THE EMPEROR, clad in black, hobbled into the great hall. An audience of powerful men had assembled in the Palace of Brussels on October 25, 1555, to listen to a speech by the Holy Roman emperor Charles V. At the time, he ruled over much of Europe as well as wide swaths of the New World. A few years before, Titian had painted his portrait, astride a war horse, encased in armor, brandishing a lance. But now, at fifty-five, he had become toothless and blank-stared. As he made his way to the front of the hall, he had to lean on both a cane and Prince William of Orange. Trailing Charles was his twenty-eight-year-old son, Philip. There was no question that they were related. Father and son alike had lower jaws that jutted far forward, leaving their mouths to hang open. Their shared look was so distinctive that anatomists later named it after their dynasty: the Habsburg jaw.
Father and son climbed together up a few steps onto a dais, where they turned and sat before the assembly. They listened to the president of the Council of Flanders announce that Charles had summoned the audience to witness his abdication. They would now have to transfer their allegiance from Charles V to Philip II, his rightful heir.
Charles then rose from the throne and put on a pair of spectacles. He read from a page of notes, reflecting on his forty-year reign. Over those decades, he had expanded his power across much of the world. In addition to Spain, he ruled the Holy Roman Empire, the Low Countries, and much of Italy. His power extended from Mexico to Peru, where his armies had recently crushed the Inca Empire. Waves of ships rolled back east across the Atlantic, arriving at Spanish ports to unload gold and silver.
Starting in the 1540s, however, Charles had begun to flag. He developed gout and hemorrhoids. His battles now ended in fewer victories and more stalemates. Charles grew depressed, sometimes so despondent he would lock himself away in a chamber. His chief consolation was his son. Charles had put Philip in charge of Spain when he was still just a teenager, and Philip had amply proven himself fit to inherit Charles’s power.
Now, in 1555, Charles was content to make him a king. As he finished his speech, he turned to Philip. “May the Almighty bless you with a son,” he said, “to whom, when old and stricken with disease, you may be able to resign your kingdom with the same good-will with which I now resign mine to you.”
It took a couple of years for all the formalities to get squared away, for Charles to retire to a monastery that he filled with clocks, and for his son to be crowned. But during all that time, the transition rolled along smoothly. No one objected to transferring their allegiance. What could be more natural, after all, than a prince succeeding his father? For anyone else to take control of the empire would have been to defy the laws of heredity.
Heredity—herencia in Spanish, hérédité in French, eredità in Italian—originally came from the Latin word hereditas. The Romans did not use their word as we typically use ours today, to describe the process by which we inherit genes and other parts of our biology. They used hereditas as a legal term, referring to the state of being an heir. “If we become heirs to a certain person,” the jurist Gaius wrote, “that person’s assets pass to us.”
It sounded simple enough, but Romans fought bitterly over heredity. Their conflicts accounted for two-thirds of all the lawsuits in Roman courts. If a wealthy man died without a will, his children would be first in line to inherit his fortune—except any daughters who had married into other families. Next in line would be the father’s brothers and their children, then more distantly related kin.
Rome’s system was one among many. Among the Iroquois, a child might have many mothers. In many South American societies, a child could have many fathers; any man who had sex with a pregnant woman was considered a parent to her unborn child. In some societies, kinship had meaning only through the father’s line, others only through the mother’s. The Apinayé of Brazil had it both ways: The men trace their ancestry back through their father’s line, while the Apinayé women trace theirs back through their mother’s. The words people used for their kin reflected how they organized relatives into a constellation of heredity; Hawaiians, for example, could use the same term for both sisters and female cousins.
Medieval Europe inherited some of Rome’s hereditary customs, but over the centuries new rules emerged. In some countries, the sons split their father’s land. In others, only the eldest inherited it. In others still, it went to the youngest son. In the early Middle Ages, daughters sometimes became heirs, too, but as the centuries passed, they were mostly shut out.
As Europe grew wealthier, new hereditary rules took hold to keep the fortunes intact. The most powerful families of all took on titles and crowns, which were handed down through hereditary succession, to a son, preferably; if not, then a daughter or perhaps a grandnephew. Sometimes the branches of a dead monarch’s family would fight for the crown, justifying their claim on heredity. But these claims became hard to judge when the memories of ancestors faded.
