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•■fiFrnff-T— nrn!^ — -^ 

H—»«*— «— ■ II ■! I I I I II I II I I II I « I IIIIM— — W»» 

This writer n^s bom Fob. 12^ 13^9, 
the same birUx-dat© as that of Abraham 
Lincoln* His first book has entitled 
''Journal of He searches into Geology and 
Natural History," published in I659f - 
a second edition of the sazae appearing 
in 1 "yh^ It is likely this is the work 
referred to by Willian H. Tlemdon as be- 
ing at Lincoln's Imnd to be dipped into 
as the Eiood mi^t incline him; othor 
works appeared from ti>30 to tiae, but 
none with titles to attract Lincoln until 
the "Origin of Species*', tJhich ^?as first 
published in book fom in 1660. TliiD, too, 
may be included in the ^rorks referred to 
by Hemdon v*ien he says: "I purclmsed the 
vorks of Spencer, Darwin, and the utter- 
enoes of other FYiglish scientists, etc." 
Lincoln wDuld ''snatch one up and peruse 
it for a little vdiile, but soon throw it 
down with the suggestion that it ^jsas en- 
tirely too heavy for an ordinary iiind to 
digest. " 

(See Hemdon' 3 Lincoln, ivage ^^56) 

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" But with regard to the material world, we can at least go so far as 
this — we can perceive that events are brought about not by insulated in- 
terpositions of Divine power, exerted in each particular case, but by the 
eetabUshment of general laws." 

W. Whewell : Bridgewater Treatise. 

" To conclude, therefore, let no man out of a weak conceit of sobriety, 
or an ill-applied moderation, think or maintain, that a man can search too 
far or be too well studied in the book of God's word, or in the book of God's 
works; divinity or philosophy; but rather let men endeavour an endless 
progress or proficience in both." 

Bacon: Advancement of Learning. 

" The only distinct meaning of the word ' natural ' is stated, fixed, or 
gettled; since what is natural as much requires and presupposes an intelli- 
gent agent to render it so, i. e. to effect it continually or at stated times, sm 
what is supernatural or miraculous does to effect it for once." 

Butler : Analogy of Revealed Religion. 

Down, Bromley, Kent, 

October \st, 1859. 















443 & 445 BROADWAY. 




I WILL here attempt to give a brief, but I fear imper- 
fect, sketch of the progress of opinion on the Origin of 
Species. The great majority of naturalists have beheved 
that species were immutable productions and liave been 
separately created : this view has been ably maintained 
by many authors. A few naturalists, and several who 
have not particularly studied natural history, believe, on 
the other hand, that species undergo modification, and 
that the existing forms of life have descended by true gen- 
eration from pre-existing forms. Passing over authors of 
the classical period, and likewise Demaillet and Bufibn, 
with whose writings I am not familiar, Lamarck was the 
first man, whose view that species undergo change ex- 
cited much attention. This justly celebrated naturalist 
published his Philosophic Zoologique in 1809, and his In- 
troduction to his Hist. Kat. des animaux sans Yert^bres 
in 1815, in which works he upholds the doctrine that spe- 
cies are descended from each other. He seems to have 
been chiefly led to this conclusion by the difficulty of dis- 
tinguishing species and varieties, — by tlie almost perfect 
gradation of the forms in certain groups, and by the anal- 
ogy of domestic productions. With respect to the means 
of modification, he attributed something to the action of 


external conditions, something to the crossing of already- 
existing forms, and mncli to use and disuse or the effects 
of habit. To this latter agency he seems to attribute all 
the beautiful adaptations in nature, — such as the long 
neck of the giraffe for browsing on the branches of trees.* 
Eut he likewise believed in a law of progressive develop- 
ment ; and as all the forms of life thus tended to progress, 
in order to account for the presence of very simple pro- 
ductions at the present day, he maintained that such forms 
were now spontaneously generated. 

Geoffroy Saint Hilaire, as is stated in his Life by his 
Son, as early as 1795, suspected 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 con- 
ditions of life, or the " monde ambiant," as the cause of 
change ; but he was cautious, and, as his son says, " C'est 
done un probl^me k reserver enti^rement a I'avenir, sup- 
pose meme que I'avenir doive avoir prise sur lui." 

In England, the Hon. and Eev. W. Herbert, after- 
wards Dean of Manchester, in his work on the Amarylli- 
dacese (1837, p. 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 w^ere created in an originally highly plastic 
condition, and that these by intercrossing and by varia- 
tion have produced all our existing species. 

* It is curious how completely my grandfather, Dr. Erasmus Darwin, 
anticipated these erroneous views in his Zoonoraia (vol. i. pp. 500-510), pub- 
lished in 1'794. 


In 1843-44, Prof. Haldeman (in the Boston (U. S.) 
Journal of Nat. Hist., vol. iv., p. 468) has ably given the 
arguments for and against the hypothesis of the develop- 
ment and modification of species : he seems to me to lean 
towards the side of change. 

The Yestiges of Creation appeared in 1844. In the 
last or tenth and much improved edition (1853, p. 155), 
the anonymous author says : ^' 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, Jirst, 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 be- 
ing few in number, and generally marked by intervals of 
organic character which we find to be a practical diflSculty 
in ascertaining affinities; second, of another impulse con- 
nected with the vital forces, tending in the course of gen- 
erations 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 be- 
lieves that organisation progresses by sudden leaps ; but 
that the effects produced by the conditions of life are 
gradual. The author 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, immedi- 
ately had a very wide circulation. In my opinion, it has 
done excellent service in calling in this country attention 
to the subject, and in removing prejudices. 

In 1846, the veteran geologist, M. J. d'Omalius d'Hal- 
loz, published in an excellent, though short, paper (Bulle- 
tins de I'Acad. Roy. Bruxelles, tom. xiii., p. 581), his opin- 
ion 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 promul- 
gated this opinion in 1831. 

M. Isidore Geoffroy Saint Hilaire, in his Lectures de- 
livered in 1850 (of which a Resume appeared in the Re- 
vue et Mag. de Zoolog., Jan. 1851), briefly gives his 
reasons for believing that specific characters " sont fixes, 
pour chaque espece, tant qu'elle se perpetue au milieu des 
memes circonstances, ils se modifient, si les circonstances 
ambiantes viennent a changer." " En resume, V observa- 
tion des animaux sauvages demontre deja la variability 
limitee des especes. Les exphiences sur les animaux sau- 
vages devenus domestiques, et sur les animaux domes- 
tiques redevenus sauvages, la demontrent plus clairement 
encore. Ces memes experiences prouvent, de plus, que 
les difi'erences produites peuvent etre de valeur gme- 

Mr. Herbert Spencer, in an Essay (originally pub- 
lished in the Leader, March, 1852, and republished in his 
Essays, 1858), has contrasted the theories of the creation 
and development of organic beings with remarkable skill 
and force. He argues from the analogy of domestic pro- 
ductions, from the changes which the embryos of many 
species undergo, from the difficulty of distinguishing spe- 


cies and varieties, and from the principle of general gra- 
dation, that species have been modified ; and he attributes 
the modification to the change of circumstances. The 
author (1855) has treated Psychology on the principle of 
the necessary acquirement of each mental power and ca- 
pacity by gradation. 

In 1852 (Revue Horticole, p. 102), M. ISTaudin, a dis- 
tinguished botanist,* has expressly stated 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 se- 
lection can act under nature. He believes, like Dean 
Herbert, that species when nascent were more plastic. 
He lays weight on what he calls the principle of finality, 
^•puissance myst^-ieuse, indeterminee ; fatalite pour les 
nns ; pour les autres, volonte providentielle, dont Faction 
incessante sur les etres vivants determine, k toutes les 
epoques de I'existence du monde, la forme, le volume et 
la duree de chacun d'eux, en raison de sa destinee dans 
I'ordre de choses dont il fait partie. C'est cette puissance 
qui harmonise chaque membre a I'ensemble en I'appropri- 
ant a la fonction qu'il doit remphr dans I'organisme gene- 
ral de la nature, fonction qui est pour lui sa raison d'etre." 

In 1853, a celebrated geologist, Count Keyserling (Bul- 
letin de la Soc. Geolog., 2d ser., torn, x., p. 357) suggested 
that as new diseases, supposed to JUave 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 afifected by circumambient molecules of 
a particular nature, and thus have given rise to new 

* M. Lecoq, another French botanist, entertains, I believe, analogous 
viewa on the modification and descent of species. 


The " Philosophy of Creation " has been treated in an 
admirable manner by the Rev. Baden Powell, in his Es- 
says on the Unity of Worlds, 1855. ITothing can be more 
striking than the manner in which he shows that the in- 
troduction 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." I 
think this work can hardly have failed to have produced 
a great effect in every philosophical mind. 

The third volume of the Journal of the Linnean So- 
ciety (August, 1858) contains papers by Mr. Wallace and 
myself, in which, as stated in the introductory remarks to 
this volume, the theory of ITatural Selection is promul- 

In June, 1859, Professor Huxley gave a lecture be- 
fore the Royal Institution on the Persistent Types of Ani- 
mal 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 dis- 
tinct act of creative power ; and it is well to recollect that 
such an assumption is as unsupported by tradition or reve- 
lation as it is opposed to the general analogy of nature. 
If, on the other hand, we view ' Persistent Types ' in rela- 
tion to that hypothesis which supposes the species living 
at anytime to be the result of the gradual modification of 
pre-existing species, — a hypothesis which, though un- 
proven, and sadly damaged by some of its supporters, is 
yet the only one to which physiology lends any counte- 
nance — their existence would seem to show that the 
amount of modification which living beings have under- 
gone during geological time is but very small in relation 


to the whole series of changes which they have suf- 

In November, 1859, the first edition of this work was 
published. In December, 1859, Dr. Hooker published 
his Introduction to the Tasmanian Flora : in the first part 
of this admirable essay he admit-s the truth of the descent 
and modification of species ; and supports this doctrine 
by many original and valuable observations. 

Chaeles Daewin 
Down, Bromley, Kent, Feb. 1860. 


INTBODV0TIOI7, ......... Page 9 



Causes of Variability— Effects of Habit— Correlation of Growtb— Inheritance— Char- 
acter 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 Domes- 
tic Productions — Circumstances favourable to Man's power of Selection, . 14 



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, . . . . , .46 



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 checlis 
to increase — Competition universal — Effects of climate — Protection from the num- 
ber of individuals — Complex relations of all animals and plants throughout nature 
— Struggle for life most severe between individuals and varieties of the same spe- 
cies ; often severe between species of the same genus — The relation of organism 
to organism the most important of all relations, . . . . .60 



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 — Cir- 
cumstances favourable and unfavourable to Natural Selection, namely, intercross- 
ing, isolation, number of individuals — Slow action — Extinction caused by Natural 
Selection — Divergence of Cliaracter, 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, ....... 77 



Effects of external conditions— Use and disuse, combined with natural selection ; 
organs of flight and of vision — Acclimatisation— Correlation of growth— Compen- 
sation and economy of growth — False correlations— Multiple, rudimentary, and 
lowly organised structures variable — Parts developed in an unusual manner are 
highly variable : specific characters more variable than generic : secondary sexual 
characters variable— Species of the same genus vaiy in an analogous manner— Re- 
versions to long lost characters — Summary, ..... 120 



Difliculties 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, ...... 154 



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, ..... 185 



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 diflerences— Causes of the sterility of first crosses and of 
hybrids— Parallelism between the eflects of changed conditions of life and crosa- 


ing— Fertility of varieties -when crossed and of their mongrel offspring not uni- 
versal—Hybrids and mongrels compared independently of their fertility— Sum- 
mary, 217 



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 palasonto- 
logical collections— On the intermittence of geological formations— On the absence 
of intermediate varieties in any one formation — On the sudden appearance of 
groups of species — On their sudden appearance in the lowest known fossiliferous 
strata, ........... 245 



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 gen- 
eral rules in their appearance and disappearance as do single species — On Extinc- 
tion — 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 de- 
velopment of ancient foi-ms — On the succession of the same types within the same 
areas— Summary of preceding and present chapters, .... 273 



Present distribution cannot be accounted for by differences in physical conditions — Im- 
portance 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, .......... 302 



Distribution of fresh-water 'productions— On the inhabitants of oceanic islands — Ab- 
sence of Batrachians and of terrestrial mammals — On the relation of the inhabit- 
ants of islands to those of the nearest mainland — On colonisation from the nearest 
source with subsequent modification — Summary of the last and present chap- 
ters, ........... 334 



Classification, groups subordinate to groups— Natural system— Rules and difficul- 
ties in classification, explained on the theory of descent with modification— Classi- 


fication 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 super- 
vening at an early age, and being inherited at a corresponding age — Rudimentary 
organs •, their origin explained — Summary, ..... 358 



Recapitulation of the difficulties on the theory of Natural Selection — Recapitulation 
of the general and special circumstances in its favour — Causes of the general 
heUef 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, .......... S9S 

Ljdex, 425 

tnttruction to Bindcr.~l!ho Diagram to front pago 133. 



"When on board H. M. S. ' Beagle ' as naturalist, I was 
mucli struck with certain facts in the distribution of the 
inhabitants of South America, and in the geological rela- 
tions of the present to the past inhabitants of that con- 
tinent. 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 some- 
thing 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 stead- 
ily pursued the same object. I hope that I may be ex- 
cused for entering on these personal details, as I give 
them to show that I have not been hasty in coming to a 

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, aud it 
is published in the third Tolume of the Journal of that 
Society. Sir C. Lyell and Dr. Hookei*, who both knew 
of my work — the latter having read my sketch of 1844 
— ^liononred 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 author- 
ities for my several statements ; and I must trust to the 
reader rej)Osing some confidence in my accuracy. Ko 
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. I^o one can feel more 
sensible than I do of the necessity of hereafter publishing 
in detail all the facts, with references, on which my con- 
clusions 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 a]3parently leading to conclusions di- 
rectly 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 judgment. 

In considering the Origin of Species, it is quite con- 
ceivable that a naturalist, reflecting on the mutual affini- 
ties 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 de- 



scended, like varieties, from other species. JSTevertheless 
such a conclusion, even if well founded, would be unsat- 
isfactory, until it could be shown how the innumerable 
species inhabiting this world have been modified, so as to 
acquire that perfection of structure and coadaptation 
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 abso- 
lutely requiring the agency of certain insects to bring pol- 
len from one flower to the other, it is equally preposterous 
to account for the structure of this parasite, with its rela- 
tions to several distinct organic beings, by the effects of 
external conditions, or of habit, or of the volition of the 
plant itself. 

It is, therefore, of the highest importance to gain a 
clear insight into the means of modification and coadapta- 
tion. At the commencement of my observations it seemed 
to me probable that a careful study of domesticated ani- 
mals and of cultivated plants would offer the best chance 
of making out this obscure problem. ISTor 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 Yariation under Domestica- 
tion. 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 ; bnt I stall, unfortunately, 
be compelled to treat tliis 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 or- 
ganic beings throughout the world, which inevitably fol- 
lows 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 pos- 
sibly 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 con 
ditions of life, will have a better chance of surviving, ano 
thus be naturally selected. From the strong principle of 
inheritance, any selected variety will tend to propagatf 
its new and modified form. 

This fundamental subject of Natural Selection will be 
treated at some length in the fourth chapter ; and wc 
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 difiiculties on the theory will be given : namely, 
first, the difficulties of transitions, or in understanding 
how a simple being or a simple organ can be changed and 
perfected into a highly developed being or elaborately con- 
structed organ ; secondly, the subject of Instinct, or the 
mental powers of animals ; thirdly. Hybridism, or the in- 
fertility of species and the fertility of varieties when inter- 
crossed ; and fourthly, the imperfection of the Geological 
Record. In the next chapter I shall consider the geolog- 
ical 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 em- 
bryonic condition. In the last chapter I shall give a 
brief recapitulation of the whole work, and a few con- 
cluding 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 in- 
numerable 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 
judgment of which I am capable, that the view which 
most naturalists entertain, and which I formerly enter- 
tained — 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 [N'atural Selection has been the main but not exclu- 
give means of modification. 




Causes of Variability — Effects of Habit — Correlation of Growth — Inheritance — Char- 
acter of Domestic Varieties — Difficulty of distinguishing between Varieties 
and Species — Origin of Domestic Varieties from"one or more Species — Domestio 
Pigeons, their Dfferences and Origin — Principle of Selection anciently followed, its 
Effects — Methodical and Unconscious Selection — Unknown Origin of our Domes- 
tio Productions — Circumstances favourable to Man's power of Selection. 

When we look to tlie 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 gen- 
erally differ much more from each other, than do the in- 
dividuals 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 produc- 
tions 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 pro- 
pounded 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 sev^eral 
generations to the new conditions of life to cause any ap- 
preciable amount of variation ; and that when the organ- 
isation 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 life, 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. Geoifroy 
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 attrib- 
uted 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 organisation, to the action of any change in the 
conditions of life. i^Tothing 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 trifiing 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 col- 
lected 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 cofinement, with the exception of the planti- 
grades 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 tlie other hand, we see 
individuals, though taken youn^ 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 unperceiyed causes as to 
fail in acting, we need not be surprised at this system, 
when it does act under confinement, acting not quite reg- 
ularly, 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 nnnatural 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 ofl'set, 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 for- 
mation ; so that, in fact, " sports " support my view, that 
variability may be largely attributed to the ovules or pol- 
len, 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 gen- 

Seedlings from the same fruit, and the young of the 
same litter, sometimes differ considerably from each other. 


tliough both the young and the parents, as Miiller has re- 
marked, 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 law^s of reproduction, and of growth, and of in- 
heritance ; for had the action of the conditions been di- 
rect, 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 agen- 
cies have produced very little direct effect, though ap- 
parently more in the case of plants. Under this point of 
view, Mr. Buckman's recent experiments on j^lants seem 
extremely valuable. When all or nearly all the individ- 
uals exposed to certain conditions are affected in the same 
way, the change at first appeal's to be directly due to 
such conditions ; but in some cases it can be shown that 
quite opposite conditions produce similar changes of struc- 
ture. ISTevertheless 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 decided influence, as in the period of 
flowering with plants when transported from one climate 
to another. In animals it has a more marked eftect ; for 
instance, I find in the domestic duck that the bones of the 
w4ng weigh less and the bones of the leg more, in propor- 
tion 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 in- 
herited 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 eftect of \ise. Xot a single domestic ani- 
mal can be named wdiich has not in some country droop- 
ing ears ; and the view suggested by some authors, that 
the drooping is due to the disuse of the muscles of the 

■£g VARIATION [Chap. L 

ear, from the animals not being mucli 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 in- 
stances 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 haA^e imper- 
fect teeth ; long-haired and coarse-haired animals are aj)t 
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 diversi- 
fied. 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, (fee. ; and it 
is really surprising to note the endless points in structure 
and constitution in which the varieties and subvarieties 
differ slightly from each other. Tlie whole organisation 
seems to have become plastic, and tends to depart in some 
small degree from that of the parental type. 

Any variation which is not inheritable is unimportant 
for us. But the number and diversity of inheritable de- 
viations 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 full- 
est and the best on this subject. 'No breeder doubts how 
strong is the tendency to inheritance : like j^roduces like is 
his fundamental belief: doubts have been thrown on this 
principle by theoretical writers alone. When any devia- 
tion of structure often appears, and we see it in the father 
and child, we cannot tell whether it may not be due to 
the same cause having acted on both ; but when amongst 
individuals, apparently exposed to the same conditions, 
any very rare deviation, due to some extraordinary com- 
bination 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 fami- 
ly. 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 diflerent indi- 
viduals 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 ex- 
clusively, 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 peculiarity first 
appears, it tends to appear in the offspring at a correspond- 
hig age, though sometimes earlier. Li many cases this 
could not be otherwise : thus the inherited peculiarities in 

20 VARIATION [Chap. L 

the horns of cattle conld appear only in the ofi'spring when 
nearly mature ; j)ecnliarities in the silkworm are known 
to appear at the corresponding caterpillar or cocoon stage. 
Bnt hereditary diseases and some other facts make me 
believe that the rnle 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 firs*: appearance of 
the j)eculiarity, 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 ofl'spring 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 difiiculty 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 Avhat the ab- 
original 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 efiects 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 efiect 
would have to be attributed to the direct action of the 


poor soil), that tliey wonld to a large extent, or even 
wholly, revert to the wild aboriginal stock. Whether or 
not the experiment would succeed, is not of great impor- 
tance 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 intercross- 
ing might check, by blending together, any slight devia- 
tions 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 con- 
ditions of life do change, variations and reversions of char- 
acter 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 char- 
acter ; by which I mean, that, although difiering 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 com]3ared with all the 
species in nature to which they are nearest allied. Wil-h 
these exceptions (and Avith that of the perfect fertility of 
varieties when crossed, — a subject hereafter to be dis- 
cussed), domestic races of the same sj)ecies 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, 

22 VARIATION [Chap. I. 

wlien 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 aborigi- 
nally 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 char- 
acters of generic value. I think it could be shown that 
this statement is hardly correct ; but naturalists difler 
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 it 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 natural species — for instance, of the many, 
foxes — inhabiting different quarters of the world. I do not 
believe, as we shall presently see, that the whole amount of 
difference between the several breeds of the dog has been 
produced under domestication ; I believe that some small 
part of the difference is due to their being descended from 
distinct species. In the case of some other domesticated 
species, there is presumptive, or even strong evidence, that 
all the breeds have descended from a single wild stock. 

It has often been assumed tJiat man has chosen for 
domestication animals and plants having an extraordi- 
nary 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 domesti- 
cated 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 rein-deer, or of cold by the common camel, 
prevented their domestication ? I cannot doubt that if 
other animals and plants, equal in number to our domes- 
ticated 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 gen- 
erations 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 ani- 
mals 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 do- 
mesticated 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 ? After the recent discov- 
eries of flint tools or celts in the superficial deposits of 
France and England, few geologists will doubt that man, 
in a sufiiciently civilized state to have manufactured 
weapons, existed at a period extremely remote as measured 
by years ; and we know that at the present day there is 
hardly any tribe so barbarian as not to have domesticated 
at least the dog. 

The whole subject must, I think, remain vague; nev- 
ertheless, I may, without here entering on any details, 
state that, from geographical and other considerations, I 
think it highl}'- probable that our domestic dogs have 
descended from several wild species. Knowing, as we 
do, that savages are very fond of taming animals, it 
seems to me unlikely, in the case of the dog-genus, which 
is distributed in a wild state throughout the world, 
that since man first appeared one single species alone 

24 VARIATION [Chap. 1. 

sliould have been domesticated. 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, (fee, 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 de- 
scended 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 Euroj)e 
alone, and several even within Great Britain. One 
author believes that there formerly existed in Great Britain 
eleven wild species of sheej) peculiar to it ! When we 
bear in mind ' that Britain has now hardly one peculiar 
mammal, and France but few distinct from those of Ger- 
many and conversely, and so with Hungary, Spain, (fee, 
but that each of these kingdoms possesses several pecu- 
liar breeds of cattle, sheep, (fee, 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 can- 
not doubt that there has been an immense amount of in- 
herited variation. Who can believe that animals closely 


resembling the Italian greylionnd, the bloodhound, the 
bnll-dog, or Blenheim spaniel, &c. — so nnlike all wild 
Canidse — ever existed freely in a state of nature ? It has 
often been loosely said that all oiir 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 inter- 
mediate 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. Tliere 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 offsj)ring 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 cross- 
ed one with another for several s^enerations, hardly two of 
them will be alike, and then the extreme difiiculty, or 
rather utter hoj)elessness, of the task becomes apparent. 
Certainly, a breed intermediate between tioo 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. Muri'ay from Per- 
sia. Many treatises in different languages have been 
published on pigeons, and some of them are very impor- 
tant, as being of considerable antiquity. I have associated 
with several eminent fanciers, and have been permitted to 
j oi n 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 differ- 
ences in their skulls. The carrier, more especially the 
male bird, is also remarkable from the wonderful develoj)- 
ment of the carunculated skin above the head, and this is 
accompanied by greatly elongated eyelids, very large ex- 
ternal 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 fiying' at a great height in 
a compact fiock, 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 singu- 
larly 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. Tlie 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 tailfeathers. The 
trumpeter and laugher, as their names express, utter a 
Tcry different coo from the other breeds. The fantail has 
thirty or even forty tailfeathers, instead of twelve or four- 
teen, 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 develop- 
ment 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 
ill a highly remarkable manner. The number of the cau- 
dal and sacral vertebra3 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 j)ro- 
portional 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 croj) 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 scutellse on the toes, the develop- 
ment of skin between the toes, are all points of structure 
which are variable. The period at which the perfect plu- 
mage 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 difl^er 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 espe- 
cially 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 diflerences 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 trifiing 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 enor- 
mous 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 geo- 
graphical sub-species, only two or three of the 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 un- 
known to ornithologists ; and this, considering their size, 
habits, and remarkable characters, seems very improba- 
ble ; or they must have become extinct in the wild state. 
But birds breeding on precipices, and good fliers, are un- 
likely 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-speci- 
fied breeds, though agreeing generally in constitution, 
habits, voice, colouring, and in most parts of their struo* 



ture, with the wild rock-pigeon, yet are certainly highly 
abnormal in other parts of their structure : we may look 
in vain throughout tJie whole great family of Columbidse 
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 in- 
tentionally or by chance picked out extraordinarily ab- 
normal 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 de- 

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. inter- 
media of Strickland, having it bluish) ; the tail has a ter- 
minal dark bar, with the bases of the outer feathers ex- 
ternally edged with white ; the Vv^ings 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. 
ISTow, 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 con- 
cur perfectly developed. Moreover, when two birds be- 
longing 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 pro- 
duced 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 mnst make one of the two follow- 
ing highly improbable suppositions. Either, firstly, that 
all the several imagined aboriginal stocks were coloured, 
and marked like the rock-pigeon, although no other ex- 
isting 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 markiogs. 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 ten- 
dency to reversion to any character derived from such 
cross will naturally become less and less, as in each suc- 
ceeding 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 gener- 
ation, this tendency, for all that we can see to the con- 
trary, 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. ]N"ow, it is difficult, 2:)erhaps im- 
possible, to bring forward one case of the hybrid offspring 
of two animals clearly distinct being themselves perfectly 
fertile. Some authors believe that long-continued domes- 
tication 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 to- 
gether, 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 per- 
fectly 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 spe- 
cies of pigeons to breed freely nnder 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 Columbidse, 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 to- 
gether, I can feel no doubt that all our domestic breeds 
have descended from the Columba livia with its geograph- 
ical sub-species. 

In favour of this view, I may add, firstly, that C. livia, 
or the rock-pigeon, has been found capable of domestica- 
tion 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 char- 
acters 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 vari- 
able ; 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 Egyptian 
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 

32 • DOMESTIC PIGEONS. [Chap. 1. 

race." Pigeons were much yahied by Akber Khan in 
Liclia, about the year 1600 ; never less than 20,000 pigeons 
were taken with the court. " The nionarchs of Iran and 
Turan sent him some very rare birds ; " and, continues 
the courtly historian, " His Majesty by crossing the breeds, 
whicli method was never practised before, has improved 
them astonishingly." About this same period the Dutch 
were as eager about pigeons as were the old Eomans. 
The paramount importance of these considerations in ex- 
plaining the immense amount of variation which pigeons 
have undergone, will be obvious when we treat of Selec- 
tion. 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 dis- 
tinct breeds, that male and female pigeons can be easily 
mated for life ; and thus different breeds can be kept to- 
gether 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 culti- 
vators 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-liorns, and 
he will laugh you to scorn. I have never met a pigeon, 
or poultry, or duck, or rabbit fancier, who was not full}'- 
(;onvinced that each main breed was descended from a 
distinct species. Yan Mons, in liis treatise on pears and 
apples, sliows how utterly he disbelieves that the several 
sorts, for instance a Kibston-pippin or Codlin-apple, could 
ever have proceeded from the seeds of the same tree. In- 


nnmerable otlier examples conld be given. The explana- 
tion, I think, is simple : from long-continued stndj they 
are strongly impressed with the dilFerences between the 
several races ; and though they well know that each race 
varies slightly, for they win their prizes by selecting snch 
slight differences, yet Ihey ignore all general arguments, 
and refuse to sum up in their minds slight differences ac- 
cumulated during many successive generations. May not 
those naturalists who, knowing far less of the laws of in- 
heritance 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 stej)S 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 sud- 
denly 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 va- 
rious 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 pur- 
pose ; when we compare the many breeds of dogs, each 
good for man in very different ways ; when we compare 


the game-cock, 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, cnli- 
nary, orchard, and flower-garden races of plants, most nse- 
ful to man at difi:erent seasons and for different pui-poses, 
or so beautiful in his eyes, we must, I think, look fui'ther 
than to mere variability. "We cannot suppose that all the 
breeds were suddenly produced as perfect and as nseful 
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 sub- 
ject, 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 agricultur- 
ists than almost any other individual, and who was him- 
self a very good judge of an animal, speaks of the prin- 
ciple of selection as " that which enables the agricultur- 
ist, 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 liad chalked out upon a wall a form per- 
fect 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 
"^ather 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 

What English breeders have actually effected is proved 
by the enormous prices given for animals with a good ped- 
igree ; 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. K 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 con- 
sists in the great effect produced by the accumulation in. 
one direction, during successive generations, of differences 
absolutely inappreciable by an uneducated eye — differ- 
ences which I for one have vainly attempted to appre- 
ciate. ISTot one man in a thousand has accuracy of eye 
and judgment 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 im- 
provements ; if he wants any of these qualities, he will as- 
suredly 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, l^o one 
supposes that our choicest productions have been pro- 
duced 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 goose- 


berry may be quoted. We see an astonisbing improve- 
ment in many florists' flowers, wben tbe flowers of tbe 
present day are compared witb 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 efi'ects of selection — namely, by compar- 
ing the diversity of flowers in the difi'erent 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 
difi'erent kinds of gooseberries difi'er in size, colour, shape, 
and hairiness, and yet the flowers present very slight 
difterences. It is not that the varieties which difi'er 
largely in some one point do not difi'er at all in other 
points ; this is hardly ever, perhaps never, the case. 
The laAvs of correlation of growth, the importance of 
which should never be overlooked, will ensure some dif- 
ferences ; but, as a general rule, I cannot doubt that the 
continued selection of slight variations, cither in the 
leaves, the flowers, or the fruit, will produce races difier- 
ing 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 tlie full 
acknowledgment of the importance of tlie principle in 
works of liigli 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 ])rinciple of selection I find dis- 
tinctly given in an ancient Chinese encyclopaedia. Ex- 
plicit rules are laid down by some of the Roman classical 
writers. From ^^assages in Genesis, it is clear that the 
colour of domestic animals was at that early period at- 
tended to. Savages now sometimes cross their dogs with 
wild canine animals, to improve the breed, and they 
formerly did so, as is attested by j^assages 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 ivitli 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 methodi- 
cal selection, with a distinct object in view, to make a new 
strain or sub-breed, superior to any thing 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 individua'. 
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. I^evertheless I cannot doubt that this process, 
continued during centuries, w^ould 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 civilized 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. Borrow 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 train- 
ing, the whole body of English race-horses 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 com- 
paring the accounts given in old pigeon treatises of car- 
riers 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 uncon- 
sciously followed, in so far that the breeders could never 
iiave expected or even have wished to have produced the 
result which ensued — namely, the production of two dis- 


tinct 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 sus- 
picion 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 difi'erence between the 
sheep possessed by these two gentlemen is so great that 
they have the appearance of being quite diflerent va- 

If there exist savages so barbarous as never to think 
of the inherited character of the oflspring of their domestic 
animals, yet any one animal particularly useful to them, 
for any special purpose, would be carefully preserved dur- 
ing famines and other accidents, to which savages are so 
liable, and such choice animals would tlius generally leave 
more off'spring 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 sufliciently distinct to be ranked at their first 
apj)earance as distinct varieties, and whether or not tvfo 
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 the 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 canot 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 j)ear 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 pre- 
served 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-hnown 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 aflbrded 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 j)erfection 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 difi'erently circum- 
stanced, 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, j)artly 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 onr 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 ab- 
normal character of our domestic races, and likewise their 
differences being so great in external characters and rela- 
tively 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. JSTo man 
would ever try to make a fantail, till he saw a pigeon with 
a tail developed in some slight degree in an unusual man- 
ner, or a pouter till he saw a pigeon with a crop of some- 
what 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 select- 
ed 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 dis- 
tinct 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 dis- 
regarded 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. ISTor must the value which would 

^2 SELECTIOiN' BY MAN. [Chap. 1. 

formerly be set on any sliglit differences in the individnals 
of tlie 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 per- 
fection 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 
fleetins: of characters, have latelv been exhibited as dis- 
tinct at our poultry-shows. 

I thmk these views further explain ivhat 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 the 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 im- 
proved individuals slowly spread in the immediate neigh- 
bourhood. But as yet they wdll 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 distmct and 
valuable, and will then probably first receive a provincial 
name. In semi-civilised countries, with little free com- 
munication, 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 fullv acknowledo-ed, 
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 insensi- 
ble changes. 

I m.ust now say a few words on the circumstances, 


favourable, or the reverse, to man's power of selection. 
A higli degree of variability is obviously favourable, as 
freely giving tlie 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 kejDt ; and hence this comes to be of the highest 
importance to success. On this principle Marshall has 
remarked, with respect to sheep of parts of Yorkshire, 
that " as they generally belong to poor people, and are 
niostlj 171 S7nall lots, thej neyer 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 keep- 
ing 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, whateyer their quality may be, will gen- 
erally be allowed to breed, and this will efi'ectually 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 qual- 
ities or structure of each individual. Unless such attention 
be paid nothing can be eflected. I have seen it graveH 
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 tlie best seedlings and bred from them, then, there 
appeared (aided by some crossing witJi distinct s]3ecies) 
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. Li 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 hav- 
ing 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 breed- 
ing ; 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 es- 
pecially from no pleasure having been felt in the display 
of distinct breeds. 

To sum up on the origin of our Domestic Paces of ani- 
mals and plants. I believe that the conditions of life, 
from their action on the reproductive system, are so far 
of the highest importanc'e 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. Yariability is governed by many un- 


known 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 in- 
finitely 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 produc- 
tions. 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 exagge- 
rated, 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 im- 
mense ; for the cultivator here quite disregards the ex- 
treme 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. 




Variability— Individual differences— Doubtful species— "Wide ranging^ mucli 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 

Before applying tlie principles arrived at in tlie last 
chapter to organic beings in a state of nature, we must 
briefly discuss wliether 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. ISTo one definition has as yet satisfied all natural- 
ists ; 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 con- 
ditions of life ; and " variations " in this sense, are sup- 
posed 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 


ill some cases be inherited for at least some few geneia- 
tions ? and in tliis case I presume tliat the form would be 
called a variety. 

Again, we have many slight differences which may 
be called individual differences, such as are known fre- 
quently to appear in the offspring from the same parents, 
or which may be presumed to have thus arisen, from be- 
ing frequently observed in the individuals of the same 
species inhabiting the same confined locality. ISTo 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 indi- 
vidual differences generally affect what naturalists con- 
sider unimportant parts ; but I could show by a long cat- 
alogue 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, 
dm-ing 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 or- 
gans, and compare them in many specimens of the same 
species. I should never have expected that the branch- 
ing 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 re- 
cently Mr. Lubbock has shown a degree of variability in 
these main nerves in Coccus, which may almost be com- 
pared 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 larvge of certain 
insects are very far from uniform. Authors sometimes 


argue in a circle when tliey state that important organs 
never vary ; for these same authors practically rank that 
character as important (as some few naturalists have hon- 
estly confessed) vdiich does not vary ; and, under this 
point of viev^, no instance of an 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 differ- 
ences, which seems to me extremely perplexing : I refer 
to those genera which have sometimes been called " pro- 
tean " or " polymorphic," in which the species present an 
inordinate amount of variation ; and hardly two natural- 
ists can agree which forms to rank as species, and which 
as varieties. We may instance Kubus, Rosa, and Hiera- 
cium amongst plants, several genera of insects, and sev- 
eral genera of Erachiopod shells. In most polymorphic 
genera some of the species have fixed and definite char- 
acters. 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 gen- 
era variations in points of structure which are of no 
service or disservice to the species, and which conse- 
quently have not been seized on and rendered definite by 
natural selection, as hereafter will be explained. 

