"Of all the creatures that were made, man is the most detestable. Of the entire brood he is the only one--the solitary one--that possesses malice. That is the basest of all instincts, passions, vices--the most hateful. He is the only creature that has pain for sport, knowing it to be pain. Also--in all the list he is the only creature that has a nasty mind."
Mark Twain's Autobiography

"Of all the animals, man is the only one that is cruel. He is the only one that inflicts pain for the pleasure of doing it. It is a trait that is not known to the higher animals."
"The Lowest Animal"

"I have been reading the morning paper. I do it every morning--knowing well that I shall find in it the usual depravities and basenesses and hypocrisies and cruelties that make up civilization, and cause me to put in the rest of the day pleading for the damnation of the human race. I cannot seem to get my prayers answered, yet I do not despair."
Letter to W. D. Howells, 4/2/1899

"Damn these human beings; if I had invented them I would go hide my head in a bag."
Letter to W. D. Howells, 1899

In a book by Charles Darwin, Twain had written: "Can any plausible excuse be furnished for the crime of creating the human race?"
The New York Times, "Hartford Museum Purchases Barrels Full of Twain's Old Books," July 31, 1997

"It is not worthwhile to try to keep history from repeating itself, for man's character will always make the preventing of the repetitions impossible."
Mark Twain in "Eruption"