SNAILS OF THE NORTH WEST 
73 
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 North 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. 
