Theory and Practice
This collection of letters, written between Darwin and Hooker whilst Darwin was preparing his barnacle book and developing his species theory, provides some insight on how Darwin grappled with the fine balance between theory and practice in natural history.
Letter 1202 — Darwin, C. R. to Hooker, J. D., 6 Oct 
Darwin writes to Hooker about his progress with barnacles. He is thinking about his species theory, but working on his barnacles. He describes “supplemental” males in detail. In working out metamorphosis, their crustacean homologies followed automatically. He also opposes appending first describer’s name to specific name.
Letter 1220 — Hooker, J. D. to Darwin, C. R., 3 Feb 1849
Hooker sends Darwin “a yarn about species” in October mail. He notes that Darwin’s complemental males in barnacles are wonderful. He warns Darwin at length to drop his battle about perpetuity of names in species descriptions.
Letter 1260 — Darwin, C. R. to Hooker, J. D., 12 Oct 1849
Darwin writes to Hooker about his species work and his barnacle work. He thinks his barnacles are becoming tedious. Careful description shows slight differences that constitute varieties, not species. He says that his approval of his plan for barnacle work over his theoretical species work had a great influence on him deciding on going on with the former and deferring the species paper.
Letter 1319 — Hooker, J. D. to Darwin, C. R., 6 & 7 Apr 1850
Hooker thinks Darwin is “too prone to theoretical considerations about species”, but is pleased Darwin took up a difficult group like barnacles. Darwin’s theories have progressed but Hooker is not converted.
Letter 1339 — Darwin, C. R. to Hooker, J. D., 13 June 
Darwin writes to Hooker on barnacles and the species theory. He thinks that systematic work would be easy “were it not for this confounded variation, which however, is pleasant to me as a speculatist though odious to me as a systematist.” He replies to the Hooker’s remarks on the Himmalaya rubi willow’s distinctness: “if my rude sketch had any small share in leading you to these observations, it has already done good & ample service, & may lay its bone in the earth in peace.”