i86 THE TREND OF THE RACE 



their lives and generally before adult age. Hamburger states 

 that in Vienna 95 per cent of the children of the poor between 

 12 and 13 years of age are infected, and he thinks that practically 

 all will be infected before they reach adult life. If it should be 

 established that most people become tuberculous at an early age, 

 the hypothesis that the parent-offspring correlation for tubercu- 

 losis is due simply to opportunities for infection will hardly suffice 

 to explain the fact. The generality of early infection is a matter 

 to be considered in interpreting the significance of the correlation. 

 If almost every one has become infected, and thus has the oppor- 

 tunity to develop tuberculosis, and if the existence of the more 

 severe forms of the disease is more closely associated with blood 

 relationship than it is with the surrounding conditions under 

 which tuberculosis is apt to become manifest, the evidence would 

 strongly point to the importance of the hereditary factor. The 

 problem is a difficult one about which there has been considerable 

 controversy, and we shall have to await further insight into the 

 subject before the precise r61e of heredity can be fully established. 

 Should the hereditary factor be a potent one it would indicate 

 that natural selection is acting to remove the stocks with a tuber- 

 cular diathesis. 



That natural selection tends to eliminate stocks with a pro- 

 clivity to other diseases is evident. Several diseases such as 

 diabetes, Blight's disease, Huntington's chorea and others which 

 are known to be transmitted are not infrequent causes of death. 

 Dwarfism, ichthyosis, xeroderma, albinism, hereditary cataract, 

 and deaf mutism, while not in themselves fatal, may lesson the 

 chances for leaving offspring and hence lead to the extinction of 

 stocks in which they occur. Haemophilia which is transmitted as 

 a sex linked character would tend inevitably to be eliminated 

 by natural selection since it greatly increases the danger from 

 any wound that causes the loss of blood. Lessen states that 18 

 out of the 37 deaths in the celebrated Mampel family were due 

 to this malady. The hereditary forms of insanity not only keep 

 their victims from propagating their kind, but they often tend 

 to shorten their lives. Brower and Bannister state that in the 



