One verse form, dactylic hexameter,epos, descends from Homer & Hesiod. It branches into modes that  invite analogies with the Three Cultural Folds that Homer incorporates into the model of daily life on the Shield of Achilles (Iliad 18). A three part division later develops also in reflection on styles of speaking & writing, viz high style, low style & what must be supposed to fall in between.
        In Homer, motifs of country life populate similes & other vignettes (e.g. the Shield of Achilles or the accounts of the Cyclops' pastoral life or the swineherd Eumaeus), while they become more central to Hesiod.
        When Theocritus shifts the focus of epos to the pastoral fringe (moving from the city, camp, & field to the former margins), he selects only three types of animals: cattle, sheep, & goats. Each has in reality its distinctive qualities & requirements for pasture & care, which allow the poet to stylize & to associate each with different inflections in poetry & love.
        To this reductive version of epos, then, Virgil replies by reopening the lines from the pastoral margins back towards the range of Hesiod & Homer, not only the georgic middle ground but the distant plane of the city, history, & war. Not by chance does Virgil in the first eclogue evoke the disruption of the old civic status & rural order through a protagonist, Meliboeus,  set by Virgil in a context underwritten by new myth, created in the story of  Tityrus' journey & meeting with a god at Rome, a story tantamount to the etiology of a new moment in the tradition of epos.