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about the theory’s shortcomings, freely admitting it was incomplete. He asked people instead to judge the idea on its broadclaims, which he was sure were correct, and not on the fine details, which he did not possess but which he was certain future generations of biologists would fill in.
    Two things stood out as glaring omissions. The first was the mechanism of variation. People clearly inherit traits from both their mother and their father: it is possible to see a mother’s red hair in a child or a father’s square jaw. But what causes the appearance of new traits from which natural selection, well,
selects
?
    The second thing missing from Darwin’s theory was the mechanism of inheritance. Darwin initially thought that information about traits was carried from generation to generation when some kind of fluid from each parent intermingled. However, just as red and yellow paint mix together to make orange paint, while losing red and yellow for ever, combining such biological fluids should blend together traits, losing some for ever. We should see people with eyes only a blend of blue and brown and never people with undiluted blue or brown eyes, something that flatly contradicts reality. Over time, the blending of such biological fluids should cause all creatures in a population to become similar, drastically reducing the variation needed for the operation of natural selection. When Darwin realised this flaw in his fluid idea, he was deeply depressed.
The mechanism of inheritance and variation
    It was a monk called Gregor Mendel in Brno, in what is now the Czech Republic, who was the first to glimpse the elusive mechanism of inheritance. Between 1856 and 1863, Mendel bred together varieties of pea plants in their tens of thousands and listed a number of traits that were inherited in their entirety. Forinstance, when Mendel bred pea plants with purple flowers with ones with white flowers, the result was not pea plants with a pinkish flower but a certain predictable fraction of white pea plants and a certain predictable fraction of purple pea plants. Characteristics are inherited equally, one from each parent, with some traits more dominant than others, Mendel found. Crucially, however, they are inherited as
particles
that can never be subdivided, not as a fluid that can be blended. Mendel, though he did not know it, had discovered what we now call genes.
    Mendel published his findings in
Proceedings of the Natural History in Brünn
in 1866. But the journal was so local and obscure that his work was not widely recognised until the twentieth century. There is a story, often repeated, that, of the 115 copies of Mendel’s pea paper, one found its way to Darwin himself. It was discovered in his library after his death, sealed and unread. It would have been a terrible tragedy if true. However, the story is mere romantic myth. Darwin had no work by Mendel in his vast collection. The two biological geniuses, each of whom possessed a crucial jigsaw piece the other lacked, missed each other not by a hair’s breadth but by a significant span of space and time.
    Mendel’s work was rediscovered only in 1900, long after Darwin’s death. Shortly afterwards, the American biologist Thomas Hunt Morgan began breeding together fruit flies. He observed that they inherited characteristics in a pattern very similar to Mendel’s pea plants. He even established that the physical elements responsible for inherited traits – genes – lay on tiny stringy structures called chromosomes. It was the birth of a new science: genetics.
    The full picture of inheritance was filled out only in the late twentieth century. The building blocks of all life are cells, tinybags of chemicals, whirring with chemical nanomachinery. 5 In the centre of every cell is a mini cell, or nucleus. And, in each nucleus, chromosomes made of DNA.
    DNA is a molecule the shape of two spiral staircases intertwined. The core, or backbone, of this double helix is made of a