3 i8 THE WILDERNESS OF THE UPPER YUKON 



are caused by "Natural Selection" of their protective 

 values. 



The white rump patch on sheep has been called a 

 directive marking, as it enables the members of a band 

 quickly to recognize one another under varied conditions. 

 If, owing to their natural color, the white sheep do not 

 need a directive marking when they are so conspicuous 

 for four months in summer, then likewise the dark sheep 

 do not need a directive marking when they are so con- 

 spicuous during the eight months when the snow is on 

 the ground. It is undoubtedly true that sheep take 

 advantage of recognition marks, just as they take advan- 

 tage of their color under conditions where it is protective. 

 But I cannot believe that these recognition marks on the 

 dark northern sheep have been developed by a process of 

 Natural Selection favoring those which possess the white 

 rump patch, since a vast majority of the sheep, being 

 white, do not have a directive marking which would be 

 advantageous to them for two-thirds of the year, while the 

 dark sheep have need of one for only one-third of the year. 

 Such a process of Natural Selection would not, under the 

 similar conditions affecting all the northern sheep, pro- 

 duce contradictory color results. 



The similarity of the sheep north of the range of Ovis 

 canadensts, except in color, indicates that all came from a 

 common ancestor. It is believed that originally American 

 wild sheep migrated from Asia. The persistence of dark 

 hairs in the pelage of all white sheep would suggest that 

 the common ancestor was a dark animal. The close 

 relation of these northern American sheep to those of 



