THE SIBERIAN MIGRATION. 195 



Reptiles and Amphibians, that the surplus population 

 of our neighbouring continents readily poured into 

 Europe when owing to changes of climate perhaps 

 they forsook their original homes. 



We observe much the same differences of origin in 

 the various groups of European Invertebrates. The 

 Central European Molluscan fauna, remarks Dr. 

 Kobelt, had already developed from the pliocene 

 in almost all its details, as regards formation of 

 species and distribution when the Ice-Age com- 

 menced (3, i. p. 162). Certain very interesting disloca- 

 tions, however, in the range of land mollusca can be 

 proved to have taken place about that time. Thus, 

 as Dr. Kobelt has pointed out, the genus Zonites, 

 which is now almost confined to the south-east of 

 Europe, occurs in inter-glacial deposits in the valley 

 of the Neckar, and even as far west as the Seine. If 

 we might judge from this single instance, a molluscan 

 migration from the east to the west seems to have 

 occurred either in early or pre-glacial times. That 

 Helix pomatia has migrated only comparatively 

 recently from the East to Western Europe is 

 rendered probable by its general range in northern 

 and western Europe, but I cannot agree with Dr. 

 Kobelt in the belief that Helix aspersa is of an 

 equally recent origin in the North. No matter 

 whether it has been found fossil or no, its range in 

 the British Islands points to its having penetrated to 

 Ireland when the latter was still connected with the 

 Continent by way of England. Its migration from 



