Inheritance of Physical and Mental Traits 



277 



Many peculiarities of the skeleton are clearly due to a 

 positive determiner that inhibits the normal development. 

 Thus the case is cited of a father with a deformed clavicle 

 or collar bone; of his seven children, five have the clavicles 

 of a more or less abnormal form. Likewise in polydactyl- 

 ism, or extra-fingeredness, there is some positive factor 

 that induces the formation of the extra toe; but normal- 

 toed persons of a polydactyl strain, being without the 

 determiner, will have all children with five toes only. 



The same is true of brachydactyly. There is something 

 that stops the growing of the fingers to the normal length, 



ION 



4N [N] (D) 6N |D] (N) (D) N 5N JN 

 FIG. 85. Pedigree of a family with diabetes insipidus. D, affected persons 



so that if the determiner gets into the zygote from either 

 side of the house, the child will be short-fingered, but not 

 otherwise. 



Diabetes is a common disease which seems to belong 

 to this category (Fig. 85). Here again two normal parents 

 may have defective children but only when the defect occurs 

 in the germ plasm of both sides of the house. 



The applications of these facts regarding abnormalities 

 and diseases that are of a positive sort have an importance 

 for eugenics. They are all characterized by this, that they 

 usually appear in each generation and do not skip genera- 



