148 LAND MAMMALS IN THE WESTERN HEMISPHERE 



Dr. Merriam's arrangement, which deals only with North 

 America without reference to the Old World, divides the land 

 into a series of transcontinental zones, which he calls the 

 Arctic, Boreal, Upper and Lower Sonoran and Tropical. 

 These zones have very irregular and sinuous boundaries, which 

 follow lines of equal temperature (isothermal lines) during the 

 breeding season. May, June and July, the tortuous boundaries 

 being conditioned by topographical features, which deflect 

 the isothermal lines. 



Fig. 54. — Polar Bear {Thalarctus maritimus) . — By permission of the N.Y. Zoolog. Soc. 



The Arctic zone is part of a circumpolar area, which is very 

 much the same in North America, Asia and Europe ; and in any 

 of these continents the fauna differs much more from that of 

 the contiguous zone to the south than from the Arctic fauna 

 of another continent. There are some local differences, but 

 the characteristic mammals of this Arctic zone are the Polar 

 Bear, Arctic Fox, Musk Ox, Barren-ground Caribou, Lemming, 



