This is one of Mr. Spencer's transitional passages. The infusorium is a moving equilibrium. Consequently it rearranges its forces for self-preservation in opposition to inimical forces of the environment, and in harmony with favoring forces of the environment. The special adjustment it displays is motion. But this is not communicated motion of a mechanical description, such as the kick given to a foot-ball. Nor, apparently, are we to regard its motions as due to a series of mechanical motions of the molecules of the environment. The action of the environment is expressed as being a stimulus. Does this mean a chemical action? Or does it refer to the action of heat and light? If so it means that the attractions and repulsions of atoms and the motions of ether and of molecules, account for the movements of the infusorium. There is certainly no "purpose" in such a theory. But then the question arises, how do we apply the theory of the moving equilibrium to such an assemblage of atoms thus acted upon to account for the fact that the assemblage of atoms endeavours to prolong its existence by defence and absorption or by absorption only? If it be said that it does not do so, and that its movements have no food object, but are simply the effect of chemical and mechanical action, then it is not an animal displaying life, inasmuch as it does not adapt means to an end--motions to the end of sustenance. If it be regarded as a moving equilibrium in this sense, it is one of the same sort as the solar system, and not one of the sort known as animals. Nevertheless, Mr. Spencer regards it as displaying life, yet very little adjusted to ends; but again he regards its actions as determined by external stimuli, without, however, explaining his meaning.