180 TREE ANCESTORS 



southward retreat of the forests. Another hnk in the chain of 

 distribution is the presence of fossil leaves Hke those of the existing 

 species in the Pliocene of the Altai mountains, north of Tibet in 

 central Asia. The extremely sHght differences between Liriodcn- 

 dron tulipifera of southeastern North America and Liriodendron 

 chinensis of southwestern China probably date from PHocene time. 

 Finally in Pleistocene time we find the fossil fruits and leaves of 

 the existing American form in Maryland, North Carolina and 

 Alabama. 



