Darwin's Theory of Sexual Selection 171 
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. Jenner 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 :— 
