The gods were once disputing whether it was possible for a living being to change its nature.
Jupiter said "Yes," but Venus said "No." So, to try the question, Jupiter turned a Cat into a Maiden, and gave her to a young man for a wife.
The wedding was duly performed and the young couple sat down to the wedding-feast.
"See," said Jupiter, to Venus, "how becomingly she behaves. Who could tell that yesterday she was but a Cat? Surely her nature is changed?"
"Wait a minute," replied Venus, and let loose a mouse into the room.
No sooner did the bride see this than she jumped up from her seat and tried to pounce upon the mouse.
"Ah, you see," said Venus, "Nature will out."
Moral of Aesop's Fable:
"Nature will out"