Authors: Carl Zimmer
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 bodyâa hand, the heart, an eyeâhad 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 environmentâthat 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 diseaseâ“the bringing forth of individuals similar to oneself and deformed by the same defect,” Mercado wrote.
In his experience with patientsâroyal and commonâMercado had seen many different kinds of hereditary diseases. Some could strike immediatelyâa child could be born deaf, for exampleâwhile 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 oneânot even Mercado himself, it seemsâcould 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.
, a fifty-five-year-old Dutchman, heavily built and sporting a graying beard, boarded a ship bound for New York. Hugo de Vries was a university professor in Amsterdam, but he was not a hothouse inhabitant of lecture halls. He spent much of his time wandering the Dutch countryside, scanning meadows for exceptional wildflowers. An English colleague once complained that
his clothes were foul and that he changed his shirt once a week.
When de Vries's ship docked in New York, he boarded a train that pushed its way across the country to California. The official reason for the journey was to visit scientists at Stanford University and the University of California at Berkeley. De Vries dutifully gave his lectures and went to the required evening banquets. But as soon as he could manage, he escaped north.
Fifty miles from San Francisco, de Vries arrived in a small farming town called Santa Rosa. With four fellow scientists in tow, he made his way from the train station to a four-acre plot ringed by low picket fences and crammed with gardens. A modest vine-covered house sat in the middle of the property, flanked by a glass-roofed greenhouse and a barn. A boxwood-lined path led from the street to the front porch of the house. Next to the path stood
a blue-and-white sign informing visitors that all interviews were limited to five minutes unless they were by appointment.
Fortunately, de Vries had one. A small, stooped man about his own age,
outfitted in a rough brown suit, came out to greet the visiting party. His name was
Burbank shared the house in the middle of the garden with his sister and mother. He had been expecting de Vries's arrival for months and set aside an evening and a day for the visit. He showed off his garden to the scientists and then took them to an eighteen-acre farm he tended in the Sonoma foothills. Those two plots of land, and the plants that sprouted from their soil, had made Burbank both rich and famous.
His results are so stupendous,” de Vries later wrote, “that they receive the admiration of the whole world.”
This was no exaggeration. Each year,
Burbank's postman brought him thirty thousand letters. Henry Ford and Thomas Edison traveled to Santa Rosa to meet him. Newspapers regularly praised Burbank, calling him “
the wizard of horticulture.” The Burbank potato, which he produced at age twenty-four, was already the standard breed for farmers across much of the United States. The Shasta daisy sprang into existence under Burbank's care, and quickly became a mainstay in middle-class flower beds. In his gardens, Burbank created thousands of different kinds of plantsâthe white blackberry, the Paradox walnut, the spineless cactus.
Such a knowledge of Nature and such ability to handle plant life would only be possible to an innately high genius,” de Vries had declared to a group of Stanford scientists on the eve of his trip to Santa Rosa. Before his meeting with Burbank, de Vries wondered how much of what was written about him was true. The
San Francisco Call
said that Burbank's flowers “thrive upon
a scale so extensive as to suggest magic rather than the sober work of science.” Sometimes Burbank's catalogs read like fairy tales. In one edition, de Vries saw that Burbank was now offering a stoneless plum. He simply couldn't believe such a thing could be created. When de Vries finally reached Santa Rosa, he asked for proof. Burbank led de Vries and his other visitors to a plum tree bowed down with blue fruit. He gave each man a plum, and when they bit down, their teeth met only soft sweetness. “Although we knew
there was no stone in the plum, we experienced a feeling of wonder and astonishment,” de Vries wrote.
De Vries was not one for much wonder. He was a scientist to his marrow, and before his trip to California, he had spent the previous two decades running experiments that helped establish the first genuine science of heredity. Not long before his visit to Burbank, it had been given a proper name: genetics.
