258 W. KUNICKI-GOLDFINGER 



number of hypothetical cytoplasmatic units, making geno- 

 typically drug-sensitive cells appear phenotypically drug- 

 resistant. These resistant cells, when transferred to a medium 

 containing streptomycin, can grow and form clones big 

 enough to permit the appearance of genotypically resistant 

 mutants. 



This may be an example of a third mechanism, besides 

 spontaneous mutations and directed adaptations, operating 

 in the evolution of new forms. In this mechanism an external 

 factor causes the semi-stable physiological change, having 

 no adaptive value in relation to the acting agent. If the en- 

 vironment of organisms is then changed, their new character 

 may have by chance an adaptive value and thus may increase 

 greatly the possibility of the formation of respective mutants. 

 It is not impossible that these processes may lead to changes 

 temporarily decreasing the resistance of bacteria to drugs, thus 

 helping the macro-organism in eliminating the initially drug- 

 resistant germs. 



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