70 INTRODUCTION. 
necessarily produced by a corresponding difference in 
the structure of the animal. This principle, which 
is exceedingly plausible at first sight, ought, if true, to 
be followed by another axiom which depends upon it, 
namely, that great diversities of the animal structure 
are attended by great differences in the shell ; and both 
ought to be confirmed by well-known facts. But, in 
truth, neither of these principles is practically of 
much value in the diagnosis of species, and every one 
familiar with shells, knows, that very considerable dif- 
ferences occur in all the particulars we have named, 
while the animal remains unchanged ; and on the other 
hand, it is equally well understood that considerable 
modifications of the animal structure may exist, without 
anything like a corresponding amount of variation in 
the shell. A remarkable instance of this occurs in the 
marine genera Lottia and Patella, in which the shells 
resemble each other very nearly, but in which a separa- 
tion of the two genera has been justified by important 
differences in the animal structure alone. In the branch 
which we are considering, though not particularly among 
the species of this country, the naturalist can find no 
differences of animal structure to sustain the generic 
distinctions which, if he relied upon variations of the 
shell, he often would found upon these external char- 
acters. 
The second remark which we make, as resulting 
from the preceding observations, is, that no one is in 
a position to establish species with confidence, even 
