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Show 264 TilE PRINCIPLES OF PArtT II. incqtmlity is greater than with any other animal~ for during twelve years, out of 6878 uirths, the ~ale buths have been as 110·1 to 100 female births. It IS, however, in Rome degree doubtful whether it is safe to infer that th same proportional 1mmbers woul~ h?ld good u~der natural conditions as nnclcr domesticatiOn; for slight ·md unknown difl'cr nces in the conditions affect to a ~ertain e tent the proportion of the sexes. Thus with manl-ind, the mal bil'ths in England are. as 1?!·5, in Hussia us 10 ·D, and with the J ews of L1vorma as 120 to 100 females. The proportion is also mysteriously atrected by tho circumstance of the births being legiti-mate or illegitimate. . For our present purpose we m:e concerned w1th ~he proportion of tho sexes, not at buth, but at. ~atur1ty, and this adds another element of doubt.; for It IS a well ascertained fact that with man a considerably larger proportion of males than of females die b~fore or dmin? birth, and durino· the first few years of mfancy. So It almo t certainly is with male lambs, and so it may be with the male~ of other animals. The males of some animal kill each other by fighting; or they drire ntch other about until they become greatly emaciated. The~ must, also, whilst wandering about in eager search for the females, be often e:s:posed to \arion" dangers. \Yith many kinds of fish the males are much smaller than the femal , and they are belie\ed often to be dc,·oured by the latter r by other fishes. With some bird' the female appear to die in larger proportion than the males: they arc also liable to be de troyed on their nests, or whil -t in charo-e of their young. With in ect' the female htnrro ar' often larger than those of the males, and\ ould con~cqucntly be more likely to be <leYour d: in ome case~ the mature female are less ncti\-e and les rapid in their mo,-ements than the males, C':IAI'. YIII. 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 proportious 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 92·6 in another year, and with greyhounds from 11.6·3 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 selection. N evertbeless with some few wild animals, the proportions seem, as shewn in the supplement, to fluctuate 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 Gertain 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 everywhere able tb 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 les3 strong or less attractive. Polygamy.-The practice of polygamy leads to the |