Noble families fought this forgetting by putting their genealogies in writing. In the Middle Ages, Venice’s Great Council created the Golden Book, which every son from the prominent old families of the republic signed on his eighteenth birthday. Only those whose names were recorded in the book could become members of the council. As unbroken lines of descent from noble ancestors became more important, leading families paid artists for visual propaganda. At first they represented heredity as vertical lines, but later they started painting simple trees. They might paint the founder of a noble lineage at the base of the tree, and his descendants perched on branches. The French gave these pictures a name in honor of their forking shape: pé de grue, meaning “crane’s foot.” In English, the word became pedigree.
By the 1400s, pedigrees had become instantly recognizable, as evidenced by a pageant that was put on in 1432 to honor Henry VI of England. The king, only ten years old at the time, had been crowned king of France. On his return to London, the city came out in force to celebrate his expanded power. Giant tableaux lined his path. He passed towers and tabernacles; Londoners dressed up as Grace, Fortune, and Wisdom, as well as a multitude of angels. The centerpiece of the citywide display was a castle constructed from green jasper, displaying a pair of trees.
One tree traced Henry’s ancestry back to the early kings of both England and France. The other was a tree that traced Jesus’s ancestry all the way back to King David and beyond. These trees were a blend of fact and fiction, of display and concealment. They represented only those supposed ancestors whose kinship bolstered Henry’s claim to power. The trees lacked siblings and cousins, bastards and wives. The most important omission of all was the House of York, Henry’s rivals to the throne. But erasing them from Henry’s tree did not erase them from history. Henry VI would be murdered at forty-nine, after which the House of York seized control of England.
When Charles V abdicated in 1555, he created a pageant of his own. Father and son stood on stage, side by side. The noblemen who sat before the emperor and his prince silently endorsed the hereditary transfer of power. Perhaps, as they listened to Charles deliver his speech, they turned their gaze from father to son and back. If they settled their gaze on the royal jaws, they would not have said that Charles had inherited his jaw from his father. They could recognize a family resemblance, but they did not explain it with the language of thrones and estates.
To account for why Charles and Philip looked alike, sixteenth-century Europeans relied largely on the teachings of ancient Greeks and Romans. The Greek physician Hippocrates argued that men and women both produced semen, and that new life formed when the two were mixed. That blending accounted for how children ended up with a mix of their parents’ characteristics. Aristotle disagreed, believing that only men produced the seeds of life. Their seeds grew on menstrual blood inside women’s bodies, developing into embryos. Aristotle and his followers believed a woman could influence the traits of her children, but only in the way the soil can influence how an acorn grows into an oak tree. “The mother is not the true parent of the child which is called hers,” the Greek playwright Aeschylus wrote. “She is a nurse who tends the growth of young seed planted by the true parent, the male.”
The classical world had less to say about why different parents passed down different traits—why some people were tall and others short, why some were dark and others pale. One widespread notion was that new differences arose through experiences—in other words, people could pass down a trait they acquired during their lives. In ancient Rome, for example, there was a prominent family called the Ahenobarbi. Their name means “red beard,” a trait that set them apart in bright contrast to Rome’s dark-haired majority. The Ahenobarbi themselves had started out dark-haired as well, according to legend. But one day, a member of the Ahenobarbi clan, a man named Lucius Domitius, was traveling home to Rome when he encountered the demigods Castor and Pollux (otherwise known as the Gemini twins). They told Domitius to deliver news to Rome that they had won a great battle. And then Castor and Pollux stroked his cheek. With that divine touch, the beard of Domitius turned the color of bronze, and he then passed down his red beard to all his male descendants.
Hippocrates provided his medical authority to another story of acquired traits, about a tribe known as the Longheads. A long head was a sign of nobility for the tribe, prompting parents to squeeze the skulls of newborns and wrap them in bandages. “Custom originally so acted that through force such a nature came into being,” Hippocrates said. Eventually, Longhead babies came into the world with their heads already stretched out.
Other Greeks told similar stories—of men who lost fingers, for example, and then fathered fingerless children. “For the seed,” Hippocrates wrote, “comes from all parts of the body, healthy seed from healthy parts, diseased seed from diseased parts.” If those parts changed during a person’s life, his or her seeds changed accordingly.
The place where people lived could also shape them, the Greeks believed, and even give them some of their national character. “The people of cold countries generally, and particularly those of Europe, are full of spirit, but deficient in skill and intelligence,” Aristotle declared. They were therefore unfit to govern themselves or others. Asians had skill and intelligence, but lacked spirit, which was why they lived under the rule of despots. “The Greeks, intermediate in geographical position, unite the qualities of both sets of peoples,” Aristotle wrote.
