* If, for some collection of related animals (say, mammals), we plot some feature (horn size, metabolic rate, longevity, etc.) against body size, we very often find that the feature does not scale with body size, but increases faster or slower. The relationship is, in modern jargon, allometric rather than isometric; indeed it is often best described by an exponential rather than a linear function on the arithmetic scale. The mathematics of allometry were first worked out by Julian Huxley in the 1920s, so Aristotle didn’t use them, but he did notice the phenomenon and try to explain it. Many have followed. Stephen Jay Gould famously used allometry to explain the monstrous antlers of the giant Irish elk, but didn’t credit Aristotle for having pioneered the area – I suppose he hadn’t read him.
* In practice he’s not always so puritanically functionalist: he’ll speak of a mole’s eye without qualification even as he tells us that moles are blinded by a layer of skin covering the eyes.
* For a diagram of Aristotle’s metabolic network see Appendix II.
* Here lies a trap for the unwary. For Aristotle, synthesis is the formation of a mixture (an agglomeration of parts) and mixis the formation of a compound (a new substance). Confusingly, the modern English cognates of these terms – synthesis and mixing – mean exactly the opposite. Translations do not always make this clear.
* The idea of an internal fire resembles, of course, our own concept of cellular respiration which is, literally, a slow combustion. But where for Aristotle the important product of the internal fire is heat itself, for us high energy bonds such as those found in ATP drive ‘concoction’ – macromolecular catabolism – and heat is just a by-product.
* For a diagram of the full CIOM model see Appendix III.
* It is anatomically wrong – the vessels that connect the lungs to the heart are the pulmonary arteries and veins, but in living animals they are filled with blood and not, as Aristotle supposes, air. It is chemically wrong – since Aristotle hasn’t read Lavoisier he does not know that combustion is a reaction that combines a component of air, oxygen, with a fuel. (In fact, he explicitly considers, and rejects, the possibility that the internal fire is nourished by air.) This leads him to the notion that the effects of air on fires (or life) must be due to cooling. It is physically wrong – the model depends on the idea that the intensity of a fire is affected by ambient temperature but, of course, this is not so. Besides the heart–lung cycle he also thinks that in blooded animals the internal fire is cooled by the brain and damped down by nutrition; that, too, is wrong.
* In Youth & Old Age, Life & Death Aristotle says that this breathing accounts for the buzzing sound that insects make, but in Historia animalium he’s clear that it’s caused by the motion of wings.
* Searching for a name for his new science, Wiener began with ‘governor’, the name given to the device that regulates a steam engine. This led him to its Latin ancestor, gubernator, and thence, via an etymological trail, to the word’s ultimate Greek ancestor kybernētēs or ‘pilot’ which he considered particularly apt since the steering devices of ships were especially good examples of negative feedback control systems. From kybernētēs came ‘cybernetics’. It was a felicitous choice since the steersman is an ancient metaphor for control used by both Plato and Aristotle in the context of political hierarchies.
* For a control diagram of the heart–lung cycle see Appendix IV.
* This definition seems to exclude egg-laying fishes which, we know, have external fertilization. Aristotle certainly knows that many male fishes sprinkle milt over newly laid eggs, but exactly what he thinks is going on during fish mating is unclear – he admits it’s an obscure business.
* Hedgehogs, bears, camels, lions, lynxes and hares do not copulate like this.
* Aristotle devotes pages to sorting out female vaginal discharges including: urine, vaginal lubrication, pathological discharges, post-partum bleeding, menstrual bleeding in humans and oestral bleeding in animals that come into heat. He argues, correctly, that the first three have little to do directly with reproduction but, incorrectly, thinks that menstruation (in humans) and oestral bleeding (in dogs and cows) are the same thing – katamēnia – that is, the sperma (seed) which the mother contributes to the embryo. These two secretions are, in fact, quite different. Only primates truly menstruate and Aristotle knew only one primate, humans, at first hand.
* Aristotle’s claim that virgin chickens lay hypēnemia (literally ‘wind eggs’) is not true now. All supermarket eggs, which are large and have perfectly formed yolks, are produced by virgin chickens. However, the first eggs produced by a pullet – a young chicken – are often small and yolkless; substitute ‘virgin’ for ‘young’ and you have Aristotle’s claim. He may also be strictly correct. Modern breeder hens are very odd birds. They have been selected for egg production for thousands of years; perhaps ancient breeds only laid the odd wind egg when virgin. Certainly, at least some bird species only start laying eggs once mated.
* Many anatids (ducks, geese and swans) have a taste for violent, coercive sex and elaborate intromittent organs to do it with. A recent report gives the Argentinian lake duck a 20cm-long, corkscrew-shaped, spine-bearing penis.
* Aristotle expects testes to be round, but fish and snake testes are elongate so he takes them to be the equivalents of the tetrapod seminal ducts, in fact, the vas deferens. The mistake is a surprising one, since he knows that fish seminal ducts fill seasonally with semen just as bird testes do, which should suggest a similar function. This is comparable to his uncertainty about the identity of fish and bird kidneys since they lack the classic tetrapod shape.
