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.

Charles Darwin’s major achievement in 1867 was the completion of his large work, The variation of animals and plants under domestication (Variation). The importance of Darwin’s network of correspondents becomes vividly apparent in his work on expression in 1867, as he continued to circulate a list of questions on human expression that he may have drawn up in late 1866. His correspondents were asked to copy the list and forward it to those who might best answer the questions, with the result that Darwin began to receive replies from different corners of the world.

The year 1866 began well for Charles Darwin, as his health, after several years of illness, was now considerably improved. In February, Darwin received a request from his publisher, John Murray, for a new edition of  Origin. Darwin got the fourth edition to the printers in July. Much to Darwin’s annoyance, however, publication was delayed by Murray, who judged that it would sell better if released later in the year. Darwin also completed the major part of what was to become Variation. Debate about Darwin’s theory of transmutation continued in 1866, with important commentaries appearing in France, Germany, and Italy.

On 7 January 1860, John Murray published the second edition of Darwin’s Origin of species, printing off another 3000 copies to satisfy the demands of an audience that surprised both the publisher and the author. It wasn't long, however, before ‘the stones began to fly’. Members of the scientific community found many difficulties in Origin in the year after its publication and Darwin despaired of making his ideas understood. Among these problems was that arising from the implications it had for human ancestry, hotly debated at the famous meeting of the British Association in Oxford that summer.

The year was marked by three deaths of personal significance to Darwin: Hugh Falconer, a friend and supporter; Robert FitzRoy, captain of the Beagle; and William Jackson Hooker, director of the Royal Botanic Gardens, Kew, and father of Darwin’s friend Joseph Hooker’s father. There was also a serious dispute between two of Darwin’s friends, John Lubbock and Charles Lyell. These events all inspired sombre reflections in Darwin’s letters. Nevertheless, all the Darwin children were thriving. When illness made work impossible, Darwin and Hooker read a number of novels, and discussed them in their letters.