Theory of Sexual Selection 



female. This Darwin saw. In order to account 

 for the phenomena of sexual dimorphism, he 

 formulated the theory of sexual selection. This 

 hypothesis is based on the assumption that there 

 is, in all species of animals, a competition among 

 the males to secure females as mates. It is not 

 difficult to understand how this competition 

 arises in polygamous species. Assuming that 

 approximately equal numbers of males and 

 females are born (an assumption which appears to 

 be justified as regards the majority of species), it 

 is clear that for every male who secures more 

 than one wife, at least one male will be obliged 

 to live in a state of single blessedness. 



But how can there be competition in the case 

 of monogamous species ? The sexes being ap- 

 proximately equal in number, there are sufficient 

 females to allow of a mate for every male. 



Such is the nature of things, said Darwin, that, 

 even under these circumstances, there is com- 

 petition among the males for females. 



" Let us take any species," he writes, on page 

 329 of The Descent of Man (Ed. 1901), "a bird 

 for instance, and divide the females inhabiting a 

 district into two equal bodies, the one consisting 

 of the more vigorous and better-nourished in- 

 dividuals, and the other of the less vigorous and 

 healthy. The former, there can be little doubt, 

 would be ready to breed in the spring before the 

 others ; and this is the opinion of Mr Jenner 



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