CHICAGO AND VTCINITT. 33 



In closing the section on rivers, all that is needed is to emphasize 

 again the idea that the life history of a river shows retrogression at 

 many points, but that the progressions outnumber the retrogressions. 

 Not only this, but retrogressive phases are relatively ephemeral. Thus 



a river system, viewed as a whole, is progressive, and through all its 



Fig. is. — Flood plain of Thorn creek near Glenwood, showing a meadow instead of a forest. 

 At the center is an uneroded island, detached from the morainic mainland, seen at the left. The 

 vegetation of the island is similar to that of the morainic upland. 



vicissitudes there is an ever- increasing area of mesophytic forest. 

 When the theoretical base level is reached, there seems to be no 

 apparent reason why mesophytic forests should not be developed 

 throughout most of the great plain. 



2. THE POND-SWAMP-PRAIRIE SERIES 



A. The pond. — There are all gradations between rapid streams and 

 completely undrained ponds, and corresponding with these various 

 •gradations are characteristic plant species. It will be convenient to 

 ■subdivide the series under discussion into two parts, the first dealing 

 with undrained ponds and swamps, the second with half-drained ponds 

 and swamps. 



