56 Merriain Geographic Distribution of Life. 



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 im- 

 passable 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 altitude 

 this lofty range maintains for a considerable 'distance.f The ex- 

 planation 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 one end or the other. In 

 their continuity, however, lofty mountain ranges do act as bar- 

 riers 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 cannot 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 dissim- 

 ilarities 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 



* Geog. Bist. of Animals, I, 1870, p. 6. 



f For 320 kilometers (200 miles) the Sierra Nevada Mountains maintain 

 an elevation of 3,100 to 4,000 meters (12,000 to 15,000 feet). 



