INTRODUCTION. 9 



a migration on a vast scale entered Europe during 

 the Pleistocene epoch the most recent of the geo- 

 logical epochs, during which great extensions of 

 glaciers occurred in the mountainous regions of 

 Europe. The latter period is known to us as the Ice 

 Age or Glacial period. This will be described more 

 fully in Chapter II., meanwhile I may mention that 

 we presume that this migration came from the east, 

 because no remains of the members of that particular 

 fauna are known from Spain, Southern Italy, Scandi- 

 navia, Ireland, or from the Balkan peninsula. The 

 number of species evidently belonging to this same 

 migration, moreover, become fewer as we proceed 

 westward, and a large proportion of them still inhabit 

 Northern Asia, though most of them are now extinct 

 in Europe. After having thoroughly studied such a 

 recent geological migration, we learn to understand 

 others better, though the more ancient they are, the 

 fewer are the traces and the more difficult are they to 

 follow. 



Then again we have to take into consideration the 

 fact, that whilst mammals, particularly the larger 

 herbivores, are forced to migrate frequently owing 

 to scarcity of food or temporary changes of climate, 

 many of the invertebrates remain practically unaffected 

 by either. Most of our land mollusca, for instance, 

 are satisfied with meagre provender, and stand ex- 

 tremes of climate well, as long as there is sufficient 

 moisture. As a result of their peculiar disposition, 

 many of them, no doubt, have survived through 



