LIBRARY OF
WELLESLEY COLLEGE
PRESENTED BY
The Publishers
CHARLES DARWIN
Science % Front is— Vol. i
}
ORIGIN OF SPECIES
BY MEANS OF NATURAL SELECTION, OR THE
PRESERVATION OF FAVORED RACES IN
THE STRUGGLE FOR LIFE
BY
CHARLES DARWIN, M.A.$ LL.D., F.R.S.
NEW YORK
P. F. COLLIER & SON
M C M V
science:
365
Oz
SCIENCE
CONTENTS
PAGE
Additions and Corrections, to the Sixth Edition . « ♦ , . 1
Historical Sketch • « * . 11
Introduction ........ . ... 25
CHAPTER I
VARIATION UNDER DOMESTICATION
Causes of Variability — Effects of Habit and the use or disuse of Parts-
Correlated Variation — Inheritance — Character of Domestic Varieties —
Difficulty of distinguishing between Varieties and Species— Origin of
Domestic Varieties from one or more Species — Domestic Pigeons,
their Differences and Origin — Principles of Selection anciently fol-
lowed, their Effects — Methodical and Unconscious Selection — Un-
known Origin of our Domestic Productions — Circumstances favora-
ble to Man's power of Selection ..».,., 81
CHAPTEE II
VARIATION UNDER NATURE
Taxability — Individual differences — Doubtful species — Wide ranging, much
diffused, and common species, vary most — Species of the larger genera
in each country vary more frequently 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 ?4
(3)
CONTENTS
CHAPTER III
STRUGGLE FOR EXISTENCE
Its bearing on natural selection — The term used in a wide sense — Geo-
metrical ratio of increase — Rapid increase of naturalized animals and
plants — Nature of the checks to increase — Competition universal —
Effects of climate — Protection from the number of individuals — Com-
plex relations of all animals and plants throughout nature — Struggle for
life most severe between individuals and varieties of the same species :
often severe between species of the same genus — The relation of organ-
ism to organism the most important of all relations . . . .98
CHAPTER IV
NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST
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 be-
tween individuals of the same species — Circumstances favorable and
unfavorable to the results of 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 small area, and to naturalization — Action of
Natural Selection, through Divergence of Character, and Extinction, on
the descendants from a common parent — Explains the grouping of all
organic beings — Advance in organization — Low forms preserved —
Convergence of character — Indefinite multiplication of species — Sum-
mary 120
CHAPTER V
LAWS OF VARIATION
Effects of changed conditions — Use and disuse, combined with natural
selection; organs of flight and of vision — Acclimatization — Correlated
CONTENTS
5
Variation — Compensation and economy of growth — False correlations
— Multiple, rudimentary, and lowly organized 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 —
Reversions to long-lost characters — Summary 190
CHAPTER VI
DIFFICULTIES OF THE THEORY
Difficulties of the theory of descent with modification — 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 — Modes 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 233
CHAPTER VII
MISCELLANEOUS OBJECTIONS TO THE THEORY OF
NATURAL SELECTION
Longevity — Modifications not necessarily simultaneous — Modifications ap-
parently of no direct service — Progressive development — Characters of
small functional importance, the most constant — Supposed incompetence
of natural selection to account for the incipient stages of useful struc-
tures— Causes which interfere with the acquisition through natural
selection of useful structures — Gradations of structure with changed
functions — "Widely different organs in members of the same class, de-
veloped from one and the same source — Reasons for disbelieving in
gTeat and abrupt modifications . . .... 288
6
CONTENTS
CHAPTER VIII
INSTINCT
Instincts comparable with habits, but different in their origin — Instincts
graduated — Aphides and ants — Instincts variable — Domestic instincts,
their origin — Natural instincts of the cuckoo, molothrus, ostrich, and
parasitic bees — Slave-making ants — Hive-bee, its cell-making instinct —
Changes of instinct and structure not necessarily simultaneous — Diffi-
culties of the theory of the Natural Selection of instincts — Neuter or
sterile insects — Summary 346
ADDITIONS AND CORRECTIONS
TO THE SIXTH EDITION
Numerous small corrections have been made in the
last and present editions on various subjects, according
as the evidence has become somewhat stronger or weaker.
The more important corrections and some additions in
the present volume are tabulated on the following pages,
for the convenience of those interested in the subject,
and who possess the fifth edition. The second edition
was little more than a reprint of the first. The third
edition was largely corrected and added to, and the
fourth and fifth still more largely. As copies of the
present work will be sent abroad, it may be of use if
I specify the state of the foreign editions. The third
French and second German editions were from the third
English, with some few of the additions given in the
fourth edition. A new fourth French edition has been
translated by Colonel Moulinie; of which the first half
is from the fifth English, and the latter half from the
present edition. A third German edition, under the super-
intendence of Professor Victor Cams, was from the fourth
English edition; a fifth is now preparing by the same
author from the present volume. The second American
edition was from the English second, with a few of the
(7)
8
ADDITIONS AND CORRECTIONS
additions given in the third; and a third American edi-
tion has been printed from the fifth English edition.
The Italian is from the third, the Dutch and three
Eussian editions from the second English edition, and
the Swedish from the fifth English edition.
Fifth
Sixth
Edition.
Edition.
Page
Page
100 v.
i. 130
158
182
220
247
225
251
230
259
Chief Additions and Corrections.
Influence of fortuitous destruction on natural selection.
On the convergence of specific forms.
Account of the Ground- Woodpecker of La Plata modified.
On the modification of the eye.
Transitions through the acceleration or retardation of the
period of reproduction.
231 260 The account of the electric organ of fishes added to.
233 263 Analogical resemblance between the eyes of Cephalopoda
and Vertebrates.
234 265 Claparede on the analogical resemblance of the hair-
claspers of the Acaridse.
248 280 The probable use of the rattle to the Rattlesnake.
248 281 Helmholtz on the imperfection of the human eye.
255 288 The first part of this new chapter consists of portions, in
a much modified state, taken from chap. iv. of the former
editions. The latter and larger part is new, and re-
lates chiefly to the supposed incompetency of natural
selection to account for the incipient stages of useful
structures. There is also a discussion on the causes
which prevent in many cases the acquisition through
natural selection of useful structures. Lastly, reasons
are given for disbelieving in great and sudden modifica-
tions. Gradations of character, often accompanied by
changes of function, are likewise here incidentally con-
sidered.
268 359 The statement with respect to young cuckoos ejecting
their foster-brothers confirmed.
2*70 362 On the cuckoo-like habits of the Molothrus.
307 v. ii. 15 On fertile hybrid moths.
319 29 The discussion on the fertility of hybrids not having been
acquired through natural selection condensed and
modified.
326 33 On the causes of sterility of hybrids, added to and cor-
rected.
ADDITIONS AND CORRECTIONS 9
Fifth
Edition.
Sixth
Edition.
Chief Additions and Corrections.
Page
Page
377 v.
ii. 87
Pyrgoma found in the chalk.
402
114
Extinct forms serving to connect existing groups.
440
156
On earth adhering to the feet of migratory birds.
463
181
On the wide geographical range of a species of Galaxias,
a fresh-water fish.
505
226
Discussion on analogical resemblances, enlarged and
modified.
516
241
Homological structure of the feet of certain marsupial
animals.
518
245
On serial homologies, corrected.
520
246
Mr. E. Ray Lankester on morphology.
521
249
On the asexual reproduction of Chironomus.
541
270
On the origin of rudimentary parts, corrected.
547
279
Recapitulation on the sterility of hybrids, corrected.
552
284
Recapitulation on the absence of fossils beneath the Cam-
brian system, corrected.
568
303
Natural selection not the exclusive agency in the modifica-
tion of species, as always maintained in this work.
572
307
The belief in the separate creation of species generally
held by naturalists, until a recent period.
"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 interpositions
of Divine power, exerted in each particular case, but by the establishment of
general laws." Whewell: Bridgewater Treatise
"The only distinct meaning of the word 'natural' is stated, fixed, or settled;
since what is natural as much requires and presupposes an intelligent agent to
render it so, i.e., to effect it continually or at stated times, as what is super-
natural or miraculous does to effect it for once."
Butler : Analogy of Revealed Religion
"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 endeavor an endless prog-
ress or proficience in both." Bacon: Advancement of Learning
Down, Beckenham, Kent,
First Edition, November 2/f., 1859
Sixth Edition, January, 187$
10
A HISTORICAL SKETCH OF THE PROGRESS OF
OPINION ON THE ORIGIN OF SPECIES
PREVIOUSLY TO THE PUBLICATION OF THE FIRST EDITION OF
THIS WORK
I will here give a brief sketch of the progress of
opinion on the Origin of Species. Until recently the
great majority of naturalists believed that species were
immutable productions, and had been separately created.
This view has been ably maintained by many authors.
Some few naturalists, on the other hand, have believed
that species undergo modification, and that the existing
forms of life are the descendants by true generation of
pre-existing forms. Passing over allusions to the subject
in the classical writers,1 the first author who in modern
1 Aristotle, in his "Physical Auscultationes" (lib. 2, cap. 8, s. 2) after re-
marking that rain does not fall in order to make the corn grow, any more than
it falls to spoil the farmer's corn when threshed out of doors, applies the same
argument to organization; and adds (as translated by Mr. Clair G-rece, who
first pointed out the passage to me), "So what hinders the different parts [of
the body] from having this merely accidental relation in nature? as the teeth,
for example, grow by necessity, the front ones sharp, adapted for dividing, and
the grinders flat, and serviceable for masticating the food ; since they were not
made for the sake of this, but it was the result of accident. And in like
manner as to the other parts in which there appears to exist an adaptation to
an end. Wheresoever, therefore, all things together (that is, all the parts of
one whole) happened like as if they were made for the sake of something, these
were preserved, having been appropriately constituted by an internal spon-
taneity ; and whatsoever things were not thus constituted, perished, and still
perish." We here see the principle of natural selection shadowed forth, but
how little Aristotle fully comprehended the principle is shown by his remarks
on the formation of the teeth.
(ii)
12
HISTORICAL SKETCH
times has treated it in a scientific spirit was Buffon.
But as his opinions fluctuated greatly at different peri-
ods, and as he does not enter on the causes or means
of the transformation of species, I need not here enter
on details.
Lamarck was the first man whose conclusions on the
subject excited much attention. This justly-celebrated
naturalist first published his views in 1801; he much
enlarged them in 1809 in his 4 4 Philosophic Zoologique,"
and subsequently, in 1815, in the Introduction to his
"Hist. Nat. des Animaux sans Verte*bres." In these
works he upholds the doctrine that all species, includ-
ing man, are descended from other species. He first did
the eminent service of arousing attention to the prob-
ability of all change in the organic, as well as in the
inorganic world, being the result of law, and not of
miraculous interposition. Lamarck seems to have been
chiefly led to his conclusion on the gradual change of
species, by the difficulty of distinguishing species and
varieties, by the almost perfect gradation of forms in
certain groups, and by the analogy of domestic produc-
tions. With respect to the means of modification, he
attributed something to the direct action of the physical
conditions of life, something to the crossing of already
existing forms, and much to use and disuse, that is, to
the effects of habit. To this latter agency he seems
to attribute all the beautiful adaptations in nature; —
such as the long neck of the giraffe for browsing on
the branches of trees. But he likewise believed in a
law of progressive development; and as all the forms
of life thus tend to progress, in order to account for
the existence at the present day of simple productions
HISTORICAL SKETCH
13
he maintains that such forms are now spontaneously
generated. 1
Geoffroy Saint-Hilaire, as is stated in his "Life,'*
written by his son, suspected, as early as 1795, that
what we call species are various degenerations of the
same type. It was not until 1828 that he published his
conviction that the same forms have not been perpetuated
since the origin of all things. Geoffroy seems to have
relied chiefly on the conditions of life, or the tlmonde
ambiant" as the cause of change. He was cautious in
drawing conclusions, and did not believe that existing
species are now undergoing modification; and, as his son
adds, uC'est done un probleme k r^server entierement
a l'avenir, suppose' me'me que l'avenir doive avoir prise
sur lui."
In 1813, Dr. W. C. Wells read before the Eoyal
Society 4 'An Account of a White female, part of whose
skin resembles that of a Negro"; but his paper was not
published until his famous "Two Essays upon Dew and
Single Vision" appeared in 1818. In this paper he dis-
1 I have taken the date of the first publication of Lamarck from Isid.
Geoffroy Sainl-Hilaire's ("Hist. Nat. Generate," torn. ii. p. 405, 1859) excel-
lent history of opinion on this subject. In this work a full account is given of
Buffon's conclusions on the same subject. It is curious how largely my grand-
father, Dr. Erasmus Darwin, anticipated the views and erroneous grounds of
opinion of Lamarck in his "Zoonomia" (vol. i. pp. 500-510), published iu 1794.
According to Isid. Geoffroy there is no doubt that Goethe was an extreme
partisan of similar views, as shown in the Introduction to a work written in
1794 and 1795, but not published till long afterward: he has pointedly remarked
("Goethe als Naturforscher, " von Dr. Karl Meding, s. 34) that the future ques-
tion for naturalists will be how, for instance, cattle got their horns, and not for
what they are used. It is rather a singular instance of the manner in which
similar views arise at about the same time that Goethe in Germany, Dr. Darwin
in England, and Geoffroy Saint-Hilaire (as we shall immediately see) in France,
came to the same conclusion on the origin of species, in the years 1794-95.
14 HISTORICAL SKETCH
tinctly recognizes the principle of natural selection, and
this is the first recognition which has been indicated;
but he applies it only to the races of man, and to cer-
tain characters alone. After remarking that negroes and
mulattoes enjoy an immunity from certain tropical dis-
eases, he observes, first, that all animals tend to vary in
gome degree, and, secondly, that agriculturists improve
their domesticated animals by selection; and then, he
adds, but what is done in this iatter case "by art,
seems to be done with equal efficacy, though more
slowly, by nature, in the formation of varieties of man-
kind, fitted for the country which they inhabit. Of the
accidental varieties of man, which would occur among
the first few and scattered inhabitants of the middle re-
gions of Africa, some one would be better fitted than
the others to bear the diseases of the country. This
race would consequently multiply, while the others would
decrease; not only from their inability to sustain the at-
tacks of disease, but from their incapacity of contend-
ing with their more vigorous neighbors. The color of
this vigorous race I take for granted, from what has
been already said, would be dark. But the same dis-
position to form varieties still existing, a darker and a
darker race would in the course of time occur: and as
the darkest would be the best fitted for the climate, this
would at length become the most prevalent, if not the
only race, in the particular country in which it had orig-
inated." He then extends these same views to the white
inhabitants of colder climates. I am indebted to Mr.
Kowley, of the United States, for having called my
attention, through Mr. Brace, to the above passage "in
Dr. Wells' work.
HISTORICAL SKETCH
15
The Hon. and Rev. W. Herbert, afterward Dean of
Manchester, in the fourth volume of the "Horticultural
Transactions," 1822, and in his work on the "Amarylli-
daceae" (1837, pages 19, 339), declares that "horticultural
experiments have established, beyond the possibility of
refutation, that botanical species are only a higher and
more permanent class of varieties." He extends the same
view to animals. The Dean believes that single species
of each genus were created in an originally highly plastic
condition, and that these have produced, chiefly by in-
tercrossing, but likewise by variation, all our existing
species.
In 1826 Professor Grant, in the concluding paragraph
in his well-known paper ("Edinburgh Philosophical Jour-
nal," vol. xiv. page 283) on the Spongilla, clearly de-
clares his belief that species are descended from other
species, and that they become improved in the course of
modification. This same view was given in his 55th
Lecture, published in the "Lancet" in 1834.
In 1831 Mr. Patrick Matthew published his work on
"Naval Timber and Arboriculture," in which he gives
precisely the same view on the origin of species as that
(presently to be alluded to) propounded by Mr. Wallace
and myself in the "Linnean Journal," and as that en-
larged in the present volume. Unfortunately the view
was given by Mr. Matthew very briefly in scattered pas-
sages in an Appendix to a work on a different subject, so
that it remained unnoticed until Mr. Matthew himself
drew attention to it in the "Gardener's Chronicle," on
April 7, 1860. The differences of Mr. Matthew's view
from mine are not of much importance: he seems to con-
sider that the world was nearly depopulated at successive
16
HISTORICAL SKETCH
periods, and then restocked; and he gives as an alterna-
tive, that new forms may be generated "without the pres-
ence of any mould or germ of former aggregates." I am
not sure that I understand some passages; but it seems
that he attributes much influence to the direct action of
the conditions of life. He clearly saw, however, the full
force of the principle of natural selection.
The celebrated geologist and naturalist, Yon Buch, in
his excellent "Description Physique des Isles Canaries"
(1836, page 147), clearly expresses his belief that varieties
slowly become changed into permanent species, which are
no longer capable of intercrossing.
Eafinesque, in his "New Flora of North America,"
published in 1836, wrote (page 6) as follows — "All species
might have been varieties once, and many varieties are
gradually becoming species by assuming constant and
peculiar characters"; but further on (page 18) he adds,
"except the original types or ancestors of the genus."
In 1843-44 Professor Haldeman ("Boston Journal of
Nat. Hist. United States," vol. iv. page 468) has ably
given the arguments for and against the hypothesis of
the development and modification of species: he seems
to lean toward the side of change.
The "Vestiges of Creation" appeared in 1844. In the
tenth and much improved edition (1853) the anonymous
author says (page 155) — "The proposition determined on
after much consideration is, that the several series of
animated beings, from the simplest and oldest up to the
highest and most recent, are, under the providence of
God, the results, first, of an impulse which has been
imparted to the forms of life, advancing them, in definite
times, by generation, through grades of organization ter-
HISTORICAL SKETCH
17
urinating in the highest dicotyledons and vertebrata, these
grades being few in number, and generally marked by
intervals of organic character, which we find to be a
practical difficulty in ascertaining affinities; second, of
another impulse connected with the vital forces, tending,
in the course of generations, to modify organic structures
in accordance with external circumstances, as food, the
nature of the habitat, and the meteoric agencies, these
being the Adaptations' of the natural theologian." The
author apparently believes that organization progresses by
sudden leaps, but that the effects produced by the condi-
tions of life are gradual. He argues with much force on
general grounds that species are not immutable produc-
tions. But I cannot see how the two supposed "impulses"
account in a scientific sense for the numerous and beauti-
ful coadaptations which we see throughout nature; I can-
not 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 this country in calling attention
to the subject, in removing prejudice, and in thus prepar-
ing the ground for the reception of analogous views.
In 1846 the veteran geologist M. J. d'Omalius d'Halloy
published in an excellent though short paper (' 4 Bulletins
de l'Acad. Roy. Bruxelles," torn. xiii. page 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 promulgated this
opinion in 1831.
18 HISTORICAL SKETCH
Professor Owen, in 1849 ("Nature of Limbs/' page 86),
wrote as follows: "The archetypal idea was manifested in
the flesh under diverse such modifications, upon this
planet, long prior to the existence of those animal species
that actually exemplify it. To what natural laws or sec-
ondary causes the orderly succession and progression of
such organic phenomena may have been committed, we,
as yet, are ignorant." In his Address to the British
Association, in 1858, he speaks (page li.) of "the axiom
of the continuous operation of creative power, or of the
ordained becoming of living things." Further on (page
xc), after referring to geographical distribution, he adds,
"These phenomena shake our confidence in the conclusion
that the Apteryx of New Zealand and the Eed Grouse
of England were distinct creations in and for those islands
respectively. Always, also, it may be well to bear in
mind that by the word 'creation' the zoologist means 'a
process he knows not what.' " He amplifies this idea by
adding that when such cases as that of the Eed Grouse
are "enumerated by the zoologist as evidence of distinct
creation of the bird in and for such islands, he chiefly
expresses that he knows not how the Red Grouse came
to be there, and there exclusively; signifying also, by
this mode of expressing such ignorance, his belief that
both the bird and the islands owed their origin to a great
first Creative Cause." If we interpret these sentences
given in the same Address, one by the other, it ap-
pears that this eminent philosopher felt in 1858 his
confidence shaken that the Apteryx and the Red
Grouse first appeared in their respective homes, "he
knew not how," or by some process "he knew not
what."
HISTORICAL SKETCH
19
This Address was delivered after the papers by Mr.
Wallace and myself on the Origin of Species, presently
to be referred to, had been read before the Linnean So-
ciety. When the first edition of this work was published,
I was so completely deceived, as were many others, by
such expressions as "the continuous operation of creative
power," that I included Professor Owen with other pale-
ontologists as being firmly convinced of the immutability
of species; but it appears ("Anat. of Vertebrates," vol.
iii. page 796) that this was on my part a preposterous
error. In the last edition of this work I inferred, and
the inference still seems to me perfectly just, from a pas-
sage beginning with the words "no doubt the type-form,"
etc. (Ibid. vol. i. page xxxv.), that Professor Owen ad-
mitted that natural selection may have done something in
the formation of a new species; but this it appears (Ibid,
vol. iii. page 798) is inaccurate and without evidence. I
also gave some extracts from a corr^ "londence between
Professor Owen and the editor of the "London Review,"
from which it appeared manifest to the editor as well as
to myself that Professor Owen claimed to have promul-
gated the theory of natural selection before I had done
so: and I expressed my surprise and satisfaction at this
announcement; but as far as it is possible to understand
certain recently published passages (Ibid. vol. iii. page
798) I have either partially or wholly again fallen into
error. It is consolatory to me that others find Professor
Owen's controversial writings as difficult to understand
and to reconcile with each other, as I do. As far as the
mere enunciation of the principle of natural selection is
concerned, it is quite immaterial whether or not Professor
Owen preceded me, for both of us, as shown in this his-
20
HISTORICAL SKETCH
torical sketch, were long ago preceded by Dr. Wells and
Mr. Matthews.
M. Isidore Geoff roy Saint- Hilaire, in his lectures de-
livered in 1850 (of which a Kesume appeared in the
"Kevue et Mag. de Zoolog.," Jan. 1851), briefly gives
his reason 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 re-
sume, V observation des animaux sauvages demontre deja
la variabilite limitee des especes. Les experiences sur les
animaux sauvages devenus domestiques, et sur les ani-
maux domestiques redevenus sauvages, la demontrent
plus clairement encore. Ces memes experiences prou-
vent, de plus, que les differences produites peuvent etre
de valeur generique." In his "Hist. Nat. Generale" (torn,
ii. page 430, 1859) he amplifies analogous conclusions.
From a circular lately issued it appears that Dr.
Freke, in 1851 ("Dublin Medical Press," page 322),
propounded the doctrine that all organic beings have
descended from one primordial form. His grounds of
belief and treatment of the subject are wholly different
from mine; but as Dr. Freke has now (1861) published
his Essay on the "Origin of Species by means of Or-
ganic Affinity," the difficult attempt to give any idea
of his views would be superfluous on my part.
Mr. Herbert Spencer, in an Essay (originally pub-
lished in the "Leader," March, 1852, and republished
in his "Essays," in 1858), has contrasted the theories of
the Creation and the Development of organic beings with
remarkable skill and force. He argues from the analogy
of domestic productions, from the changes which the em-
HISTORICAL SKETCH
21
bryos of many species undergo, from the difficulty of dis-
tinguishing species and varieties, and from the principle
of general gradation, that species have been modified; and
he attributes the modification to the change of circum-
stances. The author (1855) has also treated Psychology
on the principle of the necessary acquirement of each
mental power and capacity by gradation.
In 1852 M. ISTaudin, a distinguished botanist, expressly
stated, in an admirable paper on the Origin of Species
("Kevue Horticole, " page 102; since partly republished
in the "Nouvelles Archives du Museum," torn. i. page
171), his belief that species are formed in an analogous
manner as varieties are under cultivation; and the latter
process he attributes to man's power of selection. But
he does not show how selection acts under nature. He
believes, like Dean Herbert, that species, when nascent,
were more plastic than at present. He lays weight on
what he calls the principle of finality, "puissance mys-
terieuse, indeterminee; fatalite pour les uns; pour les au-
tres, volunte providentielle, dont Taction incessante sur
les 3tres vivants determine, a toutes les epoques de l'ex-
istence du monde, la forme, le volume, et la duree de
chacon d'eux, en raison de sa destine dans l'ordre de
choses dont il fait partie. C'est cette puissance qui har-
monise chaque membre k 1' ensemble, en l'appropriant a
la fonction qu'il doit remplir dans l'organisme general
de la nature, fonction qui est pour lui sa raison d'etre." 1
1 From references in Bronn's "Untersuchungen iiber die Entwickelungs-
G-esetze, " it appears that the celebrated botanist and paleontologist Unger pub-
lished, in 1852, his belief that species undergo development and modification.
Dalton, likewise, in Pander and Dalton's work on Fossil Sloths, expressed, in
1821, a similar belief. Similar views have, as is well known, been maintained
22
HISTORICAL SKETCH
In 1853 a celebrated geologist, Count Keyserling ("Bul-
letin de la Soc. Geol." 2d Ser., torn. x. page 357),
suggested that as new diseases, supposed to have been
caused by some miasma, have arisen and spread over the
world, so at certain periods the germs of existing species
may have been chemically affected by circumambient
molecules of a particular nature, and thus have given
rise to new forms.
In this same year, 1853, Dr. Schaaffhausen published
an excellent pamphlet ("Verhand. des Naturhist. Yereins
der Preuss. Eheinlands," etc.) in which he maintains the
development of organic forms on the earth. He infers
that many species have kept true for long periods,
whereas a few have become modified. The distinction
of species he explains by the destruction of intermediate
graduated forms. "Thus living plants and animals are
not separated from the extinct by new creations, but
are to be regarded as their descendants through contin-
ued reproduction."
A well-known French botanist, M. Lecoq, writes in
1854 ("Etudes sur Geograph. Bot.," torn. i. page 250),
"On voit que nos recherches sur la fixite ou la varia-
tion de l'espece, nous conduisent directement aux idees
emises, par deux hommes justement celebres, Geoffroy
Saint-Hilaire et Goethe." Some other passages scattered
by Oken in his mystical "Natur-Philosophie. " From other references in
Godron's work "Sur TEspece, " it seems that Bory St. Vincent, Burdach,
Poiret, and Fries, have all admitted that new species are continually being
produced.
I may add that of the thirty-four authors named in this Historical Sketch
who believe in the modification of species, or at least disbelieve in separate acts
of creation, twenty-seven have written on special branches of natural history
or geology.
HISTORICAL SKETCH
23
through M. Lecoq's large work, make it a little doubt-
ful how far he extends his views on the modification
of species.
The "Philosophy of Creation" has been treated in
a masterly manner by the Rev. Baden Powell, in his
"Essays on the Unity of Worlds," 1855. Nothing can
be more striking than the manner in which he shows
that the introduction of new species is "a regular, not
a casual phenomenon," or, as Sir John Herschel ex-
presses it, "a natural in contradistinction to a miracu-
lous process."
The third volume of the "Journal of the Linnean
Society" contains papers, read July 1st, 1858, by Mr.
Wallace and myself, in which, as stated in the intro-
ductory remarks to this volume, the theory of Natural
Selection is promulgated by Mr. W allace with admirable
force and clearness.
Yon Baer, toward whom all zoologists feel so pro-
found a respect, expressed about the year 1859 (see Prof.
Rudolph Wagner, "Zoologisch-Anthropologische Unter-
suchungen, " 1861, s. 51) his conviction, chiefly grounded
on the laws of geographical distribution, that forms now
perfectly distinct have descended from a single parent-
form.
In June, 1859, Professor Huxley gave a lecture before
the Ro}^al Institution on the "Persistent Types of Animal
Life." Referring to such cases, he remarks, "It is diffi-
cult to comprehend the meaning of such facts as these,
if we suppose that each species of animal and plant, or
each great type of organization, 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
24
HISTORICAL SKETCH
that such an assumption is as unsupported bj tradition
or revelation as it is opposed to the general analogy of
nature. If, on the other hand, we view 'Persistent
Types' in relation to that hypothesis which supposes
the species living at any time to be the result of the
gradual modification of pre-existing species — a hypothesis
which, ' though unproven, and sadly damaged by some of
its supporters, is yet the only one to which physiology
lends any countenance — their existence would seem to
show that the amount of modification which living be-
ings have undergone during geological time is but very
small in relation to the whole series of changes which
they have suffered."
In December, 1859, Dr. Hooker published his "Intro-
duction to the Australian Flora." In the first part of
this great work he admits the truth of the descent and
modification of species, and supports this doctrine by
many original observations.
The first edition of this work was published on No-
vember 24, 1859, and the second edition on January 7,
1860.
THE ORIGIN OF SPECIES
INTRODUCTION
WHEN on board H.M.S. "Beagle," as naturalist, I
was much struck with certain facts in the dis-
tribution of the organic beings inhabiting South
America, and in the geological relations of the present to
the past inhabitants of that continent. These facts, as will
be seen in the later chapters of this volume, seemed to
throw some light on the origin of species — that mystery of
mysteries, as it has been called by one of our greatest phi-
losophers. On my return home, it occurred to me, in 1837,
that something might perhaps be made out on this ques-
tion 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 sub-
ject, and drew up some short notes; these I enlarged in
1844 into a sketch of the conclusions which then seemed
to me probable: from that period to the present day
I have steadily pursued the same object. I hope that I
may be excused for entering on these personal details,
as I give them to show that I have not been hasty in
coming to a decision.
My work is now (1859) nearly finished; but as it will
take me many more years to complete it, and as my
health is far from strong, I have been urged to publish
— Science— 2 (25)
26
THE ORIGIN OF SPECIES
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. In 1858 he sent me a memoir on this
subject, with a request that I would forward it to Sir
Charles Lyell, who sent it to the Linnean Society, and it
is published in the third volume of the Journal of that
Society. Sir C. Lyell and Dr. Hooker, who both knew
of my work — the latter having read my sketch of 1844 —
honored 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 authori-
ties for my several statements; and I must trust to the
reader reposing some confidence in my accuracy. No doubt
errors will have crept in, though I hope I have always
been cautious in trusting to good authorities alone. I can
here give only the general conclusions at which I have ar-
rived, with a few facts in illustration, but which, I hope,
in most cases will suffice. No one can feel more sensible
than I do of the necessity of hereafter publishing in
detail all the facts, with references, on which my 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 apparently leading to conclu-
sions directly opposite to those at which 1 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 is here impossible.
INTRODUCTION
27
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
species had not been independently created, but had
descended, like varieties, from other species. Neverthe-
less, such a conclusion, even if well founded, would be
unsatisfactory, until it could be shown how the innumer-
able species inhabiting this world have been modified, so
as to acquire that perfection of structure and coadaptation
which justly excites our admiration. Naturalists contin-
ually refer to external conditions, such as climate, food,
etc., as the only possible cause of variation. In one
limited sense, as we shall hereafter see, this may be truej
but it is preposterous to attribute to mere external con-
ditions the structure, for instance, of the woodpecker,
with its feet, tail, beak and tongue so admirably adapted
to catch insects under the bark of tree3. In the case of
the mistletoe, which draws its nourishment from certain
trees, which has seeds that must be transported by cer-
tain birds, and which has flowers with separate sexes
absolutely requiring the agency of certain insects to bring
pollen from one flower to the other, it is equally prepos-
28
THE ORIGIN OF SPECIES
terous to account for the structure of this parasite, with
its relations to several distinct organic beings, by the
effects of external conditions, or of habit, or of the voli-
tion of the plant itself.
It is, therefore, of the highest importance to gain a
clear insight into the means of modification and coadap-
tation. At the commencement of my observations it
seemed to me probable that a careful study of domesti-
cated animals and of cultivated plants would offer the
best chance of making out this obscure problem. Nor
have I been disappointed; in this and in all other per-
plexing cases I have invariably found that our knowledge,
imperfect though it be, of variation under domestication,
afforded the best and safest clew. I may venture to ex-
press my conviction of the high value of such studies,
although they have been very commonly neglected by
naturalists.
From these considerations, I shall devote the first
chapter of this Abstract to Variation under 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 spe-
cies in a state of nature; but I shall, unfortunately, be
compelled to treat this subject far too briefly, as it can
be treated properly only by giving long catalogues of
facts. We shall, however, be enabled to discuss what
circumstances are most favorable to variation. In the
next chapter the Struggle for Existence among all organic
beings throughout the world, which inevitably follows
from the high geometrical ratio of their increase, will be
INTRODUCTION
29
considered- 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 possi-
bly 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 profita-
ble to itself, under the complex and sometimes varying
conditions of life, will have a better chance of surviving,
and thus be naturally selected. From the strong principle
of inheritance, any selected variety will tend to propagate
its new and modified form.
This fundamental subject of Natural Selection will be
treated at some length in the fourth chapter; and we
shall then see how Natural Selection almost inevitably
causes much Extinction of the less improved forms of
life, and leads to what I have called Divergence of Char-
acter. In the next chapter I shall discuss the complex
and little known laws of variation. In the five succeed-
ing chapters, the most apparent and gravest difficulties
in accepting the theory will be given; namely, first, the
difficulties of transitions, or how a simple being or a sim-
ple organ can be changed and perfected into a highly
developed being or into an elaborately constructed organ;
secondly, the subject of Instinct, or the mental powers of
animals; thirdly, Hybridism, or the infertility of species
and the fertility of varieties when intercrossed; and
fourthly, the imperfection of the Geological Record. In
the next chapter I shall consider the geological succes-
sion of organic beings throughout time; in the twelfth
and thirteenth, their geographical distribution throughout
space; in the fourteenth, their classification or mutual
affinities, both when mature and in an embryonic con-
so
THE ORIGIN OF SPECIES
dition. In the last chapter I shall give a brief recapitu-
lation of the whole work, and a few concluding remarks.
No one ought to feel surprise at much remaining as
yet unexplained in regard to the origin of species and
varieties, if he make due allowance for our profound
ignorance in regard to the mutual relations of the many
beings which live around us. Who can explain why one
species ranges widely and is very numerous, and why
another allied species has a narrow range and is rare ?
Yet these relations are of the highest importance, for
they determine the present welfare and, as I believe, the
future success and modification of every inhabitant of this
world. Still less do we know of the mutual relations of
the innumerable inhabitants of the world during the many
past geological epochs in its history. Although much
remains obscure, and will long remain obscure, I can
entertain no doubt, after the most deliberate study and
dispassionate judgment of which I am capable, that the
view which most naturalists until recently entertained and
which I formerly entertained — namely, that each species
has been independently created — is erroneous. I am
fully convinced that species are not immutable; but that
those belonging to what are called the same genera are
lineal descendants of some other and generally extinct
species, in the same manner as the acknowledged varieties
of any one species are the descendants of that species.
Furthermore, I am convinced that Natural Selection has
been the most important, but not the exclusive, means of
modification.
VARIATION UNDER DOMESTICATION
CHAPTER I
VARIATION UNDER DOMESTICATION
y Causes of Variability — Effects of Habit and the use or disuse of Parts —
Correlated Variation — Inheritance — Character of Domestic Varieties —
Difficulty of distinguishing between Varieties and Species — Origin of
Domestic Varieties from one or more Species — Domestic Pigeons,
their Differences and Origin — Principles of Selection anciently fol-
lowed, their Effects — Methodical and Unconscious Selection — Un-
known Origin of our Domestic Productions — Circumstances favora-
ble to Man's power of Selection
Causes of Variability
WHEN we compare the individuals of the same
variety or sub- variety of our older cultivated
plants and animals, one of the first points
which strikes us is, that they generally differ more from
each other than do the individuals of any one species or
variety in a state of nature. And if 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, we are driven
to conclude that this great variability is due to our
domestic productions having been raised under conditions
of life not so uniform as, and somewhat different from,
those to which the parent species had been exposed
under nature. There is, also, some probability in the
view propounded by Andrew Knight, that this variability
may be partly connected with excess of food. It seems
clear that organic beings must be exposed during several
32 THE ORIGLS OF SPECIES
generations to new conditions to cause any great amount
of variation; and that, when the organization has once
begun to vary, it generally continues varying for many
generations. No case is on record of a variable organism
ceasing to vary under cultivation. Our oldest cultivated
plants, such as wheat, still yield new varieties: our oldest
domesticated animals are still capable of rapid improve-
ment or modification.
As far as I am able to judge, after long attending to
the subject, the conditions of life appear to act in two
ways — directly on the whole organization or on certain
parts alone, and indirectly by affecting the reproductive
system. TTith respect to the direct action, we must bear
in mind that in every case, as Professor Weismann has
lately insisted, and as I have incidentally shown in my
work on '* Variation under Domestication," there are two
factors; namely, the nature of the organism, and the
nature of the conditions. The former seems to be much
the more important; for nearly similar variations some-
times arise under, as far as we can judge, dissimilar con-
ditions: and, on the other hand, dissimilar variations
arise under conditions which appear to be nearly uni-
form. The effects on the offspring are either definite or
indefinite. They may be considered as definite when all
or nearly all the offspring of individuals exposed to cer-
tain conditions during several generations are modified in
the same manner. It is extremely difficult to come to
any conclusion in regard to the extent of the changes
which nave been thus definitely induced. There can,
however, be little doubt about many slight changes —
such as size from the amount of food, color from the
nature of the food, thickness of the skin and hair from
VARIATION UNDER DOMESTICATION
33
climate, etc. Each of the endless variations which we
see in the plumage of our fowls must have had some
efficient cause; and if the same cause were to act uni-
formly during a long series of generations on many
individuals, all probably would be modified in the same
manner. Such facts as the complex and extraordinary
outgrowths which variably follow from the insertion of a
minute drop of poison by a gall-producing insect, show
us what singular modifications might result in the case of
plants from a chemical change in the nature of the sap.
Indefinite variability is a much more common result
of changed conditions than definite variability, and has
probably played a more important part in the formation
of our domestic races. ^.We see indefinite variability in
the endless slight, peculiarities which distinguish the
individuals of the same species, and which cannot be
accounted for by inheritance from either parent or from
some more remote ancestor. Even strongly-marked differ-
ences occasionally appear in the young of the same litter,
and in seedlings from the same seed-capsule. At long
intervals of time, out of millions of individuals reared in
the same country and fed on nearly the same food,
deviations of structure so strongly pronounced as to de-
serve to be called monstrosities arise; but monstrosities
cannot be separated by any distinct line from slighter
variations. All such changes of structure, whether ex-
tremely slight or strongly marked, which appear among
many individuals living together, may be considered as
the indefinite effects of the conditions of life on each
individual organism, in nearly the same manner as the
chill affects different men in an indefinite manner, ac-
cording to their state of body or constitution, causing
84
THE ORIGIN OF SPECIES
coughs or colds, rheumatism, or inflammation of various
organs.
With respect to what I have called the indirect action
of changed conditions; namely, through the reproductive
system being affected; we may infer that variability is
thus induced, partly from the fact of this system being
extremely sensitive to any change in the conditions, and
partly from the similarity, as Kolreuter and others have
remarked, between the variability which follows from the
crossing of distinct species, and that which may be ob-
served with plants and animals when reared under new
or unnatural conditions. Many facts clearly show how
eminently susceptible the reproductive system is to very
slight changes in the surrounding conditions. Nothing is
more easy than to tame an animal, and few things more
difficult than to get it to breed freely under confinement,
even when the male and female unite. How many ani-
mals there are which will not breed, though kept in an
almost free state in their native country! This is gen-
erally, but erroneously, attributed to vitiated instincts.
Many cultivated plants display the utmost vigor, and
yet rarely or never seed! In some few cases it has been
discovered that a very trifling change, such as a little
more or less water at some particular period of growth,
will determine whether or not a plant will produce seeds.
I cannot here" give the details which I have collected
and elsewhere published on this curious subject; but to
show how singular the laws are which determine the
reproduction of animals under confinement, I may men-
tion that carnivorous animals, even from the tropics, breed
in this country pretty freely under confinement, with the
exception of the plantigrades or bear family, which
VARIATION UNDER DOMESTICATION
35
seldom produce young; whereas carnivorous birds, with
the rarest exceptions, hardly ever lay fertile eggs. Many
exotic plants have pollen utterly worthless, in the same
condition as in the most sterile hybrids. When, on the
one hand, we see domesticated animals and plants, though
often weak and sickly, breeding freely under confine-
ment; and when, on the other hand, we see individuals,
though taken young from a state of nature perfectly
tamed, long-lived and healthy (of which I could give
numerous instances), yet having their reproductive sys-
tem so seriously affected by unperceived causes as to fail
to act, we need not be surprised at this system, when it
does act under confinement, acting irregularly, and pro-
ducing offspring somewhat unlike their parents. I may
add that as some organisms breed freely under the most
unnatural conditions (for instance, rabbits and ferrets
kept in hutches), showing that their reproductive organs
are not easily affected; so will some animals and plants
withstand domestication or cultivation, and vary very
slightly — perhaps hardly more than in a state of nature.
Some naturalists have maintained that all variations
are connected with the act of sexual reproduction; but
this is certainly an error; for I have given in another
work a long list of "sporting plants," as they are called
by gardeners; — that is, of plants which have suddenly
produced a single bud with a new and sometimes widely
different character from that of the other buds on the
same plant. These bud variations, as they may be
named, can be propagated by grafts, offsets, etc., and
sometimes by seed. They occur rarely under nature, but *
are far from rare under culture. As a single bud out of
the many thousands, produced year after year on the
36
THE ORIGIX OF SPECIES
same tree under uniform conditions, has been known sud-
denly to assume a new character; and as buds on distinct
trees, growing under different conditions, have sometimes
yielded nearly the same variety — for instance, buds on
peach-trees producing nectarines, and buds on common
roses producing 'moss-roses — we clearly see that the nature
of the conditions is of subordinate importance in com-
parison with the nature of the organism in determining
each particular form of variation; — perhaps of not more
importance than the nature of the spark, by which a
mass of combustible matter is ignited, has in determining
the nature of the flames.
Effects of Habit and of the Use or Disuse of Parts;
Correlated Variation; Inheritance
Changed habits produce an inherited effect, as in the
period of the flowering of plants when transported from
one climate to another. With animals the increased use
or disuse of parts has had a more marked influence;
thus I find in the domestic duck that the bones of the
wing weigh less and the bones of the leg more, in pro-
portion to the whole skeleton, than do the same bones in
the wild duck; and this change may be safely attributed
to the domestic duck flying much less, and walking
more, than its wild parents. The great and inherited
development of the udders in cows and goats in countries
where they are habitually milked, in comparison with
these organs in other countries, is probably another in-
stance of the effects of use. Not one of our domestic
animals can be named which has not in some country
drooping ears; and tne view which has. been suggested
that the drooping is due to disuse of the muscles of the
VARIATION UNDER DOMESTICATION 37
ear, from the animals being seldom much alarmed, seems
probable.
Many laws regulate variation, some few of which can
be dimly seen, and will hereafter be briefly discussed. I
will here only allude to what may be called correlated
variation. Important changes in1 the embryo or larva
will probably entail changes in the mature animal. In
monstrosities, the correlations between quite distinct parts
are very curious; and many instances are given in Isidore
Geoff roy St.-Hilaire's great work on this subject. Breeders
believe that long limbs are almost always accompanied
by an elongated head. Some instances of correlation
are quite whimsical: thus cats which are entirely white
and have blue eyes are generally deaf; but it has been
lately stated by Mr. Tait that this is confined to the
males. Color and constitutional peculiarities go together,
of which many remarkable cases could be given among
animals and plants. From facts collected by Heusinger,
it appears that white sheep and pigs are injured by cer-
tain plants, while dark-colored individuals escape: Pro-
fessor Wyman has recently communicated to me a good
illustration of this fact; on asking some farmers in Vir-
ginia how it was that all their pigs were black, they
informed him that the pigs ate the paint-root (Lachnan-
thes), which colored their bones pink, and which caused
the hoofs of all but the black varieties to drop off; and
one of the 4 'crackers" (i.e., Virginia squatters) added,
''we select the black members of a litter for raising, as
they alone have a good chance of living." Hairless dogs
have imperfect teeth; long-haired and -coarse-haired ani-
mals are apt to have, as is asserted, long or many horns;
pigeons with feathered feet have skin between their outer
88
THE ORIGIN OF SPECIES
toes; pigeons with short beaks have small feet, and those
with long beaks large feet. Hence if man goes on select-
ing, and thus augmenting, any peculiarity, he will almost
certainly modify unintentionally other parts of the struc-
ture, owing to the mysterious laws of correlation.
The results of the various, unknown, or but dimly
understood laws of variation are infinitely complex and
diversified. It is well worth while carefully to study the
several treatises on some of our old cultivated plants,
as on the hyacinth, potato, even the dahlia, etc. ; and it
is really surprising to note the endless points of structure
and constitution in which the varieties and sub-varieties
differ slightly from each other. The whole organization
seems to have become plastic, and departs in a slight
degree from that of the parental type.
Any variation which is not inherited is unimportant
for us. But the number and diversity of inheritable
deviations of structure, both those of slight and those
of considerable physiological importance, are endless. Dr.
Prosper Lucas's treatise, in two large volumes, is the
fullest and the best on this subject. No breeder doubts
how strong is the tendency to inheritance; that like pro-
duces like is his fundamental belief: doubts have been
thrown on this principle only by theoretical writers.
When any deviation 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 among individuals, apparently exposed to the
same conditions, any very rare deviation, due to some
extraordinary combination of circumstances, appears in
the parent — say, once among several million individuals
— and it reappears in the child, the mere doctrine of
VARIATION UNDER DOMESTICATION 39
chances almost compels us to attribute its reappearance
to inheritance. Every one must have heard of cases of
albinism, prickly skin, hairy bodies, etc., appearing in
several members of the same family. If strange and rare
deviations of structure are really 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 for the most part
unknown. No one can say why the same peculiarity in
different individuals of the same species, or in different
species, is sometimes inherited and sometimes not so;
why the child often reverts in certain characters to its
grandfather or grandmother or 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 im-
portance to us that peculiarities appearing in the males
of our domestic breeds are often transmitted, either exclu-
sively or in a much greater degree, to the males alone.
A much more important rule, which I think may be
trusted, is that, at whatever period of life a peculiarity
first appears, it tends to reappear in the offspring at a
corresponding age, though sometimes earlier. In many
cases this could not be otherwise; thus the inherited
peculiarities in the horns of cattle could appear only
in. the offspring when nearly mature; peculiarities in the
silkworm are known to appear at the corresponding cater-
pillar or cocoon stage. But hereditary diseases and some
other facts make me believe that the rule has a wider
«0
THE 0 RIG IS OF SPECIES
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 ex-
plaining the laws of embryology. These remarks are of
course confined to the first appearance of the peculiarity,
and not to the primary cause which may have acted on
the ovules or on the male element; in nearly the same
manner as the increased length of the horns in the off-
spring from a short-horned cow by a long-horned bull,
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, grad-
ually but invariably revert in character to their aborigi-
nal 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 endeavored to discover on what
decisive facts the above statement has so often and so
boldly been made. There would be great difficulty in
proving its truth: we may safely coDclude that very
many of the most strongly marked domestic varieties
could not possibly live in a wild state. In many cases
we do not know what the aboriginal stock was, and so
could not tell whether or not nearly perfect reversion
had ensued. It would be necessary, in order to prevent
the effects of intercrossing, that only a single variety
should have been turned loose in its new home. Never-
theless, as our varieties certainly do occasionally revert
in some of their characters to ancestral forms, it seems to
VARIATION UNDER DOMESTICATION
41
me not improbable that if we could succeed iu naturaliz-
ing, or were to cultivate, during many generations, the
several races, for instance, of the cabbage, in very poor
soil (in which case, however, some effect would have to
be attributed to the definite action of the poor soil), that
they would, to a large extent, or even wholly, revert to
the wild aboriginal stock. Whether or not the experi-
ment would succeed is not of great importance for our
line of argument; for by the experiment itself the condi-
tions of life are changed. If it could be shown that our
domestic varieties manifested a strong tendency to rever-
sion— that is, to lose their acquired characters, while kept
under the same conditions, and while kept in a consider-*
able body, so that free intercrossing might check, by
blending together, any slight deviations in their struc-
ture, 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 favor 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 unlimited num-
ber of generations, would be opposed to all experience.
Character of Domestic Varieties; difficulty of distinguishing
between Varieties and Species; origin of Domestic
Varieties from one or more Species
When we look to the hereditary varieties or races of
our domestic animals and plants, and compare them with
closely allied species, we generally perceive in each do-
mestic race, as already remarked, less uniformity of char-
acter than in true species. Domestic races often have a
somewhat monstrous character; by which I mean, that,
THE ORIGIN OF SPECIES
although differing from each other, and from other spe-
cies of the same genus, in several trifling respects, they
often differ in an extreme degree in some one part, both
when compared one with another, and more especially
when compared with the species under nature to which
they are nearest allied. With these exceptions (and with
that of the perfect fertility of varieties when crossed — a
subject hereafter to be discussed), domestic races of the
same species differ from each other in the same manner
as do the closely-allied species of the same genus in a
state of nature, but the differences in most cases are less
in degree. This must be admitted as true, for the do-
mestic races of many animals and plants have been
ranked by some competent judges as the descendants
of aboriginally distinct species, and by other competent
judges as mere varieties. If any well-marked distinction
existed between a domestic race and a species, this source
of doubt would not so perpetually recur. It has often
been stated that domestic races do not differ from each
other in characters of generic value. It can be shown
that this statement is not correct; but naturalists differ
much in determining what characters are of- generic
value; all such valuations being at present empirical.
When it is explained how genera originate under nature,
\t will be seen that we have no right to expect often
to find a generic amount of difference in our domesticated
races.
In attempting to estimate the amount of structural
difference between allied domestic races, we are soon
involved in doubt, from not knowing whether they are
descended from one or several parent species. This
point, if it could be cleared up, would be interest-
VARIATION UNDER DOMESTICATION
43
ing; if, for instance, it could be shown that the grey*
hound, bloodhound, terrier, spaniel, and bull-dog, which
we all know propagate their kind truly, were the off-
spring of any single species, then such facts would have
great weight in making us doubt about the immutability
of the many 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 domestica-
tion; I believe that a small part of the difference is due
to their being descended from distinct species. In the
case of strongly marked races of some other domesticated
species, there is presumptive or even strong evidence
that all are descended from a single wild stock.
It has often been assumed that man has chosen for
domestication animals and plants having an extraordinary
inherent tendency to vary, and likewise to withstand di-
verse climates. I do not dispute that these capacities
have added largely to the value of most of our domes-
ticated 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 en-
dure other climates? Has the little variability of the ass
and goose, or the small power of endurance of warmth
by the reindeer, or of cold by the common camel, pre-
vented their domestication? I cannot doubt that if other
animals and plants, equal in number to our domesticated
productions, and belonging to equally diverse classes and
countries, were taken from a state of nature, and could
be made to breed for an equal number of generations
under domestication, they would on an average vary as
THE ORIGIN OF SPECIES
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, it is not possible to come to any definite
conclusion, whether they are descended from one or sev-
eral wild species. The_ argument mainly relied on by
those who believe in the multiple origin of our domestic
animals is, that we find in the most ancient times, on
the monuments of Egypt, and in the lake-habitations of
Switzerland, much diversity in the breeds; and that some
of these ancient breeds closely resemble, or are even
identical with, those still existing. But this only throws
far backward the history of civilization, and shows that
animals were domesticated at a much earlier period than
has hitherto been supposed. The lake-inhabitants of
Switzerland cultivated several kinds of wheat and barley,
the pea, the poppy for oil, and flax; and they possessed
several domesticated animals. They also carried on com-
merce with other nations. All this clearly shows, as
Heer has remarked, that they had at this early age
progressed considerably in civilization; and this again
implies a long continued previous period of less ad-
vanced civilization, during which the domesticated ani-
mals, kept by different tribes in different districts, might
have varied and given rise to distinct races. Since the
discovery of flint tools in the superficial formations of
many parts of the world, all geologists believe that bar-
barian man existed at an enormously remote period; and
we know that at the present day there is hardly a tribe
so barbarous as not to have domesticated at least the
dog.
The origin of most of out domestic animals will prob-
VARIATION UNDER DOMESTICATION 45
ably forever remain vague. But I may here state that,
looking to the domestic dogs of the whole world, I have,
after a laborious collection of all known facts, come to
the conclusion that several wild species of Canidae have
been tamed, and that their blood, in some cases mingled
together, flows in the veins of our domestic breeds. In
regard to sheep and goats I can form no decided opinion.
From facts communicated to me by Mr. Blyth, on the
habits, voice, constitution, and structure of the humped
Indian cattle, it is almost certain that they are descended
from a different aboriginal stock from our European cat-
tle; and some competent judges believe that the^e latter
have had two or three wild progenitors — whether or not
these deserve to be called species. This conclusion, as
well as that of the specific distinction between the
humped and common cattle, may, indeed, be looked
upon as established by the admirable researches of Pro-
fessor Kiitimeyer. With respect to horses, from reasons
which I cannot here give, I am doubtfully inclined to
believe, in opposition to several authors, that all the
races belong to the same species. Having kept nearly
all the English breeds of the fowl alive, having bred and
crossed them, and examined their skeletons, it appears to
me almost certain that all are the descendants of the wild
Indian fowl, Gallus bankiva; and this is the conclusion
of Mr. Blyth, and of others who have studied this bird
in India. In regard to ducks and rabbits, some breeds
of which differ much from each other, the evidence is
clear that they are all 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
46
THE ORIGIN OF SPECIES
an absurd extreme by some authors. They believe that
every race which breeds true, let the distinctive characters
be ever so slight, has had its wild prototype. At this
rate there must have existed at least a score of species of
wild cattle, as many sheep, and several goats, in Europe
alone, and several even within Great Britain. One author
believes that there formerly existed eleven wild species of
sheep peculiar to Great Britain! When we bear in mind
that Britain has now not one peculiar mammal, and
France but few distinct from those of Germany, and so
with Hungary, Spain, etc., but that each of these king-
doms possesses several peculiar breeds of cattle, sheep,
etc., we must admit that many domestic breeds must have
originated in Europe; for whence otherwise could they
have been derived? So it is in India. Even in the case
of the breeds of the domestic dog throughout the world,
which I admit are descended from several wild species,
it cannot be doubted that there has been an immense
amount of inherited variation; for who will believe that
animals closely resembling the Italian greyhound, the
bloodhound, the bull-dog, pug-dog, or Blenheim spaniel,
etc. — so unlike all wild Canidae — ever existed in a state of
nature? It has often been loosely said that all our races
of dogs have been produced by the crossing of a few
aboriginal species; but by crossing we can only get forms
in some degree intermediate between their parents; and if
we account for our several domestic races by this process,
we must admit the former existence of the most extreme
forms, as the Italian greyhound, bloodhound, bull-dog,
etc., in the wild state. Moreover, the possibility of mak-
ing distinct races by crossing has been greatly exagger-
ated. Many cases are on record, showing that a race may
VARIATION UNDER DOMESTICATION 47
be modified by occasional crosses, if aided by the careful
selection of the individuals which present the desired
character; but to obtain a race intermediate between
two quite distinct races would be very difficult. Sir J.
Sebright expressly experimented with this object and
failed. The offspring from the first cross between two
pure breeds is tolerably and sometimes (as I have found
with pigeons) quite uniform in character, and everything
seems simple enough; bat when these mongrels are crossed
one with another for several generations, hardly two of
them are alike, and then the difficulty of the task be-
comes manifest.
Breeds of the Domestic Pigeon, their Differences and Origin
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 pur-
chase or obtain, and have been most kindly favored with
skins from several quarters of the world, more especially
by the Hon. W. Elliot from India, and by the Hon. C.
Murray from Persia. Many treatises in different lan-
guages have been published on pigeons, and some of
them are very important, as being of considerable antiq-
uity. I have associated with several eminent fanciers,
and have been permitted to join two of the London
Pigeon Clubs. The diversity of the breeds is something
astonishing. Compare the English carrier and the short-
faced tumbler, and see the wonderful difference in their
beaks, entailing corresponding differences in their skulls.
The carrier, more especially the male bird, is also remark-
able from the wonderful development of the carunculated
skin about the head; and this is accompanied by greatly
48
THE ORIGIN OF SPECIES
elongated eyelids, very large external orifices to the nos-
trils, 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 inherited habit of flying
at a great height in a compact flock and tumbling in the
air head over heels. The runt is a bird of great size, with
long massive beak and large feet; some of the sub-breeds
of runts have very long necks, others very long wings and
tails, others singularly short tails. The barb is allied to
the carrier, but, instead of a long beak, has a very short
and 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 short and conical beak,
with a line of reversed feathers down the breast; and it
has the habit of continually expanding, slightly, the up-
per part of the oesophagus. The Jacobin has the feathers
so much reversed along the back of the neck that they
form a hood; and it has, proportionally to its size, elon-
gated wing and tail feathers. The trumpeter and laugher,
as their names express, utter a very different coo from
the other breeds. The fantail has thirty or even forty
tail-feathers, instead of twelve or fourteen — the normal
number in all the members of the great pigeon family:
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
be specified.
In the skeletons of the several breeds, the development
of the bones of the face in length and breadth and curva-
ture differs enormously. The shape, as well as the breadth
and length of the ramus of the lower jaw, varies in a
VARIATION UNDER DOMESTICATION
49
highly remarkable manner. The caudal and sacral verte-
brae vary in number; 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 diver-
gence and relative size of the two arms of the furcula.
The proportional width of the gape of mouth, the propor-
tional length of the eyelids, of the orifice of the nostrils,
of the tongue (not always in strict correlation with the
length of beak), the size of the crop and of the upper
part of the oesophagus; the development and abortion of
the oil-gland; the number of the primary wing and caudal
feathers; the relative length of the wing and tail to each
other and to the body; the relative length of the leg and
foot; 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
plumage is acquired varies, as does the state of the down
with which the nestling birds are clothed when hatched.
The shape and size of the eggs vary. The manner of
flight, and in some breeds the voice and disposition, differ
remarkably. Lastly, in certain breeds, the males and
females have come to differ in 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 be ranked by him
as well-defined species. Moreover, I do not believe that
any ornithologist would in this case place the English
carrier, the short-faced tumbler, the runt, the barb, pouter,
and fantail in the same genus; more especially as in
each of these breeds several truly-inherited sub-breeds,
— Science — 3
50
THE ORIGIN OF SPECIES
or species, as lie would call them, could be shown
him.
Great as are the differences between the breeds of the
pigeon, I am fully convinced that the common opinion of
naturalists is correct; namely, that all are descended from
the rock-pigeon (Columba livia), including under this
term several geographical races or sub-species, which
differ from each other in the most trifling respects. As
several of the reasons which have led me to this belief
are in some degree applicable in other cases, I will here
briefly give them. If the several breeds are not varieties,
and have not proceeded from the rock-pigeon, they must
have descended from at least seven or eight aboriginal
stocks; for it is impossible to make the present domestic
breeds by the crossing of any lesser number: how, for
instance, could a pouter be produced by crossing two
breeds unless one of the parent- stocks possessed the
characteristic enormous crop ? The supposed aboriginal
stocks must all have been rock-pigeons, that is, they did
not breed or willingly perch on trees. But besides C.
livia, with its geographical sub-species, only two or three
other species of rock-pigeons are known; and these hove
not any of the characters of the domestic breeds. Hence
the supposed aboriginal stocks must either still exist in
the countries where they were originally domesticated,
and yet be unknown to ornithologists, and this, consider-
ing their size, habits, and remarkable characters, seems
improbable; or they must have become extinct in the wild
state. But birds breeding on precipices, and good fliers,
are unlikely to be exterminated^ and the common rock-
pigeon, which has the same habits with the domestic
breeds, has not been exterminated even on several of the
VARIATION UNDER DOMESTICATION
51
smaller British islets, or on the shores of the Mediter-
ranean. Hence the supposed extermination of so many
species having similar habits with the rock- pigeon seems
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 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.
i Again, all recent experience shows that it is difficult to
get wild animals 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 confine-
ment.
An argument of great weight, and applicable in sev-
eral other cases, is, that the above- specified breeds, though
agreeing generally with the wild rock-pigeon in constitu-
tion, habits, voice, coloring, and in most parts of their
structure, yet are certainly highly abnormal in other
parts; we may look in vain through the 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 intentionally or by chance
picked out extraordinarily abnormal species; and further,
that these very species have since all become extinct or
52
THE ORIGIN OF SPECIES
unknown. So many strange contingencies are improbable
in the highest degree.
Some facts in regard to the coloring of pigeons well
deserve consideration. The rock-pigeon is of a slaty-
bine, with white loins; bnt the Indian sub-species, C. in-
termedia of Strickland, has this part bluish. The tail
has a terminal dark bar, with the outer feathers exter-
nally edged at the base with white. The wings have two
black bars. Some semi- domestic breeds, and some truly
wild breeds, have, besides the two black bars, the wings
checkered with black. These several marks do not occur
together in any other species of the whole family. Now,
in every one of the domestic breeds, taking thoroughly
wejl-bred birds, all the above marks, even to the white
edging of the outer tail-feathers, sometimes concur per-
fectly developed. Moreover, when birds belonging to
two or more distinct breeds are crossed, none of which
are blue or have any of the above- specified marks, the
mongrel offspring are very apt suddenly to acquire these
characters. To give one instance out of several which I
have observed: I crossed some white fantails, which breed
very true, with some black barbs — and it so happens that
blue varieties of barbs are so rare that I never heard of
an instance in England; and the mongrels were black,
brown, and mottled. I also crossed a barb with a spot,
which is a white bird with a red tail and red spot on
the forehead, and which notoriously breeds very true;
the mongrels were dusky and mottled. I then crossed
one of the mongrel barb-fantails with a mongrel barb-
spot, and they produced a bird of as beautiful a blue
color, with the white loins, double black wing-bar, and
barred and white-edged tail-feathers, as any wild rock-
VARIATION UNDER DOMESTICATION 53
pigeon! We can understand these facts, on the well-
known principle of reversion to ancestral characters, if
all the domestic breeds are descended from the rock-
pigeon. But if we deny this, we must make one of the
two following highly improbable suppositions. Either,
first, that all the several imagined aboriginal stocks were
colored and marked like the rock-pigeon, although no
other existing species is thus colored and marked, so that
in each separate breed there might be a tendency to revert
to the very same colors and markings. Or, secondly,
that each breed, even the purest, has within a dozen, or
at most within a score, of generations, been crossed by
the rock-pigeon; I say within a dozen or twenty genera-
tions, for no instance is known of crossed descendants
reverting to an ancestor of foreign blood, removed by
a greater number of generations. In a breed which has
been crossed only once, the tendency to revert to any
character derived from such a cross will naturally become
less and less, as in each succeeding generation there will
be less of the foreign blood; but when there has been no
cross, and there is a tendency in the breed to revert to a
character which was lost during some former generation,
this tendency, for all that we can see to the contrary,
may be transmitted undiminished for an indefinite num-
ber of generations. These two distinct cases of reversion
are often confounded together by those who have written
on inheritance.
Lastly, the hybrids or mongrels from between all the
breeds of the pigeon are perfectly fertile, as I can state
from my own observations, purposely made, on the most
distinct breeds. Now, hardly any cases have been ascer-
tained with certainty of hybrids from two quite distinct
54
THE ORIGIN OF SPECIES
species of animals being perfectly fertile. Some authors
believe that long-continued domestication eliminates this
strong tendency to sterility in species. From the history
of the dog, and of some other domestic animals, this
conclusion is probably quite correct, if applied to spe-
cies closely related to each other. But to extend it so
far as to suppose that species, aboriginally as distinct as
carriers, tumblers, pouters, and fantails now are, should
yield offspring perfectly fertile inter se, would be rash
in the extreme.
From these several reasons, namely — the improbability
of man having formerly made seven or eight supposed
species of pigeons to breed freely under domestication;
— these supposed species being quite unknown in a wild
state, and their not having become anywhere feral; — these
species presenting certain very abnormal characters, as
compared with all other Columbidse, though so like the
rock-pigeon in most respects; — the occasional reappearance
of the blue color and various black marks in all the
breeds, both when kept pure and when crossed; — and
lastly, the mongrel offspring being perfectly fertile; — from
these several reasons, taken together, we may safely con-
clude that all our domestic breeds are descended from
the rock-pigeon or Columba livia with its geographical
sub-species.
In favor of this view, I may add, first, that the
wild C. livia has been found capable of domestication
in Europe and in India; and that it agrees in habits
and in a great number of points of structure with all
the domestic breeds. Secondly, that, although an En-
glish carrier or a short-faced tumbler differs immensely
in certain characters from the rock- pigeon, yet that, by
VARIATION UNDER DOMESTICATION
55
comparing the several sub-breeds of these two races, more
especially those brought from distant countries, we can
make, between them and the rock-pigeon, an almost
perfect series; so we can in some other cases, but not
with all the breeds. Thirdly, those characters which are
mainly distinctive of each breed are in each eminently
variable, 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; and the ex-
planation of this fact will be obvious when we 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 Ro-
mans, as we hear from Pliny, immense prices were given
for pigeons; "nay, they are come to this pass, that they
can reckon up their pedigree and race." Pigeons were
much valued by Akber Khan in India, about the year
1600; never less than 20,000 pigeons were taken with the
court. "The monarchs of Iran and Turan sent him some
very rare birds"; and, continues the courtly historian,
"His Majesty by crossing the breeds, which method was
never practiced before, has improved them astonishingly."
About this same period the Dutch were as eager about
pigeons as were the old Romans. The paramount impor-
tance of these considerations in explaining the immense
amount of variation which pigeons have undergone will
likewise be obvious when we treat of Selection. We
56
THE ORIGIN OF SPECIES
shall then, also, see how it is that the several breeds
so often have a somewhat monstrous character. It is
also a most favorable circumstance for the production
of distinct breeds, that male and female pigeons can
be easily mated for life; and thus different breeds can
be kept together in the same aviary.
I have discussed the probable origin of domestic
pigeons at some, yet quite insufficient, length; be-
cause when I first kept pigeons and watched the sev-
eral kinds, well knowing how truly they breed, I felt
fully as much difficulty in believing that since they had
been domesticated they had all proceeded from a common
parent, as any naturalist could in coming to a similar
conclusion in regard to the many species of finches, or
other groups of birds, in nature. One circumstance has
struck me much; namely, that nearly all the breeders
of the various domestic animals and the cultivators of
plants, with whom I have conversed, or whose treatises
I have read, are firmly convinced that the several breeds
to which each has attended are descended from so many
aboriginally distinct species. Ask, as I have asked, a
celebrated raiser of Hereford cattle, whether his cattle
might not have descended from Long-horns, or both from
a common parent- stock, and he will laugh you to scorn.
I have never met a pigeon, or poultry, or duck, or rab-
bit fancier who was not fully convinced that each main
breed was descended from a distinct species. Van Mons,
in his treatise on pears and apples, shows how utterly he
disbelieves that the several sorts, for instance a Eibston-
pippin or Codlin-apple, could ever have proceeded from
the seeds of the same tree. Innumerable other examples
could be given. The explanation, I think, is simple:
VARIATION UNDER DOMESTICATION 57
from long- continued study they are strongly impressed
with the differences between the several races; and
though they well know that each race varies slightly,
for they win their prizes by selecting such slight differ-
ences, yet they ignore all general arguments, and refuse
to sum up in their minds slight differences accumulated
during many successive generations. May not those nat-
uralists who, knowing far less of the laws of inheritance
than does the breeder, and knowing no more than he
does of the intermediate links in the long lines of de-
scent, yet admit that many of our domestic races are
descended from the same parents — may they not learn
a lesson of caution, when they deride the idea of spe-
cies in a state of nature being lineal descendants of other
species ? v
Principles of Selection anciently followed, and their
Effects
Let us now briefly consider the steps by which do-
mestic races have been produced, either from one or from
several allied species. Some effect may be attributed to
the direct and definite action of the external conditions
of life, and some to habit; but he would be a bold man
who would account by such agencies for the differences
between a dray and racehorse, a greyhound and blood-
hound, 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 teasel, with its hooks, which cannot be ri-
58
THE ORIGIN OF SPECIES
vailed by any mechanical contrivance, is only a variety
of the wild Dipsacus; and this amount of change may
have suddenly arisen in a seedling. So it has probably
been with the turnspit dog; and this is known to have
been the case with the ancon sheep. But when we com-
pare the drayhorse and racehorse, the dromedary and
camel, the various breeds of sheep fitted either for cul-
tivated land or mountain pasture, with the wool of one
breed good for one purpose, and that of another breed
for another purpose; when we compare the many breeds
of dogs, each good for man in different ways; when we
compare the gamecock, so pertinacious in battle, with
other breeds so little quarrelsome, with "everlasting lay-
ers" which never desire to sit, and with the bantam so
small and elegant; when we compare the host of agricul-
tural, culinary, orchard, and flower-garden races of plants,
most useful to man at different seasons and for different
purposes, or so beautiful in his eyes, we must, I think,
look further than to mere variability. We cannot sup-
pose that all the breeds were suddenly produced as per-
fect and as useful as we now see them; indeed, in many
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 have made 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 their breeds of cattle and sheep. In order
fully to realize what they have done, it is almost nec-
essary to read several of the many treatises devoted to
VARIATION UNDER DOMESTICATION 59
this subject, and to inspect the animals. Breeders habit-
ually speak of an animal's organization as something
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
agriculturists than almost any other individual, and who
was himself a very good judge of animals, speaks of the
principle of selection as "that which enables the agricul-
turist, 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 mold he pleases." Lord Somerville, speaking of
what breeders have done for sheep, says: "It would
seem as if they had chalked out upon a wall a form
perfect in itself, and then had given it existence." In
Saxony the importance of the principle of selection in
regard to merino sheep is so fully recognized that men
follow it as a trade: the sheep are placed on a table and
are studied, like a picture by a connoisseur; this is done
three times at intervals of months, and the sheep are
each time marked and classed, so that the very best
may ultimately be selected for breeding.
What English breeders have actually effected is
proved by the enormous prices given for animals with
a good pedigree; and these have 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 among closely allied sub-breeds. And when a
cross has been made, the closest selection is far more
indispensable even than in ordinary cases. If selection
60
THE ORIGIN OF SPECIES
consisted merely in separating some very distinct variety,
and breeding from it, the principle would be so obvious
as hardly to be worth notice; but its importance consists
in the great effect produced by the accumulation in one
direction, during successive generations, of differences ab-
solutely inappreciable by an uneducated eye — differences
which I for one have vainly attempted to appreciate.
Not one man in a thousand has accuracy of eye and
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
assuredly fail. Few would readily believe in the natural
capacity and years of practice requisite to become even
a skilful pigeon-fancier.
The same principles are followed by horticulturists;
but the variations are here often more abrupt. No one
supposes that our choicest productions have been pro-
duced by a single variation from the aboriginal stock.
"We have proofs that this has not been so in several
cases in which exact records have been kept; thus, to
give a very trifling instance, the steadily- increasing size
of the common gooseberry may be quoted. We see an
astonishing improvement in many florists' flowers, when
the flowers of the present day are compared with draw-
ings 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, likewise
VARIATION UNDER DOMESTICATION 61
followed; for hardly any one is so careless as to breed
from his worst animals.
In regard to plants, there is another means of observ-
ing the accnmulated effects of selection — namely, by com-
paring the diversity of flowers in the different varieties
of the same species in the flower-garden; the diversity of
leaves, pods, or tubers, or whatever part is valued, in the
kitchen-garden, in comparison with the flowers of the
same varieties; and the diversity of fruit of the same
species in the orchard, in comparison with the leaves and
flowers of the same set of varieties. See how different
the leaves of the cabbage are, and how extremely alike
the flowers; how unlike the flowers of the heart's-ease are,
and how alike the leaves; how much the fruit of the
different kinds of gooseberries differ in size, color, shape
and hairiness, and yet the flowers present very slight dif-
ferences. It is not that the varieties which differ largely
in some one point do not differ at all in other points;
this is hardly ever — I speak after careful observation —
perhaps never, the case. The law of correlated variation,
the importance of which should never be overlooked, will
insure some differences ; but, as a general rule, it cannot be
doubted that the continued selection of slight variations,
either in the leaves, the flowers, or the fruit, will produce
races differing from each other chiefly in these characters.
It may be objected that the principle of selection has
been reduced to methodical practice for scarcely more
than three-quarters of a century; it has certainly been
more attended to of late years, and many treatises have
been published on the subject; and the result has been,
in a corresponding degree, rapid and important. But it
is very far from true that the principle is a modern dis-
62
THE ORIGIN OF SPECIES
covery. I could give several references to works of high
antiquity, in which the full importance of the principle is
acknowledged. 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 "roguing1' of plants by nurserymen.
The principle of selection I find distinctly given in an
ancient Chinese encyclopedia. Explicit rulers are laid
down by some of the Koman classical writers. From
passages in Genesis, it is clear that the color of domestic
animals was at that early period attended to. Savages
now sometimes cross their dogs with wild canine animals,
to improve the breed, and they formerly did so, as is
attested by passages in Pliny 0 The savages in South
Africa match their draught cattle by color, as do some
of the Eskimos their teams of dogs. Livingstone states
that good domestic breeds are highly valued by the
negroes in the interior of Africa who have not associated
with Europeans. Some of these facts do not show actual
selection, but they show that the breeding of domestic
animals was carefully attended to in ancient times, and
is now attended to by the lowest savages. It would, in-
deed, have been a strange fact, had attention not been
paid to breeding, for the inheritance of good and bad
qualities is so obvious.
Unconscious Selection
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 anything of the kind
in the country. But, for our purpose, a form of P;elec-
VARIATION UNDER DOMESTICATION 63
tion, which may be called Unconscious, and which results
from every one trying to possess and breed from the best
individual animals, is more important. Thus, a man who
intends keeping pointers naturally tries to get as good
dogs as he can, and afterward breeds from his own best
dogs, but he has no wish or expectation of permanently
altering the breed. Nevertheless we may infer that this
process, continued during centuries, would improve and
modify any breed, in the same way as Bakewell, Collins,
etc., by this very same process, only carried on more
methodically, did greatly modify, even during their life-
times, the forms and qualities of their cattle. Slow and
insensible changes of this kind can never be recognized
unless actual measurements or careful drawings of the
breeds in question have been made long ago, which may
serve for comparison. In some cases, however, un-
changed, or but little changed, individuals of the same
breed exist 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 foxhound: but what
concerns us is, that the change has been effected uncon-
sciously 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.
64
THE ORIGIN OF SPECIES
By a similar process of selection, and by careful train-
ing, English racehorses have come to surpass in fleetness
and size the parent Arabs, so that the latter, by the
regulations for the Goodwood Kaces, are favored in the
weights which they carry. Lord Spencer and others have
shown how the cattle of England have increased in
weight and in early maturity, compared with the stock
formerly kept in this country. By comparing the accounts
given in various old treatises of the former and present
state of carrier and tumbler pigeons in Britain, India and
Persia, we can 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-
scious, in so far that the breeders could never have
expected, or even wished, to produce the result which
ensued — namely, the production of two distinct strains.
The two flocks of Leicester sheep kept by Mr. Buckley
and Mr. Burgess, as Mr. Youatt remarks, "have been
purely bred from the original stock of Mr. Bakewell for
upward of fifty years. There is not a suspicion existing
in the mind of any one at all acquainted with the subject
that the owner of either of them has deviated in any one
instance from the pure blood of Mr. Bakewell's flock, and
yet the difference between the sheep possessed by these
two gentlemen is so great that they have the appearance
of being quite different varieties."
If there exist savages so barbarous as never to think
of the inherited character of the offspring of their domes-
tic animals, yet any one animal particularly useful to
them, for any special purpose, would be carefully pre-
VARIATION UNDER DOMESTICATION
65
served during famines and other accidents, to which
savages are so liable, and such choice animals would thus
generally leave more offspring than the inferior ones; so
that in this case there would be a kind of unconscious
selection going on. We see the value set on animals
even by the barbarians of Tierra del Fuego, by their
killing and devouring their old women, in times of
dearth, as of less value than their dogs.
In plants the same gradual process of improvement,
through the occasional preservation of the best individ-
uals, whether or not sufficiently distinct to be ranked
at their first appearance as distinct varieties, and whether
or not two or more species or races have become blended
together by crossing, may plainly be recognized in the
increased size and beauty which we now see in the
varieties of the heart' s-ease, 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 heart' s-ease 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 culti-
vated 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 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
66
THE ORIGIN OF SPECIES
variety chanced to appear, selecting it, and so onward.
But the gardeners of the classical period, who cultivated
the best pears which they could procure, never thought
what splendid fruit we should eat; though we owe our
excellent fruit in some small degree to their having
naturally chosen and preserved the best varieties they
could anywhere find.
A large amount of change, thus slowly and uncon-
sciously accumulated, explains, as I believe, the well-
known fact that in a number of cases we cannot recog-
nize, 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 Grood Hope, nor any other region inhabited by
quite uncivilized man, has afforded us a single plant
worth culture. It is not that these countries, so rich in
species, do not by a strange chance possess the aboriginal
stocks of any useful plants, but that the native plants
have not been improved by continued selection up to a
standard of perfection comparable with that acquired by
the plants in countries anciently civilized.
In regard to the domestic animals kept by uncivilized
man, it should not be overlooked that they almost always
have to struggle for their own food, at least during
certain seasons. And in two countries very differently
circumstanced, individuals of the same species, having
slightly different constitutions or structure, would often
succeed better in the one country than in the other; and
thus by a process of "natural selection," as will hereafter
VARIATION UNDER DOMESTICATION 67
be more fully explained, two sub-breeds might be
formed. This, perhaps, partly explains why the varieties
kept by savages, as has been remarked by some authors,
have more of the character of true species than the
varieties kept in civilized countries.
On the view here given of the important part which
selection by man has played, it becomes at once obvious
how it is that our domestic races show adaptation in
their structure or in their habits to man's wants or fan-
cies. We can, I think, further understand the frequently
abnormal character of our domestic races, and likewise
their differences being so great in external characters,
and relatively so slight in internal parts or organs. Man
can hardly select, or only with much difficulty, any devia-
tion of structure excepting such as is externally visible;
and indeed he rarely cares for what is internal. He can
never act by selection, excepting on variations which are
first given to him in some slight degree by nature. No
man would ever try to make a fantail till he saw a
pigeon with a tail developed in some slight degree in an
unusual manner, or a pouter till he saw a pigeon with
a crop of somewhat unusual size; and the more abnormal
or unusual any character was when it first appeared, the
more likely it would be to catch his attention. But to
use such an expression as trying to make a fantail, is,
I have no doubt, in most cases, utterly incorrect. The
man who first selected a pigeon with a slightly larger
tail, never dreamed what the descendants of that pigeon
would become through long-continued, partly unconscious
and partly methodical selection. Perhaps the parent-bird
of all fantails had only fourteen tail-feathers somewhat
expanded, like the present Java fantail, or like individ-
68
THE ORIGIN OF SPECIES
uals of other and distinct breeds, in which as many as
seventeen tail-feathers have been counted. Perhaps the
first pouter- pigeon did not inflate its crop much more
than the turbit now does the upper part of its oesophagus
— a habit which is disregarded by all fanciers, as it is not
one of the points of the breed.
Nor let it be thought that soine great deviation of
structure would be necessary to catch the fancier's eye:
he perceives extremely small differences, and it is in
human nature to value any novelty, however slight, in
one's own possession. Nor must the value which would
formerly have been set on any slight differences in the
individuals of the same species be judged of by the
value which is now set on them, after several breeds
have fairly been established. It is known that with
pigeons many slight variations now occasionally appear,
but these are rejected as faults or deviations from the
standard of perfection in each breed. The common goose
has not given rise to any marked varieties; hence the
Toulouse and the common breed, which differ only in
color, that most fleeting of characters, have lately been
exhibited as distinct at our poultry shows.
These views appear to explain what has sometimes
been noticed — namely, that we know hardly anything
about the origin or history of any of our domestic
breeds. But, in fact, a breed, like a dialect of a lan-
guage, can hardly be said to have a distinct origin. A
man preserves and breeds from an individual with some
slight deviation of structure, or takes more care than
usual in matching his best animals, and thus improves
them, and the improved animals slowly spread in the
immediate neighborhood. But they will as yet hardly
VARIATION UNDER DOMESTICATION 69
ha ye a distinct name, and from being only slightly
valued, their history will have been disregarded. When
further improved by the same slow and gradual process,
they will spread more widely, and will be recognized as
something distinct and valuable, and will then probably
first receive a provincial name. In semi-civilized coun-
tries, with little free communication, the spreading of a
new sub-breed would be a slow process. As soon as the
points of value are once acknowledged, the principle, as
I have called it, of unconscious selection will always
tend — perhaps more at one period than at another, as
the breed rises or falls in fashion — perhaps more in one
district than in another, according to the state of civiliza-
tion of the inhabitants — slowly to add to the characteristic
features of the breed, whatever they may be. But the
chance will be infinitely small of any record having been
preserved of such slow, varying, and insensible changes.
Circumstances favorable to Man's Poiver of Selection
I will now say a few words on the circumstances,
favorable or the reverse, to man's power of selection. A
high degree of variability is obviously favorable, as freely
giving the materials for selection to work on; not that
mere individual differences are not amply sufficient, with
extreme care, to allow of the accumulation of a large
amount of modification in almost any desired direction.
But as variations manifestly useful or pleasing to man
appear only occasionally, the chance of their appearance
will be much increased by a large number of individ-
uals being kept. Hence, number is of the highest im-
portance for success. On this principle Marshall for-
merly remarked, with respect to the sheep of parts of
70
THE ORIGIN OF SPECIES
Yorkshire, "as they generally belong to poor people, and
are mostly in small lots, they never can be improved."
On the other hand, nurserymen, from keeping large
stocks of the same plant, are generally far more success-
ful than amateurs in raising new and valuable varieties.
A large number of individuals of an animal or plant can
be reared only where the conditions for its propagation
are favorable. When the individuals are scanty, all will
be allowed to breed, whatever their quality may be, and
this will effectually prevent selection. But probably the
most important element is that the animal or plant should
be so highly valued by man that the closest attention is
paid to even the slightest deviations in its qualities
or structure. Unless such attention be paid nothing can
be effected. I have seen it gravely remarked that it was
most fortunate that the strawberry began to vary just
when gardeners began to attend to this plant. No doubt
the strawberry had always varied since it was cultivated,
but the slight varieties had been neglected. As soon,
however, as gardeners picked out individual plants with
slightly larger, earlier, or better fruit, and raised seedlings
from them, and again picked out the best seedlings and
bred from them, then (with some aid by crossing distinct
species) those many admirable varieties of the strawberry
were raised which have appeared during the last half-
century.
With animals, facility in preventing crosses is an im-
portant element in the formation of new races — at least,
in a country which is already stocked with other races.
In this respect inclosure of the land plays a part. Wan-
dering savages or the inhabitants of open plains rarely
possess more than one breed of the same species. Pig
VARIATION UNDER DOMESTICATION
71
eons can be mated for life, and this is a great conven-
ience to the fancier, for thus many races may be im-
proved and kept true, though mingled in the same
aviary; and this circumstance must have largely fa-
vored the 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
easily matched, and, although so much valued by women
and children, we rarely see a distinct breed long kept
up; such breeds as we do sometimes see are almost
always imported from some other country. 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, etc., may be attributed
in main part to selection not having been brought into
play: in cats, from the difficulty in pairing them; in
donkeys, from only a few being kept by poor people,
and little attention paid to their breeding; for recently
in certain parts of Spain and of the United States this
animal has been surprisingly modified and improved by
careful selection: in peacocks, from not being very easily
reared and a large stock not kept: in geese, from being
valuable only for two purposes, food and feathers, and
more especially from no pleasure having been felt in the
display of distinct breeds; but the goose, under the con-
ditions to which it is exposed when domesticated, seems
to have a singularly inflexible organization, though it has
varied to a slight extent, as I have elsewhere described.
Some authors have maintained that the amount of
variation in our domestic productions is soon reached,
72
THE ORIGIN OF SPECIES
and can never afterward be exceeded. It would be some-
what rash, to assert that the limit has been attained in
any one case; for almost all our animals and plants have
been greatly improved in many ways within a recent
period; and this implies variation. It would be equally
rash to assert that characters now increased to their ut-
most limit could not, after remaining fixed for many
centuries, again vary under new conditions of life. No
doubt, as Mr. Wallace has remarked with much truth, a
limit will be at last reached. For instance, there must
be a limit to the fleetness of any terrestrial animal, as
this will be determined by the friction to be overcome,
the weight of body to be carried, and the power of con-
traction in the muscular fibres. But what concerns us
is that the domestic varieties of the same species differ
from each other in almost every character, which man
has attended to and selected, more than do the distinct
species of the same genera. Isidore Greoffroy St.-Hilaire
has proved this in regard to size, and so it is with color
and probably with the length of hair. With respect to
fleetness, which depends on many bodily characters,
Eclipse was far fleeter, and a drayhorse is incompara-
bly stronger, than any two natural species belonging to
the same genus. So with plants, the seeds of the differ-
ent varieties of the bean or maize probably differ more
in size than do the seeds of the distinct species in any
one genus in the same two families. The same remark
holds good in regard to the fruit of the several varieties
of the plum, and still more strongly with the melon,
as well as in many other analogous cases.
To sum up on the origin of our domestic races of ani-
mals and plants Changed conditions of life are of the
VARIATION UNDER DOMESTICATION 73
highest importance in causing variability, both by acting
directly on the organization, and indirectly by affecting
the reproductive system. It is not probable that vari-
ability is an inherent and necessary contingent, under all
circumstances. The greater or less force of inheritance
and reversion determine whether variations shall endure.
Variability is governed by many unknown laws, of which
correlated growth is probably the most important. Some-
thing, but how much we do not know, may be attributed
to the definite action of the conditions of life. Some,
perhaps a great, effect may be attributed to the increased
use or disuse of parts. The final result is thus rendered
infinitely complex. In some cases the intercrossing of
aboriginally distinct species appears to have played an
important part in the origin of our breeds. When sev-
eral breeds have once been formed in any country, their
occasional intercrossing, with the aid of selection, has, no
doubt, largely aided in the formation of new sub-breeds;
but the importance of crossing has been much exagger-
ated, both in regard to animals and to those plants which
are propagated by seed. With plants which are tempo-
rarily propagated by cuttings, buds, etc., the importance
of crossing is immense; for the cultivator may here dis-
regard the extreme variability both of hybrids and of
mongrels, and the sterility of hybrids; but plants not
propagated by seed are of little importance to us, for
their endurance is only temporary. Over all these causes
of Change, the accumulative action of Selection, whether
applied methodically and quickly, or unconsciously and
slowly but more efficiently, seems to have been the pre-
dominant Power.
— Science— ^4
7$
\
THE ORIGIN OF SPECIES
\
CHAPTEE II
VARIATION UNDER NATURE
Variability — Individual differences — Doubtful species — Wide ranging, much
diffused, and common species, vary most — Species of the larger genera
in each country vary more frequently 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
Variability
BEFORE applying the principles arrived at in the
last chapter to organic beings in a state of nature,
we must briefly discuss whether these latter are
subject to any variation. To treat this subject properly,
a long catalogue of dry facts ought to be given; but
these I shall reserve for a future work. Nor shall I
here discuss the various definitions which have been
given of the term species. No one definition has satis-
fied all naturalists; yet every naturalist knows vaguely
what he means when he speaks of a species. Generally
the term includes the unknown element of a distinct act
of creation. The term "variety" is almost equally diffi-
cult 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 gradu-
ate into varieties. By a monstrosity I presume is meant
some considerable deviation of structure, generally injuri-
ous, or not useful to the species. Some authors use the
term 11 variation" in a technical sense, as implying a
VARIATION UNDER NATURE
75
modification directly due to the physical conditions of
life; and "variations" in this sense are supposed not to
be inherited; but who can say that the dwarfed condition
of shells in the brackish waters of the Baltic, or dwarfed
plants on Alpine summits, or the thicker fur of an ani-
mal from far northward, would not in some cases be
inherited for at least a few generations? and in this case
I presume that the form would be called a variety.
It may be doubted whether sudden and considerable
deviations of structure such as we occasionally see in our
domestic productions, more especially with plants, are
ever permanently propagated in a state of nature. Al-
most every part of every organic being is so beautifully
related to its complex conditions of life that it seems as
improbable that any part should have been suddenly pro-
duced perfect, as that a complex machine should have
been invented by man in a perfect state. Under domes-
tication monstrosities sometimes occur which resemble
normal structures in widely different animals. Thus
pigs have occasionally been born with a sort of pro-
boscis, and if any wild species of the same genus had
naturally possessed a proboscis, it might have been ar-
gued that this had appeared as a monstrosity; but I
have as yet failed to find, after a diligent search, cases
of monstrosities resembling normal structures in nearly
allied forms, and these alone bear on the question. If
monstrous forms of this kind ever do appear in a state
of nature and are capable of reproduction (which is not
always the case), as they occur rarely and singly, their
preservation would depend on unusually favorable cir-
cumstances. They would, also, during the first and suc-
ceeding generations cross with the ordinary form, and
76
THE ORIGIN OF SPECIES
thus their abnormal character would almost inevitably
be lost. But I shall have to return in a future chap-
ter to the preservation and perpetuation of single or
occasional variations.
Individual Differences
The many slight differences which appear in the
offspring from the same parents, or which it may be
presumed have thus arisen, from being observed in the
individuals of the same species inhabiting the same con-
fined locality, may be called individual differences. No
one supposes that all the individuals of the same species
are cast in the same actual mold. These individual dif-
ferences are of the highest importance for us, for they
are often inherited, as must be familiar to every one;
and they thus afford materials for natural selection to
act on and accumulate, in the same manner as man accu-
mulates in any given direction individual differences in
his domesticated productions. These individual differ-
ences generally affect what naturalists consider unimpor-
tant parts; but I could show by a long catalogue of
facts, that parts which must be called important, whether
viewed under a physiological or classificatory point of
view, sometimes vary in the individuals of the same spe-
cies. I am convinced that the most experienced natural-
ist would be surprised at the number of the cases of
variability, even in important parts of structure, which
be could collect on good authority, as I have collected,
during a course of years. It should be remembered that
systematists are far from being pleased at finding varia-
bility in important characters, and that there are not
many men who will laboriously examine internal and im
VARIATION UNDER NATURE
77
portant organs, and compare them in many specimens of
the same species. It would never have been expected
that the branching of the main nerves close to the great
central ganglion of an insect would have been variable
in the same species; it might have been thought that
changes of this nature could have been effected only by
slow degrees; yet Sir J. Lubbock has shown a degree of
variability in these main nerves in Coccus, which may
almost be compared to the irregular branching of the
stem of a tree. This philosophical naturalist, I may add,
has also shown that the muscles in the larvae of certain
insects are far from uniform. Authors sometimes argue
in a circle when they state that important organs never
vary; for these same authors practically rank those parts
as important (as some few naturalists have honestly con-
fessed) which do not vary; and, under this point of view,
no instance will ever be found of an important part vary-
ing; but under any other point of view many instances
assuredly can be given.
There is one point connected with individual differ-
ences which is extremely perplexing: I refer to those
genera which have been called " protean" or u polymor-
phic," in which the species present an inordinate amount
of variation. With respect to many of these forms,
hardly two naturalists agree whether to rank them as
species or as varieties. We may instance Rubus, Rosa,
and Hieracium among plants, several genera of insects
and of Brachiopod shells. In most polymorphic genera
some of the species have fixed and definite characters.
Genera which are polymorphic in one country seem to be,
with a few exceptions, polymorphic in other countries,
and likewise, judging from Brachiopod shells, at former
78
THE ORIGIN OF SPECIES
periods of time. These facts are very perplexing, for
they seem to show that this kind of variability is inde-
pendent of the conditions of life. I am inclined to sus-
pect that we see, at least in some of these polymorphic
genera, variations which are of no service or disservice
to the species, and which consequently have not been
seized on and rendered definite by natural selection, as
hereafter to be explained.
Individuals of the same species often present, as is
known to every one, great differences of structure, inde-
pendently of variation, as in the two sexes of various
animals, in the two or three castes of sterile females or
workers among insects, and in the immature and larval
states of many of the lower animals. There are, also,
cases of dimorphism and trimorphism, both with animals
and plants. Thus, Mr. Wallace, who has lately called at-
tention to the subject, has shown that the females of
certain species of butterflies, in the Malay archipelago,
regularly appear under two or even three conspicuously
distinct forms, not connected by intermediate varieties.
Fritz Miiller has described analogous but more extraordi-
nary cases with the males of certain Brazilian Crusta-
ceans: thus, the male of a Tanais regularly occurs under
two distinct forms; one of these has strong and differ-
ently shaped pincers, and the other has antennae much
more abundantly furnished with smelling-hairs. Although
in most of these cases the two or three forms, both with
animals and plants, are not now connected by interme-
diate gradations, it is probable that they were once thus
connected. Mr. Wallace, for instance, describes a certain
butterfly which presents in the same island a great range
of varieties connected by intermediate links, and the ex*
VARIATION UNDER NATURE
79
treme links of the chain closely resemble the two forms
of an allied dimorphic species inhabiting another part of
the Malay archipelago. Thus also with ants, the several
worker-castes are generally quite distinct; but in some
cases, as we shall hereafter see, the castes are connected
together by finely graduated varieties. So it is, as I have
myself observed, with some dimorphic plants. It cer-
tainly at first appears a highly remarkable fact that the
same female butterfly should have the power of producing
at the same time three distinct female forms and a male;
and that a hermaphrodite plant should produce from
the same seed-capsule three distinct hermaphrodite forms,
bearing three different kinds of females and three or
even six different kinds of males. Nevertheless these
cases are only exaggerations of the common fact that the
female produces offspring of two sexes which sometimes
differ from each other in a wonderful manner.
Doubtful Species
The forms which possess in some considerable degree
the character of species, but which are so closely similar
to other forms, or are so closely linked to them by inter-
mediate gradations, that naturalists do not like to rank
them as distinct species, are in several respects the most
important for us. We have every reason to believe that
many of these doubtful and closely allied forms have
permanently retained their characters for a long time; for
as long, as far as we know, as have good and true
species. Practically, when a naturalist can unite by means
of intermediate links any two forms, 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
50
THE OPJGiy OF SPECIES
other as the variety. Bat cases of great difficulty, which
I will not here enumerate, sometimes arise in deciding
whether or not to rank one form as a variety of another,
even when they are closely connected by intermediate
links; nor will the commonly -assumed hybrid nature of
the intermediate forms always remove the difficulty. In
very many cases, however, one form is ranked as a vari-
ety of another, not because the intermediate links have
actually been found, bu±_because analogy leads the ob-
server to suppose either that they do now somewhere
exist, or may formerly have existed; and here a wide
door for the entry of doubt and conjecture is opened.
Hence, in determining whether a form should be
ranked as a species or a variety, the opinion of natural-
ists having sound judgment and wide experience seems
the only guide to follow. We must, however, in many
cases, decide by a majority of naturalists, for few well-
marked and well-known varieties can be named which
have not been ranked as species by at least some com-
petent judges.
That varieties of this doubtful nature are far from
uncommon cannot be disputed. Compare the several
floras of Great Britain, of France, or of the United
States, drawn up by different botanists, and see what a
surprising number of forms have been ranked by one
botanist as good species, and by another as mere vari-
eties. Mr. H. C. Watson, to whom I lie under deep obli-
gation for assistance of all kinds, has marked for me 182
British plants, which are generally considered as varieties,
but which have all been ranked by botanists as species;
and in making this list he has omitted many trifling
varieties, but which nevertheless have been ranked by
VARIATION UNDER NATURE
81
some botanists as species, and he has entirely omitted
several highly polymorphic genera. Under genera, in-
cluding the most polymorphic forms, Mr. Babington gives
251 species, whereas Mr. Bentham gives only 112 — a
difference of 139 doubtful forms! Among animals which
unite for each birth, and which are highly locomotive,
doubtful forms, ranked by one zoologist as a species and
by another as a variety, can rarely be found within the
same country, but are common in separated areas. How
many of the birds and insects in North America and
Europe, which differ very slightly from each other, have
been ranked by one eminent naturalist as undoubted spe-
cies, and by another as varieties, or, as they are often
called, geographical races! Mr. Wallace, in several valu-
able papers on the various animals, especially on the
Lepidoptera, inhabiting the islands of the great Malay
archipelago, shows that they may be classed under four
heads, namely, as variable forms, as local forms, as geo-
graphical races or sub-species, and as true representative
species. The first or variable forms vary much within
the limits of the same island. The local forms are mod-
erately constant and distinct in each separate island; but
when all from the several islands are compared together,
the differences are seen to be so slight and graduated,
that it is impossible to define or describe them, though
at the same time the extreme forms are sufficiently dis-
tinct. The geographical races or sub-species are local
forms completely fixed and isolated; but as they do not
differ from each othe/ by strongly marked and important
characters, "there is no possible test but individual opin-
ion to determine which of them shall be considered as
species and which as varieties.' ' Lastly, representative
82
THE ORIGIN OF SPECIES
species fill the same place in the natural economy of each
island as do the local forms and sub-species; but as they
are distinguished from each other by a greater amount
of difference than that between the local forms and sub-
species, they are almost universally ranked by naturalists
as true species. Nevertheless, no certain criterion can
possibly be given by which variable forms, local forms,
sub-species, and representative species can be recognized.
Many years ago, when comparing and seeing others
compare, the birds from the closely neighboring islands
of the Galapagos archipelago, one with another, and with
those from the American mainland, I was much struck
how entirely vague and arbitrary is the distinction be-
tween species and varieties. On the islets of the little
Madeira group there are many insects which are charac-
terized as varieties in Mr. Wollaston's admirable work,
but which would certainly 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 experi-
enced ornithologists consider our British red grouse as
only a strongly -marked race of a Norwegian species,
whereas the greater number rank it as an undoubted
species peculiar to Great Britain. A wide distance be-
tween the homes of two doubtful forms leads many nat-
uralists to rank them as distinct species; but what dis-
tance, it has been well asked, will suffice; if that between
America and Europe is ample, will that between Europe
and the Azores, or Madeira, or the Canaries, or between
the several islets of these small archipelagos, be suffi-
cient?
Mr. B. D. Walsh, a distinguished entomologist of the
VARIATION UNDER NATURE
83
United States, has described what he calls Phytophagic
varieties and Phytophagic species. Most vegetable-feeding
insects live on one kind of plant or on one group of
plants; some feed indiscriminately on many kinds, but
do not in consequence vary. In several cases, however,
insects found living on different plants have been ob-
served by Mr. Walsh to present in their larval or mature
state, or in both states, slight though constant differences
in color, size, or in the nature of their secretions. In
some instances the males alone, in other instances both
males and females, have been observed thus to differ in
a slight degree. When the differences are rather more
strongly marked, and when both sexes and all ages are
affected, the forms are ranked by all entomologists as
good species. But no observer can determine for an-
other, even if he can do so for himself, which of these
Phytophagic forms ought to be called species and which
varieties. Mr. Walsh ranks the forms which it may be
supposed would freely intercross, as varieties; and those
which appear to have lost this power, as species. As the
differences depend on the insects having long fed on dis-
tinct plants, it cannot be expected that intermediate links
connecting the several forms should now be found. The
naturalist thus loses his best guide in determining whether
to rank doubtful forms as varieties or species. This like-
wise necessarily occurs with closely allied organisms,
which inhabit distinct continents or islands. When, on
the other hand, an animal or plant ranges over the same
continent, or inhabits many islands in the same archipel-
ago, and presents different forms in the different areas,
there is always a good chance that intermediate forms
will be discovered which will link together the ex-
THE ORIGIN OF SPECIES
treme states; and these are then degraded to the rank
of varieties.
Some few naturalists maintain that animals never pre-
sent varieties; but then these same naturalists rank the
slightest difference as of specific value; and when the
same identical form is met with in two distant countries,
or in two geological formations, they believe that two dis-
tinct species are hidden under the same dress. The term
species thus comes to be a mere useless abstraction, im-
plying and assuming a separate act of creation. It is
certain that many forms, considered by highly-competent
judges to be varieties, resemble species so completely in
character that they have been thus ranked by other
highly-competent judges. But to discuss whether they
ought to be called species or varieties, before any defini-
tion of these terms has been generally accepted, is vainly
to beat the air.
Many of the cases of strongly-marked varieties or
doubtful species well deserve consideration; for several
interesting lines of argument, from geographical distri-
bution, analogical variation, hybridism, etc., have been
brought to bear in the attempt to determine their rank;
but space does not here permit me to discuss them.
Close investigation, in many cases, will no doubt bring
naturalists to agree 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 them. 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 attracts his attention, varieties of it will almost
universally be found recorded. These varieties, moreover,
will often be ranked by some authors as species. Look
VARIATION UNDER NATURE
85
at the common oak, how closely it has been studied; yet
a German author makes more than a dozen species out
of forms, which are almost universally considered by
other botanists to be varieties; and in this country the
highest botanical authorities and practical men can be
quoted to show that the sessile and pedunculated oaks
are either good and distinct species or mere varieties.
I may here allude to a remarkable memoir lately pub-
lished by A. de Candolle, on the oaks of the whole world.
No one ever had more ample materials for the discrimi-
nation of the species, or could have worked on them
with more zeal and sagacity. He first gives in detail all
the many points of structure which vary in the several
species, and estimates numerically the relative frequency
of the variations. He specifies above a dozen characters
which may be found varying even on the same branch,
sometimes according to age or development, sometimes
without any assignable reason. Such characters are not
of course of specific value, but they are, as Asa Gray
has remarked in commenting on this memoir, such as
generally enter into specific definitions. De Candolle then
goes on to say that he gives the rank of species to the
forms that differ by characters never varying on the same
tree, and never found connected by intermediate states.
After this discussion, the result of so much labor, he
emphatically remarks: "They are mistaken who repeat
that the greater part of our species are clearly limited,
and that the doubtful species are in a feeble minority.
This seemed to be true, so long as a genus was imperfectly
known, and its species were founded upon a few speci-
mens, that is to say, were provisional. Just as we come
to know them better, intermediate forms flow in, and
86
THE ORIGIN OF SPECIES
doubts as to specific limits augment." He also adds that
it is the best known species which present the greatest
number of spontaneous varieties and sub-varieties. Thus
Quercus robur has twenty-eight varieties, all of whicfc,
excepting six, are clustered round three sub-species,
namely, Q. pedunculata, sessiliflora, and pubescens. The
forms which connect these three sub-species are compara-
tively rare; and, as Asa Gray again remarks, if these
connecting forms which are now rare were to become
wholly extinct, the three sub-species would hold exactly
the same relation to each other as do the four or five
provisionally admitted species which closely surround the
typical Quercus robur. Finally, De Candolle admits that
out of the 300 species, which will be enumerated in his
Prodromus as belonging to the oak family, at least two-
thirds are provisional species, that is, are not known
strictly to fulfil the definition above given of a true spe-
cies. It should be added that De Candolle no longer
believes that species are immutable creations, but con-
cludes that the derivative theory is the most natural one,
"and the most accordant with the known facts in pale-
ontology, geographical botany and zoology, of anatomical
structure and classification."
When a young naturalist commences the study of a
group of organisms quite unknown to him, he is at first
much perplexed in determining what differences to con-
sider as specific, and what as varietal; for he knows
nothing of the amount and kind of variation to which
the group is subject; and this shows, at least, how very
generally there is some variation. But if he confine his
attention to one class within one country, he will soon
make up his mind how to rank most of the doubtful
VARIATION UNDER NATURE
87
forms. His general tendency will be to make many spe-
cies, for he will become impressed, just like the pigeon
or poultry fancier before alluded to, with the amount of
difference in the forms which he is continually studying;
and he has little general knowledge of analogical variation
in other groups and in other countries by which to cor-
rect his first impressions. As he extends the range of his
observations, he will meet with more cases of difficulty;
for he will encounter a greater number of closely-allied
forms. But if his observations be widely extended, he
will in the end generally be able to make up his own
mind; but he will succeed in this at the expense of ad-
mitting much variation — and the truth of this admission
will often be disputed by other naturalists. When he
comes to study allied forms brought from countries not
now continuous, in which case he cannot hope to find
intermediate links, he will be compelled to trust almost
entirely to analogy, and his difficulties will rise to a
climax.
Certainly no clear line of demarcation has as yet been
drawn between species and sub-species — that is, the forma
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 by an insensible series;
and a series impresses the mind with the idea of an
actual passage.
Hence I look at individual differences, though of small
interest to the systematist, as of the highest importance
for us, as being the first steps toward such slight varie-
ties as are barely thought worth recording in works on
88
THE ORIGIN OF SPECIES
natural history. And I look at varieties which are in
any degree more distinct and permanent as steps toward
more strongly-marked and permanent varieties; and at
the latter as leading to sub-species, and then to species.
The passage from one stage of difference to another may,
in many cases, be the simple result of the nature of the
^organism and of the different physical conditions to which
it has long been exposed; but with respect to the more
• important and adaptive characters, the passage from one
stage of difference to another may be safely attributed
to the cumulative action of natural selection, hereafter to
be explained, and to the effects of the increased use
or disuse of parts. A well-marked variety may there-
fore be called an incipient species; but whether this be-
lief is justifiable must be judged by the weight of the
various facts and considerations to be given throughout
this work.
It need not be supposed that all varieties or incipient
species attain the rank of species. They may become ex-
tinct, or they may endure as varieties for very long peri-
ods, as has been shown to be the case by Mr. Wollaston
with the varieties of certain fossil land-shells in Madeira,
and with plants by Gaston de Saporta. 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 coexist, and both rank
as independent species. But we shall hereafter return to
this subject.
From these remarks it will be seen that I look at the
term species as one arbitrarily given, for the sake of con-
venience, to a set of individuals closely resembling each
VARIATION UNDER NATURE
89
other, and that it does not essentially differ from the
term variety, which is given to less distinct and more
fluctuating forms. The term variety, again, in comparison
with mere individual differences, is also applied arbitrarily,
for convenience' sake.
Wide-ranging, much diffused, and common Species vary most
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 a
future work the discussion of these difficulties, and the
tables of the proportional numbers of the varying species.
Dr. Hooker permits me to add that after having carefully
read my manuscript, and examined the tables, he thinks
that the following statements are fairly well established.
The whole subject, however, treated as it necessarily here
is with much brevity, is rather perplexing, and allusions
cannot be avoided to the "struggle for existence," "diver-
gence of character," and other questions, hereafter to be
discussed.
Alphonse de Candolle and others have shown that
plants which have very wide ranges generally present
varieties; and this might have been expected, as they are
exposed to diverse physical conditions, and as they come
into competition (which, as we shall hereafter see, is an
equally or more important circumstance) with different
90
THE ORIGIN OF SPECIES
sets ot organic beings. But my tables further show that,
in any limited country, the species which are the most
common, that is, abound most in individuals, and the
species which are most widely diffused within their own
country (and this is a different consideration from wide
range, and to a certain extent from commonness), oftenest
give rise to varieties sufficiently well-marked to have
been recorded in botanical works. Hence it is the most
flourishing, or, as they may be called,- the dominant spe-
cies— those which range widely, are the most diffused in
their own country, and are the most numerous in indi-
viduals— which oftenest produce well-marked varieties, or,
as I consider them, incipient species. And this, perhaps,
might have been anticipated; for as varieties, in order
to become in any degree permanent, necessarily have to
struggle with the other inhabitants of the country, the
species which are already dominant will be the most
likely to yield offspring, which, though in some slight
degree modified, still inherit those advantages that en-
abled their parents to become dominant over their com-
patriots. In these remarks on predominance it should
be understood that reference is made only to the forms
which come into competition with each other, and more
especially to the members of the same genus or class
having nearly similar habits of life. With respect to the
number of individuals or commonness of species, the com-
parison of course relates only to the members of the same
group. One of the higher plants may be said to be
dominant if it be more numerous in individuals and more
widely diffused than the other plants of the same country
Thich live under nearly the same conditions. A plant of
chis kind is not the less dominant because some conferva
VARIATION UNDER NATURE
91
inhabiting the water or some parasitic fungus is infinitely
more numerous in individuals, and more widely diffused.
But if the conferva or parasitic fungus exceeds its allies
in the above respects, it will then be dominant within its
own class.
Species of the Larger Genera in each Country vary more
frequently than the Species of the Smaller Genera
If the plants inhabiting a country, as described in any
Flora, be divided into two equal masses, all those in the
larger genera (i.e., those including many species) being
placed on one side, and all those in the smaller genera
on the other side, the former will be found to include a
somewhat larger number of the very common and much
diffused or dominant species. This might have been an-
ticipated; for the mere fact of many species of the same
genus inhabiting any country, shows that there is some-
thing in the organic or inorganic conditions of that coun-
try favorable to the genus; and, consequently, we might
have expected to have found in the larger genera, or
those including many .species, a larger proportional num-
ber 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 obscu-
rity. Fresh-water and salt-loving plants generally have
very wide ranges and are much diffused, but this seems
to be connected with the nature of the stations inhabited
by them, and has little or no relation to the size of the
genera to which the species belong. Again, plants low in
the scale of organization are generally much more widely
diffused than plants higher in the scale; and here again
THE ORIGIN OF SPECIES
there is no close relation to the size of the genera. The
cause of lowly -organized plants ranging widely will be
discussed in our chapter on Geographical Distribution.
From looking at species as only strongly-marked and
well-defined varieties, I was led to anticipate that the spe-
cies of the larger genera in each country would oftener
present varieties than the species of the smaller genera;
for wherever many closely related species (i.e., species
of the same genus) have been formed, many varieties or
incipient species ought, as a general rule, to be now 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 favor-
able for variation; and hence we might expect that the
circumstances would generally be still favorable to varia-
tion. On the other hand, if we look at each species as a
special act of creation, there is no apparent reason why
more varieties should occur in a group having many
species than in one having few.
To test the truth of this anticipation I have arranged
the plants of twelve countries, and the coleopterous 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 larger proportion
of the species on the side of the larger genera presented
varieties than on the side of the smaller genera. More-
over, the species of the large genera which present any
varieties invariably present a larger average number of
varieties than do the species of the small genera. Both
these results follow when another division is made, and
when all the least genera, with from only one to four
VARIATION UNDER NATURE
species, are altogether excluded from the tables. These
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 where, if we may use the expression, the
manufactory of species has been active, we ought gen-
erally to find the manufactory still in action, more espe-
cially as we have every reason to believe the process of
manufacturing new species to be a slow one.. And this
certainly holds true, if varieties be looked at as incipient
species; for my tables clearly show as a general rule that,
wherever many species of a genus have been formed, the
species of that genus present a number of varieties, that
is of incipient species, beyond the average. It is not
that all large genera are now varying much, and are
thus increasing in the number of their species, or that
no small genera are now varying and increasing; for if
this had been so, it would have been fatal to my theory;
inasmuch as geology plainly tells us that small genera
have in the lapse of time often increased greatly in size;
and that large genera have often come to their maxima,
decline, 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 certainly
holds good.
Many of the Species included within the Larger Genera re-
semble Varieties in being very closely, but unequally,
related to each other, and in having restricted ranges
There are other relations between the species of large
genera and their recorded varieties which deserve notice.
We have seen that there is no infallible criterion by
THE ORIGIN OF SPECIES
which to distinguish species and well-marked varieties;
and when intermediate links have not been found be-
tween 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 endeavored to test
this numerically by averages, and, as far as my imperfect
results go, they confirm the view. I have also consulted
some sagacious and experienced observers, and, after de-
liberation, they concur in this view. In this respect,
therefore, the species of the larger genera resemble varie-
ties more than do the species of the smaller genera. Or
the case may be put in another way, and it may be said,
that in the larger genera, in which a number of varieties
or incipient species greater than the average are now
manufacturing, many of the species already manufac-
tured still to a certain extent resemble varieties, for they
differ from each other by less than the 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
VARIATION UNDER NATURE
95
like satellites around other species. And what are vari-
eties 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 prin-
ciple, as I call it, of Divergence of Character, we shall
see how this may be explained, and how the lesser dif-
ferences between varieties tend to increase into the greater
differences between species.
There is one other point which is worth notice.
Varieties generally have much-restricted ranges: this
statement is indeed scarcely more than a truism, for,
if a variety were found to have a wider range than
that of its supposed parent- species, their denominations
would be reversed. But there is reason to believe that
the species which are very closely allied to other species,
and in so far resemble varieties, often have much-re-
stricted ranges. For instance, Mr. H. C. Watson has
marked for me in the well-sifted London Catalogue of
plants (4th edition) 63 plants which are therein ranked
as species, but which he considers as so closely allied to
other species as to be of doubtful value: these 63 reputed
species range on an average over 6*9 of the provinces
into which Mr. Watson has divided Great Britain. Now,
in this same Catalogue, 53 acknowledged varieties are re-
corded, and these range over 7*7 provinces; whereas, the
species to which these varieties belong range over 14*3
provinces. So that the acknowledged varieties have
86
THE ORIGIN OF SPECIES
nearly the same restricted average range as have the
closely allied forms, marked for me by Mr. Watson as
doubtful species, but which are almost universally ranked
by British botanists as good and true species.
/Summary
Finally, varieties cannot be distinguished from species
— except, first, by the discovery of intermediate linking
forms; and, secondly, oy a certain indefinite amount of
difference between them; for two forms, if differing very
little, are generally ranked as varieties, notwithstanding
that they cannot be closely connected; but the amount of
difference considered necessary to give to any two forms
the rank of species cannot be defined. 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 other species. Species very closely allied
to other species apparently have restricted ranges. In
all these respects the species of large genera present
a strong analogy with varieties. And we can clearly
understand these analogies, if species once existed as
varieties, and thus originated; whereas, these analogies
are utterly inexplicable if species are independent cre-
ations.
We have, also, seen that it is the most flourishing or
dominant species of the larger genera within each class
which on an average yield the greatest number of varie-
ties; and varieties, as we shall hereafter see, tend to
become converted into new and distinct species. Thus
the larger genera tend to become larger; and throughout
VARIATION UNDER NATURE
97
nature the forms of life which are now dominant tend to
become still more dominant by leaving many modified
and dominant descendants. But by steps hereafter to be
explained, the larger genera also tend to break up into
smaller genera. And thus, the forms of life throughout
the universe become divided into groups subordinate to
groups.
—Science — 5
98
THE OEIGIN OF SPECIES
CHAPTER III
STRUGGLE FOR EXISTENCE
Its bearing on natural selection — The term used in a wide sense — Geo-
metrical ratio of increase — Rapid increase of naturalized animals and
plants — Nature of the checks to increase — Competition universal —
Effects of climate — Protection from the number of individuals — Com-
plex relations of all animals and plants throughout nature — Struggle for
life most severe between individuals and varieties of the same species:
often severe between species of the same genus — The relation of organ-
ism to organism the most important of all relations
BEFORE entering on the subject of this chapter, I
must make a few preliminary remarks, to show
how the struggle for existence bears on Natural
Selection. It has been seen in the last chapter that
among organic beings in a state of nature there is some
individual variability: indeed I am not aware that this
has ever been disputed. It is immaterial for us whether
a multitude of doubtful forms be called species or sub-
species or varieties; what rank, for instance, the two or
three hundred doubtful forms of British plants are en-
titled to hold, if the existence of any well-marked varie-
ties 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
organization to another part, and to the conditions of
life, and of one organic being to another being, been
STRUGGLE FOR EXISTENCE
99
perfected"? We see these beautiful co-adaptations most
plainly in the woodpecker and the mistletoe; and only a
little less plainly in the humblest parasite which clings
to the hairs of a quadruped or feathers of a bird; in the
structure of the beetle which dives through the water; in
the plumed seed which is wafted by the gentlest breeze;
in short, we see beautiful adaptations everywhere and in
every part of the organic world.