But genetics in 1904 was like a barely started house, more footings than walls. It still left fundamental questions about heredity unanswered. De Vries knew that he and his fellow geneticists were really just newcomers to heredity's mysteries, that other people had been plumbing them for thousands of years. He respected the wisdom of animal and plant breeders, although he also recognized much of their ancient wisdom had disappeared into unrecorded oblivion. Over the course of the 1700s and 1800s, some breeders became rich. Nations looked to them to work miracles on heredity to deliver economic salvation. And at the debut of the twentieth century, there was no greater breeder than Luther Burbank. He had dedicated decades to understanding what he called “
the inherent constitutional life force, with all its acquired habits, the sum of which is heredity.” De Vries came to Santa Rosa to learn what Burbank had learned about heredity in order to push genetics out of its infancy.
Pottery shards, ancient seeds, and the bones of livestock all indicate that the first breeders started their work in earnest around eleven thousand years ago. Plants and animals, once wild, came under the control of humans, grown for their benefit. The agricultural revolution let the population of our species explode, but it also made us precariously dependent on the heredity of what we raised. When farmers planted a new field of barley seeds, or goatherds delivered a new batch of kids, they needed each new generation of plants and animals to end up like the previous one. If corn kernels randomly became as hard as glass, or if cows were born unable to produce milk, people would starve. Learning how to steer heredity could also make farmers more prosperous. If they could raise pigs that reliably grew more pork on their bodies, they gained more wealth. And once
farmers could supply their goods to markets and trade networks, they could attract more customers for their particular breedsâtheir sweeter oranges or their more durable cowhides.
It's hard to know exactly how much early farmers understood about breeding as they carried it out. The historical record of their ideas is practically a void, but the results of their efforts were impossible to ignore. The wealth of the Habsburg kings of Spain, in fact, came in part from the mysterious art of animal breeding. The first sheep to graze the meadows of Spain were unexceptional creatures with rough wool coats. When the Moors arrived, they brought sheep with them from northern Africa, which they interbred with the resident flocks. The new cross came to be called the
Merino. For centuries, Spanish shepherds bred Merinos by the millions, every year leading them on a journey across the country. The Merinos spent each summer grazing in the Pyrenees and then traveled narrow paths for hundreds of miles to the southern lowlands to pass the winter. Over many generations of breeding, Merino wool became extraordinarily soft, lush, and silky.
Merino wool turned into a precious commodity. On their journeys, Spanish shepherds would stop to shear their sheep and sell their wool at fairs to merchants from across Europe. Henry VIII of England said he would accept nothing but Merino for his royal garments. Merino wool became so valuable to Spain that smuggling a single Merino sheep out of the country was made a crime punishable by death.
In the seventeenth century, the magnificence of Merino wool was as mysterious as the suffering of the Habsburg kings. No one at the time would have guessed they shared anything in common. Some speculated that the environment in which the Merinos lived was responsible for their wool. The cold of the mountains and the heat of the tablelands influenced their seed, in the same unknowable way the terroir of a grapevine determined the taste of its wine. More evidence for this influence came from the few cases when sheep were smuggled out of Spain. In other countries, they failed to thrive. After a few generations of crossbreeding with native flocks, the sheep no longer grew good wool.
Across Europe, the growing population was clamoring for more woolâas well as for more beef and leather from cows, for more eggs from chickens. Wheat, barley, and corn were in greater demand as well. Anyone who could steer heredity in a more profitable direction stood to make a good living. A particularly successful breeder could even become a celebrity. And no breeder in the 1700s was more famous than a portly Englishman named
Robert Bakewell. A duchess once referred to him as “
the Mr. Bakewell who invented sheep.”
Mr. Bakewell was born in 1725 on Dishley Grange, a 450-acre property that his father worked as a tenant farmer. His father encouraged him to learn new techniques by traveling to other farms around England, Ireland, and the Netherlands. He helped his father improve the farm, digging a labyrinth of channels and hatches to deliver water across the property, tripling the amount of grass that grew on it. Robert Bakewell took over Dishley Grange by the time he was thirty. A decade later the first hint of his breeding skill emerged when he won first prize at the Ashby Horse Show.
