426 ELEMENTARY ZOOLOGY 



spicuous in a dusty road. But dusty roads are precisely 

 where katydids do not rest. They alight among the 

 green leaves of a tree or shrub. The animals that live in 

 deserts are almost all obscurely mottled with gray and 

 brownish and sand-color so as to harmonize in color with 

 their habitual environment. The arctic hares and foxes 

 and grouse which live in regions of perpetual snow are 

 pure white instead of red or brown or gray like their 

 cousins of temperate and warm regions. 



These cases of an animal's color and markings har- 

 monizing with the usual environment are called instances 

 of protective resemblance ; that is, they are resemblances 

 for a purpose, that purpose being to render the animal in- 

 distinguishable from its surroundings and thus to aid it in 

 escaping its enemies. Such protective resemblances are 

 obviously of great value to animals, and, like other 

 advantageous modifications, have been produced by the 

 action of natural selection. Those individuals of a species 

 most conspicuous and hence most readily perceived by 

 enemies are the first (under ordinary circumstances) to 

 be captured and eaten. The less conspicuous live and 

 produce young like themselves. Of these young the least 

 conspicuous are again saved and so over and over again 

 through thousands of generations until this natural select- 

 ing of the protectively colored results in the production 

 of the wonderfully specialized examples of resemblance 

 to which attention is called in the following paragraphs. 



General, variable, and special protective resemblance. 

 In the brooks most fishes are dark olive or greenish 

 above and white below. To the birds and other enemies 

 which look down on them they are colored like the 

 bottom. To their fish-enemies which look up from below 

 they are like the white light above them in color and their 

 forms are not clearly seen. The green tree-frogs and 

 tree-snakes which live habitually among green foliage; 



