Relation of Mammalia to Glacial Phenomena.
The complicated glacial phenomena summed up in the preceding pages imply not merely a change from a temperate to a cold climate of extreme severity, but they show a climatal fluctuation of the sort which might be expected from the examination of the Pleistocene mammalia. When the reindeer inhabited the south of France the cold was at its maximum, and when the hippopotamus lived in England the cold was probably at its minimum. Each of these changes was probably brought about during a long series of ages, and each has left its mark in the mixed fauna of the middle zone of the map (Fig. 24).
The lowering of the temperature was probably the cause of the immigration into Europe of the Asiatic species. As the cold increased in Asia, and the warm Pleiocene climate of northern and central Europe gradually became cool, the animals which had been living in Asia for an unknown series of years poured in, a way being opened to them by the elevation of a low-lying tract of land at the head of the Caspian and the Gulf of Obi, which had probably hitherto been the bottom of a shallow sea cutting them off from Europe. It must be remarked that a change towards cold conditions has already been indicated by the ice-borne blocks of stone met with on the Pleiocene sea-shore of Suffolk. A vast migration of animals set in from Asia, analogous in every respect to that by which the European peoples arrived at their present homes, and following for the most part the same route, between the Caspian Sea and the Ural mountains (see Fig. 24, p. 111).
