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] 



