The Raven and the Swan

A Raven saw a Swan, and desired to secure for himself a like beauty of plumage. Supposing that his splendid white colour arose from his washing in the water in which he swam, the Raven left the altars in the neighbourhood of which he picked up his living, and took up his abode in the lakes and pools. But cleansing his feathers as often as he would, he could not change their colour, while through want of food he perished.

Change of habit cannot alter nature.