these states. The habits of men and the material conditions under which they live change from epoch to epoch; even when they appear to be constant they do not remain unaltered in every respect. There is therefore occasion to investigate these changes; thus arises the study of successive facts.
Of these changes the most interesting for the work of historical construction are those which tend in a common direction, so that in virtue of a series of gradual differentiations a usage or a state of society is transformed into a different usage or state, or, to speak without metaphor, cases where the men of a given period practise a habit very different from that of their predecessors without any abrupt change having taken place. This is evolution.
Evolution occurs in all human habits. In order to investigate it, therefore, it is enough to turn once more to the series of questions which we used in constructing a tabulated view of society. In respect of each of the facts, conditions, usages, persons invested with authority, official rules, the question is to be asked: What was the evolution of this fact?
This study will involve several operations: (1) the determination of the fact whose evolution is to be studied; (2) the fixing of the duration of the time during which the evolution took place (the period should be so chosen that while the transformation is obvious, there yet remains a connecting link between the initial and the final condition); (3) the establishing of the different stages of the evolution; (4) the investigation of the means by which it was brought about.
V. A series, even a complete series, of all the states of all societies and