To revert to the case of dogs, we find free interbreeding between races descended from different secondary ancestors, such as wolves, jackals, and foxes, though freer, I believe, and more permanent as the races are closer; but as the specific differences become more marked, the fertility does not abruptly cease, but rapidly diminishes. Thus Buffon's experiment shows that a hybrid cross between the dog and the wolf may be produced and perpetuated for at least three generations, and the leporine cross between the hare and rabbit is almost an established race. On the other hand, we see in the mule the last expiring trace of fertility in a cross between species which have diverged so far in different directions as the horse and the ass. The human race repeats this lesson of the animal world, and shows a graduated scale of fertility and permanence in crosses, between different types according as they are closely or distantly related. Thus if we take the two extremes, the blond white of North temperate Europe and the Negro of Equatorial Africa, the disposition to union is almost replaced by repugnance which is only overcome under special circumstances, such as slavery, and an absence of women of their own race; while the offspring, the mulatto, is everywhere a feeble folk, with deficient vitality, diminished fertility, and prone to die out, or revert to one or other of the original types. But where the types are not so extremely divergent the fertility of the cross increases, as between the brunet white of Southern Europe and the Arab or Moor with the Negro, and of the European with the native Indian of America. Perhaps the strongest argument for polgyeny is that derived from the different constitutions of different races as regards susceptibility to climatic and other influences.