45 



hind as isolated areas in the midst of the forest. Rehcs of the 

 northern coniferous forest persist in Ilhnois as tamarack swamps and 

 groves of white pine, and both are frecjuently termed "boreal islands." 

 In a similar way the detached areas^of prairie in Illinois may be re- 

 garded as western relics, although they are often miles in extent. The 

 oldest relics, that is, those toward the east or south, are regularly 

 smaller in extent and more mixed with forest species (Bonser, ipo^). 

 In the migration of the deciduous forest associations, the greatest 

 advance has always been made iir those habitats which most nearly 

 i-esemble those occupied by the climax formation, and which are 

 therefore most nearly suited to the invading vegetation. On the 

 other hand the relic associations have been left behind in those ha1)i- 

 tats, not necessarily best adapted to the itHc vegetation, which are 

 least suited to the invaders. For this I'eason the boreal associations 

 in Illinois are limited to sandstone hills and to undrained swamps, 

 while the prairies persist chiefly in the upland soils between the 

 stream courses. 



The successions by which the general mig'ration is consummated 

 are of a type different fi'om that found within the formation and 

 leading merely to the dominance of the climax vegetation, since they 

 involve associations of two and sometimes of three provinces. While 

 several descriptions of this type have been published, general con- 

 clusions have not usually been draw-n. At the present time it can 

 only be stated that the succession seems to take place betw^een equiv- 

 alent members of the different provincial successional series. Thus, 

 as shown in the following pages, the xerophytic extreme of the 

 pi"airie, the bunch-grass association, tends to give way to the corre- 

 sponding' extreme of the forest, the black oak association. In a sim- 

 ilar way the black oak association may succeed the xerophytic ex- 

 treme of the Northeastern Conifer Province, the jack-pine associa- 

 tion. We find similar relations betweeii the hydrophytic extremes, 

 and in Illinois the succession of the northeastern tamarack associa- 

 tion by the deciduous bottom-land forest may be observed. 



Northern Illinois, therefore, has been and is the scene of im- 

 portant events in the biogeographical history^ of the continent. The 

 following description of vegetation is designed to be not merely a dis- 

 cussion of static conditions, but rather a portrayal of one phase of 

 this great vegetational movement and of the consequent struggle for 

 supremacy which is still being- waged. 



In the veg-etation four distinct formations, or types of vegetation, 

 have been iTCOgnized. Each of these consists of several associations, 

 characterized by a distinct group of plants, l)y a distinct ha1;)itat, or by 



