Are you looking for something in the story that parallels this line?  If so, you need to figure out what the line means.  The waltzers, in this case, are really dancers.  "Ceased their evolutions" means they stopped dancing.  The interesting word here is "perforce".  As you can probably guess, "perforce" means necessarily or unavoidably.  So, something has made the dancers stop dancing.

You probably know that the story is very symbolic.  Your teacher probably talked to you about the clock, the black room, and the guy in the costume.  Think about what those things symbolize.  Which of these symbols causes the people to stop dancing perforce?  How does the cessation (stopping) of dancing fit in with the symbols to form a message?