132 INTRODUCTION 
ensuing spring, and thus afforded ample pasturage for 
their game. It prevented also the growth of trees 
upon the prairies, and of underwood in the forests, 
and tended to preserve them in their then existing 
condition. The conflagration of the combustible mass 
was of course fatal to the smaller animals, and in such 
tracts the mollusks were, and still are, comparatively 
rare. As, under the proprietorship of lands by the 
present population, this practice has, with few excep- 
tions, become obsolete, it may be expected that the 
land-shells will gradually spread over the tracts for- 
merly exposed to fires, and become as numerous there 
as elsewhere in the same region. Indeed, there is no 
doubt that this result has already taken place to a con- 
siderable extent, in those States where extensive prai- 
ries exist, and that they will continue to multiply 
until such lauds come under the dominion of agriculture. 
On the other hand, the accidental spreading of fires, in 
seasons of drought, over wooded tracts, may, from 
time to time, dimhiish their numbers in places subject 
to this contingency. 
Agriculture. The influence which at this time is 
producing the most marked changes in the number 
and distribution of the land-niollusks, proceeds from 
the operations of man in opening the forest for culti- 
vation, and adapting the country for the abode of 
civilization. As the forest falls, light and heat are 
admitted, the moist places are dried up, the mollusks 
are deprived of their natural protection, and brought 
