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Show 184 DARWINISM CHAP. chance of survival in tho battle of life. It is, therefore, directly under the control of natural selection, which a.cts both by the self-preservation of fertile and the self-destructiOn of inferWe stocks-except always where correlated as above, when they become useful, and therefore subject to be increased by natural selection. • Summary and Concl·uding Remarks on Hybridity. The facts which are of the greatest importance to a comprehension of this very difficult subject are those which show the extreme susceptibility of the reproductive system both in plants and animals. We have seen how both these classes of orO'anisms may be rendered infertile, by a change of conditions which does not affect their general health, by captivity, or by too close interbreeding. We have seen, also, that infertility is frequently correlated with a difference of colour, or with other characters; that it is not proportionate to divergence of structure; that it varies in reciprocal crosses between pairs of the same species ; while in the cases of dimorphic and trimorphic plants the different crosses between the same pair of individuals may be fertile or sterile at the same time. It ~tppears as if fertility depended on such a delicate adjustment of the male and female clements to each other, that, unless constantly kept up by the preservation of the most fertile individuals, sterility is always liable to arise. This preservation always occurs within the limits of each species, both because fertility is of the highest importance to the continuance of the race, and also because sterility (and to a less extent infertility) is self-destructive as well as injurious to the species. So long therefore as a species remains undivided, and in occupation of a continuous area, its fertility is kept up by natural selection ; but the moment it becomes separated, either by geographical or selective isolation, or by diversity of station or of habits, then, while each portion must be kept fertile inter se, there is nothing to prevent infertility arising between the two separated portions. As the two portions will necessarily exist under somewhat different conditions of life, and will usually have acquired some diversity of form and colour-both which circumstances we know to be either the cause of infertility or to be correlated with it1-the fact of Vll ON THE INFER'l'ILI'l'Y OW CROSSES 185 some degree of infertility usually a.ppcaring between closely allied bnt locaJly or physiologically segregated species is exactly what we should expect. 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 ferWe 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 orne evidence to show that domestication itse1£ tends to incrca ·c fertility. Among pbnt.·, wild species and varieties have been more frequently experimented on than among animals, and we accordi11gly find numerous cases in which distinct species of plants arc perfectly fertile when crossed, their hybrid offspring being also fertile inter se. vV c 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 e ·scntial to the formation of new species, in order to prevent the swamping effects of intcrcrossing, has been shown to be unsound, because the varictie. 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 :tt the conclusion th:tt the sterility or infertility of species with each other, whether manifested in the difficulty of obtaining first eros e between them or in the sterility of the hybrids thus obtained, is not a constant or necessary result of specific difference, but is incidenta.l on unknown peculiarities of tho reproductive system. These peculiarities constantly tend to arise under changed conditions owing to the extreme susceptibility of that system, and they |
