There was at this period, also, a broad isthmus between the two Americas, permitting migration north and south as well as east and west, and it is from that time that we date the arrival of many ancient South American animals, one of which still remains— the opossum. Such a condition for world-wide distribution of plants and animals seems never to have arisen again, although lesser migra tions have occurred, for "bridges" were sub merged and re-established more than once in the subsequent periods.
Finally the advancing world arrived at that comparatively recent stage, just preceding the Glacial Epoch, which is known as the Pleisto cene. All land-connections between Europe and Greenland had then sunk under the waves, leaving only Iceland, the Faroes and the Shet lands as monuments to its former situation; but now the basin of Bering Sea was once more drained and an isthmus of dry land, a thousand miles wide, united Siberia with Alaska, and this remained until the disappearance of the continental ice-cap.
It was by this broad path that America be came peopled by a large number of the many kinds of animals which formed the truly grand fauna of our country in the Pleistocene Epoch. Only those who have studied the matter realize how rich and varied this fauna was (as compared with the present paucity) in the genial time just preceding the general glacia tion, A large proportion of the animals were immigrants; and, as no bridge had existed across the Atlantic for a long time previous, they must have come over from Asia by way of Alaska.
In this way we obtained most of our north ern animals — the bighorn and the mountain goat, the bison, such deer as the moose, caribou and wapiti, the bears, the badgers, otters and other fur-bearers, foxes, wolves and a long list of lesser mammals, birds, etc. None of these
have American ancestors. In return, America gave to the Old World the horses and camels, which, originating here, passed over into Asia and on beyond, where they survived, in more favorable circumstances, the extinction that overtook their races here. These two also passed into. South America, where all the horses died out, but representatives of the camel family remain in the guanacos, vicunas and their domesticated races; and there came north the cumbersome ground-sloths and other strange early beasts, and, later, such modern ones as the puma, the porcupine and a few others.
But the strangest incident of this nature is that of the elephants which, from the Miocene onward, wandered over North America and finally penetrated to Patagonia. They devel oped as species and grew in size until at last they resulted in the huge imperial elephant, the mastodon and the mammoth, the last two of which were killed off here, as in Europe, by primitive men. All of these were, as races, immigrants; but from where? It is only within half a dozen years that this question could be answered. "Appearing?) says Dr. Scott, "sud denly in the Miocene of Europe and North' America, in which regions nothing was known that could, with any plausibility, be regarded as ancestral to them, they might as well have dropped from the moon for all that could be told concerning their history. The, exploration of the Eocene and Oligocene beds of Egypt has dispelled the mystery, and has shown that Africa was the original home of the group, whence they gradually spread to every conti nent except Australia? See ELEPHANT.
Such is the world-wide evidence of the ex istence of eland-bridges) and their lasting effects upon the plant and animal history of the earth.