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.

Similarly the hypothesis of extremely warm or subtropical conditions prevailing in interglacial times as far north as Britain, which originated with the discovery of the northerly distribution of the hippopotami and rhinoceroses, animals which we now associate with the torrid climate of Africa, is not supported by the study either of the plant life of interglacial stages or by the history of the animals themselves. It is quite probable that both the hippopotami and the rhinoceroses of the 'warm fauna' were protected by hairy covering, although not by the thick undercoating of wool which protected the woolly rhinoceros and woolly mammoth, animals favoring the borders of glaciers and flourishing during the last very cold glacial and Postglacial periods.

The combined evidence from all these great events in western Europe leads us to conclusions somewhat different from those reached by Penck as to the chronology of human culture. In the chart (Fig. 14) on the opposite page, prepared by Dr. C. A. Reeds in collaboration with the author, a new correlation of geologic, climatic, human, industrial, and faunal events is presented. The great waves of glacial advance and retreat (oblique shading) are based upon Penck's estimates of the rise and fall of the snow-line (vertical dotted lines) in the Swiss Alps. (Compare Fig. 13.) The length of these waves corresponds with the relative duration of the glacial and interglacial stages as estimated by the varying amounts of erosion and deposition of materials. The entire Palæolithic or Old Stone Age is thus seen to occupy not more than 125,000 years, or only the last quarter of the Glacial Epoch, which is estimated as extending over a period of 525,000 years. The present opinion of the leading archæologists of France and Germany, which is shared by the author, is that the Pre-Chellean industry is not older than the Third Interglacial Stage. As the Piltdown man was found in deposits containing Pre-Chellean implements, he probably lived in the last quarter of the Glacial Epoch, and not in early Pleistocene times as estimated by some British geologists. This causes us to regard the Piltdown remains as more recent than the jaw of Heidelberg, which all authorities agree is probably of Second Interglacial Age. According to our estimates the Heidelberg man is nearly twice as ancient as the Piltdown man, while Pithecanthropus (Trinil Race) is four times as ancient. Yet the Piltdown man must still be regarded as of very great antiquity, for he is four times as ancient as the final type of Neanderthal man belonging to the Mousterian industrial stage. The various archæologic and palæontologic evidences for this general correlation theory of the Glacial Epoch are fully discussed in the succeeding chapters of this volume.

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Fig. 14. Great events of the Glacial Epoch. To the left the relation of glacial and interglacial stages in Europe and North America, with the author's theory regarding the divisions of time, the beginning of the Old Stone Age, and the successive appearance in Europe of different branches of the human race. To the right the prolonged warm temperate period in Europe in the non-glaciated regions, followed by the relatively brief cold period during the past 70,000 years. Prepared by Dr. C. A. Reeds, in co-operation with the author.