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.