"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."