Chapter IX: Appendix: A

MR. GULICK'S CRITICISM OF MR. WALLACE'S VIEWS ON PHYSIOLOGICAL SELECTION.

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 principles
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 principles, 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 Accumulation 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.

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

Mr. Wallace's criticism of the theory of Physiological Selection is
unsatisfactory; (l) because he has accepted the fundamental
principle of that theory on pages 173-9, in 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,
ignoring 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 Physiological 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_ segregation, it will
disappear[60]; 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.

[60] "Positive segregation" is Mr. Gulick's term for forms of
homogamy 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 associated.

The paper likewise indicates a point which, in studying Mr. Wallace's theory, I have missed. It will be remembered 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 selection 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 selection coming into play at all. So that if, under these particular 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 accumulation 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 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
pursuers, 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 adaptation 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 selection (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, 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 divergence 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 the _environment_
presented 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[61] 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 selection 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."

[61] By Intensive Segregation Mr. Gulick means what I have called
Independent Variability.

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 independent examination.