John Lubbock was eight years old when the Darwins moved into the neighbouring property of Down House, Down, Kent; the total of one hundred and seventy surviving letters he went on to exchange with Darwin is a large number considering that the two men lived as close neighbours for most of their lives.

In 1869, when gathering data on sexual selection in humans, Darwin exchanged a short series of letters with John Beddoe, a doctor in Bristol. He was looking for evidence that racial differences that appear to have no benefit in terms of survival - and therefore could not be explained by natural selection - could instead have been acquired through choices in sexual partner. Darwin was fascinated by Beddoe's analysis of married and single women patients in Bristol Royal Infirmary by hair colour.

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?

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.