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DARWIN, AND AFTER DARWIN
m

POST-DARWINIAN QUESTIONS
ISOLATION AND PHYSIOLOGICAL SELECTION

BY THE SAME AUTHOR.

DARWIN, AND AFTER DARWIN. An Exposition of the
Darwinian Theory and a Discussion of Post-Darwinian
Questions.

1. The Darwinian Theory. With portrait of Darwin.
460 pages. 125 illustrations. Cloth, 82.00.

2. Post-Darwinian Questions. Edited by Prof. C.

Lloyd Morgan. With portrait of G. J. Romanes.
338 pages. Cloth, $1.50.

3. Post-Darwinian Questions. Isolation and Phy-

siological Selection. Ed ced by Prof. C. Lloyd
Morgan. With portrait of Mr. J. T. GuIxck. i8r
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THOUGHTS ON RELIGION. Edited by Charles Gore, M.A.,
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THE OPEN COURT PUBLISHING COMPANY,

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DARWIN, AND AFTER DARWIN

AN' EXPOSITION OF THE DARWINIAN THEORY

AND A DISCUSSION OF

POST-DARWINIAN QUESTIONS

BY THE LATE

GEORGE JOHN 5ROMANES, M.A., LL.D., F.R.S.

Honorary Fello-w of Gonville and Caius College, Cavibridge

111 -^ -1-^

POST-DARWINIAN QUESTIONS ^

ISOLATION ^ , 3

AND PHYSIOLOGICAL SELECTION ^ .

CHICAGO AND LONDON

THE OPEN COURT PUBLISHING COMPANY

Ai4

CHAPTER 1. COPYRIGHTED BY

THE OPEN COURT PUBLISHING Co
1897.

QH3CL

19)4

PREFACE

Of the six chapters which constitute this con-
cluding volume of G. J. Romanes' Darivin, and after
Darwin^ three, the first two and the last, were '.n
type at the time of his death. I have not considered
myself at liberty to make any alterations of moment
in these chapters. For the selection and arrange-
ment of all that is contained in the other three
chapters I am wholly responsible.

Two long controversial Appendices have been
omitted. Those marked A and B remain in accord-
ance with the author's expressed injunctions. In
a third, marked C, a few passages from the author's
note-books or MSS. have been printed.

The portrait of the Rev. J. Gulick, which forms the
frontispiece, was prepared for this volume before the
author's death. Mr. Gulick's chief contributions to
the theory of physiological selection are to be found
in the Linnean Society's Journal [Zoology^ vols, xx

vi Preface,

and xxiii), and in four letters to Nature (vol. xli.

P- ^?fi \ vol. xlii. pp. 28 and 369 ; and vol. xliv.

p. 29).

I have to thank Mr. Francis Galton, D.C.L., F.R.S.

and Mr. F. Howard Collins for valuable assistance

generously rendered for the sake of one whom all

who knew him held dear. For he was, if I may

echo the words of Huxley, '* a friend endeared to

me, as to so many others, by his kindly nature, and

justly valued by all his colleagues for his powers

of investigation and his zeal for the advancement of

science."

C. Lloyd Morgan.
Bristol, May 1897.

CONTENTS

CHAPTER L
Isolation i

CHAPTER 11.
Isolation [continued) ...».- 18

CHAPTER III.
Physiological Selection . . • • • • 41

CHAPFER IV.
Evidences of Physiological Selection ... 62

CHAPTER V.
Further Evidences of Physiological Selection . 81

CHAPTER VI.

A Brief History of Isolation as a Factor in

Organic Evolution loi

General Conclusions ....••. 144

Appendix A. Mr. Gulick's Criticism of Mr. Wallace's
Views on Physiological Selection . . .151

viii Contents,

VAOB

Appendix B. An Examination by Mr. Fletcher
MouLTON OF Mr. Wallace's Calculation touch-
ing THE Possibility of Physiological Selection
ever acting alone 1 57

Appendix C. Some Extracts from the Author's
Note-books 169

ISOLA TION

DARWIN, AND AFTER DARWIN.

CHAPTER I.
Isolation.

This treatise will now draw to a close by considering
what, in my opinion, is one of the most important
principles that are concerned in the process of organic
evolution — namely, Isolation. I say in my opinion
such is the case, because, although the importance of
isolation is more or less recognized by every naturalist,
I know of only one other who has perceived all that
the principle involves. This naturalist is the Rev. J.
Gulick, and to his essays on the subject I attribute
a higher value than to any other work in the field of
Darwinian thought since the date of Darwin's death ^
For it is now my matured conviction that a new point
of departure has here been taken in the philosophy of
Darwinism, and one which opens up new territories
for scientific exploration of an endlessly wide and
varied character. Indeed I believe, with Mr. Gulick,

* It will be remembered that I regard Weismann's theory of heredity,
with all its deductive consequences, as still sub judice*
III. ' B

2 Darwin, and after Darwin,

that in the principle of Isolation we have a principle
so fundamental and so universal, that even the great
principle of Natural Selection lies less deep, and
pervades a region of smaller extent. Equalled only
in its importance by the two basal principles of
Heredity and Variation, this principle of Isolation
constitutes the third pillar of a tripod on which is
reared the whole superstructure of organic evolution.

By isolation I mean simply the prevention of inter-
crossing between a separated section of a species or
kind and the rest of that species or kind. Whether
such a separation be due to geographical barriers, to
migration, or to any other stat«^ of matters leading
to exclusive breeding within the separated group,
I shall indifferently employ the term isolation for the
purpose of designating what in all cases is the same
result — namely, a prevention of intercrossing between
A and B, where A is the separated portion and B the
rest of the species or kind.

The importance of isolation as against dissimilar
forms has always been fully appreciated by breeders
fanciers, horticulturists, &c., who are therefore most
careful to prevent their pedigree productions from
intercrossing with any other stock. Isolation is indeed,
as Darwin has observed, " the corner-stone of the
breeder's art." And similarly with plants and animals
in a state of nature: unless intercrossing with allied
(i.e. dissimilar/ forms is prevented, the principle of
heredity is bound to work for uniformity, by blend-
ing the dissimilar types in one : only when there is
exclusive breeding of similarly modified forms can the
principle of heredity work in the direction of change
— i.e. of evolution.

Isolation, 3

Now, the forms of isolation — or the conditions
fvhich may lead to exclusive breeding — are manifold.
One of the most important, as well as the most obvious,
is geographical isolation ; and no one questions that
this has been an important factor in the process of
evolution, although opinions still vary greatly as to
the degree of its importance in this respect. At one
end of the series we may place the opinion of Mr.
Wallace, who denies that any of what may be termed
the evolutionary effect of geographical isolation is due
to " influence exerted by isolation per se." This effect,
he says, is to be ascribed exclusively to the fact that
a geographically isolated portion of a species must
always encounter a change of environment, and there-
fore a new set of conditions necessitating a new set of
adaptations at the hands of natural selection ^ At
the other end of the series we must place the opinion
of Moritz Wagner, who many years ago published
a masterly essay ^, the object of which was to prove
that, in the absence of geographical isolation (including
migration), natural selection would be powerless to
effect any change of specific type. For, he argued,
the initial variations on which the action of this
principle depends would otherwise be inevitably
swamped by free intercrossing. Wagner adduced
a large number of interesting facts in support of this
opinion ; but although he thus succeeded in en-
forcing the truth that geographical isolation is an
important aid to organic evolution, he failed to establish
his conclusion that it is an indispensable condition.

* Darwinism, p. 150.

' The Darwinian Theory, and the Law of Migration (Eng. Trans.,
Stanford, London, 1873).

B 2

4 Darwin, and after Darwin.

Nevertheless he may have been right — and, as I shall
presently show, I believe he was right — in his funda-
mental premiss, that in the presence of free inter-
crossing natural selection would be powerless to effect
divergent evolution. Where he went wrong was in
not perceiving that geographical isolation is not the
only form of isolation. Had it occurred to him that
there may be other forms quite as effectual for the
prevention of free intercrossing, his essay could hardly
have failed to mark an epoch in the history of Dar-
winism. But, on account of this oversight, he really
weakened his main contention, namely, that in the
presence of free intercrossing natural selection must
be powerless to effect divergent evolution. This main
contention I am now about to re-argue. At present,
therefore, we have only to observe that Wagner did it
much more harm than good by neglecting to perceive
that free intercrossing may be prevented in many other
ways besides by migration, and by the intervention of
geographical barriers.

In order that we may set out with clearer views
upon this matter, I will make one or two preliminary
remarks on the more general facts of isolation as these
are found to occur in nature.

In the first place, it is obvious that isolation
admits of degrees : it may be either total or partial ;
and, if partial, may occur in numberless grades of
efficiency. This is so manifest that I need not wait
to give illustrations. But now, in the second place,
there is another general fact appertaining to isolation
which is not so manifest, and a clear appreciation
of which is so essential to any adequate considera-
tion of the subject, that I believe the reason why

Isolation. 5

evolutionists have hitherto failed to perceive the full
importance of isolation, is because they have failed
to perceive the distinction which has now to be pointed
out. The distinction is, that isolation may be either
discriminate or indiscriminate. If it be discriminate,
the isolation has reference to the resemblance of the
separated individuals to one another; if it be indis-
criminate, it has no such reference. For example, if
a shepherd divides a flock of sheep without regard to
their characters, he is isolating one section from the
other indiscriminately ; but if he places all the white
sheep in one field, and all the black sheep in another
field, he is isolating one section from the other
discriminately. Or, if geological subsidence divides
a species into two parts, the isolation will be indis-
criminate ; but if the separation be due to one of
the sections developing, for example, a change of
instinct determining migration to another area; or
occupation of a different habitat en the same area,
then the isolation will be discriminate, so far as the
resemblance of instinct is concerned.

With the exception of Mr. Gulick, I cannot find
that any other writer has hitherto stated this
supremely important distinction between isolation as
discriminate and indiscriminate. But he has fully
as well as independently stated it, and shown in
a rnasterly way its far-reaching consequences. Indis-
criminate isolation he calls Separate Breeding, while
discriminate isolation he calls Segregate Breeding.
For the sake, however, of securing more descriptive
terms, I will coin the words Apogamy and Homogamy.
Apogamy, of course, answei's to indiscriminate isola-
tion, or separate breeding. Homogamy, on the other

6 Darwifiy and after Darv^in,

hand, answers to discriminate isolation, or segregate
breeding : only individuals belonging to the same
variety or kind are allowed to propagate. Isolation,
then, is a genus, of which Apogamy and Homogamy
are species \

Now, in order to appreciate the unsurpassed im-
portance of isolation as one of the three basal
principles of organic evolution, let us begin by
considering the discriminate species of it, or Homo-
gamy.

To state the case in the most general terms, we
may say that if the other two basal principles are
given in heredity and variability, the whole theory
of organic evolution becomes neither more nor less
than a theory of homogamy — that is, a theory of
the causes which lead to discriminate isolation, or
the breeding of like with like to the exclusion of
unlike. For the more we believe in heredity and
variability as basal principles of organic evolution,
the stronger must become our persuasion that dis-
criminate breeding leads to divergence of type, while
indiscriminate breeding leads to uniformity. This,
in fact, is securely based on what we know from the
experience supplied by artificial selection, which con-

* I may here most conveniently define the senses in which all the
following terms will be used throughout the present discussion : — Species
of isolation are, as above stnted, homogamy and apogamy, or isolation
as discriminate and indiscriminate. Forms of isolation are modes of
isolation, such as the geographical, the sexual, the instinctive, or any
other of the numerous means whereby isolation of either species may be
secured. Cases of isolation are the instances in which any of the forms
of isolation may be at work : thus, if a group of n intergenerants be
segregated into five groups, a, b, c, tf', e, then, before the segregation there
would have been one case of isolation, but after the segregation there
would be five such cases.

Isolation, 7

sists in the intentional mating of like with like to the
exclusion of unlike.

The point, then, which in the first instance must be
firmly fastened in our minds is this : — so long as there
is free intercrossing, heredity cancels variability, and
makes in favour of fixity of type. Only when as-
sisted by some form of discriminate isolation, which
determines the exclusive breeding of like with like,
can heredity make in favour of change of type, or
lead to what we understand by organic evolution.

Now the forms of discriminate isolation, or homo-
gamy, are very numerous. When, for example, any
section of a species adopts somewhat different habits
of life, or occupies a somewhat different station in
the economy of nature, homogamy arises within that
section. There are forms of homogamy on which
Darwin has laid great stress, as we shall presently
find. Again, when for these or any other reasons a
section of a species becomes in any small degree
modified as to form or colour, if the species happens
to be one where any psychological preference in
pairing can be exercised — as is very generally the
case among the higher animals — exclusive breeding
is apt to ensue as a result of such preference ; for
there is abundant evidence to show that, both in birds
and mammals, sexual selection is usually opposed to
the intercrossing of dissimilar varieties. Once more,
in the case of plants, intercrossing of dissimilar
varieties may be prevented by any slight difference in
their seasons of flowering, of topographical stations,
or even, in the case of flowers which depend on
insects for their fertilization, by differences in the
instincts and preferences of their visitors.

8 Darzvm, and after Darwin.

But, without at present going into detail with
regai'd to these different forms of discriminate isolation,
there are still two others, both of which are of much
greater importance than any that I liave hitherto
named. Indeed, these two forms are of such im-
measurable importance, that were it not for their
virtually ubiquitous operation, the process of organic
evolution could never have begun, nor, having begun,
continued.

The first of these two forms is sexual incompati-
bility— either partial or absolute — between different
taxonomic groups. If all hares and rabbits, for
example, were as fertile with one another as they
are within their own respective species, there can be
no doubt that sooner or later, and on common areas,
the two types would fuse into one. And similarly,
if the bar of sterility could be thrown down as
between all the species of a genus, or all the genera of
a family, not otherwise prevented from inter crossing,
in time all such species, or all such genera, would
become blended into a single type. As a matter
of fact, complete fertility, both of first crosses and
of their resulting hybrids, is rare, even as between
species of the same genus ; while as between genera
of the same family complete fertility does not appear
ever to occur ; and, of course, the same applies to
all the higher taxonomic divisions. On the other
hand, some degree of infertility is not unusual as
between different varieties of the same species ; and,
wherever this is the case, it must clearly aid the further
differentiation of those varieties. It will be my
endeavour to show that in this latter connexion
sexual incompatibility must be held to have taken

Isolation. 9

an immensely important part in the differentiation
of varieties into species. But meanwhile we have
only to observe that wherever such incompatibility is
concerned, it is to be regarded as an isolating agency
of the very first importance. And as it is of a
character purely physiological, I have assigned to it
the name Physiological Isolation ; while for the par-
ticular case where this general principle is concerned
in the origination of specific types, I have reserved
the name Physiological Selection.

The other most important form of discriminate
isolation to which I have alluded is Natural Selection.
To some evolutionists it has seemed paradoxical
thus to regard natural selection as a form of isola-
tion ; but a little thought will suffice to show that
such is really the most accurate way of regarding it.
For, as Mr. Gulick says, "Natural selection is the,
exclusive breeding of those better adapted to the en-
vironment : ... it is a process in which the fittest are
prevented from crossing with the less fitted, by the
exclusion of the less fitted/' Therefore it is, strictly
and accurately, a mode of isolation, where the
isolation has reference to adaptation, and is secured
in the most effectual of possible ways — i.e. by the
destruction of all individuals whose intercrossing would
interfere with the isolation. Indeed, the very term
" natural selection " shows that the principle is tacitly
understood to be one of isolation, because this name
was assigned to the principle by Darwin for the
express purpose of marking the analogy that obtains
between it and the intentional isolation which is
practised by breeders, fanciers, and horticulturists.
The only difference between " natural selection " and

lo Darwin, and after Darwin.

"artificial selection" consists in this— that under the
former process the excluded individuals must neces-
sarily perish, while under the latter they need not do
so. But clearly this difference is accidental : it is in
no way essential to the process considered as a process
of discriminate isolation. For, as far as homogamous
breeding is concerned, it can matter nothing whether
the exclusion of the dissimilar individuals is effected
by separation or by death.

Natural selection, then, is thus unquestionably
a form of isolation of the discriminate kind ; and
therefore, notwithstanding its unique importance in
certain respects, considered as a principle of organic
evolution it is less fundamental — and also less ex-
tensive—than the principle of isolation in general. In
other words, it is but a part of a much larger whole.
It is but a particular form of a general principle,
which, as just shown, presents many other forms, not
only of the discriminate, but likewise of the indiscri-
minate kind. Or, reverting to the terminology of
logic, it is a sub-species of the species Homogamy,
which in its turn is but a constituent part of the
genus Isolation.

So much then for homogamy, or isolation of the
discriminate order. Passing on now to apogamy, or
isolation of the indiscriminate kind, we may well be
disposed, at first sight, to conclude that this kind of
isolation can count for nothing in the process of evo-
lution. For if the fundamental importance of isola-
tion in the production of organic forms be due to its
segregation of like with like, does it not follow
that any form of isolation which is indiscriminate
must fail to supply the very condition on which all

Isolation. ^^Kt j i

the forms of discriminate isolation depend for their
efficacy in the causing of organic evolution ? Or, to
return to our concrete example, is it not self-evident
that the farmer who separated his stock into two
or more parts indiscriminately, would not effect any
more change in his stock than if he had left them
all to breed together?

Well, although at first sight this seems self-evident,
it is in fact untrue. For, unless the individuals which
are indiscriminately isolated happen to be a very
large number, sooner or later their progeny will come
to differ from that of the parent type, or unisolated
portion of the previous stock. And, of course, as
soon as this change of type begins, the isolation
ceases to be indiscriminate : the previous apogamy
has been converted into homogamy, with the usual
result of causing a divergence of type. The reason
why progeny of an indiscriminately isolated section
of an originally uniform stock — e.g. of a species — will
eventually deviate from the original type is, to quote
Mr Gulick. as follows : — " No two portions of a species
possess exactly the same average character, and,
therefore, the initial differences are for ever reacting
on the environment and on each other in such a way
as to ensure increasing divergence as long as the
individuals of the two groups are kept from intcr-
generating^" Or, as I stated this principle in my
essay on Physiological Selection^ published but a short
time before Mr. Gulick's invaluable contributions to
these topics : —

As a matter of fact, we find that no one individual "is like

' Divergent Evolution Ihrough Cumulative Segregation {Zool. Journal,

Linn. Soc, vol. xx. pp. 189-274).

12 Darwin, and after Darwin,

another all in all " ; which is another way of saying that a
specific type may be regarded as the average mean of all its in-
dividual variations, any considerable departure from this average
being, however, checked by intercrossing. . . . Consequently, if
from any cause a section of a species is prevented from inter-
crossing with the rest of its species, we might expect that new
varieties should arise within that section, and that in time these
varieties should pass into new species. And this is just what
we do find'.

The name which I gave to this cause of specific
change was Independent Variability, or variability in
the absence of overwhelming intercrossing. But it
now appears to me that this cause is really identi-
cal with that which was previously enunciated by
Delbceuf. Again, in his important essay on The
Influence of Isolation^ Weismann concludes, on the
basis of a large accumulation of facts, that the con-
stancy of any given specific type "does not arise
suddenly, but gradually, and is established by the
promiscuous intercrossing of all individuals." From
which, he says, it follows, that this constancy must
cease so soon as the condition which maintains it
ceases — i. e. so soon as intercrossing (Panmixia)
between all individuals ceases, or so soon as a portion
of a species is isolated from its parent stock. To
this principle he assigns the name of Amixia. But
Weismann's Amixia differs from my Independent
Variability in several important particulars ; and
on this account I have designedly abstained from

* The passage proceeds to show that in view of this consideration we
have a strong additional reason for rejettii.g the a priori dogma that all
specific characters must necessarily be useful characters. For it is evident
that any divergence of specific character which is brought about in this
way need not present any utilitarian significance— although, of course,
natural selection will ensure that it shall never be deleterious.

[

Isolation, 13

adopting his term. Here it is enough to remark
that it answers to the generic term Isolation, with-
out reference to the kind of isolation as discriminate
or indiscriminate, homogamous or apogamous. On
the other hand, my Independent Variability is merely
a re-statement of the so-called " Law of Delboeuf/'
which, in his own words, is as follows : —

One point, however, is definitely attained. It is that the
proposition, which further back we designated paradoxical,
is rigorously true. A constant cause of variation, however
insignificant it may be, changes the uniformity [of type]
little by little, and diversifies it ad infinitum. From the homo-
geneous, left to itself, only the homogeneous can proceed ; but
if there be a slight disturbance [" leger ferment "] in the
homogeneous, the homogeneity will be invaded at a single
point, differentiation will penetrate the whole, and, after
a time— it may be an infinite time— the differentiation will
have disintegrated it altogether.

In other words, the "Law," which Delboeuf has
formulated on mathematical grounds, and with express
reference to the question of segregate breeding,
proves that, no matter how infinitesimally small the
difference may be between the average qualities of
an isolated section of a species compared with the
average qualities of the rest of that species, if the
isolation continues sufficiently long, differentiation of
specific type is necessarily bound to ensue. But, to
make this mathematical law biologically complete, it
ought to be added that the time required for the
change of type to supervene (supposing apogamy to
be the only agent of change) will be governed by the
range of individual variability which the species in
question presents. A highly stable species (such as
the Goose) might require an immensely long time for

14 Darwin, and after Darwin.

apogamy alone to produce any change of type in an
isolated portion of the species, while a highly variable
species (such as the Rufif) would rapidly change in
any portion that might be indiscriminately isolated.
It was in order to recognize this additional and very
important factor that I chose the name Independent
Variability whereby to designate the diversifying
influence of merely indiscriminate isolation, or apo-
gamy. Later on Mr. Gulick published his elaborate
papers upon the divergence of type under all kinds of
isolation ; and retained my term Independent, but
changed Variability into Generation. I point this
out merely for the sake of remarking that his In-
dependent Generation is exactly the same principle
as my Independent Variability, and Delboeuf 's Mathe-
matical Law.

Now, while I fully agree with Mons. Giard where
he says, in the introductory lecture of his course on
The Factors of Evolution^, that sufficient attention
has not been hitherto given by naturalists to this
important factor of organic evolution (apogamy),
I think I have shown that among those naturalists
who have considered it there is a sufficient amount of
agreement. Per contra, I have to note the opinion
of Mr. Wallace, who steadily maintains the impossi-
bility of any cause other than natural selection (i. e.
one of the forms of homogamy) having been concerned
in the evolution of species. But at present it is enough
to remark that even Professor Ray Lankester — whose
leanings of late years have been to the side of ultra-
Darwinism, and who is therefore disposed to agree

* Rtmu Scientijique, Nov. 23, 1889,

Isolation, ^HiF 15

with Mr. Wallace wherever this is logically possible —
even Professor Ray Lankester observes : —

Mr. Wallace does not, in my judgement, give sufficient
grounds for rejecting the proposition which he indicates as
the main point of Mr. Gulick's valuable essay on Divergent
Evolution through Cuimilative Seg7egation. Mr. Gulick's
idea is that .... no two portions of a species possess exactly the
same average character, and the initial differences will, if the
individuals of the two groups are kept from intercrossing,
assert themselves continuously by heredity in such a way as
to ensure an increasing divergence of the forms belonging to
the two groups, amounting to what is recognized as specific
distinction. Mr. Gulick's idea is simply the recognition of
a permanence or persistency in heredity, which, caeteris
paribus^ gives a twist or direction to the variations of the
descendants of one individual as compared with the descendants
of another*.

Now we have seen that "Mr. Gulick's idea,''
although independently conceived by him, had been
several times propounded before ; and it is partly
implicated in more than one passage of the Origin
of Species^ where free intercrossing, or the absence of
isolation, is alluded to as maintaining the constancy
of a specific type f Moreover, it is still more fully
recognized in the last edition of the Variation of
Animals and Plants^ where a paragraph is added for
the purpose of sanctioning the principle in the
imperfect form that it was stated by Weismann ^
Nevertheless, to Mr. Gulick belongs the credit, not
only of having been the first to conceive (though the
last to publish) the "idea" in question, and of having
stated it with greater fullness than anybody else; but

• Nature, Oct. lo, 1889, p. 568. « e. g. p. 8i.

' See Chapter xxiii. vol. ii p. 263. CEdition of 188S.)

i6 Darwin, and after Darwin,

still more of having verified its importance as a factor
of organic evolution.

For, in point of fact, Mr. Gulick was led to his
recognition of the principle in question, not by any
deductive reasoning from general principles, but by
his own particular and detailed observations of the
land mollusca of the Sandwich Islands. Here there
are an immense number of varieties belonging to
several genera ; but every variety is restricted, not
merely to the same island, but actually to the same
valley. Moreover, on tracing this fauna from valley
to valley, it is apparent that a slight variation in the
occupants of valley 2 as compared with those of the
adjacent valley i, becomes more pronounced in the
next — valley 3, still more so in 4, &c., &c. Thus it
was possible, as Mr. Gulick says, roughly to estimate
the amount of divergence between the occupants of
any two given valleys by measuring the number of
miles between them.

As already stated, I have myself examined his
wonderful collection of shells, together with a topo-
graphical map of the district ; and therefore I am in
a position to testify to the great value of Mr. Gulick's
work in this connexion, as in that of the utility
question previously considered. The variations, which
affect scores of species, and themselves eventually run
into fully specific distinctions, are all more or less
finely graduated as they pass from one isolated
region to the next ; and they have reference to
changes of form and colour, which in no one case
presents any appearance of utility. Therefore — and
especially in view of the fact that, as far as he could
ascertain, the environment in the different valleys was

Isolation. 17

essentially the same — no one who examines this
collection can wonder that Mr. Gulick attributes the
results which he has observed to the influence of
apogamy alone, without any reference to utility or
natural selection.

To this solid array of remarkable facts Mr. Wallace
has nothing further to oppose than his customary
appeal to the argument from ignorance, grounded on
the usual assumption that no principle other than
natural selection can be responsible for even the
minutest changes of form or colour. For my own
part, I must confess that I have never been so deeply
impressed by the dominating influence of the a priori
method as I was on reading Mr. Wallace's criticism
of Mr. Gulick's paper, after having seen the material
on which this paper is founded. To argue that every
one of some twenty contiguous valleys in the area of
the same small island must necessarily present such
differences of environment that all the shells in each
are differently modified thereby, while in no one out
of the hundreds of cases of modification in minute
respects of form and colour can any human being
suggest an adaptive reason therefor — to argue thus
is merely to affirm an intrinsically improbable dogma
in the presence of a great and consistent array of
opposing facts.

I have laid special stress on this particular case of
the Sandwich Islands' mollusca, because the fifteen
years of labour which Mr. Gulick has devoted to their
exhaustive working out have yielded results more
complete and suggestive than any which so far have
been forthcoming with regard to the effects of isolation
in divergent evolution. But, if space permitted, it

III. C

i8 Darwin^ and after Darwin,

would be easy to present abundance of additional facts
from other sources, all bearing to the same conclusion
—namely, that as a matter of direct observation, no
less than of general reasoning, any unprejudiced mind
will concede to the principle of indiscriminate isolation
an important share in the origination of organic types.
For as indiscriminate isolation is thus seen sooner or
later to become discriminate, and as we have already
seen that discriminate isolation is a necessary condition
to all or any modification, we can only conclude that
isolation in both its kinds takes rank with heredity
and variability as one of the three basal principles
of organic evolution.

Having got thus far in the way of generalities, we
must next observe sundry further matters of com-
parative detail.

I. In any case of indiscriminate isolation, or
apogamy, the larger the bulk of the isolated section
the more nearly must its average qualities resemble
those of its parent stock ; and, therefore, the less
divergence of character will ensue in a given time
from this cause alone. For instance, if one-fourth of
a large species were to be separated from the other
three-fourths (say, by subsidence causing a discon-
tinuity of area), it would continue the specific characters
unchanged for an indefinitely long time, so far as
the influence of such an indiscriminate isolation is
concerned. But. on the other hand, if only half a
dozen individuals were to be thus separated from
the rest of their species, a comparatively short time
would be needed for their descendants to undergo
some varietal modification at the hands of apogamy.

Isolation, 19

For, in this case, the chances would be infinitely
against the average characters of the original half-
dozen individuals exactly coinciding with those of
all the rest of their species.

2. In any case of homogamy, however, it is
immaterial what proportional number of individuals
are isolated in the first instance. For the isolation is
here discriminate, or effected by the initial difference
of the average qualities themselves— a difference,
therefore, which presupposes divergence as having
already commenced, and equally bound to proceed
whether the number of intergenerants be large or
small.

It may here be remarked that, in his essay on
the Influence of Isolation, Professor Weismann fails
to distinguish between the two kinds of isolation.
This essay deals only with one of the many different
forms of isolation — the geographical — and is therefore
throughout concerned with a consideration of diversity
as arising from apogamy alone. But in dealing with
this side of the matter Weismann anticipated both
Gulick and myself in pointing out the law of inverse
proportion, which I have stated in the preceding
paragraph in what appears to me its strictly accurate
form.

3. Segregate Breeding, or homogamy, which arises
under any of the many forms of discriminate isolation,
must always tend to be aimulative. For, again to
quote Mr. Gulick, who has constituted this fact the
most prominent as it is the most original feature
of his essay, 'In the first place, every new form of
Segregation^ that now appears depends on, and is

* This term may here be taken as equivalent to Isolation.
C 2

20 Darwin^ arid after Darwin,

superimposed upon, forms of Segregation that have
been previously induced ; for when Negative Segre-
gation arises [i. e. isolation due to mutual sterility],
and the varieties of a species become less and less
fertile with one another, the complete infertility that
has existed between them and some other species
does not disappear, nor does the Positive Segregation
cease [i. e. any other form of isolation previously
existing]. ... In the second place, whenever
Segregation is directly produced by some quality of
the organism, variations that possess the endowment
in a superior degree will have a larger share in pro-
ducing the segregated forms of the next generation,
and accordingly the segregative endowment of the
next generation will be greater than that of the
present generation ; and so with each successive
generation the segregation will become increasingly
complete." And to this it may be added, in the
third place, that where the segregation (isolation) is
due to the external conditions of life under which
the organism is placed, or where it is due to natural
selection simultaneously operating in divergent lines
of evolution, the same remarks apply. Hence it
follows that discriminate isolation is, in all its forms,
cumulative.

