166 



SCIENCE 



[N. S. Vol. XXXIII. No. 840 



appetite for carbon dioxide tlian for it to 

 be bleached by the light. 



Consequently we find that man is invariably cov- 

 ered with a pigment which acts as an armor to 

 exclude the more harmful short rays, and moreover 

 the amount of pigment is in direct proportion to 

 the intensity of the light of the country to which 

 his ancestors have proved their adjustment by 

 centuries or millenniums of survival in health 

 and vigor. It is a simple matter of mathematics 

 to show that the intensity of light under the 

 zenith sun in the tropics is the greatest, and that 

 the amount of rays per unit of surface diminishes 

 as we go north in proportion to a function of the 

 latitude. In addition to this, the further from 

 the tropics we go the greater is the layer of air 

 which the rays must pass through and the more 

 of them which are absorbed. Hence we can reach 

 a latitude where there is insufficient light for 

 plant growth even if there could be sufficient 

 warmth. Yet man flourishes in these regions, 

 and so do other animals. Hence we find the 

 greatest pigment in the tropics among the Aus- 

 tralians, New Guineas, Negritos, East Indians 

 and African Negroes, some of whom are nearly 

 jet black. As we go north from the tropics we 

 iind the complexions gradually lightening, being 

 dark brown in Egypt, light brown in north Af- 

 rica states, deep olive in the Mediterranean, olive 

 in southern Europe, brunette in central Europe 

 and blond in the northwestern sections of Europe, 

 embraced by a curve passing through northern 

 Prance, northern Germany and northwest Russia. 



Undoubtedly the negro, while in the shade, is 

 able to radiate heat better than whites, and this 

 enables him to keep cool in the tropics, but puts 

 him at a disadvantage in the north where the 

 white man can keep warmer with less clothing 

 and less fire in the house. But it is a secondary 

 cause enhancing the first, because when it comes 

 to a question of light and cold, nature makes no 

 mistakes, but selects a color able to exclude the 

 light. Hence in all cold light countries, i. e., 

 steppes, plains, and the arctics, there is a pig- 

 mentation of a color in the lower end of the 

 spectrum, red or yellow, with variations of brown, 

 olive or copper. As a rule the color is markedly 

 light yellow in cold, light countries, as in North 

 China. 



In America we had every shade from the black- 

 est Indians of tropical South America through all 

 the shades of copper and brown to the very light, 

 almost white Indians of the northeastern part of 



the United States, who had conditions very sim- 

 ilar to those suited for the blonds of Europe. In 

 Italy, Spain and China, we find the same thing, 

 for the men of the south are markedly darker 

 than those of the north. 



The same law holds in France, Germany, Rus- 

 sia, Persia and India: the north is decidedly 

 blonder than the south, and the same is found in 

 the British Islands, but in a much modified way. 

 Even the Ainos in the north of Japan are said to 

 be lighter than the Japanese. 



All these red and yellow colors undoubtedly 

 enable the native to conserve his heat almost as 

 well as the white man, and at the same time 

 exclude the dangerous short waves. 



In Africa, Eimer'^^ found that in passing 

 up the Nile valley from the Delta to the 

 Soudan, the natives gradually became 

 more and more dark-skinned the further 

 south they lived. 



Woodruff^" recommends that white men 

 in the tropics should wear white outer 

 garments and black underclothes, which 

 constitutes a pretty fair imitation of a 

 dark-skinned Arab in a white burnous. 



Vernon-' points out ' ' that the diminution 

 or disappearance of pigmentation follow- 

 ing upon withdrawal of light, is best illus- 

 trated by reference to the well-known cave 

 animals. Of these, one of the most inter- 

 esting is Proteus anguineus, which is 

 found in the subterranean caves of the 

 Karst Mountains about Adelsberg. This 

 amphibian is almost white, but if kept for 

 some time in the light, it gradually be- 

 comes pigmented. Pigment cells are, in 

 fact, still present in its skin, and in all 

 probability these are directly stimulated to 

 exert their function by the action of the 

 light." If Vernon had been familiar with 

 the theorem of Le Chatelier, he would have 

 worded this last sentence very differently. 



In the case of heliotropism, the animals 



=° " Organic Evolution," 88, 1810. 



™ " Eflects of Tropical Light on White Men," 

 321, 1905. 



'" " Variation in Animals and Plants," 250, 

 1903. 



