Last month I visited the Bancroft Library at the University of California
and took photos of Twain's copy of _The Descent of Man_ and his comments on
Darwin's view of sympathy in the marginalia of the book with the permission
of the Mark Twain Project.  They might be of interest for you.  You can see
them in my homepage at http://www15.u-page.so-net.ne.jp/wf6/hhsj6225/
Best regards,
Takeshi Omiya