430 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 5, 



Eeindeer, Elks, Foxes and Wolverines make up for tlie 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 Pleis- 

 tocene 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 Konigsberg, 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 inti- 

 mate 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 Mediter- 

 ranean as far north as Kirkdale, or the Ileindeer from the north as 

 far south as the Alps. In the secular lowering of the temperature, 

 the northern animals would compete with tlie 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 elassificatory 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 wdnter 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 