Again, it may be asked, how is it that varieties, which
I have called incipient species, become ultimately 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 spe-
cies of the same genus, arise ? All these results, as we
shall more fully see in the next chapter, follow from the
struggle for life. Owing to this struggle, variations, how-
ever slight and from whatever cause proceeding, if they
be in any degree profitable to the individuals of a spe-
cies, in their infinitely complex relations to other organic
beings and to their physical conditions of life, will tend
to the preservation of such individuals, and will generally
be inherited by the 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 prin-
ciple, by which each slight variation, if useful, is pre-
served, by the term Natural Selection, in order to mark
its relation to man's power of selection. But the expres-
sion often used by Mr. Herbert Spencer of the Survival
of the Fittest is more accurate, and is sometimes equally
100
THE ORIGIN OF SPECIES
convenient. We have seen that man by selection can
certainly produce great results, and can adapt organic
beings to his own uses, through the accumulation of
slight but useful variations, given to him by the hand
of Nature. But Natural Selection, as we shall hereafter
see, is a power incessantly ready for action, and is as
immeasurably superior to man's feeble efforts as the
works of Nature are to those of Art.
We will now discuss in a little more detail the strug-
gle for existence. In my future work this subject will
be treated, as it well deserves, at greater length. The
elder De Candolle and Lyell 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 W. Herbert,
Dean of Manchester, evidently the result of his great hor-
ticultural knowledge. Nothing is easier than to admit in
words the truth of the universal struggle for life, or more
difficult — at least I have found it so — than constantly to
bear this conclusion in mind. Yet unless it be thoroughly
ingrained in the mind, the whole economy of nature, with
every fact on distribution, rarity, abundance, extinction,
and variation, will be dimly seen or quite misunderstood.
We behold the face of nature bright with gladness, we
often see superabundance of food; we do not see or we
forget that the birds which are idly singing round us
.mostly live on insects or seeds, and are thus constantly
destroying life; or we forget how largely these songsters,
or their eggs, or their nestlings, are destroyed by birds
and beasts of prey; we do not always bear in mind that,
though food may be now superabundant, it is not so at
all seasons of each recurring year.
STRUGGLE FOR EXISTENCE
101
Hie Term, Struggle for Existence, used in a large sense
I should premise that I use this term 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 prop-
erly it should be said to be dependent on the moisture.
A plant which annually produces a thousand seeds, of
which only one of an average 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
mistletoe is dependent on the apple and a few other
trees, but can only in a far-fetched sense be said to
struggle with these trees, for, if too many of these para-
sites grow on the same tree, it languishes and dies. But
several seedling mistletoes, growing close together on the
same branch, may more truly be said to struggle with
each other. As the mistletoe is disseminated by birds,
its existence depends on them; and it may metaphorically
be said to struggle with other fruit-bearing plants, in
tempting the birds to devour and thus disseminate its
seeds. In these several senses, which pass into each
other, I use for convenience' sake the general term of
Struggle for Existence.
Geometrical Ratio of Increase
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
102
THE ORIGIN OF SPECIES
several eggs or seeds, must suffer destruction during
some period of its life, and during some season or occa-
sional year, otherwise, on the principle of geometrical
increase, its numbers would quickly become so inordi-
nately great that no country could support the product.
Hence, as more individuals are produced than can possi-
bly survive, there must in every case be a struggle for
existence, either one individual with another of the same
species, or with the individuals of distinct species, or
with the physical conditions of life. It is the doctrine
of Malthus applied with manifold force to the whole
animal and vegetable kingdoms; for in this case there
can be no artificial increase of food, and no prudential
restraint from marriage. Although some species may be.
now increasing, more or less rapidly, in numbers, all
cannot do so, for the world would not hold them.
There is no exception to the rule that every organic
being naturally increases at so high a rate that, if not
destroyed, the earth would soon be covered by the
progeny of a single pair. Even slow-breeding man has
doubled in twenty -five years, and at this rate, in less
than a 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 seed-
lings next year produced two, and so on, then in twenty
years there would be a million plants. The elephant is
reckoned 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 safest to assume that
it begins breeding when thirty years old, and goes on
breeding till ninety years old, bringing forth six young in
STRUGGLE FOR EXISTENCE
103
the interval, and surviving till one hundred years old; if
this be so, after a period of from 740 to 750 years there
would be nearly nineteen million elephants alive, de-
scended from the first pair.
X But we have better evidence on this subject than
mere theoretical calculations; namely, the numerous re-
corded cases of the astonishingly rapid increase of various
animals in a state of nature, when circumstances have
been favorable to them during two or three following
seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in
several parts of the world; if the statements of the rate
of increase of slow-breeding cattle and horses in South
America, and latterly in Australia, had not been well
authenticated, they would have been 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,
such as the cardoon and a tall thistle, which are now the
commonest over the wide plains of La Plata, clothing
square leagues of surface almost to the exclusion of
every other plant, 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 dis-
covery. In such cases, and endless others could be
given, no one supposes that the fertility of the animals
or plants has been suddenly and temporarily increased in
any sensible degree. The obvious explanation is that the
conditions of life have been highly favorable, and that
there has consequently been less destruction of the old
and young, and that nearly all the young have been
104
THE ORIGIN OF SPECIES
enabled to breed. Their geometrical ratio of increase, the
result of which never fails to be surprising, simply ex-
plains their extraordinarily rapid increase and wide diffu-
sion in their new homes.
In a state of nature almost every full-grown plant
annually produces seed, and among animals there are
very few which do not annually pair. Hence we may
confidently assert that all plants and animals are tending
to increase at a geometrical ratio — that all would rapidly
stock every station in which they could anyhow exist —
and that this 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, but we do not keep in mind that thousands are
annually slaughtered for food, and that in a state of
nature an equal number would have somehow to be
disposed of.
The only difference between organisms which annually
produce eggs or seeds by the thousand, and those which
produce extremely few, is, that the slow-breeders would
require a few more years to people, under favorable con-
ditions, 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 an-
other, like the hippobosca, a single one; but this differ-
ence 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
STRUGGLE FOR EXISTENCE
105
which depend on a 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 iu any way protect its own eggs
or young, a small number may be produced, and yet the
average stock be fully kept up; but if many eggs or
young are destroyed, many must be produced, or the
species will become extinct. It would suffice to keep
up the full number of a tree, which lived on an average
for a thousand years, if a single seed were produced once
in a thousand years, supposing that this seed were never
destroyed, and could be insured to germinate in a fitting
place. So that, in all cases, the average number of any
animal or plant depends only indirectly on the number
of its eggs or seeds.
In looking at Nature, it is most necessary to keep the
foregoing considerations always in mind — never to forget
that every single organic being 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 destruc-
tion 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.
Nature of the Checks to Increase
The causes which check the natural tendency of each
species to increase are most obscure. Look at the most
vigorous species; by as much as it swarms in numbers,
106
THE ORIGIN OF SPECIES
by so much will it tend to increase still further. We
know not exactly what the checks are even in a single
instance. Nor will this surprise any one who reflects how
ignorant we are on this head, even in regard to mankind,
although so incomparably better known than any other
animal. This subject of the checks to increase has been
ably treated by several authors, and I hope in a future
work to discuss it 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, it appears that the seedlings suffer most
from germinating in ground already thickly stocked with
other plants. Seedlings, also, are destroyed in vast num-
bers 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 357 no less than 295 were destroyed,
chiefly by slugs and insects. If turf which has long been
mown, and the case would be the same with turf closely
browsed by quadrupeds, be let to grow, the more vigorous
plants gradually kill the less vigorous, though fully
grown plants; thus out of twenty species growing on a
little plot of mown 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
STRUGGLE FOR EXISTENCE 107
frequently it is not the obtaining food, but the serving
as prey to other animals, which determines the average
numbers of a species. Thus, there seems to be little
doubt that the stock of partridges, grouse and hares on
any large estate depends chiefly on the destruction of
vermin. If not one head of game were shot during the
next twenty years in England, and, at the same time,
if no vermin were destroyed, there would, in all proba-
bility, be less game than at present, although hundreds
of thousands of game animals are now annually shot. On
the other hand, in some cases, as with the elephant, none
are destroyed by beasts of prey; for 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 seem to be the most effective
of all checks. I estimated (chiefly from the greatly re-
duced numbers of nests in the spring) 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 cli-
mate 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 strug-
gle between the individuals, whether of the same or of
distinct species, which subsist on the same kind of food.
Even when climate, for instance extreme cold, acts di-
rectly, it will be the least vigorous individuals, or those
which have got least food through the advancing winter,
which will suffer most. When we travel from south to
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THE ORIGIN OF SPECIES
north, or from a damp region to a dry, we invariably
see some species gradually getting rarer and rarer, and
finally disappearing; and the change of climate being
conspicuous, we are tempted to attribute the whole effect
to its direct action. But this is a false view; we forget
that each species, even where it most abounds, is con-
stantly 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 favored by any slight change of cli-
mate, they will increase in numbers; and as each area is
already fully stocked with inhabitants, the other species
must decrease. When we travel southward and see a
Bpeciea decreasing in numbers, we may feel sure that the
cause lies quite as much in other species being favored
as in this one being hurt. So it is when we travel north-
ward, but in a somewhat lesser degree, for the number
of species of all kinds, and therefore of competitors, de-
creases northward; hence in going northward, or in as-
cending a mountain, we far oftener meet with stunted
forms, due to the directly injurious action of climate,
than we do in proceeding southward or in descending a
mountain. When we reach the Arctic regions or snow-
capped summits, or absolute deserts, the struggle for life
is almost exclusively with the elements.
That climate acts in main part indirectly by favoring
other species, we clearly see in the prodigious number of
plants which in our gardens can perfectly well endure
our climate, but which never become • naturalized, for
they cannot compete with our native plants nor resist
destruction by our native animals.
When a species, owing to highly favorable circum-
STRUGGLE FOR EXISTENCE
105
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
limiting check independent of the struggle for life. But
even some of these so-called epidemics appear to be due
to parasitic worms, which have from some cause, possibly
in part through facility of diffusion among the crowded
animals, been disproportionally favored: and here comes
in a sort of struggle between the parasite and its prey.
On the other hand, in many cases, a large stock of
individuals of the same species, relatively to the numbers
of its enemies, is absolutely necessary for its preserva-
tion. Thus we can easily raise plenty of corn and rape-
seed, etc., in our fields, because the seeds are in great
excess compared with the number of birds which feed
on them; nor can the birds, though having a superabun-
dance of food at this one season, increase in number
proportionally to the supply of seed, as their numbers
are checked during winter; but any one who has tried,
knows how troublesome it is to get seed from a few
wheat or other such plants in a garden: I have in this
case lost every single seed. This view of the necessity
of a large stock of the same species for its preservation,
explains, I believe, some singular facts in nature, such at*
that of very rare plants being sometimes extremely abun-
dant, in the few spots where they do exist; and that of
some social plants being social, that is abounding in
individuals, even on the extreme verge of their range.
For in such cases, we may believe, that a plant could
exist only where the conditions of its life were so favor-
able that many could exist together, and thus save the
species from utter destruction. I should add that fee
no
THE ORIGIN OF SPECIES
good effects of intercrossing, and the ill effects of close
interbreeding, no doubt come into play in many of these
cases; but I will not here enlarge on this subject.
Complex Relations of all Animals and Plants to each other
in the Struggle for Existence
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, 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 inclosed 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 propor-
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 effect of the introduction of a single
tree, nothing whatever else having been done, with the
exception of the land having been inclosed, so that cattle
could not enter. But how important an element inclosure
STRUGGLE FOR EXISTENCE
111
is, I plainly saw near Farnham, in Surrey. Here there
are extensive heaths, with a few clumps of old Scotch
firs on the distant hilltops: within the last ten years
large spaces have been inclosed, and sSlf-sown firs are
now springing up in multitudes, so close together that
all cannot live. When I ascertained that these young
trees had not been sown or planted, I was so much sur-
prised at their numbers that I went to several points of
view, whence I could examine hundreds of acres of the
uninclosed heath, and literally I could not see a single
Scotch fir, except the old planted clumps. But on look-
ing 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, dur-
ing 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 inclosed, it became thickly clothed with
vigorously growing young firs. Yet the heath was so
extremely barren and so extensive that no one would
ever have imagined that cattle would have so closely and
effectually searched it for food.
Here we see that cattle absolutely determine the exist-
ence of the Scotch fir; but in several parts of the world
iisects determine the existence of cattle. Perhaps Para-
guay offers the most curious instance of this; for here
neither cattle nor horses nor dogs have ever run wild,
though they swarm southward and northward in a feral
state; and Azara and Eengger have shown that this is
caused by the greater number in Paraguay of a certain
112
TEE ORIGIN OF SPECIES
fly, which lays its eggs in the navels of these animals
when first born. The increase of these flies, numerous
as they are, must be habitually checked by some means,
probably by other parasitic insects. Hence, if certain in-
sectivorous birds were to decrease in Paraguay, the para-
sitic insects would probably increase; and this would
lessen the number of the navel-frequenting flies — then
cattle and horses would become feral, and this would
certainly greatly alter (as indeed I have observed in
parts of South America) the vegetation: this again would
largely affect the insects; and this, as we have just seen
in Staffordshire, the insectivorous birds, and so onward
in ever-increasing circles of complexity. Not that under
nature the relations will ever be as simple as this. Bat-
tle within battle must be continually recurring with vary-
ing success; and yet in the long run the forces are so
nicely balanced that the face of nature remains for long
periods of time uniform, though assuredly the merest
trifle would give the victory to one organic being over
another. Nevertheless, so profound is our ignorance,
and so high our presumption, that we marvel when
we hear of the extinction of an organic being; and as
we do not see the cause, we invoke cataclysms to deso-
late the world, or invent laws on the duration cf the
forms of life!
I am tempted to give one more instance showing how
plants and animals, 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 is never visited in my garden by insects, and
consequently, from its peculiar structure, never sets a
seed. Nearly all our orchidaceous plants absolutely re-
STRUGGLE FOR EXISTENCE
113
quire the visits of insects to remove their pollen-masses
and thus to fertilize them. I find from experiments that
humble-bees are almost indispensable to the fertilization
of the heart's-ease (Yiolo tricolor), for other bees do not
visit this flower. I have also found that the visits of
bees are necessary for the fertilization of some kinds of
clover; for instance, 20 heads of Dutch clover (Trifolium
repens) yielded 2,290 seeds, but 20 other heads protected
from bees produced not one. Again, 100 heads of red
clover (T. pratense) produced 2,700 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 nectar. It has been suggested that moths may
fertilize the clovers; but I doubt whether they could do
so in the case of the red clover, from their weight not
being sufficient to depress the wing petals. Hence we
may infer as highly probable that, if the whole genus
of humble-bees became extinct or very rare in England,
the heart's-ease and red clover would become very rare,
or wholly disappear. The number of humble-bees in any
district depends in a great measure upon the number of
field-mice, which destroy their combs and nests; and Col.
Newman, who has long attended to the habits of humble-
bees, believes that "more than two-thirds of them are
thus destroyed all over England." Now the number of
mice is largely dependent, as every one knows, on the
number of cats; and Col. Newman says, "Near villages
and small towns I have found the nests of humble-bees
more numerous than elsewhere, which I attribute to the
number of cats that destroy the mice." Hence it is quite
credible that the presence of a feline animal in large
numbers in a district might determine, through the in-
114
THE ORIGIN OF SPECIES
tervention first of mice and then of bees, the frequency
of certain flowers in that district!
In the case of every species, many different checks,
acting at different periods of life, and during different
seasons or years, probably come into play; some one
check or some few being generally the most potent; bat
all will concur in determining the average number or
even the existence of the species. In some cases it can
be shown that widely-different checks act on the same
species in different districts. When we look at the plants
and bushes clothing an entangled bank, we are tempted
to attribute their proportional numbers and kinds to what
we call chance. But how false a view is this! Every
one has heard that when an American forest is cut down,
a very different vegetation springs up; but it has been
observed that ancient Indian ruins in the Southern
United States, which must formerly have been cleared
of trees, now display the same beautiful diversity and
proportion of kinds as in the surrounding virgin forest.
What a struggle must have gone on during long centu-
ries between the several kinds of trees, each annually
scattering its seeds by the thousand; what war between
insect and insect — between insects, snails, and other ani-
mals with birds and beasts of prey — all striving to
increase, all feeding on each other, or on the trees, their
seeds and seedlings, or on the other plants which first
clothed the ground and thus checked the growth of the
trees! Throw up a handful of feathers, and all fall to
the ground according to definite laws; but how simple
is the problem where each shall fall compared to that of
the action and reaction of the innumerable plants and
animals which have determined, in the course of centiu
STRUGGLE FOR EXISTENCE
115
ries, the proportional numbers and kinds of trees now
growing on the old Indian ruins!
The dependency of one organic being on another, as
of a parasite on its prey, lies generally between beings
remote in the scale of nature. This is likewise some-
times the case with those which may be strictly said to
struggle with each other for existence, as in the case
of locusts and grass-feeding quadrupeds. But the struggle
will almost invariably be most severe between the indi-
viduals of the same species, for they frequent the same
districts, require the same food, and are exposed to the
same dangers. In the case of varieties of the same
species, the struggle will generally be almost equally
severe, and we sometimes see the contest soon decided:
for instance, if several varieties of wheat be sown
together, and the mixed seed be resown, some of the
varieties which best suit the soil or climate, or are
naturally the most fertile, will beat the others and so
yield more seed, and will consequently in a few years
supplant the other varieties. To keep up a mixed stock
of even such extremely close varieties as the variously-
colored sweet peas, they must be each year harvested
separately, and the seed then mixed in due proportion,
otherwise the weaker kinds will steadily decrease in num-
ber and disappear. So again with the varieties of sheep;
it has been asserted that certain mountain-varieties will
starve out other mountain-varieties, so that they cannot
be kept together. The same result has followed from
keeping together different varieties of the medicinal
leech. It may even be doubted whether the varieties
of any of our domestic plants or animals have so exactly
the same strength, habits, and constitution, that the
116
THE ORIGIN OF SPECIES
original proportions of a mixed stock (crossing being
prevented) could be kept up for half a dozen genera-
tions, if they were allowed to struggle together, in the
same manner as beings in a state of nature, and if
the seed or young were not annually preserved in
due proportion.
Struggle for Life most severe between Individuals and
Varieties of the same Species
As the species of the same genus usually have, though
by no means invariably, much similarity in habits and
constitution, and always in structure, the struggle will
generally be more severe between them, if they come into
competition with each other, than between the species of
distinct genera. We see this in the recent extension over
parts of the United States of one species of swallow hav-
ing caused the decrease of another species. The recent
increase of the missel-thrush in parts of Scotland has
caused the decrease of the song-thrush. How frequently
we hear of one species of rat taking the place of another
species under the most different climates! In Eussia the
small Asiatic cockroach has everywhere driven before it
its great congener. In Australia the imported hive-bee
is rapidly exterminating the small, stingless native bee.
One species of charlock has been known to supplant
another species; and so in other cases. We can dimly
see why the competition should be most severe between
allied forms, which fill nearly the same place in the
economy of nature; but probably in no one case could
we precisely say why one species has been victorious
over another in the great battle of life.
A corollary of the highest importance may be deduced
STRUGGLE FOR EXISTENCE
117
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 the other organic
beings, with which it comes into competition for food or
residence, or from which it has to escape, or on which
it preys. This is obvious in the structure of the
teeth and talons of the tiger; and in that of the legs
and claws of the parasite which clings to the hair on the
tiger's body. But in the beautifully plumed seed of
the dandelion, and in the flattened and fringed legs
of the water-beetle, the relation seems at first confined
to the elements of air and water. Yet the advantage of
plumed seeds no doubt stands in the closest relation to
the land being already thickly clothed with other plants;
so that the seeds may be widely distributed and fall on
unoccupied ground. In the water-beetle, the structure of
its legs, so well adapted for diving, allows it to compete
with other aquatic insects, to hunt for its own prey, and
to escape serving as prey to other animals.
The store of nutriment laid up within the seeds of
many plants seems at first sight to have no sort of rela-
tion 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, it may be
suspected that the chief use of the nutriment in the seed
is to favor the growth of the seedlings, while struggling
with other plants growing rigorously 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
118
THE ORIGIN OF SPECIES
we can clearly see that if we wish in imagination to give
the plant the power of increasing in number, we should
have to give it some advantage over its competitors, or
over the animals which prey on it. On the confines
of its geographical range, a change of constitution with
respect to climate would clearly be an advantage to our
plant; but we have reason to believe that only a few
plants or animals range so far that they are destroyed
exclusively by the rigor of the climate. Not until we
reach the extreme confines of life, in the Arctic regions
or on the borders of an utter desert, will competition
cease. The land may be extremely cold or dry, yet there
will be competition between some few species, or between
the individuals of the same species, for the warmest or
dampest spots.
Hence we can see that when a plant or animal is
placed in a new country among new competitors, the
conditions of its life will generally be changed in an
essential manner, although the climate may be exactly
the same as in its former home. If its average numbers
are to increase in its new home, we should have to
modify it in a different way to what we should have had
to do in its native country; for we should have to give
it some advantage over a different set of competitors or
enemies.
It is good thus to try in imagination to give to any
one species an advantage over another. Probably in no
single instance should we know what to do. This ought
to convince us of our ignorance on the mutual relations
of all organic beings; a conviction as necessary as it is
difficult to acquire. All that we can do is to keep
steadily in mind that each organic being is striving to
STRUGGLE FOR EXISTENCE
119
Increase in 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
sutler great destruction. When we reflect on this strug-
gle, 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.
120
THE ORIGIN OF SPECIES
CHAPTER IV
NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST
Natural Selection : its power compared with man's selection ; its power
on characters of trilling importance; its power at all ages and on
both sexes — Sexual Selection — On the generality of intercrosses be-
tween individuals of the same species — Circumstances favorable and
unfavorable to the results of 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 small area, and to naturalization — Action of
Natural Selection, through Divergence of Character, and Extinction, .on
the descendants from a common parent- — Explains the grouping of all
organic beings — Advance in organization — Low forms preserved —
Convergence of character — Indefinite multiplication of speciss — Sum-
mary
¥ IOW WILL the struggle for existence, briefly dis-
1 I cussed in the last chapter, act in regard to varia-
tion? Can the principle of selection, which .we
have seen is so potent in the hands of man, apply under
nature? I think we shall see that it can act most effi-
ciently. Let the endless number of slight variations and
individual differences occurring in our domestic produc-
tions, and, in a lesser degree, in those under nature, be
borne in mind; as well as the strength of the hereditary
tendency. Under domestication, it may be truly said that
the whole organization becomes in some degree plastic.
But the variability, which we almost universally meet
with in our domestic productions, is not directly pro-
duced, as Hooker and Asa Gray have well remarked, by
man; he can neither originate varieties, nor prevent their
NATURAL SELECTION
121
occurrence; he can only preserve and accumulate such as
do occur. Unintentionally he exposes organic beings to
new and changing conditions of life, and variability
ensues; but similar changes of conditions might and do
occur Under nature. Let it also be borne in mind how
infinitely complex and close-fitting are the mutual rela-
tions of all organic beings to each other and to their
physical conditions of life; and consequently what in-
finitely varied diversities of structure might be of use
to each being under changing conditions of life. Can
it, then, be thought improbable, seeing that variations
useful to man have undoubtedly occurred, that other
variations useful in some way to each being in the great
and complex battle of life, should occur in the course of
many successive generations? If such do occur, can we
doubt (remembering that many more individuals are born
than can possibly survive) that individuals having any
advantage, however slight, over others, would have the
best chance of surviving and of procreating their kind?
On the other hand, we may feel sure that any variation
in the least degree injurious would be rigidly destroyed.
This preservation of favorable individual differences and
variations, and the destruction of those which are injuri-
ous, I have called Natural Selection, or the Survival of
the Fittest. Variations neither useful nor injurious would
not be affected by natural selection, and would be left
either a fluctuating element, as perhaps we see in certain
polymorphic species, or would ultimately become fixed,
owing to the nature of the organism and the nature of
the conditions.
' Several writers have misapprehended or objected to
the term Natural Selection. Some have even imagined
— Science — 6
THE ORIGIN OF SPECIES
\ that natural selection induces variability, whereas it im-
plies only the preservation of such variations as arise
and are beneficial to the being under its conditions of
life. No one objects to agriculturists speaking of the
potent effects of man's selection; and in this case the in-
dividual differences given by nature, which man for some
objects selects, must of necessity first occur. /XOthers have
objected that the term selection implies conscious choice
in the animals which become modified; and it has even
been urged that, as plants have no volition, natural se-
lection is not applicable to them! In the literal sense
of the word, no doubt, natural selection is a false term;
but who ever objected to chemists speaking of the elec-
tive affinities of the various elements? — and yet an acid
cannot strictly be said to elect the base with which it
in preference combines. It has been said that I speak
of natural selection as an active power or Deity; but
who objects to an author speaking of the attraction of
gravity as ruling the movements of the planets? Every
one knows what is meant and is implied by such meta
phorical expressions; and they are almost necessary for
brevity. So again it is difficult to avoid personifying the
word Nature; but I mean by Nature only the aggregate
action and product of many natural laws, and by laws
the sequence of events as ascertained by us. With a
little familiarity such superficial objections will be for-
gotten.
We shall best understand the probable course of natu-
ral selection by taking the case of a country undergoing
some slight physical change, for instance, of climate.
The proportional numbers of its inhabitants will almost
immediately undergo a change, and some species will
NATURAL SELECTION
123
probably become extinct. We may conclude, from what
we have seen of the intimate and complex manner in
which the inhabitants of each country are bound to-
gether, that any change in the numerical proportions
of the inhabitants, independently of the change of cli-
mate itself, would seriously affect the others. If the
country were open on its borders, new forms would
certainly immigrate, and this would likewise seriously
disturb the relations of some of the former inhabitants.
Let it be remembered how powerful the influence of a
single introduced tree or mammal has been shown to be.
But in the case of an island, or of a country partly sur-
rounded by barriers, into which new and better adapted
forms could not freely enter, we should then have places
in the economy of nature which would assuredly be bet-
ter 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 cases, slight modifications,
which in any way favored the individuals of any spe-
cies, by better adapting them to their altered conditions,
would tend to be preserved; and natural selection would
have free scope for the work of improvement.
We have good reason to believe, as shown in the first
chapter, that changes in the conditions of life give a
tendency to increased variability; and in the foregoing
cases the conditions have changed, and this would man-
ifestly be favorable to natural selection, by affording a
better chance of the occurrence of profitable variations.
Unless such occur, natural selection can do nothing.
Under the term of "variations," it must never be for-
gotten that mere individual differences are included. As
124
THE ORIGIN OF SPECIES
man can produce a great result with his domestic animals
and plants by adding up in any given direction individ-
ual differences, so could natural selection, but far more
easily from having incomparably longer time for action.
Nor do I believe that any great physical change, as of
climate, or any unusual degree of isolation to check im-
migration, is necessary in order that new and unoccupied
places should be left for natural selection to fill up by
improving some of the varying inhabitants. For as all
the inhabitants of each country are struggling together
with nicely balanced forces, extremely slight modifica-
tions in the structure or habits of one species would
often give it an advantage over others; and still further
modifications of the same kind would often still further
increase the advantage, as long as the species continued
under the same conditions of life and profited by similar
means of subsistence and defence. No country can be
named in which all the native inhabitants are now so
perfectly adapted to each other and to the physical con-
ditions under which they live, that none of them could
be still better adapted or improved; for in all countries
the natives have been so far conquered by naturalized
productions that they have allowed some foreigners to
take firm possession of .the land. And as foreigners have
thus in every country beaten some of the natives, we may
safely conclude that the natives might have been modified
with advantage, so as to have better resisted the intruders.
As man can produce, and certainly has produced, a
great result by his methodical and unconscious means
iof selection, what may not natural selection effect ? Man
&an act only on external and visible characters: Nature,
if I may be allowed to personify the natural preservation
NATURAL SELECTION
125
or survival of the fittest, cares nothing for appearances,
except in so far as they are useful to any being. She
can act on every internal organ, on every shade of con-
stitutional difference, on the whole machinery of life.
Man selects only for his own good: Nature only for
that of the being which she tends. Every selected
character is fully exercised by her, as is implied by
the fact of their selection. Man keeps the natives of
many climates in the same country; he seldom exercises
each selected character in some peculiar and fitting man-
ner; 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 the eye or
to be plainly useful to him. Under nature, the slightest
differences of structure or constitution may well turn the
nicely-balanced scale in the struggle for life, and so be
preserved. How fleeting are the wishes and efforts of
man! how short his time! and consequently how poor
will be his results, compared with those accumulated by
Nature during whole geological periods! Can we wonder,
then, that Nature's productions should be far "truer" in
character than man's productions; that they should be
infinitely better adapted to the most complex conditions
of life, and should plainly bear the stamp of far higher
workmanship ?
126
THE ORIGiy OF SPECIES
It may metaphorically be said that natural selection is
daily and hourly scrutinizing, throughout the world, the
slightest variations; rejecting those that are bad, preserv-
ing and adding up all that are good; silently and insen-
sibly working, whenever and wherever opportunity offers, at
the improvement of each organic being in relation to its
organic and inorganic conditions of life. We see nothing
of these slow changes in progress, until the hand of time
has marked the lapse of ages, and then so imperfect is
our view into long-past geological ages, that we see only
that the forms of life are now different from what they
formerly were.
In order that any great amount of modification should
be effected in a species, a variety when once formed must
again, perhaps after a long interval of time, vary or pre-
sent individual differences of the same favorable nature
as before; and these must be again preserved, and so
onward step by step. Seeing that individual differences
of the same kind perpetually recur, this can hardly be
considered as an unwarrantable assumption. But whether
it is true, we can judge only by seeing how far the hy-
pothesis accords with and explains the general phenom-
ena of nature. On the other hand, the ordinary belief
that the amount of possible variation is a strictly limited
quantity is likewise a simple assumption.
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
insects green, and bark-feeders mottled -gray ; the alpine
ptarmigan white in winter, the red gronse the color of
heather, we must believe that these tints are of service
NATURAL SELECTION
127
to these birds and insects in preserving them from dan-
ger. 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 natural selection might be effective in 'jiving the
proper color to each kind of grouse, and in keeping that
color, when once acquired, true and constant. Nor ought
we to think that the occasional destruction of an animal
of any particular color would produce little effect: we
should remember how essential it is in a flock of white
sheep to destroy a lamb with the faintest trace of black.
We have seen how the color of the hogs, which feed on
the "paint-root" in Virginia, determines whether they
shall live or die. In plants, the down on the fruit and
the color of the flesh are considered by botanists as char-
acters 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 bee-
tle, a Curculio, than those with clown; 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 colored flesh. If, with all
the aids of art, these slight differences make a great dif-
ference 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 differ-
ences would effectually settle which variety, whether a
smooth or downy, a yellow or purple fleshed fruit,
should succeed.
128
THE ORIGIN OF SPECIES
In looking at many small points of difference be-
tween species, which, as far as our ignorance permits
us to judge, seem quite unimportant, we must not forget
that climate, food, etc., have no doubt produced some
direct effect. It is also necessary to bear in mind that,
owing to the law of correlation, when one part varies,
and the variations are accumulated through natural selec-
tion, other modifications, often of the most unexpected
nature, will ensue.
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 shape, size, and flavor of the seeds of the many
varieties of our culinary and agricultural plants; in the
caterpillar and cocoon stages of the varieties of the silk-
worm; in the eggs of poultry, and in the color of the
down of their chickens; in the horns of our sheep and
cattle when nearly adult; — so in a state of nature natural
selection will be enabled to act on and modify organic
beings at any age, by the accumulation of variations
profitable at that age, and by their inheritance at a corre-
sponding 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 im-
proving by selection the down in the pods on his cotton-
trees. Natural selection may modify and adapt the larva
of an insect to a score of contingencies, wholly different
from those which concern the mature insect; and these
modifications may effect, through correlation, the structure
of the adult. So, conversely, modifications in the adult
may affect the structure of the larva; but in all cases
NATURAL SELECTION
129
natural selection will insure that they shall not be inju-
rious; for if they were so the species would become
extinct.
Natural selection will modify the structure of the
young in relation to the parent, and of the parent in
relation to the young. In social animals it will adapt the
structure of each individual for the benefit of the whole
community; if the community 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 effect may be found in works of natural
history, I cannot find one case which will bear investiga-
tion. A structure used only once in an animal's 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, used exclusively for opening the co-
coon— or the hard tip to the beak of unhatched birds,
used for breaking the egg. It has been asserted that of
the best short-beaked tumbler-pigeons a greater number
perish in the egg than are able to get out of it; so that
fanciers assist in the act of hatching. Now if nature had
to make the beak of a full-grown pigeon very short for
the bird's own advantage, the process of modification
would be very slow and there would be simultaneously
the most rigorous selection of all 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.
It may be well here to remark that with all beings
130 THE 0 RIG IX OF SPECIES
there must be much fortuitous destruction, which can
have little or no influence on the course of natural selec-
tion. For instance, a vast number of eggs or seeds are
annually devoured, and these could be modified through
natural selection only if they varied in some manner
which protected them from their enemies. Yet many of
these eggs or see ls would perhaps, if not destroyed, have
yielded individuals better adapted to their conditions of
life than any of those which happened to survive. So
again a vast number of mature animals and plants,
whether or not they be the best adapted to their condi-
tions, must be annually destroyed by accidental causes,
which would not be in the least degree mitigated by cer-
tain changes of structure or constitution which would in
other ways be beneficial to~the species. But let the de-
struction of the adults be ever so heavy, if the number
which can exist in any district be not wholly kept down
by such causes — or again let the destruction of eggs or
seeds be so great that only a hundredth or a thousandth
part are developed — yet of those which do survive, the
best adapted individuals, supposing that there is any va-
riability in a favorable direction, will tend to propagate
their kind in larger numbers than the less well adapted.
If the numbers be wholly kept down by the causes just
indicated, as will often have been the case, natural selec-
tion will be powerless in certain beneficial directions; but
this is no valid objection to its efficiency at other times
and in other ways; for we are far from having any rea-
son to suppose that many species ever undergo modifica-
tion and improvement at the same time in the same area.
NATURAL SELECTION
181
Sexual Selection
Inasmuch as peculiarities often appear under domesti-
cation in one sex and become hereditarily attached to
that sex, so no doubt it will be under nature. Thus
it is rendered possible for the two sexes to be modified
through natural selection in relation to different habits
of life, as is sometimes the case; or for one sex to
be modified in relation to the other sex, as commonly
occurs. This leads me to say a few words on what
I have called Sexual Selection. This form of selec-
tion depends, not on a struggle for existence in re-
lation to other organic beings or to external conditions,
but on a struggle between the individuals of one sex,
generally the males, for the possession of the other
sex. The result is not death to the unsuccessful com-
petitor, but few or no offspring. Sexual selection is,
therefore, less rigorous than natural selection. Generally,
the most vigorous males, those which are best fitted for
their places in nature, will leave most progeny. But in
many cases, victory depends not so much on general
vigor as on having special weapons, confined to the male
sex. A hornless stag or spurless cock would have a
poor chance of leaving numerous offspring. Sexual se-
lection, by always allowing the victor to breed, might
surely give indomitable courage, length to the spur, and
strength to the wing to strike in the spurred leg, in
nearly the same manner as does the brutal cockfighter
by the careful selection of his best cocks. How low in
the scale of nature the law of battle descends I know
not; male alligators have been described as fighting, bel-
lowing, and whirling round, like Indians in a war-dance,
132
THE ORIGIN OF SPECIES
for the possession of the females; male salmons have
been observed fighting all day long; male stag-beetles
sometimes bear wounds from the huge mandibles of other
males; the males of certain hymenopterous insects have
been frequently seen by that inimitable observer, M.
Fabre, fighting for a particular female who sits by,
an apparently unconcerned beholder of the struggle,
and then retires with the conqueror. The war is, per-
haps, severest between the males of polygamous animals,
and these seem oftenest provided with special weapons.
The males of carnivorous animals are already well armed;
though to them and to others special means of defence
may be given through means of sexual selection, as the
mane of the lion and the hooked jaw to the male salmon;
for the shield may be as important for victory as the
sword or spear.
Among 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 with the
most elaborate care, and show off in the best manner,
their gorgeous plumage; they likewise perform strange
antics before the females, which, standing by as specta-
tors, 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 a pied
peacock was eminently attractive to all his hen birds.
I cannot here enter on the necessary details; but if man
can in a short time give beauty and an elegant carriage
NATURAL SELECTION
133
to his bantams, according to his standard of beauty, I
can see no good reason to doubt that female birds, by
selecting, during thousands of generations, the most me-
lodious or beautiful males, according to their standard
of beauty, might produce a marked effect. Some well-
known laws, with respect to the plumage of male and
female birds, in comparison with the plumage of the
young, can partly be explained through the action of
sexual selection on variations occurring at different ages,
and transmitted to the males alone or to both sexes at
corresponding ages; but I have not space here to enter
on this subject.
Thus it is, as I believe, that when the males and
females of any animal have the same general habits of
life, but differ in structure, color, or ornament, such dif-
ferences have been mainly caused by sexual selection:
that is, by individual males having had, in successive
generations, some slight advantage over other males, in
their weapons, means of defence, or charms, which they
have transmitted to their male offspring alone. Yet, I
would not wish to attribute all sexual differences to this
agency: for we see in our domestic animals peculiarities
arising and becoming attached to the male sex, which
apparently have not been augmented through selection by
man. The tuft of hair on the breast of the wild turkey-
cock cannot be of any use, and it is doubtful whether it
can be ornamental in the eyes of the female bird; —
indeed, had the tuft appeared under domestication, it
would have been called a monstrosity.
134
THE ORIGIN OF SPECIES
Illustrations of the Action of Natural Selection, or the
Survival of the Fittest
In order to make it clear how, as I believe, natural
selection acts, I must beg permission to give one or two
imaginary illustrations. Let us take the case of a wolf,
which preys on various animals, securing some by craft,
some by strength, and some by fleetness; and let us sup-
pose that the fleetest prey, a deer for instance, had from
any change in the country increased in numbers, or that
other prey had decreased in numbers, during that season
of the year when the wolf was hardest pressed for
food. Under such circumstances the swiftest and slim-
mest 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
some other period of the year, when they were compelled
to prey on other animals. I can see no more reason
to doubt that this would be the result than that man
should be able to improve the fleetness of his grey-
hounds by careful and methodical selection, or by that
kind of unconscious selection which follows from each
man trying to keep the best dogs without any thought
of modifying the breed. I may add, that, according to
Mr. Pierce, there are two varieties of the wolf inhabiting
the Catskill Mountains, in the United States, one with a
light greyhound-like form, which pursues deer, and the
other more bulky, with shorter legs, which more fre-
quently attacks the shepherd's flocks.
It should be observed that, in the above illustration,
I speak of the slimmest individual wolves, and not of
any single strongly-marked variation having been pre-
NATURAL SELECTION
135
served. In former editions of this work I sometimes
spoke as if this latter alternative had frequently oc-
curred. I saw the great importance of individual dif-
ferences, and this led me fully to discuss the results of
unconscious selection by man, which depends on the
preservation of all the more or less valuable individ-
uals, and on the destruction of the worst. I saw, also,
that the preservation in a state of nature of any occa-
sional deviation of structure, such as a monstrosity,
would be a rare event; and that, if at first preserved,
it would generally be lost by subsequent intercrossing
with ordinary individuals. Nevertheless, until reading an
able and valuable article in the ''North British Review"
(1867), I did not appreciate how rarely single variations,
whether slight or strongly-marked, could be perpetuated.
The author takes the case of a pair of animals, produc-
ing during their lifetime two hundred offspring, of which,
from various causes of destruction, only two on an aver*
age survive to procreate their kind. This js, rather an
extreme estimate for most of the higher animals, but by
no means so for many of the lower organisms. He then
shows that if a single individual were born, which varied
in some manner, giving it twice as good a chance of life
as that of the other individuals, yet the chances would
be strongly against its survival. Supposing it to survive
and to breed, and that half its young inherited the favor-
able variation; still, as the Reviewer goes on to show,
the young would have only a slightly better chance of
surviving and breeding; and this chance would go on
decreasing in the succeeding generations. The justice
of these remarks cannot, I think, be disputed. If, for
instance, a bird of some kind could procure its food
136
THE ORIGIN OF SPECIES
more easily by having its beak curved, and if one were
born with its beak strongly curved, and which conse-
quently flourished, nevertheless there would be a very
poor chance of this one individual perpetuating its kind
to the exclusion of the common form; but there can
hardly be a doubt, judging by what we see taking place
under domestication, that this result would follow from
the preservation during many generations of a large
number of individuals with more or less strongly curved
beaks, and from the destruction of a still larger number
with the straightest beaks.
It should not, however, be overlooked that certain
rather strongly marked variations, which no one would
rank as mere individual differences, frequently recur
owing to a similar organization being similarly acted
on — of which fact numerous instances could be given
with our domestic productions. In such cases, if the
Varying individual did not actually transmit to its off-
spring its uCVrly-^C^uired character, it would undoubtedly
transmit to them, as long as the existing conditions re-
mained the same, a still stronger tendency to vary in the
same manner. There can also be little doubt that the
tendency to vary in the same manner has often been so
strong that all the individuals of the same species have
been similarly modified without the aid of any form of
selection. Or only a third, fifth, or tenth part of the
individuals may have been thus affected, of which fact
several instances could be given. Thus Graba estimates
that about one-fifth of the guillemots in the Faroe Islands
consist of a variety so well marked that it was formerly
ranked as a distinct species under the name of Uria lac-
rymans. In cases of this kind, if the variation were of
NATURAL SELECTION
137
a beneficial nature, the original form would soon be sup-
planted by the modified form, through the survival of
the fittest.
To the effects of intercrossing in eliminating variations
of all kinds, I shall have to recur; but it may be here
remarked that most animals and plants keep to their
proper homes, and do not needlessly wander about; we
see this even with migratory birds, which almost always
return to the same spot. Consequently each newly-
formed variety would generally be at first local, as
seems to be the common rule with varieties in a state
of nature; so that similarly modified individuals would
soon exist in a small body together, and would often
breed together. If the new variety were successful in
its battle for life, it would slowly spread from a central
district, competing with and conquering the unchanged
individuals on the margins of an ever-increasing circle.
It may be worth while to give another and more
complex illustration of the action^ of natural selection.
Certain plants excrete sweet juice,- apparently for the
sake of eliminating something injurious from the sap:
this is effected,, for instance, by glands at the base of
the stipules in some Leguminosse, and at the backs of
the leaves of the common laurel. This juice, though
small in quantity, is greedily sought by insects; but
their visits do not in any way benefit the plant. Now,
let us suppose that the juice or nectar was excreted from
the inside of the flowers of a certain number of plants
of any species. Insects in seeking the nectar would get
dusted with pollen, and would often transport it from
one flower to another. The flowers of two distinct indi-
viduals of the same species would thus get crossed; and
138
THE ORIGIN OF SPECIES
the act of crossing, as can be fully proved, gives rise to
vigorous seedlings, which consequently would have the
best chance of flourishing and surviving. The plants
which produced flowers with the largest glands or nec-
taries, excreting most nectar, would oftenest be visited
by insects, and would oftenest be crossed; and so in the
long run would gain the upper hand and form a local
variety. The flowers, also, which had their stamens and
pistils placed, in relation to the size and habits of the
particular insect which visited them, so as to favor in
any degree the transportal of the pollen, would likewise
be favored. 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 purpose
of fertilization, its destruction appears to be a simple loss
to the plant; yet if a little pollen were carried, at first
occasionally and then habitually, by the pollen-devouring
insects from flower to flower, and a cross thus effected,
although nine-tenths ,r>f the pollen were destroyed it
might still be a great gain to the plant to be thus
robbed; and the individuals which produced more and
more pollen, and had larger anthers, would be selected.
When our plant, by the above process long contin-
ued, had been rendered highly attractive to insects, they
would, unintentionally on their part, regularly carry pol-
len from flower to flower; and that they do this effect-
ually, I could easily show by many striking facts. I
will give only one, as likewise illustrating one step in
the separation of the sexes of plants. Some holly-trees
bear only male flowers, which have four stamens produc-
ing a rather small quantity of pollen, and a rudimentary
pistil; other holly-trees bear only female flowers; these
NATURAL SELECTION
189
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, 1 put the stigmas of twenty flowers, taken
from different branches, under the microscope, and on
all, without exception, there were a few pollen-grains,
and on some a profusion. As the wind had set for sev-
eral days from the female to the male tree, the pollen
could not thus have been carried. The weather had been
cold and boisterous, and therefore not favorable to bees,
nevertheless every female flower which I examined had
been effectually fertilized by the bees, which had flown
from tree to tree in search of nectar. But to return to
our imaginary case: as soon as the plant had been ren-
dered so highly attractive to insects that pollen was regu-
larly carried from flower to flower, another process might
commence. No naturalist doubts the advantage of what
has been called the 4 'physiological division of labor";
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 an-
other plant. In plants under culture and placed under
new conditions of life, sometimes the male organs and
sometimes the female organs become more or less impo-
tent; 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 advanta-
geous on the principle of the division of labor, individuals
with this tendency more and more increased would be
continually favored or selected, until at last a complete
separation of the sexes might be effected. It would take
140
THE OR1GIX OP SPECIES
up too much space to show the various steps, through
dimorphism and other means, by which the separation of
the sexes in plants of various kinds is apparently now
in progress; but I may add that some of the species of
holly in North America are, according to Asa Gray, in
an exactly intermediate condition, or, as he expresses
it, are more or less diceciously polygamous.
Let us now turn to the nectar-feeding insects; we may
suppose the plant, of which we have been slowly increas-
ing the nectar by continued selection, to be a common
plant: and that certain insects depended in main part on
its nectar for food. I could give many facts showing how
anxious bees are to save time: for instance, their habit
of cutting holes and sucking the nectar at the bases of
certain flowers, which with a very little more trouble
they can enter by the mouth. Bearing such facts in
mind, it may be believed that under certain circum-
stances individual differences in the curvature or length
of the proboscis, etc., too slight to be appreciated by us,
might profit a bee or other insect, so that certain indi-
viduals would be able to obtain their food more quickly
than others; and thus the communities to which they be-
longed would flourish and throw off many swarms in-
heriting the same peculiarities. The tubes of the corolla
of the common red and incarnate clovers (Tnfolium pra-
. tense and incarnatum) do not on a hasty glance appear
to differ in length; yet the hive-bee can easily suck the
nectar out of the incarnate clover, but not out of the
common red clover, which is visited by humble-bees
alone; so that whole fields of the red clover offer in
vain an abundant supply of precious nectar to the hive-
bee. That this nectar is much liked by the hive -bee is
NATURAL SELECTION"
141
certain; for I have repeatedly seen, but only in the au-
tumn, many hive-bees sucking the flowers through holes
bitten in the base of the tube by humble-bees. The dif-
ference in the length of the corolla in the two kinds of
clover, which determines the visits of the hive-bee, must
be very trifling; for I have been assured that when red
clover has been mown, the flowers of the second crop are
somewhat smaller, and that these are visited by many
hive-bees. I do not know whether this statement is ac-
curate; nor whether another published statement can be
trusted, namely, that the Ligurian bee, which is gen-
erally considered a mere variety of the common hive-bee,
and which freely crosses with it, is able to reach and
suck the nectar of the red clover. Thus, in a country
where this kind of clover abounded, it might be a great
advantage to the hive-bee to have a slightly longer or
differently constructed proboscis. On the other hand, as
the fertility of this clover absolutely depends on bees
visiting the flowers, if humble-bees were to become rare
in any country, it might be a great advantage to the
plant to have a shorter or more deeply divided corolla,
so that the hive-bees should be enabled to suck its flow-
ers. Thus I can understand how a flower and a bee
might slowly become, either simultaneously or one after
the other, modified and adapted to each other in the most
perfect manner, by the continued preservation of all the
individuals which presented slight deviations of structure
mutually favorable to each other.
I am well aware that this doctrine of natural selection,
exemplified in the above imaginary instances, is open to
the same objections which were first urged against Sir
Charles Lyell's noble views on 'lthe modern changes of
142
THE ORIGIN OF SPECIES
the earth, as illustrative of geology"; but we now seldom
hear the agencies which we see still at work spoken of
as trifling or insignificant, when used in explaining the
excavation of the deepest valleys or the formation of
long lines of inland cliffs. Natural selection acts only by
the preservation and accumulation of 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 banish the belief of the continued
creation of new organic beings, or of any great and sud-
den modification in their structure.
On the Intercrossing of Individuals
I must here introduce a short digression. In the case
of animals and plants with separated sexes, it is of course
obvious that two individuals must always (with the ex-
ception 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. Nevertheless
there is reason to believe that with all hermaphrodites
two individuals, either occasionally or habitually, concur
for the reproduction of their kind. This view was long
ago doubtfully suggested by Sprengel, Knight and Kol-
reuter. We shall presently see its importance; but I
must here treat the subject with extreme brevity, though
I have the materials prepared for an ample discussion.
All vertebrate animals, all insects, and some other large
groups of animals, pair for each birth. Modern research
has much diminished the number of supposed hermaphro-
dites, and of real hermaphrodites a large number pair;
that is, two individuals regularly unite for reproduction,
NATURAL SELECTION 143
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, and made so many experiments, showing, in ac-
cordance with the almost universal belief of breeders,
that with animals and plants a cross between different
varieties, or between individuals of the same variety but
of another strain, gives vigor and fertility to the off-
spring; and, on the other hand, that close interbreeding
diminishes vigor and fertility; that these facts alone
incline me to believe that it is a general law of nature
that no organic being fertilizes itself for a perpetuity of
generations; but that a cross with another individual is
occasionally — perhaps at long intervals of time — indis-
pensable.
On the belief that this is a law of nature, we can, I
think, understand several large classes of facts, such as
the following, which on any other view are inexplicable.
Every hybridizer knows how unfavorable exposure to wet
is to the fertilization of a flower, yet what a multitude of
flowers have their anthers and stigmas fully exposed to
the weather! If an occasional cross be indispensable,
notwithstanding that the plant's own anthers and pistil
stand so near each other as almost to insure self-fertiliza-
tion, the fullest freedom for the entrance of pollen from
another individual will explain the above state of ex-
posure of the organs. Many flowers, on the other hand,
144
THE ORIGIN OF SPECIES
have their organs of fructification closely inclosed, as in
the great papilionaceous or pea-family; but these almost
invariably present beautiful and curious adaptations in
relation to the visits of insects. So necessary are the
visits of bees to many papilionaceous flowers that their
fertility is greatly diminished if these visits be prevented,
rfow, it is scarcely possible for insects to fly from flower
to flower, and not to carry pollen from one to the other,
to the great good of the plant. Insects act like a camel's-
hair pencil, and it is sufficient, to insure fertilization, just
to touch with the same brush the anthers of one flower
and then the stigma of another: but it must not be sup-
posed that bees would thus produce a multitude of
hybrids between distinct species; for if a plant's own
pollen and that from another species are placed on the
same stigma, the former is so prepotent that it invariably
and completely destroys, as has been shown by Gartner,
the influence of the foreign pollen.
When the stamens of a flower suddenly spring toward
the pistil, or slowly move one after the other toward it,
the contrivance seems adapted solely to insure self-
fertilization; 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 in this very genus,
which seems to have a special contrivance for self-
fertilization, it is well known that, if closely-allied forms
or varieties are planted near each other, it is hardly pos-
sible to raise pure seedlings, so largely do they naturally
cross. In numerous other cases, far from self-fertilization
being favored, there are special contrivances which
effectually prevent the stigma receiving pollen from its
NATURAL SELECTION
145
own flower, as I could show from the works of Sprengel
and others, as well as from my own observations: for
instance, in Lobelia fulgens, there is a really beautiful and
elaborate contrivance by which all the infinitely numerous
pollen-granules are swept out of the conjoined anthers of
each flower, before the stigma of that individual flower is
ready to receive them; and as this flower is never visited,
at least in my garden, by insects, it never sets a seed,
though by placing pollen from one flower on the stigma
of another, I raise plenty of seedlings. Another species
of Lobelia, which is visited by bees, seeds freely in my
garden. In very many other cases, though there is no
special mechanical contrivance to prevent the stigma re-
ceiving pollen from the same flower, yet, as Sprengel,
and more recently Hildebrand, and others, have shown,
and as I can confirm, either the anthers burst before the
stigma is ready for fertilization, or the stigma is ready
before the pollen of that flower is ready, so that these
so-named dichogamous plants have in fact separated
sexes, and must habitually be crossed. So it is with the
reciprocally dimorphic and trimorphic plants previously
alluded to. 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 pur-
pose of self-fertilization, should be in so many cases
mutually useless to each other? How simply are these
facts explained on the view of an occasional cross with a
distinct individual being advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and
of some other plants, be allowed to seed near each other,
a large majority of the seedlings thus raised turn out, as
I have found, mongrels: for instance, I raised 233 seed-
— Science — 7
146
THE ORIGIN OF SPECIES
ling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true
to their kind, and some even of these were not perfectly
true. Yet the pistil of each cabbage-flower is surrounded
not only by its own six stamens, but by those of the
many other flowers on the same plant- and the pollen of
each flower readily gets on its own stigma without insect
agency; for I have found that plants carefully protected
from insects produce the full number of pods. How,
then, comes it that such a vast number of the seedlings
are mongrelized ? It must arise from the pollen of a
distinct variety having a prepotent effect over the 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 reversed, for a plant's own pollen
is almost always prepotent over foreign pollen; but to
this subject we shall return in a future chapter.
In the case of a large tree covered with innumerable
flowers, it may be objected that pollen could seldom be
carried from tree to tree, and at most only from flower
to flower on the same tree; and flowers on the same tree
can be considered as distinct individuals only in a limited
sense. I believe this objection to be valid, but that
nature has largely provided against it by giving to trees
a strong tendency to bear flowers with separated sexes.
When the sexes are separated, although the male and
female flowers may be produced on the same tree, pollen
must be regularly carried from flower to flower; and this
will give a better chance of pollen being occasionally
carried from tree to tree. That trees belonging to all
Orders have their sexes more often separated than other
NATURAL SELECTION
147
plants I find to be the case in this country; and at my
request Dr. Hooker tabulated the trees of New Zealand,
and Dr. Asa Gray those of the United States, and the
result was as I anticipated. On the other hand, Dr.
Hooker informs me that the rule does not hold good in
Australia; but if most of the Australian trees are dichog-
amous, the same result would follow as if they bore
flowers with separated sexes. I have made these few re-
marks on trees simply to call attention to the subject.
Turning for a brief space to animals: various terres-
trial species are hermaphrodites, such as the land-mollusca
and earth-worms; but these all pair. As yet I have not
found a single terrestrial animal which can fertilize itself.
This remarkable fact, which offers so strong a contrast
with terrestrial plants, is intelligible on the view of an
occasional cross being indispensable; for owing to the
nature of the fertilizing element there are no means,
analogous to the action of insects and of the wind with
plants, by which an occasional cross could be effected
with terrestrial animals without the concurrence of two
individuals. Of aquatic animals, there are many self-
fertilizing hermaphrodites; but here the currents of water
offer an obvious means for an occasional cross. As in
the case of flowers, I have as yet failed, after consulta-
tion with one of the highest authorities, namely, Professor
Huxley, to discover a single hermaphrodite animal with
the organs of reproduction so perfectly inclosed that ac-
cess from without, and the occasional influence of a
distinct individual, can be shown to be physically
impossible. Cirripeds long appeared to me to present,
under this point of view, a case of great difficulty; but
I have been enabled, by a fortunate chance, to prove
148
THE ORIGIN OF SPECIES
that two individuals, though both are self -fertilizing
hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange
anomaly that, both with animals and plants, some species
of the same family and even of the same genus, though
agreeing closely with each other in their whole organiza-
tion, are hermaphrodites, and some unisexual. But if,
in fact, all hermaphrodites do occasionally intercross, the
difference between them and unisexual species is, as far
as function is concerned, very small.
From these several considerations and from the many
special facts which I have collected, but which I am
unable here to give, it appears that with animals and
plants an occasional intercross between distinct individuals
is a very general, if not universal, law of nature.
Circumstances Favorable for the Production of New Forms
through Natural Selection
This is an extremely intricate subject. A great
amount of variability, under which term individual
differences are always included, will evidently be favor-
able. A large number of individuals, by giving a better
chance within any given period for the appearance of
profitable variations, will compensate for a lesser amount
of variability in each individual, and is, I believe, a
highly important element of success. Though Nature
grants long periods of time for the work of natural
selection, she does not grant an indefinite period; for as
all organic beings are striving to seize on each place in
the economy of nature, if any one species does not be-
come modified and improved in a corresponding degree
with its competitors, it will be exterminated. Unless
NATURAL SELECTION
149
favorable variations be inherited by some at least of the
offspring, nothing can be effected by natural selection.
The tendency to reversion may often check or prevent
the work; but as this tendency has not prevented man
from forming by selection numerous domestic races, why
should it prevail against natural selection ?
In the case of methodical selection, a breeder selects
for some definite object, and if the individuals be allowed
freely to intercross, his work will completely fail. But
when many men, without intending to alter the breed,
have a nearly common standard of perfection, and all try
to procure and breed from the best animals, improvement
surely but slowly follows from this unconscious process
of selection, notwithstanding that there is no separation
of selected individuals. Thus it will be under nature;
for within a confined area, with some place in the natural
polity not perfectly occupied, all the individuals varying
in the right direction, though in different degrees, will
tend to be preserved. But if the area be large, its
several districts will almost certainly present different
conditions of life; and then, if the same species under-
goes modification in different districts, the newly -formed
varieties will intercross on the confines of each. But we
shall see in the sixth chapter that intermediate varieties,
inhabiting intermediate districts, will in the long run
generally be supplanted by one of the adjoining varieties.
Intercrossing will chiefly affect those animals which unite
for each birth and wander much, and which do not breed
at a very quick rate. Hence with animals of this nature,
for instance, birds, varieties will generally be confined to
separated countries; and this I find to be the case. With
hermaphrodite organisms which cross only occasionally,
150
THE ORIGIN OF SPECIES
and likewise with animals which unite for each birth,
but which wander little and can increase at a rapid rate,
a new and improved variety might be quickly formed on
any one spot, and might there maintain itself in a body
and afterward spread, so that the individuals of the new
variety would chiefly cross together. On this principle,
nurseiymen always prefer saving seed from a large body
of plants, as the chance of intercrossing is thus lessened.
Even with animals which unite for each birth, and
which do not propagate rapidly, we must not assume that
free intercrossing would always eliminate the effects of
natural selection; for I can bring forward a considerable
body of facts showing that within the same area two
varieties of the same animal may long remain distinct,
from haunting different stations, from breeding at slightly
different seasons, or from the individuals of each variety
preferring to pair together.
Intercrossing plays a very important part in nature by
keeping the individuals of the same species, or of the
same variety, true and uniform in character. It will ob-
viously thus act far more efficiently with those animals
which unite for each birth; but, as already stated, we
have reason to believe that occasional intercrosses take
place with all animals and plants. Even if these take
place only at long intervals of time, the young thus pro-
duced will gain so much in vigor and fertility over the
offspring from long-continued self-fertilization that they
will have a better chance of surviving and propagating
their kind; and thus in the long run the influence of
crosses, even at rare intervals, will be great. With re-
spect to organic beings extremely low in the scale, which
do not propagate sexually, nor conjugate, and which can-
NATURAL SELECTION
151
not possibly intercross, uniformity of character can be
retained by them under the same conditions of life, only
through the principle of inheritance, and through natural
selection which will destroy any individuals departing
from the proper type. If the conditions of life change
and the form undergoes modification, uniformity of char-
acter can be given to the modified offspring, solely by
natural selection preserving similar favorable variations.
Isolation, also, is an important element in the modifi-
cation of species through natural selection. In a confined
or isolated area, if not very large, the organic and in-
organic conditions of life will generally be almost uni-
form; so that natural selection will tend to modify all
the varying individuals of the same species in the same
manner. Intercrossing with the inhabitants of the sur-
rounding districts will, also, be thus prevented. Moritz
Wagner has lately published an interesting essay on this
subject, and has shown that the service rendered by
isolation in preventing crosses between newly-formed
varieties is probably greater even than I supposed. But
from reasons already assigned I can by no means agree
with this naturalist, that migration and isolation are
necessary elements for the formation of new species.
The importance of isolation is likewise great in prevent-
ing, after any physical change in the conditions, such as
of climate, elevation of the land, etc., the immigration of
better adapted organisms; and thus new places in the
natural economy of the district will be left open to be
filled up by the modification of the old inhabitants.
Lastly, isolation will give time for a new variety to be
improved at a slow rate; and this may sometimes be of
much importance. If, however, an isolated area be very
152
THE ORIGIN OF SPECIES
small, either from being surrounded by barriers, or from
having very peculiar physical conditions, the total num-
ber of the inhabitants will be small; and this will retard
the production of new species through natural selection,
by decreasing the chances of favorable variations arising.
The mere lapse of time by itself does nothing, either
for or against natural selection. I state this because it
has been erroneously asserted that the element of time
has been assumed by me to play an all -important part
in modifying species, as if all the forms of life were
necessarily undergoing change through some innate law.
Lapse of time is only so far important, and its impor-
tance in this respect is great, that it gives a better chance
of beneficial variations arising and of their being selected,
accumulated, and fixed. It likewise tends to increase the
direct action of the physical conditions of life, in relation
to the constitution of each organism.
If we turn to nature to test the truth of these re-
marks, and look at any small isolated area, such as an
oceanic island, although the number of species inhabiting
it is small, as we shall see in our chapter on Geographi-
cal Distribution; yet of these species a very large propor-
tion are endemic — that is, have been produced there and
nowhere else in the wTorld. Hence an oceanic island at
first sight seems to have been highly favorable for the
production of new species. But we may thus deceive
ourselves, for to ascertain whether a small isolated area,
or a large open area like a continent, has been most
favorable for the production of new organic forms, we
ought to make the comparison within equal times; and
this we are incapable of doing.
Although isolation is of great importance in the pro-
NATURAL SELECTION
153
duction of new species, on the whole I am inclined to
believe that largeness of area is still more important,
especially for the production of species which shall prove
capable of enduring for a long period, and of spreading
widely. Throughout a great and open area, not only will
there be a better chance of favorable variations, arising
from the large number of individuals of the same species
there supported, but the conditions of life are much more
complex from the large number of already existing
species; and if some of these many species become modi-
fied 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 im-
proved, will be able to spread over the open and con-
tinuous area, and will thus come into competition with
many other forms. Moreover, great areas, though now
continuous, will often, owing to former oscillations of
level, have existed in a broken condition; so that the
good effects of isolation will generally, to a certain extent,
have concurred. Finally I conclude that, although small
isolated areas have been in some respects highly favor-
able for the production of new species, yet that the
course of modification will generally have been more
rapid on large areas; and what is more important, that
the new forms produced on large areas, which already
have been victorious over many competitors, will be those
that will spread most widely, and will give rise to the
greatest number of new varieties and species. They will
thus play a more important part in the changing history
of the organic world.
In accordance with this view, we can, perhaps, under-
stand some facts which will be again alluded to in our
.54
THE ORIGIN OF SPECIES
chapter on Geographical Distribution; for instance, the
fact of the productions of the smaller continent of Aus-
tralia now yielding before those of the larger Europaeo-
Asiatic area. Thus, also, it is that continental productions
have everywhere become so largely naturalized on islands.
On a small island, the race for life will have been less
severe, and there will have been less modification and
less extermination. Hence, we can understand how it is
that the flora of Madeira, according to Oswald Heer, re-
sembles to a certain extent the extinct tertiary flora of
Europe. All fresh-water basins, taken together, make a
small area compared with that of the sea or of the land.
Consequently, the competition between fresh-water pro-
ductions will have been less severe than elsewhere; new
forms will have been then more slowly produced, and
old forms more slowly exterminated. And it is in fresh-
water basins that we find seven genera of Ganoid fishes,
remnants of a once preponderant order: and in fresh water
we find some of the most anomalous forms now known
in the world as the Ornithorhynchus and Lepidosiren,
which, like fossils, connect to a certain extent orders at
present widely sundered in the natural scale. These
anomalous forms may be called living fossils; they hava
endured to the present day, from having inhabited a con-
fined area, and from having been exposed to less varied,
and therefore less severe, competition.
To sum up, as far as the extreme intricacy of the
subject permits, the circumstances favorable and unfavor-
able for the production of new species through natural
selection. I conclude that for terrestrial productions a
large continental area, which has undergone many oscilla-
tions of level, will have been the most favorable for the
NATURAL SELECTION
155
production of any new forms of life, fitted to endure for
a long time and to spread widely. While the area ex-
isted as a continent, the inhabitants will have been
numerous in individuals and kinds, and will have been
subjected to severe competition. When converted by
subsidence into large separate islands, there will still
have existed many individuals of the same species on
each island: intercrossing on the confines of the range of
each new species will have been checked: after physical
changes of any kind, immigration will have been pre-
vented, so that new places in the polity of each island
will have had to be filled up by the modification of the
old inhabitants; and time will have been allowed for
the varieties in each to become well modified and per-
fected. When, by renewed elevation, the islands were re-
converted into a continental area, there will again have
been very severe competition: the most favored or im-
proved varieties will have been enabled to spread: there
will have been much extinction of the less- improved
forms, and the relative proportional numbers of the vari-
ous inhabitants of the reunited continent will again have
been changed; and again there will have been a fair
field for natural selection to improve still further the
inhabitants, and thus to produce new species.
That natural selection generally acts with extreme
slowness I fully admit. It can act only when there
are places in the natural polity of a district which can
be better occupied by the modification of some of its ex-
isting inhabitants. The occurrence of such places will
often depend on physical changes, which generally take
place very slowly, and on the immigration of better
adapted forms being prevented. As some few of the
156
THE ORIGIN OF SPECIES
old inhabitants become modified, the mutual relations
of others will often be disturbed; and this will create
new places, ready to be filled up by better adapted
forms; but all this will take place very slowly. Al-
though all the individuals of the same species differ in
some slight degree from each other, it would often be
long before differences of the right nature in various
parts of the organization might occur. The result would
often be greatly retarded by free intercrossing. Many
will exclaim that these several causes are amply suffi-
cient to neutralize the power of natural selection. I do
not believe so. But I do believe that natural selection
will generally act very slowly, only at long intervals of
time, and only on a few of the inhabitants of the same
region. I further believe that these slow, intermittent
results accord well with what geology tells us of the
rate and manner at which the inhabitants of the world
have changed.
Slow though the process of selection may be, if feeble
man can do much by artificial selection, I can see no
limit to the amount of change, to the' beauty and com-
plexity of the coadaptations between all organic beings,
one with another and with their physical conditions of
life, which may have been effected in the long course
of time through nature's power of selection, that is by
the survival of the fittest.
Extinction caused by Natural Selection
This subject will be more fully discussed in our
chapter on Geology; but it must here be alluded to
from being intimately connected with natural selection.
Natural selection acts solely through the preservation of
NATURAL SELECTION
157
variations in some way advantageous, which consequently
endure. Owing to the high geometrical rate of increase
of all organic beings, each area is already fully stocked
with inhabitants; and it follows from this, that as the
favored forms increase in number, so, generally, will the
less favored decrease and become rare. Karity, as geol-
ogy tells us, is the precursor to extinction. We can see
that any form which is represented by few individuals
will run a good chance of utter extinction, during great
fluctuations in the nature of the seasons, or from a tem-
porary increase in the number of its enemies. But we
may go further than this; for, as new forms are pro-
duced, unless we admit that specific forms can go on
indefinitely increasing in number, many old forms must
become extinct. That the number of specific forms has
not indefinitely increased, geology plainly tells us; and
we shall presently attempt to show why it is that the
number of species throughout the world has not become
immeasurably great.
We have seen that the species which are most numer-
ous in individuals have the best chance of producing
favorable variations within any given period. We have
evidence of this, in the facts stated in the second chap-
ter, showing that it is the common and diffused or domi-
nant species which offer the greatest number of recorded
varieties. Hence, rare species will be less quickly modi-
fied or improved within any given period; they will con-
sequently be beaten in the race for life by the modified
and improved descendants of the commoner species.
From these several considerations I think it inevitably
iollows, that as new species in the course of time are
formed through natural selection, others will become rarer
158 THE ORIGL\ OF SPECIES
and rarer, and finally extinct. The forms which stand in
closest competition with those undergoing modification
and improvement will naturally surfer 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 vari-
ety or species, during the progress of its formation, will
generally press hardest on its nearest kindred, and tend
to exterminate them. We see the same process of exter-
mination among our domesticated productions, through
the selection of improved forms by man. Many curi-
ous instances could be given showing how quickly new
breeds of cattle, sheep, and other animals, and varieties
of flowers, take the place of older and inferior kinds. In
Yorkshire, it is historically known that the ancient black
cattle were displaced by the long-horns, and that these
"were swept away by the short-horns" (I quote the
words of an agricultural writer) ''as if by some mur-
derous pestilence."
Divergence of Character
The principle which I have designated by this term
is of high importance, and explains, as I believe, sev-
eral important facts. In the first place, varieties, even
strongly-marked ones, though having somewhat of the
character of species — as is shown by the hopeless doubts
in many cases how to rank them — yet certainly, differ far
less from each other than do good and distinct species.
Nevertheless, according to my view, varieties are species
NATURAL SELECTION
159
in the process of formation, or are, as I have called them,
incipient species. How, then, does the lesser difference
between varieties become augmented into the greater dif-
ference between species? That this does habitually hap-
pen, we must infer from most of the innumerable spe-
cies throughout nature presenting well-marked differences,
whereas varieties, the supposed prototypes and parents of
future well-marked species, present slight and ill-defined
differences. Mere chance, as we may call it, might cause
one variety to differ in some character from its parents,
and the offspring of this variety again to differ from its
parent in the very same character and in a greater de-
gree; but this alone would never account for so habitual
and large a degree of difference as that between the
species of the same genus.
As has always been my practice, I have sought light
on this head from our domestic productions. We shall
here find something analogous. It will be admitted that
the production of races so different as short-horn and
Hereford cattle, race and cart-horses, the several breeds
of pigeons, etc., could never have been effected by the
mere chance accumulation of similar variations during
many successive generations. In practice, a fancier is,
for instance, struck by a pigeon having a slightly shorter
beak; another fancier is struck by a pigeon having a
rather longer beak; and on the acknowledged principle
that "fanciers do not and will not admire a medium
standard, but like extremes," they both go on (as has
actually occurred with the sub-breeds of the tumbler-
pigeon) choosing and breeding from birds with longer
and longer beaks, or with shorter and shorter beaks.
Again, we may suppose that at an early period of his-
160
THE ORIGIN OF SPECIES
tory, the men of one nation or district required swifter
horses, while those of another required stronger and
bulkier horses. The early differences would be very
slight; but, in the course of time, from the continued
selection of swifter horses in the one case, and of
stronger ones in the other, the differences would be-
come greater, and would be noted as forming two sub-
breeds. Ultimately, after the lapse of centuries, these
sub-breeds would become converted into two well-estab-
lished and distinct breeds. As the differences became
greater, the inferior animals with intermediate characters,
being neither very swift nor very strong, would not have
been used for breeding, and will thus have tended to
disappear. Here, then, we see in man's productions the
action of what may be called the principle of divergence,
causing differences, at first barely appreciable, steadily
to increase, and the breeds to diverge in character, both
from each other and from their common parent.
But how, it may be asked, can any analogous prin-
ciple apply in nature? I believe it can and does apply
most efficiently (though it was a long time before I saw
how), from the simple circumstance that the more diver-
sified the descendants from any one species become in
structure, constitution, and habits, by so much will they
be better enabled to seize on many and widely diversified
places in the polity of nature, and so be enabled to
increase in numbers.
We can clearly discern this in the case of animals
with simple habits. Take the case of a carnivorous
quadruped, of which the number that can be supported
in any country has long ago arrived at its full average.
If its natural power of increase be allowed to act, it oftn
NATURAL SELECTION
161
succeed in increasing (the country not undergoing any
change in conditions) only by its varying descendants
seizing on places at present occupied by other animals:
some of them, for instance, being enabled to feed on new
kinds of prey, either dead or alive; some inhabiting new
stations, climbing trees, frequenting water, and some per-
haps becoming less carnivorous. The more diversified in
habits and structure the descendants of our carnivorous
animals become, the more places they will 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 effect 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 be raised in the latter than
in the former case. The same has been found to hold
good when one variety and several mixed varieties of
wheat have been sown on equal spaces of ground.
Hence, if any one species of grass were to go on vary-
ing, and the varieties were continually selected which
differed from each other in the same manner, though
in a very slight degree, as do the distinct species and
genera of grasses, a greater number of individual plants
of this species, including its modified descendants, would
succeed in living on the same piece of ground. And we
know that each species and each variety of grass is an-
nually sowing almost countless seeds; and is thus striv-
ing, as it may be said, to the utmost to increase in
number. Consequently, in the course of many thousand
generations, the most distinct varieties of any one species
162
THE ORIGIN OF SPECIES
of grass would have the best chance of succeeding and
of increasing in numbers, and thus of supplanting the
less distinct varieties; and varieties, when rendered very-
distinct from each other, take the rank of species.
The truth of the principle that the greatest amount of
life can be supported by great diversification of structure
is seen under many natural circumstances. In an ex-
tremely small area, especially if freely open to immigra-
tion, and where the contest between individual and
individual must be very severe, we always find great
diversity in its inhabitants. For instance, I found that a
piece of turf, three feet by four in size, which had been
exposed for many years to exactly the same conditions,
supported twenty species of plants, and these belonged to
eighteen genera and to eight orders, which shows how
much these plants differed from each other. So it is
with the plants and insects on small and uniform islets:
also in small ponds of fresh water. Farmers find that
they can raise most food by a rotation of plants belonging
to the most different orders: nature follows what may be
called a simultaneous rotation. Most of the animals and
plants which live close round any small piece of ground
could live on it (supposing its nature not to be in any
way peculiar), 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, the advantages of
diversification of structure, with the accompanying differ-
ences of habit and constitution, determine that the in-
habitants, 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 naturalization of
NATURAL SELECTION
163
plants through man's agency in foreign lands. It might
have been expected that the plants which would succeed
in becoming naturalized in any land would generally
have been closely allied to the indigenes; for these are
commonly looked at as specially created and adapted for
their own country. It might also, perhaps, have been
expected that naturalized plants would have belonged to
a few groups more especially adapted to certain stations
in their new homes. But the case is very different; and
Alph. de Candolle has well remarked, in his great and
admirable work, that floras gain by naturalization, pro-
portionally with the number of the native genera and
species, far more in new genera than in new species. To
give a single instance: in the last edition of Dr. Asa
Gray's "Manual of the Flora of the Northern United
States," 260 naturalized plants are enumerated, and these
belong to 162 genera. We thus see that these naturalized
plants are of a highly diversified nature. They differ,
moreover, to a large extent, from the indigenes, for out
of the 162 naturalized genera no less than 100 genera
are not there indigenous, and thus a large proportional
addition is made to the genera now living in the United
States.
By considering the nature of the plants or animals
which have in any country struggled successfully with
the indigenes, and have there become naturalized, we
may gain some crude idea in what manner some of the
natives would have to be modified, in order to gain an
advantage over their compatriots; and we may at least
infer that diversification of structure, amounting to new
generic differences, would be profitable to them.
The advantage of diversification of structure in the
164
THE ORIGIN OF SPECIES
inhabitants of the same region is, in fact, the same as
that of the physiological division of labor in the organs
of the same individual body — a subject so well elucidated
hj Milne Edwards. No physiologist doubts that a stom-
ach 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 differ-
ent habits of life, so will a greater number of individuals
be capable of there supporting themselves. A set of
animals, with their organization but little diversified,
could hardly compete with a set more perfectly diversi-
fied in structure. It may be doubted, for instance,
whether the Australian marsupials, which are divided
into groups differing but little from each other, and
feebly representing, as Mr. Waterhouse and others have
remarked, our carnivorous, ruminant, and rodent mam-
mals, could successfully compete with these well- developed
orders. In the Australian mammals, we see the process
of diversification in an early and incomplete stage of
development.
The Probable Effects of the Action of Natural Selection,
through Divergence of Character and Extinction, on
the Descendants of a Common Ancestor
After the foregoing discussion, which has been much
compressed, we may assume that the modified descendants
of any one species will succeed so much the better as
they become more diversified in structure, and are thus
enabled to encroach on places occupied by other beings.
Now let us see how this principle of benefit being
derived from divergence of character, combined with the
NATURAL SELECTION
165
principles of natural selection and of extinction, tends
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
degrees, as is so generally the case in nature, and as is
represented in the diagram by the letters standing at
unequal distances. I have said a large genus, because,
as we saw in the second chapter, on an average more
species vary in large genera than in 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 do the rare and restricted species. Let
(A) be a common, widely-diffused, and varying species,
belonging to a genus large in its own country. The
branching and diverging dotted lines of unequal lengths
proceeding from (A) may represent its varying offspring.
The variations are supposed to be extremely slight, but
of the most diversified nature; they are not supposed all
to appear simultaneously, but often after long intervals
of time; nor are they all supposed to endure for equal
periods. Only those variations which are in some way
profitable will be preserved or naturally selected. And
here the importance of the principle of benefit derived
from divergence of character comes in; for this will
generally lead to the most different or divergent varia-
tions (represented by the outer dotted lines) being pre-
served and accumulated by natural selection. When a
dotted line reaches one of the horizontal lines, and is
there marked by a small numbered letter, a sufficient
166
THE ORIGIN OF SPECIES
amount of variation is supposed to have been accumu-
lated to form it into 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 or more generations.
After a thousand generations, species (A) is supposed to
have produced two fairly well-marked varieties, namely
a1 and m\ These two varieties will generally still be
exposed to the same conditions which made their parents
variable, and the tendency to variability is in itself
hereditary; consequently they will likewise tend to vary,
and commonly in nearly the same manner as did their
parents. Moreover, these two varieties, being only slightly
modified forms, will teod to inherit those advantages
which made their parent (A) more numerous than most
of the other inhabitants of the same country; they will
also partake of those more general advantages which
made the genus to which the parent-species belonged a
large genus in its own country. And all these circum-
stances are favorable to the production of new varieties.
If, then, these two varieties be variable, the most
divergent of their variations will generally be preserved
during the next thousand generations. And after this
interval, variety a1 is supposed in the diagram to have
produced variety which will, owing to the principle
of divergence, differ more from (A) than did variety a\
Variety ml is supposed to have produced two varieties,
namely »' and s*, differing from each other, and more
considerably from their common parent (A). We may
continue the process by similar steps for any length of
time; some of the varieties, after each thousand genera-
NATURAL SELECTION
167
—-co
n
w
13
s
o
168
THE ORIGIN Of SPECIES
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 of the common parent
(A) will generally go on increasing in number and
diverging in character. In the diagram the process is
represented up to the ten-thousandth generation, and
under a condensed and simplified form up to the
fourteen-thousandth generation.
But I must here remark that I do not suppose that
the process ever goes on so regularly as is represented in
the diagram, though in itself made somewhat irregular,
nor that it goes on continuously; it is far more probable
that each form remains for long periods unaltered, and
then again undergoes modification. Nor do I suppose
that the most divergent varieties are invariably preserved:
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 per-
fectly 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 increase. In our diagram the line of suc-
cession is broken at regular intervals by small numbered
letters marking the successive forms which have become
sufficiently distinct to be recorded as varieties. But these
breaks are imaginary, and might have been inserted any-
where, after intervals long enough to allow the accumula-
tion of a considerable amount of divergent variation.
NATURAL SELECTION
169
As all the modified descendants from a common and
widely-diffused species, belonging to a large genus, will
tend to partake of the same advantages which made their
parent successful in life, they will generally go on 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 later and more highly improved branches in
the lines of descent will, it is probable, often take the
place of, and so destroy, the earlier and less improved
branches: this is represented in the diagram by some of
the lower branches not reaching to the upper horizontal
lines. In some cases no doubt the process of modifica-
tion will be confined to a single line of descent, and the
number of modified descendants will not be increased;
although the amount of divergent modification may have
been augmented. This case would be represented in the
diagram, if all the lines proceeding from (A) were re-
moved, excepting that from a1 to a10. In the same way
the English racehorse and English pointer have ap-
parently both gone on slowly diverging in character
from their original stocks, without either having given
off any fresh branches or races.
After ten thousand generations, species (A) is sup-
posed to have produced three forms, a10, /,0, and m10,
which, from having diverged in character during the
successive generations, will have come to differ largely,
but perhaps unequally, from each other and from their
common parent If we suppose the amount of change
between each horizontal line in our diagram to be ex-
cessively small, these three forms may still be only well-
marked varieties; but we have only to suppose the steps
^Science— 8
170
THE ORIGIN OF SPECIES
in the process of modification to be more numerous or
greater in amount, to convert these three forms into
doubtful or at least into well-defined species. Thus the
diagram illustrates the steps by which the small differ-
ences distinguishing varieties are increased into the larger
differences distinguishing species. By continuing the same
process for a greater number of generations (as shown in
the diagram in a condensed and simplified manner), we
get eight species, marked by the letters between a14 and
wi14, all descended from (A). Thus, as I believe, species
are multiplied and genera are formed.
In a large genus it is probable that more than one
species would vary. In the diagram I have assumed
that a second species (I) has produced, by analogous
steps, after ten thousand generations, either two well-
marked varieties (wia and z10) or two species, according
to the amount of change supposed to be represented
between the horizontal lines. After fourteen thousand
generations, six new species, marked by the letters nu to
g14, are supposed to have been produced. In any genus,
the species which are already very different in character
from each other will generally tend to produce the great-
est number of modified descendants; for these will have
the best chance of seizing on new and widely different
places in the polity of nature: hence in the diagram I
have chosen the extreme species (A), and the nearly ex-
treme 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 orig-
inal genus may for long but unequal periods continue to
transmit unaltered descendants; and this is shown in the
diagram by the dotted lines unequally prolonged upward.
NATURAL SELECTION
171
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 con-
stant tendency in the improved descendants of any one
species to supplant and exterminate in each stage of
descent their predecessors and their original progenitor.
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 states of the same species, as well as the
original parent-species itself, will generally tend to be-
come extinct. So it probably will be with many whole
collateral lines of descent, which will be conquered by
later and improved lines. If, however, the modified
offspring of a species get into some distinct country,
or become quickly adapted to some quite new station,
in which offspring and progenitor do not come into
competition, both may continue to exist.
If, then, our diagram be assumed to represent a con-
siderable amount of modification, species (A) and all the
earlier varieties will have become extinct, being replaced
by eight new species (a14 to m14); and species (I) will
be replaced by six (nl* to z14) new species.
But we may go further than this. The original spe-
cies 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
172
THE ORIGIN OF SPECIES
than to the other species; and species (I) more to G, H,
K, L, than to the others. These two species (A) and
(I) were also supposed to be very common and widely
diffused species, so that they must originally have had
some advantage over most of the other species of the
genus. Their modified descendants, fourteen in number
at the fourteen-thousandth generation, will probably have
inherited some of the same advantages: they have also
been modified and improved in a diversified manner at
each stage of descent, so as to have become adapted
to many related places in the natural economy of their
country. It seems, therefore, extremely probable that
they will have taken the places of, and thus extermi-
nated, not only their parents (A) and (I), but likewise
some of the original species which were most nearly
related to their parents. Hence very few of the original
species will have transmitted offspring to the fourteen-
thousandth generation. We may suppose that only one,
(F), of the two species (E and F) which were least
closely related to the other nine original species, has
transmitted descendants to this late stage of descent.
The new species in our diagram descended from the
original eleven species will now be fifteen in number.
Owing to the divergent tendency of natural selection, the
extreme amount of difference in character between species
a1* and 214 will be much greater than that between the
most distinct of the original eleven species. The new
species, moreover, will be allied to each other in a widely
different manner. Of the eight descendants from (A) the
three marked a14, qli, p1A, will be nearly related from
having recently branched off from a10; bx\ and /14, from
having diverged at an earlier period from a\ will be in
NATURAL SELECTION
175
some degree distinct from the three first-named species;
and lastly, o14, eM, and ra'4 will be nearly related one
to the other, but, from having diverged at the first com-
mencement of the process of modification, will be widely
different from the other five species, and may constitute
a sub-genus or a distinct genus.
The six descendants from (I) will form two sub-genera
or genera. But as the original species (I) differed largely
from (A), standing nearly at the extreme end of the orig-
inal genus, the six descendants from (I) will, owing to
inheritance alone, differ considerably from the eight de-
scendants from (A); the two groups, moreover, are sup-
posed 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 de-
scended from (I), and the eight descendants from (A),
will have to be ranked as very distinct genera, or even
as distinct sub-families.
Thus it is, as I believe, that two or more genera are
produced by descent with modification from two or more
species of the same genus. And the two or more parent-
species are supposed to be descended from some one spe-
cies of an earlier genus. In our diagram, this is indi-
cated by the broken lines, beneath the capital letters,
converging in sub-branches downward toward a single
point; this point represents a species, the supposed pro-
genitor 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 F14, which is supposed not to
have diverged much in character, but to have retained
174
THE ORIGIN OF SPECIES
the form of (F), either unaltered or altered only in a
slight degree. In this case, its affinities to the other
fourteen new species will be of a curious and circui-
tous nature. Being descended from a form which stood
between the parent-species (A) and (I), now supposed to
be extinct and unknown, it will be in some degree inter-
mediate in character between the two groups descended
from these two species. But as these two groups have
gone on diverging in character from the type of their
parents, the new species (f14) will not be directly inter-
mediate between them, but rather between types of the
two groups; and every naturalist will be able to call such
cases 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 more generations; it may also
represent a section of the successive strata of the earth's
crust including extinct remains. We shall, when we
come to our chapter on Geology, have to refer again
to this subject, and I think we shall then see that the
diagram throws light on the affinities of extinct beings,
which, though generally belonging to the same orders,
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 various remote epochs when the branch-
ing 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 the diagram we suppose the amount of change repre-
sented by each successive group of diverging dotted lines
to be great, the forms marked a14 to px\ those marked
NATURAL SELECTION
175
bl* and /M, and those marked o1* to ra14, will form three
very distinct genera. We shall also have two very dis-
tinct genera descended from (I), differing widely from
the descendants of (A). These two groups of genera will
thus form two distinct families, or orders, according to
the amount of divergent modification supposed to be rep-
resented in the diagram. And the two new families, or
orders, are descended from two species of the original
genus, and these are supposed to be descended from
some still more ancient and unknown form.
We have seen that in each country it is the species
belonging to the larger genera which oftenest present
varieties or incipient species. This, indeed, might have
been expected; for, as natural selection acts through one
form having some advantage over other forms in the
struggle for existence, it will chiefly act on those which
already have some advantage; and the largeness of any
group shows that its species have inherited from a com-
mon ancestor some advantage in common. Hence, the
struggle for the production of new and modified descend-
ants will mainly lie between the larger groups which are
all trying to increase in number. One large group will
slowly conquer another large group, reduce its numbers,
and thus lessen its chance of further variation and im-
provement. Within the same large group, the later and
more highly perfected sub-groups, from branching out
and seizing on many new places in the polity of Nature,
will constantly tend to supplant and destroy the earlier
and less improved sub-groups. Small and broken groups
and sub-groups will finally disappear. Looking to the
future, we can predict that the groups of organic beings
which are now large and triumphant, and which are least
176
THE ORIGIN OF SPECIES
broken up, that is, which have as yet suffered least ex°
tinction, will, for a long period, continue to increase.
But which groups will ultimately prevail, no man can
predict; for we know that many groups, formerly most
extensively developed, have now become extinct. Look-
ing 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, anJ leave no modified descendants; and
consequently that, of the species living at any one period,
extremely few will transmit descendants to a remote futu-
rity. I shall have to return to this subject in the chap-
ter on Classification, but I may add that as, according to
this view, extremely few of the more ancient species have
transmitted descendants to the present day, and, as all
the descendants of the same species form a class, we can
understand how it is that there exists so few classes in
each main division of the animal and vegetable king-
doms. Although few of the most ancient species have
left modified descendants, yet, at remote geological peri-
ods, the earth may have been almost as well peopled
with species of many genera, families, orders and classes
as at the present time.
On the Degree to which Organization tends to advance
Natural Selection acts exclusively by the preservation
and accumulation of variations, which are beneficial under
the organic and inorganic conditions to which each creat-
ure is exposed at all periods of life. The ultimate result
is that each creature tends to become more and more im-
proved in relation to its conditions. This improvement
inevitably leads to the gradual advancement of the organ-
NATURAL SELECTION
177
ization of the greater number of living beings throughout
the world. But here we enter on a very intricate sub-
ject, for naturalists have not defined to each other's sat-
isfaction what is meant by an advance in organization.
Among the vertebrata 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 pass through in their devel-
opment from the embryo to maturity would suffice as a
standard of comparison; but there are cases, as with cer-
tain parasitic crustaceans, in which several parts of the
structure become less perfect, so that the mature animal
cannot be called higher than its larva. Yon Baer's stan-
dard seems the most widely applicable and the best,
namely, the amount of differentiation of the parts of the
same organic being, in the adult state as I should be in-
clined to add, and their specialization for different func-
tions; or, as Milne Edwards would express it, the com-
pleteness of the division of physiological labor. But we
shall see how obscure this subject is if we look, for
instance, to fishes, among which some naturalists rank
those as highest which, like the sharks, approach nearest
to amphibians; while other naturalists rank the common
bony or teleostean fishes as the highest, inasmuch as
they are most strictly fish- like, and differ most from the
other vertebrate classes. We see still more plainly the
obscurity of the subject by turning to plants, among
which the standard of intellect is of course quite ex-
cluded; and here some botanists rank those plants as
highest which have every organ, as sepals, petals, sta-
mens, and pistils, fully developed in each flower; whereas
other botanists, probably with more truth, look at the
178
TEE ORIGLS" OF SPECIES
plants which have their several organs much modified
and reduced in number as the highest.
It we take as the standard of high organization, the
amount of differentiation and specialization of the several
organs in each being when adult (and this will include
t~e aivancer.e::: of the brain for intellectual purpose^
natural selection clearly leads toward this standard: for all
physiologists admit that the specialization of orgaDs. in-
asmuch as in this state they perform their functions bet-
ter, is an advantage to each being; and hence the accu-
mulation of variations tending toward specialization is
within the scope of natural selection. On toe 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 unoccupied or less well occupied place
in the economy of nature, that it is quite possible for
natural selection gradually to fit a being to a situation
in which several organs would be superfluous or useless:
in such cases there would be retrogression in the scale of
organization. Whether organization on the whole has
actually advanced from the remotest geological periods
to the present day will be more conveniently discussed
in our chapter on Geological Succession.
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 highly developed forms everywhere supplanted and
exterminated the lower? Lamarck, who believed in an
innate and inevitable tendency toward perfection in all
organic beings, seems to have felt this difficulty so
NATURAL SELECTION
179
strongly that he was led to suppose that new and
simple forms are continually being produced by spon-
taneous generation. Science has not as yet proved the
truth of this belief, whatever the future may reveal. On
our theory the continued existence of lowly organisms
offers no difficulty; for natural selection, or the survival
of the fittest, does not necessarily include progressive
development — it only takes advantage of such variations
as arise and are beneficial 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 intestinal worm — or even
to an earthworm, to be highly organized. If it were
no advantage, these forms would be left, by natural se-
lection, unimproved or but little improved, and might
remain for indefinite ages in their present lowly con-
dition. And geology tells us that some of the lowest
forms, as the infusoria and rhizopods, have remained for
an enormous period in nearly 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 extremely rash; for every naturalist who
has dissected some of the beings now ranked as very low
in the scale, must have been struck with their really
wondrous and beautiful organization.
Nearly the same remarks are applicable if we look to
the different grades of organization within the same great
group; for instance, in the vertebrate, to the coexistence
of mammals and fish — among mammalia, to the coexist-
ence of man and the ornithorhynchus — among fishes, to
the coexistence of the shark and the lancelet (Amphi-
oxus), which latter fish in the extreme simplicity of its
160
THE ORIGLS OF SPECIES
structure approaches the invertebrate classes. But mam-
mals and fish hardly come into competition with each
other; the advancement of the whole class of mAmmak
or of certain members in this class, to the highest grade
would not lead to their taking the place of fishes. Phys-
iologists believe that the brain must be bathed by warm
blood to be highly active, and this requires aerial respi-
ration; so that warm-blooded mammals when inhabiting
the water lie under a disadvantage in having to come
continually to the surface to breathe. With fishes, mem-
bers of the shark family would not tend to supplant the
lancelet; for the lancelet, as I hear from Fritz M tiller,
has as sole companion and competitor on the barren
sar.Lv si-: re <:: Sou:! Brazil an anomalous ar.nelii.
The three lowest orders of mammals, namely, marsu-
pials, edentata, and rodents, coexist in South America
in the same region with numerous monkeys, and prob-
a::_v interfere little wit! eac! c:!er. Although organi-
zation, on the whole, may have advanced and be still
advancing throughout the world, yet the scale will al-
ways present many degrees of perfection: for the high
advancement of certain whole classes, or of certain mem-
bers of eac! 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 here-
after see. lowly organized forms appear to have been pre-
served to the present day, from inhabiting confined or
peculiar stations, ^rlere tley lave been subjected to less
severe competi:ion. and where their scanty numbers have
retarded the chance of favorable variations arising.
/ Finally, I believe that many lowly organized forms
/ now exist throughout the world, from various causes,
NATURAL SELECTION
181
In some cases variations or individual differences of a
favorable nature may never have arisen for natural se-
lection to act on and accumulate- In no case, probably,
has time sufficed for the utmost possible amount of de-
velopment. In some few cases there has been what we
must call retrogression of organization. But the main
cause lies in the fact that under very simple conditions
of life a high organization would be of no service — pos-
sibly would be of actual disservice, as being of a more
delicate nature, and more liable to be put out of order
and injured.
Looking to the hrst dawn of life, when all organic
beings, as we may believe, presented the simplest struc-
ture, how, it has been asked, could the first steps in the
advancement or differentiation of parts have arisen ? Mr.
Herbert Spencer would probably answer that, as soon as
simple unicellular organism came, by growth or division,
to be compounded of several cells, or became attached to
any supporting surface, his law 1 4 that homologous units
of any order become differentiated in proportion as their
relations to incident forces become different" would come
into action. But as we have no facts to guide us, spec-
ulation on the subject is almost useless. It is. however,
an error to suppose that there would be no struggle for
existence, and, consequently, no natural selection, until
many forms had been produced: variations in a single
species inhabiting an isolated station might be beneficial,
and thus the whole mass of individuals might be modi-
fied, or two distinct forms might arise. But. as I re-
marked toward the close of the Introduction, no one
ought to feel surprised at much remaining as yet un-
explained on the origin of species, if we make due
132
THE OPJGIX OF SPECIES
allowance for our profound ignorance on the mutual
relations of the inhabitants of the world at the present
time, and still more so during past ages.
Convergence of Character
Mr. H. C. Watson thinks that I have overrated the
importance of divergence of character (in which, however,
he apparently believes), and that convergence, as it may
be called, has likewise played a part. If two species,
belonging to two distinct though allied genera, had both
produced a large number of new and divergent forms, it
is conceivable that these might approach each other so
closely that they would have all to be classed under the
same genus; and thus the descendants of two distinct
genera would converge into one. But it would in most
cases be extremelv rash to attribute to convergence a
close and general similarity of structure in the modified
descendants of widely distinct forms. The shape of a
crystal is determined solely by the molecular forces, and
it is not surprising that dissimilar substances should
sometimes assume the same form; but with organic
beings we should bear in mind that the form of each
depends on an infinitude of complex relations, namely
on the variations which have arisen, these being due to
causes far too intricate to be followed out — on the nature
of the variations which have been preserved or selected,
and this depends on the surrounding physical conditions,
and in a still higher degree on the surrounding organisms
with which each being has come into competition — and
lastly, on inheritance (in itself a fluctuating element)
from innumerable progenitors, all of which have had
their forms determined through equally complex rela-
NATURAL SELECTION
183
tions. It is incredible that the descendants of two
organisms, which had originally differed in a marked
manner, should ever afterward converge so closely as to
lead to a near approach to identity throughout their whole
organization. If this had occurred; we should meet with
the same form, independently of genetic connection, re-
curring in widely separated geological formations; and
the balance of evidence is opposed to any such an
admission.
Mr. Watson has also objected that the continued
action of natural selection, together with divergence of
character, would tend to make an indefinite number
of specific forms. As far as mere inorganic conditions
are concerned, it seems probable that a sufficient number
of species would soon become adapted to all considerable
diversities of heat, moisture, etc. ; but I fully admit that
the mutual relations of organic beings are more impor-
tant; and as the number of species in any country goes
on increasing, the organic conditions of life must become
more and more complex. Consequently there seems at
first sight no limit to the amount of profitable diversifi-
cation of structure, and therefore 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 and in Aus-
tralia, which support such an astonishing number of
species, many European plants have become naturalized.
But geology shows us that from an early part of the
tertiary period the number of species of shells, and that
from the middle part of this same period the number of
mammals, has not greatly or at al^. increased. What then
checks an indefinite increase in the number of specie??
184
THE ORIGIN OF SPECIES
The amount of life (I do not mean the number of specific
forms) supported on an area must have a limit, depend-
ing so largely as it does on physical conditions; there-
fore, if an area be inhabited by very many species, each
or nearly each species will be represented by few indi-
viduals; and such species will be liable to extermination
from accidental fluctuations in the nature of the seasons
or in the number of their enemies. The process of ex-
termination in such cases would be rapid, whereas the
production of new species must always be slow. Imagine
the extreme case of as many species as indidviuals in
England, and the first severe winter or very dry summer
would exterminate thousands on thousands of species.
Rare species, and each species will become rare if the
number of species in any country becomes indefinitely in-
creased, will, on the principle often explained, present
within a given period few favorable variations; conse-
quently, the process of giving birth to new specific forms
would thus be retarded. When any species becomes very
rare, close interbreeding will help to exterminate it;
authors have thought that this comes into play in
accounting for the deterioration of the Aurochs in Lithu-
ania, of Red Deer in Scotland, and of Bears in Norway,
etc. Lastly, 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 very widely; consequently, they will
tend to supplant and exterminate several species in
several areas, and thus check the inordinate increase of
specific forms throughout the world. Dr. Hooker has
NATURAL SELECTION
185
recently shown that in the S.E. corner of Australia,
where, apparently, there are many invaders from different
quarters of the globe, the endemic Australian species
have been greatly reduced in number. How much weight
to attribute to these several considerations I will not
pretend to say; but conjointly they must limit in each
country the tendency to an indefinite augmentation of
specific forms.
Summary of Chapter
If under changing conditions of life organic beings
present individual differences in almost every part of
their structure, and this cannot be disputed; if there be,
owing to their geometrical rate of increase, a severe
struggle for life at some age, season, or year, and this
certainly cannot be disputed; then, considering the in-
finite complexity of the relations of all organic beings
to each other and to their conditions of life, causing an
infinite diversity in structure, constitution, and habits, to
be advantageous to them, it would be a most extraordi-
nary fact if no variations had ever occurred useful to
each being's own welfare, in the same manner as so
many variations have occurred useful to man. But if
variations useful to any organic being ever do occur, ,
assuredly individuals thus characterized will have the
best chance of being preserved in the struggle for life;
and from the strong principle of inheritance, these will
tend to produce offspring similarly characterized. This
principle of preservation, or the survival of the fittest, I
have called Natural Selection. ''/it leads to the improve-
ment of each creature in relation to its organic and inor-
ganic conditions of life; and consequently, in most cases,
186
THE ORIGIN OF SPECIES
to what must be regarded as an advance in organization.
Nevertheless, low and simple forms will long endure if
well fitted for their simple conditions of life.
Natural selection, on the principle of qualities being
inherited at corresponding ages, can modify the egg,
seed, or young, as easily as the adult. Among many
animals, sexual selection will have given its aid to ordi-
nary selection, by assuring to the most vigorous and best
adapted males the greatest number of offspring. Sexual
selection will also give characters useful to the males
alone, in their struggles or rivalry with other males; and
these characters will be transmitted to one sex or to both
sexes, according to the form of inheritance which prevails.
Whether natural selection has really thus acted, in
adapting the various forms of life to their several condi-
tions and stations, must be judged by the general tenor
and balance of evidence given in the following chapters.
But we have already seen how it entails extinction; and
how largely extinction has acted in the world's history,
geology plainly declares. Natural selection, also, leads
to divergence of character; for the more organic beings
diverge in structure, habits, and constitution, by so much
the more can a large number be supported on the area —
of which we see proof by looking to the inhabitants of
any small spot, and to the productions naturalized in
foreign lands. Therefore, during the modification of the
descendants of any one species, and during the incessant
struggle of all species to increase in numbers, the more
diversified the descendants become, the better will be
their chance of success in the battle for life. Thus the
small differences distinguishing varieties of the same
species steadily tend to increase, till they equal the
s
NATURAL SELECTION
187
greater differences between species of the same genus, or
even of distinct genera.
We have seen that it is the common, the widely-
diffused and widely-ranging species, belonging to the
larger genera within each class, which vary most; and
these tend to transmit to their modified offspring that
superiority which now makes them dominant in their
own countries. Natural selection, as has just been re-
marked, leads to divergence of character and to much
extinction of the less improved and intermediate forms
of life. On these principles, the nature of the affinities,
and the generally well-defined distinctions between the
innumerable organic beings in each class throughout
the world, may be explained. It is a truly wonderful
fact — the wonder of which we are apt to overlook from
familiarity — that all animals and all plants throughout all
time and space should be related to each other in groups,
subordinate to groups, in the manner which we every-
where behold; namely, varieties of the same species most
closely related, species of the same genus less closely and
unequally related, forming sections and sub-genera, species
of distinct genera much less closely related, and genera
related in different degrees, forming sub-families, families,
orders, sub-classes and classes. The several subordinate
groups in any class cannot be ranked in a single file,
but seem clustered round points, and these round other
points, and so on in almost endless cycles. If species
had been independently created, no explanation would
have been possible of this kind of classification; but 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.
188
THE ORIGIN OF SPECIES
The affinities of all the beings of the same class have
sometimes been represented hy a great tree. I believe
this simile largely speaks the truth. The green and
budding twigs may represent existing species; and those
produced during former years may represent the long
succession of extinct species. At each period of growth
all the growing twigs have tried to branch out on all
sides, and to overtop and kill the surrounding twigs and
branches, in the same manner as species and groups of
species have at all times overmastered 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 young, budding
twigs; and this connection 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 flour-
ished when the tree was a mere bush, only two or three,
now grown into great branches, yet survive and bear the
other branches; so with the species which lived during
long-past geological periods, very few have left living
and modified descendants. From the first growth of the
tree, many a limb and branch has decayed and dropped
off; and these fallen branches of various sizes may repre-
sent those whole orders, families, and genera which have
now no living representatives, and which are known to us
only 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 favored and is
still alive on its summit, so we occasionally see an animal
like the Ornithorhynchus or Lepidosiren, which in some
small degree connects by its affinities two large branches
NATURAL SELECTION
189
of life, and which has apparently been saved from fatal
competition by having inhabited a protected station. As
buds give rise by growth to fresh buds, and these, if
vigorous, branch out and overtop on all sides many a
feebler branch, so by generation I believe it has been
with the great Tree of Life, which fills with its dead and
broken branches the crust of the earth, and covers the
surface with its ever- branching and beautiful ramifications.
190
THE ORIGIN OF SPECIES
CHAPTER V
LAWS OF VARIATION
Effects of changed conditions — Use and disuse, combined with natural
selection ; organs of flight and of vision — Acclimatization — Correlated
Variation — Compensation and economy of growth — False correlations
— Multiple, rudimentary, and lowly organized 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 —
Reversions to long-lost characters — Summary
I HAVE hitherto sometimes spoken as if the variations
— so common and multiform with organic beings
under domestication, and in a lesser degree with
those under nature — were due to chance. This, of
course, is a wholly incorrect expression, but it serves to
acknowledge plainly our ignorance of the cause of each
particular variation. Some authors believe it to be as
much the function of the reproductive system to produce
individual differences, or slight deviations of structure,
as to make the child like its parents. But the fact
of variations and monstrosities occurring much more fre-
quently under domestication than under nature, and the
greater variability of species having wide ranges than of
those with restricted ranges, lead to the conclusion that
variability is generally related to the conditions of life
to which each species has been exposed during several
successive generations. In the first chapter I attempted
to show that changed conditions act in two ways, directly
LAWS OF VARIATION
191
on the whole organization or on certain parts alone, and
indirectly through the reproductive system. In all cases
there are two factors, the nature of the organism, which
is much the most important of the two, and the nature
of the conditions. The direct action of changed condi-
tions leads to definite or indefinite results. In the latter
case the organization seems to become plastic, and we
have much fluctuating variability. In the former case
the nature of the organism is such that it yields readily,
when subjected to certain conditions, and all, or nearly
all, the individuals become modified in the same way.
It is very difficult to decide how far changed condi-
tions, such as of climate, food, etc., have acted in a definite
manner. There is reason to believe that in the course of
time the effects have been greater than can be proved by
clear evidence. But we may safely conclude that the
innumerable complex coadaptations of structure, which
we see throughout nature between various organic beings,
cannot be attributed simply to such action. In the fol-
lowing cases the conditions seem to have produced some
slight definite effect: E. Forbes asserts that shells at their
southern limit, and when living in shallow water, are more
brightly colored than those of the same species further
north or from a greater depth; but this certainly does
not always hold good. Mr. Gould believes that birds of
the same species are more brightly colored under a clear
atmosphere than when living near the coast or on islands,
and Wollaston is convinced that residence near the sea
affects the colors of insects. Moquin-Tandon gives a list
of plants which, when growing near the sea-shore, have
their leaves in some degree fleshy, though not elsewhere
fleshy. These slightly varying organisms are interesting
192
THE ORIGIN OF SPECIES
in as far as they present characters analogous to those
possessed by the species which are confined to similar
conditions.
When a variation is of the slightest use to any being,
we cannot tell how much to attribute to the accumulative
action of natural selection, and how much to the definite
action of the conditions of life. Thus, it is well known
to furriers that animals of the same species have thicker
and better fur the further north they live; but who
can tell how much of this difference may be due to the
warmest-clad individuals having been favored and pre-
served during many generations, and how much to the
action of the severe climate? for it would appear that
climate has some direct action on the hair of our domes-
tic quadrupeds.
Instances could be given of similar varieties being
produced from the same species under external conditions
of life as different as can well be conceived; and, on the
other hand, of dissimilar varieties being produced under
apparently the same external conditions. Again, innu-.
merable instances are known to every naturalist of
species keeping true, or not varying at all, although
living under the most opposite climates. Such considera-
tions as these incline me to lay less weight on the direct
action of the surrounding conditions than on a tendency
to vary, due to causes of which we are quite ignorant.
In one sense the conditions of life may be said not
only to cause variability, either directly or indirectly,
but likewise to include natural selection, for the condi-
tions determine whether this or that variety shall survive.
But when man is the selecting agent, we clearly see that
the two elements of change are distinct; variability is in
LAWS OF VARIATION
193
some manner excited, but it is the will of man which
accumulates the variations in certain directions; and it is
this latter agency which answers to the survival of the
fittest under nature.
Effects of the increased Use and Disuse of Parts, as
controlled by Natural Selection
From the facts alluded to in the first chapter, I think
there can be no doubt that use in our domestic animals
has strengthened and enlarged certain parts, and disuse
diminished them; and that such modifications are inher-
ited. Under free nature, we have no standard of com-
parison by which to judge of the effects of long- contin-
ued use or disuse, for we know not the parent- forms;
but many animals possess structures which can be best
explained by the effects of disuse. As Professor Owen
has remarked, there is no greater anomaly in nature than
a bird that cannot fly; yet there are several in this state.
The logger- headed duck of South America can only flap
along the surface of the water, and has its wings in
nearly the same condition as the domestic Aylesbury
duck: it is a remarkable fact that the young birds, ac-
cording to Mr. Cunningham, can fly, while the adults
have lost this power. As the larger ground-feeding birds
seldom take flight except to escape danger, it is probable
that the nearly wingless condition of several birds, now
inhabiting or which lately inhabited several oceanic isl-
ands, 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 it can defend itself by kicking its enemies, as effi-
ciently as many quadrupeds. We may believe that the
— Science— 9
194
THE ORIGIN OF SPECIES
progenitor of the ostrich genus had habits like those of
the bustard, and that, as the size and weight of its body
were increased during successive generations, its legs were
used more, and its wings less, until they became inca-
pable of flight.
Kirby has remarked (and I have observed the same
fact) that the anterior tarsi, or feet, of many male dung-
feeding beetles are often broken off; he examined seven-
teen 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.
The evidence that accidental mutilations can be inherited
is at present not decisive; but the remarkable cases ob-
served by Brown-Sequard in guinea-pigs, of the inherited
effects of operations, should make us cautious in denying
this tendency. Hence it will perhaps be safest to look at
the entire absence of the anterior tarsi in Ateuchus, and
their rudimentary condition in some other genera, not as
cases of inherited mutilations, but as due to the effects
of long-continued disuse; for as many dung-feeding bee-
tles are generally found with their tarsi lost, this must
happen early in life; therefore the tarsi cannot be of
much importance or be much used by these insects.
In some cases we might easily put down to disuse
modifications of structure which are wholly, or mainly,
due to natural selection. Mr. Wollaston has discovered
the remarkable fact that 200 beetles, out of the 550 spe-
cies (but more are now known) inhabiting Madeira, are
so far deficient in wings that they cannot fly; and that,
LAWS OF VARIATION
195
of the twenty-nine endemic genera, no less than twenty-
three have all their species in this condition! Several
facts — namely, that beetles in many parts of the world
are frequently blown to sea and perish; that the beetles
in Madeira, as observed by Mr. Wollaston, lie much con-
cealed, until the wind lulls and the sun shines; that the
proportion of wingless beetles is larger on the exposed
Desertas than in Madeira itself; and especially the ex-
traordinary fact, so strongly insisted on by Mr. Wol-
laston, that certain large groups of beetles, elsewhere
excessively numerous, which absolutely require the use
of their wings, are here almost entirely absent; — these
several considerations make me believe that the wingless
condition of so many Madeira beetles is mainly due to
the action of natural selection, combined probably with
disuse. For during many successive generations each in-
dividual 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 bettles which most readily took to
flight would oftenest have been blown to sea, and thus
destroyed.
The insects in Madeira which are not ground-feeders,
and which, as certain flower-feeding coleoptera and lepi-
doptera, must habitually use their wings to gain their
subsistence, have, as Mr. Wollaston suspects, their wings
not at all reduced, but even enlarged. This is quite
compatible with the action of natural selection. For
when a new insect first arrived on the island, the ten-
dency of natural selection to enlarge or to reduce the
wings would depend on whether a greater number of
196
THE ORIGIN OF SPECIES
individuals were saved by successfully battling with the
winds, or by giving up the attempt and rarely or never
flying. As with mariners 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
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 ro-
dent, the tucutucu, or Ctenomys, is even more subter-
ranean in its habits than the mole; and I was assured by
a Spaniard, who had often caught them, that they were
frequently blind. One which I kept alive was certainly
in this condition, the cause, as appeared on dissection,
having been inflammation of the nictitating membrane.
As frequent inflammation of the eyes must be injurious
to any animal, and as eyes are certainly not necessary to
animals having subterranean habits, a reduction in their
size, with the adhesion of the eyelids and growth of fur
over them, might in such case be an advantage; and
if so, natural selection would aid the effects of disuse.
It is well known that several animals, belonging to
the most different classes, which inhabit the caves of
Carniola 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, their loss
LAWS OF VARIATION
197
may be attributed to disuse. In one of the blind ani-
mals, namely, the cave-rat (Neotoma), two of which were
captured by Professor Silliman at above half a mile dis-
tant from the mouth of the cave, and therefore not in
the profoundest depths, the eyes were lustrous and of
large size; and these animals, as I am informed by Pro-
fessor Silliman, after having been exposed for about a
month to a graduated light, acquired a dim perception
of objects.
It is difficult to imagine conditions of life more sim-
ilar than deep limestone caverns under a nearly similar
climate; so that, in accordance with the old view of the
blind animals having been separately created for the
American and European caverns, very close similarity
in their organization and affinities might have been ex-
pected. This is certainly not the case if we look at the
two whole faunas; and with respect to the insects alone,
Schiodte has remarked: "We are accordingly prevented
from considering the entire phenomenon in any other
light than something purely local, and the similarity
which is exhibited in a few forms between the Mammoth
cave (in Kentucky) and the caves in Carniola, otherwise
than as a very plain expression of that analogy which
subsists generally between the fauna of Europe and of
North America." On my view we must suppose that
American animals, having in most cases ordinary powers
of vision, slowly migrated by successive generations from
the outer world into the deeper and deeper recesses of
the Kentucky caves, as did European animals into the
caves of Europe. We have some evidence of this grada-
tion of habit; for, as Schiodte remarks, "We accordingly
look upon the subterranean faunas as small ramifications
198
THE ORIGIN OF SPECIES
which have penetrated into the earth from the geograph-
ically limited faunas of the adjacent tracts, and which, as
they extended themselves into darkness, have been accom-
modated to surrounding 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, and whose formation is quite peculiar." These
remarks of Schiodte's, it should be understood, apply not
to the same, but to distinct species. By the time that
an animal had reached, after numberless generations, the
deepest recesses, disuse will on this view have more or
less perfectly obliterated its eyes, and natural selection
will often have effected other changes, such as an in-
crease in the length of the antennae or palpi, as a com-
pensation for blindness. Notwithstanding such modifica-
tions, we might expect still to see in the cave-animals of
America aifinities to the other inhabitants of that conti-
nent, 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 difficult to give any rational explanation of the
affinities of the blind cave-animals to the other inhabi-
tants of the two continents on the ordinary view of their
independent creation. That several of the inhabitants of
the caves of the Old and New Worlds should be closely
related, we might expect from the well-known relation-
ship of most of their other productions. As a blind spe-
cies of Bathyscia is found in abundance on shady rocks
far from caves, the loss of vision in the cave-species of
LAWS OF VARIATION
199
this one genus has probably had no relation to its dark
habitation; for it is natural that an insect already de-
prived of vision should readily become adapted to dark
caverns. Another blind genus (Anophthalmus) offers this
remarkable peculiarity, that the species, as Mr. Murray
observes, have not as yet been found anywhere except
m caves; yet those which inhabit the several caves of
Europe and America are distinct; but it is possible that
the progenitors of these several species, while they were
furnished with eyes, may formerly have ranged over both
continents, and then have become extinct, excepting in
their present secluded abodes. Far from feeling surprise
that some of the cave- animals should be very anomalous,
as Agassiz has remarked in regard to the blind fish, the
Amblyopsis, and as is the case with the blind Proteus
with reference to the reptiles of Europe, I am only sur-
prised that more wrecks of ancient life have not been
preserved, owing to the less severe competition to which
the scanty inhabitants of these dark abodes will have
been exposed.
