The Truth About Animals

Such aged eels have invariably been denied their natural instinct to migrate back to the sea by being kept captive, often as pets. An eel may seem an unconventional choice for an animal companion—not a lot of fun to be had in the snuggling department—but the Roman orator Quintus Hortensius was said to weep at the death of his, “which he kept long, and loved exceedingly.”

THE EEL’S FRESHWATER EXISTENCE may be long and gluttonous, but it is just one of the fish’s many lives, albeit the only one obvious to me and countless other naturalists over the centuries. It gives no clues about the rest of its life cycle—its birth, reproduction and death, which are shrouded by the sea—spawning an intense international quest, lasting some two thousand years, to locate the eel’s missing gonads.

Aristotle was one of the first to be stymied by the genesis of this apparently sexless fish, but he was hardly alone. Several hundred years later Pliny the Elder trialed his own imaginative ideas about eel propagation by proposing that they reproduced by rubbing themselves against rocks: “the scrapings come to life.” Hoping to have the last word on the matter, the Roman naturalist concluded with an authoritative flourish: “This is the only way they breed.” Alas, Pliny’s asexual friction was nothing but fiction.

Fantastic rumors of the eel’s reproduction bred like rabbits over the ensuing centuries. Eels were said to emerge from the gills of other fishes, from sweet morning dew (but only during certain months) and from enigmatic “electrical disturbances.” One mysterious “reverend bishop” told the Royal Society he had seen young eels that had been born on the thatching of a roof. The eggs, he claimed, had been stuck to the thatching reeds and incubated by the heat of the sun.

Not all ecclesiastical naturalists were so open-minded about such fishy tales. In his History of the Worthies of England, Thomas Fuller poured scorn on the belief widely held in the fenlands of Cambridgeshire that the illicit wives and bastard children of priests were saved from damnation by taking the form of an eel. “No doubt the first founder of so damnable an untruth hath long since received his reward,” he said.

The scientific geniuses of the Enlightenment swept aside such fanciful fables with significantly less silly—but no more accurate—theories of their own. In 1692, Antonie van Leeuwenhoek, the Dutch pioneer of microscopic worlds who discovered both bacteria and blood cells, erred towards credibility with his hypothesis that eels, like mammals, were viviparous, that is, their eggs were fertilized internally, with the females giving birth to live young. Van Leeuwenhoek at least embraced contemporary scientific method by basing his supposition on actual observations. He had stared down his magnifying lens and watched what appeared to be baby eels in what he assumed was the fish’s uterus. Unfortunately, these were actually parasitic worms squatting in the eel’s bladder, and had in fact been observed and dismissed as such by Aristotle nearly two thousand years before.

The eighteenth-century Swedish botanist and zoologist Carl Linnaeus also said that eels were viviparous, claiming to have seen what he believed to be baby eels inside an adult female. Surely no one would argue with the great father of taxonomy—a man so pedantic he even latinized his own name. But the awkward truth was that Linnaeus hadn’t actually dissected an eel but an eel imposter, a similar-looking beast now known as the eelpout—an unusually viviparous yet altogether unrelated species of fish. Which is not to say his critics had their facts any straighter. One authority who reviewed Linnaeus’s work took him to task for a case of mistaken identity—but, influenced by Aristotle, proclaimed the young eels discovered by the Swede to be parasitic worms, sending the live-birth doctrine into a vortex of inaccuracy and confusion.

Into this lofty academic fray stepped a plucky outsider. In 1862, a Scotsman by the name of David Cairncross announced to the world that he, a humble factory engineer from Dundee, had finally solved the riddle of the eel. “The reader may at once be informed that… the progenitor of the silver eel is a small beetle.” His enthusiastic, if scientifically baffling, theory—the product of sixty years of ongoing experiments, by his own reckoning—took the form of a short book, The Origin of the Silver Eel.

Cairncross began his treatise with an apology for his lack of any interest in learning the rules and norms of contemporary science. “I could not be expected to be acquainted with the names and terms used by naturalists in their classifications of the different animals, my knowledge of such books being limited,” he noted. His unconventional but highly convenient solution was to “employ names and terms of my own,” which included a reimagining of animal classification into three nonsensical classes that would have made the great Linnaeus turn in his grave.

Cairncross’s story begins at the tender age of ten, when the curious lad observed a number of “hair eels” (his term) in an open drain. “Where can they come from?” he wondered. A friend related to him a common folk belief that young eels “fall from the tails of the horses while drinking; and the water brings them to life.” The young Cairncross scoffed at this highly improbable explanation, before conjuring up his own, equally implausible idea inspired by a number of dead beetles languishing in the bottom of the same drain. Maybe the two animals were connected?

This drain-based miniature drama haunted the Scot for two decades. Then one summer, the adult Cairncross spotted a familiar-looking beetle in his garden in Dundee. He watched it intently, attempting to read its thoughts as it walked with determination towards a puddle and plunged in. The beetle, he reported, then “looks about a bit” before exiting its bath “in a very troubled state.” How Cairncross arrived at his diagnosis of the beetle’s mental state is not known. But the book’s only illustration provides the reader with valuable assistance in comprehending the insect’s next extraordinary move: entitled “the beetle in the act of parturition,” it shows Cairncross’s unlikely hero lying on its back with what appears to be a pair of lassoes emerging from its backside. The beetle, according to the Scotsman, was giving birth to two fish.

This was the eureka moment for Cairncross. He now dedicated himself to furthering his investigation by cutting open beetles, removing “hair eels” and keeping them alive for various, albeit rather limited, periods of time. He freely admitted that his theory “may seem strange” but reassured himself by looking at the behavior of “members of the vegetable kingdom.” If one species of tree can be grafted onto another, “Could not therefore the Great Creating Gardener graft a foreign nature on that of an insect?”

All manner of Frankenstein animals have been conceived in modern labs: human ears have been grafted onto mice and glow-in-the-dark fish have been created with a judicious sprinkling of jellyfish genes. But the “Great Creating Gardener” had no hand in planting them.

Had Cairncross posed his question to the scholarly community, they would have declared that his “hair eels” were yet another case of pesky parasitic worms and not the fish in an early stage of development. But the factory manager knew not of peer review. He presented his exceptional findings not to the Royal Society for serious scrutiny, but instead to a pair of farmers he bumped into one day who were perplexed by the quantities of silver eel in a ditch on their land. So he explained his theory that this profusion of eels had emerged from a beetle’s bum. “They believed me,” he announced with pride, “and rejoiced in the solution of the mystery.”

BEAVERING AWAY IN intellectual isolation for sixty-odd years, Cairncross was unaware of the radical progress being made in the hunt for the fish’s gonads. Far from Dundee, Europe’s scientific intelligentsia were gripped by “the eel question,” and they were about to reach a climax—of sorts.

Leading the charge were the Italians, who embraced the quest to locate the eel’s missing sex organs as an unlikely source of civic pride for their troubled nation. Early modern Italy was a divided mess, with huge swathes dominated by a succession of warring foreign powers and its many independent states far from unified.