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.