414 BOTANICAL GAZETTE | DECEMBER 
existing small lake areas of the northern states were covered 
by the ice during the maximum extension of the Wisconsin ice 
sheet. As there is no reason to believe that the drift sheets of 
the preceding epochs, which in many places extend beyond the 
Wisconsin terminal moraine, contained such small undrained 
depressions, it follows that the bog societies must have occupied 
other habitats. 
POSTGLACIAL NORTHWARD MIGRATION. 
With the renewal of a milder climate and ‘the consequent 
recession of the glaciers, the plant societies would gradually 
spread in the direction of continuous habitats and generally 
northward. The bog and tundra types would be the first to 
push into the barren ground left by the retreating ice. The 
area over which they spread in early postglacial times must 
have been very much more extensive even than that now 
occupied by them. In the smaller glacial depressions where 
absence of wave action would favor littoral vegetation, the bog 
plants would become firmly established. On the western and 
eastern sides of the glaciated area the tundra would be closely 
followed by the conifer forests. 
In the west the spreading of the conifers to the north was 
followed by their gradual destruction in the southwest, due to 
the increase in temperature as compared with the rainfall. It is 
possible that the rainfall in Nebraska was never any greater 
than at the present time. But the decrease in transpiration 
accompanying decrease in temperature might account for the 
presence there during glacial times of trees which cannot live 
under present conditions. The bog plants perished with the 
conifers and their southwestern boundaries today are 
with that of the forest. 
In the east, among the highlands, exceptional circumstances 
were afforded for the preservation of these northern forms. 
Many relicts still crowd the higher elevations as far south as 
western North Carolina. 
But in the northern Ohio valley, with its scant conifer vegeta- 
tion, the areas which at that time supported the bog societies 
