Moreover, the close analysis of the actions of some of the lower organisms by Jennings has shown that the tactic hypothesis is probably false in the majority of cases. This observer studied the acting of the organisms themselves and not the beginning and end of the series, and he shows that the behaviour of the organisms is far more obviously described by saying that it adopts a method of “trial and error.” Let us suppose a number of infusoria (Paramœcium) in a film of water, at one part of which is a drop of acetic acid slowly diffusing out into the surrounding medium. There is a zone of changing concentrations round the drop: if we draw imaginary contours through the points where the concentration is approximately the same (the concentric rings in the diagram), and then draw straight lines normal to these rings (the radial lines) we can construct a “field” analogous to an electric or magnetic field.
Fig. 19. The animal on approaching the field ought to orientate itself and take the direction of the “lines of force.” It does not, however, behave in this way, but only enters the field at random. Having entered, it remains within a part where the concentration is within certain limits. If it approaches the margin of this limited field it stops, swims backwards, revolves round its own axis, and then turns to the aboral side; and it repeats this series of movements whenever it approaches (by random) a region where the concentration is too high, or one where it is too low. In this, and other organisms we see then what Jennings has called a typical “avoiding reaction,” the precise nature of which depends on the “motor-system” of the animal. Its general movements are random ones, but having found a region of “optimum conditions” (conditions which are most suitable in its particular physiological state), it remains there.
Suppose (what indeed repeatedly happens) that an extensive “bed” of young mussels forms on a part of the sea bottom. In a short time the bed becomes populated by a shoal of small plaice feeding greedily on the little shellfish. In their peregrinations the fishes must repeatedly pass out beyond the borders of this feeding-ground. Usually, however, they will return, for failing to find the food they like they swim about in variable directions and so re-enter the shellfish bed.
Suppose (this was really a fine experiment made by Yerkes) a crab is confined in a box from which two paths lead out but only one of which leads to the water. The animal runs about at random, finds the wrong path, retraces it, tries again and again, and then finds the right path and gets back to the water. If the experiment is repeated the animal finds the right path again with rather less trouble, and after many trials it ends by finding it at once on every repetition of the experiment.
All this discussion of concrete cases leads up to our consideration of the modes of acting in the higher organisms. On the strictly mechanistic manner of thinking the actions of the organism in general are based on reactions of the tactic kind—inevitable reactions the nature of which is determined, and which follow a stimulus with a certainty often fatal to the organism displaying them. Accepting these tactic reactions as, in general, truly descriptive of the behaviour of the organism, we can build up a theory of instincts. In their simplest form instincts are reflexes—tactic movements. In their more complex forms they are concatenated reflexes, or tactes. A complicated instinctive action is one consisting of many individual actions, each of which is the stimulus for the next one; or, of course, it may also be complex in the sense that several simple reactions proceed simultaneously, upon simultaneous stimulation of different receptors. Now the extension of all this to movements of a “higher” grade is obvious.
Let us note in the first place, that the stimuli so far considered in all the examples quoted are simple elemental ones. There are, of course, relatively few such stimuli: gravity, conducted heat (the kinetic energy of material bodies), radiated heat (the energy of the ether), electric energy, chemical energy, and mechanical contact or pressure (including atmospheric vibrations). In all these cases we have a definite, measurable, physical quantity, with which we must relate a definite response in the form of a definite measurable, physico-chemical reaction. There should be a functionality between the stimulus and response, a definite, quantitative energy-transformation. To take a concrete example, a certain quantity of light energy falling upon the receptor organs of Loeb’s caterpillar ought to transform into another quantity of “nervous energy,” and this travelling in an analogous way to a “wave of explosion” ought to transform into an energy quantity of some kind, which initiates another “wave of explosion” in the muscle substance. All these transformations must be quantitative ones, and the energy of the individual light must be traced from the receptor organ to the points in the muscle where it disturbs a condition of false equilibrium in the substance of the latter. Nothing less than this is required to demonstrate the purely physical nature of a reaction, on the part of the organism, to an external stimulus. It may safely be said that physiological investigation has not yielded anything even approximating to such an experimental demonstration.
What are the stimuli to the actions of a higher organism? It is true that their elements are energies such as we have indicated, but these energies are integrated to form individualised stimuli (Driesch). The stimulus in an experimentally studied taxis is, perhaps, a field of parallel pencils of light rays of definite wave length; but in the action of a man, or a dog say, the stimulus is an immensely complicated disturbance of the ether, producing an image upon the retina of the animal. A sound stimulus employed in an investigation may be the relatively simple atmospheric disturbance produced by the sustained note of a syren or violin-string; but the stimulus in listening to an orchestra may consist of dozens of notes, with all their harmonies, sounding simultaneously at the rate perhaps of some hundred or two in the minute. All these are integrated by the trained listener, and one or two false ones among the multitude may entirely spoil the effect of the execution. Surely there is here something more than a mere difference in degree.
More important still is the strict functionality between stimulus and action that the theory of tactic responses imposes on itself. Putting this very precisely (but no more precisely than the theory demands), we say that ΣA = f(x, y, z), that is, the series of actions ΣA (the dependent variable) is a mathematical function of the independent variables x, y, z. Now is there anything like this functionality between the acting of the higher animal and the stimulus? Evidently there is not. We recognise someone whom we know very well by any one of a hundred different characters, mannerisms of walk, speech, dress, etc. He or she is the same person, whether seen close at hand, or afar off, or sideways, or in any one of almost infinitely different attitudes, and we respond to each of these very different physical stimuli by the same reaction of recognition: pleasure, dislike, avoidance, greeting, or whatever it may be. To a sportsman shooting wild game the stimulus may be some almost imperceptible tint or shading in cover of some kind, differing so little from its environment as hardly at all to be seen, yet, to his experience, upon this almost infinitesimal variation of stimulus depends his action with all its consequences. In Driesch’s example two polyglot friends met and one says to the other, “My brother is seriously ill,” or “Mon frère est séverèment malade,” or “mein Bruder ist ernstlich erkrankt.” Here the physical stimulus is fundamentally different in each case, but the reaction—the expressions of sympathy and concern, the discussions of mutual arrangements, etc., are absolutely the same. Or let the one friend say to the other, “My mother is seriously ill,” and in spite of the very insignificant difference between the consonantal sound br in this sentence and the corresponding sound m in the other English sentence, the reaction, that is, the subsequent conversation, and the arrangements between the two friends may be entirely different.
Putting this argument in abstract form we may say, generally, that two stimuli, which are, in the physical sense, entirely different from each other, may produce absolutely the same series of reactions; and conversely two stimuli differing from each other in quite an insignificant degree may produce entirely different reactions. It is also easy to see, by analysis of the antecedents to the actions of the intelligent animal, that these stimuli are, in the majority of cases, not elemental physical agencies, but individualised and integrated groupings of these agencies; and that the animal reacts, not to their mathematical sum, as it should do on a purely mechanistic hypothesis of action, but to the typical wholes which are expressed in these groupings.[22]