A Third Kingdom of eukaryotic life: History of an idea

Arch. Protistenkd. 148 (1997): 225-243

ARCHIV FUR

© by Gustav Fischer Verlag

PROTISTEN KUNDE

A Third Kingdom of Eukaryotic Life: History of an Idea ISEP President's Address MARK

A. RAGAN

Canadian Institute for Advanced Research, and NRC Institute for Marine Biosciences, Halifax, Nova Scotia, Canada

Protozoa and algae are not "imperfect" animals and plants, but instead represent independent lines of eukaryotic development. This appreciation came only recently, historically speaking. ARISTOTLE'S duality of plant and animal kingdoms was universally accepted for two thousand years, even by the early pioneers of protozoology such as ANTONY VAN LEEUWENHOEK, LOUIS JOBLOT and O. F. MULLER. But the relentless discovery of protozoan taxa throughout the eighteenth and early nineteenth centuries made the two-kingdom system increasingly unwieldy; thus during 1860-1866, amidst the intellectual ferment catalyzed by CHARLES DARWIN'S Origin of Species (1859), four somewhat different Third Kingdoms were proposed to accommodate the "overflow" of microscopic organisms - among them Protista (HAECKEL 1866,1:203), the basis ofthe name of our Society. The above historiography, while reasonable at first glance, is seriously misleading. In fact, the first "Third Kingdom" intermediate between plants and animals was introduced not in 1860, but fifteen centuries earlier. Compared with studies on polyps and corals, the burgeoning proto zoological bestiary played a surprisingly minor role in forcing the issue of a Third Kingdom in the eighteenth and early nineteenth centuries; indeed, EHRENBERG'S views on the internal structure of protozoa were used to justify maintaining the animal-plant dichotomy well beyond its time. Nonetheless, several late-eighteenth century scientists, most notably LINNAEUS, broached the possibility of a Third Kingdom that would include organisms we now know as protists, and the first formal proposals to this effect were made early in the nineteenth century. These precedents were unknown to, or ignored by, the authors of the 1860-1866 Third Kingdoms - among whom only HAECKEL seems seriously to have engaged Darwinian evolution.

In this essay I begin to reconstruct a historically accurate account of the idea that Protista constitute a third kingdom of eukaryotic life. Like any great idea, its semantics, meaning and intellectual context have changed dramatically over the centuries. Protista is a nineteenth-century concept, eukaryotic twentieth-century. LINNAEUS was attacked by his contemporaries for claiming that plants are alive. And with today's emphasis on monophyly, we must acknowledge that Protista is polyphyletic (or, at best, a group made paraphyletic by the exclusion of animals, plants and fungi) - not a happy situation for an aspiring Kingdom. But let us start at the beginning.

The classical era From the earliest times, tribes and peoples around the world represented their kinship with nature in totemistic and animistic ways. Gods and monsters were said to have arisen from the sky, the mountains, the ocean, or the depths of the earth. Mythologies became populated with animal deities, gods that metamorphose into animals, and witches that transform the innocent into beasts. Tales abounded of centaurs, satyrs, mermaids and werewolves; such, indeed, is the origin of our word chimaera. The boundaries of the natural kingdoms were anything but fixed. Thinkers such as ANAXIMANDER, HERACLITUS, PYTHAGORAS, PARMENIDES, EMPEDOCLES, DEMOCRITUS, GORGIAS and SOCRATES developed new modes of thinking about nature (DE SANTILLANA 1961: 21-207). This development - the beginning of scientific thought reached its apex with ARISTOTLE who, in the fourth century BCE, organized what was known about living organisms into a coherent system that reflected his

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broader view of the nature of the universe. ARISTOTLE taught that each organism is constituted of form (its organizing principle, or soul) and matter, and he divided organi sms into plants and animals. All organisms possess a nutritive soul; animal s additionally have a sensitive soul that provides sensation and indirectly accounts for movement, and humans further possess a rational soul (ARISTOTLE, De Anima: 413a-415 a) . ARISTOTLE set out an explicit method of logically dividing organisms into genera and immutable species (PANCHEN 1992: 109-114) but, so far as we know, did not actually develop an exhaustive categorization of living things. For ARISTOTLE, "Nature proceeds little by little from things lifeless to animal life in such a way that it is impossible to determine the exact line of demarcation, nor on which side thereof an intermediate form should lie. Thus , next after lifeless thing s in the upward scale come s the plant, and of plants one will differ from another as to its amount of apparent vitality; and, in a word , the whole genus of plants, whilst it is devoid of life as compared with an anim al, is endowed with life as compared with other corporeal entities. Indeed, as we ju st remarked, there is observed in plants a continuous scale of ascent toward s the anim al. So, in the sea, there are certain objects concerning which one would be at a loss to determine whether they be animal or vegetable ..." (Historia Animalium: 588b 4-13). ARISTOTLE (De Partibus Animalium: 68P 1O--681 b 13) elaborated further on these "objects". Sponges are "virtually plants and nothing more" (68 P 10-12); he did not commit himself on wheth er other sessile, apparently sensible organi sms (ascidians, holothurians) are plant or animal, but intimated no doubt that they must be one or the other. For him, there was no contradiction between, on one hand, the continuous intergradation from "lifeless objects" to animal s (68P 12-14) and, on the other, the animal-plant dichotomy. Boundaries need not be sharply drawn (HULL 1985: 43-45, 51); organisms above some (unspecified) point in the continuum possesses a sensitive soul, and are called animals. Three hundred years later, LUCRETIUS CARUS (De Rerum Natura II: 699-716) explicitly denied the possi bility of animal -plant chimaeras. PLINY THE ELDER (PLINIUS SECUNDUS) hinted at a third kingdom: "even those creatures which have not got the nature of either animal s or plants, but some third nature derived from both , possess sense-perception - I mean jelly-fish and sponges" (Historia Naturalis IX: 68). But PLINY did not carry the argument further. Organisms which share external feature s of both animals and plants eventually became known as zoophytes. This word first appeared in the latter half of the third century CE, in the writings of SEXTUS EMPIRICUS, a member of the so-called Third School of Scepticism. In about 43 BCE AENESIDEMUS OF KNossos had estab-

lished ten modes of thought, or tropes, by which judgement could be suspended, thereby leading to "an untroubled and tranquil cond ition of soul" (SEXTUS EMPIRICUS, Outlines of Pyrrhonism I: 10). SEXTUS EMPIRICUS paraphrased AENESIDEMUS 'S first trope, which establishes relativity of perception, as follow s: " ... Thefirst argument (or trope), as we said, is that which shows that the same impressions are not produced by the same objects owing to the differences in their origins and from the variety of their bodily structures. Thus, as to origin, some animals are produced without sexual union , other s by coition. And of those produced without coition , some come from the fire, like the animalcules which appear in furnaces, others from putrid water, like gnats ; other s from wine when it turns sour, like ants; others from earth, like grasshoppers; others from marsh, like frogs; others from mud, like worms; others from asses, like beetles; others from green s, like caterpillars; other s from fruits, like the gall insects in wild figs; others from rotting animal s, as bees from bulls and wasps from horses ..." (Outlines of Pyrrhonism I: 40-41 ). The term SEXTUS EMPIRICUS used, quite casually it seems, for animalcules (which appear in furnace s), was zoophyton. I ) Two hundr ed years later, this term reappeared among the Alexandrian school of neoplatonism, among whom AMMONIUS SON OF HERMIAS (AMMONIUS HERMIAE) was a dominant figure. The neoplatonists attempted to bring the teachin gs of ARISTOTLE into accord with those of PLATO. Their metaphysics involved organizing reality on the basis of triad s, and this carried over into many facets of their treatment of nature. Thu s AMMONIUS, commenting on PORPHYRY 'S Isagoge (itself a commentary on ARISTOTLE'S Categoriae), divided all being into corporeal and incorporeal; corporeal being into living and nonliving; and living ("the ensouled") into animal, plant and zoophyte: " ... for the plant has only three powers, nutrition, growth and reproduction and the animal has these [three powers] and sensory power and locomotion from place to place; and so the zoophyte is in the middle between both of these , for it has the three powers and the sense of touch , not, however, changing from place to place, these are the oysters and the sponge s; for they grow naturally on rocks ..." (AMMONIUS, In Porphyrii Isagogen 77, lines 18-24). 1) Since completing this manuscript, I have learned that scholarly opinion is divided on whether zoophyton is a correct, or incorrect, reading of this passage in SEXTUS EMPIRICUS. If incorrect, the first usage of zoophyte would be that of IAMBLICHUS (died ca early 320s to 330 AD), as preserved in a commentary by his student DEXIPPUS [BUSSE; A. (ed.): Dexippi Philosoph i Platonici in Aristoteli s Categorias Commentarium. Commentaria in Aristotelem Graeca vol. IV, pars I, Porphyrii Isagoge et in Categorias Commentarium, page 49, line 16 (Berlin 1887)]. I hope to consider these issues in greater deta il elsewhere.

