006 THE DESCENT OF MaN 



Prom the foregoing facts, more especially from neither 

 sex of certain birds changing color during either annual 

 moult, or changing so slightly that the change can hardly 

 be of any service to them, and from the females of 

 other species moulting twice yet retaining the same colors 

 throughout the year, we may conclude that the habit of 

 annually moulting twice has not been acquired in order 

 that the male should assume an ornamental character dur- 

 ing the breeding season ; but that the double moult, having 

 been originally acquired for some distinct purpose, has sub- 

 sequently been taken advantage of in certain cases for gain- 

 ing a nuptial plumage. 



It appears at first sight a surprising circumstance that 

 some closely allied species should regularly undergo a 

 double annual moult, and others only a single one. The 

 ptarmigan, for instance, moults twice or even thrice in the 

 year, and the black-cock only once ; some of the splendidly 

 colored honey-suckers (Nectarinise) of India and some sub- 

 genera of obscurely colored pipits (Anthus) have a double, 

 while others have only a single annual moult. °' But the 

 gradations in the manner of moulting, which are known to 

 occur with various birds, show us how species or whole 

 groups might have originally acquired their double annual 

 moult, or, having once gained the habit, have again lost it. 

 With certain bustards and plovers the vernal moult is far 

 from complete, some feathers being renewed, and some 

 changed in color. There is also reason to believe that 

 with certain bustards and rail-like birds, which properly 

 undergo a double moult, some of the older males retain 

 their nuptial plumage throughout the year. A few highly 

 modified feathers may merely be added during the spring 

 to the plumage, as occurs with the disk-formed tail-feathers 

 of certain drongos {Bhringa) in India, and with the elon- 

 gated feathers on the back, neck, and crest of certain 



*' On the moulting of the ptarmigan, see Gould's "Birds of Great Britain." 

 On the honey -suckers, Jerdon, "Birds of India," vol. i. pp. 359, 365, 369. 

 On the moulting of Anthus, see Blyth, in "Ibis," 1867, p. 32. 



