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].
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].
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].
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].
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].
Again, somewhere about the same time, he wrote to Moritz Wagner:—