In the book, Tuck Everlasting, what is the metaphor Tuck uses when the row boat gets stuck in the pond?

From the text:

"It goes on," Tuck repeated, "to the ocean. But this rowboat now, it's stuck. If we didn't move it out ourself, it would stay here forever, trying to get loose, but stuck. That's what us Tucks are, Winnie. Stuck so's we can't move on. We ain't part of the wheel no more. Dropped off, Winnie. Left behind. And everywhere around us, things is moving and growing and changing. You, for instance. A child now, but someday a woman. And after that, moving on to make room for the new children."