206 I-IYPSOMETRICAL DISTRIBUTION, ETC. 



I think that this should be considered rather as a portion of 

 the American region, as it has no faunal peculiarities whatever. 

 Many of the species, especially in the northern portions of the 

 territory, are identical with European species, but then a con- 

 siderable number of them are known to occur in North Asia 

 also ; they are, in fact, circumboreal. There are a few peculiar 

 species, principally in Greenland and Iceland ; both land and 

 fresh-water shells being included in these, but no Melanian or 

 Unio. 



The question of the direction whence species common to the 

 old world have been introduced into America, has not been 

 decided, although, with the increase of our knowledge of Asiatic 

 localities and species, it is becoming more probable that such 

 forms have usually come thence. This view is strengthened by 

 the occurrence of representatives of some of the Californian 

 fluviatile forms in Lake Baikal. On the other hand, the ancient 

 intercourse of Europe with Iceland, Greenland, New England, 

 etc., cannot have failed to introduce accidentally a crowd of 

 species ; some of which, indeed, are known to have spread rapidly 

 within a short period. Thus, the great snail, Limax maximus, 

 has within a ver}^ few years spread from Philadelphia, where it 

 first became acclimated in cellars, to New York and other cities; 

 adopting subterranean habits in this country rather than 

 inhabiting the surface of the earth, where the atmosphere would 

 be too dry for its existence ; it is now a commonly occurring 

 species. Identical species may have been introduced both from 

 the West and East, if of decidedly European type, but there are 

 species, as the Limnseans, for example, which may as well have 

 originated here and thence spread to the old world. Finally, we 

 do not know that identity of specific or structural character is 

 conclusive as to identity of origin. 



HYPSOMETRICAL DISTRIBUTION OF LAND AND ERESH-WATER 

 MOLLUSKS. 



Mollusks are subject to conditions of existence varying 

 according to altitude. As plants change according as the}"^ are 

 submitted to the changing climates of difterent elevations, so 

 shells are necessarily subject to like influence, some species 

 never ascending beyond a certain height, others living entirely 

 in mountainous regions. Whilst in Europe the elevation beyond 

 which mollusks cannot exist is about 8000 feet, in America and 

 Asia the}^ are collected at nearly double that altitude. 



In examining the mollusks of the Pyrenees, the Alps and 

 Auvergne, the following are the results reached : 



1. Lower Valleys. From the sea level to 3000 feet altitude 



