ON TIMBER TREES 
from their sun-loving habit. But in the main the 
tribes that escaped destruction were those that 
developed a hardiness that enabled them to with- 
stand extremes of temperature not far beyond 
the limits of the ice sheet. Others made their way 
northward again so soon as the ice sheet receded. 
And as the climate of ensuing ages, after the 
successive periods of intense refrigeration, every- 
where retained, throughout the central and eastern 
portions of America, curious reminiscences of both 
the tropical and the arctic, the plants that finally 
repopulated the devastated territories were those 
that had learned, through the strange vicissitudes 
of their ancestors, to thrive where the thermometer 
in summer might rise to the one hundred degree 
mark, and where in winter the mercury might 
freeze. 
Such are the conditions under which pines and 
oaks and willows and beeches and black walnuts 
and allied trees exist to-day in the regions of 
northern America where they flourish. 
They can withstand the glare of a tropical sun 
in summer because their ancestors reveled in a 
tropical climate. And they can withstand equally 
the arctic cold of winter because their ancestors of 
other ages were forced to subsist under arctic 
conditions. 
The versatile tree that, thanks to the racial 
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