3 i6 THE WILDERNESS OF THE UPPER YUKON 



of catching it. He asserts that white top-patterns on 

 animals (including sheep) have an obliterating effect 

 when seen from below at such an angle as to bring the 

 white against sky and cloud (and even snow). I think 

 that finally it will be admitted that Mr. Thayer correctly 

 states this optical principle. There may, however, be 

 many cases of doubt as to its practical application. 



It could only be of service to the sheep in escaping from 

 a wolf, and only at times when the latter is trying to catch 

 older sheep under conditions when the white is in such a 

 position in the wolf's line of sight as to bring the white 

 marking against a background of sky or snow. It would 

 not apply when a wolf catches young sheep (which is 

 usually the case) whose white patterns, unless the sheep 

 were running upward, are below his line of vision. 

 Nor would it always apply when sheep are seeking to 

 escape by running downward. Therefore, though the 

 advantages of these white-top patterns are true as a fact 

 of optics, I have not observed that these white markings 

 are of much practical advantage to sheep. 



Nor do I believe that white top-markings on other 

 animals have been developed because of their oblitera- 

 tive advantages to the animal. One of my reasons for this 

 disbelief consists in the fact that such advantages, if 

 they are practical, may endanger the young of animals 

 inhabiting treeless plains (and also the young of sheep), 

 and threaten the welfare of the race to an extent that off- 

 sets the advantages to the adults. The young of the ani- 

 mal as well as the enemy would see the white of the adult 

 against the sky, and still more, the white markings on 



