The Lagoon Page 28
Aristotle’s higher taxa – the megista genē – are also the basis of ours. In the first edition of Systema naturae (1735) Aristotle’s zoōtoka tetrapoda appear as the Quadrupedia (only renamed Mammalia in the tenth edition). Some other Aristotelian taxa are shuffled around or subordinated but recognizably intact: Aristotle’s ostrakoderma become Linnaeus’ Testacea; his entoma + malakostraka Linnaeus’ Insecta.
Aristotle’s influence on Linnaeus is not only apparent in his actual taxa. At least some of his taxonomic terminology, most obviously species (eidos) and genus (genos), are ultimately Aristotelian or Platonic. It is also often said that Linnaeus’ classification methods were based on the Aristotelian logic of division. Historians disagree about that, and I am inclined to doubt it. It is, however, quite clear that a complex of Platonic and Aristotelian ideas shaped how Linnaeus, and other Pre-Darwinian naturalists, saw the structure of the natural world.
Around 1260 Albert Magnus, the first modern European to study Aristotle’s zoology, expressed Aristotle’s claim that ‘nature proceeds . . . by such small steps’ in much the same terms that he did: ‘nature does not make [animal] kinds separate without making something intermediate between them; for nature does not pass from extreme to extreme nisi per medium’. By the early seventeenth century the obverse version, Natura non facit saltum (or saltus – plural), was a commonplace. In his Philosophia botanica, 1751, Linnaeus elevated it to methodological principle. ‘This is first and foremost what is required in Botany – Nature does not make jumps.’ Perhaps that is what Darwin was remembering when he quoted it.
The idea that nature does not make jumps is closely allied to another: that nature is organized into a linear scale that runs from rocks to God via plants, animals and humans. The scala naturae – the Ladder of Nature – as it came to be known, appears in the cosmic structure of The Timaeus which is nothing if not hierarchical. It is also one of Aristotle’s themes. Every natural thing may be, for him, a form and matter – eidos and hylē – compound, but the relative importance of the components varies. In rocks hylē predominates; in living things eidos does. Among living things there’s a ladder of increasing complexity too, plants to humans, running successively on uni-, bi- and tripartite souls. In The Generation of Animals Aristotle elaborates this ladder of life within the animals and underwrites it with embryology and physiology.
He begins by linking his scale of progeny ‘perfection’ (how advanced they are at birth) to their parents: ‘Nature’s rule is that perfect offspring will tend to be produced by a more perfect sort of parent.’ Parental perfection depends on intrinsic heat, hot being better than cold. Heat is reflected in the composition of their uniform parts, hot animals being more fluid and less earthy than cold ones. Heat also reflects anatomy since hot animals have lungs and more elaborate thermoregulatory devices than cold ones. Hot animals also tend to be larger, live longer and be more intelligent than cold animals. The result is a ladder of perfection that reaches from the live-bearing tetrapods down through the selachians, egg-laying fishes, crustaceans and cephalopods, larva-bearing insects and beyond to the spontaneous generators such as sponges, sea anemones and sea squirts which are little more than vegetables. Although this ladder accounts for much of the large-scale variety in their features, Aristotle is far too good a zoologist to believe that any animal can be unambiguously perched on a given rung of the ladder of zoological perfection. His attribute associations are always just ‘for the most part’.
The Ladder of Nature was adopted by Neoplatonists, Christian theologians and early modern philosophers. It underpinned Leibniz’s cosmology. Vastly expanded and much transformed from its Attic origins, it reached the apogee of its influence in the eighteenth century, which is when it appears in Systema naturae.* Linnaeus’ version of the Ladder of Nature is quite Aristotelian. Biologists forget that he classified not just plants and animals, but all Earth’s natural products – Per regna tria naturae runs the subtitle: there’s a taxonomy of stones too. The three great Kingdoms of Nature’s Empire – Animale, Vegetabile and Lapideum – are explicitly ordered by declining complexity; the book begins with Homo sapiens and ends with Ferrum – iron.
