FIG. 17.—Primula veris (Cowslip).
The meaning and use of these different forms was quite unknown till Darwin discovered, first, that cowslips and primroses are absolutely barren if insects are prevented from visiting them, and then, what is still more extraordinary, that each form is almost sterile when fertilised by its own pollen, and comparatively infertile when crossed with any other plant of its own form, but is perfectly fertile when the pollen of a long-styled is carried to the stigma of a short-styled plant, or vice versâ. It will be seen, by the figures, that the arrangement is such that a bee visiting the flowers will carry the pollen from the long anthers of the short-styled form to the stigma of the long-styled form, while it would never reach the stigma of another plant of the short-styled form. But an insect visiting, first, a long-styled plant, would deposit the pollen on the stigma of another plant of the same kind if it were next visited; and this is probably the reason why the wild short-styled plants were found to be almost always most productive of seed, since they must be all fertilised by the other form, whereas the long-styled plants might often be fertilised by their own form. The whole arrangement, however, ensures cross-fertilisation; and this, as Mr. Darwin has shown by copious experiments, adds both to the vigour and fertility of almost all plants as well as animals.
Besides the primrose family, many other plants of several distinct natural orders present similar phenomena, one or two of the most curious of which must be referred to. The beautiful crimson flax (Linum grandiflorum) has also two forms, the styles only differing in length; and in this case Mr. Darwin found by numerous experiments, which have since been repeated and confirmed by other observers, that each form is absolutely sterile with pollen from another plant of its own form, but abundantly fertile when crossed with any plant of the other form. 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. 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.