AThird Kingdom ofEukaryotic Life After the middle of the fifth century CE, teachings of pagan philosophers fell into official disfavour. Some, however, were preserved by incorporation into commentaries of Christian philosophers. Thus the usage of zoophyte introduced by AMMONIUS, a pagan, turns up in the works of Christian neoplatonists such as DAVID, ELIAS, and JOHN PHILOPONUS OF ALEXANDRIA; later still, it appears in the Fragmenta Philosophica of, and the De Animate attributed to, [SAINT] JOANNES OF DAMASCUS in the eighth century (e.g. Dialectica, ed. 1559: 493). It presumably occurs in subsequent Arabic texts and commentaries, although I am not aware of relevant scholarship. But to my knowledge, there is no evidence that Arabic (or other) scholars advanced biological classification beyond the systems of ARISTOTLE and AMMONIUS, and the zoophytes made a formal reappearance only in mid-sixteenth century Europe (below).

The Middle Ages and Renaissance Latin translations of ARISTOTLE'S works became available in Europe in about 1200 CEo Some, including his writings on natural history, fell afoul of conservative Parisian ecclesiastics, and were banned (ineffectually, it seems) in 1210, 1215 and 1231 (LINDBERG 1992: 216-218). ALBERTUS MAGNUS (1193?-1280) was a key figure in their subsequent rehabilitation, becoming "the effective founder of Christian Aristotelianism" (LINDBERG 1992: 229). Commenting on the passage in Historia Animalium quoted above, ALBERTUS MAGNUS noted that in comparison to the non-living, plants are alive, but compared to animals they are not; and that fungi inhabit a middle ground between the non-living and plants, as do sponges between plants and animals, and children between beasts and man (Quaestiones super De Animalibus VII: 2). Elsewhere ALBERTUS MAGNUS (De Principiis Motus Processivi I: 2) treated sponges, jellyfish and molluscs as animals, and remarked on sponges' plant-like mode of nutrition. His student THOMAS AQUINAS identified oysters as "the lowest of the animal kind scarcely surpass(ing) the life of plants" (Summa contra Gentiles II: 68). During the twelfth and subsequent centuries many other classical texts were reintroduced to Europe, but ARISTOTLE and PLATO remained unchallenged as arbiters of matters natural-historical. According to LOVEJOY (1936: 59), "(t)he result was the conception of the plan and structure of the world which, through the Middle Ages and down to the late eighteenth century, many philosophers, most men of science, and, indeed, most educated men, were to accept without question - the conception of creation as a scala naturae, or 'Great Chain of Being'."

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Variously building on (and occasionally arguing against) the Aristotelian tradition, PIERRE BELON, JOACHIM CAMERARIUS, ANDREAS CESALPINO, LEONHARD FUCHS, CONRAD GESNER, PHILIPPUS PARACELSUS, ANDREAS VESALIUS and others laid the foundations for modem natural and medical sciences during the sixteenth century. In De Differentiis Animalium, EDWARD WOTTON systematized ARISTOTLE'S implicit classification of animals, but departed from a strict reading by setting up within ARISTOTLE'S anaema ("bloodless animals") a new group, Zoophyta, for organisms that neque animal perfectum, neque planta, sed ambigit atque ancipiti est natura, e.g. holothurians, starfish, jellyfish, sea anemones and sponges (WOTTON 1552: 218-220). GULIELMI RONDELET (1554-1555, 2: 107) recommended zoophyta as being the Greek term that best captures the ambiguity of organisms quae nee animalium nee fruticum, sed tertiam ex utroque naturam. ULYSSE ALDROVANDI (1606: 563-564) credited zoophyta to SEXTUS EMPIRICUS, and generally followed RONDELET in treating jellyfish, sea anemones, holothurians and similar marine invertebrates as zoophytes quae inter plantam & animal natura ambigunt. ARISTOTLE'S successor THEOPHRASTUS (De Plantis I, 6:5 and IV, 6:1) had treated seaweeds and fungi as plants, and in this he was typically followed by Renaissance authors including HIERONYMUS BOCK (1539), REMBERT DODOENS (1583), ANDREAS CESALPINO (1583), JACQUES DALECHAMPS (1586-1587), ADAM ZALUZIANSKY VON ZALUZIAN (1592), GASPARD BAUHIN (1623), JEAN BAUHIN (1650-1651), and JOHN RAY (1686-1704, 1724). CESALPINO placed lichens and fungi, which he thought lacked fruiting organs, at the bottom of the plant scale nearest the minerals; ZALUZIANSKY VON ZALUZIAN (1592: II: ii and iii) considered fungi, lichens, seaweeds and sponges to be ruda et confusa. The tidy (if occasionally uncomfortable) division of all creation into discrete Aristotelian realms did not, however, go unchallenged, as tales of wondrous beasts and men - some new, some recycled from book VII of PLINY'S Historia Naturalis - found a wide audience during the Middle Ages. ALBERTUS MAGNUS'S erstwhile pupil THOMAS OF CANTIMPRE (Liber de Natura Rerum), JOHN MANDEVILLE (1465), and C[K]ONRAD "VON MEGENBERG" (1475; see also PFEIFFER 1861) wrote of dog-men, ox-men, bird-men and other "wonder-people". In the twelfth century, reports circulated in northern Europe of a Hebridean tree that produces barnacles, from which hatch out geese (LEY 1968: 97 ff.). C[K]ONRAD VON MEGENBERG reported that these geese had been consumed as vegetables during fasts, a practice finally prohibited in the mid-thirteenth century by POPE INNOCENT IV. JOHN GERARD provided his own eyewitness account of the goose-tree (1597), and a descrip-

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tion could be found in editions of ADAM LONITZER'S Kreutterbuch (1546) until the late eighteenth century. JOHN PARKINSON illustrated another animal-plant hybrid, the barometz, or "vegetable lamb", in his Paradisi in Sole (1629). While generally sceptical, ROBERT HOOKE conceded that ..." though the difference between a Plant and an Animal be very great, yet I have not hitherto met with any so cogent an Argument, as to make me positive in affirming these two to be altogether Heterogeneous, and of quite differing kinds of Nature: And besides, as there are many Zoophyts, and sensitive Plants (divers of which I have seen, which are of a middle nature, and seem to be Natures transition from one degree to another, which may be observ'd in all her other passages, wherein she is very seldom observ'd to leap from one step to another) so have we, in some Authors, Instances of Plants turning into Animals, and Animals into Plants, and the like ... (fjor we do not know, but that the Omnipotent and All-wise Creator might as directly design the structure of such a Vegetable, or such an Animal to be produc'd out of such or such a putrifaction ..." (1665: 124). Seventy years later CARL VON LINNE (1735) felt it necessary to deny the existence of the barometz and the barnacle-goose-tree. THOMAS BROWNE (1642: I: 34) focussed the Aristotelian tradition on human development in commenting that"... next we live the life of Plants, the life of Animals, the life of Men, and at last the life of Spirits". WILLIAM HARVEY (1653: 89) expressed a similar view: "About [the fourth day] the Egg beginneth to step from the life of a Plant, to the life of an Animal." This idea seems never to have resurfaced, even amongst the intracacies of OKEN and HAECKEL; but it did presage the role that ontogeny was to play in defining Kingdoms.

