide spaces
in the same quarter of the world; but we are far from having any right to
conclude that this has invariably been the case, and that large areas have
invariably been affected by the same movements. When two formations have
been deposited in two regions during nearly, but not exactly the same
period, we should find in both, from the causes explained in the foregoing
paragraphs, the same general succession in the forms of life; but the
species would not exactly correspond; for there will have been a little more
time in the one region than in the other for modification, extinction, and
immigration.

I suspect that cases of this nature have occurred in Europe. Mr. Prestwich,
in his admirable Memoirs on the eocene deposits of England and France, is
able to draw a close general parallelism between the successive stages in
the two countries; but when he compares certain stages in England with those
in France, although he finds in both a curious accordance in the numbers of
the species belonging to the same genera, yet the species themselves differ
in a manner very difficult to account for, considering the proximity of the
two areas, unless, indeed, it be assumed that an isthmus separated two seas
inhabited by distinct, but contemporaneous, faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also, shows
that there is a striking general parallelism in the successive Silurian
deposits of Bohemia and Scandinavia; nevertheless he finds a surprising
amount of difference in the species. If the several formations in these
regions have not been deposited during the same exact periods, a formation
in one region often corresponding with a blank interval in the other, and if
in both regions the species have gone on slowly changing during the
accumulation of the several formations and during the long intervals of time
between them; in this case, the several formations in the two regions could
be arranged in the same order, in accordance with the general succession of
the form of life, and the order would falsely appear to be strictly
parallel; nevertheless the species would not all be the same in the
apparently corresponding stages in the two regions.

On the Affinities of extinct Species to each other, and to living forms

Let us now look to the mutual affinities of extinct and living species. They
all fall into one grand natural system; and this fact is at once explained
on the principle of descent. The more ancient any form is, the more, as a
general rule, it differs from living forms. But, as Buckland long ago
remarked, all fossils can be classed either in still existing groups, or
between them. That the extinct forms of life help to fill up the wide
intervals between existing genera, families, and orders, cannot be disputed.
For if we confine our attention either to the living or to the extinct
alone, the series is far less perfect than if we combine both into one
general system. With respect to the Vertebrata, whole pages could be filled
with striking illustrations from our great palaeontologist, Owen, showing
how extinct animals fall in between existing groups. Cuvier ranked the
Ruminants and Pachyderms, as the two most distinct orders of mammals; but
Owen has discovered so many fossil links, that he has had to alter the whole
classification of these two orders; and has placed certain pachyderms in the
same sub-order with ruminants: for example, he dissolves by fine gradations
the apparently wide difference between the pig and the camel. In regard to
the Invertebrata, Barrande, and a higher authority could not be named,
asserts that he is every day taught that palaeozoic animals, though
belonging to the same orders, families, or genera with those living at the
present day, were not at this early epoch limited in such distinct groups as
they now are.

Some writers have objected to any extinct species or group of species being
considered as intermediate between living species or groups. If by this term
it is meant that an extinct form is directly intermediate in all its
characters between two living forms, the objection is probably valid. But I
apprehend that in a perfectly natural classification many fossil species
would have to stand between living species, and some extinct genera between
living genera, even between genera belonging to distinct families. The most
common case, especially with respect to very distinct groups, such as fish
and reptiles, seems to be, that supposing them to be distinguished at the
present day from each other by a dozen characters, the ancient members of
the same two groups would be distinguished by a somewhat lesser number of
characters, so that the two groups, though formerly quite distinct, at that
period made some small approach to each other.

It is a common belief that the more ancient a form is, by so much the more
it tends to connect by some of its characters groups now widely separated
from each other. This remark no doubt must be restricted to those groups
which have undergone much change in the course of geological ages; and it
would be difficult to prove the truth of the proposition, for every now and
then even a living animal, as the Lepidosiren, is discovered having
affinities directed towards very distinct groups. Yet if we compare the
older Reptiles and Batrachians, the older Fish, the older Cephalopods, and
the eocene Mammals, with the more recent members of the same classes, we
must admit that there is some truth in the remark.

Let us see how far these several facts and inferences accord with the theory
of descent with modification. As the subject is somewhat complex, I must
request the reader to turn to the diagram in the fourth chapter. We may
suppose that the numbered letters represent genera, and the dotted lines
diverging from them the species in each genus. The diagram is much too
simple, too few genera and too few species being given, but this is
unimportant for us. The horizontal lines may represent successive geological
formations, and all the forms beneath the uppermost line may be considered
as extinct. The three existing genera, a14, q14, p14, will form a small
family; b14 and f14 a closely allied family or sub-family; and o14, e14,
m14, a third family. These three families, together with the many extinct
genera on the several lines of descent diverging from the parent-form A,
will form an order; for all will have inherited something in common from
their ancient and common progenitor. On the principle of the continued
tendency to divergence of character, which was formerly illustrated by this
diagram, the more recent any form is, the more it will generally differ from
its ancient progenitor. Hence we can understand the rule that the most
ancient fossils differ most from existing forms. We must not, however,
assume that divergence of character is a necessary contingency; it depends
solely on the descendants from a species being thus enabled to seize on many
and different places in the economy of nature. Therefore it is quite
possible, as we have seen in the case of some Silurian forms, that a species
might go on being slightly modified in relation to its slightly altered
conditions of life, and yet retain throughout a vast period the same general
characteristics. This is represented in the diagram by the letter F14.

All the many forms, extinct and recent, descended from A, make, as before
remarked, one order; and this order, from the continued effects of
extinction and divergence of character, has become divided into several
sub-families and families, some of which are supposed to have perished at
different periods, and some to have endured to the present day.

By looking at the diagram we can see that if many of the extinct forms,
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the
uppermost line would be rendered less distinct from each other. If, for
instance, the genera a1, a5, a10, m3, m6, m9 were disinterred, these three
families would be so closely linked together that they probably would have
to be united into one great family, in nearly the same manner as has
occurred with ruminants and pachyderms. Yet he who objected to call the
extinct genera, which thus linked the living genera of three families
together, intermediate in character, would be justified, as they are
intermediate, not directly, but only by a long and circuitous course through
many widely different forms. If many extinct forms were to be discovered
above one of the middle horizontal lines or geological formations for
instance, above No. VI. but none from beneath this line, then only the two
families on the left hand (namely, a14, &c., and b14, &c.) would have to be
united into one family; and the two other families (namely, a14 to f14 now
including five genera, and o14 to m14) would yet remain distinct. These two
families, however, would be less distinct from each other than they were
before the discovery of the fossils. If, for instance, we suppose the
existing genera of the two families to differ from each other by a dozen
characters, in this case the genera, at the early period marked VI., would
differ by a lesser number of characters; for at this early stage of descent
they have not diverged in character from the common progenitor of the order,
nearly so much as they subsequently diverged. Thus it comes that ancient and
extinct genera are often in some slight degree intermediate in character
between their modified descendants, or between their collateral relations.

In nature the case will be far more complicated than is represented in the
diagram; for the groups will have been more numerous, they will have endured
for extremely unequal lengths of time, and will have been modified in
various degrees. As we possess only the last volume of the geological
record, and that in a very broken condition, we have no right to expect,
except in very rare cases, to fill up wide intervals in the natural system,
and thus unite distinct families or orders. All that we have a right to
expect, is that those groups, which have within known geological periods
undergone much modification, should in the older formations make some slight
approach to each other; so that the older members should differ less from
each other in some of their characters than do the existing members of the
same groups; and this by the concurrent evidence of our best
palaeontologists seems frequently to be the case.

Thus, on the theory of descent with modification, the main facts with
respect to the mutual affinities of the extinct forms of life to each other
and to living forms, seem to me explained in a satisfactory manner. And they
are wholly inexplicable on any other view.

On this same theory, it is evident that the fauna of any great period in the
earth's history will be intermediate in general character between that which
preceded and that which succeeded it. Thus, the species which lived at the
sixth great stage of descent in the diagram are the modified offspring of
those which lived at the fifth stage, and are the parents of those which
became still more modified at the seventh stage; hence they could hardly
fail to be nearly intermediate in character between the forms of life above
and below. We must, however, allow for the entire extinction of some
preceding forms, and for the coming in of quite new forms by immigration,
and for a large amount of modification, during the long and blank intervals
between the successive formations. Subject to these allowances, the fauna of
each geological period undoubtedly is intermediate in character, between the
preceding and succeeding faunas. I need give only one instance, namely, the
manner in which the fossils of the Devonian system, when this system was
first discovered, were at once recognised by palaeontologists as
intermediate in character between those of the overlying carboniferous, and
underlying Silurian system. But each fauna is not necessarily exactly
intermediate, as unequal intervals of time have elapsed between consecutive
formations.

It is no real objection to the truth of the statement, that the fauna of
each period as a whole is nearly intermediate in character between the
preceding and succeeding faunas, that certain genera offer exceptions to the
rule. For instance, mastodons and elephants, when arranged by Dr Falconer in
two series, first according to their mutual affinities and then according to
their periods of existence, do not accord in arrangement. The species
extreme in character are not the oldest, or the most recent; nor are those
which are intermediate in character, intermediate in age. But supposing for
an instant, in this and other such cases, that the record of the first
appearance and disappearance of the species was perfect, we have no reason
to believe that forms successively produced necessarily endure for
corresponding lengths of time: a very ancient form might occasionally last
much longer than a form elsewhere subsequently produced, especially in the
case of terrestrial productions inhabiting separated districts. To compare
small things with great: if the principal living and extinct races of the
domestic pigeon were arranged as well as they could be in serial affinity,
this arrangement would not closely accord with the order in time of their
production, and still less with the order of their disappearance; for the
parent rock-pigeon now lives; and many varieties between the rock-pigeon and
the carrier have become extinct; and carriers which are extreme in the
important character of length of beak originated earlier than short-beaked
tumblers, which are at the opposite end of the series in this same respect.

Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is the
fact, insisted on by all palaeontologists, that fossils from two consecutive
formations are far more closely related to each other, than are the fossils
from two remote formations. Pictet gives as a well-known instance, the
general resemblance of the organic remains from the several stages of the
chalk formation, though the species are distinct in each stage. This fact
alone, from its generality, seems to have shaken Professor Pictet in his
firm belief in the immutability of species. He who is acquainted with the
distribution of existing species over the globe, will not attempt to account
for the close resemblance of the distinct species in closely consecutive
formations, by the physical conditions of the ancient areas having remained
nearly the same. Let it be remembered that the forms of life, at least those
inhabiting the sea, have changed almost simultaneously throughout the world,
and therefore under the most different climates and conditions. Consider the
prodigious vicissitudes of climate during the pleistocene period, which
includes the whole glacial period, and note how little the specific forms of
the inhabitants of the sea have been affected.

On the theory of descent, the full meaning of the fact of fossil remains
from closely consecutive formations, though ranked as distinct species,
being closely related, is obvious. As the accumulation of each formation has
often been interrupted, and as long blank intervals have intervened between
successive formations, we ought not to expect to find, as I attempted to
show in the last chapter, in any one or two formations all the intermediate
varieties between the species which appeared at the commencement and close
of these periods; but we ought to find after intervals, very long as
measured by years, but only moderately long as measured geologically,
closely allied forms, or, as they have been called by some authors,
representative species; and these we assuredly do find. We find, in short,
such evidence of the slow and scarcely sensible mutation of specific forms,
as we have a just right to expect to find.

On the state of Development of Ancient Forms

There has been much discussion whether recent forms are more highly
developed than ancient. I will not here enter on this subject, for
naturalists have not as yet defined to each other's satisfaction what is
meant by high and low forms. But in one particular sense the more recent
forms must, on my theory, be higher than the more ancient; for each new
species is formed by having had some advantage in the struggle for life over
other and preceding forms. If under a nearly similar climate, the eocene
inhabitants of one quarter of the world were put into competition with the
existing inhabitants of the same or some other quarter, the eocene fauna or
flora would certainly be beaten and exterminated; as would a secondary fauna
by an eocene, and a palaeozoic fauna by a secondary fauna. I do not doubt
that this process of improvement has affected in a marked and sensible
manner the organisation of the more recent and victorious forms of life, in
comparison with the ancient and beaten forms; but I can see no way of
testing this sort of progress. Crustaceans, for instance, not the highest in
their own class, may have beaten the highest molluscs. From the
extraordinary manner in which European productions have recently spread over
New Zealand, and have seized on places which must have been previously
occupied, we may believe, if all the animals and plants of Great Britain
were set free in New Zealand, that in the course of time a multitude of
British forms would become thoroughly naturalized there, and would
exterminate many of the natives. On the other hand, from what we see now
occurring in New Zealand, and from hardly a single inhabitant of the
southern hemisphere having become wild in any part of Europe, we may doubt,
if all the productions of New Zealand were set free in Great Britain,
whether any considerable number would be enabled to seize on places now
occupied by our native plants and animals. Under this point of view, the
productions of Great Britain, may be said to be higher than those of New
Zealand. Yet the most skilful naturalist from an examination of the species
of the two countries could not have foreseen this result.

Agassiz insists that ancient animals resemble to a certain extent the
embryos of recent animals of the same classes; or that the geological
succession of extinct forms is in some degree parallel to the embryological
development of recent forms. I must follow Pictet and Huxley in thinking
that the truth of this doctrine is very far from proved. Yet I fully expect
to see it hereafter confirmed, at least in regard to subordinate groups,
which have branched off from each other within comparatively recent times.
For this doctrine of Agassiz accords well with the theory of natural
selection. In a future chapter I shall attempt to show that the adult
differs from its embryo, owing to variations supervening at a not early age,
and being inherited at a corresponding age. This process, whilst it leaves
the embryo almost unaltered, continually adds, in the course of successive
generations, more and more difference to the adult.

Thus the embryo comes to be left as a sort of picture, preserved by nature,
of the ancient and less modified condition of each animal. This view may be
true, and yet it may never be capable of full proof. Seeing, for instance,
that the oldest known mammals, reptiles, and fish strictly belong to their
own proper classes, though some of these old forms are in a slight degree
less distinct from each other than are the typical members of the same
groups at the present day, it would be vain to look for animals having the
common embryological character of the Vertebrata, until beds far beneath the
lowest Silurian strata are discovered a discovery of which the chance is
very small.

On the Succession of the same Types within the same areas, during the later
tertiary periods

Mr Clift many years ago showed that the fossil mammals from the Australian
caves were closely allied to the living marsupials of that continent. In
South America, a similar relationship is manifest, even to an uneducated
eye, in the gigantic pieces of armour like those of the armadillo, found in
several parts of La Plata; and Professor Owen has shown in the most striking
manner that most of the fossil mammals, buried there in such numbers, are
related to South American types. This relationship is even more clearly seen
in the wonderful collection of fossil bones made by MM. Lund and Clausen in
the caves of Brazil. I was so much impressed with these facts that I
strongly insisted, in 1839 and 1845, on this `law of the succession of
types,' on `this wonderful relationship in the same continent between the
dead and the living.' Professor Owen has subsequently extended the same
generalisation to the mammals of the Old World. We see the same law in this
author's restorations of the extinct and gigantic birds of New Zealand. We
see it also in the birds of the caves of Brazil. Mr Woodward has shown that
the same law holds good with sea-shells, but from the wide distribution of
most genera of molluscs, it is not well displayed by them. Other cases could
be added, as the relation between the extinct and living land-shells of
Madeira; and between the extinct and living brackish-water shells of the
Aralo-Caspian Sea.

Now what does this remarkable law of the succession of the same types within
the same areas mean? He would be a bold man, who after comparing the present
climate of Australia and of parts of South America under the same latitude,
would attempt to account, on the one hand, by dissimilar physical conditions
for the dissimilarity of the inhabitants of these two continents, and, on
the other hand, by similarity of conditions, for the uniformity of the same
types in each during the later tertiary periods. Nor can it be pretended
that it is an immutable law that marsupials should have been chiefly or
solely produced in Australia; or that Edentata and other American types
should have been solely produced in South America. For we know that Europe
in ancient times was peopled by numerous marsupials; and I have shown in the
publications above alluded to, that in America the law of distribution of
terrestrial mammals was formerly different from what it now is. North
America formerly partook strongly of the present character of the southern
half of the continent; and the southern half was formerly more closely
allied, than it is at present, to the northern half. In a similar manner we
know from Falconer and Cautley's discoveries, that northern India was
formerly more closely related in its mammals to Africa than it is at the
present time. Analogous facts could be given in relation to the distribution
of marine animals.

On the theory of descent with modification, the great law of the long
enduring, but not immutable, succession of the same types within the same
areas, is at once explained; for the inhabitants of each quarter of the
world will obviously tend to leave in that quarter, during the next
succeeding period of time, closely allied though in some degree modified
descendants. If the inhabitants of one continent formerly differed greatly
from those of another continent, so will their modified descendants still
differ in nearly the same manner and degree. But after very long intervals
of time and after great geographical changes, permitting much
inter-migration, the feebler will yield to the more dominant forms, and
there will be nothing immutable in the laws of past and present
distribution.

It may be asked in ridicule, whether I suppose that the megatherium and
other allied huge monsters have left behind them in South America the sloth,
armadillo, and anteater, as their degenerate descendants. This cannot for an
instant be admitted. These huge animals have become wholly extinct, and have
left no progeny. But in the caves of Brazil, there are many extinct species
which are closely allied in size and in other characters to the species
still living in South America; and some of these fossils may be the actual
progenitors of living species. It must not be forgotten that, on my theory,
all the species of the same genus have descended from some one species; so
that if six genera, each having eight species, be found in one geological
formation, and in the next succeeding formation there be six other allied or
representative genera with the same number of species, then we may conclude
that only one species of each of the six older genera has left modified
descendants, constituting the six new genera. The other seven species of the
old genera have all died out and have left no progeny. Or, which would
probably be a far commoner case, two or three species of two or three alone
of the six older genera will have been the parents of the six new genera;
the other old species and the other whole genera having become utterly
extinct. In failing orders, with the genera and species decreasing in
numbers, as apparently is the case of the Edentata of South America, still
fewer genera and species will have left modified blood-descendants.

Summary of the preceding and present Chapters

I have attempted to show that the geological record is extremely imperfect;
that only a small portion of the globe has been geologically explored with
care; that only certain classes of organic beings have been largely
preserved in a fossil state; that the number both of specimens and of
species, preserved in our museums, is absolutely as nothing compared with
the incalculable number of generations which must have passed away even
during a single formation; that, owing to subsidence being necessary for the
accumulation of fossiliferous deposits thick enough to resist future
degradation, enormous intervals of time have elapsed between the successive
formations; that there has probably been more extinction during the periods
of subsidence, and more variation during the periods of elevation, and
during the latter the record will have been least perfectly kept; that each
single formation has not been continuously deposited; that the duration of
each formation is, perhaps, short compared with the average duration of
specific forms; that migration has played an important part in the first
appearance of new forms in any one area and formation; that widely ranging
species are those which have varied most, and have oftenest given rise to
new species; and that varieties have at first often been local. All these
causes taken conjointly, must have tended to make the geological record
extremely imperfect, and will to a large extent explain why we do not find
interminable varieties, connecting together all the extinct and existing
forms of life by the finest graduated steps.

He who rejects these views on the nature of the geological record, will
rightly reject my whole theory. For he may ask in vain where are the
numberless transitional links which must formerly have connected the closely
allied or representative species, found in the several stages of the same
great formation. He may disbelieve in the enormous intervals of time which
have elapsed between our consecutive formations; he may overlook how
important a part migration must have played, when the formations of any one
great region alone, as that of Europe, are considered; he may urge the
apparent, but often falsely apparent, sudden coming in of whole groups of
species. He may ask where are the remains of those infinitely numerous
organisms which must have existed long before the first bed of the Silurian
system was deposited: I can answer this latter question only hypothetically,
by saying that as far as we can see, where our oceans now extend they have
for an enormous period extended, and where our oscillating continents now
stand they have stood ever since the Silurian epoch; but that long before
that period, the world may have presented a wholly different aspect; and
that the older continents, formed of formations older than any known to us,
may now all be in a metamorphosed condition, or may lie buried under the
ocean.

Passing from these difficulties, all the other great leading facts in
palaeontology seem to me simply to follow on the theory of descent with
modification through natural selection. We can thus understand how it is
that new species come in slowly and successively; how species of different
classes do not necessarily change together, or at the same rate, or in the
same degree; yet in the long run that all undergo modification to some
extent. The extinction of old forms is the almost inevitable consequence of
the production of new forms. We can understand why when a species has once
disappeared it never reappears. Groups of species increase in numbers
slowly, and endure for unequal periods of time; for the process of
modification is necessarily slow, and depends on many complex contingencies.
The dominant species of the larger dominant groups tend to leave many
modified descendants, and thus new sub-groups and groups are formed. As
these are formed, the species of the less vigorous groups, from their
inferiority inherited from a common progenitor, tend to become extinct
together, and to leave no modified offspring on the face of the earth. But
the utter extinction of a whole group of species may often be a very slow
process, from the survival of a few descendants, lingering in protected and
isolated situations. When a group has once wholly disappeared, it does not
reappear; for the link of generation has been broken.

We can understand how the spreading of the dominant forms of life, which are
those that oftenest vary, will in the long run tend to people the world with
allied, but modified, descendants; and these will generally succeed in
taking the places of those groups of species which are their inferiors in
the struggle for existence. Hence, after long intervals of time, the
productions of the world will appear to have changed simultaneously.

We can understand how it is that all the forms of life, ancient and recent,
make together one grand system; for all are connected by generation. We can
understand, from the continued tendency to divergence of character, why the
more ancient a form is, the more it generally differs from those now living.
Why ancient and extinct forms often tend to fill up gaps between existing
forms, sometimes blending two groups previously classed as distinct into
one; but more commonly only bringing them a little closer together. The more
ancient a form is, the more often, apparently, it displays characters in
some degree intermediate between groups now distinct; for the more ancient a
form is, the more nearly it will be related to, and consequently resemble,
the common progenitor of groups, since become widely divergent. Extinct
forms are seldom directly intermediate between existing forms; but are
intermediate only by a long and circuitous course through many extinct and
very different forms. We can clearly see why the organic remains of closely
consecutive formations are more closely allied to each other, than are those
of remote formations; for the forms are more closely linked together by
generation: we can clearly see why the remains of an intermediate formation
are intermediate in character.

The inhabitants of each successive period in the world's history have beaten
their predecessors in the race for life, and are, in so far, higher in the
scale of nature; and this may account for that vague yet ill-defined
sentiment, felt by many palaeontologists, that organisation on the whole has
progressed. If it should hereafter be proved that ancient animals resemble
to a certain extent the embryos of more recent animals of the same class,
the fact will be intelligible. The succession of the same types of structure
within the same areas during the later geological periods ceases to be
mysterious, and is simply explained by inheritance.

If then the geological record be as imperfect as I believe it to be, and it
may at least be asserted that the record cannot be proved to be much more
perfect, the main objections to the theory of natural selection are greatly
diminished or disappear. On the other hand, all the chief laws of
palaeontology plainly proclaim, as it seems to me, that species have been
produced by ordinary generation: old forms having been supplanted by new and
improved forms of life, produced by the laws of variation still acting round
us, and preserved by Natural Selection.

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Chapter 11 - Geographical Distribution

[* ] Present distribution cannot be accounted for by differences in physical
conditions [* ] Importance of barriers [* ] Affinity of the productions of
the same continent [* ] Centres of creation [* ] Means of dispersal, by
changes of climate and of the level of the land, and by occasional means
[* ] Dispersal during the Glacial period co-extensive with the world
----------------------------------------------------------------------------
In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the similarity
nor the dissimilarity of the inhabitants of various regions can be accounted
for by their climatal and other physical conditions. Of late, almost every
author who has studied the subject has come to this conclusion. The case of
America alone would almost suffice to prove its truth: for if we exclude the
northern parts where the circumpolar land is almost continuous, all authors
agree that one of the most fundamental divisions in geographical
distribution is that between the New and Old Worlds; yet if we travel over
the vast American continent, from the central parts of the United States to
its extreme southern point, we meet with the most diversified conditions;
the most humid districts, arid deserts, lofty mountains, grassy plains,
forests, marshes, lakes, and great rivers, under almost every temperature.
There is hardly a climate or condition in the Old World which cannot be
paralleled in the New at least as closely as the same species generally
require; for it is a most rare case to find a group of organisms confined to
any small spot, having conditions peculiar in only a slight degree; for
instance, small areas in the Old World could be pointed out hotter than any
in the New World, yet these are not inhabited by a peculiar fauna or flora.
Notwithstanding this parallelism in the conditions of the Old and New
Worlds, how widely different are their living productions!

In the southern hemisphere, if we compare large tracts of land in Australia,
South Africa, and western South America, between latitudes 25° and 35°, we
shall find parts extremely similar in all their conditions, yet it would not
be possible to point out three faunas and floras more utterly dissimilar. Or
again we may compare the productions of South America south of lat. 35° with
those north of 25°, which consequently inhabit a considerably different
climate, and they will be found incomparably more closely related to each
other, than they are to the productions of Australia or Africa under nearly
the same climate. Analogous facts could be given with respect to the
inhabitants of the sea.

A second great fact which strikes us in our general review is, that barriers
of any kind, or obstacles to free migration, are related in a close and
important manner to the differences between the productions of various
regions. We see this in the great difference of nearly all the terrestrial
productions of the New and Old Worlds, excepting in the northern parts,
where the land almost joins, and where, under a slightly different climate,
there might have been free migration for the northern temperate forms, as
there now is for the strictly arctic productions. We see the same fact in
the great difference between the inhabitants of Australia, Africa, and South
America under the same latitude: for these countries are almost as much
isolated from each other as is possible. On each continent, also, we see the
same fact; for on the opposite sides of lofty and continuous
mountain-ranges, and of great deserts, and sometimes even of large rivers,
we find different productions; though as mountain chains, deserts, &c., are
not as impassable, or likely to have endured so long as the oceans
separating continents, the differences are very inferior in degree to those
characteristic of distinct continents.

Turning to the sea, we find the same law. No two marine faunas are more
distinct, with hardly a fish, shell, or crab in common, than those of the
eastern and western shores of South and Central America; yet these great
faunas are separated only by the narrow, but impassable, isthmus of panama.
Westward of the shores of America, a wide space of open ocean extends, with
not an island as a halting-place for emigrants; here we have a barrier of
another kind, and as soon as this is passed we meet in the eastern islands
of the Pacific, with another and totally distinct fauna. So that here three
marine faunas range far northward and southward, in parallel lines not far
from each other, under corresponding climates; but from being separated from
each other by impassable barriers, either of land or open sea, they are
wholly distinct. On the other hand, proceeding still further westward from
the eastern islands of the tropical parts of the Pacific, we encounter no
impassable barriers, and we have innumerable islands as halting-places,
until after travelling over a hemisphere we come to the shores of Africa;
and over this vast space we meet with no well-defined and distinct marine
faunas. Although hardly one shell, crab or fish is common to the above-named
three approximate faunas of Eastern and Western America and the eastern
Pacific islands, yet many fish range from the Pacific into the Indian Ocean,
and many shells are common to the eastern islands of the Pacific and the
eastern shores of Africa, on almost exactly opposite meridians of longitude.

A third great fact, partly included in the foregoing statements, is the
affinity of the productions of the same continent or sea, though the species
themselves are distinct at different points and stations. It is a law of the
widest generality, and every continent offers innumerable instances.
Nevertheless the naturalist in travelling, for instance, from north to south
never fails to be struck by the manner in which successive groups of beings,
specifically distinct, yet clearly related, replace each other. He hears
from closely allied, yet distinct kinds of birds, notes nearly similar, and
sees their nests similarly constructed, but not quite alike, with eggs
coloured in nearly the same manner. The plains near the Straits of Magellan
are inhabited by one species of Rhea (American ostrich), and northward the
plains of La Plata by another species of the same genus; and not by a true
ostrich or emeu, like those found in Africa and Australia under the same
latitude. On these same plains of La Plata, we see the agouti and bizcacha,
animals having nearly the same habits as our hares and rabbits and belonging
to the same order of Rodents, but they plainly display an American type of
structure. We ascend the lofty peaks of the Cordillera and we find an alpine
species of bizcacha; we look to the waters, and we do not find the beaver or
musk-rat, but the coypu and capybara, rodents of the American type.
Innumerable other instances could be given. If we look to the islands off
the American shore, however much they may differ in geological structure,
the inhabitants, though they may be all peculiar species, are essentially
American. We may look back to past ages, as shown in the last chapter, and
we find American types then prevalent on the American continent and in the
American seas. We see in these facts some deep organic bond, prevailing
throughout space and time, over the same areas of land and water, and
independent of their physical conditions. The naturalist must feel little
curiosity, who is not led to inquire what this bond is.

This bond, on my theory, is simply inheritance, that cause which alone, as
far as we positively know, produces organisms quite like, or, as we see in
the case of varieties nearly like each other. The dissimilarity of the
inhabitants of different regions may be attributed to modification through
natural selection, and in a quite subordinate degree to the direct influence
of different physical conditions. The degree of dissimilarity will depend on
the migration of the more dominant forms of life from one region into
another having been effected with more or less ease, at periods more or less
remote; on the nature and number of the former immigrants; -- and on their
action and reaction, in their mutual struggles for life; the relation of
organism to organism being, as I have already often remarked, the most
important of all relations. Thus the high importance of barriers comes into
play by checking migration; as does time for the slow process of
modification through natural selection. Widely-ranging species, abounding in
individuals, which have already triumphed over many competitors in their own
widely-extended homes will have the best chance of seizing on new places,
when they spread into new countries. In their new homes they will be exposed
to new conditions, and will frequently undergo further modification and
improvement; and thus they will become still further victorious, and will
produce groups of modified descendants. On this principle of inheritance
with modification, we can understand how it is that sections of genera,
whole genera, and even families are confined to the same areas, as is so
commonly and notoriously the case.

I believe, as was remarked in the last chapter, in no law of necessary
development. As the variability of each species is an independent property,
and will be taken advantage of by natural selection, only so far as it
profits the individual in its complex struggle for life, so the degree of
modification in different species will be no uniform quantity. If, for
instance, a number of species, which stand in direct competition with each
other, migrate in a body into a new and afterwards isolated country, they
will be little liable to modification; for neither migration nor isolation
in themselves can do anything. These principles come into play only by
bringing organisms into new relations with each other, and in a lesser
degree with the surrounding physical conditions. As we have seen in the last
chapter that some forms have retained nearly the same character from an
enormously remote geological period, so certain species have migrated over
vast spaces, and have not become greatly modified.

On these views, it is obvious, that the several species of the same genus,
though inhabiting the most distant quarters of the world, must originally
have proceeded from the same source, as they have descended from the same
progenitor. In the case of those species, which have undergone during whole
geological periods but little modification, there is not much difficulty in
believing that they may have migrated from the same region; for during the
vast geographical and climatal changes which will have supervened since
ancient times, almost any amount of migration is possible. But in many other
cases, in which we have reason to believe that the species of a genus have
been produced within comparatively recent times, there is great difficulty
on this head. It is also obvious that the individuals of the same species,
though now inhabiting distant and isolated regions, must have proceeded from
one spot, where their parents were first produced: for, as explained in the
last chapter, it is incredible that individuals identically the same should
ever have been produced through natural selection from parents specifically
distinct.