Those forms which possess in some considerable de- 
gree 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 re- 
spects the most impoj-tant 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 hav- 
ing 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 difiiculty, 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 in- 
termediate 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 rank- 
ed as a species or a variety, the opinion of naturalists hav- 
ing sound judgment and wide experience seems the only 
guide to follow. We must, however, in many cases, de- 
cide 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 un- 
common cannot be disputed. Compare the several floras 
of Great Britain, of France or of the United States, drawn 
up by diflerent botanists, and see what a surprising num- 
ber of forms have been ranked by one botanist as good 
species, and by another as mere varieties. Mr. H. C. 
AVatson, to whom I lie under deep obligation for assist- 
ance 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 bota- 
nists 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 difl'erence 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 witHn the same conn 
try, but are common in separate 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 compar- 
ing, and seeing others compare, the birds from the sepa- 
rate islands of the Galapagos ArchijDclago, 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 characterised as varieties in Mr. AVollaston'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 Brit- 
ish red grouse as only a strongly-marked race of a Nor- 
wegian 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 Cana- 
ries, 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 

Many of the cases of strongly-marked varieties or 
doubtful s]Decies well deserve consideration ; for several 
interesting lines of argument, from geographical distribu- 
tion, analogical variation, hybridism, (fee, 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 vulgaris 
and veris. 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 Gart- 
ner, 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 doubt- 
ful whether these links arc 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 natural- 
ists 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, va- 
rieties 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, w^hich are very gener- 
ally considered as varieties ; and in this country the high- 
est 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, lie will soon make up his 
mind how to rank most of the doubtful forms. His gen- 
eral tendency will be to make many species, for he will be- 
come impressed, just like the pigeon or poultry fancier be- 
fore alluded to, with the amount of difference in the forms 
which he is continually studying ; and he has little gen- 
eral knowledge of analogical variation in other groups 
and in other countries, by which to correct his first im- 
pressions. As he extends the range of his observations, 
he will meet with more cases of difficulty ; for he will en- 
counter a greater number of closely-allied forms. But if his 
observations be widely extended, he will in the end gen- 
erally 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 entii'ely 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 

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 liis- 
tory. 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-con- 
tinued action of different physical conditions in two dif- 
ferent 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 w^ell-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 
terai species, as one arbitrarily given for the sake of con- 
venience 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 fluctuat- 
ing forms. The term variety, again, in comparison with 
mere individual diA'crences, 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 tlie discussion of these difficulties, and the 
tables themselves of the proportional numbers of the vary- 
ing 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 per- 
plexing, and allusions cannot be avoided to the " struggle 
for existence," " divergence of character," and other ques- 
tions, hereafter to be discussed. 

Al23h. De CandoUe and others have shov/n that plants 
which have very wide ranges generally present varieties ; 
and this might have been expected, as they become ex- 
posed to diverse physical conditions, and as they come 
into competition (which, as we shall hereafter see, is a far 
more important cii'cumstance) with different sets of or- 
ganic 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 cer- 
tain extent from commonness), often give rise to varieties 
sufficiently well-marked to have been recorded in botani- 
cal 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, — 
whicli oftenest produce well-marked varieties, or, as I con- 
sider them, incipient species. And this, perhaps, might 
have been anticij^ated ; for, as varieties, in order to be- 
come in any degree permanent, necessarily have to strug- 
gle with the other inhabitants of the country, tlie species 
which are already dominant will be the most likely to 
yield offspring which, though in some slight degree modi- 
fied, 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 tlie other side, a somewhat larger 
number of the very common and mnch diffused or domi- 
nant 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 he connected with the nature 
of the stations inhabited by them, and has little or no re- 
lation 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 rela- 
tion to the size of the genera. The cause of lowly-organ- 
ised plants ranging widely will be discussed in our chap- 
ter 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 in- 
cipient species ought, as a general rule, to be now form- 
ing. Where many large trees grow, we expect to find 
saplings. Where many species of a genus have been 
formed through variation, circumstances have been favour- 
able for variation ; and hence we might expect that the 
circumstances would generally be still favourable to vari- 
ation. 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 spe- 
cies, than in one having few. 


To test the truth of this anticipation I have arranged 
the plants of twelve countries, and the coleopterous in 
sects 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 large projDortion of the spe- 
cies 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 re- 
sults 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. Tliese facts are 
of plain signification on the view that species are only 
strongly marked and permanent varieties ; for wherever 
many species of the same genus have been formed, or 
w^here, 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 spe- 
cies 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. 
\Ye 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 re- 
gard to plants, and Westwood in regard to insects, that 
in large genera the amount of difference between the spe- 
cies is often exceedingly small. I have endeavoured to 
test this numerically by averages, and, as far as my im- 
perfect results go, they always confirm the view. I have 
also consulted some sagacious and most experienced ob- 
servers, 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 num- 
ber of varieties or incipient species greater than the aver- 
age 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 larger genera are related 
to each other, in the same manner as the varieties of any 
one species are related to each other. No naturalist pre- 
tends 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 im- 
portant 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. Yarieties 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 reverssd. But there is also reason to believe, that 
those sj^ecies which are very closely allied to other species, 
and in so far resemble varieties, often have much restrict- 
ed 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 valne : these 63 reputed species range 
on an average over 6.9 of the provinces into which Mr. 
Watson has divided Great Britain. ITow, in this same 
catalogue, 53 acknowledged varieties are recorded, and 
these range over Y.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 Brit- 
ish botanists as good and true species. 

Finally, then, varieties have the same general charac- 
ters 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 con- 
nect ; and except, secondly, by a certain amount of differ- 
ence, for two forms, if differing very little, are generally 
ranked as varieties, notwithstanding that intermediate link- 
insT forms have not been discovered ; but the amount of 
di&erence 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 un- 
derstand these analogies, if species have once existed as 
varieties, and have thus originated : whereas, these analo- 
gies are utterly inexplicable if each species has been in- 
dependently created. 

We have, also, seen that it is the most flourishing and 
dominant species of the larger genera which on an aver- 
age 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 domi- 
nant tend to become still more dominant by leaving many 
modified and dominant descendants. But by steps here- 
after 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 sub- 
ordinate to groups. 




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— Eflects of climate — Protection from the num- 
ber of individuals— Complex relations of all animals and plants throughout nature 
— Struggle for life most severe between individuals and varieties of the same spe- 
cies ; often severe between species of the same genus — The relation of organism 
to organism the most important of all relations. 

Before entering on tlie subject of this chapter, I must 
make a few preliminary remarks, to show how the strug- 
gle for existence bears on ISTatural Selection. It has been 
seen in the last chapter that amongst organic beings in a 
state of nature there is some individual variability ; in- 
deed 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 exist- 
ence 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 or- 
ganic being to another being, been perfected ? We see 
these beautiful co-adaptations most plainly in the wood- 
pecker and misseltoe ; 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 

Again, it may be asked, how is it that varieties, which 
I have called incipient species, become ultimately con- 
verted 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 pro- 
ceeding, 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 in- 
herited 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 IN'atural 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 accumu- 
lation of slight but useful variations, given to him by the 
hand of ISTature. But ISTatural Selection, as we shall here- 
after see, is a power incessantly ready for action, and is as 
immeasurably superior to man's feeble eftbrts, as the works 
of JSTature are to those of Art. 

AYe will now discuss a little more in 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 Lyelle have largely and philosophi- 
cally 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 ^Y. 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 thorough- 
ly 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 
l)right 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 superabun- 
dant, 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 thou- 
sand 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 misseltoe is dependent en 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 misseltoes, growing close together on 
the same branch, may more truly be said to struggle with 
each other. As the misseltoe is disseminated by birds, 
its existence depends on birds ; and it may metaphor- 
ically 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 num- 
bers would quickly become so inordinately great that no 
country could support the product. Hence as more indi- 
viduals are produced than can possibly survive, there must 
in every case be a struggle for existence, either one indi- 
vidual with another of the same species, or with the indi- 
viduals 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 caimot 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-live 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 pair 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 tliem during two or three following seasons. 
Still more striking is the evidence from our domestic ani- 
mals 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 conse- 
quently 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 natu- 
ralised 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 annu- 
ally 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 anyhow 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 wdiich annually 



produce eggs or seeds by the ttiousand, 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 it' many eggs or young are destroyed, 
many must be produced, or the species will become ex- 
tinct. It would suflice 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, sup- 
posing 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 depends only 
indirectly on the number of its eggs or seeds. 

In looking at ligature, 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 
tben 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 
spot 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 num- 
bers 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 pro- 
tected 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 cli- 
mate, 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 efi'ect 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 fa- 
voured 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 soutliward 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 some- 
what 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 inju- 
rious action of climate, than we do in proceeding south- 
wards or in descending a mountain. "When we reach the 
Arctic regions, or snow-capped summits, or absolute des- 
erts, the struggle for life is almost exclusively with the 

That climate acts in main part indirectly by favouring 
other species, we may clearly see in the prodigious num- 
ber 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 Xo highly favourable circum- 
stances, 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 lim- 
iting 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 diifusion amongst the crowded 
animals, been disproportionably favoured : and here comes 
in a sort of struggle between the parasite and its prey. 

On the other liand, 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 bird, though having a super-abundance 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 trouble- 
some 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 ex- 
treme 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 to- 
gether, 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 proper 
tional 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 eflect 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 liow important an element enclosure 
is, I plainly saw near Fariiham, in Surrey. Here there 
are extensive heaths, with a few clumps of old Scotch hrs 
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 can- 
not 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 hundred 
yards distant from one of the old clumps, I counted thirty- 
two little trees ; and one of them, with twenty-six rings 
of growth, had during many 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 elfectually searched it for food. 

Here we see that cattle absolutely determine the exist- 
ence of the Scotch fir ; but in several parts of the world 
insects determine the existence of cattle. Perhaps Para- 
guay ofiers the most curious instance of this ; for here 
neither cattle nor horses nor dogs have ever run Avild, 
though they swarm southward and northward in a feral 
state ; and Azara and Pengger 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. Tlie 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 Sontli America) the vegetation : 
this again Tvoiild 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. ISTot that in nature the relations can 
eter be as simj)le as this. Battle within battle must ever 
be recurring with varying success ; and yet in the long- 
iTin 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 pro- 
found 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 cata- 
clysms to desolate the world, or invent laws on the dura- 
tion 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 in- 
sects, and consequently, from its peculiar structure, nevei 
can set a seed. Many of our orchidaceous plants abso- 
lutely 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 fertilisa- 
tion of the heartsease (Yiola tricolor), for other bees do 
not visit this flower. From experiments which I have 
lately tried, I have found that the visits of bees are neces- 
sary for the fertilisation of some kinds of clover ; for in- 
stance, 20 heads of Dutch clover (Trifolium repens) yielded 
2290 seeds, but 20 other heads protected from bees pro- 
duced not one ; again, 100 heads of red clover (T. pratense) 
produced 2700 seeds, but the same number of protected 
heads produced not a single seed. Humble-bees alone 
visit red clover, as other bees cannot reach the nec- 
tar. Hence I have very little doubt, that if the v/hole 
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. Ilewman, 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 kaows, on the number of cats ; and Mr. ITewman 
says, " ]N"ear 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 creditable 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 flow^ers 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 temj^ted 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 sur- 
rounding virgin forests. AVliat a struggle between the 
several kinds of trees must here have gone on during long 
centuries, each annually scattering its seeds by the thou- 
sand ; what war between insect and insect — between in- 
sects, 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 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 liow 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 propor- 
tional 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 seeds be re- 
sown, 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 iu due pro- 
portion, 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. Tlie same result has fol- 
lowed 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 constitu- 
tion, 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 consti- 
tution, 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 ex- 
tension 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 Scot- 
land 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 econo- 
my 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 be- 
ings, with which it comes into competition for food or resi- 
dence, 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 aii" 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 
w^idely 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 competi- 
tors, or over the animals which preyed on it. On the 
confines of its geographical range, a change of constitu- 
tion 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. IS'ot 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 damp- 
est 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 difi*erent way to what we should have done in its native 
country ; for we should have to give it some advantage 
over a difi'erent 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 smgle 
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. 





Natural Selection^its power compared with man's selection — its power on chafactera 
of trifling importance — its power at all ages and on toth sexes — Sexual Selection — 
On the generality of intercrosses between individuals of the same species — Cir* 
cumstances favourable and unfavourable to Natural Selection, namely, intercross- 
ing, isolation, number of individuals— Slow action— Extinction caused by Natural 
Selection — Divergence of Character, related to the diversity of inhabitants of any 
email 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* 

How will tlie 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 eftectually. 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 com- 
plex battle of life, should sometimes occur in the course 
of thousands of generations? K such do occur, can we 
doubt (remembering that many more individuals are bom 
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 tlie otlier hand, we may feel sure that any variation 
in the least degree injurious would be rigidly destroyed. 
This preservation of favourable variations and the rejec- 
tion of injurious variations, I call ISTatural Selection. 
Yariations neither useful nor injurious would not be affect- 
ed by natural selection, and would be left a fluctuating 
element, as perhaps we see in the species called polymor- 

We shall best understand the probable course of na- 
tural selection by taking the case of a country undergo- 
ing some physical change, for instance, of climate. The 
proportional numbers of its inhabitants would almost im- 
mediately undergo a change, and some species might be- 
come extinct. We may conclude, from what we have 
seen of the intimate and complex manner in which the in- 
habitants of each country are bound together, that any 
change in the numerical proportions of some of the in- 
habitants, 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 cer- 
tainly 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 intro- 
duced 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 econo- 
my 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 pre- 
served ; and natural selection would thus have free scope 
for the work of improvement. 

We have reason to believe, as stated in the first chap- 
ter, 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. Kot 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 IS^ature, but far more 
easily, from having incomparably longer time at her dis- 
posal. JN^or 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 modi- 
fying 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 per- 
fectly 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 ; 
!N"ature only for that of the being which she tends. Every 


selected character is fullj exercised by lier ; and the being 
is placed under well-suited conditions of life. Man keeps 
the natives of many climates in the same country ; he sel- 
dom exercises each selected character in some peculiar 
and fitting mamier ; 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 difi'erence 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 eflbrts 
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 won- 
der, 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 varia- 
tion, even the slightest ; rejecting that which is bad, pre- 
serving and adding up all that is good ; silently and in- 
sensibly working, whenever and wherever opportunity 
ofi*ers, at the improvement of each organic being in rela- 
tion 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 lapse 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 difi'erent 
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 impor- 
tance, may thus be acted on. When we see leaf-eating in- 
sects 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 column 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 laAb with the faintest 
trace of black. In plants the down on the fruit and the 
colour of the flesh are considered by botanists as charac- 
ters 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 differ- 
ence 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 yello^v 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 di- 
rect 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 modi- 
fied through variation, and the modifications are accumu- 
lated by natural selection for the good of the being, will 
cause other modifications, often of the most unexpected 

As we see that those variations which under domesti- 
cation appear at any particular period of life, tend to re- 
appear 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 accumula- 
tion of profitable variations at that age, and by their inher- 
itance at a corresjDonding 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 cot- 
ton-trees. Natural selection may modify and adapt the 
larva of an insect to a score of contingencies, wholly dif- 
ferent 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 dif- 
ferent period of life, shall not be in the least degree inju- 
rious : for if they became so, they would cause the extinc- 
tion of the species. 

ISTatural selection will modify the structure of the 
young in relation to the parent, and of the parent in rela- 
tion to the young. In social animals it will adapt the 


structure of eacli individual for the benefit of tlie commu- 
nity ; 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 advan- 
tage, for the good of another species ; and though state- 
ments to this eflect may be found in works of natural his- 
tory, 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 rigor- 
ous 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 ap- 
pear under domestication in one sex and become heredi- 
tarily 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 difi'erent 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 Selec- 
tion. 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 com- 
petitor, but few or no ofi'spring. Sexual selection is, there- 
fore, 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, bnt on 
having special weapons, confined to the male sex. A 
hornless stag, or spnrless cock would have a poor chance 
of leaving offspring. Sexual selection hj always allowing 
the victor to breed might surely give indomitable cour- 
age, 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 alli- 
gators have been described as fighting, bellowing, and 
whirling round, like Indians in a war-dance, for the pos- 
session 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 ani- 
mals, 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 tln-ough means of sexual selec- 
tion, 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 emi- 
nently attractive to all his hen birds. It may appear 
childish to attribute any effect to such apparently weak 
mjeans : 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 gen- 
erations, 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 com- 
parison 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 modi- 
fications 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 w^hen the males and 
females of any animal have the same general habits of life, 
but differ in structure, colour, or ornament, such differ- 
ences 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 agen- 
cy : 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 ; — in- 
deed, had the tuft appeared under domestication, it would 
have been called a monstrosity. 

Illustrations of the action of Natiiral Selection. — ^In 
order to make it clear how, as I believe, natural selection 
acts, I must be^ 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 pre;y 
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 swift- 
est 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 wdiich 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 tenden- 
cies 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 rab- 
bits, 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 fonned which would either supplant or 
coexist with the parent form of wolf. Or, again, the 
wolves inhabiting a mountainous district, and those fre- 
quenting 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 inter- 
crossing 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 grevhound-like form, which j)nr- 
sues 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 elimina- 
ting something injurious from their sap : this is effected 
by glands at the base of the stipules in some Leguminosse, 
and at the beak 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 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 m 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 vis- 
ited 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 eftected, 
although nine-tenths of the pollen were destroyed, it might 
still be a great gain to the plant ; and those individuals 


wliicli produced more and more pollen, and had larger 
and larger anthers, wonld be selected. 

"When our plant, bj this process of the continued pre- 
servation or natural selection of more and more attractive 
flowers, had been rendered highly attractive to insects, 
they would, unintentionlly on their part, regularly carry 
pollen from flower to flcs^er ; 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 sta- 
mens 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 difierent branches, under the micro- 
scope, 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 j)ollen 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 efi'ectually 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. IS^o naturalist 
doubts the advantage of what has been called the " phy- 
siological 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 ntale 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 efi'ected. 

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 selec- 
tion, to be a common plant ; and that certain insects de- 
pended 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 insects, 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 (Trifo- 
lium pratense and incarnatum) do not on a hasty glance 
appear to difi'er 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 ofi'er 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 difi'erently constructed pro- 
boscis. 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 simulta- 
neously 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 Ly ell'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 clifis. I^atural selection can 
act only by the preservation and accumulation of infini- 
tesimally 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 crea- 
tion of new organic beings, or of any great and sudden 
modification in their structure. 

On the Intercrossing of Individuals. — I must here in- 
troduce a short diOT'ession. In the case of animals and 
plants with separated sexes, it is of course obvious that two 
individuals must always (with the exception of the curious 
and not well-understood cases of parthenogenesis) unite for 
each birth ; but in the case of hermaphrodites this is far 
from obvious. JSTevertheless I am strongly inclined to be- 
lieve that with all hermaphrodites two individuals, either 
occasionally or habitually, concur for the reproduction of 
their kind. This view was first suggested by Andrew 
Knight. We shall j^resently 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 hermaphro- 
dites, and of real hermaphrodites a large number pair ; 


tliat 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 reproduc- 
tion ? 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 diflerent varieties, or between individuals of the 
same variety but of another strain, gives vigour and fer- 
tility to the ofi'spring ; 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 fertilizes itself for an 
eternity of generations ; but that a cross with another in- 
dividual is occasionally — perhaps at very long intervals — 

On the belief that this is a law of nature, we can, 1 
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 
Avet is to the fertilisation of a flower, yet what a multi- 
tude of flowers have their anthers and stigmas fully ex- 
posed to the weather ! but if an occasional cross be indis- 
pensable, the fullest freedom for the entrance of pollen 
from another individual will explain this state of expo- 
sure, 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 sev- 
eral, perhaps in all, such fiowers, 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. 
Kow, 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 stig- 
ma of another with the same brush to ensure fertilisation ; 
but it must not be supposed that bees would thus pro- 
duce 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 efi'ect, that it will invariably and com- 
pletely destroy, as has been shown by Gartner, any in- 
fluence 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-fertili- 
sation ; 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 Kolreuter 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 Avell known that if very closely-allied forms or varie- 
ties 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 eflectually 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 numer- 
ous 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 tlie 
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 pre- 
vent the stigma of a flower receiving its own j^ollen, 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 in- 
dispensable ! 

K 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 difi'erent varieties 
growing near each other, and of these only Y8 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 mongrelized? 
I suspect that it must arise from the pollen of a distinct 
"variety having a prepotent efi'ect 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. 

Li the case of a gigantic tree covered with innumera- 
ble 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 lias largely provided against it bv 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 sepa- 
rated than other plants, I find to be the case in this coun- 
try ; and at my request Dr. Hooker tabulated the trees 
of l^ew 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 at- 
tention 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 oflers 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 na- 
ture of the fertilising element ; for we know of no meanSj 
analogous to the action of insects and of the wind in tlie 
case of plants, by which an occasional cross could be 
efi*ected with terrestrial animals without the concurrence 
of two individuals. Of aquatic animals, there are many 
self-fertilising hermaphrodites ; but here currents in the 
water ofi'er 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 hermaph- 
rodite animal with the organs of reproduction so per- 
fectly enclosed within the body, that access from without 
and the occasional influence of a distinct individual can 
be shown to be physically impossible. Cirripedcs long 
appeared to me to present a case of very great difficulty 


under tins point of view ; bnt I liave been enabled, by a 
fortunate cliance, elsewhere to prove that two individuals, 
though both are self-fertilising hermaphrodites, do some- 
times cross. 

It must have struck most naturalists as a strange anom- 
aly that, ill 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 hermaphro- 
dites, and some of them unisexual. But if, in fact, all 
hermaphrodites do occasionally intercross with other in- 
dividuals, the difference between hermaphrodites and uni- 
sexual 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 be- 
ings, a cross between two individuals is an obvious neces- 
sity for each birth ; in many others it occurs perhaps only 
at long intervals ; but in none, as I suspect, can self-ferti- 
lisation go on for perpetuity. 

Circumstances favourable to Natural Selection. — Tliis 
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 varia- 
bility 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 ani- 
mals, much improvement and modification surely but 
slowly follows 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 occu- 
pied 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 difi'erent 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 eflects 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 aflJ'ect those animals 
which unite for each birth, which wander much, and which 
do not breed at a very quick rate. Plence 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 main- 
tain 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 


tlie above principle, nurserymen always prefer getting seed 
from a large body of plants of tlie same variety, as the 
cbance of intercrossing with other varieties is thus les- 

Even in the case of slow-breeding animals, which unite 
for each birth, we must not overrate the ejffects of inter- 
crosses 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 con- 
vinced 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 modilication, uniformity 
of character can be given to their modified offspring, 
solely by natural selection preserving the same favourable 

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 natu- 
ral selection will tend to modify all the individuals of a 


varying species tlirouglioiit 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 be- 
come adaj)ted to, through modifications in their structure 
and constitution. Lastly, isolation, by checking immigra- 
tion and consequently competition, T^'ill give time for any 
new variety to be slowly improved ; and this may some- 
times 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 few- 
ness of individuals will greatly retard the production of 
new species through natural selection, by decreasing the 
chance of the appeai-ance 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 inhabit- 
ing 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 isl- 
and 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 considera- 
ble importance in the production of new species, on the 
whole I am inclined to believe tliat largeness of area is 
of more importance, more especially in the production of 


species, wliicli will prove capable of enduring for a long 
period, and of spreading widely. Tlirongliout a great and 
open area, not only will tliere be a better chance of 
favourable visitations arising from the large number of 
individuals of the same species there supported, but the 
conditions of life are infinitely complex from the large num- 
ber 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 com- 
petition 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 efi'ects 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 pro- 
duction 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 his- 
tory 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 produc- 
tions of the smaller continent of Australia have formerly 
yielded, and apparently are now yielding, before those of 
the larger Europaso-Asiatic area. Thus, also, it is that 
continental productions have everywhere become so largely 
naturalised on islands. On a small island, the race foV 
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 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 ex- 
terminated. 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 Ornitho- 
rhynchus 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 hav- 
ing inhabited a confined area, and from having thus been 
exposed to less severe competition. 

To sum up the circumstances favourable and unfavour- 
able 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 pro- 
duction 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 nu- 
merous in individuals and kinds, will have been subjected 
to very severe competition. When converted by subsi- 
dence into large separate islands, there will still exist 
many individuals of the same species on each island : in- 
tercrossing 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 modifi- 
cations of the old inhabitants ; and time will be allowed 
for the varieties in each to become well modified and per- 
fected. When, by renewed elevation, the islands shall 
be reconverted into a continental area, there will again be 
severe competition : the most favoured or improved vari- 
eties will be enabled to spread : there will be much ex- 


tinction 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 

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 mod- 
ification 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 dis- 
turbed. Nothing can be effected, imless favourable vari- 
ations 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 sufiicient 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 selec- 
tion accords perfectly well with what geology tells us of 
the rate and maimer 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, 1 
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 liere alluded 
to from being intimately connected with natural selection. 
ISTatural selection acts solely through the preservation of 
variations in some way advantageous, which consequently 
endure. But as from the high geometrical powers of in- 
crease 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 J for as new forms are con- 
tinually 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 maxi- 
mum 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 ex- 
tinction 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 evi- 
dence 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 im- 
proved within any given period, and they will consequent- 
ly be beaten in the race for life by the modified descend- 
ants 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 whicli stand in 
closest competition with those undergoing modification 
and improvement, will naturally sufler 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 extermmate 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 varie- 
ties 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 murder- 
ous 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 difi'er from each other far less 
than do good and distinct species. Nevertheless, accord- 
ing to my view^, varieties are species in the process of for- 
mation, or are, as I have called them, incipient species. 
How, then, does the lesser difl'erence between varieties 
become augmented into the greater difi'erence between 
species ? That this does habitually happen, we must in- 
fer from most of the innumerable species throughout na- 
ture presenting well-marked difterences; whereas varie- 


ties, the supposed protot^q^es and parents of future well- 
marked species, present slight and ill-defined differences. 
Mere cliance, as we may call it, might cause one variety 
to differ in some character from its parents, and the off- 
spring 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 ex- 
tremes," 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 supj)ose 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 continual 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 cen- 
turies, the sub-breeds would become converted into two 
well-established and distinct breeds. As the difterences 
slowly become greater, the inferior animals with interme- 
diate characters, being neither very swift nor very strong, 
will have been neglected, and will have tended to disap- 
pear. Here, then, we see in man's j)roductions the action 
of what may be called the principle of divergence, causing 
differences, at first barely appreciable, steadily to in- 
crease, and the breeds to diverge in character l)oth from 
each other and from their common parent. 

But how, it may be asked, can any analogous princi- 
ple apply in nature? I believe it can and doc^s apply 
most efiiciently, from the simj)le circumstance that the 
more diversified the descendants from any one species 


become in structure, constitution, and habits, by so mucb 
will they be better enabled to seize on many and widely 
diversified places in tlie 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 coun- 
try has long ago arrived at its full average. If its natu- 
ral 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 sta- 
tions, climbing trees, frequenting water, and some per- 
haps 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 through- 
out 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 one species of grass, and 
a similar plot 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 sever- 
al 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, includiDg 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 aimually 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 
suj)planting the less distinct varieties ; and varieties, when 
rendered very distinct from each other, take the rank of 

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 ex- 
tremely small area, especially if freely open to immigra- 
tion, and where the contest between individual and indi- 
vidual must be severe, we always find great diversity iti 
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 differ 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 groimd, 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 com- 
monly looked at as sj)ecially created and adapted for their 
own country. It might, also, perhaps have been expect- 
ed 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 admira- 
ble work, that floras gain by naturalisation, proportionally 
with the number of tlie native genera and SjDecies, 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 ISTorthern United States,' 260 natural- 
ised 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 ex- 
tent 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 

By considering the nature of the plants or animals 
which have struggled successfully with the indigenes of 
any country, and have there become naturalised, wx 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 advantao;e of diversification in the inhabitants of 
the same region is, in fact, the same as that of the physi- 
ological division of labour in the organs of the same 
individual bodies — a subject so well elucidated by Milne 
Edwards. ISTo 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 ani- 
mals, 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 represent- 


ing, as Mr. Waterhouse and others have remarked, our 
carnivorous, ruminant, and rodent mammals, could suc- 
cessfully compete with these well-pronounced orders. In 
the Australian mammals, we see the process of diversifi- 
cation 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 oc- 
cupied by other beings. 'Now let us see how this princi- 
ple of great benefit being derived from divergence of 
character, combined with the principles of natural selec- 
tion and of extinction, will tend to act. 

The accompanying diagram will aid us in understand- 
ing 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 de- 
grees, as is so generally the case in nature, and as is repre- 
sented 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 Itirge genera vary than of small genera ; 
and the varying species of the large genera j)i*esent 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 vary- 
ing species, belonging to a genus large in its own country. 
The little fan of diverging dotted lines of unequal length 
proceeding from (A), may represent its varying off'spring. 
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 peri- 
ods. 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 vari- 

14 , . '/ 

I ' 

> I 

\ I / 

> I / 

\ I / 




I K L 




T:. \ ■ . 

■'■■' 7 


Zidt.ct'SarvTU/.Ma/cr)^ I^inpp. 
^4Q Prraduai/ .1/ K 

' / / 




■•.■■..' 7: 


ing, as Mr. Watertioiise and others liave remarked, our 
carniYoroiis, rurainant, and rodent mammals, could suc- 
cessfully compete with these well-pronounced orders. In 
the Australian mammals, we see the process of diversifi- 
cation 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 oc- 
cupied by other beings. Now let us see how this princi- 
ple of great benefit being derived from divergence of 
character, combined with the principles of natural selec- 
tion and of extinction, will tend to act. 

The accompanying diagram will aid us in understand- 
ing 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 de- 
grees, as is so generally the case in nature, and as is repre- 
sented 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 Targe 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 vary- 
ing species, belonging to a genus large in its own country. 
The little fan of diverging dotted lines of unequal length 
proceeding from (A), may represent its varying ofl'spring. 
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 peri- 
ods. Only those variations which are in some way 
profitable Avill 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 vari- 

in E F 



h ' / ' 


-LL '■'4'" 

\] XIV 













A B r D E F (; H I 

D.Aliplrldii S.: C" Neil- York 

K L 


Ufh.of'.'iaTVfU/Mt'JC'^ f('iir/>/K 


ations (represented by the outer dotted lines) being pre- 
served and accumnlated bj 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 dia- 
gram, 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 «^ and 7??/\ These two varieties 
will generally continue to be exposed to the same condi- 
tions 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 inhabi- 
tants 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 cC' is supposed in the diagram to have 
produced variety «^, which will, owing to the principle of 
divergence, differ more from (A) than did variety a}. 
Variety m^ is supposed to have produced two varieties, 
namely m", and s^^ difi'ering 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 genera- 
tions, 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 tlie 
common parent (A), will generally go on increasing in 
number, and diverging in character. In tlie diagram the 
process is rej)resented np to the ten thousandth genera- 
tion, 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 in- 
creased. In our diagram the line of succession is broken 
at regular intervals by small numbered letters marking 
the successive forms which have become sufiiciently dis- 
tinct to be recorded as varieties. But these breaks are 
imaginary, and might have been inserted anywhere, aftei 
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-difiused 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 multi- 
plying 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 latter 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 tlie num- 
ber of tlie descendants will not be increased ; altbough. 
the amount of divergent modification may bave been in- 
creased in the successive generations. Tbis case would be 
represented in tbe diagram, if all the lines proceeding 
from (A) were removed, excepting tbat from a'' to a)'\ In 
tbe same way, for instance, tbe Englisb race-borse and 
Englisb pointer bave apparently botb gone on slowly 
diverging in cbaracter from tbeir original stocks, witbout 
eitber baving given off any fresb brancbes or races. 

After ten tbousand generations, species (A) is supposed 
to bave produced tbree forms, a'", /*'", and m'", wbicb, 
from baving diverged in cbaracter during tbe successive 
generations, will bave come to differ largely, but perbaps 
unequally, from eacb otber and from tbeir common parent. 
If we suppose tbe amount of cbange between eacb bori- 
zontal line in our diagram to be excessively small, tbese 
tbree forms may only still be well-marked varieties ; or 
tbey may bave arrived at tbe doubtful category of sub- 
species ; but we bave only to suppose tbe steps in tbe 
process of modification to be more numerous or greater 
in amount, to convert tbese tbree forms into well-defined 
species : tbus tbe diagram illustrates tbe steps by wbicb 
tbe small differences distinguisbing varieties are increased 
into tbe larger differences distinguisbing species. By con- 
tinuing tbe same process for a greater number of genera- 
tions (as sbown in tbe diagram in a condensed and sim- 
plified manner), we get eigbt species marked by tbe let- 
ters between cC'^ and m", all descended from (A). Tbus, 
as I believe, species are multiplied and genera are formed. 

Li a large genus it is probable tbat more tban one 
species would vary. In tbe diagram I bave assumed tbat 
a second species (I) bas produced, by analogous steps, 
after ten tbousand generations, eitber two well-marked 
varieties {w^" and s") or two species, according to tbe 
amount of cbange supposed to be represented between tbe 
borizontal lines. After fourteen tbousand generations, six 
new species, marked by tbe letters n^"^ to s'*, are sup]30sed 
to bave been produced. In eacb genus, tbe species, wbicb 
are already extremely different in cbaracter, will generally 


tend to produce the greatest number of modified descend- 
ants ; 'for these will have the best chance of filling new 
and widely difi'erent 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 con- 
tinue 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 ofi"spring 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 consid- 
erable amount of modification, species (A) and all the 
earlier varieties will have become extinct, having been 
replaced by eight new species {a^'^ to oii''^) ; and (I) will 
have been replaced by six (ti" to z"^) new species. 

But we may go further than this. The original speciea 
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 sj)ecies (I) more to G, H, 
K, L, than to tlie others. These two species (A) and (I), 
were also supposed to be very common and widely dif- 
fused species, so that they must originally have Iiad 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 inher- 
ited 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 oft'spring to the fourteen-thousandth genera- 
tion. 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 diflerence in character between species 
«^* and z^* will be much greater than that between the 
most difi'erent of the original eleven species. The new 
species, moreover, will be allied to each other in a widely 
difi'erent manner. Of the eight descendants from (A) the 
three marked a^\ q'\ p'% will be nearly related from hav- 
ing recently branched off from «" ; h^* and/"'*, from bar- 
ing diverged at an earlier period from a% will be in some 
degree distinct from the three first-named species ; and 
lastly, o'\ e'\ and m'\ will be nearly related one to the 
other, but from haviiig diverged at the first commence- 
ment of the process of modifecation, will be widely dif- 
ferent from the other five species, and may constitute a 
sub-genus or even a distinct genus. 


The six descendants from (I) will form two sii"b-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 impor- 
tant 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 dis- 
tinct 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 indi- 
cated by the broken lines, beneath the capital letters, con- 
verging 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 char- 
acter of the new species r'", which is supposed not to have 
diverged much in character, but to have retained the form 
of (F), either unaltered oi» altered only in a slight degree. 
In this case, its afiinities to the other fourteen new sj^ecies 
will be of a curious and circuitous nature. Having de- 
scended from a form which stood between the two parent- 
species (A) and (I), now supposed to be distinct and un- 
known, 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 char- 
acter from the type of their parents, the new species (r^*) 
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 snccessive 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 cC'^ to j?'*, those marked If^ 
and y ^*, and those marked d"^ to ^?i'\ 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 difi'er 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 spe- 
cies of the original genus ; and these two species are sup- 
posed to have descended from one sj)ecies 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 inci- 
j)ient 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 ad- 
vantage ; and the largeness of any group shows that its 
species have inherited from a common ancestor some ad- 
vantage in common. Hence, the struggle for the pro- 
duction of new and modified descendants, will mainly lie 


between the larger groups, wliicli are all trying to increase 
in number. One large group will slowly conquer another 
large group, reduce its numbers, and thus lessen its chance 
for further variation and improvement. "Within the same 
large group, the later and more highly j)erfected sub- 
groups, from branching out and seizing on many new 
places in the polity of ISTature, will constantly tend to 
supplant and destroy the earlier and less improved sub- 
groups. Small and broken groups and sub-groups will 
hnally tend to disa]3pear. 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 devel- 
oped, 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 ex- 
tremely 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 
wdth many species of many genera, families, orders, and 
classes, as at the present day. 

