22O HISTORY OF THE EUROPEAN FAUNA. 



the Imperial Academy of St. Petersburg, helps 

 us out of the difficulty. He found that, as a 

 rule, these so-called fossil glaciers contain seams 

 of mud and sand, and he argued that the ice had 

 formed, and is still forming at the present day, in 

 fissures of the earth. I entirely concur with this 

 view. Neither palaeontology nor the geographical 

 distribution of animals lend any support to the other 

 theory, and I think we may conclude that Brandt's 

 view in the main is probably the correct explanation 

 of the phenomena which we have discussed. Some 

 important facts of distribution are more easily explic- 

 able on this assumption. Why, for instance, should 

 the Siberian fauna of pliocene times have remained 

 in Siberia and not have migrated to Europe at that 

 time? The pliocene mammals of Siberia are mostly 

 of southern origin. Their range increased enormously 

 during the epoch throughout Northern Asia. We 

 should expect them, therefore, to have crossed the 

 Caspian plains, or even the low-lying Ural Mountains, 

 to pour into the neighbouring continent. But Pro- 

 fessor Brandt explained how they were prevented 

 from spreading west. An arm of the sea stretched 

 from the Aralo-Caspian to the Arctic Ocean, thus 

 raising an effectual barrier between the two con- 

 tinents. There is some evidence for the belief, as we 

 shall learn presently, that this marine barrier existed 

 also during the early part of the pleistocene epoch, 

 After having greatly expanded during pliocene times, 

 the fauna of Siberia gradually withdrew from the 



