The expression of conditions of existence, so often insisted on by the illustrious Cuvier, is fully embraced by the principle of natural selection. For natural selection acts by either now adapting the varying parts of each being to its organic and inorganic conditions of life; or by having adapted them during long-past periods of time: the adaptations being aided in some cases by use and disuse, being slightly affected by the direct action of the external conditions of life, and being in all cases subjected to the several laws of growth.
Notice how subtly Darwin changes Cuvier’s meaning. When Cuvier invokes the conditions of existence he is generally trying to explain the fit of an animal’s parts to each other; when Darwin does, it is to explain the fit of an animal’s parts to its environment. The difference is only one of emphasis. Anyone who seeks to understand the design of living things necessarily studies them as wholes in themselves and in their worlds; the three great students of animal design, Aristotle, Cuvier and Darwin, all kept at least an eye to both.
In the Origin Geoffroy’s Law of Compensation appears under the heading ‘correlation of growth’. ‘I mean by this expression’, says Darwin, ‘that the whole organisation is so tied together during its growth and development, that when slight variations in any one part occur, and are accumulated through natural selection, other parts become modified.’ Darwin’s idea is more general than Geoffroy’s, for he allows that the connections need not be economic. But he credits him (and Goethe) for the insight.
These concepts continue their scientific run. But they have transmuted again, for if Aristotle, Cuvier and Geoffroy all wrote on the far side of 1859, Darwin joins them in writing on the far side of 1900 or, if you prefer, 1953.* Aristotle’s principle of conditional necessity is now just as often applied, if not by that name, to molecules or even genes. The platy, Xiphophorus maculatus, and the swordtail, Xiphophorus helleri, are small Mexican live-bearing fish. If forced they will cross-mate and the hybrids, a bit weirdly, can be hybridized again. Some of these second-generation hybrids develop melanomas that spread like mould upon a grape. Natural selection has adjusted the platy’s 20,000-odd genes to work harmoniously together in the task of building a platy; the swordtail’s genes harmoniously build a swordtail. But platy genes are not designed to work with swordtail genes and so the misbegotten hybrids, whose genomes are a grab-bag of their parents’, die riddled with tumours.
They are true Empedoclean monsters. Geneticists call the genetic interactions that give rise to such effects ‘fitness epistasis’, but it’s just a translation of Cuvier’s ‘conditions of existence’ or Paley’s ‘relations’ between parts, or Aristotle’s ‘conditional necessity’. In this guise the concept wends its way through Muller and Sturtevant’s account of speciation mechanisms, Wright’s shifting-balance theory, Kondrashov’s explanation for the maintenance of sex, Kauffman’s N-K landscapes and much more. Wherever it appears, the idea is always the same: you can’t mix different animals up.
Aristotle’s ‘what nature takes from one part it gives to another’ can also be couched in genetic terms. Genes that influence apparently different parts of an animal’s body are said to have ‘pleiotropic effects’. The term applies whether they do so by virtue of sharing flows of information, matter or energy. There is a mutant strain of nematode that has a life expectancy about half as long again as a normal worm’s, but that lays far fewer eggs – the cost, apparently, of a long life. The mutation is said to have an ‘antagonistic pleiotropic effect’ for it increases one feature while decreasing another. The geneticist’s ‘pleiotropies’, Darwin’s ‘correlations of growth’, Geoffroy’s ‘Law of Compensation’ and Aristotle’s ‘what nature takes from one part it gives to another’ are all, then, related ideas. In its modern guise it underpins the evolutionary theory of life history and ageing as, in its ancient guise, it underpins Aristotle’s. Wherever it appears, it expresses the same idea: that the parts of animals are irreducibly bound one to another.
