In a case where insanity is the defence, the State must dig up and have at hand every person it can find who knew the accused at any period of his career. He will probably claim that in his youth he was kicked in a game of foot-ball and fractured his skull, that later he fell into an elevator shaft and had concussion of the brain, or that he was hit on the head by a burglar. It is usually difficult, if not impossible, to disprove such assertions, but the prosecutor must be ready, if he can, to show that foot-ball was not invented until after the defendant had attained maturity, that it was some other man who fell down the elevator shaft, and to produce the burglar to deny that the assault occurred. Naturally, complete preparation for an important trial demands the presence of many witnesses who ultimately are not needed and who are never called. Probably in most such cases about half the witnesses do not testify at all. Most of what has been said relates to the preparation for trial of cases where the accused is already under arrest when the district attorney is called into the case. If this stage has not been reached the prosecutor may well be called upon to exercise some of the functions of a detective in the first instance. A few years ago it was brought to the attention of the New York authorities that many blackmailing letters were being received bearing the name of "Lewis Jarvis." These were of a character to render the apprehension of the writer of them a matter of much importance. The letters directed that the replies be sent to a certain box in the New York post-office, but as the boxes are numerous and close together it seemed doubtful if "Lewis Jarvis" could be detected when he called for his mail. The district attorney, the police, and the post-office officials finally evolved the scheme of plugging the lock of "Lewis Jarvis's" box with a match. The scheme worked, for "Jarvis," finding that he could not use his key, went to the delivery window and asked for his mail. The very instant the letters reached his hand the gyves were upon the wrists of one of the best-known attorneys in the city.