A distinguished naturalist has objected that the con- 
tinued action of natural selection and divergence will tend 
to make an indefinite number of specific forms. As lar 
as mere inorganic conditions are concerned, it seems proba- 
ble that a sufficient number of species would soon become 


adapted to all considerable diversities of heat, moisture, 
(fee. ; but I fully admit that the mutual relations of or- 
ganic beings are the most important, and as the number 
of species in any country goes on increasing, the organic 
conditions of life will become more and more complex. 
Consequentl}^, there seems at first to be no limit to the 
amount of profitable diversification of structure, and there- 
fore no limit to the number of species which might be 
produced. "We do not know that even the most prolific 
area is fully stocked with specific forms ; at the Cape of 
Good Hope, which supports such an astonishing number 
of species, many European plants have become naturalised. 
But geology shows us, at least within the whole immense 
Tertiary period, that the number of species of shells, and 
probably of mammals, has not increased. What, then, 
checks an indefinite increase in the number of species ? 
Firstly, the amount of life (I do not mean the number of 
specific forms) supported on any area must have a limit, 
depending so largely as it does on physical conditions : 
therefore where very many species are supported, each, 
or nearly each, will be few in individuals ; and any species 
with scanty numbers would be liable to extermination 
from accidental fluctuations in the nature of seasons and 
in the number of its enemies. The process of extermina- 
tion would in such cases be rapid, whereas the process of 
the production of new species would always be slow. Im- 
agine the extreme case of as many species as individuals 
in England, and the first severe winter or very dry sum- 
mer would exterminate thousands on thousands of species ; 
and individuals of other species would take their places. 
Secondly, I suspect that when any species becomes very 
rare, close interbreeding will tend to exterminate it ; at 
least authors have thought that this comes into play in ac- 
counting for the deterioration of aurochs in Lithuania, of 
red deer in Scotland, and of bears in Norway, &c. 
Thirdly, as far as animals are concerned, some species are 
closely adapted to prey on some one other being ; but if 
this other being had been rare, it would not have been 
any advantage to the animal to have been produced in 
close relation to its prey ; therefore it would not have 


been produced by natural selection. Fourthly, when any 
species becomes few in number, the process of modifica- 
tion will be slower, for the chance of favourable variations 
arising will be lessened ; therefore if we suppose an area 
to be inhabited by very many species, each, or nearly 
each, species will be poor in individuals, and consequently 
the process of modification and of giving birth to new 
forms will be retarded. Fifthly, and this I am inclined to 
think is the most important element, a dominant species 
which has already beaten many competitors in its own 
home, will tend to spread and supplant many others. 
Alph. de Candolle has shown that those species which 
spread widely, tend generally to spread veri/ widely ; and 
consequently they will tend to exterminate several species 
in several areas, and thus check the inordinate increase 
of specific forms throughout the world. Hooker has re- 
cently shown that in the S. E. corner of Australia, where 
aj)parently there are many invaders from different quar- 
ters of the world, the endemic Australian species have 
apparently been greatly reduced in number. How much 
weight to attribute to these several causes, I do not pre- 
tend to assign ; but conjointly I think they must limit in 
each country the tendency to an indefinite augmentation 
of specific forms. 

Natural Selection acts, as we have seen, exclusively 
by the preservation and accumulation of variations, which 
are beneficial under the oi'ganic and inorganic conditions 
of life to which each creature is at each successive period 
exposed. The ultimate result will be that each creature 
will tend to become more and more improved in relation 
to its conditions of life. This improvement will, I think, 
inevitably lead to the gradual advancement of the organi- 
sation of the greater number of living beings tbrougliout 
the world. But here we enter on a very intricate sub- 
ject, for naturalists have not defined to each other's satis- 
faction what is meant by an advance in organisation. 
Amongst the Yertebrata, the degree of intellect and an 
approach in structure to man clearly come into play. It 
might be thought that the amount of change which the 
various parts and organs undergo in their development 


from the embryo to matmity would suffice as a standard 
of comparison ; but there are cases, as with certain para- 
sitic crustaceans, in which several parts of the structure 
become less j^erfect and even monstrous, so that the ma- 
ture animal cannot be called higher than its larva. Yon 
Baer's standard seems the most widely applicable and the 
best ; namely, the amount of differentiation of the differ- 
ent parts (in the adult state, as I should be inclined to 
add), and their specialisation for different functions ; or 
as Milne Edwards would express it, the completeness of 
the division of physiological labour. But we shall see 
how obscure a subject this is, if we look, for instance, to 
Fish, amongst which some naturalists rank those as high- 
est which, like the sharks, approach nearest to reptiles ; 
whilst other naturalists rank the common bony or teleos- 
tean fishes as the highest, inasmuch as they are most 
strictly fish-like, and differ most from the other vertebrate 
orders. Still more plainly we see the obscurity of the 
subject, by turning to plants, where the standard of intel- 
lect is of course quite excluded ; and here some botanists 
rank those plants as highest which have every organ, as 
sepals, petals, stamens and pistils, fully developed in each 
flower; whereas other botanists, probably with more 
truth, look at the plants which have their several organs 
much modified and somewhat reduced in number as being 
of the highest rank. 

If we look at the differentiation and specialisation of 
the several organs of each being, when adult (and this 
will include the advancement of the brain for intellectual 
purposes), as the best standard of highness of organisa- 
tion, natural selection will clearly lead towards highness ; 
for all physiologists admit that the specialisation of organs, 
inasmuch as they perform in this state their functions bet- 
ter, is an advantage to each being ; and hence the accu- 
mulation of variations tending towards specialisation is 
within the scope of natural selection. On the other hand, 
we can see, bearing in mind that all organic beings are 
striving to increase at a high ratio and to seize on every 
ill-occupied place in the economy of nature, that it is 
quite possible for natural selection gradually to fit an or- 


ganic being to a situation in which several organs -vvonld 
be superflu-ons and useless ; and in such cases there might 
be retrogression in the scale of organisation. Whether 
organisation on the whole has actuallj^ advanced from the 
remotest geological periods to the present day will be 
more conveniently discussed in our chapter on geological 

But it may be objected, that if all organic beings thus 
tend to rise in the scale, how is it that throughout the 
world a multitude of the lowest forms still exist, and how 
is it that in each great class some forms are far more 
highly-developed than others ? Why have not the more 
higldy-developed forms everywhere supplanted and exter- 
minated the lower? Lamarck, who believed in an innate 
and inevitable tendency towards peifection in all organic 
beings, seems to have felt this difficulty so strongly, that 
he was led to suppose that new and simple forms were 
continually being produced by spontaneous generation. 
I need hardly say that science in her progress has forbid 
den us to believe that living creatures are now ever pro- 
duced from inorganic matter. On my theory the present 
existence of lowly organized ^productions offers no diffi- 
culty ; for natural selection includes no necessary and 
universal law of advancement or development ; it only 
takes advantage of such variations as arise and are bene- 
iicial to each creature under its complex relations of life. 
And it may be asked, what advantage, as far as we can 
see, would it be to an infusorian animalcule — to an intes- 
tinal worm — or even to an earth-worm, to be highly or- 
ganized ? If it were no advantage, these forms would be 
left by natural selection unimproved or but little im- 
proved ; and might remain for indefinite ages in their 
present little advanced condition. And geology tells us 
that some of the lowest forms, as the infusoria and rhizo- 
pods, have remained for an enormous jDcriod in neai-ly 
their present state. But to suppose that most of the many 
now-existing low forms have not in the least advanced 
since the first dawn of life, would be rash ; for every 
naturalist who has dissected some of the beings now ranked 
as very low in the scale, must often have been struck with 
their really wondrous and beautiful organisation. 


iN'early the same remarks are applicable, if we look to 
the great existing differences in the grades of organisation 
■svitliin almost every clasSj excepting birds ; for instance, 
to the coexistence of mammals and lisli in the vertebrata, 
— or to the coexistence of man and the ornithorhynchus 
amongst mammalia, — or amongst fish, of the shark and 
Amphioxus, which latter fish in the extreme simplicity of 
its structure closely approaches the invertebrate classes. 
But mammals and fish hardly come into competition with 
each other ; the advancement of certain mammals or of 
the whole class to the highest grade of organisation would 
not lead to' their taking the place of and thus exterminat- 
ing fishes. Physiologists believe that the brain must be 
bathed by warm blood to be highl}^ active, and this re- 
quires aerial respiration ; so that warm-blooded mammals, 
when inhabiting the water, live under some disadvantages 
compared with fishes. In this latter cUiss members of the 
shark family would not, it is probable, tend to supplant 
the Amphioxus ; the struggle for existence in the case of 
the Amphioxus must lie with members of the invertebrate 
classes. The three lowest orders of mammals — namely, 
marsupials, edentata and rodents — coexist in South 
America in the same region with numerous monkeys. 
Although organisation, on the whole, may advance 
throughout the world, yet the scale of perfection will still 
present all degrees for the high advancement of certain 
v/hole classes, or of certain members of each class, does 
not at all necessarily lead to the extinction of those groups 
with which they do not enter into close competition. In 
some cases, as we shall hereafter see, lowly-organized 
forms seem to have been preserved to the present day, 
from having inhabited peculiar or isolated stations where 
they have been subjected to less severe competition ; and 
where they have not advanced in organization owing to 
their scanty individual numbers, which, as already ex- 
plained, retards the chance of favourable variations 

Finally, I believe that lowly-organised forms now 
exist in numbers throughout the world and in nearly 
every class, from various causes. In some cases favoura- 


ble variations may never have arisen for natural selection 
to act on and accumulate. In no case, perhaps, has time 
sufficed for ttie utmost possible maximum of development. 
In some few cases there may have been what we must 
call retrogression of organisation. But the main cause 
lies in the circumstance, that under very simple condi- 
tions of life, a high organisation would be of no service — 
possibly would be of actual disservice, as being of a more 
delicate nature, and more liable to be put out of order 
and thus injured. 

A difficulty, diametrically opposite to this which we 
have just been considering, might be advanced ; namely, 
looking to the dawn of life, when all organic beings, as 
we may imagine, presented the simplest structure, how 
could the first steps in advancement or in the differentia- 
tion and specialization of parts arise? I can make no 
sufficient answer, and can only say that we have no facts 
to guide us, and therefore that all speculations on this 
subject would be baseless and useless. 

Buimnary 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 characterized will have the best chance of being pre- 
served in the struggle for life ; and from the strong prin- 
ciple of inheritance they will tend to produce offspring 
similarly characteriired. This principle of preservation, 
I have called, for the sake of brevity, Natural Selection. 

Chap. IV. 3 SUMMARY. l^^ 

Natural selection, on the principle of qualities being in- 
herited 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 offsj^ring. 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 na- 
ture, 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 selec- 
tion, 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 modi- 
fication 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 difi'erences between species of the 
same genus, or even of distinct genera. 

We have seen that it is the common, the widely-difi*used, 
and widely-ranging species, belonging to the larger genera, 
which vary most ; and these will tend to transmit to their 
modified offspring that superiority Avhich 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 that the differences 
in rank of the innumerable organic beings in each class 
thi-oughout the world, as well as the nature of their affini- 
ties, 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 thronghout 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 un- 
equally 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 
siDgle tile, 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 inde- 
pendently 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 bud- 
ding twigs may represent existing species ; and those pro- 
duced during each former year may represent the long 
succession of extinct species. At each period of growth 
all the gi'owing twigs have tried to branch out on all sides, . 
and to overtop and kill the surrounding twigs and branch- 
es, 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 tliis con- 
nection 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 descend- 

Chap. IV.] SUMMARY. 119 

ants. 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 ns 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 Ornithorhynchns 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 pro- 
tected 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 Lue, which fills with its dead 
and broken branches the crust of the earth, and covers 
the surface with its ever branching and beautiful ramifi- 





Effects of external conditions — Use and disuse, combined with natural selection ; 
organs of flight and of vision— Acclimatisation— Correlation of growth— Compen- 
sation and economy of growth — False correlations — Multiple, rudimentary, and 
lowly organised structures variable — Parts developed in an unusual manner are 
highly variable : specific characters more variable than generic : secondary sexual 
characters variable — Species of the same genus vary in an analogous manner — Re- 
versions to long lost characters — Summary. 

I HAVE liitlierto sometimes spoken as if the yariations — so 
common and multiform in organic beings nncler domes- 
tication, and in a lesser degree in those in a state of nature 
— ^had been due to chance. This, of course, is a wholly in- 
correct expression, but it serves to acknowledge j)lainlj 
our ignorance of the cause of each particular variation. 
Some authors believe it to be as much the function of the 
rej^roductive 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 do- 
mestication or cultivation, than under nature, leads me to 
believe that deviations of structure are in some way due 
to the nature of tlie conditions of life to which the parents 
and tlieir 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 at- 
tribute the varying or plastic condition of the offspring. 
The male and female sexual elements seem to be affected 


before that union takes place wliicli is to form a new be- 
ing. In the case of " sporting " plants, the bnd, 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. ISTeverthe- 
less, we can here and there dimly catch a faint ray of 
light, and we may feel sure tliat 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 can- 
not 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 spealvs 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 res- 
idence 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 flesliy. Several other such cases could be 

The fact of varieties of one species, when they range 
into the zone of habitation- of other species, often acquir- 
ing in a very sliglit degree some of the characters of such 
sjDccies, accords with our view that species of all kinds are 
only well-marked and permanent varieties. Thus the spe- 
cies 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- 

j^22 LAWS OF VARIATION. [Chap. \. 

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 Avarm 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 con- 
ditions of life. Thus, it is well known to furriers that an- 
imals of the same species have thicker and better fur tlie 
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 do- 
mestic quadrupeds. 

Instances could be given of the same variety being 
produced under conditions of life as difi'erent as can well 
be conceived ; and, on the other hand, of different varie- 
ties 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 oj)posite climates. 
Such considerations as these incline me to lay very little 
weight on the direct action of the conditions of life. In- 
directly, as already remarked, they seem to play an im- 
portant 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 mid 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 cer- 
tain parts, and disuse diminishes them ; and that such 
modincations are inherited. Under free nature, we can 

Chap, V.] USE AND DISUSE. ][23 

have no standard of comparison, by wMcli to judge of the 
effects of long-contmued nse 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 arc several 
in this state. The loggerheaded 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 Ayles- 
bury duck. As the larger ground-feeding birds seldom 
take flight except to escape danger, I believe that the 
nearl}" wingless condition of several birds, which now in- 
habit 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 succes- 
sive generations the size and weight of its body, its legs 
were used more, and its wings less, until they became in- 
capable of flight. 

Ivirby has remarked (and I have observed the same 
fiict) that the anterior tarsi, or feet, of many male dung- 
feeding beetles are very often broken oft"; 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 suflicient evidence to induce us to believe that mu- 
tilations are ever inherited ; and I should prefer explain- 
ing the entire absence of the anterior tarsi in Ateuchus, 
and their rudimentary condition m some other genera, by 
the long-continued effects of disuse in their progenitors ; 
for as the tarsi are almost always lost in many dung-feed- 
ing beetles, they must be lost early in life, and therefore 
cannot be much used by these insects. 

j[24: LAWS OF VARIATION. [Chap. V. 

Ill some cases we miglit easily i^iit down to disuse mod- 
ifications of structure which are wholly, or mainly, due to 
natural selection. Mr. Wollaston has discovered the re- 
markable 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 wing- 
less beetles is larger on the exposed Dezertas than in Ma- 
deira 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 ex- 
cessively numerous, and which groups have habits of life 
almost necessitating frequent flight ; — these several con- 
siderations have made me believe that the wingless con- 
dition 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 coleoj^tera and lepidop- 
tera, must habitually use their wings to gain their subsist- 
ence, have, as Mr. AYollaston suspects, their wings not at 
all reduced, but even enlarged. This is quite compatible 
with the action of natural selection. For when a new in- 
sect 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 

Chap. V.J USE AND DISUSE. ]^25 

shipwrecked 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 con- 
dition, 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 ad- 
hesion 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 elfects of disuse. 

It is well known that several animals, belonging to the 
most diff'erent 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 tlie 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 Schicidte 
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 I^orth America 
and Europe. On my view we must suppose that Amer- 
ican 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 Euroj)e. We 
have some evidence of this gradation of habit ; for, as 
Schiodte remarks : " We accordingly look upon the subter- 
ranean Faunas as small ramifications which have penetra- 
ted into the earth from the geographically-limited Faunas 
of the adjacent tracts, and which as they extended them- 
selves into darkness, have been accommodated to surround- 
ing circumstances. 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 gener- 
ations, the deepest recesses, disuse will on this view have 
more or less j)erfectly obliterated its eyes, and natural se- 
lection, will often have effected other changes, such as an 
increase in the length of the antennse or palpi, as a com- 
pensation for blindness. ^Notwithstanding such modifica- 
tions, we might expect still to see in the cave-animals of 
America, affinities to the other inhabitants of that con- 
tinent, 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 exj^lanation 
of the affinities of the blind cave-animals to the other in- 
habitants of the two continents on the ordinary view of 


their independent creation. That several of the inhabi- 
tants of the caves of the Old and ]^ew Worlds should be 
closely related, we might expect from the well-known re- 
lationship of most of their other productions. Some, in- 
deed, of the cave-insects common to both hemispheres, 
belong to genera existing nowhere except in the caves ; 
but the progenitors of these insects may formerly have 
ranged widely over the whole area. Blind species of the 
genus Adelops now inhabit caves, and are likewise found 
in dark places under moss. Far from feeling any sur- 
prise that some of the cave-animals should be very anoma- 
lous, 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 inhabi- 
tants of these dark abodes will probably have been exposed. 

AcclwiatisaUon. — 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 genns to in- 
habit 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, acclimatisa- 
tion must be readily eflected during long-continued 
descent. It is notorious that each sj)ecies 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 j)lants 
cannot endure a damp climate. But the degree of adap- 
tation of species to the climates nnder which they live is 
often overrated. AYe may infer this from our frequent in- 
ability 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 compe- 
tition of other organic beings quite as much as, or more 
than, by adaptation to particular climates. But whether 


or not the adaptation Le generally very close, we liave 
evidence, in the case of some few plants, of their becom- 
ing, to a certain extent, naturally habituated to different 
temperatures, or becoming acclimatised : thus the pines and 
rliododendrons, raised from seed collected by Dr. Hooker 
from trees growing at different heights on the Himalaya, 
were found in this country to possess different constitu- 
tional ]30wers of resisting cold. Mr. Thwaites informs me 
that he has observed similar facts in Ceylon, and analo- 
gous observations have been made by Mr. H, C. Watson 
on European species of j)lants 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 lati- 
tudes, 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 trans- 
portation, 1 think the common and extraordinary capacity 
in our domestic animals of not only Avith standing the 
most difierent 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 ar- 
gument too far, o]i 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. Tlie 
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 zone. Hence I am inclined to look 
at adaptation to any special climate a^ 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 for- 
mer 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 both 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 spe- 
cially 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 i)i'eserve those individuals which are born with 
constitutions best adapted to their native countries. In 
treatises on many kinds of cultivated plants, certain vari- 
eties 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 con- 
stitutional difterences to habit. The case of the Jerusalem 
artichoke, which is never propagated by seed, and of 
which consequently new varieties have not been pro- 
duced, has even been advanced — for it is now as tender 
as ever it was — as proving that acclimatisation cannot be 
eftected ! 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 

130 LAWS OF VARIATION. [Chap, "V. 

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. 'Nov 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 

Correlation of Groivth. — 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. Tlie 
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 afi'ecting the 
early embryo, seriously affects the whole organisation of 
the adult. The several parts of the body which are homo- 
logous, 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 ten- 
dencies, 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 pennanent 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 nnion of 
homologous parts in normal structures, as the union of 
the petals of the corolla into a tube. Hard parts seem to 
aftect 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 pel- 
vis in the human mother influences by pressure the shape 
of the head of the child. In snakes, according to Schle- 
gel, the shape of the body and the manner of swallowing 
determine the position of several of the most important 

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 
nrst hatched, with the future colour of their plumage ; or, 
again, the relation between the hair and teeth in the 
naked Turkish dog, though here proba^ ^y homology 
comes into play ? With respect to this latte^- case of cor- 
relation, I think it can hardly be accidental, that if we 
pick out the two orders of mammalia which are most ab- 
normal in their dermal covering, 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 impor- 
tance of the laws of correlation in modifying important 
structures, independently of utility and, therefore, of nat- 
ural selection, than that of the difference between the 
outer and inner flowers in some Compositous and Umbel- 
liferous 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 dift'er in shape and sculpture ; and even the ovary 


itself, witli its accessory parts, differs, as has been de- 
scribed by Cassini. These diff'erences have been attrib- 
uted by some authors to i^ressnre, and the shape of the 
seeds in the ray-florets in some Compositse countenances 
this idea ; bnt, in the case of the corolla of the TJmbel- 
liferse, 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 nour- 
ishment from certain other parts of the flower had caused 
their abortion ; but in some Compositse there is a differ- 
ence in the seeds of the outer and inner florets without 
any difference in the corolla. Possibly, these several dif- 
ferences 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 strik- 
ing 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 uj^per 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 ad- 
vantageous in the fertilisation of plants of these two 
ordei'.^, 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 im- 
possible that they can be in any way advantageous to the 
plant : yet in the Umbelliferse these differences are of such 
apparent importance — the seeds being in some cases, ac- 
cording 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 nat- 
ural 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, miffht 2:et an advantag^e 
over those producing seeds less fitted for dispersal ; and 
this process could not possibly go on in fruit which did 
not open. 

The elder Geofi'roy and Goethe propounded, at about 
the same period, their law of compensation, or balance- 
ment 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 difiicult to get a cow to 
give much milk and to fatten readily. The same variety 
of the cabbage do not yield abundant and nutritious foli- 
age and a copious supply of oil-bearing seeds. "When 
the seeds in our fruits become atrophied, the fruit itself 
gains largely in size and cpality. In our poultry, a large 
tuft of feathers on the head is generally accompanied by 

][34: LAWS OF VARIATION. [Chap. V 

a diminished comb, and a large beard by diminished wat- 
tles. With species in a state of nature it can hardly be 
maintained that the law is of nniversal 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 con- 
ditions 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 in- 
dividual 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 cirrij^edes, 
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 Pro- 
teolepas : for the carripace in all other cirripedes consist 
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 pre- 
hensile antennae. ]N'ow the saving of a large and complex 
structure, when rendered superfluous by the parasitic hab- 
it.s of the Proteolepas, though effected by slow steps, would 
t/e a decided advantage to each successive individual of 
the species ; for in the struggle for life to which every ani- 
mal 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. 


ThuSj 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 otJier part to be largely de- 
veloped in a corresponding degree. And, conversely, 
that natural selection may perfectly well succeed in 
largely de^'eloping any organ, without requiring as a 
necessary. comj^ensation the reduction of some adjoining- 

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 vertebrge 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. Inas- 
much 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 lowiiess 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. JSTatural selection, it should 
never be forgotten, can act on each part of each being, 
solely throug-h and for its advantage. 

Eudimentary 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 rudimen- 
tary and aborted organs ; and I will here only add that 


their variability seems to be owing to tbeir 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 jpart developed in any species in an extraordinary 
degree or manner^ in co7nparison vnth the same part in 
allied species^ tends to he highly varial>le. — 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 tiie 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 proposi- 
tion 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 man- 
ner in comparison with the other sj^ecies of the same 
genus. The rule applies very strongly in the case of 
secondary sexual characters, when displayed in any un- 
usual manner. The term, secondary sexual characters, 
used by Ilimter, applies to characters which are attached 
to one sex, but are not directly connected with the act of 
rej^roduction. The rule applies to males and females ; 
but as females more rarely offer remarkable secondary 
sexual characters, it aj^plies more rarely to them. The 
rule being so plainly applicable in the case of secondary 
Bexnal characters, may be due to the great variability of 

Chap. V.] LAWS OF VARIATION. jj^g^ 

these cliaracters, whether or not displayed in any iimisnal 
manner — of which fact I think there can be little doubt. 
But that our rule is not confined to secondary sexual char- 
acters is clearly shown in the case of hermaphrodite cirri- 
pedes ; and I may here add, that I particularly attended 
to Mr. AVaterhouse'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 extreme- 
ly little even in different genera ; but in the several spe- 
cies of one genus, Pyrgoma, these valves present a mar- 
vellous 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 chai'acters of these important valves than do other 
species of distinct genera. 

As birds within the same country vary in a remark- 
ably 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 comjDare their relative degrees of variability. 

When we see any part or organ developed in a re- 
markable degree or manner in any species, the fair pre- 
sumption is that it is of high importance to that species ; 
nevertheless the part in this case is eminently liable tc 
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 sgecies have descended from 
other species, and have been modified through natural 
selection, I think we can obtain some light. In our do- 
mestic animals, if any part, or the whole animal, be neg- 

[38 LAWS OF VARIATION. [Uhap. ^ 

lected and no selection be applied, that part (for instance, 
tlie 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. Li rudimentary or- 
gans, and in those which have been but little specialised 
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 tlicre 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 no- 
toriously 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 modiiied state, as well as an innate 
tendency to further variability of all kinds, and. on tlie 
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 under- 
going modification. It further deserves notice that these 
variable characters, produced by man's selection, some- 
times 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 de- 
veloped 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 extraordi- 
nary amount of modification, since the period when the 
species branched ofli" 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 modifica- 
tion 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 re- 
maiaied for a much longer period nearly constant. And 
this, I am convinced, is the case. That the struggle be- 
tween 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 de- 
veloped 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 modifi- 
cation 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 ex- 
tended. 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 he 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 spe- 
cific 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 indii-ectly, true; I 
shall, however, have to return to this subject in our chap- 
ter 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 lias remarked with surprise that some 
important organ or part, which is generally very constant 
throughout large groups of species, has differed consider- 
ably in closely-allied species, that it has, also, been vari- 
able 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. Geofiroy 
St. Hilaire seems to entertain no doubt, that the more an 
organ normally diff'ers in the difi'erent species of the same 
group, the more subject it is to individual anomalies. 

On the ordinary view of each species having been in- 
dependently created, why should that part of the struc- 
ture, which difters from the same part in other independ- 
ently-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 vai-icties, we might surely expect to find them 
still often continuing to vary in those parts of their struc- 
ture which have varied within a moderately recent period, 
and which have thus come to diff*er. Or to state the case 

Chap. V.] LAWS OS'' VARIATION. ^^^l 

ill another manner : — tlie points in which, all tne 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 hapi^ened that natural selection will have modified 
several sjDecies, fitted to more or less widely-diftercnt 
habits, in exactly the same manner : and as tliese 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 

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 ad- 
mitted that sj^ecies 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 difierence between the males of 
gallinaceous birds, in which secondary sexual characters 
are strongly displayed, with the amount of difference be- 
tween 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 
a5 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 

j^2 LAWS 05' VALUATION. [Cha?. ^. 

gives fewer offspring to the less favoured males. What- 
ever the cause may be of the variability of secondary sex- 
ual characters, as they are highly variable, sexual selection 
will have had a wide scope for action, and may thns 
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 dif- 
ferences between the two sexes of the same species are 
generally displayed in the very same parts of the organi- 
sation in which the different species of the same genus 
differ from each other. Of this fact I will give in illus- 
tration 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 gen- 
erally common to very large groups of beetles, but in the 
Engidse, as "Westwood has remarked, the number varies 
greatly ; and the number likewise differs in the two sexes 
of the same species : again in a 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 sub- 
ject : 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 advan- 
tage 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 spe- 
cies to each otJier, or to fit the males and females to dif- 
ferent habits of life, or the males to struggle with other 
males for tlie 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 

Chap, v.] laws OF VARIATION. ^^q 

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 varia- 
bility in a part, however extraordinarily it may be devel- 
oped, if it be common to a whole gi-onp of species ; — that 
the great variability of secondary sexual characters, and 
the great amount of diflerence in these same characters 
between closely allied species ; — that secondary sexual and 
ordinarily specific difi'erences 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 Irom a com- 
mon 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 tlian parts 
which have long been inherited and have not varied, — to 
natural selection having more or less completely, accord- 
ing to the lapse of time, overmastered the tendency to re- 
version 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 pos- 
sessed by the aboriginal rock-pigeon ; these then are an- 
alogous variations in two or more distinct races. The fre- 
quent presence of fourteen or even sixteen tailfeathers in 
the pouter, may be considered as a variation representing 
the normal structure of another race, the fantail. I pre- 
sume that no one will doubt that all such analogous varia- 
tions are due to the several races of the pigeon having 

1^j_ LAWS OF VARIATIO^r. [Chap. V. 

inherited from a common parent the same constitution and 
tendency to variation, when acted on by similar miknown 
influences. In the vegetable kingdom we have a case of 
analogous variation, in the enlarged stems, or roots as com- 
monly called, of the Swedish turnip and Ruta baga, plants 
which several botanists rank as varieties produced by cul- 
tivation 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 commn 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 com- 
munity 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 other feathers external^ 
ly edged near their bases with white. As all these re- 
marks are characteristic of the parent rock-pigeon, I pre- 
sume that no one will doubt that this is a case of rever- 
sion, and not of a new yet analogous variation aj^pearing 
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 oflfspring 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 oc- 
casionally 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 re- 
tained 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 repro- 
duce the lost character might be, as was formerly remark- 
ed, 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 succes- 
sive 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 plu- 
mage 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 gener- 
ations, 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 rudi- 
ment inherited : for instance, in the common snap-dragon 
(Antirrhinum) a rudiment of a fifth stamen so often ap- 
pears, 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 

j^^g LAWS OF VARIATION. [Chap. V. 

the species, and will not be left to the mutual actions of 
the condition of life and of a similar inherited constitu- 
tion. 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 to- 
gether from simple variation. More especially we might 
have inferred this, from the blue colour and marks 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 varia- 
tions, 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 other 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, some 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 ; and this shows, unless all these forms 
be considered as independently created 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 impor- 
tant and uniform nature occasionally varying so as to ac- 
quire, in some degree, the character of the same part or 
organ in an allied species. I have collected a long list of 
Buch cases ; but here, as before, I lie under a great disad- 

Chap. V.] LAWS OF VARIATION. -[^^^ 

vantage in not being able to give them. I can only repeat 
that sucli cases certainly do occur, and seem to me very 

I will, however, give one cnrions and complex case, 
not indeed as affecting any important character, but from 
occurring in several sj^ecies 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 on the legs of 
the 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 with- 
out 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. The hemionus has 
no shoulder-stripe ; but traces of it, as stated by Mr. Blyth 
and others, occasionally appear : and I have been informed 
by Colonel Poole that the foals of this sj)ecies 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. 

AVith 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 

248 LAWa OF VARIATION. [Chap. V. 

Poole, wlio examined the breed for tlie Indian Groyern- 
ment, a horse without stripes is not considered as pnrelj- 
bred. The spine is always striped ; the legs are generally 
barred; and the shonlder-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 Mi\ 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 difi*erent breeds, in various coun- 
tries 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, AVelch ponies, cobs, the 
lanky Kattywar race, &c., inhabiting the most distant 
parts of the world. 

JS'ow let us tm-n to the efi'ects of crossing the several 
species of the horse-genus. Rollin asserts, that the com- 
mon mule from the ass and horse is particularly apt to 
have bars on its legs : according to Mr. Gosse, in certain 
parts of the United States about nine out of ten mules 
have striped legs. I once saw a mule with its legs so 
much sti-iped that any one would at first 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 
ffiven a figure of a similar mule. In four coloured draw- 

CuAP. v.] LAWS OF VAllIATIOX. -j^^q 

ings, 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 JMoreton's famous hybrid from 
a chestnut mare and male quagga, the liybrid, 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 com- 
monly 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 w^as, as we have seen, answered in 
the aflimiative. 

What now are we to say to these several facts ? We 
see several very distinct species of the horse-genus becom- 
ing, 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 be- 
come striped most strongly displayed in hybrids from be- 
tween several of the most distinct species. IS^ow observe 
the case of the several breeds of j^igeons : they are de- 
scended from a pigeon (includng two or three sub-si^ecies, 
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. "WTien tlie oldest and truest breeds of 
various colours are crossed, we see a strong tendency for 
ttie blue tint and bars and marks to reappear in the mon- 
grels. I have stated that tlie most probable hypothesis to 
account for the reappearance of very ancient characters, 
is — that there is a tendency in the young of each succes- 
sive generation to produce the long-lost character, and 
that this tendency, from unknown causes, sometimes pre- 
vails. 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 cen- 
turies, species ; and how exactly parallel is the case with 
that of the species of the horse-genus ! For myself, I ven- 
ture 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 com- 
mon 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 inde- 
j)endently created, will, I presume, assert that each spe- 
cies 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 
genns ; 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 sj)ecies 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, l^ot in one case out of a hundred can we 

Ohap, v.] summary. 1^^ 

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 com- 
parison, the same laws appear to have acted in producing 
the lesser diflerences between varieties of the same spe- 
cies, 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 modifica- 
tions. Habit in producing constitutional differences, and 
use in strengthening, and disuse in weakening and dimin- 
ishing 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 specialised to any particular 
function, so that their modifications have not been closely 
cliecked by natural selection. It is probably from this 
same cause that organic beings low in the scale of nature 
are more variable than those v/liich have their whole 
organisation more specialised, and are higher in the scale. 
Rudimentary organs, from being useless, will be dis- 
regarded 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 difi'er ; 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 genns 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 
sj)ecies 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 extra- 
ordinarily-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 
tlie same constitution from a common parent and exposed 
to similar influences will naturally tend to present ana- 
logous variations, and these same species may occasionally 
revert to some of the characters of their ancient progen- 
itors. Although new and important modifications may 
not arise from reversion and analogous variation, such 
modifications will add to the beautiful and harmonious 
diversities 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 

Chap, V.] ' SUMMARY, 


selection, of such diiferences, when beneficial to the indi- 
vidual, that gives rise to all the more important modifica- 
tions 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. 




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 alUes— 
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 daj 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 

These difficulties and objections may be classed under 
the following heads : — Firstly, why, if species have de- 
scended 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 diflerent liabits ? Can we believe that natural 
selection could produce, on the one hand, organs of trifling 
importance, such as the tail of a girafte, which serves as 
a fiy-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 ? AVhat shall we say to so marvellous 


an instinct as that wliicli leads the bee to make cells, 
which have practically anticipated the discoveries of pro- 
found 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 tJie absence or rarity of transitional varieties. — As 
natural selection acts solely by the preservation of profit- 
able modifications, each new form will tend in a fully- 
stocked country to take the place of, and finally to exter- 
minate, its own less improved parent or other less-favoured 
forms with which it comes into competition. Thus ex- 
tinction 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 per- 
fection 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 tlie 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 
suflficiently 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 sedi- 
ment is being deposited, there will be blanks in our geo- 


logical history. The crust of tlie earth is a vast museum ; 
but the natural collections have been made only at inter- 
vals of time immensely remote. 

