The main action of the Iliad covers only a few days in the tenth and final year of the Trojan War, and the Odyssey describes the return voyage of only one of the Greek heroes, although both poems assume a much broader background of Trojan myth and they contain many allusions to stories not directly covered in the poems themselves. The exceptional quality of the Homeric poems seems to have impressed itself on their audience from the beginning, and it is understandable that poets in the century following their composition should have wished to compose epics covering the elements in Trojan mythology not already covered by Homer, and, in effect, fill in the gaps. And so it came about that a cycle of epics was composed which, taken together, built up a sequence narrating the entire history of the Trojan War. Although only a few fragments have survived, we know their general contents from a series of summaries attributed to a certain Proclus, and can see how they were constructed around the Homeric epics. Thus the origins of the war and all events up to the angry withdrawal of Achilles which marks the beginning of the Iliad were
covered in a single long epic, the Cypria; and then three shorter epics (partly overlapping in content) continued where the Iliad left off, covering the final period of the war and the sack of Troy. Then the Returns told of the return voyages of the surviving Greek heroes, except for Odysseus, and last of all, the later history of Odysseus was recounted in the Telegonia , which formed an eccentric supplement to the Odyssey . Although there is reason to think that by Homeric standards the artistic quality of these poems was not high, they were of great importance from a mythographical viewpoint for the part that they played in the establishment of a canon of Trojan myth. By selecting and ordering material from the oral tradition and earlier lays, and fixing it in long poems which were transmitted to future generations, the authors of such epics made a major contribution to the formation of standard histories of adventures like the Trojan War. The account of the war in the Library is ultimately dependent on these epics for its general structure and much of its contents. Other epics composed in the seventh century or somewhat later fulfilled a similar service with regard to other mythical episodes, such as the Theban Wars, or the voyage of the Argonauts (although, as we shall see, Apollodorus followed a Hellenistic epic for that adventure).
In his Theogony , Hesiod sought to organize the traditions concerning the gods into a coherent pattern by developing the comprehensive genealogical system which forms the basis of his poem. Beginning with a mythical cosmogony presented in genealogical terms, he tells of the origin and descent of the earlier gods and the establishment of Zeus as supreme ruler, and concludes with a catalogue of Zeus marriages and his offspring by his wives and other women. A supplement was added later which includes a catalogue of the children born to goddesses by mortal men. As a story-teller, Hesiod is short-winded and often clumsy, although his material is naturally of great interest. The approach adopted by Hesiod is largely determined by the peculiar nature of his subject matter; but later, probably in the sixth century, another poet composed a continuation to his poem extending the same approach to heroic mythology. This epic, which survives in fragments only, is known, somewhat misleadingly perhaps, as the Catalogue of Women , because the origin of each line is traced to the offspring of a god by a mortal woman. Its importance for Greek mythography cannot be emphasized too strongly; for it was here that the heroic genealogies were first ordered into a coherent pan-Hellenic system. The pattern of heroic genealogy which we find in the Library is still similar in general outline (although, of course, it often reflects later developments). And the Catalogue offered far more than sequences of names; for most names suggest a story, and the relevant narratives were inserted at the appropriate points in the presentation of the genealogies. This approach was subsequently adopted by prose mythographers and, as we have observed, it is in evidence in many parts of the Library .
Historians were prominent amongst the earliest prose writers. Some concerned themselves with purely local matters, but others (including those mentioned amongst the authors most frequently cited by Apollodorus) had broader ambitions and covered the traditions associated with many parts of the Greek world. They could not extend their researches any distance into the past without engaging with what we would regard as myth; and in the present context, it is their contribution to mythography which interests us. But they regarded themselves as historians, and while they were not always totally uncritical, they were willing to accept myth and legend as reliable sources of historical truth. In this respect, they differed from the scholarly mythographers of the Hellenistic era, who were critical in their attitude to myth and regarded mythography as a separate area of investigation. These earlier authors, whose qualities must be judged from fragments and testimonies, are sometimes referred to as logographers to distinguish them from more critical historians like Herodotus and Thucydides; but since this term (which simply meant prose writers in ancient usage) can be misleading if it is thought to describe a specific school of historians, it is safer to describe them merely as historians (or mythographer-historians or chroniclers).