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THE ORIGIN OF SPECIES

THE ORIGIN
OF SPECIES

By
CHARLES DARWIN

LITERARY CLASSICS, INC.
New York

n

BOSTON COLLEGE UBRAW

CHESTNUT HILL, MA 021 67

PRINTED IN THE UNITED STATES OF AMERICA

CONTENTS

PAGE

Introduction •; •: . . . i

CHAPTER I

VARIATION UNDER DOMESTICATION

Causes of variability — Effects of habit and the use or disuse of parts — Corre-
lated 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 followed, their effects — Methodical and
unconscious selection — Unknown origin of our domestic productions — Cir-
cumstances favorable to man's power of selection 4

CHAPTER 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 2>

CHAPTEEClir'

STRUGGLE FOR EXISTENCE

Its bearing on natural selection — The term used in a wide sense — Geometrical
ratio of increase — Rapid increase of naturalized animals and plants —
Nature of the checks to increase — Competition universal — Effects of cli-
mate— Protection from the number of individuals — Complex relations of
all animals and plants throughout nature — Struggle for life most severe
between individuals and varieties of the same species: often severe between
species of the same genus — The Relation of organism to organism the most
important of all relations 39

CHAPTEf(^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 between individuals
of the same species — Circumstances favorable and unfavorable to the results
of Natural Selection, namely, intercrossing, isolation, number of individuals
— Slow action — Extinction caused by Natural Selection — Divergence of
Character, related to the diversity of inhabitants of any small area and to
naturalization — Action of Natural Selection, through Divergence of Char-
acter and Extinction, on the descendants from a common parent — Explains
the grouping of all organic beings — Advance in organization — Low forms

vi CONTENTS

PAGE

preserved — Convergence of character — Indefinite multiplication of species

— Summary 51

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, rudi-
mentary, 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 — Sum-
mary 89

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 diffi-
culty— Natura non facit saltum — Organs of small importance — Organs not
in all cases absolutely perfect — The law of Unity of Type and of the Condi-
tions of Existence embraced by the theory of Natural Selection . . . 112

CHAPTER VII

MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION

Longevity — Modifications not necessarily simultaneous — Modifications appar-
ently of no direct service — Progressive development — Characters of small
functional importance, the most constant — Supposed incompetence of nat-
ural selection to account for the incipient stages of useful structures —
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, developed from
one and the same source — Reasons for disbelieving in great and abrupt
modifications 140

CHAPTER VIII

INSTINCT

Instincts comparable with habits, but different in their origin — Instincts gradu-
ated— 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 — Difficulties of the
theory of the Natural Selection of instincts — Neuter or sterile insects — Sum-
mary 171

CONTENTS vii

PAGE

CHAPTER IX

HYBRIDISM

Distinction between the sterility of first crosses and of hybrids — Sterility various
in degree, not universal, affected by close interbreeding, removed by
domestication — Laws governing the sterility of hybrids — Sterility not a
special endowment, but incidental on other differences, not accumulated
by natural selection — Causes of the sterility of first crosses and of hybrids
— Parallelism between the effects of changed conditions of life and of
crossing — Dimorphism and Trimorphism — Fertility of varieties when
crossed and of their mongrel offspring not universal — Hybrids and mon-
grels compared independently of their fertility — Summary . . . , 196

c

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 palaeontological collec-
tions— 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 220

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 — Sunmiary of pre-
ceding and present chapter 242

CHAPTER XII

GEOGRAPHICAL DISTRIBUTION

Present distribution cannot be accounted for by differences in physical condi-
tions— Importance of barriers — Affinity of the productions of the same con-
tinent— 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 . . . 263

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

vui CONTENTS

PAGK

the inhabitants of islands to those of the nearest mainland — On coloniza-
tion from the nearest source with subsequent modification — Summary of
the last and present chapter 286

CHAPTER XIV

MUTUAL AFFINITIES OF ORGANIC BEINGS : MORPHOLOGY EMBRYOLOGY

RUDIMENTARY ORGANS

Classification, groups subordinate to groups — Natural system — Rules and diffi-
culties in classification, explained on the theory of descent with modification
— Classification of varieties — Descent always used in classification— Ana-
logical or adaptive characters — Affinities, general, complex and radiating
— Extinction separates and defines groups — Morphology, between members
of the same class, between parts of the same individual — Embryology, laws
of, explained by variations not supervening at an early age, and being
inherited at a corresponding age — Rudimentary organs, their origin ex-
plained— Summary 302

CHAPTER XV

RECAPITULATION AND CONCLUSION

Recapitulation of the objections to the theory of Natural Selection — Recapitula-
tion of the general and special circumstances in its favor — Causes of the
general 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 — Con-
cluding remarks 33^

Appendix 357

Glossary of Scientific Terms . 365

"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
supernatural 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 progress
or proficience in both." — Bacon : Advancement of Learning.

THE ORIGIN OF SPECIES

INTRODUCTION

When on board H. M. S. Beagle, as naturalist, I was much struck with cer-
tain facts in the distribution 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 latter chapters of this volume,
seemed to throw some light on the origin of species — that mystery of mys-
teries, as it has been called by one of our greatest philosophers. On my return
home it occurred to me, in 1837, that something might perhaps be made out
on this question by patiently accumulating and reflecting on all sorts of facts
which could possibly have any bearing on it. After five years work I allowed
myself to speculate on the subject, and drew up some short notes; these I en-
larged in 1 844 into a sketch of the conclusions which then seemed to me prob-
able: 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 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 can-
not here give references and authorities for my several statements ; and I must
trust to the reader reposing some confidence in my accuracy. No doubt errors
may have crept in, though I hope I have always been cautious in trusting to
good authorities alone. I can here give only the general conclusions at which
I have arrived, with a few facts in illustration, but which, I hope, in most
cases will suffice. No one can feel more sensible than I do of the necessity of
hereafter publishing in detail all the facts, with references, on which my 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 conclusions directly op-
posite to those at which I have arrived. A fair result can be obtained only by
fully stating and balancing the facts and arguments on both sides of each
question; and this is here impossible.

I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very many

2 THE ORIGIN OF SPECIES

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 conceivable that a naturalist,

o reflecting on the mutual affinities of organic beings, on their embryological
relations, their geographical distribution, geological succession, and other
such facts, might come to the conclusion that species had not been independ-
ently 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 innumerable species, inhabiting this world, have
been modified, so as to acquire that perfection of structure and coadaptation
which justly excites our admiration. Naturalists continually 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 true ; but it is pre-
posterous to attribute to mere external conditions, the structure, for instance,
of the woodpecker, with its feet, tail, beak, and tongue, so admirably adapted
to catch insects under the bark of trees. In the case of the mistletoe, which
draws its nourishment from certain trees, which has seeds that must be trans-
ported by certain birds, and which has flowers with separate sexes absolutely
requiring the agency of certain insects to bring pollen from one flower to the
other, it is equally preposterous to account for the structure of this parasite,
with its relations to several distinct organic beings, by the effects of external
conditions, or of habit, or of the volition of the plant itself.

It is, therefore, of the highest importance to gain a clear insight into the

y means of modificaticgi^and coadapta^tion. At the commencement of my obser-
vations it seemed to me probaBTe""tTiat a careful study of domesticated animals
and of cultivated plants would offer the best chance of making out this ob-
scure 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 variationjinder domestication, afforded the best and safest clew. I
may venture" to express my conviction of the high value of such studies, al"
though they have been very commonly neglected by naturalists.

From these considerations, I shall devote the first chapter of this abstract
to variation under domestication. We shall thus see that a large amount of
hereditary modification is at least possible; and, what is equally or more im-
portant, we shall see how great is the power of man in accumulating by his
selection successive slight variations. I will then pass on to the variability of
species in a state of nature; but I shall, unfortunately, be compelled to treat
this subject far too briefly, as it can be treated properly only by giving long
catalogues of facts. We shall, however, be enabled to discuss what circum-
stances are most favorable to yadatign. In the next chapter the struggle for
existence among~altl3T^nic beings throughout the world, which inevitably
follows from the high geometrical ratio of their increase, will be 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 pos-

INTRODUCTION 3

sibly survive; and as, consequently, there is a frequently recurring struggle
for existence, it follows that any being, if it vary however slightly in any
manner profitable to itself, under the complex and sometimes varying condi-
tions 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^a^TirodiSed 'fomi7

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 in-
evitably causes much extension of the less improved forms of life, and leads
to what I have called divergence of character. In the next chapter I shall dis-
cuss the complex and little known laws of variation. In the five succeeding
chapters, the most apparent and gi'avest difficulties in accepting the theory
will be given; namely, first, the difficulties of transitions, or how a simple
being or a simple organ can be changed and perfected into a highly developed
being or into an elaborately constructed organ; secondly, the subject of in-
stinct, or the mental powers of animals; thirdly, hybridism, or the infertility
of species and the fertility of varieties when intercrossed; and fourthly, the
imperfection of the geological record. In the next chapter I shall consider the
geological succession of organic beings throughout time; in the twelfth and
thirteenth, their geographical distribution throughout space ; in the fourteenth,
their classification or mutual affinities, both when mature and in an embryonic
condition. In the last chapter I shall give a brief recapitulation of the whole
work, and a few concluding remarks.

No one ought to feel surprise at much remaining as yet unexplained in rer
gard 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 five around us. V/ho 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 dispas-
sionate judgment of which I am capable, that the view which most natural-
ists until recently entertained^^d which I formerly entertained — namely,
that each species has been independently xreated — is erroneous. I am fully
convmced-Ararripecies 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 naturalselection has been themost important, but not the exclusive,
means oflnodification.

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 dis-
tinguishing between Varieties and Species — Origin of Domestic Varieties from
one or more Species — Domestic Pigeons, their Differences and Origin — Princi-
ples of Selection, anciently followed, their Effects — Methodical and Unconscious
Selection — Unknown Origin of our Domestic Productions — Circumstances favor-
able 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 gen-
erations 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 im-
provement or modification.

As far as I am able to judge, after long attending to the subject, the condi-
tions 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.
With 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 sometimes
arise under, as far as we can judge, dissimilar conditions; and, on the other
hand, dissimilar variations arise under conditions which appear to be nearly
uniform. The effects on the offspring are either definite or indefinite. They
may be considered as definite when all or nearly all the offspring of individ-
uals exposed to certain 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 have been thus definitely induced. There
can, however, be little doubt about many slight changes, such as size from the

VARIATION UNDER DOMESTICATION 5

amount of food, color from the nature of the food, thickness of the skin and
hair from 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 uniformly during a long series of generations on many in-
dividuals, all probably would be modified in the same manner. Such facts as
the complex and extraordinary outgrowths which variably follow from the in-
sertion 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 end-
less 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 differences 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 deserve to be called monstrosities arise; but mon-
strosities cannot be separated by any distinct line from slighter variations. All
such changes of structure, whether extremely slight or strongly marked, which
appear among many individuals living together, may be considered as the in-
definite 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,
according to their state of body or constitution, causing coughs or colds, rheu-
matism, or inflammation of various organs.

With respect to what I have called the indirect action of changed condi-
tions, namely, through the reproductive system being affected, we may infer
that variability is thus induced, partly from the fact of this system being ex-
tremely sensitive to any change in the conditions, and partly from the similar-
ity, as Kolreuter and others have remarked, between the variability which
follows from the crossing of distinct species, and that which may be observed
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
animals there are which will not breed, though kept in an almost free state
in their native country! This is generally, but erroneously, attributed to viti-
ated 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 sub-
ject ; but to show how singular the laws are which determine the reproduction
of animals under confinement, I may mention that carnivorous animals, even

6 THE ORIGIN OF SPECIES

from the tropics, breed in this country pretty freely under confinement, with
the exception of the plantigrades or bear family, which 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 con-
dition as in the most sterile hybrids. When, on the one hand, we see domesti-
cated animals and plants, though often weak and sickly, breeding freely under
confinement; and when, on the other hand, we see individuals, though taken
young from a state of nature perfectly tamed, long-lived and healthy (of
which I could give numerous instances) , yet having their reproductive system
so seriously aff"ected by unperceived causes as to fail to act, we need not be
surprised at this system, when it does act under confinement, acting irregu-
larly, and producing offspring somewhat unlike their parents. I may add that
as some organisms breed freely under the most unnatural conditions — for in-
stance, rabbits and ferrets kept in hutches — showing that their reproductive
organs are not easily affected; so will some animals and plants withstand do-
mestication 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 garden-
ers ; 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 many thousands
produced year after year on the same tree under uniform conditions, has been
known suddenly 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 comparison 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 com-
bustible 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 flower-
ing 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 proportion 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

VARIATION UNDER DOMESTICATION 7

they are habitually milked, in comparison with these organs in other coun-
tries, is probably another instance of the effects of use. Not one of our domes-
tic animals can be named which has not in some country drooping ears ; and
the view which has been suggested that the drooping is due to disuse of the
muscles of the ear, from the animals being seldom much alarmed, seems ,
\ probable. - -J

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 in the embryo or larva will
probably entail changes in the mature animal. In monstrosities, the correla-
tions between quite distinct parts are very curious; and many instances are
given in Isidore Geoffroy Saint-Hilaire's great work on this subject. Breeders
beUeve that long limbs are almost always accompanied by an elongated head.
Some instances of correlation are quite whimsical; thus cats which are en-
tirely 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 pecu-
liarities 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 certain plants, while dark-colored individuals
escape : Professor Wyman has recently communicated to me a good illustra-
tion of this fact; on asking some farmers in Virginia how it was that all their
pigs were black, they informed him that the pigs ate the paint-root (Lach-
nanthes), which colored their bones pink, and which caused the hoofs of all
but the black varieties to drop off: and one of the "Crackers" (i.e., Virginia
squatters) added, "We select the black members of a Utter for raising, as they-
alone have a good chance of living." Hairless dogs have imperfect teeth; long^
haired and coarse-haired animals are apt to have, as is asserted, long or many
horns ; pigeons with feathered feet have skin between their outer toes ; pigeons-
with short beaks have small feet, and those with long beaks large feet. Hence
if man goes on selecting, and thus augmenting, any peculiarity, he will almost
certainly modify unintentionally other parts of the structure, owing to the
mysterious laws of correlation.

The results of the various, unknown, or but dimly understood laws of vari'
ation 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 hya-
cinth, potato, even the dahlia, etc. ; and it is really surprising to note the end-
less points of structure and constitution in which the varieties and sub-vari-
eties 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 num-
ber and diversity of inheritable deviations of structure, both those of slight
and those of considerable physiological importance, are endless. Dr. Prosper
Lucas' treatise, in two large volumes, is the fullest and the best on this sub-
ject. No breeder doubts how strong is the tendency to inheritance; that like
produces like, is his fundamental belief; doubts have been thrown on this
principle only by theoretical writers. When any deviation of structure often

8 THE ORIGIN OF SPECIES

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 indi-
viduals, apparently exposed to the same conditions, any very rare deviation,
due to some extraordinary combination of circumstances, appears in the par-
ent— say, once among several million individuals — and it reappears in the
child, the mere doctrine of chances almost compels us to attribute its reap-
pearance 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 characteristics 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 importance to us, that peculiarities appearing in
the males of our domestic breeds are often transmitted, either exclusively or
in a much greater degree, to 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 caterpillar or cocoon stage. But hereditary diseases and some
other facts make me believe that the rule has a wider extension, and that,
when there is no apparent reason why a peculiarity should appear at any
particular age, yet that it does tend to appear in the offspring at the same
period at which it first appeared in the parent. I believe this rule to be of the
highest importance in explaining the laws of embryology. These remarks are
of course confined to the first appearance of the peculiarity, and not to 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, gradually but invariably revert in character to their aboriginal stock.
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
conclude that very many of the most strongly marked domestic varieties could

VARIATION UNDER DOMESTICATION 9

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 re-
version 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. Nevertheless, as our varieties certainly do occasionally revert in
some of their characters to ancestral forms, it seems to me not improbable
that if we could succeed in naturalizing, 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 defi-
nite action of the poor soil — that they would, to a large extent, or even wholly,
revert to the wild aboriginal stock. Whether or not the experiment would suc-
ceed is not of great importance for our line of argument; for by the experi-
ment itself the conditions of life are changed. If it could be shown that our
domestic varieties manifested a strong tendency to reversion — that is, to lose
their acquired characters, while kept under the same conditions and while
kept in a considerable body, so that free intercrossing might check, by blend-
ing together, any slight deviations in their structure, in such case, I grant that
we could deduce nothing from domestic varieties in regard to species. But
there is not a shadow of evidence in 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 number of gen-
erations, 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 per-
ceive in each domestic race, as already remarked, less uniformity of charac-
ter than in true species. Domestic races often have a somewhat monstrous
character; by which I mean, that, although differing from each other and
from other species of the same genus, in several trifling respects, they often
differ in an extreme degree in some one part, both when compared one with
another, and more especially when compared with 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 domestic 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

10 THE ORIGIN OF SPECIES

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, it 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 interesting; if, for instance, it could be
shown that the greyhound, bloodhound, terrier, spaniel, and bull-dog, which
we all know propagate their kind truly, were the offspring of any single spe-
cies, 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 the different quarters of the world. I do not believe,
as we shall presently see, that the whole amount of difference between the
several breeds of the dog has been produced under domestication; I believe
that a small part of the difference is due to their being descended from dis-
tinct 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 diverse climates. I do not dispute that these capacities have
added largely to the value of most of our domesticated productions ; but how
could a savage possibly know, when he first tamed an animal, whether it
would vary in succeeding generations, and whether it would endure other cli-
mates? 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 largely as the parent species of our existing do-
mesticated productions have varied.

In the case of most of our anciently domesticated animals and plants, it is
not possible to come to any definite conclusion, whether they are descended
from one or several 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-habita-
tions 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 commerce

VARIATION UNDER DOMESTICATION 11

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 advanced civilization, during
which the domesticated animals, 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 barbarian men 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 our domestic animals will probably 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, con-
stitution, and structure of the humped Indian cattle, it is almost certain
that they are descended from a different aboriginal stock from our European
cattle; and some competent judges believe that these 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 Professor Riitimeyer. 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 duck and wild rabbit.

The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some authors.
They believe that every race which breeds true, let the distinctive characters
be ever so slight, has had its wild prototype. At this rate there must have
existed at least a score of species of wild cattle, as many sheep, and several
goats, in Europe alone, and several even within Great Britain. One author
believes that there formerly existed 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 kingdoms 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

12 THE ORIGIN OF SPECIES

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
making distinct races by crossing has been greatly exaggerated. Many cases
are on record showing that a race may 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 every thing seems simple enough; but when these mongrels
are crossed one with another for several generations, hardly two of them are
alike, and then the difficulty of the task becomes 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 purchase 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 languages have been published on pigeons, and some of them are
very important as being of considerable antiquity. I have associated with
several eminent fanciers and have been permitted to join two of the London
Pigeon Clubs. The diversity of the breeds is something astonishing. Com-
pare the English carrier and the short-faced tumbler, and see the wonderful
difference in their beaks, entailing corresponding differences in their skulls.
The carrier, more especially the male bird, is also remarkable from the won-
derful development of the carunculated skin about the head; and this is ac-
companied by greatly elongated eyelids, very large external orifices to the
nostrils, and a wide gape of mouth. The short-faced tumbler has a beak in
outline almost like that of a finch; and the common tumbler has the singular
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 enor-
mously developed crop, which it glories in inflating, many well excite astonish-

VARIATION UNDER DOMESTICATION 13

ment 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 upper part of the oesophagus. The Jacobin has the
feathers so much reversed along the back of the neck that they form a hood ;
and it has, proportionally to its size, elongated wing and tail feathers. The
trumpeter and laugher, as their names express, utter a very different coo
from the other breeds. The fantail has thirty or even forty tail-feathers, in-
stead 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 curvature, differs enormously. The shape,
as well as the breadth and length of the ramus of the lower jaw, varies in a
highly remarkable manner. The caudal and sacral vertebrae 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 divergence and relative size of the two
arms of the furcula. The proportional width of the gape of mouth, the
proportional length of the eyeUds, 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 rela-
tive length of the wing and tail to each other and to the body; the relative
length of the leg and foot; the number of scutellae 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^ or
species, as he 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 pro-
ceeded from the rock-pigeon, they must have descended from at least seven

14 THE ORIGIN OF SPECIES

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 G. livia, with its geographical sub-species, only
two or three other species of rock-pigeons are known ; and these have not any
of the characters of the domestic breeds. Hence the supposed aboriginal
stocks must either still exist in the countries where they were originally
domesticated, and yet be unknown to ornithologists; and this, considering
their size, habits, and remarkable characters, seems improbable ; or they must
have become extinct in the wild state. But birds breeding on precipices, and
good flyers, are unlikely to be exterminated; and the common rock-pigeon,
which has the same habits with the domestic breeds, has not been ex-
terminated even on several of the smaller British islets, or on the shores of
the Mediterranean. Hence the supposed extermination of so many species
having similar habits with the rock-pigeon seems a very rash assumption.

Moreover, the several above-named domesticated breeds have been trans-
ported 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. 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 confinement.

An argument of great weight and applicable in several other cases, is,
that the above-specified breeds, though agreeing generally with the wild
rock-pigeon in constitution, 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 Golumbidae for a beak like
that of the English carrier, or that of the short-faced tumbler, or barb; for
reversed feathers like those of the Jacobin; for a crop like that of the pouter;
for tail-feathers like those of the fantail. Hence it must be assumed, not only
that half -civilized man succeeded in thoroughly domesticating several species,
but that he intentionally or by chance picked out extraordinarily abnormal
species; and further, that these very species have since all become extinct
or unknown. So many strange contingencies 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-blue, with white loins; but the Indian sub-
species, G. intermedia of Strickland, has this part bluish. The tail has a
terminal dark bar, with the outer feathers externally 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

VARIATION UNDER DOMESTICATION 15

black. These several marks do not occur together in any other species of the
whole family. Now, in every one of the domestic breeds, taking thoroughly
well-bred birds, all the above marks, even to the white edging of the outer
tail-feathers, sometimes concur perfectly developed. Moreover, when 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 f antails, 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-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 im-
probable 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 generations, for no instance is known of crossed
<iescendants 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 number of generations. These
two distinct cases of reversion are often confounded together by those who
liave 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 as-
certained with certainty of hybrids from two quite distinct species of animals
being perfectly fertile. Some authors believe that long-continued domestica-
tion 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 species closely related to each other. But to extend
_it so far as to suppose that species, aboriginally as distinct as carriers,

]6 THE ORIGIN OF SPECIES

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 Columbidae,
though so like the rock-pigeon in most respects — the occasional reappear-
ance 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 per-
fectly 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, firstly, that the wild G. 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 English carrier or a short-faced tumbler
differs immensely in certain characters from the rock-pigeon, yet that by
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 explanation 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 Romans, as we hear from Pliny, immense prices were given for
pigeons; "nay, they are come to this pass, that they can reckon up their
pedigree and race." Pigeons were much valued by Akber Khan, in India,
about the year 1600; never less than 20,000 pigeons were taken with the
court. "The monarchs of Iran and Turan sent him some very rare birds;"
and, continues the courtly historian, "His Majesty, by crossing the breeds,
which method was never practised before, has improved them astonishingly."
About this same period the Dutch were as eager about pigeons as were the
old Romans. The paramount importance of these considerations in explain-
ing the immense amount of variation which pigeons have undergone, will
likewise be obvious when we treat of selection. We 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.

VARIATION UNDER DOMESTICATION 17

I have discussed the probable origin of domestic pigeons at some, yet quite
insufficient, length; because when I first kept pigeons and watched the
several kinds, 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
rabbit fancier, who was not fully convinced that each main breed was
descended from a distinct species. Van Mons, in his treatise on pears and
apples, shows how utterly he disbelieves that the several sorts, for instance, a
Ribston-pippin or Codlin-apple, could ever have proceeded from the seeds
of the same tree. Innumerable other examples could be given. The explana-
tion, I think, is simple : from long-continued study they are strongly impressed
with the differences between the several races; and though they well know
that each race varies slightly, for they win their prizes by selecting such
slight differences, yet they ignore all general arguments, and refuse to sum
up in their minds slight differences accumulated during many successive
generations. May not those naturalists who, knowing far less of the laws
of inheritance than does the breeder, and knowing no more than he does of
the intermediate links in the long lines of descent, yet admit that many of
our domestic races are descended from the same parents — ^may they not
learn a lesson of caution, when they deride the idea of species in a state of
nature being lineal descendants of other species?

PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND THEIR EFFECTS

Let us now briefly consider the steps by which domestic 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 race-horse, a greyhound
and bloodhound, a carrier and tumbler pigeon. One of the most remarkable
features in our domesticated races is that we see in them adaptation, not
indeed to the animal's or plant's own good, but to man's use or fancy. Some
variations useful to him have probably arisen suddenly, or by one step ; many
botanists, for instance, believe that the fuller's teasel, with its hooks, which
cannot be rivalled by any mechanical contrivance, is only a variety of the
wild Dipsacus; and this amount of change may have suddenly arisen in a
seedling. So it has probably been with the turnspit dog; and this is known

18 THE ORIGIN OF SPECIES

to have been the case with the ancon sheep. But when we compare the dray-
horse and race-horse, the dromedary and camel, the various breeds of sheep
fitted either for cultivated land or mountain pasture, with the wool of one
breed good for one purpose, and that of another breed for another purpose;
when we compare the many breeds of dogs, each good for man in different
ways; when we compare the game-cock, so pertinacious in battle, with
other breeds so little quarrelsome, with "everlasting layers" which never
desire to sit, and with the bantam so small and elegant; when we compare
the host of agricultural, culinary, orchard, and flower-garden races of plants,
most useful to man at different seasons and for different purposes, or so
beautiful in his eyes, we must, I think, look further than to mere variability.
We cannot suppose that all the breeds were suddenly produced as perfect
and as useful as we now see them; indeed, in many cases, we know that this
has not been their history. The key is man's power of accumulative selec-
tion: 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 necessary to read
several of the many treatises devoted to this subject, and to inspect the
animals. Breeders habitually 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
agriculturist, not only to modify the character of his flock, but to change it
altogether. It is the magician's wand, by means of which he may summon
into life whatever form and mould he pleases." Lord Somerville, speaking
of what breeders have done for sheep, says : "It would seem as if they had
chalked out upon a wall a form perfect in itself, and then had given it
existence." 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 indis-
pensable even than in ordinary cases. If selection consisted merely in separat-

VARIATION UNDER DOMESTICATION 19

ing some very distinct variety, and breeding from it, the principle would
be so obvious as hardly to be worth notice; but its importance consists in
the great effect produced by the accumulation in one direction, during
successive generations, of differences absolutely inappreciable by an un-
educated 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 improvements; if he
wants any of these qualities, he will assuredly fail. Few would readily believe
in the natural capacity and years of practice requisite to become even a
skilful pigeon-fancier.

The same principles are followed by horticulturists ; but the variations are
here often more abrupt. No one supposes that our choicest productions have
been produced by a single variation from the aboriginal stock. We have
proofs that this 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 improve-
ment in many florists' flowers, when the flowers of the present day are com-
pared with drawings made only twenty or thirty years ago. When a race of
plants is once pretty well established, the seed-raisers do not pick out the
best plants, but merely go over their seed-beds, and pull up the "rogues," as
they call the plants that deviate from the proper standard. With animals
this kind of selection is, in fact, likewise followed; for hardly any one is so
careless as to breed from his worst animals.

In regard to plants, there is another means of observing the accumulated
effects of selection — namely, by comparing the diversity of flowers in the
different varieties of the same species in the flower garden; the diversity of
leaves, pods, or tubers, or whatever part is valued, in the kitchen-garden, in
comparison with the flowers of the same varieties; and the diversity of fruit
of the same species in the orchard, in comparison with the leaves and flowers
of the same set of varieties. See how different the leaves of the cabbage are,
and how extremely alike the flowers; how unlike the flowers of the 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 differences. It is not that the varieties which differ largely
in some one point do not differ at all in other points; this is hardly ever — I
speak after careful observation — perhaps never, the case. The law of corre-
lated 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

20 THE ORIGIN OF SPECIES

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 discovery. 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
"roguing" of plants by nurserymen. The principle of selection I find dis-
tinctly given in an ancient Chinese encyclopaedia. Explicit rules are laid
down by some of the Roman classical writers. From passages in Genesis, it
is clear that the 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. The savages in South Africa match their draught cattle by color, as
do some of the Esquimaux their team 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, indeed, 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 methodical selection, with a
distinct object in view, to make a new strain or sub-breed, superior to any-
thing of the kind in the country. But, for our purpose, a form of selection,
which may be called unconscious, and which results from every one trying
to possess and breed from the best individual animals, is more important.
Thus, a man who intends keeping pointers naturally tries to get as good dogs
as he can, and afterwards breeds from his own best dogs, but he has no wish
or expectation of permanently altering the breed. Nevertheless 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 lifetimes, 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, unchanged, 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' 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

VARIATION UNDER DOMESTICATION 21

believed, been chiefly eff"ected by crosses with the foxhound; but what con-
cerns us is, that the change has been effected unconsciously and gradually,
and yet so eff"ectually that, though the old Spanish pointer certainly came
from Spain, Mr. Borrow has not seen, as I am informed by him, any native
dog in Spain like our pointer.

By a similar process of selection, and by careful training, English race-
horses have come to surpass in fleetness and size the parent Arabs, so that
the latter, by the regulations for the Goodwood Races, 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 diff'er so greatly
from the rock-pigeon.

Youatt gives an excellent illustration of the effects of a course of selection
which may be considered as unconscious, in so far that the breeders could
never have expected or even wished, to produce the result which ensued —
namely, the production of the distinct strains. 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 domestic animals, yet any one animal
particularly useful to them, for any special purpose, would be carefully pre-
served during famines and other accidents, to which savages are so liable,
and such choice animals would thus generally leave more off"spring than
the inferior ones; so that in this case there would be a kind of unconscious
selection going on. We see the value set on animals even by the barbarians
of Tierra del Fuego, by their killing and devouring their old women, in
times of dearth, as of less value than their dogs.

In plants the same gradual process of improvement through the occasional
preservation of the best individuals, whether or not 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

22 THE ORIGIN OF SPECIES

cultivated in classical times, appears, from Pliny's description, to have been
a fruit of very inferior quality. I have seen great surprise expressed in horti-
cultural 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 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 any-
where find.

A large amount of change, thus slowly and unconsciously accumulated,
explains, as I believe, the well-known fact, that in a number of cases we
cannot recognize, 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 under-
stand how it is that neither Australia, the Gape of Good 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 circum-
stanced, individuals of the same species, having slightly different constitu-
tions or structure, would often succeed better in the one country than in
the other; and thus by a process of "natural selection," as will hereafter be
more fully explained, two sub-breeds might be formed. This, perhaps, partly
explains 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 fancies.
We can, I think, further understand the frequently abnormal character of
our domestic races, and likewise their differences being so great in external
characters, and relatively so slight in internal parts or organs. Man can
hardly select, or only with much difficulty, any deviation of structure ex-
cepting 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

VARIATION UNDER DOMESTICATION 23

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 four-
teen tail-feathers somewhat expanded, like the present Java fantail, or like
individuals of other and distinct breeds, in which as many as seventeen tail-
feathers have been counted. Perhaps the first pouter-pigeon did not inflate
its crop much more than the turbid now does the upper part of its oesophagus
- — a habit which is disregarded by all fanciers, as it is not one of the points
of the breed.

Nor let it be thought that some great deviation of structure would be
necessary to catch the fancier's eye; he perceives extremely small differences,
and it is in human nature to value any novelty, however slight, in one's
own possession. Nor must the value which would formerly 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 occa-
sionally 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 ex-
hibited as distinct at our poultry shows.

These views appear to explain what has sometimes been noticed, namely,
that w^e know hardly anything about the origin or history of any of our
domestic breeds. But, in fact, a breed, like a dialect of a language, can
hardly be said to have 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 have 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 countries, with little free com-
munication, 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 civilization of the
inhabitants — slowly to add to the characteristic features of the breed, what-
ever they may be. But the chance will be infinitely small of any record having
been preserved of such slow, varying, and insensible changes.

24 THE ORIGIN OF SPECIES

CIRCUMSTANCES FAVORABLE TO MAN S POWER 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 in-
creased by a large number of individuals being kept. Hence, number is of
the highest importance for success. On this principle Marshall formerly re-
marked, with respect to the sheep of part of Yorkshire, "As they generally
belong to poor people, and are mostly iw small lots, they never can be im-
proved." On the other hand, nurserymen, from keeping large stocks of the
same plant, are generally far more successful 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 devia-
tions 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 variations 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 important element in the
formation of new races — at least, in a country which is already stocked with
other races. In this respect enclosure of the land plays a part. Wandering
savages or the inhabitants of open plains rarely possess more than one breed
of the same species. Pigeons can be mated for life, and this is a great con-
venience to the fancier, for thus many races may be improved and kept true,
though mingled in the same aviary; and this circumstance must have largely
fa\ ored 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

VARIATION UNDER DOMESTICATION • 25

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 pur-
poses, food and feathers, and more especially from no pleasure having been
felt in the display of distinct breeds; but the goose, under the conditions 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, and can never afterward be exceeded.
It would be somewhat 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 utmost 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 the 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 charac-
ter, which man has attended to and selected, more than do the distinct
species of the same genera. Isidore Geoffroy Saint-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 dray-horse is comparably stronger, than any two natural
species belonging to the same genus. So with plants, the seeds of the different
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 animals and plants.
Changed conditions of fife are of the highest importance in causing varia-
biHty, both by acting directly on the organization, and indirectly by affecting
the reproductive system. It is not probable that variability is an inherent and
necessary contingent, under all circumstances. The greater or less force of
inheritance and reversion determine whether variations shall endure. Varia-
bility is governed by many unknown laws, of which correlated growth is
probably the most important. Something, but how much we do not know,
may be attributed to the definite action of the conditions of life. Some, per-

26 THE ORIGIN OF SPECIES

haps 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 inter-
crossing of aboriginally distinct species appears to have played an important
part in the origin of our breeds. When several 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 impor-
tance of crossing has been much exaggerated, both in regard to animals and
to those plants which are propagated by seed. With plants which are tem-
porarily propagated by cuttings, buds, etc., the importance of crossing is
immense ; for the cultivator may here disregard 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 predominant power.

CHAPTER 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 un-
equally, related to each other, and in having Restricted Ranges.

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 satisfied all naturalists; yet every naturalist
knows vaguely what he means when he speaks of a species. Generally the
term includes the unknown element of a distinct act of creation. The term
"variety" is almost equally difficult to define; but here community of descent
is almost universally implied, though it can rarely be proved. We have also
what are called monstrosities; but they graduate into varieties. By a mon-
strosity I presume is meant some considerable deviation of structure, gen-
erally injurious, or not useful to the species. Some authors use the term "vari-
ation" in a technical sense, as implying a modification directly due to the
physical conditions of life; and "variations" in this sense are supposed not to
be inherited; but who can say that the dwarfed condition of shells in the
brackish waters of the Baltic, or dwarfed plants on Alpine summits, or the
thicker fur of an animal from far northward, would not in some cases be in-
herited 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 struc-
ture, such as we occasionally see in our domestic productions, more especially
with plants, are ever permanently propagated in a state of nature. Almost
every part of every organic being is so beautifully related to its complex con-
ditions of life that it seems as improbable that any part should have been sud-
denly produced perfect, as that a complex machine should have been invented
by man in a perfect state. Under domestication monstrosities sometimes occur
which resemble normal structures in widely different animals. Thus pigs have
occasionally been born with a sort of proboscis, and if any wild species of the
same genus had naturally possessed a proboscis, it might have been argued
that this had appeared as a monstrosity; but I have as yet failed to find, after
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 preserva-
tion would depend on unusually favorable circumstances. They would, also,
during the first and succeeding generations, cross with the ordinary form,

27

THE ORIGIN OF SPECIES

and thus their abnormal character would almost inevitably be lost. But I shall
have to return in a future chapter to the preservation and perpetuation of
single or occasional variations.

INDIVIDUAL DIFFERENCES

The many slight difTerences 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 confined locality,
may be called individual differences. No one supposes that all the individuals
of the same species are cast in the same actual mould. 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 accumulates
in any given direction individual differences in his domesticated productions.
These individual differences generally affect what naturalists consider unim-
portant parts ; but I could show, by a long catalogue of facts, that parts which
must be called important, whether viewed under a physiological or classifica-
tory point of view, sometimes vary in the individuals of the same species. I
am convinced that the most experienced naturalist would be surprised at the
number of the cases he 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 variability in important characters, and that
there are not many men who will laboriously examine internal and important
organs, and compare them in many specimens of the same species. 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 spe-
cies ; 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 var-
iability 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 confessed)
which do not vary; and, under this point of view, no instance will ever be
found of an important part varying; but under any other point of view many
instances assuredly can be given.

There is one point connected with individual differences which is extremely
perplexing: I refer to those genera which have been called "protean" or
"polymorphic," in which 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 char-
acters. Genera which are polymorphic in one country seem to be, with a few

VARIATION UNDER NATURE 29

exceptions, polymorphic in other countries, and likewise, judging from Bra-
chiopod shells, at former periods of time. These facts are very perplexing, for
they seem to show that this kind of variabiHty is independent of the condi-
tions of life. I am inclined to suspect 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 defi-
nite 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, independently 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 attention to
the subject, has shown that the females of certain species of butterflies, in the
Malayan Archipelago, regularly appeared under two or even three conspicu-
ously distinct forms, not connected by intermediate varieties. Fritz Miiller
has described analogous but more extraordinary cases with the males of cer-
tain Brazilian Crustaceans : thus, the male of a Tanais regularly occurs under
two distinct forms; one of these has strong and differently shaped pincers,
and the other has antennae much more abundantly furnished with smelHng-
hairs. Although in most of these cases, the two or three forms, both with ani-
mals and plants, are not now connected by intermediate 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 extreme 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 here-
after see, the castes are connected together by finely graduated varieties. So
it is, as I have myself observed, with some dimorphic plants. It certainly 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 an 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 spe-
cies, but which are so closely similar to other forms, or are so closely linked
to them by intermediate gradations, that naturalists do not like to rank them
as distinct species, are in several respects the most important for us. We have
every reason to believe that many of these doubtful and closely allied forms
have permanently retained their characters for a long time; for as long, as

30 THE ORIGIN OF SPECIES

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 other as the variety. But cases of great diffi-
culty, 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 variety of another, not because the
intermediate links have actually been found, but because analogy leads the
observer to suppose either that they do now somewhere exist, or may formerly
have existed; and here a wide door for the entry of doubt and conjecture is
opened.

Hence, in determining whether a form should be ranked as a species or a
variety, the opinion of naturalists having sound judgment and wide experi-
ence 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 varieties. 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 va-
rieties, but which nevertheless have been ranked by some botanists as species,
and he has entirely omitted several highly polymorphic genera. Under genera,
including the most polymorphic forms, Mr. Babington gives 251 species,
whereas Mr. Bentham gives only 112 — a difference of 139 doubtful forms!
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 species, and by another as varieties, or, as
they are often called, geographical races! Mr. Wallace, in several valuable
papers on the various animals, especially on the Lepidoptera, inhabiting the
islands of the great Malayan Archipelago, shows that they may be classed
under four heads, namely, as variable forms, as local forms, as geographical
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
moderately constant and distinct in each separate island; but when all from
the several islands are compared together, the differences are seen to be so

VARIATION UNDER NATURE 31

slight and graduated that it is impossible to define or describe them, though
at the same time the extreme forms are sufficiently distinct. The geographical
races or sub-species are local forms completely fixed and isolated ; but as they
do not differ from each other by strongly marked and important characters,
"There is no possible test but individual opinion to determine which of them
shall be considered as species and which as varieties." Lastly, representative
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
sa 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 between species and varie-
ties. On the islets of the little Madeira group there are many insects which
are characterized as varieties in Mr. Wollaston's admirable work, but which
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 experienced ornithol-
ogists consider our British red grouse as only a strongly marked race of a
Norwegian species, whereas the greater number rank it as an undoubted spe-
cies peculiar to Great Britain. A wide distance between the homes of two
doubtful forms leads many naturalists to rank them as distinct species; but
what distance, 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 archipelagoes, be
sufficient?

Mr. B. D. Walsh, a distinguished entomologist of the 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
observed 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 another, even if he can do so for himself,
which of these Phytophagic forms ought to be called species and which varie-
ties. Mr. Walsh ranks the forms which it may be supposed would freely inter-
cross, as varieties; and those which appear to have lost this power, as species.
As the differences depend on the insects having long fed on distinct plants, it

32 THE ORIGIN OF SPECIES

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 likewise neces-
sarily 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 archipelago, and presents dif-
ferent forms in the different areas, there is always a good chance that inter-
mediate forms will be discovered which will link together the extreme states;
and these are then degraded to the rank of varieties.

Some few naturalists maintain that animals never present 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 distinct species are hidden
under the same dress. The term species thus comes to be a mere useless ab-
straction, implying 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 definition of these terms has been generally
accepted, is vainly to beat the air.

Many of the cases of strongly marked varieties or doubtful species well de-
serve consideration; for several interesting lines of argument, from geograph-
ical distribution, 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 at-
tention, varieties of it will almost universally be found recorded. These varie-
ties, moreover, will often be ranked by some authors as species. Look at the
common oak, how closely it has been studied; yet a German author makes
more than a dozen species out of forms, which are almost universally con-
sidered by other botanists to be varieties ; and in this country the highest bo-
tanical 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 published by A. de Can-
dolle, on the oaks of the whole world. No one ever had more ample materials
for the discrimination of the species, or could have worked on them with
more zeal and sagacity. He first gives in detail all the many points of struc-
ture 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.

VARIATION UNDER NATURE 33

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 im-
perfectly known, and its species were founded upon a few specimens, that is
to say, were provisional. Just as we come to know them better, intermediate
forms flow in, and 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 which, excepting six, are clustered round three sub-species, namely, Q.
pedunculata, sessiliflora, and pubescens. The forms which connect these three
sub-species are comparatively rare; and, as Asa Gray again remarks, if these
connecting forms which are now rare were to become totally 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 spe-
cies, 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 species. It should be added
that De Candolle no longer believes that species are immutable creations, but
concludes that the derivative theory is the most natural one, "and the most
accordant with the known facts in palaeontology, 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 consider 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 forms. His general tendency will be to
make many species, for he will become impressed, just like the pigeon or poul-
try fancier before alluded to, with the amount of difference in the forms
which he is continually studying; and he has little general knowledge of ana-
logical variation in other groups and in other countries by which to correct
his first impressions. As he extends the range of his observations he will meet
with more cases of difficulty; for he will encounter a greater number of closely
allied forms. But if his observations be widely extended he will in the end
generally be able to make up his own mind ; but he will succeed in this at the
expense of admitting much variation, and the truth of this admission will
often be disputed by other naturalists. When 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 anal-
ogy, and his difficulties will rise to a climax.

Certainly no clear line of demarcation has as yet been drawn between spe-

34 THE ORIGIN OF SPECIES

cies and sub-species — that is, the forms which in the opinion of some natu-
ralists 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 varie-
ties 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 sys-
tematist, as of the highest importance for us, as being the first steps toward
such slight varieties as are barely thought worth recording in works on natural
history. And I look at varieties which are in any degree more distinct and
permanent, as steps 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 some 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, here-
after to be explained, and to the effects of the increased use or disuse of parts.
A well-marked variety may therefore be called an incipient species; but
whether this belief 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 extinct, or they may endure as varieties
for very long periods, as has been shown to be the case by Mr. WoUaston
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 co-exist, and both rank as independent species. But we shall here-
after return to this subject.

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

WroE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST

Guided by theoretical considerations, I thought that some interesting re-
sults 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 con-
vinced me that there were many difficulties, as did subsequently Dr. Hooper,
even in stronger terms. I shall reserve for a future work the discussion of these y

VARIATION UNDER NATURE 35

difficulties, and the tables of the proportional numbers of the varying species.
Dr. Hooper permits me to add that after having carefully read my manu-
script, and examined the tables, he thinks that the following statements are
fairly well established. The whole subject, however, treated as it necessarily
here is with much brevity, is rather perplexing, and allusions cannot be
avoided to the "struggle for existence," "divergence of character," and other
questions, hereafter to be discussed.

Alphonso 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 com-
petition (which, as we shall hereafter see, is an equally or more important
circumstance) with different sets of 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 individuals — which oftenest produce well-
marked varieties, or, as I consider them, incipient species. And this, perhaps,
might have been anticipated ; for, as varieties, in order to become in any de-
gree permanent, necessarily have to struggle with the other inhabitants of the
country, the species which are already dominant will be the more likely to
yield offspring, which, though in some slight degree modified, still inherit
those advantages that enabled their parents to become dominant over their
compatriots. 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 comparison 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, which live under nearly the same conditions. A
plant of this kind is not the less dominant because some conferva 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.

I

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 num-

36 THE ORIGIN OF SPECIES

ber of the very common and much diffused or dominant species. This might
have been anticipated, for the mere fact of many species of the same genus
inhabiting any country, shows that there is something in the organic or in-
organic conditions of that country favorable to the genus; and, consequently,
we might have expected to have found in the larger genera, or those includ-
ing many species, a larger proportional number of dominant species. But so
many causes tend to obscure this result, that I am surprised that my tables
show even a small majority on the side of the larger genera. I will here allude
to only two causes of obscurity. Fresh-water and salt-loving plants 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 Httle or no rela-
tion 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 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 species of the larger genera in each country
would oftener present varieties, than the species of the smaller genera; for
wherever many closely related species {i.e., species of the same genus) have
been formed, many varieties or incipient species ought, as a general rule, to be
now forming. Where many large trees grow, we expect to find saplings.
Where many species of a genus have been formed through variation, circum-
stances have been favorable for variation; and hence we might expect that
the circumstances would generally still be favorable to variation. On the
other hand, if we look at each species as a special act of creation, there is no
apparent reason why more varieties should occur in a group having many
species, than in one having few.

To test the truth of this anticipation I have arranged the plants of twelve
countries, and the coleopterous insects of two districts, into two nearly equal
masses, the species of the larger genera on one side, and those of the smaller
genera on the other side, and it has invariably proved to be the case that a
larger proportion of the species on the side of the larger genera presented
varieties, than on the side of the smaller genera. Moreover, the species of the
large genera which present any varieties, invariably present a larger average
number of varieties than do the species of the small genera. Both these results
follow when another division is made, and when all the least genera, with
from only one to four 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 manu-
factory of species has been active, we ought generally to find the manufactory
still in action, more especially as we have every reason to believe the process
of manufacturing new species to be a slow one. And this certainly 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

VARIATION UNDER NATURE 37

species of that genus present a number of varieties, that is, of incipient species,
beyond the average. It is not that all large genera are now varying much, and
are thus increasing in the number of their species, or that no small genera
are now varying and increasing; for if this had been so, it would have been
fatal to my theory; inasmuch as geology plainly tells us that small genera
have in the lapse of time often increased greatly in size ; and that large genera
have often come to their maxima, declined, and disappeared. All that we
want to show is, that where many species of a genus have been formed, on an
average many are still forming; and this certainly holds good.

MANY OF THE SPECIE3 INCLUDED W^ITHIN THE LARGER GENERA RESEMBLE
VARIETIES IN BEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH
OTHER, AND IN HAVIITG 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 infal-
lible criterion by which to distinguish species and well-marked varieties ; and
when intermediate links have not been found between doubtful forms, natu-
ralists are compelled to come to a determination by the amount of difference
between them, judging by analogy whether or not the amount suffices to raise
one or both to the rank of species. Hence the amount of difference is one
very important criterion in settling whether two forms should be ranked as
species or varieties. Now Fries has remarked in regard to plants, and West-
wood in regard to insects, that in large genera the amount of difference be-
tween the species 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 deliberation, they concur in this view. In this respect, therefore,
the species of the larger genera resemble varieties, more than do the species
of the smaller genera. Or the case may be put in another way, and it may
be said, that in the larger genera, in which a number of varieties or incipient
species greater than the average are now manufacturing, many of the species
already manufactured still to a certain extent resemble varieties, for they
differ from each other by 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 pretends that all the species of a genus are equally distinct from
each other; they may generally be divided into sub-genera, or sections, or
lesser groups. As Fries has well remarked, little groups of species are gener-
ally clustered like satellites around other species. And what are varieties but
groups of forms, unequally related to each other, and clustered round certain
forms — that is, round their parent species? Undoubtedly there is one most
important point of difference between varieties and species, namely, that the
amount of difference between varieties, when compared with each other or
with their parent species, is much less than that between the species of the
same genus. But when we come to discuss the principle, as I call it, of di-

38 THE ORIGIN OF SPECIES

vergence of character, we shall see how this may be explained, and how the
lesser differences between varieties tend to increase into the greater differ-
ences 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 restricted ranges. For instance,
Mr. H. G. Watson has marked for me in the well-sifted London Catalogue
of Plants (4th edition) sixty- three plants which are therein ranked as spe-
cies, but which he considers as so closely allied to other species as to
be of doubtful value: these sixty-three reputed species range on an aver-
age over 6.9 of the provinces into which Mr. Watson has divided Great
Britain. Now, in this same catalogue, fifty-three acknowledged varieties are
recorded, and these range over 7.7 provinces; whereas, the species to which
these varieties belong range over 14.3 provinces. So that the acknowledged
varieties have 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, by a certain in-
definite 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 alUed to other species apparently
have restricted ranges. In all these respects the species of large genera present
a stropg analogy with varieties. And we can clearly understand these anal-
ogies, if species once existed as varieties, and thus originated; whereas, these
analogies are utterly inexplicable if species are independent creations.

V/e 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 num-
ber of varieties; and varieties, as we shall hereafter see, tend to become con-
verted into new and distinct species. Thus the larger genera tend to become
larger; and throughout nature the forms of life which are now dominant
tend to become still more dominant by leaving many modified and dominant
descendants. But, by steps hereafter to be explained, the larger genera also
tend to break up into smaller genera. And thus the forms of life throughout
the universe become divided into groups subordinate to groups.

CHAPTER III

Struggle for Existence

Its Bearing on Natural Selection — ^The Term used in a Wide Sense — Geometrical
Ratio of Increase — Rapid Increase of Naturalized Animals and Plants — Nature
of the Checks to Increase — Competition Universal — Effects of Climate — Protec-
tion from the number of Individuals — Complex Relations of all Animals and
Plants throughout Nature — Struggle for Life most Severe between Individuals
and Varieties of the Same Species: often severe between Species of the Same
Genus — The Relation of Organism to Organism the most Important of all
Relations.

Before entering on the subject of this chapter I must make a few prelim-
inary remarks to show how the struggle for existence bears on natural selec-
tion. 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 varieties be admitted. But
the mere existence of individual variability and of some few well-marked
varieties, though necessary as the foundation for the work, helps us but little
in understanding how species arise in nature. How have all those exquisite
adaptations of one part of the organization to another part, and to the con-
ditions of life, and of one organic being to another being, been perfected?
We see these beautiful coadaptations 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 in-
cipient species, become ultimately converted into good and distinct species,
which in most cases obviously differ from each other far more than do the
varieties of the same species? How do those groups of species, which consti-
tute what are called distinct genera and which differ from each other more
than do the species of the same genus, arise? All these results, as we shall
more fully see in the next chapter, follow from the struggle for life. Owing
to this struggle, variations, however slight and from whatever cause proceed-
ing, if they be in any degree profitable to the individuals of a species, in their
infinitely complex relations to other organic beings and to their physical con-
ditions of life, will tend to the preservation of such individuals, and will gen-
erally 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
principle, by which each slight variation, if useful, is preserved, by the term
natural selection, in order to mark its relation to man's power of selection.

39

40 THE ORIGIN OF SPECIES

But the expression often used by Mr. Herbert Spencer, of the Survival of the
Fittest, is more accurate, and is sometimes equally convenient. We have seen
that man by selection can certainly produce great results, and can adapt or-
ganic beings to his own uses, through the accumulation of slight but useful
variations, given to him by the hand of Nature. But Natural Selection, we
shall hereafter see, is a power incessantly ready for action, and is as immeas-
urably superior to man's feeble efforts as the works of Nature are to those
of Art.

