CHAP. XV,] 
THE NEARCTIC REGION. 
131 
Baird says, that its fauna is almost identical with that of the 
Gila River, and has hardly any relation to that of Upper 
California. It possesses a considerable number (about twenty) 
of peculiar species of birds, but all belong to genera character- 
istic of the present sub-region ; and there is no resemblance to 
the birds of Mazatlan, just across the gulf in the Neotropical 
region. 
Reptiles, Amphibia, and Fishes. — A large number of snakes 
and lizards inhabit this sub-region, but they have not yet 
been classified with sufficient precision to enable us to make 
much use of them. Among lizards, Iguanidse, Geckotidse, 
Scincidm, and Zonuridee, appear to be numerous ; and many 
new genera of doubtful value have been described. Among 
snakes, Calamariidae, Colubridse, and Crotalidse are represented. 
Among Amphibia, Siredon , one of the Proteidse, is peculiar. 
The rivers and lakes of the Great Central Basin, and the 
Colorado River, contain many peculiar forms of Cyprinidae. 
III. The Eastern or Alleghany Sub-region. 
This sub-region contains examples of all that is most charac- 
teristic of Nearctic zoology. It is for the most part an undu- 
lating or mountainous forest-clad country, with a warm or 
temperate climate, but somewhat extreme in character, and 
everywhere abounding in animal and vegetable life. To the west, 
across the Mississippi, the country becomes more open, gradually 
rises, becomes much drier, and at length merges into the arid 
plains of the central sub -region. To the south, in Georgia, 
Florida, and Louisiana, a sub-tropical climate prevails, and 
winter is almost unknown. To the north, in Michigan and New 
England, the winters are very severe, and streams and lakes are 
frozen for months together. These different climates, however, 
produce little effect on the forms of animal life ; the species to 
some extent change as we go from north to south, but the same 
types everywhere prevail. This portion of the United States, 
having been longest inhabited by Europeans, has been more 
thoroughly explored than other parts of North America; and to 
this more complete knowledge its superior zoological richness 
