SNAILS OF THE NORTH WEST 78 



ultimate effect would be somewhat similar. In reality it 

 would not be quite the same, because in the latter case it is 

 conceivable that the whole Rocky Mountain chain, which lay 

 to the west of the flooded territory, could have supported a 

 fairly rich fauna and flora. If a huge glacier really existed 

 in the Keewatin district, it is inconceivable to suppose that 

 any but the most scanty fauna and flora could have found a 

 stray asylum on the few isolated " Nunatakkr " projecting 

 beyond the Rocky Mountain ice-fields. 



The mollusks do not favour either theory. The mammals, 

 on the other hand, show clearly a geologically recent invasion 

 of Old World forms from the west, which is more compatible 

 with what I may call the marine theory. 



Mr. Walker * contends that the Glacial Epoch was a great 

 factor influencing the distribution of the Norfch American 

 molluscan fauna. I can only perceive signs of local destruc- 

 tion. The distribution of the land and fresh-water mollusks 

 in North America took place evidently long before this Glacial 

 Epoch, and was not seriously influenced by the latter. 



Both theories will be more fully dealt with in the next 

 chapters in which the animals, principally of Alaska, will be 

 described. These animals afford us more material for our 

 inquiries, being of great importance to the just appreciation 

 and understanding of the varied origin and development of 

 the American fauna. 



* Walker, Bryant, " Origin and Distribution of Mollusca," p. 53. 



