ADAPTATIONS 433 



destroyed. Therefore, each species profits by the sacrifice 

 the other makes in the education of the bird. In the case of 

 the unprotected mimic, however, the burden of education rests 

 wholly on the protected model. 



868. Color Changes. The color of animals is in the main 

 inherited, but it may often be more or less modified in the 

 individual by the environment. Animals kept in darkness 

 tend to become paler, and if the young flat fishes are illuminated 

 from below there is a tendency for the underside to develop 

 pigment where normally there is none. The color of cater- 

 pillars may be determined somewhat by the color of the sur- 

 rounding objects. If they are surrounded by dark objects 

 they tend to become darker also. Other animals, like many 

 fishes, frogs, tree toads, lizards and cuttlefishes, change color 

 rapidly and repeatedly. The color of a cuttlefish changes in a 

 flash. This is due to the action of contractile pigment cells. 

 When the cells expand the animal takes on the color of the 

 chromatophores, dark brown or orange or a combination of 

 these colors, as the different cells are stimulated. The action 

 is controlled through the nervous system, but it is not clear 

 what service it plays in the animal economy. The chameleon 

 and other lizards, as well as many frogs and fishes, change 

 color more slowly, but by a similar mechanism. In these 

 cases the color assumed is determined by the surroundings. 

 If the animal is blind the changes do not occur, and it is known 

 that the stimulus is received through the eye and transmitted 

 by the nervous system without, however, any voluntary 

 control by the animal. In these cases the colors assumed are 

 protective. 



869. Luminescence. There are two important physiological 

 processes which are practically wanting in all animals above 

 fishes, but are quite common among lower forms. These are 

 the light-producing organs and organs for generating electricity. 

 The production of light or phosphorescence occurs among all 



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