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that the general difficulty from intercrossing previously con-
sidered is here presented in a special and aggravated form ?
At aU events, I know that, after having duly and impartially
considered the matter, to me it does appear that unless the
swamping effects of intercrossing 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 perfectly sound where it treats of the
divergence of species^ and of their further divergence vaX.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 selection alone could produce
any divergence of specific character, so long as all the in-
dividuals on an overcrowded area occupy that area t(^ther.
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

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opinions on Isolation. 127

that are needed by natural selection in order to produce diver-
gence. Therefore, it appears to me that, nnder 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 diversityy multiplication^ 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. Gulick, 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 ? 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 ?

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128 Darwin, and after Darwin.

If the advantage of freedom from competition in any given
variation depends on the 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. Gulick'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 cimning 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 difterentiation 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 new factor is introduced, that

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opinions on Isolation. 129

of isolation or segregation. And there is no questioning the
foct 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 implicitly assumed '.

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

It is evident, then, as Romanes claims, that natural selection
alone tends to monotypic 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

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
apc^amy 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

' Animai Life and InteUigitHce, pp. 98, 99 (i890-x89X).
* 7%$ Fmtton ofEvoMim (1891).

ni. K

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130 Darwtn, 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 slightest 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

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

* Darwinism, p. 151. ^ Ibid.

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opinions 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 difTerences 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, then 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 %

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132 DarwtHy and after Darwin.

change. But although, for the reason above stated,
they arc 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

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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 ^." 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

^ Z^. ^'/., p. 151.

^ Namely, Lycoina d^miHi, JU pUniit^ Argynmis pales, Erekia

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134 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 dQ[ree 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 spedes, the
probability is that in a given time their descendants
would not have diverged from the parent tyyt 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 affirm that apogamy must in all
cases produce diveigence, we are amply justified
in afiirming 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 therefoie

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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, '' AU seem to vary immediately on attaining new ground, assuming
many aspects in different districts.**

Unquestionably these statements 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 investi^ted for any decisive judgement to be formed
upon them.

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136 Darwin, and after Darwin.

change to be, ^ as a general rule," the (Mimordial
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. lay.

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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

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138 Darwin, and after Darwin.

elaborately calculated out the high ratio in which the di£kr-
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 xliiL p. 127.)

I think it is to be r^retted 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 differentiation, 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 undifTerentiated 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 invertebrata),
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 Nahtr*
correspondence is the supposition on which his own theory of the origin
of spedes by oross-infertUity is founded. But in the original statement
of that theory itself, it is everywhere "supposed" that when species are

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opinions an 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 Monist 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 th$ Linnaean Society
(vol. XIX. pp. 337-411).

Having arrived at these results with r^ard to the
theory of Isolation in general and of Physiological
Isolation in particular, I arrive also at the end of this

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