
Igor | PHYSIOGRAPHIC ECOLOGY OF CHICAGU 79 
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 changing landscape. As the years pass by, one plant society 
must necessarily be supplanted by another, though the one 
passes into the other by imperceptible gradations. Here then 
is a classification both genetic and dynamic, a classification 
which has a place for all possible ecological factors. It is based 
on the normal physiographic changes of a region and hence 
should be called a physiographic classification. One thing 
more must be recognized, and that is that environmental influ- 
ences are normally cumulative. A plant society is not a prod- 
uct 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 bor- 
der lines or zones of tension, for here, rather than at the center 
of the society, one can best interpret the changes that are tak- 
ing place. Of course the order of succession referred to above 
is a vertical or historical one. One plant society is said to fol- 
low another if it is actually superimposed upon the one preced- 
ing. 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 
Succession are well shown in peat swamps and along shores and 
flood plains. Along a sandy shore it is only by studying the 
horizontal succession that one can get any idea of the vertical, 
Since all fossil traces of preceding plant societies have passed 
away. In peat swamps one can sometimes verify the results of a 
horizontal zonal study by investigating the fossil remains beneath 
