76 Remarks on Mollusca and Shells. 



garded as identical, and many of them are really so. But 

 the closer the comparison, the more it tends to diminish 

 rather than increase the identical species. The same is 

 found true in regard to other classes of animals. In fact, 

 the doctrine of the local limitation of animals, even now, 

 meets with so few apparent exceptions, that we admit it as 

 an axiom in zoology, that species strongly resembling each 

 other, derived from widely diverse localities, especially if a 

 continent intervenes, and if no known or plausible means of 

 communication can be assigned, should be assumed as dif- 

 ferent, until their identity can be proved. Much study of 

 living specimens must be had before the apparent exceptions 

 can be brought under the rule. Some shells have undoubt- 

 edly a very extensive range. The species of Cyprwa are 

 remarkable for this, and more than any other genus would 

 lead us to conclude that oceans present no limitations. Even 

 among them, however, new distinctions are constantly ap- 

 pearing. There are also some shells which may be called 

 cosmopolite, at least they are erratic, and will be found 

 wherever their pabulum is found. Thus, Helix cellaria, at- 

 taching itself to water casks, is found in most seaports in 

 all parts of the world. Helix similaris is found wherever 

 the coffee plant grows ; and Helix vitrinoides in like manner 

 accompanies the Arum esculentum or taro. Bulimus octona, 

 or a closely allied species, is a parasite of the Banana. But 

 exceptions of this kind confirm rather than militate against 

 the conclusion. 



3. Local Aspect of Species, and Characteristic Form of Regions. 



There is a certain local aspect, or peculiar facies, which 

 impresses itself upon us the more we study local collections ; 

 just as we learn by a very little observation, to distinguish 

 men of different nations and neighbourhoods. Thus we dis- 

 tinguish the loose, horny, colourless structure of the northern 

 marine species ; the stony, corroded, livid New Zealand- 

 ers ; the polished, absolutely perfect specimens from the coral 

 seas. Certain forms are so characteristic of certain regions, 

 that we never expect to find them elsewhere. Thus we look 

 for Clausilia in Europe and Asia; for Achatina in Africa; 



