PROTECTIVE PIGMENTS AND APPEARANCES 353 



(Fig. 167). The tree toad and the partridge become lost to the 

 eye, as well as the sand flea and the underwing moth (Fig. 168). 

 It is familiar to all of us 

 that desert animals are 

 frequently tawny in their 

 color, whereas arctic ani- 

 mals are frequently white. 

 There can be no doubt 

 that in relation to certain 

 enemies the resemblance 

 between an animal's color 

 and the background color 

 is often a real protection. 

 At the same time, there 

 is danger of exaggerat- 

 ing the importance of 

 this resemblance to the 

 organisms, and there is a 

 corresponding danger of 

 trying to prove too much 

 from this resemblance. 

 Thus, the whiteness of 

 arctic animals is appar- 

 ently due in many cases 

 not to the whiteness of 

 the surroundings but to 

 the low temperature. 



FIG. 169. The walking stick 



This animal has startled many a person by walking 

 away from a hand stretched out to grasp a leaf or 

 twig. The insect is related to the locust and katy- 

 did, but it has no wings. Its body and legs are 



very long in proportion to thickness, and the en- 

 largements at the joints and the irregularity of 

 outline increase the resemblance to bare twigs. 

 Moreover, the color of the animal changes with 

 the seasons, from a bright green in the spring 

 to a deep brown in the fall, thus matching its 

 surroundings very closely 



The color of an animal 

 is often due to the char- 

 acter of the wastes pro- 

 duced by the chemical 

 changes going on in the 

 protoplasm. The character of the waste, in turn, will depend 

 upon the nature of the food. A change in diet will therefore in 

 many cases result in a change of color. This is shown in the 



