108 BOTANICAL GAZETTE [FEBRUARY 
though in this case, the margin occupies the center of the original 
pond. Near Starved rock an extinct oxbow lake on the flood 
plain of the Illinois river contains an extensive patch of Sphag- 
num and Osmunda, among the most characteristic plants of 
undrained swamps. There are many undrained swamps, some 
with tamaracks, in the Calumet valley. The future of these 
swamps is like that of other swamps, and will be described in 
the next section. fig. 78 shows a morainic island in the Thorn 
creek flood plain; the stream has meandered but has thus far left 
this detached fragment of the morainic mainland with a large 
part of its original flora. 
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 outnum- 
ber the retrogressions. Not only this, but retrogressive phases 
are relatively ephemeral. Thus a river system, viewed asa whole, 
is progressive, and through all its 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 
‘ss the great plain. 
[Zo be concluded.) 