Acclimatiza Hon
Habit is hereditary with plants, as in the period of
flowering, in the time of sleep, in the amount of rain
requisite for seeds to germinate, etc., and this leads me
to say a few words on acclimatization. As it is ex-
tremely common for distinct species belonging to the
same genus to inhabit hot and cold countries, if it be
true that all the species of the same genus are descended
from a single parent form, acclimatization must be readily
effected during a long course of descent. It is notorious
that each species is adapted to the climate of its own
home: species from an arctic or even from a temperate
200
THE ORIGIN OF SPECIES
region cannot endure a tropical climate, or conversely.
So again, many succulent plants cannot endure a damp
climate. But the degree of adaptation of species to the
climates under which they live is often overrated. We
may infer this from our frequent inability to predict
whether or not an imported plant will endure our
climate, and from the number of plants and animals
brought from different countries which are here perfectly
healthy. We have reason to believe that species in a
state of nature are closely limited in their ranges by
the competition of other organic beings quite as much
as, or more than, by adaptation to particular climates.
But whether or not this adaptation is in most cases very
close, we have evidence with some few plants of their
becoming, to a certain extent, naturally habituated to dif-
ferent temperatures: that is, they become acclimatized;
thus the pines and rhododendrons, raised from seed col-
lected by Dr. Hooker. from the same species growing at
different heights on the Himalaya, were found to possess
in this country different constitutional powers of resisting
cold. Mr. Thwaites informs me that he has observed
similar facts in Ceylon; analogous observations have been
made by Mr. H. C. Watson on European species of
plants brought from the Azores to England; and I could
give other cases. In regard to animals, several authentic
instances could be adduced of species having largely ex-
tended, within historical times, their range from warmer
to cooler latitudes, and conversely; but we do not posi-
tively know that these animals were strictly adapted to
their native climate, though in all ordinary cases we
assume such to be the case; nor do we know that they
have subsequently become specially acclimatized to their
LAWS OF VARIATION
201
new homes, so as to be better fitted for them than they
were at first.
As we may infer that our domestic animals were
originally chosen by uncivilized man because they were
useful and because they bred readily under confinement,
and not because they were subsequently found capable
of far- extended transportation, the common and extraor-
dinary capacity in our domestic animals of not only
withstanding the most different climates, but of being
perfectly fertile (a far severer test) under them, may
be used as an argument that a large proportion of other
animals now in a state of nature could easily be brought
to bear widely different climates. We must not, how-
ever, push the foregoing argument too far, on account
of the probable origin of some of our domestic animals
from several wild stocks; the blood, for instance, of a
tropical and arctic wolf may perhaps be mingled in our
domestic breeds. The rat and mouse cannot be consid-
ered 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; for they live under
the cold climate of Faroe in the north and of the Falk-
lands in the south, and on many an island in the torrid
zones. Hence adaptation to any special climate may be
looked at as a quality readily grafted on an innate wide
flexibility of constitution, common to most animals. On
this view, the capacity of enduring the most different
climates by man himself and by his domestic animals,
and the fact of the extinct elephant and rhinoceros hav-
ing formerly endured 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 as examples of
202
THE ORIGIN OF SPECIES
a very common flexibility of constitution, brought, under
peculiar circumstances, into action.
How much of the acclimatization 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
an obscure question. That habit or custom has some in-
fluence I must believe, both from analogy and from the
incessant advice given in agricultural works, even in
the ancient Encyclopedias of China, to be very cautious
in transporting animals from one district to another.
And as it is not likely that man should have succeeded
in selecting so many breeds and sub-breeds with constitu-
tions specially fitted for their own districts, the result
must, I think, be due to habit. On the other hand,
natural selection would inevitably tend to preserve those
individuals which were born with constitutions best
adapted to any country which they inhabited. In trea-
tises on many kinds of cultivated plants, certain varieties
are said to withstand certain climates better than others;
this is strikingly shown in works on fruit-trees published
in the United States, in which certain varieties are ha-
bitually recommended for the Northern and others for the
Southern States; and as most of these varieties are of
recent origin, they cannot owe their constitutional differ-
ences to habit. The case of the Jerusalem artichoke,
which is never propagated in England by seed, and of
which consequently new varieties have not been pro-
duced, has even been advanced, as proving that acclima-
tization cannot be effected, for it is now as tender as
ever it was! The case, also, of the kidney-bean has
been often cited for a similar purpose, and with much
LAWS OF VARIATION
203
greater weight; but until some one will sow, during a
score of generations, his kidney-beans so early that
a very large proportion are destroyed by frost, and then
collect seed from the few survivors, with care to prevent
accidental crosses, and then again get seed from these
seedlings, with the same precautions, the experiment can-
not be said to have been tried. Nor let it be supposed
that differences in the constitution of seedling kidney-
beans never appear, for an account has been published
how much more hardy some seedlings are than others;
and of this fact I have myself observed striking instances.
On the whole, we may conclude that habit or use and
disuse, have, in some cases, played a considerable part
in the modification of the constitution and structure; but
that the effects have often been largely combined with,
and sometimes overmastered by, the natural selection of
innate variations.
Correlated Variation
I mean by this expression that the whole organization
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 be-
come modified. This is a very important subject, most
imperfectly understood, and no doubt wholly different
classes of facts may be here easily confounded together.
We shall presently see that simple inheritance often gives
the false appearance of correlation. One of the most
obvious real cases is, that variations of structure arising
in the young or larvae naturally tend to affect the struc-
ture of the mature animal. The several parts of the
body which are homologous, and which, at an early em-
204
THE ORIGIN OF SPECIES
brjonic period, are identical in structure, and which are
necessarily exposed to similar conditions, seem eminently
liable to vary in a like 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 by
some anatomists to be homologous with the limbs. These
tendencies, I do not doubt, may be mastered more or less
completely by natural selection; thus a family of stags
once existed with an antler only on one side; and if this
had been of any great use to the breed, it might probably
have been rendered permanent by selection.
Homologous parts, as has been remarked by some
authors, tend to cohere; this is often seen in monstrous
plants: and nothing is more common than the union of
homologous parts in normal structures, as in the union
of the petals into a tube. Hard parts seem to affect the
form of adjoining soft parts; it is believed by some
authors that with birds the diversity in the shape of the
pelvis causes the remarkable diversity in the shape of
their kidneys. Others believe that the shape of the
pelvis in the human mother influences by pressure
the shape of the head of the child. In snakes, accord-
ing to Schlegel, the form of the body and the manner of
swallowing determine the position and form of several
of the most important viscera.
The nature of the bond is frequently quite obscure.
M. Isid. Geoffroy St.-Hilaire has forcibly remarked that
certain malconformations frequently, and that others
rarely, co-exist, without our .being able to assign any
reason. What can be more singular than the relation
in cats between complete whiteness and blue eyes with
LAWS OF VARIATION
205
deafness, or between the tortoise-shell color and the fe-
male sex; or in pigeons between their feathered feet and
skin between the outer toes, or between the presence
of more or less down on the young pigeon when first
hatched, with the future color of its plumage; or, again,
the relation between the hair and teeth in the naked
Turkish dog, though here no doubt homology comes into
play? With respect to this latter case of correlation, I
think it can hardly be accidental that the two orders of
mammals which are most abnormal in their dermal cov-
ering, viz., Cetacea (whales) and Edentata (armadillos,
scaly ant-eaters, etc.), are likewise on the whole the
most abnormal in their teeth; but there are so many
exceptions to this rule, as Mr. Mivart has remarked,
that it has little value.
I know of no case better adapted to show the impor-
tance of the laws of correlation and variation, indepen-
dently of utility and therefore of natural selection, than
that of the difference between the outer and inner flowers
in some Compositous and Umbelliferous plants. Every
one is familiar with the difference between the ray and
central florets of, for instance, the daisy, and this differ-
ence is often accompanied with the partial or complete
abortion of the reproductive organs. But in some of
these plants, the seeds also differ in shape and sculpture.
These differences have sometimes been attributed to the
pressure of the involucra on the florets, or to their
mutual pressure, and the shape of the seeds in the
ray-florets of some Composite countenances this idea;
but with the Umbelliferse, it is by no means, as Dr.
Hooker informs me, the species with the densest heads
which most frequently differ in their inner and outer
206
THE ORIGIX OF SPECIES
flowers. It might have been thought that the develop-
ment of the ray-petals by drawing nourishment from the
reproductive organs causes their abortion; but this can
hardly be the sole cause, for in some Composite the
seeds of the outer and inner florets differ, without any
difference in the corolla. Possibly these several differ-
ences may be connected with the different flow of nutri-
ment toward the central and external flowers: we know,
at least, that with irregular flowers those nearest to the
axis are most subject to peloria, that is. to become abnor-
mally symmetrical. I may add, as an instance of this
fact, and as a striking case of correlation, that in many
pelargoniums the two upper petals in the central flower
of the truss often lose their patches of darker color: and
when this occurs, the adherent nectary is quite aborted;
the central flower thus becoming peloric or regular.
When the color is absent from only one of the two
upper petals, the nectary is not quite aborted but is
much shortened.
With respect to the development of the corolla,
Sprengel's idea that the ray-florets serve to attract in-
sects, whose agency is highly advantageous or necessary
for the fertilization of these plants, is highly probable;
and if so, natural selection may have come into play.
But with respect to the seeds, it seems impossible that
their differences in shape, which are not always correlated
with any difference in the corolla, can be in any way
beneficial: yet in the Umbellifera? these differences are
of such apparent importance — the seeds being sometimes
orthospermous in the exterior flowers and ccelosperm-
ous in the central flowers — that the elder De Candolle
founded his main divisions in the order on such char-
LAWS OF VARIATION
207
acters. Hence modifications of structure, viewed by sys-
tematists as of high value, may be wholly due to the
laws of variation and correlation, without being, as far
as we can judge, of the slightest service to the species.
We may often falsely attribute to correlated variation
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 in some neces-
sary manner correlated. Some other correlations are ap-
parently due to the manner in which natural selection
can alone act. For instance, Alph. de Candolle has
remarked that winged seeds are never found in fruits
which do not open; I should explain this rule by the
impossibility of seeds gradually becoming winged through
natural selection, unless the capsules were open; for in
this case alone could the seeds, which were a little better
adapted to be wafted by the wind, gain an advantage
over others less well fitted for wide dispersal.
Compensation and Economy of Growth
The elder Geoffroy and Goethe propounded, at about
the same time, their law of compensation or balancement
of growth; or, as Goethe expressed it, "in order to spend
on one side, nature is forced to economize on the other
side." 1 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 ex-
iv3
THE LRIGLS OF SPECIES
cess, to another part: thus it is diffieolt to get a cow to
give much milk and to fatten readily. The same varie-
ties of the cabbage do not yield abundant and nutritious
foliage and a copious supply of oil-bearing see Is. When
the seeds in our fruits become atrophied, the fruit itself
gains largely in size and quailty. In our poultry, a
larre tuft of feathers on the head is generally accom-
panied by a diminished comb, and a large beard by
diminished wattles. With species in a state of nature -
it can hardly be maintained that the law is of universal
application: but many good observers, more especially
botanists, believe in its truth. I will not, however, here
give any instances, for I see hardly any way of distin-
guishing ":: f:~ ~-n the effects, or. the ere rani, 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 grow;h in another and adjoining part.
I suspect, also, that some of the cases of compensa-
tion which have been advanced, and likewise some other
facts, may be merged under a more general principle;
namely. :rat natural selection is continually trying to
economize every part of the organization. If under
charged conditions of life a structure, "before useful,
becomes less useful, its diminution will be favored, for
it will profit the individual not to have its nutriment
wasted in building up a useless structure. I can thus
only understand a fact with which I was much struck
when examining cirri peds, and of which many analogous
instances could be given: namely, that when a cirriped
is parasitic within another cirriped and is thus protected.
LAWS OF VARIATION
209
it loses more or less completely its own shell or carapace.
This is the case with the male Ibla, and in a truly ex-
traordinary manner with the Proteolepas: for the carapace
in all other cirripeds consists of the three highly-impor-
tant 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 at-
tached to the bases of the prehensile antennae. Now the
saving of a large and complex structure, when rendered
superfluous, would be a decided advantage to each suc-
cessive individual of the species; for in the struggle for
life to which every animal is exposed each would have
a better chance of supporting itself, by less nutriment
being wasted.
Thus, as I believe, natural selection will tend in the
long run to reduce any part of the organization, as soon
as it becomes, through changed habits, superfluous, with-
out by any means causing some other part to be largely
developed in a corresponding degree. And, conversely,
'hat natural selection may perfectly well succeed in largely
ieveloping an organ without requiring as a necessary com-
pensation the reduction of some adjoining part.
Multiple, Rudimentary, and Lowly- Organized Structures
are Variable
It seems to be a rule, as remarked by Is. Geoflroy
S*.-Hilaire, both with varieties and species, that when
any part or organ is repeated many times in the same
individual (as the vertebrae in snakes, and the stamens
in polyandrous flowers) the number is variable; whereas
the same part or organ, when it occurs in lesser num-
210
THE ORIGIX OF SPECIES
bers, is constant. The same author as well as some
botanists have further remarked that multiple parts are
extremely liable to vary in structure. As "vegetative
repetition," to use Prof. Owen's expression, is a sign of
low organization, the foregoing statements accord with
the common opinion of naturalists, that beings which
stand low in the scale of nature are more variable than
those which are higher. I presume that lowness here
means that the several parts of the organization have
been but little specialized 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 not have preserved or
rejected each little deviation of form so carefully as when
the part has to serve for some one special purpose. In
the same way that a knife which has to cut all sorts of
things may be of almost any shape; while a tool for
some particular purpose must be of some particular shape.
Natural selection, it should never be forgotten, can act
solely through and for the advantage of each being.
Eudimentary parts, as it is generally admitted, are apt
to be highly variable. We shall have to recur to this
subject; and I will here only add that their variability
seems to result from their uselessness, and consequently
from natural selection having had no power to check
deviations in their structure.
A Part developed in any Species in an extraordinary
degree or manner, in comparison with the same Part
in allied Species, tends to be highly variable
Several years ago I was much struck by a remark, to
the above effect, made by Mr. Waterhouse. Professor
LAWS OF VARIATION
211
Owen, also, seems to have come to a nearly similar con-
clusion. It is hopeless to attempt to convince any one
of the truth of the above proposition without giving the
long array of facts which I have collected, and which
cannot possibly be here introduced. I can only state my
conviction that it is a rule of high generality. I am
aware of several causes of error, but I hope that I have
made due allowance for them. It should be understood
that the rule by no means applies to any part, however
unusually developed, unless it be unusually developed in
one species or in a few species in comparison with the
same part in many closely allied species. Thus, the wing
of a bat is a most abnormal structure in the class of
mammals, but the rule would not apply here, because
the whole group of bats possesses wings; it would apply
only if some one species had wings developed in a re-
markable manner in comparison with the other species of
the same genus. The rule applies very strongly in the
case of secondary sexual characters, when displayed in
any unusual manner. The term, secondary sexual char-
acters, used by Hunter, relates to characters which are
attached to one sex, but are not directly connected with
the act of reproduction. The rule applies to males and
females; but more rarely to the females, as they seldom
offer remarkable secondary sexual characters. The rule
being so plainly applicable in the case of secondary sex-
ual characters, may be due to the great variability of
these characters, whether or not displayed in any unusual
manner — of which fact I think there can be little doubt.
But that our rule is not confined to secondary sexual
characters is clearly shown in the case of hermaphrodite
cirripeds; I particularly attended to Mr. Waterhouse's
212
THE ORIGIN OF SPECIES
remark, while investigating this Order, and I am fully
convinced that the rule almost always holds good. I
shall, in a future work, give a list of all the more re-
markable cases; I will here give only one, as it illus-
trates the rule in its largest application. The opercular
valves of sessile cirripeds (rock barnacles) are, in every
sense of the word, very important structures, and they
differ extremely little even in distinct genera; but in the
several species of one genus, Pyrgoma, these valves pre-
sent a marvellous amount of diversification; the homol-
ogous valves in the different species being sometimes
wholly unlike in shape; and the amount of variation
in the individuals of the same species is so great, that
it is no exaggeration to state that the varieties of the
same species difTer more from each other in the charac-
ters derived from these important organs, than do the
6pecies belonging to other distinct genera.
As with birds the individuals of the same species,
inhabiting the same country, vary extremely little, I
have particularly attended to them; and the rule cer-
tainly seems to hold good in this class. I cannot make
out that it applies to plants, and this would have seri-
ously shaken my belief in its truth, had not the great
variability in plants made it particularly difficult to com-
pare their relative degrees of variability.
When we see any part or orgai^ developed in a
remarkable degree or manner in a species, the fair pre-
sumption is that it is of high importance to that species:
nevertheless it is in this case eminently liable to varia-
tion. 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
LAWS OF VARIATION 213
the view that groups of species are descended from some
other species, and have been modified through natural
selection, I think we can obtain some light. First let
me make some preliminary remarks. If, in our domestic
animals, any part or the whole animal be neglected, and
no selection be applied, that part (for instance, the comb
in the Dorking fowl) or the whole breed will cease to
have a uniform character: and the breed may be said to
be degenerating. In rudimentary organs, and in those
which have been but little specialized for any particular
purpose, and perhaps in polymorphic groups, we see a
nearly parallel case; for in such cases, natural selection
either has not or cannot have come into full play, and
thus the organization is left in a fluctuating condition.
But what here more particularly concerns us is, that
those points in our domestic animals, which at the pres-
ent time are undergoing rapid change by continued selec-
tion, are also eminently liable to variation. Look at the
individuals of the same breed of the pigeon, and see
what a prodigious amount of difference there is in the
beaks of tumblers, in the beaks and wattle of carriers,
in the carriage and tail of fantails, etc., these being the
points now mainly attended to by English fanciers.
Even in the same sub-breed, as in that of the short-
faced tumbler, it is notoriously difficult to breed nearly
perfect birds, many departing widely from the standard.
There may truly be said to be a constant struggle going
on between, on the one hand, the tendency to reversion to
a less perfect state, as well as an innate tendency to new
variations, and, on the other hand, the power of steady
selection to keep the breed true. In the long run selec-
tion gains the day, and we do not expect to fail so com-
214
THE ORIGIN OF SPECIES
pletely as to breed a bird as coarse as a common tum-
bler pigeon from a good sliort-faced strain. But as long
as selection is rapidly going on, much variability in the
parts undergoing modification may always be expected.
Now let us turn to nature. When a part has been
developed in an extraordinary manner in any one spe-
cies, compared with the other species of the same genus,
we may conclude that this part has undergone an ex-
traordinary amount of modification since the period when
the several species branched off from the common pro-
genitor of the genus. This period will seldom be remote
in any extreme degree, as species rarely endure for more
than one geological period. An extraordinary amount
of modification implies an unusually large and long-
continued amount of variability, which has continually
been accumulated by natural selection for the benefit
of the species. But as the variability of the extraor-
dinarily developed part or organ has been so great and
long- continued within a period not excessively remote,
we might, as a general rule, still expect to find more
variability in such parts than in other parts of the organ-
ization which have remained for a much longer period
nearly constant. And this, I am convinced, is the case.
That the struggle between natural selection on the one
hand, and the tendency to reversion and variability on
the other hand, will in the course of time cease; and
that the most abnormally developed organs may be made
constant, I see no reason to doubt. Hence, when an
organ, however abnormal it may be, has been trans-
mitted 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 our theory,
LAWS OF VARIATION
215
for an immense period in nearly the same state; and thus
it has come not to be more variable than any other
structure. It is only in those cases in which the modi-
fication 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 vary-
ing in the required manner and degree, and by the con-
tinued rejection of those tending to revert to a former
and less-modified condition.
Specific Characters more Variable than Generic Characters
The principle discussed under the last heading may
be applied to our present subject. It is notorious taat
specific characters are more variable than generic. To
explain by a simple example what is meant: if in a large
genus of plants some species had blue flowers and some
had red, the color would be only a specific character,
and no one would be surprised at one of the blue spe-
cies varying into red, or conversely; but if all the species
had blue flowers, the color would become a generic char-
acter, and its variation would be a more unusual circum-
stance. I have chosen this example because the explana-
tion which most naturalists would advance is not here
applicable, namely, that specific characters are more vari-
able than generic, because they are taken from parts of
less physiological importance than those commonly used
for classing genera. I believe this explanation is partly,
yet only indirectly, true; I shall, however, have to return
to this point in the chapter on Classification. It would
be almost superfluous to adduce evidence in support of
216
THE ORIGIN OF SPECIES
the statement, that ordinary specific characters are. more
variable than generic; but with respect to important char-
acters, I have repeatedly noticed, in works on natural
history, that when an author remarks with surprise that
some important organ or part, which is generally very
constant throughout a large group of species, differs con-
siderably in closely -allied species, it is often variable in
the individuals of the same 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 mtj remain the same. Something of the
came kind applies to monstrosities: at least Is. Geoi-
froy St.-Hilaire apparently entertains no doubt that the
more an organ normally differs in the different species
of the same group the more subject it is to anomalies
in the individuals.
On the ordinary view of each species having been
independently created, why should that part of the
structure, which differs from the same part in other
independently -created species of the same genus, be more
variable than those parts which are closely alike in the
several species? I do not see that any explanation can
be given. But on the view that species are only strongly
marked and fixed varieties, we migb+ expect often to find
them still continuing to vary in tnose parts of their
structure which have varied within a moderately recent
period, and which have thus come to differ. Or to state
the case in another manner: — the points in which all the
species of a genus resemble each other, and in which
they differ from allied genera, are called generic charac-
ters; and these characters may be attributed to inheri-
LAWS OF VARIATION
217
tance from a common progenitor, for it can rarely have
happened that natural selection will have modified sev-
eral distinct species, fitted to more or less widely-different
habits, in exactly the same manner: and as these so-
called generic characters have been inherited from before
the period when the several 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 char-
acters have varied and come to differ since the period
when the species branched off from a common progen-
itor, it is probable that they should still often be in some
degree variable — at least more variable than those parts
of the organization which have for a very long period
remained constant.
Secondary Sexual Characters Variable
I think it will be admitted by naturalists, without my
entering on details, that secondary sexual characters are
highly variable. It will also be admitted that species
of the same group differ from each other more widely
in their secondary sexual characters than in other parts
of their organization: compare, for instance, the amount of
difference between the males of gallinaceous birds, in
which secondary sexual characters are strongly displayed,
with the amount of difference between the females. The
cause of the original variability of these characters is not
manifest; but we can see why they should not have been
rendered as constant and uniform as others, for they are
— Science — 10
218
THE ORIGIN OF SPECIES
accumulated by sexual selection, which is less rigid in
its action than ordinary selection, as it does not entail
death, but only gives fewer offspring to the less favored
males. Whatever the cause may be of the variability of
secondary sexual characters, as they are highly variable,
sexual selection will have had a wide scope for action,
and may thus have succeeded in giving to the species of
the same group a greater amount of difference in these
than in other respects.
It is a remarkable fact that the secondary differences
between the two sexes of the same species are generally
displayed in the very same parts of the organization in
which the species of the same genus differ from each
other. Of this fact I will give in illustration the first
two instances 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 common
to very large groups of beetles, but in the Engidae, as
Westwood has remarked, the number varies greatly; and
the number likewise differs in the two sexes of the same
species. Again in the fossorial hymenoptera, the neura-
tion of the wings is a character of the highest impor-
tance, 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. Sir J.
Lubbock has recently remarked that several minute crus-
taceans offer excellent illustrations of this law. "In
Pontella, for instance, the sexual characters are afforded
mainly by the anterior antennae and by the fifth pair of
legs: the specific differences also are principally given
by these organs." This relation has a clear meaning on
LAWS OF VARIATION
219
my view: I look at all the species of the same genus
as having as certainly descended from a common pro-
genitor as have the two sexes of any one species. Con-
sequently, whatever part of the structure of the common
progenitor, or of its early descendants, became variable,
variations of this part would, it is highly probable, be
taken advantage of by natural and sexual selection, in
order to fit the several species to their several places
in the economy of nature, and likewise to fit the two
sexes of the same species to each other, or to fit the
males to struggle with other males for the possession of
the females.
Finally, then, I conclude that the greater variability
of specific characters, or those which distinguish species
from species, than of generic characters, or those which
are possessed by all the species; — that the frequent ex-
treme 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 slight degree of
variability in a part, however extraordinarily it may be
developed, if it be common to a whole group of species;
— that the great variability of secondary sexual characters,
and their great difference in closely allied species; — that
secondary sexual and ordinary specific differences are
generally displayed in the same parts of the organization
— are all principles closely connected together. All being
mainly due to the species of the same group being the
descendants of a common progenitor, from whom they
have inherited much in common — to parts which have
recently and largely varied being more likely still to go
on varying than parts which have long been inherited
220
THE ORIGIN OF SPECIES
and have not varied — to natural selection having more
or less completely, according to the lapse of time, over-
mastered the tendency to reversion and to further vari-
ability— 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 having
been thus adapted for secondary sexual, and for ordinary
purposes.
Distinct Species present analogous Variations, so that a
Variety of one Species often assumes a Character proper
to 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 the pigeon, in countries widely apart, present sub-
varieties with reversed feathers on the head, and with
feathers on the feet — characters not possessed by the
aboriginal rock-pigeon; these then are analogous varia-
tions in two or more distinct races. The frequent pres-
ence of fourteen or even sixteen tail-feathers in the pouter
may be considered as a variation representing the normal
structure of another race, the fantail. I presume that no
one will doubt that all such analogous variations are due
to the several races of the pigeon having inherited from
a common parent the same constitution and tendency to
variation, when acted on by similar unknown influences.
In the vegetable kingdom we have a case of analogous
variation, in the enlarged stems, or as commonly called
roots, of the Swedish turnip and Ruta-baga plants which
several botanists rank as varieties produced by cultiva-
tion from a common parent: if this be not so, the case
LAWS OF VARIATION
221
will then be one of analogous variation in two so-called
distinct species; and to these a third may be added,
namely, the common turnip. According to the ordinary
view of each species having been independently created,
we should have to attribute this similarity in the enlarged
stems of these three plants, not to the vera causa of com-
munity of descent, and a consequent tendency to vary in
a like manner, but to three separate yet closely related
acts of creation. Many similar cases of analogous varia-
tion have been observed by Naudin in the great gourd-
family, and by various authors in our cereals. Similar
cases occurring with insects under natural conditions have
lately been discussed with much ability by Mr. Walsh, who
has grouped them under his law of Equable Variability.
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, white loins, a
bar at the end of the tail, with the outer feathers exter-
nally edged near their basis with white. As all these
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 appearing
in the several breeds. We may, I think, confidently come
to this conclusion, because, as we have seen, these colored
marks are eminently liable to appear in the crossed off-
spring of two distinct and differently colored 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 naving been lost for many, probably
222
THE ORIGIN OF SPECIES
for hundreds, of generations. But when a breed has been
crossed only once by some other breed, the offspring occa-
sionally show for many generations a tendency to revert
in character to the foreign breed — some say, for a dozen
or even a score of generations. After twelve generations,
the proportion of blood, to use a common expression,
from one ancestor, is only 1 in 2,0±8; and yet, as we
see, it is generally believed that a tendency to reversion
is retained by this remnant of foreign blood. In a breed
which has not been crossed, but in which both parents
have lost some character which their progenitor possessed,
the tendency, whether strong or weak, to reproduce the
lost character might, as was formerly remarked, for all
that we can see to the contrary, be 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 one
individual suddenly takes after an ancestor removed by
some hundred generations, but that in each successive
generation the character in question has been lying latent,
and at last, under unknown favorable conditions, is de-
veloped. With the barb-pigeon, for instance, which very
rarely produces a blue bird, it is probable that there
is a latent tendency in each generation to produce blue
plumage. The abstract improbability of such a tendency
being transmitted through a vast number of generations
is not greater than that of quite useless or rudimentary
organs being similarly transmitted. A mere tendency to
produce a rudiment is indeed sometimes thus inherited.
As all the species of the same genus are supposed to
be descended from a common progenitor, it might be ex-
pected that they would occasionally vary in an analogous
LAWS OF VARIATION
223
manner; so that the varieties of two or more species
would resemble each other, or that a variety of one spe-
cies would resemble in certain characters another and
distinct species — this other species being, according to our
view, only a well-marked and permanent variety. But
characters exclusively due to analogous variation would
probably be of an unimportant nature, for the preserva-
tion of all functionally important characters will have
been determined through natural selection, in accordance
with the different habits of the species. It might further
be expected that the species of the same genus would
occasionally exhibit reversions to long-lost characters. As,
however, we do not know the common ancestor of any
natural group, we cannot distinguish between reversionary
and analogous characters. If, for instance, we did not
know that the parent rock-pigeon was not feather-footed
or turn-crowned, we could not have told whether such
characters in our domestic breeds were reversions or only
analogous variations; but we might have inferred that the
blue color was a case of reversion from the number of
the markings, which are correlated with this tint, and
which would not probably have all appeared together
from simple variation. More especially we might have
inferred this, from the blue color and the several marks
so often appearing when differently colored breeds are
crossed. Hence, although under nature it must generally
be left doubtful what cases are reversions to formerly
existing characters, and what are new but analogous vari-
ations, yet we ought, on our theory, sometimes to find
the varying offspring of a species assuming characters
which are already present in other members of the same
group And this undoubtedly is the case.
224
THE ORIGIN OF SPECIES
The difficulty in distinguishing variable species h
largely due to the varieties mocking, as it were, other
species of the same genus. A considerable catalogue,
also, could be given of forms intermediate between two
other forms, which themselves can only doubtfully be
ranked as species; and this shows, unless all these
closely allied forms be considered as independently cre-
ated species, that they have in varying assumed some
of the characters of the others. But the best evidence
of analogous variations is afforded by parts or organs
which are generally constant in character, but which oc-
casionally vary so as to resemble, in some degree, the
same part or organ in an allied species. I have collected
a long list of such cases; but here, as before, I lie under
the great disadvantage of not being able to give them.
I can only repeat that such cases certainly occur, and
seem to me very remarkable.
I will, however, give one curious and complex case,
not indeed as affecting any important character, but from
occurring in several species of the same genus, partly
under domestication and partly under nature. It is a
case almost certainly of reversion. The ass sometimes
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. The stripe on the
shoulder is sometimes double, and is very variable in
length and outline. A white ass, but not an albino, has
been described without either spinal or shoulder stripe:
and these stripes are sometimes very obscure, or actually
quite lost, in dark-colored asses. The koulan of Pallas
is said to have been seen with a double shoulder-stripe.
LAWS OF VARIATION
225
Mr. Blyth has seen a specimen of the hemionus with a
distinct shoulder-stripe, though it properly has none; and
I have been informed by Colonel Poole that the foals of
this species are generally striped on the legs, and faintly
on the shoulder. The quagga, though so plainly barred
like a zebra over the body, is without bars on the legs;
but Dr. Gray has figured one specimen with very distinct
zebra-like bars on the hocks.
With respect to the horse, I have collected cases in
England of the spinal stripe in horses of the most dis-
tinct breeds, and of all colors: transverse bars on the
legs are not rare in duns, mouse-duns, and in one in-
stance in a chestnut; a faint shoulder-stripe may some-
times 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 carthorse with a double stripe
on each shoulder and with leg-stripes; I have myself
seen a dun Devonshire pony, and a small dun Welsh
pony has been carefully described to me, both with three
parallel stripes on each shoulder.
In the northwest part of India the Kattywar breed
of horses is so generally striped that, as I hear from
Colonel Poole, who examined this breed for the Indian
Government, a horse without stripes is not considered as
purely-bred. The spine is always striped; the legs are
generally barred; and the shoulder-stripe, which is some-
times double and sometimes treble, is common; the side
of the face, moreover, is sometimes striped, The stripes
are often plainest in the foal; and sometimes quite disap-
pear 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
226
THE ORIGIN OF SPECIES
by Mr. W. W. Edwards, that with the English racehorse
the spinal stripe is much commoner in the foal than m
the full-grown animal. I have myself recently bred a
foal from a bay mare (offspring of a Turkoman horse
and a Flemish mare) by a bay English racehorse; this
foal when a week old was marked on its hinder quarters
and on its forehead with numerous, very narrow, dark,
zebra-like bars, and its legs were feebly striped: all the
stripes soon disappeared completely. Without here enter-
ing on further details, I may state that I have collected
cases of leg and shoulder stripes in horses of very differ-
ent breeds in various countries from Britain to Eastern
China; and from Norway in the north to the Malay
Archipelago in the south. In all parts of the world
these stripes occur far oftenest in duns and mouse- duns;
by the term dun a large range of color is included, from
one between brown and black to a close approach to
cream- col or.
I am aware that Colonel Hamilton Smith, who has
written on this subject, believes that the several breeds
of the horse are descended from several aboriginal spe-
cies— one of which, the dun, was striped; and that the
above- described appearances are all due to ancient crosses
with the dun stock. But this view may be safely re-
jected; for it is highly improbable that the heavy Bel-
gian carthorse, Welsh ponies, Norwegian cobs, the lanky
Kattywar race, etc., inhabiting the most distant parts of
the world, should all have been crossed with one sup-
posed aboriginal stock.
Now let us turn to the effects of crossing the several
species of the horse-genus. Eollin asserts that the com-
mon mule from the ass and horse is particularly apt to
LAWS OF VARIATION'
227
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 striped that any one might have thought that it
was a hybrid-zebra; and Mr. W. C. Martin, in his excel-
lent treatise on the horse, has given a figure of a similar
mule. In four colored drawings, which I have seen, of
hybrids between the ass and zebra, the legs were much
more plainly barred than the rest of the body; and in
one of them there was a double shoulder-stripe. In Lord
Morton's famous hybrid, from a chestnut mare and male
quagga, the hybrid, and even the pure offspring subse-
quently produced from the same mare by a black Ara-
bian sire, were much more plainly barred across the legs
than is even the pure quagga. Lastly, and this is an-
other 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 only occasionally has stripes on
his 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 Devonshire and Welsh ponies, 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
jtripe of color appears from what is commonly called
chance, that I was led solely from the occurrence of the
race-stripes on this hybrid from the ass and hemionus to
ask Colonel Poole whether such face-stripes ever occurred
in the eminently striped Kattywar breed of horses, and
was, as we have seen, answered in the affirmative.
What now are we to say to these several facts? We
THE ORIGL\ VF SPECIES
see several distinct species of the horse-genus becoming,
bj 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 coloring
of the other species of the genus. The appearance of
the stripes is not accompanied by any change of form
or by any other new character. We see this tendency to
become striped most strongly displayed in hybrids from
between several of the most distinct species. Now ob-
serve the case of the several breeds of pigeons: they are
descended from a pigeon (including two or three sub-
species or geographical races) of a bluish color, with
certain bars and other marks; and when any breed as-
sumes by simple variation a bluish tint, these bars and
other marks invariably reappear; but without any other
change of form or character. When the oldest and tru-
est breeds of various colors are crossed, we see a strong
tendency for the blue tint and bars and marks to re-
appear in the mongrels. I have stated that the most
probable hypothesis to account for the reappearance of
very ancient characters, is — that there is a tendency in
the young of each successive generation to produce the
long-lost character, and that this tendency, from unknown
causes, sometimes prevails. And we have just seen that
in several species of the horse-genus the stripes are either
plainer or appear more commonly in the young than in
the old. Call the breeds of pigeons, some of which have
bred true for centuries, species; and how exactly parallel
is the case with that jf the species of the horse-genus I
For myself, I venture confidently to look back thousands
on thousands of generations, and I see an animal striped
LAWS OF VARIATION
229
like a zebra, but perhaps otherwise very differently
constructed, the common parent of our domestic horse
(whether or not it be descended from one or more wild
stocks), of the ass, the hernionus, quagga, and zebra.
He who believes that each equine species was in-
dependently created, will, I presume, assert that each
species has been created with a tendency to vary, both
under nature and under domestication, in this particular
manner, so as often to become striped like the other spe-
cies of the genus; and that each has been created with
a strong tendency, when crossed with species inhabiting
distant quarters of the world, to produce hybrids resem-
bling in their stripes, not their own parents, but other
species of the genus. To admit this view is, as it seems
to me, to reject a real for an unreal, or at least for an
unknown, cause. It makes the works of God a mere
mockery and deception; I would almost as soon believe
with the old and ignorant cosmogonists, that fossil shells
had never lived, but had been created in stone so as to
mock the shells living on the sea-shore.
Summary
Our ignorance of the laws of variation is profound.
Not in one case out of a hundred can we pretend to
assign any reason why this or that part has varied. But
whenever we have the means of instituting a comparison,
the same laws appear to have acted in producing the
lesser differences between varieties of the same species,
and the greater differences between species of the same
genus. Changed conditions generally induce mere fluc-
tuating variability, but sometimes they cause direct and
definite effects; and these may become strongly marked
230
THE ORIGIN OF SPECIES
in the course of time, though we have not sufficient evi-
dence on this head. Habit in producing constitutional
peculiarities and use in strengthening and disuse in weak-
ening and diminishing organs, appear in many cases to
have been potent in their effects. Homologous parts tend
to vary in the same manner, 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 will
be saved. Changes of structure at an early age may
affect parts subsequently developed; and many cases of
correlated variation, the nature of which we are unable
to understand, undoubtedly occur. Multiple parts are
variable in number and in structure, perhaps arising
from such parts not having been closely specialized
for any particular function, so that their modifications
have not been closely checked by natural selection. It
follows, probably from this same cause, that organic be-
ings low in the scale are more variable than those stand-
ing higher in the scale, and which have their whole or-
ganization more specialized. Rudimentary organs, from
being useless, are not regulated by natural selection, and
hence are variable. Specific characters — that is, the char-
acters whicn 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 part3 or organs being still variable, because they
have recently varied and thus come to differ; but we
LAWS OF VARIATION
231
have also seen in the second chapter that the same prin-
ciple applies to the whole individual; for in a district
where many species of a genus are found — that is, where
there has been much former variation and differentiation,
or where the manufactory of new specific forms has been
actively at work — an that district and among these spe-
cies we now find, on an, average, most varieties. Sec-
ondary sexual characters are highly variable, and such
characters differ much in the species of the same group.
Variability in the same parts of the organization has
generally been taken advantage of in giving secondary
sexual differences to the two sexes of the same species,
and specific differences to the several species of the same
genus. Any part or organ developed to an extraordinary
size or in an extraordinary manner, in comparison with
the same part or organ in the allied species, must have
gone through an extraordinary amount of modification
since the genus arose; and thus we can understand why
it should often still be variable in a much higher degree
than other parts; for variation is a long-continued and
slow process, and natural selection will in such cases not
as yet have had time to overcome the tendency to further
variability and to reversion to a less modified state. But
when a species with any extraordinarily-developed organ
has become the parent of many modified descendants —
which on our view must be a very slow process, requir-
ing a long lapse of time — in this case, natural selection
has succeeded in giving a fixed character to the organ,
in however extraordinary a manner it may have been
developed. Species inheriting nearly the same constitu-
tion from a common parent, and exposed to similar in-
fluences, naturally tend to present analogous variations,
232
THE ORIGIN OF SPECIES
or these same species may occasionally revert to some
of the characters of their ancient progenitors. Although
new and important modifications may not arise from re-
version and analogous variation, such modifications will
add to the beautrful and harmonious diversity of nature.
Whatever the cause may be of each slight difference
between the offspring and their parents — and a cause for
each must exist — we have reason to believe that it is the
steady accumulation of beneficial differences which has
given rise to all the more important modifications of
structure in relation to the habits of each species.
DIFFICULTIES OF THE THEORY 233
CHAPTEE VI
DIFFICULTIES OF THE THEORY
Difficulties of the theory of descent with modification — 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 — Modes 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 the reader has arrived at this part
of my work, a crowd of difficulties will have
occurred to him. Some of them are so serious
that to this day I can hardly reflect on them without
being in some degree 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 the theory.
These difficulties and objections may be classed under
the following heads:
First, why, if species have descended from other spe-
cies by fine gradations, do we not everywhere see innu-
merable 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 other animal
with widely different habits and structure ? Can we be-
234 THE ORIGIN OF SPECIES
lieve that natural selection could produce, on the one
hand, an organ of trifling importance, such as the tail of
a giraffe, which serves as a fly-flapper, and, on the other
hand, an organ so wonderful as the eye ?
Thirdly, can instincts be acquired and modified
through natural selection ? What shall we say to the
instinct which leads the bee to make cells, and which
has practically anticipated the discoveries of profound
mathematicians ?
Fourthly, how can we account for species, when
crossed, being sterile and producing sterile offspring,
whereas, when varieties are crossed, their fertility is
unimpaired ?
The first two heads will here be discussed; some mis-
cellaneous objections in the following chapter; Instinct
and Hybridism in the two succeeding chapters.
On the Absence or Rarity of Transitional Varieties
As natural selection acts solely by the preservation of
profitable modifications, each new form will tend in a
fully-stocked country to take the place of, and finally to
exterminate, its own less improved parent-form and other
less-favored forms with which it comes into competition.
Thus extinction and natural selection go hand in hand.
Hence, if we look at each species as descended from some
unknown form, both the parent and all the transitional
varieties will generally have been exterminated by the
very process of the formation and perfection of the new
form.
But, as by this theory innumerable transitional forms
must have existed, why do we not find them imbedded
in countless numbers in the crust of the earth ? It will
DIFFICULTIES OF THE THEORY
235
be more convenient to discuss this question in the chapter
on the Imperfection of the Geological Record; and I will
here only state that I believe the answer mainly lies in
the record being incomparably less perfect than is gener-
ally supposed. The crust of the earth is a vast museum;
but the natural collections have been imperfectly made,
and only at long intervals of time.
But it may be urged that when several closely-allied
species inhabit the same territory, we surely ought to
find at the present time many transitional forms. Let us
take a simple case: in travelling from north to south over
a continent, we generally meet at successive intervals with
closely allied or representative species, evidently filling
nearly the same place in the natural economy of the land.
These representative species often meet and interlock; and
as the one becomes rarer and rarer, the other becomes
more and more frequent, till the one replaces the other.
But if we compare these species where they intermingle,
they are generally as absolutely distinct from each other
in every detail of structure as are specimens taken from
the metropolis inhabited by each. By my theory these
allied species are 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-form
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 imbedded there in a fossil condition. But in
the intermediate region, having intermediate conditions
of life, why do we not now find closely-linking interme-
236
THE ORIGIN OF SPECIES
diate varieties? This difficulty for a long time quite
confounded me. But I think it can be in large part
explained.
In the first place we should be extremely cautious in
inferring, because an area is now continuous, that it has
been continuous during a long period. Geology would
lead us to believe that most continents have been broken
up into islands even during the later tertiary periods;
and in such islands distinct species might have been
separately formed without the possibility of intermediate
varieties existing in the intermediate zones. By changes
in the form of the land and of climate, marine areas now
continuous must often have existed within recent times
in a far less continuous and uniform condition than at
present. But I will pass over this way of escaping from
the difficulty; for I believe that many perfectly defined
species have been formed on strictly continuous areas;
though I do not doubt that the formerly broken condi-
tion of areas now continuous ha3 played an important
part in the formation of new species, more especially
with freely-crossing and wandering animals.
In looking at species as they are now distributed over
a wide area, we generally find them tolerably numerous
over a large territory, then becoming somewhat abruptly
rarer and rarer on the confines, and finally disappearing.
Hence the neutral territory between two representative
species is generally narrow in comparison with the 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 E. Forbes in sounding the depths of the sea with the
DIFFICULTIES OF THE THEORY
237
dredge. To those who look at climate and the physical
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
metropolis, would increase immensely in numbers, were
it not for other competing species; that nearly all either
prey on or serve as prey for others; in short, that each
organic being is either directly or indirectly related in
the most important manner to other organic beings — we
see that the range of the inhabitants of any country by
no means exclusively depends on insensibly changing
physical conditions, but in a large part on the presence
of other species, on which it lives, or by which it is
destroyed, or with which it comes into competition; and
as these species are already defined objects, not blending
one into another by insensible gradations, the range of
any one species, depending as it does on the range of
others, will tend to be sharply defined. Moreover, each
species on the confines of its range, where it exists in
lessened numbers, will, during fluctuations in the number
of its enemies or of its prey, or in the nature of the
seasons, be extremely liable to utter extermination; and
thus its geographical range will come to be still more
sharply defined.
As allied or representative species, when inhabiting a
continuous area, are generally distributed in such a man-
ner that each has a wide range, with a comparatively
narrow neutral territory between them, in which they
become rather suddenly rarer and rarer; then, as varieties
do not essent; dly differ from species, the same rule will
probably apply to both; and if we take a varying species
238
THE ORIGIN OF SPECIES
inhabiting a very large area, we shall have to adapt two
varieties to two large areas, and a third variety to a nar-
row intermediate zone. The intermediate variety, conse-
quently, 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 s;enus Balanus. And it would
appear from information given me by Mr. Watson, Dr.
Asa Gray, and Mr. Wollaston, that generally, when vari-
eties intermediate between two other forms occur, they
are much rarer numerically than the forms which they
connect. Now, if we may trust these facts and infer-
ences, and conclude that varieties linking two other vari-
eties together generally have existed in lesser numbers
than the forms which they connect, then we can under-
stand 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 extermi-
nated than one existing in large numbers; and in this
particular case the intermediate form would be eminently
liable to the inroads of closely-allied forms existing on
both sides of it. But it is a far more important consider-
ation that during the process of further modification, by
which two varieties are supposed to be converted and
perfected into two distinct species, the two which exist
in larger numbers, from inhabiting larger areas, will have
a great advantage over the intermediate variety, which
DIFFICULTIES OF THE THEORY
239
exists in smaller numbers in a narrow and intermediate
zone. For forms existing in larger numbers will have
a better chance, within any given period, of presenting
further favorable variations for natural selection to seize
on, than will the rarer forms which exist in lesser num-
bers. 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 im-
proved. It is the same principle which, as I believe,
accounts for the common species in each country, as
shown in the second chapter, presenting on an average
a greater number of well-marked varieties than do the
rarer species. I may illustrate what I mean by supposing
three varieties of sheep to be kept, one adapted to an
extensive mountainous region; a second to a compara-
tively narrow, hilly tract; and a third to the 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
favor of the great holders on the mountains or on the
- plains improving their breeds more quickly than the
small holders on the intermediate narrow, hilly tract;
and consequently the improved mountain or plain breed
will soon take the place of the less improved hill
breed; and thus the two breeds, which originally existed
in greater numbers, will come into close contact with
each other, without the interposition of the supplanted,
intermediate hill variety.
To sum up, I believe that species come to be tolerably
well-defined objects, and do not at any one period present
an inextricable chaos of varying and intermediate links;
first, because new varieties are very slowly formed, for
240
THE ORIGIN OF SPECIES
variation is a slow process, and natural selection can do
nothing until favorable individual differences or variations
occur, and until a place in the natural polity of the coun-
try can be better rilled by some modification of some one
or more of its inhabitants. And such new places will
depend on slow changes of climate, or on the occasional
immigration of new inhabitants, and, probably, in a still
more important degree, on some of the old inhabitants
becoming slowly modified, with the new forms thus pro-
duced and the old ones acting and reacting on each
other. So that, in any one region and at any one time,
we ought to see only a few species presenting slight
modifications of structure in some degree permanent; and
this assuredly we do see.
Secondly, areas now continuous must often have ex-
isted within the recent period as isolated portions, in
which many forms, more especially among 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 within each
isolated portion of the land, but these links during the
process of natural selection will have been supplanted
and exterminated, so that they will no longer be found
in a living state.
Thirdly, when two or more varieties have been formed
in different 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 gen-
erally have had a short duration. For these intermedi-
ate varieties will, from reasons already assigned (namely
DIFFICULTIES OF THE THEORY
241
from what we know of the actual distribution of closely
allied or representative species, and likewise of acknowl-
edged varieties), exist in the intermediate zones in lesser
numbers than the varieties which they tend to connect.
From this cause alone the intermediate varieties will be
liable to accidental extermination; and. during the process
of further modification through natural selection, they will
almost certainly be beaten and supplanted by the forms
which they connect; for these from existing in greater
numbers will, in the aggregate, present more varieties,
and thus be further improved through natural selection
and gain further advantages.
Lastly, looking not to any one time, but to all time,
if my theory be true, numberless intermediate varieties,
linking closely together all the species of the same group,
must assuredly have existed; but the very process of nat-
ural selection constantly tends, as has been so often re-
marked, to exterminate the parent-forms and the interme-
diate links. Consequently evidence of their former existence
could be found only among fossil remains, which are pre-
served, as we shall attempt to show in a future chapter,
in an extremely imperfect and intermittent recordc
On the Origin and Transitions of Organic Beings with
peculiar Habits and Structure
It has been as£ed by the opponents of such views as
I hold, how, for instance, could a land carnivorous animal
have been converted into one with aquatic habits; for
how could the animal in its transitional state have sub-
sisted ? It would be easy to show that there now exist
carnivorous animals presenting close intermediate grades
from strictly terrestrial to aquatic habits; and as each
-Science — 11
242
THE ORIGIN OF SPECIES
exists by a struggle for life, it is clear that each must be
well adapted to its place in nature. Look at the Mustela
vison of North America, which has webbed feet, and
which resembles an otter in its fur, short legs, and form
of tail. During the summer this animal dives for and
preys on fish, but during the long winter it leaves the
frozen waters, and preys, like other polecats, on mice and
land animals. If a different case had been taken, and it
had been asked how an insectivorous quadruped could
possibly have been converted into a flying bat, the ques-
tion would have been far more difficult to answer Yet
I think such difficulties have little weight.
Here, as on other occasions, I lie under a heavy dis-
advantage, for, out of the many striking cases which I
have collected, I can give only one or two instances of
transitional habits and structures in allied species; and
of diversified habits, either constant or occasional, in the
same species. And it seems to me that nothing less than
a long list of such cases is sufficient to lessen the diffi-
culty in any particular case like that of the bat.
Look at the family of squirrels; here we have the
finest gradation from animals with their tails only slightly
flattened, and from others, as Sir J. Richardson has 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
expanse of skin, which serves as a parachute and allows
them to glide through the air to an astonishing distance
from tree to tree. We cannot doubt that each structure
is of use to each kind of squirrel in its own country, by
enabling it to escape birds or beasts of prey, to collect
DIFFICULTIES OF THE THEORY
243
food more quickly, or, as there is reason to believe, to
lessen 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 possible
conditions. Let the climate and vegetation change, let
other competing rodents or new beasts of prey immigrate,
or old ones become modified, and all analogy would lead
us to believe that some at least of the squirrels would
decrease in numbers or become exterminated, unless they
also became modified and improved in structure in a cor-
responding manner. Therefore, I can see no difficulty,
more especially under changing conditions of life, in the
continued preservation of individuals with fuller and
fuller flank-membranes, each modification being useful,
each being propagated, until, by the accumulated effects
of this process of natural selection, a perfect so-called
flying squirrel was produced.
Now look at the Galeopithecus or so-called flying
lemur, which formerly was ranked among bats, but is
now believed to belong to the Insectivora. An extremely
wide flank-membrane stretches from the corners of the
jaw to the tail, and includes the limbs with the elongated
fingers. This flank-membrane is furnished with an ex-
tensor muscle. Although no graduated links of structure,
fitted for gliding through the air, now connect the Galeo-
pithecus with the other Insectivora, yet there is no diffi-
culty in supposing that such links formerly existed, and
that each was developed in the same manner as with the
less perfectly gliding squirrels; each grade of structure
having been useful to its possessor. Nor can I see any
insuperable difficulty in further believing that the mem-
brane-connected fingers and forearm of the Galeopithecus
2U
THE ORIGIX OF SPECIES
might have been greatly lengthened by natural selection;
and this, as far as the organs of flight are concerned,
would have converted the animal into a bat. In certain
bats in which the wing-membrane extends from the top
of the shoulder to the tail and includes the hind-legs, we
perhaps see traces of an apparatus originally fitted for
gliding through the air rather than for flight.
If about a dozen genera of birds were to become
extinct, who would have ventured to surmise that birds
might have existed which used their wings solely as
flappers, like the logger-headed duck (Micropterus of
Eyton); as fins in the water and as 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 be
the result of disuse, indicate the steps by which birds
actually acquired their perfect power of flight; but they
serve to show what diversified means of transition are
at least possible.
Seeing that a few members of such water-breathing
classes as the Crustacea and Mollusca are adapted to live
on the land; and seeing that we have flying birds and
mammals, flying insects of the most diversified types,
and formerly had flying reptiles, it is conceivable that
flying-fish, which now glide far through the air, slightly
rising and turning by the aid of their flattering fins, might
have been modified into perfectly winged animals. If
DIFFICULTIES OF THE THEORY
245
this had been effected, who would have ever imagined
that in an early transitional state they had been the in-
habitants of the open ocean, and had used their incipient
organs of flight exclusively, as far as we know, to escape
being devoured by other fish?
When we see any structure highly perfected for any
particular habit, as the wings of a bird for flight, we
should bear in mind that animals displaying early transi-
tional grades of the structure will seldom have survived
to the present day, for they will have been supplanted
by their successors, which were gradually rendered more
perfect through natural selection. Furthermore, we may
conclude that transitional states between structures fitted
for very different habits of life will rarely have been
developed at an early period in great numbers and under
many subordinate forms. Thus, to return to our imagi-
nary illustration of the flying-fish, it does not seem prob-
able that fishes capable of true flight would have been
developed under many subordinate forms, for taking prey
of many kinds in many ways, on the land and in the
water, until their organs of flight had come to a high
stage of perfection, so as to have given them a decided
advantage over other animals in the battle for life.
Hence the chance of discovering species with transi-
tional grades of structure in a fossil condition will al-
ways be less, from their having existed in lesser num-
bers, than in the case of species with fully developed
structures.
I will now give two or three instances both of diver-
sified and of changed habits in the individuals of the
same species. In either case it would be easy for natu-
ral selection to adapt the structure of the animal to its
246
THE ORIGIN OP SPECIES
changed habits, or exclusively to one of its several hab-
its. It is, however, difficult to decide, and immaterial
for us, whether habits generally change first and struc-
ture afterward; or whether slight modifications of struc-
ture lead to changed habits; both probably often occur-
ring almost simultaneously. Of cases of changed habits
it will suffice merely to allude to that of the many Brit-
ish insects which now feed on exotic plants, or ex-
clusively on artificial substances. Of diversified habits
innumerable instances could be given: I have often
watched a tyrant flycatcher (Saurophagus sulphuratus)
in South America, hovering over one spot and then
proceeding to another, like a kestrel, and at other times
standing stationary on the margin of water, and then
dashing into it like a kingfisher at a fish. In our own
country the larger titmouse (Parus major) may be seen
climbing branches, almost like a creeper; it sometimes,
like a shrike, kills small birds by blows on the head;
and I have many times seen and heard it hammering the
seeds of the yew on a branch, and thus breaking them
like a nuthatch. In North America the black bear was
seen by Hearne swimming for hours with widely open
mouth, thus catching, almost like a whale, insects in the
water.
As we sometimes see individuals following habits dif-
ferent from those proper to their species and to the other
species of the same genus, we might expect that such
individuals would occasionally give rise to new species,
having anomalous habits, and with their structure either
slightly or considerably modified from that of their type.
And such instances occur in nature. Can a more strik-
ing instance of adaptation be given than that of a wood-
DIFFICULTIES OF THE THEORY 247
pecker for climbing trees and seizing insects in the
chinks of the bark ? Yet in North America there are
woodpeckers which feed largely on fruit, and others with
elongated wings which chase insects on the wing. On
the plains of La Plata, where hardly a tree grows, there
is a woodpecker (Colaptes campestris) which has two toes
before and two behind, a long pointed tongue, pointed
tail-feathers, sufficiently stiff to support the bird in a
vertical position on a post, but not so stiff as in the
typical woodpeckers, and a straight strong beak. The
beak, however, is not so straight or so strong as in
the typical woodpeckers, but it is strong enough to bore
into wood. Hence this Colaptes in all the essential parts
of its structure is a woodpecker. Even in such trifling
characters as the coloring, the harsh tone of the voice,
and undulatory flight, its close blood-relationship to our
common woodpecker is plainly declared; yet, as I can
assert, not only from my own observations, but from
those of the accurate Azara, in certain large districts it
does not climb trees, and it makes its nest in holes
in banks! In certain other districts, however, this same
woodpecker, as Mr. Hudson states, frequents trees, and
bores holes in the trunk for its nest. I may mention as
another illustration of the varied habits of this genus,
that a Mexican Colaptes has been described by De Saus-
sure as boring holes into hard wood in order to lay up
a store of acorns.
Petrels are the most aerial and oceanic of birds, but
in the quiet sounds of Tierra del Fuego, the Puffinuria
berardi, in its general habits, in its astonishing power of
diving, in its manner of swimming and of flying when
made to take flight, would be mistaken by any one for
248
THE ORIGIN OF SPECIES
an auk or a grebe: nevertheless it is essentially a petrel,
but with many parts of its organization profoundly mod-
ified in relation to its new habits of life; whereas the
woodpecker of La Plata has had its structure only
slightly modified. In the case of the water-ouzel, the
acutest observer by examining its dead body would never
have suspected its sub-aquatic habits; yet this bird, which
is allied to the thrush family, subsists by diving — using
its wings under water, and grasping stones with its feet.
All the members of the great order of Hymenopterous
insects are terrestrial, excepting the genus Proctotrupes,
which Sir John Lubbock has discovered to be aquatic in
its habits; it often enters the water and dives about by
the use not of its legs buL of its wings, and remains
as long as four hours beneath the surface; yet it ex-
hibits no modification in structure in accordance with
its abnormal habits.
He who believes that each being has been created as
we now see it, must occasionally have felt surprise when
he has met with an animal having habits and structure
not 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 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 ocean.
On the other hand, grebes and coots are eminently
aquatic, although their toes are only bordered by mem-
brane. What seems plainer than that the long toes, not
furnished with membrane, of the Grallatores are formed
for walking over swamps and floating plants? — the water-
hen and landrail are members of this order, yet the first
DIFFICULTIES OF THE THEORY
249
is nearly as aquatic as the coot, and the second nearly
as terrestrial as the quail or partridge. In such cases,
and many others could be given, habits have changed
without a corresponding change of structure. The
webbed feet of the upland goose may be said to
have become almost 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 may say, that in these cases it has pleased the
Creator to cause a being of one type to take the place
of one belonging to another type; but this seems to me
only restating the fact in dignified language. He who
believes in the struggle for existence and in the principle
of natural selection will acknowledge that every organic
being is constantly endeavoring to increase in numbers;
and that if any one being varies ever so little, either
in habits or structure, and thus gains an advantage over
some other inhabitant of the same country, it will seize
on the place of that inhabitant, however different that
may be from its own place. Hence it will cause him no
surprise that there should be geese and frigate-birds with
webbed feet, living on the dry land and 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 hardly a tree grows; that
there should be diving thrushes and diving Hymenoptera,
and petrels with the habits of auks.
250
THE ORIGIN OF SPECIES
Organs of extreme Perfection and Complication
To suppose that the eye with all its inimitable con-
trivances for adjusting the focus to .different distances,
for admitting different amounts of light, and for the
correction of spherical and chromatic aberration, could
have been formed by natural selection, seems, I freely
confess, absurd in the highest degree. When it was first
said that the sun stood still and the world turned round,
the common-sense of mankind declared the doctrine false;
but the old saying of Vox populi vox Dei, as every phi-
losopher knows, cannot be trusted in science. Eeason
tells me, that if numerous gradations from a simple and
imperfect eye to one complex and perfect can be shown
to exist, each grade being useful to its possessor, as is
certainly the case; if, further, the eye ever varies and the
variations be inherited, as is likewise certainly the case;
and if such variations should be useful to any animal
under changing conditions of life, then the difficulty of
believing that a perfect and complex eye could be formed
by natural selection, though insuperable by our imagina-
tion, should not be considered as subversive of the the-
ory. How a nerve comes to be sensitive to light, hardly
concerns us more than how life itself originated; but I
may remark that, as some of the lowest organisms, in
which nerves cannot be detected, are capable of perceiv-
ing light, it does not seem impossible that certain sensi-
tive elements in their sarcode should become aggregated
and developed into nerves, endowed with this special
sensibility.
In searching for the gradations through which an
organ in any species has been perfected, we ought
DIFFICULTIES OF THE THEORY
251
to look exclusively to its lineal progenitors; but this
is scarcely ever possible, and we are forced to look to
other species and genera of the same group, that is to
the collateral descendants from the same parent-form,
in order to see what gradations are possible, and for
the chance of some gradations having been transmitted
in an unaltered or little altered condition. But the state
of the same organ in distinct classes may incidentally
throw light on the steps by which it has been per-
fected.
The simplest organ which can be called an eye con-
sists of an optic nerve, surrounded by pigment-cells and
covered by translucent skin, but without any lens or
other refractive body. We may, however, according to
M. Jourdain, descend even a step lower and find aggre-
gates of pigment-cells, apparently serving as organs of vis-
ion, without any nerves, and resting merely on sarcodic
tissue. Eyes of the above simple nature are not capable
of distinct vision, and serve only to distinguish light
from darkness. In certain starfishes, small depressions
in the layer of pigment which surrounds the nerve are
filled, as described by the author just quoted, with trans-
parent gelatinous matter, projecting with a convex sur-
face, like the cornea in the higher animals. He suggests
that this serves not to form an image, but only to con-
centrate the luminous rays and render their perception
more easy. In this concentration of the rays we gain
the first and by far the most important step toward the
formation of a true, picture-forming eye; for we have
only to place the naked extremity of the optic nerve,
which in some of the lower animals lies deeply buried
in the body, and in some near the surface, at the right
252
THE 0RIG1X OF SPECIES
distance from the concentrating apparatus, and an image
will be formed on it.
In the great class of the Articulata, we may start
from an optic nerve simply coated with pigment, the
latter sometimes forming a sort of pupil, but destitute
of a lens or other optical contrivance. With insects it
is now known that the numerous facets on the cornea of
their great compound eves form true lenses, and that the
cones include curiously modified nervous filaments. But
these organs in the Articulata are so much diversified
that Miiller formerly made three main classes with seven
subdivisions, besides a fourth main class of aggregated
simple eyes.
When we reflect on these facts, here given much too
briefly, with respect to the wide, diversified, and gradu-
ated range of structure in the eyes of the lower animals;
and when we bear iu mind how small the number of all
living forms must be in comparison with those which
have become extinct, the difficulty ceases to be very
great in believing that natural selection may have con-
verted the simple apparatus of an optic nerve, coated
with pigment and invested by transparent membrane, into
an optical instrument as perfect as is possessed by any
member of the Articulate Class.
He who will go thus far, ought not to hesitate to go
one step further, if he finds on finishing this volume that
large bodies of facts, otherwise inexplicable, can be ex-
plained bv the theory of modification through natural
selection; he ought to admit that a structure even as
perfect as an eagle's eye might thus be formed, although
in this case he does not know the transitional states. It
has been objected that, in order to modify the eye and
DIFFICULTIES OF THE THEORY
253
still preserve it as a perfect instrument, many changes
would have to be effected simultaneously, which, it is
assumed, could not be done through natural selection;
but as I have attempted to show in my work on the
variation of domestic animals, it is not necessary to
suppose that the modifications were all simultaneous,
if they were extremely slight and gradual. Different
kinds of modification would, also, serve for the same
general purpose: as Mr. Wallace has remarked, 11 if a
lens has too short or too long a focus, it may be
amended either by an alteration of curvature or an al-
teration of density; if the curvature be irregular, and
the rays do not converge to a point, then any increased
regularity of curvature will be an improvement. So the
contraction of the iris and the muscular movements of
the eye are neither of them essential to vision, but only
improvements which might have been added and per-
fected at any stage of the construction of the instru-
ment." Within the highest division of the animal king-
dom, namely, the Vertebrata, we can start from an eye
so simple that it consists, as in the lancelet, of a little
sack of transparent skin, furnished with a nerve and
lined with pigment, but destitute of any other apparatus.
In fishes and reptiles, as Owen has remarked, "the range
of gradations of dioptric structures is very great." It is
a significant fact that even in man, according to the high
authority of Virchow, the beautiful crystalline lens is
formed in the embryo by an accumulation of epidermic
cells, lying in a sack-like fold of the skin; and the vit-
reous body is formed from embryonic sub-cutaneous tis-
sue. To arrive, however, at a just conclusion regarding
the formation of the eye, with all its marvellous yet not
2o4
TEE ORIGIX OF SPECIES
absolutely perfect characters, it is indispensable that the
reason should conquer the imagination; but I have felt
the difficulty far too keenly to be surprised at others
hesitating to extend the principle of natural selection
to so startling a length.
It is scarcely possible to avoid comparing the eye
with a telescope. We know that this instrument has
been perfected by the long-continued efforts of the high-
est human intellects: and we natarally infer that the eye
has been formed by a somewhat analogous process. But
may not this inference be presumptuous ? Have we any
right to assume that the Creator works by intellectual
powers like those of man ? If we must compare the eye
to an optical instrument, we ought in imagination to take
a thick layer of transparent tissue, with spaces filled with
fluid, and with a nerve sensitive to light beneath, and
then suppose every part of this layer to be continually
changing slowly in density, so as to separate into layers
of different densities and thicknesses, placed at different
distances from each other, and with the surfaces of each
layer slowly changing in form. Further we must suppose
that there is a power, represented by natural selection or
the survival of the fittest, always intently watching each
slight alteration in the transparent layers: and carefully
preserving each which, under varied circumstances, in
any way or in any degree, tends to produce a dis-
tincter image. We must suppose each new state of the
instrument to be multiplied by the million; each to be
preserved until a better one is produced, and then the
old ones to be all destroyed. In living bodies, variation
will cause the slight alterations, generation will multiply
them almost infinitely, and natural selection will pick out
DIFFICULTIES OF THE THEORY
255
with unerring skill each improvement. Let this process
go on for millions of years; and during each year on
millions of individuals of many kinds; and may we not
believe that a living optical instrument might thus be
formed as superior to one of glass as the works of the
Creator are to those of man ?
Modes of Transition
If it could be demonstrated that any complex organ
existed, which could not possibly have been formed by
numerous, successive, slight modifications, my theory
would absolutely break down. But I can find out no
such case. No doubt many organs exist of which we
do not know the transitional grades, more especially if
we look to much-isolated species, round which, accord-
ing to the theory, there has been much extinction. Or
again, if we take an organ common to all the members
of a class, for in this latter case the organ must have
been originally formed at a remote period, since which
all the many members of the class have been developed;
and in order to discover the early transitional grades
through which the organ has passed, we should have to
look to very ancient ancestral forms, long since become
extinct.
We should be extremely cautious in concluding that
an organ could not have been formed by transitional gra-
dations of some kind. Numerous cases could be given
among the lower animals of the same organ performing
at the same time wholly distinct functions; thus in the
larva of the dragon-fly and in the fish Cobites the ali-
mentary canal respires, digests, and excretes. In the
Hydra, the animal may be turned inside out, and the
256
THE ORIGIN OF SPECIES
exterior surface will then digest and the stomach respire.
In such cases natural selection might specialize, if any
advantage were thus gained, the whole or part of an
organ which had previously performed two functions for
one function alone, and thus by insensible steps greatly
change its nature. Many plants are known which regu-
larly produce at the same time differently constructed
flowers; and if such plants were to produce one kind
alone, a great change would be effected with compara-
tive suddenness in the character of the species. It is,
however, probable that the two sorts of flowers borne
by the same plant were originally differentiated by finely
graduated steps, which may still be followed in some
few cases.
Again, two distinct organs, or the same organ under
two very different forms, may simultaneously perform
in the same individual the same function, and this is
an extremely important means of transition: to give one
instance — there are fish with gills or branchiae that
breathe the air dissolved in the water at the same
time that they breathe free air in their swimbladders,
this latter organ being divided by highly vascular parti-
tions and having a ductus pneumaticus for the supply of
air. To give another instance from the vegetable king-
dom: plants climb by three distinct means, by spirally
twining, by clasping a support with their sensitive ten-
drils, and by the emission of aerial rootlets; these three
means are usually found in distinct groups, but some few
species exhibit two of the means, or even all three, com-
bined in the same individual. In all such cases one of
the two organs might readily be modified and perfected
so as to perform all the work, being aided during the
DIFFICULTIES OF THE THEORY
257
progress of modification by the other organ; and then
this other organ might be modified for some other and
quite distinct purpose, or be wholly 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 widely
different purpose, namely, respiration. The swimbladder
has, also, been worked in as an accessory to the auditory
organs of certain fishes. Ali physiologists admit that the
swimbladder is homologous, or ''ideally similar" in posi-
tion and structure with the lungs of the higher verte-
brate animals: hence there is no reason to doubt that
the swimbladder has actually been converted into lungs,
or an organ used exclusively for respiration.
According to this view it may be inferred that all
vertebrate animals with true lungs are descended by
ordinary generation from an ancient and unknown pro-
totype, which was furnished with a floating apparatus
or swimbladder. We can thus, as I infer from Owen's
interesting description of these parts, understand the
strange fact that every particle of food and drink which
we swallow has to pass over the orifice of the trachea,
with some risk of falling into the lungs, notwithstanding
the beautiful contrivance by which the glottis is closed.
In the higher Vertebrata the branchiae have wholly dis-
appeared— but in the embryo the slits on the sides of the
neck and the iooplike course of the arteries still mark
their former position. Bat it is conceivable that the now
utterly lost branchiae might have been gradually worked
in by natural selection for some distinct purpose: for in-
stance, Landois has shown that the wings of insects are
258
THE ORIGIN OF SPECIES
developed from the tracheae; it is therefore highly proba-
ble that in this great class organs which once served for
respiration have been actually converted into organs for
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 another instance.
Pedunculated cirripeds 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 cirripeds have
no branchiae, the whole surface of the body and of the
sack, together with the small frena, serving for respira-
tion. The Balanidae or sessile cirripeds, on the other
hand, have no ovigerous frena, the eggs lying loose at the
bottom of the sack, within the well-inclosed shell; but
they have, in the same relative position with the frena,
large, much-folded membranes, which freely communicate
with the circulatory lacunae of the sack and body, and
which have been considered by all naturalists to act as
branchiae. Now I think no one will dispute that the
ovigerous frena in the one family are strictly homologous
with the branchiae of the other family; indeed, they grad-
uate into each other. Therefore it need not be doubted
that the two little folds of skin, which originally served
as ovigerous frena, but which, likewise, very slightly
aided in the act of respiration, have been gradually con-
verted by natural selection into branchiae, simply through
an increase in their size and the obliteration of their
adhesive glands. If all pedunculated cirripeds had become
extinct, and they have suffered far more extinction than
have sessile cirripeds, who would ever have imagined that
DIFFICULTIES OF THE THEORY 259
the branchiae in this latter family had originally existed
as organs for preventing the ova from being washed out
of the sack ?
There is another possible mode of transition, namely,
through the acceleration or retardation of the period of
reproduction. This has lately been insisted on by Prof.
Cope and others in the United States. It is now known
that some animals are capable of reprod action at a very
early age, before they have acquired their perfect char-
acters; and if this power became thoroughly well devel-
oped in a species, it seems probable that the adult stage
of development would sooner or later be lost; and in
this case, especially if the larva differed much from
the mature form, the character of the species would be
greatly changed and degraded. Again, not a few ani-
mals, after arriving at maturity, go on changing in char-
acter during nearly their whole lives. With mammals,
for instance, the form of the skull is often much altered
with age, of which Dr. Murie has given some striking
instances with seals; every one knows how the horns of
stags become more and more branched, and the plumes
of some birds become more finely developed, as they
grow older. Prof. Cope states that the teeth of certain
lizards change much in shape with advancing years; with
crustaceans not only many trivial, but some important
parts assume a new character, as recorded by Fritz Muller,
after maturity. In all such cases — and many could be
given — if the age for reproduction were retarded, the
character of the species, at least in its adult state, would
be modified; nor is it improbable that the previous and
earlier stages of development would in some cases be
hurried through and finally lost. Whether species have
260
THE ORIGIN OF SPECIES
often or ever been modified through this comparatively
sudden mode of transition I can form no opinion; but
if this has occurred, it is probable that the differences
between the young and the mature, and between the
mature and the old, were primordially acquired by
graduated steps.
Special Difficulties of the Theory of Natural Selection
Although we must be extremely cautious in conclud-
ing that any organ could not have been produced by
successive, small, transitional gradations, yet undoubtedly
serious cases of difficulty occur.
One of the most serious is that of neuter insects,
which are often 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; for it is impossible to
conceive by what steps these wondrous organs have been
produced. But this is not surprising, for we do not even
know of what use they are. In the Gymnotus and Tor-
pedo they no doubt serve as powerful means of defence,
and perhaps for securing prey; yet in the Hay, as ob-
served by Matteucci, an analogous organ in the tail mani-
fests but little electricity, even when the animal is greatly
irritated; so little, that it can hardly be of any use for
the above purposes. Moreover, in the Bay, besides the
organ just referred to, there is, as Dr. E. M'Donnell has
shown, another organ near the head, not known to be
electrical, but which appears to be the real homologue
of the electric battery in the Torpedo. It is generally
admitted that there exists between these organs and ordi-
nary muscle a close analogy, in intimate structure, in the
DIFFICULTIES OF THE THEORY
261
distribution of the nerves, and in the manner m which
the j are acted on by various reagents. It should, also, be
especially observed that muscular contraction is accom-
panied by an electrical discharge; and, as Dr. Badcliffe
insists, 4 'in the electrical apparatus of the torpedo during
rest there would seem to be a charge in every respect
like that which is met with in muscle and nerve during
rest, and the discharge of the torpedo, instead of being
peculiar, may be only another form of the discharge which
attends upon the action of muscle and motor nerve.' 9
Beyond this we cannot at present go in the way of
explanation; but as we know so little about the uses
of these organs, and as we know nothing about the habits
and structure of the progenitors of the existing electric
fishes, it would be extremely bold to maintain that no
serviceable transitions are possible by which these organs
might have been gradually developed.
These organs appear at first to offer another and far
more serious difficulty; for they occur in about . a dozen
kinds of fish, of which several are widely remote in their
affinities. When the same organ is found in several mem-
bers of the same class, especially if in members having
very different habits of life, we may generally attribute
its presence to inheritance from a common ancestor; and
its absence in some of the members to loss through disuse
or natural selection. So that, if the electric organs had
been inherited from some one ancient progenitor, we
might have expected that all electric fishes would have
been specially related to each other; but this is far from
the case. Nor does geology at all lead to the belief that
most fishes formerly possessed electric organs, which their
modified descendants have now lost. But when we look
262
THE ORIGIN OF SPECIES
at the subject more closely, we find in the several fishes
provided with electric organs, that these are situated in
different parts of the body — that they differ in construc-
tion, as in the arrangement of the plates, and, according
to Pacini, in the process or means by which the electricity
is excited — and lastly, in being supplied with nerves pro-
ceeding from different sources, and this is perhaps the
most important of all the differences. Hence in the sev-
eral fishes furnished with electric organs, these cannot
be considered as homologous, but only as analogous in
function. Consequently there is no reason to suppose
that they have been inherited from a common progenitor;
for had this been the case they would have closely resem-
bled each other in all respects. Thus the difficulty of an
organ, apparently the same, arising in several remotely
allied species, disappears, leaving only the lesser yet still
great difficulty; namely, by what graduated steps these
organs have been developed in each separate group of
fishes.
The luminous organs which occur in a few insects,
belonging to widely different families, and which are
situated in different parts of the body, offer, under our
present state of ignorance, a difficulty almost exactly
parallel with that of the electric organs. Other similar
cases could be given; for instance in plants, the very
curious contrivance of a mass of pollen-grains, borne on
a foot-stalk with an adhesive gland, is apparently the
same in Orchis and Asclepias — genera almost as remote
as is possible among flowering plants; but here again the
parts are not homologous. In all cases of beings, far
removed from each other in the scale of organization,
which are furnished with similar and peculiar organs, it
DIFFICULTIES OF THE THEORY 263
will be found that although the general appearance and
function of the organs may be the same, yet fundamental
differences between them can always be detected. For
instance, the eyes of cephalopods or cuttle-fish and of
vertebrate animals appear wonderfully alike; and in such
widely sundered groups no part of this resemblance can
be due to inheritance from a common progenitor. Mr.
Mivart has advanced this case as one of special difficulty,
but I am unable to see the force of his argument. An
organ for vision must be formed of transparent tissue,
and must include some sort of lens for throwing an image
at the back of a darkened chamber. Beyond this super-
ficial resemblance, there is hardly any real similarity
between the eyes of cuttle-fish and vertebrates, as may be
seen by consulting Hensen's admirable memoir on these
organs in the Cephalopoda. It is impossible for me here
to enter on details, but I may specify a few of the points
of difference. The crystalline lens in the higher cuttle-
fish consists of two parts, placed one behind the other
like two lenses, both having a very different structure
and disposition to what occurs in the vertebrata. The
retina is wholly different, with an actual inversion of
the elemental parts, and with a large nervous ganglion
included within the membranes of the eye. The relations
of the muscles are as different as it is possible to con-
ceive, and so in other points. Hence it is not a little
difficult to decide how far even the same terms ought to
be employed in describing the eyes of the Cephalopoda
and Yertebrata. It is, of course, open to any one to deny
that the eye in either case could have been developed
through the natural selection of successive slight varia-
tions; but if this be admitted in the one case, it is
264
THE ORIGIN OF SPECIES
clearly possible in the other; and fundamental differences
of structure in the visual organs of two groups might
have been anticipated, in accordance with this view of
their manner of formation. As two men have sometimes
independently hit on the same invention, so in the sev-
eral foregoing cases it appears that natural selection,
working for the good of each being, and taking advan-
tage of all favorable variations, has produced similar or-
gans, as far as function is concerned, in distinct organic
beings, which owe none of their structure in common
to inheritance from a common progenitor.
Fritz Mtiller, in order to test the conclusions arrived
at in this volume, has followed out with much care a
nearly similar line of argument. Several families of crus-
taceans include a few species possessing an air-breathing
apparatus and fitted to live out of the water. In two of
these families, which were more especially examined by
Miiller, and which are nearly related to each other, the
species agree most closely in all important characters;
namely in their sense organs, circulating system, in the
position of the tufts of hair within their complex stom-
achs, and lastly in the whole structure of the water-
breathing branchise, even to the microscopical hooks by
which they are cleansed. Hence it might have been ex-
pected that in the few species belonging to both families
which live on the land, the equally-important air-breath-
ing apparatus would have been the same; for why should
this one apparatus, given for the same purpose, have
been made to differ, while all the other important organs
were closely similar or rather identical?
Fritz Miiller argues that this close similarity in so
many points of structure must, in accordance with the
DIFFICULTIES OF THE THEORY
265
views advanced by me, be accounted for by inheritance
from a common progenitor. But as the vast majority of
the species in the above two families, as well as most
other crustaceans, are aquatic in their habits, it is im-
probable in the highest degree that their common pro-
genitor should have been adapted for breathing air.
Miiller was thus led carefully to examine the appa-
ratus in the air-breathing species; and he found it to
differ in each in several important points, as in the po-
sition of the orifices, in the manner in which they are
opened and closed, and in some accessory details. Now
such differences are intelligible, and might even have been
expected, on the supposition that species belonging to
distinct families had slowly become adapted to live more
and more out of water, and to breathe the air. For these
species, from belonging to distinct families, would have
differed to a certain extent, and in accordance with the
principle that the nature of each variation depends on
two factors, viz., the nature of the organism and that
of the surrounding conditions, their variability assuredly
would not have been exactly the same. Consequently
natural selection would have had different materials or
variations to work on, in order to arrive at the same
functional result; and the structures thus acquired would
almost necessarily have differed. On the hypothesis of
separate acts of creation the whole case remains unintelli-
gible. This line of argument seems to have had great
weight in leading Fritz Miiller to accept the views main-
tained by me in this volume.
Another distinguished zoologist, the late Professor
Claparede, has argued in the same manner, and has ar-
rived at the same result. He shows that there are para-
—SCIENCE — 12
266
THE ORIGIN OF SPECIES
sitic mites (Acaridae), belonging to distinct sub-families
and families, which are furnished with hair-claspers.
These organs must have been independently developed,
as they could not have been inherited from a common
progenitor; and in the several groups they are formed by
the modification of the fore-legs — of the hind-legs — of the
maxillae or lips — and of appendages on the under side of
the hind part of the body.
In the foregoing cases, we see the same end gained
and the same function performed, in beings not at all
or only remotely allied, by organs in appearance, though
not in development, closely similar. On the other hand,
it is a common rule throughout nature that the same end
should be gained, even sometimes in the case of closely-
related beings, by the most diversified means. How dif-
ferently constructed is the feathered wing of a bird and
the membrane-covered wing of a bat; and still more so
the four wings of a butterfly, the two wings of a fly,
and the two wings with the elytra of a beetle. Bivalve
shells are made to open and shut, but on what a number
of patterns is the hinge constructed — from the long row
of neatly interlocking teeth in a Nucula to the simple
ligament of a Mussel! Seeds are disseminated by their
minuteness — by their capsule being converted into a light
balloon-like envelope — by being imbedded in pulp or
flesh, formed of the most diverse parts, and rendered
nutritious, as well as conspicuously colored, so as to
attract and be devoured by birds — by having hooks and
grapnels of many kinds and serrated awns, so as to ad-
here to the fur of quadrupeds — and by being furnished
with wings and plumes, as different in shape as they are
DIFFICULTIES OF THE THEORY 26?
elegant in structure, so as to be wafted by every breeze.
I will give one other instance; for this subject of the
same end being gained by the most diversified means
well deserves attention. Some authors maintain that or-
ganic beings have been formed in many ways for the
sake of mere variety, almost like toys in a shop, but
such a view of nature is incredible. With plants hav-
ing separated sexes, and with those in which, though
hermaphrodites, the pollen does not spontaneously fall
on the stigma, some aid is necessary for their fertiliza-
tion. With several kinds this is effected by the pollen-
grains, which are light and incoherent, being blown by
the wind through mere chance on to the stigma; and this
is the simplest plan which can well be conceived. An
almost equally simple, though very different, plan occurs
in many plants in which a symmetrical flower secretes a
few drops of nectar, and is consequently visited by in-
sects; and these carry the pollen from the anthers to the
stigma.
From this simple stage we may pass through an inex-
haustible number of contrivances, all for the same pur-
pose and effected in essentially the same manner, but en-
tailing changes in every part of the flower. The nectar
may be stored in variously shaped receptacles, with the
stamens and pistils modified in many ways, sometimes
forming trap-like contrivances, and sometimes capable of
neatly adapted movements through irritability or elas-
ticity. From such structures we may advance till we
come to such a case of extraordinary adaptation as that
lately described by Dr. Criiger in the Coryanthes. This
orchid has part of its labellum or lower lip hollowed
out into a great bucket, into which drops of almost pure
268
THE ORIGIN OF SPECIES
water continually fall from two secreting horns which
stand above it; and when the bucket is half full, the
water overflows by a spout on one side. The basal part
of the labellum stands over the bucket, and is itself
hollowed out into a sort of chamber with two lateral
entrances; within this chamber there are curious fleshy
ridges. The most ingenious man, if he had not witnessed
what takes place, could never have imagined what pur-
pose all these parts serve. But Dr. Criiger saw crowds
of large humble-bees visiting the gigantic flowers of this
orchid, not in order to suck nectar, but to gnaw of! the
ridges within the chamber above the bucket; in doing
this they frequently pushed each other into the bucket,
and their wings being thus wetted they could not fly-
away, but were compelled to crawl out through the pas-
sage formed by the spout or overflow. Dr. Criiger saw a
"continual procession" of bees thus crawling out of their
involuntary bath. The passage is narrow, and is roofed
over by the column, so that a bee, in forcing its way
out, first rubs its back against the viscid stigma and
then against the viscid glands of the pollen-masses. The
pollen-masses are thus glued to the back of the bee
which first happens to crawl out through the passage
of a lately expanded flower, and are thus carried away.
Dr. Criiger sent me a flower in spirits of wine, with a
bee which he had killed before it had quite crawled out
with a pollen-mass still fastened to its back. When the
bee, thus provided, flies to another flower or to the same
flower a second time, and is pushed by its comrades
into the bucket and then crawls out by the passage,
the pollen-mass necessarily comes first into contact with
the viscid stigma, and adheres to it, and the flower is
DIFFICULTIES OF THE THEORY 269
fertilized. Now at last we see the full use of every part
of the flower, of the water-secreting horns, of the bucket
half full of water, which prevents the bees from flying
away, and forces them to crawl out through the spout,
and rub against the properly placed viscid pollen-masses
and the viscid stigma.
The construction of the flower in another closely allied
orchid, namely the Catasetum, is widely different, though
serving the same end; and is equally curious. Bees visit
these flowers, like those of the Coryanthes, in order to
gnaw the labellum; in doing this they inevitably touch a
long, tapering, sensitive projection, or, as I have called
it, the antenna. This antenna, when touched, transmits a
sensation or vibration to a certain membrane which is
instantly ruptured; this sets free a spring by which the
pollen-mass is shot forth, like an arrow, in the right
direction, and adheres by its viscid extremity to the
back of the bee. The pollen-mass of the male plant
(for the sexes are separate in this orchid) is thus carried
to the flower of the female plant, where it is brought
into contact with the stigma, which is viscid enough to
break certain elastic threads, and retaining the pollen,
fertilization is effected.
How, it may be asked, in the foregoing and in innu-
merable other instances, can we understand the graduated
scale of complexity and the multifarious means for gain-
ing the same end. The answer no doubt is, as already
remarked, that when two forms vary, which already differ
from each other in some slight degree, the variability
will not be of the same exact nature, and consequently
the results obtained through natural selection for the
same general purpose will not be the same. We should
270
THE ORIGIN OF SPECIES
also bear in mind that every highly developed organism
has passed through many changes; and that each modi-
fied structure tends to be inherited, so that each modifi-
cation will not readily be quite lost, but may be again
and again further altered. Hence the structure of each
part of each species, for whatever purpose it may serve,
is the sum of many inherited changes, through which the
species has passed during its successive adaptations to
changed habits and conditions of life.
Finally, then, although in many cases it is most dim-
cult even to conjecture by what transitions organs have
arrived at their present state; yet, considering how small
the proportion of living and known forms is to the ex-
tinct and unknown, I have been astonished how rarely
an organ can be named, toward which no transitional
grade is known to lead. It certainly is true that new
organs, appearing as if created for some special purpose,
rarely or never appear in any being; — as indeed is shown
by that old, but somewhat exaggerated, canon in natural
history of "Natura non facit saltum." We meet with
this admission in the writings of almost every experi-
enced naturalist; or as Milne Edwards has well expressed
it, Nature is prodigal in variety, but niggard in innova-
tion. Why, on the theory of Creation, should there be
so much variety and so little real novelty? Why should
all the parts and organs of many independent beings,
each supposed to have been separately created for its
proper place in nature, be so commonly linked together
by graduated steps? Why should not Nature take a
sudden leap from structure to structure ? On the theory
of natural selection, we can clearly understand why she
should not; for natural selection acts only by taking
DIFFICULTIES OF THE THEORY
271
advantage of slight successive variations; she can never
take a great and sudden leap, but must advance by short
and sure, though slow steps.
Organs of little apparent Importance, as affected by Natural
Selection
As natural selection acts by life and death — by the
survival of the fittest, and by the destruction of the less
well-fitted individuals — I have sometimes felt great diffi-
culty in understanding the origin or formation of parts
of little importance; almost as great, though of a very
different kind, as in the case of the most perfect and
complex organs.
In the first place, we are much too ignorant, in regard
to the whole economy of any one organic being, to say
what slight modifications would be of importance or not.
In a former chapter I have given instances of very
trifling characters, such as the down on fruit and the
color of its flesh, the color of the skin and hair of
quadrupeds, which, from being correlated with consti-
tutional differences or from determining the attacks of
insects, might assuredly be acted on by natural selec-
tion. The tail of the giraffe looks like an artificially
constructed fly-flapper; and it seems at first incredible
that this could have been adapted for its present purpose
by successive slight modifications, each better and better
fitted, for so trifling an object as to drive away flies; yet
we should pause before being too positive even in this
case, for we know that the distribution and existence
of cattle and other animals in South America absolutely
depend on their power of resisting the attacks of insects:
so that individuals which could by any means defend
272
THE ORIGIN OF SPECIES
themselves from tnese small enemies would be able to
range into new pastures and thus gain a great advan-
tage. It is not that the larger quadrupeds are actually
destroyed (except in some rare cases) by flies, but they
are incessantly harassed and their strength reduced, so
that they are more subject to disease, or not so well
enabled in a coming dearth to search for food, or to
escape from beasts of prey.
Organs now of trifling importance have probably in
some cases been of high importance to an early pro-
genitor, and, after having been slowly perfected at a
former period, have been transmitted to existing species
in nearly the same state, although now of very slight
use; but any actually injurious deviations in their struct
ure would of course have been checked by natural selec-
tion. 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 fly -flapper, an organ of
prehension, or as an aid in turning, as in the case
of the dog, though the aid in this latter respect must
be slight, for the hare, with hardly any tail, can double
still more quickly.
In the second place, we may easily err in attributing
importance to characters, and in believing that they have
been developed through natural selection. We must by
no means overlook the effects of the definite action of
changed conditions of life — of so-called spontaneous varia-
DIFFICULTIES OF THE THEORY
273
tions, which seem to depend in a quite subordinate degree
on the nature of the conditions — of the tendency to re-
version to long-lost characters — of the complex laws of
growth, such as of correlation, compensation, of the press-
ure of one part on another, etc. — and finally of sexual
selection, by which characters of use to one sex are often
gained and then transmitted more or less perfectly to the
other sex, though of no use to this sex. But structures
thus indirectly gained, although at first of no advantage
to a species, may subsequently have been taken advan-
tage of by its modified descendants, under new conditions
of life and newly acquired habits.
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 color
was a beautiful adaptation to conceal this tree-frequenting
bird from its enemies; and consequently that it was a
character of importance, and had been acquired through
natural selection; as it is, the color is probably in chief
part due to sexual selection. A trailing palm in the
Malay Archipelago climbs the loftiest trees by the aid of
exquisitely constructed hooks clustered around the ends
of the branches, and this contrivance, no doubt, is of the
highest service to the plant; but as we see nearly similar
hooks on many trees which are not climbers, and which,
as there is reason to believe from the distribution of the
thorn-bearing species in Africa and South America, serve
as a defence against browsing quadrupeds, so the spikes
on the palm may at first have been developed for this
object, and subsequently have been improved and taken
advantage of by the plant, as it underwent further modi-
fication and became a climber, The naked skin on the
274
THE ORIGIN OF SPECIES
head of a vulture is generally considered as a direct
adaptation for wallowing in putridity; and so it may be,
or it may possibly be due to the direct action of putrid
matter; but we should be very cautious in drawing any
such inference, when we see that the skin on the head of
the clean -feeding male Turkey is likewise naked. The
sutures in the skulls of young mammals have been ad-
vanced 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 cause of each slight
variation or individual difference; and we are immediately
made conscious of this by reflecting on the differences
between the breeds of our domesticated animals in differ-
ent countries — more especially in the less civilized coun-
tries where there has been but little methodical selection.
Animals kept by savages in different countries often have
to struggle for their own subsistence, and are exposed to
a certain extent to natural selection, and individuals with
slightly different constitutions would succeed best under
different climates. With cattle susceptibility to the at-
tacks of flies is correlated with color, as is the liability
to be poisoned by certain plants; so that even color would
be thus subjected to the action of natural selection. Some
observers are convinced that a damp climate affects the
growth of the hair, and that with the hair the horns are
correlated. Mountain breeds always differ from lowland
breeds; and a mountainous country would probably affect
DIFFICULTIES OF THE THEORY
275
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 the head would
probably be affected. The shape, also, of the pelvis might
affect by pressure the shape of certain parts of the young
in the womb. The laborious breathing necessary in high
regions tends, as we have good reason to believe, to in-
crease the size of the chest; and again correlation would
come into play. The effects oi lessened exercise together
with abundant food on the whole organization is probably
still more important; and this, as H. von Kathusius has
lately shown in his excellent Treatise, is apparently one
chief cause of the great modification which the breeds of
swine have undergone. But we are far too ignorant to
speculate on the relative importance of the several known
and unknown causes of variation; and I have made these
remarks only to show that, if we are unable to account
for the characteristic differences of our several domestic
breeds, which nevertheless are generally admitted to have
arisen through ordinary generation from one or a few
parent-stocks, we ought not to lay too much stress on
our ignorance of the precise cause of the slight analogous
differences between true species.
Utilitarian Doctrine, how far true: Beauty, how acquired
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 many structures have been created for the sake of
beauty, to delight man or the Creator (but this latter
^oint is beyond the scope of scientific discussion), or for
276
THE ORIGIN OF SPECIES
the sake of mere variety, a view already discussed. Such
doctrines, if true, would be absolutely fatal to my theory.
I fully admit that many structures are now of no direct
use to their possessors, and may never have been of any
use to their progenitors; but this does not prove that
they were formed solely for beauty or variety. No
doubt the definite action of changed conditions, and the
various causes of modifications, lately specified, have all
produced an effect, probably a great effect, independently
of any advantage thus gained. But a still more impor-
tant consideration is that the chief part of the organiza-
tion of every living creature is due to inheritance; and
consequently, though each being assuredly is well fitted
for its place in nature, many structures have now no
very close and direct relation to present habits of life.
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 similar
bones in the arm of the monkey, in the foreleg 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
webbed feet no doubt were as useful to the pro-
genitor of the upland goose and of the frigate-bird
as they now are to the most aquatic of living birdsr
So we may believe that the progenitor of the seal did
not possess 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 were originally developed, on
the principle of utility, probably through the reduction
of more numerous bones in the fin of some ancient fish*
DIFFICULTIES OF THE THEORY
277
like progenitor of the whole class. It is scarcely possi-
ble to decide how much allowance ought to be made for
such causes of change as the definite action of external
conditions, so-called spontaneous variations, and the com-
plex laws of growth; but with these important excep-
tions, we may conclude that the structure of every liv-
ing creature either now is, or was formerly, of some
direct or indirect use to its possessor.
With respect to the belief that organic beings have
been created beautiful for the delight of man — a belief
which it has been pronounced is subversive of my whole
theory — I may first remark that the sense of beauty ob-
viously depends on the nature of the mind, irrespective
of any real quality in the admired object; and that the
idea of what is beautiful is not innate or unalterable.
We see this, for instance, in the men of different races
admiring an entirely different standard of beauty in their
women. If beautiful objects had been created solely for
man's gratification, it ought to be shown that before man
appeared there was less beauty on the face of the earth
than since he came on the stage. Were the beautiful
volute and cone shells of the Eocene epoch, and the
gracefully sculptured ammonites of the Secondary period,
created that man might ages afterward admire them in
his cabinet? Few objects are more beautiful than the
minute siliceous cases of the diatomaceae: were these cre-
ated that they might be examined and admired under the
higher powers of the microscope? The beauty in this
latter case, and in many others, is apparently wholly
due to symmetry of growth. Flowers rank among the
most beautiful productions of nature; but they have been
rendered conspicuous in contrast with the green leaves,
278 THE ORIGIN OF SPECIES
and in consequence at the same time beautiful, so that
they may be easily observed by insects. I have come
to this conclusion from finding it an invariable rule
that when a flower is fertilized by the wind it never
has a gay ly -colored corolla. Several plants habitually
produce two kinds of flowers; one kind open and col-
ored so as to attract insects; the other closed, not
colored, destitute of nectar, and never visited by in-
sects. Hence we may conclude that, if insects had
not been developed on the face of the earth, our
plants would not have been decked with beautiful
flowers, but would have produced only such poor flow-
ers as we see on our fir, oak, nut and ash trees, on
grasses, spinach, docks, and nettles, which are all fer-
tilized through the agency of the wind. A similar line
of argument holds good with fruits; that a ripe straw-
berry or cherry is as pleasing to the eye as to the pal-
ate— that the gayly-colored fruit of the spindle-wood tree
and the scarlet berries of the holly *,re beautiful objects
— will be admitted by every one. But this beauty serves
merely as a guide to birds and beasts, in order that the
fruit may be devoured and the matured seeds dissemi-
nated: I infer that this is the case from having as yet
found no exception to the rule that seeds are always
thus disseminated when imbedded within a fruit of any
kind (that is within a fleshy or pulpy envelope), if it
be colored of any brilliant tint, or rendered conspicuous
by being white or black.
On the other hand, I willingly admit that a great
number of male animals, as all our most gorgeous birds,
some fishes, reptiles, and mammals, and a host of mag-
nificently colored butterflies, have been rendered beauti-
DIFFICULTIES OF THE THEORY
279
ful for beauty's sake; but this has been effected through
sexual selection, that is, by the more beautiful males hav-
ing been continually preferred by the females, and not
for the delight of man. So it is with the music of birds.
We may infer from all this that a nearly similar taste
for beautiful colors and for musical sounds runs through
a large part of the animal kingdom. When the female
is as beautifully colored as the male, which is not rarely
the case with birds and butterflies, the cause apparently
lies in the colors acquired through sexual selection hav-
ing been transmitted to both sexes, instead of to the
males alone. How the sense of beauty in its simplest
form — that is, the reception of a peculiar kind of pleas-
ure from certain colors, forms, and sounds — was first de-
veloped in the mind of man and of the lower animals
is a very obscure subject. The same sort of difficulty is
presented, if we inquire how it is that certain flavors
and odors give pleasure, and others displeasure. Habit
in all these cases appears to have come to a certain
extent into play; but there must be some fundamental
cause in the constitution of the nervous system in each
species.
Natural selection cannot possibly produce any modifi-
cation in a species exclusively for the good of another
species; though throughout nature one species incessantly
takes advantage of, and profits by, the structures of oth-
ers. But natural selection can and does often produce
structures for the direct injury of other animals, 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
280
THE ORIGIN OF SPECIES
part of the structure of any one species had been formed
for the exclusive good of another species, it would an-
nihilate 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
effect, I cannot find even one which seems to me of any
weight. It is admitted that the rattlesnake has a poison-
fang for its own defence, and for the destruction of its
prey; but some authors suppose that at the same time
it js furnished with a rattle for its own injury, namely,
to warn its prey. I would almost as soon believe that
the cat curls the end of its tail, when preparing to
spring, in order to warn the doomed mouse. It is a
much more probable view that the rattlesnake uses its
rattle, the cobra expands its frill, and the puff-adder
swells while hissing so loudly and harshly, in order to
alarm the many birds and beasts which are known to at-
tack even the most venomous species. Snakes act on the
same principle which makes the hen ruffle her feathers
and expand her wings when a dog approaches her chick-
ens; but I have not space here to enlarge on the many
ways by which animals endeavor to frighten away their
enemies.
Natural selection will never produce in a being any
structure more injurious than beneficial to that being, 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
DIFFICULTIES OF THE THEORY
281
to be injurious, it will be modified; or if it be not so,
the being will become extinct as myriads have become
extinct.
Natural selection tends only to make each organic
being as perfect as, or slightly more perfect than, the
other inhabitants of the same country with which it
comes into competition. And we see that this is the
standard of perfection attained under nature. The en-
demic productions of New Zealand, for instance, are
perfect one compared with another; but they are now
rapidly yielding before the advancing legions of plants
and animals introduced from Europe. Natural selection
will not produce absolute perfection, nor do we always
meet, as far as we can judge, with this high, standard
under nature. The correction for the aberration of light
is said by Miiller not to be perfect even in that most
perfect organ, the human eye. Helmholtz, whose judg-
ment no one will dispute, after describing in the strong-
est terms the wonderful powers of the human eye, adds
these remarkable words: "That which we have discov-
ered, in the way of inexactness and imperfection in the
optical machine and in the image on the retina, is as
nothing in comparison with the incongruities which we
have just come across in the domain of the sensations.
One might say that nature has taken delight in accumu-
lating contradictions in order to remove all foundation
from the theory of a pre-existing harmony between the
external and internal worlds." If our reason leads us
to admire with enthusiasm a multitude of inimitable con-
trivances in nature, this same reason tells us, though we
may easily err on both sides, that some other contriv-
ances are less perfect. Can we consider the sting of the
282
THE ORIGIN OF SPECIES
bee as perfect, which, when used against many kinds ot
enemies, cannot be withdrawn, owing to the backward
serratures, and thus inevitably causes the death of the
insect by tearing out its viscera ?
If we look at the sting of the bee, as having existed
in a remote progenitor, as a boring and serrated instru-
ment, like that in so many members of the same great
order, and that it has since been modified but not per-
fected for its present purpose, with the poison originally
adapted for some other object, such as to produce galls,
since 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 social community, it will fulfil
all the requirements of natural selection, though it may
cause the death of some few members. If we admire the
truly wonderful power of scent by which the males of
many insects find their females, can we admire the pro-
duction for this single purpose of thousands of drones,
which are utterly useless to the community for any other
purpose, and which are ultimately slaughtered by their
industrious and sterile sisters? It may be difficult, but
we ought to admire the savage instinctive hatred of the
queen -bee, which urges her to destroy the young queens,
her daughters, as soon as they are 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
orchids and many other plants are fertilized through in-
sect agency, can we consider as equally perfect the elabo-
DIFFICULTIES OF THE THEORY
283
ration of dense clouds of pollen by our fir trees, so that a
few granules may be wafted by chance on to the ovules?
Summary : the Law of Unity of Type and of the Conditions
of Existence embraced by the Theory of Natural Selection
We have in this chapter discussed some of the diffi-
culties and objections which may be urged against the
theory. Many of them are serious; but I think that in
the discussion light has been thrown on several facts,
which on the belief of independent acts of creation are
utterly obscure. We have seen that species at any one
period are not indefinitely variable, and are not linked
together by a multitude of intermediate gradations, partly
because the process of natural selection is always very
slow, and at any one time acts only on a few forms; and
partly because the very process of natural selection im-
plies the continual supplanting and extinction of preced-
ing and intermediate gradations. Closely allied species,
now living on a continuous area, must often have been
formed when the area was not continuous, and when the
conditions of life did not insensibly graduate away from
one part to another. When two varieties are formed in
two districts of a continuous area, an intermediate variety
will often be formed, fitted for an intermediate zone; but
from reasons assigned, the intermediate variety will usu-
ally exist in lesser numbers than the two forms which it
connects; consequently the two latter, during the course
of further modification, from existing in greater numbers,
will have a great advantage over the less numerous in-
termediate variety, and will thus generally succeed in
supplanting and exterminating it.
We have seen in this chapter how cautious we should
284
THE ORIGIN OF SPECIES
be in concluding that the most different habits of life
could not graduate into each other; that a bat, for in-
stance, could not have been formed by natural selection
from an animal which at first only glided through the air.
We have seen that a species under new conditions of
life may change its habits; or it may have diversified
habits, with some 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
enough to stagger any one, yet in the case of any
organ, if we know of a long series of gradations in
complexity, each good for its possessor, then, under
changing conditions of life, there is no logical impossi-
bility 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 extremely cautious in concluding that none
can have existed, for the metamorphoses of many organs
show what wonderful changes in function are at least pos-
sible. For instance, a swimbladder has apparently been
converted into an air-breathing lung. The same organ
having performed simultaneously very different functions
and then having been in part or in whole specialized for
one function, and two distinct organs having performed
at the same time the same function, the one having been
perfected while aided by the other, must 'tften have
largely facilitated transitions.
DIFFICULTIES OF THE THEORY
285
"We have seen that, in two beings widely remote from
each other in the natural scale, organs serving for the
same purpose and in external appearance closely similar
may have been separately and independently formed; but
when such organs are closely examined, essential differ-
ences in their structure can almost always be detected;
and this naturally follows from the principle of natural
selection. On the other hand, the common rule through-
out nature is infinite diversity of structure for gaining
the same end; and this again naturally follows from the
same great principle.
In many cases we are far too ignorant to be enabled
to assert that a part or organ is so unimportant for the
welfare of a species that modifications in its structure
could not have been slowly accumulated by means of
natural selection. In many other cases, modifications are
probably the direct result of the laws of variation or
of growth, independently of any good having been thus
gained. But even such structures have often, as we may
feel assured, been subsequently taken advantage of, and
still further modified, for the good of species under new
conditions of life. We may, also, believe that a part
formerly of high importance has frequently been retained
(as the tail of an aquatic animal by its terrestrial descend-
ants), though it has become of such small importance
that it could not, in its present state, have been acquired
by means of natural selection.
Natural selection can produce nothing in one species
for the exclusive good or injury of another; though it
may well produce parts, organs, and excretions highly
useful or even indispensable, or again highly injurious to
another species, but in all cases at the same time useful
286
THE ORIGIN OF SPECIES
to the possessor. In each well-stocked country natural
selection acts through the competition of the inhabitants,
and consequently leads to success in the battle for life,
only in accordance with the standard of that particular
country. Hence the inhabitants of one country, generally
the smaller one, often yield to the inhabitants of another
and generally the larger country. For in the larger coun-
try there will have existed more individuals and more
di versified forms, and the competition will have been
severer, and thus the standard of perfection will have
been rendered higher. Natural selection will not neces-
sarily lead to absolute perfection; nor, as far as we can
judge by our- limited faculties, can absolute perfection be
everywhere predicated.
On the theory of natural selection we can clearly un-
derstand the full meaning of that old canon in natural
history, "Xatura non facit saltum." This canon, if we
look to the present inhabitants alone of the world, is not
strictly correct; but if we include all those of past times,
whether known or unknown, it must on this theory be
strictly true.
It is generally acknowledged that all organic beings
have been formed on two great laws — Unity of Type and
the Conditions of Existence. By unity of type is meant
that fundamental agreement in structure which we see in
organic beings of the same class, and which is quite
independent of their habits of life. On my theory, unity
of type is explained by unity of descent. The expression
of conditions of existence, so often insisted on by the
illustrious Cuvier, is fully embraced by the principle of
natural selection. For natural selection acts by either
now adapting the varying parts of each being to its
DIFFICULTIES OF THE THEORY
287
organic and inorganic conditions of life; or by having
adapted them during past periods of time: the adaptations
being aided in many cases by the increased use or disuse
of parts, being affected by the direct action of the external
conditions of life, and subjected in all cases to the sev-
eral laws of growth and variation. Hence, in fact, the
law of the Conditions of Existence is the higher law; as
it includes, through the inheritance of former variations
and adaptations, that of Unity of Type.
288
THE ORIGIN OF SPECIES
CHAPTER VII
MISCELLANEOUS OBJECTIONS TO THE THEORY
OF NATURAL SELECTION
Longevity — Modifications not necessarily simultaneous — Modifications ap-
parently of no direct service — Progressive development — Characters of
small functional importance, the most constant — Supposed incompetence
of natural selection to account for the incipient stages of useful struc-
tures— Causes which interfere with the acquisition through natural
selection of useful structures — Gradations of structure with changed
functions — Widely different organs in members of the same class, de-
veloped from one and the same source — Reasons for disbelieving in
great and abrupt modifications
I WILL devote this chapter to the consideration of
various miscellaneous objections which have been
advanced against my views, as some of the previous
discussions may thus be made clearer; but it would be
useless to discuss all of them, as many have been made
by writers who have not taken the trouble to understand
the subject. Thus a distinguished German naturalist has
asserted that the weakest part of my theory is, that I
consider all organic beings as imperfect: what I have
really said is that all are not as perfect as they might
have been in relation to their conditions; and this is
shown to be the case by so many native forms in many
quarters of the world having yielded their places to in-
truding foreigners. Nor can organic beings, even if they
were at any one time perfectly adapted to their conditions
of life, have remained so, when their conditions changed,
OBJECTIONS TO THE THEORY 289
unless they themselves likewise changed, and no one will
dispute that the physical conditions of each country, as
well as the numbers and kinds of its inhabitants, have
undergone many mutations.
A critic has lately insisted, with some parade of
mathematical accuracy, that longevity is a great advan-
tage to all species, so that he who believes in natural
selection " must arrange his genealogical tree' 1 in such a
manner that all the descendants have longer lives than
their progenitors! Cannot our critic conceive that a bien-
nial plant or one of the lower animals might range into
a cold climate and perish there every winter; and yet,
owing to advantages gained through natural selection,
survive from year to year by means of its seeds or ova?
Mr. E. Eay Lankester has recently discussed this subject,
and he concludes, as far as its extreme complexity allows
him to form a judgment, that longevity is generally re-
lated to the standard of each species in the scale of
organization, as well as to the amount of expenditure
in reproduction and in general activity. And these con-
ditions have, it is probable, been largely determined
through natural selection.
It has been argued that, as none of the animals and
plants of Egypt, of which we know anything, have
changed during the last three or four thousand years,
so probably have none in any part of the world. But,
as Mr. G. H. Lewes has remarked, this line of argu-
ment proves too much, for the ancient domestic races
figured on the Egyptian monuments, or embalmed, are
closely similar or even identical with those now living;
yet all naturalists admit that such races have been pro-
duced through the modification of their original types.
—Science — 13
290
THE ORIGIN OF SPECIES
The many animals which have remained unchanged since
the commencement of the glacial period would have been
an incomparably stronger case, for these have been ex-
posed to great changes of climate and have migrated
over great distances; whereas, in Egypt, during the last
several thousand years, the conditions of life, as far as
we know, have remained absolutely uniform. The fact
of little or no modification having been effected since
the glacial period would have been of some avail against
those who believe in an innate and necessary law of de-
velopment, but is powerless against the doctrine of natu-
ral selection or the survival of the fittest, which implies
that when variations or individual differences of a ben-
eficial nature happen to arise, these will be preserved;
but this will be effected only under certain favorable
circumstances.
The celebrated paleontologist, Bronn, at the close of
his German translation of this work, asks, how, on the
principle of natural selection, can a variety live side by
side with the parent species? If both have become
fitted for slightly different habits of life or conditions,
they might live together; and if we lay on one side
polymorphic species, in which the variability seems to be
of a peculiar nature, and all mere temporary variations,
such as size, albinism, etc., the more permanent varieties
are generally found, as far as I can discover, inhabiting
distinct stations — such as high land or low land, dry or
moist districts. Moreover, in the case of animals which
wander much about and cross freely, their varieties seem
to be generally confined to distinct regions.
Bronn also insists that distinct species never differ
from each other in single characters, but in many parts,
OBJECTIONS TO THE THEORY
291
and he asks, how it always comes that many parts of the
organization should have been modified at the same time
through variation and natural selection? But there is no
necessity for supposing that all the parts of any being
have been simultaneously modified. The most striking
modifications, excellently adapted for some purpose,
might, as was formerly remarked, be acquired by suc-
cessive variations, if slight, first in one part and then in
another; and as they would be transmitted all together,
they would appear to us as if they had been simulta-
neously developed. The best answer, however, to the
above objection is afforded by those domestic races which
have been modified, chiefly through man's power of se-
lection, for some special purpose. Look at the race and
dray horse, or at the gre}rhound and mastiff. Their
whole frames and even their mental characteristics have
been modified; but if we could trace each step in the
history of their transformation — and the latter steps can
be traced— we should not see great and simultaneous
changes, but first one part and then another slightly
modified and improved. Even when selection has been
applied by man to some one character alone — of which
our cultivated plants offer the best instances — it will in-
variably be found that although this one part, whether
it be the flower, fruit, or leaves, has been greatly
changed, almost all the other parts have been slightly
modified. This may be attributed partly to the principle
of correlated growth, and partly to so-called spontaneous
variation.
A much more serious objection has been urged by
Bronn, and recently by Broca, namely, that many char-
acters appear to be of no service whatever to their
292
TEE ORIGiy OF SPECIES
possessors, and therefore cannot have been influenced
through natural selection. Bronn adduces the length of
the ears and tails in the different species of hares and
mice — the complex folds of enamel in the teeth of many
animals, and a multitude of analogous cases. ^Yith respect
to plants, this subject has been discussed by Nageli in
an admirable essay. He admits that natural selection has
effected much, but he insists that the families of plants
differ chiefly from each other in morphological characters,
which appear to be quite unimportant for the welfare of
the species. He consequently believes in an innate ten-
dency toward progressive and more perfect development.
He specifies the arrangement of the cells in the tissues,
and of the leaves on the axis, as cases in which natural
selection could not have acted. To these may be added
the numerical divisions in the parts of the flower, the
position of the ovules, the shape of the seed, when not
of any use for dissemination, etc.
There is much force in the above objection. Never-
theless, we ought, in the first place, to be extremely
cautious in pretending to decide what structures now
are. or have formerly been, of use to each species. In
the second place, it should always be borne in mind
that when one part is modified, so will be other parts,
through certain dimly seen causes, such as an increased
or diminished flow of nutriment to a part, mutual press-
ure, an early developed part affecting one subsequently
developed, and so forth — as well as through other causes
which lead to the many mysterious cases of correlation,
which we do not in the least understand. These agencies
may be all grouped together, for the sake of brevity,
under the expression of the laws of growth. Iq the
OBJECTIONS TO THE THEORY 293
third place, we have to allow for the direct and definite
action of changed conditions of life, and for so-called
spontaneous variations, in which the nature of the con-
ditions apparently plays a quite subordinate part. Bud-
variations, such as the appearance of a moss-rose on a
common rose, or of a nectarine on a peach tree, offer
good instances of spontaneous variations; but even in
these cases, if we bear in mind the power of a minute
drop of poison in producing complex galls, we ought not
to feel too sure that the above variations are not the
effect of some local change in the nature of the sap, due
to some change in the conditions. There must be some
efficient cause for each slight individual difference, as
well as for more strongly marked variations which oc-
casionally arise; and if the unknown cause were to act
persistently, it is almost certain that all the individuals
of the species would be similarly modified.
In the earlier editions of this work I underrated, as
it now seems probable, the frequency and importance of
modifications due to spontaneous variability. But it is
impossible to attribute to this cause the innumerable
structures which are so well adapted to the habits of
life of each species. I can no more believe in this, than
that the well-adapted form of a racehorse or greyhound,
which, before the principle of selection by man was well
understood, excited so much surprise in the minds of the
older naturalists, can thus be explained.
It may be worth while to illustrate some of the fore-
going remarks. With respect to the assumed inutility of
various parts and organs, it is hardly necessary to ob-
serve that even in the higher and best-known animals
many structures exist which are so highly developed
294:
THE ORIGIX OF SPECIES
that no one doubts that they are of importance, yet
their use has not been, or has only recently been, as-
certained. As Bronn gives the length of the ears and
tail in the several species of mice as instances, though
trifling ones, of differences in structure which can be
of no special use, I may mention that, according to Dr.
