The Descent of Man

tions to which they will almost always have
been exposed will cause them to undergo a
certain amount of fluctuating variability. In this
case sexual selection, which depends on an
element liable to change—the taste or admiration
of the female—will have had new shades
of colour or other differences to act on and accumulate;
and as sexual selection is always at
work, it would (from what we know of the results
on domestic animals of man's unintentional
selection), be surprising if animals inhabiting
separate districts, which can never cross
and thus blend their newly-acquired characters,
were not, after a sufficient lapse of time, differently
modified. These remarks likewise apply
to the nuptial or summer plumage, whether
confined to the males, or common to both
sexes.
Although the females of the above closelyallied
or representative species, together with
their young, differ hardly at all from one anot-