In his round-up of animal variety Aristotle touches on the biogeography of sheep. He reports that in Pontus (the Black Sea littoral) the rams don’t have horns, but in Libya there is a sheep with long horns and both the rams and ewes have them;* that Sauromatic (Bosphorus) sheep have hard wool; that on Naxos they have very large gall bladders but on Euboea they have none; that flat-tailed sheep tolerate winter’s cold better than long-tailed sheep and short-fleeced sheep better than shaggy-fleeced, but that crisp-haired sheep suffer most. Syria is home to some singularly outré domesticates:
In Syria sheep’s tails are one and a half foot wide and goats’ ears about a foot long, in some cases touching down below at ground level. Cattle also have humps on their shoulders, like camels.
It’s not, by itself, a very important observation – just one more piece of natural history lore among thousands. But one wonders: what did Aristotle think these fat-tailed sheep, long-eared goats and hump-backed cattle were? Were they, for him, just local varieties of the same basic sheep, goats and cows grazing on any Greek farm, or were they something quite different? It doesn’t seem like a very momentous question, but it is. For upon its answer turns nothing less than one’s vision of the order and stability of life:
Hottentots say great tailed sheep aboriginal at Cape & a thinner tailed kind farther inland . . . Capt Davis in 1598 found cattle in Table Bay with hump on their back & big-tailed sheep.
They have the same data: fat-tailed sheep and hump-backed cattle in exotic locales, very different from anything chewing the cud at home. Yet it’s not just the data that matter; it’s what you see in them. The second passage is from Darwin’s Transmutation Notebooks. It’s 1837 or 1838, and he’s just discovered evolution.
POTAMIÁ VALLEY, LESBOS, JUNE 2011
LXX
THE FIRST CHAPTER of The Origin of Species might have been about the glories of the Brazilian rainforest in which the twenty-three-year-old wandered in frank religious ecstasy. Or it might have been about the Kentish countryside where green serenity conceals a vicious struggle for light and life. Or it might have been about the Galapagos, in evolution’s Origin Myth the fons et origo of the theory itself. Darwin could even have just abstracted the four volumes on barnacles that he’d published only a few years before and told how their cyprid larvae proves their link to shrimps and crabs; he might have described the weirder species with microscopic males (‘mere bags of spermatozoa’) and gigantic probosciform penises. All this, after all, is the problem; it’s what he’s trying to explain, and you’d think he’d want to grab the reader by showing how wonderful it is. But he doesn’t. Prosaically, he begins with pigeons.
He argues that all the pigeon breeds in the world are descended from the common rock pigeon, Columbia livia. Thousands of generations of selection by the hand of man have divided and transformed them and that’s what happens in nature too. Know the pigeon, understand the pigeon, and all the rest follows. Darwin’s argument is so familiar that no biologist can look at a pigeon, sheep, goat or goldfish without reading in their feathers or feet or fins an evolutionary tale be it grand or grotesque, or both. Each fat-tailed sheep or a hump-back cow tells a narrative of origin, migration and change that began thousands of years ago in the deserts of the Middle East and the civilizations of the Indus, that spans the mountains of Asia Minor, skirts the scrubby coastal hills of the Levant, continues down the Great African Rift across countless kilometres of veldt, reaches a geographic terminus in Table Bay, and yet – for evolution itself does not stop – continues to this day.
But it’s not quite Darwin’s story. It is only in the last few decades that molecular genetics and archaeology have truly traversed the meandering genealogies of our farmyard animals to their remote and antique origins in the wild.* Darwin’s real point was more profound. He wanted to show that species are variable and that some of this variation can be inherited. Nature generates heritable variation – and how:
The diversity of the breeds is something astonishing. Compare the English carrier and the short-faced tumbler, and see the wonderful difference in their beaks, entailing corresponding differences in their skulls. The carrier, more especially the male bird, is also remarkable from the wonderful development of the carunculated skin about the head, and this is accompanied by greatly elongated eyelids, very large external orifices to the nostrils, and a wide gape of mouth. The short-faced tumbler has a beak in outline almost like that of a finch; and the common tumbler has . . . [etc.]
