But more than this. The importance of the widespread phenomena of prepotency to the theory of physiological selection does not consist merely in thus supplying the last link in the chain of evidence touching the origin of species by selective fertility, or "elective capacity." These phenomena are of further importance as showing how in plants, at all events, physiological selection appears to be frequently capable of differentiating specific types without the necessary assistance of any other form of homogamy. In my original statement of the theory, I was careful to insist upon the great value, as differentiating agents, of even small degrees of other forms of homogamy when co-operating with physiological selection. But I also stated my belief that in many cases selective fertility is presumably of itself capable of splitting a specific type; and the reason why I still believe this is, that I do not otherwise understand these phenomena of prepotency. I cannot believe that in all the innumerable cases where they arise, they have been super-induced by some prior morphological changes going on in some other part of the organism, or by "prolonged exposure to uniform conditions of life," on the part of two well-nigh identical forms which have arisen intimately commingled in exactly the same environment, and under the operation of a previously universal intercrossing. Even if such a thing could be imagined as happening occasionally, I feel it difficult to imagine that it can happen habitually, and yet this view must be held by those who would attribute prepotency to natural selection. It must never be forgotten that the relatively enormous changes as to size, structure, habit, &c., which are presented by our domesticated plants as results of artificial selection, do not entail the physiological character of cross-sterility in any degree, save possibly in some small number of cases. Although in wild species any correspondingly small percentage of cases (where natural selection happens to hit upon parts of the organism modifications of which produce the physiological change by way of correlation) would doubtless be the ones to survive on common areas, still it is surely incredible that such an accidental association between natural selection and cross-infertility is so habitually the means of specific differentiation as the facts of prepotency (together with the observations of Jordan and Nägeli) would necessarily demand.