DISTRIBUTION OF MOLLUSCA GENERALLY. 261 



island group, when surrounded by deep water, has its own dis- 

 tinctive molluscan fauna. This we see in such islands as Cuba, 

 Jamaica, Hayti, Madagascar, New Caledonia, the Philippines, &c., 

 where the faunal peculiarity manifests itself not only among species, 

 but extends largely to genera. Even the very much smaller islands 

 of Malta, Cos, Naxos, Corfu, Zante, Lesbos, Rhodes, &c., in the 

 Mediterranean, have a full share of species which are not found 

 elsewhere. The Atlantic island groups of the Azores, Canaries, 

 and Cape Verde are still more marked in their individuality. Both 

 species and genera are here largely restricted to the several groups, 

 and of the forms which ally them with the European fauna a large 

 number are such as have been introduced by man. The com- 

 mon Holarctic fresh-water genus Unio is entirely wanting, and, 

 indeed, in the Azores there is not a single fluviatile form repre- 

 sented. Of the eighteen species of Bulimus which constitute the 

 entire non-marine molluscan fauna of the Galapagos Islands ten are 

 restricted to single islands of the group. The genus Achatinella, 

 which is restricted to the Sandwich Islands, has there some two 

 hundred and eighty-eight species or varieties, or very nearly three- 

 fourths the number of the entire molluscan fauna of the region. 



This faunal individuality, the result of a long-continued isola- 

 tion of the various island groups, permitting of a gradual but 

 steady evolution of new and independent forms, does not extend 

 to Great Britain, whose separation from the continent has been 

 effected in a comparatively recent period, and after the constitution 

 of the contiguous faunas. There appear to be but two molluscan 

 species (Limnaaa involuta and Assiminea Grayana) that are distinc- 

 tive of this island group. Similarly, the identity existing between 

 the Peninsular and North African faunas points, apart from all 

 other evidence, to the recent formation of the Strait of Gibraltar. 



The natural barriers which interpose themselves to the free 

 migration of the terrestrial and fluviatile Mollusca, such as inter- 

 cepting land and water areas, elevated mountain-chains, or deserts, 

 are apparently much more numerous than those affecting marine 

 forms, and account for the comparatively limited range of most 

 species. Yet it is remarkable how far certain forms or types of 

 forms have spread. Thus, the two most important genera of land 

 and fresh-water mollusks, Helix and Unio, and among fresh-water 

 pulmonates, the genera Limnsea, Physa, Ancylus, and Planorbis, 



