Volume Iii Part 6 (1/2)

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.