Darwin needed to understand inheritance. Heritable variation was the fuel on which his evolutionary engine ran, so he needed to know its laws and its limits. He batted the problem about for decades. The tentative jottings of the Transmutation Notebooks metamorphosed into the confident claims of the Origin which, in 1868, spawned the sprawlingly aporetic Variation of Animals and Plants under Domestication. It was all a failure, the greatest of his scientific life. Yet we now know that Darwin was right to suppose that every species is rich in heritable variation. Indeed, the great lesson of post-Darwinian biology is that diversity goes all the way down. Some of this phenotypic variety is caused by variety in genes; some by variety in environment; much of it is caused by both in ways that are so complex that we can scarcely disentangle them at all.
Darwin grasped some of this. Did Aristotle? Many scholars have thought not. Aristotle, they argue, believed that a scientist’s task was to enumerate the ‘essential’ features of the creatures he studied. ‘Essential’ features do not vary among individuals, or vary only accidentally (an amputee is still obviously a man, even if he is no longer actually bipedal). In seeking the essence of every form Aristotle ignored the variety that individuals show and placed it beyond science’s remit. However different Socrates and Callias – or two sheep – may look, for Aristotle they are ‘one in form’ and that’s the end of it.
It isn’t. Yes, he does want to understand the typical, functional – ‘essential’ – features of his kinds. But he also has a parallel research agenda to understand the useless variety that riddles even the smallest of them, the kinds that can be described as atoma eidē – ‘indivisible forms’. He doesn’t have a term for this kind of variation, so I call it informal variation by analogy to the modern biologist’s intra-specific variation. In Illyria and Paeonia (Balkans), says Aristotle, the pigs have solid hooves like those of a horse rather than the cloven feet that most pigs have. This sounds like an Aristotelian bizarrerie, but it isn’t, for Darwin says that such pigs exist in England. Aristotle and Darwin are both clear that they’re not talking about two different kinds or species of pigs but rather variants of the regular pig.*
So the various sheep, pigs, horses and cattle distributed across the limits of the world are just different manifestations of particular forms. Every domesticated animal, says Aristotle, has a wild equivalent; should they be classified into different kinds? No, such a division would be unnatural. Humans are a unity too. Aristotle knows that Ethiopians have black skin and curly hair, yet he takes it for granted that they and Greeks share the same indivisible form.
Darwin claimed that most of the variation visible in domesticated animals is heritable. Aristotle, by contrast, gives most informal variation to the direct effects of the environment. Some places are hot, others are cold; some are wet, others are dry, and such differences make for differences in appearance. The depths of the sea are cold so the sea urchins that live there have long spines. Africa is dry but the Black Sea littoral is damp so the Ethiopians have curls but the Scythians’ and Thracians’ locks are lank. Egypt’s torrid climate ensures that the naturally cold animals – snakes, lizards and the Red Sea’s turtles – that live there are very big. On the other hand, scarcity of food means that Egypt’s dogs, wolves, foxes and hares are rather small. Bees are more ‘uniformly coloured’ than hornets and wasps because they have a relatively monotonous diet. All this informal variation is devoid of functional significance. It is not for the sake of anything, but is just the product of material natures
, the physical properties of tissues moulded by the vagaries of the world.
