Crime Distinguished from Law-breaking--The General Liability to Crime--Preventive Agencies--Plan of the Work--Different Types of Murders and Robberies--Crime Developed by Civilisation--The Police the Shield and Buckler of Society--Difficulty of Disappearing under Modern Conditions--The Press an Aid to the Police: the Cases of Courvoisier, Müller, and Lefroy--The Importance of Small Clues--"Man Measurement" and Finger-Prints--Strong Scents as Clues--Victims of Blind Chance: the Cases of Troppmann and Peace--Superstitions of Criminals--Dogs and other Animals as Adjuncts to the Police--Australian Blacks as Trackers: Instances of their Almost Superhuman Skill--How Criminals give themselves Away: the Murder of M. Delahache, the Stepney Murder, and other Instances--Cases in which there is Strong but not Sufficient Evidence: the Burdell and Various Other Murders: the Probable Identity of "Jack the Ripper"--Undiscovered Murders: the Rupprecht, Mary Rogers, Nathan, and other Cases: Similar Cases in India: the Burton Crescent Murder: the Murder of Lieutenant Roper--The Balance in Favour of the Police. I.--THE CAUSES OF CRIME. Crime is the transgression by individuals of rules made by the community. Wrong-doing may be either intentional or accidental--a wilful revolt against law, or a lapse through ignorance of it. Both are punishable by all codes alike, but the latter is not necessarily a crime. To constitute a really criminal act the offence must be wilful, perverse, malicious; the offender then becomes the general enemy, to be combated by all good citizens, through their chosen defenders, the police. This warfare has existed from the earliest times; it is in constant progress around us to-day, and it will continue to be waged until the advent of that Millennium in which there is to be no more evil passion to agitate mankind. [Illustration: TYPES OF MALE CRIMINALS. (_From Photographs preserved at the Black Museum, New Scotland Yard._)]