236 The Descent of Man. rAETlI, 



with our rule, these do uot begin to be developed before the age 

 of siy months, as I am assured by Mr. Eartlett, and even at this 

 age, the two sexes can hardly be distinguished." 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 samewliat 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 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 aUke, 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 



** In the common peacock (Pava 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. vmticus) 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 Mergus cu' 



have been developed earlier in life cuUafus we have, liowever, 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 year, of both species, com- speculum, which is pure white in 



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



was no ditference 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 



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



Qeen observed subsequently. ferences ; see Audubon, * Ornitho* 



" 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 



