
1901] GENETIC DEVELOPMENT OF FORESTS 295 
its advance, so as to bring about an arctic vegetation where there 
formerly existed a temperate floral formation. 
Erosive forces also come into play. The advance and retreat 
of the ice sheet changed the physiographic features. Drainage 
lines were dammed up and lakes were formed. It is the belief 
of glaciologists that the great lakes, at least in part, were 
formed in this way. Oscillations in the change of the front of 
the ice sheet changed the levels of lakes and left old beaches, 
other sand formations, and cliffs, thus bringing into existence 
new topographic forms and new soil conditions. With the final 
retreat of the ice sheet the normal processes of water erosion 
began again. Drainage lines became reestablished, and with the 
aid of decaying vegetation the lakes began to be filled up. The 
longer an area of land has been free from the ice sheet, the more 
nearly have its drainage lines become firmly established. It is 
evident then that the edaphic conditions in a region recently 
vacated by the ice will be decidedly different from those that 
have been exposed to the erosive force of water. The more 
nearly a region reaches base level the more stable will be 
the edaphic conditions. The recent work of Cowles’ on the 
Chicago region shows clearly the successive stages in the 
advance toward the climax condition. He was also the first ® 
to bring out clearly the dynamic conditions due to physio- 
graphic changes. It is evident that the existing plant societies 
must not be looked upon as fixed, but rather as changing from 
year to year; sometimes slowly, as in the case of swamps; or 
sometimes with rapidity, as in the case of dune societies. 
II. THE SAND SOCIETIES. 
The islands of North Manitou and Beaver at the northern 
end of Lake Michigan, the adjoining mainland on the Michigan 
LEs, H. C.: The physiographic ecology of Chicago and vicinity; a study of 
the i, development, and classification of plant societies. Bor. GAZ. 31: 73-108, 
145-1 Igol. 
“cows s, H. C.: The ecological relations of the vegetation of the sand dunes of 
e Michigan Bot. Gaz. 27:95-117, 167-202, 281-308, 361-391. 1899. /dem: 
The physiographic ecology of Northern Michigan. Science 12: 708, 709. 1900. 
