Synthesized answer
The book is concerned with the expression of emotions in man and animals, distinguishing this from physiognomy, which studies character through the permanent form of features [1]. While many works have been written on expression, the author found older treatises to be of little service [1].
The painter Le Brun's "Conferences," published in 1667, is identified as the best-known ancient work on the subject and contains some good remarks [1, 2]. The passages do not elaborate further on the author's specific key concepts beyond this initial framing of the subject matter and its historical context.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: The expression of emotions in man and animals by Charles Darwin Description: Many works have been written on Expression, but a greater number on Physiognomy, - that is, on the recognition of character through the study of the permanent form of the features. With this latter subject I am not here concerned. The older treatises,[1] which I have consulted, have been of little or no service to me. The famous `Conferences'[2] of the painter Le Brun, published in 1667, is the best known ancient work, and contains some good remarks.