The fern commonly called Cystopteris fragilis (L.) Bernh. var. mackayi Laws. (Dryopteridaceae) has been interpreted as either a variety of C. fragilis or as a hybrid resulting from introgression between C. fragilis var. fragilis and the predominantly southern hemispheric taxon C. diaphana (Bory) Blasdell. Neither of these two interpretations correctly reflects its evolutionary relationships; rather, var. mackayi appears to be a distinct species on the basis of its range, habitat, morphology, and cytology. Cystopteris tenuis (Michx.) Desv. is the earliest legitimate name for this plant at the specific level.