117 



wrong ideas. In the first place, as they and their ancestors had hved 

 for generations in a forested country, the forest came to be re- 

 garded as the only possible natural covering, and any other type of 

 vegetation was considered extraordinary. In the second place, they 

 did not at first recognize that the forests were everywhere encroach- 

 ing slowly upon the prairies, or that the encroachment became meas- 

 urable as soon as the prairie fires were checked. The prairie is not 

 an extraordinary thing, to be explained only by some strange or 

 fanciful causes ; it owes its origin to ages of arid climate in the west 

 and southwest (Harvey, ipoS: 84). The forest also owes its origin 

 to ages of humid climate in the east and southeast (Adams, ip02). 

 These great climatic types acting upon the plant world through evo- 

 lution and elimination, gradually developed the two extreme types 

 of vegetation, each of which was especially adapted to its own en- 

 vironment. After the close of the glacial period migration of each 

 of these types brought them in contact in Illinois and the neighboring 

 states, and a struggle for supremacy began between them. The out- 

 come is decided mainly by two sets of factors ; first, the control of 

 the environment by the vegetation, and second, the climatic condi- 

 tions of temperature and rainfall. In the first case, the prairie vege- 

 tation, by virtue of its close sod, tends to prevent the proper germina- 

 tion and growth of the forest-tree seedlings (Harvey, ipo8: 86; Rob- 

 bins and Dodds, ipo8: 35). Prairie fires, following the advent of 

 man, also tend to restrict the growth of the forest. On the other hand, 

 the forest has control of the light supply for the herbaceous layers and 

 the well-established trees are resistant to fire. Above all, the climatic 

 conditions are favorable to forest ( Schimper, jpoj." 162-173; Tran- 

 seau, ipo^). The balance has been in general in favor of the forest 

 and it has advanced slowly upon the prairie.* The greatest speed of 

 advance has been along the lines of least resistance, the watercourses, 

 and has resulted in long strips of forest, paralleling the streams, and 

 usually widest on the east side of streams or marshes where they were 

 better protected from fire. In the sand regions the forest distribu- 

 tion is not regulated in that way, because of the absence of small 

 streams, but it does show a possible relation to fires. Where the 

 sand lies in disconnected ridges, separated by strips of moist or 

 swampy groun,d acting as fire-breaks, as in the Havana, Ambov, and 

 Kankakee areas, there is a good growth of forest on the higher 

 ground. Where the sand lies in large continuous masses, as in the 



* It is probable that at certain places and during certain periods the influence 

 of fires has turned the balance in favor of the prairie, but this has not interfered 

 with the general advance of the forest. 



