Chap. VIII. Sexual Selection. 213 



would tend to inherit similar instincts and constitutions. It 

 must be borne in mind that it would have been impossible to 

 change very materially the time of sexual maturity in the 

 females, without at the same time interfering with the period of 

 the production of the young— a period which must be determined 

 by the seasons of the year. On the whole there can be no doubt 

 that with almost all animals, in which the sexes are separate, 

 there is a constantly recurrent struggle between the males for 

 the possession of the females. 



Our difficulty in regard to sexual selection lies in understand- 

 ing how it is that the males which conquer other males, or those 

 which prove the most attractive to the females, leave a greater 

 number of offspring to inherit their superiority than their 

 beaten and less attractive rivals. Unless this result does follow, 

 the characters which give to certain males an advantage over 

 others, could not be perfected and augmented through sexual 

 selection. When the sexes exist in exactly equal numbers, the 

 worst-endowed males will (except where polygamy prevails), 

 ultimately find females, and leave as many offspring, as well 

 fitted for their general habits of life, as the best-endowed males. 

 From various facts and considerations, I formerly inferred that 

 with most animals, in which secondary sexual characters are 

 well developed, the males considerably exceeded the females in 

 number ; but this is not by any means always true. If the 

 males were to the females as two to one, or as three to two, or 

 even in a somewhat lower ratio, the whole affair would be 

 simple ; for the better-armed or more attractive males would 

 leave the largest number of offspring. But after investigating, 

 as far as possible, the numerical proportion of the sexes, I do not 

 believe that any great inequality in number 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 opinibn 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 

 eucceed in rearing the largest number of fine offspring. 7 The 

 males, as we have seen, are generally ready to breed before the 



7 Here is excellent evidence on an experienced ornithologist. Mr. 

 tiia character of the offspring from J. A. Allen, in speaking (' Mammals 



