I hadn’t heard that. Twain/Clemmons was a very witty guy.
This is a theme in THE DAMNED HUMAN RACE, included in the paperback edition of LETTERS FROM THE EARTH, which I located with difficulty because the spine is so tattered!
The theme is expressed succinctly as follows, “One is obliged to concede that in true loftiness of character, Man cannot claim to approach even the meanest of the Higher Animals.”
Of course this is a reversal of the common reference to the “lower animals”.