Darwin's Island Part 3

The eighteenth century was a vintage era for 'wild children', those raised - metaphorically or otherwise - by wolves, in the fashion of Romulus and Remus. The naturalist Linnaeus cla.s.sified them as h.o.m.o ferus - h.o.m.o ferus - wild men - whose nature would reveal what made thinking humans, wild men - whose nature would reveal what made thinking humans, h.o.m.o sapiens h.o.m.o sapiens, different. Most of the supposed examples were fakes, but a few were not.

In 1797, a young boy was found alone and almost naked in the forests of the Aveyron, in south-central France. He was captured, escaped, recaptured and escaped again, but in time he emerged from the woods under his own volition. He was about twelve years old, unable to speak and savage in his behaviour. A vicious scar on his throat hinted that his parents had tried, but failed, to kill their aggravating child. The lad appeared to have been without contact with others for almost his whole life and showed no obvious signs of joy, fear or grat.i.tude when at last he met members of his own species. Here, perhaps, was an opportunity to investigate the springs of emotion.

A young student, Jean-Marc-Gaspard Itard, heard the story and saw the chance to test Rousseau's ideas. He took the forlorn boy to Paris and set out to try to raise him to the spiritual level of his fellow citizens.

Itard had trained as a tradesman, but took up medicine at the time of the French Revolution and later became a pioneer in the study of diseases of the ear, nose and throat. In stark contrast to Rousseau he was convinced that the essence of the human condition lay in the ability to sense the feelings of others and, armed with that talent, to build a society in which pa.s.sions could be kept in check for the good of all. In his 'Historical Account of the Discovery and Education of a Savage Man' he set out his theory that 'MAN can find only in the bosom of society the eminent station that was destined for him in nature . . . that moral superiority which has been said to be natural to man, is merely the result of civilization'.

The doctor took young Victor - whom he named after one of the few sounds, 'o' (as in the French word for water), he was able to recognise - into his household and attempted to train him to express, and respond to, emotions. He was soon disappointed. The boy was 'insensible to every species of moral affection, his discernment was never excited but by the stimulus of gluttony; his pleasure, an agreeable sensation of the organs of taste; his intelligence, a susceptibility of producing incoherent ideas, connected with his physical wants; in a word, his whole existence was a life purely animal'.

Itard laboured for five years with both kindness and cruelty (the latter based on his charge's fear of heights) to transform the boy from monster into Frenchman, but with little success. Victor's behaviour stayed strange: he was obsessed with the sound of cracking walnuts but ignored gunshots close to his ears, and loved to rock water back and forth in a cup. He never learned to speak and showed no grat.i.tude for food or shelter. The sole sign he made of any response to the sentiments of others was that, when Itard's housekeeper was in tears after the death of her husband, Victor appeared to comfort her. Apart from that he stayed apart from his fellow men.

His protector insisted that the young man's failure to adapt to the inner worlds of those around him and to express his own feelings arose because he had been rescued too late to pick up the skills needed, but that view was too optimistic. The lad would nowadays be diagnosed as deeply autistic; as unable to respond to, or give, the signs - the smiles or frowns or conversations - that bind people to their parents, to their friends and to the community in which they live. The dire effects of the illness show how the expression of our own emotions and our response to those of others makes us what we are.

The term 'autism' was invented in the 1940s to describe a condition in which children fail to interact or to smile or express sentiments apart from anger and unhappiness. They speak with difficulty or not at all and are filled with obsessions about particular foods, places or clothes. About a third suffer from epilepsy. Three out of four of those with a grave form of the illness struggle to cope with society throughout their lives. Autism shades from the severe disturbance shown by Itard's Wild Boy himself, through Asperger's syndrome, in which the language problems are less marked, to general problems in the development of normal conduct. Often, the problem is noticed when parents become concerned by their child's depression or rage. Some autists, once unkindly referred to as idiots savants idiots savants, have remarkable talents in drawing or in particular mathematical tasks, but their gifts do no more than disguise their deeper problems. Once, the illness was said to be rare, with one child in two thousand affected, but now the diagnosis is made far more often, with an incidence of one in a hundred in Britain.

Autists cannot understand the signals of their fellows or make the full complement of their own. All children have that difficulty in their earliest years. As Darwin wrote in the Sketch of an Infant Sketch of an Infant, 'No one can have attended to very young children without being struck at the unabashed manner in which they fixedly stare without blinking their eyes at a new face; an old person can look in this manner only at an animal or inanimate object. This, I believe, is the result of young children not thinking in the least about themselves, and therefore not being in the least shy, though they are sometimes afraid of strangers.' For most infants such self-absorption soon pa.s.ses but an autistic child is locked into that phase for life. Many, when they look at other people, ignore the eyes, the flags of sentiment. They are just as unconcerned when someone else gazes long and hard at them.

The Expression of the Emotions used the blush as a prime example of a social cue but embarra.s.sment plays a lesser part in life today. Yawns - unacceptable in a nineteenth-century parlour - are more frequent. We do not know why we open our mouths when tired or bored (although the book discusses the gesture as a threat in baboons). Yawn and the world yawns with you and even to read about it can spark the gesture off, as about half the readers of this book can now attest. The habit begins at about the age of six. Not, however, for children with autism, for a yawn sparks off far fewer responses among them than among the general population. Such failures of empathy lie behind many of their problems. used the blush as a prime example of a social cue but embarra.s.sment plays a lesser part in life today. Yawns - unacceptable in a nineteenth-century parlour - are more frequent. We do not know why we open our mouths when tired or bored (although the book discusses the gesture as a threat in baboons). Yawn and the world yawns with you and even to read about it can spark the gesture off, as about half the readers of this book can now attest. The habit begins at about the age of six. Not, however, for children with autism, for a yawn sparks off far fewer responses among them than among the general population. Such failures of empathy lie behind many of their problems.

Psychologists talk of 'theory of mind', the ability to infer the mental state of others from smiles, frowns, gestures and speech. People with autism have little or no insight into the inner world of their fellows and cannot express their own internal universe in a way that makes much sense to those around them. They are blind to the messages written on another's countenance and find it hard to separate gestures of anger, fear, sadness or joy. Like chimpanzees (but unlike dogs) some autistic children cannot understand what is meant when their parent or doctor points at an object. They are denied even that simple social talent.

