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.
The above account shows the great richness of the vertebrate life during the Pleistocene; furthermore, this abundance evidently existed during the early stages of the epoch. It constituted the materials on which that combination of conditions which we call environment had to work during Pleistocene times. The comparison shows that the result was an impoverishment of the vertebrate fauna. Genera and families, even orders, were wiped out of existence, and these included some of the noblest animals that have graced the face of the earth, the elephants, the mastodons, tapirs, many species of bison, horses, saber-tooth cats, huge tigers, and gigantic wolves. The following nine or ten families became either wholly extinct or continued to exist only in other more hospitable lands: the Megatheriidæ, including several genera of ground-sloths; the Hoplophoridæ or glyptodons; the Caviidæ, which embraced one or more species of huge capybaras; the Elephantidæ, under which are arranged three or four species of elephants and three genera of mastodons; the Equidæ, represented by a dozen or more species of horses; the Camelidæ, of which there were several Pleistocene species and probably three or four genera; the Hyænidæ, of which there appears to have been at least one genus, with one species; the Tapiridæ, including three or four species; and probably the Rhinocerotidæ. Besides these, the subfamily of Felidæ known as Machairodontinæ, embracing those wonderful carnivores the saber-tooth tigers, was suppressed. The Dasypodidæ, which included some armadillos 5 or 6 feet long, are now represented by only one small species in Texas. Of the Tagassuidæ, to which belonged several genera and stately species of peccaries, there exists now in North America north of Mexico but one species, an animal of only moderate size.
VIII. On Evolution During the Pleistocene.
We have seen that the Pleistocene fauna was very different from that which existed when white men first entered the country; also that the difference has in large part been due to the destruction of species, genera, and families. We may now inquire whether or not the loss has been to any considerable extent compensated by the development of new forms. Many of our existing genera and species have been found in the collections that represent the earliest Pleistocene known to us. The writer believes it would be unsafe to say that any living species that one might select may not hereafter be discovered in early Pleistocene collections. It is probably true, however, that some of those small changes by which we distinguish one species from another have been produced. Some small but persistent differences might, for example, have arisen in the teeth or in the form of the skull of a group of muskrats which would justify us in regarding it as forming a new species. It is extremely doubtful that any new genus of vertebrates has been developed since the first interglacial stage. Matthew has concluded (Science, n. s., vol. XL, pp. 232–235) that the evolution of the mammals during the Pleistocene amounts to about one-tenth of that achieved during the Pliocene. The present writer regards this as a liberal estimate.
This failure to evolve new genera and species is not necessarily to be attributed to the shortness of the Pleistocene period; it may have been due rather to the unfavorable conditions. In what direction could an animal make progress when, after being subjected for some thousands of years to one set of conditions, it was compelled for some other thousands to endure just the opposite conditions? If life in front of a glacier for some centuries led to the development of a coat of hair on an elephant, that coat would probably disappear during the succeeding interglacial stage, and in the end, if the elephant had not perished, he would be where he began.