The movements of laughter are subject to the laws of movement in general, Repetition and Habit. They tend to perfect themselves by practice; and the result probably involves a strengthening and an expansion of the wide-ranging organic commotion which makes up the reaction. A child of four will laugh on being tickled much more vigorously than one of two.[121] Moreover, the effect of repeated exercises of the function would seem, as already {189} hinted, to involve the setting up in the motor-centres, from which the discharge in laughter issues, a condition of high instability, so that a very slight application of the stimulus, or (as in the case of tickling) the mere threat to apply this, suffices to evoke the reaction. Lastly, this work of organisation will plainly involve a fixing of the connection in the brain-centres between the effect of the stimulation and the motor reaction. We say that the impulse of laughter has become associated with a definite kind of sense-presentation. The instant response of a child to the threatening fingers is a clear example of the result of such an associative co-ordination. Other examples are seen when a particular sight or sound takes on permanently a funny character. A child that has come to regard a figure in a picture book or an odd sound made by the nurse as funny will laugh whenever this recurs or is spoken of, provided that the mood of the moment is favourable. This is a noteworthy illustration of the way in which the action of the novel and unexpected—which, as we all allow, has a large _rôle_ in the excitation of laughter—may be replaced by that of an antagonistic force, namely, habit, which itself appears to secure the hilarious response. It may be added that so far as Habit comes in, reducing the importance of the initial psychical stage, and rendering the reaction automatic, the theory of Lange and James applies fairly well. The feeling of genial hilarity is in this case largely the reflex mental effect of the movements themselves, including the whole organic commotion brought about. Coming now to the development of the psychical element in laughter, we may, by way of introduction, refer to certain principles which ought to be useful. (_a_) To begin with, any variety of emotional reaction {190} excited by a particular kind of presentation appears, as it is repeated, to undergo a process of development, taking on more of fulness and complexity. A feeling of attachment to a person or to a place, or of admiration for a cherished work of art, grows fuller and deeper with the establishment of a relation of intimacy. Dimly realised resonances of former like experiences melt into, and deepen the feeling, and new elements are woven into it by associative complication, and by growing reflection. This increasing complexity affects both the ideational basis of the emotion and the closely connected emotional tone itself.[122]