Disease and Immunity 99 



are developed (acquired) powers which he did 

 not have. 



Measles is a common disease of children, and 

 is normally not serious. But when it is first in- 

 troduced into tribes or races of men not before 

 subjected to it, it is a deadly scourge. Measles 

 is not serious to the white child because he in- 

 herits a resistance which enables him to overcome 

 it with small inconvenience. It is deadly when 

 first introduced among Indians or other tribes 

 because they do not inherit such resistance. But 

 resistance to this disease is something which is 

 acquired by the exertions or efforts used in over- 

 coming it, and the white child inherits the exact 

 thing which his ancestors for generations had 

 acquired. It is quite certain that the child can- 

 not inherit something which its ancestors did 

 not have, and it is also certain that ancestors 

 could not come into possession of powers of 

 resistance in any other way than by exercising 

 such powers as they had before. 



In Europe prior to the 19th century, smallpox 

 would ravage a district and then disappear com- 

 pletely for many years. By the time it came 

 back again, those who were immune or partly 