Perhaps Aristotle’s most important legacy is one that I have not touched on at all, but that also runs throughout the history of zoology. It is his insistence that the organic world is structured into natural classes that our classifications should not tear apart. For the moderns – Linnaeus and almost all systematists since – this idea became the search for a Natural System of classification. Darwin told us what such a system means and why it exists. ‘I believe’, he wrote, ‘that propinquity of descent, – the only known cause of the similarity of organic beings, – is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications.’ Now the problem is to recover the shape of that hidden bond, the topology of the great Tree of Life. It’s being revealed by scientists using very fast search algorithms that feed on terabytes of DNA sequences. Now the animals are divided into three great Super-Phyla (plus a few basal groups such as sponges) which, in turn, are divided into thirty-odd Phyla which, in turn, are divided into ever smaller groups unto species, which although not exactly innumerable can scarcely be said to have been numbered for there may be anywhere between 3 and 100 million of them on earth. The leaves on Darwin’s great tree are almost uncountable.
The great tree, a metaphor for the history of life, serves as a metaphor for the history of ideas too. That nature does not make jumps; that there is a Ladder of Nature; that natural groups of animals exist; that those groups should be defined by the homology and analogy of organs; that organs are shaped by their functional and economic relations – all these ideas, I claim, can be found in Aristotle. They have also structured modern zoology for much of its history; they still do. We may wonder, however, whether they are the same ideas.
It all depends, of course, what you mean by ‘the same’. Ideas are the organs of our thought, and like the organs of a cuttlefish and a tetrapod, they may be ‘the same’ by virtue of common descent or ‘the same’ by virtue of being independent solutions to similar needs. Aristotle himself was fond of remarking that the same ideas have occurred to many men at many times. (If that seems trite, no doubt that is because it is self-reflexively true.) For the cluster of ideas that I have discussed here, however, I believe that a good case can be made for identity by descent, intellectual homology if you will. Linnaeus, Geoffroy and Cuvier and their predecessors read Aristotle; Darwin read them; we’ve read Darwin. The genealogical thread is clear.
Among historians the tracing of conceptual genealogies across the ages – the ‘history of ideas’ (ideas pure and simple) rather than ‘intellectual history’ (their social and cultural context) – is rather unfashionable. They point out that thinkers of every age appropriate their predecessors’ terms and concepts and apply them to their own ends; and that they do so even when the underlying structure of their thought has transformed their sense entirely. Philosophers call this process ‘conceptual shifting’ and delight to spot it as terriers do rats. Scientists – always sloppy with terminology, forever pushing new theories – are notorious for it. The ever-mutable meanings of ‘analogy’ and ‘homology’ are a case in point. Aristotle is addicted to it too – his eidos and psychē are expressly not Plato’s.
Historians are right to stress this particularity, though not to the extent of denying the logic of modification by descent, a logic that applies with equal force to the realm of ideas as it does to life itself. In a way, it’s just a matter of how you look at it. Focus on the cuttlefish at home in its cuttlefish world and its weird geometry appears as a solution to its own cuttlefish problems. But take a broader view, and it looks more like a small twist on a basic plan that was laid down a long time ago.
That zoologists drew their ideas from, struggled against or simply used Aristotle, and that they did so for centuries, seems incomprehensible to us now. Darwin eclipsed his predecessors; he became to us what Aristotle was to them: an authority to inspire or merely invoke. Yet, though we have forgotten their ultimate source, Aristotle’s ideas, transformed and applied in ways that he could not have imagined, re
main with us.
XCIII
ARISTOTLE NEVER MADE the evolutionary leap. Of course he didn’t. After all, he did not stand, as Darwin did, on the shoulders of Linnaeus, Buffon, Goethe, Cuvier, Geoffroy, Grant and Lyell. He heard no transformist whisperings from Paris and Edinburgh. He saw neither the mockingbirds of the Galapagos nor the fossilized giants of the Argentinian pampas. That he had the materials for an evolutionary theory at hand is, of course, evident only in hindsight. We may read Aristotle in Darwin, but not Darwin in Aristotle. By the same token Aristotle’s system cannot be anti-Darwinian. His opponents were the physiologoi and Plato, none of whom were evolutionists in the Darwinian sense. Many of them were, however, evolutionists in a much looser sense for they gave naturalistic accounts of the origin or transformation of species. Aristotle, radically, rejected them all.
