214 THE DISPERSAL OF SHELLS. 



remarked that they are all local species, and their 

 presence in this country can hardly be said to be con- 

 spicuous, for an observer, unguided by published 

 records, might well collect for years, or for a life-time, 

 without finding them, while on the other hand, D. 

 polymorpha is " so generally distributed that it cannot 

 fail to attract the attention of the collector." ^ Dr. 

 Morch, it is true, has argued that there is no reason for 

 supposing the creature's non-existence in the west of 

 Europe previously to 1824, maintaining that it is 

 SdiTidQx'sPiitnaJiuviatilzs^hut eminent conchologists have 

 denied that this is the case.'- In the errata given at the 

 end of his volume, Jeffreys corrects the statement 

 respecting the specimen referred to by D'Orbigny, which, 

 it appears, was recent, not fossil. The animal is known, 

 however, to have occurred anciently in North Germany, 

 in the diluvium of East and West Prussia, etc., but it is 

 there supposed to have been driven away, by unfavour- 

 able climatic conditions or otherwise, and to have 

 subsequently regained its former range by immigration.^ 

 This species has frequently been referred to as illus- 

 trating the rapidity with which a mollusc may become 

 naturalized and diffused over a large area, and it is 

 even said to flourish in this country ;, under certain con- 

 ditions, greatly in excess of any native form.^ In 



' Harting's " Rambles," 1875, P- 4i- 



^ See O. A. L. Morch, "Ann. and Mag. Nat. Hist.," (3), xix. 

 (1867), pp. 82.4. 



^ See "Ann. and Mag. Nat. Hist.," (6), ix..(i892), p. 160. 



* See H. H. Higgins, "Address to Liverpool Nat. Field-club, 

 30 January, 1891," p. 7. 



