Why is the ruling class the ruling class?

My memory grows feeble in my dotage but I seem to recall 19th century social theorists like Mosca and Michels contending that oligarchy was the natural state of affairs in human society. Why this inevitability? On the left we like to argue that it's because of the lack of scruple among the rulers. And this is doubtless a contributing factor. Though the lower orders hardly seem to be overburdened with scruple either. One explanation might be that the rulers are smarter. The plebs sordida are a stupid lot. Orwell's "Animal Farm" describes a post-revolutionary situation where the smart animals -- the pigs -- rapidly become the new governing elite. Most of the rest of the animals can't even remember the second letter of the alphabet. I've been musing about the (relative) smartness of the ruling class for some time. Yesterday evening I tried once again to initiate a political discussion -- which despite my efforts rapidly deteriorated into speculation about whether Tim Pawlenty would have Michele Bachmann as running mate should he run for president. Faced with this wall of obdurate stupidity -- which is utterly incapable of grasping the structural aspects of our situation -- I become despondent. Ah well, maybe we can teach them "Four legs good, two legs bad."