enced, in so great a degree as has often been
supposed, the character of the offspring in future times; although in some
cases it probably has had a considerable effect.

Domestication, as a general rule, increases the prolificness of animals and
plants. It eliminates the tendency to sterility which is common to species
when first taken from a state of nature and crossed. On this latter head we
have no direct evidence; but as our races of dogs, cattle, pigs, &c., are
almost certainly descended from aboriginally distinct stocks, and as these
races are now fully fertile together, or at least incomparably more fertile
than most species when crossed, we may with much confidence accept this
conclusion.

Abundant evidence has been given that crossing adds to the size, vigour,
and fertility of the offspring. This holds good when there has been no
previous close interbreeding. It applies to the individuals of the same
variety but belonging to different families, to distinct varieties,
sub-species, and partially even to species. In the latter case, though size
is often gained, fertility is lost; but the increased size, vigour, and
hardiness of many hybrids cannot be accounted for solely on the principle
of compensation from the inaction of the reproductive system. Certain
plants, both of pure and hybrid origin, though perfectly healthy, have
become self-impotent, apparently from the unnatural conditions to which
they have been exposed; and such plants, as well as others in their normal
state, can be stimulated to {175} fertility only by crossing them with
other individuals of the same species or even of a distinct species.

On the other hand, long-continued close interbreeding between the nearest
relations diminishes the constitutional vigour, size, and fertility of the
offspring; and occasionally leads to malformations, but not necessarily to
general deterioration of form or structure. This failure of fertility shows
that the evil results of interbreeding are independent of the augmentation
of morbid tendencies common to both parents, though this augmentation no
doubt is often highly injurious. Our belief that evil follows from close
interbreeding rests to a large extent on the experience of practical
breeders, especially of those who have reared many animals of the kinds
which can be propagated quickly; but it likewise rests on several carefully
recorded experiments. With some animals close interbreeding may be carried
on for