4. The next point to be noted is, that the cumu-
lative divergence of type thus induced can take
place only in as many different lines as there are
different cases of isolation. This is a point which
Mr. Gulick has not expressly noticed ; but it is one
that ought to be clearly recognized. Seeing that
isolation secures the breeding of similar forms by
exclusion (immediate or eventual) of those which are

Isolation,

21

dissimilar, and that only in as far as it does this
can it be a factor in organic evolution, it follows that
the resulting segregation, even though cumulative,
can only lead to divergence of organic types in as
many directions as there are cases of isolation. For
any one group of intergenerants only serial trans-
formation is possible, even though the transformation
be cumulative through successive generations in the
single line of change. But there is always a probabiHty
that during the course of such serial transformation
in time, some other case of isolation may supervene,
so as to divide the previously isolated group of inter-
generants into two or more further isolated groups.
Then, of course, opportunity will be furnished for
divergent transformation in space— and this in as
many different lines as there are now different
homogamous groups.

That this must be so is further evident, if we
reflect that the evolutionary power of isolation
depends, not only on the preventiitg of intercross-
ing between the isolated portion of a species and
the rest of that species, but also upon the permitting
of intercrossing between all individuals of the isolated
portion, whereby the peculiar average of qualities which
they as a whole present may be allowed to assert itself
in their progeny — or, if the isolation has been from the
first discriminate, whereby the resulting homogamy
may thus be allowed to assert itself Hence any
one case of either species of isolation, discriminate or
indiscriminate, can only give "rise to what Mr. Gulick
has aptly called '• monotypic evolution," or a chain-
like series of types arising successively in time, as
distinguished from what he has called " polytypic

22 Darwin, and after Darwin,

evolution," or an arborescent multiplication of types
arising simultaneously in space.

For example, let us again take the geographical
form of isolation. Where a single small intergenerant
group of individuals is separated from the rest of
its species — say, on an oceanic island — monotypic
evolution may take place through a continuous and
cumulative course of independent variation in a
single line of change : all the individuals composing
any one given generation will closely resemble one
another, although the type may be progressively
altering through a long series of generations. But if
the original species had had two small colonies
separated from itself (one on each of two different
islands, so giving rise to two cases of isolation), then
polytypic evolution would have ensued to the extent
of there having been two different lines of evolu-
tion g()ing on simultaneously (one upon each of
the two islands concerned). Similarly, of course, if
there had been three or four such colonies, there
would have been three or four divergent lines of
evolution, and so on.

5. In the cases of isolation just supposed there
is only one form of isolation ; and it is thus shown
that under one form of isolation there may be as
many lines of divergence as there are separate cases
of such isolation. But now suppose that there are
two or more forms of isolation -for instance, that
on the same oceanic island the original colony has
begun to segregate into secondary groups under
the influence of natural selection, sexual selection,
physiological selection, or any of the other forms
of isolation — then there will be as many lines of

Isolation. 23

divergent evolution going on at the same time (and
here on the same area) as there are forms of isolation
affecting the oceanic colony. And this because each
of the forms of isolation has given rise to a different
case of isolation.

Now, inasmuch as different forms of isolation, when
thus superadded one to another, constitute different
cases of isolation^ we may lay down the following
general law as applying to all the forms of isola-
tion— namely, The number of possible directions in
zvhich divergent evolution can occur, is never greater
than, though it may be equal to. the number of cases
of efficient isolation — or the number of efficiently
separated groups of inter gener ants.

6. We have now to consider with some care the
particular and highly important form of isolation
that is presented by natural selection. For while
this form of isolation resembles all the other forms
of the discriminate kind in that it secures homo-
gamy, there are two points in which it differs from
all of them, and one point in which it differs from
most of them.

Natural selection differs from all the other known
forms of isolation (whether discriminate or indis-
criminate) in that it has exclusive reference to
adaptations on the one hand, and, on the other hand,
necessitates not only the elimination, but the de-
struction of the excluded individuals. Again, natural
selection differs from most of the other forms of
isolation in that, unless assisted by some other
form, it can never lead to polytypic, but only
to monotypic evolution. The first two points of
difference are here immaterial ; but the last is one

24 Darwin, and after Darwin,

of the highest importance, as we shall immediately
perceive.

In nearly all the other forms of isolation, polytypic
or divergent evolution may arise under the influence
of that form alone, or without the necessary co-
operation of any other form. This we have already
seen, for example, in regard to geographical isolation,
under which there may be as many different lines
of transmutation going on simultaneously as there
are different cases of isolation — say, in so many
different oceanic islands. Again, in regard to physio-
logical isolation the same remark obviously applies ;
for it is evident that even upon the same geographical
area there may be as many different lines of trans-
mutation going on simultaneously as there are cases
of this form of isolation. The bar of mutual sterility,
whenever and wherever it occurs, must always render
polytypic evolution possible. And so it is with almost
all the other forms of isolation: that is to say, one
form does not necessarily require the assistance of
another form in order to create an additional case
of isolation. But it is a peculiarity of natural selec-
tion, considered as a form of isolation, that it does
necessarily require the assistance of some other form
before it can give rise to an additional case of isola-
tion; and therefore before it can give rise to any
divergence of character in ramifying lines, as distin-
guished from transformation of characters in a single
line. Or, in other words, natural selection, when act-
ing alone, can never induce polytypic evolution, but
only monotypic.

That this important conclusion is a necessary
deduction from the theory of natural selection itself,

Isolation. 25

a very few words will be enough to show. For,
according to the theory, survival of the fittest is
a form of isolation which acts through utility, by
destroying all the individuals whom it fails to isolate.
Hence it follows that survival of the fittest is a form
of isolation which, if acting alone, cannot possibly
effect divergent evolution. For, in the first place,
there is nothing in this form of isolation to ensure
that the fitter individuals should fail to interbreed
with the less fit which are able to survive ; and, in
the second place, in all cases where the less fit are
not sufficiently fit to be suffered to breed, they are
exterminated — i.e. not permitted to form a distinct
variety of their own. If it be said that survival of
the fittest may develop simultaneously two or more
lines of useful change, the answer is that it can
only do this if each of the developing varieties is
isolated from the others by some additional form
of isolation ; for, if not, there can be no commence-
ment of utilitarian divergence^ since whatever number
of utilitarian changes may be in course of simul-
taneous development, they must in this case be all
blended together in a single line of specific trans-
mutation. Nay, even if specific divergence has
actually been commenced by natural selection when
associated with some other form of homogamy, if
the latter should afterwards be withdrawn, natural
selection would then be unable to maintain even so
much divergence of character as may already have
been attained : free intercrossing between the two
collateral, and no longer isolated branches, would
ensure their eventual blending into a common stock.
Therefore, I repeat, natural selection, when acting

26 Darwin f and after Darivin,

alone, can never induce polytypic evolution, but
only monotypic.

Now I regret to say that here, for the first and
only time throughout the whole course of the
present treatise, I find myself in seeming opposition
to the views of Darwin. For it was the decidedly
expressed opinion of Darwin that natural selection
is competent to effect polytypic, or divergent, evo-
lution. Nevertheless, I believe that the opposition
is to a large extent only apparent, or due merely to
the fact that Darwin did not explicitly state certain
considerations which throughout his discussion on
"divergence of character" are seemingly implied.
But, be this as it may, I have not even appeared
to desert his leadership on a matter of such high im-
portance without having duly considered the question
in all its bearings, and to the utmost limit of my
ability. Moreover, about two years after the publica-
tion of my first paper ^ upon the subject, Mr. Gulick
followed, at somewhat greater length, in the same line
of dissent. Like all the rest of his work, this is so
severely logical in statement, as well as profoundly
thought out in substance, that I do not see how it
is possible for any one to read impartially what he
has written, and then continue to hold that natural
selection, if unassisted by any other form of isola-
tion, can possibly effect divergence of character —
or polytypic as distinguished from monotypic evo-
lution '^.

I may here quote from Mr. Gulick's paper three
propositions, serving to state three large and general

' Zool Journal Lin. Soc, vol. xix. pp. 337 411.
^ Ibid., vol. XX. pp. 202-212.

Isolation. 27

bodies of observable fact, which severally and collec-
tively go to verify, with an overwhelming mass of
evidence, the conclusion previously reached on grounds
of general reasoning.

The facts of geographical distribution seem to me to justify
the following statements : —

(i) A species exposed to different conditions in the different
parts of the area over which it is distributed, is not repre-
sented by divergent forms when free interbreeding exists
between the inhabitants of the different districts. In other
words, Diversity of Natural Selection without Separation does
not produce divergent evolution.

(2) We find many cases in which areas, corresponding in
the character of the environment, but separated from each
other by important barriers, are the homes of divergent forms
of the same or allied species.

(3) In cases where the separation has been long continued,
and the external conditions are the most diverse in points
that involve diversity of adaptation, there we find the most
decided divergences in the organic forms. That is, where
Separation and Divergent Selection have long acted, the
results are found to be the greatest.

The 1st and 3rd of these propositions will probably be
disputed by few, if by any. The proof of the 2nd is
found wherever a set of closely allied organisms iii so
distributed over a territory that each species and variety
occupies its own narrow district, within which it is shut by
bariiers that restrain its distribution while each species of
the environing types is distributed over the whole territory.
The distribution of terrestrial molluscs on the Sandwich
Islands presents a great body of facts of this kind.

CHAPTER 11,

Isolation [continued),

I WILL now recapitulate the main doctrines which
have been set forth in the forei^oing chapter, and then
proceed to consider the objections which have been
advanced against them.

It must be remembered that by isolation I mean
exactly what Mr. Gulick does by " Segregation,'"
and approximately what Professor Weismann does
by " Amixia " — i. e. the prevention of intercrossing.

Isolation occurs in very many forms besides the
geographical, as will be more fully shown at the end
of this chapter ; and in all its forms it admits of
degrees.

It also occurs in two very different species or
kinds — namely, discriminate and indiscriminate. These
I have called respectively Homogamy and Apogamy.
This all-important distinction has been clearly recog-
nized by Mr. Gulick, as a result of his own thought
and observation, independently of anything that I have
published upon the subject.

In view of this distinction Isolation takes rank with
Heredity and Variability as one of the most funda-
mental principles of organic evolution. For, if these

Isolation, 29

other two principles be granted, the whole theory
of descent resolves itself into an inquiry touching the
causes, forms, and degrees of Homogamy.

Save in cases where very large populations are
concerned, apogamy must sooner or later give rise
per se to homogamy, owing to the Law of Delboeuf,
which is the principle that I have called Indepen-
dent Variability, and Gulick has called Independent
Generation. But of course this does not hinder that
under apogamy various other causes of homogamy
are likely to arise — in particular natural selection.

That natural selection differs from most of the other
forms of isolation in not being capable of causing
divergent or polytypic evolution must at once become
evident, if we remember that the only way in which
isolation of any form can cause such evolution is by
partitioning a given group of intergenerants into two
or more groups, each of which is able to survive as
thus separated from the other, and so to carry on the
evolution in divergent lines. But the distinguishing
peculiarity of natural selection, considered as a form
of isolation, is that it effects the isolation by killiug
off all the individuals which it fails to isolate : con-
sequently, this form of isolation differs from other
forms in prohibiting the possibility of any ramification
of a single group of intergenerants into two or more
groups, for the purpose of carrying on the evolution
in divergent lines. Therefore, under this form of
isolation alone, evolution must proceed, palm-like, in
a single line of growth. So to speak, the successive
generations continuously ascend to higher things on
the steps supplied by their own "dead selves"; but
in doing so they must climb a single ladder, no

30 Darwin, and after Darwin.

rung of which can be allowed to bifurcate in the
presence of the uniformity secured for that generation
by the free intercrossing of the most fit. Even
though beneficial variations may arise in two or more
directions simultaneously, and all be simultaneously
selected by survival of the fittest, the effect of free
intercrossing (in the absence of any other form of
isolation) will be to fuse all these beneficial variations
into one common type, and so to end in monotypic
evolution as before. In order to secure polytypic
evolution, intercrossing between the different bene-
ficial variants which may arise must be prevented ;
and there is nothing to prevent such intercrossing in
the process of natural selection per se. In order that
the original group of intergenerants should be divided
and sub-divided into two or more groups of inter-
generants, some additional form of isolation must
necessarily supervene — when, of course, polytypic
evolution will result. And, as Mr. Gulick has shown,
the conclusion thus established by deductive reason-
ing is verified inductively by the facts of geographical
distribution.

How, then, are we to account for the fact that
Darwin attributed to natural selection the power to
cause divergence of character? The answer is suffi-
ciently simple. He does so by tacitly invokinfC the aid
of some other form of homogamy in every case. If wt
carefully read pp. 86-97 of the Origin of Species, where
this subject is under consideration, we shall find that
in every one of the arguments and illustrations which
are adduced to prove the power of natural selection X,o
eftect " divergence of character," he either pre-supposes
or actually names some other form of homogamy as

I

Isolation. 31

the originating cause of the diversity that is afterwards
presented to natural selection for further intensification.
To give only one example. At the starting-point of
the whole discussion the priority of such other forms
of homogamy is assumed in the following words : —

But how, it may be asked, can any analogous principle
[to that of diversity caused by artificial selection] 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.

Now, without question, so soon as segregate
breeding in two or more lines of homogamy has been
in any sufficient degree determined by some " change
of structure, constitution, or habits " natural selection
will forthwith proceed to increase the divergence in
as many different lines as there are thus yielded dis-
criminately isolated sections of the species. And this
fact it must have been that Darwin really had before
his mind when he argued that diversification of char-
acter is caused by natural selection, through the benefit
gained by the diversified forms being thus " enabled
to increase in number.'* Nevertheless he does not ex-
pressly state the essential point, that although diversi-
fication of character, when once begun^ is \.h\xs promoted
by natural selection, which forthwith proceeds to cul-
tivate each of the resulting branches, yet diversifica-
tion of character can never be originated by natural
selection. The change of "structure," of "constitution,"
of " habits," of " station," of geographical area, of reci-

32 DarwiUy and after Darwin,

procal fertility, and so on — this change, whatever it
may have been, must clearly have been antecedent to
any operation of natural selection through the benefit
which arose from the change. Therefore the change
miist in all cases have been due, in the first instance,
to some other form of isolation than the superadded
form which afterwards arose from superior fitness
in the possession of superior benefit — although, so
long as the prior form of isolation endured, or con-
tinued to furnish the necessary condition to the co-
operation of survival of the fittest, survival of the
fittest would have continued to increase the divergence
of character in as many ramifying lines as there were
thus given to its action separate cases of isolation
by other means.

In short, as divergence of character must in all cases
be due to a prevention of intercrossing, and as in the
process of natural selection there is, ex hypothesis
nothing to prevent the intercrossing until the diver-
gence has already arisen, to suppose that natural
selection alone can have caused the divergence, is to
suppose that natural selection can have caused the
conditions of its own activity, which is absurd.

Seeing, then, that even in cases where any" benefit "
arises from divergence of character, such benefit can
arise only after the divergence has already commenced,
and seeing that on this as on other accounts previously
mentioned it is plainly impossible to attribute the
origin of such divergence to natural selection, we find
that natural selection must be in all cases assisted
by some other form of isolation, if it is to be con-
cerned in polytypic as distinguished from monotypic
evolution. But this does not hinder that, when it

Isolation, 33

IS so assisted, natural selection may become— and,
I believe, does become — the most efficient of all
the forms of isolation in promoting divergence of
character. For, in the first place, of all the forms
of isolation natural selection is probably the most
energetic in promoting monotypic evolution; so that
under the influence of such isolation monotypic
evolution probably advances more rapidly than
it does under any other form of isolation. In the
second place, when polytypic evolution has been
begun by any of these other forms of isolation, and
natural selection then sets to work on each of the
resulting branches, although natural selection is thus
engaged in as many different acts of monotypic evolu-
tion as there are thus separate cases supplied to it by
these other forms of isolation, the joint result of all
these different acts is to hurry on the polytypic
evolution which was originally started by the other
forms of isolation. So to speak, natural selection is
the forcing heat, acting simultaneously on each of the
separate branches which has been induced to sprout
by other means ; and in thus rapidly advancing the
growth of all the branches, it is still entitled to be
regarded as the most important single cause of diver-
sification in organic nature, although we must hence-
forth cease to regard it as in any instance the
originating cause — or even so much as the sustaining
cause.

So much by way of summary and recapitulation.
I "will now briefly consider the only objections
which, so far as I can see, admit of being brought
against the foregoing doctrine of Isolation as held
by Mr. Gulick and myself. These possible objections

TTI D

34 Darwin^ and after Darwin.

are but two in number — although but one of them
has been hitherto adduced. This, therefore, I will
take first.

Mr. Wallace, with his customary desire to show
that natural selection is everywhere of itself capable
of causing organic evolution, seeks to minimize the
swamping effects of free intercrossing, and the conse-
quent importance of other forms of isolation. His
argument is as follows.

Alluding to the researches of Mr. J. A. Allen,
and others, on the amount of variation presented
by individuals of a species in a state of nature,
Mr. Wallace shows that, as regards any given part of
the animal under consideration, there is always to
be found a considerable range of individual variation
round the average mean which goes to constitute the
specific character of the type. Thus, for example,
Mr. Allen says of American birds, "that a varia
tion of from fifteen to twenty per cent, m general size,
and an equal degree of variation in the relative size
of different parts, may be ordinarily expected among
specimens from the same species and sex, taken at
the same locality, while in some cases the variation
is even greater than this." Now, Mr. Wallace \s under
the impression that these facts obviate the difficulty
which arises from the presence of free intercrossing —
the difficulty, that is, against the theory of natural
selection when natural selection is supposed to have
been the exclusive means of modification. For, as
he says, '* if less size of body would be beneficial,
then, as half the variations in size are above and
half bdow the mean or existing standard of the
species, there would be ample beneficial variations " ;

¥

Isolation, 35

and similarly with regard to longer or shorter legs,
wings, tails, &c., darker or lighter colour, and so on
through all the parts of any given organism.

Well, although I have no wish at all to disparage
the biological value of these actual measurements
of the range of individual variation, I • must point
out that they are without any value at all in the
connexion which Mr. Wallace adduces them. We
did not require these measurements to tell us the
broad and patent fact that " no being on this earthly
ball is like another all in all " — or, in less Tenny-
sonian words, that as regards every specific structure
there is a certain amount of individual variability
round an average mean. Indeed, in my own paper
on Physiological Selection — against which Mr. Wal-
lace is here specially arguing- I expressly said, as
previously remarked, " that a specific type may
be regarded as the average meaii of all individual
variations^ The fact of such individual variability
round a specific mean has always been well known
to anatomists ; it constitutes one of the basal pillars
of the whole Darwinian theory ; and is besides a
matter of universal recognition as regards human
stature, features, and so forth. The value of Mr
Allen's work consists in accurately measuring the
amount or range of individual variation ; but the
question of its amount or range is without relevancy
in the present connexion. For the desirability of
isolation as an aid to natural selection even where
monotypic evolution is concerned, does not arise
with any reference to the amount or range of variation :
it arises with reference to the nuniber of variations
which are — or are not — similar and simultaneons. If

D 'I

36 Darwin^ and after Darwin.

there be a sufficient number which are both similar
and simultaneous, the desirability of any co-operating
form of isolation is correspondingly removed^ because
natural selection may then have sufficient material
wherewith to overcome the adverse influence of free
intercrossing,- and so of itself to produce monotypic
evolution. Now, variations may be numerous, similar,
and simultaneous, either on account of some common
cause acting on many individuals at the same time,
or on account of the structures in question being
more or less variable round a specific mean. In
the latter case — which is the only case that Mr.
Allen's measurements have to do with— the law of
averages will of course determine that half the whole
number of variations in any given structure, in any
given generation, will be above the mean line. But,
equally of course, no one has ever denied that where,
for either of these reasons, natural selection is pro-
vided with sufficient material, it is correspondingly
capable of improving the specific type without
the assistance of any other form of homogamy ;
so to speak, they protect themselves by their very
numbers, and their superiority over others leads to
their survival and accumulation. But what is the
result ? The result can only be monotypic evolution.
No matter how great the number, or how great the
range, of variations round an average specific mean,
out of such material natural selection can never
produce polytypic evolution : it may change the type
to any extent during successive generations, and
in a single line of change ; but it cannot branch
the type, unless some other form of homogamy
intervenes. Therefore, when Mr. Wallace adduces

Isolation. 37

the well-known fact that all structures vary more
or less round a specific mean as proof that natural
selection need not be incommoded by free inter-
crossing, but can of itself produce all the known
phenomena of specific evolution, he fails to perceive
that his argument refers only to one aspect of such
evolution (viz. the transformation of species in time),
and does not apply to the aspect with which alone
my paper on Physiological Selection was concerned
(viz. the multiplication of species in space).

The same thing may be shown in this way. It is
perfectly obvious that where the improvement of type
in a linear series is concerned (monotypic evolution),
free intercrossing, far from being a hindrance to the
process, is the very means by zvhich the process is
accomplished. Improvement here ascends by suc-
cessive steps, in successive generations, simply because
of the general intercrossing of the generally most fit
with the result that the species, as a whole, gradually
becomes transformed into another species, as a whole.
Therefore, it would be mere fatuity in any one to
adduce free intercrossing as a ^'difficulty" against
natural selection alone being competent to produce
evolution of this kind. But where the kind of
evolution is that whereby the species is differentiated
— where it is required, for instance, to produce different
structures in different portions of the species, such as
the commencement of a fighting spur on the wing of
a duck, or novel characters of any sort in different
groups of the species — free intercrossing is no longer
a condition to, but an absolute preventive of, the
process ; and, therefore, unless checked as between
each portion of the species by some form of homo-

38 Darwin^ and after Darwin.

gamy other than natural selection, it must effectually
inhibit any segregation of specific types, or divergence
of character.

Hence it is that, while no Darwinian has ever
questioned the power of unaided selection to cause
improvement of character in successive generations, in
common now with not a few other Darwinians I have
emphatically denied so much as the abstract possi-
bility of selection alone causing a divergence of char-
acter in two or more simultaneous lines of change.

And, although these opposite views cannot be
reconciled, I am under the impression that they do
admit of be^ng explained. For I take them to
indicate a continued failure to perceive the all-im-
portant distinction between evolution as monotypic
and polytypic. Unless one has fully grasped this
distinction, and constantly holds it in mind, he is
not in a position to understand the " difficulty " in
question ; nor can he avoid playing fast and loose
with natural selection as possibly the sole cause of
evolution, and as necessarily requiring the co-operation
of some other cause. But if he once clearly perceives
that "evolution" is a logical genus, of which the mono-
typic and the polytypic forms are species, he will
immediately escape from his confusion, and find that
while the monotypic form may be caused by natural
selection alone the polytypic form can never be
so caused.

The second difficulty which I have to mention as at
first sight attaching to the views of Mr. Gulick and
myself on the subject of Isolation is, that in an iso-
lated section of a species Mr. Francis Galton's law of

Isolation. 39

regression in the average character of offspring to
the typical character of the group through reversion
or atavism {^Natural Inheritance, p. 97) must have
the effect of neutralizing the segregative influence of
mere apogamy. That such, however, cannot be the
case has been well shown by Mr. Gulick in his paper
on Intensive Segregation, Without at all disputing
the validity of Mr. Galton's law, he proves that " it can
hold in full force only where there is free crossing,
otherwise no divergent race could ever be formed by
any amount of selection and independent breeding^"
This is so self-evident that I need not quote his demon-
stration of the point.

In conclusion, then, and having regard to the
principle of isolation as a whole, or in all the many and
varied forms in which this principle obtains, I trust that
I have redeemed the promise with which I set out —
viz. to show that in relation to the theory of descent
this principle is of an importance second to no other,
not even excepting heredity, variability, and the
struggle for existence. This has now been fully
shown, inasmuch as we have clearly seen that the im-
portance of the struggle for existence, and consequent
survival of the fittest, arises just because survival
of the fittest is a form, and a very stringent form, of
isolation ; while, as regards both heredity and vari-
ability, we are now in a position to see that the more
fully we recognize their supreme importance as
principles concerned in organic evolution, the more
must we also recognize that any rational theory of
such evolution becomes, in the last resort, a theory

' Zool. Journal Lin. Soc, vol. xxiii. p. 313.

40 Darwin^ and after Darwin,

of the different modes in which efficient isolation can
be secured. For, in whatever degree the process of
organic evolution has been dependent upon heredity
with variability, in that degree must it also have been
dependent upon the means of securing homogamy,
whereby alone the force of heredity can be made to
expend itself in the innumerable directions of pro-
gressive change, instead of continually neutralizing the
force of variability by promiscuous intercrossing.

CHAPTER III.

Physiological Selection.

So far we have been concerned with the principle
of Isolation in general. We have now to consider
that form of isolation which arises in consequence of
mutual infertility between the members of any group
of organisms and those of all other similarly isolated
groups occupying simultaneously the same area.

Against the view that natural selection is a sufficient
explanation of the origin of species, there are two
fatal difficulties: one, the contrast between natural
species and domesticated varieties in respect of cross-
sterility ; the other, the fact that natural selection
cannot possibly give rise to polytypic as distinguished
from monotypic evolution. Now it is my belief that
the theory of physiological selection fully meets both
these difficulties. Indeed I hold this to be undeniable
in a formal or logical sense : the only question is as
to the evidence which can be adduced for the theory
in a practical or biological sense. Therefore in this
chapter, where the theory has first of all to be stated,
I shall restrict the exposition as much as possible
to the former, leaving for subsequent consideration the
biological side.

42 Darwin^ and after Darwin

The following is a brief outline sketch of this
theory ^.

Of all parts of those variable objects which we
call organisms, the most variable is the reproductive
system ; and the variations may carry with them func-
tional changes, which may be either in the direction
of increased or of diminished fertility. Consequently
variations in the way of greater or less fertility fre-
quently take placC; both in plants and animals ; and
probably, if we had adequate means of observing this
point we should find that there is no one variation
more common. But of course where infertility arises
— whether as a result of changed conditions of life, or,
as we say, spontaneously — it immediately becomes
extinguished, seeing that the individuals which it
affects are less able (if able at all) to propagate and
to hand on the variation. If, however, the variant,
while showing some degree of infertility with the parent
form, continues to be as fertile as before when mated
with similar variants^ under these circumstances there
is no reason why such differential fertility should not
be perpetuated.

Stated in another form this suggestion enables us
to regard many, if not most, species as the records of
variations in the reproductive systems of their ancestors.
When variations of a non-useful kind occur in any
of the other systems or parts of organisms, they are,
as a rule, immediately extinguished by intercrossing.
But whenever they arise in the reproductive system
in the way here suggested, they tend to be preserved
as new natural varieties, or incipient species. At
first the difference would only be in respect of the

* Stt Nintteenth Century^ January, 1887, pp. 61, 6a.

Physiological Selection, 43

reproductive systems ; but eventually, on account of
independent variation, other differences would super-
vene, and the variety would take rank as a true
species.

Now we must remember that physiological isola-
tion is not like those other forms of isolation (e. g
geographical) which depend for their occurrence on
accidents of the environment, and which may therefore
take place suddenly in a full degree of complete-
ness throughout a large section of a species. Physio-
logical isolation depends upon distinctive characters
belonging to organisms themselves ; and it would
be opposed to the whole theory of descent with
progressive modification to imagine that absolute
sterility usually arises in a single generation between
two sections of a perfectly fertile species. Therefore
evolutionists must believe that in most, if not in all
cases — could we trace the history, say of any two
species, which having sprung from a single parent
stock on a common area, are now absolutely sterile
with one another - we should find that this mutual
sterility had been itself a product of gradual evolution.
Starting from complete fertility within the limits of a
single parent species, the infertility between derivative
or divergent species, at whatever stage hi their evolution
this began to occur, must usually at first have been well-
nigh imperceptible, and thenceforth have proceeded
to increase stage by stage.

But, if it be true that physiological isolation between
genetically allied groups must usually itself have been
the product of a gradual evolution ; and if, when
fully evolved, it constitutes a condition of the first
importance to any further differentiation of these

44 Darwin^ and after Darwin,

groups (by preventing fusion again into one group,
more or less resembling the original parent form), do
we not perceive at least a strong probability that
in the lower stages of its evolution such mutual in-
fertility must have acted as a segregating influence
between the diverging types, in a degree proportional
to its own development ? The importance of mutual
sterility as a condition to divergent evolution is not
denied, when this sterility is already present in an
absolute degree ; and we ha\e just seen that, before
it can have attained to this absolute degree, it must
presumably^ and as a rzde, itself have been the subject
of a gradual development. Does it not therefore
become, on merely antecedent grounds, in a high
degree probable, that from the moment of its in-
ception this isolating agency must have played the
part of a segregating cause, in a degree propor-
tional to that of its completeness as a physiological
character ?

Whoever answers this question in the affirmative
will have gone most of the way towards accepting, on
merely antecedent grounds, the theory of physiological
selection. And therefore it is that I have begun this
statement of the theory by introducing it upon these
grounds, thereby hoping to show how extremely simple
— how almost self-evident — is the theory which it will
now be my endeavour to substantiate. I may here
add that the theory was foreshadowed by Mr. Belt
in 1874 ^ clearly enunciated in its main features by
Mr. Catchpool in 1884 2, and very fully thought out
by Mr. Gulick during a period of about fifteen years,

' Nicaragua, p. 207.
• Nature, vol. xxxi. p. 4.

Physiological Selection, 45

although he did not publish until a year after the
appearance of my own paper in 1 886 V

I must next proceed to state some of the leading
features of physiological selection in further detail.

It has already been shown that Darwin clearly
perceived that the very general occurrence of some
degree of infertility between allied species cannot
possibly be attributed to the direct agency of natural
selection. His explanation was that the slight struc-
tural modifications entailed by the transformation of
one specific type into another, so react upon the
highly delicate reproductive system of the changing
type as to render it in some degree infertile with
its parent type. Now the theory of physiological
selection begins by traversing this view. It does
not, however, deny that in some cases the morpho-
logical may be the prior change ; but it strenuously
denies that this must be so in all cases. Indeed,
according to my statement in 1886, the theory inclines
to the view that, as a rule, the physiological change
is prior. At the same time, the theory, as I have
always stated it, maintains that it is immaterial whether,
"in the majority of instances,^' the physiological change
has been prior to the morphological, or vice versa ;
since in either case the physiological change will
equally make for divergence of character.

