280 



Heredity and Eugenics 



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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. — Beexh.ardt. 



Some e)'e diseases are clearly due to a defect in tlie 

 determiner. Sucli is the case with an inflammatory con- 

 dition of tlie retina in which pigment is deposited and the 



patient ultimately loses his sight 

 (Fig. 89). This disease fre- 

 Cjuently appears in the children 

 of two normal persons who are 

 cousins, and consec^uently 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 frecjuently 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 

 haA'e 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. This 

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

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



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t]6[^t]6i ? 



Fig. 88. — Pedigree showing 

 inheritance of a tendency toward 

 colds, catarrh, and respiratory- 

 diseases. 



