Page:Popular Science Monthly Volume 59.djvu/297

Rh 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 geological data show. On reaching America, the snails spread southward. 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 River. This would prevent the eastward spread of the snail emigrants from Asia. Since that time, the increasing height of the Rocky Mountains, and the arid conditions of much of the mountain region, with its poverty in deciduous trees, would be, and is to-day, an effectual bar to the eastward distribution 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.