Color in Nature 333 



face cells, the cuticula or elsewhere, or else lodged in spe- 

 cial cells known as chromatophores, the absorption of cer- 

 tain rays of light by these pigments, and the reflection of 

 their complementary rays causing the various colors. Pig- 

 ments develop through the action of an oxidizing ferment 

 upon a color-forming substance or chromogen, and many 

 different pigments may be merely different stages in the modi- 

 fication of a single chromogen. Thus the brown and black 

 pigments of animals pass through yellow, orange and red 

 stages, before attaining their final color. 



The influence of external factors in producing more or 

 less permanent color changes in animals has been discussed 

 in a previous chapter, dealing with the influence of the en- 

 vironment upon the development of the individual. Tem- 

 porary changes in the hue or color of animals may result'in 

 response to external stimuli. The chameleon is the class- 

 ical example of this. Temperature and light appear to be 

 the controlling stimuli although their effects differ in differ- 

 ent species. Fear may affect the color of an animal. Thus 

 it is possible to cause a frog to ''turn pale with fear" by 

 continually disturbing it with a stick or otherwise. The color 

 changes in these cases are due to changes in the distribution 

 of the granules of pigment in the chromatophores; when the 

 pigment is distributed throughout the cell the color is darker, 

 when concentrated around the nucleus the reverse is true. 



One of the most remarkable cases of color adaptation 

 known is that of the flatfish. Symmetrical both in form and 

 color in its early stages this fish soon turns on its side and 

 thereafter lies on the bottom of the sea. Accompanying this 

 change in life the eyes, fins and mouth shift to the upper 

 side of the body, and the lower side loses its color. But, 

 as the English naturalist Cunningham has shown, the color 

 will return to the lower side in fish kept in an aquarium 

 which is lighted from below. Living on the bottom the flat- 

 fish finds itself from time to time on differently colored back- 

 grounds, now on white and now on dark sand, and again 

 on gravel of various shades and patterns. In an extensive 

 series of experiments Sumner has shown that this species 

 adjusts its color to match that of its background with won- 

 derful accuracy; and that further this change is affected in 

 some unknown way through the nervous system in response 

 to sight, for if the eye be removed the power of adjustment 

 is lost with it. 



The physical colors of animals are due to the form of the 

 body surface, causing refraction and the formation of "me- 

 tallic" coloring, or interference of the reflected light rays, 

 thus producing the wonderful iridescence characteristic of 



