
Igor | PHYVSIOGRAPHIC ECOLOGY OF CHICAGO 8 
or culminating type. These stages may be slow or rapid; some 
habitats may be mesophytic from the start; undrained lakes and 
swamps fill up and become mesophytic with great rapidity, 
whereas granite hills might take many centuries or even geological 
epochs in being reduced to the mesophytic level. Again the 
stages may be direct or tortuous; we have already seen how the 
first consequences of stream erosion may be to make mesophytic 
areas xerophytic. So, too, in flood plains, the meanderings of 
the river may cause retrogressions to the hydrophytic condition 
such as are seen in oxbow lakes, or the river may lower its bed 
and the mesophytic flood plain become a xerophytic terrace. 
But through all these changes and counterchanges the great 
mesophytic tendency is clearly seen; mesophytic areas may be 
lost here and there but many more are gained, so that the 
approach to the mesophytic base level is unmistakable. More- 
over, the retrogressive phases are relatively ephemeral, while the 
progressive phases often take long periods of time for their full 
development, especially in their later stages. 
The above phenomena postulate congenial climates and more 
or less static crustal conditions. It is obvious, however, that 
€roSive processes in a desert region do not result in a mesophytic 
flora; the same is true of alpine and arctic climates. Again, the 
climate of all regions is doubtless changing, as it has changed 
in past ages. So, too, there are crustal movements up and down. 
In other words the condition of equilibrium is never reached, 
and when we say that there is an approach to the mesophytic 
forest, we speak only roughly and approximately. As a matter 
of fact we have a variable approaching a variable rather than a 
constant. These conditions do not destroy the validity ofa 
physiographic classification, but rather they require an enlarge- 
ment of conception. Retrogressive phases, 2. ¢., away from the 
mesophytic and toward the hydrophytic or xerophytic, must be 
_ included, as well as progressive phases away from the hydrophytic 
oF xerophytic and toward the mesophytic. In this way all pos- 
 Sible conditions are accounted for. For example, upward crustal 
movements make hills more xerophytic and swamps more 
o 
