Abstract

Evolutionary relations are better represented by new classifications than by the traditional two kingdoms. tionary directions (Fig. 1). In time the system seemed not reasonable but axiomatic; suggestions of other kingdoms were regarded as the idiosyncrasies of individuals. There were such suggestions, however, as the limitations of the two-kingdom system became more evident. I have reviewed proposals for other kingdoms in more detail elsewhere (5). Limitations of the Two-Kingdom System There are those who consider questions in science which have no unequivocal, experimentally determined answer scarcely worth discussing. Such feeling, along with conservatism, may have been responsible for the long and almost unchallenged dominance of the system of two kingdoms-plants and animals-in the broad classification of organisms. The unchallenged position of these kingdoms has ended, however; alternative systems are being widely considered (1-18) and are appearing in many introductory biology texts (19-24). My purpose in this article is to discuss the merits of two classifications which depart from the traditional two kingdoms, the systems of Copeland (1-3) and Whittaker (4, 5). Two-Kingdom System Man is terrestrial, and he sees around him two major groups of organisms of very different adaptation to nutrition on land-the photosynthetic, rooted, higher plants, and the food-ingesting, motile, higher animals. So distinct in way of life, direction of evolution, and kind of body organization are these groups that a concept of dichotomy-plants versus animals-is almost inescapable if they are considered by themselves. The two groups became the nuclei around which concepts of the plant and animal kingdoms were developed by early naturalists. The kingdoms have been part of the formal classification of living things since Linnaeus (25). Mosses, liverworts, and macroscopic algae are clearly plants in their photosynthetic and nonmotile way of life, and (though the photosynthetic process itself was not understood by early naturalists) these forms were grouped