222 SEXUAL SELECTION : BIEDS. Part IL 



will not pretend to say ; but the case is too remarkable 

 to be passed over without notice. 



We have now seen in numerous instances under all 

 six classes, that an intimate relation exists between the 

 plumage of the young and that of the adults, either of 

 one sex or both sexes. These relations are fairly well 

 explained on the principle that one sex — this being in 

 the great majority of cases the male — first acquired 

 throuo^h variation and sexual selection bright colours 

 or other ornaments, and transmitted them in various 

 wavs, in accordance with the recoirnised laws of inhe- 

 ritance. Why variations have occurred at different 

 periods of life, even sometimes with the species of the 

 same group, we do n()t know ; but with respect to 

 th.e form of transmission, one important determining 

 cause seems to have been the age at which the varia- 

 tions first appeared. 



From the principle of inheritance at corresponding 

 aaes, and from anv 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 A-ariations occurring at or near the period of 

 reproduction have been preserved, it follows that the 

 plumage of the young will often have been left unmo- 

 difie 1, or but little modified. AVe 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 both are 

 brightly coloured, at least during the breeding-season, 

 whilst the young are invariably less brightly coloured 

 than the adults, or are quite dull-coloured; for no in- 

 stance is known, as far as I can discover, of the young 

 of dull-coloured species displaying bright colours, or 