* Since the testes aren’t, in fact, counterweights, Aristotle’s ingenious explanation for the looping vas deferens must be wrong. So what’s the true function of the loop? The anwer, curiously, is that it doesn’t have one. It’s a contingent, non-adaptive product of a mammalian evolution history in which internal testicles descending from the abdominal cavity, their ancestral position, to between the loins, happened to take an inefficient route. Here Aristotle’s teleology over-reaches itself. At least it does so if the standard evolutionary account is correct.
* In his Essays Montaigne quotes Aristotle to the effect that a ‘man . . . should touch his wife prudently and soberly, lest if he caresses her too lasciviously the pleasure should transport her outside the bounds of reason’. I don’t know where he got this dismal bit of advice from, but it certainly wasn’t Aristotle.
* Evolutionary biologists have also puzzled over the function of the female orgasm. Male orgasms are an obvious adaptation, a direct inducement to reproduce, but women, however much they may like one, don’t need an orgasm to conceive. If, then, the female orgasm has a function, it must be a rather subtle one, and there are many ingenious accounts of what that might be. Some biologists have even argued that it has no adaptive function at all, but is merely a developmental by-product of selection for male pleasure – the genital equivalent of male nipples. This will strike most of us as implausible.
* When Aristotle claims that the smell of a cock partridge can cause a hen to ‘conceive’ he means only that it can induce her to produce wind eggs that never complete development. An experienced pheasant breeder has told me that this isn’t so – young hens produce wind eggs regardless of the presence of a male, but I wonder whether the influence of male pheromones on partridge oogenesis deserves further study.
* Aristotle mentions three fishes that might reproduce without males: the khannos, erythrinos and psētta. The khannos is the comber Serranus cabrilla; the erythrinos is probably the anthias, Anthias anthias; the identity of the psētta is a mystery, but is thought to be a textual error for the perkē, the painted comber, Serranus scriba. If these identifications are correct, then all three of Aristotle’s ‘female-only’ kinds are members of the Serranidae. In 1787, Cavolini showed that S. cabrilla and S. scriba are simultaneous hermaphrodites; Anthias is a protogynous (female-first) hermaphrodite in which males are rare. In the simultaneous hermaph
rodites the testes are small and hard to see. Aristotle, having failed to find the testes, or any males, therefore moots the possibility that these fish kinds reproduce without sex.
* Perhaps Aristotle is describing the female grasshopper, or some other orthopteran, who, during copulation, bends her long, pointed ovipositor to meet the more modest genitals of the smaller male perched on her back. Even so, it’s unclear why such an arrangement forbids true insemination.
* Aristotle thinks that an excess of female seed causes conjoined twinning. But he also wants to argue here that an excess of male seed doesn’t. The argument is based on an ingenious, but incorrect, theory of the causes of conjoined twinning. Aristotle is, however, right to suppose that the second-in-line male often succeeds in fertilizing the eggs. The phenomenon is known as ‘last male precedence’, is found in many bird species and is due to sperm competition.
* This model of fertilization persisted for centuries through the history of developmental biology. It did so even after 1677, which is when Leeuwenhoek reported the spermatic ‘animalcules’ that he had seen under his microscope. Fabricius supposed that semen did its mysterious work via an ‘irradiant or spiritous faculty’; Harvey, scorning his teacher’s terminology, said it works by ‘contagion’. Both could just as easily have said it works by pneuma or bubbles. Even von Baer’s model of fertilization was still very Aristotelian. (It was von Baer who called sperm spermatozoa – itself a name redolent of their ambiguity.) It was only in 1875 that Oskar Hertwig definitively showed that the embryo begins with the fusion of a sperm and egg nucleus – and did so by looking at a very Aristotelian creature, the edible sea urchin, Paracentrotus lividus. That, however, required a microscope. Proof that chromosomes are the carriers of inherited information had to await Thomas Hunt Morgan’s 1910 fruitfly experiments – and even then there were sceptics. In 1928 William Bateson, one of Mendel’s earliest champions, and the man who coined the word ‘genetics’, was still arguing that inheritance operates by a system of intra-nuclear ‘vibrations’ – that is to say, movements.
* The whole passage, paraphrased and using modern anatomical terms, runs like this: the three-day-old embryo has a heart that beats and from which two blood vessels, the left and right vitelline arteries, ramify into the capillaries of the yolk sac. The body, head and eyes can be seen. In the ten-day-old embryo, the head is still bigger than the body, the eyes are large enough to dissect out and several membranes – the chorion, allantois, amnion and yolk sac – can be seen. These membranes are separated from each other by fluid-filled spaces; the amniotic sac is vascularized; the yolk has become more liquid and there is less albumin. The stomach and other viscera can be seen. The twenty-day-old chick has down all over its body and it is bent so that its head is next to its leg and covered by a wing. The allantois now contains excreted material and its connection to the chick has been severed; the yolk sac has been almost entirely absorbed within its stomach. The chick sleeps, wakes up, moves, looks up and chirps; it is about to hatch. So much for the development of birds.
