PAPERS ON BIOLOGY AND AGRICULTURE 167 



development, peculiar only to the Rock River region but that 

 it would gradually spread over more or less of the county 

 outside of that specifically dealt with in this paper. This 

 oak-maple climax is to be seen well in portions of the Cart- 

 wright woods on the west side of the Rock north of Oregon 

 and in the east shore part of the McCormack woods north 

 of Byron. The association has become in large part inde- 

 pendent of physiographic diversity, that is to say, it is 

 found not only in small ravine heads for example, which af- 

 ford especially favorable mesophytic conditions, but 

 spread over larger physiographic areas that involve much 

 diversity of habitat. The floral composition of the associa- 

 tion in such examples of the climax will be treated in more 

 detail later. From the viewpoint of geological time, of 

 course, the mesophytism of the Rock River woodlands re- 

 gion is a temporary effect. Geologically later, when lateral 

 erosion of the river has approached planation, mesophytism 

 will have disappeared. 



For contrast, in order to get an impression of the course 

 of the plant succession, and some idea of the probable ef- 

 fectiveness of the factors influencing it, let us consider an 

 association representing the other extreme, a xeroph\i:ic 

 woodland. Although not especially concerned with the 

 prairie groves as they remain in the county today, chiefly 

 in the form of degenerate farm woodlots, they will, never- 

 theless, serve the purpose. Their mesoph}i:ism when white 

 settlers first came into the county about 1840 has already 

 been noticed. While not applicable to all prairie groves 

 eighty years ago many of them certainly comprised an as- 

 semblage of plants whose mesophytism is indicated by the 

 presence of such species as maples, walnuts and elms, be- 

 sides the oaks. Today, after less than a century of occupa- 

 tion by the whites, these groves or woodlots generally con- 

 tain an association whose retrogression is indicated by their 

 composition of oaks and hickory. The t>T)ical species are 

 Quercus macrocarpa, Q. ellipsoidalis, Q. velutina, Q. alba, 

 Hicoria ovata. Since the variations comprise almost all 

 possible combinations of these species no more definite 

 statement is necessary. In this connection there must be 

 comipared what has happened when the course of the plant 



