Attention should first be called to the fact that, preceding the epoch we have now entered, the glacial and interglacial forces operating over the great peninsula of western Europe had left their impress chiefly on the glaciated areas and only to a minor degree on the free, non-glaciated areas. Until toward the close of Third Interglacial times no traces of northern much less of arctic forests and animals are discovered anywhere, except along the borders of the ice-fields. It would appear as if the animal and plant life of Europe were, in the main, but slightly affected by the first three glaciations. We cannot entertain for a moment the belief that in glacial times all the warm flora and fauna migrated southward and then returned, because there is not a shred of evidence for this theory. It is far more in accord with the known facts to believe that all the southern and eastern forms of life had become very hardy, for we know how readily animals now living in the warm earth belts are acclimatized to northern conditions. [Illustration: FIG. 52. Human types and culture stages of the last third of the Glacial Epoch. Theoretic estimates of the geologic and time divisions and introduction of human races during the Third Interglacial, Fourth Glacial, and Postglacial Stages (see Fig. 14, p. 41). Prepared by the author with the aid of C. A. Reeds.] If, on the other hand, we depend solely on the testimony of the life conditions, we might conclude that the Pre-Chellean flint workers reached western Europe either in Second Interglacial times, or during the third glaciation, or again during Third Interglacial times. Let us consider this evidence of the fossil mammals more closely. [Illustration: FIG. 53. Distribution of the principal Pre-Chellean and Chellean industrial stations in western Europe.]