No historian worthy of the name can narrate the events even of a limited period without at least an inferential reference to the world-historic import of these events. Just in proportion as one fails to take a sweeping general view, the force of his facts is weakened; any narrow period of history, on which the attention is fixed, assumes, for the time being, a disproportionate interest, and is necessarily seen quite out of perspective. It is only when the limited period is considered in reference to other periods that it can be made to assume anything like its proper status. Something of this has been understood by all writers from the earliest times, and accordingly we find that very few of the ancient authors failed to take at least a sweeping view of contemporaneous events, even when detailing specifically the incidents of a restricted period; and often, as in the case of Herodotus, the space devoted to the history of events not strictly cognate to the main story is quite out of proportion to that reserved for the main story itself. Thus in a certain sense the history of Herodotus is a world history, inasmuch as it deals more or less comprehensively with practically all nations known to the Greeks of that time. Thucydides, as we have seen, confines himself much more closely to a precise text; yet even he devotes an introductory book to a summary of the past history of the Greeks as a preparation for the full understanding of the Peloponnesian War. But, after all, a somewhat sharp distinction should be drawn between histories such as these, which ostensibly describe the incidents of a particular period, and more comprehensive treatises, which set the explicit task of dealing with the history of all nations in all times. Of the works of this latter class,--World Histories proper,--the oldest one that has come down to us is at the same time probably the most comprehensive in scope, and the most extensive in point of matter, of any that was written in ancient times. This is the so-called Historical Library of Diodorus the Sicilian. Diodorus was a Greek, a native of Sicily, who lived during the time of Julius Cæsar and of Augustus. He set himself the explicit task of writing a comprehensive history of the world, and he devoted thirty years to the accomplishment of this task. This history, as originally written, comprised forty books, which treated of the entire history of mankind from the earliest times to the age of Augustus. Diodorus recognised the vagueness of early chronology, and he made no attempt to estimate the exact age of the world, but he computes the time covered by what he considers the historic period proper, in the following terms: