The assumption that in the behavior of original groups of men war arose as a natural result of the life of the group seems to be an allowable hypothesis. Whether warlike conduct came by some modification of the habits brought up from animal life as instinctive reactions, or whether man invented warfare from some strong motive peculiar to human life, and produced it intelligently, so to speak, under stress of circumstances may have to remain an open question so far as conclusive evidence is concerned. What we lack is a knowledge of the type and form of the instincts of man in his first stages, and the degree and kind of intelligence he had. But the reconstructed pre-human history of man so far as we can make it seems to show, as we have already suggested, that early man could have had no definite herd instincts or pack instincts such as some of the animals have, that his habits were plastic and guided by intelligence rather than by impulse. His social life, his predaceous habits, the habit of killing large game, his warfare must have been a gradual acquisition, and from the beginning have been very different as regards motive and development from animal behavior which judged externally may seem to be like it in character and to have the same ends. There are already inherent in any group of human individuals that fits into such knowledge of man past and present as we have, all the necessary motives of warfare in some form. There are the reactions of anger made to any threat or injury, fear, the predaceous impulse and habit, originating in hunger, the motives arising in sexual rivalry. These motives are the source of behavior toward both members of the group and outsiders. There is no absolute distinction between these objects. It is of the nature of man to be both aggressive and social. One instinct or motive did not come from the other, since there are emotions and desires at every stage that tend, some of them to unite and some to disrupt, the group. The sense of difference of kind and the fear of the strange on the one hand, and the effect of propinquity and practical necessity in the conduct in regard to the familiar on the other make the reactions different in degree in the two spheres but not different in kind. There is no aggressive instinct or war motive that is directed exclusively toward the outsider. Certain tendencies toward violence and strife, modified and controlled within the group, become unrestrained when directed toward the stranger. Among these motives are those of sexual rivalry, fear, anger, desire, and the play motive as an expression of any instinctive habits of aggression that may have been phyletically established.