The incidents of a Fable ought also to have a real foundation in nature. This rule may be infringed by ascribing to creatures appetites and passions that are not consistent with their known characters. “A Fox should not be said to long for Grapes.”[9] The rule of Horace will hold universally—
Sed non ut placidis cœant immitia: non ut
Serpentes avibus geminentur, tigribus agni.
Delphinum Sylvis appingit Fluctibus aprum.
—Horace, I. 13.
To join the wild with creatures that are tame,
Serpents with birds, or tygers with the lamb,
Paint whales in woods, and wild boars in the sea,
Ah, what a motley piece the whole would be!