trated, comprising the region north of a line passing from Yorkshire eastwards through Hamburg and Russia; 2d, the middle zone, common to both groups of animals, extending south of this line as far as the Pyrenees, the Alps, and Hungary; and lastly, the southern, into which the northern animals never penetrated, or Spain, Italy, and Greece. It must further be observed that the range of the mountainous species, such as the ibex, into the region of the southern animals—into the Apennines, the Sierra Nevada—the Atlas, and the mountains of Sardinia, of Crete, and Anatolia,—shows that the distribution of the Pleistocene mammalia was regulated by climate rather than by physical barriers.
From this distribution we may infer that the climate was severe in the north and warm in the south, while in the middle zone, comprising France, Germany, and the greater part of Britain, the winters were cold, and the summers warm as in middle Asia and North America, where large tracts of land extend from the Polar region towards the equator, and offer no barrier to the swinging to and fro of the animals. In the summer time the southern species would pass northwards, and in the winter time the northern would swing southwards, and thus occupy at different times of the year the same tract of ground, as is now the case with the elks and reindeer.
It must not, however, be supposed that the southern animals migrated from the Mediterranean area as far north as Yorkshire in the same year, or the northern as far south as the Mediterranean. There were, as we shall see presently, secular changes of climate in Pleistocene Europe, and while the cold was at its maximum the arctic animals arrived at the southern limit, and while it was at its