It all seems clear cut. It wasn’t. In the eighteenth century naturalists struggled, as Aristotle had, to classify rock-like plants and plant-like animals. Successive editions of Systema naturae record their efforts. In the first edition, 1735, the lowest of the low animals are in the Order Zoophyta, literally ‘Plant-Animals’. It contains the sluggish, barely sensate creatures – sea cucumbers, sea stars, medusae and sea anemones – that had worried Aristotle. (It also contains, rather weirdly, the cuttlefish.) Sponges, corals, gorgonians and bryozoans aren’t even animals; they’re plants and, within them, the lowest of the low. They belong to the Order Lithophyta, literally ‘Rock-Plants’. Over the next fifty years they are all upgraded. By the last, posthumous edition of 1788–93, Aristotle’s plant–animal dualizers have full-animal status. The Order Zoophyta still exists but now it contains all the creatures – corals, gorgonians, bryozoans and, of course, sponges – that once inhabited the Lithophyta. Rock-Plants have become Animal-Plants. Linnaeus found these ambiguities attractive. He defined the Zoophyta as ‘Composite animals efflorescing like vegetables’ and said that this was where the boundaries of the three Kingdoms met.
Of the many naturalists – Trembley, Peyssonnel, B. de Jussieu to name just three – who sorted the plant-animals out, one deserves special mention. John Ellis was a London merchant who liked to press sea life into artistic arrangements. Fascinated by the materials of his art he took to studying them. In 1765 he went down to Brighton by the sea, placed a living sponge in a glass bowl and saw that it sucked water in and out through its ‘little tubes’. That, he said, in a letter to the Royal Society, is how a sponge receives its nourishment and discharges its excrement, from which it follows that sponges must be animals too.
If we consult the ancients, we shall find that in the days of Aristotle the persons who made it their business to collect these substances [sponges] perceived a particular sensation, like shrinking, when they pulled them from the rocks; and, in the time of Pliny, the same opinion continued that they have a kind of feeling or animal life in them; but after that no attention was paid to this kind of knowledge . . .
He felt, with some justice, that he had vindicated Aristotle. Few were convinced. Sponges only really became animals in 1826 when the Edinburgh zoologist Robert Grant demonstrated their motile larvae.
Thus the Platonic–Aristotelian vision of nature as a ladder of perfection and its influence. Yet there is another vision of nature’s order that can also be found in Aristotle. Throughout much of his biology he does not speak of a Ladder of Nature, but only of his great, natural groups of creatures, all of which do much the same kinds of things – eat, sense, move, reproduce – but do them in very different ways using very different devices. Just as both these visions appear at different places in Aristotle’s texts, both also appear in post-seventeenth-century zoology. Sometimes they even coexist, albeit uneasily.
Even as the Ladder of Nature triumphed, naturalists such as Pallas were protesting that animals, in all their diversity, could not, should not, be forced into a linear scale. In 1812 Cuvier divided the animals into four great groups that he called embranchements: Vertebrata, Articulata, Mollusca, Radiata. ‘It formed no part of my design to arrange the animated tribes according to perceived superiority,’ he wrote, ‘nor do I conceive such a plan to be practical.’ He elaborated his scheme in Le règne animal, 1817. Bold, clear, detailed and synoptic, it made Cuvier famous. It is here that he celebrated Aristotle as his great precursor who, he said, had left hardly anything for his successors to do. Cuvier’s classification, however, doesn’t look very Aristotelian at all. The great division between the bloodless and blooded animals (reformulated by Lamarck as animaux sans vertèbres and animaux à vertèbres) is abolished entirely; the hierarchy of Classes, Orders, Families and Genera becomes enormously expanded; fe
w of Aristotle’s megista gēne survive intact. There is, however, an Aristotelian element to the scheme. Just as Aristotle delineated each of his megista genē as a complex of functioning parts, so Cuvier delineated his embranchements. It is precisely this element that would give rise to one of the most bitterly fought and consequential battles in the history of zoology.
