Darwin wrote to many people on his account of beauty. In this collection of letters, written after the Duke of Argyll’s address to the Royal Society of Edinburgh (1864), Darwin used birds, flowers and butterflies as examples to illustrate his ideas on beauty.
Letter 4752 — Darwin, C. R. to Lyell, Charles, 22 Jan 
Darwin writes to King’s College, London Professor of geology, Charles Lyell, criticising the Duke of Argyll’s address [to the Royal Society of Edinburgh (1864)] on beauty and sexual selection. He discusses humming birds and orchids as examples.
Letter 4939 — Shaw, James to Darwin, C. R., 20 Nov 1865
Scottish school teacher and writer James Shaw praises Darwin’s theory. He believes beauty in nature is caused by sexual selection, but there is quite a long discussion on beauty in the natural world.
Letter 4943 — Darwin, C. R. to Shaw, James, 30 Nov 1865
Darwin writes to James Shaw. He is gratified that Shaw defends views of Origin. He thinks beauty of flowers is solely to attract insects.
Letter 5003f — Shaw, James to Darwin, C. R., [6--10 Feb 1866]
James Shaw transcribes a newspaper report of the paper he read to the Dumfriesshire and Galloway Natural History and Antiquarian Society on 6 February. In it Shaw defends Darwin’s account of Beauty against the Duke of Argyll’s criticisms.
Letter 5004 — Darwin, C. R. to Shaw, James, 11 Feb 
Darwin thanks James Shaw for the abstract of his paper on beauty. He discusses beauty of birds and butterflies, noting in particular that butterflies offer an excellent instance of beauty being displayed in conspicuous parts.
Letter 5060 — Shaw, James to Darwin, C. R., 19 Apr 1866
James Shaw fills a letter to Darwin with anecdotes about appreciation of beauty by animals.
Letter 5565 — Kingsley, Charles to Darwin, C. R., 6 June 1867
Clergyman Charles Kingsley writes to Darwin criticising the Duke of Argyll’s book [Reign of law (1867)], particularly on beauty and sexual selection. However, he believes Darwin overlooks God’s intention to instruct man by nature’s beauty.
Letter 5648 — Darwin, C. R. to Wallace, A. R., 12–13 Oct 
Darwin thinks naturalist A. R. Wallace’s remarks on hideous objects and on flowers not being made beautiful except when of practical use to them are good. He thinks the Duke of Argyll’s argument on beauty is not candid.