MANNER OF PRODUCTION OF MUTATIONS 539 



work with Droso-phila polar caps, shows, even on the most conservative 

 estimates, an order of magnitude several times as high (about three times) 

 for the amount of ultraviolet energy which must be absorbed by the chro- 

 mosomes, in order on the average to produce one mutation, as for the 

 amount which must be absorbed by them when X rays are applied, in 

 order to produce the same effect. As noted later, it is chiefly the ultra- 

 violet absorbed by the chromosomes themselves which produces the muta- 

 tions in the work thus far carried out on multicellular organisms; hence it 

 is appropriate to limit in this way the reckoning for ultraviolet mutations 

 induced in the polar caps. Moreover, a similar limitation may without 

 much error be made for the X-ray results, as shown by the prevailing 

 spatial restriction of the efi'ect in Drosophila, discussed on pp. 518-525. 



On the basis of this result, it can be concluded that the X-ray activa- 

 tions are inordinately more effective, even for a given total amount of 

 energy, than the ultraviolet activations. This corresponds with results 

 obtained by various investigators for the relative efficiencies of these 

 agents in inactivating bacteria, viruses, and enzymes. Since X rays dis- 

 tribute only about half their energy in the form of mere excitations (not 

 counting excitations which result secondarily, from degradation of ioniza- 

 tions), it would seem at first sight to follow that, when ionizing radiation 

 is used, a far greater role in mutagenesis must be played by the ioniza- 

 tions than by the atomic changes which originated as excitations. It 

 must be borne in mind, however, that the excitations produced by ion- 

 izing radiation and by ultraviolet are, on the whole, of different types, 

 the ultraviolet being more selective, and so it is conceivable, even if not 

 very likely, that some of the former excitations are much more efficient in 

 mutagenesis than the latter. Moreover, if two or more ultraviolet exci- 

 tations do cooperate in causing mutations, then the crowding of excita- 

 tions occurring in the tracks of ionizing particles, by giving a local density 

 of them corresponding to an extremely high dose of ultraviolet, would 

 cause the X-ray-induced excitations (both the original ones and those 

 resulting from the degradation of ionizations) to be much more efficient 

 mutagenically than an equal number of the more scattered ultraviolet- 

 induced excitations. For these reasons the ultraviolet comparison does 

 not yet definitely allow the conclusion that, when ionizing radiation is 

 used, the excitations have a far less important role in mutagenesis than 

 the ionizations. 



At the same time, if it is admitted that only one or even two of these 

 inefficient ultraviolet activations can, on occasion, break a chromosome 

 or cause a point mutation, it would seem quite unnecessary to require 

 two or more ionizations, working in cooperation, to produce an equivalent 

 effect. It would thereby become probable that, at least with ionizing 

 radiation, the ionizations act individually rather than as either small or 

 large clusters in producing breaks and mutations. This agrees with the 



