The reason why varieties do not usually exhibit a similar amount of infertility is not difficult to explain. The popular conclusions on this matter have been drawn chiefly from what occurs among domestic animals, and we have seen that the very first essential to their becoming domesticated was that they should continue fertile under changed conditions of life. During the slow process of the formation of new varieties by conscious or unconscious selection, fertility has always been an essential character, and has thus been invariably preserved or increased; while there is some evidence to show that domestication itself tends to increase fertility.
Among plants, wild species and varieties have been more frequently experimented on than among animals, and we accordingly find numerous cases in which distinct species of plants are perfectly fertile when crossed, their hybrid offspring being also fertile inter se. We also find some few examples of the converse fact—varieties of the same species which when crossed are infertile or even sterile.
The idea that either infertility or geographical isolation is absolutely essential to the formation of new species, in order to prevent the swamping effects of intercrossing, has been shown to be unsound, because the varieties or incipient species will, in most cases, be sufficiently isolated by having adopted different habits or by frequenting different stations; while selective association, which is known to be general among distinct varieties or breeds of the same species, will produce an effective isolation even when the two forms occupy the same area.
From the various considerations now adverted to, Mr. Darwin arrived at the conclusion that the sterility or infertility of species with each other, whether manifested in the difficulty of obtaining first crosses between them or in the sterility of the hybrids thus obtained, is not a constant or necessary result of specific difference, but is incidental on unknown peculiarities of the reproductive system. These peculiarities constantly tend to arise under changed conditions owing to the extreme susceptibility of that system, and they are usually correlated with variations of form or of colour. Hence, as fixed differences of form and colour, slowly gained by natural selection in adaptation to changed conditions, are what essentially characterise distinct species, some amount of infertility between species is the usual result.
Here the problem was left by Mr. Darwin; but we have shown that its solution may be carried a step further. If we accept the association of some degree of infertility, however slight, as a not unfrequent accompaniment of the external differences which always arise in a state of nature between varieties and incipient species, it has been shown that natural selection has power to increase that infertility just as it has power to increase other favourable variations. Such an increase of infertility will be beneficial, whenever new species arise in the same area with the parent form; and we thus see how, out of the fluctuating and very unequal amounts of infertility correlated with physical variations, there may have arisen that larger and more constant amount which appears usually to characterise well-marked species.
The great body of facts of which a condensed account has been given in the present chapter, although from an experimental point of view very insufficient, all point to the general conclusion we have now reached, and afford us a not unsatisfactory solution of the great problem of hybridism in relation to the origin of species by means of natural selection. Further experimental research is needed in order to complete the elucidation of the subject; but until these additional facts are forthcoming no new theory seems required for the explanation of the phenomena.
FOOTNOTES:
[51] Darwin's Animals and Plants under Domestication, vol. ii. pp. 163-170.
[52] For a full account of these interesting facts and of the various problems to which they give rise, the reader must consult Darwin's volume on The Different Forms of Flowers in Plants of the same Species, chaps, i.-iv.
[53] See Nature, vol. xxi. p. 207.