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 Rhynchaea 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 



