228 HISTORY OF THE EUROPEAN FAUNA. 



the fauna from Siberia. Only in deposits later 

 than the lower continental boulder-clay do we find 

 traces of a Siberian migration. The time of maxi- 

 mum glaciation had then passed away ; the great 

 glacier which was believed to have invaded the 

 lowlands of Northern Europe had again retreated, 

 before the Siberian mammals made their appearance 

 in Germany. 



It has been stated above (p. 226) that while the 

 Russian boulder-clay was being laid down, the Aralo- 

 Caspian probably had some communication with the 

 White Sea. 



But how can this view be reconciled with the 

 existence of a huge mer de glace in the northern 

 plains of Russia? The existence of the ice-sheet 

 has been conjured up in order to explain the 

 presence of the boulder-clay. But not long ago a 

 very different interpretation of the origin of this clay 

 was given; and one, I may say, which explains the 

 history of the Siberian and the European fauna in a 

 more satisfactory manner than is done by the ice- 

 sheet hypothesis. It is that the boulder-clay is not 

 the product of land-ice, but has been deposited by a 

 sea with floating icebergs. Thus the latter hypothesis 

 does not deny the existence of glaciers, but allows 

 .the mud to be deposited on the floor of a turbid 

 sea, instead of beneath an immense mer de glace. I 

 need hardly mention that this view, which was 

 formerly universally accepted by geologists, is now 

 scouted by almost every authority, both British and 



