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2. Nearctic—those not found beyond the limits of the Nearctic Region.
3. Neogean—those found in the New, but not in the Old World.
4. Arctic and Palæogean—those which are found in the Old World, and only in the Canadian Sub-region of the New; and, finally
5. Neogean and Palæogean - containing the most widely distributed forms.
SECTION V.-THE WESTERN OR ARID SUB-REGION
This Sub-region is, on the whole, the richest of the three, both as regards the total number of genera found within its limits and also as regards the number of genera peculiar to it, which amounts to seven out of fifty-three, as compared with four out of forty in the Canadian, and one out of forty in the Eastern.
The Western Sub-region shares with the Eastern the only representative of the Marsupials found in North America, the Virginian Opossum. Just extending, too, to within its limits occurs the only member of the Order Edentata, the Nine-banded Armadillo (Tatusia novemcincta). Among the Ungulates, the Prong-buck (Antilocapra) is restricted to this Sub-region, and the genus Cervus of the cold Temperate Sub-region is replaced by the purely American genus Cariacus, of which the Black-tailed Deer is the representative.
This Sub-region is also more particularly the home of the American Bison, which, however, ranged even in historic times eastward nearly to the Atlantic seaboard. Among the Rodents there are no less than five endemic genera, of which, perhaps, the best known is Cynomys, the Prairiedog. The other endemic genera all belong to the family Geomyidæ, which contains a number of small Rodents known as Pocket-gophers.
Among the Carnivora there are no genera in the Western Sub-region which are not more or less widely distributed, the greater number of them, both in the case of this order and of the last, being also found in the Old World. Three genera of Moles belonging to the next order, Insectivora, though confined to North America, extend into the Eastern Sub-region. The Bats of the Arid Sub-region include among their members two genera (each with one species) which occur only in California, while four other genera are found only in the New World. One of these, Macrotus, is of special interest, since it contains the only member of a very large and well-marked family (Phyllostomatidae), which extends into the Nearctic Region from South America.
This Sub-region, as compared with the Canadian, contains a far larger proportion of Neotropical genera, and, in addition, is characterized by the absence of a number of the Palæarctic forms found only in the Canadian Sub-region. Such, for instance, are Rangifer (the Reindeer), Alces (the Elk), Haploceros (the mountain goat), Gulo (the glutton), and many others.
The following table, which gives a summary of the genera of this Sub-region, differs only from the summary of the Canadian Sub-region in the omission of the heading “Arctic and Palæarctic,” since practically all the genera common to this Sub-region and the Palæarctic Region are also found in the Canadian Sub-region.
SECTION VI.—THE EASTERN OR HUMID SUB-REGION
So far as peculiar forms go, the Eastern Sub-region is quite the least peculiar of the three. It contains only one genus strictly confined within its limits; this is Neofiber, with a single species commonly known as the “Round-tailed Musk-rat.” This Rodent is found only in Florida, and is much less completely aquatic in its habits than the true Musk-rat (Fiber), which is spread over the rest of North America.
Taking the orders seriatim, the Marsupials are represented by the widely spread Virginian Opossum; but the Edentates do not reach the Region at all. Of the Ungulates only the genus Cariacus (the Virginian Deer) occurs, the Bison (though formerly inhabiting this Sub-region) not having been seen east of the Mississippi for the last forty or fifty years.
The Rodents, as in the other Sub-regions, make up the great mass of the mammalian genera, numbering seventeen in all, including Neofiber.
The Carnivores, Insectivores, and Bats do not differ very markedly from those of the Western Sub-region.
On the whole the Eastern Sub-region is not a very well-marked division; it differs from the Canadian chiefly in the non-existence of the numerous northern Palæogean types found there, and from the Western Sub-region in the absence of a good many characteristic desert-haunting forms, and also of several of the South American genera, which have spread up northwards from the Nearctic Region into the Western Sub-region, but which have not reached the more distant Eastern.
The following table gives a summary of the genera of this Sub-region, from which it will be seen that the total number (forty) of genera is inarkedly less than the corresponding number in the Arid Sub-region.
SECTION VII.- THE PAST HISTORY OF THE NEARCTIC
During the last twenty years the wonderful discoveries of American palæontologists have thrown a flood of light, not only on the past history of the Nearctic Region, but also on the evolution of many of the mammalian groups themselves. It is, therefore, very necessary, when reviewing the geographical distribution of the present mammalian fauna, to shortly recapitulate the more important results and conclusions arrived at from their writings.
A very useful and comprehensive summary of this work will be found in a paper by Professor Zittel (3), lately published in the Geological Magazine. The beds which contain the remarkably perfect remains above alluded to are found only in the western part of North America. Here, apparently, there existed throughout the Tertiary Epoch a series of great fresh-water lakes, on the sides and the bottoms of which were formed an almost continuous series of deposits with the remains of the animals of the surrounding districts embedded in them. interest of these discoveries lies in the fact that we can here trace the gradual formation and evolution of several of the mammalian orders as they at present exist. In the oldest beds the mammals resemble one another so closely that it is often impossible to assign them very definitely to any of the existing orders, although the germs of the commencing distinctive characters can even here be traced.
In the later beds the various groups gradually differentiate themselves, until in the most modern of