il.88 The Descent of Man. I'abt II 



somewhat losemble the iidulto of the corresponding sex, the resi mblance 

 gradually hecomiTig miire and more complete. 



Jn considering this last case, if as before we take the plumage of the 

 young as our guide, it 'soulil appear that both sexes have been rendered 

 beautiful indepemlently; ami nut that one sex has p.irtially transferred 

 its beauty to the other. The mil. apparently has acquired liis bright 

 colours through sexual selection in the same manner as, for instance, 

 the peacock or pheasant in our first class of onses ; and the female in 

 the same nianuer as f.lie female Riiynchasa or Turnix in our second 

 class of cases. But there is mucli difficulty in understanding how this 

 could have been eificted at the same tinje wiih tlie two sexes of the 

 fame species. Mr. Salvin states, as we have seen in the eighth chapter, 

 that with cert tin humniing-l.irds the males greatly exceed the females 

 in number, whilst with other species inhabiting the same country the 

 females greatly exored the males. If, then, we might assume that 

 during sonte former lengthened period the males of the Juan Fernandez 

 species had greatly exceeded the fi'males in mmber, but that during 

 another lengthened period the females had fur eioeeded the males, we 

 could undi-rstand how the males at one tiae, and the females at 

 another, niijht iiave bien rendrred beautiful by the selection of the 

 brighter-coloured individuals of either sex ; both sexes transmitting 

 their characters tn their y..ung at a rather earlier age than usual. 

 Whether this is the true explanation I will not pielend to say ; but the 

 case is too remarkable to be passed over witi.out notice. 



We have noT7 seen in all six classes, that an intimate relation 

 exists between the plumage of the young and the adults, either 

 of one sex or both. These relations are fairly well explained 

 on the principle that one sex — this being in the great majority 

 of eases the male — first acquired through variation and sexual 

 selection bright colours or other ornaments, and transmitted 

 them in various ways, in accordance with the recognised laws of 

 inheritance. Why -variations have occurred at different periods 

 of Ufe, even sometimes with species of the same group, we do not 

 know, but with respect to the form of transmission, one important 

 determining cause seems to be the age at which the variations 

 first appear. 



From the principle of inheritance at corresponding ages, and 

 from any variations in colour which occurred in the males at an 

 early age not being then selected — on the contrary being often 

 eliminated as dangerous — whilst similar variations occurring 

 at or near the period of reproduction have been preserved, it 

 follows that the plumage of the young will often have been left 

 unmodified, or but little modified. We thus get some insight 

 into the colouring of the progenitors of our existing species. In 

 a vast number of species in five out of our six classes of cases, 

 the adults of one sex or of both are bright coloured, at least 

 during the breeding-season, whilst the young are invariably less 

 brightly coloured than the adults, or are quite dull coloured 



