THE EVIDENCE OF SNAILS. 287 



with abundance of sheltering windfalls and rotting boles to give them 

 refuge, and decaying leaves to feed the fungi and tender herbage, which 

 are the food of snails. 



It is not unlikely, then, that, in the distant past, when a kinder 

 climate allowed the forests of the temperate zone to extend to the 

 Arctic shores in Alaska and Siberia, the snails went with them; and, 

 if we assume a very moderate elevation in the region of Bering Strait, 

 connecting Alaska and Asia by a land bridge, there would be no bar 

 whatever to the spread of forest trees and the emigration of snails 

 from one continent to the other. 



The distribution southward of the snails and other inhabitants of 

 the forest would be merely a question of time in the absence of 

 barriers in the form of lofty mountains, deserts or arms of the sea, 

 running across their path. 



There are good reasons for believing that the dart-bearing snails 

 originated in the Orient, and, if so, their migration was eastward 

 to America. In all probability, the slowly upbuilding land mass in 

 western America had none of the higher land snails before the advent 

 of the Asiatic snails in the later Cretaceous, as it was profoundly 

 isolated in earlier mesozoic and preceding time, so far as existing geo- 

 logical data show. On reaching America, the snails spread south- 

 ward. Why, then, it may be asked, do we not have the descendants of 

 the Asiatic dart-bearers in eastern North America? There are several 

 reasons. During the Cretaceous period an inland sea extended from 

 the Gulf of Mexico, through the Dakotas and northward to the Arctic 

 Ocean, in the neighborhood of the Mackenzie Eiver.* This would pre- 

 vent the eastward spread of the snail emigrants from Asia. Since that 

 time, the increasing height of the Rocky Mountains, and the arid condi- 

 tions of much of the mountain region, with its poverty in deciduous 

 trees, would be, and is to-day, an effectual bar to the eastward dis- 

 tribution of the Pacific slope snails. 



In the Far West, however, no barriers prevented the southward 

 spread of the dart-bearing Helices. They pushed south to Mexico, 

 and, perhaps later, to the Andean region of South America. Ther^ 

 was also undoubtedly a land bridge connecting an Antillean continent 

 or archipelago with Central America, over which the dart-bearers 

 passed to the Antilles. This connection is shown by many other 

 groups of land snails also, the distribution of which can be explained 

 in no other manner. 



* Dawson maps the Cretaceous inland sea as extending to the Arctic Ocean. 

 During the earlier Cretaceous it also reached the Pacific, though an archipelago 

 probably extended north to Alaska ; but in the Laramie there was a broad 

 belt of land to the westward of the Cretaceous sea or lakes. See map, which 

 represents the probable extent of the sea at the beginning of the Laramie. 



