And now in our series of changing scenes comes all at once that strange blank which we hope one day to fill up; and when we look again the large reptiles are gone, the birds are spreading far and wide, and we come upon those early and primitive forms of insect-eaters, gnawers, monkeys, grass-feeders, and large flesh-eaters, whose descendants, together with those of the earlier marsupials, are henceforward to spread over the earth. We need scarcely carry our pictures much farther. We have seen how, in these early times, the flesh-feeders and grass-feeders were far less perfectly fitted for their lives than they are now;[198] how the horse has only gradually acquired his elegant form; the stag his branching antlers; and the cat tribe their scissor-like teeth, powerful jaws, and muscular limbs; while the same history of gradual improvement applies to nearly all the many forms of milk-givers.
But there is another kind of change which we must not forget, which has been going on all through this long history, namely, alterations in the level and shape of the continents and islands, as coasts have been worn away in some places and raised up or added to in others, so that different countries have been separated from or joined to each other. Thus Australia, now standing alone, with its curious animal life, must at some very distant time have been joined to the mainland of Asia, from which it received its low forms of milk-givers, and since then, having become separated from the great battlefield of the Eastern Continent, has been keeping for us, as it were in a natural isolated zoological garden, the strange primitive Platypus and Echidna, and Marsupials of all kinds and habits.
So too, Africa, no doubt for a long time cut off by a wide sea which prevented the larger and fiercer animals from entering it, harboured the large wingless ostriches, the gentle lemurs, the chattering monkeys, the scaly manis, and a whole host of insect-eaters; while South America, also standing alone, gave the sloths and armadilloes, the ant-bears, opossums, monkeys, rheas, and a number of other forms, the chance of establishing themselves firmly before stronger enemies came to molest them. These are only a few striking examples which help us to see how, if we could only trace them out, there are reasons to be found why each animal or group of animals now lives where we find it, and has escaped destruction in one part of the world when it has altogether disappeared in others.
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So, wandering hither and thither, the backboned family, and especially the milk-givers, took possession of plains and mountain ranges, of forests and valleys, of deserts and fertile regions. But still another question remains—How has it come to pass that large animals which once ranged all over Europe and Northern Asia,—mastodons, tusked tapirs, rhinoceroses, elephants, sabre-toothed tigers, cave-lions, and hippopotamuses in Europe,[199] gigantic sloths and llamas in North America, and even many huge forms in South America, have either been entirely destroyed or are represented now only by scattered groups here and there in southern lands? What put an end to the “reign of the milk-givers,” and why have they too diminished on the earth as the large fish, the large newts, and the large lizards did before them?
To answer this question we must take up our history just before the scene at the head of our last chapter,[200] which the reader may have observed does not refer, as the others have done, to the animals in the chapter itself. Nevertheless it has its true place in the series, for it tells of a time when the great army of milk-givers had its difficulties and failures as well as all the other groups, only these came upon them not from other animals but from the influence of snow and ice.
For we know that gradually from the time of tropical Europe, when all the larger animals flourished in our country, a change was creeping very slowly and during long ages over the whole northern hemisphere. The climate grew colder and colder, the tropical plants and animals were driven back or died away, glaciers grew larger and snow deeper and more lasting, till large sheets of ice covered Norway and Sweden, the northern parts of Russia, Germany, England, Holland, and Belgium, and in America the whole of the country as far south as New York. Then was what geologists call the “Glacial Period;” and whether the whole country was buried in ice, or large separate glaciers and thick coverings of snow filled the land, in either case the animals, large and small, must have had a bad time of it.
True, there were probably warmer intervals in this intense cold, when the more southern animals came and went, for we find bones of the hippopotamus, hyæna, and others buried between glacial beds in the south of England. But there is no doubt that at this time numbers of land animals must have perished, for in England alone, out of fifty-three known species which lived in warmer times, only twelve survived the great cold, while others were driven southwards never to return, and the descendants of others came back as new forms, only distantly related to those which had once covered the land.
Moreover, when the cold passed away and the country began again to be covered with oak and pine forests where animals might feed and flourish, we find that a new enemy had made his appearance. Man—active, thinking, tool-making man—had begun to take possession of the caves and holes of the rocks, making weapons out of large flints bound into handles of wood, and lighting fires by rubbing wood together, so as to protect himself from wild beasts and inclement weather.
In America and in England alike, as well as in Northern Africa, Asia Minor, and India, we know that man was living at this time among animals, many of them of species which have since become extinct, and with his rude weapons of jagged flint was conquering for himself a place in the world.