A broad line of distinction has been drawn between "the old or _artistic_ type of history," and the new or _sociological_ type which belongs to the present century. The ancient historians represented the former type. They prized literary form. They aimed to interweave moral and political reflections. Polybius often interrupts his narrative to introduce remarks of this sort. But they were not, as a rule, diligent and accurate in their researches. And, above all, they had no just conception of society as a whole, and of the complex forces out of which the visible scene springs. The Greeks were the masters in this first or artistic form of history. The French Revolution was one stimulus to a profounder and more comprehensive method of studying history. The methods and investigations of natural science have had a decided influence in the same direction. THE SOURCES OF HISTORY.--History must depend for credence on credible evidence. In order to justify belief, one must either himself have seen or heard the facts related, or have the testimony, direct or indirect, of witnesses or of well-informed contemporaries. The sources of historic knowledge are mainly comprised in _oral tradition_, or in some form of _written records_. _Tradition_ is exposed to the infirmities of memory, and to the unconscious invention and distortion which grow out of imagination and feeling. Ordinarily, bare tradition, not verified by corroborative proofs, can not be trusted later than the second generation from the circumstances narrated. It ceases to be reliable when it has been transmitted through more than two hands. In the case of a great and startling event, like a destructive convulsion of nature or a protracted war, the authentic story, though unwritten, of the central facts, at least, is of much longer duration. There may be visible monuments that serve to perpetuate the recollection of the occurrences which they commemorate. _Institutions_ may exist--popular festivals and the like--which keep alive the memory of past events, and, in certain circumstances, are sufficient to verify them to generations far removed in time. Events of a stirring character, when they are embodied in _songs_ of an early date, may be transmitted orally, though in a poetic dress. Songs and legends, it may be added, even when they do not suffice to verify the incidents to which they refer, are valuable as disclosing the sentiments and habits of the times when they originated, or were cherished. The central fact, the nucleus of the tradition, may be historical when all the details belonging with it have been effaced, or have been superseded by other details, the product of imagination. The historical student is to distinguish between traditionary tales which are _untrustworthy throughout_, and traditions which have _their roots in fact_. Apart from oral tradition, the sources of historical knowledge are the following:--