The Lagoon Page 26

  Such an analysis is, however, incomplete. He may weave his webs of conditional necessity as wide or as deep as he pleases, but he must also give a final explanation for why a given animal has one set of features rather than another. I’ve suggested that Aristotle often doesn’t do so. He often doesn’t bother to relate an animal’s specific features to an ultimate cause, or if he does, only sketches it in a cursory way. When explaining life-history variation, however, these two kinds of explanation – conditional and final – come together brilliantly, for he argues that whether a bird invests in its parts, or in reproduction, depends on its bios or lifestyle. Raptors, he argues, need powerful wings, large feathers and massive claws in order to catch their prey; partridges and pigeons, which feed on grain and fruit, don’t. So raptors invest in wings and claws, have little nutrition left over for reproduction and so lay few eggs; partridges and pigeons do not invest in wings and claws, have lots of nutrition left over, and lay many.

  In his analysis of animal life-history variation, Aristotle uses quantitative data to descry the great patterns, parses out causal from accidental associations, and then explains the causal associations as the best possible compromise between physiological necessity and teleological need – that is, between the demands of their bodies and the demands of their world. It is, I think, his most complete and successful analysis of the function of any complex of animal parts; of how the nature of each animal finds the best of the possibilities that are available to it. Birds and tetrapods, however, embrace only a small fraction of possible life histories. His best analysis of why creatures reproduce as they do comes when he talks about fish.

  LXXXIV

  IN THEOPHRASTUS’ YEAR, SUMMER brings the rose campion, carnations, lilies, spike lavender, sweet marjoram and a delphinium called ‘regret’, of which there are two kinds, one with a flower like the larkspur, the other with white flowers that is used at funerals. The iris also blooms then and so does the soapwort, which has, he says, a beautiful, but scentless, flower.

  That is how summer starts. But by late July the Aegean archipelagos are scorched earth. In the olive groves cicadas cling to branches and sing for mates; in the pine forests firefighters slump in trucks and watch for incendiaries. (Theophrastus says that Pyrrha’s forest burnt and then regrew; it has doubtless done so many times since.) The rivers that feed the Lagoon run dry. The Vouváris always flows but even its spring is a stagnant pool and the waterfall at Pessa is a trickle. Its mullets are targets for rapier-billed herons while its eels have found sanctuary in the estuary’s mud. Its terrapins imitate stones. The phrygana covering the volcanic western shore, once soft, multi-hued and scented, is now just a threadbare cloak of brittle thorns.

  Even as the land bakes, the sea waxes. The summer wind that the locals call the boukadora (the ‘wind that goes inside’) blows into the Lagoon from the open sea and turns its surface into foam. In Kalloni’s depths the glutted spontaneous generators are spawning. But no one eats spermy oysters and mussels (sea-urchin gonads are another thing); it’s fish now, especially the polyvalent sardella. Kalloni’s pilchards are usually eaten salted as sardeles pastes, but now is the time to eat them fresh. Aristotle says that in summer the fish come into the Lagoon to spawn. If he means the pilchards, then he is only partly right for, although they have migrated in from the open sea, they did so as larvae. Kalloni isn’t their breeding ground, it’s their nursery, and by August they’re grown and fat and are heading out to their spawning grounds in the Aegean itself. Traversing the Lagoon’s narrow mouth, they’re intercepted by a wall of nets and landed by the ton.

  Aristotle’s analysis of bird and tetrapod life history, insightful though it is, doesn’t take into account the one factor that evolutionary biologists suppose is most important in shaping animal life histories: the pattern of age-specific mortality – that is, whether the risk of death weighs most heavily on juveniles or adults. Considering fishes he makes good the omission. He says that it is their function to be very prolific. Of course, it is the function of every living thing (spontaneous generators aside) to reproduce, but for fishes the goal is especially exigent because of their high rate of infant mortality: ‘The majority of the externally laid embryos are destroyed, and that is the reason why fish, as a kind, produce many offspring. For nature uses number to combat destruction.’

