Of these changes the most interesting for the work of historical construction are those which tend in a common direction,[194] 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 of all their evolutions would not be enough to exhaust the subject-matter of history. There remains a number of unique facts which we cannot pass over, because they explain the origin of certain states of society, and form the starting-points of evolutions. How could we study the institutions or the evolution of France if we ignored the conquest of Gaul by Cæsar and the invasion of the Barbarians? This necessity of studying unique facts has caused it to be said that history cannot be a science, for every science has for its object that which is general. History is here in the same situation as cosmography, geology, the science of animal species: it is not the abstract knowledge of the general relations between facts, it is a study which aims at _explaining_ reality. Now, reality exists but once. There has been but a single evolution of the world, of animal life, of humanity. In each of these evolutions the successive facts have not been the product of abstract laws, but of the concurrence, at each moment, of several circumstances of different nature. This concurrence, sometimes called chance, has produced a series of accidents which have determined the particular course taken by evolution.[195] Evolution can only be understood by the study of these accidents; history is here on the same footing as geology or palæontology.