The Problems of Plant Distribution 251 



found than is the case in the northern regions. 

 Even where common genera occur, there has been 

 the development of new species, owing to climatic 

 differences, and very few species are really common 

 to these two regions except where these species also 

 occur in the northern zone. Thus while Europe and 

 Atlantic North America possess many genera in 

 common, like the oaks, elms, walnuts, larches, 

 asters, goldenrods, gentians, violets, etc., they are 

 with few exceptions represented by quite distinct 

 species. All of these may safely be considered to 

 be the common descendants of Tertiary ancestors, 

 which through isolation have become specifically dis- 

 tinct. On the other hand, there are numerous types 

 which belong to one or the other of the two regions 

 but are absent from the other. Thus Europe has 

 no magnolias, tulip-trees, gums, sassafras, hick- 

 ories, trilliums, milk-weeds, mandrakes, and very 

 many more familiar American plants; while on the 

 other hand, America possesses no daffodils, tulips, 

 snowdrops, foxgloves, heaths, brooms, and many 

 other beautiful flowers, which adorn the woods 

 and meadows of the Old World. 



Within the north temperate zone are enormous 

 areas showing far greater differences of conditions 

 than are found in the regions of the North, and in 

 consequence their floras are far more varied. In the 

 southern portions of this zone there is frequently an 

 invasion of tropical types, and the limits between 

 the temperate and tropical floras are very vague. 



