Protozoa, Protista, Protoctista: what's in a name?

Abstract

A rose may still smell like a rose, but Juliet missed the point. The utility of taxonomic nomenclature lies the wealth of biological information that it conveys. This biological information is based on relatedness, and the post-Darwinian era relatedness is usually meant the genealogical sense. However, the fascination with nomenclature goes beyond this, for nomenclature is tempered by a curious amalgamation of pragmatism, priority, prejudice, sociology, and occasionally even humor. The division between the plant and animal kingdoms would appear to be the most basic taxonomic division. Yet, early workers realized the difficulties associated with classifying organisms that had characteristics of plants as well as animals, such as mobile photosynthetic organisms, and attached or otherwise sessile carnivores. Aristotle had conveyed this sense of taxonomic futility by pointing out that in the sea, there are certain objects concerning which one would be at a loss to determine whether they be animal or vegetable (Aristotle 1941:635). With the discovery of microbes, the class of taxonomically ambiguous organisms broadened. Still, taxonomists seemed more comfortable making what some admitted were arbitrary taxonomic decisions rather than challenging the plant/animal dichotomy. Then, within a decade, five workers from both sides of the Atlantic proposed the erection of another kingdom: Richard Owen (1859, 1860, 1861) designated it the Protozoa, John Hogg (1860) designated it the Regnum Primigenum (Protoctista), Thomas B. Wilson and John Cassin (1863) designated it the Primalia, and Ernst Haeckel (1866) designated it the Protista. It is only within the last twenty years, however, that the use of a separate kingdom has been widely adopted. This paper focuses on the uneasy taxonomic and phylogenetic position of the unicellular eukaryotes with respect to the animal and plant kingdoms. The emphasis will be on these early proposals for a separate kingdom and the responses they elicited.