1 68 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 : (i) 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 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 pozver 

 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 par 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- 

 tion, since unarmed, unornamented, or unattractive males 

 would succeed equally well in the battle for life and in leaving 



