456 PLANT DISTRIBrTION 



barriers. It is true that there are numerous plants in 

 North America which could readily grow in South Amer- 

 ica but do not spread from the one place to the other be- 

 cause the warm moist climate of the intervening zones, 

 together with the plants which inhabit them, make it 

 impossible to cross. 



Biological Barriers. — In considering the treeless 

 prairie and the adjacent forested areas one might suppose 

 that tliey are distinct because the species found in each 

 could not live in the other but this is daily proved untrue 

 by the thriving groves planted about the farms of the 

 prairie states and by the meadows of the forest region. 

 The truth — in part, at least — seems to be that the 

 plants oif one region can actually grow in the other in 

 many instances but they are not so well adapted to soil 

 and climate as the natives and so are unable to compete 

 successfully with them. Here again it is to be noted 

 that it is not a question of actual barriers but rather 

 one of a failure in competition due to imperfect adapta- 

 tion. Of course a grassy prairie lying between two 

 similar forested areas would act as a barrier to hinder 

 species spreading from one to the other; for if they were 

 unable to live in the grass-land they would be unlikely 

 to reach the other area suitable to their growth. 



The Great Plant Realms. — When the world as a 

 whole is regarded on a basis of the principles set forth 

 above and the actual distribution of the species of plants 

 and animals is considered, it is found that it can be 

 divided into five major divisions or Realms, as follows: 

 ( 1 ) The Holarctic Realm, including the whole of Europe, 

 most of North America and Asia, and Africa north of 

 the Sahara Desert; (2) The Neo-tropical Realm, includ- 

 ing South America, Central America, and the West 

 Inrlios; (3) The Oriental Realm, including Asia south 

 of the Himalayas, Borneo, Java, and the Philippines; 

 (4) The Australian Realm, including Australia, New 

 Guinea, Tasmania, and several smaller islands; (5) The 



