| Quote #1
With the brow of a philosopher above and the jaw of a sensualist below, the man must have started with great capacities for good or for evil. But one could not look upon his cruel blue eyes [...] and the threatening, deep-lined brow without reading Nature's plainest danger-signals. (Empty House.1.68)
Some scientists in this period thought that you could tell a lot about a person's personality, intelligence, etc. through certain physical features. Watson here slightly romanticizes this scientific view and assesses the personality traits indicated by a criminal's physical features.
| Quote #2
"There are some trees, Watson, which grow to a certain height, and then suddenly develop some unsightly eccentricity. You will see it often in humans." (Empty House.2.19)
Holmes's tree metaphor is in response to a discussion about what turns someone to a life of crime. For Holmes, a criminal nature can appear in a person late in life and rather unexpectedly.
| Quote #3
"But he had not that supreme gift of the artist, the knowledge of when to stop. He wished to improve that which was already perfect - to draw the rope tighter yet round the neck of his unfortunate victim - and so he ruined all." (Norwood Builder.207)
It's interesting that Watson describes Holmes as an artist, and Holmes describes good criminals as potential artists as well. For Holmes, work can and should be done well whether it's crime or crime-solving. Holmes respects "artistic" ability, regardless of where it's put to use.