February, 1931 



EVOLUTION 



Page fifteen 



Radium rays, like X-rays, produce mutations, because they 

 too, being short-wave-length higli-frequency electromagnetic 

 waves of great energy content, release high-speed electrons, and 

 the cosmic rays, which are still more extreme in these same re- 

 spects, and so release electrons of still higher speed, must neces- 

 sarily act likewise. For, as Hanson has shown in experiments 

 with radium, the number of mutations produced depends simp- 

 ly on the number of electrons released and the speed and dis- 

 tance they travel (i.e., on the total energy of ionization) re- 

 gardless of the source of the electrons. Oliver, too, in experi- 

 ments with X-rays in our laboratory, has obtained evidence 

 that the number of mutations produced is directly proportional 

 to the dosage of radiation used, and Stadler's work points in 

 the same direction. This being true, there being no evidence of 

 a minimal or "threshold" dosage, we are forced to conclude 

 that the minute amounts of natural radiation present almost 

 everywhere in nature — some of it of terrestrial origin, derived 

 from the radium and other radioactive substances in earth, 

 water and air, and a smaller part of it of cosmic origin, appar- 

 ently derived from the diifuse and distant factories of matter — 

 all this natural radiation tnust be producing some mutations in 

 the living things on the earth. These mutations must be very 

 scattered and very infrequent in proportion to the total non- 

 mutated population, just because the amount of natural short- 

 wave-length radiation is very small at any one place, but, con- 

 sidering the extent of the earth and the multiplicity of living 

 things, the total number of mutations so produced per year 

 must be very considerable. It can, therefore, scarcely be denied 

 that in this factor we have found at least one of the natural 

 causes of mutation, and hence of evolution. 



To Find Other Factors 



And yet, despite the fact that radioactive processes are 

 capable of causing the changes that lie at the basis of evolu- 

 tion, and do undoubtedly cause some of them, it is probable 

 that they themselves are responsible for only a minute fraction 

 of the mutations that occur in nature, and that their value, 

 for the theory of evolution, lies rather in their illustrating the 

 general type of the phenomena to which mutations belong, and 

 in providing us with all kinds of mutations in abundance for 

 purposes of analysis. X-ray, cosmic rays, and the radiation 

 from radioactive substances are not themselves the cause of the 

 vast majority of natural mutations, if the calculations which 

 L. M. Mott-Smith and I have recently made are correct, for 

 these calculations indicate that the amount of such radiation 

 in nature is far below what would be needed to produce the 

 numbers of mutations which are observed to occur. There are 

 probably, then, other factors, working in a similar way to 

 X-rays, that are the actually operative causes. 



The attempt to find these other factors is proceeding. The 

 natural phenomena most closely related to the radiation which 

 we know to be effective are rays of the ultraviolet region of 

 the spectrum (of wave length too short to affect our eyes). 

 Though preliminary tests conducted by Altenburg have not 

 demonstrated an effect of such rays, it is too early to be 

 certain of a conclusion here, for a mixture of wave-lengths was 

 tried, none of which, however, were of the extremely short 

 type, and it might still be tru that some particular wave- 



lengths, especially the very short ones (more nearly like 

 X-rays) , would be found effective. It is to be recalled in this 

 connection that Gurwitsh and other Russian investigators claim 

 that rays of exactly this kind are produced in living matter — 

 a kind of ultra-phosphorescence — stimulating cells to divide. 

 If the random motions of the atoms and molecules of the 

 cell, striking against one another, are capable, when "favorable 

 hits" occur, of causing mutations, as we have supposed the 

 X-ray "hits" to do, then raising the temperature should cause 

 a rise in the mutation frequency, just as it causes a rise in the 

 rate of other chemical reactions. E.xperiments by Altenburg 

 and myself indicate that it does so; this work requires large 

 numbers and is now being repeated. On the other hand, more 

 specific chemical influences have so far failed. Returning to 

 the investigation of the possible effectiveness of poisons and 

 other influences than X-rays, I have, during the past two 

 years, tried out a number of drastic treatments, using a re- 

 fined genetic technique similar to that in the X-ray experiments, 

 which would have allowed of the detection of lethals and other 

 mutations with far greater ease, and therefore in greater 

 abundance, than in the inconclusive experiments of the past. 

 Included among the treatments were heavy doses of manganese 

 and of lead salts, which had been claimed by J. W. H. Harri- 

 son (on the basis of what appeared to me genetically uncon- 

 vincing data) to produce visible mutations in butterflies. There 

 was also included a repetition of the experiments reported by 

 Morgan, who at one time suspected that he had been able to 

 cause visible mutations in the germ-cells of red-eyed flies by 

 injuring their eyes with a hot needle, an operation which was 

 followed by a release of the optic pigment and its distribution 

 throughout the body. But our trials of all these and of a num- 

 ber of other chemical agencies have given negative results, 

 and it is becoming evident that a mere derangement of cell 

 functionings does not ordinarily cause mutations. 



Another method of studying the way in which mutations 

 are produced is to investigate the possible effect of other 

 conditions applied along with radiation. There is now evidence 

 that some other conditions, internal and perhaps also external, 

 accompanying an X-ray treatment, can affect the sensitivity 

 of the cells to that treatment. Thus Stadler finds that the 

 sprouting cells of seedlings have mutations produced in them 

 in much greater abundance, by a given dose of X-rays, than 

 do the dormant cells of seeds, though some mutation^ are 

 produced in both. Yet in the case of flies both Hanson and 

 Harris, working independently, find that the genes of growing 

 immature male germ-cells are far less sensitive to the mutating 

 effect of radium or X-rays than are the dormant genes in ma- 

 ture spermatozoa. I find that the genes in the spermatozoa 

 of the adult male are also more sensitive than those in the 

 germ-cells of the female, or than those in the germ-cells of the 

 larval male. It is probable that part of these apparent differ- 

 ences are due to a relatively lower multiplication rate of most 

 mutated immature germ-cells, as compared with the non-mu- 

 tated ones. There seems to be even more difference in their 

 sensitivity to the gene-rearranging effect of the rays than in 

 their sensitivity to the transmuting effect on individual genes. 

 The activity of metabolism, however, varied by starving, and 

 by feeding and mating the female, had no perceptible in- 

 fluence in my experiments, and, as both Stadler and I have 

 found independently on barley and flies respectively, extremes 



