black swan 的词源

14世纪晚期，Juvenal ["Sat." vi. 164]中用于形容“极其稀有或不存在的事物”，但真正的黑天鹅后来在澳大利亚被发现（Chenopsis atratus）。

"Do you say no worthy wife is to be found among all these crowds?" Well, let her be handsome, charming, rich and fertile; let her have ancient ancestors ranged about her halls; let her be more chaste than all the dishevelled Sabine maidens who stopped the war—a prodigy as rare upon the earth as a black swan! yet who could endure a wife that possessed all perfections? I would rather have a Venusian wench for my wife than you, O Cornelia, mother of the Gracchi, if, with all your virtues, you bring me a haughty brow, and reckon up Triumphs as part of your marriage portion. [Juvenal]

“你说在这些人群中找不到配得上的妻子吗？”好吧，让她美丽、迷人、富有并且多产；让她的古老祖先环绕在她的大厅；让她比所有那些阻止战争的蓬头垢面的萨宾少女更加贞洁——在地球上如黑天鹅般稀有的奇迹！然而，谁能忍受一个拥有所有完美的妻子？如果你，哦，科尔内莉亚，格拉古的母亲，带给我一个傲慢的眉头，并把凯旋作为你婚姻财产的一部分，我宁愿要一个金星的女孩作为我的妻子，而不是你，尽管你拥有所有的美德。

White crow在英语中也是类似的比喻. Blue dahlia在19世纪也被用来形容“稀有和闻所未闻的事物”。