We are thus brought to the question which has been largely discussed by
naturalists, namely, whether species have been created at one or more points
of the earth's surface. Undoubtedly there are very many cases of extreme
difficulty, in understanding how the same species could possibly have
migrated from some one point to the several distant and isolated points,
where now found. Nevertheless the simplicity of the view that each species
was first produced within a single region captivates the mind. He who
rejects it, rejects the vera causa of ordinary generation with subsequent
migration, and calls in the agency of a miracle. It is universally admitted,
that in most cases the area inhabited by a species is continuous; and when a
plant or animal inhabits two points so distant from each other, or with an
interval of such a nature, that the space could not be easily passed over by
migration, the fact is given as something remarkable and exceptional. The
capacity of migrating across the sea is more distinctly limited in
terrestrial mammals, than perhaps in any other organic beings; and,
accordingly, we find no inexplicable cases of the same mammal inhabiting
distant points of the world. No geologist will feel any difficulty in such
cases as Great Britain having been formerly united to Europe, and
consequently possessing the same quadrupeds. But if the same species can be
produced at two separate points, why do we not find a single mammal common
to Europe and Australia or South America? The conditions of life are nearly
the same, so that a multitude of European animals and plants have become
naturalised in America and Australia; and some of the aboriginal plants are
identically the same at these distant points of the northern and southern
hemispheres? The answer, as I believe, is, that mammals have not been able
to migrate, whereas some plants, from their varied means of dispersal, have
migrated across the vast and broken interspace. The great and striking
influence which barriers of every kind have had on distribution, is
intelligible only on the view that the great majority of species have been
produced on one side alone, and have not been able to migrate to the other
side. Some few families, many sub-families, very many genera, and a still
greater number of sections of genera are confined to a single region; and it
has been observed by several naturalists, that the most natural genera, or
those genera in which the species are most closely related to each other,
are generally local, or confined to one area. What a strange anomaly it
would be, if, when coming one step lower in the series, to the individuals
of the same species, a directly opposite rule prevailed; and species were
not local, but had been produced in two or more distinct areas!

Hence it seems to me, as it has to many other naturalists, that the view of
each species having been produced in one area alone, and having subsequently
migrated from that area as far as its powers of migration and subsistence
under past and present conditions permitted, is the most probable.
Undoubtedly many cases occur, in which we cannot explain how the same
species could have passed from one point to the other. But the geographical
and climatal changes, which have certainly occurred within recent geological
times, must have interrupted or rendered discontinuous the formerly
continuous range of many species. So that we are reduced to consider whether
the exceptions to continuity of range are so numerous and of so grave a
nature, that we ought to give up the belief, rendered probable by general
considerations, that each species has been produced within one area, and has
migrated thence as far as it could. It would be hopelessly tedious to
discuss all the exceptional cases of the same species, now living at distant
and separated points; nor do I for a moment pretend that any explanation
could be offered of many such cases. But after some preliminary remarks, I
will discuss a few of the most striking classes of facts; namely, the
existence of the same species on the summits of distant mountain-ranges, and
at distant points in the arctic and antarctic regions; and secondly (in the
following chapter), the wide distribution of freshwater productions; and
thirdly, the occurrence of the same terrestrial species on islands and on
the mainland, though separated by hundreds of miles of open sea. If the
existence of the same species at distant and isolated points of the earth's
surface, can in many instances be explained on the view of each species
having migrated from a single birthplace; then, considering our ignorance
with respect to former climatal and geographical changes and various
occasional means of transport, the belief that this has been the universal
law, seems to me incomparably the safest.

In discussing this subject, we shall be enabled at the same time to consider
a point equally important for us, namely, whether the several distinct
species of a genus, which on my theory have all descended from a common
progenitor, can have migrated (undergoing modification during some part of
their migration) from the area inhabited by their progenitor. If it can be
shown to be almost invariably the case, that a region, of which most of its
inhabitants are closely related to, or belong to the same genera with the
species of a second region, has probably received at some former period
immigrants from this other region, my theory will be strengthened; for we
can clearly understand, on the principle of modification, why the
inhabitants of a region should be related to those of another region, whence
it has been stocked. A volcanic island, for instance, upheaved and formed at
the distance of a few hundreds of miles from a continent, would probably
receive from it in the course of time a few colonists, and their
descendants, though modified, would still be plainly related by inheritance
to the inhabitants of the continent. Cases of this nature are common, and
are, as we shall hereafter more fully see, inexplicable on the theory of
independent creation. This view of the relation of species in one region to
those in another, does not differ much (by substituting the word variety for
species) from that lately advanced in an ingenious paper by Mr Wallace, in
which he concludes, that `every species has come into existence coincident
both in space and time with a pre-existing closely allied species.' And I
now know from correspondence, that this coincidence he attributes to
generation with modification.

The previous remarks on `single and multiple centres of creation' do not
directly bear on another allied question, namely whether all the individuals
of the same species have descended from a single pair, or single
hermaphrodite, or whether, as some authors suppose, from many individuals
simultaneously created. With those organic beings which never intercross (if
such exist), the species, on my theory, must have descended from a
succession of improved varieties, which will never have blended with other
individuals or varieties, but will have supplanted each other; so that, at
each successive stage of modification and improvement, all the individuals
of each variety will have descended from a single parent. But in the
majority of cases, namely, with all organisms which habitually unite for
each birth, or which often intercross, I believe that during the slow
process of modification the individuals of the species will have been kept
nearly uniform by intercrossing; so that many individuals will have gone on
simultaneously changing, and the whole amount of modification will not have
been due, at each stage, to descent from a single parent. To illustrate what
I mean: our English racehorses differ slightly from the horses of every
other breed; but they do not owe their difference and superiority to descent
from any single pair, but to continued care in selecting and training many
individuals during many generations.

Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of `single
centres of creation,' I must say a few words on the means of dispersal.

Means of Dispersal

Sir C. Lyell and other authors have ably treated this subject. I can give
here only the briefest abstract of the more important facts. Change of
climate must have had a powerful influence on migration: a region when its
climate was different may have been a high road for migration, but now be
impassable; I shall, however, presently have to discuss this branch of the
subject in some detail. Changes of level in the land must also have been
highly influential: a narrow isthmus now separates two marine faunas;
submerge it, or let it formerly have been submerged, and the two faunas will
now blend or may formerly have blended: where the sea now extends, land may
at a former period have connected islands or possibly even continents
together, and thus have allowed terrestrial productions to pass from one to
the other. No geologist will dispute that great mutations of level have
occurred within the period of existing organisms. Edward Forbes insisted
that all the islands in the Atlantic must recently have been connected with
Europe or Africa, and Europe likewise with America. Other authors have thus
hypothetically bridged over every ocean, and have united almost every island
to some mainland. If indeed the arguments used by Forbes are to be trusted,
it must be admitted that scarcely a single island exists which has not
recently been united to some continent. This view cuts the Gordian knot of
the dispersal of the same species to the most distant points, and removes
many a difficulty: but to the best of any judgement we are not authorised in
admitting such enormous geographical changes within the period of existing
species. It seems to me that we have abundant evidence of great oscillations
of level in our continents; but not of such vast changes in their position
and extension, as to have united them within the recent period to each other
and to the several intervening oceanic islands. I freely admit the former
existence of many islands, now buried beneath the sea, which may have served
as halting places for plants and for many animals during their migration. In
the coral-producing oceans such sunken islands are now marked, as I believe,
by rings of coral or atolls standing over them. Whenever it is fully
admitted, as I believe it will some day be, that each species has proceeded
from a single birthplace, and when in the course of time we know something
definite about the means of distribution, we shall be enabled to speculate
with security on the former extension of the land. But I do not believe that
it will ever be proved that within the recent period continents which are
now quite separate, have been continuously, or almost continuously, united
with each other, and with the many existing oceanic islands. Several facts
in distribution, such as the great difference in the marine faunas on the
opposite sides of almost every continent, the close relation of the tertiary
inhabitants of several lands and even seas to their present inhabitants, a
certain degree of relation (as we shall hereafter see) between the
distribution of mammals and the depth of the sea, these and other such facts
seem to me opposed to the admission of such prodigious geographical
revolutions within the recent period, as are necessitated in the view
advanced by Forbes and admitted by his many followers. The nature and
relative proportions of the inhabitants of oceanic islands likewise seem to
me opposed to the belief of their former continuity with continents. Nor
does their almost universally volcanic composition favour the admission that
they are the wrecks of sunken continents; if they had originally existed as
mountain-ranges on the land, some at least of the islands would have been
formed, like other mountain-summits, of granite, metamorphic schists, old
fossiliferous or other such rocks, instead of consisting of mere piles of
volcanic matter.

I must now say a few words on what are called accidental means, but which
more properly might be called occasional means of distribution. I shall here
confine myself to plants. In botanical works, this or that plant is stated
to be ill adapted for wide dissemination; but for transport across the sea,
the greater or less facilities may be said to be almost wholly unknown.
Until I tried, with Mr Berkeley's aid, a few experiments, it was not even
known how far seeds could resist the injurious action of sea-water. To my
surprise I found that out of 87 kinds, 64 germinated after an immersion of
28 days, and a few survived an immersion of 137 days. For convenience sake I
chiefly tried small seeds, without the capsule or fruit; and as all of these
sank in a few days, they could not be floated across wide spaces of the sea,
whether or not they were injured by the salt-water. Afterwards I tried some
larger fruits, capsules, &c., and some of these floated for a long time. It
is well known what a difference there is in the buoyancy of green and
seasoned timber; and it occurred to me that floods might wash down plants or
branches, and that these might be dried on the banks, and then by a fresh
rise in the stream be washed into the sea. Hence I was led to dry stems and
branches of 94 plants with ripe fruit, and to place them on sea water. The
majority sank quickly, but some which whilst green floated for a very short
time, when dried floated much longer; for instance, ripe hazel-nuts sank
immediately, but when dried, they floated for 90 days and afterwards when
planted they germinated; an asparagus plant with ripe berries floated for 23
days, when dried it floated for 85 days, and the seeds afterwards
germinated: the ripe seeds of Helosciadium sank in two days, when dried they
floated for above 90 days, and afterwards germinated. Altogether out of the
94 dried plants, 18 floated for above 28 days, and some of the 18 floated
for a very much longer period. So that as 64/87 seeds germinated after an
immersion of 28 days; and as 18/94 plants with ripe fruit (but not all the
same species as in the foregoing experiment) floated, after being dried, for
above 28 days, as far as we may infer anything from these scanty facts, we
may conclude that the seeds of 14/100 plants of any country might be floated
by sea-currents during 28 days, and would retain their power of germination.
In Johnston's physical Atlas, the average rate of the several Atlantic
currents is 33 miles per diem (some currents running at the rate of 60 miles
per diem); on this average, the seeds of 14/100 plants belonging to one
country might be floated across 924 miles of sea to another country; and
when stranded, if blown to a favourable spot by an inland gale, they would
germinate.

Subsequently to my experiments, M. Martens tried similar ones, but in a much
better manner, for he placed the seeds in a box in the actual sea, so that
they were alternately wet and exposed to the air like really floating
plants. He tried 98 seeds, mostly different from mine; but he chose many
large fruits and likewise seeds from plants which live near the sea; and
this would have favoured the average length of their flotation and of their
resistance to the injurious action of the salt-water. On the other hand he
did not previously dry the plants or branches with the fruit; and this, as
we have seen, would have caused some of them to have floated much longer.
The result was that 18/98 of his seeds floated for 42 days, and were then
capable of germination. But I do not doubt that plants exposed to the waves
would float for a less time than those protected from violent movement as in
our experiments. Therefore it would perhaps be safer to assume that the
seeds of about 10/100 plants of a flora, after having been dried, could be
floated across a space of sea 900 miles in width, and would then germinate.
The fact of the larger fruits often floating longer than the small, is
interesting; as plants with large seeds or fruit could hardly be transported
by any other means; and Alph. de Candolle has shown that such plants
generally have restricted ranges.

But seeds may be occasionally transported in another manner. Drift timber is
thrown up on most islands, even on those in the midst of the widest oceans;
and the natives of the coral-islands in the Pacific, procure stones for
their tools, solely from the roots of drifted trees, these stones being a
valuable royal tax. I find on examination, that when irregularly shaped
stones are embedded in the roots of trees, small parcels of earth are very
frequently enclosed in their interstices and behind them, so perfectly that
not a particle could be washed away in the longest transport: out of one
small portion of earth thus completely enclosed by wood in an oak about 50
years old, three dicotyledonous plants germinated: I am certain of the
accuracy of this observation. Again, I can show that the carcasses of birds,
when floating on the sea, sometimes escape being immediately devoured; and
seeds of many kinds in the crops of floating birds long retain their
vitality: peas and vetches, for instance, are killed by even a few days'
immersion in sea-water; but some taken out of the crop of a pigeon, which
had floated on artificial salt-water for 30 days, to my surprise nearly all
germinated.

Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the ocean.
We may I think safely assume that under such circumstances their rate of
flight would often be 35 miles an hour; and some authors have given a far
higher estimate. I have never seen an instance of nutritious seeds passing
through the intestines of a bird; but hard seeds of fruit will pass
uninjured through even the digestive organs of a turkey. In the course of
two months, I picked up in my garden 12 kinds of seeds, out of the excrement
of small birds, and these seemed perfect, and some of them, which I tried,
germinated. But the following fact is more important: the crops of birds do
not secrete gastric juice, and do not in the least injure, as I know by
trial, the germination of seeds; now after a bird has found and devoured a
large supply of food, it is positively asserted that all the grains do not
pass into the gizzard for 12 or even 18 hours. A bird in this interval might
easily be blown to the distance of 500 miles, and hawks are known to look
out for tired birds, and the contents of their torn crops might thus readily
get scattered. Mr Brent informs me that a friend of his had to give up
flying carrier-pigeons from France to England, as the hawks on the English
coast destroyed so many on their arrival. Some hawks and owls bolt their
prey whole, and after an interval of from twelve to twenty hours, disgorge
pellets, which, as I know from experiments made in the Zoological Gardens,
include seeds capable of germination. Some seeds of the oat, wheat, millet,
canary, hemp, clover, and beet germinated after having been from twelve to
twenty-one hours in the stomachs of different birds of prey; and two seeds
of beet grew after having been thus retained for two days and fourteen
hours. Freshwater fish, I find, eat seeds of many land and water plants:
fish are frequently devoured by birds, and thus the seeds might be
transported from place to place. I forced many kinds of seeds into the
stomachs of dead fish, and then gave their bodies to fishing-eagles, storks,
and pelicans; these birds after an interval of many hours, either rejected
the seeds in pellets or passed them in their excrement; and several of these
seeds retained their power of germination. Certain seeds, however, were
always killed by this process.

Although the beaks and feet of birds are generally quite clean, I can show
that earth sometimes adheres to them: in one instance I removed twenty-two
grains of dry argillaceous earth from one foot of a partridge, and in this
earth there was a pebble quite as large as the seed of a vetch. Thus seeds
might occasionally be transported to great distances; for many facts could
be given showing that soil almost everywhere is charged with seeds. Reflect
for a moment on the millions of quails which annually cross the
Mediterranean; and can we doubt that the earth adhering to their feet would
sometimes include a few minute seeds? But I shall presently have to recur to
this subject.

As icebergs are known to be sometimes loaded with earth and stones, and have
even carried brushwood, bones, and the nest of a land-bird, I can hardly
doubt that they must occasionally have transported seeds from one part to
another of the arctic and antarctic regions, as suggested by Lyell; and
during the Glacial period from one part of the now temperate regions to
another. In the Azores, from the large number of the species of plants
common to Europe, in comparison with the plants of other oceanic islands
nearer to the mainland, and (as remarked by Mr H. C. Watson) from the
somewhat northern character of the flora in comparison with the latitude, I
suspected that these islands had been partly stocked by ice-borne seeds,
during the Glacial epoch. At my request Sir C. Lyell wrote to M. Hartung to
inquire whether he had observed erratic boulders on these islands, and he
answered that he had found large fragments of granite and other rocks, which
do not occur in the archipelago. Hence we may safely infer that icebergs
formerly landed their rocky burthens on the shores of these mid-ocean
islands, and it is at least possible that they may have brought thither the
seeds of northern plants.

Considering that the several above means of transport, and that several
other means, which without doubt remain to be discovered, have been in
action year after year, for centuries and tens of thousands of years, it
would I think be a marvellous fact if many plants had not thus become widely
transported. These means of transport are sometimes called accidental, but
this is not strictly correct: the currents of the sea are not accidental,
nor is the direction of prevalent gales of wind. It should be observed that
scarcely any means of transport would carry seeds for very great distances;
for seeds do not retain their vitality when exposed for a great length of
time to the action of seawater; nor could they be long carried in the crops
or intestines of birds. These means, however, would suffice for occasional
transport across tracts of sea some hundred miles in breadth, or from island
to island, or from a continent to a neighbouring island, but not from one
distant continent to another. The floras of distant continents would not by
such means become mingled in any great degree; but would remain as distinct
as we now see them to be. The currents, from their course, would never bring
seeds from North America to Britain, though they might and do bring seeds
from the West Indies to our western shores, where, if not killed by so long
an immersion in salt-water, they could not endure our climate. Almost every
year, one or two land-birds are blown across the whole Atlantic Ocean, from
North America to the western shores of Ireland and England; but seeds could
be transported by these wanderers only by one means, namely, in dirt
sticking to their feet, which is in itself a rare accident. Even in this
case, how small would the chance be of a seed falling on favourable soil,
and coming to maturity! But it would be a great error to argue that because
a well-stocked island, like Great Britain, has not, as far as is known (and
it would be very difficult to prove this), received within the last few
centuries, through occasional means of transport, immigrants from Europe or
any other continent, that a poorly-stocked island, though standing more
remote from the mainland, would not receive colonists by similar means. I do
not doubt that out of twenty seeds or animals transported to an island, even
if far less well-stocked than Britain, scarcely more than one would be so
well fitted to its new home, as to become naturalised. But this, as it seems
to me, is no valid argument against what would be effected by occasional
means of transport, during the long lapse of geological time, whilst an
island was being upheaved and formed, and before it had become fully stocked
with inhabitants. On almost bare land, with few or no destructive insects or
birds living there, nearly every seed, which chanced to arrive, would be
sure to germinate and survive.

Dispersal during the Glacial period

The identity of many plants and animals, on mountain-summits, separated from
each other by hundreds of miles of lowlands, where the Alpine species could
not possibly exist, is one of the most striking cases known of the same
species living at distant points, without the apparent possibility of their
having migrated from one to the other. It is indeed a remarkable fact to see
so many of the same plants living on the snowy regions of the Alps or
Pyrenees, and in the extreme northern parts of Europe; but it is far more
remarkable, that the plants on the White Mountains, in the United States of
America, are all the same with those of Labrador, and nearly all the same,
as we hear from Asa Gray, with those on the loftiest mountains of Europe.
Even as long ago as 1747, such facts led Gmelin to conclude that the same
species must have been independently created at several distinct points; and
we might have remained in this same belief, had not Agassiz and others
called vivid attention to the Glacial period, which, as we shall immediately
see, affords a simple explanation of these facts. We have evidence of almost
every conceivable kind, organic and inorganic, that within a very recent
geological period, central Europe and North America suffered under an Arctic
climate. The ruins of a house burnt by fire do not tell their tale more
plainly, than do the mountains of Scotland and Wales, with their scored
flanks, polished surfaces, and perched boulders, of the icy streams with
which their valleys were lately filled. So greatly has the climate of Europe
changed, that in Northern Italy, gigantic moraines, left by old glaciers,
are now clothed by the vine and maize. Throughout a large part of the United
States, erratic boulders, and rocks scored by drifted icebergs and
coast-ice, plainly reveal a former cold period.

The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained with remarkable clearness by Edward
Forbes, is substantially as follows. But we shall follow the changes more
readily, by supposing a new glacial period to come slowly on, and then pass
away, as formerly occurred. As the cold came on, and as each more southern
zone became fitted for arctic beings and ill-fitted for their former more
temperate inhabitants, the latter would be supplanted and arctic productions
would take their places. The inhabitants of the more temperate regions would
at the same time travel southward, unless they were stopped by barriers, in
which case they would perish. The mountains would become covered with snow
and ice, and their former Alpine inhabitants would descend to the plains. By
the time that the cold had reached its maximum, we should have a uniform
arctic fauna and flora, covering the central parts of Europe, as far south
as the Alps and Pyrenees, and even stretching into Spain. The now temperate
regions of the United States would likewise be covered by arctic plants and
animals, and these would be nearly the same with those of Europe; for the
present circumpolar inhabitants, which we suppose to have everywhere
travelled southward, are remarkably uniform round the world. We may suppose
that the Glacial period came on a little earlier or later in North America
than in Europe, so will the southern migration there have been a little
earlier or later; but this will make no difference in the final result.

As the warmth returned, the arctic forms would retreat northward, closely
followed up in their retreat by the productions of the more temperate
regions. And as the snow melted from the bases of the mountains, the arctic
forms would seize on the cleared and thawed ground, always ascending higher
and higher, as the warmth increased, whilst their brethren were pursuing
their northern journey. Hence, when the warmth had fully returned, the same
arctic species, which had lately lived in a body together on the lowlands of
the Old and New Worlds, would be left isolated on distant mountain-summits
(having been exterminated on all lesser heights) and in the arctic regions
of both hemispheres.

Thus we can understand the identity of many plants at points so immensely
remote as on the mountains of the United States and of Europe. We can thus
also understand the fact that the Alpine plants of each mountain-range are
more especially related to the arctic forms living due north or nearly due
north of them: for the migration as the cold came on, and the re-migration
on the returning warmth, will generally have been due south and north. The
Alpine plants, for example, of Scotland, as remarked by Mr H. C. Watson, and
those of the Pyrenees, as remarked by Ramond, are more especially allied to
the plants of northern Scandinavia; those of the United States to Labrador;
those of the mountains of Siberia to the arctic regions of that country.
These views, grounded as they are on the perfectly well-ascertained
occurrence of a former Glacial period, seem to me to explain in so
satisfactory a manner the present distribution of the Alpine and Arctic
productions of Europe and America, that when in other regions we find the
same species on distant mountain-summits, we may almost conclude without
other evidence, that a colder climate permitted their former migration
across the low intervening tracts, since become too warm for their
existence.

If the climate, since the Glacial period, has ever been in any degree warmer
than at present (as some geologists in the United States believe to have
been the case, chiefly from the distribution of the fossil Gnathodon), then
the arctic and temperate productions will at a very late period have marched
a little further north, and subsequently have retreated to their present
homes; but I have met with no satisfactory evidence with respect to this
intercalated slightly warmer period, since the Glacial period.

The arctic forms, during their long southern migration and re-migration
northward, will have been exposed to nearly the same climate, and, as is
especially to be noticed, they will have kept in a body together;
consequently their mutual relations will not have been much disturbed, and,
in accordance with the principles inculcated in this volume, they will not
have been liable to much modification. But with our Alpine productions, left
isolated from the moment of the returning warmth, first at the bases and
ultimately on the summits of the mountains, the case will have been somewhat
different; for it is not likely that all the same arctic species will have
been left on mountain ranges distant from each other, and have survived
there ever since; they will, also, in all probability have become mingled
with ancient Alpine species, which must have existed on the mountains before
the commencement of the Glacial epoch, and which during its coldest period
will have been temporarily driven down to the plains; they will, also, have
been exposed to somewhat different climatal influences. Their mutual
relations will thus have been in some degree disturbed; consequently they
will have been liable to modification; and this we find has been the case;
for if we compare the present Alpine plants and animals of the several great
European mountain-ranges, though very many of the species are identically
the same, some present varieties, some are ranked as doubtful forms, and
some few are distinct yet closely allied or representative species.

In illustrating what, as I believe, actually took place during the Glacial
period, I assumed that at its commencement the arctic productions were as
uniform round the polar regions as they are at the present day. But the
foregoing remarks on distribution apply not only to strictly arctic forms,
but also to many sub-arctic and to some few northern temperate forms, for
some of these are the same on the lower mountains and on the plains of North
America and Europe; and it may be reasonably asked how I account for the
necessary degree of uniformity of the sub-arctic and northern temperate
forms round the world, at the commencement of the Glacial period. At the
present day, the sub-arctic and northern temperate productions of the Old
and New Worlds are separated from each other by the Atlantic Ocean and by
the extreme northern part of the Pacific. During the Glacial period, when
the inhabitants of the Old and New Worlds lived further southwards than at
present, they must have been still more completely separated by wider spaces
of ocean. I believe the above difficulty may be surmounted by looking to
still earlier changes of climate of an opposite nature. We have good reason
to believe that during the newer Pliocene period, before the Glacial epoch,
and whilst the majority of the inhabitants of the world were specifically
the same as now, the climate was warmer than at the present day. Hence we
may suppose that the organisms now living under the climate of latitude 60°,
during the Pliocene period lived further north under the Polar Circle, in
latitude 66°-67°; and that the strictly arctic productions then lived on the
broken land still nearer to the pole. Now if we look at a globe, we shall
see that under the Polar Circle there is almost continuous land from western
Europe, through Siberia, to eastern America. And to this continuity of the
circumpolar land, and to the consequent freedom for intermigration under a
more favourable climate, I attribute the necessary amount of uniformity in
the sub-arctic and northern temperate productions of the Old and New Worlds,
at a period anterior to the Glacial epoch.

Believing, from reasons before alluded to, that our continents have long
remained in nearly the same relative position, though subjected to large,
but partial oscillations of level, I am strongly inclined to extend the
above view, and to infer that during some earlier and still warmer period,
such as the older Pliocene period, a large number of the same plants and
animals inhabited the almost continuous circumpolar land; and that these
plants and animals, both in the Old and New Worlds, began slowly to migrate
southwards as the climate became less warm, long before the commencement of
the Glacial period. We now see, as I believe, their descendants, mostly in a
modified condition, in the central parts of Europe and the United States. On
this view we can understand the relationship, with very little identity,
between the productions of North America and Europe, a relationship which is
most remarkable, considering the distance of the two areas, and their
separation by the Atlantic Ocean. We can further understand the singular
fact remarked on by several observers, that the productions of Europe and
America during the later tertiary stages were more closely related to each
other than they are at the present time; for during these warmer periods the
northern parts of the Old and New Worlds will have been almost continuously
united by land, serving as a bridge, since rendered impassable by cold, for
the inter-migration of their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as the
species in common, which inhabited the New and Old Worlds, migrated south of
the Polar Circle, they must have been completely cut off from each other.
This separation, as far as the more temperate productions are concerned,
took place long ages ago. And as the plants and animals migrated southward,
they will have become mingled in the one great region with the native
American productions, and have had to compete with them; and in the other
great region, with those of the Old World. Consequently we have here
everything favourable for much modification, for far more modification than
with the Alpine productions, left isolated, within a much more recent
period, on the several mountain-ranges and on the arctic lands of the two
Worlds. Hence it has come, that when we compare the now living productions
of the temperate regions of the New and Old Worlds, we find very few
identical species (though Asa Gray has lately shown that more plants are
identical than was formerly supposed), but we find in every great class many
forms, which some naturalists rank as geographical races, and others as
distinct species; and a host of closely allied or representative forms which
are ranked by all naturalists as specifically distinct.

As on the land, so in the waters of the sea, a slow southern migration of a
marine fauna, which during the Pliocene or even a somewhat earlier period,
was nearly uniform along the continuous shores of the Polar Circle, will
account, on the theory of modification, for many closely allied forms now
living in areas completely sundered. Thus, I think, we can understand the
presence of many existing and tertiary representative forms on the eastern
and western shores of temperate North America; and the still more striking
case of many closely allied crustaceans (as described in Dana's admirable
work), of some fish and other marine animals, in the Mediterranean and in
the seas of Japan, areas now separated by a continent and by nearly a
hemisphere of equatorial ocean.

These cases of relationship, without identity, of the inhabitants of seas
now disjoined, and likewise of the past and present inhabitants of the
temperate lands of North America and Europe, are inexplicable on the theory
of creation. We cannot say that they have been created alike, in
correspondence with the nearly similar physical conditions of the areas; for
if we compare, for instance, certain parts of South America with the
southern continents of the Old World, we see countries closely corresponding
in all their physical conditions, but with their inhabitants utterly
dissimilar.

But we must return to our more immediate subject, the Glacial period. I am
convinced that Forbes's view may be largely extended. In Europe we have the
plainest evidence of the cold period, from the western shores of Britain to
the Oural range, and southward to the Pyrenees. We may infer, from the
frozen mammals and nature of the mountain vegetation, that Siberia was
similarly affected. Along the Himalaya, at points 900 miles apart, glaciers
have left the marks of their former low descent; and in Sikkim, Dr Hooker
saw maize growing on gigantic ancient moraines. South of the equator, we
have some direct evidence of former glacial action in New Zealand; and the
same plants, found on widely separated mountains in this island, tell the
same story. If one account which has been published can be trusted, we have
direct evidence of glacial action in the southeastern corner of Australia.

Looking to America; in the northern half, ice-borne fragments of rock have
been observed on the eastern side as far south as lat. 36°-37°, and on the
shores of the Pacific, where the climate is now so different, as far south
as lat. 46°; erratic boulders have, also, been noticed on the Rocky
Mountains. In the Cordillera of Equatorial South America, glaciers once
extended far below their present level. In central Chile I was astonished at
the structure of a vast mound of detritus, about 800 feet in height,
crossing a valley of the Andes; and this I now feel convinced was a gigantic
moraine, left far below any existing glacier. Further south on both sides of
the continent, from lat. 41° to the southernmost extremity, we have the
clearest evidence of former glacial action, in huge boulders transported far
from their parent source.

We do not know that the Glacial epoch was strictly simultaneous at these
several far distant points on opposite sides of the world. But we have good
evidence in almost every case, that the epoch was included within the latest
geological period. We have, also, excellent evidence, that it endured for an
enormous time, as measured by years, at each point. The cold may have come
on, or have ceased, earlier at one point of the globe than at another, but
seeing that it endured for long at each, and that it was contemporaneous in
a geological sense, it seems to me probable that it was, during a part at
least of the period, actually simultaneous throughout the world. Without
some distinct evidence to the contrary, we may at least admit as probable
that the glacial action was simultaneous on the eastern and western sides of
North America, in the Cordillera under the equator and under the warmer
temperate zones, and on both sides of the southern extremity of the
continent. If this be admitted, it is difficult to avoid believing that the
temperature of the whole world was at this period simultaneously cooler. But
it would suffice for my purpose, if the temperature was at the same time
lower along certain broad belts of longitude.

