This posting was transferred from my original blog
Worth pointing out that we don't actually have a hereditary monarchy. Since William I, succession by the hereditary heir is not really that common:
Not the hereditary heir to the previous king:
William I, William II, Henry I, Stephen, Henry II, Henry IV, Edward IV, Richard III, Henry VII, William II/Mary II, George I, George VI (12)
Took over while a previous king was still alive:
Edward III, Henry IV, Edward IV, William II/Mary II, George IV, George VI, Elizabeth II (7)
Didn't take over on death of previous king, but had to wait:
Charles II (1)
Total with "dodgy" successions: 17
Heir succeeding on death of previous monarch:
Richard I, John, Henry III, Edward I, Edward II, Richard II, Henry V, Henry VI, Henry VIII, Edward VI, Mary I, Elizabeth I, James I, Charles I, James II, Anne, George II, George III, William IV, Victoria, Edward VII, George V, Edward VIII, Elizabeth II (24)
So that's 23-16 in favour of the hereditary succession in neat order (Elizabeth II is in both categories in that she succeeeded George VI conventionally, but Edward VIII was still alive on their coronations).
Hardly the overwhelming victory for the hereditary principle that you might anticipate from some people's rather prejudiced views of history. Admittedly, I do get some help from the Normans (five successive kings without a conventional succession), but it's still only 2:1 in favour of the hereditary principle if you start from Henry II instead of William I.