SOME ANIMALS OF YUKON TERRITORY 317 



the young would be observed by the enemy from above 

 rather than from below, and hence offer an effective target 

 for attack. Not only would the attack be more apt to 

 be directed against the young, but even when directed 

 unsuccessfully at the adult (attacks fail far oftener than 

 they succeed), during the immediate flight the white top- 

 markings would deceive not only the enemy, but also the 

 young who might lose their parents and remain unpro- 

 tected against a pursuing enemy who could see the white 

 marking of the young against a dark background. 



The dark sheep have light rumps. Mr. Thayer would 

 argue that these dark sheep have excellent concealing 

 colors except sometimes on the snow, but even on the 

 snow, at the final dash of the enemy, their backs would 

 be turned in sudden flight and the rumps would blend 

 with the sky or snow background and thus conceal them 

 at the critical moment. But since there is fully as much 

 snow on the mountains in the regions inhabited by dark 

 sheep as in those inhabited by white sheep, and it remains 

 on the surface practically as long, why has one species 

 been developed white and the other with white only on 

 the rump ? If enemies cause the coloration, then the 

 same enemies should act to develop the colors of all the 

 northern sheep. Therefore, the same cause has produced 

 not only two opposite colors, pure white and black, but 

 also, and over vast areas, a variety of intermediate colors. 

 Under numerous conditions the protective values of the 

 opposite colors are contradictory. 



As applicable to the northern sheep, therefore, we 

 must dismiss theories suggesting that animal colors 



