456 THE DESCENT OF MAN. 



their development following on some slight change in the elective 

 afBnities of her constituent tissues. 



A few words must be added on changes of plumage in relation 

 to the season of the year. From reasons formerly assigned there 

 can be little doubt that the elegant plumes, long pendant feathers, 

 crests, fee, of egrets, herons, and many other birds, which are 

 developed and retained only during the summer, serve for orna- 

 mental and nuptial purposes, though common to both sexes. The 

 female is thus rendered more conspicuous during the period of in- 

 cubation 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 off inconvenient ornaments during the win- 

 ter. But this view cannot be extended to the many waders, whose 

 summer and winter plumages differ very little in color. With 

 defenseless species, in which both sexes, or the males alone, be- 

 come 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 cer- 

 tainly at first appears highly probable that the second moult has 

 been gained for the special purpose of throwing off these orna- 

 ments. We must, however, remember that many birds, such as 

 some of the Birds of Paradise, the Argus pheasant and peacock, do 

 not cast their plumes during the winter; and It can hardly be 

 maintained that the constitution of these birds, at least of the Gal- 

 linacese, renders 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 covering; and that 

 variations in the plumage occurring during the summer were ac- 

 cumulated through sexual selection, and transmitted to the off- 

 spring at the same season of the year; that such variations were 

 inherited either by both sexes or by the males alone, according to 

 the form of inheritance which prevailed. This appears more 

 probable than that the species in all cases originally tended to re- 

 tain their ornamental plumage during the winter, but were saved 



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



