324 BOTANICAL GAZETTE [MAY 
North Manitou island. But the succession of plant societies is 
much slower on sand than on clay soil, hence the prevalence of 
pine forests on the former. The reasons for believing that ulti- 
mately these pine forests will give way to the deciduous have 
already been given. 
It is the belief of the writer that with some modifications the 
principles brought out in the foregoing pages will hold for all 
regions climatically capable of supporting a tree vegetation. 
The region under discussion is physiographically young. If the 
terms of physiography be adopted for ecology, a region having 
scanty vegetation may be considered young, while from this 
there will be all stages through maturity to old age, viz.,a meso- 
phytic climax forest. It would appear to the casual observer 
that northern Michigan is a region physiographically young and 
ecologically old. But no inconsiderable portion of the area of 
both Michigan and Wisconsin is occupied by swamps and lakes 
(including the great lakes). These will remain hydrophytic and 
hence ecologically young until filled up or drained. This con- 
dition is not attained until the region reaches maturity. The 
whole region, from an ecological standpoint, therefore, cannot 
reach maturity until it does so physiographically. Thus the 
development of the forest is in a measure held back until the 
normal physiographic processes extinguish the swamps and lakes. 
When these are eliminated, the region will reach old age from the 
vegetation standpoint much sooner than it does physiograph- 
ically, because the climate is favorable for the extension of meso- 
phytic forests into altitudes which would otherwise support only 
a xerophytic society. 
In an area where the climate is more severe for tree growth 
the life-history stages are less rapid, for even though the region 
may be nearer base level the climax stages are more restricted. 
For example, Cowles* has shown that in the Chicago region, as 
a rule, the river bottoms contain the mesophytic plant societies, 
while the clay hills have only attained a semi-mesophytic forest 
” Bot. GAZ. 31 : 88 ff. 1901. ; 

