Even for such an accomplished writer of short stories as Jane Yolen, "De Natura Unicorni" is exceptionally fine work. She uses historical fact to ground her tale in reality. The blowhard Brother Bartholomaeus is an historical figure whose depiction of unicorns could have been taken for reality by his contemporaries—as if such animals might be found if one looked in the right place. Yolen further gives her tale a sound foundation by providing an understated version of a medieval court, with Duke William and his family and followers all fulfilling traditional roles like those in medieval English society. Into this mixture of real-life medieval folkloric beliefs and realistic castle life, Yolen adds modern touches that are subtle but likely to ring true to modern readers, adding to the verisimilitude of the setting and events. For instance, she notes that unicorns are virtually extinct&mdash...