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).

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.  

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.

In May 1856, Darwin began writing up his 'species sketch’ in earnest. During this period, his working life was completely dominated by the preparation of his 'Big Book', which was to be called Natural selection. Using letters are the main source for much of his research he amassed data, carried out breeding experiments, and struggled with statistical analysis. Several of his experiments: seeds would not germinate; beans failed to cross; newly-hatched molluscs refused to do what he hoped.  Most significant in terms of Darwin’s future, however, was the beginning of his correspondence with Alfred Russel Wallace.

At the start of 1869, Darwin was hard at work making changes and additions for a fifth edition of  Origin. He may have resented the interruption to his work on sexual selection and human evolution, but he spent forty-six days on the task. Much of the remainder of the year was spent researching and revising chapters for  Descent, and gathering additional material on emotional expression. Yet the scope of Darwin’s interests remained extremely broad, many letters throughout the year touching on subjects such as South American geology, barnacle morphology, insectivorous plants, and earthworms, subjects that had exercised Darwin for decades.

The quantity of Darwin’s correspondence increased dramatically in 1868 due largely to his ever-widening research on human evolution and sexual selection.Darwin’s theory of sexual selection as applied to human descent led him to investigate aspects of the structure and behaviour of other animals more extensively, and to further this programme, he re-established links with specialists who had provided assistance. Considerable correspondence was generated by the long-awaited publication of Variation in animals and plants under domestication. Having been advertised by the publisher John Murray as early as 1865, the two-volume work appeared in January 1868.

One of the real pleasures afforded in reading Charles Darwin’s correspondence is the discovery of areas of research on which he never published, but which interested him deeply. We can gain many insights about Darwin’s research methods by following these ‘letter trails’ and observing how correspondence served as a vital research tool for him.

The years 1858 and 1859 were, without doubt, the most momentous of Darwin’s life. From a quiet rural existence filled with steady work on his ‘big book’ on species, he was jolted into action by the arrival of an unexpected letter from Alfred Russel Wallace. This letter led to the first announcement of Darwin’s and Wallace’s respective theories of organic change at the Linnean Society of London in July 1858 and prompted the composition and publication, in November 1859, of Darwin’s major treatise On the origin of species by means of natural selection. By the end of 1859, Darwin’s work was being discussed in publications as diverse as The Times and the English Churchman, and Darwin himself was busy as never before: answering letters, justifying and explaining his views to friends, relations, and ‘bitter opponents’. The correspondence shows vividly just how distressed Darwin was during the days leading up to the Linnean meeting. On 18 June 1858, his eldest daughter, Henrietta Emma,  was stricken with diphtheria, then a little-known and frightening illness. Several days later, their 18-month-old baby, Charles Waring, came down with scarlet fever. His condition deteriorated rapidly in the space of a few days and the Darwins were shocked by his unexpected death on 28 June.