j6 BASIS OF INVESTIGATION. 



gate use in a given period of time and independent of the number of 

 ancestors within that time. This may be made still clearer by re- 

 membering that use-inheritance means the inheritance of acquired 

 functional capacity, and that unless an individual acquires a func- 

 tional capacity above the average of its ancestors it will not have 

 an acquirement which it can transmit. 



As use really means surplus use on the part of the individual, 

 or use more than the normal use, it is evident that the functional 

 capacity acquired by use is made up of two factors, viz., functional 

 activity of the individual and time. We may assume for conven- 

 ience that the capacity acquired is proportional to the time occu- 

 pied in its acquirement. Thus if an individual acquire m capacity 

 in time t, then -capacity 2/» will be acquired in time 2t. While not 

 strictly true, this is approximately true during the period within 

 which the individual may continue to acquire functional capacity. 



AGE AT COMPLETE DEVELOPMENT. 



Under uniform conditions the healthy man usually attains his 

 best physical development between twenty-five and thirty, and main- 

 tains it to some time between forty and fifty. Occasionally he comes 

 to physical maturity at an earlier age, and sometimes he retains his 

 strength beyond fifty and even beyond sixty. Under conditions 

 which are not uniform he may, by physical training at a particu- 

 lar time, reach his greatest development at any age between twenty- 

 five and sixty, or even seventy. When a man who has passed the 

 age of twenty-five takes up systematic physical culture, the func- 

 tional capacity of his muscles will develop rapidly under the stimu- 

 lus of muscular activity. Within a few months or a year he reaches 

 a physical strength and development beyond which further training 

 will not carry him. In this we have a case of use consisting of 



