INTRODUCTION. xl 
of Cyprea exanthema, cervina and cervinetta, &c. A large proportion 
of the shells inhabiting the eastern and western shores of the Atlantic, 
have been 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. 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. 
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 neyer expect to find them elsewhere. Thus, we look for Clausilia 
in Europe and Asia; for Achatina in Africa; for Cylindrella in the 