On this view of the whole world, or at least of broad longitudinal belts,
having been simultaneously colder from pole to pole, much light can be
thrown on the present distribution of identical and allied species. In
America, Dr Hooker has shown that between forty and fifty of the flowering
plants of Tierra del Fuego, forming no inconsiderable part of its scanty
flora, are common to Europe, enormously remote as these two points are; and
there are many closely allied species. On the lofty mountains of equatorial
America a host of peculiar species belonging to European genera occur. On
the highest mountains of Brazil, some few European genera were found by
Gardner, which do not exist in the wide intervening hot countries. So on the
Silla of Caraccas the illustrious Humboldt long ago found species belonging
to genera characteristic of the Cordillera. On the mountains of Abyssinia,
several European forms and some few representatives of the peculiar flora of
the Cape of Good Hope occur. At the Cape of Good Hope a very few European
species, believed not to have been introduced by man, and on the mountains,
some few representative European forms are found, which have not been
discovered in the intertropical parts of Africa. On the Himalaya, and on the
isolated mountain-ranges of the peninsula of India, on the heights of
Ceylon, and on the volcanic cones of Java, many plants occur, either
identically the same or representing each other, and at the same time
representing plants of Europe, not found in the intervening hot lowlands. A
list of the genera collected on the loftier peaks of Java raises a picture
of a collection made on a hill in Europe! Still more striking is the fact
that southern Australian forms are clearly represented by plants growing on
the summits of the mountains of Borneo. Some of these Australian forms, as I
hear from Dr. Hooker, extend along the heights of the peninsula of Malacca,
and are thinly scattered, on the one hand over India and on the other as far
as Japan.

On the southern mountains of Australia, Dr. F. Müller has discovered several
European species; other species, not introduced by man, occur on the
lowlands; and a long list can be given, as I am informed by Dr. Hooker, of
European genera, found in Australia, but not in the intermediate torrid
regions. In the admirable `Introduction to the Flora of New Zealand,' by Dr.
Hooker, analogous and striking facts are given in regard to the plants of
that large island. Hence we see that throughout the world, the plants
growing on the more lofty mountains, and on the temperate lowlands of the
northern and southern hemispheres, are sometimes identically the same; but
they are much oftener specifically distinct, though related to each other in
a most remarkable manner.

This brief abstract applies to plants alone: some strictly analogous facts
could be given on the distribution of terrestrial animals. In marine
productions, similar cases occur; as an example, I may quote a remark by the
highest authority, Prof. Dana, that `it is certainly a wonderful fact that
New Zealand should have a closer resemblance in its crustacea to Great
Britain, its antipode, than to any other part of the world.' Sir J.
Richardson, also, speaks of the reappearance on the shores of New Zealand,
Tasmania, &c., of northern forms of fish. Dr Hooker informs me that
twenty-five species of Algae are common to New Zealand and to Europe, but
have not been found in the intermediate tropical seas.

It should be observed that the northern species and forms found in the
southern parts of the southern hemisphere, and on the mountain-ranges of the
intertropical regions, are not arctic, but belong to the northern temperate
zones. As Mr. H. C. Watson has recently remarked, `In receding from polar
towards equatorial latitudes, the Alpine or mountain floras really become
less and less arctic.' Many of the forms living on the mountains of the
warmer regions of the earth and in the southern hemisphere are of doubtful
value, being ranked by some naturalists as specifically distinct, by others
as varieties; but some are certainly identical, and many, though closely
related to northern forms, must be ranked as distinct species.

Now let us see what light can be thrown on the foregoing facts, on the
belief, supported as it is by a large body of geological evidence, that the
whole world, or a large part of it, was during the Glacial period
simultaneously much colder than at present. The Glacial period, as measured
by years, must have been very long; and when we remember over what vast
spaces some naturalised plants and animals have spread within a few
centuries, this period will have been ample for any amount of migration. As
the cold came slowly on, all the tropical plants and other productions will
have retreated from both sides towards the equator, followed in the rear by
the temperate productions, and these by the arctic; but with the latter we
are not now concerned. The tropical plants probably suffered much
extinction; how much no one can say; perhaps formerly the tropics supported
as many species as we see at the present day crowded together at the Cape of
Good Hope, and in parts of temperate Australia. As we know that many
tropical plants and animals can withstand a considerable amount of cold,
many might have escaped extermination during a moderate fall of temperature,
more especially by escaping into the warmest spots. But the great fact to
bear in mind is, that all tropical productions will have suffered to a
certain extent. On the other hand, the temperate productions, after
migrating nearer to the equator, though they will have been placed under
somewhat new conditions, will have suffered less. And it is certain that
many temperate plants, if protected from the inroads of competitors, can
withstand a much warmer climate than their own. Hence, it seems to me
possible, bearing in mind that the tropical productions were in a suffering
state and could not have presented a firm front against intruders, that a
certain number of the more vigorous and dominant temperate forms might have
penetrated the native ranks and have reached or even crossed the equator.
The invasion would, of course, have been greatly favoured by high land, and
perhaps by a dry climate; for Dr. Falconer informs me that it is the damp
with the heat of the tropics which is so destructive to perennial plants
from a temperate climate. On the other hand, the most humid and hottest
districts will have afforded an asylum to the tropical natives. The
mountain-ranges north-west of the Himalaya, and the long line of the
Cordillera, seem to have afforded two great lines of invasion: and it is a
striking fact, lately communicated to me by Dr. Hooker, that all the
flowering plants, about forty-six in number, common to Tierra del Fuego and
to Europe still exist in North America, which must have lain on the line of
march. But I do not doubt that some temperate productions entered and
crossed even the lowlands of the tropics at the period when the cold was
most intense, when arctic forms had migrated some twenty-five degrees of
latitude from their native country and covered the land at the foot of the
Pyrenees. At this period of extreme cold, I believe that the climate under
the equator at the level of the sea was about the same with that now felt
there at the height of six or seven thousand feet. During this the coldest
period, I suppose that large spaces of the tropical lowlands were clothed
with a mingled tropical and temperate vegetation, like that now growing with
strange luxuriance at the base of the Himalaya, as graphically described by
Hooker.

Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial period
from the northern and southern temperate zones into the intertropical
regions, and some even crossed the equator. As the warmth returned, these
temperate forms would naturally ascend the higher mountains, being
exterminated on the lowlands; those which had not reached the equator, would
re-migrate northward or southward towards their former homes; but the forms,
chiefly northern, which had crossed the equator, would travel still further
from their homes into the more temperate latitudes of the opposite
hemisphere. Although we have reason to believe from geological evidence that
the whole body of arctic shells underwent scarcely any modification during
their long southern migration and re-migration northward, the case may have
been wholly different with those intruding forms which settled themselves on
the intertropical mountains, and in the southern hemisphere. These being
surrounded by strangers will have had to compete with many new forms of
life; and it is probable that selected modifications in their structure,
habits, and constitutions will have profited them. Thus many of these
wanderers, though still plainly related by inheritance to their brethren of
the northern or southern hemispheres, now exist in their new homes as
well-marked varieties or as distinct species.

It is a remarkable fact, strongly insisted on by Hooker in regard to
America, and by Alph. de Candolle in regard to Australia, that many more
identical plants and allied forms have apparently migrated from the north to
the south, than in a reversed direction. We see, however, a few southern
vegetable forms on the mountains of Borneo and Abyssinia. I suspect that
this preponderant migration from north to south is due to the greater extent
of land in the north, and to the northern forms having existed in their own
homes in greater numbers, and having consequently been advanced through
natural selection and competition to a higher stage of perfection or
dominating power, than the southern forms. And thus, when they became
commingled during the Glacial period, the northern forms were enabled to
beat the less powerful southern forms. Just in the same manner as we see at
the present day, that very many European productions cover the ground in La
Plata, and in a lesser degree in Australia, and have to a certain extent
beaten the natives; whereas extremely few southern forms have become
naturalised in any part of Europe, though hides, wool, and other objects
likely to carry seeds have been largely imported into Europe during the last
two or three centuries from La Plata, and during the last thirty or forty
years from Australia. Something of the same kind must have occurred on the
intertropical mountains: no doubt before the Glacial period they were
stocked with endemic Alpine forms; but these have almost everywhere largely
yielded to the more dominant forms, generated in the larger areas and more
efficient workshops of the north. In many islands the native productions are
nearly equalled or even outnumbered by the naturalised; and if the natives
have not been actually exterminated, their numbers have been greatly
reduced, and this is the first stage towards extinction. A mountain is an
island on the land; and the intertropical mountains before the Glacial
period must have been completely isolated; and I believe that the
productions of these islands on the land yielded to those produced within
the larger areas of the north, just in the same way as the productions of
real islands have everywhere lately yielded to continental forms,
naturalised by man's agency.

I am far from supposing that all difficulties are removed on the view here
given in regard to the range and affinities of the allied species which live
in the northern and southern temperate zones and on the mountains of the
intertropical regions. Very many difficulties remain to be solved. I do not
pretend to indicate the exact lines and means of migration, or the reason
why certain species and not others have migrated; why certain species have
been modified and have given rise to new groups of forms, and others have
remained unaltered. We cannot hope to explain such facts, until we can say
why one species and not another becomes naturalised by man's agency in a
foreign land; why one ranges twice or thrice as far, and is twice or thrice
as common, as another species within their own homes.

I have said that many difficulties remain to be solved: some of the most
remarkable are stated with admirable clearness by Dr. Hooker in his
botanical works on the antarctic regions. These cannot be here discussed. I
will only say that as far as regards the occurrence of identical species at
points so enormously remote as Kerguelen Land, New Zealand, and Fuegia, I
believe that towards the close of the Glacial period, icebergs, as suggested
by Lyell, have been largely concerned in their dispersal. But the existence
of several quite distinct species, belonging to genera exclusively confined
to the south, at these and other distant points of the southern hemisphere,
is, on my theory of descent with modification, a far more remarkable case of
difficulty. For some of these species are so distinct, that we cannot
suppose that there has been time since the commencement of the Glacial
period for their migration, and for their subsequent modification to the
necessary degree. The facts seem to me to indicate that peculiar and very
distinct species have migrated in radiating lines from some common centre;
and I am inclined to look in the southern, as in the northern hemisphere, to
a former and warmer period, before the commencement of the Glacial period,
when the antarctic lands, now covered with ice, supported a highly peculiar
and isolated flora. I suspect that before this flora was exterminated by the
Glacial epoch, a few forms were widely dispersed to various points of the
southern hemisphere by occasional means of transport, and by the aid, as
halting-places, of existing and now sunken islands, and perhaps at the
commencement of the Glacial period, by icebergs. By these means, as I
believe, the southern shores of America, Australia, New Zealand have become
slightly tinted by the same peculiar forms of vegetable life.

Sir C. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alterations of climate on
geographical distribution. I believe that the world has recently felt one of
his great cycles of change; and that on this view, combined with
modification through natural selection, a multitude of facts in the present
distribution both of the same and of allied forms of life can be explained.
The living waters may be said to have flowed during one short period from
the north and from the south, and to have crossed at the equator; but to
have flowed with greater force from the north so as to have freely inundated
the south. As the tide leaves its drift in horizontal lines, though rising
higher on the shores where the tide rises highest, so have the living waters
left their living drift on our mountain-summits, in a line gently rising
from the arctic lowlands to a great height under the equator. The various
beings thus left stranded may be compared with savage races of man, driven
up and surviving in the mountain-fastnesses of almost every land, which
serve as a record, full of interest to us, of the former inhabitants of the
surrounding lowlands.

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Chapter 12 - Geographical Distribution -- continued

[* ] Distribution of fresh-water productions [* ] On the inhabitants of
oceanic islands [* ] Absence of Batrachians and of terrestrial Mammals [* ]
On the relations of the inhabitants of islands to those of the nearest
mainland [* ] On colonisation from the nearest source with subsequent
modification [* ] Summary of the last and present chapters
----------------------------------------------------------------------------
As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not have
ranged widely within the same country, and as the sea is apparently a still
more impassable barrier, that they never would have extended to distant
countries. But the case is exactly the reverse. Not only have many
fresh-water species, belonging to quite different classes, an enormous
range, but allied species prevail in a remarkable manner throughout the
world. I well remember, when first collecting in the fresh waters of Brazil,
feeling much surprise at the similarity of the fresh-water insects, shells,
&c., and at the dissimilarity of the surrounding terrestrial beings,
compared with those of Britain.

But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having become
fitted, in a manner highly useful to them, for short and frequent migrations
from pond to pond, or from stream to stream; and liability to wide dispersal
would follow from this capacity as an almost necessary consequence. We can
here consider only a few cases. In regard to fish, I believe that the same
species never occur in the fresh waters of distant continents. But on the
same continent the species often range widely and almost capriciously; for
two river-systems will have some fish in common and some different. A few
facts seem to favour the possibility of their occasional transport by
accidental means; like that of the live fish not rarely dropped by
whirlwinds in India, and the vitality of their ova when removed from the
water. But I am inclined to attribute the dispersal of fresh-water fish
mainly to slight changes within the recent period in the level of the land,
having caused rivers to flow into each other. Instances, also, could be
given of this having occurred during floods, without any change of level. We
have evidence in the loess of the Rhine of considerable changes of level in
the land within a very recent geological period, and when the surface was
peopled by existing land and fresh-water shells. The wide difference of the
fish on opposite sides of continuous mountain-ranges, which from an early
period must have parted river-systems and completely prevented their
inosculation, seems to lead to this same conclusion. With respect to allied
fresh-water fish occurring at very distant points of the world, no doubt
there are many cases which cannot at present be explained: but some
fresh-water fish belong to very ancient forms, and in such cases there will
have been ample time for great geographical changes, and consequently time
and means for much migration. In the second place, salt-water fish can with
care be slowly accustomed to live in fresh water; and, according to
Valenciennes, there is hardly a single group of fishes confined exclusively
to fresh water, so that we may imagine that a marine member of a fresh-water
group might travel far along the shores of the sea, and subsequently become
modified and adapted to the fresh waters of a distant land.

Some species of fresh-water shells have a very wide range, and allied
species, which, on my theory, are descended from a common parent and must
have proceeded from a single source, prevail throughout the world. Their
distribution at first perplexed me much, as their ova are not likely to be
transported by birds, and they are immediately killed by sea water, as are
the adults. I could not even understand how some naturalised species have
rapidly spread throughout the same country. But two facts, which I have
observed and no doubt many others remain to be observed throw some light on
this subject. When a duck suddenly emerges from a pond covered with
duck-weed, I have twice seen these little plants adhering to its back; and
it has happened to me, in removing a little duck-weed from one aquarium to
another, that I have quite unintentionally stocked the one with fresh-water
shells from the other. But another agency is perhaps more effectual: I
suspended a duck's feet, which might represent those of a bird sleeping in a
natural pond, in an aquarium, where many ova of fresh-water shells were
hatching; and I found that numbers of the extremely minute and just hatched
shells crawled on the feet, and clung to them so firmly that when taken out
of the water they could not be jarred off, though at a somewhat more
advanced age they would voluntarily drop off. These just hatched molluscs,
though aquatic in their nature, survived on the duck's feet, in damp air,
from twelve to twenty hours; and in this length of time a duck or heron
might fly at least six or seven hundred miles, and would be sure to alight
on a pool or rivulet, if blown across sea to an oceanic island or to any
other distant point. Sir Charles Lyell also informs me that a Dyticus has
been caught with an Ancylus (a fresh-water shell like a limpet) firmly
adhering to it; and a water-beetle of the same family, a Colymbetes, once
flew on board the `Beagle,' when forty-five miles distant from the nearest
land: how much farther it might have flown with a favouring gale no one can
tell.

With respect to plants, it has long been known what enormous ranges many
fresh-water and even marsh-species have, both over continents and to the
most remote oceanic islands. This is strikingly shown, as remarked by Alph.
de Candolle, in large groups of terrestrial plants, which have only a very
few aquatic members; for these latter seem immediately to acquire, as if in
consequence, a very wide range. I think favourable means of dispersal
explain this fact. I have before mentioned that earth occasionally, though
rarely, adheres in some quantity to the feet and beaks of birds. Wading
birds, which frequent the muddy edges of ponds, if suddenly flushed, would
be the most likely to have muddy feet. Birds of this order I can show are
the greatest wanderers, and are occasionally found on the most remote and
barren islands in the open ocean; they would not be likely to alight on the
surface of the sea, so that the dirt would not be washed off their feet;
when making land, they would be sure to fly to their natural fresh-water
haunts. I do not believe that botanists are aware how charged the mud of
ponds is with seeds: I have tried several little experiments, but will here
give only the most striking case: I took in February three table-spoonfuls
of mud from three different points, beneath water, on the edge of a little
pond; this mud when dry weighed only 6 3/4 ounces; I kept it covered up in
my study for six months, pulling up and counting each plant as it grew; the
plants were of many kinds, and were altogether 537 in number; and yet the
viscid mud was all contained in a breakfast cup! Considering these facts, I
think it would be an inexplicable circumstance if water-birds did not
transport the seeds of fresh-water plants to vast distances, and if
consequently the range of these plants was not very great. The same agency
may have come into play with the eggs of some of the smaller fresh-water
animals.

Other and unknown agencies probably have also played a part. I have stated
that fresh-water fish eat some kinds of seeds, though they reject many other
kinds after having swallowed them; even small fish swallow seeds of moderate
size, as of the yellow water-lily and Potamogeton. Herons and other birds,
century after century, have gone on daily devouring fish; they then take
flight and go to other waters, or are blown across the sea; and we have seen
that seeds retain their power of germination, when rejected in pellets or in
excrement, many hours afterwards. When I saw the great size of the seeds of
that fine water-lily, the Nelumbium, and remembered Alph. de Candolle's
remarks on this plant, I thought that its distribution must remain quite
inexplicable; but Audubon states that he found the seeds of the great
southern water-lily (probably, according to Dr Hooker, the Nelumbium luteum)
in a heron's stomach; although I do not know the fact, yet analogy makes me
believe that a heron flying to another pond and getting a hearty meal of
fish, would probably reject from its stomach a pellet containing the seeds
of the Nelumbium undigested; or the seeds might be dropped by the bird
whilst feeding its young, in the same way as fish are known sometimes to be
dropped.

In considering these several means of distribution, it should be remembered
that when a pond or stream is first formed, for instance, on a rising islet,
it will be unoccupied; and a single seed or egg will have a good chance of
succeeding. Although there will always be a struggle for life between the
individuals of the species, however few, already occupying any pond, yet as
the number of kinds is small, compared with those on the land, the
competition will probably be less severe between aquatic than between
terrestrial species; consequently an intruder from the waters of a foreign
country, would have a better chance of seizing on a place, than in the case
of terrestrial colonists. We should, also, remember that some, perhaps many,
fresh-water productions are low in the scale of nature, and that we have
reason to believe that such low beings change or become modified less
quickly than the high; and this will give longer time than the average for
the migration of the same aquatic species. We should not forget the
probability of many species having formerly ranged as continuously as
fresh-water productions ever can range, over immense areas, and having
subsequently become extinct in intermediate regions. But the wide
distribution of fresh-water plants and of the lower animals, whether
retaining the same identical form or in some degree modified, I believe
mainly depends on the wide dispersal of their seeds and eggs by animals,
more especially by fresh-water birds, which have large powers of flight, and
naturally travel from one to another and often distant piece of water.
Nature, like a careful gardener, thus takes her seeds from a bed of a
particular nature, and drops them in another equally well fitted for them.

On the Inhabitants of Oceanic Islands

We now come to the last of the three classes of facts, which I have selected
as presenting the greatest amount of difficulty, on the view that all the
individuals both of the same and of allied species have descended from a
single parent; and therefore have all proceeded from a common birthplace,
notwithstanding that in the course of time they have come to inhabit distant
points of the globe. I have already stated that I cannot honestly admit
Forbes's view on continental extensions, which, if legitimately followed
out, would lead to the belief that within the recent period all existing
islands have been nearly or quite joined to some continent. This view would
remove many difficulties, but it would not, I think, explain all the facts
in regard to insular productions. In the following remarks I shall not
confine myself to the mere question of dispersal; but shall consider some
other facts, which bear on the truth of the two theories of independent
creation and of descent with modification.

The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and Wollaston for insects. If we look to the large size and
varied stations of New Zealand, extending over 780 miles of latitude, and
compare its flowering plants, only 750 in number, with those on an equal
area at the Cape of Good Hope or in Australia, we must, I think, admit that
something quite independently of any difference in physical conditions has
caused so great a difference in number. Even the uniform county of Cambridge
has 847 plants, and the little island of Anglesea 764, but a few ferns and a
few introduced plants are included in these numbers, and the comparison in
some other respects is not quite fair. We have evidence that the barren
island of Ascension aboriginally possessed under half-a-dozen flowering
plants; yet many have become naturalised on it, as they have on New Zealand
and on every other oceanic island which can be named. In St. Helena there is
reason to believe that the naturalised plants and animals have nearly or
quite exterminated many native productions. He who admits the doctrine of
the creation of each separate species, will have to admit, that a sufficient
number of the best adapted plants and animals have not been created on
oceanic islands; for man has unintentionally stocked them from various
sources far more fully and perfectly than has nature.

Although in oceanic islands the number of kinds of inhabitants is scanty,
the proportion of endemic species (i.e. those found nowhere else in the
world) is often extremely large. If we compare, for instance, the number of
the endemic land-shells in Madeira, or of the endemic birds in the Galapagos
Archipelago, with the number found on any continent, and then compare the
area of the islands with that of the continent, we shall see that this is
true. This fact might have been expected on my theory for, as already
explained, species occasionally arriving after long intervals in a new and
isolated district, and having to compete with new associates, will be
eminently liable to modification, and will often produce groups of modified
descendants. But it by no means follows, that, because in an island nearly
all the species of one class are peculiar, those of another class, or of
another section of the same class, are peculiar; and this difference seems
to depend on the species which do not become modified having immigrated with
facility and in a body, so that their mutual relations have not been much
disturbed. Thus in the Galapagos Islands nearly every land-bird, but only
two out of the eleven marine birds, are peculiar; and it is obvious that
marine birds could arrive at these islands more easily than land-birds.
Bermuda, on the other hand, which lies at about the same distance from North
America as the Galapagos Islands do from South America, and which has a very
peculiar soil, does not possess one endemic land bird; and we know from Mr.
J. M. Jones's admirable account of Bermuda, that very many North American
birds, during their great annual migrations, visit either periodically or
occasionally this island. Madeira does not possess one peculiar bird, and
many European and African birds are almost every year blown there, as I am
informed by Mr. E. V. Harcourt. So that these two islands of Bermuda and
Madeira have been stocked by birds, which for long ages have struggled
together in their former homes, and have become mutually adapted to each
other; and when settled in their new homes, each kind will have been kept by
the others to their proper places and habits, and will consequently have
been little liable to modification. Madeira, again, is inhabited by a
wonderful number of peculiar land-shells, whereas not one species of
sea-shell is confined to its shores: now, though we do not know how
seashells are dispersed, yet we can see that their eggs or larvae, perhaps
attached to seaweed or floating timber, or to the feet of wading-birds,
might be transported far more easily than land-shells, across three or four
hundred miles of open sea. The different orders of insects in Madeira
apparently present analogous facts.

Oceanic islands are sometimes deficient in certain classes, and their places
are apparently occupied by the other inhabitants; in the Galapagos Islands
reptiles, and in New Zealand gigantic wingless birds, take the place of
mammals. In the plants of the Galapagos Islands, Dr. Hooker has shown that
the proportional numbers of the different orders are very different from
what they are elsewhere. Such cases are generally accounted for by the
physical conditions of the islands; but this explanation seems to me not a
little doubtful. Facility of immigration, I believe, has been at least as
important as the nature of the conditions.

Many remarkable little facts could be given with respect to the inhabitants
of remote islands. For instance, in certain islands not tenanted by mammals,
some of the endemic plants have beautifully hooked seeds; yet few relations
are more striking than the adaptation of hooked seeds for transportal by the
wool and fur of quadrupeds. This case presents no difficulty on my view, for
a hooked seed might be transported to an island by some other means; and the
plant then becoming slightly modified, but still retaining its hooked seeds,
would form an endemic species, having as useless an appendage as any
rudimentary organ, for instance, as the shrivelled wings under the soldered
elytra of many insular beetles. Again, islands often possess trees or bushes
belonging to orders which elsewhere include only herbaceous species; now
trees, as Alph. de Candolle has shown, generally have, whatever the cause
may be, confined ranges. Hence trees would be little likely to reach distant
oceanic islands; and an herbaceous plant, though it would have no chance of
successfully competing in stature with a fully developed tree, when
established on an island and having to compete with herbaceous plants alone,
might readily gain an advantage by growing taller and taller and overtopping
the other plants. If so, natural selection would often tend to add to the
stature of herbaceous plants when growing on an island, to whatever order
they belonged, and thus convert them first into bushes and ultimately into
trees.

With respect to the absence of whole orders on oceanic islands, Bory St.
Vincent long ago remarked that Batrachians (frogs, toads, newts) have never
been found on any of the many islands with which the great oceans are
studded. I have taken pains to verify this assertion, and I have found it
strictly true. I have, however, been assured that a frog exists on the
mountains of the great island of New Zealand; but I suspect that this
exception (if the information be correct) may be explained through glacial
agency. This general absence of frogs, toads, and newts on so many oceanic
islands cannot be accounted for by their physical conditions; indeed it
seems that islands are peculiarly well fitted for these animals; for frogs
have been introduced into Madeira, the Azores, and Mauritius, and have
multiplied so as to become a nuisance. But as these animals and their spawn
are known to be immediately killed by sea-water, on my view we can see that
there would be great difficulty in their transportal across the sea, and
therefore why they do not exist on any oceanic island. But why, on the
theory of creation, they should not have been created there, it would be
very difficult to explain.

Mammals offer another and similar case. I have carefully searched the oldest
voyages, but have not finished my search; as yet I have not found a single
instance, free from doubt, of a terrestrial mammal (excluding domesticated
animals kept by the natives) inhabiting an island situated above 300 miles
from a continent or great continental island; and many islands situated at a
much less distance are equally barren. The Falkland Islands, which are
inhabited by a wolf-like fox, come nearest to an exception; but this group
cannot be considered as oceanic, as it lies on a bank connected with the
mainland; moreover, icebergs formerly brought boulders to its western
shores, and they may have formerly transported foxes, as so frequently now
happens in the arctic regions. Yet it cannot be said that small islands will
not support small mammals, for they occur in many parts of the world on very
small islands, if close to a continent; and hardly an island can be named on
which our smaller quadrupeds have not become naturalised and greatly
multiplied. It cannot be said, on the ordinary view of creation, that there
has not been time for the creation of mammals; many volcanic islands are
sufficiently ancient, as shown by the stupendous degradation which they have
suffered and by their tertiary strata: there has also been time for the
production of endemic species belonging to other classes; and on continents
it is thought that mammals appear and disappear at a quicker rate than other
and lower animals. Though terrestrial mammals do not occur on oceanic
islands, aërial mammals do occur on almost every island. New Zealand
possesses two bats found nowhere else in the world: Norfolk Island, the Viti
Archipelago, the Bonin Islands, the Caroline and Marianne Archipelagoes, and
Mauritius, all possess their peculiar bats. Why, it may be asked, has the
supposed creative force produced bats and no other mammals on remote
islands? On my view this question can easily be answered; for no terrestrial
mammal can be transported across a wide space of sea, but bats can fly
across. Bats have been seen wandering by day far over the Atlantic Ocean;
and two North American species either regularly or occasionally visit
Bermuda, at the distance of 600 miles from the mainland. I hear from Mr.
Tomes, who has specially studied this family, that many of the same species
have enormous ranges, and are found on continents and on far distant
islands. Hence we have only to suppose that such wandering species have been
modified through natural selection in their new homes in relation to their
new position, and we can understand the presence of endemic bats on islands,
with the absence of all terrestrial mammals.

Besides the absence of terrestrial mammals in relation to the remoteness of
islands from continents, there is also a relation, to a certain extent
independent of distance, between the depth of the sea separating an island
from the neighbouring mainland, and the presence in both of the same
mammiferous species or of allied species in a more or less modified
condition. Mr. Windsor Earl has made some striking observations on this head
in regard to the great Malay Archipelago, which is traversed near Celebes by
a space of deep ocean; and this space separates two widely distinct
mammalian faunas. On either side the islands are situated on moderately deep
submarine banks, and they are inhabited by closely allied or identical
quadrupeds. No doubt some few anomalies occur in this great archipelago, and
there is much difficulty in forming a judgment in some cases owing to the
probable naturalisation of certain mammals through man's agency; but we
shall soon have much light thrown on the natural history of this archipelago
by the admirable zeal and researches of Mr Wallace. I have not as yet had
time to follow up this subject in all other quarters of the world; but as
far as I have gone, the relation generally holds good. We see Britain
separated by a shallow channel from Europe, and the mammals are the same on
both sides; we meet with analogous facts on many islands separated by
similar channels from Australia. The West Indian Islands stand on a deeply
submerged bank, nearly 1000 fathoms in depth, and here we find American
forms, but the species and even the genera are distinct. As the amount of
modification in all cases depends to a certain degree on the lapse of time,
and as during changes of level it is obvious that islands separated by
shallow channels are more likely to have been continuously united within a
recent period to the mainland than islands separated by deeper channels, we
can understand the frequent relation between the depth of the sea and the
degree of affinity of the mammalian inhabitants of islands with those of a
neighbouring continent, an explicable relation on the view of independent
acts of creation.

All the foregoing remarks on the inhabitants of oceanic islands, namely, the
scarcity of kinds -- the richness in endemic forms in particular classes or
sections of classes, the absence of whole groups, as of batrachians, and of
terrestrial mammals notwithstanding the presence of aërial bats, the
singular proportions of certain orders of plants, herbaceous forms having
been developed into trees, &c., seem to me to accord better with the view of
occasional means of transport having been largely efficient in the long
course of time, than with the view of all our oceanic islands having been
formerly connected by continuous land with the nearest continent; for on
this latter view the migration would probably have been more complete; and
if modification be admitted, all the forms of life would have been more
equally modified, in accordance with the paramount importance of the
relation of organism to organism.

I do not deny that there are many and grave difficulties in understanding
how several of the inhabitants of the more remote islands, whether still
retaining the same specific form or modified since their arrival, could have
reached their present homes. But the probability of many islands having
existed as halting-places, of which not a wreck now remains, must not be
overlooked. I will here give a single instance of one of the cases of
difficulty. Almost all oceanic islands, even the most isolated and smallest,
are inhabited by land-shells, generally by endemic species, but sometimes by
species found elsewhere. Dr. Aug. A. Gould has given several interesting
cases in regard to the land-shells of the islands of the Pacific. Now it is
notorious that land-shells are very easily killed by salt; their eggs, at
least such as I have tried, sink in sea-water and are killed by it. Yet
there must be, on my view, some unknown, but highly efficient means for
their transportal. Would the just-hatched young occasionally crawl on and
adhere to the feet of birds roosting on the ground, and thus get
transported? It occurred to me that land-shells, when hybernating and having
a membranous diaphragm over the mouth of the shell, might be floated in
chinks of drifted timber across moderately wide arms of the sea. And I found
that several species did in this state withstand uninjured an immersion in
sea-water during seven days: one of these shells was the Helix pomatia, and
after it had again hybernated I put it in sea-water for twenty days, and it
perfectly recovered. As this species has a thick calcareous operculum, I
removed it, and when it had formed a new membranous one, I immersed it for
fourteen days in sea-water, and it recovered and crawled away: but more
experiments are wanted on this head.

