144 THE PROBLEMS OF EVOLUTION 



struction, but not yet of the appearance of useful 

 changes. 



Nevertheless we must grant the possibility that 

 mutations of slight degree, even like those observed 

 in Drosophila, may occasionally be of use under 

 peculiar environmental conditions. The fact that 

 survival value has not yet been associated with 

 observed mutations is not in itself conclusive and 

 our admission of possibility is no more unsound 

 than the admission in the case of natural selection 

 that slight variations may sometimes be of use to 

 the individual. Indeed, since natural selection ex- 

 plains evolution beyond the existing characters of 

 a species only when taken in conjunction with 

 mutation, the two are essentially the same thing. 

 When we arrive at this point in our analysis, only 

 one thing remains to be done; we must explain the 

 occurrence of mutations. 



We now have absolute evidence on this point. 

 X-rays are capable of effecting a change in the 

 arrangement of inorganic compounds, as in the 

 photographic plate, and the success of geneticists 

 in producing mutations by the same agency is 

 apparently due to the same type of action. Why 

 should not the chemical organization of the gene, 

 relatively stable though it is, be altered by forces 

 capable of penetrating all of the defenses which 

 shield it from the outer world .^ 



The favorable acceptance of mutations produced 