But it may be urged tliat when several closely-allied 
species inhabit the same territory we surely ought to find 
at the j^resent 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 sj)ecies, 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 inter- 
mediate region, having intermediate conditions of life, why 
do we not now find closely-linking intermediate varieties ? 
This difiiculty 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 inter- 
mediate 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 tins way of escap- 
ing from the difficulty ; for I believe that many perfectly 
delined 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 impor- 
tant 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 terri- 
tory 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 ^^hysical 
conditions of life as the all-important elements of distri- 
bution, 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 metro- 
polis, 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 spe- 
cies, on which it depends, or by which it is destroyed, or 
with which it comes into competition ; and as these spe- 
cies 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 de- 
fined. Moreover, each species on the confines of its 
range, where it exists in lessened numbers, will, during 


fluctuations in tlie nnmber of its enemies or of its prey, 
or in the seasons, be extremely liable to utter extermina- 
tion ; and thus its geographical range will come to be 
still more sharply denned. 

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 com- 
paratively 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 aj)ply to both ; and if we in imagina- 
tion 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 varie- 
ties intermediate between two other forms occur, they 
are much rarer numerically than the forms which they 
connect. ISTow, 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 exter- 
minated than one existing in large numbers ; and in tliis 
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 modifi- 
cation, by which two varieties are supposed on my theory 


to be converted and perfected into two distinct species, 
the two wliicli exist in larger numbers from inhabiting 
larger areas, will have a great advantage over the inter- 
mediate 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 varia- 
tions 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 s]3ecies. 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 conse- 
quently 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 gi-eater num- 
bers, will come into close contact with each other, with- 
out the interposition of the supplanted, intermediate hill- 

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 imtil 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 clianges of climate, or on the occasional immigration 
of new inhabitants, and, probably, in a still more impor- 
tant 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 struc- 
ture in some degree permanent; and this assm'edly we 

Secondly, areas now continuous must often have ex- 
isted 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 com- 
mon parent, must formerly have existed in each broken 
portion of the land, but these links will have been sup- 
planted and exterminated during the process of natural 
selection, so that they will no longer exist in a living 

Thirdly, when two or more varieties have been formed 
in difi'erent portions of a strictly continuous area, inter- 
mediate varieties will, it is probable, at first have been 
formed in the intermediate zones, but they will generally 
have ha,d a short duration. For these intermediate vari- 
eties 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 num- 
bers 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 fur- 
ther 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, link- 
ing most closely all the sjDecies of the same grouj) together^ 
must assuredly have existed ; but the very process of 
natural selection constantly tends, as has been so often re- 
marked, to exterminate the parent-forms and the inter- 
mediate links. Consequently evidence of their fomer ex- 
istence 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 i/ransitions of organic teings 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 ex- 
ist 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 N^orth 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 pole-cats on mice and land 
animals. If a different case had been taken, and it had 
been asked how an insectivorous quadruped could pos- 
sibly 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 

Here, as on other occasions, I lie under a heavy dis- 
advantage, for out of the many striking cases wh'icli I 
have collected, I can give only one or two instauccs of 
transitional habits and structures in closely allied species 
of the same genus; and of diversified habits, eitlicr con- 
stant or occasional, in the same species. And it eoems to 
me that nothing less than a long list of such cases is suf- 
ficient to lessen the difficulty in any particular case like 
that of t]ie 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 re- 
marked, 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 ex- 
panse of skin, which serves as a parachute and allows 
them to glide through the air to an astonishing distance 
from tree to tree. vVe 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, un- 
less they also became modified and improved in stnicture 
in a corresponding manner. Tlierefore, I can see no dif- 
ficulty, more especially under changing conditions of life, 
in the continued preservation of individuals with fuller 
and fuller flank-membranes, each modification being use- 
ful, each being propagated, until by the accumulated 
effects of this process of natural selection, a perfect so- 
called flying squirrel was produced. 

ITow look at the Galeopithecus or flying lemur, which 
formerly was falsely ranked amongst bats. It has an ex- 
tremely 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 con- 
nect the Galeopithecus with the other Lemuridte, 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 eacii grade of structure had been useful to its pos- 
sessor. Nor can I see any insuperable difficulty in fur- 
ther believing it possible that the membrane-connected 
lingers 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 ex- 
tended from the top of the shoulder to the tail, including 
the hind-legs, we perhaps see traces of an apparatus orig- 
inally 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 sur- 
mised that birds might have existed which used their 
wings solely as flappers, like the logger-headed duck 
(Micropterus of Ey ion) ; 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 diversifled 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 flyiug- 
fish, which now glide far through the air, slightly rising 
and turning by the aid of their fluttering flns, mic^ht have 
been modified into perfectly winged animals. If'this 1 ad 
been cftected, who would have ever imagined that in an 
early transitional state they had been inhabitants of the 
open ocean, and had used their incipient organs of flight 
exclusivclv, as far as we know, to escape being devoured 
by other fish ? 


When we see any structure Mglily perfected for any 
particular habit, as the wings of a bird for flight, we 
should bear in mind that animals displaying early tran- 
sitional grades of the structure will seldom continue to 
exist to the present day, for they will have been supplant- 
ed by the very process of perfection through natural se- 
lection. Furthermore, we may conclude that transitional 
grades between stiaictures fitted for very difi'erent habits 
of life will rarely have been develoj)ed 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 subordi 
nate 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 

I will now give two or three instances of diversified 
and of changed habits in the individuals of the same spe- 
cies. When either case occurs, it would be easy for na- 
tural selection to fit the animal, by some modification of 
its structure, for its changed habits, or exclusively for one 
of its several diflPerent habits. But it is difficult to tell, 
and immaterial for us, whether habits generally change 
first and structure afterwards ; or whether slight modifica- 
tions 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 exclu- 
sively on artificial substances. Of diversified habits innu- 
merable 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 station- 
ary on the margin of water, and then dashing like a king- 


fisher at a fish. In our own country the large 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 E'orth America 
the black bear was seen by Plearne swimming for hours 
with widely open mouth, thus catching, like a whale, in- 
sects 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 w^e 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, w^e might ex- 
pect, on my theory, that such individuals would occasion- 
ally have given rise to new species, having anomalous 
habits, and with their structure either slightly or consid- 
erably modified from that of their projDcr type. And such 
instances do occur in nature. Can a more striking in- 
stance of adaptation be given than that of a woodpecker 
for climbing trees and for seizing insects in the chinks of 
the bark ? Yet in ]N"orth 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 woodpeck- 
er, which in every essential part of its organisation, even 
in its colouring, in the harsh tone of its voice, and undu- 
latory 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 aerial 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 un- 
willingly it takes flight, would be mistaken by any one 
for an auk or grebe; nevertheless, it is essentially a 


petrel, but witli many parts of its organisation profoundly 
modified. On the other hand, the acutest observer by 
examhiing 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 surprised 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 swim- 
ming? 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, al- 
though 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 
been 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 se- 
lection, will acknowledge that every organic being is con- 
stantly endeavouring to increase in numbers ; and that if 
any one being vary ever so little, either in habits or struc- 
ture, and thus gain an advantage over some other inhab- 
itant of the country, it will seize on the place of that in- 
habitant, however difi'erent 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 admit- 
ing 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 use- 
ful to its possessor, can be shown to exist ; if, further, the 
eye does vary ever so slightly, and the variations be in- 
herited, which is certainly the case ; and if any variation 
or modification in the organ be ever useful to an animal 
under clianging conditions of life, then the difficulty of 
believing that a perfect and complex eye could be formed 
by natural selection, though insuperable by our imagina- 
tion, 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 exclu- 
sively 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 gra- 
dations having been transmitted from the earlier stages 
of descent, in an unaltered or little altered condition. 
Amongst existing Yertebrata, we find but a small amount 


of gradation in tlie structure of the eye, and from fossil 
species we can learn nothing on this head. In this great 
class we should jjrobably have to descend far beneath the 
lowest known fossiliferons stratum to discover the earlier 
stages, bj 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 gra- 
dations of structure, branchino; off in two fundamentally 
different lines, can be shown to exist, until we reach a 
moderately high stage of perfection. In certain crusta- 
ceans, for instance, there is a double cornea, the inner one 
divided into facets, within each of w^hich 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 im- 
perfectly 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 pro- 
portion 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 aj)paratus of an optic nerve merely 
coated with pigment and invested by transparent mem- 
brane, into an optical instrument as perfect as is ]30ssessed 
by any member of the great Articulate class. 

He who will go thus far, if lie finds 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 na- 
tural 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 ex- 
tending the principle of natural selection to such startling 


It is scarcely possible to avoid comparing the eye to 
a telescope. Ave know that this instrument has been 
perfected by the long-continued efforts of the highest hu- 
man intellects ; and we naturally infer that the eye has 
been formed by a somewhat analogous process. But may 
not this inferejice be presumptuous ? Have we any right 
to assume that the Creator works by intellectual powers 
like those of man ? K 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 diflerent 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 trans- 
parent 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 mul- 
tiplied by the million ; and each to be preserved till a 
better be produced, and then the old ones to be destroyed. 
In livmg bodies, variation will cause the slight alterations, 
generation will multiply them almost infinitely, and na- 
tural selection will pick out with unerring skill each im- 
provement. Let this process go on for millions on mil- 
lions of years ; and during each year on millions of indi- 
viduals 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. JN'o 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 
ray theory, there has been much extinction. Or again, if 


we look to an organ common to all tlie members of a 
large class, for in this latter case the organ mnst have 
been first formed at an extremely remote period, since 
which all the many members of the class have been de- 
veloped ; 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 grada- 
tions of some kind. ISTumerous cases could be given 
amongst the lower animals of the same organ perform- 
ing 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 in^de out, and the ex- 
terior 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 in- 
stance, 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 per- 
fected 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 
diflerent 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 swiinbladder. 
All physiologists admit that the swimbladder is homolo- 
gons, 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 swimbladdei 
into a lung, or organ used exclusively for respiration. 

I can, indeed, hardly doubt that all vertebrate ani- 
mals having true lungs have descended by ordinary gen- 
eration from an ancient prototype, of which we know 
nothing, furnished with a floating apparatus or swim- 
bladder. 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 closeo. 
In the higher Yertebrata the branchiae have Avholly disap- 
peared — 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 ut- 
terly lost branchise 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 ^^robable 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, in- 
cluding the small frena, serving for respiration. The 
Balanidge or sessile cirripedes, on the other hand, have no 


ovigeroiis frena, the eggs lying loose at the bottom of the 
sack, in the well-enclosed shell ; but they have large 
folded branchiae. IS^ow I think no one will dispute that 
the ovigerous frena in the one family are strictly homolo- 
gous 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 se- 
lection 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 con- 
cluding that any organ could not possibly have been pro- 
duced by successive transitional gradations, yet, un- 
doubtedly, 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 Kays have an organ 
closely analogous to the electric apparatus, and yet do 
not, as Matteucci 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 mem- 
bers 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 natu- 
ral selection. But if the electric organs had been inherited 
from one ancient progenitor thus j)rovided, 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 pres- 
ence of luminous organs in a few insects, belonging to 
difierent families and orders, ofi!*ers a parallel case of 
difiiculty. 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. Li all 
these cases of two very distinct species furnished with 
apparently the same anomalous organ, it should be ob- 
served that, although the general appearance and function 
of the organ may be the same, yet some fundamental 
diflerence can generally be detected. I am inclined to 
believe that in nearly the same way as two men have 
sometimes indei^endently hit on the very same invention, 
so natural selection, working for the good of each being 
and taking advantage of analogous variations, has some- 
times modified in very nearly the same manner two parts 
in two organic beings, which owe but little of their struc- 
tm*e 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 liv- 
ing 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 " l^atura non facit saltum." 
We meet with tliis 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 separate- 
ly created for its proper place in nature, be so invariably 
linked together by graduated steps? Why should not 
ISTature have taken a leap from structure to structure ? 
On the theory of natural selection, Tve can clearly under- 
stand 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 short- 
est and slowest steps. 

Organs of little wpparent importance. — As natural se- 
lection acts by life and death, — by the preservation of 
individuals with any favorable variation, and by the de- 
struction of those with any unfavorable deviation of 
structure, — I have sometimes felt much difficulty in un- 
derstanding the origin of simple parts, of which the im- 
portance 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 com- 
plex as the eye. 

In the first place, we are much too ignorant in regard 
to the wliole 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 modi- 
fications, each better and better, for so trifiing an object 
as driving away flies ; yet we should pause before being 
too positive even in this case, for we know that the distri- 
bution 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 tliemselves from these small enemies, 
would be able to range into new pastures and thus gain a 



great advantage. It is not tliat tlie larger quadrupeds are 
actually destroyed (except in some rare cases) by the flies, 
but they are incessantly harassed and their strength re- 
duced, 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 inju- 
rious deviations in their structures 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 
swimbladders 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 fl^^-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 im- 
portance to characters which are really of very little im- 
portance, 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 ; tliat correlation of 
growth will have had a most important influence in mod- 
ifying various structures ; and, finally, tliat 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 de- 
scendants of the species under new conditions of life and 
with newly acquired habits. 


To give a few instances to illustrate these latter re- 
marks. 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-frequent- 
ing 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 Ar- 
chipelago climbs the loftiest trees by the aid of exquisitely 
constructed hooks clustered around the ends of the branch- 
es, 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 under- 
going 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 draw- 
ing 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 imme- 
diately made conscious of this by reflecting on the diifer- 
ence in the breeds of our domesticated animals in diflerent 
countries, — more especially in the less civilized comitries 
where there has been but little artificial selection. Care- 
ful observers are convinced that a damp climate aflects 
the growth of the hair, and that with the hair the horns 


are correlated. Mountain breeds always differ from low- 
land 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 neces- 
sary in high regions would, we have some reason to be- 
lieve, increase the size of the chest ; and again correlation 
would come into play. Animals kept by savages in dif- 
ferent countries often have to struggle for their own sub- 
sistence, 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 coloui 
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 be- 
tween 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 tbeory. Yet I fully ad- 
mit that many structures are of no direct use to their j)os- 
sessors. 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 for- 
merly 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 
du-ect 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 conse- 
quently, though each being assuredly is well fitted for its 
place in nature, many structures now have no direct rela- 
tion 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 use- 
ful 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 sev- 
eral 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 nse to some ancestral form, or as 
being now of special use to the descendants of this foi'm 
— either directly or indirectly through the complex laws 
of growth, 

Natural selection cannot possibly produce any modifi- 
cation in any one species exclusively for the good of 
another species ; though throughout nature one species 
incessantly takes advantage of, and profits by, the struc- 
ture 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 pro- 
duced through natural selection. Although many state- 
ments may be found in works on natural history to this 
eifect, 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 injur}", 
namely, to warn its prey to escape. I would almost as 
60on 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 

Natural selection will never produce in a being any- 
thing 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 myi'iads have be- 
come extinct. 

E'atural 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 nnder nature. The endemic 
productions of I^ew Zealand, for instance, are perfect one 
compared with another ; but they are now rapidly yield- 
ing before the advancing legions of plants and animals 
introduced from Europe. Natural selection will not pro- 
duce 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. K 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 indus- 

Chap. VI,] . SUMMARY. JgJ_ 

trious 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 fertil- 
ised 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 dis- 
cussed 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 spe- 
cies 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 con- 
tinuous 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 con- 
tinuous 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 num- 
bers 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 ad- 
vantage over the less numerous intermediate variety, and 


will thus generally succeed in supplanting and exterminat- 
ing it. 

We have seen in tliis chapter ho"w cautious we should 
he in concluding that the most different habits of life could 
not graduate into each other ; that a hat, 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 condi- 
tions of life change its habits, or have diversified habits, 
with some habits very unlike those of its nearest con- 
geners. 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 com- 
plexity, 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 concludmg that none could have existed, 
for the homologies of many organs and their intermediate 
estates show that wonderful metamorj)hoses in function are 
at least possible. For instance, a swimbladder has ap- 
parently been converted into an air-breathing lung. The 
same organ having performed simultaneously very different 
functions, and then having been specialised for one func- 
tion ; 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 struc- 
ture could not have been slowly accumulated by means 
of natural selection, But we may confidently believe 

OnAP. VI.l SUMMARY. 183 

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. 

JS^atural selection will ^^I'oduce 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. ISTatural selection in each well-stocked country, 
must act chiefiy through the comj)etition of the inhabit- 
ants one with another, and consequently will produce per- 
fection, 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. 
E'atural 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, " Katura 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 acknowleged 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 tlie same class, and wliicli is quite inde- 
pendent of their habits of life. On my theory, imity of 
type is explained by nnity of descent. The expression 
of conditions of existence, so often insisted on by the 
ilhistrious 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 dui-ing long-past periods of time : the 
adaptations being aided in some cases by use and disuse, 
being slightly aflected 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. 

Chip. VII.] INSTINCT. 185 



Instiricts comparable with habits, but different in their origin— Instincts graduated— 
Aphides and ants— Instincta 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. 

The subject of instinct miglit liave 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 diver- 
sities of instinct and of the other mental qualities of ani- 
mals 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 under- 
stands 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 w^e 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. 

•j^gg IITSTIl^CT. [Chap. VII 

Frederick Cuvier and several of the older metaphysi- 
cians have compared instinct with habit. This compar- 
ison gives, I think, a remarkably accurate notion of the 
frame of mind under which an instinctive action is per- 
formed, 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 modi- 
fied by the will or reason. Habits easily become associ- 
ated 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 Avith a cater- 
pillar, which makes a very complicated hammock ; for if 
he took a caterpillar which had completed its hammock 
up to, say, the sixth stage of constniction, and j)ut it into 
a hammock completed up only to the third stage, the cat- 
erpillar 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 embar- 
rassed, and, in order to complete its hammock, seemed 
forced to start from the third stage, where it had left ofi", 
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 piano- 
forte at three years old with wonderfully little practice, 
had played a tune with no practice at all, he 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 tlien transmitted by inheritance to succeeding gene- 
rations. 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 
sj)ecies, under its present conditions of life. Under changed 
conditions of life, it is at least possible that slight modifi- 
cations 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 difiiculty 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 w^onderful instincts have 
originated. As modifications of corporeal structure arise 
from, and are increased by, use or habit, and are dimin- 
ished or lost by disuse, so I do not doubt it has been 
with instincts. But I believe that the eftects of habit are 
of quite subordinate importance to the efi'ects of the natu- 
ral selection of what may be called. accidental variations 
of instincts ; — that is of variations produced by the same 
tmknown causes which produce slight deviations of bodily 

JSTo complex instinct can possibly be produced through 
natural selection, except by the slow and gradual accu- 
mulation 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 ])ossible ; 
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 

288 INSTINCT. [Chap. VII. 

" ITatura 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 
difierent instincts at different periods of life, or at different 
seasons of the year, or when placed under different cir- 
cumstances, &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 con- 
formably 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 pre- 
vented 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 Avith 
its antennse 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 man- 
ner, 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 
mstinctively excrete for the sole good of the ants. Al- 
though I do not believe that any animal in the world per- 
forms an action for the exclusive good of another of a 

OniP. Vltl INSTINCT. 189 

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 in- 
dispensable for the action of natural selection, as many in- 
stances as possible ought to have been here given ; but 
want of space prevents me. I can only assert, that in- 
stincts certainly do vary — for instance, the migratory in- 
stinct, both in extent and direction, and in its total loss. 
So it is with the nests of birds, which vary partly in de- 
pendence 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 sever- 
al 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 sho^vn, by various animals inhabiting 
desert islands ; and we may see an inst^ance 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 
Korway, 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 spe- 
cies, give rise, through natural selection, to quite new in- 
stincts. But I am well aware that these general statements, 
without facts given in detail, can produce but a feeble 


effect on the reader's raind. I can only repeat my assur- 
ance, that I do not speak without good evidence. 

The possibility, or even j)robability, of inherited varia- 
tions 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 va- 
riations have played in modifying the mental qualities of 
our domestic animals. A number of curious and authen- 
tic 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 certainly far less fixed or invariable than nat- 
ural instincts ; but they have been acted on by far less 
rigorous selection, and have been transmitted for an in- 
comparably shorter period, under less fixed conditions of 

- How strongly these domestic instincts, habits, and dis- 
positions are mherited, 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 bnll-dog 
has affected for many generations the courage and obsti- 
nacy 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 parent- 
age 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, l^o 
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 train- 
ing 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 : the act of 
pointing is probably, as many have thought, only the ex- 
aggerated pause of an animal preparing to spring on its 
prey. When the first tendency to point was once dis- 
played, 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 

J^92 INSTINCT. [Chap. VII. 

of the tame rabbit ; but I do not suppose that domestic 
rabbits have ever been selected for tameness ; and I pre- 
sume that we must attribute the whole of the inherited 
change from extreme wildness to extreme tameness, sim- 
ply to habit and long-continued close confinement. 

l^atural instincts are lost under domestication : a re- 
markable 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 pre- 
vents our seeing how universally and largely the minds 
of our domestic animals have been modified by domestica- 
tion. 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 ! J^o doubt they occasionally do make an attack, 
and are then beaten ; and if not cured, they are destroy- 
ed ; so that habit, with some degree of selection, has prob- 
ably 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 tur- 
keys) from under her, and conceal themselves in the sur- 
rounding grass or thickets ; and this is evidently done for 
the instinctive purpose of allowing, as we see in wild 
ground-birds, their mother to fiy away. But this instinct 
retained by our chickens has become useless under do- 
mestication, 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 

Chap. VII.] OF THE CUCKOO. j[93 

by habit, and partly by man selecting and accunmlating 
during successive generations, peculiai mental habits and 
actions, which, at lirst appeared from what we must in 
our ignorance call an accident. In some cases compul- 
sory 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 incon- 
veniently 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 Ameri- 
can cuckoo occasionally lays her eggs in other birds' 
nests ; but I hear on the high authority of Dr. Brewer, 
that this is a mistake. ISTevertheless, I could give several 
instances of various birds which have been known occa- 
eionally 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 

-j^g^ INSTINCT. [Chap. VIL 

occasionally she laid an egg in another bird's nest. If 
the old bird profited by this occasional habit, or if the 
yonng were made more vigorous by advantage having 
been taken of the mistaken maternal instinct of another 
bird, than by their own mother's care, encnmbered as she 
can hardly fail to be by having eggs and yonng of dif- 
ferent ages at the same time ; then the old birds or the 
fostered yonng 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 Gallinacess ; and this per- 
haps 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 himting I picked up no less than 
twenty lost and Avasted eggs. 

Many bees are parasitic, and always lay their eggs in 
the nests of bees of other kinds. Tliis case is more remark- 
able than that of the cuckoo ; for these bees have not 
only their instincts but their structure modified in accord- 
ance with their parasitic habits ; for they do not j^ossess 
the pollen-collecting apparatus which would be necessary 
if they had to store food for their own young. Some 


species, likewise, of Spliegidse (wasp-like insects) are 
parasitic on other species ; and M. Fabre lias lately shown 
good reason for believing that althongh the Tachy tes nigra 
generally makes its own burrow and stores it with para- 
lysed 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 selec- 
tion 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 ex- 

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 larvse. "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 larvse 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 extra- 
ordinary 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. 

Another species (F. sanguinea) was also discovered by P. 
Huber to be a slave-making ant. This species is found in 


the soutliern parts of England, and its habits have beer, 
attended to bj Mr. F. Smith, of the British Museum, to 
whom I am much indebted for information on this and 
other subjects. Although fully trustmg 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 ex- 
cused 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 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 commu- 
nities, 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 ap- 
pearance is very great. When the nest is slightly . dis- 
turbed, the slaves occasionally come out, and like their 
masters are much agitated and defend the nest : when the 
nest is much disturbed and the larvae and pup^ are ex- 
posed, 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 num- 
ber, 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 com- 
munity with an unusually large stock of slaves, and I ob- 
served a few slaves mingled with their masters leaving the 
nest, and inarching 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. Accord- 
ing to Huber, who had ample opportunities for observa- 
tion, 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 mas- 
ters 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 witnessed a migration of F. san- 
guinea from one nest to another, and it was a most inter- 
esting spectacle to behold the masters carefully carrying 
their slaves in their jaws, instead of being carried by 
them, as in the case of the F. rufescens. 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. san- 
guinea. The latter ruthlessly killed their small oppo- 
nents, and carried their dead bodies as food to their nest, 
twenty-nine yards distant ; but they were prevented from 
getting any pupse to rear as slaves. I then dug up a 
small parcel of the pupse 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 1 
have seen it ferociously attack other ants. In one in- 
stance I found to my surj)rise an indej3endent community 
of F. flava under a stone beneath a nest of the slave- 
making F. sanguinea ; and when I had accidentally dis- 
tui'bed both nests, the little ants attacked theii* big neigh- 

•£Qg INSTINCT. [Chap. VIL 

bours witli surprising courage. 'Now I was curious to 
ascertain wliether 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 pup 83 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 oif the pupae. 

One evening I visited another community of F. san- 
guinea, 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 pupae ;• 
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 mo- 
tionless with its own pupa in its mouth on the top of a 
spray of heath, an image of despair, over its ravaged home. 
Such are the facts, though they did not need confir- 
mation 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 continental 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 numer- 
ous slaves. Formica sanguinea, on the other hand, 
possesses much fewer slaves, and in the early part of 
tlie summer extremely few : the masters determine 
when and where a new nest shall be formed, and when 
ihey 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 larvse. 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 pupse originally stored as food might become devel- 
oped ; and the ants thus imintentionally 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 pupse 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 
s]3ecies 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 
2:ive 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 mathe- 
maticians 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 precious wax in their con- 
struction. It has been remarked that a skilful workman, 

200 IN-STINCT. [Chap. VII. 

witli fitting tools and measures, would find it very difii- 
cult to make cells of wax of the true form, though this is 
perfectly efi'ected 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 in- 

I was led to investigate this subject by Mr. Water- 
house, 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 his theory. Let us look to the great prin- 
ciple of gradation, and see whether I^ature does not re- 
veal 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 known, 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 perfec- 
tion of the cells of the hive-bee and the simplicity of those 
of the humble-bee, we have the cells of the Mexican Me- 
lipona 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 
Iiatched, 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 ii'- 
regular mass. But the important point to notice, is that 


these cells are always made at tliat 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 in- 
tersect. 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 con- 
struction 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 : — 

K 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 4/ 2, or radius x 
1-4:1421 (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 intersec- 
tion 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 

202 INSTINCT. [Chap. VH. 

rliombs ; 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 sui-prising, seeing that she already does 
so to a certain extent, and seeing what perfectly cylindri- 
cal 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 al- 
ready 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 dis- 
tance, that she always describes her spheres so as to in- 
tersect largely ; and then she unites the points of intersec- 
tion by perfectly flat surfaces. We have further to sup- 
pose, 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 modifica- 
tions of instincts in themselves not very wonderful, — ^hard- 
ly more wonderful than those wdiich 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. Follow- 
ing 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 cir- 
cular pits in it ; and as they deepened these little pits, 
they made them wider and wider until they were convert- 
ed 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 wherevei 
several bees had begun to excavate these basins near to- 
gether, 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 tlie 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 be- 
tween 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, col- 
oured with Vermillion. The bees instantly began on both 
sides to excavate little basius near to each other, in the 
same w^ay as before ; but the ridge of w^ax 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 vermillion wax having been left ungnaw^ed, 
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 sides of the ridge of vermil- 
lion wax, as they circularly gnawed aw^ay and deepened 
the basins on both sides, in order to have succeeded in thus 
leaving flat plates between the basins, by stojDping work 
along the intermediate planes or planes of intersection. 

2Q4 IN8TINCT. [Chap. VII. 

Considering liow 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 strij) 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-com- 
menced 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 \)^(ioviiQ perfectly flat : it was absolutely impossi- 
ble, from the extreme thinness of the little rhombic plate, 
that they could have effected 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 vei-milion wax, 
we can clearly see that if the bees were to build for them- 
selves 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 en- 
deavouring to make equal spherical hollows, but never 
allowing the spheres to break into each other. xS"ow 
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 pyra- 
midal 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 com- 
plete the upper edges of the rhombic plates, until the 
hexagonal walls are commenced. Some of these statements 
differ from those made b}^ the justly celebrated elder Huber, 


but I am convinced of their acciiracv ; and if I had space. 
I conld show that thej 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 T 
have seen, strictly correct ; the first commencement hav- 
ing always been a little hood of wax ; but I will not here 
enter on these details. We see how important a part ex- 
cavation 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, fiexures may some- 
times 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 oif, 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 com- 
pleted, being thus crowned by a strong coping of wax, 
the bees can cluster and crawl over the comb without in- 
iuring the delicate hexagonal walls, which are only about 
one four-hundreth 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 difiiculty of understand- 
ing how the cells are made, that a multitude of bees all 
work together ; one bee after working a short time at one 

205 INSTINCT. [Chap. VII. 

cell going to another, so that, as Hiiber has stated, a score 
of individuals work even at the commencement of the first 
cell. I was able practically to show this fact, hy 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 difi'used 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. Tlie 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 ungnawecl, the planes of intersection between 
these spheres. It was really curious to note in cases of 
difficulty, as when two j)ieces 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 tlie ex- 


tremc margin of wasp-combs are sometimes strictly hex- 
agonal ; but I have not space here to enter on this sub- 
ject. I^or 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 cyl- 
inders, 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 
otiier, strike the planes of intersection, and so make an 
isolated liexagon : 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 profit- 
able 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 con- 
struction 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 
bee^ being supported. Hence the saving of wax by 
largely saving honey must be a most important element 

208 INSTINCT. [Chap. VII. 

of success in any family of bees. Of course the success of any 
species of bee may be dependent on the number of its para- 
sites 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 cir- 
cumstance determined, as it probably often does determine, 
the numbers of a humble-bee which could exist in a country. 
But let us suppose (differently to what really is the case) that 
the community lived throus^hout the winter, and conse- 
quently 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 j)^i't 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 Melij)ona, if she were to 
make her cells closer together, and more regular in every 
w^ay than at present; for then, as we have seen, the spher- 
ical 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 Imown 
instincts, that of the hive-bee, can be explained by natural 
selection having taken ad v^antage 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 secre- 
tion of wax, having succeeded best, and having transmitted 
by inheritance its newly acquired eonomical 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 pos- 
sibly have originated ; cases, in which no intermediate 
gradations are known to exist ; cases of instinct of appar- 
ently 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 ster- 
ile females in insect communities : for these neuters often 
difler 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 ren- 
dered 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 profit- 
able to the community, that a number should have been 
annually born capable of work, but incapable of procrea- 
tion, I can see no very great difficulty in this being efi'ect- 
ed by natural selection. But I must pass over this pre- 

210 INSTINCT. [Chap. VIL 

liminary 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 prodi- 
gious difference in this respect between the workers and 
the perfect females, would have been far better exempli- 
fied 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 bom with some slight profitable 
modification of structure, this being inherited by its off- 
spring, 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 ac- 
quired 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 repro- 
ductive system is active, as in the nuptial plumage of 
many birds, and. in the hooked jaws of the male salmon. 
"VVe have even slight difterences in the horns of difterent 
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 les- 
ed, or, as I believe, disappears, wlien 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-flavored 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 modifica- 
tion 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 
trasmitted 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 diflference between the fertile aj^d 
sterile females of the same species has been proginced, 
which we see in many social insects. 

But we have not as yet touched on the climax of the difiS.- 
culty ; 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 ex- 
traordinarily 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 

212 INSTINCT. tCHAP. vn. 

they have an enormously developed abdomen which se- 
cretes a sort of honey, supplying the place of that excreted 
by the aphides, or the domestic cattle as they may be 
called, which onr Em-oj^ean 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 
inodification 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 
Ei^rope have been carefully examined. Mr. F. Smith has 
shown how surprisingly the neuters of several British 
ants difi'er 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 aftirm 
that the eyes are far more rudimentary in the smaller 
workers than can be accounted for merely by their pro- 

Ohap. Vli.3 K-EUTER lls*SECTS. 213 

portionally 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 com- 
munity, 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 con- 
dition ; 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 illus- 
tration : 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, more- 
over, of the working ants of the several sizes differ 
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 se= 


lection, by acting on the fertile pare^its, could form a spe- 
cies 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 inter- 
mediate 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 instniments, a per- 
fect 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 selec- 
tion. But I am bound to confess, that, with all my faith 
m 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 accu- 
mulation of numerous, slight, and as we must call them 

Chap. Vll.] BITMMARY. 215 

accidental, yariations, whicli are in any manner profitable, 
without exercise or habit having come into play. For no 
amount of exercise, or habit, or yolition, in the utterly 
sterile members of a community could possibly haye 
affected the structure or instincts of the fertile members, 
which alone leaye 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 chajDter 
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. ]^o one will dispute that 
instincts are of the highest importance to each animal. 
Therefore I can see no difiiculty, under changing condi- 
tions 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 difiiculty, 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 inhab- 
iting distant parts of the world and living under consider- 
ably 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 pecu- 

216 INSTINCT. [Chap. Vll, 

liar manner as does our British thrush : how it is that the 
Hornbills of Africa and India, though belonging to allied 
but distinct genera, have the same extraordinary instinct 
of plastering up and imprisoning their hens whilst sitting 
on their eggs in a hole in a tree, with only a small hole 
left in the plaster, through which the males feed the 
hens and the young when hatched : how it is that the 
male wrens (Troglodytes) of Korth 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 imagiuation it is far more satisfactory to look at such 
instiucts as the young cuckoo ejecting its foster brothers, 
— ants making slaves, — the larvae of ichneumonidae feed- 
ing within the live bodies of caterpillars, — ^not as es23ecial- 
ly endowed or created instincts, but as small consequences 
of one general law, leading to the advancement of all or- 
ganic beings, namely, multiply, vary, let the strongest 
live and the weakest die. 




Distinction between the sterility of iirst crosses and of hybrids— Sterility various in 
degree, not universal, afiected by close interbreeding, removed by domestication — 
Laws governing the sterility of hybrids— Sterility not a special endowment, but 
incidental on other diiJerences— Causes of the sterility of first crosses and of 
hybrids— Parallelism between the effects of changed conditions of life and cross- 
ing-Fertility of varieties when crossed and of their mongrel offspring not uni- 
versal—Hybrids and mongrels compared independently of their fertility— Sum- 

The view generally entertained by naturalists is that spe- 
cies, wlien 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 hard- 
ly 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 con- 
tinued preservation of successive profitable degrees of 
sterility. I hope, however, to be able to show that ste- 
rility 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 con- 
founded together; namely, the sterility of two species 
when first crossed, and the sterility of the hybrids pro- 
duced from them. 

Pure species have of course their organs of reproduc- 
tion in a perfect condition, yet when intercrossed they 

2;|^g HYBRIDISM. IChap. VIII. 

produce either few or no offspring. Hybrids, on tliG 
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 micro- 
scope 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 imper- 
fectly 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 sev- 
eral memoirs and works of those two conscientious and 
admirable observers, Kolreuter and Gartner, who almost 
devoted their lives to this subject, without l)eing deeply 
impressed with the high generality of some degree of ste- 
rility. Kolreuter 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 tliem as va- 
rieties. Gartner also, makes the rule equally universal ; 
and he disputes the entire fertility of Kolreuter's ten cases. 
But in these and in many other cases, Gartner is obliged 
carefully to count the seeds, in order to show that there is 
any degree of sterility. He always compares the maxi- 
mum number of seeds produced by two species when 
crossed and by their hybrid offspring, Avitli the average 
number produced by both pure-parent sj^ecies 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 cas- 


tratecl, and, what is often more important, must be se- 
cluded in order to prevent pollen being brought to it by 
insects from other j^lants. I^early all the plants experi- 
mentised on bj Gartner 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 doubt- 
ed ; for Gartner 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 
Leguminosse, in which there is an acknowledged difiiculty 
in the manipulation) half of these twenty plants had their 
fertility in some degree impaired. Moreover, as Gartner 
during several years repeatedly crossed the primrose and 
cowslip, which we have such good reasons to believe to 
be varieties, and only once or twice succeeded in getting 
fertile seed ; as he found the common red and blue j)im- 
pernels (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 Gartner believes. 

It is certain, on the one hand, that the sterility of va- 
rious 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 
difiicult 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, Kolreuter and Gartner, should have 
arrived at diametrically opposite conclusions in regard to 
the very same species. It is also most instructive to com- 
pare — 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 sj)e- 
cies or varieties, with the evidence from fertility adduced 
by different hybridisers, or by the same author, from ex- 
periments made during different years. It can thus be 
shown that neither sterility nor fertility affords any clear 


distinction between species and varieties ; but tliat the 
evidence from this source graduates away, and is doubt- 
ful in tlie same degree as is the evidence derived from 
other constitutional and structural differences. 

