SOME ANIMALS OF YUKON TERRITORY 321 



same genera extend their ranges and reach different con- 

 ditions of climate and environment, or become isolated, 

 they tend to differentiate more or less according to their 

 plasticity and the degree or character of the change in 

 environment. As a result, geographical races are formed, 

 gradually changing from one area to another, until the 

 extremes may be widely different. Near the border-line 

 between the geographical races there is an intermingling 

 and where two forms come close together no doubt 

 interbreeding, which results in intergrades referable to the 

 race which they most closely resemble on either side of 

 the border. But, within the geographical areas, the differ- 

 ing characters are sufficiently uniform to mark a race, 

 though there may be individual variation. Where species 

 of the same genera differ and intergrades are unknown, 

 it is either because the intergrades have not been found 

 or they have not survived. 



Sometimes the difference in environment, physical and 

 climatic, is so subtle as to be indeterminate. It is pos- 

 sible that the variation in the colors of the sheep is thus 

 produced, by subtle and indeterminate changes of envi- 

 ronment, to a much greater extent than the facts seem 

 to me to indicate. A wider and more thorough famili- 

 arity with the facts of intergradation might cause me 

 to change my present belief, that at some time in the past 

 either the intergrading forms between the Dall and the 

 Stone sheep disappeared, or that the extremes of dark 

 and white were produced by long isolation, in different 

 environments, owing to the interposition of barriers be- 

 tween sheep of similar color; or from some other cause; 



