Who was Stapleton? Why did no one know that he was a Baskerville? (chapter 15)

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. Born Rodger Baskerville, he has used several aliases in the past, including Mr. Vandeleur, and once managed a school. He is discovered to be a nephew of Sir Charles Baskerville, meaning he is therefore Sir Henry's cousin.