284 HISTORY OF THE EUROPEAN FAUNA. 



ranean Sea broke across the ^Egean region, and that 

 the Northern Sea retired from the plains of Eastern 

 Russia to admit the typical Siberian fauna and flora 

 into our continent (vide pp. 189-241). 



I cannot close this chapter without referring to the 

 active distributional centre or I might say, centre of 

 origin of species situated in South-eastern Europe. 

 No group of animals is more instructive in eluci- 

 dating the paths of migration from this centre than 

 the terrestrial mollusca. Wherever the original home 

 of the genus Clausilia may have been in early Tertiary 

 times, it is certain that the most active centre of 

 origin is now, and has been for a considerable 

 time past, in South-eastern Europe. One of the 

 earliest migrants from that modern centre of this 

 interesting genus is Clausilia bidentata, which is the 

 only species found in Southern Spain, and one of 

 the two met with in Ireland, and which has been 

 observed in high altitudes in the Alps and in 

 Scandinavia. As we go eastward from Western 

 Europe the number of species of Clausilia^ as we 

 have seen, increases until we reach a maximum in 

 the Balkan peninsula and the region of the Caucasus. 

 Limax, Agriolimax, and Amalia, three genera of 

 slugs, likewise appear to have originated in the same 

 region and spread over Europe from there. Some 

 species like Limax maximus and L. marginatus are 

 very ancient, and probably commenced their wander- 

 ings in early Tertiary times. In this manner many 

 animals of European origin have joined the Oriental 



