DEPENDENCE ON ASSOCIATION. 99 



actions form themselves into recurrent sequences, — 

 repeating themselves in groups with an established 

 regular order in each group. Those stimuli, arising 

 from chemical- changes in the process of nutrition, 

 are always present ; and though they have their varia- 

 tions, yet, in the healthy organism, they remain 

 generally the same, save that they are subject more 

 or less to recurrent periods of greater intensity. 

 The stimuli causing muscular motion, or protoplas- 

 mic contraction, are also frequently repeated. In 

 the animal kingdom, the muscular motions on which 

 the life of the individual depends, are repeated daily 

 and many hourly or even momentarily. Even the 

 stimuli causing the process of reproduction are, in 

 most animals, repeated many times during the life 

 of an individual. 



We have already observed it to be a nervous 

 property of organisms, that a series of actions fre- 

 quently repeated tends to become more and more 

 readily performed. The repetition of the series 

 establishes a co-ordination in the mechanisms gov- 

 erning the actions, so that each action seems to 

 have some guiding stimulus in the action preced- 

 ing ; and though some of the stimuli which were 

 originally necessary to the separate acts of the 

 series may be withdrawn, yet under the influence of 

 the remaining stimuli the whole series will be 

 executed with its usual regularity. Series of actions 



