COMPARISON WITH "SPONTANEOUS" MUTATIONS 299 



temperature increase by about 5 times instead of by only the 2 or 3 

 times usual for chemical reactions. 



To this latter line of reasoning, however, we must interpose several 

 words of warning. According to one possible criticism, low reaction 

 rates are not always or entirely caused, as assumed above, by the fact 

 that sufficiently high energy levels are so seldom attained by the reac- 

 tive particles. They may equally well be caused, especially in a complex 

 medium like protoplasm, by infrequency of encounters of just the right 

 structural (that is, "qualitative") types for the production of the reac- 

 tion, despite the fact that the energy level remains low. This infre- 

 quency of effective encounters may simply be due to rarity (low con- 

 centration) of the reactive substances or, what amounts to nearly the 

 same thing, to a highly specific and unusual type of encounter being 

 necessary. However, the high temperature coefficient would not be ex- 

 plained in this way without bringing in some accessory assumption, such 

 as that the reactive (mutagenic) substances were present in higher con- 

 centration at a higher temperature. As another alternative, a low reac- 

 tion rate for mutation (long half life) may be due to a coincidence of 

 two or more unusual events being required for its occurrence, even 

 though both these events took place at a relatively low energy level. 

 In that case the temperature coefficient could be as high as the product 

 of the coefficients of the two or more participating events, and so it 

 would simulate the coefficient to be expected for a high-energy-level re- 

 action. Thus w^e see that, although a high energy level, of some 1.5 ev, 

 seems to be the simplest way of interpreting the finding of a low rate of 

 mutation combined with a high temperature coefficient, it is by no means 

 the only plausible possibility. 



That the occurrence of mutation does not depend merely upon a given 

 energy level being reached at a given point, but also upon the confor- 

 mation of the material, is indicated by a comparison of the energy levels 

 for spontaneous and ultraviolet-induced gene mutations. Even the high 

 energy level of some 1.5 ev proposed by Delbriick for spontaneous mu- 

 tations falls far short of the 10 ev or more (accumulated in units of 

 about 5 ev each) which, as we shall see in the next section, is probably 

 necessary for mutation by ultraviolet. This seems to mean that, from 

 an energetics standpoint, the chemical pathway to mutation by ultra- 

 violet activation of purines or pyrimidines is far less efficient than the 

 spontaneous pathway or pathways. 



However, we cannot yet exclude the opposite possibility that, instead 

 of involving lower energy, the spontaneous mutation process may in- 

 volve even higher energy than that of Delbriick's hypothesis. Living 

 matter can on occasion attain energy levels higher than those found in 



