Chap. VIII. 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 Crossoptilon 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 



29 In some other species of the Duck Family the speculum in the 

 two sexes diners 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 white 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. hi. 1835, p. 249-250. 



u 2 



