ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 423 



and opalescent tints of the inner surface of molluscan shells. 

 We pass also that coloration of purely physiological value, 

 even though it be adaptive, such as the black color of 

 hibernating arctic insects, adapted to absorbing the maxi- 

 mum amount of heat during the short summer season ; and 

 the white color of winter animals of the same region, secur- 

 ing them against the too great loss of their own animal heat 

 supply through radiation. We study that adjust- 

 ment of color and form that has to do with being 

 seen, that doubtless did not exist in the beginning, that 

 came into existence when animals began to hunt with their 

 eyes ; that has been developed along with the perfecting of 

 organs of vision. 



There are some coloration phenomena so very common 

 and widely distributed that they may be studied in abun- 

 dant examples anywhere. These are resemblance, -flash 

 colors, warning coloration, and mimicry. 



Resemblance. Few animals can afford to be conspicuous 

 only those possessed of sufficient swiftness or agility to 

 escape their ordinary enemies, or those possessing special 

 means of defense. For most animals, to be conspicuous is 

 to invite destruction. Hence the majority of animals, 

 although conspicuous enough when exhibited in museums, 

 are in their proper environment to be found only by careful 

 searching. 



Natural selection furnishes a simple and satisfactory 

 explanation of resemblance. The less fitly colored, being 

 the more conspicuous, are the ones eliminated by enemies, 

 leaving those that are better concealed by their coloration to 

 survive and perpetuate in their descendants their own 

 inconspicuousness. 



The most general and fundamental phenomenon of 

 resemblance is the darker pigmentation of the side of the 

 body that is uppermost and the shading off lighter below, to 



