HUMAN INHERITANCE 275 



Retrogressive variation of eye pigmentation leads from 

 "heavily pigmented iris (back and front)" through more 

 faintly pigmented conditions to "iris pigmented only be- 

 hind," the ultimate recessive, blue. Spotting with white, 

 affecting skin and hair pigmentation, or affecting only the 

 pigmentation of the iris (Bond, 1912) are unit-character 

 variations completely parallel with those of rodents. Nearly 

 all other known Mendelizing characters in man are more or 

 less pathological. They include a variety of hereditary mal- 

 formations or "diseases" affecting skin, eye, skeleton, kid- 

 neys or nervous system. (See Table 33.) 



Many characters (mostly loss variations) are probably 

 Mendelian in inheritance, but not enough is known concern- 

 ing their behavior to permit of a positive statement in the 

 matter. (See Table 33, 4.) 



In Section 5 of Table 33 are included many important 

 characters known to be to some extent hereditary, but in 

 accordance with what law is still uncertain. Especially im- 

 portant are such characters as general mental ability, mental 

 capacity in special directions, hereditary epilepsy and in- 

 sanity, and longevity. It would be a mistake to cover up 

 our present ignorance concerning the inheritance of these 

 characters by classifying them either as unifactorial or as 

 multifactorial. We shall presently examine into the evidence 

 that the more important of these are inherited. 



Hair-form. This character has been studied by Dr. and 

 Mrs. Davenport, whose findings may be briefly summarized. 

 Hair having a circular cross-section is straight. But if the 

 hair is elliptical in cross-section, it has a tendency to become 

 curly. Grades of departure from the straight condition are 

 formed with increase in flattening of the hair in cross-section 

 as follows: (1) straight, (2) wavy, (3) curly, (4) kinky (Afri- 

 cans). Crosses produce intermediates or show imperfect 

 dominance of curliness, with segregation more or less com- 

 plete in later generations. 



Hair and shin-color. Hair-color is in general correlated 

 with skin-color, the darkest shades of hair-color being found 



