FIGURE 15.1. In this Hogarth print, a midwife tends to Mary Toft. While her newborn rabbits scurry about, one joyous onlooker exclaims, “A great birth.”
For several reasons, heredity posed huge problems to early scientists. Armed with countless observations, many of which seemed to contradict one another, they found themselves adrift in an ocean of anecdotes. Blue-eyed parents always had blue-eyed children, for instance. So far, so good. But brown-eyed parents sometimes had brown-eyed children and sometimes did not. Why was that? Was there something special about the color blue? Something suspect about brown-eyed parents?
Parents surely passed traits to their children, but no one could imagine how that worked. A mother’s broad chin might reappear in perfect miniature on her young daughter, but then another child born to the same woman would look completely different. Aristotle noted the case of a father who had been branded on the arm and had a son with the same sort of mark on his arm. Was there a law lurking there? Two millennia later, Darwin was still examining the same riddle. “I have been assured by three medical men of the Jewish faith,” he wrote, “that circumcision, which has been practiced for so many ages, has produced no inherited effect.”
Matters grew more confusing still when you followed a particular trait as it worked its way through several generations in one family. Aristotle puzzled over the case of a white woman who had gone to bed with “an Aethiop” (i.e., a black man). She gave birth to a pale-skinned daughter, but, to Aristotle’s bewilderment, “the son of that daughter was an Aethiop.”
If these early scientists had known that there was a science of heredity—if they had known, say, that there were strict rules that governed the color of a child’s eyes—then they would quickly have seen that both mother and father played key roles in forming the new baby. But in the absence of such insight, it was easy to make up after-the-fact explanations to justify whatever beliefs you happened to hold. That baby looks like his grandfather because the mother spent years taking care of the grandfather.
That seemed good enough because, for thousands of years, heredity had been seen more as a collection of intriguing observations than as a mystery in search of an explanation. Our forebears thought of heredity in roughly the way that we think of trees or clouds. We know there are broad patterns—oak trees look different from maples—but no one knows precisely how the branches of a given tree will twist and turn or even how many branches there will be. We know that cumulus clouds, which feature in every child’s drawing, look fat and puffy, while cirrus clouds are thin and wispy. But no one claims to know—or even seeks to know—just how many clouds will appear in the sky today or precisely how they will change shape as the day passes.
DOCTORS AND SCIENTISTS ACCEPTED THE MOST FAR-FETCHED stories, at least as possibilities. Eyewitness testimony commanded special attention. Odd as our forebears’ credulity sounds to us—Where was the proof?—we can still find echoes of it in courtroom trials today, where eyewitness testimony carries far more emotional punch than technical evidence about tire tracks and bits of fiber.
One renowned doctor—this was Fortunio Liceti, a professor of medicine at the University of Padua (and a friend and colleague of Galileo)—wrote about “a scoundrel who coupled with a cow.” From this dubious union, “there resulted a boy resembling a complete man in every regard, except for his sharing the cow’s inclination to graze on the grass and to chew its cud.”
Such lurid stories were supercharged versions of familiar tales. In books of natural history, hybrids popped up everywhere. They were not mythological figures like centaurs but real, flesh-and-blood creatures. The camelopard—in reality quite likely a giraffe—was perhaps best-known. That strange-looking beast supposedly came to be when a camel and a leopard mated.
Far more disturbing were the countless stories of human-animal offspring. From ancient times down through the 1700s, the most renowned scholars and physicians passed along such reports. They appeared not in Renaissance versions of the National Enquirer but in the most earnest and learned venues. The philosopher John Locke discussed the troubling case of one pig/human hybrid in his Essay Considering Human Understanding, one of the most admired works of its era. Locke pondered a variety of ethical dilemmas. Would destroying the “monster” count as murder? Could the creature attend church?
FIGURE 15.2. The original caption of this drawing (from 1573) explained, “A monster, half-man, half-swine.”
