And yet the course of true love does not run quite as smoothly as one would under the circumstances expect.
A reference to Shakespeare's A Midsummer Night's Dream: "The course of true love never did run smooth."

…there was thrust out an evil yellow face, a terrible animal face, all seamed and scored with vile passions.
Like Dr. Mortimer, Watson apparently believes that there is a criminal "type"—a throwback to a less evolved human being who is closer to our animal origins.

Late nineteenth-century criminology focused on the vain hope that criminals could be recognized by certain physical traits. Cesare Lombroso (1835-1909), Italian scientist, first developed his theory of the "born," or "atavistic," criminal in 1872. Lombroso believed that this type of criminal was a throwback to an earlier stage of evolution—the Neanderthal—both physically and psychologically, and therefore lacked an ethical sense. The criminal's face and body were marked by what Lombroso called "stigmata."

Other researchers often had trouble reproducing Lombroso's results, probably because his methods were slipshod. In The Female Offender Lombroso complains that no one but he could find 33 "criminal anomalies" in the dimensions of the skull of Charlotte Corday, murderer of Marat.

Lombroso's measurements of the skulls of female criminals and prostitutes	Skull of Charlotte Corday