FORCING INTELLIGENCE 75 



on the growth of human intelligence can be fully 

 shown, mention must be made of the fact that some 

 currents of energy in passing through the nervous 

 system produce changes in it. It is not known 

 whether these changes are more chemical or more 

 structural, but that they endure for periods of vary- 

 ing lengths, sometimes even from childhood into old 

 age, is evidenced by the phenomena of memory, 

 habit, and character. Some of them merely facilitate 

 the passage of subsequent similar currents through 

 the system. Others endow it with the power of repro- 

 ducing movements and experiences like those caused 

 by the passage of the currents which originally pro- 

 duced the changes. These last- mentioned results 

 are usually not entirely accomplished by one passage 

 of currents of a certain kind, but require sometimes 

 less, sometimes more numerous repetitions for effi- 

 ciency. Much depends on the incident force with 

 which the first currents entered the system, and on 

 the degree of complexity in the movements and 

 experiences or both which they provoke. The 

 changes which reproduce experiences and move- 

 ments constitute the physical basis of memory, 

 habit, and character. 



For memory cannot be rationally explained, 

 except as the result of the activity of structures, 

 which by occurrence of such currents of energy as 

 produce experience have been modified in such 

 manner that, when excited subsequently, they recall 

 and repeat these experiences. Nor can habit be 



