PRELIMINARY CONSIDERATIONS. 47 



peculiar, and which we may consider to have had 

 their origin there. Almost the whole of the British 

 fauna is composed of streams of migrants which came 

 from the north, south, and east, though many of 

 these immigrant species have since their arrival been 

 more or less distinctly modified into varieties or local 

 races. 



The eminent French conchologist Bourguignat (a, p. 

 352) was of opinion that, as far as terrestrial mollusca 

 were concerned, there are in Europe three principal 

 centres of creation or dispersion all situated in 

 mountainous countries and not in the plains. He 

 distinguished the Spanish, Alpine, and Tauric centres, 

 and believed that almost all species known from 

 Europe had originated in one of these three, and that 

 each of them possessed quite a distinct type of its 

 own. This theory seems to agree very well with the 

 facts of distribution. Let us take, for instance, the 

 genus Clausilid) a pretty turret-shaped snail, which 

 abounds on old ruined walls. Only two species, viz., 

 CL laminata and CL bidentata, are met with in Ireland. 

 In England we find the same species with the addition 

 of two others, CL biplicata and Cl. Rolphii. Crossing 

 over the Channel to Belgium, these four species occur 

 again, and also several others not known in England. 

 In Germany the list of Clausilice mounts up to twenty- 

 five species, including all those found in the British 

 Islands. As we proceed eastward the number of 

 species of this genus increases steadily, and when we 

 reach the Caucasus or the Balkan Peninsula the con- 



