size
and vigour of the offspring and their sterility. Moreover it has been
clearly proved that mongrels which are perfectly fertile gain these same
advantages as well as sterile hybrids.

The evil consequences of long-continued close interbreeding are not so
easily recognised as the good effects from crossing, for the deterioration
is gradual. Nevertheless it is the general opinion of those who have had
most experience, especially with animals which propagate quickly, that evil
does inevitably follow sooner or later, but at different rates with
different animals. No doubt a false belief may widely prevail like a
superstition; yet it is difficult to suppose that so many acute and
original {143} observers have all been deceived at the expense of much cost
and trouble. A male animal may sometimes be paired with his daughter,
granddaughter, and so on, even for seven generations, without any manifest
bad result; but the experiment has never been tried of matching brothers
and sisters, which is considered the closest form of interbreeding, for an
equal number of generations. There is good reason to believe that by
keeping the members of the same family in distinct bodies, especially if
exposed to somewhat different conditions of life, and by occasionally
crossing these families, the evil results may be much diminished, or quite
eliminated. These results are loss of constitutional vigour, size, and
fertility; but there is no necessary deterioration in the general form of
the body, or in other good qualities. We have seen that with pigs
first-rate animals have been produced after long-continued close
interbreeding, though they had become extremely infertile when paired with
their near relations. The loss of fertility, when it occurs, seems never to
be absolute, but only relative to animals of the same blood; so that this
sterility is to a certain extent analogous with that of self-impotent
plants which cannot be fertilised by their own pollen, but are perfectly
fertile with pollen of any other plant of the same species. The fact of
infertility of this peculiar nature being one of the results of
long-continued interbreeding, shows that interbreeding does not act merely
by combining and augmenting various morbid tendencies common to both
parents; for animals with such tendencies, if not at the time actually ill,
can generally propagate their kind. Although offspring descended from the
nearest blood-relations are not necessarily de