276 GENETICS AND EUGENICS 



only in persons with dark skin. Whole races of mankind 

 have only black hair and dark skin (known as "black, brown, 

 red or yellow"). A dark skin is an adaptation to life in a 

 tropical country or one having much intense sunlight. Fair- 

 skinned races are unable to endure life in the tropics unless 

 the body is protected from the direct rays of the sun. Dark- 

 skinned races, however, have a natural protection against 

 the effects of direct sunlight. From an evolutionary stand- 

 point the white races are possibly retrogressive variations, 

 ''loss" variations. In a population of Europeans, the darker 

 shades of hair and skin-color are either completely or incom- 

 pletely dominant. It is not at all uncommon to find a 

 mixture of dark-haired and light-haired children in the 

 same family, provided one or both parents are dark-haired, 

 but when both parents are light-haired, the children are all 

 light-haired. This result shows that the lighter shades of 

 hair-color are recessive in relation to the darker shades. An 

 exact estimate is often difficult to make because persons with 

 light hair in childhood often have much darker hair when 

 adult, and further, the hair may later become gray or even 

 white, which makes direct comparison with the hair of 

 younger persons impossible. 



Extremely pale conditions of hair, skin and eye pigmenta- 

 tion are known as albinism and occur in all races, even in 

 negroes and American Indians. Albinism is clearly a reces- 

 sive character in relation to normal pigmentation. The vari- 

 ous shades of blonds probably correspond physiologically 

 and as regards inheritance with the graded series of albino 

 allelomorphs found in guinea-pigs. Each darker shade is 

 dominant to the lighter shades, any two in the entire series 

 being allelomorphs of each other. This is known to be the 

 case in rodents and probably holds for European races of 

 mankind. In other races of mankind blond variations are 

 rare, even more so than extreme albinism. Here again we 

 have a condition parallel with that found in most rodents, in 

 which the albino variation is known, but not other members 

 of the graded series of retrogressive allelomorphs. 



