Papers on biology and agriculture 263 



The idea of Lesquereux 4 * that all the prairies of the 

 Mississippi Valley have been formed by the slow recess 

 of sheets of water of various extent, first transformed 

 into swamps and by and by drained and dried" is no 

 longer tenable. It is true that glacial Lake Chicago and 

 glacial Lake Maumee, as well as many smaller lakes, be- 

 came swamps and later either prairies or forests, but 

 there is no evidence that all of Illinois and the neighbor- 

 ing states, or even a large part of them, were ever covered 

 by water since the ice age. The "prairie fire" theoiy 

 has still less in its favor. Prairie fires may check the 

 advance of the forests but it is not likely that forests de- 

 stroyed by fire are ever replaced by prairies. Such areas 

 usually are invaded by weeds followed by briers, then 

 scrub, then forest. 



Prairies are treeless because conditions, at the time of 

 their origin, favored the invasion of a grass vegetation 

 rather than a tree vegetation. The prairies of Illinois 

 arose from a tundra which bordered the ice sheet and in- 

 vaded the drift as the ice retreated. This tundra was 

 invaded by prairie grasses except along streams and 

 morainal ridges where the uneven topography checked 

 the wind velocity and reduced evaporation so that tree 

 seedlings could grow. Forests have been steadily in- 

 vading the prairies since the ice age but the rate of ad- 

 vance is so slow on the relatively fiat surface of Illinois 

 that large areas still remain as prairies. In the states 

 to the north and east invasion has been more rapid be- 

 cause of rougher topography and most of these states 

 have been forested. 



