6(3 INTRODUCTION. 



our experience in respect to many of our native spe- 

 cies ; for individuals from various and distant localities, 

 especially from those differing in climate, soil, or geo- 

 logical structure, are rarely identical in external char- 

 acter, but almost always present striking differences in 

 the size, thickness, or weight of the shell ; the eleva- 

 tion or depression of the spire ; the smoothness or rough- 

 ness of the epidermis ; the prominence and number 

 of the striae of increase ; the diameter of the open axis 

 of the shell, known as the umbihcus ; and in the number 

 and magnitude of the tubercles, folds, and other tes- 

 taceous deposits wliich are often formed in the aperture, 

 and upon the columella of the shell. These modifica- 

 tions are so constant, in some species, that the practised 

 eye can thereby distinguish the stations, or rather the 

 section of country, from which individuals exhibiting them 

 are respectively derived. The same remark is true 

 as regards the marine moUusks used for food. The 

 dealers recognize the localities of some of them hy 

 variations which often escape the naturalist ; and they 

 sometimes know that distinctions wliich he considers 

 structural and constant, are due only to physical influ- 

 ences. It is well known that there is a tendency in 

 nature to continue, to successive generations, those mod- 

 ifications of form which have, in the first place, been 

 introduced by accidental causes, and thus to continue, 

 for a time, what have been called permanent varieties. 

 But these, it is behoved, return, sooner or later, to their 

 original type. 



