Bonds of friendship were very important in science in a period when strong institutional structures were largely absent. Darwin had a small circle of scientific friends with whom he shared work openly and who he relied upon to spot weaknesses and to sharpen his thinking. He also looked to this circle for support in times of uncertainty, controversy, or personal loss. Letter writing was not only a means of sustaining such friendships over distance, it was also a medium through which intimacy and trust could be established in the absence of face-to-face contact. His correspondence with Joseph Hooker and Asa Gray illustrates how close personal ties could be built gradually through the exchange of scientific knowledge and the free expression of theoretical differences.
This section contains two sets of letters. The first is between Darwin and his friend Kew botanist J. D. Hooker. The second is between Darwin and Harvard botanist Asa Gray.
Darwin and Hooker
Letter 714 — Darwin, C. R. to Hooker, J. D., [13 or 20 Nov 1843]
Darwin knows Cambridge botanist J. S. Henslow has sent some of Darwin’s South American plants to his friend Kew botanist J. D. Hooker for examination and he is curious about Hooker’s thoughts.
Letter 729 — Darwin, C. R. to Hooker, J. D., [11 Jan 1844]
Darwin begins with an assessment of his views on Hooker. He relates some queries on ratios of species to genera on southern islands, some observations on distribution of Galapagos organisms, South American fossils, and facts he has gathered that led him to conclusion that species are not immutable. He admits to Hooker “it is like confessing a murder”.
Letter 736 — Darwin, C. R. to Hooker, J. D., 23 Feb 
Darwin begins with a charming statement about his friendship with Hooker. He has just completed Volcanic islands and sends queries on Galapagos flora in particular and island floras in general, as well as on the relationship of wide-ranging species to wide-ranging genera.
Darwin and Gray
Letter 1674 — Darwin, C. R. to Gray, Asa, 25 Apr 
Darwin opens by reminding Harvard botanist Asa Gray they met most recently at Kew. Darwin is collecting facts on variation and questions Gray on the alpine flora of the USA. He sends a list of plants from Gray’s Manual of botany  and asks him to append the ranges of the species.
Letter 1685 — Gray, Asa to Darwin, C. R., 22 May 1855
Gray recalled meeting Darwin three years earlier at Hooker’s. Gray has filled up Darwin’s paper [see 1674]. He discusses the distribution and relationships of alpine flora in the USA.
Letter 2125 — Darwin, C. R. to Gray, Asa, 20 July 
Darwin writes a challenging letter to Gray, saying: “But my letter has been horribly egotistical: but your letters always so greatly interest me; & what is more they have in simple truth been of the utmost value to me.” Darwin believes species have arisen, like domestic varieties, with much extinction, and that there are no such things as independently created species. Explains why he believes species of the same genus generally have a common or continuous area; they are actual lineal descendants. Darwin discusses fertilisation in the bud and the insect pollination of papilionaceous flowers. His theory explains why, despite the risk of injury, cross-fertilisation is usual in the animal and vegetable kingdoms, even in hermaphrodites.
This collection of letters between Darwin and Hooker, while Darwin was writing his barnacle books and developing his species theory, provides some insight on their friendship and their approach to information exchange.
Letter 1202 — Darwin, C. R. to Hooker, J. D., 6 Oct 
Darwin catches up on personal matters as well as makes progress with barnacles. He describes “supplemental” males in detail. In working out metamorphosis, their crustacean homologies followed automatically. On the issue of nomenclature reform, Darwin opposes appending first describer’s name to specific name.
Letter 1220 — Hooker, J. D. to Darwin, C. R., 3 Feb 1849
In this gossipy letter, Hooker relates personal matters. Hooker has received Darwin’s earlier letter . He thanks Darwin for saving his correspondence. He sent “a yarn about species” in October mail, and some “puerile” letters printed in Athenæum. He requests Darwin extract anything valuable from his letters to Darwin and Lyell for Athenæum. He mentioned Darwin’s work on complemental males in barnacles is wonderful, but warns Darwin to drop his battle about perpetuity of names in species descriptions.
Letter 1260 — Darwin, C. R. to Hooker, J. D., 12 Oct 1849
Darwin opens by discussing their correspondence. He then moves on to a discussion of the great dam across Yangma valley as a lateral glacial moraine. He reports on the Birmingham BAAS meeting and details of water-cures. He notes that barnacles are becoming tedious; careful description shows slight differences constitute varieties, not species. He ends with a discussion of lamination of gneiss.
Letter 1319 — Hooker, J. D. to Darwin, C. R., 6 & 7 Apr 1850
Hooker apologises for the delay between letters. He says he used to think Darwin “too prone to theoretical considerations about species,” hence was pleased Darwin took up a difficult group like barnacles. Darwin’s theories have progressed but Hooker is not converted. Sikkim has not cleared up his doubts about Darwin’s doctrines. In his second letter he talks about his visit with Falconer.
Letter 1339 — Darwin, C. R. to Hooker, J. D., 13 June 
Darwin writes to Hooker from his water cure in Malvern. He writes on Himalayan stratigraphy. He believes Hooker’s observations of glacial action are the first ever done east of Urals. He also talks about barnacles and the species theory, and he is impressed with variation. Here we see the effect of Darwin’s species sketch on Hooker’s view of willow systematics.