
ies) 
Tgo1 | GENETIC DEVELOPMENT OF FORESTS 32 
VII. CONCLUSIONS 

The life-history of the vegetation of four sets of physio- 
graphic formations has thus been traced. It has been shown 
that in each series the climax plant growth is a deciduous-hem- 
lock combination. These physiographic formations have been 
treated separately because usually they are distinct from one 


a 
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IG. 18.—A white birch clearing growth on a fossil beach near Marquette. The 
undergrowth consists of Preris aguilina, Diervilla trifida, etc. 
another. In some instances, however, clay is mixed with rock 
débris or underlies the sand. The only effect such a mixture 
has is to hasten the succession of stages, for the advance toward 
the climax is more rapid on clay than on other soils. Princi- 
pally for this reason the clay soils on the whole are already 
covered with the climax forest. In many instances the sandy 
soils have likewise attained the last stage in the life-history. 
Especially is this so where free from the influence of man as on 