But it was with sheep that Bakewell would become famous. He and his neighbors reared a humdrum local breed known as Old Leicester. The animals were heavy, long, and flat-sided. They grew rough wool, and their mutton, a coarse-grained meat with little flavor, brought no excitement to the dinner table. But when Bakewell looked at an Old Leicester sheep, he saw a New Leicester sheep waiting to emerge. The generating powers inside the animals could, with the proper guidance, produce a breed that could make sideboards groan with huge cuts of delicious muttonâwhile requiring relatively little feed. Bakewell was a man of his mechanical age, engineering woolen meat-making machines.
Unlike an engineer, however, Bakewell did not understand the natural processes he was trying to manipulate. He could only guess, picking out ewes from his flock that approached his vision. Bakewell believed that the traits he could see on the outside of a sheep were linked to qualities on the inside, ones that could be passed down to offspring.
“He asserts,” a visitor to Dishley Grange wrote, “
the smaller the bones, the
truer will be the make of the beastâthe quicker she will fatâand her weight, we may easily conceive, will have a larger proportion of valuable meat.”
Bakewell traveled England inspecting rams and brought home a select few to breed with his ewes. When he crossed these sheep, they did not instantly produce a uniform supply of New Leicester lambs. Instead, their litters were a hodgepodge, made up of lambs of different sizes and shapes. But Bakewell did not lose faith in his vision. He turned his exacting eye to his lambs. He picked out ones to mate with one another, or with other sheep he bought from other farms. These cycles of inspection and selection went on for years, during which time Bakewell turned his farm into a primitive laboratory. He herded his sheep into houses and sheds kept as clean as horse stables so that he could experiment on their heredity in secret. He measured his sheep and weighed them every week until slaughter.
He chalked his data on slates and then transferred them to ledgers, which sadly were later lost.
In time, the sheep began to accord with the animal that gamboled in Bakewell's mind. He stopped touring England to buy rams. Instead, he employed a strategy known as in-and-in breeding. Bakewell mated cousin to cousin, brother to sister, father to daughter. Other farmers thought him mad because they believed inbreeding invariably led to disaster. That might be true for other farmers, but not for Bakewell. He was able to make sure that all the qualities he wanted in his sheep became fixed in his flock, but none of the deformities that might ruin his new breed.
After fifteen years, Old Leicester had at last become New Leicester. People found
Bakewell's new breedâwith its broad, barrel-shaped body; its straight, short, flat back; its small head; and its short, small-boned legsâpeculiarly pleasing to the eye. New Leicester mutton might not have the fine flavor that aristocrats clamored for. One critic even declared it “
only fit to glide down the throat of a Newcastle coal-heaver.” But Bakewell didn't care about epicurean snobs. “
My people want fat mutton and I gave it to them,” he declared.
He was fibbing a bit. With a flock of just a few hundred New Leicester, Bakewell couldn't feed the millions of hungry English. Instead, he sold his
sheep to other breeders, who started their own New Leicester flocks. They paid him dearly. They were even willing to do something that had previously been unheard-of: They would rent his rams for their services. Bakewell sent the rams to their appointments in two-wheel sprung carriages, suspended inside from slings. He claimed the right to take the best lambs produced by his rented rams, improving his own flock even more.
Dishley Grange itself became a destination for travelers, who came from as far as Russia to see Bakewell's work and learn about the astonishing methods of “
this prince of breeders.” Bakewell welcomed visits. He turned his house into a museum of heredity, filling it with sheep skeletons and brine-pickled joints, demonstrating the transformation he had brought about in his animals. It was great public relations. Bakewell's visitors wrote letters and books about his experiments. One French nobleman declared that Bakewell “
had been making observations, and studying how to bring into being his fine breed of animals with as much care as one would put into the study of mathematics or any of the sciences.”
In fact, Bakewell didn't leave behind a single measurement of a sheep. He published no law of heredity to explain his success. Bakewell lived at the turning point in the history of heredity, when people recognized it as something to be understood and manipulated, while still relying on the intuitions of their farming ancestors to steer it. Looking back at Bakewell's work, we can't help but turn our attention to what it lackedâthe data and statistics that are essential to studying heredity today. But in his own time, Bakewell had an enormous impact, showing the world how much heredity could be stretched and sculpted. As one of his visitors wrote, “
He has convinced the unbelievers of the truth of his sheepish doctrine.”