Darwin’s close relationship with John Stevens Henslow, the professor of botany at Cambridge, is well known. His letters to Henslow on the Beagle voyage show the young Darwin hard at work trying to impress his Cambridge mentor and establish a degree of independence, while remaining deferential to the elder naturalist. Although Darwin never held a university post, he encouraged and shaped the work of many naturalists, initiating collaborations, offering suggestions for experiments, or inspiring whole research programmes.
This collection of letters documents Henslow’s mentoring while Darwin was on the Beagle voyage and afterwards.
Letter 152 — Darwin, C. R. to Henslow, J. S., 3 Dec 
Darwin expresses confusion on board the Beagle at the definite prospect of sailing. He gives directions for sending mail to Montevideo. He talks of being a sort of Protégé of Henslow’s and it is Henslow’s “bounden duty to lecture me”.
Letter 196 — Henslow, J. S. to Darwin, C. R., 15 & 21 Jan 
Henslow acknowledges receipt of two letters from Darwin and a box of specimens. He mentions attendance at the BAAS meeting and a gift to him of a small living near Oxford, as well as some political news. He congratulates Darwin on the work he has done. The specimens are of great interest and he gives advice on packing, labelling, and future collecting. He suggests that – as a precaution – Darwin sends home a copy of his notes on the specimens.
Letter 249 — Henslow, J. S. to Darwin, C. R., 22 July 1834
Henslow notes that Darwin’s cargo is safe; the fossils have been sent to William Clift. Henslow asks for dried plants (those sent were all of greatest interest). Henslow sends news of Cambridge and mutual friends.
Letter 251 — Darwin, C. R. to Henslow, J. S., 24 July & 28 Oct & 7 Nov 1834
Darwin is excited by Henslow’s high opinion of his collections. He discusses his notes and some new discoveries. He includes a summary of events since leaving Falklands and the geology of Patagonia. The Corallines at Tierra del Fuego convince him of the artificiality of arrangement of their families by Lamarck and Cuvier. The geological expedition in the Andes ends with serious illness and specimens are sent to Henslow.
Letter 272 — Darwin, C. R. to Henslow, J. S., [10–13] Mar 1835
Darwin tells Henslow the termination of the voyage has been decided – September 1836. He writes of the earthquake of Concepción. Given his geological observations (since November), he can now prove both sides of the Andes have recently risen to considerable heights. He writes of his zoological collection and plans to cross the Cordilleras.
Letter 1189 — Darwin, C. R. to Henslow, J. S., 2 July 
Darwin criticises the lecturing system in education and its emphasis on classics. He has forgotten all his classical knowledge. He asks Cambridge botanist J. S Henslow’s help in naming cirripedes, on which he is working. He believes he has made “some very curious points”.
Darwin as Mentor
This section contains two sets of letters between Darwin and two naturalists he mentored. The first is between Darwin and his neighbour, John Lubbock and the second is between Darwin and German naturalist Hermann Müller.
Darwin and Lubbock
Letter 1585 — Darwin, C. R. to Lubbock, John, [Sept 1854]
Darwin sends Lubbock a beetle he cannot identify. He is reading J. O. Westwood [Introduction to the modern classification of insects (1839–40)] and it has reawakened his passion for entomology.
Letter 1720 — Darwin, C. R. to Lubbock, John, 19 [July 1855]
Darwin congratulates Lubbock on finding musk-ox fossil.
Letter 1979 — Darwin, C. R. to Lubbock, John, 27 Oct 
Darwin provides detailed constructive, comments on Lubbock’s paper on Daphnia, [“An account of methods of reproduction in Daphnia and of the structure of the ephippium”, Philos. Trans. R. Soc. Lond. 147 (1857): 79–100].
Darwin and Müller
Letter 5457 — Müller, H. L. H. to Darwin, C. R., 23 Mar 1867
Müller explains how Origin turned him away from a Linnean interpretation of flowers and mosses. He is glad that Darwin appreciates his continuing work on mosses, in support of natural selection, and plans to repeat Darwin’s orchid experiments. He sends an interpretation of the floral anatomy of Lopezia miniata.
Letter 5471 — Darwin, C. R. to Müller, H. L. H., 29 Mar 
Darwin learns that German botanist Fritz Müller is Hermann Müller’s brother.
Letter 5481 — Müller, H. L. H. to Darwin, C. R., 1 Apr 
Müller thanks Darwin for the “Climbing plants” offprint and for references on fertilisation of flowers. Considering the bounty of work already done, he is looking for something original to do. He mentions that Subularia does not grow in Westphalia.
Letter 5657 — Müller, H. L. H. to Darwin, C. R., 23 Oct 1867
Müller thanks Darwin for the German version of Origin  and the portrait. He discusses how dipterous insects are adapted to eating pollen rather than only to sucking nectar. He describes such adaptations in two dipteran species.
Letter 5770 — Müller, H. L. H. to Darwin, C. R., Jan 
Müller thanks Darwin for his photograph and reminding him of Delpino’s work. He intends to start experimenting with mosses to determine which differences in structure are affected by altered conditions of life, and says he would be happy to share his observations.