|William Fitzwilliam, 4th Earl Fitzwilliam|
William Wentworth-Fitzwilliam, 4th Earl Fitzwilliam was a British Whig statesman of the late 18th and early 19th centuries. In 1782 he inherited the estates of his uncle Charles Watson-Wentworth, 2nd Marquess of Rockingham, making him one of the richest people in Britain. He played a leading part in Whig politics until the 1820s.
In discussing the choice of naming Mr. Darcy's sister Georgiana, Stephen Derry relates:
Fitzwilliam Darcy has been associated with the Whigs, as his names recall those of two prominent Whig noblemen, Robert D’Arcy, fourth Earl of Holdernesse (1718-1788), and William Fitzwilliam, fourth Earl Fitzwilliam (1748-1833), who both held high ministerial office. Donald Greene considered that Darcy’s arrogance might have been “a satire on an aspect of Whiggism most obnoxious to Pittite lories,” (Persuasion #11)Derry also says that Pemberley is based on Chatsworth based on it's size and description. That was in 1989, and today many are claiming that the majestic house is instead based on the home of the 4th Earl Fitzwilliam, Wentworth Woodhouse.
Fitzwilliam was the son of William Fitzwilliam, 3rd Earl Fitzwilliam, by his wife Lady Anne, daughter of Thomas Watson-Wentworth, 1st Marquess of Rockingham. He inherited the two earldoms of Fitzwilliam (in the Peerages of Great Britain and of Ireland) in 1756 at the age of eight on the death of his father.
He supported John Wilkes (subject of an earlier RMM) in his fight to keep the seat he was elected to and supported the American Colonies in their dispute with Britain. On 8 July 1776 he asked Lord Rockingham to arrange for a remonstrance to be sent to the King when war broke out in America, so the Americans would see "that there is still in the country a body of men of the first rank and importance, who would still wish to govern them according to the old policy".
On his uncle Lord Rockingham's death on 1 July 1782 he inherited Wentworth House, the largest mansion in the country, and his substantial estates, making him one of the greatest landowners in the country. The Wentworth estate in south Yorkshire was made up of 14,000 acres (57 km2) of farm land, woods and mines yielding nearly £20,000 annually in rents.
Fitzwilliam had a very prominent career in politics, you can read more about his career on his Wikipedia page. More information on the Earl Fitzwilliam. For the article in Persuasions written by Stephen Derry you can go here.