MULLER 



the genes descended from those origi- 

 nally present would have become 

 changed. After these genes have once 

 mutated, however, their previous mu- 

 tability no longer holds. In addition 

 to this "banking house method" there 

 are also methods, emplo\'ed by Alten- 

 burg and myself, for— as it were— auto- 

 matically sweeping up wide areas of 

 the streets and sifting the collections 

 for the valuables. By these special gen- 

 etic methods of reaping mutations 

 we have recently shown that the ordi- 

 nary genes of Drosophila— unlike the 

 mutable genes above— would usually 

 require at least a thousand years— 

 probably very much more— before half 

 of them became changed. This puts 

 their stability about on a par with, if 

 not much higher than, that of atoms of 

 radium— to use a fairly familiar anal- 

 ogy. Since, even in these latter experi- 

 ments, many of the mutations prob- 

 ably occurred within a relatively few 

 rather highly mutable genes, it is 

 likely that most of the genes have a 

 stability far higher than this result 

 suggests. 



The above mutation rates are mere 

 first gleanings— we have yet to find 

 how difi'erent conditions affect the 

 occurrence of mutations. There had 

 so far been only the negative findings 

 that mutation is not confined to one 

 sex (Muller and Altenburg, 1919; 

 Zeleny, 1921), or to any one stage in 

 the liJFe cycle (Bridges, 1919; Muller, 

 1920; Zeleny, 1921), Zeleny's finding 

 that bar-mutation is not influenced by 

 recency of origin of the gene (1921), 

 and the as yet inconclusive differences 

 found by Altenburg and myself for 

 mutation rate at different tempera- 

 tures (1919), until at this year's meet- 

 ing of the botanists Emerson an- 

 nounced the definite discovery of the 

 influence of a genetic factor in corn 

 upon the mutation rate in its allelo- 

 morph, and Anderson the finding of 



113 



an influence upon mutation in this 

 same gene, caused by developmental 

 conditions— the mutations from white 

 to red of the mutable gene studied 

 occurring far more frequently in the 

 cells of the more mature ear than in 

 those of the younger ear. These two 

 results at least tell us decisively that 

 mutation is not a sacred, inviolable, 

 unapproachable process: it may be 

 altered. These are the first steps; the 

 way now lies open broad for explora- 

 tion. 



It is true that I have left out of 

 account here the reported findings by 

 several investigators, of genetic vari- 

 ations caused by treatments with vari- 

 ous toxic substances and with certain 

 other unusual conditions. In most of 

 these cases, however, the claim has 

 not been made that actual gene 

 changes have been caused: the results 

 have usually not been analyzed gen- 

 etically and were in fact not analyza- 

 ble genetically; they could just as well 

 be interpreted to be due to abnor- 

 malities in the distribution of genes— 

 for instance, chromosome abnormal- 

 ities like those which Mavor has re- 

 cently produced with X-rays— as to be 

 due to actual gene mutations. But even 

 if they were due to real genie differ- 

 ences, the possibility has in most cases 

 by no means been excluded (1) that 

 these genie differences were present in 

 the stock to begin with, and merely 

 became sorted out unequally, through 

 random segregation; or (2) that other, 

 invisible genie differences were pres- 

 ent which, after random sorting out, 

 themselves caused differences in muta- 

 tion rate between the different lines. 

 Certain recent results by Altenburg 

 and myself suggest that genie differ- 

 ences, affecting mutation rate, may be 

 not uncommon. To guard against 

 either of these possibilities it would 

 have been necessary to test the stocks 

 out by a thorough course of inbreed- 