' Zool. Journal ^ Lin. Soc.^ vol. xix. pp. 337-411 (1886); and for
Mr. Gulick's papers, ibid., vol. xx. pp. 189-274 (1887), vol. xxiii.
pp. 312-380 (1889). Mr. Gulick has recently drawn my attention, in
a private letter, to the fact that as early as 1872 a paper of his was
read at the British Association, bearing the title Diversity of Evolution
under one set of External Conditions, and that heie the principle of
physiological segregation is stated. Although it does not appear that
Mr. Gulick then appreciated the great importance of this principle, it
entitles him to claim priority.

46 Darwifiy and after Darwin.

To show this clearly the best way will be to consider
the two cases separately, taking first that in which
the physiological change has priority. In this case
our theory regards any morphological changes which
afterwards supervene as due to the independent varia-
bility which will sooner or later arise under the
physiological isolation thus secured. But to what-
ever causes the subsequent morphological changes
may be due, the point to notice is that they are as
a general rule, consequent upon the physiological
change. For in whatever degree such infertility arises
between two sections of a species occupying the same
area, in that degree is their interbreeding prevented,
and, therefore, opportunity is given for a subsequent
divergence of type, whether by the influence of inde-
pendent variability alone, or also by that of natural
selection, as now acting more or less independently
on each of the partially separated groups. In short,
all that was said in the foregoing chapters with respect
to isolation in general, here applies to physiological
isolation in particular ; and by supposing such isola-
tion to have been the prior change, we can as well un-
derstand the subsequent appearance of morphological
divergence on continuous areas, as in other forms
of isolation we can understand such divergence on
discontinuous areas, seeing that even a moderate
degree of cross-infertility may be as effectual for
purposes of isolation as a high mountain-chain, or
a thousand miles of ocean.

Here, then, are two sharply-defined theories to
explain the very general fact of there being some
greater or less degree of cross-infertility between allied
species. The older, and hitherto current theory,

Physiological Selection. 47

supposes the cross-infertility to be but an accident
of specific divergence, which, therefore, has nothing
to do with causing the divergence. The newer theory,
on the other hand, supposes the cross-infertility to
have often been a necessary cofidition to the diverg-
ence having begun at all. Let us now consider which
theory has most evidence in its favour.

First of all we have to notice the very general
occurrence of the fact in question. For when we
include the infertility of hybrids, as well as first
crosses, the occurrence of some degree of infertility
between allied species is so usual that Mr. Wallace
recommends experiments to ascertain whether careful
observation might not prove, even of species which
hybridize, *' that such species, when crossed with their
near allies, do always produce offspring which are
more or less sterile inter se^T This seems going too
far, but nevertheless it is the testimony of a highly
competent naturalist to the very general occurrence of
an association between the morphological differentia-
tion of species and the fact of a physiological isolation.
Now I regard it as little short of self-evident that this
general association between mutual infertility and
innumerable secondary, or relatively variable mor-
phological distinctions, is due to the former having
been an original and a necessary condition to the
occurrence of the latter, in cases where intercrossing
has not been otherwise prevented.

The importance of physiological isolation, when
once fully developed, cannot be denied, for it is evident
that if such isolation could be suddenly destroyed
between two allied species occupying a common area,

* Darwinism, p. 169.

48 Darwin J and after Darwin,

they would sooner or later become fused into a
common type — supposing, of course, no other form
of isolation to be present. The necessity then for
this physio] oj^ical form of isolation in maintainmg
a specific differentiation which has been already at-
tained cannot be disputed. Yet it has been regarded
as '* Darwinian heresy" to suggest that it can have
been of any important service during the process of
attainment^ or while the specific differentiation is
being advanced, and this notwithstanding that the
physiological change must presumably have developed
pari passu with the morphological, and notwithstand-
ing that in countless cases the former is associated
with every conceivable variety of the latter.

Again, why should the physiological change be
thus associated with every conceivable variety of
morphological change ? Throughout the length and
breadth of both vegetable and animal kingdoms we
find this association, in the great majority of cases,
where new species arise. Therefore, on the supposi-
tion that in all such cases the physiological change
has been adventitiously induced by the morpho-
logical changes, we have to face an apparently unan-
swerable question — Why should the reproductive
mechanism of all organic beings have been thus
arranged, as it were, to chang;e in immediate response
to the very slightest alteration in the complex har-
mony of *• somatic '* processes, which now more than
ever is recognized as exercising so comparatively
little influence on the hereditary endowments of this
mechanism ? Consider the difference between a worm
and the bird that is eating it, an oak tree and the
gall-insect that is piercing it : are we to suppose that

Physiological Selection, 49

in all cases, no matter how greatly the types differ,
they must agree in this, that when any parts of
these complex structures change, ever so slightly,
the reproductive system is almost certain to be
adventitiously affected, yet always thus affected in
the same peculiar way?

If it be answered that the reproductive system is
known to be very sensitive to slight changes in the
external conditions of life, the answer proves too
much. For though this is true, yet our opponents
must acknowledge that the reproductive system is
not so sensitive, in this particular respect, as their
interpretation of the origin of specific infertility
requires. The proof of this point is overwhelming,
for there is the evidence from the entire range of our
domesticated productions, both vegetable and animal.
Here the amount of structural change, which has been
slowly accumulated by artificial selection, is often
much greater in amount, and incomparably more
rapid, than that which has been induced between
allied species by natural selection ; and yet there is
scarcely any indication of the reproductive system
having been affected in the particular way that our
opponents* theory requires. There are many in-
stances of its having been affected in sundry other
ways (chiefly, however, without any accompanying
morphological change) ; but among all the thousands
of our more or less enormously modified artificial
types, there is scarcely one instance of such a peculiar
sexual relation between the modified descendants of
a common type as so usually obtains between allied
species in nature. Yet in all other respects evolu-
tionists are bound to believe that the process of

III. E

50 Darwin, and after Dai^n,

modification has been in both cases strictly analogous
Why then this conspicuous difference with respect to
the reproductive system ?

The answer is simple. It has never been the object
of breeders or of horticulturists to select variations
in the direction of cross-infertility, for the swamping
effects of intercrossing are much more easily and
rapidly prevented by artificial isolation. Consequently,
although they have been able to modify natural types
in so many directions and in such high degrees with
regard io morphology, there has been no accompanying
physiological modification of the kind required. But
in nature there is no such thing as artificial, i.e. in-
tentional, isolation. Consequently, on common areas
it must usually happen that those changes of mor-
phology which are associated with cross-infertility
are the only ones which can arise. Hence the very
remarkable contrast between our domesticated varie-
ties and natural species with regard to cross-infertility
is just what the present theory would expect, or,
indeed, require. But on any other theory it has
hitherto remained inexplicable.

In particular, the contrast in question has consti-
tuted one of the main difficulties with which the theory
of natural selection has hitherto had to contend, not
only in the popular mind, but also in the judgement
of naturalists, including the joint-authors of the theory
themselves. Thus Darwin says : —

The fertility of varieties is, with reference to my theory,
of equal importance with the sterility of species, for it seems
to make a broad and clear distinction between varieties and

species ^

Origin of Species, p. 236.

Physiological Selection, 51

And Mr. Wallace says : —

One of the greatest, or perhaps we may say the greatest, of
all the difficulties in the way of accepting the theory of natural
selection as a complete explanation of the origin of species, has
been the remarkable difference between varieties and species in
respect of fertility when crossed \

Now, in view of this conspicuous contrast, Darwin
suggested that species in a state of nature '" will have
been exposed during long periods of time to more
uniform conditions than have domesticated varieties,
and [that] this may well make a wide difference in the
result." Now we have to remember that species, living
and extinct, are numbered by millions, and represent
every variety of type, constitution, and habits ; is
it probable, then, that this one peculiarity of the
reproductive system should be due, in so many cases,
to some merely incidental effect produced on that
system by uniform conditions of life? Again, ex
hypothesis at the time when a variety is first forming,
the influence exercised by uniform conditions of life
(whatever in different cases this may happen to be)
cannot be present as regards that variety : yet this is
just the time when its infertility with the parent (or
allied) form is most likely to have arisen ; for it is
just then that the nascent variety would otherwise
have been most liable to extinction by free inter-
crossing— even supposing that in the presence of such
intercrossing the variety could ever have come into
existence at all.

Mr. Wallace meets the difficulty by arguing that
sterility between allied species may have been brought
about by the direct influence of natural selection.

* Darwinism, p. 15a.
E 2

52 Darwin, and after Darwin,

But, as previously remarked, this view is expressly
opposed to that of Darwin, who held that Wallace's
contention is erroneous.

It will be seen, then, that both Darwin, and Wallace,
fully recognize the necessity of finding some explana-
tion of the infertility of allied species, over and above
the mere reaction of morphological differentiation on
the physiology of the reproductive system, and they
both agree in suggesting additional causes, though
they entirely disagree as to what these causes are.
Now, the theory of physiological selection likewise
suggests an additional cause— or, rather, a new ex-
planation— and one which is surely the most probable.
For what is to be explained? The very general
association of a certain physiological peculiarity with
that amount of morphological change which dis-
tinguishes species from species, of whatever kind the
change may be, and in whatever family of the animal
or vegetable kingdom it may occur. Well, the theory
of physiological selection explains this very general
association by the simple supposition that, at least
in a large number of cases, it was the physiological
peculiarity which first of all led to the morphological
divergence, by interposing the bar of sterility between
two sections of a previously uniform species ; and by
thus isolating the two sections one from another,
started each upon a subsequently independent course
of divergent evolution.

Or, to put it in another way, if the occurrence of
this physiological peculiarity has been often the only
possible means of isolating two sections of a species
occupying a common area, and thus giving rise to
a divergence of specific type (as obviously must have

Physiological Selection, 53

been the case wherever there was an absence of any
other form of isolation), it is nothing less than a
necessary consequence that many allied species should
now present the physiological peculiarity in question.
Thus the association between the physiological pecu-
liarity and the morphological divergence is explained
by the simple hypothesis, that the former has acted
as a necessary condition to the occurrence of the
latter. In the absence of other forms of isolation,
the morphological divergence could not have taken
place at all, had not • the physiological peculiarity
arisen ; and hence it is that we now meet with so
many cases where such divergence is associated with
this peculiarity.

So far we have been considering the physiological
change as historically the prior one. Here, at first
sight, it may seem that the segregative power of
physiological selection must end ; for it may well
seem impossible that the physiological chani^e can
ever be necessary for the divergence of morphological
varieties into true species in cases where it has not
been the prior change, but has only set in after mor-
phological changes have proceeded far enough to have
already constituted definite varieties. A little thought,
however, will show that physiological selection is quite
as potent a condition to the differentiation of species
when it occurs after varietal divergence has begun, as
it is when it occurs before the divergence — and hence
that it really makes no difference to the theory of
physiological selection whether, in particular cases, the
cross-infertility arises before or after any structural or
other modifications with which it is associated.

54 Darwin, and after Darwin.

For the theory does not assert that all varieties
have been due to physiological selection. There are
doubtless many other causes of the origin of varieties
besides cross-infertility with parent forms; but, as
a general rule, it does not appear that they are by
themselves capable of carrying divergence beyond
a merely varietal stage. In order to carry divergence
to the stage of producing species, it appears to be
a general condition that sooner or later, cross-infertility
should arise — seeing that, when varieties do succeed
in becoming species, we almost invariably find that,
as a matter of fact, cross-infertility has arisen. Hence,
if cross-infertility has thus usually been a necessary
condition to a varietal divergence becoming specific,
it can make no material difference when the incipient
infertility arose.

It may be asked, however, whether I suppose that;
when the physiological change is subsequent, it is
directly caused by change of structure, size, colour, &c.,
or that it arises, so to speak, accidentally, from other
causes which may have affected the sexual system in
the required way. To this question I may briefly
reply, that, looking to the absence of any influence
exercised on the reproductive systems of our domesti-
cated plants and animals by the great and varied
changes which so many of these forms present, it
would seem that among natural varieties such qlosely
analogous changes are presumably not the usual causes
of the physiological change, even where the latter are
subsequent to the former. Nevertheless, I do not
deny that in some of these cases changes of structure,
size, colour, &c., may be the causes of the physiological
change by reacting on the sexual system in the re-

Physiological Selection, 55

quired way. But in such cases free intercrossing will
have prevented the perpetuation of any morphological
changes, save those which have the power of so re-
acting on the reproductive system as to produce the
physiological change, and thus to protect themselves
against the full and adverse power of free intercrossing.
We know that slight or initial changes of structure,
colour, &c., frequently occur as varieties, and yet that
on common areas very few of these varieties become
distinct species : free intercrossing prevents any such
further divergence of character. But if in the course
of many such abortive attempts, as it were, to produce
a new species, nature happens to hit upon a structural
or a colour variation which is capable of reacting on
the sexual system in the particular way required, then
this variation will be enabled to protect itself against
free intercrossing in proportion to its own development.
Or, in other words, the more it develops as a morpho-
logical change, the more will it increase the physio-
logical change ; while the more the physiological
change is thus increased, the more will it in turn
promote the morphological. By such action and
reaction the development of each furthers the develop-
ment of the other, till from an almost imperceptible
variety, apparently quite fertile with its parent form,
there arises a distinct species absolutely sterile with
its parent form. In such cases, therefore, it is still
the physiological conditions which have selected the
particular morphological changes capable of so react-
ing on the reproductive system as to produce cross-
infertility, and thus to protect themselves against the
destructive power of free intercrossing. So to spe^k,
free intercrossing is alv\ays on the watch to level

56 Darwin, and after Darwin,

down any changes which natural selection, or any
other cause of varietal divergence, may attempt to
produce ; and therefore, in order to produce — or to
increase — such divergence in the absence of any other
form of isolation, natural selection must hit upon such
changes of structure, form, or colour, as are so cor-
related with the reproductive system as to create the
physiological isolation that is required.

To show how the principle of selective fertility
may be combined with what apparently is the most
improbable form of isolation for this purpose — the
geographical — I quote the following suggestion made
by Professor Lloyd Morgan in his Animal Life and
Intelligence : —

Suppose two divergent local varieties were to arise in
adjacent areas, and were subsequently (by stress of competition
or by geographical changes) driven together into a singe
area. ... If their unions be fertile, the isolation will be an-
nulled by intercrossing— the two varieties will form one mean or
average variety. But if the unions be infertile, the isolation
will be preserved, and the two varieties will continue separate.
Suppose now, and the supposition is by no means an improb-
able one, that this has taken place again and again in the
evolution of species ; then it is clear that those varietal forms
which had continued to be fertile together would be swamped
by intercrossing ; while those varietal forms which had become
infertile would remain isolated. Hence, in the long run, iso-
lated forms occupying a common area would be infertile,
(p. 107.)

If then cross-sterility may thus arise even in associ-
ation with geographical isolation, may it not also
arise in its absence ? And may it not thus give rise
to the differentiation of varieties on account of this
physiological isolation alone?

Physiological Selection, 57

Only two further points need be mentioned to
make this statement of physiological selection as
complete as the present rhum^ of its main principles
requires.

The first is, that, as Mr. Wallace remarks, '* every
species has come into existence coincident both in
space and time with a pre-existing and closely allied
species." I regard this as important evidence that
physiological selection is one of the natural causes
concerned. For the general fact implied is that every
species has come into existence on an area occupied
by its parent type, and therefore under circumstances
which render it imperative that intercrossing with that
type should be prevented. In the case of monotypic
evolution by natural selection alone, intercrossing
with the parent type is prevented through the gradual
extinction of that type by successive generations of
the developing type. But in the case of polytypic
evolution, intercrossing with the parent type can
only be prevented by some form of isolation other
than natural selection ; and here it is evident that
cross-infertility with the parent type must be as
efficient to that end as any other form of isolation
that can be imagined. Consequently we might
almost have expected beforehand, that in a large
proportional number of cases cross-infertility should
have been the means employed. And the fact that
this is actually the case so far corroborates the only
theory which is able to explain it.

The second point is this.

It appears to be comparatively rare for any cause
of specific divergence to prove effectual on common
areas, unless it sooner or later becomes associated with

58 Darwifif and after Darwin.

some degree of cross-infertility. But through this
association, the segregating influence of both the
causes concerned is, as Mr. Gulick has shown, greatly
increased. For instance, if the segregating influence
of some degree of cross-infertility be associated with
that of any othef form of isolation, then, not only
will the two segregating influences be added, but
multiplied together. And thus, by their mutual
action and reaction, divergent evolution is promoted
at a rapidly increasing rate.

I will now summarize the main points of the theory
of physiological isolation in a categorical form.

1. If no other form of isolation be present, specific
divergence can only take place when some degree of
cross-infertility has previously arisen between two or
more sections of a species.

2. When such cross-infertility has arisen it may
cause specific divergence, either {a) by allowing in-
dependent variability in each of the physiologically
isolated groups ; {b) by becoming associated with any
other cause of diflerentiation already operating ; or
{c) by both these means combined.

3. As some degree of cross-infertility generally
obtains between allied species, we are justified in
concluding that this has been the most frequent
— or, at any rate, the most effective— kind of isola-
tion where the origin of species is concerned ; and
therefore the kind with which, in the case of
species-formation, natural selection, or any other
cause of specific divergence, has been most usually
associated.

4. Where varietal divergence has begun in the

Physiological Selection, 59

absence of cross-infertility, such divergence seems, as
a general rule, to have been incapable of attaining to a
specific value.

5. Therefore, in the. vast majority of such cases, it
must have been those varietal changes of structure,
size, colour, &c., which happened to have afterwards
been assisted by the reproductive change that were
on this account selected as successful candidates for
specific differentiation.

6. It follows, that it makes no difference to the
general theory of physiological selection in what pro-
portion of cases the physiological change has been
the initial change ; for, whether prior or subsequent
to the varietal changes with which it becomes associ-
ated, its presence has been equally important as a
condition to specific divergence.

7. When physiological isolation becomes associated
with natural selection, or any other form of homogamy,
the segregative power of both is augmented. More-
over, so great is the augmentation that even very
moderate degrees of physiological isolation — them-
selves capable of effecting little or nothing — become
very powerful when associated with moderate degrees
of any other kind of homogamy, and vice versa.

8. The theory of physiological selection effec-
tually explains the divergent evolution of specific
types and the cross-infertility of such types when
evolved.

To prevent, if possible, the continuance of certain
misunderstandings with regard to my original state-
ment of the new theory, let me here disclaim some
yiews which have been assigned to me. They are :

6o Darwin^ and after Darwin,

1. That the theory of physiological selection is
opposed to the theory of natural selection. Far from
this being so, it is — at all events in my own opinion — a
very important aid to it, in preventing free intercross-
ing on a common area, and thus allowing divergent
evolution to occur within that area.

2. That, in advancing the theory of physiological
selection as *' an additional suggestion on the origin
of species," I wish to represent it as being the
originating cause of all species. What I hold is, that
all species must have owed their origin to isolation^ in
some form or other ; but that as physiological selection
is only one among many other forms of isolation (in-
cluding natural selection), and as it can only act on
common areas, a large number of species must have
been formed without its aid.

3. That I imagine physiological varieties always
to arise " sporadically," or as merely individual
*' sports " of the reproductive system. On the con-
trary, I expressly stated that this is not the way in
which I suppose the " physiological variation " to
arise, when giving origin to a new species ; but that
it arises, whenever it is effectual, as a " collective
variation" affecting a number of individuals simul-
taneously, and therefore characterizing " a whole race,
or strain.^'

4. That I suppose physiological selection always to
act alone. This I have never supposed. The essential
point is, not that the physiological isolation is un-
associated with other forms of isolation, but that
unless associated with some degree of physiological
isolation, no one of the other forms is capable of
originating species on common areas with any approach

I

Physiological Selection, 6i

to frequency. This proposition is the essence of
the new theory, and I take it to be proved, not only
by general deductive reasoning which shows that
it must be so, but also by the fact of an otherwise
inexplicable association between specific divergence
on common areas and some more or less considerable
degree of mutual infertility.

CHAPTER IV.
Evidences of Physiological Selection.

I WILL now give an outline sketch of the evidences
in favour of the theory which has been ict forth in
the preceding chapter, stating first what is the nature
of the verification which it requires.

The theory is deduced from a highly general
association between distinctive specific characters
of any kind and a relatively constant specific
character of a particular kind — namely, sexual
exclusiveness. For it is from this highly general
association that the theory infers that this relatively
constant specific character has been at least one of
the needful conditions to the development of the
other specific characters with which it is found
associated. Hence the necessary verification must
begin by showing the strength of the theory on these
merely deductive, or antecedent, grounds. It may
then proceed to show how far the facts of organic
nature corroborate the theory in other and inde-
pendent ways.

First, let it be carefully observed that here we have
to do only with Xk^o^ fact of selective fertility, and with
its consequences as supposed by the theory : we have

I

Evidences of Physiological Selection, 63

nothing to do either with its causes or its degrees.
Not with its causes, because in this respect the
theory of physiological selection is in just the same
position as that of natural selection : it is enough for
both if the needful variations are provided, without
its being incumbent on either to explain the causes
which produce them. Not with its degrees, because,
in the first place, it can only be those degrees of
variation which in particular cases are supposed
adequate to induce specific divergence, that fall
within the scope of the theory ; and because, in the
second place, degrees which are adequate only to
induce — or to assist in inducing - varietal divergence,
must always tend to increase, or pass into higher
degrees.

Antecedent Standing of the Theory,

The antecedent standing or logical basis of the
theory has already been in large measure displayed
in the preceding chapter; for it was impossible to
state the theory without thereby showing in how
considerable a degree it is self-evident. A brief
recapitulation is therefore all that is here necessary

It has been shown that divergent or polytypic
evolution "on common areas is inexplicable by natural
selection alone. Hence the question arises: What
form of isolation has, under such circumstances,
rendered possible divergent evolution? In answer
to this question the theory of physiological selection
suggests that variations in the reproductive function
occur in such a way as to isolate more or less
perfectly from each other different sections of a
species. While cross-fertility remains unimpaired

64 DarwiUy and after Darwin,

among the members of each section, there is more or
less cross-infertility when members of either section
mate with those of the other. Thus a physiological
barrier is interposed between the two sections ; and
any divergences of structure, colouring, or instinct
arising in the members of either section will not in
any way be affected by such divergences as arise
among the members of the other.

In support of this suggestion, it has been shown in
the preceding chapter that the very general association
of cross-infertility with specific differentiation points
most strongly to the inference that the former has
usually been an indispensable condition to the
occurrence of the latter. It cannot be denied that
in many cases the specific distinction is now main-
tained by means of that sexual isolation which cross-
infertility confers : it is therefore probable that such
isolation has been instrumental in securing its initial
attainment.

This probability is strengthened by the observed
fact that the general association in question is
conspicuously absent in the case of domesticated
varieties, notwithstanding that their multitudinous
and diverse varietal characters usually equal, and
frequently surpass, specific characters in their degrees
of divergence.

Since, then, it would seem to be impossible for
divergent evolution on common areas to take place
in the absence of some mode of isolation ; since
cross- infertility appears to be the only possible mode
under the given circumstances ; and since among
domesticated varieties, where isolation is otherwise
secured by artificial means, cross-infertility is usually

I

Evidences of Physiological Selection. 65

absent, the logical foundations of the theory of

physiological selection would seem to be securely laid.

We may therefore pass to more special lines of

evidence.

Evidence from Geographical Distribution.

Darwin has adduced very good evidence to show
that large areas, notwithstanding the disadvantages
which (on his theory) must arise from free inter-
crossing, are what he terms better manufactories of
species than smaller areas, such as oceanic islands.
On the other hand, as a matter of fact, oceanic
islands are comparatively rich in peculiar species.
These two statements, however, are not incompatible.
Smaller areas are, as a rule, rich in peculiar species
relatively to the number of their inhabitants ; but
it does not follow that they are rich in species as
contrasted with larger areas containing very many
more inhabitants. Therefore, the rules are, that
large areas turn out an absolutely greater number
of specific types than small areas ; although, relatively
to the number of individuals or amount of population,
the small areas turn out a larger number of species
than the large areas.

Now, these two complementary rules admit of
being explained as Darwin explains them. Small
and isolated areas are rich in species relatively to
the amount of population, because, as we have before
seen, this population has been permitted to develop
an independent history of its own, shielded from
intercrossing with parent forms, and from competition
with exotic forms ; while, at the same time, the
homogamy thus secured, combined with change of

III. F

66 Darwin, and after Darwin.

environment, will give natural selection an improved
chance of finding new points of departure for its
operation. On the other hand, large and continuous
areas are favourable to the production of numerous
species, first, because they contain a large population,
thus favouring the occurrence of numerous variations ;
and, secondly, because the large area furnishes
a diversity of conditions in its different parts, as to
food, climate, attitude, &c., and thus so many
different opportunities for the occurrence of sundry
forms of homogamy. Now, it is obvious that of all
these sundry forms of homogamy, physiological
selection must have what may be termed a first-rate
opportunity of assisting in the manufacture of species
on large areas. For not only is it upon large and
continuous areas that the antagonistic effects of
intercrossing are most pronounced (and, therefore,
that the influence of pliysiological selection must be
most useful in the work of species-making) ; but here
also the diversity in the external conditions of life,
which the large area supplies to different parts of
the extensive population, cannot fail to furnish physio-
1( gical selection with a greater abundance of that
particular variation in the reproductive system on
which its action depends. Again, and of still more
importance, on large areas there are a greater number
of species already differentiated from one another
as such ; thus a greater number of already sexually
differentiated forms are presented for further differen-
tiation at the hands of physiological selection. For
all these reasons, therefore, we might have expected,
upon the new theory, that large and continuous areas
would be good manufactories of species.

Evidences of Physiological Selection. 67

Again, Darwin has shown that not only large
areas, but likewise "dominant" genera within those
areas, are rich in species. By dominant genera he
meant those which are represented by numerous
individuals, as compared with other genera inhabiting
the same area. This general rule he explains by the
consideration that the qualities which first led to the
form being dominant must have been useful ; that
these would be transmitted to the otherwise varying
offspring; and, therefore, that when these offspring
had varied sufficiently to become new species, they
would still enjoy their ancestral advantages in the
struggle for existence. And this, doubtless, is in part
a true explanation ; but I also think that the reason
why dominant genera are rich in species, is chiefly
because they everywhere present a great number of
individuals exposed to relatively great differences in
their conditions of life : or, in other words, that they
furnish the best raw material for the manufacture of
species by physiological selection, as explained in
the last paragraph. For, if the fact of dominant
genera being rich in species is to be explained only
by natural selection, it appears to me that the useful
qualities which have already led to the dominance
of the ancestral type ought rather to have proved
inimical to its splitting up into a number of sub-
ordinate types. If already so far " in harmony with
its environment " as to have become for this reason
dominant, one would suppose that there is all the
more reason for its not undergoing change by the
process of natural selection. Or, at least, I do not
see why the fact of its being in an unusual degree
of harmony with its environment should in itself

F z

68 Darwin, and after Darwin,

constitute any unusual reason for its modification by
survival of the fittest. On the other hand, as just
observed, I do very plainly see why such a reason
is furnished for the modifying influence of physio-
logical selection.

Let us next turn to another of Darwin's general
rules with reference to distribution. He took a great
deal of trouble to collect evidence of the two following
facts, namely, (i) that "species of the larger genera
in each country vary more frequently than the species
of the smaller genera"; and (2) that " many of the
species included within the larger genera resemble
varieties in being very closely, but unequally, related
to each other, and in having restricted ranges ^"
By larger genera he means genera containing many
species ; and he accounts for these general facts by
the principle, '* that where many species of a genus
have been formed, on an average many are still
forming." But how forming? If we say by natural
selection alone, we should expect to find the multi-
tudinous species differing from one another in respect
of features presenting well-marked adaptive meanings ;
yet this is precisely what we do not find. For
Darwin's argument here is that " in large genera the
amount of difference between the species is often
exceedingly small, so that in this respect the species
of the larger genera resemble varieties more than do
the species of the smaller genera." Therefore the
argument, while undoubtedly a very forcible one in
favour of the fact of evolution, appears to me scarcely
consistent with the view of this evolution being due
solely to natural selection. On the other hand, the

^ Origin of species, ^^. ^^, /{I.

Evidences of Physiological Selection. 69

argument tells strongly (though unconsciously) in
favour of physiological selection. For the larger a
genus, or the greater the number of its species, the
greater must be the opportunity for the occurrence
of that particular kind of variation on which the
principle of physiological selection depends. The
species of a genus may be regarded as so many
varieties which have already been separated from one
another physiologically ; therefore each of them may
now constitute a new starting-point for a further and
similar separation — particularly as, in virtue of their
previous segregation, many are now exposed to
different conditions of life. Thus, it seems to me,
we can well understand why it is that genera already
rich in species tend to grow richer ; while such is not
the case in so great a degree with genera that are
poor in species. Moreover, we can well understand
that, multiplication of species being as a rule, and in
the first instance, determined by changes in the repro-
ductive system, wherever a large number of new
species are being turned out, the secondary differences
between them should be '* often exceedingly small " —
a general correlation which, so far as I can see, we
are not able to understand on the theory of natural
selection.

The two subsidiary facts, that very closely allied
species have restricted ranges, and that dominant
species are rich in varieties, both seem to tell more
in favour of physiological than of natural selection.
For "very closely allied species "is but another name
for species which scarcely differ from one another
at all except in their reproductive systems ; and,
therefore, the more restricted their ranges, the more

70 DarwtHj and after Darwin,

certainly would they have become fused by inter-
crossing with one another, had it not been for the
barrier of sterility imposed by the primary distinc-
tion. Or rather, I should say, had it not been
for the original occurrence of this barrier, these now
closely-allied species could never have become species.
Again, that dominant species should be rich in varie-
ties is what might have been expected ; for the
greater the number of individuals in a species, the
greater is the chance of variations taking place in
all parts of the organic type, and particularly in the
reproductive system, seeing that this system is the
most sensitive to small changes in the conditions
of life, and that the greater the number of indi-
viduals composing a specific type, the more certainty
there is of some of them encountering such
changes. Hence, the richness of dominant species
in varieties is, I believe, mainly due to the greater
opportunity which such species afford of some degree
of cross-infertility arising between their constituent
members.

