There is only one letter documented from Twain to Croly in Union
Catalog database.  It was printed in _Mark Twain's Letters, Vol. 5_,
pp. 341-343, (Univ. of Cal. Press, 1997).  It does not contain the
quote attributed to Twain at:

http://www.historicpages.com/texts/dgcat9.htm


Barb