An Army in action is a special instance of what may be termed a _dynamic crowd_--that is, an assemblage of men united for common action and inspired by the same ideas and desires. Throughout history, certain characteristics have been exhibited by assemblages so united, however differently they were composed, and no matter under what conditions, or towards what ends, they were acting. Popular assemblies in epochs of change, spiritual ebullitions such as have marked the origin, or revival, of religions, political parties, and even juries, boards, and committees, all show similar phenomena. But the most striking instances of dynamic crowds are mobs collected for action. Such mobs have usually worked harm, when they must be called criminal mobs, but have often been violent with good intentions, as when the Paris mob took the Bastille. Other instances of non-criminal mobs actuated by high enthusiasm are those roused for the First Crusade by Peter the Hermit, or that incited by Mark Antony, at the funeral of Cæsar, to avenge his murder. Of nature akin to crowds are unions of persons holding similar opinions, even when not actually in touch with each other, or physically assembled. Such unions have produced, and will yet produce, some of the greatest changes in history, like the rise of Christianity, the Renaissance, the Reformation, and the French Revolution. In all ages, systems of religious thought have created such dynamic aggregates, often world-wide and endowed with persistent vitality. The most striking examples are the unions of nations and races which form the Mohammedan world, or the Roman Catholic Church. But a combination of armed men for the purpose of fighting has always been the commonest and strongest form of organized assemblage. Such a combination is the highest example of a dynamic crowd, and has effected the most rapid and striking changes which the world has undergone. The essential underlying character of a “dynamic crowd” of any sort is that it possesses _religion_ in the sense defined by a great French psychologist, as “placing all the resources of the spirit, all the submission of the will, all the ardour of fanaticism, at the service of a Cause, which then becomes the guide and end of all the ideas and actions of the assemblage.”