4-62 The Descent of Man. Part IL 



those vihere fertile females, whilst young, acquire the characters 

 of the male, through variation or some unknown cause.'^ But 

 all these cases have so much in common that they depend, 

 according to the hypothesis of pangenesis, on gemmules derived 

 from each part of the male being present, though latent, in the 

 female ; their development following on some slight change in 

 the elective affinities of her constituent tissues. 



h few words must he 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, cre:-ts, &c., of egrets, herons, and many other birds, 

 which are developed and retained only during the summer, 

 serve for ornamental and nuptial purposes, 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 them- 

 selves. 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 ofif 

 inconvenient ornaments during the winter. But this view 

 cannot be extended to the many waders, whoso summer and 

 winter plumages differ very little in colour. With defenceless 

 species, in which both sexes, or the males alone, becomi' 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 gainec. 

 for the special purpose of throwing off these ornaments. Wc 

 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 Gallinacese, 

 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 colours during the winter, have acquired this 

 habit on account of the inconvenience or danger which they 

 would otherwise have suffered. 



'^ Of these latter cases Jfr. Blyth also recorded a similar case (' Oi-nikh. 



has lecorded (Translation of Cuvier's Biog.' vol. ». p. 519) with Lyranga 



' Rfegne Animal,' p. 158) various . astiva. 



instances with Lanius, Ruticilla, " See Gould's 'Birds of (trc«l 



l.inai'ia, and Anas. Audubon ha:; Britain.' 



