Darwin's Theory of Sexual Selection oak 
ences amongst animals. Other attempts to explain these 
differences have been equally unsuccessful. Thus Wallace 
accounts for them as due to the excessive vigor of the male, 
but Darwin’s reply to Wallace appears to show that this is 
not the cause of the difference. He points out that, while 
the hypothesis might appear plausible in the case of color, 
it is not so evident in the case of other secondary sexual 
characters, such, for instance, as the musical apparatus of the 
males of certain insects, and the difference in the size of the 
larynx of certain birds and mammals. 
Darwin’s theory served to draw attention to a large num- 
ber of most interesting differences between the sexes, and, 
even if it prove to be a fiction, it has done much good in 
bringing before us an array of important facts in regard to 
differences in secondary sexual characters. More than this I 
do not believe it has done. The theory meets with fatal ob- 
jections at every turn. 
In a later chapter the question will be more fully discussed 
as to the sense in which these secondary sexual differences 
may be looked upon as adaptations. 
