162 ILLINOIS STATE ACADEMY OF SCIENCE 



ful though purely artificial separation into 1) Primary 

 Succession; that resulting from natural causes, and 2) 

 Secondary Succession; that resulting from disturbance by 

 man. This is wholly a classification of convenience. The 

 different categories may, in many cases, overlap consider- 

 ably in so far as the laws of physics and of chemistry are 

 concerned. But this is of no great importance in a qualita- 

 tive investigation. Nor need it interfere at all with quanta- 

 tive investigation of vegetation, where an endeavor is made 

 to seek out physical and chemical relations in plant life or 

 to advance our present defective state of knowledge concern- 

 ing quantitative methods of investigating such phenomena. 



A simple classification of convenience for the purposes of 

 the qualitative investigation in hand is as follows : 



Activities of men, 



Climatic fluctuations, 



Fires, 



Plant diseases, 



Principles of plant succession. 



The first of these categories may obviously be split up into 

 such heads as : 



Clearing, 



Cutting, 



Grazing, 



Burning, 



Weed introduction, 

 Crop raising. 

 Industrial developments. 

 Artificial planting. 



Since no complete handling of all these elements is at all 

 possible it will be well to dispose here of a few that are either 

 of lesser importance or are practically impenetrable, reserv- 

 ing the more important or more feasible for a later discus- 

 sion, after the plant ecology had undergone consideration. 



The height of water in the Rock River and its volume of 

 flow is influenced not only by natural causes but by indus- 

 trial development. The water is artificially ponded often be- 

 hind the dam at Rockford in Winnebago County to the north 



