120 HISTORY OF THE HUMAN BODY 



resembling the sandy or muddy bottom on which the fish 

 lies. 



This principle is a general one and applies to the distribu- 

 tion of pigment, whether in the integument itself or in its 

 accessory structures, a fact that may be readily seen by com- 

 paring a form with a naked skin, like a frog or a porpoise, 

 with one covered with hair. In terrestrial mammals the dif- 

 ference is not always a marked one and is apt to be greater in 

 short-legged forms that creep close to the ground ; in birds, 

 which in the majority of cases expose nearly the entire surface 

 to the light, the body may apparently be of uniform color, but 

 here the deeper pigmentation is confined to the exposed sur- 

 faces of the feathers, while the portions which are shielded 

 from the sun are less deeply colored or even without pigment. 

 The white ventral surface of aquatic birds like snipe and gulls 

 is a protective coloring and comes under another principle. 

 In man the distribution of pigment is also unequal, the darker 

 areas being, in all races, the back and the dorsal aspect of arms 

 and legs, while the chest and abdomen, the ventral aspect of 

 the limbs, and especially the palms and soles, are lighter in 

 color. This distribution, it will be noticed, corresponds to 

 the influence of the light, when man assumes the quadrupedal, 

 and not the usual human, position. Aside from these general 

 areas, a deep local pigmentation occurs in the axilla and groin, 

 about the anus and the external genitals, and upon the nip- 

 ples and areola, where it is evident that the pigmentation is 

 for some other purpose and can bear no relation to the distribu- 

 tion of light. In this general connection between a darker 

 color and the increase of the sun's light and heat, there must 

 be some physiological advantage which a dense pigmentation 

 gives its possessor, an advantage which seems to be a real one 

 whenever there is a chance for comparison between a black 

 man and a white man in the tropics in regard to their relative 

 power of enduring the heat of the sun. Although the subject 

 is still an obscure one, it seems probable that the presence of a 

 dense layer of pigment in the stratum germinativum effectually 

 prevents the direct action of the light upon the surface capil- 



