Secondary Sexual Characters 443 



and jealousy." Human song has also been the result of court- 

 ship and rivalry. 



The theory demands that the females select the most beautiful 

 males, and this implies either that the taste or appreciation of the 

 female must always improve in advance of the improvement in 

 the secondary sexual characters of the male, or that the female 

 is gifted with this appreciation from the beginning. If the for- 

 mer view is held, one may ask how has her taste been improved ; 

 if the latter is held, one may ask how she has come to be so gifted. 

 The theory of sexual selection can give no answer to either alter- 

 native, yet cannot justify itself without some such assumptions. 

 Other objections might be urged, but these will suffice, I think, 

 to show how improbable it is that the secondary sexual char- 

 acters can be accounted for on the principle of female selection. 



Wallace, who suggested the theory of natural selection si- 

 multaneously with Darwin, has never accepted the theory of 

 female selection, and has attempted to explain the facts in a 

 different way. In his essay on "Tropical Nature," published 

 in 1878, he first developed his view, but has given the subject 

 much fuller treatment in his recent book on " Darwinism." He 

 assumes that there is a tendency for the males of most animals, 

 especially birds and insects, to develop a greater intensity of 

 color, and this "tendency" is ascribed to the greater "vitality" 

 of the male. In the female, especially amongst birds, the de- 

 velopment of color is rigorously kept down by natural selec- 

 tion, especially in those species that build open or exposed nests. 

 In such cases the female sitting on the nest would be exposed to 

 the attacks of enemies were she brightly colored. In support 

 of this latter view, Wallace points out, the species in which both 

 sexes are equally colored, the kingfishes, woodpeckers, toucans, 

 parrots, turacos, hangnests, and starlings, make their nests in 

 holes in trees, or in the ground, or build a dome over them. The 

 female is in consequence not exposed on the nest. Conversely, 

 when the male is brilliantly colored and the female plain the nest 

 is open. The color, Wallace states, must have adapted itself to 

 the instinct, and not the instinct to the color, because habits 



