Why didn't Holmes and Watson have Stapleton arrested after Seldon's death?
In Ch.12 Selden is attacked by the hound because he is wearing the clothes of Sir Henry Baskerville which he had given to his housekeeper Barrymore, Selden's brother in law. While trying to escape from the hound Selden falls down a steep cliff and dies. Holmes and Watson who discover his body are convinced that Stapleton is responsible for his death. Watson remarks, "Why should we not seize him (Stapleton) atonce?" To which Holmes replies. "Our case is not complete...It is not what we know, but what we can prove." A little later Holmes remarks "We could prove nothing against him," because he (Stpleton) is not acting through a human agent but through an animal and that they "should be laughed out of court" if they tried to produce the hound as evidence in court. Holmes conclusively states that, "there was no direct connection between the hound and the man's (Selden's) death."
Holmes' opinion is that only after establishing a definite connection between Stapleton, the hound, and his victims should Stapleton be arrested and produced in court.