THE SIBERIAN MIGRATION. 211 



smaller burrowing mammals would not have chosen 

 a wood to live in, if they could possibly help it. 

 Prairies, or sand-dunes with short grass or shrubs, 

 such as abound in Europe near the sea-coast, would 

 suit these species perfectly. If we suppose Northern 

 Germany to have been covered by sea (p. 156) during 

 part of the Pleistocene Epoch, forests would probably 

 not have grown there for a very considerable time 

 afterwards, owing to the excessive salinity of the soil, 

 but a tract of sandy country would have been left 

 on the retreat of the sea. Possibly a slight change 

 of climate in the original home of these steppe-species 

 may have reduced their habitable area, and thus 

 caused their migration into Europe. 



But this migration problem cannot be solved 

 without tracing the mammals to their place of 

 origin and investigating their early history. This I 

 shall attempt to do presently; meanwhile, it would 

 be interesting to note whether other groups of 

 animals support Professor Nehring's steppe-theory. 



Among groups other than mammals, the most 

 important, for the purpose of drawing conclusions 

 as to former physical conditions and climate, are the 

 mollusca. Their remains have been well preserved, 

 and are easily identified. Though Professor Nehring 

 argues that the molluscs found along with the small 

 mammals harmonise perfectly with the assumption 

 of a steppe-climate (p. 212), I cannot at all agree 

 with him. He enumerates the following sixteen 

 species as having been discovered by him : 



