Synthesized answer
The provided passages do not contain information about what questions remain unanswered in the book. The text focuses on introducing the subject of the expression of emotions in man and animals and mentions some of the historical works that have been written on the topic, such as Le Brun's `Conferences' [2]. It also states that the author is not concerned with physiognomy, which is the recognition of character through the study of the permanent form of features [1].
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.