
80 BOTANICAL GAZETTE [FEBRUARY 
We may now outline the main features of a physiographic 
classification of plant societies. Speaking in the large, the tend- 
ency of the erosive processes is to reduce the inequalities of the 
topography and produce a base level. This base level may not 
soon be reached, though geological history furnishes instances 
of extensive base leveling. Crustal movements interfere with 
the erosive agencies and a mature base level topography may 
become rejuvenated by a great uplift of the land, or sinking on 
the other hand may check the rapid action of erosion. Yet 
even with the crustal movements there go these topographic 
changes and with them the plant societies must change. Put- 
ting the facts of physiography in the terms of ecology, the con- 
ditions become more and more mesophytic as the centuries pass. 
In a young topography, such as the recently glaciated areas of 
Michigan, Wisconsin, and Minnesota, there is a great variety of 
topographic conditions and of plant societies. Among these are 
many hydrophytic lakes and swamps and many xerophytic hills. 
The hills are being denuded and the swamps and lakes are being 
filled, so that the hydrophytic and xerophytic areas are becoming 
more and more restricted, while the mesophytic areas are becom- 
ing more and more enlarged. In passing from youth to old age 
then, a region gradually loses its hydrophytic areas and also its 
xerophytic areas, though in the latter case there is usually at first 
an increase in the xerophytic areas which is due to the working 
back of the young streams into the hills. The latter conditions 
are well shown in Iowa; in the comparatively recent Wisconsin 
drift of north-central Iowa the topography is much less diversi- 
fied and there are fewer xerophytic areas than in the older Iowan 
drift farther south, which has been greatly dissected by stream 
erosion. Later, however, the inequalities are removed, and we 
find great mesophytic flood plain areas, such as are seen along 
the lower Mississippi. 
_ From what has been stated it will be seen that the ultimate 
stage of a region is mesophytic. The various plant societies 
pass in a series of successive types from their original condition 
to the mesophytic forest, which may be regarded as the climax 

