FERNS, FOSSILS AND FUEL 
matic conditions highly favorable to plant growth at 
both times; for otherwise it would be inconceivable that 
coal beds, which are nothing but accumulated carbonized 
plant material, could have been deposited to such a vast 
extent. 
There is another fact which we must not overlook. 
The coal beds of the Pennsylvanian period, as well as those 
of the Tertiary, extend from near the north pole down 
into the temperate zone, throughout North America, 
Europe, and Asia, including Alaska, Greenland, Spitz- 
bergen, Siberia, and many Arctic islands. Apparently 
the north pole must have been in about the same place 
in those times as it is now. This puts a very awkward 
question before us. We can explain a mild temperature 
from pole to pole, but it is impossible to explain by any 
stretch of the imagination how the polar night was 
illuminated to permit a constant plant growth. 
We know very well that plants cannot grow without 
light; yet the trees of the Pennsylvanian period do not 
even have annual rings indicating seasonal changes of 
growth. How could a tropical or semi-tropical vegeta¬ 
tion more abundant than that of our times have grown 
in the polar darkness? The only theory which can explain 
it is the assumption of continental movements. If the 
continents in Pennsylvanian times were arranged in a 
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