COMPARISON WITH "SPONTANEOUS" MUTATIONS 297 



the present author found a rate of visible mutations 5 times that found 

 by a group of specially trained graduate students. In view of this, it is 

 evident that unless the same person, who must be one with special apti- 

 tude for such work, has had a chance to accumulate data of this kind on 

 spontaneous mutations at specified loci over a period of years, and to 

 obtain corresponding data on mutations induced by radiation, the com- 

 parison will be dubious, either on quantitative or on qualitative grounds 

 or both. This then poses a condition which present Droso-phila projects, 

 with their short terms, can hardly meet. The same stricture applies 

 also to the determination of the spectra of both spontaneous and induced 

 mutations to be obtained under given conditions, as in different cell 

 stages, on application of chosen chemicals, in various physiological states, 

 and in the presence of particular genes. 



But, although no such exact comparison of spectra has been possible, 

 nevertheless there is at least rough agreement between spontaneous and 

 irradiated samples in regard to their ratios of visible to lethal gene muta- 

 tions. Indeed, the data of Timofeeff-Ressovsky (72, 73) on this point 

 appeared to show a really good agreement, but since in this work the 

 results for spontaneous mutations had to be obtained over a consider- 

 able period, as is usually the case, the errors due to probable personality 

 differences, referred to above, must make us hesitate to place reliance 

 on the exact figures obtained. To turn back to a consideration of in- 

 dividual loci, although again really quantitative conclusions are impos- 

 sible, it is in general to be noted that gene mutations which are obtained 

 relatively readily without treatment, such as white eye, garnet eye, and 

 cut wing, are comparatively frequent after irradiation also, whereas cer- 

 tain others, such as scute and achaete, which have seldom been found 

 spontaneously, are similarly rare (although they were carefully looked 

 for) in the irradiated material. At the same time, we must acknowl- 

 edge that even in untreated material, as in the comparisons of "high- 

 mutation-rate lines" with other lines reported by Neel (60) and by Ives 

 (29), the rates for one or two loci (especially that of yellow body) are 

 sometimes found to have been disproportionately changed. If this can 

 be true even without artificial treatment, and if the result was not caused 

 by unstable alleles of the particular genes in question having been present 

 in some lines (a very likely possibility in these cases), then it would 

 hardly seem likely that irradiation, a much stronger influence, would 

 raise the mutation rates of all loci to just the same extent. This is a 

 matter that merits further analysis, if similar cases of high mutation 

 rate are found in the future, by transference of genes at the loci in ques- 

 tion to different genetic backgrounds. 



Yet, whatever the answer may be to the question last raised, it is 



