Authors: Carl Zimmer
The Burbanks hoped Luther would become a doctor, but at school he showed little proficiency in Latin or Greek. He was more interested in the books about natural history that his cousin, an amateur naturalist, gave him. They took walks around the countryside, where his cousin instructed him on the landscape, from the rocks to the plants that grew over them. Luther developed a fierce desire, he later said, “to know,
not second-hand, but first-hand, from Nature herself, what the rules of this exciting game of Life were.”
In 1868, when Luther Burbank was nineteen, all daydreams about nature and medicine were cut short. His father suddenly died, forcing his family to sell off their farm and move away. Burbank had to support his mother and sisters by farming rented fields. “
Nature was calling me to the land, and when there came to me my share of my father's modest estate I could no longer resist the call,” Burbank later remembered.
He decided that he had to do more than stick seeds in the ground. He
needed to change the seeds themselves. When Burbank sold produce in town markets, he could see how some farmers made more money because they used better breeds. Their customers preferred bigger fruits, tastier vegetables. Farmers who planted early-growing breeds could start selling produce earlier in the year. Burbank got a grand ambition: to use the rules of the game of Life to create entirely new breeds.
In the 1860s, the concept of heredity had not penetrated the United States very far.
The textbooks Burbank read in school didn't even use the word. Instead, they offered a jumble of folk explanations for why people resemble their ancestors. Burbank's physiology textbook informed him that
if a woman “has a small, taper waist, either hereditary or acquired, this form may be impressed on her offspring;âthus illustrating the truthfulness of scripture, âthat the sins of the parents shall be visited upon the children unto the third and fourth generation.'”
One day Burbank spotted a new two-volume book at the Lancaster town library on animal and plant breeds. Desperate for help with his experiments, he dipped into it, and before long he had devoured the entire work. After he finished, Burbank felt as if he had been given the keys to heredity's locks. He was ready to create new kinds of crops the world had never seen. “I think
it is impossible for most people to realize the thrills of joy I had in reading this most wonderful work,” Burbank later said.
The Variation of Animals and Plants Under Domestication,
was written by a British naturalist named Charles Darwin. In it, Darwin cast heredity as a scientific question in urgent need of an answer. But the answer he offered would turn out to be spectacularly wrong.
he Variation of Animals and Plants Under Domestication
served as a sequel to its far better-known predecessor,
The Origin of Species
. In that earlier book, Darwin had presented the outlines of his theory of evolution. In every species and strain, Darwin observed, individuals varied from one another. Some of those variations may help some individuals survive and reproduce. The next generation will inherit those successful variations,
and will pass them down in turn. Darwin called this process natural selection, and he argued that, over many generations, it could turn varieties into separate species. Over even longer periods, it could produce radically new forms of life.
The Origin of Species
became one of the most influential books ever written, opening up millions of minds to the fact that life has been evolving into new species for billions of years and is continuing to evolve today. Yet Darwin knew that in the book he had glossed over some of the most important parts of evolution. While its logic was straightforward enough, Darwin couldn't explain the biology that made it possible. Yes, individuals varied, but why? Yes, offspring resembled their parents, but why? Anyone who would answer those questions would first have to explain what heredity really is.
The laws governing inheritance,” Darwin conceded, “are quite unknown.”
Three decades earlier, when Darwin was twenty-eight, he began
jotting down notes and questions in a series of notebooks. In their pages, we can see the slow metamorphosis of his ideas about the diversity of life. From the beginning, he already recognized the importance and mystery of heredity. When two breeds were crossed, he wondered, why did the offspring sometimes look more like one breed than the other? Why did they sometimes look like neither parent?
In search of answers, Darwin read everything he could find about heredity. Dissatisfied by what naturalists had to say, he turned to breeders for help. He read
Bakewell's famous rules for producing better sheep and cows. He printed up
a short pamphlet entitled
Questions About the Breeding of Animals
and sent it out in 1839 to England's leading breeders. He asked them what happened when they crossed different species or varietiesâwhether hybrids were produced and, if so, whether their offspring were sterile. He asked how reliably traits were passed down from generation to generation, whether animals inherited the behaviors of their parents, whether the disuse of some body part might lead it to dwindle away.
The information Darwin got back from the breeders still wasn't enough.
So he became a breeder himself. Filling his greenhouse with plants, Darwin became expert at crossing orchids. He bought rabbits so that he could compare their dimensions to wild hares. He built a pigeon house at the end of his yard and stocked it with rare breeds. He went to club meetings of pigeon breeders, and even attended the annual poultry show in Birmingham, known as “the Olympic Game of the Poultry World.” Darwin marveled at the way the breeders could spot tiny variations from one pigeon to the next, and how they used those differences to produce extravagant new breeds. Pigeons, Darwin declared to his friend Charles Lyell in 1855, were “
the greatest treat, in my opinion, which can be offered to human being.”
Darwin turned to humans for clues to heredity as well, but he mainly studied how they went mad. Doctors had long puzzled over the causes of insanity. Some blamed alcohol, others sorrow, others sin, others masturbation. But some considered insanity to be a hereditary disease. In eighteenth-century France, a fierce debate broke out about whether hereditary diseases even existed, and the French doctors of the mindâ
alienists, as they were then knownâstarted gathering data to prove they did. They filled out entrance forms when people were admitted to asylums, and they studied national censuses. Madness, the alienists decided, clearly ran in families. “Of all illnesses,” the French alienist Ãtienne Esquirol said in 1838, “
mental alienation is the most eminently hereditary.”
