The Descent of Man

family the speculum differs in a greater degree
in the two sexes; but I have not been able to
discover whether its full development occurs
later in life in the males of such species, than in
the male of the common duck, as ought to be
the case according to our rule. With the allied
Mergus cucullatus we have, however, a case of
this kind: the two sexes differ conspicuously in
general plumage, and to a considerable degree
in the speculum, which is pure white in the
male and greyish-white in the female. Now the
young males at first entirely resemble the females,
and have a greyish-white speculum, which
becomes pure white at an earlier age than that
at which the adult male acquires his other and
more strongly-marked sexual differences: see
Audubon, 'Ornithological Biography,' vol. iii.
1835, pp. 249-250.) Between such extreme cases
of close sexual resemblance and wide dissimilarity,
as those of the Crossoptilon and peacock,
many intermediate ones could be given, in