406- THE DESCENT OF MAN 



male, or to the male having retained, or perhaps recovered, 

 the primordial colors of the group. It also deserves notice 

 that in those groups in which the sexes differ the females 

 usually somewhat resemble the males, so that when the 

 males are beautiful to an extraordinary degree, the females 

 almost invariably exhibit some degree of beauty. From 

 the many cases of gradation in the amount of difference be- 

 tween the sexes, and from the prevalence of the same general 

 type of coloration throughout the whole of the same group, 

 we may conclude that the causes have generally been the 

 same which have determined the brilliant coloring of the 

 males alone of some species, and of both sexes of other 

 species. 



As so many gorgeous butterflies inhabit the tropics, it 

 has often been supposed that they owe their colors to the 

 great heat and moisture of these zones; but Mr. Bates' has 

 shown, by the comparison of various closely allied groups 

 of insects from the temperate and tropical regions, that this 

 view cannot be maintained; and the evidence becomes con- 

 clusive when brilliantly colored males and plain-colored 

 females of the same species inhabit the same district, feed 

 on the same food, and follow exactly the same habits of life. 

 Even when the sexes resemble each other, we can hardly 

 believe that their brilliant and beautifully arranged colors 

 are the purposeless result of the nature of the tissues and 

 of the action of the surrounding conditions. 



With animals of all kinds, whenever color has been modi- 

 fied for some special purpose, this has been, as far as we can 

 judge, either for direct or indirect protection, or as an attrac- 

 tion between the sexes. With many species of butterflies 

 the upper surfaces of the wings are obscure; and this in all 

 probability leads to their escaping observation and danger. 

 But butterflies would be particularly liable to be attacked 

 by their enemies when at rest; and most kinds while rest- 

 ing raise their wings vertically over their backs, so that the 



« "The Naturalist on the Amazons," vol. i., 1863, p. 19. 



