The Journal of Heredity 



ments, has a contrary action. And 

 Stadler, in his great work on the pro- 

 duction of mutations in cereals, started 

 independently of our own, has obtained 

 evidence that in this material X-radiation 

 in the doses used is unable to produce a 

 sensible rise in the gene mutation fre- 

 quency, though numerous chromosome 

 breakages do arise, leading to both gross 

 and minute rearrangements of chromo- 

 some parts. Either the genes are more 

 resistant in this material to permanent 

 changes by X-rays as compared with 

 their responsiveness to thermal agita- 

 tion, or a break or loss must usually be 

 produced by X-rays along with the gene 

 change. The milder ultraviolet quanta, 

 on the other hand, do produce gene mu- 

 tations like the natural ones in these 

 plants. 



Such variations in effectiveness are, I 

 believe, to have been expected. They do 

 not shake our conclusion as to the acci- 

 dental, quantum character of the event 

 which usually initiates a gene mutation. 

 But they ,f;ive rise to the hope that, 

 through further study of them, more 

 may be learned concerning the nature 

 of the mutation process, as well as of 

 the genetic material that undergoes the 

 changes. 



Controlled Mutation? 



No one can answer the question 

 whether some special means may not be 

 found whereby, through the application 

 of molar influences, such as specific anti- 

 bodies, individual genes could be changed 

 to order. Certainly the search for such 

 influences, and for increasing control of 

 things on a microscopic and submicro- 

 scopic scale as well, must be carried 

 further. But there is as yet no good evi- 

 dence that anything of the sort has been 

 done artificially, or that it occurs natu- 

 rally. Even if possible, there could be 

 no generalized method of control of 

 gene composition without far greater 

 knowledge than we now have of the in- 

 timate chemical structure and the mode 

 of working of the most complicated and 

 diverse substances that exist, namely, 

 nucleoproteins, proteins in general, and 

 enzymes. The works of Sumner. North- 



rup and Stanley, together with those of 

 other protein chemists, point the way in 

 this direction, but everyone will agree 

 that it is a long and devious system of 

 roads which is beginning here. 



It is true that some cases are known 

 of mutable genes which change selec- 

 tively in response to special conditions. 

 Such cases may be very informative in 

 shedding light on gene structure, but 

 we have as yet no indication that the al- 

 terations of these genes, which in the 

 great majority of instances known are 

 abnormal genes, have anything in com- 

 mon with ordinary natural mutations. It 

 is also true that cases are known among 

 bacteria and viruses of the induction of 

 particular kinds of hereditary changes 

 by application of particular substances, 

 but here the substances applied are in 

 each case the same as those whose pres- 

 ence is later found to have been induced, 

 and so there is every reason to infer that 

 they have in fact become implanted in 

 some way, that is, that we do not really 

 have a specifically induced mutation. 



So far, then, we have no means, or 

 prospect of means, of inducing given 

 mutations at will in normal material ; 

 though the production of mutations in 

 abundance at random may be regarded 

 as a first step along such a path, if there 

 is to be such a path. So long as we 

 cannot direct mutations, then, selection 

 is indispensable, and progressive change 

 in the hereditary constitution of a living 

 thing can be made only with the aid of a 

 most thoroughgoing selection of the mu- 

 tations that occur since, being non-adap- 

 tive except by accident, an overwhelming 

 majority is always harmful. For a sen- 

 sible advance, usually a considerable 

 number of rare steps must be accu- 

 mulated in this painful selective pro- 

 cess. By far the most of these are indi- 

 vidually small steps, but, as species and 

 race crossings have shown, there may be 

 a few large distinctive steps that have 

 been, as Huxley terms it. "buiTered", by 

 small changes that readjust the organ- 

 ism to them. Not only is this accumula- 

 tion of many rare, mainly tiny changes 

 the chief means of artificial animal and 

 plant improvement, but it is, even more. 



