The Descent of Man

country, never unite; others thus circumstanced
frequently unite but never produce offspring;
others again produce some offspring, but fewer
than in a state of nature; and as bearing on the
above cases of man, it is important to remark
that the young are apt to be weak and sickly, or
malformed, and to perish at an early age.
Seeing how general is this law of the susceptibility
of the reproductive system to changed conditions
of life, and that it holds good with our
nearest allies, the Quadrumana, I can hardly
doubt that it applies to man in his primeval
state. Hence if savages of any race are induced
suddenly to change their habits of life, they
become more or less sterile, and their young
offspring suffer in health, in the same manner
and from the same cause, as do the elephant
and hunting-leopard in India, many monkeys
in America, and a host of animals of all kinds,
on removal from their natural conditions.