The most striking and important fact for us in regard to the inhabitants of
islands, is their affinity to those of the nearest mainland, without being
actually the same species. Numerous instances could be given of this fact. I
will give only one, that of the Galapagos Archipelago, situated under the
equator, between 500 and 600 miles from the shores of South America. Here
almost every product of the land and water bears the unmistakeable stamp of
the American continent. There are twenty-six land birds, and twenty-five of
those are ranked by Mr Gould as distinct species, supposed to have been
created here; yet the close affinity of most of these birds to American
species in every character, in their habits, gestures, and tones of voice,
was manifest. So it is with the other animals, and with nearly all the
plants, as shown by Dr. Hooker in his admirable memoir on the Flora of this
archipelago. The naturalist, looking at the inhabitants of these volcanic
islands in the Pacific, distant several hundred miles from the continent,
yet feels that he is standing on American land. Why should this be so? why
should the species which are supposed to have been created in the Galapagos
Archipelago, and nowhere else, bear so plain a stamp of affinity to those
created in America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate, or in the
proportions in which the several classes are associated together, which
resembles closely the conditions of the South American coast: in fact there
is a considerable dissimilarity in all these respects. On the other hand,
there is a considerable degree of resemblance in the volcanic nature of the
soil, in climate, height, and size of the islands, between the Galapagos and
Cape de Verde Archipelagos: but what an entire and absolute difference in
their inhabitants! The inhabitants of the Cape de Verde Islands are related
to those of Africa, like those of the Galapagos to America. I believe this
grand fact can receive no sort of explanation on the ordinary view of
independent creation; whereas on the view here maintained, it is obvious
that the Galapagos Islands would be likely to receive colonists, whether by
occasional means of transport or by formerly continuous land, from America;
and the Cape de Verde Islands from Africa; and that such colonists would be
liable to modifications; the principle of inheritance still betraying their
original birthplace.

Many analogous facts could be given: indeed it is an almost universal rule
that the endemic productions of islands are related to those of the nearest
continent, or of other near islands. The exceptions are few, and most of
them can be explained. Thus the plants of Kerguelen Land, though standing
nearer to Africa than to America, are related, and that very closely, as we
know from Dr. Hooker's account, to those of America: but on the view that
this island has been mainly stocked by seeds brought with earth and stones
on icebergs, drifted by the prevailing currents, this anomaly disappears.
New Zealand in its endemic plants is much more closely related to Australia,
the nearest mainland, than to any other region: and this is what might have
been expected; but it is also plainly related to South America, which,
although the next nearest continent, is so enormously remote, that the fact
becomes an anomaly. But this difficulty almost disappears on the view that
both New Zealand, South America, and other southern lands were long ago
partially stocked from a nearly intermediate though distant point, namely
from the antarctic islands, when they were clothed with vegetation, before
the commencement of the Glacial period. The affinity, which, though feeble,
I am assured by Dr. Hooker is real, between the flora of the south-western
corner of Australia and of the Cape of Good Hope, is a far more remarkable
case, and is at present inexplicable: but this affinity is confined to the
plants, and will, I do not doubt, be some day explained.

The law which causes the inhabitants of an archipelago, though specifically
distinct, to be closely allied to those of the nearest continent, we
sometimes see displayed on a small scale, yet in a most interesting manner,
within the limits of the same archipelago. Thus the several islands of the
Galapagos Archipelago are tenanted, as I have elsewhere shown, in a quite
marvellous manner, by very closely related species; so that the inhabitants
of each separate island, though mostly distinct, are related in an
incomparably closer degree to each other than to the inhabitants of any
other part of the world. And this is just what might have been expected on
my view, for the islands are situated so near each other that they would
almost certainly receive immigrants from the same original source, or from
each other. But this dissimilarity between the endemic inhabitants of the
islands may be used as an argument against my views; for it may be asked,
how has it happened in the several islands situated within sight of each
other, having the same geological nature, the same height, climate, &c.,
that many of the immigrants should have been differently modified, though
only in a small degree. This long appeared to me a great difficulty: but it
arises in chief part from the deeply-seated error of considering the
physical conditions of a country as the most important for its inhabitants;
whereas it cannot, I think, be disputed that the nature of the other
inhabitants, with which each has to compete, is at least as important, and
generally a far more important element of success. Now if we look to those
inhabitants of the Galapagos Archipelago which are found in other parts of
the world (having on one side for the moment the endemic species, which
cannot be here fairly included, as we are considering how they have come to
be modified since their arrival), we find a considerable amount of
difference in the several islands. This difference might indeed have been
expected on the view of the islands having been stocked by occasional means
of transport a seed, for instance, of one plant having been brought to one
island, and that of another plant to another island. Hence when in former
times an immigrant settled on any one or more of the islands, or when it
subsequently spread from one island to another, it would undoubtedly be
exposed to different conditions of life in the different islands, for it
would have to compete with different sets of organisms: a plant, for
instance, would find the best-fitted ground more perfectly occupied by
distinct plants in one island than in another, and it would be exposed to
the attacks of somewhat different enemies. If then it varied, natural
selection would probably favour different varieties in the different
islands. Some species, however, might spread and yet retain the same
character throughout the group, just as we see on continents some species'
spreading widely and remaining the same.

The really surprising fact in this case of the Galapagos Archipelago, and in
a lesser degree in some analogous instances, is that the new species formed
in the separate islands have not quickly spread to the other islands. But
the islands, though in sight of each other, are separated by deep arms of
the sea, in most cases wider than the British Channel, and there is no
reason to suppose that they have at any former period been continuously
united. The currents of the sea are rapid and sweep across the archipelago,
and gales of wind are extraordinarily rare; so that the islands are far more
effectually separated from each other than they appear to be on a map.
Nevertheless a good many species, both those found in other parts of the
world and those confined to the archipelago, are common to the several
islands, and we may infer from certain facts that these have probably spread
from some one island to the others. But we often take, I think, an erroneous
view of the probability of closely allied species invading each other's
territory, when put into free intercommunication. Undoubtedly if one species
has any advantage whatever over another, it will in a very brief time wholly
or in part supplant it; but if both are equally well fitted for their own
places in nature, both probably will hold their own places and keep separate
for almost any length of time. Being familiar with the fact that many
species, naturalised through man's agency, have spread with astonishing
rapidity over new countries, we are apt to infer that most species would
thus spread; but we should remember that the forms which become naturalised
in new countries are not generally closely allied to the aboriginal
inhabitants, but are very distinct species, belonging in a large proportion
of cases, as shown by Alph. de Candolle, to distinct genera. In the
Galapagos Archipelago, many even of the birds, though so well adapted for
flying from island to island, are distinct on each; thus there are three
closely-allied species of mocking-thrush, each confined to its own island.
Now let us suppose the mocking-thrush of Chatham Island to be blown to
Charles Island, which has its own mocking-thrush: why should it succeed in
establishing itself there? We may safely infer that Charles Island is well
stocked with its own species, for annually more eggs are laid there than can
possibly be reared; and we may infer that the mocking-thrush peculiar to
Charles Island is at least as well fitted for its home as is the species
peculiar to Chatham Island. Sir C. Lyell and Mr. Wollaston have communicated
to me a remarkable fact bearing on this subject; namely, that Madeira and
the adjoining islet of Porto Santo possess many distinct but representative
land-shells, some of which live in crevices of stone; and although large
quantities of stone are annually transported from Porto Santo to Madeira,
yet this latter island has not become colonised by the Porto Santo species:
nevertheless both islands have been colonised by some European land-shells,
which no doubt had some advantage over the indigenous species. From these
considerations I think we need not greatly marvel at the endemic and
representative species, which inhabit the several islands of the Galapagos
Archipelago, not having universally spread from island to island. In many
other instances, as in the several districts of the same continent,
pre-occupation has probably played an important part in checking the
commingling of species under the same conditions of life. Thus, the
south-east and south-west corners of Australia have nearly the same physical
conditions, and are united by continuous land, yet they are inhabited by a
vast number of distinct mammals, birds, and plants.

The principle which determines the general character of the fauna and flora
of oceanic islands, namely, that the inhabitants, when not identically the
same, yet are plainly related to the inhabitants of that region whence
colonists could most readily have been derived, the colonists having been
subsequently modified and better fitted to their new homes, is of the widest
application throughout nature. We see this on every mountain, in every lake
and marsh. For Alpine species, excepting in so far as the same forms,
chiefly of plants, have spread widely throughout the world during the recent
Glacial epoch, are related to those of the surrounding lowlands; thus we
have in South America, Alpine humming-birds, Alpine rodents, Alpine plants,
&c., all of strictly American forms, and it is obvious that a mountain, as
it became slowly upheaved, would naturally be colonised from the surrounding
lowlands. So it is with the inhabitants of lakes and marshes, excepting in
so far as great facility of transport has given the same general forms to
the whole world. We see this same principle in the blind animals inhabiting
the caves of America and of Europe. Other analogous facts could be given.
And it will, I believe, be universally found to be true, that wherever in
two regions, let them be ever so distant, many closely allied or
representative species occur, there will likewise be found some identical
species, showing, in accordance with the foregoing view, that at some former
period there has been intercommunication or migration between the two
regions. And wherever many closely-allied species occur, there will be found
many forms which some naturalists rank as distinct species, and some as
varieties; these doubtful forms showing us the steps in the process of
modification.

This relation between the power and extent of migration of a species, either
at the present time or at some former period under different physical
conditions, and the existence at remote points of the world of other species
allied to it, is shown in another and more general way. Mr. Gould remarked
to me long ago, that in those genera of birds which range over the world,
many of the species have very wide ranges. I can hardly doubt that this rule
is generally true, though it would be difficult to prove it. Amongst
mammals, we see it strikingly displayed in Bats, and in a lesser degree in
the Felidae and Canidae. We see it, if we compare the distribution of
butterflies and beetles. So it is with most fresh-water productions, in
which so many genera range over the world, and many individual species have
enormous ranges. It is not meant that in world-ranging genera all the
species have a wide range, or even that they have on an average a wide
range; but only that some of the species range very widely; for the facility
with which widely-ranging species vary and give rise to new forms will
largely determine their average range. For instance, two varieties of the
same species inhabit America and Europe, and the species thus has an immense
range; but, if the variation had been a little greater, the two varieties
would have been ranked as distinct species, and the common range would have
been greatly reduced. Still less is it meant, that a species which
apparently has the capacity of crossing barriers and ranging widely, as in
the case of certain powerfully-winged birds, will necessarily range widely;
for we should never forget that to range widely implies not only the power
of crossing barriers, but the more important power of being victorious in
distant lands in the struggle for life with foreign associates. But on the
view of all the species of a genus having descended from a single parent,
though now distributed to the most remote points of the world, we ought to
find, and I believe as a general rule we do find, that some at least of the
species range very widely; for it is necessary that the unmodified parent
should range widely, undergoing modification during its diffusion, and
should place itself under diverse conditions favourable for the conversion
of its offspring, firstly into new varieties and ultimately into new
species.

In considering the wide distribution of certain genera, we should bear in
mind that some are extremely ancient, and must have branched off from a
common parent at a remote epoch; so that in such cases there will have been
ample time for great climatal and geographical changes and for accidents of
transport; and consequently for the migration of some of the species into
all quarters of the world, where they may have become slightly modified in
relation to their new conditions. There is, also, some reason to believe
from geological evidence that organisms low in the scale within each great
class, generally change at a slower rate than the higher forms; and
consequently the lower forms will have had a better chance of ranging widely
and of still retaining the same specific character. This fact, together with
the seeds and eggs of many low forms being very minute and better fitted for
distant transportation, probably accounts for a law which has long been
observed, and which has lately been admirably discussed by Alph. de Candolle
in regard to plants, namely, that the lower any group of organisms is, the
more widely it is apt to range.

The relations just discussed, namely, low and slowly-changing organisms
ranging more widely than the high, some of the species of widely-ranging
genera themselves ranging widely, such facts, as alpine, lacustrine, and
marsh productions being related (with the exceptions before specified) to
those on the surrounding low lands and dry lands, though these stations are
so different the very close relation of the distinct species which inhabit
the islets of the same archipelago, and especially the striking relation of
the inhabitants of each whole archipelago or island to those of the nearest
mainland, are, I think, utterly inexplicable on the ordinary view of the
independent creation of each species, but are explicable on the view of
colonisation from the nearest and readiest source, together with the
subsequent modification and better adaptation of the colonists to their new
homes.

Summary of last and present Chapters

In these chapters I have endeavoured to show, that if we make due allowance
for our ignorance of the full effects of all the changes of climate and of
the level of the land, which have certainly occurred within the recent
period, and of other similar changes which may have occurred within the same
period; if we remember how profoundly ignorant we are with respect to the
many and curious means of occasional transport, a subject which has hardly
ever been properly experimentised on; if we bear in mind how often a species
may have ranged continuously over a wide area, and then have become extinct
in the intermediate tracts, I think the difficulties in believing that all
the individuals of the same species, wherever located, have descended from
the same parents, are not insuperable. And we are led to this conclusion,
which has been arrived at by many naturalists under the designation of
single centres of creation, by some general considerations, more especially
from the importance of barriers and from the analogical distribution of
sub-genera, genera, and families.

With respect to the distinct species of the same genus, which on my theory
must have spread from one parent-source; if we make the same allowances as
before for our ignorance, and remember that some forms of life change most
slowly, enormous periods of time being thus granted for their migration, I
do not think that the difficulties are insuperable; though they often are in
this case, and in that of the individuals of the same species, extremely
grave.

As exemplifying the effects of climatal changes on distribution, I have
attempted to show how important has been the influence of the modern Glacial
period, which I am fully convinced simultaneously affected the whole world,
or at least great meridional belts. As showing how diversified are the means
of occasional transport, I have discussed at some little length the means of
dispersal of fresh-water productions.

If the difficulties be not insuperable in admitting that in the long course
of time the individuals of the same species, and likewise of allied species,
have proceeded from some one source; then I think all the grand leading
facts of geographical distribution are explicable on the theory of migration
(generally of the more dominant forms of life), together with subsequent
modification and the multiplication of new forms. We can thus understand the
high importance of barriers, whether of land or water, which separate our
several zoological and botanical provinces. We can thus understand the
localisation of sub-genera, genera, and families; and how it is that under
different latitudes, for instance in South America, the inhabitants of the
plains and mountains, of the forests, marshes, and deserts, are in so
mysterious a manner linked together by affinity, and are likewise linked to
the extinct beings which formerly inhabited the same continent. Bearing in
mind that the mutual relations of organism to organism are of the highest
importance, we can see why two areas having nearly the same physical
conditions should often be inhabited by very different forms of life; for
according to the length of time which has elapsed since new inhabitants
entered one region; according to the nature of the communication which
allowed certain forms and not others to enter, either in greater or lesser
numbers; according or not, as those which entered happened to come in more
or less direct competition with each other and with the aborigines; and
according as the immigrants were capable of varying more or less rapidly,
there would ensue in different regions, independently of their physical
conditions, infinitely diversified conditions of life, there would be an
almost endless amount of organic action and reaction, and we should find, as
we do find, some groups of beings greatly, and some only slightly modified,
some developed in great force, some existing in scanty numbers in the
different great geographical provinces of the world.

On these same principles, we can understand, as I have endeavoured to show,
why oceanic islands should have few inhabitants, but of these a great number
should be endemic or peculiar; and why, in relation to the means of
migration, one group of beings, even within the same class, should have all
its species endemic, and another group should have all its species common to
other quarters of the world. We can see why whole groups of organisms, as
batrachians and terrestrial mammals, should be absent from oceanic islands,
whilst the most isolated islands possess their own peculiar species of
aërial mammals or bats. We can see why there should be some relation between
the presence of mammals, in a more or less modified condition, and the depth
of the sea between an island and the mainland. We can clearly see why all
the inhabitants of an archipelago, though specifically distinct on the
several islets, should be closely related to each other, and likewise be
related, but less closely, to those of the nearest continent or other source
whence immigrants were probably derived. We can see why in two areas,
however distant from each other, there should be a correlation, in the
presence of identical species, of varieties, of doubtful species, and of
distinct but representative species.

As the late Edward Forbes often insisted, there is a striking parallelism in
the laws of life throughout time and space: the laws governing the
succession of forms in past times being nearly the same with those governing
at the present time the differences in different areas. We see this in many
facts. The endurance of each species and group of species is continuous in
time; for the exceptions to the rule are so few, that they may fairly be
attributed to our not having as yet discovered in an intermediate deposit
the forms which are therein absent, but which occur above and below: so in
space, it certainly is the general rule that the area inhabited by a single
species, or by a group of species, is continuous; and the exceptions, which
are not rare, may, as I have attempted to show, be accounted for by
migration at some former period under different conditions or by occasional
means of transport, and by the species having become extinct in the
intermediate tracts. Both in time and space, species and groups of species
have their points of maximum development. Groups of species, belonging
either to a certain period of time, or to a certain area, are often
characterised by trifling characters in common, as of sculpture or colour.
In looking to the long succession of ages, as in now looking to distant
provinces throughout the world, we find that some organisms differ little,
whilst others belonging to a different class, or to a different order, or
even only to a different family of the same order, differ greatly. In both
time and space the lower members of each class generally change less than
the higher; but there are in both cases marked exceptions to the rule. On my
theory these several relations throughout time and space are intelligible;
for whether we look to the forms of life which have changed during
successive ages within the same quarter of the world, or to those which have
changed after having migrated into distant quarters, in both cases the forms
within each class have been connected by the same bond of ordinary
generation; and the more nearly any two forms are related in blood, the
nearer they will generally stand to each other in time and space; in both
cases the laws of variation have been the same, and modifications have been
accumulated by the same power of natural selection.

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Chapter 13 - Mutual Affinities of Organic Beings: Morphology: Embryology:
Rudimentary Organs

[* ] CLASSIFICATION, groups subordinate to groups [* ] Natural system [* ]
Rules and difficulties in classification, explained on the theory of descent
with modification [* ] Classification of varieties [* ] Descent always used
in classification [* ] Analogical or adaptive characters [* ] Affinities,
general, complex and radiating [* ] Extinction separates and defines groups
[* ] MORPHOLOGY, between members of the same class, between parts of the
same individual [* ] EMBRYOLOGY, laws of, explained by variations not
supervening at an early age, and being inherited at a corresponding age [* ]
RUDIMENTARY ORGANS; their origin explained [* ] Summary
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From the first dawn of life, all organic beings are found to resemble each
other in descending degrees, so that they can be classed in groups under
groups. This classification is evidently not arbitrary like the grouping of
the stars in constellations. The existence of groups would have been of
simple signification, if one group had been exclusively fitted to inhabit
the land, and another the water; one to feed on flesh, another on vegetable
matter, and so on; but the case is widely different in nature; for it is
notorious how commonly members of even the same subgroup have different
habits. In our second and fourth chapters, on Variation and on Natural
Selection, I have attempted to show that it is the widely ranging, the much
diffused and common, that is the dominant species belonging to the larger
genera, which vary most. The varieties, or incipient species, thus produced
ultimately become converted, as I believe, into new and distinct species;
and these, on the principle of inheritance, tend to produce other new and
dominant species. Consequently the groups which are now large, and which
generally include many dominant species, tend to go on increasing
indefinitely in size. I further attempted to show that from the varying
descendants of each species trying to occupy as many and as different places
as possible in the economy of nature, there is a constant tendency in their
characters to diverge. This conclusion was supported by looking at the great
diversity of the forms of life which, in any small area, come into the
closest competition, and by looking to certain facts in naturalisation.

I attempted also to show that there is a constant tendency in the forms
which are increasing in number and diverging in character, to supplant and
exterminate the less divergent, the less improved, and preceding forms. I
request the reader to turn to the diagram illustrating the action, as
formerly explained, of these several principles; and he will see that the
inevitable result is that the modified descendants proceeding from one
progenitor become broken up into groups subordinate to groups. In the
diagram each letter on the uppermost line may represent a genus including
several species; and all the genera on this line form together one class,
for all have descended from one ancient but unseen parent, and,
consequently, have inherited something in common. But the three genera on
the left hand have, on this same principle, much in common, and form a
sub-family, distinct from that including the next two genera on the right
hand, which diverged from a common parent at the fifth stage of descent.
These five genera have also much, though less, in common; and they form a
family distinct from that including the three genera still further to the
right hand, which diverged at a still earlier period. And all these genera,
descended from (A), form an order distinct from the genera descended from
(I). So that we here have many species descended from a single progenitor
grouped into genera; and the genera are included in, or subordinate to,
sub-families, families, and orders, all united into one class. Thus, the
grand fact in natural history of the subordination of group under group,
which, from its familiarity, does not always sufficiently strike us, is in
my judgement fully explained.

Naturalists try to arrange the species, genera, and families in each class,
on what is called the Natural System. But what is meant by this system? Some
authors look at it merely as a scheme for arranging together those living
objects which are most alike, and for separating those which are most
unlike; or as an artificial means for enunciating, as briefly as possible,
general propositions, that is, by one sentence to give the characters
common, for instance, to all mammals, by another those common to all
carnivora, by another those common to the dog-genus, and then by adding a
single sentence, a full description is given of each kind of dog. The
ingenuity and utility of this system are indisputable. But many naturalists
think that something more is meant by the Natural System; they believe that
it reveals the plan of the Creator; but unless it be specified whether order
in time or space, or what else is meant by the plan of the Creator, it seems
to me that nothing is thus added to our knowledge. Such expressions as that
famous one of Linnaeus, and which we often meet with in a more or less
concealed form, that the characters do not make the genus, but that the
genus gives the characters, seem to imply that something more is included in
our classification, than mere resemblance. I believe that something more is
included; and that propinquity of descent, the only known cause of the
similarity of organic beings, is the bond, hidden as it is by various
degrees of modification, which is partially revealed to us by our
classifications.

Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification either
gives some unknown plan of creation, or is simply a scheme for enunciating
general propositions and of placing together the forms most like each other.
It might have been thought (and was in ancient times thought) that those
parts of the structure which determined the habits of life, and the general
place of each being in the economy of nature, would be of very high
importance in classification. Nothing can be more false. No one regards the
external similarity of a mouse to a shrew, of a dugong to a whale, of a
whale to a fish, as of any importance. These resemblances, though so
intimately connected with the whole life of the being, are ranked as merely
`adaptive or analogical characters;' but to the consideration of these
resemblances we shall have to recur. It may even be given as a general rule,
that the less any part of the organisation is concerned with special habits,
the more important it becomes for classification. As an instance: Owen, in
speaking of the dugong, says, `The generative organs being those which are
most remotely related to the habits and food of an animal, I have always
regarded as affording very clear indications of its true affinities. We are
least likely in the modifications of these organs to mistake a merely
adaptive for an essential character.' So with plants, how remarkable it is
that the organs of vegetation, on which their whole life depends, are of
little signification, excepting in the first main divisions; whereas the
organs of reproduction, with their product the seed, are of paramount
importance!

We must not, therefore, in classifying, trust to resemblances in parts of
the organisation, however important they may be for the welfare of the being
in relation to the outer world. Perhaps from this cause it has partly
arisen, that almost all naturalists lay the greatest stress on resemblances
in organs of high vital or physiological importance. No doubt this view of
the classificatory importance of organs which are important is generally,
but by no means always, true. But their importance for classification, I
believe, depends on their greater constancy throughout large groups of
species; and this constancy depends on such organs having generally been
subjected to less change in the adaptation of the species to their
conditions of life. That the mere physiological importance of an organ does
not determine the classificatory value, is almost shown by the one fact,
that in allied groups, in which the same organ, as we have every reason to
suppose, has nearly the same physiological value, its classificatory value
is widely different. No naturalist can have worked at any group without
being struck with this fact; and it has been most fully acknowledged in the
writings of almost every author. It will suffice to quote the highest
authority, Robert Brown, who in speaking of certain organs in the
Proteaceae, says their generic importance, `like that of all their parts,
not only in this but, as I apprehend, in every natural family, is very
unequal, and in some cases seems to be entirely lost.' Again in another work
he says, the genera of the Connaraceae `differ in having one or more ovaria,
in the existence or absence of albumen, in the imbricate or valvular
aestivation. Any one of these characters singly is frequently of more than
generic importance, though here even when all taken together they appear
insufficient to separate Cnestis from Connarus.' To give an example amongst
insects, in one great division of the Hymenoptera, the antennae, as Westwood
has remarked, are most constant in structure; in another division they
differ much, and the differences are of quite subordinate value in
classification; yet no one probably will say that the antennae in these two
divisions of the same order are of unequal physiological importance. Any
number of instances could be given of the varying importance for
classification of the same important organ within the same group of beings.

Again, no one will say that rudimentary or atrophied organs are of high
physiological or vital importance; yet, undoubtedly, organs in this
condition are often of high value in classification. No one will dispute
that the rudimentary teeth in the upper jaws of young ruminants, and certain
rudimentary bones of the leg, are highly serviceable in exhibiting the close
affinity between Ruminants and Pachyderms. Robert Brown has strongly
insisted on the fact that the rudimentary florets are of the highest
importance in the classification of the Grasses.

Numerous instances could be given of characters derived from parts which
must be considered of very trifling physiological importance, but which are
universally admitted as highly serviceable in the definition of whole
groups. For instance, whether or not there is an open passage from the
nostrils to the mouth, the only character, according to Owen, which
absolutely distinguishes fishes and reptiles the inflection of the angle of
the jaws in Marsupials -- the manner in which the wings of insects are
folded mere colour in certain Algae mere pubescence on parts of the flower
in grasses the nature of the dermal covering, as hair or feathers, in the
Vertebrata. If the Ornithorhynchus had been covered with feathers instead of
hair, this external and trifling character would, I think, have been
considered by naturalists as important an aid in determining the degree of
affinity of this strange creature to birds and reptiles, as an approach in
structure in any one internal and important organ.

The importance, for classification, of trifling characters, mainly depends
on their being correlated with several other characters of more or less
importance. The value indeed of an aggregate of characters is very evident
in natural history. Hence, as has often been remarked, a species may depart
from its allies in several characters, both of high physiological importance
and of almost universal prevalence, and yet leave us in no doubt where it
should be ranked. Hence, also, it has been found, that a classification
founded on any single character, however important that may be, has always
failed; for no part of the organisation is universally constant. The
importance of an aggregate of characters, even when none are important,
alone explains, I think, that saying of Linnaeus, that the characters do not
give the genus, but the genus gives the characters; for this saying seems
founded on an appreciation of many trifling points of resemblance, too
slight to be defined. Certain plants, belonging to the Malpighiaceae, bear
perfect and degraded flowers; in the latter, as A. de Jussieu has remarked,
`the greater number of the characters proper to the species, to the genus,
to the family, to the class, disappear, and thus laugh at our
classification.' But when Aspicarpa produced in France, during several
years, only degraded flowers, departing so wonderfully in a number of the
most important points of structure from the proper type of the order, yet M.
Richard sagaciously saw, as Jussieu observes, that this genus should still
be retained amongst the Malpighiaceae. This case seems to me well to
illustrate the spirit with which our classifications are sometimes
necessarily founded.

Practically when naturalists are at work, they do not trouble themselves
about the physiological value of the characters which they use in defining a
group, or in allocating any particular species. If they find a character
nearly uniform, and common to a great number of forms, and not common to
others, they use it as one of high value; if common to some lesser number,
they use it as of subordinate value. This principle has been broadly
confessed by some naturalists to be the true one; and by none more clearly
than by that excellent botanist, Aug. St. Hilaire. If certain characters are
always found correlated with others, though no apparent bond of connexion
can be discovered between them, especial value is set on them. As in most
groups of animals, important organs, such as those for propelling the blood,
or for aërating it, or those for propagating the race, are found nearly
uniform, they are considered as highly serviceable in classification; but in
some groups of animals all these, the most important vital organs, are found
to offer characters of quite subordinate value.

We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for our classifications of
course include all ages of each species. But it is by no means obvious, on
the ordinary view, why the structure of the embryo should be more important
for this purpose than that of the adult, which alone plays its full part in
the economy of nature. Yet it has been strongly urged by those great
naturalists, Milne Edwards and Agassiz, that embryonic characters are the
most important of any in the classification of animals; and this doctrine
has very generally been admitted as true. The same fact holds good with
flowering plants, of which the two main divisions have been founded on
characters derived from the embryo, on the number and position of the
embryonic leaves or cotyledons, and on the mode of development of the
plumule and radicle. In our discussion on embryology, we shall see why such
characters are so valuable, on the view of classification tacitly including
the idea of descent.

Our classifications are often plainly influenced by chains of affinities.
Nothing can be easier than to define a number of characters common to all
birds; but in the case of crustaceans, such definition has hitherto been
found impossible. There are crustaceans at the opposite ends of the series,
which have hardly a character in common; yet the species at both ends, from
being plainly allied to others, and these to others, and so onwards, can be
recognised as unequivocally belonging to this, and to no other class of the
Articulata.

Geographical distribution has often been used, though perhaps not quite
logically, in classification, more especially in very large groups of
closely allied forms. Temminck insists on the utility or even necessity of
this practice in certain groups of birds; and it has been followed by
several entomologists and botanists.

Finally, with respect to the comparative value of the various groups of
species, such as orders, sub-orders, families, sub-families, and genera,
they seem to be, at least at present, almost arbitrary. Several of the best
botanists, such as Mr Bentham and others, have strongly insisted on their
arbitrary value. Instances could be given amongst plants and insects, of a
group of forms, first ranked by practised naturalists as only a genus, and
then raised to the rank of a sub-family or family; and this has been done,
not because further research has detected important structural differences,
at first overlooked, but because numerous allied species, with slightly
different grades of difference, have been subsequently discovered.

All the foregoing rules and aids and difficulties in classification are
explained, if I do not greatly deceive myself, on the view that the natural
system is founded on descent with modification; that the characters which
naturalists consider as showing true affinity between any two or more
species, are those which have been inherited from a common parent, and, in
so far, all true classification is genealogical; that community of descent
is the hidden bond which naturalists have been unconsciously seeking, and
not some unknown plan of creation, or the enunciation of general
propositions, and the mere putting together and separating objects more or
less alike.

