AND ENVIRONMENT 25 



really thought the whole matter over carefully will 

 recognize that the soil has to be considered as well as 

 the bacillus. Does every one of those 94 % of Viennese 

 children have an actively tuberculous father or mother ? 

 Clearly and obviously not. Infection from parents 

 cannot be the chief source of the spread of the disease 

 among children. Whatever may be the state of affairs 

 in the future, to-day we are compelled to regard 

 tuberculosis as an almost ubiquitous disease through 

 which the great bulk of the urban population passes 

 in childhood, and having regard to the broad facts of 

 heredity, as I have laid them before you, to hold that 

 certain stocks are relatively more immune ^ than others, 

 or acquire this immunity more readily as a result of 

 attacks in childhood. 



Let us now consider for a moment that a population is 

 composed of stocks with varying degrees of immunity and 

 that it is subject to the presence of an infectious disease. 

 Whither does this idea lead us ? Clearly each case will 

 infect a certain number of others, and these again will 

 spread the infection to another circle. In each circle 

 those beyond a certain intensity of immunity recover, 

 those below it die. There is thus a selection of the less 

 immune at each stage of the disease, and the population 

 by the law of heredity, since the non-immune die earher, 

 is left gradually more and more immune. I am taking 

 here the case of constitutional immunity, not acquired 

 immunity. Now the whole problem thus stated admits 

 of mathematical treatment on the basis of the theory of 



^ Two such high authorities as Drs. Newsholme and Bashford have 

 recently denied flatly the existence of relative immunity in animals ; but 

 careful German work seems to be indicating the existence of this relative 

 immunity in both mice and men, and finding out in what characters it can 

 be summed up. 



