Downing Street and CCHQ will be cock-a-hoop over the defection of Amjad Bashir from UKIP to the Conservatives.  And amidst the thrust-and-parry-and-counter-thrust of the duel between the two parties, his move is certainly significant: compensation of a kind for the departure of Mark Reckless and Douglas Carswell, and nicely timed in this run-up to May’s poll.  Who’s next, either way?

But how much difference do defectors make even in the medium-term?  In a magisterial article for this site last year, Lewis Baston read out a roll-call of them.  The reasons for their departures are mostly forgotten today.  Who remembers why Sir Richard Acland, the Duchess of Atholl or Desmond Donnelly left their parties?

Furthermore, some defectors leave parties without a clear-cut reason at all: this is arguably true of Winston Churchill, at least the second time round.  Others famous for crossing the floor never actually did so.  Peel could have said – as so many others have done – that he didn’t leave his party: it left him.  Gladstone didn’t quit the Conservatives for the Liberals, but travelled via the Peelites.

The defections which turn out to matter usually involve more than the man who undertakes them.  The “Gang of Four” who left Labour to found the SDP were forerunners of the New Labour project.  The trickle of MPs from the Conservatives to Labour during the Blair and Brown years – Alan Howarth, Quentin Davies, Peter Temple-Morris, Sean Woodward – were a sign of the latter’s dominance.

Perhaps the biggest defection of all was Joe Chamberlain’s, which helped to take a current of Liberal thinking into the great river of conservatism.  Maybe Carswell’s defection will turn out to be a forerunner of something comparable – or Bashir’s a sign that the rise of UKIP has peaked.  But the truth is that for all the palaver they stir at the time, most defections make no difference to anything much at all.