The fact that the effect of tickling becomes so well defined by, or soon after, the end of the second month, proves pretty conclusively that it is an inherited reflex; and the evolutionist naturally asks what it means, what its significance has been in the life of our ancestors. Dr. Stanley Hall carries back evolutional speculation very far, and suggests that in tickling we may have the oldest stratum of our psychic life, that it is a survival of a process in remote animal progenitors for which touch was the only {179} sense. He supposes that in these circumstances even light or “minimal” touches, say those coming from the movements of small parasites, being unannounced by sight or other far-reaching sense, would be accompanied by disproportionately strong reactions. He does not attempt to explain how laughter grew out of these reactions. He does indeed call them reactions “of escape,” but he does not follow up the idea by hinting that the violent shakings of the body by laughter, when it came, helped to get rid of the little pesterers. In truth, this ingenious thinker hardly appears to make the explanation of the laughter of tickling, as distinguished from the other reactions, the subject of a special inquiry.[114] A more serious attempt to explain the evolution of the laughter of tickling has been made by Dr. Louis Robinson. He, too, hints at the vestigial survival of experiences of parasites, but appears to think that these account only for the disagreeable effects which are brought about when the hairy orifices of the nostril and the ear are tickled. This limitation strikes one as a little arbitrary. The reaction of laughter, which Dr. L. Hill called forth when he made his fingers run up the arm of his infant, is surely suggestive of a vestigial reflex handed down from ages of parasitic pestering.[115] With regard to the laughing reaction, which, as we have seen, he considers to involve a distinct mode of stimulation, he suggests that it is an inherited form of that common mode of play among young animals, which consists in an exchange of good-natured and make-believe attacks and defences, or a sort of game of sham-fight. {180} In support of this theory he lays stress on the fact that susceptibility to tickling is shared in by the young of a number of species of animals standing high in point of intelligence, including not only the higher apes, but the dog and the horse. He adds that, in general, there is a concomitance between the degree of playfulness of a young creature and that of its ticklishness, though lambs and kids which are not ticklish are allowed to be an awkward exception.