BIRDS. 503 



male has four central, while the female has the six exterior 

 feathers ornamented with white tips. What makes the 

 case more curious is that, although the coloring in the tail 

 differs remarkably in both sexes of many kinds of hum- 

 ming-birds, Mr. Grould does not know a single species, 

 besides the Urosticte, in which the male has the four central 

 feathers tipped with white. 



The Duke of Argyll, in commenting on this case,* 

 passes over sexual selection, and asks: ** What explanation 

 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 abrapt, as in tlie case 

 recently given of the humming-birds near Bogota, in which 

 certain individuals alone have the " central tail-feathers 

 tipped with beautiful green." In the female of the Urosticte 

 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-tuffs, certainly add, as the Duke of Argyll admits, to 

 the beauty of the male ; and whiteness is apparently appre- 

 cited by other birds, as may be inferred from such cases as 

 the snow-white male of the bell-bird. The statement made 

 by Sir R. Heron should not be forge tten, 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 Urosticte 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 

 splendor of these feathers. AVe liave, moreover, good 

 evidence that Iiumming-birds take especial pains in dis- 



*" The Reign of Law," 1867, p. 247. 



