1012] on the Use, of Pedigrees. 469 



a number of defectives who will cost society about twice as much as 

 the estimated annual charge of the Commissioners' scheme, while the 

 misery and degradation involved are fearful to contemplate. 



When we pass from the pedigrees of the feeble-minded to those of 

 the insane we again meet with clear evidence of heredity. Even the 

 particular type of delusion, and the action based on it, are inherited. 

 Thus in Shde No. 14 we have suicidal melancholia manifested in the 

 family A in a tendency to drown themselves, while in the family B it 

 takes the form of shooting. When a marriage between representa- 

 tives of the two families took place the ten children were in a difficulty. 

 Two drowned themselves, one shot himself, two, as a compromise, 

 took poison ; two more, hesitating too long, were confined safely in 

 asylums ; while three are described as normal. 



Pedigrees might be given to illustrate the inheritance of almost 

 any quality, physical or mental. Even the tendency to produce 

 twins is hereditary (Shde No. 15). 



Special interest is now felt in tuberculosis, which has been proved 

 to be an infective disease. This result has somewhat tended to ob- 

 scure the no less certain fact that the special liability to the infection 

 which makes some individuals succumb is definitely hereditary. 

 Countless pedigrees might be given to show this inheritance. Here 

 is one (SUde No. 16). In present conditions the germs of tubercle 

 are so wide-spread that nearly everyone is infected. But most of us 

 are resistent to a moderate attack. Only those specially liable contract 

 the disease. Those liable fall into two groups. In one the predis- 

 position to tuberculosis is an isolated weakness in an otherwise sound 

 stock which may be of great value to the nation ; in the other it is 

 but one of many forms, in which radical unsoundness shows itself. 



Hitherto we have dealt with the facts of the pedigrees as they 

 came, and made no attempt to discover any definite laws of inheri- 

 tance. I expect that of late years this room has often resounded 

 with the name of Mendel, and I will give but the shortest account of 

 the phenomena associated with his name. 



It is found that certain qualities are inherited as definite units 

 which are either present or absent. With regard to such qualities 

 we are not a vague blend of all our ancestors, but are composed of 

 various units, picked here and there by chance from our ancestral tree. 



As an illustration we may take Mendel's original experiments on 

 green peas. A dwarf variety crossed with a tall variety gave seed- 

 lings which were all tall. Hence tallness is said to be dominant and 

 dwarf ness recessive. But, when seK-fertihzed, these ofi'spring proved 

 different from the original pure-bred tall parent. One quarter of 

 their seeds gave rise to pure tall plants which bred true, one quarter 

 to pure dwarf plants which also bred true, while the remaining 

 half gave " tails," outwardly indistinguishable from the first group, 

 but, when self-fertihsed, reproduced the mixed descendants of the 

 second generation, 



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