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.