The Descent of Man

A much more probable view is suggested by
the analogy of the lower animals. The reproductive
system can be shewn to be susceptible
to an extraordinary degree (though why we
know not) to changed conditions of life; and
this susceptibility leads both to beneficial and
to evil results. A large collection of facts on this
subject is given in chap. xviii. of vol. ii. of my
'Variation of Animals and Plants under Domestication,'
I can here give only the briefest abstract;
and every one interested in the subject
may consult the above work. Very slight changes
increase the health, vigour, and fertility of
most or all organic beings, whilst other changes
are known to render a large number of animals
sterile. One of the most familiar cases, is that of
tamed elephants not breeding in India; though
they often breed in Ava, where the females are
allowed to roam about the forests to some extent,
and are thus placed under more natural
conditions. The case of various American monkeys,
both sexes of which have been kept for