82 DARWINISM AND THE PROBLEMS OF LIFE 



I 



How could natural selection lead to a different colouring 

 of the two sexes ? 



Darwin put these questions long ago, and as an 

 answer to them he framed his theory of sexual 

 selection as the second great agency in the formation 

 of species. We shall understand this best if we compare 

 it with natural selection. 



Both by natural and sexual selection certain animals 

 have a better prospect than others of leaving offspring 

 and so preserving their species in the next generation. 

 In the one case it is those animals that have been 

 able to escape destruction longest, thus reaching the 

 period of reproduction, while the others die off before 

 it comes. In the second case it is the males (sexual 

 selection affects only the one sex) that attain to union 

 with a female, as many of them cari never reach love, 

 and therefore never reproduce. Natural selection, 

 therefore, determines which animals will reach the 

 period of reproduction ; and among the selected males 

 sexual selection then chooses those that are actually 

 to reproduce. In the one case the "bad" are 

 destroyed ; in the other case they are condemned 

 to sterility. In both cases their kind perishes with 

 them. 



Thus in natural and sexual selection a few are 

 chosen out of a large number of animals. This is 

 possible for natural selection because more animals 

 are produced than can live ; in sexual selection also 

 choice is only possible if the males are so numerous 

 that there are not enough females for all, and some 

 must go without. As a fact, we do find this pre- 



