Cambridge University Library houses the world's largest and most significant collection of the personal papers of Charles Robert Darwin (1809-1882), the British naturalist who was the first fully to articulate a theory of how all living things have evolved from a common ancestor. The single largest category of material in the Darwin archive, however, is not his experiment notes or theoretical notebooks but letters: more than 9,000 of the 15,000 letters Darwin is known to have written and received in his lifetime are in the Cambridge collection. It was largely through letters that Darwin gathered the data that first allowed him to develop his theories, and later to illustrate his arguments, and it was through discussion in letters that he honed his ideas and built strategic relationships with colleagues and supporters: they are an integral part of Darwin's scientific legacy.

Darwin corresponded with around 2,000 people from all around the world and all walks of life. His correspondents included other leading scientists and thinkers, such as the geologist Charles Lyell, the philosopher Herbert Spencer, the zoologist Thomas Henry Huxley, and the other man whose name is linked to species theory, Alfred Russel Wallace. No single set of letters was more important to Darwin, however, or is more important now, than those exchanged with his closest friend, the botanist Joseph Dalton Hooker (1817-1911). At around 10% of Darwin's surviving correspondence, the 1,200 letters published here provide a structure within which all the other letters can be explored. They are a connecting thread that spans forty years of Darwin's mature working life from 1843 until his death in 1882. They bring into sharp focus every aspect of Darwin's scientific work throughout that period, and illuminate the mutual friendships he and Hooker shared with other scientists, but they also provide a window of unparalleled intimacy into the personal lives of the two men.

Their correspondence began in 1843 when Hooker, just returned from James Clark Ross's Antarctic expedition, and already an admirer of the older man, was approached about working on Darwin's collection of plants from the Beagle voyage. Just the previous year Darwin had written out his first coherent account of the main elements of his species theory, and within a few months Hooker was admitted into the small and select group of those with whom Darwin felt able to discuss his emerging ideas. In perhaps his most famous letter of all , Darwin wrote to Hooker in January 1844 of his growing conviction that species "are not ... immutable" - an admission he likened, half jokingly, to "confessing a murder". When Alfred Russel Wallace (1823-1913) sent Darwin a letter in 1858 outlining an almost identical theory to his own, it was Hooker, together with Charles Lyell, who engineered the simultaneous publication of papers by both men, and secured Darwin's claim to the theory of "modification through descent" by means of the mechanism Darwin called "natural selection".

It was also to Hooker that Darwin, writing furiously in the succeeding months, sent batches of the manuscript of On the Origin of Species for comment, and Hooker continued to be a sounding board for successive publications.

Much of the most important experimental work conducted by Darwin after the publication of Origin was on variation and adaptation in plants, in particular the mechanisms by which various plants are nourished, reproduce, and colonise. Hooker, who after ten years as assistant director of the Royal Botanic Gardens, Kew, succeeded his father as director in 1865, was perfectly placed to provide Darwin with exotic species, and to help him build vital global networks of well-informed correspondents.

Hooker was a frequent visitor to Darwin at his home in Downe, Kent, and became a great favourite of Darwin's children. The two men shared their experience of attending the birth of their children: Darwin advocated the use of chloroform which he thought as "composing to oneself as well as to the patient". It was to Darwin that Hooker wrote just an hour after the death of his six year-old daughter, Maria , knowing that his friend, who had lost both a ten year-old daughter and a baby son, would all too clearly understand his grief. Those letters are amongst the most poignant in the collection.

Of the many hundreds of letters that passed between Darwin and Hooker, over 1,200 are still known to survive, and all but a handful are in the Cambridge Darwin archive. Darwin's son Francis incorporated many extracts in two published editions of his father's letters, in 1888 and 1902, the second of which he dedicated to Hooker "in remembrance of his lifelong friendship with Charles Darwin". At some time between those two editions, Hooker returned Darwin's letters to the family, retaining copies for himself; those copies now form part of the Hooker archive at the Royal Botanic Gardens, Kew. Both sides of the original correspondence, bound into several large volumes, arrived in Cambridge University Library, in 1948, together with the bulk of the Darwin archive, following transfer of ownership from the Darwin family, supported by funding from The Pilgrim Trust.

The Cambridge Digital Library, and the Darwin Correspondence Project , are very pleased to be able to publish images of almost the entire correspondence between Charles Darwin and Joseph Hooker, together with transcriptions of more than 1,000 of their letters. These are being published simultaneously through CUDL and on the The Darwin Correspondence Project website .

It is because of the vital importance of the letters to a full understanding of Darwin’s life and work that the University Library is host to the Darwin Correspondence Project. The Project exists to research and publish all of Darwin's surviving letters, reuniting those in the Library’s collection with others from collections around the world. The letters not only provide an invaluable insight into Darwin's mind throughout his working life, but also offer modern readers of all ages an engaging and accessible route into his published writings.

Transcripts of Darwin's letters are normally made available online four years after publication in The Correspondence of Charles Darwin . We are grateful to Cambridge University Press for allowing us to make available here all the texts of the Darwin-Hooker correspondence so far published.

We are also grateful to the Darwin family for permission to publish, and for their continuing support of both Cambridge University Library and the Darwin Correspondence Project.

Digitisation of the letters has been made possible by a National Science Foundation grant to the Darwin Correspondence Project, through the American Council of Learned Societies.

• The Darwin Correspondence Project
• Darwin Papers
• Darwin's Library
• Darwin Online
• Darwin Manuscripts Project
• Sir Joseph Dalton Hooker (Kew)
• Joseph Dalton Hooker Website
• Wallace Letters Online