Darwin s Theory of Sexual Selection i 7 1 



commonly exists. In most cases 'sexual selection appears 

 to have been effective in the following manner. 



" Let us take any species, 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 

 individuals, 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. Tenner Weir, who has carefully attended to 

 the habits of birds during many years. There can also be 

 no doubt that the most vigorous, best-nourished and earliest 

 breeders would on an average succeed in rearing the largest 

 number of fine offspring. The males, as we have seen, are 

 generally ready to breed before the females ; the strongest, 

 and with some species the best-armed of the males, drive 

 away the weaker ; and the former would then unite with 

 the more vigorous and better-nourished females, because 

 they are the first to breed. Such vigorous pairs would 

 surely rear a larger number of offspring than the retarded 

 females, which would be compelled to unite with the con- 

 quered and less powerful males, supposing the sexes to be 

 numerically equal ; and this is all that is wanted to add, in 

 the course of successive generations, to the size, strength 

 and courage of the males, or to improve their weapons." 



I shall comment later on the points here raised, but we 

 should not let this opportunity pass without noticing, that even 

 if the pairing were to follow according to the method here 

 imagined, still the argument breaks down at the critical 

 point, for there is no evidence that the more precocious 

 females would rear a larger number of offspring than the 

 more normal females, or even those that breed somewhat 

 later. 



The greater eagerness of the males which has been ob- 

 served in so many different classes of animals is accounted 

 for as follows : — 