We will now discuss in a little more detail the struggle for existence. In my
future work this subject will be treated, as it well deserves, at greater length.
The elder De Candolle and Lyell have largely and philosophically shown
that all organic beings are exposed to severe competition. In regard to plants,
no one has treated this subject with more spirit and ability than W. Herbert,
Dean of Manchester, evidently the result of his great horticultural knowl-
edge. Nothing is easier than to admit in words the truth of the universal
struggle for life, or more difficult — at least I 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 misunder-
stood. We behold the face of nature bright with gladness, we often see super-
abundance 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 de-
stroying 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 sea-
sons of each recurring year.

THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE

I should premise that I use this term in a large and metaphorical sense, in-
cluding dependence of one being on another, and including (which is more
important) not only the life of the individual, but success in leaving progeny.
Two canine animals, in a time of dearth, may be truly said to struggle with
each other which shall get food and live. But a plant on the edge of a desert
is said to struggle for life against the drought, though more properly it should
be said to be dependent on the moisture. A plant which annually produces
a thousand seeds, of which 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 parasites 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

STRUGGLE FOR EXISTENCE

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 Hfe-
time produces several eggs or seeds, must suffer destruction during some
period of its life, and during some season or occasional year; otherwise, on
the principle of geometrical increase, its numbers would quickly become so
inordinately great that no country could support the product. Hence, as more
individuals are produced than can possibly survive, there must in every case
be a struggle for existence, either one individual with another of the same
species, or with the individuals of distinct species, or with the physical condi-
tions 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 arti-
ficial 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 in-
creases at so high a rate, that, if not destroyed, the earth would soon be cov-
ered 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 unpro-
ductive as this — and their seedlings next year produced two, and so on, then
in twenty years there would be a million plants. The elephant is reckoned
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 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 descended from
the first pair.

But we have better evidence on this subject than mere theoretical calcula-
tions, namely, the numerous recorded cases of the astonishingly rapid increase
of various animals in a state of nature, when circumstances have been favor-
able 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 com-
mon 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 com-
monest over the wide plains of La Plata, clothing square leagues of surface

42 THE ORIGIN OF SPECIES

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 Himalayas, which have been imported
from America since its discovery. 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 ex-
planation 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 enabled to breed. Their geometrical ratio of
increase, the result of which never fails to be surprising, simply explains their
extraordinarily rapid increase and wide diffusion 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 de-
struction 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 num-
ber 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 another, like the hippobosca, a single one. But this
difference does not determine how many individuals of the two species can
be supported in a district. A large number of eggs is of some importance to
those species which depend on a fluctuating amount of food, for it allows
them rapidly to increase in numbers. But the real importance of a large num-
ber of eggs or seed is to make up for much destruction at some period of life;
and this period in the great majority of cases is an early one. If an animal
can in any way protect its own eggs or young, a small number may be pro-
duced, 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 aver-
age 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 considera-
tions always in mind — never to forget that every single organic being may be
said to be striving to the utmost to increase in nimibers; that each lives by a

STRUGGLE FOR EXISTENCE 43

struggle at some period of its life; that heavy destruction inevitably falls
either on the young or old during each generation or at recurrent intervals.
Lighten any check, mitigate the destruction ever so little, and the numbers 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, 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 au-
thors, 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 in-
variably 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 numbers by various enemies ; for instance,
on a piece of ground three feet long and two wide, dug and cleared, and
where there could be no choking from other plants, I marked all the seed-
lings 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 grown on a little plot
of mown turf (three feet by four), nine species perished, from the other spe-
cies being allowed to grow up freely.

The amount of food for each species, of course, gives the extreme limit to
which each can increase ; but very frequently it is not the obtaining food, but
the serving as prey to other animals, which determines the average number
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 probabiHty, be less game than at present, although hundreds of thou-
sands 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 protected by
its dam.

Climate plays an important part in determining the average numbers of a
species, and periodical seasons of extreme cold or drought seem to be the
most effective of all checks. I estimated (chiefly from the greatly reduced

44 THE ORIGIN OF SPECIES

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 mortahty
from epidemics with man. The action of climate seems at first sight to be
quite independent of the struggle for existence; but in so far as climate
chiefly acts in reducing food, it brings on the most severe struggle between
the individuals, whether of the same or of distinct species, which subsist on
the same kind of food. Even when climate, for instance, extreme cold, acts
directly, it will be the least vigorous individuals, or those which have got least
food through the advancing winter, which will suffer the most. When we
travel from south to north, or from a damp region to a dry, we invariably
see some species gradually getting rarer and rarer, and finally disappearing;
and the change of climate being conspicuous, we are tempted to attribute
the whole effect to its direct action. But this is a false view; we forget that
each species, even where it most abounds, is constantly suffering enormous
destruction at some period of its life, from enemies or from competitors for
the same place and food ; and if these enemies or competitors be in the least
degree favored by any slight change of climate, they will increase in num-
bers; and as each area is already fully stocked with inhabitants, the other
species must decrease. When we travel southward and see a species decreasing
in numbers, we may feel sure that the cause lies quite as much in other spe-
cies 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, decreases northward, or in ascending a moun-
tain, we far oftener meet with stunted forms, due to the directly injurious ac-
tion of climate, than we do in proceeding southward or in descending a
mountain. When we reach the arctic regions, or snow-capped summits, or ab-
solute 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 per-
fectly 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 circumstances, increases inor-
dinately 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
preservation. 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 super-

STRUGGLE FOR EXISTENCE 45

abundance of food at this one season, increase in number proportionally to
the supply of seed, as their numbers are checked during the winter; but any
one who has tried knows how troublesome it is to get seed from a few wheat or
other such plants in a garden; I have in this case lost every single seed. This
view of the necessity of a large stock of the same species for its preservation,
explains, I believe, some singular facts in nature, such as that of very rare
plants being sometimes extremely abundant, in the few spots where they do
exist; and that of some social plants being social, that is abounding in indi-
viduals, 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
favorable that many could exist together, and thus save the species from
utter destruction. I should add that the 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 enclosed twenty-five years previ-
ously 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 plan-
tations, which were not to be seen on the heath; and the heath was fre-
quented 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 enclosed,
so that cattle could not enter. But how important an element enclosure is, I
plainly saw near Farnham, in Surrey. Here there are extensive heaths, with
a few clumps of old Scotch firs on the distant hilltops: within the last ten
years large spaces have been enclosed, and self-sown firs are now springing
up in multitudes, so close together that all cannot live. When I ascertained
that these young trees had not been sown or planted, I was so much surprised
at their numbers that I went to several points of view, whence I could ex-
amine hundreds of acres of the unenclosed heath, and literally I could not
see a single Scotch fir, except the old planted clumps. But on looking closely
between the stems of the heath, I found a multitude of seedlings and little

46 THE ORIGIN OF SPECIES

trees which had been perpetually browsed down by the cattle. In one square
yard, at a point some hundred yards distant from one of the old clumps, I
counted thirty- two little trees; and one of them, with twenty-six rings of
growth, had, during many years, tried to raise its head above the stems of
the heath, and had failed. No wonder that, as soon as the land was enclosed,
it became thickly clothed with vigorously growing young firs. Yet the heath
was so extremely barren and so extensive that no one would ever have
imagined that cattle would have so closely and effectually searched it for food.

Here we see that cattle absolutely determine the existence of the Scotch
fir; but in several parts of the world insects determine the existence of cattle.
Perhaps Paraguay offers the most curious instance of this; for here neither
cattle nor horses nor dogs have ever run wild, though they swarm southward
and northward in a feral state; and Azara and Rengger have shown that this
is caused by the greater number in Paraguay of a certain fly, which lays its
eggs in the navels of these animals when first born. The increase of these
flies, numerous as they are, must be habitually checked by some means, prob-
ably by other parasitic insects. Hence, if certain insectivorous birds were to
decrease in Paraguay, the parasitic 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. Battle within
battle must be continually recurring with varying 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 in-
voke cataclysms to desolate the world, or invent laws on the duration of the
forms of lifel

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 rela-
tions. 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-
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 (Viola 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, twenty heads of Dutch
clover (Trifolium repens) yielded 2,290 seeds, but twenty other heads, pro-
tected from bees, produced not one. Again, one hundred 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

STRUGGLE FOR EXISTENCE 47

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 Colonel 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 Colonel
Newman says, "Near villages and small towns I have found the nests of
humble-bees more numerous than elsewhere, which I attribute to the num-
ber of cats that destroy the mice." Hence it is quite credible that the presence
of a feline animal in large numbers in a district might determine, through
the intervention first of mice and then of bees, the frequency of certain
flowers in that district!

In the case of every species, many different checks, acting at different
periods of life, and during different seasons or years, probably come into
play; some one check or some few being generally the most potent; but all
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 forests. What a struggle must have gone on during
long centuries 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 animals, 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
centuries, the proportional numbers and kinds of trees now growing on the
old Indian ruins!

The dependency of one organic being on another, as of a parasite on its
prey, lies generally between beings remote in the scale of nature. This is
likewise sometimes 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

48 THE ORIGIN OF SPECIES

the individuals of the same species, for they frequent the same districts,
require the same food, and are exposed to the same dangers. In the case of
varieties of the same species, the struggle will generally be almost equally
severe, and we sometimes see the contest soon decided : for instance, if several
varieties of wheat be sown together and the mixed seed be resown, some of
the varieties which best suit the soil or climate, or are naturally the most
fertile, will beat the others and so yield more seed, and will consequently
in a few years supplant the other varieties. To keep up a mixed stock of
even such extremely close varieties as the variously colored sweet-pease, they
must be each year harvested separately, and the seed then mixed in due
proportion, otherwise the weaker kinds will steadily decrease in number and
disappear. So again with the varieties of sheep; it has been asserted that
certain mountain varieties will starve out other mountain varieties, so th^t
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 original propor-
tions of a mixed stock (crossing being prevented) could be kept up for half
a dozen generations, 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 in-
variably, 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, having caused the decrease of another species. The recent in-
crease of the missel-thrush in parts of Scotland has caused the decrease of
the song-thrush. How frequently we hear of one species of rat taking the
place of another species under the most different climates! In Russia the
small Asiatic cockroach has everywhere driven before it its great congener.
In Australia the imported hive-bee is rapidly exterminating the small, sting-
less 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 iniportance may be deduced from the fore-
going 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

STRUGGLE FOR EXISTENCE 49

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 the 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 relation to other plants. But from the strong
growth of young plants produced from such seeds, as pease 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 vigorously all around.

Look at a plant in the midst of its range ! Why does it not double or quad-
ruple 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 dryer districts. In this case we can clearly see that if
we wish in imagination to give the plant the power of increasing in numbers,
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 diflferent 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 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
increase in a geometrical ratio; that each, at some period of its life, during

50 THE ORIGIN OF SPECIES

some season of the year, during each generation, or at intervals, has to
struggle for life and to suffer great destruction. When we reflect on this
struggle we may console ourselves with the full belief that the war of nature
is not incessant, that no fear is felt, that death is generally prompt, and that
the vigorous^ the healthy, and the happy survive and multiply.

CHAPTER IV

Natural Selection; or the Survival of the Fittest

Natural Selection — Its Power compared with Man's Selection — Its Power on Charac-
ters of Trifling Importance — Its Power at All Ages and on Both Sexes — Sexual
Selection — On the Generality of Intercrosses between Individuals of the Same
Species — Circumstances Favorable and Unfavorable to the Results of Natural
Selection, namely. Intercrossing, Isolation, Number of Individuals — Slow Action
— Extinction caused by Natural Selection — Divergence of Character, related to
the Diversity of Inhabitants of any Small Area and to 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 — Summary.

How will the struggle for existence, briefly discussed in the last chapter, act
in regard to variation? Can the principle of selection, which we have seen
is so potent in the hands of man, apply under nature? I think we shall see
that it can act most efficiently. '/Let the endless number of slight variations
and individual differences occurring in our domestic productions, and, in a
lesser degree, in those under nature, be borne in mind; as well as the
strength of the hereditary tendency .j Under domestication, it may truly be
said that the whole organization becomes in some degree plastic. But the
variability, which we almost universally meet with in our domestic produc-
tions, is not directly produced, as Hooker and Asa Gray have well remarked, /^
by man; he can neither originate varieties nor prevent their occurrence; he
can only preserve and accumulate such as do occur. Unintentionally he ex-
poses 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 relations of all organic beings to each other and to their physical
conditions of life; and consequently what infinitely 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 in-
dividuals having any advantage, however slight, over others, would have the
best chance of surviving and procreating their kind?. On the other hand, we
may feel sure that any variation in the least degree injiirious would be rigidly
destroyed. This preservation of favorable individual differences and varia-
tions, and the destruction of those which are injurious, I have called Natural
Selection, or the Survival of the Fittest. Variations neither useful nor in-
jurious 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.

51

52 THE ORIGIN OF SPECIES

Several writers have misapprehended or objected to the term Natural
Selection. Some have even imagined that natural selection induces varia-
bility, whereas it implies 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 individual differences given by nature, which man for some object
selects, must of necessity first occur. Others 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 selec-
tion 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 speak-
ing of the elective affinities of the various elements? — and yet an acid
cannot strictly be said to elect the base with which it in preference com-
bines. 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 metaphorical expressions; and they are almost neces-
sary 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 forgotten.

We shall best understand the probable course of natural 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 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 climate itself, would seriously aflfect 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 surrounded by barriers, into which
new and better adapted forms could not freely enter, we should then have
places in the economy of nature which would assuredly be better filled up
if some of the original inhabitants were in some manner modified; for, had
the area been open to immigration, these same places would have been
seized on by intruders. In such cases, slight modifications, which in any
way favored the individuals of any species, by better adapting them to their
I 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
manifestly be favorable to natural selection, by affording a better chance

NATURAL SELECTION 53

of the occurrence of profitable variations. Unless such occur, natural selec-
tion can do nothing.' Under the term of "variations," it must never be for-
gotten that mere individual differences are included. As man can produce
a great result with his domestic animals and plants by adding up in any
given direction individual 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 immigration, is necessary in order that new and un-
occupied 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 modi-
fications 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 con-
tinued 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
conditions 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 con-
quered 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 intrudei^.

As man can produce, and certainly has produced, a great result by his
methodical and unconscious means of selection, what may not natural
selection effect? Man can act only on external and visible characters;
Nature, if I may be allowed to personify the natural preservation 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
constitutional difference, on the whole machinery of life. Man selects only
for his own good; Nature, only for that of the being which she tends. Every
selected character is fully exercised by her, 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
manner; he feeds a long and a short-beaked pigeon on the same food; he
does not exercise a long-backed or long-legged quadruped in any peculiar
manner; he exposes sheep with long and short wool to the same climate;
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 conse-

54 THE ORIGIN OF SPECIES

quently how poor will be his results, compared with those accumulated by-
Nature during whole geological periods! Can we wonder, then, that Na-
ture's productions should be far "truer" in character than man's produc-
tions; that they should be infinitely better adapted to the most complex
conditions of life, and should plainly bear the stamp of far higher
workmanship ?

It may metaphorically be said that natural selection is daily and hourly
I scrutinizing, throughout the world, the slightest variations; rejecting those
\ that are bad, preserving and adding up all that are good ; silently and in-
■' sensibly working, ]whenever and wherever opportunity offers, at the im-
provement 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 agesj| 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 present individual differences of the same favorable
nature as before; and these must again be 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 hypothesis
accords with and explains the general phenomena 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 importance, 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 grouse the color of heather, we must believe that these
tints are of service to these birds and insects in preserving them from
danger. Grouse, if not destroyed at some period of their lives, would in-
crease 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 giving 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 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 characters of the
most trifling importance; yet we hear from an excellent horticulturist,
Downing, that in the United States the smooth-skinned fruits suffer far

NATURAL SELECTION 55

more from a beetle, a Curculio, than those with down; that purple plums
suffer far more from a certain disease than yellow plums; whereas another
disease attacks yellow-fleshed peaches far more than those with other
colored flesh. If, with all the aids of art, these slight differences make a
great difference in cultivating the several varieties, assuredly, in a state of
nature, where the trees would have to struggle with other trees and with a
host of enemies, such differences would effectually settle which variety,
whether a smooth or downy, a yellow or a purple fleshed fruit, should
succeed.

In looking at many small points of difference between species, which, as
far as our ignorance permits us to judge, seem 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 correla-
tion, when one part varies and the variations are accumulated through
natural selection, other modifications, often of the most unexpected nature,
will ensue.

As we see that those variations which, under domestication, appear at
any particular period of life, tend to reappear in the offspring at the same
period; for instance, in the 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 silkworm; 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 accumula-
tion of variations profitable at that age, and by their inheritance at a
corresponding age. If it profit a plant to have its seeds more and more
widely disseminated by the wind, I can see no greater difficulty in this
being effected through natural selection, than in the cotton-planter in-
creasing and improving by selection the down in the pods on his cotton-
trees. Natural selection may modify and adapt the larva of an insect to a
score of contingencies, wholly different from those which concern the
mature insect; and these modifications may affect, 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 will insure that
they shall not be injurious: 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 advantage, for the good of another species; and though statements
to this effect may be found in works of natural history, I cannot find one
case which will bear investigation. A structure used only once in an animal's
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

56 THE ORIGIN OF SPECIES

exclusively for opening the cocoon — or the hard tip to the beak of un-
hatched birds, used for breaking the eggs. 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 raost 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 there must be much
fortuitous destruction, which can have little or no influence on the course
of natural selection. 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 seeds 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 certain changes of structure or constitution
which would in other ways be beneficial to the species. But let the destruc-
tion 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 destruc-
tion 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 variability 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 selection 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 reason to suppose that many
species ever undergo modification and improvement at the same time in
the same area.

SEXUAL SELECTION

Inasmuch as peculiarities often appear under domestication 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 com-
monly occurs. This leads me to say a few words on what I have called
sexual selection. This form of selection depends, not on a struggle for
existence in relation to other organic beings or to external conditions, but
on a struggle between the individuals of one sex, generally the males, for

NATURAL SELECTION 57

the possession of the otherjex. The result is not death to the unsuccessful
competitor, but few or no offspring. Sexual selection is, therefore^, less
rigorous than natural selection. Generally, the most vigorous males, those
which are best fitted for their places in nature, will leave most progeny. But
in many oases 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 selection,
by always allowing the victor to breed, might surely give indomitable
courage, length of 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,
bellowing, and whirling round, like Indians in a war-dance, for the posses-
sion 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, perhaps, severest between the males
of polygamous animals, and . these seem oftenest provided with special
weapons. The males of carnivorous animals are already well armed; though
to them and to others, special means of defence may be given through
means of sexual selection, as the mane 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 spectators, at last choose the most
attractive partner. Those who have closely attended to birds in confine-
ment well know that they often take individual preferences and dislikes:
thus Sir R. Heron has described how a pied peacock was eminently attrac-
tive 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 to his bantams,
according to his standard of beauty, I can see no good reason to doubt that
female birds, by selecting, during thousands of generations, the most
melodious or beautiful males, according to their standard of beauty, might
produce a marked effect. 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.

58 THE ORIGIN OF SPECIES

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 orna-
ment, such differences have been mainly caused by sexual selection : that is,
by individual males having had, in successive generations, some slight ad-
vantage 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.

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 suppose that the fleetest prey,
a deer for instance, had from any change in the country increased in num-
bers, or that other prey had decreased in numbers, during that season of
the year when the wolf was hardest pressed for food. Under such circum-
stances the swiftest and slimmest wolves have the best chance of surviving,
and so being preserved or selected, provided always that they retain
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 im-
prove the fleetness of his greyhounds 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 in-
habiting the Gatskill Mountains, in the United States, one with a light
greyhound-like form, which pursues deer, and the other more bulky, with
shorter legs, which more frequently attacks the shepherd's flocks.

It should be observed that in the above illustration, I speak of the slim-
mest individual wolves, and not of any single strongly marked variation
having been preserved. In former editions of this work I sometimes spoke
as if this latter alternative had frequently occurred. I saw the great im-
portance of individual differences, and this led me fully to discuss the re-
sults of unconscious selection by man, which depends on the preservation
of all the more or less valuable individuals, 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

NATURAL SELECTION 59

intercrossing with ordinary individuals. Nevertheless, until reading an able
and valuable article in the North British Review (1867), I did not ap-
preciate how rarely single variations, whether slight or strongly marked,
could be perpetuated. The author takes the case of a pair of animals, pro-
ducing during their lifetime two hundred offspring, of which, from various
causes of destruction, only two on an average survive to procreate their
kind. This is 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 favorable variation; still, as the re-
viewer 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 more
easily by having its beak curved, and if one were born with its beak strongly
curved, and which consequently flourished, nevertheless there would be a
very poor chance of this one individual perpetuating its kind to the exclu-
sion 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 produc-
tions. In such cases, if the varying individual did not actually transmit to
its offspring its newly acquired character, it would undoubtedly transmit
to them, as long as the existing conditions remained 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
lacrymans. In cases of this kind, if the variation were of a beneficial nature,
the original form would soon be supplanted 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

60 THE ORIGIN OF SPECIES

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 success-
ful in its battle for life, it would slowly spread from a central district, com-
peting 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 Legu-
minosae, 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 individuals of the same species would thus get
crossed ; and 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 nectaries, 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 of 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 continued, had been rendered
highly attractive to insects, they would, unintentionally on their part,
regularly carry pollen from flower to flower: and that they do this effectu-
ally 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 producing a
rather small quantity of pollen, and a rudimentary pistil; other holly- trees
bear only female flowers; these have a full-sized pistil, and four stamens
with shrivelled anthers, in which not a grain of pollen can be detected.
Having found a female tree exactly sixty yards from a male tree, I put the
stigmas of twenty flowers, taken from different branches, under the micro-

NATURAL SELECTION 61

scope, and on all, without exception, there were a few pollen-grains, and
on some a profusion. As the wind had set for several days from the female
to the male tree, the pollen could not thus have been carried. The weather
had been cold and boisterous and therefore not favorable to bees, neverthe-
less 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 render.ed so
highly attractive to insects that pollen was regularly carried from flower
to flower, another process might commence. No naturalist doubts the ad-
vantage of what has been called the "physiolofflcal diyisioii_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 another plant. In plants under culture and placed under new
conditions of life, sometimes the male organs and sometimes the female
organs become more or less impotent; now if we suppose this to occur in
ever so slight a degree under nature, then, as pollen is already carried
regularly from flower to flower, and as a more complete separation of the
sexes of our plant would be advantageous on the principle of the division
of 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 up too much space to show the
various steps, through dimorphism and other means, by which the separa-
tion 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, ac-
cording to Asa Gray, in an exactly intermediate condition, or, as he ex-
presses it, are more less dioeciously polygamous.

Let us now turn to the nectar-feeding insects; we may suppose the plant,
of which we have been slowly increasing the nectar by continued selection,
to be a common plant; and that certain insects depended in main part on
its nectar for food. I could give many facts showing how anxious bees are
to save time: for instance, their habit of cutting holes and sucking the
nectar at the bases of certain flowers, which with a very little more trouble
they can enter by the mouth. Bearing such facts in mind, it may be believed
that under certain circumstances 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 individuals would be able to obtain
their food more quickly than others; and thus the communities to which
they belonged would flourish and throw off many swarms inheriting the
same peculiarities. The tubes of the corolla of the common red or incarnate
clovers (Trifolium pratense and incarnatum) do not on a hasty glance
appear to differ in length; yet the hive-bee can easily suck the nectar out of
the incarnate clover, but not out of the common red clover, which is
visited by humble-bees alone, so that whole fields of the red clover offer in
vain an abundant supply of precious nectar to the hive-bee. That this nectar
is much liked by the hive-bee is certain; for I have repeatedly seen, but only
in the autumn, many hive-bees sucking the flowers through holes bitten in

62 THE ORIGIN OF SPECIES

the base of the tube by humble-bees. The difference 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 accurate; nor whether another published statement can be trusted,
namely, that the Ligurian bee, which is generally 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 flowers. Thus I can under-
stand 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 "the modern changes of
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 smalMsiSFi^^^ 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 sudden modification in their structure.

ON THE INTERCROSSING OF INDIVIDUALS

I must here introduce a short digression. In the case of animals and
plants with separated sexes, it is of course obvious that two individuals
must always (with the exception of the curious and not well understood
cases of parthenogenesis) unite for each birth; but in the case of hermaph-
rodites this is far from obvious. Nevertheless there is reason to believe
that with all hermaphrodites two individuals, either occasionally or habitu-
ally, concur for the reproduction of their kind. This view was long ago
doubtfully suggested by Sprengel, Knight, and Kolreuter. We shall pres-
ently 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. Modem research has much diminished the number of

NATURAL SELECTION 63

supposed hermaphrodites, and of real hermaphrodites a large number
pair; that is, two individuals regularly unite for reproduction, which is all
that concerns us. But still there are many hermaphrodite animals which
certainly do not habitually pair, and a vast majority of plants are her-
maphrodites. What reason, it may be asked, is there for supposing in
these cases that two individuals ever concur in reproduction? As it is im-
possible here to enter on details, I must trust to some general considera-
tions alone.

In the first place, I have collected so large a body of facts, and made so
many experiments, showing, in accordance with the almost universal be-
lief 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 offspring; 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 — indispensable.

On the belief that this is a law of nature, we can, I think, understand
several large classes of facts, such as the following, which on any other
view are inexplicable. Every hybridizer knows how 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 occa-
sional cross be indispensable, notwithstanding that the plant's own anthers
and pistil stand so near each other as almost to insure self-fertilization, the
fullest freedom for the entrance of pollen from another individual will
explain the above state of exposure of the organs. Many flowers, on the
other hand, have their organs of fructification closely enclosed, 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. Now, 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-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 supposed 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

64 THE ORIGIN OF SPECIES

well known that, if closely-allied forms or varieties are planted near each
other, it is hardly possible to raise pure seedlings, so largely do they natu-
rally cross. In numerous other cases, far from self-fertilization being
favored, there are special contrivances which effectually prevent the stigma
receiving pollen from its 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 con-
joined 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 receiving pollen from the same flower, yet, as Sprengel
and more recently Hildebrand and others have shown, and as I can con-
firm, 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 habit-
uaUy 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 purpose of self-fertilization, should be in so
many cases mutually useless to each other! How simply are these facts ex-
plained 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 seed-
lings thus raised turn out, as I found, mongrels: for instance, I raised 233
seedling cabbages from some plants of different varieties growing near
each other, and of these only 78 were true to their kind, and some even of
these were not perfectly true. Yet the pistil of each cabbage-flower is sur-
rounded 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 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

NATURAL SELECTION 65

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 occa-
sionally carried from tree to tree. That trees belonging to all orders have
their sexes more often separated than other plants, I find to be the case in
this country; and at my request Dr. Hooker tabulated the trees of New
Zealand, and Dr. Asa Gray those of the United States, and the result was
as I anticipated. On the other hand. Dr. Hooker informs me that the rule
does not hold good in Australia; but if most of the Australian trees are
dichogamous, the same result would follow as if they bore flowers with
separated sexes. I have made these few remarks on trees simply to call at-
tention to the subject.

Turning for a brief space to animals: various terrestrial 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"~Temafkable fact, which off'ers 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 eflfected 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 consultation with one of the highest authorities, viz.. Professor
Huxley, to discover a single hermaphrodite animal with the organs of re-
production so perfectly enclosed that access from without, and the occa-
sional influence of a distinct individual, can be shown to be physically
impossible. Cirripedes 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 that two individuals, though both of self-fertilizing her-
maphrodites, 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 or-
ganization, are hermaphrodites, and some unisexual. But if, in fact, all
hermaphrodites do occasionally intercross, the difl'erence 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.

66 THE ORIGIN OF SPECIES

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 favorable. 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 become
modified and improved in a corresponding degree with its competitors it
will be exterminated. Unless 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 separa-
tion of selected individuals. Thus it will be under nature; for within a
•confined area, with some place in the natural poUty not perfectly occupied,
all the individuals varying in the right direction, though in different de-
grees, 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 undergoes 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, and likewise
for 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 nurserymen always prefer saving seed
from a large body of plants, as the chance of intercrossing is tRus lessened.

Even with animals which unite for each birth, and which do not propa-

NATURAL SELECTION 67

gate rapidly, we must not assume that free intercrossing would always
eliminate the efTects of natural selection; for I can bring forward a con-
siderable body of facts showing that within the same area two varieties of
the same animal may long remain distinct, from haunting different sta-
tions, 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 in-
dividuals of the same species, or of the same variety, true and uniform in
character. It will obviously thus act far more efficiently with those animals
which unite for each birth; but, as already stated, we have reason to be-
lieve that occasional intercrosses take place with all animals and plants.
Even if these take place only at long intervals of time, the young thus
produced will gain so much in vigor and fertility over the offspring from
long-continued self-fertiKzation, that they will have a better chance of sur-
viving and propagating their kind; and thus in the long-run the influence
of crosses, even at rare intervals, will be great. With respect to organic
beings extremely low in the scale, which do not propagate sexually, nor
conjugate, and which cannot 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 de-
stroy any individuals departing from the proper type. If the conditions of
life change, and the form undergoes modification, uniformity of character
can be given to the modified offspring, solely by natural selection preserv-
ing similar favorable variations.

Isolation also is an important element in the modification of species
through natural selection. In a confined or isolated area, if not very large,
the organic and inorganic conditions of life will generally be almost uni-
form; so that natural selection will tend to modify all the varying indi-
viduals of the same species in the same manner. Intercrossing with the
inhabitants of the surrounding 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 Kkewise great in preventing, 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 some-
times be of much importance. If, however, an isolated area be very small,
either from being surrounded by barriers, or from having very peculiar
physical conditions, the total number of the inhabitants will be small; and
this will retard the production of new species through natural selection,
by decreasing the chances of favorable variations arising.

68 THE ORIGIN OF SPECIES

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 importance 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 condi-
I tions of life, in relation to the constitution of each organism.

If we turn to nature to test the truth of these remarks, and look at any
small isolated area, such as an oceanic island, although the number of
species inhabiting it is small, as we shall see in our chapter on Geographical
Distribution; yet of these species a very large proportion are endemic, —
that is, have been produced there and nowhere else in the world. 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 ascer-
tain 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 production 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. Through-
out 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 modified and improved, others will have to
be improved in a corresponding degree, or they will be exterminated. Each
new form, also, as soon as it has been much improved, will be able to
spread over the open and continuous area, and will thus come into com-
petition with many other forms. Moreover, great areas, though now con-
tinuous, 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 favorable for the produc-
tion 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, understand some facts
which will be again alluded to in our chapter on Geographical Distribu-

NATURAL SELECTION 69

tion; for instance, the fact of the productions of the smaller continent of
Australia now yielding before those of the larger Europaso- 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 ex-
termination. Hence, we can understand how it is that the flora of Madeira,
according to Oswald Heer, resembles 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 competi-
tion between fresh-water productions will have been less severe than else-
where, 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 w^e 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
have endured to the present day, from having inhabited a confined 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 unfavorable for the production of new species
through natural selection. I conclude that for terrestrial productions a
large continental area, which has undergone many oscillations of level,
will have been the most favorable for the production of many new forms
of life, fitted to endure for a long time and to spread widely. While the
area existed as a continent, the inhabitants will have been numerous in
individuals and kinds, and will have been subjected to severe competition.
When converted by subsistence into large separate islands, there will still
have existed many individuals of the same species on each island: inter-
crossing on the confines of the range of each new species will have been
checked: after physical changes of any kind, immigration will have been
prevented, 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 reconverted into a
continental area, there will again have been very severe competition; the
most favored or improved 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 various 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

70 THE ORIGIN OF SPECIES

existing 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
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. Although 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 sufficient 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 in-
habitants 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 complexity 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 varia-
tions 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. Rarity, as geology tells us, is the precursor to
extinction. We can see that any form which is represented by few indi-
viduals will run a good chance of utter extinction, during great fluctua-
tions in the nature of the seasons, or from a temporary increase in the
number of its enemies. But we may go further than this; for, as new forms
are produced, 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 numerous 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 chapter,
showing that it is the common and diffused or dominant species which

NATURAL SELECTION 71

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

DIVERGENCE OF CHARACTER

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

72 THE ORIGIN OF SPECIES

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 ex-
tremes," 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 history, 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 con-
tinued selection of swifter horses in the one case, and of stronger ones in the
other, the differences would become 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-established and distinct breeds. As
the differences became greater, the inferior animals with intermediate char-
acters, 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 principle 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 diversified
the descendants from any one species become in structure, constitution, and
habits, by so much will they be better enabled to seize on many and widely
diversified places in the polity of nature, and so be enabled to increase in
numbers.

We can clearly discern this in the case of animals with simple habits. Take
the case of a carnivorous quadruped, of which the number that can be sup-
ported in any country has long ago arrived at its full average. If its natural
power of increase be allowed to act, it can 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 in-
stance, being enabled to feed on new kinds of prey, either dead or alive ; some
inhabiting new stations, climbing trees, frequenting water, and some perhaps
becoming less carnivorous. The more diversified in habits and structure the
descendants of our carnivorous animals become, the more places they will
be enabled to occupy. What applies to one animal will apply throughout all
time to all animals — that is, if they vary — for otherwise natural selection can
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 the
former case. The same has been found to hold good when one variety and

NATURAL SELECTION 73

several mixed varieties of wheat have been sown on equal spaces of ground.
Hence, if any one species of grass were to go on varying, and the varieties
were continually selected which differed from each other in the same man-
ner, 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 annually sowing
almost countless seeds; and is thus striving, as it may be said, to the utmost
to increase in number. Consequently, in the course of many thousand gener-
ations, the most distinct varieties of any one species of grass would have the
best chance of succeeding and of increasing in numbers, and thus of supplant-
ing the less distinct varieties ; and varieties, when rendered very distinct from
each other, take the rank of species.

The truth of the principle that the greatest amount of life can be supported
by great diversification of structure, is seen under many natural circumstances.
In an extremely small area, especially if freely open to immigration, and
where the contest between individual and individual must be 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 in-
sects on small and uniform islets : also in small ponds of fresh water. Farmers
find that they can raise more food by a rotation of plants belonging to the
most different orders : nature follows what may be called a simultaneous ro-
tation. Most of the animals and plants which Hve 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 differences of habit and
constitution, determine that the inhabitants, which thus jostle each other
most closely, shall, as a general rule, belong to what we call different genera
and orders.

The same principle is seen in the naturalization of 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 spe-
cially 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 CandoUe has well remarked, in his great and
admirable work, that floras gain by naturalization, proportionally with the
number of the native genera and species, far more in new genera than in new
species. To give a single instance: in the last edition of Dr. Asa Gray's "Man-
ual of the Flora of the Northern United States," 260 naturalized plants are
enumerated, and these belong to 162 genera. We thus see that these natural-

74 THE ORIGIN OF SPECIES

ized plants are of a highly diversified nature. They differ, moreover, to a large
extent, from the indigenes, for out of the 1 62 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 coun-
try struggled successfully with the indigenes, and have there become natural-
ized, 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 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 by Milne
Edwards. No physiologist doubts that a stomach 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 ani-
mals and plants are diversified for different habits of life, so will a greater
number of individuals be capable of there supporting themselves. A set of
animals, with their organization but little diversified, could hardly compete
with a set more perfectly diversified in structure. It may be doubted, for in-
stance, whether the AustraUan marsupials, which are divided into groups
differing but Httle from each other, and feebly representing, as Mr. Water-
house and others have remarked, our carnivorous, ruminant, and rodent
mammals, 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 improvement.

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 prin-
ciple of benefit being derived from divergence of character, combined with
the principles of natural selection and of extinction, tends to act.

The accompanying diagram will aid us in understanding this rather per-
plexing subject. Let A to L represent the species of a genus large in its own
country; these species are supposed to resemble each other in unequal de-
grees, as is so generally the case in nature, and as is represented in the dia-
gram 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 most widely diffused, vary more than do the

NATURAL SELECTION 75

rare and restricted species. Let (A) be a common, widely- diffused, and vary-
ing 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 simulta-
neously, 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 profit-
able 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 variations (represented
by the outer dotted lines) being preserved and accumulated by natural selec-
tion. When a dotted line reaches one of the horizontal lines, and is there
marked by a small numbered letter, a sufficient amount of variation is sup-
posed to have been accumulated 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 diagram 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 a^
and m^. These two varieties will generally still be exposed to the same condi-
tions 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 tend to inherit those advantages
which made their parent (A) more numerous than most of the other inhabit-
ants of the same country; they will also partake of those more general ad-
vantages which made the genus to which the parent species belonged, a large
genus in its own country. And all these circumstances are favorable to the
production of new varieties.

If, then, these two varieties be variable, the most divergent of their varia-
tions will generally be preserved during the next thousand generations. And
after this interval, variety a^ is supposed in the diagram to have produced
variety a^, which will, owing to the principle of divergence, differ more from
(A) than did variety a^. Variety m^ is supposed to have produced two varie-
ties, namely m^ 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 gen-
erations, 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 gen^
eration.

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 some-
what irregular, nor that it goes on continuously; it is far more probable that

76

THE ORIGIN OF SPECIES

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78 THE ORIGIN OF SPECIES

each form remains for long periods unaltered, and then again undergoes
modification. Nor do I suppose that the most divergent varieties are invari-
ably 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
perfectly occupied by other beings; and this will depend on infinitely com-
plex 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 succession is broken at regular intervals by small
numbered letters marking the successive forms which have become sufficiently
distinct to be recorded as varieties. But these breaks are imaginary, and
might have been inserted anywhere, after intervals long enough to allow the
accumulation of a considerable amount of divergent variation.

As all the modified descendants from a common and widely-diffused spe-
cies, 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 multiply-
ing in number as well as diverging in character: this is represented in the
diagram by the several divergent branches proceeding from ( A) . The modi-
fied 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 modification will be confined to a single
line of descent, and the number of modified descendants will not be in-
creased ; although the amount of divergent modification may have been aug-
mented. This case would be represented in the diagram, if all the lines pro-
ceeding from (A) were removed, excepting that from cD- to a}-^. In the same
way the English race-horse and English pointer have apparently both gone
on slowly diverging in character from their original stocks, without either
having given off any fresh branches or races.

After ten thousand generations, species (A) is supposed to have produced
three forms, a^°, /^^, and m^°, which, from having diverged in character dur-
ing 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 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 differences distinguishing varieties are increased into the
larger differences distinguishing species. By continuing the same process for
a greater number of generations (as shown in the diagram in a condensed
and simplified manner) , we get eight species, marked by the letters between
a}-^ and m^^, all descended from ( A) . Thus, as I believe, species are multi-
plied, and genera are formed.

NATURAL SELECTION 79

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 (w^^ and z^^) or two species, according to the amount of change
supposed to be represented between the horizontal lines. After fourteen thou-
sand generations, six new species, marked by the letters n^* to z^^, are sup-
posed 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 greatest 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
extreme species (I), as those which have largely varied, and have given rise
to new varieties and species. The other nine species (marked by capital
letters) of our original genus, may for long but unequal periods continue to
transmit unaltered descendants; and this is shown in the diagram by the
dotted lines unequally prolonged upward.

But during the process of modification, represented in the diagram, an-
other of our principles, namely that of extinction, will have played an im-
portant part. As in each fully stocked country natural selection necessarily
acts by the selected form having some advantage in the struggle for life over
other forms, there will be a constant tendency in the improved descendants
of any one species to supplant and exterminate in each stage of descent their
predecessors and their original progenitor. For it should be remembered that
the competition will generally be rnost 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 become 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 considerable amount of
modification, species (A) and all the earlier varieties will have become ex-
tinct, being replaced by eight new species (a^* to m"), and species (I) will
be replaced by six (n^* to 2^*) new species.

But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally the
case in nature; species (A) being more nearly related to B, C, and D than to
the other species; and species (I) more to G, H, K, L, than to the others.
These two species (A and I) were also supposed to be very common and
widely diffused species, so that they must originally have had some ad-
vantage over most of the other species of the genus. Their modified descend-
ants, fourteen in number at the fourteen-thousandth generation, will prob-
ably have inherited some of the same advantages; they have also been modi-

80 THE ORIGIN OF SPECIES

fied 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 exterminated, not only their parents (A) and (I) , but like-
wise some of the original species which were most nearly related to their
parents. Hence very few of the original species will have transmitted off-
spring 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 spe-
cies, will now be fifteen in number. Owing to the divergent tendency of natu-
ral selection, the extreme amount of difference in character between species
a}^ and z'^^ 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 a^*, q^^, p^^, will be nearly related from having recently
branched off from a^^; b^^ and /^*, from having diverged at an earlier period
from a^, will be in some degree distinct from the three first-named species;
and lastly, o^*, e^^, and m^* will be nearly related one to the other, but, from
being diverged at the first commencement of the process of modification, will
be widely different from the other five species, and may constitute a sub-
genus or 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 original genus, the six descendants from (I) will, owing
to inheritance alone, differ considerably from the eight descendants from
(A) ; the two groups, moreover, are supposed to have gone on diverging in
different directions. The intermediate species, also (and this is a very impor-
tant consideration), which connected the original species (A) and (I), have
all become, except (F), extinct, and have left no descendants. Hence the six
new species descended 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 species
of an earlier genus. In our diagram this is indicated by the broken lines be-
neath the capital letters, converging in sub-branches downward toward a
single point: this point represents a species, the supposed progenitor of our
several new sub-genera and genera.

It is worth while to reflect for a moment on the character of the new spe-
cies F^*, which is supposed not to have diverged much in character, but to
have retained the form of (F), either unaltered or altered only in a slight
degree. In this case its affinities to the other fourteen new species will be of
a curious and circuitous nature. Being descended from a form that stood be-
tween the parent-species (A) and (I), now supposed to be extinct and un-

NATURAL SELECTION 81

known, it will be in some degree intermediate 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 (f^^) will not be directly intermediate between them, but rather be-
tween 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 repre-
sent a thousand generations, but each may represent a million or more gen-
erations; 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 exist-
ing groups; and we can understand this fact, for the extinct species lived at
various remote epochs when the branching lines of descent had diverged less.

I see no reason to limit the process of modification, as now explained, to
the formation of genera alone. If, in the diagram, we suppose the amount of
change represented by each successive group of diverging dotted lines to be
great, the forms marked a}^ to p^*, those marked 6^* and /^*, and those marked
o^* to 772^% will form three very distinct genera. We shall also have two very
distinct 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
represented in the diagram. And the two new families, or orders, are de-
scended 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 common ancestor
some advantage in common. Hence, the struggle for the production of new
and modified descendants will mainly lie between the larger groups which are
all trying to increase in number. One large group will slowly conquer another
large group, reduce its number, and thus lessen its chance of further variation
and improvement. Within the same large group, the later and more highly
perfected sub-groups, from branching out and seizing on many new places
in the polity of nature, will constantly tend to supplant and destroy the earlier
and less improved sub-groups. Small and broken groups and sub-groups will
finally disappear. Looking to the future, we can predict that the groups of
organic beings which are now large and triumphant, and which are least
broken up, that is, which have as yet suffered least extinction, 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 exten-

82 THE ORIGIN OF SPECIES

sively developed, have now become extinct. Looking still more remotely to
the future, we may predict that, owing to the continued and steady increase
of the larger groups, a multitude of smaller groups will become utterly ex-
tinct, and leave no modified descendants; and consequently, that, of the spe-
cies living at any one period, extremely few will transmit descendants to a
remote futurity. I shall have to return to this subject in the chapter on classi-
fication, 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 exist so few classes in each main division of the animal
and vegetable kingdoms. Although few of the most ancient species have left
modified descendants, yet, at remote geological periods, 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 creature is exposed at all periods of life. The ultimate result is
that each creature tends to become more and more improved in relation to
its conditions. This improvement inevitably leads to the gradual advance-
ment of the organization of the greater number of living beings throughout
the world. But here we enter on a very intricate subject, for naturalists have
not defined to each other's satisfaction what is meant by an advance in or-
ganization. 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 develop-
ment from embryo to maturity would suffice as a standard of comparison;
but there are cases, as with certain parasitic crustaceans, in which several
parts of the structure become less perfect, so that the mature animal cannot
be called higher than its larva. Von Baer's standard 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 inclined to add,
and their specialization for different functions; or, as Milne Edwards would
express it, the completeness 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 excluded; and here some botanists rank those
plants as highest which have every organ, as sepals, petals, stamens, and pis-
tils, fully developed in each flower; whereas other botanists, probably with

NATURAL SELECTION 83

more truth, look at the plants which have their several organs much modi-
fied and reduced in number, as the highest.

If we take as the standard of high organization, the amount of differentia-
tion and specialization of the several organs in each being when adult (and
this will include the advancem.ent of the brain for intellectual purposes),
natural selection clearly leads toward this standard : for all physiologists ad-
mit that the specialization of organs, inasmuch as in this state they perform
their functions better, is an advantage to each being; and hence the accumu-
lation of variations tending toward specialization is within the scope of natu-
ral selection. On the other hand, we can see, bearing in mind that all organic
beings are striving to increase at a high ratio and to seize on every unoc-
cupied 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 Suc-
cession.

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 every-
where 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 strongly that he was led to suppose that new
and simple forms are continually being produced by spontaneous 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 rela-
tions 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
earth-worm, to be highly organized. If it were no advantage, these forms
would be left, by natural selection, unimproved or but little improved, and
might remain for indefinite ages in their present lowly condition. 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 ver-
tebrata, to the co-existence of mammals and fish — among mammalia, to the

84 THE ORIGIN OF SPECIES

co-existence of man and the Ornithorhynchus — among fishes, to the co-exist•^
ence of the shark and the lancelet (Amphioxus), which latter fish in the ex-
treme simplicity of its structure approaches the invertebrate classes. But
mammals and fish hardly come into competition with each other; the ad-
vancement of the whole class of mammals, or of certain members in this class,
to the highest grade, would not lead to their taking the place of fishes. Physi-
ologists believe that the brain must be bathed by warm blood to be highly
active, and this requires aerial respiration; so that warm-blooded mammals
when inhabiting the water lie under a disadvantage in having to come con-
tinually to the surface to breathe. With fishes, members of the shark family
would not tend to supplant the lancelet; for the lancelet, as I hear from Fritz
Miiller, has as sole companion and competitor on the barren sandy shore of
South Brazil, an anomalous annelid. The three lowest orders of mammals,
namely, marsupials, edentata, and rodents, co-exist in South America in the
same region with numerous monkeys, and probably interfere little with each
other. Although organization, on the whole, may have advanced and be
still advancing throughout the world, yet the scale will always present many
degrees of perfection; for the high advancement of certain whole classes, or
of certain m^embers of each class, does not at all necessarily lead to the ex-
tinction of those groups with which they do not enter into close competition.
In some cases, as we shall hereafter see, lowly organized forms appear to
have been preserved to the present day, from inhabiting confined or peculiar
stations, where they have been subjected to less severe competition, and
where their scanty numbers have retarded the chance of favorable varia-
tions arising.

Finally, I believe that many lowly organized forms now exist throughout
the world, from various causes. In some cases, variations or individual dif-
ferences of a favorable nature may never have arisen for natural selection
to act on and accumulate. In no case, probably, has time sufficed for the
utmost possible amount of development. 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, — possibly 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 first dawn of life, when all organic beings, as we may be-
lieve, presented the simplest structure, how, it has been asked, could the first
step 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 "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, speculation on the subject is almost useless. It is, however, an error to sup-
pose that there would be no struggle for existence, and consequently no natu-
ral selection, until many forms had been produced: variations in a single
species inhabiting an isolated station might be beneficial, and thus the whole

NATURAL SELECTION 85

mass of individuals might be modified, or two distinct forms might arise.
But, as I remarked toward the close of the introduction, no one ought to feel
surprise at much remaining as yet unexplained on the origin of species, if we
make due 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. G. Watson thinks that I have overrated the importance of diver-
gence of character (in which, however, he apparently believes) , and that con-
vergence, as it may be called, has likewise played a part. If two species be-
longing to two distinct though allied genera, had both produced a large
number of new and divergent forms, it is conceivable that these might ap-
proach 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 extremely rash to attribute to con-
vergence a close and general similarity of structure in the modified descend-
ants 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 condi-
tions, 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 relations. 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 con-
nection, recurring in widely separated geological formations; and the bal-
ance of evidence is opposed to any such admission.

Mr. Watson has also objected that the continued action of natural selec-
tion, together with divergence of character, would tend to make an indefi-
nite number of specific forms. As far as mere inorganic conditions are con-
cerned, it seems probable that a sufficient number of species would soon be-
come adapted to all considerable diversities of heat, moisture, etc.; but I
fully admit that the mutual relations of organic beings are more important;
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 no limit to the amount of profitable diversification 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 Australia, which support such an

86 THE ORIGIN OF SPECIES

astonishing number of species, many European plants have become natural-
ized. 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 all increased. What then checks
an indefinite increase in the number of species? The amount of life ( I do
not mean the number of specific forms) supported on an area must have a
limit, depending so largely as it does on physical conditions ; therefore, if an
area be inhabited by very many species, each or nearly each species will be
represented by few individuals; and such species will be liable to extermina-
tion from accidental fluctuations in the nature of the seasons or in the num-
ber of their enemies. The process of extermination 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 individuals 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 increased, will, on the principle
often explained, present within a given period few favorable variations; con-
sequently, the process of giving birth to new specific forms would thus be re-
tarded. 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 Lithuania, 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 CandoUe 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 recently
shown that in the south-east corner of Australia, where, apparently, there are
many invaders from diff"erent 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 dif-
ferences 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 infinite 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 extraordinary 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.^ut if variations useful to any organic being ever do occur,

NATURAL SELECTION 87

ssuredlv individuals thus characterized will have the best chance of being
ireserved in the struggle for life; and from the strong principle of inheritance,
hese will tend to produce offspring similarly characterized. This principle
,f preservation, or the survival of the fittest, I have called natural selection.:'
t leads to the improvement of each creature in relation to its organic and
nor^anic conditions of life; and consequently, in most cases, to what must be
•e^arded as an advance in organization. Nevertheless, low and simple forms
Yill lonff endure if well fitted for their simple conditions of life.^

Natural selection, on the principle of qualities being inherited at corre-
;ponding ages, can modify the egg, seed, or young, as easily as the adult.
\mon^ many animals sexual selection will have given its aid to ordinary
;election by assuring to the most vigorous and best adapted males the great-
est number of offspring. Sexual selection will also give characters useful to
■he males alone in their struggles or rivalry with other males; and these char-
acters will be transmitted to one sex or to both sexes, according to the lorm
Df inheritance which prevails. , . , . i

Whether natural selection has really thus acted m adapting the various
forms of life to their several conditions and stations, must be judged by the
general tenor and balance of evidence given in the following chapters. But
we have aheady 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 num-
ber be supported on the area, of which we see proof by looking to the inhabit-
ants of any small spot, and to the productions naturaHzed m 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 nTthTbatde for life:\Thus the small differences distinguishing varie-
ties of the same species, steadily tend to increase, till they equal the greater
differences between species of the same genus, or even of distinct genera.

We have seen that it is the common, the widely diffused, and widely rang-
ing 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 remarked, leads to divergence of character and to much
extinction of the less improved and intermediate forms of hfe. On these prin-
ciples, 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 everywhere 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

88 THE ORIGIN OF SPECIES

sub-familiesj 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 diver-
gence of character, as we have seen illustrated in the diagram.

The affinities of all the beings of the same class have sometimes been rep-
resented by a great tree. I believe this simile largely speaks the truth. The
green and budding twigs may represent existing species ; and those produced
during 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 ex-
tinct and living species in groups subordinate to groups. Of the many twigs
which flourished when thc'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 free, many
a limb and branch has decayed and dropped off; and these fallen branches
of various sizes may represent those whole orders, families, and genera which
have now no living representatives, and which are known to us only in a fos-
sil 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 Orni--
thorhynchus or Lepidosiren, which in some small degree connects by its af-
finities two large branches of life, and which has apparently been saved from
fatal competition by having inhabited a protected station. As buds give rise
by growth to fresh buds, and these, if vigorous, branch out and overtop on
all sides many a feebler branch, so by generation I believe it has been with
the great Tree of Life, which fills with its dead and broken branches the
crust of the eartfi, and covers the surface with its ever-branching and beauti-»
ful ramifications.

