THE NATURE OF THE GENE 389 



wastage of hitting a single gene twice. Work on these lines is pro- 

 ceeding.^ 



We derive, from the work outlined above, a picture of the mutation 

 process as one involving a single event of ionization. Now in a compli- 

 cated molecule, an ionization is not to be regarded simply as the loss of 

 an electron but rather as a transference of an electron to a new position. 

 Such transferences can also occur spontaneously, and the mutation 

 process deduced from X-ray work can therefore also be used to account 

 for spontaneous mutation. The agent which brings about the spon- 

 taneous transference of an electron is heat. The atoms and electrons in 

 a molecule can be represented as vibrating about their equilibrium 

 positions, the energy of the vibration being the heat energy. The 

 magnitude of the vibration is only statistically constant, and is subject 

 to random fluctuations which in extreme cases may carry an electron 

 over its Hmits of stability, when it will, as it were, click over into a new 

 position in the molecule and begin vibrating around a new equilibrium. 

 The frequency with which this occurs naturally increases as the tem- 

 perature, and therefore the average amplitude of the vibrations, is 

 raised. It is a general rule that the rarer a process is, the greater will be 

 its dependence on temperature, other things being equal; and it is thus 

 understandable that the natural mutation rate has a temperature 

 coeflicient higher than that of the developmental processes, and in- 

 creases with the rise of temperature even when measured in terms of 

 life cycles. On the other hand, if the mutation process is more frequent 

 for any reason, the effect of temperature should be less. And in fact we 

 find no effect of temperature on the mutation rate (measured in life 

 cycles) of mutable genes or on X-ray induced mutation; 'diat is to say, 

 these processes have temperature coefficients of the same order as those 

 of the processes of development. 



The scheme of mutation therefore applies both to the spontaneous 

 and the induced process. A mutation occurs by a single alteration in the 

 equilibrium position of an atom or electron in a molecule; and diis 

 change may occur spontaneously as a result of heat vibrations or under 

 radiation as an ionization. 



It has been mentioned that chromosome aberrations are also pro- 

 duced by X-rays. The relation between the dosage of radiation and 

 the number of aberrations is still not entirely clear. ^ On a priori 

 grounds, two mechanisms have been suggested for the formation of 

 translocations and inversions; on the one hand, they may arise when 

 ^ Cf. Nagai and Locker 1938. - Rev. Muller 1938. 



