Autograph Letter Signed ("C. Darwin"), 4 pp recto and verso, 8vo (conjoined leaves), Down Bromley, Kent, November 29, [1857], to Charles Spence Bate, with autograph transmittal envelope. Light soiling to envelope, letter fine with just a couple of spots and slight fold separation at outer margin.

FINE SCIENTIFIC CONTENT: DARWIN PEPPERS A COLLEAGUE WITH QUESTIONS ABOUT THE REPRODUCTIVE ACT AMONG BARNACLES. Darwin first became intrigued by barnacles on a beach off the coast of Chile in January, 1835. However it was not until 1846 that he began the lengthy researches that resulted in the authoritative 4-volume Cirripedia, published from 1851-1854. In studying barnacles, Darwin was "the first person to classify a group of organisms according to the principle of 'common descent,' the idea that related animals and plants all descend from a common ancestor. This was a central idea in Darwin's theory of evolution and probably the only proposal to come from The Origin which was more or less immediately and widely adopted by the scientific community" ("Charles Darwin & Evolution," Christ's College, Cambridge, 2009).

In the present letter, Darwin shows that his fascination with the creatures has not abated one jot over the years. He has had it on hearsay (John Lubbock told Darwin about a friend of Bate's) that an unknown man has actually observed the sexual act in a group of barnacles and Darwin can hardly wait for the details. Darwin letters regarding sexual intercourse are rare. Indeed, it is not uncommon to be fascinated by barnacle sex. The barnacle is the creature with proportionally the largest penis in the animal kingdom.
Balanus is a genus of barnacles which includes the common barnacle. The recipient, Charles Spence Bate (1819-1889), was a zoologist and dentist. Richard Bishop's detailed response C.S. Bate's forwarding of Darwin's letter can be read on the Darwin Correspondence Database.