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 — Com-
pensation and Economy of Growth — False Correlations — Multiple, Rudimentary,
and Lowly Organized Structures Variable — Parts developed in an Unusual Man-
ner are highly Variable: Specific Characters more Variable than Generic; Sec-
ondary 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 mul-
tiform 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 func-
tion 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 frequently under domes-
tication than under nature, and the greater variability of species having wide
ranges than of those with restricted ranges, lead to the conclusion that vari-
ability is generally related to the conditions of life to which each species has
been exposed during several successive generations. In the first chapter I at-
tempted to show that changed conditions act in two ways, directly on the
whole organization or on certain parts alone, and indirectly through the re-
productive system. In all cases there are two factors, the nature of the organ-
ism, which is much the most important of the two, and the nature of the
conditions. The direct action of changed conditions leads to definite or in-
definite 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 condi-
tions, and all, or nearly all, the individuals become modified in the same way.
It is very difficult to decide how far changed conditions, 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 following
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 from farther
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 in-
sects. Moquin-Tandon gives a list of plants which, when growing near the
seashore, have their leaves in some degree fleshy, though not elsewhere

89

90 THE ORIGIN OF SPECIES

fleshy. These slightly varying organisms are interesting in as far as they pre-
sent characters analogous to those possessed by the species which are con-
fined 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
farther north they live: but who can tell how much of this difference may be
due to the warmest clad individuals having been favored and preserved
during many generations, and how much to the action of the severe climate?
Tor it would appear that climate has some direct action on the hair of our
domestic 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 appar-
ently the same external conditions. Again, innumerable instances are known
to every naturalist, of species keeping true, or not varying at all, although
living under the most opposite climates. Such considerations as these incline
me to lay 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
conditions 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 some manner excited, but it is the will of man
which accumulates the variations in a certain direction; 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 cer-
tain parts, and disuse diminished them; and that such modifications are in-
herited. Under free nature we have no standard of comparison by which to
judge of the effects of long-continued use or disuse, for we know not the
parent-forms; but many animals possess structures which can be best ex-
plained 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 sev-
eral 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 condi-
tion as the domestic Aylesbury duck: it is a remarkable fact that the young
birds, according 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 islands, tenanted by no

LAWS OF VARIATION 91

beasts of prey, has been caused by disuse. The ostrich indeed inhabits con-
tinents, and is exposed to danger from which it cannot escape by flight, but
it can defend itself, by kicking its enemies, as efficiently as many quadrupeds.
We may believe that the 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 incapable of flight.

Kirby has remarked (and I have observed the same fact) that the anterior
tarsi, or feet, of many male dung-feeding beetles are often broken off; he
examined seventeen specimens in his own collection, and not one had even
a relic left. In the Onites apelles the tarsi are so habitually lost that the insect
has been described as not having them. In some other genera they are
present, but in a rudimentary condition. In the Ateuchus or sacred beetle of
the Egyptians, they are totally deficient. The evidence that accidental
mutilations can be inherited is at present not decisive; but the remarkable
cases observed 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 beetles 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 dis-
covered the remarkable fact that 200 beetles, out of the 550 species (but
more are now known) inhabiting Madeira, are so far deficient in wings that
they cannot fly; and that, of the twenty-nine endemic genera, no less than
twenty- three 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
concealed, 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 extraordinary fact, so strongly insisted on by Mr. Wollas-
ton, 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 individual beetle which flew least, either from its wings
having been ever so little less perfectly developed or from indolent habit,
will have had the best chance of surviving from not being blown out to sea;
and, on the other hand, those beetles which most readily took to flight 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 lepidoptera, must habitually use their

92 THE ORIGIN OF SPECIES

wings to gain their subsistence, have, as Mr. Wollaston suspects, their wings
not at all reduced, but even enlarged. This is quite compatible with the
action of natural selection. For when a new insect first arrived on the island,
the tendency of natural selection to enlarge or to reduce the wings, would
depend on whether a greater number of individuals were saved by success-
fully 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 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 rodent, the tucotuco, or
Ctenomys, is even more subterranean in its habits than the mole; and I
was assured by a Spaniard, who had often caught them, that they were
frequently blind. One which I kept alive was certainly in this condition, the
cause, as appeared on dissection, having been inflammation of the nictitating
membrane. As frequent inflammation of the eyes must be injurious to any
animal, and as eyes are certainly not necessary to animals having subter-
ranean 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 Camiola. and Kentucky, are blind. In
some of the crabs the footstalk for the eyes 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 may be attributed
to disuse. In one of the blind animals, namely, the cave-rat (Neotoma),
two of which were captured by Professor Silliman at above half a mile
distance 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 Professor 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 similar than deep lime-
stone caverns under a nearly similar climate; so that, in accordance with the
old view of the blind animals having been separately created for the Ameri-
can and European caverns, very close similarity in their organization and
affinities might have been expected. 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

LAWS OF VARIATION 93

Europe and of North America." On my view we must suppose that Ameri-
can 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 gradation of habit; for, as Schiodte
remarks: "We accordingly look upon the subterranean faunas as small
ramifications which have penetrated into the earth from the geographically
limited faunas of the adjacent tracts, and which, as they extended them-
selves into darkness, have been accommodated to 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 increase in the length of the
antennae or palpi, as a compensation for blindness. Notwithstanding such
modifications, we might expect still to see in the cave-animals of America,
affinities to the other inhabitants of that continent, and in those of Europe
to the inhabitants of the European continent. And this is the case with some
of the American cave-animals, as I hear from Professor Dana; and some
of the European cave-insects are very closely allied to those of the surround-
ing country. It would be difficult to give any rational explanation of the
affinities of the blind cave-animals to the other inhabitants of the two con-
tinents on the ordinary view of their independent creation. That several of
the inhabitants of the caves of the Old and New Worlds should be closely
related, we might expect from the well-known relationship of most of their
other productions. As a blind species of Bathyscia is found in abundance on
shady rocks far from caves, the loss of vision in the cave species of this one
genus has probably had no relation to its dark habitation; for it is natural
that an insect already deprived 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 in 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 surprised 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.

94 THE ORIGIN OF SPECIES

ACCLIMATIZATION

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 extremely com-
mon 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 de-
scended 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 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 over-
rated. 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 per-
fectly 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 different temperatures; that is, they become ac-
climatized; thus the pines and rhododendrons, raised from seed collected
by Dr. Hooker from the same species growing at different heights on the
Himalayas, 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 authen-
tic instances could be adduced of species having largely extended, within
historical times, their range from warmer to colder latitudes, and con-
versely; but we do not positively 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 acclimated to their 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 extraordinary capacity in
our domestic animals of not only withstanding the most different climates,
but of being perfectly fertile (a far severer test) under them, may be used
as an argument that a large proportion of other animals now in a state of
nature could easily be brought to bear widely different climates. We must
not, however, push the foregoing argument too far, on account of the

LAWS OF VARIATION 95

probable origin of some of our domestic animals from several wild stocks;
the bloodj for instance, of a tropical and arctic wolf may perhaps be
mingled in our domestic breeds. The rat and mouse cannot be considered
as domestic animals, but they have been transported by man to many parts
of the world, and now have a far wider range than any other rodent; for
they live under the cold climate of Faroe in the north, of the Falklands in
the south, and on many an island in the torrid zones. Hence adaptation to
any special climate may be looked at as 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 rhi-
noceros having 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 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 hav-
ing different innate constitutions, and how much to both means combined,
is an obscure question. That habit or custom has some influence, I must
believe, both from analogy and from the incessant advice given in agricul-
tural 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 constitutions 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 con-
stitutions best adapted to any country which they inhabited. In treatises on
many kinds of cultivated plants, certain varieties are said to withstand cer-
tain climates better than others; this is strikingly shown in works on fruit-
trees published in the United States, in which certain varieties are habitually
recommended for the Northern and others for the Southern States; and
as most of these varieties are of recent origin, they cannot owe their con-
stitutional differences 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 produced, has even been advanced, as proving that
acclimatization 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 pur-
pose, and with much greater weight; but until some one will sow, during
a score of generations, his kidney-beans so early that a very large propor-
tion are destroyed by frost, and then collect seed from the few survivors,
with care to prevent accidental crosses, and then again get seed from these
seedlings, with the same precautions, the experiment cannot be said to have
been 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.

96 THE ORIGIN OF SPECIES

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 constitu-
tion 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 under-
stood, and no doubt wholly different classes of facts may be here easily con-
founded together. We shall presently see that simple inheritance often gives
the false appearance of correlation. One of the most real obvious cases is
that variations of structure arising in the young or larvse naturally tend to
affect the structure of the mature animal. The several parts which are
homologous, and which, at an early embryonic 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, ac-
cording 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. Is. Geoffroy Saint-
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 deafness, or between the tortoise-shell color and the
female sex; or in pigeons, between their feathered feet and skin betwixt
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 the teeth in the naked Turkish dog,
though here no doubt homology comes into play? With respect to this

LAWS OF VARIATION 97

latter case of correlation, I think it can hardly be accidental that the two
orders of mammals which are most abnormal in their dermal covering, viz.,
cetacea (whales) and edentata (armadilloes, 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 importance of the laws of
correlation and variation, independently of utility, and therefore of natural
selection, than that of the difference between the outer and inner flowers
in some compositous and umbelliferous plants. Every one is familiar with
the difference between the ray and central florets of, for instance, the daisy,
and this difference is often accompanied with the partial or complete abor-
tion 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 compositae
countenances this idea; but with the umbelliferae 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 flowers. It might have been
thought that the development 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 compositae the seeds of the outer and inner
florets differ, without any difference in the corolla. Possibly these several
differences may be connected with the different flow of nutriment 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 abnormally 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 insects, 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 umbelliferae these differences are of such apparent importance —
the seeds being sometimes orthospermous in the exterior flowers and
ccelospermous in the central flowers — that the elder De Candolle founded
his main divisions in the order on such characters. Hence modifications of
structure, viewed by systematists 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.

98 THE ORIGIN OF 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 necessary
manner correlated. Some other correlations are apparently 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 ex-
pressed it, "in order to spend on one side, nature is forced to economize on
the other side." I think this holds true to a certain extent with our domestic
productions: if nourishment flows to one part or organ in excess, it rarely
flows, at least in excess, to another part; thus it is difficult to get a cow to
give much milk and to fatten readily. The same varieties of the cabbage do
not yield abundant and nutritious foliage and a copious supply of oil-
bearing seeds. When the seeds in our fruits become atrophied, the fruit
itself gains largely in size and quality. In our poultry, a large tuft of feathers
on the head is generally accompanied by a diminished comb, and a large
beard by diminished wattles. With species in a state of nature it can hardly
be maintained that the law is of universal application; but many good
observers, more especially botanists, believe in its truth. I will not, how-
ever, here give any instances, for I see hardly any way of distinguishing
between the eff'ects, on the one hand, of a part being largely developed
through natural selection and another and adjoining part being reduced
by the same process or by disuse, and, on the other hand, the actual with-
drawal of nutriment from one part owing to the excess of growth in
another and adjoining part.

I suspect, also, that some of the cases of compensation which have been
advanced, and likewise some other facts, may be merged under a more
general principle, namely, that natural selection is continually trying to
economize every part of the organization. If, under changed conditions of
life, a structure, before useful, becomes less useful, its diminution will be
favored, for it wiU profit the individual not to have its nutriment wasted in
building up a useless structure. I can thus only understand a fact with

LAWS OF VARIATION 99

which I was much struck when examining cirripedes, and of which many
analogous instances could be given: namely, that when a cirripede is
parasitic within another cirripede, and is thus protected, it loses more or
less completely its own shell or carapace. This is the case with the male Ibla,
and in a truly extraordinary manner with the Proteolepas : for the carapace
in all other cirripedes consists of the three highly important anterior seg-
ments of the head enormously developed, and furnished with great nerves
and muscles; but in the parasitic and protected Proteolepas, the whole
anterior part of the head is reduced to the merest rudiment attached to
the bases of the prehensile antennae. Now the saving of a large and complex
structure, when rendered superfluous, would be a decided advantage to each
successive individual of the species; for in the struggle for life to which
every animal is exposed, each would have a better chance of supporting
itself, by less nutriment being wasted.

Thus, as I beHeve, natural selection will tend in the long-run to reduce
any part of the organization, as soon as it becomes, through changed habits,
superfluous, without by any means causing some other part to be largely
developed in a corresponding degree. And conversely, that natural selection
may perfectly well succeed in largely developing an organ without requiring
as a necessary compensation the reduction of some adjoining part.

MULTIPLE, RUDIMENTARY, AND LOWLY ORGANIZED
STRUCTURES ARE VARIABLE

It seems to be a rule, as remarked by Is. Geoffroy Saint-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 numbers, is constant. The same author, as
well as some botanists, have further remarked that multiple parts are
extremely liable to vary in structure. As "vegetable repetition," to use Pro-
fessor Owen's expression, is a sign of low organization, the foregoing state-
ments 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.

Rudimentary parts, as is generally admitted, are apt to be highly variable.

100 THE ORIGIN OF SPECIES

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 Owen, also, seems to have come to a nearly
similar conclusion. 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 allowances for them. It
should be understood that the rule by no means applies to any part, how-
ever 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 the 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 remarkable manner in comparison with the other species of
the same genus. The rule applies very strongly in the case of secondary
sexual characters, when displayed in any unusual manner. The term, sec-
ondary sexual characters, used by Hunter, relates to characters which are
attached to one sex, but are not directly connected with the act of reproduc-
tion. 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 sexual characters, may
be due to the great variability of these characters, whether or not displayed
in any unusual manner — of which fact I think there can be little doubt.
But that our rule is not confined to secondary sexual characters is clearly
shown in the case of hermaphrodite cirripedes; I particularly attended to
Mr. Waterhouse' s 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 remarkable cases. I will here give only one, as it
illustrates the rule in its largest application. The opercular valves of sessile
cirripedes (rock barnacles) are, in every sense of the word, very important
structures, and they differ extremely little even in distinct genera; but in
the several species of one genus, Pyrgoma, these valves present a marvellous
amount of diversification; the homologous valves in the different species
being sometimes wholly unlike in shape; and the amount of variation in
the individuals of the same species is so great that it is no exaggeration to
state that the varieties of the same species differ more from each other in
the characters derived from these important organs, than do the species
belonging to other distinct genera.

LAWS OF VARIATION 101

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 certainly seems to hold good in this class. I cannot make out that
it applies to plants, and this would have seriously shaken my belief in its
truth, had not the great variability in plants made it particularly difficult
to compare their relative degrees of variability.

When we see any part or organ developed in a remarkable degree or
manner in a species, the fair presumption is that it is of high importance
to that species: nevertheless it is in this case eminently liable to variation.
Why should this be so? On the view that each species has been inde-
pendently created, with all its parts as we now see them, I can see no ex-
planation. But on 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 par-
ticularly concerns us is, that those points in our domestic animals, which
at the present time are undergoing rapid change by continued selection, 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 con-
stant 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 selection gains the day, and we do not expect to fail so com-
pletely as to breed a bird as coarse as a common tumbler pigeon from a
good short-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 extraor-
dinary manner in any one species, compared with the other species of the
same genus, we may conclude that this part has undergone an extraordinary
amount of modification since the period when the several species branched
off from the common progenitor 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 con-

102 THE ORIGIN OF SPECIES

tinually been accumulated by natural selection for the benefit of the species.
But as the variability of the extraordinarily developed part or organ has
been so great and long-continued, within a period not excessively remote,
we might, as a general rule, still expect to find more variability in such
parts than in other parts of the organization 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 transmitted in approximately the same condi-
tion to many modified descendants, as in the case of the wing of the bat,
it must have existed, according to our theory, 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 modification has
been comparatively recent and extraordinarily great that we ought to find
the generative variability, as it may be called, still present in a high degree.
For in this case the variability will seldom as yet have been fixed by the
continued selection of the individuals varying in the required manner and
degree, and by the continued rejection of those tending to revert to a former
and less modified condition.

SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC CHARACTERS

The principle discussed under the last heading may be applied to our
present subject. It is notorious that 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 species varying into red, or conversely; but if all the species had
blue flowers, the color would become a generic character, and its variation
would be a more unusual circumstance. I have chosen this example because
the explanation which most naturalists would advance is not here ap-
plicable, namely, that specific characters are more variable than generic,
because they are taken from parts of less physiological importance than
those commonly used for classing genera. I believe this explanation is
partly, yet only indirectly, true; I shall, however, have to return to this
point in the chapter on Classification. It would be almost superfluous to
adduce evidence in support of the statement, that ordinary specific charac-
ters 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,

LAWS OF VARIATION 103

often becomes variable, though its physiological importance may remain
the same. Something of the same kind applies to monstrosities: at least Is.
Geoffroy Saint-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 might expect often to find them still
continuing to vary in those parts of their structure which have varied
within a moderately recent period, and which have thus come to differ.
Or to state the case in another manner: the points in which all the species
of a genus resemble each other, and in which they differ from allied genera,
are called generic characters; and these characters may be attributed to
inheritance from a common progenitor, for it can rarely have happened
that natural selection will have modified several distinct species, fitted to
more or less widely different habits, in exactly the same manner: and as
those 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 characters have varied and come to differ since the period
when the species branched off from a common progenitor, it is probable
that they should still often be in some degree variable — at least more vari-
able 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 ad-
mitted 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 dis-
played, 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 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.

104 THE ORIGIN OF SPECIES

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 two first 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 num-
ber 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 neuration of the wings is a charac-
ter of- the highest importance, because common to large groups; but in
<:ertain 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 crustaceans 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
my view: I look at all the species of the same genus as having as certainly
descended from a common progenitor, as have the two sexes of any one
species. Consequently, whatever part of the structure of the common pro-
genitor, 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 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 extreme
variability of any part which is developed in a species in an extraordinary
manner in comparison with the same part in its congeners; and the 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 difference in closely allied species;
that secondary sexual and ordinary specific differences are generally dis-
played 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 in-
herited much in common, to parts which have recently and largely varied
being more likely still to go on varying than parts which have long been
inherited and have not varied, to natural selection having more or less
completely, according to the lapse of time, overmastered the tendency to
reversion and to further variability, to sexual selection being less rigid than
ordinary selection, and to variations in the same parts having been ac-

LAWS OF VARIATION 105

cumulated 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 variations in two or more distinct races. The
frequent presence of fourteen or even sixteen tail-feathers in the pouter may
be considered as a variation representing the normal structure of another
race, the fantail. I presume that no one will doubt that all such analogous
variations are due to the several races of the pigeon having inherited from a
common parent the same constitution and tendency to variation, when
acted on by similar unknown influences. In the vegetable kingdom we have
a case of analogous variation, in the enlarged stems, or as commonly called
roots, of the Swedish turnip and ruta-baga, plants which several botanists
rank as varieties produced by cultivation from a common parent: if this be
not so, the case will then be one of analogous variation in two so-called
distinct species; and to these a third may be added, namely, the common
turnip. According to the ordinary view of each species having been in-
dependently created, we should have to attribute this similarity in the
enlarged stems of these three plants, not to the vera causa of community
of descent, and consequent tendency to vary in a like manner, but to three
separate yet closely related acts of creation. Many similar cases of analogous
variation 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 equal 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
externally edged near their basis with white. As all these marks are charac-
teristic of the parent rock-pigeon, I presume that no one will doubt that
this is a case of reversion, and not of a new yet analogous variation appear-
ing in the several breeds. We may, I think, confidently come to this conclu-
sion, 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
having been lost for many^ probably for hundreds of generations. But when

106 THE ORIGIN OF SPECIES

a breed has been crossed only once by some other breed, the offspring
occasionally 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 expres-
sion, from one ancestor, is only one in 2048; 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 f
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 developed. 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 rudi-
mentary 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 expected that they would occasionally vary
in an analogous manner; so that the varieties of two or more species would
resemble each other, or that a variety of one species 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 on an
unimportant nature, for the preservation of all functionally important char-
acters 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 doubt-
ful, what cases are reversions to formerly existing characters, and what are
new but analogous variations, yet we ought, on our theory, sometimes to

LAWS OF VARIATION 107

find the varying oflfspring of a species assuming characters which are already
present in other members of the same group. And this undoubtedly is

the case.

The difficulty in distinguishing variable species is largely due to the
varieties mocking, as it were, other species of the same genus. A con-
siderable 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 created 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 occasionally vary so as to resemble, in some degree, the same
part or organ in an allied species. I have collected a long Hst 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 affect-
ing 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 trans-
verse 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 outUne. A white ass, but not sm 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. 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 distinct breeds and of all colors; transverse bars
on the legs are not rare in duns, mouse-duns, and in one instance in a
chestnut; a faint shoulder-stripe may sometimes be seen in duns, and I
have seen a trace in a bay horse. My son made a careful exaniination and
sketch for me of a dun Belgian cart-horse 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 north-west part of India the Kattywar breed of horses is so gen-
erally 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

108 THE ORIGIN OF SPECIES

the shoulder-stripe, which is sometimes 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 disappear in old horses.
Colonel Poole has seen both gray and bay Kattywar horses striped when
first foaled. I have also reason to suspect, from information given me by
Mr. W. W. Edwards, that with the English race-horse the spinal stripe is
much commoner in the foal than in the full-grown animal. I have myself
recently bred a foal from a bay mare (offspring of a Turkoman horse and a
Flemish mare) by a bay English race-horse. 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 entering on further details I
may state that I have collected cases of leg and shoulder-stripes in horses of
very different breeds in various countries from Britain to Eastern China,
and from Norway in the north to the Malay Archipelago in the south. In
all parts of the world these stripes occur far oftenest in duns and mouse-
duns. By the term dun a large range of color is included, from one between
brown and black to a close approach to cream color.

I am aware that Colonel Hamilton Smith, who has written on this sub-
ject, believes that the several breeds of the horse are descended from
several aboriginal species, one of which, the dun, was striped; and that the
above-described appearances are all due to ancient crosses with the dun
stock. But this view may be safely rejected, for it is highly improbable that
the heavy Belgian cart-horse, Welsh ponies, Norwegian cobs, the lanky
Kattywar race, etc., inhabiting the most distant parts of the world, should
all have been crossed with one supposed aboriginal stock.

Now let us turn to the effects of crossing the several species of the horse
genus. Rollin asserts that the common mule from the ass and horse is
particularly apt to have bars on its legs ; 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 excellent
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 subsequently produced from the same mare by a black
Arabian sire, were much more plainly barred across the legs than is even
the pure quagga. Lastly, and this is another most remarkable case, a hybrid
has been figured by Dr. Gray (and he informs me that he knows of a second
case) from the ass and the hemionus; and this hybrid, though the ass 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

LAWS OF VARIATION 109

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

He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a tendency
to vary, both under nature and under domestication, in this particular
manner, so as often to become striped like the other species of the genus;
and that each has been created with a strong tendency, when crossed with
species inhabiting distant quarters of the world, to produce hybrids 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 seashore.

no THE ORIGIN OF SPECIES

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 fluctuating
variability, but sometimes they cause direct and definite effects; and these
may become strongly marked in the course of time, though we have not
sufficient evidence on this head. Habit in producing constitutional peculiari-
ties, and use in strengthening and disuse in weakening 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 spe-
cialized 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 beings low in the scale are more variable than those
standing higher in the scale, and which have their whole organization more
specialized. Rudimentary organs, from being useless, are not regulated by
natural selection, and hence are variable. Specific characters — that is, the
characters which have come to differ since the several species of the same
genus branched off from a common parent — are more variable than generic
characters, or those which have long been inherited, and have not differed
within this same period. In these remarks we have referred to special parts
or organs being still variable, because they have recently varied and thus
come to differ; but we have also seen in the second chapter that the same
principle applies to the whole individual; for in a district where many
species of 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 — in that district and among these species,
we now find, on an average, most varieties. Secondary sexual characters
are highly variable, and such characters differ ranch 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

LAWS OF VARIATION 111

allied species, must have gone through an extraordinary amount of modifi-
cation 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 varia-
tion 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
an extraordinarily developed organ has become the parent of many modified
descendants — which in our view must be a very slow process, requiring 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 constitution from
a common parent, and exposed to similar influences, naturally tend to
present analogous variations, 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 reversion and analogous varia-
tion, such modifications will add to the beautiful and harmonious diversity
of nature.

Whatever the cause may be of each slight difference between the off-
spring 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 rela-
tion to the habits of each species.

CHAPTER VI

Difficulties of the Theory

Difficulties of the Theory of Descent with Modification — Absence or Rarity of Transi-
tional 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 species by fine
gradations, do we not everywhere see innumerable transitional forms?
Why is not all nature in confusion, instead of the species being, as we see
them, well defined?

Secondly, is it possible that an animal having, for instance, the structure
and habits of a bat, could have been formed by the modification of some
other animal with widely different habits and structure? Can we believe
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 selec-
tion? What shall we say to the instinct which leads the bee to make cells,
and which has practically anticipated the discoveries of profound mathe-
maticians?

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

The two first heads here will be discussed; some miscellaneous 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 modifica-
tions, 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 transi-
tional varieties will generally have been exterminated by the very process
of the formation and perfection of the new form.

112

DIFFICULTIES OF THE THEORY 113

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 be more convenient to discuss this question m the chapter
on the Imperfection of the Geological Record; and I will here only state
that I beheve the answer mainly lies in the record bemg incom^rably_^ .Less
perfect than is gen^Slysuppo^^
b^QEEjoiSniaOonecg^^

intervals of time^ . . , ,•.. *u

-^ut 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 m 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 alHed 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 m each region
though they must have existed there, and may be imbedded there m a fossil
condition. But in the intermediate region, having intermediate conditions ot
life why do we not now find closely linking intermediate varieties.'' I his
difficulty for a long time quite confounded me. But I think it can be m large

part explained. , . . . r • i_

In the first place we should be extremely cautious m 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 m such islands
distinct species might have been separately formed without the possibility ot
intermediate varieties existing in the intermediate zones. By changes m the
form of the land and of climate, marine areas now continuous must of ten have
existed within recent times in a far less continuous and umform 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 ion
of areas now continuous, has played an important part in the formation
of new species, more especially with freely crossing and wandering ammals.
In looking at species as they are now distributed over a wide area we gen-
erally 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

114 THE ORIGIN OF SPECIES

narrow in comparison with the territory proper to each. We see the same
fact in ascending mountains, and sometimes it is quite remarkable how
abruptly, as Alph. de Candolle has observed, a common alpine species dis-
appears. The same fact has been noticed by E. Forbes in sounding the depths
of the sea with the dredge. To those who look at climate and the physical
conditions of life as the all-important elements of distribution, these facts
ought to cause surprise, as climate and height or depth graduate away in-
sensibly. But when we bear in mind that almost every species, even in its
metropolis, would increase immensely in numbers, were it not for other com-
peting 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 blend-
ing 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 manner that each has a wide range, with a
comparatively narrow neutral territory between them, in which they become
rather suddenly rarer and rarer; then, as varieties do not essentially differ
from species, the same rule will probably apply to both; and if we take a
varying species inhabiting a very large area, we shall have to adapt two varie-
ties to two large areas, and a third variety to a narrow intermediate zone. The
intermediate variety, consequently, will exist in lesser numbers from inhabit-
ing a narrow and lesser area; and practically, as far as I can make out, this
rule holds good with varieties in a state of nature. I have met with striking
instances of the rule in the case of varieties intermediate between well-marked
varieties in the genus Balanus. And it would appear from information given
me by Mr. Watson, Dr. Asa Gray and Mr. Wollaston, that generally, when
varieties intermediate between two other forms occur, they are much rarer
numerically than the forms which they connect. Now, if we may trust these
facts and inferences, and conclude that varieties linking two other varieties
together generally have existed in lesser numbers than the forms which they
connect, then we can understand why intermediate varieties should not endure
for very long periods: why, as a general rule, they should be exterminated
and disappear, sooner than the forms which they originally linked together.

For any form existing in lesser numbers would, as already remarked, run
a greater chance of being exterminated than one existing in large numbers;
and in this particular case the intermediate form would be eminently liable
to the inroads of closely allied forms existing on both sides of it. But it is a

DIFFICULTIES OF THE THEORY 115

far more important consideration, that during the process of further modifi-
cation, by which two varieties are supposed to be converted and perfected
into two distinct species, the two which exist in larger numbers, from inhabit-
ing larger areas, will have a great advantage over the intermediate variety,
which exists in smaller numbers in a narrow and intermediate zone. For
forms existing in larger numbers will 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 numbers. Hence, the more
common forms, in the race for life, will tend to beat and supplant the less
common forms, for these will be more slowly modified and improved. It is the
same principle which, as I believe, accounts for the common species in each
country, as shown in the second chapter, presenting on an average a greater
number of well-marked varieties than do the rarer species. I may illustrate
what I mean by supposing three varieties of sheep to be kept, one adapted
to an extensive mountainous region; a second to a comparatively narrow,
hilly tract; and a third to the wide plains at the base; and that the inhabit-
ants 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 hold-
ers on the mountains or on the plains, improving their breeds more quickly
than the small holders on the intermediate narrow, hilly tract; and conse-
quently the improved mountain or plain breed will soon take the place of the
less improved hill breed; and thus the two breeds, which originally existed
in 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
variation is a slow process, and natural selection can do nothing until favor-
able individual dififerences or variations occur, and until a place in the natu-
ral polity of the country can be better filled by some modification of some
one or more of its inhabitants. And such new places will depend on slow
changes of climate, or on the occasional immigration of new inhabitants,
and, probably, in a still more important degree, on some of the old inhabit-
ants becoming slowly modified, with the new forms thus produced and the
old one 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 modifica-
tions of structure in some degree permanent; and this assuredly we do see.

Secondly, areas now continuous must often have existed 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 common parent, must formerly have existed within each isolated por-
tion 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.

116 THE ORIGIN OF SPECIES

Thirdly, when two or more varieties have been formed in different por-
tions of a strictly continuous area, intermediate varieties will, it is probable
at first have been formed in the intermediate zones, but they will generall)
have had a short duration. For these intermediate varieties will, from reason*
already assigned (namely from what we know of the actual distribution oi
closely allied or representative species, and likewise of acknowledged varie-
ties), 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 at 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 natural
selection constantly tends, as has been so often remarked, to exterminate the
parent-forms and the intermediate links. Consequently evidence of their
former existence could be found among fossil remains, which are preserved,
as we shall attempt to show in a future chapter, in an extremely imperfect
and intermittent record.

ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH PECULIAR HABITS

AND STRUCTURE

It has been asked by the opponents of such views as I hold, how, for in-
stance, 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 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
pole-cats, on mice and land animals. If a different case had been taken, and
it had been asked how an insectivorous quadruped could possibly have been
converted into a flying bat, the question would have been far more difficult
to answer. Yet I think such difficulties have little weight.

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

Look at the family of squirrels; here we have the finest gradation from

DIFFICULTIES OF THE THEOPvY 117

animals with their tails only slightly flattened, and from others, as Sir J.
Richardson has remarked, with the posterior part of their bodies rather
wide and with the skin on their flanks rather full, to the so-called flying squir-
rels ; 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 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 become modified and improved in
structure in a corresponding manner. Therefore, I can see no difficulty, more
especially under changing conditions of life, in the continued preservation of
individuals with fuller and fuller flank-membranes, each modification being
useful, each being propagated, until, by the accumulated effects of this proc-
ess of natural selection, a perfect so-called flying squirrel was produced.

Now look at the Galeopithecus or so-called flying lemur, which was for-
merly 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-mem-
brane is furnished with an extensor muscle. Although no graduated links of
structure, fitted for gliding through the air, now connect the Galeopithecus
with the other Insectivora, yet there is no difficulty 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 be-
lieving that the membrane connecting fingers and forearm of the Galeopithe-
cus 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 struc-
ture of each of these birds is good for it, under 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 per-

118 THE ORIGIN OF SPECIES

haps 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 Crus-
tacea 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 Ijad flying reptiles, it is conceivable that flying-fish, which now glide
far through the air, slightly rising and turning by the aid of their fluttering
fins, might have been modified into perfectly winged animals. If this had
been effected, who would have ever imagined that in an early transitional
state they had been the inhabitants of the open ocean, and had used their
incipient organs of flight exclusively, so far as we know, to escape being de-
voured 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 display-
ing early transitional 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. Further-
more, 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
imaginary illustration of the flying-fish, it does not seem probable that fishes
capable of true flight would have been developed under many subordinate
forms, for taking prey of many kinds in many ways, on the land and in the
water, until their organs of flight had come to a high stage of perfection, so
as to have given them a decided advantage over other animals in the battle
for life. Hence the chance of discovering species with transitional grades of
structure in a fossil condition will always be less, from their having existed in
lesser numbers, than in the case of species with fully developed structures.

I will now give two or three instances, both of diversified and of changed
habits, in the individuals of the same species. In either case it would be easy
for natural selection to adapt the structure of the animal to its changed
habits, or exclusively to one of its several habits. It is, however, difficult to
decide and immaterial for us, whether habits generally change first and struc-
ture afterward; or whether slight modifications of structure lead to changed
habits; both probably often occurring almost simultaneously. Of cases of
changed habits it will suffice merely to allude to that of the many British in-
sects which now feed on exotic plants, or exclusively on artificial substances.
Of diversified habits innumerable instances could be given: I have often
watched a tyrant flycatcher (Saurophagus sulphuratus) in South America,
hovering over one spot and then proceeding to another, like a kestrel, and at
other times standing stationary on the margin of water, and then dashing
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,

DIFFICULTIES OF THE THEORY 119

and thus breaking them Hke a nuthatch. In North America the black bear
was seen by Hearne swimming for hours with widely open mouth, thus
catching, almost hke a whale, insects in the water.

As we sometimes see individuals following habits different 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, hav-
ing 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 striking instance of adaptation be given than that of a woodpecker 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 woodpeck-
ers, 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 charac-
ters 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 wood-
pecker, 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 Saussure 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 astonish-
ing power of diving, in its manner of swimming and of flying when made to
take flight, would be mistaken by any one for an auk or a grebe; nevertheless
it is essentially a petrel, but with many parts of its organization profoundly
modified 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 but of its wings, and remains as long as four
hours beneath the surface; yet it exhibits no modification in structure in ac-
cordance 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

120 THE ORIGIN OF SPECIES

and structure not in agreement. What can be plainer than that the webbec
feet of ducks and geese are formed for swimming? Yet there are upland geesi
with webbed feet which rarely go near the water; and no one, except Audui
bon, 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 eminenth
aquatic, although their toes are only bordered by membrane. What seemj
plainer than that the long toes, not furnished with membrane of the Gralla4
tores, are formed for walking over swamps and floating plants? The water
hen and landrail are members of this order, yet the first is nearly as aquatici
as the coot, and the second is nearly as terrestrial as the quail or partridges
In such cases, and many others could be given, habits have changed without ai
corresponding change of structure. The webbed feet of the upland goose may
be said to have become almost rudimentary in function, though not in struc-:
ture. 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 alight-
ing on the water, that there should be long-toed corncrakes, living in mead-
ows 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.

ORGANS OF EXTREME PERFECTION AND COMPLICATION

To suppose that the eye with all its inimitable contrivances for adjusting
the focus to different distances, for admitting different amounts of light, and
for the correction of spherical and chromatic aberration, could have been
formed by natural selection, seems, I freely confess, absurd in the highest
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 philosopher knows, cannot be trusted
in science. Reason 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

DIFFICULTIES OF THE THEORY 121

formed by natural selection, though insuperable by our imagination, should
not be considered as subversive of the theory. How a nerve comes to be sensi-
tive 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 perceiving light, it does not seem impossible that
certain sensitive elements in their sarcode should become aggregated and de-
veloped into nerves, endowed with this special sensibility.

In searching for the gradations through which an organ in any species has
been perfected, we ought 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 perfected.

The simplest organ which can be called an eye consists of an optic nerve,
surrounded by pigment-cells and covered by transluc-ent skin, but without
any lens or other refractive body. We may, however, according to M. Jour-
dain, descend even a step lower and find aggregates of pigment-cells, appar-
ently serving as organs of vision, 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 star-fishes,
small depressions in the layer of pigment which surrounds the nerve are filled,
as described by the author just quoted, with transparent gelatinous matter,
projecting with a convex surface, like the cornea in the higher animals. He
suggests that this serves not to form an image, bui only to concentrate 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 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 lens or other optical contrivance. With insects it is now known
that the numerous facets on the cornea of their great compound eyes form
true lenses, and that the cones include curiously modified nervous filaments.
But these organs in the Articulata are so much diversified that Miiller for-
merly 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 graduated range of structure in the eyes of the
lower animals; and when we bear in mind how small the number of all liv-
ing 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
converted the simple apparatus of an optic nerve, coated with pigment and

122 THE ORIGIN OF SPECIES

invested by transparent membrane, into an optical instrument as perfect as
is possessed by any member of the Articulata 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 explained by 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 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 re-
marked, "If a lens has too short or too long a focus, it may be amended either
by an alteration of curvature, or an alteration 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 instrument." Within the highest
division of the animal kingdom, namely, the Vertebrata, we can start from
an eye so simple, that it consists, as in the lancelet, of a little sack of trans-
parent 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 graduation of dioptic structures is very great. It is a significant fact that
even in man, according to the high authority of Virchow, the beautiful crys-
talline lens is formed in the embryo by an accumulation of epidermic cells,
lying in a sac-like fold of the skin; and the vitreous body is formed from
embryonic subcutaneous tissue. To arrive, however, at a just conclusion re-
garding the formation of the eye, with all its marvellous yet not 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 highest human intellects ; and we naturally infer that the eye has been
formed by a somewhat analogous process. But may not this inference be
presumptuous? Have we any right to assume that the Creator works by in-
tellectual powers like those of men? 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 be-
neath, 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 thick-
nesses, placed at different distances from each other, and with the surfaces of

DIFFICULTIES OF THE THEORY 123

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 care-
fully preserving each which, under varied circumstances, in any way or de-
gree, tends to produce a distincter 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 alteration, generation will multiply
them almost infinitely, and natural selection will pick out 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, around which, according
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 gradations 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 alimentary canal respires, digests and excretes. In
the Hydra, the animal may be turned inside out, and the exterior surface will
then digest and the stomach respire. In such cases natural selection might
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 regularly produce at the same time diff"erently constructed flowers;
and if such plants were to produce one kind alone, a great change would be
eff"ected with comparative 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 fol-
lowed 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 func-
tion, and this is an extremely important means of transition: to give one

124 THE ORIGIN OF SPECIES

instance — there are fish wdth gills or branchiae that breathe the air dissolved
in the water, at the same time that they breathe free air in their swim-blad-
ders, this latter organ being divided by highly vascular partitions and ha\dng
a ductus pneumaticus for the supply of air. To give another instance from the
vegetable kingdom: plants climb by three distinct means, by spirally twin-
ing, by clasping a support with their sensitive tendrils, 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, combined
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 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 swim-bladder in fishes is a good one, because it
shows us clearly the highly important fact that an organ originally con-
structed for one purpose, namely, flotation, may be converted into one for
a widely different purpose, namely respiration. The swim-bladder has, also,
been worked in as an accessory to the auditory organs of certain fishes. All
physiologists admit that the swim-bladder is homologous, or "ideally similar"
in position and structure with the lungs of the higher vertebrate animals:
hence there is no reason to doubt that the swim-bladder 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 un-
known prototype, which was furnished with a floating apparatus or swim-
bladder. "VVe 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
w^e swallow has to pass over the orifice of the trachea, with some risk of fall-
ing into the lungs, notvvithstanding the beautiful contrivance by which the
glottis is closed. In the higher vertebrata the branchiae have wholly disap-
peared— but in the embrv'o the slits on the sides of the neck and the loop-
like course of the arteries still mark their former position. But it is conceiv-
able that the now utterly lost branchiae might have been gradually worked
in by natural selection for some distinct purpose : for instance, Landois has
shown that the wings of insects are developed from the trachea; it is there-
fore highly probable 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 conversions from one function to another, that I will give an-
other instance. Pedunculated cirripedes have two minute folds of skin, called
by me the ovigerous frena, which serve, through the means of a sticky secre-
tion, to retain the eggs until they are hatched within the sack. These cirri-
pedes have no branchiae, the whole surface of the body and of the sack, to-
gether with the small frena, serving for respiration. The Balanidae or sessile
cirripedes, on the other hand, have no ovigerous frena, the eggs lying loose at
the bottom of the sack, within the well-enclosed shell; but they have, in the

DIFFICULTIES OF THE THEORY 125

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 graduate into
each other. Therefore it need not be doubted that the two httle folds of skin,
which originally served as ovigerous frena, but which, likewise, very slightly
aided in the act of respiration, have been gradually converted by natural
selection into branchiae, simply through an increase in their size and the
obliteration of their adhesive, glands. If all pedunculated cirripedes had be-
come extinct, and they have suffered far more extinction than have sessile
cirripedes, who would ever have imagined that the branchiae in this latter
family had originally existed as organs for preventing the ova from being
washed out of the sack?

There is another possible mode of transition, namely, through the accelera-
tion or retardation of the period of reproduction. This has lately been in-
sisted on by Professor Cope and others in the United States. It is now known
that some animals are capable of reproduction at a very early age, before
they have acquired their perfect characters; and if this power became thor-
oughly well developed 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 animals, after arriving at
maturity, go on changing in character 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.
Professor Cope states that the teeth of certain lizards change much in shape
with advancing years. With crustaceans not only many trivial, but some im-
portant, parts assume a new character, as recorded by Fritz Miiller, after
maturity. In all such cases — and many could be given — if the age for repro-
duction were retarded, the character of the species, at least in its adult state,
would be modified; nor is it improbable that the previous and earher stages
of development would in some cases be hurried through and finally lost.
Whether species have 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 concluding 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

126 THE ORIGIN OF SPECIES

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 torpedo they no
doubt serve as powerful means of defence, and perhaps for securing prey;
yet in the ray, as observed by Matteucci, an analogous organ in the tail tin
manifests 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 ray, besides the organ just referred to, there is, as Dr. R. McDonnell 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 ordinary muscle '
a close analogy, in intimate structure, in the distribution of the nerves, and I
in the manner in which they are acted on by various reagents. It should, also, ,
be especially observed that muscular contraction is accompanied by an elec-
trical discharge; and, as Dr. Radcliffe insists, "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 the rest, and the dis- •
charge of the torpedo, instead of being peculiar, may be only another form i
of the discharge which attends upon the action of muscle and motor nerve." '
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 pos-
sible by which these organs might have been gradually developed.

These organs appear at first to offer another and far more serious diffi-^
culty; 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
members 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 modffied descendants have now lost.
But when we look 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 construction, 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 proceeding from different
sources, and this is perhaps the most important of all the differences. Hence
in the several fishes furnished with electric organs, these cannot be consid-
ered as homologous, but only as analogous in function. Consequently there
is no reason to suppose that they have been inherited from a common pro-

DIFFICULTIES OF THE THEORY 127

genitor; for had this been the case they would have closely resembled 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 As-
clepias, 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 will be found that although the general appearance
and function of the organs may be the same, yet fundamental differences be-
tween 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 superficial resemblance, there is hardly any real similarity between the
eyes of cuttle-fish and vertebrates, as may be seen by consulting Hensen's ad-
mirable 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 conceive, 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 Vertebrata.
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 variations;
but if this be admitted in the one case it is 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 inven-
tion, so in the several foregoing cases it appears that natural selection, work-
ing for the good of each being, and taking advantage of all favorable varia-
tions, has produced similar organs, as far as function is concerned, in dis-
tinct organic beings, which owe none of their structure in common to in-
heritance from a common progenitor.

128 THE ORIGIN OF SPECIES

Fritz Miiller, 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 crustaceans include a few species, possessing an air-breathing ap-
paratus 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 systems, in the position of the tufts of hair
within their complex stomachs, and lastly in the whole structure of the water-
breathing branchiae, even to the microscopical hooks by which they are
cleansed. Hence it might have been expected that in the few species belong-
ing 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 struc-
ture must, in accordance with the 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 improbable in the highest degree that their common
progenitor should have been adapted for breathing air. Miiller was thus led
carefully to examine the apparatus in the air-breathing species ; and he found
it to differ in each in several important points, as in the position 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 unintelligible. This line of argument seems to have had
great weight in leading Fritz Miiller to accept the views maintained by me
in this volume.

Another distinguished zoologist, the late Professor Claparede, has argued
in the same manner, and has arrived at the same result. He shows that there
are parasitic mites (Acaridae) , belonging to distinct sub-families and families,
which are furnished with hair-claspers. These organs must have been inde-
pendently 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.

DIFFICULTIES OF THE THEORY 129

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 appear-
ance, 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 differently constructed is the feathered wing of a bird and the 'mem-
brane-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 Nuci jla
to the simple ligament of a Mussel! Seeds are disseminated by their minuce-
ness, by their capsule being converted into a light balloon-like envelope, by
being embedded 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 adhere to the fur of quadrupeds, and by being furnished with
wings and plumes, as different in shape as they are elegant in structure, so
as to be wafted by every breeze. I will give one other instance : for this sub-
ject of the same end being gained by the most diversified means well deserves
attention. Some authors maintain that organic 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 having separated sexes, and with
those in which, though hermaphrodites, the pollen does not spontaneously
fall on the stigma, some aid is necessary for their fertilization. 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 diff'erent plan occurs in many plants in which a symmetrical
flower secretes a few drops of nectar, and is consequently visited by insects;
and these carry the pollen from the anthers to the stigma.

From this simple stage we may pass through an inexhaustible number of
contrivances, all for the same purpose and effected in essentially the same
manner, but entailing 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 elasticity. 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 water continually fall from two secreting
horns which stand above it; and when the bucket is half -full, the water over-
flows 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 in-
genious man, if he had not witnessed what takes place, could never have
imagined what purpose all these parts serve. But Dr. Criiger saw crowds of

130 THE ORIGIN OF SPECIES

large humble-bees visiting the gigantic flowers of this orchid, not in order to
suck nectar, but to gnaw off the ridges within the chamber above the bucket;
in doin^■ this they frequently pushed each other into the bucket, and their
wings yeing thus wetted they could not fly away, but were compelled to crawl
out through the passage formed by the spout or overflow. Dr. Griiger saw a
"coninual 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
iorcjn^ its way out, first rubs its back against the viscid stigma and then
agKifist the viscid glands of the pollen-masses. The pollen-masses are thus
§ted to the back of the bee which first happens to crawl out through the
pli-sage of a lately expanded flower, and are thus carried away. Dr. Griiger
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 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 theni 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 Goryanthes, in order to
gnaw the labellum; in doing this they inevitably touch a long, tapering, sen-
sitive 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 Hke 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 retain the pollen, thus effecting fertilization.

How, it may be asked, in the foregoing and in innumerable other in-
stances, can we understand the graduated scale of complexity and the multi-
farious means for gaining 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 also bear in mind that every
highly developed organism has passed through many changes; and that each .
modified structure tends to be inherited, so that each modification 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 suni of many inherited changes, through which the species has passed
during its successive adaptations to changed habits and conditions of life.

DIFFICULTIES OF THE THEORY 131

Finally then, although in many cases it is most difficult 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 extinct 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 ap-
pear 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 experienced naturalist; or as Milne
Edwards has well expressed it, "Nature is prodigal in variety, but niggard in
innovation." Why, on the theory of Creation, should there be so much variety
and so little real novelty? Why should all the parts and organs of many in-
dependent beings, each supposed to have been separately created for its own
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 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 difficulty 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 econ-
omy 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 triffing
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 as-
suredly be acted on by natural selection. The tail of the giraffe looks like an
artificially constructed fly-flapper; and it seems at first incredible that this
could have been adapted for its present purpose by successive slight modifi-
cations, 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 themselves
from these small enemies, would be able to range into new pastures and thus
gain a great advantage. It is not that the larger quadrupeds are actually
destroyed (except in some rare cases) by ffies, but they are incessantly
harassed and their strength reduced, so that they are more subject to disease.

132 THE ORIGIN OF SPECIES

or not so well enabled in a coming dearth to search for food, or to escape
from beasts of prey.

Organs now of trifling importance have probably in some cases been of
high importance to an early progenitor, and, after having been slowly per-
fected 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 structure would of course have been checked by natural
selection. Seeing how important an organ of locomotion the tail is in most
aquatic animals, its general presence and use for many purposes in so many
land animals, which in their lungs or modified swim-bladders betray their
aquatic origin, may perhaps be thus accounted for. A well-developed tail
having been formed in an aquatic animal, it might subsequently come to be
worked in for all sorts of purposes, as a fly-flapper, an organ of prehension, or
as an aid in turning, as 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 char-
acters, 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 variations, which
seem to depend in a quite subordinate degree on the nature of the condi-
tions, of the tendency to reversion to long-lost characters, of the complex
laws of growth, such as of correlation, comprehension, of the pressure 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 per-
fectly to the other sex, though of no use to the sex. But structures thus
indirectly gained, although at first of no advantage to a species, may sub-
sequently have been taken advantage 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 con-
trivance, 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 quad-
rupeds, 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 modification and became a climber. The
naked skin on the head of a vulture is generally considered as a direct
adaptation for wallowing in putridity; and so it may be, or it may possibly

DIFFICULTIES OF THE THEORY 133

be due to the direct action of putrid matter; but we should be very cautious
in drawing any such inference, when we see that the skin on the head of
the clean-feeding male turkey is likewise naked. The sutures in the skulls of
young mammals have been advanced as a beautiful adaptation for aiding
parturition, and no doubt they facilitate, or may be indispensable for this
act: but as sutures occur in the skulls of young birds and reptiles, which
have only to escape from a broken egg, we may infer that this structure has
arisen from the laws of growth, and has been taken advantage of in the
parturition of the higher animals.

We are profoundly ignorant of the 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 different countries, more especially in the less civilized countries, 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 attacks 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 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 increase the size of the chest; and again correlation would come into
play. The effects of lessened exercise, together with abundant food, on the
whole organization is probably still more important; and this, as H. von
Nathusius 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

134 THE ORIGIN OF SPECIES

that many structures have been created for the sake of beauty, to delight
man or the Creator (but this latter point is beyond the scope of scientific
discussion) , or for 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 many
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, independ-
ently of any advantage thus gained. But a still more important considera-
tion is that the chief part of the organization 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 fore leg of the horse, in the wing of the bat, and in the
flipper of the seal, are of special use to these animals. We may safely
attribute these structures to inheritance. But webbed feet no doubt were as
useful to the progenitor of the upland goose and of the frigate-bird, as
they now are to the most aquatic of living birds. 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-like progenitor of the
whole class. It is scarcely possible to decide how much allowance ought to
be made for such causes of change, as the definite action of external condi-
tions, so-called spontaneous variations, and the complex laws of growth;
but with these important exceptions, we may conclude that the structure
of every living 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 beauti-
ful for the delight of man — a belief which it has been pronounced is sub-
versive of my whole theory — I may first remark that the sense of beauty
obviously 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 created that they might be

DIFFICULTIES OF THE THEORY 135

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, and in consequence at the same time beautiful, so that they
may be easily observed by insects. I have come to this conclusion from find-
ing it an invariable rule that when a flower is fertilized by the wind it never
has a gayly-colored corolla. Several plants habitually produce two kinds of
flowers; one kind open and colored so as to attract insects; the other closed,
not colored, destitute of nectar, and never visited by insects. 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 flowers as we see on our fir, oak, nut,
and ash trees, on grasses, spinach, docks and nettles, which are all fertilized
through the agency of the wind. A similar line of argument holds good
with fruits; that a ripe strawberry or cherry is as pleasing to the eye as to
the palate — that the gayly-colored fruit of the spindlewood tree and the
scarlet berries of the holly are 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 disseminated. I infer
that this is the case from having as yet found no exception to the rule that
seeds are always thus disseminated when embedded 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 magnificently colored butterflies, have been rendered beauti-
ful for beauty's sake. But this has been eff"ected through sexual selection,
that is, by the more beautiful males having 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 having 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 pleasure from certain colors,
forms, and sounds — ^was first developed 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 modification in a species
exclusively for the good of another species, though throughout nature

136 THE ORIGIN OF SPECIES

one species incessantly takes advantage of and profits by the structures of
others. 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 part of the structure
of any one species had been formed for the exclusive good of another
species, it would annihilate my theory, for such could not have been pro-
duced through natural selection. Although many statements may be found
in works on natural history to this effect, I cannot find even one which
seems to me of any weight. It is admitted that the rattlesnake has a poison
fang for its own defence and for the destruction of its prey; but some
authors suppose that at the same time it is 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 attack 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 chickens. 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 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 endemic productions of New
Zealand, for instance, are perfect, one compared with another; but they
are now rapidly yielding before the advancing legions of plants and animals
introduced from Europe. Natural selection will not produce absolute per-
fection, 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 judgment no one will dispute, after describing in
the strongest terms the wonderful powers of the human eye, adds these
remarkable words: "That which we have discovered in the way of inexact-
ness and imperfection in the optical machine and in the image on the
retina, is as nothing in comparison with the incongruities which we have

DIFFICULTIES OF THE THEORY 137

just come across in the domain of the sensations. One might say that nature
has taken deUght in accumulating 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 contrivances in nature, this same reason tells us,
though we may easily err on both sides, that some other contrivances are
less perfect. Can we consider the sting of the bee as perfect, which, when
used against many kinds of 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 progeni-
tor, as a boring and serrated instrument, like that in so many members of
the same great order, and that it has since been modified but not perfected
for its present purpose, with the poison originally adapted for some other
object, such as to produce galls, since intensified, we can perhaps under-
stand 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 won-
derful power of scent by which the males of many insects find their females,
can we admire the production for this single purpose of thousands of
drones, which are utterly useless to the community for any other 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 daugh-
ters, as soon as they are born, or to perish herself in the combat; for un-
doubtedly 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 principles of natural selection. If we admire the several
ingenious contrivances by which orchids and many other plants are fertilized
through insect agency, can we consider as equally perfect the elaboration
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 difficulties 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 Hnked together by a multitude of intermediate gradations, partly be-
cause 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 implies the continual supplanting and extinction of pre-

138 THE ORIGIN OF SPECIES

ceding and intermediate gradations. Closely allied species, now living on
a continuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly graduate
away from one part to another. When two varieties are formed in two
districts of a continuous area, an intermediate variety will often be formed,
fitted for an intermediate zone; but from reasons assigned, the intermediate
variety will usually exist in lesser numbers than the two forms which it
connects; consequently the two latter, during the course of further modifi-
cation, from existing in greater numbers, will have a great advantage over
the less numerous intermediate variety, and will thus generally succeed in
supplanting and exterminating it.

