ON SPECIFIC THERAPEUTICS. 87 



power of resistance. We must further note 

 that every such resistance is specific. Thus 

 the fuchsin-fast strains are resistant only to this 

 substance, but not to atoxyl and trypan-red, and 

 vice versa. This resistance extends, however, to 

 chemically different substances of the same 

 group, so that we can in any individual case 

 decide to which group a given new trypano- 

 cidal agent belongs. Thus, e.g. t the atoxyl-fast 

 strain is resistant also to a number of related 

 substances. Among hundreds of trypanocidal 

 bodies, it is thus possible to single out those 

 substances which contain arsenic. 



It has further been shown that the resistance 

 to trypan-red and trypan-blue is a mutual one, 

 in spite of considerable chemical differences 

 between these two substances, which in fact 

 have nothing in common with one another 

 except that they contain naphthalin rings, which 

 are substituted in a certain position (3, 6) by 

 sulphuric acid radicals. 



Thus we possess in this specific resistance, so 

 to speak, a therapeutic sieve, a cribmm therapeu- 

 ticum, with which we can undertake the classifi- 

 cation of any new chemo-therapeutic substance. 

 If such a substance is found to have a destruc- 

 tive effect on our three different resistant strains, 



