"We have therefore distinguished four kinds of action: _Reflex Action_, which is purely physical and independent of association, and which is the last link in all the derived varieties; _Lower Instinctive Action_, which is caused by the first introduction of association, and is hardly to be distinguished in its phenomena from the last;... _Higher Instinctive Action_, which involves perception of qualities or objects;... and finally, _Voluntary_ or _Intentional Action_, such as we find it in man.... Though we have separated these classes from each other for clearness of description, there is no distinct line to be drawn anywhere between them. Each fades insensibly into the next.... Evolution, we must remember, does not advance by stages; these are merely marks that we make ourselves, like the constellations in astronomy, for convenience of study. "Finally, we must remark that the last two kinds of Action ever tend to relapse into the second, which subjectively is a mere form of the first. Association of all kinds tends to become organic. By this we mean that, as the connection becomes more definitely marked and easy, the perpetual radiation which occurs as the current passes the different points on its path, disappears; and the whole current passes unimpaired. First, the radiation caused by the changes disappears, and reasoning becomes instinct, as in doing a mathematical example from mere memory of the different steps. Secondly, the radiation from the different nervous centres also disappears, and the current which ends in action becomes not only unreasoning but unperceived, as in walking or reading aloud while thinking of something else.... "Long habituation has two effects: it increases the number of trains connected with each object, and also the length of each. If we suppose the simpler emotions to have, by this time, become organic or apparently simple states of consciousness, a continuance of association tends to connect them together in bundles, as they themselves were originally bundles of elementary pleasures and pains. Hence the emotions become organized in their turn so as to form higher emotions, and eventually, when association has completed its work,... this organization ends in one supreme emotion, which is the head of the emotional or sensitive side of the consciousness....