Wood Floor Help

I own an 1830’s home in Kentucky. The original poplar floors upstairs have some decent sized gaps. I want to fill them in as they are pretty large and snag our feet when walking across them. What’s the best way to tackle this?

On a side note, It looks like the planks had some kind of filler originally (second photo). What would that have likely been? Just grime from over the years or an actual filler?