282 WIDE DISTRIBUTION OF FRESH-WATER MOLLUSCA chap. 



of fresh-water Mollusca in New Zealand, one of the most isolated 

 districts known, only one is peculiar. In South Africa and the 

 Antilles no genus is peculiar. In the latter case, this fact is 

 remarkable, when we consider that the same sub-region has at 

 least ten peculiar genera of operculate land Mollusca alone. 



To give a few instances of the distribution of particular 

 species: — 



Limnaea stagnalis L. occurs in the whole of Europe, and 

 northern Asia to Amoorland, Turkestan, Afghanistan, North 

 Persia, and Kashmir ; Greenland, North America from the 

 Atlantic to the Pacific, and from North Canada and British 

 Columbia as far south as Texas. The distribution of L. i^eregra 

 Miill., L. truncatula MiilL, and L. palustris Miill. is almost 

 equally wide. 



Planorhis alhus occurs in the whole of Europe, and northern 

 Asia to Amoorland, Kamschatka, and Japan ; Turkestan, the 

 Altai-Baikal district, Alaska and Greenland, North Canada, and 

 the whole of eastern North America. 



The distribution of Anodonta anatina L., Cyclas cornea L., 

 and Plsidium pusillum Gmel. is almost equally wide. 



It is evident that the accidental means of transport mentioned 

 above are insufficient to account for the facts as we find them ; 

 we are therefore compelled to seek for further exjDlanation. 

 Anything in the nature of a current furnishes a ready means 

 of transport for Mollusca which have obtained a footing in the 

 upper waters of a river, and there is no difficulty in imagining 

 the gradual spread of species, through the agency of floods or 

 otherwise, over a whole river system, when once established at 

 any point upon it. The feeble clinging power of newly-hatched 

 Limnaea has often been noticed as contributinor to the chances 

 of their range of distribution becoming extended. Fresh- water 

 Mollusca, too, or their ova, are exceedingly likely, from their 

 extreme abundance, to be transported by water-birds, which fly 

 without alighting from one piece of water to another. Again, 

 the isolation of one river system from another is, in many 

 instances, by no means well marked or permanent, and a very 

 slight alteration of level will frequently have the effect of 

 diverting the supplies of one watershed into another. When we 

 know what enormous oscillations in level have taken place over 

 practically the whole surface of the globe, we can recognise the 



