James Sully,
An Essay on Laughter: Its Forms…
“ He adds that, in general, there is a concomitance 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.If tickling is a playing at fighting we may expect it, like other kinds of play, to mimic serious forms of assault. Now we know that the first rude attacks of man, so far as we can gather from the movements of a passionate infant, took the forms of striking, tearing with the nails and biting. Tickling may be said to be a sort of mild pretence at clawing. ”
