232 SEXUAL selection: birds. PartII. 



covered with snow. On the otlier hand we have reason 

 to believe that wliiteness has been gained by many birds 

 as a sexual ornament. We may therefore conclude that 

 an early progenitor of the Arclea asha and the Bujohus 

 acquired a white plumage for nuptial purposes, and 

 transmitted this colour to their young ; so that the 

 young and the old became white like certain existing 

 egrets ; the whifeness liaving afterwards been retained 

 by the young whilst exchanged by the adults for more 

 strongly pronounced tints. But if we could look still 

 further backwards in time to the still earlier progenitors 

 of these two species, we should probably see the adults 

 dark-coloured. I infer that this would be the case, from 

 the analogy of many other birds, which are dark whilst 

 young, and when adult are white ; and more especially 

 from the case of the Ardea c/ularis, the colours of which 

 are the reverse of those of A. asha, for the young are 

 dark-coloured and the adults white, the young having 

 retained a former state of plumage. It appears there- 

 fore that the progenitors in their adult condition of the 

 Ardea asha, the Bui^hus, and of some allies, have under- 

 gone, during a long line of descent, the following changes 

 of colour ; firstly a dark shade, secondly pure white, 

 and thirdly, owing to another change of fashion (if I 

 may so express myself), their present slaty, reddish, or 

 golden-buff tints. These successive changes are in- 

 telligible only on the principle of novelty having been 

 admired by birds for the sake of novelty. 



Summary of the Four Chapters on Birds. — Most male 

 birds are highly pugnacious during the breeding-season, 

 and some possess weapons especially adapted for fight- 

 ing with their rivals. But the most pugnacious and the 

 best-armed males rarely or never depend for success 

 solely on their power to drive away or kill their rivals, 



