In the story of Downton Abbey, it was taxes and a medieval inheritance system that was the real root of the families troubles. Both together, these two things threatened to ruin the life that Earl of Grantham's ancestors had built up over many years.
Robert Crawley and his wife had daughters and no sons and because of the laws of primogeniture, none of them could inherit their father's title, house, or land. It was Robert's first cousin James and his son Patrick who stood to inherit, but when they died on the Titanic, it was Matthew Crawley, Robert's third cousin once removed, who became heir presumptive.
Primogeniture is centuries old however the taxation of a person's wealth at the time of their death dates from the the
end of the 17th century.