Schobl, the external ears of the common mouse are sup-
plied in an extraordinary manner with nerves, so that
they no doubt serve as tactile organs; hence the length
of the ears can hardly be quite unimportant. We shall,
also, presently see that the tail is a highly useful pre-
hensile organ to some of the species; and its use would
be much influenced by its length.
With respect to plants, to which, on account of
Nageli's essay, I shall confine myself in the follow-
ing remarks, it will be admitted that the flowers of
orchids present a multitude of curious structures, which
a few years ago would have been considered as mere
morphological differences without any special function;
but they are now known to be of the highest impor-
tance for the fertilization of the species through the aid
of insects, and have probably been gained through natu-
ral selection. No one until lately would have imagined
that in dimorphic and trimorphic plants the different
lengths of the stamens and pistils, and their arrange-
ment, could have been of any service, but now we know
this to be the case.
In certain whole groups of plants the ovules stand
erect, and in others they are suspended; and within the
same ovarium of some few plants, one ovule holds the
former and a second ovule the latter position. These
positions seem at first purely morphological, or of no
OBJECTIONS TO THE THEORY
295
physiological signification; but Dr. Hooker informs me
that within the same ovarium, the upper ovules alone
in some cases, and in other cases the lower ones alone
are fertilized; and he suggests that this probably depends
on the direction in which the pollen-tubes enter the ova-
rium. If so, the position of the ovules, even when one
is erect and the other suspended within the same ova-
rium, would follow from the selection of any slight de-
viations in position which favored their fertilization and
the production of seed.
Several plants belonging to distinct orders habitually
produce flowers of two kinds — the one open of the ordi-
nary structure, the other closed and imperfect. These
two kinds of flowers sometimes differ wonderfully in
structure, yet may be seen to graduate into each other
on the same plant. The ordinary and open flowers can
be intercrossed; and the benefits which certainly are de-
rived from this process are thus secured. The closed
and imperfect flowers are, however, manifestly of high
importance, as they yield with the utmost safety a large
stock of seed, with the expenditure of wonderfully little
pollen. The two kinds of flowers often differ much, as
just stated, in structure. The petals in the imperfect
flowers almost always consist of mere rudiments, and the
pollen-grains are reduced in diameter. In Ononis col-
umnar five of the alternate stamens are rudimentary; and
in some species of Viola three stamens are in this state,
two retaining their proper function, but being of very
small size. In six out of thirty of the closed flowers in
an Indian violet (name unknown, for the plants have
never produced with me perfect flowers), the sepals are
reduced from the normal number of five to three. In
296
THE ORIGIN OF SPECIES
one section of the Malpighiaceae the closed flowers,
according to A. de Jussieu, are still further modified,
for the five stamens which stand opposite to the sepals
are all aborted, a sixth stamen standing opposite to a
petal being alone developed; and this stamen is not
present in the ordinary flowers of these species; the style
is aborted; and the ovaria are reduced from three to
two. Now although natural selection may well have had
the power to prevent some of the flowers from expand-
ing, and to reduce the amount of pollen, when rendered
by the closure of the flowers superfluous, yet hardly
any of the above special modifications can have been
thus determined, but must have followed from the laws
of growth, including the functional inactivity of parts,
during the progress of the reduction of the pollen and
the closure of the flowers.
It is so necessary to appreciate the important effects
of the laws of growth that I will give some additional
cases of another kind; namely, of differences in the same
part or organ, due to differences in relative position on
the same plant. In the Spanish chestnut, and in certain
fir trees, the angles of divergence of the leaves differ,
according to Schacht, in the nearly horizontal and in the
upright branches. In the common rue and some other
plants, one flower, usually the central or terminal one,
opens first, and has five sepals and petals, and five divi-
sions to the ovarium; while all the other flowers on the
plant are tetramerous. In the British Adoxa the upper-
most flower generally has two calyx-lobes with the other
organs tetramerous, while the surrounding flowers gener-
ally have three calyx-lobes with the other organs pan-
tamerous. In many Composite and Umbelliferse (and in
OBJECTIONS OF THE THEORY
297
some other plants) the circumferential flowers have their
corollas much more developed than those of the centre,
and this seems often connected with the abortion of the
reproductive organs. It is a more curious fact, previously
referred to, that the achenes or seeds of the circumference
and centre sometimes differ greatly in form, color, and
other characters. In Carthamus and some other Com-
positae the central achenes alone are furnished with a
pappus; and in Hyoseris the same head yields achenes
of three different forms. In certain Umbelliferae the
exterior seeds, according to Tausch, are orthospermous,
and the central one ccelospermous, and this is a character
which was considered by De Candolle to be in other
species of the highest systematic importance. Prof.
Braun mentions a Fumariaceous genus, in which the
flowers in the lower part of the spike bear oval, ribbed,
one-seeded nutlets; and in the upper part of the spike,
lanceolate, two-valved, and two-seeded siliques. In these
several cases, with the exception of that of the well-
developed ray-florets, which are of service in making the
flowers conspicuous to insects, natural selection cannot, as
far as we can judge, have come into play, or only in a
quite subordinate manner. All these modifications follow
from the relative position and inter-action of the parts;
and it can hardly be doubted that if all the flowers and
leaves on the same plant had been subjected to the same
external and internal condition as are the flowers and
leaves in certain positions, all would have been modified
in the same manner.
In numerous other cases we find modifications of
structure, which are considered by botanists to be gener-
ally of a highly important nature, affecting only some of
298
TEE OMGIX OF SPECIES
the flowers on the same plant, or occurring on distinct
plants, which grow close together under the same condi-
tions. As these variations seem of no special use to the
plants, they cannot have been influenced by natural selec-
tion. Of their cause we are quite ignorant; we cannot
even attribute them, as in the last class of cases, to any
proximate agency, such as relative position. I will give
only a few instances. It is so common to observe, on the
same plant, flowers indifferently tetramerous, pentamerous,
etc., that I need not give examples; but as numerical
variations are comparatively rare when the part3 are few,
I may mention that, according to De Candolle, the flowers
of Papaver bracteatum offer either two sepals with four
petals (which is the common type with poppies), or three
sepals with six petals. The manner in which the petals
are folded in the bud is in most groups a very constant
morphological character; but Professor Asa Gray states
that with some species of Mimulus, the aestivation is
almost as frequently that of the Rhinanthidese as of the
Antirrhinideae. to which latter tribe the genus belongs,
Aug. St.-Hilaire gives the following cases: the genus
Zanthoxylon belongs to a division of the Rutacese with a
single ovary, but in some species flowers may be found
on the same plant, and even in the same panicle, with
either one or two ovaries. In Helianthemum the capsule
has been described as unilocular or 3-locular: and in H.
mutabile, "Une lame, plus ou moins large, s'^tend entre
le pericarpe et le placenta." In the flowers of Saponaria
officinalis. Dr. Masters has observed instances of both
marginal and free central placentation. Lastly, St.-
Hilaire found toward the southern extreme of the range
of Gomphia oleaeformis two forms which he clid not at
OBJECTIONS TO THE THEORY
299
first doubt were distinct species, but he subsequently saw
them growing on the same bush; and he then adds,
"Voila" done dans un m£me individu des loges et un
style qui se rattachent tan tot a un axe verticale et
tantot a un gynobase."
We thus see that with plants many morphological
changes may be attributed to the laws of growth and the
inter-action of parts, independently of natural selection.
But with respect to Nageli's doctrine of an innate ten-
dency toward perfection or progressive development, can
it be said in the case of these strongly pronounced
variations that the plants have been caught in the act of
progressing toward a higher state of development? On
the contrary, I should infer from the mere fact of the
parts in question differing or varying greatly on the same
plant that such modifications were of extremely small im-
portance to the plants themselves, of whatever importance
they may generally be to us for our classifications. The
acquisition of a useless part can hardly be said to raise
an organism in the natural scale; and in the case of the
imperfect, closed flowers above described, if any new
principle has to be invoked, it must be one of retro-
gression rather than of progression; and so it must be
with many parasitic and degraded animals. We are igno-
rant of the exciting cause of the above specified modifi-
cations; but if the unknown cause were to act almost
uniformly for a length of time, we may infer that the
result would be almost uniform; and in this case all
the individuals of the species would be modified in the
same manner.
From the fact of the above characters being unim-
portant for the welfare of the species, any slight varia-
600
THE ORIGIN OF SPECIES
tions which occurred in them would not have been
accumulated and augmented through natural selection.
A structure which has been developed through long-
continued selection, when it ceases to be of service to
a species, generally becomes variable, as we see with
rudimentary organs; for it will no longer be regulated
by this same power of selection. But when, from the
nature of the organism and of the conditions, modifica-
tions have been induced which are unimportant for the
welfare of the species, they may be, and apparently often
have been, transmitted in nearly the same state to
numerous, otherwise modified, descendants. It cannot
have been of much importance to the greater number
of mammals, birds, or reptiles, whether they were clothed
with hair, feathers, or scales; yet hair has been trans-
mitted to almost all mammals, feathers to all birds, and
scales to all true reptiles. A structure, whatever it may
be, which is common to many allied forms, is ranked by
us as of high systematic importance, and consequently
is often assumed to be of high vital importance to the
species. Thus, as I am inclined to believe, morphological
differences, which we consider as important — such as the
arrangement of the leaves, the divisions of the flower or
of the ovarium, the position of the ovules, etc. — first
appeared in many cases as fluctuating variations, which
sooner or later became constant through the nature of
the organism and of the surrounding conditions, as well
as through the intercrossing of distinct individuals, but
not through natural selection; for as these morphological
characters do not affect the welfare of the species, any
slight deviations in them could not have been governed
or accumulated through this latter agency. It is a strange
OBJECTIONS TO THE THEORY 301
result which we thus arrive at; namely, that characters
of slight vital importance to the species are the most
important to the systematist; but, as we shall hereafter
see when we treat of the genetic principle of classifica-
tion, this is by no means so paradoxical as it may at
first appear.
Although we have no good evidence of the existence
in organic beings of an innate tendency toward progres-
sive development, yet this necessarily follows, as I have
attempted to show in the fourth chapter, through the
continued action of natural selection. For the best
definition which has ever been given of a high standard
of organization is the degree to which the parts have
been specialized or differentiated; and natural selection
tends toward this end, inasmuch as the parts are thus
enabled to perform their functions more efficiently.
A distinguished zoologist, Mr. St. George Mivart, has
recently collected all the objections which have ever been
advanced by myself and others against the theory of
natural selection, as propounded by Mr. Wallace and
myself, and has illustrated them with admirable art
and force. When thus marshalled, they make a for-
midable array; and as it forms no part of Mr. Mivart's
plan to give the various facts and considerations opposed
to his conclusions, no slight effort of reason and memory
;s left to the reader, who may wish to weigh the evidence
on both sides. When discussing special cases, Mr. Mivart
passes over the effects of the increased use and disuse of
parts, which I have always maintained to be highly im-
portant, and have treated in my "Variation under Domes-
tication" at greater length than, as I believe, any other
502
THE ORIGIN OF SPECIES
writer. He likewise often assumes that I attribute
nothing to variation, independently of natural selection,
whereas in the work just referred to I have collected a
greater number of well-established cases than can be
found in any other work known to me. My judgment
may not be trustworthy, but after reading with care Mr.
Mivart's book, and comparing each section with what I
have said on the same head, I never before felt so
strongly convinced of the general truth of the conclusions
here arrived at, subject, of course, in so intricate a sub-
ject, to much partial error.
All Mr. Mivart's objections will be, or have been,
considered in the present volume. The one new point
which appears to have struck many readers is, * 4 that nat-
ural selection is incompetent to account for the incipient
stages of useful structures." This subject is intimately
connected with that of the gradation of characters, often
accompanied by a change of function — for instance, the
conversion of a swimbladder into lungs — points which
were discussed in the last chapter under two headings.
Nevertheless, I will here consider in some detail several
of the cases advanced by Mr. Mivart, selecting those
which are the most illustrative, as want of space pre-
vents me from considering all.
The giraffe, by its lofty stature, much elongated neck,
fore-legs, head and tongue, has its whole frame beauti-
fully adapted for browsing on the higher branches of
trees. It can thus obtain food beyond the reach of the
other Ungulata or hoofed animals inhabiting the same
country; and this must be a great advantage to it during
dearths. The Niata cattle in South America show ^s
how small a difference in structure may make, during
OBJECTIONS TO THE THEORY 303
such periods, a great difference in preserving an ani-
mal's life. These cattle can browse as well ?,s others
on grass, but from the projection of the lower jaw they
cannot, during the often recurrent droughts, browse on
the twigs of trees, reeds, etc., to which food the common
cattle and horses are then driven; so that at these times
the Niatas perish, if not fed by their owners. Before
coming to Mr. Mivart's objections, it may be well to
explain once again how natural selection will act in all
ordinary cases. Man has modified some of his animals,
without necessarily having attended to special points of
structure, by simply preserving and breeding from the
fleetest individuals, as with the racehorse and greyhound,
or as with the gamecock, by breeding from the victorious
birds. So under nature with the nascent giraffe, the in-
dividuals which were the highest browsers and were able
during dearths to reach even an inch or two above the
others will often have been preserved; for they will have
roamed over the whole country in search of food. That
the individuals of the same species often differ slightly
in the relative lengths of all their parts may be seen in
many works of natural history, in which careful measure-
ments are given. These slight proportional differences,
due to the laws of growth and variation, are not of the
slightest use or importance to most species. But it will
have been otherwise with the nascent giraffe, considering
its probable habits of life; for those individuals which
had some one part or several parts of their bodies rather
more elongated than usual would generally have sur-
vived. These will have intercrossed and left offspring,
either inheriting the same bodily peculiarities, or with a
tendency to vary again in the same manner; while the
304
THE ORIGIN OF SPECIES
individuals less favored in the same respects will have
been the most liable to perish.
We here see that there is no need to separate single
pairs, as man does, when he methodically improves a
breed: natural selection will preserve and thus separate
all the superior individuals, allowing them freely to in-
tercross, and will destroy all the inferior individuals. By
this process long-continued, which exactly corresponds
with what I have called unconscious selection by man,
combined no doubt in a most important manner with the
inherited effects of the increased use of parts, it seems
to me almost certain that an ordinary hoofed quadruped
might be converted into a giraffe.
To this conclusion Mr. Mivart brings forward two
objections. One is that the increased size of the body
would obviously require an increased supply of food,
and he considers it as "very problematical whether the
disadvantages thence arising would not, in times of scar-
city, more than counterbalance the advantages." But as
the giraffe does actually exist in large numbers in South
Africa, and as some of the largest antelopes in the
world, taller than an ox, abound there, why should we
doubt that, as far as size is concerned, intermediate gra-
dations could formerly have existed there, subjected as
now to severe dearths? Assuredly the being able to reach,
at each stage of increased size, to a supply of food, left
untouched by the other hoofed quadrupeds of the coun-
try, would have been of some advantage to the nascent
giraffe. Nor must we overlook the fact that increased
bulk would act as a protection against almost all beasts
of prey excepting the lion; and against this animal its
tall neck — and the taller the better — would, as Mr.
OBJECTIONS TO THE THEORY
305
Chauncey Wright has remarked, serve as a watch-
tower. It is from this cause, as Sir S. Baker re-
marks, that no animal is more difficult to stalk than
the giraffe. This animal also use3 its long neck as a
means of offence or defence, by violently swinging its
head armed with stump-like horns. The preservation of
each species can rarely be determined by any one advan-
tage, but by the union of all, great and small.
Mr. Mivart then asks (and this is his second objec-
tion), if natural selection be so potent, and if high
browsing be so great an advantage, why has not any
other hoofed quadruped acquired a long neck and lofty
stature, besides the giraffe, and, in a lesser degree, the
camel, guanaco, and macrauchenia ? Or, again, why has
not any member of the group acquired a long proboscis?
With respect to South Africa, which was formerly inhab-
ited by numerous herds of the giraffe, the answer is not
difficult, and can best be given by an illustration. In
every meadow in England in which trees grow, we see
the lower branches trimmed or planed to an exact level
by the browsing of the horses or cattle; and what advan-
tage would it be, for instance, to sheep, if kept there, to
acquire slightly longer necks? In every district some
one kind of animal will almost certainly be able to
browse higher than the others; and it is almost equally
certain that this one kind alone could have its neck
elongated for this purpose, through natural selection and
the effects of increased use. In South Africa the com-
petition for browsing on the higher branches of the
acacias and other trees must be between giraffe and
giraffe, and not with the other ungulate animals.
Why, in other quarters of the world, various animals
606
THE ORIGIN OF SPECIES
belonging to this same order have not acquired either an
elongated neck or a proboscis, cannot be distinctly an-
swered; but it is as unreasonable to expect a distinct
answer to such a question, as why some event in the
history of mankind did not occur in one country, while
it did in another. We are ignorant with respect to the
conditions which determine the numbers and range of
each species; and we cannot even conjecture what
changes of structure would be favorable to its in-
crease in some new country. We can, however, see
in a general manner that various causes might have
interfered with the development of a long neck or pro-
boscis. To reach the foliage at a considerable height
(without climbing, for which hoofed animals are singu-
larly ill -constructed) implies greatly increased bulk of
body; and we know that some areas support singularly
few large quadrupeds, for instance South America, though
it is so luxuriant; while South Africa abounds with them
to an unparalleled degree. Why this should be so we
do not know; nor why the later tertiary periods should
have been much more favorable for their existence than
the present time. Whatever the causes may have been,
we can see that certain districts and times would have
been much more favorable than others for the develop-
ment of so large a quadruped as the giraffe.
In order that an animal should acquire some structure
specially and largely developed, it is almost indispensa-
ble that several other parts should be modified and
coadapted. Although every part of the body varies
slightly, it does not follow that the necessary parts
should always vary in the right direction and to the
right degree. With the different species of our domesti-
OBJECTIONS TO THE THEORY
307
cated animals we know that the parts vary in a different
manner and degree; and that some species are much more
variable than others. Even if the fitting variations did
arise, it does not follow that natural selection would be
able to act on them, and produce a structure which ap-
parently would be beneficial to the species. For instance,
if the number of individuals existing in a country is de-
termined chiefly through destruction by beasts of prey —
by external or internal parasites, etc. — as seems often to
be the case, then natural selection will be able to do
little, or will be greatly retarded, in modifying any par-
ticular structure for obtaining food. Lastly, natural se-
lection is a slow process, and the same favorable condi-
tions must long endure in order that any marked effect
should thus be produced. Except by assigning such gen-
eral and vague reasons, we cannot explain why, in many
quarters of the world, hoofed quadrupeds have not ac-
quired much elongated necks or other means for browsing
on the higher branches of trees.
Objections of the same nature as the foregoing have
been advanced by many writers. In each case various
causes, besides the general ones just indicated, have
probably interfered with the acquisition through natu-
ral selection of structures which it is thought would be
beneficial to certain species. One writer asks, why has
not the ostrich acquired the power of flight? But a mo-
ment's reflection will show what an enormous supply of
food would be necessary to give to this bird of the
desert force to move its huge body through the air.
Oceanic islands are inhabited by bats and seals, but by
no terrestrial mammals; yet as some of these bats are
peculiar species, they must have long inhabited their
808
THE ORIGIN OF SPECIES
present homes. Therefore Sir C. Lyell asks, and as-
signs certain reasons in answer, why have not seals and
bats given birth on such islands to forms fitted to live
on the land? But seals would necessarily be first con-
verted into terrestrial carnivorous animals of considerable
size, and bats into terrestrial insectivorous animals; for
the former there would be no prey; for the bats ground-
insects would serve as food, but these would already be
largely preyed on by the reptiles or birds, which first
colonize and abound on most oceanic islands. Gradations
of structure, with each stage beneficial to a changing spe-
cies, will be favored only under certain peculiar condi-
tions. A strictly terrestrial animal, by occasionally hunt-
ing for food in shallow water, then in streams or lakes,
might at last be converted into an animal so thoroughly
aquatic as to brave the open ocean. But seals would
not find on oceanic islands the conditions favorable to
their gradual reconversion into a terrestrial form. Bats,
as formerly shown, probably acquired their wing3 by at
first gliding through the air from tree to tree, like the
so-called flying squirrels, for the sake of escaping from
their enemies, or for avoiding falls; but when the power
of true flight had once been acquired, it would never be
reconverted back, at least for the above purposes, into
the less efficient power of gliding through the air. Bats
might, indeed, like many birds, have had their wings
greatly reduced in size, or completely lost, through dis-
use; but in this case it would be necessary that they
should first have acquired the power of running quickly
on the ground, by the aid of their hind legs alone, so as
to compete with birds or other ground animals; and for
such a change a bat seems singularly ill-fitted. Thess
OBJECTIONS TO THE THEORY
309
conjectural remarks have been made merely to show that
a transition of structure, with each step beneficial, is a
highly complex affair; and that there is nothing strange
in a transition not having occurred in any particular
case.
Lastly, more than one writer has asked, why have
some animals had their mental powers more highly de-
veloped than others, as such development would be
advantageous to all? Why have not apes acquired the
intellectual powers of man? Various causes could be
assigned; but as they are conjectural, and their relative
probability cannot be weighed, it would be useless to give
them. A definite answer to the latter question ought not
to be expected, seeing that no one can solve the simpler
problem why, of two races of savages, one has risen
higher in the scale of civilization than the other; and
this apparently implies increased brain-power.
We will return to Mr. Mivart's other objections. In-
sects often resemble for the sake of protection various
objects, such as green or decayed leaves, dead twigs, bits
of lichen, flowers, spines, excrement of birds, and living
insects; but to this latter point I shall hereafter recur.
The resemblance is often wonderfully close, and is not
confined to color, but extends to form, and even to the
manner in which the insects hold themselves. The cater-
pillars which project motionless like dead twigs from the
bushes on which they feed, offer an excellent instance of
a resemblance of this kind. The cases of the imitation
of such objects as the excrement of birds are rare and
exceptional. On this head, Mr. Mivart remarks, "As,
according to Mr. Darwin's theory, there is a constant
tendency to indefinite variation, and as the minute incipi-
310
THE ORIGIN OF SPECIES
ent variations will be in all directions, they must tend to
neutralize each other, and at first to form such unstable
modifications that it is difficult, if not impossible, to see
how such indefinite oscillations of infinitesimal beginnings
can ever build up a sufficiently appreciable resemblance
to a leaf, bamboo, or other object, for Natural Selection
to seize upon and perpetuate."
But in all the foregoing cases the insects in their
original state no doubt presented some rude and acci-
dental resemblance to an object commonly found in the
stations frequented by them. Nor is this at all improba-
ble, considering the almost infinite number of surrounding
objects and the diversity in form and color of the hosts
of insects which exist. As some rude resemblance is
necessary for the first start, we can understand how it
is that the larger and higher animals do not (with the
exception, as far as I know, of one fish) resemble for
the sake of protection special objects, but only the sur-
face which commonly surrounds them, and this chiefly
in color. Assuming that an insect originally happened to
resemble in some degree a dead twig or a decayed leaf,
and that it varied slightly in many ways, then all the
variations which rendered the insect at all more like any
such object, and thus favored its escape, would be pre-
served, while other variations would be neglected and
ultimately lost; or, if they rendered the insect at all less
like the imitated object, they would be eliminated. There
would indeed be force in Mr. Mivart's objection, if we
were to attempt to account for the above resemblances,
independently of natural selection, through mere fluctuat-
ing variability; but as the case stands there is none.
Nor can I see any force in Mr. Mivart's difficulty
OBJECTIONS TO THE THEORY
311
with respect to 4 'the last touches of perfection in the
mimicry"; as in the case given by Mr. Wallace, of a
walking-stick insect (Ceroxylus laceratus), which resem-
bles "a stick grown over by a creeping moss or j linger-
mannia." So close was this resemblance that a native
Dyak maintained that the foliaceous excrescences were
really moss. Insects are preyed on by birds and other
enemies, whose sight is probably sharper than ours, and
every grade in resemblance which aided an insect to
escape notice or detection would tend toward its pres-
ervation; and the more perfect the resemblance so much
the better for the insect. Considering the nature of the
differences between the species in the group which in-
cludes the above Ceroxylus, there is nothing improbable
in this insect having varied in the irregularities on its
surface, and in these having become more or less green-
colored; for in every group the characters which differ
in the several species are the most apt to vary, while the
generic characters, or those common to all the species,
are the most constant.
. The Greenland whale is one of the most wonderful
animals in the world, and the baleen, or whale-bone, one
of its greatest peculiarities. The baleen consists of a row,
on each side of the upper jaw, of about 300 plates or
laminae, which stand close together transversely to the
longer axis of the mouth. Within the main row there
are some subsidiary rows. The extremities and inner
margins of all the plates are frayed into stiff bristles,
which clothe the whole gigantic palate, and serve to
strain or sift the water, and thus to secure the minute
prey on which these great animals subsist. The middle
S12
THE ORIGIN OF SPECIES
and longest lamina in the Greenland whale is ten, twelve,
or even fifteen feet in length; but in the different species
of Cetaceans there are gradations in length; the middle
lamina being in one species, according to Scoresby, four
feet, in another three, in another eighteen inches, and in
the Balaenoptera rostrata only about nine inches in length.
The quality of the whalebone also differs in the different
species.
With respect to the baleen, Mr. Mivart remarks that
if it "had once attained such a size and development as
to be at all useful, then its preservation and augmentation
within serviceable limits would be promoted by natural
selection alone. But how to obtain the beginning of such
useful development ?" In answer, it may be asked, why
should not the early progenitors of the whales with baleen
have possessed a mouth constructed something like the
lamellated beak of a duck? Ducks, like whales, subsist
by sifting the mud and water; and the family has some-
times been called Criblatores, or sifters. I hope that I
may not be misconstrued into saying that the progenitors
of whales did actually possess mouths lamellated like the
beak of a duck. I wish only to show that this is not
incredible, and that the immense plates of baleen in the
Greenland whale might have been developed from such
lamellae by finely graduated steps, each of service to its
possessor.
The beak of a shoveller-duck (Spatula clypeata) is a
more beautiful and complex structure than the mouth
of a whale. The upper mandible is furnished on each
side (in the specimen examined by me) with a row or
comb formed of 188 thin, elastic lamellae, obliquely
bevelled so as to be pointed, and placed transversely
OBJECTIONS TO THE THEORY
313
to tlie longer axis of the mouth. They arise from the
palate, and are attached by flexible membrane to the sides
of the mandible. Those standing toward the middle are
the longest, being about one-third of an inch in length,
and they project -14 of an inch beneath the edge. At
their bases there is a short subsidiary row of obliquely
transverse lamellae. In these several respects they resem-
ble the plates of baleen in the mouth of a whale. But
toward the extremity of the beak they differ much, as
they project inward, instead of straight downward. The
entire head of the shoveller, though incomparably less
bulky, is about one-eighteenth of the length of the head
of a moderately large Balaenoptera rostrata, in which spe-
cies the baleen is only nine inches long; so that if we
were to make the head of the shoveller as long as that
of the Balaenoptera, the lamellae would be six inches in
length — that is, two- thirds of the length of the baleen
in this species of whale. The lower mandible of the
shoveller- duck is furnished with lamellae of equal length
with those above, but finer; and in being thus furnished
it differs conspicuously from the lower jaw of a whale,
which is destitute of baleen. On the other hand, the
extremities of these lower lamellae are frayed into fine
bristly points, so that they thus curiously resemble the
plates of baleen. In the genus Prion, a member of the
distinct family of the Petrels, the upper mandible alone
is furnished with lamellae which are well developed and
project beneath the margin; so that the beak of this bird
resembles in this respect the mouth of a whale.
From the highly developed structure of the shoveller's
beak we may proceed (as I have learned from information
and specimens sent to me by Mr. Salvin), without any
— Science — 14
814
THE ORIGIN OF SPECIES
great break, as far as fitness for sifting is concerned,
through the beak of the Merganetta armata, and in some
respects through that of the Aix sponsa, to the beak of
the common duck. In this latter species, the lamellae are
much coarser than in the shoveller, and are firmly at-
tached to the sides of the mandible; they are only about
50 in number on each side, and do not project at all
beneath the margin. They are square-topped, and are
edged with translucent hardish tissue, as if for crushing
food. The edges of the lower mandible are crossed by
numerous fine ridges, which project very little. Although
the beak is thus very inferior as a sifter to that of the
shoveller, yet this bird, as every one knows, constantly
uses it for this purpose. There are other species, as I
hear from Mr. Salvin, in which the lamellae are consider-
ably less developed than in the common duck; but I do
not know whether they use their beaks for sifting the
water.
Turning to another group of the same family. In the
Egyptian goose (Chenalopex) the beak closely resembles
that of the common duck; but the lamellae are not so
numerous, nor so distinct from each other, nor do they
project so much inward; yet this goose, as I am informed
by Mr. E. Bartlett, 44 uses its bill like a duck by throwing
the water out at the corners." Its chief food, however,
is grass, which it crops like the common goose. In this
latter bird, the lamellae of the upper mandible are much
coarser than in the common duck, almost confluent, about
27 in number on each side, and terminating upward in
teeth-like knobs. The palate is also covered with hard
rounded knobs. The edges of the lower mandible are
serrated with teeth much more prominent, coarser, and
OBJECTIONS TO THE THEORY
315
sharper than in the duck. The common goose does not
sift the water, but uses its beak exclusively for tearing
or cutting herbage, for which purpose it is so well fitted
that it can crop grass closer than almost any other ani-
mal. There are other species of geese, as I hear from
Mr. Bartlett, in which the lamella are less developed
than in the common goose.
We thus see that a member of the duck family, with
a beak constructed like that of the common goose and
adapted solely for grazing, or even a member with a beak
having less well-developed lamellae, might be converted
by small changes into a species like the Egyptian goose
— this into one like the common duck — and, lastly, into
one like the shoveller, provided with a beak almost
exclusively adapted for sifting the water; for this bird
could hardly use any part of its beak, except the hooked
tip, for seizing or tearing solid food. The beak of a
goose, as 1 may add, might also be converted by small
changes into one provided with prominent, recurved
teeth, like those of the Merganser (a member of the
same family), serving for the widely different purpose
of securing live fish.
Eeturning to the whales. The Hyperoodon bidens is
destitute of true teeth in an efficient condition, but its
palate is roughened, according to Lacep&de, with small, un-
equal, hard points of horn. There is, therefore, nothing
improbable in supposing that some early Cetacean form
was provided with similar points of horn on the palate,
but rather more regularly placed, and which, like the
knobs on the beak of the goose, aided it in seizing or
tearing its food. If so, it will hardly be denied that the
points might have been converted through variation and
316
THE ORIGIN OF SPECIES
natural selection into lamellae as well-developed as those
of the Egyptian goose, in which case they would have
been used both for seizing objects and for sifting the
water; then into lamellae like those of the domestic duck;
and so onward, until they became as well constructed as
those of the shoveller, in which case they would have
served exclusively as a sifting apparatus. From this
stage, in which the lamellae would be two-thirds of the
length of the plates of baleen in the Balaenoptera ros-
trata, gradations, which may be observed in still-existing
Cetaceans, lead us onward to the enormous plates of
baleen in the Greenland whale. Nor is there the least
reason to doubt that each step in this scale might have
been as serviceable to certain ancient Cetaceans, with
the functions of the parts slowly changing during the
progress of development, as are the gradations in
the beaks of the different existing members of the duck-
family. We should bear in mind that each species of
duck is subjected to a severe struggle for existence, and
that the structure of every part of its frame must be well
adapted to its conditions of life.
The Pleuronectidae, or Flat-fish, are remarkable for
their asymmetrical bodies. They rest on one side — in
the greater number of species on the left, but in some
on the right side; and occasionally reversed adult speci-
mens occur. The lower, or resting-surface, resembles at
first sight the ventral surface of an ordinary fish: it is
of a white color, less developed in many ways than the
upper side, with the lateral fins often of smaller size.
But the eyes offer the most remarkable peculiarity; for
they are both placed on the upper side of the head.
During early youth, however, they stand opposite to each
OBJECTIONS TO THE THEORY
317
other, and the whole body is then symmetrical, with both
sides equally colored. Soon the eye proper to the lower
side begins to glide slowly round the head to the upper
side; but does not pass right through the skull, as was
formerly thought to be the case. It is obvious that
unless the lower eye did thus travel round, it could not
be used by the fish while living in its habitual position on
one side. The lower eye would, also, have been liable to
be abraded by the sandy bottom. That the Pleuronectidas
are admirably adapted by their flattened and asymmetrical
structure for their habits of life is manifest from several
species, such as soles, flounders, etc., being extremely
common. The chief advantages thus gained seem to be
protection from their enemies, and facility for feeding
on the ground. The different members, however, of the
family present, as Schiodte remarks, "a long series of
forms exhibiting a gradual transition from Hippoglossus
pinguis, which does not in any considerable degree alter
the shape in which it leaves the ovum, to the soles,
which are entirely thrown to one side."
Mr. Mivart has taken up this case, and remarks that a
sudden spontaneous transformation in the position of the
eyes is hardly conceivable, in which I quite agree with
him. He then adds: "If the transit was gradual, then
how such transit of one eye a minute fraction of the
journey toward the other side of the head could benefit
the individual is, indeed, far from clear. It seems, even,
that such an incipient transformation must rather have
been injurious." But he might have found an answer to
this objection in the excellent observations published in
1867 by Malm. The Pleuronectidse, while very young
and still symmetrical, with their eyes standing on oppo-
318
THE ORIGIN OF SPECIES
site sides of the head, cannot long retain a vertical
position, owing to the excessive depth of their bodies,
the small size of their lateral fins, and to their being
destitute of a swimbladder. Hence, soon growing tired,
they fall to the bottom on one side. While thus at rest
they often twist, as Malm observed, the lower eye up-
ward, to see above them; and they do this so vigorously
that the eye is pressed hard against the upper part of
the orbit. The forehead between the eyes consequently
becomes, as could be plainly seen, temporarily contracted
in breadth. On one occasion Malm saw a young fish
raise and depress the lower eye through an angular dis-
tance of about seventy degrees.
We should remember that the skull at this early age
is cartilaginous and flexible, so that it readily yields to
muscular action. It is also known with the higher
animals, even after early youth, that the skull yields
and is altered in shape, if the skin or muscles be per-
manently contracted through disease or some accident.
With long-eared rabbits, if one ear lops forward and
downward, its weight drags forward all the bones of the
skull on the same side, of which I have given a figure.
Malm states that the newly-hatched young of perches,
salmon, and several other symmetrical fishes, have the
habit of occasionally resting on one side at the bottom;
and he has observed that they often then strain their
lower eyes so as to look upward; and their skulls are thus
rendered rather crooked. These fishes, however, are soon
able to hold themselves in a vertical position, and no
permanent effect is thus produced. With the Pleuronec-
tidae, on the other hand, the older they grow the more
habitually they rest on one side, owing to the increasing
OBJECTIONS TO THE THEORY
319
flatness of their bodies, and a permanent effect is thus
produced on the form of the head, and on the position
of the eyes. Judging from analogy, the tendency to dis-
tortion would no doubt be increased through the prin-
ciple of inheritance. Schiddte believes, in opposition to
some other naturalists, that the Pleuronectidae are not
quite symmetrical even in the embryo; and if this be
so, we could understand how it is that certain species,
while young, habitually fall over and rest on the left
side, and other species on the right side. Malm adds,
in confirmation of the above view, that the adult Tra-
chypterus arcticus, which is not a member of the Pleuro-
nectidae, rests on its left side at the bottom, and swims
diagonally through the water; and in this fish, the two
sides of the head are said to be somewhat dissimilar.
Our great authority on Fishes, Dr. Giinther, concludes his
abstract of Malm's paper by remarking that 4 1 the author
gives a very simple explanation of the abnormal condi-
tion of the Pleuronectoids."
We thus see that the first stages of the transit of the
eye from one side of the head to the other, which Mr.
Mivart considers would be injurious, may be attributed
to the habit, no doubt beneficial to the individual and to
the species, of endeavoring to look upward with both
eyes while resting on one side at the bottom. We may
also attribute to the inherited effects of use the fact of
the mouth in several kinds of flat-fish being bent toward
the lower surface, with the jaw bones stronger and more
effective on this, the eyeless side of the head, than on
the other, for the sake, as Dr. Traquair supposes, of
feeding with ease on the ground. Disuse, on the other
hand, will account for the less developed condition of the
820
THE ORIGIN OF SPECIES
whole inferior half of the body, including the lateral
fins; though Yarrell thinks that the reduced size of these
fins is advantageous to the fish, as "there is so much
less room for their action than with the larger fins
above." Perhaps the lesser number of teeth in the pro-
portion of four to seven in the upper halves of the two
jaws of the plaice, to twenty-five to thirty in the lower
halves, may likewise be accounted for by disuse. From
the colorless state of the ventral surface of most fishes
and of many other animals we may reasonably suppose
that the absence of color in flat-fish on the side, whether
it be the right or left, which is undermost, is due to the
exclusion of light. But it cannot be supposed that the
peculiar speckled appearance of the upper side of the
sole, so like the sandy bed of the sea, or the power in
some species, as recently shown by Pouchet, of changing
their color in accordance with the surrounding surface,
or the presence of bony tubercles on the upper side of
the turbot, are due to the action of the light. Here
natural selection has probably come into play, as well
as in adapting the general shape of the body of these
fishes, and many other peculiarities, to their habits of
life. We should keep in mind, as I have before insisted,
that the inherited effects of the increased use of parts,
and perhaps of their disuse, will be strengthened by
natural selection. For all spontaneous variations in the
right direction will thus be preserved; as will those
individuals which inherit in the highest degree the effects
of the increased and beneficial use of any part. How
much to attribute in each particular case to the effects of
use, and how much to natural selection, it seems impos-
sible to decide.
OBJECTIONS TO THE THEORY
321
I may give another instance of a structure which ap-
parently owes its origin exclusively to use or habit. The
extremity of the tail in some American monkeys has
been converted into a wonderfully perfect prehensile
organ, and serves as a fifth hand. A reviewer who
agrees with Mr. Mivart in every detail remarks on this
structure: "It is impossible to believe that in any num-
ber of ages the first slight incipient tendency to grasp
could preserve the lives of the individuals possessing it,
or favor their chance of having and of rearing offspring."
But there is no necessity for any such belief. Habit, and
this almost implies that some benefit great or small is
thus derived, would in all probability suffice for the
work. Brehm saw the young of an African monkey
(Cercopithecus) clinging to the under surface of their
mother by their hands, and at the same time they hooked
their little tails round that of their mother. Professor
Henslow kept in confinement some harvest mice (Mus
messorius) which do not possess a structurally prehensile
tail; but he frequently observed that they curled their
tails round the branches of a bush placed in the cage,
and thus aided themselves in climbing. I have received
an analogous account from Dr. Gunther, who has seen a
mouse thus suspend itself. If the harvest mouse had
been more strictly arboreal, it would perhaps have had
its tail rendered structurally prehensile, as is the case with
some members of the same order. Why Cercopithecus,
considering its habits while young, has not become thus
provided, it would be difficult to say. It is, however,
possible that the long tail of this monkey may be of
more service to it as a balancing organ in making its
prodigious leaps than as a prehensile organ.
322
THE ORIGIN OF SPECIES
The mammary glands are common to the whole class
of mammals, and are indispensable for their existence;
they must, therefore, have been developed at an ex-
tremely remote period, and we can know nothing posi-
tively about their manner of development. Mr. Mivart
asks: *'Is it conceivable that the young of any animal
was ever saved from destruction by accidentally sucking
a drop of scarcely nutritious fluid from an accidentally
hypertrophied cutaneous gland of its mother? And even
if one was so, what chance was there of the perpetuation
of such a variation?" But the case is not here put
fairly. It is admitted by most evolutionists that mam-
mals are descended from a marsupial form; and if so, the
mammary glands will have been at first developed within
the marsupial sack. In the case of the fish (Hippo-
campus) the eggs are hatched, and the young are reared
for a time, within a sack of this nature; and an Ameri-
can naturalist, Mr. Lockwood, believes, from what he has
seen of the development of the young, that they are
nourished by a secretion from the cutaneous glands of
the sack. Now with the early progenitors of mammals,
almost before they deserved to be thus designated, is it
not at least possible that the young might have been
similarly nourished? And in this case, the individuals
which secreted a fluid, in some degree or manner the most
nutritious, so as to partake of the nature of milk, would
in the long run have reared a larger number of well-
nourished offspring than would the individuals which
secreted a poorer fluid; and thus the cutaneous glands,
which are the homologues of the mammary glands, would
have been improved or rendered more effective. It ac-
cords with the widely extended principle of specializa-
OBJECTIONS TO THE THEORY
323
tion, that the glands over a certain space of the sack
should have become more highly developed than the
remainder; and they would then have formed a breast,
but at first without a nipple, as we see in the Ornitho-
rhynchus, at the base of the mammalian series. Through
what agency the glands over a certain space became more
highly specialized than the others, I will not pretend to
decide, whether in part through compensation of growth,
the effects of use, or of natural selection.
The development of the mammary glands would have
been of no service, and could not have been effected
through natural selection, unless the young at the same
time were able to partake of the secretion. There is no
greater difficulty in understanding how young mammals
have instinctively learned to suck the breast than in
understanding how unhatched chickens have learned to
break the eggshell by tapping against it with their spe-
cially adapted beaks; or how, a few hours after leaving
the shell, they have learned to pick up grains of food.
In such cases the most probable solution seems to be,
that the habit was at first acquired by practice at a more
advanced age, and afterward transmitted to the offspring
at an earlier age. But the young kangaroo is said not to
suck, only to cling to the nipple of its mother, who has
the power of injecting milk into the mouth of her help*
less, half-formed offspring. On this head Mr. Mivart re-
marks: 4 'Did no special provision exist, the young one
must infallibly be choked by the intrusion of the milk
into the windpipe. But there is a special provision. The
larynx is so elongated that it rises up into the posterior
end of the nasal passage, and is thus enabled to give free
entrance to the air for the lungs, while the milk passes
S24
THE ORIGIN OF SPECIES
harmlessly on each side of this elongated larynx, and so
safely attains the gullet behind it." Mr. Mivart then
asks how did natural selection remove, in the adult kan-
garoo (and in most other mammals, on the assumption
that they are descended from a marsupial form), "this at
least perfectly innocent and harmless structure?" It may
be suggested in answer that the voice, which is certainly
of high importance to many animals, could hardly have
been used with full force as long as the larynx entered
the nasal passage; and Professor Flower has suggested to
me that this structure would have greatly interfered with
an animal swallowing solid food.
We will now turn for a short space to the lower
divisions of the animal kingdom. The Echinodermata
(star-fishes, sea-urchins, etc.) are furnished with remark-
able organs, called pedicellarisa, which consist, when well
developed, of a tridactyle forceps — that is, of one formed
of three serrated arms, neatly fitting together and placed
on the summit of a flexible stem, moved by muscles.
These forceps can seize firm hold of any object; and
Alexander Agassiz has seen an Echinus or sea-urchin
rapidly passing particles of excrement from forceps to
forceps down certain lines of its body, in order that its
shell should not be fouled. But there is no doubt that,
besides removing dirt of all kinds, they subserve other
functions; and one of these apparently is defence.
With respect to these organs, Mr. Mivart, as on so
many previous occasions, asks: "What would be the
utility of the first rudimentary beginnings of such struct-
ures, and how could such incipient buddings have ever
preserved the life of a single Echinus?" He adds, "not
even the sudden development of the snapping action could
OBJECTIONS TO THE THEORY
325
have been beneficial without the freely movable stalk,
nor could the latter have been efficient without the snap-
ping jaws, yet no minute merely indefinite variations
could simultaneously evolve these complex co-ordinations
of structure; to deny this seems to do no less than to
affirm a startling paradox." Paradoxical as this may
appear to Mr. Mivart, tridactyle forcepses, immovably
fixed at the base, but capable of a snapping action,
certainly exist on some star-fishes; and thi3 is intel-
ligible if they serve, at least in part, as a means of
defence. Mr. Agassiz, to whose great kindness I am
indebted for much information on the subject, informs
me that there are other star-fishes, in which one of the
three arms of the forceps is reduced to a support for the
other two; and again, other genera in which the third
arm is completely lost. In Echinoneus, the shell is de-
scribed by M. Perrier as bearing two kinds of pedicel-
lariaa, one resembling those of Echinus, and the other
those of Spatangu-; and such cases are always interest-
ing as affording the means of apparently sudden tran-
sitions, through the abortion of one of the two states
of an organ.
With respect to the steps by which these curious
organs have bsen evolved, Mr. Agassiz infers, from his
own researches and those of Miiller, that both in star-
fishes and sea-urchins the pedicellariae must undoubtedly
be looked at as modified spines. This may be inferred
from their manner of development in the individual, as
well as from a long and perfect series of gradations in
different species and genera, from simple granules to
ordinary spines, to perfect tridactyle pedicellarise. The
gradation extends even to the manner in which ordinary
328
THE ORIGIN OF SPECIES
spines and the pedicellariae with their supporting calcare-
ous rods are articulated to the shell. In certain genera
of star-fishes, 4 'the very combinations needed to show
that the pedicellariae are only modified branching spines"
may be found. Thus we have fixed spines, with three
equidistant, serrated, movable branches, articulated to
near their bases; and higher up, on the same spine,
three other movable branches. Now when the latter
arise from the summit of a spine they form in fact
a rude tridactyle pedicel! aria, and such may be seen on
the same spine together with the three lower branches.
In this case the identity in nature between the arms of
the pedicellaria3 and the movable branches of a spine is
unmistakable. It is generally admitted that the ordinary
spines serve as a protection; and if so, there can be no
reason to doubt that those furnished with serrated and
movable branches likewise serve for the same purpose;
and they would thus serve still mere effectively as soon
is by meeting together they acted as a prehensile or
snapping apparatus. Thus every gradation, from an ordi
nary fixed spine to a fixed pedicellaria, would be of
service.
In certain genera of star-fishes these organs, instead
of being fixed or borne on an immovable support, are
placed on the summit of a flexible and muscular, though
short, stem; and in this case they probably subserve some
additional function besides defence. In the sea-urchins
the steps can be followed by which a fixed spine be-
comes articulated to the shell, and is thus rendered mov-
able. I wish I had space here to give a fuller abstract
of Mr. Agassiz's interesting observations on the develop-
ment of the pedicellariae. All possible gradations, as he
OBJECTIONS TO THE THEORY
327
adds, may likewise be found between the pedicellariae of
the star-fishes and the hooks of the Ophiurians, another
group of the Echinodermata; and again between the pedi-
cellariae of sea-urchins and the anchors of the Holothuriae,
also belonging to the same great class.
Certain compound animals, or zoophytes as they have
been termed, namely the Polyzoa, are provided with cu-
rious organs called avicularia. These differ much in
structure in the different species. In their most perfect
condition, they curiously resemble the head and beak
of a vulture in miniature, seated on a neck and capa-
ble of movement, as is likewise the lower jaw or man-
dible. In one species observed by me all the avicularia
on the same branch often moved simultaneously back-
ward and forward, with the lower jaw widely open,
through an angle of about 90°, in the course of five
seconds; and their movement caused the whole polyzoary
to tremble. When the jaws are touched with a needle
they seize it so firmly that the branch can thus be
shaken.
Mr. Mivart adduces this case, chiefly on account of
the supposed difficulty of organs, namely the avicularia
of the Polyzoa and the pedicellariae of the Echinoder-
mata, which he considers as 4 4 essentially similar," having
been developed through natural selection in widely dis-
tinct divisions of the animal kingdom. But, as far as
structure is concerned, I can see no similarity between
tridactyle pedicellariae and avicularia. The latter resem-
ble somewhat more closely the chelae or pincers of Crus-
taceans; and Mr. Mivart might have adduced with equal
appropriateness this resemblance as a special difficulty; or
328
THE ORIGIN OF SPECIES
even their resemblance to the head and beak of a bird.
The avicularia are believed by Mr. Busk, Dr. Smitt, and
Dr. Nitsche — naturalists who have carefully studied this
group — to be homologous with the zooids and their cells
which compose the zoophyte; the movable lip or lid of
the cell corresponding with the lower and movable man-
dible of the avicularium. Mr. Busk, however, does not
know of any gradations now existing between a zooid
and an avicularium. It is therefore impossible to con-
jecture by what serviceable gradations the one could
have been converted into the other: but it by no means
follows from this that such gradations have not existed.
As the chelae of Crustaceans resemble in some degree
the avicularia of Polyzoa, both serving as pincers, it may
be worth while to show that with the former a long
series of serviceable gradations still exists. In the first
and simplest stage, the terminal segment of a limb shuts
down either on the square summit of the broad penulti-
mate segment, or against one whole side; and is thus
enabled to catch hold of an object; but the limb still
serves as an organ of locomotion. We next find one
corner of the broad penultimate segment slightly promi-
nent, sometimes furnished with irregular teeth; and
against these the terminal segment shuts down. By an
increase in the size of this projection, with its shape, as
well as that of the terminal segment, slightly modified
and improved, the pincers are rendered more and more
perfect, until we have at last an instrument as efficient
as the chelae of a lobster; and all these gradations can
be actually traced.
Besides the avicularia, the Polyzoa possess curious
organs called vibracula. These generally consist of long
OBJECTIONS TO THE THEORY
329
bristles, capable of movement and easily excited. In one
species examined by me the vibracula were slightly
curved and serrated along the outer margin; and all of
them on the same polyzoary often moved simultaneously;
so that, acting like long oars, they swept a branch
rapidly across the object-glass of my microscope. When
a branch was placed on its face, the vibracula became
entangled, and they made violent efforts to free them-
selves. They are supposed to serve as a defence, and
may be seen, as Mr. Busk remarks, "to sweep slowly and
carefully over the surface of the polyzoary, removing
what might be noxious to the delicate inhabitants of the
cells when their tentacula are protruded.' ' The avicu-
laria, like the vibracula, probably serve for defence, but
they also catch and kill small living animals, which it
is believed are afterward swept by the currents within
reach of the tentacula of the zooids. Some species are
provided with avicularia and vibracula; some with avicu-
laria alone, and a few with vibracula alone.
It is not easy to imagine two objects more widely
different in appearance than a bristle or vibraculum, and
an avicularium like the head of a bird; yet they are
almost certainly homologous and have been developed
from the same common source; namely, a zooid with its
cell. Hence we can understand how it is that these
organs graduate in some cases, as I am informed by
Mr. Busk, into each other. Thus with the avicularia of
/everal species of Lepralia, the movable mandible is so
much produced and is so like a bristle, that the presence
of the upper or fixed beak alone serves to determine its
avicularian nature. The vibracula may have been directly
developed from the lips of the cells, without having
330
THE ORIGIN OF SPECIES
passed through the avicularian stage; but it seems more
probable that they have passed through this stage, as,
during the early stages of the transformation, the other
parts of the cell with the included zooid could hardly
have disappeared at once. In many cases the vibracula
have a grooved support at the base, which seems to
represent the fixed beak; though this support in some
species is quite absent. This view of the development of
the vibracula, if trustworthy, is interesting; for supposing
that all the species provided with avicularia had become
extinct, no one with the most vivid imagination would
ever have thought that the vibracula had originally
existed as part of an organ, resembling a bird's head or
an irregular box or hood. It is interesting to see two
such widely different organs developed from a common
origin; and as the movable lip of the cell serves as a
protection to the zooid, there is no difficulty in believing
that all the gradations by which the lip became con-
verted first into the lower mandible of an avicularium
and then into an elongated bristle, likewise served as
a protection in different ways and under different cir-
cumstances.
In the vegetable kingdom Mr. Mivart only alludes to
two cases; namel}7, the structure of the flowers of orchids,
and the movements of climbing plants. With respect to
the former, he says, "the explanation of their origin
is deemed thoroughly unsatisfactory — utterly insufficient
to explain the incipient, infinitesimal beginnings of struc-
tures which are of utility only when they are consider-
ably developed." As I have fully treated this subject
in another work, I will here give only a few detaib on
OBJECTIONS TO THE THEORY
331
one alone of the most striking peculiarities of the
flowers of orchids; namely, their pollinia. A pollinium
when highly developed consists of a mass of pollen-
grains, affixed to an elastic foot-stalk or caudicle, and
this to a little mass of extremely viscid matter. The
pollinia are by this means transported by insects from
one flower to the stigma of another. In some orchids
there is no caudicle to the pollen-masses, and the grains
are merely tied together by fine threads; bat as these are
not confined to orchids, they need not here be considered;
yet I may mention that at the base of the orchidaceous
series, in Cypripedium, we can see how the threads were
probably first developed. In other orchids the threads
cohere at one end of the pollen-masses; and this forms
the first or nascent trace of a caudicle. That this is the
origin of the caudicle, even when of considerable length
and highly developed, we have good evidence in the
aborted pollen-grains which can sometimes be detected
imbedded within the central and solid parts.
With respect to the second chief peculiarity, namely,
the little mass of viscid matter attached to the end of the
caudicle, a long series of gradations can be specified,
each of plain service to the plant. In most flowers
belonging to other orders the stigma secretes a little
viscid matter. Now in certain orchids similar viscid
matter is secreted, but in much larger quantities by one
alone of the three stigmas; and this stigma, perhaps in
consequence of the copious secretion, is rendered sterile.
When an insect visits a flower of this kind, it rubs off
some of the viscid matter and thus at the same time
drags away some of the pollen-grains. From this simple
condition, which differs but little from that of a multitude
832 THE ORIGIN OF SPECIES
of common flowers, there are endless gradations — to
species in which the pollen-mass terminates in a very
short, free caudicle — to others in which the caudicle
becomes firmly attached to the viscid matter, with the
sterile stigma itself much modified. In this latter case
we have a pollinium in its most highly developed and
perfect condition. He who will carefully examine the
flowers of orchids for himself will not deny the existence
of the above series of gradations — from a mass of pollen-
grains merely tied together by threads, with the stigma
differing but little from that of an ordinary flower, to a
highly complex pollinium, admirably adapted for trans-
portal by insects; nor will he deny that all the gradations
in the several species are admirably adapted in relation
to the general structure of each flower for its fertilization
by different insects. In this, and in almost every other
case, the inquiry may be pushed further backward; and
it may be asked, how did the stigma of an ordinary
flower become viscid? But as we do not know the full
history of any one group of beings, it is as useless to
ask, as it is hopeless to attempt answering, such questions.
We will now turn to climbing plants. These can be
arranged in a long series, from those which simply twine
round a support to those which I have called leaf-
climbers, and to those provided with tendrils. In these
two latter classes the stems have generally, but not
always, lost the power of twining, though they retain the
power of revolving, which the tendrils likewise possess.
The gradations from leaf -climbers to tendril-bearers are
wonderfully close, and certain plants may be indifferently
placed in either class. But in ascending the series from
simple twiners to leaf-climbers, an important quality is
OBJECTIONS TO THE THEORY
333
added; namely, sensitiveness to a touch, by which means
the foot-stalks of the leaves or flowers, or these modified
and converted into tendrils, are excited to bend round
and clasp the touching object. He who will read my
memoir on these plants will, I think, admit that all the
many gradations in function and structure between simple
twiners and tendril-bearers are in each case beneficial in
a high degree to the species. For instance, it is clearly
a great advantage to a twining plant to become a leaf-
climber; and it is probable that every twiner which
possessed leaves with long foot-stalks would have been
developed into a leaf-climber, if the foot-stalks had pos-
sessed in any slight degree the requisite sensitiveness to
a touch.
As twining is the simplest means of ascending a
iupport, and forms the basis of our series, it may natu-
rally be asked how did plants acquire this power in an
incipient degree, afterward to be improved and increased
through natural selection? The power of twining depends,
first, on the stems while young being extremely flexible
(but this is a character common to many plants which are
not climbers); and, secondly, on their continually bending
to all points of the compass, one after the other in suc-
cession, in the same order. By this movement the stems
are inclined to all sides, and are made to move round
and round. As soon as the lower part of a stem strikes
against any object and is stopped, the upper part still
goes on bending and revolving, and thus necessarily
twines round and up the support. The revolving move-
ment ceases after the early growth of each shoot. As in
many widely separated families of plants, single species
and single genera possess the power of revolving, and
S34
THE ORIGIN OF SPECIES
have thus become twiners, they must have independently
acquired it, and cannot have inherited it from a common
progenitor. Hence I was led to predict that some slight
tendency to a movement of this kind would be found to
be far from uncommon with plants which did not climb;
and that this had afforded the basis for natural selection
to work on and improve. When I made this prediction,
I knew of only one imperfect case; namely, of the young
flower-peduncles of a Maurandia which revolved slightly
and irregularly, like the stems of twining plants, but
without making any use of this habit. Soon afterward
Fritz Miiller discovered that the young stems of an
Alisma and of a Linum — plants which do not climb and
are widely separated in the natural system — revolved
plainly, though irregularly; and he states that he has
reason to suspect that this occurs with some other plants.
These slight movements appear to be of no service to the
plants in question; anyhow, they are not of the least use
in the way of climbing, which is the point that concerns
us. Nevertheless we can see that if the stems of these
plants had been flexible, and if under the conditions to
which they are exposed it had profited them to ascend
to a height, then the habit of slightly and irregularly
revolving might have been increased and utilized through
natural selection until they had become converted into
well -developed twining species.
With respect to the sensitiveness of the foot-stalks of
the leaves and flowers, and of tendrils, nearly the same
remarks are applicable as in the case of the revolving
movements of twining plants. As a vast number of spe-
cies, belonging to widely distinct groups, are endowed
with this kind of sensitiveness, it ought to be found in a
OBJECTIONS TO THE THEORY
335
nascent condition in many plants which have not become
climbers. This is the case: I observed that the young
flower peduncles of the above Maurandia curved them-
selves a little toward the side which was touched. Mor-
ren found in several species of Oxalis that the leaves and
their foot-stalks moved, especially after exposure to a hot
sun, when they were gently and repeatedly touched, or
when the plant was shaken. I repeated these observa-
tions on some other species of Oxalis with the same
result; in some of them the movement was distinct, but
was best seen in the young leaves; in others it was ex-
tremely slight. It is a more important fact that, according
to the high authority of Hofmeister, the young shoots and
leaves of all plants move after being shaken; and with
climbing plants it is, as we know, only during the early
stages of growth that the foot-stalks and tendrils are
sensitive.
It is scarcely possible that the above slight move-
ments, due to a touch or shake, in the young and grow-
ing organs of plants, can be of any functional importance
to them. But plants possess, in obedience to various
stimuli, powers of movement, which are of manifest im-
portance to them; for instance, toward and more rarely
from the light — in opposition to, and more rarely in the
direction of, the attraction of gravity. When the nerves
and muscles of an animal are excited by galvanism or by
the absorption of strychnine, the consequent movements
may be called an incidental result, for the nerves and
muscles have not been rendered specially sensitive to
these stimuli. So with plants it appears that, from
having the power of movement in obedience to certain
stimuli, they are excited in an incidental manner by a
336
THE ORIGIN OF SPECIES
touch, or by being shaken. Hence there is no great
difficulty in admitting that, in the case of leaf -climbers
and tendril-bearers, it is this tendency which has been
taken advantage of and increased through natural selec-
tion. It is, however, probable, from reasons which I have
assigned in my memoir, that this will have occurred only
with plants which had already acquired the power of
revolving, and had thus become twiners.
I have already endeavored to explain how plants
became twiners, namely, by the increase of a tendency
to slight and irregular revolving movements, which were
at first of no use to them; this movement, as well as that
due to a touch or shake, being the incidental result of
the power of moving, gained for other and beneficial
purposes. Whether, during the gradual development of
climbing plants, natural selection has been aided by the
inherited effects of use, I will not pretend to decide; but
we know that certain periodical movements, for instance
the so-called sleep of plants, are governed by habit.
I have now considered enough, perhaps more than
enough, of the cases, selected with care by a skilful nat-
uralist, to prove that natural selection is incompetent to
account for the incipient stages of useful structures; and
I have shown, as I hope, that there is no great difficulty
on this head. A good opportunity has thus been afforded
for enlarging a . little on gradations of structure, often
associated with changed functions — an important subject,
which was not treated at sufficient length in the former
editions of this work. I will now briefly recapitulate the
foregoing cases.
With the giraffe, the continued preservation of the
OBJECTIONS TO THE THEORY
337
individuals of some extinct high-reaching ruminant, which
had the longest necks, legs, etc., and could browse a
little above the average height, and the continued de-
struction of those which could not browse so high, would
have sufficed for the production of this remarkable quad-
ruped; but the prolonged use of all the parts together
with inheritance will have aided in an important manner
in their co-ordination. With the many insects which
imitate various objects, there is no improbability in the
belief that an accidental resemblance to some common
object was in each case the foundation for the work of
natural selection, since perfected through the occasional
preservation of slight variations which made the resem-
blance at all closer; and this will have been carried on
as long as the insect continued to vary, and as long as
a more and more perfect resemblance led to its escape
from sharp-sighted enemies. In certain species of whales
there is a tendency to the formation of irregular little
points of horn on the palate; and it seems to be quite
within the scope of natural selection to preserve all favor-
able variations, until the points were converted first into
lamellated knobs or teeth, like those on the beak of a
goose — then into short lamellae, like those of the domestic
ducks — and then into lamellae as perfect as those of the
shoveller-duck — and finally into the gigantic plates of
baleen, as in the mouth of the Greenland whale. In the
family of the ducks, the lamellae are first used as teeth,
then partly as teeth and partly as a sifting apparatus,
and at last almost exclusively for this latter purpose.
With such structures as the above lamellae of horn or
whalebone, habit or use can have done little or nothing,
as far as we can judge, toward their development. On
—Science — 15
338
THE ORIGIN OF SPECIES
the other hand, the transportal of the lower eye of a flat-
fish to the upper side of the head, and the formation of
a prehensile tail, may be attributed almost wholly to con-
tinued use, together with inheritance. With respect to
the mammae of the higher animals, the most probable
conjecture is that primordially the cutaneous glands over
the whole surface of a marsupial sack secreted a nutri-
tious fluid; and that these glands were improved in func-
tion through natural selection, and concentrated into a
confined area, in which case they would have formed
a mamma. There is no more difficulty in understanding
how the branched spines of some ancient Echinoderm,
which served as a defence, became developed through
natural selection into tridactyle pedicellariae, than in un-
derstanding the development of the pincers of crustaceans,
through slight, serviceable modifications in the ultimate
and penultimate segments of a limb, which was at first
used solely for locomotion. In the avicularia and vibrac-
ula of the Polyzoa we have organs widely different in
appearance developed from the same source; and with
the vibracula we can understand how the successive gra-
dations might have been of service. With the pollinia
of orchids, the threads which originally served to tie
together the pollen-grains can be traced cohering into
caudicles; and the steps can likewise be followed by
which viscid matter, such as that secreted by the stigmas
of ordinary flowers, and still subserving nearly but not
quite the same purpose, became attached to the free ends
of the caudicles; — all these gradations being of manifest
benefit to the plants in question. With respect to climb-
ing plants, I need not repeat what has been so lately
said.
OBJECTIONS TO THE THEORY
339
It has often been asked, if natural selection be so
potent why has not this or that structure been gained
by certain species, to which it would apparently have
been advantageous ? But it is unreasonable to expect a
precise answer to such questions, considering our igno-
rance of the past history of each species, and of the con-
ditions which at the present day determine its numbers
and range. In most cases only general reasons, but in
some few cases special reasons, can be assigned. Thus to
adapt a species to new habits of life many co-ordinated
modifications are almost indispensable, and it may often
have happened that the requisite parts did not vary in
the right manner or to the right degree. Many species
must have been prevented from increasing in numbers
through destructive agencies, which stood in no relation
to certain structures, which we imagine would have been
gained through natural selection from appearing to us
advantageous to the species. In this case, as the struggle
for life did not depend on such structures, they could not
have been acquired through natural selection. In many
cases complex and long-enduring conditions, often of a
peculiar nature, are necessary for the development of
a structure; and the requisite conditions may seldom
have concurred. The belief that any given structure,
which we think, often erroneously, would have been
beneficial to a species, would have been gained under
ail circumstances through natural selection, is opposed to
what we can understand of its manner of action. Mr.
Mivart does not deny that natural selection has effected
something; but he considers it as "demoustrably insuffi-
cient" to account for the phenomena which I explain by
its agency. His chief arguments have now been consid-
340
THE ORIGIN OF SPECIES
ered, and the others will hereafter be considered. They
seem to me to partake little of the character of demon-
stration, and to have little weight iD comparison with
those in favor of the power of natural selection, aided by
the other agencies often specified. I am bound to add,
that some of the facts and arguments here used by me
have been advanced for the same purpose in an able arti-
cle lately published in the "Medico-Chirurgical Review."
At the present day almost all naturalists admit evolu-
tion under some form. Mr. Mivart believes that species
change through "an internal force or tendency," about
which it is not pretended that anything is known. That
species have a capacity for change will be admitted by
all evolutionists; but there is no need, as it seems to me,
to invoke any internal force beyond the tendency to ordi-
nary variability, which through the aid of selection by
man has given rise to many well-adapted domestic races,
and which through the aid of natural selection would
equally well give rise by graduated steps to natural races
or species. The final result will generally have been, as
already explained, an advance, but in some few cases a
retrogression, in organization.
Mr. Mivart is further inclined to believe, and some
naturalists agree with him, that new species manifest
themselves "with suddenness and by modifications appear-
ing at once." For instance, he supposes that the differ-
ences between the extinct three-toed Hipparion and the
horse arose suddenly. He thinks it difficult to believe
that the wing of a bird "was developed in any other way
than by a comparatively sudden modification of a marked
and important kind"; and apparently he would extend
the same view to the wings of bats and pterodactyles.
OBJECTIONS TO THE THEORY
341
This conclusion, which implies great breaks or discon-
tinuity in the series, appears to me improbable in the
highest degree.
Every one who believes in slow and gradual evolu-
tion will of course admit that specific changes may have
been as abrupt and as great as any single variation which
we meet with under nature, or even under domestication.
But as species are more variable when domesticated or
cultivated than under their natural conditions, it is not
probable that such great and abrupt variations have often
occurred under nature as are known occasionally to arise
under domestication. Of these latter variations several
may be attributed to reversion; and the characters which
thus reappear were, it is probable, in many cases at
first gained in a gradual manner. A still greater num-
ber must be called monstrosities, such as six-fingered
men, porcupine men, Ancon sheep, Niata cattle, etc.;
and as they are widely different in character from nat-
ural species, they throw very little light on our subject.
Excluding such cases of abrupt variations, the few which
remain would at best constitute, if found in a state of*
nature, doubtful species, closely related to their parental
types.
My reasons for doubting whether natural species have
changed as abruptly as have occasionally domestic races,
and for entirely disbelieving that they have changed in
the wonderful manner indicated by Mr. Mivart, are as fol-
lows. According to our experience, abrupt and strongly
marked variations occur in our domesticated productions,
singly and at rather long intervals of time. If such oc-
curred under nature, they would be liable, as formerly
explained, to be lost by accidental causes of destruction
342
THE ORIGIN OF SPECIES
and by subsequent intercrossing; and so it is known to
be under domestication, unless abrupt variations of this
kind are specially preserved and separated by the care of
man. Hence in order that a new species should suddenly
appear in the manner supposed by Mr. Mivart, it is al-
most necessary to believe, in opposition to all analogy,
that several wonderfully changed individuals appeared
simultaneously within the same district. This difficulty,
as in the case of unconscious selection by man, is avoided
on the theory of gradual evolution, through the preserva-
tion of a large number of individuals, which varied more
or less in any favorable direction, and of the destruction
of a large number which varied in an opposite manner.
That many species have been evolved in an extremely
gradual manner there can hardly be a doubt. The spe-
cies and even the genera of many large natural families
are so closely allied together that it is difficult to distin-
guish not a few of them. On every continent in pro-
ceeding from north to south, from lowland to upland,
etc., we meet with a host of closely related or repre-
sentative species; as we likewise do on certain distinct
continents, which we have reason to believe were for-
merly connected. But in making these and the follow-
ing remarks, I am compelled to allude to subjects here-
after to be discussed. Look at the many outlying islands
round a continent, and see how man}* of their inhabitants
can be raised only to the rank of doubtful species. So
it is if we look to past times, and compare the species
which have just passed away with those still living
within the same areas; or if we compare the fossil spe-
cies imbedded in the sub-stages of the same geological
formation. It is indeed manifest that multitudes of spe-
OBJECTIONS TO THE THEORY
343
cies are related in the closest manner to other species
that still exist, or have lately existed; and it will hardly
be maintained that such species have been developed in
an abrupt or sudden manner. Nor should it be forgotten,
when we look to the special parts of allied species, in-
stead of to distinct species, that numerous and wonder-
fully fine gradations can be traced, connecting together
widely different structures.
Many large groups of facts are intelligible only on the
principle that species have been evolved by very small
steps. For instance, the fact that the species included in
the larger genera are more closely related to each other,
and present a greater number of varieties than do the
species in the smaller genera. The former are also
grouped in little clusters, like varieties round species;
and they present other analogies with varieties, as was
shown in our second chapter. On this same principle
we can understand how it is that specific characters are
more variable than generic characters; and how the parts
which are developed in an extraordinary degree or man-
ner are more variable than other parts of the same spe-
cies. Many analogous facts, all pointing in the same
direction, could be added.
Although very many species have almost certainly
been produced by steps not greater than those separating
fine varieties, yet it may be maintained that some have
been developed in a different and abrupt manner. Such
an admission^ however, ought not to be made without
strong evidence being assigned. The vague and in some
respects false analogies, as they have been shown to be
by Mr. Chauncey Wright, which have been advanced in
favor of this view, such as the sudden crystallization of
344
THE ORIGIN OF SPECIES
inorganic substances, or the falling of a facetted spheroid
from one facet to another, hardly deserve consideration.
One class of facts, however, namely, the sudden appear-
ance of new and distinct forms of life in our geological
formations, supports at first sight the belief in abrupt
development. But the value of this evidence depends
entirely on the perfection of the geological record, in
relation to periods remote in the history of the world.
If the record is as fragmentary as many geologists stren-
uously assert, there is nothing strange in new forms
appearing as if suddenly developed.
Unless we admit transformations as prodigious as
those advocated by Mr. Mivart, such as the sudden
development of the wings of birds or bats, or the sud-
den conversion of a Hipparion into a horse, hardly any
light is thrown by the belief in abrupt modifications on
the deficiency of connecting links in our geological for-
mations. But against the belief in such abrupt changes
embryology enters a strong protest. It is notorious that
the wings of birds and bats, and the legs of horses or
other quadrupeds, are indistinguishable at an early em-
bryonic period, and that they become differentiated by
insensibly fine steps. Embryological resemblances of all
kinds can be accounted for, as we shall hereafter see,
by the progenitors of our existing species having varied
after early youth, and having transmitted their newly
acquired characters to their offspring, at a correspond-
ing age. The embryo is thus left almost unaffected, and
serves as a record of the past condition of the species.
Hence it is that existing species during the early stages
of their development so often resemble ancient and ex-
tinct forms belonging to the same class. On this view
OBJECTIONS TO THE THEORY
345
of the meaning of embryological resemblances, and in-
deed on any view, it is incredible that an animal should
have undergone such momentous and abrupt transforma-
tions as those above indicated; and yet should not bear
even a trace in its embryonic condition of any sudden
modification; every detail in its structure being developed
by insensibly fine steps.
He who believes that some ancient form was trans-
formed suddenly through an internal force or tendency
into, for instance, one furnished with wings, will be al-
most compelled to assume, in opposition to all analogy,
that many individuals varied simultaneously. It cannot
be denied that such abrupt and great changes of struct-
ure are widely different from those which most species
apparently have undergone. He will further be com-
pelled to believe that many structures beautifully adapted
to all the other parts of the same creature and to the
surrounding conditions, have been suddenly produced;
and of such complex and wonderful co-adaptations he
will not be able to assign a shadow of an explanation.
He will be forced to admit that these great and sudden
transformations have left no trace of their action on the
embryo. To admit all this is, as it seems to me, to enter
into the realms of miracle, and to leave those of Science.
346
THE ORIGIN OF SPECIES
CHAPTER VIII
INSTINCT
Instincts comparable with habits, but different in their origin — Instincts
graduated — Aphides and ants — Instincts variable — Domestic instincts,
their origin — Natural instincts of the cuckoo, molotnrus, ostrich, and
parasitic bees — Slave-making ants — Hive-bee, its cell-making instinct —
Changes of instinct and structure not necessarily simultaneous — Diffi-
culties of the theory of the Natural Selection of instincts — Neuter or
sterile insects — Summary
MANY instincts are so wonderful that their devel-
opment will probably appear to the reader a dif-
ficulty sufficient to overthrow my whole theory.
I may here premise, that I have nothing to do with the
origin of the mental powers, any more than I have with
that of life itself. We are concerned only with the
diversities of instinct and of the other mental faculties
in animals of 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 we ourselves require experience
to enable us to perform, when performed by an animal,
more especially by a very young one, without experience,
and when performed by many individuals in the same
way, without their knowing for what purpose it is per-
INSTINCT
347
formed, is usually said to be instinctive. But I could
show that none of these characters are universal. A lit-
tle dose of judgment or reason, as Pierre Huber ex-
presses it, often comes into play, even with animals
low in the scale of nature.
Frederick Cuvier and several of the older metaphy-
sicians have compared instinct with habit. This compari-
son gives, I think, an accurate notion of the frame of
mind under which an instinctive action is performed, but
not necessarily 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, 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 any-
thing by rote, he is generally forced to go back to re-
cover the habitual train of thought; so P. Huber found
it was with a caterpillar, which makes a very complicated
hammock; for if he took a caterpillar which had com-
pleted its hammock up to, say, the sixth stage of con-
struction, and put it into a hammock completed up only
to the third stage, the caterpillar simply re-performed the
fourth, fifth, and sixth stages of construction. If, how-
ever, 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 deriving any ben-
348
THE ORIGIN OF SPECIES
efit from this, it was much embarrassed, and in order to
complete its hammock seemed forced to start from the
third stage, where it had left off, and thus tried to com-
plete the already finished work.
If we suppose any habitual action to become inherited
— and 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 distin-
guished. If Mozart, instead of playing the pianoforte
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
a serious error to suppose that the greater number of
instincts have been acquired by habit in one generation,
and then transmitted by inheritance to succeeding genera-
tions. 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 acquired by habit.
It will be universally admitted that instincts are as
important as corporeal structures for the welfare of each
species, under its present conditions of life. Under
changed conditions of life, it is at least possible that
slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary
ever so little, then I can see no difficulty in natural
selection preserving and continually accumulating varia-
tions of instinct to any extent that was profitable. It is
thus, as I believe, that all the most complex and won-
derful instincts have originated. As modifications of cor-
poreal structure arise from, and are increased by, use or
habit, and are diminished or lost by disuse, so I do not
INSTINCT
349
doubt it has been with instincts. But I believe that the
effects of habit are in many cases of subordinate impor-
tance to the effects of the natural selection of what may
be called spontaneous variations of instincts; — that is, of
variations produced by the same unknown causes which
produce slight deviations of bodily structure.
No complex instinct can possibly be produced through
natural selection, except by the slow and gradual 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 possible; and this we certainly can do. I have been
surprised to find, making allowance for the instincts of
animals having been but little observed except in Europe
and North America, and for no instinct being known
among extinct species, how very generally gradations,
leading to the most complex instincts, can be discov-
ered. Changes of instinct may sometimes be facilitated
by the same species having different instincts at different
periods of life, or at different seasons of the year, or
when placed under different circumstances, etc.; in which
case either the one or the other instinct might be pre-
served by natural selection. And such instances of di-
versity of instinct in the same species can be shown
to occur in nature.
Again, as in the case of corporeal structure, and con-
formably to my theory, the instinct of each species is
350
THE ORIGIN OF SPECIES
good for itself, bat 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 perform-
ing an action for the sole good of another, with which
I am acquainted, is that of aphides voluntarily yielding,
as was first observed by Huber, their sweet excretion
to ants: that they do so voluntarily, the following facts
show. I removed all the ants from a group of about
a dozen aphides on a dock-plant, and prevented their
attendance during several hours. After this interval,
I felt sure that the aphides would want to excrete. I
watched them for some time through a lens, but not one
excreted; I then tickled and stroked them with a hair in
the same manner, as well as I could, as the ants do with
their antennae; but not one excreted. Afterward I al-
lowed an ant to visit them, and it immediately seemed,
by its eager way of running about, to be well aware
what a rich flock it had discovered; it then began to
play with its antennas on the abdomen first of one aphis
and then of another; and each, as soon as it felt the
antennae, immediately lifted up its abdomen and excreted
a limpid drop of sweet juice, which was eagerly devoured
by the ant. Even the quite young aphides behaved in
this manner, showing that the action was instinctive and
not the result of experience. It is certain, from the ob-
servations of Huber, that the aphides show no dislike
to the ants: if the latter be not present they are at last
compelled to eject their excretion. But as the excretion
is extremely viscid, it is no doubt a convenience to the
aphides to have it removed; therefore probably they do
not excrete solely for the good of the ants. Although
there is no evidence that any animal performs an action
INSTINCT
351
for the exclusive good of another species, yet each tries
to take advantage of the instincts of others, as each takes
advantage of the weaker bodily structure of other species.
So again certain instincts cannot be considered as abso-
lutely 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
instances as possible ought to be given; but want of
space prevents me. I can only assert that instincts cer-
tainly do vary — for instance, the migratory instinct, both
in extent and direction, and in its total loss. So it is
with the nests of birds, which vary partly in dependence
on the situations chosen, and on the nature and temper-
ature of the country inhabited, but often from causes
wholly unknown to us: Audubon has given several re-
markable cases of differences in the nests of the same
species in the northern and southern United States.
Why, it has been asked, if instinct be variable, has
it not granted to the bee "the ability to use some other
material when wax was deficient"? But what other natu-
ral material could bees use? They will work, as I have
seen, with wax hardened with vermilion or softened with
lard. Andrew Knight observed that his bees, instead of
laboriously collecting propolis, used a cement of wax and
turpentine, with which he had covered decorticated trees.
It has lately been shown that bees, instead of searching
for pollen, will gladly use a very different substance,
namely oatmeal. Fear of any particular enemy is cer-
tainly an instinctive quality, as may be seen in nestling
S52
THE ORIGIN OF SPECIES
birds, though it is strengthened by experience, and by
the sight of fear of the same enemy in other animals.
The fear of man is slowly acquired, as I have elsewhere
shown, by the various animals which inhabit desert isl-
ands; and we see an instance of this even in England, in
the greater wildness of all our large birds in comparison
with 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 uninhab-
ited islands large birds are not more fearful than small;
and the magpie, so wary in England, is tame in Norway,
as is the hooded crow in Egypt.
That the mental qualities of animals of the same kind,
born in a state of nature, vary much, could be shown by
many facts. Several cases could also be adduced of occa-
sional and strange habits in wild animals, which, if ad-
vantageous to the species, might have given rise, through
natural selection, to new instincts. But I am well aware
that these general statements, without the facts in detail,
will produce but a feeble effect on the reader's mind. I
can only repeat my assurance that I do not speak with-
out good evidence.
Inherited Changes of Habit or Instinct in Domesticated
Animals
The possibility, or even probability, 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 be enabled to see the part which habit
and the selection of so-called spontaneous variations have
played in modifying the mental qualities of our domestic
animals. It is notorious how much domestic animals vary
INSTINCT
353
in their mental qualities. With cats, for instance, one
naturally takes to catching rats, and another mice, and
these tendencies are known to be inherited. One cat,
according to Mr. St. John, always brought home game-
birds, another hares or rabbits, and another hunted on
marshy ground and almost nightly caught woodcocks or
snipes. A number of curious and authentic instances
could be given of various shades of disposition and of
taste, and likewise of the oddest tricks, associated with
certain frames of mind or periods of time, being in-
herited. But let us look to the familiar case of the
breeds of the 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 mas-
ter 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 behold one kind of wolf, when young and without
any training, as soon as it scented its prey, stand mo-
tionless 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,
354
THE ORIGIN OF SPECIES
are certainly far less fixed than natural instincts; but
they have been acted on by far less rigorous selection,
and have been transmitted for an incomparably shorter
period, under less fixed conditions of life.
How strongly these domestic instincts, habits, and dis-
positions are inherited, and how curiously they become
mingled, is well shown when different breeds of dogs are
crossed. Thus it is known that a cross with a bulldog
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 par-
entage 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 is not true. No one
would ever have thought of teaching, or probably could
have taught, the tumbler-pigeon to tumble — an action
which, as I have witnessed, is performed by young birds
that have never seen a pigeon tumble. We may believe
that some one pigeon showed a slight tendency to this
strange habit, and that the long-continued selection of the
best individuals in successive generations made tumblers
what they now are; and near Glasgow there are house
tumblers, as I hear from Mr. Brent, which cannot fly
eighteen inches high without going head over heels. It
INSTINCT
355
may be doubted whether any one would have thought of
training a dog to point, had not some one dog naturally
shown a tendency in this line; and this is known occa-
sionally to happen, as I once saw, in a pure terrier: the
act of pointing is probably, as many have thought, only
the exaggerated pause of an animal preparing to spring
on its prey. When the first tendency to point was once
displayed, methodical selection and the inherited effects of
compulsory training in each successive generation would
soon complete the work; and unconscious selection is still
in progress, as each man tries to procure, without intend-
ing to improve the breed, dogs which stand and hunt
best. On the other hand, habit alone in some cases has
sufficed; hardly any animal is more difficult to tame than
the young of the wild rabbit; scarcely any animal is
tamer than the young of the tame rabbit; but I can
hardly suppose that domestic rabbits have often been
selected for tameness alone; so that we must attribute
At least the greater part of the inherited change from
extreme wildness to extreme tameness to habit and long-
continued close confinement.
Natural 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 largely and how permanently the
minds of our domestic animals have been modified. It
is scarcely possible to doubt that the love of man has
become instinctive in the dog. All wolves, foxes, jack-
als, 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
856
THE ORIGIN OF SPECIES
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 civilized dogs, even when quite
young, require to be taught not to attack poultry, sheep,
and pigs! No doubt they occasionally do make an attack,
and are then beaten; and if not cured, they are destroyed;
so that habit and some degree of selection have probably
concurred in civilizing 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 origi-
nally instinctive in them; for I am informed by Captain
Hutton that the young chickens of the parent-stock, the
Gallus bankiva, when reared in India under a hen, are
at first excessively wild. So it is with young pheasants
reared in England under a hen. It is not that chickens
have lost all fear, but fear only of dogs and cats, for if
the hen gives the danger-chuckle, they will run (more
especially young turkeys) from under her, and conceal
themselves in the surrounding grass or thickets; and this
is evidently done for the instinctive purpose of allowing,
as we see in wild ground-birds, their mother to fly away.
But this instinct retained by our chickens has become
useless under domestication, for the mother-hen has
almost lost by disuse the power of flight.
Hence, we may conclude that under domestication
instincts have been acquired, and natural instincts have
been lost, partly by habit, and partly by man selecting
and accumulating, during successive generations, peculiar
mental habits and actions, which at first appeared from
what we must in our ignorance call an accident. In
some cases compulsory habit alone has sufficed to pro-
INSTINCT
357
duce 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 habit and selection have probably
concurred.
Special Instincts
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 — namely,
the instinct which leads the cuckoo to lay her eggs in
other birds' nests; the slave-making instinct of certain
ants; and the cell-making power of the hive-bee. These
two latter instincts have generally and justly been ranked
by naturalists as the most wonderful of all known
instincts.
Instincts of the Cuckoo. — It is supposed by some natu-
ralists that the more immediate cause of the instinct of
the cuckoo is that she lays her eggs, not daily, but at
intervals of two or three days; so that, if she were to
make her own nest and sit on her own eggs, those first
laid would have to be left for some time unincubated, or
there would be eggs and young birds of different ages in
the same nest. If this were the case, the process of
laying and hatching might be inconveniently long, more
especially as she migrates 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 both asserted and denied that the American
cuckoo occasionally lays her eggs in other birds' nests;
but I have lately heard from Dr. Merrell of Iowa that
358
THE ORIGIN OF SPECIES
he once found in Illinois a young cuckoo together with
a young jay in the nest of a Blue jay (Grarrulus cris-
tatus); and as both were nearly full feathered, there
could be no mistake in their identification. I could
also give several instances of various birds which have
been known occasionally to lay their eggs in other birds'
nests. Now let us suppose that the ancient progenitor
of our European cuckoo had the habits of the American
cuckoo, and that she occasionally laid an egg in another
bird's nest. If the old bird profited by this occasional
habit through being enabled to migrate earlier or through
any other cause; or if the young were made more vigor-
ous by advantage being taken of the mistaken instinct of
another species than when reared by their own mother,
incumbered as she could hardly fail to be by having
eggs and young of different ages at the same time; then
the old birds or the fostered young would gain an ad-
vantage. And analogy would lead us to believe that
the young thus reared would be apt to follow by inheri-
tance 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 more successful in rearing their
young. By a continued process of this nature, I believe
that the strange instinct of our cuckoo has been gen-
erated. It has, also, recently been ascertained on suffi-
cient evidence, by Adolf Muller, that the cuckoo occa-
sionally lays her eggs on the bare ground, sits on them,
and feeds her young. This rare event is probably a
case of reversion to the long-lost, aboriginal instinct
of nidification.
It has been objected that I have not noticed other
related instincts and adaptations of structure in the
INSTINCT
359
cuckoo, which are spoken of as necessarily co-ordinated.
But in all cases speculation on an instinct known to us
only in a single species is useless, for we have hitherto
had no facts to guide us. Until recently the instincts of
the European and of the non-parasitic American cuckoo
alone were known; now, owing to Mr. Kamsay's observa-
tions, we have learned something about three Australian
species which lay their eggs in other birds' nests. The
chief points to be referred to are three: first, that the
common cuckoo, with rare exceptions, lays only one egg
in a nest, so that the large and voracious young bird
receives ample food. Secondly, that the eggs are remark-
ably small, not exceeding those of the skylark — a bird
about one-fourth as large as the cuckoo. That the small
size of the egg is a real case of adaptation we may infer
from the fact of the non-parasitic American cuckoo lay-
ing full-sized eggs. Thirdly, that the young cuckoo, soon
after birth, has the instinct, the strength, and a properly
shaped back for ejecting its foster-brothers, which then
perish from cold and hunger. This has been boldly
called a beneficent arrangement, in order that the young
cuckoo may get sufficient food, and that its foster broth-
ers may perish before they had acquired much feeling!
Turning now to the Australian species; though these
birds generally lay only one egg in a nest, it is not rare
to find two and even three eggs in the same nest. In
the Bronze cuckoo the eggs vary greatly in size, from
eight to ten lines in length. Now if it had been of an
advantage to this species to have laid eggs even smaller
than those now laid, so as to have deceived certain foster-
parents, or, as is more probable, to have been hatched
within a shorter period (for it is asserted that there is a
860
THE 0R1GLS OF SPECIES
relation between the size of eggs and the period of their
incubation), then there is no difficulty in believing that
a race or species might have been formed which would
have laid smaller and smaller eggs; for these would have
been more safely hatched and reared. Mr. Ramsay re-
marks that two of the Australian cuckoos, when they lay
their eggs in an open nest, manifest a decided preference
for nests containing eggs similar in color to their own.
The European species apparently manifests some tendency
toward a similar instinct, but not rarely departs from it,
as is shown by her laying her dull and pale-colored eggg
in the nest of the Hedge- warbler with bright greenish-
blue eggs. Had our cuckoo invariably displayed the
above instinct, it would assuredly have been added to
those which it is assumed must all have been acquired
together. The eggs of the Australian Bronze cuckoo
vary, according to Mr. Ramsay, to an extraordinary de-
gree in color; so that in this respect, as well as in
size, natural selection might have secured and fixed any
advantageous variation.
In the case of the European cuckoo, the offspring
of the foster-parents are commonly ejected from the nest
within three days after the cuckoo is hatched; and as the
latter at this age is in a most helpless condition, Mr.
Gould was formerly inclined to believe that the act of
ejection was performed by the foster-parents themselves.
But he has now received a trustworthy account of a
young cuckoo which was actually seen, while still blind
and not able even to hold np its own head, in the act
of ejecting its foster brothers. One of these was replaced
in the nest by the observer, and was again thrown oat.
With respect to the means by which this strange and
INSTIXCT
361
odious instinct was acquired, if it were of great impor-
tance for the young cuckoo, as is probably the case, to
receive as much food as possible soon after birth, I can
see no special difficulty in its having gradually ac-
quired, during successive generations, the blind desire,
the strength and structure necessary for the work of
ejection; for those young cuckoos which had such habits
and structure best developed would be the most securely
reared. The first step toward the acquisition of the
proper instinct might have been mere unintentional rest-
lessness on the part of the young bird, when somewhat
advanced in age and strength; the habit having been
afterward improved, and transmitted to an earlier age.
I can see no more difficulty in this than in the un-
hatched young of other birds acquiring the instinct to
break through their own shells; or than in young
snakes acquiring in their upper jaws, as Owen has re-
marked, a transitory sharp tooth for cutting through the
tough egg-shell. For if each part is liable to individual
variations at all ages, and the variations tend to be in-
herited at a corresponding or earlier age — propositions
which cannot be disputed — then the instincts and struct-
ure of the young could be slowly modified as surely as
those of the adult; and both cases must stand or fall
together with the whole theory of natural selection.
Some species of Molothrus, a widely distinct genus of
American birds, allied to our starlings, have parasitic
habits like those of the cuckoo; and the species present
an interesting gradation in the perfection of their in-
stincts. The sexes of Molothrus badius are stated by
an excellent observer, Mr. Hudson, sometimes to live
promiscuously together in flocks, and sometimes to pair.
—Science — 16
362
THE ORIGIN OF SPECIES
They either build a nest of their own, or seize on one
belonging to some other bird, occasionally throwing out
the nestlings of the stranger. They either lay their eggs
in the nest thus appropriated, or oddly enough build one
for themselves on the top of it. They usually sit on
their own eggs and rear their own young; but Mr. Hud-
son says it is probable that they are occasionally para-
sitic, for he has seen the young of this species following
old birds of a distinct kind and clamoring to be fed by
them. The parasitic habits of another species of Molo-
thrus, the M. bonariensis, are much more highly devel-
oped than those of the last, but are still far from perfect.
This bird, as far as it is known, invariably lays its eggs
in the nests of strangers; but it is remarkable that sev-
eral together sometimes commence to build an irregular
untidy nest of their own, placed in singularly ill-adapted
situations, as on the leaves of a large thistle. They
never, however, as far as Mr. Hudson has ascertained,
complete a nest for themselves. They often lay so many
eggs — from fifteen to twenty — in the same foster-nest that
few or none can possibly be hatched. They have, more-
over, the extraordinary habit of pecking holes in the
eggs, whether of their own species or of their foster-
parents, which they find in the appropriated nests. They
drop also many eggs on the bare ground, which are thus
wasted. A third species, the M. pecoris of North Amer-
ica, has acquired instincts as perfect as those of the
cuckoo, for it never lays more than one egg in a
foster-nest, so that the young bird is securely reared.
Mr. Hudson is a strong disbeliever in evolution, but he
appears to have been so much struck by the imperfect
instincts of the Molothrus bonariensis that he quotes my
INSTINCT
363
words, and asks, 14 Must we consider these habits, not
as especially endowed or created instincts, but as small
consequences of one general law, namely, transition?"
"Various birds, as has already been remarked, occa-
sionally lay their eggs in the nests of other birds. This
habit is not very uncommon with the Gallinaceae, and
throws some light on the singular instinct of the ostrich.
In this family several hen-birds 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 with the cuckoo, at intervals of
two or three days. The instinct, however, of the Ameri-
can ostrich, as in the case of the Molothrus bonariensis,
has not as yet been perfected; for a surprising number
of eggs lie strewed over the plains, so that in one day's
hunting I picked up no less than twenty lost and wasted
eggs.
Many bees are parasitic, and regularly lay their eggs
in the nests of other kinds of bees. This case is more
remarkable than that of the cuckoo; for these bees have
not only had their instincts but their structure modified
in accordance with their parasitic habits; for they do not
possess the pollen-collecting apparatus which would have
been indispensable if they had stored up food for their
own young. Some species of Sphegidse (wasp- like in-
sects) are likewise parasitic; and M. Fabre has lately
shown good reason for believing that, although the
Tachytes nigra generally makes its own burrow and
stores it with paralyzed prey for its own larvae, yet
that, when this insect finds a barrow already made and
stored by another sphex, it takes advantage of the prize,
THE ORIGIN OF SPECIES
and becomes for the occasion parasitic. In this case, as
with that of the Molothrus or cuckoo, 1 can see no diffi-
culty in natural selection making an occasional habit per-
manent, if of advantage to the species, and if the insect
whose nest and stored food are feloniously appropriated
be not thus exterminated.
Slave-making instinct. — This remarkable instinct was
first discovered in the Formica (Polyerges) rufescens
by Pierre Huber, a better observer even than his
celebrated father. This ant is absolutely dependent
on its slaves; without their aid, the species would cer-
tainly become extinct in a single year. The males
and fertile females do no work of any kind, and the
workers or sterile females, though most energetic and
courageous in capturing slaves, do no other work.
They are incapable of making their own nests, or of
feeding their own larvae. When the old nest is found
inconvenient, and they have to migrate, it is the slaves
which determine the migration, and actually carry their
masters in their jaws. So utterly helpless are the mas-
ters, that when Huber shut up thirty of them without a
slave, but with plenty of the food which they like best,
and with their own larvae and pupae to stimulate them to
work, they did nothing; they could not even feed them-
selves, and many perished of hunger. Huber then intro-
duced a single slave (F. fusca), and she instantly set to
work, fed and saved the survivors; made some cells and
tended the larvae, and put all to rights. What can be
more extraordinary than these well-ascertained facts? If
we had not known of any other slave-making ant, it
would have been hopeless to speculate how so wonderful
an instinct could have been perfected.
INSTINCT
365
Another species, Formica sanguinea, was likewise first
discovered by P. Huber to be a slave-making ant. This
species is found in the southern parts of England, and
its habits have been attended to by Mr. F. Smith, of the
British Museum, to whom I am much indebted for infor-
mation on this and other subjects. Although fully trust-
ing to the statements of Huber and Mr. Smith, I tried to
approach the subject in a sceptical frame of mind, as any
one mav well be excused for doubting the existence of
so extraordinary an instinct as that of making slaves.
Hence, I will give the observations which I made in
some little detail. I opened fourteen nests of F. san-
guinea, and found a few slaves in all. Males and fertile
females of the slave species (F. fusca) are found only in
their own proper communities, and have never been ob-
served in the nests of F. sanguinea. The slaves are
black and not above half the size of their red masters,
so that the contrast in their appearance is great. When
the nest is slightly disturbed 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 pupae are exposed, the slaves work energetically to-
gether 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 watched for many hours sev-
eral nests in Surrey and Sussex, and never saw a slave
either leave or enter a nest. As, during these months,
the slaves are very few in number, I thought that they
might behave differently when more numerous; but Mr.
Smith informs me that he has watched the nests at vari-
ous hours during May, June, and August, both in Surrey
S66
THE ORIGIN OF SPECIES
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 con-
stantly seen bringing in materials for the nest, and food
of all kinds. During the year 1860, however, in the
month of July, I came across a community with an un-
usually large stock of slaves, and I observed a few slaves
mingled with their masters leaving the nest, and march-
ing along the same road to a tall Scotch firtree, twenty-
five yards distant, which they ascended together, prob-
ably in search of aphides or cocci. According to Huber,
who had ample opportunities for observation, the slaves
in Switzerland habitually work with their masters in
making the nest, and they alone open and close the doors
in the morning and evening; and, as Huber expressly
states, their principal office is to search for aphides.
This difference in the usual habits of the masters and
slaves in the two countries probably depends merely on
the slaves being captured in greater numbers in Switzer-
land than in England.
One day I fortunately witnessed a migration of F.
sanguinea from one nest to another, and it was a most
interesting spectacle to behold the masters carefully car-
rying their slaves in their jaws instead of being carried
by them, as in the case of 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 re-
pulsed by an independent community of the slave-species
(F. fusca); sometimes as many as three of these ants
clinging to the legs of the slave-making F. sanguinea.
INSTINCT
367
The latter ruthlessly killed their small opponents, and
carried their dead bodies as food to their nest, twenty-
nine yards distant; but they were prevented from getting
any pupae to rear as slaves. I then dug up a small par-
cel of the pupae of F. fusca from another nest, and put
them down on a bare spot near the place of combat; they
were eagerly seized and carried off by the tyrants, who
perhaps fancied that, after all, they had been victorious
in their late combat.
At the same time I laid on the same place a small
parcel of the pupae of another species, F. flava, with a
few of these little yellow ants still clinging to the frag-
ments of their 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 coura-
geous, and I have seen it ferociously attack other ants. In
one instance I found to my surprise an independent com-
munity of F. flava under a stone beneath a nest of the
slave-making F. sanguinea; and when I had accidentally
disturbed both nests, the little ants attacked their big
neighbors with surprising courage. Now I was curious
to ascertain whether F. sanguinea could distinguish the
pupae of F. fusca, which they habitually make into
slaves, from those of the little and furious F. flava,
which they rarely capture, and it was evident that they
did at once distinguish them; for we have seen that
they eagerly and instantly seized the pupae of F. fusca,
whereas they were much terrified when they came across
the pupae, or even the earth from the nest, of F. flava,
and quickly ran away; but in about a quarter of an
hour, shortly after all the little yellow ants had crawled
away, they took heart and carried off the pupae.
368
THE ORIGIN OF SPECIES
One evening I visited another community of F.
sanguinea, and found a number of these ants returning
home and entering their nests, carrying the dead bodies
of F. fusca (showing that it was not a migration) and
numerous pupae. I traced a long file of ants burdened
with booty, for about forty yards back, to a very thick
clump of heath, whence I saw the last individual of F.
sanguinea emerge, carrying a pupa; but I was not able
to find the desolated nest in the thick heath. The nest,
however, must have been close at hand, for two or three
individuals of F. fusca were rushing about in the greatest
agitation, and one was perched motionless with its own
pupa in its mouth on the top of a spray of heath, 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 numerous
slaves. Formica sanguinea, on the other hand, possesses
much fewer slaves, and in the early part of the summer
extremely few: the masters determine when and where a
new nest shall be formed, and when they migrate, the
masters carry the slaves. Both in Switzerland and Eng-
land the slaves seem to have the exclusive care of the
larvaB, and the masters alone go on slave- making expedi-
tions. In Switzerland the slaves and masters work to-
gether, making and bringing materials for the nest; both,
but chiefly the slaves, tend, and milk, as it may be called,
INSTINCT
369
their aphides; and thus both collect food for the com-
munity. In England the masters alone usually leave the
nest to collect building materials and food for themselves,
their slaves and larva?. 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 the pupse
of other species, if scattered near their nests, it is possi-
ble that such pupae originally stored as food might be-
come developed; and the foreign ants thus unintentionally
reared would then follow their proper instincts, and do
what work they could. If their presence proved useful
to the species which had seized them — if it were more
advantageous to this species to capture workers than to
procreate them — the habit of collecting pupae, originally
for food, might by natural selection be strengthened and
rendered permanent for the very different purpose of
raising slaves. When the instinct was once acquired, if
carried out to a much less extent even than in our
British F. sanguinea, which, as we have seen, is less
aided by its slaves than the same species in Switzerland,
natural selection might increase and modify the instinct —
always supposing each modification to be of use to the
species — until an ant was formed as abjectly dependent
on its slaves as is the Formica rufescens.
Cell-making instinct of the Hive- Bee. — I will not here
enter on minute details on this subject, but will merely
give an outline of the conclusions at which I have
arrived. He must be a dull man who can examine the
exquisite structure of a comb, so beautifully adapted to
570
THE ORIGIN OF SPECIES
its end, without enthusiastic admiration. "We hear from
mathematicians that bees have practically solved a recon-
dite 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 construction. It has been remarked that a skilful
workman with fitting tools and measures would find it
very difficult to make cells of wax of the true form,
though this is effected by a crowd of bees working in a
dark hive. Granting whatever instincts you please, 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
simple instincts.
I was led to investigate this subject by Mr. TVater-
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
principle of gradation, and see whether Nature does not
reveal to us her method of work. At one end of a short
series we have humble-bees, which use their old cocoons
to hold honey, sometimes adding to them short tubes of
wax, and likewise making separate and very irregular
rounded cells of wax. At the other end of the series we
have the cells of the hive-bee, placed in a double layer:
each cell, as is well known, is a hexagonal prism, with
the basal edges of its six sides bevelled so as to join an
inverted pyramid, of three rhombs. These rhombs have
certain angles, and the three which form the pyramidal
INSTINCT
371
base of a single cell on one side of the comb enter into
the composition of the bases of three adjoining cells
on the opposite side. In the series between the extreme
perfection of the cells of the hive-bee and the simplicity
of those of the humble-bee we have the cells of the
Mexican Melipona domestica, carefully described and
figured by Pierre Huber. The Melipona itself is inter-
mediate in structure between the hive and humble-bee,
but more nearly related to the latter; it forms a nearly
regular waxen comb of cylindrical cells, in which the
young are hatched, and, in addition, some large cells of
wax for holding honey. These latter cells are nearly
spherical and of nearly equal sizes, and are aggregated
into an irregular mass. But the important point to notice
is that these cells are always made at that degree of
nearness to each other that they would have intersected
or broken into each other if the spheres had been com-
pleted; but this is never permitted, the bees building
perfectly flat walls of wax between the spheres which
thus tend to intersect. Hence, each cell consists of an
outer spherical portion, and of two, three, or more flat
surfaces, according as the cell adjoins two, three, or more
other cells. When one cell rests on 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
H aber has remarked, is manifestly a gross imitation of
the three sided pyramidal base of the cell of the hive-
bee. As in the cells of the hive-bee, so here, the three
plane surfaces in any one cell necessarily enter into the
construction of three adjoining cells. It is obvious that
the Melipona saves wax, and, what is more important,
572
THE ORIGIX OF SPECIES
labor, by this manDer of building; for the flat walls be-
tween 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 hare been as perfect as
the comb of the hive-bee. Accordingly, I wrote to Pro-
fessor Miller of Cambridge, and this geometer has kindly
read over the following statement, drawn up from his
information, and tells me that it is strictly correct:
If a number of equal spheres be described with their
centres placed in two parallel layers; with the centre of
each sphere at the distance of radius X ^ 2, or radius
X 1-41421 (or at some lesser distance), from the centres
of the six surrounding spheres in the same layer; and at
the same distance from the centres of the adjoining
spheres in the other and parallel layer; then, if planes
of intersection between the several spheres in both layers
be formed, there will result a double layer of hexagonal
prisms united together by pyramidal bases formed of
three rhombs; and the rhombs and the sides of the
hexagonal prisms will have every angle identically the
same with the best measurements which have been made
of the cells of the hive-bee. But I hear from Prof.
Wyman, who has made numerous careful measurements,
that the accuracy of the workmanship of the bee has
been greatly exaggerated; so much so, that whatever the
typical form of the cell may be, it is rarely, if ever,
realized
INSTINCT
373
Hence we may safely conclude that, if we conld
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
have the power of forming her cells truly spherical, and
of equal sizes; and this would not be very surprising,
seeing that she already does so to a certain extent, and
seeing what perfectly cylindrical burrows many insects
make in wood, apparently by turning round on a fixed
point. We must suppose the Melipona to arrange her
cells in level layers, as she already does her cylindrical
cells; and we must further suppose, and this is the
greatest difficulty, that she can somehow judge accurately
at what distance to stand from her fellow-laborers when
several are making their spheres; but she is already so
far enabled to judge of distance that she always describes
her spheres so as to intersect to a certain extent; and
then she unites the points of intersection by perfectly
flat surfaces. By such modifications of instincts which in
themselves are not very wonderful — hardly more wonder-
ful than those which guide a bird to make its nest — I
believe that the hive-bee has acquired, through natural
selection, her inimitable architectural powers.
But this theory can be tested by experiment. Follow-
ing the example of Mr. Tegetmeier, I separated two
combs, and put between them a long, thick, rectangular
strip of wax: the bees instantly began to excavate minute
circular pits in it; and as they deepened these little pits,
they made them wider and wider until they were con-
verted into shallow basins, appearing to the eye perfectly
true or parts of a sphere, and of about the diameter of a
374
THE ORIGIN OF SPECIES
cell. It was most interesting to observe that, wherever
several bees had begun to excavate these basins near
together, they had begun their work at such a distance
from each other that by the time the basins had ac-
quired the above-stated width (i.e., about the width of
an ordinary cell), and were in depth about one-sixth
of the diameter of the sphere of which they formed a
part, the rims of the basins intersected or broke into
each other. As soon as this occurred, the bees ceased to
excavate, and began to build up flat walls of wax on the
lines of intersection between the basins, so that each
hexagonal prism was built upon the scalloped 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, rectangu-
lar piece of wax, a thin and narrow, knife-edged ridge,
colored with vermilion. The bees instantly began on
both sides to excavate little basins near to each other,
in the same way as before; but the ridge of wax was so
thin that the bottoms of the basins, if they had been
excavated to the same depth as in the former experiment,
would have broken into each other from the opposite
sides. The bees, however, did not suffer this to happen,
and they stopped their excavations in due time; so that
the basins, as soon as they had been a little deepened,
came to have flat bases; and these flat bases, formed by
thin little plates of the vermilion wax left ungnawed,
were situated, as far as the eye could judge, exactly
along the planes of imaginary intersection between the
basins on the opposite sides of the ridge of wax. In
some parts, only small portions, in other parts, large
portions of a rhombic plate were thus left between the
INSTINCT
375
opposed basins, but the work, from the unnatural state
of things, had not been neatly performed. The bees
must have worked at very nearly tbe same rate in circu-
larly gnawing away and deepening the basins on both
sides of the ridge of vermilion wax, in order to have
thus succeeded in leaving flat plates between the basins,
by stopping work at the planes of intersection.
Considering how flexible thin wax is, I do not see
that there is any difficulty in the bees, while at work on
the two sides of a strip of wax, perceiving when they
have gnawed the wax away to the proper thinness, and
then stopping their work. In ordinary combs it has ap-
peared to me that the bees do not always succeed in
working at exactly the same rate from the opposite sides;
for I have noticed half-completed rhombs at the base of
a just commenced cell, -which were slightly concave cn
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 in-
stance, I put the comb back into the hive, and allowed
the bees to go on working for a short time, and again
examined the cell, and I found that the rhombic plate
had been completed, and had become perfectly fiat: it was
absolutely impossible, from the extreme thinness of the
little plate, that they could have effected this by gnawing
away the convex side; and I suspect that the bees in
such cases stand on opposite sides and push and bend
the ductile and warm wax (which, as I have tried, is
easily done) into its proper intermediate plane and thus
flatten it.
From the experiment of the ridge of vermilion wax
we can see that, if the bees were to build for themselves a
S76
THE ORIGiy OF SPECIES
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 endeavor-
ing to make equal spherical hollows, but never allowing
the spheres to break into each other. Now bees, as may
be clearly seen by examining the edge of a growing
comb, do make a rough, circumferential wall or rim all
round the comb: and they gnaw this away from the op-
posite sides, always working circularly as they deepen
each cell. They do not make the whole three-sided
pyramidal base of any one cell at the same time, but
only that one rhombic plate which stands on the extreme
growing margin, or the two plates as the case may be;
and they never complete the upper edges of the rhombic
plates until the hexagonal walls are commenced. Some
of these statements differ from those made by the justly
celebrated elder Huber, but I am convinced of their
accuracy; and if I had space. I could show that they are
conformable with my theory.
Huber's statement, that the very first cell is excavated
out of a little parallel-sided wall of wax. is not, as far
as I have seen, strictly correct: the first commencement
havine alwavs been a little hood of wax: bat I will not
<— .
here enter on details. We see how important a part
excavation plays in the construction of the cells; but it
would be a great error to suppose that the bees cannot
build up a rough wall of wax in the proper position —
that is. along the plane of intersection between two ad-
joining spheres. I have several specimens showing
clearly that they can do this. Even in the rude circum-
ferential rim or wall of wax round a growing comb,
flexures may sometimes be observed, corresponding in
LSST1XCT
37?
position to the planes of the rhombic basal plates of
future cells. But the rough wall of wax has in every
case to be finished off, by being largely gnawed away
on both sides. The manner in which the bees build is
curious; they always make the first rough wall from ten
to twenty times thicker than the excessively thin finished
wall of the cell, which will ultimately be left. We shall
understand how they work, by supposing masons first to
pile up a broad ridge of cement, and then to begin cut-
ting 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 bat always
crowned by a gigantic coping. From all the cells, both
those just commenced and those completed, being thus
crowned by a strong coping of wax, the bees can cluster
and crawl over the comb without injuring the delicate
hexagonal walls. These walls, as Professor Miller has
kindly ascertained for me, vary greatly in thickness;
being, on an average of twelve measurements made near
the border of the comb, gh of an inch in thickness;
whereas the basal rhomboidal plates are thicker, nearly
in the proportion of three to two, having a mean thick-
ness, from twenty-one measurements, of tW of an inch.
By the above singular manner of building, strength is
continually given to the comb, with the utmost ultimate
economy of wax.
It seems at first to add to the difficulty of understand-
ing how the cells are made, that a multitude of bees all
work together; one bee after working a short time at one
cell going to another, so that, as Huber has stated, a
THE ORIGIN OF SPECIES
score of individuals work even at the commencement of
the first cell. I was able practically to show this fact,
by covering the edges of the hexagonal walls of a single
cell, or the extreme margin of the circumferential rim of
a growing comb, with an extremely thin layer of melted
vermilion wax; and I invariably found that the color was
most delicately diffused by the bees — as delicately as a
painter could have done it with his brush — by atoms of
the colored wax having been taken from the spot on
which it had been placed and worked into the growing
edges of the cells all round. The work of construction
seems to be a sort of balance struck between many bees,
all instinctively standing at the same relative distance
from each other, all trying to sweep equal spheres, and
then building up, or leaving ungnawed, the planes of
intersection between these spheres. It was really curi-
ous to note in cases of difficulty, as when two pieces
of comb met at an angle, how often the bees would
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 downward, 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 com-
pleted 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
INSTINCT
379
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 sub-
versive of the foregoing theory; namely, that the cells on
the extreme margin of wasp-combs are sometimes strictly
hexagonal; but I have not space here to enter on this
subject. Nor does there seem to me any great difficulty
in a single insect (as in the case of a queen-wasp) mak-
ing hexagonal cells, if she were to 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, sweep-
ing spheres or cylinders, and building up intermediate
planes.
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 succes.
sion of modified architectural instincts, all tending toward
the present perfect plan of construction, could have prof-
ited the progenitors of the hive-bee ? I think the answer
is not difficult: cells constructed like those of the bee or
the wasp gain in strength, and save much in labor and
space, and in the materials of which they are constructed.
With respect to the formation of wax, it is known that
oees are often hard pressed to get sufficient nectar, and I
am informed by Mr. Tegetmeier that it has been experi-
mentally proved that from twelve to fifteen pounds of
380 THE ORIGIN OF SPECIES
dry sugar are consumed by a hive of bees for the secre-
tion of a pound of wax; so that a prodigious quantity of
fluid nectar must be collected and consumed by the bees
in a hive for the secretion of the wax necessary for the
construction of their combs. Moreover, many bees have
to remain idle for many days during the process of se-
cretion.
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 num-
ber of bees being supported. Hence the saving of wax
by largely saving honey and the time consumed in col-
lecting the honey must be an important element of suc-
cess to any family of bees. Of course the success of
the species may be dependent on the number of its
enemies, or parasites, or on quite distinct causes, and
so be altogether independent of the quantity of honey
which the bees can collect. But let us suppose that this
latter circumstance determined, as it probably often has
determined, whether a bee allied to our humble-bees
could exist in large numbers in any country; and let us
further suppose that the community lived through the
winter, and consequently required a store of honey: there
can in this case be no doubt that it would be an advan-
tage to our imaginary humble-bee if a slight modification
in her instincts 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 labor
and wax.
Hence it would continually be mo^e and more ad-
vantageous to our humble-bees if they were to make
their cells more and more regular, nearer together, and
INSTINCT
381
aggregated into a mass, like the cells of the Melipona;
for in this case a large part of the bounding surface of
each cell would serve to bound the adjoining cells, and
much labor and wax would be saved. Again, from the
same cause, it would be advantageous to the Melipona
if she were to make her cells closer together, and more
regular in every way than at present; for then, as we
have seen, the spherical surfaces would wholly disappear
and 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 economizing
labor and wax.
Thus, as I believe, the most wonderful of all known
instincts, that of the hive-bee, can be explained by natu-
ral selection having taken advantage of numerous succes-
sive, slight modifications of simpler instincts; natural se-
lection 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 the con-
struction of cells of due strength and of the proper size
and shape for the larvae, this being effected with the
greatest possible economy of labor and wax; that indi-
vidual swarm which thus made the best cells with least
labor, and least waste of honey in the secretion of wax.
332 THE ORIGIN OF SPECIES
Laving sucee-edei best, and having transmitted their
newly acquired economical instincts to new
which in their tarn will hare had the best chance
c: 5 e e :~ iz.e s::zzz\z ::r exigence.
(Mjtrtim to the Theory of Natural Selection as appliei to
Instincts: Sevier and Sterile Insects
It has been objected to the foregoing view of the
origin of instincts that '"the variations of structure and
of insure* mas; have been simultaneous and accurately
adjusted to each other, as a modification in the one
without am immediate corresponding change in the other
would have been fatal/* The force of this objection rests
entirely on the assumption that the changes in the inmim In
and structure are abrupt To take as an illustration the
case of the larger titmouse (Paras major) alluded to in a
previous chapter: this bird often holds the seeds of the
yew between it3 feet on a branch, and hammers with its
beak till it gets at the keraeL Now what special diffi-
culty would there be in natural selection preserving all
tike slight individual variations in the shape of the beaky
which were better and better adapted to break open the
seeis, until a beak was formed as well constructed for
this purpose as that of the nuthatch, at the same time
that habit, or compulsion, or spontaneous variations of
taste, led the bird to become more and more of a seed-
eater?
In this case the beak is supposed to be slowly
mc-Iided by narxrai selection, subsequently to, but in
accordance with, slowly changing habits or taste: but let
toe feet of the titmouse vary and grow larger from cor-
relation with the beak, or from any other unknown cause,
INSTINCT
383
and it is not improbable that such larger feet would lead
the bird to climb more and more until it acquired the
remarkable climbing instinct and power of the nuthatch.
In this case a gradual change of structure is supposed to
lead to changed instinctive habits. To take one more
case: few instincts are more remarkable than that which
leads the swift of the Eastern Islands to make its nest
wholly of inspissated saliva. Some birds build their nests
of mud, believed to be moistened with saliva; and one of
the swifts of North America makes its nest (as I have
seen) of sticks agglutinated with saliva, and even with
flakes of this substance. Is it then very improbable that
the natural selection of individual swifts, which secreted
more and more saliva, should at last produce a species
with instincts leading it to neglect other materials, and
to make its nest exclusively of inspissated saliva? And
so in other cases. It must, however, be admitted that
in many instances we cannot conjecture whether it was
instinct or structure which first varied.