The French alienists investigated how madness could be hereditaryâwhat it had in common with other hereditary diseases like gout or scrofula. They contemplated the underlying mystery: the process by which traitsâboth illnesses and ordinary traitsâwere passed down through the generations. Along the way, their language experienced a subtle yet profound shift. At first French alienists only used the adjective
in order to describe diseases inherited from ancestors. But in the early 1800s, they began using the noun
. Heredity was becoming a thing unto itself.
In his research into madness, Darwin plowed through a two-volume tome called
Treatise on Natural Inheritance
, published in 1850 by the French alienist Prosper Lucas. Darwin framed its pages with notes in the margins.
In English, he began to follow Lucas's example. Again and again, he wrote down the word
Darwin was not drawn to heredity purely out of intellectual curiosity. Marrying his first cousin Emma led him to worry what fate they might deliver to their children. He read reports from alienists about how the children of first-cousin marriages were prone to madness. His anxiety only grew as his own health failed. In his twenties, he had been fit enough to take a voyage around the planet, but after his return he developed
a constellation of disorders. He vomited violently, he suffered from boils and eczema, his fingers went numb, and his heart often raced. He described himself in 1857 as a “
wretched contemptible invalid.” Three of Darwin's ten children died young, and the others suffered from bouts of poor health.
It is the great drawback to my happiness, that they are not very robust,” he wrote to a friend in 1858. “Some of them seem to have inherited my detestable constitution.”
Darwin put only a little of his research on heredity in
The Origin of Species.
he saved that profound matter for a book of its own. When he began focusing his thoughts on heredity, however, he decided that all the details he had been collecting about pigeons and insanity would not be enough. He would also have to figure out the physical process that accounted for all
the strange ways in which animals and plants reproduced.
At the time, Mendel was raising pea plants and hawkweed, but Darwinâlike most scientists of his dayâdidn't even know who Mendel was. Instead, Darwin drew his inspiration from other biologists who had made a profound discovery of their own: that all of life is made of cells.
To Darwin, the central question of inheritance was what sort of substances the cells of parents transmitted to an embryo so that its cells came to resemble theirs. Whatever made muscles strong was stored in muscle cells. Whatever made brains wise or defective must be stored in brain cells.
Perhaps, Darwin thought, the cells throughout the body cast off “
minute granules or atoms.” He dubbed these imaginary specks gemmules.
Once released by cells, gemmules coursed through the body, gradually piling up in the sexual organs. When the gemmules from both parents combined in a fertilized egg, they enabled it to develop into a blend of cells from both parents.
Darwin wanted a catchy name for this imaginary process. Maybe
something that combined
Darwin asked his son George, then a student at the University of Cambridge, to ask classics professors there for a name. George came back with outlandish suggestions like
. Darwin settled on
Pangenesis set Darwin apart from most naturalists of his day. They explained heredity as a blending of traitsâakin to mixing blue and yellow paint to produce green. Darwin looked at heredity instead as the result of distinct particles. They never fused and never lost their separate identities. Darwin readily admitted that pangenesis was “
merely a provisional hypothesis or speculation.” Yet it offered Darwin great powers of explanation. “
It has thrown a flood of light on my mind in regard to a great series of complex phenomena,” he said.
Darwin could explain why children sometimes resembled one parent more than the other with pangenesis: Some gemmules were stronger than others. The gemmules that gave rise to newborn babies were a mixture of particles that had accumulated over generations, from parents, grandparents, and on back through time. A gemmule might be overshadowed by stronger ones for thousands of years, only to leap forward and revive some ancient feature. And as experiences altered cells, they would also alter their gemmules. As a result, a trait acquired in life could be passed down to future generations.
On that last point, Darwin was simply following in a tradition that reached back over two thousand years to the writings of Hippocrates. Earlier in the nineteenth century, Darwin's predecessor, the French naturalist Jean-Baptiste Lamarck, had offered the first detailed theory of evolution, and he had made the inheritance of acquired characteristics a crucial part. A giraffe striving to reach leaves on a high branch would force a vital fluid into its neck, stretching it. Its offspring would then be born with that longer
neck, and over many generations this stretching produced the neck of the giraffes we see today.
Darwin saw gemmules as acting like Lamarck's vital fluid. In his research, Darwin discovered that
improved breeds of cattle grew small lungs and livers compared to free-ranging breeds. He saw this as the result of pangenesis. Farmers fed these breeds better food and expected less work from them. As a result, they didn't need to work their lungs or livers, and the organs produced different gemmules as a result.
To Darwin, cattle and other domesticated animals put pangenesis on an impressive display. In just a few thousand years, humans had altered the heredity of animals and plants in endless ways, producing greyhounds and corgis and Saint Bernards, racehorses and draft horses, apples, wheat, and corn. Breeders such as Bakewell selected individuals to breed, unknowingly choosing which animals could pass their gemmules to future generations. They crossed different strains to combine gemmules in new combinations. Breeders had exploited the same laws of inheritance that had made the evolution of all speciesâeven ourselvesâpossible.
“Man, therefore, may be
said to have been trying an experiment on a gigantic scale,” Darwin wrote, “and it is an experiment which nature during the long lapse of time has incessantly tried.”
Sitting in Massachusetts, young Luther Burbank read
The Variation of Animals and Plants Under Domestication
with a rush of marvel and relief. He might be a novice farmer, but now he felt he was part of something far bigger. The same biology that gave rise to all living thingsâtheir variation, their selection, and their heredityânow felt like clay he could shape with his hands. Darwin declared that variation emerged from crossbreeding, which mixed gemmules of different origins together in new combinations. By selecting which plants to breed again, Burbank could eventually produce a new variety that reliably passed down its traits to future generations.
While I had been struggling along with my experiments, blundering
on half-truths and truths,” Burbank later wrote, “the great master had been reasoning out causes and effects for me and setting them down in orderly fashion, easy to understand.”