98 Disease and Immunity 



been a periodical plague from the 9th century on, 

 yet "smallpox was probably never more prevalent 

 than during the 18th century," and never more 

 deadly. Recently previous generations acquired 

 resistance by vaccination, and observable in- 

 heritance of resistance comes first in the imme- 

 diate descendants of those who acquired it. The 

 offspring inherited the exact thing that the 

 parent acquired, and there is no appearance of 

 such inheritance until after the acquirement be- 

 came general by repeated vaccinations. Nine 

 hundred years of selective death-rate applied to 

 the entire population of a whole continent pro- 

 duced a minus quantity. Less than one hundred 

 years of acquirement before reproducing has 

 brought forth something very definite. 



When a person suffers from the measles, he 

 does so because he did not have the power neces- 

 sary to resist the attack. But in fighting the 

 disease and overcoming it he develops a power 

 he did not have before, and thereafter he is 

 immune from attack. The same thing is true 

 of many other diseases. The thing to be noted is 

 that by exercising the powers which he had there 



