One of his main collaborators was Dr James Crichton-Browne, one of the leading Victorian psychiatrists, who happened to be the medical director of the West Riding Lunatic Asylum in Wakefield.
In the book Darwin concludes that there are only six basic facial expressions. He did it by painstaking research and by sending detailed questionnaires to fellow naturalists and doctors, like Dr Crichton-Browne, in order to accumulate a substantial amount of information. He found that these six expressions are common to all humans, regardless of their race or place of origin. They are essentially inherited. They are the emotions of happiness, sadness, fear, surprise, anger and disgust. You will recognise them easily enough, because they are automatic rather than learned.
Interesting research on facial expressions has just been done in California. They selected 63 sculptures - crafted between 3,500 and 600 years ago in Mexico and Central America - which conveyed many facial expressions across five varieties of emotions. They then asked 325 people, with an average age of 35 years to view images of only the faces of these sculptures. Another 100 participants were asked to see how well the emotions accorded with written descriptions of the sculptures. They found very high agreement and concluded that five types of expressions were universal across thousands of years, and were common to both ancient civilisation and modern western culture. Yet in the new normal when we have to wear face masks we are limiting the facial expressions we communicate with to the upper face. There is a serious point, because lip reading and interpreting facial expressions are important for people with hearing difficulties. There is certainly a case for using transparent masks. Indeed, I understand that the government is going to deliver clear face masks to frontline NHS and social care workers to support better care for people who use lip reading and facial expressions to communicate. Perhaps it is something we should all keep in mind.