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the tableland of Mexico, the Neotropical Region extends as a narrow strip along the Gulf of Mexico and the Pacific, as far north as the Rio Grande on the former coast, and to about Guaymas on the latter.
There are no islands of any importance belonging to this Region that need be mentioned here. The Pacific islands on the west coast of Mexico have few, if any, mammals. The large islands of Newfoundland and Vancouver are of the true continental type, being separated from the mainland only by quite shallow water. The Antilles, or West India islands, belong entirely to the Neotropical Region, and have been already considered in a previous chapter.
SECTION II.-GENERAL VIEW OF THE MAMMAL-FAUNA OF THE NEARCTIC REGION
On referring to the table given at the end of the first chapter, it will be seen that the total number of genera, and also of species contained in the Nearctic Region, is considerably less than in any of the other Regions hitherto treated of. This may be explained partly by the fact that the whole of this Region is practically outside the tropics, whereas the other Regions previously described lie to a great extent within the tropical zone, which is very favourable to the development of a rich and varied fauna.
Out of the nine Orders into which the terrestrial Mammals have been divided, two only are not represented in this Region. These are the Primates, at the head of the list, and the Monotremes, at the extreme end, the latter being confined to the Australian Region. The Marsupials
are represented by one species only, the well-known Virginian opossum (Fig. 29), which is found with slight modifications from the Southern States of North America
southwards over the greater part of South America. This animal would, perhaps, judging merely by the present distribution of life, be considered to have intruded into the Nearctic Region from South America, where alone members of this family still survive; but, on examining its past
history, we ascertain that the genus Didelphys was formerly found both in Europe and in North America during Eocene and Miocene times, so that it is possible that the Virginian
opossum may be a survivor rather than an intruder in North America.
The next order, the Edentates, is represented in this
Region only by a single species of armadillo (Tatusia novem-cincta), which almost certainly came up from the south, and is only met with just inside the southern borders.
Among the Ungulates, the most remarkable form is the prong-buck (Antilocapra), which forms a distinct family of that order, and is entirely confined to this Region (Fig. 30, p. 156). It is allied in some respects to the Antelopes of the
Old World, but it is unique among all the hollow-horned ruminants from the fact that it sheds its horns every year.
Two other genera, belonging to the family Bovidæ, are confined to the Nearctic Region; these are the Rocky Mountain goat (Haploceros), found only in the Rocky
Mountains (Fig. 31, p. 157); and the Musk Ox (Ovibos), which ranges over the barren grounds at the extreme north of the continent, and spreads into Greenland (see Fig. 32). The latter, however, was also found in the northern parts of the Old World until a comparatively recent epoch, geologically speaking.