But I must explain my meaning more fully. I believe that the arrangement of
the groups within each class, in due subordination and relation to the other
groups, must be strictly genealogical in order to be natural; but that the
amount of difference in the several branches or groups, though allied in the
same degree in blood to their common progenitor, may differ greatly, being
due to the different degrees of modification which they have undergone; and
this is expressed by the forms being ranked under different genera,
families, sections, or orders. The reader will best understand what is
meant, if he will take the trouble of referring to the diagram in the fourth
chapter. We will suppose the letters A to L to represent allied genera,
which lived during the Silurian epoch, and these have descended from a
species which existed at an unknown anterior period. Species of three of
these genera (A, F, and I) have transmitted modified descendants to the
present day, represented by the fifteen genera (a14 to z14) on the uppermost
horizontal line. Now all these modified descendants from a single species,
are represented as related in blood or descent to the same degree; they may
metaphorically be called cousins to the same millionth degree; yet they
differ widely and in different degrees from each other. The forms descended
from A, now broken up into two or three families, constitute a distinct
order from those descended from I, also broken up into two families. Nor can
the existing species, descended from A, be ranked in the same genus with the
parent A; or those from I, with the parent I. But the existing genus F14 may
be supposed to have been but slightly modified; and it will then rank with
the parent-genus F; just as some few still living organic beings belong to
Silurian genera. So that the amount or value of the differences between
organic beings all related to each other in the same degree in blood, has
come to be widely different. Nevertheless their genealogical arrangement
remains strictly true, not only at the present time, but at each successive
period of descent. All the modified descendants from A will have inherited
something in common from their common parent, as will all the descendants
from I; so will it be with each subordinate branch of descendants, at each
successive period. If, however, we choose to suppose that any of the
descendants of A or of I have been so much modified as to have more or less
completely lost traces of their parentage, in this case, their places in a
natural classification will have been more or less completely lost, as
sometimes seems to have occurred with existing organisms. All the
descendants of the genus F, along its whole line of descent, are supposed to
have been but little modified, and they yet form a single genus. But this
genus, though much isolated, will still occupy its proper intermediate
position; for F originally was intermediate in character between A and I,
and the several genera descended from these two genera will have inherited
to a certain extent their characters. This natural arrangement is shown, as
far as is possible on paper, in the diagram, but in much too simple a
manner. If a branching diagram had not been used, and only the names of the
groups had been written in a linear series, it would have been still less
possible to have given a natural arrangement; and it is notoriously not
possible to represent in a series, on a flat surface, the affinities which
we discover in nature amongst the beings of the same group. Thus, on the
view which I hold, the natural system is genealogical in its arrangement,
like a pedigree; but the degrees of modification which the different groups
have undergone, have to be expressed by ranking them under different
so-called genera, sub-families, families, sections, orders, and classes.

It may be worth while to illustrate this view of classification, by taking
the case of languages. If we possessed a perfect pedigree of mankind, a
genealogical arrangement of the races of man would afford the best
classification of the various languages now spoken throughout the world; and
if all extinct languages, and all intermediate and slowly changing dialects,
had to be included, such an arrangement would, I think, be the only possible
one. Yet it might be that some very ancient language had altered little, and
had given rise to few new languages, whilst others (owing to the spreading
and subsequent isolation and states of civilisation of the several races,
descended from a common race) had altered much, and had given rise to many
new languages and dialects. The various degrees of difference in the
languages from the same stock, would have to be expressed by groups
subordinate to groups; but the proper or even only possible arrangement
would still be genealogical; and this would be strictly natural, as it would
connect together all languages, extinct and modern, by the closest
affinities, and would give the filiation and origin of each tongue.

In confirmation of this view, let us glance at the classification of
varieties, which are believed or known to have descended from one species.
These are grouped under species, with sub-varieties under varieties; and
with our domestic productions, several other grades of difference are
requisite, as we have seen with pigeons. The origin of the existence of
groups subordinate to groups, is the same with varieties as with species,
namely, closeness of descent with various degrees of modification. Nearly
the same rules are followed in classifying varieties, as with species.
Authors have insisted on the necessity of classing varieties on a natural
instead of an artificial system; we are cautioned, for instance, not to
class two varieties of the pine-apple together, merely because their fruit,
though the most important part, happens to be nearly identical; no one puts
the swedish and common turnips together, though the esculent and thickened
stems are so similar. Whatever part is found to be most constant, is used in
classing varieties: thus the great agriculturist Marshall says the horns are
very useful for this purpose with cattle, because they are less variable
than the shape or colour of the body, &c.; whereas with sheep the horns are
much less serviceable, because less constant. In classing varieties, I
apprehend if we had a real pedigree, a genealogical classification would be
universally preferred; and it has been attempted by some authors. For we
might feel sure, whether there had been more or less modification, the
principle of inheritance would keep the forms together which were allied in
the greatest number of points. In tumbler pigeons, though some sub-varieties
differ from the others in the important character of having a longer beak,
yet all are kept together from having the common habit of tumbling; but the
short-faced breed has nearly or quite lost this habit; nevertheless, without
any reasoning or thinking on the subject, these tumblers are kept in the
same group, because allied in blood and alike in some other respects. If it
could be proved that the Hottentot had descended from the Negro, I think he
would be classed under the Negro group, however much he might differ in
colour and other important characters from negroes.

With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, or
that of a species, the two sexes; and how enormously these sometimes differ
in the most important characters, is known to every naturalist: scarcely a
single fact can be predicated in common of the males and hermaphrodites of
certain cirripedes, when adult, and yet no one dreams of separating them.
The naturalist includes as one species the several larval stages of the same
individual, however much they may differ from each other and from the adult;
as he likewise includes the so-called alternate generations of Steenstrup,
which can only in a technical sense be considered as the same individual. He
includes monsters; he includes varieties, not solely because they closely
resemble the parent-form, but because they are descended from it. He who
believes that the cowslip is descended from the primrose, or conversely,
ranks them together as a single species, and gives a single definition. As
soon as three Orchidean forms (Monochanthus, Myanthus, and Catasetum), which
had previously been ranked as three distinct genera, were known to be
sometimes produced on the same spike, they were immediately included as a
single species. But it may be asked, what ought we to do, if it could be
proved that one species of kangaroo had been produced, by a long course of
modification, from a bear? Ought we to rank this one species with bears, and
what should we do with the other species? The supposition is of course
preposterous; and I might answer by the argumentum ad hominem, and ask what
should be done if a perfect kangaroo were seen to come out of the womb of a
bear? According to all analogy, it would be ranked with bears; but then
assuredly all the other species of the kangaroo family would have to be
classed under the bear genus. The whole case is preposterous; for where
there has been close descent in common, there will certainly be close
resemblance or affinity.

As descent has universally been used in classing together the individuals of
the same species, though the males and females and larvae are sometimes
extremely different; and as it has been used in classing varieties which
have undergone a certain, and sometimes a considerable amount of
modification, may not this same element of descent have been unconsciously
used in grouping species under genera, and genera under higher groups,
though in these cases the modification has been greater in degree, and has
taken a longer time to complete? I believe it has thus been unconsciously
used; and only thus can I understand the several rules and guides which have
been followed by our best systematists. We have no written pedigrees; we
have to make out community of descent by resemblances of any kind. Therefore
we choose those characters which, as far as we can judge, are the least
likely to have been modified in relation to the conditions of life to which
each species has been recently exposed. Rudimentary structures on this view
are as good as, or even sometimes better than, other parts of the
organisation. We care not how trifling a character may be let it be the mere
inflection of the angle of the jaw, the manner in which an insect's wing is
folded, whether the skin be covered by hair or feathers if it prevail
throughout many and different species, especially those having very
different habits of life, it assumes high value; for we can account for its
presence in so many forms with such different habits, only by its
inheritance from a common parent. We may err in this respect in regard to
single points of structure, but when several characters, let them be ever so
trifling, occur together throughout a large group of beings having different
habits, we may feel almost sure, on the theory of descent, that these
characters have been inherited from a common ancestor. And we know that such
correlated or aggregated characters have especial value in classification.

We can understand why a species or a group of species may depart, in several
of its most important characteristics, from its allies, and yet be safely
classed with them. This may be safely done, and is often done, as long as a
sufficient number of characters, let them be ever so unimportant, betrays
the hidden bond of community of descent. Let two forms have not a single
character in common, yet if these extreme forms are connected together by a
chain of intermediate groups, we may at once infer their community of
descent, and we put them all into the same class. As we find organs of high
physiological importance those which serve to preserve life under the most
diverse conditions of existence are generally the most constant, we attach
especial value to them; but if these same organs, in another group or
section of a group, are found to differ much, we at once value them less in
our classification. We shall hereafter, I think, clearly see why
embryological characters are of such high classificatory importance.
Geographical distribution may sometimes be brought usefully into play in
classing large and widely-distributed genera, because all the species of the
same genus, inhabiting any distinct and isolated region, have in all
probability descended from the same parents.

We can understand, on these views, the very important distinction between
real affinities and analogical or adaptive resemblances. Lamarck first
called attention to this distinction, and he has been ably followed by
Macleay and others. The resemblance, in the shape of the body and in the
fin-like anterior limbs, between the dugong, which is a pachydermatous
animal, and the whale, and between both these mammals and fishes, is
analogical. Amongst insects there are innumerable instances: thus Linnaeus,
misled by external appearances, actually classed an homopterous insect as a
moth. We see something of the same kind even in our domestic varieties, as
in the thickened stems of the common and swedish turnip. The resemblance of
the greyhound and racehorse is hardly more fanciful than the analogies which
have been drawn by some authors between very distinct animals. On my view of
characters being of real importance for classification, only in so far as
they reveal descent, we can clearly understand why analogical or adaptive
character, although of the utmost importance to the welfare of the being,
are almost valueless to the systematist. For animals, belonging to two most
distinct lines of descent, may readily become adapted to similar conditions,
and thus assume a close external resemblance; but such resemblances will not
reveal will rather tend to conceal their blood-relationship to their proper
lines of descent. We can also understand the apparent paradox, that the very
same characters are analogical when one class or order is compared with
another, but give true affinities when the members of the same class or
order are compared one with another: thus the shape of the body and fin-like
limbs are only analogical when whales are compared with fishes, being
adaptations in both classes for swimming through the water; but the shape of
the body and fin-like limbs serve as characters exhibiting true affinity
between the several members of the whale family; for these cetaceans agree
in so many characters, great and small, that we cannot doubt that they have
inherited their general shape of body and structure of limbs from a common
ancestor. So it is with fishes.

As members of distinct classes have often been adapted by successive slight
modifications to live under nearly similar circumstances, to inhabit for
instance the three elements of land, air, and water, we can perhaps
understand how it is that a numerical parallelism has sometimes been
observed between the sub-groups in distinct classes. A naturalist, struck by
a parallelism of this nature in any one class, by arbitrarily raising or
sinking the value of the groups in other classes (and all our experience
shows that this valuation has hitherto been arbitrary), could easily extend
the parallelism over a wide range; and thus the septenary, quinary,
quaternary, and ternary classifications have probably arisen.

As the modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages, which made the groups to which they
belong large and their parents dominant, they are almost sure to spread
widely, and to seize on more and more places in the economy of nature. The
larger and more dominant groups thus tend to go on increasing in size; and
they consequently supplant many smaller and feebler groups. Thus we can
account for the fact that all organisms, recent and extinct, are included
under a few great orders, under still fewer classes, and all in one great
natural system. As showing how few the higher groups are in number, and how
widely spread they are throughout the world, the fact is striking, that the
discovery of Australia has not added a single insect belonging to a new
order; and that in the vegetable kingdom, as I learn from Dr. Hooker, it has
added only two or three orders of small size.

In the chapter on geological succession I attempted to show, on the
principle of each group having generally diverged much in character during
the long-continued process of modification, how it is that the more ancient
forms of life often present characters in some slight degree intermediate
between existing groups. A few old and intermediate parent-forms having
occasionally transmitted to the present day descendants but little modified,
will give to us our so-called osculant or aberrant groups. The more aberrant
any form is, the greater must be the number of connecting forms which on my
theory have been exterminated and utterly lost. And we have some evidence of
aberrant forms having suffered severely from extinction, for they are
generally represented by extremely few species; and such species as do occur
are generally very distinct from each other, which again implies extinction.
The genera Ornithorhynchus and Lepidosiren, for example, would not have been
less aberrant had each been represented by a dozen species instead of by a
single one; but such richness in species, as I find after some
investigation, does not commonly fall to the lot of aberrant genera. We can,
I think, account for this fact only by looking at aberrant forms as failing
groups conquered by more successful competitors, with a few members
preserved by some unusual coincidence of favourable circumstances.

Mr. Waterhouse has remarked that, when a member belonging to one group of
animals exhibits an affinity to a quite distinct group, this affinity in
most cases is general and not special: thus, according to Mr. Waterhouse, of
all Rodents, the bizcacha is most nearly related to Marsupials; but in the
points in which it approaches this order, its relations are general, and not
to any one marsupial species more than to another. As the points of affinity
of the bizcacha to Marsupials are believed to be real and not merely
adaptive, they are due on my theory to inheritance in common. Therefore we
must suppose either that all Rodents, including the bizcacha, branched off
from some very ancient Marsupial, which will have had a character in some
degree intermediate with respect to all existing Marsupials; or that both
Rodents and Marsupials branched off from a common progenitor, and that both
groups have since undergone much modification in divergent directions. On
either view we may suppose that the bizcacha has retained, by inheritance,
more of the character of its ancient progenitor than have other Rodents; and
therefore it will not be specially related to any one existing Marsupial,
but indirectly to all or nearly all Marsupials, from having partially
retained the character of their common progenitor, or of an early member of
the group. On the other hand, of all Marsupials, as Mr. Waterhouse has
remarked, the phascolomys resembles most nearly, not any one species, but
the general order of Rodents. In this case, however, it may be strongly
suspected that the resemblance is only analogical, owing to the phascolomys
having become adapted to habits like those of a Rodent. The elder De
Candolle has made nearly similar observations on the general nature of the
affinities of distinct orders of plants.

On the principle of the multiplication and gradual divergence in character
of the species descended from a common parent, together with their retention
by inheritance of some characters in common, we can understand the
excessively complex and radiating affinities by which all the members of the
same family or higher group are connected together. For the common parent of
a whole family of species, now broken up by extinction into distinct groups
and sub-groups, will have transmitted some of its characters, modified in
various ways and degrees, to all; and the several species will consequently
be related to each other by circuitous lines of affinity of various lengths
(as may be seen in the diagram so often referred to), mounting up through
many predecessors. As it is difficult to show the blood-relationship between
the numerous kindred of any ancient and noble family, even by the aid of a
genealogical tree, and almost impossible to do this without this aid, we can
understand the extraordinary difficulty which naturalists have experienced
in describing, without the aid of a diagram, the various affinities which
they perceive between the many living and extinct members of the same great
natural class.

Extinction, as we have seen in the fourth chapter, has played an important
part in defining and widening the intervals between the several groups in
each class. We may thus account even for the distinctness of whole classes
from each other for instance, of birds from all other vertebrate animals by
the belief that many ancient forms of life have been utterly lost, through
which the early progenitors of birds were formerly connected with the early
progenitors of the other vertebrate classes. There has been less entire
extinction of the forms of life which once connected fishes with
batrachians. There has been still less in some other classes, as in that of
the Crustacea, for here the most wonderfully diverse forms are still tied
together by a long, but broken, chain of affinities. Extinction has only
separated groups: it has by no means made them; for if every form which has
ever lived on this earth were suddenly to reappear, though it would be quite
impossible to give definitions by which each group could be distinguished
from other groups, as all would blend together by steps as fine as those
between the finest existing varieties, nevertheless a natural
classification, or at least a natural arrangement, would be possible. We
shall see this by turning to the diagram: the letters, A to L, may represent
eleven Silurian genera, some of which have produced large groups of modified
descendants. Every intermediate link between these eleven genera and their
primordial parent, and every intermediate link in each branch and sub-branch
of their descendants, may be supposed to be still alive; and the links to be
as fine as those between the finest varieties. In this case it would be
quite impossible to give any definition by which the several members of the
several groups could be distinguished from their more immediate parents; or
these parents from their ancient and unknown progenitor. Yet the natural
arrangement in the diagram would still hold good; and, on the principle of
inheritance, all the forms descended from A, or from I, would have something
in common. In a tree we can specify this or that branch, though at the
actual fork the two unite and blend together. We could not, as I have said,
define the several groups; but we could pick out types, or forms,
representing most of the characters of each group, whether large or small,
and thus give a general idea of the value of the differences between them.
This is what we should be driven to, if we were ever to succeed in
collecting all the forms in any class which have lived throughout all time
and space. We shall certainly never succeed in making so perfect a
collection: nevertheless, in certain classes, we are tending in this
direction; and Milne Edwards has lately insisted, in an able paper, on the
high importance of looking to types, whether or not we can separate and
define the groups to which such types belong.

Finally, we have seen that natural selection, which results from the
struggle for existence, and which almost inevitably induces extinction and
divergence of character in the many descendants from one dominant
parent-species, explains that great and universal feature in the affinities
of all organic beings, namely, their subordination in group under group. We
use the element of descent in classing the individuals of both sexes and of
all ages, although having few characters in common, under one species; we
use descent in classing acknowledged varieties, however different they may
be from their parent; and I believe this element of descent is the hidden
bond of connexion which naturalists have sought under the term of the
Natural System. On this idea of the natural system being, in so far as it
has been perfected, genealogical in its arrangement, with the grades of
difference between the descendants from a common parent, expressed by the
terms genera, families, orders, &c., we can understand the rules which we
are compelled to follow in our classification. We can understand why we
value certain resemblances far more than others; why we are permitted to use
rudimentary and useless organs, or others of trifling physiological
importance; why, in comparing one group with a distinct group, we summarily
reject analogical or adaptive characters, and yet use these same characters
within the limits of the same group. We can clearly see how it is that all
living and extinct forms can be grouped together in one great system; and
how the several members of each class are connected together by the most
complex and radiating lines of affinities. We shall never, probably,
disentangle the inextricable web of affinities between the members of any
one class; but when we have a distinct object in view, and do not look to
some unknown plan of creation, we may hope to make sure but slow progress.

Morphology

We have seen that the members of the same class, independently of their
habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term `unity of
type;' or by saying that the several parts and organs in the different
species of the class are homologous. The whole subject is included under the
general name of Morphology. This is the most interesting department of
natural history, and may be said to be its very soul. What can be more
curious than that the hand of a man, formed for grasping, that of a mole for
digging, the leg of the horse, the paddle of the porpoise, and the wing of
the bat, should all be constructed on the same pattern, and should include
the same bones, in the same relative positions? Geoffroy St Hilaire has
insisted strongly on the high importance of relative connexion in homologous
organs: the parts may change to almost any extent in form and size, and yet
they always remain connected together in the same order. We never find, for
instance, the bones of the arm and forearm, or of the thigh and leg,
transposed. Hence the same names can be given to the homologous bones in
widely different animals. We see the same great law in the construction of
the mouths of insects: what can be more different than the immensely long
spiral proboscis of a sphinx-moth, the curious folded one of a bee or bug,
and the great jaws of a beetle? yet all these organs, serving for such
different purposes, are formed by infinitely numerous modifications of an
upper lip, mandibles, and two pairs of maxillae. Analogous laws govern the
construction of the mouths and limbs of crustaceans. So it is with the
flowers of plants.

Nothing can be more hopeless than to attempt to explain this similarity of
pattern in members of the same class, by utility or by the doctrine of final
causes. The hopelessness of the attempt has been expressly admitted by Owen
in his most interesting work on the `Nature of Limbs.' On the ordinary view
of the independent creation of each being, we can only say that so it is;
that it has so pleased the Creator to construct each animal and plant.

The explanation is manifest on the theory of the natural selection of
successive slight modifications, each modification being profitable in some
way to the modified form, but often affecting by correlation of growth other
parts of the organisation. In changes of this nature, there will be little
or no tendency to modify the original pattern, or to transpose parts. The
bones of a limb might be shortened and widened to any extent, and become
gradually enveloped in thick membrane, so as to serve as a fin; or a webbed
foot might have all its bones, or certain bones, lengthened to any extent,
and the membrane connecting them increased to any extent, so as to serve as
a wing: yet in all this great amount of modification there will be no
tendency to alter the framework of bones or the relative connexion of the
several parts. If we suppose that the ancient progenitor, the archetype as
it may be called, of all mammals, had its limbs constructed on the existing
general pattern, for whatever purpose they served, we can at once perceive
the plain signification of the homologous construction of the limbs
throughout the whole class. So with the mouths of insects, we have only to
suppose that their common progenitor had an upper lip, mandibles, and two
pair of maxillae, these parts being perhaps very simple in form; and then
natural selection will account for the infinite diversity in structure and
function of the mouths of insects. Nevertheless, it is conceivable that the
general pattern of an organ might become so much obscured as to be finally
lost, by the atrophy and ultimately by the complete abortion of certain
parts, by the soldering together of other parts, and by the doubling or
multiplication of others, variations which we know to be within the limits
of possibility. In the paddles of the extinct gigantic sea-lizards, and in
the mouths of certain suctorial crustaceans, the general pattern seems to
have been thus to a certain extent obscured.

There is another and equally curious branch of the present subject; namely,
the comparison not of the same part in different members of a class, but of
the different parts or organs in the same individual. Most physiologists
believe that the bones of the skull are homologous with that is correspond
in number and in relative connexion with the elemental parts of a certain
number of vertebrae. The anterior and posterior limbs in each member of the
vertebrate and articulate classes are plainly homologous. We see the same
law in comparing the wonderfully complex jaws and legs in crustaceans. It is
familiar to almost every one, that in a flower the relative position of the
sepals, petals, stamens, and pistils, as well as their intimate structure,
are intelligible in the view that they consist of metamorphosed leaves,
arranged in a spire. In monstrous plants, we often get direct evidence of
the possibility of one organ being transformed into another; and we can
actually see in embryonic crustaceans and in many other animals, and in
flowers, that organs which when mature become extremely different, are at an
early stage of growth exactly alike.

How inexplicable are these facts on the ordinary view of creation! Why
should the brain be enclosed in a box composed of such numerous and such
extraordinarily shaped pieces of bone? As Owen has remarked, the benefit
derived from the yielding of the separate pieces in the act of parturition
of mammals, will by no means explain the same construction in the skulls of
birds. Why should similar bones have been created in the formation of the
wing and leg of a bat, used as they are for such totally different purposes?
Why should one crustacean, which has an extremely complex mouth formed of
many parts, consequently always have fewer legs; or conversely, those with
many legs have simpler mouths? Why should the sepals, petals, stamens, and
pistils in any individual flower, though fitted for such widely different
purposes, be all constructed on the same pattern ?

On the theory of natural selection, we can satisfactorily answer these
questions. In the vertebrata, we see a series of internal vertebrae bearing
certain processes and appendages; in the articulata, we see the body divided
into a series of segments, bearing external appendages; and in flowering
plants, we see a series of successive spiral whorls of leaves. An indefinite
repetition of the same part or organ is the common characteristic (as Owen
has observed) of all low or little-modified forms; therefore we may readily
believe that the unknown progenitor of the vertebrata possessed many
vertebrae; the unknown progenitor of the articulata, many segments; and the
unknown progenitor of flowering plants, many spiral whorls of leaves. We
have formerly seen that parts many times repeated are eminently liable to
vary in number and structure; consequently it is quite probable that natural
selection, during a long-continued course of modification, should have
seized on a certain number of the primordially similar elements, many times
repeated, and have adapted them to the most diverse purposes. And as the
whole amount of modification will have been effected by slight successive
steps, we need not wonder at discovering in such parts or organs, a certain
degree of fundamental resemblance, retained by the strong principle of
inheritance.

In the great class of molluscs, though we can homologise the parts of one
species with those of another and distinct species, we can indicate but few
serial homologies; that is, we are seldom enabled to say that one part or
organ is homologous with another in the same individual. And we can
understand this fact; for in molluscs, even in the lowest members of the
class, we do not find nearly so much indefinite repetition of any one part,
as we find in the other great classes of the animal and vegetable kingdoms.

Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae: the jaws of crabs as metamorphosed legs; the stamens and pistils
of flowers as metamorphosed leaves; but it would in these cases probably be
more correct, as Professor Huxley has remarked, to speak of both skull and
vertebrae, both jaws and legs, &c., as having been metamorphosed, not one
from the other, but from some common element. Naturalists, however, use such
language only in a metaphorical sense: they are far from meaning that during
a long course of descent, primordial organs of any kind vertebrae in the one
case and legs in the other have actually been modified into skulls or jaws.
Yet so strong is the appearance of a modification of this nature having
occurred, that naturalists can hardly avoid employing language having this
plain signification. On my view these terms may be used literally; and the
wonderful fact of the jaws, for instance, of a crab retaining numerous
characters, which they would probably have retained through inheritance, if
they had really been metamorphosed during a long course of descent from true
legs, or from some simple appendage, is explained.

Embryology

It has already been casually remarked that certain organs in the individual,
which when mature become widely different and serve for different purposes,
are in the embryo exactly alike. The embryos, also, of distinct animals
within the same class are often strikingly similar: a better proof of this
cannot be given, than a circumstance mentioned by Agassiz, namely, that
having forgotten to ticket the embryo of some vertebrate animal, he cannot
now tell whether it be that of a mammal, bird, or reptile. The vermiform
larvae of moths, flies, beetles, &c., resemble each other much more closely
than do the mature insects; but in the case of larvae, the embryos are
active, and have been adapted for special lines of life. A trace of the law
of embryonic resemblance, sometimes lasts till a rather late age: thus birds
of the same genus, and of closely allied genera, often resemble each other
in their first and second plumage; as we see in the spotted feathers in the
thrush group. In the cat tribe, most of the species are striped or spotted
in lines; and stripes can be plainly distinguished in the whelp of the lion.
We occasionally though rarely see something of this kind in plants: thus the
embryonic leaves of the ulex or furze, and the first leaves of the
phyllodineous acaceas, are pinnate or divided like the ordinary leaves of
the leguminosae.

The points of structure, in which the embryos of widely different animals of
the same class resemble each other, often have no direct relation to their
conditions of existence. We cannot, for instance, suppose that in the
embryos of the vertebrata the peculiar loop-like course of the arteries near
the branchial slits are related to similar conditions, in the young mammal
which is nourished in the womb of its mother, in the egg of the bird which
is hatched in a nest, and in the spawn of a frog under water. We have no
more reason to believe in such a relation, than we have to believe that the
same bones in the hand of a man, wing of a bat, and fin of a porpoise, are
related to similar conditions of life. No one will suppose that the stripes
on the whelp of a lion, or the spots on the young blackbird, are of any use
to these animals, or are related to the conditions to which they are
exposed.

The case, however, is different when an animal during any part of its
embryonic career is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on, the
adaptation of the larva to its conditions of life is just as perfect and as
beautiful as in the adult animal. From such special adaptations, the
similarity of the larvae or active embryos of allied animals is sometimes
much obscured; and cases could be given of the larvae of two species, or of
two groups of species, differing quite as much, or even more, from each
other than do their adult parents. In most cases, however, the larvae,
though active, still obey more or less closely the law of common embryonic
resemblance. Cirripedes afford a good instance of this: even the illustrious
Cuvier did not perceive that a barnacle was, as it certainly is, a
crustacean; but a glance at the larva shows this to be the case in an
unmistakeable manner. So again the two main divisions of cirripedes, the
pedunculated and sessile, which differ widely in external appearance, have
larvae in all their several stages barely distinguishable.

The embryo in the course of development generally rises in organisation: I
use this expression, though I am aware that it is hardly possible to define
clearly what is meant by the organisation being higher or lower. But no one
probably will dispute that the butterfly is higher than the caterpillar. In
some cases, however, the mature animal is generally considered as lower in
the scale than the larva, as with certain parasitic crustaceans. To refer
once again to cirripedes: the larvae in the first stage have three pairs of
legs, a very simple single eye, and a probosciformed mouth, with which they
feed largely, for they increase much in size. In the second stage, answering
to the chrysalis stage of butterflies, they have six pairs of beautifully
constructed natatory legs, a pair of magnificent compound eyes, and
extremely complex antennae; but they have a closed and imperfect mouth, and
cannot feed: their function at this stage is, to search by their
well-developed organs of sense, and to reach by their active powers of
swimming, a proper place on which to become attached and to undergo their
final metamorphosis. When this is completed they are fixed for life: their
legs are now converted into prehensile organs; they again obtain a
well-constructed mouth; but they have no antennae, and their two eyes are
now reconverted into a minute, single, and very simple eye-spot. In this
last and complete state, cirripedes may be considered as either more highly
or more lowly organised than they were in the larval condition. But in some
genera the larvae become developed either into hermaphrodites having the
ordinary structure, or into what I have called complemental males: and in
the latter, the development has assuredly been retrograde; for the male is a
mere sack, which lives for a short time, and is destitute of mouth, stomach,
or other organ of importance, excepting for reproduction.

We are so much accustomed to see differences in structure between the embryo
and the adult, and likewise a close similarity in the embryos of widely
different animals within the same class, that we might be led to look at
these facts as necessarily contingent in some manner on growth. But there is
no obvious reason why, for instance, the wing of a bat, or the fin of a
porpoise, should not have been sketched out with all the parts in proper
proportion, as soon as any structure became visible in the embryo. And in
some whole groups of animals and in certain members of other groups, the
embryo does not at any period differ widely from the adult: thus Owen has
remarked in regard to cuttle-fish, `there is no metamorphosis; the
cephalopodic character is manifested long before the parts of the embryo are
completed;' and again in spiders, `there is nothing worthy to be called a
metamorphosis.' The larvae of insects, whether adapted to the most diverse
and active habits, or quite inactive, being fed by their parents or placed
in the midst of proper nutriment, yet nearly all pass through a similar
worm-like stage of development; but in some few cases, as in that of Aphis,
if we look to the admirable drawings by Professor Huxley of the development
of this insect, we see no trace of the vermiform stage.

How, then, can we explain these several facts in embryology, namely the very
general, but not universal difference in structure between the embryo and
the adult; of parts in the same individual embryo, which ultimately become
very unlike and serve for diverse purposes, being at this early period of
growth alike; of embryos of different species within the same class,
generally, but not universally, resembling each other; of the structure of
the embryo not being closely related to its conditions of existence, except
when the embryo becomes at any period of life active and has to provide for
itself; of the embryo apparently having sometimes a higher organisation than
the mature animal, into which it is developed. I believe that all these
facts can be explained, as follows, on the view of descent with
modification.

