"At some future period, not very distant as measured by centuries, the civilized races of man will almost certainly exterminate, and replace, the savage races throughout the world. At the same time the anthropomorphous apes, as Professor Schaaffhausen has remarked, will no doubt be exterminated. The break between man and his nearest allies will then be wider, for it will intervene between man in a more civilized state, as we may hope, even than the Caucasian, and some ape as low as a baboon, instead of as now between the Negro or Australian and the gorilla."
We now consider this to be not merely morally repugnant, but factually incorrect (and, yes, I did mean to put those two clauses in that order). However, when Darwin wrote this passage, such racist statements were commonplace. There was nothing even vaguely controversial about the idea that white men were better than Negroes. This poses an interesting question: what moral or political positions do we hold today that will, or might, be as utterly repugnant to future generations as Darwin's casual racism is to us? I can think of a few:
It is currently considered perfectly acceptable for Walmart to declare that they try to "Buy American", and for British politicians to claim that products made in this country should "proudly display the Union Flag". This is exactly as immoral as racism, for exactly the same reason. (Steve Landsburg has been pointing this out for a while - see, eg, this video). I can only hope that there will come a time in the future when such blatant tribalism: the utterly deplorable idea that I should care more about a complete stranger who happens to have been born on the right side of some arbitrary line, is considered every bit as heinous as the utterly deplorable idea that I should care more about some complete stranger who happens to have been born with the right colour skin.
It is currently considered immoral to ingest certain substances. So much so that we have passed laws banning the ingestion of said substances. This strikes me as bizarre. It seems eminently likely that, at the very least, the substances which we consider it immoral to ingest will change over time.
There are people who seriously suggest that it would be immoral to switch the law regarding transplantation from a default "no" to a default "yes". This is plainly ridiculous, as is the idea that a market in organs is immoral. However, I can actually imagine future generations going one step further, and being utterly horrified at the idea that we let people die from lack of transplants when there were perfectly good organs rotting away inside human carcasses: as if the dead people had some use for them! Compulsory post-mortem organ donation is certainly a plausible moral imperative.
We bring sentient beings into existence, feed them, house them, and kill them just in order to eat their dead bodies. I can easily see how a future society in which either meat has just dropped rapidly out of fashion, or in which meat production no longer requires the participation of sentient beings, might hold us to account.
I am not the first person to have noticed that children are essentially treated as non-people in our society. I can easily imagine the way we allow parents to completely dictate their children's lives could be considered morally repugnant. Also relating to children: it seems quite plausible that our current attitude to Child Sexual Activity is somewhat misguided (see, eg Rind Et Al) - the very fact that I'm almost afraid to write that statement in public without a string of caveats should tell us, at least, that society's current attitudes to CSA are too emotional to be completely rational.
Almost all of the examples I've managed to come up with are examples in which I'm fairly sure that the current mainstream moral opinion is actually wrong, or at least in which I'm fairly sure it could be wrong. Clearly this is a failure of imagination on my part - there is almost certainly at least one thing that wouldn't even occur to me that will be seriously considered to be a moral issue in, say, 250 years' time - anyone have any ideas what it might be?