[108] Darwin, Expression of the Emotions, p. 133.
[109] A. Lehmann, in his interesting account of the development of the emotions and their expression in the individual, suggests that the first imperfect smile of the infant, which expresses the pleasure of sweetness, is genetically related to the movements of sucking (Hauptgesetze des menschl. Gefühlslebens, ss. 295, 296).
[110] Darwin, Expression of the Emotions, pp. 134, 135.
[111] As pointed out above, the French e sound seems to be the common one in children’s laughter. Preyer tells us that the corresponding sound in German (ä) occurs in the first infantile babble (Development of Intellect, p. 239).
[112] Expression of the Emotions, pp. 132–3.
[113] See the article already quoted on “The Psychology of Tickling, Laughing,” etc., p. 33.
[114] See the article already quoted.
[115] Dr. Robinson considers that another agreeable effect of tickling may be an inherited echo of the caresses of man’s progenitors.
[116] Stanley Hall also suggests that the most ticklish parts, which, according to his inquiries, are the sole of the foot, the throat, etc., are the “most vulnerable”. But he does not explain what he means by vulnerable here, and certainly does not appear to use the word in the sense given it by Dr. L. Robinson.
[117] Groos deals with the teasing of animals under the head of “Fighting Plays” (Play of Animals, p. 136 ff.).