These characteristic features, taken in conjunction with the absolutely peculiar groups before enumerated, demonstrate that the Nearctic region cannot with propriety be combined with any other. Though not very rich, and having many disadvantages of climate and of physical condition, it is yet sufficiently well characterized in its zoological features to rank as one of the well-marked primary divisions of the earth's surface.
There is one other consideration bearing on this question which should not be lost sight of. In establishing our regions we have depended wholly upon their now possessing a sufficient number and variety of animal forms, and a fair proportion of peculiar types; but when the validity of our conclusion on these grounds is disputed, we may supplement the evidence by an appeal to the past history of the region in question. In this case we find a remarkable support to our views. During the whole Tertiary period, North America was, zoologically, far more strongly contrasted with South America than it is now; while, during the same long series of ages, it was always clearly separated from the Eastern hemisphere or the Palæarctic region by the exclusive possession of important families and numerous genera of Mammalia, as shown by our summary of its extinct fauna in Chapter VII. Not only may we claim North America as now forming one of the great zoological regions, but as having continued to be one ever since the Eocene period.
Insects.
In describing the Palæarctic and Neotropical regions, many of the peculiarities of the insect-fauna of this region have been incidentally referred to; and as a tolerably full account of the distribution of the several families is given in the Fourth Part of our work (Chapter XXI.), we shall treat the subject very briefly here.
Lepidoptera.—The butterflies of the Nearctic region have lately been studied with much assiduity, and we are now able to form some idea of their nature and extent. Nearly 500 species belonging to about 100 genera have been described; showing that the region, which a few years ago was thought to be very poor in species of butterflies, is really much richer than Europe, and probably about as rich as the more extensive Palæarctic region. There is, however, very little speciality in the forms. A considerable number of Neotropical types enter the southern States; but there are hardly any peculiar genera, except one of the Lycænidæ and perhaps a few among the Hesperidæ, The most conspicuous feature of the region is its fine group of Papilios, belonging to types (P. turnus and P. troilus) which are characteristically Nearctic. It is also as rich as the Palæarctic region in some genera which we are accustomed to consider as pre-eminently European; such as Argynnis, Melitæa, Grapta, Chionabas, and a few others. Still, we must acknowledge, that if we formed our conclusions from the butterflies alone, we could hardly separate the Nearctic from the Palæarctic region. This identity probably dates from the Miocene period; for when our existing arctic regions supported a luxuriant vegetation, butterflies would have been plentiful; and as the cold came on, these would move southwards both in America and Europe, and, owing to the long continuance of the generic types of insects, would remain little modified till now.
Coleoptera.—Only a few indications can be given of the peculiarities of the Nearctic coleoptera. In Cicindelidæ the region possesses, besides the cosmopolite Cicindela, four other genera, two of which—Amblychile and Omus—are peculiar to the West Coast and the Rocky Mountains. Of Carabidæ it possesses Dicælus, Pasimachus, Eurytrichus, Sphæroderus, Pinacodera, and a number of smaller genera, altogether peculiar to it; Helluomorpha, Galerita, Callida, and Tetragonoderus, in common with South America; and a large number of characteristic European forms.
The Lucanidæ are all of European types. The region is poor in Cetoniidæ, but has representatives of the South American Euphoria, as well as of four European genera. Of Buprestidæ it has the South American Actenodes; a single species of the Ethiopian and Eastern Belionota, in California; and about a dozen other genera of European and wide distribution.
Among Longicorns it possesses fifty-nine peculiar genera, representatives of five Neotropical, and thirteen Palæarctic genera; as well as many of wider distribution. Prionus is the chief representative of the Prionidæ; Leptura and Crossidius of the Cerambycidæ; Leptostylus, Liopus, Graphidurus, and Tetraopes, of the Lamiidæ, the latter genus being confined to the region.
Terrestrial and Fluviatile Mollusca.
The land-shells of temperate North America almost all belong to the Inoperculate or Pulmoniferous division; the Operculata being represented only by a few species of Helicina and Truncatella, chiefly in the Southern States. According to Mr. Binney's recent "Catalogue of the Terrestrial Air-breathing Mollusks of North America," the fauna consists of the following genera:—Glandina (6 sp.); Macrocyclis (5 sp.); Zonites (37 sp.); Vitrina (4 sp.); Limax (5 sp.); Arion (3 sp.); Ariolimax (3 sp.); Prophysaon (1 sp.); Binneia (1 sp.); Hemiphillia (1 sp.); Patula (16 sp.); Helix (80); Holospira (2 sp.); Cylindrella (2 sp.); Macroceramus (2 sp.); Bulimulus (8 sp.); Cionella (2 sp.); Stenogyra (4 sp.); Pupa (19 sp.); Strophia (1 sp.); Vertigo (6 sp.); Liguus (1 sp.); Orthalicus (2 sp.); Punctum (1 sp.); Succinea (26 sp.); Tebennophorus (1 sp.); Pallifera (1 sp.); Veronicella (2 sp.).