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
have originated; cases, in which no intermediate grada-
tions are known to exist; cases of instincts of 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 progenitor, and consequently must be-
lieve that they were independently acquired through nat-
ural selection. I will not here enter on these several
cases, but will confine myself to one special difficulty,
384:
THE ORIGIN OF SPECIES
which at first appeared to me insuperable, and actually
fatal to the whole theory. I allude to the neuters or
sterile females in insect-communities; for these neuters
often differ widely in instinct and in structure from both
the males and fertile females, and yet, from being sterile,
they cannot propagate their kind.
The subject well deserves to be discussed at great
length, but I will here take only a single case, that of
working or sterile ants. How the workers have been
rendered sterile is a difficulty; but not much greater than
that of any other striking modification of structure; for it
can be shown that some insects and other articulate ani-
mals in a state of nature occasionally become sterile; and
if such insects had been social, and it had been profitable
to the community that a number should have been an-
nually born capable of work, but incapable of procrea-
tion, I can see no especial difficulty in this having been
effected through natural selection. But I must pass over
this preliminary difficulty. The great difficulty lies in
the working ants differing widely from both the males
and the fertile females in structure, as in the shape of
the thorax, and in being destitute of wings and some-
times of eyes, and in instinct. As far as instinct alone
is concerned, the wonderful difference in this respect be-
tween the workers and the perfect females would have
been better exemplified by the hive-bee. If a working
ant or other neuter insect had been an ordinary animal,
I should have unhesitatingly assumed that all its char-
acters had been slowly acquired through natural selec-
tion; namely, by individuals having been born with slight
profitable modifications, which were inherited by the
offspring; and that these again varied and again were
INSTINCT
385
selected, and so onward. But with the working ant we
have an insect differing greatly from its parents, yet
absolutely sterile; so that it could never have trans-
mitted successively acquired modifications of structure
or instinct to its progeny. It may well be asked how
is it possible to reconcile this case with the theory of
natural selection ?
First, let it be remembered that we have innumerable
instances, both in our domestic productions and in those
in a state of nature, of all sorts of differences of inherited
structure which are correlated with certain ages, and with
either sex. We have differences correlated not only with
one sex, but with that short period when the reproduc-
tive system is active, as in the nuptial plumage of many
birds, and in the hooked jaws of the male salmon. We
have even slight differences in the horns of different
breeds of cattle in relation to an artificially imperfect
state of the male sex; for oxen of certain breeds have
longer horns than the oxen of other breeds, relatively
to the length of the horns in both the bulls and cows
of these same breeds. Hence I can see no great diffi-
culty in any character becoming correlated with the
sterile condition of certain members of insect-commu-
nities: 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 less-
ened, or, as I believe, disappears, when it is remem-
bered that selection may be applied to the family, as
well as to the individual, and may thus gain the desired
end. Breeders of cattle wish the flesh and fat to be
well marbled together: an animal thus characterized has
—Science — 17
386
THE ORIGIN OF SPECIES
been slaughtered, but the breeder has gone with confi-
dence to the same stock and has succeeded. Such faith
may be placed in the power of selection, that a breed
of cattle, always yielding oxen with extraordinarily long
horns, could, it is probable, be formed by carefully watch-
ing which individual bulls and cows, when matched, pro-
duced oxen with the longest horns; and yet no one ox
would ever have propagated its kind. Here is a better
and real illustration: according to M. Yerlot, some varie-
ties of the double annual Stock, from having been long
and carefully selected to the right degree, always produce
a large proportion of seedlings bearing double and quite
sterile flowers; but they likewise yield some single and
fertile plants.
These latter, by which alone the variety can be
propagated, may be compared with the fertile male
and female ants, and the double sterile plants with the
neuters of the same community. As with the varieties
of the stock, so with social insects, selection has been
applied to the family, and not to the individual, for the
sake of gaining a serviceable end. Hence we may con-
clude that slight modifications of structure or of instinct,
correlated with the sterile condition of certain members of
the community, have proved advantageous: consequently
the fertile males and females have flourished, and trans-
mitted to their fertile offspring a tendency to produce
sterile members with the same modifications. This proc-
ess must have been repeated many times, until that
prodigious amount of difference between the fertile and
sterile females of the same species has been produced
which we see in many social insects.
But we have not as yet touched on the acme of the
INSTINCT
387
difficulty; namely, the fact that the neuters of several
ants differ, not only from the fertile females and males,
but from each other, sometimes to an almost incredible
degree, and are thus divided into two or even three
castes. The castes, moreover, do not commonly graduate
into each other, but are perfectly well denned; being as
distinct from each other as are any two species of the
same genus, or rather as any two genera of the same
family. Thus in Eciton there are working and soldier
neuters, with jaws and instincts extraordinarily different:
in Cryptocerus, the workers of one caste alone carry a
wonderful sort of shield on their heads, the use of which
is quite unknown: in the Mexican Myrmecocystus, the
workers of one caste never leave the nest; they are fed
by the workers of another caste, and they have an
enormously developed abdomen which secretes a sort of
honey, supplying the place of that excreted by the
aphides, or the domestic cattle as they may be called,
which our European auts guard and 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 the theory. In the simpler case of
neuter insects all of one caste, which, as I believe, have
been rendered different from the fertile males and females
through natural selection, we may conclude from the anal-
ogy of ordinary variations that the successive, slight,
profitable modifications did not first arise in all the
neuters in the same nest, but in some few alone; and
that by the survival of the communities with females
which produced most neuters having the advantageous
modification, all the neuters ultimately came to be thus
583
THE 0 RIG IS OF SPECIES
characterized. According to this view, we ought occa-
sionally to find in the same nest neuter insects, present-
ing gradations of structure: and this we do find, even
not rarely, considering how few neuter insects out of
Europe have been carefully examined. Mr. F. Smith has
shown that the neuters of several British ants differ sur-
prisingly from each other in size and sometimes in color;
and that the extreme forms can be linked together by
individuals taken out of the same nest: I have myself
compared perfect gradations of this kind. It sometimes
happens that the larger or the smaller sized workers are
the most numerous: or that both large and small
are numerous, while those of an intermediate size are
scanty in numbers. Formica flava has larger and smaller
workers, with some few of intermediate size; and. in
this species, as Mr. F. Smith has observed, the larger
workers have simple eyes (ocelli), which though small can
be plainly distinguished, whereas the smaller workers have
their ocelli rudimentary. Having carefully dissected
several specimens of these workers, I can affirm that the
eyes are far more rudimentary in the smaller workers
than can be accounted for merely by their proportion-
ally 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 here we have two bodies of sterile workers in the
same nest, differing not only in size, but in their organs
of vision, yet connected by some few members in an
intermediate condition. I may digress by adding that
if the smaller workers had been the most useful to the
community, and those males and females had been con-
tinually selectel which produced more and more of
IXSTLXCT
389
the smaller workers, until all the workers were in this
condition; we should then have had a species of ant
with neuters in nearly the same condition as those of
Mjrmica. 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 ex-
pect occasionally to find gradations of important struc-
tures between the different castes of neuters in the same
species that I gladly availed myself of Mr. F. Smith's
offer of numerous specimens from the same nest of the
driver ant (Anomma) of West Africa. The reader will
perhaps best appreciate the amount of difference in these
workers by my giving not the actual measurements, but
a strictly accurate illustration: the difference was the same
as if we were to see a set of workmen building a house,
of whom many were five feet four inches high, and many
sixteen feet high; but we must in addition suppose that
the larger workmen had heads four instead of three times
as big as those of the smaller men, and jaws nearly five
times as big. The jaws, moreover, of the working ants
of the several sizes differed wonderfully in shape, and in
the form and number of the teeth. But the important
fact for us is that, though the workers can be grouped
into castes of different sizes, yet they graduate insensibly
into each other, as does the widely-different structure of
their jaws.
I speak confidently on this latter point, as Sir J.
Lubbock made drawings for me, with the camera lucida,
of the jaws which I dissected from the workers of the
several sizes. Mr. Bates, in his interesting 4 •Naturalist
on the Amazons,' has described analogous cases.
390
THE ORIGIN OF SPECIES
With these facts before me, I believe that natural
selection, by acting on the fertile ants or parents, could
form a species which should regularly produce neuters,
all of large size with one form of jaw, or all of small
size with widely different jaws; or lastly, and this is the
greatest 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
first been formed, as in the case of the driver ant, and
then the extreme forms having been produced in greater
and greater numbers, through the survival of the parents
which generated them, until none with an intermediate
structure was produced.
An analogous explanation has been given by Mr.
Wallace of the equally complex case of certain Malayan
Butterflies regularly appearing under two or even three
distinct female forms; and by Fritz Miiller, of certain
Brazilian crustaceans likewise appearing under two widely
distinct male forms. But this subject need not here be
discussed.
I have now explained how, as I believe, the wonder-
ful 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 ants, on the same principle that the
division of labor is useful to civilized man. Ants, how-
ever, work by inherited instincts and by inherited organs,
or tools, while man works by acquired knowledge and
manufactured instruments. But I must confess that, with
all my faith in natural selection, I should never have
anticipated that this principle could have been efficient
INSTINCT
391
in so high a degree, had not the case of these neuter
insects led me to this conclusion. I have, therefore, dis-
cussed 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 may
be effected by the accumulation of numerous slight,
spontaneous variations which are in any way profitable,
without exercise or habit having been brought into play.
For peculiar habits confined to the workers or sterile
females, however long they might be followed, could not
possibly affect the males and fertile females, which alone
leave descendants. I am surprised that no one has
hitherto advanced this demonstrative case of neuter
insects, against the well-known doctrine of inherited
habit, as advanced by Lamarck.
Summary
I have endeavored in this chapter briefly to show that
the mental qualities of our domestic animals vary, and
that the variations are inherited. Still more briefly 1
have attempted to show that instincts vary slightly in a
state of nature. No one will dispute that instincts are
of the highest importance to each animal. Therefore
there is no real difficulty, under changing conditions of
life, in natural selection accumulating to any extent
slight modifications of instinct which are in any way
useful. In many cases habit or use and disuse have
probably come into play. I do not pretend that the facts
S92
THE ORIGIN OF SPECIES
given in this chapter strengthen in any great degree my
theory; but none of the cases of difficulty, to the best
of my judgment, annihilates it. On the other hand, the
fact that instincts are not always absolutely perfect and
are liable to mistakes; that no instinct can be shown
to have been produced for the good of other animals,
though animals take advantage of the instincts of others;
that the canon in natural history, of "Natur non facit
sal turn," 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 cor-
roborate 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 distinct, species, when inhabiting dis-
tant parts of the world and living under considerably
different conditions of life, yet often retaining nearly the
same instincts. For instance, we can understand, on
the principle of inheritance, how it is that the thrush
of tropical South America lines its nest with mud, in
the same peculiar manner as does our British thrush;
how it is that the Hornbills of Africa and India have
the same extraordinary instinct of plastering up and
imprisoning the females in a hole in a tree, with only
a small hole left in the plaster through which the males
feed them and their young when hatched; how it is that
the male wrens (Troglodytes) of North America build
1 ' cock- nests," to roost in, like the males of our Kitty-
wrens — a habit wholly unlike that of any other known
bird.
Finally, it may not be a logical deduction, but to
my imagination it is far more satisfactory to look at
INSTINCT
393
such instincts as the young cuckoo ejecting its foster-
brothers — ants making slaves — the larvae of ichneumonidaa
feeding within the live bodies of caterpillars — not as
specially endowed or created instincts, but as small con-
sequences of one general law leading to the advancement
of all organic beings — namely, multiply, vary, let the
strongest live and the weakest die.
BHD OF VOL. L OP "TME ORIGIN OF SPBOIB©,?
THE ORIGIN OF SPECIES
PART TWO
CONTENTS
CHAPTER IX
HYBRIDISM
Distinction between the sterility of first crosses and of hybrids — Sterility
various in degree, not universal, affected by close interbreeding, re-
moved by domestication — Laws governing the sterility of hybrids-
Sterility not a special endowment, but incidental on other differences,
not accumulated by natural selection — Causes of the sterility of first
crosses and of hybrids — Parallelism between the effects of changed con-
ditions of life and of crossing — Dimorphism and trimorphism — Fertility
of varieties when crossed and of their mongrel offspring not universal —
Hybrids and mongrels compared independently of their fertility —
Summary
CHAPTER X
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD
On the absence of intermediate varieties at the present day— On the nature
of extinct intermediate varieties; on their number — On the lapse of
time, as inferred from the rate of denudation and of deposition — On the
lapse of time as estimated by years — On the poorness of our paleontolog-
ical collections — On the intermittence of geological formations — On the
denudation of granitic areas — 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 —
Antiquity of the habitable earth .... ... 64
(3)
4
CONTENTS
CHAPTER XI
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS
On the slow and successive appearance of new species — On their different
rates of change — Species once lost do not reappear — Groups of species
follow the same general rules in their appearance and disappearance as
do single species — On Extinction — On simultaneous changes in the
forms of life throughout the world — On the affinities of extinct species
to each other and to living species — On the state of development of
ancient forms — On the succession of the same types within the same
areas— Summary of preceding and present chapters . M
CHAPTER XII
GEOGRAPHICAL DISTRIBUTION
Present distribution cannot be accounted for by differences in physical
conditions — Importance of barriers — Affinity of the productions of the
same continent — Centres of creation — Means of dispersal, by changes
of climate and of the level of the land, and by occasional means —
Dispersal during the Glacial period — Alternate Glacial periods in the
North and South 13?
CHAPTER XIII
geographical distribution — continued
Distribution of fresh-water productions — On the inhabitants of oceanic
islands — Absence of Batrachians and of terrestrial Mammals — On the
relation of the inhabitants of islands to those of the nearest mainland —
On colonization from the nearest source with subsequent modification
— Summary of the last and present chapters 180
CONTENTS
CHAPTEK XIV
5
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:
EMBRYOLOGY: RUDIMENTARY ORGANS
Classification, groups subordinate to groups — Natural system — Rules
and difficulties in classification, explained on the theory of descent with
modification — Classification of varieties — Descent always used in classi-
fication— Analogical or adaptive characters — Affinities, general, com-
plex, and radiating — Extinction separates and defines groups—
Morphology, between members of the same class, between parts of
the same individual — Embryology, laws of, explained by variations not
supervening at an early age, and being inherited at a corresponding age
— Rudimentary organs; their origin explained — Summary . . 211
CHAPTEK XV
RECAPITULATION AND CONCLUSION
Recapitulation of the objections to the theory of Natural Selection — Re-
capitulation of the general and special circumstances in its favor —
Causes of the general belief in the immutability of species — How far the
theory of Natural Selection may be extended — Effects of its adoption
on the study of Natural History — Concluding remarks .... 276
Glossary of Scientific Terms ... .... 317
Index
. 337
THE ORIGIN OF SPECIES
CHAPTER IX
HYBRIDISM
Distinction between the sterility of first crosses and of hybrids — Sterility
various in degree, not universal, affected by close interbreeding, re-
moved by domestication — Laws governing the sterility of hybrids —
Sterility not a special endowment, but incidental on other differences,
not accumulated by natural selection — Causes of the sterility of first
crosses and of hybrids — Parallelism between the effects of changed con-
ditions of life and of crossing — Dimorphism and trimorphism — Fertility
of varieties when crossed and of their mongrel offspring not universal —
Hybrids and mongrels compared independently of their fertility —
Summary
THE view commonly entertained by naturalists is that
species, when intercrossed, have been specially en-
dowed with sterility, in order to prevent their
confusion. This view certainly seems at first highly prob-
able, for species living together could hardly have been
kept distinct had they been capable of freely crossing.
The subject is in many ways important for us, more
especially as the sterility of species when first crossed,
and that of their hybrid offspring, cannot have been ac-
quired, as I shall show, by the preservation of successive
profitable degrees of sterility. It is an incidental result
of differences in the reproductive systems of the parent-
species.
In treating this subject, two classes of facts, to a large
extent fundamentally different, have generally been con-
(7)
s
THE ORIGIN OF SPECIES
founded; namely, the sterility of species when first crossed,
and the sterility of the hybrids produced from them.
Pure species have of course their organs of reproduc-
tion in a perfect condition, yet when intercrossed they
produce either few or no offspring. Hybrids, on the
other hand, have their reproductive organs functionally
impotent, as may be clearly seen in the state of the male
element in both plants and animals; though the formative
organs themselves are perfect in structure, as far as the
microscope reveals. In the first case the two sexual
elements which go to form the embryo are perfect; in the
second case they are either not at all developed, or are
imperfectly developed. This distinction is important,
when the cause of the sterility, which is common to the
two cases, has to be considered. The distinction probably
has been slurred over, owing to the sterility in both cases
being looked on as a special endowment, beyond the prov-
ince of our reasoning powers.
The fertility of varieties, that is of the forms known
or believed to be descended from common parents, when
crossed, and likewise the fertility of their mongrel off-
spring, is, with reference to my theory, of equal impor-
tance with the sterility of species; for it seems to make a
broad and clear distinction between varieties and species.
Degrees of Sterility
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 being deeply
impressed with the high generality of some degree of
HYBRIDISM
9
sterility. Rolreuter makes the rule universal; but then
he cuts the knot, for in ten cases in which he found two
forms, considered by most authors as distinct species,
quite fertile together, he unhesitatingly ranks them as
varieties. 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 first
crossed, and the maximum produced by their hybrid off-
spring, with the average number produced by both pure
parent-species in a state of nature. But causes of serious
error here intervene: a plant, to be hybridized, must be
castrated, and, what is often more important, must be se-
cluded in order to prevent pollen being brought to it by
insects from other plants. Nearly all the plants experi-
mented on by Gartner were potted, and were kept in a
chamber in his house. That these processes are often
injurious to the fertility of a plant cannot be doubted;
for Gartner gives in his table about a score of cases of
plants which he castrated, and artificially fertilized with
their own pollen, and (excluding all cases such as the
Leguminosae, in which there is an acknowledged difficulty
in the manipulation) half of these twenty plants had their
fertility in some degree impaired. Moreover, as Gartner
repeatedly crossed some forms, such as the common red
and blue pimpernels (Anagallis arvensis and ccerulea),
which the best botanists rank as varieties, and found
them absolutely sterile, we may doubt whether many
species are really so sterile, when intercrossed, as he
believed.
10
THE ORIGIN OF SPECIES
It is certain, on the one hand, that the sterility of
various species when crossed is so different in degree and
graduates away so insensibly, and, on the other hand,
that the fertility of pure species is so easily affected by
various circumstances, that for all practical purposes it
is most difficult to say where perfect fertility ends and
sterility begins. I think no better evidence of this can
be required than that the two most experienced observ-
ers who have ever lived, namely Kolreuter and Gartner,
arrived at diametrically opposite conclusions in regard to
some of the very same forms. It is also most instructive
to compare — but I have not space here to enter on de-
tails— the evidence advanced by our best botanists on the
question whether certain doubtful forms should be ranked
as species or varieties, with the evidence from fertility
adduced by different hybridizers, or by the same observer
from experiments made during different years. It can
thus be shown that neither sterility nor fertility affords
any certain distinction between species and varieties.
The evidence from this source graduates away, and is
doubtful in the same degree as is the evidence derived
from other constitutional and structural differences.
In regard to the sterility of hybrids in successive
generations; though Gartner was enabled to rear some
hybrids, carefully guarding them from a cross with either
pure parent, for six or seven, and in one case for ten
generations, yet he asserts positively that their fertility
never increases, but generally decreases greatly and sud-
denly. With respect to this decrease, it may first be no-
ticed that when any deviation in structure or constitution
is common to both parents, this is often transmitted in
an augmented degree to the offspring; and both sexual
HYBRIDISM
11
elements in hybrid plants are already affected in some
degree. But I believe that their fertility has been dimin-
ished in nearly all these cases by an independent cause;
namely, by too close interbreeding. I have made so
many experiments and collected so many facts, showing
on the one hand that an occasional cross with a distinct
individual or variety increases the vigor and fertility of
the offspring, and on the other hand that very close in-
terbreeding lessens their vigor and fertility, that I cannot
doubt the correctness of this conclusion. 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, if left to themselves, will generally be fertilized
during each generation by pollen from the same flower;
and this would probably be injurious to their fertility,
already lessened by their hybrid origin. I am strength-
ened in this conviction by a remarkable statement re-
peatedly made by Gartner, namely, that if even the less
fertile hybrids be artificially fertilized with hybrid pollen
of the same kind, their fertility, notwithstanding the fre-
quent ill effects from manipulation, sometimes decidedly
increases, and goes on increasing. Now, in the process of
artificial fertilization, pollen is as often taken by chance
(as 1 know from my own experience) from the anthers of
another flower as from the anthers of the flower itself
which is to be fertilized; so that a cross between two
flowers, though probably often on the same plant, would
be thus effected. Moreover, whenever complicated experi-
ments are in progress, so careful an observer as Gartner
would have castrated his hybrids, and this would have
12
THE ORIGIN OF SPECIES
insured in each generation a cross with pollen from a
distinct flower, either from the same plant or from an-
other plant of the same hybrid nature. And thus, the
strange fact of an increase of fertility in the successive
generations of artificially fertilized hybrids, in contrast
with those spontaneously self-fertilized, may, as I be-
lieve, be accounted for by too close interbreeding having
been avoided.
Now let us turn to the results arrived at by a third
most experienced hybridizer, namely, the Hon. and Rev.
W. Herbert. He is as emphatic in his conclusion that
some hybrids are perfectly fertile — as fertile as the pure
parent-species — as are Kolreuter and Gartner that some
degree of sterility between distinct species is a universal
law of nature. He experimented on some of the very
same species as did Gartner. The difference in their re-
sults may, I think, be in part accounted for by Herbert's
great horticultural skill, and by his having hot- houses at
his command. Of his many important statements I will
here give only a single one as an example, namely, that
* 4 every ovule in a pod of Grinum capense fertilized by
C. revolutum produced a plant, which I never saw to
occur in a case of its natural fecundation." So that here
we 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 singu-
lar fact, namely, that individual plants of certain species
of Lobelia, Yerbascum and Passiflora can easily be fer-
tilized by pollen from a distinct species, but not by
pollen from the same plant, though this pollen can be
proved to be perfectly sound by fertilizing other plants
or species. In the genus Hippeastrum, in Corydalis ae
HYBRIDISM
13
shown by Professor Hildebrand, in various orchids as
shown by Mr. Scott and Fritz Miiller, all the individuals
are in this peculiar condition. So that, with some spe
cies, certain abnormal individuals, and in other species
all the individuals, can actually be hybridized much more
readily than they can be fertilized by pollen from the
same individual plant! To give one instance, a bulb of
Hippeastrum aulicum produced four flowers; three were
fertilized by Herbert with their own pollen, and the
fourth was subsequently fertilized by the pollen of a
compound hybrid descended from three distinct species:
the result was that 4 'the ovaries of the first three flowers
soon ceased to grow, and after a few days perished en-
tirely, whereas the pod impregnated by the pollen of the
hybrid made vigorous growth and rapid progress to ma-
turity, and bore good seed, which vegetated freely."
Mr. Herbert tried similar experiments during many
years, and always with the same result. These cases
serve to show on what slight and mysterious causes the
lesser or greater fertility of a species 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, Rhododen-
dron, etc., 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, "reproduces itself as
perfectly as if it had been a natural species 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
14
THE ORIGIN OF SPECIES
of them are perfectly fertile. Mr. C. Noble, for instance,
informs me that he raises stocks for grafting from a
hybrid between Rhod. ponticum and catawbiense, and that
this hybrid "seeds as freely as it is possible to imagine."
Had hybrids when fairly treated always gone on decreas-
ing in fertility in each successive generation, as Gartner
believed to be the case, the fact would have been notori-
ous to nurserymen. Horticulturists raise large beds of
the same hybrid, and such alone are fairly treated, for by
insect agency the several individuals are allowed to cross
freely with each other, and the injurious influence of
close interbreeding is thus prevented. Any one may
readily convince himself of the efficiency of insect-
agency by examining the flowers of the more sterile
kinds of hybrid Rhododendrons, which produce no
pollen, for he will find on their stigmas plenty of pollen
brought from other flowers.
In regard to animals, much fewer experiments have
been carefully tried than with plants. If our systematic
arrangements can be trusted, that is, if the genera of
animals are as distinct from each other as are the genera
of plants, then we may infer that animals more widely
distinct in the scale of nature can be crossed more easily
than in the case of plants; but the hybrids themselves
are, I think, more sterile. 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 dis-
tinct species of finches, but, as not one of these 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
HYBRIDISM
15
respect to the fertility in successive generations of the
more fertile hybrid animals, I hardly know of an in-
stance in which two families of the same hybrid have
been raised at the same time from different parents, so
as to avoid the ill effects of close interbreeding. On the
contrary, brothers and sisters have usually been crossed
in each successive generation, in opposition to the 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 increas-
ing.
Although I know of hardly any thoroughly well-
authenticated cases of perfectly fertile hybrid animals, I
have reason to believe that the hybrids from Cervulus
vaginalis and Eeevesii, -and from Phasianus colchicus
with P. torquatus, are perfectly fertile. M. Quatrefages
states that the hybrids from two moths (Bombyx cynthia
and arrindia) were proved in Paris to be fertile inter se
for eight generations. It has lately been asserted that
two such distinct species as the hare and rabbit, when
they can be got to breed together, produce offspring
which are highly fertile when crossed with one of the
parent-species. The hybrids from the common and Chi-
nese geese (A. cygnoides), species which are so different
that they are generally ranked in distinct genera, have
often bred in this country with either pure parent, and
in one single instance they have bred inter se. This was
effected by Mr. Eyton, who raised two hybrids from the
same parents, but from different hatches; and from these
two birds he raised no less than eight hybrids (grand-
children of the pure geese) from one nest. In India,
however, these cross-bred geese must be far more fertile;
— Science — 18
16
THE ORIGIN OF SPECIES
for I am assured by two eminently capable judges,
namely, Mr. Blyth and Capt. Hutton, that whole flocks
of these crossed geese are kept in various parts of the
country; and as they are kept for profit, where neither
pure parent-species exists, they must certainly be highly
or perfectly fertile.
With our domesticated animals, the various races when
crossed together are quite fertile; yet in many cases they
are descended from two or more wild species. From this
fact we must conclude either that the aboriginal parent-
species at first produced perfectly fertile hybrids, or that
the hybrids subsequently reared under domestication be-
came quite fertile. This latter alternative, which was first
propounded by Pallas, seems by far the most probable,
and can, indeed, hardly be doubted. It is, for instance,
almost certain that our dogs are descended from several
wild stocks; yet, with perhaps the exception of certain
indigenous domestic dogs of South America, all are quite
fertile together; but analogy makes me greatly doubt
whether the several aboriginal species would at first have
freely bred together and have produced quite fertile
hybrids. So again I have lately acquired decisive evi-
dence that the crossed offspring from the Indian humped
and common cattle are inter se perfectly fertile; and from
the observations by Rutimeyer on their important osteo-
logical differences, as well as from those by Mr. Blyth
on their differences in habits, voice, constitution, etc.,
these two forms must be regarded as good and distinct
species. The same remarks may be extended to the two
chief races of the pig. We must, therefore, either give
tip the belief of the universal sterility of species when
crossed; or we must look at this sterility in animals, not
HYBRIDISM
17
as an indelible characteristic, but as one capable of being
removed by domestication.
Finally, considering 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 laws
governing the sterility of first crosses and of hybrids.
Our chief object will be to see whether or not these laws
indicate that species have been specially endowed with
this quality, in order to .prevent their crossing and blend-
ing together in utter confusion. The following conclu-
sions are drawn up chiefly from Gartner's admirable work
on the hybridization of plants. I have taken much
pains to ascertain how far they apply to animals, and,
considering how scanty our knowledge is in regard to
hybrid animals, I have been surprised to find how gener-
ally 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; but only the
barest outline of the facts can here be given. When
pollen from a plant of one family is placed on the
stigma of a plant of a distinct family, it exerts no
more influence than so much inorganic dust. From this
absolute zero of fertility, the pollen of different species,
applied to the stigma of some one species of the same
18
THE ORIGIX OF SPECIES
genus, yields a perfect gradation in the number of seeds
produced, up to nearly complete or even quite complete
fertility; and, as we have seen, in certain abnormal cases,
even to an excess of fertility, beyond that which the
plant's own pollen produces. So, in hybrids themselves,
there are some which never have produced, and probably
never would produce, even with the pollen of the pure
parents, 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 fertilization. From this
extreme degree of sterility we have self-fertilized hybrids
producing a greater and greater number of seeds up to
perfect fertility.
The hybrids raised from two species which are very
difficult to cross, and which rarely produce any offspring,
are generally very sterile; but the parallelism between
the difficulty of making a first cross, and the sterility of
the hybrids thus produced — two classes of facts which
are generally confounded together — is by no means strict.
There are many cases in which two pure species, as in
the genus Verbascum, can be united with unusual facil-
ity, 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 ex-
treme 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,
HYBRIDISM
19
is more easily affected by unfavorable conditions than is
that of pure species. But the fertility of first crosses is
likewise innately variable; for it is not always the same
in degree when the same two species are crossed under
the same circumstances; it depends in part upon the con-
stitution of the individuals which happen to have been
chosen for the experiment. So it is with hybrids, for
their degree of fertility is often found to differ greatly in
the several individuals raised from seed out of the same
capsule and exposed to the same conditions.
By the term systematic affinity is meant the general
resemblance between species in structure and constitution.
Now the fertility of first crosses, and of the hybrids pro-
duced from them, is largely governed by their systematic
affinity. This is clearly shown by hybrids never having
been raised between species ranked by systematists in
distinct families; and on the other hand, by very closely
allied species generally uniting with facility. But the
correspondence between systematic affinity and the facil-
ity of crossing is by no means strict. A multitude of
cases could be given of very closely allied species which
will not unite, or only with extreme difficulty; and on
the other hand of very distinct species which unite with
the utmost facility. In the same family there may be
a genus, as Dianthus, in which very many species can
most readily be crossed; and another genus, as Silene, in
which the most persevering efforts have failed to produce
between extremely close species a single hybrid. Even
within the limits of the same genus, we meet with this
same difference; for instance, the many species of Nico-
tiana have been more largely crossed than the species of
almost any other genus; but Gartner found that N. acu-
20
THE ORIGIN OF SPECIES
minata, which is not a particularly distinct species, obsti-
nately failed to fertilize, or to be fertilized by no less
than eight other species of Nicotiana. Many analogous
facts could be given.
No one has been able to point out what kind or what
amount of difference, in any recognizable 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 perennial plants, deciduous and evergreen trees,
plants inhabiting different stations and fitted for ex-
tremely different climates, can often be crossed with
ease.
By a reciprocal cross between two species, I mean
the case, for instance, of a female-ass being first crossed
by a stallion, and then a mare by a male-ass; 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, that is of any difference in their
structure or constitution, excepting in their reproductive
systems. The diversity of the result in reciprocal crosses
between the same two species was long ago observed by
Kolreuter. To give an instance: Mirabilis jalapa can
easily be fertilized 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 fertilize reciprocally M. long-
HYBRIDISM
21
iflora with the pollen of M. jalapa, 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 ob-
served it even between closely related forms (as Mat-
thiola annua and glabra) which many botanists rank only
as varieties. It is also a remarkable fact that hybrids
raised from reciprocal crosses, though of course com-
pounded of the very same two species, the one species
having first been used as the father and then as the
mother, though they rarely differ in external characters,
yet generally differ in fertility in a small and occasion-
ally in a high degree.
Several other singular rules could be given from
Gartner: for instance, some species have a remarkable
power of crossing with other species; other species of the
same genus have a remarkable power of impressing their
likeness on their hybrid offspring; but these two powers
do not at all necessarily go together. There are certain
hybrids which, instead of having, as is usual, an in-
termediate character between their two parents, always
closely resemble one of them; and such hybrids, though
externally so like one of their pure parent-species, are
with rare exceptions extremely sterile. So again, among
hybrids which are usually intermediate in structure be-
tween their parents, exceptional and abnormal individuals
sometimes are born which closely resemble one of their
pure parents; and these hybrids are almost always utterly
sterile, even when the other hybrids raised from seed
from the same capsule have a considerable degree of fer-
22
THE ORIGIN OF SPECIES
tility. These facts show how completely the fertility of
a hybrid may be independent of its external resemblance
to either pure parent.
Considering the several rules now given, which govern
the fertility of first crosses and of hybrids, we see that
when forms, which must be considered as good and dis-
tinct 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 favorable and unfavorable condi-
tions, is innately variable; that it is by no means always
the same in degree in the first cross and in the hybrids
produced from this cross; that the fertility of hybrids is
not related to the degree in which they resemble in ex-
ternal appearance either parent; and lastly, that the facil-
ity of making a first cross between any two species is
not always governed by their systematic affinity or degree
of resemblance to each other. This latter statement is
clearly proved by the difference in the result of recipro-
cal crosses between the same two species, for, according
as the one species or the other is used as the father or
the mother, there is generally some difference, and oc-
casionally the widest possible difference, in the facility
of effecting a union. The hybrids, moreover, produced
from reciprocal crosses often differ in fertility.
Now do these complex and singular rules indicate
that species have been endowed with sterility simply to
prevent their becoming confounded in nature ? I think
not. For why should the sterility be so extremely dif-
ferent in degree, when various species are crossed, all of
which we must suppose it would be equally important
to keep from blending together? Why should the de-
HYBRIDISM
23
gree of sterility be innately variable in the individuals of
the same species ? Why should some species cross with
facility, and yet produce very sterile hybrids: and other
species cross with extreme difficulty, and yet produce
fairly fertile hybrids? Why should there often be so
great a difference in the result of a reciprocal cross
between the same two species ? Why, it may even be
asked, has the production of hybrids been permitted ?
To grant to species the special power of producing hy-
brids, and then to stop their further propagation by dif-
ferent degrees of sterility, not strictly related to the
facility of the first union between their parents, seems
a strange arrangement.
The foregoing rules and facts, on the other hand,
appear to me clearly to indicate that the sterility both
of first crosses and of hybrids is simply incidental or
dependent on unknown differences in their reproductive
systems: the differences being of so peculiar and limited
a nature, that, in reciprocal crosses between the same
two species, the male sexual element of the one will
often freely act on the female sexual element of %he
other, but not in a reversed direction. It will be ad-
visable to explain a little more fully by an example
what I mean by sterility being incidental on other dif-
ferences, and not a specially endowed quality. As the
capacity of one plant to be grafted or budded on another
is unimportant for their 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 inci-
dental 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
24
THE ORIGIX OF SPECIES
of growth, in the hardness of their wood, in the period
of the flow or nature of their sap, etc. ; but in a multi-
tude of cases we can assign no reason whatever. Great
diversity in the size of two plants, one being woody and
the other herbaceous, one being evergreen and the other
deciduous, and adaptation to widely different climates,
do not always prevent the two grafting together. As
in hybridization, so with grafting, the capacity is limited
by systematic affinity, for no one has been able to graft
together trees 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 hybridiza-
tion, 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 facil-
ity 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 Sageret 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 a 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.
HYBRIDISM
25
We have seen that the sterility of hybrids, which
have their reproductive organs in an imperfect condi-
tion, is a different case from the difficulty of uniting
two pure species which have their reproductive organs
perfect; yet these two distinct classes of cases run to a
large extent parallel. Something analogous occurs in
grafting; for Thouin found that three species of Ro-
binia, which seeded freely on their own roots, and which
could be grafted with no great difficulty on a fourth
species, when thus grafted were rendered barren. On
the other hand, certain species of Sorbus when grafted
on other species yielded twice as much fruit as when on
their own roots. We are reminded by this latter fact of
the extraordinary cases of Hippeastrum, Passiflora, etc.,
which seed much more freely when fertilized with the
pollen of a distinct species than when fertilized with
pollen from the same plant.
We thus see, that, although there is a clear and great
difference between the mere adhesion of grafted stocks,
and the union of the male and female elements in the
act of reproduction, yet that there is a rude degree of
parallelism in the results of grafting and of crossing 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 on unknown dif-
ferences in their vegetative systems, so I believe that
the still more complex laws governing the facility of first
crosses are incidental on unknown differences in their re-
productive systems. These differences in both cases fol-
low to a certain extent, as might have been expected,
systematic affinity, by which term every kind of resem-
blance and dissimilarity between organic beings is at-
26
THE ORIGIN OF SPECIES
tempted to be expressed. The facts by do means seem
to indicate that the greater or lesser difficulty of either
grafting or crossing various species has been a special
endowment; although, in the case of crossing, the diffi-
culty is as important for the endurance and stability of
specific forms as in the case of grafting it is unimportant
for their welfare.
Origin and Causes of the Sterility of first Crosses and
of Hybrids
At one time it appeared to me probable, as it has to
others, that the sterility of first crosses and of hybrids
might have been slowly acquired through the natural
selection of slightly lessened degrees of fertility, which,
like any other variation, spontaneously appeared in cer-
tain individuals of one variety when crossed with those
of another variety. For it would clearly be advantageous
to two varieties or incipient species if they could be kept
from blending, on the same principle that, when man is
selecting at the same time two varieties, it is necessary
that he should keep them separate. In the first place, it
may be remarked that species inhabiting distinct regions
are often sterile when crossed; now it could clearly have
been of no advantage to such separated species to have
been rendered mutually sterile, and consequently this
could not have been effected through natural selection;
but it may perhaps be argued, that, if a species was ren-
dered sterile with some one compatriot, sterility with other
species would follow as a necessary contingency. In the
second place, it is almost as much opposed to the theory
of natural selection as to that of special creation, that in
reciprocal crosses the male element of one form should
HYBRIDISM
27-
have been rendered utterly impotent on a second form,
while at the same time the male element of this second
form is enabled freely to fertilize the first form; for this
peculiar state of the reproductive system could hardly
have been advantageous to either species.
In considering the probability of natural selection
having come into action, in rendering species mutually
sterile, the greatest difficulty will be found to lie in the
existence of many graduated steps from slightly lessened
fertility to absolute sterility. It may be admitted that it
would profit an incipient species if it were rendered in
some slight degree sterile when crossed with its parent
form or with some other variety; for thus fewer bastard-
ized and deteriorated offspring would be produced to
commingle their blood with the new species in process
of formation. But he who will take the trouble to re-
flect on the steps by which this first degree of sterility
could be increased through natural selection to that high
degree which is common with so many species, and which
is universal with species which have been differentiated
to a generic or family rank, will find the subject extraor-
dinarily complex. After mature reflection it seems to me
that this could not have been effected through natural
selection. Take the case of any two species which, when
crossed, produced few and sterile offspring; now, what is
there which could favor the survival of those individuals
which happened to be endowed in a slightly higher de-
gree with mutual infertility, and which thus approached
by one small step toward absolute sterility? Yet an ad-
vance of this kind, if the theory of natural selection be
brought to bear, must have incessantly occurred with
many species, for a multitude are mutually quite barren.
28
THE ORIGIN OF SPECIES
With sterile neuter insects we have reason to believe that
modifications in their structure and fertility have been
slowly accumulated by natural selection, from an advan-
tage having been thus indirectly given to the community
to which they belonged over other communities of the
same species; but an individual animal not belonging
to a social community, if rendered slightly sterile when
crossed with some other variety, would not thus itself
gain any advantage or indirectly give any advantage to
the other individuals of the same variety, thus leading
to their preservation.
But it would be superfluous to discuss this question
in detail; for with plants we have conclusive evidence
that the sterility of crossed species must be due to some
principle quite independent of natural selection. Botri
Gartner and Kolreuter have proved that in genera in-
cluding numerous species, a series can be formed from
species which when crossed yield fewer and fewer seeds,
to species which never produce a single seed, bu* vet are
affected by the pollen of certain other specie©, for the
germen swells. It is here manifestly impossible to select
the more sterile individuals, which have already ceased
to yield seeds; so that this acme of sterility, when the
germen alone is affected, cannot have been gained
through selection; and from the laws governing the
various grades of sterility being so uniform throughout
the animal and vegetable kingdoms, we may infer that
the cause, whatever it may be, is the same or nearly the
same in all cases.
We will now look a little closer at the probable
nature of the differences between species which induce
HYBRIDISM
29
sterility in first crosses and in hybrids. In the case of
first crosses, the greater or less difficulty in effecting
a union and in obtaining offspring apparently depends
on several distinct causes. There must sometimes be a
physical impossibility in the male element reaching the
ovule, as would be the case with a plant having a pistil
too long for the pollen tubes to reach the ovarium. It
has also been observed that when the pollen of one spe-
cies is placed on the stigma of a distantly allied species,
though the pollen-tubes protrude, they do not penetrate
the stigmatic surface. Again, the male element may
reach the female element but be incapable of causing
an embryo to be developed, as seems to have been the
case with some of Thuret's experiments on Fuci. No
explanation can be given of these facts, any more than
why certain trees cannot be grafted on others. Lastly,
an embryo may be developed, and then perish at an
early period. This latter alternative has not been suffi-
ciently attended to; but I believe, from observations
communicated to me by Mr. Hewitt, who has had great
experience in hybridizing pheasants and fowls, that the
early death of the embryo is a very frequent cause of
sterility in first crosses. Mr. Salter has recently given
the results of an examination of about 500 eggs produced
from various crosses between three species of Gallus and
their hybrids; the majority of these eggs had been fertil-
ized, and in the majority of the fertilized eggs the em-
bryos had either been partially developed and had then
perished, or had become nearly mature, but the young
chickens had been unable to break through the shell.
Of the chickens which were born, more than four-fifths
died within the first few days, or at latest weeks, 4 'with-
30
THE ORIGIN OF SPECIES
out any obvious cause, apparently from mere inability
to live"; so that from the 500 eggs only twelve chickens
were reared. With plants, hybridized embryos probably
often perish in a like manner; at least it is known that
hybrids raised from very distinct species are sometimes
weak and dwarfed, and perish at an early age; of which
fact Max Wichura has recently given some striking cases
with hybrid willows. It may be here worth noticing that
in some cases of parthenogenesis the embryos within the
eggs of silk moths which had not been fertilized pass
through their early stages of development and then perish
Jike the embryos produced by a cross between distinct
species. Until becoming acquainted with these facts, I
was unwilling to believe in the frequent early death of
hybrid embryos; for hybrids, when once born, are gen-
erally healthy and long-lived, as we see in the case of
the common mule. Hybrids, however, are differently cir-
cumstanced before and after birth: when born and living
in a country where their two parents live, they are gen-
erally placed under suitable conditions of life. But a
hybrid partakes of only half of the nature and constitu-
tion of its mother; it may therefore before birth, as long
as it is nourished within its mother's womb, or within
the egg or seed produced by the mother, be exposed to
conditions in some degree unsuitable, and consequently
be liable to perish at an early period; more especially as
all very young beings are eminently sensitive to injurious
or unnatural conditions of life. But after all, the cause
more probably lies in some imperfection in the original
act of impregnation, causing the embryo to be imper-
fectly developed, rather than in the conditions to which
it is subsequently exposed.
HYBRIDISM
31
In regard to the sterility of hybrids, in which the
sexual elements are imperfectly developed, the case is
somewhat different. I have more than once alluded to
a large body of facts showing that, when animals and
plants are removed from their natural conditions, they
are extremely liable to have their reproductive systems
seriously affected. This, in fact, is the great bar to the
domestication of animals. Between the sterility thus su-
perinduced 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 oc-
curs in various degrees; in both, the male element is the
most liable to be affected; but sometimes the female more
than the male. In both, the. tendency goes to a certain
extent with systematic affinity, for whole groups of ani-
mals and plants are rendered impotent by the same un-
natural conditions; and whole groups of species tend to
produce sterile hybrids. On the other hand, one species
in a group will sometimes resist great changes of condi-
tions with unimpaired fertility; and certain species in a
group will produce unusually fertile hybrids. No one
can tell, till he tries, whether any particular animal
will breed under confinement, or any exotic plant seed
freely under culture; nor can he tell till he tries whether
any two species of a genus will produce more or less
sterile hybrids. Lastly, when organic beings are placed
during several generations under conditions not natural
to them, they are extremely liable to vary, which seems
to be partly due to their reproductive systems having
been specially affected, though in a lesser degree than
when sterility ensues. So it is with hybrids, for their
32
THE ORIGIN OF SPECIES
offspring in successive generations are eminently liable
to vary, as every experimentalist has observed.
Thus we see that when organic beings are placed
under new and unnatural conditions, and when hybrids
are produced by the unnatural crossing of two species,
the reproductive system, independently of the general
state of health, is affected in a very similar manner. In
the one case, the conditions of life have been disturbed,
though often in so slight a degree as to be inappreciable
by us; in the other case, or that of hybrids, the external
conditions have remained the same, but the organization
has been disturbed by two distinct structures and consti-
tutions, including of course the reproductive systems,
having been blended into one. For it is scarcely possible
that two organizations should be compounded into one,
without some disturbance occurring in the development,
or periodical action, or mutual relations of the different
parts and organs one to another or to the conditions of
life. When hybrids are able to breed inter se, they
transmit to their offspring from generation to generation
the same compounded organization, and hence we need
not be surprised that their sterility, though in some
degree variable, does not diminish; it is even apt to
increase, this being generally the result, as before ex-
plained, of too close interbreeding. The above view of
the sterility of hybrids being caused by two constitutions
being compounded into one has been strongly maintained
by Max Wichura.
It must, however, be owned that we cannot under-
stand, on the above or any other view, several facts with
respect to the sterility of hybrids; for instance, the
unequal fertility of hybrids produced from reciprocal
HYBRIDISM
83
crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either
pure parent. Nor do I pretend that the foregoing re-
marks 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 organization having been disturbed by two
organizations being compounded into one.
A similar parallelism holds good with an allied yet
very different class of facts. It is an old and almost
universal belief, founded on a considerable body of evi-
dence which I have elsewhere - given, 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, etc., from
one soil or climate to another, and back again. During
the convalescence of animals, great benefit is derived
from almost any change in their habits of life. Again,
both with plants and animals, there is the clearest evi-
dence that a cross between individuals of the same
species, which differ to a certain extent, gives vigor and
fertility to the offspring; and that close interbreeding
continued during several generations between the nearest
relations, if these be kept under the same conditions of
life, almost always leads to decreased size, weakness,
or sterility.
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
34
THE ORIGIN OF SPECIES
between the males and females of the same species, which
have been subjected to slightly different conditions, or
which have slightly varied, give vigor and fertility to
the offspring. But, as we have seen, organic beings long
habituated to certain uniform conditions under a state
of nature, when subjected, as under confinement, to a
considerable change in their conditions, very frequently
are rendered more or less sterile; and we know that
a cross between two forms, that have become widely or
specifically different, produce hybrids which are almost
always in some degree sterile. I am fully persuaded that
this double parallelism is by no means an accident or an
illusion. He who is able to explain why the elephant
and a multitude of other animals are incapable of breed-
ing when kept under only partial confinement in their
native country will be able to explain the primary cause
of hybrids being so generally sterile. He will at the
same time be able to explain how it is that the races
of some of our domesticated animals, which have often
been subjected to new and not uniform conditions, are
quite fertile together, although they are descended from
distinct species, which would probably have been sterile
if aboriginally crossed. The above two parallel series of
facts seem to be connected together by some common
but unknown bond, which is essentially related to the
principle of life; this principle, according to Mr. Herbert
Spencer, being that life depends on, or consists in, the
incessant action and reaction of various forces, which, as
throughout nature, are always tending toward an equilib-
rium; and when this tendency is slightly disturbed by any
change, the vital forces gain in power.
HYBRIDISM
35
Reciprocal Dimorphism and Trimorphism
This subject may be here briefly discussed, and will
be found to throw some light on hybridism. Several
plants belonging to distinct orders present two forms,
which exist in about equal numbers and which differ in
no respect except in their reproductive organs; one form
having a long pistil with short stamens, the other a short
pistil with long stamens; the two having differently sized
pollen grains. With trimorphic plants there are three
forms likewise differing in the lengths of their pistils
and stamens, in the size and color of the pollen-grains,
and in some other respects; and as in each of the three
forms there are two sets of stamens, the three forms
possess altogether six sets of stamens and three kinds of
pistils. These organs are so proportioned in length
to each other that half the stamens in two of the forms
stand on a level with the stigma of the third form.
Now I have shown, and the result has been confirmed by
other observers, that, in order to obtain full fertility with
these plants, it is necessary that the stigma of the one
form should be fertilized by pollen taken from the
stamens of corresponding height in another form. So
that with dimorphic species two unions, which may be
called legitimate, are fully fertile; and two, which may
be called illegitimate, are more or less infertile. With
trimorphic species six unions are legitimate, or fully
fertile — and twelve are illegitimate, or more or less in-
fertile.
The infertility which may be observed in various
dimorphic and trimorphic plants, when they are illegiti-
mately fertilized, that is, by pollen taken from stamens
36
THE ORIGIN OF SPECIES
not corresponding in height with the pistil, differs much
in degree, up to absolute and utter sterility; just in the
same manner as occurs in crossing distinct species. As
the degree of sterility in the latter case depends in an
eminent degree on the conditions of life being more or
less favorable, so I have found it with illegitimate unions.
It is well known that if pollen of a distinct species be
placed on the stigma of a flower, and its own pollen
be afterward, even after a considerable interval of time,
placed on the same stigma, its action is so strongly pre-
potent that it generally annihilates the effect of the
foreign pollen; so it is with the pollen of the several
forms of the same species, for legitimate pollen is
strongly prepotent over illegitimate pollen, when both
are placed on the same stigma. I ascertained this by
fertilizing several flowers, first illegitimately, and twenty-
four hours afterward legitimately, with pollen taken from
a peculiarly colored variety, and all the seedlings were
similarly colored; this shows that the legitimate pollen,
though applied twenty-four hours subsequently, had
wholly destroyed or prevented the action of the pre-
viously applied illegitimate pollen. Again, as in making
reciprocal crosses between the same two species, there is
occasionally a great difference in the result, so the same
thing occurs with trimorphic plants; for instance, the
mid-styled form of Lythrum salicaria was illegitimately
fertilized with the greatest ease by pollen from the longer
stamens of the short-styled form, and yielded many
seeds; but the latter form did not yield a single seed
when fertilized by the longer stamens of the mid-styled
form.
In all these respects, and in others which might be
HYBRIDISM
87
added, the forms of the same undoubted species when
illegitimately united behave in exactly the same manner
as do two distinct species when crossed. This led me
carefully to observe during four years many seedlings,
raised from several illegitimate unions. The chief result
is that these illegitimate plants, as they may be called,
are not fully fertile. It is possible to raise from dimor-
phic species both long-styled and short-styled illegitimate
plants, and from trimorphic plants all three illegitimate
forms. These can then be properly united in a legitimate
manner. When this is done, there is no apparent reason
why they should not yield as many seeds as did their
parents when legitimately fertilized. But such is not the
case. They are all infertile, in various degrees; some
being so utterly and incurably sterile that they did not
yield during four seasons a single seed or even seed-
capsule. The sterility of these illegitimate plants, when
united with each other in a legitimate manner, may be
strictly compared with that of hybrids when crossed
inter se. If, on the other hand, a hybrid is crossed with
either pure parent-species, the sterility is usually much
lessened: and so it is when an illegitimate plant ia
fertilized by a legitimate plant. In the same manner
as the sterility of hybrids does not always run parallel
with the difficulty of making the first cross between the
two parent-species, so the sterility of certain illegitimate
plants was unusually great, while the sterility of the
union from which they were derived was by no means
great. With hybrids raised from the same seed-capsule
the degree of sterility is innately variable, so it is in a
marked manner with illegitimate plants. Lastly, many
hybrids are profuse and persistent flowerers, while other
88
THE ORIGIN OF SPECIES
and more sterile hybrids produce few flowers, and are
weak, miserable dwarfs; exactly similar cases occur with
the illegitimate offspring of various dimorphic and tri-
morphic plants..
Altogether there is the closest identity in character
and behavior between illegitimate plants and hybrids. It
is hardly an exaggeration to maintain that illegitimate
plants are hybrids, produced within the limits of the
same species by the improper union of certain forms,
while ordinary hybrids are produced from an improper
union between so-called distinct species. We have also
already seen that there is the closest similarity in all
respects between first illegitimate unions and first crosses
between distinct species. This will perhaps be made
more fully apparent by an illustration; we may suppose
that a botanist found two well-marked varieties (and such
occur) of the long-styled form of the trimorphic Lythrum
salicaria, and that he determined to try by crossing
whether they were specifically distinct. He would find
that they yielded only about one-fifth of the proper
number of seed, and that they behaved in all the other
above specified respects as if they had been two distinct
species. But to make the case sure, he would raise
plants from his supposed hybridized seed, and he would
find that the seedlings were miserably dwarfed and utterly
sterile, and that they behaved in all other respects like
ordinary hybrids. He might then maintain that he had
actually proved, in accordance with the common view,
that his two varieties were as good and as distinct
species as any in the world; but he would be completely
mistaken.
The facts now given on dimorphic and trimorphic
HYBRIDISM
39
plants are important, because they show us, first, that
the physiological test of lessened fertility, both in first
crosses and in hybrids, is no safe criterion of specific
distinction; secondly, because we may conclude that there
is some unknown bond which connects the infertility of
illegitimate unions with that of their illegitimate off-
spring, and we are led to extend the same view to first
crosses and hybrids; thirdly, because we find, and this
seems to me of especial importance, that two or three
forms of the same species may exist and may differ in
no respect whatever, either in structure or in constitu-
tion, relatively to external conditions, and yet be sterile
when united in certain ways. For we must remember
that it is the union of the sexual elements of individuals
of the same form, for instance, of two long-styled forms,
which results in sterility; while it is the union of the
sexual elements proper to two distinct forms which is
fertile. Hence the case appears at first sight exactly the
reverse of what occurs, in the ordinary unions of the
individuals of the same species and with crosses between
distinct species. It is, however, doubtful whether this
is really so; but I will not enlarge on this obscure
subject.
We may, however, infer as probable, from the con-
sideration of dimorphic and trimorphic plants, that the
sterility of distinct species when crossed, and of their
hybrid progeny, depends exclusively on the nature of
their sexual elements, and not on any difference in their
structure or general constitution. We are also led to this
same conclusion by considering reciprocal crosses, in
which the male of one species cannot be united, or can
be united with great difficulty, with the female of a
—Science — 19
fO
THE ORIGIX OF SPECIES
second species, while the converse cross can be effected
with perfect facility. That excellent observer, Gartner,
likewise concluded that species when crossed are sterile
owing to differences confined to their reproductive systems.
Fertility of Varieties when Crossed, and of their Mongrel
Offspring, not universal
It may be urged, as an overwhelming argument, that
there must be some essential distinction between species
and varieties, inasmuch as the latter, however much they
may differ from each other in external appearance, cross
with perfect facility, and yield perfectly fertile offspring.
"With some exceptions, presently to be given, I fully
admit that this is the rule. But the subject i9 surrounded
by difficulties, for, looking to varieties produced under
nature, if two forms hitherto reputed to be 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, which are considered by most
botanists as varieties, are said by Gartner to be quite
sterile when crossed, and he consequently ranks them as
undoubted species. If we thus argue in a circle, the
fertility of all varieties produced under nature will
assuredly have to be granted.
If we turn to varieties, produced, or supposed to have
been produced, under domestication, we are still involved
in some doubt. For when it is stated, for instance, that
certain South American indigenous domestic dogs do not
readily unite with European dogs, the explanation which
will occur to every one. and probably the true one, is
that they are descended from aboriginally distinct species.
HYBRIDISM
±1
Nevertheless the perfect fertility of so many domestic
races, differing widely from each other in appearance, for
instance, those of the pigeon, or of the cabbage, is a
remarkable fact; more especially when we reflect how
many species there are, which, though resembling each
other most closely, are utterly sterile when intercrossed.
Several considerations, however, render the fertility of
domestic varieties less remarkable. In the first place, it
may be observed that the amount of external difference
beween two species is no sure guide to their degree of
mutual sterility, so that similar differences in the case of
varieties would be no sure guide. It is certain that with
species the cause lies exclusively in differences in their
sexual constitution. Now the varying conditions to
which domesticated animals and cultivated plants have
been subjected, have had so little tendency toward modi-
fying the reproductive system in a manner leading to
mutual sterility, that we have good grounds for admitting
the directly opposite doctrine of Pallas; namely, that
such conditions generally eliminate this tendency; so
that the domesticated descendants of species which, in
their natural state, probably would have been in some
degree sterile when crossed, become perfectly fertile
together. With plants, so far is cultivation from giving
a tendency toward sterility between distinct species that
in several well-authenticated cases already alluded to,
certain plants have been affected in an opposite manner,
for they have become self-impotent while still retaining
the capacity of fertilizing, and being fertilized by, other
species. If the Pallasian doctrine of the elimination of
sterility through long-continued domestication be ad-
mitted, and it can hardly be rejected, it becomes in the
42
THE ORIGIN OF SPECIES
highest degree improbable that similar conditions long-
continued should likewise induce this tendency: though
in certain cases, with species having a peculiar constitu-
tion, sterility might occasionally be thus caused. Thus,
as I believe, we can understand why with domesticated
animals varieties have not been produced which are
mutually sterile; and why with plants only a few such
cases, immediately to be given, have been observed.
The real difficulty in our present subject is not, as it
appears to me, why domestic varieties have not become
mutually infertile when crossed, but why this has so
generally occurred with natural varieties, as soon as they
have been permanently modified in a sufficient degree to
take rank as species. We are far from precisely knowing
the cause; nor is this surprising, seeing how profoundly
ignorant we are in regard to the normal and abnormal
action of the reproductive system. But we can see that
species, owing to their struggle for existence with numer-
ous competitors, will have been exposed during long
periods of time to more uniform conditions than have
domestic varieties; and this may well make a wide differ-
ence in the result. For we know how commonly wild
animals and plants, when taken from their natural condi-
tions and subjected to captivity, are rendered sterile; and
the reproductive functions of organic beings which have
always lived under natural conditions would probably in
like manner be eminently sensitive to the influence of an
unnatural cross. Domesticated productions, on the other
hand, which, as shown by the mere fact of their domes-
tication, were not originally highly sensitive to changes
in their conditions of life, and which can now generally
resist with undiminished fertility repeated changes of con-
HYBRIDISM
43
ditions, might be expected to produce varieties, which
would be little liable to have their reproductive powers
injuriously affected by the act of crossing with other
varieties which had originated in a like manner.
I have as yet spoken as if the varieties of the same
species were invariably fertile when intercrossed. But it
is impossible to resist the evidence of the existence of
a certain amount of sterility in the few following cases,
which I will briefly abstract. The evidence is at least
as good as that from which we believe in the sterility
of a multitude of species. The evidence is, also, de-
rived from hostile witnesses, who in all other cases con-
sider 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 with
red seeds growing near each other in his garden; and
although these plants have separated sexes, they never
naturally crossed. He then fertilized thirteen flowers of
the one kind with pollen of the other; but only a single
head produced any seed, and this one head produced
only five grains. Manipulation in this case could not
have been injurious, as the plants have separated sexes.
No one, I believe, has suspected that these varieties of
maize are distinct species; and it is important to notice
that the hybrid plants thus raised were themselves
perfectly fertile; so that even Gartner did not venture to
consider the two varieties as specifically distinct.
Girou de Buzareingues crossed three varieties of
gourd, which like the maize has separated sexes, and
he asserts that their mutual fertilization is by so much
the less easy as their differences are greater. How far
these experiments may be trusted I know not; but the
44
THE 0R1G1X OF SPECIES
forms experimented on are ranked by Sageret, who
mainly founds his classification by the test of infertility,
as varieties, and Naudin has come to the same conclu-
sion.
The following case is far more remarkable, and seems
at first incredible: but it is the result of an astonishing
number of experiments made during many years on nine
species of Yerbascum, by so good an observer and so
hostile a witness as Gartner; namely, that the yellow and
white varieties when crossed produce less seed than the
similarly colored varieties of the same species. Moreover,
he asserts that, when yellow and white varieties of one
species are crossed with yellow and white varieties of a
distinct species, more seed is produced by the crosses
between the similarly colored flowers than between those
which are differently colored. Mr. Scott also has experi-
mented on the species and varieties of Yerbascum; and
although unable to confirm Gartner's results on the cross-
ing of the distinct species, he finds that the dissimilarly
colored varieties of the same species yield fewer seeds, in
the proportion of 86 to 100, than the similarly colored
varieties. Yet these varieties differ in no respect except
in the color of their flowers; and one variety can some-
times be raised from the seed of another.
Kolreuter, whose accuracy has been confirmed by
every subsequent observer, has proved the remarkable
Tact, that one particular variety of the common tobacco
was more fertile than the other varieties, when crossed
with a widely distinct species. He experimented on five
forms which are commonly reputed to be varieties, and
which he tested by the severest trial, namely, by recip-
rocal crosses, and he found their mongrel offspring per-
HYBRIDISM
45
fectly fertile. But one of these five varieties, when used
either as the father or mother, and crossed with the
Nicotiana glutinosa, always yielded hybrids not so ster-
ile as those which were produced from the four other
varieties when crossed with N. glutinosa. Hence the
reproductive system of this one variety must have been
in some manner and in some degree modified.
From these facts it can no longer be maintained that
varieties when crossed are invariably quite fertile. From
the great difficulty of ascertaining the infertility of varie-
ties in a state of nature, for a supposed variety, if proved
to be infertile in any degree, would almost universally be
ranked as a species; — from man attending only to exter-
nal characters in his domestic varieties, and from such
varieties not having been exposed for very long periods
to uniform conditions of life; — from these several con-
siderations we may conclude that fertility does not con-
stitute a fundamental distinction between varieties and
species when crossed. The general sterility of crossed
species may safely be looked at, not as a special ac-
quirement or endowment, but as incidental on changes
of an unknown nature in their sexual elements.
Hybrids and Mongrels compared, independently of their
fertility
Independently of the question of fertility, the offspring
of species and of varieties when crossed may be com-
pared in several other respects. Gartner, whose strong
wish it was to draw a distinct line between species and
varieties, could find very few, and, as it seems to me,
quite unimportant differences between the so-called hy-
46
THE ORIGIN OF SPECIES
brid offspring of species, and the so-called mongrel off-
spring of varieties. And, on the other hand, they agree
most closely in many important respects.
I shall here discuss this subject with extreme brevity.
The most important distinction is, that in the first gen-
eration mongrels are more variable than hybrids; but
Gartner admits that hybrids from species which have
long been cultivated are often variable in the first gen-
eration; 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 distinct species; and this shows that the difference
in the degree of variability graduates away. When
mongrels and the more fertile hybrids are propagated
for several generations, an extreme amount of variability
in the offspring in both cases is notorious; but some few
instances of both hybrids and mongrels long retaining a
uniform character could be given. The variability, how-
ever, in the successive generations of mongrels is, per-
haps, greater than in hybrids.
This greater variability in mongrels than in hybrids
does not seem at all surprising. For the parents of
mongrels are varieties, and mostly domestic varieties
(very few experiments having been tried on natural
varieties), and this implies that there has been recent
variability, which would often continue and would aug-
ment that arising from the act of crossing. The slight
variability of hybrids in the first generation, in contrast
with that in the succeeding generations, is a curious fact
and deserves attention. For it bears on the view which
I have taken of one of the causes of ordinary variabil-
ity; namely, that the reproductive system, from being
HYBRIDISM
47
eminently sensitive to changed conditions of life, fails
under these circumstances to perform its proper function
of producing offspring closely similar in all respects to
the parent-form. Now hybrids in the first generation are
descended from species (excluding those long-cultivated)
which have not had their reproductive systems in any
way affected, and they are not variable; but hybrids
themselves have their reproductive systems seriously
affected, and their descendants are highly variable.
But to return to our comparison of mongrels and
hybrids: 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.
Moreover, Gartner expressly states that hybrids from long
cultivated plants are more subject to reversion than hy-
brids from species in their natural state; and this prob*
ably explains the singular difference in the results arrived
at by different observers: thus Max Wichura doubts
whether hybrids ever revert to their parent-forms, and
he experimented on uncultivated species of willows; while
Naudin, on the other hand, insists in the strongest terms
on the almost universal tendency to reversion in hybrids,
and he experimented chiefly on cultivated plants. Gart-
ner further states that when any two species, although
most closely allied to each other, are crossed with a third
species, the hybrids are widely different from each other;
whereas if two very distinct varieties of one species are
crossed with another species, the hybrids do not differ
much. But this conclusion, as far as I can make out,
is founded on a single experiment; and seems directly
opposed to the results of several experiments made by
Kolreuter.
48
THE ORIGIS OF SPECIES
Such alone are the unimportant differences which
Gartner is able to point out between hybrid and mon-
grel plants. On the other hand, the degrees and kinds
of resemblance in mongrels and in hybrids to their re-
spective parents, more especially in hybrids produced
from nearly related species, follow according to Gartner
the same laws. "When two species are crossed, one has
sometimes a prepotent power of impressing its likeness
on the hybrid. So I believe it to be with varieties of
plants; and with animals one variety certainly often has
this prepotent power over another variety. Hybrid plants
produced from a reciprocal cross generally resemble each
other closely; and so it is with mongrel plants from a
reciprocal cross. Both hybrids and mongrels can be re-
duced 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 much complicated, partly
owing to the existence of secondary • sexual characters;
but more especially owing to prepotency in transmitting
likeness running more strongly in one sex than in the
other, both when one species is crossed with another,
and when one variety is crossed with another variety.
For instance, I think those authors are right who main-
tain that the ass has a prepotent power over the horse,
so that both the mule and the hinny resemble more
closely the ass than the horse; but that the prepotency
runs more strongly in the male than in the female
ass, 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.
HYBRIDISM
49
Much stress has been laid by some authors on the
supposed fact, that it is only with mongrels that the
offspring are not intermediate in character, but closely
resemble one of their parents; but this does sometimes
occur with hybrids, yet I grant much less frequently
than with mongrels. Looking to the cases which I have
collected of cross-bred animals closely resembling one
parent, the resemblances seem chiefly confined to char-
acters almost monstrous in their nature, and which have
suddenly appeared — such as albinism, melanism, defi-
ciency of tail or horns, or additional fingers and toes;
and do not relate to characters which have been slowly
acquired through selection. A tendency to sudden re-
versions to the perfect character of either parent would,
also, be much more likely to occur with mongrels, which
are descended from varieties often suddenly produced and
semi-monstrous in character, than with hybrids, which
are descended from species slowly and naturally pro-
duced.
On the whole, I entirely agree with Dr. Prosper
Lucas, who, after arranging an enormous body of facts
with respect to animals, comes to the conclusion that the
laws of resemblance of the child to its parents are the
same, whether the two parents differ little or much from
each other, namely, in the union of individuals of the
same variety, or of different varieties, or of distinct
species.
Independently of 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
50
THE ORIGIN OF SPECIES
produced by secondary laws, this similarity would be an
astonishing fact. But it harmonizes perfectly with the
view that there is no essential distinction between species
and varieties.
Summary
First crosses between forms, sufficiently distinct to be
ranked as species, and their hybrids, are very generally,
but not universally, sterile. The sterility is of all de-
grees, and is often so slight that the most careful ex-
perimentalists have arrived at diametrically opposite con-
clusions in ranking forms by this test. The sterility is
innately variable in individuals of the same species, and
is eminently susceptible to the action of favorable and
unfavorable conditions. The degree of sterility does not
strictly follow systematic affinity, but is governed by sev-
eral curious and complex laws. It is generally different,
and sometimes widely different in reciprocal crosses be-
tween the same two species. It is not always equal in
degree in a first cross and in the hybrids produced from
this cross.
In the same manner as in grafting trees, the capacity
in one species or variety to take on another is incidental
on differences, generally of an unknown nature, 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 their crossing and blending in nature, than to
think that trees have been specially endowed with vari-
ous and somewhat analogous degrees of difficulty in
HYBRIDISM
51
being grafted together in order to prevent their inarch-
ing in our forests.
The sterility of first crosses and of their hybrid
progeny has not been acquired through natural selection.
In the case of first crosses it seems to depend on several
circumstances; in some instances in chief part on the
early death of the embryo. In the case of hybrids, it
apparently depends on their whole organization having
been disturbed by being compounded from two distinct
forms; the sterility being closely allied to that which so
frequently affects pure species, when exposed to new and
unnatural conditions of life. He who will explain these
latter cases will be able to explain the sterility of hybrids.
This view is strongly supported by a parallelism of auother
kind; namely, that, first, slight changes in the conditions
of life add to the vigor and fertility of all organic beings;
and, secondly, that the crossing of forms, which have
been exposed to slightly different conditions of life or
which have varied, favors the size, vigor and fertility
of their offspring. The facts given on the sterility of the
illegitimate unions of dimorphic and trimorphic plants and
of their illegitimate progeny, perhaps render it probable
that some unknown bond in all cases connects the degree
of fertility of first unions with that of their offspring.
The consideration of these facts on dimorphism, as well
as of the results of reciprocal crosses, clearly leads to the
conclusion that the primary cause of the sterility of
crossed species is confined to differences in their sexual
elements. But why, in the case of distinct species, the
sexual elements should so generally have become more or
less modified, leading to their mutual infertility, we do
not know; but it seems to stand in some close relation
52
THE ORIGIN OF SPECIES
to species having been exposed for long periods of time
to nearly uniform conditions of life.
It is not surprising that the difficulty in crossing any
two species, and the sterility of their hybrid offspring,
should in most cases correspond, even if due to distinct
causes: for both depend on the amount of difference
between the species which are crossed. Nor is it sur-
prising that the facility of effecting a first cross, and the
fertility of the hybrids thus 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 subjected to experiment;
for systematic affinity includes resemblances of all kinds.
First crosses between forms known to be varieties, or
sufficiently alike to be considered as varieties, and their
mongrel offspring, are very generally, but not, as is so
often stated, invariably fertile. Nor is this almost uni-
versal and perfect fertility surprising, when it is remem-
bered 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 that they have not been long exposed to
uniform conditions of life. It should also be especially
kept in mind that long-continued domestication tends to
eliminate sterility, and is therefore little likely to induce
this same quality. Independently of the question of
fertility, in all other respects there is the closest general
resemblance between hybrids and mongrels — in their
variability, in their power of absorbing each other by
repeated crosses, and in their inheritance of characters
HYBRIDISM
53
from both parent-forms. Finally, then, although we are
as ignorant of the precise cause of the sterility of first
crosses and of hybrids as we are why animals and plants
removed from their natural conditions become sterile, yet
the facts given in this chapter do not seem to me
opposed to the belief that species aboriginally existed
as varieties.
54
THE ORIGIN OF SPECIES
CHAPTER X
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD
On the absence of intermediate varieties at the present day— On the nature
of extinct intermediate varieties; on their number — On the lapse of
time, as inferred from the rate of denudation and of deposition — On the
lapse of time as estimated by years — On the poorness of our paleontology
ical collections — On the intermittence of geological formations — On the
denudation of granitic areas — 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 —
Antiquity of the habitable earth
IN THE sixth chapter I enumerated the chief objec-
tions 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 innumer-
able 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 favorable for their presence, namely, on an exten-
sive and continuous area with graduated physical con-
ditions. I endeavored to show that the life of each
species depends in a more important manner on the
presence of other already defined organic forms, than on
climate, and, therefore, that the really governing condi-
tions of life do not graduate away quite insensibly like
heat or moisture. I endeavored, also, to show that inter-
mediate varieties, from existing in lesser numbers than
IMPERFECTION OF GEOLOGICAL RECORD 55
the forms which they connect, will generally be beaten
out and exterminated during the course of further modi-
fication and improvement. The main cause, however, of
innumerable intermediate links not now occurring every-
where throughout nature, depends on the very process of
natural selection, through which new varieties continually
take the places of aud supplant their parent-forms. But
just in proportion as this process of extermination has
acted on an enormous scale, so must the number of
intermediate varieties, which have formerly existed, be
truly enormous. Why then is not every geological for-
mation and every stratum full of such intermediate links ?
Geology assuredly does not reveal any such finely-
graduated organic chain; and this, perhaps, is the most
obvious and serious objection which can be urged against
the theory. The explanation lies, as I believe, in the
extreme imperfection of the geological record.
In the first place, it should always be borne in mind
what sort of intermediate forms must, on the theory,
have formerly existed. I have found it difficult, when
looking at any two species, to avoid picturing to myself
forms directly intermediate between them. But this is a
wholly false view; we should always look for forms
intermediate between each species and a common but
unknown progenitor; and the progenitor will generally
have differed in some respects from all its modified de-
scendants. To give a simple illustration: the fantail and
pouter pigeons are 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
56
THE ORIGIN OF SPECIES
pouter; none, for instance, combining a tail somewhat
expanded with a crop somewhat enlarged, the character-
istic features of these two breeds. These two breeds,
moreover, have become so much modified that, if we had
no historical or indirect evidence regarding their origin,
it would not have been possible to have determined,
from a mere comparison of their structure with that of
the rock-pigeon, C. livia, whether they had descended
from this species or from some other allied form, such
as C. oenas.
So, with natural species, if we look to forms very
distinct, for instance, to the horse and tapir, we have
no reason to suppose that links directly intermediate
between them ever existed, but between each and an
unknown common parent. The common parent will have
had in its whole organization much general resemblance
to the tapir and to the horse; but in some points of
structure may have differed considerably from both, even
perhaps more than they differ from each other. Hence,
in all such cases, we should be unable to recognize the
parent-form of any two or more species, even if we
closely compared the structure of the parent with that of
its modified descendants, unless at the same time we had
a nearly perfect chain of the intermediate links.
It is just possible by the 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, while its descendants had
undergone a vast amount of change; and the principle
of competition between organism ?,nd organism, between
IMPERFECTION OF GEOLOGICAL RECORD 57
ohild and parent, will render this a very rare event; for
in all cases the new and improved forms of life tend to
supplant 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 natural and domestic 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 forms; and so on backward, always con-
verging to the common ancestor of each great class. So
that the number of intermediate and transitional links,
between all living and extinct species, must have been
inconceivably great. But assuredly, if this theory be
true, such have lived upon the earth.
On the Lapse of Time, as inferred from the rate of Depo-
sition and extent of Denudation
Independently of our not finding fossil remains of
such infinitely numerous connecting links, it may be
objected that time cannot have sufficed for so great an
amount of organic change, all changes having been
effected slowly. It is hardly possible for me to recall to
the reader who is not a practical geologist the facts
leading the mind feebly to comprehend the lapse of
time. He who can read Sir Charles Lyell's grand work
on the Principles of Geology, which the future historian
will recognize as having produced a revolution in natural
science, and yet does not admit how 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
68
THE ORIGIN OF SPECIES
formations, and to mark how each author attempts to
give aD inadequate idea of the duration of each forma-
tion, or even of each stratum. We can best gain some
idea of past time by knowing the agencies at work, and
learning how deeply the surface of the land has been
denuded, and how much sediment has been deposited.
As Lyell has well remarked, the extent and thickness
of our sedimentary formations are the result and the
measure of the denudation which the earth's crust has
elsewhere undergone. Therefore a man should examine
for himself the great piles of superimposed strata, and
watch the rivulets bringing down mud, and the waves
wearing away the sea-cliffs, in order to comprehend
something about the duration of past time, the monu-
ments of which we see all around us.
It is good to wander along the coast, when formed of
moderately hard rocks, and mark the process of degrada-
tion. The tides in most cases reach the cliffs only for
a short time twice a day. and the waves eat into them
only when they are charged with sand or pebbles; for
there is good evidence that pure water effects nothing
in wearing away rock. At last the base of the cliff is
undermined, huge fragments fall clown, and these, re-
maining fixed, have to be worn away atom by atom,
until, after being reduced in size, they can be rolled
about by the waves, and then they are more quickly
ground into pebbles, sand, or mud. But how often do
we see along the bases of retreating cliffs rounded bowl-
ders, sll thickly clothed by marine productions, showing
how little they are abraded and how seldom they are
rolled about! Moreover, if we follow for a few miles
any line of rocky cliff, which is undergoing degradation,
IMPERFECTION OF GEOLOGICAL RECORD 59
we find that it is only here and there, along a short
length or round a promontory, that the cliffs are at the
present time suffering. The appearance of the surface
and the vegetation show that elsewhere years have
elapsed since the waters washed their base.
We have, however, recently learned from the observa-
tions of Ramsay, in the van of many excellent observers
— of Jukes, Geikie, Croll, and others — that subaerial deg-
radation is a much more important agency than coast-
action, or the power of the waves. The whole surface
of the land is exposed to the chemical action of the air
and of the rain-water with its dissolved carbonic acid,
and in colder countries to frost; the disintegrated matter
is carried down even gentle slopes during heavy rain,
and to a greater extent than might be supposed, espe-
cially in arid districts, by the wind; it is then trans-
ported by the streams and rivers, which when rapid
deepen their channels, and triturate the fragments. On
a rainy day, even in a gently undulating country, we
see the effects of subaerial degradation in the muddy
rills which flow down every slope. Messrs. Ramsay and
Whitaker have shown, and the observation is a most
striking one, that the great lines of escarpment in the
Wealden district and those ranging across England, which
formerly were looked at as ancient sea-coasts, cannot have
been thus formed, for each line is composed of one and
the same formation, while our sea-cliffs are everywhere
formed by the intersection of various formations. This
being the case, we are compelled to admit that the es-
carpments owe their origin in chief part to the rocks
of which they are composed having resisted subaerial
denudation better than the surrounding surface; this sur-
60
THE ORIGIS OF SPECIES
face consequently has been gradually lowered, with the
lines of harder rock left projecting. Nothing impresses
the mind with the vast duration of time, according to
our ideas of time, more forcibly than the conviction thus
gained that subaerial agencies which apparently have so
little power, and which seem to work so slowly, have
produced great results.
When thus impressed with the slow rate at which the
land is worn away through subaerial and littoral action,
it is good, in order to appreciate the past duration of
time, to consider, on the one hand, the masses of rock
which have been removed over many extensive areas,
and on the other hand the thickness of our sedimentary
formations. I remember having been much struck when
viewing volcanic islands, which have been worn by the
waves and pared all round into perpendicular cliffs of
one or two thousand feet in height; for the gentle slope
of the lava-streams, due to their formerly liquid state,
showed at a glance how far the hard, rocky beds had
once extended into the open ocean. The same story is
told still more plainly by faults — those gTeat 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, and
it makes no great difference whether the upheaval was
sudden, or, as most geologists now believe, was slow and
effected by many starts, the surface of the land has been
so completely planed down that no trace of these vast
dislocations is externally visible. The Craven fault, for
instance, extends for upward of 30 miles, and along this
line the vertical displacement of the strata varies from
600 to 3,000 feet Professor Ramsay has published an
IMPERFECTION OF GEOLOGICAL RECORD
61
account of a downthrow in Anglesea of 2,300 feet; and
he informs me that he fully believes that there is one in
Merionethshire of 12,000 feet; yet in these cases there
is nothing on the surface of the land to show such
prodigious movements; the pile of rocks on either side
of the crack having been smoothly swept away.
On the other hand, in all parts of the world the piles
of sedimentary strata are of wonderful thickness. In the
Cordillera I estimated one mass of conglomerate at ten
thousand feet; and although conglomerates have probably
been accumulated at a quicker rate than finer sediments,
yet from being formed of worn and rounded pebbles,
each of which bears the stamp of time, they are good
to show how slowly the mass must have been heaped
together. Professor Ramsay has given me the maximum
thickness, from actual measurement in most cases, of the
successive formations in different parts of Great Britain;
and this is the result:
Feet
Paleozoic strata (not including igneous beds) 57,154
Secondary strata 13,190
Tertiary strata 2,240
— making altogether 72.581 feet; that is, very nearly
thirteen and three-quarters British miles. Some of the
formations, which are represented in England by thin
beds, are thousands of feet in thickness on the Conti-
nent. Moreover, between each successive formation we
have, in the opinion of most geologists, blank periods of
enormous length. So that the lofty pile of sedimentary
rocks in Britain gives but an inadequate idea of the time
which has elapsed during their accumulation. The con-
sideration of these various facts impresses the mind al-
S2
THE 0RIG1S OF SPECIES
most in the same manner as does the vain endeavor
to grapple with the idea of eternity.
Nevertheless this impression is partly false. Mr. Croll,
in an interesting paper, remarks that we do not err "in
forming too great a conception of the length of geologi-
cal periods," but in estimating them by years. When
geologists look at large and complicated phenomena, and
then at the figures representing several million years, the
two produce a totally different effect on the mind, and
the figures are at once pronounced too small. In regard
to subaerial denudation, Mr. Croll shows, by calculating
the known amount of sediment annually brought down
by certain rivers, relatively to their areas of drainage,
that 1,000 feet of solid rock, as it became gradually dis-
integrated, would thus be removed from the mean level
of the whole area in the course of six million years.
This seems an astonishing result, and some considerations
lead to the suspicion that it may be too large, but even
if halved or quartered it is still very surprising. Few of
us, however, know what a million really means: Mr. Croll
gives the following illustration: take a narrow strip of
paper. 83 feet 4 inches in length, and stretch it along
the wall of a large hall; then mark off at one end the
tenth of an inch. This tenth of an inch will represent
one hundred years, and the entire strip a million years.
But let it be borne in mind, in relation to the subject
of this work, what a hundred years implies, represented
as it is by a measure utterly insignificant in a hall of the
above dimensions. Several eminent breeders, during a
single lifetime, have so largely modified some of the
higher animals, which propagate their kind much more
slowly than most of the lower animals, that they have
IMPERFECTION OF GEOLOGICAL RECORD 63
formed what well deserves to be called a new sub-breed.
Few men have attended with due care to any one strain
for more than half a century, so that a hundred years
represents the work of two breeders in succession. It is
not to be supposed that species in a state of nature ever
change so quickly as domestic animals under the guid-
ance of methodical selection. The comparison would be
in every way fairer with the effects which follow from
unconscious selection, that is the preservation of the most
useful or beautiful- animals, with no intention of modify-
ing the breed; but by this process of unconscious selec-
tion various breeds have been sensibly changed in the
course of two or three centuries.
Species, however, probably change much more slowly,
and within the same country only a few change at the
same time. This slowness follows from all the inhabi-
tants of the same country being already so well adapted
to each other that new places in the polity of nature do
not occur until after long intervals, due to the occurrence
of physical changes of some kind, or through the immi-
gration of new forms. Moreover, variations or individual
differences of the right nature, by which some of the in-
habitants might be better fitted to their new places under
the altered circumstances, would not always occur at
once. Unfortunately we have no means of determining,
according to the standard of years, how long a period
it takes to modify a species; but to the subject of time
we must return.
On the Poorness of Paleontological Collections
Now let us turn to our richest geological museums,
and what a paltry display we behold I That our collec-
~-Sciexce— 20
64
THE ORIGIN OF SPECIES
tions are imperfect is admitted by every one. The
remark of that admirable paleontologist, Edward Forbes,
should never be forgotten, namely, that very many fossil
species are known and named from single and oftei;
broken specimens, or from a few specimens collected on
some one spot. Only a small portion of the surface of
the earth has been geologically explored, and no part
with sufficient care, as the important discoveries made
every year in Europe prove. No organism wholly soft
can be preserved. Shells and bones decay and disappear
when left on the bottom of the sea, where sediment is
not accumulating. We probably take a quite erroneous
view, when we assume that sediment is being deposited
over nearly the whole bed of the sea, at a rate suf-
ficiently quick to imbed 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 immense interval of time, by another
and later formation, without the underlying bed having
suffered in the interval any wear and tear, seem expli-
cable only on the view of the bottom of the sea not
rarely lying for ages in an unaltered condition. The re-
mains which do become imbedded, 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 water mark seem to
be rarely preserved. For instance, the several species of
the Chthamalinae (a sub-family of sessile cirripeds) coat
the rocks all over the world in infinite numbers: thev
are all strictly littoral, with the exception of a single
IMPERFECTION OF GEOLOGICAL RECORD
Mediterranean species, which inhabits deep water, and
this has been found fossil in Sicily, whereas not one
other species has hitherto been found in any tertiary
formation: yet it is known that the genus Chthamalus
existed during the Chalk period. Lastly, many great
deposits requiring a vast length of time for their accu-
mulation are entirely destitute of organic remains, with-
out our being able to assign any reason: one of the most
striking instances is that of the Flysch formation, which
consists of shale and sandstone, several thousand, occa-
sionally even six thousand, feet in thickness, and extend-
ing for at least 300 miles from Vienna to Switzerland;
and although this great mass has been most carefully
searched, no fossils, except a few vegetable remains,
have been found.
With respect to the terrestrial productions which lived
during the Secondary and Paleozoic periods, it is super-
fluous to state that our evidence is fragmentary in an
extreme degree. For instance, until recently not a land-
shell was known belonging to either of these vast peri-
ods, with the exception of one species discovered by Sir
C. Lyell and Dr. Dawson in the carboniferous strata of
North America; but now land-shells have been found
in the lias. In regard to mammiferous remains, a glance
at the historical table published in Lyell's Manual will
bring home the truth, how accidental and rare is their
preservation, far better than pages of detail. Nor is
their rarity surprising, when we remember how large a
proportion of the bones of tertiary mammals have been
discovered either in caves or in lacustrine deposits; and
that not a cave or true lacustrine bed is known belonging
to the age of our secondary or paleozoic formations.
66
TEE 0RIG1S OF SPECIES
But the imperfection in the geological record largely
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. This doctrine has been emphatically admitted by
many geologists and paleontologists, who, like E. Forbes,
entirely disbelieve in the change of species. When we
see the formations tabulated in written works, or when
we follow them in nature, it is difficult to avoid believ-
ing that they are closely consecutive. But we know, for
instance, from Sir R. Murchison's great work on Russia,
what wide gaps there are in that country between the
superimposed formations; so it is in North America, and
in many other parts of the world. The most skilful
geologist, if his attention had been confined exclusively
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 accu-
mulated. 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 rnineralogical composition of
consecutive formations, generally implying great changes
in the geography of the surrounding lands, whence the
sediment was derived, accord with the belief of vast
intervals of time having elapsed between each formation.
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
IMPERFECTION OF GEOLOGICAL RECORD 67
miles of the South American coasts, which have been
upraised several hundred feet within the recent period,
than the absence of any recent deposits sufficiently ex-
tensive 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 poorly developed that
no record of several successive and peculiar marine
faunas will probably be preserved to a distant age. A
little reflection will explain why, along the rising coast
of the western side of South America, no extensive for-
mations with recent or tertiary remains can anywhere be
found, though the supply of sediment must for ages have
been great, from the enormous degradation of the coast-
rocks and from muddy streams entering the sea. The
explanation, no doubt, is, that the littoral and sub-littoral
deposits are continually worn away, as soon as they are
brought up by the slow and gradual rising of the land
within the grinding action of the coast- waves.
We may, I think, conclude that sediment must be ac-
cumulated in extremely thick, solid, or extensive masses,
in order to withstand the incessant action of the waves,
when first upraised and during successive oscillations of
level, as well as the subsequent subaerial degradation.
Such thick and extensive accumulations of sediment may
be formed in two ways: either in profound depths of the
isea, in which case the bottom will not be inhabited by so
many and such varied forms of life as the more shallow
seas; and the mass when upraised will give an imperfect
record of the organisms which existed in the neighbor-
hood during the period of its accumulation. Or, sedi-
ment may be deposited to any thickness and extent over
a shallow bottom, if it continue slowly to subside. In
THE ORIGIN OF SPECIES
this ratter case, as long as the rate of subsidence and
the supply of sediment nearly balance each other, the sea
will remain shallow and favorable for many and varied
forms, and thus a rich fossiliferous formation, thick
enough, when upraised, to resist a large amount of
denudation, may be formed.
I am convinced that nearly all our ancient formations,
which are throughout the greater part of their thickness
rich in fossils, have thus been formed during subsidence.
Since publishing my views on this subject in 1845, I
have watched the progress of Geology, and have been
surprised to note how author after author, in treating of
this or that great formation, has come to the conclusion
that it was accumulated during subsidence. I may add,
that the only ancient tertiary formation on the west coast
of South America, which has been bulky enough to re-
sist such degradation as it has as yet suffered, but which
will hardly last to a distant geological age, was deposited
during a downward oscillation of level, and thus gained
considerable thickness.
All geological facts tell us plainly that each area has
undergone numerous slow oscillations of level, and appar-
ently these oscillations have affected wide spaces. Conse-
quently, formations rich in fossils and sufficiently thick
and extensive to resist subsequent degradation, will have
been formed over wide spaces during periods of sub-
sidence, but only where the supply of sediment was suffi-
cient to keep the sea shallow and to imbed and preserve
the remains before they had time to decay. On the other
hand, as long as the bed of the sea remains stationary,
thick deposits cannot have been accumulated in the shal-
low parts which are the most favorable to life. Still less
IMPERFECTION OF GEOLOGICAL RECORD 69
can this have happened during the alternate periods of
elevation; or, to speak more accurately, the beds which
were then accumulated will generally have been de-
stroyed by being upraised and brought within the limits
of the coast-action.
These remarks apply chiefly to littoral and sublittoral de-
posits. In the case of an extensive and shallow sea, such
as that within a large part of the Malay Archipelago, where
the depth varies from 30 or 40 to 60 fathoms, a widely
extended formation might be formed during a period of
elevation, and yet not suffer excessively from denudation
during its slow upheaval; but the thickness of the for-
mation could not be great, for owing to the elevatory
movement it would be less than the depth in which it
was formed; nor would the deposit be much consoli-
dated, nor be capped by overlying formations, so that
it would run a good chance of being worn away by
atmospheric degradation and by the action of the sea
during subsequent oscillations of level. It has, how-
ever, been suggested by Mr. Hopkins, that if one part
of the area, after rising and before being denuded, sub-
sided, the deposit formed during the rising movement,
though not thick, might afterward become protected by
fresh accumulations, and thus be preserved for a long-
period.
Mr. Hopkins also expresses his belief that sedimentary
beds of considerable horizontal extent have rarely been
completely destroyed. But all geologists, excepting the
few who believe that our present metamorphic schists
and plutonic rocks once formed the primordial nucleus
of the globe, will admit that these latter rocks have
been stripped of their covering to an enormous extent.
70
THE ORIGIN OF SPECIES
For it is scarcely possible that such rocks could have
been solidified and crystallized while uncovered; but if
the metamorphic action occurred at profound depths of
the ocean, the former protecting mantle of rock may
not have been very thick. Admitting then that gneiss,
mica-schist, granite, diorite, etc., were once necessarily
covered up, how can we account for the naked and ex-
tensive areas of such rocks in many parts of the world,
except on the belief that they have subsequently been
completely denuded of all overlying strata? That such
extensive areas do exist cannot be doubted: the granitic
region of Parime is described by Humboldt as being
at least nineteen times as large as Switzerland. South
of the Amazon, Boue colors an area composed of rocks
of this nature as equal to that of Spain, France, Italy,
part of Germany, and the British Islands, all conjoined.
This region has not been carefully explored, but, from
the concurrent testimony of travellers, the granitic area
is very large: thus, Yon Eschwege gives a detailed sec-
tion of these rocks, stretching from Rio de Janeiro for
260 geographical miles inland in a straight line; and I
travelled for 150 miles in another direction, and saw
nothing but granitic rocks. Numerous specimens, col-
lected along the whole coast from near Rio Janeiro to
the mouth of the Plata, a distance of 1,100 geographi-
cal miles, were examined by me, and they all belonged to
this class. Inland, along the whole northern bank of the
Plata, I saw, besides modern tertiary beds, only one small
patch of slightly metamorphosed rock, which alone could
have formed a part of the original capping of the granitic
series. Turning to a well-known region, namely, to the
United States and Canada, as shown in Professor H. D.
IMPERFECTION OF GEOLOGICAL RECORD 71
Rogers's beautiful map, I have estimated the areas by
cutting out and weighing the paper, and I find that the
metamorphic (excluding 4 'the semi-metamorphic,,) and
granitic rocks exceed, in the proportion of 19 to 12 -5,
the whole of the newer Paleozoic formations. In many
regions the metamorphic and granitic rocks would be
found much more widely extended than they appear to
be, if all the sedimentary beds were removed which
rest unconformably on them, and which could not have
formed part of the original mantle under which they
were crystallized. Hence it is probable that in some
parts of the world whole formations have been com-
pletely denuded, with not a wreck left behind.
One remark is here worth a passing notice. During
periods of elevation the area of the land and of the
adjoining shoal parts of the sea will be increased, and
new stations will often be formed: — all circumstances
favorable, as previously explained, for the formation
of new varieties and species; but during such peri-
ods there will generally be a blank in the geological
record. On the other hand, during subsidence, the in-
habited area and number of inhabitants will decrease
(excepting on the shores of a continent when first broken
up into an archipelago), and consequently during subsi-
dence, though there will be much extinction, few new
varieties or species will be formed; and it is during
these very periods of subsidence that the deposits which
are richest in fossils have been accumulated.
THE ORIGIN OF SPECIES
On the Absence of Numerous Intermediate Varieties in any
Single Formation
From these several considerations it cannot be doubted
that the geological record, viewed as a whole, is ex-
tremely imperfect; but if we confine our attention to any
one formation, it becomes much more difficult to under-
stand why we do not therein find closely graduated vari-
eties between the allied species which lived at its com-
mencement and at its close. Several cases are on record
of the same species presenting varieties in the upper and
lower parts of the same formation; thus, Trautschold
gives a number of instances with Ammonites; and Hil-
gendorf has described a most curious case of ten grad-
uated forms of Planorbis multiformis in the successive
. beds of a fresh-water formation in Switzerland. Although
each formation has indisputably req aired a vast number
of years for its deposition, several reasons can be given
why each should not commonly include a graduated
series of links between the species which lived at its
commencement and close; but I cannot assign due pro-
portional weight to the following considerations.
Although each formation may mark a very long lapse
of years, each probably is short compared with the period
requisite to change one species into another. I am aware
that two paleontologists, whose opinions are worthy of
much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation
is twice or thrice as long as the average duration of
specific forms. But insuperable difficulties, as it seems
to me, prevent us from coming to any just conclusion
IMPERFECTION OF GEOLOGICAL RECORD
73
on this head. When we see a species first appearing
in the middle of any formation, it would be rash in the
extreme to infer that it had not elsewhere previously
existed. So again when we find a species disappearing
before the last layers have been deposited, it would be
equally rash to suppose that it then became extinct. We
forget how small the area of Europe is compared with
the rest of the world; nor have the several stages of the
same formation throughout Europe been correlated with
perfect accuracy.
We may safely infer that with marine animals of all
kinds there has been a large amount of migration due to
climatal and other changes; and when we see a species
first appearing in any formation, the probability is that
it only then first immigrated into that area. It is well
known, for instance, that several species appear somewhat
earlier in the paleozoic beds of North America than in
those of Europe; time having apparently been required
for their migration from the American to the European
seas. In examining the latest deposits in various quar-
ters of the world, it has everywhere been noted that
some few still existing species are common in the de-
posit, but have become extinct in the immediately sur-
rounding sea; or, conversely, that some are now abun-
dant in the neighboring sea, but are rare or absent in
this particular deposit. It is an excellent lesson to reflect
on the ascertained amount of migration of the inhabitants
of Europe during the glacial epoch, which forms only a
part of one whole geological period; and likewise to re-
flect on the changes of level, on the extreme change
of climate, and on the great lapse of time, all included
within this same glacial period. Yet it may be doubted
T4
THE ORIGIN OF SPECIES
whether, in any quarter of the world, sedimentary depos-
its, including fossil remains, have gone on accumulating
within the same area during the whole of this period. It
is not, for instance, probable that sediment was deposited
during the whole of the glacial period near the mouth of
the Mississippi, within that limit of depth at which ma-
rine animals can best flourish: for we know that great
geographical changes occurred in other parts of America
during this space of time. When such beds as were de-
posited in shallow water near the mouth of the Missis-
sippi during some part of the glacial period shall have
been upraised, organic remains will probably first appear
and disappear at different levels, owing to the migrations
of species and to geographical changes. And in the dis-
tant future, a geologist, examining these beds, would be
tempted to conclude that the average duration of life
of the imbedded fossils had been less than that of the
glacial period, instead of having been really far greater,
that is, extending from before the glacial epoch to the
present day.
In order to get a perfect gradation between two forms
in the upper and lower parts of the same formation, the
deposit must have gone on continuously accumulating
during a long period, sufficient for the slow process of
modification; hence the deposit must be a very thick
one; and the species undergoing change must have lived
in the same district throughout the whole time. But we
have seen that a thick formation, fossiliferous throughout
its entire thickness, can accumulate only during a period
of subsidence; and to keep the depth approximately the
same, which is necessary that the same marine species
may live on the same space, the supply of sediment
IMPERFECTION OF GEOLOGICAL RECORD 75
must nearly counterbalance the amount of subsidence.
But this same movement of subsidence will tend to sub-
merge the area whence the sediment is derived, and thus
diminish the supply, while the downward movement con-
tinues. In fact, this nearly exact balancing between the
supply of sediment and the amount of subsidence is
probably a rare contingency; for it has been observed
by more than one paleontologist that very thick deposits
are usually barren of organic remains, except near their
upper or lower limits.
It would seem that each separate formation, like the
whole pile of formations in any country, has generally
been intermittent in its accumulation. When we see, as
is so often the case, a formation composed of beds
of widely different mineralogical composition, we may
reasonably suspect that the process of deposition has
been more or less interrupted. Nor will the closest in-
spection of a formation give us any idea of the length of
time which its deposition may have consumed. Many
instances could be given of beds only a few feet in
thickness, representing formations, which are elsewhere
thousands of feet in thickness, and which must have re-
quired an enormous period for their accumulation; yet
no one ignorant of this fact would have even suspected
the vast lapse of time represented by the thinner forma-
tion. Many cases could be given of the lower beds of a
formation having been upraised, denuded, submerged, and
then recovered by the upper beds of the same formation
— facts, showing what wide, yet easily overlooked, inter-
vals have occurred in its accumulation. In other cases
we have the plainest evidence in great fossilized trees,
still standing upright as they grew, of many long inter-
THE ORIGIN OF SPECIES
vals of time and changes of level during the process of
deposition, which would not have been suspected, had
not the trees been preserved: thus Sir C. Lyell and Dr.
Dawson found carboniferous beds 1,400 feet thick in
Nova Scotia, with ancient root-bearing strata, one above
the other, at no less than sixty-eight different levels.
Hence, when the same species occurs at the bottom,
middle, and top of a formation, the probability is that it
has not lived on the same spot during the whole period
of deposition, but has disappeared and reappeared, per-
haps many times, during the same geological period.
Consequently if it were to undergo a considerable amount
of modification during the deposition of any one geologi-
cal formation, a section would not include all the fine
intermediate gradations which must on our theory have
existed, but abrupt, though perhaps slight, changes of
form.
It is all-important to remember that naturalists have
no golden rule by which to distinguish species and
varieties; they grant some little variability to each
species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank
both as species, unless they are enabled to connect them
together by the closest intermediate gradations; and this,
from the reasons just assigned, we can seldom hope to
effect in any one geological section. Supposing B and C
to be two species, and a third, A, to be found in an
older and underlying bed; even if A were strictly inter-
mediate between B and C, it would simply be ranked as
a third and distinct species, unless at the same time it
could be closely connected by intermediate varieties with
either one or both forms. Nor should it be forgotten, as
IMPERFECTION OF GEOLOGICAL RECORD 77
before explained, that A might be the actual progenitor
of B and C, and yet would not necessarily be strictiy
intermediate between them in all respects. So that we
might obtain the parent-species and its several modified
descendants from the lower and upper beds of the same
formation, and unless we obtained numerous transi-
tional gradations, we should not recognize their blood-
relationship, and should consequently rank them as
distinct species.
It is notorious on what excessively slight differences
many paleontologists have founded their species; and
they do this the more readily if the specimens come
from different sub- stages of the same formation. Some
experienced conchologists are now sinking many of the
very fine species of D'Orbigriy and others into the rank
of varieties; and on this view we do find the kind of
evidence of change which on the theory we ought to
find. Look again at the later tertiary deposits, which
include many shells believed by the majority of natural-
ists to be identical with existing species; but some excel-
lent naturalists, as Agassiz and Pictet, maintain that all
these tertiary species are specifically distinct, though the
distinction is admitted to be very slight; so that here,
unless we believe that these eminent naturalists have
been misled by their imaginations, and that these late
tertiary species really present no difference whatever from
their living representatives, or unless we admit, in oppo-
sition to the judgment of most naturalists, that these
tertiary species are all truly distinct from the recent, we
have evidence of the frequent occurrence of slight modifi-
cations of the kind required. If we look to rather wider
iutervals of time, namely, tc distinct but consecutive
78
THE ORIGIS OF SPECIES
stages of the same great formation, we find that the im-
bedded fossils, though universally ranked as specifically
different, yet are far more closely related to each other
than are the species found in more widely separated for-
mations; so that here again we have undoubted evidence
of change in the direction required by the theory; but
to this latter subject I shall return in the following
chapter.
With animals and plants that propagate rapidly and
do not wander much, there is reason to suspect, as we
have formerly seen, that their varieties are generally at
first local; and that such local varieties do not spread
widely and supplant their parent-forms until they have
been modified and perfected in some considerable degree-
According to this view, the chance of discovering in a
formation in any one country all the early stages of
transition between any two forms, is small, for the suc-
cessive 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 probable that those which had the widest range, far
exceeding the limits of the known geological formations
in Europe, have oftenest given rise, first to local varieties
and ultimately to new species; and this again would
greatly lessen the chance of our being able to trace the
stages of transition in any one geological formation.
It is a more important consideration, leading to the
same result, as lately insisted on by Dr. Falconer,
namely, that the period during which each species under-
went modification, though long as measured by 3*ears,
IMPERFECTION OF GEOLOGICAL RECORD 79
was probably short in comparison with that during which
ic remained without undergoing any change.
It should not be forgotten that, at the present day,
with perfect specimens for examination, two forms can
seldom be connected by intermediate varieties, and thus
proved to be the same species, until many specimens are
collected from many places; and with fossil species this
can rarely be done. We shall, perhaps, best perceive the
improbability of our being enabled to connect species by
numerous, fine, intermediate, fossil links, by asking our-
selves whether, for instance, geologists at some future
period will be able to prove that our different breeds of
cattle, sheep, horses, and dogs are descended from a
single stock or from several aboriginal stocks; or, again,
whether certain sea-shells inhabiting the shores of North
America, which are ranked by some conchologists as
distinct species from their European representatives, and
by other conchologists as only varieties, are really varie-
ties, or are, as it is called, specifically distinct. This
could be effected by the future geologist only by his
discovering in a fossil state numerous intermediate grada-
tions; and such success is improbable in the highest
degree.
It has been asserted over and over again, by writers
who believe in the immutability of species, that geology
yields no linking forms. This assertion, as we shall see
in the next chapter, is certainly erroneous. As Sir
J. Lubbock has remarked, "Every species is a link be-
tween other allied forms." If we take a genus having
a score of species, recent and extinct, and destroy four-
fifths of them, no one doubts that the remainder will
stand much more distinct from each other. If the ex-
80
THE OR1GIX OF SPECIES
treme forms in the genus happen to have been thus
destroyed, the genus itself will stand more distinct from
other allied genera. What geological research has not
revealed, is the former existence of infinitely numerous
gradations, as fine as existing varieties, connecting to-
gether nearly all existing and extinct species. But this
ought not to be expected; yet this has been repeatedly
advanced as a most serious objection against my views.
It may be worth while to sum up the foregoing re-
marks on the causes of the imperfection of the geologi-
cal record under an imaginary illustration. The Malay
Archipelago is about the size of Europe from the North
Cape to the Mediterranean, and from Britain to .Russia;
and therefore equals all the geological formations which
have been examined with any accuracy, excepting those
of the United States of America. I fully agree with Mr
Godwin-Austen, that the present condition of the Malay
Archipelago, with its numerous large islands separated by
wide and shallow seas, probably represents the former
state of Europe, while most of our formations were
accumulating. The Malay Archipelago is one of the
richest regions 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 preserved
in an extremely imperfect manner in the formations
which we suppose to be there accumulating. Not many
of the strictly littoral animals, or of those which lived
on naked submarine rocks, would be imbedded; and
those imbedded in gravel or sand would not endure to
IMPERFECTION OF GEOLOGICAL RECORD 81
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.
Formations rich in fossils of many kinds, and of
thickness sufficient to last to an age as distant in futurity
as the secondary formations lie in the past, would gener-
ally be formed in the archipelago only during periods of
subsidence. These periods of subsidence would be sepa-
rated from each other by immense intervals of time,
during which the area would be either stationary or
rising; while rising, the fossiliferous formations on the
steeper shores would be destroyed, almost as soon as
accumulated, by the incessant coast-action, as we now
see on the shores of South America. Even throughout
the extensive and shallow seas within the archipelago,
sedimentary beds could hardly be accumulated of great
thickness during the periods of elevation, or become
capped and protected by subsequent deposits so as to
have a good chance of enduring to a very distant future.
During the periods of subsidence, there would probably
be much extinction of life; during the periods of eleva-
tion, there would be much variation, but the geological
record would then be less perfect.
It may be doubted whether the duration of any one
great period of subsidence over the whole or part of the
archipelago, together with a contemporaneous accumula-
tion of sediment, would exceed the average duration of
the same specific forms; and these contingencies are
indispensable for the preservation of all the transitional
gradations between any two or more species. If such
gradations were not all fully preserved, transitional
S2
THE OR1G1S OF SPECIES
varieties would merely appear as so many new, though
closely allied species. It is also probable that each great
period of subsidence would be interrupted by oscillations
of level, and that slight ciimatal changes would inter-
vene during such lengthy periods: and in these cases
the inhabitants of the archipelago would migrate, and
no closely consecutive record of their modifications could
be preserved in any one formation.
Very many of the marine inhabitants of the archipel-
ago now range thousands of miles beyond its confines;
and analogy plainly leads to the belief that it would be
chiefly these far-ranging species, though only some of
them, which would oftenest produce new varieties: and
the varieties would at first be local or confined to one
place, but if possessed of any decided advantage, or
when further modified and improved, they would slowly-
spread and supplant their parent-forms. When such
varieties returned to their ancient homes, as they would
differ from their former state in a nearly uniform, though
perhaps extremely slight degree, and as they would be
found imbedded in slightly different sub-stages of the
same formation, they would, according to the principles
followed by many paleontologists, be ranked as new and
distinct species.
I: then there be some degree of truth in these re-
marks, we have no right to expect to find, in our
geological formations, an infinite number of those fine
transitional forms which, on our theory, 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, and such assuredly we do find —
some more distantly, some more closely, related to each
IMPERFECTION OF GEOLOGICAL RECORD 83
other; and these links, let them be ever so close, if
found in different stages of the same formation, would,
by many paleontologists, be ranked as distinct species.
But I do not pretend that I should ever have suspected
how poor was the record in the best preserved geological
sections, had not the absence of innumerable transitional
links between the species which lived at the commence-
ment and close of each formation pressed so hardly on
my theory.
On the sudden Appearance of whole Groups of allied Species
The abrupt manner in which whole groups of species
suddenly appear in certain formations has been urged by
several paleontologists — for instance, by Agassiz, Pictet,
and Sedgwick — as a fatal objection to the belief in the
transmutation of species. If numerous species, belonging
to the same genera or families, have really started into
life at once, the fact would be fatal to the theory of
evolution through natural selection. For the development
by this means of a group of forms, all of which are
descended from some one progenitor, must have been an
extremely slow process; and the progenitors must have
lived long before their modified descendants. But we
continually overrate the perfection of the geological
record, and falsely v infer, because certain genera or
families have not been found beneath a certain stage,
that they did not exist before that stage. In all cases
positive paleontological evidence may be implicitly
trusted; negative evidence is worthless, as experience
has so often shown.' We continually forget how large
the world is, compared with the area over which our
geological formations have been carefully examined; we
THE ORIGIN OF SPECIES
forget that groups of species may elsewhere have long
existed, and have slowly multiplied, before they invaded
the ancient archipelagoes of Europe and the United
States. We do not make due allowance for the intervals
of time which have elapsed between our consecutive
formations — longer perhaps in many 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 parent-form: and in the succeed-
ing formation such groups or species will appear as if
suddenly created.
I may here recall a remark formerly made, namely,
that it might require a long succession of ages to adapt
an organism to some new and peculiar line of life, for
instance, to fly through the. air: and consequently that
the transitional forms would often long remain confined
to some one region; but that, when this adaptation had
once been effected, and a few species had thus acquired
a great advantage over other organisms, a comparatively
short time would be necessary to produce many divergent
forms, which would spread rapidly and widely, throughout
the world. Professor Pictet, in his excellent Eeview of
this work, in commenting on early transitional forms,
and taking birds as an illustration, cannot see how the
successive modifications of the anterior limbs of a sup-
posed prototype could possibly have been of any advan-
tage. But look at the penguins of the Southern Ocean;
have not these birds their front limbs in this precise
intermediate state of "neither true arms nor true
wings"? Yet these birds hold their place victoriously
in the battle for life; for they exist in infinite numbers
and of many kinds. I do not suppose that we here see
IMPERFECTION OF GEOLOGICAL RECORD 85
the real transitional grades through which the wings of
birds have passed; but what special difficulty is there in
believing that it might profit the modified descendants of
the penguin, first to become enabled to flap along the
surface of the sea like the logger-headed duck, and ulti-
mately to rise from its surface and glide through the air ?
I will now give a few examples to illustrate the fore-
going remarks, and to show how liable we are to error
in supposing that whole groups of species have suddenly
been produced. Even in so short an interval as that
between the first and second editions of Pictet's great
work on Paleontology, published in 1844-46 and in
1853-57, the conclusions on the first appearance and
disappearance of several groups of animals have been
considerably modified; and a third edition would require
still further changes. I may recall the well-known fact
that in geological treatises, published not many years
ago, mammals were always spoken of as having abruptly
come in at the commencement of the tertiary series.
And now one of the richest known accumulations of
fossil mammals belongs to the middle of the secondary
series; and true mammals have 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, as far back as the miocene 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 no less than at least thirty
different birdlike animals, some of gigantic size, existed
during that period? Not a fragment of bone has been
pi
THE OE1GLS OF SPECIES
discovered in these beds. Not long ago, paleontologists
maintained that the whole class of birds came suddenly
into existence during the eocene period: but now we
know, on the authority of Professor Owen, that a bird
certainly lived during the deposition of the upper green-
sand; and still more recently, that strange bird, the
Archeopteryx. with a long lizard-like tail, bearing a pair
of feathers on each joint, and with its wings furnished
with two free claws, has been discovered in the oolitic
slates of Soleuhofen. Hardly any recent discovery shows
more forcibly thau this how little we as yet know of
the former inhabitants of the world.
I may give another instance, which, from having
passed under my own eyes, has much struck me. In a
memoir on Fossil Sessile Cirripeds, I stated that, from
the large number of existing and extinct tertiary specie*;
from the extraordinary abundance of the individuals of
many species all over the world, from the Arctic regions
to the equator, inhabiting various zones of depths from
the upper tidal limits to 50 fathoms; from the perfect
manner in which specimens are preserved in the oldest
tertiary beds: from the ease with which even a fragment
of a valve can be recognized; from all these circum-
stances. I inferred that, had sessile cirripeds existed
during the secondary periods, they would certainly have
been preserved and discovered; and as not one species
had then been discovered in beds of this age. I concluded
that this great group had been suddenly developed at the
commencement of the tertiary series. This was a sore
trouble to me, adding as I then thought one more
instance of the abrupt appearance of a great group of
species. But my work had hardly been published, when
D1PERFECTI0X OF GEOLOGICAL RECORD
87
& skilful paleontologist, M. Bosquet, sent me a drawing
of a perfect specimen of an unmistakable sessile cirri ped,
which he had himself extracted from the chalk of Bel-
gium. And, as if to make the case as striking as pos
sible, this cirriped was a Chthamalus, a very common,
large, and ubiquitous genus, of which not one species
has as jet been found even in any tertiary stratum.
Still more recently, a Pyrgoma, a member of a distinct
sub-family of sessile cirripeds, has been discovered by
Mr. "Woodward in the upper chalk; so that we now have
abundant evidence of the existence of this group of
animals during the secondary period.
The case most frequently insisted on by paleontol-
ogists of the apparently sudden appearance of a whole
group of species, is that of the teleostean fishes, low
down, according to Agassiz. in the Chalk period. This
group includes the large majority of existing species.
But certain Jurassic and Triassic forms are now com-
monly admitted to be teleostean; and even some paleo-
zoic forms have thus been classed by one high authority.
If the teleosteans had really appeared suddenly in the
northern hemisphere at the commencement of the chalk
formation, the fact would have been highly remarkable;
but it would not have formed an insuperable difficulty,
unless it could likewise have been shown that at the
same period the species were suddenly and simultane-
ously developed in other quarters of the world. It is
almost superfluous to remark that hardly any fossil-fish
are known from south of the equator; and by running
through Pictet's Paleontology it will be seen that very
few species are known from several formations in Eu-
rope. Some few families of fish now have a confined
— Science — 21
THE ORIGiy OF SPECIES
range; the teleostean fishes might formerly have had a
similarly confined range, and. after having been largely
developed in some one sea, have spread widely. Nor
have we any right to suppose that the seas of the world
have always been so freely open from south to north as
they are at present. Even at this day, if the Malay
Archipelago were converted into land, the tropical parts
of the Indian Ocean would form a large and perfectly
inclosed basin, in which any great group of marine ani-
mals might be multiplied; and here they would remain
confined, until some of the species became adapted to a
cooler climate, and were enabled to double the Southern
capes of Africa or Australia, and thus reach other and
distant seas.
From these considerations, from our ignorance of the
geology of other countries beyond the confines of Europe
and the United States, and from the revolution in our
paleontologicai knowledge effected by the discoveries of
the last dozen years, it seems to me to be about as rash
to dogmatize on the succession of organic forms through-
out the world, as it would be for a naturalist to land for
five minutes on a barren point in Australia, and then
to discuss the number and range of its productions.
On the sudden Appearance of Groups of allied Species in
the lowest known Fossiliferous Strata
There is another and allied difficulty, which is much
more serious. I allude to the manner in which species
belonging to several of the main divisions of the animal
kingdom suddenly appear in the lowest known f ossifer-
ous rocks. Most of the arguments which have convinced
me that all the existing species of the same group are
IMPERFECTION OF GEOLOGICAL RECORD 89
descended from a single progenitor apply with equal
force to the earliest known species. For instance, it
cannot be doubted that all the Cambrian and Silurian
trilobites are descended from some one crustacean, which
must have lived long before the Cambrian age, and
which probably differed greatly from any known ani-
mal. Some of the most ancient animals, as the Nau-
tilus, Lingula, etc., do not differ much from living
species; and it cannot on our theory be supposed that
these old species were the progenitors of all the species
belonging to the same groups which have subsequently
appeared, for they are not in any degree intermediate
in character.