* Aristotle does better than Leonardo, who infamously sketched a human foetus attached to a cow’s cotyledonary placenta. On the other hand, he does not have a technical term for the placenta.
* One that stood until very recently. In the past few years, transcriptomic data have shown that the very earliest stages of embryos are also quite variable. It is now thought that the embryos of different, related species are most conserved at some intermediate stage. In vertebrates, this is around the time of somite formation and neurogenesis. After that, the pattern is as Aristotle and von Baer said it is.
* Looking around his kitchen for a chemical analogy to explain early embryogenesis, Aristotle, strikingly, reaches for one based on enzymes (fig-juice and rennet both contain proteases). He hints at the idea of catalysis for he thinks that the active ingredients do not become part of the product; he fails to grasp it for he also thinks that they become consumed in the reaction.
* The kordylos is obviously a newt eft or frog tadpole, but it’s unclear that Aristotle knows that it is a larva; he may think that it is a ‘dualizing’ adult like a seal or a dolphin. When Aristotle speaks of the minute organ that dictates the future development of the animal, zoologists will be irresistibly reminded of the endocrine organs – the hypothalamus, pituitary or thyroid – that control amphibian metamorphosis. Aristotle probably means the heart – he usually does.
* That, however, was really just a lucky guess or, perhaps, a programmatic statement. It certainly wasn’t an empirical generalization. Karl von Baer discovered the minuscule, if not actually microscopic, mammalian ovum only in 1827. Less fortunate than Harvey in his patrons, he found it by dissecting a colleague’s dog.
* Not to be confused with ‘epigenetics’ in the modern sense – that is, chemical modifications of DNA, or chromosomal structure, that result in altered patterns of gene expression.
* Perhaps this long-horn is another species, the Barbary sheep, Ammotragus lervia, since modern North African Berber sheep are notably hornless.
* Darwin suspects that hump-back cattle may, in fact, be derived from a distinct species of ancestral bovid; they are now thought to descend from a distinct subspecies, Bos primigenius indicus, where European cattle are derived from B. p. taurus.
* A syndactylous mutant, of the sort known in Louisiana as ‘mule-foot hogs’.
* In another passage he hints that fleece colour might be heritable, albeit in a strange way – namely, that the colour of the veins under the ram’s tongue predicts the colour of its offspring. I doubt that this is true; at least some farmers that I asked about it seemed puzzled.
* In the Politics Aristotle suggests that the state should regulate marriage and the production of children with a view to raising healthy children; he even recommends that deformed children be killed. But nowhere does he adduce hereditary arguments for such laws so his argument is not eugenic.
* Done deliberately, this is known to botanists as a ‘common garden’ experiment and is used for exactly the purposes that Theophrastus invokes: to unravel the contribution of heredity and environment to phenotypic variation.
* The Hippocratic theory is so similar to Darwin’s that modern scholars use his label with no sense of anachronism even though it was more sophisticated. In The Variation of Animals and Plants under Domestication, Darwin says that William Ogle had told him that Aristotle had known, and rejected, a very similar theory to his own. Since Darwin never read much Aristotle, and certainly not The Generation of Animals, there’s no doubt he hit upon it independently.
* This is his version of the ‘Jewish foreskin’ argument: if acquired characteristics are inherited, then why, after millennia of circumcision, are Jews still born with foreskins?
* I considered the possibility that people are inherently biased to see a resemblance between sons and fathers and daughters and mothers. So, in a small experiment, I asked thirty-five parents to rate their fifty-five children for ancestral resemblance (father, mother, paternal grandfather, etc.) in a variety of features (nose, eye shape, hair colour, etc.). From this I constructed, for each child, a ‘paternal’ and ‘maternal’ similarity score. It was not a very powerful test; even so the scores for boys and girls were indistinguishable, so the bias, if present, must be small. People seem to think that a child’s features can come from either parent regardless of its sex. Of course, some of my subjects will have had a dim recollection of high-school Mendelian genetics, and so it’s possible that perception was different in ancient Greece, but I doubt it.
* The term ‘blending inheritance’ is usually associated with the theory proposed, in a hostile 1867 review of Darwin’s Origin, by the Scottish engineer Fleeming Jenkin. But it was Francis Galton who drew a clear distinction between particulate and blending inheritance. Of course, a continuous trait distribution does not necessarily imply particulate rather than blending inheritance for, as R. A. Fisher famously showed in 1918, a continuous distribution is compatibl
e with particulate inheritance if we assume that many particles contribute to the phenotype. That, indeed, was the basis of the reconciliation of the Biometricians and the Mendelians and the explanation for why many traits (skin colour, height) can be continuous and yet be controlled by particulate genes. However, the Hippocratic author (i) hasn’t read Fisher and (ii) clearly talks about fluids rather than particles, so his theory must be a blending one.
* The data are improbable, but not implausible, for the inheritance of human skin pigmentation is complex. In general, however, one would expect the daughter to be coffee coloured and the grandson to be fairer than that.
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