She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity by Carl Zimmer

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 ‘proofs’ collapse,” Weismann said.
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    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 cell—known as a mother cell—made new copies of all its chromosomes—which 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”—rare 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