The Descent of Man

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