Darwin's Theory of Sexual Selection 201 
section dealing with cases in which the young in their first 
plumage differ from each other according to sex, the young 
males resembling more or less closely the adult males, and 
the young females more or less closely the adult females: 
“Two humming-birds belonging to the genus Eustepha- 
nus, both beautifully colored, inhabit the small island of Juan 
Fernandez, and have always been ranked as specifically dis- 
tinct. But it has lately been ascertained that the one which 
is of a rich chestnut-brown color with a golden-red head, is 
the male, whilst the other, which is elegantly variegated with 
green and white with a metallic-green head, is the female. 
Now the young from the first somewhat resemble the adults 
of the corresponding sex, the resemblance gradually becom- 
ing more and more complete. 
“In considering this last case, if as before we take the plu- 
mage of the young as our guide, it would appear that both 
sexes have been rendered beautiful independently ; and not 
that one sex has partially transferred its beauty to the other. 
The male apparently has acquired his bright colors through 
sexual selection in the same manner as, for instance, the pea- 
cock or pheasant in our first class of cases; and the female 
in the same manner as the female Rhynchzea or Turnix in 
our second class of cases. But there is much difficulty in 
understanding how this could have been effected at the same 
time with the two sexes of the same species. Mr. Salvin 
states, as we have seen in the eighth chapter, that with cer- 
tain humming-birds the males greatly exceed the females in 
number, whilst with other species inhabiting the same coun- 
try the females greatly exceed the males. If, then, we might 
assume that during some former lengthened period the males 
of the Juan Fernandez species had greatly exceeded the 
females in number, but that during another lengthened 
period’ the females had far exceeded the males, we could 
understand how the males at one time, and the females at 
another, might have been rendered beautiful by the selection 