Autists also find it harder to tell people apart or to recognise a photograph of themselves. A certain group of brain cells is activated when monkeys or men see or copy the movements of others or observe an expression of pain, fear or disgust. They are also involved in the shared response to a yawn or a smile. These mirror neurons, as they are called, are almost silent in children with the severe form of the disease. Perhaps they are part of the system that helps us see into the souls of those around us. In their failure they condemn people with autism to a world whose other denizens act in a mysterious and unpredictable way.

n.o.body knows what causes autism and the condition has no cure, even if some of its symptoms such as insomnia or depression can be treated. The illness is four times more frequent in boys than girls, but shows no fit with race, social cla.s.s or parental education. Infection, immune problems, vaccines, heavy metals, drug use while pregnant, Caesarean births and defective family structure in Freudian mode (the child psychologist Bruno Bettelheim spoke of 'refrigerator mothers') have all been blamed but those claims do not stand up. Some say that the brain of a typical autistic child grows too fast too soon, but then slows down. The amygdalae - those detectors of fear - are overactive in some patients, but many other parts of the brain have also been implicated. Problems with serotonin, that universal alibi for disorders of emotion, may be to blame, for some autistic children synthesise the stuff less well than normal. Certain drugs used against depression can help, as a further hint of a tie between social isolation and the emotional universe.

Genes are without doubt involved in some patients, even if not more than a tenth or so of cases can be ascribed to a definite genetic cause. If an identical twin has autism its sib is at a seven-in-ten risk while the figure risk for non-identicals is far lower. The incidence increases by twenty times above average in the brothers and sisters of those with autism and some among them are tactless, aloof or silent but are not diagnosed as ill.

Such behaviour sometimes presents itself as part of a larger medical problem. Fragile-X syndrome is the commonest cause of mental disability among boys. It comes from a huge multiplication of a short segment of DNA upon the X chromosome. Some patients have symptoms quite like those of autism and some individuals diagnosed with that condition may in fact have the chromosomal abnormality. Other deletions, duplications or reversals of a segment of chromosome are behind other cases of the illness. Often, these arise anew in the children compared with their parents. Some badly affected patients have problems with a gene involved in the transmission of impulses between nerves. A few may have errors elsewhere in the DNA - and dozens of genes, with a variety of tasks, have been blamed. One candidate belongs to a group of genes that is multiplied in number in humans compared with all other mammals, is active in the brain and is damaged in at least a few autistic children. In spite of such hints the biology of autism remains obscure and there are likely to be several explanations for a condition that is not a single disease but many.

Autistic children are an experiment in emotion. Their isolation is mental rather than physical, for they are cut off by an inability to respond to the flow of information that pa.s.ses between others. A world full of autists could not function, for all societies depend on a silent dialogue in which every member's intentions are overtly or otherwise expressed. Civilisation turns on the ability to bear another's company.

Those who break civilisation's rules must be punished; and part of that invariably involves the manipulation of a criminal's mental state. Prisons are, of their nature, places in which social interactions are forcibly reduced. Solitary confinement is a penalty far more severe than mere imprisonment, for it is autism imposed: a permanent denial of what it means to be human, inflicted upon someone who once experienced the full range of human emotion. The penalty is bitter indeed and is much appealed to by punitive societies, from medieval England to the modern United States. Charles d.i.c.kens visited such a penitentiary in Philadelphia and wrote that 'I hold this slow and daily tampering with the mysteries of the brain to be immeasurably worse than any torture of the body.' The infamous 'Supermax' at Marion, Illinois, a jail built to hold the most violent offenders, together with political prisoners such as Black Panthers and members of the American Indian Movement, allowed almost n.o.body out of their cells for twenty years, even to exercise. It closed in 2007, but some of its forty and more replacements are just as brutal. Some even feed their inmates on tasteless 'Nutraloaf ' further to reduce their contact with the world of the senses. Many inmates - like autists - become anxious, agitated and angry, and may end in insanity, killing themselves should the chance arise.

If Zacarias Moussaoui, sentenced to life in solitary for his supposed ties to the Twin Towers outrage, were allowed reading material in his soundproofed Colorado cell he might learn something from both d.i.c.kens and Darwin about why he feels such hatred for those who do not share his views. As books are not available, he may wish instead to spend his solitary hours in contemplation of the expression of a condemned prisoner as the electricity pa.s.ses through his head, which is said - in an echo of the great naturalist's own observations - to be of ungovernable horror.

CHAPTER IV.

THE TRIUMPH OF THE WELL-BRED.

Charles Darwin was worried about his plans for marriage. Perhaps the whole idea was a mistake because of the time that would be wasted on family life at the expense of science. His diary records how he agonised over the pros and cons of matrimony, and his decision to 'Marry, marry, marry!' And marry, in the end, he did.

His spouse was his cousin, Emma Wedgwood. In falling for a relative he stuck to a clan tradition. The Darwins, like many among the Victorian upper crust, had long preferred to share a bed with their kin. Charles's grandfather Josiah Wedgwood set up home with his third cousin Sarah Wedgwood. Their daughter, Susannah, chose Robert Darwin, Charles's father. Charles's uncle - Emma's father - had nine offspring, four of whom married cousins. The evolutionist's own marriage was in the end happy, with ten children (and when his wife was in her forties he wrote that 'Emma has been very neglectful of late for we have not had a child for more than one whole year'). Even so, in Queen Victoria's fecund days the Darwin-Wedgwood dynasty did less well than most, for among the sixty-two uncles, cousins and aunts (Emma and Charles included) who descended from Josiah, thirty-eight had no progeny that survived to adulthood.

Six years after his wife's last confinement Darwin began to think about the dangers of inbreeding, in particular as they applied to his own choice of spouse. His concern was picked up from another of his cousins, Francis Galton, the founder of eugenics, who had pointed out the potential dangers of marriage within the clan.

Charles was anxious about his children. His tenth and last, Charles the younger, died while a baby; he was 'backward in walking & talking, but intelligent and observant'. Henrietta had a digestive illness not unlike that of her father and took to her bed for years, and he feared that his son Leonard was 'rather slow and backward' (which did not prevent his later marriage to his own cousin or his acceptance of the Presidency of the Eugenics Society), while Horace had 'attacks, many times a day, of shuddering & gasping & hysterical sobbing, semi-convulsive movements, with much distress of feeling'. His second daughter, Elizabeth, 'shivers & makes as many extraordinary grimaces as ever'. George's problem was an irregular pulse, which hinted at 'some deep flaw in his const.i.tution' and, worst of all, his beloved Annie expired at the age of ten, throwing her parents into despair. As he wrote, 'When we hear it said that a man carries in his const.i.tution the seeds of an inherited disease there is much literal truth in the expression.' Once he even wrote to a friend that 'We are a wretched family & ought to be exterminated.' Might his own illness and that of his sons and daughters be due to his own and his ancestors' choice of a relative as life-partner? Was inbreeding a universal threat?