The theories of Aristotle and other ancient writers were preserved by Arab scholars, from whom Europeans learned of them in the Middle Ages. In the 1200s, the philosopher Albertus Magnus declared that the temperature and humidity of people’s birthplace determined the color of their skin. Indians were especially good at math, Albertus thought, because the influence of the stars was especially strong in India.
But over the next three centuries, Europeans developed a new explanation for the link between one generation and the next: They were joined by blood. Even today, Westerners still use the word blood to talk about kinship, as if the two were equivalent in some obvious way. But other cultures thought of kinship in terms of other substances. On the Malaysian island of Langkawi, to pick just one counterexample, people traditionally believed that children gained kinship through what they ate. They consumed the same milk as their siblings, and they later ate the same rice grown from the same soil. These beliefs are so strong among the Langkawi that if children from two families nurse from the same woman, a marriage between them would be considered incest.
The European concept of blood gave ancestry a different form. It sealed off kinship from the outside world. A child was born with the blood of its parents coursing through its veins, and inherited all that went with it. Philip II was fit to inherit his father’s crown because he had royal blood, which came from his father, and his grandfather before that. Genealogies became bloodlines, serving as proof that noble families were not tainted with lower-class blood. The Habsburgs were especially protective of their royal blood, only marrying other members of their extended family. Charles V married Isabella of Portugal, for example; they were both grandchildren of King Ferdinand and Queen Isabella of Spain.
Before long, Europeans even began to sort animals according to their blood. Of all birds, falcons had the noblest blood, and falconry was thus suitable to be the sport of kings. If a falcon mated with a less noble bird, the chicks were called bastards. Noblemen also became connoisseurs of dogs and horses, paying fortunes for pure-blooded breeds. For animals no less than people, inheriting noble blood meant inheriting noble traits like bravery and strength.
No experience could hide the virtue carried in the blood of man or beast. In a medieval romance called Octavian, the Roman emperor of the same name unknowingly fathers a child named Florentine, who ends up being raised by a butcher. Even in that lowly household, Florentine’s noble blood cannot be masked. His adoptive father sends him to the market to sell two oxen, and Florentine trades them instead for a sparrow hawk.
In the 1400s, people began to use a new word to define a group of animals that shared the same blood: a race. A Spanish manual from around 1430 offered breeders tips for providing a “good race” of horse. Their stallion must “be good and beautiful and of good coat and the mare that she be large and well formed and of good coat.” Before long, people were assigned to races as well. A priest named Alfonso Martínez de Toledo declared in 1438 that it’s easy to tell the difference between men belonging to good and bad races. It doesn’t matter how they’re raised, Martínez de Toledo said. Imagine that the son of a laborer and the son of a knight are reared together on some isolated mountain away from their parents. The laborer’s son would end up enjoying working in a farm field, Martínez de Toledo promised, while the knight’s son would take pleasure only in riding horses and sword fighting.
“The good man of good race always returns to his origins,” he wrote, “whereas the miserable man, of bad race or lineage, no matter how powerful or how rich, will always return to the villainy from which he descends.”
In the late 1400s, Jews in Spain found themselves defined as a race of their own. For centuries, Jews across Europe had been tormented for all sorts of concocted crimes against Christians. In fifteenth-century Spain, thousands of Jews tried to escape this persecution by converting to Christianity, becoming so-called conversos. The self-proclaimed “Old Christians” remained hostile, rejecting the idea that Jews could escape their sinful inheritance with a mere oath. Nor could their children, for that matter, because Jewish immorality was carried in their blood and embedded in their seed, passed down from one generation to the next. “From the days of Alexander up till now, there has never been a treasonous act that did not involve a Jew or his descendants,” the Spanish historian Gutierre Díaz de Games declared in 1435.
Spanish writers began referring to unconverted Jews and conversos alike as the Jewish “race.” Christian men were warned not to have children with a woman of the Jewish race, in the same way that a fine stallion shouldn’t be bred with a mare from a lower caste. In 1449 the Spanish city of Toledo began turning this hostility into law, decreeing that even a trace of Jewish blood disqualified a subject from holding office or marrying a true Christian.
