Chap. VIII. 
SEXUAL SELECTION. 
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 107T in one 
year to 92*6 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. 
Eor 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 
