CHAP. XV.] THE NEARCTIC REGION. 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 Palyearctic 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 Palsearctic region by its Pro- 

 cyonidae, or racoons, Hesperomys, or vesper mice, and Diddphys, 

 or opossums, among Mammalia ; by its Vireonidse, or greenlets, 

 Mniotiltidse, or wood- warblers, Icteridte, or hang-nests, Tyran- 

 nidse, or tyrant shrikes, and Trochilidoe, 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, Iguanidte, and Cinosternum, 

 among reptiles ; and by its Siluridse, and Lepidosteidee, 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 Sciuroptenis, or flying squirrels ; Tamias, or ground- 

 squirrels ; and Lacjomys, or marmots, among mammals ; its 

 numerous Paridee, or tits, and Tetraonidse, or grouse, among 

 birds ; its Trionychidse among reptiles ; its Proteidae, and Sala- 

 mandridse, among Amphibia ; and its Gasterosteidae, Atherinidae, 

 Esocidse, Umbridse, Accipenseridae, 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 



