FERNS, FOSSILS AND FUEL 
fats and cellulose and proteins. The plant oils, after 
having been buried for millions of years, are liberated 
by the oil driller, refined into gasoline, and burned in 
the carburetor of an automobile to provide its motive 
power. Even oil of animal derivation is indirectly of 
solar origin, since animals depend on plants for their food. 
Only a limited reservoir of fossil fuel is at the dis¬ 
posal of the human race, but it is so large that our gen¬ 
eration and the next one will not have to bother about 
its exhaustion. Perhaps by the time it is gone, man will 
have discovered other great sources of energy. 
Important as our contemporary vegetation is for our 
well-being, the plants of the past have influenced human 
life to at least as large a degree. They have added 
greatly to its comforts, and have immeasurably increased 
human power. The history of plant life on earth is not 
a matter of purely scientific interest; it is one of great 
practical importance as well. 
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