480 Proceedings of the Royal Irish Academy. 



Russia, Greece, or any of the Mediterranean islands, except Sicily, 

 where it is present, according to Doderlein ^24). Then there are 

 forms -vrhich, though they have come to the British Islands from 

 Southern Europe, have probahly not originated there, but in Central 

 or Southern Asia. Certaia of these Asiatic species have joined what 

 I call this " SoutTiern Migration,'''' but subsequently they have invaded 

 Europe along with the Siberian migration. In most of these instances, 

 however, the members of the earlier southern migration belong to a 

 different variety from those of the later one, or exhibit such racial 

 cliaracters, that naturalists are able to distinguish them from one 

 another, and thus differentiate between the two migrations. On p. 452, 

 I referred to two of such cases, viz. those of the European hare and 

 the bullfinch, and I shall mention others directly. 



I have more than once drawn attention to the important role played 

 bv the land mollusca in elucidating former changes of land and sea. 

 1 he mollusca of islands are of great importance in studying geo- 

 graphical distribution. A knowledge of the moUuscan fauna of such 

 islands as Ireland, Sardinia, and Corsica will help us to solve many of 

 the problems associated with their former continental connexions. 

 Especially is this the case with the slugs. As the sea forms an 

 impassable barrier to slugs, being deadly both to themselves and their 

 eggs, the occurrence of the same species on an island and the adjoin- 

 ing mainland, proves that these were formerly connected by land. 



The chief centres of the creation of species in the Holarctic region 

 are all in southern latitudes, as Dr. Simroth has pointed out (77, 

 p. 20). One active creative-centre lay, according to this learned 

 malacologist, in South-western Europe, another in the Caucasus. 

 Tliis agrees perfectly with the data which we possess of the geo- 

 graphical range of slugs. For instance, the genus Arion undoubtedly 

 originated in South-western Europe. Most of the species are stiU 

 confined to the Spanish peninsula, and if we proceed south, east, or 

 north, the number of species gradually decreases, and outside Europe 

 and Ifortherii Africa the genus is quite unknown. If we suppose the 

 French west-coast to have been continued north as far as the Irish, an 

 Avion, proceeding to migrate from its original home in South-western 

 Europe, would have had about as far to go to Ireland as to Germany, 

 and, indeed, an equal number of species inhabit both countries. But 

 in the more distant Piussia, Scandinavia, and Turkey the numbers of 

 species of Arion are much fewer. 



Then we have the peculiar genus Geomalacus almost confined to 

 the Spanish peninsula, which genus, owing to its discontinuous distri- 



