IMMUNITY TO DISEASE 105 



nection between their easily acquired immunity and an 

 hereditary character secured by stringent selection acting 

 upon many generations of ancestors seems evident. Here, 

 however, even the negroes that have continued to live in 

 the malarial country from generation to generation without 

 interruption have not attained complete immunity. The 

 negro is still subject to an attack of the disease, particularly 

 during childhood. Ought it not to happen that complete 

 immunity should be attained by the race if the selection 

 is sufficiently stringent and lasts long enough ? According 

 to the theory here advanced, the answer is, that complete 

 immunity may never be attained, but the race may develop 

 the hereditary capacity of acquiring immunity to the disease 

 in so high a degree that the mortality caused through indi- 

 viduals succumbing in the process of acquiring immunity 

 will be so small that it does not materially affect the race. 

 Indeed, as individuals must always vary, some must inherit 

 less capacity for acquiring immunity, and so there must 

 remain some mortality as long as the selection, that is, so 

 long as the disease, continues, in order to eliminate unfavour- 

 able variations. 



Examples of hereditary power of acquiring immunity exist 

 in this country. Almost every child has measles, but it is 

 comparatively rare that the same individual has more than 

 one attack of the disease. Neither is the disease usually a 

 serious one in the case of English children. There are of 

 course some serious and even fatal cases, but these again are 

 comparatively rare. We, as a race, have been subject to 

 selection with regard to measles for a very long period. What 

 happens when measles attacks a race which has never been 

 subjected to any selection with regard to it ? In the Polyne- 

 sian Islands, and in several other cases where measles was an 

 unknown disease, the result of its introduction by Europeans 

 has been very fatal indeed. Large numbers, not only of the 

 native children, but of the native adults, died of it, and to 

 them it was just as serious and fatal as the old gaol fever 

 (typhus) was to the English a couple of centuries ago. 



