THE GEOGRAPHICAL DISTRIBUTION OF 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. 



Canadian aubregion 

 Transition zone 



Sonoran region 



Neotropical region 



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



