The variation of the organism’s response is made possible by the existence of higher nerve centers, that is, of connecting neurons of a higher order, more remote from the sensory and motor points of the body. Let us imagine the proverbial reaction of a child to the sight of a flame, and discuss the successive stages of development by the help of figure 15. (1) The visual stimulation starts a nervous process from _s_{1}_, which passes through the bulb and spinal cord into the muscles of the arm at _m_{1}_. A small part of the current may branch off at _a_ and, instead of passing down towards _b_, take the direction of _v_. But the resistance in this direction is for the present so high that only an insignificant part of the process can take this way, and so no corresponding motor response is noticeable. [Illustration: FIG. 15.--“A BURNT CHILD FEARS THE FIRE.”] (2) While all this is still going on and the child’s arm is still moving forward, the heat of the flame acts as a pain stimulus at _s_{2}_. The nervous process produced passes over _c_ and _d_ to the muscles at _m_{2}_, whose contraction results in the arm’s being pulled back. This results in a third stimulation at _s_{3}_, which we need not trace farther here. But not the whole of the nervous process passes from _c_ down to _d_. A part of it, of considerable absolute magnitude because of the intensity of stimulation, passes from _c_ up to _p_ and thence over _k_ down to _d_ and finally also into _m_{2}_. This process going from _p_ to _k_, according to a general law of nervous activity, tends to attract other, weaker nervous processes, if the neuron connections make this possible. Consequently the nervous process from _s_{1}_ to _a_ is now turned mostly into the path _a-v-p_ and only an insignificant part of it continues to go from _a_ towards _b_. The consequence is that the resistance of the path _a-v-p-k-d_ is soon reduced to less than the resistance of the path _a-b_. The great significance of this fact becomes clear in the third stage of development. (3) At some later time the flame again acts as a visual stimulus. But now, because of the change of resistance just explained, the nervous process takes for the most part the path over _a-v-p-k-d_, and the reaction follows at _m_{2}_ instead of at _m_{1}_. The child has learned to avoid the flame. The child, when seeing the flame, is conscious of the pain, as imagery, without having to receive the actual stimulation at _s_{2}_. Thus the inflexible regularity of reaction gives place to another type of reaction, an adaptation, not only to those conditions which at the time make their impression upon the organism, but also to those conditions which are mere future possibilities. The experience of the past guides the organism into the future. QUESTIONS 117. What is the twofold connection into which instinctive movement enters with consciousness?