At the other end of our continent a train of events not wholly dissimilar was in motion. Even in the latter part of the Pliocene some South American edentates, such as _Megalonyx_, _Mylodon_, and _Glyptotherium_, had reached Texas. Probably a little later the bridge had become widened so that other edentates and a few genera of South American hystricine rodents swarmed into our southern borders. At the same time a host of carnivores, tapirs, horses, camels, peccaries, deer, and cricetine and sciurine rodents made their way into South America. It is now certain that the land bridge over which the interchange took place did not include the West Indies. Possibly there yet remained along the western coast of Central America some of the border, now submerged, which Schuchert (Bull. Geol. Soc. Amer., vol. XX, plates 96 to 100) represents as being present during the Tertiary.
VI. THE SOURCES OF THE VERTEBRATES OF THE PLEISTOCENE.
The Pleistocene vertebrate fauna of North America has been derived from three sources. One component had descended from the animals which occupied the continent during the late Tertiary, but even these were of mixed derivation. A few appear to have filtered in from South America during the Pliocene; others had come from Asia during Tertiary invasions; but a large element was native to the country. As such may be taken the camels, the peccaries, the three-toed horses, the prong-horn antelope, the deer of the genus _Odocoileus_.
Upon a continent of vast extent and great fertility, possessing unbounded variety of climate and habitat, all these animals were thrown together to struggle for their existence. We must depend upon the imagination to picture what the result would have been if nature had pursued a course which might have been predicted. What the result in reality was, we shall see.
VII. THE RICHNESS OF THE PLEISTOCENE VERTEBRATE LIFE.
It will be profitable to consider briefly the character of the Pleistocene vertebrate fauna. The writer has compiled a list of the species which have, so far as he knows, been collected and described up to this time. There are in all 637 species; of these, 387 belong to the mammals, 154 to the birds, 26 only to the reptiles, 7 to the amphibians, 56 to the bony fishes, and 7 to the group of sharks and rays. Certainly these form only a part of the species which existed. At present there are known in our existing fauna north of Mexico 693 species of mammals, excluding the cetaceans—somewhat more than twice the number of known Pleistocene species. It is, however, rather in the great variety of forms that the Pleistocene excelled. Following Gerrit S. Miller’s Land Mammals of North America, 1912, we find in our present fauna north of Mexico 29 families; in the Pleistocene there are now known 37 families, not including the cetaceans. In our existing mammalian fauna there are recognized 111 genera; in the Pleistocene, with hardly half as many species recorded, 138 genera are counted. In order to realize more vividly the variety of Pleistocene forms, we have only to recall the animals then present, now absent, namely, the great ground-sloths, the glyptodons, the numerous species of horses, tapirs, numerous peccaries, camels, the extinct relatives of the musk-oxen, extinct bisons, elephants, mastodons of three or four genera, the giant beaver, and the saber-tooth tigers. Among the birds, reptiles, batrachians, and fishes, there were few striking forms, and these were mostly among the birds and the tortoises.