Voyage of the barnacle: Darwin paid his dues as a scientist by exploring a miniature universe of marine animals

Natural History, June, 2003 by Richard Milner

Darwin and the Barnacle: The Story of One Tiny Creature and History's Most Spectacular Scientific Breakthrough by Rebecca Stott W.W. Norton & Company, 2003; $24.95

As a city boy, I once supposed that fossils were as rare as large meteorites and could be encountered only in museums. Eventually I learned that they are almost everywhere: Mesozoic ammonite shells from ancient oceans populate the polished marble of skyscraper lobbies; microscopic plankton skeletons inhabit every piece of chalk; herds of fossil rhinoceroses lie beneath Nebraskan farms.

In 1811 James Parkinson, the English physician and amateur geologist chiefly remembered for identifying the disease that bears his name, marveled that his European contemporaries lived literally surrounded by fossils. In volume one of his work Organic Remains of a Former World, he noted that extinct marine organisms "have become the chief constituent parts of the limestone, which forms the humble cottage of the peasant; and of the marble which adorns the splendid palace of the prince." But what Parkinson found even more astonishing was that no one seemed to share his intense curiosity about how and when those organisms had found their way into the building materials of hovels and mansions alike.

Billions of primitive marine animals still share the planet with us today. One of the more ubiquitous of them is the barnacle--the small invertebrate that clings to whales as well as to dock pilings and shoreline rocks. In her entrancing book Darwin and the Barnacle, Rebecca Stott, a professor of English at the University of Cambridge, quotes Charles Dickens's Little Dorrit on this creature's omnipresence: "Wherever there was a square yard of ground in British occupation under the sun or moon, with a public post upon it, sticking to that post was a Barnacle." Perhaps most human activity directed at barnacles has been devoted to that despised sailor's task--scraping them off ships' hulls.

Stott begins her tale by recalling childhood visits to the seashore, where she first encountered cone-shaped barnacle shells. Each one held a "bizarre inhabitant, a cream-coloured shrimplike creature, upside down, glued to the rock by its head, fishing for plankton through the hole in its cone with its feathery feet." Stott also came across the stalked barnacles that cluster on driftwood, which some consider a seafood delicacy. She began to wonder: Just what kind of critters are barnacles? Are they mollusks? Crustaceans? How many kinds are there? Where did they come from? How far back can one trace their ancestry? By the time she summoned the courage to order barnacles in a seafood restaurant, she associated them with Charles Darwin, whom she realized was obsessed with the odd creatures.

In 1831, when twenty-two-year-old Darwin set sail as a fledgling naturalist on HMS Beagle, no one knew much about barnacles--and few cared. But young Darwin was, as his uncle described him, "a man of enlarged curiosity." In 1835 he collected a conch shell on a Chilean beach and noticed that there were hundreds of tiny holes in it, which interested him more than the species of the shell itself. He suspected that some small creature had made the holes, although he could see none. Later, under a microscope, he spotted the culprit: a minuscule, soft-bodied inhabitant cemented into the hole by its head and waving its jointed legs in the air. Anatomically it resembled an acorn barnacle. But that creature was defined by its cone-shaped shell.

Darwin had discovered something as yet unknown to science: a rare burrowing barnacle with no shell-house of its own. The questions raised by this creature's anomalies would occupy him for years. As Stott reveals:

   Darwin will carry this Chilean barnacle on
   a journey around the world, from the
   South American beach back to London,
   preserved in ajar of wine spirits. When he
   has finished finding homes for all the
   1,529 species he has collected ... on the
   Beagle, he will return to the puzzle that the
   creature's strange anatomy presents; and
   then he will write this Chilean barnacle's
   evolutionary biography--a puzzle that will
   take him eight years to think through.

Eight years, from 1846 until 1854, devoted entirely to barnacles? By 1842 Darwin had already sketched out his theory of evolution by natural selection. But he pushed it all aside, squirreling it away to work on the barnacle riddle. What was so compelling about these invertebrates that Darwin chose to postpone the completion of his major work--Origin of Species--for their sake?

Hundreds of books have touched on diverse aspects of Darwin's discoveries: his encounters with finches on the Galapagos Islands; his elucidation of sexual selection, orchid pollination, and the formation of coral reefs; his treatise on the evolution of emotional expression. Barnacle anatomy and classification, however, is an arcane technical field that most Darwin scholars have treated only superficially. Now, at last, Rebecca Stott, albeit a nonspecialist in barnacles, has had the courage and tenacity to make Darwin's barnacles--and their importance--accessible to the rest of us.