So your grave, middle-aged family practitioner vanishes into thin air, my dear Watson, and there emerges a young fellow under thirty, amiable, unambitious, absent-minded, and the possessor of a favourite dog, which I should describe roughly as being larger than a terrier and smaller than a mastiff." I laughed incredulously as Sherlock Holmes leaned back in his settee and blew little wavering rings of smoke up to the ceiling. "As to the latter part, I have no means of checking you," said I, "but at least it is not difficult to find out a few particulars about the man's age and professional career." From my small medical shelf I took down the Medical Directory and turned up the name. There were several Mortimers, but only one who could be our visitor. I read his record aloud. "Mortimer, James, M.R.C.S., 1882, Grimpen, Dartmoor, Devon. House-surgeon, from 1882 to 1884, at Charing Cross Hospital. Winner of the Jackson prize for Comparative Pathology, with essay entitled 'Is Disease a Reversion?' Corresponding member of the Swedish Pathological Society. Author of 'Some Freaks of Atavism' (Lancet 1882). Do We Progress?' (Journal of Psychology, March, 1883).