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.