The answer is simple. It has never been the object of breeders or of horticulturists to select variations in the direction of cross-infertility, for the swamping effects of intercrossing are much more easily and rapidly prevented by artificial isolation. Consequently, although they have been able to modify natural types in so many directions and in such high degrees with regard to _morphology_, there has been no accompanying physiological modification of the kind required. But in nature there is no such thing as artificial, i.e. intentional, isolation. Consequently, on common areas it must usually happen that those changes of morphology which are associated with cross-infertility are the only ones which can arise. Hence the very remarkable contrast between our domesticated varieties and natural species with regard to cross-infertility is just what the present theory would expect, or, indeed, require. But on any other theory it has hitherto remained inexplicable. In particular, the contrast in question has constituted one of the main difficulties with which the theory of natural selection has hitherto had to contend, not only in the popular mind, but also in the judgement of naturalists, including the joint-authors of the theory themselves. Thus Darwin says:-- The fertility of varieties is, with reference to my theory, of equal importance with the sterility of species, for it seems to make a broad and clear distinction between varieties and species[20]. [20] _Origin of Species_, p. 136. And Mr. Wallace says:-- One of the greatest, or perhaps we may say the greatest, of all the difficulties in the way of accepting the theory of natural selection as a complete explanation of the origin of species, has been the remarkable difference between varieties and species in respect of fertility when crossed[21]. [21] _Darwinism_, p. 152.