DISTRIBUTION AND ORIGIN. 253 



The British. Isles, geologists tell us, once formed a portion of the 

 European Continent. The character and distribution of their land 

 and freshwater mollusks, which are Continental species in dimi- 

 nished number and variety, go to support that view. The great 

 Caucasian province, of which they are outlying fragments, embraces 

 the whole of Europe and Russian Asia, with a portion of Japan, 

 the whole of North Africa, Asia Minor, Arabia, Persia, Thibet, 

 and a portion of India. The specific centre of its molluscan popu- 

 lation is about the region of Hungary and the Caucasus. In 

 India and beyond Thibet the species commingle with those of 

 another province, the Malayan, which embraces Siam, Cambojia, 

 Malacca and the Indian Archipelago, China, probably, in part, for 

 we know little or nothing as yet of either the fauna or flora of that 

 country, and Corea, with the remaining portion of Japan, and 

 whose specific centre is about the region of Borneo, and the Mo- 

 lucca and Philippine groups. A partial subdivision of the Cau- 

 casian province of distribution is indicated in Southern Europe by 

 the chain of the Pyrenees, Alps, and Carpathians ; and the districts 

 north and south of that mountain-barrier have been designated the 

 Germanic and Lusitanian regions. It will, however, be seen that the 

 land and freshwater mollusks on either side of this chain are of 

 the same ideal type. 



Edward Forbes, as all naturalists are aware, propounded a theory, 

 supported in a most comprehensive manner by established geo- 

 logical theories, to the effect that the British Isles were peopled 

 with animals and plants from the European continent, partly by 

 transmission on floating masses of ice, chiefly by transmission 

 through migration before the land became isolated. His theory 

 was based on the general and traditional bebef of mankind that 

 all the individuals of a species have descended from a single pro- 

 genitor (or pair) ; and the point in the geographical province at 

 which it might be assumed there was originally the most nume- 

 rous and highly developed assemblage of progenitors, he called its 

 specific centre. During the sixteen years which have elapsed since 

 this theory was broached, and more especially since the loss of its 

 illustrious founder, great progress has been made in the accession 

 and collation of materials indispensable for its consideration. The 

 desire which it has elicited among naturalists to inquire into the 

 origin of species, has stimulated a more carefid registration of 

 habitats, and more detailed comparisons of local faunas and floras ; 

 and a vast amount of additional information has been collected 



