Since every individual creature has his needs that can be satisfied only by preying in some way upon other animals of his own species or others, the motives for strife are original in organic life. Every animal lives in a world of which he is suspicious, and rightly so. He is suspicious toward the members of his own kind and group, and toward all strangers he shows watchfulness and fear. There are two motives, therefore, of a highly practical nature that contribute to a general state of unfriendliness in animal life. Both the motives of conflict within the group, the habit of aggression and its complement, fear, and the jealousy and display motive (the display itself probably having originated as a show of ferocity on the part of males) must have been transferred to relations between groups as a natural result of the proximity of groups to one another, although this process is not quite so simple as this would imply, since in part the outside groups are produced by these very same antagonistic motives in the group, for example the driving out of young males because of sexual jealousy. The presence of other groups must have excited all the motives of warfare at a very early stage, and this contrast had the effect of stimulating the social feeling of the group and developing control of impulses on the part of individuals within the group toward one another. So the motives of combat, as shown within the group and toward outsiders, developed, so to speak, by a dialectic process.