152 
SEXUAL selection: birds. 
Part D. 
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 Uros- 
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-tufts, certainly 
add, as the Duke of Argyll admits, to the beauty 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 E. 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 IJrosticte 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 sola 
