520 EVOLUTION, GENETICS, AND EUGENICS 



Studied separately before we shall know how each is inherited. Care 

 must be taken, too, to distinguish between congenital deafness and 

 blindness — that which inheres in the germ plasm — and those forms, 

 due to accident or contagious disease, which are acquired modifications 

 and so not heritable. Thus measles often produces deafness as one 

 of its after effects. Persons so rendered deaf would not transmit the 

 affliction to their children any more than they would transmit blind- 

 ness if the eyes of the parents were put out by accident. 



Feeble-mindedness apparently behaves as a Mendelian recessive. 

 Goddard's studies of the family pedigree of the inmates of the Vine- 

 land, New Jersey, institution for the care of the feeble-minded gives 

 us an abundance of material to show the heritability of this defect and 

 its relation to alcoholism, insanity, syphilis, etc. Briefly, syphilitic 

 infection is a fairly common cause of feeble-mindedness in children. 

 There is a higher percentage of feeble-mindedness in the offspring 

 of alcohoUc parents than among those of parents not addicted to it. 

 There seems little or no causal relation between feeble-mindedness 

 and insanity. But aside from feeble-mindedness that may be pro- 

 duced by such causes or by occasional accidents such as falls, 

 blows on the head, there is the great mass of feeble-mindedness that 

 is wholly a matter of heredity. 



If a feeble-minded individual comes from parents both of whom 

 are congenitally feeble-minded or who both have a great deal of feeble- 

 mindedness in their ancestry, such a one is taken to be a pure recessive 

 as far as this character is concerned, and his germ cells have a double 

 dose of the factor for feeble-mindedness (FF). When two such per- 

 sons mate, their offspring would be expected to be all feeble-minded, 

 for all eggs and sperm contain the factor F, and when any egg is fertil- 

 ized the person produced is an FF individual. Out of 144 such mat- 

 ings resulting in 482 offspring whose records are known, Goddard 

 found that 476 were feeble-minded. This type of mating as well as 

 others cited below are illustrated in the family pedigrees shown on 

 pages 463 and 464, selected from Goddard's book. 



If a person comes from parents one of whom is entirely normal and 

 one is feeble-minded with many feeble-minded ancestors, it is probable 

 that such an individual is a hybrid with germ cells that, as far as this 

 one character is concerned, can be designated NF. Such a person 

 will pass for normal, since feeble-mindednes= is recessive. If such a 

 one mates with the type described above (FF), it would be expected 

 that half the offspring would be normal, half feeble-minded. Out of 