Here is another general fact, also first noticed by
Darwin, and one which he experiences some difficulty
in explaining on the theory of natural selection. He
says : —

In travelling from north to south over a continent, we generally
meet at successive intervals with closely-allied or representative
species, evidently filling the same place in the economy of the
land. These representative species often meet and interlock,
and as one becomes rarer and rarer, the other becomes more and
more frequent, till the one replaces the other. But if we com-
pare 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 of each. ... In the

I

Evidences of Physiological Seleciton. 71

intermediate region, having intermediate conditions of life, why
do we not now find closely- linking intermediate varieties ? This
difficulty for a long time quite confounded me. But I think it
can in large part be explained '.

His explanation is that, " as the neutral territory
between two representative species is generally narrow
in comparison with the territory proper to each,
. . . and as varieties do not essentially differ from
species, the same rule will probably apply to both ; and,
therefore, if we take a varying species inhabiting
a very large area, we shall have to adapt two varieties
to two large areas, and a third variety to a narrow
intermediate zone.'* It is hence argued that this
third or intermediate variety, on account of its existing
ill lesser numbers, will probably be soon overrun and
exterminated by the larger populations on either side
of it. But how is it possible " to adapt two varieties
to two large areas, and a third [transitional] variety
to a narrow intermediate zone," in the face of free
intercrossing on a continuous area? Let A, B, and
C represent the three areas in question. According to

A

B

C

the argument, variety A passes first into variety B^
and then into variety C, while variety B eventually
becomes exterminated by the inroads both from
A and C. But how can all this have taken place
with nothing to prevent intercrossing throughout the
entire area A,B^C? I confess that to me it seems this
argument can only hold on the supposition that the
analogy between varieties and species extends to the

* Ori^'n of species, ed. 6, pp. 134, 135*

72 Darwin J and after Darwin,

reproductive system ; or. in a sense more absolute
than the argument has in view, that "varieties do
not essentially differ from the species" which they
afterwards form, but from the first show some
degree of infertility towards one another. And, if so,
we have of course to do with the principles of physio-
logical selection.

That in all such cases of species distribution these
principles have played an important part in the
species-formation, appears to be rendered further
probable from the suddenness of transition on the
area occupied by contiguous species, as well as from
the completeness of it— i. e. the absence of connecting
forms. For these facts combine to testify that the
transition was originally due to that particular change
in the reproductive systems of the forms concerned,
which still enables those forms to *' interlock " without
intercrossing. On the other hand, neither of these
facts appears to me compatible with the theory of
species-formation by natural selection alone.

But this leads us to another general fact, also
mentioned by Darwin, and well recognized by all
naturalists, namely, that closely allied species, or
species differing from one another in trivial details,
usually occupy contiguous areas ; or, conversely stated,
that contiguity of geographical position is favourable
to the appearance of species closely allied to one
another. Now, the large body of facts to which
I here allude, but need not at present specify, appear
to me to constitute one of the strongest of all my
arguments in favour of physiological selection. Take,
for instance, a large continental area, and follow across
it a chain of species, each link of which differs from

Evidences of Physiological Selection. 73

those on either side of it by the minute and trivial
distinctions of a secondary kind, but all the links
of which differ from one another in respect of the
primary distinction, so that no one member of the
series is perfectly fertile with any other member. Can
it be supposed that in every case this constant
primary distinction has been superinduced by the
secondary distinctions, distributed as they are over
different parts of all these kindred organisms, and
yet nowhere presenting any but a trifling amount of
morphological change ?

For my own part, I cannot believe — any more
than Darwin could believe — that all these numerous,
diverse, and trivial changes have always had the
accidental effect of inducing the same peculiar change
in the reproductive system, and so producing it with-
out any reference to the process of specific divergence.
Nor can I believe, as Darwin incidentally and pro-
visionally suggested, that prolonged exposure to
uniform conditions of life have so generally induced
an equally meaningless result. I can only believe
that all the closely allied species inhabiting our
supposed continent, and differing from one another
in so many and such divers points of small detail, are
merely so many records of the fact that selective
fertility has arisen among their ancestry, and has
thus given as many opportunities for the occurrence
of morphological differentiations as it has furnished
cases of efficient isolation. Of course, I do not deny
that many, or probably most, of these trivial morpho-
logical differentiations have been produced by natural
selection on account of their utility : I merely deny
that they could have been so produced on this

74 Darwin J and after Darwin,

common area, but for the sexual isolation with which
every distinct set of them is now found to be asso-
ciated.

Evidence from Topographical Distribution of
Species.

By topographical distribution I mean the distri-
bution of organisms with reference to comparatively
small areas, as distinguished from larger regions with
reference to which the term geographical distribution
is appropriate.

It will be at once apparent that a study of the
topographical distribution of organic types is of even
more importance for us than a study of their geogra-
phical distribution. For while the former study is
conducted, as it were, with a low power of our
observing microscope, the latter is conducted with
a high power. The larger facts of geographical
distribution yield, indeed, all the general characters
which we might expect them to yield, on the theory
that divergence of specific types on common areas
has been in chief part determined by physiological
conditions. But for the purpose of testing this
theory in a still more exacting manner, it is of the
first importance to consider the more detailed facts
of topographical distribution, since we here come to
closer quarters with the problem of specific differen-
tiation. Therefore, as we have already considered
this problem under the most general points of view,
we will now consider it under more special points
of view.

It is self-evident, as we have seen in the preceding

Evidences of Physiological Selection. 75

section, that the greater the number of individuals
of the same species on a given area, the less must
be the power of natural selection to split that species
into two or more allied types ; because, the more
crowded the population, the greater must be the
uniformitarian effect of free intercrossing. This ob-
vious fact has been insisted upon by several previous
writers on Darwinism ; and the only reason why it
has not been recognized by all naturalists is, that so
few of them have observed the all-important dis-
tinction between monotypic and polytypic evolution.
The denser the population, and therefore the greater
the intercrossing and the severer the struggle for
existence within the species, the better will it be
for transmutation of the species by natural selection ;
but the worse it will be for differentiation of the
jpecies by this form of homogamy. On the other
hand, if physiological selection be entertained as
a form of homogamy, the denser the population, the
better opportunity it will have of differentiating the
species, first, because a greater number of individuals
will be present in which the physiological change
may arise, and, secondly, because, if it does arise, the
severity of the struggle for existence will then give
natural selection a better chance of acting rapidly
and effectually on each of the isolated sections.

Hence, where the question is whether selective
fertility has played any large or general part in the
differentiation of specific types, the best criterion we
can apply is to ascertain whether it is a general
rule that closely allied species occur in intimate
association, so that their individual members con-
stitute, as it were, a single population, or, on the

76 Darwin^ and after Darzvtn,

other hand, whether they occur rather on different
sides of physical barriers. If they occur intimately
associated, the form of homogamy to which their
,differentiation was due must have presumably been
the physiological form ; whereas, if they are proved
to be correlated with physical barriers, the form of
homogamy which was concerned in their differen-
tiation must presumably have been the geographical
form.

Now, at first this consideration was a trouble to
me, because Moritz Wagner had strenuously argued
— and supported his argument by a considerable
wealth of illustration — that allied species are always
found correlated with physical barriers or discon-
tinuous areas. Weismann's answer, indeed, had
shown that Wagner's statement was much too general :
nevertheless, I was disappointed to find that so
much could be said in favour of the geographical
(or topographical) form of isolation where closely
allied species are concerned. Subsequently, however,
I read the writings of. Nageli on this subject, and
in them I find a very different state of matters
represented.

Seeing as clearly as Wagner that it is impossible
under any circumstances for natural selection to
cause specific differentiation unless assisted by some
other forms of homogamy, but committing the same
oversight as Wagner and Weismann in supposing
that the only other form of homogamy in nature is
geographical isolation, Nageli, with great force of
reasoning, and by many examples, founded his argu-
ment against the theory of natural selection on the
ground that in the vegetable kingdom closely allied

I

I

Evidences of Physiological Selection. 77

species are most frequently found in intimate asso-
ciation with one another, not, that is to say. in any
way isolated by means of physical barriers. This
argument is everywhere logically intact ; and. as he
sustains it by a large knowledge of topographical
botany, his indictment against natural selection as
a cause of specific differentiation appeared to be
insurmountable. And, in point of fact, it was in-
surmountable; so that the whole problem of the
origin of species by differentiation on common areas
has hitherto been left in utter obscurity. Nor is there
now any escape from this obscurity, unless we enter-
tain the " supplementary factor " of selective fertility.
And, apparently, the only reason why this has not
been universally recognized, is because Darwinians
have hitherto failed to perceive the greatness of the
distinction between the differentiation and the trans-
mutation of species ; and hence have habitually met
such overwhelming difficulties as Nageli presented by
an illogical confounding of these two totally distinct
things.

But if the idea of selective fertility had ever
occurred to Nageli as a form of segregation which
gives rise to specific differentiation, I can have no
doubt that so astute and logical a thinker would
have perceived that his whole indictment against
natural selection was answered. For it is incredible
that he should not have perceived how this physio-
logical form of homogamy (supposing it to arise before
or during, and not after the specific differentiation)
would perform exactly the same function on a con-
tinuous area, as he allowed that " isolation " does on
a discontinuous one.

78 Darwin, and after Darwin,

However, be this as it may, there cannot be any
question touching the innmense value of his facts and
arguments as evidence in favour of physiological
selection — albeit this evidence was given uncon-
sciously, or, as it were, prophetically. Therefore
I will here quote a few examples of both, from his
paper Du Dhjeloppement des Espkes Sociales ^.

After stating the theory of natural selection, he
says that if the theory is (of itself) a true explanation
of the origin (or divergence) of specific forms, it
ought to follow that

two closely allied forms, derived the one from the other,
would necessarily occupy two different geographical areas [or
topographical stations], since otherwise they would soon become
blended. Until they had already become sufficiently consolidated
as distinct species to render mutual intercrossing highly impro-
bable, they could not be intermingled without disadvantage
[to differentiadon]. Had Darwin endeavoured to support his
hypothesis by facts, he would, at least in the vegetable kingdom,
have found little to favour his cause. I can cite many hundreds
of cases, in which species in every stage of development have
been found closely mingling with one another, and not in any
way isolated. Therefore, I do not think that one can rightly
speak of natural selection in the Darwinian sense in the
vegetable kingdom ; and, in my estimation, there is a great
difference between the formation of species by nature and the
production of stock by a breeder. ... (p. 212).

Of the two kinds of distribution (i. e. growing apart and
growing together), Synoicy (or growing together) is by far
the most usual in nature. I reckon that out of a hundred
allied vegetable forms, at least ninety-five would be found to be
synoical (p. 219).

This is a most important point. That so enormous

* Archives des Sciences physiques et naiurelles iijtnhs'e),\o\. liii. (1875),
pp. 211-236.

Evidences of Physiological Selection. 79

a proportion of vegetable species should have origi-
nated in intimate association with their parent or
sister types, is clearly unintelligible on the theory of
natural selection alone ; there obviously must be some
other form of homogamy which, whether or not in
all places associated with natural selection, is the
primary condition to the differentiation. Such,
I hold with Nageli, is a logical necessity ; and this
whether or not I am right in believing the other
form of homogamy in question to be selective fertility.
But I go further and say, Surely there can be no
rational question that this other form of homogamy
must have been, at any rate as a highly general rule,
the one which I have assigned. For how is it that
in these ninety-five per cent, of cases, where vegetable
species are growing intimately associated with their
nearest allies, there is no hybridizing, or blending
and relapsing to the original undifferentiated types?
We know well the answer. These are fully differen-
tiated species, and, as such, are protected from mutual
intercrossing by the barrier of mutual sterility. But
now, if this bar is thus necessary for preserving the
specific distinctions when they have been fully
developed, much more must it have been so to admit
of their development ; or, otherwise stated, since we
know that this barrier is associated with " synoical "
species, and since we clearly perceive that were it
withdrawn these species would soon cease to exist,
can we reasonably doubt that their existence (or
origin) is due to the previous erection of this
barrier? If synoical species were comparatively
rare, the validity of such reasoning might be open
to question ; or, even if we should not doubt it in

8o Darwin^ and after Darwin,

such cases, at any rate we might well doubt the
importance or extent of selective fertility as a factor
in the origination of species. But the value of
Nageli's writings on the present subject consists in
showing that synoical species constitute so over-
whelming a majority of the vegetable kingdom, that
here, at all events, it appears imp<«sible to rate too
highly the importance of the principle I have called
physiological selection.

CHAPTER V.

Further Evidences of Physiological
Selection.

Evidence from Topographical Distribution of
Varieties,

In the last section we have considered the topo-
graphical distribution of closely allied species. I now
propose to go still further into matters of detail by
considering the case of natural varieties. And here
we come upon a branch of our inquiry where we may
well expect to meet with the most crucial tests of
our theory. For if it should appear that these nascent
species more or less resemble fully developed species
in presenting the feature of cross- infertility, the theory
would be verified in the most direct and conclusive
manner possible. These nascent species may be
called embryo species, which are actually in course
of differentiation from their parent-type ; and there-
fore, if they do not exhibit the feature in relation
to that type which the present theory infers to be
necessary for the purposes of differentiation, the
theory must be abandoned. On the other hand, if
they do exhibit this feature, it is just the feature
which the theory predicted as one that would be
found highly characteristic of such embryo types,

III. G

82 Darwin, and after Darwin,

Contrariwise, the theory of natural selection can have
no reason to form any such anticipation ; or rather
its anticipation would necessarily require to be the
exact opposite. For, according to this theory, the
cross-infertility of allied species is due, either to
correlation with morphological changes wliich are
being produced by the selection, or else, as Darwin
supposed, to " prolonged exposure to uniform con-
ditions of life " ; and thus, in either case, the sterility
variation ought to be, as a general rule at all events,
subsequent to the specific differentiation, and, ac-
cording to Darwin's view, long subsequent. Thus
we ought not to find that the physiological change
is ever, on any large or general scale, the initial
change ; nor ought we to find that it is, on any
such scale, even so much as a contemporary change :
there ought, in fact, to be no constant or habitual
association between divergence of embryo-types and
the concurrence of cross-infertility.

Now, it will be my endeavour to prove that
there is an extraordinarily general association between
varietal divergence and cross-infertility, wherever
common areas are concerned ; and in as far as this
can be proved, I take it that the evidence will make
wholly in favour of physiological selection as the
prime condition to specific divergence, while at the
same time they will make no less wholly, and quite
independently, against natural selection as the unaided
cause of such divergence.

I shall begin with some further quotations from
Nageli.

Species may be synoical at all stages of relationship. We
come across varieties, scarcely distinguishable from one another,

Evidences of Physiological Selection, 83

growing in the same locality (as, for example, the . Cirsitim
heterophyllum^ with smooth or jagged leaves, the Hieracium
sylvaticum, with or without caulinary leaves) ; again, we meet
other varieties more accentuated (as the H. hoppeanum, with
under ligules of white or red, the Campanula^ with white or lilac
flowers, &c.), other varieties even more marked, which might
almost be elevated to the rank of species [Hieracium alpinwn^
with hairs and glands, and the new form H. holadenium^ which
has only glands. Campanula rotundifolia with smooth and hairy
leaves), or forms still more distinct, up to well-defined species.
I could enumerate endless examples at all stages.

It will be seen that in my definition of synoicy I do not mean
to assert that all allied forms are invariably found together, but
that they are much more often seen in groups than singly.
Take, for instance, nine forms closely related [A to /). A^ E, H
will be found side by side at one point, B, D at another, C, F
at a third, &c. These facts are plainly opposed to the theory of
isolation and amixia, and make, on the contrary, in favour of the
social development of species (Joe. cil., p. 221).

Not to multiply quotations to the same general effect,
I will supply but one other, referring to a particular
case.

At one spot {Roihwand) much exposed to the sun, and
difficult of access, I remarked two closely allied forms, so nearly
related to H. villosum that this would seem to be an interme-
diary form between the two. One of these {H. villosissimum^
is distinguished by its tongue and thick pubescence, its tolerably
large capitula, and by the lengthened and separated scales of
the involucrum ; the other, on the contrary {H. elongalwn)^ is
less pubescent, has smaller capitula, and more compact scales
on the involucrum than H. villosum. Both are finally distin-
guishable from the type by their longer stalks, which are more
decidedly aphyllous, and by their later flowering. At the spot
where I found them the two forms were closely intermingled,
and each was represented by a considerable number of plants.
I did not find them anywhere else on the mountain, nor could
I find at the spot where these were growing a single specimen
of the true H. villosum, nor a single hybrid from these two

G 2

84 Darwin, and after Darwin,

I concluded that these two new forms had, by joining their
forces, expelled the H. villosum from its primitive abode, but
had not succeeded in displacing one another. As to their origin,
they had evidently developed in two different directions from
a common point of departure, namely H. villosum. They had
succeeded, not only in separating themselves from the original
form, but also in preventing any intermediary form from inter-
posing. I thought myself therefore justiried in considering this
as a case of varieties which have come into existence subsequently
to the Glacial epoch. The morphological characteristics rf the
three forms are sufficiently distinct for them to be designated as
species by a good many writers. They are better defined than
some of MM. Frolich and Fries' weaker species, and as well
defined as some of MM. Koch and Grisebach's (p. 222).

Now it is clear, without comment, that all this is
exactly as it ought to be, if allied species have been
differentiated on common areas by selective fertility.
For if, as Nageli elsewhere says, "one meets forms
in nature associated with one another, and severally
distinguished by every possible degree of differen-
tiation," not only as Nageli adds, does this general
fact lead to the inference that species are (usually)
developed when plants grow intimately associated
together; but as certainly it leads to the further
inference that such development must be due to
a prior development of cross-infertility between the
diverging varietal forms, cross-infertility which is
therefore afterwards so characteristic of the allied
species, when these are found, in their fully dif-
ferentiated condition, still occupying the same area
in large and intimately mingled populations.

To my mind there could not be any inference more
strongly grounded than this, because, with the one
exception of the physiological form, no other form
of homogamy can be conceived which shall account

Evidences of Physiological Seleclion. 85

for the origin and permanence of these synoical
varieties, in all degrees of differentiation up to well-
defined synoical species. Least of all, as we have
seen, can natural selection alone have had anything
to do with such a state of matters ; while, as we have
likewise seen, in all its details it is exactly the state
of matters which the theory of physiological selection
requires.

Nevertheless, although this inference is so strongly
grounded, we ought to remember that it is only an
inference. In order fully to verify the theory of
physiological selection, we ought to prove by experi-
ment the fact of cross -infertility between these synoical
varieties, as we learn that it afterwards obtains between
synoical species. It is to be regretted that the theory
of physiological selection did not occur to the mind
of Nageli, because he would then, no doubt, have
ascertained this by actual experiment. As it is, the
great value of his observations goes no further than
establishing a strong presumption, that it must be
selective fertility which causes the progressive dif
ferentiation of synoical varieties; and also that, il
so, this must be the principal factor in the differentia-
tion of vegetable species, seeing that some ninety-five
per cent, are of synoical origin.

Evidence front Experimental Research.

My paper on Physiological Selection pointed out that
the whole theory would have to stand or fall with the
experimental proof of the presence or the absence of
cross-infertility between varieties of the same species
growing on common areas. From the facts and
considerations which we have hitherto been dealing

86 Darwin^ and after Darwin.

with, it did indeed appear to me that there was the
strongest conceivable ground for inferring that cross-
infertility between such varieties would be found by
experiment to be a phenomenon of highly gene-
ral occurrence— amply sufficient ground to prove
that allied species on common areas for the most part
owed their origin to this character of mutual sterility,
and not vice versa as previously supposed. At that
time I was not aware that any experiments had been
made in this direction. Soon after the paper was
published, however, my attention was directed to a
laborious research which had been directed to this
very point, and carried on for more than thirty years,
by M. Jordan ^ This had not attracted the general
notice which it undoubtedly deserved ; and I have
since ascertained that even Darwin began to look
into it only a few months before his death.

Having devoted his life to closely observing in
divers stations multitudes of different species of plants
— annuals and perennials, bulbous and aquatic, trees
and shrubs — M. Jordan has been able to satisfy him-
self, and the French school of botanists to which this
line of observation has given rise, that in most cases
(or "nearly everywhere"), when a Linnean species
is indigenous to a country and is there of common
occurrence, this species within that district is repre-
sented by more or less numerous and perfectly constant
varieties. These varieties are constituted by such
minute differences of morphological character that

* Retnarques snr le fait de T existence en sociiti h Vitat sauvage des
esphes vegetales ajfmes et sur d" ant res /aits relatifs h la question de
lesphe, par Alexis Joidan ; lues au congres c'e TAssocialion Fiancaise
pour rAvancement des Sciences, a™* session, Lyon, seance de a8 Aout,
1873.

Evidences of Physiological Selection. 87

their very existence eluded the observation of botanists,
until M. Jordan began to search specially for them as
the special objects of his scrutiny. Moreover, these
varieties of a Linnean species occupy common areas,
and there grow in intimate association with one
another, or as M. Jordan says, '' pele-mele!' So far,
be it noticed, Jordan was proceeding on exactly the
same lines as Nageli ; only he carried his observa-
tions over a still wider range of species on the one
hand, and into a still minuter search for varieties
on the other. But the all-important point for us is,
that he further proceeded to test by experiment the
physiological relations between these morphological
varieties ; and found, in many hundreds of cases,
that they not only came true to seed (i. e. are hereditary
and not merely climatic), but likewise cross-sterile
inter se. For these reasons, M. Jordan, who is
opposed to the theory of evolution, regards all such
varieties as separately created species ; and the
inspiring motive of his prolonged investigations has
been a desire to multiply these proofs of creative
energy. But it clearly makes no difference, so far
as evolutionists are concerned with them, whether
all this multitude of sexually isolated forms be de-
nominated species or varieties.

The points which are of importance to evolu-
tionists— and of the first order of importance in the
present connexion— may be briefly summarized as
follows : —

(i) The research embraces large numbers of species,
belonging to very numerous and very varied orders
of plants; (2) in the majority of cases — although not
all — indigenous species which are of common occur-

88 Darwin^ and after Darwin.

rence present constant varieties ; (3) these varieties,
nevertheless, may be morphologically so sh'ght as to
be almost imperceptible ; (4) they occupy common
areas and grow in intimate association ; (5) although
many of them have undergone so small an amount
of morphological change, they have undergone a sur-
prising amount of physiological change ; for (6) not
only do very many of these varieties come true to
seed ; but, (7) when they do, they are always more or
less cross-infertile inter se.

Now, it is self-evident that every one of these seven
points is exactly what the theory of physiological
selection requires, while there is not one of them
which it does not require. For if the theory be
sound, we should expect to find large numbers of
species belonging to numerous and varied orders
of plants presenting constant varieties on common
areas ; we should expect this to be a highly general,
though not a universal, rule ; and we should expect
it to apply only to species which are indigenous. More-
over, we should expect these varieties, although but
slightly difterentiated morphologically, to present a
great differentiation physiologically — and this in the
special direction of selective fertility, combined, of
course, with heredity.

On the other hand, as I have said, this catalogue
of evidences leaves nothing to be supplied. It gives
us all the facts — and no more than all the facts —
which my paper on Physiological Selection anticipated
as the eventual result of a prolonged experimental
research. And if I have to regret my ignorance of
these facts when that paper was published, at any
rate it now furnishes the best proof that my anticipa-

Evidences of Physiological Selection, 89

tions were not guided by the results of a verification
which had already been supplied. These anticipations
were deduced exclusively from the theory itself, as
representing what ought to be the case if the theory
were true ; and, I must confess, if I had then been
told that they had already been realized — that it
had actually been found to be a general rule that
endemic species present constant and hereditary
varieties, intimately commingled on common areas,
morphologically almost indistinguishable, but physio-
logically isolated by selective fertility— I should
have felt that the theory had been verified in
advance. For there are only two alternatives :
either these things are due to physiological selection,
or else they are due — as M. Jordan himself believes
— to special creation. Which is equivalent to say-
ing that, for evolutionists, the facts must be held
to verify the former theory in as complete a manner
as it is logically possible for the theory to be
verified.

Evidence from Prepotency,

We have now to consider the bearing of what is
called ''prepotency'* on the theory of physiological
selection.

Speaking of the vast number of species of Com-
positae, Darwin says : —

There can be no doubt that if the pollen of all these species
could be simultaneously or successively placed on the stigma of
any one species, this one would elect with unerring certainty its
own pollen. This elective capacity, is all the more wonderful, as
it must have been acquired since the many species of this great
group of plants branched off from a common progenitor.

90 DarwiUy and after Darwin,

Darwin is here speaking of elective affinity in
its fully developed form, as absolute cross-sterility
between fully differentiated species. But we meet
with all lower degrees of cross-infertility — sometimes
between "incipient species," or permanent varieties,
and at other times between closely allied species.
It is then known as "prepotency" of the pollen
belonging to the same variety or species over the
pollen of the other variety or species, when both sets
of pollen are applied to the same stigma. Although
in the absence of the prepotent pollen the less potent
will fertilize the seed, yet, such is the appetency for
the more appropriate pollen, that even if this be
applied to the stigma some considerable time after
the other, it will outstrip or overcome the other in
fertilizing the ovules, and therefore produce the
same result on the next generation as if it had been
applied to the mother plant without any admixture
of the less potent pollen, although in some cases such
incipient degrees of cross-infertility are further shown
by the number or quality of the seeds being fewer
or inferior.

Now, in different varieties and in different allied
species, all degrees of such prepotency have been
noticed by many observers, from the faintest per-
ceptible amount up to complete impotency of the
alien pollen — when, of course, there is absolute
sterility between the two varieties or allied species.
The inference is obvious. In this graduated scale
of prepotency — beginning with an experimentally
almost imperceptible amount of sexual differentia-
tion between two varieties, and ending in an absolute
partitioning of two allied species — we have the only

Evidences of Physiological Selection. 91

remaining fact that is required to complete the case
in favour of the present theory. We are here brought
back to the very earliest stages of physiological differ-
entiation or to the stages whicli lie behind Jordan's
" Physiological Species " ; and therefore, when taken
in conjunction with his results, the phenomena 01
prepotency may be said to give us the complete and
final demonstration of one continuous development,
which, beginning in an almost imperceptible amount
of cross-infertility, ends in absolute cross-sterility.
The " elective capacity " to which Darwin alludes as
having been "acquired" by all the species of Com-
positae since they "branched off from a common
progenitor," is thus seen among innumerable other
species actually in process of acquisition ; and so
we can perfectly well understand, what is otherwise
unintelligible, that closely allied species 01 plants
occur, in ninety-five per cent, of cases, intimately asso-
ciated on common areas, while exhibiting towards one
another the character of mutual sterility.

But more than this. The importance of the wide-
spread phenomena of prepotency to the theory of
physiological selection does not consist merely in
thus supplying the last link in the chain of evidence
touching the origin of species by selective fertility,
or "elective capacity." These phenomena are of
further importance as showing how in plants, at all
events, physiological selection appears to be frequently
capable of differentiating specific types without the
necessary assistance of any other form of homogamy.
In my original statement of the theory, I was careful
to insist upon the great value, as differentiating agents,
of even small degrees of other forms of homogamy

92 Darwin, and after Darwin,

when co-operating with physiological selection. But
I also stated my belief that in many cases selective
fertility is presumably of itself capabl-e of splitting
a specific type ; and the reason why I still believe
this is, that I do not otherwise understand these pheno-
mena of prepotency. I cannot believe that in all the.
innumerable cases where they arise, they have been
super-induced by some prior morphological changes
going on in some other part of the organism, or by
"prolonged exposure to uniform conditions of life,"
on the part of two wellnigh identical forms which
have arisen intimately commingled in exactly the
same environment, and under the operation of a pre-
viously universal intercrossing. Even if such a thing
could be imagined as happening occasionally, I feel
it difficult to imagine that it can happen habitually,
and yet this view must be held by those who would
attribute prepotency to natural selection.

It must never be forgotten that the relatively
enormous changes as to size, structure, habit, &c.,
which are presented by our domesticated plants as
results of artificial selection, do not entail the physio-
logical character of cross-sterility in any degree,
save possibly in some small number of cases. Although
in wild species any correspondingly small percentage
of cases (where natural selection happens to hit upon
parts of the organism modifications of which produce
the physiological change by way of correlation) would
doubtless be the ones to survive on common areas,
still it is surely incredible that such an accidental
association between natural selection and cross-
infertility is so habitually the means of specific
differentiation as the facts of prepotency (together

Evidences of Physiological Selection, 93

with the observations of Jordan and Nageli) would
necessarily demand.

Moreover, this view of the matter is still further
corroborated by certain other facts and considerations.
For example, the phenomena of prepotency (whether
as between varieties or between closely allied species)
are found to occur when the two forms occupy a
common area, i.e. are growing intermingled with
one another. Therefore, but for this physiological
differentiation, there could be absolutely nothing to
prevent free intercrossing. Yet the fact that hybrids
are so comparatively rare in a state of nature— a fact
which Sir Joseph Hooker has pointed out to me as
olhervvise inexplicable —proves the efficacy of even
a low degree of such differentiation in preventing
the physiologically-differentiated forms from inter-
crossing. Even in cases where there is no difficulty
in producing artificial hybrids or mongrels between
species or varieties growing on common areas, it is
perfectly astonisliing what an extremely small per-
centage of the hybrid or mongrel forms are found to
occur in nature. And there can be no question that
this is due to the very efficient manner in which
prepotency does its work— efficient, I mean, from
the point of view of the new theory ; for upon any
other theory prepotency is a meaningless pheno-
menon, which, notwithstanding its frequent occur-
rence, plays no part whatever in the process of organic
evolution.

I attach considerable importance to the phenomena
of prepotency in view of the contrast which is pre-
sented between plants and animals in the relation of
their species to physical barriers. For animals —

94 Darwin, and after Darwin.

and especially the higher animals — appear to depend
for their specific differentiations upon such barriers
much more than in the case with plants. This is no
more than we should expect ; for, in accordance with
our theory, selective fertility is not so likely to work
alone in the case of the higher animals which mate
together, as in plants which are fertilized through the
agency of wind or insects. In the former case there
is no opportunity given for the first rise of cross-
infertility, in the form of prepotency ; and even where
selective fertility has gained a footing in other ways,
the chances against the suitable mating of "physio-
logical complements " must be much greater than it
is in the latter case. Hence, among the higher animals,
selective fertility ought much more frequently to be
found in association with other forms of homogamy
than it is among plants. And this is exactly what
we find. Thus it seems to me that this contrast
between the comparative absence and presence of
physical barriers, where allied species of plants and of
higher animals are respectively concerned, is entitled
to be taken as a further corroboration of our theory.
For while it displays exactly such a general corre-
lation as this theory would expect, the correlation is
one which cannot possibly be explained on any other
theory. It is just where physiological selection can
be seen to have the best opportunity of acting (viz.
in the vegetable kingdom) that we find the most
unequivocal evidence of its action ; while, on the
other hand, it is just where it can be seen to have
the least opportunity of asserting itself (viz. among
the higher animals) that we find it most associated
with, and therefore assisted by, other forms of homo-

Evidences of Physiological Selection, 95

gamy, i. e. not only geographical isolation, but also
by sexual preference in pairing, and the several
other forms of homogamy, which Mr. GuHck has
shown to arise in different places as the result of
intelligence.