The late seventeenth and eighteenth centuries JOHN RAY famously restated the Aristotelian scala naturae in his preface to Methodus Plantarum Nova (1682: [ii]): "Nature, as the saying goes, makes no jumps and passes from extreme to extreme only through a mean. She always produces species intermediate between higher and lower types, species of doubtful classification linking one type with another and having something in common with both - as for example the socalled zoophytes between plants and animals". The continuum was finding mathematical respectability in the calculus of ISAAC NEWTON and GOTTFRIED LEIBNITZ; the subsequent hundred years were to be the age of continuum in biology (although not yet in geology). Free-living freshwater protozoa were first observed by VAN LEEUWENHOEK in the summer of 1674 (DOBELL 1932: 220); apparently unimpressed, he later com-

mented that "these little animals were the most wretched creatures that I have ever seen" (DOBELL 1932: 118). LEEUWENHOEK observed Eimeria in the autumn of 1674; Paramecium, Astasia, Vorticella and Chilodon(ella) in 1678, Giardia in 1681, and opalinids in 1683. From the first, he referred to his dierken as being animals, with muscles, tendons, joints and tails (DOBELL 1911: 305), although admittedly no alternative terminology was available. BERNARD DE FONTENELLE (1687, "Third Evening"), popularizing the principle of plenitude, wrote of drops of water crowded with petits po issons ou petits serpents que l' on n' aurait jamais soupconnes d'y habiter. New species of infusoria continued to be discovered through the eighteenth century. In the first monograph on microscopic organisms, JOBLOT (1718) illustrated what is now known as Paramecium, and demonstrated that infusoria were not produced abiogenically. Noctiluca, Pelomyxa, Vorticella, Stentor, Cyclidium, Encheiys, Lachrymaria, Volvox and Actinosphaerium were described in the succeeding decades (CORLISS 1992), and in 1765 HEINRICH AUGUST WRISBERG introduced the term Infusoria. Infusoria may have been the terrae australes of the animal world (BONNET 1803: 235), but they continued to be classified among the animals, typically as worms (LINNAEUS 1758: 820-821) or insects (DE REAUMUR 1742; ROSEL VON ROSENHOF 1755: 617-621). BENTLEY GLASS (1959: 33) wrote that LEEUWENHOEK'S observations, plus new species brought back from the voyages of discovery, meant that "old systems of classification were splitting at the seams" by the late 1600s. But so long as infusoria remained petits poissons ou petits serpents (or little worms and insects), the seams of biological systematics would remain intact, and the "old systems" outlived LEEUWENHOEK by a century. But the world of "insects" was to change dramatically. In 1723 JEAN ANDRE PEYSSONAL discovered that "flowers of coral" were in fact minute animals; to protect him from ridicule, RENE-ANTOINE DE REAUMUR condensed PEYSSONAL'S report and published it anonymously (DE REAUMUR 1729; RITTERBUSH 1964: 129). But soon thereafter, studies on the "water insect" Hydra (TREMBLEY 1743, 1744) "threw the biological world of France and England into a ferment" (CROCKER 1959: 116). Hydra perfectly epitomized the conceptual animalplant, on the one hand budding, regenerating, sometimes green in colour, yet on the other hand capable of movement and responsive to touch; and zoophytes were immediately acclaimed as the true link between plants and animals, the final confirmation of the Great Chain (BUFFON, 1750: II, 8-9; LA METTRIE, ed. 1936). CHARLES BONNET abandoned his quadripartite division of nature (BONNET ed. 1781: 42) in which vegetables ("inanimate structured beings") were distinguished

AThird Kingdom ofEukaryotic Life from animals ("animate structured beings"), and replaced it with a bipartite system of living vs nonliving (BONNET ed. 1783: 235; cf. ANDERSON 1982: 48-54). LAMARCK (1801: 2-3) later adopted the same dichotomy. JEAN-BAPTISTE ROBINET (1766: 1-2) rejected even BONNET'S modest demarcation, declaring that "there are only individuals, and no kingdoms or classes or genera or species". Various third kingdoms (zoophytes, cryptogams, fungi) could have been recognized within the continuum, and indeed NATALIS JOSEPH NECKER (1783: 103-104) proposed for fungi a Regnum mesymale intermediate between plants and minerals. Nonetheless, the boundaries of such additional kingdoms would necessarily be elusive; and with the widespread optimism that further "missing links" necessarily exist amongst the plenitude of nature, third kingdoms could only become increasingly difficult to define and defend.

LINNAEUS

Some of the first hesitant steps toward erecting a third kingdom of life came not from France, but from the pen of CARL VON LINNE. LINNAEUS recognized a scala naturae (RITTERBUSH 1964: 113-114; LINDROTH 1983: 16-20), and he quoted (without attribution) JOHN RAY 'S exact words "Natura ... non facit saltus" (LINNAEUS 1751: aphorism 77). Like ARISTOTLE, he divided the continuum based on a principle of superaddition ("Minerals crescunt. Vegetables crescunt et vivunt. Animals crescunt, vivunt et sentiunt. Therefrom are the limits among these Kingdoms established": LINNAEUS 1735). By mid-career, though, LINNAEUS clearly realized that the scala naturae could not be a simple linear continuum: "Nature herself associates and joins Minerals and Plants and Animals; but she does not join the most perfect plants with the least perfect animals, but combines imperfect animals and imperfect plants", e.g. seaweeds with diatoms, fucoids with bryozoa and sertularians, seaweeds with tubularia and polyps, etc. (LINNAEUS 1751: aphorism 153). Although LINNAEUS did not elaborate, his formulation suggests a bifurcating relationship with multiple points of association between simple animals and simple plants. Relationships within the vegetable kingdom were likewise complex; plants show affinities on all sides, not as a simple continuum but rather as territories in a geographical map (LINNAEUS 1751: aphorism 77). This map was later realized by P. D. GISEKE, editor of LINNAEUS'S posthumous Praelectiones (LINNAEUS 1792: 623-627; PANCHEN 1992: 17). As LINNAEUS developed his concept of metamorphosis, he was led to characterize zoophytes as "composite animals that generate blossoms and have a vegetative

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stalk" (LINNAEUS 1758: 799) that are "dually constituted of animal and plant" (1758: 643), characterizations expanded upon the following year in Animalia Composita (LINNAEUS 1759: 8-9). Within Animals the sixth and last class was Vermes (worms), and within Vermes the two orders most proximate to plants were lithophytes (corals), which inhabit a mineralized structure, and the zoophytes, whose medulla belongs to the vegetable kingdom, the flower-like animal to the animal kingdom (LINNAEUS 1759: 7-8; 1767: 390-393; BASTER 1761). The zoophyte medulla thus literally metamorphoses from plant to animal. A few years later LINNAEUS corresponded with OITo VON MONCHHAUSEN who, as subsequently described in his wide-ranging Der Hausvater (1768: 899-901), had observed that Ustilago maydis and other fungi propagate via "living dust". Under favourable conditions this "dust" yields up small "worms" which, in tum, re-form the fungal body. For these and other "zweifelhafte Producte der Natur" (polyps, corals, fungi and lichens) VON MUNCHHAUSEN proposed a new kingdom, Regnum neutrum or "Das Mittelreich" (1766: 253-259, 745-753), constituting the middle ground among plants, animals and minerals. LINNAEUS (1767: 404) commented that "as Zoophytes blossom forth from vegetables into Animalcules, so conversely do Fungi mature from Animalcules into Vegetables, if indeed the genesis of Fungi be confirmed to be so". The same year, RUDOLPH ERIC RASPE and JOHN ELLIS reported similar observations on other fungi (RITTERBUSH 1964: 139-140). LINNAEUS concluded that fungi might indeed have to be referred to a third kingdom, Regnum chaoticum (LINNAEUS 1767: 396). LINNAEUS classified some simple organisms (Volvox. later adding Furia and Chaos) first as Cryptogamia Fungi among the vegetables (LINNAEUS 1735), then as Vermes Zoophyta among the animals (LINNAEUS 1758: 820-821). But he questioned whether "Moleculae vivae, animalia infusoria" were "living bodies informed by organs, or something rather different, such as a salt, an oil, etc." which likewise belonged in the Chaotic kingdom (LINNAEUS 1767: 395-396). In reply, O. F. MOLLER (1786: vi-vii) avowed that infusoria have hearts and intestines, and that he himself on hundreds of occasions had observed their "spontaneous" and "voluntary" movement, their "judgment" in choosing direction, their avoidance of threats and crowding - observations that "annihilate the neutral and chaotic kingdom which brief observation of the infusoria has fashioned". Other would-be members of this kingdom, including the vegetable fly (RITTERBUSH 1964: 138-139) were revealed as spurious, and save for a short-lived reappearance (for zoophytes minus nematoid worms: RUDOLPHI 1819: 572) , Regnum chaoticum was abandoned.

