forms also the separating line for the geological zones of the Pliocene epoch. All these are facts which render quite inevitable the idea of an origin of the higher organic species of to-day through descent. But still, from another side, animal geography, though it does not yet speak for a common pedigree of the whole animal world, as the facts just mentioned also do not, still at least speaks for a descent of related, though at present separated, genera and species from common forefathers. The continents of the Old and New World are so constructed that toward the North Pole they approach one another very closely, and toward the South Pole they withdraw from one another. Without doubt there existed in the North, through long periods of time, a land-connection of America with Asia and with Europe. Now, both continents have their more or less characteristic animal world, and these characteristics are distributed over the two halves of the globe in the following extremely remarkable way: The fauna of the Old and the New World, in those groups of animal genera which live only in the warmer or tropic zones or only south of the equator, and have no associates of genera or families in the higher North, is in each hemisphere entirely characteristic, and differs in a {71} marked way from the fauna of the other half of the globe. For instance, the rhinoceros, the hippopotamus, the giraffe, the antelope with undivided horns, the hedgehog, the mole proper, are only inhabitants of the Old World, whence also the horse originally came, the striped ones in Africa and the non-striped in Asia; on the other hand, the lemur, the ant-eater, the armadillo, and others, are limited to South America. The apes of the Old World have five molar teeth on each side of the jaw, narrow noses, tails usually short and never prehensile, and fleshy protuberances for sitting; the apes of the New World have six molar teeth, flat noses, and long prehensile tails. And on the contrary, where closely related species are found on both parts of the globe, they belong only to genera of which single species live or have lived in the far North; as, for instance, the rein-deer, still common to the Old and the New World in this very North which once formed a bridge between the two halves of the earth. The same is true in regard to cattle, the deer, the cat, the dog, the hare. Similar facts can also be shown of other animal classes. The farther the different species of these genera withdraw from the North Pole, the greater become the differences between the species on the one half of the globe and the analogous species of the other. Compare on this point K. E. von Baer's "Studien aus dem Gebiete der Naturwissenchaften, ueber Darwin's Lehre," ("Studies from the Realm of Natural Science upon Darwin's Teachings"), p. 356 f. If we add, further, the before mentioned fact, that those genera which are exclusively peculiar to one or the other continent, have their related predecessors in the