The ending of Middlemarch is a problem for a lot of readers and critics: is it a happy ending? This is an honest question – we learn in the "Finale" chapter what happens to all the major characters: Mary and Fred live happily ever after; Lydgate and Rosamond live unhappily, but at least non-adulterously, until Lydgate dies and Rosamond is free to remarry.
That's all fine, and what we would have predicted. But what about Dorothea? She becomes Mrs. Ladislaw, and Will becomes an important politician. Wasn't Dorothea destined for greater things? The narrator tells us that many of her friends thought so: "many who knew her, thought it a pity that so substantive and rare a creature should have been absorbed into the life of another, and be only known in a certain circle as a wife and mother" (8.finale.13). Her brother-in-law, Sir James Chettam, always used to say that Dorothea should have been a "queen," and yet she ends up playing the conventional Victorian role of "wife and mother." Are we supposed to be disappointed that she doesn't lead an "epic" life?
Take a look at the final lines of the novel – Eliot says that Dorothea's "full nature, like that river of which Cyrus broke the strength, spent itself in channels which had no great name on the earth." In other words, Dorothea's lofty, idealistic nature is like a river that gets dammed up and redirected and turned into lots of smaller canals. So, instead of being a wide and powerful river, her energies get turned into narrower channels. Is this a bad thing? Seems like it could be – but read on.
The narrator goes on to say that Dorothea's "effect […] on those around her was incalculably diffusive: for the growing good of the world is partly dependent on unhistoric acts." So, the good thing about the river that gets redirected into many narrow channels is that it provides water to lots of people, even if they don't realize what a great and powerful river it used to be. It's the same way with Dorothea: her "effect" on the people around her is "incalculably diffusive." And this is important, too: it might not have been an "epic" life in the sense that she didn't complete any of her grand plans, but just because her "acts" were "unhistoric" (i.e., not worthy of being recorded in history books alongside great battles and lives of kings and queens), doesn't make them any the less important in the long run.
The ending of Middlemarch might not be "happy" in the traditional sense – after all, the final words are "unvisited tombs" – but maybe it's not as pessimistic as it at first might seem. We're all performing "unhistoric acts" everyday, and although we might not see all the consequences of what we do, the effects still could be as "incalculably diffusive" as Dorothea's.