126 JOURNAL OF SCIENCE. 



Classes 2, 3, and 4 exhibit a decided tendency to transmit deafness to 

 their offspring. 



Now, there is a law of heredity that may afford great comfort to 

 many of the deaf, — the law of reversion. There is a very strong- 

 tendency in offspring to revert to the normal type of the race. It 

 requires constant selection from generation to generation on both 

 sides to perpetuate any abnormal peculiarity. There will always, 

 therefore, be a tendency to produce hearing children rather than 

 deaf, excepting in cases where both parties to a marriage come from 

 families belonging to Classes 2, 3, and 4. 



Probabilities for Your Guidance. 



Whatever may be the character of the deafness in your own case, 

 you will probably diminish your liability to have deaf offspring (1) by 

 marrying a hearing person in whose family there is no deafness ; (2) 

 marrying a deaf person (not born deaf) who has no deaf relatives 

 (Class 1), or a hearing brother or sister of such a person. 



On the other hand, you will probably increase your liability to 

 have deaf offspring (1) by marrying a deaf person (not born deaf) who 

 has deaf relatives (Class 2), or a hearing brother or sister of such a 

 person ; (2) by marrying a deaf person (born deaf) who has no deaf 

 relatives (Class 3), or a hearing brother or sister of such a person ; (3) 

 by marrying a deaf person (born deaf) who has deaf relatives (Class 

 4), or a hearing brother or sister of such a person. 



Of course, if you yourself were born deaf, or have deaf relatives, 

 it is perfectly possible that in any event some of your children may 

 be deaf. Still, I am inclined to think, that, if you marry a member of 

 a family in which there is no deafness (or only a single case of non- 

 congenital deafness), you will not only have fewer deaf children than 

 if you married into a family containing a congenital deaf-mute, or a 

 number of deaf persons, but the deafness of your children will not 

 tend so strongly to be handed down to the grandchildren. The ten- 

 dency to inheritance will be weakened in the one case, and intensified 

 in the other : that is, in the former case your deaf child will have a 

 less tendency to transmit his defect to his children than you yourself 

 possess ; in the latter case a greater tendency. 



Take the case of a family in which three or four children are 

 born deaf. 



Now, suppose that all the members of this family and their deaf 

 descendants are careful to marry only into families which are free 

 from deafness, or which contain only single cases of non-congenital 

 deafness. Then the probabilities are that at each generation the 

 percentage of children born deaf will be less, and the proportion of 

 hearing children greater, until finally the deaf tendency disappears, 

 and all the descendants will hear. 



On the other hand, suppose that the members of this family and 

 their deaf descendants marry into families containing a congenital 

 deaf-mute, or containing several deaf persons. Then the probabilities 

 are that at each generation the percentage of children born deaf will 

 increase, and the proportion of hearing children will be less, until 

 finally the tendency to produce hearing offspring disappears, and all 



