SEXUAL SELECTION. 231 



conspicuously from each other in almost every part of their plum- 

 age, 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 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, whilst 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 in- 

 termediate 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 con- 

 dition, it is doubtful whether the period of development can deter- 

 mine 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, 

 whilst in the other they are alike, are developed at the same rela- 

 tive age in the cocoon. Nor do we know whether all the scales 

 are simultaneously developed on the wings of the same species 

 of butterfly, in which certain colored marks are confined to one 

 sex, whilst 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 succession 

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

 females, and acquire their distinctive masculine characters only 



life than in the common peacock; but M. Hogt of Amsterdam In- 

 forms me, that with young birds of the previous year, of both species, 

 compared on April 23rd, 1869, there was no difference in the deivelop- 

 ment of the spurs. The spurs, however, were as yet represented 

 merely by slight knobs or elevations. I presume that I should have 

 been informed if any difference in the rate of development had been 

 observed subsequently. 



'^ In some other species of the Duck 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 

 cuoullatus we have, however, a case of this kind: the two sexes dif- 

 fer 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 differences; see Audu- 

 bon, 'Ornithological Biography,' vol. iii. 1835, pp. 249-250. 



