Ovid works his way through his subject matter often in an apparently arbitrary fashion; however, the connection between all the seemingly unconnected stories is that all of them talk about transformation. Change as the only permanent aspect of nature is the certainty that underlies the work of Ovid, who jumps from one transformation tale to another, sometimes retelling what had come to be seen as central events in the world of Greek myths and sometimes straying in odd directions. The poem is often called a mock-epic. It is written in dactylic hexameter, the form of the great heroic and nationalistic epic poems, both those of the ancient tradition (the Iliad and Odyssey) and of Ovid's own day (the Aeneid). It begins with the ritual invocation of the muse," and makes use of traditional epithets and circumlocutions. But instead of following and extolling the deeds of a human hero, it leaps from story to story sometimes in very cunning ways, and, because of the clever ways in which it connects the stories, the Metamorphoses were once called the "Thousand and One Nights of the Ancient World"."