INTRODUCTION. 9 



cause the wearing down of the hills and the filling up of the hollows. 

 These two processes, denudation and deposition, working in harmony 

 produce planation ; the inequalities are brought down to a base level. 

 The chief agent in all these activities is water, and no fact is better es- 

 tablished thaii the gradual eating back of the rivers into the land and 

 the wearing away of coast lines ; the material thus gathered fills up 

 lakes, forms the alluvium of flood plains, or is taken to the sea. Vege- 

 tation plays a part in all these processes, the peat deposits adding 

 greatly to the rapidity with which lakes and swamps are filled, while 

 the plant covering of the hills, on the contrary, greatly retards the 

 erosive processes. Thus the hollows are filled more rapidly than the 

 hills are worn away. As a consequence of all these changes, the slopes 

 and soils must change ; so, too, the plant societies, which are replaced 

 in turn by others that are adapted to the new conditions. 



There must be, then, an order of succession of plant societies, just 

 as there is an order of succession of topographic forms in the chang- 

 ing landscape. As the years pass by, one plant society must necessa- 

 rily be supplanted by another, though the one passes into the other by 

 imperceptible gradations. One thing more must be recognized, and 

 that is that environmental influences are normally cumulative. A plant 

 society is not a product of present conditions alone, but the past is 

 involved as well. For example, a hydrophytic plant society may be 

 seen growing in a mesophytic soil ; the author has seen a mesophytic 

 tamarack swamp which can be explained only in this way. We have 

 in this phenomenon a lagging of effects behind their cumulative 

 causes, just as the climax of the heat in summer comes long after the 

 solstice. 



In a classification like this great emphasis is placed on border lines 

 or zones of tension, for here, rather than at the center of the society, 

 one can best interpret the changes that are taking place. Of course 

 the order of succession referred to above is a vertical or historical one. 

 One plant society is said to follow another if it is actually superimposed 

 upon the one preceding. In many cases, if not in most, there is a 

 horizontal order of succession at the present time that resembles the 

 vertical succession of which we now have only the topmost member. 

 Instances of similarity between vertical and horizontal orders of suc- 

 cession are well shown in peat swamps and along shores and flood 

 plains. Along a sandy shore it is oply by studying the horizontal suc- 

 cession that one can get any idea of the vertical, since all fossil traces 



