We need not, however, go so far back in the zooelogical series to explain the origin and significance of tickling in the human species.  Sir J.Y.  Simpson suggested, in an elaborate study of the position of the child in the womb, that the extreme excitomotory sensibility of the skin in various regions, such as the sole of the foot, the knee, the sides, which already exists before birth, has for its object the excitation and preservation of the muscular movements necessary to keep the foetus in the most favorable position in the womb.[10] It is, in fact, certainly the case that the stimulation of all the ticklish regions in the body tends to produce exactly that curled up position of extreme muscular flexion and general ovoid shape which is the normal position of the foetus in the womb.  We may well believe that in this early developed reflex activity we have the basis of that somewhat more complex ticklishness which appears somewhat later.

The mental element in tickling is indicated by the fact that even a child, in whom ticklishness is highly developed, cannot tickle himself; so that tickling is not a simple reflex.  This fact was long ago pointed out by Erasmus Darwin, and he accounted for it by supposing that voluntary exertion diminishes the energy of sensation.[11] This explanation is, however, inadmissible, for, although we cannot easily tickle ourselves by the contact of the skin with our own fingers, we can do so with the aid of a foreign body, like a feather.  We may perhaps suppose that, as ticklishness has probably developed under the influence of natural selection as a method of protection against attack and a warning of the approach of foreign bodies, its end would be defeated if it involved a simple reaction to the contact of the organism with itself.  This need of protection it is which involves the necessity of a minimal excitation producing a maximal effect, though the mechanism whereby this takes place has caused considerable discussion.  We may, it is probable, best account for it by invoking the summation-irradiation theory of pain-pleasure, the summation of the stimuli in their course through the nerves, aided by capillary congestion, leading to irradiation due to anastomoses between the tactile corpuscles, not to speak of the much wider irradiation which is possible by means of central nervous connections.

Prof.  C.L.  Herrick adopts this explanation of the phenomena of tickling, and rests it, in part, on Dogiel’s study of the tactile corpuscles ("Psychological Corollaries of Modern Neurological Discoveries,” Journal of Comparative Neurology, March, 1898).  The following remarks of Prof.  A. Allin may also be quoted in further explanation of the same theory:  “So far as ticklishness is concerned, a very important factor in the production of this feeling is undoubtedly that of the summation of stimuli.  In a research of Stirling’s, carried on under Ludwig’s direction, it was shown that