On the Broad Classification of Organisms

A system of broad classification which recognized a plant kingdom of four divisions and an animal kingdom of ten to fifteen phyla was for many years stable and standardized. Significant changes have occurred, or are now proposed. Among these, three major lines of development are discussed: a. Classification of the algae has been fundamentally revised; seven or more algal series are distinguished primarily by characteristics of cells. The phylum concept, long established in zoological classification, has been brought into botanical classification in the systems of Pascher (1931) and Tippo (1942), in which the major algal series, the Bryophyta, and the Tracheophyta are regarded as phyla. b. Many authors have advocated recognition of kingdom of lower organisms, to meet the difficulty of dividing these between the plant and animal kingdoms. Two major possibilities for such a third kingdom are the Protista of Haeckel (1866, 1894), essentially identified with the unicellular organisms, and the Proctoctista of Hogg (1860) and Copeland (1947, 1956), comprising the nucleate, "acellular" organisms including protozoa, algae, and fungi. c. Study of the fungi has led to the view that these are probably derived from colorless flagellates as a line of evolution independent of true plants. The bacteria are better regarded as an ancient complex of many nutritive types, than as a group derived from the blue-green algae. In natural communities bacteria and fungi together form a major functional group (reducers) distinct from the green plants (producers) and animals (consumers). It is consequently appropriate to conceive the broad relations of the living world in terms of three modes of nutrition and directions of evolution rather than two-the photosynthetic of the green plants, the ingestive of the animals, and the absorptive of the bacteria and fungi. These three directions of evolution appear on three major levels of organization-the Monera, or bacteria and blue-green algae, which lack nuclear membranes; the Eunucleata, or unicellular organisms with nuclear membranes; and the multicellular and multinucleate higher plants, animals, and fungi. On this basis four kingdoms are here proposed: The Protista, or unicellular organisms; the Plantae, or multicellular plants; the Fungi; and the Animalia or multicellular animals. Among the Protista the subkingdoms Monera and Eunucleata and distinguished. Among the higher organisms the less widely successful lines of evolution into the multicellular and multinucleate conditions are recognized as the subkingdoms Rhodophyta and Phaeophyta among the plants, Myxomycota among the fungi, and Parazoa and Mesozoa among the animals. Other alternatives to the traditional two-kingdom system are discussed. Despite the general acceptance of the two-kingdom system, these alternatives have value in expressing current understanding of the broad relations among organisms. They should be judged in comparison with the two-kingdom system and with one another for their relative success in embodying these relations in a "natural" classification.