"Really, Watson, you excel yourself," said Holmes, pushing back his chair and lighting a cigarette. "I am bound to say that in all the accounts which you have been so good as to give of my own small achievements you have habitually underrated your own abilities. It may be that you are not yourself luminous, but you are a conductor of light. Some people without possessing genius have a remarkable power of stimulating it. I confess, my dear fellow, that I am very much in your debt."
Holmes' backhanded compliment underlies the bizarre relationship between the detective and his crony, Watson. Holmes enjoys leading Watson on, letting him think he has the right answer, when in fact the detective himself is holding all the cards. In as much as Watson serves as a stand-in for the readers, Holmes' coy little encouragement tempts us to try our hand at detection as well, even if we will never be as good at solving mysteries as Sherlock Holmes.
The moon was shining bright upon the clearing, and there in the centre lay the unhappy maid where she had fallen, dead of fear and of fatigue. But it was not the sight of her body, nor yet was it that of the body of Hugo Baskerville lying near her, which raised the hair upon the heads of these three daredevil roysterers, but it was that, standing over Hugo and plucking at his throat, there stood a foul thing, a great, black beast, shaped like a hound, yet larger than any hound that ever mortal eye has rested upon.
The appearance of the old-time manuscript signals a shift in the narrative's format from Watson's straight-up reporting to the land of make-believe, manuscripts and a redoubled dubiousness. Mortimer reads the manuscript in a folk tale style, suggesting the importance of supernatural and fantasy in adding suspense to the plot.
Throughout the novel, Doyle is careful to distance his legendary detective from any implication of inaccuracy, even going so far as to couch the gathering of clues in a Watson-only space up in Devonshire. Here, the mystery that serves so important a function in the novel appears third-hand, and in a manuscript read by Mortimer. The reading of the manuscript serves both to allow the detective some critical distance and to make the mystery that much more ominous.
Over the rocks, in the crevice of which the candle burned, there was thrust out an evil yellow face, a terrible animal face, all seamed and scored with vile passions. Foul with mire, with a bristling beard, and hung with matted hair, it might well have belonged to one of those old savages who dwelt in the burrows on the hillsides. The light beneath him was reflected in his small, cunning eyes which peered fiercely to right and left through the darkness, like a crafty and savage animal who has heard the steps of the hunters.
Physiognomy has a long and illustrious history, from Chaucer's gummy-toothed travelers up through the early twentieth century. The assumption that physical features match personality and temperament comes through strongly in this quote, where the sinful convict ends up looking like a beady-eyed rat.
Interestingly, Doyle's picture of a debaucherous man who looks the part resonates with another novel of the same period, The Picture of Dorian Gray. Oscar Wilde and Arthur Conan Doyle received commissions from the same publisher on the same night in 1889, Wilde for Dorian and Doyle for The Sign of Four. Doyle's physiognomy is also expressive of a classist sentiment, whereby the uneducated, ignoble criminal looks "like a crafty and savage animal" while the evil nobleman looks just like everybody else.
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.