If it be admitted that the females prefer, or are unconsciously excited
by the more beautiful males, then the males would slowly but surely be
rendered more and more attractive through sexual selection. That it is
this sex which has been chiefly modified, we may infer from the fact
that, in almost every genus where the sexes 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 evident that the brilliant
colours, top-knots, fine plumes, etc., of many male birds cannot have
been acquired as a protection; indeed, they sometimes lead to danger.
That they are not due to the direct and definite action of the
conditions of life, we may feel assured, because the females have been
exposed to the same conditions, and yet often differ from the males to
an extreme degree. Although it is probable that changed conditions
acting during a lengthened period have in some cases produced a
definite effect on both sexes, or sometimes on one sex alone, the more
important result will have been an increased tendency to vary or to
present more strongly-marked individual differences; and such
differences will have afforded an excellent ground-work for the action
of sexual selection.
The laws of inheritance, irrespectively of selection, appear to have
determined whether the characters acquired by the males for the sake of
ornament, for producing various sounds, and for fighting together, have
been transmitted to the males alone or to both sexes, either
permanently, or periodically during certain seasons of the year. Why
various characters should have been transmitted sometimes in one way
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