the habit. We shall look for the extreme cases, the first and the last appearance of the form, the doctrine, the usage, the institution, the group. But it will not be enough to note the two isolated cases, the earliest and the most recent; we must ascertain the period in which it was really active.
The formula of an evolution ought to indicate the successive variations in the habit, giving in each case precise limits of extent and duration. Then, by comparing all the variations, it will be possible to determine the general course of the evolution. The general formula will indicate when and where the evolution began and ended, and the nature of the change which it effected. All evolutions present common features which enable them to be divided into stages. Every habit (usage or institution) begins by being the spontaneous act of several individuals; when others imitate them it becomes a usage. Similarly social functions are in the first instance performed by persons who undertake them spontaneously, when these persons are recognised by others they acquire an official status. This is the first stage; individual initiative followed by general imitation and recognition. The usage becomes traditional and is transformed into an obligatory custom or rule; the persons acquire a permanent status and are invested with powers of material or moral constraint. This is the stage of tradition and authority; very often it is the last stage, and continues till the society is destroyed. The usage is relaxed, the rules are violated, the persons in authority cease to be obeyed; this is the stage of revolt and decomposition. Finally, in certain civilised societies, the rule is criticised, the persons in authority are censured, by the