362 THE HOLARCTIC REGION. [CHAP. IX. 



of the latter, and occupying the summits of the Coast Ranges in 

 California and of many of the desert ranges of the Great Basin. 



"The Transition zone, as its name indicates, is a zone of 

 overlapping of Canadian and Sonoran types. Many Canadian 

 genera and species here reach the southern limits of their distribu- 

 tion, and many Sonoran genera and species their northern limits. 

 But a single mammalian genus (Synaptomys) is restricted to the 

 Transition zone, and future research may show that it inhabits the 

 Canadian region also." 



FIG. 71. MUSQUASH (Fiber zibet hi cus). 



The writer adds, however, that there are a considerable number 

 of species mostly rodents restricted to this zone. The follow- 

 ing Canadian genera, namely, Condylura, Urotrichus, Ursus, 

 Arctomys, Tamias, Fiber 1 , Zapus, Erethizon, Cervus, and Ovis, 

 almost or completely disappear in this zone ; while the following 

 intruders from the Sonoran, namely Scalops, Bassariscus, Spilo- 

 gale, Perognathus, Thomomys, Geoitiys, Cynomys and Antilocapra 

 do not range further north, several of them, indeed, only intruding 

 into the zone in a small area in the west. 



1 Penetrates the Sonoran along the lines of streams where cool currents of 

 air are carried down. 