Consequently, if the theory be true, it is indisputable
that before the lowest Cambrian stratum was deposited
long periods elapsed, as long as, or probably far longer
than, the whole interval from the Cambrian age to the
present day; and that during these vast periods the world
swarmed with living creatures. Here we encounter a
formidable objection; for it seems doubtful whether the
earth, in a fit state for the habitation of living creatures,
has lasted long enough. Sir W. Thompson concludes
that the consolidation of the crust can hardly have oc-
curred less than 20 or more than 400 million years ago,
but probably not less than 98 or more than 200 million
years. These very wide limits show how doubtful the
data are; and other elements may have hereafter to be in-
troduced into the problem. Mr. Croll estimates that about
60 million years have elapsed since the Cambrian period,
but this, judging from the small amount of organic
change since the commencement of the Glacial epoch,
appears a very short time for the many and great mu-
THE ORIGIN OF SPECIES
tations of life which have certainly occurred since the
Cambrian formation; and the previous 140 million years
can hardly be considered as sufficient for the develop-
ment of the varied forms of life which already existed
during the Cambrian period. It is, however, probable,
as Sir William Thompson insists, that the world at a
very early period was subjected to more rapid and vio-
lent changes in its physical conditions than those now
occurring; and such changes would have tended to in-
duce changes at a corresponding rate in the organisms
which then existed.
To the question why we do not find rich fossiliferous
deposits belonging to these assumed earliest periods prior
to the Cambrian system, I can give no satisfactory an-
swer. Several eminent geologists, with Sir R. Murchison
at their head, were until recently convinced that we
beheld in the organic remains of the lowest Silurian
stratum the first dawn of life. Other highly competent
judges, as Lyell and E. Forbes, have disputed this con-
clusion. We should not forget that only a small portion
of the world is known with accuracy. Not very long
ago M. Barrande added another and lower stage, abound-
ing with new and peculiar species, beneath the then
known Silurian system; and now, still lower down in
the Lower Cambrian formation, Mr. Hicks has found
in South Wales beds rich in trilobites, and containing
various mollusks and annelids. The presence of phos-
phatic nodules and bituminous matter, even in some of
the lowest azoic rocks, probably indicates life at these
periods; and the existence of the Eozoon in the Lauren-
tian formation of Canada is generally admitted. There
are three great series of strata beneath the Silurian sys-
IMPERFECTION OF GEOLOGICAL RECORD
91
tern in Canada, in the lowest of which the Eozoon is
found. Sir W. Logan states that their " united thickness
may possibly far surpass that of all the succeeding rocks,
from the base of the paleozoic series to the present time.
We are thus carried back to a period so remote that the
appearance of the so-called Primordial fauna (of Bar-
rande) may by some be considered as a comparatively
modern event." The Eozoon belongs to the most lowly
organized of all classes of animals, but is highly organ-
ized for its class; it existed in countless numbers, and,
as Dr. Dawson has remarked, certainly preyed on other
minute organic beings, which must have lived in great
numbers. Thus the words, which I wrote in 1859, about
the existence of living beings long before the Cambrian
period, and which are almost the same with those since
used by Sir W. Logan, have proved true. Nevertheless,
the difficulty of assigning any good reason for the ab-
sence of vast piles of strata rich in fossils beneath the
Cambrian system is very great. It does not seem prob-
able that the most ancient beds have been quite worn
away by denudation, or that their fossils have been
wholly obliterated by metamorphic action, for if this
had been the case we should have found only small
remnants of the formations next succeeding them in age,
and these would always have existed in a partially met-
amorphosed condition. But the descriptions which we
possess of the Silurian deposits over immense territories
in Russia and in North America do not support the
view, that the older a formation is the more invariably
it has suffered extreme denudation and metamorphism.
The case at present must remain inexplicable; and
may be truly urged as a valid argument against the
92
THE ORIGIN OF SPECIES
views here entertained. To show that it may hereafter
receive some explanation, I will give the following hy-
pothesis. From the nature of the organic remains which
do not appear to have inhabited profound depths, in the
several formations of Europe and of the United States;
and from the amount of sediment, miles in thickness, of
which the formations are composed, we may infer that
from first to last large islands or tracts of land, whence
the sediment was derived, occurred in the neighborhood
of the now existing continents of Europe and North
America. This same view has since been maintained
by Agassiz and others. But we do not know what was
the state of things in the intervals between the several
successive formations; whether Europe and the United
States during these intervals existed as dry land, or as
a submarine surface near land, on which sediment was
not deposited, or as the bed of an open and unfathom-
able sea.
Looking to the existing oceans, which are thrice as
extensive as the land, we see them studded with many
islands; but hardly one truly oceanic island (with the;
exception of New Zealand, if this can be called a truly
oceanic island) is as yet known to afford even a remnant
of any paleozoic or secondary formation. Hence we may
perhaps infer that, during the paleozoic and secondary
periods, neither continents nor continental islands existed
where our oceans now extend; for had they existed,
paleozoic and secondary formations would in all prob-
ability have been accumulated from sediment derived
from their wear and tear; and these would have been
at least partially upheaved by the oscillations of level,
which must have intervened during these enormously
IMPERFECTION OF GEOLOGICAL RECORD 95
long periods. If then we may infer anything from these
facts, we may infer that, where our oceans now extend,
oceans have extended from the remotest period of which
we have any record; and, on the other hand, that where
continents now exist, large tracts of land have existed,
subjected no doubt to great oscillations of level, since the
Cambrian period. The colored map appended to my vol-
ume on Coral Reefs led me to conclude that the great
oceans are still mainly areas of subsidence, the great
archipelagoes still areas of oscillations of level, and the
continents areas of elevation. But we have no reason to
assume that things have thus remained from the begin-
ning of the world. 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 long antecedent to the Cambrian
epoch, continents may have existed where oceans are
now spread out; and clear and open oceans may have
existed where our continents now stand. Nor should we
be justified in assuming that if, for instance, the bed of
the Pacific Ocean were now converted into a continent
we should there find sedimentary formations in a recog-
nizable condition older than the Cambrian strata, suppos-
ing such to have been formerly deposited; for it might
well happen that strata which had subsided some miles
nearer to the centre of the earth, and which had been
pressed on by an enormous weight of superincumbent
water, might have undergone far more metamorphic ac-
tion 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 naked meta-
94
THE ORIGIN OF SPECIES
morphic rocks, which must have been heated under great
pressure, have always seemed to me to require some spe-
cial explanation; and we may perhaps believe that we
see in these large areas the many formations long an-
terior to the Cambrian epoch in a completely metamor-
phosed and denuded condition.
The several difficulties here discussed — namely, that,
though we find in our geological formations many links
between the species which now exist and which formerly
existed, we do not find infinitely numerous fine tran-
sitional forms closely joining them all together; the sud-
den manner in which several groups of species first
appear in our European formations; the almost entire
absence, as at present known, of formations rich in fos-
sils beneath the Cambrian strata — are all undoubtedly of
the most serious nature. We see this in the fact that the
most eminent paleontologists, namely, Cuvier, Agassiz,
Barrande, Pictet, Falconer, E. Forbes, etc., and all our
greatest geologists, as Lyell, Murchison, Sedgwick, etc.,
have unanimously, often vehemently, maintained the im-
mutability of species. But Sir Charles Lyell now gives
the support of his high authority to the opposite side;
and most geologists and paleontologists are much shaken
in their former belief. Those who believe that the geo-
logical record is in any degree perfect will undoubtedly
at once reject the theory. For my part, following out
Ly ell's metaphor, I look at the geological record as a
history of the world imperfectly kept, and written in
a changing dialect; of this history we possess the last
volume alone, relating only to two or three countries.
Of this volume, only here and there a short chapter has
been preserved; and of each page, only here and there
IMPERFECTION OF GEOLOGICAL RECORD 95
a few lines. Each word of the slowly-changing language,
more or less different in the successive chapters, may
represent the forms of life, which are intombed in our
consecutive formations, and which falsely appear to have
been abruptly introduced. On this view, the difficul-
ties above discussed are greatly diminished, or even
disappear.
96
THE 0R1Q1N OF SPEClES
CHAPTER XI
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS
On the slow and successive appearance of new species — On their different
rates of change — Species once lost do not reappear — Groups of species
follow the same general rules in their appearance and disappearance as
do single species — On Extinction — On simultaneous changes in the
forms of life throughout the world — On the affinities of extinct species
to each other and to living species — On the state of development of
ancient forms — On the succession of the same types within the same
areas — Summary of preceding and present chapter
ET us now see whether the several facts and laws
relating to the geological succession of organic
beings accord best with the common view of
the immutability of species, or with that of their slow
and gradual modification, through variation and natural
selection.
New species have appeared very slowly, one after
another, both on the land and in the waters. Lyell
has shown that it is hardly possible to resist the evi-
dence on this head in the case of the several tertiary
stages; and every year tends to fill up the blanks be-
tween the stages, and to make the proportion between
the lost and existing forms more gradual. In some of
the most recent beds, though undoubtedly of high an-
tiquity if measured by years, only one or two species
are extinct, and only one or two are new, having ap-
peared there for the first time, either locally, or, as far
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 97
as we know, on the face of the earth. The secondary
formations are more broken; but, as Bronn has re-
marked, neither the appearance nor disappearance of
the many species imbedded in each formation has
been simultaneous.
Species belonging to different genera and classes have
not changed at the same rate, or in the same degree.
In the older tertiary beds a few living shells may still
be found in the midst of a multitude of extinct forms.
Falconer has given a striking instance of a similar fact,
for an existing crocodile is associated with many lost
mammals and reptiles in the sub-Himalayan deposits.
The Silurian Lingula differs but little from the living
species of this genus; whereas most of the other Silurian
Mollusks and all the Crustaceans have changed greatly.
The productions of the land seem to have changed at a
quicker rate than those of the sea, of which a striking
instance has been observed in Switzerland. There is
some reason to believe that organisms high in the scale
change more quickly than those that are low: though
there are exceptions to this rule. The amount of organic
change, as Pictet has remarked, is not the same in each
successive so-called formation. Yet if we compare any
but the most closely related formations, all the species
will be found to have undergone some change. When a
species has once disappeared from the face of the earth,
we have no reason to believe that the same identical
form ever reappears. The strongest apparent exception
to this latter rule is that of the so-called "colonies" of
M. Barrande, which intrude for a period in the midst
of an older formation, and then allow the pre-existing
fauna to reappear; but Ly ell's explanation, namely, that
98
THE ORIGIN OF SPECIES
it is a case of temporary migration from a distinct gee
graphical province, seems satisfactory.
These several facts accord well with our theory, which
includes no fixed law of development, causing all the
inhabitants of an area to change abruptly, or simul-
taneously, or to an equal degree. The process of modi-
fication must be slow, and will generally affect only a
few species at the same time; for the variability of each
species is independent of that of all others. Whether
such variations or individual differences as may arise will
be accumulated through natural selection in a greater or
less degree, thus causing a greater or less amount of
permanent modification, will depend on many complex
contingencies — on the variations being of a beneficial
nature, on the freedom of intercrossing, on the slowly
changing physical conditions of the country, on the immi-
gration of new colonists, and 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 identical form much
longer than others; or, if changing, should change in a
less degree. We find similar relations between the exist-
ing inhabitants of distinct countries; for instance, the
land-shells and coleopterous insects of Madeira have
come to differ considerably from their nearest allies on
the continent of Europe, whereas the marine shells and
birds have remained unaltered. We can perhaps under-
stand the apparently quicker rate of change in terrestrial
and in more highly organized productions compared with
marine and lower productions, by the more complex rela-
tions of the higher beings to their organic and inorganic
conditions of life, as explained in a former chapter.
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 99
When many of the inhabitants of any area have become
modified and improved, we can understand, on the prin-
ciple of competition, and from the all-important relations
of organism to organism in the struggle for life, that any
form which did not become in some degree modified and
improved would be liable to extermination. Hence we
see why all the species in the same region do at last, if
we look to long enough intervals of time, become modi-
fied, for otherwise they would become extinct.
In members of the same class the average amount of
change, during long and equal periods of time, may,
perhaps, be nearly the same; but as the accumulation of
enduring formations, rich in fossils, depends on great
masses of sediment being deposited on subsiding areas,
our formations have been almost necessarily accumulated
at wide and irregularly intermittent intervals of time; con-
sequently the amount of organic change exhibited by the
fossils imbedded 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 an ever slowly changing
drama.
We can clearly understand why a species when once
lost should never reappear, even if the very same condi-
tions 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 place of another species in the economy of
nature, and thus supplant it; yet the two forms — the old
and the new — would not be identically the same; for both
would almost certainly inherit different characters from
their distinct progenitors; and organisms already differing
100
THE ORIGIN OF SPECIES
would vary in a different manner. For instance, it is
possible, if all our fantail pigeons were destroyed, that
fanciers might make a new breed hardly distinguishable
from the present breed; but if the parent rock-pigeon
were likewise destroyed, and under nature we have every
reason to believe that parent-forms are generally sup-
planted and exterminated by their improved offspring,
it is incredible that a fantail, identical with the existing
breed, could be raised from any other species of pigeon,
or even from any other well-established race of the
domestic pigeon, for the successive variations would
almost certainly be in some degree different, and the
newly -formed variety would probably inherit from its
progenitor some characteristic differences.
Groups 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, when it has
once disappeared, never reappears; that is, its existence,
as long as it lasts, is continuous. I am aware that there
are some apparent exceptions to this rule, but the ex-
ceptions 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 the theory. For all the species of
the same group, however long it may have lasted, are the
modified descendants one from the other, and all from a
common progenitor. In the genus Lingula, for instance,
the species which have successively appeared at all ages
must have been connected by an unbroken series of
generations, from the lowest Silurian stratum to the
present day.
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 101
We have seen in the last chapter that whole groups
of species sometimes falsely appear to have been abruptly
developed; and I have attempted to give an explanation
of this fact, which if true would be fatal to my views.
But such cases are certainly exceptional; the general rule
being a gradual increase in number, until the group
reaches its maximum, and then, sooner or later, a gradual
decrease. If the number of the species included within
a genus, or the number of the genera within a family,
be represented by a vertical line of varying thickness,
ascending through 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 upward,
often keeping of equal thickness for a space, and ulti-
mately 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 conform-
able with the theory, for the species of the same genus,
and the genera of the same family, can increase only
slowly and progressively; the process of modification and
the production of a number of allied forms necessarily
being a slow and gradual process — one species first giving
rise to two or three varieties, these being slowly converted
into species, which in their turn produce by equally slow
steps other varieties and species, and so on, like the
branching of a great tree from a single stem, till the
group becomes large.
On Extinction
We have as yet only spoken incidentally of the dis-
appearance of species and of groups of species. On the
102
THE ORIGIN OF SPECIES
theory of natural selection, the extinction of old forms
and the production of new and improved forms are inti-
mately connected together. The old notion of all the
inhabitants of the earth having been swept away by
catastrophes at successive periods is very generally given
up, even by those geologists, as Elie de Beaumont,
Murchison, Barrande, etc., whose general views would
naturally lead them to this conclusion. On the contrary,
we have every reason to believe, from the study of the
tertiary formations, that species and groups of species
gradually disappear, one after another, first from one
spot, then from another, and finally from the world. In
some few cases, however, as by the breaking of an isth-
mus and the consequent irruption of a multitude of new
inhabitants into an adjoining sea, or by the final subsi-
dence of an island, the process of extinction may have
been rapid. Both single species and whole groups of
species last for very unequal periods; some groups,
as we have seen, have endured from the earliest known
dawn of life to the present day; some have disappeared
before the close of the paleozoic period. No fixed law
seems to determine the length of time during which any
single species or any single genus endures. There is
reason to believe that the extinction of a whole group
of species is generally a slower process than their pro-
duction: if their appearance and disappearance be repre-
sented, as before, by a vertical line of varying thickness,
the line is found to taper more gradually at its upper
end, which marks this progress of extermination, than at
its lower end, which marks the first appearance and the
early increase in number of the species. In some cases,
however, the extermination of whole groups, as of am-
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 103
monites, toward the close of the secondary period, has
been wonderfully sudden.
The extinction of species has been involved in the
most gratuitous mystery. Some authors have even sup-
posed that, as the individual has a definite length of life,
so have species a definite duration. No one can have
marvelled more than I have done at the extinction of
species. When I found in La Plata the tooth of a horse
imbedded with the remains of Mastodon, Megatherium,
Toxodon, and other monsters, which all co-existed with
still living shells at a very late geological period, I was
filled with astonishment; for, seeing that the horse, since
its introduction by the Spaniards into South America,
has run wild over the whole country and has increased
in numbers at an unparalleled rate, I asked myself what
could so recently have exterminated the former horse
under conditions of life apparently so favorable. But
my astonishment was groundless. Professor Owen soon
perceived that the tooth, though so like that of the
existing horse, belonged to an extinct species. Had this
horse been still living, but in some degree rare, no
naturalist would have felt the least surprise at its rarity;
for rarity is the attribute of a vast number of species of
all classes, in all countries. If we ask ourselves why
this or that species is rare, we answer that something is
unfavorable in its conditions of life; but what that some-
thing is we can hardly ever tell. On the supposition of
the fossil horse still existing as a rare species, we might
have felt certain, from the analogy of all other mammals,
even of the slow-breeding elephant, and from the history
of the naturalization of the domestic horse in South
America, that under more favorable conditions it would
104
THE ORIG1X OF SPECIES
in a very few years have stocked the whole continent.
But we could not have told what the unfavorable condi-
tions were which checked its increase, whether some one
or several contingencies, and at what period of the horse's
life, and in what degree they severally acted. If the
conditions had gone on, however slowly, becoming less
and less favorable, we assuredly should not have per-
ceived the fact, yet the fossil horse would certainly have
become rarer and rarer, and finally extinct; — its place
being seized on by some more successful competitor.
It is most difficult always to remember that the in-
crease of every creature is constantly being checked by
unperceived hostile agencies; and that these same unper-
ceived agencies are amply sufficient to cause rarity, and
finally extinction. So little is this subject understood
that I have heard surprise repeatedly expressed at such
great monsters as the Mastodon and the more ancient
Dinosaurians having become extinct; as if mere bodily
strength gave victory in the battle of life. Mere size,
on the contrary, would in some cases determine, as has
been remarked by Owen, quicker extermination from the
greater amount of requisite food. Before man inhabited
India or Africa, some cause must have checked the con-
tinued increase of the existing elephant. A highly
capable judge, Dr. Falconer, believes that it is chiefly
insects which, from incessantly harassing and weakening
the elephant in India, check its increase; and this was
Bruce* s conclusion with respect to the African elephant
in Abyssinia. It is certain that insects and blood-sucking
bats determine the existence of the larger naturalized
quadrupeds in several parts of South America.
TVe see in many cases in the more recent tertiary
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 105
formations that rarity precedes extinction; and we know
that this has been the progress of events with those
animals which have been exterminated, either locally or
wholly, through man's agency. I may repeat what I
published in 1845, namely, that to admit that species
generally become rare before they become extinct — to
feel no surprise at the rarity of a species, and yet
to marvel greatly when the species ceases to exist, is
much the same as to admit that sickness in the indi-
vidual is the forerunner of death — to feel no surprise at
sickness, but, when the sick man dies, to wonder and to
suspect that he died by some deed of violence.
The theory of natural selection is grounded on the
belief that each new variety, and ultimately each new
species, is produced and maintained by having some ad-
vantage over those with which it comes into competition;
and the consequent extinction of the less-favored forms
almost inevitably follows. It is the same with our
domestic productions; when a new and slightly improved
variety has been raised, it at first supplants the less
improved varieties in the same neighborhood; when much
improved it is transported far and near, like our short-
horn cattle, and takes the place of other breeds in other
countries. Thus the appearance of new forms and the
disappearance of old forms, both those naturally and
those artificially produced, are bound together. In flour-
ishing groups, the number of new specific forms which
have been produced within a given time has at some
periods probably been greater than the number of the
old specific forms which have been exterminated; but
we know that species have not gone on indefinitely
increasing, at least during the later geological epochs,
106
THE ORIGIN OF SPECIES
so that, looking to later times, we may believe that the
production of new forms has caused the extinction of
about the same number of old forms.
The competition will generally be most severe, as
formerly explained and illustrated by examples, between
the forms which are most like each other in all respects.
Hence the improved and modified descendants of a
species will generally cause the extermination of the
parent-species; and if many new forms have been devel-
oped from any one species, the nearest allies of that
species, i.e., the species of the same genus, will be the
most liable to extermination. Thus, as I believe, a
number of new species descended from one species, that
is, a new genus, comes to supplant an old genus, belong-
ing to the same family. But it must often have hap-
pened that a new species belonging to some one group
has seized on the place occupied by a species belonging
to a distinct group, and thus have caused its extermina-
tion. If many allied forms be developed from the suc-
cessful intruder, many will have to yield their places;
and it will generally be the allied forms which will
suffer from some inherited inferiority in common. But
whether it be species belonging to the same or to a
distinct class which have yielded their places to other
modified and improved species, a few of the sufferers
may often be preserved for a long time, from being fitted
to some peculiar line of life, or from inhabiting some
distant and isolated station, where they will have escaped
severe competition. For instance, some species of Tri-
gonia, a great genus of shells in the secondary formations,
survive in the Australian seas; and a few members of
the great and almost extinct group of Ganoid fishes still
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS lOT
inhabit our fresh waters. Therefore the utter extinction
of a group is generally, as we have seen, a slower proc-
ess than its production.
With respect to the apparently sudden extermination
of whole families or orders, as of Trilobites at the close
of the paleozoic period and of Ammonites at the
close of the secondary period, we must remember what
has been already said on the probable wide intervals of
time between our consecutive formations; and in these
intervals there may have been much slow extermination.
Moreover, when, by sudden immigration or by unusually
rapid development, many species of a new group have
taken possession of an area, many of the older species
will have been exterminated in a correspondingly rapid
manner; and the forms which thus yield their places will
commonly be allied, for they will partake of the same
inferiority in common.
Thus, as it seems to me, the manner in which single
species and whole groups of species become extinct ac-
cords well with the theory of natural selection. We need
not marvel at extinction; if we must marvel, let it be at
our own 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 inordi-
nately, and that some check is always in action, yet
seldom perceived by us, the whole economy of nature
will be utterly obscured. Whenever we can precisely
say why this species is more abundant in individuals
than that; why this species and not another can be
naturalized in a given country; then, and not until then,
we may justly feel surprise why we cannot account for
10S
THE ORIGIN OF SPECIES
the extinction of any particular species or group
species.
On the Forms of Life changing almost simultaneously
throughout the World
Scarcely any paleontological discovery is more striking
than the fact that the forms of life change almost simul-
taneously throughout the world. Thus our European
Chalk formation can be recognized in many distant
regions, under the most different climates, where not
a fragment of the mineral chalk itself can be found;
namely, in North America, in equatorial South America,
in Tierra del Fuego, at the Cape of Good Hope, and in
the peninsula of India. For at these distant points, the
organic remains in certain beds present an unmistakable
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 geoera, aDd some-
times are similarly characterized 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, occur in
the same order at these distant points of the world.
In the several successive paleozoic formations of Russia,
Western Europe, and North America, a similar parallel-
ism in the forms of life has been observed by several
authors; so it is, according to Lyell, with the European
and North American tertiary deposits. Even if the few
fossil species which are common to the Old and New
Worlds were kept wholly out of view, the general paral-
lelism in the successive forms of life, in the paleozoic
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 109
and tertiary stages, would still be manifest, and the
several formations could be easily correlated.
These observations, however, relate to the marine
inhabitants of the world: we have not sufficient data to
judge whether the productions of the land and of fresh
water at distant points change in the same parallel man-
ner. We may doubt whether they have thus changed: if
the Megatherium, Mylodon, Macrauchenia, and Toxodon
had been brought to Europe from La Plata, without any
information in regard to their geological position, no one
would have suspected that they had coexisted with sea-
shells all still living; 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 year, or to the same century, or even that it has a
very strict geological sense; for if all the marine animals
now living in Europe, and all those that lived in Europe
during the pleistocene period (a very remote period as
measured by years, including the whole glacial epoch)
were compared with those now existing in South America
or in Australia, the most skilful naturalist would hardly
be able to say whether the present or the pleistocene
inhabitants of Europe resembled most closely those of
the southern hemisphere. So, again, several highly com-
petent observers maintain that the existing productions
of the United States are more closely related to those
which lived in Europe during certain late tertiary stages
than to the present inhabitants of Europe; and if this
110
THE ORIGIN OF SPECIES
be so, it is evident that fossiliferous beds now deposited
on the shores of North America would hereafter be
liable to be classed with somewhat older European beds.
Nevertheless, looking to a remotely future epoch, there
can be little doubt that all the more modern marine
formations, namely, the upper pliocene, the pleistocene
and strictly modern beds of Europe, North and South
America, and Australia, from containing fossil remains
in some degree allied, and from not including those forms
which are found only in the older underlying deposits,
would be correctly ranked as simultaneous in a geological
sense.
The fact of the forms of life changing simultaneously,
in the above large sense, at distant parts of the world,
has greatly struck those admirable observers, MM. de
Verneuil and d'Archiac. After referring to the parallel-
ism of the paleozoic forms of life in various parts of
Europe, they addr "If, struck by this strange sequence,
we turn our attention to North America, and there dis-
cover a series of analogous phenomena, it will appear
certain that all these modifications of species, their ex-
tinction, and the introduction of new ones, cannot be
owing to mere changes in marine currents or other causes
more or less local and temporary, but depend on general
laws which govern the whole animal kingdom." M.
Barrande has made forcible remarks to precisely the
same effect. It is, indeed, quite futile to look to changes
of currents, climate, or other physical conditions, as the
cause of these great mutations in the forms of life
throughout the world, under the most different climates.
We must, as Barrande has remarked, look to some
special law. We shall see this more clearly when we
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS HI
treat of the present distribution of organic beings, and
find how slight is the relation between the physical
conditions of various countries and the nature of their
inhabitants.
This great fact of the parallel succession of the forms
of life throughout the world is explicable on the theory
of natural selection. New species are formed by having
some advantage over older forms; and the forms, which
are already dominant, or have some advantage over the
other forms in their own country, give birth to the great-
est number of new varieties or incipient species. We
have distinct evidence on this head, in the plants which
are dominant, that is, which are commonest and most
widely diffused, producing the greatest number of new
varieties. It is also natural that the dominant, varying,
and far-spreading species, which have already invaded to
a certain extent the territories of other species, should
be those which would have the best chance of spreading
still further, and of giving rise in new countries to other
new varieties and species. The process of diffusion would
often be very slow, depending on climatal and geograph-
ical changes, on strange accidents, and on the gradual
acclimatization of new species to the various climates
through which they might have to pass, but in the
course of time the dominant forms would generally suc-
ceed in spreading and would ultimately prevail. The
diffusion would, it is probable, be slower with the ter-
restrial inhabitants of distinct continents than with the
marine inhabitants of the continuous sea. We might
therefore expect to find, as we do find, a less strict de-
gree of parallelism in the succession of the productions
of the land than with those of the sea.
— Science— 22
112
THE ORlGiy OF SPECIES
Thus, as it seems to me, the parallel, and, taken in
a large sense, simultaneous, succession of the same forms
of life throughout the world accords well with the prin-
ciple of new species having been formed by dominant
species spreading widely and varying; the new species
thus produced being themselves dominant, owing to their
having had some advantage over their already dominant
parents, as well as over other species, and again spread-
ing, varying, and producing new forms. The old forms
which are beaten, and which yield their places to the new
and victorious forms, will generally be allied in groups,
from inheriting some inferiority in common ; and there-
fore, as new and improved groups spread throughout
the world, old groups disappear from the world; and the
succession of forms everywhere tends to correspond 'both
in their first appearance and final disappearance.
There is one other remark connected with this subject
worth making. I have given my reasons for believing
that most of our great formations, rich in fossils, were
deposited during periods of subsidence; and that blank
intervals of vast duration, as far as fossils are concerned,
occurred during the periods when the bed of the sea was
either stationary or rising, and likewise when sediment
was not thrown down quickly enough to imbed and pre-
serve organic remains. During these long and blank
intervals I suppose that the inhabitants of each region
underwent a considerable amount of modification and ex-
tinction, and that there was much migration from other
parts of the world. As we have reason to believe that
large areas are affected by the same movement, it is
probable that strictly contemporaneous formations have
often been accumulated over very wide spaces in the
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 113
same quarter of the world; but we are very far from
having any right to conclude that this has invariably
been the case, and that large areas have invariably been
affected by the same movements. When two formations
have been deposited in two regions during nearly, but
not exactly, the same period, we should find in both,
from the causes explained in the foregoing paragraphs,
the same general succession in the forms of life; but the
species would not exactly correspond; for there will have
been a little more time in the one region than in the
other for modification, extinction, and immigration.
I suspect that cases of this nature occur 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 be-
longing to the same genera, yet the species themselves
differ in a manner very difficult to account for, consider-
ing the proximity of the two areas — unless, indeed, it be
assumed that an isthmus separated two seas inhabited by
distinct, but contemporaneous, faunas. Ly ell 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
surprising amount of difference in the species. If the
several formations in these regions have not been de-
posited during the same exact periods — a formation in
one region often corresponding with a blank interval
in the other — and if in both regions the species have
THE ORIGIN OF SPECIES
gone on slowly changing during the accumulation of the
several formations and during the long intervals of time
between them; in this case the several formations in the
two regions could be arranged in the same order, in ac-
cordance with the general succession of the forms of life,
and the order would falsely appear to be strictly parallel;
nevertheless the species would not be all the same in the
apparently corresponding stages in the two regions.
On the Affinities of Extinct Species to each other, and
to Living Forms
Let us now look to the mutual affinities of extinct
and living species. All fall into a few grand classes;
and this fact is at once explained on the principle of
descent. The more ancient any form is, the more, as a
general rule, it differs from living forms. But, as Buck-
land long ago remarked, extinct species can all be classed
either in still existing groups, or between them. That
the extinct forms of life help to fill up the intervals be-
tween existing genera, families, and orders, is certainly
true; but as this statement has often been ignored or
even denied, it may be well to make some remarks on
this subject, and to give some instances. If we confine
our attention either to the living or to the extinct species
of the same class, the series is far less perfect than if we
combine both into one general system. In the writings
of Professor Owen we continually meet with the expres-
sion of generalized forms, as applied to extinct animals;
and in the writings of Agassiz, of prophetic or synthetic
types; and these terms imply that such forms are in fact
intermediate or connecting links. Another distinguished
paleontologist, M. Gaudry, has shown in the most strik-
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 115
ing manner that many of the fossil mammals discovered
by him in Attica serve to break down the intervals be-
tween existing genera. Cuvier ranked the Euminants and
Pachyderms as two of the most distinct orders of mam-
mals: but so many fossil links have been disintombed
that Owen has had to alter the whole classification, and
has placed certain pachyderms in the same sub-order
with ruminants; for example, he dissolves by gradations
the apparently wide interval between the pig and the
camel. The Ungulata or hoofed quadrupeds are now
divided into the even- toed or odd-toed divisions; but
the Macrauchenia of South America connects to a cer-
tain extent these two grand divisions. No one will deny
that the Hipparion is intermediate between the existing
horse and certain older ungulate forms. What a wonder-
ful connecting link in the chain of mammals is the
Typotherium from South America, as the name given
to it by Professor Grervais expresses, and which cannot
be placed in any existing order. The Sirenia form a
very distinct group of mammals, and one of the most re-
markable peculiarities in the existing dugong and laman-
tine is the entire absence of hind limbs without even
a rudiment being left; but the extinct Halitherium had,
according to Professor Flower, an ossified thighbone "ar-
ticulated to a well-defined acetabulum in the pelvis,'1
and it thus makes some approach to ordinary hoofed
quadrupeds, to which the Sirenia are in other respects
allied. The cetaceans or whales are widely different
from all other mammals, but the tertiary Zeuglodon
and Squalodon, which have been placed by some nat-
uralists in an order by themselves, are considered
by Professor Huxley to be undoubtedly cetaceans,
116
THE ORIGIN OF SPECIES
"and to constitute connecting links with the aquatic
carnivora. 1 '
Even the wide interval between birds and reptiles has
been shown by the naturalist just quoted to be partially
bridged over in the most unexpected manner, on the one
hand, by the ostrich and extinct Archeopteryx, and on
the other hand, by the Compsognathus. one of the Dino-
saurians — that group which includes the most gigantic of
all terrestrial reptiles. Turning to the Invertebrata, Bar-
raude asserts — a higher authority could not be named
— that he is every day taught that, although paleozoic
animals can certainly be classed under existing groups,
jet that at this ancient period the groups were not so
distinctly separated from each other as they now are.
Some writers have objected to any extinct species,
or group of species, being considered as intermediate
between any two living species, or groups of species.
If by this term it is meant that an extinct form is
directly intermediate in all its characters between two
living forms or groups, the objection is probably valid.
But in a natural classification many fossil species cer-
tainly stand between living species, and some extinct
genera between living genera, even between genera be-
longing 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 by a score of char-
acters, the ancient members are separated by a somewhat
lesser number of characters; so that the two groups for-
merly made a somewhat nearer approach to each other
than they now do.
It is a common belief that the more ancient a form
GEOLOGICAL SUCCESSIOX OF ORGANIC BEIXGS 117
is, by so much the more it tends to connect by some
of its characters groups now widely separated from each
other. This remark no doubt must be restricted to those
groups which have undergone much change in the course
of geological ages: and it would be difficult to prove the
truth of the proposition, for every now and then even a
living animal, as the Lepidosiren. is discovered having
affinities directed toward very distinct groups. Yet if we
compare the older Eeptiles and Batrachians. the older
Fish, the older Cephalopods. and the eocene Mammals,
with the more recent members of the same classes, we
must admit that there is 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 in italics
represent genera, and the dotted lines diverging from
them the species in each genus. The diagram is much
too simple, too few genera and too few species being
given, but this is unimportant for us. The horizontal
lines may represent successive geological formations, and
all the forms beneath the uppermost line may be con-
sidered as extinct. The three existing genera a14, q1A, p1*,
will form a small family; 6M and f1* a closely allied
family or sub-family; and o14, e14, to1*, a third family.
These three families, together with the many extinct
genera on the several lines of descent diverging from the
parent-form (A) will form an order, for all will have
inherited something in common from their ancient pro-
genitor, On the principle of the continued tendency to
divergence of character, which was formerly illustrated
118
THE ORIGIN OF SPECIES
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 most not,
however, assume that divergence of character is a neces-
sary contingency; it depends solely on the descendants
from a species being thas enabled to seize on many and
different places in the economy of nature. Therefore it
is quite possible, as we have seen in the case of some
Silurian forms, that a species might go on being slightly
modified in relation to its slightly altered conditions of
life, and yet retain throughout a vast period the same
general characteristics. This is represented in the dia-
gram by the letter F14.
All the many forms, extinct and recent, descended
from (A), make, as before remarked, one order; and this
order, from the continued effects of extinction and diver-
gence of character, has become divided into several sub-
families and families, some of which are supposed to
have perished at different periods, and some to have
endured to the present day.
By looking at the diagram we can see that if many
of the extinct forms supposed to be imbedded in the
successive formations were discovered at several points
low down in the series, the three existing families on the
uppermost line woul l be rendered less distinct from each
other. If, for instance, the genera a\ a', a18, /9, m*, m*,
m*. 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 certain
pachyderms. Yet he who objected to consider as inter-
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 11^
mediate the extinct genera, which thus link together the
living genera of three families, would be partly justified,
for they are intermediate, not directly, but only by a
long and circuitous course through many widely different
forms. If many extinct forms were to be discovered
above one of the middle horizontal lines or geological
formations — for instance, above ISTo. VI. — but none from
beneath this line, then only two of the families (those on
the left hand, a14, etc., and 514, etc.) would have to be
united into one; and there would remain two families,
which would be less distinct from each other than they
were before the discovery of the fossils. So again if the
three families formed of eight genera (a14 to ?n14), on
the uppermost line, be supposed to differ from each
other by half a dozen important characters, then the
families which existed at the period marked VL would
certainly have differed from each other by a less number
of characters; for they would at this early stage of de-
scent have diverged in a less degree from their common
progenitor. Thus it comes that ancient and extinct
genera are often in a greater or less degree intermediate
in character between their modified descendants, or be-
tween their collateral relations.
Under nature, the process will be far more compli-
cated than is represented in the diagram; for the groups
will have been more numerous; they will have endured
-for extremely unequal lengths of time, and will have
been modified in various degrees. As we possess only
the last volume of the geological record, and that in a
very broken condition, we have no right to expect,
except in rare cases, to fill up the wide intervals in the
natural system, and thus to unite distinct families or
120
THE ORIGIN OF SPECIES
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 forma-
tions make some slight approach to each other; so that
the older members should differ less from each other in
some of their characters than do the existing members of
the same groups; and this, by the concurrent evidence
of our best paleontologists, is frequently 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,
are explained in a satisfactory manner. And they are
wholly inexplicable on any other view.
On this same theory, it is evident that the fauna
during any one great period in the earth's history will
be intermediate in general character between that which
preceded and that which succeeded it. Thus the species
which lived at the sixth great stage of descent in the
diagram are the modified offspring of those which lived
at the fifth stage, and are the parents of those which
became still more modified at the seventh stage; hence
they could hardly fail to be nearly intermediate in char-
acter between the forms of life above and below. We
must, however, allow for the entire extinction of some
preceding forms, and in any one region for the immigra-
tion of new forms from other regions, and for a large
amount of modification during the long and blank inter-
vals between the successive formations. Subject to these
allowances, the fauna of each geological period undoubt-
edly is intermediate in character between the preceding
and succeeding faunas. I need give only one instance,
namely, the manner in which the fossils of the Devonian
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 121
system, when this system was first discovered, were at
once recognized by paleontologists as intermediate in
character between those of the overlying carboniferous
and underlying Silurian systems. 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 inter-
mediate in character between the preceding and succeed-
ing faunas, that certain genera offer exceptions to the
rule. For instance, the species of mastodons and ele-
phants, when arranged by Dr. Falconer in two series — in
the first place according to their mutual affinities, and
in the second place according to their periods of exist-
ence— do not accord in arrangement. The species ex-
treme in character are not the oldest or the most recent;
nor are those which are intermediate in character, inter-
mediate in age. But supposing for an instant, in this
and other such cases, that the record of the first appear-
ance and disappearance of the species was complete,
which is far from the case, we have no reason to believe
that forms successively produced necessarily endure for
corresponding lengths of time. A very ancient form may
occasionally have lasted much longer than a form else-
where subsequently produced, especially in the case of
terrestrial productions inhabiting separated districts. To
compare small things with great; if the principal living
and extinct races of the domestic pigeon were arranged
in serial affinity, this arrangement would not closely
accord with the order in time of their production, and
even less with the order of their disappearance; for the
parent rock-pigeon still lives; and many varieties between
122
TEE ORlGiy OF SPECIES
the rock-pigeon and the carrier have become extinct; and
carriers which are extreme in the important character of
length of beak originated earlier than short-beaked tum-
blers, which are at the opposite end of the series in this
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 paleontologists, 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
formation, though the species are distinct in each stage.
This fact alone, from its generality, seems to have shaken
Professor Pictet in his belief in the immutability of
species. He who is acquainted with the distribution
of existing species over the globe will not attempt to
account for the close resemblance of 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 simul-
taneously throughout the world, and therefore under
the most different climates and conditions. Consider the
prodigious vicissitudes of climate during the pleistocene
period, which includes the whole glacial epoch, and note
how little the specific forms of the inhabitants of the sea
have been affected.
On the theory of descent, the full meaning of the
fossil remains from closely consecutive formations being
closely related though ranked as distinct species is ob-
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 123
vious. 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
expect to find, as I attempted to show in the last chapter,
in any one or in any two formations, all the intermediate
varieties between the species which appeared at the com-
mencement and close of these periods: but we ought to
find after intervals, very long as measured by years, but
only moderately long as measured geologically, closely
allied forms, or, as they have been called by some
authors, representative species; and these assuredly we
do find. We find, in short, such evidence of the slow
and scarcely sensible mutations of specific forms as
we have the right to expect.
On the State of Development of Ancient compared with
Living Forms
We have seen in the fourth chapter that the degree
of differentiation and specialization of the parts in organic
beings, when arrived at maturity, is the best standard,
as yet suggested, of their degree of perfection or high-
ness. We have also seen that, as the specialization of
parts is an advantage to each being, so natural selection
will tend to render the organization of each being more
specialized and perfect, and in this sense higher; not but
that it may leave many creatures with simple and unim-
proved structures fitted for simple conditions of life, and
in some cases will even degrade or simplify the organiza-
tion, yet leaving such degraded beings better fitted for
their new walks of life. In another and more general
manner, new species become superior to their predeces-
sors; for they have to beat in the struggle for life all
124
THE ORIGIN OF SPECIES
the older forms, with which they come into close com-
petition. We may therefore conclude that if under a
nearly similar climate the eocene inhabitants of the world
could be put into competition with the existing inhab-
itants, the former would be beaten and exterminated by
the latter, as would the secondary by the eocene, and
the paleozoic by the secondary forms. So that by this
fundamental test of victory in the battle for life, as well
as by the standard of the specialization of organs, modern
forms ought, on the theory of natural selection, to stand
higher than ancient forms. Is this the case? A large
majority of paleontologists would answer in the affirma-
tive; and it seems that this answer must be admitted
as true, though difficult of proof.
It is no valid objection to this conclusion that certain
Brachiopods have been but slightly modified from an
extremely remote geological epoch; and that certain land
and fresh-water shells have remained nearly the same,
from the time when, as far as is known, they first ap-
peared. It is not an insuperable difficulty that Foram-
inifera have not, as insisted on by Dr. Carpenter, pro-
gressed in organization since even the Laurentian epoch;
for some organisms would have to remain fitted for sim-
ple conditions of life, and what could be better fitted for
this end than these lowly organized Protozoa ? Such ob-
jections as the above would be fatal to my view, if it
included advance in organization as a necessary contin-
gent. They would likewise be fatal, if the above Foram-
inifera, for instance, could be proved to have first come
into existence during the Laurentian epoch, or the above
Brachiopods during the Cambrian formation; for in this
case there would not have been time sufficient for the
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 125
development of these organisms up to the standard which
they had then reached. When advanced up to any given
point, there is no necessity, on the theory of natural se-
lection, for their further continued progress; though they
will, during each successive age, have to be slightly mod-
ified, so as to hold their places in relation to slight
changes in their conditions. The foregoing objections
hinge on the question whether we really know how
old the world is, and at what period the various forms
of life first appeared; and this may well be disputed.
The problem whether organization on the whole has
advanced is in many ways excessively intricate. The
geological record, at all times imperfect, does not extend
far enough back to show with unmistakable clearness
that within the known history of the world organization
has largely advanced. Even at the present day, looking
to members of the same class, naturalists are not unani-
mous which forms ought to be ranked as highest: thus,
some look at the selaceans or sharks, from their approach
in some important points 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 pre-
ponderant in number; but formerly selaceans and ganoids
alone existed; and in this case, according to the standard
of highness chosen, so will it be said that fishes have
advanced or retrograded in organization. To attempt to
compare members of distinct types in the scale of high-
ness seems hopeless; who will decide whether a cuttle-
fish be higher than a bee — that insect which the great
Von Bare believed to be "in fact more highly organized
than a fish, although upon another type"? In the com-
126
THE ORIGIN OF SPECIES
plex straggle for life it is quite credible that crustaceans,
not very high in their own class, might beat cephalopods,
the highest mollusks; and such crustaceans, though not
highly 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 these inherent diffi-
culties in deciding which forms are the most advanced in
organization, we ought not solely to compare the highest
members of a class at any two periods — though undoubt-
edly this 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 molluskoidal ani-
mals, namely, cephalopods and brachiopods, swarmed in
numbers; at the present time both groups are greatly
reduced, while others, intermediate in organization, have
largely increased: consequently some naturalists maintain
that mollusks were formerly more highly developed than
at present; but a stronger case can be made out on the
opposite side, by considering the vast reduction of brachi*
opods, and the fact that our existing cephalopods, though
few in number, are more highl}r organized than their an-
cient representatives. We ought also to compare the rela-
tive proportional numbers at any two periods of the high
and low classes throughout the world: if, for instance, at
the present day fifty thousand kinds of vertebrate ani-
mals exist, and if we knew that at come former period
only ten thousand kinds existed, we ought to look at
this increase in number in the highest class, which im-
plies a great displacement of lower forms, as a decided
advance in the organization of the world. We thus see
how hopelessly difficult it is to compare with perfect fair-
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 127
ness, under such extremely complex relations, the stand-
ard of organization of the imperfectly-known faunas of
successive periods.
We shall appreciate this difficulty more clearly by
looking to certain existing faunas and floras. From the
extraordinary manner in which European productions have
recently spread over New Zealand, and have seized on
places which must have been previously occupied by the
indigenes, we must believe, that if all the animals and
plants of Great Britain were set free in New Zealand,
a multitude of British forms would in the course of time
become thoroughly naturalized there, and would extermi-
nate many of the natives. On the other hand, from the
fact *hat hardly a single inhabitant of the southern hemi-
sphere has become wild in any part of Europe, we may
well doubt whether, if all the productions of New Zea-
land were set free in Great Britain, any considerable
number would be enabled to seize on places now occu-
pied by our native plants and animals. Under this point
of view, the productions of Great Britain stand much
higher in the scale than those of New Zealand. Yet
the most skilful naturalist, from an examination of the
species of the two countries, could not have foreseen
this result.
Agassiz and several other highly competent judges
insist that ancient animals resemble to a certain extent
the embryos of recent animals belonging to the same
classes; and that the geological succession of extinct
forms is nearly parallel with the embryological devel-
opment of existing forms. This view accords admirably
well with our theory. In a future chapter I shall at-
tempt to show that the adult differs from its embryo,
126
THE ORIGIN OF SPECIES
owing to variations having supervened at a not early
age, and having been inherited at a corresponding age.
This process, while it leaves the embryo almost un-
altered, 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 pic-
ture, preserved by nature, of the former and less mod-
ified condition of the species. This view may be true,
and yet may never be capable of proof. Seeing, for
instance, that the oldest known mammals, reptiles, and
fishes strictly belong to their proper classes, though some
of these old forms are in a slight degree less distinct
from each other than are the typical members of the
same groups at the present day, it would be vain to look
for animals having the common embryological character
of the Vertebrata, until beds rich in fossils are discov-
ered far beneath the lowest Cambrian strata — a discovery
of which the chance is small.
On the Succession of the same Types within the same Areas,
during the later Tertiary periods •
Mr. Clift many years ago showed that the fossil mam-
mals from the Australian caves were closely allied to the
living marsupials of that continent. In South America,
a similar relationship is manifest, even to an uneducated
eye, in the gigantic pieces of armor, like those of the
armadillo, found in several parts of La Plata; and Pro-
fessor Owen has shown in the most striking manner that
most of the fossil mammals, buried there in such num-
bers, are related to South American types. This rela-
tionship is even more clearly seen in the wonderful col-
lection of fossil bones made by MM. Lund and Clausen
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 129
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 wonder-
ful relationship in the same continent between the dead
and the living." Professor Owen has subsequently ex-
tended the same generalization to the mammals of the
Old World. We see the same law in this author's res-
torations of the extinct and gigantic birds of New Zea-
land. 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
mollusks, it is not well displayed by them. Other cases
could be added, as the relation between the extinct
and living land-shells of Madeira; and between the
extinct and living brackish water-shells of the Aralo-
Caspian Sea.
Now what does this remarkable law of the succession
of the same types within the same areas mean ? He
would be a bold man who, after comparing the present
climate of Australia and of parts of South America
under the same latitude, would attempt to account, on
the one hand, through dissimilar physical conditions, for •
the dissimilarity of the inhabitants of these two conti-
nents; and, on the other hand, through similarity of
conditions, for the uniformity of the same types in each
continent during the later tertiary periods. Nor can it
be pretended that it is an immutable law that marsupials
should have been chiefly or solely produced in Australia;
or that Edentata and other American types should have
been solely produced in South America. For we know
that Europe in ancient times was peopled by numerous
marsupials; and I have shown in the publications above
130
THE ORIGIN OF SPECIES
alluded to that in America the law of distribution of
terrestrial mammals was formerly different from what it
now is. North America formerly partook strongly of the
present character of the southern half of the continent;
and the southern half was formerly more closely allied
than it is at present to the northern half. In a similar
manner we know, from Falconer and Cautley's discover-
ies, that Northern India was formerly more closely re-
lated in its mammals to Africa than it is at the present
time. Analogous facts could be given in relation to the
distribution of marine animals.
On the theory of descent with modification, the great
law of the long enduring, but not immutable, succession
of the same types within the same areas is at once ex-
plained; for the inhabitants of each quarter of the world
will obviously tend to leave in that quarter, during the
next succeeding period of time, closely allied though
in some degree modified descendants. If the inhabitants
of one continent formerly differed greatly from those of
another continent, so will their modified descendants still
differ in nearly the same manner and degree. But after
very long intervals of time, and after great geographical
changes, permitting much intermigration, the feebler will
yield to the more dominant forms, and there will be
nothing immutable in the distribution of organic beings.
It may be asked in ridicule, whether I suppose that
the megatherium and other allied huge monsters, which
formerly lived in South America, have left behind them
the sloth, armadillo, and anteater, as their degenerate
descendants. This cannot for an instant be admitted.
These huge animals have become wholly extinct, and
have left no progeny. But in the caves of Brazil there
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 131
are many extinct species which are closely allied in size
and in all other characters to the species still living in
South America; and some of these fossils may have been
the actual progenitors of the living species. It must not
be forgotten that, on our theory, all the species of the
same genus are the descendants of some one species; so
that, if six genera, each having eight species, be found
in one geological formation, and in a succeeding forma-
tion there be six other allied or representative genera
each with the same number of species, then we may con-
clude that generally only one species of each of the older
genera has left modified descendants, which constitute the
new genera containing the several species; the other
seven species of each old genus having died out and
left no progeny. Or, and this will be a far commoner
case, two or three species in two or three alone of the
six older genera will be the parents of the new genera:
the other species and the other old genera having become
utterly extinct. In failing orders, with the genera and
species decreasing in numbers as is the case with the
Edentata of South America, still fewer genera and species
will leave modified blood-descendants.
Summary of the preceding and present Chapters
I have attempted to show that the geological record
is extremely imperfect; that only a small portion of the
globe has been geologically explored with care; that
only certain classes of organic beings have been largely
preserved in a fossil state; that the number both of
specimens and of species, preserved in our museums,
is absolutely as nothing compared with the number of
generations which must have passed away even during a
132
THE ORIGIN OF SPECIES
single formation; that, owing to subsidence being almost
necessary for the accumulation of deposits rich in fossil
species of many kinds, and thick enough to outlast
future degradation, great intervals of time must have
elapsed between most of our successive formations; that
there has probably been more extinction during the periods
of subsidence, and more variation during the periods of
elevation, and during the latter the recon! will have been
least perfectly kept; that each single formation has not
been continuously deposited; that the duration of each
formation is probably short compared with the average
duration of specific forms; that migration has played an
important part in the first appearance of new forms
in any one area and formation; that widely ranging
species are those which have varied most frequently, and
have oftenest given rise to new species; that varieties have
at first been local; and lastly, although each species must
have passed through numerous transitional stages, it is
probable that the periods, during which each underwent
modification, though many and long as measured by
years, have been short in comparison with the periods
during which each remained in an unchanged condition.
These causes, taken conjointly, will to a large extent
explain why — though we do find many links — we do not
find interminable varieties, connecting together all extinct
and existing forms by the finest graduated steps. It
should also be constantly borne in mind that any linking
variety between two forms which might be found would
be ranked, unless the whole chain could be perfectly
restored, as a new and distinct species; for it is not
pretended that we have any sure criterion by which
species and varieties can be discriminated.
GEOLOGICAL SUCCESSION' OF ORGANIC BEINGS 133
He who rejects this view of the imperfection of the
geological record will rightly reject the whole theory.
For he may ask in vain where are the numberless tran-
sitional links which must formerly have connected the
closely allied or representative species found in the suc-
cessive stages of the same great formation ? He may
disbelieve in the immense intervals of time which must
have elapsed between our consecutive formations; he
may overlook how important a part migration has played,
when the formations of any one great region, as those of
Europe, are considered: he may urge the apparent, but
often falsely apparent, sudden coming in of whole groups
of species. He may ask where are the remains of those
infinitely numerous organisms which must have existed
long before the Cambrian system was deposited ? We
now know that at least one animal did then exist: but
I can answer this last question only by supposing that
where our oceans now extend they have extended for
an enormous period, and where our oscillating continents
now stand they have stood since the commencement of
the Cambrian system; but that, long before that epoch,
the world presented a widely different aspect; and that
the older continents, formed of formations older than any
known to us, exist now only as remnants in a metamor-
phosed condition, or lie still buried under the ocean.
Passing from these difficulties, the other great leading
facts in paleontology agree admirably with the theory of
descent with modification through variation and natural
selection. We can thus understand how it is that new
species come in slowly and successively; how species of
different classes do not necessarily change together, or at
the same rate, or m the same degree; yet in the long
184
THE ORIGIN OF SPECIE*
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 belonging
to large and dominant groups tend to leave many modi-
fied descendants, which form new sub-groups and groups.
As these are formed, the species of the less vigorous
groups, from their inferiority inherited from a common
progenitor, tend to become extinct together, and to leave
no modified offspring on the face of the earth.- But the
utter extinction of a whole group of species has some-
times been a 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 it is that dominant forms
which spread widely and yield the greatest number of
varieties tend to people the world with allied, but modi-
fied, descendants; and these will generally succeed in
displacing the groups which are their inferiors in the
struggle for existence. Hence, after long intervals of
time, the productions of the world appear to have
changed simultaneously.
We can understand how it is that all the forms of
life, ancient and recent, make together a few grand
classes. We can understand, from the continued ten-
dency to divergence of character, why, the more ancient
a form is, the more it generally differs from those now
GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 135
living; why ancient and extinct forms often tend to fill
up gaps between existing forms, sometimes blending two
groups, previously classed as distinct, into one; but more
commonly bringing them only a little closer together.
The more ancient a form is, the more often it stands in
some degree intermediate between groups now distinct;
for the more ancient a form is, the more nearly it will
be related to, and consequently resemble, the common
progenitor of groups since become widely divergent.
Extinct forms are seldom directly intermediate between
existing forms; but are intermediate only by a long
and circuitous course through other extinct and different
forms. We can clearly see why the organic remains of
closely consecutive formations are closely allied; for they
are closely linked together by generation. We can clearly
see why the remains of an intermediate formation are
intermediate in character.
The inhabitants of the world at each successive period
in its history have beaten their predecessors in the race
for life, and are, in so far, higher in the scale, and their
structure has generally become more specialized; and this
may account for the common belief held by so many
paleontologists that organization on the whole has pro-
gressed. Extinct and ancient animals resemble to a
certain extent the embryos of the more recent animals
belonging to the same classes, and this wonderful fact
receives a simple explanation according to our views.
The succession of the same types of structure within the
same areas during the later geological periods ceases
to be mysterious, and is intelligible on the principle of
inheritance.
If then the geological record be as imperfect as many
—Science — 23
136
THE ORIGIN OF SPECIES
believe, and it may at least be asserted that the record
cannot be proved to be much more perfect, the main
objections to the theory of natural selection are greatly
diminished or disappear. On the other hand, all the
chief laws of paleontology 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, the products of Variation
and the Survival of the Fittest.
GEOGRAPHICAL DISTRIBUTION 137
CHAPTEE XII
GEOGRAPHICAL DISTRIBUTION"
Present distribution cannot be accounted for by differences in physical
conditions — Importance of barriers — Affinity of the productions of the
same continent — Centres of creation — Means of dispersal, by changes
of climate and of the level of the land, and by occasional means —
Dispersal during the Glacial period — Alternate Glacial periods in the
North and South
IN considering the distribution of organic beings over
the face of the globe, the first great fact which
strikes us is, that neither the similarity nor the dis-
similarity of the inhabitants of various regions can be
wholly accounted for by 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 arctic and northern temperate parts,
all authors agree that one of the most fundamental divi-
sions in geographical distribution is that between the
New and Old Worlds; yet if we travel over the vast
American continent, from the central parts of the United
States to its extreme southern point, we meet with the
most diversified conditions; humid districts, arid deserts,
lofty mountains, grassy plains, forests, marshes, lakes,
and great rivers, under almost every temperature. There
is hardly a climate or condition in the Old World which
cannot be paralleled in the New — at least as closely as
138
THE ORIGIN OF SPECIES
the same species generally require. No doubt small areas
can be pointed out in the Old World hotter than any in
the New World; but these are not inhabited by a fauna
different from that of the surrounding districts; for it is
rare to find a group of organisms confined to a small
area, of which the conditions are peculiar in only a
slight degree. Notwithstanding this general parallelism
in the conditions of the Old and New Worlds, how
widely different are their living productions!
In the southern hemisphere, if we compare large
tracts of land in Australia, South Africa, and western
South America, between latitudes 25° and 35°, we shall
find parts extremely similar in all their conditions, yet
it would not be possible to point out three faunas and
floras more utterly dissimilar. Or, again, we may com-
pare the productions of South America south of latitude
35° with those north of 25°, which consequently are sep-
arated by a space of ten degrees of latitude, and are
exposed to considerably different conditions; yet they
are 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 re-
gions. We see this in the great difference in nearly r.ll
the terrestrial productions of the New and Old Worlds,
excepting in the northern parts, where the land almost
joins, and where, under a slightly different climate, there
might have been free migration for the northern temper-
GEOGRAPHICAL DISTRIBUTION
139
ate forms, as there now is for the strictly arctic produc-
tions. We see the same fact in the great difference
between the inhabitants of Australia, Africa, and South
America under the same latitude; for these countries are
almost as much isolated from each other as is possible.
On each continent, also, we see the same fact; for on the
opposite sides of lofty and continuous mountain-ranges, of
great deserts and even of large rivers, we find different
productions; though as mountain-chains, deserts, etc., are
not as impassable, or likely to have endured so long,
as the oceans separating continents, the differences are
very inferior in degree to those characteristic of distinct
continents.
Turning to the sea, we find the same law. The
marine inhabitants of the eastern and western shores
of South America are very distinct, with extremely few
shells, Crustacea, or echinodermata in common; but Dr.
Giinther has recently shown that about thirty per cent
of the fishes are the same on the opposite sides of the
Isthmus of Panama; and this fact has led naturalists to
believe that the isthmus was formerly open. Westward
of the shores of America, a wide space of open ocean
extends, with not an island as a halting-place for emi-
grants; 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 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 almost wholly distinct. On the other
hand, proceeding still further westward from the eastern
140
THE ORIGIN OF SPECIES
islands of the tropical parts of the Pacific, we encounter
no impassable barriers, and we have innumerable islands
as halting-places, or continuous coasts, until, after travel-
ling 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 so few marine animals
are common to the above-named three approximate faunas
of Eastern and Western America and the eastern Pacific
Islands, yet many fishes range from the Pacific into the
Indian Ocean, and many shells are common to the east-
ern 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
statement, is the affinity of the productions of the same
continent or of the same sea, though the species them-
selves are distinct at different points and stations. It is
a law of the widest generality, and every continent offers
innumerable instances. Nevertheless the naturalist, in
travelling, for instance, from north to south, never fails
to be struck by the manner in which successive groups
of beings, specifically distinct, though nearly related, re-
place each other. He hears, from closely allied yet dis-
tinct kinds of birds, notes nearly similar, and sees their
nests similarly constructed, but not quite alike, with eggs
colored in nearly the same manner. The plains near the
Straits of Magellan are inhabited by one species of Rhea
(American ostrich), and northward the plains of La Plata
by another species of the same genus; and not by a true
ostrich or emu, like those inhabiting Africa and Aus-
tralia under the same latitude. In these same plains of
La Plata we see the agouti and bizcacha, animals having
nearly the same habits as our hares and rabbits, and be-
GEOGRAPHICAL DISTRIBUTION
longing to the same order of Eodents, but they plainly
display an American type of structure. We ascend the
lofty peaks of the Cordillera, and we find an alpine spe-
cies of bizcacha; we look to the waters, and we do not
find the beaver or musk-rat, but the coypu and capy-
bara, rodents of the South 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 are essen-
tially American, though they may be all peculiar species.
We may look back to past ages, as shown in the last
chapter, and we find American types then prevailing on
the American continent and in the American seas. We
see in these facts some deep organic bond, throughout
space and time, over the same areas of land and water,
independently of physical conditions. The naturalist must
be dull who is not led to inquire what this bond is.
The bond is simply inheritance, that cause which
alone, as far as we positively know, produces organisms
quite like each other, or, as we see in the case of vari-
eties, nearly alike. The dissimilarity of the inhabitants
of different regions may be attributed to modification
through variation and natural selection, and probably in
a subordinate degree to the definite influence of different
physical conditions. The degrees of dissimilarity will de-
pend on the migration of the more dominant forms of
life from one region into another having been more or
less effectually prevented, at periods more or less remote;
— on the nature and number of the former immigrants; —
and on the action of the inhabitants on each other in
leading to the preservation of different modifications; the
relation of organism to organism in the struggle for lifo
142
THE ORIGIS OF SPECIES
being, as 1 have already often remarked, the most im-
portant of all relations. Thus the high importance of
barriers comes into play by checking migration; as does
time for the slow process of modification through natural
selection. Widely-ranging species, abounding in individu-
als which have already triumphed over many competitors
in their own widely-extended homes, will have the best
chance of seizing on new places, when they spread into
new countries. In their new homes they will be exposed
to new conditions, and will frequently undergo further
modification and improvement; and thus they will be-
come still further victorious, and will produce groups
of modified descendants. On this principle of inheritance
with modification we can understand how it is that sec-
tions of genera, whole genera, and even families, are con-
fined to the same areas, as is so commonly and notori-
ously the case.
There is no evidence, as was remarked in the last
chapter, of the existence of any law of necessary devel-
opment. As the variability of each species is an inde-
pendent property, and will be taken advantage of by
natural selection only so far as it profits each individual
in its complex struggle for life, so the amount of modi-
fication in different species will be no uniform quantity.
If a number of species, after having long competed with
each other in their old home, were to migrate in a body
into a new and afterward isolated country, they would
be little liable to modification; for neither migration nor
isolation in themselves effect anything. These principles
come into play only by bringing organisms into new re-
lations with each other and in a lesser degree with the
surrounding physical conditions. As we have seen in
GEOGRAPHICAL DISTRIBUTION
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 or at all modified.
According to these views, it is obvious that the sev-
eral species of the same genus, though inhabiting the
most distant quarters of the world, must originally have
proceeded from the same source, as they are descended
from the same progenitor. In the case of those species
which have undergone during whole geological periods
little modification, there is not much difficulty in believ-
ing that they have migrated from the same region; for
during the vast geographical and climatal changes which
have supervened since ancient times, almost any amount
of migration is possible. But in many other cases, in
which we have reason to believe that the species of a
genus have been produced within comparatively recent
times, there is great difficulty on this head. It is also
obvious that the individuals of the same species, though
now inhabiting distant and isolated regions, must have
proceeded from one spot, where their parents were first
produced: for, as has been explained, it is incredible
that individuals identically the same should have been
produced from parents specifically distinct.
Single Centres of supposed Creation
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 many cases of extreme
difficulty in understanding how the same species could
possibly have migrated from some one point to the sev-
144
THE ORIGIN OF SPECIES
eral distant and isolated points where now found. Never-
theless the simplicity of the view that each species was
first produced within a single region captivates the mind.
He who rejects it, rejects the vera causa of ordinary
generation with subsequent migration, and calls in the
agency of a miracle. It is universally admitted that
in most cases the area inhabited by a species is continu-
ous; and that 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 have been easily passed
over by migration, the fact is given as something re-
markable and exceptional. The incapacity of migrating
across a wide sea is more clear in the case of terrestrial
mammals than perhaps with any other organic beings;
and, accordingly, we find no inexplicable instances of the
same mammals inhabiting distant points of the world.
No geologist feels any difficulty in Great Britain pos-
sessing the same quadrupeds with the rest of Europe,
for they were no doubt once united. But if the same
species can be produced at two separate points, why do
we not find a single mammal common to Europe and
Australia or South America? The conditions of life are
nearly the same, so that a multitude of European animals
and plants have become naturalized in America and Aus-
tralia; 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 wide and broken interspaces. The
great and striking influence of barriers of all kinds is
intelligible only on the view that the great majority of
GEOGRAPHICAL DISTRIBUTION
145
species have been produced on one side, and have not
been able to migrate to the opposite side. Some few
families, many sub-families, very many genera, and a
still greater number of sections of genera, are confined
to a single region; and it has been observed by several
naturalists that the most natural genera, or those genera
in which the species are most closely related to each
other, are generally confined to the same country, or if
they have a wide range that their range is continuous.
What a strange anomaly it would be, if a directly oppo-
site rule were to prevail, when we go down one step
lower in the series, namely, to the individuals of the
same species, and these had not been, at least at first,
confined to some one region!
Hence it seems to me, as it has to many other
naturalists, that the view of each species having been
produced in one area alone, and having subsequently
migrated from that area as far as its powers of migra-
tion and subsistence under past and present conditions
permitted, is the .most probable. Undoubtedly many
cases occur in which we cannot explain how the same
species could have passed from one point to the other.
But the geographical and climatal changes which have
certainly occurred within recent geological times must
have rendered discontinuous the formerly continuous
range of many species. So that we are reduced to
consider whether the exceptions to continuity of range
are so numerous and of so grave a nature, that we
ought to give up the belief, rendered probable by gen-
eral considerations, that each species has been produced
within one area and has migrated thence as far as it
could. It would be hopelessly tedious to discuss all the
146
THE ORIGIN OF SPECIES
exceptional cases of the same species, now living at dis-
tant and separated points, nor do I for a moment pretend
that any explanation could be offered of many instances.
But, after some preliminary remarks, I will discuss a few
of the most striking classes of facts; namely, the ex-
istence 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 chap-
ter), the wide distribution of fresh- water productions; and
thirdly, the occurrence of the same terrestrial species on
islands and on the nearest mainland, though separated by
hundreds of miles of open sea. If the existence of the
same species at distant and isolated points of the earth's
surface can in many instances be explained on the view
of each species having migrated from a single birthplace;
then, considering our ignorance with respect to former
climatal and geographical changes and to the various oc-
casional means of transport, the belief that a single birth-
place is the law seems to me incomparably the safest.
In discussing this subject, we shall be enabled at the
same time to consider a point equally important for us,
namely, whether the several species of a genus which
must on our theory all be descended from a common
progenitor can have migrated, undergoing modification
during their migration, from some one area. If, when
most of the species inhabiting one region are different
from those of another region, though closely allied to
them, it can be shown that migration from the one re-
gion to the other has probably occurred at some former
period, our general view will be much strengthened; for
the explanation is obvious on the principle of - descent
with modification. A volcanic island, for instance, up-
GEOGRAPHICAL DISTRIBUTION
147
heaved and formed at the distance of a few hundreds
of miles from a continent, would probably receive from
it in the course of time a few colonists, and their de-
scendants, though modified, would still be related by
inheritance to the inhabitants of that continent. Cases
of this nature are common, and are, as we shall here-
after see, inexplicable on the theory of independent cre-
ation. This view of the relation of the species of one
region to those of another does not differ much from
that advanced by Mr. Wallace, who concludes that
44 every species has come into existence coincident both
in space and time with a pre-existing closely allied spe-
cies/* And it is now well known that he attributes this
coincidence to descent with modification.
The question of single or multiple centres of creation
differs from another though allied question — namely,
whether all the individuals of the same species are
descended from a single pair, or single hermaphrodite,
or whether, as some authors suppose, from many in-
dividuals simultaneously created. With organic beings
which never intercross, if such exist, each species must
be descended from a succession of modified varieties that
have supplanted each other, but have never blended with
other individuals or varieties of the same species; so
that, at each successive stage of modification, all the
individuals of the same form will be descended from
a single parent. But in the great majority of cases,
namely, with all organisms which habitually unite for
each birth, or which occasionally intercross, the individ-
uals of the same species inhabiting the same area will
be kept nearly uniform by intercrossing; so that many
individuals will go on simultaneously changing, and the
148
THE ORIGIN OF SPECIES
whole amount of modification at each stage will not be
due to descent from a single parent. To illustrate what
I mean: our English racehorses differ 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 the selecting and training of many
individuals during each generation.
Before discussing the three classes of facts, which
I have selected as presenting the greatest amount of
difficulty on the theory of * 'single centres of creation,"
I must say a few words on the means of dispersal.
Means of Dispersal
Sir C. Lyeii and other authors have ably treated this
subject. I can give here only the briefest abstract of the
more important facts. Change of climate must have had
a powerful influence on migration. A region now impas-
sable to certain organisms from the nature of its climate
might have been a highroad for migration, when the
climate was different. 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 sub-
merged, and the two faunas will now blend together,
or may formerly have blended. Where the sea now
extends, land may at a former period have connected
islands or possibly even continents together, and thus
have allowed terrestrial productions to pass from one to
the other. No geologist disputes that great mutations of
level have occurred within the period of existing organ-
isms Edward Forbes insisted that all the islands in the
GEOGRAPHICAL DISTRIBUTION
149
Atlantic must have been recently connected with Europe
or Africa, and Europe likewise with America. Other au-
thors have thus hypothetically bridged over every ocean,
and united almost every island with some mainland. If
indeed the arguments used by Forbes are to be trusted,
it must be admitted that scarcely a single island exists
which has not recently been united to some continent.
This view cuts the Gordian knot of the dispersal of the
same species to the most distant points, and removes
many a difficulty; but to the best of 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 in the level of the land or sea; but not of
such vast changes in the position and extension of our
continents, as to have united them within the recent
period to each other and to the several intervening
oceanic islands. I freely admit the former existence of
many islands, now buried beneath the sea, which may
have served as halting-places for plants and for many
animals during their migration. In the coral-producing
oceans such sunken islands are now marked by rings of
coral or atolls standing over them. Whenever it is fully
admitted, as 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 most of our continents which now stand
quite separate have been continuously, or almost contin-
uously, united with each other and with the many exist-
150
THE ORIGIN OF SPECIES
ing oceanic islands. Several facts in distribution — such
as the great difference in the marine faunas on the oppo-
site sides of almost every continent — the close relation of
the tertiary inhabitants of several lands and even seas to
their present inhabitants — the degree of affinity between
the mammals inhabiting islands with those of the nearest
continent, being in part determined (as we shall hereafter
see) by the depth of the intervening ocean — these and
other such facts are opposed to the admission of such
prodigious geographical revolutions within the recent
period as are necessary on the view advanced by
Forbes and admitted by his followers. The nature and
relative proportions of the inhabitants of oceanic islands
are likewise opposed to the belief of their former con-
tinuity with continents. Nor does the almost universally
volcanic composition of such islands favor the admission
that they are the wrecks of sunken continents; — if they
had originally existed as continental mountain ranges,
some at least of the islands would have been formed,
like other mountain summits, of granite, metamorphic
schists, old fossiliferous and other rocks, instead of con-
sisting of mere piles of volcanic matter.
I must now say a few words on what are called acci-
dental means, but which more properly should be called
occasional means of distribution. I shall here confine
myself to plants. In botanical works, this or that plant
is often stated to be ill adapted for wide dissemination;
but the greater or less facilities for transport across the
sea may be said to be almost wholly unknown. Until
I tried, with Mr. Berkeley's aid, a few experiments, it
was not even known how far seeds could resist the inju-
rious action of sea- water. To my surprise I found that,
GEOGRAPHICAL DISTRIBUTION
151
out of 87 kinds, 64 germinated after an immersion of
28 days, and a few survived an immersion of 187 days.
It deserves notice that certain orders were far more in-
jured than others: nine Leguminosae were tried, and, with
one exception, they resisted the salt water badly; seven
species of the allied orders, Hydrophyllaceae and Pole-
moniaceae, were all 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 have been floated across wide spaces of
the sea, whether or not they were injured by the salt
water. Afterward I tried some larger fruits, capsules,
etc., and some of these floated for a long time. It is
well known what a difference there is in the buoyancy
of green and seasoned timber; and it occurred to me
that floods would often wash into the sea dried plants
or branches with seed-capsules or fruit attached to them.
Hence I was led to dry the stems and branches of 94
plants with ripe fruit, and to place them on sea-water.
The majority sank quickly, but some which, while 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 afterward
when planted germinated; an asparagus-plant with ripe
berries floated for 23 days, when dried it floated for 85
days, and the seeds afterward germinated; the ripe seeds
of Helosciadium sank in two days, when dried they
floated for above 90 days, and afterward 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 U kinds of seeds ger-
minated after an immersion of 28 days; and as M distinct
152
THE ORIGIN OF SPECIES
species with ripe fruit (but not all the same species as
in the foregoing experiment) floated, after being dried,
for above 28 days, we may conclude, as far as anything
can be inferred from these scanty facts, that the seeds
of rift kinds of plants of any country might be floated
by sea-currents during 28 days, and would retain their
power of germination. In Johnston's Physical Atlas, the
average rate of the several Atlantic currents is 33 miles
per diem (some currents running at the rate of 60 miles
per diem); on this average, the seeds of rift plants be-
longing to one country might be floated across 924 miles
of sea to another country, and when stranded, if blown
by an inland gale to a favorable spot, would germinate.
Subsequently to my experiments, M. Martens tried
similar ones, but in a much better manner, for he placed
the seeds in a box in the actual sea, so that they were
alternately wet and exposed to the air like really floating
plants. He tried 98 seeds, mostly different from mine;
but he chose many large fruits and likewise seeds from
plants which live near the sea; and this would have
favored both the average length of their flotation and
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 H of his seeds of different
kinds floated for 42 days, and were then capable of ger-
mination. 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. There-
fore it would perhaps be safer to assume that the seeds
of about rift plants of a flora, after having been dried,
GEOGRAPHICAL DISTRIBUTION
15S
could be floated across a space of sea 900 miles in
width, and would then germinate. The fact of the
larger fruits often floating longer than the small is
interesting; as plants with large seeds or fruit which,
as Alph. de Candolle has shown, generally have restricted
ranges, could hardly be transported by any other means.
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
that when irregularly shaped stones are imbedded in the
roots of trees, small parcels of earth are frequently in-
closed in their interstices and behind them — so perfectly
that not a particle could be washed away during the
longest transport: out of one small portion of earth thus
completely inclosed by the roots of an oak about 50 years
old, three dicotyledonous plants germinated: I am cer-
tain 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
many kinds of seeds 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 sea-water for 30 days, to my surprise nearly
all germinated.
Living birds can hardly fail to be highly effective
agents in the transportation of seeds. I could give many
facts showing how frequently birds of many kinds are
blown by gales to vast distances across the ocean. We
154
THE ORIGIN OF SPECIES
may 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 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 were tried, germinated. But the following
fact is more important: the crops of birds do not secrete
gastric juice, and do not, as I know by trial, injure in
the least the germination of seeds; now, after a bird has
found and devoured a large supply of food, it is posi-
tively asserted that all the grains do not pass into the
gizzard for twelve or even eighteen 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. Some hawks and owls bolt their
prey whole, and, after an interval of from twelve to
twenty hours, disgorge pellets, which, as I know from
experiments made in the Zoological Gardens, include
seeds capable of germination. Some seeds of the oat,
wheat, millet, canary, hemp, clover, and beet germinated
after having been from twelve to twenty-one hours in the
stomachs of different birds of prey; and two seeds of
beet grew after having been thus retained for two days
and fourteen hours. Fresh-water fish, I find, eat seeds
of many land and water plants; fish are frequently de-
voured by birds, and thus the seeds might be transported
from place to place. I forced many kinds of seeds into
GEOGRAPHICAL DISTRIBUTION
155
the stomachs of dead fish, and then gave their bodies to
fishing-eagles, storks, and pelicans; these birds, after an
interval of many hoars, either rejected the seeds in
pellets or passed them in their excrement; and several
of these seeds retained the power of germination. Certain
seeds, however, were always killed by this process.
Locusts are sometimes blown to great distances from
the land; I myself caught one 370 miles from the coast
of Africa, and have heard of others caught at greater
distances. The Rev. R. T. Lowe informed Sir C. Lyell
that in November, 1844, swarms of locusts visited the
island of Madeira. They were in countless numbers, as
thick as the flakes of snow in the heaviest snowstorm,
and extended upward as far as could be seen with a
telescope. During two or three days they slowly careered
round and round in an immense ellipse, at least five or
six miles in diameter, and at night alighted on the taller
trees, which were completely coated with Jhem. They
then disappeared over the sea, as suddenly as they had
appeared, and have not since visited the island. Now,
in parts of Natal it is believed by some farmers, though
on insufficient evidence, that injurious seeds are intro-
duced into their grass-land in the dung left by the great
flights of locusts which often visit that country. In con-
sequence of this belief Mr. Weale sent me in a letter
a small packet of the dried pellets, out of which I ex-
tracted under the microscope several seeds, and raised
from them seven grass plants, belonging to two species,
of two genera. Hence a swarm of locusts, such as that
which visited Madeira, might readily be the means of
introducing several kinds of plants into an island lying
far from the mainland.
156
THE ORIGIN OF SPECIES
Although the beaks and feet of birds are generally
clean, earth sometimes adheres to them: in one case I
removed sixty-one grains, and in another case twenty two
grains of dry argillaceous earth from the foot of a part-
ridge, and in the earth there was a pebble as large as
the seed of a vetch. Here is a better case: the leg of
a woodcock was sent to me by a friend, with a little
cake of dry earth attached to the shank, weighing only
nine grains; and this contained a seed of the toad-rush
(Juncus bufonius) which germinated and flowered. Mr.
Swaysland, of Brighton, who during the last forty years
has paid close attention to our migratory birds, informs
me that he has often shot wagtails (Motacillae), wheat-
ears, and whinchats (Saxicolae), on their first arrival on
pur shores, before they had alighted; and he has several
times noticed little cakes of earth attached to their feet.
Many facts could be given showing how generally soil is
charged with seeds. For instance, Prof. Newton sent me
the leg of a red-legged partridge (Caccabis rufa) which
had been wounded and could not fly, with a ball of hard
earth adhering to it, and weighing six and a half ounces.
The earth had been kept for three years, but when
broken, watered and placed under a bell glass, no less
than 82 plants sprung from it: these consisted of
12 monocotyledons, including the common oat, and at
least one kind of grass, and of 70 dicotyledons, which
consisted, judging from the young leaves, of at least
three distinct species. With such facts before us, can we
doubt that the many birds which are annually blown by
gales across great spaces of ocean, and which annually
migrate — for instance, the millions of quails across the
Mediterranean— must occasionally transport a few seeds
GEOGRAPHICAL DISTRIBUTION
157
imbedded in dirt adhering to their feet or beaks? But
I shall have to recur to this subject.
As icebergs are known to be sometimes loaded with
earth and stones, and have even carried brushwood,
bones, and the nest of a land-bird, it can hardly be
doubted that they must occasionally, as suggested by
Lyell, have transported seeds from one part to another of
the arctic and antarctic regions; and during the Glacial
period from one part of the now temperate regions to
another. In the Azores, from the large number of plants
common to Europe, in comparison with the species on
the other islands of the Atlantic which stand nearer to
the mainland, and (as remarked by Mr. H. C. Watson)
from their somewhat northern character in comparison
with the latitude, I suspected that these islands had been
partly stocked by ice-borne seeds, during the Glacial
epoch. At my request Sir C. Lyell wrote to M. Hartung
to inquire whether he had observed erratic bowlders 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 ice-
bergs formerly landed their rocky burdens on the shores
of these mid-ocean islands, and it is at least possible
that they may have brought thither some few seeds
of northern plants.
Considering that these several means of transport, and
that other means, which without doubt remain to be dis-
covered, have been in action year after year for tens of
thousands of years, it would, I think, be a marvellous
fact if many plants had not thus become widely trans-
ported. These means of transport are sometimes called
accidental, but this is not strictly correct: the currents
158
THE ORIGIN OF SPECIES
of the sea are not accidental, nor is the direction of
prevalent gales of wind. It should be observed that
scarcely any means of transport would carry seeds for
very great distances: for seeds do not retain their vitality
when exposed for a great length of time to the action of
sea- water; nor could they be long carried in the crops
or intestines of birds. These means, however, would
suffice for occasional transport across tracts of sea some
hundred miles in breadth, or from island to island, or
from a continent to a neighboring island, but not from
one distant continent to another. The floras of distant
continents would not by such means become mingled;
but would remain as distinct as they now are. The cur-
rents, from their course, would never bring seeds from
North America to Britain, though they might and do
bring seeds from the West Indies to our western shores,
where, if not killed by their very long immersion in salt
water, they could not endure our climate. Almost every
year, one or two land-birds are blown across the whole
Atlantic Ocean, from North America to the western
shores of Ireland and England; but seeds could be
transported by these rare wanderers only by one means,
namely, by dirt adhering to their feet or beaks, which is
in itself a rare accident. Even in this case, how small
would be the chance of a seed falling on favorable soil,
and coming to maturity! But it would be a great error
to argue that because a well-stocked island, like Great
Britain, has not, as far as is known (and it would be
very difficult to prove this), received within the last few
centuries, through occasional means of transport, immi-
grants from Europe or any other continent, that a
poorly-stocked island, though standing more remote from
GEOGRAPHICAL DISTRIBUTION
159
the mainland, would not receive colonists by similar
means. Out of a hundred kinds of seeds or animals
transported to an island, even if far less well-stocked
than Britain, perhaps not more than one would be so
well fitted to its new nome as to become naturalized.
But this is no valid argument against what would be
effected by occasional means of transport, during the long
lapse of geological time, while the island was being
upheaved, and before it had become fully stocked with
inhabitants. On almost bare land, with few or no de-
structive insects or birds living there, nearly every seed
which chanced to arrive, if fitted for the climate, would
germinate and survive.
Dispersal during the Glacial Period
The identity of many plants and animals, on mountain-
summits, separated from each other by hundreds of miles
of lowlands, where Alpine species could not possibly ex-
ist, is one of the most striking cases known of the same
species living at distant points, without the apparent
possibility of their having migrated from one point to
the other. It is indeed a remarkable fact to see so many
plants of the same species living on the snowy regions
of the Alps or Pyrenees, and in the extreme northern
parts of Europe; but it is far more remarkable that the
plants on the White Mountains, in the United States of
America, are all the same with those of Labrador, and
nearly all the same, as we hear from Asa Gray, with
those on the loftiest mountains of Europe. Even as long
ago as 1747, such facts led Gmelin to conclude that the
same species must have been independently created at
many distinct points; and we might have remained in
—Science — 24
160
THE ORIGIN OF SPECIES
this same belief, had not Agassiz and others called vivid
attention to the Glacial period, which, as we shall imme-
diately see, affords a simple explanation of these facts.
We have evidence of almost every conceivable kind,
organic and inorganic, that, within a very recent geologi-
cal period, central Europe and North America suffered
under an arctic climate. The ruins of a house burned
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 bowlders, of the
icy streams with which their valleys were lately filled.
S6 greatly has the climate of Europe changed that, in
Northern Italy, gigantic moraines, left by old glaciers,
are now clothed by the vine and maize. Throughout a
large part of the United States, erratic bowlders and
scored rocks plainly reveal a former cold period.
The former influence of the glacial climate on the
distribution of the inhabitants of Europe, as explained
by Edward Forbes, is substantially as follows. But we
shall follow the changes more readily, by supposing
a new glacial period slowly to come on, and then pass
away, as formerly occurred. As the cold came on, and
as each more southern zone became fitted for the inhabi-
tants of the north, these would take the places of the
former inhabitants of the temperate regions. The latter,
at the same time, would travel further and further south-
ward, 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 an
arctic fauna and flora, covering the central parts of
GEOGRAPHICAL DISTRIBUTION
161
Europe, as far south as the Alps and Pj^renees, and even
stretching into Spain. The now temperate regions of the
United States would likewise be covered by arctic plants
and animals, and these would be nearly the same with
those of Europe; for the present circumpolar inhabitants,
which we suppose to have everywhere travelled south-
ward, are remarkably uniform round the world.
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, always ascending, as the warmth increased and
the snow still further disappeared, higher and higher,
while their brethren were pursuing their northern jour-
ney. Hence, when the warmth had fully returned,
the same species, which had lately lived together on the
European and North American lowlands, would again be
found in the arctic regions of the Old and New Worlds,
and on many isolated mountain-summits far distant from
each other.
Thus we can understand the identity of many plants
at points so immensely remote as the mountains of the
United States and those of Europe. We can thus also
understand the fact that the Alpine plants of each
mountain -range are more especially related to the arctic
forms living due north or nearly due north of them: for
the first migration when the cold came on, and the
remigration on the returning warmth, would generally
have been due south and north. The Alpine plants, for
example, of Scotland, as remarked by Mr. H. C. Watson,
and those of the Pyrenees, as remarked by Kamond, are
162
THE ORIGIN OF SPECIES
more especially allied to the plants of northern Scandi-
navia; those of the United States to Labrador; those of
the mountains of Siberia to the arctic regions of that
country. These views, grounded as they are on the
perfectly well -ascertained occurrence of a former Glacial
period, seem to me to explain in so satisfactory a manner
the present distribution of the Alpine and Arctic produc-
tions of Europe and America, that when in other regions
we find the same species on distant mountain-summits we
may almost conclude, without other evidence, that a
colder climate formerly permitted their migration across
the intervening lowlands, now become too warm for
their existence.
As the arctic forms moved first southward and after-
ward backward to the north, in unison with the changing
climate, they will not have been exposed during their
long migrations to any great diversity of temperature;
and as they all migrated in a body together, their mutual
relations will not have been much disturbed. Hence, in
accordance with the principles inculcated in this volume,
these forms will not have been liable to much modifica-
tion. But with the 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 far distant from each other, and have
survived there ever since; they will also, in all proba-
bility, have become mingled with ancient Alpine species,
which must have existed on the mountains before the
commencement of the Glacial epoch, and which during
the coldest period will have been temporarily driven down
GEOGRAPHICAL DISTRIBUTION
163
to the plains: they will, also, have been subsequently
exposed to somewhat different climatal influences. Their
mutual relations will thus have been in some degree dis-
turbed; consequently they will have been liable to modi-
fication; and they have been modified; for if we compare
the present Alpine plants and animals of the several
great European mountain-ranges one with another, though
many of the species remain identically the same, some
exist as varieties, some as doubtful forms or sub-species,
and some as distinct yet closely allied species represent-
ing each other on the several ranges.
In the foregoing illustration I have assumed that at
the commencement of our imaginary Glacial period, the
arctic productions were as uniform round the polar re-
gions as they are at the present day. But it is also nec-
essary to assume that many sub- arctic and some few tem-
perate forms were the same round the world, for some
of the species which now exist on the lower mountain-
slopes and on the plains of North America and Europe
are the same; and it may be asked how I account for
this degree of uniformity in the sub- arctic and temperate
forms round the world, at the commencement of the real
Glacial period. At the present day, the sub-arctic and
northern temperate productions of the Old and New
Worlds are separated from each other by the whole At-
lantic Ocean and by the northern part of the Pacific. '
During the Glacial period, when the inhabitants of the
Old and New Worlds lived further southward than they
do at present, they must have been still more completely
separated from each other by wider spaces of ocean; so
that it may well be asked how the same species could
then or previously have entered the two continents. The
164
THE ORIGIN OF SPECIES
explanation, I believe, lies in the nature of the climate
before the commencement of the Glacial period. At this,
the newer Pliocene period, the majority of the inhabitants
of the world were specifically the same as now, and we
have good reason to believe that the climate was warmer
than at the present day. Hence we may suppose that
the organisms which now live under latitude 60°, lived
during the Pliocene period further north under the Polar
Circle, in latitude 66°-67°; and that the present arctic
productions then lived on the broken land still nearer
to the pole. Now, if we look at a terrestrial globe, we
see under the Polar Circle that there is almost continu-
ous land from western Europe, through Siberia, to east-
ern America. And this continuity of the circumpolar
land, with the consequent freedom under a more favor-
able climate for intermigration, will account for the sup-
posed uniformity of the sub- arctic and temperate produc-
tions of the Old and New Worlds, at a period anterior
to the Glacial epoch.
Believing, from reasons before alluded to, that our
continents have long remained in nearly the same relative
position, though subjected to great oscillations of level, I
am strongly inclined to extend the above view, and to
infer that during some still earlier and still warmer
period, such as the older Pliocene period, a large num-
ber of the same plants and animals inhabited the almost
continuous circumpolar land; and that these plants and
animals, both in the Old and New Worlds, began slowly
to migrate southward as the climate became less warm,
long before the commencement of the Glacial period.
We now see, as I believe, their descendants, mostly in
a modified condition, in the central parts of Europe and
GEOGRAPHICAL DISTRIBUTION
165
the United States. On this view we can understand the
relationship, with very little identity, between the pro-
ductions of North America and Europe — a relationship
which is highly remarkable, considering the distance of
the two areas and their separation by the whole Atlantic
Ocean. We can further understand the singular fact re-
marked on by several observers that the productions of
Europe and America during the later tertiary stages were
more closely related to each other than they are at the
present time; for during these warmer periods the north-
ern 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 intermigration
of their inhabitants.
During the slowly decreasing warmth of the Pliocene
period, as soon as the species in common, which inhab-
ited the New and Old Worlds, migrated south of the
Polar Circle, they will have been completely cut off
from each other. This separation, as far as the more
temperate productions are concerned, must have taken
place long ages ago. As the plants and animals mi-
grated southward, they will have become mingled in the
one great region with the native American productions,
and would have had to compete with them; and in the
other great region, with those of the Old World. Con-
sequently we have here everything favorable for much
modification — for far more modification than with the
Alpine productions, left isolated, within a much more
recent period, on the several mountain- ranges and on
the arctic lands of Europe and North America. Hence
it has come, that when we compare the now living pro-
ductions of the temperate regions of the New and Old
166
THE ORIGIN OF SPECIES
Worlds, we find very few identical species (though Asa
Gray has lately shown that more plants are identical
than was formerly supposed), but we find in every great
class many forms, which some naturalists rank as geo-
graphical races, and others as distinct species; and a host
of closely allied or representative forms which are ranked
by all naturalists as specifically distinct.
As on the land, so in the waters of the sea, a slow
southern migration of a marine fauna, which, during the
Pliocene or even a somewhat earlier period, was nearly
uniform along the continuous shores of the Polar Circle,
will account, on the theory of modification, for many
closely allied forms now living in marine areas com-
pletely sundered. Thus, I think, we can understand the
presence of some closely allied, still existing and extinct
tertiary forms, on the eastern and western shores of tem-
perate North America; and the still more striking fact of
many closely allied crustaceans (as described in Dana's
admirable work), some fish and other marine animals,
inhabiting the Mediterranean and the seas of J apan — these
two areas being now completely separated by the breadth
of a whole continent and by wide spaces of ocean.
These cases of close relationship in species either now
or formerly inhabiting the seas on the eastern and west-
ern shores of North America, the Mediterranean and
Japan, and the temperate lands of North America and
Europe, are inexplicable on the theory of creation. We
cannot maintain that such species have been created
alike, in correspondence with the nearly similar phys-
ical conditions of the areas; for if we compare, for in-
stance, certain parts of South America with parts of
South Africa or Australia, we see countries closely
GEOGRAPHICAL DISTRIBUTION 167
similar in all their physical conditions, with their
inhabitants utterly dissimilar.
Alternate Glacial Periods in the North and South
But we must return to our more immediate subject.
I am convinced that Forbes' s view may be largely ex-
tended. In Europe we meet with the plainest evidence
of the Glacial period, from the western shores of Britain
to the Oural range, and southward to the Pyrenees. We
may infer, from the frozen mammals and nature of the
mountain vegetation, that Siberia was similarly affected.
In the Lebanon, according to Dr. Hooker, perpetual snow
formerly covered the central axis, and fed glaciers which
rolled 4,000 feet down the valleys. The same observer
has recently found great moraines at a low level on the
Atlas range in North Africa. Along the Himalaya, at
points 900 miles apart, glaciers have left the marks of
their former low descent; and in Sikkim, Dr. Hooker saw
maize growing on ancient and gigantic moraines. South-
ward of the Asiatic continent, on the opposite side of the
equator, we know, from the excellent researches of Dr.
J. Haast and Dr. Hector, that in New Zealand immense
glaciers formerly descended to a low level; and the same
plants found by Dr. Hooker on widely separated moun-
tains in this island tell the same story of a former cold
period. From facts communicated to me by the Rev. W.
B. Clarke, it appears also that there are traces of for-
mer glacial action on the mountains of the southeastern
corner of Australia.
Looking to America; in the northern half, ice- borne
fragments of rock have been observed on the eastern side
of the continent, as far south as latitude 36°-37°, and on
168
THE ORIGIN OF SPECIES
the shores of the Pacific, where the climate is now so
different, as far south as latitude 46°. Erratic bowlders
have, also, been noticed on the Rocky Mountains. In
the Cordillera of South America, nearly under the equa-
tor, glaciers once extended far below their present level.
In Central Chile I examined a vast mound of detritus
with great bowlders, crossing the Portillo Valley, which
there can hardly be a doubt once formed a huge mo-
raine; and Mr. D. Forbes informs me that he found in
various parts of the Cordillera, from latitude 13° to 30°
S., at about the height of 12,000 feet, deeply- furrowed
rocks, resembling those with which he was familiar in
Norway, and likewise great masses of detritus, including
grooved pebbles. Along this whole space of the Cordil-
lera true glaciers do not now exist even at much more
considerable heights. Further south on both sides of the
continent, from latitude 41° to the southernmost extrem-
ity, we have the clearest evidence of former glacial ac-
tion, in numerous immense bowlders transported far from
their parent source.
From these several facts, namely, from the glacial
action having extended all round the northern and south-
ern hemispheres — from the period having been in a geo-
logical sense recent in both hemispheres — from its having
lasted in both during a great length of time, as may be
inferred from the amount of work effected — and lastly
from glaciers having recently descended to a low level
along the whole line of the Cordillera, it at one time
appeared to me that we could not avoid the conclusion
that the temperature of the whole world had been simul-
taneously lowered during the Glacial period. But now
Mr. Croll, in a series of admirable memoirs, has at-
GEOGRAPHICAL DISTRIBUTION
169
tempted to show that a glacial condition of climate is
the result of various physical causes, brought into oper-
ation by an increase in the eccentricity of the earth's
orbit. All these causes tend toward the same end; but
the most powerful appears to be the indirect influence
of the eccentricity of the orbit upon oceanic currents.
According to Mr. Croll, cold periods regularly recur
every ten or fifteen thousand years; and these at long
intervals are extremely severe, owing to certain contin-
gencies, of which the most important, as Sir C. Lyell
has shown, is the relative position of the land and water.
Mr. Croll believes that the last great Glacial period oc-
curred about 240,000 years ago, and endured with slight
alterations of climate for about 160,000 years. With re-
spect to more ancient Glacial periods, several geologists
are convinced from direct evidence that such occurred
during the Miocene and Eocene formations, not to men-
tion still more ancient formations. But the most impor-
tant result for us, arrived at by Mr. Croll, is that when-
ever the northern hemisphere passes through a cold
period the temperature of the southern hemisphere is
actually raised, with the winters rendered much milder,
chiefly through changes in the direction of the ocean-
currents. So conversely it will be with the northern
hemisphere, while the southern passes through a Glacial
period. This conclusion throws so much light on geo-
graphical distribution that I am strongly inclined to trust
in it; but I will first give the facts, which demand an
explanation.
In South America, Dr. Hooker has shown that besides
many closely allied species, between forty and fifty of
the flowering plants of Tierra del Fuego, forming no in-
170
THE ORIGIN OF SPECIES
considerable part of its scanty flora, are common to North
America and Europe, enormously remote as these areas
in opposite hemispheres are from each other. On the
lofty mountains of equatorial America a host of peculiar
species belonging to European genera occur. On the
Organ Mountains of Brazil, some few temperate Euro-
pean, some Antarctic, and some Andean genera were
found by Gardner, which do not exist in the low inter-
vening hot countries. On the Silla of Caracas, the
illustrious Humboldt long ago found species belonging
to genera characteristic of the Cordillera.
In Africa, several forms characteristic of Europe, and
some few representatives of the flora of the Cape of Good
Hope, occur on the mountains of Abyssinia. At the Cape
of Good Hope a very few European species, believed not
to have been introduced by man, and on the mountains
several representative European forms are found, which
have not been discovered in the intertropical parts of
Africa. Dr. Hooker has also lately shown that several
of the plants living on the upper parts of the lofty island
of Fernando Po and on the neighboring Cameroon Moun-
tains, in the Gulf of Guinea, are closely related to those
on the mountains of Abyssinia, and likewise to those of
temperate Europe. It now also appears, as I hear from
Dr. Hooker, that some of these same temperate plants
have been discovered by the Eev. K. T. Lowe on the
mountains of the Cape de Yerde Islands. This exten-
sion of the same temperate forms, almost under the
equator, across the whole continent of Africa and to
the mountains of the Cape de Yerde Archipelago, is one
of the most astonishing facts ever recorded in the
distribution of plants.
GEOGRAPHICAL DISTRIBUTION
171
On the Himalaya, and on the isolated mountain-ranges
of the peninsula of India, on the heights of Ceylon, and
on the volcanic cones of Java, many plants occur, either
identically the same or representing each other, and at
the same time representing plants of Europe not found
in the intervening hot lowlands. A list of the genera of
plants collected on the loftier peaks of Java raises a
picture of a collection made on a hillock in Europe!
Still more striking is the fact that peculiar Australian
forms are represented by certain plants growing on the
summits of the mountains of Borneo. Some of these
Australian forms, as I hear from Dr. Hooker, extend
along the heights of the peninsula of Malacca, and are
thinly scattered on the one hand over India, and on the
other hand 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 lowlands;
and a long list can be given, as I am informed by
Dr. Hooker, of European genera found in Australia, but
not in the intermediate torrid regions. In the admirable
"Introduction to the Flora of New Zealand," by
Dr. Hooker, analogous and striking facts are given in
regard to the plants of that large island. Hence we see
that certain plants growing on the more lofty mountains
of the tropics in all parts of the world, and on the tem-
perate plains of the north and south, are either the same
species or varieties of the same species. It should, how-
ever, be observed that these plants are not strictly arctic
forms; for, as Mr. H. C. Watson has remarked, "in re-
ceding from polar toward equatorial latitudes, the Alpine
or mountain floras really become less and less Arctic."
172
THE ORIGIN OF SPECIES
Besides these identical and closely allied forms, many
species inhabiting the same widely sundered areas belong
to genera not now found in the intermediate tropical
lowlands.
These brief remarks apply to plants alone; but sonu
few analogous facts could be given in regard to terres-
trial animals. In marine productions, similar cases like-
wise occur; as an example, I may quote a statement by
the highest authority, Prof. Dana, that 4 'it is certainly
a wonderful fact that New Zealand should have a closei
resemblance in its Crustacea to Great Britain, its antipode,
than to any other part of the world." Sir J. Kichardson.
also, speaks of the reappearance on the shores of Nevt
Zealand, Tasmania, etc., of northern forms of fish.
Dr. Hooker informs me that twenty-five species of Algae
are common to New Zealand and to Europe, but have
not been found in the intermediate tropical seas.
From the foregoing facts, namely the presence of
temperate forms on the highlands across the whole
of equatorial Africa, and along the peninsula of India,
to Ceylon and the Malay Archipelago, and in a less
well-marked manner across the wide expanse of tropical
South America, it appears almost certain that at some
former period, no doubt during the most severe part of a
glacial period, the lowlands of these great continents were
everywhere tenanted under the equator by a considerable
number of temperate forms. At this period the equa«
torial climate at the level of the sea was probably about
the same with that now experienced at the height of
from five to six thousand feet under the same latitude,
or perhaps even rather cooler. During this, the coldest
period, the lowlands under the equator must have been
GEOGRAPHICAL DISTRIBUTION
173
clothed with a mingled tropical and temperate vegetation,
like that described by Hooker as growing luxuriantly at
the height of from four to five thousand feet on the
lower slopes of the Himalaya, but with perhaps a still
greater preponderance of temperate forms. So again in
the mountainous island of Fernando Po, in the Gulf of
Guinea, Mr. Mann found temperate European forms
beginning to appear at the height of about five thousand
feet. On the mountains of Panama, at the height of only
two thousand feet, Dr. Seemann found the vegetation
like that of Mexico, "with forms of the torrid zone
harmoniously blended with those of the temperate."
Now let us see whether Mr. Croll's conclusion, that,
when the northern hemisphere suffered from the extreme
cold of the great Glacial period, the southern hemisphere
was actually warmer, throws any clear light on the
present apparently inexplicable distribution of various
organisms in the temperate parts of both hemispheres,
and on the mountains of the tropics. The Glacial period,
as measured by years, must have been very long; and
when we remember over what vast spaces some natural-
ized plants and animals have spread within a few cen-
turies, this period will have been ample for any amount
of migration. As the cold became more and more in-
tense, we know that Arctic forms invaded the temperate
regions; and, from the facts just given, there can hardly
be a doubt that some of the more vigorous, dominant
and widest-spreading temperate forms invaded the equa-
torial lowlands. The inhabitants of these hot lowlands
would at the same time have migrated to the tropical
and subtropical regions of the south, for the southern
hemisphere was at this period warmer. On the decline
i74
THE ORIGIN OF SPECIES
of * the Glacial period, as both hemispheres gradually re-
covered their former temperatures, the northern temperate
forms living on the lowlands under the equator would
have been driven to their former homes or have been
destroyed, being replaced by the equatorial forms return-
ing from the south. Some, however, of the northern
temperate forms would almost certainly have ascended
any adjoining high land, where, if sufficiently lofty, they
would have long survived like the Arctic forms on the
mountains of Europe. They might have survived, even
if the climate was not perfectly fitted for them, for
the change of temperature must have been very slow,
and plants undoubtedly possess a certain capacity for ac-
climatization, as shown by their transmitting to their
offspring different constitutional powers of resisting heat
and cold.
In the regular course of events the southern hemi-
sphere would in its turn be subjected to a severe Glacial
period, with the northern hemisphere rendered warmer;
and then the southern temperate forms would invade the
equatorial lowlands. The northern forms which had
before been left on the mountains would now descend
and mingle with the southern forms. These latter, when
the warmth returned, would return to their former homes,
leaving some few species on the mountains, and carrying
southward with them some of the northern temperate
forms which had descended from their mountain fast-
nesses. Thus, we should have some few species identi-
cally the same in the northern and southern temperate
zones and on the mountains of the intermediate tropical
regions. But the species left during a long time on these
mountains, or in opposite hemispheres, would have to
GEOGRAPHICAL DISTRIBUTION
175
compete with many new forms and would be exposed
to somewhat different physical conditions; hence they
would be eminently liable to modification, and would
generally now exist as varieties or as representative
species; and this is the case. We must, also, bear in
mind the occurrence in both hemispheres of former
Glacial periods; for these will account, in accordance
with the same principles, for the many quite distinct
species inhabiting the same widely separated areas, and
belonging to genera not now found in the intermediate
torrid zones.
It is a remarkable fact strongly insisted on by Hooker
in regard to America, and by Alph. de Candolle in
regard to Australia, that many more identical or slightly
modified species have migrated from the north to the
south than in a reversed direction. We see, however,
a few southern forms on the mountains of Borneo and
Abyssinia. I suspect that this preponderant migration
from the north to the south is due to the greater extent
of land in the north, and to the northern forms having
existed in their own nomes in greater numbers, and
having consequently been advanced through natural selec-
tion and competition to a higher stage of perfection, or
dominating power, than the southern forms. And thus,
when the two sets became commingled in the equatorial
regions, during the alternations of the Glacial periods,
the northern forms were the more powerful and were
able to hold their places on the mountains, and afterward
to migrate southward with the southern forms; but not
so the southern in regard to the northern forms. In the
same manner at the present day we see that very many
European productions cover the ground in La Plata, New
176
THE ORIGIN OF SPECIES
Zealand, and to a lesser degree in Australia, and have
beaten the natives; whereas extremely few southern forma
have become naturalized in any part of the northern
hemisphere, though hides, wool, and other objects likely
to carry seeds have been largely imported into Europe
during the last two or three centuries from La Plata and
during the last forty or fifty years from Australia. The
Keilgherrie Mountains in India, however, offer a partial
exception ; for here, as I hear from Dr. Hooker, Austra-
lian forms are rapidly sowing themselves and becoming
naturalized. Before the last great Glacial period, no
doubt the intertropical mountains were stocked with en-
demic Alpine forms; but these have almost everywhere
yielded to the more dominant forms generated in the
larger areas and more efficient workshops of the north.
In many islands the native productions are nearly
equalled, or even outnumbered, by those which have
become naturalized; and this is the first stage toward
their extinction. Mountains are islands on the land, and
their inhabitants have yielded to those produced within
the larger areas of the north, just in the same way as
the inhabitants of real islands have everywhere yielded
and are still yielding to continental forms naturalized
through man's agency.
The same principles apply to the distribution of
terrestrial animals and of marine productions, in the
northern and southern temperate zones, and on the in-
tertropical mountains. When, during the height of the
Glacial period, the ocean-currents were widely different
to what they now are, some of the inhabitants of the
temperate seas might have reached the equator; of these
a few would perhaps at once be able to migrate south-
GEOGRAPHICAL DISTRIBUTION
177
ward, by keeping to the cooler currents, while others
might remain and survive in the colder depths until the
southern hemisphere was in its turn subjected to a gla-
cial climate and permitted their further progress; in
nearly the same manner as, according to Forbes, iso-
lated spaces inhabited by Arctic productions exist to the
present day in the deeper parts of the northern temperate
seas.
I am far from supposing that all the difficulties in
regard to the distribution and affinities of the identical
and allied species, which now live so widely separated in
the north and south, and sometimes on the intermediate
mountain-ranges, are removed on the views above given.
The exact lines of migration cannot be indicated. We
cannot say why certain species and not others have mi-
grated; why certain species have been modified and have
given rise to new forms, while others have remained un-
altered. We cannot hope to explain such facts, until we
can say why one species and not another becomes natu-
ralized by man's agency in a foreign land; why one spe-
cies ranges twice or thrice as far, and is twice or thrice
as common, as another species within their own homes.
Various special difficulties also remain to be solved;
for instance, the occurrence, as shown by Dr. Hooker,
of the same plants at points so enormously remote as
Kerguelen Land, New Zealand, and Fuegia; but icebergs,
as suggested by Lyell, may have been concerned in their
dispersal. The existence at these and other distant points
of the southern hemisphere, of species, which, though
distinct, belong to genera exclusively confined to the
south, is a more remarkable case. Some of these species
are so distinct that we cannot suppose that there has
178
THE ORIGIN OF SPECIES
been time since the commencement of the last Glacial
period for their migration and subsequent modification
to the necessary degree. The facts seem to indicate
that distinct species belonging to the same genera have
migrated in radiating lines from a common centre; and
I am inclined to look in the southern, as in the northern
hemisphere, to a former and warmer period, before the
commencement of the last Glacial period, when the Ant-
arctic lands, now covered with ice, supported a highly
peculiar and isolated flora. It may be suspected that
before this flora was exterminated, during the last Glacial
epoch, a few forms had been already widely dispersed to
various points of the southern hemisphere by occasional
means of transport, and by the aid, as halting- places,
of now sunken islands. Thus the southern shores of
America, Australia, and New Zealand may have become
slightly tinted by the same peculiar forms of life.
Sir C. Lyell in a striking passage has speculated,
in language almost identical with mine, on <the effects
of great alterations of climate throughout the world on
geographical distribution. And we have now seen that
Mr. Croll's conclusion that successive Glacial periods in
the one hemisphere coincide with warmer periods in the
opposite hemisphere, together with the admission of the
slow modification of species, explains a multitude of
facts in the distribution of the same and of the allied
forms of life in all parts of the globe. The living waters
have flowed during one period from the north and during
another from the south, and in both cases have reached
the equator; but the stream of life has flowed with
greater force from the north than in the opposite di-
rection, and has consequently more freely inundated the
GEOGRAPHICAL DISTRIBUTION 179
south. As the tide leaves its drift in horizontal lines,
rising higher on the shores where the tide rises highest,
so have the living waters left their living drift on our
mountain summits, in a line gently rising from the Arctic
lowlands to a great altitude 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 serves
as a record, full of interest to us, of the former inhabi-
tants of the surrounding lowlands.
180
THE ORIGIN OF SPECIES
CHAPTEE XIII
GEOGRAPHICAL DISTRIBUTION — continued
Distribution of fresh -water productions — On the inhabitants of oceanic
islands — Absence of Batrachians and of terrestrial Mammals — On the
relation of the inhabitants of islands to those of the nearest mainland —
On colonization from the nearest source with subsequent modification
— Summary of the last and present chapters
S lakes and river-systems are separated from each
other by barriers of land, it might have been
thought that fresh-water productions would not
have ranged widely within the same country, and as
the sea is apparently a still more formidable barrier,
that they would never have extended to distant coun-
tries. But the case is exactly the reverse. Not only
have many fresh- water species, belonging to different
classes, an enormous range, but allied species prevail
in a remarkable manner throughout the world. When
first collecting in the fresh waters of Brazil, I well re-
member feeling much surprise at the similarity of the
fresh- water insects, shells, etc., and at the dissimilarity
of the surrounding terrestrial beings, compared with those
of Britain.
But the wide-ranging power of fresh-water productions
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
Fresh- water Productions
GEOGRAPHICAL DISTRIBUTION
181
from stream to stream, within their own countries; and
liability to wide dispersal would follow from this capacity
as an almost necessary consequence. We can here con-
sider only a few cases; of these, some of the most diffi-
cult to explain are presented by fish. It was formerly
believed that the same fresh-water species never existed
on two continents distant from each other. But Dr.
Gunther has lately shown that the Galaxias attenuatus
inhabits Tasmania, New Zealand, the Falkland Islands,
and the mainland of South America. This is a wonder-
ful case, and probably indicates dispersal from an Ant-
arctic centre during a former warm period. This case,
however, is rendered in some degree less surprising by
the species of this genus having the power of crossing
by some unknown means considerable spaces of open
ocean; thus there is one species common to New Zea-
land and to the Auckland Islands, though separated by
a distance of about 230 miles. On the same continent
fresh-water fish often range widely, and as if capri-
ciously; for in two adjoining river-systems some of the
species may be the same, and some wholly different.
It is probable that they are occasionally transported
by what may be called accidental means. Thus fishes
still alive are not very rarely dropped at distant points
by whirlwinds; and it is known that the ova retain their
vitality for a considerable time after removal from the
water. Their dispersal may, however, be mainly attrib-
uted to changes in the level of the land within the
recent period, causing rivers to flow into each other.
Instances, also, could be given of this having occurred
during floods, without any change of level. The wide
difference of the fish on the opposite sides of most
182
THE ORIGIN OF SPECIES
mountain-ranges, which are continuous, and which con-
sequently must from an early period have completely
prevented the inosculation of the river-systems on the
two sides, leads to the same conclusion. Some fresh-
water fish belong to very ancient forms, and in such
cases there will have been ample time for great geo-
graphical changes, and consequently time and means for
much migration. Moreover Dr. Giinther has recently
been led by several considerations to infer that with
fishes the same forms have a long endurance. Salt-
water fish can with care be slowly accustomed to live
in fresh water; and, according to Valenciennes, there
is hardly a single group of which all the members are
confined to fresh water, so that a marine species belong-
ing to a fresh-water group might travel far along the
shores of the sea, and could, it is probable, become
adapted without much difficulty to the fresh waters of
a distant land.
Some species of fresh-water shells have very wide
ranges, and allied species which, on our 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 the
ova, as well as the adults, are immediately killed by
sea-water. I could not even understand how some nat-
uralized species have spread rapidly throughout the same
country. But two facts, which I have observed — and
many others no doubt will be discovered — throw some
light on this subject. When ducks suddenly emerge
from a pond covered with duck-weed, I have twice
seen these little plants adhering to their backs; and
GEOGRAPHICAL DISTRIBUTION
183
it has happened to me, in removing a little duck-weed
from one aquarium to another, that I have unintention-
ally stocked the one with fresh-water shells from the
other. But another agency is perhaps more effectual: I
suspended the feet of a duck 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
mollusks, 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 if blown across the
sea to an oceanic island, or to any other distant point,
would be sure to alight on a pool or rivulet. Sir Charles
Lyell informs me that a Dytiscus has been caught with
an Ancylus (a fresh-water shell like a limpet) firmly ad-
hering 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
further it might have been blown by a favoring gale
no one can tell.
With respect to plants, it has long been known what
enormous ranges many fresh-water, and even marsh
species, have, both over continents and to the most re-
mote oceanic islands. This is strikingly illustrated, ac-
cording to Alph. de Candolle, in those large groups
of terrestrial plants which have very few aquatic mem-
bers; for the latter seem immediately to acquire, as if
in consequence, a wide range. I think favorable means
^Science — 25
184
THE ORIGIN OF SPECIES
of dispersal explain this fact. I have before mentioned
that earth occasionally 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 wander more than those of any other; and they
are occasionally found on the most remote and barren
islands of the open ocean; they would not be likely to
alight on the surface of the sea, so that any dirt on their
feet would not be washed off; and when gaining the
land, they would be sure to fly to their natural fresh-
water haunts. I do not believe that botanists are aware
how charged the mud of ponds is with seeds; I have
tried several little experiments, but will here give only
the most striking case: I took in February three table-
spoonfuls of mud from three different points, beneath
water, on the edge of a little pond: this mud when dried
weighed only 6| ounces; I kept it covered up in my
study for six months, pulling up and counting each
plant as it grew; the plants were of many kinds, and
were altogether 537 in number; and yet the viscid mud
was all contained in a breakfast cup! Considering these
facts, I think it would be an inexplicable circumstance
if water-birds did not transport the seeds of fresh-water
plants to unstocked ponds and streams, situated at very
distant points. The same agency may have come into
play with the eggs of some of the smaller fresh- water
animals.
Other and unknown agencies probably have also
played a part. I have stated that fresh-water fish eat
some kinds of seeds, though they reiect many other
kinds after having swallowed tucm\ even small fish
GEOGRAPHICAL DISTRIBUTION
185
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 many hours after-
ward in pellets or in the excrement. When I saw the
great size of the seeds of that fine water-lily, the Nelum-
bium, and remembered Alph. de Candolle's remarks on
the distribution of this plant, I thought that the means
of its dispersal must remain inexplicable; but Audubon
states that he found the seeds of the great south-
ern water-lily (probably, according to Dr. Hooker, the
Nelumbium luteum) in a heron's stomach. Now this
bird must often have flown with its stomach thus well
stocked to distant ponds, and then getting a hearty meal
of fish, analogy makes me believe that it would have re-
jected the seeds in a pellet in a fit state for germination.
