INTRODUCTION 7 



in vegetation. Zonation and alternation, on the other hand, 

 are static phenomena, characteristic of the vegetative cover- 

 ing as it is and will be when it becomes more and more 

 static. Thus, the relation of plants to the surface of the 

 globe finds its basal expression in the one great principle or 

 phenomenon of association, for which indeed vegetation is 

 but another name. In its dynamic aspect, developmentally, 

 association is controlled by two laws, that of invasion and of 

 succession, while in its static aspect, structurally, it con- 

 forms to the laws of zonation and of alternation. 



In the analysis of vegetation, it is imperative that the 

 twofold nature of its origin be kept clearly in mind. Just 

 as a species of today is the result of accumulated ancestral 

 facts modified to a certain degree by existing physical 

 factors, so a particular vegetation of the present time, indeed 

 all vegetation, is the result of a complex physical fact, the 

 habitat or environment, acting upon a biological or histor- 

 ical fact, the antecedent vegetation. Since the basis of 

 vegetation is twofold, physical and historical, the principles 

 manifested in it must correspond to the one or the other. 



In the following treatment, the fundamental facts of vege- 

 tation are considered separately in a somewhat detailed and 

 systematic manner. The arrangement under each is the 

 same. The conception of each principle and its relation to 

 the others are first discussed; then the consideration of 

 causes, categories, manifestations and results follows in the 

 logical order. A short statement of the historical develop- 

 ment of each subject is given in the proper place. Because 

 of its bearing upon the origin and modification of formations, 

 the neglected doctrine, polyphylesis, is considered in con- 

 nection with invasion. 



