FERNS, FOSSILS AND FUEL 
was much the same on both continents. The great ice 
shields which covered large portions of them both during 
the glacial period killed a considerable number of plant 
types, leaving only the hardy alpine or arctic species, 
which could exist near glaciers and ice fields. After the 
ice sheets disappeared, the remnant of the southern types 
could return. But in Europe they encountered an insuper¬ 
able barrier to their migration northward in the Mediter¬ 
ranean and Black Seas, and in the mountain chains 
which in Europe run mostly east and west; all of which 
were a result of glacial action. In North America, on 
the other hand, the mountain ranges run in a north-south 
direction, and there are no sea barriers; so the plants’ 
return to the north was unobstructed. Consequently, we 
now have distinctly different floras in northern Europe 
and North America, respectively. 
In the conclusions we draw from plants concerning the 
climate of geologic times we assume that the biological 
reactions of plants were the same then as in the present 
age. We apply the information derived from living 
plants to those of the past. Our entire knowledge of 
the structural and external conditions of fossil plants 
justifies this assumption. Fossil plants, indeed, have fre¬ 
quently been called the thermometer of geologic times. 
Besides the plant evidence, we also have other, but less 
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