CHAP. XV.] 
THE NEAECTIC KEGION. 
121 
genera of these groups, a proportion of about two-sevenths. 
This is the smallest proportion of peculiar 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 Nearetic 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 Palaearctic region by its Pro- 
eyonidse, or racoons, Hesperomys , or vesper mice, and Didelphys, 
or opossums, among Mammalia ; by its Yireonidse, or greenlets, 
Mniotiltidae, or wood-warblers, Icteridse, or hang-nests, Tyran- 
nidae, 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, Iguanidae, and Cinosternum , 
among reptiles ; and by its Siluridse, and Lepidosteidse, among 
fishes. Prom 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 Arvicolm, or 
voles; its Seiuropterus, or flying squirrels; Tarnias, 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- 
mandridse, among Amphibia ; and its Gasterosteidge, Atherinidse, 
Esocidse, Umbridse, Aceipenseridae, and Polydontidse, 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 
