THE CREATION OF SPECIES 



an abnormal male may marry quite outside the 

 family with impunity, but a normal daughter may 

 transmit the defect to her sons. But such a woman 

 may marry with impunity if all of her brothers are 

 without defect and there are more than two of them. 

 A defective male should abstain from having chil- 

 dren, for some of his sons, at least, will probably be 

 defective." 



The principle that a person belonging to a family 

 in which any conspicuous physical infirmity is heredi- 

 tary should religiously avoid marrying into another 

 family having the same defect is one that cannot be 

 too forcibly insisted on. Indeed, with regard to a 

 large number of defects this might be said to be the 

 crux of the entire matter. A good illustration is 

 found in the infirmity of congenital deafness. It 

 would appear that the congenital defect that produces 

 deaf-mutism acts as a recessive trait, and thus it 

 may come to pass that a deaf mute married to a 

 normal person has normal children; normality show- 

 ing the same dominance here that is shown by black- 

 ness in the case of the guinea pig and dark eyes as 

 against blue eyes. 



But if, on the other hand, both parents are con- 

 genitally deaf, it was found by Fay that 26 per cent, 

 of the offspring are deaf. When the partners belong 

 to the same deaf mute strain that is to say are re- 

 lated the percentage of marriage yielding deaf 

 mute offspring rises to 45, and the proportion of 

 deaf offspring to 30 per cent. Moreover the closer 

 the relationship of the parents the larger the pro- 

 portion of deaf children. Among the cases investigat- 



205 



