894 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
American mammals all but eight, or three-fourths, occur also in Eurasia, 
and but a single family is restricted to cold-temperate America. This 
family (the Aplodontide) is the sole representative of a group approach- 
ing extinction, and the accident of its survival (in a single genus and 
two closely related species) in a very limited area along our west coast 
san hardly be construed as of much faunal significance. Contrasted 
with this one family (which ought not to be counted) and eight genera 
of Boreal North American mammals not occurring in Eurasia, tropical 
North America (Central America and part of Mexico, exclusive of the 
West Indies) has no less than eight families and fifty-three genera not 
belonging to the immediately adjoining Sonoran region of the southern 
United States and the plateau of Mexico. 
THE SONORAN NOT A TRANSITION REGION. 
Before leaving this part of the subject reference should be made to 
the view recently advanced by some naturalists, notably by Angelo 
Heilprin, that the Sonoran region is itself a “Transition region” 
between the Boreal and Tropical faunas and floras. The incorrect- 
ness of this hypothesis is easily demonstrated, for it rests upon the 
assumption that the Sonoran region is a mixture of Boreal and Tropical 
forms. The contrary has just been shown to be the case, the hiatus 
between the Sonoran and Boreal on the one hand and the Sonoran and 
Tropical on the other being not only immense, but vastly greater than 
that between Boreal America and Eurasia. 
DIFFERENTIATION OF LIFE FROM THE NORTH SOUTHWARD. 
Animals and plants inhabiting the Arctic regions are usually spe- 
cifically identical throughout Arctic America, Greenland, and the polar 
parts of Eurasia and outlying islands, while as they diverge from the 
pole southward they tend to split up intomany species; in other words, 
Boreal species are more stable and persistent than those inhabiting 
warmer countries. The explanation of this fact is obvious. The iden- 
tity of climate and environment throughout the Arctic zone tends to 
preserve identity of specific characters, giving rise to a homogeneous 
fauna and flora, while the diversity of physical conditions and climatic 
influences prevailing in an increasing degree at greater distances from 
the pole exerts a powerful influence upon the various forms of life, 
producing first local geographic races or sub-species, then species, and 
finally groups of species constituting well-marked sub-genera and even 
genera, giving rise to greatly diversified faunas and floras. Thus 
among mammals the polar or ice bear (Thalarctos maritimus) has no 
very near relative, and is replaced in the tundras by the brown and 
barren-ground bears (Ursus arctos and richardsoni), which run into 
several more or less distinct forms, as the snow bear (U. isabellinus), 
Syrian bear (U. syriacus), and hairy-eared bear (U. piscator). Besides 
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