  To produce so many progeny, egg-laying fishes have a plethora of special features. Females are bigger than males so that they can hold all their embryos. As evidence of this need he points to some small fishes whose uteri seem to be just one mass of eggs. He also cites the belonē that literally bursts under pressure from them, though nevertheless survives the experience. He means the pipefish that lives in the eelgrass beds near the head of the Lagoon and broods its embryos in a pouch.* This is also why most fish eggs are only ‘perfected’ – fertilized – once they’ve been laid. Were they to become ‘perfected’ in the uterus, there would not be space for them all. Fish eggs are generally small, but once they’re fertilized the embryos and larvae also grow very quickly to ‘prevent the destruction of their kind which would occur as a result of their spending a long time over the period of their formation’. Finally, some fishes, such as the glanis (Aristotle’s catfish) care for their young to prevent them being eaten.

  To compensate for the high mortality of their embryos and larvae, egg-laying fish, then, have a whole suite of interlocking adaptations: high fecundity, small eggs, reverse-size sexual dimorphism, altricial development, rapid growth and parental care. Those selachians that, by contrast, give birth to live young have no need to be so fecund because their young are large and relatively perfect at birth and so ‘have a better chance of escaping destruction’. When I began this book, I said that scientists make poor historians for they tend to read their own theories into the past, but I also ventured that a scientist might see things that the classicists have missed, precisely because he does. Aristotle’s analysis of why tetrapods, birds and fishes have such varied numbers of offspring is as good example as any of this tension. For all that has been written about Aristotle’s biology, these passages have been, as far as I can tell, ignored. Yet any evolutionary biologist reading The Generation of Animals would seize on them for they are, or seem to be, about life-history theory, the part of adaptationist biology that considers the ultimate currency in which reproductive success is counted. Moreover, the structure of Aristotle’s analysis is instantly recognizable. Like ours, it is an analysis of the solutions that animals have found to the varied contingencies of their environments and how bodily economics shapes the form that those solutions take. Our theory is, of course, couched in equations, but that’s just a matter of expression; there are other, deeper differences (on which more later). The real question is whether Aristotle’s analysis is as important to him, as fundamental, as its analogue is to us. I think the answer must be that it is. For, at the bottom of Aristotle’s explanation of organismic diversity, is the claim that the ultimate purpose and desire of each and every creature is to reproduce: the cycle must turn again. So it was for him; so it is for us.

  In Skala, to celebrate the catch, there’s a panagyri where, for two nights, you can feast for free on piles of grilled pilchards and drink and dance, silver scales flashing like sequins on your feet. The music is self-consciously traditional; most of the songs are the songs of Asia Minor and dwell on Constantinople, the lost heavenly city. (When Smyrna burned in 1922, it was to Lesbos that the Greek refugees first came and where many stayed.) But one song was all about fish:

  I got into my new boat and set off from Agios Giorgos. I found some young boys, sailors, fishing: ‘You fishermen, do you have fish, lobsters and squid?’ ‘We have salted sardellas, like beautiful girls. Come aboard, pick them up, weigh them, Take a rope, string them up, and pay as much as you like!’*

  LXXXV

  IN THE VILLAGES SURROUNDING Kalloni, autumn is when old men reclaim their rights. Scandinavian girls and Dutch families no longer occupy their favourite c
hairs in the kafeneons; even the English nature-walkers, invariably in couples, have gone home. Old men can then sip ouzo, play backgammon and vociferously debate the issues of the day without having the proprietor, who rather liked the tourists, telling them to keep the noise down and not wave their sticks about.

  Life expectancy in the islands is often said to be high – Ikaria, due south of Lesbos, has even been touted as a kind of Aegean Shangri-La where nonagenarians bound about like goats. That’s an exaggeration, though there’s some evidence that elderly Ikarian women have high survival rates. What is true is that Greeks (and Italians and Spaniards) have, for all their love of cigarettes, higher life expectancies than the citizens of most Northern European countries, and that this appears to be due, at least in part, to a ‘Mediterranean diet’ rich in vegetables, fruits and nuts, olive oil and legumes and low in meat.

