
82 BOTANICAL GAZETTE [ FEBRUARY 
mesophytic, whereas downward movements make hills more 
mesophytic and swamps more hydrophytic. Thus in the upward 
movement of hills and the downward movement of swamps, 
physiographic processes are more or less neutralized and we may 
speak of retrogressive tendencies ; in the other two cases physio- 
graphic processes are accelerated and we have more rapid prog- 
ress toward the mesophytic climax. !faclimate grows colder or 
more arid, we find retrogressive tendencies toward the xerophytic 
condition, while in a climate that is getting more moist or more 
genial the mesophytic tendencies of the erosive processes are 
accelerated. Furthermore, climatic and crustal changes are 
commonly so slow in comparison with physiographic changes, 
that it is usually difficult to decipher their tendencies. We can be 
far more sure, in other words, with relation to the past and future 
of a topographic form and its plant societies, so far as erosion is 
concerned, than we can as to the actual effect that changing 
climatic and crustal conditions are making. 
One other modification of the physiographic theory is 
necessary, as has been clearly shown by recent field studies. 
While changes in plant societies are certain to follow changes 
in topography, it does not necessarily follow that plant societies 
remain the same if topographic conditions remain unchanged. 
In other words, changes may take place in plant societies more 
rapidly than in the topography. A cycle of vegetation may be 
much shorter than a cycle of erosion. One of the most interest- 
ing cases of this is seen in a growing river system. In the 
ravine stage there may be a rapid change from the xerophytic 
to the mesophytic plant societies on the slopes. As the valley 
widens xerophytic conditions appear on the slopes once more. 
This first and relatively short-lived mesophytic condition may 
be called a temporary climax, in distinction to the more perma- 
nent climax of the base level. 
In a study of plant societies such as this, it must be recog- 
nized that orders of succession are not the same in various 
regions. There is probably a close analogy between the various 
society life histories where climatic conditions are the same, but 

