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 let them know 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 police headquarters, 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 than a sleuth from 300 Mulberry Street, New York, following a forger to Australasia or Polynesia.
The best agencies absolutely decline to touch divorce and matrimonial cases of any sort.

It does not do a detective agency any good to have its men constantly upon the witness stand subject to attack, with a consequent possible reflection upon their probity of character or truthfulness.