250 HEREDITY IN RELATION TO EUGENICS 



once, a century ago, may produce another not less note- 

 worthy again. 



A feature of positive unit characters, which from their 

 very nature tend to reappear in each generation is that of 

 anticipation. This means that the trait appears at an eariier 

 age in each generation. Nettleship (1910, pp. 23-25) has re- 

 ferred to some striking cases of this. Thus he gives three 

 pedigrees of hereditary glaucoma and diabetes illustrating 

 this law. In one case the average known age in successive 

 generations for the incidence of glaucoma is 66 and 48 years; 

 in another family 71, 45, and 23 years; in still another, 47 

 and 20. In the case of diabetes deaths occurred, on the 

 average, at 69, 35 and 26 years. Nettleship explains this 

 result ''by assuming that certain defects, taints or vices of 

 the system, say of the blood, are not only hereditary in the 

 true or germinal sense, but able to produce toxic agents in 

 the embryo which have an evil influence upon all its cells, 

 and thus so lower their power of resistance that the innate 

 hereditary factor has freer play and is likely to manifest 

 itself earlier." 



The law of segregation of traits, the disproof of the blend- 

 ing hypothesis, is of the utmost importance since it shows 

 how a strain may get completely rid of an undesirable trait. 

 If the undesirable character is a positive one, like polydac- 

 tylism, it will disappear if the normal children alone have 

 offspring. If it is a negative character its complete and 

 certain elimination is not so easy to be assm-ed of, but off- 

 spring without the undesirable trait are easily secured if 

 marriage be always with germ plasm that is without the 

 defect. Thus a simpleton married into a mentally strong 

 strain will probably have mentally well endowed offspring. 

 Here is where the beneficence of heredity clearly appears. 



But do traits never arise de novo is often asked. If you 

 deny it, how do you accoimt for the presence of great men 



