152 SEXUAL selection: birds. Part II. 



passes over sexual selection, and asks, " What explana- 

 " tion does the law of natural selection give of such 

 " specific varieties as these ? " He answers " none 

 "whatever;" and I quite agree with him. But can 

 this be so confidently said of sexual selection ? Seeing 

 in how many ways the tail-feathers of humming-birds 

 differ, why should not the four central feathers have 

 varied in this one species alone, so as to have acquired 

 white tips ? The variations may have been gradual, or 

 somewhat abrupt as in the case recently given of the 

 humming-birds near Bogota, in which certain indi- 

 viduals alone have the " central tail-feathers tipped 

 " with beautiful green." In the female of the XJros- 

 ticte I noticed extremely minute or rudimental white 

 tips to the two outer of the four central black tail- 

 feathers ; so that here we have an indication of change 

 of some kind in the plumage of this species. If we grant 

 the possibility of the central tail-feathers of the male 

 varying in whiteness, there is nothing strange in such 

 variations having been sexually selected. The white 

 tips, together with the small white ear-tuits, certainly 

 add, as the Duke of Argyll admits, to the beantj^ of the 

 male ; and whiteness is apparently appreciated by other 

 birds, as may be inferred from such cases as the snow- 

 white male of the Bell-bird. The statement made by 

 Sir K. Heron should not be forgotten, namely that his 

 peahens, when debarred from access to the pied peacock, 

 would not unite with any other male, and during that 

 season produced no offspring. Nor is it strange that 

 variations in the tail-feathers of the Urgsticte should 

 have been specially selected for the sake of ornament, 

 for the next succeeding genus in the family takes its 

 name of Metallura from the splendour of these feathers. 

 Mr. Gould, after describing the peculiar plumage of the 

 Urosticte, adds, '•' that ornament and variety is the sole 