In considering these several means of distribution, it
should be remembered that when a pond or stream
is first formed, for instance, on a rising islet, it will be
unoccupied; and a single seed or egg will have a good
chance of succeeding. Although there will always be a
struggle for life between the inhabitants of the same
pond, however few in kind, yet as the number even in
a well-stocked pond is small in comparison with the num-
ber of species inhabiting an equal area of land, the com-
petition between them will probably be less severe than
between terrestrial species; consequently an intruder from
the waters of a foreign country would have a better
chance of seizing on a new place than in the case of
terrestrial colonists. We should also remember that many
186
THE ORIGIN OF SPECIES
fresh -water productions are low in the scale of nature,
and we have reason to believe that such beings become
modified more slowly than the high; and this will give
time for the migration of aquatic species. We should
not forget the probability of many fresh- water forms
having formerly ranged continuously over immense areas,
and then having become extinct at intermediate points.
But the wide distribution of fresh -water plants and of
the lower animals, whether retaining the same identical
form or in some degree modified, apparently depends in
main part on the wide dispersal of their seeds and eggs
by animals, more especially by fresh- water birds, which
have great powers of flight, and naturally travel from one
piece of water to another.
On the Inhabitants of Oceanic Islands
We now come to the last of the three classes of facts
which I have selected as presenting the greatest amount
of difficulty with respect to distribution, on the view that
not only all the individuals of the same species have
migrated from some one area, but that allied species,
although now inhabiting the most distant points, have
proceeded from a single area — the birthplace of their
early progenitors. I have already given my reasons for
disbelieving in continental extensions within the period
of existing species, on so enormous a scale that all the
many islands of the several oceans were thus stocked
with their present terrestrial inhabitants. This view re-
moves many difficulties, bat it does not accord with all
the facts in regard to the productions of islands. In the
following remarks I shall not confine myself to the mere
question of dispersal, but shall consider some other cases
GEOGRAPHICAL DISTRIBUTION
187
bearing 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 Candolle admits this for plants,
and Wollaston for insects. New Zealand, for instance,
with its lofty mountains and diversified stations, extend-
ing over 780 miles of latitude, together with the outlying
islands of Auckland, Campbell and Chatham, contain
altogether only 960 kinds of flowering plants; if we com-
pare this moderate number with the species which swarm
over equal areas in southwestern Australia or at the
Cape of Good Hope, we must admit that some cause,
independently of different physical conditions, has given
rise to so great a difference in number. Even the uni-
form county of Cambridge has 847 plants, and the little
island of Anglesea 764, but a few ferns and a few intro-
duced 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 less than half a dozen flowering
plants; yet many species have now become naturalized on
it, as they have in New Zealand and on every other
oceanic island which can be named. In St. Helena there
is reason to believe that the naturalized plants and
animals have nearly or quite exterminated many native
productions. He who admits the doctrine of the creation
of each separate species will have to admit that a suffi-
cient number of the best adapted plants and animals
were not created for oceanic islands; for man has unin-
tentionally stocked them far more fully and perfectly
than did nature.
188
THE ORIGiy OF SPECIES
Although in oceanic islands the species are few in
number, the proportion of endemic kinds (z.e., those
found nowhere else in the world) is often extremely
large. If we compare, for instance, the number of en-
demic land-shells in Madeira, or of endemic birds in the
Galapagos Archipelago, with the number found on any
continent, and then compare the area of the island with
that of the continent, we shall see that this is true.
This fact might have been theoretically expected, for, as
already explained, species occasionally arriving after long
intervals of time in the new and isolated district, and
having to compete with new associates, would be emi-
nently liable to modification, and would often produce
groups of modified descendants. But it by no means
follows that, because in an island nearly all the species
of one class are peculiar, those of another class, or of
another section of the same class, are peculiar; and this
difference seems to depend partly on the species which
are not modified having immigrated 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, with which the insular forms
have intercrossed. It should be borne in mind that the
offspring of such crosses would certainly gain in vigor; so
that even an occasional cross would produce more effect
than might have been anticipated. I will give a few
illustrations of the foregoing remarks: in the Galapagos
Islands there are 26 land birds; of these 21 (or perhaps
23) are peculiar, whereas of the 11 marine birds only
2 are peculiar; and it is obvious that marine birds could
arrive at these islands much more easily and frequently
than land birds. Bermuda, on the other hand, which lies
GEOGRAPHICAL DISTRIBUTION
139
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 a single endemic
land bird; and we know from Mr. J. M. Jones's admi-
rable account of Bermuda that very many North Ameri-
can birds occasionally or even frequently visit this island.
Almost every year, as I am informed by Mr. E. Y. Har-
court, many European and African birds are blown to
Madeira; this island is inhabited by 99 kinds, of which
one alone is peculiar, though very closely related to a
European form; and three or four other species are con-
fined to this island and to the Canaries. So that the
Islands of Bermuda and Madeira have been stocked from
the neighboring continents with, birds, which for long
ages have there struggled together, and have become
mutually co-adapted. Hence when settled in their new
homes each kind will have been kept by the others to
its proper place and habits, and will consequently have
been but little liable to modification. Any tendency to
modification will also have been checked by intercrossing
with the unmodified immigrants, often arriving from the
mother-countiy. Madeira again is inhabited by a won-
derful number of peculiar land-shells, whereas not one
species of sea-shell is peculiar to its shores: now, though
we do not know how sea-shells are dispersed, yet we can
see that their eggs or larvae, perhaps attached to sea-weed
or floating timber, or to the feet of wading-birds, might
be transported across three or four hundred miles of open
sea far more easily than land-shells. The different or-
ders of insects inhabiting Madeira present nearly parallel
cases.
Oceanic islands are sometimes deficient in animals
190
THE ORIGIN OF SPECIES
of certain whole classes, and their places are occupied
by other classes; thus in the Galapagos Islands reptiles,
and in New Zealand gigantic wingless birds, take, or
recently took, the place of mammals. Although New
Zealand is here spoken of as an oceanic island, it is
in some degree doubtful whether it should be so ranked;
it is of large size, and is not separated from Australia
by a profoundly deep sea; from its geological character
and the direction of its mountain-ranges, the Rev. W. B.
Clarke has lately maintained that this island, as well as
New Caledonia, should be considered as appurtenances
of Australia. Turning to plants, Dr. Hooker has shown
that in the Galapagos Islands the proportional numbers
of the different orders are very different from what they
are elsewhere. All such differences in number, and the
absence of certain whole groups of animals and plants,
are generally accounted for by supposed differences in
the physical conditions of the islands; but this explana-
tion is not a little doubtful. Facility of immigration
seems to have been fully as important as the nature
of the conditions.
Many remarkable little facts could be g:ven with re-
spect to the inhabitants of oceanic islands. For instance,
in certain islands not tenanted by a single mammal some
of the endemic plants have beautifully hooked seeds; yet
few relations are more manifest than that hooks serve
for the transportal of seeds in trie wool or fur of quadru-
peds. But a hooked seed might be carried to an island
by other means; and the plant then becoming modified
would form an endemic species, still retaining its hooks,
which would form a useless appendage like the shrivelled
wings under the soldered wing- covers of many insular
GEOGRAPHICAL DISTRIBUTION
191
beetles. Again, islands often possess trees or bushes
belonging to orders which elsewhere include only her-
baceous species; now trees, as Alph. de Candolle has
shown, generally have, whatever the cause may be, con-
fined ranges. Hence trees would be little likely to reach
distant oceanic islands; and a herbaceous plant, which
had no chance of successfully competing with the many
fully developed trees growing on a continent, might,
when established on an island, gain an advantage over
other herbaceous plants by growing taller and taller and
overtopping them. In this case, natural selection would
tend to add to the stature of the plant, to whatever
order it belonged, and thus first convert it into a bush
and then into a tree.
Absence of Batrachians and Terrestrial Mammals on
Oceanic Islands
With respect to the absence of whole orders of
animals on oceanic islands, Bory St. Vincent long ago
remarked that Batrachians (frogs, toads, newts) are never
found on any of the many islands with which the great
oceans are studded. I have taken pains to verify this
assertion, and have found it true, with the exception of
New Zealand, New Caledonia, the Andaman Islands, and
perhaps the Salomon Islands and the Seychelles. But I
have already remarked that it is doubtful whether New
Zealand and New Caledonia ought to be classed as
oceanic islands; and this is still more doubtful with
respect to the Andaman and Salomon groups and the
Seychelles. This general absence of frogs, toads, and
newts on so many true oceanic islands cannot be ac-
counted for by their physical conditions: indeed it seems
192
THE ORIGIN OF SPECIES
that islands are peculiarly fitted for these animals; for
frogs have been introduced into Madeira, the Azores,
and Mauritius, and have multiplied so as to become a
nuisance. But as these animals and their spawn are im-
mediately killed (with the exception, as far as known,
of one Indian species) by sea- water, there would be great
difficulty in their transportal across the sea, and therefore
we can see why they do not exist on strictly oceanic isl-
ands. Bat 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-
fully searched the oldest voyages, and have not found a
single instance, free from doubt, of a terrestrial mammal
(excluding domesticated animals kept by the natives) in-
habiting an island situated above 300 miles from a con-
tinent or great continental island; and many islands sit-
uated at a much less distance are equally barren. The
Falkland Islands, which are inhabited by a wolf-like fox,
come nearest to an exception; but this group cannot be
considered as oceanic, as it lies on a bank in connection
with the mainland at the distance of about 280 miles;
moreover, icebergs formerly brought bowlders to its west-
ern shores, and they may have formerly transported
foxes, as now frequently happens in the arctic regions.
Yet it cannot be said that small islands will not support
at least small mammals, for they occui in many parts of
the world on very small islands, when lying close to a
continent; and hardly an island can be named on which
our smaller quadrupeds have not become naturalized and
greatly multiplied. It cannot be said, on the ordinary
view of creation, that there has not been time for the
GEOGRAPHICAL DISTRIBUTION
193
creation of mammals; many volcanic islands are suffi-
ciently ancient, as shown by the stupendous degradation
which they have suffered, and by their tertiary strata;
there has also been time for the production of endemic
species belonging to other classes; and on continents it
is known that new species of mammals appear and dis-
appear at a quicker rate than other and lower animals.
Although terrestrial mammals do not occur on oceanic
islands, aerial mammals do occur on almost every island.
New Zealand possesses two bats found nowhere else in
the world: Norfolk 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 ter-
restrial mammal can be transported across a wide space
of sea, but bats can fly across. Bats have been seen
wandering by day far over the Atlantic Ocean; and two
North American species either regularly or occasionally
visit Bermuda, at the distance of 600 miles from the
mainland. I hear from Mr. Tomes, who has specially
studied this family, that many species have enormous
ranges, and are found on continents and on far-distanv
islands. Hence we have only to suppose that such wan-
dering species have been modified in their new homes
in relation to their new position, and we can understand
the presence of endemic bats on oceanic islands, with the
absence of all other terrestrial mammals.
Another interesting relation exists, namely, between
the depth of the sea separating islands from each other
or from the nearest continent, and the degree of affinity
194
THE ORIGIN OF SPECIES
of their mammalian inhabitants. Mr. Windsor Earl has
made some striking observations on this head, since
greatly extended by Mr. Wallace's admirable researches,
in regard to the great Malay Archipelago, which is trav-
ersed near Celebes by a space of deep ocean, and this
separates two widely distinct mammalian faunas. On
either side the islands stand on a moderately shallow
submarine bank, and these islands are inhabited by the
same or by closely allied quadrupeds. I have not as
yet had time to follow up this subject in all quarters
of the world; but, as far as I have gone, the relation
holds good. For instance, Britain is separated by a
shallow channel from Europe, and the mammals are the
same on both sides; and so it is with all the islands
near the shores of Australia. The West Indian Islands,
on the other hand, stand on a deeply submerged bank,
nearly 1,000 fathoms in depth, and here we find Ameri-
can forms, but the species and even the genera are quite
distinct. As the amount of modification which animals
of all kinds undergo partly depends on the lapse of
time, and as the islands which are separated from each
other or from the mainland by shallow channels are
more likely to have been continuously united within a
recent period than the islands separated by deeper chan-
nels, we can understand how it is that a relation exists
between the depth of the sea separating two mammalian
faunas and the degree of their affinity — a relation which
is quite inexplicable on the theory of independent acts
of creation.
The foregoing statements in regard to the inhabitants
of oceanic islands — namely, the fewness of the species,
with a large proportion consisting of endemic forms —
GEOGRAPHICAL DISTRIBUTION
195
the members of certain groups, but not those of other
groups in the same class, having been modified — the
absence of certain whole orders, as of batrachians and
of terrestrial mammals, notwithstanding the presence of
aerial bats — the singular proportions of certain orders
of plants — herbaceous forms having been developed into
trees, etc. — seem to me to accord better with the belief
in the efficiency of occasional means of transport, carried
on during a long course of time, than with the belief in
the former connection of all oceanic islands with the
nearest continent; for on this latter view it is probable
that the various classes would have immigrated more
uniformly, and from the species having entered in a
body their mutual relations would not have been much
disturbed, and consequently they would either have not
been modified, or all the species in a more equable
manner.
I do not deny that there are many and serious dif-
ficulties in understanding how many of the inhabitants
of the more remote islands, whether still retaining the
same specific form or subsequently modified, have reached
their present homes. But the probability of other islands
having once existed as halting-places, of which not a
wreck now remains, must not be overlooked. I will
specify one difficult case. 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 — striking instances of which
have been given by Dr. A. A. Gould in relation to the
Paoific. Now it is notorious that land- shells are easily
killed by sea-water; their eggs, at least such as I have
tried, sink in it and are killed. Yet there must be some
196
THE ORIGIN OF SPECIES
unknown but occasionally efficient means for their trans-
portal. Would the just-hatched young sometimes adhere
to the feet of birds roosting on the ground, and thus
get transported? It occurred to me that land-shells, when
hibernating and having a membranous diaphragm over
the mouth of the shell, might be floated in chinks of
drifted timber across moderately wide arms of the sea.
And I find that several species in this state withstand
uninjured an immersion in sea- water during seven days:
one shell, the Helix pomatia, after having been thus
treated and again hibernating was put into sea-water
for twenty days, and perfectly recovered. During this
length of time the shell might have been carried by a
marine current of average swiftness to a distance of 660
geographical miles. As this Helix has a thick calcareous
operculum, I removed it, and when it had formed a new
membranous one, I again immersed it for fourteen days
in sea-water, and again it recovered and crawled away.
Baron Aucapitaine has since tried similar experiments:
he placed 100 land-shells, belonging to ten species, in a
box pierced with holes, and immersed it for a fortnight
in the sea. Out of the hundred shells, twenty-seven re-
covered. The presence of an operculum seems to have
been of importance, as out of twelve specimens of Cyclo-
stoma elegans, which is thus furnished, eleven revived.
It is remarkable, seeing how well the Helix pomatia re-
sisted with me the salt water, that not one of fifty-four
Tspecimens belonging to four other species of Helix tried
by Aucapitaine recovered. It is, however, not at all
probable that land-shells have often been thus trans-
ported; the feet of birds offer a more probable method.
GEOGRAPHICAL DISTRIBUTION
197
On the Relations of the Inhabitants of Islands to those of
the nearest Mainland
The most striking and important fact for us is the
affinity of the species which inhabit islands to those of
the nearest mainland, without being actually the same.
Numerous instances could be given. The Galapagos
Archipelago, situated under the equator, lies at the dis-
tance of between 500 and 600 miles from the shores
of South America. Here almost every product of the
land and of the water bears the unmistakable stamp of
the American continent. There are twenty -six land-
birds; of these, twenty-one, or perhaps twenty-three,
are ranked as distinct species, and would commonly be
assumed to have been here created; yet the close affinity
of most of these birds to American species is manifest in
every character, in their habits, gestures, and tones of
voice. So it is with the other animals, and with a large
proportion of the plants, as shown by Dr. Hooker in his
admirable Flora of this archipelago. The naturalist, look-
ing at the inhabitants of these volcanic islands in the
Pacific, distant several hundred miles from the continent,
feels that he is standing on American land. Why should
this be so? why should the species which are supposed
to have been created in the Galapagos Archipelago, and
nowhere else, bear so plainly the stamp of affinity to
those created in America? There is nothing in the con-
ditions of life, in the geological nature of the islands, in
their height or climate, or in the proportions in which
the several classes are associated together, which closely
resembles the conditions of the South American coast;
in fact, there is a considerable dissimilarity in all these
193
THE ORIGIN OF SPECIES
respects. On the other hand, there is a considerable de-
gree of resemblance in the volcanic natnre of the soil,
in the climate, height, and size of the islands, between
the Galapagos and Cape de Yerde Archipelagoes: but what
an entire and absolute difference in their inhabitants!
The inhabitants of the Cape de Yerde Islands are related
to those of Africa, like those of the Galapagos to Amer-
ica. Facts such as these admit of no sort of explanation
on the ordinary view of independent creation; whereas on
the view here maintained it is obvious that the Gala-
pagos Islands would be likely to receive colonists from
America, whether by occasional means of transport or
(though I do not believe in this doctrine) by formerly
continuous land, and the Cape de Yerde Islands from
Africa; 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 the nearest large island. The exceptions are few,
and most of them can be explained. Thus although
Kerguelen Land stands nearer to Africa than to America,
the plants are related, and that very closely, as we know
'from Dr. Hooker's account, to those of America: but on
the view that this island has been mainly stocked by
seeds brought with earth and stones on icebergs, drifted
by the prevailing currents, this anomaly disappears. New
Zealand in it3 endemic planes is much more closely
related to Australia, the nearest mainland, than to any
other region: and this is what might have been expected;
but it is also plainly related to South America, which,
GEOGRAPHICAL DISTRIBUTION
199
although the next nearest continent, is so enormously
remote that the fact becomes an anomaly. But this
difficulty partially disappears on the view that New Zea-
land, South America, and the other southern lands have
been stocked in part from a nearly intermediate though
distant point, namely, from the antarctic islands, when
they were clothed with vegetation, during a warmer
tertiary period, before the commencement of the last
Glacial period. The affinity, which, though feeble, I am
assured by Dr. Hooker is .real, between the flora of the
southwestern corner of Australia and of the Cape of
Good Hope, is a far more remarkable case; but this
affinity is confined to the plants, and will, no doubt,
some day be explained.
The same law which has determined the relationship
between the inhabitants of islands and the nearest main-
land is sometimes displayed on a small scale, but in a
most interesting manner, within the limits of the same
archipelago. Thus each separate island of the Galapagos
Archipelago is tenanted, and the fact is a marvellous
one, by many distinct species; but these species are re-
lated to each other in a very much closer manner than
to the inhabitants of the American continent, or of any
other quarter of the world. This is what might have
been expected, for islands situated so near to each other
would almost necessarily receive immigrants from the
same original source, and from each other. But how is
it that many of the immigrants have been differently
modified, though only in a small degree, in islands
situated within sight of each other, having the same
geological nature, the same height, climate, etc. ? This
long appeared to me a great difficulty: but it arises in
200
THE ORIGIN OF SPECIES
chief part from the deeply-seated error of considering the
physical conditions of a country as the most important;
whereas it cannot be disputed that the nature of the
other species with which each has to compete is at least
as important, and generally a far more important, element
of success. Now if we look to the species which inhabit
the Galapagos Archipelago, and are likewise found in
other parts of the world, we find that they differ con-
siderably in the several islands. This difference might
indeed have been expected if the islands have 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, though all
proceeding from the same general source. Hence, when
in former times an immigrant first settled on one of the
islands, or when it subsequently spread from one to
another, it would undoubtedly be exposed to different
conditions in the different islands, for it would have
to compete with a different set of organisms; a plant, for
instance, would find the ground best fitted for it occupied
by somewhat different species in the different islands, and
would be exposed to the attacks of somewhat different
enemies. If then it varied, natural selection would
probably favor different varieties in the different islands.
Some species, however, might spread and yet retain the
same character throughout the group, just as we see
some species spreading widely throughout a continent
and remaining the same.
The really surprising fact in this case of the Galapagos
Archipelago, and in a lesser degree in some analogous
cases, is that each new species, after being formed in any
one island, did not spread quickly to the other islands.
GEOGRAPHICAL DISTRIBUTION
201
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 sup-
pose that they have at any former period been continu-
ously united. The currents of the sea are rapid and
sweep between the islands, and gales of wind are extraor-
dinarily rare; so that the islands are far more effectually
separated from each other than they appear on a map.
Nevertheless some of the species, both of those found
in other parts of the world and of those confined to the
archipelago, are common to the several islands; and we
may infer from their present manner of distribution that
they have spread from one island to the others. But
we often take, I think, an erroneous view of the proba-
bility of closely-allied species invading each other's
territory, when put into free intercommunication. Un-
doubtedly, if one species has any advantage 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,
both will probably hold their separate places for almost
any length of time. Being familiar with the fact that
many species, naturalized through man's agency, have
spread with astonishing rapidity over wide areas, we are
apt to infer that most species would thus spread; but we
should rememoer that the species which become natural-
ized in new countries are not generally closely allied to
the aboriginal inhabitants, but are very distinct forms,
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, differ on the
different islands; thus there are three closely-allied species
202
THE ORIGIN OF 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 and
young birds hatched than can possibly be reared; and we
may infer that the mocking-thrush peculiar to Charles
Island is at least as well fitted for its home as is the
species peculiar to Chatham Island. Sir C. Lyell and
Mr. Woilaston 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 species of 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 colonized
by the Porto Santo species; nevertheless both islands
have been colonized by European land-shells, which no
doubt had some advantage over the indigenous species.
From these considerations I think we need not greatly
marvel at the endemic species which inhabit the several
islands of the Galapagos Archipelago not having all
spread from island to island. On the same continent,
also, preoccupation has probably played an important
part in checking the commingling of the species which
inhabit different districts with nearly the same physical
conditions. Thus, the southeast and southwest corners of
Australia have nearly the same physical conditions, and
are united by continuous land, yet they are inhabited by
a vast number of distinct mammals, birds, and plants;
so it is, according to Mr. Bates, with the butterflies and
GEOGRAPHICAL DISTRIBUTION
203
other animals inhabiting the great, open, and continuous
valley of the Amazons.
The same principle which governs the general char-
acter of the inhabitants of oceanic islands, namely, the
relation to the source whence colonists could have been
mostly easily derived, together with their subsequent
modification, is of the widest application throughout
nature. We see this on every mountain-summit, in every
lake and marsh. For Alpine species, excepting in as far
as the same species have become widely spread during
the Glacial epoch, are related to those of the surrounding
lowlands; thus we have in South America, Alpine
humming-birds, Alpine rodents, Alpine plants, etc., all
strictly belonging to American forms; and it is obvious
•that a mountain, as it became slowly upheaved, would be
colonized from the surrounding lowlands. So it is with
the inhabitants of lakes and marshes, excepting in so far
as great facility of transport has allowed the same forms
to prevail throughout large portions of the world. We
see this same principle in the character of most of the
blind animals inhabiting the caves of America and of
Europe. Other analogous facts could be given. It will,
I believe, be found universally true, that wherever in
two regions, let them be ever so distant, many closely
allied or representative species occur, there will likewise
be found some identical species; and wherever many
closely-allied species occur, there will be found many
forms which some naturalists rank as distinct species, and
others as mere varieties; these doubtful forms showing
us the steps in the progress of modification.
The relation between the power and extent of migra-
tion in certain species, either at the present or at some
THE ORIGIN OF SPECIES
former period, and the existence at remote points of the
world of closely-allied species, is shown in another and
more general way. Mr. Gould remarked to me long
ago that in those genera of birds which range over
the world, many of the species have very wide ranges.
I can hardly doubt that this rule is generally true,
though difficult of proof. Among mammals, we see it
strikingly displayed in Bats, and in a lesser degree in
the Felidaa and Canidas. We see the same rule in the
distribution of butterflies and beetles. So it is with most
of the inhabitants of fresh water, for many of the genera
in the most distinct classes range over the world, and
many of the species have enormous ranges. It is not
meant that all, but that some of the species have very
wide ranges in the genera which range very widely. Nor
is it meant that the species in such genera have on an
average a very wide range; for this will largely depend
on how far the process of modification has gone; for in-
stance, two varieties of the same species inhabit America
and Europe, and thus the species has an immense range;
but, if variation were to be carried a little further, the
two varieties would be ranked as distinct species, and
their range would be greatly reduced. Still less is it
meant that species which have 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 accord-
ing to the view that all the species of a genus, though
distributed to the most remote points of the world, are
GEOGRAPHICAL DISTRIBUTION
205
descended from a single progenitor, we ought to find,
and I believe as a general rule we do find, that some
at least of the species range very widely.
We should bear in mind that many genera in all
classes are of ancient origin, and the species in this case
will have had ample time for dispersal and subsequent
modification. There is also reason to believe from geo-
logical evidence that within each great class the lower
organisms change at a slower rate than the higher; con-
sequently they will have had a better chance of ranging
widely and of still retaining the same specific character.
This fact, together with that of the seeds and eggs of
most lowly organized forms being very minute and better
fitted for distant transportal, probably accounts for a law
which has long been observed, and which has lately
been discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms stands the
more widely it ranges.
The relations just discussed — namely, lower organisms
ranging more widely than the higher — some of the
species of widely- ranging genera themselves ranging
widely — such facts as alpine, lacustrine, and marsh pro-
ductions being generally related to those which live on
the surrounding low lands and dry lands — the striking
relationship between the inhabitants of islands and those
of the nearest mainland — the still closer relationship of
the distinct inhabitants of the islands in the same ar-
chipelago— are inexplicable on the ordinary view of the
independent creation of each species, but are explicable
if we admit colonization from the nearest or readiest
source, together with the subsequent adaptation of the
colonists to their new homes.
206
THE ORIGIN OF SPECIES
Summary of the last and present Chapters
In these chapters I have endeavored to show that if
we make due allowance for our ignorance of the full
effects of changes of climate and of the level of the land,
which have certainly occurred within the recent period,
and of other changes which have probably occurred — if
we remember how ignorant we are with respect to the
many curious means of occasional transport — if we bear
in mind, and this is a very important consideration, how
often a species may have ranged continuously over a wide
area, and then have become extinct in the intermediate
tracts — the difficulty is not insuperable in believing that
all the individuals of the same species, wherever found,
are descended from common parents. And we are led
to this conclusion, which has been arrived at by many
naturalists under the designation of single centres of
creation, by various general considerations, more espe-
cially from the importance of barriers of all kinds, and
from the analogical distribution of sub-genera, genera,
and families.
With respect to distinct species belonging to the same
genus, which on our theory have spread from one parent-
source; if we make the same allowances as before our
ignorance, and remember that some forms of life have
changed very slowly, enormous periods of time having
been thus granted for their migration, the difficulties are
far from insuperable; though in this case, as in that of
the individuals of the same species, they are often great.
As exemplifying the effects of climatal changes on
distribution, I have attempted to show how important a
part the last Glacial period has played, which affected
GEOGRAPHICAL DISTRIBUTION
207
even the equatorial regions, and which, during the
alternations of the cold in the north and south, allowed
the productions of opposite hemispheres to mingle, and
left some of them stranded on the mountain-summits in
all parts of the world. 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 all the individuals of the
same species, and likewise of the several species belong-
ing to the same genus, have proceeded from some one
source; then all the grand leading facts of geographical
distribution are explicable on the theory of migration,
together with subsequent modification and the multipli-
cation of new forms. We can thus understand the high
importance of barriers, whether of land or water, in not
only separating, but in apparently forming the several
zoological and botanical provinces. We can thus under-
stand the concentration of related species within the
same areas; 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 des-
erts, are linked together in so mysterious a manner, and
are likewise linked to the extinct beings which formerly
inhabited the same continent. Bearing in mind that the
mutual relation of organism to organism is of the highest
importance, we can see why two areas having nearly
the same physical conditions should often be inhabited
by very different forms of life; for according to the
length of time which has elapsed since the colonists en-
tered one of the regions, or both; according to the nature
^Science — 26
208
THE ORIGIN OF SPECIES
of the communication whicb allowed certain forms and
not others to enter, either in greater or lesser numbers;
according or not as those which entered happened to
come into more or less direct competition with each
other and with the aborigines; and according as the
immigrants were capable of varying more or less rapidly,
there would ensue in the two or more regions, independ-
ently of their physical conditions, infinitely diversified
conditions of life — there would be an almost endless
amount of organic action and reaction — and we should
find some groups of beings greatly, and some only
slightly modified — some developed in great force, some
existing in scanty numbers — and this we do find in the
several great geographical provinces of the world.
On these same principles we can understand, as I have
endeavored to show, why oceanic islands should have few
inhabitants, but that of these, a large proportion should
be endemic or peculiar; and why, in relation to the
means of migration, one group of beings should have
all its species peculiar, and another group, even within
the same class, should have all its species the same with
those in an adjoining quarter of the world. We can see
why whole groups of organisms, as batrachians and ter-
restrial mammals, should be absent from oceanic islands,
while the most isolated islands should possess their own
peculiar species of aerial mammals or bats. We can see
why, in islands, there should be some relation between
the presence of mammals, in a more or less modified
condition, and the depth of the sea between such islands
and the mainland. We can clearly see why all the in-
habitants of an archipelago, though specifically distinct
on the several islets, should be closely related to each
GEOGRAPHICAL DISTRIBUTION
209
other; and should likewise be related, but less closely,
to those of the nearest continent, or other source whence
immigrants might have been derived. We can see why,
if there exist very closely allied or representative species
in two areas, however distant from each other, some
identical species will almost always there be found.
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 govern-
ing at the present time the differences in different areas.
We see this in many facts. The endurance of each spe-
cies and group of species is continuous in time; for the
apparent exceptions to the rule are so few that they may
fairly be attributed to our not having as yet discovered
in an intermediate deposit certain forms which are absent
in it, but which occur both above and below: so in
space, it certainly is the general rule that the area inhab-
ited by a single species, or by a group of species, is con-
tinuous, and the exceptions, which are not rare, may, as
I have attempted to show, be accounted for by former
migrations under different circumstances, or through occa-
sional means of transport, or by the species having be-
come extinct in the intermediate tracts. But iu time and
space species and groups of species have their points of
maximum development. Groups of species, living during
the same period of time, or living within the same area,
are often characterized by trifling features in common, as
of sculpture or color. In looking to the long succession
of past ages, as in looking to distant provinces through-
out the world, we find that species in certain classes dif-
fer little from each other, while those in another class,
210
THE ORIGIN OF SPECIES
or only in a different section of the same order, differ
greatly from each other. In both time and space the
lowly organized members of each class generally change
less than the highly organized ; but there are in both
cases- marked exceptions to the rule. According to our
theory, these several relations throughout time and space
are intelligible; for whether we look to the allied forms
of life which have changed during successive ages, or to
those which have changed after having migrated into dis-
tant quarters, in both cases they are connected by the
same bond of ordinary generation; in both cases the laws
of variation have been the same, and modifications have
been accumulated by the same means of natural selection.
CLASSIFICATION
211
CHAPTER, XIV*
MUTUAL AFFINITIES OF ORGANIC BEINGS : MORPHOLOGY:
EMBRYOLOGY: RUDIMENTARY ORGANS
Classification, groups subordinate to groups — Natural system — Rules
and difficulties in classification, explained on the theory of descent with
modification — Classification of varieties — Descent always used in classi-
fication— Analogical or adaptive characters— Affinities, general, com-
plex, and radiating—Extinction separates and defines groups —
Morphology, between members of the same class, between parts of
the same individual — Embryology, laws of, explained by variations not
supervening at an early age, and being inherited at a corresponding age
—Rudimentary organs; their origin explained — Summary
Classification
FROM the most remote period in the history of the
world organic beings have been found to resemble
each other in descending degrees, so that they can
be classed in groups under groups. This classification is
not arbitrary like the grouping of the stars in constella-
tions. The existence of groups would have been of
simple significance, if one group had been exclusively
fitted to inhabit the land, and another the water; one to
feed on flesh, another on vegetable matter, and so on;
but the case is widely different, for it is notorious
how commonly members of even the same sub-group
have different habits. In the second and fourth chap-
ters, on Variation and on Natural Selection, I have
attempted to show that within each country it is the
widely ranging, the much diffused and common, that is
212
THE ORIGIN OF SPECIES
the dominant species, belonging to the larger genera in
each class, which vary most. The varieties, or incipient
species, thus produced, ultimately become converted into
new and distinct species; and these, on the principle of
inheritance, tend to produce other new and dominant
species. Consequently the groups which are now large,
and which generally include many dominant species, tend
to go on increasing in size. I further attempted to show
that from the varying descendants of each species trying
to occupy as many and as different places as possible in
the economy of nature, they constantly tend to diverge
in character. This latter conclusion is supported by ob-
serving the great diversity of forms which, in any small
area, come into the closest competition, and by certain
facts in naturalization.
I attempted also to show that there is a steady
tendency, in the forms which are increasing in number
And diverging in character, to supplant and exterminate
ihe preceding, less divergent and less improved forms.
I request the reader to turn to the diagram illustrating
the action, as formerly explained, of these several prin-
ciples; and he will see that the inevitable result is, that
the modified descendants proceeding from one progenitor
become broken up into groups subordinate to groups.
In the diagram each letter on the uppermost line may
represent a genus including several species; and the
whole of the genera along this upper line form to-
gether one class, for all are descended from one ancient
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
lub-family, distinct from that containing the next two
CLASSIFICATION
213
genera on the right hand, which diverged from a com-
mon parent at the fifth stage of descent. These five
genera have also much in common, though less than
when grouped in sub-families; and they form a family
distinct from that containing the three genera still further
to the right hand, which diverged at an 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 progeni-
tor grouped into genera; and the genera into sub-families,
families, and orders, all under one great class. The
grand fact of the natural subordination of organic beings
in groups under groups, which, from its familiarity, does
not always sufficiently strike us, is in my judgment thus
explained. No doubt organic beings, like all other ob-
jects, can be classed in many ways, either artificially by
single characters, or more naturally by a number of char-
acters. We know, for instance, that minerals and the
elemental substances can be thus arranged. In this case
there is of course no relation to genealogical succession,
and no cause can at present be assigned for their falling
into groups. But with organic beings the case is dif-
ferent, and the view above given accords with their nat-
ural arrangement in group under group; and no other
explanation has ever been attempted.
Naturalists, as we have seen, try to arrange tne spe-
cies, genera, and families in each class on what is called
the Natural System. But what is meant by this system?
Some authors look at it merely as a scheme for arranging
together those living objects which are most alike, and for
separating those which are most unlike; or as an artificial
method of enunciating, as briefly as possible, general
214
THE ORIGIN OF SPECIES
propositions — that is, by one sentence to give the char-
acters 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 in-
genuity and utility of this system are indisputable. But
many naturalists think that something more is meant by
the Natural System; they believe that it reveals the plan
of the Creator; but unless it be specified whether order
in time or space, or both, or what else is meant by the
plan of the Creator, it seems to me that nothing is thus
added to our knowledge. Expressions such as that fa-
mous one by Linnaeus, which we often meet with in a
more or less concealed form, namely, that the characters
do not make the genus, but that the genus gives the
characters, seem to imply that some deeper bond is in-
cluded in our classifications than mere resemblance. I
believe that this is the case, and that community of de-
scent— the one known cause of close similarity in organic
beings — is the bond, which, though observed by various
degrees of modification, is partially revealed to us by our
classifications.
Let us now consider the rules followed in classifica-
tion, and the difficulties which are encountered on the
view that classification either gives some unknown plan
of creation, or is simply a scheme for enunciating general
propositions and of placing together the forms most like
each other. It might have been thought (and was in
ancient times thought) that those parts of the structure
which determined the habits of life, and the general
place of each being in the economy of nature, would be
of very high importance in classification. Nothing can
CLASSIFICA TION
215
be more false. No one regards the external similarity of
a mouse to a shrew, of a dugong to a whale, of a whale
to a fish, as of any importance. These resemblances,
though so intimately connected with the whole life of
the being, are ranked as merely "adaptive or analogical
characters"; but to the consideration of these resem-
blances we shall recur. It may even be given as a
general rule that the less any part of the organization
is concerned with special habits, the more important it
becomes for classification. As an instance: Owen, in
speaking of the dugong, says, "The generative organs,
being those which are most remotely related to the habits
and food of an animal, I have always regarded as afford-
ing very clear indications of its true affinities. We are
least likely in the modifications of these organs to mis-
take a merely adaptive for an essential character." With
plants how remarkable it is that the organs of vegetation,
on which their nutrition and life depend, are of little
signification; whereas the organs of reproduction, with
their product the seed and embryo, are of paramount
importance! So again in formerly discussing certain
morphological characters which are not functionally im-
portant, we have seen that they are often of the highest
service in classification. This depends on their con-
stancy throughout many allied groups; and their constancy
chiefly depends on any slight deviations not having been
preserved and accumulated by natural selection, which
acts only on serviceable characters.
That the mere physiological importance of an organ
does not determine its classificatory value, is almost
proved by the fact, that in allied groups, in which the
same organ, as we have every reason to suppose, has
216
THE ORIGIN OF SPECIES
nearly vhe same physiological value, its classificatory
value is widely different. No naturalist can have worked
long at any group without being struck with this fact;
and it has been fully acknowledged in the writings of
almost every author. It will suffice to quote the highest
authority, Eobert Brown, who, in speaking of certain
organs in the Proteaceae, says their generic importance,
"like that of all their parts, not only in this, but, as I
apprehend, in every natural family, is very unequal, and
in some cases seems to be entirely lost." Again, in
another work, he says the genera of the Connaraceae
**'differ in having one or more ovaria, in the existence or
absence of albumen, in the imbricate or valvular aestiva-
tion. Any one of these characters singly is frequently of
more than generic importance, though here even when
all taken together they appear insufficient to separate
Cnestis from Connarus." To give an example among
insects: in one great division of the Hymenoptera, the
antennae, as Westwood has remarked, are most constant
in structure; in another division they differ much, and
the differences are of quite subordinate value in classifi-
cation ; yet no one will say that the antennae in these two
divisions of the same order are of unequal physiological
importance. Any number of instances could be given of
the varying importance for classification of the same im-
portant organ within the same group of beings.
Again, no one will say that rudimentary or atrophied
organs are of high physiological or vital importance; yet,
undoubtedly, organs in this condition are often of much
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
CLASSIFICATION
217
serviceable in exhibiting the close affinity between rumi-
nants and pachyderms. Robert Brown has strongly in-
sisted on the fact that the position of the rudimentary
florets is of the highest importance in the classification
of the grasses.
Numerous instances could be given of characters
derived from parts which must be considered of very
trifling physiological importance, but which are univer-
sally admitted as highly serviceable in the definition of
whole groups. For instance, whether or not there is an
open passage from the nostrils to the mouth, the only
character, according to Owen, which absolutely distin-
guishes fishes and reptiles — the inflection of the angle of
the lower jaw in Marsupials — the manner in which the
wings of insects are folded — mere color in certain Algae —
mere pubescence on parts of the flower in grasses — the
nature of the dermal covering, as hair or feathers, in
the Vertebrata. If the Ornithorhynchus had been covered
with feathers instead of hair, this external and trifling
character would have been considered by naturalists as an
important aid in determining the degree of affinity of this
strange creature to birds.
The importance, for classification, of trifling char-
acters, mainly depends on their being correlated with
many other characters of more or less importance. The
value indeed of an aggregate of characters is very evi-
dent in natural history. Hence, as has often been re-
marked, a species may depart from its allies in several
characters, both of high physiological importance, and of
almost universal prevalence, and yet leave us in no doubt
where it should be ranked. Hence, also, it has been
found that a classification founded on any single char-
218
THE ORIGIN OF SPECIES
acter, however important that may be, has always failed;
for no part of the organization is invariably constant.
The importance of an aggregate of characters, even when
none are important, alone explains the aphorism enunci-
ated by Linnaeus, namely, that the characters do not give
the genus, but the genus gives the characters; for this
seems founded on the appreciation of many trifling points
of resemblance, too slight to be denned. Certain plants,
belonging to the Malpighiaceae, bear perfect and degraded
flowers; in the latter, as A. de Jussieu has remarked,
"the greater number of the characters proper to the
species, to the genus, to the family, to the class, dis-
appear, and thus laugh at our classification." When
Aspicarpa produced in France, during several years,
only these degraded flowers, departing so wonderfully in
a number of the most important points of structure from
the proper type of the order, yet M. Richard sagaciously
saw, as Jussieu observes, that this genus should still be
retained among the Malpighiaceae. This case well illus-
trates the spirit of our classifications.
Practically, when naturalists are at work, they do not
trouble themselves about the physiological value of the
characters which they use in defining a group or in
allocating any particular species. If they find a character
nearly uniform, and common to a great number of forms,
and not common to others, they use it as one of high
value; if common to some lesser number, they use it as
of subordinate value. This principle has been broadly
confessed by some naturalists to be the true one; and by
none more clearly than by that excellent botanist, Aug.
St.-Hilaire. If several trifling characters are always
found in combination, though no apparent bond of con-
CLASSIFICATION
219
nection can be discovered between them, especial value is
set on them. As in most groups of animals, important
organs, such as those for propelling the blood, or for
aerating it, or those for propagating the race, are found
nearly uniform, they are considered as highly serviceable
in classification; but in some groups all these, the most
important vital organs, are found to offer characters of
quite subordinate value. Thus, as Fritz Miiller has lately
remarked, in the same group of crustaceans, Cvpridina is
furnished with a heart, while in two closely allied genera,
namely, Cypris and Cytherea, there is no such organ;
one species of Cypridina has well-developed branchiae,
while another species is destitute of them.
We can see why characters derived from the embryo
should be of equal importance with those derived from
the adult, for a natural classification of course includes
all ages. 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 embryological char-
acters are the most important of all; and this doctrine
has very generally been admitted as true. Nevertheless,
their importance has sometimes been exaggerated, owing
to the adaptive characters of larvas not having been
excluded; in order to show this, Fritz Miiller arranged
by the aid of such characters alone the great class of
crustaceans, and the arrangement did not prove a natural
one. But there can be no doubt that embryonic, ex-
cluding larval characters, are of the highest value for
classification, not only with animals but with plants.
220
THE ORIGIN OF SPECIES
Thus the main divisions of flowering plants are founded
on differences in the embryo — on the number and position
of the cotyledons, and on the mode of development of
the plumule and radicle. We shall immediately see why
these characters possess so high a value in classification,
namely, from the natural system being genealogical in its
arrangement.
Our classifications are often plainly influenced by
chains of affinities. Nothing can be easier than to define
a number of characters common to all birds; but with
crustaceans any such definition has hitherto been found
impossible. There are crustaceans at the opposite ends
of the series which have hardly a character in common;
yet the species at both ends, from being plainly allied
to others, and these to others, and so onward, can be
recognized as unequivocally belonging to this, and to no
other class of the Articulata.
Geographical distribution has often been used, though
perhaps not quite logically, in classification, more
especially in very large groups of closely allied forms,
Temminck insists on the utility or even necessity of this
practice in certain groups of birds; and it has been fol-
lowed by several entomologists and botanists.
Finally, with respect to the comparative value of the
various groups of species, such as orders, sub-orders,
families, sub-families, and genera, they seem to be, at
least at present, almost arbitrary. Several of the best
botanists, such as Mr. Bentham and others, have strongly
insisted on their arbitrary value. Instances could be
given, among plants and insects, of a group first ranked
by practiced naturalists as only a genus, and then raised
to the rank of a sub- family or family; and this has been
CLASSIFICATION
221
done, not because further research has detected important
structural differences, at first overlooked, but because
numerous allied species with slightly different grades of
difference have been subsequently discovered.
All the foregoing rules and aids and difficulties in
classification may be explained, if I do not greatly
deceive myself, on the view that the Natural System
is founded on descent with modification; — that the char-
acters which naturalists consider as showing true affinity
between any two or more species are those which have
been inherited from a common parent, all true classifi-
cation being genealogical; — that community of descent
is the hidden bond which naturalists have been uncon-
sciously seeking, and not some .unknown, plan of crea-
tion, 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 each other,
must be strictly genealogical in order to be natural; but
that the amount of difference in the several branches
or groups, though allied in the same degree in blood to
their common progenitor, may differ greatly, being due
to the different degrees of modification which they have
undergone; and this is expressed by the forms being
ranked under different genera, families, sections, or
orders. The reader will best understand what is meant,
if he will take the trouble to refer to the diagram in the
fourth chapter. We will suppose the letters -A to L to
represent allied genera existing during the Silurian epoch,
and descended from some still earlier form. In three of
222
THE ORIGIX OF SPECIES
these genera (A, F, and I), a species has transmitted
modified descendants to the present day, represented by
the fifteen genera (a1* to z1*) on the nppermost horizontal
line. Xow all these modified descendants from a single
species are related in blood or descent in the same de-
gree; they may metaphorically be called cousins to the
same millionth degree; yet they differ widely and in
different degrees from each other. The forms descended
from A, now broken up into two or three families, con-
stitute a distinct order from those descended from I, also
broken up into two families. Xor can the existing species,
descended from A, be ranked in the same genus with the
parent A; or those from I, with the parent L But the
existing genus F1* 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 organisms belong
to Silurian genera. So that the comparative value of the
differences between these organic, beings, which are all
related to each other in the same degree in blood, has
come to be widely different. Nevertheless their genea-
logical arrangement remains strictly true, not only at the
present time, but at each successive period of descent.
All the modified descendants from A will have inherited
something in common from their common parent, as will
all the descendants from I; so will it be with each sub-
ordinate branch of descendants at each successive stage.
If. however, we suppose any descendant of A, or of I,
to have become so much modified as to have lost all
traces of its parentage, in this case its place in the nat-
ural system will be lost, as seems to have occurred with
some few existing organisms. All the descendants of the
genus F, along its whole line of descent, are supposed
CLASSIFICATION
223
to have been but little modified, and they form a single
genus. But this genus, though much isolated, will still
occupy its proper intermediate position. The representa-
tion of the groups, as here given in the diagram on a flat
surface, is much too simple. The branches ought to
have diverged in all directions. If the names of the
groups had been simply written down in a linear series,
the representation would have been still less natural; and
it is notoriously not possible to represent in a series, on
a flat surface, the affinities which we discover in nature
among the beings of the same group. Thus, the natural
system is genealogical in its arrangement, like a pedigree:
but the amount of modification which the different groups
have undergone has to be expressed by ranking them
under different so-called genera, sub-families, families,
sections, orders, and classes.
It may be worth while to illustrate this view of
classification, by taking the case of languages. If we
possessed a perfect pedigree of mankind, a genealogical
arrangement of the races of man would afford the best
classification of the various languages now spoken
throughout the world; and if all extinct languages,
and all intermediate and slowly changing dialects, were
to be included, such an arrangement would be the only
possible one. Yet it might be that some ancient lan-
guages had altered very little and had given rise to few
new languages, while others had altered much, owing to
the spreading, isolation, and state of civilization of the
several codescended races, and had thus given rise to
many new dialects and languages. The various degrees
of difference between the languages of the same stock
would have to be expressed by groups subordinate to
224
THE ORIGIN OF SPECIES
groups; but the proper or even the only possible ar-
rangement would still be genealogical; and this would
be strictly natural, as it would connect together all lan-
guages, extinct and recent, 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 known or believed
to be descended from a single species. These are
grouped under the species, with the sub-varieties under
the varieties; and in some cases, as with the domestic
pigeon, with several other grades of difference. Nearly
the same rules are followed as in classifying species.
Authors have insisted on the necessity of arranging vari-
eties on a natural instead of an artificial system; we are
cautioned, for instance, not to class two varieties of the
pineapple together, merely because their fruit, though
the most important part, happens to be nearly identical;
no one puts the Swedish and common turnip together,
though the esculent and thickened stems are so similar.
Whatever part is found to be most constant is used in
classing varieties: thus the great agriculturist Marshall
says the horns are very useful for this purpose with
cattle, because they are less variable than the shape or
color of the body, etc. ; whereas with sheep the horns are
much less serviceable, because less constant. In class-
ing varieties I apprehend that if we had a real pedigree,
a genealogical classification would be universally pre-
ferred; and it has been attempted in some cases. For
we might feel sure, whether there had been more or less
modification, that the principle of inheritance would keep
the forms together which were allied in the greatest
number of points. In tumbler pigeons, though some of
CLASSIFICATION
225
the sub- varieties differ in the important character of the
length of the beak, jet all are kept together from having
the common habit of tumbling; but the short- faced breed
has nearly or quite lost this habit: nevertheless, without
any thought on the subject, these tumblers are kept in
the same group, because allied in blood and alike
in some other respects.
With species in a state of nature, every naturalist
has in fact brought descent into his classification; for
he includes in his lowest grade, that of 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 predicted in common of the
adult males and hermaphrodites of certain cirripeds, and
yet no one dreams of separating them. As soon as the
three Orchidean forms, Monacanthus, Myanthus, and
Catasetum, which had previously been ranked as three
distinct genera, were known to be sometimes produced
on the 'same plant, they were immediately considered as
varieties; and now I have been able to show that they
are the male, female, and hermaphrodite forms of the
same species. The naturalist includes as one species
the various larval stages of the same individual, how-
ever much they may differ from each other and from
the adult, as well as the so-called alternate generations
of Steenstrup, which can only in a technical sense be
considered as the same individual. He includes monsters
and varieties, not from their partial resemblance to the
parent-form, but because they are descended from it.
As descent has universally been used in classing
together the individuals of the same species, though
the males and females and larvae are sometimes ex-
a eonsHiefMie amooiit 01 momma
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belieire it has been unconsciously used;
Call I ■Zlirr^liZ.i llr frTrril nles *T_d
LsTt zzi sttzt^zz z-rL:zzi~r.s. ire
:;:r ~r ;l>:^r :z^z^.:i^ sr'ziz:'iz ire ir_e
to biTe been modified, in relation to the ei
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CLASSIFICATION
227
We can understand why a species or a group of
species may depart from its allies, in several of its most
important characteristics, and yet be safely classed with
them. This may be safely done, and is often done, as
long as a sufficient number of characters, let them be
ever so unimportant, betrays the hidden bond of com-
munity of descent. Let two forms have not a single
character in common, yet, if these extreme forms are
connected together by a chain of intermediate groups,
we may at once infer their community of descent, and
we put them all into the same class. As we find organs
of high physiological importance — those which serve to
preserve life under the most diverse conditions of exist-
ence— are generally the most constant, we attach especial
value to them; but if these same organs, in another
group or section of a group, are found to differ much,
we at once value them less in our classification. We
shall presently see why embryological characters are of
such high classificatory importance. Geographical distri-
bution may sometimes be brought useful Jy into play in
classing large genera, because all the species of the same
genus, inhabiting any distinct and isolated region, are in
all probability descended from the same parents.
Analogical Resemblances
We can understand, on the above views, the very
important distinction between real affinities and analogical
or adaptive resemblances. Lamarck first called attention
to this subject, and he has been ably followed by
Macleay and others. The resemblance in the shape of
the body and in the finlike anterior limbs between du-
228
THE ORIGIN OF SPECIES
gongs and whales, and between these two orders of mam-
mals and fishes, are analogical. So is the resemblance
between a mouse and a shrewmouse (Sorex), which belong
to different orders; and the still closer resemblance, in-
sisted on by Mr. Mivart, between the mouse and a small
marsupial animal (Antechinus) of Australia. These latter
resemblances may be accounted for, as it seems to me,
by adaptation for similarly active movements through
thickets and herbage, together with concealment from
enemies.
Among insects there are innumerable similar instances;
thus Linnaeus, misled by external appearances, actually
classed a homopterous insect as a moth. We see some-
thing of the same kind even with our domestic varieties,
as in the strikingly similar shape of the body in the
improved breeds of the Chinese and common pig, which
are descended from distinct species; and in the similarly
thickened stems of the common and specifically distinct
Swedish turnip. The resemblance between the greyhound
and the racehorse is hardly more fanciful than the analo-
gies which have been drawn by some authors between
widely different animals.
On the view of characters being of real importance
for classification, only in so far as they reveal descent,
we can clearly understand why analogical or adaptive
characters, although of the utmost importance to the
welfare of the being, are almost valueless to the syste-
matise For animals, belonging to two most distinct lines
of descent, may have become adapted to similar condi-
tions, and thus have assumed a close external resem-
blance; but such resemblances will not reveal — will
rather tend to conceal their blood-relationship. We can
CLASSIFICATION
229
thus also understand the apparent paradox, that the very
same characters are analogical when one group is com-
pared with another, but give true affinities when the
members of the same group are compared together: thus,
the shape of the body and fin like limbs are only analogi-
cal when whales are compared with fishes, being adapta-
tions in both classes for swimming through the water;
but between the several members of the whale family,
the shape of the body and the finlike limbs offer char-
acters exhibiting true affinity; for as these parts are so
nearly similar throughout the whole family, we cannot
doubt that they have been inherited from a common
ancestor. So it is with fishes.
Numerous cases could be given of striking resem-
blances in quite distinct beings between single parts or
organs, which have been adapted for the same functions.
A good instance is afforded by the close resemblance of
the jaws of the dog and Tasmanian wolf or Thylacinus —
animals which are widely sundered in the natural system.
But this resemblance is confined to general appearance,
as in the prominence of the canines, and in the cutting
shape of the molar teeth. For the teeth really differ
much: thus the dog has on each side of the upper jaw
four pre-molars and only two molars; while the Thyla-
cinus has three pre-molars and four molars. The molars
also differ much in the two animals in relative size and
structure. The adult dentition is preceded by a widely
different milk dentition. Any one may of course deny
that the teeth in either case have been adapted for
tearing flesh, through the natural selection of successive
variations; but if this be admitted in the one case, it is
unintelligible to me that it should be denied in the
230
THE ORIGIN OF SPECIES
other. I am glad to find that so high an authority as
Professor Flower has come to this same conclusion.
The extraordinary cases given in a former chapter, of
widely different fishes possessing electric organs — of wide-
ly different insects possessing luminous organs — and of
orchids and asclepiads having pollen-masses with viscid
disks, come under this same head of analogical resem-
blances. But these cases are so wonderful that they
were introduced as difficulties or objections to our
theory. In all such cases some fundamental difference
in the growth or development of the parts, and generally
in their matured structure, can be detected. The end
gained is the same, but the means, though appearing
superficially to be the same, are essentially different.
The principle formerly alluded to under the term of
analogical variation has probably in these cases often
come into play: that is, the members of the same class,
although only distantly allied, have inherited so much in
common in their constitution that they are apt to vary
under similar exciting causes in a similar manner; and
this would obviously aid in the acquirement through
natural selection of parts or organs, strikingly like each
other, independently of their direct inheritance from a
common progenitor.
As species belonging to distinct classes have often
been adapted by successive slight modifications to live
under nearly similar circumstances — to inhabit, for in-
stance, the three elements of land, air, and water — we
can perhaps understand how it is that a numerical paral-
lelism has sometimes been observed between the sub-
groups of distinct classes. A naturalist, struck with a
parallelism of this nature, by arbitrarily raising or sinking
CLASSIFICA TION
231
the value of the groups in several classes (and all our
experience shows that their valuation is as yet arbitrary),
could easily extend the parallelism over a wide range;
and thus the septenary, quinary, quaternary and ternary
classifications have probably arisen.
There is another and curious class of cases in which
close external resemblance does not depend on adaptation
to similar habits of life, but has been gained for the sake
of protection. I allude to the wonderful manner in
which certain butterflies imitate, as first described by
Mr. Bates, other and quite distinct species. This excel-
lent observer has shown that in some districts of South
America, where, for instance, an Ithomia abounds in
gaudy swarms, another butterfly, namely, a Leptalis,
is often found mingled in the same flock; and the latter
so closely resembles the Ithomia in every shade and
stripe of color, and even in the shape of its wings, that
Mr. Bates, with his eyes sharpened by collecting during
eleven years, was, though always on his guard, continu-
ally deceived. When the mockers and the mocked are
caught and compared, they are found to be very different
in essential structure, and to belong not only to distinct
genera, but often to distinct families. Had this mimicry
occurred in only one or two instances, it might have
been passed over as a strange coincidence. But, if we
proceed from a district where one Leptalis imitates an
Ithomia, another mocking and mocked species belonging
to the same two genera, equally close in their resem-
blance, may be found. Altogether no less than ten
genera are enumerated which include species that imitate
other butterflies. The mockers and mocked always in-
habit the same region; we never find an imitator living
^Science— 27
232
THE ORIGiy OF SPECIES
remote from the form which it imitates. The mocker3
are almost invariably rare insects; the mocked in almost
every case abound in swarms. In the same district in
which a species of Leptalis closely imitates an Ithomia
there are sometimes other Lepidoptera mimicking the
same Ithomia: so that in the same place, species of three
genera of butterflies and even a moth are found all
closely resembling a butterfly belonging to a fourth
genus. It deserves especial notice that many of
the mimicking forms of the Leptalis, as well as of the
mimicked forms, can be shown by a graduated series to
be merely varieties of the same species; while others are
undoubtedly distinct species. Bat why, it may be asked,
are certain forms treated as the mimicked and others
as the mimickers? Mr. Bates satisfactorily answers this
question, by showing that the form which is imitated
keeps the usual dress of the group to which it belongs,
while the counterfeiters have changed their dress and do
not resemble their nearest allies.
We are next led to inquire what reason can be as-
signed for certain butterflies and moths so often assuming
the dress of another and quite distinct form; why, to the
perplexity of naturalists, has nature condescended to
the tricks of the stage? Mr. Bates has, no doubt, hit
on the true explanation. The mocked forms, which
always abound in numbers, must habitually escape de-
struction to a large extent, otherwise they couid not
exist in such swarms; and a large amount of evidence
has now been collected, showing that they are distasteful
to birds and other insect-devouring animals. The mock-
ing forms, on the other hand, that inhabit the same dis-
trict, are comparatively rare, and belong to rare groups;
CLASSIFICA TION
233
hence they must suffer habitually from some danger, for
otherwise, from the number of eggs laid by all butter-
flies, they would in three or four generations swarm over
the whole country. Now if a member of one of these
persecuted and rare groups were to assume a dress so
like that of a well-protected species that it continually
deceived the practiced eyes of an entomologist, it would
often deceive predaceous birds and insects, and thus often
escape destruction. Mr. Bates may almost be said to
have actually witnessed the process by which the mimick-
ers have come so closely to resemble the mimicked; for
he found that some of the forms of Leptalis which mimic
so many other butterflies varied in an extreme degree.
In one district several varieties . occurred, and of these
one alone resembled to a certain extent the common
Ithomia of the same district. In another district there
were two or three varieties, one of which was much com-
moner than the others, and this closely mocked another
form of Ithomia. From facts of this nature, Mr. Bates
concludes that the Leptalis first varies; and when a
variety happens to resemble in some degree any common
butterfly inhabiting the same district, this variety, from
its resemblance to a flourishing and little-persecuted
kind, has a better chance of escaping destruction from
predaceous birds and insects, and is consequently oftener
preserved — "the less perfect degrees of resemblance
being generation after generation eliminated, and only
the others left to propagate their kind." So that here
we have an excellent illustration of natural selection.
Messrs. Wallace and Trimen have likewise described
several equally striking cases of imitation in the Lepidop-
tera of the Malay Archipelago and Africa, and with
2S4 THE ORIGIX OF SPECIES
some other insects. Mr. Wallace has also detected one
such case with birds, but we have none with the larger
qua drupe ds. The much greyer frequency of im::a:::a
with insects than with other animals is probably the
consequence of their small size; insects cannot defend
themselves, excepting indeed the kinds furnished with a
sting, and I have never heard of an instance of such
kinds mocking other insects, though they are mocked;
insects cannot easily escape by flight from the larger
animals which prey on them: therefore, speaking meta-
phorically, they are reduced, like most weak creatures,
to trickery and dissimularlon.
It should be observed that the process of imitation
probably never commenced between forms widely dis-
similar in color. But starting with s::e:ies alrea It 5 : me-
what like each other, the closes: resemblance, if bene-
ficial, could readily be gained by the above means; and
if the imitated form was subsequently and gradually
modified through any agency, the imitating form would
be led along the same track, and thus be altered to
alm:s: iiv extent, so that i: mi^rh: ultimately assume
an appearance or coloring wholly unlike that of the other
members of the family to which it belonged- There is,
however, some difficulty on this head, for it is necessary
to suppose in some cases that ancient members belonging
to several distinct groups, before they had diverged to
their present extent, accidentally resembled a member of
another and protected group in a sufficient degree to
afford some slight protection; this having given the
basis for the subsequent acquisition of the most per-
fect resemblance.
CLASSIFICATION
235
On the Nature of the Affinities connecting Organic Beings
As the modified descendants of dominant species,
belonging to the larger genera, tend to inherit the
advantages which made the groups to which they be-
long large and their parents dominant, they are almost
sure to spread widely, and to seize on more and more
places in the economy of nature. The larger and more
dominant groups within each class 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 in-
cluded under a few great orders, and under still fewer
classes. As showing how few the higher groups are in
number, and how widely they are spread throughout
the world, the fact is striking that the discovery of Aus-
tralia has not added an insect belonging to a new class;
and that in the vegetable kingdom, as I learn from Dr.
Hooker, it has added only two or three families of small
size.
In the chapter on Geological Succession I attempted
to show, on the principle of each group having generally
diverged much in character during the long-continued
process of modification, how it is that the more ancient
forms of life often present characters in some degree in-
termediate between existing groups. As some few of the
old and intermediate forms have transmitted to the pres-
ent day descendants but little modified, these constitute
our so-called osculant or aberrant species. The more
aberrant any form is, the greater must be the number
of connecting forms which have been exterminated and
utterly lost. And we have some evidence of aberrant
THE ORIGIN OF SPECIES
groups having suffered severely from extinction, for they
are almost always represented by extremely few species;
and such species as do occur are generally very distinct
from each other, which again implies extinction. The
genera Ornithorhynchus and Lepidosiren, for example,
would not have been less aberrant had each been rep-
resented by a dozen species instead of as at present
by a single one, or by two or three. We can, I think,
account for this fact only by looking at aberrant groups
as forms which have been conquered by more successful
competitors, with a few members still preserved under
unusually favorable conditions.
Mr. Waterhouse has remarked that, when a member
belonging to one group of animals exhibits an affinity to
a quite distinct group, this affinity in most cases is gen-
eral and not special; thus, according to Mr. Waterhouse,
of all Rodents, the bizcacha is most nearly related to
Marsupials; but in the points in which it approaches this
order, its relations are general, that is, not to any one
marsupial species more than to another. As these points
of affinity are believed to be real and not merely adap-
tive, they must be due in accordance with our view to
inheritance from a common progenitor. Therefore we
must suppose either that all Rodents, including the biz-
cacha, branched off from some ancient Marsupial, which
will naturally have been more or less intermediate in
character with respect to all existing Marsupials; or that
both Rodents and Marsupials branched off from a com-
mon progenitor, and that both groups have since un-
dergone much modification in divergent directions. On
either view we must suppose that the bizcacha has re-
tained, by inheritance, more of the characters of its
CLASSIFICA TION
237
ancient progenitor than have other Rodents; and there-
fore 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 com-
mon progenitor, or of some early member of the group.
On the other hand, of all Marsupials, as Mr. Waterhouse
has remarked, the Phascolomys resembles most nearly,
not any one species, but the general order of Rodents.
In this case, however, it may be strongly suspected that
the resemblance is only analogical, owing to the Phas-
colomys having become adapted to habits like those of a
Rodent. The elder De Candolle has made nearly similar
observations on the general nature of the affinities of dis-
tinct families of plants.
On the principle of the multiplication and gradual
divergence in character of the species descended from a
common progenitor, together with their retention by in-
heritance of some characters in common, we can under-
stand the excessively complex and radiating affinities by
which all the members of the same family or higher
group are connected together. For the common progen-
itor of a whole family, now broken up by extinction
into distinct groups and sub-groups, will have transmitted
some of its characters, modified in various ways and de-
grees, to all the species; and they 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 predeces-
sors. As it is difficult to show the blood-relationship
between -the numerous kindred of any ancient and noble
family even by the aid of a genealogical tree, and almost
impossible to do so without this aid, we can understand
238
THE ORIGIN OF SPECIES
the extraordinary difficulty which naturalists have experi-
enced in describing, without the aid of a diagram, the
various affinities which they perceive between the many
living and extinct members of the same great natural
class.
Extinction, as we have seen in the fourth chapter,
has played an important part in defining and widening
the intervals between the several groups in each class.
We may thus account for the distinctness of whole
classes from each other — for instance, of birds from all
other vertebrate animals — by the belief that many ancient
forms of life have been utterly lost, through which the
early progenitors of birds were formerly connected with
the early progenitors of the other and at that time less
differentiated vertebrate classes. There has been much
less extinction of the forms of life which once connected
fishes with batrachians. There has been still less within
some whole classes, for instance the Crustacea, for here
the most wonderfully diverse forms are still linked to-
gether by a long and only partially broken chain of affin-
ities. Extinction has only defined the 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, still a natural classi-
fication, or at least a natural arrangement, would be pos-
sible. "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, with every link in each branch and sub-
branch still alive; and the links not greater than those
between existing varieties. In this case it would be
CLASSIFICATION
239
quite impossible to give definitions by which the several
members of the several groups could be distinguished
from their more immediate parents and descendants. Yet
the arrangement in the diagram would still hold good
and would be natural; for, on the principle of inheri-
tance, all the forms descended, for instance, from A,
would have something in common. In a tree we can
distinguish this or that branch, though at the actual fork
the two unite and blend together. We could not, as I
have said, define the several groups; but we could pick
out types, or forms, representing most of the characters
of each group, whether large or small, and thus give a
general idea of the value of the differences between them.
This is what we should be driven to, if we were ever to
succeed in collecting all the forms in any one class which
have lived throughout all time and space. Assuredly we
shall never succeed in making so perfect a collection:
nevertheless, in certain classes, we are tending toward
this end; 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
follows from the struggle for existence, and which almost
inevitably leads to extinction and divergence of character
in the descendants from any one parent- species, explains
that great and universal feature in the affinities of all
organic beings, namely, their subordination in group under
group. We use the element of descent in classing the
individuals of both sexes and of all ages under one spe-
cies, although they may have but few characters in com-
mon; we use descent in classing acknowledged varieties,
240
THE ORIGIN OF SPECIES
however different they may be from their parents; and I
believe that this element of descent is the hidden bond
of connection which naturalists have sought under the
term of the Natural System. On this idea of the natural
system being, in so far as it has been perfected, genea-
logical in its arrangement, with the grades of difference
expressed by the terms genera, families, orders, etc., we
can understand the rules which we are compelled to fol-
low in our classification. We can understand why we
value certain resemblances far more than others; why
we use rudimentary and useless organs, or others of tri-
fling physiological importance; why, in finding the rela-
tions between one group and another, we summarily re-
ject 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 within a few great classes;
and how the several members of each class are connected
together by the most complex and radiating lines of affin-
ities. We shall never, probably, disentangle the inextri-
cable web of the affinities between the members of any
one class; but when we have a distinct object in view,
and do not look to some unknown plan of creation,
we may hope to make sure but slow progress.
Professor Haeckel, in his "Generelle Morphologic'7 and
in other works, has recently brought his great knowledge
and abilities to bear on what he calls phylogeny, or the
lines of descent of all organic beings. In drawing up
the several series he trusts chiefly to embryological char-
acters, but receives aid from homologous and rudimentary
organs, as well as from the successive periods at which
the various forms of life are believed to have first ap-
MORPHOLOGY
241
peared in our geological formations. He has thus boldlj
made a great beginning, and shows us how classification
will in the future be treated.
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 organization. This resem-
blance is often expressed by the term "unity of type";
or by saying that the several parts and organs in the
different species of the class are homologous. The whole
subject is included under the general term of Morphol-
ogy. This is one of the most interesting departments of
natural history, and may almost 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 similar bones, in
the same relative positions? How curious it is, to give a
subordinate though striking instance, that the hind feet
of the kangaroo, which are so well fitted for bounding
over the open plains — those of the climbing, leaf eating
koala, equally well fitted for grasping the branches of
trees — those of the ground-dwelling, insect or root eating,
bandicoots — and those of some other Australian mar-
supials— should all be constructed on the same extraordi-
nary type, namely, with the bones of the second and
third digits extremely slender and enveloped within the
same skin, so that they appear like a sing/e toe furnished
with two claws. Notwithstanding this similarity of pat-
tern, it is obvious that the hind feet of these several
242
THE ORIGIN OF SPECIES
animals are used for as widely different purposes as it is
possible to conceive. The case is rendered all the more
striking by the American opossums, which follow nearly
the same habits of life as some of their Australian
relatives, having feet constructed on the ordinary plan.
Professor Flower, from whom these statements are taken,
remarks in conclusion: 4 'We may call this conformity to
type, without getting much nearer to an explanation
of the phenomenon"; and he then adds, "but is it not
powerfully suggestive of true relationship, of inheritance
from a common ancestor?"
Geoffroy St.-Hilaire has strongly insisted on the high
importance of relative position or connection in homol-
ogous parts; they may differ to almost any extent in
form and size, and yet remain connected together in the
same invariable order. We never find, for instance,
the bones of the arm and forearm, or of the thigh and
leg, transposed. Hence the same names can be given to
the homologous bones in widely different animals. We
see the same great law in the construction of the mouths
of insects: what can be more different than the immensely
long spiral proboscis of a sphinx-moth, the curious folded
one of a bee or bug, and the great jaws of a beetle? —
yet all these organs, serving for such widely different
purposes, are formed by infinitely numerous modifications
of an upper lip, mandibles, and two pairs of maxillae.
The same law governs the construction of the mouths
and limbs of crustaceans. So it is with the flowers
of plants.
Nothing can be more hopeless than to attempt to ex-
plain this similarity of pattern in members of the same
class, by utility or by the doctrine of final causes. The
MORPHOLOGY
243
hopelessness of the attempt has been expressly admitted
by Owen in his most interesting work on the "Nature of
Limbs." On the ordinary view of the independent crea-
tion of each being, we can only say that so it is; — that
it has pleased the Creator to construct all the animals
and plants in each great class on a uniform plan; but
this is not a scientific explanation.
The explanation is to a large extent simple on the
theory of the selection of successive slight modifications —
each modification being profitable in some way to the
modified form, but often affecting by correlation other
parts of the organization. In changes of this nature,
there will be little or no tendency to alter the original
pattern, or to transpose the parts. The bones of a limb
might be shortened and flattened to any extent, becoming
at the same time enveloped in thick membrane, so as
to serve as a fin; or a webbed hand might have all its
bones, or certain bones, lengthened to any extent, with
the membrane connecting them increased, so as to serve
as a wing; yet all these modifications would not tend to
alter the framework of the bones or the relative connec-
tion of the parts. If we suppose that an early progenitor
— the archetype as it may be called — of all mammals,
birds, and reptiles, had its limbs constructed on the ex-
isting general pattern, for whatever purpose they served,
we can at once perceive the plain signification of the
homologous construction of the limbs throughout the class.
So with the mouths of insects, we have only to suppose
that their common progenitor had an upper lip, mandi-
bles, and two pairs of maxillae, these parts being perhaps
very simple in form; and then natural selection will
account for the infinite diversity in the structure and
244
THE ORIGIN OF SPECIES
functions of the mouths of insects. Nevertheless, it is
conceivable that the general pattern of an organ might
become so much obscured as to be finally lost, by the
reduction and ultimately by the complete abortion of
certain parts, by the fusion of other parts, and by the
doubling or multiplication of others — variations which we
know to be within the limits of possibility. In the pad-
dles of the gigantic extinct sea-lizards, and in the mouths
of certain suctorial crustaceans, the general pattern seems
thus to have become partially obscured.
There is another and equally curious branch of our
subject; namely, serial homologies, or the comparison of
the different parts or organs in the same individual,
and not of the same parts or organs in different members
of the same class. Most physiologists believe that the
bones of the skull are homologous — 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 all the higher vertebrate classes are
plainly homologous. So it is with the wonderfully com-
plex jaws and legs of crustaceans. It is familiar to
almost every one that in a flower the relative position
of the sepals, petals, stamens, and pistils, as well as their
intimate structure, are intelligible on the view that they
consist of metamorphosed leaves, arranged in a spire. In
monstrous plants, we often get direct evidence of the
possibility of one organ being transformed into another;
and we can actually see, during the early or embryonic
stages of development in flowers, as well as in crustaceans
and many other animals, that organs which when mature
become extremely different are at first exactly alike.
How inexplicable are the cases of serial homologies
MORPHOLOGY
245
on the ordinary view of creation! Why should the brain
be inclosed in a box composed of such numerous and
such extraordinarily shaped pieces of bone, apparently
representing vertebras? As Owen has remarked, the
benefit derived from the yielding of the separate pieces
in the act of parturition by mammals will by no means
explain the same construction in the skulls of birds and
reptiles. Why should similar bones have been created to
form the wing and the leg of a bat, used as they are for
such totally different purposes, namely, flying and walk-
ing? Why should one crustacean, which has an extremely
complex mouth formed of many parts, consequently
always have fewer legs; or conversely, those with many
legs have simpler mouths ? Why should the sepals,
petals, stamens and pistils, in each flower, though fitted
for such distinct purposes, be all constructed on the
same pattern ?
On the theory of natural selection, we can, to a cer-
tain extent, answer these questions. We need not here
consider how the bodies of some animals first became
divided into a series of segments, or how they be-
came divided into right and left sides, with corresponding
organs, for such questions are almost beyond investiga-
tion. It is, however, probable that some serial structures
are the result of cells multiplying by division, entailing
the multiplication of the parts developed from such cells.
It must suffice for our purpose to bear in mind that an
indefinite repetition of the same part or organ is the
common characteristic, as Owen has remarked, of all low
or little specialized forms; therefore the unknown pro-
genitor of the Yertebrata probably possessed many verte-
brae; the unknown progenitor of the Articulata, many
246
THE ORIGIN OF SPECIES
segments; and the unknown progenitor of flowering
plants, many leaves arranged in one or more spires. We
have also formerly seen that parts many times repeated
are eminently liable to vary, not only in number, but in
form. Consequently such parts, being already present
in considerable numbers, and being highly variable,
would naturally afford the materials for adaptation to
the most different purposes; yet they would generally
retain, through the force of inheritance, plain traces of
their original or fundamental resemblance. They would
retain this resemblance all the more, as the variations,
which afforded the basis for their subsequent modifica-
tion through natural selection, would tend from the first
to be similar; the parts being at an early stage of growth
alike, and being subjected to nearly the same conditions.
Such parts, whether more or less modified, unless their
common origin became wholly obscured, would be serially
homologous.
In the great class of mollusks, though the parts in
distinct species can be shown to be homologous, only a
few serial homologies, such as the valves of Chitons, can
be indicated; that is, we are seldom enabled to say that
one part is homologous with another part in the same
individual. And we can understand this fact; for in
mollusks, 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.
But morphology is a much more complex subject than
it at first appears, as has lately been well shown in a
remarkable paper by Mr. E. Kay Lankester, who has
drawn an important distinction between certain classes of
MORPHOLOGY
247
cases which have all been equally ranked by naturalists
as homologous. He proposes to call the structures which
resemble each other in distinct animals, owing to their
descent from a common progenitor with subsequent modi-
fication, homogenous; and the resemblances which cannot
thus be accounted for, he proposes to call homoplastic.
For instance, he believes that the hearts of birds and
mammals are as a whole homogenous — that is, have been
derived from a common progenitor; but that the four
cavities of the heart in the two classes are homoplastic
— that is, have been independently developed. Mr.
Lankester also adduces the close resemblance of the
parts on the right and left sides of the body, and in
the successive segments of the same individual animal;
and here we have parts commonly called homologous,
which bear no relation to the descent of distinct species
from a common progenitor. Homoplastic structures are
the same with those which I have classed, though in a
very imperfect manner, as analogous modifications or
resemblances. Their formation may be attributed in part
to distinct organisms, or to distinct parts of the same
organism, having varied in an analogous manner; and in
part to similar modifications having been preserved for
the same general purpose or function — of which many
instances have been given.
Naturalists frequently speak of the skull as formed
of metamorphosed vertebras; the jaws of crabs as meta-
morphosed legs; the stamens and pistils in flowers as
metamorphosed leaves; but it would in most cases be
more correct, as Professor Huxley has remarked, to speak
of both skull and vertebras, jaws and legs, etc., as hav-
ing been metamorphosed, not one from the other, as they
248
THE ORIGIN OF SPECIES
now exist, but from some common and simpler element.
Most naturalists, however, use such language only in a
metaphorical sense; they are far from meaning that dur-
ing a long course of descent, primordial organs of any
kind — vertebrae in the one case and legs in the other —
have actually been converted into skulls or jaws. Yet
so strong is the appearance of this having occurred, that
naturalists can hardly avoid employing language having
this plain signification. According to the views here
maintained, such language may be used literally; and
the wonderful fact of the jaws, for instance, of a crab
retaining numerous characters which they probably would
have retained through inheritance, if they had really been
metamorphosed from true though extremely simple legs,
is in part explained.
Development and Embryology
This is one of the most important subjects in the
whole round of natural history. The metamorphoses of
insects, with which every one is familiar, are generally
effected abruptly by a few stages; but the transformations
are in reality numerous and gradual, though concealed.
A certain ephemerous insect (Chloeon) during its devel-
opment, moults, as shown by Sir J. Lubbock, above
twenty times, and each time undergoes a certain amount
of change; and in this case we see the act of metamor-
phosis performed in a primary and gradual manner.
Many insects, and especially certain crustaceans, show
us what wonderful changes of structure can be effected
during development. Such changes, however, reach their
acme in $he so-called alternate generations of some of the
lower animals. It is, for instance, an astonishing fact
EMBRYOLOGY
24^
that a delicate branching coralline, studded with polypi
and attached to a submarine rock, should produce, first
by budding and then by transverse division, a host of
huge floating jelly-fishes; and that these should produce
eggs, from which are hatched swimming animalcules,
which attach themselves to rocks and become devel-
oped into branching corallines; and so on in an end-
less cycle. The belief in the essential identity of the
process of alternate generation and of ordinary meta-
morphosis has been greatly strengthened by Wagner's
discovery of the larva or maggot of a fly, namely the
Cecidomyia, producing asexually other larva?, and these
others, which finally are developed into mature males
and females, propagating their kind in the ordinary
manner by eggs.
It may be worth notice that when Wagner's remark-
able discovery was first announced, I was asked how was
it possible to account for the larvae of this fly having
acquired the power of asexual reproduction. As long as
the case remained unique no answer could be given.
But already Grimm has shown that another fly, a Chi-
ronomus, reproduces itself in nearly the same manner,
and he believes that this occurs frequently in the Order.
It is the pupa, and not the larva, of the Chironomus
which has this power; and Grimm further shows that
this case, to a certain extent, "unites that of the Ceci-
domyia with the parthenogenesis of the Coccidae" ; — the
term parthenogenesis implying that the mature females
of the Coccidae are capable of producing fertile eggs
without the concourse of the male. Certain animals be-
longing to several classes are now known to have the
power of ordinary reproduction at an unusually early
250
THE ORIGIN OF SPECIES
age; and we have only to accelerate parthenogenetic re-
production by gradual steps to an earlier and earlier age
— Chironomus showing us an almost exactly intermediate
stage, viz., that of the pupa — and we can perhaps account
for the marvellous case of the Cecidomyia.
It has already been stated that various parts in the
same individual, which are exactly alike during an early
embryonic period, become widely different and serve for
widely different purposes in the adult state. So again it
has been shown that generally the embryos of the most
distinct species belonging to the same class are closely
similar, but become, when fully developed, widely dis-
similar. A better proof of this latter fact cannot be
given than the statement by Yon Baer that 4 4 the em-
bryos of mammalia, of birds, lizards, and snakes, prob-
ably also of chelonia, are in their earliest states exceed-
ingly like one another, both as a whole and in the mode
of development of their parts j so much so, in fact, that
we can often distinguish the embryos only by their size.
In my possession are two little embryos in spirit, whose
names I have omitted 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 similarity 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 had existed in the earliest stage of their develop-
ment 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 funda-
mental form." The larvae of most crustaceans, at corre-
sponding stages of development, closely resemble each
EMBR YOLOG Y 251
other, however different the adults may become; and so
it is with very many other animals. A trace of the law
of embryonic resemblance occasionally lasts till a rather
late age: thus birds of the same genus, and of allied
genera, often resemble each other in their immature plu-
mage; as we see in the spotted feathers in the young of
the thrush group. In the cat tribe, most of the species
when adult are striped or spotted in lines; and stripes or
spots can be plainly distinguished in the whelp of the
lion and the puma. We occasionally though rarely see
something of the same kind in plants; thus the first
leaves of the ulex or furze, and the first leaves of
the phyllodineous acacias, are pinnate or divided like
the ordinary leaves of the leguminosae.
The points of structure, in which the embryos of
widely different animals within the same class resemble
each other, often have no direct relation to their con-
ditions of existence. We cannot, for instance, suppose
that in the embryos of the vertebrata the peculiar loop-
like courses 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 similar bones in the hand of a man,
wing of a bat, and fin of a porpoise, are related to sim-
ilar conditions of life. No one supposes that the stripes
on the whelp of a lion, or the spots on the young black-
bird, are of any use to these animals.
The case, however, is different when an animal during
any part of its embryonic career is active, and has to
252
THE ORIGIN OF SPECIES
provide for itself. The period of activity may come on
earlier or later ia 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. In
how important a manner this has acted, has recently
been well shown by Sir J. Lubbock in his remarks on
the close similarity of the larvae of some insects belong-
ing to very different orders, and on the dissimilarity of
the larvae of other insects within the same order, accord-
ing to their habits of life. Owing to such adaptations,
the similarity of the larvae of allied animals is sometimes
greatly obscured; especially when there is a division of
labor during the different stages of development, as when
the same larva has during one stage to search for food,
and during another stage has to search for a place of
attachment. Cases can even be given of the larvae of
allied species, or groups of species, differing more from
each other than do the adults. In most cases, however,
the larvae, though active, still obey, more or less closely,
the law of common embr}'onic resemblance. Cirri peds
afford a good instance of this; even the illustrious Cuvier
did not perceive that a barnacle was a crustacean: but a
glance at the larva shows this in an unmistakable man-
ner. So again the two main divisions of cirripeds, the
pedunculated and sessile, though differing widely in ex-
ternal appearance, have larvae in all their stages barely
distinguishable.
The embryo in the course of development generally
rises in organization; I use this expression, though I am
aware that it is hardly possible to define clearly what is
meant by the organization being higher or lower. But
nc one probably will dispute that the butterfly is higher
EMBRYOLOGY
253
than the caterpillar. In some cases, however, the mature
animal must be considered as lower in the scale than the
larva, as with certain parasitic crustaceans. To refer
once again to cirri peds: the larvae in the first stage
have three pairs of locomotive organs, a simple single
eye, and a probosciformed mouth, with which they feed
largely, for they increase much in size. In the second
stage, answering to the chrysalis stage of butterflies, they
have six pairs of beautifully constructed natatory legs, a
pair of magnificent compound eyes, and extremely com-
plex antennae; but they have a closed and imperfect
mouth, and cannot feed: their function at this stage is to
search out by their well-developed organs of sense, and
to reach by their active powers .of swimming, a proper
place on which to become attached and to undergo their
final metamorphosis. When this is completed they are
fixed for life: their legs are now converted into prehen-
sile organs; they again obtain a well-constructed mouth;
but they have no antennae, and their two eyes are now
reconverted into a minute, single, simple eye-spot. In
this last and complete state, cirripeds may be considered
as either more highly or more lowly organized than they
were in the larval condition. But in some genera the
larvae become developed into hermaphrodites having the
ordinary structure, and into what I have called comple-
mental males; and in the latter the development has as-
suredly been retrograde, for the male is a mere sack,
which lives for a short time and is destitute of mouth,
stomach, and every other organ of importance, excepting
those for reproduction.
We are so much accustomed to see a difference in
structure between the embryo and the adult that we are
254
THE ORIGIN OF SPECIES
tempted to look at this difference as in some necessary
manner contingent on growth. But there is no reason
why, for instance, the wing of a bat, or the fin of a por-
poise, should not have been sketched out, with all their
parts in proper proportion, as soon as any part became
visible. In some whole groups of animals and in certain
members of other groups this is the case, and the embryo
does not at any period differ widely from the adult: thus
Owen has remarked in regard to cuttle-fish, 4 'There is no
metamorphosis; the cephalopodic character is manifested
long before the parts of the embryo are completed."
Land -shells and fresh- water crustaceans are born having
their proper forms, while the marine members of the
same two great classes pass through considerable and
often great changes during their development. Spiders,
again, barely undergo any metamorphosis. The larvae of
most insects pass through a worm-like stage, whether
they are active and adapted to diversified habits, or are
inactive from being placed in the midst of proper nutri-
ment or from being fed by their parents; but in some
few cases, as in that of Aphis, if we look to the ad-
mirable drawings of the development of this insect, by
Professor Huxley, we see hardly any trace of the vermi-
form stage.
Sometimes it is only the earlier developmental stages
which fail. Thus Fritz Miiller has made the remarkable
discovery that certain shrimp-like crustaceans (allied to
Penceus) first appear under the simple nauplius-form, and
after passing through two or more zoea-stages, and then
through the mysis-stage, finally acquire their mature
structure: now in the whole great malacostracan order,
to which these crustaceans belong, no other member is
EMBRYOLOGY
255
as yet known to be first developed under the nauplius-
form, though many appear as zoeas; nevertheless Miiller
assigns reasons for his belief, that if there had been no
suppression of development all these crustaceans would
have appeared as nauplii.
How, then, can we explain these several facts in
embryology — namely, the very general, though not uni-
versal, difference in structure between the embryo and
the adult; — the various parts in the same individual em-
bryo, which ultimately become very unlike and serve for
diverse purposes, being at an early period of growth
alike;— the common, but not invariable, resemblance be-
tween the embryos or larvae of the most distinct species
in the same class: — the embryo often retaining, while
within the egg or womb, structures which are of no
service to it, either at that or at a later period of life;
on the other hand, larvae, which have to provide for
their own wants, being perfectly adapted to the surround-
ing conditions;— and lastly the fact of certain larvae
standing higher in the scale of organization than the
mature animal into which they are developed? I believe
that all these facts can be explained, as follows.
It is commonly assumed, perhaps from monstrosities '
affecting the embryo at a very early period, that slight
variations or individual differences necessarily appear at
an equally early period. We have little evidence on this
head, but what we have certainly points the other way;
for it is notorious that breeders of cattle, horses, and
various fancy .animals cannot positively tell, until some
time after birth, what will be the merits or demerits of
their young animals. We see this plainly in our own
children; we cannot tell whether a child will be tall or
— Science— 28
256
THE ORIGIN OF SPECIES
short, or what its precise features will be. The question
is not, at what period of life each variation may have
been caused, but at what period the effects are displayed.
The cause may have acted, and I believe often has
acted, on one or both parents before the act of genera-
tion. It deserves notice that it is of no importance to 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, whether most of its characters
are acquired a little earlier or later in life. It would not
signify, for instance, to a bird which obtained its food
by having a much-curved beak whether or not while
young it possessed a beak of this shape, as long as it
was fed by its parents.
I have stated, in the first chapter, that at whatever
age a variation first appears in the parent, it tends to
reappear at a corresponding age in the offspring. Certain
variations can only appear at corresponding ages; for
instance, peculiarities in the caterpillar, cocoon, or imago
states of the silk-moth: or, again, in the full-grown
horns of cattle. But variations, which, for all that we
can see might have first appeared either earlier or later
in life, likewise tend to reappear at a corresponding
age in the offspring and parent. I am far from meaning
that this is invariably the case, and I could give several
exceptional cases of variations (taking the word in the
largest sense) which have supervened at an earlier age in
the child than in the parent.
These two principles, namely, that slight variations
generally appear at a not very early period of life, and
are inherited at a corresponding not early period, explain,
as I believe, all the above specified leading facts in em-
EMBRYOLOGY
257
bryology. But first let us look to a few analogous cases
in our domestic varieties. Some authors who have writ-
ten on Dogs, maintain that the greyhound and bulldog,
though so different, are really closely allied varieties,
descended from the same wild stock; hence I was curiou3
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 acquired nearly their full amount of pro-
portional difference. So, again, I was told that the foals
of cart and race-horses — breeds which have been almost
wholly formed by selection under domestication — differed
as much as the full-grown animals; but having had care-
fuls measurements made of the dams and of three-day-old
colts of race and heavy cart-horses, I find that this is by
no means the case.
As we have conclusive evidence that the breeds of
the Pigeon are descended from a single wild species,
I compared the young within twelve hours after being
hatched; I carefully measured the proportions (but will
not here give the details) of the beak, width of mouth,
length of nostril and of eyelid, size of feet and length of
leg, in the wild parent- species, in pouters, fantails, runts,
barbs, dragons, carriers, and tumblers. Now some of
these birds, when mature, differ in so extraordinary a
manner in the length and form of beak, and in other
characters, that they would certainly have been ranked
as distinct genera if found in a state of nature. But
when the nestling birds of these several breeds were
placed in a row, though most of them could just bo
THE ORIGIN OF SPECIES
distinguished, the proportional differences in the above
specified points were incomparably less than in the full-
grown birds. Some characteristic points of difference —
for instance, that of the width of mouth — could hardly
be detected in the young. But there was one remarkable
exception to this rule, for the young of the short-faced
tumbler differed from the young of the wild rock-pigeon
and of the other breeds in almost exactly the same pro-
portions as in the adult state.
These facts are explained by the above two principles.
Fanciers select their dogs, horses, pigeons, etc., for breed-
ing, when nearly grown up: they are indifferent whether
the desired qualities are acquired earlier or later in life,
if the full-grown animal possesses them. And the cases
just given, more especially that of the pigeons, show that
the characteristic differences which have been accumulated
by man's selection, and which give value to his breeds,
do not generally appear at a very early period of life,
and are inherited at a corresponding not early period.
But the case of the short-faced tumbler, which when
twelve hours old possessed its proper characters, proves
that this is not the universal rule; for here the character-
istic differences must either have appeared at an earlier
period than usual, or, if not so, the differences must
have been inherited, not at a corresponding, but at an
earlier age.
Now let us apply these two principles to species in a
state of nature. Let us take a group of birds, descended
from some ancient form and modified through natural
selection for different habits. Then, from the many
slight successive variations having supervened in the
several species at a not early age, and having been
EMBRYOLOGY
259
inherited at a correspond iag age, the young will have
been but little modified, and they will still resemble
each other much more closely than do the adults — just
as we have seen with the breeds of the pigeon. We
may extend this view to widely distinct structures and
to whole classes. The fore- limbs, for instance, which
once served as legs to a remote progenitor, may have
become, through a long course of modification, adapted in
one descendant to act as hands, in another as paddles,
in another as wings; but on the above two principles
the fore-limbs will not have been much modified in the
embryos of these several forms; although in each form
the fore-limb will differ greatly in the adult state.
"Whatever influence long-continued use or disuse may
have had in modifying the limbs or other parts of any
species, this will chiefly or solely have affected it when
nearly mature, when it was compelled to use its full
powers to gain its own living; and the effects thus pro-
duced will have been transmitted to the offspring at a
corresponding nearly mature age. Thus the young will
not be modified, or will be modified only in a slight
degree, through the effects of the increased use or disuse
of parts.
With some animals the successive variations may have
supervened at a very early period of life, or the steps
may have been inherited at an earlier age than that at
which they first occurred. In either of these ca^es, the
young or embryo will closely resemble the mature j arent-
form, as we have seen with the short-faced tumbler.
And this is the rule of development in certain whole
groups, or in certain sub-groups alone, as with cuttle-fish,
land-shells, fresh-water crustaceans, spiders, and some
2dO
THE ORIGIN OF SPECIES
members of the great class of insects. With respect to
the final cause of the young in such groups not passing
through any metamorphosis, we can see that this would
follow from the following contingencies; namely, from the
young having to provide at a very early age for their
own wants, and from their following the same habits of
life with their parents; for in this case it would be in-
dispensable for their existence that they should be modi-
fied in the same manner as their parents. Again, with
respect to the singular fact that many terrestrial and
fresh-water animals do not undergo any metamorphosis,
while marine members of the same groups pass through
various transformations, Fritz Muller has suggested that
the process of slowly modifying and adapting an animal
to live on the land or in fresh water, instead of in the
sea, would be greatly simplified by its not passing through
any larval stage; for it is not probable that places well
adapted for both the larval and mature stages, under such
new and greatly changed habits of life, would commonly
be found unoccupied or ill-occupied by other organisms.
In this case the gradual acquirement at an earlier and
earlier age of the adult structure would be favored by
natural selection; and all traces of former metamorphoses
would finally be lost.
If, on the other hand, it profited the young of an
animal to follow habits of life slightly different from
those of the parent-form, and consequently to be con-
structed on a slightly different plan, or if it profited a
larva already different from its parent to change still
further, then, on the principle of inheritance at corre-
sponding ages, the young or the larvae might be rendered
by natural selection more and more different from their
EMBRYOLOGY
parents to any conceivable extent. Differences in the
larva might, also, become correlated with successive stages
of its development; so that the larva, in the first stage,
might come to differ greatly from the larva in the second
stage, as is the case with many animals. The adult
might also become fitted for sites or habits, in which
organs of locomotion or of the senses, etc., would be
useless; and in this case the metamorphoses would
be retrograde.
From the remarks just made we can see how by
changes of structure in the young, in conformity with
changed habits of life, together with inheritance at corre-
sponding ages, animals might come to pass through stages
of development perfectly distinct from the primordial
condition of their adult progenitors. Most of our best
authorities are now convinced that the various larval and
pupal stages of insects have thus been acquired through
adaptation, and not through inheritance from some ancient
form. The curious case of Sitaris — a beetle which passes
through certain unusual stages of development — will illus-
trate how this might occur. The first larval form is
described by M. Fabre as an active, minute insect, fur-
nished with six legs, two long antennas, and four eyes.
These larvae are hatched in the nests of bees; and when
the male-bees emerge from their burrows, in the spring,
which they do before the females, the larvae spring on
them, and afterward crawl on to the females while
paired with the males. As soon as the female bee de-
posits her eggs on the surface of the honey stored in
the cells, the larvae of the Sitaris leap on the eggs
and devour them. Afterward they undergo a complete
change; their eyes disappear; their legs and antennae be-
262
THE ORIGIN OF SPECIES
come rudimentary, and they feed on honey; so that they
now more closely resemble the ordinary larvae of insects:
ultimately they undergo a further transformation, and
finally emerge as the perfect beetle. Now, if an insect,
undergoing transformations like those of the Sitaris, were
to become the progenitor of a whole new class of insects,
the course of development of the new class would be
widely different from that of our existing insects; and
the first larval stage certainly would not represent the
former condition of any adult and ancient form.
On the other hand, it is highly probable that with
many animals the embryonic or larval stages show us,
more or less completely, the condition of the progenitor
of the whole group in its adult state. In the great class
of the Crustacea, forms wonderfully distinct from each
other, namely, suctorial parasites, cirripeds, entomostraca,
and even the malacostraca, appear at first as larvae under
the nauplius-form; and as these larvae live and feed
in the open sea, and are not adapted for any peculiar
habits of life, and from other reasons assigned by Fritz
Miiller, it is probable that at some very remote period
an independent adult animal, resembling the Nauplius>
existed, and subsequently produced, along several diver-
gent lines of descent, the above-named great Crustacean
groups. So again it is probable, from what we know of
the embryos of mammals, birds, fishes, and reptiles, that
these animals are the modified descendants of some
ancient progenitor, which was furnished in its adult state
with branchiae, a swimbladder, four finlike limbs, and
a long tail, all fitted for an aquatic life.
As all the organic beings, extinct and recent, which
have ever lived, can be arranged within a few great
EMBRYOLOGY
263
classes; and as all within each class have, according to
our theory, been connected together by fine gradations,
the best, and, if our collections were nearly perfect, the
only possible arrangement, would be genealogical; descent
being the hidden bond of connection which naturalists
have been seeking under the term of the Natural Sys-
tem. 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. In two or more groups of animals, however much
they may differ from each other in structure and habits
in their adult condition, if they pass through closely
similar embryonic stages, we may feel assured that they
all are descended from one parent form, and are therefore
closely related. Thus, community in embryonic structure
reveals community of descent; but dissimilarity in em-
bryonic development does not prove discomm unity of
descent, for in one of two groups the developmental
stages may have been suppressed, or may have been
so greatly modified through adaptation to new habits of
life as to be no longer recognizable. Even in groups,
in which the adults have been modified to an extreme
degree, community of origin is often revealed by the
structure of the larvas; we have seen, for instance, that
cirripeds, though externally so like shell-fish, are at
once known by their larvse to belong to the great class
of crustaceans. As the embryo often shows us more or
less plainly the structure of the less modified and ancient
progenitor of the group, we can see why ancient and
extinct forms so often resemble m their adult state the
embryos of existing species of the same class. Agassiz
believes this to be a universal law of nature; and we
264
THE ORIGIN OF SPECIES
may hope hereafter to see the law proved true. It can,
however, be proved true only in those cases in which
the ancient state of the progenitor of the group has not
been wholly obliterated, either by successive variations
having supervened at a very early period of growth, or
by such variations having been inherited at an earlier age
than that at which they first appeared. It should also be
borne in mind that the law may be true, but yet, owing
to the geological record not extending far enough back
in time, may remain for a long period, or forever, in-
capable of demonstration. The law will not strictly hold
good in those cases in which an ancient form became
adapted in its larvae state to some special line of life,
and transmitted the same larval state to a whole group
of descendants; for such larval will not resemble any still
more ancient form in its adult state.
Thus, as it seems to me, the leading facts in em-
bryology, which are second to none in importance, are
explained on the principle of variations in the many
descendants from some one ancient progenitor having
appeared at a not very early period of life, and hav-
ing been inherited at a corresponding period. Embry-
ology rises greatly in interest, when we look at the
embryo as a picture, more or less obscured, of the pro-
genitor, either in its adult or larval state, of all the
members of the same great class.
Rudimentary, Atrophied, and Aborted Organs
Organs or parts in this strange condition, bearing the
plain stamp of inutility, are extremely common, or even
general, throughout nature. It would be impossible to
RUDIMENTARY ORGANS
265
name one of the higher animals in which some part or
other is not in a rudimentary condition. In the mam-
malia, for instance, the males possess rudimentary mam-
ma3; in snakes one lobe of the lungs is rudimentary; in
birds the "bastard- wing" may safely be considered as a
rudimentary digit, and in some species the whole wing is
so far rudimentary that it cannot be used for flight. What
can be more curious than the presence of teeth in fetal
whales, which when grown up have not a tooth in their
heads; or the teeth, which never cut through the gums,
in the upper jaws of unborn calves?
Eudimentary organs plainly declare their origin and
meaning in various ways. There are beetles belonging
to closely allied species, or even to the same identical
species, which have either full-sized and perfect wings,
or mere ' rudiments of membrane, which not rarely lie
under wing-covers firmly soldered together; and in these
cases it is impossible to doubt that the rudiments rep-
resent wings. Rudimentary organs sometimes retain their
potentiality: this occasionally occurs with the mamma? of
male mammals, which have been known to become well
developed and to secrete milk. So again, in the udders
in the genus Bos, there are normally four developed and
two rudimentary teats; but the latter in our domestic
cows sometimes become well developed and yield milk.
In regard to plants the petals are sometimes rudimentary,
and sometimes well-developed in the individuals of the
same species. In certain plants having separated sexes
Kolreuter found that by crossing a species, in which the
male flowers included a rudiment of a pistil, with a
hermaphrodite species, having of coarse a well-developed
pistil, the rudiment in the hybrid offspring was much in
266
THE ORIGIN OF SPECIES
creased in size; and this clearly shows that the rudimen-
tary and perfect pistils are essentially alike in nature. An
animal may possess various parts in a perfect state, and
yet they may in one sense be rudimentary, for they are
useless: thus the tadpole of the common Salamander or
Water-newt, as Mr. G. H. Lewes remarks, "has gills,
and passes its existence in the water; but the Salamandra
atra, which lives high up among the mountains, bring3
forth its young full-formed. This animal never lives in
the water. Yet if we open a gravid femaie, we find
tadpoles inside her with exquisitely feathered gills; and
when placed in water they swim about like the tadpoles
of the water-newt. Obviously this aquatic organization
has no reference to the future life of bae animal, nor
has it any adaptation to its embryonic condition; it
has solely reference to ancestral adaptations, it repeats
a phase in the development of its progenitors."
An organ, serving for two purposes, may becom3
rudimentary or utterly aborted for one, even the more
important purpose, and remain perfectly efficient for the
other. Thus in plants, the office of the pistil is to allow
the pollen-tubes to reach the ovules within the ovarium.
The pistil consists of a stigma supported on a style; but
in some Composite, the male florets, which of course
cannot be fecundated, have a rudimentary pistil, for it
is not crowned with a stigma; but the style remains well
developed and is clothed in the usual manner with hairs,
which serve to brush the pollen out of the surrounding
and conjoined anthers. Again, an organ may become
rudimentary for its proper purpose, and be used for a
distinct one: in certain fishes the swimbladder seems to
be rudimentary for its proper function of giving buoy-
RUDUIEXTARY ORGANS
267
ancj, but has become converted into a nascent breathing
organ or lung. Many similar instances could be given.
Useful organs, however little they may be developed,
unless we have reason to suppose that they were formerly
more highly developed, ought not to be considered as
rudimentary. They may be in a nascent condition, and
in progress toward further development. Rudimentary
organs, on the other hand, are either quite useless, such
as teeth which never cut through the gums, or almost
useless, such as the wings of an ostrich, which serve
merely as sails. As organs in this condition would for-
merly, when still less developed, have been of even less
use than at present, they cannot formerly have been
produced through variation and natural selection, which
acts solely by the preservation of useful modifications.
They have been partially retained by the power of in-
heritance, and relate to a former state of things. It
is, however, often difficult to distinguish between rudi-
mentary and nascent organs; for we can judge only
by analogy whether a part is capable of further devel-
opment, in which case alone it deserves to be called
nascent. Organs in this condition will always be some-
what rare; for beings thus provided will commonly have
been supplanted by their successors with the same organ
in a more perfect state, and consequently will have be-
come long ago extinct. The wing of the penguin is of
high service, acting as a fin; it may, therefore, represent
the nascent state of the wing: not that I believe this to
be the case; it is more probably a reduced organ, modi-
fied for a new function: the wing of the Apteryx, on
the other hand, is quite useless, and is truly rudimen-
tary. Owen considers the simple filamentary limbs of the
THE 0R1G1X OF SPECIES
Lepidosiren as the ''beginnings of organs which attain
fall functional development in higher vertebrates"'; but,
according to the view lately advocated by Dr. Gunther,
they are probably remnants, consisting of the persistent
axis of a fin. with the lateral rays or branches aborted.
The mammary glands of the Ornithorhynchus may be
considered, in comparison with the udders of a cow,
as in a nascent condition. The ovigerous frena of cer-
tain cirripeds. which have ceased to give attachment to
the ova and are feebly developed, are nascent branchiae.
Rudimentary organs in the individuals of the same
species are very liable to vary in the degree of their
development and in other respects. In closely allied spe-
cies, also, the extent to which the same organ has been
reduced occasionally differs much. This latter fact is
well exemplified in the stare of the wings of female
moths belonging to the same family. Rudimentary or-
gans may be utterly aborted: and this implies, that, in
certain animals or plants, parts are entirely absent which
analogy would lead us to expect to find in them, and
which are occasionally found in monstrous individuals.
Thus in most of the Scrophulariacea? the fifth stamen is
utterly aborted; yet we may conclude that a fifth stamen
once existed, for a rudiment of it is found in many spe-
cies of the family, and this rudiment occasionally be-
comes perfectly developed, as may sometimes be seen
in the common snap-dragon. In tracing the homologies
of any part in different members of the same class, noth-
ing is more common, or, in order fully to understand the
relations of the parts, more useful than the discovery of
rudiments. This is well shown in the drawings given by
Owen of the lesr-bones of the horse, ox, and rhinoceros.
RUDIMENTARY ORGANS
269
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 afterward wholly
disappear. It is also, I believe, a universal rule, that a
rudimentary part is of greater size in the embryo rela-
tively to the adjoining parts 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
rudimentary organs in the adult are often said to have
retained their embryonic condition.
I have now given the leading facts with respect to
rudimentary organs. In reflecting on them, every one
must be struck with astonishment; for the same reason-
ing power which tells us that most parts and organs are
exquisitely adapted for certain purposes tells us with
equal plainness that these rudimentary or atrophied
organs are imperfect and useless. In works on natural
history, rudimentary organs are generally said to frave
been created 4 4 for the sake of symmetry/' or in order 4 4 to
complete the scheme of nature.7 ' But this is not an ex-
planation, merely a restatement of the fact. Nor is it con-
sistent with itself: thus the boa-constrictor has rudiments
of hind-limbs and of a pelvis, and if it be said that these
bones have been retained 4 'to complete the scheme of
nature," why, as Professor Weismann asks, have they
not been retained by other snakes, which do not possess
even a vestige of these same bones? What would ba
thought of an astronomer who maintained that the satel-
lites revolve in elliptic courses round their planets 44for
the sake of symmetry," because the planets thus revolve
round the sun? An eminent physiologist accounts for
the presence of rudimentary organs by supposing that
270
THE ORIGIN OF SPECIES
they serve to excrete matter in excess, or matter injuri-
ous to the system; but can we suppose that the minute
papilla, which often represents the pistil in male flowers,
and which is formed of mere cellular tissue, can thus
act? Can we suppose that rudimentary teeth, which are
subsequently absorbed, are beneficial to the rapidly grow-
ing embryonic calf by removing matter so precious as
phosphate of lime? When a man's fingers have been
amputated, imperfect nails have been known to appear
on the stumps, and I could as soon believe that these
vestiges of nails are developed in order to excrete horny
matter, as that the rudimentary nails on the fin of the
manatee have been developed for this same purpose.
On the view of descent with modification, the origin
of rudimentary organs is comparatively simple; and we
can understand to a large extent the laws governing their
imperfect development. We have plenty of cases of rudi-
mentary organs in our domestic productions — as the stump
of a tail in tailless breeds — the vestige of an ear in ear-
less breeds of sheep — the reappearance of minute dan-
gling horns in hornless breeds of cattle, more especially,
according to Youatt, in young animals — and the state
of the whole flower in the cauliflower. We often see
rudiments of various parts in monsters; but I doubt
whether any of these cases throw light on the origin of
rudimentary organs in a state of nature, further than by
showing that rudiments can be produced; for the balance
of evidence clearly indicates that species under nature do
not undergo great and abrupt changes. But we learn
from the study of our domestic productions that the
disuse of parts leads to their reduced size; and that
the result is inherited.
RUDIMENTARY ORGANS
271
It appears probable that disuse has been the main
agent in rendering organs rudimentary. It would at first
lead by slow steps to the more and more complete reduc-
tion of a part, until at last it became rudimentary — as in
the case of the eyes of animals inhabiting dark caverns,
and of the wings of birds inhabiting oceanic islands,
which have seldom been forced by beasts of prey to
take flight, and have ultimately lost the power of flying.
Again, an organ, useful under certain conditions, might
become injurious under others, as with the wings of
beetles living on small and exposed islands; and in this
case natural selection will have aided in reducing the
organ, until it was rendered harmless and rudimentary.
Any change in structure and function, which can be
effected by small stages, is within the power of natural
selection; so that an organ rendered, through changed
habits of life, useless or injurious for one purpose, might
be modified and used for another purpose. An organ
might, also, be retained for one alone of its former func-
tions. Organs, originally formed by the aid of natural
selection, when rendered useless may well be variable,
for their variations can no longer be checked by natural
selection. All this agrees well with what we see under
nature. Moreover, at whatever period of life either dis-
use or selection reduces an organ, and this will generally
be when the being has come to maturity and has to
exert its full powers of action, the principle of inheri-
tance at corresponding ages will tend to reproduce the
organ in its reduced state at the same mature age, but
will seldom affect it in the embryo. Thus we can under-
stand the greater size of rudimentary organs in the em-
bryo relatively to the adjoining parts, and their lesser
272
THE ORIGIN OF SPECIES
relative size in the adult. If, for instance, the digit of
an adult animal was used less and less during many
generations, owing to some change of habits, or if an
organ or gland was less and less functionally exercised,
we may infer that it would become reduced in size in
the adult descendants of this animal, but would retain
nearly its original standard of development in the embryo.
There remains, however, this difficulty. After an
organ has ceased being used, and has become in conse-
quence much reduced, how can it be still further reduced
in size until the merest vestige is left; and how can it
be finally quite obliterated ? It is scarcely possible that
disuse can go on producing any further effect after the
organ has once been rendered functionless. Some addi-
tional explanation is here requisite which I cannot give.
If, for instance, it could be proved that every part of
the organization tends to vary in a greater degree toward
diminution than toward augmentation of size, then we
should be able to understand how an organ which has
become useless would be rendered, independently of the
effects of disuse, rudimentary and would at last be wholly
suppressed; for the variations toward diminished size
would no longer be checked by natural selection. The
principle of the economy of growth, explained in a
former chapter, by which the materials forming any part,
if not useful to the possessor, are saved as far as is pos-
sible, will perhaps come into play in rendering a useless
part rudimentary. But this principle will almost neces-
sarily be confined to the earlier stages of the process of
reduction; for we cannot suppose that a minute papilla,
for instance, representing in a male flower the pistil of the
female flower, and formed merely of cellular tissue, could
SUMMARY
273
be further reduced or absorbed for the sake of econo-
mizing nutriment.
Finally, as rudimentary organs, by whatever steps
they may have been degraded into their present useless
condition, are the record of a former state of things, and
have been retained solely through the power of inheri-
tance— we can understand, on the genealogical view of
classification, how it is that systematists, in placing
organisms in their proper places in the natural system,
have often found rudimentary parts as useful as, or even
sometimes more useful than, parts of high physiological
importance. Rudimentary organs may be compared with
the letters in a word, still retained in the spelling, but
become useless in the pronunciation, but which serve as
a clew for its derivation. On the view of descent with
modification, we may conclude that the existence of
organs in a rudimentary, imperfect, and useless condition,
or quite aborted, far from presenting a strange difficulty,
as they assuredly do on the old doctrine of creation,
might even have been anticipated in accordance with the
views here explained.
Summary
In this chapter I have attempted to show that the
arrangement of all organic beings throughout all time in
groups under groups — that the nature of the relationships
by which all living and extinct organisms are united by
complex, radiating, and circuitous lines of affinities into a
few grand classes — the rules followed and the difficulties
encountered by naturalists in their classifications — the
value set upon characters, if constant and prevalent,
whether of high or of the most trifling importance, or,
274
THE ORIGIN OF SPECIES
as with rudimentary organs, of no importance — the wide
opposition in value between analogical or adaptive char-
acters, and characters of true affinity; and other such,
rules; — all naturally follow if we admit the common
parentage of allied forms, together with their modifica-
tion through variation and natural selection, with the
contingencies of extinction and divergence of character.
In considering this view of classification, it should be
borne in mind that the element of descent has been
universally used in ranking together the sexes, ages,
dimorphic forms, and acknowledged varieties of the same
species, however much they may differ from each other
in structure. If we extend the use of this element of
descent — the one certainly known cause of similarity in
organic beings — we shall understand what is meant by
the Natural System: it is genealogical in its attempted
arrangement, with the grades of acquired difference
marked by the terms, varieties, species, genera, families,
orders, and classes.
On this same view of descent with modification most
of the great facts in Morphology become intelligible —
whether we look to the same pattern displayed by the
different species of the same class in their homologous
organs, to whatever purpose applied; or to the serial and
lateral homologies in each individual animal and plant.
On the principle of successive slight variations, not
necessarily or generally supervening at a very early
period of life, and being inherited at a correspond-
ing period, we can understand the leading facts in
Embryology; namely, the close resemblance in the indi-
vidual embryo of the parts which are homologous, and
which when matured become widely different in structure
SUMMARY
275
and function; and the resemblance of the homologous
parts or organs in allied though distinct species, though
fitted in the adult state for habits as different as is
possible. Larvae are active embryos, which have been
specially modified in a greater or less degree in relation
to their habits of life, with their modifications inherited
at a corresponding early age. On these same principles —
and bearing in mind that when organs are reduced in
size, either from disuse or through natural selection, it
will generally be at that period of life when the being-
has to provide for its own wants, and bearing in mind
how strong is the force of inheritance — the occurrence of
rudimentary organs might even have been anticipated.
The importance of embryological characters and of rudi-
mentary organs in classification is intelligible, on the
view that a natural arrangement must be 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,
with which this world is peopled, are 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 ii
were unsupported by other facts or arguments.
276
THE ORIGIN OF SPECIES
CHAPTER XV
RECAPITULATION AND CONCLUSION
Recapitulation of the objections to the theory of Natural Selection — Re-
capitulation of the general and special circumstances in its favor —
Causes of the general belief in the immutability of species — How far the
theory of Natural Selection may be extended — Effects of its adoption
on the study of Natural History — Concluding remarks
S THIS whole volume is one long argument, it
may be convenient to the reader to have the
leading facts and inferences briefly recapitulated.
That many and serious objections may be advanced
against the theory of descent with modification through
variation and natural selection, I do not deny. I have
endeavored to give to them their full force. Nothing
at first can appear more difficult to believe than that the
more complex organs and instincts 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. Nev-
ertheless, this difficulty, though appearing to our imag-
ination insuperably great, cannot be considered real if
we admit the following propositions, namely, that all
parts of the organization and instincts offer, at least,
individual differences — that there is a struggle for ex-
istence leading to the preservation of profitable devia-
tions of structure or instinct — and, lastly, that gradations
RECAPITULATION
277
in the state of perfection of each organ may have existed
each good of its kind. 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 among broken and failing groups of or-
ganic beings, which have suffered much extinction; but
we see so many strange gradations in nature that we
ought to be extremely cautious in saying that any organ
or instinct, or any whole structure, could not have
arrived at its present state by many graduated steps.
There are, it must be admitted, cases of special difficulty
opposed to the theory of natural selection; and one of
the most curious of these is the existence in the same
community of two or three defined castes of workers or
sterile female ants; but I have attempted to show how
these difficulties 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 ninth chapter,
which seem to me conclusively to show that this sterility
is no more a special endowment than is the incapacity of
two distinct kinds of trees to be grafted together; but
that it is incidental on differences confined to the repro-
ductive systems of the intercrossed species. We see the
truth of this conclusion in the vast difference in the re-
sults of crossing the same two species reciprocally — that
is, when one species is first used as the father and then
as the mother. Analogy from the consideration of di-
morphic and trimorphic plants clearly leads to the same
278
THE ORIGIN OF SPECIES
conclusion, for when the forms are illegitimately united,
they yield few or no seed, and their offspring are more
or less sterile; and these forms belong to the same un-
doubted species, and differ from each other in no respect
except in their reproductive organs and functions.
Although the fertility of varieties when intercrossed
and of their mongrel offspring has been asserted by so
many authors to be universal, this cannot be considered
as quite correct after the facts given on the high au-
thority of Gartner and Kolreuter. Most of the varieties
which have been experimented on have been produced
under domestication; and as domestication (I do not
mean mere confinement) almost certainly tends to elim-
inate that sterility which, judging from analogy, would
have affected the parent-species if intercrossed, we ought
not to expect that domestication would likewise induce
sterility in their modified descendants when crossed. This
elimination of sterility apparently follows from the same
cause which allows our domestic animals to breed freely
under diversified circumstances; and this again apparently
follows from their having been gradually accustomed to
frequent changes in their conditions of life.