Aristotle’s environmentalist view of geographic variation is puzzling. Did he not understand that the varied features of domestic animals are inherited? Presumably he’d only read about fat-tailed Syrian sheep and mule-footed Balkan hogs, but surely any farmer could have told him that long-hair and crisp-hair sheep are breeds? He informs us that in some districts the sheep are white and in others black, and that this may be due to the water.* That’s absurd. In Greece sheep flocks are a chequerboard of white and black and every shepherd must have known, as my Corinthian highlander certainly did, that fleece colour is inherited. It’s also not as though the principles of selective breeding were unknown in fourth-century Greece, for in The Republic Plato discusses how to breed a better sheepdog. Of course, being Plato, that’s only by way of introducing his real interest, how to breed a better human. Granted Aristotle did well to ignore Plato’s eugenic fantasies,* but he had at least one other colleague, his closest friend, whose grasp on the causes of intra-specific variety was really rather subtle – and whose data were much better than his own.
LXXI
HE SEEMS LIKE THE classic epigone. If Aristotle is all pyrotechnics, Theophrastus is candle-powered. His theories aren’t as bold; they don’t run as deep; they seem mostly borrowed from his friend. When telling us about his plants, Theophrastus never names Aristotle – but he’s always there. Yet Theophrastus shouldn’t be underrated. That difference in temperament is also a difference in method. Theophrastus is more cautious, less quarrelsome, more empirical, less theory-driven. He carries less metaphysical baggage. This isn’t just because his Metaphysics is in fragments, where Aristotle’s survives intact. When Theophrastus considers alternative explanations and gives you the evidence for each, you don’t get the sense – pervasive with Aristotle – that he’s already stacked the deck. ‘Divine Speech’ is not the very model of a modern scientist – but he’s closer.
Thracian wheat, Theophrastus says, sprouts late and takes three months to mature; in other places wheat sprouts early and matures in two – why? One obvious explanation is that there’s something different about Thracian air, water or soil. He analyses the effects of soils, water and winds on plant growth at length. In Lesbos, a river near Pyrrha is so nutritious that its waters actually kill plants, and people who bathe in it become covered in some scaly stuff. (He must mean the mineral-rich hot springs of Lisvori that emerge just west of the Lagoon.) Although animals, too, are affected by environment, he says that they are less so than plants, since their connection to the soil is less direct.
The allusion is to Aristotle’s Egyptian animals. In fact, the whole model is very Aristotelian. But then he points out that if you grow Thracian wheat in other places it still sprouts late, and if you grow early-sprouting wheat in Thrace it still sprouts early.* Each variety of wheat, he concludes, has its own ‘special nature’. He seems to think that the differences between the wheat varieties are fixed, that they are heritable. But, speaking generally, both the environment and hereditary qualities can affect the growth of a plant: ‘For when something comes about as a result of two or more things possessing power, the whole necessarily varies with the differences in its sources (and this also happens in animals; for animals get differences due not only to the male and the female parent, but also to the country and air, in short, their food).’ Or, to put it in Francis Galton’s terms, it’s a matter of both nature and nurture.
Here, at least, Theophrastus is closer to the phenomena than Aristotle. We sense that the student was a gardener, but that his teacher just peered over the farmyard fence. But the two scientists complement each other. Theophrastus’ theories are thin. How is variation inherited? He doesn’t really say. Aristotle does.
LXXII
HE MAY ATTRIBUTE CURLY hair and straight hair to the effects of climate, but Aristotle knows, of course, that children inherit at least some of their parents’ peculiarities. He had at least two of his own, a daughter and a son. Of all the scientific problems that he tackled, the inheritance of sub-specific – informal – variation is among the hardest. Its phenomena are elusive: to accurately describe how children resemble their parents requires a grasp of probability; to accurately describe the inside of a cuttlefish does not. And observation, by itself, can’t crack a genetic problem: difficult experiments involving the rearing and measuring of many individuals over many generations are required. Darwin, who conducted just such experiments, and even tried his hand at ratios, made no headway at all.
