112 INTRODUCTION. 



"which further north occupy the country. Helix fallax, 

 if it be considered a legitimate species, is another simi- 

 lar instance. It appears first, sparingly, north of the 

 Ohio, where it can hardly be distmguished from the 

 analogous Helix tridentata ; but in the sandy parts of 

 the south it is greatly multiplied, and entirely takes the 

 place of the latter species. 



But, although distribution is mainly influenced by 

 climate, there are many other minor causes which 

 gi-eatly modify its effect, and determine the number, 

 both of genera and species, in particular localities, and 

 even over large tracts of country. Among these are 

 the following. 



Crcological Structure. The influence of this cause 

 is very important, and admits of no doubt. The soils 

 formed from the debris of calcareous strata are those 

 which support the terrestrial mollusks in the greatest 

 numbers, and on which individuals attain the greatest 

 size and beauty. It would seem, therefore, that the 

 calcareous matter of the soil is in some way essential 

 to the perfection of the shell. Soils formed from pri- 

 mary rocks, on the other hand, are those on which they 

 are least numerous, and where the shell is in general 

 smaller and less ponderous. We doubt not that indi- 

 viduals brought from primitive and from calcareous 

 regions may be respectively distinguished by the greater 

 or less development of, and the amount of calcareous 

 matter in, the shell. As to the other geological form- 