In regard to the sterility of hybrids in successive gen- 
erations ; though Grartner was enabled to rear some hy- 
brids, carefully guiding them from a cross with either 
pure parent, for six or seven, and in one case for ten gen- 
erations, yet he asserts positively that their fertility never 
increased, but generally greatly decreased. I do not 
doubt that this is usually tlie 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 fertili- 
ty, that I cannot doubt the correctness of this almost uni- 
versal 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 Gartner, 
namely, that if even the less fertile hybrids be artificially 
fertilised with hybrid pollen of the same kind, their fer- 
tility, notwithstanding the frequent ill eflects of manipu- 
lation, sometimes decidedly increases, and goes on increas- 
ing. 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 efi'ected. Moreover, whenever complicat- 
ed experiments are in progress, so careful an observer as 


Gartner would have castrated his hybrids, and this would 
have insured in each generation a cross with a 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 be- 
lieve, be accounted for by close interbreeding having been 

]^ow let us turn to the results arrived at by the third 
most experienced hybridiser, namely, the Hon. and Kev. 
"W. Herbert. He is as emphatic in his conclusion that 
some hybrids are perfectly fertile — as fertile as the pure 
parent species — as are Kobeuter and Gartner 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 Gartner. The difi'erence in their re- 
sults 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, notwithstand- 
ing 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 ac- 
tually be hybridised much more readily than they can be 
self-fertilised ! For instance, a bulb of Hippeastrum auli- 
cum produced four flowers ; three were fertilised by Her- 
bert with their own pollen, and the fourth was subse- 


quently fertilised by tlie 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 Yerbascum. Although the plants in 
these experiments appeared perfectly healthy, and al- 
though 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, Rhodo- 
dendron, &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 sj^ecies from the 
mountains of Chile." I have taken some pains to ascer- 
tain 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 be- 
tween lihod. 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 

Chap. VIII.] STERILITY. 223 

fertility in each successive generation, as Gartner believes 
to be the case, the fact would have been notorious to nur- 
serymen. 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 in- 
jurious 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 ani- 
mals are as distinct from each other, as are the genera of 
plants, then we may infer that animals more widely sep- 
arated 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 difi'erent parents, so as 
to avoid the ill efi'ects of close interbreeding. On the 
contrary, brothers and sisters have usually been crossed 
in each successive generation, in opposition to the con- 
stantly 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 

224 HYBRIDISM. [Chap. VHl. 

tlie case of any pure animal, wbicli 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 j)erfectly fertile hybrid animals, I 
have some reason to believe that the hybrids from Cer- 
vulus vaginalis and Reevesii, and from Phasianus colchicus 
with P. torquatus and with P. versicolor are perfectly fer- 
tile. There is no doubt that these three pheasants, namely, 
the common, the true ring-necked, and the Japan, inter- 
cross, and are becoming blended together in the woods of 
several parts of England. 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 sin- 
gle 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 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 intercross- 
ing. On this view, the original 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 it 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 indig- 
enous domestic dogs of South America, all are quite 
fertile together ; and analogy makes me greatly doubt, 
whether tlie several aboriginal species would at first have 


freely bred together and have produced quite fertile 
hybrids. So a^ain there is reason to believe that our 
European and tne 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 ani- 
mals, w^e must either give up the belief of the almost 
universal sterility of distinct species of animals when 
crossed ; or we must look at steriKty, not as an indelible 
characteristic, but as one capable of being removed by 

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 can- 
not, 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 pre- 
vent their crossing and blending together in utter con- 
fusion. The following rules and conclusions are chiefly 
drawn up from Gartner's admirable work on the hybridi- 
sation 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 fer- 
tility, 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 abso- 
lute zero of fertility, the pollen of difi'erent species of 

226 HYBRIDISM. [Chap. VIII. 

the same genus applied to the stigma of some one sj^ecies, 
yields a perfect gradation in the number of seeds pro* 
duced, "up to nearly complete or even quite complete fer- 
tility ; and, as we have seen, in certain abnormal cases, even 
to an excess of fertility, beyond that which the plant's o^oi 
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 steril- 
ity 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 gener- 
ally very sterile ; but the parallelism between the dif- 
ficulty 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 sj)ecies 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 aftected 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 consti- 
tution 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 difter greatly in the 
several individuals raised from seed out of the same 
capsule and exposed to exactly the same conditions. 


Bj the term systematic affinity is meant, the resem- 
blance 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 be- 
tween 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 be- 
tween 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 
efibrts 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 in- 
stace, the many species of Nicotiana have been more 
largely crossed than the species of almost any other genus ; 
but Gartner found that N. acuminata, which is not a par- 
ticularly distinct species, obstinately failed to fertilise, or 
to be fertilised by, no less than eight other species of ISTi- 
cotiana. Yery many analogous facts could be given. 

IsTo 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 differ- 
ences in every part of the flower, even in the pollen, in 
the fruit, and in the cotyledons, can be crossed. Annual 
and pereimial plants, deciduous and evergreen trees, plants 
inhabiting different stations and fitted for extremely dif- 
ferent 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 

228 HYBRIDISM, [Chap. VIII. 

with a female-ass, and tlien a male-ass witli 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 con- 
nected 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 Kolreuter. 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 Kolreuter 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. Gartner, 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 compound- 
ed of the very same two species, the one species having 
first been used as the father and then as the mother, gen- 
erally difter in fertility in a small, and occasionally in a high 

Several other singular i-ules could be given from Gart- 
ner : 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 like- 
ness on their hybrid oft'spring ; but these two powers do 
not at all necessarily go together. There are certain hy- 
brids which instead of having, as is usual, an intermediate 
character between their two parents, always closely re- 
semble one of them ; and such hybrids, though externally 


SO like one of tlieir pure parent species, are with rare ex- 
ceptions extremely sterile. So again amongst hybrids 
which are usually intermediate in structure between their 
parents, exceptional and abnormal individuals sometimes 
are born, wliicJi 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 
sliow how completely fertility in the hybrid is indepen- 
dent of its external resemblance to either pure parent. 

Considering the several rules now given, which gov- 
ern 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 con- 
ditions, is innately variable. Tliat 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 afiinity or 
degree of resemblance to each other. This latter state- 
ment 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 difi'erence, and occasionally the widest ])ossible dif- 
ference, in the facility of effecting an union. The hybrids 
moreover, produced from reciprocal crosses often differ in 

JS'ow 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 differ- 
ent in degree, when various species are crossed, all of 
which we must suppose it would be equally important to 
keep from blending together ? Why shoiild the degree 
of sterility be innately variable in the individuals of'tho 

230 HYBRIDISM, [Chap. VIIl. 

same species ? Why should some species cross with facil- 
ity, 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 diflerence in the result of a reciprocal cross be- 
tween the same two species ? Why, it may even be ask- 
ed, has the production of hybrids been j)ermitted ? 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 ar- 

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 de- 
pendent on unknown differences, chiefly in the reproduc- 
tive systems of the species which are crossed. The differ- 
ences 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 direc- 
tion. It will be advisable to explain a little more fully 
by an example what I mean by sterility being incidental 
on other difl'erences, and not a s]3ecially 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 diver- 
sity 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 togctlier 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 Gartner 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 some- 
times 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 paral- 
lel. Something analogous occurs in grafting ; for Thouin 
found that three species of Eobinia, 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 
Sorbns, when grafted on other species, yielded twice as 
much frnit as when on their own roots. We are remind- 
ed by this lattpr tact of the extraordinary case of Hippe- 
astrum, Lobelia, etc., 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 funda- 

232 HYBRIDISM. [Ghap. Vni. 

mental difference between the mere adliesion of grafted 
stocks, and tlie 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 dis- 
tinct 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 or unknown differ- 
ences 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 ex- 
pected, systematic afiinity, by which every kind of resem- 
blance and dissimilarity between organic beings is at- 
tempted to be expressed. The fact by no means seems to 
me to indicate that the greater or lesser difliculty of either 
grafting or crossing together various species has been a 
special endowment ; although in the case of crossing, the 
difliculty is as important for the endurance and stability 
of specific forms, as in the case of grafting it is unimjDor- 
tant for their welfare. 

Causes of the Sterility of first Crosses andof Hyhrids. — • 
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 sex- 
ual elements are perfect, whereas in ]iybrids they are im- 
perfect. Even in first crosses, the greater or lesser difli- 
culty in effecting an union apparently depends on several 
distinct causes. Tliere must sometimes be a physical 
impossibility in the male element reaching the ovule, as 
would be the case with a plant having a justil too long for 
the pollen tubes to reach tlie 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 clement may reach the female clement, 
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 he 
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 commmiicated 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 un- 
willing 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 
difi*erently 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 con- 
sequently be liable to perish at an early period ; more 
especially as all very young beings seem eminently sensi- 
tive 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 diflferent. I have more than once alluded to a large 
body of facts, which I have collected, showing that where 
animals and plants are removed from their natural con- 
ditions, they are extremely liable to have their reproduc- 
tive systems seriously afi'ected. This, in fact, is the great 
bar to the domestication of animals. Between the ster- 
ility 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 occm*s in various degrees ; in both, the male 
element is the most liable to be efi'ected ; but sometimes 
the female more than the male. Li both, the tendency 
goes to a certain extent with systematic affinity, for whole 
groups of animals and plants are rendered impotent by the 

234: HYBRIDISM. [Chap. VIII. 

same iiimatural conditions ; and whole groups of sj)ecies 
teiad 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. Ko 
one can tell, till he tries, whether any particular animal 
will breed under confinement or any plant seed freel}' 
under culture ; nor can he tell, till he tries, whether an;y 
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 be- 
lieve, to their reproductive systems having been specially 
eflected, 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 experi- 
mentalist has observed. 

Thus we see that when organic beings are placed under 
new and unnatural conditions, and when hybrids are pro- 
duced by the unnatural crossing of two species, the re- 
productive 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 sliglit a degree as to be innappreciable 
by us ; in the other case, or that of liybrids, the external 
conditions have remained the same, but the organisation, 
has been disturbed by two different structures and consti- 
tutions having been blended into one. For it is scarcely 
possible that two organisations should be compounded 
into one, without some disturbance occurring in the devel- 
opment, or periodical action, or mutual relation of the 
different parts and organs one to another, or to the condi- 
tions of life. When liybrids are able to breed intei' se, they 
transmit to their offspring from generation to generation 
the same com])ounded 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 under- 
stand, excepting on vague hypotheses, several facts with 
respect to the sterility of hybrids ; for instance, the im- 


equal fertility of hybrids produced from reciprocal cross- 
es ; or the increased sterility in those hybrids which oc- 
casionally 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 or- 
ganisation 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 vigor and fertility to the oft"- 
spring. 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 
inter-breeding continued during several generations be- 
tween the nearest relations, especially if these be kept 
under the same conditions of life, always induces weak- 
ness 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 f>articular nature, often render 

236 HYBRIDISM. [Chap. VIII. 

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 hy- 
brids which are generally sterile in some degree. I can- 
not 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 Hon- 
grel offsirrhig. — It may be urged, as a most forcible argu- 
ment, that there must be some essential distinction be- 
tween spepies 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 inva- 
riably the case, and that the case offers a great difficulty ; 
probably something here remains unexplained. But if 
we look to varieties produced under nature, we are imme- 
diately 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 bota- 
nists as varieties, are said by Gartner not to be quite fer- 
tile when crossed, and he consequently ranks them as un- 
doubted 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 domes- 
tic dogs do not readily cross with European dogs, the ex- 
phmation which will occur to every one, and probably the 
true one, is that these dogs have descended from several 
aboriginally distinct species. I^evertheless 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, how- 
ever, render the fertility of domestic varieties less remark- 
able than at first appears. In the first place, we must 
remember how ignorant we are regarding the precise 
cause of sterility, when species are crossed and when spe- 
cies arc removed from their natural conditions. On this 
latter head I have not had space to adduce the many re- 
markable facts which could have been given ; with re- 
spect to sterility from crossing, reflect on the difierence in 
the result of reciprocal crosses,— reflect on the singular 
cases in which a plant can be more easily fertilised by 
foreign pollen than by its own. When we think over 
such cases and on that of the difi'erent coloured varieties 
of Yerbascum presently to be given, Ave must feel how 
ignorant we are, and how little likely it is that we should 
luiderstand why certain forms are fertile and other forms 
sterile when intercrossed. It can, in the second place, be 
clearly shown that mere external dissimilarity between 
two species does not determine their greater or lesser de- 
gree of sterility when crossed; and we may apply the 
same rule to domestic varieties. In the third place, some 
eminent naturalists believe that a long course of domesti- 
cation tends to eliminate sterility in the successive genera- 
tions 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 
the most important consideration, new races of animals 
and plants are produced under domestication by man's 
methc^dical and imconscious power of selection, for his 
own use and pleasure : he neither wishes to select, nor 
could select, slight diflerences in the reproductive system, 
or other constitutional difi'erences correlated with the re- 
productive system. Domestic productions have become 
less closely adapted to climate and to the other physical 
conditions of life, than are those in a state of nature. Man 
supplies his several varieties with the same food ; treats 
them nearly in the same manner, and does not wish to 


alter their general habits of life. iJ^ature acts uniformly 
and slowly during vast periods of time on the whole or- 
ganisation, in any way w'hich may be for each creature's 
owm good ; and thus she may, either directly, or more 
probably indirectly, through correlation, modily the re- 
productive system in the several descendants from any 
one species. Seeing this difference in the process of se- 
lection, 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 fol- 
lowing cases, which I will briefly abstract. The evidence 
is at least as good as that from w^hich 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. Gartner kept during several years a dwarf 
kind of maize with yellow seeds, and a tall variety wdth 
red seeds,, growing near each other in his garden ; and al- 
though these plants have separated sexes, they never nat- 
urally crossed. He then fertilised 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. Xo 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 Gartner did not venture to consider the two 
varieties as specifically distinct. 

Girou de Buzareingues crossed three varieties of gourd, 
which like the uiaize has separated sexes, and he asserts 
that their mutual fertilisation is by so much the less easy 
as their difterences are greater. How far these experi- 
ments may be trusted, I know^ not; but the forms exj^er- 
imentised 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 astonish- 


ing number of experiments made during many years on 
nine species of Ycrbascum, by so good an observer and 
so hostile a witness, as Gartner : namely, that yellow and 
white varieties of the same species of Yerbascum when 
intercrossed produce less seed, than do either coloured 
varieties when fertilised with pollen from their own col- 
oured 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 pro- 
duced by the crosses between the same coloured flowers, 
than between those which are differently coloured. Yet 
these varieties of Yerbascum present no other difi'erence 
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. 

Kolreuter, 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 l!^icotiana glu- 
tinosa, always yielded hybrids not so sterile as those 
which were produced from the four other varieties when 
crossed with N. glutinosa. Hence the reproductive sys- 
tem of this one variety must have been in some manner 
and in some degree modified. 

From these facts ; from the great difficulty of ascer- 
taining the infertility of varieties in a state of nature, for 
a supposed variety if infertile in any degree would gener- 
ally be ranked as species; from man selecting only ex- 
ternal 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 repro- 
ductive system ; from these several considerations and 
facts, I do not think that the very general fertility of 

240 HYBRIDISM. [CSAP. Vlli, 

Yarieties 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, steril- 
ity 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 re23roductive systems 
of the forms which are crossed. 

Hyhvids and Mongrels compared^ independently oftJieir 
fertility. — Independently of the question of fertility, the ofi'- 
spring of species when crossed and of varieties when crossed 
may be compared in several other respects. Gartner, whose 
strong wish was to draw a marked line of distinc- 
tion between species and varieties, could find very few 
and, as it seems to me, quite unimportant difi'erences be- 
tween the so-called hybrid oflspring of species, and the 
so-called mongrel offs]3ring of varieties. And, on the 
other hand, they agree most closely in very many im- 
portant respects. 

I shall here discuss this subject with extreme brevity. 
The most important distinction is, that in the first gener- 
ation mongrels are more variable than hybrids ; but Gart- 
ner 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. 
Gartner further admits that hybrids between very closely 
allied species are more variable than those from very dis- 
tinct species ; and this shows that the difierence in the de- 
gree of variability, graduates away. When mongrels and 
the more fertile hybrids are proj)agated for several gener- 
ations an extreme amount of variability in their ofifspring 
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 gener- 
ations 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 moii2:rels are varieties, and mostlv domestic varieties 
(very few experiments having been tried on natural van- 


eties), and this implies in most cases that there has been re- 
cent variability ; and therefore we might expect that such 
variability would often continue and be superadded to that 
arising from the mere act of crossing. The slight degree of 
variability in hybrids from the lirst cross or in the lirst gen- 
eration, in contrast with their extreme variability in the 
succeeding generations, is a curious fact and deserves atten- 
tion. 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 ren- 
dered either impotent or at least incapable of its proper 
function of producing offspring identical with the parent- 
form. ISTow 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 hy- 
brids : Gartner 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. Gartner fur- 
ther insists that when any two species, although most close- 
ly 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 sin- 
gle experiment ; and seems directly opj^osed to the results 
of several experiments made by Kolreuter. 

These alone are the unimportant differences, which 
Gartner 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 ac- 
cording to Gartner 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 j^lants. With animals one variety certainly 
often has this prepotent power over another variety. Hy- 

242 HYBRIDISM. [Chap. Vni. 

brid 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 charac- 
ters ; but more especially owing to prepotency in trans- 
mitting 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 main- 
tain 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 sup- 
posed 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 "^vith mongrels. Look- 
ing to the cases which I have collected of cross-bred ani- 
mals closely resembling one parent the resemblances seem 
chiefly conlined to characters almost monstrous in their 
nature, and which have suddenly appeared — such as 
albinism, melanism, deficiency of tail or horns, or addi- 
tional fingers and toes ; and do not relate to characters 
which have been slowly acquired by selection. Conse- 
quently, sudden reversions to the perfect character of 
either parent would be more likely to occur with mon- 
grels, 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 con- 
clusion, that the laws of resemblance of the child to its 

Chap. VIII.] SUMMARY. 243 

parents are tlie same, whether the two parents differ much 
or little from each other, namely in the nnion of individu- 
als of the same variety, or of different varieties, or of dis- 
tinct 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 sec- 
ondary laws, this similarity would be an astonishing fact. 
But it harmonises perfectly with the view that there is no 
essential distinction between species and varieties. 

Summary of Chapter. — First crosses between forms suf- 
ficiently 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 care- 
ful experimentalists who have ever lived, have come to dia- 
metrically 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 afiinity, but is governed by sev- 
eral 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 spe- 
cially endowed with various and somewhat analogous de- 
grees 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 

"24:4: HYBRIDISM. [Chap. VIll. 

depend on several circumstances ; in some cases largel v 
on tlie early death of the embryo. The sterility of hy- 
brids, which have their reproductive systems imperfect, 
and which have had this system and their whole organisa- 
tion distm'bed by being compounded of two distinct spe- 
cies, seems closely allied to that sterility which so fre- 
quently 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 aj)parently favourable to the 
vigour and fertility of all organic beings. It is not sur- 
prising 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. ISTor is it surpris- 
ing 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 attemj)ts to express all kinds of resemblance be- 
tween all species. 

First crosses between fonns known to be varieties, or 
sufficiently alike to be considered as varieties, and their 
mongrel offspring, are very generally, but not quite uni- 
versally, fertile. I^or 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 dif- 
ferences 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. 




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 palfeonto- 
logical collections— On the intermittence of geological formations— On the absence 
of intermediate varieties in any one formation — On the sudden appearance of 
groups of species— On their sudden appearance in the lowest known fossiliferous 

In the sixth chapter I eniimerated 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 mamier on the presence of other already defined 
organic forms, than on climate ; and, therefore, that the 
really governing conditions of life do not graduate aw^ay 
quite insensibly like heat or moisture. I endeavoured, 
also, to show that inteiTtiediate 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 w^hich 
new varieties continually take the places of and extermi- 
nate their parent-forms. But just in proportion as this 


process of extermination lias acted on an enormous scale, 
so must tlie number of intermediate varieties, wHcli have 
formerly existed on the earth, be truly enormous. "Why 
then is not every geological formation and every stra- 
tum 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 objec- 
tion which can be urged against my theory. The expla- 
nation lies, as I believe, in the extreme imperfection of 
the geological record. 

Li 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 difiicult, 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 in- 
termediate between each species and a common but un 
known progenitor ; and the progenitor will generally 
have diftered in some respects from all its modified de- 
scendants. -^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 ex- 
panded with a crop somewhat enlarged, the characteristic 
features of these two breeds. These two breeds, more- 
over, 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 dis- 
tinct, for instance to the horse and tapir, we have no 
reason to suppose that links ever existed directly interme- 
diate 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 sliould 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 de- 
scendants, unless at the same time we had a nearly per- 
fect 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 pa- 
rent, will render this a very rare event ; for in all cases 
the new and improved forms of life will tend to supj^lant 
the old and unimproved forms. 

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 in- 
termediate 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 find- 
ing 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 Prin- 
ciples 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 clitfs 
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 pebblesj 
sand, or mud But how often do we see along the bases 
of retreating cliffs, rounded boulders, all thickly clothed 
by marine productions, shewing how little they are abrad- 
ed and how seldom they are railed 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 appear- 
ance of the surface and the vegetation show that else- 
where years have elapsed since the waters w^ashed their 

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 suflered. And what an amount 
of degradation is implied by the sedimentary deposits of 
many countries ! Professor Kamsay has given me the 
maximum thickness, in most cases from actual measure- 
ment, in a few cases from estimate, of each formation in 
different parts of Great Britain ; and this is the result : — 

Palceozoic strata (not including igneous beds) .. .. ' .. 57,154 

Secondary strata 13,190 

Tertiary strata 2,240 

— making altogether ^2,584 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 esti- 
mated that sediment is deposited by the great Mississippi 
Eiver at the rate of only 600 feet in a hundred thousand 
years. This estimate may be quite erroneous ; yet, con- 
sidering 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 
Jiccumulation of the degraded matter, probablv offers the 


best evidence of tlie lapse of time. I remember having 
been much struck with the evidence of denudation, when 
viewing volcanic islands, which have been worn by the 
Vn' aves and pared all round into perpendicular cliffs of one 
or two tliousand 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. Kam- 
say has published an account of a downthrow in Angle- 
sea 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 prodi- 
gious movements ; the pile of rocks on the one or other 
side having been smoothly swept away. The considera- 
tion 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 intermediate hilly country and look on the one 
hand at the Xorth Downs, and on the other hand at the 
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 south- 


ern Downs is about 22 miles, and the thickness of the 
several formations is on an average about 1100 feet, as I 
am informed bj Prof. Ramsay, But if, as some geologists 
suppose, a range of older rocks underlies the AVeald, 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 ajDplied to the w^estern extremity of the district. If, 
then, we knew^ the rate at Avhich tlie 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 w^e 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 exce]3ting on the 
most exposed coasts ; though no doubt the degradation 
of a lofty cliff would be much more rapid from the breakage 
of the fallen fragments. On the other hand, I do not be- 
lieve 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 re- 
sisting attrition form a breakwater at the base. We may 
at least confidently believe that no rocky coast 500 feet in 
height commonly yields at the rate of a foot per century ; 
for this would be the same in amount as a cliff' one yard in 
height retreating twelve yards in twenty-two years ; and 
no one, I think, who has carefully observed the shape of 
old fallen fragments at the base of cliffs, will admit any 
near approach to such rapid wearing away. Hence, un- 
der ordinary circumstances, I should infer that for a cliff 
500 feet in height, a denudation of one inch per century 
for the whole length would be a sufficient allowance. At 
this rate, on the above data, the denudation of the Weald 
must have required 306,662,400 years ; or say three hun- 


dred million years. But perhaps it would be safer to al- 
low two or three inches per century, and this would reduce 
the number of years to one hundred and fifty or one hun- 
dred 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 sm^face 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- 

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 ! ISTow.turn 
to our richest geological museums, and what a paltry dis- 
play we behold ! 

On the poorness of our Palceontological collections. — 
That our palseontological collections are very imperfect, is 
admitted by every one. The remark of that admirable 

* I have left the foreo^oing passages as they stand in the second edition, 
but I confess that an able and justly severe article, since published in the 
Saturday Review (Dee. 24th, 1859), shows that I have been rash. I have 
not sufficiently allowed for the softness of the strata underlying the chalk ; 
the remarks made are more truly applicable to denuded areas composed of 
hard rocks. Xor have I allowed for the denudation going on on both sides 
of the ancient AVeald-Bay ; but the circumstance of the denudation having 
taken place within a protected bay would prolong the process. It has long 
been my habit to observe the shape and state of surface of the fragments at 
the bases of lofty retreating cliffs, and I can find no words too strong to ex- 
press my conviction of tlie extreme slowness with which they are worn away 
and removed. I beg the reader to observe that I have expressly stated 
that we cannot know ut what rate the sea wears away a line of cliff: I as- 
sumed the one inch per century in order to gain some crude idea of the lapse 
of years ; but I always supposed that the reader would double or quadruple 
or increase in any proportion which seemed to him fair the probable rate of 
denudation per century. But I own that I have been rash and unguarded 
in the calculation. 


palaeontologist, tlie 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 coflected on some one spot. Only 
a small portion of the surface of the earth has been geo- 
logically 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 charged with 
carbonic acid. Some of the many kinds of animals which 
live on the beach between high and low watermark seem 
to be rarely preserved. For instance, the several species 
of the Chthamalinse (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 Medi- 
terranean 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 moUuscan genus Chiton offers a par- 
tially analogous case. 

With respect to the terrestrial productions which lived 
during the Secondary and Palaeozoic periods, it is super- 
fluous 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. In regard to 
mammiferons remains, a single glance at the historical 
table j)nblished 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. JSTor is their 
rarity surprising, when we remember how large a propor- 
tion of the bones of tertiary mammals have been dis- 
covered 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 natiu-e, it is difficult to avoid 
believing that they are closely consecutive. But we 
know, for instance from Sir B. Murchison's great work on 
Russia, what wide gaps there are in that country between 
the superimposed formations ; so it is in ISTorth 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 geograj)hy 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 up- 
raised 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 re- 
flection 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 explana- 
tion, 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- weaves. 

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 oscilla- 
tions of level. Such thick and extensive accumulations 
of sediment may be formed in two ways ; either, in pro- 
found 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 ac- 
cumulated 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 sed- 
iment nearly balance each other, the sea will remain shal- 
low and favourable for life, and thus a fossiliferous forma- 
tion 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 subsi- 
dence. 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 de- 
posited 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 appar- 
ently these oscillations have affected wide spaces. Con- 
sequently formations rich in fossils and sufficiently thick 
and extensive to resist subsequent degradation, may have 
been formed over wide spaces during periods of subsi- 
dence, but only where the supply of sediment was suffi- 
cient 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 de- 
stroyed 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 adjoin- 
ing shoal parts of the sea will be increased, and new sta- 
tions will often be formed ; — all circumstances most fa- 
vourable, 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 pro- 
ductions on the shores of a continent when first broken 
up into an archipelago), and consequently during subsi- 
dence, 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 diflicult to understand, Avhy 
we do not therein find closely graduated varieties between 
the allied sjDecies 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 indis- 
putably requii'ed 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 pro- 
portional weight to the following considerations. 

Although each formation may mark a very long lapse 
of years, each perhaps is short compared with the j)eriod 
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 con- 
cluded 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. 
\Vhen 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. AVe forget how small 
the area of Europe is compared with the rest of the world ; 


nor haye the several stages of the same formation throngh- 
out Europe been correlated with perfect accm-acy. 

AVith marine animals of all kinds, we maj safely infer 
a large amount of migration during climatal and other 
changes ; and when we see a species hrst appearing in any 
formation, the probability is that it only then first immi- 
grated into that area. It is well known, for instance, that 
several species appeared somewhat earlier in the palseozoic 
beds of North America than in those of Eiiroj)e ; 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 spe- 
cies 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 excel- 
lent lesson to reflect on the ascertained amount of migra- 
tion of the inhabitants of Europe during the Glacial period, 
w^hich forms only a j)art of one whole geological period ; 
aud likewise to reflect on the great changes of level, on 
the inordinately great change of climate, on the pro- 
digious lapse of time, all included within this same gla- 
cial 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 durmg the whole of this period. It is not, for 
instance, probable that sediment was deposited durino- the 
whole of the glacial period near the mouth of the Missis- 
sippi, 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. AVhen 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 re- 
mains will probably at first appear and disappear at. dif- 
ferent levels, owing to the migration of species and to 
geographical changes. And in the distant future, a geol- 
ogist examining these beds might be tempted to conclude 
that the averao-e duration of life of the embedded fossils 


had been less tlian that of the glacial period, instead of 
having been really far greater, that is extending from be- 
fore the glacial ejjocli to the present day. 

Ill order to get a perfect gradation between two forms 
in the npper 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 modi- 
fication will have had to live on the same area throughout 
this whole time. But we have seen that a thick fossil- 
iferous 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 spe- 
cies to live on the same space, the suj)ply 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 con- 
tinues. In fact, this nearly exact balancing between the 
supply of sediment and the amount of subsidence is prob- 
ably 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 
wliole 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 sed- 
iment 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 in- 
stances 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 ; vet no one ia'norant of this 

260 IMPERFECTlOis- OF THE • [Chap. IX. 

fact would have suspected the vast lapse of time represent- 
ed bj 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 accu- 
mulation. 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 N"ova 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 forma- 
tion, the probability is, that they have not lived on the 
same spot during the whole period of deposition, but have 
disappeared and reap]Deared, 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-imjDortant to remember that naturalists have 
no golden rule by which to distinguish species and varie- 
ties ; they grant some little variability to each species, 
but when they meet with a somewhat greater amoimt of 
difference between any two forms, they rank both as 
species, unless they are enabled to connect them together. 
by close intei-mediate 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 


tniglit be tlie 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 
difterent sub-stages of the same formation. Some ex- 
perienced 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 evi- 
dence of change which on my theory we ought to find. 
Moreover, if we look to rather wide intervals, namely, the 
distinct but consecutive stages of the same great forma- 
tion, 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 former- 
ly 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 often- 


est 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 sel- 
dom 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 eftected by palaeontologists. 
"We shall, perhaps, best perceive the improbability of our 
being enabled to connect species by numerous, fine, inter- 
mediate, fossil links, by asking ourselves whether, for in- 
stance, geologists at some future period will be able to 
prove, that our diflerent breeds of cattle, sheep, horses, 
and dogs have descended from a single stock or from sev- 
eral aboriginal stocks ; or, again, whether certain sea- 
shells inhabiting the shores of I^orth 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 efi'ected only by the 
future geologist discovering in a fossil state numerous in- 
termediate gradations ; and such success seems to me im- 
probable 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 con- 
necting them together by numerous, fine, intermediate 
varieties ; and this not having been efi'ected, is j^robably 
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 Achipelago 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 may have been 
examined with any accuracy, excepting those of the United 


States of America. I fully agi-ee witli Mr. Godwin- Aus- 
ten, that the present condition of the Malay Archipelago, 
with its nnmerous 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 terres- 
trial productions of the archipelago would be ]3reserved 
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 em- 
bedded ; 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 forma- 
tions 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 w^ould be 
either stationary or rising ; whilst rising, each fossiliferous 
formation would be destroyed, almost as soon as accumu- 
lated, by the incessant coast-action, as we now see on the 
shores of South America. During the periods of subsi- 
dence 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 

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 accumula- 
tion of sediment, would exceed the average duration of the 
same specific forms ; and these contingencies are indis- 


pensable for tlie preservation of all the transitional grada- 
tions between any two or more species. If such grada- 
tions 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 cli- 
matal changes would intervene during such lengthy 
periods ; and in these cases the inhabitants of the archi- 
pelago would have to migrate, and no closely consecutive 
record of their modifications could be preserved in any 
one formation. 

Yery many of the marine inhabitants of the archipe- 
lago 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 im- 
proved, 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 umform, though perhaps extremely 
slight degree, they would, according to the principles fol- 
lowed by many palgeontologists, be ranked as new and 
distinct species. 

If then, there be some degree of truth in these re- 
marks, we have no right to expect to find in our geologi- 
cal 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 diflerent 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 com- 


mencement and close ot* each formatioiij 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 trans- 
mutation 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 de- 
scent 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 descend- 
ants. But we continually over-rate the perfection of the 
geological record, and falsely infer, because certain genera 
or families have not been found within 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 spe- 
cies will a23pear 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 in- 
stance to fly through the air; but that when this had been 
eflected, and a few species had thus acquired a great ad- 
vantage 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 re- 
marks, and to show how liable we are to error in suppos- 
ing that whole groups of species have suddenly been pro- 
duced. I may recall the well-known fact that in geological 
treatises, jjublished 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, 
for its thickness, 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 oc- 
curred 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. Had it not 
been for the rare accident of the preservation of footsteps 
in the new red sandstone of the United States, who would 
have ventured to suppose that, besides reptiles, no less 
than at least thirty kinds of birds, some of gigantic size, 
existed during that period ? Not a fragment of bone has 
been discovered in these beds. Notwithstanding that 
the number of joints shown in the fossil impressions cor- 
respond with the number in the several toes of living birds' 
feet, some authors doubt whether the animals which left 
the impressions were really birds. ^ Until quite recently 
these authors might have maintained, and some have 
maintained, that the whole class of birds came suddenly 
into existence during an early tertiary period ; but now 
we know, on the authority of Professor Owen (as may be 
seen in Lyell's ' Manual '), that a bird certainly lived dur- 
ing the deposition of the upper green-sand. 

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 spe- 
cies all over the world, from the Arctic regions to the 


equator, inliabiting 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 cirrij)edes existed dm-ing the sec- 
ondary periods, they would certainly have been preserved 
and discovered ; and as not one species has been dis- 
covered 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 ap- 
pearance of a great group of species. But my work had 
hardly been published, when a skilful palseontologist, M. 
Bosquet, sent me a drawing of a perfect specimen of an 
unmistakeable sessile cirripede, which he had himself ex- 
tracted 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 pro- 
genitors of our many tertiary and existing species. 

The case most frequently insisted on by palaeontolo- 
gists 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 ma- 
jority 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 afiinities are as yet imperfectly 
known, are really teleostean. Assuming, however, that 
the Avhole 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 diificulty 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 superfiuous to 


remark that hardly any fossil-fish are known from south 
of the equator ; and by running through Pictet's Palaeon- 
tology 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. JS^or have we any right to 
suppose that the seas of the world have always been so 
freely open from north to south as they are at present. 
Even at this day, if the Malay Archipelago were con- 
verted 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 multij)lied ; 
and here they would remain confined, until some of the 
species became adapted to a cooler climate, and were en- 
abled 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 be- 
yond the confines of Europe and the United States ; and 
from the revolution in our palseontological ideas on many 
points, which the discoveries of even the last dozen years 
have efi'ected, 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 groujps of Allied Species 
in the loioest known fossiliferoxis strata. — ^There is another 
and allied difficulty, w^hich is much graver. I allude t£> 
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 Silu- 


rian age, and wMcli probably differed greatly from any 
known animal. Some of tlie most ancient Silurian ani- 
mals, as the Kantilus, Lingula, &c., do not differ mucb 
from living species ; and it cannot on my theory be sup- 
posed, 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 be- 
tween them. If, moreover, they had been the progenitors 
of these orders, they would almost certainly have been 
long ago supplanted and exterminted by their numerous 
and improved descendants. 

