220 THE DISPERSAL OF SHELLS. 



successful molluscan colonists in the world. The floating 

 of rafts of timber for commercial purposes from place to 

 place upon rivers and canals has doubtless greatly 

 facilitated its dispersal, and it may frequently attach it- 

 self to the bottoms of canal-boats ; possibly, also, from 

 having free-roving larvae which live on the surface for a 

 time, it possesses considerable facilities for dispersal in- 

 dependently of man, and, as just mentioned, it has 

 found its way, in this country, to unnavlgable water ; 

 every naturalist, the Rev. J. Dalton has observed, can 

 " cite examples of Drcissena polyvwrpJia turning up 

 profusely in waters where it had hitherto been un- 

 known, and whither its progress could not be traced." ' 

 The creature is able to seize quickly upon new and un- 

 occupied water, and this must have greatly aided it in the 

 struggle with other forms. As already noted it was found 

 in immense numbers in new docks at Goole within three 

 or four years after they were opened. Some time ago, 

 a canal near Northampton was diverted by a railway 

 company, and after three or four years the stones form- 

 ing the new banks were found to be entirely covered 

 with the shells of this species.- In the neighbour- 

 hood of Gloucester, also, it is said to have appeared 

 within a few years after the opening of the Glouces- 

 ter and Berkeley Canal, and to have subsequently 

 increased greatly, so as to almost line the banks, from 

 the edge of the water to a considerable depth, from 



' J. Dalton, "Zoologist," xix. (1861), pp. 7318-9. 

 * W. D. Crick, as cited by Taylor and Roebuck, " Journ. North- 

 ants. Nat. Hist. Soc," iii. (1885), p. 212. 



