CLIMATE AND RACE — COON 283 



sj)onse to liglit and losing it when the light is gone is probably the 

 original genetic situation with man. 



The physiological advantages of the second and third types of pig- 

 ment are easy enough to see. They concern entirely, as far as we know, 

 ultraviolet radiation. The UV scale runs from about 2,400 to 3,900 

 Angstrom units, where it joins the lower end of the range of visible 

 light. Actually, although shorter waves are produced artificially by 

 lamp makers, all solar radiation under about 2,900 units is filtered out 

 by the earth's atmosphere and has nothing to do with the adaptive 

 character of the human skin (Luckeish, 1946, pp. 59-72). Through 

 the remaining thousand-unit range, UV radiation penetrates exposed 

 skin to irradiate some of the subcutaneous fats, thus producing 

 vitamin D, which is of benefit to the system. 



However, those rays which are concentrated in an extremely narrow 

 peak near the short end of the range, and centered at 2,907 units, can 

 damage the unpigmented skin if the sky is clear, the sun overhead, 

 and if the exposure is prolonged past a critical time limit. Sunburn, 

 erythema, prickly heat, and sunstroke can follow. Ilovv^ever, the 

 hazard carries its own cure, for if the skin is exposed for short periods 

 it will tan. The pigment so acquired absorbs the UV radiation con- 

 centrated at this critical peak and converts it into radiant heat, which 

 the skin then loses through the normal processes of radiation, convec- 

 tion, and sweating, along with other heat produced by the metabolism 

 of food within the body. The pigment granules do not interfere with 

 UV penetration along the rest of the scale, and thus vitamin D pro- 

 duction can continue. Tanned skin is thus useful in regions where the 

 peak of UV radiation is seasonal, since in the season of reduced light 

 the skin bleaches and permits the maximum of irradiation. 



In contrast to the genetic capacity for change inherent in skin that 

 tans, black skin is constant. In the distant and naked past, it must 

 have had a clear advantage in the Tropics over tannable skin. That 

 advantage remains to be discovered experimentally. Geographically 

 speaking, peoples with black skin who are known to have lived in their 

 present habitats since the rather mobile dawn of history live in regions 

 close to the Equator where UV is strongest. They inhabit the forests 

 and adjacent grasslands of central Africa. The second great center 

 is Melanesia, including Papua and northern Australia. They also in- 

 clude the extinct (in the full-blooded state) Tasmanians. In between 

 Africa and Melanesia fringes of land and islands hold connecting 

 links; southern India, Ceylon, the Andamans, the Malay Peninsula, 

 and the Lesser Sundas contain black-skinned peoples, as do some of 

 the islands of the Philippines. 



Except for Tasmania, whose inhabitants had obviously migrated 

 there from a region of lower latitude, these areas are all within 20° 



