THE MEANING OF " IMMUNITY " 309 



the former and more chronic than the latter. But since native 

 West Africans suffer more from it as children than as adults, and 

 since adult negro immigrants from non-malarious countries suffer 

 more at first than the native adults and more than they themselves 

 do later, it is evident that individuals of a race that has long been 

 exposed to it are capable of acquiring a considerable degree of 

 immunity. Apparently, average Englishmen gain no such increase 

 of resisting power. 1 Nevertheless, judging from the direction taken 

 by the evolution, it is probable that members even of a race that 

 has had no previous experience have some power of acquiring 

 resisting power a power which, though it does not afford much 

 protection to a majority of individuals, yet has selection value. 

 Similarly, opium lies midway between alcohol and tobacco. The 

 individuals of all races are capable of increasing their power of 

 resisting opium ; that is they are able to acquire the power of 

 taking greatly increased doses without immediate poisoning, 

 without intoxication. But individuals of races that have long 

 experienced it tend to take it in less excess, tend more to be 

 satisfied with non-intoxicating doses than races to which the 

 drug is new. Experience of opium, therefore, tends to place a 

 race in the same position of advantage that it occupies from the 

 first with respect to tobacco. 



518. It should be noted that the term ' immunity/ when applied 

 to microbic diseases and to narcotics, has not identical meanings. 

 Immunity to disease implies a physical power of resisting infection. 

 Immunity to a narcotic is a mental phenomenon implying such an 

 insusceptibility to its charm that the individual is not tempted to 

 excessive indulgence. Nevertheless, the analogy between diseases 

 and narcotics is very close. 



519. We see, then, that alcohol and opium act as agents of 

 selection in the same way precisely as lethal and prevalent diseases. 

 They eliminate particular types of individuals. Individuals differ 

 in their susceptibility. The more susceptible are eliminated and 

 as a consequence the race undergoes protective evolution. When 

 thinking of narcotics, the reader must bear in mind evolution in 

 general. All progressive evolution depends on selection, all 

 stringent selective elimination results in evolution. When the 

 whole of the facts are taken into account it is not believable that 

 alcohol and opium, which are so enormously destructive of life 

 and so plainly selective, can be an exception to the rule. It has 

 been objected that, since alcohol awakens unlike sensations in 



1 See 393. 



