Page:George Archdall Reid 1896 The present evolution of man.djvu/363

Rh manufacture of strong preparations of alcoliol, wlien once their use became prevalent, evolution against them was much more rapid than was the evolution against alcohol, that is, in the case of such easily manufactured poisons, Narcotic Selection was doubtless more severe in the beginning than in the case of alcohol, and therefore evolution against them must have been more rapid.

When subjected to the assaults of some zymotic diseases—e.g. measles and chicken-pox—the average individual, even of a race that has had no experience of them, is able to vary in such a manner that with comparative ease he becomes highly resistant to them, i.e. he acquires immunity against them. But when subjected to the assaults of certain other diseases—e.g. tuberculosis, leprosy, and malaria—the average individual, even of a race that has long been familiar Avith them, is not able appreciably to vary in such a manner as to render him more resistant, no matter how often or how long he be subjected to their assaults. Racial differences in relation to such diseases as these latter as a consequence depend solely on inborn powers of making resistance, not on inborn powers of acquiring capabilities of making resistance. Between diseases against which powers of resistance may be easily acquired by the individual, and diseases against which powers of resistance cannot in the least be acquired, lie all the other zymotic diseases, scarlatina, typhoid, diphtheria, small-pox, typhus, &c.; some of which resemble chicken-pox in the ease in which the individual is able to undergo protective reaction against them, and others tuberculosis in the difficulty or impossibility of any such protective reaction. The worst, the most death-dealing' of diseases are those against which little or no protective reaction can be undergone by the individual—e.g. tuberculosis; and against which, there-