We have seen in this chapter how cautious we should be in concluding
that the most different habits of life could not graduate into each other;
that a bat, for instance, could not have been formed by natural selection
from an animal which at first 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 congeners. Hence we can understand, bearing in mind that
each organic being is trying to live wherever it can live, how it has arisen
that there are upland geese with webbed feet, ground woodpeckers, diving
thrushes, and petrels with the habits of auks.

Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is enough to stagger any one; yet in the case of
any organ, if we know of a long series of gradations in complexity, each
good for its possessor, then under changing conditions of life, there is no
logical impossibility in the acquirement of any conceivable degree of perfec-
tion through natural selection. In the cases in which we know of no inter-
mediate or transitional states, we should be extremely cautious in conclud-
ing that none can have existed, for the metamorphoses of many organs
show what wonderful changes in function are at least possible. For instance,
a swim-bladder has apparently been converted into an air-breathing lung.
The same organ having performed simultaneously very different functions,
and then having been 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 often have
largely facilitated transitions.

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 appear-
ance closely similar may have been separately and independently formed;
but when such organs are closely examined, essential differences 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
throughout 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

DIFFICULTIES OF THE THEORY 139

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 descendants), 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 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
country there will have existed more individuals and more diversified forms,
and the competition will have been severer, and thus the standard of per-
fection will have been rendered higher. Natural selection will not necessarily
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 understand the full
meaning of that old canon in natural history, "Natura 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 selec-
tion. For natural selection acts by either now adapting the varying parts of
each being to its organic and inorganic conditions of life; or by having
adapted them during 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 several laws of growth and variation. Hence, in fact, the law of the
Conditions of Existence is the higher law; as it includes, through the in-
heritance of former variations and adaptations, that of Unity of Type.

CHAPTER VII

Miscellaneous Objections to the Theory of Natural Selection

Longevity — Modifications not necessarily Simultaneous — Modifications apparently
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 Structures — Causes which interfere
with the Acquisition through Natural Selection of Useful Structures — Grada-
tions of Structure with Changed Functions — Widely Different Organs in Mem-
bers of the Same Class, developed 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 wTiters who have not taken
the trouble to understand the subject. Thus a distinguished German
naturahst 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 showTi to be the case by so many native forms in many quarters of the
world having yielded their places to intruding foreigners. Nor can organic
beings, even if they were at any one time perfectly adapted to their condi-
tions of Life, have remained so, when their conditions changed, unless they
themselves likewise changed; and no one will dispute that the physical
conditions of each country, as well as the number and kinds of its in-
habitants, have undergone many mutations.

A critic has lately insisted, with some parade of mathematical accuracy,
that longevity is a great advantage to all species, so that he who believes
in natural selection "must arrange his genealogical tree" in such a manner
that all the descendants have longer Hves than their progenitors! Cannot
our critics conceive that a biennial 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. Ray 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 related 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 conditions 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 argument 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 produced through the modi-

140

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 141

fication of their original types. 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 exposed 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 Httle 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
development, but is powerless against the doctrine of natural selection or
the survival of the fittest, which implies that when variations or individual
differences of a beneficial nature happen to arise, these will be preserved;
but this will be effected only under certain favorable circumstances.

The celebrated palaeontologist, Bronn, at the close of his German transla-
tion 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 Hve 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; 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 sup-
posing 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 successive variations, if
slight, first in one part and then in another; and as they would be trans-
mitted all together, they would appear to us as if they had been simultane-
ously 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 selection, for some special purpose. Look at the race and
dray horse, or at the greyhound and mastiff. Their w^hole 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 cul-
tivated plants offer the best instances — it will invariably 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.

142 THE ORIGIN OF SPECIES

A much more serious objection has been urged by Bronn, and recently
by Broca, namely, that many characters appear to be of no service what-
ever to their 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. With 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 tendency 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. Nevertheless, we ought, in
the first place, to be extremely cautious in pretending to decide what struc-
tures 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 in-
creased or diminished flow of nutriment to a part, mutual pressure, 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. In the 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 conditions 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 eff'ect 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 occasionally 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 prob-
able, 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 race-
horse or greyhound, which before the principle of selection by man was

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 143

well understood, excited so much surprise in the minds of the older natural-
ists, can thus be explained.

It may be worth while to illustrate some of the foregoing remarks. With
respect to the assumed inutility of various parts and organs, it is hardly
necessary to observe that even in the higher and best-known animals many
structures exist, which are so highly developed that no one doubts that
they are of importance, yet their use has not been, or has only recently
been, ascertained. 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 supplied 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 prehensile 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 con-
fine myself in the following remarks, it will be admitted that the flowers of
the 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 importance
for the fertilization of the species through the aid of insects, and have
probably been gained through natural selection. No one until lately would
have imagined that in dimorphic and trimorphic plants the different
lengths of the stamens and pistils, and their arrangement, 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 posi-
tions seem at first purely morphological, or of no physiological signification;
but Dr. Hooker informs me that within the same ovarium, the upper ovules
alone in some cases, and in others the lower ones alone are fertilized; and
he suggests that this probably depends on the direction in which the pollen-
tubes enter the ovarium. If so, the position of the ovules, even when one is
erect and the other suspended within the same ovarium, would follow the
selection of any slight deviations in position which favored their fertiliza-^
tion, and the production of seed.

Several plants belonging to distinct orders habitually produce flowers of
two kinds — the one open, of the ordinary structure, the other closed and
imperfect. These two kinds of flowers sometimes differ wonderfully in struc-
ture, 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 derived 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 won-
derfully little pollen. The two kinds of flowers often differ much, as just
stated, in structure. The petals in the imperfect flowers almost always con-

144 THE ORIGIN OF SPECIES

sist of mere rudiments, and the pollen-grains are reduced in diameter. In
Ononis columnse 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 one section of the Malpighiacese 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 this species; the style is aborted;
and the ovaria are reduced from three to two. Now although natural selec-
tion may well have had the power to prevent some of the flowers from
expanding, 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 eff'ects 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 diff"erences 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 divisions to the ovarium; while all
the other flowers on the plant are tetramerous. In the British Adoxa the
uppermost flower generally has two calyx-lobes with the other organs
tetramerous, while the surrounding flowers generally have three calyx-lobes
with the other organs pentamerous. In many compositae and umbelliferae
(and in some other plants) the circumferential flowers have their corollas
much more developed than those of the centre; and this seems often con-
nected 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 compositae the central achenes alone are fur-
nished 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. Professor Braun mentions a Fuma-
riaceous 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

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 145

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
interaction 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 generally of a highly important nature, affect-
ing only some of the flowers on the same plant, or occurring on distinct
plants, which grow close together under the same conditions. As these
variations seem of no special use to the plants, they cannot have been in-
fluenced by natural selection. 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 varia-
tions are comparatively rare when the parts 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 Antirrhinidese,
to which latter tribe the genus belongs. Aug. Saint-Hilaire gives the follow-
ing cases : the genus Zanthoxylon belongs to a division of the Rutaceae 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 Helian-
themum the capsule has been described as unilocular or trilocular; and
in H. mutabile, "Une lame plus ou moins large s'etend 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,
Saint-Hilaire found toward the southern extreme of the range of Gomphia
oleaeformis two forms which he did not at first doubt were distinct species,
but he subsequently saw them growing on the same bush; and he then
adds, "Voila done dans un meme individu des loges et un style qui se
rattachent tantot a un axe vertical et tantot a un gynobase."

We thus see that with plants many morphological changes may be at-
tributed to the laws of growth and the interaction of parts, independently
of natural selection. But with respect to Nageli's doctrine of an innate
tendency 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
diff"ering or varying greatly on the same plant, that such modifications were
of extremely small importance to the plants themselves, of whatever im-
portance they may generally be to us for our classifications. The acquisition

146 THE ORIGIN OF SPECIES

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 retrogression rather than
of progression; and so it must be with many parasitic and degraded animals.
We are ignorant of the exciting cause of the above specified modifications;
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 unimportant for the welfare
of the species, any slight variations 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 condi-
tions, modifications have been induced which are unimportant for the wel-
fare of the species, they may be, and apparently often have been, trans-
mitted in nearly the same state to numerous, otherwise modified, de-
scendants. 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 transmitted 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, morpho-
logical differences, which we consider as important — such as the arrange-
ment of the leaves, the divisions of the flower or of the ovarium, the posi-
tion of the ovules, etc., first appeared in many cases as fluctuating varia-
tions, which sooner or later became constant through the nature of the
organism and of the surrounding conditions, as well as through the inter-
crossing 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 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 classification, this is by no means so para-
doxical as it may at first appear.

Although we have no good evidence of the existence in organic beings
of an innate tendency toward progressive 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.

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 147

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 formidable 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 is 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 important, and have
treated in my "Variation under Domestication" at greater length than, as I
believe, any other 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 subject, 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, "That natural selection is incompetent to account for the in-
cipient stages of useful structures." This subject is intimately connected
with that of the gradation of the characters, often accompanied by a
change of function, for instance, the conversion of a swim-bladder 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 ad-
vanced by Mr. Mivart, selecting those which are the most illustrative, as
want of space prevents me from considering all.

The giraffe, by its lofty stature, much elongated neck, fore legs, head,
and tongue, has its whole frame beautifully 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 us how small a difference in structure may make, during
such periods, a great difference in preserving an animal's life. These cattle
can browse as well as others on grass, but from the projection of the lower
jaw they cannot, 3uring 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 race-horse and greyhound, or as with the game-cock, by breed-
ing from the victorious birds. So under nature with the nascent giraffe, the

148 THE ORIGIN OF SPECIES

individuals which were the highest browsers and were able during dearth
to reach even an inch or two above the others, will often have been pre
served; for they will have roamed over the whole country in search of fooc
That the individuals of the same species often differ slightly in the relativ
lengths of all their parts may be seen in many works of natural history h
which careful measurements are given. These slight proportional differ
ences, due to the laws of growth and variation, are not of the slightest us(
or importance to most species. But it will have been otherwise with th(
nascent giraffe, considering its probable habits of life; for those individual'
which had some one part or several parts of their bodies rather more
elongated than usual, would generally have survived. These will have inter-
crossed and left offspring, either inheriting the same bodily peculiarities oi
with a tendency to vary again in the same manner; while the individual^
less favored m 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 intercross
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 disad^
vantages thence arising would not, in times of scarcity, more than counter-
balance the advantages." But as the giraffe does actually exist in large num-
bers m 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 con-
cerned, intermediate gradations could formerly have existed there sub-
jected 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 country, 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
hon; and against this animal, its tall neck— and the taller the better-
would, as Mr. Chauncey Wright has remarked, serve as a watch-tower It
is from this cause, as Sir S. Baker remarks, that no animal is more difficult
to stalk than the giraffe. This animal also uses 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
advantage, but by the union of all, great and small.

^ Mr. Mivart then asks (and this is his second objection), if natural selec-
tion 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

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 149

jnacrauchenia? Or, again, why has not any member of the group acquired
a long proboscis? With respect to South Africa, which was formerly in-
habited 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 advantage 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 belonging to this
same order have not acquired either an elongated neck or a proboscis, can-
not be distinctly answered; 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 increase in some new country. We can, however, see in
a general manner that various causes might have interfered with the de-
velopment of a long neck or proboscis. To reach the foliage at a considerable
height (without climbing, for which hoofed animals are singularly 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 development of so large a quadruped as
the giraffe.

In order that an animal should acquire some structure specially and
largely developed, it is almost indispensable 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-
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 apparently
would be beneficial to the species. For instance, if the number of individuals
existing in a country is determined chiefly through destruction by beasts of
prey — by external or internal parasites, etc. — as seems often to be the case,

150 THE ORIGIN OF SPECIES

then natural selection will be able to do little, or will be greatly retarded,
in modifying any particular structure for obtaining food. Lastly, natural
selection is a slow process, and the same favorable conditions must long
endure in order that any marked effect should thus be produced. Except
by assigning such general and vague reasons, we cannot explain why, in
many quarters of the world, hoofed quadrupeds have not acquired mudi
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 natural
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 moment'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, i
but by no terrestrial mammals; yet as some of these bats are peculiar'
species, they must have long inhabited their present homes. Therefore Sir '
C. LyeU asks, and assigns 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 converted 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 species, will be favored
only under certain peculiar conditions. A strictly terrestrial animal, by
occasionally hunting 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 wings 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 eflicient power of gliding through
the air. Bats might, indeed, like many birds, have had their wings greatly
reduced in size, or completely lost, through disuse; 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. These 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

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 151

their mental powers more highly developed than others, as such develop-
ment would be advantageous to all? Why have not apes acquired the in-
tellectual powers of man? Various causes could be assigned; but as they
jare 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. Insects 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. Dar-
win's theory, there is a constant tendency to indefinite variation, and as the
minute incipient 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 oscillation, of in-
finitesimal beginnings can ever build up a sufficiently appreciable resem-
blance 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 accidental resemblance to an object commonly
found in the stations frequented by them. Nor is this at all improbable,
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 surface 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 preserved, 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
fluctuating variability; but as the case stands there is none.

Nor can I see any force in Mr. Mivart's difficulty with respect to "the
last touches of perfection in the mimicry"; as in the case given by Mr.
Wallace, of a walking-stick insect (Ceroxylus laceratus), which resembles

152 THE ORIGIN OF SPECIES

"a stick grown over by a creeping moss or jungermannia." So close was tin
resemblance, that a native Dyak maintained that the foliaceous excrescence
were really moss. Insects are preyed on by birds and other enemies whos
sight is probably sharper than ours, and every grade in resemblance whici
aided an insect to escape notice or detection, would tend toward its pres
ervation; and the more perfect the resemblance, so much the better fo
the insect. Considering the nature of the differences between the species i:
the group which includes the above Ceroxylus, there is nothing improbabl
in this insect having varied in the irregularities on its surface, and in thes
having become more or less green-colored; for in every group the charac
ters which differ in the several species are the most apt to vary, while tb
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 whalebone, one of its greatest peculiarities. The baleei
consists of a row, on each side of the upper jaw, of about 300 plates o.
laminse, which stand close together transversely to the longer axis of the
mouth. Within the main row there are some subsidiary rows. The extremitie
and inner margins of all the plates are frayed into stiff bristles, which clotb
the whole gigantic palate, and serve to strain or sift the water, and thus t(
secure the minute prey on which these great animals subsist. The middk
and longest lamina in the Greenland whale is ten, twelve, or even fifteer
feet in length; but in the different species of Cetaceans there are gradation!
in length; the middle lamina being in one species, according to Scoresby
four feet, in another three, in another eighteen inches, and in the Balaenop-
tera 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 preserva-
tion and augmentation within serviceable limits would be promoted by
natural selection alone. But how to obtain the beginning of such useful de-
velopment?" In answer, it may be asked, why should not the early progeni-
tors of the whales with baleen have possessed a mouth constructed some-
thing like the lamellated beak of a duck? Ducks, like whales, subsist by
sifting the mud and w^ater; and the family has sometimes been called
Crihlatores, 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 de-
veloped from such lamellae by finely graduated steps, each of service to its'
possessor.

The beak of the shoveller-duck (Spatula clypeata) is a more beautiful
and complex structure than the mouth of the 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 lamellse, obliquely beveled so as to be
pointed, and placed transversely to the longer axis of the mouth. They

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 153

* 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 fourteen one-
hundredths of an inch beneath the edge. At their bases there is a short
subsidiary row of obliquely transverse lamellae. In these several respects

' they resemble 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 incompa-
rably less bulky, is about one-eighteenth of the length of the head of a
moderately large Bal^noptera rostrata, in which species 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 Balasnoptera, 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 these 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 lamellse 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 informiation and specimens sent to me
by Mr. Salvin), without any 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 lamella are much coarser than in the shoveller, and
are firmly attached to the sides of the mandible; they are only about fifty
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 a 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 w^hich the lamellae are considerably less developed
than in the common duck; but I do not know \vhether they use their beaks
for sifting the water.

Turning to another group of the same family. In the Eg\^tian 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 imvard; yet this goose, as I am informed by Mr. E.
Bartlett, "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 twenty-seven in number on

154 THE ORIGIN OF SPECIES

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 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 animal. There
are other species of geese, as I hear from Mr. Bartlett, in which the lamellse
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 a common goose and adapted solely for grazing, or even a
member with a beak having less well developed lamellae, might be con-
verted by small changes into a species like the Egyptian goose — this into
one like the common duck — and, lastly, into one like the shoveller, pro-
vided 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 I 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.

Returning to the whales. The Hyperoodon bidens is destitute of true
teeth in an efficient condition, but its palate is roughened, according to
Lacepede, with small, unequal, hard points of horn. There is, therefore,
nothing improbable in supposing that some early Cetacean form was pro-
vided with similar points of horn on the palate, but rather more regularly
placed, and which, like the knobs on the beak of the goose, aiding it in
seizing or tearing its food. If so, it will hardly be denied that the points
might have been converted through variation and 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 rostrata, 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 struc-
ture 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 specimens
occur. The lower, or resting-surface, resembles at first sight the ventral

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 155

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 op-
posite to each 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
lying in its habitual position on one side. The lower eye would, also, have
been liable to be abraded by the sandy bottom. That the Pleuronectidae
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 spontane-
ous 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
cleatr. 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 Pleu-
ronectidae, while very young and still symmetrical, with their eyes stand-
ing on opposite 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 swim-bladder. 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 upward, 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
distance 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 permanently contracted through
disease or some accident. With long-eared rabbits, if one ear flops 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,

156 THE ORIGIN OF SPECIES

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 t
permanent effect is thus produced. With the Pleuronectidae, on the other
hand, the older they grow the more habitually they rest on one side, owing
to the increasing 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 distortion would no doubt be increased
through the principle of inheritance. Schiodte 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 Trachypterus arcticus, which is not a
member of the Pleuronectidae, 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
"the author gives a very simple explanation of the abnormal condition 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 rest-
ing 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 to-
ward the lower surface, with the jawbones 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 whole in-
ferior half of the body, including the lateral fins; though Yarrel 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 proportion 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

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 157

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 impossible to decide.

I may give another instance of a structure which apparently 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 number 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 confine-
ment some harvest mice (Mus messorius) which do not possess a structur-
ally prehensile tail; but he frequently observed that they curled their tails
round the branches of a bush placed in the cage, and thus aided them-
selves in climbing. I have received an analogous account from Dr. Giinther,
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.

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 extremely remote period, and we can know nothing posi-
tively about their manner of development. Mr. Mivart asks: "Is it con-
ceivable 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 mammals
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 (Hippocampus) the eggs are hatched, and the young are reared

158 THE ORIGIN OF SPECIES

for a time, within a sack of this nature; and an American 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 deserve 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 accords with the widely extended principle of
specialization, 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
Ornithorhynchus, 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 egg-shell by tapping against it with their specially
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 helpless, half-
formed offspring. On this head Mr. Mivart remarks: "Did no special pro-
vision 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 harmlessly on each side of this elongated larynx, and so
safely attains the gullet behind it." Mr. Mivart then asks, how did natural
selection removed in the adult kangaroo (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.

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 159

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 remarkable organs called pedicellarias, which consist, when well de-
veloped, 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 re-
moving 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 oc-
casions, asks: "What would be the utility of the first rudimentary beginnings
of such structures, and how could such incipient buddings have ever pre-
served the life of a single Echinus?" He adds, "Not even the sudden de-
velopment of the snapping action could have been beneficial without the
freely movable stalk*, nor could the latter have been efficient without the
snapping jaws, yet no minute, merely indefinite variations could simultane-
ously evolve these complex coordinations 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
this is intelligible 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 described by M. Perrier as bearing two kinds of
pedicellarias, one resembling those of Echinus, and the other those of
Spatangus; and such cases are always interesting as affording the means
of apparently sudden transitions, through the abortion of one of the two
states of an organ.

With respect to the steps by which these curious organs have been evolved,
Mr. Agassiz infers from his own researches and those of Mr. 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 diflferent species and genera, from simple granules to
ordinary spines, to perfect tridactyle pedicellariae. The gradation extends
even to the manner in which ordinary spines and the pedicellariae,
with their supporting calcareous rods, are articulated to the shell. In certain
genera of star-fishes, "the very combinations needed to show that the pedi-
cellariae are only modified branching spines" may be found. Thus we have
fixed spines, with three equi-distant, 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,

160 THE ORIGIN OF SPECIES

in fact, a rude tridactyle pedicellaria, 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 pedicellariae 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 more effectively as soon as
by meeting together they acted as a prehensible or snapping apparatus.
Thus every gradation, from an ordinary 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 becomes articulated to the shell, and is
thus rendered movable. I wish I had space here to give a fuller abstract
of Mr. Agassiz's interesting observations on the development of the pedi-
cellariae. All possible gradations, as he adds, may likewise be found be-
tween the pedicellariae of the star-fishes and the hooks of the Ophiurians,
another group of the Echinodermata ; and again between the pedicellariae
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 curious 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 minia-
ture, seated on a neck and capable of movement, as is likewise the lower
jaw or mandible. In one species observed by me, all the avicularia on the
same branch often moved simultaneously backward and forward, with the
lower jaw widely open, through an angle of about go degrees, 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 Echinodermata, which he considers as "essentially similar," having been
developed through natural selection in widely distinct divisions of the
animal kingdom. But, as far as structure is concerned, I can see no similarity
between tridactyle pedicellariae and avicularia. The latter resembles some-
what more closely the chelae or pincers of Crustaceans; and Mr. Mivart
might have adduced, with equal appropriateness, this resemblance as a
special difficulty, or 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 hornologous with
the zooids and their cells which compose the zoophyte, the movable lip or
lid of the cell corresponding with the lower and movable mandible of the

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 161

avicularium. Mr. Busk, however, does not know of any gradations now
existing between a zooid and an avicularium. It is therefore impossible to
conjecture by what serviceable gradations the one could have been con-
verted into the other, but it by no means follows from this that such grada-
tions have not existed.

As the chelas 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 penultimate 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 prominent, 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 possesses curious organs called vibrac-
ula. These generally consist of long 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 themselves. 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 avicularia, like the vibracula, probably serve for defence, but they ^Iso
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 avicularia 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 under-
stand 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 several 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 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,

162 THE ORIGIN OF SPECIES

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 mov-
able 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 converted
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
circumstances.

In the vegetable kingdom Mr. Mivart only alludes to two cases, namely
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 structures which are of utility
only when they are considerably developed." As I have fully treated this
subject in another work, I will here give only a few details on 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; but 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 embedded 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 belong-
ing 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 of common flowers, there

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 163

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 transportal 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 like-
wise possess. The gradations from leaf -climbers to tendril bearers are won-
derfully 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 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 possessed in any slight degree the requisite sensitiveness to
a touch.

As twining is the simplest means of ascending a support, and forms the
basis of our series, it may naturally 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, firstly, 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 succession, 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 bend-
ing and revolving, and thus necessarily twines round and up the support.

164 THE ORIGIN OF SPECIES

The revolving movement 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 have thus become twiners, they must
have independently acquired it, and cannot have inherited it from a com-
mon 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 twin-
ing 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
species, belonging to widely distinct groups, are endowed with this kind of
sensitiveness, it ought to be found in a 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 themselves a little toward
the side which was touched. Morren 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 observations 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 extremely 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 movements, due to a touch
or shake in the young and growing organs of plants, can be of any func-
tional importance to them. But plants possess, in obedience to various
stimuli, powers of movement, which are of manifest importance 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

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 165

of strychnine, the consequent movements may be called an incidental re-
sult, 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 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
selection. 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 de-
velopment of climbing plants, natural selection has been aided by the
inherited effects of use, I will not pretend to decide; but we know that cer-
tain 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 naturalist 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 strange 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 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
destruction of those which could not browse so high, would have sufficed
for the production of this remarkable quadruped; 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 resem-
blance to some common object was in each case the foundation for the
work of natural selection, since perfected through the occasional preserva-
tion of slight variations which made the resemblance 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 favorable
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

166 THE ORIGIN OF SPECIES

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 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 continued use, together with inherit-
ance. 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 nutritious fluid; and that these glands were
improved in function 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 understanding the
development of the pincers of crustaceans through slight, serviceable
modifications in the ultimate and penultimate segments of a limb which
was first used solely for locomotion. In the avicularia and vibracula of the
Polyzoa we have organs widely different in appearance developed from
the same source; and with the vibracula we can understand how the suc-
cessive gradations 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 flow-
ers, and still subserving nearly but not quite the same purpose, became at-
tached to the free ends of the caudicles — all these gradations being of
manifest benefit to the plants in question. With respect to climbing plants,
I need not repeat what has been so lately said.

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 ignorance of the past
history of each species, and of the conditions 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 indis-
pensable, 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 advan-
tageous 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

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 167

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 all 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 some-
thing; but he considers it as "demonstrably insufficient" to account for the
phenomena which I explain by its agency. His chief arguments have now
been considered, and the others will hereafter be considered. They seem to
me to partake little of the character of demonstration, and to have little
weight in 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 article lately published in the "Medico-Chirurgical
Review."

At the present day almost all naturalists admit evolution 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 ordinary 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 modi-
fications appearing at once." For instance, he supposes that the differences
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. This conclusion, which implies great
breaks or discontinuity in the series, appears to me improbable in the
highest degree.

Every one who believes in slow and gradual evolution, 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 domestica-
tion. 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
rise 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 number

168 THE ORIGIN OF SPECIES

must be called monstrosities, such as six-fingered men, porcupine men,
Ancon sheep, Niata cattle, etc.; and as they are widely different in charac-
ter from natural species, they throw very little light on our subject. Ex-
cluding 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 follows: According to our experience, abrupt and strongly marked
variations occur in our domesticated productions, singly and at rather long
intervals of time. If such occurred under nature, they would be liable, as
formerly explained, to be lost by accidental causes of destruction 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 almost necessary to
believe, in opposition to all analogy, that several wonderfully changed
individuals appeared simultaneously within the same district. This diffi-
culty, as in the case of unconscious selection by man, is avoided on the
theory of gradual evolution, through the preservation 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 species and even the genera of many
large natural families are so closely allied together that it is difficult to
distinguish not a few of them. On every continent, in proceeding from
north to south, from lowland to upland, etc., we meet with a host of closely
related or representative species; as we likewise do on certain distinct
continents, which we have reason to beHeve were formerly connected. But
in making these and the following remarks, I am compelled to allude to
subjects hereafter to be discussed. Look at the many outlying islands round
a continent, and see how many of their inhabitants can be raised only to
the rank of doubtful species. So it is if we look to past times, and com-
pare the species which have just passed away with those still living within
the same area; or if we compare the fossil species embedded in the sub-
stages of the same geological formation. It is indeed manifest that multi-
tudes of species 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, instead
of to distinct species, that numerous and wonderfully 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

OBJECTIONS TO THE THEORY OF NATURAL SELECTION 169

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
manner are more variable than other parts of the same species. 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 main-
tained 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 ad-
vanced in favor of this view, such as the sudden crystallization of 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 appearance 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 geo-
logical record, in relation to periods remote in the history of the world. If
the record is as fragmentary as many geologists strenuously 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 sudden conversion of a Hipparion into a horse, hardly any light is
thrown by the belief in abrupt modifications on the deficiency of connect-
ing links in our geological formations. 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 undistin-
guishable at an early embryonic 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 corresponding 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 extinct forms belonging
to the same class. On this view of the meaning of embryological resem-
blances, and indeed on any view, it is incredible that an animal should
have undergone such momentous and abrupt transformations 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 transformed suddenly
through an internal force or tendency into, for instance, one furnished with

170 THE ORIGIN OF SPECIES

wings, will be almost compelled to assume, in opposition to all analogy,
that many individuals varied simultaneously. It cannot be denied that such
abrupt and great changes of structure 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 coadaptations, 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.

CHAPTER VIII

Instinct

Instincts Comparable with Habits, but Different in Their Origin — Instincts Gradu-
ated— 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 — Difficulties of the Theory of the
Natural Selection of Instincts — Neuter or Sterile Insects — Summary.

Many instincts are so wonderful that their development will probably
appear to the reader a difficulty 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 understands what is meant, when it is said that instinct impels
the cuckoo to migrate and to lay her eggs in other birds' nests. An action,
which we ourselves require experience to enable us to perform, when per-
formed by an animal, more especially by a very young one, without experi-
ence, and when performed by many individuals in the same way, without
their knowing for what purpose it is performed, is usually said to be in-
stinctive. But I could show that none of these characters are universal. A
little dose of judgment or reason, as Pierre Huber expresses it, often comes
into play, even with animals low in the scale of nature.

Frederick Guvier and several of the older metaphysicians have com-
pared instinct with habit. This comparison gives, I think, an accurate
notion of the frame of mind under which an instinctive action is per-
formed, but not necessarily of its origin. How unconsciously many habitual
actions are performed, indeed not rarely in direct opposition to our con-
scious will! yet they may be modified by the will or reason. Habits easily
become associate 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 anything by rote, he is generally forced to go back to
recover the habitual train of thought; so P. Huber found it was with a
caterpillar, which makes a very complicated hammock; for if he took a
caterpillar which had completed its hammock up to, say, the sixth stage of
construction, and put it into a hammock completed up only to the third
stage, the caterpillar simply re-performed the fourth, fifth and sixth stages
of construction. If, however, a caterpillar were taken out of a hammock
made up, for instance, to the third stage, and were put into one finished

171

172 THE ORIGIN OF SPECIES

up to the sixth stage^ so that much of its work was already done for it, far
from deriving any benefit 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 complete 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
distinguished. If Mozart, instead of playing the piano-forte at three years
old with wonderfully little practice, had played a tune with no practice at
all, he might truly be said to have done so instinctively. But it would be 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 generations. It can be clearly shown that the most wonderful
instincts with which we are acquainted, namely, those of the hive-bee and
of many ants, could not possibly have been acquired by habit.

It will be universally admitted that instincts are as important as cor-
poreal structures for the welfare of each species, under its present condi-
tions 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 vari-
ations of instinct to any extent that was profitable. It is thus, as I believe,
that all the most complex and wonderful instincts have originated. As modi-
fications of corporeal structure arise from, and are increased by, use or
habit, and are diminished or lost by disuse, so I do not doubt it has been
with instincts. But I believe that the effects of habit are in many cases of
subordinate importance 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.
4n|ij No complex instinct can possibly be produced through natural selection,
*' except by the slow and gradual accumulation of numerous slight, yet
profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which each
complex instinct has been acquired — for these could be found only in the
lineal ancestors of each species — ^but we ought to find in the collateral lines
of descent some evidence of such gradations; or we ought at least to be able
to show that gradations of some kind are possible; and this we certainly
can do. I have been surprised to find, making allowance for the instincts of
animals having been but little observed, except in Europe and North
America, and for no instinct being known among extinct species, how very
generally gradations, leading to the most complex instincts, can be dis-
covered. 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 preserved by natural

. INSTINCT 173

selection. And such instances of diversity of instinct in the same species can
be shown to occur in nature.

Again, as in the case of corporeal structure, and conformably to my
theory, the instinct of each species is good for itself; but has never, as far
as we can judge, been produced for the exclusive good of others. One of the
strongest instances of an animal apparently performing an action for the
sole good of another, with which I am acquainted, is that of aphides
voluntarily yielding, 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 allowed an ant to visit them,
and it immediately seemed, by its eager way of running about to be well
aware what a rich flock it had discovered; it then began to play with its
antennae on the abdomen first of one aphis and then of another; and each,
as soon as it felt the antennae, immediately lifted up its abdomen and ex-
creted 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 observations 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 con-
venience 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 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 absolutely perfect; but as details on this
and other such points are not indispensable, they may be here passed over.
As some degree of variation in instincts under a state of nature, and the
inheritance of such variations, are indispensable for the action of natural
selection, as many instances as possible ought to be given; but want of
space prevents me. I can only assert that instincts certainly do vary—for
instance, the migratory instinct, both in extent and direction, and m its
total loss. So it is with the nests of birds, which vary partly in independence
on the situations chosen, and on the nature and temperature of the country
inhabited, but often from causes wholly unknown to us. Audubon has given
several remarkable cases of differences in 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 natural 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

174 . THE ORIGIN OF SPECIES

laboriously collecting propolis, used a cement of wax and turpentine, with
which he had covered decorated 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 certainly an instinctive
quality, as may be seen in nestling birds; though it is strengthened by ex-
perience, 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 islands; 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 uninhabited islands large birds are not more fearful than small; and
the magpie, so wary in England, is tame in Norway, as is the hooded crow
in Egypt.

That the 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 occasional and strange habits in wild animals, which,
if advantageous to the species, might have given rise, through natural
selection, to new instincts. But I am well aware that these general state-
ments, 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 without
good evidence.

INHERITED CHANGES OF HABIT OR INSTINCT IN DOMESTICATED ANIMALS

The possibility, or even probability, of inherited variations of instinct in
a state of nature will be strengthened by briefly considering a few cases
under domestication. We shall thus be enabled to see the part which habit
and the selection of so-called spontaneous variations have played in modi-
fying the mental qualities of our domestic animals. It is notorious how
much domestic animals vary in their mental qualities. With cats, for in-
stance, 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 minds or periods of time, being inherited. 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 striking instances) 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

INSTINCT 175

can no more know that he points to aid his master, than the white butterfly
knows why she lays her eggs on the leaf of the cabbage — I cannot see that
these actions differ essentially from true instincts. If we were to behold one
kind of wolf, when young and without any training, as soon as it scented its
prey, stand motionless like a statue, and then slowly crawl forward with a
peculiar gait; and another kind of wolf rushing round, instead of at, a herd
of deer, and driving them to a distant point, we should assuredly call these
actions instinctive. Domestic instincts, as they may be called, are certainly
far less fixed than natural instincts; but they have been acted on by far
less rigorous selections, and have been transmitted for an incomparably
shorter period, under less fixed conditions of life.

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

Domestic instincts are sometimes spoken of as actions which have be-
come 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 in-
dividuals in successive generations made tumblers what they now are; and
near Glasgow there are house-tumblers, as I hear from Mr. Brent, which
cannot fly eighteen inches high without going head over heels. It may be
doubted whether any one would have thought of training a dog to point,
had not some one dog naturally shown a tendency in this line; and this is
known occasionally to happen, as I once saw, in a pure terrier: 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 com-
pulsory training in each successive generation would soon complete the
work; and unconscious selection is still in progress, as each man tries to
procure, without intending 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
Jiardly suppose that domestic rabbits have often been selected for tameness

176 THE ORIGIN OF SPECIES

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 remarkable instance of
this is seen in those breeds of fowls which very rarely or never become
"broody," that is, never wish to sit on their eggs. Familiarity alone prevents
our seeing how 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, jackals, and
species of the cat genus, when kept tame, are most eager to attack poultry,
sheep, and pigs; and this tendency has been found incurable in dogs which
have been brought home as puppies from countries such as Tierra del
Fuego and Australia, where the savages do not keep these domestic animals.
How rarely, on the other hand, do our 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 originally instinctive in them, for I am in-
formed by Captain Hutton that the young chickens of the parent stock,
the Gallus bankiva, when reared in India under a hen, are at first exces-
sively 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 produce 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

INSTINCT 177

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 naturalists 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 unin-
cubated, 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 occa-
sionally lays her eggs in other birds' nests; but I have lately heard from Dr.
Merrill, of Iowa, that he once found in Illinois a young cuckoo, together with a
young jay, in the nest of a blue jay (Garrulus cristatus) ; 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 emigrate earlier or through any other cause; or if the young were made
more vigorous by advantage being taken of the mistaken instinct of another
species than when reared by their own mother, encumbered 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 advantage.
And analogy would lead us to believe that the young thus reared would be
apt to follow by inheritance the occasional and aberrant habit of their
mother, and in their turn would be apt to lay their eggs in other birds'
nests, and thus be more successful in rearing their young. By a continued
process of this nature, I believe that the strange instinct of our cuckoo has
been generated. It has, also, recently been ascertained on sufficient evidence,
by Adolf Miiller, that the cuckoo occasionally lays her eggs on the bare
ground, sits on them and feeds her young. This rare event is probably the
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 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 Ameri-
can cuckoo alone were known; now, owing to Mr. Ramsay's observations,
we have learned something about three Australian species, which lay their

178 THE ORIGIN OF SPECIES

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. Sec-
ondly, that the eggs are remarkably small, not exceeding those of the sky-
lark— 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 laying 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-brothers
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 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 remarks that two of the Australian
cuckoos, when they lay their eggs in an open nest, manifest a decided prefer-
ence 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 eggs 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 degree 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 per-
formed by the foster-parents themselves. But he has now received a trust-
worthy account of a young cuckoo which was actually seen, while still blind
and not able even to hold up 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 out. With respect to the means by which this strange and
odious instinct was acquired, if it were of great importance 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 acquired,

INSTINCT 179

during successive generations, the blind desire, the strength, and structure
necessary for the work of ejection; for those 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 restlessness 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 unhatched 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 remarked, 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 inherited at a corresponding or earlier
age — propositions which cannot be disputed — then the instincts and struc-
ture 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 some-
times to pair. They either build a nest of their own or seize on one belong-
ing 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. Hudson says it is probable
that they are occasionally parasitic, 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 Molothrus, the M. bonariensis, are
much more highly developed 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 several together sometimes com-
mence to build an irregular untidy nest of their own, placed in singular
ill-adapted situations, as on the leaves of a large thistle. They never, how-
ever, 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, moreover, 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 America, 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
words, and asks, "Must we consider these habits, not as especially endowed

180 THE ORIGIN OF SPECIES

or created instincts, but as small consequences of one general law, namely,
transition?"

Various birds, as has already been remarked, occasionally lay their eggs
in the nests of other birds. This habit is not very uncommon with the
Gallinacese, 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 American ostrich, as in the case of the Molothrus bonarien-
sis, 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 Sphegidae (wasp-like
insects) 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 burrow already made and stored by another sphex, if
takes advantage of the prize, and becomes for the occasion parasitic. In this
case as with that of the Molothrus or cuckoo, I can see no difficulty in
natural selection making an occasional habit permanent, if of advantage to
the species, and if the insect whose nest and stored food are feloniously
appropriated, be not thus exterminated.

SLAVE-MAKING INSTINCT

This remarkable instinct was first discovered in the Formica (Polyerges)
rufescens by Pierre Huber, a better observer even than his celebrated father.
This ant is absolutely dependent on its slaves; without their aid, the species
would certainly become extinct in a single year. The males and fertile
females do no work 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 masters, that when Huber shut up
thirty of them without a slave, but with plenty of food which they liked
best, and with their own larvae and pupae to stimulate them to work, they
did nothing; they could not even feed themselves, and many perished of
hunger. Huber then introduced a single slave (F. fusca), and she instantly
set to work, fed and saved the survivors; made some cells and tended the

INSTINCT 181

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.

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 information on this and
other subjects. Although fully trusting to the statements of Huber and Mr.
Smith, I tried to approach the subject in a sceptical frame of mind, as any
one may well be excused for doubting the 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. sanguinea, 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 observed in the nests of F. sanguinea. The slaves are black and not
above half the size of their red masters, so that the contrast in their appear-
ance 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 together 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 several nests in Surrey and Sussex, and never saw a slave
either leave or enter a nest. As, during these months, the slaves are very
few in number, I thought that they might behave differently when more
numerous; but Mr. Smith informs me that he has watched the nests at
various hours during May, June, and August, both in Surrey and Hamp-
shire, and has never seen the slaves, though present in large numbers in
August, either leave or enter the nest. Hence, he considers them as strictly
household slaves. The masters, on the other hand, may be constantly seen

! bringing in materials for the nest, and food of all kinds. During the year
i860, however, in the month of July, I came across a community with an
unusually large stock of slaves, and I observed a few slaves mingled with
their masters leaving the nest, and marching along the same road to a tall
Scotch fir-tree, twenty-five yards distant, which they ascended together,
probably in search of aphides or cocci. According to Huber, who had
ample opportunities for observation, 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

i of the masters and slaves in the two countries, probably depends merely on
the slaves being captured in greater numbers in Switzerland than in England.
One day I fortunately witnessed a migration of F. sanguinea from one
nest to another, and it was a most interesting spectacle to behold the mas-
ters carefully carrying their slaves in their jaws instead of being carried by

182 THE ORIGIN OF SPECIES

them, as in the case of F. rufescens. Another day my attention was struck j
by about a score of the slave-makers haunting the same spot, and evidently
not in search of food; they approached and were vigorously repulsed by
an independent community of the slave-species (F. fusca) ; sometimes as
many as three of these ants cHnging to the legs of the slave-making F.
sanguinea. The latter ruthlessly killed their small opponents and carried
their dead bodies as food to their nest, twenty-nine yards distant; but they
were prevented from getting any pupae to rear as slaves. I then dug up a -i
small parcel of the pupae of F. fusca from another nest, and put them down
on a bare spot near the place of combat; they were eagerly seized and
carried ofT by the tyrants, who perhaps fancied that, after all, they had been
victorious in their late combat.

At the same time I laid on the same place a small parcel of the pupae of ^
another species, F. flava, with a few of these little yellow ants still clinging
to the fragments of 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 courageous, and I have seen it ferociously attack other
ants. In one instance I found to my surprise an independent community of
F. flava under a stone beneath a nest of the slave-making F. sanguinea; and
when I had accidentally disturbed both nests, the little ants attacked their
big 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 dis-
tinguish them; for we have seen that they eagerly and instantly seize the
pupae of F. fusca, whereas they were much terrified when they came across
the pupae, or even the earth from the nest, of F. flava, and quickly ran
away; but in about a quarter of an hour, shortly after all the little yellow
ants had crawled away, they took heart and carried off the pupae.

One evening I visited another community of F. sanguinea, and found a
number of these ants returning home and entering their nests, carrying the
dead bodies of F. fusca (showing that it was not a migration) and numer- i
ous pupae. I traced a long file of ants burdened with booty, for about forty I
yards back, to a very thick clump of health, 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 confirmation by me, in i
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 3
continental F. rufescens. The latter does not build its own nest, does not t
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

INSTINCT 183

slaves. Formica sanguinea, on the other hand, possesses much fewer slaves,
and in the early part of the summer extremely few: the masters determine
when and where a new nest shall be formed, and when they migrate, the
masters carry the slaves. Both in Switzerland and England the slaves seem
to have the exclusive care of the larvae, and the masters alone go on slave-
making expeditions. In Switzerland the slaves and masters work together,
making and bringing materials for the nest; both, but chiefly the slaves,
tend and milk, as it may be called, their aphides; and thus both collect
food for the community. In England the masters alone usually leave the
nest to collect building materials and food for themselves, their slaves and
larvae. So that the masters in this country receive much less service from
their slaves than they do in Switzerland.

, By what steps the instinct of F. sanguinea originated I will not pretend
ito conjecture. But as ants which are not slave-makers will, as I have seen,
carry off the pupae of other species, if scattered near their nests, it is possible
that such pupae originally stored as food might become 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 its end, without enthusiastic admiration. We hear from mathe-
maticians that bees have practically solved a recondite problem, and have
made their cells of the proper shape to hold the greatest possible amount
of honey, with the least possible consumption of precious wax in their con-
struction. It has been remarked that a skilful workman 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
first appears: all this beautiful work can be shown, I think, to follow from
a few simple instincts.

184 THE ORIGIN OF SPECIES

I was led to investigate this subject by Mr. Waterhouse, who has shown ii
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 modifi-
cation 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 an 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 base of a
single cell on one side of the comb enter into the composition of the bases
of three adjoining cells on the opposite side. In the series between the
extreme perfection of the cells of the hive-bee and the simplicity of those
of the humble-bee we have the cells of the Mexican Melipona domestica,
carefully described and figured by Pierre Huber. The Melipona itself is
intermediate in structure between the hive and humble-bee, but more nearly
related to the latter; it forms a nearly regular waxen comb of cylindrical
cells, in which the young are hatched, and, in addition, some large cells of
wax for holding honey. These latter cells are nearly spherical and of nearly
equal sizes, and are aggregated into an irregular mass. But the important
point to notice is, that these cells are always made at that degree of nearness
to each other that they would have intersected or broken into each other if
the spheres had been completed; but this is never permitted, the bees build-
ing perfectly flat walls of wax between the spheres which thus tend to in-
tersect. Hence, each cell consists of an outer spherical portion, and of two,
three, or more 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 Huber 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, labor, by this manner of building; for
the flat walls between the adjoining cells are not double, but are of the
same thickness as the outer spherical portions, and yet each flat portion i
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 1
equal sizes, and had arranged them symmetrically in a double layer, the re--
suiting structure would have been as perfect as the comb of the hive-bee..
Accordingly I wrote to Professor Miller of Cambridge, and this geometer''
has kindly read over the following statement, drawn up from his informa
tion, and tells me that it is strictly correct : —

INSTINCT 185

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 V 2j or radius X 141 421 (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 Professor 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.

Hence we may safely conclude that, if we could slightly modify the in-
stincts already possessed by the Melipona, and in themselves not very won-
derful, 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 them-
selves are not very wonderful — hardly more wonderful than those which
guide a bird to make its nest — I believe that the hive-bee has acquired,
through natural selection, her inimitable architectural powers.

But this theory can be tested by experiment. Following the example of
Mr. Tegetmeier, I separated two combs, and put between them a long,
thick, 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 converted into shallow basins, appearing
to the eye perfectly true or parts of a sphere, and of about the diameter of a
cell. It was most interesting to observe that, wherever several bees had be-
gun to excavate these basins near together, they had begun their work at
such a distance from each other that by the time the basins had acquired
the above-stated width {i.e.y 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

186 THE ORIGIN OF SPECIES

I

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, rectangular piece of wax, a
thin and narrow, knife-edged ridge, colored with vermilion. The bees in-
stantly 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 side 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 opposed basins, but the work, from the unnatural
state of things, had not been neatly performed. The bees must have worked
at very nearly the same rate in circularly 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, per-
ceiving when they have gnawed the wax away to the proper thinness, and
then stopping their work. In ordinary combs it has appeared to me that
the bees do not always succeed in working at exactly the same rate from the
opposite sides; for I have noticed half -completed rhombs at the base of a
just commenced cell, which were slightly concave on one side, where I I
suppose that the bees had excavated too quickly, and convex on the opposed
side where the bees had worked less quickly. In one well-marked instance, I
put the comb back into the hive, and allowed the bees to go on working for '
a short time, and again examined the cell, and I found that the rhombic
plate had been completed, and had become perfectly fiat: it was absolutely
impossible, from the extreme thinness of the little plate, that they could
have eff'ected 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 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 endeavoring 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;

INSTINCT 187

and they gnaw this away from the opposite sides, always working circularly
as they deepen each cell. They do not make the whole three-sided pyramidal
base of any one cell at the same time, but only that one rhombic plate
iwhich 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 con-
formable with my theory.

Ruber's statement, that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly correct; the
first commencement having always been a little hood of wax; but I will not
here enter on details. We see how important a part excavation plays in the
construction of the cells; but it would be a great error to suppose that the
bees cannot build up a rough wall of wax in the proper position — that is,
along the plane of intersection between two adjoining spheres. I have
several specimens showing clearly that they can do this. Even in the rude
circumferential rim or wall of wax round a growing comb, flexures may
sometimes be observed, corresponding in position to the planes of the rhom-
bic 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 man-
ner in which the bees build is curious ; they always make the first rough wall
from ten to twenty times thicker than the excessively thin finished wall of the
cell, which will ultimately be left. We shall understand how they work, by
supposing masons first to pile up a broad ridge of cement, and then to begin
cutting it away equally on both sides near the ground, till a smooth, very
thin wall is left in the middle; the masons always piling up the cut away
cement, and adding fresh cement on the summit of the ridge. We shall
thus have a thin wall steadily growing upward, but always crowned by a
gigantic coping. From all the cells, both those just commenced and those
completed, being thus crowned by a strong coping of wax, the bees can
cluster and crawl over the comb without injuring the delicate hexagonal
walls. 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, tI^ of an inch in thickness; whereas the
basal rhomboidal plates are thicker, nearly in the proportion of three to
two, having a mean thickness, from twenty-one measurements, of -fi? 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 understanding how the cells
are made, that a multitude of bees all work together; one bee after work-
ing a short time at one cell going to another, so that, as Huber has stated, a
score of individuals work even at the commencement of the first cell. I was
able practically to show this fact, by covering the edges of the hexagonal
walls of a single cell, or the extreme margin of the circumferential rim of a
growing comb, with an extremely thin layer of melted vermilion wax; and

188 THE ORIGIN OF SPECIES

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 try-
ing to sweep equal spheres, and then building up, or leaving ungnawed, the
planes of intersection between these spheres. It was really curious to note in
cases of difficulty, as when two pieces of comb met at an angle, how often
the bees would pull down and rebuild in different ways the same cell, some-
times 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 completed cells. It suffices that the bees should be enabled to stand
at their proper relative distances from each other and from the walls of the
last completed cells, and then, by striking imaginary spheres, they can build
up a wall intermediate between two adjoining spheres; but as far as I have
seen, they never gnaw away and finish off the angles of a cell till a large
part both of that cell and of the adjoining cells has beenbuilt. 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 subversive of the foregoing theory; namely,
that the cells on the extreme margin of wasp-combs are sometimes strictly
hexagonal; but I have not space here to enter on this subject. Nor does
there seem to me any great difficulty in a single insect (as in the case of a
queen-wasp) making hexagonal cells, if she 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, sweeping spheres or cylinders, and building up intermediate
planes.

