170 



THE AMERICAN NATURALIST 



[Vol. XL VI 



deciduous forest region of the southern Appalachians. Among 

 the widespread Miocene genera which still exist in America 

 may be mentioned Magnolia, Liriodendron, Vitis, Sassafras, 

 Aralia, Nyssa. Indeed it is supposed that some of the Miocene 

 species still survive. Among these are the southern Cypress 

 (Taxodium distichum), Balsam Poplar (Populus balsamifera) , 

 Sweet Gum (Liquidambar styraciflua) , Black "Walnut (Juglans 

 nigra) and many others. 



With the redistribution of the forest flora due to the glacial 

 epoch, many of these types disappeared from western America 

 and Europe but have survived in eastern America and eastern 

 Asia. It is clear also that genera now confined to the Old World 

 also existed at that time in North America. A long list of the 

 Cretaceous and Tertiary genera of North America shows, for 

 example such extra- American genera as Banksia (Australia), 

 Casuarina (Australia and Malaysia), Encephalartos (South 

 Africa), Eucalyptus (Australia), Ginkgo (China), Hedera (Eu- 

 rope, Asia), Sterculia (tropics of the Old World and Austral- 



During Tertiary times the eastern and western parts of the 

 continent were again united, and there seems to have been a 

 fairly uniform northern flora throughout the Atlantic and Pa- 

 cific regions, although the separation of the Atlantic and Pacific 

 floras was already indicated, the difference in climate no doubt 

 being influenced by the changes in elevation caused by the up- 

 lifting of the great western mountain masses, inducing a drier 

 climate in the interior of the continent. It seems probable that 

 to the north of the great forest belt extending across the con- 

 tinent was an arctic flora, remnants of which are found in the 

 Alpine regions of the higher mountains where they presumably 

 took refuge after the migrations subsequent to the great glacial 

 epoch. 



It is pretty generally agreed that the present distribution of 

 plants in America is mainly the result of the advance of the 

 great glacial sheet from the north. In the southward retreat 

 of the great forest belt there was a division which, it has been 

 suggested, was caused by the presence of the dry plains of the 

 interior, which were unfitted for forest growth, and thus acted 

 as a wedge separating the western forest, predominantly of 

 coniferous trees, from the eastern forest mainly deciduous in 

 character. "Whether or not this is the true explanation, the fact 



