Jove spat white snow upon the wintry Alps.The biggest mistake however is made by those who believe that everything is appropriate in prose which is permitted to the poets, whose only standard is pleasure and who are often forced into Tropes by the necessities of metre. Personally, I would not say 'shepherd of the people,' on Homer's authority, in a speech, nor speak of birds' 'swimming' through the air, though Vergil uses this very beautifully for the bees and for Daedalus. Metaphor ought either to occupy a vacant space or, if it replaces something else, to be more effective than the word it banishes.
-from "Metaphor," Book 8.6, Quintilian, The Orator's Education, IV, Books 6-8 (Loeb Classical Library No. 127), Cambridge: Loeb Classical Library, 2002.