The Descent of Man

played almost as important a part with reptiles
as with birds; and the less conspicuous colours
of the females in comparison with the males
cannot be accounted for, as Mr. Wallace believes
to be the case with birds, by the greater
exposure of the females to danger during incubation.
CHAPTER XIII.
SECONDARY SEXUAL CHARACTERS OF
BIRDS.
Sexual differences—Law of battle—Special
weapons—Vocal organs— Instrumental music—Love-antics
and dances—Decorations,
permanent and seasonal—Double and single
annual moults—Display of ornaments by the
males.
Secondary sexual characters are more diversified
and conspicuous in birds, though not per-