A double and parallel series of facts seems to throw
much light on the sterility of species, when first crossed,
and of their hybrid offspring. On the one side, there ia
good reason to believe that slight changes in the condi-
tions of life give vigor and fertility to all organic beings.
We know also that a cross between the distinct individ-
uals of the same variety, and between distinct varieties,
increases the number of their offspring, and certainly
gives to them increased size and vigor. This is chiefly
owing to the forms which are crossed having been ex-
RECAPITULATION
279
posed to somewhat different conditions of life; for I have
ascertained by a laborious series of experiments that if all
the individuals of the same variety be subjected during
several generations to the same conditions, the good de-
rived from crossing is often much diminished or wholly
disappears. This is one side of the case. On the other
side, we know that species which have long been exposed
to nearly uniform conditions, when they are subjected
under confinement to new and greatly changed condi-
tions, either perish, or, if they survive, are rendered
sterile, though retaining perfect health. This does not
occur, or only in a very slight degree, with our domes-
ticated productions, which have long been exposed to
fluctuating conditions. Hence when we find that hybrids
produced by a cross between two distinct species are few
in number, owing to their perishing soon after conception
or at a very early age,, or if surviving that they are ren-
dered more or less sterile, it seems highly probable that
this result is due to their having been in fact subjected
to a great change in their conditions of life, from being
compounded of two distinct organizations. He who will
explain in a definite manner why, for instance, an ele-
phant or a fox will not breed under confinement in its
native country, while the domestic pig or dog will breed
freely under the most diversified conditions, will at the
same time be able to give a definite answer to the ques-
tion why two distinct species, when crossed, as well as
their hybrid offspring, are generally rendered more or
less sterile, while two domesticated varieties when crossed
and their mongrel offspring are perfectly fertile.
Turning to geographical distribution, the difficulties
encountered on the theory of descent with modification
—Science — 29
280
THE ORIGIX OF SPECIES
are serious enough. All the individuals of the same spe-
cies, and all the species of the same genus, or even
higher group, are descended from common parents: and
therefore, in however distant and isolated parts of the
world they may now be found, they must in the course
of successive generations have travelled from some one
point to all the others. We are often wholly unable
even to conjecture how this could have been effected.
Yet, as we have reason to believe that some species have
retained the same specific form for very long periods of
time, immensely long as measured by years, too much
stress ought not to be laid on the occasional wide dif-
fusion of the same species; for during very long periods
there will always have been a good chance for wide mi-
gration by many means. A broken or interrupted range
may often be accounted for by the extinction of the spe-
cies in the intermediate regions. It cannot be denied
that we are as yet very ignorant as to the full extent of
the various climatal and geographical changes which have
affected the earth during modern periods; and such
changes will often have facilitated migration. As an
example, I have attempted to show how potent has been
the influence of the Glacial period on the distribution of
the same and of allied species throughout the world.
We are as yet profoundly ignorant of the many occa-
sional means of transport. With respect to distinct spe-
cies of the same genus inhabiting 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 the species of the same
genus is in some degree lessened.
RECAPITULA TION
281
As according to 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 are our existing varieties, it may
be asked, Why do we not see these linking forms all
around us? Why are not all organic beings blended
together in an inextricable chaos? With respect to ex-
isting forms, we should remember that we have no right
to expect (excepting in rare cases) to discover directly
connecting links between them, but only between each
and some extinct and supplanted form. Even on a wide
area, which has during a long period remained continu-
ous, and of which the climatic and other condition of life
change insensibly in proceeding from a district occupied
by one species into another district occupied by a closely
allied species, we have no just right to expect often to
find intermediate varieties in the intermediate zones. For
we have reason to believe that only a few species of a
genus ever undergo change; the other species becoming
utterly extinct and leaving no modified progeny. Of the
species which do change, only a few within the same
country change at the same time; and all modifications
are slowly effected. I have also shown that the inter-
mediate varieties which probably at first existed in the
intermediate zones would be liable to be supplanted by
the allied forms on either hand; for the latter, from ex-
isting in greater numbers, would generally be modified
and improved at a quicker rate than the intermediate
varieties, which existed in lesser numbers; so that the
intermediate varieties would, in the long run, be sup-
planted and exterminated.
On this doctrine of the extermination of an infinitude
282
THE ORIGIN OF SPECIES
of connecting links, between the living and extinct in-
habitants of the world, and at each successive period
between the extinct and still older species, why is not
every geological formation charged with such links?
Why does not every collection of fossil remains afford
plain evidence of the gradation and mutation of the
forms of life? Although geological research has un-
doubtedly revealed the former existence of many links,
bringing numerous forms of life much closer together,
it does not yield the infinitely many fine gradations be-
tween past and present species required on the theory;
and this is the most obvious of the many objections
which may be urged against it. Why, again, do whole
groups of allied species appear, though this appearance is
often false, to have come in suddenly on the successive
geological stages? Although we now know that organic
beings appeared on this globe, at a period incalculably
remote, long before the lowest bed of the Cambrian
system was deposited, why do we not find beneath
this system great piles of strata stored with the re-
mains of the progenitors of the Cambrian fossils? For
on the theory, such strata must somewhere have been
deposited at these ancient and utterly unknown epochs
of the world's history.
I can answer these questions and objections only on
the supposition that the geological record is far more
imperfect than most geologists believe. The number of
specimens in all our museums is absolutely as nothing
compared with the countless generations of countless
species which have certainly existed. The parent-form of
any two or more species would not be in all its char-
acters directly intermediate between its modified offspring,
RECAPITULATION 283
any more than the rock- pigeon is directly intermediate in
crop and tail between its descendants, the pouter and
fantail pigeons. We should not be able to recognize a
species as the parent of another and modified species, if
we were to examine the two ever so closely, unless we
possessed most of the intermediate links; and owing to
the imperfection of the geological record, we have no just
right to expect to find so many links. If two or three,
or even more linking forms were discovered, they would
simply be ranked by many naturalists as so many new
species, more especially if found in different geological
sub-stages, let their differences be ever so slight. !N umer-
ous 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 whether or not these doubtful
forms ought to be called varieties? Only a small portion
of the world has been geologically explored. Only
organic beings of certain classes can be preserved in
a fossil condition, at least in any great number. Many
species when once formed never undergo any further,
change, but become extinct without leaving modified de-
scendants; and the periods during which species have
undergone modification, though long as measured by
years, have probably been short in comparison with the
periods during which they retained the same form. It
is the dominant and widely ranging species which vary
most frequently and vary most, and varieties are often at
first local — both causes rendering the discovery of inter-
mediate links in any one formation less likely. Local
varieties will not spread into other and distant regions
until they are considerably modified and improved; and
284 THE ORIGIN OF SPECIES
when they have spread, and are discovered in a geo-
logical formation, they appear as if suddenly created
there, and will be simply classed as new species. Most
formations have been intermittent in their accumulation;
and their duration has probably been shorter than the
average duration of specific forms. Successive formations
are in most cases separated' from each other by blank
intervals of time of great length; for fossiliferous forma-
tions thick enough to resist future degradation can as a
general rale be accumulated only where much sediment
is deposited on the subsiding bed of the sea. During
the alternate periods of elevation and of stationary level
the record will generally be blank. During these lat-
ter periods there will probably be more variability in
the forms of life; during periods of subsidence, more
extinction.
With respect to the absence of strata rich in fossils
beneath the Cambrian formation, I can recur only to
the hypothesis given in the tenth chapter; namely, that
though our continents and oceans have endured for an
enormous period in nearly their present relative positions,
we have no reason to assume that this has always been
the case; consequently formations much older than any
now known may lie buried beneath the great oceans.
With respect to the lapse of time not having been suffi-
cient since our planet was consolidated for the assumed
amount of organic change, and this objection, as urged
by Sir William Thompson, is probably one of the
gravest as yet advanced, I can only say, first, that we
do not know at what rate species change as measured
by years, and, secondly, that many philosophers are
not as yet willing to admit that we know enough
RECAPITULATION
285
of the constitution of the universe and of the inte-
rior of our globe to speculate with safety on its past
duration.
That the geological record is imperfect all will admit;
but that it is imperfect to 'the degree required by our
theory, few will be inclined to admit. If we look to
long enough intervals of time, geology plainly declares
that species have all changed; and they have changed
in the manner required by the theory, for they have
changed slowly and in a graduated manner. We
ciear/y see this in the fossil remains from consecutive
formations invariably being much more closely related
to each other than are the fossils from widely separated
formations.
Such is the sum of the several chief objections and
difficulties which may be justly urged against the theory;
and I have now briefly recapitulated the answers and ex-
planations which, as far as I can see, may be given. 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 ques-
tions on which we are confessedly ignorant; nor do we
know how ignorant we are. We do not know all the
possible transitional gradations between the simplest and
the most perfect organs; it cannot be pretended that we
know all the varied means of Distribution during the
long lapse of years, or that we know how imperfect is
the Geological Eecord. Serious as these several objec-
tions are, in my judgment they are by no means suffi-
cient to overthrow the theory of descent with subsequent
modification.
Z86
THE ORIGIN OF SPECIES
Now let us turn to the other side of the argument.
Under domestication we see much variability, caused, or
at least excited, by changed conditions of life; but often
in so obscure a manner that we are tempted to consider
the variations as spontaneous. Variability is governed by
many complex laws — by correlated growth, compensation,
the increased use and disuse of parts, and the definite
action of the surrounding conditions. There is much
difficulty in ascertaining how largely our domestic pro-
ductions have been modified; but we may safely infer
that the amount has been large, and that modifications
can be inherited for long periods. As long as the condi-
tions of life remain the same, we have reason to believe
that a modification, which has already been inherited for
many generations, may continue to be inherited for an
almost infinite number of generations. On the other
hand, we have evidence that variability, when it has
once come into play, does not cease under domestication
for a very long period; nor do we know that it ever
ceases, for new varieties are still occasionally produced
by our oldest domesticated productions.
Variability is not actually caused by man; he only
unintentionally exposes organic beings to new conditions
of life, and then nature acts on the organization and
causes it to vary. But man can and does select the
variations given to him by nature, and thus accumulates
them in any desired manner. He thus adapts animals
and plants for his own benefit or pleasure. He may do
this methodically, or he may do it unconsciously by
preserving the individuals most useful or pleasing to him
without any intention of altering the breed. It is certain
that he can largely influence the character of a breed by
RECAPIT ULA TION
287
selecting, in each successive generation, individual differ-
ences so slight as to be inappreciable except by an
educated eye. This unconscious process of selection has
been the great agency in the formation of the most dis-
tinct and useful domestic breeds. That many breeds
produced by man have to a large extent the character
of natural species is shown by the inextricable doubts
whether many of them are varieties or aboriginally
distinct species.
There is no reason why the principles which have
acted so efficiently under domestication should not have
acted under nature. In the survival of favored individ-
uals and races, during the constantly-recurrent Struggle
for Existence, we see a powerful and ever-acting form of
Selection. The struggle for existence inevitably follows
from the high geometrical ratio of increase which is com-
mon to all organic beings. This high rate of increase
is proved by calculation — by the rapid increase of many
animals and plants during a succession of peculiar
seasons, and when naturalized in new countries. More
individuals are born than can possibly survive. A grain
in the balance may determine which individuals shall
live and which shall die — which variety or species
shall increase in number, and which shall decrease, or
finally become extinct. As the individuals of the same
species come in all respects into the closest competition
with each other, the struggle will generally be most severe
between them; it will be almost equally severe between
the varieties of the same species, and next in severity
between the species of the same genus. On the other
hand, the struggle will often be severe between beings
remote in the scale of nature. The slightest advantage in
288
THE ORIGIN OF SPECIES
certain individuals, at any age or during any season, over
those with which they come into competition, or better
adaptation in however slight a degree to the surround-
ing physical conditions, will, in the long run, turn the
balance.
With animals having separated sexes, there will be in
most cases a struggle between the males for the posses-
sion of the females. The most vigorous males, or those
which have most successively struggled with their con-
ditions of life, will generally leave most progeny. But
success will often depend on the males having special
weapons, or means of defence, or charms; and a slight
advantage will lead to victory.
As geology plainly proclaims that each land has
undergone great physical changes, we might have ex-
pected to find that organic beings have varied under
nature, in the same way as they have varied under
domestication. And if there has been 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 incapable of proof,
that the amount of variation under nature is a strictly
limited quantity. Man, though acting on external char-
acters alone and often capriciously, can produce within a
short period a great result by adding up mere individual
differences in his domestic productions; and every one
admits that species present individual differences. But,
besides such differences, all naturalists admit that natural
varieties exist which are considered sufficiently distinct to
be worthy of record in systematic works. No one has
drawn any clear distinction between individual differences
and slight varieties*, or between more plainly marked
RECAPITULATION
289
yarieties and sub-species, and species. On separate con-
tinents, and on different parts of the same continent
when divided by barriers of any kind, and on outlying
islands, what a multitude of *forms exist, which some
experienced naturalists rank as varieties, others as geo-
graphical races or sub-species, and others as distinct,
though closely allied species!
If, then, animals and plants do vary, let it be ever
so slightly or slowly, why should not variations or indi-
vidual differences, which are in any way beneficial, be
preserved and accumulated through natural selection,
or the survival of the fittest? If man can by patience
select variations useful to him, why, under changing and
complex conditions of life, should not variations useful
to nature's living products often arise, and be preserved
or selected? What limit can be put to this power, act-
ing during long ages and rigidly scrutinizing the whole
constitution, structure, and habits of each creature —
favoring the good and rejecting the bad? I can see no
limit to this power, in slowly and beautifully adapting
each form to the most complex relations of life. The
theory of natural selection, even if we look no further
than this, seems to be in the highest degree prob-
able. I have already recapitulated, as fairly as I
could, the opposed difficulties and objections: now let
us turn to the special facts and arguments in favor of
the theory.
On the view that species are only strongly marked
and permanent varieties, and that each species first ex-
isted as a variety, we can see why it is that no line of
demarcation can be drawn between species commonly
290
THE ORIGIN OF SPECIES
supposed to have been produced by special acts of crea-
tion and varieties which are acknowledged to have been
produced by secondary laws. On this same view we can
understand how it is that in a region where many species
of a genus have been produced, and where they now
flourish, these same species should present many vari-
eties; for where the manufactory of species has been
active, we might expect, as a general rule, to find it
still in action; and this is the case if varieties be
incipient species. Moreover, the species of the larger
genera, which afford the greater number of varieties or
incipient species, retain to a certain degree the character
of varieties; for they differ from each other by a less
amount of difference than do the species of smaller
genera. The closely allied species also of the larger gen-
era apparently have restricted ranges, and in their
affinities they are clustered in little groups round other
species — in both respects resembling varieties. These are
strange relations on the view that each species was inde-
pendently created, but are intelligible if each existed first
as a variety.
As each species tends by its geometrical rate of repro-
duction to increase inordinately in number; and as the
modified descendants of each species will be enabled to
increase by as much as they become more diversified in
habits and structure, so as to be able to seize on many
and widely different places in the economy of nature;
there will be a constant tendency in natural selection to
preserve the most divergent offspring of any one species.
Hence, during a long-continued course of modification,
the slight differences characteristic of varieties of the
same species tend to be augmented into the greater
REC APITULA TION
291
differences characteristic of the species of the same genus.
New and improved varieties will inevitably supplant and
exterminate the older, less improved, and intermediate
varieties; and thus species are rendered to a large extent
defined and distinct objects. Dominant species belonging
to the larger groups within each class 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 go
on increasing in size, for the world would not hold them,
the more dominant groups beat the less dominant. This
tendency in the large groups to go on increasing in size
and diverging in character, together with the inevitable
contingency of much extinction, explains the arrangement
of all the forms of life in groups subordinate to groups,
all within a few great classes, which has prevailed
throughout all time. This grand fact of the group-
ing of all organic beings under what is called the
Natural System, is utterly inexplicable on the theory
of creation.
As natural selection acts solely by accumulating slight,
successive, favorable variations, it can produce no great
or sudden modifications; it can act only by short and
slow steps. Hence, the canon of "Natura non facit
salt urn," which every fresh addition to our knowledge
tends to confirm, is on this theory intelligible. We can
see why throughout nature the same general end is
gained by an almost infinite diversity of means, for every
peculiarity when once acquired is long inherited, and
structures already modified in many difiierent ways have
to be adapted for the same general purpose. We can,
in short, see why nature is prodigal in variety, though
292
THE ORIGIN OF SPECIES
niggard in innovation. But why this should be a law
of nature if each species has been independently created
no man can explain.
Many other facts are, as it seems to me, explicable
on this theory. How strange it is that a bird, under
the form of a woodpecker, should prey on insects on the
ground; that upland geese, which rarely or never swim,
should possess webbed feet; that a thrush-like bird
should dive and feed on sub-aquatic insects; and that a
petrel should have the habits and structure fitting it for
the life of an auk! and so in endless other cases. But
on the view of each species constantly trying to increase
in number, with natural selection always ready to adapt
the slowly varying descendants of each to any unoccupied
or ill-occupied place in nature, these facts cease to be
strange, or might even have been anticipated.
We can to a certain extent understand how it is that
there is so much beauty throughout nature; for this may
be largely attributed to the agency of selection. That
beauty, according to our sense of it, is not universal,
must be admitted by every one who will look at some
venomous snakes, at some fishes, and at certain hideous
bats with a distorted resemblance to the human face.
Sexual selection has given the most brilliant colors, ele-
gant patterns, and other ornaments to the males, ami
sometimes to both sexes of many birds, butterflies, and
other animals. With birds it has often rendered the
voice of the male musical to the female, as welJ as
to our ears. Flowers and fruit have been rendered con-
spicuous by brilliant colors in contrast with the green
foliage, in order that the flowers may be easily seen,
visited, and fertilized by insects, and the seeds dissem-
RECAPITU LA TION
293
inated by birds. How it comes that certain colors,
sounds and forms should give pleasure to man and the
lower animals — that is, how the sense of beauty in its
simplest form was first acquired — we do not know any
more than how certain odors and flavors were first ren-
dered agreeable.
As natural selection acts by competition, it adapts and
improves the inhabitants of each country only in relation
to their co-inhabitants; so that we need feel no surprise
at the species of any one country, although on the ordi-
nary view supposed to have been created and specially
adapted for that country, being beaten and supplanted
by the naturalized productions from another land. Nor
ought we to marvel if all the contrivances in nature be
not, as far as we can judge, absolutely perfect, as in the
case even of the human eye; or if some of them be ab-
horrent to our ideas of fitness. We need not marvel at
the sting of the bee, when used against an enemy, caus-
ing the bee's own death; at drones being produced in
such great numbers for one single act, and being then
slaughtered by their sterile sisters; at the astonishing
waste of pollen by our fir trees; at the instinctive hatred
of the queen -bee for her own fertile daughters; at ich-
neumonidae feeding within the living bodies of caterpil-
lars; or at other such cases. The wonder indeed is, on
the theory of natural selection, that more cases of the
want of absolute perfection have not been detected.
The complex and little known laws governing the
production of varieties are the same, as far as we can
judge, with the laws wnich have governed the produc-
tion of distinct species. In both cases physical condi-
tions seem to have produced some direct and definite
294
THE ORIGIN OF SPECIES
effect, but how much we cannot say. Thus, when vari-
eties enter any new station, they occasionally assume
some of the characters proper to the species of that
station. With both varieties and species, use and dis-
use seem to have produced a considerable effect; for it
is impossible 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
tucutuco, which is occasionally blind, and then at cer-
tain moles, which are habitually blind and have their
eyes covered with skin; or when we look at the blind
animals inhabiting the dark caves of America and Eu-
rope. With varieties and species, correlated variation
seems to have played an important part, so that when
one part has been modified other parts have been neces-
sarily modified. With both varieties and species rever-
sions to long-lost characters occasionally occur. How in-
explicable on the theory of creation is the occasional
appearance of stripes on the shoulders and legs of the
several species of the horse-genus and of their hybrids!
How simply is this fact explained if we believe that
these species are all descended from a striped progenitor,
in the same manner as the several domestic breeds of
the pigeon are descended from the blue and barred
rock-pigeon !
On the ordinary view of each species having been
independently created, why should specific characters,
or those by which the species of the same genus differ
from each other, be more variable than generic characters
in which they all agree? Why, for instance, should the
color of a flower be more likely to vary in any one spe-
RECAPITULA TION
295
cies of a genus, if the other species possess differently
colored flowers, than if all possessed the same colored
flowers? If species are only well-marked varieties,
of which the characters have become in a high de-
gree permanent, we can understand this fact; for they
have already varied since they branched off from a com-
mon progenitor in certain characters, by which they have
come to be specifically distinct from each other; therefore
these same characters would be more likely again to vary
than the generic characters which have been inherited
without change for an immense period. It is inexpli-
cable on the theory of creation why a part developed in
a very unusual manner in one species alone of a genus,
and therefore, as we may naturally infer, of great impor-
tance to that species, should be eminently liable to varia-
tion; but, on our view, this part has undergone, since
the several species branched off from a common progeni-
tor, an unusual amount of variability and modification,
and therefore we might expect the part generally to be
still variable. But a part may be developed in the most
unusual manner, like the wing of a bat, and yet not
be more variable than any other structure, if the part
be common to many subordinate forms, that is, if it has
been inherited for a very long period; for in this case
it will have been rendered constant by long-continued
natural selection.
Glancing at instincts, marvellous as some are, they
offer no greater difficulty than do corporeal structures on
the theory of the natural selection of successive, slight,
but profitable modifications. We can thus understand
why nature moves by graduated steps in endowing dif-
ferent animals of the same class with their several in-
296
THE ORIGIN OF SPECIES
stincts. I have attempted to show how much light the
principle of gradation throws on the admirable architect-
ural powers of the hive-bee. Habit no doubt often comes
into play in modifying instincts; but it certainly is not
indispensable, as we see in the case of neuter insects,
which leave no progeny to inherit the effects of long-
continued habit. On the view of all the species of the
same genus having descended from a common parent, and
having inherited much in common, we can understand
how it is that allied species, when placed under widely
different conditions of life, yet follow nearly the same
instincts; why the thrushes of tropical and temperate
South America, for instance, line their nests 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 not perfect and liable
to mistakes, and at many instincts causing other animals
to suffer.
If species be only well-marked and permanent vari-
eties, 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 ab-
sorbed into each other by successive crosses, and in other
such points — as do the crossed offspring of acknowledged
varieties. This similarity would be a strange fact, if spe-
cies had been independently created and varieties had
been produced through secondary laws.
If we admit that the geological record is imperfect
to an extreme degree, then the facts which the record
does give strongly support the theory of descent with
modification. New species have come on the stage slowly
and at successive intervals, and the amount of change,
RECAPITULATION
29?
after equal intervals of time, is widely different in dif-
ferent 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 inevi-
tably follows from the principle of natural selection; for
old forms are supplanted by new and improved forms.
Neither single species nor groups of species reappear
when the chain of ordinary generation is once 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 fos-
sils in the formations above and below, is simply ex-
plained by their intermediate position in the chain of
descent. The grand fact that all extinct beings can be
classed with all recent beings, naturally follows from the
living and the extinct being the offspring of common
parents. As species have generally diverged in charac-
ter during their long course of descent and modification,
we can understand why it is that the more ancient forms,
or early progenitors of each group, so often occupy a
position in some degree intermediate between existing
groups. Kecent forms are generally looked upon as be*
ing, on the whole, higher in the scale of organization
than ancient forms; and they must be higher, in so far
as the later and more improved forms have conquered
the older and less improved forms in the struggle for
life; they have also generally had their organs more spe-
cialized for different functions. This fact is perfectly
compatible with numerous beings still retaining simple
298
THE ORIGIN OF SPECIES
and but little improved structures, fitted for simple con-
ditions of life; it is likewise compatible with some forms
having retrograded in organization, by having become at
each stage of descent better fitted for new and degraded
habits of life. Lastly, the wonderful law of the long
endurance of allied forms on the same continent — of mar-
supials in Australia, of edentata in America, and other
such cases — is intelligible, for within the same country
the existing and the extinct will be closely allied by
descent.
Looking to geographical distribution, if we admit that
there has been during the long course of ages much mi-
gration from one part of the world to another, owing
to former climatal and geographical changes and to the
many occasional and unknown means of dispersal, then
we can understand, on the theory of descent with 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 has struck every traveller, namely,
that on the same continent, under the most diverse con-
ditions, 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 are the
descendants of the same progenitors and early colonists.
On tbis same principle of former migration, combined in
most cases with modification, we can understand, by the
aid of the Glacial period, the identity of some few plants,
RECAPITULA TION
299
and the close alliance of many others, on the most dis-
tant mountains, and in the northern and southern tem-
perate zones; and likewise the close alliance of some of
the inhabitants of the sea in the northern and southern
temperate latitudes, though separated by the whole m in-
tertropical ocean. Although two countries may present
physical conditions as closely similar as the same species
ever require, we need feel no surprise at their inhabitants
being widely different, if they have been for a long
period completely sundered from each other; for as the
relation of organism to organism is the most important of
all relations, and as the two countries will have received
colonists at various periods and in different proportions,
from some other country or from each other, the course
of modification in the two areas will inevitably have
been different.
On the view of migration, with subsequent modifica-
tion, we see why oceanic islands are inhabited by only
few species, but of these, why many are peculiar or en-
demic forms. We clearly see why species belonging to
those groups of animals which cannot cross wide spaces
of the ocean, as frogs and terrestrial mammals, do not
inhabit oceanic islands; and why, on the other hand, new
and peculiar species of bats, animals which can traverse
the ocean, are often found on islands far distant from
any continent. Such cases as the presence of peculiar
species of bats on oceanic islands and the absence of all
other terrestrial mammals are facts utterly inexplicable
on the theory of independent acts of creation.
The existence of closely allied or representative species
in any two areas, implies, on the theory of descent with
modification, that the same parent-forms formerly inhab-
300 THE ORIGIN OF SPECIES
ited both areas: and we almost invariably find that
wherever many closely allied species inhabit two areas,
some identical species are still common to both. Where-
ever many closely allied yet distinct species occur, doubt-
ful forms and varieties belonging to the same groups
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 the striking relation of nearly
all the plants and animals of the Galapagos Archipelago,
of Juan Fernandez, and of the other American islands,
to the plants and animals of the neighboring American
mainland; and of those of the Cape de Verde Archi-
pelago and of the 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 can be arranged within a few great classes,
in groups subordinate to groups, and with the extinct
groups often falling in between the recent groups, is in-
telligible on the theory of natural selection with its con-
tingencies of extinction and divergence of character. On
these same principles we see how it is, that the mutual
affinities of the forms 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 beings, are of hardly any importance in classi-
fication; why characters derived from rudimentary parts,
though of no service to the beings, are often of high
classificatory value; and why embryological characters are
often the most valuable of all. The real affinities of »11
RECAPITULATION
301
organic beings, in contradistinction to their adaptive re-
semblances, are due to inheritance or community of de-
scent. The Natural System is a genealogical arrange-
ment, with the acquired grades of difference marked by
the terms varieties, species, genera, families, etc. ; and
we have to discover the lines of descent by the most
permanent characters whatever they may be and of how-
ever slight vital importance.
The similar framework of bones in the hand of a man,
wing of a bat, fin of the porpoise, and leg of the horse
— the same number of vertebrae forming the neck of the
giraffe and of the elephant — and innumerable other such
facts, at once explain themselves on the theory of de-
scent with slow and slight successive modifications. The
similarity of pattern in the wing and in the leg of a
bat, though used for such different purpose — in the jaws
and legs of a crab — in the petals, stamens, and pistils of
a flower, is likewise, to a large extent, intelligible on the
view of the gradual modification of parts or organs, which
were aboriginally alike in an early progenitor in each of
these classes. On the principle of successive variations
not always supervening at an early age, and being in-
herited at a corresponding not early period of life, we
clearly see why the embryos of mammals, birds, reptiles,
and fishes should be so closely similar, and 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 of 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
have reduced organs when rendered useless under changed
302
THE ORIGIN OF SPECIES
habits or conditions of life; and we can understand on
this view the meaning of rudimentary organs. But disuse
and selection will generally act on each creature, when it
has come to maturity and has to play its full part in the
struggle for existence, and will thus have little power on
an organ during early life; hence the organ will not be
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 pro-
genitor having well-developed teeth; and we may believe
that the teeth in the mature animal were formerly re-
duced by disuse, owing to the tongue and palate, or lips,
having become excellently fitted through natural selection
to browse without their aid; whereas in the calf the teeth
have been left unaffected, and on the principle of inheri-
tance at corresponding ages have been inherited from a
remote period to the present day. On the view of each
organism with all its separate parts having been specially
created, how utterly inexplicable is it that organs bearing
the plain stamp of inutility, such as the teeth in the
embryonic calf or the shrivelled wings under the soldered
wing-covers of many beetles, should so frequently occur.
Nature may be said to have taken pains to reveal her
scheme of modification, by means of rudimentary organs,
of embryological and homologous structures, but we are
too blind to understand her meaning.
Conclusion
I have now recapitulated the facts and considerations
which have thoroughly convinced me that species have
been modified, during a long course of descent. This has
been effected chiefly through the natural selection of
CONCLUSION
803
numerous successive, slight, favorable variations; aided
in an important manner by the inherited effects of che
use and disuse of parts; and in an unimportant manner,
that is, in relation to adaptive structures, whether past or
present, by the direct action of external conditions, and
by variations which seem to us in our ignorance to arise
spontaneously. It appears that I formerly underrated the
frequency and value of these latter forms of variation, as
leading to permanent modifications of structure indepen-
dently of natural selection. But as my conclusions have
lately been much misrepresented; and it has been stated
that I attribute the modification of species exclusively to
natural selection, I may be permitted to remark that in
the first edition of this work, and subsequently, I placed
in a most conspicuous position — namely, at the close of
the Introduction — the following words: "I am convinced
that natural selection has been the main but not the
exclusive means of modification." This has been of no
avail. Great is the power of steady misrepresentation;
but the history of science shows that fortunately this
power does not long endure.
It can hardly be supposed that a false theory would
explain, in so satisfactory a manner as does the theory of
natural selection, the several large classes of facts above
specified. It has recently been objected that this is an
unsafe method of arguing; but it is a method used in
judging of the common events of life, and has often been
used by the greatest natural philosophers. The undula-
tory theory of light has thus been arrived at; and the
belief in the revolution of the earth on its own axis was
until lately supported by hardly any direct evidence. It
is no valid objection that science as yet throws no light
—Science — 30
804
THE ORIGIN OF SPECIES
on the far higher problem of the essence or origin of
life. Who can explain what is the essence of the attrac-
tion of gravity ? No one now objects to following out
the results consequent on this unknown element of
attraction; notwithstanding that Leibnitz formerly accused
Newton of introducing "occult qualities and miracles into
philosophy."
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 impres-
sions are, to remember that the greatest discovery ever
made by man, namely, the law of the attraction of
gravity, was also attacked by Leibnitz, "as subversive
of natural, and inferentially of revealed, religion." A
celebrated author and divine has written to me that "he
has gradually learned 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- development 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, until recently did nearly all
the most eminent living naturalists and geologists disbe-
lieve in 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
clear distinction has been, or can be, drawn between
species and well-marked varieties. It cannot be main-
tained that species when intercrossed are invariably
sterile, and varieties invariably fertile; or that sterility
is a special endowment and sign of creation. The belief
CONCLUSION
805
that species were immutable productions was almost
unavoidable as long as the history of the world was
thought to be of short duration; and now that we have
acquired some idea of the lapse of time, we are too apt
to assume, without proof, that the geological record is so
perfect that it would have afforded us plain evidence of
the mutation of species, if they had undergone mutation.
But the chief cause of our natural unwillingness to
admit that one species has given birth to other and
distinct species, is that we are always slow in admitting
great changes of which we do not see the steps. The
difficulty is the same as that felt by so many geologists,
when Lyell first insisted that long lines of inland cliffs
had been formed, and great valleys excavated, by the
agencies which we see still at work. The mind cannot
possibly grasp the full meaning of the term of even a
million years; it cannot add up and perceive the full
effects of many slight variations, accumulated during an
almost infinite number of generations.
Although I am fully convinced of the truth of the
views given in this volume under the form of an abstract,
I by no means expect to convince experienced naturalists
whose minds are stocked with a multitude of facts all
viewed, during a long course of years, from a point of
view directly opposite to mine. It is so easy to hide
our ignorance under such expressions as the "plan of
creation," "unity of design," etc., 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 cer-
tain number of facts will certainly reject the theory. A
few naturalists, endowed with much flexibility of mind,
306
THE ORIG1X OF SPECIES
and who have alreacry begun to doubt the immutability
of species, may be influenced by this volume; but I look
with confidence to the future — to young and rising
naturalists, who will be able to view both sides of the
question with impartiality. Whoever is led to believe
that species are mutable will do good service by con-
scientiously expressing his conviction; for thus only can
the load of prejudice by which this subject is over-
whelmed be removed.
Several eminent naturalists have of late published
their belief that a multitude of reputed species in each
genus are not real species; but that other species are
real, that is, have been independently created. This seems
to me a strange conclusion to arrive at. They admit that
a multitude of forms, which till lately they themselves
thought were special creations, and which are still thus
looked at by the majority of naturalists, and which con-
sequently have all the external characteristic features of
true species — they admit that these have been produced
by variation, but they refuse to extend the same view to
other and slightly different forms. Nevertheless they
do not pretend that they can define, or even conjecture,
which are the created forms of life, and which are those
produced by secondary laws. They admit variation as a
vera causa in one case, they arbitrarily reject it in
another, without assigning any distinction in the two
cases.
The day will come when this will be given as a
curious illustration of the blindness of preconceived
opinion. These authors seem no more startled at a
miraculous act of creation than at an ordinary birth.
But do they really believe that at innumerable periods
CONCLUSION
307
m 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 in-
finitely numerous kinds of animals and plants created
as eggs or seed, or as full grown? and in the case of
mammals, were they created bearing the false mark3
of nourishment from the mother's womb? Undoubtedly
some of these same questions cannot be answered by
those who believe in the appearance or creation of only
a few forms of life, or of some one form alone. It has
been maintained by several authors that it is as easy to
believe in the creation of a million beings as of one; but
Maupertius' philosophical axiom ""of least action" leads
ihe mind more willingly to admit the smaller number;
and certainly we ought not to believe that innumerable
beings within each great class have been created with
plain, but deceptive, marks of descent from a single
parent.
As a record of a former state of things, I have re-
tained in the foregoing paragraphs, and elsewhere, several
sentences which imply that naturalists believe in the
separate creation of each species; and I have been much
censured for having thus expressed myself. But un-
doubtedly this was the general belief when the first
edition of the present work appeared. I formerly spoke
to very many naturalists on the subject of evolution, and
never once met with any sympathetic agreement. It is
probable that some did then believe in evolution, but
they were either silent, or expressed themselves so am-
biguously that it was not easy to understand their mean-
ing. Now things are wholly changed, and almost every
308
THE ORIGIN OF SPECIES
naturalist admits the great principle of evolution. There
are, however, some who still think that species have
suddenly given birth, through quite unexplained means,
to new and totally different forms: but, as I have
attempted to show, weighty evidence can be opposed to
the admission of great and abrupt modifications. Under
a scientific point of view, and as leading to further in-
vestigation, but little advantage is gained by believing
that new forms are suddenly developed in an inexplicable
manner from old and widely different forms, over the old
belief in the creation of species from the dust of the
earth.
It may be asked how far I extend the doctrine of the
modification of species. The question is difficult to
answer, because the more distinct the forms are which
we consider, by so much the arguments in favor of
community of descent become fewer in number and less
in force. But some arguments of the greatest weight
extend very far. All the members of whole classes are
connected together by a chain of affinities, and all can
be classed on the same principle, in groups subordinate
to groups. Fossil remains sometimes tend to fill up very
wide intervals between existing orders.
Organs in a rudimentary condition plainly show that
an early progenitor had the organ in a fully developed
condition; and this in some cases implies an enormous
amount of modification in the descendants. Throughout
whole classes various structures are formed on the same
pattern, and at a very early age the embryos closely
resemble each other. Therefore I cannot doubt that
the theory of descent with modification embraces all the
members of the same great class or kingdom. X be-
CONCLUSION
309
lieve that animals are descended from at most only
four or five progenitors, and plants from an equal or
lesser number.
Analogy would lead me one step further, namely, to
the belief that all animals and plants are descended from
some one prototype. But analogy may be a deceitful
guide. Nevertheless all living things have much in
common, in their chemical composition, their cellular
structure, their laws of growth, and their liability to
injurious influences. We see this even in so trifling a
fact as that the same poison often similarly affects plants
and animals; or that the poison secreted by the gall-fly
produces monstrous growths on the wild rose or oak-
tree. With all organic beings, "excepting perhaps some
of the very lowest, sexual reproduction seems to be
essentially similar. With all, as far as is at present
known, the germinal vesicle is the same; so that all
organisms start 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 inter-
mediate in character that naturalists have disputed to
which kingdom they should be referred. As Professor
Asa Gray has remarked, " the spores and other repro-
ductive bodies of many of the lower algae may claim to
have first a characteristically animal, and then an une-
quivocally vegetable existence." Therefore, on the prin-
ciple of natural selection with divergence of character,
it does not seem incredible that, from some such low and
intermediate form, both animals and plants may have
been developed; and, if we admit this, we must likewise
admit that all the organic beings which have ever lived
on this earth may be descended from some one primordial
310
THE ORIGIN OF SPECIES
form. But this inference is chiefly grounded on analogy,
and it is immaterial whether or not it be accepted. No
doubt it is possible, as Mr. G. H. Lewes has urged, that
at the first commencement of life many different forms
were evolved; but if so, we may conclude that only a
very few have left modified descendants. For, as I have
recently remarked in regard to the members of each great
kingdom, such as the Vertebrata, Articulata, etc., we
have distinct evidence in their embryological, homolo-
gous, and rudimentary structures, that within each
kingdom all the members are descended from a single
progenitor.
When the views advanced by me in this volume, and
by Mr. Wallace, or when analogous views on the origin
of species are generally admitted, we can dimly foresee
that there will be a considerable revolution in natural
history. Systematists will be able to pursue their labors
as at present; but they will not be incessantly haunted
by the shadowy doubt whether this or that form be a
true species. This, I feel sure, and I speak after expe-
rience, will be no slight relief. The endless disputes
whether or not some fifty species of British brambles are
good species will cease. Systematists will have only to
decide (not that this will be easy) whether any form be
sufficiently constant and distinct from other forms to be
capable of definition; and if definable, whether the differ-
ences be sufficiently important to deserve a specific name.
This latter point will become a far more essential con-
sideration than it is at present; for differences, however
slight, between any two forms, if not blended by inter-
mediate gradations, are looked at by most naturalists as
sufficient to raise both forms to the rank of species.
CONCLUSION
311
Hereafter we shall be compelled to acknowledge that
the only distinction between species and well-marked
varieties is, that the latter are known, or believed, to
be connected at the present day by intermediate grada-
tions, whereas species were formerly thus connected.
Hence, without rejecting the consideration of the present
existence of intermediate gradations between any two
forms, we shall be led to weigh more carefully and to
value higher the actual amount of difference between
them. It is quite possible that forms now generally
acknowledged to be merely varieties may hereafter be
thought worthy of specific names; and in this case scien-
tific and common language will come into accordance.
In short, we shall have to treat species in the same
manner as those naturalists treat genera who admit that
genera are merely artificial combinations made for con-
venience. This may not be a cheering prospect; but
we shall at least be freed from the vain search for the
undiscovered and undiscoverable essence of the term
species.
The other and more general departments of natural
history will rise greatly in interest. The terms used by
naturalists, of affinity, relationship, community of type,
paternity, morphology, adaptive characters, rudimentary
and aborted organs, etc., will cease to be metaphorical,
and will have a plain signification. When we no longer
look at an organic being as a savage looks at a ship, as
something wholly beyond his comprehension; when we
regard every production of nature as one which has had
a long history; when we contemplate every complex
structure and instinct as the summing up of many con-
trivances, each useful to the possessor, in the same way
812 THE ORIGIN OF SPECIES
as any great mechanical invention is the summing up of
the labor, 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 — does the study of natural history be-
come !
A grand and almost untrodden field of inquiry will
be opened, on the causes and iaws of variation, on cor-
relation, on the e5ect3 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 more important
and interesting subject for study than one more species
added to the infinitude of already recorded species. Our
classifications will come to be, as far as they can be so
made, genealogies; and will then truly give what may
be called the plan of creation. The rules for classifying
will no doubt become simpler when we have a definite
object in view. We possess no pedigrees or armorial
bearings; and we have to discover and trace the many
diverging lines of descent in our natural genealogies, by
characters of any kind which have long been inherited.
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
picture of the ancient forms of life. Embryology will
often reveal to us the structure, in some degree obscured,
of the prototypes of each great class.
When we can feel assured that all the individuals
of the same species, and all the closely allied species
of most genera, have within a not very remote period
CONCLUSION
318
descended from one parent, and have migrated from
some one birthplace; and when we better know the
many means of migration, then, by the light which
geology now throws, and will continue to throw, on
former changes of climate and of the level of the land,
we shall surely be enabled to trace in an admirable man-
ner the former migrations of the inhabitants of the whole
world. Even at present, by comparing the differences
between the inhabitants of the sea on the opposite sides
of a continent, and the nature of the various inhabitants
on 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 I
with its imbedded remains must not be looked at as a 1
well-filled museum, but as a poor collection made at \
hazard and at rare intervals. The accumulation of each I
great fossiliferous formation will be recognized as having
depended on an unusual concurrence of favorable circum-
stances, and the blank intervals between the successive 1
stages as having been of vast duration. But we shall be
able to gauge with some security the duration of these
intervals by a comparison of the preceding and succeed-
ing organic forms. We must be cautious in attempting
to correlate as strictly contemporaneous two formations,
which do not include many identical species, by the gen-
eral succession of the forms of life. As species are pro-
duced and exterminated by slowly acting and still exist-
ing causes, and not by miraculous acts of creation; and
as the most important of all causes of organic change ia
one which is almost independent of altered and perhapa
814
THE ORIGIN OF SPECIES
suddenly altered physical conditions, namely, the mutual
relation of organism to organism — the improvement of
one organism entailing the improvement or the extermi-
nation of others; it follows that the amount of organic
change in the fossils of consecutive formations probably
serves as a fair measure of the relative, though not ac-
tual, lapse of time. A number of species, however, keep-
ing in a body might remain for a long period unchanged,
while within the same period several.. hn*i& species by
migrating into new countries and coming into competition
with foreign associates, might become modified; so that
we must not overrate the accuracy of organic change
as a measure of time.
In the future I see open fields for far more important
researches. Psychology will be securely based on the
foundation already well laid by Mr. Herbert Spencer,
that of the necessary acquirement of each mental power
and capacity by gradation. Much light will be thrown
on the origin of man and his history.
Authors of the highest eminence seem to be fully satis-
fied with the view that each species 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 pres-
ent inhabitants of the world should have been due to
secondary causes, like those determining the birth and
death of the individual. When I view all beings not
as special creations, but as the lineal descendants of some
few beings which lived long before the first bed of the
Cambrian system was deposited, they seem to me to be-
come ennobled. Judging from the past, we may safely
infer that not one living species will transmit its unal-
CONCLUSION
315
tered likeness to a distant futurity. And of the species
now living very few will transmit progeny of any kind
to a far distant futurity; for the manner in which all
organic beings are grouped shows that the greater num-
ber of species in each genus, and all the species in many
genera, have left no descendants, but have become ut-
terly extinct. We can so far take a prophetic glance
into futurity ^s to foretell that it will be the common
and widely-. species, belonging to the larger and
dominant groups within each class, which will ultimately
prevail and procreate new and dominant species. As all
the living forms of life are the lineal descendants of
those which lived long before the Cambrian epoch, we
may feel certain that the ordinary succession by genera-
tion 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 great length.
And as natural selection works solely by and for the
good of each being, all corporeal and mental endowments
will tend to progress toward perfection.
It is interesting to contemplate a tangled bank, clothed
with many plants of many kinds, with birds singing on
the bushes, with various insects flitting about, and with
worms crawling through the damp earth, and to reflect
that these elaborately constructed forms, so different from
each other, and dependent upon each other in so complex
a manner, have all been produced by laws acting around
us. These laws, taken in the largest sense, beiDg Growth
with Reproduction; Inheritance which is almost implied
by reproduction; Variability from the indirect and direct
action of the conditions of life, and from use and disuse:
a Ratio of Increase so high as to lead to a Struggle for
816
THE ORIGIN OF SPECIES
\ Life, and as a consequence to Natural Selection, entail-
1 ing Divergence of Character and the Extinction of less-
\ improved forms. Thus, from the war of nature, from
famine and death, the most exalted object which we
are capable of conceiving, namely, the production of the
higher animals, directly follows. There is grandeur in
this view of life, with its several powers, having been
originally breathed by the Creator into ^wfew forms or
into one; and that, while this planet iH^gone cycling
on according to the fixed law of gravity, from so sim-
ple a beginning endless forms most beautiful and most
wonderful have been and are being evolved.
GLOSSARY
OF THE PRINCIPAL SCIENTIFIC TERMS USED IN THE
PRESENT VOLUME"'
Aberrant. — Forms or groups of animals or plants which
deviate in important characters from their nearest allies,
so as not to be easily included in the same group with
them, are said to be aberrant.
Aberration (in Optics). — In the refraction of light by a
convex lens the rays passing through different parts of
the lens are brought to a focus at slightly different dis-
tances— this is called spherical aberration; at the same
time the colored rays are separated by the prismatic
action of the lens and likewise brought to a focus at dif-
ferent distances — this is chromatic aberration.
Abnormal. — Contrary to the general rule.
Aborted. — An organ is said to be aborted when its devel-
opment has been arrested at a very early stage.
Albinism. — Albinos are animals in which the usual color-
ing matters characteristic of the species have not been
produced in the skin and its appendages. Albinism is
the state of being an albino.
Alg^e. — A class of plants including the ordinary sea-weeds
and the filamentous fresh- water weeds.
1 I am indebted to the kindness of Mr. W. S. Dallas for this Glossary, which
has been given because several readers have complained to me that some of the
terms used were unintelligible to them. Mr. Dallas has endeavored to give
the explanations of the terms in as popular a form as possible.
(317)
318
GLOSSARY
Alternation of Generations.— This term is applied to
a peculiar mode of reproduction which prevails among
many of the lower animals, in which the egg produces a
living form quite different from its parent, but from
which the parent-form is reproduced by a process of
budding, or by the division of the substance of the first
product of the egg.
Ammonites. — A group of fossil, spiral, chambered shells,
allied to the existing pearly Nautilus, but having the
partitions between the chambers waved in complicated
patterns at their junction with the outer wall of the shell.
Analogy. — That resemblance of structures which depends
upon similarity of function, as in the wings of insects
and birds. Such structures are said to be analogous, and
to be analogues of each other.
Animalcule. — A minute animal: generally applied to
those visible only by the microscope.
Annelids. — A class of worms in which the surface of the
body exhibits a more or less distinct division into rings
or segments, generally provided with appendages for
locomotion and with gills. It includes the ordinary
marine worms, the earthworms, and the leeches.
Antenna. — Jointed organs appended to the head in In-
sects, Crustacea and Centipeds, and not belonging to
the mouth.
Anthers. — The summits of the stamens of flowers, in which
the pollen or fertilizing dust is produced.
Aplacentalia, Aplacentata, or Aplacental Mammals.
See Mammalia.
Archetypal. — Of or belonging to the Archetype, or ideal
primitive form upon which all the beings of a group
seem to be organized.
Articulata. — A great division of the Animal Kingdom
characterized generally by having the surface of the
body divided into rings called segments, a greater or less
number of which are furnished with jointed legs (such as
Insects, Crustaceans and Centipeds).
GLOSSARY
Asymmetrical. — Having the two sides unlike.
Atrophied. — Arrested in development at a very early stage.
Balanus. — The genus including the common Acorn-shells
which live in abundance on the rocks of the sea-coast.
Batrachians. — A class of animals allied to the Keptiles,
but undergoing a peculiar metamorphosis, in which the
young animal is generally aquatic and breathes by gills,
{Examples, Frogs, Toads, and Newts.)
' Bowlders. — Large transported blocks of stone generally
imbedded in clays or gravels.
Brachiopoda. — A class of marine Mollusca, or soft-bodied
animals, furnished with a bivalve shell, attached to
submarine objects by a stalk which passes through an
aperture in one of the valves, and furnished with fringed
arms, by the action of which food is earned to the
mouth.
Branchiae. — Gills or organs for respiration in water.
Branchial. — Pertaining to gills or branchiae.
Cambrian System. — A Series of very ancient Paleozoic
rocks, between the Laurentian and the Silurian. Until
recently these were regarded as the oldest fossiliferous
rocks.
Caxid^e.— The Dog-family, including the Dog, Wolf, Fox,
Jackal, etc.
Carapace. — The shell enveloping the anterior part of the
body in Crustaceans generally; applied also to the hard
shelly pieces of the Cirripeds.
Carboniferous. — This term is applied to the great for-
mation which includes, among other rocks, the coal-
measures. It belongs to the oldest, or Paleozoic,
system of formations.
Caudal. — Of or belonging to the tail.
Cephalopods. — The highest class of the Mollusca, or soft-
bodied animals, characterized by having the mouth
surrounded by a greater or less number of fleshy awns
82j
GLQ6SABY
or tentacles, which, in most living species, are furnished
with sucking-cups. (Examples. Cuttle-fish. Nautilus.)
Cetacea. — An order of Mammalia, including the Whales,
Dolphins^ etc., having the form of the body fishlike, the
skin naked, and only the fore-limbs developed.
Cheloxia. — An order oi Reptiles including the Turtles,
Tortoises, etc.
Cibripeds. — An order of Crustaceans including the Bar-
nacles and Acorn-shells. Their young resemble those of
many other Crustaceans in form: but when mature they
are always attached to other objects, either directly or
by means of a stalk, and their bodies are inclosed by
a calcareous shell composed of several pieces, two of
which can open to give issue to a bunch of curled,
jointed tentacles, which represent the limbs.
Coccus. — The genus of Insects including the Cochineal.
In these the male is a minute, winged fly, and the female
generally a motionless, berry -like mass.
Cocoo>\ — A case usually of silky material, in which insects
are frequently enveloped during the second or restmg-
stage (pupa) of their existence. The term k 'cocoon-
stage' ? is here used as equivalent to "pupa -stage."
C<ELOSPERirous.— A term applied to those fruits of the
Umbelliferae which have the seed hollowed on the
inner face.
Coleoptera. — Beetles, an order of Insects, having a biting
mouth and the first pair of wings more or less horny,
forming sheaths for the second pair, and usually meeting
in a straight line down the middle of the back.
Column. — A peculiar organ in the flowers of Orchids, in
which the stamens, style and stigma (or the reproductive
parts) are united.
Composite or Compositous Plants.— Plants in which the
inflorescence consists of numerous small flowers (florets)
brought together into a dense head, the base of which is
inclosed by a common envelope. {Examples, the Daisy,
Dandelion, etc.)
GLOSSARY
321
Conferva. — The filamentous weeds ot /resh water.
Conglomerate. — A rock made up of fragments of rock or
pebbles, cemented together by some other material.
Corolla. — The second envelope of a flower, usually com-
posed of colored, leaf-like organs (petals), which may
be united by their edges either in the basal part or
throughout.
Correlation. — The normal coincidence of one phenome-
non, character, etc., with another.
Corymb. — A bunch of flowers in which those springing
from the lower part of the flower stalk are supported on
long stalks so as to be nearly on a level with the upper
ones.
Cotyledons. — The first or seed-leaves of plants.
Crustaceans. — A class of articulated animals, having the
skin of the body generally more or less hardened by the
deposition of calcareous matter, breathing by means of
gills. (Examples, Crab, Lobster, Shrimp, etc.)
Curculio. — The old generic term for the Beetles known as
Weevils, characterized by their four-jointed feet, and
by the head being produced into a sort of beak, upon the
sides of which the antennae are inserted.
Cutaneous. — Of or belonging to the skin.
Degradation. — The wearing down of land by the action of
the sea or of meteoric agencies.
Denudation. — The wearing away of the surface of the
land by water.
Devonian System or formation. — A series of Paleozoic
rocks, including the Old Red Sandstone.
Dicotyledons or Dicotyledonous Plants. — A class of
plants characterized by having two seed-leaves, by the
formation of new wood between the bark and the old
wood (exogenous growth), and by the reticulation of
the veins of the leaves. The parts of the flowers are
generally in multiples of five.
Differentiation. — The separation or discrimination of
322
GLOSSARY
parts or organs which in simpler forms oi life are more
or les3 united.
Dimorphic. — Having two distinct forms. — Dimorphism is
the condition of the appearance of the same species
under two dissimilar forms.
Dicecio us.— Having the organs of the sexes upon distinct
individuals.
Diorite. — A peculiar form of Greenstone.
Dorsal. — Of or belonging to the back.
Edentata. — A peculiar order of Quadrupeds, characterized
by the absence of at least the middle incisor (front) teeth
in both jaws. (Examples, the Sloths and Armadillos.-)
Elytra. — The hardened fore-wings of Beetles, serving as
sheaths for the membranous hind-wings, which consti-
tute the true organs of flight.
Embryo. — The young animal undergoing development
within the egg or womb.
Embryology. — The study of the development of the
embryo.
Endemic. — Peculiar to a given locality.
Entomostraca. — A division of the class Crustacea, having
all the segments of the body usually distinct, gills at-
tached to the feet or organs of the mouth, and the feet
fringed with fine hairs. They are generally of small
size.
Eocene. — The earliest of the three divisions of the Tertiary
epoch of geologists. Rocks of this age contain a small
proportion of shells identical with species now living.
Ephemerous iNSECTS. — lnsect3 allied to the May-fly.
Fauna. — The totality of the animals naturally inhabiting
a certain country or region, or which have lived during a
given geological period.
FELID.E. — The Cat family.
Feral. — Having become wild from a state of cultivation or
domestication.
GLOSSARY
323
Fetal. — Of or belonging to the feetus, or embryo in course
of development.
Flora. — The totality of the plants growing naturally in a
country, or during a given geological period.
Florets. — Flowers imperfectly developed in some respects,
and collected into a dense spike or head, as in the
Grasses, the Dandelion, etc.
Foraminifera. — A class of animals of very low organiza-
tion, and generally of small size, having a jelly-like
body, from the surface of which delicate filaments can
be given off and retracted for the prehension of external
objects, and having a calcareous or sandy shell, usually
divided into chambers, and perforated with small
apertures.
Fossiliferous. — Containing fossils.
Fossorial. — Having a faculty of digging. The Fossorial
Hymenoptera are a group of Wasp-like Insects, which
burrow in sandy soil to make nests for their young.
Frenum (pi. Frena). — A small band or fold of skin.
Fungi (sing. Fungus). — A class of cellular plants of
which Mushrooms, Toadstools, and Moulds are familiar
examples.
Furcula. — The forked bone formed by the union of the
collar-bones in many birds, such as the common Fowl.
Gallinaceous Birds. — An order of Birds of which the
common Fowl, Turkey, and Pheasant are well-known
examples.
Gallus. — The genus of birds which includes the common
Fowl.
Ganglion. — A swelling or knot from which nerves are
given off as from a centre.
Ganoid Fishes. — Fishes covered with peculiar enamelled
bony scales. Most of them are extinct.
Germinal Vesicle. — A minute vesicle in the eggs of
animals, from which the development of the embryo
proceeds.
S24
GLOSSARY
Glacial Period. — A period of great cold and of enormous
extension of ice upon the surface of the earth. It is
believed that glacial periods have occurred repeatedly
during the geological history of the earth, but the term
is generally applie l to the close of the Tertiary epoch,
when nearly the whole oi Europe was subjected to an
arctic climate.
Gland. — An organ which secretes or separates some peculiar
product from the blood or sap of animals or plants.
Glottis. — The opening of the windpipe into the oesophagus
or gullet.
Gneiss. — A rock approaching granite in composition, but
more or less laminated, and really produced by the alter-
ation of a sedimentary deposit after its consolidation.
Grallatoees. — The so-called VTading-birds (Storks, Cranes,
Snipes, etc.). which are generally furnished with long
legs, bare of feathers above the heel, and have no mem-
branes between the toes.
Granite. — A rock consisting essentially of crystals of
felspar and mica in a mass of quartz.
Habitat. — The locality in which a plant or animal naturally
lives.
Hemiptera. — An order or sub-order of Insects, character-
ized by the possession of a jointed beak or rostrum, and
by having the fore-wing3 horny in the basal portion and
membranous at the extremity, where they cross each
other. This group includes the various species of
Bugs.
Hermaphrodite. — Possessing the organs of both sexes.
Homology. — Tnat relation between parts which results from
their development from corresponding embryonic parts,
eitner in different animals, as in the case of the arm of
man, the fore-leg of a quadruped, and the wing of a
bird; or in the same individual, as in the case of the fore
and hind legs in quadrupeds, and the segments or rings
and their appendages of which the body of a worm, a
GLOSSARY
325
centiped, etc., is composed. The latter is called serial
homology. The parts which stand in such a relation to
each other are said to be homologous, and one such part
or organ is called the homologue of the other. In differ-
ent plants the parts of the flower are homologous, and
in general these parts are regarded as homologous with
leaves.
Homoptera. — An order or sub-order of Insects having (like
the Hemiptera) a jointed beak, but in which the fore-
wings are either wholly membranous or wholly leathery.
The Cicadce, Frog-hoppers, and Aphides, are well-known
examples.
Hybrid. — The offspring of the union of two distinct species.
Hymenoptera. — An order of Insects possessing biting jaws
and usually four membranous wings in which there are
a few veins. Bees and Wasps are familiar examples of
this group.
Hypertrophied. — Excessively developed.
Ichneuaionidje. — A family of Hymenopterous insects, the
members of which lay their eggs in the bodies or eggs
of other insects.
Imago. — The perfect (generally winged) reproductive state
of an insect.
Indigens. — The aboriginal animal or vegetable inhabitants
of a country or region.
Inflorescence. — The mode of arrangement of the flowers
of plants.
Infusoria. — A class of microscopic Animalcules, so called
from their having originally been observed in infusions
of vegetable matters. They consist of a gelatinous ma-
terial inclosed in a delicate membrane, the whole or part
of which is furnished with short vibrating hairs (called
cilia), by means of which the animalcules swim through
the water or convey the minute particles of their food to
the orifice of the mouth.
Insectivorous. Feeding on Insects.
326
GLOSSARY
Invertebrata, or Invertebrate Animals.— Those ani-
mals which do not possess a backbone or spinal column.
Lacunae. — Spaces left among the tissues in some of the
lower animals, and serving in place of vessels for the
circulation of the fluids of the body.
Lamellated. — Furnished with lamellae or little plates.
Larva (pi. Larvje). — The first condition of an insect at its
issuing from the egg. when it is usually in the form of a
grub, caterpillar, or maggot.
Larynx. — The upper part of the windpipe opening into the
gullet.
Laurentian. — A group of greatly altered and very ancient
rocks, which is greatly developed along the course of the
St. Laurence, whence the name. It is in these that the
earliest known traces of organic bodies have been found.
LeguminoSjE. — An order of plants represented by the com-
mon Peas and Beans, having an irregular flower in which
one petal stands up like a wing, and the stamens and pis-
til are inclosed in a sheath formed by two other petals.
The fruit is a pod (or legume).
Lemuridjs. — A group of four-handed animals, distinct from
the Monkeys and approaching the Insectivorous Quadru-
peds in some of their characters and habits. Its members
have the nostrils curved or twisted, and a claw instead of
a nail upon the first finger of the hind hands.
Lepidoptera. — An order of Insects, characterized by the
possession of a spiral proboscis, and of four large more
or less scaly wings. It includes the well-known Butter-
flies and Moths.
Littoral. — Inhabiting the sea-shore.
Loess. —A marly deposit of recent (Post-Tertiary) date,
which occupies a great part of the valley of the Rhine.
Malacostraca. — The higher division of the Crustacea,
including the ordinary Crabs, Lobsters, Shrimps, etc.,
together with the Woodlice and Sand-hoppers.
GLOSSARY
327
Mammalia. — The highest class of animals, including the
ordinary hairy quadrupeds, the Whales, and Man, and
characterized by the production of living young which
are nourished after birth by milk from the teats {Mammas,
Mammary gtanas) oi the mother. A striking difference
in embryonic development has led to the division of this
class into two great groups; in one of these, when the
embryo has attained a certain stage, a vascular connec-
tion, called the placenta, is formed between the embryo
and the mother; in the other this is wanting, and the
young are produced in a very incomplete state. The
former, including the greater part of the class, are called
Placental mammals ; the latter, or Aplacental mammals, in-
clude the Marsupials and Monotremes {Ornithorhynchus).
Mammiferous.— Having mammae or teats (see Mammalia).
Mandibles, in Insects. — The first or uppermost pair of
jaws, which are generally solid, horny, biting organs.
In Birds the term is applied to both jaws with their
horny coverings. In Quadrupeds the mandible is prop-
erly the lower jaw.
Marsupials. — An order of Mammalia in which the young
are born in a very incomplete state of development, and
carried by the mother, while sucking, in a ventral pouch
(marsupium), such as the Kangaroos, Opossums, etc. (see
Mammalia).
Maxillae, in Insects. — The second or lower pair of jaws,
which are composed of several joints and furnished with
peculiar jointed appendages called palpi, or feelers.
Melanism. — The opposite of albinism; an undue develop-
ment of coloring material in the skin and its appendagese
Metamorphic Rocks. — Sedimentary rocks which have un-
dergone alteration, generally by the action of heat, sub-
sequently to their deposition and consolidation.
Mollusca. — One of the great divisions of the Animal King-
dom, including those animals which have a soft body,
usually furnished with a shell, and in which the nervous
ganglia, or centres, present no definite general arrange-
—Science — 31
828
GLOSSARY
ment. They are generally known under the denomina
nation of "shell-fish"; the cuttle-fish, and the common
snails, whelks, oysters, mussels, and cockles, may serve
as examples of them.
Monocotyledons, or Monocotyledonous Plants. —
Plants in which the seed sends up only a single
seed-leaf (or cotyledon); characterized by the absence
of consecutive layers of wood in the stem (endogenous
growth), by the veins of the leaves being generally
straight, and by the parts of the flowers being gen-
erally in multiples of three. (Examples, Grasses, Lilies,
Orchids, Palms, etc.)
Moraines. — The accumulations of fragments of rock
brought down by glaciers.
Morphology. — The law of form or structure independent
of function.
Mysis-stage. — A stage in the development of certain Crus-
taceans (Prawns), in which they closely resemble the
adults of a genus (Mysis) belonging to a slightly lower
group.
Nascent. — Commencing development.
Natatory. — Adapted for the purpose of swimming.
Nauplius-form. — The earliest stage in the development
of many Crustacea, especially belonging to the lower
groups. In this stage the animal has a short body, with
indistinct indications of a division into segments, and
three pairs of fringed limbs. This form of the common
fresh-water Cyclops was described as a distinct genus
under the name of Nauplius.
Neuration. — The arrangement of the veins or nervures in
the wings of Insects.
Neuters. — Imperfectly developed females of certain social
insects (such as Ants and Bees), which perform all the
labors of the community. Hence they are also called
vjorhers.
Nictitating Membrane. — A semi-transparent membrane,
GLOSSARY
329
which can he drawn across the eye in Birds and Kep-
tiles, either to moderate the ejects of a strong light or to
sweep particles of dust, etc., from the surface of the eye.
Ocelli. — The simple eyes or sternmata of Insects, usually
situated on the crown of the head between the great
compound eyes,
(Esophagus.— The gullet.
Oolitic. — A great series of secondary rocks, so called from
the texture of some of its members, which appear to be
made up of a mass of small egg-like calcareous bodies..
Operculum. — A calcareous plate employed by many Mol-
lusca to close the aperture of their shell. The opercular
valves of Cirripeds are those which close the aperture of
the shell.
Orbit. — The bony cavity for the reception of the eye.
Organism.— An organized being, whether plant or animal.
Orthospermous.— A term applied to those fruits of the
Umbeiiiferae which have the seed straight.
Osculant. — Forms or groups apparently intermediate
between and connecting other groups are said to
bs osculant
Ova.— Eggs.
Ovarium or Ovary (in plants). — The lower part of the
pistil or female organ of the Sower, containing the
ovules or incipient seeds; by growth after the other
organs of the flower have fallen it usually becomes
converted into the fruit.
Ovigsrous. —Egg-bearing.
Ovules (of plants).— The seeds in the earliest condition.
Pachyderms. — A group of Mammalia, so called from their
thick skins, and including the Elephant, Ehinoceros,
Hippopotamus, etc.
Paleozoic. — The oldest system of fossiliferous rocks.
Palpi. — Jointed appendages to some of the organs of the
mouth in Insects and Crustacea.
330
GLOSSARY
PapilionacEuE.— An order of Plants (see Leguminos^:). —
The flowers of these plants are called papilionaceous,
or butterfly-like, from the fancied resemblance of the
expanded superior petals to the wings of a butterfly.
Parasite. — An animal or plant living upon or in, and at
the expense of, another organism.
Parthenogenesis. — The production of living organisms
from unimpregnated eggs or seeds.
Pedunculated. — Supported upon a stem or stalk. The
pedunculated oak has its acorns borne upon a footstool.
Peloria or Pelorism. — The appearance of regularity of
structure in the flowers of plants which normally bear
irregular flowers.
Pelvis.— The bony arch to which the hind limbs of ver-
tebrate animals are articulated.
Petals. — The leaves of the corolla, or second circle of or-
gans in a flower. They are usually of delicate texture
and brightly colored.
Phyllodineous. — Having flattened, leaf-like twigs or leaf-
stalks instead of true leaves.
Pigment. — The coloring material produced generally in the
superficial parts of animals. The cells secreting it are
called pigment-cells.
Pinnate.— Bearing leaflets on each side of a central stalk.
Pistils. — The female organs of a flower, which occupy a
position in the centre of the other floral organs. The
pistil is generally divisible into the ovary or germen,
the style and the stigma.
Placentalia, Placentata, or Placental Mammals. — See
Mammalia.
Plantigrades. — Quadrupeds which walk upon the whole
sole of the foot, like the Bears.
Plastic. — Readily capable of change.
Pleistocene Period. — The latest portion of the Tertiary
epoch.
Plumule (in plants). — The minute bud between the seed-
leaves of newly-germinated plants.
GLOSSARY
331
Plutonic Rocks. — Rocks supposed to have been produced
by igneous action in the depths of the earth.
Pollen. — The male element in flowering plants; usually
a fine dust produced by the anthers, which, by contact
with the stigma, effects the fecundation of the seeds.
This impregnation is brought about by means of tubes
(pollen -tubes) which issue from the pollen-grains adher-
ing to the stigma, and penetrate through the tissues until
they reach the ovary.
Polyandrous (flowers). — Flowers having many stamens.
Polygamous Plants. — Plants in which some flowers are
unisexual and others hermaphrodite. The unisexual
(male and female) flowers may be on the same or on
different plants.
Polymorphic. — Presenting many forms.
Polyzoary. — The common structure formed by the cells
of the Polyzoa, such as the well-known Sea-mats.
Prehensile. — Capable of grasping.
Prepotent. — Having a superiority of power.
Primaries. — The feathers forming the tip of the wing of
a bird, and inserted upon that part which represents the
hand of man.
Processes. — Projecting portions of bones, usually for the
attachment of muscles, ligaments, etc.
Propolis. — A resinous material collected by the Hive-Bees
from the opening buds of various trees.
Protean. — Exceedingly variable.
Protozoa. — The lowest great division of the Animal King-
dom. These animals are composed of a gelatinous mate-
rial, and show scarcely any trace of distinct organs. The
Infusoria, Foraminifera, and Sponges, with some other
forms, belong to this division.
Pupa (pi. Pup^:). — The second stage in the development of
an Insect, from which it emerges in the perfect (winged)
reproductive form. In most insects the pupal stage is
passed in perfect repose. The chrysalis is the pupal
state of butterflies.
332
GLOSSARY
Radicle. — The minute root of an embryo plant.
Ramus. — One-half of the lower jaw in the Mammalia. The
portion which rises to articulate with the skull is called
the ascending ramus.
Range. — The extent of country over which a plant or ani^
mal is naturally spread. Range in time expresses the
distribution of a species or group through the fossilifer-
ous beds of the earth's crust.
Retina. — The delicate inner coat of the eye, formed by
nervous filaments spreading from the optic nerve, and
serving for the perception of the impressions produced
by light.
Retrogression. — Backward development. When an ani-
mal, as it approaches maturity, becomes less perfectly
organized than might be expected from its early stages
and known relationships, it is said to undergo a retrograde
development or metamorphosis .
Rhizopods. — A class of lowly organized animals (Protozoa),
having a gelatinous body, the surface of which can be
protruded in the form of rootlike processes or filaments,
which serve for locomotion and the prehension of
food. The most important order is that of the Foram-
inifera.
Rodents. — The gnawing Mammalia, such as the Rats, Rab-
bits, and Squirrels. They are especially characterized
by the possession of a single pair of chisel-like cutting
teeth in each jaw, between which and the grinding teeth
there is a great gap.
Rubus.— The Bramble Genus.
Rudimentary. — Very imperfectly developed.
Ruminants. — The group of Quadrupeds which ruminate
or chew the cud, such as oxen, sheep, and deer. They
have divided hoofs, and are destitute of front teeth in
the upper jaw.
Sacral. — Belonging to the sacrum, or the bone composed
usually of two or more united vertebrae to which the
sides of the pelvis in vertebrate animals are attached.
GLOSSARY
333
Sarcode. — The gelatinous material of which the bodies of
the lowest animals (Protozoa) are composed.
Scutell^e. — The horny plates with which the feet of birds
are generally more or less covered, especially in front.
Sedimentary Formations. — Rocks deposited as sediments
from water.
Segments. — The transverse rings of which the body of an
articulate animal or Annelid is composed.
Sepals. — The leaves or segments of the calyx, or outermost
envelope of an ordinary flower. They are usually green,
but sometimes brightly colored.
Serratures. — Teeth like those of a saw.
Sessile. — Not supported on a stem or footstalk.
Silurian System. — A very ancient system of fossiliferous
rocks belonging to the earlier part of the Paleozoic series.
Specialization. — The setting apart of a particular organ
for the performance of a particular function.
Spinal Cord. — The central portion of the nervous system
in the Vertebrata, which descends from the brain through
the arches of the vertebras, and gives off nearlv all the
nerves to the various organs of the body.
Stamens. — The male organs of flowering plants, standing
in a circle within the petals. They usually consist of a
filament and an anther, the anther being the essential
part in which the pollen, or fecundating dust, is formed.
Sternum. — The breast-bone.
Stigma. — The apical portion of the pistil in flowering
plants.
Stipules. — Small leafy organs placed at the base of the
footstalks of the leaves in many plants.
Style. — The middle portion of the perfect pistil, which
rises like a column from the ovary and supports the
stigma at its summit.
Subcutaneous. — Situated beneath the skin.
Suctorial. — Adapted for sucking.
Sutures (in the skull). — The lines of junction of the bones
of which the skull is composed.
534
GLOSSARY
Tarsus (pi. Tarsi).— The jointed feet of articulate animals,
such as Insects.
Teleostean Fishes.— Fishes of the kind familiar to us in
the present day, having the skeleton usually completely
ossified and the scales horny.
Tentacula or Tentacles.— Delicate fleshy organs of
prehension or touch possessed by many of the lower
animals.
Tertiary. — The latest geological epoch, immediately pre-
ceding the establishment of the present order of things.
Trachea. — The windpipe or passage for the admission of
air to the lungs.
Tridactyle. — Three-fingered, or composed of three movable
parts attached to a common base.
Trilobites. — A peculiar group of extinct Crustaceans,
somewhat resembling the Woodlice in external form,
and, like some of them, capable of rolling themselves
up into a ball. Their remains are found only in the
Paleozoic rocks, and most abundantly in those of
Silurian age.
Trimorphic. — Presenting three distinct forms.
Umbelliferous. — An order of plants in which the flowers,
which contain five stamens and a pistil with two styles,
are supported upon footstalks which spring from the top
of the flower stem and spread out like the wires of an
umbrella, so as to bring all the flowers in the same head
(umbel) nearly to the same level. (Examples , Parsley
and Carrot.)
Ungulata. — Hoofed quadrupeds.
Unicellular. — Consisting of a single celL
Vascular. — Containing blood-vessels.
Vermiform. — Like a worm.
Vertebrata, or Vertebrate Animals.— The highest
division of the animal kingdom, so called from the
presence in most cases of a backbone composed of
GLOSSARY
335
numerous joints or vertebrae, which constitutes the centre
of the skeleton and at the same time supports and pro-
tects the central parts of the nervous system.
Whorls. — The circles or spiral lines in which the parts of
plants are arranged upon the axis of growth.
Workers.— See Neuters.
Zoea-stage. — The earliest stage in the development of
many of the higher Crustacea, so called from the name
of Zoea applied to these young animals when they were
supposed to constitute a peculiar genus.
Zooids. — In many of the lower animals (such as the Corals,
Medusae, etc.) reproduction takes place in two ways,
namely, by means of eggs and by a process of budding
with or without separation from the parent of the product
of the latter, which is often very different from that of
the egg. The individuality of the species is represented
by the whole of the form produced between two sexual
reproductions; and these forms, which are apparently
individual animals, have been called zooids.
INDEX
A
Aberrant groups, ii. 236
Abyssinia, plants of, ii. 175
Acclimatization, i. 199
Adoxa, i. 296
Affinities of extinct species, ii. 115
of organic beings, ii. 235
Agassiz, on Amblyopsis, i. 199
, on the latest tertiary forms, ii.
77
, on groups of species suddenly
appearing, ii. 94
, on prophetic forms, ii. 114
, on embryological succession, ii.
127
, on the Glacial period, ii. 159
, on embryological characters, ii.
219
, on parallelism of embryological
development and geological succes-
sion, ii. 263
, Alex., on pedicellarise, i. 324
Algae of New Zealand, ii. 172
Alligators, males, fighting, i. 131
Alternate generations, ii. 248
Amblyopsis, blind fish, i. 199
America, North, productions allied to
those of Europe, ii. 165
, , bowlders and glaciers of,
ii. 167-168
, South, on modern formations on
west coast, ii. 67
Ammonites, sudden extinction of, ii.
107
Anagallis, sterility of, ii. 9
Analogy °f variations, i. 223
Ancylus, ii. 183
Andaman Islands inhabited by a toad,
ii. 191
Animals, not domesticated from being
variable, i. 43
, domestic, descended from sev-
eral stocks, i. 45
, , acclimatization of, i. 201
of Australia, i. 164
with thicker fur in cold climates,
i. 192
, blind, in caves, i. 196-199
, extinct, of Australia, ii. 128
Anomma, i. 389
Antarctic islands, ancient flora of, ii.
199
Antechinus, ii. 228
Ants attending aphides, i. 350
, slave-making instinct, i. 364
, neuters, structure of, i. 387
Apes, not having acquired intellectual
powers, i. 309
Aphides, attended by ants, i. 350
Aphis, development of, ii. 254
Apteryx, i. 244
Arab horses, i. 64
Aralo-Caspian Sea, ii. 129
Archeopteryx, ii. 86
Archiac, M. de, on the succession of
species, ii. 110
Artichoke, Jerusalem, i. 202
Ascension, plants of, ii. 187
Asclepias, pollen of, i. 262
Asparagus, ii. 151
Aspicarpa, ii. 218
Asses, striped, i. 224
340
INDEX
Cirripeds capable of crossing, i. 147-
148
, carapace aborted, i. 210
, their ovigerous frena, i. 258
, fossil sessile, ii. 86
, larvae of, ii. 252
Claparede, Prof., on the hair-claspers
of the Acaridae, i. 265-266
Clarke, Rev. W. B. , on old glaciers in
Australia, ii. 167
Classification, ii. 211
Clausen and Lund, on fossils of Brazil,
ii. 128
Clift, Mr., on the succession of types,
ii. 128
Climate, effects of, in checking in-
crease of beings, i. 107
, adaptation of, to organisms, i.
200
Climbing plants, i. 256
, development of, i. 332
Ckr *>r visited by bees, i. 140-141
Cobites, intestine of, i. 255
Cockroach, Asiatic, i. 116
Collections, paleontological, poor, ii. €3
Color, influenced by climate, i. 191
, in relation to attack by flies,
i. 274
Columba livia, parent of domestic
pigeons, i. 50
Colymbetes (water-beetle), ii. 183
Compensation of growth, i. 207
Composite, flowers and seeds of, i.
205
— — , outer and inner florets of, i. 297
, male flowers of, ii. 266
Conclusion, general, ii. 302
Conditions, slight changes in, favor-
able to fertility, ii. 33
Convergence of genera, i. 182
Coot, i. 248
Cope, Prof., on the acceleration or
retardation of the period of repro-
duction, i. 259
Coral-islands, seeds drifted to, ii. 158
Corn-crake, i. 249
Correlated variation in domeotic pro-
ductions, i. 37
Coryanthes, i. 267
Coypu, ii. 141
Creation, single centres of, ii. 143
Crinum capense and revolutum, ii.
12
Croll, Mr., on subaerial denudation,
ii. 59, 62
, on the age of our oldest forma-
tions, ii. 89
, on alternate Glacial periods in
the North and South, ii. 168-169
Crosses, reciprocal, ii. 20
Crossing of domestic animals, impor-
tance in altering breeds, i. 46-47
, advantages of, i. 142-143
, unfavorable to selection, i. 149
Cruger, Dr., on Coryanthes, i. 267
Crustacea of New Zealand, ii. 172
Crustacean, blind, i. 197
air-breathers, i. 264
Crustaceans, their chelae, i. 327
Cryptocerus, i. 387
Ctenomys, blind, i. 196
Cuckoo, instinct of, i. 346, 357
Cunningham, Mr., on the flight of the
logger-headed duck, i. 193
Currants, grafts of, ii. 24
Currents of sea, rate of, ii. 152
Cuvier, on fossil monkeys, ii. 85
, Fred., on instinct, i. 347
Cyclostoma elegans, resisting salt wa-
ter, ii. 196
D
Dana, Prof., on blind cave-animals, i.
198
, on relations of crustaceans of
Japan, ii. 166
, on crustaceans of New Zealand,
ii. 172
INDEX
341
Dawson, Dr., on eozoon, it 91
De Candolle, Aug. Pyr., on struggle
for existence, i. 100
, on uinbelliferae, i. 206
, on general affinities, ii 237
De Candoile, Alph., on the variability
of oaks, i 85
« , on widely-ranging plants being
variable, i. 89
— , on naturalization, L 163
«= , on winged seeds, i. 207
— , on Alpine species suddenly be*
coining rare, i. 236
— . on distribution of plants with
large seeds, ii. 153
, on vegetation of Australia, ii.
175
, on fresh- water plants, ii. 183
, on insular plants, ii. 187
, on low plants, widely dispersed,
ii. 205
Degradation of rocks, ii. 58
Denudation, rate of, ii. 60
— of oldest rocks, ii. 91
of granitic areas, ii. 70
Development of ancient forms, ii. 123
Devonian system, ii. 120-121
Dianthus, fertility of crosses, ii. 18
Dimorphism in plants, i. 78; ii. 35
Dirt on feet of birds, ii. 156
Dispersal, mean3 of, ii. 148
during Glacial period, ii. 159
Distribution, geographical, ii 137
, means of, ii. 148
Disuse, effect of, under nature, i 193
Divergence of character, i. 158
Diversification of means for same gen-
eral purpose, i 266
Division, physiological, of labor, L
164
Dog, resemblance of jaw to that of
the Thylacinus, ii. 229
Dogs, hairless, with imperfect teeth,
i 37
Dogs descended from several wild
stocks, i. 46
, domestic instincts of, i. 354
, inherited civilization of, i. 354
, fertility of breeds together, ii. 16
, of crosses, ii. 41
, proportions of body in different
breeds, when young, ii. 257
Domestication, variation under, i 31
Double flowers, i. 386
Downing, Mr., on fruit-trees in Amer-
ica, i. 127
Dragon flies, intestines of, i. 255
Drift-timber, ii 153
Driver ant (Anomma), i 389
Drones killed by other bees, i. 282
Duck, domestic, wings of, reduced, i
36
— — , beak of, i. 312
, logger-headed, i. 244
Duckweed, ii. 182
Dugong. affinities of, ii 215
Dung-beetles with deficient tarsi, i
194
Dytiscus. ii. 183
E
Eael, Mr. W., on the Malay Archi-
pelago, ii. 194
Ears, drooping, in domestic arr'mala,
i 36
, rudimentary, ii 270
Earth seeds in roots of trees, ii 153
charged with seeds, ii 156
Echinodermata, their pediceiiarie, i
324
Eeiton, i 387
Economy of organization, i 207
Edentata, teeth and hair, L 205
, fossil species of, ii 298
Edwards, Milne, on physio-logical divi-
sion of labor, i 164
, on gradations of structure, L
270
INDEX
Goethe, on compensation of growth,
i. 207
Gomphia oleseformis, i. 298
Gooseberry, grafts of, ii. 24
Gould, Dr. Aug. A., on land-shells, ii.
195
, Mr., on colors of birds, i. 191
, on instincts of cuckoo, i. 360
, on distribution of genera of
birds, ii. 204
Gourds, crossed, ii. 43-44
Graba, on the Uria lacrymas, i. 136
Grafting, capacity of, ii. 23-26
Granite, areas of denuded, ii. 70
Grasses, varieties of, i. 161
Gray, Dr. Asa, on the variability of
oaks, i. 85
, on man not causing variability,
i. 120
, on sexes of the holly, i. 140
, on trees of the United States, i.
147
, on naturalized plants in the
United States, i. 163
, on rarity of intermediate vari-
eties, i. 238
, on aestivation, i. 298
, on Alpine plants, ii. 159
— , Dr. J. E., on striped mule, i. 225
Grebe, I 248
Grimm, on asexual reproduction, ii.
249
Groups, aberrant, ii. 236-237
Grouse, colors of, i. 127
, red, a doubtful species, i. 82
Growth, compensation of, i. 207
Gunther, Dr., on flat-fish, i. 319
. , on prehensile tails, i. 321
, on the fishes of Panama, ii.
139
, on the range of fresh -water
fishes, ii. 181
mm — , on the limbs of Lepidosiren, i;.
268
H
Haast, Dr., on glaciers of New Zea-
land, ii. 167
Habit, effect of, under domestication,
i. 36
, effect of, under nature, i. 194
, diversified, of same species, i.
245
Haeckel, Prof., on classification and
the lines of descent, ii. 240
Hair and teeth, correlated, i. 205
Halitherium, ii. 115
Harcourt, Mr. E. V., on the birds of
Madeira, ii. 189
Hartung, M., on bowlders in the
Azores, ii. 157
Hazel-nuts, ii. 151
Hearne, on habits of bears, i. 246
Heath, changes in vegetation, caused
by cattle, i. 110
Hector, Dr., on glaciers of New Zea-
land, ii. 167
Heer, Oswald, on ancient cultivated
plants, i. 44
, on plants of Madeira, i. 154
Helianthemum, i. 298
Helix pomatia, ii. 196
, resisting salt water, ii. 196
Helmholtz, M., on the imperfection of
the human eye, i. 281
Helosciadium, ii. 151
Hemionus, striped, i. 227
Hensen, Dr., on the eyes of Cephalo-
pods, i. 263
Herbert, "W., on struggle for existence,
i. 100
, on sterility of hybrids, ii 12
Hermaphrodites crossing, i. 142-143
Heron eating seed, ii. 185
Heron, Sir R., on peacocks, i. 132
Heusinger, on white animals poisoned
by certain plants, i. 37
Hewitt, Mr., on sterility of first
crosses, ii. 29
INDEX
345
Hildebrand, Prof., on the self-sterility
of Corydalis, ii. 12-13
Hilgendorf, on intermediate varieties,
ii. 72
Himalaya, glaciers of, ii. 167
, plants of, ii. 171
Hippeastrum aulicum, ii. 13
Hippocampus, i. 322
Hofmeister, Prof., on the movements
of plants, i. 335
Holly-trees, sexes of, i. 138
Hooker, Dr., on man not causing
variability, i. 120
, on trees of New Zealand, i.
147
, on acclimatization of Himalayan
trees, i. 200
— , on flowers of umbelliferae, i. 205
— , on the position of ovules, i.
295
, on glaciers of Himalaya, ii. 167
, on glaciers of the Lebanon, ii.
167
, on plants of Tierra del Fuego,
ii. 169-170
, on plants of mountains of Fer-
nando Po, ii. 170
, on Australian plants, ii. 171, 199
, on algae of New Zealand, it
171-172
, on vegetation at the base of the
Himalaya, ii. 173
, on relations of flora of America,
ii. 176
, on flora of the Antarctic lands,
ii. 177, 198
, on the plants of the Galapagos,
ii. 190, 197
Hooks, on palms, i. 273
, on seeds, on islands, ii. 190
Hopkins, Mr., on denudation, ii. 69
Hornbill, remarkable instinct of, i. 392
Horns, rudimentary, ii. 270
Horse, fossil, in La Plata, ii. 103
Horse, proportions of, when young,
ii. 257
Horses destroyed by flies in Paraguay,
i. 111-112
, striped, i. 225
Horticulturists, selection applied by,
i. 60
Huber, on cells of bees, i. 376
, P., on reason blended with in-
stinct, i. 347
, on habitual nature of instincts,
i. 347
, on slave-making ants, i. 364
, on Melipona domestica, i. 371
Hudson, Mr., on the Ground-Wood-
pecker of La Plata, i. 247
, on the Molothrus, i. 361
Humble-bees, cells of, i. 370
Hunter, J., on secondary sexual char-
acters, i. 211
Hutton, Captain, on crossed geese, ii. 16
Huxley, Prof., on structure of her-
maphrodites, i. 147
, on the affinities of the Sirenia,
ii. 115
, on forms connecting birds and
reptiles, ii. 116
, on homologous organs, ii. 247
, on the development of aphis,
ii. 254
Hybrids and mongrels compared, ii.
45
Hybridism, ii. 7
Hydra, structure of, i. 255-256
Hymenoptera, fighting, i. 132
Hymenopterous insect, diving, L 248
Hyoseris, i. 297
1
Ibla, i. 209
Icebergs transporting seeds, ii. 157
Increase, geometrical ratio of, i. 101
Individuals, numbers favorable to se-
lection, i. 148
346
INDEX
Individuals, many, whether simulta-
neously created, ii. 147
Inheritance, laws of., i. 39
, at corresponding ages, i. 39, 128
Insects, color of, fitted for their sta-
tions, i. 126
, seaside, colors of, i. 191
, blind, in caves, i. 197
, luminous, i. 262
, their resemblance to certain ob-
jects, i. 309
, neuter, i. 387
Instinct, i. 346
, not varying simultaneously with
structure, i. 384-385
Instincts, domestic, i. 352
Intercrossing, advantages of, i. 143;
ii. 33
Islands, oceanic, ii. 186
Isolation favorable to selection, i. 151
J
Japan, productions of, ii. 166
Java, plants of, ii. 171
Jones, Mr. J. M., on the birds of
Bermuda, ii. 189
Jonrdain, M., on the eye-spots of star-
fishes, i. 251
Jukes, Prof., on subaerial denudation,
ii. 59
Jussieu, on classification, ii. 218
K
Kentucky, caves of, i. 197
Kergueien Land, flora of, ii. 177, 198
Kidney-bean, acclimatization of, i.
202-203
Kidneys of birds, i. 204
Kirby, on tarsi deficient in beetles, t
194
Knight, Andrew, on cause of varia-
tion, i. 31
Kolreuter, on intercrossing, i. 142
, on the barberry, i. 144
, on sterility of hybrids, ii. 9-10
, on reciprocal crosses, ii. 20
, on crossed varieties of nicotian a,
ii. 44
, on crossing male and hermaph-
rodite flowers, ii. 265
Ii
Lamarck, on adaptive characters, ii
227
Lancelot (Amphioxus), i. 179
, eyes of, i. 253
Landois, on the development of the
wings of insects, i. 257-258
Land-shells, distribution of, ii. 195
, of Madeira, naturalized, ii. 202
, resisting salt water, ii. 196
Languages, classification of, ii. 223
Lankester, Mr. E. Bay, on longevity,
i. 289
, on homologies, ii. 246
Lapse, great, of time, ii. 57
Larvae, ii. 250-252
Laurel, nectar secreted by the leaves,
i. 137
Lauren tian formation, ii. 90
Laws of variation, i. 190
Leech, varieties of, i. 115
Leguminosae, nectar secreted by
glands, i. 137-138
Leibnitz's attack on Newton, ii. 304
Lepidosiren, i. 154; ii. 117
, limbs in a nascent condition, ii.
267-268
Lewes, Mr. €r. H., on species not
having changed in Egypt, L 289
, on the Salamandra atra, ii. 266
, on many forms of life having
been at first evolved, ii. 310
Life, struggle for, i. 100
Lingula. Silurian, 'i. 89
INDEX
347
Linnaeus, aphorism of, ii. 214
Lion, mane of, i. 132
, young of, striped, ii. 251
Lobelia fulgens, i. 112, 145
, sterility of crosses, ii. 12
Lock wood, Mr., on the ova of the
Hippocampus, i. 322
Locusts transporting seeds, ii. 155
Logan, Sir W., on Laurentian forma-
tion, ii. 91
Lowe, Rev. R. T., on locusts visiting
Madeira, ii. 155
Lowness of structure connected with
variability, i. 209
, related to wide distribution, ii.
205
Lubbock, Sir J., on the nerves of
coccus, i. 77
, on secondary sexual characters,
i. 218
, on a diving hymenopterous in-
sect, i. 248
, on affinities, ii. 79
, on metamorphoses, ii. 248, 252
Lucas, Dr. P., on inheritance, i. 38
, on resemblance of child to
parent, ii. 49
Lund and Clausen, on fossils of Brazil,
ii. 128
Lyell, Sir ft, on the struggle for ex-
istence, i. 100
, on modern changes of the earth,
i. 141-142
, on terrestrial animals not having
been developed on islands, i. 308
, on a carboniferous land-shell,
ii. 65
— , on strata beneath Silurian sys-
tem, ii. 90
-= — , on the imperfection of the geo-
logical record, ii. 94
— — , on the appearance of species,
ii. 94
— . on Barrande's colonies, ii. 97-98
| Lyell, Sir ft, on tertiary formations of
Europe and North America, ii. 108
, on parallelism of tertiary forma-
tions, ii. 113
, on transport of seeds by ice-
bergs, ii. 157
, on great alterations of climate,
ii. 178
, on the distribution of fresh-
water shells, ii. 183
, on land-shells of Madeira, ii. 202
Lyell and Dawson, on fossilized trees
in Nova Scotia, ii. 76
Lythrum salicaria, trimorphic, ii. 38
HI
Macleay, on analogical characters,
ii. 227
Macrauchenia, ii. 115
M'Donnell, Dr., on electric organs,
i. 260
Madeira, plants of, i. 154
, beetles of, wingless, i. 195
, fossil land-shells of, ii. 129
, birds of, ii. 189
Magpie tame in Norway, i. 352
Males fighting, i. 131-132
Maize, crossed, ii. 43
Malay Archipelago compared with Eu-
rope, ii. 80-81
, mammals of, ii. 194
Malm, on flat-fish, i. 317-318
Malpighiacese, small imperfect flowers
of, i. 296
, perfect and degraded flowers of,
ii. 218
Mammas, their development, i. 322
, rudimentary, ii. 265
Mammals, fossil, in secondary forma-
tion, ii. 85
, insular, ii. 192
Man, origin of, ii. 314
Manatee, rudimentary nails of, ii. 270
348
INDEX
Marsupials of Australia, i. 164
, structure of their feet, ii. 241
, fossil species of, ii. 128
Martens, M.s experiment on seeds, ii
152
Martin, Mr. W. ft, on striped mules,
i. 22^
Masters, Dr., on Saponaria, i. 298
Matteueci, on the electric organs of
rays, i. 260
Matthioia, reciprocal crosses of, ii. 21
Maurandia, i. 334-335
Means of dispersal, ii. 148
Melipona domes tica, i, 371
Merrell, Dr., on the American cuckoo,
i. 357-358
Motamorphism of oldest rocks, ii. 91
Mice destroying bees, i. 113
, acclimatization of, i. 201
- — , tails of, i. 321
Miller, Prof., on the cells of bees, i.
372, 377
Mirabiiis, crosses of, ii. 20
Missel -thrush, i. 116
Mistletoe, complex relations of, i. 27
Mivart, Mr., on the relation of hair
and teeth, i. 205
=- — , on the eyes of cephaiopods, i.
263
, various objections to Natural
Selection, i. 301
, on abrupt modifications, i. 340
, or* the resemblance of the mouse
ano antechinus, ii. 228
Mocking-thrush of the Galapagos, ii.
202
Modification of species not abrupt, ii.
308
Moles, blind, i. 196
Molothrus, habits of, i. 361
Mongrels, fertility and sterility of, ii.
40
— and hybrids compared, ii. 46
Monkeys, fossil, ii. 85
j Monachanthus, ii 225
i Mods, Van, on the origin of fruit-
trees, i. 56
Monstrosities, i. 74
Moqum-Tandon, on sea-side plants, i.
191
Morphology, ii. 241
Morren, on the leaves of Oxalis, i. 335
Moths, hybrid, ii. 15
Mozart, musical powers of, i, 348
Mud, seeds in, ii. 184
Mules, striped, i. 227
Muller, Adolf, on the instincts of the
cuckoo, i. 358
Muller, Dr. Ferdinand, on Alpine Aus-
tralian plants, ii. 171
Muller, Fritz, on dimorphic crustace-
ans, i. 78, 390
, on the lancelet, i. ISO
, on air-breathing crustaceans, i.
264-265
, on climbing plants, i. 334
— — , on the self-sterility of orchids,
ii. 13
— , on embryology in relation to
classification, ii. 219
— — , on the metamorphoses of crus-
taceans, ii. 254, 262
, on terrestrial and fresh-water
organisms not undergoing any meta-
morphosis, ii. 260
Multiplication of species net indefinite,
j. 182-183
Murchison, Sir R., on the formations
of Russia, ii. 66
, on azoic formations, ii. 90
, on extinction, ii. 102
Murie, Dr., on the modification of the
skull in old age, i. 259
Murray, Mr. A. , on cave-insects, i. 199
Mustela vison, i 242
Myanthus, ii. 225
Myrmecocystus (Mexican), i. 387
Myrmica, eyes of, i. 389
INDEX
849
Nageli, on morphological characters,
i. 292
Nails, rudimentary, ii. 270
Nathusius, Von, on pigs, i. 275
Natural history, future progress of, ii.
311
selection, i. 120
system, ii. 213
Naturalization -of forms distinct from
the indigenous species, i. 162-163
Naturalization in New Zealand, L
281
Naudin, on analogous variations in
gourds, i. 221
— , on hybrid gourds, ii. 44
— , on reversion, ii. 47
Nautilus, Silurian, ii. 89
Nectar of plants, i. 137
Nectaries, how formed, i. 138
Nelumbium luteum, ii. 185
Nests, variations in, i. 351, 383, 392
Neuter insects, i. 387-388
Newman, Col., on humble-bees, i. 113
New Zealand, productions of, not per-
fect, i. 281
, naturalized products of, ii. 127
, fossil birds of, ii. 129
, glaciers of, ii. 167
, crustaceans of, ii. 172
, algae of, ii. 172
, number of plants of, ii. 187
, flora of, ii. 198
Newton, Sir I., attacked for irreligion,
ii. 304
, Prof., on earth attached to a
partridge's foot, ii. 156
Nicotiana, crossed varieties of, ii. 45
, certain species very sterile, ii. 20
Nitsche, Dr., on the Polyzoa, i. 328
Noble, Mr., on fertility of Rhododen-
dron, ii. 14
Nodules, phosphatic, in azoic rocks,
ii. 90
O
Oaks, variability of, i. 85
Onites, appelles, i. 194
Ononis, small imperfect flowers of, i.
295
Orchids, fertilization of, i. 267
, the development of their Sowers,
i. 330
, forms of, ii. 225
Orchis, pollen of, i. 262
Organization, tendency to advance, i.
176
Organs of extreme perfection, i. 250
, electric, of fishes, i. 260
of little importance, i. 271
, homologous, ii. 242
, rudiments of, and nascent, ii.
264-265
Ornithorhynchus, i. 154; ii. 217
— , mammas of, i. 323
Ostrich not capable of flight, i. 307
, habit of laying eggs together, i.
363
, American, two species of, ii.
140
Otter, habits of, how acquired, i. 242
Ouzel, water, i. 248
Owen, Prof., on birds not flying, i. 193
, on vegetative repetition, i. 210
— , on variability of unusually de-
veloped parts, i. 211
, on the eyes of fishes, i. 253
, on the swimbladder of fishes, i
257
, on fossil horse of La Plata, ii.
103
— , on generalized form, ii 114
, on relation of ruminants and
pachyderms, ii. 115
— , on fossil birds of New Zealand,
ii. 128
- — , on succession of types, ii. 129
. on affinities of the dogong, ii
315
850
INDEX
Owen, Prof., on homologous organs,
ii. 243
, on the metamorphosis of cepha-
lopoda, ii. 254
P
Pacific Ocean, faunas of, ii. 139
Pacini, on electric organs, i. 262
Paley, on no organ formed to give
pain, i. 280
Pallas, on the fertility of the domes-
ticated descendants of wild stocks,
ii. 16
Paim with hooks, i. 273
Papaver bracteatum, i. 298
Paraguay, cattle destroyed by flies, L
111-112
Parasites, i. 361
Partridge, with ball of earth attached
to foot, ii. 156
Parts greatly developed, variable, i.
210
Parus major, i. 246
Parfsiflora, ii. 12
Peaches in United States, i. 12?
Pear, grafts of, ii. 24
Pedieellariae, i. 324-325
Pelagomium, flowers of, L 206
, sterility of, ii. 13
Pelvis of women, i. 204
Peioria, i. 206
Period, glacial, ii. 159
Petrels, habits of, L 24?
Phasi&aus colchieus, and ?. torquatus,
fertility of hybrids, ii. 15
Pheasant, yoking, wild, i. 356
Pictet, Prof., on groups of species
suddenly appearing, ii. 83
— — , on rate of organic change, ii. 97
— — , on continuous succession of gen-
en*, iL 100
— , on change in latest tertiary
forms, ii. 77
Pictet, Prof., on close alliance of fosaila
in consecutive formations, ii. 122
— , on early transitional links, ii.
84
Pierce, Mr., on varieties of wolves, i.
134
Pigeons with feathered feet and akin
between toes, i. 37-38
, breeds described, and origin of,
i. 47
, breeds of, hew produced, i.
67-68, 70
, tumbler, not being able to get
out of egg, i. 129
, reverting to blue color, i. 223
, instinct of tumbling, L 354
, young of, ii. 257-258
Pigs, black, not affected by the paint
root, i. 37
, modified by want of exercise, u
275
Pincers of crustaceans, i. 327
Pistil, rudimentary, ii. 265
Plants, poisonous, not affecting cer-
tain colored animals, i. 37
, selection applied to, i. 65
gradual improvement oL i. 65-
66
, not improved in barbarous coun-
tries, i. 66
— , dimorphic, i, 78 ; U. 35
— — , destroyed by insects, i. 106
, in midst of range, have to strug«
gle with other plants, L 117-118
, nectar of, i. 137
, fleshy, on sea-shores, i. 191
, climbing, i. 256, 332
— — — , fresh-water, distribution of, ii
183
,* low in scale, widely distributed^
ii. 205
Pieuronectidse, their structure, i. 317
Plumage, laws of change in sexes of
birds, i. 133
INDEX
351
Plums in the United States, i. 127
Pointer dog, origin of, i. 63
, habits of, i. 355
Poison not affecting certain colored
animals, i. 37
, similar effect of, on animals and
plants, ii. 309
Pollen of fir-trees, i. 283
transported by various means, i.
267, 278
Pollinia, their development, i. 331
Polyzoa, their avicularia, i. 328
Poole, Col., on striped hemionus, i.
227
Potamogeton, ii. 185
Pouchet, on the colors of flat-fish, i.
320
Pre3twich, Mr. , on English and French
eocene formations, ii. 113
Proctotrupes, i. 248
Proteolepas, i. 209
Proteus, i. 199
Psychology, future progress of, ii. 314
Pyrgoma, found in the chalk, ii. 87
<*
Quagga, striped, i. 227
Quatrefages, M., on hybrid moths, ii.
15
Quercus, variability of, i. 85
Quince, grafts of, ii. 24
R
Rabbits, disposition of young, i. 355
Races, domestic, characters of, i. 41
Race horses, Arab, i. 64
, English, ii. 148
Radcliffe, Dr., on the electrical organs
of the torpedo, i. 261
Ramond, on plants of Pyrenees, ii. 161
Ramsay, Prof., on subaerial denuda-
tion, ii. 59
Science — •
Ramsay, Prof., on faults, ii. 60-61
, on thickness of the British for-
mations, ii. 61-62
Ramsay, Mr. , on instincts of cuckoo,
i. 360
Ratio of increase, i. 101
Rats supplanting each other, i. 116
, acclimatization of, i. 201
, blind, in cave, i. 197
Rattlesnake, i. 280
Reason and instinct, i. 346
Recapitulation, general, ii. 276
Reciprocity of crosses, ii. 20
Record, geological, imperfect, ii. 54
Rengger, on flies destroying cattle, i.
Ill
-Reproduction, rate of, i. 102
Resemblance, protective, of insects, i.
309-310
to parents in mongrels and hy-
brids, ii. 47
Reversion, law of inheritance, i. 40
, in pigeons, to blue color, i. 223
Rhododendron, sterility of, ii. 13-14
Richard, Prof., on Aspicarps, ii. 218
Richardson, Sir J., on structure of
squirrels, i. 242
, on fishes of the southern hemi-
sphere, ii. 172
Robinia, grafts of, ii. 25
Rodents, blind, i. 196
Rogers, Prof., Map of North America,
ii. 70-71
Rudimentary organs, ii. 264
Rudiments important for classification,
ii. 216
Riitimeyer, on Indian cattle, i. 45 ; ii.
16
S
Salamandra ata, ii. 266
Saliva used in nests, i. 383
Salvia, Mr., on the beaks of ducks, i.
314
L. 1—32
352
INDEX
Sageret, on grafts, ii. 24
Salmons, males righting, and hooked
jaws of, i. 132
Salt water, how far injurious to seeds,
ii. 150
not destructive to land-shells, ii.
196
Salter, Mr., on early death of hybrid
embryos, ii. 29
Saurophagus sulphuratus (tyrant fly-
catcher), i. 246
Schacht, Prof. , on Phyllotaxy, i. 296
Schiddte, on blind insects, i. 198
, on flat-fish, i. 317
Schlegel, on snakes, i. 204
Schobl, Dr., on the ears of mice, i.
294
Scott, Mr. J., on the self -sterility of
orchids, ii. 13
, on the crossing of varieties of
verbascum, ii. 44
Sea-water, how far injurious to seeds,
ii. 150
not destructive to land-shells, ii.
196
Sebright, Sir J., on crossed animals,
i. 47
Sedgwick, Prof. , on groups of species
suddenly appearing, ii. 83
Seedlings destroyed by insects, i. 106
Seeds, nutriment in, i. 117
, winged, i. 207
, means of dissemination, i. 266-
267, 278; ii. 154
, power of resisting salt water, ii.
150-151
, in crops and intestines of birds,
ii. 154-155
, eaten by fish, ii. 154, 184
, in mud, ii. 184
, hooked, on islands, ii. 190
Selection of domestic products, i. 57
, principle not of recent origin, i.
62
Selection, unconscious, i. 62-63
, natural, i. 120
, sexual, i. 131
, objections to term, i. 121-122
, natural has not induced sterility,
ii. 26
Sexes, relations of, i. 131-132
Sexual characters variable, i. 217
selection, i. 131
Sheep, Merino, their selection, i. 59
, two sub-breeds, unintentionally
produced, i. 64
, mountain varieties of, i. 115
Shells, colors of, i. 191
, hinges of, i. 266
• , littoral, seldom imbedded, ii. 64
, fresh-water, long retain the
same forms, ii. 124
, fresh-water, dispersal of, ii. 182
, of Madeira, ii. 189
, land, distribution of, ii. 189
, land, resisting salt water, ii. 196
Shrew-mouse, ii. 228
Silene, infertility of crosses, ii. 19
Silliman, Prof., on blind rat, i. 197
Sirenia, their affinities, ii. 115
Sitaris, metamorphosis of, ii. 262
Skulls of young mammals, i. 274; ii.
245
Slave-making instinct, i. 364
Smith, Colonel Hamilton, on striped
horses, i. 226
, Mr. Fred. , on slave-making ants,
i. 365
, on neuter ants, i. 388
Smitt, Dr., on the Polyzoa, i. 328
Snake with tooth for cutting through
egg-shell, i. 361
Somerville, Lord, on selection of sheep,
i. 59
Sorbus, grafts of, ii. 25
Sorex (shrew-mouse), ii. 228
Spaniel, King Charles's breed, i. 63
Specialization of organs, i. 178
INDEX
853
Species, polymorphic, i. *7*7
, dominant, i. 90
, common, variable, i. 89
in large genera variable, i. 92
, groups of, suddenly appearing,
ii. 83-88
beneath Silurian formations, ii.
90
successively appearing, ii. 96
changing simultaneously through-
out the world, ii. 108
Spencer, Lord, on increase in size of
cattle, i. 64
, Herbert, Mr., on the first steps
in differentiation, i. 181
, on the tendency to an equilib-
rium in all forces, ii. 34
Sphex, parasitic, i. 363
Spiders, development of, ii. 254
Sports in plants, i. 35
Sprengel, 0. C, on crossing, i. 142
— — , on ray-florets, i. 206
Squalodon, ii. 115"
Squirrels, gradations in structure, i.
242
Staffordshire, heath, changes in, i. 110
Stag-beetles, fighting, i. 132
Starfishes, eyes of, i. 251
, their pedicellariae, i. 326
Sterility from changed conditions of
life, i. 34
of hybrids, ii. 9
, laws of, ii. 17
, causes of, ii. 26
, from unfavorable conditions, ii.
32
not induced through natural se-
lection, ii. 27
St. Helena, productions of, ii. 187
St.-Hilaire, Aug., on variability of cer-
tain plants, i. 298
, on classification, ii. 218
St. John, Mr., on habits of cats, i. 353
Sting of bee, i. 282
Stocks, aboriginal, of domestic ani-
mals, i. 45-46
Strata, thickness of, in Britain, ii. 61
Stripes on horses, i. 225
Structure, degrees of utility of, i. 276
Struggle for existence, i. 98
Succession, geological, ii. 96
of types in same areas, ii. 128
Swallow, one species supplanting an-
other, i. 116
Swaysland, Mr., on earth adhering to
the feet of migratory birds, ii. 156
Swifts, nests of, i. 383
Swimbladder, i. 256
Switzerland, lake habitations of, i. 44
System, natural, ii. 213
T
Tail of giraffe, i. 271
of aquatic animals, i. 272
, prehensile, i. 321
, rudimentary, ii. 270
Tanais, dimorphic, i. 78
Tarsi, deficient, i. 194
Tausch, Dr., on umbelliferae, i. 297
Teeth and hair correlated, i. 205
, rudimentary, in embryonic calf,
ii. 265, 302
Tegetmeier, Mr., on cells of bees, i.
373, 379
Temminck, on distribution aiding
classification, ii. 220
Tendrils, their development, i. 332
Thomson, SirW., on the age of the
habitable world, ii. 89
, on the consolidation of the crust
of the earth, ii. 284
Thouin, on grafts, ii. 25
Thrush, aquatic species of, i. 248
, mocking, of the Galapagos, ii.
201-202
, young of, spotted, ii. 251
, nest of, i. 392
354
INDEX
Thuret, M., on crossed fuci, ii. 21
Thwaites, Mr., on acclimatization i.,
200
Thylacinus (Tasmanian wolf), ii. 229
Tierra del Fuego, dogs of, i. 355-356
■ , plants of, ii. 177
Timber, drift, ii. 153
Time, lapse of, ii. 57
by itself not causing modifica-
tion, i. 150
Titmouse (Parus major), i. 246
Toads on islands, ii. 191
Tobacco, crossed varieties of, ii. 44
Tomes, Mr., on the distribution of
bats, ii. 193
Transitions in varieties rare, i. 234
Traquair, Dr., on flat-fish, i. 319
Trautschold, on intermediate varieties,
ii. 72
Trees on islands belong to peculiar
orders, ii. 191
with separated sexes, i. 146
Trifolium pratense, i. 113, 140
repens, i. 113
incarnatum, i. 140
Trigonia, ii. 106
Trilobites, ii 89
, sudden extinction of, ii. 107
Trimen, Mr., on imitating-insects, ii.
233
Trimorphism in plants, i. 78; ii. 35
Troglodytes, i. 392
Tucutuco, blind, i. 196
Tumbler pigeons, habits of, hereditary,
i. 354
Tumbler, short-faced, young of, ii. 258
Turkey-cock, tuft of hair on breast, i.
133
— — , naked skin on head, i. 274
, young of, instinctively wild, i.
356
Turnip and cabbage, analogous varia
tions of, i. 220-221
Type, unity of, i. 286-287
Types, succession of, in same areas,
ii. 128
Typotherium, ii. 115
U
Udders enlarged by use, i. 36
, rudimentary, ii. 265
Ulex, young leaves of, ii. 251
Umbelliferae, flowers and seeds of, i.
205-206
, outer and inner florets of, i.
296-297
Unity of type, i. 286-287
Uria lacrymans, i. 136
Use, effects of, under domestication,
i. 36
, effects of, in a state of nature,
i. 193
Utility, how far important in the con-
struction of each part, i. 275
V
Valenciennes, on fresh- water fish,
ii. 182
Variability of mongrels and hybrids,
ii. 45
Variation under domestication, i. 31
caused by reproductive system
being affected by conditions of life,
i. 34
under nature, i. 74
i laws of, i. 190
, correlated, i. 37, 203, 274
Variations appear at corresponding
ages, i. 39-40, 128
analogous in distinct species, i.
219
Varieties, natural, i. 73
, struggle between, i. 116
, domestic, extinction of, i. 158
, transitional, rarity of, i. 234
, when crossed, fertile, ii. 40
INDEX
355
Varieties, when crossed, sterile, ii. 43
, classification of, ii. 224
Verbascum, sterility of, ii. 12
, varieties of crossed, ii. 44
Yerlot, M., on double stocks, i. 386
Verneuil and d'Archiac, MM. de, on
the succession of species, ii. 110
Yibracula of the Polyzoa, I 328
Viola, small imperfect flowers of, i.
295
, tricolor, i. 113
Virchow, on the structure of the crys-
talline lens, i. 253
Virginia, pigs of, i. 127
Volcanic islands, denudation of, ii. 60
Vulture, naked skin on head, i. 273-
274
W
Wading-birds, ii. 184
"Wagner, Dr., on Cecidomyia, ii. 249
Wagner, Moritz, on the importance of
isolation, i. 151
"Wallace, Mr., on origin of species, i.
26
, on the limit of variation under
domestication, i. 72
, on dimorphic lepidoptera, i. 78,
390
, on races in the Malay Archipel-
ago, i. 81
, on the improvement of the eye,
t 253
, on the walking-stick insect, i.
311
, on laws of geographical distri-
bution, ii. 147
, on the Malay Archipelago, ii.
194
, on mimetic animals, ii. 233
"Walsh, Mr. B. D., on phytophagic
forms, i. 82-83
, on equal variability, i. 221
Water, fresh, productions of, ii. 180
Water-hen, i. 248
Waterhouse, Mr., on Australian mar-
supials, i. 164
, on greatly developed parts being
variable, i. 210
, on the cells of bees, i. 370
, on general affinities, ii. 236
Water-ouzel, i. 248
Watson, Mr. H. C, on range of varie-
ties of British plants, i. 80, 95
, on acclimatization, i. 158
, on flora of Azores, ii. 157
, on Alpine plants, ii. 161
, on rarity of intermediate varie-
ties, i. 238
, on convergence, i. 182
, on the indefinite multiplication
of species, i. 183
Weale, Mr., on locusts transporting
seeds, ii. 155
Web of feet in water-birds, i. 249
Weismann, Prof., on the causes of
variability, i. 32
, on rudimentary organs, ii. 269
West Indian Islands, mammals of, ii.
194
Westwood, Mr., on species in large
genera being closely allied to oth-
ers, i. 94
, on the tarsi of Engidse, i. 218
, on the antennas of hymenopter-
ous insects, ii. 216
Whales, i. 311
Wheat, varieties of, i. 161
White Mountains, flora of, ii. 159
Whitaker, Mr., on lines of escarpment,
ii. 59 •
Wichura, Max, on hybrids, ii. 30, 32,
47
Wings, reduction of size, i. 194-195
of insects homologous with bran-
chiae, i. 257-258
, rudimentary, in insects, ii. 265
Wolf crossed with dog, i. 354
356
INDEX
Wolf of Falkland Isles, ii. 192
Wollaston, Mr., on varieties of insects,
i. 82
, on fossil varieties of shells in
Madeira, i. 88
, on colors of insects on sea-shore,
i. 191
, on wingless beetles, i. 195
, on rarity of intermediate varie-
ties, i. 238
, on insular insects, ii. 187
, on land-shells of Madeira natu-
ralized, ii. 202
Wolves, varieties of, i. 134
Woodcock with earth attached to leg,
ii. 156
Woodpecker, habits of, i. 246-247
, green color of, i. 273
Woodward, Mr., on the duration of
specific forms, ii. 72
, on Pyrgoma, ii. 87
, on the continuous succession of
genera, ii. 100
Woodward, Mr., on the succession of
types, ii. 129
World, species changing simultaneous-
ly throughout, ii. 108
Wrens, nest of, i. 392
Wright, Mr. Chauncey, on the giraffe,
i. 304-305
, on abrupt modifications, i. 343-
344
Wyman, Prof., on correlation of color
and effects of poison, i. 37
, on the cells of the bee, i. 372
Y
Youatt, Mr., on selection, i. 59
, on sub-breeds of sheep, i. 64
, on rudimentary horns in young
cattle, ii. 270
Z
Zanthoxylon, i. 298
Zebra, stripes on, i. 224-225
Zeuglodon, tertiary, ii. 115-116
1
Date j^ue
• — E
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I
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ftf T 1 ft
1933
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Library Bureau Cat. No. 1137
JUNE 1956
QH365 .02 1905
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iiiii
mi iF
3 5002 00030 5545
Darwin, Charles
Origin of species by means of natural se
/ QH
/ 36
/ 02
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