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THE NEARCTIC REGION
(PLATE VI, p. 176) SECTION 1.-BOUNDARIES OF THE NEARCTIC REGION
With the Nearctic Region we enter upon a subject on which there has been a great deal of controversy among
the students of geographical distribution. As was pointed out in the introductory article of this volume, a certain number of writers maintain that this Region does not contain a sufficient quantity of distinctive and indigenous forms to entitle it to separation from the Palæarctic Region. What should constitute a sufficient number of distinctive forms depends, of course, largely on the individual opinions of the writers, but if allowance be made for the undoubted similarities of the extreme northern parts of the Old and New Worlds, which together may be considered as forming a kind of intermediate district, the facts and figures given below will, we think, convince every one that the land-surfaces of the Palæarctic and Nearctic Regions have now, and have had in the past, quite sufficiently distinct faunas to warrant their division into two primary Regions.
The boundaries of the Nearctic Region are comparatively simple. They embrace the whole of North America as far as the southern limit of the tableland of Mexico, with which Greenland may be included. On either side of 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