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Unease with the scala naturae If nature were a single linear chain with zoophytes at the plant-animal interface, where did infusoria belong? Although LINNAEUS had briefly treated infusoria as zoophytes (above), most eighteenth-century naturalists did not (e.g. PALLAS 1766; ELLIS & SOLANDER 1786). There were other problems too. If polyps truly linked plants and animals in a linear chain, it became difficult to account for "plantlike" features of more-perfect animals. LINNAEUS (1758: 819) had written of the "stirps ... floriferis" of the tapeworm Taenia, and the regenerative abilities of worms and starfish were well-known. "Sensitive plants" such as Mimosa were surely at the apex of the plant scale nearest the polyps, and authorities from CESALPINO (1583) to BONNET (1781) placed seaweeds and fungi at the bottom, just above the minerals; this made VON MONCHHAUSEN'S observations (above), and those on algal zoospores (below), all the more puzzling. At midcentury, VITALIANO DONATI (1750: xxi-xxii) suggested that a parallel scala naturae may occur among marine creatures; earthworms populate the animal-plant interface in the terrestrial chain, hydras and similar zoophytes in the aquatic. BONNET knew of DONATI'S work, BONNET'S relative TREMBLEY visited DONATI in Turin in 1755 (BAKER 1952: 142-144), and a German translation of Della Storia Naturae was published in 1753, dedicated to PIERRE MAUPERTUIS; but with few exceptions (e.g. DE BLAINVILLE 1825: 6), there was little appetite for parallel series. Few other options remained open. DENIS DIDEROT (ed. 1937: 134) ignored plants altogether in writing that "(o)ne must begin by classifying beings, from the inert molecule, if there is one, to the living molecule [i.e., cell], to the microscopic animal, to the plant-animal, to the animal, to man". JOSEPH PRIESTLEY (1779: 1, 342) speculated that the coating of algae that appears on the sides of water-filled vessels "can neither be of an animal or vegetable nature, but a thing sui generis, and which ought, therefore, to be characterized by some peculiar name".

The end of the scala naturae: France Although sometimes criticized for its vestiges of scholasticism, the Histoire Naturelle, Generale et Particuliere (1749-1767) of GEORGES-LoUIS LECLERC, COMTE DE BUFFON enjoyed immense popularity in its time. In this work (particularly its Premier Discours) BUFFON waged an ultimately unsuccessful attack on Linnean (indeed all) biological systematics, which he viewed as artificially and arbitrarily dismembering the continuum that links all individuals in nature (SLOAN 1976). As mentioned above, he acknowledged the existence of " ... beings that are nei-

ther animals, nor vegetables, nor minerals, and that we will vainly attempt to place in one or the other" (cited by WILSON & CASSIN 1864: 120). Upon BUFFON'S death in 1788, LOUIS-JEAN-MARIE DAUBENTON, MARIE-JEAN-ANTOINE-NICHOLAS CONDORCET and other French naturalists led an ultimately successful attack upon Buffonian histoire naturelle. Although DAUBENTON (1800) rejected a linear scala naturae, he nonetheless agreed that "(t)he polyps, the acetabularian, the infusory animals, have they not an organization that differs sufficiently from that of most animals to be given another name? Are the seaweeds, fungi, molds and lichens true plants? ... there exists a great quantity of organized beings that are neither true plants nor true animals. It will only be through observations and meditations that we will be able to distinguish clearly the true plants and the true animals from other organized beings that differ enough to merit another determination and another rank in the methodical division of the products of nature" (DAUBENTON 1800: 277). The years immediately following brought political upheaval, abolition of the Academie des Sciences, and the establishment of the Museum Nationale d'Histoire Naturelle (1793) with the establishment of specialized chairs in zoology. JEAN-BAPTISTE-PIERRE-ANTOINE DE MONET DE LAMARCK, previously distinguished for his classificatory work with plants, was elected to the chair of "insects, worms, and microscopic animals" (CORSI 1988: 13); he could scarcely have avoided playing a central role in further developing the idea of a third kingdom of life. LAMARCK accepted a single gradual scale of organization along which the major families are continuous and "regularly spaced", from which genera and especially species ramified as lateral branches (LAMARCK 1815: I, 330; ed. 1984b: 416). In 1800 he referred to coral-forming polyps as zoophytes (LAMARCK ed. 1984b: 420), and stated that "the polyps are in relation to other animals what the cryptogamic plants are to plants of other classes" (LAMARCK ed. 1984a: 37; ed. 1984b: 416-417). Later he rejected the term zoophyte as absurd, obnoxious (LAMARCK ed. 1984a: 200) and very inappropriate (LAMARCK 1815: 390) for organisms that are "completely animal". LAMARCK initially grouped eight infusorial genera (Trichoda, Trichocerca, Cercaria, Colpoda, Vibrio, Proteus, Volvox, Monas) as Polypes Amorphes ou Microscopiques (LAMARCK 1801: 390-397). Subsequently he moved Les Infusoires from the polyps to Animaux Apathiques (LAMARCK ed. 1984a: 130, 134-135; 1815: 381, 392-450; 1835: 337-437). In his famous prototree (LAMARCK ed. 1984a: 179) the infusorians, polyps and radiate animals stand apart as a separate, much abbreviated series that appeared long before the second animal series was established (LAMARCK 1815: 455).

AThird Kingdom ofEukaryotic Life WILLIAM SHARP MACLEAY (1821: 199) quoted LAMARCK as stating that "it is certain that if the vegetable kingdom could be shown to connect itself or pass into the animal kingdom by any points of their respective series, it would be by those alone which are the most simple in their organization; so that the passage from the least perfect plants to the least perfect animals would be quite insensible. All naturalists have perceived this truth; and in fact it is in such a point, namely, where organization is the most simple, that animals appear to approach nearest to plants. Now, if the chasm which separates the kingdoms at these points be imperceptible, we shall be obliged to admit that instead of forming a chain, plants and animals present two distinct branches, united at their base like the two branches of the letter V." LAMARCK, however, did not accept that there were any such points. GEORGES CUVIER was the first to classify animals entirely on the basis of comparative functional anatomy. By 1812 he recognized four major embranchements, each seemingly modelled (semblent avoir ite modeles) on its own structural plan. Within each branch there might be occasional series of limited scope, but unlike GEOFFROY SAINT-HILAIRE (1818) he denied the existence of an unbroken scala naturae, calling the idea not only "erroneous" but "unimaginably harmful" (1817: 1, xx-xxi). Each organism and taxon must be considered in its own terms, through its characteristics and organization, not in terms of archetypes (CUVIER & VALENCIENNES 1828: 1, 568-569). Relatively simple forms coexist with relatively complex ones within each branch, and no smooth series exists even among classes (cf. AGASSIZ 1859: 31-33). CUVIER established Les Infusoires as the fifth and final order in his fourth embranchement, Les Zoophytes ou Animaux rayonnes (1817: 4, 89); CARIUS (1827: 35, 46-47), DUJARDIN (1841) and many others followed his example. AGASSIZ (1859: 145-146) later commented that since the structural plan of infusoria was little-known in CUVIER'S day, CUVIER'S classification of Infusoria was actually based on their simplicity. CUVIER exercised substantial power in the French, and especially Parisian, scientific community, and his quadripartite division of animaly was widely accepted. An exception was HENRI DE BLAINVILLE, who put forward a "rationalist" tripartite division, replete with novel terminology, based entirely on external features. BLAINVILLE (1816: 107; ApPEL 1980: 300, 308) nonetheless recognized infusoria (Agastraires) as the last class in his animal system. JEAN-BAPTISTE-GEORGES-MARIE BORY DE SAINT-VINCENT viewed nature as a network of links, within which some organisms may be intrinsically unassignable to one major group or the other (BORY 1824b: 658). Such organisms specifically included sponges, polyps, corals

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and other zoophytes, as well as vorticellids and diatoms, for which he established Regne Psychodiare, the kingdom of "two-souled" organisms (BORY 1824b: 657-663; 1828: 329-335). BORY wrote that the very existence of zoophytes had dispirited many naturalists, but had prompted quelques bons esprits to seek to establish a third kingdom; LAMARCK had sensed, but had not established, his improperly delimited Animaux apathiques as such (BORY 1824b: 659-660). BORY'S Psychodiare contained three classes (Ichnozoaires, Phytozoaires, Lithozoaires). He supposed that members of this kingdom (psychodies) appeared in creation before plants and animals tdurent apparoitre les premiers dans le creation: BORY 1824b: 661-662). For BORY as for LINNAEUS, the psychodies vegetans et vivans successivement (BORY 1824b: 659; 1825: 247). BORY did not intentionally include infusoria in Regne Psychodiare. Like GMELIN (LINNAEUS 1788: I (4): 3021-3028) and LAMARCK (1815: I: 407-450), BORY retained infusorial animalcules in Regne Animal, calling them Microscopiques (BORY 1824a: 515-543; 1826: 533-546). This group encompassed 82 genera in 17 families and five orders (Gymnodes, Trichodes, Stomoblephares, Rotiferes, Crustodes); the Gymnodes included bacteria, green algae, monads, amoebae, ciliates and cercaria. Among the Microscopique are les etres par lesquels la nature semble s' etre essayee aproduire la vie; he viewed these organisms as the sources of diverse higher animal classes, and of rudimentary and primitive vegetation (BORY 1826: 534), a concept that strongly presages HAECKEL'S monera (below).

