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the deposits the genera can all be definitely assigned to existing orders.
The earliest mammals that have been found in North America come from the Trias of North Carolina, but neither these nor any of the other mammalian remains of the Secondary Period tend to assist the geographical problems involved, or are of importance in the present juncture.
With the oldest Tertiary beds an entirely new fauna appears, and furnishes us with remains of forms belonging to various orders of which no traces can be found in the earlier Secondary deposits. The following is a short list of these deposits, together with their European equivalents, so far as they can be ascertained :
LOWER FOCUE (Puerco beds of New Mexico.
Wasatch beds of Wyoming, Utah, and New
Mexico. Mid-EOCENE . . Bridger beds of Wyoming. UPPER EOCENE. . Uintah beds of Utah and Wyoming. LOWER MIOCENE . White-river beds of Nebraska, Dakota, Colo
rado, and Wyoming. MID-MIOCENE . . John-Daybeds of Oregon, Nevada, and
Washington. PLIOCENE ... Loup-Fork beds of Nebraska, Colorado, Wyo
ming, Kansas, and New Mexico. LATE PLIOCENE . Equus beds of Western and South-Western
States, and Megalonyx beds of Eastern
In the lowest Eocene beds, not only the Genera, but even the Orders of mammals are in almost every case different from those at present existing. The greater number of these belong to two orders—Creodontia and Condylarthra; the former the progenitors of the modern Carnivora, the latter of the existing Ungulata. These two, and indeed the other orders to which the mammals of this fauna have been assigned, all show considerable points of resemblance to one another, first in the possession of five toes on both limbs, which are provided with neither claws nor hoofs, but with a structure somewhat intermediate between the two, and, secondly, in their extremely small cerebral cavity. A similar, though much more incomplete Fauna has been found in certain beds of a corresponding age in Europe, the Genera of their fossil mammals being in most cases identical with those of the Nearctic Region.
In the next stage, the Wasatch beds, which correspond in age nearly to the London clay of England, a further development of the same fauna is found, with, however, the commencement of certain of the modern Orders; such, for instance, as the Perissodactyla (or Odd-toed Ungulates), the Rodents, the Insectivores, and the Lemurs. Here, too, so far as the scanty remains found in Europe allow us to form a comparison, there is a close similarity between the faunas of the two Regions.
In the succeeding “Bridger beds” of Mid-Eocene age is found the earliest evidence of the still surviving genus Didelphys (the Opossum). Here also marine Mammals and Bats appear for the first time. But comparison of these remains with European forms is even more difficult than in the last case, owing to the scarcity of such fossils in beds of the same age in Europe.
In the Uintah beds of the Upper Eocene we first begin to find very distinct traces of differentiation between the European and the North American faunas, although a good many of the Genera met with are still common to the two Regions.
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 Camelus, 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 palæontology 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 Palæarctic 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 Palæarctic 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 Palæarctic element, is far more prevalent in the extreme north.
LIST OF THE PRINCIPAL AUTHORITIES REFERRED TO
IN CHAPTER VI.
(1) ALLEN, J. A.—“The Geographical Distribution of North American Mammals.” Bull. Amer. Mus. 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. I. 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).