The letters are between Darwin and his bff, botanist Joseph Dalton Hooker, with whom he worked out the theory of evolution, edited The Origin of Species, and talked about travel, literature, and family life. ("Did you administer the Chloroform?" Darwin asked Hooker upon hearing about the birth of Hooker's first daughter. Darwin was a fan of chloroform for both father and mother during births.) The two exchanged 1,400 letters over their lifetimes, making up 10 percent of Darwin's surviving correspondence.