What does the white whale symbolize? And why?

Reading Comprehension: Read the quoted parts carefully and answer the questions in English. Write your answers in the corresponding space.Almost simultaneously, with a mighty volition of ungraduated, instantaneous swiftness, the White Whale darted through the weltering sea. But when Ahab cried out to the steersman to take new turns with the line, and hold it so; and commanded the crew to turn round on their seats, and tow the boat up to the mark; the moment the treacherous line felt that double strain and tug, it snapped in the empty air! 
What does the white whale symbolize? And why?
【正确答案】：

THE WHITE WHALE SYMBOLIZES NATURE FOR MELVILLE,FOR IT IS COMPLEX,UNFATHOMABLE,MALIGNANT.AND BEAUTIFUL AS WELL.

【题目解析】：对于麦尔维尔来说，白鲸象征着自然，因为它是复杂的、深不可测的、邪恶的，而且很美丽。