His first statement of concern came three years after The Origin The Origin, as an afterword to his book On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects, and on the Good On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing Effects of Intercrossing. The last paragraph of that hefty work, most of it devoted to botanical minutiae, ends: 'Nature thus tells us, in the most emphatic manner, that she abhors perpetual self-fertilisation. This conclusion seems to be of high importance, and perhaps justifies the lengthy details given in this volume. For may we not further infer as probable, in accordance with the belief of the vast majority of the breeders of our domestic productions, that marriage between near relatives is likewise in some way injurious,-that some unknown great good is derived from the union of individuals which have been kept distinct for many generations?'

The idea that children born to related parents might suffer harm was already in the air. The first study of its risks came in 1851 when Sir William Wilde (father of Oscar) found, in work years ahead of its time, an increased incidence of deafness among the progeny of cousins. Sir Arthur Mitch.e.l.l, the Deputy Commissioner in Lunacy for Scotland, had earlier claimed that in the inbred fishing communities of north-east Scotland the average hat size was six and seven-eighths, a quarter-inch less than that of their more open-minded agricultural neighbours; proof, he thought, of the malign effects of the marriage of kin upon the mental powers.

s.e.x within the household has a venerable history. The Pharaohs lived through generations of the habit in an attempt to preserve the bloodline of a G.o.d. Akhenaten, who lived around 1300 BC, first married his cousin Nefert.i.ti, then a lesser wife, Kiya, and then three of his own daughters by Nefert.i.ti and then (perhaps) his own mother. The story is confused by difficulties with sorting out quite who was who (and one of his supposed wives was in fact male), but incestuous affairs were without doubt common in ancient Egypt. Cleopatra herself may have been the scion of ten generations of brother-sister unions. The practice is condemned in Leviticus, where the Children of Israel were enjoined that 'After the doings of the land of Egypt, wherein ye dwelt, shall ye not do.'

The belief that the children of cousins are bound to be unfit, and the desire of all rulers to control their citizens' private lives, still fuels a jaundiced view of the joys of s.e.x within the household. In 2008, a British government minister, in reference to the Pakistani population of Bradford, made the quite unjustified claim that 'If you have a child with your cousin the likelihood is there will be a genetic problem.' Many of his fellow citizens share that vague Galtonian sense that inbreeding is harmful. Most of their alarm rests on anecdote rather than on science.

All states are interested in how their subjects behave in the bedroom. For years, England based its marital rules on those of the Church of England, which descend from those of the Israelites, themselves established to put an end to the habits of the Pharaohs. In 1907, after hundreds of hours of parliamentary discussion, the statutes were at last clarified. The new legislation removed absurd anomalies such as the biologically senseless law that forbade a widower to marry his dead wife's sister but it also firmed up the prohibition against s.e.x with close kin, be it father with daughter, or brother with sister.

Politicians often act on the basis of prejudice. Darwin did not. When faced with a scientific question - about s.e.x or anything else - he set out not to speculate but to discover. To learn more about inbreeding he turned again to plants. The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom appeared in 1876. It gives an account of experiments on a wide variety of hermaphrodite plants forced to mate with themselves. His verdict was clear: 'The first and most important of the conclusions which may be drawn from the observations given in this volume, is that cross-fertilisation is generally beneficial, and self-fertilisation injurious.' It was 'as unmistakably plain that innumerable flowers are adapted for cross-fertilisation, as that the teeth and talons of a carnivorous animal are adapted for catching prey'. The exchange of genes between unrelated individuals was the rule and selfing an expensive exception. What was true of plants must, he imagined, apply to animals, men and women included. appeared in 1876. It gives an account of experiments on a wide variety of hermaphrodite plants forced to mate with themselves. His verdict was clear: 'The first and most important of the conclusions which may be drawn from the observations given in this volume, is that cross-fertilisation is generally beneficial, and self-fertilisation injurious.' It was 'as unmistakably plain that innumerable flowers are adapted for cross-fertilisation, as that the teeth and talons of a carnivorous animal are adapted for catching prey'. The exchange of genes between unrelated individuals was the rule and selfing an expensive exception. What was true of plants must, he imagined, apply to animals, men and women included.

Flowering plants have long been known to have s.e.xual habits more inventive than our own. Charles's grandfather Erasmus's poem The Loves of the Plants The Loves of the Plants is a work of science in two hundred pages of Arcadian verse. Many lines deal with the balance between male and female interests ('Each wanton beauty, tricked in all her grace, Shakes the bright dew-drops from her blushing face; In gay undress displays her rival charms, And calls her wandering lovers to her arms' - in other words, this species needs a pollinator). His descendant asked deeper questions in plainer prose. He found that many plants live in a reproductive universe that would have shocked the shepherds of Arcady. They have a system of choice that transcends the familiar preferences of one gender for its opposite. Most retain their original nature as hermaphrodites, with male and female parts in the same flower, but they span the range from obligate self-fertilisers to others that make an absolute demand for pollen from another individual. Many among the latter group have to ensure that they do not accept genes from their close kin, and have imposed additional and refined laws of s.e.xual choice upon their mates. is a work of science in two hundred pages of Arcadian verse. Many lines deal with the balance between male and female interests ('Each wanton beauty, tricked in all her grace, Shakes the bright dew-drops from her blushing face; In gay undress displays her rival charms, And calls her wandering lovers to her arms' - in other words, this species needs a pollinator). His descendant asked deeper questions in plainer prose. He found that many plants live in a reproductive universe that would have shocked the shepherds of Arcady. They have a system of choice that transcends the familiar preferences of one gender for its opposite. Most retain their original nature as hermaphrodites, with male and female parts in the same flower, but they span the range from obligate self-fertilisers to others that make an absolute demand for pollen from another individual. Many among the latter group have to ensure that they do not accept genes from their close kin, and have imposed additional and refined laws of s.e.xual choice upon their mates.

