This volume is part of the definitive edition of letters written by and to Charles Darwin, the most celebrated naturalist of the nineteenth century. It is already an important source for students and scholars in many academic disciplines. 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 18 includes letters from 1870, as well as a supplement of more than a hundred recently discovered or redated letters from before 1870. During 1870 Darwin was making final preparations for publication of Descent of Man, as well as continuing his research on expression in humans and animals.