During 1867 Darwin intensified lines of research that were to result in two important publications, Descent of Man and Selection in Relation to Sex and Expression of the Emotions in Man and Animals. Darwin circulated a questionnaire on human expression, asking his established contacts to pass it on to their acquaintances, with the result that he began to receive letters from an even more diverse and far-flung network of correspondents than had previously been the case. Convinced that human descent was strongly influenced by sexual selection, he also started to ask his correspondents about sexual differences in animals and birds. At the same time, he was working on the proof-sheets of another major work, Variation of Animals and Plants under Domestication, while negotiating almost weekly with French, German, and Russian translators. For information on the Charles Darwin Correspondence Project, see http://www.lib.cam.ac.uk/Departments/Darwin.
