Synthesized answer
The central thesis of this text, as indicated by its title, is to explore "The expression of emotions in man and animals" [1]. The author, Charles Darwin, explicitly states that his concern is with this subject of expression [Passage 1].
The provided passages do not elaborate further on the specific arguments or findings that constitute the central thesis beyond its stated topic. The author mentions consulting older treatises and a famous work by Le Brun, but does not detail how these contributed to or formed his thesis [Passage 1]. Therefore, while the subject of the thesis is clear, the detailed argument or conclusion is not present in the given text.
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.