TWENTY-FIVE YEARS OF BACTERIOLOGY 147 



could appreciate that the overcoming and healing of in- 

 fection is never a mere passive process, and the action of 

 healing agents in the body does not occur, as the older 

 therapeutics believed, precisely as would happen if the 

 parasitic agent could be exposed to the effects of drugs, 

 say in a test tube. Moreover, it was always evident that 

 such effective drugs as quinine and mercury must be em- 

 ployed sparingly, because while they were able to injure 

 and thus to lead to the destruction of the microbes in- 

 ducing malaria and syphilis, they were likewise capable 

 of injuring the component cells of the body itself. 



The outstanding instance in which experimental chemo- 

 therapy has registered a great success is in connection with 

 the organic compounds of arsenic, which have been 

 adapted to the overcoming of infection induced on the 

 one hand by spirochetes and on the other by trypanosomes. 

 That arsenical compounds possess therapeutically active 

 properties against these two classes of parasitic diseases 

 as represented on the one hand by syphilis and on the 

 other by African sleeping sickness is not entirely a re- 

 cent discovery ; but until the systematic investigations of 

 Ehrlich were instituted, which ultimately yielded salvar- 

 san, knowledge was fragmentary, medical practice based 

 on it ineffective, and the road to progress obscure. Now 

 the outlook is wholly changed, and there is going forward 

 an active and either already successful or at least highly 

 promising search for new drugs or chemicals, directed 

 against both the bacterial and the protozoal parasitic mi- 

 crobes. This territory so newly opened to exploration in 

 which organic chemists and pathologists should pool inter- 

 ests in order to move forward, is of almost infinite 

 possibility, since the number of chemicals is nearly limit- 

 less which can be produced and so fashioned as to injure 

 and subdue as it were the parasitic invader, and at the 

 same time, pass over and leave little influenced the ad- 

 jacent body cells. But the conditions of the search are 

 intricate since, as just indicated, a useful drug must 