As natural selection acts only by the accumulation of slight modifications
of structure or instinct, each profitable to the individual under its condi-
tions of life, it may reasonably be asked, how a long and graduated succession
of modified architectural instincts, all tending toward the present perfect
plan of construction, could have profited 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 forma-
tion of wax, it is known that bees 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 dry sugar are con-
sumed by a hive of bees for the secretion of a pound of wax; so that a

INSTINCT 189

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 dur-
ing the process of secretion. A large store of honey is indispensable to sup-
port a large stock of bees during the winter; and the security of the hive is
known mainly to depend on a large number of bees being supported. Hence
the saving of wax by largely saving honey, and the time consumed in col-
lecting the honey, must be an important element of success to any family
of bees. Of course the success of the species may be dependent on the num-
ber of its enemies, or parasites, or on quite distinct causes, and so be alto-
gether 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 com-
munity lived through the winter, and consequently required a store of
honey: there can in this case be no doubt that it would be an advantage to
our 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 more and more advantageous to our
humble-bees, if they were to make their cells more and more regular, nearer
together, and aggregated into a mass, like the cells of the Melipona; for in
this case a large part of the bounding surface of each cell would serve to
bound 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 dis-
appear 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 natural selection having taken advantage of
numerous, successive, slight modifications of simpler instincts; natural
selection having, by slow degrees, more and more perfectly led the bees to
sweep equal spheres at a given distance from each other in a double layer,
and to build up and excavate the wax along the planes of intersection; the
bees, of course, no more knowing that they swept their spheres at one par-
ticular 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 construction 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
individual swarm which thus made the best cells with least labor, and
least waste of honey in the secretion of wax, having succeeded best, and
having transmitted their newly-acquired economical instincts to new swarms.

190 THE ORIGIN OF SPECIES

which in their turn will have had the best chance of succeeding in the
struggle for existence.

objections to the theory of natural selection as applied to
instincts: neuter and sterile insects

It has been objected to the foregoing view of the origin of instincts, that
"the variations of structure and of instinct must have been simultaneous
and accurately adjusted to each other, as a modification in the one with-
out an 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 instincts and structure are abrupt. To take as an illustration
the case of the larger titmouse (Parus major), alluded to in a previous
chapter; this bird often holds the seeds of the yew between its feet on a
branch, and hammers with its beak till it gets at the kernel. Now what
special difficulty would there be in natural selection preserving all the
slight individual variations in the shape of the beak, which were better and
better adapted to break open the seeds, until a beak was formed, as well
constructed for this purpose as that of the nut-hatch, 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 modified by natural selection, subsequently to, but in ac-
cordance with, slowly changing habits or taste; but let the feet of the
titmouse vary and grow larger from correlation with the beak, or from any
other unknown cause, 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 nut-hatch. 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 in-
stincts 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 gradations
are known to exist; cases of instincts of such trifling importance, that they
could hardly have been acted upon 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 pro-

■i

INSTINCT 191

genitor, and consequently must believe that they were independently ac-
quired through natural selection. I will not here enter on these several cases,
but will confine myself to one special difficulty, which at first appeared to
me insuperable and actually fatal to 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 animals in a state of nature occasionally
become sterile; and if such insects had been social, and it had been profita-
ble to the community that a number should have been annually bom
capable of work, but incapable of procreation, I can see no especial diffi-
culty 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 struc-
ture, as in the shape of the thorax, and in being destitute of wings and
sometimes of eyes, and in instinct. As far as instinct alone is concerned, the
wonderful difference in this respect between 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 un-
hesitatingly assumed that all its characters had been slowly acquired
through natural selection; namely, by individuals having been born with
slight profitable modifications, which were inherited by the offspring, and
that these again varied and again were 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 transmitted successively ac-
quired modifications of structure or instinct to its progeny. It may well be
asked how it is 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 reproductive system is active, as in the
nuptial plumage of many birds, and in the hooked jaws of the male salmon.
We have even slight differences in the horns of different breeds of cattle in
relation to an artificially imperfect state of the male sex, for oxen of certain
breeds have longer horns than the oxen of other breeds, relatively to the
length of the horns in both the bull^ and cows of these same breeds. Hence,
I can see no great difficulty in any character becoming correlated with the
sterile condition of certain members of insect communities; the difficulty
lies in understanding how such correlated modifications of structure could
have been slowly accumulated by natural selection.

192 THE ORIGIN OF SPECIES

This difficulty, though appearing insuperable, is lessened, or, as I believe,
disappears, when it is remembered that selection may be applied to the
family, as well as to the individual, and may thus gain the desired end.
Breeders of cattle wish the flesh and fat to be well marbled together. An
animal thus characterized has been slaughtered, but the breeder has gone
with confidence 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 care-
fully watching which individual bulls and cows, when matched, produced
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. Verlot,
some varieties of the double annual stock, from having been long and care-
fully 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 ap-
plied to the family, and not to the individual, for the sake of gaining a
serviceable end. Hence, we may conclude that slight modifications of struc-
ture 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 transmitted to their fertile off"-
spring a tendency to produce sterile members with the same modifications.
This process must have been repeated many times, until that prodigious
amount of diff"erence between the fertile and sterile females of the same
species has been produced which we see in many social instincts.

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

INSTINCT 193

different from the fertile males and females through natural selection, we
may conclude from the analogy 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 com-
munities with females which produced most neuters having the advantage-
ous modification, all the neuters ultimately came to be thus characterized.
According to this view we ought occasionally to find in the same nest neuter
insects, presenting gradations of structure; and this we do find, even not
rarely, considering how few neuter insects out of Europe have been care-
fully examined. Mr. F. Smith has shown that the neuters of several British
ants differ surprisingly 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 rudi-
mentary in the smaller workers than can be accounted for merely by their
proportionately 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 continually
selected, which produced more and more of 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 Myrmica. For the
workers of Myrmica have not even rudiments of ocelli, though the male

, and female ants of this genus have well-developed ocelli.

I I may give one other case: so confidently did I expect occasionally to
find gradations of important structures 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 sup-
pose 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

194 THE ORIGIN OF SPECIES

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 dif-
ferent 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 "Naturalist on the Amazons," has described analogous cases.

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 sur-
vival of the parents which generated them, until none with an intermediate
structure were 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 Bra-
zilian crustaceans likewise appearing under two widely distinct male forms.
But this subject need not here be discussed.

I have now explained how, I believe, the wonderful fact of two dis-
tinctly 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 com-
munity of ants, on the same principle that the division of labor is useful
to civilized man. Ants, however, work by inherited instincts, and by in-
herited organs or tools, while man works by acquired knowledge and manu-
factured 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 in so high a degree, had not the case of these neuter instincts
led me to this conclusion. I have, therefore, discussed this case, at some
little but wholly insufficient length, in order to show the power of natural
selection, and likewise because this is by far the most serious special diffi-
culty 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 vari-
ations, which are in any way profitable, without exercise or habit having)
been brought into play. For peculiar habits, confined to the workers of j
sterile females, however long they might be followed, could not possiblyj
affect the males and fertile females, which alone leave descendants. I ai
surprised that no one has hitherto advanced this demonstrative case ol
neuter insects, against the well-known doctrine of inherited habit, as ad-
vanced by Lamarck.

INSTINCT 195

SUMMARY

I have endeavored in this chapter briefly to show that the mental quahties
of our domestic animals vary, and that the variations are inherited. Still
more briefly I have attempted to show that instincts vary slightly in a state
of nature. No one will dispute that instincts are of the highest importance
to each animal. Therefore, there is no real difficulty, under changing con-
ditions of life, in natural selection accumulating to any extent slight modi-
fications 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 given in this chapter strengthen in any great degree my theory; but
none of the cases of difficulty, to the best of my judgment, annihilate it.
On the other hand, the fact that instincts are not always absolutely perfect
and are Hable 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 "Natura non
facit saltum," is applicable to instincts as well as to corporeal structure, and
is plainly explicable on the foregoing views, but is otherwise inexplicalDle —
all tend to corroborate the theory of natural selection.

This theory is also strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but distinct, species,
when inhabiting distant parts of the world and living under considerable
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 horn-
bills 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 "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 such instincts as the young cuckoo ejecting its foster-brothers, ants mak-
ing slaves, the larvae of ichneumonidae feeding within the live bodies of
caterpillars, not as specially endowed or created instincts, but as small
consequences of one general law leading to the advancement of all organic
beings — namely, multiply, vary, let the strongest live and weakest die.

CHAPTER IX

Hybridism

Distinction between the Sterility of First Crosses and of Hybrids — Sterility Various
in Degree, not Universal, affected by Close Interbreeding, removed by Domesti-
cation— Laws governing the Sterility of Hybrids — Sterility not a Special Endow-
ment, 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 Conditions 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 inter-
crossed, have been specially endowed with sterility, in order to prevent their
confusion. This view certainly seems at first highly probable, for species liv-
ing 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 acquired, 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 lunda-
mentally different, have generally been confounded; 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 reproduction in a perfect con-
dition, yet when intercrossed they produce either few or no offspring. Hy-
brids, on the other hand, have their reproductive organs functionally impo-
tent, as may by clearly seen in the state of the male element in both plants
and animals; though the formative organs themselves are perfect in struc-
ture, 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 im-
portant, 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 province 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 offspring, is, with reference to my theory, of equal importance
with the sterility of species; for it seems to make a broad and clear distinc-
tion 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 several memoirs and works of those two con-

196

HYBRIDISM 197

scientious and admirable observers, Kolreuter and Gartner, who almost de-
voted their lives to this subject, without being deeply impressed with the
high generality of some degree of sterility. Kolreuter makes the rule univer-
sal ; 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 un-
hesitatingly 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
maximum number of seeds produced by two species when first crossed, and
the maximum produced by their hybrid offspring, with the average number
produced by both pure parent-species in a state of nature. But causes of seri-
ous error here intervene; a plant, to be hybridized, must be castrated, and,

I what is often more important, must be secluded in order to prevent pollen
being brought to it by insects from other plants. Nearly all the plants ex-
perimented 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, can-
not 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 (ex-
cluding all cases such as the Leguminosse, 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 abso-
lutely sterile, we may doubt whether many species are really so sterile, when
intercrossed, as he believed.

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

jperfect fertility ends and sterility begins. I think no better evidence of this
can be required than that the two most experienced observers who have ever
lived, namely, Kolreuter and Gartner, arrived at diametrically opposite con-
clusions 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 details — the evidence
advanced by our best botanists on the question whether certain doubtful
forms should be ranked as species or varieties, with the evidence from fertility
adduced by different 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 genera-
tions, yet he asserts positively that their fertility never increases, but generally

198 THE ORIGIN OF SPECIES

decreases greatly and suddenly. With respect to this decrease, it may first be
noticed that when any deviation in structure or constitution is common to
both parents, this is often transmitted in an augmented degree to the off-
spring; and both sexual elements in hybrid plants are already affected in
some degree. But I believe that their fertility has been diminished 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 interbreeding lessens their vigor and fertility, that I cannot doubt
the correctness of this conclusion. Hybrids are seldom raised by experimental-
ists in great numbers; and as the parent-species, or other allied hybrids,
igenerally 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 strengthened in this conviction by a re-
markable statement repeatedly 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 frequent ill effects from manipula-
tion, sometimes decidedly increases, and goes on increasing. Now, in the
process of artificial fertilization, pollen is as often taken by chance ( as I know
from my own experience) from the anthers of another flower, as from the
anthers of the flower itself which is to be fertilized; so that a cross between
two flowers, though probably often on the same plant, would be thus effected.
Moreover, whenever complicated experiments are in progress, so careful an
observer as Gartner would have castrated his hybrids, and this would have
insured in each generation a cross with pollen from a distinct flower, either
from the same plant or from another plant of the same hybrid nature. And
thus, the strange fact of an increase of fertility in the successive generations
of artificially fertilized hybrids, in contrast with those spontaneously self-
fertilized, may, as I believe, 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 diff'erence in their results
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 state-
ments I will here give only a single one as an example, namely, that "every
ovule in a pod of Crinum 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.

HYBRIDISM 199

This case of the Crinum leads me to refer to a singular fact, namely, that
individual plants of certain species of Lobeha, Verbascum, and Passiflora,
can easily be fertilized by the 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,
as 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 species 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 auHcum 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 "the ovaries of the three first flowers soon ceased to grow, and
after a few days perished entirely, whereas the pod impregnated by the pollen
of the hybrid made vigorous growth and rapid progress to maturity, and bore
good seed, which vegetated freely." 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, Rho-
dodendron, 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
Chili." I have taken some pains to ascertain the degree of fertility of some of
the complex crosses of Rhododendrons, and I am assured that many of them
are perfectly fertile. Mr. C. Noble, for instance, informs me that he raises
stocks for grafting from a hybrid between Rhod. ponticum and catawbiense,
and that this hybrid "seeds as freely as it is possible to imagine." Had hybrids,
when fairly treated, always gone on decreasing in fertility in each successive
generation, as Gartner believed to be the case, the fact would have been
notorious 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 most 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 them-

200 THE ORIGIN OF SPECIES

selves 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 distinct species of finches, but, as not one of these breeds freely in con-
finement, we have no right to expect that the first crosses between them and
the canary, or that their hybrids, should be perfectly fertile. Again, with
respect to the fertility in successive generations of the more fertile hybrid
animals, I hardly know of an instance in which two families of the same
hybrid have been raised at the same time from different parents, so as to
avoid the ill effects of close interbreeding. On the contrary, brothers and
sisters have usually been crossed in each successive generation, in opposition
to the constantly repeated admonition of every breeder. And in this case, it
is not at all surprising that the inherent sterility in the hybrids should have
gone on increasing.

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 Reevesii, 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 to-
gether, produce offspring which are highly fertile when crossed with one of
the parent-species. The hybrids from the common and Chinese geese (A.
cygnoides) species, which are so different that they are generally ranked in
distinct genera, have often bred in this country with either pure parent, and
in one single instance they have bred inter se. This was effected by Mr. Eyton,
who raised two hybrids from the same parents, but from different hatches;
and from these two birds he raised no less than eight hybrids (grandchildren
of the pure geese) from one nest. In India, however, these cross-bred geese
must be far more fertile; for I am assured by two eminently capable judges,
namely, Mr. Blyth and Captain 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 subse-
quently reared under domestication became 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 stock; yet, with per-
haps 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-

HYBRIDISM 201

dence that the crossed offspring from the Indian humped and common cattle
are inter se perfectly fertile; and from the observations by Riitimeyer on
their important osteological 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 ex-
tended to the two chief races of the pig. We must, therefore, either give
up the belief of the universal sterility of species when crossed; or we must
look at this sterility in animals, not 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 cannot,
under our present state of knowledge, be considered as absolutely universal.

LAW^S GOVERNING THE STERILITY OF FIRST GROSSES 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 blending together in utter
confusion. The following conclusions 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
generally the same rules apply to both kingdoms.

It has been already remarked, that the degree of fertility, both of first
crosses and of hybrids, graduates from zero to perfect fertility. It is surpris-
ing 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 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, be-
yond that which the plant's own pollen produces. So in hybrids themselves,
there are some which never have produced, and probably never would pro-
duce, 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 ex-
treme 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

202 THE ORIGIN OF SPECIES

which rarely produce any offspring, are generally very sterile ; but the parallel-
ism 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 facility,
and produce numerous hybrid offspring, yet these hybrids are remarkably
sterile. On the other hand, there are species which can be crossed very rarely,
or with extreme difficulty, but the hybrids, when at last produced, are very
fertile. Even within the limits of the same genus, for instance in Dianthus,
these two opposite cases occur.

The fertility, both of first crosses and of hybrids, is more easily affected
by 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 constitution of the individuals which happen to
have been chosen for the experiment. So it is with hybrids, for their degree
of fertility is often found to differ greatly in the several individuals raised
from seed out of the same capsule and exposed to 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 produced from them, is largely governed by their systematic
affinity. This is clearly shown by hybrids never having been raised between
species ranked by systematists in distinct families; and on the other hand,
by very closely allied species generally uniting with facility. But the cor-
respondence between systematic affinity and the facility of crossing is by
no means strict. A multitude of cases could be given of very closely allied
species which will not unite, or only with extreme difficulty; and on the
other hand of very distinct species which unite with the utmost facility.
In the same family there may be a genus, as Dianthus, in which very many
species can most readily be crossed; and another genus, as Silene, in which
the most persevering efforts have failed to produce between extremely close
species a single hybrid. Even within the limits of the same genus, we meet
with this same difference; for instance, the many species of Nicotiana have
been more largely crossed than the species of almost any other genus; but
Gartner found that N. acuminata, which is not a particularly distinct species,
obstinately 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 ap-
pearance, and having strongly marked differences in every part of the flower,
even in the pollen, in the fruit, and in the cotyledons, can be crossed. Annual
and perennial plants, deciduous and evergreen trees, plants inhabiting
different stations, and fitted for extremely different climates, can often be
crossed with ease.

By a reciprocal cross between two species, I mean the case, for instance,

HYBRIDISM 203

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 difTerence 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 system-
atic affinity, that is, of any difference in their structure or constitution,
excepting in their reproductive systems. The diversity of the result in re-
ciprocal 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. longiflora 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 observed it even
between closely related forms (as Matthiola 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 compounded 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 occasionally 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 like-
ness on their hybrid offspring; but these two powers do not at all necessarily
go together. There are certain hybrids which, instead of having, as is usual,
an intermediate character between their two parents, always closely re-
semble 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 between
their parents, exceptional and abnormal individuals sometimes are bom,
which closely resemble one of their pure parents; and these hybrids are
almost always utterly sterile, even when the other hybrids raised from seed
from the same capsule have a considerable degree of fertility. These facts
show how completely 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 con-
sidered as good and distinct species, are united, their fertility graduates
from zero to perfect fertility, or even to fertility under certain conditions
in excess ; that their fertility, besides being eminently susceptible to favorable
and unfavorable conditions, is innately variable; that it is by no means
always the same in degree in the first cross and in the hybrids produced
from this cross; that the fertility of hybrids is not related to the degree in
which they resemble in external appearance either parent; and lastly, that

204 THE ORIGIN OF SPECIES

the facility of making a first cross between any two species is not always
governed by their systematic affinity or degree of resemblance to each other.
This latter statement is clearly proved by the difference in the result of re-
ciprocal crosses between the same two species, for, according as the one
species or the other is used as the father or the mother, there is generally
some difference, and occasionally the widest possible difference, in the
facility of effecting an union. The hybrids, moreover, produced from re-
ciprocal crosses often differ in fertility.

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

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 the other,
but not in a reversed direction. It will be advisable to explain a little more
fully, by an example, what I mean by sterility being incidental on other 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 incidental on differences
in the laws of growth of the two plants. We can sometimes see the reason
why one tree will not take on another, from differences in their rate of
growth, in the hardness of their wood, in the period of the flow or nature
of their sap, etc. ; but in a multitude of cases we can assign no reason what-
ever. 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.

HYBRIDISM 205

But this capacity, as in hybridization, is by no means absolutely governed
by systematic affinity. Although many distinct genera within the same family
have been grafted together, in other cases species of the same genus will
not take on each other. The pear can be grafted far more readily on the
quince, which is ranked as a distinct genus, than on the apple, which is a
member of the same genus. Even different varieties of the pear take with
different degrees of facility on the quince; so do different varieties of the
apricot and peach on certain varieties of the plum.

As Gartner found that there was sometimes an innate difference in dif-
ferent 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 an
union is often very far from equal, so it sometimes is in grafting. The common
gooseberry, for instance, cannot be grafted on the currant, whereas the
currant will take, though with difficulty, on the gooseberry.

We have seen that the sterility of hybrids which have their reproductive
organs in an imperfect condition, is a 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 Robinia,
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 re-
minded by this latter fact of the extraordinary cases of hippeastrum, passi-
flora, 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 paral-
lelism in the results of grafting and of crossing distinct species. And as we
must look at the curious and complex laws governing the facility with
which trees can be grafted on each other as incidental on unknown 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 dif-
ferences in their reproductive systems. These differences in both cases follow,
to a certain extent, as might have been expected, systematic affinity, by which
term every kind of resemblance and dissimilarity between organic beings is
attempted to be expressed. The facts by no 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 difficulty
is as important for the endurance and stability of specific forms as in the
case of grafting it is unimportant for their welfare.

206 THE ORIGIN OF SPECIES

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 certain 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 con-
sequently this could not have been effected through natural selection; but it
may perhaps be argued, that, if a species was rendered sterile with some one
compatriot, sterility with other species would follow as a necessary con-
tingency. 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 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 tlie re-
productive 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
He in the existence of many graduated steps, from slightly lessened fertiUty
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 bastardized 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
reflect on the steps by which this first degree of steriHty 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 extraordinarily 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 degree with mutual infertility, and which thus approached
by one small step toward absolute sterility? Yet an advance of this kind, if
the theory of natural selection be brought to bear, must have incessantly oc-
curred with many species, for a multitude are mutually quite barren. With
sterile neuter insects we have reason to beheve that modifications in their
structure and fertility have been slowly accumulated by natural selection,

HYBRIDISM 207

from an advantage having been thus indirectly given to the community to
which they belonged over other communities of the same species; but an in-
dividual 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. Both Gart-
ner and Kolreuter have proved that in genera including 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, but yet are affected by
the pollen of certain other species, 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 af-
fected, cannot have been gained through selection; and from the laws gov-
erning 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 sterility in first crosses and in hybrids. In the
case of first crosses, the greater or less difficulty in effecting an 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
species is placed on the stigma of a distantly alhed 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 em-
bryo to be developed, as seems to have been the case with some of Thuret's
experiments on Fuci. No explanation can be given of these facts, any more
than why certain trees cannot be grafted on others. Lastly, an embryo may
be developed, and then perish at an early period. This latter alternative has
not been sufficiently attended to; but I believe, from observations communi-
cated 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
fertilized; and in the majority of the fertilized eggs, the embryos had either
been partially developed and had then perished, or had become nearly ma-
ture, 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, "without any obvious cause, apparently from
mere inability to live" ; so that from the 500 eggs only twelve chickens were

208 THE ORIGIN OF SPECIES

I

reared. With plants, hybridized embryos probably often perish in a like man-
ner; 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 like 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 em-
bryos; for hybrids, when once born, are generally healthy and long-lived, as
we see in the case of the common mule. Hybrids, however, are differently
circumstanced before and after birth: when born and living in a country
where their two parents live, they are generally placed under suitable con-
ditions 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 imperfectly developed, rather than
in the conditions to which it is subsequently exposed.'

In regard to the sterility of hybrids, in which the sexual elements are im-
perfectly 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 superinduced and
that of hybrids, there are many points of similarity. In both cases the sterility
is independent of general health, and is often accompanied by excess of size
or great luxuriance. In both cases the sterility occurs in various degrees; in
both, the male element is the most liable to be affected; but sometimes the
female more than the male. In both, the tendency goes to a certain extent
with systematic affinity, for whole groups of animals and plants are rendered
impotent by the same unnatural conditions; and whole groups of species
tend to produce sterile hybrids. On the other hand, one species in a group
will sometimes resist great changes of conditions with unimpaired fertility;
and certain species in a group will produce unusually fertile hybrids. No one
can tell till he tries, whether any particular animal will breed under confine-
ment, 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

HYBRIDISM 209

is with hybrids, for their 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 un-
natural conditions, and when hybrids are produced by the unnatural cross-
ing 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 constitutions, including of course the repro-
ductive 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 explained, 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 understand, on the above or
any other view, several facts with respect to the sterility of hybrids; for in-
stance, the unequal fertility of hybrids produced from reciprocal crosses; or
the increased sterility in those hybrids which occasionally and exceptionally
resemble closely either pure parent. Nor do I pretend that the foregoing
remarks go to the root of the matter; no explanation is offered why an organ-
ism, 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 evidence, 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 evidence 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

210 THE ORIGIN OF SPECIES

life benefit all organic beings, and on the other hand, that slight crosses,
that is, crosses between the males and females of the same species, which
have 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 con-
ditions, 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 breeding when kept under only partial con-
finement 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, ac-
cording 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 equilibrium; and when this tendency
is slightly disturbed by any change, the vital forces gain in power.

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 dif-
fering 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 i
the stigma of the third form. Now I have shown, and the result has beeni
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 lesss
infertile. With trimorphic species six unions are legitimate, or fully fertile, ,
and twelve are illegitimate, or more or less infertile.

HYBRIDISM 211

The infertility which may be observed in various dimorphic and trimor-
phic plants^ when they are illegitimately fertilized, that is, by pollen taken
from stamens 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 de-
pends in an eminent degree on the conditions of life being more or less favor-
able, 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 prepotent that it generally annihi-
lates 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 previously applied illegitimate pollen. Again,
as in making reciprocal crosses between the same two species, there is occa-
sionally a great difference in the result, so the same thing occurs with trimor-
phic plants; for instance, the mid-styled form of Lythrum salicaria was
illegitimately fertilized with the greatest ease by pollen from the longer sta-
mens 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 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 care-
fully to observe during four years many seedlings, raised from several illegiti-
mate unions. The chief result is that these illegitimate plants, as they may
be called, are not fully fertile. It is possible to raise from dimorphic 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 fer-
tilized. But such is not the case. They are all infertile, in various degrees;
some being so utterly and incurably sterile th^t they did not yield during
four seasons a single seed or even seed-capsule. The sterility of these illegiti-
mate 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 usu-
ally much lessened and so it is when an illegitimate plant is 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 that sterility of certain illegitimate plants was unusu--
ally great, while the sterility of the union from which they were derived was

212 THE ORIGIN OF SPECIES

by no means great. With hybrids raised from the same seed-capsule the de-
gree of sterility is innately variable, so it is in a marked manner with illegiti-
mate plants. Lastly, many hybrids are profuse and persistent flowerers, while
other and more sterile hybrids produce few flowers, and are weak, miserable
dwarfs; exactly similar cases occur with the illegitimate offspring of various
dimorphic and trimorphic 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 per-
haps 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 be-
haved in all the other above specified respects as if they had been two dis-
tinct species. But to make the case sure, he would raise plants from his sup-
posed 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 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 chat of their illegitimate
offspring, 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 constitution, relatively to external condi-
tions, 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 consideration 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 constitu-

HYBRIDISM 213

tion. 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 second species, while the converse cross
can be effected with perfect facility. That excellent observer, Gartner, like-
wise concluded that species when crossed are sterile owing to differences con-
fined 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 es-
sential distinction between species and varieties, inasmuch as the latter, how-
ever much they may differ from each other in external appearance, cross
with perfect facility, and yield perfectly fertile offspring. With some excep-
tions, presently to be given, I fully admit that this is the rule. But the sub-
ject is 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 con-
sequently 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. Nevertheless the perfect fertility of so many do-
mestic 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 considera-
tions, however, render the fertility of domestic varieties less remarkable. In
the first place, it may be observed that the amount of external difference be-
tween 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 sex-
ual constitution. Now the varying conditions to which domesticated animals
and cultivated plants have been subjected, have had so little tendency toward
modifying 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 per-
fectly fertile together. With plants, so far is cultivation from giving a tend-

214 THE ORIGIN OF SPECIES

ency toward sterility between distinct species, that in several well-authenti-
cated cases already alluded to, certain plants have been affected in an oppo-
site 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 admitted, and it can hardly be rejected, it becomes in the highest degree
improbable that similar conditions long-continued should likewise induce
this tendency; though in certain cases, with species having a peculiar consti-
tution, sterility might occasionally be thus caused. Thus, as I believe, we can
understand why, with domesticated animals, varieties have not been pro-
duced 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 numerous competitors, will have been exposed during
long periods of time to more uniform conditions, than have domestic vari-
eties; and this may well make a wide difference in the result. For we know
how commonly wild animals and plants, when taken from their natural con-
ditions and subjected to captivity, are rendered sterile; and the reproductive
functions of organic beings which have always lived under natural condi-
tions 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 domestication, were not originally highly
sensitive to changes in their conditions of life, and which can now generally
resist with undiminished fertility repeated changes of conditions, might be
expected to produce varieties, which would be little liable to have their re-
productive powers injuriously affected by the act of crossing with other vari-
eties 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 ex-
istence of a certain amount of sterility in the few following cases, which I will
briefly abstract. The evidence is at least as good as that from which we believe
in the sterility of a multitude of species. The evidence is also derived from
hostile witnesses, who in all other cases consider fertility and sterility as safe
criterions of specific distinction. 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

HYBRIDISM • 215

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 im-
portant 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 separate sexes, and he asserts that their mutual fertilization is by so much
the less easy as their differences are greater. How far these experiments may
be trusted, I know not ; but the forms 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 conclusion.

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 Verbascum, by so good an observer and so hos-
tile 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 experimented
on the species and varieties of Verbascum; and although unable to confirm
Gartner's results on the crossing of the distinct species, he finds that the dis-
similarly colored varieties of the same species yield fewer seeds, in the pro-
portion of eighty-six to one hundred, than the similarly colored varieties.
Yet these varieties differ in no respect, except in the color of their flowers;
and one variety can sometimes be raised from the seed of another.

Kolreuter, whose accuracy has been confirmed by every subsequent ob-
server, has proved the remarkable fact that one particular variety of the com-
mon 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 reciprocal crosses, and he found their mongrel offspring perfectly 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 sterile as
those which were produced from the four other varieties when crossed with
N. glutinosa. Hence, the reproductive system of this one variety must have
been in some manner and in some degree modified.

From these facts it can no longer be maintained that varieties when crossed
are invariably quite fertile. From the great difficulty of ascertaining the in-
fertility of varieties 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 external 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 considerations we may conclude that
fertility does not constitute a fundamental distinction between varieties and

216 THE ORIGIN OF SPECIES

species when crossed. The general sterility of crossed species may safely be
looked at, not as a special acquirement 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 compared 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 hybrid offspring of species, and the so-called mongrel
offspring 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 generation mongrels are more variable than
hybrids; but Gartner admits that hybrids from species which have long been
cultivated are often variable in the first generation; and I have myself seen
striking instances of this fact. Gartner further admits that hybrids between
very closely allied species are more variable than those from very distinct
species; and this shows that the difference in degree of variability graduates
away. When mongrels and the more fertile hybrids are propagated for sev-
eral 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, however, in
the successive generations of mongrels is, perhaps, 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 augment 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 variability, namely, that
the reproductive system, from being eminently sensitive to changed condi-
tions 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 the repro-
ductive 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, Gart-
ner expressly states that the hybrids from long cultivated plants are more
subject to reversion than hybrids from species in their natural state; and this
probably explains the singular difference in the results arrived at by differ-

HYBRIDISM 217

ent 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 uni-
versal tendency to reversion in hybrids, and he experimented chiefly on culti-
vated plants. Gartner 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.

Such alone are the unimportant differences which Gartner is able to point
out between hybrid and mongrel plants. On the other hand, the degrees and
kinds of resemblance in mongrels and in hybrids to their respective parents,
more especially in hybrids produced from nearly related species, follow, ac-
cording to Gartner, the same laws. When two species are crossed, one has
sometimes a prepotent power of impressing its likeness on the hybrid. So I
believe it to be with varieties of plants; and with animals, one variety cer-
tainly 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 mon-
grels can be reduced to either pure parent form by repeated crosses in suc-
cessive generations with either parent.

These several remarks are apparently applicable to animals, but the sub-
ject 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 maintain 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
an offspring of the male ass and mare, is more like an ass than is the hinny,
which is the offspring of the female ass and stallion.

Much stress has been laid by some authors on the supposed fact, that 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 characters almost mon-
strous in their nature, and which have suddenly appeared — such as albinism,
melanism, deficiency of tail or horns, or additional fingers and toes; and do
not relate to characters which have been slowly acquired through selection.
A tendency to sudden reversions to the perfect character of either parent
would, also, be much more likely to occur with mongrels, which are de-
scended from varieties often suddenly produced and semi-monstrous in char-

218 THE ORIGIN OF SPECIES

acter, than with hybrids, which are descended from species slowly and nat-
urally produced. On the whole, I entirely agree with Dr. Prosper Lucas,
who, after arranging an enormous body of facts with respect to animals,
comes to the conclusion that the laws of resemblance of the child to its
parents are the same, whether the two parents differ 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 produced by secondary laws, this
similarity would be an astonishing fact. But it harmonizes perfectly with the
view that there is no essential distinction between species and varieties.

SUMMARY OF CHAPTER

First crosses between forms, sufficiently distinct to be ranked as species, ,
and their hybrids, are very generally, but not universally, sterile. The sterility
is of all degrees, and is often so slight that the most careful experimentalists ;
have arrived at diametrically opposite conclusions in ranking forms by this
test. The sterility is innately variable in individuals of the same species, and
is eminently susceptible to action of favorable and unfavorable conditions.
The degree of sterility does not strictly follow systematic affinity, but is gov-
erned by several curious and complex laws. It is generally different, and
sometimes widely different, in reciprocal crosses between the same two
species. It is not always equal in degree in a first cross and in the hybrids pro-
duced 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 un-
known nature, in their vegetative systems, so in crossing, the greater or less
facility of one species to unite with another is incidental on unknown differ-
ences in their reproductive systems. There is no more reason to think that
species have been specially endowed with various degrees of sterility to pre-
vent their crossing and blending in nature, than to think that trees have
been specially endowed with various and somewhat analogous degrees of
difficulty in being grafted together in order to prevent their inarching in our
forests.

The sterility of first crosses and of their hybrid progeny has not been ac- i
quired through natural selection. In the case of first crosses it seems to de- '
pend 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 i[
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 another kind;

HYBRIDISM 219

namely, that, firstly, 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 primal cause of the sterility of crossed species is confined to differ-
ences in their sexual elements. But why, in the case of distinct species, the
sexual elements should so generally have become more or less modified, lead-
ing to their mutual infertility, we do not know; but it seems to stand in some
close relation 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 surprising that the facility of effect-
ing a first cross, and the fertility of the hybrids thus produced, and the ca-
pacity of being grafted together — though this latter capacity evidently de-
pends 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 universal
and perfect fertility surprising, when it is remembered 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. Independ-
ently 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 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 condi-
tions become sterile, yet the facts given in this chapter do not seem to me
opposed to the belief that species aboriginally existed as varieties.

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 in-
ferred from the Rate of Denudation and of Deposition — On the Lapse of Time
as estimated by Years — On the Poorness of our Palaeontological 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 objection^ which might be justly-
urged against the views maintained in this volume. Most of them have now
been discussed. One, namely, the distinctness of specific forms and their not
being blended together by innumerable transitional links, is a very obvious
difficulty. I assigned reasons why such links do not commonly occur at the
present day under the circumstances apparently most favorable for their
presence, namely, on an extensive and continuous area with graduated
physical conditions. I endeavored to show that the life of each species de-
pends in a more important manner on the presence of other already defined
organic forms, than on climate, and, therefore, that the really governing
conditions of life do not graduate away quite insensibly like heat or moisture.
I endeavored, also, to show that intermediate varieties, from existing in
lesser numbers than the forms which they connect, will generally be beaten
out and exterminated during the course of further modification and improve-
ment. The main cause, however, of innumerable intermediate links not now
occurring everywhere throughout nature, depends on the very process of
natural selection, through which new varieties continually take the places
of and supplant their parent-forms. But just in proportion as this process of
extermination has acted on an enormous scale, so much the number of in-
termediate varieties, which have formerly existed, be truly enormous. Why
then is not every geological formation and every stratum full of such inter-
mediate 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 inter-
mediate 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 descendants. 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

220

THE IMPERFECTION OF THE GEOLOGICAL RECORD 221

should have no varieties directly intermediate between the fantail and
pouter; none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds. These two
breeds, moreover, have become so much modified, that, if we had no historical
or indirect evidence regarding their origin, it would not have been possible
to have determined, from a mere comparison of their structure with that of
the rock-pigeon, 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 inter-
mediate 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 de-
scendants, unless at the same time we had a nearly perfect chain of the inter-
mediate 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 prin-
ciple of competition between organism and organism, between child and
parent, will render this a very rare event; for in all cases the new and im-
proved 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
converging to the common ancestor of each great class. So that the number
of intermediate and transitional links, between all living and extinct species,
must have been inconceivably great. But assuredly, if this theory be true^
such have lived upon the earth.

ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF
DEPOSITION 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,

222 THE ORIGIN OF SPECIES

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 formations, and to mark how each author attempts to give an
inadequate idea of the duration of each formation, 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. There-
fore 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 monuments 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 degradation. The tides in most cases reach
the cliffs only for a short time twice a day, and the waves eat into them only
when they are charged with sand or pebbles; for there is good evidence that
pure water effects nothing in wearing away rock. At last the base of the cliff
is undermined, huge fragments fall down, and these, remaining fixed, have
to be worn away atom by atom, until 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
cHffs rounded bowlders, all thickly clothed by marine productions, showing
how little they are abraded, and how seldom they are rolled about! More-
over, if we follow for a few miles any line of rocky cliff, which is undergoing
degradation, we find that it is only here and there, along a short length or
round a promontory, that the clifTs 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 observations of Ramsay, in
the van of many excellent observers — of Jukes, Geikie, Croll, and others,
that subaerial degradation 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, especially in arid districts, by the wind; it is then
transported 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 seacoasts, cannot have been thus formed, for each

THE IMPERFECTION OF THE GEOLOGICAL RECORD 223

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 escarpments owe their origin in
chief part to the rocks of which they are composed, having resisted subaerial
denudation better than the surrounding surface; this surface 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 great cracks along which the strata have been upheaved on one side,
or thrown down on the other, to the height or depth of thousands of feet;
for since the crust cracked, 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 thirty miles, and along this line
the vertical displacement of the strata varies from 600 to 3,000 feet. Pro-
fessor Ramsay has published an 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 10,000 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.

Palaeozoic strata (not including igneous beds) 57jI54

Secondary strata 13,190

Tertiary strata 2,240

224 THE ORIGIN OF SPECIES

— making altogether 72,584 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 Continent.
Moreover, between each successive formation we have, in the opinion of most
geologists, blank periods of enormous length. So that the lofty pile of sedi-

< mentary rocks in Britain gives but an inadequate idea of the time which has
elapsed during their accumulation. The consideration of these various facts
impresses the mind almost 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 geological 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 disintegrated,
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 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,
eighty-three feet four 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 hun-
dred years implies, represented as it is by a measure utterly insignificant in a
hall of the above dimensions. Several eminent breeders, during a single life-
time, 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
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 guidance of methodical selection. The com-
parison 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 modifying the breed; but by this process of
unconscious selection, 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

J all the inhabitants 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 IMPERFECTION OF THE GEOLOGICAL RECORD 225

the immigration of new forms. Moreover, variations or individual differences
of the right nature, by which some of the inhabitants 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 PAL/^ONTOLOGICAL COLLECTIONS

Now let us turn to our richest geological museums, and what a paltry
display we behold! That our collections are imperfect, is admitted by every
one. The remark of that admirable palaeontologist, Edward Forbes, should
never be forgotten, namely, that very many fossil species are known and
named from single and often broken specimens, or from a few specimens
collected on some one spot. Only a small portion of the surface of the earth
has been geologically explored, and no part with sufficient care, as the im-
portant 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 sufficiently quick to embed and
preserve fossil remains. Throughout an enormously large proportion of the
ocean, the bright blue tint of the water bespeaks its purity. The many cases
on record of a formation conformably covered, after an immense interval
of time, by another and later formation, without the underlying bed having
suffered in the interval any wear and tear, seem explicable only on the view
of the bottom of the sea not rarely lying for ages in an unaltered condition.
The remains which do become embedded, if in sand or gravel, will, when
the beds are upraised, generally be dissolved by the percolation of rain water
charged with carbolic 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 Chthamalinse (a sub-family of sessile
cirripedes) coat the rocks all over the world in infinite numbers: they are
all strictly littoral, with the exception of a single 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 ac-
cumulation, are entirely destitute of organic remains, without 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, oc-
casionally even six thousand, feet in thickness, and extending 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 Sec-

226 THE ORIGIN OF SPECIES

ondary and Palaeozoic periods, it is superfluous 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 periods, 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 be-
longing to the age of our secondary or palaeozoic formations.

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
palaeontologists, 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 believing that they are closely
consecutive. But we know, for instance, from Sir R. Murchison's great
work on Russia, what wide gaps there are in that country between the
superimposed formations; so it is in North America, and in many other
parts of the world. The most skilful geologist, if his attention had been
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 accumulated. And if, in every separate territory, hardly any
idea can be formed of the length of time which has elapsed between the
consecutive formations, we may infer that this could nowhere be ascertained.
The frequent and great changes in the mineralogical composition of consecu-
tive formations, generally implying great changes in the geography of the sur-
rounding 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
miles of the South American coasts, which have been upraised several
hundred feet within the recent period, than the absence of any recent deposits
sufficiently extensive to last for even a short geological period. Along the
whole west coast, which is inhabited by a peculiar marine fauna, tertiary
beds are so 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 formations with recent or tertiary remains can anywhere be
found, though the supply of sediment must for ages have been great, from
the enormous degradation of the coast rocks and from the muddy streams

THE IMPERFECTION OF THE GEOLOGICAL RECORD 227

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 accumulated in ex-
tremely 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 ex-
tensive accumulations of sediment may be formed in two ways; either in
profound depths of the sea, 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 neighborhood during the period of its accumulation. Or
sediment may be deposited to any thickness and extent over a shallow bottom,
if it continue slowly to subside. In this latter case, as long as the rate of sub-
sidence 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 through-
out 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 resist 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 apparently these oscillations have affected wide
spaces. Consequently formations rich in fossils and sufficiently thick and
extensive to resist subsequent degradation will have been formed over wide
spaces during periods of subsidence, but only where the supply of sediment
was sufficient to keep the sea shallow and to embed and preserve the remains
before they had time to decay. On the other hand, as long as the bed of the
sea remains stationary, thick deposits cannot have been accumulated in the
shallow parts, which are the most favorable to life. Still less can this have
happened during the alternate periods of elevation; or, to speak more accu-
rately, the beds which were then accumulated will generally have been
destroyed by being upraised and brought within the limits of the coast-action.

These remarks apply chiefly to littoral and sub-littoral deposits. 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 thirty or forty to sixty
fathoms, a widely extended formation might be formed during a period of
elevation, and yet not suffer excessively from denudation during its slow

228 THE ORIGIN OF SPECIES

upheaval; but the thickness of the formation 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 consolidated, 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, however, been suggested by Mr. Hopkins, that if
one part of the area, after rising and before being denuded, subsided, the
deposit formed during the rising movement, though not thick, might after-
ward 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. 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
extensive areas of such rocks in any 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 Von Eschwege gives a detailed section 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, collected along the whole coast, from
near Rio de 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. Turn-
ing to a well-known region, namely, to the United States and Canada, as
shown in Professor H. D. Rogers's beautiful map, I have estimated the areas
by cutting out and weighing the paper, and I find that the metamorphic
(excluding the "semi-metamorphic" ) and granite rocks exceed, in the pro-
portion of 19 to 12.5, the whole of the newer Palaeozoic formations. In many
regions the metamorphic and granite 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.

THE IMPERFECTION OF THE GEOLOGICAL RECORD 229

Hence, it is probable that in some parts of the world whole formations have
been completely 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 in-
creased, and new stations will often be formed — all circumstances favorable,
as previously explained, for the formation of new varieties and species; but
during such periods there will generally be a blank in the geological record.
On the other hand, during subsidence, the inhabited area and number of
inhabitants will decrease (excepting on the shores of a continent when first
broken up into an archipelago) , and consequently, during subsidence, 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.

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 extremely imperfect; but if we confine our
attention to any one formation, it becomes much more difficult to understand
why we do not therein find closely graduated varieties between the allied
species which lived at its commencement and at its close. 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 Hilgendorf has described a most curious case of ten
graduated forms of Planorbis multiformis in the successive beds of a fresh-
water formation in Switzerland. Although each formation has indisputably
required 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 can-
not assign due proportional 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 palaeontologists, whose opinions are worthy
of much deference, namely Bronn and Woodward, have concluded that tlic
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 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 de-
posited, 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

230 THE ORIGIN OF SPECIES

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 palaeozoic beds of North
America than in those of Europe; time having apparently been required
for their migration from the American to the European seas. In examining
the latest deposits, in various quarters of the world, it has everywhere been
noted, that some few still existing species are common in the deposit, but
have become extinct in the immediately surrounding sea; or, conversely, that
gome are now abundant 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
reflect 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 whether, in any quarter of the world, sedimentary deposits,
including fossil remains, have gone on accumulating within the same area
during the whole of this period. It is not, for instance, probable that sediment
was deposited during the whole of the glacial period near the mouth of the
Mississippi, within that limit of depth at which marine animals can 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 Mississippi during some part
of the glacial period shall have been upraised, organic remains will probably
first appear and disappear at different levels, owing to the migrations of
species and to geographical changes. And in the distant future, a geologist,
examining these beds, would be tempted to conclude that the average dura-
tion of life of the embedded fossils had been less than that of the glacial
period, instead of having been really far greater, that is, extending from
before the glacial epoch to the present day.

In order to get a perfect gradation between two forms in the upper anc
lower parts of the same formation, the deposit must have gone on con-'
tinuously 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 must
nearly counterbalance the amount of subsidence. But this same movement
of subsidence will tend to submerge the area whence the sediment is derived,
and thus diminish the supply, while the downward movement continues. In
fact, this nearly exact balancing between the supply of sediment and the
amount of subsidence is probably a rare contingency; for it has been observed
by more than o^e palaeontologist that very thick deposits are usually barren
of organic remains, except near their upper or lower limits.

THE IMPERFECTION OF THE GEOLOGICAL RECORD 231

It would seem that each separate formation, Hke the whole pile of forma-
tion 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
inspection 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 thou-
sands of feet in thickness, and which must have required 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 formation. Many
cases could be given of the lower beds of a formation having been upraised,
denuded, submerged, and then re-covered by the upper beds of the same
formation — facts, showing what wide, yet easily overlooked, intervals have
occurred in its accumulation. In other cases we have the plainest evidence
in great fossilized trees, still standing upright as they grew, of many long in-
tervals 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
eighty-six 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, perhaps many times, during the same geological period. Con-
sequently if it were to undergo a considerable amount of modification during
the .deposition of any one geological 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 before
explained, that A might be the actual progenitor of B and C, and yet would
not necessarily be strictly 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 ob-
tained numerous transitional 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 palaeontologists

232 THE ORIGIN OF SPECIES

have founded their species; and they do this the more readily if the specimens
come from different sub-stages of the same formation. Some experienced
conchologists are now sinking many of the very fine species of D'Orbigny
and others into the rank of varieties; and on this view we do find the kind
of evidence of change which on the theory we ought to find. Look again at
the later tertiary deposits, which include many shells believed by the majority
of naturalists to be identical with existing species; but some excellent
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
opposition to the judgment of most naturalists, that these tertiary species are
all truly distinct from the recent, we have evidence of the frequent occur-
rence of slight modifications of the kind required. If we look to rather wider
intervals of time, namely, to distinct but consecutive stages of the same great
formation, we find that the embedded 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 formations; 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-form until they have been modified and perfected
in some considerable degree. According to this view, the chance of discover-
ing in a formation in any one country all the early stages of transition be-
tween any two forms, is small, for the successive changes are supposed to have
been local or confined to some one spot. Most marine animals have a wide
range; and we have seen that with plants it is those which have the widest
range, that oftenest present varieties; so that, with shells and other marine
animals, it is 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 transi-
tion 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 underwent modification, though long as measured by years, was
probably short in comparison with that during which it 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

THE IMPERFECTION OF THE GEOLOGICAL RECORD 233

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 de-
scended 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 rep-
resentatives, and by other conchologists as only varieties, are really varieties,
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
gradations; 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 between 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 extreme 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 together 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 remarks on the causes of
the imperfection of the geological 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, except-
ing 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 rep-
resents the former state of Europe, while most of our formations were ac-
cumulating. 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 terrestrial 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 embedded ; and those embedded in gravel or sand would not endure
to a distant epoch. Wherever sediment did not accumulate on the bed of the
sea, or where it did not accumulate at a sufficient rate to protect organic
bodies from decay, no remains could be preserved.

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 generally be formed in the archipelago only during periods of
subsidence. These periods of subsidence would be separated from each other

234 THE ORIGIN OF SPECIES

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 sub-
sidence over the whole or part of the archipelago, together with a con-
temporaneous accumulation of sediment, would exceed the average duration
of the same specific forms; and these contingencies are indispensable for the
preservation of all the transitional gradations between any two or more
species. If such gradations were not all fully preserved, transitional 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 climatical changes would intervene
during such lengthy periods; and in these cases the inhabitants of the archi-
pelago would migrate, and no closely consecutive record of their modifica-
tions could be preserved in any one formation.

Very many of the marine inhabitants of the archipelago now range thou-
sands 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 of tenest 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 sup-
plant 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 shght degree, and as they would be found em-
bedded in slightly different sub-stages of the same formation, they would,
according to the principles followed by many palaeontologists, be ranked as.
new and distinct species.

If then there be some degree of truth in these remarks, we have no right
to expect to find, in our geological formations, an infinite number of those
fine transitional forms which, on 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 other; and these links, let
them be ever so close, if found in different stages of the same formation,
would, by many palaeontologists, 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

THE IMPERFECTION OF THE GEOLOGICAL RECORD 235

transitional links between the species which lived at the commencement and
close of each formation, pressed so hardly on my theory.

ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES

The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palaeontologists — for instance,
by Agassiz, Pictet, and 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 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 palaeontological
evidence may be implicitly trusted; negative evidence is worthless, as ex-
perience 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 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 consecu-
tive 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 succeeding
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 transi-
tional 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
review 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 supposed prototype could possibly have been of any
advantage. 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 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

236 THE ORIGIN OF SPECIES

flap along the surface of the sea like the logger-headed duck, and ultimately
to rise from its surface and glide through the air?

I will now give a few examples to illustrate the foregoing 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 Palaeontology, pub-
lished 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. Guvier used to ursje that no
monkey occurred in any tertiary stratum; but now extinct species have been
discovered in India, South America, and ia 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 bird-like ani-
mals, some of gigantic size, existed during that period? Not a fragment of
bone has been discovered in these beds. Not long ago, palaeontologists main-
tained that the whole class of birds came suddenly into existence during the
eocene period ; but now we know, on the authority of Prof essol* 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 Solenhofen.
Hardly any recent discovery shows more forcibly than 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 Girripedes, I stated
that, from the large number of existing and extinct tertiary species; from
the extraordinary abundance of the individuals of many species all over the
world, from the arctic regions to the equator, inhabiting various zones of
depths, from the upper tidal limits to fifty 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 cir-
cumstances, I inferred that, had sessile cirripedes existed during the sec-
ondary periods, they would certainly have been preserved and discovered;
and as not one species had then been discovered in beds of this age, I con-
cluded that this great group had been suddenly developed at the commence-
ment 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 a skilful palaeon-
tologist, M. Bosquet, sent me a drawing of a perfect specimen of an unmis-

THE IMPERFECTION OF THE GEOLOGICAL RECORD 237

takable sessile cirripede, which he had himself extracted from the chalk of
Belgium. And, as if to make the case as striking as possible, this cirripede
was a Chthamalus, a very common, large, and ubiquitous genus, of which
not one species has as yet been found even in any tertiary stratum. Still more
recently, a Pyrgoma, a member of a distinct sub-family of sessile cirripedes,
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 palaeontologists, 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 commonly admitted to be teleostean; and even some
palaeozoic 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 simultaneously 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 Palaeontology it will be
seen that very few species are known from several formations in Europe.
Some few families of fish now have a confined range; the teleostean 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 enclosed basin, in which any great group
of marine animals might be multiplied; and here they would remain
confined, until some of the species became adapted to a cooler climate, and
were enabled to double the southern capes of Africa or Australia and thus
reach other and distant seas.

From these 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 palaeontological 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 throughout 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

238 THE ORIGIN OF SPECIES

the animal kingdom suddenly appear in the lowest known fossiliferous rocks.
Most of the arguments which have convinced me that all the existing species
of the same group are 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 animal. Some of the most ancient
animals, as the Nautilus, 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 occurred less than twenty or more than four
hundred million years ago, but probably not less than ninety-eight or more,
than two hundred million years. These very wide hmits show how doubtful
the data are; and other elements may have hereafter to be introduced inta
the problem. Mr. Croll estimates that about sixty 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 great mutations of life, which have certainly
occurred since the Cambrian formation; and the previous one hundred
and forty million years can hardly be considered as sufficient for the de-
velopment 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 induce 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 answer. 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 Hfe. Other highly
competent judges, as Lyell and E. Forbes, have disputed this conclusion.
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,
abounding 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 South Wales beds rich in trilobites, and containing various

THE IMPERFECTION OF THE GEOLOGICAL RECORD 239

mollusks and annelids. The presence of phosphatic nodules and bituminous
matter, even in some of the lowest azotic rocks, probably indicates life at
these periods; and the existence of the Eozoon in the Laurentian formation
of Canada is generally admitted. There are three great series of strata be-
neath the Silurian system 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 palaeozoic series
to the present time. We are thus carried to a period so remote, that the ap-
pearance of the so-called primordial fauna (of Barrande) 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 organized 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 absence of vast piles of
strata rich in fossils beneath the Cambrian system is very great. It does not
seem probable that the most ancient beds have been quite worn away by
denudation, or that their fossils have been wholly obHterated by metamorphic
action, for if this had been the case we should have found only small rem-
nants of the formations next succeeding them in age, and these would always
have existed in a partially metamorphosed 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 forma-
tion is, the more invariably it has suffered extreme denudation and meta-
morphism.

