GEOGEAPHY OF SHELLS. 33 



voyages, being equalled only by tlieir ready assimilation 

 with opposite climates. 



It seems, also, as if some unkno^Ti causes affected not only 

 the hue but the size of shells. Perou and Lesueur remark that 

 those pertaining to the Haliofis gigantea of Van Diemen^s 

 Land, and the Phasia7ieUa, diminish in size as they follow 

 the coasts of New Holland northward of EJing George^s 

 Sound, and that they entirely disappear beyond them. 



Obstacles without doubt exist to the migration of various 

 species, such as continuous Hues of continents stretching 

 from north to south. ^Vere it otherwise, the Nautilus, and 

 Argonauta, and lanthina would everywhere abound; but 

 the reverse occiu's ; and hence the molluscous inhabitants 

 of shells that pertain to the West Indies cannot enter the 

 Pacific without passing through the inclement regions of 

 Cape Horn. Currents, also, flowing permanently in certain 

 directions, with the influx at prescribed points of vast quan- 

 tities of fresh water, limit the extension of many species. 

 Those which love deep water are arrested in their progress by 

 shoals; others, fitted by their construction for shallow seas, 

 cannot migrate across those yawning chasms that open in the 

 bed of ocean. The nature also of the ground exercises an 

 important influence both on land and water shells. Some 



D 



