The Descent of Man

explained on the principle of protection, for on
the Pampas, which are equally open, though
covered by green grass, and where the birds
would be equally exposed to danger, many
brilliant and conspicuously coloured species
are common. I have sometimes speculated
whether the prevailing dull tints of the scenery
in the above named countries may not have
affected the appreciation of bright colours by
the birds inhabiting them.), who shews that in
the United States many species of birds gradually
become more strongly coloured in proceeding
southward, and more lightly coloured in
proceeding westward to the arid plains of the
interior. Both sexes seem generally to be affected
in a like manner, but sometimes one sex
more than the other. This result is not incompatible
with the belief that the colours of birds are
mainly due to the accumulation of successive
variations through sexual selection; for even
after the sexes have been greatly differentiated,
climate might produce an equal effect on both