viii Preface 



intelligence is apparently increased by mental 

 activity, it must be assumed that the work 

 involved is brain work. 



Work is measured by force, velocity, and 

 time. The product of force and velocity 

 may be represented by degree of activity, 

 and by dividing the amount of work per- 

 formed in some long period, as a century, by 

 the number of years in the period, it is possi- 

 ble to determine the average activity per 

 year. The year would then be the unit of 

 time in measuring the amount of brain work 

 performed in any period greater or less than 

 a century. 



In any long period covering several genera- 

 tions, the degree of mental activity becomes 

 highly variable. It is zero during gestation 

 and is maximum during adult life. The aver- 

 age activity, and consequently the energy- 

 storing work, is increased by increasing the 

 proportion of maximum to the zero activity. 

 Other things being equal, the amount of 

 energy stored in the brain by doing brain 

 work will be increased by lengthening the 



