Having laboured for nearly five years on human evolution, sexual selection, and the expression of emotions, Darwin was able to devote 1873 almost exclusively to his beloved plants. He resumed work on the digestive powers of sundews and Venus fly traps, and the comparative fertility and vigour of self- and cross-pollinated species, work that would culminate in two books, Insectivorous plants (1875) and Cross and self fertilisation (1876). Darwin's son Francis became increasingly involved in this botanical research, eventually renouncing plans for a medical career to become his father's scientific secretary.

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.