The cross-fertilisation book was a first step in the scientific study of s.e.x. Fifteen years earlier its author had noted that 'We do not even in the least know the final cause of s.e.xuality; why new beings should be produced by the union of the two s.e.xual elements, instead of a process of parthenogenesis.' Not much has changed. We are still not certain how the habit persists in the face of its obvious drawbacks in terms of cost, stress and more - in Dr Johnson's famous words, its expense d.a.m.nable, its position ridiculous and its pleasure fleeting. Why mate, when virgin birth ensures that your own genes have a guaranteed chance of survival? Parthenogenesis - virgin birth - guarantees that all the genes of those who indulge in it reach the next generation. It looks like the obvious solution but parthenogenesis remains rare - a few lizards and fish, and about one species of flowering plant in a thousand. For a hermaphrodite, a bout of s.e.x with oneself also ensures that the DNA is not diluted with that of an unrelated individual. For creatures with separate males and females incest - s.e.x with a close relative - is quite effective at keeping genes in the family, but that too is frowned upon.

Plants hint at the answer. Plenty among them have, like men and women, separate s.e.xes. Some - like the strawberry or the dandelion - have gone to the opposite extreme for they are parthenogens and propagate themselves with shoots, roots or broken fragments. The majority of the flowering kinds have taken a lesser step towards as.e.xuality for they are hermaphrodites that bear male and female functions on the same individual.

In spite of the chance they have for s.e.x with themselves some hermaphrodites insist on exchanging genes with a stranger. Others in that situation are, in contrast, happy to self-fertilise and will accept genes from a different flower on the same individual, or evolve flowers with both male and female structures that can fertilise themselves. That pattern marks a real step towards the abandonment of s.e.x.

Animals, too, have often tried to give up that habit. Some, like praying mantises and certain lizards, are true parthenogens, while a few such as snails and worms are hermaphrodites. Others go in for more bizarre forms of close copulation. The habits of mites would astonish any p.o.r.nographer. A certain parasite of locusts gives birth to two types of male. The first clambers back into his mother and fertilises her. With their help she then produces a second brood, with a few males included - and those males then have s.e.x with their sisters. In another mite, a mother has s.e.x with her grandson, the scion of her own daughter, the daughter herself a child of the mother herself and a son. Other mites confine themselves to brother-sister pairs - but they copulate before they are born.

Darwin saw that, when it comes to s.e.x, plants are more convenient subjects for experiment than animals. Self-pollination in hermaphrodites marks a biological indulgence more extreme than the matings between cousins that so concerned him, or between sons and mothers, fathers and daughters and brothers and sisters. He set out to explore how often it took place, what it did to the health of his subjects and, with luck, to learn a little about the importance of s.e.xual reproduction in general.

Within a few months of starting work as a planned pollinator in the greenhouse at Downe he found that the effects of inbreeding could be dramatic. With the help of a botanical condom - a fine mesh to keep out insects - and a small paintbrush he could himself, like a bee, move male cells to the female parts of a flower and could arrange that the plant received its own genes, or those of another individual.

First, Darwin noted that certain species would self-fertilise, while others refused to do so even when obliged to try. Among those that did, he discovered - somewhat to his alarm - that the habit did damage later generations. His initial experiments were on toadflax, a common yellow-flowered weed. In the wild, outcrossing was the rule. In the greenhouse, he could force his subjects to self, and soon found a large, and unexpected, effect upon the next generation. The progeny of such crosses were smaller and less vigorous than were those of plants allowed to mate with another. At first he supposed that his inbred offspring were weakly because of some disease, or because they were grown in unsuitable soil. That was not so, for however well they were treated they stayed feeble. Darwin ran through a variety of species - carnations, tobacco, peas, monkey-flowers, morning glory, foxgloves and many other garden and wild flowers. With statistical help from Francis Galton he discovered that, almost without exception, those grown from crossed seed were taller, healthier and more productive than were those from self-fertilised. Some experiments went on for several generations, and the effects of s.e.x with a relative got worse with time. The inbreds suffered most of all when life was hard: when they were crowded, had to compete with their outcrossed kin or were moved from the greenhouse to the rigours of the open air. The malign influence of selfing applied almost as much to species that went in for it in nature as to those that almost never did so.

Consistent as most of his results were, once or twice, out of thousands tested, the descendants of a self-fertilised individual were healthy and vigorous. Sometimes they even outgrew their compet.i.tors. A particular selfed morning glory (a species normally damaged by inbreeding) he referred to as 'Hero' because its line flourished under that mode of reproduction. Why, he did not understand.

Even so, in most plants in the wild, s.e.x with a close relative appeared to be impossible, an expensive error or - at best - a doctrine of last resort. The implications for humans were, perhaps, alarming.

Darwin's views were, we now know, too inflexible. Almost all hermaphrodite plants have at least the potential to fertilise themselves and many do so as a matter of course (he also denied the importance of selfing in animals and was again mistaken: I myself once worked on hermaphrodite slugs, who manage quite well with s.e.x within their own skins). Only about one in five hermaphrodite plants prefers to self as a general rule. Others that normally outcross will self when no alternative is available and only a minority avoids the habit altogether. The notion that grew up in the century or so after his book that plants could be divided into two distinct types based on s.e.xual habits is wrong. In fact, they go from obligate selfers to determined outcrossers, but most are happy to adopt either practice as conditions change. Some shift from s.e.xual to as.e.xual in different places or as the seasons move on. For others selfing is a side-effect of s.e.x, a kind of green onanism when a bee, as it flits from flower to flower, fertilises one flower with pollen from another on the same individual.

The decline in fitness of self-fertilised individuals and the occasional appearance of healthy lineages both emerge from the simple rules of inheritance (which were not known to the nineteenth century) and from the existence of large amounts of hidden genetic damage in most plants and animals. A hermaphrodite that bears two different versions of a particular gene - for example, a pea in which the DNA that codes for seed shape has the instructions for round seeds paired with another set for wrinkled - will, after self-fertilisation, produce half the next generation with the round-wrinkled mix, a quarter with round alone and a quarter with just wrinkled. If they are again selfed, the pure round and pure wrinkled plants will have offspring identical to themselves while those with both kinds of instruction will repeat the proportions that emerged in the previous generation - a quarter pure round, a quarter pure wrinkled and the remainder with a single copy of each variant. As selfing goes on, a smaller and smaller proportion of the population retains both versions of the shape gene. Generations of such crosses hence lead to the emergence of 'pure lines', within which every individual is identical: all with round seeds or all with wrinkled. The same is true for every other variant in each line. In time, each will contain different combinations of genes for seed shape, colour, height and so on.