  That would have interested Aristotle. ‘We must investigate the reasons for some animals being long lived and others short lived and the length and shortness of life in general.’ So begins the treatise known as The Length and Shortness of Life. From Historia animalium the relevant data include, among much else: a report on the ephēmeron or mayfly which emerges from little sacks in the River Hypanis near the summer solstice by the Cimmerian Bosphorus and lives only for a day,* the (suspect) claim that the elephant lives for centuries and the observation that most winged insects die in autumn. Perhaps he is thinking of the cicadas, whose desiccated corpses litter the silent olive groves by summer’s end.

  Summarizing, Aristotle notes that plants tend to live longer than animals; large animals tend to live longer than small ones, blooded longer than bloodless, terrestrial longer than marine. No one feature predicts longevity very well, but together they say something about the relative fragility of kinds. There are plenty of exceptions: some plants (annuals) are very short lived; some bloodless animals (bees) are long lived; some large animals (horses) do not live as long as smaller ones (men). As so often, he descries the great patterns and notes the exceptions.

  He wonders whether it is possible to give a general explanation for death in all its forms, and whether the fact that some individuals and kinds live for so much longer than others has one explanation or many. These are, he admits, difficult questions. He therefore designs his theory of ageing to cope with both the general patterns and the exceptions. Indeed, he discusses the exceptions precisely to get simplistic explanations (‘big animals live long because they’re big’) out of the way and so open the door to a much more sophisticated account.

  To explain lifespan diversity Aristotle begins with the observation that living things are warm and moist. They are particularly so when young; the old are cold and dry and so are the dead. ‘This’, says Aristotle with conviction, ‘is an observed fact.’ He then argues that animals differ in the quantity and quality or heat of their moisture. Using these variables he cooks his explanations. Large animals and plants have (relatively) more hot-moist matter, and so live longer, than small ones. The exceptional lifespans (for their size) of humans and bees are due to the same cause. Marine bloodless animals (invertebrates) may be always wet (they live in the sea), but even so they’re short lived because of the low-heat-content stuff of which they’re made. Talk of heat content seems vague, but it’s all about fat. Of the various uniform parts, fat has a very high heat content and resistance to decay. (Of all foods, olive oil keeps in the kitchen.) Fat, in Aristotle’s view, is life-promoting stuff.

  None of this, however, explains why animals should age at all. Aristotelian animals are continually sustained by nutrition and have complex regulatory devices for keeping their metabolisms in check. As we age, something must deprive animals of the warmth and moisture that they need to stay alive. Aristotle thinks it is reproduction that deprives the body not only of the material it needs to grow some parts but of life itself. This gives him another way of explaining lifespan variation. It’s a matter not just of how much hot-moist matter animals start with but of how fast they spend it. Salacious animals, he says, age more quickly than continent animals. Mules, which are sterile, live longer than horses or donkeys; the cock-sparrow, an unusually lecherous bird, doesn’t live as long as his hen. Plants pay the same cost of reproduction. Annual plants die each autumn because they expend all their nutrition on seed. Aristotle seems to see a body as a bank account that is continually filled by the income of nutrition, but that is drained even more swiftly by the expense of maintenance and reproduction, and that, once overdrawn, dies. This is biological economics with a bite.*

  Turning to plants, Aristotle argues that the reason that they generally live longer than animals is partly due to their oiliness. But he often gives competing explanations for natural phenomena if he thinks the facts merit them, so he argues that plants are also long lived because they are capable of regeneration: ‘For plants are always being reborn; that is why they live so long.’ Roots, trunks and branches may die, but new parts grow up beside them. Moreover, as demonstrated by cuttings, ‘the plant possesses potential root and stalk in every part of it’ – indeed, a cutting is ‘in a sense a part of the [parent] plant’. And although he knows, or thinks he knows, that some animals can regenerate organs – snakes and lizards can regenerate their tails and nestling swallows their eyes – only plants can be continually reborn; only they have the ‘vital principle in every part’.* By ‘vital principle’ Aristotle means the soul.*

