Authors: Carl Zimmer
Weismann, like many other biologists of his generation, was taking advantage of powerful new microscopes and ingenious chemical stains to document life at the cellular scale. He observed how eggs developed into embryos, how some of their cells turned into eggs or sperm, which came together to make new embryos.
In addition to mapping cells, Weismann and his colleagues could also peer within them. In animals and plants, they could see a pouch inside each cell, which came to be known as the nucleus. Whenever a cell divided, its nucleus turned into a pair as well. But when a sperm fertilized an egg, the two nuclei seemed to fuse into a single one.
What lurked within the nucleus, Weismann and other scientists could not say for sure. It seemed to contain threadlike structures that were
duplicated each time a cell divided. But some studies suggested that when eggs developed, they lost half of the normal supply of threads.
Weismann wove together his own observations and those of other scientists into one powerful model of life. He divided the body into two types of cells: germ cells (sperm and eggs) and somatic cells (everything else). Once germ cells developed in an embryo, they carried inside of them a mysterious substance he called germ-plasm that could give rise to new life.
This substance transfers its hereditary tendencies from generation to generation,” Weismann said. Germ cells had a kind of immortality, because their germ-plasm could survive for millions of years. Somatic cells, on the other hand, were doomed to die along with the body in which they were trapped.
If Weismann's so-called germ line theory was right, then Darwin's pangenesis had to be wrong. Darwin envisioned germ cells as wide-mouthed pots into which gemmules from throughout the body could pour. Weismann envisioned a barrier sealing off the germ cells, isolating them from any influence from the somatic cells.
It also meant that the inheritance of acquired traitsâtaken as a fact by Hippocrates, Lamarck, and Darwin alikeâwas impossible. An animal's somatic cells might be altered by experiences, but there was no way for those changes to get communicated to its germ cells. “
Ever since I began to doubt the transmission of acquired characters,” Weismann said, “I have been unable to meet with a single instance which could shake my conviction.”
When Weismann turned against the inheritance of acquired characters in the late 1800s, it was still popular. In 1887, a certain “Dr. Zacharias” brought tailless cats to the annual meeting of German Naturalists. Dr. Zacharias claimed the mother of the cats had lost her tail when she was run over by a wagon. Other researchers did surgery on the spinal cord of guinea pigs, causing them to have seizures. Their pups had seizures as well. Mendel's mentor, Carl NÃ¤geli, claimed that the thick coat of mammals in arctic regions had developed in a reaction to the cold air, and then became inherited. Swans and other waterfowl were born with webbed feet thanks to the habit of their ancestors to strike the water with outstretched toes.
To Weismann, none of these stories about acquired characters was proof of inheritance. They could simply be coincidences. The guinea pigs might not have inherited their seizures; instead, they might have developed infections. If a cat lost her tail and then gave birth to tailless cats, the scientific thing to do would be to track down the father and see if he had a tail or not. There was no need to invoke acquired characters to explain why musk ox have thick fur. Natural selection favored individuals that, for whatever reason, had warmer coats that made them less likely to freeze to death.
In 1887, Weismann decided to do what the advocates of acquired characters never did, and run an experiment. He set out to test the idea that mutilations could be passed down. He ran the study on white mice, cutting their tails before letting them mate. The female mice got pregnant and delivered litters. And none of their pups had a shortened tail. Weismann repeated the procedure on their pups, and their grand-pups, and so on over the course of five generations. He produced 901 new mice. They all grew normal tails.
On its own, Weismann admitted, the experiment might not destroy the theory of acquired characters, but it added more weight to all the other reasons to question it. Lamarck's followers claimed proof based on far less evidence.
All such âproofs' collapse,” Weismann said.
Weismann reconfigured how scientists thought about heredity, an accomplishment all the more impressive for all the details of heredity he did not yet know about. After he introduced his germ-line theory, other researchers looked more closely at the multiplying threads in the nucleus of cells. They were dubbed chromosomes.