Creationism and evolution are rivalrous siblings. Both propose that the past was a very different place. Both propose that the creatures that we see in the world were not always there but have an origin in time. In the Greeks it’s not always easy to tell them apart. The physiologoi may have rejected the myths but, as I said, the divine can often be found lurking somewhere in their thought. Xenophanes of Colophon (fl. 525 BC) is said to have argued that all living creatures originate from earth and water, though we don’t know how he got them to do so. We know Empedocles’ zoogony in confusing detail. First, there are those separated body parts, then their fusion into various improbable forms, then the selection process and, finally, the survivors sort themselves out by habitat. Democritus, too, evidently gave a naturalistic zoogony, but we know nothing about how it worked except that it ran on atoms.
The Pre-Socratic zoogonies are not, usually, transformist. Empedocles’ creatures, having acquired their features, stick with them. But Anaximander of Miletus (fl. 525 BC) appears to have believed that humans are related to fish. The sources disagree how. One says that Anaximander held that humans originally resembled fish, another that humans arose from fish; another that they were born from a galeos. That may be a reference to the smooth dogfish, Aristotle’s leios galeos, which nurtures its embryos in the womb via a placenta and umbilical cord and then gives birth to pups.
And then there is The Timaeus. Let us, for just a moment, treat Plato’s origin myth with the seriousness that it doesn’t deserve. Animals are degenerate humans. The gods transformed the silly, if harmless, men who studied the heavens (astronomers) into birds. Men who used their hearts rather than their heads became land animals: their forelimbs were drawn to earth; their heads deformed for lack of use. Truly stupid men acquired earth-bound bodies and many legs (centipedes?); the utterly thick were made legless (snakes or worms). Vicious men plumbed greater depths. Unworthy of breathing air, they were condemned to live as fish and snails in the muddy waters. Or else they became women.
Anaximander derives humans from fish, Plato fish from humans. The two theories have an appealing progressivist/degenerationist symmetry. Aristotle mentions neither. Indeed, he says very little about origin-of-life or species theories. When attacking Empedocles he, rightly or wrongly, treats his zoogony as embryology. But he was aware of them. In The Generation of Animals, discussing spontaneous generation, he says that if, ‘as some allege’, all animals, even men, were originally ‘earth-born’, they would have spontaneously generated from the earth as larvae – and he is thinking of the eel’s gēs entera. Who, exactly, alleges? Anaxagoras? Xenophanes? Democritus? Diogenes? It doesn’t really matter: he’s just toying with the idea, pointing out that if there had been a zoogony, his nutritional physiology shows how it would have worked. As far as he’s concerned, it never happened. As far as he’s concerned, all sexually reproducing animal kinds have always existed and always will.
Our conceptual world is structured on a Manichean conflict between creationism and evolution. The conceptual world of the Greeks, before and after Aristotle, was structured on a conflict between creationist and naturalistic explanations for the origin of its living inhabitants. For Aristotle, there’s not much to choose between them. Both fail to grasp one of the most salient features of the biological world: its regularity.
For Aristotle, the origin of any individual of a given sexual kind requires the existence of two others of the same kind. To make a sparrow you first need two other sparrows. His slogan, ‘a human being gives rise to a human being’, applies, mutatis mutandis, to all sexual kinds. Only parents – more precisely, the father – can supply the form, the eidos, required to make a new individual. This theory, taken literally, implies an eternal regress of sparrows. Aristotle takes it literally.
Aristotle’s theory of sexual reproduction, and its metaphysical basis, is incompatible with any zoogenic or transformist theory. His theory of inheritance is too. I have argued that Aristotle has a dual-inheritance system. The formal system is the father’s unique contribution to the embryo and transmits the logos – the set of functional features that will enable its offspring to live in its environment and, if it is male, reproduce its form in turn. The informal system, due to both parents, is responsible for variation among individuals of a kind – Socrates’ v. Callias’ nose – and encodes accidental variety. This division of labour between these two inheritance systems has profound consequences. Aristotle is perfectly prepared to allow that an individual can have a new mutation that gives him some novel feature, a snub nose say; but he does not – pace Socrates, who thought his snub so useful – seem to allow that it can be adaptive. In his view, all errors of development, inherited or not, are either devoid of functional import (odd-shaped noses) or deleterious (missing organs). The production of females aside, he never even hints that a mutation might benefit an animal. In his world every creature is, within the limits of its physiology, perfectly adapted to its environment; there is no room for improvement. Were he to meet Darwin he would ask – and ask rightly – where are these ‘favourable variations’ of which you speak? When a father’s sperma fails to concoct the embryo, all I see is death, deformity or, at best, a girl. Darwin would have been unable to answer. Happily for him, his successors can – though not without difficulty.
