INTRODUCTION. Q'J 
Now the naturalist, occupying perhaps an insulated 
position, and familiar with a given species only as it 
exists in a single section, where its characters are uni- 
formly the same, cannot a priori suppose it to assume 
other appearances elsewhere ; and when a strongly- 
marked variety is presented to him, he of course con- 
siders it to be a new species, closely allied indeed to 
the one with which he is acquainted, but yet distinct. 
He hastens to make it known by publication, and thus 
falls into an error similar to those which have given 
occasion for these remarks. It is the local naturalist 
especially, who is most likely to commit this kind of 
mistake ; for in the paucity of materials for comparison 
usually at his command, he cannot have the means of 
arriving at a more correct judgment. 
These remarks lead to certain inferences, which, if 
well founded, are worthy of serious attention, and which, 
combined with other considerations, ought to govern 
the conduct of naturalists. In the first place, we see 
that the diagnosis of species which rests exclusively upon 
external characters is, from their mere uncertainty, not 
wholly to be relied upon. The shell is an extraneous 
product of the animal, a substance foreign to it in 
some respects, and formed only for shelter in times of 
danger, and the protection of the soft parts from ex- 
ternal injuries. It possesses no vitality, and its charac- 
ters are, therefore, as we have seen, subject to change 
and even to obliteration, to a degree that never happens 
to organs partaking of the vitality of the animal. Sci- 
