The difficulty of assigning all things to the Lapides, Plantae or Animalia became increasingly evident in the mid- to late 19th century, especially between the latter two. Organisms, usually microscopic, combining features of both of the latter were discovered in the 19th century. As a result, it is now well known that John Hogg (1860) created a fourth kingdom, the Regnum Primigenum or Protoctista, meaning ‘first beings' because they were believed to have arisen before plants and animals. It is ironic that two of the main examples Hogg used to bolster his arguments were not valid illustrations of the problem. One example was the freshwater green sponge, Spongilla, an animal that was known to give off oxygen in the light. In fact, the photosynthesis is due to symbiotic ‘algae’. Another puzzle at the time was the nature of diatoms. Twenty-two years earlier, the great German microscopist Christian Gottfried von Ehrenberg had dogmatically asserted that they were tiny, ‘complete’ little animals (Infusionsthierchen als Volkomene Organismen), containing multiple stomachs (actually vacuoles) and other organs such as gonads (nuclei) and digestive glands (plastids). The ‘cell theory’ had not yet been formulated, and so there seemed to be no reason why such organs could not get smaller and smaller. In the 1840s, as the fundamental unitary nature of cells was being independently proposed by Mathias Schleiden and Theodore Schwann, Felix Dujardin showed that the cytoplasm (sarcode) of foraminiferans was not multicellular and Carl Theodor von Seibold established that they and other ‘Protozoa’ were unicellular.
As Whittaker & Margulis (1978) noted, many early classifications were essentially ‘top-down’ views of the living world, tracing plants and animals downward into plant-like and animal-like ‘lower organisms’. The German evolutionist, embryologist, microscopist, philosopher, artist and long-jump champion of Jena University, Ernst Haeckel, who was the strongest 19th century advocate of the distinctness of unicellular organisms, had a ‘bottom-up’ view, looking at the diversity of the living world from the earliest cells. He proposed that many groups evolved separately from the plant and animal lineages and, in 1866, only 7 years after the publication of Darwin's On the Origin of Species in 1859, named them members of the Protistenreich or Protist Kingdom, the Protista. Collectively, they were referred to as ‘protists' [see the review by Rothschild (1989) for the origins and uses of such names and Ragan (1997) for a more detailed history of the concept]. Although his tree showed them as a sister group to animals and plants, the choice of name and the text indicated that he thought they arose before animals and plants. At first, he included sponges and fungi but, in his later publications (Haeckel, 1894, 1904), he explicitly restricted Protista to predominantly unicellular organisms or colonies not forming tissues. Bacteria were (understandably) included, first as Mychota and later as Monerans. Corliss (1998a, b) has recently reviewed the development of the concept of kingdom Protista extensively. Ernst Haeckel is clearly the ‘Father of Protistology’ as we know it.
Clifford Dobell (1911) seemed to champion this approach in his polemic, The Principles of Protistology, but he was mainly concerned with countering the reductionist view of protists as mere unicells, emphasizing their functional completeness as organisms (like Ehrenberg), and his scope in this and later publications was clearly traditionally protozoological. Further, he championed an ‘acellular’ view of protists, rather than unicellular. A debate about the unicellularity of protists carried on for nearly half a century, summarized by Corliss (1989a), but electron microscopy unequivocally showed that, with the possible exception of multinucleate cells, they are homologous with the single cells of multicellular eukaryotes, but do everything with only one such unit. In fact, the SET made clear that eukaryotic ‘cells' with mitochondria are dyadic (digenomic) entities compared with prokaryotes and that those with plastids are triads (trigenomic; Taylor, 1974). This genomic complexity, arising from intracellular symbioses, was believed to be the source of major evolutionary novelty by Ivan Wallin (1927) in the same work in which he proposed a symbiotic origin for the mitochondrion. He viewed this as a new form of speciation termed ‘symbionticism’, considering it to be more important in evolution than mutation. These concepts have been evaluated by Taylor (1980a, 1983, 1987a), who concluded that he failed to convince others because of his excessive ‘overselling’ of symbionticism, and Sapp (1995). Using Mereschkowsky's term ‘symbiogenesis’, Margulis & Cohen (1994) discussed its possible potential for innovation in a paleo-evolutionary context, and Margulis & Sagan (2002) have promoted its importance as a speciation mechanism to a wider audience.
Herbert F. Copeland tried to bring the Protoctista back formally in 1938 and, in an extensive, thorough book (Copeland, 1956), offered a complete, unified reclassification of ‘lower organisms’, but his efforts were to no avail. Werner Rothmaler's system, with Protobionta as a comparable category, was also greeted largely with indifference (Rothmaler, 1948). Algae, fungi and protozoa were far too entrenched in their traditional departments to change radically and were formally supported by the International Codes of Botanical and Zoological Nomenclature. Twenty years later, the ecologist Robert H. Whittaker took the protistological baton and ran with it, reviving Haeckel's Kingdom Protista, together with a separate kingdom for Fungi (Whittaker, 1959), following it with a major article introducing his Five Kingdom system in the usually conservative pages of Science (Whittaker, 1969). However, both these and later versions, such as that with Margulis (Whittaker & Margulis, 1978), still retained polyphyletic groups such as zoo- and phytoflagellates as precursor branches of animal or plant protist lineages. Emphasis was on macroclassification rather than on actual protist affinities.
These bold, perceptive individuals kept the protistan concept going but couldn't accomplish a general fundamental change in biologists' thinking. After all, all biologists received the same training in ‘lower organisms’, usually strong in only the material taught by one or other department. However, when Paul Weisz responded to the demand for an integrative biology text to be used at the introductory university level, he responded with The Science of Biology, in which he followed Copeland and Whittaker and used Protists as a major group (Kingdom) in place of Protozoa, Algae and Fungi and uniting the two former. However, no sooner had students passed beyond first year, their conceptual framework regressed 50 years, back to the traditional categories of Algae, Protozoa and Fungi.
Protozoa were traditionally classified as the simplest phylum of the Invertebrata, even by the most authoritative invertebratologist of the first half of the 20th century, the American Libbie Hyman. As recently as 1991, at the insistence of the editor, protozoa still were (Corliss, 1991, with a caveat that they were included only for ‘completeness’). Protist groups are still being added to the Treatise on Invertebrate Paleontology. In a contribution on Protozoa for a symposium on ‘Invertebrate classification and phylogeny’, organized by Hyman in 1959, John Corliss concluded that: ‘the outlook for a good, defensible understanding of any phylogenetic relationships involving Protozoa is as discouraging as it ever was' (Corliss, 1959, p. 169).
The desire for change was strong in the late 1950s but the data needed were not yet at hand.