For ages Asia appears to have served as a vast and fecund nursery for
new mammals from which North Temperate and Arctic America have been
supplied. The last and comparatively recent land bridge, across which
came the ancestors of our moose, elk, caribou, prong-horned antelope,
mountain goats, mountain sheep, musk-oxen, bears, and many other
mammals, was in the far Northwest, where Bering Straits now form a
shallow channel only 28 miles wide separating Siberia from Alaska.
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