The ban spread across Spain, expanding its scope along the way. Jewish blood now barred people from getting university degrees, inheriting estates, or even entering some parts of the country. In order to define Jews as a separate race, the majority of Spain had to define itself as a race of its own. Noble families now claimed that their genealogies extended back to the Visigoths. They boasted of the cleanliness of their blood, known as limpieza de sangre. They extolled the pale skin of Old Christians, which revealed the sangre azul—blue blood—coursing in the vessels beneath. The phrase would survive for centuries and cross the Atlantic, becoming a label for upper-class New Englanders.
Official certificates of purity were required for marriages between powerful Spanish families and for lucrative government posts. The Spanish Inquisition would follow up with their own detective work, getting testimony from relatives and neighbors. The inquisitors would investigate any rumor of Jewish ancestry—a report that an ancestor worked as a clothes merchant or a moneylender could be enough to arouse suspicion. The discovery of even a single Jew in one’s ancestry could spell doom. Wealthy families would hire special race researchers, called linajudos, to marshal proof of their limpieza de sangre. Of course, just about every noble family actually did have some Jewish ancestry. The linajudos grew rich by inventing chronicles that left it out.
The label of race emerged around the time that Europeans began colonizing other parts of the world. They discovered more people to whom they could attach the label.
“I have found no monsters,” Christopher Columbus wrote in a letter from the Caribbean in 1493. Instead of Cyclopses or Amazons, he encountered people, whom he named Indians. Columbus was not sure what to make of them at first. They seemed to flout Aristotle’s rule about skin color: Even though they lived under a fierce sun, their skin was not black like that of Africans. They lacked clothes, steel, or weapons. Yet Columbus was impressed by their skill in building and piloting canoes. “A galley could not compete with them by rowing, because they travel incredibly fast,” he said. “They are of subtle intelligence and can find their way around those seas.”
While Columbus may have found some things to admire in the Native Americans he encountered, he didn’t hesitate to force them into slavery. He dispatched some to work on farms or in mines; he sent hundreds more to Spain to be sold, although most died during the voyage across the Atlantic. Conquistadors and settlers followed Columbus’s example. While some theologians pleaded that they treat Native Americans more humanely, others justified slavery by race. They declared Native Americans to be natural slaves, incapable of reason and designed by God to serve European masters.
“For them there is no tomorrow and they are content that they have enough to eat and drink for a week,” wrote the Spanish jurist Juan Matienzo. “Nature proportioned their bodies so that they should have strength for personal service,” said another scholar. “The Spaniards, on the other hand, are delicately proportioned, and were made prudent and clever, so that they should be able to lead a political and civil life.”
Yet Native Americans suffered so badly from new diseases and hard labor that their population collapsed. In response, Charles V outlawed their slavery, although many ended up as impoverished peasants toiling on haciendas. Now a new supply of workers had to be imported to take their place: African slaves.
For centuries, a vigorous slave trade had moved people out of sub-Saharan Africa into Europe, the Near East, and South Asia. The enslavers justified the practice by dehumanizing the enslaved. In 1377, the Tunisian scholar Ibn Khaldun, declared that Africans—as well as Slavs, another enslaved population—“possess attributes that are quite similar to those of dumb animals.” But Khaldun still subscribed to a Hippocratic view of heredity. The black Africans who moved north into the cold climate of Europe, Khaldun claimed, were “found to produce descendants whose colour gradually turned white.”
Muslims first brought African slaves into Spain in the eighth century, and their numbers grew as Portuguese traders captured Africans and brought them back to Europe. And yet the social boundaries between slavery and freedom remained loose. Some slaves of African ancestry gained their freedom and spent the rest of their lives alongside Europeans. Some joined the crews that sailed with Columbus to the New World.
As slave traders began shipping their cargo straight to Brazil, Peru, and Mexico, Europeans developed more enduring justifications of slavery. Some declared it a curse that Africans inherited from their biblical ancestors. Theologians had long claimed that Africans were the descendants of Ham, one of Noah’s sons. After Ham saw his father naked, Noah cursed him, declaring that Ham’s own son, Canaan, would never know freedom. “A slave of slaves shall he be to his brothers,” Noah said.
In the 1400s, European scholars revived the story of Ham, casting it as the foundation of a distinct race, its cursed essence marked by dark skin. In 1448, the Portuguese scholar Gomes Eanes de Azurara wrote that because of Ham’s sin, “his race should be subject to all the other races of the world. And from this race these blacks are descended.”
