296 HEREDITY AND DISEASE 



in many ancestors of a given child, and that the chances of 

 their being transmitted are therefore greater. 



Even when there is reason to believe that an offspring has inherited 

 a predisposition to a particular disease, it does not necessarily 

 follow that this item in the inheritance must he expressed in develop- 

 ment. 



5. Predispositions may dwindle away. — There is, at least, 

 some evidence to show that hereditary tendencies to particular 

 diseases may dwindle until it becomes almost permissible to say 

 that they have been eradicated. 



The biological interpretation of this is twofold : (i) that 

 interbreeding with an untainted stock may result in an over- 

 powering of the vicious tendency or in a re-habilitation of the 

 normal ; and (2) that in the course of selection the more severely 

 tainted tend to die out, thus leaving the race relatively stronger. 

 The biological caution is that we must not infer from non- 

 reappearance, or from non-expression— e.g'. in healthier con- 

 ditions of function and environment — that the evil tendency has 

 ceased to be inherited. Prof. Hamilton says (igoo, p. 301), 

 " My firm conviction is that if a vicious line be introduced it 

 may die out, and probably does in most cases die out by inter- 

 breeding with a series of pure stocks ; but that no reliance can 

 be placed upon its not recurring atavistically, it may be, genera- 

 tions after." 



§ 9. Immunity 



Immunity to a disease may be inborn or acquired. It may 

 be acquired in various ways : by having the disease and surviving 

 it — thus, recovery from smallpox usually confers an immunity 

 which lasts for years ; by being inoculated with the modified 

 virus of the disease — thus, vaccination confers immunity from 

 small-pox ; by being inoculated with a very minute quantity of 

 the virus ; by being inoculated with the metabolic products or 

 toxins of the microbes ; by having injections of the blood or 



