696 THE DESCENT OF MAN 



many other birds, which are developed and retained only 

 during the summer, serve for ornamental and nuptial pur- 

 poses, though common to both sexes. The female is thus 

 rendered more conspicuous during the period of incubation 

 than during the winter; but such birds as herons and egrets 

 would be able to defend themselves. As, however, plumes 

 would probably be inconvenient, and certainly of no use 

 during the winter, it is possible that the habit of moulting 

 twice in the year may have been gradually acquired through 

 natural selection for the sake of casting ofE inconvenient 

 ornaments during the winter. But this view cannot be 

 extended to the many waders, whose summer and winter 

 plumages differ very little in color. With defenceless 

 species, in which both sexes, or the males alone, become 

 extremely conspicuous during the breeding season — or when 

 the males acquire at this season such long wing or tail- 

 feathers as to impede their flight, as with Cosmetornis and 

 Vidua — it certainly at first appears highly probable that the 

 second moult has been gained for the special purpose of 

 throwing off these ornaments. We must, however, remem- 

 ber that many birds, such as some of the Birds of Paradise, 

 the Argus pheasant and peacock, do not cast their plum^ 

 during the winter; and it can hardly be maintained thafe 

 the constitution of these birds, at least of the Gallinaceae, 

 fenders a double moult impossible, for the ptarmigan moults 

 thrice in the year." Hence it must be considered as doubt- 

 ful whether the many species which moult their ornamental 

 plumes or lose their bright colors during the winter, have 

 acquired this habit on account of the inconvenience or 

 danger which they would otherwise have suffered. 



I conclude, therefore, that the habit of moulting twice 

 in the year was in most or all cases first acquired for some 

 distinct purpose, perhaps for gaining a warmer winter cov- 

 ering; and that variations in the plumage occurring' during 

 the summer were accumulated through sexual selection and 



»» See Gould's "Birds of Great Britain." 