  Aristotle, then, believes that lifespan can be influenced by a variety of mechanisms. In another treatise, on Youth & Old Age, Life & Death, he gives a theory more tailored to vertebrates. Here he asserts that death is always due to the exhaustion of vital heat. Blooded animals have particularly active metabolisms and so are especially susceptible to the vagaries of the chemical conflict within them – all that concoction. That is why they have such elaborate homeostatic devices. The reason they die, then, is because these devices, in particular their thermoregulatory systems, fail. He even defines the life cycle in thermoregulatory terms: ‘Youth is when the primary organ for cooling grows, old age is when it is destroyed. The middle period is the prime of life.’ The destruction of the ‘cooling organs’ – lungs and gills – occurs because, as the animal ages, they become more ‘earthy’, less flexible, and finally simply seize up. Should that happen, metabolic meltdown ensues followed quickly by heat-death. Or, as Aristotle, puts it, the animal ‘suffocates’.

  Aristotle notes that the words for old age (gēras) and earth (geēron) are similar.* The etymology is false and, in any case, does not explain why lungs and gills become more ‘earthy’ with age. Perhaps he thinks that they accrete earth rather as smokers’ lungs accrete tar. Or perhaps he thinks that, losing warmth and moisture, they just become relatively earthier. The latter idea appeals since it links his two material explanations for ageing – and in fact he explain wrinkling skin in exactly this way.

  Yet there is an interesting difference between the two theories. Where the cost-of-reproduction theory is deterministic – there is a simple cause-and-effect relationship between the depletion of fat reserves and risk of death – the homeostasis-failure theory has a stochastic element. This is evident in passages in which Aristotle claims that old creatures are more susceptible to variation in their external environments, their health or the state of the internal fire. Old animals die from even trivial ailments as ‘a brief and tiny flame is extinguished by a slight movement’. Small animals are especially vulnerable because they have ‘but little margin in either direction’. It’s a picture of creatures beset with metabolic challenges that cause the vital heat to wax and wane in ways that if large or young they might well survive, but when old or small tip them over the edge.*

  It’s unclear why Aristotle thinks that ageing must be explained in different ways in different taxa. However, his various explanations all depend, in one way or another, on a creature’s metabolism and the devices that regulate it – that is, the workings of the nutritive soul. And that, in turn, implies that a creat
ure’s lifespan is not, for the most part, a matter of chance: it’s written in its form; it is part of what makes it one kind of animal rather than another.

  The peculiar fascination of Aristotle’s theory – or theories – of ageing is that they are answers to still-unanswered questions. The proximate cause – or is it causes? – of senescence are, for us, hardly less mysterious than they were for him. There is, of course, no shortage of scientists prepared to assert with confidence no less sublime than his that they know ageing’s secret, though if they do then they have failed to convince their colleagues of it. But, then, many of their explanations have scarcely more empirical content than his, and some have rather less.

  There is, however, one question to which our answer is better than his. Aristotelian and modern science both demand teleological or, if you prefer, adaptive explanations for most visible and ubiquitous biological phenomena. Hearts, feathers, teeth and genitals are adaptations; they exist for the sake of survival and reproduction. But what can the purpose of ageing be? Death has no obvious utility.

  Aristotle sidesteps the question. He says that it is just the ‘nature’ of living things on earth to age and die. All that remains to be discussed is how and when. Darwin sidestepped it too. He said even less. The omission was glaring. August Weissmann, Darwin’s German disciple, tried to fill the gap – and it’s as if he’s rebutting Aristotle. ‘I believe’, he wrote, ‘that life is endowed with a limited duration, not because it is contrary to its nature to be unlimited [italics mine], but because the unlimited existence of individuals would be a luxury without corresponding advantage.’ He then argued that old animals, worn and torn, are useless, even harmful, to the species and so evolution has devised ageing just to get them out of the way.