SOME ANIMALS OF YUKON TERRITORY 31 1 



With the eye of the artist, no doubt, Mr. Thayer has an- 

 alyzed remarkably well the obliterative colors of the 

 animal kingdom in their relations to lights and shades, 

 and of blending under certain conditions with their back- 

 ground. His analysis reveals some important discoveries. 

 His book presents an array of facts animal coloration in 

 relation to matching certain backgrounds, analyzed ac- 

 cording to the laws of the science of optics which I fully 

 believe must revolutionize not only many of the precon- 

 ceived notions on the subject, but must also destroy some 

 of the theories which, in the attempt to determine the 

 causes of animal coloration, have been widely accepted. 

 It seems to be a necessary inference from Mr. Thayer's 

 writings that concealing coloration has been exclusively 

 produced by the relations between predatory animals 

 and their prey.* 



It may be granted (though by no means to the extent 

 and under all the circumstances that Mr. Thayer asserts) 

 that all animals more or less, according to certain con- 

 ditions, blend with their background. The question is, 

 What are the causes which produce such harmony ? 



Let us apply Mr. Thayer's theory to the sheep. 



All the northern sheep live in regions where the snow- 

 fall is practically the same. Above timber-line, where 

 they continually live, the country is in general the same, 



* Mr. Thayer, while repeatedly asserting his own positive belief that the Con- 

 cealing Colors of animals result exclusively from a process of Natural Selection 

 which has established the most favorable balance of color between predatory 

 animals and their prey, admits in at least two places in his book the possibility of 

 other causes. In fact, he writes me that while he deeply believes the cause to be 

 Natural Selection, he has given a wrong idea to any one who thinks that he 

 lays stress on its being the modus operandi of all color matching. He emphasizes 

 only the color matching itself. C. SHELDON. 



