W-e-e-e-e-e-e-ll, that's more a clue than a source, but a good possibility,
deserving of a beer if not the hard stuff. Henderson's text reads "Certain
passages in his books on the subject of slavery, as the historian Lecky has
declared, are the truest things that have ever been expressed on the subject
which vexed a continent and plunged a nation in bloody, fratricidal strife."
Henderson himself (not Lecky, as I'd implied --sorry) then goes on to cite
HF and LonM as having particularly vivid examples of what he's talking
about. I may be reading him wrong, but the way Henderson puts it seems to
indicate he assumes his readers will be familiar with Lecky's opinion, or at
least that it had been stated in public or in print.

I was initially confident the source was buried in H's appendix (an
excellent bibliography by 1910 standards), but Lecky's name does not appear
under any of those entries. Henderson had interviewed Twain at length while
preparing this book, and I've wondered if perhaps Twain showed H some letter
to T from Lecky (none in Machlis), or even if H perhaps discussed with Twain
his (T's) readings of Lecky and then later on garbled his (H's) notes of
that conversation.

Kevin Mac Donnell
Austin TX