A word of warning may be added. We have repeatedly spoken of biological and psychological ends. By this we mean what seems to the observer, as an interpreter of natural processes, the purpose and object of their existence. But the word “end” is often used in such a way as to imply foresight and contrivance on the part of a rational being. We have not used it in this sense. Whether the whole of nature, including animal behaviour, is driven onwards to definite ends by an underlying Cause, is a metaphysical question. It is not one on which science has any right to express an opinion one way or the other. Science deals with the phenomena; the causes of their being lie outside her province.
CHAPTER VII
THE EVOLUTION OF ANIMAL BEHAVIOUR
I.—The Physiological Aspect
At the outset of our inquiry, we used the word “behaviour” in a wide and comprehensive sense. Thus broadly used, I said, the term in all cases indicates and draws attention to the reaction of that which we speak of as behaving in response to certain surrounding forces or circumstances which evoke the behaviour. The behaviour of living cells is dependent on changes in their environment; that of deciduous trees, as they put forth their leaves in the spring or shed them in the autumn, is related to the change of the season; instinctive, intelligent, and emotional behaviour are called forth in response to those circumstances which exercise a constraining influence at the moment of action. Used in this comprehensive sense, the term “behaviour” neither implies nor excludes the presence of consciousness. We know from our own experience, however, that consciousness does in some cases accompany behaviour, and we infer that in many other cases it may be present. But we need a criterion of its presence to guide our inferences, and this criterion we found in the ability of living beings to profit by experience. In Dr. Stout’s phraseology, if a thing seems to acquire meaning for such a being, and the behaviour is guided in accordance with such acquired meaning, we infer the presence of consciousness as supplying conditions effective in determining its course. Still this does not exclude, nay, rather it presupposes the presence of sentience at a lower stage of evolution, a sentience which is as yet ineffective since the process of conscious coalescence has not begun, or has not been carried far enough.
In foregoing chapters we have constantly held the problems of evolution in view, and in special sections directed attention to them. But the subject is so central to modern thought and discussion, that some further consideration of certain aspects of the evolutionary process and products will fitly serve to bring our inquiry to a conclusion.
We must accept, as a datum from the physiological point of view, the fact that protoplasm does respond to stimuli,—that it possesses the fundamental property of irritability. It is a substance that is in a state of unstable equilibrium. Its tendency to pass to a condition of more stable equilibrium is that in and through which organic behaviour in its very simplest expression is possible. And this, with progressive complication, runs through the whole gamut of animal behaviour, and eventually passes over into the sphere of consciousness. “The tendency to equilibrium,” writes Dr. Stout,[187] “is the physiological correlate of what on the psychical side we call conation,—the striving aspect of consciousness.” But, protoplasm at the outset—or as near the outset as we can get—is, in technical phrase, differentially responsive. The nature of the stimulus and the nature of the conditions decide what the nature of the response shall be. And even in that jelly-like speck of living matter, the Amœba, the responses conspire to a biological end. If they did not so conspire, we should not have the phenomena of life. The mere act of living, building up from food-stuff and oxygen an unstable substance which “explodes” and contracts under stimulation, implies that the processes which thus conspire are related in such a manner as to fulfil and secure their end. In higher unicellular animals, such as the Paramecium, the relations are less simple; but in them the continuance of that sum of organic behaviour which we call life, is secured only on the condition that these less simple relations are duly preserved, and that the vital processes conspire with sufficient unity of biological purpose. And when we pass to the higher creatures in which many cells unite to form one animal, the very word “unite” indicates that the vital processes of all must conspire with sufficient unity of biological purpose to insure the continued life of the whole.
Now, in all the higher and more active animals a nervous system is developed, which has for its purpose and end the preservation and furtherance of unity amid circumstances of progressively increasing diversity. And in the course of its evolution an added means of preserving and furthering the essential unity is provided in consciousness, which, through the coalescence of scattered units of sentience, leads behaviour to acquire a new and higher unanimity of purpose. Thus a mental evolution is engrafted on the organic evolution which precedes it. But every step in this mental evolution presupposes a step in organic evolution. And such is the complexity of structure and process in all the higher animals that much of the business of behaviour is relegated to quasi-independent nervous centres, which perform this business automatically, and will continue to perform it, with much subsidiary unity of end, when they are left to themselves and all connection with the supremely unifying sensorium has been severed.
Before proceeding to give some examples of this fact, and to indicate its bearing on our interpretation of behaviour, it may be well to state distinctly that no attempt is or will be here made to trace in detail the course of the evolution of animal behaviour through the ascending grades of life, nor, indeed, to prove that there has been any such evolution. Evolution by natural genesis is here assumed as the only hypothesis with which science has any concern. If it be false, then have the labours of workers and thinkers, since Darwin and Mr. Herbert Spencer worked and thought, been vain. Special creation is not a scientific hypothesis, but a reference of biological and mental phenomena to an ultimate cause, which lies beyond and altogether apart from the scope of scientific inquiry. The fundamental assumption of the man of science is, that any natural event he may select for detailed study has natural conditions and antecedents. And it is only in such detailed study—taking this or that particular occurrence and endeavouring to ascertain what were its related antecedents—that advance in the evolutionary interpretation of nature can be secured.
Such advance has been secured by the labours of those physiologists who have established by careful experiment the quasi-independent action of subsidiary nerve-centres as constituents of the nervous system as a whole. In such animals as the crayfish and the lobster the central nervous system consists of a chain of “ganglia,” or nerve-knots, which are connected together by nerve-strands. If these strands be cut between the thorax, which carries the walking limbs, and the abdomen or hinder portion of the body, the nerve-connection between these parts is severed. If the forepart be irritated through its sense-organs, the limbs of that part will respond; but, whereas an unmutilated crayfish, subjected to such irritation, would give a vigorous flap of the tail, this does not take place in the crippled animal.[188] Still, if the abdomen be irritated, it will respond by a strong and swift contraction. The two portions of the body are each capable of acting independently with well co-ordinated movements, but no longer of working together with unity of purpose. In the hinder portion the abdominal limbs, or swimmerets, all swing backwards and forwards simultaneously with rhythmic strokes; they act in concert. Sever now the connections between their ganglia, and each pair of limbs will continue to swing rhythmically but not with concerted rhythm. We have isolated a number of quasi-independent centres, and rendered them really independent. Each is concerned with its own proper co-ordination, but can no longer combine with others in a wider co-ordination. Mr. Hyde[189] has shown that in the king-crab, Limulus, when the nerve-chain is severed just in front of the abdominal region, the rhythmic respiratory movements of the abdominal segments still proceed regularly and co-ordinately. Even when only a fraction of the nerve-cord, separated by severances in front and behind, is left, corresponding with a single abdominal segment, the rhythmic action of that segment continues; but it is no longer synchronous with that of adjacent segments similarly isolated.