The end of the scala naturae: England ERASMUS DARWIN proposed that all organic life up to and including man could have arisen from "one living filament" or a "microscopic animalcule" in the sea (E. DARWIN 1801: II, 233-244; 1803: Canto I, lines 295-334). His views depart from those of BONNET, however, in that DARWIN did not position infusoria within a linear series between animals and plants: "... (a)t the same time new microscopic animalcules would immediately commence wherever there was warmth and moisture, and some organic matter, that might induce putridity. These situated on dry land, and immersed in dry air, may gradually acquire new powers to preserve their existence; and by innumerable successive reproductions for some thousands, or perhaps millions of ages, may at length have produced many of the vegetable and animal inhabitants which now people [sic] the earth" (E. DARWIN 1803: Canto I, note at line 327). It seems but a short step from ERASMUS DARWIN'S vague formulation to a bifurcated chain in which man is not

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the sole Crown of Creation. Elsewhere, he wrote that plants have sensation and volition (DARWIN 1800). Even more than LINNAEUS, DARWIN came under scathing attack from his critics. In a 560-page response, the psychologist THOMAS BROWN (1798: 245-262) objected to (among numerous other perceived faults) DARWIN'S attribution of animal properties to plants; and JOHN MASON GOOD (1831: 201) mocked him for deriving man from "the race of oysters". A "barbarous state of semi-civilization ... far worse than absolute ignorance" brought on by servile, superstitiously intolerant disciples: such was MACLEAY'S (1819: xxi) characterization of the legacy of LINNAEUS. More explicitly than ERASMUS DARWIN, MACLEAY proposed a bifurcation in nature. "Organized Matter may be generally described as ramifying into two branches which represent the animal and vegetable kingdoms, and which touch one another very nearly, if not precisely, at those points where the organization of each is the least complicated", specifically linking algae and infusoria, groups which differed only in the supposed presence in the latter of an intestinal canal. Thus we are "forced to acknowledge that like the letter V plants and animals present two distinct branches united at their base". The "great object" of Horae Entomologicae was to "trace one of the ramifications of this dichotomous tree to its extreme fibres" (MACLEAY 1821: 194-200). MACLEAY divided the animal kingdom into five "tribes", one of which "cannot in the present state of knowledge be better described than as masses of a transparent homogeneous, mobile, and sensible pulp ... This last division I propose to name Acrita" (MACLEAY 1821: 201-202, 224). Except for their locomotion and irritability, the acrite animals might be classified among the lower plants (MACLEAY 1821: 215). He apportioned the acrita (monads, infusoria, voticellids, various polyps, rotifers, and Taenia) into five subgroupings, and upon arranging them in a cycle, discovered correspondences that convinced him that his system truly conformed to nature. He recognized similar cycles for each of the other four animal groups, and moreover could arrange all five cycles into a supercycle describing the animal kingdom (MACLEAY 1821: 203-210, 318). Proceeding halfway around the animal supercycle clockwise from the infusoria regenerated LAMARCK'S series of inarticulated animals (LAMARCK 1815: 1,457), while proceeding in the other direction gave LAMARCK'S series of articulates (MACLEAY 1821: 318, 331-333). Unlike THOMAS BROWNE (1658) and LINNAEUS (1766), who before him had "discovered" fivefold arrays in Nature, MACLEAY (1821: 321-323) seemed almost apologetic about the recurrence of this number. His system of five-membered cycles, with bridging ("osculant") groups and manifold analogies between parts, collectively consituted the so-called quinarian system

(MACLEAY 1821: 318) that briefly found vogue in England (SWAINSON 1835: 201 ff.; HULL 1988: 92-96). He sketched, but did not develop in detail, an analogous supercylce for plants (MACLEAY 1821: 210-212).

Germany: from Naturphilosophie to EHRENBERG

N aturphilosophie, "the scientific incarnation of German romanticism" (GOULD 1977: 35), grew out of philosophical and literary traditions that emphasized themes of growth and development (BOEHME), a vital spirit (VAN HELMONT), monadology (LEIBNIZ), death and regeneration (KANT), the unity of nature (GOETHE), and man as the self-conscious embodiment of the World Spirit (SCHELLING). In his Ideen zur Philosophie der Geschichte der Menschheit (1784: 265), JOHANN GOTTFRIED VON HERDER embraced the Great Chain of Being and gave it a naturphilosophischen twist: "(f)rom stones to crystals, from crystals to metals, from these to plants, from plants to animals and from animals to man, we see the form of organization ascend; and with it the powers and propensities of the creature become more various, until finally they all, so far as possible, unite in the form of man" (translation by LOVEJOY 1959: 208-209). JOHANN WOLFGANG VON GOETHE (1807: 254) wrote that "... from a kinship in which plants and animals can scarcely be distinguished, have emerged creatures that gradually perfected themselves in two opposite directions, such that the plant attains its glory in the tree, durable and immobile, the animal in man, of the highest mobility and freedom." The Danish naturalist OTTO FRIDERICH MULLER (1786: xxvi-xxvii) collected seventeen genera of (mostly) unicellular organisms under the heading Infusoria; in so doing, he initiated a new phase of protozoology (DUJARDIN 1841: 4, 9-11). The two pages immediately preceding the introduction of Infusoria are replete with Naturphilosophie: vital fluid and living spirits; the cycle of life, death, decomposition and rebirth; the unity of nature (MOLLER 1786: xxiv-xxv). The appearance of GOTTFRIED TREVIRANUS'S Biologie, oder Philosophie der lebenden Natur (1802-1822) was a landmark in the emergence of biology from natural history. Large sections of this work treated biogeography, reproduction, physiology and nutrition, movement and the nervous system, as well as Naturphilosophie (although ontogeny was afforded relatively little coverage). TREVIRANUS recognized separate kingdoms (Reiche) of animals, zoophytes, and plants. Nitrogen predominates in animals, whose parts have dissimilar textures and structure; plants have a predominance of carbon, with body parts of homogeneous texture and structure. Nitrogen predominates in zoophytes also, but

AThird Kingdom ofEukaryotic Life their body parts, like those of plants, are homogeneous (TREVIRANUS 1802: 1: 165). Zoophytes were "the Urformen from which all the organisms of the higher classes had arisen by gradual development" (quoted by HAECKEL 1924a: 100-101). TREVIRANUS further divided zoophytes into animalplants (sea stars, actinoids, sea feathers, corals, gorgonians and Infusionsthiere) and plant-animals (fungi, confervae, fuci, lichens, hepatics, mosses, ferns and Najadales) (TREVIRANUS 1802: 1, 399-425). Infusoria represented the "minimum" of animal organization, as sponges (Schwdmme), algae and lichens did of plant organization (p. 447). He recognized an "unbroken gradation" in both the animal and plant kingdoms, but only for the totality of diverse organs and for the size and number of certain specific parts; in other respects, he noted, nature instead shows oppositions or broken steps (pp. 447-448). Although a few decades later these observations could have illuminated an analysis of descent, TREVIRANUS instead mobilized them in elaborating his idea that "not a single, but thousands and many thousands of chains, which with infinite art have been interlaced into the tightest knots, comprise the totality of Nature" (p. 475). Like TREVIRANUS, LORENz OKEN embraced an ascending scale of organic perfection while denying the existence of an actual linear series in nature. He interpreted the natural world in reference to animals (element-animals, mineral-animals, plant-animals, and animal-animals), animals in reference to man ("the whole Animal Kingdom ... is naught else than Man disintegrated": OKEN 1847: 2), and man in reference to God (and viceversa). OKEN arranged animals into two grades and four or five cycles; within the first grade and cycle, Infusoria were variously analogized with "the mucous primary vesicle", "a galvanic point", the vitellus (pp. 189-190), a simple stomach (p. 491) or intestine (p. 498), "the animal semen of the planet, the animal dissolved" (p. 512), monads (p. 570), and "viscera or visceral nerves" (p. 655); plants and animals are only "metamorphoses of infusoria" (p. 189), and death "is only a continuous growth through retrogression into the organic primary matter or Infusoria" (OKEN 1805; 1847: 493). Although every animal must have at least one mouth, hence at least one stomach (OKEN 1847: 513), OKEN seemed reluctant to force this idea too literally upon protozoa (section 3149, pp. 511-512); he later characterized only the rotifers, among infusoria, as actually having (as opposed to being) an intestine (p. 571). OKEN'S and CUVIER'S systems of animal classification were compared in detail in OKEN'S Isis (1817); as we have seen, CUVIER included Les Infusoires among his Animaux rayonnes ou Zoophytes, while OKEN treated Infusorien, Korallen and Zoophyten as orders within class Geschlechtsthiere, or sexual animals. In response,