It is commonly assumed, perhaps from monstrosities often affecting the
embryo at a very early period, that slight variations necessarily appear at
an equally early period. But we have little evidence on this head indeed the
evidence rather points the other way; for it is notorious that breeders of
cattle, horses, and various fancy animals, cannot positively tell, until
some time after the animal has been born, what its merits or form will
ultimately turn out. We see this plainly in our own children; we cannot
always tell whether the child will be tall or short, or what its precise
features will be. The question is not, at what period of life any variation
has been caused, but at what period it is fully displayed. The cause may
have acted, and I believe generally has acted, even before the embryo is
formed; and the variation may be due to the male and female sexual elements
having been affected by the conditions to which either parent, or their
ancestors, have been exposed. Nevertheless an effect thus caused at a very
early period, even before the formation of the embryo, may appear late in
life; as when an hereditary disease, which appears in old age alone, has
been communicated to the offspring from the reproductive element of one
parent. Or again, as when the horns of cross-bred cattle have been affected
by the shape of the horns of either parent. For the welfare of a very young
animal, as long as it remains in its mother's womb, or in the egg, or as
long as it is nourished and protected by its parent, it must be quite
unimportant whether most of its characters are fully acquired a little
earlier or later in life. It would not signify, for instance, to a bird
which obtained its food best by having a long beak, whether or not it
assumed a beak of this particular length, as long as it was fed by its
parents. Hence, I conclude, that it is quite possible, that each of the many
successive modifications, by which each species has acquired its present
structure, may have supervened at a not very early period of life; and some
direct evidence from our domestic animals supports this view. But in other
cases it is quite possible that each successive modification, or most of
them, may have appeared at an extremely early period.

I have stated in the first chapter, that there is some evidence to render it
probable, that at whatever age any variation first appears in the parent, it
tends to reappear at a corresponding age in the offspring. Certain
variations can only appear at corresponding ages, for instance,
peculiarities in the caterpillar, cocoon, or imago states of the silk-moth;
or, again, in the horns of almost full-grown cattle. But further than this,
variations which, for all that we can see, might have appeared earlier or
later in life, tend to appear at a corresponding age in the offspring and
parent. I am far from meaning that this is invariably the case; and I could
give a good many cases of variations (taking the word in the largest sense)
which have supervened at an earlier age in the child than in the parent.

These two principles, if their truth be admitted, will, I believe, explain
all the above specified leading facts in embryology. But first let us look
at a few analogous cases in domestic varieties. Some authors who have
written on Dogs, maintain that the greyhound and bulldog, though appearing
so different, are really varieties most closely allied, and have probably
descended from the same wild stock; hence I was curious to see how far their
puppies differed from each other: I was told by breeders that they differed
just as much as their parents, and this, judging by the eye, seemed almost
to be the case; but on actually measuring the old dogs and their six-days
old puppies, I found that the puppies had not nearly acquired their full
amount of proportional difference. So, again, I was told that the foals of
cart and race-horses differed as much as the full-grown animals; and this
surprised me greatly, as I think it probable that the difference between
these two breeds has been wholly caused by selection under domestication;
but having had careful measurements made of the dam and of a three-days old
colt of a race and heavy cart-horse, I find that the colts have by no means
acquired their full amount of proportional difference.

As the evidence appears to me conclusive, that the several domestic breeds
of pigeon have descended from one wild species, I compared young pigeons of
various breeds, within twelve hours after being hatched; I carefully
measured the proportions (but will not here give details) of the beak, width
of mouth, length of nostril and of eyelid, size of feet and length of leg,
in the wild stock, in pouters, fantails, runts, barbs, dragons, carriers,
and tumblers. Now some of these birds, when mature, differ so
extraordinarily in length and form of beak, that they would, I cannot doubt,
be ranked in distinct genera, had they been natural productions. But when
the nestling birds of these several breeds were placed in a row, though most
of them could be distinguished from each other, yet their proportional
differences in the above specified several points were incomparably less
than in the full-grown birds. Some characteristic points of difference for
instance, that of the width of mouth -- could hardly be detected in the
young. But there was one remarkable exception to this rule, for the young of
the short-faced tumbler differed from the young of the wild rock-pigeon and
of the other breeds, in all its proportions, almost exactly as much as in
the adult state.

The two principles above given seem to me to explain these facts in regard
to the later embryonic stages of our domestic varieties. Fanciers select
their horses, dogs, and pigeons, for breeding, when they are nearly grown
up: they are indifferent whether the desired qualities and structures have
been acquired earlier or later in life, if the full-grown animal possesses
them. And the cases just given, more especially that of pigeons, seem to
show that the characteristic differences which give value to each breed, and
which have been accumulated by man's selection, have not generally first
appeared at an early period of life, and have been inherited by the
offspring at a corresponding not early period. But the case of the
short-faced tumbler, which when twelve hours old had acquired its proper
proportions, proves that this is not the universal rule; for here the
characteristic differences must either have appeared at an earlier period
than usual, or, if not so, the differences must have been inherited, not at
the corresponding, but at an earlier age.

Now let us apply these facts and the above two principles which latter,
though not proved true, can be shown to be in some degree probable to
species in a state of nature. Let us take a genus of birds, descended on my
theory from some one parent-species, and of which the several new species
have become modified through natural selection in accordance with their
diverse habits. Then, from the many slight successive steps of variation
having supervened at a rather late age, and having been inherited at a
corresponding age, the young of the new species of our supposed genus will
manifestly tend to resemble each other much more closely than do the adults,
just as we have seen in the case of pigeons. We may extend this view to
whole families or even classes. The fore-limbs, for instance, which served
as legs in the parent-species, may become, by a long course of modification,
adapted in one descendant to act as hands, in another as paddles, in another
as wings; and on the above two principles namely of each successive
modification supervening at a rather late age, and being inherited at a
corresponding late age the fore-limbs in the embryos of the several
descendants of the parent-species will still resemble each other closely,
for they will not have been modified. But in each individual new species,
the embryonic fore-limbs will differ greatly from the fore-limbs in the
mature animal; the limbs in the latter having undergone much modification at
a rather late period of life, and having thus been converted into hands, or
paddles, or wings. Whatever influence long-continued exercise or use on the
one hand, and disuse on the other, may have in modifying an organ, such
influence will mainly affect the mature animal, which has come to its full
powers of activity and has to gain its own living; and the effects thus
produced will be inherited at a corresponding mature age. Whereas the young
will remain unmodified, or be modified in a lesser degree, by the effects of
use and disuse.

In certain cases the successive steps of variation might supervene, from
causes of which we are wholly ignorant, at a very early period of life, or
each step might be inherited at an earlier period than that at which it
first appeared. In either case (as with the short-faced tumbler) the young
or embryo would closely resemble the mature parent-form. We have seen that
this is the rule of development in certain whole groups of animals, as with
cuttle-fish and spiders, and with a few members of the great class of
insects, as with Aphis. With respect to the final cause of the young in
these cases not undergoing any metamorphosis, or closely resembling their
parents from their earliest age, we can see that this would result from the
two following contingencies; firstly, from the young, during a course of
modification carried on for many generations, having to provide for their
own wants at a very early stage of development, and secondly, from their
following exactly the same habits of life with their parents; for in this
case, it would be indispensable for the existence of the species, that the
child should be modified at a very early age in the same manner with its
parents, in accordance with their similar habits. Some further explanation,
however, of the embryo not undergoing any metamorphosis is perhaps
requisite. If, on the other hand, it profited the young to follow habits of
life in any degree different from those of their parent, and consequently to
be constructed in a slightly different manner, then, on the principle of
inheritance at corresponding ages, the active young or larvae might easily
be rendered by natural selection different to any conceivable extent from
their parents. Such differences might, also, become correlated with
successive stages of development; so that the larvae, in the first stage,
might differ greatly from the larvae in the second stage, as we have seen to
be the case with cirripedes. The adult might become fitted for sites or
habits, in which organs of locomotion or of the senses, &c., would be
useless; and in this case the final metamorphosis would be said to be
retrograde.

As all the organic beings, extinct and recent, which have ever lived on this
earth have to be classed together, and as all have been connected by the
finest gradations, the best, or indeed, if our collections were nearly
perfect, the only possible arrangement, would be genealogical. Descent being
on my view the hidden bond of connexion which naturalists have been seeking
under the term of the natural system. On this view we can understand how it
is that, in the eyes of most naturalists, the structure of the embryo is
even more important for classification than that of the adult. For the
embryo is the animal in its less modified state; and in so far it reveals
the structure of its progenitor. In two groups of animal, however much they
may at present differ from each other in structure and habits, if they pass
through the same or similar embryonic stages, we may feel assured that they
have both descended from the same or nearly similar parents, and are
therefore in that degree closely related. Thus, community in embryonic
structure reveals community of descent. It will reveal this community of
descent, however much the structure of the adult may have been modified and
obscured; we have seen, for instance, that cirripedes can at once be
recognised by their larvae as belonging to the great class of crustaceans.
As the embryonic state of each species and group of species partially shows
us the structure of their less modified ancient progenitors, we can clearly
see why ancient and extinct forms of life should resemble the embryos of
their descendants, our existing species. Agassiz believes this to be a law
of nature; but I am bound to confess that I only hope to see the law
hereafter proved true. It can be proved true in those cases alone in which
the ancient state, now supposed to be represented in many embryos, has not
been obliterated, either by the successive variations in a long course of
modification having supervened at a very early age, or by the variations
having been inherited at an earlier period than that at which they first
appeared. It should also be borne in mind, that the supposed law of
resemblance of ancient forms of life to the embryonic stages of recent
forms, may be true, but yet, owing to the geological record not extending
far enough back in time, may remain for a long period, or for ever,
incapable of demonstration.

Thus, as it seems to me, the leading facts in embryology, which are second
in importance to none in natural history, are explained on the principle of
slight modifications not appearing, in the many descendants from some one
ancient progenitor, at a very early period in the life of each, though
perhaps caused at the earliest, and being inherited at a corresponding not
early period. Embryology rises greatly in interest, when we thus look at the
embryo as a picture, more or less obscured, of the common parent-form of
each great class of animals.

Rudimentary, atrophied, or aborted organs

Organs or parts in this strange condition, bearing the stamp of inutility,
are extremely common throughout nature. For instance, rudimentary mammae are
very general in the males of mammals: I presume that the `bastard-wing' in
birds may be safely considered as a digit in a rudimentary state: in very
many snakes one lobe of the lungs is rudimentary; in other snakes there are
rudiments of the pelvis and hind limbs. Some of the cases of rudimentary
organs are extremely curious; for instance, the presence of teeth in foetal
whales, which when grown up have not a tooth in their heads; and the
presence of teeth, which never cut through the gums, in the upper jaws of
our unborn calves. It has even been stated on good authority that rudiments
of teeth can be detected in the beaks of certain embryonic birds. Nothing
can be plainer than that wings are formed for flight, yet in how many
insects do we see wings so reduced in size as to be utterly incapable of
flight, and not rarely lying under wing-cases, firmly soldered together!

The meaning of rudimentary organs is often quite unmistakeable: for instance
there are beetles of the same genus (and even of the same species)
resembling each other most closely in all respects, one of which will have
full-sized wings, and another mere rudiments of membrane; and here it is
impossible to doubt, that the rudiments represent wings. Rudimentary organs
sometimes retain their potentiality, and are merely not developed: this
seems to be the case with the mammae of male mammals, for many instances are
on record of these organs having become well developed in full-grown males,
and having secreted milk. So again there are normally four developed and two
rudimentary teats in the udders of the genus Bos, but in our domestic cows
the two sometimes become developed and give milk. In individual plants of
the same species the petals sometimes occur as mere rudiments, and sometimes
in a well-developed state. In plants with separated sexes, the male flowers
often have a rudiment of a pistil; and Kölreuter found that by crossing such
male plants with an hermaphrodite species, the rudiment of the pistil in the
hybrid offspring was much increased in size; and this shows that the
rudiment and the perfect pistil are essentially alike in nature.

An organ serving for two purposes, may become rudimentary or utterly aborted
for one, even the more important purpose;, and remain perfectly efficient
for the other. Thus in plants, the office of the pistil is to allow the
pollen-tubes to reach the ovules protected in the ovarium at its base. The
pistil consists of a stigma supported on the style; but in some Compositae,
the male florets, which of course cannot be fecundated, have a pistil, which
is in a rudimentary state, for it is not crowned with a stigma; but the
style remains well developed, and is clothed with hairs as in other
compositae, for the purpose of brushing the pollen out of the surrounding
anthers. Again, an organ may become rudimentary for its proper purpose, and
be used for a distinct object: in certain fish the swim-bladder seems to be
rudimentary for its proper function of giving buoyancy, but has become
converted into a nascent breathing organ or lung. Other similar instances
could be given.

Rudimentary organs in the individuals of the same species are very liable to
vary in degree of development and in other respects. Moreover, in closely
allied species, the degree to which the same organ has been rendered
rudimentary occasionally differs much. This latter fact is well exemplified
in the state of the wings of the female moths in certain groups. Rudimentary
organs may be utterly aborted; and this implies, that we find in an animal
or plant no trace of an organ, which analogy would lead us to expect to
find, and which is occasionally found in monstrous individuals of the
species. Thus in the snapdragon (antirrhinum) we generally do not find a
rudiment of a fifth stamen; but this may sometimes be seen. In tracing the
homologies of the same part in different members of a class, nothing is more
common, or more necessary, than the use and discovery of rudiments. This is
well shown in the drawings given by Owen of the bones of the leg of the
horse, ox, and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in the upper
jaws of whales and ruminants, can often be detected in the embryo, but
afterwards wholly disappear. It is also, I believe, a universal rule, that a
rudimentary part or organ is of greater size relatively to the adjoining
parts in the embryo, than in the adult; so that the organ at this early age
is less rudimentary, or even cannot be said to be in any degree rudimentary.
Hence, also, a rudimentary organ in the adult, is often said to have
retained its embryonic condition.

I have now given the leading facts with respect to rudimentary organs. In
reflecting on them, every one must be struck with astonishment: for the same
reasoning power which tells us plainly that most parts and organs are
exquisitely adapted for certain purposes, tells us with equal plainness that
these rudimentary or atrophied organs, are imperfect and useless. In works
on natural history rudimentary organs are generally said to have been
created `for the sake of symmetry,' or in order `to complete the scheme of
nature;' but this seems to me no explanation, merely a restatement of the
fact. Would it be thought sufficient to say that because planets revolve in
elliptic courses round the sun, satellites follow the same course round the
planets, for the sake of symmetry, and to complete the scheme of nature? An
eminent physiologist accounts for the presence of rudimentary organs, by
supposing that they serve to excrete matter in excess, or injurious to the
system; but can we suppose that the minute papilla, which often represents
the pistil in male flowers, and which is formed merely of cellular tissue,
can thus act? Can we suppose that the formation of rudimentary teeth which
are subsequently absorbed, can be of any service to the rapidly growing
embryonic calf by the excretion of precious phosphate of lime? When a man's
fingers have been amputated, imperfect nails sometimes appear on the stumps:
I could as soon believe that these vestiges of nails have appeared, not from
unknown laws of growth, but in order to excrete horny matter, as that the
rudimentary nails on the fin of the manatee were formed for this purpose.

On my view of descent with modification, the origin of rudimentary organs is
simple. We have plenty of cases of rudimentary organs in our domestic
productions, as the stump of a tail in tailless breeds, the vestige of an
ear in earless breeds, -- the reappearance of minute dangling horns in
hornless breeds of cattle, more especially, according to Youatt, in young
animals, and the state of the whole flower in the cauliflower. We often see
rudiments of various parts in monsters. But I doubt whether any of these
cases throw light on the origin of rudimentary organs in a state of nature,
further than by showing that rudiments can be produced; for I doubt whether
species under nature ever undergo abrupt changes. I believe that disuse has
been the main agency; that it has led in successive generations to the
gradual reduction of various organs, until they have become rudimentary, as
in the case of the eyes of animals inhabiting dark caverns, and of the wings
of birds inhabiting oceanic islands, which have seldom been forced to take
flight, and have ultimately lost the power of flying. Again, an organ useful
under certain conditions, might become injurious under others, as with the
wings of beetles living on small and exposed islands; and in this case
natural selection would continue slowly to reduce the organ, until it was
rendered harmless and rudimentary.

Any change in function, which can be effected by insensibly small steps, is
within the power of natural selection; so that an organ rendered, during
changed habits of life, useless or injurious for one purpose, might easily
be modified and used for another purpose. Or an organ might be retained for
one alone of its former functions. An organ, when rendered useless, may well
be variable, for its variations cannot be checked by natural selection. At
whatever period of life disuse or selection reduces an organ, and this will
generally be when the being has come to maturity and to its full powers of
action, the principle of inheritance at corresponding ages will reproduce
the organ in its reduced state at the same age, and consequently will seldom
affect or reduce it in the embryo. Thus we can understand the greater
relative size of rudimentary organs in the embryo, and their lesser relative
size in the adult. But if each step of the process of reduction were to be
inherited, not at the corresponding age, but at an extremely early period of
life (as we have good reason to believe to be possible) the rudimentary part
would tend to be wholly lost, and we should have a case of complete
abortion. The principle, also, of economy, explained in a former chapter, by
which the materials forming any part or structure, if not useful to the
possessor, will be saved as far as is possible, will probably often come
into play; and this will tend to cause the entire obliteration of a
rudimentary organ.

As the presence of rudimentary organs is thus due to the tendency in every
part of the organisation, which has long existed, to be inherited we can
understand, on the genealogical view of classification, how it is that
systematists have found rudimentary parts as useful as, or even sometimes
more useful than, parts of high physiological importance. Rudimentary organs
may be compared with the letters in a word, still retained in the spelling,
but become useless in the pronunciation, but which serve as a clue in
seeking for its derivation. On the view of descent with modification, we may
conclude that the existence of organs in a rudimentary, imperfect, and
useless condition, or quite aborted, far from presenting a strange
difficulty, as they assuredly do on the ordinary doctrine of creation, might
even have been anticipated, and can be accounted for by the laws of
inheritance.

Summary

In this chapter I have attempted to show, that the subordination of group to
group in all organisms throughout all time; that the nature of the
relationship, by which all living and extinct beings are united by complex,
radiating, and circuitous lines of affinities into one grand system; the
rules followed and the difficulties encountered by naturalists in their
classifications; the value set upon characters, if constant and prevalent,
whether of high vital importance, or of the most trifling importance, or, as
in rudimentary organs, of no importance; the wide opposition in value
between analogical or adaptive characters, and characters of true affinity;
and other such rules; all naturally follow on the view of the common
parentage of those forms which are considered by naturalists as allied,
together with their modification through natural selection, with its
contingencies of extinction and divergence of character. In considering this
view of classification, it should be borne in mind that the element of
descent has been universally used in ranking together the sexes, ages, and
acknowledged varieties of the same species, however different they may be in
structure. If we extend the use of this element of descent, the only
certainly known cause of similarity in organic beings, we shall understand
what is meant by the natural system: it is genealogical in its attempted
arrangement, with the grades of acquired difference marked by the terms
varieties, species, genera, families, orders, and classes.

On this same view of descent with modification, all the great facts in
Morphology become intelligible, whether we look to the same pattern
displayed in the homologous organs, to whatever purpose applied, of the
different species of a class; or to the homologous parts constructed on the
same pattern in each individual animal and plant.

On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being inherited at
a corresponding period, we can understand the great leading facts in
Embryology; namely, the resemblance in an individual embryo of the
homologous parts, which when matured will become widely different from each
other in structure and function; and the resemblance in different species of
a class of the homologous parts or organs, though fitted in the adult
members for purposes as different as possible. Larvae are active embryos,
which have become specially modified in relation to their habits of life,
through the principle of modifications being inherited at corresponding
ages. On this same principle and bearing in mind, that when organs are
reduced in size, either from disuse or selection, it will generally be at
that period of life when the being has to provide for its own wants, and
bearing in mind how strong is the principle of inheritance the occurrence of
rudimentary organs and their final abortion, present to us no inexplicable
difficulties; on the contrary, their presence might have been even
anticipated. The importance of embryological characters and of rudimentary
organs in classification is intelligible, on the view that an arrangement is
only so far natural as it is genealogical.

Finally, the several classes of facts which have been considered in this
chapter, seem to me to proclaim so plainly, that the innumerable species,
genera, and families of organic beings, with which this world is peopled,
have all descended, each within its own class or group, from common parents,
and have all been modified in the course of descent, that I should without
hesitation adopt this view, even if it were unsupported by other facts or
arguments.

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Chapter 14 - Recapitulation and Conclusion

[* ] Recapitulation of the difficulties on the theory of Natural Selection
[* ] Recapitulation of the general and special circumstances in its favour
[* ] Causes of the general belief in the immutability of species [* ] How
far the theory of natural selection may be extended [* ] Effects of its
adoption on the study of Natural history [* ] Concluding remarks
----------------------------------------------------------------------------
As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.

That many and grave objections may be advanced against the theory of descent
with modification through natural selection, I do not deny. I have
endeavoured to give to them their full force. Nothing at first can appear
more difficult to believe than that the more complex organs and instincts
should have been perfected not by means superior to, though analogous with,
human reason, but by the accumulation of innumerable slight variations, each
good for the individual possessor. Nevertheless, this difficulty, though
appearing to our imagination insuperably great, cannot be considered real if
we admit the following propositions, namely, -- that gradations in the
perfection of any organ or instinct, which we may consider, either do now
exist or could have existed, each good of its kind, -- that all organs and
instincts are, in ever so slight a degree, variable, -- and, lastly, that
there is a struggle for existence leading to the preservation of each
profitable deviation of structure or instinct. The truth of these
propositions cannot, I think, be disputed.

It is, no doubt, extremely difficult even to conjecture by what gradations
many structures have been perfected, more especially amongst broken and
failing groups of organic beings; but we see so many strange gradations in
nature, as is proclaimed by the canon, `Natura non facit saltum,' that we
ought to be extremely cautious in saying that any organ or instinct, or any
whole being, could not have arrived at its present state by many graduated
steps. There are, it must be admitted, cases of special difficulty on the
theory of natural selection; and one of the most curious of these is the
existence of two or three defined castes of workers or sterile females in
the same community of ants but I have attempted to show how this difficulty
can be mastered. With respect to the almost universal sterility of species
when first crossed, which forms so remarkable a contrast with the almost
universal fertility of varieties when crossed, I must refer the reader to
the recapitulation of the facts given at the end of the eighth chapter,
which seem to me conclusively to show that this sterility is no more a
special endowment than is the incapacity of two trees to be grafted
together, but that it is incidental on constitutional differences in the
reproductive systems of the intercrossed species. We see the truth of this
conclusion in the vast difference in the result, when the same two species
are crossed reciprocally; that is, when one species is first used as the
father and then as the mother.

The fertility of varieties when intercrossed and of their mongrel offspring
cannot be considered as universal; nor is their very general fertility
surprising when we remember that it is not likely that either their
constitutions or their reproductive systems should have been profoundly
modified. Moreover, most of the varieties which have been experimentised on
have been produced under domestication; and as domestication apparently
tends to eliminate sterility, we ought not to expect it also to produce
sterility.

The sterility of hybrids is a very different case from that of first
crosses, for their reproductive organs are more or less functionally
impotent; whereas in first crosses the organs on both sides are in a perfect
condition. As we continually see that organisms of all kinds are rendered in
some degree sterile from their constitutions having been disturbed by
slightly different and new conditions of life, we need not feel surprise at
hybrids being in some degree sterile, for their constitutions can hardly
fail to have been disturbed from being compounded of two distinct
organisations. This parallelism is supported by another parallel, but
directly opposite, class of facts; namely, that the vigour and fertility of
all organic beings are increased by slight changes in their conditions of
life, and that the offspring of slightly modified forms or varieties acquire
from being crossed increased vigour and fertility. So that, on the one hand,
considerable changes in the conditions of life and crosses between greatly
modified forms, lessen fertility; and on the other hand, lesser changes in
the conditions of life and crosses between less modified forms, increase
fertility.

Turning to geographical distribution, the difficulties encountered on the
theory of descent with modification are grave enough. All the individuals of
the same species, and all the species of the same genus, or even higher
group, must have descended from common parents; and therefore, in however
distant and isolated parts of the world they are now found, they must in the
course of successive generations have passed from some one part to the
others. We are often wholly unable even to conjecture how this could have
been effected. Yet, as we have reason to believe that some species have
retained the same specific form for very long periods, enormously long as
measured by years, too much stress ought not to be laid on the occasional
wide diffusion of the same species; for during very long periods of time
there will always be a good chance for wide migration by many means. A
broken or interrupted range may often be accounted for by the extinction of
the species in the intermediate regions. It cannot be denied that we are as
yet very ignorant of the full extent of the various climatal and
geographical changes which have affected the earth during modern periods;
and such changes will obviously have greatly facilitated migration. As an
example, I have attempted to show how potent has been the influence of the
Glacial period on the distribution both of the same and of representative
species throughout the world. We are as yet profoundly ignorant of the many
occasional means of transport. With respect to distinct species of the same
genus inhabiting very distant and isolated regions, as the process of
modification has necessarily been slow, all the means of migration will have
been possible during a very long period; and consequently the difficulty of
the wide diffusion of species of the same genus is in some degree lessened.

As on the theory of natural selection an interminable number of intermediate
forms must have existed, linking together all the species in each group by
gradations as fine as our present varieties, it may be asked, Why do we not
see these linking forms all around us? Why are not all organic beings
blended together in an inextricable chaos? With respect to existing forms,
we should remember that we have no right to expect (excepting in rare cases)
to discover directly connecting links between them, but only between each
and some extinct and supplanted form. Even on a wide area, which has during
a long period remained continuous, and of which the climate and other
conditions of life change insensibly in going from a district occupied by
one species into another district occupied by a closely allied species, we
have no just right to expect often to find intermediate varieties in the
intermediate zone. For we have reason to believe that only a few species are
undergoing change at any one period; and all changes are slowly effected. I
have also shown that the intermediate varieties which will at first probably
exist in the intermediate zones, will be liable to be supplanted by the
allied forms on either hand; and the latter, from existing in greater
numbers, will generally be modified and improved at a quicker rate than the
intermediate varieties, which exist in lesser numbers; so that the
intermediate varieties will, in the long run, be supplanted and
exterminated.

On this doctrine of the extermination of an infinitude of connecting links,
between the living and extinct inhabitants of the world, and at each
successive period between the extinct and still older species, why is not
every geological formation charged with such links? Why does not every
collection of fossil remains afford plain evidence of the gradation and
mutation of the forms of life? We meet with no such evidence, and this is
the most obvious and forcible of the many objections which may be urged
against my theory. Why, again, do whole groups of allied species appear,
though certainly they often falsely appear, to have come in suddenly on the
several geological stages? Why do we not find great piles of strata beneath
the Silurian system, stored with the remains of the progenitors of the
Silurian groups of fossils? For certainly on my theory such strata must
somewhere have been deposited at these ancient and utterly unknown epochs in
the world's history.

I can answer these questions and grave objections only on the supposition
that the geological record is far more imperfect than most geologists
believe. It cannot be objected that there has not been time sufficient for
any amount of organic change; for the lapse of time has been so great as to
be utterly inappreciable by the human intellect. The number of specimens in
all our museums is absolutely as nothing compared with the countless
generations of countless species which certainly have existed. We should not
be able to recognise a species as the parent of any one or more species if
we were to examine them ever so closely, unless we likewise possessed many
of the intermediate links between their past or parent and present states;
and these many links we could hardly ever expect to discover, owing to the
imperfection of the geological record. Numerous existing doubtful forms
could be named which are probably varieties; but who will pretend that in
future ages so many fossil links will be discovered, that naturalists will
be able to decide, on the common view, whether or not these doubtful forms
are varieties? As long as most of the links between any two species are
unknown, if any one link or intermediate variety be discovered, it will
simply be classed as another and distinct species. Only a small portion of
the world has been geologically explored. Only organic beings of certain
classes can be preserved in a fossil condition, at least in any great
number. Widely ranging species vary most, and varieties are often at first
local, -- both causes rendering the discovery of intermediate links less
likely. Local varieties will not spread into other and distant regions until
they are considerably modified and improved; and when they do spread, if
discovered in a geological formation, they will appear as if suddenly
created there, and will be simply classed as new species. Most formations
have been intermittent in their accumulation; and their duration, I am
inclined to believe, has been shorter than the average duration of specific
forms. Successive formations are separated from each other by enormous blank
intervals of time; for fossiliferous formations, thick enough to resist
future degradation, can be accumulated only where much sediment is deposited
on the subsiding bed of the sea. During the alternate periods of elevation
and of stationary level the record will be blank. During these latter
periods there will probably be more variability in the forms of life; during
periods of subsidence, more extinction.

With respect to the absence of fossiliferous formations beneath the lowest
Silurian strata, I can only recur to the hypothesis given in the ninth
chapter. That the geological record is imperfect all will admit; but that it
is imperfect to the degree which I require, few will be inclined to admit.
If we look to long enough intervals of time, geology plainly declares that
all species have changed; and they have changed in the manner which my
theory requires, for they have changed slowly and in a graduated manner. We
clearly see this in the fossil remains from consecutive formations
invariably being much more closely related to each other, than are the
fossils from formations distant from each other in time.

Such is the sum of the several chief objections and difficulties which may
justly be urged against my theory; and I have now briefly recapitulated the
answers and explanations which can be given to them. I have felt these
difficulties far too heavily during many years to doubt their weight. But it
deserves especial notice that the more important objections relate to
questions on which we are confessedly ignorant; nor do we know how ignorant
we are. We do not know all the possible transitional gradations between the
simplest and the most perfect organs; it cannot be pretended that we know
all the varied means of Distribution during the long lapse of years, or that
we know how imperfect the Geological Record is. Grave as these several
difficulties are, in my judgement they do not overthrow the theory of
descent with modification.

Now let us turn to the other side of the argument. Under domestication we
see much variability. This seems to be mainly due to the reproductive system
being eminently susceptible to changes in the conditions of life so that
this system, when not rendered impotent, fails to reproduce offspring
exactly like the parent-form. Variability is governed by many complex laws,
-- by correlation of growth, by use and disuse, and by the direct action of
the physical conditions of life. There is much difficulty in ascertaining
how much modification our domestic productions have undergone; but we may
safely infer that the amount has been large, and that modifications can be
inherited for long periods. As long as the conditions of life remain the
same, we have reason to believe that a modification, which has already been
inherited for many generations, may continue to be inherited for an almost
infinite number of generations. On the other hand we have evidence that
variability, when it has once come into play, does not wholly cease; for new
varieties are still occasionally produced by our most anciently domesticated
productions.

Man does not actually produce variability; he only unintentionally exposes
organic beings to new conditions of life, and then nature acts on the
organisation, and causes variability. But man can and does select the
variations given to him by nature, and thus accumulate them in any desired
manner. He thus adapts animals and plants for his own benefit or pleasure.
He may do this methodically, or he may do it unconsciously by preserving the
individuals most useful to him at the time, without any thought of altering
the breed. It is certain that he can largely influence the character of a
breed by selecting, in each successive generation, individual differences so
slight as to be quite inappreciable by an uneducated eye. This process of
selection has been the great agency in the production of the most distinct
and useful domestic breeds. That many of the breeds produced by man have to
a large extent the character of natural species, is shown by the
inextricable doubts whether very many of them are varieties or aboriginal
species.

