“To unknown yokes their brawny necks they yield, And, like tame oxen, plough the wondering field. The Colchians stare; the Grecians shout, and raise Their champion’s courage with inspiring praise. Emboldened now, on fresh attempts he goes, With serpent’s teeth the fertile furrows sows; The glebe, fermenting with enchanted juice, Makes the snake’s teeth a human crop produce.”

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This is generally interpreted as referring to the natural aspiration of the soul for higher things; characterized in Purgatorio XXI 1, as