Publications of the Nuttall Ornithological Club, No. 9. 127 pp. Avian classification and the data on which the taxonomy is based. Table of Contents: Applicability of the Biological Species Concept; Analytical Table of Species-level Taxa; Taxonomic Comments; Discussion and Conclusions: Problems of Species Delimitation; Polymorphism, Phenotypic Similarity; Hybridization; Evolutionary Intermediacy; Ecological Speciation; Recent Trends in Avian Classification.