To make a pure line is not easy, for if the original population contained hidden variants harmless in single dose but harmful in double (as many do), its descendants pay the price when such variants are exposed in double copy. The more damage is concealed, the smaller the chances of success. The effect can be spectacular. In loblolly pines, natives of the southern United States, just a fiftieth of eggs fertilised by pollen from the same plant survive. As a result it is almost impossible to make an inbred line.

Lines of genetically identical plants are at the centre of modern agriculture. Thousands have been produced for use as crops or as garden flowers. Wheat, rice, barley, tomatoes and more - all have their reproductive lives controlled by farmers and almost all are the descendants of a few survivors from vast numbers of inbred lines who paid a fatal price for the damaged genes hidden within their ancestors. The survivors were those who drew lucky in the s.e.xual lottery. Darwin's discussions with breeders told him that the first few generations of kin mating caused the effects to get worse and worse - because, we now know, more and more harmful genes emerge in double copy as time goes on. Now and again, as in the famous morning glory 'Hero', an inbred line wins because it inherits, by chance, genes that increase, rather than reduce, its ability to survive. Its descendants thrive and may, in time, be used in their millions on farms or in gardens.

Farmers, consciously or not, have built on that observation and the same is true in the wild. In some species, well-adapted inbreds emerge to cope with the horrors of Nature. Millions of identical individuals then fill the landscape. The advantages of self-fertilisation often depend on how predictable life may be. The practice is commoner when the struggle to survive involves starvation or bad weather, which come back more or less unchanged each year. Perhaps, by chance, a set of genes emerges that deals well with food shortage or with cold. It then pays to stick to that well-adapted combination rather than to mix it with other genes during a spasm of s.e.x. Selfing is more frequent in cold and starved northern forests and when life is short or when few mates are available. For hermaphrodites whose pollinators are grounded by bad weather, or if the season is so bad that it becomes hard to make a decent flower, a shift to selfing also makes sense, for the choice is between an unhealthy brood or none at all.

If - as in the tropics - the main enemies are parasites or predators, who themselves use s.e.x to shift their own tactics by scrambling up their genes, that tactic does not work. For a plant to stick to a single strategy is to play poker with the same hand each time against an opponent who reshuffles on each deal. Sooner or later the other player will draw an unbeatable combination, and the selfer will go bust.

The rules that apply to plants work for animals too. When - in an echo of the Down House experiments - wild mice are mated brother with sister in the laboratory and the offspring released into nature almost none survive. Inbred animals do not often die of obvious genetic disease, but their parenthood can much weaken them. Song sparrows on a small island off Canada's west coast have been ringed for years and their pedigrees worked out in detail. Those born to close relatives are at more risk of death in bad weather than are other birds. The island of Soay, in the St Kilda group, is famous for its native sheep, which have been there since Viking times. The animals are filled with worms, and those with the heaviest burden suffer most of all when faced with a vicious Scottish winter. Lambs born to close relatives have more parasites, and are at more risk of death in storms, than others. Once purged with medicine, though, the inbred animals survive the tempest as well as do their outbred kin, as proof that the worms are to blame. The finches on the Galapagos also pay the price for s.e.x with close relatives, as do wild shrews, red deer, seals, toads, bats and many other creatures.

Endangered species are, given the shortage of mates, at particular risk. Planned parenthood can help. The Florida panther was once on its last legs, with many animals plagued with kinky tails and undescended t.e.s.t.i.c.l.es because of the exposure of genetic damage in the tiny and inbred population that remained. In 1995, eight female Texas cougars (a related species) were introduced, and now the natives, with the help of their relatives' genes, have fought off their tail and t.e.s.t.i.c.l.e problems and returned to genetic health. The situation in zoos is just as bad. A mere hundred or so Arabian oryx were left in the wild by the 1960s, and most were soon killed by hunters. Two males and a female were rescued, and a dozen or so additional individuals were already in captivity. The entire world population of two thousand individuals, domestic and nominally wild, now descends from those few founders - and the most inbred individuals still have fewer young than average.

If s.e.x within the family is bad for plants and animals, what might it do to people? Many are convinced of its dangers and many societies try to limit the practice. Every one of the United States has some restriction on marriage between relatives and cousin marriage is a criminal offence in eight states and in a further twenty-two is at least illegal, although the rules are often ignored. An attempt to ban such alliances was defeated in Maryland as recently as the year of the millennium. In a nod to the eugenic agenda, Wisconsin restricts them to couples in which the wife or husband is infertile, or where the wife is over fifty-five. The traditions of particular groups can lead to a reluctant acceptance of difference; uncle-niece marriages are allowed in Rhode Island but only for Jews, and male Native Americans in Colorado are allowed to marry their step-daughters even if elsewhere the habit is, in spite of the lack of any shared genes, against the law. Europe is just as confused. In Cyprus the Church prohibits marriage between second cousins but in Belgium incest is not even mentioned in the statute books. Britain, too, forbids incest and even prohibits the - rare - s.e.xual liaisons between grandfather and granddaughter. Sweden, in contrast, permits half-sibs - children with one parent in common - to enter into marital bliss should they wish. The nation has even considered the legalisation of wedlock between brother and sister.

It is, needless to say, impossible to carry out planned crosses with men and women, but Charles Darwin came up with another way to test the dangers of inbreeding. First, he tried to get questions about cousin matrimony included in the 1871 census. He pointed out that 'the marriages of cousins are objected to from their supposed injurious consequences; but this belief rests on no direct evidence. It is therefore manifestly desirable that the belief should either be proved false, or should be confirmed, so that in this latter case the marriages of cousins might be discouraged.' His request was debated in the Commons but thrown out as 'the grossest cruelty', for it would cause children to be 'anatomised by science' (and from a parliamentary point of view the issue was almost traitorous, for the Queen herself had wed her cousin). A query about 'lunatics, imbeciles and idiots' was allowed but was dropped for the next census a decade later as most people refused to answer it. Darwin was annoyed by his failure to persuade Parliament to ask a scientific question and complained about 'ignorant members of our legislature'. His son George was even more caustic about 'the scornful laughter of the House, on the ground that the idle curiosity of philosophers was not to be satisfied'.

