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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