Charles Darwin depended on correspondence to collect data from all over the world, and to discuss his emerging ideas with scientific colleagues, many of whom he never met in person. In January of 1868, Darwin's Variation under Domestication was published. The first printing of 1,500 copies rapidly sold out so that the publisher, John Murray, ordered a second printing. Responses to this new book, added to Darwin's continuing research into sexual selection and the expression of the emotions, increased the quantity of Darwin's correspondence to such an extent that that we present the letters he wrote and received during this period in chronological order across two volumes. Notes and appendices put them into context, explain references, and provide information on related works. For information on the Charles Darwin Correspondence Project, founded by Frederick Burkhardt, the editor of this set, visit http: //www.darwinproject.ac.uk.