The great mind grew cynical in the end, too battered by the slaughters of the civil warand the seeming winning of the world by railroad barons and other rich, mindlessly cruel capatlists throughout the world.
He kept his sense of humour until the end, but there is nothing child-like in the late writing that he wouldn't allow them to publish until 100 years after his death. This book is filled with scraps of how he viewed mankind as opposed to animals, which he saw as better than man in every way possible.
He was a bit of a misanthrope in the end, living with daughters after the death of beloved wife. He was famous and sought out, but he was too bitter to care. He was known for ignoring people who came to talk to him, though he was said to always take hours out of his day to play with any stray cat that came along. Loving animals is easier than humans, because we can basically understand why they do what they do, because they have limited language games/cultural--biological patterns to choose from. They bite, they purr, they bark, they want to go out... etc.
In 'On Man and Beast,' the stories range from balls out comparisions of humans and pets, to stories about the devil appearing to some kids and creating villages to show creation with tiny people, and then slaughtering them all just to make a point. Other stories are a bit more pathetic, like the man who beats his dog so bad it has an eye hanging out, but when the man is injured, the dog does all it can to help him. Lucky for us, most of the stories are funnier than that one.
This is an incredibly enlightened view for his time, when most animals were still thought of as nothing but a meat or a ride. People have always loved animals, but it is obvious that it would be difficult to allow a lot of emotions to develop for a pig that is going to be eaten, or a cat that is simplyt there to eat the rats on the farm, etc... I have written in here before about the expanding of the human ethical circle that is taking place now. Back in Twain's time, native americans, blacks, women, the Chinese -- basically, anyone who was not white and comely or rich -- was demonized. The xenophobia of our mammalian nature was rampant, and culture had yet to alter the genetics of a chimp that had to distrust any chimp not from their tribe (at least the ones run by males).
How did we change? One study showed that a group of apes lost all of their alpha males, because they were the fighters for garbage back about twenty years ago, and they all caught a disease and died. Now, though they are xenophobic, without these males the females created an entirely different tribe. More about sharing than fighting for a place in the heirarchy. This state of affairs last until the present day. Basically, their environment caused their culture to change, and they found a better way to live.
This has happened to humans over and over, too fast for any of us to sit down and find causation, like this chimp study was able to do. Mark Twain, in the end, seems to have lost hope in humans. A lot of this was the result of his being fed romantic notions when he was a child -- a universe complete with a god, and the truth was almost too much for him to bear.