GEOGRAPHICAL DISTRIBUTION. l\l 



The general effect of climate, irrespective of other 

 influences, is imdoubtecUj a reduction of the number, 

 both of genera and species, towards the north ; and a 

 manifest diminution of the number, size, and perfection 

 of individuals. Thus, while in the northern sections 

 Criandina, Ilelicina, Cyclostoma^ and the poljgyral forms 

 of Helix entirely disappear, and only one of the native 

 species of true Bulimus remains, the single genus Vitrina 

 begins to be observed. At the same time, although in 

 the southern sections the genera are more numerous, 

 and only two northern genera, TehenjiojjJiorus and Vi- 

 trina, are knoA\Ti to disappear, yet, the great genus 

 Helix becomes less numerous in species, thus indicating 

 that its focus is in the more temperate portion of the 

 central region. This result however, is affected b}- 

 elevation and other causes as well as latitude, species 

 being continued on the Appalachian table lands, as far 

 south as Georgia and Alabama, when they have already 

 disappeared on either side in the same latitude. And 

 while the northern species show a tendency gradually 

 to run out, towards the south, their places are supplied 

 to some extent by other forms. Thus, the poh/c/p'al 

 Selices, which form a very distinct division of this 

 genus, and by some have been thought to possess char- 

 acters sufficiently marked to constitute a genus by them- 

 selves, occurring but rarely in the latitude of the Ohio 

 river, become more common towards the south, until, on 

 the borders of the Gulf of Mexico, they exist in vast 

 numbers, to the exclusion of nearly all tlio species. 



