SEXUAL SELECTION 811 



developed very early in life, long before the other orna- 

 ments, which are confined 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, while the curled tail-feathers and other ornaments of 

 the male are developed later." 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 our two 

 rules in their order of development. 



As most insects emerge from the pupal state in a mature 

 condition, it is doubtful whether the period of development 

 can determine the transference of their characters to one or 

 to both sexes. But we do not know that the colored scales, 

 for instance, in two species of butterflies, in one of which 

 the sexes differ in color, while in the other they are alike, 

 are developed at the same relative age in the cocoon. ITor 

 do we know whether all the scales are simultaneously devel- 

 oped on the wings of the "same species of butterfly, in which 

 certain colored marks are confined to one sex, while others 

 are common to both sexes. A difference of this kind in 

 the period of development is not so improbable as it may 

 at first appear; for with the Orthoptera, which assume their 

 adult state not by a single metamorphosis, but by a suc- 

 cession of moults, the young males of some species at first 

 resemble the females, and acquire their distinctive masculine 

 characters only at a later moult. Strictly analogous cases 

 occur at the successive moults of certain male crustaceans. 



*^ In some other species of the Duck family the speculum differs in a greater 

 degree in the two sexes; but I ha-?e not been able to discover whether its full 

 development occurs later in life iu 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 grayish white in the female. 

 Now the young males at first entirely resemble the females, and have a grayish 

 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 differ- 

 ences: see Audubon, "Ornithological Biography," vol. ili., 1835, pp. 249-250. 



