THE PRESENT EVOLUTION OF MAN — MENTAL 369 



that a man. may be strong against alcohol, but weak 

 against opium, or vice versa; just as he may be strong 

 against tuberculosis, but weak against malaria, or vice 

 versa. In which case the reason why men do not 

 commonly indulge in both alcohol and opium must be, 

 that when the craving for one is awakened it is so 

 strong as to exclude, to put out of sight, so to speak, the 

 craving for the other, especially when the latter has not 

 been awakened, as the sexual instinct is awakened, by a 

 particular known object. I take it that the mental effects, 

 the mental paresis, produced by one strong narcotic is in 

 some measure much the same as that induced by any 

 other, just as the systemic effects, the systemic prostra- 

 tion, produced by the toxins of one virulent zymotic 

 disease is, to some extent, the same as that induced by 

 any other ; and, therefore, that the craving for one 

 powerful narcotic is in some measure satisfied by 

 indulgence in any other. 



But just as a man may be strong or weak against one 

 zymotic disease at the same time that he is weak or 

 strong against another, so he may be strong or weak 

 aaainst one narcotic at the same time that he is weak 

 or strong against another. In other words, while one 

 narcotic aq-ainst which his race has undergone evolution 

 may awaken in him only a feeble desire for intoxica- 

 tion, — i. c. a desire for a slight degree of intoxication only, 

 — another narcotic of different chemical composition, 

 against which his race has not undergone evolution, may 

 awaken in him a much stronger desire — i. e. a desire for 

 much deeper intoxication. It follows that a race which 

 has undergone evolution against one powerful narcotic 

 — i. e. a narcotic powerful enough to produce mental 

 paresis even in those habituated to it — may find the act 

 of indulgence in that narcotic a protection or preventative 

 against indulgence in other powerful narcotics, against 

 which it has not undergone evolution ; from which it 



