The Descent of Man

Before treating of the sexual characters with
which we are here more particularly concerned,
I may just allude to certain differences between
the sexes which apparently depend on differences
in their habits of life; for such cases,
though common in the lower, are rare in the
higher classes. Two humming-birds belonging
to the genus Eustephanus, which inhabit the
island of Juan Fernandez, were long thought to
be specifically distinct, but are now known, as
Mr. Gould informs me, to be the male and female
of the same species, and they differ slightly
in the form of the beak. In another genus of
humming-birds (Grypus), the beak of the male
is serrated along the margin and hooked at the
extremity, thus differing much from that of the
female. In the Neomorpha of New Zealand,
there is, as we have seen, a still wider difference
in the form of the beak in relation to the
manner of feeding of the two sexes. Something
of the same kind has been observed with the
goldfinch (Carduelis elegans), for I am assured