230 HISTORY OF THE EUROPEAN FAUNA. 



place, as the climate renders Northern Siberia 

 almost uninhabitable for mammals at the present 

 day, how much more severe must it have been 

 during the time of the maximum glaciation in 

 Europe. As the then existing fauna was not 

 driven into Europe, where could it possibly have 

 survived ? Secondly, how can we reconcile the 

 contemporaneous existence of a great inland sea 

 (the Aralo-Caspian) containing survivals of mild 

 Sarmatic times with an immense glacier almost 

 touching it on its northern shores? How did 

 one of the most characteristic species of that sea, 

 Dreyssensia polymorpha, come to make its appearance 

 in the lower boulder-clay of Prussia and then dis- 

 appear in the upper? And finally, how are we to 

 explain the sudden appearance of a Siberian fauna 

 after the deposition of the lower boulder-clay, except 

 by the removal of a barrier which had prevented 

 their egress from Siberia? 



If we assume that the continental boulder-clay of 

 Russia has been formed in the manner so ably 

 explained by Murchison, de Verneuil, and von Key- 

 serling, viz., by a sea with floating icebergs, the 

 temperature of Siberia might have been higher than 

 at present, and have supported a fauna in more 

 northern latitudes. 



The contemporaneousness of the deposits of this 

 sea with those of the Aralo-Caspian is also rendered 

 more intelligible. If we suppose, moreover, the con- 

 nection between the Aralo-Caspian and the White 