This theory of inheritance obviously closes the door to evolution by natural selection. That troubles us but not Aristotle, for he never argued with Darwin. Could Aristotle have developed a theory of evolution? Perhaps. He’d have to throw some of his own theory overboard and the result wouldn’t necessarily be Darwinian.
In middle age Linnaeus became convinced that new, stable species of plants could arise, and had arisen, by hybridization. Aristotle may have believed this too. In the Metaphysics he says that mules are ‘unnatural’. In the zoology he doesn’t. He certainly believes that, in general, only animals of the same kind copulate and produce offspring,* but he also says that animals of different kinds can sometimes mate and produce offspring; or at least they can do so when they are not too different in form, size and gestation period.* He gives an elaborate explanation for why mules are sterile, but clearly thinks it’s an exception since his hybridization limits are otherwise generous. He thinks that crosses between different kinds of hounds, wolves and dogs, foxes and dogs, horses and asses, and various raptors all yield fertile progeny. He moots the possibility that the ‘Indian dog’ is the F2 progeny of a male tiger and a dog bitch (if the tiger doesn’t eat the dog), and that the weird rhinobatos (guitarfish, Rhinobatos rhinobatos) is the progeny of the equally weird rhinē (angelshark, Squatina squatina) and a batos (probably a skate, Rajiformes) – but here he’s on uncertain ground and knows it.
That new animal kinds might arise from hybridization is inconsistent with Aristotle’s oft-stated claim that the form of a kind comes from the father. If a hybrid is to have the functional features of both parents, as the rhinobatos presumably has, then its eidos must come from both. As I read him, Aristotle doesn’t believe that, but there are enough difficulties in his texts to make it plausible that, at some time, he did.
But, had Aristotle taken the road to evolution,
I think he’d have taken the road that Geoffroy Saint-Hilaire took. In the second volume of his Philosophie anatomique, 1822, Geoffroy laid the foundation of teratology, the science of monsters. He noticed how teratological deformities have a certain order and how they often resemble some normal species or other. He named one human deformity Aspalasoma because its urogenital anatomy resembled a mole’s, aspalax. Such observations became transformist musings. ‘Nothing is monstrous, and all nature is one’ was one of his more gnomic sayings.
This is in the spirit of Book IV of The Generation of Animals: ‘Even what is unnatural does, in a way, conform with nature.’ Monsters are unnatural, but mostly because they are rare. Aristotle’s impulse is to naturalize them by explaining them in terms of the normal processes of embryogenesis. Indeed, the ‘cause of monstrosities is very close and, in a way similar to, the cause of deformed animals . . .’ By ‘deformed animals’ Aristotle means here naturally deformed creatures. Moles are deformed because they are blind; seals are deformed because they have flippers instead of proper limbs; lobsters are deformed because they have asymmetrical claws. They violate, in some way, the norms of the wider kinds to which they belong. In drawing this parallel, he means only that the moving causes of unnatural and natural deformity are the same. Unlike Geoffroy, he does not mean that deformity gives rise to new species. Aristotle never took the evolutionary leap.
He might have. Plato showed him how. Moral vice obviously won’t transform a man into a fish, but a mutation, a lysis, might. Or at least it might transform a human into a tetrapod. Sometimes Aristotle’s language suggests as much. There is a passage in The Parts of Animals in which he’s explaining why tetrapods walk on four feet rather than two. He says that tetrapods have relatively heavy upper torsos compared to man. This excessive top hamper has two consequences. First, it causes their bodies to become unstable and hence lurch towards the ground; second, it inhibits the soul’s activity centred on the heart. For these two reasons tetrapods developed – egeneto – bent over and then, for the sake of stability, nature gave them forefeet instead of arms.
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