Reindeer, Elks, Foxes and Wolverines make up for the poverty of vegetation by the rich abundance of animal life. Enormous flights of Swans, Geese, and Ducks arrive in the spring, and seek deserts, where they may moult and build their nests in safety. Ptarmigan run in troops among the bushes; little Snipes are busy among the brooks and in the morasses; the social crows seek the neighbourhood of new habitations; and when the sun shines in spring one may even sometimes hear the cheerful note of the Finch, and in autumn that of the Thrush." The hypothesis of a series of conditions in Europe, in Pleistocene times, similar to those of Northern Asia or of Northern America, would amply satisfy the difficulty of the case. In the Pleistocene winter the northern animals would pass southwards, and in the summer the southern forms would creep northwards; and to this swinging to and fro of the animals, according to the seasons, the peculiar intermixture of their remains, over what may be called the debatable ground of Central Europe, may be accounted for, the head quarters of the northern animals being to the south-east of a line drawn from Yorkshire and Königsberg, and the head quarters of the southern being the regions bordering the Mediterranean.
It must be borne in mind that this mode of explaining the intimate association of the mammalia of the north and south, in the same deposit of the same river, does not imply that in one season a migration took place from the head quarters of each of these groups to the extreme point to which the remains of the animals of which it is composed occur, such as the Hippopotamus from the Mediterranean as far north as Kirkdale, or the Reindeer from the north as far south as the Alps. In the secular lowering of the temperature, the northern animals would compete with the southern for their feeding-grounds, according to the season. And this competition, if the climatal conditions were stationary, might be carried on, over a very small area, for a very long time—the debatable ground being a narrow band between the invaders and the animals in possession. There were probably many such pauses. Nor does it imply that there were no reversions to a warm, or temperate, state after the glacial conditions had begun in Northern Europe. One such reversion at least is proved by the physical evidence to have taken place; but it has left no impression on the mammal fauna sufficiently marked for classificatory purposes. The Middle-Pleistocene mammalia may be the result of such a reversion, since the mixture of forms brought about by the southward advance of the northern animals would be the same as that of their retreat. The predominance of the northern over the southern animals in Central Europe implies that the winter cold during the Pleistocene age was more severe than it is now; and this conclusion is corroborated by the condition of the river-deposits in which they are found in France and Britain. The contortions of the gravels and the angularity of the pebbles are, according to Mr. Prestwich, only explicable on the theory of ice having been formed in our rivers in far larger quantities than at the present day. The large plateaux of brick-earths were probably deposited by floods, caused by the sudden melting of the winter
