SEXUAL SELECTION 637 



when it discovers and flies down to a carcass floating ou 

 the sea or cast upon the beach, will be seen from a great 

 distance, and will guide other birds of the same and other 

 species to the prey; but as this would be a disadvantage 

 to the first finders, the individuals which were- the whitest 

 or blackest would not thus procure more food than the less 

 strongly colored individuals. Hence conspicuous colors 

 cannot have been gradually acquired for this purpose 

 through natural selection. 



As sexual selection depends on so fluctuating an element 

 as taste, we can understand how it is that, within the same 

 group of birds having nearly the same habits, there should 

 exist white or nearly white, as well as black or nearly 

 black species — for instance, both white and black cocka- 

 toos, storks, ibises, swans, terns, and petrels. Piebald birds 

 likewise sometimes occur in the same groups together with 

 black and white species; for instance, the black-necked 

 swan, certain terns, and the common magpie. That a 

 strong contrast in color is agreeable to birds we may con- 

 clude by looking through any large collection, for the sexes 

 often differ from each other in the male having the pale 

 parts of a purer white, and the variously colored dark parts 

 of still darker tints than the female. 



It would even appear that mere novelty or slight changes 

 for the sake of change have sometimes acted on female 

 birds as a charm, like changes of fashion with us. Thus 

 the males of some parrots can hardly be said to be more 

 beautiful than the females, at least according to our taste, 

 but they differ in such points as in having a rose-colored 

 collar instead of "a bright emeraldine narrow green collar"; 

 or in the male having a black collar instead of "a yellow 

 demi-collar in front," with a pale roseate instead of a plum- 

 blue head." And so many male birds have elongated tail- 



or largely white, and that many others are hlaok. So that here again conspicu- 

 ous colors may possibly aid the sexes in finding each other during the breeding 

 season. 



" See Jerdon on the genus Palseornis, "Burds of India," voL L pp. 258-260. 



