Preface ix 



time between generations. If this energy- 

 is the foundation of intelligence, then intel- 

 lectual power is built up through the genera- 

 tions in the offspring of comparatively old 

 rather than comparatively young parents. 

 Also, if this is the energy of intelligence, in- 

 tellectual power is built up in the offspring 

 of parents who were mentally active rather 

 than in the offspring of parents who were 

 mentally inactive. 



Without at present going further into the 

 reasoning by which this energy is applied to 

 the successive generations of living organisms, 

 it will be said that the age of the parents at 

 birth of offspring is taken as the factor time 

 in measuring work performed, and records 

 for the kind and rate of work are taken to 

 determine as nearly as possible the degree of 

 activity. These are applied directly to in- 

 dividual cases and groups of cases to see if 

 intelligence-producing energy is proportional 

 to the amount of energy-storing work which 

 was performed. The steps to this end 

 lead through a consideration of muscular 