One thick volume by an eminent French physician named Ambroise Paré—chief surgeon to two French kings—devoted a chapter to well-documented mix-and-match creatures.* Paré called his compendium On Monsters and Marvels, and it is itself a hybrid of medieval credulity and modern skepticism.
THROUGH THE AGES, EVERY QUESTION TO DO WITH HEREDITY HAD been difficult. Then the doctrine of preformation appeared, and the difficult became almost impossible. If parents don’t form their children, who were created when the Earth was new, why should there be any resemblance at all between parent and child? Look more closely, and the problems grew even more formidable. If God created the complete set of Russian dolls at the beginning of time, how could he have known to give baby number 1,000,000 a pointy nose and curly hair just like her father’s? What if the little girl’s mother had married a man with a snub nose and hair as straight as straw?
The philosopher Immanuel Kant, whose prose typically fell somewhere in a range between dense and impenetrable, for once put the difficulty clearly. “If the woman had been with another man,” he wrote, “she still would have produced the same children.” (For spermists, Kant added, the same point would apply vice versa.) That left ovists and spermists caught in the same trap, neither of them able to explain how a baby could inherit traits from both parents.
Perhaps sensing their own vulnerability on the whole topic of heredity, both sides refrained from pushing too hard. Ovists, as we have seen, attributed nearly every puzzling observation to “maternal influence.” This was a weak argument, although it helped a bit that the dolls within a doll were so astonishingly small; perhaps a tiny push could reshape so tiny a body, much as the flick of a finger might set a pebble flying, though it would not budge a boulder.
Spermists resorted to the ancient woman-as-field analogy, but that didn’t get them far, either. The attempt was to invoke something like the winemakers’ notion of terroir, where the same grape makes markedly different wine depending on particular conditions of soil, rainfall, and sunlight. “This is so poor,” one writer scoffed in 1707, that you might as well argue that “an Orange-Tree transplanted from Sevil to England would bear Apples.”
Hybrid births, in which the parents are from different species altogether, posed a particularly daunting challenge for ovists and spermists alike. Take mules, which are the offspring of male donkeys and female horses. Mules have been familiar since ancient times. But according to preformationist doctrine, donkeys should give birth to an endless line of donkeys, and horses to horses. How could it be, then, that one day a horse gave birth to a different sort of animal?
The everyday explanation was perfectly simple: someone decided, one day, to breed a donkey and a horse. But preformationists had to tie themselves in knots to explain where this long-eared interloper had come from. Had God foreseen at the beginning of time just which Russian doll would be not a horse but a mule?
IN THE ABSENCE OF ANY BETTER THEORY THAN PREFORMATION, ovists and spermists did their best to ignore uncomfortable questions. Sometimes this strategy cost them. Leeuwenhoek, for instance, had veered near one of Gregor Mendel’s key findings centuries ahead of the father of genetics. Then he turned his back on his own insight.
In a letter to the Royal Society in 1683, Leeuwenhoek had mentioned some rabbit breeders he had met. He had peppered them with questions. Often, he learned, they mated wild rabbits with domestic ones. The wild rabbits were small, gray males, the domestic rabbits large, white females. All the offspring were small and gray, like their fathers.
Leeuwenhoek pursued the matte
r further. What happened, he asked the breeders, if the females were not white? Again, so long as the father was gray, all the offspring were gray, no matter whether the mother was black-and-white or solid black. “Indeed,” Leeuwenhoek exclaimed, “it has never been seen that any such young rabbit had a single white hair or any other hair than gray.”
What could it mean? In line with his spermist views, Leeuwenhoek concluded triumphantly that the moral was plain: only the male is important. But if Leeuwenhoek had gone on to perform a follow-up experiment—this time mating white, domestic males with wild, gray females—all the offspring would again have been gray, like their mothers. Leeuwenhoek would have been confused, but if he had concluded that gray always prevailed he would have been the first to discover a dominant genetic trait.
