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].