A famous sentence with the rest of the letter attached
In a letter dated by its editors to 11 January 1844, Charles Darwin told the botanist Joseph Dalton Hooker that he was becoming convinced species were not immutable. His comparison to 'confessing a murder' is memorable. The rest of the letter matters too: it contains questions about plants, islands, collections and observations, alongside an account of the facts he had been gathering.
The document establishes what Darwin wrote to Hooker in that exchange. Turning the phrase into a complete explanation of his personality, fears or later publication decisions would ask a single letter to carry much more.
Sources: s1
Read across the correspondence
The Darwin Correspondence Project presents letters as evidence of scientific work and networks as well as personal life. Its Hooker collection spans a long relationship; the project describes their correspondence as a thread connecting decades of Darwin's work. Reading across that sequence can test an interpretation that one dramatic sentence merely suggests.
Sources: s2
Leave room between the evidence and the narrator
The National Archives explains why primary sources still need interpretation and attention to perspective. In biography, a useful discipline is to separate three statements: the person wrote this; other evidence suggests this; the narrator proposes this explanation.
A lively account need not erase those boundaries. Describe the particular request, disagreement or joke that survives on the page. Attribute an inference about motive, and show what might challenge it. For invented characters in the lost-world stories, interior thoughts are available to the writer. For a real historical person, they need evidence or an explicit admission that we do not know.
Sources: s3