But there is another set of plants, the trimorphic, in which the styles and stamens have each three forms--long, medium, and short, and in these it is possible to have eighteen different crosses. By an elaborate series of experiments it was shown that the six legitimate unions--that is, when a plant was fertilised by pollen from stamens of length corresponding to that of its style in the two other forms--were all abundantly fertile; while the twelve illegitimate unions, when a plant was fertilised by pollen from stamens of a different length from its own style, in any of the three forms, were either comparatively or wholly sterile.[52] We have here a wonderful amount of constitutional difference of the reproductive organs within a single species, greater than usually occurs within the numerous distinct species of a genus or group of genera; and all this diversity appears to have arisen for a purpose which has been obtained by many other, and apparently simpler, changes of structure or of function, in other plants. This seems to show us, in the first place, that variations in the mutual relations of the reproductive organs of different individuals must be as frequent as structural variations have been shown to be; and, also, that sterility in itself can be no test of specific distinctness. But this point will be better considered when we have further illustrated and discussed the complex phenomena of hybridity. _Cases of the Fertility of Hybrids, and of the Infertility of Mongrels._ I now propose to adduce a few cases in which it has been proved, by experiment, that hybrids between two distinct species are fertile _inter se_; and then to consider why it is that such cases are so few in number. The common domestic goose (Anser ferns) and the Chinese goose (A. cygnoides) are very distinct species, so distinct that some naturalists have placed them in different genera; yet they have bred together, and Mr. Eyton raised from a pair of these hybrids a brood of eight. This fact was confirmed by Mr. Darwin himself, who raised several fine birds from a pair of hybrids which were sent him.[53] In India, according to Mr. Blyth and Captain Hutton, whole flocks of these hybrid geese are kept in various parts of the country where neither of the pure parent species exists, and as they are kept for profit they must certainly be fully fertile. Another equally striking case is that of the Indian humped and the common cattle, species which differ osteologically, and also in habits, form, voice, and constitution, so that they are by no means closely allied; yet Mr. Darwin assures us that he has received decisive evidence that the hybrids between these are perfectly fertile _inter se_.