180 LAND REGIONS. 



the Cape (introduced), and nearly all the northern portion of 

 North America. Helix cellaria inhabits Europe and the Northern 

 United States, and has been carried abroad with the roots of 

 plants, or attached to water-casks, and naturalized at the Cape 

 and New Zealand. Testacella maiigei has been transported from 

 the Canary Islands to England. Helix similaris, a native of 

 Eastern Asia, lives in South Africa, Australia, Polynesia, Brazil, 

 the West Indies, etc., wherever the coftee-tree is cultivated. 

 Ennea bicolor has a very similar distribution, by human agency. 

 The Canary Islands, Azores and Madeira support a considerable 

 number of Hluropean Helices, introduced with cultivated plants : 

 even St. Helena has a half-dozen acclimated European species. 



Several species of fresh-water pulmonates, Limn^a, Ph3^sa, 

 Planorbis, are common to both continents : some of them well 

 distributed over either, others apparently originating in one and 

 introduced into the other. American species thus occvir in 

 England in isolated localities, and others known to be of Euro- 

 pean origin have from time to time been made known as inhabiting 

 the United States. 



The best known and most instructive instance of the spread 

 of a bivalve mollusk is that of Dreissena jjoh/morpha. Pallas 

 discovered it in 1*769 at the mouth of the River Volga, which 

 empties into the Caspian Sea. It was subsequently found in 

 the rivers flowing into the Black Sea, the Dneiper, Danube, etc. 

 From the latter it is supposed to have been carried into Germany 

 by the pontoon trains during the wars of Napoleon, and became 

 known as a German mollusk in 1814 ; as English, toward 1824. 

 In London the}^ have caused trouble by growing in the water- 

 pipes, and in 1834 they appeared in Edinburgh. They were 

 discovered in Belgium in 1833, and their march into various 

 parts of Prance has been heralded by various local collectors at 

 periods extending from 1838 to 1866 : so that in twenty-five 

 3^ears this mollusk spread throughout all the great hydrographic 

 basins of France. If, says Fischer, this extension had occurred 

 several centuries ago, it would have been impossible to ascertain 

 the original locality of the species, except by reference to its 

 fossil remains : these show that it did not exist in the quarter- 

 naries of Western Europe, bvit that it occurred in the chalk of 

 the Steppes. 



The Old World and America may be regarded as provinces of 

 paramount importance, having no indigenous species in common 

 (except a few in the extreme north), and each possessing many 

 chai'acteristic genera. 