Evidence from Special Cases,
Hitherto I have been considering, from the most
general point of view, the most widespread facts
and broadest principles which serve to substantiate
the theory of physiological selection. I now pass
to the consideration of one of those special cases in
which the theory appears to have been successfully
applied.

Professor Le Conte has adduced the fossil snails
of Steinheim as serving to corroborate the theory of
physiological selection ^

The facts are these. The snail population of this
lake remain for a long time uniform and unchanged.
Then a small percentage of individuals suddenly began
to vary as regards the form of their shells, and this in
two or three directions at the same time, each affected
individual, however, only presenting one of the varia-
tions. But after all these variations had begun to
affect a proportionally large number of individuals,
some individuals occur in which two or more of the
variations are blended together, evidently, as Weis-
mann says, by intercrossing of the varieties so blended.
Later still, both the separate varieties and their
blended progeny became more and more numerous,
and eventually a single blended type, comprising
in itself all the initial varieties, supplanted the

* Evolution andits Relations to Eeligious Thought , 8cc. pp. 2?6-7.

96 Darwin^ and after Darwin.

parent form. Then another long period of stability
ensued until another eruption of new variations took
place ; and these variations, after having affected
a greater and greater number of individuals, eventu-
ally blended together by intercrossing and sup-
planted their parent form. So the process went on,
comparatively short periods of variation alternating
with comparatively long periods of stability, the
variations, moreover, always occurring suddenly in
crops, then multiplying, blending together, and in
their finally blended type eventually supplanting their
parent form.

Now, the remarkable fact here is that whenever the
variations arose, they only intercrossed between them-
selves, they did not intercross with their parent form ;
for, if they had, not only could they never have
survived (having been at first so few in number and
there having been no geographical barriers in the
small lake), but we should have found evidence of
the fact in the half-bred progeny. Moreover, natural
selection can have had nothing to do with the process,
because not only are the variations in the form of the
shells of no imaginable use in themselves ; but it
would be preposterous to suppose that at each of these
'variation periods" several different variations should
always have occurred simultaneously, all of which were
of some hidden use, although no one of them ever
occurred during any of the prolonged periods of
stability. How, then, are we to explain the fact that
the individuals composing each crop of varieties, while
able to breed among themselves, never crossed with
their parent form ? These varieties, each time that
they arose, were intimately commingled with their

Evidences of Physiological Selection. 97

parent form, and would certainly have been re-
absorbed into it had intercrossing in that direction
been possible. With Professor Le Conte, therefore,
I conclude that there is only one conceivable answer
to this question. Each crop of varieties must have
been protected from intercrossing with their parent
form.

They must have been the result of a variation, which
rendered the affected individuals sterile with their
parent form, whilst leaving them fertile amongst them-
selves. The progeny of these individuals would then
have dispersed through the lake, physiologically isolated
from the parent population, and especially prone to
develop secondary variations as a direct result of the
primary variation. Thus, as we might expect, two or
three variations arose simultaneously, as expressions
of so many different lines of family descent from the
original or physiological variety ; these were every-
where prevented from intercrossing with their parent
form, yet capable of blending whenever they or their
ever-increasing progeny happened to meet. Thus,
without going into further details, we are able by
the theory of physiological selection to give an ex-
planation of all these facts, which otherwise remain
inexplicable.

In view of the evidence which has now been pre-
sented, I will now ask five questions which must be
suitably answered by critics of the theory of physio-
logical selection.

I. Can you doubt that the hitherto insoluble pro-
blem of inter-specific sterility would be solved, sup-
posing cross-infertility were proved to arise before or

III II

98 Darwin^ and after Darwin,

during the process of specific differentiation, instead
of after that process had been fully completed ?

2. Can you doubt, after duly considering the cir-
cumstances under which allied species of plants have
been differentiated — viz. in ninety-five per cent, of
cases intimately commingled on common areas, and
therefore under identical environments— that cross-
infertility must have arisen before or during the
specific differentiation ?

3. Can you doubt, after duly considering the facts
of prepotency on the one hand and those of Jordan's
physiological varieties on the other, that cross-infer-
tility does arise before or during the specific differen-
tiation ?

4. If you cannot express a doubt upon any of these
points, can you explain why you refuse to accept the
theory of the origin of species by means of physio-
logical selection, together with the explanation which
this theory affords of the continued cross-fertility of
domesticated varieties ?

5. Supposing this theory to be true, can you con-
ceive of any other classes of facts which, either
quantitatively or qualitatively, could more directly or
more effectually prove its truth than those which have
now been adduced ?

On these five heads I entertain no doubt. I am
convinced that the theory of physiological selection is
the only one that can explain the facts of inter-specific
sterility on the one hand, and, on the other hand, the
contrast which these facts display to the unimpaired
fertility of our domesticated varieties.

In conclusion, it seems desirable once more to insist
that there is no antagonism or rivalry between the

Evidences of Physiological Selection. 99

theories of natural and of physiological selection. For
which purpose I will quote the final paragraph of my
original paper.

So much, then, for the resemblances and the differences
between the two theories. It only remains to add that the two
are complementary. I have already shown some of the respects
in which the newer theory comes to the assistance of the older,
and this in the places where the older has stood most in need of
assistance. In particular, I have shown that segregation of the
fit entirely relieves survival of the fittest from the difficulty under
which it has hitherto laboured of explaining why it is that sterility
is so constantly found between species, while so rarely found
between varieties which differ from one another even more than
many species ; why so many features of specific distinction are
useless to the species presenting them ; and why it is that
incipient varieties are not obliterated by intercrossing with parent
forms. Again, we have seen that physiological selection, by
preventing such intercrossing, enables natural selection to
promote diversity of character, and thus to evolve species in
ramifying branches instead of in linear series— a work which I
cannot see how natural selection could possibly perform unless
thus aided by physiological selection. Moreover, we have seen
that although natural selection alone could not induce sterility
between allied types, yet when this sterility is given by physio-
logical selection, the forms which present it would be favoured in
the struggle for existence ; and thus again the two principles are
found playing, as it were, into each other's hands. And here, as
elsewhere, 1 believe that the co-operation enables the two prin-
ciples to effect very much more in the way of species-making
than either of them could effect if working separately. On the
one hand, without the assistance of physiological selection,
natural selection would, I believe, be all but overcome by the
adverse influences of free intercrossing — influences all the more
potent under the very conditions which are required for the
multiplication of species by divergence of character. On the
other hand, without natural selection, physiological selection
would be powerless to create any differences of specific type,
other than those of mutual sterility and trivial details of structure,

H 2

loo Darwin, and after Darwin,

form, and colour— differences wholly without meaning from a
utilitarian point of view. But in their combination these two
principles appear to me able to accomplish what neither can
accomplish alone — namely, a full and satisfactory explanation of
the origin of speciesi.

CHAPTER VI.

A Brief History of Opinions on Isolation
AS A Factor of Organic Evolution.

This historical sketch must begin with a considera-
tion of Darwin's opinions on the subject ; but as these
were considerably modified from time to time during
a period of thirty years by the publications of other
naturalists, it will be impossible to avoid cross-
references as between his writings and theirs. It
may also be observed that the Life and Letters of
Charles Darwin was not published until the year
1887, so that the various opinions which I shall
quote from the letters, and which show some con-
siderable approximation in his later years to the
views which have been put forward by Mr. Gulick
and myself, were not before us at the time when our
papers were read.

The earliest allusion that I can find to geographical
isolation in the writings of Darwin occurs in a
correspondence with Sir Joseph Hooker, as far back
as 1844. He there says : —

I cannot give my reasons in detail ; but the most general
conclusion which the geographical distribution of all organic

I02 Darwirif and after Darwin,

beings appears to me to indicate is, that isolation is the chief
concomitant or cause of the appearance of new forms (I well
know there are some staring exceptions) \

And again : —

With respect to original creation or production of new forms,
I have said that isolation appears the chief element *.

Next, in the earlier editions of the Origin of Species
this view is abandoned, and in its stead we meet
with the opinion that geographical isolation lends
a certain amount of assistance to natural selection,
by preventing free intercrossing. But here we must
note two things. First, the distinction between mono-
typic and polytypic evolution is not defined. Secondly,
the levelling effect of free intercrossing in nature, and
hence its antagonism to divergence of character by
natural selection, is not sufficiently recognized ; while,
on the other hand, and in consequence of this, the
importance of isolation as a factor of evolution is
underrated — not only in its geographical, but likewise
in all its other forms.

Taking these two points separately, the only
passages in Darwin's writings, so far at least as I
can find, in which any distinction is drawn between
evolution as monotypic and polytypic, are those in
which he deals with a somewhat analogous distinction
between artificial selection as intentional and un-
conscious. He says, for example : —

In the case of methodical selection, a breeder selects for some
definite object, and if the individuals be allowed freely to inter-
cross, his work will completely fail. But when many men,
without intending to alter the breed, have a nearly common

' Life and Letters y vol. ii. p, a8. * Ibid.

opinions on Isolation. 103

standard of perfection, and all try to procure and breed from the
best animals, improvement surely but slowly follows from this
unconscious process of selection, notwithstanding that there is no
separation of selected individuals. Thus it will be under nature \

Here we have what may perhaps be regarded as a
gh'mmering of the distinction between monotypic and
polytypic evolution. But that it is only a glimmering
is proved by the immediately ensuing sentences, which
apply this analogy of unconscious selection not to the
case of monotypic, but to that of polytypic evolution.
So likewise, in the succeeding discussion on "divergence
of character," the analogy is again resorted to for the
purpose of showing how polytypic evolution may occur
in nature.

Thus far, then, it may be said that we have scarcely
so much as a glimmering of the distinction between
monotypic and polytypic evolution ; and as the same
discussion (with but a few verbal alterations) runs
through all the editions of the Origin^ it may well be
asked why I should have alluded to such passages in
the present connexion. Well, I have done so because
it is apparent that, during the last years of his life, the
distinction between selection as "methodical" and
" unconscious " enabled Darwin much more clearly to
perceive that between evolution as monotypic and
polytypic. Thus in 1868 he wrote to Moritz Wagner
(who, as we shall presently see, entirely failed to
distinguish between monotypic and polytypic evolu-
tion), expressing his belief —

That in many large areas all the individuals of the same
species have been slowly modified, in the same manner, for
instance, as the English racehorse has been improved, that is,

^ Origin of Species, p. 80, 6th ed. (1872).

I04 Darwin^ and after Darwin.

by the continued selection of the fleetest individuals, without
any separation. But I admit that by this process two or
more new species could hardly be formed within the same limited
area ^

Again, in 1876 he wrote another letter to Wagner,
in which the following passage occurs : —

I believe that all the individuals of a species can be slowly
modified within the same district, in nearly the same manner as
man effects by what I have called the process of unconscious
selection. I do not believe that one species will give birth to
two or more new species as long as they are mingled together
within the same district *.

Two years later he wrote to Professor Semper : —

There are two different classes of cases, it appears to me,
viz. those in which species becomes slowly modified in the
same country, and those cases in which a species splits into two,
or three, or more new species ; and, in the latter case, I should
think nearly perfect separation would greatly aid in their
"specification," to coin a new word^

Now, these passages show a very much clearer
perception of the all-important distinction between
monotypic and polytypic evolution than any which
occur in the Origin of Species ; and they likewise
show that he was led to this perception through what
he supposed to be a somewhat analogous distinction
between " unconscious " and " methodical " selection
by man. The analogy, I need hardly say, is radically
unsound ; and it is a curious result of its unsoundness
that, whereas in the Origin of Species it is adduced
to illustrate the process of polytypic evolution, as
previously remarked, in the letters above quoted we

^ Life and Letters, vol. iii. p. 158.
^ Ibid. p. 159. ^ Ibid. p. 160.

opinions on Isolation, 105

find it adduced to illustrate the process df monotypic
evolution. But the fact of this analogy being unsound
does not affect the validity of the distinction between
monotypic and polytypic evolution to which it led
Darwin, in his later years, so clearly to express ^

Turning next to the second point which we have to
notice, it is easy to show that in the earlier editions
of his works Darwin did not sufficiently recognize
the levelling effects of free intercrossing, and conse-
quently failed to perceive the importance of isolation
(in any of its forms) as a factor of organic evolution.
This may be most briefly shown by quoting his own
more matured opinion upon the subject. Thus, with
reference to the swamping effects of intercrossing, he
wrote to Mr. Wallace in 1867 as follows: —

I must have expressed myself atrociously : I meant to say
exactly the reverse of what you have understood. F. Jenkin
argued in the North British Review against single variations
being perpetuated, and has convinced me, though not in quite
so broad a manner as here put. I always thought individual
differences more important ; but I was blind, and thought that
single variations might be preserved much oftener than I now
see is possible or probable. I mentioned this in my former

^ The analogy is radically unsound because unconscious selection
differs from methodical selection only in the degree of "separation"
which it effects. These two forms of selection do not necessarily differ
from one another in regard to the number of characters which are being
simultaneously diversified ; for while it may be the object of methodical
selection to breed for modification of a single character alone, it may,
on the other hand, be the result of unconscious selection to diversify an
originally uniform stock, as Darwin himself observes with regard to
horse-breeding. The real distinction between monotypic and polytypic
evolution is, not at all with reference to the degree of isolation (i. e.
amount of "separation"), but to the number of cases in which any
efficient degree of it occurs (i. e. whether in but a single case, or in two
or more cases).

io6 Darwiftj and after Darwin,

note merely because I believed that you had come to a similar
conclusion, and I like much to be in accord with you. I believe
I was mainly deceived by single variations offering such simple
illustrations, as when man selects [i.e. isolates]*.

Again, somewhere about the same time, he wrote
to Moritz Wagner : —

Although I saw the effects of isolation in the case of islands
and mountain-ranges, and knew of a few instances of rivers,
yet the greater number of your facts were quite unknown to me.
I now see that, from the want of knowledge, I did not make
nearly sufficient use of the views which you advocate*.

Now it would be easy to show the justice of these
self-criticisms by quoting longer passages from earlier
editions of the Origin of Species \ but as this, in view
of the above passages, is unnecessary, we may next
pass on to another point.

The greatest oversight that Wagner made in his
otherwise valuable essays on geographical isolation,
was in not perceiving that geographical isolation is only
one among a number of other forms of isolation :
and, therefore, that although it is perfectly true, as
he insisted, that polytypic evolution cannot be effected
by natural selection alone, it is very far from true,
as he further insisted, that geographical isolation is
the only means whereby natural selection can be
assisted in this matter. Hence it is that, when
Darwin said he had not himself " made nearly
sufficient use " of geographical isolation as a factor
of specific divergence, he quite reasonably added that
he could not go so far as Wagner did in regarding
such isolation as a condition, si7te qtia non^ to diver-
gent evolution in all cases. Nevertheless, he adds

* Life and Letters, vol. iii. pp. 157-8. » Ibid. pp. 157-8.

opinions on Isolation. 107

the important words, " I almost wish I could believe
in its importance to the same extent with you ; for
you well show, in a manner which never occurred to
me, that it removes many difficulties and objections."
These words are important, because they show that
Darwin had come to feel the force of the " difficulties
and objections " with regard to divergent evolution
being possible by means of natural selection alone,
and how readily they could be removed by assuming
the assistance of isolation. Hence, it is much to be
deplored that Wagner presented a single kind of
isolation (geographical) as equivalent to the principle
of isolation in general. For he thus failed to present
the complete — and, therefore, the true — philosophy
of the subject to Darwin's mind ; and in this, as
in certain other respects which I shall notice later
on, served rather to confuse than to elucidate the
matter as a whole.

To sum up. Although in his later years, as shown
by his correspondence, Darwin came to recognize
more fully the swamping effects of free intercrossing,
and the consequent importance of " separation " for
the prevention of these effects, and although in this
connexion he likewise came more clearly to dis-
tinguish between the " two cases " of monotypic
and polytypic evolution, it is evident that he never
worked out any of these matters — " thinking it pru-
dent," as he wrote with reference to them in 1878,
*'now I am growing old, to work at easier subjects ^"
Therefore he never clearly saw, on the one hand,
that free intercrossing, far from constituting a " diffi-
culty " to monotypic evolution by natural selection,

' Life and Letters^ vol. iii. p. i6l.

io8 Darwin^ and after Darwin,

is the very means whereby natural selection is in
this case enabled to operate; or, on the other
hand, that, in the case of polytypic evolution, the
" difficulty " in question is so absolute as to render
such evolution, by natural selection alone, absolutely
impossible. Hence, although in one sentence of the
Origin of Species he mentions three forms of isolation
(besides the geographical form) as serving in some
cases to assist natural selection in causing "diver-
gence of character " (i. e. polytypic evolution ^), on
account of not perceiving how great and how sharp
is the distinction between the two kinds or " cases "
of evolution, he never realized that, where " two or
more new species" are in course of differentiation,
some form of isolation other than natural selection
must necessarily be present, whether or not natural
selection be likewise so. The nearest approach which
he ever made to perceiving this necessity was in one
of his letters *to Wagner above quoted, where, after
again appealing to the erroneous analogy between
monotypic evolution and " unconscious selection/' he
says : — " But I admit that by this process (i. e. un-
conscious selection) two or more new species could
hardly be formed within the same limited area: some
degree of separation, if not indispensable, would be
highly advantageous ; and here your facts and views
will be of great value." But even in this passage the
context shows that by " separation " he is thinking
exclusively o{ geographical SQ'^2iX^.\h.ov\, which he rightly
enough concludes (as against Wagner) need certainly

* Page 8r. The three forms of isolation mentioned are, "from
haunting different stations, from breeding at slightly different seasons, or
from the individuals of each variety preferring to pair together."

opinions on Isolation, 109

not be " indispensable." Had he gone a step further,
he must have seen that separation, in some form
or another, is " indispensable " to polytypic evolution.
Instead of taking this further step, however, two years
later he wrote to Semper as follows : —

I went as far as I could, perhaps too far, in agreement with
Wagner [i. e. in the last edition of the Origin of Species] ; since
that time I have seen no reason to change my mind ; but then
I must add that my attention has been absorbed on other
subjects \

And he seems to have ended by still failing to
perceive that the explanation which he gives of
*' divergence of character" in the Origin of Species,
can only hold on the unexpressed assumption that
free intercrossing is in some way prevented at the
commencement, and throughout the development, of
each diverging type.

Lastly, we have to consider Darwin's opinion touching
the important principle of " Independent Variability."
This, it will be remembered, is the principle which
ensures that when a portion (not too large) of a
species is prevented from interbreeding with the rest
of the species, sooner or later a divergence of type
will result, owing to the fact that the average qualities
of the separated portion at the time of its separation
cannot have been exactly the same as the average
qualities of the specific type as a whole. Thus the
state of Amixia, being a state of what Mr. Gulick
calls Independent Generation, will of itself — i. e. even
if unassisted by natural selection— induce divergence
of type, in a ratio that has been mathematically
calculated by Delbceuf.

* Life and Letters^ vol. iii. p. 159.

no Darwifiy and after Darwin.

Darwin wrote thus to Professor Weismann in

1872:—

I have now read your essay with very great interest. Your
view of the origin of local races through " Amixia " is altogether
new to me, and seems to throw an important light on an obscure
question \

And in the last edition of the Variation of Animals
and Plants he adds the following paragraph : —

This view may throw some light on the fact that the domestic
animals which formerly inhabited the several districts in Great
Britain, and the half-wild cattle lately kept in several British
parks, differed slightly from one another; for these animals were
prevented from wandering over the whole country and inter-
crossing, but would have crossed freely within each district or
park ^

Now, although I allow that Darwin never attri-
buted to this principle of Amixia, or Independent
Variability, anything like the degree of importance
to which, in the opinion of Delboeuf, Gulick, Giard,
and myself, it is entitled, the above passage appears
to show that, as soon as the "view" was clearly
* suggested " to his mind, he was so far from being
unfavourably disposed towards it, that he added
a paragraph to the last edition of his Variation for
the express purpose of countenancing it. Never-
theless, later on the matter appears to have entirely
escaped his memory ; for in 1878 he wrote to Semper,
that he did " not see at all more clearly than I did
before, from the numerous cases which he [Wagner]
has brought forward, how and why it is that a long
i.'i'olated form should almost always become slightly
modified ^." I think this shows entire forgetfulness

* Life and Letters , vol. iii. p. 155. ' Variation^ &c., vol. ii. p. 262.
' Life and Letters^ vol. iii. p. 161.

opinions on Isolation. iii

of the principle in question, because, if the latter is
good for explaining the initial divergence of type as
between separated stocks of " domesticated animals,"
much more miist it be competent to explain the
further divergence of type which is " almost always "
observable in the case of " a long isolated form "
under nature. The very essence of the principle
being that, when divergence of type has once begun,
this divergence must ipso facto proceed at an ever-ac-
celerating pacC; it is manifestly inconsistent to entertain
the principle as explaining the first commencement of
divergence, and then to ignore it as explaining the
further progress of divergence. Hence, I can only
conclude that Darwin had forgotten this principle
altogether when he wrote his letter to Semper in J878
— owing, no doubt, as he says in the sentence which
immediately follows, to his having " not attended
much of late years to such questions."

So much, then, for Darwin's opinions. Next in
order of time we must consider Moritz Wagner's
essays on what he called the "Law of Migration^."
The merit of these essays was, first, the firm ex-
pression of opinion upon the swamping effects of free
intercrossing ; and, second, the production of a large
body of facts showing the importance of geographical
isolation in the prevention of these effects, and in
the consequent differentiation of specific types. On
the other hand, the defect of these essays was, first,
not distinguishing between evolution as monotypic
and polytypic ; and, second, not perceiving that geo-

* Die Darwin sche Theorie und das Migrationsgesetz (1868) : Ueber
den Einjluss der geographischen Isolirung, &c. (1870).

112 Darwin^ and after Darwin,

graphical isolation is only one among a number of
other forms of isolation. From these two radical
oversights— which, however, were shared by all other
writers of the time, with the partial exception of
Darwin himself, as previously shown — there arose the
following and most lamentable errors.

Over and over again Moritz Wagner insists, as con-
stituting the fundamental doctrine of his attempted
reform of Darwinism, that evolution by natural
selection is impossible, unless natural selection be
assisted by geographical isolation, in order to prevent
the swamping effects of intercrossing ^. Now, if instead
of " evolution " he had said "divergence of type,"
and if instead of " geographical isolation " he had
said " prevention of intercrossing," he would have
enunciated the general doctrine which it has been the
joint endeavour of Mr. Gulick and myself to set forth.
But by not perceiving that " evolution " is of two
radically different kinds — polytypic and monotypic —
he entirely failed to perceive that, while for one of its
kinds the prevention of intercrossing is an absolute
necessity, for the other of its kinds the permission of
intercrossing is a necessity no less absolute. And,
again, in missing the fact that geographical isolation

^ For instance, speaking of common, or continuous areas, he says : —
"In this case a constant variety, or new species, cannot be produced,
because the free crossing of a new variety with the old unaltered stock
will always cause it to revert to the original type; in other words, will
destroy the new form. The formation of a real variety, which Darwin,
as we know, regards as the commencement of a new species, will only
succeed when a few individuals, having crossed the barrier of their
habitat, are able to separate themselves for a long time from the old
stock," And the last sentence, given as a summary of his whole
doctrine, is — "The geographical isolation of the form, a necessary
consequence of migration, is the cause of its typical character."

opinions on Isolation, 113

is but one of the many ways whereby intercrossing
may be prevented, he failed to perceive that, even
as regards the case of polytypic evolution, he greatly
erred in representing this one form of isolation as
being universally a necessary condition to the process.
The necessary condition to this process is, indeed, the
prevention of intercrossing by some means or another ;
but his unfortunate insistence on geographical separa-
tion as the only possible means to this end — especially
when coupled with his no less unfortunate disregard
of monotypic evolution — caused him to hinder rather
than to advance a generalization which he had only
grasped in part. And this generalization is, as now
so repeatedly stated, that while the form of isolation
which we know as natural selection depends for its
action upon the intercrossing of all the individuals
which it isolates (i. e. selects), when acting alone
it can produce only monotypic evolution ; but that
when it is supplemented by any of the other
numerous forms of isolation, it is furnished with
the necessary condition to producing polytypic
evolution — and this in as many lines of divergent
change as there may be cases of this efficient
separation.

Nevertheless, while we must lament these short-
comings on the part of Wagner, we ought to re-
member that he rendered important services in the
way of calling attention to the swamping effects of
free intercrossing, and, still more, in that of showing
the high importance of geographical isolation as a
factor of organic evolution. Therefore, although \\\ an
elaborate criticism of his views Weismann was easily
able to di.spose of his generalizations in the imperfect

III. T

li4 Darwin, and after Darwin,

form that they presented, I do not think it was just in
Weismann to remark, " if Wagner had confined himself
to the statement that geographical isolation materi-
ally assists the process of natural selection, and
thus also promotes the origination of new species, he
would have met with little or no opposition ; but then,
of course, in saying this much, he would not have
been saying anything new." No doubt, as I have
just shown, he ought thus (as well as in other and
still more important respects not perceived by Prof.
Weismann) to have limited his statement ; but, had
he done so, it does not follow that he would not have
been saying anything new. For, in point of fact, in
as far as he said what was true, he did say a great
deal that was also new. Thus, most of what he said
of the principle of separation (apogamy) was as new
as it was true, although, as we have seen, he said it
to very little purpose on account of his identifying
this principle as a whole with that of but one of its
forms. Again, notwithstanding this great error, or
oversight, he certainly showed of the particular form
in question — viz. geographical isolation — that it was
of considerably more importance than had previously
been acknowledged. And this was so far a valuable
contribution to the general theory of descent.

Prof. Weismann's essay, to which allusion has just
been made \ was, however, in all respects a great
advance upon those of Wagner. It was not only
more comprehensive in its view of the whole subject
of geographical isolation, but likewise much more
adequate in its general treatment thereof Its prin-

* Ueber den EinJIuss der holirung auf die Artbildung (1872).

opinions on Isolation. 115

cipal defects, in my judgement, were, first, the in-
ordinately speculative character of some of its parts,
and, second, the restriction of its analysis to but one
form of isolation — a defect which it shares with the
essays of Wagner, and in quite as high a degree.
Furthermore, although this essay had the great merit
of enunciating the principle of Amixia, it did so in
a very inefficient manner. For not only was this
principle adduced with exclusive reference to geo-
graphical isolation, but even in regard to this one
kind of isolation it was presented in a highly in-
consistent manner, as I will now endeavour to show.

Weismann was led to perceive the principle in
question by the consideration that new specific char-
acters, when they first appear, do not all appear
together in the same individuals : they appear one
in one individual, another in another, a third in a
third, &c. ; and it is only in the course of succes-
sive generations that they all become blended in
the same individuals by free intercrossing. Hence, the
eventually emerging constant or specific type is the
resultant of all the transitory or varietal types, when
these have been fused together by intercrossing.
From which Weismann deduces what he considers
a general law — namely, that "the constancy of a
specific type does not arise suddenly, but gradually;
and it is established by the promiscuous crossing
of all individuals^." From which again it follows,
that this constancy must cease so soon as the condition
which maintains it ceases — i. e. so soon as free inter-
crossing is prevented by the geographical isolation
of a portion of the species from its parent stock.

* Loc. cit., p. 43.
I s

ii6 Darwin, and after Darwin,

Now, to begin with, this statement of the principle
in question is not a good statement of it. There was
no need while stating the doctrine that separation
induces differentiation, to found the doctrine on any-
such highly speculative basis. In point of fact, there
is no real evidence that specific types do attain
their constancy in the way supposed ; nor, for the
purposes of the doctrine in question, is it necessary
that there should be. For this doctrine does not
need to show how the constancy has been attained;
it only has to show that the constancy is maintained
by free intercrossing, with the result that when free
intercrossing is by any means prevented, divergence
of character ensues. In short, the correct way of
stating the principle is that which has been adopted
by Delbceuf and Gulick — namely, the average char-
acters of a separated portion of a species are not
likely to be the same as those of the whole species ;
with the result that divergence of type will be set
up in the separated portion by intercrossing within
that portion. Or the principle may be presented
as I presented it under the designation of " Inde-
pendent Variability" — namely, "a specific type may
be regarded as the average mean of all individual
variations, any considerable departure from this
average mean being, however, checked by inter-
crossing," with the result that when intercrossing
is prevented between a portion of a species and
the rest of the species, " this population is permitted
to develop an independent history of its own, shielded
from intercrossing with its parent form ^"

Not only, however, is Weismann's principle of

' Physiological Selection, pp. 348, 389.

opinions on Isolation, 117

"Amfxia" thus very differently stated from that
of my ''Independent Variability" (apogamy), or
Gulick's *' Independent Generation " ; but, apparently
owing to this difference of statement, the principle
itself is not the same. In particular, while Weismann
holds with us that when new characters arise in
virtue of the mere prevention of intercrossing with
parent forms these new characters will be of non-
utilitarian kind^ he appears to think that divergence
of character under such circumstances is not likely to
go on to a specific value. Now, it is of importance
to observe why he arrives at this conclusion, which is
not only so different from that of Delboeuf, Gulick,
and myself, but apparently so inconsistent with his
own recognition of the diversifying effect of " Amixia"
as regards the formation oi permanent varieties. For,
as we have already seen while considering Darwin's
views on this same principle of " Amixia," it is highly
inconsistent to recognize its diversifying effect up to
the stage of constituting fixed varieties, and then not
to recognize that, so much divergence of character
having been already secured by the isolation alone,
much more must further divergence continue, and
continue at an ever accelerating pace -as Delboeuf
and Gulick have so well shown. What, then, is the
explanation of this apparent inconsistency on Weis-
mann's part? The explanation evidently is that,
owing to his erroneous statement of the principle, he
misses the real essence of it. For, in the first place,
he does not perceive that this essence consists in an
initial difference of average characters on the part of
the isolated colony as compared with the rest of their
» Loc. cit., p. 54.

ii8 Darwin^ and after Darwin.

species. On the contrary, he loses himself in a maze
of speculation about all species having had what he
calls " variation-periods," or eruptions of general varia-
bility alternating with periods of repose — both being
as unaccountable in respect of their causation as they
are hypothetical in respect of their occurrence. From
these speculations he concludes, that isolation of a
portion of a species will then only lead to divergence
of character when the isolation happens to coincide
with a " variation-period " on the part of the species
as a whole, and that the divergence will cease so
soon as the "variation-period " ceases. Again, in the
second place as previously remarked, equally with
Wagner whom he is criticizing, he fails to perceive
that geographical isolation is not the only kind of
isolation, or the only possible means to the prevention
of free intercrossing. And the result of this oversight
is, that he thinks amixia can act but comparatively
seldom upon sufficiently small populations to become
a factor of much importance in the differentiation of
species. Lastly, in the third place, owing to his
favourite hypothesis that all species pass through
a "variation-period," he eventually concludes that the
total amount of divergence of type producible by
isolation alone (even in a small population) can never
be greater than that between the extremes of varia-
tion which occur within the whole species at the date
of its partition (p. 75). In other words, the possibility
of change due to amixia alone is taken to be limited
by the range of deviation from the general specific
average, as manifested by different individual varia-
tions, before the species was divided. Thus the
doctrine of amixia fails to recognize the law of

opinions on Isolation, 119

Delboeuf, or the cumulative nature of divergence of
type when once such divergence begins in a separated
section. Therefore, in this all-important — and, indeed,
essential — respect, amixia differs entirely from the
principle which has been severally stated by Delboeuf,
Gulick, and myself.

