A BRIEF HISTORY OF OPINIONS ON ISOLATION AS A FACTOR OF ORGANIC
EVOLUTION.
This historical sketch must begin with a consideration 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 considerable 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 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)[27].
[27] _Life and Letters_, vol. ii. p. 28.
And again:--
With respect to original creation or production of new forms, I
have said that isolation appears the chief element[28].
[28] _Ibid._
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 monotypic 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 unconscious. 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
intercross, his work will completely fail. But when many men,
without intending to alter the breed, have a nearly common
standard of perfection, and all try to procure and breed from the
best animals, improvement surely but slowly follows from this
unconscious process of selection, notwithstanding that there is no
separation of selected individuals. Thus it will be under
nature[29].
[29] _Origin of Species_, p. 80, 6th ed. (1872).
Here we have what may perhaps be regarded as a glimmering 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 evolution), 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, 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[30].
[30] _Life and Letters_, vol. iii. p. 158.
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[31].
[31] _Ibid._ p. 159.
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[32].
[32] _Ibid._ p. 160.
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 find it adduced
to illustrate the process of 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[33].
[33] 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).
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
consequently 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 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][34].
[34] _Life and Letters_, vol. iii. pp. 157-8.
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[35].
[35] _Ibid._ pp. 157-8.
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, _sine qua non_,
to divergent evolution in all cases. Nevertheless, he adds 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 distinguish between the "two cases" of monotypic
and polytypic evolution, it is evident that he never worked out any of
these matters--"thinking it prudent," as he wrote with reference to them
in 1878, "now I am growing old, to work at easier subjects[36]."
Therefore he never clearly saw, on the one hand, that free
intercrossing, far from constituting a "difficulty" to _monotypic_
evolution by natural selection, 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
"divergence of character" (i. e. polytypic evolution[37]), 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.
unconscious 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 of _geographical_
separation, which he rightly enough concludes (as against Wagner) need
certainly 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:--
[36] _Life and Letters_, vol. iii. p. 161.
[37] Page 81. 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."
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[38].
[38] _Life and Letters_, vol. iii. p. 159.
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 Delboeuf.
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[39].
[39] _Life and Letters_, vol. iii. p. 155.
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 intercrossing,
but would have crossed freely within each district or park[40].
[40] _Variation_, &c., vol. ii. p. 262.
Now, although I allow that Darwin never attributed 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. Nevertheless, 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 isolated
form should almost always become slightly modified[41]." I think this
shows entire forgetfulness 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 must 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-accelerating pace, 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 1878--owing, no doubt, as he says
in the sentence which immediately follows, to his having "not attended
much of late years to such questions."
[41] _Life and Letters_, vol. iii. p. 161.
* * * * *
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[42]." The merit of these essays was, first, the firm expression
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
geographical 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.
[42] _Die Darwin'sche Theorie und das Migrationsgesetz_ (1868):
_Ueber den Einfluss der geographischen Isolirung_, &c. (1870).
Over and over again Moritz Wagner insists, as constituting 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[43]. 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 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 separation 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.
[43] 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."
Nevertheless, while we must lament these shortcomings on the part of
Wagner, we ought to remember 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 in an
elaborate criticism of his views Weismann was easily able to dispose of
his generalizations in the imperfect 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 materially 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[44], 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 principal defects, in my judgement, were, first,
the inordinately 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 _geographical_ isolation, but even in regard to this one kind of
isolation it was presented in a highly inconsistent manner, as I will
now endeavour to show.
[44] _Ueber den Einfluss der Isolirung auf die Artbildung_ (1872).
Weismann was led to perceive the principle in question by the
consideration that new specific characters, 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 successive 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[45]." 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 intercrossing is prevented by the
geographical isolation of a portion of the species from its parent
stock.
[45] _Loc. cit._, p. 43.
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
Delboeuf and Gulick--namely, the average characters 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 "Independent
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 intercrossing," 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[46]."
[46] _Physiological Selection_, pp. 348, 389.
Not only, however, is Weismann's principle of "Amixia" 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[47], 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 of _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 Weismann'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 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 variability 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 variation 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 variations, before the species was divided. Thus
the doctrine of amixia fails to recognize the law of 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.
