262 HISTORY OF THE EUROPEAN FAUNA. 



is of Asiatic origin, or whether some of the species 

 have migrated from Europe to Asia, I am not pre- 

 pared to say; but there can be no doubt that Helix 

 pomatia has reached Western Europe from the east. 



On the whole, the number of mollusca which we 

 might point to as having migrated to Europe is not 

 large, the great majority being indigenous to our 

 continent. However, some of the other groups of 

 invertebrates differ very materially in that respect 

 from the mollusca. I cannot leave the consideration 

 of the mollusca without referring to the fact that 

 there appears to be a very important centre of 

 distribution in South-eastern Europe. It is from 

 this centre that many species have spread north 

 and south, east and west Take, for example, the 

 genus Clausilia, a small land-shell shaped like a 

 pointed round tower, and abundant on old walls 

 and tree trunks. In England we have four species 

 of Clausilia> in Ireland only two. In the greater 

 part of Spain only our common Cl. bidentata occurs. 

 As we go east the number of species rapidly 

 increases. A maximum is reached in South-eastern 

 Europe, where hundreds of different kinds are found. 

 Towards Northern Europe a similar decrease of 

 species takes place. So far the history of the 

 Clausilice seems perfectly simple. An active centre 

 of origin appears to exist in South-eastern Europe, 

 from which the species radiate out in all directions. 

 But when we come to look more closely into the 

 extra-European distribution of the genus, and 



