Sa a i reas en ie Sia Gi a ma ice eae a Sie ieee 
1912] GLEASON—PRAIRIE GROVE 49 
at Bur Oak was benefited by a peculiar and unusual topography, 
and was virtually the only portion of an extensive forest system 
to be spared. 
The origin of the prairie as a type of vegetation cannot, however, 
be referred to prairie fires as a cause, as was frequently supposed by 
early authors and occasionally even in recent years. A prairie 
fire presupposes a prairie, and in prairie fires we have merely one 
factor which has been of assistance in the maintenance or extension 
of the prairie in its struggle against forest invasion. In the last 
half century, since the cessation of prairie fires, the forests have 
again begun an advance into the prairie, but, as is well known, 
their route is chiefly up the streams, and the migration is limited to 
a comparatively small number of mobile species. Because of 
increasing cultivation, this migration is very irregular and can 
never lead to any serious modification in the vegetation of the 
region. 
In conclusion, the conditions in Bur Oak Grove serve to indicate 
the last three periods in the vegetational history of the state: 
1. Period of forest advance, leading to a great development of 
forests in areas of physiographic diversity. 
2. Period of prairie fires, following the advent of man and leading 
to the restriction of the forest to protected areas and the corre- 
sponding extension of the prairie. 
3. Period of civilization and the virtual cessation of the struggle 
between forest and prairie. , 
UNIVERSITY OF MICHIGAN 
teO. Bot, Garden 
1912 
