Page fourteen 



EVOLUTION 



February, 1931 



place, and there has been no sign in them of any tendency to 

 revert back to the originally normal condition. They have their 

 own, new norm; they are real, new variants. The new forms 

 are permanent, in so far as the word permanent may be applied 

 legitimately to living things. And when crossed to other forms, 

 the new differences obey the same laws of Mendelian and chro- 

 mosomal inheritance as do the gene-differences existing be;tween 

 natural varieties. 



VII. THE NATURE AND SIGNIFICANCE OF 

 THE GENETIC EFFECT OF RADIATION 



It might perhaps be contended in some quarters that while 

 the artificial mutations may be similar in some respects to 

 natural ones, and even identical with some natural ones, yet 

 they may not be similar to those particular natural mutations 

 which may be termed "progressive:" the mutant genes resulting 

 from which survive, multiply and thus become a part of the 

 heritage of an evolving species. Such claimants would hold 

 that the X-ray action is necessarily destructive, causing only 

 loss and injury, and that thus it can work only harm, or at 

 least can cause no indefinite amount of progress in organization. 

 Such a contention would rest upon a misconception of the ac- 

 tion of the X-ray, for it can be shown that the speeding electron 

 is capable of imparting energy to other atoms through which it 

 goes, and that the resulting chemical changes may be of a 

 synthetic character as well as otherwise. 



It is evident, as my wife has suggested, that if the change 

 induced by X-ray from, say, a gene designated as large A, to a 

 mutant gene of different composition, designated as small a, 

 has really involved a destructive process or a loss, then the 

 opposite change, from small a to large A, must, conversely, 

 involve a constructive process or a gain. With this question in 

 mind. Professor J. T. Patterson and I have been engaged in 

 some extensive irradation experiments involving particular 

 characters. The character which we have used most is the re- 

 cessive mutant character termed "forked bristles" (f), as com- 

 pared with the dominant normal straight bristles (F) . The evi- 

 dence is now positive and convincing that the X-rays not only 

 induce the mutation of straight bristles to the recessive forked, 

 but also the precisely opposite type of change: namely, forked 

 bristles to the dominant straight, and abundant controls have 

 shown that it is really the X-rays which are the inducing agent. 

 Since giving this lecture, the author has received several very 

 important articles by N. W. Timofeef-Ressovsky, describing 

 numerous cases of the production of mutations in opposite direc- 

 tions by means of X-rays. Various different gene-loci were in- 

 volved, and in the case of one locus (that of white eye) the 

 mutations of, and to, various different allelomorphs, were ob- 

 served. The mutations arising as a result of X-raying are, there- 

 fore, not merely destructive changes, not merely losses. If some 

 are losses, others, then, are gains. Doubtless, as in the case of 

 most chemical reactions, most mutations too are changes involv- 

 ing substitutions and rearrangements, not mere losses or gains. 



Not Mere Curiosities 



It should be mentioned that, in addition to the changes in in- 

 dividual genes which X-rays bring about, they also cause — 

 with considerable frequency, as Altenburg and I have shown — 

 breakages of entire chromosomes or strings of genes, accom- 

 panied by reattachments of the broken-off fragments to differ- 



ent chromosomes or to the chromosome-remainder from which 

 they were broken, at a different point from before. The re- 

 arrangements of genes thus resulting can be analyzed by breed- 

 mg tests, and at the same time checked up by studies of the 

 chromosomes as seen through the microscope — an undertaking 

 which Dr. Painter and I have been cooperatively engaged upon 

 during the past two years. There is evidence that such rear- 

 rangements of chromosome parts, as well as mutations in in- 

 dividual genes, have occurred repeatedly in natural evolution. 

 The question may now be raised: to what extent can all these 

 results be regarded as mere curiosities: effects confined to the 

 mature sfjerm-cells of the fruit-fly, and of little significance 

 elsewhere? In this connection, it may first be pointed out that 

 my results in producing gene mutations in the fruit-fly were 

 immediately confirmed by "Weinstein, working at Columbia 

 University, later by others (Hanson, Patterson, Harris, Oliver) 

 at this laboratory, and more recently by Serebrovsky ;ind his 

 colleagues in Russia and by Dr. and Mrs. Timofeef-Ressovsky 

 in Berlin. In my own work, the treatments were not confined to 

 sperm-cells, but were also applied to the female, and it was 

 found that both the mature eggs and the immature female 

 germ-cells (oogonia) were susceptible to the mutation effect. 

 Harris has recently extended the finding to the immature germ- 

 cells of the adult male. Paterson has found the early germ-cells 

 of both male and female larvae are likewise susceptible and 

 also the larval somatic cells. The latter finding, which has 

 recently been announced also by Timofeef-Ressovsky, opens up 

 a whole realm of interesting possibilities in the production of 

 mutant areas of the adult body, derived from cells of the treat- 

 ed embryo — such effects as might result, for instance, in an 

 individual with eyes of different colors, or with parts of the 

 same eye different. Casteel has been making an anatomical 

 analysis of these latter effects through microscopic sections of 

 the eye. The production of mutations by X-rays is thus a gen- 

 eral effect for Drosophila, producible in all kinds of cells in 

 that organism. What, now, of the generality of the effect on 

 other organisms? 



Results Amply Confirmed 



I need not, perhaps, remind the general reader of the fact 

 that all the principles of heredity so far discovered in the fruit- 

 fly — the favorite experimental object of many modern genet- 

 icists — -have proved applicable to animals and plants in general. 

 It is more to the point to mention that investigators elsewhere, 

 working on other organisms, have now reported results of the 

 same kind as those now in question. Thus, Stadlet. at the 

 University of Missouri, was independently attempting to in- 

 duce gene-mutations in barley and corn by means of X-rays 

 and radium at the same time that I was doing my first experi- 

 ments along these lirfts on flies, and he has found indubitable 

 evidence of the production of gene-mutations in monocotyle- 

 donous plants by both these means. Following my work on flies, 

 Whiting has obtained positive results by the use of X-rays on 

 wasps. Blakeslee, Buchholtz and the others of this group have 

 a mass of interesting mutation results from X-rays and radium 

 applied to the Jimsonweed, Datura, that extend the findings ^ 

 concerning lethal as well as visible mutations to dicotyledonous 

 plants. With these so widely separated bits of the living world 

 sampled and all responding positively, it is a reckless critic who 

 still would doubt the probable generality of the phenomenon. 



