This volume is part of the definitive edition of letters written by and to Charles Darwin, the most celebrated naturalist of the nineteenth century. Notes and appendixes put these fascinating and wide-ranging letters in context, making the letters accessible to both scholars and general readers. 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. The letters are published chronologically: volume 20 includes letters from 1872, the year in which The expression of the emotions in man and animals was published, making ground-breaking use of photography. Also in this year, the sixth and final edition of On the origin of species was published, and Darwin resumed his work on carnivorous plants and plant movement, finding unexpected similarities between the plant and animal kingdoms.