240 Discussion 



improved the growi^h rate in the presence of the drug and so were 

 selected. Hence, when Dr. Eagle subcultures such a colony and finds that 

 it gives a new curve of resistance, this does not tell us whether the 

 parental cell of this colony was genetically different from sister cells that 

 failed to yield colonies. It seems to me that it will take an elaborate new 

 kind of experimentation, along lines initiated by Dr. Hughes, to analyse 

 the role of mutation and that of physiological adaptation in the region of 

 small differences in resistance. 



Eagle: You have answered my question, at least in terms of your own 

 opinion: that the term "step" is a misnomer, because we don't know 

 either how many mutational steps there have actually been or their 

 individual magnitude. 



