204 THE PROBLEMS OF EVOLUTION 



any slight fluctuations of the environment. Others 

 are expressed only under special conditions. The 

 latter result in the acquired characters which have 

 evolutionary significance. They include the strik- 

 ing responses to mechanical stimuli, light rays, 

 unusual foods, and the various other conditions 

 which have been previously listed, and they may 

 be either adaptive or non-adaptive, according to 

 the part which they play in the life of the individ- 

 ual. According to one possible interpretation of 

 these characters, a given hereditary power is either 

 used or not used during individual life; according 

 to another, a given gene responds positively in 

 various ways to various stimuli. It is satisfactory 

 to use the expression use and disuse in this connec- 

 tion. This interpretation of gene action may be 

 applied equally well to the variable development 

 of heritable characters in connection with fluctu- 

 ating conditions in the internal environment. 



Elsewhere I have considered in detail the several 

 kinds of acquired characters and have shown that 

 all of them may be explained in this one way.^ 

 Whether the condition is an acquired immunity, a 

 degree of muscular development, or pigmentation 

 of the skin, it is the result of use or disuse of some 

 inherent functional capacity. This is the basis 

 for acquired characters generally as far as they 

 concern us. 



2 Am. Nat., Vol. LXI, p. 251, 1927. 



