'My heart & soul care for worms & nothing else in this world,' Darwin wrote to his old Shrewsbury friend Henry Johnson on 14 November 1880. Darwin became fully devoted to earthworms in the spring of the year, just after finishing the manuscript of Movement in plants, his most ambitious botanical book. Both projects explored the complexities of movement, forms of sensitivity, and the ability of organisms to adapt to varying conditions.

Darwin spent a considerable part of 1879 in the eighteenth century. His journey back in time started when he decided to publish a biographical account of his grandfather Erasmus Darwin to accompany a translation of an essay on Erasmus's evolutionary ideas by the German science writer Ernst Krause. Darwin's preoccupation with his own roots ran alongside a botanical interest in roots, as he and his son Francis carried out their latest experiments on plant movement for the book they intended to publish on the subject. They concentrated on radicles-the embryonic roots of seedlings-and determined that the impetus for movement derived from the sensitivity of the tips. Despite this breakthrough, when Darwin first mentioned the book to his publishers, he warned that it was 'dry as dust'.

In 1878, Darwin devoted most of his attention to the movements of plants. He investigated the growth pattern of roots and shoots, studying the function of specific organs in this process. Working closely with his son Francis, Darwin devised a series of experiments to trace these subtle movements over long periods of time, often using household materials. Francis spent an extended period in Würzburg at Julius Sachs's botanical institute, one of most advanced plant laboratories in Europe.

Ever since the publication of Expression, Darwin's research had centred firmly on botany. The year 1877 was no exception. The spring and early summer were spent completing Forms of flowers, his fifth book on a botanical topic. He then turned to the mysterious role of the waxy coating (or 'bloom') on leaves and fruit, and to the movement of plants, focusing especially on the response of leaves to changing conditions. He also worked intermittently on earthworms, for the most part gathering observations made by others.

1876 was the year in which the Darwins became grandparents for the first time. And tragically lost their daughter-in-law, Amy, who died just days after her son's birth. All the letters from 1876 are now published in volume 24 of The Correspondence of Charles Darwin, and a new chapter to Darwin's 'Life in letters' has been added here.

'I am getting sick of insectivorous plants', Darwin confessed in January 1875. He had worked on the subject intermittently since 1859, and had been steadily engaged on a book manuscript for nine months; January also saw the conclusion of a bitter dispute with the zoologist St George Jackson Mivart. In April and early May, Darwin was occupied with a heated debate over vivisection, and at the end of the year, he campaigned vigorously on behalf of a young zoologist, whose blackballing by the Linnean Society infuriated him: 'I have not felt so angry for years.'

'My career', Darwin wrote towards the end of 1872, 'is so nearly closed. . . What little more I can do, shall be chiefly new work', 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 difficult a subject, as evolution'.

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.

In his private life also, Darwin was in a nostalgic frame of mind, picking up the threads with schoolfriends, old friends from student days, and Beagle shipmates. As the year went on, he exchanged reminiscences, and laments about advancing age and poor health, with family friends from childhood, some of whom he had not been in touch with for many years.

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 1861 marked an important change in the direction of Darwin's work. He had weathered the storm that followed the publication of Origin, and felt cautiously optimistic about the ultimate acceptance of his ideas. The letters from this year provide an unusually detailed and intimate understanding of Darwin's problem-solving method of work. The beginning of the American Civil War and the possibility of British involvement brings an unusally political flavour to the correspondence with tensions increasingly evident between two of his closest friends and supporters, Joseph Hooker and the American Asa Gray.

In May 1856, Darwin began writing up his 'species sketch' in earnest. During this period, his working life was completely dominated by the preparation of his 'Big Book', which was to be called Natural selection. Using letters are the main source for much of his research he amassed data, carried out breeding experiments, and struggled with statistical analysis. Several of his experiments: seeds would not germinate; beans failed to cross; newly-hatched molluscs refused to do what he hoped. Most significant in terms of Darwin's future, however, was the beginning of his correspondence with Alfred Russel Wallace.