28o 



Heredity and Eugenics 



Some eye diseases are clearly due to a defect in the 

 determiner. Such is the case with an inflammatory con- 

 dition of the retina in which pigment is deposited and the 



patient ultimately loses his sight 

 (Fig. 89). This disease fre- 

 quently appears in the children 

 of two normal persons who are 

 cousins, and consequently both 

 carry the defect in their germ 

 cells. 



Deaf -mutism also is due to a 

 defect; but the nature of the 

 defect is different in different 

 cases. Deaf-mutism is so varied 

 that frequently two unrelated 

 deaf mutes may have hearing 

 children (Fig. 90). But if the deaf-mute parents be 

 cousins, the chances that the 

 deafness is due to the same 

 unit defect are increased and 

 all of the children will probably 

 be deaf. 



We come now to consider 

 mental peculiarities, and here 

 at once enter a , vast field in 

 which surprising discoveries 

 have been made in recent years, 

 and which point to the cause of 

 many of our social difficulties and the way out. 

 First, consider the facts of feeble-mindedness 



Fig. 87. — -Pedigree of Thorn- 

 sen's disease (black symbols). Ap- 

 pears in cousin marriages even 

 from affected parents; hence due to 

 a defect. Squares indicate males; 

 circles, females. — Bernhardt. 



DtC 



/tfSfrC 



6 



DtO 



C0/</s 





fyjfarr/i 



6 



t6 

 5 



aneum. £///>/A /hnfaf re^r /Omt/>^J fonsi/'tts 



/onj/y/^ii 



/rouiit fraui/e 



Fig. 88.— Pedigree showing 

 inheritance of a tendency toward 

 colds, catarrh, and respiratory 

 diseases. 



This 



term is a lumber-room and comprises various mental defi- 

 ciences, such as inability to count, to repeat phrases, to 



