208 THE PROBLEMS OF EVOLUTION 



tional capacity of the chromosomes than with the 

 idea that fixed chromosomal fmictions may pro- 

 duce a cumulative effect in the cytoplasm. If 

 increased pigmentation is due to cumulative de- 

 posit of pigment, the constant renewal of epider- 

 mal cells would seem inimical to the process; if it 

 is due to an increased capacity for the formation 

 of melanin by the melanoblasts, the newly formed 

 epidermal cells would find an increasing supply of 

 melanin as the organism responds to light. The 

 latter alternative is in harmony with the known 

 facts and indicates an increase in the functional 

 capacity of the melanoblasts, which would involve 

 the nucleus as well as the cytoplasm. 



This idea is the theory of use and disuse in the 

 chromosomes which I have previously expressed. 

 In brief it is no more than the extension of the 

 principle of use and disuse, which has been an 

 accepted source of modification of living substance 

 in macroscopic units, to the minute bodies which 

 have been found so important in heredity. Its 

 value is limited, like the value of all other theories 

 of evolutionary method, but it has the unique 

 possibility of explaining a chromosomal modifica- 

 tion as a result of organic activity on the basis of 

 a recognized peculiarity of living substance. It 

 suggests a means whereby the existing latitude of 

 organic response may shift in harmony with the 

 demands made upon the organism by environ- 