Researchers determined that a somatic cell carried pairs of chromosomes. (We humans have twenty-three pairs, for example.) A duplicating cellâknown as a mother cellâmade new copies of all its chromosomesâwhich it bequeathed evenly between two daughter cells. But when germ cells arose in an embryo, they ended up with only one set of chromosomes. Fertilization brought an egg and sperm together, creating a new set of pairs.
A new generation of scientists then asked how inheriting chromosomes determined the different forms that life could take. Hugo de Vries was among them.
De Vries had trained as a botanist, and at first heredity had meant little to him. He studied how plants grew, stretching their stems and sending out tendrils. His work caught the attention of Darwin, who recounted young de Vries's work in a book about plants. Darwin sent him a complimentary copy and then invited him to visit his estate when de Vries visited England in 1878.
We talked for a short time about all kinds of things, the country house (which is very large and beautiful), the surroundings (also very beautiful), politics, my journey etc.,” de Vries eagerly wrote his grandmother that night. “Thereafter Darwin took me to his room and we talked about scientific subjects. At first about tendrils, in connection with our former correspondence.”
Darwin took de Vries on a tour of his garden, handing him a peach along the way. Later, de Vries gushed to his grandmother that he “was received so kindly and cordially as I never had dared hope for.”
When de Vries returned home to the Netherlands, he and Darwin kept up the correspondence about plants. But in a letter he wrote Darwin in 1881, de Vries abruptly changed the subject. Now he was consumed with heredity.
“I have always been especially interested in your hypothesis of Pangenesis,” de Vries told Darwin, “and have collected a series of facts in favour of it.”
De Vries roamed the countryside for “sports of nature”ârare plants that sprouted weird growths or displayed odd colors. He wanted to create an herbarium of monstrosities, he later told a friend. By breeding them, he hoped to prove Darwin's theory of pangenesis right.
When Weismann unveiled the concept of the germ line, de Vries recognized its importance. As a botanist, though, he found it parochial. Plants, like animals, were made of cells that contained nuclei, and inside those nuclei were chromosomes. When plant cells divided, they also made a new set of chromosomes. But plants did not wall off their germ cells early in
development. An apple tree would grow for years before producing germ cells that could give rise to pollen grains or seeds. A cutting from a willow could grow into an entire tree, complete with roots, branches, and leaves. A hidden potential to produce new plants must be spread throughout their cells, de Vries thought. While pangenesis might have its problems, he thought it had to be the foundation of any true understanding of heredity.
Darwin died in 1882, leaving de Vries to search for that understanding without the guidance of his guru. He began running experiments with his monsters. He crossed them with ordinary plants, and sometimes their bizarre traits turned up in later generations. De Vries came up with a theory of his own: Every cell contained invisible particles that were responsible for passing traits from one generation to the next. Under some circumstances, the particles in somatic cells could guide the development of a new organism. In honor of Darwin, de Vries called the particles pangenes.
In 1889, de Vries published
in which he distilled over a decade's worth of work
Hardly any scientists took notice of it. One of the few who did advised de Vries not to mention pangenesis again.
De Vries did not give up. In the 1890s, he noticed that monstrosities crossed with regular flowers produced regular ratios of offspring. De Vries thought that flowers could have different numbers of pangenes in them, and those numbers were what determined traits in their offspring.
Despite his struggles with these ratios, de Vries became convinced that pangenes were real, and that their changes were what made evolution possible. Pangenes could abruptly change in a process he called mutation, and flowers that inherited a mutation abruptly became a new species. De Vries's mutation theory was pushing him far from Darwin, who had argued for the gradual evolution of species through tiny steps.
One day early in 1900, de Vries got a letter from a friend who was familiar with his obsession with hybrid plants. His friend thought de Vries might be interested in
a thirty-five-year-old paper by “a certain Mendel.” When de Vries scanned the paper, he was stunned that a Moravian monk he had never heard of had found the same patterns he had. He had even come up with a theory of invisible hereditary factors to account for it.
