Jennings : A Botanical Survey of Presque Isle. 307 



obviously the most logical method of procedure is to trace the devel- 

 opment from the initial formation, through the successive stages to the 

 climax- formation. By this ' ' developmental method ' ' the structure and 

 the correlations of the individual associations, of whatever rank, and 

 the continuity of the succession, as a whole, can be discussed side by 

 side and in a logical and natural manner. Where, however, but few 

 stages of the succession are in evidence the treatment of the vegeta- 

 tional associations from the standpoint of habitat is, of course, much 

 simpler; it may be difficult or even impossible to ascertain the whole 

 succession. 



Presque Isle offers exceptional opportunities for a developmental 

 study of the vegetation. In the course of not more than five miles 

 there is a continuous series of formations representing stages in several 

 different successions, the climax-stages having been reached in a 

 couple of the successions, at least in those where the habitat represents 

 an age of between five and six hundreds of years. For these reasons 

 it has been deemed best to follow the developmental method in the 

 discussion of the ecological structure of the vegetation. 



Ecology, as at present understood, is largely a development of the 

 last decade. Warming's ecological plant geography "^ was the first suc- 

 cessful systematic classification of the ecological plant-formations 

 of the world. His classification was based mainly upon the watery 

 content of the soil, the "Societies" being either hydrophytic, 

 mesophytic, xerophytic, or halophytic. Schimper in his great work ■* 

 went a step farther and pointed out that halophytes are essentially 

 xerophytes and that ecological associations are to be properly classi- 

 fied, not according to the physical watery content of the soil, but ac- 

 cording to the physiological availability of this water for plants. 



Following these classifications many workers have made detailed 

 studies of generally more or less localized areas. Among these workers 

 Cowles has recognized the importance of the developmental method 

 of classification based upon physiography and has followed out this 

 idea in his work on the sand-dunes of Lake Michigan " and on the 



^^ Warming, E. " Plantesamfund," Copenhagen, 1895. (Knoblauch's German 

 translation, Berlin, 1896.) 



2* Schimper, A. F. W. " Pflanzengeographie auf physiologischer Grundlage," 

 Jena, 1898. (English translation by Fisher, Groom and Balfour, Oxford, 1903.) 



^"Cowles, H. C. "The Ecological Relations of the Vegetation of the Sand 

 Dunes of Lake Michigan." Bot. Gaz., 27 : 95-117, 167-202, 281-303 and 361- 

 391, February, March, April and May, 1899. 



