The dynasties are disappearing. The latest proof came when Caroline Kennedy, the only daughter of John and Jackie Kennedy, dramatically removed herself as a possible replacement for Hillary Rodham Clinton - a dynast by marriage - in the Senate seat from New York once held by her Uncle Bobby.
Caroline's announcement came just two days after a seizure in the Capitol had served as a reminder that her surviving uncle, Ted Kennedy, the veteran senator from Massachusetts, is battling a serious illness, a malignant brain tumor.
And it came just two weeks after the heir apparent to the Bush family dynasty, former Gov. Jeb Bush, had taken himself out of consideration for the Senate seat that will become vacant next year in Florida.
Jeb is young enough that he could have another bite at the apple, running in 2012 or a later year to succeed his father and his brother, the two George Bushes, as president. But the last time Jeb's name was on a ballot was in 2002 - and the lapse of a decade is a lifetime in politics.
As for the Kennedys, where there once seemed to be a limitless supply of them - handsome, energetic and ambitious - they now can count only one federal officeholder in the younger generation, Sen. Kennedy's son, Patrick, a congressman from Rhode Island. Patrick is enormously popular at home, but his reputation in Washington has been shaped more by his personal problems than his political accomplishments.
These two families have written their way into the history books, along with such tribes as the Adamses, the Lees, the Roosevelts, the Tafts, the Harrisons, the Byrds and the Frelinghuysens.
My friend Stephen Hess, a political historian who has written a fine book about these and other "leading families," offers no sweeping generalizations about their rise and fall.
There is almost always an ancestor with the talent and drive to lift his sights beyond what others can envisage. Until now, those pioneers have mostly been males. Joseph P. Kennedy and Prescott Bush made their fortunes on Wall Street before turning to government service and instilling the ambition in their sons.
But it will not be long before the inheritance shifts to the maternal line, given the pace with which women are moving into higher office in both federal and state governments.
For now, though, women and men alike are inheriting the political gene mainly from their fathers - as witness Kansas. Nancy Landon Kassebaum Baker, its former Republican senator, is the daughter of Alf Landon, the state's former governor and the 1936 Republican presidential nominee. Kathleen Sebelius, now the Democratic governor, learned politics from her father, John Gilligan, who was once the governor of Ohio.
I see no inevitability in the fading of particular dynasties. Some children may receive too close-up a view of the costs of public life, the wear-and-tear on marriages and families. But others are unfazed. Jerry Brown, the former governor of California, saw the father whose name he bears, Edmund G. "Pat" Brown Sr., defeated in his third term bid for governor by Ronald Reagan. But Jerry keeps running, currently serving as state attorney general, and likely trying for governor again next year.
And speaking of ambitious attorneys general, New York's Andrew Cuomo, the son of Mario Cuomo, the former governor, has been poised to swoop in and claim the New York Senate seat that had seemed to be ticketed for Caroline Kennedy - at least until The New York Times ran verbatim excerpts from its interview with her, a transcript so studded with "you knows" and broken sentences as to invite a Tina Fey imitation.
Mario Cuomo, like Pat Brown, saw his career end in defeat - something his son desperately wants to avenge. But Andrew was passed over for the Senate seat when Gov. David Paterson picked Rep. Kirsten Gillibrand on Friday.
My favorite surviving dynasts are Mark and Tom Udall, the Democratic cousins just elected to the Senate from Colorado and New Mexico, respectively. They are the sons of Morris "Mo" Udall, the courageous and marvelously humorous congressman from Arizona, and his brother, Stewart, who left the House to become John Kennedy's secretary of interior - two of the best friends the environment and public lands have ever had.
That's the kind of legacy we can always use.
E-mail nationally syndicated columnist David Broder at firstname.lastname@example.org.