In chapter 14, why does Holmes say they can't arrest Stapleton?

In Chapter Fourteen, Watson believes they have enough evidence to arrest Stapleton, but Holmes does not agree. Holmes notes that hearing a hound doesn't mean the hound was responsible for either death (Sir Charles or Selden). As far as Holmes is concerned, Selden's death remains the result of a fall, and Sir Charles' death is attributed a fright. He cannot lonk a motive, thus, he disagrees with Watson.