THE EVIDENCE FOR EVOLUTION 



735 



eastern China were regions untouched by the glaciations and here the 

 alHgators and magnolia trees survived. Because the alligators and mag- 

 nolias of the two regions have been separated for several million years, 

 they have had the opportunity to accumulate different random muta- 

 tions. They are thus slightly different but closely related species of the 

 same genera. 



316. The Biogeographic Realms 



Careful studies of the distribution of plants and animals over the 

 earth have revealed the existence of six major biogeographic realms, each 

 characterized by the presence of certain unique organisms. These realms 

 were originally defined on the basis of the distribution of mammals, but 

 they have proved to be valid for many other kinds of animals and plants 

 as well. The various parts of each realm may be widely separated and 

 have quite different conditions of climate and topography, but it has 



Figure 35.7. A polar projection map of the world showing the biogeographic realms. 

 (After Matthew.) 



