The key to this great prehistoric chronology is found in palæontology. The arctic tundra rodents especially are the most invaluable timekeepers; according to Schmidt(5) there is no doubt whatever that the _Upper Rodent Layer_, composed of the animals of the second invasion from the arctic tundras, corresponds, on the one hand, with the beginning of the Magdalenian industry and, on the other, with the renewed glacial advance in the Alpine region, known as the _Bühl_ stage, and probably also with that in the north. The _Upper Rodent Layer_ of Magdalenian times is found in the remarkably complete succession of deposits at the stations of Schweizersbild and Kesslerloch, which are more recent in time than the 'low terraces' bordering the neighboring River Rhine. The fossil animals prove that after the extreme cold of early Magdalenian times the tundra fauna gradually gave way to a wide-spread steppe fauna. Along the Rhine and the Danube the banded lemmings become less frequent; the jerboas, hamsters, and susliks of the steppes become more abundant. Exactly similar changes are observed in Dordogne. In Longueroche, on the Vézère, there occur for the first time in western Europe great numbers of rabbits (_Lepus cuniculus_); numerous hares (_Lepus timidus_) are also observed at the type station of La Madeleine, especially in the uppermost and lowermost culture layers. These small rabbits probably came from the Mediterranean region and denote a slight elevation of temperature. But it is only in the very highest Magdalenian layers that the animal life of western Europe begins to approach that of recent times, namely, that of the prehistoric forest and meadow faunæ. MAMMALIAN LIFE OF MAGDALENIAN TIMES Thus it is very important to keep in mind that during Magdalenian times there were both cold and moist periods favorable to tundra life and cold and arid periods favorable to steppe life. In the latter were deposited the sheets of 'upper loess.' The mammalian life of Magdalenian times is of interest not only in connection with the climate and environment of the Crô-Magnon race, but with the development of their industry and especially of their art. It is noteworthy that the imposing forms of animal life, the mammoth among the tundra fauna and the bison among the meadow fauna, made a very strong impression and were the favorite subjects of the draftsmen and colorists; but the eye was also susceptible to the beauty of the reindeer, the stag, and the horse and to the grace of the chamois. The artists and sculptors have preserved the external appearance of more than thirty forms of this wonderful mammalian assemblage, which accord exactly with the fossil records preserved in the fire-hearths of the grottos and shelters, and with the deposits assembled by beasts and birds of prey in the uninhabited caverns.