"Roman satirists may be thought of as functioning within a triangle of which the apices are (a) attack, (b) entertainment, and (c) preaching. If a poem rests too long on apex (a) it passes into lampoon or invective; if it lingers on (b) it changes into some form of comedy; and if it remains on (c) it becomes a sermon." Niall Rudd, Themes in Roman Satire
What is striking and original about Rudd's application of this theoretical structure for satire is the fact that he sees a good deal of movement within individual pieces; the effect is on of hovering and flitting, like a bird that never alights. (Which is why my bird traps on the ground keep turning up with nothing more than handfuls of feathers, I suppose - time to construct a bow and arrow.)