200 Light 



pattern and presumably are not easily recognized by their natural 

 enemies. Any such visual deception will, of course, not affect preda- 

 tors that locate their prey exclusively by smell, sound, or other meth- 

 ods. Some species when disturbed produce a flash of some brightly 

 colored or contrasting part that is concealed when the animals are 

 quiet, and the startling effect may serve to distract or to frighten off 

 pursuers. 



Another type of protective resemblance is mimicry— a phenomenon 

 that is particularly well developed among the insects. Here one 

 species closely resembles in color and form another totally unrelated 

 species sometimes in an entirely different order. The mimic is be- 

 lieved to derive protection from the fact that it is mistaken by preda- 

 tors for the species it resembles. If the model species is distasteful or 

 harmful as a food organism and hence is avoided by the predators, 

 the mimic may also escape unmolested. Extraordinarily close 

 mimicry certainly exists between insect species, but the reality of the 

 benefit of the mimicry, its mode of operation, and its evolutionary 

 origin are controversial subjects which still await conclusive investi- 

 gation. For a further discussion of the far-reaching problems of pro- 

 tective coloration the reader may consult more extensive treatments 

 of the subject such as that by Cott ( 1940 ) . 



Some animals are able to change their color or pattern sometimes 

 within a matter of minutes or even seconds. Such changes in ap- 

 pearance occur characteristically as adaptations to the background, 

 and are found among reptiles, amphibians, fishes, crustaceans, insects, 

 cephalopods, and other invertebrates. The mechanisms by which 

 color changes are brought about have been summarized by Prosser 

 (1950, Ch. 21). In many instances they involve the nature of the 

 light received from the background through the eyes, but in other in- 

 stances, they are activated by direct radiation. A flounder changes 

 its general color tone and also the pattern of the black and white 

 patches of its skin as it moves from one type of bottom to another. 

 Such changes in coloration tending to match the background furnish 

 an obvious advantage in concealment (Summer, 1935). In other in- 

 stances color changes serve as protection from high illumination, take 

 part in thermoregulation, or are associated with breeding, as in cer- 

 tain lizards, fishes, and squids. The seasonal color changes of the 

 varying hare, weasel, and ptarmigan from brown in summer to white 

 in winter are obviously related to the conspicuousness of such animals 

 asjainst bare ground or snow-covered landscape. 



The different, and usually more brilliant, coloration of the males 

 of many birds and of some other animals is familiar to everyone. In 



