On the origin of species by means of natural selection …so begins the title of Darwin's most famous book, and the reader would rightly assume that such a thing as 'species' must therefore exist and be subject to description. But the title continues, …or the preservation of favoured races in the struggle for life. This is more ambiguous, especially to the modern reader, for whom race carries a different and highly charged meaning. In the context of natural history, when Darwin used the term here, he simply meant 'variety', as in 'a fast-growing race of wheat'.

The effects of cross and self fertilisation in the vegetable kingdom, published on 10 November 1876, was the result of a decade-long project to provide evidence for Darwin's belief that ''Nature thus tells us, in the most emphatic manner, that she abhors perpetual self-fertilisation' (Orchids, p. 359).

The power of movement in plants, published on 7 November 1880, was the final large botanical work that Darwin wrote. It was the only work in which the assistance of one of his children, Francis Darwin, is mentioned on the title page. The research for this book is well documented in correspondence, partly because Francis visited the botanical institute at Würzburg for two summers and exchanged letters with his father about their research while he was away from home.

In Autumn of 1875, Francis Darwin was busy researching aggregation in the tentacles of Drosera rotundifolia (F. Darwin 1876). This phenomenon occurs when coloured particles within either protoplasm or the fluid in the cell vacuole (the cell sap) cluster together. Darwin senior had theorized in Insectivorous Plants that these aggregated cellular masses consisted of living protoplasm. Inspired by his investigation of Drosera, Francis set out to examine the cup-like receptacles formed by the leaves of Dipsacus sylvestris (synonym D. fullonum), commonly known as fuller's teasel.

Darwin's work on insectivorous plants began by accident. While on holiday in the summer of 1860, staying with his wife's relatives in Hartfield, Sussex, he went for long walks on the heathland and became curious about the large number of insects caught by the common sundew (Drosera rotundifolia). He reported to his friend Joseph Dalton Hooker: 'I amused myself with a few observations on the insect-catching power of Drosera; & I must consult you some time whether my "twaddle" is worth communicating to Linnean Soc.' Although he continued to think of his studies of carnivorous plants as a guilty pleasure, this encounter began a long-running research project that showed some of the connections between plants and animals.

Darwin published four volumes on barnacles, the crustacean sub-class Cirripedia, between 1851 and 1854, two on living species and two on fossil species. Written for a specialist audience, they are among the most challenging and least read of Darwin's works and are often dismissed as a necessary detour on the road to the development of the theory of evolution through natural selection. These volumes reveal, in fact, how observation, experiment, and classification both informed and were informed by Darwin's species theory. Every aspect, from the choice of subject to the production of the volumes, reveals something about Darwin's approach to scientific investigation. While appearing to conform to long-established practice, Darwin introduced a new approach to systematics that challenged the purely morphological methodology of his predecessors.

The Darwin Correspondence Project was co-sponsor of Biodiversity and its Histories, which brought together scholars and researchers in ecology, politics, geography, anthropology, cultural history, and history and philosophy of science, to explore how aesthetic, economic, and moral value came to be attached to the diversity of life on earth. The conference included a session on 'Darwin and evolutionary theory' involving past and present members of the Project.

We are grateful to the speakers for permission to make their talks available here.

In a letter to Henslow in March 1835 Darwin remarked that he had done 'very little' in zoology; the 'only two novelties' he added, almost as an afterthought, were a new mollusc and a 'genus in the family Balanidæ' - a barnacle - but it was an oddity. Who, he wondered 'would recognise a young Balanus in this ill-formed little monster?' Darwin put his specimens away for over a decade, and when he returned to the puzzling little creature in October 1846, he planned only to write a paper on the anomalous 'Mr Arthrobalanus', as it was now known to him. The work took him eight years.

Darwin played an important role in the controversy over vivisection that broke out in late 1874. Public debate was sparked when the Royal Society for the Prevention of Cruelty to Animals brought an unsuccessful prosecution against a French physiologist who had performed vivisection on dogs.

Earthworms were the subject of a citizen science project to map the distribution of earthworms across Britain (BBC Today programme, 26 May 2014). The general understanding of the role earthworms play in improving soils and providing nutrients for plants to flourish can be traced back to the last book Darwin wrote, snappily-titled 'The formation of vegetable mould through the action of worms with observation on their habits', which was published in 1881. Despite Darwin's fears that a book on earthworms might prove a failure, it became a best seller.