George Darwin set out to build on his father's work. From the records of Burke's Landed Gentry Burke's Landed Gentry and the and the Pall Mall Gazette Pall Mall Gazette, together with a circular sent to lunatic asylums, he worked out the frequency of cousin marriage in various groups. Such unions were, it transpired, twice as common among n.o.blemen as among the proletariat. His enquiry to the superintendents of asylums as to how many of those in their care were the scions of related parents was, as they pointed out, unlikely to pay off because of the mental state of their charges. Even so, George found no increase in the level of inbreeding among the patients compared with that of the general population (even if the Deputy Commissioner in Lunacy for Scotland did a.s.sure him that most of his nation's idiots were the children of relatives).

After his mixed success with lunatics, the young man went on to study the inmates of Oxford and Cambridge colleges. He chose the boat-race crews - 'a picked body of athletic men' - and asked how many had been born of cousins. After a correction for a falsified answer from the stroke of Corpus Christi College Cambridge, he found that there was indeed a slight shortage of such inbred individuals among top oarsmen compared with the general population. The same was true among sporting boys in the princ.i.p.al schools for the upper and middle cla.s.ses. In both cases the numbers were small and the evidence not altogether persuasive.

Then George had an idea that might produce large amounts of information on the s.e.x lives of the British people. He used surnames - an inherited character - to estimate the extent of marriages among relatives. Two people with the same name, particularly a rare t.i.tle such as 'Darwin' or 'Wedgwood', are, he realised, more liable than average to descend from a common ancestor. Indeed, Sir William Wilde had already found that the parents of deaf children had a higher chance of a shared surname.

George Darwin himself did little with the idea, but surnames have now been a.n.a.lysed in their millions. They give a new insight into mating patterns. In most places, they pa.s.s, like the Y chromosome, down the male line. The fit between the two is real, but for several reasons not precise. Common names ('Jones' - which means 'son of John' - included) originate many times in different places. To confuse the issue further, children are adopted into new families, or agree to change their name for testamentary advantage. Some people take up a new tag because they do not like the label they were given and illegitimacy, too, is a problem. These frailties weaken the link between shared names and shared genes.

Even so, a random set of a hundred pairs of British men who each had the same surname showed a real tendency for them to have a common set of genetic variants on the Y. Two males who bore the same rare tag were far more liable to share a Y chromosome than were two sharing a frequent name. Names hence provide a real insight into genetic history - and the police are already interested in tracking down criminals by using DNA to search for surnames.

The failure with the census, and his son's ambiguous results from idiots and Etonians, suggested to Darwin that perhaps the effects of human inbreeding were less dire than he had feared. Honest as always, he admitted that 'my son George has endeavoured to discover by a statistical investigation whether the marriages of first cousins are at all injurious, although this is a degree of relationship which would not be objected to in our domestic animals; and . . . he has come to the conclusion that on the whole points to its being very small'. He removed his comment on its harmful effects from the second edition of the Orchids Orchids book. book.

The effects of cousin marriage on health were too small to be picked up by Darwin but the ma.s.s of information now available, from official records, from surnames and from patterns of shared genes, makes it clear that its influence cannot be ignored.

European aristocrats, like those of ancient Egypt, have long married their kin. In a world in which n.o.body mates with a relative, and with the births of parents and children separated by twenty to thirty years, each reader of these pages could have had, at the time of Charles Darwin's birth seven generations ago in 1809, a hundred and twenty-eight different ancestors - two multiplied by itself seven times. For almost everyone that figure is too high, for some marriage among those who have no idea that their spouse is a distant relative is inevitable. Even so, social pressure for matrimony within the household can do a lot to reduce that number. Alfonso, the Infante of Spain, who died in the 1960s, had just twelve - rather than more than a hundred - ancestors seven generations back, King Alfonso XII, a contemporary of Darwin, had sixteen, while plenty of others in that n.o.ble line had between fifteen and twenty great-great-great-great-grandparents - far fewer than expected in a s.e.xually open society. The Spaniards are still keen on the pastime, with levels of cousin marriage well above the European average, and their nation has the most inbred villages on the continent, Asturias in the north being the most inward-looking place of all.

The habit also thrives elsewhere. Many of India's thousands of communities insist on alliances within their own group. The Dravidian Hindus of the south have encouraged s.e.x between cousins, or between uncles and nieces, for two thousand years, and continue to do so, with one marriage in ten between a man and his brother's daughter. In large parts of Africa and the Middle East, a fifth of all alliances - and in some places even more - is between close relatives. The practice is also common in the world of Islam. The Prophet discouraged the idea, but he did marry off his daughter Fatima to her cousin Ali, which in the eyes of the faithful legitimises the habit. The tradition is not, contrary to popular belief, central to that religion but has become part of its culture. In Bombay, one Hindu marriage in fifteen is between close relatives, but for Muslims the figure is three times as high. Among the nomadic Qashqai people of Iran, the incidence of cousin marriage is three in four.

The picture in the West is quite different. Most of us rarely meet our relatives, let alone sleep with them. In most places less than one marriage in a hundred is between cousins. The English have long been among the least inbred nations in Europe. George Darwin had imagined that farm labourers 'would hold together very closely' but he was surprised to find how few weddings among kin there were in the rural districts of his time. England has never had a peasant cla.s.s that sits on its own land for centuries and falls for the girl next door for lack of a better candidate. John Bull was a tenant. His master often forced him off and provided, as an unexpected bonus, a wider choice of s.e.xual partners. In modern Britain, husbands and wives are, on the average, sixth cousins. Most share no more than a great-great-great-great-great-grandparent in common - someone who lived before Charles Darwin was born - and most have no idea at all that they are related. Only on islands, real or metaphorical, do we go in for the habit. Many Northern Irish travellers and a fifth of the people of parts of the Hebrides choose a first or second cousin as a mate. On Darwin's island today, faith is a better barrier to s.e.x than are miles. In Bradford, the Pakistani community is among the most inbred in the world, with its British-born children having an even higher chance of marrying a cousin (often one still in Pakistan) than did their parents.

Social pressures can also reduce the extent of inbreeding. In many places the prospect of prison for s.e.x with a cousin has, no doubt, often put paid to romance. In hunter-gatherer days, men and women roamed the landscape to about the same extent. With agriculture, life changed. The geography of men and women as seen in the patterns of Y-chromosome and mitochondrial genes shows that the males tended to stay at home while their mates came from elsewhere. The son inherits the capital and does not wish to move or share with a neighbour. He prefers a wife from far away who is, as an incidental, unlikely to share his genes.