By an unparalleled coincidence, two other scientists studying inheritance, William Bateson and Carl Correns, also stumbled across Mendel's work at about the same time. They all realized that they had been scooped. And they also recognized just how important Mendel's experiments had been. Before 1900, scientists didn't have the right frame of mind to appreciate them. It took Darwin and Galton establishing heredity as a scientific question. It took Weismann and others to look closely at cells to ask how heredity was transmitted.
De Vries, Bateson, and Correns all began sharing the belated news about Mendel. Bateson emerged as the leader of the campaign: He and his colleagues demonstrated that animals could display the same ratios as plants. Even certain hereditary diseases in people fit the pattern.
A British doctor named Archibald Garrod noticed that a condition he called alkaptonuriaâwhich turned urine blackâtended to run in families. Sometimes when two seemingly healthy parents started a family, about a quarter of their children fell ill. That ratio fit Mendel's predictions: The parents must be carriers, each carrying a recessive factor.
whole problem of heredity has undergone a complete revolution,” Bateson declared. Mendel's discoveries could at last mature into a true science. Bateson christened it genetics.
No sooner was genetics born, however, than it was hurled into battle. Some scientists felt that Mendel must have made a mistake. Some tried to get his neat ratios of hybrids and failed. Other critics found it inconceivable that physical particles could be inherited and give rise to every trait in an organism.
De Vries went his own way. He accepted that Mendel's results were genuine, but he came to doubt they mattered much to big evolutionary changes. Those could only come about through the appearance of major new mutations. Evolution didn't creep forward, de Vries believed. It leaped.
De Vries unpacked this idea in his sprawling two-volume work,
The Mutation Theory
, in 1903. His theory that new mutations could produce
new species in a single leap proved sensational. It finally earned de Vries the fame that had escaped him in earlier years. When he came to the United States to give lectures about his mutation theory, newspapers put his face on their front pages. It was on one of those tours that de Vries paid his first visit to Luther Burbank, in 1904.
By then, Burbank no longer considered himself simply a plant breeder. The honors that scientists had heaped on him persuaded him he was a genius of heredity. When scientists visited Burbank, he would regale them with a grand theoryâ“
perhaps as original as Darwin's,” he modestly declaredâthat the universe consisted of what he called “organized lightning.” The scientists who listened to Burbank's ramblings politely nodded, said that they were unqualified to judge, and hoped they could gain access to his legendary garden.
De Vries traveled to Burbank's garden to find support for his mutation theory. His own evening primroses produced mutants from time to time, but he had yet to find another species that displayed mutations so clearly. De Vries's gigantic theory had come to rest on precious little evidence, like an elephant trying to ride a bicycle. Maybe Burbank's new varieties were, in fact, a wealth of new mutants.
Between bites of Burbank's stoneless plums, de Vries interrogated his host. Burbank had become wary of sharing his secrets by then. He would sometimes force his workers to empty their pockets to make sure they weren't smuggling out his prize seeds. If they chatted across the picket fence with a passerby, he would fire them. With de Vries, Burbank was more forthcoming. He explained how he had crossed plums, selecting the ones with smaller and smaller stones. He described how he set about breeding cacti without spines as a new source of food for cattle. He searched for varieties to cross, each missing different parts of their spines. Over generations, they became soft enough for Burbank to stroke over his cheek.
De Vries left Santa Rosa impressed by Burbank's passion. “
The sole aim of all his labors is to make plants that will add to the general welfare of his fellow beings,” de Vries wrote later. As a scientific mission, however, the
journey ended up a disappointment. De Vries hoped his visit would shed light on how plants acquired new traits. “Burbank's experience did not throw any light on this question,” he concluded.
De Vries's time with Burbank marked the high-water mark in the careers of both men. When he traveled to Santa Rosa, de Vries had become famous as one of the founders of modern genetics and as the author of a controversial new theory about mutations that seemed to overthrow Darwin. Burbank, meanwhile, had become a celebrity as both a mystic of nature and a keen businessman. Things would never be so good for either of them again.