168 Evolution and Adaptation 
ary sexual differences between the sexes by the principle of 
sexual selection. 
There are two ways in which Darwin supposes sexual se- 
lection to act: (1) through competition of the individuals of 
the same sex with each other, — the strongest or best-equipped 
for fighting or for finding the individuals of the other sex 
gaining an advantage ; (2) through selection by the individu- 
als of one sex of certain preferred individuals of the other sex. 
The first category is natural selection applied to the members| 
of one sex in competition with each other, although 1 the ‘result 
‘does not lead to the death of the unsuccessful individual, 
but excludes it from leaving progeny. In the second cate- 
gory a new element is introduced, namely, the selective power 
of the individuals of one sex, usually ‘the female. “It is this 
part that adds a distinctly new element to Darwin’s other 
two theories of selection, and it is this part that we naturally 
think of. as the _theory of sexual selection sar excellence. 
Darwin makes, however, no sharp distinction between these 
two sides of his theory, but includes both under the heading 
of sexual selection. 
In order to get the theory itself before us in as concrete 
form as possible, let us examine some of the cases that 
Darwin has given to show how he supposes the process to 
be carried out. 
“There are many other structures and instincts which must 
have been developed through sexual selection — such as the 
weapons of offence and the means of defence of the males for 
fighting with and driving away their rivals —their courage 
and pugnacity — their various ornaments — their contrivances 
for producing vocal or instrumental music — and their glands 
for emitting odors, most of these latter structures serving 
only to allure or excite the female. It is clear that these 
characters are the result of sexual and not of ordinary selec-. 
HOD, since _unarmed, unornamented, or lnatttactive: moles 
