LUTHER BURBANK 
And it was only such plants as could migrate 
with relative celerity that were able to maintain 
existence and escape extermination by fleeing 
southward when the era of cold succeeded to the 
warm era in the arctic regions and when the arctic 
chill gradually spread southward and encom- 
passed all the higher and middle latitudes of the 
northern hemisphere. 
The plants that chanced to flee southward along 
the land surface that we now term Europe found 
their further flight checked when they reached the 
stretches of mountains extending east and west 
that we now term the Alps. Here thousands of 
species made a last stand and ultimately perished. 
But the plants that were fortunate enough to 
choose the other avenues of escape, passing down 
across the land surface that we now term America 
and Asia, were not obstructed in their flight. In- 
deed, the long ranges of the Appalachians and 
Rockies and Sierras in particular served, as it 
were, to guide the line of march and aid the flight. 
So the American species made their way to the 
region of the gulf, and some of them even to the 
southern continent. And when the ice sheet finally 
receded, they were able to make their way north- 
ward again, though never to their former habitat; 
whereas Europe was treeless until the plant life of 
Asia spread westward to re-people it. 
[186] 
