Another principle of the animal economy, which is near ly allied to association, is habit, which consists in a pecu liar state of the system, induced by the frequent repetition of the same act. The force of habit is too well known to be insisted upon, and we have perpetual illustration of the truth of the remark, that habit is a second nature ; for there is scarcely any impression, however disagreeable, to which we do not become reconciled by habit. The effects of ha bit are the most observable in those operations which recur only after certain intervals, such as taking food and going to rest. When the usual period arrives, we experi ence the accustomed sensations; but, if we powerfully resist the calls of hunger, or the inclination for sleep, the hunger or drowsiness cease, and some time elapses before they are again experienced, thus showing that they do not altogether depend upon the state of the stomach, or the exhaustion of the nervous system. It is difficult to assign a cause for these periodical accessions of habitual feelings, and more especially, why they should observe the diurnal period of twenty-four hours. It would be curious to in quire, whether, by any voluntary effort, we could so far change our modes of life, as to go through the ordinary routine of the diurnal actions in a longer or shorter space of time, for example, in thirty, or eighteen hours. A re markable effect of habit upon the corporeal organs is to blunt or diminish impressions of all kinds, so that not only do we acquire, in a great measure, an indifference to those that were originally agreeable; but we even become, with in certain limits, insensible to pain. There are, on the contrary, trains of actions which were originally almost in different, that, by habit, become essential to our enjoy ment, and sometimes almost so to the continuance of life.
Another very important principle in the animal econo my is the tendency which we have to imitation. Strictly speaking, imitation is rather a complicated series of ac tions than a principle or faculty, yet it seems to depend upon a peculiar condition of the system, or some of its or gans, which is not easily to be referred to any more gene ral principle. The tendency to imitation appears to be natural to the constitution ; for we observe, in children, that one of the first symptoms of intellect which they dis play, is to imitate the actions of those about them. This has generally been regarded as an ultimate fact, a circum stance which we are unable to explain, although we can not doubt of its existence. We may go so far in our at tempts at accounting for this series of facts as to assume, that when an action has been once performed, the repeti tion of it becomes more easy, and farther, that it is more easy to imitate an action that passes under our observa tion than to invent a new one. But although the physi cal cause of imitation be obscure, the final cause is ob vious ; it is in this way that we acquire the first rudiments of our knowledge, and profit by the information of those who have preceded us. In this way it is that we learn to form the articulate sounds which constitute speech, which is the basis of all education.
Speech is altogether a voluntary act, and affords an ex ample of a process composed of a train of actions, the connexion between which is obscure, although the differ ent steps of the operation are easily explained. The me
chanism of speech has been accurately examined and sa tisfactorily explained by the writers on anatomy ; it is suf ficient for our purpose to remark, that it depends upon a number of complicated motions of various muscles be longing to the tongue and lips, while the inflections of the voice are produced by another set of muscles connected with the upper part of the larynx.
The power which we possess of imitating articulate sounds and the inflections of the voice, is so far different from the ordinary kinds of imitation, as the parts concern ed are concealed from the eye, so as to render it very dif ficult for us to conceive how we acquire a knowledge of the muscles necessary to perform the proper actions. We can only judge of them through the medium of the ear, and without knowing in what the actual change consists, yet by an act of volition we produce a certain state of the muscles of the part, which enables us to form the same tone or articulation. It has been conjectured that, by re peated trials, we discover what sensations in the tongue or larynx are accompanied by particular sounds, and when ever we wish to produce these sounds, we excite the same sensations. But even admitting the truth of this hypo thesis, it scarcely explains the difficulty.
Some of the most remarkable actions of the animal body are those which we refer to the effect of sympathy. There is scarcely a motion or affection that takes place in any part which does not give rise to a motion of affection of some other part, in some cases depending, as it appears, upon mere contiguity, at other times being conveyed through the medium of the nerves, and frequently, as far as we can judge, connected merely by association. But there are also cases in which none of these causes can be supposed to operate ; where no association can be traced, and where there is nothing in the physical structure of the body which can account for the effect. We may in stance as an example the act of sneezing. Here, in order to remove an irritation from the nose, we force a current of air through the nostrils, which is accomplished by the involuntary contraction of the diaphragm. As this act takes place in the newly born child, and is independent of the will, it cannot be referred to association, and we are not acquainted with any circumstance in the structure of the nervous system which can account for this connexion.' But we not only observe the effects of sympathy as ope rating between different parts of the same individual, but also as affecting different individuals. And this not mere ly in the intellectual operations, but in the physical actions of the system ; for it is well known, that by witnessing pain or suffering in another, the body sometimes actually be comes affected in a similar manner. The tendency to fainting, which is experienced by the sight of blood, or by being present at surgical operations, and still more, those cases in which convulsions are excited by the sight of an individual labouring under such diseases, are to be regard ed as examples of this kind of transferred sympathy. To this head likewise must be referred many of the examples of enthusiam and fanaticism, where violent gestures are propag,tted through large assemblies of people, in such a manner as to prove that they are, at least in a certain de gree, beyond the control of the will.