The only complete poem of the genealogical group is the “Theogony”, which traces from the beginning of things the descent and vicissitudes of the families of the gods. Like the “Works and Days” this poem has no dramatic plot; but its unifying principle is clear and simple. The gods are classified chronologically: as soon as one generation is catalogued, the poet goes on to detail the offspring of each member of that generation. Exceptions are only made in special cases, as the Sons of Iapetus (ll. 507-616) whose place is accounted for by their treatment by Zeus. The chief landmarks in the poem are as follows: after the first 103 lines, which contain at least three distinct preludes, three primeval beings are introduced, Chaos, Earth, and Eros–here an indefinite reproductive influence. Of these three, Earth produces Heaven to whom she bears the Titans, the Cyclopes and the hundred-handed giants. The Titans, oppressed by their father, revolt at the instigation of Earth, under the leadership of Cronos, and as a result Heaven and Earth are separated, and Cronos reigns over the universe. Cronos knowing that he is destined to be overcome by one of his children, swallows each one of them as they are born, until Zeus, saved by Rhea, grows up and overcomes Cronos in some struggle which is not described. Cronos is forced to vomit up the children he had swallowed, and these with Zeus divide the universe between them, like a human estate. Two events mark the early reign of Zeus, the war with the Titans and the overthrow of Typhoeus, and as Zeus is still reigning the poet can only go on to give a list of gods born to Zeus by various goddesses. After this he formally bids farewell to the cosmic and Olympian deities and enumerates the sons born of goddess to mortals. The poem closes with an invocation of the Muses to sing of the ‘tribe of women’.