88 EXPERIMENTAL RESEARCHES 



it necessarily belongs to a fourth chemical 

 group. One can, of course, materially simplify 

 this examination by employing strains that 

 have been rendered resistant to all the groups 

 hitherto known. Thus I have, together with 

 Dr. Browning, obtained a strain which is resis- 

 tant to atoxyl, trypan-red and fuchsin. By 

 using such a strain, a single animal experiment 

 suffices for the purpose of establishing any new 

 group of such chemo-therapeutic agents. For, 

 if one injects the substance in question to such a 

 trebly resistant mouse, the trypanosomes will 

 either go on proliferating then the substance 

 must belong to one of the three known groups 

 or the trypanosomes are injured ; in that case 

 we have before us a new type. As you see, we 

 are therefore enabled by means of these resis- 

 tant strains to separate substances of different 

 modes of action from one another, and thus to 

 solve a problem which hitherto could not even 

 be suggested. A way is thus shown us by 

 which we may enter upon the consideration of 

 the most intricate problems of pharmacodyna- 

 mics and, if I may say so, obtain a fuller know- 

 ledge " de sedibus et causis pharmacorum" 



I also wish to lay especial stress upon my 

 view that the drugs, also, are attracted by and 