There is no obvious reason why the principles which have acted so
efficiently under domestication should not have acted under nature. In the
preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful and
ever-acting means of selection. The struggle for existence inevitably
follows from the high geometrical ratio of increase which is common to all
organic beings. This high rate of increase is proved by calculation, by the
effects of a succession of peculiar seasons, and by the results of
naturalisation, as explained in the third chapter. More individuals are born
than can possibly survive. A grain in the balance will determine which
individual shall live and which shall die, -- which variety or species shall
increase in number, and which shall decrease, or finally become extinct. As
the individuals of the same species come in all respects into the closest
competition with each other, the struggle will generally be most severe
between them; it will be almost equally severe between the varieties of the
same species, and next in severity between the species of the same genus.
But the struggle will often be very severe between beings most remote in the
scale of nature. The slightest advantage in one being, at any age or during
any season, over those with which it comes into competition, or better
adaptation in however slight a degree to the surrounding physical
conditions, will turn the balance.

With animals having separated sexes there will in most cases be a struggle
between the males for possession of the females. The most vigorous
individuals, or those which have most successfully struggled with their
conditions of life, will generally leave most progeny. But success will
often depend on having special weapons or means of defence, or on the charms
of the males; and the slightest advantage will lead to victory.

As geology plainly proclaims that each land has undergone great physical
changes, we might have expected that organic beings would have varied under
nature, in the same way as they generally have varied under the changed
conditions of domestication. And if there be any variability under nature,
it would be an unaccountable fact if natural selection had not come into
play. It has often been asserted, but the assertion is quite incapable of
proof, that the amount of variation under nature is a strictly limited
quantity. Man, though acting on external characters alone and often
capriciously, can produce within a short period a great result by adding up
mere individual differences in his domestic productions; and every one
admits that there are at least individual differences in species under
nature. But, besides such differences, all naturalists have admitted the
existence of varieties, which they think sufficiently distinct to be worthy
of record in systematic works. No one can draw any clear distinction between
individual differences and slight varieties; or between more plainly marked
varieties and subspecies, and species. Let it be observed how naturalists
differ in the rank which they assign to the many representative forms in
Europe and North America.

If then we have under nature variability and a powerful agent always ready
to act and select, why should we doubt that variations in any way useful to
beings, under their excessively complex relations of life, would be
preserved, accumulated, and inherited? Why, if man can by patience select
variations most useful to himself, should nature fail in selecting
variations useful, under changing conditions of life, to her living
products? What limit can be put to this power, acting during long ages and
rigidly scrutinising the whole constitution, structure, and habits of each
creature, --- favouring the good and rejecting the bad? I can see no
limit to this power, in slowly and beautifully adapting each form to the
most complex relations of life. The theory of natural selection, even if we
looked no further than this, seems to me to be in itself probable. I have
already recapitulated, as fairly as I could, the opposed difficulties and
objections: now let us turn to the special facts and arguments in favour of
the theory.

On the view that species are only strongly marked and permanent varieties,
and that each species first existed as a variety, we can see why it is that
no line of demarcation can be drawn between species, commonly supposed to
have been produced by special acts of creation, and varieties which are
acknowledged to have been produced by secondary laws. On this same view we
can understand how it is that in each region where many species of a genus
have been produced, and where they now flourish, these same species should
present many varieties; for where the manufactory of species has been
active, we might expect, as a general rule, to find it still in action; and
this is the case if varieties be incipient species. Moreover, the species of
the large genera, which afford the greater number of varieties or incipient
species, retain to a certain degree the character of varieties; for they
differ from each other by a less amount of difference than do the species of
smaller genera. The closely allied species also of the larger genera
apparently have restricted ranges, and they are clustered in little groups
round other species -- in which respects they resemble varieties. These are
strange relations on the view of each species having been independently
created, but are intelligible if all species first existed as varieties.

As each species tends by its geometrical ratio of reproduction to increase
inordinately in number; and as the modified descendants of each species will
be enabled to increase by so much the more as they become more diversified
in habits and structure, so as to be enabled to seize on many and widely
different places in the economy of nature, there will be a constant tendency
in natural selection to preserve the most divergent offspring of any one
species. Hence during a long-continued course of modification, the slight
differences, characteristic of varieties of the same species, tend to be
augmented into the greater differences characteristic of species of the same
genus. New and improved varieties will inevitably supplant and exterminate
the older, less improved and intermediate varieties; and thus species are
rendered to a large extent defined and distinct objects. Dominant species
belonging to the larger groups tend to give birth to new and dominant forms;
so that each large group tends to become still larger, and at the same time
more divergent in character. But as all groups cannot thus succeed in
increasing in size, for the world would not hold them, the more dominant
groups beat the less dominant. This tendency in the large groups to go on
increasing in size and diverging in character, together with the almost
inevitable contingency of much extinction, explains the arrangement of all
the forms of life, in groups subordinate to groups, all within a few great
classes, which we now see everywhere around us, and which has prevailed
throughout all time. This grand fact of the grouping of all organic beings
seems to me utterly inexplicable on the theory of creation.

As natural selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modification; it
can act only by very short and slow steps. Hence the canon of `Natura non
facit saltum,' which every fresh addition to our knowledge tends to make
more strictly correct, is on this theory simply intelligible. We can plainly
see why nature is prodigal in variety, though niggard in innovation. But why
this should be a law of nature if each species has been independently
created, no man can explain.

Many other facts are, as it seems to me, explicable on this theory. How
strange it is that a bird, under the form of woodpecker, should have been
created to prey on insects on the ground; that upland geese, which never or
rarely swim, should have been created with webbed feet; that a thrush should
have been created to dive and feed on sub-aquatic insects; and that a petrel
should have been created with habits and structure fitting it for the life
of an auk or grebe! and so on in endless other cases. But on the view of
each species constantly trying to increase in number, with natural selection
always ready to adapt the slowly varying descendants of each to any
unoccupied or ill-occupied place in nature, these facts cease to be strange,
or perhaps might even have been anticipated.

As natural selection acts by competition, it adapts the inhabitants of each
country only in relation to the degree of perfection of their associates; so
that we need feel no surprise at the inhabitants of any one country,
although on the ordinary view supposed to have been specially created and
adapted for that country, being beaten and supplanted by the naturalised
productions from another land. Nor ought we to marvel if all the
contrivances in nature be not, as far as we can judge, absolutely perfect;
and if some of them be abhorrent to our ideas of fitness. We need not marvel
at the sting of the bee causing the bee's own death; at drones being
produced in such vast numbers for one single act, and being then slaughtered
by their sterile sisters; at the astonishing waste of pollen by our
fir-trees; at the instinctive hatred of the queen bee for her own fertile
daughters; at ichneumonidae feeding within the live bodies of caterpillars;
and at other such cases. The wonder indeed is, on the theory of natural
selection, that more cases of the want of absolute perfection have not been
observed.

The complex and little known laws governing variation are the same, as far
as we can see, with the laws which have governed the production of so-called
specific forms. In both cases physical conditions seem to have produced but
little direct effect; yet when varieties enter any zone, they occasionally
assume some of the characters of the species proper to that zone. In both
varieties and species, use and disuse seem to have produced some effect; for
it is difficult to resist this conclusion when we look, for instance, at the
logger-headed duck, which has wings incapable of flight, in nearly the same
condition as in the domestic duck; or when we look at the burrowing
tucutucu, which is occasionally blind, and then at certain moles, which are
habitually blind and have their eyes covered with skin; or when we look at
the blind animals inhabiting the dark caves of America and Europe. In both
varieties and species correction of growth seems to have played a most
important part, so that when one part has been modified other parts are
necessarily modified. In both varieties and species reversions to long-lost
characters occur. How inexplicable on the theory of creation is the
occasional appearance of stripes on the shoulder and legs of the several
species of the horse-genus and in their hybrids! How simply is this fact
explained if we believe that these species have descended from a striped
progenitor, in the same manner as the several domestic breeds of pigeon have
descended from the blue and barred rock-pigeon!

On the ordinary view of each species having been independently created, why
should the specific characters, or those by which the species of the same
genus differ from each other, be more variable than the generic characters
in which they all agree? Why, for instance, should the colour of a flower be
more likely to vary in any one species of a genus, if the other species,
supposed to have been created independently, have differently coloured
flowers, than if all the species of the genus have the same coloured
flowers? If species are only well-marked varieties, of which the characters
have become in a high degree permanent, we can understand this fact; for
they have already varied since they branched off from a common progenitor in
certain characters, by which they have come to be specifically distinct from
each other; and therefore these same characters would be more likely still
to be variable than the generic characters which have been inherited without
change for an enormous period. It is inexplicable on the theory of creation
why a part developed in a very unusual manner in any one species of a genus,
and therefore, as we may naturally infer, of great importance to the
species, should be eminently liable to variation; but, on my view, this part
has undergone, since the several species branched off from a common
progenitor, an unusual amount of variability and modification, and therefore
we might expect this part generally to be still variable. But a part may be
developed in the most unusual manner, like the wing of a bat, and yet not be
more variable than any other structure, if the part be common to many
subordinate forms, that is, if it has been inherited for a very long period;
for in this case it will have been rendered constant by long-continued
natural selection.

Glancing at instincts, marvellous as some are, they offer no greater
difficulty than does corporeal structure on the theory of the natural
selection of successive, slight, but profitable modifications. We can thus
understand why nature moves by graduated steps in endowing different animals
of the same class with their several instincts. I have attempted to show how
much light the principle of gradation throws on the admirable architectural
powers of the hive-bee. Habit no doubt sometimes comes into play in
modifying instincts; but it certainly is not indispensable, as we see, in
the case of neuter insects, which leave no progeny to inherit the effects of
long-continued habit. On the view of all the species of the same genus
having descended from a common parent, and having inherited much in common,
we can understand how it is that allied species, when placed under
considerably different conditions of life, yet should follow nearly the same
instincts; why the thrush of South America, for instance, lines her nest
with mud like our British species. On the view of instincts having been
slowly acquired through natural selection we need not marvel at some
instincts being apparently not perfect and liable to mistakes, and at many
instincts causing other animals to suffer.

If species be only well-marked and permanent varieties, we can at once see
why their crossed offspring should follow the same complex laws in their
degrees and kinds of resemblance to their parents, -- in being absorbed into
each other by successive crosses, and in other such points, -- as do the
crossed offspring of acknowledged varieties. On the other hand, these would
be strange facts if species have been independently created, and varieties
have been produced by secondary laws.

If we admit that the geological record is imperfect in an extreme degree,
then such facts as the record gives, support the theory of descent with
modification. New species have come on the stage slowly and at successive
intervals; and the amount of change, after equal intervals of time, is
widely different in different groups. The extinction of species and of whole
groups of species, which has played so conspicuous a part in the history of
the organic world, almost inevitably follows on the principle of natural
selection; for old forms will be supplanted by new and improved forms.
Neither single species nor groups of species reappear when the chain of
ordinary generation has once been broken. The gradual diffusion of dominant
forms, with the slow modification of their descendants, causes the forms of
life, after long intervals of time, to appear as if they had changed
simultaneously throughout the world. The fact of the fossil remains of each
formation being in some degree intermediate in character between the fossils
in the formations above and below, is simply explained by their intermediate
position in the chain of descent. The grand fact that all extinct organic
beings belong to the same system with recent beings, falling either into the
same or into intermediate groups, follows from the living and the extinct
being the offspring of common parents. As the groups which have descended
from an ancient progenitor have generally diverged in character, the
progenitor with its early descendants will often be intermediate in
character in comparison with its later descendants; and thus we can see why
the more ancient a fossil is, the oftener it stands in some degree
intermediate between existing and allied groups. Recent forms are generally
looked at as being, in some vague sense, higher than ancient and extinct
forms; and they are in so far higher as the later and more improved forms
have conquered the older and less improved organic beings in the struggle
for life. Lastly, the law of the n='448'> long endurance of allied forms on
the same continent, --- of marsupials in Australia, of edentata in
America, and other such cases, -- is intelligible, for within a confined
country, the recent and the extinct will naturally be allied by descent.

Looking to geographical distribution, if we admit that there has been during
the long course of ages much migration from one part of the world to
another, owing to former climatal and geographical changes and to the many
occasional and unknown means of dispersal, then we can understand, on the
theory of descent with modification, most of the great leading facts in
Distribution. We can see why there should be so striking a parallelism in
the distribution of organic beings throughout space, and in their geological
succession throughout time; for in both cases the beings have been connected
by the bond of ordinary generation, and the means of modification have been
the same. We see the full meaning of the wonderful fact, which must have
struck every traveller, namely, that on the same continent, under the most
diverse conditions, under heat and cold, on mountain and lowland, on deserts
and marshes, most of the inhabitants within each great class are plainly
related; for they will generally be descendants of the same progenitors and
early colonists. On this same principle of former migration, combined in
most cases with modification, we can understand, by the aid of the Glacial
period, the identity of some few plants, and the close alliance of many
others, on the most distant mountains, under the most different climates;
and likewise the close alliance of some of the inhabitants of the sea in the
northern and southern temperate zones, though separated by the whole
intertropical ocean. Although two areas may present the same physical
conditions of life, we need feel no surprise at their inhabitants being
widely different, if they have been for a long period completely separated
from each other; for as the relation of organism to organism is the most
important of all relations, and as the two areas will have received
colonists from some third source or from each other, at various periods and
in different proportions, the course of modification in the two areas will
inevitably be different.

On this view of migration, with subsequent modification, we can see why
oceanic islands should be inhabited by few species, but of these, that many
should be peculiar. We can clearly see why those animals which cannot cross
wide spaces of ocean, as frogs and terrestrial mammals, should not inhabit
oceanic islands; and why, on the other hand, new and peculiar species of
bats, which can traverse the ocean, should so often be found on islands far
distant from any continent. Such facts as the presence of peculiar species
of bats, and the absence of all other mammals, on oceanic islands, are
utterly inexplicable on the theory of independent acts of creation.

The existence of closely allied or representative species in any two areas,
implies, on the theory of descent with modification, that the same parents
formerly inhabited both areas; and we almost invariably find that wherever
many closely allied species inhabit two areas, some identical species common
to both still exist. Wherever many closely allied yet distinct species
occur, many doubtful forms and varieties of the same species likewise occur.
It is a rule of high generality that the inhabitants of each area are
related to the inhabitants of the nearest source whence immigrants might
have been derived. We see this in nearly all the plants and animals of the
Galapagos archipelago, of Juan Fernandez, and of the other American islands
being related in the most striking manner to the plants and animals of the
neighbouring American mainland; and those of the Cape de Verde archipelago
and other African islands to the African mainland. It must be admitted that
these facts receive no explanation on the theory of creation.

The fact, as we have seen, that all past and present organic beings
constitute one grand natural system, with group subordinate to group, and
with extinct groups often falling in between recent groups, is intelligible
on the theory of natural selection with its contingencies of extinction and
divergence of character. On these same principles we see how it is, that the
mutual affinities of the species and genera within each class are so complex
and circuitous. We see why certain characters are far more serviceable than
others for classification; -- why adaptive characters, though of paramount
importance to the being, are of hardly any importance in classification; why
characters derived from rudimentary parts, though of no service to the
being, are often of high classificatory value; and why embryological
characters are the most valuable of all. The real affinities of all organic
beings are due to inheritance or community of descent. The natural system is
a genealogical arrangement, in which we have to discover the lines of
descent by the most permanent characters, however slight their vital
importance may be.

The framework of bones being the same in the hand of a man, wing of a bat,
fin of the porpoise, and leg of the horse, -- the same number of vertebrae
forming the neck of the giraffe and of the elephant, -- and innumerable
other such facts, at once explain themselves on the theory of descent with
slow and slight successive modifications. The similarity of pattern in the
wing and leg of a bat, though used for such different purposes, -- in the
jaws and legs of a crab, -- in the petals, stamens, and pistils of a flower,
is likewise intelligible on the view of the gradual modification of parts or
organs, which were alike in the early progenitor of each class. On the
principle of successive variations not always supervening at an early age,
and being inherited at a corresponding not early period of life, we can
clearly see why the embryos of mammals, birds, reptiles, and fishes should
be so closely alike, and should be so unlike the adult forms. We may cease
marvelling at the embryo of an air-breathing mammal or bird having branchial
slits and arteries running in loops, like those in a fish which has to
breathe the air dissolved in water, by the aid of well-developed branchiae.

Disuse, aided sometimes by natural selection, will often tend to reduce an
organ, when it has become useless by changed habits or under changed
conditions of life; and we can clearly understand on this view the meaning
of rudimentary organs. But disuse and selection will generally act on each
creature, when it has come to maturity and has to play its full part in the
struggle for existence, and will thus have little power of acting on an
organ during early life; hence the organ will not be much reduced or
rendered rudimentary at this early age. The calf, for instance, has
inherited teeth, which never cut through the gums of the upper jaw, from an
early progenitor having well-developed teeth; and we may believe, that the
teeth in the mature animal were reduced, during successive generations, by
disuse or by the tongue and palate having been fitted by natural selection
to browse without their aid; whereas in the calf, the teeth have been left
untouched by selection or disuse, and on the principle of inheritance at
corresponding ages have been inherited from a remote period to the present
day. On the view of each organic being and each separate organ having been
specially created, how utterly inexplicable it is that parts, like the teeth
in the embryonic calf or like the shrivelled wings under the soldered
wing-covers of some beetles, should thus so frequently bear the plain stamp
of inutility! Nature may be said to have taken pains to reveal, by
rudimentary organs and by homologous structures, her scheme of modification,
which it seems that we wilfully will not understand.

I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have changed, and are still slowly
changing by the preservation and accumulation of successive slight
favourable variations. Why, it may be asked, have all the most eminent
living naturalists and geologists rejected this view of the mutability of
species? It cannot be asserted that organic beings in a state of nature are
subject to no variation; it cannot be proved that the amount of variation in
the course of long ages is a limited quantity; no clear distinction has
been, or can be, drawn between species and well-marked varieties. It cannot
be maintained that species when intercrossed are invariably sterile, and
varieties invariably fertile; or that sterility is a special endowment and
sign of creation. The belief that species were immutable productions was
almost unavoidable as long as the history of the world was thought to be of
short duration; and now that we have acquired some idea of the lapse of
time, we are too apt to assume, without proof, that the geological record is
so perfect that it would have afforded us plain evidence of the mutation of
species, if they had undergone mutation.

But the chief cause of our natural unwillingness to admit that one species
has given birth to other and distinct species, is that we are always slow in
admitting any great change of which we do not see the intermediate steps.
The difficulty is the same as that felt by so many geologists, when Lyell
first insisted that long lines of inland cliffs had been formed, and great
valleys excavated, by the slow action of the coast-waves. The mind cannot
possibly grasp the full meaning of the term of a hundred million years; it
cannot add up and perceive the full effects of many slight variations,
accumulated during an almost infinite number of generations.

Although I am fully convinced of the truth of the views given in this volume
under the form of an abstract, I by no means expect to convince experienced
naturalists whose minds are stocked with a multitude of facts all viewed,
during a long course of years, from a point of view directly opposite to
mine. It is so easy to hide our ignorance under such expressions as the
`plan of creation,' `unity of design,' &c., and to think that we give an
explanation when we only restate a fact. Any one whose disposition leads him
to attach more weight to unexplained difficulties than to the explanation of
a certain number of facts will certainly reject my theory. A few
naturalists, endowed with much flexibility of mind, and who have already
begun to doubt on the immutability of species, may be influenced by this
volume; but I look with confidence to the future, to young and rising
naturalists, who will be able to view both sides of the question with
impartiality. Whoever is led to believe that species are mutable will do
good service by conscientiously expressing his conviction; for only thus can
the load of prejudice by which this subject is overwhelmed be removed.

Several eminent naturalists have of late published their belief that a
multitude of reputed species in each genus are not real species; but that
other species are real, that is, have been independently created. This seems
to me a strange conclusion to arrive at. They admit that a multitude of
forms, which till lately they themselves thought were special creations, and
which are still thus looked at by the majority of naturalists, and which
consequently have every external characteristic feature of true species, --
they admit that these have been produced by variation, but they refuse to
extend the same view to other and very slightly different forms.
Nevertheless they do not pretend that they can define, or even conjecture,
which are the created forms of life, and which are those produced by
secondary laws. They admit variation as a vera causa in one case, they
arbitrarily reject it in another, without assigning any distinction in the
two cases. The day will come when this will be given as a curious
illustration of the blindness of preconceived opinion. These authors seem no
more startled at a miraculous act of creation than at an ordinary birth. But
do they really believe that at innumerable periods in the earth's history
certain elemental atoms have been commanded suddenly to flash into living
tissues? Do they believe that at each supposed act of creation one
individual or many were produced? Were all the infinitely numerous kinds of
animals and plants created as eggs or seed, or as full grown? and in the
case of mammals, were they created bearing the false marks of nourishment
from the mother's womb? Although naturalists very properly demand a full
explanation of every difficulty from those who believe in the mutability of
species, on their own side they ignore the whole subject of the first
appearance of species in what they consider reverent silence.

It may be asked how far I extend the doctrine of the modification of
species. The question is difficult to answer, because the more distinct the
forms are which we may consider, by so much the arguments fall away in
force. But some arguments of the greatest weight extend very far. All the
members of whole classes can be connected together by chains of affinities,
and all can be classified on the same principle, in groups subordinate to
groups. Fossil remains sometimes tend to fill up very wide intervals between
existing orders. Organs in a rudimentary condition plainly show that an
early progenitor had the organ in a fully developed state; and this in some
instances necessarily implies an enormous amount of modification in the
descendants. Throughout whole classes various structures are formed on the
same pattern, and at an embryonic age the species closely resemble each
other. Therefore I cannot doubt that the theory of descent with modification
embraces all the members of the same class. I believe that animals have
descended from at most only four or five progenitors, and plants from an
equal or lesser number.

Analogy would lead me one step further, namely, to the belief that all
animals and plants have descended from some one prototype. But analogy may
be a deceitful guide. Nevertheless all living things have much in common, in
their chemical composition, their germinal vesicles, their cellular
structure, and their laws of growth and reproduction. We see this even in so
trifling a circumstance as that the same poison often similarly affects
plants and animals; or that the poison secreted by the gall-fly produces
monstrous growths on the wild rose or oak-tree. Therefore I should infer
from analogy that probably all the organic beings which have ever lived on
this earth have descended from some one primordial form, into which life was
first breathed.

When the views entertained in this volume on the origin of species, or when
analogous views are generally admitted, we can dimly foresee that there will
be a considerable revolution in natural history. Systematists will be able
to pursue their labours as at present; but they will not be incessantly
haunted by the shadowy doubt whether this or that form be in essence a
species. This I feel sure, and I speak after experience, will be no slight
relief. The endless disputes whether or not some fifty species of British
brambles are true species will cease. Systematists will have only to decide
(not that this will be easy) whether any form be sufficiently constant and
distinct from other forms, to be capable of definition; and if definable,
whether the differences be sufficiently important to deserve a specific
name. This latter point will become a far more essential consideration than
it is at present; for differences, however slight, between any two forms, if
not blended by intermediate gradations, are looked at by most naturalists as
sufficient to raise both forms to the rank of species. Hereafter we shall be
compelled to acknowledge that the only distinction between species and
well-marked varieties is, that the latter are known, or believed, to be
connected at the present day by intermediate gradations, whereas species
were formerly thus connected. Hence, without quite rejecting the
consideration of the present existence of intermediate gradations between
any two forms, we shall be led to weigh more carefully and to value higher
the actual amount of difference between them. It is quite possible that
forms now generally acknowledged to be merely varieties may hereafter be
thought worthy of specific names, as with the primrose and cowslip; and in
this case scientific and common language will come into accordance. In
short, we shall have to treat species in the same manner as those
naturalists treat genera, who admit that genera are merely artificial
combinations made for convenience. This may not be a cheering prospect; but
we shall at least be freed from the vain search for the undiscovered and
undiscoverable essence of the term species.

The other and more general departments of natural history will rise greatly
in interest. The terms used by naturalists of affinity, relationship,
community of type, paternity, morphology, adaptive characters, rudimentary
and aborted organs, &c., will cease to be metaphorical, and will have a
plain signification. When we no longer look at an organic being as a savage
looks at a ship, as at something wholly beyond his comprehension; when we
regard every production of nature as one which has had a history; when we
contemplate every complex structure and instinct as the summing up of many
contrivances, each useful to the possessor, nearly in the same way as when
we look at any great mechanical invention as the summing up of the labour,
the experience, the reason, and even the blunders of numerous workmen; when
we thus view each organic being, how far more interesting, I speak from
experience, will the study of natural history become!

A grand and almost untrodden field of inquiry will be opened, on the causes
and laws of variation, on correlation of growth, on the effects of use and
disuse, on the direct action of external conditions, and so forth. The study
of domestic productions will rise immensely in value. A new variety raised
by man will be a far more important and interesting subject for study than
one more species added to the infinitude of already recorded species. Our
classifications will come to be, as far as they can be so made, genealogies;
and will then truly give what may be called the plan of creation. The rules
for classifying will no doubt become simpler when we have a definite object
in view. We possess no pedigrees or armorial bearings; and we have to
discover and trace the many diverging lines of descent in our natural
genealogies, by characters of any kind which have long been inherited.
Rudimentary organs will speak infallibly with respect to the nature of
long-lost structures. Species and groups of species, which are called
aberrant, and which may fancifully be called living fossils, will aid us in
forming a picture of the ancient forms of life. Embryology will reveal to us
the structure, in some degree obscured, of the prototypes of each great
class.

When we can feel assured that all the individuals of the same species, and
all the closely allied species of most genera, have within a not very remote
period descended from one parent, and have migrated from some one
birthplace; and when we better know the many means of migration, then, by
the light which geology now throws, and will continue to throw, on former
changes of climate and of the level of the land, we shall surely be enabled
to trace in an admirable manner the former migrations of the inhabitants of
the whole world. Even at present, by comparing the differences of the
inhabitants of the sea on the opposite sides of a continent, and the nature
of the various inhabitants of that continent in relation to their apparent
means of immigration, some light can be thrown on ancient geography.

The noble science of Geology loses glory from the extreme imperfection of
the record. The crust of the earth with its embedded remains must not be
looked at as a well-filled museum, but as a poor collection made at hazard
and at rare intervals. The accumulation of each great fossiliferous
formation will be recognised as having depended on an unusual concurrence of
circumstances, and the blank intervals between the successive stages as
having been of vast duration. But we shall be able to gauge with some
security the duration of these intervals by a comparison of the preceding
and succeeding organic forms. We must be cautious in attempting to correlate
as strictly contemporaneous two formations, which include few identical
species, by the general succession of their forms of life. As species are
produced and exterminated by slowly acting and still existing causes, and
not by miraculous acts of creation and by catastrophes; and as the most
important of all causes of organic change is one which is almost independent
of altered and perhaps suddenly altered physical conditions, namely, the
mutual relation of organism to organism, -- the improvement of one being
entailing the improvement or the extermination of others; it follows, that
the amount of organic change in the fossils of consecutive formations
probably serves as a fair measure of the lapse of actual time. A number of
species, however, keeping in a body might remain for a long period
unchanged, whilst within this same period, several of these species, by
migrating into new countries and coming into competition with foreign
associates, might become modified; so that we must not overrate the accuracy
of organic change as a measure of time. During early periods of the earth's
history, when the forms of life were probably fewer and simpler, the rate of
change was probably slower; and at the first dawn of life, when very few
forms of the simplest structure existed, the rate of change may have been
slow in an extreme degree. The whole history of the world, as at present
known, although of a length quite incomprehensible by us, will hereafter be
recognised as a mere fragment of time, compared with the ages which have
elapsed since the first creature, the progenitor of innumerable extinct and
living descendants, was created.

In the distant future I see open fields for far more important researches.
Psychology will be based on a new foundation, that of the necessary
acquirement of each mental power and capacity by gradation. Light will be
thrown on the origin of man and his history.

Authors of the highest eminence seem to be fully satisfied with the view
that each species has been independently created. To my mind it accords
better with what we know of the laws impressed on matter by the Creator,
that the production and extinction of the past and present inhabitants of
the world should have been due to secondary causes, like those determining
the birth and death of the individual. When I view all beings not as special
creations, but as the lineal descendants of some few beings which lived long
before the first bed of the Silurian system was deposited, they seem to me
to become ennobled. Judging from the past, we may safely infer that not one
living species will transmit its unaltered likeness to a distant futurity.
And of the species now living very few will transmit progeny of any kind to
a far distant futurity; for the manner in which all organic beings are
grouped, shows that the greater number of species of each genus, and all the
species of many genera, have left no descendants, but have become utterly
extinct. We can so far take a prophetic glance into futurity as to foretel
that it will be the common and widely-spread species, belonging to the
larger and dominant groups, which will ultimately prevail and procreate new
and dominant species. As all the living forms of life are the lineal
descendants of those which lived long before the Silurian epoch, we may feel
certain that the ordinary succession by generation has never once been
broken, and that no cataclysm has desolated the whole world. Hence we may
look with some confidence to a secure future of equally inappreciable
length. And as natural selection works solely by and for the good of each
being, all corporeal and mental endowments will tend to progress towards
perfection.

It is interesting to contemplate an entangled bank, clothed with many plants
of many kinds, with birds singing on the bushes, with various insects
flitting about, and with worms crawling through the damp earth, and to
reflect that these elaborately constructed forms, so different from each
other, and dependent on each other in so complex a manner, have all been
produced by laws acting around us. These laws, taken in the largest sense,
being Growth with Reproduction; inheritance which is almost implied by
reproduction; Variability from the indirect and direct action of the
external conditions of life, and from use and disuse; a Ratio of Increase so
high as to lead to a Struggle for Life, and as a consequence to Natural
Selection, entailing Divergence of Character and the Extinction of
less-improved forms. Thus, from the war of nature, from famine and death,
the most exalted object which we are capable of conceiving, namely, the
production of the higher animals, directly follows. There is grandeur in
this view of life, with its several powers, having been originally breathed
into a few forms or into one; and that, whilst this planet has gone cycling
on according to the fixed law of gravity, from so simple a beginning endless
forms most beautiful and most wonderful have been, and are being, evolved.

----------------------------------------------------------------------------
Glossary

     I am indebted to the kindness of Mr. W. S. Dallas for this
     Glossary, which has been given because several readers have
     complained to me that some of the terms used were unintelligible
     to them. Mr. Dallas has endeavoured to give the explanations of
     the terms in as popular a form as possible.

