Moreover, this view of the matter is still further corroborated by certain other facts and considerations. For example, the phenomena of prepotency (whether as between varieties or between closely allied species) are found to occur when the two forms occupy a common area, i.e. are growing intermingled with one another. Therefore, but for this physiological differentiation, there could be absolutely nothing to prevent free intercrossing. Yet the fact that hybrids are so comparatively rare in a state of nature--a fact which Sir Joseph Hooker has pointed out to me as otherwise inexplicable--proves the efficacy of even a low degree of such differentiation in preventing the physiologically-differentiated forms from intercrossing. Even in cases where there is no difficulty in producing artificial hybrids or mongrels between species or varieties growing on common areas, it is perfectly astonishing what an extremely small percentage of the hybrid or mongrel forms are found to occur in nature. And there can be no question that this is due to the very efficient manner in which prepotency does its work--efficient, I mean, from the point of view of the new theory; for upon any other theory prepotency is a meaningless phenomenon, which, notwithstanding its frequent occurrence, plays no part whatever in the process of organic evolution. I attach considerable importance to the phenomena of prepotency in view of the contrast which is presented between plants and animals in the relation of their species to physical barriers. For animals--and especially the higher animals--appear to depend for their specific differentiations upon such barriers much more than in the case with plants. This is no more than we should expect; for, in accordance with our theory, selective fertility is not so likely to work alone in the case of the higher animals which mate together, as in plants which are fertilized through the agency of wind or insects. In the former case there is no opportunity given for the first rise of cross-infertility, in the form of prepotency; and even where selective fertility has gained a footing in other ways, the chances against the suitable mating of "physiological complements" must be much greater than it is in the latter case. Hence, among the higher animals, selective fertility ought much more frequently to be found in association with other forms of homogamy than it is among plants. And this is exactly what we find. Thus it seems to me that this contrast between the comparative absence and presence of physical barriers, where allied species of plants and of higher animals are respectively concerned, is entitled to be taken as a further corroboration of our theory. For while it displays exactly such a general correlation as this theory would expect, the correlation is one which cannot possibly be explained on any other theory. It is just where physiological selection can be seen to have the best opportunity of acting (viz.