Today's Quintilian: 8.6.17-18

XVII. sunt et durae, id est a longinqua similitudine ductae, ut "capitis nives" et "Iuppiter hibernas cana nive conspuit Alpes". In illo vero plurimum erroris, quod ea quae poetis, qui et omnia ad voluptatem referunt et plurima vertere etiam ipsa metri necessitate coguntur, permissa sunt convenire quidam etiam prorsae putant. XVIII. At ego in agendo nec "pastorem populi" auctore Homero dixerim nec volucres per a‰ra "nare", licet hoc Vergilius in apibus ac Daedalo speciosissime sit usus. Metaphora enim aut vacantem locum occupare debet aut, si in alienum venit, plus valere eo quod expellit.

17. Some are harsh, that is, based on a resemblance not sufficiently close, as "The snows of the head," and,

Jupiter hibernas canâ nive conspuit Alpes,

Jove over the Alps spits forth the wintry snows.
But the greatest source of error in regard to this subject is that some speakers think whatever is allowed to poets (who make it their sole object to please and are obliged by the necessity of the meter to adopt many metaphorical expressions) is permissible also to those who express their thoughts in prose. 18. But I, in pleading, would never say the "shepherd of the people" on the authority of Homer, nor speak of "birds rowing with their wings," though Virgil, in writing of bees and of Daedalus, has used that phrase with great happiness. For a metaphor ought either to occupy a place that is vacant, or, if it takes possession of the place of something else, to appear to more advantage in it than that which it excludes.