Cunningham's Theory 



brilliant plumage, spurs on the legs, and combs, 

 wattles, or other excrescences on the head. In 

 the Columbidae per contra the males are not 

 polygamous, but pair for life, the males do not 

 fight, and share equally with the females in 

 parental duties. 



" Corresponding with this contrast of sexual 

 habits is the contrast of sexual dimorphism, 

 which is virtually absent in the Columbidse. 



" I think, then, the only scientific explanation 

 is that the difference of habits is the cause of 

 the sexual dimorphism, and that the special 

 sexual habits which occur in some species but 

 not in others are the causes of the sexual char- 

 acters. . . . The habits in question always involve 

 certain definite stimulations applied to those parts 

 of the body whose modification constitutes the 

 somatic sexual characters. The stimulations are 

 confined, as the characters are confined, to one 

 sex, to one period of life, to one season of the 

 year, to those animals which have the characters, 

 to those parts of the body which are modified." 

 Mr Cunningham believes that these stimulations 

 cause hypertrophy or excessive growth of the 

 part affected, and that this peculiarity is trans- 

 mitted to the offspring. And thus he supposes 

 all the ornaments and excrescences of the males 

 of various species to have arisen. 



As evidence in favour of his view, he points 

 out that these excrescences are, in many species, 



