CHAPTER X. 

 THE SONORAN REGION. 



Limits Characteristics of Mammalian Fauna Extinct Groups of Mammals 

 characteristic of Western Arctogaea Distinctness of the Region Dual 

 Origin of Groups. 



As stated in the introductory chapter, wherever one zoological 

 region of the globe has no definite physical barrier by which 

 it is separated from the next well-marked region, there must 

 always occur an intermediate tract where the characteristic types 

 of the faunas of the two regions inosculate and intermingle. That 

 this is the case with that area of North America denominated the 

 Sonoran region has been indicated at the close of the preceding 

 chapter, and the existence of the Transition zone, which seems, 

 on the whole, to pertain to the Holarctic region, unfortunately 

 prevents the Sonoran from being defined with the precision which 

 would be possible had this area a high mountain-barrier on its 

 northern frontier. 



In a map of the small dimensions of the one accompanying 

 this volume it is impossible to show with any 



Limits. 



attempt at accuracy the complex nature of the 

 northern boundary of this region, which will, however, be found 

 accurately laid down in the map illustrating Dr Merriam's memoir 1 . 

 According to the latter writer, " the Sonoran region as a whole 

 stretches across the continent from the Atlantic to the Pacific, 

 covering nearly the whole country south of latitude 43, and 

 reaching northward on the Great Plains and Great Basin to about 

 latitude 48. It is invaded from the north by three principal 

 intrusions of Canadian 2 forms along the three great mountain- 



1 Appendix, No. 19. 



2 Boreal in the original. 



