SEVEN 
PLANTS AND MAN 
D URING the glacial epoch, a new factor was intro¬ 
duced into the evolution of plants. Slight at 
first, it increased more and more as time went on, until 
in our own day it has become of truly revolutionary sig¬ 
nificance. Its effect upon the life of the plant kingdom 
is comparable to that of the differentiation effected by 
climates. 
Man’s influence upon plants has been both destructive 
and constructive. On the one hand, he has destroyed 
most of the great primitive forests of North America, 
of northern Europe, and of China, and he has seriously 
reduced even some of the luxuriant African forests. 
Many of these areas formerly covered by majestic 
forest trees man has replanted with orchards and with 
cultivated fields of grain and vegetables; and in numer¬ 
ous instances the plants that he cultivates have been 
brought long distances from their native soil. Most of 
the cereals, vegetables, and fruits that are now grown 
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