I $6 The Descent of Man. Part II. 



with our rule, these do not begin to be developed before the age 

 of six months, as I am assured by Mr. Bartlett, and even at this 

 age, the two sexes can hardly be distinguished. 44 The male and 

 female Peacock differ conspicuously 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 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. 45 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 coloured scales, for instance, 

 in two species of butterflies, 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 of the same 

 species of butterfly, in which certain coloured marks are confined 



44 In the common peacock (Pavo sexes; but I have not been able to 

 cristatus) the male alone possesses discover whether its full develop- 

 spurs, whilst both sexes of the Java ment occurs later in life in the 

 Peacock (P. muticus) offer the un- males of such species, than in the 

 usual case of being furnished with male of the common duck, as ought 

 spurs. Hence I fully expected that to be the case according to our 

 in the latter species they would rule. With the allied Alergus cu- 

 have been developed earlier in life cullatus we have, however 1 , a case of 

 than in the common peacock; but this kind: the two sexes differ con- 

 M. Hegt of Amsterdam informs me, spicuously in general plumage, and 

 that with young birds of the pre- to a considerable degree in the 

 vious yeai-, of both species, com- speculum, which is pure white in 

 pared on April 23rd, 1869, there the male and greyish- white in the 

 was no difference in the develop- female. Now the young males at 

 ment of the spurs. The spurs, first entirely resemble the females, 

 however, were as yet represented and have a greyish-white speculum, 

 merely by slight knobs or eleva- which becomes pure white at an 

 tions. I presume that I should earlier age than that at which the 

 have been informed if any difference adult male acquires his other and 

 in the rate of development had more strongly-marked sexual dif- 

 aeen observed subsequently. ferences : see Audubon, ' Ornitho* 



45 In some other species of the logical Biography,' vol. iii. 1835, 

 Duck family the speculum differs pp. 249-250. 



.u a greater degree in the two 



