black swan 的詞源

典故指“极其稀有或不存在的事物”，14世纪晚期，来自于尤维纳利斯 ["Sat." vi. 164]，但真正的事物后来在澳大利亚出现 (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一个值得的妻子吗？" 好吧，让她既美丽，又迷人，富有而多产；让她有古老的祖先环绕在她的大厅中；让她比所有那些阻止战争的蓬头垢面的萨宾女子更端贞——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，格拉古斯的母亲，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，我宁愿要一个维纳斯的女孩做我的妻子，而不是你，尽管你拥有所有美德。

White crow在英语中也是一个类似的典型。 Blue dahlia在19世纪也被用来指“稀有且闻所未闻的事物。”