Snails and Their Relations to the Soil 1 



By Harley J. Van Cleave 



Department of Zoology, University of Illinois, Urbana 



[With 1 plate] 

 THE CHANGING EARTH 



Popularly, streams and lakes or ponds are very commonly thought 

 of as relatively permanent features of the earth's countenance, even 

 though physiography teaches that they are but temporary aspects in 

 a series of successional changes between water and dry land. By 

 erosion, soils are washed into lakes and ponds from the surrounding 

 land. Ultimately, the shallow water at the margin with its fixed vege- 

 tation encroaches upon the open waters which, at the same time, are 

 becoming shallower through the accumulation of silt at the bottom 

 of the lake. If these processes continue, the lake is gradually trans- 

 formed into a marsh or swamp land and there results an intermediate 

 stage between the purely aquatic habitat and dry land. Snails of 

 many species thrive in the shallow waters at the margin of a lake or 

 swamp where vegetation is abundant enough to provide them with a 

 constant food supply. They live their relatively limited life span, 

 and after death the empty shells accumulate in the ooze at the bottom 

 of the pond. The lake bottom thus becomes an admixture of soil and 

 organic debris, washed in from the surrounding land, and the resistant 

 remains of the organisms that once lived in the lake. Ultimately, in 

 the processes of nature or by will of man, the lake may revert to dry 

 land. Then the content of the old lake bottom is exposed for easy 

 inspection and the shells which have been accumulating for ages are 

 exposed to view. The interpretation of these deposits of limy shells 

 involves an understanding of the varied conditions under which snails 

 live and likewise consideration of the agencies responsible for their 

 accumulation in the soil, where their influence on agriculture has long 

 been recognized. Regarding the significance of lime in agriculture, 

 Truog (1938) has said, "It is not too much to say that it must be 

 considered the very backbone of profitable crop production, soil con- 



1 Reprinted by permission from the Biologist, vol. 34, Nos. 1-2, September-December 

 1951. 



273 



