For an explanation of habit we must turn to the nervous system. Before knowledge of the nervous system was so fully developed analogies for habit were found in many simple physical and biological phenomena. Change in tendencies as a result of action characterizes practically all things, particularly animate be, lugs. Streams change their channels when some trickle finds a new course; "as the twig is bent so the tree is inclined)); a scar once formed will persist through all the changes in tissue due to new growth. In this metaphorical sense habit is a universal law of nature. Now we place the permanent effects of learning upon which habit depends in the nervous system. In simplest terms all action of a higher animal is due to a nervous impulse which starts in some sensory stimulation and finds its way through the nervous system to some muscle. The nervous units that intervene are living cells or neurones each with two proc esses extending in opposite directions. One of the filaments of a sensory neurone extends to a sense organ, the other inward to come into contact with other neurones. The last unit in the link has a process extending to a muscle. In the simplest case a filament of a sensory neurone may come into contact with a filament from a motor neurone and an im pulse pass from one directly to the other and produce the movement. Usually if not always the sensory neurone is in contact with several motor neurones and in that case the impulse will pass to the motor neurone which is least separated from it, at the point of contact which offers the least resistance. Certain of these points of connection offer little resistance at birth and the reflexes or first acts will follow that path. After birth new connections are constantly formed and old connections come through use to offer a diminished resistance. It is this opening of new pathways that con stitutes the formation of habits. In all more complicated acts more than two neurones are involved but that does not change the prin ciple. Habits are formed when the connections between the sensory and motor neurones have been developed. It is at the points of con nection between neurones that the action wears a path, in terms of the older metaphorical con nections. As a result of frequent use the sen sory stimulus that has become associated with the movement will always call out that move. ment.
In the formation of the habits the first move ments are tentative and apparently succeed only by chance. The child and animal both learn to make the first movement by a process of trial and error. The impulse from the sensory impression may be pictured as spreading more or less at random through the nervous system, and calling out one response after another. The movements that are not successful or do not give pleasant responses are not repeated, those which are successful are repeated until the habit is formed and the given situation always excites the same movement. After the separate movements have been learned so that they can be made consciously they may be combined into larger units by habits. There is some trial and error in learning to combine them at this stage. The boy makes many false starts when he first attempts to tie a cravat and the whole requires much time and constant care. With repetition less and less thought is
required until in the adult the whole operation is carried out without effort and finally even without awareness of the separate movements. In the continuous or compound act one element of the movement is excited by the sensations that come from the preceding element. In walking, the stimulus to one movement comes from the preceding movement; in tying -the cravat completion of one movement arouses the next. When hesitation comes and the sensation from one movement is lost before the next starts, one must go back to the beginning be fore the following one can be made. In the final automatic stage thinking of what is to be done is a hindrance rather than a help as it emphasizes impressions and ideas that are not necessary to start the movements. After one movement has been learned or has become habitual it is linked with others to constitute a larger whole. Where at first a number of stimuli or acts of consciousness are needed to start the separate later elements one will evoke an entire series. A man starts to dress and one garment after another is put on without thought. One cue is sufficient to carry out the whole process. This leaves the man free to attend to other matters. Sometimes a wrong element will be introduced by habit. The absent-minded man may undress and go to bed when he starts to dress for dinner, but mistakes of this sort are sufficiently infrequent tojnake habit an advantage rather than a hindrance—in fact habit is indispensable.
Granted the supreme importance of habit for the individual and for society it is essential that right rules be followed to develop suitable habits. First, one must appreciate the immense importance of habit, that every act leaves its mark upon the nervous system, and that these marks in the long run make the man. When a habit is recognized as good every occasion pos sible should be utilized for executing the acts that are to make the habit. Especially is it necessary that no exception be permitted, no set of circumstances that demand the act should be allowed to pass without its perform ance. Each time a partly established habit is permitted to lapse,— each time some other re sponse is made when it should have been,—the habit is weakened. In actions that are likely to prove valuable in moments of stress it is well according to James to make an effort to do the disagreeable at times to keep alive the capacity for doing the unpleasant when it shall be necessary. Little acts of self-denial recommended to maintain the moral tone, just as a little unnecessary exercise is needed to keep the muscles of the sedentary worker ready for an emergency and to maintain the physical health. In general it is important for all de velopment to choose good habits and to check the bad. Gradually through repetition a stock of habits will be acquired that constitute a large part of what is ordinarily regarded as the man and will determine his manners, his morals, as well as his efficiency. Consult Carpenter, Wil liam Benjamin, 'Mental Physiology' (Chap. VIII, James, William, 'Habit,' also 'Principles of Psychology' (Vol. I, chap. iv, NeW York 1890) ; Hering, Ewald, 'On Mem (4th ed., 1913).