354 GENERAL ZOOLOGY 



the highly complex vertebrate eye, all represent specialized end 

 organs for the perception of light stimulation. In the lower forms 

 these organs do not form images of external objects but serve 

 simply as light receptors which distinguish intensities of light. 

 The image-forming function of eyes of higher forms is an acquisi- 

 tion that came with the evolution of the lens and accompanying 

 structures, which in the case of the vertebrate eye is a cameralike 

 organ capable of projecting images of external objects on a sensi- 

 tive retina. 



There is undoubtedly a relation between light and the forma- 

 tion of pigment in the skin, but there are difficulties involved in 

 reaching a clearcut conclusion regarding the relationship, as has 

 been indicated in the discussion of the pigment of cave sala- 

 manders. In addition to the facts brought out in that discussion, 

 it might be noted that light, in some cases at least, is necessary 

 for the complete development of pigment in the skin. Thus, if 

 frogs or young salamanders are allowed to develop in the dark, 

 the formation of pigment is retarded. Flounders are normally 

 pigmented above and are white on the undersurface of the 

 body, which ordinarily is not exposed to the direct action 

 of light. If young flounders are allowed to develop in tanks 

 illuminated from below, it has been found that pigment develops 

 on the undersurface as well as above. Human races native to 

 tropical regions have deeply pigmented skin and dark-brown 

 or black-colored eyes, which certainly act as protective adapta- 

 tions against the intense illumination of the sun. The dark eye 

 of the Eskimos similarly is a protection from the intense reflec- 

 tion of a snow-covered environment. White-skinned races 

 developed in regions of less intense light. Skin color seems to 

 be a purely environmental phenomenon, not necessarily corre- 

 lated with other traits such as intellectual vigor. 



Moisture. — Metabolism requires that the concentration of 

 the body fluids be maintained at a certain value. If an undue 

 amount of water is lost through the integument or in other ways, 

 death follows. Aquatic and semiaquatic animals, such as 

 fishes and amphibians, are so highly adapted to a moist environ- 

 ment that they cannot live for any length of time in a medium of 

 dry air because under these conditions there is no check on the 

 loss of water through the integument. Such uncontrollable loss 

 does not occur in terrestrial animals because of a difference in the 



