106 THE PROBLEMS OF EVOLUTION 



stated briefly as follows: Use and disuse are to be 

 regarded as the degree of exercise of functional 

 powers. It has been established in many cases 

 such as muscular activity, bone development, and 

 immunization against poisons, that conditions of 

 environment which demand a given response re- 

 sult, if long continued, in the modification of the 

 functional capacity of the part activated. Use 

 brings about increase of functional capacity and 

 disuse, decrease even to degeneration. There 

 seem to be limits of stimulation beyond which 

 harm may result, rather than adaptation. Since 

 chromosomes and their contained genes are living 

 units, albeit immune from the methods of observa- 

 tion and experiment which may be brought to 

 bear upon macroscopic parts, and since they are 

 active in controlling the activities of the cell 

 throughout its life, there is every reason to con- 

 clude that they share in this capacity for increase 

 or decrease of their functional powers as a result 

 of the degree of activity demanded of them. I am 

 unable to find any material, logical or otherwise, 

 in refutation of this view. Its value, of course, 

 rests upon the uniform maintenance of the chro- 

 mosome complex throughout the body, including 

 the germinal epithelium. 



With this discussion of some of the significant 

 facts and problems of heredity, it is apparent that 

 the nature of the interaction of heritage and en- 



