The Descent of Man

Throughout the animal kingdom, when the
sexes differ in external appearance, it is, with
rare exceptions, the male which has been the
more modified; for, generally, the female retains
a closer resemblance to the young of her
own species, and to other adult members of the
same group. The cause of this seems to lie in
the males of almost all animals having stronger
passions than the females. Hence it is the males
that fight together and sedulously display their
charms before the females; and the victors
transmit their superiority to their male offspring.
Why both sexes do not thus acquire the
characters of their fathers, will be considered
hereafter. That the males of all mammals eagerly
pursue the females is notorious to every one.
So it is with birds; but many cock birds do not
so much pursue the hen, as display their plumage,
perform strange antics, and pour forth
their song in her presence. The male in the few
fish observed seems much more eager than the
female; and the same is true of alligators, and