In this case the pollen of the two forms cannot be distinguished under the microscope (whereas that of the two forms of Primula differs in size and shape), yet it has the remarkable property of being absolutely powerless on the stigmas of half the plants of its own species.

The crosses between the opposite forms, which are fertile, are termed by Mr.Darwin "legitimate," and those between similar forms, which are sterile, "illegitimate"; and he remarks that we have here, within the limits of the same species, a degree of sterility which rarely occurs except between plants or animals not only of different _species_ but of different _genera_.
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.