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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, citingthe 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 namedLuis 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, Mercadoclaimed, 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