CHAP, v.] MAMMALIA AND GLACIAL PHENOMENA. 121 



Relation of Mammalia to Glacial Phenomena. 



The complicated glacial phenomena summed up in 

 the preceding pages imply not merely a change from a 

 temperate to a cold climate of extreme severity, but they 

 show a climatal fluctuation of the sort which might be 

 expected from the examination of the Pleistocene mam- 

 malia. When the reindeer inhabited the south of France 

 the cold was at its maximum, and when the hippo- 

 potamus lived in England the cold was probably at its 

 minimum. Each of these changes was probably brought 

 about during a long series of ages, and each has left its 

 mark in the mixed fauna of the middle zone of the map 

 (Fig. 24). 



The lowering of the temperature was probably the 

 cause of the immigration into Europe of the Asiatic 

 species. As the cold increased in Asia, and the warm 

 Pleiocene climate of northern and central Europe gra- 

 dually became cool, the animals which had been living in 

 Asia for an unknown series of years poured in, a way 

 being opened to them by the elevation of a low-lying 

 tract of land at the head of the Caspian and the Gulf of 

 Obi, which had probably hitherto been the bottom of a 

 shallow sea cutting them off from Europe. It must be 

 remarked that a change towards cold conditions has 

 already been indicated by the ice-borne blocks of stone 

 met with on the Pleiocene sea-shore of Suffolk. A vast 

 migration of animals set in from Asia, analogous in every 

 respect to that by which the European peoples arrived 

 at their present homes, and following for the most part 

 the same route, between the Caspian Sea and the Ural 

 mountains (see Fig. 24, p. 111). 



