The frequent and extreme climatic changes, of which we find such abundant evidence in the Pleistocene, caused extensive migrations and dispersions of animals and plants, and the rapid 3E succession of Arctic and temperate forms in the same region. Many land bridges between different continents, or between continents and what are now islands, were not severed until the end of the Pleistocene, permitting migrations which are no longer possible. The extension of the ice-sheets brought with them Arctic floras and faunas, which retreated in the Interglacial times, while temperate animals and plants spread northward to replace them. These conditions produced a very severe struggle for existence and were fatal to a great many large mammals, causing numerous extinctions over the larger part of the world.

Pleistocene Plants are almost all of the same species as those now living, but they are often very differently distributed. The Glacial cold greatly impoverished the European forests, which in the Pliocene had many kinds of trees now found only in North America or in eastern Asia. Owing to the east and west trend of the European mountains, the forests could not retire before the ice, and return, as they did in the United States, where no mountain barriers shut them off from the warm latitudes of the south. When the ice-sheets melted and the climate was ameliorated, the Arctic flora and fauna were forced to retreat in their turn; they did so not only by following the retiring ice-front, but also by ascending the mountains as the latter were cleared of ice. Thus, high mountains in the northern hemisphere have on their upper slopes an Arctic flora and fauna, separated, perhaps, by hundreds of miles from the nearest similar colony. For example, the higher parts of the White Mountains have plants which do not occur on the lowlands until Labrador is reached, and the snowy Alps have truly Arctic plants and animals.

In Europe the disappearance of the ice-sheets was followed by a dry climate, when a fauna like that of the Russian Steppes extended to western Europe.

Of Pleistocene animals it is only the mammals that require mention. Here also we find the same mingling of northern and southern forms, and association of types now widely separated. North America had Mastodons (i.e. an extinct type of elephant which had smaller and simpler grinding teeth than the true elephants), Elephants, Horses, Tapirs, the first Bisons (which had migrated from the Old World, as had several kinds of Deer and the Musk-ox), Peccaries and huge Llamas, Wolves, great Cats as large as lions, Sabre-tooth Tigers, and the first Bears, also immigrants. A great variety of Rodents is found, most of them kinds which still inhabit the country, but mingled with these are South American forms like the Cavies and Water Hog (Hydro-chcerus), and the giant Beaver {Castoroides) is an altogether peculiar form. Enormous Ground Sloths {Megatherium, Mylodon, and Megalonyx) and Glyptodonts show that the way of migration from the south was still open.

In South America were an astonishing number of huge Edentates: Sloths nearly as large as elephants, Ant-eaters, and a marvellous variety of giant Armadillos. Some of the immigrants from the north, which are now extinct, still lingered in the Pleistocene, such as the Mastodons and Horses, as did also some of the peculiarly South American hoofed animals, Typotheria, Toxodontia, and Litopterna, the ancestors of which may be traced back almost continuously to the Notostylops beds of the Paleocene.

Europe was the meeting-ground of mammalian types now widely scattered. Together with Arctic forms like the Reindeer, Musk-ox, Mammoth (Hairy Elephant), Hairy Rhinoceros, and the Lemming {Myodes) were found southern animals, such as the Hippopotamus, several kinds of Elephants and Rhinoceroses, Lions, and Hyaenas, and likewise species allied to those still living in Europe, such as the huge Cave Bear, the gigantic Irish Deer {Megaceros), and great Oxen. Northern Africa was joined to Europe by way of Malta and Sicily, and probably at Gibraltar also, permitting frequent intermigrations. The junction of Ireland with Great Britain and of both with the continent continued until after the ice-sheets had disappeared, so that these islands, and especially Great Britain, were stocked by the continental animals and plants.

In the Pleistocene of India are found many animals which now live only in Africa, such as, the Baboon, Spotted Hyaena, etc.

Australia had a Pleistocene mammalian fauna composed, with the exception of the Wild Dog (Canis dingo), of Marsupials, allied to those which still inhabit that region, but many of them were of vastly greater size than the living forms.

The Pleistocene Mammals are remarkable for the great size which distinguishes many of them, and it is just these which have passed away, leaving a world that is " zoologically impoverished," but is nevertheless a much more agreeable place of residence without them. Further we note, (1) that the Pleistocene mammals are in general like the smaller forms which have succeeded them in the same regions, but (2) that in Europe and North America there was a commingling of types now found only in widely separated regions.

In Europe Man first appears in the early Pleistocene. It is altogether probable that the human race originated in Asia, quite aside from the doubtful testimony of the Pliocene Pithecanthropus, and reached Europe by migration. The most ancient European men, such as the "Man of Spy " in Belgium, are of a much lower type physically than those of the later Pleistocene; only in the Recent age do human remains and implements become at all common and so the Recent or Postglacial time is frequently called the Human Period, the description of which is rather in the province of prehistoric Archaeology than in that of Geology. Whether Man reached North America in the Pleistocene is still an open question, though there is no reason why he should not have accompanied the Old World mammals in their frequent migrations, and there is some evidence that he did and that a race older than the American Indian occupied this continent. This evidence, however, is not altogether conclusive and has been subjected to a vigorous destructive criticism, so that many authorities are altogether sceptical.

On the other hand, the undoubted presence of human bones in the Pleistocene of South America, associated with a mammalian fauna which is almost entirely extinct, lends additional strength to the position of those who contend that Man had reached North America before the ice finally disappeared.