It was crucial to Darwin’s theories of species change that naturally occurring variations could be inherited.  But at the time when he wrote Origin, he had no explanation for how inheritance worked – it was just obvious that it did.  Darwin’s attempt to describe how heredity might work, his 'provisional hypothesis of Pangenesis’, was published in 1868 in his book, Variation of animals and plants under domestication

The year 1871 was an extremely busy and productive one for Darwin, with the publication in February of his long-awaited book on human evolution, Descent of man. The other main preoccupation of the year was the preparation of his manuscript on expression. Darwin continued to investigate the mechanisms of various emotions, such as grief, shame, and astonishment, drawing on the expertise of physicians and physiologists, as well as zoo-keepers, pet owners, parents of young children, and professional photographers. The year  also brought a significant milestone for the family, as Darwin’s eldest daughter Henrietta was married in August. 

Charles Darwin’s major achievement in 1867 was the completion of his large work, The variation of animals and plants under domestication (Variation). The importance of Darwin’s network of correspondents becomes vividly apparent in his work on expression in 1867, as he continued to circulate a list of questions on human expression that he may have drawn up in late 1866. His correspondents were asked to copy the list and forward it to those who might best answer the questions, with the result that Darwin began to receive replies from different corners of the world.