The matter may be looked at from another point of view. If Mylodon, Megalonyx, and Glyptotherium are referred to the Middle Pliocene, we shall probably have them recorded as living in Texas before they existed in South America. It is true that Santiago Roth (Neues Jahrb., Min. Beil., Bd., vol. XXVI, table opposite p. 144) states that Glyptodon occurs in the Lower Pampas beds, and these he refers to the Upper Miocene; but the writer believes that Wilckens (Neues Jahrb, Min. Beil., Bd., vol. XXI, p. 193) is more nearly correct in placing them in the Pliocene. While the opinion may be correct that, when no obstacles intervene, the time required for mammals to spread over even a continent constitutes but a small part of a geological age, yet in making their way from South America, especially from Argentina, along the narrow bridge that appears to have been offered them, probably over mountain ranges, and across rivers and gorges, and in the face of the competing fauna advancing from the north, some of which were wolves and saber-tooth tigers, the slowly plodding and inoffensive edentates would have encountered too many hindrances to be able to make the journey in a short time.
The writer, therefore, ventures to range the Blanco immediately below the Pleistocene. On about the same level may be placed the Tulare-Etchegoin and the Thousand Creek formations of Merriam (Bull. Dept. Geol. Univ. Calif., vol. X, pp. 425, 429).
III. The Historical Divisions of the Pleistocene.
The writer accepts the divisions of the Pleistocene which the geologists appear to have established. Formerly it was believed that North America had been subjected to a single glacial epoch; now it seems to be proved that there have occurred five such glacial epochs, or stages, and that there have intervened four interglacial stages of mild climate. The interglacial stages are italicized. The Nebraskan stage is the earliest, the Wisconsin the latest: Wisconsin, Peorian, Iowan, Sangamon, Illinoian, Yarmouth, Kansan, Aftonian, Nebraskan.
The characteristics of the various stages will be briefly discussed. The stages are not equally well understood and at present do not seem to be of equal importance in their relation to vertebrate paleontology.
IV. Elevations of the Continent Immediately Preceding or Accompanying the Opening of the Pleistocene.
In pursuing the study of the Pleistocene, one soon realizes that this period was one of great geological activity. Ranges of mountains, if not begun anew, were at least raised to greater altitudes. The Cascade Range appears to have begun to rear its head at the beginning of the epoch, or even a little later. Here and there the crust of the earth was ruptured and great sheets of lava were poured out over the land. Ice caps repeatedly accumulated over large areas in North America and Europe, and in their movements southward transported vast amounts of earthy débris and turned the courses of great streams. Apparently at times the rainfall was greatly increased. The rivers, quickened by greater slope and the increased volume of water, cut their channels deeper and in the mountains excavated profound gorges. Through elevation of the land North America was, late in the Pliocene or early in the Pleistocene, put into easy communication with Asia and South America, so that vertebrated animals passed freely to and fro. A part of these activities probably belonged to the latter part of the Pliocene. In the more elevated regions of the eastern United States, through the chemical, rupturing, and transporting properties of water, rocks were dissolved and their disintegrated materials produced what has been designated the Lafayette formation; but it is possible that this belongs to the early Pleistocene.
V. Connections with Asia and South America.
Mention has just been made of a land connection with Asia at some time about the beginning of the Pleistocene. The evidence for this may be called circumstantial rather than direct. The geological evidence has not been developed. If any deposits containing marine fossils had been laid down along the Asiatic and Alaskan coasts during a time of elevation, they would now be covered by the sea. Our evidence for the connection is derived from the distribution of the vertebrate animals. During the early Pleistocene our country was invaded by a host of mammals whose home was originally in Asia. These included elephants, bisons, elk, goats, bears, wolves, and foxes, besides many mammals of smaller size. It is the presence in America of the smaller animals, many genera of rodents of Asiatic origin, that shows that there must have been a land connection. These could not have made their passage across Bering Strait on the ice, as it might be imagined the larger animals did.
The way between the two continents had more than once before been open, but it was during the early Pleistocene that modern Asiatic genera entered North America in great numbers. Exactly where the land bridge between the two countries was situated is not certain; it may be that a large part of the area now occupied by Bering Sea was then dry land. Arldt (Entwicklung der Kontinente, plate 21) represents a connection extending from the northern border of Alaska southward to include the Aleutian Islands. Where narrowest, this bridge, as represented by the author named, extended from latitude 60° to 70°, a distance of about 700 miles. In such case the cold currents from the Arctic Ocean would have been prevented from entering the Pacific, while the Japan Current would have warmed up the southern side of the bridge. The route was then open on the north for the boreal animals of Asia to enter Alaska; while on the south the genera inhabiting the more temperate part of eastern Asia would have had free access to the American shore. Once on the continent, the boreal mammals might have spread along the shores of the Arctic Ocean and those of the temperate parts of Asia have made their way up the Yukon Valley, or possibly along the Pacific coast, to the warmer regions toward the south. We do not need to suppose that even during the first glacial, or Nebraskan, stage the climate of that part of North America was as inclement as now.