26 OUTLINE OF PLANT GEOGRAPHY 



the tropics of the old and new worlds are widely sundered by 

 the great oceans, and as might be anticipated, the tropical veg- 

 etation of the East and West is composed for the most part of 

 very different elements. 



There are very few common species, and to a great extent, 

 the genera and even families differ. This is evidently the result 

 of a very long period of isolation combined with the more intense 

 competition which must have developed under conditions so 

 favorable to rapid growth and multiplication. 



While the superficial aspect of the equatorial forests of the 

 old and new worlds is quite similar, nearer inspection will show 

 a very great difference in the species of which they are composed. 



The Boreal Regions 



Under the name Boreal or Holarctic is included the whole 

 of the regions lying between the Tropic of Cancer and the North 

 Pole, but we may recognize three divisions, more or less well 

 defined, the Arctic, Sub-arctic and North Temperate. 



As might be expected from the intimate connection between 

 the northern continents their vegetation has much in common, 

 so that many common trees of temperate Eurasia and America, 

 for example, are obviously related. Pines and firs, oaks, beeches, 

 maples, willows and poplars, are characteristic of the temperate 

 regions of both continents, and the same is true of very many 

 shrubs and herbaceous plants. 



This uniformity in the vegetation is especially marked in 

 the arctic and sub-arctic zones, many species occurring through- 

 out these regions, while in other cases, Eurasian species are 

 represented in America by different, but closely related ones. 



Southward the differences become more and more pronounced, 

 owing to isolation and different climatic conditions, as well as 

 to an intrusion of tropical or subtropical types, which differ 

 widely in the old and new worlds. 



The history of the origin and distribution of the boreal floras 

 is revealed by a study of the abundant fossil plant remains in 

 the Cretaceous and Tertiary rocks of both the eastern and western 

 hemispheres. 



Originating, apparently, in the far north, there is found in 



