278 LOCALISATION OF GENERA AND SPECIES chap. 



wich Islands, in spite of the enormous intervening distance, are 

 not very different from those of Natal, but the land MoUusca of 

 the two countries are as widely different as is possible to imagine. 



Land Mollusca are, as has been remarked, fettered to the soil. 

 Quadrupeds, birds, fishes, and reptiles are provided with organs of 

 motion which enable them to overpass barriers of various kinds. 

 Even plants, although themselves incapable of motion, may be 

 conveyed in every direction by means of seeds, which are either 

 wafted by the wind or adhere to the skin of animals. But the 

 Mollusca have no such regular means of transport, and are, in a 

 large number of instances, limited to districts of a certain char- 

 acter of soil, or producing certain kinds of vegetation. 



The localisation, both of genera and species, occurs all over 

 the world. The genus Achatinella^ w^hich is peculiar to the 

 Sandwich Islands, is found there in a profusion of species. It 

 lives in the mountain valleys which radiate from the central 

 ridge of each island, and each valley is characterised by its own 

 peculiar set of species. The great carnivorous Glandina is_ 

 restricted to Central America and the adjacent parts of the two! 

 continents, with one or two species in Southern Europe. Bulimus 

 proper is restricted to South America; Achatina to Africa south 

 of the Sahara ; Tornatellina to the Pacific Islands ; Cochlostyla 

 to the Philippines ; Cylindrella and Bulimulus are peculiar to 

 the New World ; Buliminus^ Nanina^ Scarahus^ and Cassidula to 

 the Old. 



Extreme cases of this restriction of habitat sometimes occur. 

 Thus Limnaea involuta is found only in a single small mountain 

 tarn in Ireland ; Clausilia scalaris along a narrow strip of lime- 

 stone in Malta ; Strophia naria is confined to a few square rods 

 on an island that is itself a mere dot in the Caribbean Sea ; the 

 genus Camptonyx occurs only in the neighbourhood of Mt. Girnar, I 

 in Gujerat ; and Lantzia in moss on the top of a mountain in I 

 Bourbon. f 



Attempts to colonise snails in strange localities have usually 

 resulted in failure, especially when the attempt has involved . 

 serious changes of environment. The common Cochlicella acuta 4 

 of our coasts resists all endeavours to establish it beyond a cer- , 

 tain distance from the sea. Snails brought from the Riviera and 

 placed under almost similar conditions of climate on our own ^ 

 southern coasts have lived for a while, but have very rarely taken } 



i\ 