XCI
IN OCTOBER 1829 TWO tyro anatomists, Meyranx and Laurencet, submitted a manuscript to the French Académie des Sciences purporting to show that if one took a tetrapod and folded it in half backwards so that its tail touches its head (an exercise performed only on paper, I believe), it looked a lot like a cuttlefish. I do not know if their demonstration was inspired by Aristotle’s analysis of cuttlefish geometry in Historia animalium and The Parts of Animals, for their manuscript appears to have vanished. In any event, the cuttlefish – blameless in itself – was the casus belli for a clash between two worldviews.
The protagonists were Georges Cuvier and his colleague at the Muséum d’Histoire Naturelle in Paris, Étienne Geoffroy Saint-Hilaire. Geoffroy, the elder of the two, had nurtured Cuvier’s career (had indeed got him his job), but by 1830 the younger man had eclipsed his mentor. Cuvier’s Leçons d’anatomie comparée had revitalized comparative anatomy; his Le règne animal was the standard for classification; his Recherches sur les ossements fossiles de quadrupèdes established the fact of extinction in the fossil record; his Histoire naturelle des poissons dwarfed everything else that had ever been written about fish. Napoleon had put him on the council of the Imperial University; the restored Bourbons made him a baron and then a peer of France – but to list Cuvier’s works, titles, jobs and honours would take pages. Geoffroy’s major contribution, by contrast, was the two-volume Philosophie anatomique (1818–22), an idiosyncratic collection of essays on comparative themes and teratology that espoused a Naturphilosophie-influenced ‘transcendental morphology’.
The seeds of the dispute lay in Cuvier’s 1812 classification. Following his hero Aristotle, Cuvier claimed that, within each of his four great embranchements, animals had fundamentally the same structures, shaped into their various forms by the contingencies of function. Animals in different embranchements, on the other hand, had merely analogous organs. Each embranchement was separated from any other by a gulf, one across which nature did not, could not, jump.
Geoffroy disagreed. One of nature’s romantics, he was inclined to see unities where others saw differences. There was, he said, a grand Unity of Plan that spanned all the animals, a unity that transcended the walls of Cuvier’s embranchements. Considering the exoskeleton of an insect and the vertebrae of a fish, Geoffroy proposed that they were one and the same structure. To be sure, insects have an exoskeleton (hard parts surrounding soft) while fish have an endoskeleton (soft parts surrounding hard) but where other anatomists saw this as ample reason to keep them distinct, he explained with the simple confidence of the true visionary that ‘every animal lives within or without its vertebral column’. Not content with this application of his all-revealing system, he went on to show how the whole anatomy of a lobster was really very similar to that of a vertebrate – if you only flipped it upside down. Where lobsters carry their major nerve cords on their ventral sides (bellies) and their major blood vessels on their backs, the reverse is true for vertebrates (as indeed it is).
All this pained Cuvier for the violence it did to his embranchements; no, it outraged him. For years he stewed and sniped. When, in 1829, Meyranx and Laurencet submitted their paper to the Académie, Geoffroy was delighted. The wall between another two of Cuvier’s embranchements, the Vertebrata and Mollusca, had been breached. He urged immediate publication. It was too much for Cuvier. Leaping to the defence of his much violated embranchements, he denounced the cuttlefish paper in session. It was all Geoffroy’s fault; he didn’t really blame the young men. Geoffroy replied, and for three months in 1830 the two zoologists were embattled at the Académie. The fracas went public; Goethe and Balzac championed Geoffroy, but the consensus was that Cuvier had destroyed him on points.
It is sometimes said that it was a debate about evolution, and it is true that Geoffroy was flirting with the idea. At the time, however, it was more a debate about the power and meaning of Aristotelian science. Geoffroy looked through the weird geometry of the cuttlefish to the unity that lay beneath, and argued that all its organs were the same as a vertebrate’s, merely rearranged. For Cuvier, this was wrong in too many different ways. It was anatomically wrong: cephalopods, he showed forensically, have a variety of organs that vertebrates don’t; it was conceptually wrong: there could be no identity across nature’s great gulfs; it was historically wrong: a perversion of Aristotle’s doctrines. Meyranx and Laurencet, a hapless pair, never did get their paper published. Cuvier, however, published his rebuttal.
