Environmental Factors 221 



Differences in intellectual and emotional characteristics 

 are of sufficient concern and practical importance to have 

 received attention for a long time. Our democratic tradition 

 has made us overlook these variations, or it has made us as- 

 sume that they are either of no great consequence or merely 

 incidental to differences in living conditions. The accumu- 

 lation of data points to the more fundamental fact that 

 among the many traits with respect to which individuals 

 differ some are of constitutional or hereditary source. It is 

 true today as it was in the past that sows' ears cannot be con- 

 verted into silk purses. The differences, whether important 

 or trivial, indicate a complex population varying constantly 

 in many directions, and more plastic than most species of 

 living things. 



Environmental Factors 



As to the causes of these variations we know very little. 

 Some indeed can be traced to definite environmental or de- 

 velopmental agencies. Today there is no excuse, for exam- 

 ple, for a child to be rickety even if rickets has in the past 

 " run in the family." We know definitely that a defective 

 calcium metabolism resulting in deficient bones can be cor- 

 rected by suitable diet or by appropriate irradiation with 

 ultra-violet rays. Many people assume that pigmentation 

 is related to outer conditions. We are all familiar with the 

 effects of sunlight upon the color of the skin. For large parts 

 of the population in all parts of the world there has become 

 embodied in the folklore the assumption that dark peoples 

 are dark because they live in tropical regions and that the 

 pale faces are light because they are sheltered from the sun. 

 To this day most people who have given the matter any 

 thought are continuously puzzled by the question whether 

 dark races are dark because their ancestors had been exposed 

 to excessive sunlight or whether they live in the tropics be- 

 cause they are pigmented and can tolerate the excessive 

 sunlight. 



