Whatever the specific causes of war may be, war is of course possible only because there exists a mechanism or instinct or feeling, because of which great groups of people act as a unit in the common interests of all. We usually speak of this collective action as the result of _social instincts_ or a general _social instinct_. It is the place of this "instinct" in the causes and moods of war that we must consider. War is a social phenomenon: it is a movement directed toward an object, but the force that drives the movement is of course social. Several writers, among them MacCurdy (37), Murray (90), and Trotter (82), have dealt with this social aspect of war, and have interpreted war as a herd reaction. All these theories are simple. Trotter maintains that in man there are four instincts and no more: self-preservative, reproductive nutritional, and herd instincts. The peculiarity of the herd instinct is that it does not itself have definite motor expression, but serves to intensify and direct the other instincts. This herd instinct is a tendency, so to speak, which can confer instinctive sanction upon any other part of the field of action or belief. The herd instinct, for example, gives instinctive quality to the social organization and social proclivities of three different types of society, which appear as national characters. These are the wolf, the sheep, and the bee types. The aggressive type of social organization is represented by the Roman and now by the German civilization. This is a declining type, but it was because moral equality could not be tolerated in Germany that the rulers were obliged to cause Germany to revert to the primitive aggressive form of gregariousness. China would be a good example of Trotter's herd of the sheep type, for here the defensive instinct seems to be the dominating social reaction. War becomes, in such a herd, a great stimulus when, and only when, it is a threat to the whole nation, and when, therefore, the individual fears for the whole herd rather than for himself.