where the rivers once flowed. The vastness of the excavation of the valley from the level of the old river bed 600 ft. up on the sloping hill-side to its present low-lying bed in the floor of the valley–gives us some measure of the time which has elapsed in the process. No one can tell, at present, the limit in the past of Palæolithic man. The period of time over which his existence extended, as indicated by the trimmed flints undoubtedly made by human workmanship, is a matter of hundreds of thousands of years. In Western Europe races came and went, succeeded one another and disappeared, either migrating or absorbed or more rarely destroyed by the later invaders. Naturally enough, in the later deposits of rivers and in the higher layers of earth and limestone cake which fill many caves to the depth of 30 or 40 ft. we find the remains of man's workmanship more abundantly than in the older deposits. We can broadly distinguish in the Palæolithic epoch three (perhaps four) periods, separated by the occurrence of great extensions of the northern or arctic ice cap of such a volume as to cover North Europe and North America, and the simultaneous extension of the glaciers of the mountains of Europe. This period of the alternating extension and retreat of the great northern glaciers is known as the Glacial period, or Ice Age. The _latest_ Palæolithic men are subsequent to it–that is, post-Glacial. We can distinguish several successive ages of these post-Glacial Palæolithic men, altogether distinct from and anterior to the Neolithic men. In the earlier of these ages many of the great animals of the Glacial period–now extinct or withdrawn to other regions–still survived in Europe. The mammoth survived, but was fast dying out in the south and centre of France, and we find its outline scratched on ivory and on bone by the early post-Glacial men. The lion still survived in Europe, also the hyena, the bear and the rhinoceros. The reindeer seems to have been especially abundant, and to have been associated with the men of this period. The horse was very abundant, and was largely eaten by the earlier post-Glacial people. From the first these men show extraordinary artistic skill, and have left in their caves many carvings on ivory, bone and stone. In the oldest deposits of the post-Glacial age the carvings are complete all-round sculptures of small size or carvings in low relief, all of rough primitive workmanship. Larger life-size sculptures in rock are also found. In later deposits we find better sculpture and also engraving on flat pieces of bone and ivory, and also on stone. This art persisted, and attained its greatest perfection in the latest deposits of all in which the work of Palæolithic man is found. The reindeer persisted through this post-Glacial period (hence often called "the reindeer period") until the gradual increase of temperature and change of herbage and forest led to its migration northwards and to the relative abundance of the red deer.