The case at present must remain inexplicable, and may be truly urged
as a valid argument against the views here entertained. To show that it
may hereafter receive some explanation, I will give the following hypothesis.
From the nature of the organic remains which do not appear to have in-
habited 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
unfathomable 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 palaeozoic or second-

240 THE ORIGIN OF SPECIES

ary formation. Hence, we may perhaps infer, that during the palaeozoic and I
secondary periods, neither continents nor continental islands existed where
our oceans now extend ; for had they existed, palaeozoic and secondary forma-
tions would in all probability have been accumulated from sediment derived
from their wear and tear; and these would have been at least partially up-
heaved by the oscillations of level, which must have intervened during these
enormously long periods. If, then, we may infer anything from these facts,
we may infer that, where our oceans now extend, oceans have extended
from the remotest period of which we have any record; and on the other
hand, that where continents now exist, large tracts of land have existed,
subjected, no doubt, to great oscillations of level, since the Cambrian period.
The colored map appended to my volume on Coral Reefs led me to con-
clude 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 beginning of the world. Our continents seem to have been formed
by a preponderance, during many oscillations of level, of the force of eleva-
tion. But may not the areas of preponderant movement have changed in
the lapse of ages? At a period long antecedent to the Cambrian epoch, con-
tinents 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 recognizable condition, older than the Cambrian strata, sup-
posing such to have been formerly deposited; for it might well happen that
strata which had subsided some miles nearer to the centre of the earth, and
which had been pressed on by an enormous weight of superincumbent water,
might have undergone far more metamorphic action than strata which
have always remained nearer to the surface. The immense areas in some
parts of the world, for instance in South America, of naked metamorphic
rocks, which must have been heated under great pressure, have always seemed
to me to require some special explanation; and we may perhaps believe that
we see in these large areas the many formations long anterior to the Cambrian
epoch in a completely metamorphosed 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 transi-
tional forms closely joining them all together, the sudden 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 fossils beneath
the Cambrian strata, are all undoubtedly of the most serious nature. We see
this in the fact that the most eminent palaeontologists, 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 immutability of species. But Sir Charles Lyell
now gives the support of his high authority to the opposite side, and most

THE IMPERFECTION OF THE GEOLOGICAL RECORD 241

geologists and palaeontologists are much shaken in their former belief. Those
who believe that the geological record is in any degree perfect, will un-
doubtedly at once reject the theory. For my part, following out Lyell's meta-
phor, 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 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 entombed in our consecutive formations, and which falsely
appear to have been abruptly introduced. On this view the difficulties above
discussed are greatly diminished or even disappear.

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 Chap-
ter.

Let 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 im-
mutability 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 evidence on this head in the case of the several tertiary stages; and every
year tends to fill up the blanks between the stages, and to make the propor-
tion between the lost and existing forms more gradual. In some of the most
recent beds, though undoubtedly of high antiquity if measured by years,
only one or two species are extinct, and only one or two are new, having
appeared there for the first time, either locally, or, as far as we know,' on
the face of the earth. The secondary formations are more broken; but, as
Bronn has remarked, neither the appearance nor disappearance of the many
species embedded 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 dis-
appeared 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 Lyell's explanation, namely, that it is a

242

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 243

case of temporary migration from a distinct geographical 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 ab-
ruptly, or simultaneously, or to an equal degree. The process of modifica-
tion 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 accu-
mulated through natural selection in a greater or lesser degree, thus causing
a greater or lesser 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 immigration 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 existing 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 understand the apparently quicker rate of change
in terrestrial and in more highly organized productions compared with
marine and lower productions, by the more complex relations of the
higher beings to their organic and inorganic conditions of life, as explained
in a former chapter. When many of the inhabitants of any area have become
modified and improved, we can understand, on the principle 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 modified, 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 ac-
cumulation 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; consequently the amount of organic change exhibited by the fossils
embedded in consecutive formations is not equal. Each formation, on this
view, does not mark a new and complete act of creation, but only an occa-
sional 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 conditions of life, organic and inorganic,
should recur. For though the ofTspring 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

244 THE ORIGIN OF SPECIES

would almost certainly inherit different characters from their distinct pro-
genitors; and organisms already differing 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 t
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 de- I
gree 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 disappearance 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 exceptions are surprisingly few, so few that E. Forbes, Pictet,
and Woodward (though all strongly opposed to such views as I maintain)
admit its truth; and the rule strictly accords with the theory. For all the spe-
cies of the same group, however long it may have lasted, are the modified de-
scendants 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.

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 ultimately thins out in the upper beds, marking the decrease and final
extinction of the species. This graciual increase in number of the species of
a group is strictly tconformable 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.

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 245

ON EXTINCTION

We have as yet only spoken incidentally of the disappearance of species
and of groups of species. On the theory of natural selection, the extinction
of old forms and the production of new and improved forms are intimately
connected together. The old notion of all the inhabitants of the earth hav-
ing been swept away by catastrophes at successive periods is very gener-
ally given up, even by those geologists, as Elie de Beaumont, Murchison,
Barrande, etc., whose general views would naturally lead them to this con-
clusion. On the contrary, we have every reason to believe, from the study
of the tertiary formations, that species and groups of species gradually dis-
appear, 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 isthmus
and the consequent irruption of a multitude of new inhabitants into an ad-
joining sea, or by the final subsidence 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 palaeozoic period. No fixed law seems to determine
the length of time during which any single species or any single genus en-
dures. There is reason to believe that the extinction of a whole group of
species is generally a slower process than their production: if their ap-
pearance and disappearance be represented, as before, by a vertical line of
varying thickness, the line is found to taper more gradually at its upper end,
which marks the progress of extermination, than at its lower end, which
marks the first appearance and the early increase in number of the species.
In some cases, however, the extermination of whole groups, as of Ammonites,
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 supposed 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 embedded with the remains of Mastodon,
Megatherium, Toxodon, and other extinct monsters, which all co-existed
with still living shells of 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 ap-
parently so favorable. But my astonishment was groundless. Professor Owen
soon perceived that the tooth, though so like that of the existing horse, be-
longed to an extinct species. Had this horse been still living, but in some
degree rare, no naturalist would have felt the least surprised 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

246 THE ORIGIN OF SPECIES

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 in a very few years have stocked the whole
continent. But we could not have told what the unfavorable conditions
were which checked its increase, whether some one or several contingencies,
and at ^vhat 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 perceived the fact, yet the fossil
horse would certainly have become rarer and rarer, and finally extinct — its
place being seized on by some more successful competitor.

It is most difficult ahyays to remember that the increase of every creature-
is constantly being checked by unperceived hostile agencies; and that these-
same unperceived 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 \dctory in the battle of life. Mere size, on the contrary, w^ould 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 continued increase of the exist-
ing elephant. A highly capable judge. Dr. Falconer, believes that it is chiefly
insects, which, from incessantly harassing and weaking the elephant in India,
check its increase; and this was Bruce's conclusion with respect to the African
elephant in Abyssina. It is certain that insects and bloodsucking bats de-
termine the existence of the larger naturalized quadrupeds in several parts
of South America.

We see in many cases in the more recent tertiary formations, that rarity
precedes extinction; and we know that this has been the progress of events
with those animals which have been exterminated, either locally or wholly,
through man's agency. I may repeat what I published in 1845, namely, that
to admit that species generally become rare before they become extinct —
to feel no surprise at the rarity of a species, and yet to marvel greatly when
the species ceases to exist, is much the same as to admit that sickness in the
indi\ddual 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 advantage 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

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 247

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 flourishing 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 ex-
terminated; but we know that species have not gone on indefinitely increas-
ing, at least during the later geological epochs, so that, looking to later times,
we may believe that the production of new forms has caused the extinction
of about the same number of old forms.

The competition will generally be most severe, as formerly explained and
illustrated by examples, between the forms which are most like each other
in all respects. Hence the improved and modified descendants of a species
will generally cause the extermination of the parent-species; and if many
new forms have been developed from any one species, the nearest allies of
that species, i.e., the species of the same genus, will be the most liable to
extermination. Thus, as I beheve, 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 happened 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 extermination. If
many allied forms be developed from the successful 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 suff"erers may
often be preser\'ed 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 Trigonia,
a great genus of shells in the secondar)^ formations, survive in the Australian
seas; and a few members of the great and almost extinct group of Ganoid
fishes still inhabit our fresh waters. Therefore, the utter extinction of a group
is generally, as we have seen, a slower process than its production.

With respect to the apparently sudden extermination of whole famihes
or orders, as of Trilobites at the close of the palaeozoic period, and of Am-
monites at the close of the secondan- period, we must remember what has
been already said on the probable wide inter\'als of time between our con-
secutive 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 alHed, 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 accords well with the theor)' of natural

248 THE ORIGIN OF SPECIES

selection. We need not marvel at extinction; if we must marvel, let it be a
our own presumption in imagining for a moment that we understand th(
many complex contingencies on which the existence of each species depends
If we forget for an instant that each species tends to increase inordinately
and that some check is always in action, yet seldom perceived by us, the
whole economy of nature will be utterly obscured. Whenever we can pre
cisely say why this species is more abundant in individuals than that; wh^
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 foi
the extinction of any particular species or group of species.

ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY
THROUGHOUT THE WORLD

Scarcely any palseontological discovery is more striking than the fact that^
the forms of life change almost simultaneously throughout the world. Thus'
our European Chalk formation can be 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 penin-
sula 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 sec-
tions of genera, and sometimes 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 palaeozoic formations of Russia, Western Europe,
and North America, a similar parallelism in the forms of life has been ob-
served 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 parallelism in the successive forms of life, in the palaeozoic and
tertiary stages, would still be manifest, and the several formations could j
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 manner.
We may doubt whether they have thus changed: if the Megatherium,
Mylodon, Macrauchenia, and Toxodon had been brought to Europe from i
La Plata, without any information in regard to their geological position,
no one would have suspected that they had co-existed with sea-shells all
still living; but as these anomalous monsters co-existed with the Masto-
don and Horse, it might at least have been inferred that they had lived dur-
ing one of the later tertiary stages.

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 249

When the marine forms of life are spoken of as having changed simul-
taneously 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 re-
mote 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 competent observers main-
tain 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 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. Never-
theless, 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 ob-
servers, MM. de Verneuil and d'Archiac. After referring to the parallelism
of the palaeozoic forms of life in various parts of Europe, they add: "If,
struck by this strange sequence, we turn our attention to North America,
and there discover a series of analogous phenomena, it will appear certain
that all these modifications of species, their extinction, and the introduc-
tion of new ones, cannot be owing to mere changes in marine currents or
other causes more or less local and temporary, but depend on general laws
which govern the whole animal kingdom." M. Barrande has made forcible
remarks to precisely the same effect. It is, indeed, quite futile to look to
changes of currents, climate, or other physical conditions, as the cause of
these great mutations in the forms of life throughout the world, under the
most different climates. We must, as Barrande has remarked, look to some
special law. We shall see this more clearly when we treat of the present
distribution of organic beings, and find how slight is the relation between the
physical conditions of various countries and the nature of their inhabitants.

This great fact of the parallel succession of the forms of life throughout
the world, is explicable on the theory of natural selection. New species are
formed by 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 greatest 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, produc-
ing the greatest number of new varieties. It is also natural that the dominant.

250 THE ORIGIN OF SPECIES

varying and far-reaching 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 farther, 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 geographical changes, on strange ac-
cidents, 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 succeed in spreading and would ulti-
mately 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 degree of parallelism in the succession of the productions of the land
than with those of the sea.

Thus, as it seems to me, the parallel, and, taken in a large sense, simultane-
ous, succession of the same forms of life throughout the world, accords well
with the principle of new species having been formed by dominant species
spreading widely and varying; the new species thus produced being them-
selves 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 inter-
vals 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 embed and preserve
organic remains. During these long and blank intervals I suppose that the
inhabitants of each region underwent a considerable amount of modification
and extinction, and that there was much migration from other parts of the
world. As we have reason to believe that large areas are affected by the same
movement, it is probable that strictly contemporaneous formations have
often been accumulated over very wide spaces in the same quarter of the
world; but we are 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 gen-
eral succession in the forms of life; but the species would not exactly cor-
respond; for there will have been 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

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 251

admirable Memoirs on the eocene deposits of England and France, is able
to draw a close general parallelism between the successive stages in the two
countries; but when he compares certain stages in England with those in
France, although he finds in both a curious accordance in the numbers of
the species belonging to the same genera, yet the species themselves differ in
a manner very difficult to account for considering the proximity of the two
areas, unless, indeed, it be assumed that an isthmus separated two seas in-
habited by distinct but contemporaneous faunas. Lyell has made similar
observations on some of the later tertiary formations. Barrande, also, shows
that there is a striking general parallelism in the successive Silurian deposits
of Bohemia and Scandinavia; nevertheless he finds a surprising amount of
difference in the species. If the several formations in these regions have not
been deposited during the same exact periods — a formation in one region
often corresponding with a blank interval in the other — and if in both
regions the species have gone on slowly changing during the accumulation
of the several formations and during the long intervals of time between
them; in this case the several formations in the two regions could be ar-
ranged in the same order, in accordance with the general succession of the
forms of life, and the order would falsely appear to be strictly parallel; nev-
ertheless the species would not be all the same in the apparently correspond-
ing 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 prin-
ciple of descent. The more ancient any form is, the more, as a general rule, it
differs from living forms. But, as Buckland long ago remarked, 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 between 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 expression of generalized forms, as ap-
plied to extinct animals; and in the writings of Agassiz, of prophetic or syn-
thetic types ; and these terms imply that such forms are, in fact, intermediate
or connecting links. Another distinguished palaeontologist, M. Gaudry, has
shown in the most striking manner that many of the fossil mammals discov-
ered by him in Attica serve to break down the intervals between existing
genera. Cuvier ranked the Ruminants and Pachyderms as two of the most
distinct orders of mammals; but so many fossil links have been disentombed
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

252 THE ORIGIN OF SPECIES

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
certain extent these two grand divisions. No one will deny that the Hipparion
is intermediate between the existing horse and certain other ungulate forms.
What a wonderful connecting link in the chain of mammals is the Typo-
therium from South America, as the name given to it by Professor Gervais
expresses, and which cannot be placed in any existing order. The Sirenia
form a very distinct group of the mammals, and one of the most remarkable
peculiarities in existing dugong and lamentin is the entire absence of hind
limbs, without even a rudiment being left; but the extinct Halitherium had,
according to Professor Flower, an ossified thigh-bone "articulated to a well-
defined acetabulum in the pelvis," 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
naturalists in an order by themselves, are considered by Professor Huxley to
be undoubtedly cetaceans, "and to constitute connecting links with the
aquatic carnivora."

Even the wide interval between birds and reptiles has been shown by the
naturalists 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 Dinosaurians — that group
which includes the most gigantic of all terrestrial reptiles. Turning to the
Invertebrata, Barrande asserts (a higher authority could not be named) that
he is every day taught that, although palaeozoic animals can certainly be
classed under existing groups, yet 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 inter-
mediate in all its characters between two living forms or groups, the objec-
tion 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 belonging to distinct families. The most com-
mon 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 characters, the ancient members are separated by a some-
what lesser number of characters, so that the two groups formerly made a
somewhat nearer approach to each other than they now do.

It is a common belief, that the more ancient a form is, by so much the
more it tends to connect by some of its characters groups now widely sepa-
rated 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 affini-

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 253

ties directed toward very distinct groups. Yet if we compare the older reptiles
and Batrachians, the older fish, the older cephalopods, and the eocene mam-
mals, with the 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 considered
as extinct. The three existing genera a^*, q^^, p^^, will form a small family;
6^*, and /^*, a closely allied family or sub-family; and o^*, e^^, w}^, 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
progenitor. On the principle of the continued tendency to divergence of char-
acter, which was formerly illustrated by this diagram, the more recent any
form is, the more it will generally differ from its ancient progenitor. Hence,
we can understand the rule that the most ancient fossils differ most from
existing forms. We must not, however, assume that divergence of character
is a necessary contingency; it depends solely on the descendants from a species
being thus enabled to seize on many and different places in the economy of
nature. Therefore it is quite possible, as we have seen in the case of some
Silurian forms, that a species might go on being slightly modified in relation
to its slightly altered conditions of life, and yet retain throughout a vast
period the same general characteristics. This is represented in the diagram
by the letter f *.

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

By looking at the diagram we can see that if many of the extinct forms
supposed to be embedded in the successive formations, were discovered at
several points low down in the series, the three existing families on the upper-
most fine would be rendered less distinct from each other. If, for instance,
the genera a}-, a°, a^^, f, 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-
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 dif-
ferent forms. If many extinct forms were to be discovered above one of the

254 THE ORIGIN OF SPECIES

middle horizontal lines or geological formations — for instance, above No.
VI. — ^but none from beneath this line, then only two of the families (those
on the left hand, «^*, etc., and b^^, 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 {a^^ to m^*), on the uppermost line, be sup-
posed to differ from each other by half-a-dozen important characters, then
the families which existed at a period marked VI. would certainly have dif-
fered from each other by a less number of characters ; for they would at this
early stage of descent have diverged in a less degree from their common
progenitor. Thus it comes that ancient and extinct genera are often in a
greater or lesser degree intermediate in character between their modified
descendants, or between their collateral relations.

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

Thus, on the theory of descent with modification, the main facts with re-
spect to the mutual aflfinities 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 off-
spring of those which lived at the fifth stage, and are the parents of those
which became still more modified at the seventh stage; hence they could
hardly fail to be nearly intermediate in character between the forms of life
above and below. We must, however, allow for the entire extinction of some
preceding forms, and in any one region for the immigration of new forms
from other regions, and for a large amount of modification during the long
and blank intervals between the successive formations. Subject to these allow-
ances, the fauna of each geological period undoubtedly is intermediate in
character, between the preceding and succeeding faunas. I need give only
one instance, namely, the manner in which the fossils of the Devonian sys-
tem, when this system was first discovered, were at once recognized by palaeon-
tologists as intermediate in character between those of the overlying car-
boniferous and underlying Silurian systems. But each fauna is not neces-

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 255

sarily exactly intermediatej as unequal intervals of time have elapsed be-
tween consecutive formations.

It is no real objection to the truth of the statement that the fauna of each
period as a whole is nearly intermediate in character between the preceding
and succeeding faunas, that certain genera offer exceptions to the rule. For
instance, the species of mastodons and elephants, when arranged by Dr. Fal-
coner in two series — in the first place according to their mutual affinities, and
in the second place according to their periods of existence — do not accord
in arrangement. The species extreme in character are not the oldest or the
most recent; nor are those which are intermediate in character, intermediate
in age. But supposing for an instant, in this and other such cases, that the
record of the first appearance and disappearance of the species was complete,
which is far from the case, we have no reason to believe that forms suc-
cessively produced necessarily endure for corresponding lengths of time. A
very ancient form may occasionally have lasted much longer than a form
elsewhere subsequently produced, especially in the case of terrestrial pro-
ductions 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 disappear-
ance; for the parent rock-pigeon still lives; and many varieties between the
rock-pigeon and the carrier have become extinct; and carriers which are
extreme in the important character of length of beak originated earlier than
short-beaked tumblers, which are at the opposite end of the series in this
respect.

Closely connected with the statement, that the organic remains from an
intermediate formation are in some degree intermediate in character, is the
fact, insisted on by all palaeontologists, that fossils from two consecutive 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 re-
semblance of the organic remains from the several stages of the Chalk for-
mation, 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 im-
mutability 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
simultaneously throughout the world, and therefore under the most different
climates and conditions. Consider the prodigious vicissitudes of climate dur-
ing 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 dis-
tinct species, is obvious. As the accumulation of each formation has often
been interrupted, and as long blank intervals have intervened between sue-

256 THE ORIGIN OF SPECIES

cessive 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 commencement and
close of these periods : but we ought to find after intervals, very long as meas-
ured 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.

t

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 highness. 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 unimproved structures fitted for sim-
ple conditions of life, and in some cases will even degrade or simplify the
organization, 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 predecessors ; for they have to beat in the struggle for life all
the older forms, with which they come into close competition. We may there-
fore conclude that if under a nearly similar climate the eocene inhabitants
of the world could be put into competition with the existing inhabitants, the
former would be beaten and exterminated by the latter, as would the second-
ary by the eocene, and the palaeozoic 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 palaeontologists would answer in the affirmative; 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 appeared. It is not an insuperable
difficulty that Foraminifera 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 simple conditions of life, and what could be
better fitted for this end than these lowly organized Protozoa? Such objec-
tions as the above would be fatal to my view, if it included advance in organ-
ization as a necessary contingent. They would likewise be fatal, if the above
Foraminifera, 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 257

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 selection, for their further continued process; though they
will, during each successive age, have to be slightly modified, 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
unanimous 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 teleos-
teans; the latter at the present day are largely preponderant in number; but
formerly selaceans and gamoids 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 highness seems hopeless; who will decide whether a
cuttle-fish be higher than a bee — that insect which the great Von Baer be-
Heved to be "in fact more highly organized than a fish, although upon an-
other type"? In the complex struggle for life it is quite credible that crusta-
ceans, 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 difficulties 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 un-
doubtedly 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 two
periods. At an ancient epoch the highest and lowest molluscoidal animals,
namely, cephalopods and brachiopods, swarmed in numbers; at the present
time both groups are greatly reduced, while others, intermediate in organ-
ization, 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 brachiopods, and the fact that our existing cephalopods, though few in
number, are more highly organized than their ancient representatives. We
ought also to compare the relative 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 animals exist, and if we knew
that at some former period only ten thousand kinds existed, we ought to look
at this increase in number in the highest class, which implies a great dis-

258 THE ORIGIN OF SPECIES

placement 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-
ness, under such extremely complex relations, the standard of organization
of the imperfectly known faunas of successive periods.

We shall appreciate this difficulty more clearly by looking to certain ex-
isting 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 be-
come thoroughly naturalized there, and would exterminate many of the
natives. On the other hand, from the fact that hardly a single inhabitant of
the southern hemisphere has become wild in any part of Europe, we may
well doubt whether, if all the productions of New Zealand were set free in
Great Britain, any considerable number would be enabled to seize on places
now occupied by our native plants and animals. Under this point of view,
the productions of Great Britain stand much higher in the scale than those of
New Zealand. Yet the most skilful naturalists, 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 ani-
mals 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 development of existing forms. This view
accords admirably well with our theory. In a future chapter I shall attempt
to show that the adult differs from its embryo, 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 unaltered, continually
adds, in the course of successive generations, more and more difference to
the adult. Thus the embryo comes to be left as a sort of picture, preserved by
nature, of the former and less modified 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 dis-
tinct 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 dis-
covered 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 mammals from the
Australian caves were closely allied to the living marsupials of that continent.
In South America a similar relationship is manifest, even to an uneducated
eye, in the gigantic pieces of armor, like those of the armadillo, found in sev-

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 259

eral parts of La Plata; and Professor Owen has shown in the most striking
manner that most of the fossil mammals, buried there in such numbers, are
related to South American types. This relationship is even more clearly seen
in the wonderful collection of fossil bones made by MM. Lund and Clausen
in the caves of Brazil. I was so much impressed with these facts that I strongly
insisted, in 1839 and 1845, on this "law of the succession of types/' — on
"this wonderful relationship in the same continent between the dead and the
living." Professor Owen has subsequently extended the same generalization
to the mammals of the Old World. We see the same law in this author's
restorations of the extinct and gigantic birds of New Zealand. We see it also
in the birds of the caves of Brazil. Mr. Woodward has shown that the same
law holds good with sea-shells, but, from the wide distribution of most mol-
lusks, 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 compar-
ing the present climate of Australia and of parts of South America, under
the same latitude, would attempt to account, on the one hand through dis-
similar physical conditions, for the dissimilarity of the inhabitants of these
two continents; 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 numer-
ous marsupials ; and I have shown, in the publications above alluded to, that
in America the law of distribution of terrestrial mammals was formerly dif-
ferent from what it now is. North America formerly partook strongly of the
present character of the southern half of the continent; and the southern
half was formerly more closely allied, than it is at present, to the northern
half. In a similar manner we know, from Falconer and Cautley's discoveries,
that Northern India was formerly more closely related in its mammals to
Africa than it is at the present time. Analogous facts could be given in rela-
tion to the distribution of marine animals.

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

260 THE ORIGIN OF SPECIES

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 ant-eater, as their degenerate
descendants. This cannot for an instant be admitted. These huge animals
have become wholly extinct, and have left no progeny. But in the caves of
Brazil there are many extinct species which are closely allied in size and in 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 for-
mation there be six other allied or representative genera, each with the same
number of species, then we may conclude 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 com-
moner 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 imper-
fect; that only a small portion of the globe has been geologically explored
with care; that only certain classes of organic beings have been largely pre-
served in a fossil state; that the number both of specimens and of species,
preserved in our museums, is absolutely as nothing compared with the num-
ber of generations which must have passed away even during a single forma-
tion; 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 record 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 J
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 often-
est given rise to new species ; that varieties have at first been local ; and lastly,
although each species must have passed through numerous transitional stages, i
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.]

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 261

These causes, taken conjointly, will to a large extent explain why — though
we do find many links — we do not find interminable varieties, connecting to-
gether 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 per-
fectly 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.

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 num-
berless transitional links which must formerly have connected the closely
allied or representative species, found in the successive 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 ques-
tion 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 metamorphosed condition, or lie still buried under
the ocean.

Passing from these difficulties, the other great leading facts in palaeontology
agree admirably with the theory of descent with modification through varia-
tion 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 neces-
sarily change together, or at the same rate, or in the same degree; yet in
the long run that all undergo modification to some extent. The extinction
of old forms is the almost inevitable consequence of the production of new
forms. We can understand why, when a species has once disappeared, it
never reappears. Groups of species increase in numbers slowly, and endure
for unequal periods of time; for the process of modification is necessarily
slow, and depends on many complex contingencies. The dominant species
belonging to large and dominant groups tend to leave many modified de-
scendants, 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 modi-
fied offspring on the face of the earth. But the utter extinction of a whole
group of species has sometimes 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.

262 THE ORIGIN OF SPECIES

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 modified, descendants; and these will generally succeed in displac-
ing 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
tendency to divergence of character, why, the more ancient a form is, the
more it generally differs from those now living; why ancient and extinct
forms often tend to fill up gaps between existing forms, sometimes blending
two groups, previously classed as distinct, into one; but more commonly
bringing them only a little closer together. The more ancient a form is, the
more ofteh it stands in some degree intermediate between groups now dis-
tinct; 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 organicj
remains of closely consecutive formations are closely allied; for they arq
closely linked together by generation. We can clearly see why the remains ol
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 speciaUzed; and this
may account for the common belief held by so many palaeontologists, that
organization on the whole has progressed. Extinct and ancient animals re-
semble 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 structures 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 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 palaeon-
tology plainly proclaim, as it seems to me, that species have been produced by
ordinary generation: old forms having been supplanted by new and im-
proved forms of life, the products of Variation and the Survival of the Fittest.

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.

In considering the distribution of organic beings over the face of the
globe, the first great fact which strikes us is, that neither the similarity nor
the dissimilarity of the inhabitants of various regions can be wholly ac-
counted 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 divisions in geographical distribution is that between the
New and the Old Worlds; yet if we travel over the vast American con-
tinent, 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 con-
dition in the Old World which cannot be paralleled in the New — at least
so closely as 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. Notwithstand-
ing this general parallelism in the conditions of Old and New Worlds, how
widely different are their living productions!

In the southern hemisphere, if we compare large tracts of land in Aus-
tralia, South Africa, and western South America, between latitudes 25 and
35 degrees, 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 dis-
similar. Or, again, we may compare the productions of South America south
of latitude 35 degrees with those north of 25 degrees, which consequently are
separated by a space of ten degrees of latitude, and are exposed to con-
siderably different conditions; yet they are incomparably more closely re-
lated 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 im-
portant manner to the differences between the productions of various re-
gions. We see this in the great difference in nearly all the terrestrial pro-
ductions of the New and Old Worlds, excepting in the northern parts,

263

264 THE ORIGIN OF SPECIES

%vhere the land almost joins, and where, under a sHghtly different climate,
there might have been free migration for the northern temperate forms, as
there now is for the strictly arctic productions. \Ve see the same fact in the
great difference ber^veen the inhabitants of AustraUa, .\frica. and South
America, under the same latiuide: 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 loft)^ and continuous mountain-
ranges, of great deserts and even of large rivers, we find different produc-
tions: though as mountain-chains, deserts, etc., are not as impassible, or
likely to have endured so long, as the oceans separating continents, the
dififerences are ver\' inferior in degree to those characteristic of distinct
continents.

Turning to the sea, we find the same la\v. The marine inhabitants of the
eastern and \s-estem shores of South America are ver)' distinct, with ex-
tremely few shells, Crustacea, or echinodermata in common: but Dr.
Giinther has recentiy shown that about thirt\- per cent of the fishes are the
same on the opposite sides of the isthmus of Panama: and this fact has led
naturalists to beUeve that the isthmus was formerly open. \Vest\s'ard of the
shores of America, a wide space of open ocean extends, with not an island
as a halting-place for emigrants: here we have a barrier of another kind,
and as soon as this is passed we meet in the eastern islands of the Pacific
with another and totally distinct faima. So that three marine faimas range
northward and southward in parallel lines not far from each other, imder
corresponding climate: but from being separated from each other by im-
passable barriers, either of land or open sea, they are almost wholly distinct.
On the other hand, proceeding still farther west^vard from the eastern
islands of the tropical parts of the Pacific, we encounter no impassable
barriers, and we have innimierable islands as halting-places, or continuous
coasts, until, after travelling over a hemisphere, ^ve come to the shores of
Africa; and over this vast space we meet \dth no well-defined and distinct
marine faimas. Although so fe^v marine animals are coromon 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 eastern islands of the
Pacific and the eastern shores of Africa on almost exactly opposite meridians
of lon^tude.

A third great fact, partly included in the foregoing statement, is the
afl&nit\- of the productions of the same continent or of the same sea, though
the species themselves are distinct at different points and stations. It is a
law of the widest generality', and ever\' continent offers innimierable in-
stances. 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, replace each other.
He hears from closely aUied yet distinct 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

GEOGR.\PHIC.\L DISTRIBUTION 265

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 Australia under the
same latitude. On these same plains of La Plata we see the agouti and
bizcacha, animals having nearly the same habits as our hares and rabbits,
and belonging to the same order of rodents, but they plainly display an
American t\-pe of structure. \Ve ascend the loft)' peaks of the Cordillera,
and we find an alpine species of bizcacha; we look to the waters, and we
do not find the beaver or muskrat, but the co\-pu and cap\i>ara, rodents of
the South American t\pe. Innumerable other instance could be given. If
we look to the islands off the American shore, however much they may
differ in geological structure, the inhabitants are essentially American,
though they may be all peculiar species. ^Ve may look back to past ages, as
shown in the last chapter, and we find American t\-pes then prevailing oh
the American continent and in the American seas. ^Ve 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 wc
positively know^, produces organisms quite like each other, or, as we see in
the case of varieties, nearly alike. The dissimilarity^ of the inhabitants of
different regions may be attributed to modification through \'ariation and
natural selection, and probably in a subordinate degree to the definite in-
fluence of different physical conditions. The degrees of dissimilarity will
depend 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 preser\^ation of different modifications: the relation of organism to
organism in the struggle for life beiug, as I have already often remarked,
the most important of aU relations. Thus the high importance of barriers
comes into play by checking migration; as does time for the slow process ctf
modification through natural selection. ^Videly ranging species, ^x)unding
in indi\iduals, which have already triumphed over many competitors in
their own \sidelv extended homes, wdll have the best chance of seizing on
new places, when they spread out into new^ countries. In their new homes
they will be exposed to new conditions, and will frequentiy undergo further
modification and improvement; and thus they will become stiU further
victorious, and \Nill produce groups of modified descendants. On tbi^ prin-
ciple of inheritance with modification we can understand how it is that
sections of genera, whole genera, and e\^n famiHes, are confined to the
same areas, as is so commonly and notoriously the case.

There is no e\-idence, as was remarked in the last chapter, of die existence
of any law of necessary- development. As the variability- of each species is
an independent property', and will be taken advantage of by natural selec-
tion, only so far as it profits each individual in its complex stru^le for life.

266 THE ORIGIN OF SPECIES

so the amount of modification in different species will be no uniform
quantity. If a number of species, after having long competed with eack
other in their old home, were to migrate in a body into a new and after-J
ward isolated country, they would be little liable to modification; forr
neither migration nor isolation in themselves effect any thing. These:
principles come into play only by bringing organisms into new relations:
with each other and in a lesser degree with the surrounding physical condi-
tions. As we have seen in the last chapter that some forms have retained
nearly the same character from an enormously remote geological period^
so certain species have migrated over vast spaces, and have not become?
greatly or at all modified.

According to these views, it is obvious that the several species of the
^jne 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 under-
gone, during whole geological periods, little modification, there is not much-:
difficulty in believing that they have migrated from the same region; for
during the vast geographical and climatical 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 mi-
grated from some one point to the several distant and isolated points where
now found. Nevertheless the simplicity of the view that each species was
first produced within a single region captivates the mind. He who rejects
i% rejects the vera causa of ordinary generation with subsequent migration,
and calls in the agency of a miracle. It is universally admitted, that in most
cases the area inhabited by a species is continuous; and 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 remarkable 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, ac-
cordingly, we find no inexplicable instances of the same mammals inhabit-
ing distant points of the world. No geologist feels any difficulty in Great

GEOGRAPHICAL DISTRIBUTION 267

Britain possessing 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 Australia; and some of the aboriginal plants
are identically the same at these distant points of the northern and southern
hemispheres. The answer, as I believe, is, that mammals have not been able
to migrate, whereas some plants, from their varied means of dispersal, have
migrated across the 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 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, 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 opposite 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 sub-
sequently migrated from that area as far as its powers of migration and
subsistence under past and present conditions permitted, is the most prob-
able. Undoubtedly many cases occur in which we cannot explain how the
same species could have passed from one point to the other. But the geo-
graphical and climatical 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
general considerations, that each species has been produced within one
area, and has migrated thence as far as it could. It would be hopelessly
tedious to discuss all the exceptional cases of the same species, now living
at distant and separated points, nor do I for a moment pretend that any
explanation could be offered of many instances. But, after some preliminary
remarks, I will discuss a few of the most striking classes of facts, namely,
the existence of the same species on the summits of distant mountain ranges,
and at distant points in the Arctic and Antarctic regions; and secondly (in
the following chapter), the wide distribution of fresh-water productions;
and thirdly, the occurrence of the same terrestrial species on islands and
on the 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

268 THE ORIGIN OF SPECIES

species having migrated from a single birthplace, then, considering ouri
ignorance with respect to former climatical and geographical changes, and;
to the various occasional means of transport, the belief that a single birth4
place is the law seems to me incomparably the safest.

In discussing this subject we shall be enabled at the same time to con-
sider 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 migra-
tion 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 region 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, upheaved and formed at
the distance of a few hundreds of miles from a continent, would probably
receive from it in the course of time a few colonists, and their descendants,
though modified, would still be 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 creation. 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 "every
species has come into existence coincident both in space and time with a
pre-existing closely allied species." And it is now well known that he at-
tributes this coincidence to descent with modification.

The question of single or multiple centres of creation differs from another
though allied question; namely, whether all individuals of the same species
are descended from a single pair, or single hermaphrodite, or whether, as
some authors suppose, from many individuals simultaneously created. With
organic beings which never intercross, if such exist, each species must be de-
scended 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 in-
dividuals of the same form will be descended from a single parent. But in
the great majority of cases, namely, with all organisms which habitually
mnite for each birth, or which occasionally intercross, the individuals of the
same species inhabiting the same area will be kept nearly uniform by inter-
crossing; so that many individuals will go on simultaneously changing, and
the whole amount of modification at each stage will not be due to descent
from a single parent. To illustrate what I mean: our English race-horses
differ from the horses of every other breed ; but they do not owe their differ-
ence 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.

GEOGRAPHICAL DISTRIBUTION 269

MEANS OF DISPERSAL

Sir C. LyeU and other authors have ably treated this subject. I can give
here only the briefest abstract of the more important facts. Change of cHmate
must have had a powerful influence on migration. A region now impassable
to certain organisms from the nature of its climate, might have been a high
road for migration, when the climate was different. I shall, however, pres-
ently have to discuss this branch of the subject in some detail. Changes of
level in the land must also have been highly influential: a narrow isthmus
now separates two marine faunas; submerge it, or let it formerly have been
submerged, and the two faunas will now blend 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 al-
lowed terrestrial productions to pass from one to the other. No geologist
disputes that great mutations of level have occurred within the period of
existing organisms. Edward Forbes insisted that all the islands in the Atlantic
must have been recently connected with Europe or Africa, and Europe like-
wise with America. Other authors 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
geographical 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 migra-
tion. 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 birth-
place, 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 continuously united with each
other, and with the many existing oceanic islands. Several facts in distribu-
tion— such as the great difference in the marine faunas on the opposite sides
of almost every continent — the close relation of the tertiary inhabitants of
several lands and even seas to their present inhabitants — the degree of
affinity between the mammals inhabiting islands with those of the nearest

270 THE ORIGIN OF SPECIES

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 behef of their former continuity
of 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
consisting of mere piles of volcanic matter.

I must now say a few words on what are called accidental 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 injurious action of sea- water.
To my surprise I found that out of eighty-seven kinds, sixty-four germinated
after an immersion of twenty-eight days, and a few survived an immersion
of 137 days. It deserves notice that certain orders were far more injured than
others; nine Leguminosae were tried, and, with one exception, they resisted
the salt water badly; seven species of the allied orders, Hydrophyllaceae
and Polemoniaceae, were all killed by a month's immersion. For convenience'
sake I chiefly tried small seeds without the capsules or fruit; and as all of
these sunk in a few days, they could not have been floated across wide spaces
of the sea, whether or not they were injured by 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 diff'erence 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 ninety-four plants with
ripe fruit, and to place them on sea- water. The majority sunk quickly, but
some which, while green, floated, for a very short time, when dried, floated
much longer; for instance, ripe hazel-nuts sunk immediately, but when dried
they floated for ninety days, and afterward when planted germinated; an
asparagus plant with ripe berries floated for twenty-three days, when dried
it floated for eighty-five days, and the seeds afterward germinated; the ripe
seeds of Helosciadium sunk in two days, when dried they floated for above
ninety days, and afterward germinated. Altogether, out of the ninety-four
dried plants, eighteen floated for above twenty-eight days; and some of the
eighteen floated for a very much longer period. So that as ^%7 kinds of
seeds germinated after an immersion of twenty-eight days; and as ■^%4:
distinct species with ripe fruit (but not all the same species as in the fore-
going experiment) floated, after being dried, for above twenty-eight days,

GEOGRAPHICAL DISTRIBUTION 271

we may conclude, as far as anything can be inferred from these scanty
facts, that the seeds of ^%oo kinds of plants of any country might be floated
by sea-currents during twenty-eight days, and would retain their power of
germination. In Johnston's Physical Atlas, the average rate of the several
Atlantic currents is thirty- three miles per diem (some currents running at
the rate of sixty miles per diem) ; on this average, the seeds of ^%oo
plants belonging 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 ninety-eight 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 ^%s oi his seeds of different kinds floated for forty-two
days, and were then capable of germination. But I do not doubt that plants
exposed to the waves would float for a less time than those protected from
violent movement as in our experiments. Therefore, it would perhaps be
safer to assume that the seeds of about ^%oo plants of a flora, after having
been dried, could be floated across a space of sea 900 miles in width, and
would then germinate. The facts 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 embedded
in the roots of trees, small parcels of earth are frequently enclosed 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 fifty years old, three dicot-
yledonous plants germinated: I am certain of the accuracy of this observa-
tion. 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: pease 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 thirty days, to my surprise nearly all germinated.

Living birds can hardly fail to be highly effective agents in the transporta-
tion of seeds. I could give many facts showing how frequently birds of many

272 THE ORIGIN OF SPECIES

kinds are blown by gales to vast distances across the ocean. We may safely
assume that under such circumstances their rate of flight would often be
thirty-five miles an hour; and some authors have given a far higher estimate, il
I have never seen an instance of nutritious seeds passing through the in|'
testines 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 ii
my garden twelve kinds of seeds, out of the excrement of small birds, an(
these seemed perfect, and some of them, which were tried, germinated. Bui
the following fact is more important : the crops of birds do not secrete gastric i:
juice, and do not, as I know by trial, injure in the least the germination off
seeds; now, after a bird has found and devoured a large supply of food, it it
is positively asserted that all the grains do not pass into the gizzard for r
twelve or even eighteen hours. A bird in this interval might easily be blown)
to the distance of five hundred miles, and hawks are known to look out for''
tired birds, and the contents of their torn crops might thus readily get scat-
tered. 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
experiment 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 devoured by
birds, and thus the seeds might be transported from place to place. I forced
many kinds of seeds into the stomachs of dead fish, and then gave their
bodies to fishing-eagles, storks and pelicans; these birds, after an interval of
many hours, either rejected the seeds in pellets or passed them in their excre-
ment; 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 im-
mense ellipse, at least five or six miles in diameter, and at night alighted on
the taller trees, which were completely coated with them. They then dis-
appeared 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 introduced into their
grass-land in the dung left by the great flights of locusts which often visit
that country. In consequence of this belief Mr. Weale sent me in a letter a
small packet of the dried pellets, out of which I extracted 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

GEOGRAPHICAL DISTRIBUTION 273

visited Madeira, might readily be the means of introducing several kinds
of plants into an island lying far from the mainland.

Although the beaks and feet of birds are generally clean, earth some-
times 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
partridge, 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), wheatears, and whin-
chats (Saxicolae), on their first arrival on our 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, Professor 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 eighty- two plants sprung from it:
these consisted of twelve monocotyledons, including the common oat, and
at least one kind of grass, and of seventy 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 occasion-
ally transport a few seeds embedded 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 Atlan-
tic, which, stand nearer to the mainland and (as remarked by Mr. H. G.
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 G. 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 icebergs 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,

274 THE ORIGIN OF SPECIES

which without doubt remain to be discovered, 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 transported. These means
of transport are sometimes called accidental; but this is not strictly correct:
the currents of the sea are not accidental, nor is the direction of prevalent r
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 thei/"
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 con-
tinent to a neighboring island, but not from one distant continent to an-
other. The floras of distant continents would not by such means become
mingled; but would remain as distinct as they now are. The currents, from
their course, would never bring seeds from North America to Britain,
though they might and do bring seeds from the West Indies to our western
shores, where, if not killed by 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, immigrants from Europe or any other continent, that a
poorly stocked island, though standing more remote from the mainland,
would not receive colonists by similar means. 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 home,
as to become naturalized. But this is no valid argument against what would
be efTected 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 destructive 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, sepa-
rated from each other by hundreds of miles of lowlands, where alpine
species could not possibly exist, is one of the most striking cases known of
the same species living at distant points, without the apparent possibility
of their having migrated from one point to the other. It is indeed a re-
markable fact to see so many plants of the same species living on the snowy

GEOGRAPHICAL DISTRIBUTION 275

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 Moun-
tains, 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 1 747, 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 this same
belief, had not Agassiz and others called vivid attention to the Glacial
period, which, as we shall immediately see, affords a simple explanation of
these facts. We have evidence of almost every conceivable kind, organic
and inorganic, that, within a very recent geological period. Central Europe
and North America suffered under an arctic climate. The ruins of a house
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. So
greatly has the climate of Europe changed, that in Northern Italy, gigantic
moraines, left by old glaciers, are now clothed by the vine and maize.
Throughout a large part of the United States, erratic 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
inhabitants of the north, these would take the places of the former in-
habitants of the temperate regions. The latter, at the same time, would
travel farther and farther southward, unless they were stopped by barriers,
in which case they would perish. The mountains would become covered
with snow and ice, and their former alpine inhabitants would descend to
the plains. By the time that the cold had reached its maximum, we should
have an arctic fauna and flora, covering the central parts of Europe, as far
south as the Alps and Pyrenees, and even stretching into Spain. The now
temperate regions of the United States would likewise be covered by arctic
plants and animals, and these would be nearly the same with those of
Europe; for the present circumpolar inhabitants, which we suppose to have
everywhere travelled southward, are remarkably uniform round the world.

As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more temperate
regions. And as the snow melted from the bases of the mountains, the arctic
forms would seize on the cleared and thawed ground, always ascending, as
the warmth increased and the snow still further disappeared, higher and
higher, while their brethren were pursuing their northern journey. 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.

276 THE ORIGIN OF SPECIES

Thus we can understand the identity of many plants at points so im-
mensely 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 moun-
tain range are more especially related to the arctic forms living due north I
or nearly due north of them: for the first migration when the cold camei
on, and the re-migration 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
Ramond, are more especially allied to the plants of Northern Scandinavia;
those of the United States, to Labrador; those of the mountains of Siberia,
to the arctic regions of that country. These views, grounded as they are on
the perfectly well-ascertained occurrence of a former Glacial period, seem
to me to explain in so satisfactory a manner the present distribution of the
alpine and arctic productions of Europe and America, that when in other
regions we find the same species on distant mountain summits, we may
almost conclude, without other evidence, that a colder climate formerly
permitted their migration across the intervening lowlands, now become too
warm for their existence.

As the arctic forms moved first southward and afterward backward to the
north, in unison with the changing climate, they will not have been ex-
posed 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 in-
culcated in this volume, these forms will not have been liable to much
modification. But with the alpine productions, left isolated from the mo-
ment of the returning warmth, first at the bases and ultimately on the sum-
mits 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 probability, have become mingled with ancient Alpine
species, which must have existed on the mountains before the commence-
ment of the Glacial epoch, and which during the coldest period will have
been temporarily driven down to the plains; they will, also, have been sub-
sequently exposed to somewhat different climatical influences. Their mutual
relations will thus have been in some degree disturbed; consequently they
will have been liable to modification; 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
of sub-species, and some as distinct yet closely allied species representing
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 regions as they are at the present day. But it is also neces-
sary to assume that many sub-arctic and some few temperate forms were
the same round the world, for some of the species which now exist on the

GEOGRAPHICAL DISTRIBUTION 277

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 commence-
ment 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 Atlantic Ocean and by the northern part of
the Pacific. During the Glacial period, when the inhabitants of the Old
and New Worlds lived farther 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 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 in-
habitants 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 degrees, lived during the Pliocene period farther north, under the Polar
Circle, in latitude 66-67 degrees; 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 Eastern America. And
this continuity of the circumpolar land, with the consequent freedom under
a more favorable climate for intermigration, will account for the supposed
uniformity of the sub-arctic and temperate productions of the Old and
New Worlds, at a period anterior to the Glacial epoch.

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

During the slowly decreasing warmth of the Pliocene period, as soon as

278 THE ORIGIN OF SPECIES

the species in common, which inhabited 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 con-
cerned, must have taken place long ages ago. As the plants and animals
migrated 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.
Consequently 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 productions of the temperate regions of the
New and Old Worlds, we find very few identical species (though Asa Gray
has lately shown that more plants are identical than was formerly sup-
posed), but we find in every great class many forms, which some naturalists
rank as geographical races, and others as distinct species; and a host of
closely allied or representative forms which are ranked by all naturalists
as specifically distinct.

As on the land, so in the waters of the sea, a slow southern migration of
a marine fauna, which, during the Pliocene or even a somewhat earlier
period, was nearly uniform along the continuous shores of the Polar Circle,
will account, on the theory of modification, for many closely allied forms
now living in marine areas completely 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 temperate North Amer-
ica; 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 Japan — 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 in-
habiting the seas on the eastern and western 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 physical conditions of the areas; for if we compare, for in-
stance, certain parts of South America with parts of South Africa or Aus-
tralia, we see countries closely 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' view may be largely extended. In Europe we meet with the plainest
evidence of the Glacial period, from the western shores of Britain to the
Ural range, and southward to the Pyrenees. We may infer from the frozen

GEOGRAPHICAL DISTRIBUTION 279

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.
Southward 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 mountains in
this island tell the same story of a former cold period. From facts com-
municated to me by the Rev. W. B. Clarke, it appears also that there are
traces of former glacial action on the mountains of the south-eastern
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 thirty-six and thirty-seven degrees, and on the shores of the Pacific,
where the climate is now so different, as far south as latitude forty-six
degrees. Erratic bowlders have, also, been noticed on the Rocky Mountains.
In the Cordillera of South America, nearly under the equator, glaciers
once extended far below their present level. In Central Chili 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 moraine; and Mr.
D, Forbes informs me that he found in various parts of the Cordillera, from
latitude thirteen to thirty degrees south, 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 Cordillera true glaciers do not now exist even
at much more considerable heights. Further south, on both sides of the
continent, from latitude forty-one degrees to the southern-most extremity,
we have the clearest evidence of former glacial action, in numerous im-
mense bowlders transported far from their parent source.

From these several facts, namely, from the glacial action having extended
all round the northern and southern hemispheres — from the period having
been in a geological 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 de-
scended 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 tempera-
ture of the whole world had been simultaneously lowered during the Glacial
period. But now, Mr. Croll, in a series of admirable memoirs, has at-
tempted to show that a glacial condition of climate is the result of various
physical causes, brought into operation 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

280 THE ORIGIN OF SPECIES

upon oceanic currents. According to Mr. Croll, cold periods regularly recur
every ten or fifteen thousand years; and these at long intervals are ex-
tremely severe, owing to certain contingencies, of which the most impor-
tant, as Sir C. Lyell has shown, is the relative position of the land and
water. Mr. Croll believes that the last great glacial period occurred about
240,000 years ago, and endured, with slight alterations of climate, for
about 160,000 years. With respect to more ancient glacial periods, several
geologists are convinced, from direct evidence, that such occurred during
the miocene and eocene formations, not to mention still more ancient
formations. But the most important result for us, arrived at by Mr. Croll,
is that whenever the northern hemisphere passes through a cold period thei
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 geographical 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 inconsiderable part of its scanty flora, are common to North
America and Europe, enormously remote as these areas in opposite hem-
ispheres are from each other. On the lofty mountains of equatorial Amer-
ica a host of peculiar species belonging to European genera occur. On the
Organ Mountains of Brazil some few temperate European, some antarctic,
and some Andean genera were found by Gardner which do not exist in the
low intervening hot countries. On the Silla of Caracas the illustrious Hum-
boldt long ago found species belonging to genera characteristic of the
Cordillera.

In Africa, several forms characteristic of Europe, and some few repre-
sentatives 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, be-
lieved not to have been introduced by man, and on the mountains several
representative European forms, are found which have not been discovered
in the inter-tropical 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 Mountains, 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 Rev. R. T. Lowe on the mountains of the Cape Verde Islands. This
extension of the same temperate forms, almost under the equator, across
the whole continent of Africa and to the mountains of the Cape Verde
archipelago, is one of the most astonishing facts ever recorded in the dis-
tribution of plants.

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

GEOGRAPHICAL DISTRIBUTION 281

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 temperate plains of the north and south, are either the same species
or varieties of the same species. It should, however, be observed that these
plants are not strictly arctic forms; for, as Mr. H. C. Watson has remarked,
"in receding from polar toward equatorial latitudes, the alpine or moun-
tain flora really become less and less arctic." 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 low-
lands.

