This is the work in which Darwin documents the universal nature of facial expressions. Published in the spring of 1872, and after he was diverted to complete publication of the 10-years of revisions to Origin of Species, Darwin proved that living creatures share certain "states of mind".
The proof was drawn from scientists' responses to his questionnaires propounded around the world, and with hundreds of photographs of actors, babies and "imbeciles" in an asylum. He also described his own observations, with particular empathy for the grief following a family death.
Here's the point: Animal Life SHARES feelings. Young and old, across widely different races and species, "express the same state of mind by the same movements."
{Darwin showed that ALL the so-called Races of humankind are not only emotionally identical, but we are very much the same as ANIMALS! This scientific evidence was in 100 years before the Nazis made the fiction of Race "differences" a matter of State Policy. The science clearly justifies criminalizing the mistreatment of people or animals.}
The evidence of shared evolution, and shared feelings, is in contrast to the ideology current in Darwin's day. Charles Bell's Anatomy and Physiology of Expression claimed that certain muscles in the face were divinely created to express man's exquisite feelings in a manner unique to those bearing God's Image.
The proofs, tackled by his daughter Henrietta and son Leo, needed major revision, which made Darwin "sick of the subject, and myself, and the world". It was to be one of the first books with photographs, with seven heliotype plates. The publisher Murray, warned that the plates "would poke a terrible hole in the profits". But Darwin was PROVING the fact that all animals share feelings. The Expression of Emotions in Man and Animals sold over 5,000 copies, a matter of popularity, profits, and proof.