DESERT SANDS 347 



In reality, this universal sombre tint of desert animals 

 is a beautiful example of the imperious working of our 

 modern Dciis ex viacJiind, natural selection. The more 

 uniform in hue is the environment of any particular region, 

 the more uniform in hue must be all its inhabitants. In 

 the arctic snows, for example, we find this principle pushed 

 to its furthest logical conclusion. There, everything is and 

 must be white — hares, foxes, and ptarmigans alike ; and 

 the reason is obvious — there can be no exception. Any 

 brown or black or reddish animal who ventured north 

 would at once render himself unpleasantly conspicuous in 

 the midst of the uniform arctic whiteness. If he were a 

 brown hare, for example, the foxes and bears and birds of 

 prey of the district would spot him at once on the white 

 fields, and pounce down upon him forthwith on his first 

 appearance. That hare would leave no similar descendants 

 to continue the race of brown hares in arctic regions after 

 him. Or, suppose, on the other hand, it were a brown fox 

 who invaded the domain of eternal snow. All the hares 

 and ptarmigans of his new district would behold him 

 coming from afar and keep well out of his way, while he, 

 poor creature, would never be able to spot them at all 

 among the white snow-fields. He would starve for want 

 of prey, at the very time when the white fox, his neighbour, 

 was stealing unperceived with stealthy tread upon the 

 hares and ptarmigans. In this way, from generation to 

 generation of arctic animals, the blacker or browner have 

 been constantly weeded out, and the greyer and whiter 

 have been constantly encouraged, till now all arctic 

 animals alike are as spotlessly snowy as the snow around 

 them. 



In the desert much the same causes operate, in a 



slightly different way, in favour of a general greyness or 



brownness as against pronounced shades of black, white, 

 23 



