62 Discussion 



other hand, we have a much smaller sample, but nevertheless a few quite 

 unambiguous cases, of kinetic changes in enzyme balance which are 

 sufficiently self-regenerative so that they fit into the type of picture that 

 Prof. Hinshelwood elaborated. Therefore, there is no need to argue any 

 further as to the fact that these mechanisms do exist. If there is any 

 point in the controversy, it is : What happens in this specific situation, 

 what happens in the next specific situation? I suggest that if we are 

 going into details of experiments, we decide beforehand that there will 

 be two or three chosen as exemplifying the type of analysis that must be 

 done, and then we can concentrate on trying to see to what extent 

 mutational occurrence has been ruled out in any one case; to what 

 extent have directed adapted changes (which may have more or less 

 permanence) been ruled out or ruled in. But to say that something might 

 have happened here and something else there is no longer the point 

 since we have mutual agreement of examples of both extreme types of 

 behaviour. 



Yudkin: We are really having this discussion on two levels. Dr. Ponte- 

 corvo said that we are agreed — and I wish we were — that we accept that 

 these two phenomena which are most under discussion can occur. What 

 I am a little concerned about is that from time to time we read that "It 

 has now been proved " that mechanism A has occurred and that therefore 

 we may assume that mechanism B does not. 



Slonimski: It cannot be said that mechanism A has been proved; all 

 that one can say is that mechanism B has been disproved. On logical 

 grounds a hypothesis can only be disproved, but not proved. 



Yudkin: I take exactly the opposite view: I think nothing has been 

 disproved, but certain things have been proved. 



Pollock: I agree with Prof. Yudkin's last remark. I do not know of 

 anj^one who has denied the existence of so-called spontaneous mutations 

 appearing in certain cases, but I am not so sure about examples of 

 induced heritable adaptive changes. 



Lederberg : Novick has been carrying the two states of lactase formation 

 in Esch. coli for what could be considered to be an indefinite number^ 

 many hundreds — of generations. 



Pollock: I feel fairly certain that they do exist, but if we are going to 

 adopt your suggestion of taking a few isolated examples, then it is 

 important to find one which we can get to grips with on the adaptation 

 side. 



Lederberg: The one that has satisfied me without any doubt that there 

 can be such a mechanism is the one that Cohn and Novick showed. 

 That one can be carried on indefinitely. 



Davis: Dr. Demerec has pointed out that in the presence of a concen- 

 tration of streptomycin that did not kill the cells, but allowed them to 

 grow extremely slowly, after a while resistant mutants appeared. In 

 trying to decide whether the amount of background growth would be 

 sufficient to account for the number of resistant clones, it might be 

 important to take into account some work reported by Novick and 

 Szilard a few years ago. It is generally assumed that the rate of appear- 

 ance of a given mutation in a given strain has a fixed value per generation. 



