NATURE OF THE GENETIC EFFECTS 413 



perature by the molecular and submolecular particles in the protoplasm 

 in their collisions with the genes, and that any such superthreshold colli- 

 sion results in a mutation, then it can be calculated, as was done by 

 Delbriick, that the mutation frequency of any given gene is so low — one 

 mutation in some thousands of years, according to the present writer 

 (1923) — as to indicate a correspondingly high energy threshold for the 

 reaction. In fact, the threshold thus calculated from the mutation fre- 

 quency turns out to be of just the right order of magnitude to result in 

 the observed Qw of 5 to 6. This correspondence with the observations 

 lent support to Delbrlick's interpretation. 



Alternatively, it might be supposed that the mutation process requires 

 contact between the gene and one or more special substances, perhaps 

 also in a special way. In that case there would not necessarily be a par- 

 ticularly high energy threshold, for the given collisions might be very 

 rare. As thermal agitation would in any case play a role in bringing 

 about the required contacts, there would still be, other things being equal, 

 an increase in mutation frequency with rise in temperature. But the 

 increase caused in this way should be proportionate only to the speeding 

 up of the life cycle and of general metabolism, thus following a Qm of 2 to 

 3, unless the mutation process required the concatenation of more acci- 

 dental events than did the other processes (i.e., unless it was essentially 

 multimolecular) . 



In a case of this kind, however, "other things" might not be equal. 

 That is, a difference in temperature might in addition influence the 

 mutation frequency in another way, namely, by occasioning a difference 

 in the concentration of substances which, directly or indirectly, affect the 

 occurrence of mutation. Since substances favoring mutation would not 

 necessarily become more concentrated with rise in temperature, it could 

 not be predicted whether this influence, if it existed, would be in the 

 direction of raising or of lowering the Qio. It might, in fact, work in one 

 direction in some types of organisms and contrariwise in others, and it 

 might work differently according to just which temperatures were being 

 studied. 



In the above connection it should be noted that the lower and higher 

 temperatures used (approximately 17° and 27°C) were both within the 

 range that may be considered normal and innocuous to the organism. 

 But, if this range had been transgressed in either direction, the circum- 

 stances affecting mutation frequency might well have been altered 

 markedly. This is to be inferred from the considerations indicating that 

 all organisms have been subjected to prolonged selection for a constitu- 

 tion which under normal conditions results in a lower mutation rate than 

 would otherwise obtain in them. In consequence of this situation, 

 marked departures from normal conditions, leading to disturbances in the 

 biochemical organization, would on the whole tend to result in a higher 



