Page:Early Man in Britain and His Place in the Tertiary Period.djvu/138

110 remote parts of the world, points with unerring certainty to great geographical and climatal changes. Nearly all the temperate, northern, and mountainous species can be traced to northern and central Asia, and for their migration it is necessary to suppose that a very close connection With Asia was caused by the elevation of land at the close of the Pleiocene age.

This, probably, is indicated by the low tract of country uniting the northern end of the Caspian with the Sea of Aral, and reaching through the salt steppes of Ishim into the valley of the Irtisch, and thence to the Gulf of Obi and the Arctic Sea. Britain also formed part of the mainland, and the bottom of the Pleiocene sea (Fig. 10) became the feeding-grounds of the animals which have left their remains in the forest bed of Nor- folk and Suffolk, as well as in the Dogger Bank off Yar- mouth. Ireland also must have been united to Britain to have allowed of their finding their way so far to the west. The elevation above the present sea-level, necessary to account for this distribution of the animals, is not less than 600 feet or 100 fathoms. At this depth the soundings show the presence of a line of submarine cliffs which form the margin of the plateau of the British Isles, and which mark the probable Atlantic coast-line of north-western Europe, during a large part of the Pleistocene age, as represented in the accompanying map (Fig. 24).

The invasion also of Europe by southern animals, whose headquarters are in Africa, proves an intimate connection between the two continents. The Straits of Gibraltar could not have been in existence when the African elephant ranged as far north as Madrid, and the