COMPARISON WITH "SPONTANEOUS" MUTATIONS 301 



proportions, so circumscribed, that it strikes only a single one of two 

 near-by, similar loci in the same nucleus"; that is, when one gene mu- 

 tates, its allele of identical composition, usually only a small fraction 

 of a micron away, remains unaffected. 



Thus the determination of just which gene mutates in a given case, 

 and to which allele, must be a consequence of the ultramicroscopic ac- 

 cidents of thermal agitation rather than of the chemical nature of the 

 material reacting with the gene. And it may be concluded [Muller (39), 

 p. 43] that "mutations are not caused by some general pervasive influ- 

 ence, but are due to 'accidents' occurring on a molecular scale. When 

 the molecular or atomic motions chance to take a particular form, to 

 which the gene is vulnerable, then the mutation occurs." Since the time 

 when this statement was made, it has become possible to add that the 

 similarity of radiation mutations to spontaneous mutations, in regard to 

 the kinds of effects produced, their relative numbers, and their random 

 distribution in space and time, lends strong support to this viewpoint. 

 In this connection it is especially noteworthy that in the genesis of the 

 radiation mutations, unlike that of the spontaneous ones, the acci- 

 dents are initiated by a fast particle the path of which can have had no re- 

 lation to cellular needs or metabolic processes, and that nevertheless the 

 spontaneous mutations appear as random as those produced by radiation, 

 and essentially similar to them. If in the natural accidents that cause 

 spontaneous gene mutations different kinds of protoplasmic substances 

 differed much from one another in regard to the types of mutations they 

 favored, then we should hardly expect spontaneous gene mutations as a 

 group to agree as much as they do with the group of gene mutations 

 produced by the absorption of high-energy radiation. Hence it seems 

 likely that a thermally occasioned encounter of the right kind to produce 

 one mutation w^ould also, according to w'hich gene and gene-part hap- 

 pened to be involved, have sufficed to produce practically any other mu- 

 tation. Thus there seems to be very little difference in the type of 

 process, or amount of energy necessary, for the occurrence of different 

 kinds of gene mutations, and the same general sort of chemical substitu- 

 tion may well be involved in all cases. If we had more data on muta- 

 tional spectra w^e might make this inference more secure. 



Our inference that the mutations of different genes can be occasioned 

 by chemical encounters of the same type by no means implies, con- 

 versely, that the type of chemical encounter to which a gene is exposed 

 is of no account in the determination of whether or not a mutation will 

 be produced in it. That is, the occurrence of spontaneous mutations 

 does not depend solel3^ on the energy level reached, as has sometimes 

 been assumed, but also on the energy being conveyed in an appropriate 



