Of special importance for our main question is the great invasion of Europe by Central Asiatic animals; immigrants direct from the Asiatic steppes pushed westward “as in a migration of nations,” among them the wild ass, saiga antelope, bobac, Asiatic porcupine, zizel, jumping mouse, whistling hare, and musk shrew-mouse. According as the glaciers and inland ice grew or shrank, the animals of the glacial period advanced more or less far to the North or retired more to the South, extending or reducing their range of distribution. The Glacial Period was no invariable climatic phenomenon. It is perfectly certain that a first Glacial Period with a low yearly temperature, under the influence of which the ice-masses, with their moraines, advanced a long way from the North and from the high mountains, so that in Germany, for instance, only a comparatively narrow strip remained free and habitable for higher forms of life between the two opposing rivers of ice--was succeeded by at least one period of warmer climate, and that certainly not a short one. The mean yearly temperature had increased so much that the ice-masses melted to a considerable extent, and had to retire far to the North and into the high valleys of the Alps. In this warmer Interglacial Period, as it is called, the Drift animals advanced far to the North, especially the mammoth, which, with the exception of the greater part of Scandinavia and Finland (districts which remained covered with ice during the Interglacial Period), is distributed throughout the drift strata of the whole of Europe and North Africa, and as far as Lake Baikal and the Caspian Sea in Northern Asia. Even the older Drift fauna, so far as it had not yet died out or retired, returned to its old habitats, so that the Interglacial fauna of Central Europe appear very similar to the Preglacial fauna. A long-sustained decrease of temperature led once more to the growth of the ice, which in this second Glacial Period almost reconquered the territory it had won at first. In consequence of these oscillations in the climatic conditions of the Drift Era as a whole, we have to distinguish the Preglacial Era and the Interglacial Era, as warmer sub-periods of the Drift, from the real Glacial Periods. The latter appear as a first, or earlier, and a second, or later Glacial Period, as remains of which the zone of the older moraines and the zone of the later ones clearly mark the limits of the former glaciation. [Illustration: Alpine Hares The Chamois The Ibex The Marmot Dando TYPES OF ANIMALS SURVIVING IN CENTRAL EUROPE FROM THE DRIFT PERIOD Many of the animal forms that were very frequent in the Drift Period appear now in Central Europe only as Alpine dwellers, living on the borders of eternal snow. Such are the ibex, chamois, marmot, and Alpine hare. ] [Sidenote: Breaking up of the Earth]