Here, again, the question how far animals are susceptible of the effect becomes important. I have already alluded to Darwin’s remark, that if a young chimpanzee is tickled, more particularly under the armpits, he responds by a kind of laughter. The sound is of a chuckling or laughing kind. The emission of these sounds is accompanied by retraction of the corners of the mouth, and sometimes by a slight amount of wrinkling in the lower eyelids.[112] Dr. Louis Robinson publishes other observations of the effect of tickling on the young of anthropoid apes. He tells us that a young chimpanzee when tickled for some time under the armpits would roll over on his back showing all his teeth and accompanying the simian grin by defensive movements, just as a child does. A young ourang at the Zoological Gardens (London) behaved in a very similar way. The young of other animals, too, betray some degree of ticklishness. Stanley Hall remarks that a dog will retract the corners of his mouth and thus go some way towards smiling if tickled over the ribs.[113] Dr. Robinson finds that horses and pigs are also ticklish; and he thinks that these animals have specially ticklish regions, which correspond to a considerable extent to those which have been ascertained in the case of the child.
We may now refer to the first appearances of the tickling reflex in the child. As pointed out above, the response by defensive movements appears shortly after birth, whereas {178} the earliest instance of a response by laughter occurs in the second, or in the first half of the third month. It is to be noted that this date is distinctly later than that of the first laughter of pleasure, though it is not far removed from that of the first clear appearance of the laughter of gaiety or jubilation.
These chronological facts bear out the theory that the laughter of a tickled child has a distinct psychical antecedent. On this point Dr. L. Robinson writes to me as follows: “I have never been able to succeed in eliciting laughter from young infants under three months old by means of tickling, unless one also smiled and caught their attention in some such way”. This evidently points to the influence of mental agencies even in the first stages of laughter from tickling.
With respect to the parts in which the tickling first excites laughter, different observers appear to have reached dissimilar results. Preyer distinctly speaks of the tickling of the sole of the foot as provoking laughter in the second month. Whether he tried other parts he does not say. Dr. Leonard Hill tells me that one of his children first responded to tickling when the titillation attacked the palm of the hand, or ran up the arm. Responses to the tickling of the neck and soles of the feet came later.
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.
If tickling is a playing at fighting we may expect it, like other kinds of play, to mimic serious forms of assault. Now we know that the first rude attacks of man, so far as we can gather from the movements of a passionate infant, took the forms of striking, tearing with the nails and biting. Tickling may be said to be a sort of mild pretence at clawing. Dr. Robinson tells us that about 10 per cent. of the children he has examined pretended to bite when they were tickled, just as a puppy will do.