Who fathered Agnes' daughter?

I found three reviews which say that Nathaniel is maybe, probably or likely the father of Agnes’ daughter but none that says he is not the father. The BookRags study notes take a firm position that The Darter is the father:

“His (The Darter) wife, Sophie, was the woman who had once dated Nathaniel and called herself Agnus. They had had an affair and had married after The Darter impregnated her.”

However, I think that Nathaniel is the father based on this passage on page 266. This was as Nathaniel was thinking about his visit to The Darter at his house. “I imagine the two of them (The Darter and Agnes) – with what? Envy? Relief? Guilt that I had not known what I was responsible for till now?”

I believe that the “what I was responsible for till now” refers to finding out that he got Agnes pregnant. This also makes the story tie together much better too.