X BARRIERS TO DISPERSAL 28 1 



stretches, with scarcely a break, from the west coast of Africa 

 to the extreme east coast of Asia. The Mediterranean offers no 

 effectual barrier; shells of southern Europe are found in pro- 

 fusion in Morocco, Tunis, and Egypt, while all through Siberia 

 to the extreme of Kamschatka the same types, and even the 

 same species, of MoUusca occur. 



A detailed examination of the means, other than voluntary, 

 by which Mollusca are transported from one place to another 

 hardly comes within the scope of this work. Ocean currents, 

 rivers, floods, cyclonic storms of wind, birds, and even beetles and 

 frogs, play a part, more or less considerable, in carrying living 

 Mollusca or their ova, either separately or in connexion with float- 

 ing debris of every kind, to a distance from their native home. 

 Accidental locomotion, of one or other of these kinds, combined 

 with the well-known tenacity of life in many species (p. 37), 

 may have contributed to enlarge the area of distribution in many 

 cases, especially in the tropics, where the forces of nature are 

 more vigorous than in our latitudes. The ease with which 

 species are accidentally spread by man increases the probability 

 of such cases occurring without the intervention of human 

 agency, and numbers of instances may be collected of their 

 actual occurrence.^ 



A point, however, which more concerns us here is to remark 

 on the exceedingly wide distribution of the prevailing forms of 

 fresh-water Mollusca. It might have been expected that the 

 area of distribution in the fresh-water forms would be greatly 

 restricted, since they cannot migrate across the land from one 

 piece of water to another, and since the barriers between pond 

 and pond, lake and lake, and one river system and another are, 

 as far as they are concerned, all but insuperable. We might 

 have expected, therefore, as Darwin and Wallace have remarked, 

 to find a great multiplicity of species confined to very restricted 

 areas, since the possibility of communication with the parent 

 stock appears, in any given case, to be so exceedingly remote. 



As is well known, the exact reverse occurs. The range, not 

 merely of genera, but even of individual species, is astonishingly 

 wide. This is especially the case with regard to the Pulmonata 

 and Pelecypoda. The genera Limnaea, Flanorhis, Physa, Ancylus, 

 Unio, and Cyclas are world-wide. Out of about ten genera of 

 1 Mr. H. W. Kew, The Dispersal of Shells, has brought together a very large series. 



