[88] Among previous writers on the subject M. Dugas seems to be the one who has had the clearest apprehension of the essentially playful character of laughter (op. cit., chap. vi., especially p. 115 seq.).
[89] Karl Groos connects both the tusslings and the tearings of young animals with the instinct of sex-competition (Play of Animals, p. 35 ff.).
[90] The Psychological Review, 1899, p. 91.
[91] Descent of Man, Part I., chap. iii.
[92] Animal Life and Intelligence, p. 407. The author strikes me as almost excessively cautious in accepting these evidences of canine jocosity.
[93] W. Preyer, Die Seele des Kindes, p. 197.
[94] Quoted by Lloyd Morgan, loc. cit.
[95] Expression of Emotions, p. 208; cf. p. 132 ff.
[96] See Darwin, The Descent of Man, Part I., chap. iii.
[97] So in The Expression of the Emotions, pp. 211, 212. In the notes contributed to Mind, vol. ii. (1877), p. 288, two infants are spoken of, one of which smiled when forty-five, the other when forty-six days old.