The individual boards vary from 3-1/4 to 3-1/2 inches, with a few
narrower ones where they fit up against the doorways. I don't see how
the gaps will close, because where there is filler, it runs the length
of the plank, with the filler popping out in bits and pieces only here
and there (in other words, I think the remaining filler would prevent
the gap closing with humidity). Also, where the filler is chipping
out, the missing bits don't extend the entire depth of the plank; it's
just creating little crevices in the thin line of filler.
Not all of the boards have this; some of them fit together tightly. I
suspect that when "whoever" decided to finish the wood in order to use
the subfloor as an exposed wood floor, the planks had shrunk here and
there since the original construction, and they filled in the gaps
where that had happened before finishing the floor.
It really is not very noticeable, so is not particularly unattractive.
I am a little concerned that the filler below where it is chipped does
not have the surface finish, so might take on water when I wash the
floors (although I do this with a nearly dry mop). My bigger concern
is simply that I'm a new homeowner and I want to take care of my house
properly. It's stood here for more than 100 years -- I don't want it
to crumble down on my watch <g>!