Upon the whole, then, we must say that although
Professor Weismann was the first to recognize the
diversifying influence of merely indiscriminate isolation
per se (apogamy). he did so only in part. He failed
to distinguish the true essence of the principle, and by
overlaying it with a mass of hypothetical speculation,
concealed even more of it than he revealed.

The general theory of Isolation, as independently
worked out by Mr. Gulick and myself, has already
been so fully explained, that it will here be sufficient
merely to enumerate its more distinguishing features.
These are, first, drawing the sharpest possible line
between evolution as monotypic and polytypic ;
second, showing that while for the former the peculiar
kind of isolation which is presented by natural
selection suffices of itself to transform a specific type,
in order to work for the latter, or to branch a specific
type, natural selection must necessarily be assisted by
some other kind of isolation ; third, that even in the
absence of natural selection, other kinds of isolation
may be sufficient to efTect specific divergence through
independent generation alone ; fourth, that, neverthe-
less, natural selection, where present, will always
accelerate the process of divergence ; fifth, that
monotypic evolution by natural selection depends
upon the presence of intercrossing, quite as much as

I20 Darwin^ and after Darwin.

polytypic evolution (whether with or without natural
selection) depends upon the absence of it ; sixth, that,
having regard to the process of evolution throughout
all taxonomic divisions of organic nature, we must
deem the physiological form of isolation as the most
important, with the exception only of natural
selection. • •

The only difference between Mr. Gulick's essays
and my own is, that, on the one hand, he has
analyzed much more fully than I have the various
forms of isolation ; while, on the other hand, I have
considered much more fully than he has the particular
form of physiological isolation which so frequently
obtains between allied species. This particular form
of physiological isolation I have called " physiological
selection," and claim for it so large a share in the
differentiation of specific types as to find in it a
satisfactory explanation of the contrast between
natural species and artificial varieties in respect of
cross- infertility.

Mr. Wallace, in his Darwinism^ has done good
service by enabling all other naturalists clearly to
perceive how natural selection alone produces mono^
typic evolution — namely, through the free intercross-
ing of all individuals vt^hich have not been eliminated by
the isolating process of natural selection itself. For
he very lucidly shows how the law of averages must
always ensure that in respect of any given specific
character, half the individuals living at the same time
and place will present the character above, and half
below its mean in the population as a whole. Con^
sequently, if it should ever be of advantage to a species

opinions on Isolation, 121

that this character should undergo either increase or
decrease of its average size, form, colour, &c., there
will always be, in each succeeding generation, a suffi-
cient number of individuals — i. e. half of the whole —
which present variations in the required direction,
and which will therefore furnish natural selection
with abundant material for its action, without the
need of any other form of isolation. It is to be
regretted, however, that while thus so clearly pre-
senting the fact that free intercrossing is the very
means whereby natural selection is enabled to effect
monotypic evolution, he fails to perceive that such
intercrossing must always and necessarily render it
impossible for natural selection to effect polytypic
evolution. A little thought might have shown him
that the very proof which he gives of the necessity
of intercrossing where the transmutation of species
is concerned, furnishes, measure for measure, as good
a proof of the necessity of its absence where the multi-
plication of species is concerned. In justice to him,
however, it may be added, that this distinction be-
tween evolution as monotypic and polytypic (with
the important consequence just mentioned) still con-
tinues to be ignored also by other well-known evo-
lutionists of the " ultra-Darwinian " school. Professor
Meldola, for example, has more recently said that in
his opinion the "difficulty from intercrossing " has been
in large part — if not altogether— removed by Mr.
Wallace's proof that natural selection alone is capable
of effecting [monotypic] evolution ; while he regards
the distinction between monotypic and polytypic
evolution as mere " verbiage V*

^ Nature t vol. xliii. p. 410, and vol. xliv. p. 29.

122 Darwin^ and after Darwin.

It is in relation to my presentment of the im-
possibility of natural selection alone causing poly-
typic evolution, that Mr. Wallace has been at the
pains to show how the permission of intercrossing
(panmixia) is necessary for natural selection in its
work of causing monotypic evolution. And not only
has he thus failed to perceive that the "difficulty"
which intercrossing raises against the view of natural
selection being of itself capable of causing polytypic
evolution in no way applies to the case of monotypic ;
but as regards this " difficulty," where it does apply,
he says: —

Professor G. J. Romanes has adduced it as one of the
difficulties which can alone be overcome by his theory of physio-
logical selection ^

This, however, is a misapprehension. I have by
no means represented that the difficulty in question
can alone be overcome by this theory. What I have
represented is, that it can be overcome by any of the
numerous forms of isolation which I named, and
of which physiological selection is but one. And
although, where common areas are concerned^ I believe
that the physiological form of isolation is the most
important form, this is a very different thing from
entertaining the supposition which Mr. Wallace here
assigns to me.

I may take this opportunity of correcting a some-
what similar misunderstanding which has been more
recently published by Professor W. A. Herd man, of
Liverpool ; and as the case which he gives is one ot

* Darwinism, p. 143.

opinions on Isolation. 123

considerable interest in itself, I will quote his remarks
in extenso. In his Opening Address to the Liverpool
Biological Society^ Professor Herd man said : —

Some of you will doubtless remember that in last year's
address, while discussing Dr. Romanes' theory of physiological
selection, I quoted Professor Flemming Jenkin's imaginary case
of a white man wrecked upon an island inhabited by negroes,
given as an illustration of the supposed swamping effect by
free intercrossing of a marked variety with the parent species.
I then went on to say in criticism of the result at which Jenkin
arrived, viz. that the characteristics of the white man would be
stamped out by intercrossing with the black : —

"Two influences have, 1 think, been ignored, viz. atavism,
or reversion to ancestral characters, and the tendency of the
members of a variety to breed with one another. Keeping to
the case described above, I should imagine that the numbers of
intelligent young mulattoes produced in the second, third, fourth,
and few succeeding generations would to a large extent inter-
marry, the result of which would be that a more or less white
aristocracy would be formed on the island, including the king
and all the chief people, the most intelligent men and the bravest
warriors. Then atavism might produce every now and then
a much whiter individual— a reversal to the characteristics of
the ancestral European— who, by being highly thought of in
the whitish aristocracy, would have considerable influence
on the colour and other characteristics of the next generation.
Now such a white aristocracy would be in precisely the same
circumstances as a fa- Durable variety competing with its parent
species," &c.

You may imagine then my pleasure when, a few months after
writing the above, I accidentally found, in a letter ^ written by the
celebrated African traveller Dr. David Livingstone to Lord
Granville, and dated " Unyanyembe, July ist, 1872," the follow-
ing passage : —

" About five generations ago, a white man came to the high-
lands of Basango, which are in a line east of the watershed.

' In Appendix to H. M. Stanley's A^<?w I found Livingstone, 2nd ed.
London, 187a, p. 715.

124 Darwifif and after Darwin,

He had six attendants, who all died, and eventually their head-
man, called Charura, was elected chief by the Basafigo. In
the third generation he had sixty able-bodied spearmen as lineal
descendants. This implies an equal number of the other sex.
They are very light in colour, and easily known, as no one is
allowed to wear coral beads such as Charura brought except the
royal family. A book he brought was lost only lately. The
interest of the case lies in its connexion with Mr. Darwin's
celebrated theory on the ' origin of species,' for it shows that an
improved variety, as we whites modestly call ourselves, is not so
liable to be swamped by numbers as some have thought."

Here we have a perfect fulfilment of what I last year, in
ignorance of this observation of Livingstone's, predicted as being
likely to occur in such a case. We have the whitish aristocracy
in a dominant condition, and evidently in a fair way to spread
their characteristics over a larger area and give rise to a marked
variety, and it had clearly struck Livingstone fourteen years
before the theory of physiological selection had been heard of,
just as it must strike us now, as an instance telling strongly
against the " swamping " argument as used by Flemming Jenkin
and Romanes.

Here we have a curious example of one writer
supporting the statements of another, while appear-
ing to be under the impression that he is controvert-
ing those statements. Both Professor Herdman's
imaginary case, and its realization in Livingstone's
account, go to show " the tendency of the members
of a variety to breed with one another." This is
what I have called " psychological selection," and,
far from "ignoring'* it, I have always laid stress
upon it as an obviously important form of isolation
or prevention of free intercrossing. But it is a form
of isolation which can only occur in the higher animals,
and, therefore, the whole of Professor Herdman's
criticism is merely a restatement of my own views
as already published in the paper which he is

opinions on Isolation: 125

criticizing. For all that his argument goes to prove
is, first, the necessity for some form of isolation if
the overwhelming effects of intercrossing are to be
obviated ; and, secondly, the manifest consequence
that where the psychological form is unavailable (as
in many of the lower animals and in all plants),
some other form must be present if divergent evolu-
tion is taking place on a common area.

Seeing that so much misunderstanding has been
shown with reference to my views on " the swamp-
ing effects of intercrossing," and seeing also that
this misunderstanding extends quite as much to Mr.
Gulick's views as to my own, I will here supply
brief extracts from both our original papers, for the
double purpose of showing our complete agreement,
and of leaving it to be judged whether we can
fairly be held responsible for the misunderstanding
in question. After having supplied these quotations,
I will conclude this historical sketch by considering
what Mr. Wallace has said in reply to the views
therein presented. I will transcribe but a single
passage from our papers, beginning with my own.

Any theory of the origin of species in the way of descent must
be prepared with an answer to the question. Why have species
multiplied'i How is it that, in the course of evolution, species
have not simply become transmuted in linear series instead of
ramifying into branches? This question Mr. Darwin seeks to
answer "from the simple circumstance that the more diversified
the' descendants from any one species becomes in structure,
constitution, and habits, by so much will they be better enabled
to seize on many and widely diversified places in the economy
of nature, and so be enabled to increase in numbers." And he
proceeds to illustrate this principle by means of a diagram,

126 Darwin^ and after Darwin.

showing the hypothetical divergence of character undergone by
the descendants of seven species. Thus, he attributes divergence
of character exclusively to the influence of natural selection.

Now, this argument appears to me unassailable in all save
one particular; but this is a most important particular: the
argument wholly ignores the fact of intercrossing with parent
forms. Granting to the argument that intercrossing with parent
forms is prohibited, and nothing can be more satisfactory. The
argument, however, sets out with showing that it is in limited
areas, or in areas already overstocked with the specific form in
question, that the advantages to be derived from diversification
will be most pronounced. It is where they "jostle each other
most closely " that natural selection will set a premium upon
any members of the species which may depart from the common
type. Now, inasmuch as this jostling or overcrowding of
individuals is a needful condition to the agency of natural
selection in the way of diversifying character, must we not fee)
that the general difficulty from intercrossing previously con-
sidered is here presented in a special and aggravated form ?
At all events, I know that, after having duly and impartially
considered the matter, to me it does appear that unless the
swamping effects of intercro sing with the parent form on an
overcrowded area is in some way prevented to begin with,
natural selection could never have any material supplied by
which to go on with. Let it be observed that I regard Mr.
Darwin's argument as perfect y sound where it treats of the
divergence of species^ and of their further divergence m.\.o genera ;
for in these cases the physiological barrier is known to be
already present. But in applying the argument to explain
the divergence of individuals into varieties, it seems to me that
here, more than anywhere else, Mr. Darwin has strangely lost
sight of the formidable difficulty in question; for in this
particular case so formidable does the difficulty seem to me,
that I cannot believe that natural selecticm alone could produce
any divergence of specific character, so long as all the in-
dividuals on an overcrowded area occupy that area together.
Yet, if any of them quit that area, and so escape from the
unifying influence of free intercrossing, these individuals also
escape from the conditions which Mr. Darwin names as those

opinions on Isolation, 127

that are needed by natural selection in order to produce diver-
jjence. Therefore, it appears to me that, under the circum-
stances supposed, natural selection alone could not produce
divergence ; the most it could do would be to change the whole
specific type in some one direction, and thus induce trans-
mutation of species in a linear series, each succeeding member
of which might supplant its parent form. But in order to
secure diversity^ mult ip I i cation^ or ramification of species,
it appears to me obvious that the primary condition required is
that of preventing intercrossing with parent forms at the origin
of each branch, whether the prevention be from the first
absolute, or only partial.

Now for Mr. GuUck, a portion of whose more
lengthy discussion of the subject, however, is all that
I need quote

Having found that the evolution of the fitted is secured through
the prevention of crossing between the better fitted and the less
fitted, can we believe that the evolution of a special race,
regularly transmitting a special kind of fitness, can be realized
without any prevention of crossing with other races that have
no power to transmit that special kind of fitness.? Can we
suppose that any advantage, derived from new powers that
prevent severe competition with kindred, can be permanently
transmitted through succeeding generations to one small section
of the species while there is free crossing equally distributed
between all the families of the species 1 Is it not apparent that
the terms of this supposition are inconsistent with the funda-
mental laws of heredity ? Does not inheritance follow the lines
of consanguinity \ and when consanguinity is widely diffused,
can inheritance be closely limited ? When there is free crossing
between the families of one species, will not any peculiarity
that appears in one family either be neutralized by crosses
with families possessing the opposite quality, or, being preserved
by natural selection, while the opposite quality is gradually
excluded, will not the new quality gradually extend to all the
branches of the species ; so that, in this way or in that, increas-
ing divergence of form will be prevented ?

128 DarwiUy and after Darwin.

If the advantage of freedom from competition in any given
variation depends on tiie possession, in some degree, of new
adaptations to unappropriated resources, there must be some
cause that favours the breeding together of those thus specially
endowed, and interferes in some degree with their crossing
with other variations, or, failing this, the special advantage will
in succeeding generations be lost. As some degree of Inde-
pendent Generation is necessary for the continuance of the
advantage, it is evident that the same condition is necessary
for the accumulation through Natural Selection of the powers
on which the advantage depends. The advantage of divergence
of character cannot be retained by those that fail to retain the
divergent character; and divergent character cannot be retained
by those that are constantly crossing with other kinds ; and the
prevention of free crossing between those that are equally
successful is in no way secured by Natura Selection.

So much, then, as expressive of Mr. GuHck's
opinion upon this subject. To exactly the same
effect Professor Lloyd. Morgan has recently published
his judgement upon it thus : —

That perfectly free intercrossing, between any or all of the
individuals of a given group of animals, is, so long as the
characters of the parents are blended in the offspring, fatal to
divergence of character, is undeniable. Through the elimination
of less favourable variations, the swiftness, strength, and
cunning of a race may be gradually improved. But no form of
elimination can possibly differentiate the group into swift,
strong, and cunning varieties, distinct from each other, so long
as all three varieties freely interbreed, and the characters of
the parents blend in the offspring. Elimination may and does
give rise to progress in any given group, as a group ; it does
not and cannot give rise to differentiation and divergence, so
long as interbreeding with consequent interblending of characters
be freely permitted. Whence it inevitably follows, as a matter
of simple logic, that where divergence has occurred, inter-
crossing and interbreeding must in some way have been
lessened or prevented. Thus a neVv factor is introduced, that

opinions on Isolation, 129

of isolation or segregation. And there is no questioning the
fact that it is of great importance. Its importance, indeed, can
only be denied by denying the swamping effects of intercrossing,
and such denial implies the tacit assumption that interbreeding
and interblending are held in check by some form of segregation.
The isolation explicitly denied is impUcitly assumed '.

Similarly, and still more recently, Professor
Le Conte writes : —

It is evident, then, as Romanes claims, that natural selection
alone tends to tnonotypic evolution. Isolation of some sort
seems necessary to polytypic evolution. The tree of evolution
under the influence of natural selection alone grows palm-like
from its terminal bud. Isolation was necessary to the starting
of lateral buds, and thus for the profuse ramification which is its
most conspicuous character*.

In order to complete this historical review, it only
remains to consider Mr. Wallace's utterances upon the
subject.

It is needless to say that he stoutly resists the
view of Weismann, Delboeuf, Gulick, and myself, that
specific divergence can ever be due — or, as I under-
stand him, even so much as assisted — by this prin-
ciple of indiscriminate isolation (apogamy). It will be
remembered, however, that Mr. Gulick has adduced
certain general principles and certain special facts
of geographical distribution, in order to prove that
apogamy eventually leads to divergence of character,
provided that the isolated section of the species does
not contain any very large number of individuals.
Now, Mr. Wallace, without making any reference to
this argument of Mr. Gulick, simply states the reverse
—namely, that, as a matter of fact, indiscriminate

* Animal Life and Intelligence, pp. 98, 99 (1890-1891).
■ The Factors of Evolution (1891).

ni. K

130 DarwiUy and after Darwin,

isolation is not found to be associated with diverg-
ence of character. For, he says, " there is an entire
absence of change, where, if this were a vera causa,
we should expect to find it^.'* But the only case
which he gives is that of Ireland.

This, he says, furnishes *' an excellent test case, for
we know that it [Ireland] has been separated from
Britain since the end of the glacial epoch: . . . yet
hardly one of its mammals, reptiles, or land molluscs
has undergone the sliglitest change^." Here, how-
ever, Mr. Wallace shows that he has failed to under-
stand '*the views of those who, like Mr. Gulick,
believe isolation itself to be a cause of modification
of species " ; for it belongs to the very essence of these
views that the efficiency of indiscriminate isolation as
a " vera causa " of organic evolution varies inversely
with the number of individuals (i. e. the size of the
species-section) exposed to its influence. Therefore,
far from being "an excellent test case," the case
of Ireland is unsatisfactory. If we are in search of
excellent test cases, in the sense intended by Mr.
Wallace, we ought not to choose a large island,
which from the time of its isolation must have con-
tained large bulks of each of the geographically
separated species concerned ; we ought to choose
cases where as small a number as possible of the
representatives of each species were in the first
instance concerned. And, when we do this, the
answer yielded by any really " excellent test case " is
unequivocal.

No better test case of this kind has ever been
furnished than that of Mr. Gulick's land -shells,

' Darwinism, p. 151. "^ Ibid.

Optntons on Isolation, 131

which Mr. Wallace is specially considering in the
part of his book where the sentence above quoted
occurs. How, then, does he meet this case? He
meets it by assuming that in all the numerous
adjacent valleys of a small island there must be
as many differences of environment, each of which
is competent to induce slight varietal changes on
the part of its occupants by way of natural selection,
although in no one case can the utility of these
slight changes be surmised. Now, against this ex-
planation there are three overwhelming considerations.
In the first place, it is purely gratuitous, or offered
merely in order to save the hypothesis that there
can be no other cause of even the most trivial change
in species than that which is furnished by natural
selection. In the second place, as Mr. Gulick writes
to me in a private letter, " if the divergence of
Sandwich Island land molluscs is wholly due to
exposure to different environments, as Mr. Wallace
argues on pages 147-150, tl.en there must be com-
pletely occult influences in the environment that
vary progressively with each successive mile. This
is so violent an assumption that it throws doubt
on any theory that requires such support." In the
third place, the assumption that the changes in
question must have been due to natural selection,
is wholly incompatible with the facts of isolation
elsewhere — namely, in those cases where (as in that
of Ireland) a large section of species, instead of
a small section, has been indiscriminately isolated.
Mr. Wallace, as we have seen, inadvertently alludes
to these " many other cases of isolation " as evidence
against apogamy being per se a cause of specific

K 2

132 Darwin^ and after Darwin.

change. But although, for the reason above stated,
they are without relevancy in this respect, they
appear to me fatal to the explanation which he gives
of specific changes under apogamy where only small
sections of species are concerned. For example, can
it be rationally maintained that there are more
differences of environment between every two of
the many contiguous valleys of a small island,
such as Mr. Gulick describes, than there are in
the incomparably larger area of the whole of
Ireland? But, if not, and if natural selection is
able to work such *' occult " wonders in each succes-
sive mile on the Sandwich Islands, why has it so
entirely lost this magic power in the case of Ireland
— or in the " many other cases of isolation " to
which Mr. Wallace refers? On his theory there
is no coherent answer to be given to this question,
while on our theory the answer is given in the
very terms of the theory itself The facts are
plainly just what the theory requires that they
should be ; and therefore, if they were not as they
are, the theory would be deprived of that confirma-
tion which it now derives from them.

Thus, in truth, though in an opposite way, the
case of Ireland is, as Mr. Wallace says, "an excel-
lent test case," when once the theory of apogamy
as a '' vera causa'' of specific change is understood;
and the effect of applying the test is fully to corro-
borate this theory, while at the same time it as
fully negatives the other. For the consideration
whereby Mr. Wallace seeks to explain the inactivity
of natural selection in the case of Ireland is not
" coherent." What he says is, " That changes have

opinions on Isolation, 133

not occurred through natural selection, is perhaps
due to the less severe struggle for existence, owing
to the smaller number of competing species V But
even with regard to molluscs alone, there is a greatly
larger number of species in Ireland than occurs in
any one valley of the Sandwich Islands ; while if we
have regard to all the other classes of animal life,
comparison entirely fails.

Much more to the point are certain cases which
were adduced long ago by Weismann in his essay
previously considered. Nevertheless, although this
essay was published as far back as 1872, and,
although it expressly deals with the question of
divergence of character through the mere prevention of
intercrossing (Amixia), Mr. Wallace nowhere alludes
to these cases per contra, which are so much more
weighty than his own "test case" of Ireland. Of
such are four species of butterflies, belonging to three
genera ^, which are identical in the polar regions and
in the Alps, notwithstanding that the sparse Alpine
populations have been presumably separated from
their parent stocks since the glacial period; or of
certain species, of fresh water crustaceans (Apus\ the
representatives of which are compelled habitually to
form small isolated Colonies in widely separated
ponds, and nevertheless exhibit no divergence of
character, although apogamy has probably lasted for
centuries. These cases are unquestionably of a very
cogent nature, and appear of themselves to prove
that apogamy alone is not invariably capable of

» JLocHt., p. 151.

'-' Namely, Lycaena dmzelii, L. pheretes, Argynnis pales, Erebia
mania.

£34 Darwin^ and after Darwin.

inducing divergence - at any rate, so rapidly as we
might expect. There appears, however, to be
another factor, the presence or absence of which
makes a great difference. This as stated in the text,
is the degree in which a specific type is stable or
unstable — liable or not liable to vary. Thus, for
example, the Goose is what Darwin calls an *' inflex-
ible " type as compared with most other domesticated
birds. Therefore, if a lot of geese were to be indis-
criminately isolated from the rest of their species, the
probability is that in a given time their descendants
would not have diverged from the parent type to such
an extent as would a similar lot of ducks under
similar circumstances: the more stable specific type
would require a longer time to change under the
influence of apogamy alone. Now, the butterflies
and crustaceans quoted by Weismann may be of a
highly stable type, presenting but a small range
of individual variability ; and, if so, they would
naturally require a long time to exhibit any change
of type under the influence of apogamy alone. But,
be this as it may, Weismann himself adduces these
cases merely for the sake of showing that there are
cases which seem to tell against the general prin-
ciple of modification as due to apogamy alone — i.e.
the general principle which, under the name amixia,
he is engaged in defending. And the conclusion
at which he himself arrives is, that while it would
be wrong to aflirm that apogamy must in all
cases produce divergence, we are amply justified
in affirming that in many cases it may have done
so ; while there is good evidence to prove that in
not a few cases it has done so, and therefore

opinions on Isolation. 135

should be accepted as one of the factors of organic
evolution ^.

My view from the very first has been that variations
in the way of cross-infertility are of frequent occur-
rence (how, indeed, can they be otherwise, looking
to the complex conditions that have to be satisfied
in every case of full fertility ?) ; and, therefore,
however many of such variations are destined to die
out, whenever one arises, " under suitable conditions,"
"it must inevitably tend to be preserved as a new
natural variety, or incipient species." Among the
higher animals — which are " comparatively few in
number "—I think it probable that some slight change
of form, colour, habit, &c., must be usually needed
either to " superinduce," or, which is quite a dif-
ferent thing, to coincide with the physiological change.
But in the case of plants and the lower inverte-
brata, I see no reason for any frequent concomitance
of this kind ; and therefore believe the physiological

* Since the above was written, I have heard of some cases which seem
to present greater difficulties to our theory than those above quoted.
These refer to some of the numerous species of land mollusca which
inhabit the isolated rocks near Madeira (Dezertas). My informant is
Dr. Grabham, who has himself investigated the matter, and reports
as follows : —

" It is no uncommon thing to meet with examples of the same species,
sub-fossil, recent, and living upon one spot, and presenting no variation
in the long record of descent." Then, after naming these examples, he
adds, "All seem to vary immediately on attaining new ground, assuming
many aspects in diffcient districts."

Unquestionably these stntements support, in a very absolute manner,
Mr. Wallace's opinion, while making directly against my own. It is
but fair, however, to add that the cases are not numerous (some half-
dozen at the most, and all within the limits of a single genus\ and that,
even in the opinion of my informant himself, the facts have not hitherto
been sufficiently investigated for any decisive judgement to be formed
upon them.

13$ Darwin, and after Darwin.

change to be, "as a general rule," the primordial
change. At the same time, I have always been
careful to insist that this opinion had nothing to do
with "the essence of physiological selection ''; seeing
that "it was of no consequence" to the theory in
what proportional number of cases the cross-sterility
had begun per se, had been superinduced by morpho-
logical changes, or only enabled to survive by
happening to coincide with any other form of
homogamy. In short, *' the essence of physiological
selection" consists in all cases of the diversifying effect
of cross-infertility, whensoever and howsoever it may
happen in particular cases to have been caused.

Thus I emphatically reaffirm that '' from the first
I have always maintained that it makes no essen-
tial difference to the theory in what proportional
number of cases they [the physiological variations]
have arisen 'alone in an otherwise undifferentiated
species'"; therefore, "even if I am wrong in sup-
posing that physiological selection can ever act
alone, the principle of physiological selection, as I
have stated it, is not thereby affected. And this
principle is, as Mr. Wallace has re-stated it, 'that
some amount of infertility characterizes the distinct
varieties which are in process of differentiation into
species ' — infertility whose absence, ' to obviate the
effects of intercrossing, may be one of the usual
causes of their failure to become developed into
distinct species.'*'

These last sentences are quoted from the corre-
spondence in Nature'^, and to them Mr. Wallace replied
by saying, "if this is not an absolute change of front,

' Vol. xliii. p. 1 37.

opinions on Isolation, 137

words have no meaning '*; that '• if this is *the whole
essence of physiological selection,' then physiological
selection is but a re-statement and amplification of
Darwin's views "; that such a '* change of front " is
incompatible, not only with my term " physiological
selection," but also with my having '' acknowledged
that Mr. Catchpool had ' very clearly put forward the
theory of physiological selection'"; and much more
to the same effect.

Now, to begin with, it is due to Mr. Catchpool to
state that his only publication upon this subject is
much too brief to justify Mr. Wallace's inference, that
he supposes variations in the way of cross-infertility
always to arise " alone in an otherwise undifferentiated
species." What Mr. Catchpool's opinion on this
point may be, I have no knowledge ; but, whatever it
is, he was unquestionably the first writer who "clearly
stated the leading principles" of physiological selec-
tion, and this fact I am very glad to have " acknow-
ledged." In my correspondence with Mr. Wallace,
however, I not only named Mr. Catchpool : I also
named — and much more prominently — Mr. Gulick.
For even if I were to grant (which I am far indeed
from doing) that there was any want of clearness in
my own paper touching the point in question, I have
now repeatedly shown that it is simply impossible
for any reader of Mr. Gulick's papers to misunder-
stand his views with regard to it. Accordingly,
I replied to Mr. Wallace in Nature by saying: —

Not only have I thus from the first fully recognized the
sundry other causes of specific change with which the physio-
logical variations may be associated ; but Mr. Gulick has gone
into this side of our common theory much more fully, and

139 Darwin, and after Darwin,

elaborately calculated out the high ratio in which the differ-
entiating agency of any of these other causes must be increased
when assisted by — i. e. associated with -even a moderate degree
of the selective fertility, and vice versa. Therefore, it is simply
impossible for Mr. Wallace to show that " our theory " differs
from his in this respect. Yet it is the only respect in which his
reply alleges any difference. (Vol. xHii. p. 127.)

I think it is to be regretted that, in his answer to
this, Mr. Wallace alludes only to Mr. Catchpool, and
entirely ignores Mr. Gulick — whose elaborate calcula-
tions above alluded to were communicated to the
Linnaean Society by Mr. Wallace himself in 1887.

The time has now come to prove, by means of
quotations, that I have from the first represented
the " principle," or " essence," of physiological selec-
tion to consist in selective fertility furnishing a need-
ful condition to specific difi*erentiation, in at least
a large proportional number of allied species which
afterwards present the reciprocal character of cross-
sterility ; that I have never represented variations
in the way of this selective fertility as necessarily
constituting the initial variations, or as always arising
"alone, in an otherwise undifferentiated species";
and that, although I have uniformly given it as my
opinion that these variations do in some cases thus
arise (especially among plants and lower invertebrala),
I have as uniformly stated "that it makes no differ-
ence to the theory in what proportional number of
cases they have done so " — or even if, as Mr. Wallace
supposes, they have never done so in any case at all '.

