408 GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 
four genera of mammals alone,* and the North American Tropical from 
the Sonoran by ten families and upwards of fifty genera; while the 
American Boreal differs from the Eurasian Boreal by the possession of 
but a single family and only eight genera. 
MOUNTAINS AS BARRIERS TO DISPERSION. 
Wallace makes the surprising statement that on the two sides of the 
Rocky mountains in America “almost all the mammalia, birds, and 
insects are of distinct species,”{—a statement that is wholly untrue, as 
has been long known to American naturalists. In another place he 
makes the general statement that mountains, “ when rising to a great 
height in unbroken ranges, form an impassable barrier to many groups.” 
No instance of this kind is known in North America. Even in the high 
Sierra in California nearly all of the families, genera, and species occur 
on the east slope as well as on the west, notwithstanding the great alti- 
tude this lofty range maintains for a considerable distance.t The expla- 
nation of the similarity or identity of the species on the two sides of all 
our mountain systems is that similar or identical climatic zones occur 
on both sides, between which avenues of communication exist or have 
existed by means of passes, either through the ranges themselves or at 
oneend or the other. In their continuity, however, lofty mountain 
ranges do act as barriers to the spread of species from lower levels, but 
they do so indirectly by their effects upon climate—by interposing an 
arctic zone in which the species of lower latitudes can not live. On the 
other hand, this same arctic-alpine climate enables many polar species 
to thrive in regions two or three thousand miles south of their normal 
continental homes. 
The great Himalaya has little or no influence in bringing about the 
really enormous differences that exist between the faunas and floras of 
the plains on its two sides, for these dissimilarities are due primarily to 
the great difference of temperature resulting from unequal base level, 
the Thibetan plateau on the north being several thousand feet higher 
than the plain on the south. 
THE SO-CALLED EASTERN, CENTRAL, AND WESTERN PROVINCES AND 
THE EVIDENCE ON WHICH THEY ARE BASED. 
Wallace, in common with most recent writers, divides the United 
States into eastern, central, or Rocky mountain, and Pacifie or Cali- 
*These genera’are: Didelphis, Dicotyles, Cariacus, Antilocapra, Cynomys, Reithrodon- 
tomys, Onychomys, Oryzomys, Sigmodon, Neotoma, Geomys, Thomomys, Dipodomys, 
Perodipus, Microdipodops, Perognathus, Heteromys, Felis, Urocyon, Procyon, Bassariscus, 
Taxidea, Conepatus, Mephitis, Spilogale, Notiosorex, Scalops, Corynorhinus, Huderma, 
Antrozous, Nycticejus, Molossus, Nyctinomous, and Otopterus. Five of these genera 
have each a species reaching a short distance into the southern edge of the Boreal 
region, namely, Cariacus, Neotoma, Felis, Procyon, and Mephitis. 
t Geog. Dist. of Animals, 1876, 1, p. 6. 
} For 320 kilometers (200 miles) the Sierra Nevada mountains maintain an eleya- 
tion of 3,100 to 4,600 meters (12,000 to 15,000 feet). 
