As with most great minds of the 19th century, MT put down thoughts on an
enormous range of topics.  I am continually surprized at some of the
things to be found in the Notebooks, Letters and Autobiography.  You may
be able to find an answer to your query in "The science fiction of Mark
Twain" / edited by David by David Ketterer. Hamden, Conn. : Archon Books,
1984. Also try "What is man? and other essays" UC Press.

Cheers,
Rhio H. Barnhart
Head, Music Department Library
University of California, Davis