Consequently, if my theory be true, it is indisputable 
that before the lowest Silui-ian stratum was dej)osited, 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 quiie unknown, 
periods of time, the world swarmed with living crea- 

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. Mur- 
chison at their head, are convinced that we see in the or- 
ganic 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, aboundinsj 
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 nod- 
ules 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 somewhat 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 rem- 
nants of the formations next succeeding them in age, and 


these ought to be very generally in a metoniorphosed con- 
dition. Bnt the descrij^tions which we now possess of the 
Silurian deposits over immense territories in Kussia and 
in North America, do not support the view, that the older 
a formation is, the more it has sufiered 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 ap- 
pear 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 w^hich 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 exist- 
ing continents of Europe and iNorth 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 sedi- 
ment 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 for- 
mation. 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 forma- 
tions would in all probability have been accumulated from 
sediment derived from their wear and tear ; and would have 
been at least partially uj)heaved by the oscillations of level, 
which we may fairly conclude must have intervened dur- 
ing these enormously long periods. If then we may infer 
anything from these facts, Ave may infer that where our 
oceans now extend, oceans have extended from the remot 
est period of wliich 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 Keefs, led me to 
conclude that the great oceans are still mainly areas of 
subsidence, the great archipelagoes still areas of oscilla- 
tions of level, and the continents areas of elevation. But 
have we any right to assume that things have thus re- 
mained 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 silu- 
rian epoch, continents may have existed where oceans are 
now spread out ; and clear and open oceans may have ex- 
isted where our continents now stand. JSTor 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 superincum- 
bent 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 metamor- 
phic 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 con- 

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 appear in our European formations ; the almost 
entire absence, as at present known, of fossiliferous for- 


mations beneath the Silurian strata, are all undoubtedly 
of the gravest nature. We see this in the plainest manner 
bj the fact that all the most eminent palaeontologists, 
namely Cuvier, Owen, Agassiz, Barrande, Falconer, E. 
Forbes, &c., and all our greatest geologists, as Ljell, Mur- 
chison, Sedgwick, &c., have unanimously, often vehemently, 
maintained the immutability of species. But I have rea- 
son 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 difler from these great 
authorities, to whom, with others, we owe all our knowl- 
edge. 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 given 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 imper- 
fectly kept, and written in a changing dialect ; of this 
history we j)ossess 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 diflerent in the inter- 
rujDted succession of chapters, may represent the appar- 
ently abruptly changed forms of life, entombed in our 
consecutive, but widely sej^arated, formations. On this 
view, the difficulties above discussed are greatly diminish- 
ed, or even disappear. 




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 gen- 
eral rules in their appearance and disappearance as do single species— On Extinc- 
tion—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 de- 
velopment of ancient forms— On the succession of the same types within the same 
areas— Summary of preceding and present chapters. 

Let us now see whether the several facts and rules re- 
lating to the geological succession of organic beings, better 
accord with the common view of the immutability of spe- 
cies, or with that of their slow and gradual modification, 
through descent and natural selection. 

'New species have appeared very slowly, one after an- 
other, 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. Li some of the most recent beds, though un- 
doubtedly 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 lo- 
3ally, 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 
liavG been many and most gradual. The secondary for- 
mations are more broken ; but, as Bronn has remarked, 
neither the appearance nor disappearance of their many 
now extinct species has been simultaneous in each sepa- 
rate 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 snll be found 
in the midst of a multitude of extinct forms. Falconer 
has given a striking instance of a similar fact, in an ex- 
isting crocodile associated with many strange and lost 
mammals and reptiles in the sub-Himalayan dejDosits. 
The Silurian Lingula differs but little from the living spe- 
cies 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 geologi- 
cal formations; so that between each two consecutive 
formations, the forms of life have seldom changed in ex- 
actly the same degree. Yet if we compare any but the 
most closely related formations, all the species will be 
found to have imdergone some change. When a species 
has once disapj)eared from the face of the earth, we have 
reason to believe that the same identical form never re- 
a23pears. 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 forma- 
tion, and then allow the pre-existing fauna to reappear ; 
but Ly ell's explanation, namely, that it is a case of tem- 
porary migration from a distinct geographical province, 
seems to me satisfactory. 

These several facts accord well with my theory. I be- 
lieve in no fixed law of development, causing all the in- 
habitants of a country to change abruptly, or simulta- 
neously, or to an equal degree. Tlie process of modifica- 
tion 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 nat- 
ural 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, de- 
pends on many complex contingencies, — on the variability 
Leing 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 come into competition. Hence it is by no means 
surprising that one species should retain the same identi- 
cal 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 coleop- 
terous insects of Madeira having come to difier consider- 
ably from their nearest allies on the continent of Europe, 
whereas the marine shells and birds have remained un- 
altered. 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 inhab- 
itants 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 in- 
tervals 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, per- 
haps, 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 sub- 
siding, our formations have been almost necessarily accu- 
mulated 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 slowlj changing 

We can clearly understand why a species when once 
lost should never reappear, even if the very same con- 
ditions 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 difi'erent characters 
from their distinct progenitors. Eor 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 ex- 
terminated by its improved ofi'spring, 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 j)igeon, for 
newly-formed fantail would be almost sure to inherit 
from its new progenitor some slight characteristic difi:er- 

Grouj)S of species, that is, genera and families, follow 
the same general rules in their appearance and disappear- 
ance as do single species, changing more or less quickly, 
and in a greater or lesser degree. A group does not re- 
appear 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 
Buccession of ages, so long must its members have contin- 

Chap. X.] EXTINCTIONS^. 277 

uonsly 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 continu- 
ously 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 be- 
ing a gradual increase in number, till the group reaches its 
maximum, and then, sooner or later, it gradually de- 
creases. 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, tliese being slowly con- 
verted 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. 

O71 Extinction. — "We have as yet spoken only inci- 
dentally of the disappearance of species and of groups 
of species. On the theory of natural selection the extinc- 
tion of old forms and the production of new and improved 
forms are intimately connected together. Tlie old notion 
of all the inhabitants of the earth having been swept 
away at successive periods by catastrophes, is very gene- 


rally given up, even by those geologists, as Elie de Beau- 
mont, Murchison, Barrande, (fee., whose general views 
wonld 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 sj)ot, 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, hav- 
ing endured from the earliest known dawn of life to the 
present day ; some having disappeared before the close 
of the palaeozoic period, ffo fixed law seems to deteiTaine 
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, whicli marks the first appearance and increase in 
numbers of the species. In some cases, however, the 
extermination of whole groups of beings, as of ammon- 
ites 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 Masto- 
don, 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 Span- 
iards 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 exter- 
minated the former horse under conditions of life appar- 

Chap. X.] EXTINCTION". 279 

ently so favourable. But liow utterly groundless was my 
astonishment ! Professor Owen soon ]3erceived that the 
tooth, though so like that of the existing horse, belonged 
to an extinct species. Had this horse be^n 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 
otlier 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 con- 
ditions 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 sever- 
ally acted. If the conditions had gone on, however slow- 
ly, 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 ex- 
tinct ; — its place being seized on by some more successful 

It is most difficult always to remember that the increase 
of every living being is constantly being checked by unper- 
ceived injurious agencies ; and that these same unperceived 
agencies are amply sufficient to cause rarity, and finally 
extinction. AVe 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 184:5, 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 deatli — to feel no surprise at sickness, but when ttie sick 
man dies, to wonder and to snspect that he died by some 
unknown deed of violence. 

The theoiy of natural selection is grounded on the 
belief that each new variety, and ultimately each new 
species, is j)roduced and maintained by having some ad- 
vantage 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 do- 
mestic productions : when a new and slightly improved 
variety has been raised, it at first suj)plants the less im- 
proved varieties in the same neighbourhood ; when much 
improved it is transported far and near, like our short- 
horned 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 pro- 
duction 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 ex- 
termination. 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 haj^pcDcd that a new species belong- 
ing to some one group will have seized on the place occu- 
pied by a species belonging to a distinct group, and thus 
caused its extermination ; and if many allied forms be 

Chap. X.] EXTINCTION. 281 

developed from the successful intruder, many will have to 
yield their places ; and it will generally be allied forms, 
which will sufier from some inherited inferiority in com- 
mon. 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 
sufl'erers 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 com- 
petition. 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 

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 be- 
tween our consecutive formations ; and in these intervals 
there may have been much slow extermination. More- 
over, 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 exterminat-ed in a 
correspondingly rapid manner many of the old inhabit- 
ants ; 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 per- 
ceived by us, the whole economy of nature will be utterly 


obscured. Whenever we can precisely say why this spe- 
cies is more abundant in individuals than that • why this 
species and not another can be naturalised m a given 
country ; then, and not till then, we may justly feel sur- 
prise why we cannot account for the extinction of this par- 
ticular species or group of species. 

On the Forms of Life changing almost simultaneously 
throughout the World. — Scarcely any palseontological dis- 
covery 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 l^orth 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 
poiuts, 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 character- 
ised 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 forma- 
tions 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 JSTorth American tertiary deposits. 
Even if the few fossil species which are common to the 
Old and jS^ew 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 in- 
habitants of distant parts of the world : we have not suffi- 


cient data to judge whether the productions of the land 
and of fresh water change at distant points in the same 
parallel manner. We may donbt whether thej have thns 
changed : if the Megatherium, Mylodon, Macrauchenia, 
and Toxodon had been brought to Europe from La Plata, 
without any information in regard to their geological po- 
sition, no one would have suspected that they had coex- 
isted 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 later 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 thou- 
sandth 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, includ- 
ing the whole glacial epoch), were to be comj^ared 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 Eu- 
rope resembled most closely those of the southern hemis- 
phere. 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 fossil- 
iferous beds deposited at the present day on the shores of 
ISTorth America would hereafter be liable to be classed 
with somewhat older Euro]3ean beds. ISTevertheless, look- 
ing to a remotely future epoch, there can, I think, be little 
doubt that all the more modern marine formations, name- 
ly, the upper pliocene, the pleistocene and strictly modern 
beds, of Europe, ISTorth 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 Yerneuil 
and d'Archiac. After referring to the parallelism of the 
palaeozoic forms of life in various parts of Euroj^e, they 
add, " If struck by this strange sequence, we turn our at- 
tention 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 intro- 
duction 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 in- 

This great fact of the parallel succession of the forms 
of life throughout the world, is explicable on the theory 
of natural selection. ISTew species are formed by new va- 
rieties 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 aremost widely diflused, 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 exten't 
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 cliniatal 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 lower with the terrestrial inhab- 
itants of distinct continents than with the marine inhab- 
itants 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 en- 
counter still more dominant s]3ecies, and then their trium- 
phant 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 appear- 
ance of favourable variations, and that severe competition 
with many already existing forms, w^ould be highly fa- 
vourable, 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 dom- 
inant species on the land, and another for these 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 pro- 
longed and severe ; and some from one birthplace and 
some from the other might be victorious. Bat in the 
course of time, the forms dominant in tlie 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 througlioiit the world, accords well witli the principle 
of new s]jecies having been formed by dominant species 
spreading widely and varying ; the nevv species thns pro- 
duced being themselves dominant owing to inheritance, 
and to having already had some advantage over their 
parents or over other species ; these again spreading, vary- 
ing, and producing new species. The forms which are 
beaten and which yield their places to the new and victo- 
rious forms, will generally be allied in groups, from in- 
heriting some inferiority in common ; and therefore as 
new and improved groups spread throughout the world, 
old groups will disappear from the world ; and the succes- 
sion of forms in both ways will everywhere tend to cor- 

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 j)eriods when the 
bed of the sea was either stationary or rising, and like- 
wise when sediment was not thrown do^vn 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 modi- 
fication and extinction, and that there was much migration 
from other parts of the world. As we have reason to be- 
lieve 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 de- 
posited 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 forma- 
tions. Barrande, also, shows that there is a striking 
general parallelism in the successive Silurian deposits of 
Bohemia and Scandinavia ; nevertheless he finds a sur- 
prising amount of difference in the species. If the several 
formations in these regions have not been deposited during 
the same exact periods, — a foiTQation 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 
aiTanged 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 othei\ 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 diflers 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 be- 
tween existing genera, families, and orders, cannot be dis- 


puted. 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 Yertebrata, whole pages conld be filled 
with striking illustrations from our great palaeontologist) 
Owen, showing how extinct animals fall in between exist- 
ing groups. Cuyier ranked the Ruminants and Pachy- 
derms, 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 grada- 
tions 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 palseozoic animals, though belong- 
ing to the same orders, families^ or genera with those 
living at the present day, were not at this epoch limited 
In such distinct groups as they now are. 

Some, writers have objected to any extract 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 char- 
acters between two living forms, the objection is probably 
valid. But I apprehend that in a perfectly natural classi- 
fication 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 mem- 
bers of the same two groups would be distiaguished 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 


whicli 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 veiy distinct groups. Yet if 
we compare the older Eeptiles and Batrachians, the older 
Fish, the older Cej)halopods, 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 upper- 
most line may be considered as extinct. The three exist- 
ing genera, ^^*, ^^*, ^^*, will form a small family ; J'*, and 
f^\ a closely allied family or sub-family ; and 6>^*, 6^*, m^^^ 
a third family. Tliese 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 sj)ecies might go on being 
slightly modified in relation to its slightly altered condi- 
tions of life, and yet retain throughout a vast period the 


same general characteristics. This is represented in the 
diagram by the letter f^\ 

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 diver- 
gence of character, has become divided into several sub- 
families and families, some of wliich 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 «\ d^ a^\ /" ^, m^, 011% 
r)%\ 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 Ibrms. If many extinct 
forms were to be discovered above one of the middle 
horizontal lines or geological formations — for instance, 
above !No. YI. — but none from beneath this line, then 
only the two families on the left hand (namely, a^\ etc., 
and &^\ (fee.) would have to be united into one family ; 
and the two other families (namely, a^^ ^of^* now includ- 
ing five genera, and 0'* to iii"^) 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 YL, would differ by a lesser number of characters ; 
for at this early stage of descent they have not diverged 
in character from tlie common progenitor of the order, 
nearly bo much as they subsequently diverged. Tluis it 


comes that ancient and extinct genera are often in some 
sliglit 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 'svill 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 diflfer 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 palseontologists 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 modi- 
fied offspring of those which lived at the fifth stage, and 
are the parents of those which became still more modi- 
fied 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 un- 


doubtedly is intermediate in cliaracter, between the pre- 
ceding and succeeding faunas. I need give only one 
instance, namely, the manner in wliicli the fossils of the 
Devonian system, when this system was first discovered, 
were at once recognised by paleontologists 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 ofter exceptions to the rule. For in- 
stance, mastodons and elephants, when arranged by Dr. 
Falconer in two series, first according to their mutual afii- 
nities and then according to their periods of existence, do 
not accord in arrangement. The species extreme in char- 
acter are not the" oldest, or the most recent ; nor are those 
which are intermediate in character, intermdiate 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 succssively produced necessarily endure for 
corresponding lengths of time ; a very ancient form might 
occasionally last much longer than a form elsewhere sub- 
sequently 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 ot their production, 
and still less with the order of their disappearance ; for the 
parent rock-pigeon now lives ; and many varieties be- 
tween the rock-pigeon and the carrier have become ex- 
tinct ; and carriers which are extreme in the important 
character of length of beak have originated earlier than 
short-beaked tumblers, which are at the opjDOsite end of 
the series in this same respect. 

Closely connected with the statement, that the organic 
remains from an intermediate formation are in some de- 


gree intermediate in character, is the fact, insisted on by 
all palaeontologists, that fossils from two consecutive for- 
mations 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 for- 
mation, thongh the species are distinct in each stage. 
Tliis 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 ac- 
count 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 diflfer- 
ent climates and conditions. Consider the prodigious vi- 
cissitudes of climate during the pleistocene period, which in- 
cludes the whole glacial period, and note how little the spe- 
cific 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 inter- 
vened between successive formations, we ought not to ex- 
pect 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 commence- 
ment 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, repre- 
sentative species ; and these we assuredly do find. We 
find, in short, such evidence of the slow and scarcely sen- 
sible mutation of specific forms, as we have a just right to 
expect to find. 

0)1 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 en- 
ter on this subject, for naturalists have not as yet defined 
to each other's satisfaction what is meant by high and low 
forms. The best definition probably is, that the higher 
forms have their organs more distinctly specialised for 
different functions ; and as such division of physiological 
labour seems to be an advantage to each being, natural 
selection will constantly tend in so far to make the later 
and more modified forms higher than their early progeni- 
tors, or than the slightly modified descendants of such pro- 
genitors. In a more general 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 advan- 
tage 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 afifect- 
ed in a marked and sensible manner the organisation of 
the more recent and victorious forms of life, in compari- 
son with the ancient and beaten forms ; but I can see no 
way of testing this sort of progress. Crustaceans, for in- 
stance, 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 
ISTew Zealand, and have seized on places which must have 
been previously occupied, we may believe, if all the ani- 
mals and plants of Great Britain were set free in I^ew 
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 Is'ew Zealand, 
and from hardly a single inhabitant of the southern hem- 
isphere having l)ccome wild in any part of Europe, we 
may doubt, if all the productions of jN^ew 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, tlie 
productions of Great Britain may be said to be higher 
than those of 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 cer- 
tain 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 embryo-logical develop- 
ment 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 con- 
dition of each animal. This view may be true, and yet 
it may never be capable of full proof. Seeing, for in- 
stance, 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 Yertebrata, until beds far beneath the lowest Silurian 
strata are discovered — a discovery of which the chance is 

verv small. 


On the Succession of the same Types within the same 
areas, cluring the later tertiary periods . — Mr. Clift many 
years ago showed that the fossil mammals from the Aus- 
tralian caves were closely allied to the living marsupials 


of that continent. In Sontli America, a similar relation- 
ship is manifest, even to an uneducated eye, in the gi- 
gantic 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 restoration of the extinct and gigantic l)ird3 
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 be- 
tween 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 dissimilari- 
ty 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. 
Nov 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 Somth America. 
For we know that Euro]3e in ancient times was peopled by 
numerous marsupials ; and I liave shown in the publica- 
tions above alluded to, that in America the law of distri- 


bution of terrestrial mammals was formerly different from 
what it now is. l^orth. America formerly partook strong- 
ly of the present character of the southern half of the con- 
tinent ; 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 Cantley's dis- 
coveries, 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 modiJElcation, 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 ob- 
viously tend to leave in that quarter, during the next suc- 
ceeding period of time, closely allied though in some de- 
gree modiiied descendants. If the inhabitants of one con- 
tinent formerly differed greatly from those of another con- 
tinent, 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 im- 
mutable 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 be- 
come 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 spe- 
cies 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 suc- 
ceeding formation there be six other allied or representa- 
tive genera with the same number of species, then we 


may conclude that only one species of eacli 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 t^vo 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 be- 
come 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. 

SuriiTnary of the preceding and present Chapters. — I 
have attempted to show that the geological record is ex- 
tremely imperfect ; that only a small portion of the globe 
has been geologically explored with care ; that only cer- 
tain 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 gen- 
erations which must have passed away even during a 
single formation ; that, owing to subsidence being neces- 
sary 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 pe- 
riods 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 aver- 
age 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 in- 

Chap. X..1 SUMMARY. 299 

terminable 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 geo- 
logical 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 im- 
portant a part migration must have played, when the for- 
mations 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 in- 
finitely numerous organisms w^hich raust 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 over since the Silurian epoch ; but that long before 
that period, the world may have presented a wholly dif- 
ferent 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 

Passing from these difiiculties, all the other great 
leading facts in palaeontology seem to me simply to fol- 
low on the theory of descent wdth modification through 
natural selection. We can thus understand how it is that 
new species come in slowly and successively ; how species 
of difterent 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 conse- 
quence of the production of new forms. We can under- 
stand 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 tlie 
larger dominant groups tend to leave many modified de- 
scendants, and thus new sub-groups and groups are 
formed. As these are formed, the species of the less 
vigorous groups, from tlieir 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 domi- 
nant 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. AVhy ancient and extinct 
forms often tend to fill up gaps between existing forms, 
sometimes blending two groups previously classed as dis- 
tinct into one ; but more commonly only bringing them a 
little closer together. The more ancient a form is, the 
more often, aj^parently, it displays characters in some de- 
gree 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 difierent forms. We can 
clearly see why the organic remains of closely consecutive 

CuAP. X.] SUMMARY. - 3()J_ 

formations are more closelj allied to eacli other, tlian are 
those of remote formations ; for the forms are more closely 
linlied 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 natnre ; 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 in- 

If then the geological record be as imperfect as I be- 
lieve it to be, and it may at least be asserted that the rec- 
ord 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 ITatural Selection. 




Present distribution cannot he accounted for by differences in physical conditions— Im- 
portance 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 
■\vith the world. 

In considering tlie distribution of organic beings over tbe 
face of the globe, tbe first great fact wbicli strikes lis is, 
tbat neither the similarity nor the dissimilarity of the in- 
habitants 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 divi- 
sions in geographical distribution is that between the 
Kew 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 l^ew — 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 Kew World, yet 


tliese are not inhabited by a peculiar fauna or flora. I^ot- 
withstandin^ this parallelism in the conditions of the Old 
and New Worlds, how widely diflerent 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 pro- 
ductions of South America south of lat. 35^ with those 
north of 25°, which consequently inhabit a considerably 
difi'erent 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 ter- 
restrial productions of the ITew and Old Worlds, except- 
ing 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 inhabit- 
ants of Australia, Africa, and South America under the 
same latitude : for these countries are almost as much iso- 
lated from each other as is possible. On each continent, 
al^o, 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 pro- 
ductions ; 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 conti- 

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 en- 
counter 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 In- 
dian 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 difi'erent points and stations. It is a law of the widest 
generality, and every continent ofi*ers innumerable in- 
stances. iSTevertheless the naturalist in travelling, for in 
stance, 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 near- 
ly 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 in- 


habited Ly one species of Rliea (American ostricli), 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 lati- 
tude. 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 Kodents, but they plainly display an American 
type of structure. We ascend the lofty peaks of the Cor- 
dillera and we find an alpine species of bizcacha ; we look 
to the waters, and we do not find the beaver and 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 inhab- 
itants of different regions may be attributed to modifica- 
tion through natural selection, and in a quite subordinate 
degree to the du-ect influence of different physical condi- 
tions. The degree of dissimilarity will depend on the mi- 
gration 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 num- 
ber 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 tlie slow process of modifica- 
tion throngli 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 frequent- 
ly 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 fami- 
lies 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 ad- 
vantage of by natural selection, only so far as it profits 
the individual in its complex struggle for life, so the de- 
gree 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 prin- 
ciples 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 mod- 
ification, there is not much difiiculty in believing that 
they may have migrated from the same region ; for dm*- 


ing the vast geographical and climatal changes which 
will have supervened since ancient times, almost any 
amount of migration is ^^ossible. But in many other 
cases, in which we have reason to believe that the species 
of- a genus have been produced within comparatively re- 
cent 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 ex- 
treme 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. 
JSTevertheless 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 ordi- 
nary 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 continu- 
ous ; 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. Tlie capacity of migrating across the sea is 
more distinctly limited in terrestrial mammals, than per- 
haps in any other organic beings ; and, accordingly, we 
find no inexplicable cases of the same mammal inhabiting 
distant points of the world. ISTo geologist will feel any 
difficulty in such cases as Great Britain having been for- 
merly united to Europe, and consequently possessing the 
same quadrupeds. But if the same species can be pro- 
duced 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 be- 
come 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 intersj)ace. 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 
sj)ecies 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 natu- 
ralists, 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 spe- 
cies, a directly opposite rule j^revailed ; 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 natu- 
ralists, that the view of each species having been pro- 
duced in one area alone, and having subsequently mi- 
grated from that area as far as its powers of migration 
and subsistence under past and present conditions per- 
mitted, 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 cer- 
tainly occurred within recent geological times, must have 
interrupted or rendered discontinuous the formerly con- 
tinuous 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 con- 
siderations, that each species has been produced within 


one area, and has migrated thence as far it could. It 
would be hopelessly tedious to discuss all the exceptional 
cases of the same species, now living at distant and sep- 
arated points ; nor do I for a moment j)retend 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 fresh-water 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 dis- 
tant and isolated j)oints 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 geographi- 
cal changes and various occasional means of transport, the 
belief that this has been the universal lavf, 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 inhab- 
ited by their progenitor. If it can be shown to be almost 
invariablv the case, that a reofion, of which most of its in- 
habitants 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 clear- 
ly 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, w^ould prob- 
ably 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 fnlly see, inexplicable on the theory 
of independent creation. This view of the relation of 
sj)ecies 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. "Wal- 
lace, 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 ques- 
tion, — namely whether all the individuals of the same 
species have descended from a single pair, or single her- 
maphrodite, or whether, as some authors suppose, from 
many individuals simultaneously created. "With those or- 
ganic beings which never intercross (if such exist), the 
species, on my theory, must have descended from a suc- 
cession 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 in- 
tercross, I believe that during the slow process of modifi- 
cation the individuals of the species will have been kept 
nearly uniform by intercrossing ; so that many individ- 
uals will liave gone on sijnultaneously changing, and the 
whole amount ot modification will not have been due, at 
each stage, to descent from a single parent. To illustrate 
what I mean : our English race-horses 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 in- 
dividuals during many generations. 

Before discussing the three classes of facts, which I 
have selected as presenting the greatest amomit of diffi- 


culty 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 migra- 
tion : 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, l^o geologist will dispute that great mutations 
of level have occurred within the period of existing or- 
ganisms. 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 main- 
land. K 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 conti- 
nent. 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 my judgment we 
are not authorized in admitting such enormous geograph- 
ical 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 tlieir migration. In 
the coral-producing oceans sucli 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 distribu- 
tion, — such as the great difi'erence 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 distri- 
bution 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 on 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 uni- 
versally 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 acci- 
dental 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 trans2)ort across the sea, the greater or less facilities 
may be said to be almost wholly unknown. Until I tried, 
witii 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. It deserves notice 
that certain orders were far more" affected than others ; 
nine Leguminosse were tried, and with one exception all 
resisted the salt-water badly : seven species of the allied 
orders Hydrophyllaceae and Polemoniacese were killed 
by a month's immersion. 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 Avhat 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 fniit, and to place them on sea water. 
The majority sank quickly, but some which w^hilst 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 f ^ seeds germinated after an immer- 
sion of 28 days ; and as ^f 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 yVo 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 tlie rate of 60 miles per 
diem) ; on this average, the seeds of jW plants belonging 
to one country might be floated across 924 miles of sea to 
another country ; and when stranded, if blown to a fa- 
vourable 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 difi'erent 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 -^f 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 yVo plants of a flora, 
after having been dried, could be floated across a sj^ace 
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 
Dot a particle could be washed away in the longest trans- 
port : 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 germi- 

Living birds can hardly fail to be highly efi'ective 
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 ex- 
periments made in the Zoological Gardens, include seeds 
capable of germination. Some seeds of the oat, wheat, 


millet, canary, liemjD, clover, and beet germinated after 
having been from twelve to twentj-one hours in the 
stomachs of difierent birds of prey ; and two seeds of 
beet grew after having been thus retained for two days 
and fourteen hours. Fresh-water 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 fish- 
ing-eagles, storks, and pehcans ; these birds after an in- 
terval 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 bii'ds 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. Re- 
flect for a moment on the millions of quails which annual- 
ly 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 

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-bome 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 bur- 
thens 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 some- 
times called accidental, but this is not strictly correct : 
the currents of the sea are not accidental, nor is the direc- 
tion of prevalent gales of wind. It should be observ- 
ed that scarcely any means of transport would carry seeds 
for very great distances ; for seeds do not retain their vi- 
tality when exposed for a great length of time to the 
action of sea-water ; nor could they be long carried in the 
crops or intestines of birds. These means, however, would 
suliice 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 conti- 
nents 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 E'orth 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 im- 
mersion 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 occa- 
sional means of transport, immigrants from Europe or any 
other continent, that a poorly-stocked island, thougn 
standing more remote from the mainland, would not re- 
ceive 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 argu- 
ment against what would be efi'ected by occasional means 
of transport, during the long lapse of geological time, 
whilst an island was being upheaved and formed, and be- 
fore 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, sepa- 
rated 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 possibiHty 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 re- 
markable, 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 con- 
clude that the same sj^ecies must have been independently 
created at several distinct points ; and we might have re- 
mained 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 conceiv- 
able kind, organic and inorganic, that within a very 
recent geological period, central Europe and JSTorth 
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 ]^orthern Italy, gigantic moraines, left 
by old glaciers, are now clothed by the vine and maize. 
Throughout a large part of the TJnited 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 dis- 
tribution 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 aw^ay, as formerly occurred. As the cold came 
on, and as each more southern zone became fitted for arc- 
tic 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 in- 
habitants would descend to the plains. By the time that 
the cold had reached its maximum, we should have a uni' 
form 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 witli 
those of Europe ; for the present circumpolar inhabitants, 
which we suppose to have everywhere travelled south- 
ward, are remarkably uniform round the world. We 
may suppose that the Glacial period came on a little 


earlier or later in ISTorth 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 re- 
treat 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, al- 
ways ascending higher and higher, as the warmth in- 
creased, whilst their brethren were pursuing their north- 
ern 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 under- 
stand the fact that the Alj)ine 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 re- 
marked 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 Al- 
pine and Arctic productions of Europe and America, that 
when in other regions we find the same species on distant 
mountain-summits, we may also conclude without other 
evidence, that a colder climate permitted their former 
migration across the low intervening tracts, since become 
too warm for theii' existence. 

If the climate, since the Glacial period, has ever been 


ill 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 Gnathodou), 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 inter- 
calated 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 modifica- 
tion. 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 sur- 
vived 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 commence- 
ment 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 com- 
mencement 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 louver mountains and on the plains of JSTorth 
America and Europe; 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 J^ew Worlds are separated 
from each other by the Atlantic Ocean and by the ex- 
treme northern part of the Pacific. During the Glacial 
period, when the inhabitants of the Old and Xew "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 oppo- 
site 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 or- 
ganisms 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 inter -migration 
under a more favourable climate, I attribute the necessary 
amount of uniformity in the sub-arctic and northern tem^ 
perate productions of the Old and ISTew 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 pe- 


riod, such as the older Pliocene period, a large number of 
the same plants and animals inhabited the almost contimi- 
ous circnmpolar 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 con- 
dition, 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 
ISTorth 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 ob- 
servers, 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 j)eriods 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. Tliis separation, as far as the more temperate pro- 
ductions 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 AYorld. Consequently we have here every-thing 
favourable for much modification, — for far more modifica- 
tion than with the Alpine productions, left isolated, with- 
in 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 pro- 
ductions 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, wliich 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 nat- 
uralists 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 sun- 
dered. Thus, I think, we can understand the presence of 
many existing and tertiary representative forms on the 
eastern and western shores of temperate ISTorth 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 con- 
tinent and by nearly a hemisphere of equatorial ocean. 

These cases of relationship, without identity, of the in- 
habitants of seas now disjoined, and likewise of the past 
and present inhabitants of the temperate lands of l^orth 
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 conti- 
nents of the Old World, we see countries closely corre- 
sponding in all their physical conditions, but with their in- 
habitants utterly dissimilar. 

But we must return to our more immediate subject, 
the Glacial period. I am convinced that Porbes's view 
may be largely extended. In Europe we have the plain- 
est evidence of the cold period, from the western shores 
of Britain to the Oural range, and southward to the Py- 
renees. We may infer, from the frozen mammals and 
nature of the mountain vegetation, that Sibei'ia was simi- 
larly aff'ected. Along the Himalaya, at points 900 miles 
apart, glaciers have left the marks of their former low 
descent ; and in Sikkim, Pr, Hooker saw maize growing 



on gigantic ancient moraines. Sontli of the equator, we 
have some direct evidence of former glacial action in ISTew 
Zealand ; and the same plants, found on widely separated 
mountains in this island, tell the same stoiy. If one ac- 
count which has been published can be trusted, we have 
direct evidence of glacial action in the south-eastern cor- 
ner 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 Pa- 
cific, where the climate is now so difi'erent, 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 con- 
tinent, 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 an- 
other, 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 simul- 
taneous on the eastern and western sides of ISTorth Amer- 
ica, in the Cordillera under the equator and under the 
warmer temperate zones, and on both sides of the south- 
ern 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 

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 incon- 
siderable 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 sj)ecies belonging 
to European genera occur. On the highest mountains of 
Brazil, some few European genera were found by Gard- 
ner, which do not exist in the wide intervening hot coun- 
tries. So on the Silla of Caraccas the illustrious Hum- 
boldt long ago found species belonging to genera charac- 
teristic of the Cordillera. On the mount^ns 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 sj)ecies, be- 
lieved 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 intertropi- 
cal parts of Africa. On the Himalaya, and on the isolat- 
ed 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 repre- 
senting each other, and at the same time representing 
plants of Europe, not found in the intervening hot low- 
lands. 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 grow- 
ing on the summits of the mountains of Borneo. Some 
of these Australian forms, as I hear from Dr. Hooker, ex- 
tend along the heights of the peninsula of Malacca, and 


are thinly scattered, on the one hand over India and on 
the other as far north as Japan. 

On the southern mountains of Australia, Dr. F. 
Miiller has discovered several European species ; other 
species, not introduced by man, occur on the lov\^lands ; 
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 ' In- 
troduction 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 through- 
out 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 

This brief abstract applies to plants alone : some strict- 
ly 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 high- 
est 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. Kichardson, also, speaks 
of the re-appearance on the shores of New Zealand, Tas- 
mania, (fee, of northern forms of fish. Dr. Hooker in- 
forms me that twenty-five species of Algse 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 hemi- 
sphere, and on the mountain-ranges of the intertropical 
regions, are not arctic, but belong to the northern tempe- 
rate 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 arc- 
tic' Many of the forms living on the mountains of the 
warmer regions of the earth and in the southern hemi- 
sphere are of doubtful value, being ranked by some natu- 


ralists as specifically distinct, by others as varieties ; but 
some are certainly identical, and many, thongli closely 
related to northern forms, must be ranked as distinct 

IS^ow let lis see what light can be thrown on the fore- 
going 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, simulta- 
neously 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 natural- 
ised plants and animals have spread within a few centu- 
ries, this period will have been ample for any amount of 
migration. As the cold came slowly on, all tlie 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 Aus- 
stralia. As we know that many troj)ical plants and ani- 
mals 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 cer- 
tain extent. On the other hand, the temperate produc- 
tions, 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 tem- 
perate plants, if protected from the inroads of competi 
tors, 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 equar(/i. The in- 


Yasion would, of course, have been greatly favoured hj 
high land, and perhaps by a dry climate ; for Dr. Fal- 
coner 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 produc- 
tions entered and crossed even the lowlands of the tropics 
at the period when the cold was most intense, — when arc- 
tic forms had migrated some twenty-five degi'ees of lati- 
tude from their native country and covered the land at 
the foot of the Pyrenees. At this period of extreme cold, 
I beheve 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 tem- 
perate vegetation, like that now growing with strange 
luxuriance at the base of the Himalaya, as graphically de- 
scribed by Hooker. 

Thus, as I believe, a considerable number of plants, a 
few terrestrial animals, and some marine productions, mi- 
grated during the Glacial period from the northern and 
southern temperate zones into the intertropical regions, 
and some even crossed the equator. As the warmth re- 
turned, these temperate forms would naturally ascend 
the higher mountains, being exterminated on the low- 
lands ; 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 hemi- 
sphere. Although we have reason to believe from geo- 


logical evidence tliat the whole body of arctic shells imder- 
went 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 j)]ainly 
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 re- 
gard 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, how- 
ever, a few southern vegetable forms 'on the mountains of 
Borneo and Abyssinia. I suspect that this prcjDonderant 
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 hav- 
ing consequently been advanced through natural selection 
and competition to a higher stage of perfection or domi- 
nating power, than the southern forms. And thus, when 
they became commingled during the Glacial period, the 
northern forms were enabled to beat the less 2^o"^erful 
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 nat- 
uralised in any j^art of Europe, though hides, wool, and 
other objects likely to carry seeds, have been largely im- 
ported 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 isl- 
ands 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 ex- 
tinction. 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 every- 
where lately yielded to continental forms, naturalised by 
man's agency. 

