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. The police detective usually lacks the training, education, and social experience to make him effective in dealing with the class of elite criminals who make high society their field. Yet, of course, it is this class of crooks who most excite our interest and who fill the pages of popular detective fiction. The headquarters man has no time nor inclination to follow the sporting duchess and the fictitious earl who accompanies her in their picturesque wanderings around the world. He is busy inside the confines of his own country. Parents or children may disappear, but the mere seeking of oblivion on their part is no crime and does not concern him except by special dispensation on the part of his superiors. Divorced couples may steal their own children back and forth, royalties may inadvertently involve themselves with undesirables, governmental information exude from State portals in a peculiar manner, business secrets pass into the hands of rivals, racehorses develop strange and untimely diseases, husbands take long and mysterious trips from home--a thousand exciting and worrying things may happen to the astonishment, distress, or intense interest of nations, governments, political parties, or private individuals, which from their very nature are outside the purview of the regular police. Here, then, is the field of the secret agent or private detective, and here, forsooth, is where the detective of genuine deductive powers and the polished address of the so-called "man of the world" is required. There are two classes of cases where a private detective must needs be used, if indeed any professional assistance is to be called in: first, where the person whose identity is sought to be discovered or whose activities are sought to be terminated is not a criminal or has committed no crime, and second, where, though a crime has been committed, the injured parties cannot afford to undertake a public prosecution.