H2 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 distinguished 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 
greatly 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. 
Geological 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 hi, the shell. As to the other geological form- 
