The Descent of Man

differ, the males differ much more from one
another than do the females; this is well shewn
in certain closely-allied representative species,
in which the females can hardly be distinguished,
whilst the males are quite distinct. Birds in
a state of nature offer individual differences
which would amply suffice for the work of
sexual selection; but we have seen that they
occasionally present more strongly marked
variations which recur so frequently that they
would immediately be fixed, if they served to
allure the female. The laws of variation must
determine the nature of the initial changes, and
will have largely influenced the final result. The
gradations, which may be observed between
the males of allied species, indicate the nature
of the steps through which they have passed.
They explain also in the most interesting manner
how certain characters have originated,
such as the indented ocelli on the tail-feathers
of the peacock, and the ball-and-socket ocelli on
the wing-feathers of the Argus pheasant. It is