Who is Stapleton? How do he and Watson meet?

Mr. Jack Stapleton, a "naturalist" who studies butterflies and plants out on the moor, presents himself as an eccentric but is actually the mystery's insidious villain. As Watson is walking back to Baskerville, Stapleton "the naturalist" overtakes him (163). Having learned about Watson from Dr. Mortimer, Stapleton shares his own theory about Sir Charles's death: the man's anxieties had grown so great that the appearance of a random dog led to his death. Stapleton also surprises Watson by asking about Holmes's opinions on the matter; he insists that the detective is well-known even on the moor.