EVOLUTION IN NATURE 145 



by X-rays and other forms of radiation has been 

 based hitherto on the satisfaction arising from the 

 experimental production of heritable characters. 

 Unfortunately another point of view is possible 

 which is much less favorable to the current evalua- 

 tion of these changes. We have considered among 

 the results of environmental factors acting on the 

 organism two classes of effects, viz., mechanical 

 modifications such as mutilations and the shaping 

 of the skull by restriction during infancy, and 

 modifications due to a functional response of the 

 organism to the conditions surrounding it. X-rays 

 are a distinctly environmental factor; to which 

 class of characters do their effects belong? It is 

 possible, of course, to regard a chemical reorganiza- 

 tion as response, but the nature of X-ray actions in 

 general suggests very strongly that here is a modifi- 

 cation of the other type, a condition forced upon 

 the organism regardless of its functional powers. 

 Certainly some genes may be less susceptible than 

 others to the rays, but this variation is not neces- 

 sarily a matter of function in the biological sense. 

 The attitude that X-ray mutations are actually a 

 result of damage to the genes has much to recom- 

 mend it. Since these rays penetrate all defenses of 

 the chromosomes and act upon them directly, they 

 cannot fail to leave their impress upon the heritage 

 if they cause any change at all, and their action 

 may be so delicate that the gene is not destroyed. 