I am far from supposing that all difficulties are re- 
moved on the view here given in regard to the range and 
affinities of the allied species v^hich live in the northern 
and southern temperate zones and on the mountains of 
the intertropical regions. Yery many difficulties remain 
to be solved. I do not pretend to indicate the exact lines 
and means of migration, or the reason why certain spe- 
cies 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 spe- 
cies 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 Ker- 
guelen 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 difiiculty. 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 modi- 
fication 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 north- 
ern hemisphere, to a former and warmer period, before 
the commencement of the Glacial period, when the an- 
tarctic 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 hemi- 
sphere 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 

Sir C. Lyell in a striking passage has speculated, in 
language almost identical with mine, on the efi'ects of 

freat alternations of climate on geographical distribution, 
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 liave the 
living waters left their living drift on our mountain- 
Bummits, 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 surround- 
ing lowlands. 




Distribution of frcBli-water productions — On the inhabitants of oceanic islands — Ab» 
eence of Batrachiails and of terrestrial mammals — On the relation of the inhabit- 
ants of islands to those of the nearest mainland — On colonisation from the nearest 
source with subsequent modification — Summary of the last and present chap- 

As lakes and river-systems are separated from eacli other 
by barriers of land, it might have been tbonglit that fresh- 
water productions wonld 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. 
IS^ot only have many fresh-water species, belonging to 
quite different classes, an enormous range, but allied spe- 
cies 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 simi- 
larity of the fresh-water insects, shells, &c., and at the 
dissimilarity of the surrounding terrestrial beings, com- 
pared with those of Britain. 

But this power in fresh- water productions of ranging 
widely, though so unexjDected, 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 liabil- 
ity 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- 

Chap. XIl.l FRESH- WATER tROBtlCTlONS. 33^ 

systems will have some fish in common and some different. 
A few facts seem to favour tlie possibility of their occa- 
sional 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 geolog- 
ical period, and when the surface was peopled by existing 
land and fresh-water shells. Tlie wide difi'erence 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 conse- 
quently 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 
Yalenciennes, 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 

Some species of fresh-water shells have a very wide 
range, and allied species, which, on my theory, are de- 
scended 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 imme- 
diately killed by sea-water, as are the adults. I could 
not even understand how some naturalised species have 


rapidly spread througliout the same country. But two 
facts, whicli 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 the 
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 she'll 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 j^lants, 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 
w^ide range. I think favourable means of dispersal ex- 
plain this fact. I have before mentioned that earth occa- 
sionally, 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 occasion- 
ally 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 Feb- 
ruary three table-spoonfuls of mud from three different 
points, beneath water, on the edge of a little pond ; this 
mud when dry weighed only 6f 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 53Y 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 flsh 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. Wlien I saw the great size of 
the seeds of that fine water-lily, the ]N"elumbium, and re- 
membered Alph. de CandoUe's remarks on this plant, I 
thought that its distribution must remain quite inexplica- 
ble ; but Audubon states that he found the seeds of the 


great southern water-lilj (probably, according to Dr. 
Hooker, the Kelumbmm lutenm) 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 stom- 
ach a pellet containing the seeds of the !N"elumbium un- 
digested ; or the seeds might be dropped by the bird 
whilst feeding its young, in the same way as fish are 
known sometimes to be drop]3ed. 

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 unoccu- 
pied ; 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 w^ith those on the land, the com- 
petition will probably be less severe between aquatic 
than between terrestrial species ; consequently an intrud- 
er 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 fomerly 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 main- 
ly 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 Inhcobitants of Oceanic Islands. — We now 
come to tlie last of the three classes of facts, which I have 
selected as presenting the greatest amount of difficult j, 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, 
notwithstandmg that in the course of time thej have come 
to inhabit distant points of the globe. I have already 
stated that I cannot honestly admit Forbes's view on con- 
tinental 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 conti- 
nental areas : Alph. de CandoUe admits this for plants, 
and Wollaston for insects. If we look to the large size 
and varied stations of ]N^ew Zealand, extending over 'TSO 
miles of latitude, and compare its flowering plants, only 
^50 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 
84:7 plants, and the little island of Angiesea Y64, 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 flow- 
ering plants ; yet many have become naturalised on it, 
as they have on IN^ew 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 sepa- 
rate sj)ecies, will have to admit, that a sufficient nnmber 
of the best adapted plants and animals have not been 
created on oceanic islands ; for man has nnintentionally 
stocked them from various sources far more fully and per- 
fectly 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 ex- 
tremely 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, sj)ecies occasionally arriving after 
long intervals in a new and isolated district, and having to 
compete with new associates, will be eminently hable 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 difi'erence seems to depend on the spe- 
cies which do not become modified having immigrated 
with facility and in a body, so that their mutual relations 
have not been much disturbed ; and partly on the frequent 
arrival of unmodified immigrants from the mother-country, 
and the consequent intercrossing with them. With re- 
spect to . the effects of this intercrossing, it should be re- 
membered that the ofi'spring of such crosses would almost 
certainly gain in vigour ; so that even an occasional cross 
would produce more effect than might at first have been 
anticipated. To give a few examples : in the Galapagos 
Islands there are twenty-six land-birds, of these twenty- 
one (or perhaps twenty-three) are peculiar, whereas of the 
eleven marine birds only two are j)eculiar ; and it is ob- 
vious 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 admira- 
ble account of Bermuda, that very many E^ortli 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 blbwn there, as I am 
informed by Mr. E. Y. 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 sea-shells are dispersed, yet we can 
see that their eggs or larvse, 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 difi'erent 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 ISTew 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 
difi'erent orders are very difi'erent from what they are 
elsewhere. Such cases are generally accounted for by 
the physical conditions of the islands ; but this explana- 
tion seems to me not a little doubtful. Facility of immi- 
gration, I believe, has been at least as important as the 
nature of the conditions. 

Many remarkable little facts could be given with re- 
spect 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, wonld form an en- 
demic species, having as useless an appendage as any ru- 
dimentary organ, — for instance, as the shrivelled wings 
under the soldered elytra of many insular beetles. Again, 
islands ofteo possess trees or bushes belonging to orders 
which elsewhere include only herbaceous species ; now 
trees, as Alph. de CandoUe 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, w^ien established on an island and having to com- 
pete with herbaceous plants alone, might readily gain an 
advantage by growing taller and taller and overtopping 
the other ^^lants. 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. Yincent long ago remarked that Batra- 
chians (frogs, toads, newts) have never been found on any 
of the many islands with which the great oceans are stud- 
ded. I have taken pains to verify this assertion, and I 
have found it strictly true. I have, however, been as- 
sured that a frog exists on the mountains of the great 
island of Kew 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 Mauri- 
tius, 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 difiiculty 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 care- 
fullj 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 domesti- 
3ated animals kept by the natives) inhabiting an island 
situated above 800 miles from a continent or great con- 
tinental 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 excep- 
tion ; 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 fre- 
quently 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 degrada- 
tion 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, aerial mammals 
do occur on almost every island. ISTew Zealand possesses 
two bats found nowhere else in the world : I^orfolk Island, 
the Yiti 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 tlie distance of 600 miles 
from the mainland. I hear from Mr. Tomes, who has 
specially studied this family, that many of the same spe- 
cies have enormous ranges, and are found on continents 
and on far distant islands. Hence we have only to sup- 
pose 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 terres- 
trial 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 neighouring 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 steep 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 difiiculty in forming 
a judgment in some cases owing to the probable naturali- 
sation 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 simi- 
lar 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 chan- 
nels are more likely to have been continuously nnited 
within a recent period to the mainland than islands sepa- 
rated by deeper channels, we can understand the freqnent 
relation between the depth of the sea and the degree of 
affinity of the mammalian inliabitants of islands with 
those of a neighbouring continent, — an inexplicable rela- 
tion 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 ^^resence of aerial 
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 occa- 
sional 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 con- 
tinuous land with the nearest continent ; for on this latter 
view the migration would probably have been more com- 
plete ; and if modification be admitted, all the forms of 
life would have been more equally modified, in accord- 
ance with the ]3aramount importance of the relation of or- 
ganism 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 spe- 
cific form or modified since their arrival, could have 
reach-ed 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 diffi- 
culty. 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. 
Kow 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 mv 


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 w4de arms 
of the sea. And I foimd that several species did in this 
state withstand uninjured an immersion in sea-water dur- 
ing seven days : one of these shells was the Helix poma- 
tia, 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. 

Tlie 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. I^umerous instances could he 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-one or perhaps twenty-three of 
these are ranked as distinct species, and are supj^osed to 
have been created here; yet the close affinity of most of 
these birds to American species in every character, in tlieir 
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 inhabi- 
tants of these volcanic islands in the Pacific, distant several 
hundred miles from the continent, yet feels that he is stand- 
ing on American land. Why should tliis 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 wliicli the several classes are associated to- 
gether, which resembles closely the conditions of the South 
American coast : in fact there is a considerable dissimilar- 
ity 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, be- 
tween the Galapagos and Cape de Yerde Archipelagos : 
but what an entire and absolute difterence in their inhab- 
itants ! The inhabitants of the Cape de Yerde 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 ; 
"^vhereas on the vie-w 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 Yerde 
Islands from Africa ; and that such colonists would be 
liable to modification ; — 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 then 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 prevail- 
ing currents, this anomaly disappears. ISTew Zealand in 
its endemic plants is much more closely related to Austra- 
lia, 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 difiiculty almost disaj)- 
pears 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, wliicli, tliougli 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 archi- 
pelago, 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 most 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 origi- 
nal 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 ; where- 
as 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. ]^ow if we look to those inhabitants 
of the Galapagos Archipelago which are found in other 
parts of the world (laying 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 tlie view of tlie 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 difierent 
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 
occuj)ied 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 arcliipelago, and gales of wind are extraor- 
dinarily 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 archi- 
pelago, 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 spe- 
cies 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 sepa- 
rate 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 pecu- 
liar 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 : never- 
theless both islands have been colonised by some European 
land-shells, which no doubt had some advantage over the 
indigenous S]Decies. 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 sonth-west corners of Australia have nearly the same 
physical conditions, and are united by continnons land, 
yet they are inhabited by a vast number of distinct mam- 
mals, birds, and plants. 

Tlie 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 hav- 
ing 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 up- 
heaved, would naturally be colonised from the surround- 
ing 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 inhabit- 
ing the caves of America and of Europe. Other analo- 
gous facts could be given. And it will, I believe, be 
universally found to be true, that wherever in two re- 
gions, 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 migra- 
tion of a species, either at the present time or at some 
former period under difi'erent 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 oyer 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 Felidse and Canidse. 
"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 aver- 
age 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 dis- 
tinct 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 s^dc- 
cies of a genus having descended from a single j)arent, 
though now distributed to the most remote points of the 
world, we ought to And, 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 
diflusion, and should place itself under diverse conditions 
favourable for the conversion of its ofl'spring, 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 modiiied 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 them- 
selves 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 archi- 
pelago or island to those of the nearest mainland, — are, 1 
think, utterly inexplicable on the ordinary view of the in- 
dependent 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, whicE 
have certainly occurred within the recent period, and of 
other similar changes which may have occnrred within 
the same period ; if we remember how profoundly igno- 
rant 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 in- 
dividuals of the same species, wherever located, have de- 
scended 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 cen- 
tres 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 gi-anted 
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 aflected 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 moi'e dominant 
forms of life), together with subsequent modification and 


Chap. XII.] SUMMARY. qkK 

the multiplication of new forms. We can thus under- 
stand the high importance of barriers, whether of land or 
water, which separate our several zoological and botan- 
ical j^rovinces. 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 Imked 
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 difierent 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 hap- 
j^ened 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 raj^idly, 
there would ensue in difierent regions, independently of 
their physical conditions, infinitely diversified conditions 
of life, — there would be an almost endless amount of or- 
ganic action and reaction, — and we should find, as we do 
find, some groups of beings greatly, and some only slight- 
ly modified, — some developed in great force, some exist- 
ing in scanty numbers — in the difierent great geographi- 
cal provinces of the world. 

On these same principles, we can understand, as I 
have endeavoured to show, why oceanic islands should 
have few inliabitants, 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 an- 
other 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 aerial mam- 
mals or bats. We can see why there should be some re- 
lation 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, thongh specifically dis- 
tinct on the several islands, should be closely related to 
each other, and likewise be related, bnt less closely, to 
those of the nearest continent or other source whence im- 
migrants 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 varie- 
ties, of doubtful species, and of distinct but representative 

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 excep- 
tions to the rule are so few, that they may faii-ly be at- 
tributed to our not having as yet discovered in an inter- 
mediate 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 spe- 
cies, or by a group of sj^ecies, is continuous ; and the ex- 
ceptions, which are not rare, may, as I have attemjDted 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 grouj)s of species have their points of maximum de- 
velopment. Groups of species, belonging either to a cer- 
tain period of time, or to a certain area, are often charac- 
terised 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 diflerent 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 con- 
nected by the same bond of ordinary generation ; and the 
more nearly any two forms are related in blood, the near- 
er 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. 




Classification, groups subordinate to groups— Natural system— Rules and difficul- 
ties in classification, explained on the theory of descent -with modification — Classi- 
fication 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 super- 
vening at an early age, and being inherited at a corresponding age— Rudimentary 
ORGANS •, their origin explained — Summary. 

From the first dawn of life, all organic beings are fonnd 
to resemble eacb other in descending degrees, so that they 
can be classed in groups nnder groups. This classification 
is evidently not arbitrary like the grouj)ing of the stars in 
constellations. The existence of groups would have been 
of simj)le 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 diff'erent in nature ; for it is notori- 
ous how commonly members of even the same sub-group 
have difi'erent habits. In our second and fourth chapters, 
on Variation and on I^atural Selection, I have attempted 
to show that it is the widely ranging, the much diftused 
and common, that is the dominant species belonging to 
the larger genera, which vary most. The varieties, or in- 
cipient species, thus produced ultimately become convert- 
ed, 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 domi- 
nant species, tend to go on increasing indefinitely in size. 
I further attempted to show that from the varying de- 


scendants 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 cer- 
tain facts in naturalisation. 

I attempted also to show that there is a constant ten- 
dency 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 princi- 
ples, 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 repre- 
sent a genus including several species ; and all the genera 
on this line form together one class, for all have descend- 
ed 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 in- 
cluding the next two genera on the right hand, which 
diverged from a common parent at the lifth stage of de- 
scent. These five genera have also much, though less, in 
common ; and they form a family distinct from that includ- 
ing 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, 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 judgment fully explained. 

"Naturalists try to arrange the species, genera, and 
families in each class, on what is called the ^Natural Sys- 
tem. But what is meant by this system ? Some authors 

3gQ CLASSlFlCA'TlON. [Chap. XIll. 

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 I^atural 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 pro- 
pinquity of descent, — the only known cause of the simi- 
larity of organic beings,-— is the bond, hidden as it is by 
various degrees of modification, which is partially revealed 
to us by oui' classifications. 

Let us now consider the rules followed in classification, 
and the difliculties which are encountered on the view 
that classification either gives some unknown plan of crea- 
tion, or is simply a scheme for enunciating general pro- 
positions 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 de- 
termined the habits of life, and the general place of each 
being in the economy of nature would be of very high 
importance in classification. [N^othing can be more false. 
Ko 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 intimate- 
ly 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 re- 
motely 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 de- 
pends, are of little signification, excepting in the first main 
divisions ; whereas the organs of reproduction, with their 
product the seed, are of paramount importance ! 

"VVe must not, therefore, in classifying, trust to resem- 
blances 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 resem- 
blances in organs of high vital or physiological impor- 
tance. 1^0 doubt this view of the classificatory importance 
of organs which are important is generally, but by no 
means always, true. But their importance for classifica- 
tion, I believe, depends on their greater constancy through- 
out 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. Tliat the mere physiological importance of an 
organ does not determine its 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, ^o 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 authori- 
ty, 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 uneqiial, and in some cases 
seems to be entirely lost." Again in another work he 
says, the genera of the Connaraceae " difter 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 im- 
portance, 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 re- 
marked, are most constant in structure ; in another divi- 
sion 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 im- 
portance 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 imj)ortance ; 
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 ser- 
viceable in exhibiting the close affinity between Rumi- 
nants and Pachyderms. Robert Brown has strongly in- 
sisted on the fact that the rudimentary florets are of the 
highest importance in the classification of the Grasses. 

IS'umerous instances could be given of characters de- 
rived from parts which must be considered of very trifling 
physiological importance, but which are universally ad- 
mitted 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 infiection of the angle of the jaws in 
Marsupials — the manner i]i which the wings of insects arc 
folded — mere colour in certain Algae — mere pubescence 
on the parts of the flower in grasses — the nature of the 
deraial covering, as hair or feathers, in the Yertebrata. 


If the Ornitliorhynclnis 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 uni- 
versal 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 im- 
portant, alone explains, I think, that saying of Linngeus, 
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 
Malpighiacese, 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. 
Eichard sagaciously saw, as Jussieu observes, that this 
genus should still be retained amongst the Malpighiacese. 
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 allo- 
cating any particular species. K they find a character 


nearly uniform, and common to a great nnmber 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. Tliis principle has been broadly con- 
fessed 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 cor- 
related with others, though no apparent bond of connexion 
can be discovered between them, es]3ecial value is set on 
them. As in most groups of animals, important organs, 
such as those for propelling the blood, or for aerating it, 
or those for propagating the race, are found nearly uni- 
form, they are considered as highly serviceable in classifica- 
tion ; but in some groups of animals all these, the most im- 
portant 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 ani- 
mals ; 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 develoj^ment 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 afiinities. 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 com- 
mon ; 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 espe- 
cially in very large groups of closely allied forms. Tem- 
minck insists on the utility or even necessity of this prac- 
tice 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, fam- 
ilies, 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 be- 
cause further research has detected important structural 
differences, at first overlooked, but because numerous al- 
lied species, with slighty difi'erent grades of difl*erence, 
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 classifica- 
tion 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 be- 
lieve 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 tlie several branches or 
gro-iips, 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 or- 
ders. 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 ex- 
isted at an unknown anterior period. Species of three of 
these genera (A, F, and I) have transmitted modified de- 
scendants to the present day, represented by the fifteen 
genera {p}^ to 0") on the uppermost horizontal line. JN'ow 
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 dif- 
ferent 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, de- 
scended from A, be ranked in the same genus with the 
parent A ; or those from I, with the parent I. But the 
existing genus F'* 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 arrange' 
ment 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 descend- 
ants 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 tlieir 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 occu23y its proper intermediate position ; for F origi- 
nally 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. 
K 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 repre- 
sent 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 difi'erent groups have 
undergone, have to be expressed by ranking them under 
difi'erent so-called genera, sub-families, families, sections, 
orders, and classes. 

It may be worth while to illustrate this view of classi- 
fication, by taking the case of languages. If we possessed 
a perfect pedigree of mankind, a genealogical arrange- 
ment of the races of man would afi'ord the best classifica- 
tion 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 sub- 
ordinate to groups ; but the proj^er or even only possible 
arrangement would still be genealogical ; and this would 
be strictly natural, as it would connect together all lan- 
guages, 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 difierence 
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. Xearly 
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 j^uts the Swedish and common 
turnips together, though the esculent and thickened stems 
are so similar. Whatever part is found to be most con- 
stant, is used in classing varieties : thus the great agri- 
culturist Marshall says the horns are very useful for tliis 
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 pedi- 
gree, 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 num- 
ber of points. In tumbler j^igeons, though some sub- 
varieties difi*er 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 lias nearly or quite lost this habit ; neverthe- 


less, wittLont any reasoning or thinking on tlie 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 IS'egro, 
I think he would be classed under the Kegro group, how- 
ever much he might differ in colour and other iniportant 
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 natural- 
ist 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 be- 
cause they 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, Myan- 
thus, and Catasetum), which had previously been ranked 
as three distinct genera, were known to be sometimes jyvo- 
duced on the same spike, they were immediately included 
as a single specieS; But it may be asked, what ought wo 
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 sup- 
position is of course preposterous ; and I might answer by 
the argumentum ad Jiominem, 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 


tlie bear genus. Tbe 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 to- 
gether 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 consider- 
able amount of modification, may not this same element 
of descent have been unconsciously used in grouping spe- 
cies 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 sj)ecies has been recently exposed. 
Rudimentary structures on this view are as good as, or 
even sometimes better than, other parts of the organisa- 
tion. 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 through- 
out many and different species, especially those having 
very different habits of life, it assumes high value ; for we 
can account for its j)i'esence 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 cloysification. 

We can understand why a species or a group of species 


may depart, in several of its most important character- 
istics, 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 unim- 
portant, 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 com- 
munity 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 classifi- 
cation. We shall hereafter, I think, clearly see why em- 
bryological characters are of such high classificatory im- 
portance. 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 impor- 
tant 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. Tlie resemblance of the greyhound and racehorse 
is hardly more fanciful than the analogies which have 
been drawn by some authors between very distinct ani- 
mals. On my view of characters being of real impor- 
tance 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 sys- 
tematist. 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 proj)er 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 
comiDared 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 theii' 
general shape of body and structure of limbs from a com- 
mon ancestor. So it is with fishes. 

As members of distinct classes have often been adapt- 
ed 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 under- 
stand how it is that a numerical parallelism has some- 
times 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 ex- 
perience shows that this valuation has hitherto been arbi- 
trary), 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, be- 
longing to the larger genera, tend to inherit the advan- 
tages, which made the groups to which they belong large 
and their parents dominant, tliey are almost sure to 
spread widely, and to seize cm 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 sys- 
tem. 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 

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 trans- 
mitted 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 suflered severely from 
extinction, for they are generally represented by extreme- 
ly few species ; and such species as do occur are generally 
very distinct from each otlier, which again implies extinc- 
tion. 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 in- 
vestigation, 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 pre- 
served by some unusual coincidence of favourable circum- 

Mr. "Waterhouse has remarked that, when a member 


belonging to one group of animals exhibits an affinity to 
a quite distinct group, tbis affinity in most cases is gen- 
eral and not special : thus, according to Mr. Waterbonse, 
of all Rodents, tbe bizcacba is most nearly related to Mar- 
supials ; but in tbe points in wbicb it approaches tbis 
order, its relations are general, and not to any one mar- 
supial species more tban to another. As tbe points of 
affinity of tbe bizcacba 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 bizcacba, branched 
off from some very ancient Marsupial, which will liave 
bad 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 bizcacba 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 j)rogenitor, or of an early 
member of the group. On the other band, of all Marsu- 
pials, as Mr. Waterbouse has remarked, the pbascolomys 
resembles most nearly, not any one species, but the gene- 
ral order of Rodents. In tbis case, however, it may be 
strongly suspected that the resemblance is only analogi- 
cal, owing to the pbascolomys having become adapted to 
habits like those of a Rodent. Tbe elder De Candolle 
has made nearly similar osbservations on tbe general 
nature of the affinities of distinct orders of plants. 

On the principle of the multiplication and gradual di- 
vergence in character of the species descended from a com- 
mon 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 con- 
nected together. For the common parent of a whole 
family of species, now broken up by extinction into dis- 



tinct groiij^s 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 nu- 
merous 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 extraor- 
dinary difficulty which naturalists have experienced in 
describing, without the aid of a diagram, the various 
affinities which they perceive between the many Kving 
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 ver- 
tebrate 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. Ex- 
tinction 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 

3>7g CLASSIFICATION [Chap. Xlll. 

their primordial parent, and every intermediate link in 
eacli branch and sub-branch of their descendants, may be 
sujDposed to be still alive ; and the links to be as fine as 
those between the finest varieties. In this case it wonld 
be qnite impossible to give any definition by which the 
several members of the several gronps conld be distin- 
guished 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 foniis 
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 difi'erences 
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-spe- 
cies, 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, al- 
though having few characters in common, under one spe- 
cies ; we use descent in classing acknowledged varieties, 
however difi'erent they may be from their j^arent ; and I 
believe this element of descent is the hidden bond of con- 
nexion which naturalists have sought under the teiTQ of 
the Natural System. On this idea of the natural system 

Chap. XIIL] ' MORPHOLOGY. 3^7 

being, in so far as it has been perfected, genealogical in 
its arrangement, witb the grades of difference between tlie 
descendants from a common parent, expressed by the 
terms genera, families, orders, &c., we can understand the 
rules which w^e are compelled to follow in our classifica- 
tion. We can understand why we value certain resem- 
blances 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 mem- 
bers of each class are connected together by the most com- 
plex 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 

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 re- 
semblance is often expressed by the term " unity of type ; " 
or by saying that the several parts and organs in the dif- 
ferent 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 ? Geoftroy 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. 

Q'7g ■ MORPHOLOGY. [Chap. XIU, 

We never find, for instance, the bones of the arm and 
forearm, or of the thigh and leg, transposed. Hence the 
same names can he given to the homologous bones in 
widely different animals. AYe 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, serv- 
ing for such different purposes, are formed by infinitely 
numerous modifications of an upper lip, mandibles, and 
two pairs of maxillae. Analogous laws govern the con- 
struction of the mouths and limbs of crustaceans. So it is 
with the flowers of plants. 

l^othing can be more hopeless than to attempt to ex- 
plain 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 ' jSTature of 
Limbs.' On the ordinary view of the independent crea- 
tion of each being, we can only say that so it is ; — that it 
has so pleased the Creator to construct each animal and 

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 con- 
necting 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 relatiye connexion of the several parts. If we sup- 
pose 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 selec- 
tion will account for the infinite diversity in structure and 
function of the mouths of insects. i^NTevertheless, 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 difierent members of a class, but of the 
difierent 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 connection 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 compariug 
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 j)istils, 
as well as their intimate structure, are intelligible on the 
view that they consist of metamorphosed leaves, arranged 
in a spire. In monstrous plants, we often get direct evi- 
dence 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 diflfer- 
ent, 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 sucli numerous and such, extraordinarily 
shaped pieces of bone ? As Owen has remarked, the 
benefit derived from the yielding of the several pieces in 
the act of parturition of mammals, will by no means ex- 
plain 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 comj)lex mouth formed of many 
parts, consequently always have fewer legs ; or con- 
versely, those with many legs have simpler mouths? 
Why should the sepals, petals, stamens, and pistils 
in any individual flower, though fitted for such wide- 
ly 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 appen- 
dages ; 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 pro- 
genitor of the articulata, many segments ; and the un- 
known progenitor of flowering plants, many spiral whorls 
of leaves. We have formerly seen that parts many times 
repeated are eminently liable fo vary in number and 
structure ; consequently it is quite probable that natural 
selection, during a long-continued course of modificatioUj 
should have seized on a certain number of the primordi- 
ally 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 discover- 
ing in such parts or organs, a certain degree of fundamen- 
tal resemblance, retained by the strong ]3rinciple of in- 

Chap. XIII.] EMBRYOLOGY. 33]^ 

In the great class of molluscs, thougli we can homolo- 
gise the parts of one species with those of another and 
distinct species, we can indicate but few serial homolo- 
gies ; that is, we are seldom enabled to say that one part 
or organ is homologous with another in the same indi- 
vidual. And we can understand this fact ; for in mol- 
luscs, 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. 

^N'aturalists frequently speak of the skull as formed of 
metamorphosed vertebrae : the jaws of crabs as metamor- 
phosed legs ; the stamens and pistils of flowers as meta- 
morphosed 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. l!Taturalists, 
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 modi- 
fied into skulls or jaws. Yet so strong is the appearance 
of a modification of this nature having occurred, that nat- 
uralists 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 

EiiibryoTogy. — It has already been casually remarked 
that certain organs in the individual, which when mature 
become widely diflerent and serve for difi'erent 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 
statement by Yon Baer, namely, that, " The embryos of 


mammalia, of birds, lizards, and snakes, probably also 
of cbelonia, are in tbeir earliest states exceedingly like one 
anotber, botb as a wbole and in tbe mode of development 
of tbeir parts ; so mucb so, in fact, that we can often dis- 
tinguish tbe embryos only by tbeir size. In my possession 
are two little embr^^os in spirit, whose names I have omit- 
ted to attach, and at present I am quite unable to say to 
what class they belong. They may be lizards, or small 
birds, or very young mammalia, so complete is the simi- 
larity in the mode of formation of the head and trunk in 
these animals. The extremities, however, are still absent 
in these embryos. But even if they existed in the earliest 
stage of their development, we should learn nothing ; for the 
feet of lizards and mammals, the wings and feet of birds, no 
less than the hands and feet of man, all arise from the same 
fundamental form." 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 whelj) 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 acacias, are 
pinnate or divided like the ordinary leaves of the legumi- 

The points of structure, in which the embryos of wide- 
ly difi'erent 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 larvse or 
active embryos of allied animals is sometimes much obscur- 
ed ; and cases could be given of the larvae of two species, 
or of two groups of species, difl'ering quite as much, or even 
more, from each other than do their adult parents. In most 
cases, however, the larvse, though active, still obey more or 
less closely the law of common embryonic resemblance. Cir- 
ripedes 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 probos- 
ciformed 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 

384 EMBRYOLOGY. [Chap. Xlll. 

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: 
tlieir function at this stage is, to search by their well-de- 
veloped 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 rejDroduction. 

We are so much accustomed to see dififerences in struc- 
ture between the embryo and the adult, and likewise a 
close similarity in the embryos of widely different animals 
withm 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 pro- 
portion, 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 metamor- 
phosis ; the cephalopodic character is manifested long be- 
fore the parts of the embryo are completed ; " and again 
in spiders, " there is nothing worthy to be called a meta- 
morphosis." ThelarvfB 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 nutri- 

Othp Xr\i,] EMBRYOLOGY. 3§5 

nient, jet nearly all pass tlirougli a similar worm-like 
stage of development ; but in some few cases, as in that 
of Apliis, if we look to the admirable drawings by Profes- 
sor Huxley of the development of this insect, we see no 
trace of the vermiform stage. 

How, then, can we explain these several facts in em- 
bryology, — namely the very general, but not universal 
difference in structure between the embryo and the adult ; 
— of parts in the same individual embryo, which ulti- 
mately become very unlike and serve for diverse purposes, 
being at this early period of growth alike ; — of embryos 
of different species witliin 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, excej^t when the embryo becomes at any 
period of life active and has to provide for itself ; of the 
embryo apparently having sometimes a higher organisa- 
tion 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 noto- 
rious 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 ulti- 
mately turn out. We see this plainly in our own chil- 
dren ; 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. ^STevertheless an 
effect thus caused at a very early period, even before the 
formation of the embrj^o, may appear late in life ; as when 


an hereditary disease, whicli appears in old age alone, 
has been communicated to the offspring from the repro- 
ductive 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 If 
quite possible, that each of the many successive modifica- 
tions, by which each species has acquired its present struc- 
ture, 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 varia- 
tions 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 ofl'spring 
and parent. I am far from meaning that this is invari- 
ably the case ; and I could give a good many cases of va- 
riations (taking the word in the largest sense) which have 
supervened at an earlier age in the child than in tho 

Tliese 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 bull- 


dog, though appearing so different, are really varieties 
most closely allied, and have probably descended from 
the same wild stock ; hence I was cm-ions 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 differ- 
ence. 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 care- 
fully 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 nest- 
ling 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 dif- 
ference — 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. 

3gg EMBRYOLOGY. [Chap. XllL 

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 selec- 
tion, have not generally first appeared at an early period 
of life, and have been inherited by the offspring at a cor- 
responding 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. 

]S[ow let us apply these facts and the above two prin- 
ciples — which latter, though not j^roved 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- 
sj)ecies, 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 prin- 
ciples — namely of each successive modification superven- 
ing at a rather late age, and being inherited at a corre- 
sponding late age — 'the fore-limbs in the embryps 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 difi'er 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 infiuence 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. Where- 
as 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 in- 
herited at an earlier period than that at which it first 
appeared. In either case (as with the short-faced tumbler) 
the yoimg or embryo would closely resemble the mature 
parent-form. We have seen that this is the rule of de- 
velopment 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 

390 EMBRYOLOGY. [Chap. XHl. 

degree different from those of their parent, and conse- 
quently 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 larvse, 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 meta- 
morphosis 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 oiir collections were nearly perfect, 
the only possible arrangement, would be genealogical; 
descent being on my view the hidden bond of connection 
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 struc- 
ture of its progenitor. In two groups of animals, however 
much they may at present differ from each other in struc- 
ture 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, com- 
munity in embryonic structure reveals community of 
descent. It will reveal this community of descent, how- 
ever 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 larvse as 
belonging to the great class of crustaceans. As the em- 
bryonic state of each sj^ecies and group of species partially 
shows us the structure of their less modified ancient pro- 
genitors, we can clearly see why ancient and extinct 


forms of life should resemble the embryos of tlieir de- 
scendants, — onr 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 varia- 
tions 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 sup- 
posed 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 embryol- 
ogy, which are second in importance to none in natural 
history, are explained on the principle of slight modifi- 
cations 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. Embryol- 
ogy rises greatly in interest, when we thus look at the 
embryo as a picture, more or less obscured, of the com- 
mon 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 mammse 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 rudi- 
mentary 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 rudi- 
mentary 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 lias even been stated on good 
autliority that rndiments of teetli 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 un- 
mistakeable : 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 mem- 
brane ; and here it is impossible to doubt, that the rudi- 
ments represent wmgs. Kudimentary organs sometimes 
retain their potentiality, and are merely not developed : 
this seems to be the case with the mammae of male mam- 
mals, 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 Kolreuter found that by cross- 
ing such male plants with an hermaphrodite species, the 
rudiment of the jjistil in the hybrid ofispring 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 rudi- 
mentary or utterly aborted for one, even the more impor- 
tant 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 Compositse, 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 compositse, for the pur- 
pose 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 nearly 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. 

Organs, however little developed, if of use, should not be 
called rudimentary ; they cannot properly be said to be in an 
atrophied condition ; they may be called nascent, and may 
hereafter be developed to any extent by natural selection. 
Rudimentary organs, on the other hand, are essentially use- 
less, as teeth which never cut through the gums ; in a still 
less developed condition, they would be of still less use. 
They cannot, therefore, under their present condition, have 
been formed by natural selection, which acts solely by the 
preservation of useful modifications ; they have been re- 
tained, as we shall see, by inheritance, and relate to a former 
condition of their possessor. It is difficult to know what are 
nascent organs ; looking to the future, we cannot of course 
tell how any part will be developed, and whether it is now 
nascent ; looking to the past, creatures with an organ in a 
nascent condition will generally have been supplanted and 
exterminated by their successors with the organ in a more 
perfect and developed condition. The wing of the penguin 
is of high service, and acts as a fin ; it may, therefore, repre- 
sent the nascent state of the wings of birds ; not that I be- 
lieve this to be the case, it is more probably a reduced or- 
gan, modified for a new function : the wing of the Apteryx 
is useless, and is truly rudimentary. The mammary glands 
of the Ornithorhynchus may, perhaps, be considered, in 
comparison with the udder of a cow, as in a nascent state. 
The ovigerous frena of certain cirripedes, which are only 
slightly developed and which have ceased to give attach- 
ment to the ova, are nascent branchiae. 

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 difi'ers much. This latter fact 


is well exemplified in the state of the wings of the female 
moths in certain groups. Rudimentary organs may be ut- 
terly aborted ; and this implies, that we find in an ani- 
mal 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 ru- 
diment of a fifth stamen ; but this may sometimes be 
seen. In tracing the homologies of the same part in dif- 
ferent 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 ru- 
dimentary 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 or- 
gans, are imperfect and useless. In works on natural his- 
tory rudimentary organs are generally said to have been 
created " for the sake of symmetry," or in order " to com- 
plete the scheme of nature ;" but this seems to me no ex- 
planation, 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 physiolo- 
gist 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 supjDose 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 ru- 
dimentary teeth which are subsequently absorbed, can be 
of any service to the rapidly gr(3wing 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 especial- 
ly, according to Youatt, in young animals, — and the state 
of the whole flower in the cauliflower. We often see ru- 
diments 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 reduc- 
tion of various organs, until they have become ru-diment- 
ary, — 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 inju- 
rious under others, as with the wings of beetles living on 
small and exposed islands ; and in this case natural selec- 
tion would continue slowly to reduce the organ, until it 
was rendered harmless and rudimentary. 

