252 PROBLEMS OF HUMAN EVOLUTION 



many cases sudden. It does not gradually become less and 

 less in a series of generations till it finally disappears, but the 

 evil often begins with the birth of an individual in whom the 

 sense is lacking. Very often his parents have in perfection the 

 sense which is missing in him. Possibly if accurate experiments 

 were made, some qualification of this statement might be neces- 

 sary. It might be found that one or other parent had not 

 perfect hearing in both ears. 1 Generally, upon investigation, it is 

 found that the deaf man has some deaf relatives. Nevertheless 

 what I have pointed out in the first part of the book is a fact, 

 the importance of which cannot be reduced by any slight paring 

 down that is possible, viz., that an organ which has required 

 thousands of generations to build up may be lost suddenly and 

 completely through failure of heredity. 



The working of heredity is very curious. If a deaf-mute is 

 the first in his family who has suffered from the defect, if he has 

 no deaf relatives, he is not likely to transmit the defect to his 

 children, though it is quite possible that he may. Reversion 

 will probably set in and reinstate the lost sense. If, on the 

 other hand, there is a strong tendency to deaf- dumbness 

 in the family, then it is very likely to be inherited. More- 

 over, a parent, who has absolutely perfect hearing, if he 

 comes of a strain which is unsound in this point, is likely to 

 transmit deafness, far more likely than a completely deaf man, 

 who springs from a stock previously untainted. In the light of 

 these facts, that have become known only in recent years, we 

 can understand a great deal that to Darwin was obscure and 

 perplexing. Deaf-mutism, the facts at his disposal seemed to 

 show, was very rarely inherited and he was at a loss to under- 

 stand the reason. " When a male or female deaf-mute marries 

 a sound person," he writes, "their children are most rarely 

 affected. Even when both parents have been deaf-mutes, as in 

 the case of forty-one marriages in the United States and of six 

 in Ireland, only two deaf and dumb children were produced." 2 

 Certainly the power of reversion is extraordinary, but it is 



1 Marriages of the Deaf in America, p. 7. 



2 Darwin : Animals and Plants under Domestication, vol. ii. p. 22. 



