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the Mississippi and west of the Sierra Nevada, bear but a small proportion to its whole area. Again, we know that at a very recent period geologically, it was subjected to a very severe Glacial epoch, which wrapped a full half of it in a mantle of ice, and exterminated a large number of animals which previously inhabited it. Taking all this into account, we need not be surprised to find the Nearctic region somewhat less rich and varied in its forms of life than the Palaearctic or the Australian regions, with which alone it can fairly be compared. The wonder rather is that it should be so little inferior to them in this respect, and that it should possess such a variety of groups, and such a multitude of forms, in every class of animals. Zoological characteristics of the Nearctic Region.—Temperate North America possesses representatives of 26 families of Mammalia, 48 of Birds, 18 of Reptiles, 11 of Amphibia, and 18 of Fresh-water Fish. The first three numbers are considerably less than the corresponding numbers for the Palaearctic region, while the last two are greater—in the case of fishes materially so, a circumstance readily explained by the wonderful group of freshwater lakes and the noble southward-flowing river system of the Mississippi, to which the Palaearctic region has nothing comparable. But although somewhat deficient in the total number of its families, this region possesses its full proportion of peculiar and characteristic family and generic forms. No less than 13 families or sub-families of Vertebrata are confined to it, or just enter the adjacent Neotropical region. These are:-three of mammalia, Antilocaprinae, Saccomyidae and Haploodontidae; one of birds, Chamaeidae; one of reptiles, Chirotidae; two of amphibia, Sirenidae and Amphiumidae; and the remaining six of fresh-water fishes. The number of 'peculiar or characteristic genera is perhaps more important for our purpose; and these are very considerable, as the following enumeration will show. Mammalia.-Of the family of moles (Talpidae) we have 3 peculiar genera: Condylura, Scapanus, and Scalops, as well as the remarkable Urotrichus, found only in California and Japan. In the weasel family (Mustelidae) we have Lataa, a peculiar kind of otter; Taxidea, allied to the badgers; and one of the
remarkable and characteristic skunks is separated by Dr. J. E. Gray as a genus—Spilogale. In the American family Procyonidae, a peculiar genus (Bassaris) is found in California and Texas, extending south along the mountains of Mexico and Guatemala. Eumetopias, and Halicyon, are seals confined to the west coast of North America. The Bovidae, or hollow-horned ruminants, contain three peculiar forms; Antilocapra, the remarkable prong-buck of the Rocky Mountains; Aplocerus, a goatlike antelope; and Ovibos, the musk-sheep, confined to Arctic America and Greenland. Among the Rodents are many peculiar genera: Neotoma, Sigmodon, and Fiber, belong to the Muridae, or rats; Jaculus to the Dipodidae, or jerboas. The very distinct family Saccomyidae, or pouched rats, which have peculiar cheek pouches, or a kind of outer hairy mouth, consists of five genera all confined to this region, with one of doubtful affinities in Trinidad and Central America. In the squirrel family (Sciuridae), Cynomys, the prairie-dogs, are peculiar; and Tamias, the ground squirrel, is very characteristic, though found also in North Asia. Haploodon, or sewellels, consisting of two species, forms a distinct family; and Erethizon is a peculiar form of tree porcupine (Cercolabidae). True mice and rats of the genus Mus are not indigenous to North America, their place being supplied by a distinct genus (Hesperomys), confined to the American continent. Birds—The genera of birds absolutely peculiar to the Nearctic region are not very numerous, because, there being no boundary but one of climate between it and the Neotropical region, most of its characteristic forms enter a short distance within the limits we are obliged to concede to the latter. Owing also to the severe winter-climate of a large part of the region (which we know is a comparatively recent phenomenon), a large proportion of its birds migrate southwards, to pass the winter in the West-Indian islands or Mexico, some going as far as Guatemala, and a few even to Venezuela. In our chapter on extinct animals, we have shown, that there is good reason for believing that the existing union of North and South America is a quite recent occurrence; and that the separation was effected by an arm of the sea across what is now Nicaragua, with perhaps another at Panama. This would leave Mexico and Guatemala joined to North America, and forming part of the Nearctic region, although no doubt containing many Neotropical forms, which they had received during earlier continental periods ; and these countries might at other times have been made insular by a strait at the isthmus of Tehuantepec, and have then developed some peculiar species. The latest climatal changes have tended to restrict these Neotropical forms to those parts where the climate is really tropical; and thus Mexico has attained its present strongly marked Neotropical character, although deficient in many of the most important groups of that region. .
In view of these recent changes, it seems proper not to draw any decided line between the Nearctic and Neotropical regions, but rather to apply, in the case of each genus, a test which will show whether it was probably derived at a comparatively recent date from one region or the other. The test referred to, is the existence of peculiar species of the genus, in what are undoubtedly portions of ancient North or South America. If, for example, all the species of a genus occur in North America, some, or even all, of them, migrating into the Neotropical region in winter, while there are no peculiar Neotropical species, then we must class that genus as strictly Nearctic; for if it were Neotropical it would certainly have developed some peculiar resident forms. Again, even if there should be one or two resident species peculiar to that part of Central America north of the ancient dividing strait, with an equal or greater number of species ranging over a large part of Temperate North America, the genus must still be considered Nearctic. Examples of the former case, are Helminthophaga and Myiodioctes, belonging to the Mniotiltidæ, or wood-warblers, which range over all Temperate North America to Canada, where all the species are found, but in each case one of the species is found in South America, probably as a winter migrant. Of the latter, are Ammodramus and Junco (genera of finches), which range over the whole United States, but each have one peculiar species in Guatemala. These
may be claimed as exclusively Nearctic genera, on the ground that Guatemala was recently Nearctic; and is now really a transition territory, of which the lowlands have been invaded and taken exclusive possession of by a Neotropical fauna, while the highlands are still (in part at least) occupied by Nearctic forms. In his article on “Birds,” in the new edition of the “Encyclopædia Britannica” (now publishing), Professor Newton points out, that the number of peculiar genera of Nearctic birds is much less than in each of the various sub-divisions' of the Neotropical region; and that the total number of genera is also less, while the bulk of them are common either to the Neotropical or Palaearctic regions. This is undoubtedly the case if any fixed geographical boundary is taken; and it would thus seem that the “Nearctic” should, in birds, form a sub-region only. But, if we define “Nearctic genera” as above indicated, we find a considerable amount of speciality, as the following list will show. The names not italicised are those which are represented in Mexico or Guatemala by peculiar species:—
LIST of TYPical, NEARCTIC GENERA of LAND BIRDs.
1. Oreoscoptes 17. Phoenopepla 33. Empidias
The above are all groups which are either wholly Nearctic or typically so, but entering more or less into the debatable ground of the Neotropical region; though none possess any peculiar species in the ancient Neotropical land south of Nicaragua. But we have, besides these, a number of genera which we are accus