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 wUl 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 lands 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, diminish 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-mollusks, proceeds from 

 the operations of man m 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 



