The Descent of Man

when it discovers and flies down to a carcase
floating on the sea or cast up on the beach, will
be seen from a great distance, and will guide
other birds of the same and other species, to the
prey; but as this would be a disadvantage to the
first finders, the individuals which were the
whitest or blackest would not thus procure
more food than the less strongly coloured individuals.
Hence conspicuous colours cannot
have been gradually acquired for this purpose
through natural selection.
As sexual selection depends on so fluctuating
an element as taste, we can understand how it
is that, within the same group of birds having
nearly the same habits, there should exist white
or nearly white, as well as black, or nearly black
species,—for instance, both white and black
cockatoos, storks, ibises, swans, terns, and petrels.
Piebald birds likewise sometimes occur in
the same groups together with black and white
species; for instance, the black-necked swan,