THE SIBERIAN MIGRATION. 243 



fauna. Recent geological ages have witnessed the arrival in 

 Southern Europe of mammals now almost confined to the arctic 

 and subarctic regions. In Siberia, on the other hand, many 

 southern species penetrated, apparently about the same time, 

 to the extreme northern limits of that country. The greatest 

 authority on the Siberian fossil fauna, Tcherski, believes that 

 this took place in pliocene times, the gradual retreat occupy- 

 ing the whole of the Glacial period. If this were correct, the 

 retreat from the Arctic Regions would have occurred at 

 the same time when, according to our European authorities, 

 Professors Nehring and Geikie, the much more southern 

 parts of our continent were already uninhabitable. But 

 Siberia could not have supported the large mammals at all 

 at a time when Europe was uninhabitable, as it would be 

 difficult to conceive under what geographical conditions the 

 climate of the latter was arctic and that of the former temperate. 

 If the whole fauna was driven into Southern Asia, how is it that 

 the Siberian invasion of Europe occurred immediately after the 

 deposition of the lower boulder-clay, that is to say, after the 

 earlier part of the Glacial period? The difficulty can be met by 

 the supposition that both Europe and Siberia had a temperate 

 climate at that time. This view is supported by certain 

 evidences, fully described, of a connection between the Caspian 

 and the White Sea, which would have had the effect of influ- 

 encing the climate. The Siberian fauna would thus have been 

 prevented from spreading westward in Pliocene and early 

 Glacial times. But on the disappearance of the marine con- 

 nection, a way would have been opened into our continent, 

 which again had an effect on the climate. The latter would 

 have become sensibly colder and thus have reduced the 

 habitable area of the Siberian fauna. 



Such geographical conditions would have been incompatible 

 with a great northern mer de glace, and the boulder-clay in 

 Northern Europe could not have represented a ground moraine 

 but is a marine deposit. The sea is supposed to have covered 



