44 



FORESTS AND MANKIND 



saved from extinction. Many 

 that were unable to migrate 

 in this way, or could not cross 

 barriers of water and moun- 

 tains became extinct. Some, 

 like the sequoia, were almost 

 totally engulfed, but in a few 

 protected places managed to 

 survive. Then the ice moved 

 back and the great wealth of 

 plant life in the south gradu- 

 ally followed the retreating 

 glaciers northward. Years 

 passed, centuries, and another 

 sheet of ice came down out of 

 the north. And as each succes- 

 sive blanket of ice returned, 

 there were corresponding 

 waves of plant and animal life 

 moving back and forth and 

 ever changing as the climate 

 and other conditions changed. 

 It must have brought multi- 

 tudes of new species into ex- 

 istence just as it must have ex- 



EXTINCT SPECIES OF TULIP POPLAR 



Specimens i, 2 and 3 are fossil leaves 

 of a species closely resembling our own 

 tulip poplar (Liriodendron tulipifera), 

 whose leaf is shown as number 4. The 

 others have become extinct. 