It’s no surprise, then, that Aristotle’s data on inherited variation are poor. Even so, it’s surprising just how poor they are. True, he mentions a few cases of inherited variation, but they’re just confused hearsay and he misses much of what he could have seen. No Darwin, he ignores domesticated animals. Of course, he doesn’t cross anything (though there are some intriguing passages on hybrids). He devotes pages to variation in human eye and hair colour, but gives no indication that they can be inherited. He’s fascinated by teratology – dwarfism, hermaphroditism, conjoined twinning, anomalous genitals, extra appendages – and says that such deformities are often inherited, but sometimes not, which, although certainly true, doesn’t get us very far. All in all, Aristotle’s grasp of the facts of inheritance is only slightly more sophisticated than the musings of any newly minted father:
Some children resemble their parents, others do not. Some resemble their fathers, others their mothers, some in the whole body, some in each individual part, some their parents, some their ancestors, some just a general person. Males may resemble the father, females the mother. Some, though, resemble no relative but do resemble a human being. Some do not even resemble a human being in form but, actually, a monster.
Mendelian ratios are not even a distant dream.
Yet, however weak these data may be, they give Aristotle a list of phenomena to explain, namely, why a child: (i) sometimes takes after its parents; (ii) sometimes takes after its ancestors; (iii) sometimes doesn’t take after a relation, but just looks human; (iv) sometimes doesn’t look human, but just looks monstrous. Also: (v) why boys usually, but not always, take after their fathers and girls after their mothers; and (vi) why the different features of a child may take after different parents or ancestors. Naturally, Aristotle has a theory to explain all this; and, just as naturally, he must first dispose of someone else’s.
Aristotle often doesn’t name his opponents, but occasionally we know who they were anyway since we have a text containing the very argument that’s aroused his ire. A fifth-century tract called On Generation contains a brief account of a theory of inheritance that Aristotle evidently read. It belongs to the Corpus Hippocraticum but Hippocrates certainly didn’t write it. The theory is especially interesting since it crops up in the nineteenth century too. Aristotle effectively demolishes it and so, with one blow, takes two scalps separated by more than two millennia, one of which is Darwin’s.
The Hippocratic model is simple. A father’s seed originates in his body parts: his hands, heart and all his other organs and tissues give off fluids that travel via the blood vessels to the penis where they are churned, heated and ejaculated. Something similar happens in mothers. The parental seeds mix in the uterus and an embryo is formed which has the features of both parents, weighted by their contributions. It’s a superficially persuasive idea. The direct physical connection between body parts and seed neatly explains how the characteristics of the parents’ bodies are transmitted to the seed and so to their offspring. Democritus seems to have adopted a version of it, but probably had particles rather than fluids as his units of transmission. In 1868 Darwin published the same idea, with a few elaborations, and called it ‘pangenesis’.*
Aristotle took pangenesis seriously. ‘Hippocrates’ sketched several arguments for it; Aristotle repeats this evidence and even adds to it – but only so that he can knock it down. Over a dozen pages of digressive dialectic he offers fifteen separate objections. On
e of them turns on the great question of nineteenth-century genetics: can acquired characteristics be inherited?
‘Hippocrates’ argued that if some part of a parent is crippled, then the semen that comes from that part will be weak and the child will be crippled in the same way. Aristotle sees that if this were true then ‘children [would be] born which resemble their parents in respect not only of congenital characteristics but also of acquired ones’. (In his translation of The Generation of Animals, 1942, Peck says: ‘It will be seen that this translation, in spite of its sound of modernity, is a close representation of the original.’) Aristotle even speaks of a man from Chalcedon who was branded on his arm and whose child had a faint version of the same mark. The middle-aged Darwin proposed his version of pangenesis precisely because he thought that acquired characteristics might matter in evolution after all. Aristotle, however, will have none of it: ‘In fact the children of the disabled are not necessarily disabled just as children do not necessarily resemble their parents.’ Pangenesis also implies that if you were to prune some part of a plant then its offspring should grow up ready pruned, but they don’t.* Mutilations should be inherited, but they aren’t: the relationship between the parents’ bodies and the genetic content of their seeds must be much less direct.
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