Although the gradual decrease and ultimate extinction of the races of man is a highly complex problem, depending on many causes which differ in different places and at different times, it is the same problem as that presented by the extinction of one of the higher animals--of the fossil horse, for instance, which disappeared from South America, soon afterward to be replaced, within the same districts, by countless troops of the Spanish horse. The New-Zealander seems conscious of this parallelism, for he compares his future fate with that of the native rat, now almost exterminated by the European rat. Though the difficulty is great to our imagination, and really great, if we wish to ascertain the precise causes and their manner of action, it ought not to be so to our reason, as long as we keep steadily in mind that the increase of each species and each race is constantly checked in various ways; so that, if any new check, even a slight one, be superadded, the race will surely decrease in number; and decreasing numbers will sooner or later lead to extinction; the end, in most cases, being promptly determined by the inroads of conquering tribes. XI. SEXUAL SELECTION AS AN AGENCY TO ACCOUNT FOR THE DIFFERENCES BETWEEN THE RACES OF MAN. [Descent of Man, page 198.] We have thus far been baffled in all our attempts to account for the differences between the races of man; but there remains one important agency, namely, sexual selection, which appears to have acted powerfully on man, as on many other animals. I do not intend to assert that sexual selection will account for all the differences between the races. An unexplained residuum is left, about which we can only say, in our ignorance, that as individuals are continually born with, for instance, heads a little rounder or narrower, and with noses a little longer or shorter, such slight differences might become fixed and uniform, if the unknown agencies which induced them were to act in a more constant manner, aided by long-continued intercrossing. Such variations come under the provisional class, alluded to in our second chapter, which for the want of a better term are often called spontaneous. Nor do I pretend that the effects of sexual selection can be indicated with scientific precision; but it can be shown that it would be an inexplicable fact if man had not been modified by this agency, which appears to have acted powerfully on innumerable animals. It can further be shown that the differences between the races of man, as in color, hairiness, form of features, etc., are of a kind which might have been expected to come under the influence of sexual selection. STRUGGLE OF THE MALES FOR THE POSSESSION OF THE FEMALES. [Descent of Man, page 213.] 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.