158 " Dynamic Evolution 



Given a person of fair natural intelligence 

 and one who follows some business requiring 

 him to think and think hard, as the law or 

 engineering. At twenty he has a certain 

 amount of mental power, due partly to his 

 inheritance and partly to the extent to which 

 he has exercised his brains. At thirty his 

 mental power has considerably increased, 

 that increase being due to the mental work 

 he performed in the intervening ten years. 

 At forty there has been a further increase of 

 mental power, and this development of men- 

 tal power continues up to some uncertain 

 age which is different for different persons. 

 This difference in the mental power at differ- 

 ent ages is common knowledge and was 

 recognized in the Constitution of the United 

 States. A man cannot vote before the age of 

 21, cannot be a Representative in Congress 

 before the age of 25, cannot be a Senator 

 before 30, or a President before 35. A 

 man under forty years of age could get 

 very few votes for the office of President 

 of the United States. His work involves 



