 This material was of varying quality and comprehensiveness—while some of it was "finely worked", in other cases Ovid may have been working from limited material.In the case of an oft-used myth such as that of Io in Book I, which was the subject of literary adaptation as early as the 5th century BC, and as recently as a generation prior to his own, Ovid reorganises and innovates existing material in order to foreground his favoured topics and to embody the key themes of the "Metamorphoses".Scholars have found it difficult to place the "Metamorphoses" in a genre.