LUMBER TREES 119 



habit of the individual. Plants put forth mobile 

 seeds, and devise many strange ways of insuring 

 their wide dissemination. They are always seek- 

 ing new territories, and, granted proper condi- 

 tions, always finding them. 



And it is 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 ulti- 

 mately perished. 



But the plants that were fortunate enough to 

 choose the other avenues of escape, passing down 

 across the land surfaces that we now term 

 America and Asia, were not obstructed in their 

 flight. 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. 



