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