42 



throughout its range.* In this respect the present status of systematic 

 ecology resembles that of pre-Linnaean taxonomy, a maze of de- 

 tached facts waiting for a Linnaeus to collate and correlate them 

 into a foundation for future investigation. 



The associations recognized in the field have been grouped into 

 formations, characterized partly by uniformity in the physiognomy 

 of the vegetation, and partly by uniformity of environment, to which 

 the physiognomy is in some extent due. Fomiations correspond 

 somewhat to genera in taxonomy, and like them may be limited or 

 comprehensive in their scope, this depending solely upon individual 

 opinion. As far as possible they have been made to coincide with the 

 popular idea of the different types of vegetation. The four fonna- 

 tions are all generally known through the sand regions and given the 

 names used here, with the exception of the forest, which is col- 

 loquially known as "timber" or, in some places, as "black-jack." The 

 latter term applies to the particular association rather than to the 

 forest formation in general. The areal extent of a foniiation is ap- 

 proximately coincident with one of the phytogeographical provinces 

 of North America, and fonnations with the same distribution are 

 placed in the same province. It is thus seen that the differentiation 

 of both minor and major ecological groups depends principally upon 

 the plants themselves, the associations being distinguished by the 

 specific composition, the formations by the general appearance, and 

 the province by the distribution of the vegetation. This is an ex- 

 tension of the idea already expressed, that the most important feature 

 of the association is not the habitat but the plant. It is believed that 

 the regional classification of associations is really genetic and dynamic, 

 bringing together those which are most closely related by origin and 

 succession. 



The Vegetation 



The area covered by the state of Illinois occupies a unique posi- 

 tion in respect to the vegetation of the continent, marking the region 

 of closest approximation of four great floral and vegetational prov- 

 inces. (See maps in Schimper, lOO^; Engler, ipo2; Transeau, ipoj, 

 ipo^^; Merriam, i8gS; Sargent, 1884.) 



The Austroriparian Province (Merriam, i8p8: 45) enters the 

 state at the extreme southern end, and well-developed examples of 

 its dominant hydrophytic vegetation, the cypress swamp (Ta.vodiuui 

 distichnm), extend northward into the lower valley of the Wabash 



*The nearest approach to this has been made by Transeau (/poj; 1903-6) in his 

 studies of boe floras. 



