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A great advance is found in the Fauna of the Whiteriver beds of Miocene age. In this case the mammals can almost be referred to existing orders, but comparatively few of the genera are common to the Old and New Worlds; and it appears that, whilst during the older Eocene there was a considerable emigration of New World forms into Europe, in Miocene times the stream was reversed, and North America received the greater number of its immigrants from this side of the Atlantic. This immigration continued during the Middle Miocene epoch, the Fauna of which has been well preserved in the JohnDay beds of the extreme west. At the same time many endemic Families and forms are also met with, especially as regards the early predecessors of the Camel Family, which apparently had its origin and early development in the Nearctic Region, though now entirely absent from it . In the John-Day beds, we also find, for the first time, remains of the modern genera, Rhinoceros, Sciurus, Hesperomys, and Lepus.
The succeeding "Loup-Fork beds" contain additional recent genera, some of which, such as Equus and Cannelus, are now no longer found in the Nearctic Region, while others, such as Canis, Mustela, and Lutra, still remain there. On the whole, however, the Fauna of this epoch is still further removed from that of the corresponding period of the Old World than that of the preceding.
A little later, in the so-called "Equus-beds" of the Western States, and in the contemporaneous "Megalonyxbeds" of the Eastern, we first find a number of Neotropical forms, such as Mylodon (a gigantic Sloth), Glyptodon (a gigantic Armadillo), Hydrochoerus (the Capybara), and Toxodon, a member of a peculiar extinct family of Ungulates.
The occurrence of all these animals indubitably proves that now for the first time a connection had been formed between the continents of North and South America. Before this epoch, no trace of a Neotropical admixture can be anywhere detected in the Nearctic mammal-fauna.
Thus the evidence of palseontology in every way supports the deductions drawn from a study of the distribution of recent forms, namely, that the bulk of the present Nearctic fauna has been mainly derived from the Old World, although at times the Region has been sufficiently isolated and sufficiently extensive for the independent evolution of its own characteristic forms. In accordance with these deductions, the present remaining inhabitants of the Nearctic Region may be divided into three categories, as follows: (1) The Endemic Fauna, the bulk of which has had, at some considerably remote geological period, a common origin with that of the Palsearctic Region, although it has enjoyed ample time to develop and differentiate itself on its own lines. (2) A Neotropical constituent, which first appeared in the Nearctic Region in Pliocene times. (3) A comparatively modern Palsearctic fragment, in which not only the genera, but frequently the species, are identical in both Regions. This portion of the fauna has probably reached the Nearctic Region by the passage which must have existed in comparatively modern times across Behring Straits. Consequently, while the Neotropical element is the stronger in the south, this last, the Pahearctic element, is far more prevalent in the extreme north.
List Of The Principal Authorities Referred To
(1) Allen, J. A.—" The Geographical Distribution of North American Mammals." Bull. Amer. Mug. Nat. Hist., iv., p. 199. 1892.
(2) Merriam, C. H.—" The Geographical Distribution of Life in North America, with Special Reference to the Mammalia." Proc. Biol. Soc. Washington, vii., p. 1. 1892.
(3) Zittel, Karl Von.—"The Geographical Development, Descent, and Distribution of the Mammalia." Geol. Mag., 3rd ser., x., p. 401. 1893. (Translated from the S. B. k. Bayer. Akad. Wiss., xxiii., p. 137. 1893).