INTROD UC TION. 1 1 



phytic with great rapidity, whereas granite hills might take many cen- 

 turies, 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 meander- 

 ings 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 unmis- 

 takable. Moreover, 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 statements 

 made in this paragraph have reference only to such regions as the one 

 in which Chicago is located. In desert regions, and also in arctic or 

 alpine districts, the ultimate stage cannot, of course, be mesophytic 

 under the present climatic conditions. 



