Aristocracy, as Webby informs us, actually originally meant "rule by the best citizens". But nowadays Webby's third meaning - "a form of government in which the supreme power is vested in the principal persons of a state, or in a privileged order" - sounds truer to more modern ears. Because wouldn't a government of the best be a meritocracy? Not in the eyes of the aristocrats - their right to rule stemmed, they thought, from their being just inherently better than everyone else. In the end, when the aristocracy found themselves under attack from the values of the modern world, their ideology became in many cases a racial one, pitting a trans-national super-race of aristocrats against the inferior, servile lower races - which meant everyone else. In fact, one of the very first theories of racism was developed in defence of the aristocracy.
The difference between meritocracy and aristocracy is indeed large. Meritocracy is something that could not really exist on a large scale until the modern age, with the development of capitalism and the bureaucracy of the modern state. These developments created organizational structures that value their own efficiency above all else, and so seek out the best people to populate themselves for achieving their ends. And that's what we mean by meritocracy - rule by those who have merit, where merit is defined by the organizations we want to advance in. So that might be administering health care or selling cars. Of course, it doesn't always work like that, and corruption exists - but when this happens everyone realizes that the appropriate response is outrage.
That sort of thing couldn't exist when such organizational structures didn't exist, and when ends were defined rather differently. Aristocracy - "rule by the best" - didn't mean the best at doing anything in particular, but quite literally the best people. The ideology of the European aristocract provided for his own rule by asserting that as the most refined, cultured, and virtuous man, he and others like him were the ones who ought to be in charge. His right to rule others stemmed, in his view, from his exceptional rule over himself, because he was moderate and able to control his passions; his energy was exerted in controlled ways according to the culture and traditions of his ancestors, unlike the undisciplined rabble.
Rule of the laziest?
The prerequisites for becoming this sort of man were property and leisure. Property, because it gave one a stable place in the world and meant that the aristocrat would never have to reduce himself to working for an income; a fate worse than death, because working for an income means you're bound by the lowest common denominator of existence, a slave to necessity. Not having to do this means, in turn, that you have leisure.
In the modern world, leisure is seen as opposed to work; and in the most consistent expressions of the ideas that underpin modern society, i.e. those found in America, leisure is seen as basically being a bad thing - just listen to the boasts that America is more efficient than France because people take fewer holidays. This wasn't always the case.
The Ancient Greeks had two words for leisure - aergia, which meant basically laziness as we would understand it; and skhole, which meant something quite different. Skhole is the leisure - the time off from having to look after the bare necessities of life - that you were thought to need to attend to political matters, to be what we would call a "gentleman" - a man who is gentle because he does not make himself rough through work. The Bolsheviks used to decide who to kill by looking at how smooth the skin of their hands was - but I digress. We would not be so extreme, but it is again clear that the "gentleman" understood in these terms has no place - at least, no place of power and influence - in the modern world.
But having skhole and hence refinement, and time to spend in judgement of weighty matters, was what the aristocracy derived their right to rule from. The benefits of such logic are clear - they essentially ossify power in those families that are rich enough to have leisure and pass their property down to their children. And the reason that property was passed intact to the first born son rather than split into many parts was to maintain the family name undiluted, with all the eggs in one basket and each generation as propertied as the last.
The end is nigh!
The rise of capitalism and a new idea of virtue eventually destroyed this aristocratic order entirely. The idea that the best type of man was the one who made money through his own hard work and industry was the opposite of having leisure in which to pursue personal refinement. And as the ideas of equality of rights and opportunity spread, the aristocracy could only lose out. Because they didn't produce wealth, but only seemed to consume it, they began to be seen as parasites on the people, fleecing them and giving nothing back; their superior claim to virtue and refinement couldn't protect them once the rules of the game changed.
This is really what "right-wing" originally meant - a commitment to inequality, to the belief that all men are not created equal, and some are better than others. And as the aristocracy found themselves under attack from the ideas of equality, they had to strike back by coming up with various ways that they were better than everyone else. The most extreme version came from a French aristocrat, the Comte de Gobineau. Gobineau - who claimed to be descended from the Norse God Odin - came up with with a race theory to satisfy aristocratic feelings of superiority.
Aristocrats had always had a trans-national character in Europe, and a nobleman in France would for the longest periods of history have had more in common with one in Germany - or even Russia - than his own peasants, whose language he might not even speak. Gobineau took it all a step further. The bourgeoisie - the new, rising class of traders and merchants who were displacing the aristocracy - were, he said, racially entirely different to the aristocracy. Through the veins of the bourgeoisie flowed the blood of Gallic-Roman slaves, whereas the aristocracy was Germanic: "of the race of Gods", in fact; and so they had the right to rule. And Gobineau was convinced that because the blood of the aristocrats was being diluted, western civilization was ruined.
All that was really ruined, of course, was the old aristocratic splendour; and some still lament that, but their numbers are hardly significant. What is, I would say, most interesting about this tale is the fact that it eventually ended in racism: so many tales of decline do. A group of people stripped of everything - their sense of superiority, the justification they used to peddle for their status and privilege - eventually has nothing to fall back on but the blood in their veins, and the vicious slander than it is worth more than the blood of another human being. The Nazis took Gobineau's theory, modified it so that the whole German people were one giant aristocracy and every other European people a pitiful mass beneath them, and set about recapturing lost German glory on this basis. Finally, the ideology of inequality which started with the aristocrats was blasted out of existence in Berlin, 1945.