294 THE SHEEP AND ITS COUSINS 



these slender-horned bighorns is that they are essen- 

 tially a northern type — occurring in America, in 

 British Columbia, and Alaska — which is common 

 to north-eastern Asia and America. This indi- 

 cates that the bighorns, like most other wild 

 sheep, had their origin in Asia, whence — alone 

 among their kindred — they effected an entrance 

 into the Western Hemisphere by way of what is 

 now Bering Strait. The Alaskan and British 

 Columbian races have retained many of the charac- 

 teristics of their Asiatic forbears, whereas when 

 they wandered far south they became differentiated 

 into the heavy-horned type represented by the 

 Rocky Mountain bighorn and its immediate local 

 phases. 



At one time American naturalists recognised 

 three distinct American species of thin-horned big- 

 horns, but these are now known to intergrade,* and 

 they are consequently classed as local phases of 

 one and the same species, whether we regard it as 

 identical with or distinct from O. canadensis. 



Of these American races, O. c. dalli, described 

 in 1884, from the Upper Yukon district of Alaska, 

 is, except for accidental staining, pure white at 

 all times of the year. Near Dawson City occurs an 

 unstable grey phase, O. c. fannini, connecting the 

 white bighorn with the black bighorn {jO. c. stonei) 



* See Charles Sheldon, The Wilderness of the Upper Yukon, 

 New York, 191 1. 



