Besides these animals there were associated with man at this time the Musk Sheep and the Reindeer, both now confined to the regions of the far north, but during the Glacial period ranging into southern France, and mingling their bones with those both of man and of the southern species already enumerated. [Illustration: Fig. 92.--Musk-sheep (_Ovibos moschatius_).] The Wolverine, the Arctic Fox, the Marmot, the Lemming--all now confined to colder regions--at that time mingled on the plains of central Europe with the species mentioned as belonging now to Africa and southern Asia. The Ibex, also, and the Snowy Vole and Chamois descended to the plains from their mountain-heights, and joined in the strange companionship of animals from the north and from the south. Besides these extremes there were associated with man during the Glacial period numerous representatives of the temperate group of existing animals, such as the bison, the horse, the stag, the beaver, the hare, the rabbit, the otter, the weasel, the wild-cat, the fox, the wolf, the wild boar, and the brown bear. [Illustration: Fig. 93.--Reindeer.] To account for this strange intermingling of arctic and torrid species of animals, especially in Europe, during man's occupancy of the region in glacial times, various theories have been resorted to, but none of them can be said to be altogether satisfactory. One hypothesis is that the bones of these diverse animals became mingled by reason of the great range of the annual migration of the species. The reindeer, for example, still performs extensive annual migrations. In summer it ventures far out upon the _tundras_ of North America and Siberia to feed upon the abundant vegetation that springs up like magic under the influence of the long days of sunshine; while, as winter approaches, it returns to the forests of the interior. Or in other places this animal and his associates, like birds of passage, move northward in summer to escape the heat, and southward in the winter to escape the extreme cold. Many of the other animals also are more or less migratory in their habits. Thus it is thought that during the Glacial period, when man occupied northern France and southern England, the reindeer, the musk sheep, the arctic fox, and perhaps the hippopotamus and some other animals, annually vibrated between northern England and southern France, a slight elevation of the region furnishing a land passage from England to the continent; while the chamois and other Alpine species vibrated as regularly between the valleys in winter and the mountain-heights in summer. The habits of these species are such that it is not difficult to see how in their case this migration could have taken place.