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.