that a man may be strong against alcohol, but weak against opium, or vice versâ; 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 prostration, 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 against one narcotic at the same time that he is weak or strong against another. In other words, while one narcotic against which his race has undergone evolution may awaken in him only a feeble desire for intoxication,—i.e. 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