Any change in function, which can be efi'ected by 
insensibly small steps, is within the power of natural se- 


lection ; 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 ij:s full powers of action, the principle 
of inheritance at corresponding ages will reproduce the 
organ in its reduced state at the same age, and consequent- 
ly will seldom affect or reduce it in the embryo. Thus 
we can understand the greater relative size of rudiment- 
ary 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 com- 
plete 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 systema- 
tists have found rudimentary parts as useful as, or even 
sometimes more useful than, parts of high physiological 
importauce. Rudimentary organs may be compared with 
the letters in a word, still retained in the spelling, but be- 
come 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 difliculty, 
as they assuredly do on the ordinary doctrine of creation, 

Chap. XIIL] SUMMARY. 397 

migtit even have been anticipatedj 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 com- 
plex, radiating, and circuitous lines of affinities into one 
grand system ; the rules followed and the difficulties en- 
countered by naturalists in their classifications ; the value 
set upon characters, if constant and prevalent, whether 
of high vital importance, or of the most trifling impor- 
tance, or, as in rudimentary organs, of no importance ; 
the wide opposition in value between analogical or adap- 
tive characters, and the 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 contmgencies of extinc- 
tion 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 difi'erent they may be in struc- 
ture. If w^e 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 natu- 
ral system : it is genealogical in its attempted arrange- 
ment, with the grades of acquired diflerence marked by 
the terms varieties, species, genera, families, orders, and 

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 
dififerent sj)ecies of a class ; or to the homologous parts 
constructed on the same pattern in each individual ani- 
mal and plant. 

On the principle of successive slight variations, not 
necessarily or generally supervening at a very early period 

398 8UMMART. [Chap. XIIL 

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 func- 
tion ; 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. 
Larvse 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 se- 
lection, 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 oc- 
currence 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 rudiment- 
ary 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 argu- 




Kecapitulation of the difficulties on the theory of Natural Selection— Recapitulation 
of the general and special circumstances in its favour — Causes of the general 
helief in the immutability of species — How far the theory of natural selection may 
be extended— Eflects of its adoption on the study of Natural History — Concluding 

As this whole volume is one long argument, it maj be 
convenient to the reader to have the leading facts and in- 
ferences 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 or- 
gans 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 insupera- 
bly great, cannot be considered real, if we admit the follow- 
ing propositions, namely, — that gradations in the perfec- 
tion 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 strug 
gle for existence leading to the preservation of each prof- 
itable 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 or- 


gauic beings ; but we see so many strange gradations in 
nature, as is proclaimed by the canon, " Shatura 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 spe- 
cies 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 recapitula- 
tion of the facts given at the end of the eighth chapter, 
which seem to me conclusivel}^ 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 inci- 
dental on constitutional dift'erences 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 remem- 
ber 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 do- 
mestication ; and as domestication apparently tends to 
eliminate sterility, we ought not to expect it also to pro- 
duce sterility. 

The sterility of hybrids is a very difierent 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 con- 
dition. As we continually see that organisms of all kinds 


are rendered in some degree sterile from theii' constitutions 
having been disturbed by sligbtlj different and new con- 
ditions 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 varie- 
ties acquire from being crossed increased vigour and fer- 
tility. 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 spe- 
cies, and all the species of the same genus, or even higher 
group, must have desoended from common parents ; and 
therefore, in however distant and isolated parts of the 
world they are now found, they must in the course of suc- 
cessive 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 rea- 
son 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 ; foi 
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 foi 
by the extinction of the species in the intermediate re- 
gions. It cannot be denied that we are as yet very igno- 
rant of the full extent of the various climatal and geo- 
graphical changes which have affected the earth during 
modern periods ; and such changes will obviously haA^e 
greatly facilitated migration. As an example, I have at- 
tempted to show how potent has been the influence of the 


Glacial period on the distribution both of the same and 
of representative species thronghont the world. We are 
as yet profoundly ignorant of the many occasional means 
of transport. With resj)ect 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 

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 ? AV^ith respect to existing forms, we should re- 
member 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 sup- 
planted 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 dis- 
trict 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 in- 
termediate 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 in- 
habitants of the wo rid, and at each successive period be- 


tween 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 aj)pear, though certainly they often falsely 
appear, to have come in suddenly on the several geologi- 
cal stages ? Why do we not find great piles of strata be- 
neath the Silurian system, stored with the remains of the 
progenitors of the Silurian groups of fossils ? For cer- 
tainly 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 beheve. 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 in- 
tellect. The number of specimens in all our museums is 
absolutely as nothing compared with the countless gen- 
erations of countless s]3ecies 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 and present states ; 
and these many links we could hardly ever expect to dis- 
cover, owing to the imperfection of the geological record, 
j^umerous 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 natural- 
ists 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 num- 
ber. "Widely ranging species vary most, and varieties 
are often at first local,- — both canses rendering the discov- 
ery 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. Many formations have been in- 
termittent 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 dej)osited on the subsiding bed of the sea. 
During the alternate periods of elevation and of station- 
ary 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 ex- 

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 geologi- 
cal record is imperfect all will admit ; but that it is im- 
perfect 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 wliich 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 close- 
ly related to each other, than are the fossils from forma- 
tions 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 ex- 
planations 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 whicli we 
are confessedly ignorant ; nor do we know how ignorant 
we are. We do not know all tlie possible transitional 
gradations between tlie 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 
judgment they do not overthrow the theory of descent 
with modification, 

Kow let us turn to the other side of the argument. 
Under domestication we see much variability. Tliis 
seems to be mainly due to the reproductive system being 
eminently susceptible to changes in the conditions of life ; 
80 that this system, when not rendered impotent, fails to 
reproduce ofispring exactly like the parent-form. Ya- 
riability is governed by many complex laws, — by correla- 
tion 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 pro- 
ductions have imdergone ; 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 
modification, which has already been inlierited 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 domesti- 
cated 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 natm-e, 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 preserv- 


ing the individuals most useful to liim atthetime, 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 sj^ecies, is shown by the 
inextricable doubts whether very many of them are va- 
rieties 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 in- 
crease 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 nat- 
uralisation, as explained in the third chapter. More in- 
dividuals 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 in- 
crease 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 varie- 
ties 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 de- 
gree to the surrounding physical conditions, will turn the 

With animals having separate 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 de- 
fence, or on the charms of the males ; and the slightest 
advantage will lead to victory. 

As geology plainly proclaims that each land has un- 
dergone great physical changes, w^e 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 exter- 
nal characters alone and often capriciously, can produce 
within a short period a great result by adding \\]y mere 
individual differences in his domestic productions ; and 
every one admits that there are at least individual differ- 
ences in species under nature. But, besides such differ- 
ences, all naturalists have admitted the existence of varie- 
ties, which they think sufficiently distinct to be worthy of 
record in systematic works. 'No one can draw any clear 
distinction between individual difi'erences and slight varie- 
ties ; or between more plainly marked varieties and sub- 
species, and species. Let it be observed how naturalists 
differ in the rank which they assign to the many repre- 
sentative forms in Europe and l^orth America. 

If then we have under nature variability and a ^^owe)*- 
ful 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 pre- 
served, 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 poAver, in slowly and 
beautifully adapting each form to the most complex rela- 
tions of life. The theory of natural selection, even if we 
look no further than this, seems to me to be in itself prob- 
able. I have already recapitulated, as fairly as I could, 
the opposed difficulties and objections ; now let us turn to 
the special acts and arguments in favour of the theory. 

On the view that sj)ecies 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 demarca- 
tion can be drawn between species, commonly supposed to 
have been produced by special acts of creation, and varie- 
ties 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. More- 
over, the species of the larger genera, which aflord the 
greater number of varieties or incipient species, retain to 
a certain degree the character of varieties ; for they difi'er 
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 respect they resemble varieties. These are 
strange relations on the view of each s]3ecies having been 
independently created, but are intelligible if all species 
first existed as varieties. 

As each species tends by its geometrical ratio of repro- 
duction to increase inordinately in number ; and as the 
modified descendants of each species will be enabled to in- 
crease by so much the more as they become more diversi- 
fied in habits and structure, so as to be enabled to seize on 
many and widely diflerent places in the economy of na- 
ture, there will be a constant tendency in natural selection 
to preserve the most divergent offspring of any one spe- 
cies. Hence during a long-continued course of modifica' 


tion, the slight differences characteristic of varieties of the 
same species, tend to he augmented into the greater differ- 
ences characteristic of species of the same genus. New 
and improved varieties will inevitably supplant and exter- 
minate the older, less improved and intermediate varie- 
ties ; and thus species are rendered to a large extent 
defined and distinct objects. Dominant species belong- 
ing 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 tend- 
ency 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 gi'oups, 
all within a few great classes, which we now see every- 
where 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 sim- 
ply intelligible. We can plainly see why nature is prod- 
igal in variety, though niggard in innovation. But why 
this should be a law of nature if each species has been in- 
dependently 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 a woodpecker, should have been created to prey on in- 
sects 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 tlie 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 nnoccu]3ied 
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 su]3posed to have been specially 
created and adapted for that country, being beaten and 
supplanted by the naturalised productions from another 
land. Xor 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 j)ollen by 
our fir-trees ; at the instinctive hatred of the queen bee 
for her own fertile daughters ; at ichneumonidee 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 perfec- 
tion have not been observed. 

The complex and little known laws governing varia- 
tion 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 efi'ect ; yet when varieties enter 
any zone, they occasionally assume some of the characters 
of the species proper to that zone. Li both varieties and 
species, use and disuse seem to have produced some efiect ; 
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 tueu- 
tucu, 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 

Chap. S.1V.] RECAPITULATIOIir. ^][|^ 

inhabiting the dark caves of America and Europe. In 
both varieties and species correlation of growth seems to 
have j^layed 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 charac- 
ters occur. How inexplicable on the theory of creation is 
the occasional appearance of stripes on the shonlder and 
legs of the several species of the horse-genns and in their 
hybrids ! How simply is this fact explained if we believe 
that these S2:)ecies have descended from a striped progeni- 
tor, 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, w^hy should the specific characters, 
or those by which the species of the same genus dififer 
from each other, be more variable than the generic char- 
acters 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 va- 
rieties, of which the characters have become in a high 
degree permanent, we can understand this fact ; for they 
have already varied since they branched oft' from a com- 
mon 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 sj)ecies 
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 modifi- 
cation, 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 
to 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 imderstand 
why nature moves by graduated steps in endowing differ- 
ent 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 indis- 
pensable, 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 differ- 
ent conditions of life, yet should follow nearly the same 
instincts ; why the thrush of South America, for instaijce, 
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 varie- 
ties, 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. 

K 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 inter- 
vals 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 prin- 
ciple of natural selection ; for old forms will be supplanted 
by new and improved forms. ISTeither 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 de- 
scendants will often be intermediate in character in com- 
parison 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 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 natu- 
rally be allied by descent. 

Looking to geographical distribution, if we admit that 


there lias been during the long course of ages much 
migration from one part of the world to another, owing to 
fonner climatal and geographical changes, and to the 
many occasional and unknown means of dispersal, then 
we can understand, on the theory of descent with modi- 
fication, 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 colo- 
nists. On this same principle of former migration, com- 
bined 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. Al- 
though two areas may present the same physical condi- 
tions 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 rela- 
tions, 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 modifica- 
tion, 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 pecu- 
liar species of bats, and the absence of all other mammals, 
on oceanic islands, are utterly inexplicable on the tlieory 
of independent acts of creation. 

The existence of closely allied or representative spe- 
cies in any two areas, implies, on the theory of descent 
with modification, that the same parents formerly in- 
habited both areas ; and we almost invariably find that 
wherever many closely allied species inhabit two areas, 
some identical species common to both still exist. Wher- 
ever many closely allied yet distinct species occur, many 
doubtful forms and varieties of the same S23ecies 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 Gala- 
pagos archipelago, of Juan Fernandez, and of the other 
American islands being related in the most striking man- 
ner to the plants and animals of the neighbouring Ameri- 
can mainland ; and those of the Cape de Yerde archipel- 
ago 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 ex- 
tinction and divergence of character. On these same 
principles we see how it is, that the mutual af&nities 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 wliy embiyological characters are the most 
vahiable 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 bj the most per- 
manent characters, however slight their vital imj)ortance 
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 vertebrse 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 suc- 
cessive T ariations 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 nnder 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 matu- 
rity and has to play its full part in the struggle for exist- 
ence, and will thus have little power of acting on an or- 
gan 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 progen- 

Chap. XIV.] CONCLUSION. ^^fj 

itor having well-developed teetli ; and we may Relieve, 
that the teeth in the mature animal were reduced, during 
successive generations, by disuse, or by the tongue and 
j)alate 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 j^rinciple 
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 ! ]N"ature may be said to have taken pains to re- 
veal, by rudimentary organs and by homologous struc- 
tures, her scheme of modification, which it seems that we 
wilfully will not understand. 

1 have now recapitulated the chief facts and consider- 
ations which have thoroughly convinced me that species 
have been modified, during a long course of descent, by 
the preservation or the natural selection of many succes- 
sive slight favourable variations. I cannot believe that 
a false theory would explain, as it seems to me that the 
theory of natural selection does explain, the several large 
classes of facts above specified. I see no good reason why 
the views given in this volume should shock the religious 
feelings of any one. A celebrated author and divine has 
written to me that " he has gradually learnt to see that it 
is just as noble a conception of the Deity to believe that 
He created a few original forms capable of self-develop- 
ment into other and needful forms, as to believe that He 
required a fresh act of creation to supply the voids caused 
by the action of His laws." 

Why, it may be asked, have nearly 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 varia- 
tion in the course of long ages is a limited quantity; no 

418 CONCLUSION. [Chap. XIV. 

clear distinction has been, or can be, drawn between spe- 
cies 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, with- 
out proof, that the geological record is so perfect that it 
would have afforded us plain evidence of the mutation of 
epecies, 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 geolo- 
gists, 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 crea- 
tion," " 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 mfluenced by this volume ; but I look 
with confidence to the future, to young and rising natu- 

Chap. XIV.] CON^CLUSION. 4;[9 

ralists, who will be able to view both sides ol the question 
with impartiality. Whoever is led to believe that species 
are mutable will do good service by conscientiously ex- 
pressing his conviction ; for only thus can the load of 
prejudice by which this subject is overwhelmed be re- 

Several eminent naturalists have of late published their 
belief that a multitude of reputed species in each genus 
are not real species ; bat 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 thua 
looked at by the majority of naturalists, and which conse- 
quently have every external characteristic feature of true 
S]3ecies, — 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 Avhich are those pro- 
duced by secondary laws. They admit variation as a 
liera cau^a in one case, they arbitrarily reject it in an- 
other, 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 kiuds 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 i 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. 

420 COXCLUSIOX. [Chap. XIV. 

It may be asked liow far I extend the doctrine of the 
modification of species. The question is difficult to an- 
swer, 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 con- 
nected together by chains of afiinities, 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 ru- 
dimentary condition ]3lainly show that an early progenitor 
had the organ in a fully developed state ; and this in some 
instances necessarily implies an enormous amount of mod- 
ification 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 

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 com- 
mon, in their chemical composition, their germinal vesicles, 
their cellular structure, and their laws of growth and re- 
production. We see this even in so trifling a circumstance 
as that the same poison often similarly affects j^lants and 
animals ; or that the poison secreted by the gall-fly pro- 
duces 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 con- 
siderable revohition in natural historv. Svstematists 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 sufiiciently constant and 
distinct from other forms, to be capable of definition ; and 
if definable, whether the differences be sufficiently imjDor- 
tant 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 blinded 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 interme- 
diate gradations, whereas species were formerly thus con- 
nected. 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 be- 
tween them. It is quite possible that forms now general- 
ly 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 lan- 
guage 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 es- 
sence 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 abortive organs, etc., will cease to be metaphorical, 

^22 C0XCLUSI02s\ [Chap. XIV. 

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 ]DOssessor, 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 correla- 
tion 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 impor- 
tant and interesting subject for study than one more spe- 
cies 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 classify- 
ing will no doubt become sim]3ler 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. 
Eudimentary 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 fanci- 
fully be called living fossils, will aid us in forming a pic- 
ture of the ancient forms of life. Embryology will reveal 
to us the structure, in some degree obscured, of the pro- 
totypes 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 birth- 
place; and when we better know the manv means of mi- 

Chap. XIV.] CONCLUSION. 423 

gration, then, by the liglit wliicli 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 ex- 
treme 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 
guage 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 sud- 
denly altered physical conditions, namely, the mutual re- 
lation 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 

424: CDXCLUSION. [Chap. XIV, 

accuracy of organic cliarige as a measure of time. During 
earlj periods of the eartli's history, vrhen the forms of 
h'fe were probably fewer and simpler, the rate of change 
was i^robably slow^er ; and at the first dawn of life, when 
very few forms of the simj^lest structure existed, the rate 
of change may have been slow in an extreme degree. 
The w^liole history of the Avorld, as at present known, al- 
though of a length quite incomprehensible by us, will 
hereafter be recognised as a mere fragment of time, com- 
pared Avith 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. Fsychology 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 sat 
isfied with the view that each sj)ecies has been independ- 
ently 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 w^orld should have been due to second- 
ary causes, like those determining the birth and death of 
the individual. When I view all beings not as sj^ecial 
creations, but as the lineal descendants of some few be- 
ings which lived long before the first bed of the Silm-ian 
system w^as de230sited, they seem to me to become en- 
nobled. Judging from the past, w^e 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 w^ill transmit progeny of any kind to a far distant 
futurity ; for the manner in wdiich all orgaidc 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 proplietic glance into futurity as to foretel 
tliat it wdll be the common and widely-spread species, be- 
longing to the larger and dominant groups, which will 
ultimately prevail and procreate new and dominant spe- 


cies. As all the living forms of life are the lineal descend- 
ants of those which lived long before the Silurian epoch, 
we may feel certain that the ordinary succession by gen- 
eration 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 inapprecia- 
ble length. And as natural selection works solely by and 
for the good of each being, all corporeal and mental en- 
dowments will tend to progress towards perfection. 

It is interesting to contemplate an entangled bank, 
clothed with many plants of many kinds, with birds sing- 
ing on the bushes, with various insects flitting about, and 
with worms crawling through the damp earth, and to re- 
flect that these elaborately constructed forms, so difi*erent 
from each other, and dependent on each other in so com- 
plex 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 ; Yariability 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 Strug- 
gle 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 a grandeur in this 
view of life, wdth its several powers, having been original- 
ly 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. 


The following additions and alterations, prepared bj the author ex- 
pressly for this edition, were received too late to be incorporated in 
their proper places. 

Page 46, eight lines from bottom, after " not generally propagated," 
insert : 

If it could be shown that monstrosities were even propa- 
gated for a succession of generations in a state of nature, 
moditications might be effected (with the aid of natural 
selection) more abruptly than I am inclined to believe 
they are. 

Page 79, six lines from bottom, after word " nature," insert paren- 
thesis — reading the whole sentence thus : 

Man can act only on external and visible characters : na- 
ture (if I may be allowed for brevity-sake to personify the 
natural preservation of favoured individuals during the 
struggle for existence) cares nothing for appearances, ex- 
cept in so far as they may be useful to any being. 

Page 168, first line, after "structure of the eye," insert: 

(though in tlie fish Amphioxus, the eye is in an extremely 
simple condition without a lens) 

Page 108, sixth line from top, omit all seventeen lines beginning 
with "In the Articulata." and ending with '• living crustaceans, and 
bearing in mind," and insert as follows : 

[In the great kingdom of the Articulata we can start 



from an optic nerve, simply coated witli pigment, whicli 
sometimes forms a sort of pupil, but is destitute of a lens 
or any other optical mechanism. From this rndimentaiy 
eye which can distinguish light from darkness, but noth- 
ing else, tiiere is an advance towards perfection along two 
lines of structure, which Miiller thought were fundamen- 
tally different ; namely, lirstly, stemmata, or the so-called 
"simple eyes," whicli have a lens and cornea; and sec- 
ondly, " compound eyes," which seem to act mainly or 
solely by excluding all the rays from eacli point of the 
viewed object, except the pencil that comes in a line per- 
pendicular to the convex retina. In the class of com- 
pound eyes, besides endless differences in the form, pro- 
portion, number and position of the transparent cones 
coated by pigment which act by exclusion, we have addi- 
tions of a more or less perfect concentrating apparatus ; 
thus in the eyes of the meloe the facets of the cornea are 
" slightly convex both externally and internally ; that is, 
lens-shaped." * In many crustaceans there are two cornea, 
the external smooth and the internal divided into facets, 
within the substance of which, as Milne Edwards says, 
"renflemens lenticulaires paraissent s'etre developpes ; " 
and sometimes these lenses can be detached in a layer dis- 
tinct from the cornea. The transparent cones, which were 
supposed by Miiller to act solely by excluding the diverg- 
ent pencils of light, usually adhere to the cornea, but not 
rarely they are detached from it, and have their free ends 
convex, and in this case they must act, I presume, as con- 
verging lenses. Altogether, so diversified is the structure 
of the compound eyes, that Miiller makes three main 
classes, with no less than seven subdivisions of structure. 
He makes a fourth main class — namely, " aggregates " of 
stemmata; and he adds that "this is the transition-form 
between the mosaic-like compound eyes, unprovided with 
a concentrating apparatus and organs of vision with such 
an apparatus." 

AVith these facts, here too briefly and imperfectly 
given, which show how much graduated diversity'- there is 
in the eye of our existing articulata, and bearing in mind, 
&c., &c. 



Page 1G9, tenth line from top, after " transparent tissue," add : 

•with spaces filled with fluid and with a nerve sensitive, 
&c., &c. 

Page 169, sixteenth line from top, after "there is a power," insert 
in brackets : 

[natural selection] 

Page 170, twenty-first line from top, after " insensible steps," in- 

Certain plants, as some Legmninosse, Yiolacese, (fee, bear 
two kinds of flowers ; one having the normal structure of 
the order, the other kind being degraded, though some- 
times more fertile than the perfect flowers ; if the plant 
ceased to bear its perfect flowers, and this did occur dur- 
ing several years with an imported specimen of Aspicarpa 
in France, a great and sudden transition would in fact be 
efl'ected in the nature of the plant. . • 

Pnges 293 and 294. Omit thirty lines, beginning, " On the state 
of Development ^^^ and ending >with "class, may have beaten the highest 
molluscs," and insert as follows: 

On the state of Development of ancient compared with 
living Forms. — We have seen in the Fourth Chapter that 
the degree of difl'erentiation and specialisation of the parts 
of all organic beings, when come to maturity, is the best 
standard as 3"et suggested of their degree of jDcrfection or 
highness. We have also seen that, as the specialisation 
of parts or organs is an advantage to each being, so natu- 
ral selection will constantly tend thus to render the or- 
ganisation of each more specialised and perfect, and in 
this sense higher ; not but that it may, and will, leave 
many creatures fltted for simple conditions of life with 
simple and unimproved structures. In another and more 
general manner we can see that, on the theory of natural 
selection, the more recent forms will tend to be higher 
than their progenitors ; 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. So that by this fundamental test of 
victory in the battle for life, as well as by the standard of 
the specialisation of organs, modern forms ought, on the 
theory of natural selection, to stand higher than an- 
cient forms. Is this the case? A large majority of pa- 
laeontologists would certainly answer in the- affirmative ; 
but in my imperfect judgment, I can, after having read 
the discussions on this subject by Lyell and by Hooker in 
regard to plants, concur only to a cretain limited extent. 
Nevertheless, it may be anticipated that the evidence Avill 
be rendered more decisive by future geological research. 

The problem is in many ways excessively intricate. 
The geological record, at all times imperfect, does not ex- 
tend far enough back, as I believe, to show with unmis- 
takable clearness that within the known history of the 
world organisation has largely advanced. Even at the 
present day, looking to members of the same class, natu- 
ralists are not unanimous which forms are highest ; thus 
some look at the Selaceans or sharks from their approach 
in some important point of structure to reptiles as the 
highest fish ; others look at the teleosteans as the highest. 
The ganoids stand intermediate between the selaceans and 
teleosteans ; the latter, at the present day, are largely 
preponderant in number, but formerly selaceans and ga- 
noids alone existed; and in this case, according to the 
standard of highness chosen, so -will it be said that fishes 
have advanced or have retrograded in organisation. To 
attempt to compare in the scale of highness members of 
distinct types seems hopeless ; who will decide whether a 
cuttle-fish be higher than a bee ? that insect which the 
great Yon Baer believed to be " in fact more highly or- 
ganised than a fish, although upon another type." lii the 
complex struggle for life it is quite credible that crusta- 
ceans, for instance, not very high in their own class, might 
beat the cephalopods or highest molluscs ; and such eras- 



taceans, thongli not liighly developed, would stand very 
high in the scale of invertebrate animals, if judged by the 
most decisive of all trials, — the law of battle. 

Besides this inherent difficnlty in deciding which forms 
are the most advanced in oi'ganization, we ought not 
solely to compare the highest members of a class at any 
two distant periods, — though undoubtedly tliis is one and 
perhaps the most important element in striking a balance 
— but we ought to compare all the members, high and 
low, at the two periods. At an ancient epoch the highest 
and lowest molluscs — namely, cephalopods and brachio- 
pods — swarmed in numbers ; at the present time, both 
these orders have been greatly reduced, whereas otlier or- 
ders, intermediate in grade of organisation, have largely 
increased ; consequently some naturalists have niaintained 
that molluscs were formerly more highly developed than 
at present ; but a stronger case can be made out on the 
other side by considering the vast reduction at the present 
day of the lowest molluscs ; more especially as the exist- 
ing cephalopods, though so few in number, are more 
highly organised than their ancient representatives. We 
ought, also, to consider the relative proportional numbers 
of the high and low classes in the population of the world 
at the two periods; if, for instance, at the present day 
there were fifty thousand kinds of vertebrate animals, and 
if we had reason to believe that at some former period 
there were only ten thousand kinds, we ought to look at 
this increase in number of the highest class, which implies 
a great displacement of lower forms, as a decided advance 
in tlie organisation of the world, whether or not it were 
the higher vertebrata wh?ch had thus largely increased. 
We can thus see how hopelessly difficult it will appa- 
rently forever be to compare wi^li perfect fairness under 
such extremely complex relations the standard of organi- 
sation of the imperfectly known faunas of successive pe- 
riods of the earth's history. 

We shall appreciate (under one important point of 
view) this difficulty the more clearly, by looking to the 
case of certain existing faunas and floras. From the ex- 
traordinary manner in which European productions have 
recently spread over l^ew Zealand, &c., &c. 


Page 417, 25th line, after "facts above specified," insert: 

It is no valid objection that science as jet throws no 
light on the origin of Lite. Who can explain what is the 
essence of the Attraction of gravity ? Althongh Leibnitz 
accused Newton of introducing "occult qualities and 
miracles into philosophy;" yet this unknown element of 
attraction is now universally looked at as a vera causa 
perfectly well established.] 

[I see no good reason why the views given in this 
volume should shock the religious feelings of any one. It 
is satisfactory, as showing how transient such impressions 
are, to remember that the greatest discovery ever made 
by man^ namely, the law of gravity, was attacked by 
Leibnitz, "as subversive of natural and inferentially of 
revealed religion," A celebrated author and divine, &c., 

Page 420, fifteen lines from top, after "deceitful guide," omit 
whole remainder of paragraph, and insert, instead, as follows : 

Nevertheless, all living things have much in common ; 
in their chemical composition, their cellular structtn*e, 
their laws of growth, and their liability to injurious inllu- 
ences. We" see this 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 mon- 
strous growths on the wild rose or oak tree. In all or- 
ganic beings the union of a male and female elemental 
cell seems occasionally to be necessary for the production 
of a new being. In all, as far as is at present known, the 
germinal vesicle is the same. So that every individual 
organic being starts from a common origin. If we look 
even to the two main divisions — namely, to the animal 
and vegetable kingdoms — certain low forms are so far 
intermediate in character that naturalists have disputed 
to which kingdom they should be referred ; and on the 
principle of natural selection with divergence of charac- 
ter, it does not seem utterly incredible that from some 
such intermediate production both animals and plants 
might possibly have been developed. Therefore I should 
inter that probably all the organic beings which have 



ever lived on tliis earth have descended from some one 
primordial form, into which life was first breathed by the 
Creator. But this inference is chiefly grounded on anal- 
ogy, and it is immaterial whether or not it be accepted. 
The case is different with the members of each great class, 
as the Yertebrata or Articulata ; for here, as has just been 
remarked, we have in the laws of homology and embry- 
ology, &c., some distinct evidence that all have descended 
from a single primordial parent.] 


Aberbant groups, 373. 
Abj-ssioia, plants of, 326. 
Acclimatisation, 127. 
Afliaities of extinct species, 287. 

of organic beings, 358. 
Agassiz on Amblyopsis, 127. 

on groups of species suddenly ap- 
pearing, 264, 267. 

on embryological succession, 295. 

on the glacial period, 319. 

ou embryological characters, 364. 

on the embryos of vertebrata, 382. 

on parallelism of embryological de- 
velopment and geological succes- 
sion, 390. 
Algas of New Zealand, 327. 
Alligators, males, fighting, 84. 
Amblyopsis, blind fish, 127. 
America, North, productions allied to 
those of Europe, 323. 

boulders and glaciers of, 325. 

South, no modern formations on 
west coast, 254. 
Ammonites, sudden extinction of, 281. 
Anagallis, sterility of, 219. 
Analogy of variations, 143. 
Ancylus, 336, 

Animals, not domesticated from being 
variable, 23. 

domestic, descended from several 
stocks, 24. 

acclimatisation of, 129. 

of Australia, 108. 

■with thicker fur in cold climates, 122. 

blind, in caves, 125. 

extinct, of Australia, 296. 
Anomma, 213. 

Antarctic islands, ancient flora of, 347. 
Antirrhinum, 145. 
Ants attending aphides, 188. 

slave-making instinct, 195. 
Ants, neuter, structure of, 209. 
Aphides attended by ants, 188. 
Aphis, development of, 384. 
Apteryx, 163. 
Arab horses, 38. 
Aralo-Caspian Sea, 296. 
Archiac, M. de, on succession of species, 

Artichoke, Jerusalem, 129. 

Ascension, plants of, 339. 

Asclepias, pollen of, 173. 

Asparagus, 313. 

Aspicarpa, 363. 

Asses, striped, 147. 

Ateuchus, 123. 

Audubon on habits of frigate-bird, 169 

on variation in birds'-nests, 189. 

on heron eating seeds, 338. 
Australia, animals of, 108, 

dogs of, 192. 

extinct animals of, 296. 

European plants in, 327. 
Azara on flies destroying cattle, 70. 
Azores, flora of, 316. 

Babington, Mr., on British plants, 49. 

Balancement of growth, 133. 

Bamboo with hooks, 176. 

Barberry, flowers of, 92. 

Barrande, M., on Silurian colonies, 274. 

on the succession of species, 284. 

on parallelism of palaeozoic forma- 
tions, 287. 

on affinities of ancient species, 288. 
Barriers, importance of, 303. 
Batrachians on islands, 342. 
Bats, how structure acquired, 163. 

distribution of, 343. 
Bear, catching water-insects, 165. 
Bee, sting of, ISO. 

queen, killing rivals, 180. 
Bees fertilising flowers, 71. 

hive, not sucking the red clover, 

hive, cell-making instinct, 200. 

humble, cells of, 200. 

parasitic, 195. 
Beetles, wingless, in Madeira, 124. 

with deficient tarsi, 123. 
Bentham, Mr., on British plants, 49. 

on classification, 365. 
Berkeley, Mr., on seeds in salt-water, 31Z 
Bermuda, birds of, 341. 
Birds acquiring fear, 189. 

annually cross the Atlantic, 317. 

colour of, on continents, 121. 

fossil, in caves of Brazil, 296. 



Birds of Madeira, Bermuda, and Gala- 
pagos, 340. 

song of males, 84. 

transporting seeds, 315. 

waders, 337. 

wingless, 123, 163. 

with traces of embryonic teeth, 391. 
Bizcacha, 305. 

affinities of, 373. 
Bladder for s-wimming in fish, 170. 
Blindness of cave animals, 126. 
Blvtb, Mr., on distinctness of Indian cat- 
tle, 23. 

on striped Hemionus, 147. 

on crossed geese, 224. 
Boar, shoulder-pad of, 84. 
Borrow, Mr., on the Spanish pointer, 38. 
Bory St. Vincent on Batrachians, 342. 
Bosquet, M., on fossil Chthamalus, 266. 
Boulders, erratic, on the Azores, 316. 
Branch^, 170. 
Brent, Mr., on house-tumhlers, 191. 

on hawks killing pigeons, 315. 
Brewer, Dr., on American cuckoo, 193. 
Britain, mammals of, 344. 
Bronn on duration of specific forms, 257. 
Brown, Robert, on classification, 361. 
Buckman on variation in plants, 17. 
Buzareingues on sterility of varieties, 238. 

Cabbage, varieties of, crossed, 93. 

Calceolaria, 222. 

Canary-birds, sterility of hybrids, 223. 

Cape de Verde islands, 347. 

Cape of Good Hope, plants of, 102, 326. 

Carrier-pigeons killed by hawks, 315. 

Cassini on flowers of compositae. 131. 

Catasetum, 369. 

Cats, with blue eyes, deaf, 18. 

variation in habits of, 86. 

curling tail when going to spring, 179. 
Cattle destroying fir-trees, 69. 

destroyed by flies in La Plata, 70. 

breeds of, locally extinct, 103. 

fertility of Indian and European 
breeds, 225. 
Cave, inhabitants of, blind, 125. 
Centres of creation, 307. 
Cephalopodae, development of, 384. 
Cervulus, 224. 

Cetacea, teeth and hair, 131. 
Ceylon, plants of, 326. 
Chalk formation, 282. 
Characters, divergence of, 103. 

sexual, varialjle, 141. 

adaptive or analogical, 371. 
Charlock, 74. 
Checks to increase, 66. 

mutual, 69. 
Chickens, instinctive tameness of, 192. 
Chthamalinae, 253. 

Chthamalus, cretacean species of, 266. 
Circumstances favourable to selection of 
domestic products, 42. 

to natural selection, 95. 
Cirripedes capable of crossing, 95. 

carapace aborted, 134. 

their ovigerous freua, 172. 

fossil, 266. 

larvje of, 383. 

Classification, 360. 

Clift, Mr., on the succession of types, 295. 
Climate, effects of, in checking increase ol 
beings, 67. 

adaptation of, to organisms, 127. 
Cobites, intestine of, 170. 
Cockroach, 74. 

Collections, palseontological, poor, 252. 
Colour, influenced by climate, 121. 

in relation to attacks by flies, 177. 
Columbalivia, parent of domestic pigeons, 

Colymbetes, 336. 
Compensation of growth, 134. 
Compositae, outer and inner florets of, 131. 

male flowers of, 392. 
Conclusion, general, 416. 
Conditions, slight changes in, favourable 

to fertility, 235. 
Coot, 166. 
Coral-islands, seeds drifted to, 315. 

reefs, indicating movements of earth, 
Corn-crake, 166. 

Correlation of growth in domestic produc- 
tions, 18. 

of growth, 130, 177. 
Cowslip, 51. 

Creation, single centres of, 307. 
Crinum, 221. 
Crosses, reciprocal, 228. 
Crossing of domestic animals, importance 
in altering breeds, 25. 

advantaees of, 91. 

unfavourable to selection, 96. 
Crustacea of New Zealand, 327. 
Crustacean, blind, 125. 
Cryptocerus, 211. 
Ctenomys, blind, 125. 
Cuckoo, instinct of, 193. 
Currants, grafts of, 231. 
Currents of sea, rate of, 313. 
Cuvier on conditions of existence, 184. 

on fossil monkeys, 265. 

Fred., on instinct, 186. 

Dana, Prof., on blind cave-animals, 126. 

on relations of crustaceans of Japan, 

on crustaceans of New Zealand, 327. 
De Candolle on struggle for existence, 61. 
on umbel