‘My career’, Darwin wrote towards the end of 1872, ‘is so nearly closed. … What little more I can do, shall be chiefly new work’ (letter to Francis Galton, 8 November [1872]), and the tenor of his correspondence throughout the year is one of wistful reminiscence, coupled with a keen eye to the crafting of his legacy. Bracketed by the publication in February of the sixth edition of On the origin of species, intended to be Darwin's last, and of Expression of the emotions in man and animals in November, the year marked the culmination of a programme of publication that can be traced back to his never-completed ‘big book’, Natural selection, begun in 1856. Coming hard on the heels of The descent of man and selection in relation to sex, published in 1871, these books brought a strong if deceptive sense of a job now done: Darwin intended, he declared to Alfred Russel Wallace, to see whether he could now occupy himself without writing anything more on ‘so diYcult a subject, as evolution’ (letter to A. R. Wallace, 27 July [1872]).

By the end of the year Darwin was immersed in two of the studies that would characterise his final decade: the powers of movement and digestion in plants, and the role of earthworms in shaping the environment. The former led to a series of books and papers, and the latter formed the subject of Darwin's last book, The formation of vegetable mould through the action of worms, published in the year before his death. Despite Darwin's declared intention to take up new work, both represent returns to lines of enquiry begun many years before.