[47] _Loc. cit._, p. 54.
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 effect specific divergence through independent
generation alone; fourth, that, nevertheless, 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 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 monotypic evolution--namely, through the free intercrossing of
all individuals which 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. Consequently, if it should ever be of advantage to a
species 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 sufficient 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 presenting 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 _multiplication_ of species is concerned. In justice
to him, however, it may be added, that this distinction between
evolution as monotypic and polytypic (with the important consequence
just mentioned) still continues to be ignored also by other well-known
evolutionists 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[48]."
[48] _Nature_, vol. xliii. p. 410, and vol. xliv. p. 29.
It is in relation to my presentment of the impossibility of natural
selection alone causing polytypic 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 physiological
selection[49].
[49] _Darwinism_, p. 143.
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 somewhat similar
misunderstanding which has been more recently published by Professor W.
A. Herdman, of Liverpool; and as the case which he gives is one of
considerable interest in itself, I will quote his remarks in extenso. In
his _Opening Address to the Liverpool Biological Society_, Professor
Herdman 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, I 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 intermarry, 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
favourable 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[50] written by the
celebrated African traveller Dr. David Livingstone to Lord
Granville, and dated "Unyanyembe, July 1st, 1872," the following
passage:--
[50] In Appendix to H. M. Stanley's _How I found Livingstone_, 2nd
ed. London, 1872, p. 715.
"About five generations ago, a white man came to the highlands of
Basañgo, which are in a line east of the watershed. He had six
attendants, who all died, and eventually their headman, called
Charura, was elected chief by the Basañgo. 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 appearing to be under the impression that he is
controverting 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 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
evolution is taking place on a common area.
* * * * *
Seeing that so much misunderstanding has been shown with reference to my
views on "the swamping 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_? 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, 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 feel that the general difficulty from
intercrossing previously considered 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 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 into _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 individuals 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 that are needed by natural
selection in order to produce divergence. Therefore, it appears to
me that, under the circumstances 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 transmutation of species in a linear series, each succeeding
member of which might supplant its parent form. But in order to
secure _diversity_, _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 fundamental 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, increasing divergence of form will be
prevented?
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 Independent 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 Natural
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 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, intercrossing and interbreeding must in
some way have been lessened or prevented. Thus a new factor is
introduced, that 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
implicitly assumed[51].
[51] _Animal Life and Intelligence_, pp. 98, 99 (1890-1891).
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[52].
[52] _The Factors of Evolution_ (1891).
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 understand him, even so much as assisted--by this
principle 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 isolation is not found to be
associated with divergence 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[53]." But the only case which he gives is that of
Ireland.
[53] _Darwinism_, p. 151.
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[54]." Here, however, Mr. Wallace shows
that he has failed to understand "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 contained 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.
[54] _Ibid._
No better test case of this kind has ever been furnished than that of
Mr. Gulick's land-shells, 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 explanation 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 completely
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 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
successive 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 confirmation which it now derives from them.
Thus, in truth, though in an opposite way, the case of Ireland is, as
Mr. Wallace says, "an excellent 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 corroborate 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 not occurred through natural selection, is perhaps due to the less
severe struggle for existence, owing to the smaller number of competing
species[55]." 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.
[55] _Loc. cit._, p. 151.
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[56], 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 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
"inflexible" type as compared with most other domesticated birds.
Therefore, if a lot of geese were to be indiscriminately 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 principle 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 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 should be accepted
as one of the factors of organic evolution[57].
[56] Namely, _Lycaena denzelii_, _L. pheretes_, _Argynnis pales_,
_Erebia mante_.
[57] 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 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 investigated for any
decisive judgement to be formed upon them.
My view from the very first has been that variations in the way of
cross-infertility are of frequent occurrence (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 different thing, to _coincide_ with
the physiological change But in the case of plants and the lower
invertebrata. I see no reason for any frequent concomitance of this
kind; and therefore believe the physiological 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
morphological 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 essential 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 supposing 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 correspondence in _Nature_[58],
and to them Mr. Wallace replied by saying, "if this is not an absolute
change of front, 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.
[58] Vol. xliii. p. 127.