     EDITOR'S NOTE: This glossary did not appear in the first edition
     and has been reproduced here directly from the sixth edition

[* ] ABERRANT
     Forms or groups of animals or plants which deviate in important
     characters from their nearest allies, so as not to be easily included
     in the same group with them, are said to be aberrant.
[* ] ABERRATION (in Optics)
     In the refraction of light by a convex lens the rays passing through
     different parts of the lens are brought to a focus at slightly
     different distances, this is called spherical aberration; at the same
     time the coloured rays are separated by the prismatic action of the
     lens and likewise brought to a focus at different distances, this is
     chromatic aberration.
[* ] ABNORMAL
     Contrary to the general rule.
[* ] ABORTED
     An organ is said to be aborted, when its development has been arrested
     at a very early stage.
[* ] ALBINISM
     Albinos are animals in which the usual colouring matters characteristic
     of the species have not been produced in the skin and its appendages.
     Albinism is the state of being an albino.
[* ] ALGAE
     A class of plants including the ordinary sea-weeds and the filamentous
     fresh-water weeds.
[* ] ALTERNATION OF GENERATIONS
     This term is applied to a peculiar mode of reproduction which prevails
     among many of the lower animals, in which the egg produces a living
     form quite different from its parent, but from which the parent-form is
     reproduced by a process of budding, or by the division of the substance
     of the first product of the egg.
[* ] AMMONITES
     A group of fossil, spiral, chambered shells, allied to the existing
     pearly Nautilus, but having the partitions between the chambers waved
     in complicated patterns at their junction with the outer wall of the
     shell.
[* ] ANALOGY
     That resemblance of structures which depends upon similarity of
     function, as in the wings of insects and birds. Such structures are
     said to be analogous, and to be analogues of each other.
[* ] ANIMALCULE
     A minute animal: generally applied to those visible only by the
     microscope.
[* ] ANNELIDS
     A class of worms in which the surface of the body exhibits a more or
     less distinct division into rings or segments, generally provided with
     appendages for locomotion and with gills. It includes the ordinary
     marine worms, the earthworms, and the leeches.
[* ] ANTENNÆ
     Jointed organs appended to the head in Insects, Crustacea and
     Centipedes, and not belonging to the mouth.
[* ] ANTHERS
     The summits of the stamens of flowers, in which the pollen or
     fertilising dust is produced.
[* ] APLACENTALIA, APLACENTATA or Aplacental Mammals
     See Mammalia.
[* ] ARCHETYPAL
     Of or belonging to the Archetype, or ideal primitive form upon which
     all the beings of a group seem to be organised.
[* ] ARTICULATA
     A great division of the Animal Kingdom characterised generally by
     having the surface of the body divided into rings called segments, a
     greater or less number of which are furnished with jointed legs (such
     as Insects, Crustaceans and Centipedes).
[* ] ASYMMETRICAL
     Having the two sides unlike.
[* ] ATROPHIED
     Arrested in development at a very early stage.
[* ] BALANUS
     The genus including the common Acorn-shells which live in abundance on
     the rocks of the sea-coast.
[* ] BATRACHIANS
     A class of animals allied to the Reptiles, but undergoing a peculiar
     metamorphosis, in which the young animal is generally aquatic and
     breathes by gills. (Examples, Frogs, Toads, and Newts.)
[* ] BOULDERS
     Large transported blocks of stone generally imbedded in clays or
     gravels.
[* ] BRACHIOPODA
     A class of marine Mollusca, or soft-bodied animals, furnished with a
     bivalve shell, attached to submarine objects by a stalk which passes
     through an aperture in one of the valves, and furnished with fringed
     arms, by the action of which food is carried to the mouth.
[* ] BRANCHIÆ
     Gills or organs for respiration in water.
[* ] BRANCHIAL
     Pertaining to gills or branchiæ.
[* ] CAMBRIAN SYSTEM
     A Series of very ancient Palæozoic rocks, between the Laurentian and
     the Silurian. Until recently these were regarded as the oldest
     fossiliferous rocks.
[* ] CANIDÆ
     The Dog-family, including the Dog, Wolf, Fox, Jackal, &c.
[* ] CARAPACE
     The shell enveloping the anterior part of the body in Crustaceans
     generally; applied also to the hard shelly pieces of the Cirripedes.
[* ] CARBONIFEROUS
     This term is applied to the great formation which includes, among other
     rocks, the coal-measures. It belongs to the oldest, or Palæozoic,
     system of formations.
[* ] CAUDAL
     Of or belonging to the tail.
[* ] CEPHALOPODS
     The highest class of the Mollusca, or Soft-bodied animals,
     characterised by having the mouth surrounded by a greater or less
     number of fleshy arms or tentacles, which, in most living species, are
     furnished with sucking-cups. (Examples, Cuttle-fish, Nautilus.)
[* ] CETACEA
     An order of Mammalia, including the Whales, Dolphins, &c., having the
     form of the body fish-like, the skin naked, and only the fore-limbs
     developed.
[* ] CHELONIA
     An order of Reptiles including the Turtles, Tortoises, &c.
[* ] CIRRIPEDES
     An order of Crustaceans including the Barnacles and Acorn-shells. Their
     young resemble those of many other Crustaceans in form; but when mature
     they are always attached to other objects, either directly or by means
     of a stalk, and their bodies are enclosed by a calcareous shell
     composed of several pieces, two of which can open to give issue to a
     bunch of curled, jointed tentacles, which represent the limbs.
[* ] COCCUS
     The genus of Insects including the Cochineal. In these the male is a
     minute, winged fly, and the female generally a motionless, berry-like
     mass.
[* ] COCOON
     A case usually of silky material, in which insects are frequently
     enveloped during the second or resting-stage (pupa) of their existence.
     The term `cocoon-stage' is here used as equivalent to `pupa-stage.'
[* ] COELOSPERMOUS
     A term applied to those fruits of the Umbelliferæ which have the seed
     hollowed on the inner face.
[* ] COLEOPTERA
     Beetles, an order of Insects, having a biting mouth and the first pair
     of wings more or less horny, forming sheaths for the second pair, and
     usually meeting in a straight line down the middle of the back.
[* ] COLUMN
     A peculiar organ in the flowers of Orchids, in which the stamens, style
     and stigma (or the reproductive parts) are united.
[* ] COMPOSITÆ or COMPOSITOUS PLANTS
     Plants in which the inflorescence consists of numerous small flowers
     (florets) brought together into a dense head, the base of which is
     enclosed by a common envelope. (Examples, the Daisy, Dandelion, &c.)
[* ] CONFERVÆ
     The filamentous weeds of fresh water.
[* ] CONGLOMERATE
     A rock made up of fragments of rock or pebbles, cemented together by
     some other material.
[* ] COROLLA
     The second envelope of a flower usually composed of coloured, leaf-like
     organs (petals), which may be united by their edges either in the basal
     part or throughout.
[* ] CORRELATION
     The normal coincidence of one phenomenon, character, &c., with another.
[* ] CORYMB
     A bunch of flowers in which those springing from the lower part of the
     flower stalk are supported on long stalks so as to be nearly on a level
     with the upper ones.
[* ] COTYLEDONS
     The first or seed-leaves of plants.
[* ] CRUSTACEANS
     A class of articulated animals, having the skin of the body generally
     more or less hardened by the deposition of calcareous matter, breathing
     by means of gills. (Examples, Crab, Lobster, Shrimp, &c.)
[* ] CURCULIO
     The old generic term for the Beetles known as Weevils, characterised by
     their four-jointed feet, and by the head being produced into a sort of
     beak, upon the sides of which the antennæ are inserted.
[* ] CUTANEOUS
     Of or belonging to the skin.
[* ] DEGRADATION
     The wearing down of land by the action of the sea or of meteoric
     agencies.
[* ] DENUDATION
     The wearing away of the surface of the land by water.
[* ] DEVONIAN SYSTEM or formation
     A series of Palæozoic rocks, including the Old Red Sandstone.
[* ] DICOTYLEDONS or DICOTYLEDONOUS PLANTS
     A class of plants characterised by having two seed-leaves, by the
     formation of new wood between the bark and the old wood (exogenous
     growth) and by the reticulation of the veins of the leaves. The parts
     of the flowers are generally in multiples of five.
[* ] DIFFERENTIATION
     The separation or discrimination of parts or organs which in simpler
     forms of life are more or less united.
[* ] DIMORPHIC
     Having two distinct forms. Dimorphism is the condition of the
     appearance of the same species under two dissimilar forms.
[* ] DIOECIOUS
     Having the organs of the sexes upon distinct individuals.
[* ] DIORITE
     A peculiar form of Greenstone.
[* ] DORSAL
     Of or belonging to the back.
[* ] EDENTATA
     A peculiar order of Quadrupeds, characterised by the absence of at
     least the middle incisor (front) teeth in both jaws. (Examples, the
     Sloths and Armadillos.)
[* ] ELYTRA
     The hardened fore-wings of Beetles, serving as sheaths for the
     membranous hind-wings, which constitute the true organs of flight.
[* ] EMBRYO
     The young animal undergoing development within the egg or womb.
[* ] EMBRYOLOGY
     The study of the development of the embryo.
[* ] ENDEMIC
     Peculiar to a given locality.
[* ] ENTOMOSTRACA
     A division of the class Crustacea, having all the segments of the body
     usually distinct, gills attached to the feet or organs of the mouth,
     and the feet fringed with fine hairs. They are generally of small size.
[* ] EOCENE
     The earliest of the three divisions of the Tertiary epoch of
     geologists. Rocks of this age contain a small proportion of shells
     identical with species now living.
[* ] EPHEMEROUS INSECTS
     Insects allied to the May-fly.
[* ] FAUNA
     The totality of the animals naturally inhabiting a certain country or
     region, or which have lived during a given geological period.
[* ] FELIDÆ
     The Cat-family.
[* ] FERAL
     Having become wild from a state of cultivation or domestication.
[* ] FLORA
     The totality of the plants growing naturally in a country, or during a
     given geological period.
[* ] FLORETS
     Flowers imperfectly developed in some respects, and collected into a
     dense spike or head, as in the Grasses, the Dandelion, &c.
[* ] FOETAL
     Of or belonging to the foetus, or embryo in course of development.
[* ] FORAMINIFERA
     A class of animals of very low organisation, and generally of small
     size, having a jelly-like body, from the Surface of which delicate
     filaments can be given off and retracted for the prehension of external
     objects, and having a calcareous or sandy shell, usually divided into
     chambers, and perforated with small apertures.
[* ] FOSSILIFEROUS
     Containing fossils.
[* ] FOSSORIAL
     Having a faculty of digging. The Fossorial Hymenoptera are a group of
     Wasp-like Insects, which burrow in sandy soil to make nests for their
     young.
[* ] FRENUM (pl
     FRENA). A small band or fold of skin.
[* ] FUNGI (Sing
     FUNGUS). A class of cellular plants, of which Mushrooms, Toadstools,
     and Moulds, are familiar examples.
[* ] FURCULA
     The forked bone formed by the union of the collarbones in many birds,
     such as the common Fowl.
[* ] GALLINACEOUS BIRDS
     An order of Birds of which the common Fowl, Turkey, and Pheasant, are
     well-known examples.
[* ] GALLUS
     The genus of birds which includes the common Fowl.
[* ] GANGLION
     A swelling or knot from which nerves are given off as from a centre.
[* ] GANOID FISHES
     Fishes covered with peculiar enamelled bony scales. Most of them are
     extinct.
[* ] GERMINAL VESICLE
     A minute vesicle in the eggs of animals, from which development of the
     embryo proceeds.
[* ] GLACIAL PERIOD
     A period of great cold and of enormous extension of ice upon the
     surface of the earth. It is believed that glacial periods have occurred
     repeatedly during the geological history of the earth, but the term is
     generally applied to the close of the Tertiary epoch, when nearly the
     whole of Europe was subjected to an arctic climate.
[* ] GLAND
     An organ which secretes or separates some peculiar product from the
     blood or sap of animals or plants.
[* ] GLOTTIS
     The opening of the windpipe into the oesophagus or gullet.
[* ] GNEISS
     A rock approaching granite in composition, but more or less laminated,
     and really produced by the alteration of a sedimentary deposit after
     its consolidation.
[* ] GRALLATORES
     The so-called Wading-birds (Storks, Cranes, Snipes, &c.), which are
     generally furnished with long legs, bare of feathers above the heel,
     and have no membranes between the toes.
[* ] GRANITE
     A rock consisting essentially of crystal of felspar and mica in a mass
     of quarts.
[* ] HABITAT
     The locality in which a plant or animal naturally lives.
[* ] HEMIPTERA
     An order or sub-order of Insects, characterised by the possession of a
     jointed beak or rostrum, and by having the fore-wings horny in the
     basal portion and membranous at the extremity, where they cross each
     other. This group includes the various species of Bugs.
[* ] HERMAPHRODITE
     Possessing the organs of both sexes.
[* ] HOMOLOGY
     That relation between parts which results from their development from
     corresponding embryonic parts, either in different animals, as in the
     case of the arm of man, the foreleg of a quadruped, and the wing of a
     bird; or in the same individual, as in the case of the fore and hind
     legs in quadrupeds, and the segments or rings and their appendages of
     which the body of a worm, a centipede, &c., is composed. The latter is
     called serial homology. The parts which stand in such a relation to
     each other are said to be homologous, and one such part or organ is
     called the homologue of the other. In different plants the parts of the
     flower are homologous, and in general these parts are regarded as
     homologous with leaves.
[* ] HOMOPTERA
     An order or sub-order of Insects having (like the Hemiptera) a jointed
     beak, but in which the fore-wings are either wholly membranous or
     wholly leathery. The Cicadoe, Frog-hoppers, and Aphides, are well-known
     examples.
[* ] HYBRID
     The offspring of the union of two distinct species.
[* ] HYMENOPTERA
     An order of insects possessing biting jaws and usually four membranous
     wings in which there are a few veins. Bees and Wasps are familiar
     examples of this group.
[* ] HYPERTROPHIED
     Excessively developed.
[* ] ICHNEUMONIDÆ
     A family of Hymenopterous insects, the members of which lay their eggs
     in the bodies or eggs of other insects.
[* ] IMAGO
     The perfect (generally winged) reproductive state of an insect.
[* ] INDIGENS
     The aboriginal animal or vegetable inhabitants of a country or region.
[* ] INFLORESCENCE
     The mode of arrangement of the flowers of plants.
[* ] INFUSORIA
     A class of microscopic Animalcules, so called from their having
     originally been observed in infusions of vegetable matters. They
     consist of a gelatinous material enclosed in a delicate membrane, the
     whole or part of which is furnished with short vibrating hairs (called
     cilia), by means of which the animalcules swim through the water or
     convey the minute particles of their food to the orifice of the mouth.
[* ] INSECTIVOROUS
     Feeding on Insects.
[* ] INVERTEBRATA, or INVERTEBRATE ANIMALS
     Those animals which do not possess a backbone or spinal column.
[* ] LACUNÆ
     Spaces left among the tissues in some of the lower animals, and serving
     in place of vessels for the circulation of the fluids of the body.
[* ] LAMELLATED
     Furnished with lamellæ or little plates.
[* ] LARVA (pl
     LARVÆ). The first condition of an insect at its issuing from the egg,
     when it is usually in the form of a grub, caterpillar, or maggot.
[* ] LARYNX
     The upper part of the windpipe opening into the gullet.
[* ] LAURENTIAN
     A group of greatly altered and very ancient rocks, which is greatly
     developed along the course of the St. Laurence, whence the name. It is
     in these that the earliest known traces of organic bodies have been
     found.
[* ] LEGUMINOSÆ
     An order of plants represented by the common Peas and Beans, having an
     irregular flower in which one petal stands up like a wing, and the
     stamens and pistil are enclosed in a sheath formed by two other petals.
     The fruit is a pod (or legume).
[* ] LEMURIDÆ
     A group of four-handed animals, distinct from the Monkeys and
     approaching the Insectivorous Quadrupeds in some of their characters
     and habits. Its members have the nostrils curved or twisted, and a claw
     instead of a nail upon the first finger of the hind hands.
[* ] LEPIDOPTERA
     An order of Insects, characterised by the possession of a spiral
     proboscis, and of four large more or less scaly wings. It includes the
     well-known Butterflies and Moths.
[* ] LITTORAL
     Inhabiting the seashore.
[* ] LOESS
     A marly deposit of recent (Post-Tertiary) date, which occupies a great
     part of the valley of the Rhine.
[* ] MALACOSTRACA
     The higher division of the Crustacea, including the ordinary Crabs,
     Lobsters, Shrimps, &c., together with the Woodlice and Sand-hoppers.
[* ] MAMMALIA
     The highest class of animals, including the ordinary hairy quadrupeds,
     the Whales, and Man, and characterised by the production of living
     young which are nourished after birth by milk from the teats (Mammoe,
     Mammary glands) of the mother. A striking difference in embryonic
     development has led to the division of this class into two great
     groups; in one of these, when the embryo has attained a certain stage,
     a vascular connection, called the placenta, is formed between the
     embryo and the mother; in the other this is wanting, and the young are
     produced in a very incomplete state. The former, including the greater
     part of the class, are called Placental mammals; the latter, or
     Aplacental mammals, include the Marsupials and Monotremes
     (Ornithorhynchus).
[* ] MAMMIFEROUS
     Having mammæ; or teats (See MAMMALIA).
[* ] MANDIBLES, in Insects
     The first or uppermost pair of jaws, which are generally solid, horny,
     biting organs. In Birds the term is applied to both jaws with their
     horny coverings. In Quadrupeds the mandible is properly the lower jaw.
[* ] MARSUPIALS
     An order of Mammalia in which the young are born in a very incomplete
     state of development, and carried by the mother, while sucking, in a
     ventral pouch (marsupium), such as the Kangaroos, Opossums, &c. (see
     MAMMALIA).
[* ] MAXILLÆ, in Insects
     The second or lower pair of jaws, which are composed of several joints
     and furnished with peculiar jointed appendages called palpi, or
     feelers.
[* ] MELANISM
     The opposite of albinism; an undue development of colouring material in
     the skin and its appendages.
[* ] METAMORPHIC ROCKS
     Sedimentary rocks which have undergone alteration, generally by the
     action of heat, subsequently to their deposition and consolidation.
[* ] MOLLUSCA
     One of the great divisions of the Animal Kingdom, including those
     animals which have a soft body, usually furnished with a shell, and in
     which the nervous ganglia, or centres, present no definite general
     arrangement. They are generally known under the denomination of
     `shell-fish;' the cuttle-fish, and the common snails, whelks, oysters,
     mussels, and cockles, may serve as examples of them.
[* ] MONOCOTYLEDONS, or MONOCOTYLEDONOUS PLANTS
     Plants in which the seed sends up only a single seed-leaf (or
     cotyledon); characterised by the absence of consecutive layers of wood
     in the stem (endogenous growth), by the veins of the leaves being
     generally straight, and by the parts of the flowers being generally in
     multiples of three. (Examples, Grasses, Lilies, Orchids, Palms, &c.)
[* ] MORAINES
     The accumulations of fragments of rock brought down by glaciers.
[* ] MORPHOLOGY
     The law of form or structure independent of function.
[* ] MYSIS-STAGE
     A stage in the development of certain Crustaceans (Prawns), in which
     they closely resemble the adults of a genus (Mysis) belonging to a
     slightly lower group.
[* ] NASCENT
     Commencing development.
[* ] NATATORY
     Adapted for the purpose of swimming.
[* ] NAUPLIUS-FORM
     The earliest stage in the development of many Crustacea, especially
     belonging to the lower groups. In this stage the animal has a short
     body, with indistinct indications of a division into segments, and
     three pairs of fringed limbs. This form of the common fresh-water
     Cyclops was described as a distinct genus under the name of Nauplius.
[* ] NEURATION
     The arrangement of the veins or nervures in the wings of Insects.
[* ] NEUTERS
     Imperfectly developed females of certain social insects (such as Ants
     and Bees), which perform all the labours of the community. Hence they
     are also called workers.
[* ] NICTITATING MEMBRANE
     A semi-transparent membrane, which can be drawn across the eye in Birds
     and Reptiles, either to moderate the effects of a strong light or to
     sweep particles of dust, &c., from the surface of the eye.
[* ] OCELLI
     The simple eyes or stemmata of Insects, usually situated on the crown
     of the head between the great compound eyes.
[* ] OESOPHAGUS
     The gullet.
[* ] OOLITIC
     A great series of secondary rocks, so called from the texture of some
     of its members, which appear to be made up of a mass of small egg-like
     calcareous bodies.
[* ] OPERCULUM
     A calcareous plate employed by many Mollusca to close the aperture of
     their shell. The opercular valves of Cirripedes are those which close
     the aperture of the shell.
[* ] ORBIT
     The bony cavity for the reception of the eye.
[* ] ORGANISM
     An organised being, whether plant or animal.
[* ] ORTHOSPERMOUS
     A term applied to those fruits of the Umbelliferæ which have the seed
     straight.
[* ] OSCULANT
     Forms or groups apparently intermediate between and connecting other
     groups are said to be osculant.
[* ] OVA
     Eggs.
[* ] OVARIUM or OVARY (in plants)
     The lower part of the pistil or female organ of the flower, containing
     the ovules or incipient seeds; by growth after the other organs of the
     flower have fallen, it usually becomes converted into the fruit.
[* ] OVIGEROUS
     Egg-bearing.
[* ] OVULES (of plants)
     The seeds in the earliest condition.
[* ] PACHYDERMS
     A group of Mammalia, so called from their thick skins, and including
     the Elephant, Rhinoceros, Hippopotamus, &c.
[* ] PALÆOZOIC
     The oldest system of fossiliferous rocks.
[* ] PALPI
     Jointed appendages to some of the organs of the mouth in Insects and
     Crustacea.
[* ] PAPILIONACEÆ
     An order of Plants (see LEGUMINOSÆ). The flowers of these plants are
     called papilionaceous, or butterfly-like, from the fancied resemblance
     of the expanded superior petals to the wings of a butterfly.
[* ] PARASITE
     An animal or plant living upon or in, and at the expense of, another
     organism.
[* ] PARTHENOGENESIS
     The production of living Organisms from unimpregnated eggs or seeds.
[* ] PEDUNCULATED
     Supported upon a stem or stalk. The pedunculated oak has its acorns
     borne upon a footstalk.
[* ] PELORIA or PELORISM
     The appearance of regularity of structure in the flowers of plants
     which normally bear irregular flowers.
[* ] PELVIS
     The bony arch to which the hind limbs of Vertebrate animals are
     articulated.
[* ] PETALS
     The leaves of the corolla, or second circle of organs in a flower. They
     are usually of delicate texture and brightly coloured.
[* ] PHYLLODINEOUS
     Having flattened, leaf-like twigs or leafstalks instead of true leaves.
[* ] PIGMENT
     The colouring material produced generally in the superficial parts of
     animals. The cells secreting it are called pigment-cells.
[* ] PINNATE
     Bearing leaflets on each side of a central stalk.
[* ] PISTILS
     The female organs of a flower, which occupy a position in the centre of
     the other floral organs. The pistil is generally divisible into the
     ovary or germen, the style and the stigma.
[* ] PLACENTALIA, PLACENTATA, or Placental Mammals
     See MAMMALIA.
[* ] PLANTIGRADES
     Quadrupeds which walk upon the whole sole of the foot, like the Bears.
[* ] PLASTIC
     Readily capable of change.
[* ] PLEISTOCENE PERIOD
     The latest portion of the Tertiary epoch.
[* ] PLUMULE (in plants)
     The minute bud between the seed-leaves of newly-germinated plants.
[* ] PLUTONIC ROCKS
     Rocks supposed to have been produced by igneous action in the depths of
     the earth.
[* ] POLLEN
     The male element in flowering plants; usually a fine dust produced by
     the anthers, which, by contact with the stigma effects the fecundation
     of the seeds. This impregnation is brought about by means of tubes
     (pollen-tubes) which issue from the pollen-grains adhering to the
     stigma, and penetrate through the tissues until they reach the ovary.
[* ] POLYANDROUS (flowers)
     Flowers having many stamens.
[* ] POLYGAMOUS PLANTS
     Plants in which some flowers are unisexual and others hermaphrodite.
     The unisexual (male and female) flowers, may be on the same or on
     different plants.
[* ] POLYMORPHIC
     Presenting many forms.
[* ] POLYZOARY
     The common structure for the Polyzoa, such as the well-known Sea-mats.
[* ] PREHENSILE
     Capable of grasping.
[* ] PREPOTENT
     Having a superiority of power.
[* ] PRIMARIES
     The feathers forming the tip of the wing of a bird, and inserted upon
     that part which represents the hand of man.
[* ] PROCESSES
     Projecting portions of bones, usually for the attachment of muscles,
     ligaments, &c.
[* ] PROPOLIS
     A resinous material collected by the Hive-Bees from the opening buds of
     various trees.
[* ] PROTEAN
     Exceedingly variable.
[* ] PROTOZOA
     The lowest great division of the Animal Kingdom. These animals are
     composed of a gelatinous material, and show scarcely any trace of
     distinct organs. The Infusoria, Foraminifera, and Sponges, with some
     other forms, belong to this division.
[* ] PUPA (pl
     PUPÆ). The second stage in the development of an Insect, from which it
     emerges in the perfect (winged) reproductive form. In most insects the
     pupal stage is passed in perfect repose. The chrysalis is the pupal
     state of butterflies.
[* ] RADICLE
     The minute root of an embryo plant.
[* ] RAMUS
     One half of the lower jaw in the Mammalia. The portion which rises to
     articulate with the skull is called the ascending ramus.
[* ] RANGE
     The extent of country over which a plant or animal is naturally spread.
     Range in time expresses the distribution of a species or group through
     the fossiliferous beds of the earth's crust.
[* ] RETINA
     The delicate inner coat of the eye, formed by nervous filaments
     spreading from the optic nerve, and serving for the perception of the
     impressions produced by light.
[* ] RETROGRESSION
     Backward development. When an animal, as it approaches maturity,
     becomes less perfectly organised than might be expected from its early
     stages and known relationships, it is said to undergo a retrograde
     development or metamorphosis.
[* ] RHIZOPODS
     A class of lowly organised animals (protozoa), having a gelatinous
     body, the surface of which can be protruded in the form of root-like
     processes or filaments, which serve for locomotion and the prehension
     of food. The most important order is that of the Foraminifera.
[* ] RODENTS
     The gnawing Mammalia, such as the Rats, Rabbits, and Squirrels. They
     are especially characterised by the possession of a single pair of
     chisel-like cutting teeth in each jaw, between which and the grinding
     teeth there is a great gap.
[* ] RUBUS
     The Bramble Genus.
[* ] RUDIMENTARY
     Very imperfectly developed.
[* ] RUMINANTS
     The group of Quadrupeds which ruminate or chew the cud, such as oxen,
     sheep, and deer. They have divided hoofs, and are destitute of front
     teeth in the upper jaw.
[* ] SACRAL
     Belonging to the sacrum, or the bone composed usually of two or more
     united vertebræ to which the sides of the pelvis in Vertebrate animals
     are attached.
[* ] SARCODE
     The gelatinous material of which the bodies of the lowest animals
     (Protozoa) are composed.
[* ] SCUTELLÆ
     The horny plates with which the feet of birds are generally more or
     less covered, especially in front.
[* ] SEDIMENTARY FORMATIONS
     Rocks deposited as sediments from water.
[* ] SEGMENTS
     The transverse rings of which the body of an articulate animal or
     Annelid is composed.
[* ] SEPALS
     The leaves or segments of the calyx, or outermost envelope of an
     ordinary flower. They are usually green, but sometimes brightly
     coloured.
[* ] SERRATURES
     Teeth like those of a saw.
[* ] SESSILE
     Not supported on a stem or footstalk.
[* ] SILURIAN SYSTEM
     A Very ancient system of fossiliferous rocks belonging to the earlier
     part of the Palæozoic series.
[* ] SPECIALISATION
     The setting apart of a particular organ for the performance of a
     particular function.
[* ] SPINAL CHORD
     The central portion of the nervous system in the Vertebrata, which
     descends from the brain through the arches of the vertebræ, and gives
     off nearly all the nerves to the various organs of the body.
[* ] STAMENS
     The male organs of flowering plants, standing in a circle within the
     petals. They usually consist of a filament and an anther, the anther
     being the essential part in which the pollen, or fecundating dust, is
     formed.
[* ] STERNUM
     The breast-bone.
[* ] STIGMA
     The apical portion of the pistil in flowering plants.
[* ] STIPULES
     Small leafy organs placed at the base of the footstalks of the leaves
     in many plants.
[* ] STYLE
     The middle portion of the perfect pistil, which rises like a column
     from the ovary and supports the stigma at its summit.
[* ] SUBCUTANEOUS
     Situated beneath the skin.
[* ] SUCTORIAL
     Adapted for sucking.
[* ] SUTURES (in the skull)
     The lines of junction of the bones of which the skull is composed.
[* ] TARSUS (pl
     TARSI). The jointed feet of articulate animals, such as Insects.
[* ] TELEOSTEAN FISHES
     Fishes of the kind familiar to us in the present day, having the
     skeleton usually completely ossified and the scales horny.
[* ] TENTACULA or TENTACLES
     Delicate fleshy organs of prehension or touch possessed by many of the
     lower animals.
[* ] TERTIARY
     The latest geological epoch, immediately preceding the establishment of
     the present order of things.
[* ] TRACHEA
     The windpipe or passage for the admission of air to the lungs.
[* ] TRIDACTYLE
     Three-fingered, or composed of three movable parts attached to a common
     base.
[* ] TRILOBITES
     A peculiar group of extinct Crustaceans, somewhat resembling the
     Woodlice in external form, and, like some of them, capable of rolling
     themselves up into a ball. Their remains are found only in the
     Palæozoic rocks, and most abundantly in those of Silurian age.
[* ] TRIMORPHIC
     Presenting three distinct forms.
[* ] UMBELLIFERÆ
     An order of plants in which the flowers, which contain five stamens and
     a pistil with two styles, are supported upon footstalks which spring
     from the top of the flower stem and spread out like the wires of an
     umbrella, so as to bring all the flowers in the same head (umbel)
     nearly to the same level. (Examples, Parsley and Carrot).
[* ] UNGULATA
     Hoofed quadrupeds.
[* ] UNICELLULAR
     Consisting of a single cell.
[* ] VASCULAR
     Containing blood-vessels
[* ] VERMIFORM
     Like a worm.
[* ] VERTEBRATA: or VERTEBRATE ANIMALS
     The highest division of the animal kingdom, so called from the presence
     in most cases of a backbone composed of numerous joints or vertebroe,
     which constitutes the centre of the skeleton and at the same time
     supports and protects the central parts of the nervous system.
[* ] WHORLS
     The circles or spiral lines in which the parts of plants are arranged
     upon the axis of growth.
[* ] WORKERS
     See neuters.
[* ] ZOEA-STAGE
     The earliest stage in the development of many of the higher Crustacea,
     so called from the name of Zoea applied to these young animals when
     they were supposed to constitute a peculiar genus.
[* ] ZOOIDS
     In many of the lower animals (such as the Corals, Medusæ, &c.)
     reproduction takes place in two ways, namely, by means of eggs and by a
     process of budding with or without separation from the parent of the
     product of the latter, which is often very different from that of the
     egg. The individuality of the species is represented by the whole of
     the form produced between two sexual reproductions; and these forms,
     which are apparently individual animals, have been called zooids.