Charles Darwin's health improved substantially in 1866 under a dietary and exercise regime prescribed by his physician Henry Bence Jones. With renewed vigour, he worked steadily on his manuscript of Variation of Plants and Animals under Domestication, submitting all but the final chapter to his publisher in December. He also worked on the fourth, and much revised, edition of Origin which was delivered to printers in July, and preparations were begun for a third German edition of Origin. His improved health allowed him a more active social life. At Down, Darwin entertained a number of scientific colleagues whom he had known previously only through correspondence. He also made his first appearance in London scientific society in many years, touring the Zoological Gardens at Regent's Park, and appearing at a soiree at the Royal Society.

Volume 13 contains letters for 1865, the year Charles Darwin published his long paper on climbing plants and continued work on his book, The Variation of Plants and Animals under Domestication. 1865 was also the year when Robert FitzRoy committed suicide; Joseph Dalton Hooker became director of the Royal Botanic Gardens, Kew; and Charles Lyell and John Lubbock quarrelled over an alleged incident of plagiarism. The volume includes a supplement of over 100 letters discovered or redated since the series began publication, including a fascinating collection written when Darwin was 12.