320 THE WILDERNESS OF THE UPPER YUKON 



white sheep on the north and the dark sheep on the 

 south. Gradually these interbreeds extended their range 

 (always in limited numbers, for the areas are not suitable 

 for abundance of sheep) both toward the north where the 

 dark colors are swamped by the preponderance of white 

 sheep, and toward the south where dark sheep prepon- 

 derate and swamp the light colors. Thus traces of the 

 dark pattern exist, rapidly dying out, among the white 

 sheep of the north, while the opposite is true in the south. 

 As soon as the interbreeds reach the continuous ranges 

 the variable colors are rapidly lost, and the result is a 

 tendency toward uniformity in color within narrow limits. 

 In the regions of uniform dark color, individuals are 

 exceptionally found which have either reverted in color 

 or have strayed there. Sheep do not cross the divide 

 of the Mackenzie River water-shed, hence the sheep in 

 the continuous ranges on the Mackenzie side remain pure 

 white, notwithstanding the fact that they range parallel 

 with the variables on the Yukon side. 



Between the regions where typical Ovis dalli and 

 typical Ovis stonei exist a distance north and south of 

 approximately six hundred and fifty miles, and of vari- 

 able width east and west there is no geographical area 

 in which the color of the sheep is uniform. Within that 

 intermediate region the uniformity of either species is 

 destroyed, and the individuals must be referred to one 

 or the other according to their color leanings. 



It is the generally accepted view that all species are the 

 products of evolution. True intergradation is so common 

 that it is only necessary to mention it. As species of the 



