To trace the steps of this elaboration we must commence with a modification of the foregoing abstract scheme. The unit reflex arc is an undue simplification. In no developed organism do we find a single receptor linked with only one effector. Every recep tor has a path to various effectors, every effector is at the behest of many receptors, the system so comprising a network of con verging and diverging arcs. In this way the effects of a simple stimulus may irradiate a group of effectors producing extremely complex movements. These movements, in their turn, or their external effects, may provide the adequate stimulus for a further series of reactions. Thereby complex co-ordinated movements (simultaneous or successive) are secured, the orderly arrange ment of which is further guaranteed by a system of inhibitions. A dominant reflex being evoked, all reactions of an antagonistic nature are automatically prevented from occurring.
The guiding principle of the reflex thus directly explains the more uniform and the more mechanical functions, simple "in voluntary" actions such as blinking, sneezing, and withdrawing the limbs from harmful stimuli. Such rhythmic functions as breath ing and orderly series of movements in processes such as swallow ing all fall easily under the general scheme. Many constituent acts in locomotion and in instinctive behaviour are similarly explained. But as yet no clear light is thrown on many important features of animal behaviour. All is exceedingly wooden. We may test the theory in more detail by inquiring into four such characteristics as variability, apparent spontaneity, educability and intelligence.
Clearly the conduct of an organism is far from comparable to the antics of a mechanical doll. Even the best established reflex has its refractory phase and is subject to in hibition. Variability takes several forms. A stimulus repeated may fail to elicit any overt response at all, different responses may be called out at different times, and the same response may be provoked in various ways and in greatly varying degrees. The organization by which a receptor may distribute the outcome of stimulation among many diverging tracts, and by which im pulses separate in origin may converge upon a final common path to some effector, provides a structural basis for such van ability. The problem, however, is far from being merely structural, nor are the prevailing paths of conduction, and their variations, adequately explained by the original distribution of resistance and accidental variation in conductivity, which changes in temper ature or nutrition, for example, might produce. The variations in question subserve self-conservation too efficiently to be wholly explained along these lines. They suggest the presence of some specific regulative machinery.
The fact that the uniformities of behaviour are only approxi mate, and formulated as tendencies, not as invariable laws, sug gests that we are only inadequately acquainted with the operative conditions. Generally, responses seem to depend upon multiple stimulation, or upon some receptive "pattern." Were this wholly
a matter of the external receptors there would still be unex pected failures of the organism to respond to an apparently adequate stimulus, on account of certain contributory factors being overlooked, or on account of other factors working in an an tagonistic sense. But sole dependence on the external receptors is (if real) the exception, not the rule. The majority of move ments depend on the co-operation of internal receptive organs inaccessible to direct observation. The dog reacts to food, but only if he is hungry, and "hunger" may for the present be conceived as implying only a certain organic state. The "touchi ness" of an organism "out of sorts" is proverbial. Changes in organic state would seem generally to disorganize (or adaptively re-organize) the conditions of collateral internal stimulation upon which our normal conduct depends To these factors must be added the progressive ripening and decay of instincts, involving no doubt concomitant modification of neural structure. The senile cat is unresponsive to things that fill a kitten with delight.
If spontaneity be taken to imply absolute independence of causal process, the notion can find no place in a system of thought which aims at explanation. On a less extreme interpretation, it has a certain descriptive value in relation to vital process. The causal relations between a self-conserving sys tem and its environment are essentially reciprocal and cyclical in nature. It is therefore inappropriate to endow either with the priority which the notion of stimulus and response on the one hand, and of absolute spontaneity on the other, inevitably sug gest. But the two notions are mutually corrective. The organism reacts to stimuli, but such reactions are frequently conditioned by preparatory adjustments, and themselves consist, as it were, in the search for further stimulation. The organism gives as much as it receives, and if it does not receive, it will go out to find. Absence of stimuli is the occasion of exploratory movements. to which the concept of spontaneity would most directly seem to refer. But whilst worthy of emphasis in this sense the notion will serve to justify neither the conception of action from a purely central origin, nor the assumption of an intruding "mind," to sup plement on occasion the function of the receptors. For facts which appear to call for such an interpretation the objective psycholo gist can find a ready explanation, in principle if not in detail. We wake to an alarm, but if the clock has stopped we wake "of our own accord." In this spontaneous awakening internal organic stimuli probably play their part, but even a purely cyclical pro cess of sleeping and waking would explain the facts. Spontaneity here refers to a causal process immanent in the organism. Many apparent cases of this sort, moreover, are probably due to variable factors delaying a response to prior stimulation. The influence of contributory internal stimulations invoked in the case of vari ability is also relevant here.