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GEORG AUGUST GOLDFUSS (1818) proposed a revised grouping of animals also based on concepts of Naturphilosophie. His first class was "Urthiere. Protozoa", containing four orders, Infusoria, Phytozoa, Lithozoa, and Medusinae. Order Infusoria contained four families, Monades, Vorticellae, Brachioni, and Polypi. Although Naturphilosophie continued to influence scientific thought not only in Germany but also in France (e.g. VIREY 1835: 413-427) and England (RICHARD OWEN, below), by the mid-1820s German science was becoming more experimental, less metaphysically speculative. KARL ERNST VON BAER expressed the occasional naturphilosophische sentiment (e.g. 1828: 263-264), but through careful observation of animal development came to recognize four different types (positional relationships of elements and organs), each of which may be realized to different degrees of perfection; for VON BAER, the combination of degree of perfection with type determined class. There four types were, to be sure, based on an underlying common plan, but he believed that they became established via separate developmental schemata (VON BAER 1826: 739-740). Consequently, he rejected the Great Chain of Being (VON BAER 1826: 759). VON BAER extended his animal types to include infusoria. He placed "certain discoid Infusoria" together with Rhizostomidae, jellyfish, starfish, and perhaps some others in his "peripheral or radiate" type (VON BAER 1828: 209). Other unidentified Infusoria, "coiled, or at least neither symmetrical nor peripheral," were of the molluscan type (1828: 199); these may not have been protozoa at all. Like BORY DE SAINT-VINCENT, VON BAER felt that any of the principal forms of invertebrates could be traced through "a more or less uninterrupted series down to the Protozoa" (1826: 738; DUJARDIN 1841: 21). VON BAER (1826: 736, 739) complained about lack of character-based definition of Infusoria, specifically criticizing O. F. MULLER for having based genera on "inessential organs, hairs and spines". PAVEL FEODOROVICH HORANINOW (1834, 1843) published two universal classifications somewhat reminiscent of OKEN'S universal system but without its cosmic philosophizing, with details drawn eclectically from more than fifty diverse authorities (THEOPHRASTUS, LINNAEUS, GOETHE, LAMARCK, CUVIER, OKEN ...). In Primae Lineae Systematis Naturae (1834) he proposed two kingdoms, Regnum ampho-anorganicum seu molecularium and Regnum organicum seu cellularium, each having four divisions of unstated taxonomic rank (Aether or Ignis, Aqua, Aerem and Circulum corporum anorganicorum solidorum discedens; Vegetabilia, Phytozoa, Animalia and Homo sapiens). His Phytozoa contained various seaweeds, Characeae, diatoms, fungi, sponges, polyps and cnidarians, while he placed Infusoria as the first class within Animalia mollia or Gastero-

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/ CC'~wr.n~ ·u,),II/

/

\ -;

Fig. 1. HORANINOW'S universal system of nature (1834). Beginning at numeral I slightly below and to the left of centre, the inward spiral of organ ismal group s reads: Plantae sporophorae, Pseudospermae, Coccophorae, Plantae spermophorae , Algae, Fungi , Polypara , Acalephae, Infusoria, Entozoa, Radiata , Mollus ca, Annulata , Arachnida , Insecta, Crustacea, Pisces , Amphibia, Aves, Mammalia , HOMO.

AThird Kingdom ofEukaryotic Life zoa (Fig. 1). In Tetractys Naturae (1843), HORANINOW elevated his two former kingdoms to orbits or worlds (Orbis elementaris, molecularis seu anorganicus and Orbis organicus seu cellularis), and his eight unranked divisions to kingdoms (Regnum Aethereum, Regnum Aqueum, Regnum Aereum and Regnum Minerale; Regnum Vegetabile, Regnum Amphorganicum, Regnum Animale and Regnum Hominis). Regnum Amphorganicum held four classes of zoophytes and phytozoans (Fungi, Algae, Polyparii and Acalephae), while Infusoria (EHRENBERG'S Polygastrica) again appeared as Class I within the first circle (Gasterozoa) of Regnum Animale. CHRISTIAN GOTTFRIED EHRENBERG (1834: 6) likewise rejected the Great Chain as "false philosophical speculation". He established "animal-corals" and "plantcorals" as separate orders within Anthozoa, without interpreting Anthozoa as straddling the line between animals and plants. He objected to what he saw as CUVIER'S acceptance of an organizational scala in the animal kingdom, and to CUVIER'S grouping of radiates based in part on their structural simplicity (WINSOR

1976: 33). In corals, jellyfish and echinoderms EHRENBERG found only the "merest traces or hints of structure" (WINSOR 1976: 34), although these traces were of great significance in his argument against CUVIER. But in Die Infusionsthiere als vollkommene Organismen, EHRENBERG (1838) "discovered" complete organ systems, including alimentary systems (with stomachs), in diverse protozoa (particularly ciliates) and rotifers. Both OKEN and CUVIER had argued that the alimentary system is a defining feature of animals: " ... hence is derived the first character of animals, or their alimentary canal, from which their nutritive fluid penetrates all other parts through pores or vessels, which are a kind of internal roots" (CUVIER 1837: I, 9). Thus EHRENBERG retained ciliates (and some other unicellular eukaryotes) as class Polygastrica within his Division Racemifera, animals with branched intestines (1837: 181-256). The Great Chain of Being had been broken up, OKEN'S analogies abandoned, and CUVIER'S Radiata substantially rearranged; but with "multiple stomachs", protozoa remained more firmly than ever within the animal kingdom.

Infusoria as both animals and plants Speculations that plants might metamorphose into animals resurfaced with the discovery that algae liberated motile cells from which, subsequently, algae were regenerated. Despite the Aristotelian dictum that movement per se did not suffice to demonstrate animality, FRANZ UNGER (1843), FRIEDRICH TRAUGOTT KOTZING

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(1844), JULIUS VON FLOTOW (1844) and others interpreted these zoospores as animal entities within the algal life cycle. JOHN HOGG (1839: 379), CARL THEODOR VON SIEBOLD (1844, 1845) and GUSTAV THURET (1851) disputed this interpretation, but it was not until NATHANAEL PRINGSHEIM (1855) observed the fertilization process that zoospores were accepted as fully algal. Even so, the idea of coexisting or alternating animal and plant identities for these organisms persisted (BERKELEY 1857: 85-89; HUXLEY ed. 1897: 194; BERGSON 1911: 112). KOTZING (1843: 4; 1844) argued further that the production of zoospores demonstrates that in their early developmental stages, all beings are both animal and plant; in higher organisms one of the two natures eventually wins out, whereas in lower organisms they remain equally balanced. KOTZUNG also believed that simple algae (Protococcus, Lyngbya) actually changed into (verwandeln sich in, ilbergehen in) morphologically more-complex algae, an idea disputed by CARL NAGELI (1847: 97-100). RUDOLF LEUCKART omitted most protozoa from consideration in his Die Morphologie und die Yerwandischaftsverhdlmisse der wirbellosen Thiere (1848: 10-11) because, given KOTZING'S theory, many protozoa could be degenerate forms, lower plants, or spores. LOUIS AGASSIZ felt that Infusoria was "an unnatural combination of the most heterogeneous beings" that should be "divided and scattered, partly among plants, in the class of Algae, and partly among animals, in the classes of Acephala (Vorticellae) of Worms (Paramecium and Opalina), and of Crustacea (Rotifera)", He had seen "a Planaria lay eggs out of which Paramecium was born ... while the Opalina is hatched from Distoma eggs" (AGASSIZ 1859: 200-202, 259). BORY'S suggestion of a distinct middle ground was not widely accepted. The true nature of diatoms and desmids remained in dispute beyond midcentury, with distinguished naturalists claiming one or both as animals (BAILEY, BAKER, BORY himself, CARMICHAEL, DALYRIMPLE, ECKHARD, EHRENBERG, JABEZ HOGG, MENEGHINI, O. F. MOLLER, SCHLEIDEN, VON BAER) or plants (AGASSIZ, BERKELEY, BRAUN, DE BREBISSON, COHN, DILLWYN, DUJARDIN, GEGENBAUR, W. H. HARVEY, JOHN HOGG, HORANINOW, KOTZING, LYNGBYE, MEYER, NAGELI, PRITCHARD, RABENHORST, VON SIEBOLD, SMITH), or leaving the question unresolved (LINDLEY, THURET). Diatoms and desmids were sometimes grouped together, e.g. as Bacillaria (DUJARDIN 1841: 670-674). JOHN RALFS first considered desmids to be animals and diatoms vegetables, but by 1848 had reversed his position; desmids, he felt, have as strong a claim to algal status as Conjugatae or Palmellae, but "the proper station of the Diatomaceae [is] very doubtful. They have at least as much right to a place in the

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A. RAGAN

animal as in the vegetable kingdom; and perhaps the safest course would be that adopted by several celebrated continental naturalists, who regard them as belonging to a distinct and intermediate group, and partaking almost equally of the characters of animal and of vegetable" (RALFS 1848: 17). JABEZ HOGG (1859: 304-305; 1869: 417) suggested that desmids "... would be more correctly placed in a median, or Molluscian sub-kingdom" between plants and animals.

