If the present habits of these animals be any index to their mode of life in the pleistocene age, their presence in the area north of the Alps and Pyrenees implies that the climate in France, Germany, and Britain was severe, or analogous to that which they now enjoy on the tops of lofty mountains, or in the northern Asiatic steppes, or the high northern latitudes of America. But this conclusion is diametrically opposed to that which is based on the evidence of the southern group of animals.[261] And the remains of the two groups of animals are so associated together in the caves, and river-deposits of Europe, north of the Pyrenees, that it is impossible to deny the fact that it was the common feeding-ground of both during the same era.[262] _Probable Cause of Association of Northern and Southern Groups._ Must we then infer that in the pleistocene age the present habits of the musk-sheep, the reindeer, chamois, or ibex, were so changed as to allow them to flourish side by side with the hippopotamus, or _vice versâ_? Was the climate colder than it is now in Europe, or was it hotter? How was this singular association of northern and southern species brought about? The problem may be solved if we refer to the present distribution of animals in northern Asia and North America. As the winter comes on the arctic species gradually retreat southwards, and occupy the summer feeding-grounds of the elk, red-deer, and other creatures which are unable to endure the extreme severity of an arctic winter. In the spring the latter pass northwards, to enjoy the summer herbage of that area, which had been the winter-quarters of the arctic group of animals. Thus there is a continued swinging to and fro, over the same region, of the arctic and the temperate animals, and their remains must necessarily become more or less associated in the river-deposits, as well as in caves, where these last happen to occur. In northern Asia, and in America, the only boundary between the northern and temperate zoological provinces is that constituted by the fluctuating annual temperature, and there are no great hilly barriers running east and west, to prevent free migration to the north or south. If reference be made to the map, Fig. 126, it will be seen that these conditions were amply satisfied in the pleistocene age. There were no physical barriers to migration, from the shores of the Mediterranean, as far north as Ireland. If the winter cold were severe, the reindeer and musk-sheep might advance as far south as the Pyrenees, and if the summer heat were intense there would be nothing to forbid the hippopotamus and the African carnivores advancing northwards. It seems to me that this is the only hypothesis which will satisfy all the facts of the case. The traces of glaciers and snow-fields where they are no longer found prove that the winter was severe; while the warmth of the summer seems to be sufficiently demonstrated by the presence of African species.