But the work of even the “national” agencies is not of the kind which the novel-reading public generally associates with detectives—that is to say, it rarely deals with the unravelling of “mysteries,” except the identity of passers of fraudulent paper and occasional murderers. The protection of the banks is naturally the most important work that such an agency can perform. The National Bankers’ Association consists of eleven thousand members. “Pinkerton’s Bank and Bankers’ Protection” has a large organization of subscribers. These devote themselves to identifying and running down all criminals whose activities are dangerous to them. Here the agency and the police work hand in hand, exchanging photographs of crooks and suspects and keeping closely informed as to each other’s doings. Yet there is no official connection between any detective agency and the police of any city. It is an almost universal rule that a private detective shall not make an arrest. The reasons for this are manifold. In the first place, the private detective has neither the general authority nor the facilities for the manual detention of a criminal. A blue coat and brass buttons, to say nothing of a night stick, are often invaluable stage properties in the last act of the melodrama. And as the criminal authorities are eventually to deal with the defendant anyway, it is just as well if they come into the case as soon as may be. It goes without saying, of course, that a detective _per se_ has no more right to make an arrest than any private citizen—nor has a policeman, for that matter, save in exceptional cases. The officer is valuable for his dignity, avoirdupois, “bracelets,” and other accessories. The police thus get the credit of many arrests in difficult cases where all the work has been done by private detectives, and it is good business for the latter to keep mum about it. One of the chief assets of the big agency is its accumulated information concerning all sorts of professional criminals. Its galleries are quite as complete as those of the local head-quarters, for a constant exchange of art objects is going on with the police throughout the world. And as the agency is protecting banks all over the United States it has greater interest in _all_ bank burglars as a class than the police of any particular city who are only concerned with the burglars who (as one might say) burgle in their particular burg. Thus, you are more likely to find a detective from a national agency following a forger to Australasia or Polynesia than you are a sleuth from 300 Mulberry Street, New York.