THE GEOGRAPHICAL DISTRIBUTION OF MAMMALS 153 



the mole-shrews (Urotrichus) do not extend south of the 

 Transition zone. Probable intruders from the south into 

 the Boreal subregion are the pumas, or "mountain lions," 

 which just enter the subregion, the Canada Lynx (Lynx 

 rufus) and one species of skunks (Mephitis), the Raccoon 

 (Procyon lotor), Badger (Taxidea americana) and the Ameri- 

 can deer (Odocoileus) . A large number of rodents are char- 

 acteristically Boreal : marmots, or woodchucks (Marmota), the 

 Sewellel (Aplodontia rufa), lemmings (My odes), Jumping Mouse 

 (Zapus), the Canada Porcupine (Erethizon dor solus) and the 

 pikas, "tailless or whistling hares" (Ochotona). Boreal ro- 

 dents that enter the Sonoran are the chipmunks (Tamias), 

 beavers (Castor), meadow-mice (Microtus), the Muskrat (Fiber 

 zibethicus). The white-footed mice (Sitomys) and the wood- 

 rats (Neotoma) are southern rodents that reach or enter the 

 Boreal. 



Between the Boreal subregion and the Sonoran region is 

 the Transition zone, which follows all the complex windings of 

 the boundary lines. It covers most of New England, New 

 York, Pennsylvania and southern Ontario ; passing through 

 southern Michigan and Wisconsin, it bends northward over 

 Minnesota and covers most of North Dakota, Manitoba and 

 the plains of the Saskatchewan, then turns abruptly south- 

 ward and includes eastern Montana and parts of South Dakota 

 and Nebraska. Crossing Wyoming, it follows around the 

 northern edge of the Great Basin to the plains of the Columbia. 

 The three great mountain-systems carry the zone far to the 

 south and arms of it extend along the Appalachians to northern 

 Georgia, along the Rockies to New Mexico, and it follows the 

 Sierras to southern California. "The Transition zone, as 

 its name indicates, is a zone of overlapping Boreal and Sono- 

 ran types. Many Boreal genera and species here reach the 

 extreme southern limits of their distribution and many Sonoran 

 genera and species their northern limits. But a single mam- 

 malian genus (Synaptomys) [one of the field mice] is restricted 



