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e necessary, for with these plants fertility is gained only by the union of distinct species, or of hybrids of distinct parentage. These facts all point to the same general conclusion, namely, that good is derived from a cross between individuals, which either innately, or from exposure to dissimilar conditions, have come to differ in sexual constitution. Exotic animals confined in menageries are sometimes in nearly the same state as the above-described self-impotent plants; for, as we shall see in the following chapter, certain monkeys, the larger carnivora, several finches, geese, and pheasants, cross together, quite as freely as, or even more freely than, the individuals of the same species breed together. Cases will, {142} also, be given of sexual incompatibility between certain male and female domesticated animals, which, nevertheless, are fertile when matched with any other individual of the same kind. In the early part of this chapter it was shown that the crossing of distinct forms, whether closely or distantly allied, gives increased size and constitutional vigour, and, except in the case of crossed species, increased fertility, to the offspring. The evidence rests on the universal testimony of breeders (for it should be observed that I am not here speaking of the evil results of close interbreeding), and is practically exemplified in the higher value of cross-bred animals for immediate consumption. The good results of crossing have also been demonstrated, in the case of some animals and of numerous plants, by actual weight and measurement. Although animals of pure blood will obviously be deteriorated by crossing, as far as their characteristic qualities are concerned, there seems to be no exception to the rule that advantages of the kind just mentioned are thus gained, even when there has not been any previous close interbreeding. The rule applies to all animals, even to cattle and sheep, which can long resist breeding in-and-in between the nearest blood-relations. It applies to individuals of the same sub-variety but of distinct families, to varieties or races, to sub-species, as well as to quite distinct species. In this latter case, however, whilst size, vigour, precocity, and hardiness are, with rare exceptions, gained, fertility, in a greater or less degree, is lost; but the gain cannot be exclusively attributed to the principle of compensation; for there is no close parallelism between the increased
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