BASIS OF INVESTIGATION. 75 



The amount of use necessary to bring the individual to, and main- 

 tain it at, the average functional capacity of the species to which it 

 belongs may be called average use, and the amount necessary for 

 any particular individual may be called normal use. Before an in- 

 dividual can arrive at the average functional capacity of the species 

 to which it belongs it must have passed from the adolescent to the 

 adult stage. The distance into the adult stage which an individual 

 must pass to arrive at the average for its species depends partly upon 

 its inherited functional capacity and partly upon the degree of its 

 functional activity. Hence the acquired functional capacity of an 

 individual is represented by the absolute use minus the normal use. 

 In this connection it should be remembered that the normal use for 

 a particular individual is not a fixed aggregate of use, but varies 

 with the age of the individual. Thus, after an individual has ar- 

 rived at the average functional capacity of its species it must con- 

 tinue its normal use for the purpose of maintaining itself at this 

 standard. If the individual fails to continue this normal use it falls 

 below the standard and we have a case of disuse. Ordinarily the 

 normal use and the average use do not differ much, but when an 

 individual possesses an organ endowed by heredity with great func- 

 tional capacity the normal use is much less than the average use, 

 and, conversely, when an individual inherits less than the average 

 functional capacity the normal use becomes greater than the average 

 use. 



THE MEASURE OF USE-INHERITANCE. 



From our definition of the word use, it is evident that to have 

 use-inheritance in a descendant, there must have been, on the part 

 of the ancestor, an aggregate use greater than the normal use for 

 such ancestor. Stated in other words, use-inheritance is to be meas- 

 ured by the amount of use for each ancestor and not by the aggre- 



