forms must some day be supplemented by a parallel study of the real habits.
It is much more difficult to embrace in a single formula a habit which is composed of real actions, as is the case with economic phenomena, private life, politics; for we have to find in the different actions those common characteristics which constitute the habit; or, if this work has already been done in the documents, and condensed into a formula (the most common case), we must criticise this formula in order to make sure that it really represents a homogeneous habit.
The same difficulty occurs in constructing the formula for a group; we have to describe the characteristic common to all the members of the group and to find a collective name which shall exactly designate it. In documents there is no lack of names of groups; but, as they have their origin in usage, many of them correspond but ill to the real groups; we have to criticise these names to fix their precise meaning, sometimes to correct their application.
This first operation should yield formulæ expressive of the conventional and real characteristics of all the habits of the different groups.
In order to fix the precise extent of a habit we shall seek the most distant points where it appears (this will give the area of distribution), and the region where it is most common (the centre). Sometimes the operation takes the form of a map (for example the map of the tumuli and the dolmens of France). It will also be necessary to indicate the groups of men who practised each habit, and the sub-groups in which it was most pronounced.
The formula should also indicate the duration of