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.^

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.
^ See tlie article already quoted.
^ Dr. Kobinson considers that another agreeable efiect of tickling may-
be an inherited echo of the caresses of man's progenitors.
180 THE ORIGIN OF LAUGHTER

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 con-
comitance 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.