Leeuwenhoek was done in by ideological blinders, but his mistake was a natural one. We are all quick to settle for explanations that confirm our assumptions. All the more so when we can slide new facts neatly into place inside an existing, ready-made theory. Leeuwenhoek was too far ahead of his time. When he pondered the riddle of gray and white rabbits, he was musing about genetics two hundred years before the word even came into existence.
AGAIN AND AGAIN IN THE SEX AND BABIES MYSTERY, WE CAN SEE our forebears on the brink of grasping a clue that required concepts and a vocabulary they had not formulated. The riddle of heredity provides a tantalizing example, and not just for Leeuwenhoek. William Harvey, who lived before the preformation era, almost sorted it out. Trying to account for family resemblances, he wrote that “there is no part of the future offspring actually in being, but all parts are indeed present in it potentially.”
This sounds like mumbo jumbo, but Harvey was stumbling to find words to convey a genuine insight. Look at a woman’s freckled nose, Harvey had said, in effect. Years from now, she might have a child with an identical freckled nose of his own. That has to mean that somewhere there is a ghostly realm where that freckled nose lies waiting to make its appearance. The problem was that Harvey could not bring into focus what he had dimly glimpsed. Nor could anyone else in his era.
Harvey did not believe in a simple physical explanation of inheritance; he rejected the ancient suggestion that a baby inherited his mother’s strong chin or his father’s big ears from miniature chin and ear particles that somehow joined together as the embryo grew. But the baby did have his mother’s chin. How could that be? With the idea of a genetic code completely out of reach, Harvey could only gesture toward an answer.
We can now describe, with hindsight’s aid, what he had struggled to make out. Parents pass to their offspring not building blocks but instructions. But that insight belonged to a far-off future. In the early decades of the 1700s, scientists trying to solve the sex and babies mystery found themselves snarling in frustration. Their theory of the case relied on preformation, the doctrine of Russian dolls. That doctrine seemed unassailable. But, as we have seen, preformation made the simplest facts of heredity impossible to explain.
The impasse lasted until 1740. Then, one summer morning in Holland, a young man took two boys for a walk.
SIXTEEN
“ALL IN PIECES, ALL COHERENCE GONE”
THE BOYS HAD SPENT THE MORNING HAPPILY SPLASHING THROUGH the ponds on their parents’ grand estate, scooping up prizes in glass jars to examine later. Without a magnifying glass, their finds didn’t look like much. Green flecks, mostly, that floated in their jars.
Their tutor, a young Swiss naturalist named Abraham Trembley, believed that ponds and meadows made as good a classroom as any indoor space. Now Trembley and his two charges, one six years old and the other just three, gazed intently at the debris they had scooped up. Trembley peered through a magnifying glass. Was this quarter-inch-long green tube a plant—that was the boys’ vote—or were those wavy, slow-moving tentacles not branches but arms? Plant or animal?
Trembley set to work to find out. The experiments he carried out, for two little boys and with hardly more equipment than a pair of scissors, would stun him and turn the world of science upside down. Trembley shot to fame. Today he is all but forgotten, and nobody pays attention to fresh-water polyps. In the 1700s, his discoveries were considered the most important of the age.
His first observations gave little hint of the revelations that lay ahead. Trembley saw early on that his pond creatures could crawl along the glass walls of their jars and shorten and lengthen their bodies. Then, one day when a water flea swam by, Trembley saw the polyp shoot out an arm, grab the flea, and stuff it into its “head.” Okay, then—these were animals, not plants. That seemed noteworthy, perhaps, though scarcely earthshaking. To seal the deal with one final, simple experiment, Trembley took scissors in hand and cut one of the quarter-inch long organisms in two. If the polyp was a plant, he reasoned, then there was a chance it would survive; some plants grow from cuttings. But if it was an animal, splitting it would surely kill it.
Trembley watched and waited. After a week, both head and tail were thriving. (The head was the end with tentacles.) After two weeks, the head had grown into a full, new creature. So had the tail! One creature had become two. Trembley repeated the experiment. Same result. He tried again, now making several cuts. He watched in disbelief as each tiny snippet cut from the original creature grew into a full-fledged, independent animal, indistinguishable from its “parent.”
