Human Variability 217 



tion of a large number of influences (see Fig. 38) . From the 

 fairest blond to perfectly black hair, we find a graded series, 

 and another series from reddish tint to very dark brown and 

 black. The unpigmented hair and skin of the Albino repre- 

 sent a mutation and are found more frequently among dark- 

 skinned peoples than among Caucasiand. This albinism is 

 parallel to similar phenomena among other mammals, and the 

 conditions of its hereditary transmission have been pretty well 

 worked out. Pigmentation of the eyes, while often associated 

 with pigmentation of the skin and hair, is in some strains 

 an independent variable. We must recognize the existence 

 of several distinct races or subspecies characterized by dis- 

 tinct skin color; but we must see also that there is a consid- 

 erable range of variation within each race. 



The shape of the head, that is, the proportion of the 

 height, length and width, shows a wide range of variability. 

 There is a tendency for each race or for each fairly uniform 

 population to approach a mode in the ratio of length to 

 width, or length to height. But within a given population, 

 these ratios are quite variable. 



Most of the people you meet have two arms and two 

 legs. It is very rarely that one meets a person with a third 

 arm or leg. The absence of one arm or leg is not so rare. 

 We do not, however, exclude from our category of human 

 beings a person who manifests an excess or shortage of one 

 of these useful appendages. At the end of most human arms 

 is the hand consisting of a palm and five digits. Occasionally 

 a child is born with a deficiency in these digits. Varying in 

 the opposite direction, there are individuals with supernum- 

 erary digits (Fig. 55). Sometimes two fingers are grown 

 together so that they cannot be separated; sometimes the 

 skin extends between two fingers or two toes. These varia- 

 tions we usually look upon as abnormalities, even where they 

 do not interfere with the effective use of the organs. They 

 are abnormal in the sense that they depart from the typical 

 structure. Each finger normally has three distinct joints, 

 except the thumb, which has two. There are individuals 



