Besides those movements which cause locomotion those which effect seizure must no less certainly become thus adjusted. The molecular changes caused by absorption of nutritive matter from organic substance in contact, or from adjacent organic substance, initiate motions which are indefinite where the organization is low, and which become more definite with the advance of organization. At the outset, while the undifferentiated protoplasm is everywhere absorbent and everywhere contractile, the changes of form initiated by the physical stimulation of adjacent nutritive matter are vague, and ineffectually adapted to utilization of it; but gradually, along with the specialization into parts that are contractile and parts that are absorbent, these motions become better adapted; for necessarily individuals in which they are least adapted disappear faster than those in which they are most adapted. Recognizing this necessity we have here especially to recognize a further necessity. The relation between these stimulations and adjusted contractions must be such that increase of the one causes increase of the other; since the directions of the discharges being once established, greater stimulation causes greater contraction, and the greater contraction causing closer contact with the stimulating agent, causes increase of stimulus and is thereby itself further increased. And now we reach the corollary which more particularly concerns us. Clearly as fast as an accompanying sentiency arises, this cannot be one that is disagreeable, prompting desistance, but must be one that is agreeable, prompting persistence. The pleasurable sensation must be itself the stimulus to the contraction by which the pleasurable sensation is maintained and increased; or must be so bound up with the stimulus that the two increase together. And this relation which we see is directly established in the case of a fundamental function, must be indirectly established with all other functions; since non-establishment of it in any particular case implies, in so far, unfitness to the conditions of existence. In two ways then, it is demonstrable that there exists a primordial connection between pleasure-giving acts and continuance or increase of life, and, by implication, between pain-giving acts and decrease or loss of life. On the one hand, setting out with the lowest living things, we see that the beneficial act and the act which there is a tendency to perform, are originally two sides of the same; and cannot be disconnected without fatal results. On the other hand, if we contemplate developed creatures as now existing, we see that each individual and species is from day to day kept alive by pursuit of the agreeable and avoidance of the disagreeable.