In favor of the latter theory is the strikingly close succession of the Lower Palæolithic cultures in the valley of the Somme, followed by an equally close succession from Acheulean to Magdalenian times, as, for example, in the station of Castillo. It does not appear possible that a vast interval of time, such as that of the third glaciation, separated the Chellean from the Mousterian culture. On the other hand, in favor of the greater antiquity of the Pre-Chellean and Chellean cultures may be urged their alleged association in several localities with very primitive mammals of early Pleistocene type, namely, the Etruscan rhinoceros, Steno's horse, and the saber-tooth tiger, as witnessed in Spain and in the deposits of the Champs de Mars, at Abbeville. It is true, moreover, that at points distant from the great ice-fields, like the valley of the Somme and that of the Marne, we have no other means of separating glacial from interglacial times than that afforded by the deposition and erosion of the 'terraces'; in fact, the interpretation of the age of the cultures may be similar to that applied to the age of the mammalian fauna. There are no proofs of periods of severe cold in western Europe in any country remote from the glaciers until the very cold steppe-tundra climate immediately preceding the fourth glaciation swept the entire land and drove out the last of the African-Asiatic mammals. GEOGRAPHIC CHANGES The migrations of mammals and of races of men into western Europe from the Eurasiatic continent on the east and from Africa on the south were favored or interrupted by the periods of elevation or of subsidence of the coastal borders of the Ægean, Mediterranean, and North Seas, and also of the Iberian and British coast-lines. The maximum period of elevation of the coastal borders, as represented in the accompanying map (Fig. 12), never occurred in all portions of the continent of Europe at the same time, because there were oscillations both on the northern and southern coasts of Europe and Africa. The early Pleistocene, especially the period of the First Interglacial Stage, was one of elevation remarkable for the broad land bridges which brought the animal life of Europe, Africa, and Asia together. The Mediterranean coast rose 300 feet. Land bridges from Africa were formed at Gibraltar and over to the island of Sicily, so that for the time there was a free migration of mammalian life north and south. It is to this that western Europe owes the majestic mammals of Asiatic and African life which dominated the native fauna.