Two months had gone by, and the case had to some extent passed from our minds. Then one morning there came an enigmatic note slipped into our letter box. ‘Dear me, Mr. Holmes. Dear me!’ said this singular epistle. There was neither superscription nor signature. I laughed at the quaint message; but Holmes showed unwonted seriousness.

'Deviltry, Watson!' he remarked, and sat long with a clouded brow.

John Watson, The Valley of Fear

I have a theory that the individual represents in his development the whole procession of his ancestors, and that such a sudden turn to good or evil stands for some strong influence which came into the line of his pedigree. The person becomes, as it were, the epitome of the history of his own family.
I have been guilty of several monographs. They are all upon technical subjects. Here, for example, is one ‘Upon the Distinction between the Ashes of the Various Tobaccos’. In it I enumerate a hundred and forty forms of cigar, cigarette, and pipe tobacco, with coloured plates illustrating the difference in the ash.

Sherlock Holmes, The Sign of the Four