SEXUAL COLOUR-DIMORPHISM IN BIRDS. 29 



appendages will result from the ordinary action of natural selection in 

 preserving the most healthy and vigorous individuals, and the still fur- 

 ther selective agency of the sexual struggle in giving to the very strong- 

 est and most energetic the parentage of the next generation In 



many groups in which this superabundant energy is at a maximum, 

 the development of dermal appendages and brilliant colours has gone 

 on increasing till it has resulted in a great diversity between the sexes, 

 and in most of these cases there is evidence to show that natural selection 

 has caused the female to retain the primitive and more sober colours of 

 the group for the purposes of protection." 



Wallace is able to adduce much evidence in favour of this theory, 

 and his writings on the subject doubtless form a most valuable contribu- 

 tion to our knowledge of sexual dimorphism ; but it is, I venture to 

 say, absurd to pretend that the theory offers a complete explanation of 

 the phenomena in question. 



In the first place, it fails to explain why some species are sexually 

 dimorphic as regards plumage, while some are not. 



Were all birds which nest in holes or construct covered nests sexual- 

 ly monomorphic as regards plumage, and were all those which build 

 open nests, and of which only one sex performs the work of incubation, 

 sexually dimorphic, then, Wallace's theory would explain every- 

 thing. The need of protection of the sitting bird would of course 

 account for its duller plumage. 



Unfortunately for Wallace, many birds which nest in holes are sexu- 

 ally dimorphic, while many which nest in the open, and of which only 

 one sex performs the duties of incubation, are sexually monomorphic. 

 Further, there are some sexually dimorphic species, which build open 

 nests, and of which both the male and female birds sit alternatively on 

 the eggs. Darwin mentions the case of Pyranga (estiva, one of the 

 most splendid birds in the United States, where the male is vermillion, 

 and the female light greenish brown. As Darwin remarks, " if 

 brilliant colours had been extremely dangerous to birds whilst sitting 

 on their open nests, the males in these cases would have suffered 

 greatly." 



Again, the sexual dimorphism of many species is so slight, that I do 

 not think that it can possibly be accounted for by the greater need of 

 the female for protection, 



