Photocopies of about 6,000 more have been acquired from private collections and over 200 institutions in more than 20 countries. It took ten years to carry out a systematic search for additional material, to obtain copies, and to transcribe all known letters into an electronic format. This resulted in the creation of the extensive computerised archive which now forms the basis for editorial work. The many undated letters were also given a provisional dating, and the identification of unknown correspondents attempted. To date, the project has located, both in libraries and in private collections, a total of more than 15,000 letters exchanged by Darwin and hundreds of correspondents around the world between 1821 and his death in 1882.
New letters continue to be discovered at the rate of about sixty a year.
When complete, The Correspondence of Charles Darwin will comprise approximately 30 volumes. It provides, for the first time, the full, authoritative texts of Darwin's letters, edited according to modern textual editorial principles and practice. It is already an important source for students and scholars in the fields of history, history of science, literature, philosophy, sociology, and the natural sciences. Footnotes and appendixes explain references, identify people, and direct attention to related published work, making the letters accessible not only to historians and scientists but also to general readers. Darwin's work was to an unusual extent dependent upon letter-writing.