THE GEOGRAPHICAL DISTRIBUTION OF MAMMALS 



161 



to the Transition zone. ... A number of species, however, 

 seem to be nearly or quite confined to this zone " (Merriam). 

 The most characteristic portion of North America, zoolog- 

 ically speaking, is the Sonoran region of Dr. Merriam, the 

 Warm Temperate of Dr. Allen. It crosses the continent 

 from ocean to ocean, its northern boundary following for most 

 of the way the 43d parallel of latitude, but over the Great 



Fig. 73. — Opossum (Didelphis niarsupialis).- 

 N.Y. Zoolog. Soc. 



-By permission of the 



Plains and Great Basin, on each side of the Rocky Mountains 

 and the high plateaus, it extends to lat. 48°. On the south, 

 it takes in the greater part of Mexico, covering all of the table- 

 land of that country, the lowlands of which belong to the South 

 American or Neotropical region. The Sonoran is invaded from 

 the north by the long branches from the Boreal and Transition 

 zones, which follow the three great mountain-systems in the 

 manner already explained, and the Mexican plateau permits 

 the similar invasion of Neotropical territory by the Sonoran 

 fauna. Characteristic Sonoran genera, none of which extend 

 into the Boreal, are the opossums (Didelphis), in the southern 

 part a peccsiTy (Tagassu) or "Wild Texas Pig," representative 

 of a family of swine quite different from the true pigs of the Old 



