294 PHYSIOLOGICAL GENETICS 



equation covering all cases of effect by a definite dose upon X 

 areas oul of n possible areas. He finds that the empirical equa- 

 tion is identical with the general one if the number of hits is one. 

 From this he concludes t hat one hit suffices to produce a gene 

 imitation (as bad also been assumed by others). He proceeds to 

 compare the consequences of the empirical facts with the different 

 theories of biological action of radiation. The fact that the effect 

 is independent of the wave length leads him to the conclusion 

 that the decisive effect is the formation of a pair of ions by the 

 hit, because this formation is independent of wave length by 

 definition (ionization = measure of dosage). One excitation, 

 then, is supposed to be necessary for a gene mutation. 



These facts are used by Delbrueck for the construction of a 

 model. It has to be supposed that the gene is a very stable 

 molecule, i.e., a combination of atoms with definite positions and 

 electronic conditions. Such a system might be changed in 

 different ways, the most important of which is the dissipation of 

 energy of excitation of an electron. This leads to an ionization in 

 the neighbourhood, i.e., a rearrangement of the atomic complex by 

 a single elementary process in the sense of the quantum theory. 

 A comparison of the energetic consequences of this view with the 

 facts of mutation furnishes a qualitative agreement of both. 



The general conclusion is drawn that mutation consists in a 

 change of equilibrium of the atoms of a molecule, produced by the 

 influx of energy from the outside or the oscillations of temperature 

 energy always present. In detail, this process is assumed to 

 show parallel behavior to photochemic processes. The primary 

 process of absorption of a quantum might lead to very different 

 secondary processes, such as simple steric rearrangements or 

 dissociation of definite bonds setting free a reactive radical. A 

 new residue may be attached from the surroundings, and thus 

 the whole molecule may be changed. From these deliberations 

 it follows that the gene itself must be represented by such an 

 indivisible atomic combination or molecule. The authors make 

 it clear that these conclusions are independent of any assumption 

 as to whether a gene is a unit or a part of a whole. We shall later 

 show that the theory of the gene as a unit is no longer tenable. 

 This will not prevent the application of this theory, especially 

 the part which assumes that one of the secondary consequences 

 of the excitation is an atomic dissociation. This theory is then 



