Chap. VIII. SEXUAL SELECTIOX. 265 



and would not be so well able to escape from danger. 

 Hence, with animals in a state of nature, in order to 

 judge of the proportions of the sexes at maturity, we 

 must rely on mere estimation ; and this, except perhaps 

 when the inequality is strongly marked, is but little 

 trustworthy. Nevertheless, as far as a judgment can 

 be formed, we may conclude from the facts given in the 

 supplement, that the males of some few mammals, of 

 many birds, of some fish and insects, considerably 

 exceed in number the females. 



The proportion between the sexes fluctuates slightly 

 during successive years : thus with race-horses, for every 

 100 females born, the males varied from 107*1 in one 

 year to 92*0 in another year, and with greyhounds from 

 116*3 to 95*3. But had larger numbers been tabulated 

 throughout a more extensive area than England, these 

 fluctuations would probably have disappeared ; and such 

 as they are, they would hardly suffice to lead under 

 a state of nature to the effective action of sexual selec- 

 tion. Nevertheless with some few wild animals, the 

 proportions seem, as shewn in the supplement, to fluc- 

 tuate either during different seasons or in different 

 localities in a sufficient degree to lead to such action. 

 For it should be observed that any advantage gained 

 during certain years or in certain localities by those 

 males which were able to conquer other males, or were 

 the most attractive to the females, would probably be 

 transmitted to the offspring and would not subsequently 

 be eliminated. During the succeeding seasons, when 

 from the equality of the sexes every male was every- 

 where able to procure a female, the stronger or more 

 attractive males previously produced would still have 

 at least as good a chance of leaving offspring as the 

 less strong or less attractive. 



Polygamy. — The practice of polygamy leads to the 



