Friendship and trust
- Letter 714 — Darwin, C. R. to Hooker, J. D., [13 or 20 Nov 1843]
- Letter 729 — Darwin, C. R. to Hooker, J. D., [11 Jan 1844]
- Letter 736 — Darwin, C. R. to Hooker, J. D., 23 Feb [1844]
- Letter 1674 — Darwin, C. R. to Gray, Asa, 25 Apr [1855]
- Letter 1685 — Gray, Asa to Darwin, C. R., 22 May 1855
- Letter 2125 — Darwin, C. R. to Gray, Asa, 20 July [1857]
Friendship and information exchange
- Letter 1202 — Darwin, C. R. to Hooker, J. D., 6 Oct [1848]
- Letter 1220 — Hooker, J. D. to Darwin, C. R., 3 Feb 1849
- Letter 1260 — Darwin, C. R. to Hooker, J. D., 12 Oct 1849
- Letter 1319 — Hooker, J. D. to Darwin, C. R., 6 & 7 Apr 1850
- Letter 1339 — Darwin, C. R. to Hooker, J. D., 13 June [1850]
Darwin as mentor
- Letter 1585 — Darwin, C. R. to Lubbock, John, [Sept 1854]
- Letter 1720 — Darwin, C. R. to Lubbock, John, 19 [July 1855]
- Letter 1979 — Darwin, C. R. to Lubbock, John, 27 Oct [1856]
- Letter 5457 — Müller, H. L. H. to Darwin, C. R., 23 Mar 1867
- Letter 5471 — Darwin, C. R. to Müller, H. L. H., 29 Mar [1867]
- Letter 5481 — Müller, H. L. H. to Darwin, C. R., 1 Apr [1867]
- Letter 5657 — Müller, H. L. H. to Darwin, C. R., 23 Oct 1867
- Letter 5770 — Müller, H. L. H. to Darwin, C. R., Jan [1868]
Darwin’s mentors
- Letter 1189 — Darwin, C. R. to Henslow, J. S., 2 July [1848]
- Letter 196 — Henslow, J. S. to Darwin, C. R., 15 & 21 Jan [1833]
- Letter 152 — Darwin, C. R. to Henslow, J. S., 3 Dec [1831]
- Letter 249 — Henslow, J. S. to Darwin, C. R., 22 July 1834
- Letter 251 — Darwin, C. R. to Henslow, J. S., 24 July & 28 Oct & 7 Nov 1834
- Letter 272 — Darwin, C. R. to Henslow, J. S., [10–13] Mar 1835
Crossing class boundaries
- Letter 1751 — Darwin, C. R. to Tegetmeier, W. B., 31 Aug [1855]
- Letter 1788 — Darwin, C. R. to Tegetmeier, W. B., [2 Dec 1855]
- Letter 3139 — Tegetmeier, W. B. to Darwin, C. R., 4 May [1861]
- Letter 3800 — Scott, John to Darwin, C. R., [11 Nov 1862]
- Letter 3805 — Darwin, C. R. to Scott, John, 12 Nov [1862]
- Letter 4463 — Scott, John to Darwin, C. R., 14 Apr [1864]
- Letter 4468 — Darwin, C. R. to Hooker, J. D., 19 [Apr 1864]
- Letter 4469 — Hooker, J. D. to Darwin, C. R., 20 Apr 1864
- Letter 4471 — Darwin, C. R. to Hooker, J. D., 25 Apr [1864]
- Letter 4611 — Darwin, C. R. to Gray, Asa, 13 Sept [1864]
Gender
- Letter 4170 — Becker, Lydia to Darwin, C. R., 18 May 1863
- Letter 4258 — Becker, Lydia to Darwin, C. R., 31 July [1863]
- Letter 4260a — Darwin, C. R. to Becker, L. E., 2 Aug [1863]
- Letter 4441 — Becker, Lydia to Darwin, C. R., 30 Mar 1864