These brief remarks apply to plants alone; but some few analogous facts
could be given in regard to terrestrial animals. In marine productions,
similar cases likewise occur; as an example, I may quote a statement by the
highest authority. Professor Dana, that "it is certainly a wonderful fact
that New Zealand should have a closer resemblance in its Crustacea to
Great Britain, its antipode, than to any other part of the world." Sir J.
Richardson, also, speaks of the reappearance on the shores of New Zealand,
Tasmania, 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 penin-
sula of India, to Ceylon and the Malay Archipelago, and in a less well-
marked manner across the wide expanse of tropical South America, it ap-
pears 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 tem-
perate forms. At this period the equatorial 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

282 THE ORIGIN OF SPECIES

equator must have been 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 Hima-
laya, 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 naturalized plants and ani-
mals have spread within a few centuries, this period will have been ample
for any amount of migration. As the cold became more and more intense,
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 equatorial
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 of the
Glacial period, as both hemispheres gradually recovered their former tem-
perature, 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 returning 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 acclimatization, as shown by
their transmitting to their offspring different constitutional powers of resist-
ing heat and cold.

In the regular course of events the southern hemisphere 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 fastnesses. Thus, we should have some few species

GEOGRAPHICAL DISTRIBUTION 283

identically 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 compete with many new forms, and would be exposed to some-
what different physical conditions: hence, they would be eminently liable
to modification, and would generally now exist as varieties or as representa-
tive 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 homes
in greater numbers, and having consequently been advanced through
natural selection and competition to a higher stage of perfection, or
dominating power, than the southern forms. And thus, when 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 Zea-
land, and to a lesser degree in Australia, and have beaten the natives;
whereas extremely few southern forms 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 Neilgherrie Mountains in India, however, offer a
partial exception; for here, as I hear from Dr. Hooker, Australian forms
are rapidly sowing themselves, and becoming naturalized. Before the last
great Glacial period, no doubt the inter-tropical mountains were stocked
with endemic 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

284 THE ORIGIN OF SPECIES

of marine productions, in the northern and southern temperate zones, and
on the inter- tropical 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-
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 glacial climate and permitted their further progress; in nearly
the same manner as, according to Forbes, isolated 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 distribu-
tion 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 migrated; why certain species have been modified and have
given rise to new forms, while others have remained unaltered. We cannot
hope to explain such facts, until we can say why one species and not another
becomes naturalized by man's agency in a foreign land; why one species
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 oc-
currence, as shown by Dr. Hooker, of the same plants at points so enor-
mously 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 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 be-
longing to the same genera have migrated in radiating lines from a com-
mon centre; and I am inclined to look in the southern, as in the northren
hemisphere, to a former and warmer period, before the commencement of
the last Glacial period, when the antarctic 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, Aus-
tralia, and New Zealand may have become slightly tinted by the same
peculiar forms of life.

Sir G. Lyell in a striking passage has speculated, in language almost
identical with mine, on the effects of great alterations of climate through-
out the world on geographical distribution. And we have now seen that

GEOGRAPHICAL DISTRIBUTION 285

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 direction, and has consequently more freely inundated the
south. As the tide leaves its drift in horizontal Hnes, 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 low-
lands to a great altitude under the equator. The various beings thus left
stranded may be compared with savage races of man, driven up and sur-
viving in the mountain fastnesses of almost every land, which serves as a
record, full of interest to us, of the former inhabitants of the surrounding
lowlands.

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.

FRESH-WATER PRODUCTIONS

As LAKES and river systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently a
still more formidable barrier, that they would never have extended to dis-
tant countries. 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 remember 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
from stream to stream, within their own countries; and liability to wide dis-
persal would follow from this capacity as an almost necessary consequence.
We can here consider only a few cases; of these, some of the most difficult
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 wonderful case, and probably indicates dispersal from
an antarctic 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 Zealand and to the Auck-
land Islands, though separated by a distance of about 230 miles. On the
same continent fresh-water fish often range widely, and as if capriciously;
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 dis-
persal may, however, be mainly attributed to changes in the level of the
land within the recent period, causing rivers to flow into each other. In-

286

GEOGRAPHICAL DISTRIBUTION 287

stances, also, could be given of this having occurred during floods, without
any change of level. The wide diff'erences of the fish on the opposite sides
of most mountain-ranges, which are continuous and consequently 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 geographical 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 belonging 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 descended from a common parent, and
must have proceeded from a single source, prevail throughout the world.
Their distribution at first perplexed me much, as their ova are not likely to
be transported by birds; and the ova, as well as the adults, are immediately
killed by sea-water. I could not even understand how some naturalized
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 it has happened to me, in removing a little duck-weed
from one aquarium to another, that I have unintentionally 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 ex-
tremely 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 in-
forms me that a dytiscus has been caught with an ancylus (a fresh- water
shell like a limpet) firmly adhering to it; and a water-beetle of the same
family, a colymbetes, once flew on board the Beagle, when forty-five miles
distant from the nearest land : how much farther it might have 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 remote oceanic islands. This is strikingly illustrated, according

288 THE ORIGIN OF SPECIES

to Alph. de Candolle, in those large groups of terrestrial plants, which have
very few aquatic members; for the latter seem immediately to acquire, as if
in consequence, a wide range. I think favorable means 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 tablespoonfuls of
mud from three different points, beneath water, on the edge of a little
pond; this mud when dried weighed only six and three-fourths 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 reject many
other kinds after having swallowed them; even small fish swallow seeds of
moderate size, as of the yellow water-lily and Potamogeton. Herons and
other birds, century after century, have gone on daily devouring fish; they
then take flight and go to other waters, or are blown across the sea; and
we have seen that seeds retain their power of germination, when rejected
many hours afterward in pellets or in the excrement. When I saw the great
size of the seeds of that fine water-lily, the Nelumbium, 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 southern 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 rejected the seeds in the pellet in a fit state
for germination.

In considering these several means of distribution, it should be remem-
bered 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

GEOGRAPHICAL DISTRIBUTION 289

in a well-stocked pond is small in comparison with the number of species
inhabiting an equal area of land, the competition between them will prob-
ably 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 re-
member that many 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 dis-
tribution, 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 removes
many difficulties, but 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 bear-
ing on the truth of the two theories of independent creation and of descent
with modification.

The species of all kinds which inhabit oceanic islands are few in number
compared with those on equal continental areas: Alph. de Candolle admits
this for plants, and WoUaston for insects. New Zealand, for instance, with
its lofty mountains and diversified stations, extending 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 uniform county of Cambridge has
847 plants, and the little island of Anglesea 764, but a few ferns and a few
introduced plants are included in these numbers, and the comparison in
some other respects is not quite fair. We have evidence that the barren

290 THE ORIGIN OF SPECIES

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 sufficient number of the best-adapted plants and animals were not created
for oceanic islands; for man has unintentionally stocked them far more
fully and perfectly than did nature.

Although in oceanic islands the species are few in number the propor-
tion of endemic kinds {i.e.y those found nowhere else in the world) is often
extremely large. If we compare, for instance, the number of endemic 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 eminently
liable to modification, and would often produce groups of modified de-
scendants. 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 twenty-six land birds; of these, twenty-one (or
perhaps twenty- three) are peculiar, whereas of the eleven marine birds
only two are peculiar; and it is obvious that marine birds could arrive at
those islands much more easily and frequently than land birds. Bermuda,
on the other hand, which lies at about the same distance from North
America as the Galapagos Islands do from South America, and which has
a very peculiar soil, does not possess a single endemic land bird; and we
know from Mr. J. M. Jones' admirable account of Bermuda, that very
many North American birds occasionally or even frequently visit this island.
Almost every year, as I am informed by Mr. E. Harcourt, many European
and African birds are blown to Madeira; this island is inhabited by ninety-
nine kinds, of which one alone is peculiar, though very closely related to a
European form; and three or four other species are confined 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

GEOGRAPHICAL DISTRIBUTION 291

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 arriv-
ing from the mother-country. Madeira again is inhabited by a wonderful
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 dis-
persed, 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 orders of insects inhabiting Madeira present nearly
parallel cases.

Oceanic islands are sometimes deficient in animals 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 re-
cently 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 ac-
counted for by supposed differences in the physical conditions of the islands;
but this explanation 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 given with respect to the in-
habitants 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 the wool or fur of quadrupeds. 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 beetles. Again, islands often possess trees or
bushes belonging to orders which elsewhere include only herbaceous species;
now trees, as Alph. de Candolle has shown, generally have, whatever the
cause may be, confined ranges. Hence trees would be little likely to reach
distant oceanic islands; and an herbaceous plant, 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 over-topping 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.

292 THE ORIGIN OF SPECIES

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 Solomon 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 Solomon groups and the Seychelles. This gen-
eral absence of frogs, toads, and newts on so many true oceanic islands
cannot be accounted for, by their physical conditions : indeed, it seems that
islands are peculiarly fitted for these animals; for frogs have been intro-
duced into Madeira, the Azores, and Mauritius, and have multiplied so as
to become a nuisance. But as these animals and their spawn are immediately
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 islands. But
why, on the theory of creation, they should not have been created there, it
would be very difficult to explain.

Mammals offer another and similar case. I have carefully searched the
oldest voyages, and have not found a single instance, free from doubt, of
a terrestrial mammal (excluding domesticated animals kept by the natives)
inhabiting an island situated about 300 miles from a continent or great
continental island; and many islands situated at a much less distance are
equally barren. The Falkland Islands, which are inhabited by a wolf-like
fox, come nearest to an exception; but this group cannot be considered as
oceanic, as it lies on a bank in connection with the mainland at a distance
of about 280 miles; moreover, icebergs formerly brought bowlders to its
western 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 occur 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 creation of
mammals; many volcanic islands are sufficiently ancient, as shown by the
stupendous degradation which they have suffered, and by their tertiary
strata: there has also been time for the production of endemic species be-
longing to other classes ; and on continents it is known that new species of .^^
mammals appear and disappear 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 Viti Archj-

GEOGRAPHICAL DISTRIBUTION 293

pelago, the Bonin Islands, the CaroHne and Marianne Archipelagoes, and
Mauritius, all possess their peculiar bats. Why, it may be asked, has the
supposed creative force produced bats and no other mammals on remote
islands? On my view this question can easily be answered; for no terrestrial
mammal can be transported across a wide space of sea, but bats can fly
across. Bats have been seen wandering by day far over the Atlantic Ocean;
and two North American species, either regularly or occasionally, visit
Bermuda, at the distance of 600 miles from the mainland. I hear from Mr.
Tomes, who has specially studied this family, that many species have enor-
mous ranges, and are found on continents and on far distant islands. Hence,
we have only to suppose that such wandering 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 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 Archi-
pelago, which is traversed 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 one thousand fathoms in depth, and here we find American 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 channels,
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 rela-
tion 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 — 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

294 THE ORIGIN OF SPECIES

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 im-
migrated more uniformly, and from the species having entered in a body,
their mutual relations would not have been much disturbed, and, conse-
quently, 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 difficulties in understand-
ing 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 over-
looked. I will specify one difficult case. Almost all oceanic islands, even the
most isolated and smallest, are inhabited by land-shells, generally by en-
demic species, but sometimes by species found elsewhere, striking instances
of whidh have been given by Dr. A. A. Gould in relation to the Pacific. 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
unknown, but occasionally efficient, means for their transportal. 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 membraneous 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 un-
injured 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 mem-
braneous 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 recovered. The presence of an operculum
seems to have been of importance, as out of twelve specimens of Cyclostoma
elegans, which is thus furnished, eleven revived. It is remarkable, seeing
how well the Helix pomatia resisted with me the salt water, that not one of
fifty-four specimens belonging to four other species of Helix tried by Aucapi-
taine recovered. It is, however, not at all probable that land-shells have
often been thus transported; the feet of birds offer a more probable method.

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

GEOGRAPHICAL DISTRIBUTION 295

actually the same. Numerous instances could be given. The Galapagos
Archipelago, situated under the equator, lies at the distance 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 com-
monly 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 ad-
mirable Flora of this archipelago. The naturalist, looking 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 conditions
of life, in the geological nature of the islands, in their height or climate, or
in the proportions in which the several classes are associated together, which
closely resembles the conditions of the South American coast. In fact, there
is a considerable dissimilarity in all these respects. On the other hand, there
is a considerable degree of resemblance in the volcanic nature of the soil,
in the climate, height, and size of the islands, between the Galapagos and
Cape Verde Archipelagoes: but what an entire and absolute difference in
their inhabitants! The inhabitants of the Cape Verde Islands are related
to those of Africa, like those of the Galapagos to America. 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 Verde 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 near-
est 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 its endemic plants is much more closely related
to Australia, the nearest mainland, than to any other region : and this is
what might have been expected; but it is also plainly related to South
America, which, although the next nearest continent, is so enormously re-
mote, that the fact becomes an anomaly. But this difficulty partially dis-
appears on the view that New Zealand, South America, and the other

296 THE ORIGIN OF SPECIES

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 commence-
ment 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 inhabit-
ants of islands and the nearest mainland, 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 related 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
meat 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,
the 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 chief part from the deeply-seated error of con-
sidering 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 Gala-
pagos Archipelago, and are likewise found in other parts of the world, we
find that they diflfer considerably in the several islands. This difference
might indeed have been expected if the islands have been stocked by occa-
sional 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.
But the islands, though in sight of each other, are separated by deep arms

GEOGRAPHICAL DISTRIBUTION 297

of the sea, in most cases wider than the British Channel, and there is no
reason to suppose that they have at any former period been continuously
united. The currents of the sea are rapid and deep between the islands, and
gales of wind are extraordinarily rare; so that the islands are far more
effectually separated from each other than they appear on a map. Never-
theless, 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 the present manner of distribution that they have
spread from one island to the others. But we often take, I think, an errone-
ous view of the probability of closely allied species invading each other's
territory, when put into free intercommunication. Undoubtedly, if one
species has any advantage 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 remem-
ber that the species which become naturalized 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 Can-
dolle, 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 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. Wollaston have communicated to
me a remarkable fact bearing on this subject; namely, that Madeira and
the adjoining islet of Porto Santo possess many distinct but representative
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 Archi-
pelago not having all spread from island to island. On the same continent,
also, pre-occupation 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 south-east and south-west corners of
Australia have nearly the same physical conditions, and are united by con-
tinuous land, yet they are inhabited by a vast number of distinct mammals,

298 THE ORIGIN OF SPECIES

birds, and plants; so it is, according to Mr. Bates, with the butterflies and
other animals inhabiting the great, open, and continuous valley of the
Amazons.

The same principle which governs the general character of the inhabit-
ants of oceanic islands, namely, the relation to the source whence colonists
could have been most easily derived, together with their subsequent modifi-
cation, 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 the 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 like-
wise 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 show-
ing us the steps in the progress of modification.

The relation between the power and extent of migration in certain
species, either at the present or at some 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 Felidae and Canidae. We see the same rule in
the distribution of butterflies and beetles. So it is with most of the inhabit-
ants 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 instance, 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

GEOGRAPHICAL DISTRIBUTION 299

more important power of being victorious in distant lands in the struggle
for life with foreign associates. But according to the view that all the species
of a genuSj though distributed to the most remote points of the world, are
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 geologi-
cal evidence, that within each great class the lower organisms change at a
slower rate than the higher; consequently 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 them-
selves ranging widely — such facts, as alpine, lacustrine, and marsh produc-
tions 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 archipelago — are inex-
plicable 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.

SUMMARY OF THE LAST AND PRESENT CHAPTERS

In these chapters I have endeavored to show that if we make due allow-
ance 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 trans-
port— 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 in-
superable in believing that all the individuals of the same species, wherever
found, are descended from common parents. And we are led to this con-
clusion, which has been arrived at by many naturalists under the designa-
tion of single centres of creation, by various general considerations, more
especially from the importance of barriers of all kinds, and from the
analogical distribution of subgenera, 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 allow-

300 THE ORIGIN OF SPECIES

ances as before for 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 climatical changes on distribution, I have
attempted to show how important a part the last Glacial period has played,
which affected even the equatorial regions, and which, during the alterna-
tions of the cold in the north and the 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 belonging 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
multiplication 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 in-
habitants of the plains and mountains, of the forests, marshes and deserts,
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
entered one of the regions, or both; according to the nature of the com-
munication which allowed certain forms and not others to enter, either in
greater or lesser numbers; according or not as those which entered hap-
pened to come into more or less direct competition with each other and
with the aborigines; and according as the immigrants were capable of vary-
ing more or less rapidly, there would ensue in the two or more regions, in-
dependently of their physical conditions, infinitely diversified conditions of
life; there would be an almost endless amount of organic action and reac-
tion, 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,

GEOGRAPHICAL DISTRIBUTION 301

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 terrestrial 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 inhabitants of an
archipelago, though specifically distinct on the several islets, should be
closely related to each 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 succes-
sion of forms in past times being nearly the same with those governing at
the present time the differences in different areas. We see this in many
facts. The endurance of each species and group of species is continuous in
time; for the 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 in-
habited by a single species, or by a group of species, is continuous, and the
exceptions, which are not rare, may, as I have attempted to show, be ac-
counted for by former migrations under different circumstances, or through
occasional means of transport, or by the species having become extinct in
the intermediate tracts. Both in time and space species and groups of
species have their points of maximum development. Groups of species, liv-
ing 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 throughout the world, we find that species in certain classes differ
little from each other, while those in another class, 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 distant 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.

CHAPTER XIV

Mutual Affinities of Organic Beings: Morphology — Embryology — Rudimen-
tary Organs

Classification, Groups Subordinate to Groups — ^Natural System — ^Rules and Difficulties
in Classification, explained on the Theory of Descent with Modification — Classi-
fication of Varieties — Descent always used in Classification — Analogical or
Adaptive Characters — Affinities, General, Complex, and Radiating — Extinction
separates and defines Groups — Morphology, between Members of the Same Class, ,
between Parts of the Same Individual — Embryology, Laws of, explained by Vari- '
ations 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 constellations. 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 notori-
ous how commonly members of even the same sub-group have different
habits. In the second and fourth chapters, on Variation and on Natural
Selection, I have attempted to show that within each country it is the widely
ranging, the much difl["used and common, that is the dominant species, be-
longing 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 pro-
duce 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 descend-
ants of each species trying to occupy as many and as different places as pos-
sible in the economy of nature, they constantly tend to diverge in character.
This latter conclusion is supported by observing 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 exter-
minate the 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 principles; and he will see that the inevitable result is, that
the modified descendants proceeding from one progenitor become broken
up into groups subordinate to groups. In the diagram each letter on the up-
permost line may represent a genus including several species; and the whole
of the genera along this upper line form together one class, for all are de-
scended from one ancient parent, and, consequently, have inherited some-

302

MUTUAL AFFINITIES OF ORGANIC BEINGS 303

thing in common. But the three genera on the left hand have, on this same
principle, much in common, and form a sub-family distinct from that con-
taining the next two 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 grooiped in sub-families; and they form a
family distinct from that containing the three genera still farther 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 progenitor, 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 suffi-
ciently strike us, is in my judgment thus explained. No doubt organic beings,
like all other objects, can be classed in many ways, either artificially by single
characters, or more naturally by a number of characters. We know, for in-
stance, 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 diff'erent, and the view above given accords with their
natural arrangement in group under group; and no other explanation has
ever been attempted.

Naturalists, as we have seen, try to arrange the species, genera, and families
in each class, on what is called the Natural System. But what is meant by this
system? Some authors look at it merely as a scheme for arranging together
those Hving 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 propositions — that is, by one sentence to give the characters com-
mon, for instance, to all mammals, by another those common to all carnivora,
by another those common to the dog-genus, and then, by adding a single
sentence, a full description is given of each kind of dog. The ingenuity and
utility of this system are indisputable. But many naturalists think that some-
thing 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 famous
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 included in our clas-
sifications than mere resemblance. I believe that this is the case, and that
community of descent — the one known cause of close similarity in organic
beings — is the bond, which, though observed by various degrees of modifica-
tion, is partially revealed to us by our classifications.

Let us now consider the rules followed in classification, and the difficulties
which are encountered on the view that classification either gives some un-
known plan of creation, or is simply a scheme for enunciating general propo-
sitions and of placing together the forms most like each other. It might have

304 THE ORIGIN OF SPECIES

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 classi-
fication. Nothing can be more false. No one regards the external similarity
of a mouse to a shrew, of a dugong to a whale, of a whale to a fish, as of
any importance. These resemblances, though so intimately connected with
the whole life of the being, are ranked as merely "adaptive or analogical
characters": but to the consideration of these resemblances we shall recur.
It may even be given as a general rule, that the less any part of the organiza-
tion is concerned with special habits, the more important it becomes for clas-
sification. As an instance: Owen, in speaking of the dugong, says, "The
generative organs, being those which are most remotely related to the habits
and food of an animal, I have always regarded as affording very clear indi-
cations of its true affinities. We are least likely in the modifications of these
organs to mistake 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 reproduc-
tion, with their product the seed and embryo, are of paramount importance!
So again, in formerly discussing certain morphological characters which are
not functionally important, we have seen that they are often of the highest
service in classification. This depends on their constancy 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 nearly the
same physiological value, its classificatory value is widely different. No natu-
ralist 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, Robert Brown, who, in speaking
of certain organs in the Proteaceae, says their generic importance, "like that
of all their parts, not only in this, but, as I apprehend, in every natural
family, is very unequal, and in some cases seems to be entirely lost." Again,
in another work he says, the genera of the Connaracese "differ in having
one or more ovaria, in the existence or absence of albumen, in the imbricate
or valvular aestivation. Any one of these characters singly is frequently of
more than generic importance, though here, even when all taken together,,
they appear insufficient to separate Cnestis from Connarus." To give an
example among insects : in one great division of the Hymenoptera, the anten-
nae, as Westwood has remarked, are most constant in structure; in another
division they differ much, and the differences are of quite subordinate value
in classification ; yet no one will say that the antennae in these two divisions of
the same order are of unequal physiological importance. Any number of in-
stances could be given of the varying importance for classification of the same^
important organ within the same group of beings.

MUTUAL AFFINITIES OF ORGANIC BEINGS 305

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 rudi-
mentary teeth in the upper jaws of young ruminants, and certain rudimen-
tary bones of the leg, are highly serviceable in exhibiting the close affinity
between ruminants and pachyderms. Robert Brown has strongly insisted on
the fact that the position of the rudimentary florets is of the highest im-
portance in the classification of the grasses.

Numerous instances could be given of characters derived from parts which
must be considered of very trifling physiological importance, but which are
universally admitted as highly serviceable in the definition of whole groups.
For instance, whether or not there is an open passage from the nostrils to the
mouth, the only character, according to Owen, which absolutely distinguishes
fishes and reptiles — the inflection of the angle of the 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 Ornithorhyn-
chus 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 characters, mainly depends
on their being correlated with many other characters of more or less im-
portance. The value indeed of an aggregate of characters is very evident in
natural history. Hence, as has often been remarked, a species may depart
from its allies in several characters, both of high physiological importance,
and of almost universal prevalence, and yet leave us in no doubt where it
should be ranked. Hence, also, it has been found that a classification founded
on any single character, however important that may be, has always failed;
for no part of the organization is invariably constant. The importance of an
aggregate of characters, even when none are important, alone explains the
aphorism enunciated by Linnaeus, namely, that the characters do not give
the genus, but the genus gives the character; for this seems founded on the
appreciation of many trifling points of resemblance, too slight to be defined.
Certain plants belonging to the Malpighiaceae bear perfect and degraded
flowers; in the latter, as A. de Jussieu has remarked, "The greater number of
the characters proper to the species, to the genus, to the family, to the class,
disappear, and thus laugh at our classification." 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 illustrates 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

306 THE ORIGIN OF SPECIES

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. Saint-Hilaire. If several trifling characters are
always found in combination, though no apparent bond of connection 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, Cypridina 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 Ed-
wards and Agassiz, that embryological characters are the most important
of all; and this doctrine has very generally been admitted as true. Neverthe-
less, their importance has sometimes been exaggerated, owing to the adap-
tive characters of larvae 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, excluding larval, characters, are of the
highest value for classification, not only with animals but with plants. Thus
the main divisions of flowering plants are founded on diflferences 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. Noth-
ing 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 unequivo-
cally belonging to this, and to no other class of the Articulata.

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

MUTUAL AFFINITIES OF ORGANIC BEINGS 307

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 practised naturalists as only a genus, and then raised to the rank
of a sub-family or family; and this has been done, not because further re-
search has detected important structural differences, at first overlooked, but
because numerous allied species, with slightly difTerent 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 characters 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 unconsciously seeking, and not some unknown
plan of creation, or the enunciation of general propositions, and the mere
putting together and separating objects more or less alike.

But I must explain my meaning more fully. I believe that the arrangement
of the groups within each class, in due subordination and relation to 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 and L to represent allied genera existing during
the Silurian epoch, and descended from some still earlier form. In three of
these genera (A, F, and I) a species has transmitted modified descendants
to the present day, represented by the fifteen genera {a^^ to z^^) on the up-
permost horizontal line. Now, all these modified descendants from a single
species are related in blood or descent in the same degree. They may
metaphorically be called cousins to the same millionth degree, yet they differ
widely and in different degrees from each other. The forms descended from
A, now broken up into two or three families, constitute a distinct order from
those descended from I, also broken up into two families. Nor can the
existing species descended from A be ranked in the same genus with the
parent A, or those from I with the parent I. But the existing genus /^* 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 diff'erent. Nevertheless, their genealogical
arrangement remains strictly true, not only at the present time, but at each

308 THE ORIGIN OF SPECIES

successive period of descent. All the modified descendants from A will have
inherited something in common from their common parents, as will all the
descendants from I; so will it be with each subordinate branch of de-
scendants 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 natural 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 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 representation
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 repre-
sentation 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 modifica-
tion which the different groups have undergone has to be expressed by
ranking them under different so-called genera, sub-families, families, sec-
tions, 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 classifica-
tion 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 languages 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 groups; but the proper or even the
only possible arrangement would still be genealogical; and this would be
strictly natural, as it would connect together all languages, 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 varieties 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

MUTUAL AFFINITIES OF ORGANIC BEINGS 309

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 classing varieties, I apprehend
that if we had a real pedigree, a genealogical classification would be uni-
versally preferred; 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 alUed in the greatest
number of points. In tumbler pigeons, though some of the sub-varieties differ
in the important character of the length of the beak, yet all are kept to-
gether 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 predicated in common of the adult males and hermaphrodites
of certain cirripedes, and yet no one dreams of separating them. As soon as
the three Orchidean forms, Monachanthus, 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, however 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 larvas are sometimes
extremely different; and as it has been used in classing varieties which have
undergone a certain, and sometimes a considerable, amount of modification;
may not this same element of descent have been unconsciously used in
grouping species under genera, and genera under higher groups, all under
the so-called natural system? I believe it has been unconsciously used; and
thus only can I understand the several rules and guides which have been
followed by our best systematists. As ^ve have no written pedigrees, we are
forced to trace community of descent by resemblances of any kind. There-
fore, we choose those characters which are the least likely to have been
modified, in relation to the conditions of life to which each species has been
recently exposed. Rudimentar)^ structures on this \'iew are as good, or even
sometimes better than other parts of the organization. We care not how

310 THE ORIGIN OF SPECIES

trifling a character may be — let it be the mere inflection of the angle of the
jaw, the manner in which an insect's wing is folded, whether the skin be
covered by hair or feathers — if it prevail throughout many and different
species, especially those having very different habits of life, it assumes high
value; for we can account for its presence in so many forms with such dif-
ferent habits, only by inheritance from a common parent. We may err in this
respect in regard to single points of structure, but when several characters,
let them be ever so trifling, concur throughout a large group of beings having
different habits, we may feel almost sure, on the theory of descent, that these
characters have been inherited from a common ancestor; and we know that
such aggregated characters have especial value in classification.

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, betray the
hidden bond of community of descent. Let two forms have not a single
character in common, yet, if these extreme forms are connected together by a
chain of intermediate groups, we may at once infer their community of
descent, and we put them all into the same class. As we find organs of high
physiological importance — those which serve to preserve life under the most
diverse conditions of existence — are generally the most constant, we attach
especial value to them ; but if these same organs, in another group or section
of a group, are found to differ much, we at once value them less in our
classification. We shall presently see why embryological characters are of
such high classificatory importance. Geographical distribution may some-
times be brought usefully 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 resemblances in the shape of the body and in the
fin-like anterior limbs between dugongs and whales, and between these two
orders of mammals and fishes, are analogical. So is the resemblance between
a mouse and a shrew-mouse (Sorex), which belong to different orders; and
the still closer resemblance, insisted 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 con-
cealment from enemies.

Among insects there are innumerable similar instances; thus Linnaeus,
misled by external appearances, actually classed an homopterous insect as a
moth. We see something of the same kind even with our domestic varieties,

MUTUAL AFFINITIES OF ORGANIC BEINGS 311

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 race-
horse is hardly more fanciful than the analogies 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 systematist. For animals, belonging
to two most distinct lines of descent, may have become' adapted to similar
conditions, and thus have assumed a close external resemblance; but such
resemblances will not reveal — will rather tend to conceal their blood-
relationship. We can thus also understand the apparent paradox, that the
very same characters are analogical when one group is compared 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
analogical when whales are compared with fishes, being adaptations in
both classes for swimming through the water; but between the several mem-
bers of the whale family, the shape of the body and the fin-like limbs offer
characters 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 resemblances in quite distinct
beings between single parts of 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
Thylacinus 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 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 widely different insects possessing
luminous organs — and of orchids and asclepiads having pollen-masses with
viscid disks, come under this same head of analogical resemblances. 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

312 THE ORIGIN OF SPECIES

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 alHed, have inherited so much in common
in their constitution, that they are apt to vary under siroilar exciting causes in.
a similar manner; and this would obviously aid in the acquirement through
natural selection of parts or organs, strikingly Hke each other, independently
of their direct inheritance from a common progenitor.

As species belonging to distinct classes have often been adapted by suc-
cessive slight modifications to live under nearly similar circumstances — to
inhabit, for instance, the three elements of land, air, and water — ^we can
perhaps understand how it is that a numerical parallelism has sometimes
been observed between the sub-groups of distinct classes. A naturalist,
struck with a parallelism of this nature, by arbitrarily raising or sinking 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 re-
semblance 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 excellent 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,
continually 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, be-
longing to the same two genera, equally close in their resemblance, may be
found. Altogether no less than ten genera are enumerated, which include
species that imitate other butterflies. The mockers and mocked always
inhabit the same region; we never find an imitator living remote from the
form which it imitates. The mockers are almost invariably rare insects; the
mocked in almost every case abounds 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.

MUTUAL AFFINITIES OF ORGANIC BEINGS 313

forms, can be shown by a graduated series to be merely varieties of the same
species; while others are undoubtedly distinct species. But 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 re-
semble their nearest allies.

We are next led to inquire what reason can be assigned for certain butter-
flies 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 destruction to a large extent, otherwise they could 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
mocking forms, on the other hand, that inhabit the same district, are com-
paratively rare, and belong to rare groups ; 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 practised 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 v>^itnessed the process by which the mimickers 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 dis-
trict. In another district there were two or three varieties, one of which was
much commoner 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 re-
semblance 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 Lepidoptera of the Malay Archipelago and
Africa, and with some other insects. Mr. Wallace has also detected one such
case with birds, but we have none with the larger quadrupeds. The much
greater frequency of imitation with insects than with other animals, is
probably the consequence of their small size; insects cannot defend them-
selves, 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

314 THE ORIGIN OF SPECIES

mocked; insects cannot easily escape by flight from the larger animals which
prey on them; therefore, speaking metaphorically, they are reduced, like
most weak creatures, to trickery and dissimulation.

It should be observed that the process of imitation probably never com-
menced between forms widely dissimilar in color. But, starting with species
already somewhat like each other, the closest resemblance, if beneficial, could
readily be gained by the above means, and if the imitated form was sub-
sequently and gradually modified through any agency, the imitating form
would be led along the same track, and thus be altered to almost any extent,
so that it might ultimately assume an appearance or coloring wholly unlike
that of the other members of the family to which it belonged. There is, how-
ever, 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 perfect
resemblance.

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 belong large and their parents dominant, they are almost sure to spread
widely, and to seize on more and more places in the economy of nature.
The larger and more dominant groups 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 included 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
Australia 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 prin-
ciple 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 intermediate between
existing groups. As some few of the old and intermediate forms have
transmitted to the present day descendants but little modified, these con-
stitute 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 evidence of aberrant groups
having suffered severely from extinction, for they are almost always rep-
resented by extremely few species, and such species as do occur are generally
very distinct from each other, which again implies extinction. The genera

MUTUAL AFFINITIES OF ORGANIC BEINGS 315

Ornithorhynchus and Lepidosiren, for example, would not have been less
aberrant had each been represented 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 general and not special; thus, according to Mr. Waterhouse,
of all Rodents, the bizcacha is most nearly related to Marsupials; but in the
points in which it approaches this order, its relations are general, 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 adaptive, 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 bizcacha,
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 common
progenitor, and that both groups have since undergone much modification
in divergent directions. On either view we must suppose that the bizcacha
has retained, by inheritance, more of the characters of its ancient progenitor
than have other Rodents; and therefore it will not be specially related to
any one existing Marsupial, but indirectly to all or nearly all Marsupials,
from having partially retained the character of their common progenitor, or
of 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 Phascolomys having become adapted to habits like those of a Rodent.
The elder De Candolle has made nearly similar observations on the general
nature of the affinities of distinct families of plants.

On the principle of the multiplication and gradual divergence in char-
acter of the species descended from a common progenitor, together with
their retention by inheritance of some characters in common, we can under-
stand the excessively complex and radiating affinities by which all the mem-
bers of the same family or higher group are connected together. For the
common progenitor 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 degrees, to all the species; and they will con-
sequently be related to each other by circuitous lines of affinity of various
lengths (as may be seen in the diagram so often referred to), mounting up
through many predecessors. As it is difficult to show the blood relationship
between the numerous kindred of any ancient and noble family even by the
aid of a genealogical tree, and almost impossible to do so without this aid,
we can understand the extraordinary difficulty which naturalists have ex-

316 THE ORIGIN OF SPECIES

perienced in describing, without the aid of a diagram, the various affinities
which they perceive between the many Hving 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 progeni-
tors 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 together by a long and only partially broken chain of affinities. Ex-
tinction 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 classification, or at least a natural
arrangement, would be possible. We shall see this by turning to the diagram;
the letters, A to L, may represent eleven Silurian genera, some of which have
produced large groups of modified descendants, with every link in each
branch and sub-branch still alive; and the links not greater than those be-
tween existing varieties. In this case it would be 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 inheritance, 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
species, although they may have but few characters in common; we use

MUTUAL AFFINITIES OF ORGANIC BEINGS 317

descent in classing acknowledged varieties, 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, genealogical 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 follow in our classification. We can under-
stand why we value certain resemblances far more than others; why we use
rudimentary and useless organs, or others of trifling physiological im-
portance; why, in finding the relations between one group and another, we
summarily reject analogical or adaptive characters, and yet use these same
characters within the limits of the same group. We can clearly see how it is
that all living and extinct forms can be grouped together within a few great
classes; and how the several members of each class are connected together
by the most complex and radiating lines of affinities. We shall never,
probably, disentangle the inextricable web of the affinities between the mem-
bers 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 Hackel in his "Generelle Morphologic," 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 characters, 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 appeared
in our geological formations. He has thus boldly 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 resemblance is often expressed by the term "unity of type"; or by saying
that the several parts and organs in the different species of the class are
homologous. The whole subject is included under the general term of
Morphology. 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 a 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 marsupials — should all be constructed on the same extraor-

318 THE ORIGIN OF SPECIES

dinary 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 single
toe furnished with two claws. Notwithstanding the similarity of pattern, it
is obvious that the hind feet of these several 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 : "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 Saint-Hilaire has strongly insisted on the high importance of
relative position or connection in homologous 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 Hmbs
of crustaceans. So it is with the flowers of plants.

Nothing can be more hopeless than to attempt to explain this similarity
of pattern in members of the same class, by utility or by the doctrine of final
causes. The hopelessness of the attempt has been expressly admitted by Owen
in his most interesting work on the "Nature of Limbs." On the ordinary view
of the independent creation of each being, we can only say that so it is; that
it has 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 being profitable in some way to the
modified form, but often affecting by correlation other parts of the organiza-
tion. 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 modifica-
tions would not tend to alter the framework of the bones or the relative
connection of the parts. If we suppose that an early progenitor — the arche-
type, as it may be called — of all mammals, birds, and reptiles, had its limbs
constructed on the existing general pattern, for whatever purpose they
served, we can at once perceive the plain signification of the homologous

MUTUAL AFFINITIES OF ORGANIC BEINGS 319

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,
mandibles, and two pairs of maxillae, these parts being perhaps very simple in
form; and then natural selection will account for the definite diversity in
the structure and 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 paddles 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 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 complex 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 be-
come extremely diff'erent are at first exactly alike.

How inexplicable are the cases of serial homologies on the ordinary
view of creation! Why should the brain be enclosed in a box composed of
such numerous and such extraordinarily shaped pieces of bone, apparently
representing vertebrae? 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 walking? Why should one crustacean, which has an extremely complex
mouth formed of many parts, consequently always have fewer legs; or con-
versely, those with many legs have simpler mouths? Why should the sepals,
petals, stamens, and pistils, in 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 certain 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 became divided
into right and left sides, with corresponding organs, for such questions are
almost beyond investigation. It is, however, probable that some serial struc-

320 THE ORIGIN OF SPECIES

tures are the result of cells multiplying by division, entailing the multiplica-
tion 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 progenitor of the Vertebrata prob-
ably possessed many vertebrae; the unknown progenitor of the Articulata,
many 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 num-
bers, 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 varia-
tions, which afforded the basis for their subsequent modification 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. Ray
Lankester, who has drawn an important distinction between certain classes of
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 modifi-
cation, 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 de-
veloped. 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 organ-
isms, 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

MUTUAL AFFINITIES OF ORGANIC BEINGS 321

same general purpose or function, of which many instances have been given.
Naturalists frequently speak of the skull as formed of metamorphosed
vertebrae; the jaws of crabs as metamorphosed 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
vertebrae, jaws and legs, etc., as having been metamorphosed, not one from
the other, as they 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 during 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 met-
amorphosed 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 eflfected abruptly by a few stages; but the transformations are in
reality numerous and gradual, though concealed. A certain ephemerous
insect (Chloeon) during its development 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 metamorphosis 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 the so-called alternate genera-
tions of some of the lower animals. It is, for instance, an astonishing fact
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 them-
selves to rocks and become developed into branching corallines ; and so on in
an endless cycle. The belief in the essential identity of the process of alternate
generation and of ordinary metamorphosis has been greatly strengthened
by Wagner's discovery of the larva or maggot of a fly, namely the Cecidomyia,
producing asexually other larvae, 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 remarkable 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

322 THE ORIGIN OF SPECIES

shown that another fly, a Chironomus, 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
Cecidomyia, with the parthenogenesis of the Goccidae"; the term par-
thenogenesis implying that the mature females of the Coccidse are capable
of producing fertile eggs without the concourse of the male. Certain animals
belonging to several classes are now known to have the power of ordinary
reproduction at an unusually early age; and we have only to accelerate
parthenogenetic reproduction 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 dissimilar. A better proof of this latter fact cannot be given than the
statement by Von Baer that "the embryos of mammalia, of birds, lizards and
snakes, probably also of chelonia, are in the earliest states exceedingly like
one another, both as a whole and in the mode of development of their parts;
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 development we
should learn nothing, for the feet of lizards and mammals, the wings and feet
of birds, no less than the hands and feet of man, all arise from the same
fundamental form." The larvae of most crustaceans, at corresponding stages
of development, closely resemble each 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 plumage; 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 some-
thing 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 leguminosse.

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 conditions of existence. We cannot, for instance, suppose that in the

MUTUAL AFFINITIES OF ORGANIC BEINGS 323

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 similar conditions of life. No one supposes that the stripes on the
whelp of a Hon, or the spots on the young blackbird, 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 provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on, the
adaptation of the larva to its conditions of life is just as perfect and as beauti-
ful 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 belonging to very different orders, and
on the dissimilarity of the larvae of other insects within the same order,
according 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 embryonic re-
semblance. Cirripedes afford a good instance of this; even the illustrious
Cuvier did not percTeive that a barnacle was a crustacean; but a glance at
the larva shows this in an unmistakable manner. So again the two main
divisions of cirripedes, the pedunculated and sessile, though differing widely
in external 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 organization being higher or lower. But no one
probably will dispute that the butterfly is higher than the caterpillar. In some
cases, however, the mature animal must be considered as lower in the scale
than the larva, as with certain parasitic crustaceans. To refer once again
to cirripedes: the larvae in the first stage have three pairs of 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 chrysahs stage of butterflies, they have six pairs of beautifully con-
structed natatory legs, a pair of magnificent compound eyes, and extremely
complex antennae; but they have a closed and imperfect mouth and cannot
feed: their function at this stage is, to search 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

324 THE ORIGIN OF SPECIES

into prehensile organs; they again obtain a well-constructed mouth; but they
have no antenna, and their two eyes are now reconverted into a minute,
single, simple eye-spot. In this last and complete state, cirripedes 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
complemental males; and in the latter the development has assuredly been
retrograde, for the male is a mere sac, 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 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 porpoise, 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,
"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 wormlike stage, whether they are
active and adapted to diversified habits, or are inactive from being placed in
the midst of proper nutriment, or from being fed by their parents; but in
some few cases, as in that of Aphis, if we look to the admirable drawings of
the development of this insect, by Professor Huxley, we see hardly any trace
of the vermiform 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 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 universal, difference in structure between the
embryo and the adult; the various parts in the same individual embryo,
which ultimately become very unlike, and serve for diverse purposes, being
at an early period of growth alike; the common, but not invariable, re-
semblance between the embryos or larvae of the most distinct species in the
same class; the embryo often retaining, while within the egg or womb,

MUTUAL AFFINITIES OF ORGANIC BEINGS 325

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 surrounding 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 neces-
sarily 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 and demerits of their
young animals. We see this plainly in our own children; we cannot tell
whether a child will be tall or 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 generation. 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 in-
stance, 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
embryology. But first let us look to a few analogous cases in our domestic
varieties. Some authors who have written on dogs maintain that the grey-
hound and bull-dog, though so different, are really closely allied varieties,
descended from the same wild stock, hence I was curious to see how far
their puppies differed from each other. I was told by breeders that they
differed just as much as their parents, and this, judging by the eye, seemed
almost to be the case; but on actually measuring the old dogs and their
six-days-old puppies, I found that the puppies had not acquired nearly their
full amount of proportional difference. So, again, I was told that the foals

326 THE ORIGIN OF SPECIES

of cart and race horses — breeds which have been almost wholly formed
by selection under domestication — differed as much as the full-grown ani-
mals; but having had careful measurements made of the dams and of three-
days-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 de-
scended 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 be distinguished, the proportional differences in the above specified
points were incomparably less than in the full-grown birds. Some character-
istic 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 exacdy the same ]
proportions as in the adult stage. i

These facts are explained by the above two principles. Fanciers select I
their dogs, horses, pigeons, etc., for breeding, 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 i
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 in-
herited 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 characteristic 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 modi-
fied 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 inherited at a corresponding 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 modi-
fication, adapted in one descendant to act as hands, in another as paddles,

MUTUAL AFFINITIES OF ORGANIC BEINGS 327

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 produced 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 cases, the young
or embryo will closely resemble the mature parent-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 members of the great class of
insects. With respect to the final cause of the young in such groups not pass-
ing 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 indispensable for their
existence that they should be modified 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 Miiller 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 constructed 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 in-
heritance at corresponding ages, the young or the larvae might be rendered
by natural selection more and more different from their parents to any con-
ceivable 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 metamorphosis would be retrograde.

328 THE ORIGIN OF SPECIES

From the remarks just made we can see how by changes of structure in
the young, in conformity with changed habits of hfe, together with inheri-
tance at corresponding 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 curi-
ous case of Sitaris — a beetle which passes through certain unusual stages of
development — ^will illustrate how this might occur. The first larval form is
described by M. Fabre, as an active, minute insect, furnished with six legs,
two long antennae, 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 deposits 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
become 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, suc-
torial parasites, cirripedes, 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 re-
mote period an independent adult animal, resembling the Nauplius, existed,
and subsequently produced, along several divergent lines of descent, the
above-named great Crustacean groups. So again, it is probable, from what
we knov/ 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 swim-bladder, four fin-Hke
limbs, and a long tail, all fitted for an aquatic fife.

As all the organic beings, extinct and recent, which have ever lived, can
be arranged within a few great 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 arrange-
ment would be genealogical; descent being the hidden bond of connection
which naturalists have been seeking under the term of the Natural System.

MUTUAL AFFINITIES OF ORGANIC BEINGS 329

On this view we can understand how it is that, in the eyes of most naturaHsts,
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 embryonic development does not prove dis-
community 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, com-
munity of origin is often revealed by the structure of the larvae; we have
seen, for instance, that cirripedes, though externally so like shell-fish, are
at once known by their larvae 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 in their adult state the embr^^os of existing
species of the same class. Agassiz believes this to be a universal law of
nature; and we may hope hereafter to see the law proved true. It can, how-
ever, 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, incapable of demonstration. The law
will not strictly hold good in those cases in which an ancient form became
adapted in its larval state to some special line of life, and transmitted the
same larval state to a whole group of descendants; for such larvae will not
resemble any still more ancient form in its adult state.

Thus, as it seems to me, the leading facts in embryology, 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 having been inherited at a corresponding
period. Embryology rises greatly in interest, when we look at the embryo as
a picture, more or less obscured, of the progenitor, 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 name one of the higher animals in which some part or
other is not in a rudimentary condition. In the mammalia, for instance, the
male possesses rudimentary mammae; in snakes one lobe of the lungs is

330 THE ORIGIN OF SPECIES

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 foetal 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?

Rudimentary 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 rudi-
ments represent wings. Rudimentary organs sometimes retain their potential-
ity: this occasionally occurs with the mammae 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 some-
times 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 an hermaphrodite species, having of course a well-developed
pistil, the rudiment in the hybrid offspring was much increased in size; and
this clearly shows that the rudimentary 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 re-
marks, "has gills, and passes its existence in the water; but the Salamandra
atra, which lives high up among the mountains, brings forth its young full-
formed. This animal never lives in the water. Yet if we open a gravid female,
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 the 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 become rudimentary or utterly
aborted for one, even the more important purpose, and remain perfectly
efficient for the other. Thus, in plants, the office of the pistil is to allow
the pollen tubes to reach the ovules within the ovarium. The pistil consists
of a stigma supported on a style; but in some Compositae, 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 cer-
tain fishes the swim-bladder seems to be rudimentary for its proper function

MUTUAL AFFINITIES OF ORGANIC BEINGS 331

of giving buoyancy, 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 formerly, 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 inheritance, and relate to a former state of things. It is, how-
ever, often difficult to distinguish between rudimentary and nascent organs;
for we can judge only by analogy whether a part is capable of further de-
velopment, in which case alone it deserves to be called nascent. Organs in
this condition will always be somewhat 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 become 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, modified for a new function;
the wing of the Apteryx, on the other hand, is quite useless, and is truly
rudimentary. Owen considers the simple filamentary limbs of the Lepidosiren
as the "beginnings of organs which attain full functional development in
higher vertebrates"; but, according to the view lately advocated by Dr.
Giinther, they are probably remnants, consisting of the persistent axis of a
fin, with the lateral rays or branches aborted. The mamniary glands of the
Ornithorhynchus may be considered, in comparison with the udders of a
cow, as in a nascent condition. The ovigerous frena of certain cirripedes,
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 species, also, the extent to which the same organ has been reduced
occasionally differs much. This latter fact is well exemplified in the state
of the wings of female moths belonging to the same family. Rudimentary
organs 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 Scrophulariaceae 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 species 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 difTerent members of the
same class, nothing is more common, or, in order fully to understand the

332 THE ORIGIN OF SPECIES

relations of the parts, more useful than the discovery of rudiments. This
is well shown in the drawings given by Owen of the leg bones of the horse,
ox and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in the upper
jaws of whales and ruminants, can often be detected in the embryo, but
afterward wholly disappear. It is also, I believe, a universal rule, that a
rudimentary part is of greater size in the embryo relatively to the adjoining
parts, than in the adult; so that the organ at this early age is less rudimen-
tary, or even cannot be said to be in any degree rudimentary. Hence rudi-
mentary 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 reasoning power which tells us that most parts and organs are ex-
quisitely adapted for certain purposes, tells us with equal plainness that
these rudimentary or atrophied organs are imperfect and useless. In works
on natural history, rudimentary organs are generally said to have been cre-
ated "for the sake of symmetry," or in order "to complete the scheme of
nature." But this is not an explanation, merely a restatement of the fact. Nor
is it consistent 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
"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 be thought of an astronomer who main-
tained that the satellites revolve in elliptic courses round their planets "for
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 they serve to excrete matter in excess, or matter injurious
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 sub-
sequently absorbed, are beneficial to the rapidly growing 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 rudirrientary 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 rudimentary organs in our domestic productions, as the stump of a tail
in tailless breeds, the vestige of an ear in earless breeds of sheep — the re-
appearance of minute dangling horns in hornless breeds of cattle, more
especially, according to Youatt, in young animals — and the state of the whole
flower in the cauliflower. We often see rudiments of various parts in monsters ;
but I doubt whether any of these cases throw light on the origin of rudi-

MUTUAL AFFINITIES OF ORGANIC BEINGS 333

mentary 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.

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 reduction 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 efTected 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 functions. 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
disuse 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 inheritance 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 understand the greater size of rudimentary
organs in the embryo relatively to the adjoining parts, and their lesser
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 consequence 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 pro-
ducing any further effect after the organ has once been rendered function-
less. Some additional 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, rudimen-
tary, and would at last be wholly suppressed ; for the variations toward dimin-
ished size would no longer be checked by natural selection. The principle of

334 THE ORIGIN OF SPECIES

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 possible,
will perhaps come into play in rendering a useless part rudimentary. But
this principle will almost necessarily be confined to the earlier stages of the
process of reduction; for we cannot suppose that a minute papilla, for in-
stance, representing in a male flower the pistil of the female flower, and
formed merely of cellular tissue, would be further reduced or absorbed for
the sake of economizing nutriment.

Finally, as rudimentary organs, by whatever steps they may have been de-
graded into their present useless condition, are the record of a former state
of things, and have been retained solely through the power of inheritance —
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 desce.nt with modification, we may con-
clude 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 antici-
pated 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, as with rudimentary
organs, of no importance — the wide opposition in value between analogical
or adaptive characters, and characters of true affinity; and other such rules;
— all naturally follow if we admit the common parentage of allied forms, to-
gether with their modification 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, dimor-
phic 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 ele-
ment 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 dif-
ference marked by the terms, varieties, species, genera, families, orders, and
classes.

MUTUAL AFFINITIES OF ORGANIC BEINGS 335

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
organsj 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 individual embryo of the parts which are homologous,
and which when matured become widely different in structure and function ;
and the resemblance of the homologous parts or organs in allied though dis-
tinct species, though fitted in the adult state for habits as different as is pos-
sible. Larvae are active embryos, which have been specially modified in a
greater or less degree in relation to their habits of life, with their modifica-
tions 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
rudimentary organs in classification is ihtelligible, 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 it were unsupported by other facts or arguments.

CHAPTER XV

Recapitulation and Conclusion

Recapitulation of the Objections to the Theory of Natural Selection — ^Recapitulation
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.

As THIS whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.

That many and 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 varia-
tions, each good for the individual possessor. Nevertheless, this difficulty,
though appearing to our imagination insuperably great, cannot be consid-
ered 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 existence leading to the preservation of profitable deviations
of structure or instinct — and, lastly, that gradations 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 organic 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 species when first crossed,
which forms so remarkable a contrast with the almost universal fertility of
varieties when crossed, I must refer the reader to the recapitulation of the
facts given at the end of the ninth chapter, which seem to me conclusively
to show that this sterility is no more a special endowment than is the inca-
pacity of two distinct kinds of trees to be grafted together; but that it is inci-
dental on differences confined to the reproductive systems of the inter-crossed
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 consider-
ation of dimorphic and trimorphic plants clearly leads to the same conclu-

336

RECAPITULATION AND CONCLUSION 337

sion, 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 undoubted 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 authority 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 eliminate that sterility
which, judging from analogy, would have affected the parent-species if in-
tercrossed, 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 ap-
parently 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 is 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 individuals of the same variety, and between dis-
tinct 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 exposed to somewhat different conditions of life; for I
have ascertained by a laborious series of experiments that if all the individ-
uals of the same variety be subjected during several generations to the same
conditions, the good derived from crossing is often much diminished or
w^holly 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 con-
ditions, either perish, or if they survive, are rendered sterile, though retain-
ing perfect health. This does not occur, or only in a very slight degree, with
our domesticated 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 rendered 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 elephant or a fox will not breed under confine-
ment in its native country, whilst 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 question why two distinct species, when crossed, as
well as their hybrid offspring, are generally rendered more or less sterile.

