436 THE DESCENT OP MAN. 



shown to many persons, and all have admitted that they are 

 beautiful, some thinking them even more so than the ball-and- 

 socket ocelli. As the secondary plumes become lengthened 

 through sexual selection, and as the elliptic ornaments increased 

 in diameter, their colors apparently became less bright; and 

 then the ornamentation of the plumes had to be gained by an 

 improvement in the pattern and shading; and this process was 

 carried on until the wonderful ball-and-socket ocelli were finally 

 developed. Thus we can understand — and in no other way as 

 it seems to me — the present condition and origin of the orna- 

 ments on the wing-feathers of the Argus pheasant. 



From the light afforded by the principle of gradation — from 

 what we know of the laws of variation — from the changes which 

 have taken place in many of our domesticated birds — and, lastly, 

 from the character (as we shall hereafter see more clearly) of the 

 immature plumage of young birds — we can sometimes indicate 

 with a certain amount of confidence, the probable steps by which 

 the males have acquired their brilliant plumage and various or- 

 naments; yet in many cases we are involved in complete darkness. 

 Mr. Gould several years ago pointed out to me a humming-bird, 

 the Urosticte benjamlni, 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, whilst the female has the 

 six exterior feathers ornamented 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 spe- 

 cies alone, so as to have acqu.red 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 beau- 



»= 'The Reign of Law,' 1867, p. 247. 



