136 INTRODUCTION. 



tion 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 in the interior, and that they are par- 

 ticularly deficient immediately upon the sea-shore, upon 

 what may be termed sea-hanks. The influence of the 

 sea, as well as of large bodies of fresh water, tends to 

 mehorate 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 mfluence 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 

 sihceous 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 their 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 



