英语故事:The Raven and the Swan乌鸦和天鹅

A RAVEN saw a Swan and desired to secure for himself the same  beautiful plumage. Supposing that the Swan's splendid white  color arose from his washing in the water in which he swam, the  Raven left the altars in the neighborhood where he picked up his  living, and took up residence in the lakes and pools. But  cleansing his feathers as often as he would, he could not change  their color, while through want of food he perished.   
Change of habit cannot alter Nature.