Chap. I. SERVICEABLE ASSOCIATED HABITS. 29 



III. The principle of actions due to the constitution 

 of the Nervous System, independently from the first of 

 the Will, and independently to a certain extent of Habit. 

 — When the sensorium is strongly excited, nerve-force 

 is generated in excess, and is transmitted in certain 

 definite directions, depending on the connection of the 

 nerve-cells, and partly on habit: or the supply of nerve- 

 force may, as it appears, be interrupted. Effects are thus 

 produced which we recognize as expressive. This third 

 principle may, for the sake of brevity, be called that of 

 the direct action of the nervous system. 



With respect to our first Principle, it is notorious 

 how powerful is the force of habit. The most complex 

 and difficult movements can in time be performed with- 

 out the least effort or consciousness. It is not posi- 

 tively known how it comes that habit is so efficient 

 in facilitating complex movements; but physiologists 

 admit 2 " that the conducting power of the nervous 

 fibres increases with the frequency of their excitement." 

 This applies to the nerves of motion and sensation, as 

 well as to those connected with the act of thinking. 

 That some physical change is produced in the nerve-cells 

 or nerves which are habitually used can hardly be doubt- 

 ed, for otherwise it is impossible to understand how the 

 tendency to certain acquired movements is inherited. 

 That they are inherited we see with horses in certain 

 transmitted paces, such as cantering and ambling, which 

 are not natural to them, — in the pointing of young 

 pointers and the setting of young setters — in the peculiar 



2 Miiller, ' Elements of Physiology,' Eng. translat. vol. 

 ii. p. 939. See also Mr. H. Spencer's interesting- specula- 

 tions on the same subject, and on the genesis of nerves, 

 in his ' Principles of Biology,' vol. ii. p. 346; and in his 

 ' Principles of Psychology,' 2nd edit. pp. 511—557. 



