This theory would plainly illustrate Mr. Herbert Spencer’s principle, that states of feeling affect the voluntary muscles in the order of increasing calibre, the smaller being called into play by feelings of lower intensity, the larger by those of higher intensity. But this theory is not enough. We must take into account also the order of frequency of use, and of consequent liability to discharge in the connected nerve-centres. It seems probable that the muscles engaged in the movements of the mouth and those exercised in phonation would, for these reasons, be specially liable to be acted upon. These wider tendencies would, according to the above hypothesis, be assisted by special associations. These would secure the combination of the two groups of movements, which I have assumed to have been employed independently as utterances of pleasurable feeling: namely, those involved in smiling, and those underlying the first happy reiterated sounds of a quasi-infantile babbling. {176} One element in the laugh, its explosive vigour, seems unaccounted for on this hypothesis. Here, I think, the effect of relief from strain, which is so common a factor in human laughter, may be called in. The earliest laughter of the child seems to illustrate this element. For example, that which occurs during tickling, in a game of bo-peep, and at the sight of the mother making faces may be said to arise from a serious attitude suddenly dissolved. Perhaps the first great laugh was produced by man or by his proximate progenitor, when relief came after fear or the strain of battle. So far as primitive laughter was the outcome of such concentrated energy seeking relief, this circumstance would help to account for the prolongation as well as for the strength of the sounds. Our conjecture cannot lay claim to be a hypothesis. It makes no attempt to explain the precise forms of the changes which enter both into the smile and into the laugh. At best, it is only a rough hint as to a possible mode of genesis. I have here treated of the genesis of laughter under its more general aspect as an expression of pleasurable states of feeling. We have seen, however, that within the first three months of life another and clearly specialised variety of laughter emerges, namely, that called forth by tickling. It follows from our analysis of the effect of tickling that it is one of the earliest manifestations, in a clear form, of the laughter of fun or of play. As such, it demands special attention in any attempt to explain the development of laughter. As a specialised reaction having a clearly marked reflex form, it is natural to ask whether laughter in response to tickling is not inherited, and, if so, how it arose in the evolution of the race. And we find that suggestions have {177} been made for explaining the genesis of this curious phenomenon. We will first glance again at the facts, and then examine the hypotheses put forward for explaining them.