The most forceful and persistent critic of the term 'natural selection' was the co-discoverer of the process itself, Alfred Russel Wallace. Wallace seized on Herbert Spencer's term 'survival of the fittest', explicitly introduced as an alternative way of expressing 'natural selection' in the October 1864 instalment of Spencer's Principles of biology (Spencer 1864-7, 1: 444-5, 2: 48, et passim). Wallace was so taken with it that he went through his own copy of the first edition of Origin neatly crossing through every occurrence of 'natural selection' and pencilling 'survival of the fittest' in the margin (the copy is now in Cambridge University Library, Keynes.M.2.27).

As he was first developing his ideas, among the potential problems Darwin recognised with natural selection was how to account for developmental change that conferred no apparent advantage. He proposed a 'mysterious law' of 'correlation of growth' where advantageous changes in one part of an organism dragged associated, but otherwise useless, changes in other parts along in their wake. One apparent example of correlation in seemingly unrelated characteristics, was an association between deafness in cats, and the colour of their fur and eyes.

Darwin came to regard 'bear' as a 'word of ill-omen'. In the first edition of Origin he told the story of a black bear seen swimming for hours with its mouth wide open scooping insects from the water 'like a whale'. He went on to imagine that natural selection might produce increasingly aquatic bears 'with larger and larger mouths' until a creature 'as monstrous as a whale' emerged. His meaning was widely misunderstood, and he ended up regretting ever including it.

Origin is 157 years old. (Probably) the most famous book in science was published on 24 November 1859. To celebrate we have uploaded hundreds of new images of letters, bringing the total number you can look at here to over 9000 representing more than 2000 letters, written both before and after the book appeared. You can now see examples of letters to Darwin from nearly 250 different people, and letters he wrote to 150 more.

…the wicked book which you have been so long a-hatching.

How do new species arise? This was the ancient question that Charles Darwin tackled soon after returning to England from the Beagle voyage in October 1836. Darwin realised a crucial (and cruel) fact: far more individuals of each species were born than could possibly survive.

Although natural selection could explain the differences between species, Darwin realised that (other than in the reproductive organs themselves) it could not explain the often marked differences between the males and females of the same species. So what accounted for these 'secondary sexual characteristics'? The longer manes in male lions and beards in male humans? Antlers or horns being so much smaller, or completely absent, in some female deer or cattle?

It was crucial to Darwin's theories of species change that naturally occurring variations could be inherited. But at the time when he wrote Origin, he had no explanation for how inheritance worked - it was just obvious that it did. Darwin's attempt to describe how heredity might work, his 'provisional hypothesis of Pangenesis', was published in 1868 in his book, Variation of animals and plants under domestication

In a later account of how he had come to the evolutionary ideas published in Origin, Darwin wrote: 'Of all the minor points, the last which I appreciated was the importance & cause of the principle of Divergence' (to Ernst Haeckel, [after 10] August - 8 October [1864]).

The original manuscript about varieties that Wallace composed on the island of Gilolo and sent to Darwin from the neighbouring island of Ternate (Brooks 1984) has not been found. It was sent to Darwin as an enclosure in a letter (itself missing), and was subsequently sent by Darwin to Charles Lyell (letter to Charles Lyell, 18 [June 1858]). The only known version of the text is the one published in Journal of the Proceedings of the Linnean Society (Zoology) 3 (1859): 45-62, and this text is reprinted below.