PAPERS ON BIOLOGY AND AGRICULTURE 161 



The Rock River Woodlands are the vegetation response 

 of this particular part of the world to the conditions of in- 

 land plant succession in the depressions of the erosion topo- 

 graphy of a river system that may be considered, in general 

 at least, as in its mid-phase of ravines, bluffs, and flood 

 plains, A varied assortment of habitats for plant life is a 

 corollary. Early xerophj-tic ravine stages through to ex- 

 tremely mesophjlic late ones in both clay and loams are 

 present, with the rock bluff phase of river action in sand- 

 stone and limestone, the depositional phase of flood plains, 

 and depositing and eroding shores. This is the structure 

 that has already been described as the Rock River system in 

 Ogle County, with its vegetational history as already briefly 

 outlined. Further, into all this there has been projected an 

 element of powerful effect in the changes wrought by it 

 directly and indirectly on the topography and the vegeta- 

 tion. This element is that of the various activities of men. 

 It is in its effects from minor to cataclysmic and from slow 

 to the swiftest of all factors. 



FACTORS OF THE COMPLEX 



In the chains of causation leading to certain effects it is 

 customary today to stress particularly the physical and 

 chemical alterations in the surroundings, that is to say in 

 the kinds and the rates of processes making up the condi- 

 tions under which things exist and change. From the scien- 

 tific standpoint this is, of course, desirable since it is always 

 the more fundamental that is needed for establishing prin- 

 ciples of application that approach universality. Today, 

 however, the whole story, so to speak, of many occurrences 

 in nature cannot be expressed in physical and chemical 

 terms. The development of vegetation, for example, cannot 

 possibly be told in such terms alone, whatever may be pos- 

 sible at some later time. An account of the plant ecology of 

 any locality must necessarily use other means of expression 

 also. So, for a qualitative investigation of vegetation, an 

 artificial classification of the "factors" or elements of the 

 different complexes involved is desirable. A common, very 

 general one, of great use in the past but now somewhat out- 

 worn, is the division into topographic and biotic factors. 

 Another recognition of this is to be seen in the still very use- 



