The letters Darwin exchanged with John Stevens Henslow, professor of Botany and Mineralogy at Cambridge University, were among the most significant of his life. It was a letter from Henslow that brought Darwin the invitation to sail round the world as companion to captain Robert FitzRoyof HMS Beagle, and during the voyage it was Henslow who received the vast numbers of specimens Darwin sent home, and sent practical advice about how best to prepare, preserve, and ship them. Despite Henslow's reservations about the evolutionary ideas put forward in Origin the two men remained friends to the end of Henslow's life.

As an author, friend and correspondent, Charles Lyell played a crucial role in shaping Darwin's scientific life. Born to a wealthy gentry family in Scotland in 1797, Lyell had a classical and legal education but by the 1820s had become entranced by the popular and exciting subject of geology. Geologists had already revealed a succession of unexpected forms of animals and plants and demonstrated the need for a vastly longer time scale than that allowed for by traditional Biblical criticism. Lyell believed, however, that the subject remained beset by speculation and uncertainty

Dubbed "Darwin's bulldog" for his combative role in controversies over evolution, Huxley was a leading Victorian zoologist, science popularizer, and education reformer. He was born in Ealing, a small village west of London, in 1825. With only two years of formal education, he apprenticed in medicine and then entered the navy, serving as assistant surgeon on H.M.S. Rattlesnake in the South Pacific (1846-1851). He pursued natural history alongside his medical duties, concentrating on the anatomy of marine invertebrates.

Wallace was a leading Victorian naturalist, with wide-ranging interests from biogeography and evolutionary theory to spiritualism and politics. He was born in 1823 in Usk, a small town in south-east Wales, and attended a grammar school in Hertford. At the age of 13, he was forced to leave school and enter a trade because of financial hardship. He joined an older brother in London as a builder's apprentice, and the following year started work as a land surveyor with another brother, travelling to different parts of England and Wales and collecting plants.

John Lubbock was eight years old when the Darwins moved into the neighbouring property of Down House, Down, Kent; the total of one hundred and seventy surviving letters he went on to exchange with Darwin is a large number considering that the two men lived as close neighbours for most of their lives.

The 1400 letters exchanged between Darwin and Joseph Dalton Hooker (1817-1911) account for around 10% of Darwin's surviving correspondence and 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 and bring into sharp focus every aspect of Darwin's scientific work throughout that period. They 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.

Darwin's longest running and most significant exchange of correspondence dealing with the subjects of design in nature and religious belief was with the Harvard botanist Asa Gray. Gray was one of Darwin's leading supporters in America. He was also a devout Presbyterian. The vigorous and yet civil and humble manner in which the two men debated matters of intense personal belief and social consequence serves as a model of constructive engagement. The entire extant correspondence, consisting of about 300 letters written between 1854 and 1881, is now available for the first time.