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 selection, and this fact I am very glad to have
"acknowledged." 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 misunderstand _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 physiological
variations may be associated; but Mr. Gulick has gone into this
side of our common theory much more fully, and elaborately
calculated out the high ratio in which the differentiating 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. xliii. 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 calculations 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
selection to consist in selective fertility furnishing a needful
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
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 invertebrata), I have as uniformly
stated "that it makes no difference 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[59]. 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 the Linnaean Society_ (vol. xix. pp. 337-411).
[59] 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 of species by cross-infertility is founded. But in the
original statement of that theory itself, it is everywhere
"supposed" that when species are originated by cross-infertility,
the _initial_ change _is_ the physiological change. In his original
statement of that theory, therefore, he literally 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.
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 view of the whole history of Darwinism, what must strike us
as the really significant fact is the astonishing unanimity which has
been so rapidly attained with regard to matters of such immeasurable
importance. 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 unequivocally 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 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 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 Darwinian.
This primary difference of opinion leads deductively to certain
secondary differences. For if a man starts with the premiss that natural
selection must necessarily 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 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 I 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 ADAPTATIONS WHEREVER THESE OCCUR; AND THEREFORE IS ONLY INCIDENTALLY,
OR LIKEWISE, A THEORY OF THE ORIGIN OF SPECIES 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 SELECTION IS COMPETENT TO PRODUCE SUCH SLIGHT DEGREES OF
MODIFICATION AS GO TO CONSTITUTE 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 THAN NATURAL
SELECTION.
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 ADAPTATIONS; FOR, EVEN
UPON THIS ERRONEOUS SUPPOSITION, 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--_I.E._ OF THE MOST
GENERAL OF ALL "SPECIFIC CHARACTERS."
6. WITHOUT ISOLATION, OR THE PREVENTION OF FREE INTERCROSSING, ORGANIC
EVOLUTION IS IN NO CASE POSSIBLE. THEREFORE, IT IS ISOLATION THAT _HAS_
BEEN "THE EXCLUSIVE MEANS OF MODIFICATION," OR, MORE CORRECTLY, THE
UNIVERSAL 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.
7. ISOLATION MAY BE EITHER DISCRIMINATE OR INDISCRIMINATE. WHEN
DISCRIMINATE, IT HAS REFERENCE TO RESEMBLANCES BETWEEN INDIVIDUALS
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 DISCRIMINATE; 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:--
(_A_) THE ISOLATION IS WITH REFERENCE TO SUPERIORITY OF FITNESS; (_B_)
IS EFFECTED BY DEATH OF THE EXCLUDED INDIVIDUALS; AND (_C_) UNLESS
ASSISTED BY SOME OTHER FORM OF ISOLATION, CAN ONLY EFFECT MONOTYPIC AS
DISTINGUISHED FROM POLYTYPIC EVOLUTION.
10. 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 NUMBER OF CASES OF EFFICIENT ISOLATION; BUT, EXCEPTING
NATURAL SELECTION, ANY ONE FORM OF ISOLATION NEED NOT NECESSARILY
REQUIRE THE CO-OPERATION 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 EVOLUTION 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, I
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.
_APPENDICES_
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.
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"[62]. On the other hand, as 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 physiological 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 overturns 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.
[62] 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-evidently unjustifiable (seeing that
utility is not the only possible means 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.
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 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 untrustworthy. 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 mathematicians 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 consideration. 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 considerations
which it has already been my endeavour to present[63].
[63] In our _Nature_ correspondence of 1890-1891, 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 disquisition 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.
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 survival
of a _peculiarity_ of _type_. No one has ever said that an
_individual_ is _assisted_ by the possession of selective
fertility: 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_ increase 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
opposite sex lessens the chance of his having progeny. Whether or
not he is more or less favourably situated than his _confreres_ 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[64].
[64] 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 performance is necessary for the perpetuation of their
species.--G. J. R.
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 selective 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
selective fertility would be so fatal to individuals that _no_ type
which presents it could be formed or perpetuated--a conclusion
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. _It exists as a recognized phenomenon._ Hence all these
numerical proofs that it would lead to extinction, 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 regard 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 understand you
to want me to examine and criticize the attempted 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
arguments 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
individuals 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.