178 “BOTANICAL GAZETTE [MARCH 
The physiographic theory here presented is the result of 
several years of field study devoted chiefly to testing the current 
theories and to developing new ones. The classification is based 
on the fundamental notion that a true theory must be “genetic 
and dynamic; the plant societies must be grouped according to 
origins and relationships, and the idea of constant,change must 
be strongly emphasized. 
The laws that govern changes in plant societies are mainly 
physiographic ; whether we have broad flood plains, xerophytic 
hills, or undrained swamps depends on the past and present of 
the ever-changing topography. Nor is topographic change 
haphazard. Modern physiography has made a logical classifica- 
tion of dynamic surface forces, and has found a progressive 
tendency toward a definite end. Denudation of the uplands and 
deposition in the lowlands results in an ultimate planation, known 
as the base level. Wherever hills are being eroded, or lakes 
filled, or coastal plains enlarged, it is obvious that there must 
be changing plant societies, in other words, a definite order of 
succession of plant groups. These changes, too, are cumulative ; 
a topographic form will have plants that are relicts of an older 
stage, as well as those that are typical of the new conditions, 
showing that the supplanting of one plant society by another is 
slow and gradual. The full effect of a given environment may 
not be felt till that environment has gone. 
Using ecological terms in place of those of physiography, 
soil conditions tend to become more mesophytic as the base level 
develops. A young topography is rich in xerophytic hills and 
in hydrophytic lakes and swamps. There may be local retro- 
gressions toward xerophytic or even hydrophytic plant societies, 
forming eddies, as it were, but the great movement is ever pro- 
gressive and toward the mesophytic condition. Though instances 
of vast planation are found in geological history, the ultimate 
mesophytic base level is seldom reached, since crustal move- 
ments interfere with physiographic processes. So far as plants 
are concerned, however, a physiographic terminology may still 
be used, since all possible crustal changes are either toward or 

