136 INTRODUCTION. 
tiou of individuals. If these effects were the result of 
the causes named, we might expect to find them occur- 
ring in similar localities elsewhere ; but, in this country 
at least, the observed facts do not support such a con- 
clusion. On the contrary, so far as the author's per- 
sonal observation and inquiry have extended, it has 
appeared that the land-shells are much less numerous 
near the sea than ha the interior, and that they are par- 
ticularly deficient immediately upon the sea-shore, upon 
what may be termed sea-banks. The influence of the 
sea, as well as of large bodies of fresh water, tends to 
meliorate the climate of the country bordering upon 
them, and hence, perhaps, the sea-shore of Great Britain 
is a more favorable habitat than the interior, although 
this influence modifies the climate of the whole island in 
a greater or less degree. Our experience, too, as to 
the presence of sand, is entirely opposite to that noticed 
in Great Britain, sandy banks near the sea and else- 
where being destitute of shells, and all soils in which 
siliceous sand predominates supporting them only in 
small numbers. Indeed, pure sand creates a mechan- 
ical obstacle which these animals cannot overcome : its 
loose dry particles adhere to then wet mucous surface, 
incrust them entirely, and prevent their locomotion. 
They vainly endeavor to free themselves from it, by 
secreting more mucus, and in this way soon become 
exhausted, and die. 
There is, however, a class of facts in the United 
States, which at first view seems to confirm the English 
