On Christmas Day 1866, Bartholomew Sulivan sat down to write a typically long and chatty letter to his old friend, Charles Darwin, commiserating on shared ill-health, glorying in the achievements of their children, offering to collect plant specimens, and reminiscing about their time together on board HMS Beagle:

William Yarrell was a London businessman, a stationer and bookseller, who became an expert on British birds and fish, writing standard reference works on both.  He was a member of several science and natural history societies, including the Linnean Society, and was a founder member of both the Zoological Society of London and the Entomological Society of London. 

Hermann (Heinrich Ludwig Hermann) Müller, was born in Mühlberg near Erfurt in 1829. He was the younger brother of Fritz Müller (1822–97). Following the completion of his secondary education at Erfurt in 1848, he studied natural sciences at Halle and Berlin, focusing on botany, zoology, and geology. In 1852, he qualified as a teacher, but returned home to recover from illness before making his first of many trips to the Alps in the summer of 1853. He spent a probationary year teaching at a secondary school in Berlin and then half a year as a substitute teacher in Schwerin.

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.

Conrad Martens was born in London, the son of an Austrian diplomat. He studied landscape painting under the watercolourist Copley Fielding (1789–1855), who also briefly taught Ruskin. In 1833 he was on board the Hyacinth, headed for India, but en route in Rio de Janeiro, learned that Robert FitzRoy, captain of HMS Beagle, was looking for a replacement after Augustus Earle, the Beagle’s original artist, had become seriously ill.

The phases of Charles Darwin’s career have often been defined by the books that he read, from Lyell’s Principles of Geology during the Beagle voyage to Malthus’s Essay on the Principle of Population during his London years. The book that encouraged him to pursue a scientific voyage in the first place was the Personal Narrative of Alexander von Humboldt’s travels in Central and South America between 1799 and 1804.

One of the early leaders of geology in Britain, Adam Sedgwick  was born in the Yorkshire village of Dent in 1785. Attending Trinity College Cambridge, he was ordained as clergyman and in 1818 was appointed to the Woodwardian Chair of Geology, which offered a small stipend. Despite having little prior knowledge of the subject, Sedgwick soon commenced fieldwork, offered regular annual lectures, and joined with John Stevens Henslow, William Whewell and others to build up the University's reputation in the sciences.

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

Fritz Müller, a German who spent most of his life in political exile in Brazil, described Darwin as his second father, and Darwin's son, Francis, wrote that, although they never met 'the correspondence with Müller, which continued to the close of my father's life, was a source of very great pleasure to him. My impression is that of all his unseen friends Fritz Müller was the one for whom he had the strongest regard.'