Close inbreeding can - as Darwin had found in the greenhouse - impose a real burden. All human populations contain damaged genes, manifest only when inherited in double copy. The children of cousins may pay the price for that legacy. As Bagehot wrote: 'It has been said, not truly, but with an approximation to truth, that in 1802 every hereditary monarch was insane' and inbreeding was at least in part to blame. Every reader of these pages carries at least one gene in single dose that would kill them if present in double copy. Most inborn faults are hidden by normal versions of the same gene; for example, one British child in two thousand five hundred is born with two doses of the damaged gene that leads to cystic fibrosis, but one Briton in twenty-five has a single copy. If relatives mate and if their common ancestor bore a faulty piece of DNA, the chance that each partner will inherit that fault by virtue of shared descent goes up. Their children are then at increased risk of receiving two damaged versions.

The malign effects of s.e.x within a closed pool were noticed long ago. Umar Ibn Al-Khattab, the second Caliph and direct follower of Muhammad, advised members of a certain tribe to marry out because, he believed, they had become weak and unhealthy through their habit of s.e.x with kin. Its power became manifest in 1908 when Sir Archibald Garrod identified an inborn illness called alkaptonuria as a condition in which two copies of a faulty piece of DNA were needed to show their effects. An enzyme that breaks down certain food substances is damaged. Symptoms include dark ear wax, smelly urine and, later in life, heart, skeletal and other problems. The disease is rare, with just one case in every twenty thousand births, but Garrod found that more than half of the parents of his patients were cousins. The same is true for other such conditions. In France, with one marriage in five hundred between first cousins, the incidence of cystic fibrosis is over-represented by seven times in the children of such matings.

In an unfortunate coincidence, certain places with a lot of s.e.x among kin - North Africa, the Middle East and the Indian subcontinent - also have a high incidence of inherited blood diseases that protect against malaria. Sickle-cell is carried in single dose by almost half the members of some African populations and related errors are almost as common in other places. Each is dangerous when inherited in double copy.

In Saudi Arabia, where in some villages eight out of ten people marry their cousins, such diseases are common. A fifth of all admissions to children's wards are due to hereditary disorders. Many families are unaware of the dangers and the devotion to cousin marriage remains. Doctors now advise those at risk to screen pregnancies in the hope that damaged foetuses might be picked up before they are born and the government insists that engaged couples are tested to see if both carry a blood disorder. No overt pressure is brought to bear, but the incidence of marriages within the family has dropped by a fifth.

Even in places without high levels of inborn disease children born to cousins die younger than usual. Such unions in Utah Mormons - not themselves an unusually inbred population - from the mid-nineteenth to the mid-twentieth century led to a notable increase in ill health. The effects became worse as the infants grew older, perhaps because death in old age has a stronger genetic component than do the accidents of infection or starvation that killed the pioneers' babies. Heart disease is also more frequent among the children of cousins. A study of half a million pregnancies in the modern United States suggests that the death rate of the sons and daughters of cousins rises by about 5 per cent above average. The products of uncle-niece marriages, a pattern frequent in India and elsewhere, do even worse and while incest - s.e.x between sibs, or parents and children - is rare the offspring pay a high price. A German brother and sister, adopted at birth and strangers until they met as adults, had four children, two of whom were severely affected. A study of thirty or so Canadian children born to such parents also suggests that almost half inherit some abnormality.

A more subtle, but more marked, effect of within-family s.e.x has emerged in Iceland. Among a hundred and fifty thousand couples born between 1800 and 1965, partners who were close relatives had more, rather than fewer, children than average. Even so, the proportion of the children of first and second cousins who themselves reproduced (and hence the number of grandchildren born to the pair of relatives) was well below average, in part because many of those first-generation progeny died young. Charles Darwin and his cousin Emma may have been testimony to that effect, for seven of their ten sons and daughters expired before their time or lived on but stayed childless. Close mating may be more harmful to a family's prospects than was once supposed.

Continued inbreeding leads to a decrease in variability within a lineage. The effects soon extend across the entire genome. The overall level of DNA variation is lower in people who emerge from a limited pool of ancestors and, as a result, the level of inherited diversity gives an insight into the extent of inbreeding. Many illnesses - diabetes, heart disease and more - are more frequent, and more severe, among those so revealed to have a history of s.e.x with kin. In Bradford, some members of the Pakistani community are uniform in long stretches of their genome. They pay the price in terms of health and even their general liability to infection goes up. In Darwin's day, childhood death came in the main from contagious disease. His beloved Annie died of tuberculosis (although the diagnosis was the obscure 'bilious fever with typhoid character') but her plight may, as he feared, in part have been due to her parents' marital history.

s.e.x is, needless to say, more complicated in the bedroom than in the greenhouse and the simple fit of health with kinship does not always hold. In some places, relatives marry to keep wealth within the household, which means that there must be some wealth around in the first place. In parts of India and Pakistan, cousin marriages are commoner among the affluent than among the poorest, for the latter have no financial incentive to set up home with a relative. The effects of cash outweigh those of genes. As a result, the children of cousins are less, rather than more, liable to suffer ill health. As is true for the sheep on Soay, the environment also plays a part. In the poor and embittered j.a.pan of the 1940s, cousin marriage had a large influence on infant welfare, but twenty years later the effect had almost disappeared.

For both plants and people, s.e.x (when not with oneself) must involve another party. Almost always, he or she must choose or be chosen from a pool of potential mates. The process calls for hard decisions. Some are obvious. Whites tend to marry whites and blacks blacks, while the rich marry the rich and the tall the tall and, some say, men tend to find a wife who looks rather like their mother. Plenty of religions make it hard to find a partner outside the creed. Language, place of birth, education and more also affect the choice of candidate. All this means that for any man or woman the number of possible mates is far smaller than it might appear to be. That observation is familiar enough, but biology sharpens up human s.e.xual decisions in ways both obvious and less so.

The fundamental question about s.e.x is: why bother? The habit is expensive for s.e.x reduces the number of potential mates. It imposes the simple rule that only some individuals - those of a different gender from oneself - are available for copulation. Self-fertile plants circ.u.mvent that demand, but s.e.xual creatures (ourselves included) have their choices much restricted by it. Often the rules become even more stringent. Evolution generates laws that ensure that no longer can any male mate with any female (or vice versa). They ensure, as a result, that an individual of one s.e.x will be accepted by no more than a fraction of those of the other. s.e.x becomes more s.e.xual than before. The process is driven by the need to avoid inbreeding.

