DEPENDENCE ON ASSOCIATION. 89 



molecular conditions. The progress of the change 

 may be compared to the wearing away of the 

 roughness of the machinery and the reduction of 

 friction and resistance to a minimum. Also, new 

 arrangement and co-ordination of the nervous mech- 

 anisms have been accomplished, for where at first 

 each separate part of an action required a distinct 

 impulse to start it, after practice the whole of a 

 complicated series of actions may be produced by 

 one initial impulse ; for example, a trill played on 

 the piano: — just as if at first five impulses had 

 to be sent from the brain to the five fingers, and 

 afterward only one impulse should be sent from the 

 brain to the arm, which should then send on the five 

 secondary impulses to the fingers. The change of 

 structure resulting from frequent repetition of the 

 same series of nervous molecular changes is in gen- 

 eral a permanent one ; for when an art or habit is 

 once acquired the skill persists, and even after long 

 lapse of disuse is more readily revived, or acquired 

 more readily the second time than the first. 



Besides rendering individual actions easier of exe- 

 cution, repetition has the effect of uniting successive 

 actions when frequently performed in the same 

 succession. A series of actions often repeated be- 

 come thus so associated that when the first action 

 of the series is performed the rest will generally 



