[Chap. Lll PLANTS OF THE PAST 735 



the ice. Later the flora spread in the opposite direction when the second 

 glacier disappeared. Thus even in Canada some plants survived the 

 glacial epochs, and the present vegetation of Canada is not wholly com- 

 posed of species that migrated northward from the area south of the 

 last glacial invasion. 



Moreover, the unglaciated area in southwestern Wisconsin, so often 

 shown on glacial maps as a small triangular area surrounded by ice, 

 was never completely surrounded by any one of the glaciers. This area 

 was always larger than the State of Illinois and most of the time it was 

 fully open to the southwest. This was consequently a nearly continuous 

 source of plant migrants to the Lake Superior shores and region of the 

 headwaters of the Mississippi. There were thus local sources of plants 

 scattered about the northern part of the continent. Except in the moun- 

 tains the vegetation was disturbed very little 50 to 100 miles south of 

 the ice margin. 



South of the glacial boundaries, forests extended across the sagebrush 

 lands between the Sierra and the Rocky Mountains and eastward over 

 what is now plains and prairie grasslands. Farther east the drainage 

 of the Great Lakes through the St. Lawrence River was at times 

 blocked by the ice, and much of the adjacent lowland was covered by 

 extensive lakes. The beaches of these lakes are still recognizable, and 

 many of them have been accurately mapped. As the ice-fields slowly 

 disappeared, numerous smaller lakes were invaded by bog vegetation, 

 so common about the lakes farther north today. Bog plants occupied 

 many of these lake sites until the present century, when most of them 

 were destroyed by agricultural practices. 



As the lakes slowly filled with peat, pollen carried by the wind from 

 the plants of the upland as well as those of the bog fell into them and 

 were preserved. Studies of the distribution of pollen in the successive 

 layers of the peat have yielded a fairly accurate record of the successive 

 associations of plants that lived on the surrounding uplands from gla- 

 cial times to the present (Figs. 368 and 369). 



Summary. On the basis of the fossil record, it appears that plants have 

 been living upon the earth for a billion years or more. The first plants 

 were exceedingly small and simple in structure. Their physiology proba- 

 bly was complex, for no living systems are known to have a simple 

 physiology. Through a slow process of evolution, which appears to be 

 dependent solely upon changes in hereditary units of matter in living 

 cells ( Chapters XXXIX and XL ) , new kinds of plants have, at all times 



