There is, however, no proof of the departure of these supposedly less hardy mammals nor of the spread over Europe of the more hardy arctic and steppe types until the advent of the Fourth Glacial Stage. Then, for the first time, all western Europe north of the Pyrenees experienced a general fall of temperature, and conditions of climate prevailed such as are now found in the arctic tundra regions of the north and in the high steppes of central Asia, which are swept by dry and cold winter winds. Fluctuations of temperature, of moisture, and of aridity in Pleistocene time, are evidenced not only by the rise and fall of the snow-line and the advance and retreat of the ice-caps but also by the appearance of plant and animal life in the periods of the 'loess' deposition, indicating the following cycles of climatic change as witnessed from beginning to end of the Third Interglacial Stage: IV. Glacial maximum, cold and moist climate, arctic and cold steppe fauna and flora. Cool and dry steppe climate, wide-spread deposition of 'loess.' Interglacial maximum, a long period of warm temperate forest and meadow conditions. Glacial retreat, cool and moist climate bordering the glacial regions. III. Glacial maximum, cold and humid climate bordering the glaciers, favorable to arctic and subarctic plant and animal life. That great fields of ice and advancing glaciers alone do not constitute proof of very low temperatures is shown at the present time in southeastern Alaska, where very heavy snowfall or precipitation causes the accumulation of vast glaciers, although the mean annual temperature is only 10° Fahr. (5.56° C.) lower than that of southern Germany. Neumayr(69) estimated that during the Ice Age there was a general lowering of temperature in Europe of not more than 6° C. (10.8° Fahr.), and held that even during the glacial advances a comparatively mild climate prevailed in Great Britain. Martins(70) estimated that a lowering of the temperature to the extent of 4° C. (7.2° Fahr.) would bring the glaciers of Chamonix down to the level of the plain of Geneva. Penck estimates that, all the atmospheric conditions remaining the same as at present, a fall of temperature to the extent of 4° to 5° C. would be sufficient to bring back the Glacial Epoch in Europe. These moderate estimates entirely agree with our theory that animals of African and Asiatic habit flourished in western Europe to the very close of the Third Interglacial Stage, and that then for the first time the warm fauna, or _faune chaude_, gradually disappeared.