THE GEOGRAPHICAL DISTRIBUTION OP MAMMALS 



147 



daries between the regions which make up North America 

 should be very sharply drawn. It is not surprising, therefore, 

 to find a transition zone, extending all across the continent, 

 in which the Holarctic and Sonoran faunas mingle, or that 

 Central America should, in considerable measure, be transi- 

 tional to South America, though zoologically a part of the latter. 



Boreal zone or 



Canadian subregion 



Transition zone 

 Sonoran region 

 Neotropical region 



Fig. 53. —Zoological Divisions of North America. (After Merriam.) 



