[Illustration: FIG. 12. Europe in the period of maximum continental elevation, in which the coast-lines are widely extended, connecting Africa and Europe--including Great Britain and Ireland--in a single vast peninsula, and affording free migration routes for animal and human races north and south, as well as east and west. The ocean boundaries are more remote and the interior seas are greatly reduced in area. After Obermaier.] In general, the _elevation_ of the continent took place during interglacial, the _subsidence_ during glacial times, but Great Britain appears to have been almost continuously elevated and a part of the continent, and was certainly so during the Third Interglacial, Fourth Glacial, and Postglacial Stages, because there was a free migration of animal life and of human culture. The Lower Palæolithic peoples of Pre-Chellean and Chellean times wandered at will from the valley of the Somme to the not far distant valley of the Thames, interchanging their weapons and inventions. The close proximity of these stations is well illustrated in the admirable map (Fig. 56) prepared under the direction of Lord Avebury (Sir John Lubbock). The relation which _elevation_ and _subsidence_ respectively bear to the glacial and interglacial stages is believed to be as follows: ELEVATION, emergence of the coast-lines from the sea, broad land connections facilitating migration, retreat of the glaciers, deepening of the river-valleys, and cutting of terraces. Arid continental climate and deposition of 'loess.' SUBSIDENCE, submergence of the coast-lines and advance of the sea, interruption of land connections and of migration routes, advance of the glaciers, filling of the river-valleys with the products of glacial erosion, the sand and gravel materials of which the 'terraces' are composed, and subglacial erosion of the loam, from which in arid periods the 'loess' is derived. Subsidence was the great feature of closing glacial times both in Europe and America. During the Fourth Glacial and Postglacial Stages the Black and Caspian Seas and the eastern portion of the Mediterranean were deeply depressed, while the British Isles were still connected with France, but by a narrower isthmus than that of early interglacial times. The scattered stations of Upper Palæolithic culture found in the British Isles include one Aurignacian, one Solutrean, two Magdalenian, and two Azilian; this shows that travel communication with the continent continued throughout that period, in all probability by means of a land connection. In late Neolithic times the English Channel was formed, Great Britain became isolated from Europe, and Ireland lost its land connection first with Wales and then with Scotland. CHANGES OF CLIMATE