* This refers to what I understand Mr. Wallace to say in the Nature
correspondence is the supposition on which his own theory of the origin
cf species by cross-infertility is founded. But in the original statement
of that theory itself, it is everywhere "supposed" that when species are

opinions on Isolation. 139

These statements (all of which are contradictory
of the only points of difference alleged) have already
been published in my article in the Mo7iist of
October, 1890. And although Mr. Wallace, in his
reply to that article, ignores my references to the
" original paper," it is scarcely necessary to quote the
actual words of the paper itself, since the reader who
is further interested in this controversy can readily
refer to it in the Journal of the Linnaean Society
(vol. xix. pp. 337-411).

Having arrived at these results with regard to the
theory of Isolation in general and of Physiological
Isolation in particular, I arrive also at the end of this
work. And if, while dealing with the post-Darwinian
period, I have imparted to any general reader the
impression that there is still a great diversity of
expert opinion ; I must ask him to note that points
with reference to which disagreement still exists
are but very subordinate to those with regard to
which complete agreement now prevails. The noise
of wrangling disputations which has so filled the
camp of evolutionists since the death of their
captain, is apt to hide from the outside world the
solid unanimity that prevails with regard to all
the larger and more fundamental questions, which
were similarly the subjects of warfare in the past
generation. Indeed, if we take a fair and general

originated by cross-infertility, the initial change is the physiological
change. In his original statement of that theory, therefore, he liteially
went further than I had gone in my "original paper," with reference to
supposing the physiological change to be the initial change. I do not
doubt that this is due to some oversight of expression ; but it is curious
that, having made it, he should still continue his endeavour to fix exactly
the same oversight upon me.

140 DarwtHy and after Darwin,

view of the whole history of Darwinism, what must
strike us as the really significant fact is the astonish-
ing unanimity which has been so rapidly attained
with regard to matters of such immeasurable impor-
tance. It is now but little more than thirty years
since the publication of the Origin of Species \ and
in that period not only have all naturalists unequi-
vocally embraced the doctrine of descent considered
as a fact ; but, in one degree or another, they have
all as unequivocally embraced the theory of natural
selection considered as a method. The only points
with regard to which any difference of opinion still
exist, have reference to the precise causation of that
mighty stream of events which, under the name of
organic evolution, we have now all learnt to accept as
scientifically demonstrated. But it belongs to the
very nature of scientific demonstration that, where
matters of great intricacy as well as of high generality
are concerned, the process of demonstration must be
gradual, even if it be not always slow. It is only by
the labours of many minds working in many directions
that, in such cases., truth admits of being eventually
displayed. Line upon line, precept upon precept,
here a little and there a little — such is the course of
a scientific revelation ; and the larger the subject-
matter, the more subtle and the more complex the
causes, the greater must be the room for individual
differences in our reading of the book of Nature.
Now, if all this be -true, must we not feel that in the
matter of organic evolution the measure of agreement
which has been attained is out of all proportion to
the differences which still remain — differences which,
although of importance in themselves, are insignificant

opinions on Isolation, 141

when compared with those which once divided the
opinions of not a few still living men ? And if we are
bound to feel this, are we not bound further to feel
that the very intensity of our disputations over these
residual matters of comparative detail, is really the
best earnest that can be given of the determination
of our quest — determination which, like that of our
fathers, cannot fail to be speedily rewarded by the
discovery of truth ?

Nevertheless, so long as this noise of conflict is
in the Senate, we cannot wonder if the people are
perplexed. Therefore, in conclusion, I may ask it to
be remembered exactly what are the questions — and
the only questions — which still divide the parties.

Having unanimously agreed that organic evolution
is a fact and that natural selection is a cause, or
a factor in the process, the primary question in debate
is whether natural selection is the only cause, or
whether it has been assisted by the co-operation of
other causes. The school of Weismann maintain that
it is the only cause ; and therefore deem it worse
than useless to search for further causes. With this
doctrine Wallace in effect agrees, excepting as regards
the particular case of the human mind. The school
of Darwin, on the other hand — to which I myself
claim to belong — believe that natural selection has
been to a considerable extent supplemented by other
factors ; and, therefore, although we further believe
that it has been the "main" factor, we agree with
Darwin himself in strongly reprobating all attempts
to bar a priori the progress of scientific investigation
touching what, if any, these other factors may be.
Lastly, there are several more or less struggling

142 Darwin, and after Darwin,

schools, chiefly composed of individual members who
agree with each other only to the extent of holding
that the causal agency of natural selection is not so
great as Darwin supposed. The Duke of Argyll,
Mr. Mivart, and Mr. Geddes may be named in this
connexion ; together with the self-styled neo-
Lamarckians, who seek to magnify the Lamarckian
principles at the expense of the distinctively Dar-
winian.

This primary di(Terence of opinion leads deductively
to certain secondary differences. For if a man starts
with the premiss that natural selection must neces-
sarily be the "exclusive" cause of organic evolution,
he is likely to draw conclusions which another man
would not draw who starts with the premiss that
natural selection is but the " main " cause. Of these
subordinate differences the most important are those
which relate to the possible transmission of acquired
characters, to the necessary (or only general) utility
of specific characters, and to the problem touching the
inter-sterility of allied species. But we may well
hope that before another ten years shall have passed,
even these .still outstanding questions will have been
finally settled ; and thus that within the limits of an
ordinary lifetime the theory of organic evolution will
have been founded and completed in all its parts, to
stand for ever in the world of men as at once the
greatest achievement in the history of science, and the
most splendid monument of the nineteenth century.

In the later chapters of the foregoing treatise I have
sought to indicate certain matters of general principle,
which many years of study specially devoted to this
great movement of contemporary thought have led

opinions on Isolation, 143

me to regard as almost certainly sound in themselves,
and no less certainly requisite as complements of the
Darwinian theory. I will now conclude by briefly
summarizing these matters of general principle in the
form of twelve sequent propositions. And, in doing
so, I may ask it to be noticed that the system which
these propositions serve to express may now claim
at the least, to be a strictly logical system. For the
fact that, not merely in its main outlines, but likewise
in its details, it has been independently constructed
by Mr. Gulick, proves at any rate this much ; seeing
that, where matters of such intricacy are concerned,
nothing but accurate reasoning from a common
foundation of data could possibly have yielded so
exact an agreement. The only difference between us
is, that Mr. Gulick has gone into much further detail
than I have ever attempted in the way of classifying
the many and varied forms of isolation ; while 1 have
laid more special stress upon the physiological form,
and found in it what appears to me a satisfactory
solution of "the greatest of all the difficulties in the
way of accepting the theory of natural selection as
a complete explanation of the origin of species" —
namely, " the remarkable difference between varieties
and species when crossed."

GENERAL CONCLUSIONS.

1. Natural Selection is primarily a theory

OF THE cumulative DEVELOPMENT OF ADAPTA-
tions wherever these occur ; and therefore
is only incidentally, or likewise, a theory
of the origin of speciiis in cases where allied
species differ from one another in respect of
peculiar characters, which are also adaptive
characters.

2. Hence, it does not follow from the
theory of natural selection that all
species— much less all specific characters —
must necessarily have owed their origin to
natural selection; since it cannot be proved
deductively from the theory that no " means
of modification " other than natural selec-
tion is competent to produce such slight
degrees of modification as go to consti-
tute diagnostic distinctions between closely
allied species; while, on the other hand,
there is an overwhelming mass of evidence

TO PROVE THE ORIGIN OF "A LARGE PROPORTIONAL
NUMBER OF SPECIFIC CHARACTERS " BY CAUSES OF
MODIFICATION OTHER TPIAN NATURAL SELECTION.

General Conclusions, 145

3. Therefore, and upon the whole, as
Darwin so emphatically held, "Natural
selection has been the main, but not the
exclusive means of modification."

4. Even if it were true that all species

AND ALL specific CHARACTERS MUST NECESSARILY
OWE THEIR ORIGIN TO NATURAL SELECTION, IT
WOULD STILL REMAIN ILLOGICAL TO DEFINE THE
THEORY OF NATURAL SELECTION AS INDIFFERENTLY
A THEORY OF SPECIES OR A THEORY OF ADAPTA-
TIONS ; FOR, EVEN UPON THIS ERRONEOUS SUPPO-
SITION, SPECIFIC CHARACTERS AND ADAPTIVE
CHARACTERS WOULD REMAIN VERY FAR INDEED
FROM BEING CONTERMINOUS— MOST OF THE MORE
IMPORTANT ADAPTATIONS WHICH OCCUR IN
ORGANIC NATURE BEING THE COMMON PROPERTY
OF MANY SPECIES.

5. In no CASE CAN NATURAL SELECTION HAVE
BEEN THE CAUSE OF MUTUAL INFERTILITY
BETWEEN ALLIED, OR ANY OTHER, SPECIES— /.tf. OF
THE MOST GENERAL OF ALL "SPECIFIC CHAR-
ACTERS."

6. Without Isolation, or the prevention of

FREE intercrossing, ORGANIC EVOLUTION IS IN
NO CASE POSSIBLE. THEREFORE, IT IS ISOLATION
THAT kas BEEN "THE EXCLUSIVE MEANS OF

modification," or, more correctly, the uni-
versal condition to it. therefore, also,
Heredity and Variability being given, the

WHOLE theory OF ORGANIC EVOLUTION BECOMES
A THEORY OF THE CAUSES AND CONDITIONS WHICH
LEAD TO ISOLATION. •

IIL L

146 Darwin, and ajter Darwin,

7. Isolation may be either discriminate

OR INDISCRIMINATE. WhEN DISCRIMINATE, IT
HAS REFERENCE TO RESEMBLANCES BETWEEN IN-
DIVIDUALS CONSTITUTING THE ISOLATED COLONY
OR GROUP; WHEN INDISCRIMINATE, IT HAS NO
SUCH REFERENCE. In THE FORMER CASE THERE
ARISES HOMOGAMY, . AND IN THE LATTER CASE
THERE ARISES ApOGAMY.

8. Except where very large populations
are concerned, indiscriminate isolation
always tends to become increasingly discrim-
inate ; and, in the measure that it does so,
apogamy passes into homogamy, by virtue of
Independent Variability.

9. Natural Selection is one among many
other forms of discriminate isolation, and

presents IN THIS RELATION THE FOLLOWING
PECULIARITIES :—(rt!) THE ISOLATION IS WITH
REFERENCE TO SUPERIORITY OF FITNESS; {b) IS
EFFECTED BY DEATH OF THE EXCLUDED INDI-
VIDUALS; AND {c) UNLESS ASSISTED BY SOME
OTHER FORM OF ISOLATION, CAN ONLY EFFECT
MONOIYPIC AS DISTINGUISHED FROM POLYTYPIC
EVOLUTION.

TO. It is a GENERAL LAW OF ORGANIC EVOLUTION
THAT THE NUMBER OF POSSIBLE DIRECTIONS IN
WHICH DIVERGENCE MAY OCCUR CAN NEVER BE
MORE THAN EQUAL TO THE NUM15ER OF CASES OF
EFFICIENT ISOLATION ; BUT, EXCEPTING NATURAL
SELECTION, ANY ONE FORM OF ISOLATION NEED
NOT NKCESSARILY R?:QUIRE THE CO-OPERATION

General Conclusions. 147

OF ANOTHER FORM IN ORDER TO CREATE AN
ADDITIONAL CASE OF ISOLATION, OR TO CAUSE
POLYTYPIC AS DISTINGUISHED FROM MONOTYPIC
EVOLUTION.

11. Where common areas and polytypic evo-
lution ARE concerned, THE MOST GENERAL AND

most efficient form of isolation has been
the physiological, and this whether the
mutual infertility has been the antecedent
or the consequent of morphological changes
on the part of the organisms concerned, and
whether or not these changes are of an
adaptive character.

12. This form of isolation— which, in
regard to incipient species, j have called
Physiological Selection— may act either

ALONE OR in CONJUNCTION WITH OTHER FORMS OF
isolation ON COMMON AREAS: IN THE FORMER
CASE ITS AGENCY IS OF MOST IMPORTANCE AMONG
PLANTS AND THE LOWER CLASSES OF ANIMALS;
IN THE LATTER CASE ITS IMPORTANCE CONSISTS
IN ITS GREATLY INTENSIFYING THE SEGREGATIVE
POWER OF WHATEVER OTHER FORM OF ISOLATION
IT MAY BE WITH WHICH IT IS ASSOCIATED.

L %

APPENDICES

APPENDIX A.

Mr. Gulick's Criticism of Mr. Wallace's Views on
Physiological Selfxtion.

I HAVE received from Mr. Gulick the results of his
consideration of Mr. Wallace's criticism. As these results
closely resemble those which I have myself reached, and
as they were independently worked out on the other side of
the globe, I deem it desirable to publish them here for the
sake of comparison.

In his covering letter Mr. Gulick writes: —

Mr. Wallace has most certainly adopted the fundamental prin-
ciples of our theory, and in an arbitrary way attempted to claim
the results produced by these principles as the effects of natural
selection. He takes our principles, which in the previous
chapter he has combated ; but he makes such disjointed use of
them that I am not willing to recognize his statement as an
intelligible exposition of our theory. ... I have endeavoured to
indicate at what points Mr. Wallace has deserted his own prin-
ciples, and at what points he has failed to make the best use of
ours. To bring out these points distinctly has been no easy
task; but if you regard this paper on The Preservation and Ac-
cumulation of Cross-infertility as giving any help in elucidating
the true principles, and in showing Mr. Wallace's position in
regard to them, I shall be satisfied. Please make any use of it
that may seem desirable, and then forward it to Professor
Dana.

152 Darwin, and after Darwin.

The following is a general summary of Mr. Gulick's
results : —

Mr. Wallace's criticism of the theory of Physiological Selec-
tion is unsatisfactory ; (i) because he has accepted the funda-
mental principle of that theory on pages 173-9, »" that he
maintains that without the cross-infertility the incipient species
there considered would be swamped ; (2) because he assumes
that physiological selection pertains simply to the infertility
of first crosses, and has nothing to do with the infertility of
mongrels and hybrids; (3) because he assumes that infertility
between first crosses is of rare occurrence between species of
the same genus, ignor ng the fact that in many species of plants
the pollen of the species is pre- potent on the stigma of the same
species when it has to compete with the pollen of other species
of the same genus ; (4) because he not only ignores Mr. Romanes'
statement that cross-infertility often affects "a whole race or
strain," but he gratuitously assumes that the theory of Physio-
logical Selection excludes this "racial incompatibility" (which
Mr. Romanes maintains is the more probable form), and bases his
computation on the assumption that the cross-infertility is not
associated with any other form of segregation ; (5) because he
claims to show that **all infertility not correlated with some
useful variation has a constant tendency to effect its own
elimination," while his computation only shows that, if the cross-
infertility is not associated with some form of positive segre-
gation, it will disappear * ; and (6) because he does not observe
that the positive segregation may be secured by the very form of
the physiological incompatibility. . . . Without here entering
into any computation, it is evident that, e. g. the prepotency of
pollen of each kind with its own kind, if only very slight, will
prevent cross-fertilization as effectually as a moderate degree of
instinctive preference in the case of an animal.

' *• Positive segregation " is Mr. Gulick's term for forms of homo-
gamy other than that which is due to selective fertility. Of these other,
or " positive " forms, natural selection is one ; but as it is far from
being the only one, the criticism points out that utility is not the
only conserving principle with which selective fertility may be asso-
ciated.

Appendix A, 153

The paper likewise indicates a point which, in studying
Mr. Wallace's theory, I have missed. It will be remem-
bered that the only apparent difference between his theory
and mine has been shown to consist in this — that while
I was satisfied to state, in a general way, that natural selec-
tion is probably able to increase a selective fertility which
has already been begun by other causes, Mr. Wallace
has sought to exhibit more in detail the precise conditions
under which it can do so. Now, Mr. Gulick shows that
the particular conditions which Mr. Wallace describes, even
if they do serve to promote an increase of cross-infertility,
are conditions which preclude the possibility of natural selec-
tion coming into play at all. So that if, under these parti-
cular conditions, a further increase of cross-infertility does
take place, it does not take place in virtue of natural selection.
To me it appears that this criticism is sound; and, if so,
it disposes of even the one very subordinate addition to
our theory which Mr. Wallace " claims " as the most
"distinctive" part of his.

The following is the criticism in question : —

On pages 173-186 Mr. Wallace maintains that "Natural
selection is, in some probable cases at all events, able to
accumulate variations in infertility between incipient species"
(p. 174); but his reasoning does not seem to me conclusive.
Even if we grant that the increase of this character [cross-
infertility] occurs by the steps which he describes, it is not
a process of accwmdation by natural selection. In order to be a
means of cumulative modification of varieties, races, or species,
selection, whether artificial or adaptational [i.e. natural], must
preserve certain forms of an intergenerating stock, to the
exclusion of other forms of the same stock. Progressive
change in the size of the occupants of a poultry-yard may be
secured by raising only bantams the first, only common fowls
the second, and only Shanghai fowls the third year ; but this
is not the form of selection that has produced the different
races of fowls. So in nature, rats may drive out and supplant

154 Darwin^ and after Darwin,

mice ; but this kind of selection modifies neither rats nor mice.
On the other hand, if certain variations of mice prevail over
others, through their superior success in escaping their pur-
suers, then modification begins. Now, turning to page 175, we
find that, in the illustrative case introduced by Mr. Wallace,
the commencement of infertility between the incipient species
is in the relations to each other of two portions of a species
that are locally segregated from the rest of the species, and
partially segregated from each other by different modes of
life. These two local varieties, being by the terms of his
supposition better adapted to the environment than the freely
interbreeding forms in other parts of the general area, increase
till they supplant these original forms. Then, in some limited
portion of the general area, there arise two still more divergent
forms, with greater mutual infertility, and with increased adap-
tation to the environment, enabling them to prevail throughout
the whole area. The process here described, if it takes place,
is not modification by natural selection.

On the other hand, it is modification by physiological
selection. For, among the several other forms of isolation
which are called into requisition, the physiological (i.e.
ever accumulating cross- infertility) is supposed to play an
important part. That the modification is not modification
by natural selection may perhaps be rendered more
apparent by observing, that in as far as any other mode
of isolation is involved or supposed, so far is the possible
agency of natural selection eliminated as between the two
or more otherwise isolated sections of a species ; and yet it is
modes of isolation other than that furnished by natural selec-
tion (i.e. perishing of the less fit), that Mr. Wallace here
supposes to have been concerned — including, as I have
before shown, the physiological form, to which, indeed, he
really assigns most importance of all. Or, as Mr. Gulick
states the matter in his independent criticism: —

In the supposed case pictured by Mr. Wallace, the principle
by which the two segregating forms are kept from crossing,

Appendix A, 155

and so are eventually preserved as permanently distinct forms,
is no other than that which Mr. Romanes and myself have
discussed under the terms Physiological Selection and Segregate
Fecundity. Not only is Mr. Wallace's exposition of the diverg-
ence and the continuance of the same in accord with these
principles which he has elsewhere rejected, but his whole
exposition is at variance with his own principle, which, in the
previous chapter, he vigorously maintains in opposition to my
statement that many varieties and species of Sandwich Island
land molluscs have arisen, while exposed to the same environment,
in the isolated groves of the successive valleys of the same
mountain range. If he adhered to his own theory, " the greater
infertility between the two forms in one portion of the area"
would be attributed to a difference between i\\Q environment ^le,-
sented in that portion and that presented in the other portions ;
and the difficulty would be to consistently show how this
greater infertility could continue unabated when the varieties
thus characterized spread beyond the environment on which
the character depends. But, without power to continue, the
process which he describes would not take place. Therefore, in
order to solve the problem of the origin and increase of
infertility between species, he tacitly gives up his own theory,
and adopts not only the theory of Physiological Selection but
that of Intensive Segregation* through Isolation, though he
still insists on calling the process natural selection ; for on
page 183 he says, "No form of infertility or sterility between
the individuals of a species can be increased by natural selec-
tion unless correlated with some useful variation, while all
infertility not so correlated has a constant tendency to effect
its own elimination. " Even this claim he seems to unwittingly
abandon when on page 184 he says: "The moment it [a
species] becomes separated either by geographical or selective
isolation, or by diversity of station or of habits, then, while
each portion must be kept fertile inter se, there is nothing
to prevent infertility arising between the two separated
portions."

' By Intensive Segregation Mr. Guliek means what I have called Inde-
pendent Variability.

15b DarwiHy and after Darzmn,

The criticism proceeds to show yet further inconsistencies
and self-contradictions in Mr. Wallace's treatment of this
subject; but it now seems needless to continue. Nor,
indeed, should I have quoted this much but for the sake
of so fully justifying my own criticism by showing the
endorsement which it has received from a completely in-
dependent examination.

APPENDIX B.

An Examination by Mr. Fletcher Moulton of Mr.
Wallace's Calculation touching the Possibility of
Physiological Selection ever acting alone.

We have seen that the only important point of difference
between Mr. Wallace's more recent views and my own on
the problem of inter-specific sterility, has reference to the
question whether variations in the way of cross-infertility can
ever arise and act " alone, in an otherwise undifferentiated
species," or whether they can never so arise and act. It
is Mr. Wallace's opinion that, even if they ever do arise
alone, at all events they can never act in differentiating a
specific type, seeing that the chances against their suitable
mating must be so great : only if they be from the first
associated with some other form of homogamy, which will
have the effect of determining their suitable mating, does
he think that they can act in the way supposed by our
theory of *' selective fertility " \ On the other hand, as

' His sentence, "all fertility not correlated with some useful variation
has a constant tendency to effect its own elimination," still further
restricts the possible action of physiological selection to cases where at
least one of the other forms of homogamy with which it is associated is
natural selection. Or, in other words, it is represented that physiological
selection must always be associated with natural selection, even if it be
likewise associated with any other form of exclusive breeding. But as
this further limitation appears to me self-evidenily unjustifiable (seeing

158 Darwin, and after Darwin,

previously and frequently stated, I have so strong a belief
in the segregating power of physiological selection, or
selective fertility, that I do not think it is necessary for
this principle to be always associated with some other form
of homogamy. From the first, indeed, I have laid great
stress (as, also, has Mr. Gulick) on the re-enforcing influence
which association with any other form of homogamy must
exercise upon the physiological form, and vice versa; but
I have also said that, in my opinion, the physiological form
may in many cases be able to act entirely alone, or without
assistance derived from any other source. The question
here is, as we have already so fully seen, a question of but
secondary importance ; since, whether or not the physio-
logical form of homogamy ever acts alone, even Mr. Wallace-
now allows, or rather argues, that it acts in combination —
and this so habitually, as well as with so much effect, that it
constitutes a usual condition to the origination of species.
Nevertheless, although the only relevancy of his numerical
computation of chances— whereby he thinks that he over-
turns my theory in toto — is such relevancy as it bears to this
question of secondary importance, I have thought it desirable
to refer the question, together with Mr. Wallace's views upon
it, to the consideration of a trained mathematician.

As this "subordinate question" depends entirely on
numerical computations involving the doctrine of chances,
I should first of all like to remark, that in reference to
biological problems of the kind now before us, I do not
myself attach much importance to a merely mathematical
analysis. The conditions which such problems involve are
so varied and complex, that it is impossible to be sure about
the validity of the data upon which a mathematical analysis is

that utility is not the only possible menns of securing effective isolation)
I here neglect it, and take the wider ground marked out above. It is
needless to say that this is giving Mr. Wallace every possible advantage,
by not holding him to his still narrower ground.

Appendix B. 159

founded. Nevertheless, for the sake of meeting these
criticisms upon their own ground, I will endeavour to show
that, even as mathematical calculations, they are quite un-
trustworthy. And, in order to do this effectually, I will quote
the results of a much more competent, as well as a much more
thorough, inquiry. I applied to Mr. Moulton for this
purpose, not only because he is one of the ablest mathe-
maticians of my acquaintance ; but also because his interest
in biology, and his knowledge of Darwinian literature,
render him well fitted to appreciate exactly, and in all their
bearings, the questions which were submitted to his con-
sideration. I need only add that his examination was
completely independent, and in no way influenced by me.
Having previously read my paper on Physiological Selection,
Mr. Gulick's paper on Divergent Evolution, and Mr. Wallace's
book on Darwinism, he was in possession of all the materials ;
and I merely requested the favour of his opinion upon the
whole case from a mathematical point of view. The
following is his reply ; and I give it in extenso, because it
serves to place in another light some of the general considera-
tions which it has already been my endeavour to present*.

After some introductory remarks on Mr. Wallace's
"adoption of the theory of physiological selection pure
and simple," and '*the pure caricature of it which he
puts forward as" mine, the letter proceeds thus: —

The reason why it is so easy to attack your theory is that
it is so easy to confuse the survival of an individual with the

* In onr A'a/Kr? correspondence of 1 890-1 891, Mr. Wallace remarked :
" If Dr. Romanes will carefully work out numerically (as I have
attempted to do) a few cases showing the preservative and accumulative
agency of pure physiological selection within an otherwise undifferentiated
species, he will do more for his theory than volumes of general disquisi-
tion or any number of assertions that it does possess this power."
Several months before this was written I had already in my hands
Mr. Moulton's letter, with its accompanying calculations.

i6o Darwifiy and after Darwin,

survival of a peculiarity of type. No one has ever said that
an individual is assisted by the possession of selective fer-
tility : that is a matter which cannot affect his chance of life.
Nor has any one said that the possession of selective fertility
in an individual will of itself mcTea.se the chance of his having
progeny that will survive, and in turn become the progenitors
of others that will survive. Taken by itself, the fact that an
individual is capable of fertility with some only of the oppo-
site sex lessens the chance of his having progeny. Whether
or not he is more or less favourably situated than his con-
freres for the battle of life must be decided by the total sum
of his peculiarities ; and the question whether or not this
selective fertility will be a hindrance must be decided by
considerations depending on the other peculiarities associated
with it.

But when we come to consider the survival or permanence
of a type or peculiarity.^ the case is quite different. It then
becomes not only a favourable circumstance, but, in my opinion,
almost a necessary condition, that the peculiarity should be
associated with selective fertility ^

Take the case of the Jews. I don't think that intermarriage
with other nations would lessen their fertility, or diminish the
number of their progeny ; nor is there any reason to think that
this progeny would be unequal to the struggle for existence.
But no one doubts that the abandonment of their voluntary
isolation (which operates so far as this is concerned as a selec-
tive fertility), would lead to the disappearance of the familiar
Jewish type. All the world would get some of it ; but as a whole
it would be "swamped."

Now although no doubt Wallace would admit all this, he
fails to give it the weight it ought to have. In discussing the
question of its operation he considers too exclusively the case
of the individual.

Of course, a type can only be perpetuated through the medium
of individuals, and all that his argument amounts to is, that

^ As, for example, in the case of sexuality in general. It is not to
the advantage of such individual male Arthropoda as perish after the
performance of the sexual act that they should perform it; but its per-
formance is necessary for the perpetuation of their species. — G. J. R.

Appendix B, i6i

selective fertility would be so fatal to individuals that no type
which presents it could be formed or perpetuated— a conclu-
sion which is not only absurd in itself, but contradicted by
his own subsequent adoption of your theory. Besides, apart
from calculations (with which I will deal when I write next),
such reasoning brings its own refutation. Selective fertility is
not in the same category as some of the other influences to
which an important share has been ascribed in the formation
of the existing types. // exists as a recognized phenomenon.
Hence all these numerical proofs that it would lead to extinc-
tion, because it is so disadvantageous to the possessor, prove
too much. They would show that the degree of selective
fertility which so frequently characterizes species is a most
onerous gift ; and that, were it not present, there would be
a vastly increased chance of fertility, which would render the
races fitter and lead to their increased survival. Why then
has it not been got rid of?

The two answers which no doubt would be given seem to
me to support rather than to make against your theory. In
the first place, Wallace might say that this infertility is an
advantage because it keeps pure a type which is specially
fitted to its surroundings, as shown by its continued existence.
But if this be so, and it is necessary to protect the developed
type, how much more necessary to protect the incipient type !
In the second place, he might say that this selective fertility
is not so disadvantageous when the species has been formed,
because the individual can choose his mate from his like ;
whereas, when it is beginning to be formed, he must mate
blindly, or without what you call " psychological selection."
But this seems to me to be wholly inapplicable to at least half
the animal, and to all the vegetable kingdom. Moreover, with re-
gard to the other half of the animal kingdom, it merely raises the
question, — How soon will such an incipient type recognize itself?
Seeing it is probable that many families [broods] will belong to
the same [incipient] type, I should not be surprised if it were
found that this sexual recognition and preference sets in very
early.

But this leads me to the question of your letter. I under-
stand you to want me to examine and criticize the attempted

III. M

i62 Darwin^ and after Darwin,

numerical arguments against or for your theory. Now it seems
to me that it will be best to take, in the first instance, the
vegetable kingdom, and with regard to it I cannot see how
there can be any numerical argument against the theory. For
we often have species side by side with others nearly allied,
but much more numerous. The condition of these is precisely
analogous to that of your incipient species. They are exposed
to fertilization from, say, ten times as numerous individuals of
the allied species. They reject this in favour of that from the
relatively few individuals of their own. Yet the two species
are in competition. I could go through the numerical argu-
ments of your assailant word for word, applying them to
such a case as this, and they would triumphantly show that
the specific fertility of the rarer kind would lead to its certain
extinction. Yet we know that this is not so.

Indeed, the too triumphant character of the logic used against
you seems to me to be capable of being turned to your use.
If cross-infertility is so intensely disadvantageous to the indi-
viduals presenting it, it cannot have been that which made
these individuals and their progeny survive. It is therefore
a burden which they have carried. But we find that it is
more or less present in all the closely allied types that occur
on common areas : therefore it must be a necessary feature
in the formation of such types ; for it cannot be an accident
that it is present in so many. In other words, it must be
the price which the individual and his progeny pay for their
formation into a type. And this is your theory pure and
simple.