Of all the powerful families in Europe, none worked as hard to keep themselves free of hereditary taint as the Habsburgs. Their blood ran blue, as their detailed genealogies could attest. To maintain their purity—and to keep the world’s greatest empire intact—the Habsburgs only married among themselves. Cousins married cousins. Uncles married nieces. And yet the more time passed, the more the Habsburgs of Spain became burdened with hereditary suffering. The Habsburg jaw was the most prominent of their afflictions. Scientists have examined the paintings of Philip II and the other Habsburg kings to make a diagnosis, and they now suspect that the Habsburgs did not actually have an enlarged lower jaw so much as a small upper jaw that failed to develop to its full size. Philip II also suffered from other troubles familiar to the Habsburg family, including asthma, epilepsy, and melancholy.
To protect the family’s power, Philip II married Maria Manuela, his first cousin. Genetically speaking, though, she was even more closely related than that. Philip’s parents, Charles and Isabella, were also first cousins, while Maria Manuela’s own parents were Charles and Isabella’s siblings. Her father was Isabella’s brother, and her mother was Charles’s sister. The result of this close union was a sickly son born in 1545, Don Carlos. The right side of his body was less developed than the left, causing him to walk with a limp. He was born with a hunchback and a kind of deformed rib cage called pigeon chest.
Don Carlos was ten when his father became king. The boy wailed inconsolably and often refused to eat. But his many troubles didn’t stop Philip from naming Don Carlos his “universal heir” at age twelve, destined to inherit all the kingdoms Philip had inherited from his own father, Charles.
By the time Don Carlos was nineteen, however, it was obvious to everyone, his father included, that something was very wrong. One visitor to the Spanish court wrote, “He is still like a child of seven years.” Philip himself agreed. “Although other children develop late,” the king wrote, “God wishes that mine lags far behind all others.”
In his early twenties, Don Carlos grew violent. He once hurled a servant out of a window for displeasing him. He wasted hundreds of thousands of ducats. He tried to kill a nobleman. Philip decided that his son’s “natural and unique temperament” would never change, and that he could not be allowed to rule. The king put on a suit of chain mail, assembled a group of armed courtiers, and stormed his son’s room. They nailed Don Carlos’s windows shut, removed all the weapons, papers, and treasure from the prince’s room, and turned it into a prison cell. Don Carlos died there a few weeks later, on July 24, 1568, at age twenty-three.
Philip II remarried—this time choosing his own niece, Anna of Austria. In 1578, they had a son, Philip III, who succeeded his father twenty years later. Philip III married a cousin of his own and ruled till 1621, whereupon his own son, Philip IV, took over. It was during Philip IV’s reign that the Spanish Empire—long the greatest power on Earth—went into decline. The Spanish army grew weak, and Portugal slipped from Philip IV’s grasp. Gold and silver continued to arrive from the New World, but it headed straight to bankers elsewhere in Europe rather than enriching the people of Spain, who suffered from plagues and famines.
Philip IV was insulated from the chaos within the confines of his huge palace. He hung masterpieces by Rubens on the walls and listened to poets sing his praises. They called him the Planet King. The endless pageantry was disturbed only by the king’s worry that his planetary throne might slip out of Habsburg hands if he didn’t produce a son and heir.
Along with the Habsburg jaw and other ailments, the dynasty began to suffer an increasing number of miscarriages and infant deaths. Although they were among the most pampered people on Earth at the time, they suffered a higher rate of infant mortality than Spanish peasant families. Philip IV’s first wife, Elisabeth of France, had a long string of miscarriages and babies who died young before her death in 1644. Their son, Balthasar Charles, managed to survive to age seventeen before dying of smallpox in 1646. The Habsburg dynasty now faced a crisis: It had no heir to succeed Philip IV after his death.
After Balthasar’s death, Philip IV married his son’s fiancée—and his own niece—Mariana. In 1651 she bore the king a daughter, Margaret Theresa, who would survive for twenty-two years. But over the following years, she had two more children who died young. In 1661, when their son, Philip Prospero, died at age four, Philip IV blamed their deaths on his lust for actresses.
When we look back to the seventeenth century, it can be hard to understand why Philip IV didn’t recognize that the heredity of his family was to blame. But hardly anyone at the time thought about heredity this way during the years of the Habsburg dynasty. One of the few exceptions was the writer Michel de Montaigne, who published an essay in 1580 called “Of the Resemblance of Children to Their Fathers.”
