"Yes; but you forget that science has often been mistaken in its conclusions," interrupted Sprague, "so that it has constantly been necessary to alter theories to fit newly acquired or better understood facts." "Granted," rejoined Sturgis, "but that is because the interpreters of the evidence are fallible; not because the evidence itself is incomplete. The same cause will always produce the same effect; the same chain of events will invariably terminate in one and the same catastrophe. The apparent deviations from this law are due to unrecognized differences in the producing causes, to additional or missing links in the chain of evidence. Therefore I hold that a criminal, however clever he may be, leaves behind him a complete trace of his every act, from which his crime may be reconstructed with absolute certainty by a competent detective." "In short, 'Murder will out!'" said a man who had been a silent listener to the conversation up to this point. He spoke with a quiet smile, which barely escaped being a polite sneer. Sturgis's keen eyes met his interlocutor's as he replied gravely: "I should hardly care to make so sweeping an affirmation, Doctor Murdock. I have merely stated that the history of every crime is indelibly written in tangible evidence. The writing is on the wall, but of course a blind man cannot see it, nor can an illiterate man understand it. Every event, however trivial, owes its occurrence to a natural cause, and leaves its indelible impress upon nature. The Indian on the trail reads with an experienced eye the story of his enemy's passage, as it has been recorded in trodden turf and broken twigs; while the bloodhound follows, with unerring judgment, a still surer though less tangible trail. The latter's quarry has left behind, at every step, an invisible, imponderable, and yet unmistakable part of itself. Perhaps my meaning can be made clear by an illustration. When a photographer in his dark room takes an exposed plate from his camera, it is apparently a blank; but in reality there is upon this plate the minutely detailed history of an event, which, in proper hands, can be brought before the least competent of observers as irrefutable evidence. Here, the actinic rays of the sunlight are the authors of the evidence; but every natural force, in one way or another, conspires with the detective to run the criminal to earth." "Unless," suggested Murdock, "the ability happens to be on the side of the quarry; in which case, the conspiracy of Nature's forces turns against the hunter." "Ah!" retorted the reporter, "the game is not an equal one. The dice are loaded. For while on the one hand, the detective, if he falls into an error, has a lifetime in which to correct it, any misstep on the part of the criminal is fatal. And who is infallible?"