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 Ant arctic 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* ex isted 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 sions of the 'land and the sea-bottom, 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 nar rowest part. There also would appear, prob ably, a second line of dry land about on the 80th parallel. Both of these, in the warm cli mate 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 ele vation of these (bridges.) The Arctic Sea would then be confined to its own basin, and enable 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 pres ent; and so the whole ocean must have been warmer than now. Hence, we find in the north ern coast-rocks of that period remains of trop ical 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, pro duced 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 Green land must have had weather in summer like that of Maryland, for it was clothed with simi lar 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 noth ing 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 sud denly 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 far to the north, but the genial climate kept it open to all sorts of migratory creatures.