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from your mother as if mixing blue paint and red paint to make purple. If this were really how heredity worked (as people vaguely thought in Darwin’s time) we’d all be a middling average, halfway between our two parents. In that case, all variation would rapidly disappear from the population (no matter how assiduously you mix purple paint with purple paint, you’ll never reconstitute the original red and blue). In fact, of course, anybody can plainly see that there is no such intrinsic tendency for variation to decrease in a population. Mendel showed that this is because when paternal genes and maternal genes are combined in a child (he didn’t use the word ‘gene’, which wasn’t coined until 1909), it is not like blending paints, it is more like shuffling and reshuffling cards in a pack. Nowadays, we know that genes are lengths of DNA code, not physically separate like cards, but the principle remains valid. Genes don’t blend; they shuffle. You could say they are shuffled badly, with groups of cards sticking together for several generations of shuffling before chance happens to split them.
    
    
    Any one of your eggs (or sperms if you are male) contains either your father’s version of a particular gene or your mother’s version, not a blend of the two. And that particular gene came from one and only one of your four grandparents; and from one and only one of your eight great-grandparents.*
    
    
    Hindsight says this should have been obvious all along. When you cross a male with a female, you expect to get a son or a daughter, not a hermaphrodite.† Hindsight says anybody in an armchair could have generalized the same all-or-none principle to the inheritance of each and every characteristic. Fascinatingly, Darwin himself was glimmeringly close to this, but he stopped just short of making the full connection. In 1866 he wrote, in a letter to Alfred Wallace:My dear Wallace
    
    I do not think you understand what I mean by the non-blending of certain varieties. It does not refer to fertility. An instance will explain. I crossed the Painted Lady and Purple sweet peas, which are very differently coloured varieties, and got, even out of the same pod, both varieties perfect but none intermediate. Something of this kind, I should think, must occur at first with your butterflies . . . Though these cases are in appearance so wonderful, I do not know that they are really more so than every female in the world producing distinct male and female offspring.
    
    
    
    
    Darwin came that close to discovering Mendel’s law of the non-blending of (what we would now call) genes.* The case is analogous to the claim, by various aggrieved apologists, that other Victorian scientists, for example Patrick Matthew and Edward Blyth, had discovered natural selection before Darwin did. In a sense that is true, as Darwin acknowledged, but I think the evidence shows that they didn’t understand how important it is. Unlike Darwin and Wallace, they didn’t see it as a general phenomenon with universal significance – with the power to drive the evolution of all living things in the direction of positive improvement. In the same way, this letter to Wallace shows that Darwin got tantalizingly close to grasping the point about the non-blending nature of heredity. But he didn’t see its generality, and in particular he failed to see it as the answer to the riddle of why variation didn’t automatically disappear from populations. That was left to twentieth-century scientists, building on Mendel’s before-his-time discovery.†
    
    
    So now the concept of the gene pool starts to make sense. A sexually reproducing population, such as, say, all the rats on Ascension Island, remotely isolated in the South Atlantic, is continually shuffling all the genes on the island. There is no intrinsic tendency for each generation to become less variable than the previous generation, no tendency towards ever more boringly grey, middling intermediates. The genes