The decade immediately following publication of On the origin of species in 1859 to the eve of publication of Descent of man in 1871 was arguably the most intense and productive of Charles Darwin's life. These were years in which the implications of the theories made public through Origin were explored and debated around the world, not only in the scientific community but in the public arena. Darwin, so far as his health would allow, set about countering criticisms with ever more detailed researches into complex mechanisms in organisms, teasing out how they could be explained as adaptations arising through the operation of natural selection. He also sought answers to the questions he knew Origin had not answered, in particular concerning the mechanisms of inheritance, and the evolutionary role of competition for sexual partners.

At the beginning of this period Darwin still intended to write the ‘Big Book’ on species of which Origin was only an abstract. As he resumed work on what had been intended as a single chapter on pigeon-breeding, however, it quickly became apparent that a detailed exposition of the production of domestic varieties of the various animals he was researching would require a separate publication. In fact as his researches deepened and widened publications expanded out of one another like Russian dolls: a planned final chapter on human origins for Variation under domestication became another two-volume work, Descent of man and selection in relation to sex, and his work on the relationship of human and animal emotions outgrew the confines of Descent and was eventually published in 1872 as Expression of the emotions in man and animals.