The Descent of Man

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 colouring
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
(53. 'The Reign of Law,' 1867, p. 247.), 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-