The Making of Species 



other characters which arise from the develop- 

 ment of these, or are correlated with them " 

 (Darwinism, p. 283). But the view that the 

 female selects the most beautiful of her suitors 

 has always seemed to Wallace "to be un- 

 supported by evidence, while it is also quite 

 inadequate to account for the facts." For 

 example, the accessory plumes of birds " usually 

 appear in a few definite parts of the body. We 

 require some cause to initiate the development in 

 one part rather than in another." 



Wallace considers that natural selection is 

 able to explain all the phenomena of sexual 

 dimorphism. He points out that, when the sexes 

 are dissimilar among birds, it is almost invariably 

 the female which is duller coloured. The reason 

 for this is, he believes, that the hen birds, while 

 sitting, "are exposed to observation and attack 

 by the numerous devourers of eggs and birds, 

 and it is of vital importance that they should be 

 protectively coloured in all those parts of the 

 body which are exposed during incubation. To 

 secure this, all the bright colours and showy 

 ornaments which decorate the male have not 

 been acquired by the female, who often remains 

 clothed in the sober hues which were probably 

 once common to the whole order to which she 

 belongs. The different amounts of colour ac- 

 quired by the females have no doubt depended 

 on peculiarities of habits and environment, and on 



322 



