524 RADIATION BIOLOGY 



this result might be explained entirely by the greater clustering of muta- 

 tions when neutrons are used. Hence there no longer remains any valid 

 reason for following these investigators in their inference that neutrons 

 are actually less efficient than X rays in the production of the primary 

 mutational changes at individual loci. Instead, the efficiency of the 

 neutrons in this respect would appear to be about the same as that of 

 X rays, despite the dense crowding of ionizations caused by neutrons. '^ 



This relation would mean that a given locus, or interlocus connection, 

 which is instigated to mutation or breakage by a given ionized or activated 

 particle or minute cluster of them, is seldom simultaneously within range 

 of another such particle or cluster that by itself would likewise have 

 succeeded in causing a mutation or break at the given locus or interlocus. 

 This is, however, very different from saying that there are no other active 

 particles simultaneously present, within a range close enough to produce 

 mutation in the given genetic material; it implies only that, even though 

 these are present, the chance of any of them proving mutagenically effec- 

 tive is very small. For, as will be explained shortly, there is reason to 

 infer that in experiments of this type there are usually several or many 

 other ionized particles or clusters within a mutagenic distance of the 

 genetic material in question in addition to the one which actually causes 

 the mutation in it. This being the case, the conclusion would follow that 

 for an active particle or cluster to be within a certain prescribed space is 

 not enough to result in its instigating a mutation. Evidently other 

 special circumstances are necessary at the same time, such as for the 

 particle to be in exactly the right strategic position and to participate in 

 the right concatenation of energy relations and chemical structure, 

 before it can effectuate a mutational event. 



Even after acceptance of this conclusion, which points to the existence 

 of factors other than mere presence within a given region that determine 

 whether an activated particle is to be mutagenic, it is still necessary to 

 conclude from the neutron results that distance from the point to be 

 affected is a factor that plays a very important role. In other words, the 

 essential result still stands that in order to be mutagenic the activated 

 particle produced by ionizing radiation in spermatozoa of Drosophila 

 or in pollen of Tradescantia must usually be within an ultramicroscopic 

 distance of the genetic material to be affected. Yet, as will be shown on 

 pp. 548-552, even this is not an absolute requirement; for in certain other 

 types of material, with ultraviolet radiation at any rate, the mutational 

 pathway has been found, on occasion, to be far longer. This being the 

 case, it would be strange if any material were entirely free from such 

 long-pathway effects. Moreover, inasmuch as a part of the effects of 

 ionizing radiation must be identical with the effects of ultraviolet, it is to 

 be expected that even ionizing radiation would be capable of exerting 



' However, new data confute all earlier estimates of the mutagenicity of neutrons. 



