570 TBE DESCENT OF MAS 



amoant of confidence, the probable steps by which the 

 males have acqaired their brilliant plumage and varioos 

 ornaments; yet in many cases we are involved in complete 

 darkness. Mr. Qonld several years ago pointed oat to me 

 a humming-bird, the Urosticte benjamini, remarkable for the 

 curious differences between the sexes. The male, besides 

 a splendid gorget, has greenish black tail-feathers, with the 

 four central ones tipped with white; in the female, as with 

 most of the allied species, the three outer tail-feathers on 

 each side are tipped with white, so that the male has the four 

 central, while the female has the six exterior feathers orna- 

 mented with white tips. What makes the case more curious 

 is that, although the coloring of the tail differs remarkably 

 in both sexes of many kinds of humming-birds, Mr. Gould 

 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 abrupt as in the case re- 

 cently 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 Uro- 

 sticte I noticed extremely minute or rudi mental 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 

 « "The Eeign of Law," 1867, p. 247. " 



