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1901] PHYSIOGRAPHIC ECOLOGY OF CHICAGO 179 
away from the mesophytic, 7. ¢., progressive or retrogressive. 
Again, climatic changes doubtless occur; even here we may use 
the general terminology, since the new conditions either favor 
or retard the general mesophytic development. This leads to 
the general view that the climax type differs with the climate. 
While the general series of physiographic changes is much the 
same everywhere, the corresponding plant societies are vastly 
different. Ina desert climate most of the societies, including 
the climax type itself, are xerophytic. Finally, there is at least 
one point where physiographic-and ecological classifications must 
diverge. Changes. in vegetation often take place where the 
topography remains the same; in other words, a cycle of vege- 
tation may be shorter than a cycle of erosion. The following 
application of these principles applies only to the Chicago region. 
The typical erosion series is based on the life history of 
rivers, and this series is the most instructive ecologically. An - 
embryonic clay ravine is essentially a little desert, though this 
character is due more to the instability of the soil than to the 
ordinary xerophytic factors. Soon landslide action becomes 
much reduced, and a xerophytic flora may appear, though ina 
remarkably short time a rich mesophytic forest is developed. 
This forest is not permanent, but may be regarded as a tempo- 
rary climax. Rock ravines, whether of limestone or sandstone, 
commonly have more vertical slopes and drip with moisture, 
favoring the growth of extreme shade plants. The stages in 
limestone and sandstone habitats are essentially the same in 
spite of great physical and chemical differences in the rock. 
As a clay ravine widens, the exposure increases; xerophytic 
herbs appear at the top of the slope, and later farther down 
toward the stream bed. Xerophytic shrubs and finally xerophytic 
trees make their appearance, notoriously Ostrya Virginica. In 
the early stages of these xerophytic bluffs trees are often found 
that look back to the ravine for their origin, while under them 
are xerophytic herbs that are better suited to the new conditions. 
As the slopes of a mesophytic rock ravine pass to a xerophytic 
rock bluff, changes in the vegetation are most pronounced. A 
