EVOLUTION OF THE COLORS OF BIRDS. 85 



the dances and tournaments which have been executed 

 for their gratification, select for their mates the males 

 that they find most pleasing, points strongly to the con- 

 clusion that the great differences in color and in orna- 

 ment between the males and females of these spiders are 

 the result of sexual selection." Indeed, anyone who 

 will attentively follow the account of these elaborate and 

 painstaking investigations of the courting habits of this 

 family of spiders, must admit that in this instance at 

 least, there is considerable reason to believe that sexual 

 selection has played an important part in the modifica- 

 tion of color. 



Among birds it is difficult if not impossible to secure 

 such definite results from observation. It is known, 

 however, that many male birds display their markings 

 before the female in a very elaborate way. Darwin 

 pointed this fact out as being especially confirmatory of 

 his theory of sexual selection. It is thus explained by 

 Wallace:^ "At pairing-time the male is in a state of 

 excitement, and full of exuberant energy. Even un- 

 ornamented birds flutter their wings or spread them out, 

 erect their tails or crests and thus give vent to the 

 nervous excitability with which they are overcharged. 

 It is not improbable that crests and other erectile 

 feathers may be primarily of use in frightening away 

 enemies, since they are generally erected when angry 

 or during combat. ^ ^'- "^ But if those portions of 

 the plumage which were originally erected under the 

 influence of anger or fear became largely developed and 

 brightly colored, the actual display under the influence 

 of jealousy or sexual excitement becomes quite intelligi- 

 ble." Mr. Wallace, in the above passage, has introduced 

 a new theory explaining the brilliant colors of male 



*l. c, p. 377. 



