For the science-fiction or fantasy writer, the rules governing the genre serve as a reminder of the importance of discipline; without discipline, imaginative literature tends toward a hermetic expression more akin to madness than to art. And finally, without labels like science fiction and fantasy, we could not have the salutary experience of seeing our expectations confounded by writers who know that, in the long run, it is the business of the imagination to break all rules.

Poul Anderson's "The Merman's Children" is a case in point. The story, which is based on a medieval Danish ballad, depicts the dying out of the folk of Faerie—including the web-footed amphibian creatures of the title—as a direct consequence of the spread of Christian civilization. Rites of exorcism figure importantly in the plot. From this brief description, one might guess the book to be either a fantasy or a...