He rarely smiled and went methodically about his business, which was to drive the Italian criminals out of the city and country.

Of course, being a marked man in more senses than one, it was practically impossible to disguise himself, and, accordingly, he had to rely upon his own investigations and detective powers, supplemented by the efforts of the trained men in the Italian branch, many of whom are detectives of a high order of ability.

If the life of Petrosino were to be written, it would be a book unique in the history of criminology and crime, for this man was probably the only great detective of the world to find his career in a foreign country amid criminals of his own race.
I have instanced Petrosino as an example of a police detective of a very unusual type, but I have known several other men on the New York Police Force of real genius in their own particular lines of work.

One of these is an Irishman who makes a specialty of get-rich-quick men, oil and mining stock operators, wire-tappers and their kin, and who knows the antecedents and history of most of them better than any other man in the country.

He is ready to take the part of either a "sucker" or a fellow crook, as the exigencies of the case may demand.
There are detectives--real ones--on the police force of all the great cities of the world to-day, most of them specialists, a few of them geniuses capable of undertaking the ferreting out of any sort of mystery, but the last are rare.