All my unspoken instincts, my vague suspicions, suddenly took shape and centred upon the naturalist. In that impassive, colourless man, with his straw hat and his butterfly-net, I seemed to see something terrible—a creature of infinite patience and craft, with a smiling face and a murderous heart.
The criminal breaks the physiognomy mold, because his physical features do not match his personality or his behavior. As a result, the criminal is an adversary worthy of Holmes' skills, because he hides his evil under a benign surface. His class and entitlement are reflected in his dress and mannerisms, as well as his intelligence and education.
We do not get a chance to evaluate the difference between the criminal's behavior and his appearance. Although Doyle openly expressed his distaste for mystery stories that "fake it," or do not give their readers all they need to know, Doyle springs the identity of the killer on us with very little fanfare. Like Watson, we are dumbfounded by Holmes' announcement that Stapleton and his sister are married, that Stapleton is in fact a Baskerville. Though Watson is ready to take Holmes' word for it, we are not as convinced. Doyle does not give us a terribly compelling picture of a wolf in sheep's clothing, so we just need to accept Watson's trust in Holmes' intuition.