"The mongrel of the third generation barks: it has pendent ears, raised tail: it is no longer wild. The mongrel of the fourth generation is entirely dog. Four generations, then, have sufficed to restore one of the two primitive types--the dog type; and four generations suffice also to restore the other type--the jackal type. Thus, when the mongrels produced from the union of two distinct species unite together, either become soon sterile, or they unite with one of the two primitive stocks, and they soon revert to this stock; in no case do they yield what may be called a new species, that is, an intermediate, durable species. "Whether, then, we consider the external causes,--the succession of time, years, ages, revolutions of the globe, or internal causes,--that is to say, the crossing of the species, the species do not alter, do not change, nor pass from one to the other; the species is fixed." Such are the conclusions of the admirable efforts of Flourens. "The imprint of each species," says Buffon, "is a type, the principal features of which are engraved in characters ineffaceable, and permanent forever; but all the accessory touches vary; no individual perfectly resembles another." XVII. Among the human families, the law of hybridity, which has been pointed out so clearly by Flourens, has also its fixed and inflexible rules; these rules have not been so well studied with men as with animals, but it is believed to have its limit at the seventh generation. At all events, the experiments of human hybridism, and acclimation in strange latitudes, have always in time ended in disaster; and that such will always be the fate of the attempted union of different races in unfavorable climes, have been the views of Humboldt, of Canning, of Guizot, and other profound statesmen. We observe among the races in savage life a natural repugnance to unite: as for instance, the negroes and the fairer people of the Philippine and Polynesian Isles show no disposition to unite; and though living side by side, in the same country, for a long period, they have not produced an intermediate race. Neither do the Eskimos nor the Red Men, neither do the Caffres nor the Hottentots mix, for in the state of nature the law of ethnic repugnance is supreme. It is only in the artificial and depraved states of society that hybrids appear, and their existence is of short and fixed duration. The apparent duration and perfection of the Coulouglis, the bipartates of the Bergers and Turks, may be an exception to the general rule. But the results of the mingling of human families, widely separated, is generally very decided.