THE NATURE OF THE GENE 293 



(1924) to interpret series of multiple allelomorphs. This is, of 

 course, only a modification of the view that the quantity of a 

 gene is changed by mutation, as such a quantitative change 

 might as well be an increase in the number of free molecules as a 

 polymerization. All the arguments applying to one theory 

 therefore apply also to the other. But some of the recent 

 developments in chemistry may lead to a reconsideration of these 

 different types of quantitative views. The work done on chain 

 molecules (Meyer, Mark, Staudinger) and their micellar con- 

 glomerations might furnish a new model of the gene, capable of 

 quantitative changes, as Goldschmidt (1932a, 1934c) and also 

 Koltzoff (1934) pointed out. No elaboration of such an idea is 

 yet available. But the same idea may lead in still another direc- 

 tion, as will be shown below. 



2. THE THEORY OF THE GENE AS BASED UPON PHYSICAL 

 CONSIDERATIONS 



The work on production of mutations by X rays, inaugurated 

 by Muller, has naturally appealed to the physicist, who was 

 induced to compare the action of X rays in physics with that 

 of the same agent upon the gene in producing a mutation. The 

 most elaborate inquiry from this standpoint has been made in 

 collaboration by the radiologist Zimmer, the theoretical physicist 

 Delbrueck, and the geneticist Timofeeff (Timofeeff , Zimmer, and 

 Delbrueck, 1935). The decisive fact for a physical analysis is the 

 proportionality between X-ray dosis (measured in "r-units") 

 and rate of mutability. This proportion is a perfectly linear one, 

 as was proved first by Hanson and Heys (1929). The second 

 important fact is that the same rule applies to radiation of 

 different length (7 rays), demonstrating that the effect is inde- 

 pendent of the wave length. From the genetic side, the impor- 

 tant points are that mutations are reversible and, further, that 

 different "genes" react differently upon radiation. Zimmer now 

 assumes that the biological effect is produced in any case by hits, 

 whichever of the different theories in this field is used. The facts 

 of proportionality to dosis, just mentioned, may be expressed 

 by the equation x = a(l — e~ kD ), in which x — number of 

 mutated genes, a = number of irradiated genes, D = dose of 

 radiation, k — a velocity constant, e = base of natural loga- 

 rithms. This empirical equation is compared with the general 