Darwin discusses such issues in his book The Different Forms of Flowers on Plants of the Same Species, The Different Forms of Flowers on Plants of the Same Species, published a year after the volume on self-fertilisation. It begins with a simple tale: the story of the children of the village of Downe, who made necklaces from cowslips. They could, they told him, use just a few of the plants, those with a long 'pin' that protruded from the flower, through which they could thread the plants together. Other flowers, instead of a pin, had no more than a short protrusion called a 'thrum' and were of no use as juvenile jewellery. The cowslip's close relative, the primrose, was much the same. published a year after the volume on self-fertilisation. It begins with a simple tale: the story of the children of the village of Downe, who made necklaces from cowslips. They could, they told him, use just a few of the plants, those with a long 'pin' that protruded from the flower, through which they could thread the plants together. Other flowers, instead of a pin, had no more than a short protrusion called a 'thrum' and were of no use as juvenile jewellery. The cowslip's close relative, the primrose, was much the same.

Pins and thrums represent an additional mechanism of s.e.x choice. Female 'pin' flowers were, Darwin found, much happier to accept pollen from male 'thrums' than they were from males of their own kind. The same applied in the opposite direction. A male needs a female, but the cowslip asks for more. The flower's form is inherited - which means that the plants decide whether or not to accept another's pollen advances on genetic grounds. Like mates only with unlike, so that a second s.e.xual filter reduces the chance of an encounter between plants that bear similar genes. It is, as a result, a precaution against inbreeding. With pins and thrums, Nature has come up with a trick to reduce the proportion of individuals with whom genes can be shared. She has, in effect, invented more and more s.e.xes.

Darwin's greenhouse experiments on cowslips have grown into a science that shows how, in both primroses and people, partners are chosen in unexpected ways and that the choice may decrease the prospects of successful mating between those who have recent ancestors in common. Thirty or so plant groups have evolved systems rather like that of the cowslip and primrose. He found others in which the flowers came in not two but three forms, each of which would not accept a partner from within their own cla.s.s.

Now we know many more examples of such physical barriers to s.e.x. Some species produce flowers that are mirror images of each other and can cross only left to right and right to left. In a bizarre system found in tropical gingers, some individuals are male in the morning and female in the afternoon while others prefer the opposite pattern. As a result they are obliged to exchange genes with those whose s.e.xual timetable is different from their own. Once again, the imperative is to avoid carnal relations with those like themselves - with kin.

In his crossing experiments, Charles Darwin had no real idea of how and why his experimental subjects accepted or rejected particular kinds of pollen when he placed it on their female organs. He referred only to the 'extreme sensitiveness and delicate affinities of the reproductive system', which is poetic rather than persuasive. In fact, as in the cowslip and the tropical ginger, they make a test of kinship before deciding whether to accept a mate. The female parts judge the hopeful male cells by comparing their genes with their own. They reject any pollen grain if the similarity is too close.

The process is at work in the many hermaphrodites that insist on outcrossing. Like sperm, a grain of pollen contains but a single set of genes. Unlike the const.i.tuents of that potent liquid, the male s.e.x cell from a flower must fight its way through a barrier of female tissue to reach the egg. To do so it grows a long pollen tube that penetrates into the appropriate part of its partner. Her protective layer bears the normal double complement of DNA. For her, to choose is simple: compare the pollen with her own tissues and if the two share too many genes, block it. For species that prefer to self-fertilise, the rule is relaxed or reversed.

For outcrossers, the system ensures that unrelated mates have the best chance of success. A new version of the ident.i.ty cues carried by pollen is almost certain to succeed, for in its novelty it charms its way into the affections of all females, none of whom bear it themselves. As the generations go on, the new gene spreads - but it begins to lose its magic as more and more females inherit it and reject males with a matching copy. Each shift in male ident.i.ty goes through the same process and in time a system emerges in which almost every individual has his or her own unique s.e.xual calling card. That allows females to make decisions about the kinship of the hopeful males and to choose those most different from themselves. Other species delay that decision until later. The pollen is allowed to fertilise the egg, but matings with close kin are not allowed to develop further.

Animals - ourselves included - have a similar set of mechanisms, with a variety of genetic ident.i.ty tests before s.e.x is allowed. Some are obvious, while others are less so.

Simple familiarity can breed contempt. Unrelated Jewish infants brought up together in kibbutzim, or Asian children betrothed and made to live together when they are tiny, may prefer to avoid s.e.xual contact when they grow up, and - in the latter case - are, after the arranged marriage, said to be less fertile and more liable to divorce than average. Brothers and sisters also tend not to fancy each other. Older sibs feel a stronger sense of aversion to their younger fellows than do the young to the old. The degree of kinship is the same, but the older child can be almost certain that the junior members of the household are the products of their own mother for they saw them cared for as babies. A younger sib, on the other hand, knows only that an older individual lives under the same roof - which could happen for other reasons. They are less repelled by the idea of s.e.x with somebody who might not, after all, be a relative. It takes fifteen years of shared residence for a younger brother or sister to build up the erotic revulsion that an older member of the family can generate by watching a few months of childcare.

Social pressures play a large part in our marital patterns, but genes are involved too. Some are obvious - people do, after all, tend to marry someone of the same skin colour as themselves - but others are more subtle.

As a boy, I kept mice in my bedroom, a fad quashed because of the awful stench. At the time that was no more than a nuisance, but in fact the aroma of mouse urine was an introduction to a new world of s.e.xual contact, through the nose. Quite unexpectedly, mice have more genes than we do. Almost all the extras are involved with the sense of smell. The genes that code for smell receptors - most of them decayed in the human race - are in full order. Mice have hundreds, which together can tell apart a vast diversity of scents. The animals choose both food and mates through the nasal pa.s.sage.

Given the choice, an inbred laboratory female mouse prefers to mate with a male from a different line. So keen is she on a new swain that a pregnant female will resorb her foetuses to render herself available. Bedding soaked with male urine has the same effect. The females a.s.sess health as well as kinship. Their acute nostrils sniff out those who carry parasites and avoid them. Perhaps - as in the wormy sheep on the Isle of Soay - the healthiest males, with the most impressive statements of their fine condition, are less inbred.

Mice live in an aggressive s.e.xual universe. Each male dominates a small patch in which h