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 year 1871 was an extremely busy and productive one for Darwin, with the publication in February of his long-awaited book on human evolution, Descent of man. The other main preoccupation of the year was the preparation of his manuscript on expression. Darwin continued to investigate the mechanisms of various emotions, such as grief, shame, and astonishment, drawing on the expertise of physicians and physiologists, as well as zoo-keepers, pet owners, parents of young children, and professional photographers. The year also brought a significant milestone for the family, as Darwin's eldest daughter Henrietta was married in August.

The year 1870 is aptly summarised by the brief entry Darwin made in his journal: 'The whole of the year at work on the Descent of Man & Selection in relation to Sex'. Descent was the culmination of over three decades of observations and reflections on human ancestry, including the origin of language, mind, morals, and religious temperament. The year was otherwise coloured by controversies, including vigorous objections to the application of natural selection to humans from Wallace and St George Jackson Mivart.

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.

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.