Although this co-operation is brought about in the first instance by the purposive direction of our efforts towards a given end, under the guidance of our visual and muscular sensations, yet when we have once learned to walk erect, we find ourselves able to maintain our balance without any exertion of which we are conscious; all that is necessary for the performance of this movement being that a certain stimulus (volitional, or some other) shall call the mechanism into activity.--But further, we have seen that special powers of sense-perception can be acquired by the habitual direction of the attention to particular classes of objects; and that special movements come to be the secondarily automatic expression of them. How nearly related these are to the preceding, we may assure ourselves by attending to the process by which an adult learns to walk on a narrow base, such as a rope or the edge of a plank. For the co-ordinating action has here to be gone through afresh under altered and more special conditions, so as to give a greater development to the balancing power; yet when this has been fully acquired, it is exerted automatically with such an immediateness and perfection, that a Blondin can cross Niagara on his rope with no more danger of falling into the torrent beneath, than any ordinary man would experience if walking without side-rails along the broad platform of the suspension bridge which spans it. Now since in those cases in which man acquires powers that are original or intuitive in the lower animals, there is the strongest reason for believing that a mechanism forms itself in him which is equivalent to that congenitally possessed by them, we seem fully justified in the belief that in those more special forms of activity which are the result of prolonged training, the sensori-motor apparatus grows to the mode in which it is habitually exercised, so as to become fit for the immediate execution of the mandate it receives: it being often found to act not only without intelligent direction, but without any consciousness of exertion, in immediate response to some particular kind of stimulus,--just as an automaton that executes one motion when a certain spring is touched, will execute a very different one when set going in some other way.