CHAPTER XLI 

 HUMAN INHERITANCE 



Human Traits. — Man, like all other animals, is 

 a composite of many characters, which depend on the 

 presence of definite genes or factors in the germ cells 

 which unite as the first step in his being. The manner 

 in which the genes bring about the development of their 

 corresponding characters is, however, in many cases 

 more complicated in man than in a vinegar fly or a plant. 

 It has already been shown in previous chapters (XVI, 

 XVII) that certain characters depend for their expression 

 on the actions of hormones. Obviously in these cases the 

 factors which control the development of the hormone- 

 producing glands will also indirectly control other more 

 obvious characters. It has recently been claimed that a 

 large number of human characters are determined in this 

 indirect fashion, though this remains yet to be proved. 

 On the other hand a great many characters appear to 

 vary more or less continuously through a wide range. 

 Examples of this sort of quantitative variation have 

 already been examined in plants and animals (Chaps. 

 XXXI-XXXIII) and shown to depend on what are 

 commonly called " cumulative factors." 



Difficulties of Studying Human Inheritance. — 

 The fact that many characters in man are controlled 

 indirectly through hormone secretion and many more 

 through cumulative factors, makes the study of man's in- 

 heritance difficult. These are not, however, the only diffi- 

 culties, for the chief ones are concerned with the impos- 

 sibility of conducting controlled experiments on man. In 

 the first place, no one lives long enough to observe much 



505 



