526 THE DESCENT OF MAN, 



in common that they depend, according to the hypothesis 

 of pangenesis, on gemmules derived from each part of the 

 male being present, though hitent, in the female; their 

 development following on some slight change in the elective 

 affinities 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, etc., of egrets, herons and 

 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 ren- 

 dered 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 off inconvenient 

 ornaments during the winter. But this yiew cannot be 

 extended to the many waders, whose summer and winter 

 plumages differ very little in color. With defenseless spe- 

 cies, 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 pur- 

 pose of throwing off these ornaments. 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 Gallinaceae, renders a double moult impossible, for the 

 ptarmigan moults thrice in the year.* Hence it must be 

 considered as doubtful 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. 



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



