The Encyclopedia Americana (1920)/Land-Bridges Across the Oceans
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Land-Bridges Across the Oceans
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LAND-BRIDGES ACROSS THE OCEANS. One of the most attractive studies in geology is that of the change in form of the continents, and in the relative spaces of ocean, especially since the continents assumed their present general shape, and especially since the beginning of the Age of Mammals, or Tertiary Period, — that geological period which closed with the Glacial Epoch.
It is plain that during this period millions of years long, many changes occurred in the level of the lands of the globe. Sometimes one or the other of the great masses was lifted, until a much larger expanse of land was out of water than before; then again it would sink until the sea overrran broad areas. Geologists know this from the fact that they find rocks which were evidently formed under salt water. By their characteristic fossils, and by other marks, they know where these rocks belong in the scale of geological succession, or time; and by plotting them on a map they can show approximately the shape any continent had at some long-past time. Of course this may not be done for any stage you may ask for, but it can for some of them. Thus in the early part of the Tertiary, while both Americas were in much their present condition, only broader in Canada, Europe was an archipelago of large islands, separated from Asia by a broad sea, and the Mediterranean extended over the Saharan deserts, leaving central and southern Africa as an island. All Persia, Syria and Arabia were then under water, so that the cold Arctic Sea flowed through into the Indian Ocean, which must have made the climate of India and East Africa very much cooler than now.
The most interesting feature of these changes, however, is that by which, now and again, the Old World was connected with the New by necks or spaces of land, known as “land-bridges”; especially as these permitted an interchange of plants and animals, giving to us many new ones from the other side of the ocean, including, finally, man himself.
No more fascinating department of natural history exists than the study of the distribution on the earth of living beings, past and present. A striking result of this study is the knowledge that, while the continents and great islands of the southern hemisphere differ from each other, and from the northern hemisphere, in their plants and animals, the several parts of the northern hemisphere are closely similar in this respect. The same families of trees — pines, spruces, cedars, oaks, maples, chestnuts, birches and so on; and the same sorts of animals — quadrupeds, birds, fishes and insects — are found in Europe and northern Asia as in North America. In fact, many of the living species are virtually identical in all three regions. It is bard to separate the Canadian marten from the Russian sable, our big-horn from the Himalayan argali, our moose and caribou from the elk and reindeer of Norway; and some, like the polar bear, fox and wolf, the raven, golden plover, crossbill, bank-swallow and others, are quite alike in both the Old and the New Worlds. This has been so, judging by the fossils in the various Tertiary strata, ever since the Age of Reptiles.
What is the explanation? None of these animals, save possibly certain birds or fishes, could get across an ocean. They must have been able to travel upon land, and it is from their presence that it seems certain that land-bridges have existed, at various times in the past, between the northern parts of America, and Europe and Asia.
Let us pause here a moment to note what North and South America have to show on this point. South America possesses a fauna which is peculiar to itself. Several large groups there are not represented in any other part of the world, and nearly all its animals in every class, are different from those elsewhere. The fossils show that the same was true in the far past; so that it looks as if that continent has been isolated ever since its life began: only Australia is more self-supplied.
But if South America has always been cut off from the rest of the world (except from North America at times), where did it get its marsupials? These were numerous there in Tertiary times, and big and little opossums still remain. The only answer is a supposition that at a period when mammals and birds were just beginning to take distinctive form in a world mainly reptilian, both the Australasian islands and South America were attached to an Antarctic continent then far broader than now. An elevation of 10,000 feet above the present level would expose dry land far beyond the Antarctic Circle and include Australia, New Zealand and Patagonia in a South-polar continent; and there is other evidence that such an “Antarctica” existed in Cretaceous and Paleocene times, and that its borders, at least, had a temperate climate.
It is supposed, also, that at the same time, and somewhat later, Brazil and Africa were connected by a ridge, or a chain of islands, since it is hard to account otherwise for the presence of monkeys in South America, which first appeared there in Miocene time, or for certain rodents like those of South Africa. Furthermore, the Atlantic is comparatively shallow and island-studded even now in that narrow part
Let us return now to North America and its oceanic bridges. These appear, from the data given in the works of Professors Osborn and Scott, to have been repeatedly established and destroyed by alternating elevations and depressions of the land and the sea-bottom, both before and during the Age of Mammals.
At the beginning of the Tertiary Period, continuous land encircled the North Pole. That this would require no very startling change from the present level may be seen by looking at a chart of the northern oceans. This shows that a broad space extends from Scotland to Greenland, where the water is nowhere more than 1,000 fathoms deep; and that the central part of this is a wide, winding plateau, named Wyville Thomson Ridge, which at the present time comes within 300 or 400 fathoms of the surface. Therefore an uplift of the bottom of the north Atlantic of less than 2,000 feet would extend our coast beyond Greenland and the Banks of Newfoundland, include the British Isles within Europe, drain the Baltic and North seas and connect the two continents by a neck of dry land about 300 miles wide in its narrowest part. There also would appear, probably, a second line of dry land about on the 80th parallel. Both of these, in the warm climate of the early Tertiary Period, would speedily have become covered with vegetation and attract and sustain wandering animals.
On the Pacific side such a rise would drain nearly the whole of Bering Sea and join Alaska to Siberia by a stretch of mountainous land a thousand miles in breadth. Even 500 feet of uplift would now close Bering Strait.
A moment's thought will show one that a decided, if slow, alteration in the climate of all northern lands must have followed the elevation of these “bridges.” The Arctic Sea would then be confined to its own basin, and unable to pour its icy currents into either the Atlantic or the Pacific Ocean. Hence the warm Gulf Stream and its Oriental counterpart, the Japan Current, would, and must, follow a solid coast right around to Europe and western America, respectively, without interference by any cold currents from the north, as at present; and so the whole ocean must have been warmer than now. Hence, we find in the northern coast-rocks of that period remains of tropical sea-animals which could not exist in waters as cold as those of the present day. At the same time solid land extended much farther toward the North Pole than at present, warding off Arctic influences to some extent.
All these circumstances, with others, produced a warm climate in the earlier half of the Tertiary, so that, as the fossil plants of that time show us, tropical conditions prevailed in the United States, and even southern Greenland must have had weather in summer like that of Maryland, for it was clothed with similar plants and hardwood trees.
One may ask: What is the evidence that enables geologists to speak so confidently of the existence of these “land-bridges,” since nothing is left of them? It is this:
In the older layers of North American Eocene rocks, all the fossils are of animals peculiar to this continent — families and species which had developed here alone. In the next later layers, however, races of animals, suddenly appear, for which no American ancestors can be found, but which are identical with those of the Old World of that time. None of these is much like any present creatures, of course, but the similarity of fossils on both sides of the oceans is so great that it is evident that these animals must then have been able to pass from one continent and colonize the other. The road lay far to the north, but the genial climate kept it open to all sorts of migratory creatures.
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 migrations have occurred, for “bridges” were submerged 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 Pleistocene. All land-connections between Europe and Greenland had then sunk under the waves, leaving only Iceland, the Faroës and the Shetlands 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 became 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 glaciation. 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 northern 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, vicuñas 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 developed 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, “suddenly 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 continent except Australia.” See Elephant.
Such is the world-wide evidence of the existence of “land-bridges” and their lasting effects upon the plant and animal history of the earth.