90 DEVELOPMENT AND HEREDITY. 



follow without any additional stimulus. The fre- 

 quent repetition of a series of nervous molecular (or 

 material) changes, tends to make the changes in 

 the series always follow each other in the same 

 order. We know, when a train of ideas has often 

 passed through the mind in a certain order, that 

 thereafter those ideas, when the first one has been 

 suggested, will always come to mind in the same 

 order. This is also well illustrated in the case of 

 recalling or humming a tune, — which we may sup- 

 pose to be correlated with as many nervous mole- 

 cular changes as there are musical notes. Every one 

 knows how the different notes follow each other, 

 associated together in measures and rhythms. The 

 principle is the same in playing a tune on an 

 instrument where the movements of the fingers 

 follow in regular order. The different movements 

 become firmly associated together. The same is 

 true of a great many kinds of actions performed 

 by artisans and skilled labourers who may be con- 

 stantly repeating the same series of actions. The 

 arts of knitting and spinning are good examples, as 

 being each a series of actions that can go on 

 indefinitely, while the attention of the operator is 

 devoted to something else. 



By the statement that the series of actions takes 

 place without requiring attention, is meant that 



