86 NORTH AMERICA 



kind. At the same time, the great diversity of the relief 

 and of the soil permits of a large variety in the form 

 of the forests and their components. As a rule the 

 broad-leaf type is found on the richer soils, and largely 

 predominates west of the Appalachians. Pine and other 

 coniferous forests are found in greater proportion in the 

 east and on porous sandy soils. 



The circum- Appalachian region offers an admirable col- 

 lection of broad-leaf and summer-green trees and shrubs 

 requiring a relatively moist climate. Indeed, the wealth 

 and variety of these forests almost challenge comparison 

 with the rich flora and luxuriant vegetation of eastern 

 Asia, in a similar climatic situation, and surpass any- 

 thing seen even in south-eastern Europe. When, in past 

 ages, the polar ice invaded the now temperate latitudes, 

 the vegetation of eastern North America and Asia found 

 shelter in the southern extensions of those continents. 

 On the retreat of the ice, the new plant population of 

 the lands thus laid bare was mostly recruited from these 

 rich southern floras. In Europe and western Asia the 

 earlier flora was stopped in its retreat by the Mediter- 

 ranean and the mountain barriers running east to west 

 and destroyed: the plant population which arose after- 

 wards was mostly drawn from the east. 



The catalogue of trees of the temperate type in eastern 

 North America includes many species of oak, walnut, 

 hickory, chestnut, birch, alder, hazel-nut, hornbeam, 

 willow, poplar, elm, magnolia, tulip-tree, maclura, 

 laurel, sassafras, plane, maple, ash, robinia (our common 

 acacia), horse-chestnut, and allied trees. Conifers are 

 represented chiefly by pines. Among the countless 

 shrubs of the undergrowth, numerous species of 

 rhododendron and magnolia are the most remarkable, 

 with kalmia, and several shrubs allied to it and to 



