216 The Descent of Man. Part IL, 



until they become greatly emaciated. They must also be often 

 exposed to various dangers, whilst wandering about in eager 

 search for the females. In many kinds of fish the males are 

 much smaller than the females, and they are believed often to be 

 devoured by the latter, or by other fishes. The females of 

 some birds appear to die earlier than the males ; they are 

 also liable to be destroyed on their nests, or whilst in charge 

 of their young. "With insects the female larvae are often larger 

 than those of the males, and would consequently be more likely 

 to be devoured. In some cases the mature females are less 

 active and less rapid in their movements than the males, and 

 could not escape so well from danger. Hence, with animals in a 

 state of nature, we must rely on mere estimation, in order to 

 judge of the proportions of the sexes at maturity ; and this is 

 but little trustworthy, except when the inequality is strongly 

 marked. 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, are considerably more numerous than the females. 



The proportion between the sexes fluctuates slightly during 

 successive years : thus with race-horses, for every 100 mares born 

 the stallions varied from 107*1 in one year* to 926 in another year, 

 and with greyhounds from 116 '3 to 95 - 3. But had larger num- 

 bers been tabulated throughout an area more extensive than 

 England, these fluctuations would probably have disappeared ; 

 and such as they are, would hardly suffice to lead to effective 

 sexual selection in a state of nature. Nevertheless, in the cases 

 of some few wild animals, as shewn in the supplement, the 

 proportions seem to fluctuate either during different seasons 

 or in different localities in a sufficient degree to lead to sucb 

 selection. For it should be observed that any advantage, 

 gained during certain years or in certain localities by those males 

 which were able to conquer their rivals, 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, 

 e very male was able to procure a female, the stronger or more at- 

 tractive males previously produced would still have at least as 

 goon a chance of leaving offspring as the weaker or less attractive. 



Polygamy. — The practice of polygamy leads to the same results 

 as would follow from an actual inequality in the number of the 

 sexes ; for if each male secures two or more females, many males 

 cannot pair ; and the latter assuredly will be the weaker or less 

 attractive individuals. Many mammals and some few birds are 