From MACLEAY to OWEN, HOGG, and WILSON & CASSIN In establishing his five tribes of animals, MACLEAY broke CUVIER'S Zoophytes ou Animaux rayonnes into two groupings. RICHARD OWEN did likewise in 1835 (OWEN 1843: 14) and 1836, recognizing two somewhat different subkingdoms within CUVIER'S radiates, Nematoneura (echinoderms, rotifers, coelemic entozoa, and ciliobrachiates) and Acrita (acelephs, nudibrachiates, parenchymous entozoa, and polygastric infusoria). He acknowledged MACLEAY'S usage of Acrita (but credited it to VIREY, an origin I have been unable to confirm), and considered his Acrita equivalent to LAMARCK'S Animaux apathiques (OWEN 1836: 47). In his sixth Hunterian lecture of 1837, OWEN (ed. 1992: 239-261) reviewed the characters by which plants and animals may be distinguished, including Ehrenbergian stomachs in infusoria. He concluded that even those organisms "on the lowest step of the Animal and Vegetable Series" can be properly placed in one or the other kingdom (OWEN ed. 1992: 256). In his Hunterian lectures of 1843 (OWEN 1843: 17), he was likewise certain that Polygastria " ... belong, in fact, to the higher division of organic nature, and manifest the distinctive properties of animals in the most striking and unequivocal manner". To be sure, OWEN found these properties to apply most clearly to the rotifers, and showed diminishing enthusiasm for stomachs in polygastria. But " ... the endowment of distinct organs of generation, for propagating their kind by fertile ova, raises the Polygastric Infusoria much above the mere organic cell" (OWEN 1843: 26). Indeed, the infusorial monad is the primary form from which the four major animal types arise (OWEN 1843: 367-371). By his Hunterian lectures of 1852 (OWEN 1855: 2), OWEN admitted that the difficulty of defining plants and animals had "increased, and seems now to be insuperable". There are "very numerous living beings, especially those that retain the form of nucleated cells, which manifest the common organic characters, but without the distinctive superadditions of either kingdom. Such organisms are the Diatomaceae, Desmideae, Protococci, Volvocinae, Vibriones, Astasieae, Thalassi-

colae and Spongiae, all of which retain the character of the organized fundamental cell, with comparatively little change or superaddition" (OWEN 1855: 8). OWEN nonetheless maintained Class Polygastria within the animal Subprovince Infusoria. In 1857, in a series of addresses before the Royal School of Mines (WILLIAMS 1971), OWEN (1858: 92, 95) elevated Protozoa to a rank equivalent with animals without, however, calling either a Kingdom. "Kingdom Protozoa" appeared abruptly in the "Contents, or systematic index" of his Palaeontology (1860). In both, OWEN defined animals by their ability to move, receive nutrition via a mouth and stomach, inhale oxygen and exhale carbon dioxide, and develop tissues based on carbon, hydrogen, oxygen and nitrogen; and plants as rooted, lacking mouth and stomach, exhaling oxygen, and having tissues based on cellulose or other complex materials. Again, "... there are numerous organisms [beings], mostly of minute size and retaining the form of nucleated cells, which manifest the common organic characters, but without the distinctive superadditions of true plants or animals. Such organisms are called 'Protozoa', and include the sponges or Amorphozoa, the Foraminifera or Rhizopods, the Polycystineae, the Diatomaceae, Desmidiae, [Gregarinae,} and most of the so-called Polygastria of EHRENBERG, or infusorial animalcules of older authors" (OWEN 1858: 92; items in square brackets added in 1860: 4). This passage was closely paraphrased, without attribution, by JABEZ HOGG (1869: 366-367). OWEN again modified this definition slightly in the second edition of Palaeontology, and replaced Protozoa with Acrita (OWEN 1861: 4); two pages later, he treated the two words as synonyms. OWEN (1861: 48) cited his earlier comment (1836: 48) that "(t)he Acrita have been termed Protozoa, as being on the first step of animal organization". But OWEN'S choice of the term Acrita is puzzling, as his Acrita of 1861 differs significantly in rank and membership from his Acrita of 1836 and 1843, and from the Acrita ofMACLEAY. In a paper read before the British Association on 28 June 1860, JOHN HOGG reviewed his own earlier ideas about the plant-animal boundary, including his arguments that sponges are vegetables intermediate between algae and fungi (HoGG 1839: 405) and that the stomach, muscles and nervous system are characteristic of animals (HoGG 1847). "Some foreign naturalists" had maintained that two kingdoms (Inanimate and Animate, or Inorganic and Organic) might suffice, but he disagreed because "for an entire century at least, three kingdoms have been most generally received" (HOGG 1860: 221). HOGG criticized OWEN'S usage of the suffix -zoa for taxa (Protozoa, Amorphozoa) which include organisms, such as sponges, whose animality is in doubt, and proposed new, more-inclusive terms (Protoctista, Amorphoctista, Spongioctiston, palaeoctistology ...).

AThird Kingdom ofEukaryotic Life Thus HOGG put forward a fourth kingdom "under the title of the Primigenal kingdom, Regnum PRIMIGENUM, continens Protoctista, i.e., Protophyta et Protozoa" to "comprise all the lower creatures, or the primary organic beings, - 'Protoctista'" (HoGG 1860: 223), i.e. OWEN'S Protozoa. He characterised this measure as provisional and perhaps even unnecessary, and stated that the new kingdom might be placed either last, or between the vegetables and animals. In a famous diagram, he portrayed the animal and vegetable kingdoms as "two lofty pyramids, which diverge from each other as they ascend, but are placed on, or united in, a common base" of the Primigenal kingdom. The intergradation of colours - yellow and blue for the most "perfect" plants and animals respectively, green for protoctists was intended to convey the difficulty of demarcating boundaries, and the ambiguity of nature of the corresponding organisms (HoGG 1860: 224-225). His depiction of nature as two pyramids strongly recalls ISIDOR BOURDON almost forty years earlier: "(t)he organized bodies form thus as two pyramids, intimately united at their base, extremely divergent at their summits" (BOURDON & BORY DE SAINT-VINCENT 1822: 369). Although HOGG's paper was read well after the publication of CHARLES DARWIN'S Origin of Species (and literally amidst the WILBERFORCE-HuXLEY debate), it contains no references to descent with modification. By contrast, the paper submitted to the Academy of Natural Scienes of Philadelphia in May 1863 by THOMAS WILSON and JOHN CASSIN admits an evolutionary interpretation (although whether according to ERASMUS DARWIN, LORENZ OKEN, or CHARLES DARWIN, may be debated): "(i)t has hitherto been assumed ... that from a point of the first manifestation of life, its progress of evolution or development is into two series or great classes of existences, - animal and vegetable, - or perhaps into one series only ... regarded as the chain of being, from the lowest vegetable to the highest animal. In our opinion it may be demonstrable, that the first assumption of life manifests itself in objects constituting a primary great class or kingdom of more simple organization than either the animal or vegetable kingdom, and possessing also an equally characteristic specialization in its structure and functions" (WILSON & CASSIN 1864: 113-114). According to WILSON and CASSIN, it is simple to recognize members of the Animalia and Vegetabilia, regardless of their grade of development; organisms for which difficulties persist simply belong to an inferior kingdom. Identification must be based on specialized "actual or reactive characters and affinities". The three kingdoms present the reproductive, nutritive, and sentient functions, and the corresponding organ systems, as characters; while all systems are probably present, albeit in rudimentary form, in all three kingdoms, "the

237

reproductive function .. becomes specialized first in a great group of organized beings of more simple structure than either Vegetables or Animals, which we regard as eminently and demonstrably a primary division or kingdom, and apply to it the name Primalia", The nutritive function (and associated organs) becomes specialized first among vegetables, and the organs of sensation only in animals. Primalia encompassed five Sub-Kingdoms, of Algae, Lichenes, Fungi, Spongiae, and Conjugata, organisms which are nonvascular and without organs of respiration or circulation; "the kingdom Primalia contains all such organisms known to exist in Nature" (WILSON & CASSIN 1864: 114-118).

