The Descent of Man

We thus see that several customs prevail with
savages which must greatly interfere with, or
completely stop, the action of sexual selection.
On the other hand, the conditions of life to
which savages are exposed, and some of their
habits, are favourable to natural selection; and
this comes into play at the same time with
sexual selection. Savages are known to suffer
severely from recurrent famines; they do not
increase their food by artificial means; they
rarely refrain from marriage (16. Burchell says
('Travels in S. Africa,' vol. ii. 1824, p. 58), that
among the wild nations of Southern Africa,
neither men nor women ever pass their lives in
a state of celibacy. Azara ('Voyages dans l'Amerique
Merid.' tom. ii. 1809, p. 21) makes precisely
the same remark in regard to the wild Indians
of South America.), and generally marry
whilst young. Consequently they must be subjected
to occasional hard struggles for existence,
and the favoured individuals will alone survive.