Chap. VilL 
SEXUAL SELECTION. 
291 
spicuously from each other in almost every part of their 
plumage, except in the elegant head-crest, which is 
common to both sexes ; and this is developed very early 
in life, long before the other ornaments which are con- 
fined to the male. The wild-duck offers an analogous 
case, for the beautiful green speculum on the wings 
is common to both sexes, though duller and somewhat 
smaller in the female, and it is developed early in life, 
whilst the curled tail-feathers and other ornaments 
peculiar to the male are developed later . 29 Between 
such extreme cases of close sexual resemblance and 
wide dissimilarity, as those of the Orossoptiion and 
peacock, many intermediate ones could be given, in 
which the characters follow in their order of develop- 
ment our two rules. 
As most insects emerge from their pupal state in a 
mature condition, it is doubtful whether the period of 
development determines the transference of their cha- 
racters to one or both sexes. But we do not know that 
the coloured scales, for instance, in two species of but- 
terflies, in one of which the sexes differ in colour, whilst 
in the other they are alike, are developed at the same 
relative age in the cocoon. Nor do we know whether 
•all the scales are simultaneously developed on the wings 
20 In some other species of the Duck Family the speculum in the 
two sexes differs in a greater degree ; but I have not been able to dis- 
cover 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 wdiite in the male and greyish-white in the female. Now the 
young males at first resemble, in all respects, the female, and have a 
greyish-white speculum, but this becomes pure white at an earlier age 
than that at which the adult male acquires his other more strongly- 
marked sexual differences in plumage : see Audubon, ‘ Ornithological 
Biography,’ vol. iii. 1835, p. 249-250. 
u 2 
