The Three Divisions of the Pleistocene Age.
As the climate in Europe changed, the Pleiocene species yielded place to those which were better adapted to the new conditions, either retreating southwards or becoming extinct. The first division of the Asiatic invaders is composed of the animals forming the temperate group above mentioned; they are found in the early Pleistocene strata, in Britain and in France, side by side with the survivals from the Pleiocene age. No arctic mammalia had as yet arrived. The next stage in the migration is that in which the temperate group of animals had for the most part replaced the Pleiocene survivals, in Britain and France, and the arctic mammalia begin to appear, but only in small numbers. This constitutes the middle Pleistocene division. The third stage in the migration is indicated by the presence in full force of the arctic species in the area north of the Alps and Pyrenees. They are not, however, met with south of this boundary, and therefore this classification does not apply to the deposits of Spain, or the other portions of the southern zone.
It must also be noted that the temperate group of the Asiatic invaders found their way over the whole of southern Europe, and along the Mediterranean shores, as far south as Palestine and the Sahara Desert—Sicily and Malta affording one line of migration southwards, and the land barrier then stretching across the Straits of Gibraltar offering another.
