The response to tickling in these regions is actively and obviously
self-defensive. The horse discharges energy in the form of a kick;
the dog wriggles and makes a counter-bite; the man makes efforts
at defense and escape.

There is strong evidence that the deep ticklish points of the body
were developed through vast periods of fighting with teeth and claws
(Fig. 9). Even puppies at play bite each other in their ticklish
points and thus give a recapitulation of their ancestral battles
and of the real battles to come (Fig. 10). The mere fact that animals
fight effectively in the dark and always according to the habit
of their species supports the belief that the fighting of animals
is not an intellectual but a reflex process. There are no rules
which govern the conduct of a fight between animals. The events
follow each other with such kaleidoscopic rapidity that the process
is but a series of automatic stimulations and physiologic reactions.
Whatever their significance, therefore, it is certain that man did
not come either accidentally or without purpose into possession
of the deep ticklish regions of his chest and abdomen.
Should any one doubt the vast power that adequate stimulation
of these regions possesses in causing the discharge of energy,
let him be bound hand and foot and vigorously tickled for an hour.
What would happen? He would be as completely exhausted as though he had
experienced a major surgical operation or had run a Marathon race.

A close analogy to the reflex process in the fighting of animals
is shown in the role played by the sexual receptors in conjugation.
Adequate stimulation of either of these two distinct groups
of receptors, the sexual and the noci, causes specific behavior--
the one toward embrace, the other toward repulsion. Again, one of