Cunningham’s Theory 
~~ brilliant plumage, spurs on the legs, and combs, 
wattles, or other excrescences on the head. In 
the Columbidz fer 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 Columbide. 
‘“T 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, 
33! 
