334 



Biulogy in America 



many boetlos and birds. Metallic colors and iridescence are 

 generally super-imposed on pigment color producing a com- 

 pound eil'ect, white being the only purely physical color that 

 we know in animals. 



The functions of animal colors are doubtless manifold, but 

 concerning them our knowledge is unfortunately very frag- 

 mentary. Omitting those internal i)igments such as haemo- 

 globin, bile pigments and the like, which are intimately re- 

 lated to the physiology of the animal, and pigments derived 

 from tlie animal's food, such as the green or yellow color of 

 some caterpillars fed on green leaves or yellow flowers re- 



One of the Flatfishes 

 Animals having remarkable powers of adjusting their appearance to 

 the bottom on which they lie. The same fish jjhotographed on different 

 backgrounds. 



Courtesy of Dr. F. B. Sumner. 



spectively ; and considering surface color only, we are struck 

 with the apparent lack of any physiological use of such color. 

 One might expect arctic animals to bo black so as to absorb 

 the maximum of heat energy from the sun, and tropical ani- 

 mals to be white, thereby reflecting the sun's rays and avoid- 

 ing absorption of heat; but the reverse is true of the for- 

 mer, Avhile the latter are widely variable in color. 



How then may the multitude of colors and markings in ani- 

 mals be explained? The follower of Danvin bases his an- 

 swer on the efficacy of selection in preserving those forms 



