THE SIBERIAN MIGRATION. 231 



Sea (Fig. 12, p. 156) to have existed at this time, we 

 possess an explanation of the method of migration of 

 the Arctic marine species into the Southern and of 

 the Caspian species (Dreyssensia) into the Northern 

 Sea. 



An inter-glacial phase is believed to have super- 

 vened after the deposition of the lower boulder-clay, 

 and it is during this period that the Siberian species 

 first appeared in Central Europe. If we assume then 

 that the retreat of the Northern Sea (Fig. 13, p. 170) 

 opened up a passage for the Siberian fauna, we have in 

 this very fact also an explanation of the extraordinarily 

 large exodus of Asiatic "mammals, because the great 

 reduction of the marine area in Northern Europe 

 would have had an important influence in lowering 

 the temperature in Asia. Only a sudden change of 

 climate in Siberia could have brought about the 

 migration of the vast hordes of Asiatic mammals 

 whose remains we find in Central and Western 

 Europe in deposits of that period. 



Throughout this work we are made acquainted 

 with facts which bear out the view that the climate 

 during the greater part of the Glacial period was 

 mild rather than intensely arctic in Europe. That 

 a huge ice-sheet could have covered Northern Europe 

 under such conditions appears to me very doubt- 

 ful. No one can deny, however, that glaciers must 

 have existed during the Glacial period in all the 

 mountainous regions of Central and Northern 

 Europe, though their existence is not incompatible 