From

EHRENBERG

to HAECKEL

Cell biology, nascent in the 1840s, had a special impact on protozoology. The recognition that animal and plant cells are fundamentally identical in structure and development (SCHWANN 1839), and that protoplasm is basically identical in both (UNGER 1855), made it increasingly futile to seek to differentiate between the simplest animals and the simplest plants. Infusorian stomachs, already bereft of observational support (VON SIEBOLD 1848: 7-10,14-19), lost theoretical justification. Defining protozoa as animals "reduced to a single cell", VON SIEBOLD (1845; 1848: 7-25) segregated rotifers into a separate taxon, yielding Hauptgruppe Protozoa in more-or-less modem sense, encompassing only infusoria and rhizopods; LEUCKART (1848) and CARL VOGT (1851) followed suit. All but one of the pieces were now in place. The major animal and plant types could be traced, morphologically and developmentally, to single-celled organisms not via a linear scale, but through a ramified tree. These simple organisms might variously be animals, plants, alternatively both, or neither; in any case, they form a communi fonte from which the animal and plant series diverge (GEGENBAUR 1860). CARL GEGENBAUR nonetheless apportioned unicellular organisms between the plant and animal kingdoms, deciding mostly on cytological grounds that only the acinetina and some amoebae should be classified as animals (HAECKEL 1862: 163-164). ERNST HAECKEL (1862: 165) refined his colleague's argument, deciding that all true rhizopods without contractile vacuoles (acyttaria and radiolaria) are animals. Four years later - seven years after CHARLES DARWIN'S Origin ofSpecies - HAECKEL took a different tack, introducing the Protista. HAECKEL began each volume of Generelle Morphologie der Organismen (1866) by quoting from GOETHE (Vol. I: "Die Natur schafft ewig neue Gestalten ..."). He dedicated Volume II to CHARLES DARWIN, GOETHE and LAMARCK, and his terminology (Urdarm, Urthiere,

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Darmthiere, Eithiere: e.g. HAECKEL 1868, 1874b) seems borrowed from OKEN. But where the Naturphilosophen invoked vitalistic force, analogy and symbolism, HAECKEL placed mechanism: "phylogenesis is the mechanical cause of ontogenesis" (HAECKEL 1874a: 5). HAECKEL'S creation of Protista as a Third Kingdom seems not to have stemmed from any perceived need on his part for a high-level taxon commensurate with the morphological diversity of these organisms. Quite to the contrary, insofar as is discernable, he deduced the necessity for such a lineage through a priori consideration of the origin of living organisms: "(v)ery numerous, varied types of monera probably also arose autogenously, all unanimously representing the simplest organismal forms conceivable, that is, completely homogeneous, formless and structureless clumps of protein ... it is highly probable that the conditions necessary for the commencement of autogenesis continued uninterrupted for a long time, and that therefore this event took place neither only once nor at a single site but, on the contrary, occurred over a long period and at many sites on the primitive earth" (1866: I, 201-202). The continued existence of very simple moneran forms such as Protamoeba suggested to him that direct autogenesis, which began in the Laurentian period (HAECKEL 1899: 48), might be continuing "before our eyes" (1866: I, 202). HAECKEL found no reason to suppose that all organismal lineages must be either animal or plant; instead, "numerous organisms, which cannot be enrolled among the animal or plant kingdoms without manifest coercion, belong to multiple independent lineages of living creatures, which have evolved independent of the lineages of the animal and plant kingdoms" (1866: I, 202). These organisms may instead be assembled into "a separate intermediate kingdom (Mittelreiche) or kingdom of primitive beings (Urwesenreiche)". Based on der obigen Deduktionen, HAECKEL proposed to include all these "independent organismal lineages, which can be assigned to neither the animal- nor the plant-kingdom with full assurance and without contradiction, under the collective name the Protists, first-produced or originalbeings (Protisten, Erstlinge oder Urwesen)" (HAECKEL 1866: I, 203). In this his method recalls not DARWIN or LAMARCK, but rather LUCRETIUS (De Rerum Natura V: 835-853). In Natiirliche Schopfungsgeschichte, HAECKEL (1868) examined alternative hypotheses for monophyletic or polyphyletic origins of protista, animals and plants from abiogenetically produced monera. Six years later (HAECKEL 1874b, c) he introduced the Gastraea as the hypothetical single common ancestral form for the metazoa (Darmthiere); protozoa are unicellular and hence pre-Gastraeal, while sponges exhibit a rudimentary intestine, hence two primary germ-lamellae (Keim-

bldtter], Ontogenetic development of the germ-lamellae, and subsequent differentiation of the coelom and the transverse axis, thus replace the typology of CUVIER and VON BAER (HAECKEL 1874c: 149-154). The composition of HAECKEL'S Protista changed somewhat between Generelle Morphologie (1866) and GastraeaTheorie (1874), latterly including "animal Monera", amoebae, gregarines, acinetes and infusoria. HAECKEL felt that the latter assemblage, Urthiere, corresponds, on the whole, with TREVIRANUS'S zoophytes, "organisms of the lowest ranks and simplest constitution, in particular another original form of life, neutral, standing in the middle between animals and plants" (HAECKEL ed. 1924a: 100). As HAECKEL'S monera form the material bridge between inanimate matter and the three Kingdoms, "the psychic phenomena of the protists form a bridge that connects the chemical processes of the inorganic world with the psychic life of the highest animals; they represent the germ of the highest psychic phenomena of the metazoa and man" (HAECKEL ed. 1992: 48,152). This is not atavistic nature-philosophy; according to MAX VERWORN (1889: 5-17), the concept can be traced to CHRISTIAN AUGUST CRUSIUS (1749). O. F. MULLER (1786: vi) had observed the "anxious concern" of infusoria as their drop of water evaporated, while EHRENBERG, rejecting "speculation and poetry" (1837: 241), had recognized not only stomachs, but intellectual qualities in infusoria (1837: 220-224). HAECKEL (ed. 1924b; ed. 1992: 132-169) explored the protistan psychoplasm, protistan soul-embryology and protistan soul-phylogeny indeed, later (1917), the souls of crystals, molecules, atoms and electrons. He discovered that ciliates have essentially the same psychic life as multicellular animals, based perhaps in the macronucleus (HAECKEL ed. 1992: 154). But alas, not even ciliates are self-conscious (VERWORN 1889: 221). From ARISTOTLE'S plant and animal souls to HAECKEL'S cell-soul and VERWORN'S cell-psyche, we have travelled full circle through some of the early chapters of protistology. From HAECKEL a better-surveyed path leads to RICHARD HERTWIG, then to MAX HARTMANN, who founded Archiv fur Protistenkunde (with FRITZ SCHAUDINN as the first editor). Historian CHARLES SINGER judged HAECKEL'S concept of Protista "untenable" (SINGER 1959: 342). To be sure, recognition of the prokaryote-eukaryote dichotomy (CHATTON 1938: 50) means that Protista (as well as Animalia and Plantae) is no longer accorded the highest taxonomic rank, and today "Third Kingdom" usually refers to Archaea (Archaebacteria). Nonetheless it is heartening to find this and other selected papers from the Eleventh Biennial Meeting of the International Society for Evolutionary Protistology in the pages of HARTMANN'S Archiv.

AThird Kingdom ofEukaryotic Life Acknowledgements: I am most grateful to GINA DOUGLAS, Archivist, Linnean Society of London, and CAROL GOCKE, Librarian, Natural History Museum (London) for their cheerful expert assistance. I thank ANITRA LAYCOCK (Department of Classics, Dalhousie University) for the translation of AMMONIUS, L. MICHAEL HARRINGTON (Department of Classics, Dalhousie University) for assistance with O. F. MULLER, PAT LEBLANC (NRC Institute for Marine Biosciences) for assistance with BORY, and DAVID M. WILLIAMS (Natural History Museum, London) for helpful discussions on EHRENBERG.

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A Third Kingdom of Eukaryotic Life

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Author's address: Prof. Dr. MARK RAGAN, Na tiona l Research Council of Canada, In stitute for Marine Biosciences, 1411 Oxford St., Halifax, N. S., B3H 3Z l, Can ad a. Issued as NRCC No . 39763.