Oral tradition is by its nature a process of continual alteration; hence in the established sciences only written transmission is accepted. Historians have no avowable motive for proceeding differently, at any rate when it is a case of establishing a particular fact. We must therefore search documents for statements derived from oral tradition in order that we may suspect them. We rarely have direct information as to statements being thus derived; authors who borrow from oral tradition are not anxious to proclaim the fact.[159] There is thus only an indirect method, and that is to ascertain that written transmission was impossible; we may then be sure that the fact reached the author only by oral tradition. We have therefore to ask the question: In this period and in this group of men was it customary to commit to writing facts of this kind? If the answer is negative, the fact considered rests on oral tradition alone. The most striking form of oral tradition is _legend_. It arises among groups of men with whom the spoken word is the only means of transmission, in barbarous societies, or in classes of little culture, such as peasants or soldiers. In this case it is the whole group of facts which is transmitted orally and assumes the legendary form. There is a legendary period in the early history of every people: in Greece, at Rome, among the Germanic and Slavonic races, the most ancient memories of the people form a stratum of legend. In periods of civilisation popular legends continue to exist in reference to events which strike the imagination of the people.[160] Legend is exclusively oral tradition. When a people has emerged from the legendary period and begun to commit its history to writing, oral tradition does not come to an end, but only applies to a narrower sphere; it is now restricted to facts which are not registered, whether because they are by their nature secret, or because no one takes the trouble to record them, such as private actions, words, the details of events. Thus arise _anecdotes_, which have been named "the legends of civilised society." Like legends they have their origin in confused recollections, allusions, mistaken interpretations, imaginings of all kinds which fasten upon particular persons and events. Legends and anecdotes are at bottom mere popular beliefs, arbitrarily attached to historical personages; they belong to folk-lore, not to history.[161] We must therefore guard against the temptation to treat legend as an alloy of accurate facts and errors out of which it is possible by analysis to extract grains of historical truth. A legend is a conglomerate in which there may be some grains of truth, and which may even be capable of being analysed into its elements; but there is no means of distinguishing the elements taken from reality from those which are the work of imagination. To use Niebuhr's expression, a legend is "a mirage produced by an invisible object according to an unknown law of refraction."