The Descent of Man

many years together in their own countries,
and yet have very rarely or never bred, is a more
apposite instance, because of their relationship
to man. It is remarkable how slight a change
in the conditions often induces sterility in a
wild animal when captured; and this is the more
strange as all our domesticated animals have
become more fertile than they were in a state of
nature; and some of them can resist the most
unnatural conditions with undiminished fertility.
(46. For the evidence on this head, see 'Variation
of Animals,' etc., vol. ii. p. 111.) Certain
groups of animals are much more liable than
others to be affected by captivity; and generally
all the species of the same group are affected in
the same manner. But sometimes a single species
in a group is rendered sterile, whilst the
others are not so; on the other hand, a single
species may retain its fertility whilst most of the
others fail to breed. The males and females of
some species when confined, or when allowed
to live almost, but not quite free, in their native