THE PRESENT EVOLUTION OF MAN — MENTAL 351 



manufacture of strong preparations of alcohol, when 

 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 manu- 

 factured 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 com- 

 parative ease he becomes highly resistant to them, i. e. 

 he acquires immunity against them. But when sub- 

 jected to the assaults of certain other diseases — e.g. 

 tuberculosis, leprosy, and malaria — the average in- 

 dividual, even of a race that has long been familiar 

 with 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 ac- 

 quired 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 re- 

 semble 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- 



