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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 spotteach 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 ofheredity 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.
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    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 nurseusually 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