c bap. VIII. 
SEXUAL SELECTION. 
291 
s picuously 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 
m 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 . 31 Between 
8 Uch extreme cases of close sexual resemblance and 
wide dissimilarity, as those of the Crossoptilou 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- 
mbers to one or both sexes. But we do not know that 
tfie coloured scales, for instance, in two species of but- 
lerflies, in one of which the sexes differ in colour, whilst 
m the other they are alike, are developed at the same 
mlative age in the cocoon. Nor do we know whether 
f dl the scales are simultaneously developed on the wings 
22 la some other species of the Duck Family the speculum in the 
sexes differs in a greater degree ; but I have not been able to dis- 
c °ver whether its full development occurs later in life in the males of 
Slle h species, than in the male of the common duck, as ought to be the 
J* 8 * according to our rule. With the allied Mergus cueuUatiis we have, 
°Wever, a case of this kind : the two sexes differ conspicuously in 
j^Uerai plumage, and to a considerable degree in the speculum, which 
^ Pure white in the male and greyish- white in the female. Now the 
5°Uiig males at first resemble, in all respects, the female, and have a 
^ re yish-white speculum, hut this becomes pure white at an earlier age 
aii that at which the adult male acquires his other more strongly- 
lurked sexual differences in plumage : see Audubon, * Ornithological 
l0 graphy,’ vol. iii. 1835, p, 243-250. 
u 2 
