140 



INTRODUCTION. 



more readily find shelter and escape observation, thus 

 being exempted from many destructive agencies. 



There are some species, upon the distribution of which, 

 climate, vegetation, geological structure and other gen- 

 eral causes, seem to exercise but very little influence, as 

 is shown by the universality of their diffusion through- 

 out the country. Helix labyrinthica, for example, is 

 equally abundant, and as much developed, upon the sides 

 of the primary hills of Vermont and Maine, in the rig- 

 orous climate of 45° north latitude, where snow covers 

 the ground three or four months of the year, as it is 20° 

 further south, upon the tertiary levels of southern 

 Florida only a few feet above the sea, where it enjoys a 

 climate of almost tropical mildness. The same remark 

 may be made of Helix minusoula, Pupa contracta, 

 P. rupicola and P. exigua, and perhaps of other mhmte 

 species. 





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