Is time slip a relatively new device? Every modern novel I read now seems to go back and forth between the past and the present. Sometimes it works; sometimes not. I'm probably the only person on the planet who couldn't cope with The Time Traveller's Wife (the ultimate time slip novel), and I had my doubts about One Day, probably for similar reasons.
But I cannot think of a single "classic" novel which works in this way. Dickens, Austen, Gaskell, Hardy, Trollope - they all move seamlessly in one direction, with only passing references to the past. When did this time slip thing start (I've done it, too, in my novels, almost without realising it)? And why?