338 ELEMENTS OF BIOLOGY 



inance of the character in this family, but one should not reach the 

 generalization that curly hair is dominant in all families. The na- 

 ture of the hair of the first generation, I in Figure 209, is not known 

 but since the six children all had curly hair and since in the fifth 

 generation, V in Figure 209, two curly-haired parents had six curly- 

 haired children, it is presumed to have been curly. 



Many other human characters are known to follow Mendelian 

 principles. It has been shown before (p. 332) how Daltonism and 

 haemophilia are sex linked. In addition, it is definitely known that 



6 ifax# ^ 6 ixax0 Dx5~ix# mxi 6 ^ 





Fig. 210. — Inheritance of fceble-mindedness in the descendants of Sam Sixty. 

 Squares represent males; circles represent females. Blacks represent feeble-minded in- 

 dividuals; blanks represent normals. Abner, the male parent in the first generation 

 shown, was the feeble-minded son of the feeble-minded Sam Sixty. Abner married 

 Rose, who was also feeble-minded. Their children and grandchildren are represented. 

 (From Publication No. 8, Ohio Board of Administration, by Kostir.) 



certain anatomical abnormalities, such as the presence of double 

 thumbs, susceptibility to tuberculosis and other diseases, stature, eye 

 color, and other characters are inherited in accord with Mendelian 

 rules, although the data are incomplete and the details have not 

 been worked out. Wide studies have been made of the inheritance 

 of feeble-mindedness; fairly complete family histories have been 

 traced by skilled investigators, which reveal that this, too, is a Men- 

 delian character, in some families apparently a dominant (Fig. 210). 

 In this case the control of the frequency of occurrence of this type 

 of defective mentality is possible, within limits; progress has been 

 attained by legal enactments designed to prevent the mating of 

 feeble-minded. The nature of inheritance of many human charac- 

 ters, including defects in mentality, as shown by collected family 



