INTRODUCTION. x j 



of Cyprea exanthema, cervina and cervinetta, &c. A large proportion 

 of the shells inhabiting the eastern and western shores of the Atlantic, 

 have heen regarded 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 limita- 

 tion of animals, even now, meets with so few apparent exceptions, that 

 we admit it as an axiom in zoology, that species strongly resem- 

 bling each other, derived from widely diverse localities, especially 

 if a continent intervenes, and if no known or plausible means of com- 

 munication can be assigned, should be assumed as different, 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 undoubtedly have a very extensive range. The species of 

 Cyprea 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 appearing. 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, attaching 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. Bulirnus octona, or a closely allied species, 

 is a parasite of the Banana. But exceptions of this kind confirm 

 rather than militate against the conclusion. 



There is a certain local aspect, a 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 distinguish the loose, horny, colourless 

 structure of the northern marine species ; the stony, corroded, livid 

 New Zealanders ; 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; for Cylindrella in the 