The more I consider the matter, the more I feel that it is
impossible to decide as to the sufificiency of selective fertility
to explain the formation of species, if we consider merely the
effect it would have on the number of individuals, as con-
trasted with what it would be if no such peculiarity had de-
veloped itself. Indeed, I may say that on pondering over
the matter I have come to the conclusion, that mere fertility
is probably a comparatively unimportant factor in the preser-
vation of the species, after a certain sufficient degree of fertility
is attained. I do not wish to be misunderstood. To a certain
point fertility is not only advantageous but necessary, in

Appendix B. 163

order to secure survival of the type; but I feel that little
reliance can be placed on calculations based on the numerical
co-efficient of fertility (i. e. the ratio of the number of offspring
to the number of parents) in determining the relative chance of
type-survival.

Take, for instance, the oak tree. It produces thousands of
acorns, almost the whole of which die without producing any
progeny. Have we any reason to believe that if the number
of acorns borne by oak trees were diminished, even so much
as to one-tenth, the race of oaks would perish ? It may
of course be said that, if all other things are equal, the pro-
babilities of survival must be increased by increased fertility
of this kind ; but I feel convinced that when numerical fertility
has attained to a high point in circumstances in which
actual increase of the race cannot take place to any substan-
tial extent, the numerical value of this fertility sinks down
into a factor of the second or third order of importance — that
is to say, into the position of a factor whose effects are only to
be considered when we have duly allowed for the full effects
of all the main factors. Until we have done that, we gain
little or nothing in the way of accuracy of conclusion by taking
into consideration the minor factors. It may be very well to
neglect the effect of the attraction of Jupiter in our early re-
searches on the motion or the Moon ; and our doing so will
not prevent the results being approximate and having consider-
able value, because we are retaining the two main factors that
establish the motion, viz. the effects of the Earth and the Sun.
But if we exclude the effect of one of these main factors, our
results would be worthless; and it would not be rendered sub-
stantially less so by the fact that we had taken Jupiter into
account in arriving at them.

You must not imagine, however, that I think it wholly profit-
less to see whether there would be any substantial effect on
numerical fertility were selective fertility to manifest itself. But
if we want to derive any assistance from calculation, it must
be by applying it with a good deal more precision and definite-
ness than anything that Wallace shows. And, in the first
place, it is useless to confuse the vegetable and animal kingdoms.
In the former you have union unaffected by choice ; in the latter,

M 2

164 Darwin^ and after Darwin,

so far at all events as the higher animals are concerned, you
have " psychological selection." In order to give you a speci-
men of what can safely be done by calculation if you take
a problem of sufficient definiteness, I have chosen the case of
a flowering plant in which a certain proportion of the race
have developed the peculiarity of being sterile with the re-
mainder, while retaining the normal fertility of the race in
unions among themselves. In order to give the greatest ad-
vantage to your critics, I have assumed that such flowers as
possess the peculiarity are not self-fertilizable ; for it is clear that
if we suppose that they are self-fertilizable, the fertility need
be very slightly affected.

As I have excluded self-fertilization, it is necessary, if we are
to get any trustworthy results, that one should consider the
mode in which fertilization will be produced. I have taken
the case of fertilization by insects, and have assumed that each
flower is visited a certain number of times by insects during
the period when fertilization is possible ; and, further, that the
insects which visit it have on the average visited a certain
number of flowers of the same species before they came there.
Of course nothing but observation can fix these latter numbers ;
but I should not be surprised at finding that they are of
considerable magnitude \ In order to make the results a little

' In this anticipation Mr. Moulton is right. The well-known botanist,
Mr. Bennett, read a most interesting paper on the subject before the
British Association in 1881. His results have since been corroborated
by other observers. In particular, Mr. R. M. Christy has recorded the
movements of 76 insects while visiting at least 2.400 flowers. {^Entomo-
logist, July 1883, and Zool. Journal Lin. Soc, August 1883.) The
following is an analysis of his results. In the case of butterflies, in
twelve observations on nearly as many species, there are recorded
altogether 99 visits to fifteen species of flowers ; and of these 99 visits 94
were constant to the same species, leaving only 5 visits to any other,
or second species. In the case of tht- hive-bee, there were 8 ii dividuals
observed : these visited altogether 258 flowers, and all the visits paid by
the same individual were i aid to the same species in each of the eight
cases. Lastly, as regards humble-bees, there were altogether obseived
55 individuals belon^mg to four species. These paid altogether 1751
visits to 94 species of flowers. Of these 1751 visits, 1605 were paid to
one species, 131 to two species, 16 to three, 6 to four, and i to five.

Appendix B. 165

more intelligible, I have grouped them under the numbers which
represent the average number of flowers that an insect visits
in a journey. This is a little more than twice as great as the
number which represents the number of flowers he has on the
average visited before coming to the individual whose fertility
we are considering.

I send you the formula and the calculation on which it is
based in an Appendix ; but as I know you have a holy horror
of algebraical formulae, I give you here a few numerical
results.

The cases I have worked out are those in which the number
of insects visiting each flower is 5, or 10, or 15 ; and I have
also taken 5, 10, and 15, to represent the number of flowers
which an insect visits each journey. This makes nine cases
in all ; and I have applied these to two instances — viz. one
in which one-fifth of the whole race have developed cross-
infertility, and the other in which one-tenth only have done so.
Taking first the instance where one-fifth have developed the
peculiarity, I find that if on the average five insects visit
a flower, and each insect on the average visits five flowers on
a journey, the fertility is diminished by about one-tenth. If,
however, the average number of flowers the insect visits is ten,
the reduction of fertility is less than one per cent. And it
becomes inappreciable if the average number is fifteen. If on
the average ten insects visit each flower, then, if each insect
visits on the average five flowers on a journey, the reduction
of fertility is a little over one per cent. ; but if it visits ten or
fifteen the reduction is inappreciable. If fifteen insects visit the
flower on an average, then, if these insects on the average visit

Adding all these results together, we find that 75 insects (butternies
and bees) visited 117 species of flowers: of these visits, 1957 were
constant to one species of flower; 136 were paid also to a second
species, 16 also to a third, 6 also to a fourth, and i also to a fifth. Or,
otherwise stated, while 1957 were absolutely constant, from such absolute
constancy there were only 159 deviations. Moreover, if we eliminate
three individual humble-bees, which paid nearly an equal number of visits
to two species (and, therefore, would have ministered to the work of
physiological selection almost as well as the others), the 159 deviations
become reduced to 72, or about four per cent, of the whole. — G. J. R.

i66 Darivin, and after Darwin,

five or more flowers on a journey, the reduction of fertility
is inappreciable.

By the term inappreciable I mean that it is not substan-
tially greater than one-tenth of one per cent.— i.e. not more
than one-thousandth.

Of course, if the proportion of individuals acquiring the
peculiarity is less, the effect on the fertility under the above
hypothesis will be greater; and it will not be counteracted so
fully unless the number of insect visits is larger, or unless the
insects visit more flowers on a journey. Thus if only one-tenth
of the race have developed the peculiarity, then, if each flower
is visited on the average by five insects who visit five flowers
on each trip, the fertility will be reduced about one-third.
If, however, the insects visit on the average ten flowers per
trip, it will be only diminished about one-tenth ; and if they
visit fifteen on each trip, it will be only diminished about
one-fortieth. If in the same case we suppose that each
flower receives ten insect visits, then, if the insects visit on an
average five flowers per trip, the fertility will be diminished
about one-eighth. If they visit ten on a trip, it will be dimi-
nished about one-hundredth, and the diminution is inappreciable
if they visit fifteen on a trip. Similarly, if a flower receives
fifteen insect visits, the diminution is about one-twenty-fifth,
if insects visit on the average five flowers on a trip ; and is
inappreciable if they visit ten or fifteen.

These figures will show you that it is exceedingly possible
that a peculiarity like this, the effect of which at first sight
would seem to be so prejudicial to fertility, may in fact have
little or no influence upon it ; and if you set against this the
overwhelming importance of such a peculiarity in segregating
the type so as to give it a chance of becoming a fixed species,
you will, I think, feel that your hypothesis has nothing to
fear from a numerical examination.

I have not examined the case of fertilization by other means ;
nor have I examined the case of fertilization in animals, where
psychological selection can come in. To obtain any useful
results, one would have to consider very carefully the circum-
stances of each case ; and at present, at all events, I do not
think it would be useful to do so. Nor have I attempted to

Appendix B. 167

show the converse of the problem— viz. the effect of swamping
where cross-fertilization is possible. I shall be very glad to
examine any one of these cases if you want me to do so ;
but I should prefer to leave it until I hear from you again.

If you contrast the results that I have given above with
those given on pages i8i to 183 of Wallace's book, you will
see the enormous difference. His calculations can only apply
to the animal kingdom in those cases in which there is only
a union between one individual of each sex ; and before you
can deal with the question of such animals, you will have to
take into consideration many elements besides that of merjB
fertility, if you wish to get any tolerably accurate result'.

The above analysis leaves nothing to be added by me.
But, in conclusion, I may once more repeat that the particular
point with which it is concerned is a point of very subor-
dinate importance. For even if Mr. Wallace's computation
of chances had been found by Mr. Moulton to have been an
adequate computation — and, therefore, even if it had been
thus proved that physiological homogamy must always be
associated with some other form of homogamy in order to
produce specific divergence — still the importance of selective
fertility as a factor of organic evolution would not have
been at all diminished. For such a result would merely
have shown that, not only " in many cases " (as I originally
said), but actually in all cases, the selective fertility which
I hold to have been so generally concerned in the differentia-
tion of species has required for this purpose the co-operation
of some among the numerous other forms of homogamy.
But inasmuch as, by hypothesis, no one of these other or
co-operating factors would of itself have been capable of
effecting specific divergence in any of the cases where its
association with selective fertility is concerned, the mathe-

* Here follows the Appendix presenting the calculations on which the
above results are founded ; but it seems unnecessary to reproduce it
on the present occasion. — G. J. R.

i68 Darwin^ and after Darwin,

matical proof that such an association is always — and not
merely often — necessary, would not have materially affected
the theory of the origin of species by means of physiological
selection. We have now seen, however, that a competent
mathematical treatment proves the exact opposite ; and, there-
fore, that Mr. Wallace's criticism fails even as regards the
very subordinate point in question.

APPENDIX C.

SOME EXTRACTS FROM THE AUTHOr's NOTE-BOOKS.

Bearing of Wetsmanm'sm on Physiological Selection. — If
in view of oiher considerations I could fully accept Professor
Weismann's theory of heredity, it would appear to me in no
small measure to strengthen my own theory of physiological
selection. For Weismann's theory supposes that all changes
of specific type must have their origin in variations of a
continuous germ-plasm. But the more the origin of species is
referred directly to variations arising in the sexual elements,
the greater is the play given to the principles of physiological
selection ; while, on the other hand, the less standing-ground
is furnished to the theory that cross-infertility between allied
species is due to " external conditions of life," " prolonged
exposure to uniform change of conditions," " structural
modifications re-acting on the sexual functions " ; or, in
short, that " somatogenetic " changes of any kind can of
themselves induce the ** blastogenetic " change of cross-
infertility between progeny of the same parental stock.

Cross-infertility and Diversity of Life. — Observe that one
great consequence of duly recognizing the importance of inter-
crossing is indefinitely to raise our estimate of the part played
by the principle of cross-infertility in diversifying organic
nature. For whenever in any line of descent the bar of

^ DoctHne of Descent and Darwinism, Eng. trans, p. 139.

170 Darwin, and after Darwin,

sterility arises, ih&re the condition is given for a new crop of
departures (species of a genus) ; and when genera are formed
by the occurrence of this bar, there natural selection and all
other equilibrating causes are supplied with new material for
carrying on adaptational changes in new directions. Thus,
owing to cross-infertility, all these causes are enabled to
work out numberless adaptations in many directions (i.e.
lines of descent) simultaneously.

Cross-infertility and Stability. — The importance of sterility
as a diagnostic feature is obvious if we consider that more
than any other feature it serves to give stability to the type ;
and unless a type is stable or constant, it cannot be ranked
as a species. That Darwin himself attributes the highest
importance to this feature as diagnostic, see Forms 0/ Flowers ,
pp. 58, 64.

Cross-infertility and Specific Differentiation. — In their
elaborate work on the many species of the genus Hieracium,
Nageli and Peter are led to the general conclusion that the
best defined species are always those which display absolute
sterility inter se; while the species which present most
difficulty to the systematist are always those which most
easily hybridize. Moreover, they find, as another general
rule applicable to the whole genus, that there is a constant
correlation between inability to hybridize and absence of
intermediate varieties, and, conversely, between ability to
hybridize and the presence of such varieties.

Cr OSS-infer tilily in Domesticated Cattle. — Mr. J. W. Crompton,
who has had a large experience as a professional cattle-
breeder, writes to me (March 2, 1887) —

"That form of barrenness, very common in some districts,
which makes heifers become what are called 'bullers'- that
is, irregularly in 'season,' wild, and failing to conceive- is
certainly produced by excess of iron in their drinking-water,
and I suspect also by a deficiency of potash in the soil."

Appendix C 171

He also informs me that pure white beasts of either sex
are so well known by experienced breeders to be comparaiively
infertile together, that they are never used for breeding
purposes, so that "in some parts of the country, where a
tendency to sterility had become so confirmed in the white
race that they utterly died out," only the coloured breeds are
now to be found. He goes on to say that if " a lot of white
heifers were put to a lot of white bulls, I think you would
probably get a fertile breed of pure white cattle. ... I think,
in short, that domestication has produced just what your
theory suggests, a new variety inclined to prove sterile with
its parent stock."

Commenting on the origin of domesticated cattle. Professor
Oscar Schmidt remarks {Doctrine of Descent, p. 139) —

" Riitimeyer's minute researches on domestic cattle have shown
that, in Europe at least, three well-defined species of the diluvial
period have contributed to their formation — Bos pritnigenius^
tongifrons, and frontosus. These species once lived geogra-
phically separate, but contemporaneously ; and they and their
specific peculiarites have perished, to rise again in our domestic
races. These races breed together with unqualified fertility.
In the form of skull and horns they recall one or other of the
extinct species; but collectively they constitute a new main species.
That from their various breeds, the three or any one of the
aboriginal species would ever emerge in a state of pristine
purity, would be an utterly ludicrous assertion."

Now, seeing that these " aboriginal species," although living
"contemporaneously," were "geographically separate," we
can well understand that their divergence of type from a
common ancestor did not require, as a condition to their
divergence, that any cross-sterility should have arisen between
them. The geographical isolation was enough to secure
immunity from mutual intercrossing, and therefore, as our
present theory would have expected as probable, morpho-
logical divergence occurred without any corresponding physio

172 DarwiUy mid after Darwin.

logical divergence, as must almost certainly have been the
case if such polytypic evolution had occurred on a common
area. Indeed, one of the two lines of experimental verifica-
tion of our theory consists in selecting cases where nearly
allied species are separated by geographical barriers, and
proving that, in such cases, there is no cross-sterility.

Fertility of Domesticated Varieties. — Some writers have
sought to explain the contrast between domesticated varieties
and natural species in respect of fertility when crossed, by
the consideration that it is only those natural species which
have proved themselves so far flexible as to continue fertile
under changed conditions of life that can have ever allowed
themselves to become domesticated. But although this
condition may well serve to explain the unimpaired fertility
under domestication of such species as for this very reason have
ever become domesticated, I fail to see how it explains the
further and altogether different fact, that this fertility continues
unimpaired between all the newly differentiated morphological
types which have been derived from the original specific type.
It is one thing that this type should continue fertile after
domestication : it is quite another thing that fertility should
continue as between all its modified descendants, even
although the amount of modification may extend much
further than that which usually obtains between different
natural species.

Testing for Cross-infertility among varieties growing on
the same area is a much more crucial line of verification than
testing for unimpaired fertility between allied species which
occupy different areas, because while in the former case we
are dealing with " incipient species" with a view to ascertain-
ing whether the divergence which they have already undergone
is accompanied by physiological isolation, in the latter case
we can never be sure that two allied species, which are now
widely disconnected geographically, have always been so

Appendix C, 173

disconnected. They may both have originated on the same
area; or one may have diverged from the other before it
migrated from that area ; or even if, when it migrated, it was
unchanged, and if in its new home it afterwards split into two
species by physiological selection, the newer species would
probably prove infertile, not only with its parent type, but
also with its grand-parent in any other part of the world.

Seebohm on Isolation. — Seebohm is so strongly influenced
by the difficulty from " the swamping effects of free intercross-
ing," that he is driven by it to adopt Asa Gray's hypothesis
of variations as teleological. Indeed, he goes as far as
Wagner, for he maintains that in no case can there be
divergence or multiplication of species without isolation.
He makes the important statement that "the more the
geographical distribution of birds is studied, the more doubtful
it seems to be that any species of bird has ever been differen-
tiated without the aid of geographical isolation" {Charadriidae^
p. 17). If this is true, it makes in favour of physiological
selection by showing the paramount importance of the
swamping effects of intercrossing, and consequent impor-
tance of isolation. But it makes against physiological
selection by showing that the geographical form of isolation
is sufficient to explain all the cases of specific differentiation
in birds. But I must remember that the latter point rests
largely on negative inference, and that birds, owing to
their highly locomotive habits, are the class of animals where
ph} siological selection is likely to be most handicapped.

Herbert on Hybridization. — Herbert tells us that when he
first astonished the Horticultural Society by laying before them
the results of his experiments on hybridization, his brother
botanists took serious alarm. For it appeared to them ihat
this "intermixture of species would confuse the labours
of botanists, and force them to work their way through
a wilderness of uncertainty." Therefore he was bluntly told

174 DarwiUy and after Darwin,

by several of these gentlemen, " I do not thank you for your
mules." Now, although naturalists have travelled far and
learnt much since those days, it appears to me that a modern
evolutionist might still turn to the horticulturist with the same
words. For assuredly he has no reason to thank the
horticulturist for his mules, until he has found a satisfactory
answer to the question why it is that natural species differ so
profoundly as regards their capacity for hybridizing.

Advance on Herberts Position. — If it be said that all my
work amounts to showing what Herbert said long ago — viz.
that the only true or natural distinction between organic types
is the sexual distinction — I answer that my work does much
more than this. For it shows that the principle of sterility
is the main condition to the differentiation, not merely of
species and genera, but also to the evolution of adaptations
everywhere, in higher as well as in lower taxonomic divisions.
Moreover, even though naturalists were everywhere to consent
to abandon specific designations, and, as Herbert advises, to
"entrench themselves behind genera," there would still re-
main the facts of what are now called specific differences (of
the secondary or morphological kind), and by whatever name
these are called, they alike demand explanation at the hands
of the evolutionist.

Fritz Muller on Cross-infertility. — Fritz Miiller writes,
" Every plant requires, for the production of the strongest
possible and most prolific progeny, a certain amount of
difference between male and female elements which unite.
Fertility is diminished as well when this degree is too low
(in relatives too closely allied) as when it is too high (in
those too little related)." Then he adds, as a general rule,
" Species which are wholly sterile with pollen of the same
stock, and even with pollen of nearly allied stocks, will
generally be fertilized very readily by the pollen of another
species. The self-sterile species of the genus Abutilon,

Appendix C. 175

which are, on the other hand, so much inclined to hybri-
dization, afTord a good example of this theory, which appears
to be confirmed also by Lobelia, Passiflora, and Oncidium "
(American Naiuralisij vol. viii, pp. 223-4, 1874).

Different groups of plants exhibit remarkable differences in
the capability of their constituent species to hybridize. — In so
far as these differences have reference only to first crosses,
they have no bearing either for or against my theory. Only
in so far as the differences extend to the production of fertile
hybrids does any question arise for me. First of all, therefore,
I must ascertain whether (or how far) there is any correlation
between groups whose species manifest aptitude to form first
crosses, and groups where first crosses manifest aptitude
to produce fertile hybrids. Next, whatever the result of this
inquiry should be, if I find that certain natural groups of
plants exhibit comparatively well-marked tendencies to form
fertile hybrids, the question will arise. Are these tendencies
correlated with paucity of species? If they are, the fact
would make strongly in favour of physiological selection.
For the fact would mean that in these natural groups, owing
to " the nature of the organisms " included under them, less
opportunity is given to physiological selection in its work of
differentiating specific types than is given by other natural
groups where the nature of the organism renders them more
prone to mutual sterility. But in prosecuting this branch
of verification, I must remember to allow for possibilities of
differential degrees of geographical isolation in the different
groups compared.

On this subject Focke writes me as follows: — "In a
natural group (family, order, genus) showing considerable
variability in the structure of the flower, we may expect
to find [or do find] a greater number of mules than in
a group whose species are only distinguished by differ-
ences m the shape of the leaves, or in growth, &c. I do not

176 DarwiUy and after Darwin,

know, however, which in this connexion of things is the
cause and which the effect. A useful ancestral structure of
the flower may be conserved by an otherwise varying pro-
geny, on condition that the progress of diversity be not
disturbed by frequent intercrossings. [Therefore, if this
condition be satisfied, the structure of the flower in different
members of the group will continue constant : here the cause
of constancy in 'the flower (however much variability there
may be in the leaves, &c.) is its original inability to hybri-
dize.] On the other hand, in species or groups ready to
hybridize [or capable of hybridizing], the fixation of a new
specific type will require some change in the structure of the
flower, and a change considerable enough to alter the con-
ditions of fertilization. [Here the reason of the zwconstancy
of the flower in different members of the group is the
original aptitude of their ancestral forms to hybridize.]
Perhaps there is something in this suggestion, but certainly
there are other efficient physiological relations, which are
at present unknown. Your theory of physiological selection
may serve to explain many difficult facts."

The Importance of Prepotency. — A. Kerner shows by means
of his own observations on sundry species of plants which
hybridize in the wild state, that they do so very much more
frequently if both, or even if only one of the parent forms be
rare in the neighbourhood. This fact can only be explained
by supposing that, even in species most prone to hybridizing
under Nature, there is some degree of prepotency of pollen
of the same species over that of the other species ; so that
where both species are common, it is correspondingly rare
that the foreign pollen gets a chance. But if there were no
prepotency, the two species would blend ; and this Kerner
supposes must actually take place wherever two previously
separated species, thus physiologically circumstanced, happen
to be brought together. (Kerner's paper is published in

Appendix C 177

Oester. Bot. Zeitschrift, XXI, 1871, where he alludes to
sundry other papers of his own advocating similar views.)

The relation of these observations to Jordan's especes affines
is obvious. We have only to suppose that some such slight
and constant difference characterizes the sexual elements of
these allied varieties as demonstrably characterizes their
morphology, and we can understand how pollen-prepotency
would keep the forms distinct — such forms, therefore, being
so many records of such prepotency.

Boih from Kerner's work, and still more from that of
Jordan and Niigeli, I conclude that (at all events in plants)
prepotency is the way in ^ which physiological selection
chiefly acts. That is to say, sudden and extreme variations in
the way of sexual incompatibility are probably rare, as com-
pared with some degree of prepotency. According as this
degree is small or great so will be the amount of the
corresponding separation. This view would show that in
plants the principle of physiological selection is one of
immensely widespread influence, causing (on the same
areas) more or less permanent varieties much below specific
rank. And when we remember on how delicate a balance
of physiological conditions complete correspondency of pollen
to ovules depends, we may be prepared to expect that the
phenomenon of prepotency is not of uncommon occurrence.

Self-fertilization and Variability. — It occurred to Count
Berg Sagniiz that, if physiological selection is a true
principle in nature, vegetable species in which self-
fertilization obtains ought to be more rich in constant
varieties than are species in which cross-fertilization rules.
For, although even in the latter case physiological isolation
may occasionally arise, it cannot be of such habitual or
constant occurrence as it must be in the former case.
Acting on this idea, Count Berg Sagnitz applied himself to
ascertain whether there is any general correlation between the

III. N

178 Darwin^ and after Darwin,

habit of self-fertilization and the fact of high variability ; and
he says that in all the cases which he has hitherto investi-
gated, the correlation in question is unmistakable.

Additional Hypothesis concerning Physiological Selection, —
In reciprocal crosses ^ x -5 is often more fertile than
B >. A. If hybrid AB is more fertile with A, and hybrid
BA with B, than vice versa, there would be given a good
analogy on which to found the following hypothesis.

Let A and B be two intergenerating groups in which
segregate fecundity is first beginning. Of the hybrids, AB
will be more fertile with A, and BA with B^ than vice versa.
The interbreeding o( AB with A will eventually modify
sexual characters of A by assimilating it to those of AB^
while the interbreeding of BA with B will similarly modify
sexual characters of B by assimilating it to those of BA.
Consequently, A will become more and more infertile with
B, while B becomes more and more infertile with A. Fewer
and fewer hybrids will thus be produced till mutual sterility
is complete.

To sustain this hypothesis it would be needful to prove
experimentally, (i) that hybrid forms AB are more fertile
with A than with B, while hybrid forms BA are more fertile
with B than with A [or, it may be possible that the opposite
relations would be found to obtain, viz. that AB would be
more fertile with B, and BA with A]; (2) that, if so,
effect of intercrossing AB with A is to make progeny more
fertile with A than with B, while effect of intercrossing BA
wiih B is to make progeny more fertile with B than with A.

Such experiments had best be tried with species where
there is already known to be a difference of fertility between
reciprocal crosses (e.g. Matlhiola annua and M. glabra, see
Origin 0/ Species, p. 244).

INDEX

A.

Allen, Mr. J. A., on variation

under nature, 34.
Amixia, 12-28, 1 10-115, 117-

133-
Apogamy, 5, 6, 10, 18, 28.

B.
Belt, on physiological selection,

44.

Berg Sagnitz, Count, on self-
fertilization and variability, 177.

Breeding, separate and segregate,

6-

Biiltei flies of polar regions and
Alps, 133.

C.

Catchpool, Mr. , on physiological

selection, 44, 137.
Cross-infertility, 46; and varietal

divergence, 82 ; and diversity of

life, 169; and stability, 170;

and specific diflferentiation, 1 70 ;

in domesticated cattle, 170;

testing for, 172; Fritz M liller on,

174.

D.

Darwin, Charles, on isolation,
a, 106; on diversity under
nature, 31 ; on the fertility of
varieties, 50 ; on the origin ot
cross-infertility, 51; on distri-
bution, 68 ; on prepotency, 89 ;
on geographical isolation, 101,
108 ; on methodical selection,

102 ; on modification in large
an ns, 103 ; on the swamping
effects of intercrossing, 105; on
independent variability, 109 ; on
domestic animals, 110.

Delbceuf, Iscw of independent
variability, 13.

Differentiation under natural se-
lection, 37.

Diversity of life and cross- infer-
tility, 169.

Domesticated cattle and cross-
infertility, 170, 172.

Evidences of physiological selec-
tion 62.

Evolution, monotypic and poly-
typic, 21, 75, 102, 107, 112,
129.

Experimental research in physio-
logical selection, 85.

F.

Fertility of domesticated varieties,

172.
FocKE, Herr, on hybridization,

'"•

Galton, Mr. Francis, law of

regression, 39.
General conclusions, 144.
Geographical distribution and

physiological selection, 65.
GiARi), M., on apogamy, 14.
Grabham, Dr., on mollusca of

Madeira, 135.

N 2

i8o

Index.

Gui.TCK. Rev. J., on natural
selection as a mode of isolation,
9; on diveigence, ii ; on segre-
gate breeding. 19; on geogra-
phical distriliution, 27; on the
prevention of intercrossing, 127;
on Mr. Wallace's criticisms, 151.

H.

Herbert, on hybridization, 173;

advance on his position , 1 74.
Herdman, Prof., on physiological

isolation, 123.
Historical sketch of opinions on

isolation, 101.
Homogamy, 5, 6 ; forms of, 7, 19,

29.
Hybridization, Herbert on, 1 73 ;

in j)lants, 175.
Hypothesis, additional, concerning

physiological selection, 178.

Independent variability, 12-29.

Isolation, defined, 2 ; forms of.
3> 6; geographical, 3; discri-
minate and indiscriminate, 5;
physiological, 9, ^1, <jo ; us
importance, 39 ; sKetcti ol
opinions on, 10 1 ; general con-
clusions, 144; Seebohm on, 173.

Jordan, M., on cross sterile
varitties of plants, 86; his re-
searches summarized, 87.

K.

Kerner, Prof. A., on prepotency,
176.

L.

Lankester, Prof. Ray, on di-
vergent evolution, 15.

Le Conte, Prof., on fossil snails
of Steinheim, 95 ; on isolation,
129.

Livingstone, Dr. David, quoted,
123.

Meldola, Prof., on difficulty
from intercrossing, 121.

Misunderstandings of physio-
logical selection, 59.

Monotypic evolution, see Evolu-
tion.

Morgan, Prof. Lloyd, on steri-
lity, 56; on isolation, 128.

MouLTON, Mr. Fletcher, an
examination of Mr. Wallace's
calculations on physiological
selection, 157.

M iJLLER, Fritz,on cross-infertility,
174.

N.

Nageli, on isolation, 76; on
synoicy, 78, 82.

Natural selection, a form 'of dis-
criminate isolation, 9, 10, 23;
leads to monotypic evolution,
24-29; difficulties of, 41, 51.

P.

Panmixia, 12.

Physiological selection, 9, 41 ;
summarized , 58 ; misunderstand-
ings of, 59; evidences of, 81-
119; and Weismannism, 169;
additional hypothesis, 1 78.

Polytypic evolution, see Evolution.

Prepotency, 89 ; importance of,
176.

Schmidt, Prof. Oscar, on do-
mesticated cattle, 171.

Seebohm on isolation, 173.

Segregation, 28.

Selection, physiological, see Physi-
ological selection.

Self fertilization and variability,
177.

Snails of Sandwich Islands, 16,
130; fossil of Steinheim, 95.

Specific differentiation and cross-
infertility, 170.

Stability and cross-infertility, 170.

Synoicy, 78.

Index.

i8i

Topographical distribution and
physiological selection, 74 ; of
varieties, 81.

Transformation, serial and di-
vergent, 21, 121.

Variability and self-fertilization,
177.

Variation in birds, 34.

Varieties, topographical distribu-
tion of, 81.

W.

Wagner, Maritz, 3 ; on geo-
graphical isolation, 76; quoted,
1 03 ; law of migration, ill.

Wallace, Mr. A. R., 3, 17;
quoted, 34, 47, 51, 57, 130-136;
criticized by Gulick, 152.

Weismann, Prof., on geographical
isolation, 76, 114-118.

Weismannism and physiologic* 1
selection, 169.

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