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,--thatgeological 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 ap proximately the shape nay 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 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 hard to separate the Canadian marten from the Russian sable, our big-horn from the Hima layan 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 tunes in the past, between the northern parts of America, and Europe and Asia.
Let us pause 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.