A ball thrown into the air is acted upon by the initial force given it, persisting as inertia of movement and tending to carry it in the same straight line, and by the constant pull of gravity downward, as well as by the resistance of the air. It moves, accordingly, in a curved path. Now the path does not represent the working of any particular force; there is simply the combination of the three elementary forces mentioned; but in a real sense, there is something in the total action besides the isolated action of three forces, namely, their joint action. In the same way, when two or more human individuals are together, their mutual relationships and their arrangement into a group are things which would not be revealed if we confined our attention to each individual separately. The significance of group behavior is greatly increased in the case of human beings by the fact that some of the tendencies to action of the individual are related definitely to other persons, and could not be aroused except by other persons acting as stimuli. An individual in complete isolation would not reveal their competitive tendencies, their tendencies towards the opposite sex, their protective tendencies towards children. This shows that the traits of human nature do not fully appear until the individual is brought into relationships with other individuals.