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CHAP. XV.] THE NEAECTIC REGION. 121
genera of these groups, a proportion of about two-sevenths.
This is the smallest proportion of p"iliar genera we have found
in any of the regions; but many of the genera are of ' such
isolated and exceptional forms that they constitute separate
families, so that we have no less than 12 families of vertebrata
confined to the region. The Pahearctic region has only 3
peculiar families, and even the Oriental region only 12; so that,
judged by this test, the Nearctic region is remarkably well
characterized. We must also remember that, owing to the
migration of many of its peculiar forms during the Glacial
period, it has recently lost some of its speciality ; and we should
therefore give some weight to the many characteristic groups it
possesses, which, though not quite peculiar to it, form important
features in its fauna, and help to separate it from the other
regions with which it has been thought to be closely allied. It
is thus well distinguished from the Paleearctic region by its Pro-
cyonidae, or racoons, Hesperomys, or vesper mice, and Didelphys,
or opossums, among Mammalia ; by its Vireonidse, or greenlets^
Mniotiltidffi, or wood-warblers, Icteridse, or hang-nests, Tyran-
nidse, or tyrant shrikes, and Trochilidse, or humming-birds,
among birds, families which, extending to its extreme northern
limits must be held to be as truly characteristic of it as of the
Neotropical region ; by its Teidse, Iguanidse, and Cinostermim,
among reptiles ; and by its Siluridse, and Lepidosteidse, among
fishes. From the Neotropical region it is still more clearly
separated, by its numerous insectivora; by its bears; its Old
World forms of ruminants ; its beaver ; its numerous Arvicolce, or
voles ; its Sciuropterus, or flying squirrels ; Tamias, or ground-
squirrels ; and Lagomys, or marmots, among mammals ; its
numerous Paridse, or tits, and Tetraonidae, or grouse, among
birds ; its Trionychidse among reptiles ; its Proteidae, and Sala-
mandridae, among Amphibia ; and its Gasterosteidse, Atherinidee,
Esocidse, Umbridae, Accipenseridse, and Polydontidae, among
fishes.
These characteristic features, taken in conjunction with the
absolutely peculiar groups before enumerated, demonstrate that
the Nearctic region cannot with propriety be combined with