Montaigne was a French courtier who retired from political life in 1571 to sit in a castle tower and reflect on vanity and happiness, on liars and friendship. While he found comfort in this solitude, pain intruded on his contemplation from time to time, thanks to his kidney stones. One day, Montaigne transformed the stones into grist for an essay.
“It is likely I inherited the gravel from my father,” Montaigne guessed, “for he died sadly afflicted by a large stone in the bladder.” Yet Montaigne had no idea how one could inherit a disease, as opposed to a crown or a farm. His father had been in perfect health when Montaigne was born, and remained so for another twenty-five years. Only in his late sixties did his kidney stones first appear, and they then tormented him for the last seven years of his life.
“While he was still so remote from the disease, how could the light trifle of his substance out of which he built me convey so deep an impress?” Montaigne wondered. “Where could the propensity have been brooding all this while?”
Simply musing in this way was a visionary act. No one in Montaigne’s day thought of traits as being distinct things that could travel down through generations. People did not reproduce; they were engendered. Life unfolded as reliably as the rising of bread or the fermenting of wine. Montaigne’s doctors did not picture a propensity lurking in parents and then being reproduced in their children. A trait could not disappear and be rediscovered, like a hidden letter. Doctors did sometimes observe certain diseases that were common in certain families. But they didn’t think very much about why that was so. Many simply turned to the Bible for guidance, citing the passage telling of God “visiting the iniquity of the fathers upon the children unto the third and fourth generation.”
Whatever Montaigne’s doctors might have said about his father’s kidney stones, he probably would have dismissed it. He hated doctors, like his father and grandfather before him. “My antipathy against their art is hereditary,” he said.
Montaigne wondered if such an inclination could be inherited, along with diseases and physical traits. But how all of that could be carried from one generation to another in a seed, Montaigne could not begin to imagine. “The doctor who can satisfy me on this point I’ll believe as many miracles of as he pleases,” he promised, “provided he does not give me—as doctors usually do—a theory more intricate and fantastic than the thing itself.”
Montaigne lived for another dozen years, apparently never meeting a doctor who could satisfy him about heredity. In the year of his death, an elderly Spanish doctor named Luis Mercado was appointed by Philip II to be Physician of the King’s Chamber. Mercado might have met Montaigne’s high standards, because he was one of the few doctors in Europe to recognize that people inherit diseases and to ask why.
For decades before his appointment to the court, Mercado had taught medicine at the University of Valladolid. A colleague there called him “modest in dress, sparing in diet, humble in character, simple in matter.” At the university, Mercado had given lectures steeped in Aristotle’s ideas. But his dedication to the ancients didn’t prevent him from making observations of his own and publishing books with new ideas about fevers and plagues. And in 1605, at age eighty, Mercado published his masterwork: De morbis hereditariis—On Hereditary Diseases. It was the first book dedicated to the subject.
Mercado sought an explanation for why diseases ran in families. He dismissed the possibility that they were divine punishments. Instead, to understand hereditary diseases, Mercado believed it was necessary to understand how new lives develop. He argued that each part of the body—a hand, the heart, an eye—had its own distinctive shape, its own balance of humors, and its own particular function. In the bloodstream, Mercado claimed, the humors from each part of the body mixed together, and a mysterious formative power shaped them into seeds. Unlike Aristotle, Mercado believed that both men and women produced seeds, which were combined through sex. The same formative power acted on those joined seeds, producing from them a new supply of humors that gave rise to a new human being that developed the same parts as its parents.
Mercado believed that this cycle of generation, combination, and development was well shielded from the outside world. The willy-nilly waves of chance could not reach the hidden seeds of human life and alter their hereditary traits. He dismissed popular notions about the power of the environment—that a mother’s imagination could alter her baby, or that dogs taught new tricks could pass them down to their puppies. A hereditary disease was like a stamp that marked a seed. The same stamp appeared on each new generation’s seeds and gave rise to the same disease—“the bringing forth of individuals similar to oneself and deformed by the same defect,” Mercado wrote.
