PAPERS ON BIOLOGY AND AGRICULTURE 189 



in the county, was said to have occurred, along with the 

 extremely diy 3-ear 1901, when the temperature, however, 

 throughout all four seasons presented nothing abnormal. 

 The seasonal monthly means for the remaining seasons pre- 

 sent remarkably uniform figures. 



It is, therefore, e\ident that there is nothing in the way 

 of climatic fluctuation of a periodic nature within ifie last 

 fifty or sixty years at least within the region of the Rock 

 River woodland and thereabouts to account for any swing- 

 ing of the successional trend between the xerophji-ic and 

 the mesophytic directions. The only marked feature would 

 be that of the rather unfavorable years of about 1879- 

 1888, when even then, not all of these were sufficiently 

 unfavorable to afford suif.cient reason for much effect upon 

 the native vegetation. The effect, especially if aided by 

 plant disease of insect attack history, would be retrogres- 

 sive. Recovery from such a setback would mean a return 

 to the former rate of progression, or something approxi- 

 mating it. A lag in the response of the plant associations 

 would probably alter the dates somewhat for vegetative 

 changes. 



The writer believes, however, that it is not any climatic 

 change, and probably not materially any plant disease at- 

 tack, that is mainly responsible for the seeming swinging 

 of the succession, nor for the marked alterations m the 

 nature of the native plant growth of the county. A general 

 observation only of the Rock River woodland region would, 

 it is believed, lead one at first to the conclusion that, instead 

 of any swinging of the successional trend, there is a pro- 

 nounced movement in the xerophytic direction. Closer ob- 

 servation and the consideration given to the climatic data 

 available bring the writer to the conclusion that the affair 

 is related mainly to man's interference with the native 

 vegetation. WTiere the activities of man have caused inter- 

 ference, there retrogression of the vegetative succession 

 has occurred and occurs today. Where man has not in- 

 terfered in the past with the native vegetation, there it has 

 advanced in the mesophytic direction, that is to say pro- 

 gression of the vegetative succession has occurred. And 

 whenever man has. after such interference, ceased his re- 



