Darwin, and After Darwin Volume Iii Part 6

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 a.s.sistance 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 pa.s.sages 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 a.n.a.logous 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 a.n.a.logy 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 a.n.a.logy 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 pa.s.sages 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 pa.s.sage 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 cla.s.ses 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 pa.s.sages 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 a.n.a.logous distinction between "unconscious" and "methodical" selection by man. The a.n.a.logy, 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 ill.u.s.trate the process of polytypic evolution, as previously remarked, in the letters above quoted we find it adduced to ill.u.s.trate the process of monotypic evolution. But the fact of this a.n.a.logy 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 a.n.a.logy 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 ill.u.s.trations, 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 pa.s.sages from earlier editions of the _Origin of Species_; but as this, in view of the above pa.s.sages, is unnecessary, we may next pa.s.s 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 a.s.sisted 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 a.s.suming the a.s.sistance 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 const.i.tuting 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 a.s.sist 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 a.n.a.logy 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 pa.s.sage 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 a.s.sumption 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 una.s.sisted by natural selection--induce divergence of type, in a ratio that has been mathematically calculated by Delbuf.

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 Delbuf, Gulick, Giard, and myself, it is ent.i.tled, the above pa.s.sage 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."