Good breeding: Darwin doubted his own family's "fitness."

Natural History, Nov, 2005 by James Moore

Brought up in a provincial market town, Charles Darwin lived for forty years in rural Kent, where he raised a large family. The English countryside was his natural habitat, a world of gentleman farmers devoted to breeding livestock, flowers, fruit--and people. His paternal grandfather, Erasmus Darwin, was a noted horticulturalist, and his maternal grandfather, Josiah Wedgwood I, who raised sheep, improved the flocks with hundreds of Merinos. "It is a beautiful part of my theory," Charles jotted, when developing his ideas on evolution, "that domesticated races ... are made by precisely [the] same means as species." Breeders decided which animals mate and which offspring survive--this was "artificial selection." Nature, in Darwin's view, did the same thing through the struggle for existence: he called it "natural selection."

Ironically, some of the problems caused by inbreeding, which Darwin had heard about from farmers, threatened to play out in his own family. In 1839, as he turned thirty, did Charles select well in choosing a mate? His betrothed, Emma Wedgwood, was his first cousin. The Darwin and Wedgwood families had intermarried for some time (I call them the Darwoods, for short). Charles's grandfather Josiah had eight children with his third cousin Sarah. Their eldest daughter, Susannah, married Robert Darwin, a noted physician; Charles was the fifth of Robert and Susannah's six children. Josiah and Sarah's second-eldest son, Josiah II, fathered nine children, four of whom, Emma Wedgwood Darwin among them, married first cousins.

From our vantage point long after Darwin's death, the results of this unintended experiment in close-cousin breeding are striking. Twenty-six children were born from these first-cousin marriages, yet nineteen of the offspring did not reproduce: five died prematurely, five were unmarried and considered somehow deficient, and nine married without issue. Indeed, among the sixty-two aunts, uncles, and cousins in the four generations founded by Josiah I and Sarah Wedgwood, thirty-eight remained childless. Just as Britain's population was booming, the fertility of Darwins and Wedgwoods seemed to be falling.

When Charles's mixed Darwood blood was added to Emma's "pure" Wedgwood, how would their children turn out? Darwin observed them tenderly, but with a breeder's eye, starting with Willy, "my little animalcule of a son," and his first daughter, Annie. A third child, Mary, died shortly after her birth, but other healthy babies followed--Henrietta, George, Elizabeth, Francis, and Leonard. Then the eldest girl, Annie, fell ill in 1850. She died a year later, soon after her tenth birthday. Another son, Horace, was born in 1851.

Darwin was devastated by Annie's death, fearing she had inherited the "wretched" illness that had plagued him since the Beagle voyage. (Historians now think Annie died of tuberculosis, while Darwin was infected by a blood parasite he acquired in the tropics.) As the other children reached the age at which Annie had become sick, he watched them anxiously. "My dread is hereditary ill-health," he confided in a letter. "Even death is better for them."

He found what he feared. Elizabeth "shivers & makes ... extraordinary grimaces"; at age ten she developed a weak, irregular pulse. Henrietta had similar symptoms at age thirteen and took to her bed for years. George's irregular pulse at age eight pointed to "some deep flaw in his constitution," his father assumed, and he spotted the same symptom at the same age in Leonard.

As the children failed--or appeared to fail--one after another, Darwin began experimenting with pigeons. He bought fancy varieties and worked out their family tree; he observed the chicks to determine the age at which slight "differences appear" that breeders could select or nature could exploit. Those variations, as Darwin wrote in the Origin of Species, usually arose "at a corresponding age in the offspring" and parent, but he knew of cases in which flaws appeared "at an earlier age in the child." The evidence for the latter lived at home. His own condition had set in about age thirty, the children's as adolescence approached, seemingly like clockwork.

In 1856 the Darwins' tenth and final child arrived without its "full share of intelligence." Baby Charles never began to talk; he shivered and grimaced, and died within two years. But the evidence that the family was blighted already seemed abundantly clear. In 1862, when Horace broke down at age eleven, "with shuddering & gasping & hysterical sobbing," his father felt he knew the cause: it was "a serious form of inheritance from my poor constitution." Now, to clinch the diagnosis, all he needed was quantitative proof that inbreeding was bad--evidence from more than his own ten offspring.

Darwin turned to breeding plants in his garden and greenhouses. His experiments proved to him what he had always feared about his family: the offspring of close or self-fertilizing "illegitimate unions" were weak and stunted compared with the offspring of "legitimate unions" between unrelated parents.