MANNER OF PRODUCTION OF MUTATIONS 571 



the result in Caldecott and Smith's work, the heat shock did not reduce 

 aberration frequency when X rays were used without infrared. More- 

 over, the heat shock did not reduce the frequency if it was applied before 

 the infrared pretreatment. 



It has been debated whether the infrared acts by promoting other types 

 of union in preference to restitution, as argued by Kaufmann, or by 

 promoting chromosome breakage itself, as argued chiefly by Hollaender 

 and Swanson. If gene mutations are regarded as being usually associated 

 with breakage, then the lack of effect of infrared on them is opposed to 

 the idea that breakage is promoted, but evidence has been presented on 

 pp. 496-507 against such an association. The relative lack of effect of 

 infrared in promoting dominant lethals in Drosophila, however, appears to 

 offer a more pertinent argument against an effect on breakage. On the 

 other hand, this seems to be contradicted by the finding that in Trades- 

 cantia there are more unrestituted breaks found after infrared has been 

 used with X rays, although actually this finding could be interpreted on 

 either view. However, the main argument offered by Swanson is based 

 on the fact that infrared remains as effective in Tradescantia when applied 

 many hours after X irradiation as when applied 1 hour after. It had 

 been deduced by Sax and others on the basis of their studies of the 

 influence of timing of irradiation, temperature, and centrifuging on the 

 frequency of different kinds of aberrations in Tradescantia that very few 

 breaks remained unjoined and unhealed after 1 hour. If this is true 

 (despite the objections raised by Lane), it is necessary to conclude that 

 infrared given later than 1 hour after X irradiation can have little 

 influence in increasing aberration frequency except by causing more 

 breaks to occur. 



This inferred promotion of breaks by infrared, an agent which does not 

 by itself produce breaks, is, on the view suggested by McElroy and 

 by Swanson and Yost, brought about by the induction in the chromosome 

 of a relatively high-energy metastable state which, in combination with a 

 different metastable state induced by X rays, can result in a break. 

 Either kind of metastable state, on this interpretation, would be returned 

 to the original state by the degree of thermal agitation arising from a heat 

 shock. Thus a heat shock interposed between an infrared and an X-ray 

 application would pre\'ent the occurrence of the combination effects 

 which give rise to the additional breaks, not produced by X rays alone. 



The question at issue cannot yet be regarded as settled. Meanwhile, 

 difficulties remain on either view. For example, on the breakage view 

 the rather special assumption must be made that neither X rays nor 

 infrared gives rise to a metastable state which, by accumulation of two 

 or more incitements of the same derivation (i.e., all from infrared or all 

 from X rays), can result in a break but that only the combination of the 

 two different metastable states can effect a break. Otherwise, infrared 



