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 > f 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 t' 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 * i. how powerful is the force of habit. The most complex ' v 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 admit2 " 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,' En£. translat. vol. ii. p. 030. See also Mr. IT. Spencer's interesting1 specula- tions on the same subject, and on the genesis of nerves, in his 'Principles of Biology,' vol. ii. p. 340; and in his * Principles of Psychology,' 2nd edit. pp. 511—557.