VERTEBRATE AND CEPHALOPOD GEOMETRY, COMPARED
Appealing to the authority of antiquity, Cuvier declared that the study of resemblance among species is ‘the object of a special science that is called comparative anatomy, but it is far from being a modern science, for its author is Aristotle’. In reply Geoffroy spoke of how he had broken those ancient bonds: ‘I did not content myself with Aristotle’s account. At first I had never failed to cite Aristotle in my works . . . but I wanted to receive more advanced instruction from the facts themselves.’ Cuvier sneered that, where Aristotle had built a monument of facts, Geoffroy was merely doing philosophy. It was not Cuvier’s best-judged line.
A semantic fog enveloped the field. Both claimed that the cuttlefish’s organs and a tetrapod’s were ‘analogous’, but they clearly meant very different things by it. Cuvier’s usage was closer to Aristotle’s; Geoffroy, in a bold move, appropriated the term to mean precisely the opposite, what Aristotle called ‘the same without qualification’ and Owen, in 1834, would call ‘homologous’. By March 1830, however, such terminological matters were no longer at issue. Nor, for that matter, were cuttlefish or classification. The protagonists were divided on something far more fundamental – how form should be explained.
Cuvier was the greatest functional anatomist of his age. It was his proud boast that he could classify an animal from only a single bone. Animal parts are correlated so that ‘the form of the tooth implies the form of the condyle; that of the shoulder blade, that of the claws, just as the equation of a curve implies all its properties’. This was the apotheosis of Aristotle’s method. Cuvier’s great explanatory principle, the Conditions of Existence that he expounded endlessly, was Aristotle’s conditional necessity elevated to a law:
Natural history has a rational principle which is particular to it, and which is usefully employed on many occasions: that of the conditions of existence, commonly known as final causes. Nothing can exist unless it unites the conditions which make its existence possible; therefore the different parts of each being must be coordinated in such a way as to make possible the whole being, not only in itself but also in its relations with those around it. The analysis of these conditions often gives rise to new general laws, as rigorously demonstrated as those of calculation or experiment.
In an age of scientific laws, Geoffroy had one of his own. Function, he declared, does not determine form; rather form determines function. Calling the vertebrate breastbone as witness, he explained the varying proportions of its parts in purely physiological terms. The hypertrophied sternal keel of a bird, to which the flight muscles attach, stunt other bones by ‘diverting to its own profit the nutritive fluid’ that might have fed them. No Cuvierian functional harmony there, just economics. He called his discovery the loi de balancement – Law of Compensation – and proclaimed it a great discovery. Goethe had already anticipated him. But Geoffroy probably got it from The Parts of Animals, for the loi de balancement is Aristotle’s ‘what nature takes from one part it gives to another’ elevated to law. The Great Cuttlefish Debate of 1830 was, then, about many things: the unity of animal life, the identities of organs, the terminology by which those identitie
s should be described and, above all, the causal explanation of organic diversity. It is testament to the scope of his thought and its protean quality that much of it was Aristotle contra Aristotle.
SKELETON OF A HUMMINGBIRD
XCII
IT WAS THE LAST great scientific debate that Aristotle attended. Its protagonists lived but two centuries ago yet conceptually they are closer to him than they are to us, for they all wrote on the far side of 1859. The Origin of Species transformed the very terms of Aristotle’s science or else rendered them obsolete: genē (and embranchements) became true families that descend from a common ancestor; dualizers ceased to dualize and became convergent solutions to adaptive problems; parts were no longer ‘analogous’ or ‘the same without qualification’, but analogous or homologous in a way, a new way, that depended on their origins in the tree of life. Geoffroy’s Unity of Plan was explained by descent by modification; Cuvier’s Conditions of Existence by natural selection.
It is sometimes said that Cuvier got his teleology from Kant, but for Kant teleology was just a ‘heuristic fiction’ and an invitation to despair. ‘There would never be’, he said, ‘a Newton capable of explaining a blade of grass.’ Cuvier was more sanguine. ‘Why should not natural history also have its Newton one day?’ (‘And now it’s got one’ was his unspoken reply.) One can feel a pang of sympathy for Cuvier. If natural history has a Newton, it’s Darwin who, in the Origin, is generous even as he puts his predecessor in his place: