Institutio Oratoria

Quintilian

Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.

He also points out that a metaphor must not be too great for its subject or, as is more frequently the case, too little, and that it must not be inappropriate. Anyone who realises that these are faults, will be able to detect instances of them only too frequently. But excess in the use of metaphor is also a fault, more especially if they are of the same species.

Metaphors may also be harsh, that is, far-fetched, as in phrases like

the snows of the head
or
  1. Jove with white snow the wintry Alps bespewed.
From Furius, an old epic poet of the second century (not Furius Bibaculus), cp. Hor. S. ii. v. 11.
v7-9 p.311
The worst errors of all, however, originate in the fact that some authors regard it as permissible to use even in prose any metaphors that are allowed to poets, in spite of the fact that tile latter aim solely at pleasing their readers and are compelled in many cases to employ metaphor by sheer metrical necessity.

For my own part I should not regard a phrase like

the shepherd of the people
as admissible in pleading, although it has the authority of Homer, nor would I venture to say that winged creatures
swim through the air,
despite the fact that this metaphor has been most effectively employed by Virgil to describe the flight of bees and of Daedalus.1 For metaphor should always either occupy a place already vacant, or if it fills the room of something else, should be more impressive than that which it displaces.