Chapter VI (1)

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._