With respect to Dr. Robinson’s hypothesis, it may be acknowledged ungrudgingly to be a brilliant piece of hypothetical construction. But, as the writer frankly confesses, the facts, here and there, do not point in its direction. A very serious objection is the fact that the sole of the foot and the palm of the hand are not taken into account in his attempt to establish a correspondence between the ticklish areas of {182} the surface and a high degree of vulnerability. In Stanley Hall’s returns it is the sole of the foot which is most frequently mentioned as a ticklish area; and, as we have seen, it was the first to give rise to laughter in the case of one child at least.[116] There is another and more serious objection to Dr. Robinson’s theory as an explanation of laughter. One may urge that the occurrence of such violent movements would, by shaking the body and by inducing fatigue much earlier than need be, pretty certainly be detrimental to that prolonged practice of skill in attack and defence, to which Dr. Robinson attaches so much importance. The supposition that tickling is a variety of play developed by natural selection among combative animals is, I think, highly probable. The play of animals, like that of children, is largely a form of social activity involving a playmate; and is apt, as we know, to take the form of attack and defence, as in chasing, throwing over, pretending to bite, etc. These playful attacks are, as we have seen, closely related to teasing; indeed, teasing may be viewed as merely a play-imitation of the first stage of combat, that of challenging or exciting to contest.[117] Tickling pretty obviously finds a fitting place among the simpler forms of playful combat which have a teasing-like character. Moreover, these forms of social play all seem to show, in a particularly clear manner, the utility referred to in the preceding chapter. {183}