In an era when living animals, especially small ones, were regarded as machines, this was unfathomable. It would be as if, in our day, someone took a blowtorch to a car, cut it into random pieces, and then looked on as each piece—this one containing a mutilated chunk of engine, that one a bit of door and a side mirror—regrew into a complete car and zoomed off down the street.
The first report on Trembley’s “little machines,” from 1741, conveyed the scientific world’s astonishment. “From each portion of an animal cut in two, three, four, ten, twenty, thirty, forty parts and, so to speak, chopped up, just as many complete animals are reborn, similar to the first.” So far as anyone could tell, the process could go on forever. “The story of the Phoenix who is reborn from his ashes, as fabulous as it might be,” declared France’s normally staid Academy of Sciences, “offers nothing more marvelous.”
Trembley sent glass jars with hydra inside to all of Europe’s leading scientists. (He named his creatures for the nine-headed beast that Hercules had managed to kill even though two new heads grew whenever he cut off one.) Skeptical but fascinated, naturalists launched into a frenzy of experiments. But whether you cut the creature in pieces or turned it inside out or put one hydra inside another, nothing fazed this bizarre animal. “These are Truths,” wrote one bowled-over English scientist, “the Belief whereof would have been looked upon some Years ago as only fit for Bedlam.”
FIGURE 16.1. Hercules battling the hydra.
More bewilderingly still, the creature performed its miracles even though it seemed not to have any working parts. Even under a microscope’s lens, one scientist complained in frustration, hydra appeared to be nothing but a stomach. Trembley’s first thought had been that the growth of a new hydra from a mere scrap might be a souped-up version of some more familiar form of regeneration, like a crab’s growing a new claw. But no, he quickly conceded, that would not do. A new claw was remarkable, but a complete new organism was uncanny.
Everyone, not just scientists, wanted in on the excitement. Sales of microscopes soared. Salons buzzed with chatter. Writers scurried to explain what it all meant or to lampoon the intellectuals who sat entranced, staring at magnified drops of water. “If ’tis cut in two, it is not dead; / Its head shoots out a tail, its tail a head,” wrote one English satirist, in a mock epic. “Cut off any part that you desire, / That part extends and makes itself entire.”
WHY SUCH ASTONISHMENT? NOT SIMPLY BECAUSE THESE ANIMALS acted like no animals ever had. More than that, these tiny creatures blasted the firmest tenets of eighteenth-century biology to smithereens. First, the most fundamental law of nature had suddenly been
violated. Here is new life arising without a hint of sex or mating. Second, biology’s reigning theory, the doctrine that new life came in the form of dolls within dolls, found itself shoved aside and completely beside the point. Does anyone seriously maintain that every single piece of cut-up hydra, no matter where it came from, contained a mini-hydra within it? Third, the all-but-universal assumption that living creatures were ingenious machines no longer made sense. Who has ever heard of machines like these?
Underlying those distinct challenges was a deeper assault on established views, one almost too unsettling to spell out. If life could shape itself from random scraps of matter, as now seemed undeniable, how did God fit into the picture? Where in these scenes of burgeoning life was the Creator’s shaping hand?
And still more trouble was coming. At virtually the same time that Trembley was slicing up hydra, another Swiss naturalist was staring, bewildered, at aphids. These, too, were seemingly insignificant little creatures destined to undermine the foundations of seventeenth-century science.
Charles Bonnet happened to be Trembley’s nephew, though he was only ten years younger. The two men had a great deal in common. Both were devout, obsessed with the natural world and with insects most of all, and in constant correspondence with each other and with a host of other scientists. It was one of the most eminent of these correspondents, a French naturalist named René Réaumur, who had goaded Bonnet into taking on a mystery that he himself had failed to crack.
The Seeds of Life Page 16