The Descent of Man

iations, so no doubt were the early progenitors
of man; the variations being formerly induced
by the same general causes, and governed by
the same general and complex laws as at present.
As all animals tend to multiply beyond
their means of subsistence, so it must have been
with the progenitors of man; and this would
inevitably lead to a struggle for existence and to
natural selection. The latter process would be
greatly aided by the inherited effects of the increased
use of parts, and these two processes
would incessantly react on each other. It appears,
also, as we shall hereafter see, that various
unimportant characters have been acquired
by man through sexual selection. An unexplained
residuum of change must be left to the
assumed uniform action of those unknown
agencies, which occasionally induce strongly
marked and abrupt deviations of structure in
our domestic productions.