In his experience with patients—royal and common—Mercado had seen many different kinds of hereditary diseases. Some could strike immediately—a child could be born deaf, for example—while others were slower to emerge, like the kidney stones that afflicted Montaigne as they had his father. In many cases, Mercado came to believe, parents impressed only a tendency toward a disease on their children. A child’s humors might be able to weaken that impression. Or a healthy parent’s seed could sometimes counter a diseased one. The defect still lurked in the child, who could then pass it down to its own children. If they didn’t inherit a countervailing seed from their other parents, the disease could flare up out of hiding.
Some hereditary diseases could be treated, Mercado argued, but only slowly and incompletely. “Let us in some secluded spot teach the deaf and dumb to speak by forming and articulating the voice,” he wrote. “By long practice many with hereditary affliction have regained their speech and hearing.”
But for the most part, a doctor could do little, because the stamp of heredity was sealed away from a physician’s reach. Mercado urged instead that people with the same defect not marry, because their children would be at greatest risk of developing the same hereditary disease. All people should seek out a spouse as different from themselves in as many individual characteristics as possible.
Mercado went remarkably far toward answering Montaigne’s questions about heredity. But the world was not ready to investigate his ideas. The Scientific Revolution was decades away, and it would take two centuries more before heredity itself would come to be seen as a scientific question. No one—not even Mercado himself, it seems—could recognize that his own royal patients were in the midst of staging their own hereditary disaster. By preserving their noble blood, they were increasing the number of disease-causing mutations in their lines. They were lowering their odds of having children, and the children who beat those odds were then at grave risk of inheriting mutations that would give them a host of diseases.
By 1660, Philip IV had been trying to produce a male heir for forty years. In that time he fathered a dozen children. Ten died young, and the surviving two were girls. As Philip grew older, the survival of the entire Habsburg dynasty fell into jeopardy. The following year, at last, the empire celebrated the birth of a son who would become king.
Charles, the new prince, was “most beautiful in features, large head, dark skin, and somewhat overplump,” according to the official gazette. Spain’s royal astrologers declared the stars at Charles’s birth to be well arranged, “all of which promised a happy and fortunate life and reign.” When Charles was only three, his father died. On his deathbed, gazing at the crucifix on the wall before him, Philip IV could console himself that he had forged a new link in heredity’s chain, leaving behind a boy king.
King Charles II of Spain proved to be the sickest Habsburg monarch of them all. “He seems extremely weak,” an ambassador wrote back to France, “with pale cheeks and very open mouth.” The ambassador observed a nurse usually carried him from place to place so that he would not have to walk. “The doctors do not foretell a long life,” the ambassador reported.
Charles II, born six decades after Mercado published On Hereditary Diseases, managed to survive to manhood, although his health remained poor and his mind weak. Famines and wars unfolded around him, but he preferred to distract himself with bullfights. The only national matter with which he concerned himself was producing an heir of his own. And even in that task, he failed.
As the years passed without his queen becoming pregnant, Charles grew more ill. “He has a ravenous stomach, and swallows all he eats whole, for his nether jaw stands so much out, that his two rows of teeth cannot meet,” a British ambassador reported, “to compensate which, he has a prodigious wide throat, so that a gizzard or liver of a hen passes down whole, and his weak stomach not being able to digest it, he voids in the same manner.”
The Spanish Inquisition blamed the lack of an heir on witches, but their trials did nothing to help the king. It became clear he would die soon. Yet Charles managed to dither for months over whom to name as his heir. Finally, in October 1700, he selected the Duke of Anjou, the grandson of the king of France. Charles worried that the empire might collapse after his death, and so he issued a demand that his heir rule “without allowing the least dismemberment nor diminishing of the Monarchy founded with such glory by my ancestors.”
But his monarchy soon began disintegrating anyway. The prospect of France and Spain forming an alliance prompted England to form an alliance of its own with many of the other great powers in Europe. Skirmishes began breaking out, both in Europe and in the New World. Eventually, the fighting would escalate into the War of Spanish Succession. The conflict would change the planet’s political landscape, leaving England ascendant and Spain broken.
Yet Charles still dreamed that his empire would remain whole. He even added a codicil to his will stating his wishes that the Duke of Anjou would marry one of his Habsburg cousins in Austria. Not long afterward, he grew so ill that he could no longer hear or speak. Charles died on November 1, 1700. He was only thirty-five. There was no child left to inherit his empire, because of invisible things Charles had inherited from his ancestors. When doctors examined the king’s cadaver, they found that his liver contained three stones. His kidneys were awash in water. His heart, they reported, was the size of a small nut.