338 THE ORIGIN OF SPECIES

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 are serious enough. All the individuals
of the same species, and all the species of the same genus, or even higher
group, are descended from common parents; and therefore, in however dis-
tant 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 diffusion of the same species; for during very long periods there will
always have been a good chance for wide migration by many means. A
broken or interrupted range may often be accounted for by the extinction of
the species in the intermediate regions. It cannot be denied that we are as
yet very ignorant as to the full extent of the various climatical and geographi-
cal changes which have affected the earth during modern periods; and such
changes will often have facilitated migration. As an example, I have at-
tempted 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 occasional means of transport.
With respect to distinct species 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.

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 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 existing
forms, we should remember that we have no right to expect (excepting in
rare cases) to discover directly connecting links between them, but only be-
tween each and some extinct and supplanted form. Even on a wide area,
which has during a long period remained continuous, and of which the cli-
matic and other conditions 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 intermediate
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

RECAPITULATION AND CONCLUSION 339

existing 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 supplanted and
exterminated.

On this doctrine of the extermination of an infinitude of connecting links,
between the living and extinct inhabitants of the world, and at each suc-
cessive period between the extinct and still older species, why is not every
geological formation charged with such links? Why does not every col-
lection of fossil remains afford plain evidence of the gradation and muta-
tion of the forms of life? Although geological research has undoubtedly re-
vealed the former existence of many links, bringing numerous forms of life
much closer together, it does not yield the infinitely many fine gradations
between 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 incal-
culably remote, long before the lowest bed of the Cambrian system was de-
posited, why do we not find beneath this system great piles of strata stored
with the remains 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 ex-
isted. The parent form of any two or more species would not be in all its
characters directly intermediate between its modified offspring, 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 recog-
nize 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 difTerences be ever so slight. Numerous existing doubtful
forms could be named which are probably varieties; but who will pretend
that in future ages so many fossil links will be discovered, that naturalists will
be able to decide 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 descend-
ants; and the periods during which species have undergone modification,

340 THE ORIGIN OF SPECIES

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 domi-
nant and widely ranging species which vary most frequently and vary most,
and varieties are often at first local — both causes rendering the discovery of
intermediate 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 when they have spread, and are discovered in a geological
formation, they appear as if suddenly created there, and will be simply classed
as new species. Most formations have been intermittent in their accumula-
tion, 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 formations
thick enough to resist future degradation can, as a general rule, be accumu-
lated 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 latter periods there will probably be
more variability in the forms of life; during periods of subsidence, more ex-
tinction.

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 enor-
mous 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 sufficient 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 ad-
vanced, I can only say, firstly, 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 of the constitution of the uni-
verse and of the interior of our globe to speculate with safety on its past du-
ration.

That the geological record is imperfect, all will admit; but that it is im-
perfect 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
clearly 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 explanations 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
questions on which we are confessedly ignorant; nor do we know how igno-

RECAPITULATION AND CONCLUSION 341

rant 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 Record. Serious as these sev-
eral objections are, in my judgment they are by no means sufficient to over-
throw the theory of descent with subsequent modification.

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 productions 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 conditions of. life remain the
same, we have reason to believe that a modification, which has already been
inherited for many generations, may continue to be inherited for an almost
infinite number of generations. On the other hand we have evidence that
variability, when it has once come into play, does not cease under domestica-
tion for a very long period; nor do we know that it ever ceases, for new
varieties are still occasionally produced by our oldest domesticated produc-
tions.

Variability is not actually caused by man ; he only unintentionally exposes
organic beings to new conditions of life, and then nature acts on the organiza-
tion 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 raay 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 selecting,
in each successive generation, individual differences so slight as to be inap-
preciable except by an educated eye. This unconscious process of selection
has been the great agency in the formation of the most distinct 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
individuals and races, during the constantly recurrent Struggle for Existence,
we see a powerful and ever-acting form of Selection. The struggle for ex-
istence inevitably follows from the high geometrical ratio of increase which
is common to all organic beings. This high rate of increase is proved by cal-
culation— by the rapid increase of many animals and plants during a suc-
cession of peculiar seasons, and when naturalized in new countries. More in-
dividuals are born than can possibly survive. A grain in the balance may

342 THE ORIGIN OF SPECIES

determine which individuals shall live, and which shall die — ^which variety
or species shall increase in number, and which shall decrease, or finally be-
come 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 certain individuals,
at any age or during any season, over those with which they come into com-
petition, or better adaptation in however slight a degree to the surrounding
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 possession of the females. The most vigorous males,
or those which have most successfully struggled with their conditions 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 advan-
tage will lead to victory.

As geology plainly proclaims that each land has undergone great physical
changes, we might have expected 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 charac-
ters 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, beside
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 varieties and sub-species and i
species. On separate continents, and on different parts of the same continent,
when divided by barriers of any kind, and on out-lying islands, what a mul-
titude of forms exist, which some experienced naturalists rank as varieties,
others as geographical 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 individual 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, acting 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

RECAPITULATION AND CONCLUSION 343

life. The theory of natural selection, even if we look no further than this,
seems to be in the highest degree probable. I have already recapitulated, as
fairly as I could, the opposed difficulties and objections: now let us turn to
the special facts and arguments in favor of the theory.

On the view that species are only strongly marked and permanent vari-
eties, and that each species first existed as a variety, we can see why it is that
no line of demarcation can be drawn between species, commonly supposed to
have been produced by special acts of creation, and varieties which are ac-
knowledged 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 pre-
sent many varieties; for where the manufactory of species has been active,
we might expect, as a general rule, to find it still in action; and this is the
case if varieties be incipient species. Moreover, the species of the 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 a larger genera 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 independently created, but are intelligible if
each existed first as a variety.

As each species tends by its geometrical rate of reproduction 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 selec-
tion to preserve the most divergent offspring of any one species. Hence, dur-
ing a long-continued course of modification, the slight differences character-
istic of varieties of the same species, tend to be augmented into the greater
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 domi-
nant. This tendency in the large groups to go on increasing in size and diverg-
ing in character, together with the inevitable contingency of much extinc-
tion, 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 grouping 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

344 THE ORIGIN OF SPECIES

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 saltum,"
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 dif-
ferent ways have to be adapted for the same general purpose. We can, in
short, see why nature is prodigal in variety, though niggard in innovation.
But why this should be a law of nature if each species has been independ-
ently 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, elegant patterns,
and other ornaments to the males, and 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 well as to our ears. Flowers and
fruit have been rendered conspicuous by brilliant colors in contrast with the
green foliage, in order that the flowers may be easily seen, visited and fer-
tilized by insects, and the seeds disseminated 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 ac-
quired, we do not know any more than how certain odors and flavors were
first rendered agreeable.

As natural selection acts by competition, it adapts and improves the in-
habitants 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 abhorrent to our ideas of fitness. We need not
marvel at the sting of the bee, when used against an enemy, causing the bee's
own death; at drones being produced in such great numbers for one single

RECAPITULATION AND CONCLUSION 345

act, and being then slaughtered by their sterile sisters; at the astonishing
waste of pollen by our fir-trees; at the instinctive hatred of the queen bee
for her own fertile daughters; at ichneumonidae feeding within the living
bodies of caterpillars; or at other such cases. The wonder, indeed, is, on the
theory of natural selection, that more cases of the want of absolute perfec-
tion have not been detected.

The complex and little known laws governing the production of varieties
are the same, as far as we can judge, with the laws which have governed the
production of distinct species. In both cases physical conditions seem to have
produced some direct and definite effect, but how much we cannot say. Thus,
when varieties 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 disuse 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 condi-
tion as in the domestic duck; or when we look at the burrowing tucu-tucu,
which is occasionally blind, and then at certain moles, which are habitually
blind and have their eyes covered with skin; or when we look at the blind
animals inhabiting the dark caves of America and Europe. 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 necessarily
modified. With both varieties and species, reversions to long-lost characters
occasionally occur. How inexplicable on the theory of creation is the occa-
sional 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 species of a genus, if the other species possess dif-
ferently 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 degree permanent, we can understand this fact; for they have
already varied since they branched off from a common progenitor in cer-
tain 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 inexplicable 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 importance to that species,
should be eminently liable to variation; but, on our view, this part has un-
dergone, since the several species branched ofT from a common progenitor,
an unusual amount of variability and modification, and therefore we might

346 THE ORIGIN OF SPECIES

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 subordi-
nate 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 diffi-
culty 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 different animals of the same
class with their severaL instincts. I have attempted to show how much light
the principle of gradation throws on the admirable architectural powers of
the hive-bee. Habit no doubt 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 i
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 tem-
perate 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 varieties, we can at once see
why their crossed offspring should follow the same complex laws in their
degrees and kinds of resemblance to their parents — in being absorbed into
each other by successive crosses, and in other such points — as do the
crossed offspring of acknowledged varieties. This similarity would be a
strange fact, if species 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, after equal intervals of
time, is widely different in different groups. The extinction of species and of
whole groups of species, which has played so conspicuous a part in the his-
tory of the organic world, almost inevitably 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
ordiriary 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 simultane-
ously throughout the world. The fact of the fossil remains of each formation
being in some degree intermediate in character between the fossils in the
formations above and below, is simply explained by their intermediate posi-
tion 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 ex-

RECAPITULATION AND CONCLUSION 347

tinct being the offspring of common parents. As species have generally di-
verged in character 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 be-
tween existing groups. Recent forms are generally looked upon as being, 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 con-
quered the older and less improved forms in the struggle for life; they have
also generally had their organs more specialized for different functions. This
fact is perfectly compatible with numerous beings still retaining simple and
but little improved structures, fitted for simple conditions of life; it is like-
wise 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 marsupials 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 dur-
ing the long course of ages much migration from one part of the world to
another, owing to former climatical and geographical changes and to the
many occasional and unknown means of dispersal, then we can understand,
on the theory of descent with modification, most of the great leading facts in
distribution. We can see why there should be so striking a parallelism in the
distribution of organic beings throughout space, and in their geological suc-
cession 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
conditions, under heat and cold, on mountain and lowland, on deserts and
marshes, most of the inhabitants within each great class are plainly related;
for they are the descendants of the same progenitors and early colonists. On
this same principle of former migration, combined in most cases with modi-
fication, we can understand, by the aid of the Glacial period, the identity of
some few plants, and the close alliance of many others, on the most distant
mountains, and in the northern and southern temperate 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 intertropical
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 com-
pletely sundered from each other; for as the relation of organism to organ-
ism 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 this view of migration, with subsequent modification^ we see why

348 THE ORIGIN OF SPECIES

oceanic islands are inhabited by only few species, but of these, why many are
peculiar or endemic 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 in-
dependent acts of creation.

The existence of closely allied representative species in any two areas, im-
plies, on the theory of descent with modification, that the same parent forms
formerly inhabited both areas : and we almost invariably find that wherever
many closely allied species inhabit two areas, some identical species are still
common to both. Wherever many closely allied yet distinct species occur,
doubtful 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 ani-
mals of the Galapagos Archipelago, of Juan Fernandez, and of the other
American islands, to the plants and animals of those of the mainland; and
of those of the Cape de Verde Archipelago, and of the other African islands
to the African mainland. It must be admitted that these facts receive no ex-
planation on the theory of creation.

The fact, as we have seen, that all past and present organic beings can be ar-
ranged within a few great classes, in groups subordinate to groups, and with the
extinct groups often falling in between the recent groups, is intelligible on the
theory of natural selection with its contingencies of extinction and divergence
of character. On these same principles we see how it is that the mutual affini-
ties 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 classification; why characters derived from rudi-
mentary parts, though of no service to the beings, are often of high classifi-
catory value; and why embryological characters are often the most valuable
of all. The real affinities of all organic beings, in contra-distinction to their
adaptive resemblances, are due to inheritance or community of descent. The
Natural System is a genealogical arrangement 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 however 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 descent 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

RECAPITULATION AND CONCLUSION 349

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 inherited 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 habits or conditions of life ; and
we can understand on this view the meaning of rudimentary organs. But dis-
use 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 in-
stance, has inherited teeth, which never cut through the gums of the upper
jaw, from an early progenitor having well-developed teeth; and we may be-
lieve that the teeth in the mature animal were formerly reduced 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 inheritance 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 modi-
fication, by means of rudimentary organs, of embryological and homologous
structures, but we are too blind to understand her meaning.

I have now recapitulated the facts and considerations which have thor-
oughly convinced me that species have been modified, during a long course of
descent. This has been effected chiefly through the natural selection of
numerous successive, slight, favorable variations ; aided in an important man-
ner by the inherited effects of the use and disuse of parts; and in an unim-
portant 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 independently 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 con-

350 THE ORIGIN OF SPECIES

vinced 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 mis-
representation; but the history of science shows that fortunately this power r
does not long endure.

It can hardly be supposed that a false theory would explain, in so satisfac-
tory 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
undulatory 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
on the far higher problem of the essence or origin of life. Who can explain
what is the essence of the attraction of gravity? No one now objects to fol-
lowing out the results consequent on this unknown element of attraction;
notwithstanding that Leibnitz formerly accused Newton of introducing "oc-
cult qualities and miracles into philosophy."

I see no good reasons why the views given in this volume should shock the
religious feelings of any one. It is satisfactory, as showing how transient such
impressions are, to remember that the greatest discovery ever made by man,
namely, the law of the attraction of gravity, was also attacked by Leibnitz,
"as subversive of natural, and inferentially of revealed, religion." A cele-
brated 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 disbelieve 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 variation in the course of long ages
is a limited quantity; no clear distinction has been, or can be, drawn be-
tween species and well-marked varieties. It cannot be maintained that species
when intercrossed are invariably sterile and varieties invariably fertile; or
that sterility is a special endowment and sign of creation. The belief that
species were immutable productions was almost unavoidable as long as the
history of the world was thought to be of short duration; and now that we
have acquired some idea of the lapse of time, we are too apt to assume, with-
out 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 still see at work. The mind cannot possibly grasp the full

RECAPITULATION AND CONCLUSION 351

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 in-
finite number of generations.

Although I am fully convinced of the truth of the views given in this vol-
ume under the form of an abstract, I by no means expect to convince ex-
perienced 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 ex-
planation when we only restate a fact. Any one whose disposition leads him
to attach more weight to unexplained difficulties than to the explanation of
a certain number of facts will certainly reject the theory. A few naturalists,
endowed with much flexibility of mind, and who have already 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 conscientiously ex-
pressing his conviction; for thus only can the load of prejudice by which this
subject is overwhelmed be removed.

Several eminent naturalists have of late published their belief that a mul-
titude 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 conse-
quently 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 varia-
tion as a vera causa in one case, they arbitrarily reject it in another, without
assigning any distinction in the two cases. The day will come when this will
be given as a curious illustration of the blindness of preconceived opinion.
These authors seem no more startled at a miraculous act of creation than at
an ordinary birth. But do they really believe that at innumerable periods in
the earth's history certain elemental atoms have been commanded suddenly
to flash into living tissues ? Do they believe that at each supposed act of crea-
tion one individual or many were produced? Were all the infinitely numerous
kinds of animals and plants created as eggs or seed, or as full grown? and in
the case of mammals, were they created bearing the false marks of nourish-
ment 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 be-
ings as of one; but Maupertuis' philosophical axiom of "least action" leads
the mind more willingly to admit the smaller number ; and certainly we ought

352 THE ORIGIN OF SPECIES

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 retained 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 cen-
sured for having thus expressed myself. But undoubtedly 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 ambiguously
that it was not easy to understand their meaning. Now, things are wholly
changed, and almost every 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 investigation, but little advantage is gained by be-
lieving 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 com-
munity of descent become fewer in number and less in force. But some argu-
ments 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 some-
times 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 impHes
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. I believe 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 ani-
mals 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 J
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

RECAPITULATION AND CONCLUSION 353

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 intermediate in character that naturalists have disputed to which king-
dom they should be referred. As Professor Asa Gray has remarked, "the
spores and other reproductive bodies of many of the lower algse may claim
to have first a characteristically animal, and then an unequivocally vegetable
existence." Therefore, on the principle of natural selection with divergence
of character, it does not seem incredible, that, from some such low and inter-
mediate 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 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, homologous, and rudimentary structures,
that within each kingdom all the members are descended from a single pro-
genitor.

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 his-
tory. 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 irue species. This, I feel sure, and I speak after experience, will
be no slight relief. The endless disputes whether or not some fifty species of
British brambles are 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 differences be sufficiently important to deserve a specific name.
This latter point will become a far more essential consideration than it is at
present; for differences, however slight, between any two forms, if not
blended by intermediate gradations, are looked at by most naturalists as suffi-
cient to raise both forms to the rank of species.

Hereafter, we shall be compelled to acknowledge that the only distinction
between species and well-marked varieties is, that the latter are known or
believed to be connected at the present day by intermediate gradations,
whereas species were formerly thus connected. Hence, without 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 scientific and common language
will come into accordance. In short, we shall have to treat species in the same
manner as those naturalists treat genera, who admit that genera are merely

354 THE ORIGIN OF SPECIES

artificial combinations made for convenience. This may not be a cheering
prospect; but we shall at least be freed from the vain search for the undis-
covered 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, com-
munity 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 contrivances, each useful to the possessor, in the same way 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 become !

A grand and almost untrodden field of inquiry will be opened, on the
causes and laws of variation, on correlation, on the effects of use and disuse,
on the direct action of external conditions, and so forth. The study of domes-
tic 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 classifica-
tions 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 pedigree or armorial bearings; and we have to discover and trace
the many diverging lines of descent in our natural genealogies, by characters
of any kind which have long been inherited. Rudimentary organs will speak
infallibly with respect to the nature of long-lost structures. Species and
groups of species which are called aberrant, and which may fancifully be
called living fossils, will aid us in forming a picture of the ancient forms of
life. Embryology will often reveal to us the structure, in some degree ob-
scured, of the prototypes of each great class.

When we can feel assured that all the individuals of the same species, and
all the closely allied species of most genera, have, within a not very remote
period, descended from one parent, and have migrated from some one birth-
place; and when we better know the many means of migration, then, by the
light which geology now throws, and will continue to throw, on former
changes of climate and of the level of the land, we shall surely be enabled to
trace in an admirable manner the former migrations of the inhabitants of the
whole world. Even at present, by comparing the differences between the in-
habitants 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 extreme imperfection
of the record. The crust of the earth, with its embedded remains, must not

RECAPITULATION AND CONCLUSION 355

be looked at as a well-filled museum, but as a poor collection made at hazard
and at rare intervals. The accumulation of each great fossiliferous formation
will be recognized as having depended on an unusual occurrence of favor-
able circumstances, and the blank intervals between the successive stages as
having been of vast duration. But we shall be able to gauge with some
security the duration of these intervals by a comparison of the preceding
and succeeding organic forms. We must be cautious in attempting to correlate
as strictly contemporaneous two formations, which do not include many
identical species, by the general succession of the forms of life. As species
are produced and exterminated by slowly acting and still existing causes,
and not by miraculous acts of creation; and as the most important of all
causes of organic change is one which is almost independent of altered and
perhaps suddenly altered physical conditions, namely, the mutual relation
of organism to organism — the improvement of one organism entailing the
improvement or the extermination of others; it follows, that the amount of
organic change in the fossils of consecutive formations probably serves as a
fair measure of the relative, though not actual lapse of time. A number of
species, however, keeping in a body might remain for a long period un-
changed, w^hile within the same period, several of these species, by migrating
into new countries and coming into competition with foreign associates,
might become modified ; so that we mixst not overrate the accuracy of organic
change as a measure of time.

In the future I see open fields for far more important researches. Psy-
chology 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 satisfied with the view
that each species has been independently created. To my mind it accords^
better with what we know of the laws impressed on matter by the Creator,
that the production and extinction of the past and present inhabitants of the
world should have been due to secondary causes, like those determining the
birth and death of the individual. When I view all beings not as special
creations, but as the lineal descendants of some few beings which lived long
before the first bed of the Cambrian system was deposited, they seem to me
to become ennobled. Judging from the past, we may safely infer that not one
living species will transmit its unaltered likeness to a distinct futurity. And of
the species now living, very few will transmit progeny of any kind to a far
distant futurity; for the manner in which all organic beings are grouped
shows that the greater number of species in each genus, and all the species in
many genera, have left no descendants, but have become utterly extinct. We
can so far take a prophetic glance into futurity as to foretell that it will be
the common and widely spread 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

356 THE ORIGIN OF SPECIES

that the ordinary succession by generation has never once been broken, and
that no cataclysm has desolated the whole world. Hence, we may look with
some confidence to a secure future of 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 de-
pendent upon each other in so complex a manner, have all been produced
by laws acting around us. These laws, taken in the largest sense, being
Growth with reproduction; Inheritance which is almost implied by reproduc-
tion; 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 Life, and as a consequence to Natural Selection, entailing
Divergence of Character and the Extinction of less improved forms. Thus,
from the war of nature, from famine and death, the most exalted object
which we are capable of conceiving, namely, the production of the higher
animals, directly follows. There is grandeur in this view of life with its
several powers, having been originally breathed by the Creator into a few
forms or into one; and that, while this planet has gone circling on according
to the fixed law of gravity, from so simple a beginning endless forms most
beautiful and most wonderful have been, and are being evolved.

APPENDIX

AN HISTORICAL SKETCH
OF THE PROGRESS OF OPINION ON THE ORIGIN OF SPECIES

PREVIOUSLY TO THE PUBLICATION OF THE
FIRST EDITION OF THIS WORK

I WILL here give a brief sketch of the progress of opinion on the Origin of
Species. Until recently the great majority of naturalists believed that species
were immutable productions, and had been separately created. This view
has been ably maintained by many authors. Some few naturalists, on the
other hand, have believed that species undergo modification, and that the
existing forms of life are the descendants by true generation of pre-existing
forms. Passing over allusions to the subject in the classical writers,"^ the first
author who in modern times has treated it in a scientific spirit was Buffon.
But as his opinions fluctuated greatly at different periods, and as he does
not enter on the causes 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 "Philosophic Zoologique,"
and subsequently, 18 15, in the Introduction to his "Hist. Nat. des Animaux
sans Vertebres." In these works he upholds the doctrine that all species,
including man, are descended from other species. He first did the eminent
service of arousing attention to the probability of all change in the organic,
as well as in the inorganic world, being the result of law, and not of miracu-
lous 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 gradations of forms in certain groups,
and by the analogy of domestic productions. With respect to the means of
modification, he attributed something to the direct action of the physical

*AristotIe, in his "Physicae Auscultationes" (lib. 2, cap. 8, s. 2), after remarking
that rain does not fall in order to make the corn grow, any more than it falls to spoil
the fanner's corn when threshed out of doors, applies the same argument to organ-
ization; and adds (as translated by Mr. Clair Grece, who first pointed out the passage
to me), "So what hinders the different parts [of the body] from having this merely
accidental relation in nature? as the teeth, for example, grow by necessity, the front
ones sharp, adapted for dividing, and the grinders flat, and serviceable for masticating
the food ; since they were not made for the sake of this, but it was the result of acci-
dent. And in like manner as to other parts in which there appears to exist an adapta-
tion to an end. Wheresoever, therefore, all things together (that is, all the parts of
one whole) happened like as if they were made for the sake of something, these were
preserved, having been appropriately constituted by an internal spontaneity ; and what-
soever things were not thus constituted, perished and still perish." We here see the
principle of natural selection shadowed forth, but how little Aristotle fully compre-
hended the principle, is shown by his remarks on the formation of the teeth.

357

358 APPENDIX

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 like-
wise believed in a law of progressive development; and as all the forms of
life thus tend to progress, in order to account for the existence at the present
day of simple productions, he maintains that such forms are now spontane-
ously generated.*

Geoffroy Saint-Hilaire, as is stated in his "Life," written by his son, sus-
pected, as early as 1 795, that what we call species are various degenerations
of the same type. It was not until 1828 that he published his conviction
that the same forms have not been perpetuated since the origin of all things.
Geoffroy seems to have relied chiefly on the conditions of life, or the "monde
ambiant" as the cause of change. He was cautious in drawing conclusions,
and did not believe that existing species are now undergoing modification;
and, as his son adds, "C'est done un probleme a reserver entierement a
I'avenir, suppose meme que I'avenir doive avoir prise sur lui."

In 18 13 Dr. W. C. Wells read before the Royal Society "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 18 18. In this paper he distinctly 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 certain
characters alone. After remarking that negroes and mulattoes enjoy an im-
munity from certain tropical diseases, he observes, firstly, that all animals
tend to vary in some degree, and, secondly, that agriculturists improve their
domesticated animals by selection; and then, he adds, but what is done in
this latter case "by art, seems to be done with equal efficacy, though
more slowly, by nature, in the formation of varieties of mankind, fitted for
the country which they inhabit. Of the accidental varieties of man, which
would occur among the first few and scattered inhabitants of the middle
regions of Africa, some one would be better fitted than others to bear the
diseases of the country. This race would consequently multiply, while the
others would decrease; not only from their inability to sustain the attacks

*I have taken the date of the first publication of Lamarck from Isidore Geoffroy
Saint-Hilaire's ("Hist. Nat. Generale," tom. ii. p. 405, 1859) excellent history of
opinion on this subject. In this work a full account is given of BufTon's conclusions
on the same subject. It is curious how largely my grandfather, Dr. Erasmus Darwin,
anticipated the views and erroneous grounds of opinion of Lamarck in his "Zoonomia"
(vol. i, pp. 509-510), published in 1794. According to Isid. Geoffroy there is no
doubt that Goethe was an extreme partisan of similar views, as shown in the introduc-
tion 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 question for naturalists will be how, for instance, cattle got their
horns, and not for what they are used. It is rather a singular instance of the manner
in which similar views arise at about the same time, that Goethe in Germany, Dr.
Darwin in England, and Geoffroy Saint-Hilaire (as we shall immediately see) in
France, came to the same conclusion on the origin of species, in the years i794~95'

APPENDIX 359

of disease, but from their incapacity of contending with their more vigorous
neighbors. The color of this vigorous race I take for granted, from what
has been aheady said, would be dark. But the same disposition to form
varieties still existing, a darker and a darker race would in the course of
time occur, and as the darkest would be the best fitted for the climate, this
would at length become the most prevalent, if not the only race, in the
particular country in which it had originated." He then extends these same
views to the white inhabitants of colder climates. I am indebted to Mr.
Rowley, of the United States, for having called my attention, through Mr.
Brace, to the above passage of Dr. Wells's work.

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 "Amaryllidacese" (1837, PP- ^9? 339)? declares that "horticultural
experiments have established, beyond the possibility of refutation, that
botanical species are only a higher and more permanent class of varieties."
He extends the same view to animals. The dean believes that single species
of each genus were created in an originally highly plastic condition, and
that these have produced, chiefly by intercrossing, but likewise by variation,
all our existing species.

In 1826 Professor Grant, in the concluding paragraph in his well-known
paper ("Edinburgh Philosophical Journal," vol. xiv. p. 283) on the Spon-
gilla, clearly declares his belief that species are descended from other
species, and that they become improved in the course of modification. This
same view was given in his Fifty-fifth Lecture, published in the "Lancet"
in 1834.

In 1 83 1 Mr. Patrick Matthew published his work on "Naval Timber and
Arboriculture," in which he gives precisely the same view on the origin of
species as that (presently to be alluded to) propounded by Mr. Wallace
and myself in the "Linnean Journal," and as that enlarged in the present
volume. Unfortunately the view was given by Mr. Matthew very briefly in
scattered passages, in an appendix to a work on a diff"erent subject, so that
it remained unnoticed until Mr, Matthew himself drew attention to it in
the "Gardeners' Chronicle," on April 7, i860. The differences of Mr.
Matthew's views from mine are not of much importance: he seems to con-
sider that the world was nearly depopulated at successive periods, and then
restocked; and he gives as an alternative, that new forms may be generated
"without the presence of any mould or germ of former aggregates." I am
not sure that I understand some passages; but it seems that he attributes
much influence to the direct action of the conditions of life. He clearly saw,
however, the full force of the principle of natural selection.

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

Rafinesque, in his "New Flora of North America," published in 1836,
wrote (p. 6) as follows: "All species might have been vaiieties once, and

360 APPENDIX

many varieties are gradually becoming species by assuming constant and
peculiar characters"; but further on (p. i8), he adds, "except the original
types or ancestors of the genus."

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

The "Vestiges of Creation" appeared in 1844. In the tenth and much
improved edition (1853) the anonymous author says (p. 155) : "The
proposition determined on after much consideration is, that the several
series of animated beings, from the simplest and oldest up to the highest
and most recent, are, under the providence of God, the results, first, of an
impulse which has been imparted to the forms of life, advancing them in
definite times, by generation, through grades of organization terminating
in the highest dicotyledons and vertebrata, these grades being few in num-
ber, and generally marked by intervals of organic character, which we find
to be a practical difficulty in ascertaining affinities; second, of another im-
pulse 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 conditions of life are gradual. He argues with much force on general
grounds that species are not immutable productions. But I cannot see how
the two supposed "impulses" account in a scientific sense for the numerous
and beautiful coadaptations which we see throughout nature; I cannot see
that we thus gain any insight how, for instance, a woodpecker has become
adapted to its peculiar habits of life. The work, from its powerful and
brilliant style, though displaying in the early editions little accurate knowl-
edge and a great want of scientific caution, immediately had a very wide
circulation. In my opinion it has done excellent service in this country in
calling attention to the subject, in removing prejudice, and in thus 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 ("Bulletins de I'Acad. Roy. Bruxelles," torn,
xiii. p. 581) his opinion that it is more probable that new species have been
produced by descent with modification than that they have been separately
created: the author first promulgated this opinion in 1831.

Professor Owen, in 1849 ("Nature of Limbs," p. 86), wrote as follows:
"The archetypal idea was manifested in the flesh under divers such modi-
fications, upon this planet, long prior to the existence of those animal
species that actually exemplify it. To what natural laws or secondary
causes the orderly succession and progression of such organic phenomena
may have been committed, we, as yet, are ignorant." In his address to the
British Association, in 1858, he speaks (p. li) of "the axiom of the continu-
ous operation of creative power, or of the ordained becoming of living

APPENDIX 361

things." Further on (p. xc), after referring to geographical distribution, he
adds, "These phenomena shake our confidence in the conclusion that the
Apteryx of New Zealand and the Red Grouse of England were distinct
creations in and for those islands respectively. Always, also, it may be well
to bear in mind that by the word 'creation' the zoologist means 'a process he
knows not what.' " He amplifies this idea by adding that when such cases
as that of the Red Grouse are "enumerated by the 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 exclu-
sively; signifying also, by this mode of expressing such ignorance, his belief
that both the bird and the islands owed their origin to a great first Creative
Cause." If we interpret these sentences given in the same address, one by
the other, it appears that this eminent philosopher felt in 1858 his con-
fidence shaken that the Apteryx and the Red Grouse first appeared in their
respective homes "he knew not how," or by some process "he knew not
what."

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

M. Isidore Geoff roy Saint-Hilaire, in his lectures delivered in 1850 (of
which a resume appeared in the "Revue 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 :

362 APPENDIX

ils se modifient, si les circonstances ambiantes viennent a changer." "En
resume, Vobservation des animaux sauvages demontre deja la variabilite
limitee des especes. Les experiences sur les animaux sauvages devenus
domestiques, et sur les animaux domestiques redevenus sauvages, la de-
montrent plus clairement encore. Ces memes experiences prouvent, de plus,
que les differences produites peuvent etre de valeur generique." In his
"Hist. Nat. Generale" (torn. ii. p. 430, 1859) he amplifies analogous con-
clusions.

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

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

In 1852 M. Naudin, a distinguished botanist, expressly stated, in an ad-
mirable paper on the Origin of Species ("Revue Horticole," p. 102; since
partly republished in the "Nouvelles Archives du Museum," tom. i. p. 171),
his belief that species are formed in an analogous manner as varieties are
under cultivation; and the latter process he attributes to man's power of
selection. But he does not show how selection acts under nature. He be-
lieves, 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 mysterieuse, indeterminee; fatalite pour les uns; pour les autres
volonte providentielle, dont Taction incessante sur les etres vivantes deter-
mine, a toutes les epoques de 1' existence du monde, la forme, le volume, et
la duree de chacun d'eux, en raison de sa destinee dans I'ordre de choses
dont il fait partie. G'est cette puissance qui harmonise chaque membre a
I'ensemble, en I'appropriant a la fonction qu'il doit remplir dans I'organisme
generale de la nature, fonction qui est pour lui sa raison d'etre."*

*From references in Bronn's "Untersuchungen iiber die Entwickelungs-Gesetze,"
it appears that the celebrated botanist and palaeontologist Unger published, in 1852,
his belief that species undergo development and modification. Dalton, likewise, in
Pander and Dalton's work on Fossil Sloths, expressed, in 182 1, a similar belief. Similar
views have, as is well known, been maintained by Oken in his mystical "Natur-
Philosophie." From other references in Godron's work "Sur I'Espece," it seems that

APPENDIX 363

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

In this same year, 1853, Dr. Schaaffhausen published an excellent
pamphlet ("Verhand. des Naturhist. Vereins der Preuss. Rheinlands," 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 destruc-
tion of intermediate graduated forms. "Thus living plants and animals are
not separated from the extinct by new creations, but are to be regarded as
their descendants through continued reproduction."

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

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

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

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

In June, 1859, Professor Huxley gave a lecture before the Royal Institu-
tion on the "Persistent Types of Animal Life." Referring to such cases, he
remarks, "It is difficult to comprehend the meaning of such facts as these,
if we suppose that each species of animal and plant, or each great type of
organization, was formed and placed upon the surface of the globe at long
intervals by a distinct act of creative power; and it is well to recollect that

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.

364 APPENDIX

such an assumption is as unsupported by tradition or revelation as it is
opposed to the general analogy of nature. If, on the other hand, we view
Tersistent Types' in relation to that hypothesis which supposes the species
living at any time to be the result of the gradual modification of pre-exist-
ing 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 modi-
fication which living beings have undergone during geological time is but
very small in relation to the whole series of changes which they have
suffered."

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

The first edition of this work was published on November 24, 1859, and
the second edition on January 7, i860.

GLOSSARY

OF THE

PRINCIPAL SCIENTIFIC TERMS USED IN THE
PRESENT VOLUMEi

Aberrant. — Forms or groups of animals or plants which deviate in important char-
acters from their nearest allies, so as not to be easily included in the same group
with them, are said to be aberrant.

Aberration (in Optics) . — In the refraction of light by a convex lens the rays passing
through different parts of the lens are brought to a focus at slightly different
distances — this is called spherical aberration; at the same time the colored rays
are separated by the prismatic action of the lens and likewise brought to a focus
at different distances — this is chromatic aberration.

Abnormal. — Contrary to the general rule.

Aborted. — ^An organ is said to be aborted, when its development has been arrested
at a very early stage.

Albinism. — ^Albinos are animals in which the usual coloring matters characteristic
of the species have not been produced in the skin and its appendages. Albinism
is the state of being an Albino.

ALGi^.^ — ^A class of plants including the ordinary sea-weeds and the filamentous fresh-
water weeds.

Alternation of Generations. — ^This term is applied to a peculiar mode of repro-
duction 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 com-
plicated patterns at their junction with the outer wall of the shell.

Analogy. — ^The resemblance of structures which depends upon similarity of func-
tion, 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 earth-
worms and the leeches.

Antenna. — Jointed organs appended to the head in Insects, Crustacea and Centi-
pedes, and not belonging to the mouth.

Anthers. — The summits of the stamens of flowers, in which the pollen or fertilizing
dust is produced.

Aplagentalia, Aplacentata or Aplagental Mammals. See Mammalia.

^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.

365

366 GLOSSARY

Archetypal. — Of or belonging to the Archetype, or ideal primitive form upon
which all the beings of a group seem to be organized.

Artigulata. — ^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, Crusta-
ceans and Centipedes).

Asymmetrical. — Having the two sides unlike.

Atrophied. — Arrested in development at a very early stage.

Balanus. — The genus including the common Acorn-shells which live in abundance

on the rocks of the seacoast.
Batrachl\ns. — ^A class of animals allied to the Reptiles, but undergoing a peculiar

metamorphosis, in which the young animal is generally aquatic and breathes by

gills. {Examples, Frogs, Toads, and Newts.)
Bowlders. — Large transported blocks of stone generally embedded in clays or

gravels.
Brachiopoda. — A class of marine Mollusca, or soft-bodied animals, furnished with a

bivalve shell, attached to submarine objects by a stalk which passes through an

aperture in one of the valves, and furnished with fringed arms, by the action

of which food is carried to the mouth.
Branchi^. — Gills or organs for respiration in water.
Branchial. — Pertaining to gills or branchiae.

Cambrian System. — ^A series of very ancient Palaeozoic rocks, between the Lau-
rentian and the Silurian. Until recently these were regarded as the oldest
fossiliferous rocks.

Canid/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 Cirripedes.

Carboniferous. — This term is applied to the great formation which includes,
among other rocks, the coal-measures. It belongs to the oldest, or Palaeozoic,
system of formations.

Caudal. — Of or belonging to the tail.

Cephalopods. — The highest class of the Mollusca, or soft-bodied animals, char-
acterized by having the mouth surrounded by a greater or less number of fleshy
arms or tentacles, which, in most living species are furnished with sucking-cups.
{Examples, Cuttle-fish, Nautilus.)

Cetacea. — An order of Mammalia, including the Whales, Dolphins, etc., having the
form of the body fish-like, the skin naked, and only the fore limbs developed.

Chelonia. — ^An order of Reptiles including the Turtles, Tortoises, etc.

Cirripedes. — An order of Crustaceans including the Barnacles and Acorn-shells.
Their young resemble those of many other Crustaceans in form; but when
mature they are always attached to other objects, either directly or by means of
a stalk, and their bodies are enclosed by a calcareous shell composed of several
pieces, two of which can open to give issue to a bunch of curled, jointed
tentacles, which represent the limbs.

Coccus. — The genus of Insects including the Cochineal. In these the male is a
minute, winged fly, and the female generally a motionless, berry-like mass.

Cocoon. — A case usually of silky material, in which insects are frequently enveloped
during the second or resting-stage (pupa) of their existence. The term "cocoon-
stage" is here used as equivalent to "pupa-stage."

GLOSSARY 367

CCELOSPERMOUS. — ^A term applied to those fruits of the Umbelliferae which have

the seed hollowed on the inner face.
CoLEOPTERA. — Bectlcs, 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,
CoMPOsiT/E or CoMPOsiTOus Plants. — Plants in which the inflorescence consists

of numerous small flowers (florets) brought together into a dense head, the base

of which is enclosed by a common envelope. {Examples, the Daisy, Dandelions,

etc.)
Conferva. — The filamentous weeds of fresh water.
Conglomerate. — ^A rock made up of fragments of rock or pebbles, cemented

together by some other material.
Corolla. — The second envelope of a flower usually composed of 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 phenomenon, character, etc., with

another.
Corymb. — A bunch of flowers in which those springing from the lower part of the

flower-stalks 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 Palaeozoic 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 parts or organs which in
simpler forms of life are more or less united.

Dimorphic. — Having two distinct forms — Dimorphism is the condition of the ap-
pearance of the same species under two dissimilar forms.

DiCECious. — 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. )

368 GLOSSARY

Elytra. — The hardened fore-wings of Beetles, serving as sheaths for the mem-
braneous hind-wings, which constitute the true organs of flight.

Embryo. — The young animal undergoing development within the egg or womb.

Embryology. — The study of the development of the embryo.

Endemic. — Peculiar to a given locality.

Entomostraca. — ^A division of the class Crustacea, having all the segments of the
body usually distinct, gills attached to the feet or organs of the mouth, and the
feet fringed with fine hairs. They are generally of small size.

Eocene. — The earliest of the three divisions of the Tertiary epoch of geologists.
Rocks of this age contain a small proportion of shells identical with species now
living.

Ephemerous Insects. — Insects allied to the May-fly.

Fauna. — ^The totality of the animals naturally inhabiting a certain country Or region,
or which have lived during a given geological period.

Feled^. — The Cat-family.

Feral. — Having become wild from a state of cultivation or domestication.

Flora. — The totality of the plants growing naturally in a country, or during a given
geological period.

Florets. — Flowers imperfectly developed in some respects, and collected into a
dense spike or head, as in the Grasses, the Dandelion, etc.

Fcetal. — Of or belonging to the foetus, or embryo in course of development.

FoRAMiNiFERA. — ^A class of animals of very low organization 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, Toad-
stools, 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.

Callus. — ^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.

Glacial Period. — ^A period of great cold and of enormous extension of ice upon the
surface of the earth. It is believed that glacial periods have occurred repeatedly
during the geological history of the earth, but the term is generally applied to
the close of the Tertiary epoch, when nearly the whole of Europe was sub-
jected to an arctic climate. •

Gland. — An organ which secretes or separates some peculiar product from the
blood or sap of animals or plants.

GLOSSARY 369

Glottis. — ^The opening of the windpipe into the oesophagus or gullet.

Gneiss. — A rock approaching granite in composition, but more or less laminated,
and really produced by the alteration of a sedimentary deposit after its con-
solidation.

Grallatores. — The so-called Wading-birds (Storks, Cranes, Snipes, etc.), which
are generally furnished with long legs, bare of feathers above the heel, and
have no membranes between the toes.

Granite. — A rock consisting essentially of 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, characterized by the possession of a
jointed beak or rostrum, and by having the fore-wings horny in the basal portion
and membraneous at the extremity, where they cross each other. This group
includes the various species of Bugs.

Hermaphrodite. Possessing the organs of both sexes.

Homology. — The relation between parts which results from their development from
corresponding embryonic parts, either in different animals, as in the case of the
arm of man, the 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 centi-
pede, 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 different plants the parts
of the flower are homologous, and in general these parts are regarded as homol-
ogous 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 membraneous 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 mem-
braneous wings in which there are a few veins. Bees and Wasps are familiar*
examples of this group.

Hypertrophied. — Excessively developed.

IcHNEUMONiD/^. — ^A family of Hymenopterous insects, the members of which lay
their eggs in the bodies or eggs of other insects.

Imago. — The perfect (generally winged) reproductive state of an insect.

Indigens. — The aboriginal animal or vegetable inhabitants of a country or region.

Inflorescence. — The mode of arrangement of the flowers of plants.

Infusoria. — A class of microscopic Animalcules, so called from their having
originally been observed in infusions of vegetable matters. They consist of a
gelatinous material enclosed in a delicate membrane, the whole or part of
which is furnished with short vibrating hairs ( called cilia ) , by means of which
the animalcules swim through the water or convey the minute particles of their
food to the orifice of the mouth.

Insectivorous. — Feeding on Insects.

Invertebrata or Invertebrate Animals. — Those animals which do not possess
a backbone or spinal column.

370 GLOSSARY

Lacuna. — 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. Larv.^). — 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. Lawrence, whence the name. It is in these
that the earliest known traces of organic bodies have been found.

Leguminos/E. — An order of plants represented by the common Pease and Beans,
having an irregular flower in which one petal stands up like a wing, and the
stamens and pistil are enclosed in a sheath formed by two other petals. The
fruit is a pod (or legume).

Lemurjd^. — A group of four-handed animals, distinct from the Monkeys, and ap-
proaching the Insectivorous Quadrupeds in some of their characters and habits.
Its members have the nostrils curved or twisted, and a claw instead of a nail
upon the first finger of the hind hands.

LEProoPTERA. — ^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
Butterflies and Moths.

Littoral. — Inhabiting the seashore.

Loess. — A marly deposit of recent (Post-Tertiary) date, which occupies a great part
of the valley of the Rhine.

Malacostraca. — The higher division of the Crustacea, including the ordinary
Crabs, Lobsters, Shrimps, etc., together with the Wood-lice and Sand-hoppers.

Mammalia. — The highest class of animals, including the ordinary hairy quadru-
peds, the Whales and Man, and characterized by the production of living
young which are nourished after birth by milk from the teats {Mammce, Mam-
mary glands) of the mother. A striking difference in embryonic development has
led to the division of this class into two great groups; in one of these, when the
embryo has attained a certain stage, a vascular connection, called the placenta,
is formed between the embryo and the mother; in the other this is wanting, and
the young are produced in a very incomplete state. The former, including the
greater part of the class, are called Placental mammals; the latter, or Aplacental
mammals, include the Marsupials and Monotremes [Ornithorhynchus) .

Mammiferous. — Having mammce or teats (see Mammalia).

Mandibles in Insects. — The first or uppermost pair of jaws, which are generally
solid, horny, biting organs. In Birds the term is applied to both jaws with their
horny coverings. In quadrupeds the mandible is properly the lower jaw.

Marsupl\ls. — An order of Mammalia in which the young are born in a very in-
complete state of development and carried by the mother, while sucking, in a
ventral pouch (marsupium), such as the Kangaroos, Opossums, etc. (see
Mammalia).

Maxilla in Insects. — ^The second or lower pair of jaws, which are composed of
several joints and furnished with peculiar jointed appendages called palpi or
feelers.

Melanism. — The opposite of Albinism; an undue development of coloring material
in the skin and its appendages.

Metamorphic Rocks. — Sedimentary rocks which have undergone alteration, gen-
erally by the action of heat, subsequent to their deposition and consolidation.

GLOSSARY 371

MoLLUSCA. — One of the great divisions of the Animal Kingdom, including those
animals which have a soft body, usually furnished with a shell, and in which
the nervous ganglia, or centres, present no definite general arrangement. They
are generally known under the denomination of "shell-fish"; the cuttle-fish, and
the common snails, whelks, oysters, mussels and cockles, may serve as examples
of them.

Monocotyledons, or Monocotyledonous Plants. — Plants in which the seed
sends up only a single seed-leaf (or cotyledon); 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 Crustaceans (Pra^vns), in
which they closely resemble the adults of a genus [My sis) 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, espe-
cially 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.

Nictitating Membrane. — A semi-transparent membrane, which can be drawn
across the eye in Birds and Reptiles, either to moderate the effects of a strong
light or to sweep particles of dust, etc., from the surface of the eye.

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 workers.

Ocelli. — The simple eyes or stemmata of Insects, usually situated on the crown of

the head between the great compound eyes.
CEsoPHAGUS. — The gullet.
OoLmc. — ^A great series of secondary rocks, so called from the texture of some of

its members, which appear to be made up of a mass of small egg-like calcareous

bodies.
Operculum. — ^A calcareous plate employed by many Mollusca to close the aperture

of their shell. The opercular valves of Cirripedes are those which close the

aperture of the shell.
Orbit. — The bony cavity for the reception of the eye.
Organism. — An organized being, whether plant or animal.
Orthospermous. — A term applied to those fruits of the Umbelliferae which have

the seed straight.
Osculant. — Forms or groups apparently intermediate between and connecting

other groups are said to be osculant.
Ova. — Eggs.
Ovarium or Ovary (in plants). — The lower part of the pistil or female organ of

the flow^er, containing the o\Tiles or incipient seeds; by growth after the other

organs of the flower have fallen, it usually becomes converted into the fruit.

372 GLOSSARY

OviGEROUS. — Egg-bearing.

Ovules (of plants). — The seeds in the earliest condition.

Pachyderms. — A group of Mammalia so called from their thick skins, and includ-
ing the Elephant, Rhinoceros, Hippopotamus, etc.

Pal/EOZoic. — The oldest system of fossiliferous rocks.

Palpi. — Jointed appendages to some of the organs of the mouth in Insects and
Crustacea.

Papilionace/e. — ^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 vertebrate animals are articu-
lated.

Petals. — The leaves of the corolla, or second circle of organs 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.

Plagentalia, 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 germi-
nated plants.

Plutonic Rocks. — Rocks supposed to have been produced by igneous action in the
depths of the earth.

Pollen. — The male element in flowering plants; usually a fine dust produced by
the anthers, which, by contact with the stigma effects the fecundation of the
seeds. This impregnation is brought about by means of tubes {pollen-tubes)
which issue from the pollen-grains adhering to the stigma, and penetrate
through the tissues until they reach the ovary.

Polyandrous (flowers). — Flowers having many stamens.

Polygamous Plants. — Plants in which some flowers are unisexual and others
hermaphrodite. The unisexual (male and female) flowers may be on the same
or on different plants.

Polymorphic. — Presenting many forms.

GLOSSARY 373

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 Kingdom. These animals are

composed of a gelatinous material and show scarcely any trace of distinct

organs. The Infusoria, Foraminifera and Sponges, with some other forms,

belong to this division.
Pupa (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.

Ramus. — One half of the lower jaw in the Mammalia. The portion which rises to

articulate with the skull is called the ascending ramus.
Radicle. — The minute root of an embryo plant.
Range. — The extent of country over which a plant or animal is naturally spread.

Range in time expresses the distribution of a species or group through the

fossiliferous beds of the earth's crust.
Retina. — The delicate inner coat of the eye, formed by nervous filaments spreading

from the optic nerve and serving for the perception of the impressions produced

by light.
Retrogression. — Backward development. When an animal, as it approaches

maturity, becomes less perfectly organized than might be expected from its

early stages and known relationships, it is said to undergo a retrograde develop-
ment or metamorphosis.
Rhizopods. — A class of lowly organized animals (Protozoa), having a gelatinous

body, tJie surface of which can be protruded in the form of root-Hke processes

or nlaments, which serve for locomotion and the prehension of food. The most

important order is that of the Foraminifera.
Rodents. — The gnawing Mammalia, such as Rats, Rabbits 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 at-
tached.

Sarcode. — The gelatinous material of which the bodies of the lowest animals
(Protozoa) are composed.

374 GLOSSARY

ScuTELL^E. — The horny plates with which the feet ' urds 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 Palaeozoic 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 vertebrae, and gives off nearly

all the nerves to the various organs of the body.
Stamens. — The male organs of flowering plants, standing in a circle within the

petals. They usually consist of a filament and an anther, the anther being the

essential part in which the pollen, or fecundating dust, is formed.
Sternum. — The breast-bone.

Stigma. — The apical portion of the pistil in flowering plants.
Stipules. — Small leafy organs placed at the base of the footstalks of the leaves

in many plants.
Style. — The middle portion of the perfect pistil, which rises like a column from

the ovary and supports the stigma at its summit.
Subcutaneous. — Situated beneath the skin.
Suctorial. — Adapted for sucking.
Sutures (in the skull). — The lines of junction of the bones of which the skull is

composed.

Tarsus (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 preceding the establishment
of the present order of things.

Trachea. — The windpipe or passage for the admission of air to the lungs.

Tridactyle. — Three-fingered, or composed of three movable parts attached to a
common base.

Trilobites. — A peculiar group of extinct Crustaceans, somewhat resembling the
Wood-lice in external form, and, like some of them, capable of rolling them-
selves up into a ball. Their remains are found only in the Palaeozoic rocks, and
most abundantly in those of Silurian age.

Trimorphic. — Presenting three distinct forms.

Umbellifer^. — An order of plants in which the flowers, which contain five stamens
and a pistil with two styles, are supported upon foot stalks 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.)

GLOSSARY 375

Ungulata. — Hoofed quadi' ^^ds.
Unicellular. — Consisting of a single cell.

Vascular. — Containing blood-vessels.

Vermiform. — Like a worm.

Vertebrata; or Vertebrate Animals. — The highest division of the Animal King-
dom, so called from the presence in most cases of a backbone composed of
numerous joints or vertebrce, which constitutes the centre of the skeleton and at
the same time supports and protects the central parts of the nervous system.

Whorls. — The circles or spiral lines in which the parts of plants are arranged upon

the axis of growth.
Workers. — See Neuters.

Zoea-stage. — The earliest stage in the development of many of the higher
Crustacea, so called from the name of Zoea applied to these young animals
when they were supposed to constitute a peculiar genus.

ZooiDS. — In many of the lower animals (such as the Corals Medusae, etc.) repro-
duction 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.

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