200 SEXUAL SELECTION: BIEDS. Part IL 



tioQS in brightness which occurred in the females or in 

 the youns: would have been of no service to them, and 

 would not have been selected ; moreover, if dangerous, 

 would have been eliminated. Thus the females and the 

 young will either have been left unmodified, or, and 

 this has much more commonly occurred, will have been 

 partially modified by receiviug through transference from 

 the males some of the successive variations. Both sexes 

 have perhaps been directly acted on by the conditions 

 of life to which they have long been exposed ; but the 

 females from not being otherwise much modified will 

 best exhibit any such effects. These changes and all 

 others will have been kept uniform by the free inter- 

 crossing of many individuals. In some cases, especially 

 with ground birds, the females and the young may pos- 

 sibly have been modified, independently of the males, 

 for the sake of protection, so as to have acquired the 

 same dull-coloured plumage. 



Class II. When the adult female is more consjncuous 

 than the adult male, the young of hoth sexes in their first 

 plumage resemhle the adult male. — This class is exactly 

 the reverse of the last, for the females are here more 

 brightly coloured or more conspicuous than the males ; 

 and the young, as far as they are known, resemble 

 the adult males instead of the adult females. But the 

 difference between the sexes is never nearly so great 

 as occurs with many birds in the first class, and the 

 cases are comparatively rare. Mr. Wallace wdio first 

 called attention to the sinsfular relation which exists 

 between the less bright colours of the males and their 

 performing the duties of incubation, lays great stress on 

 this point,^^ as a crucial test that obscure colours have 



13 'Westminster Eeview,' Jul)\ 1S67, and A. Murray, 'Journal of 

 Travel,' 1868, p. 83. 



