That people differ from one another has never been a secret, but this knowledge alone cannot form the basis of an identification system.
Personal identification requires:
1) an accepted recognition of individual uniqueness;
2) a means of permanently recording that uniqueness; and
3) a method of sorting that recorded uniqueness into a logical sequence for later retrieval and comparison.
Prior to the nineteenth century, identification systems generally involved dismemberment, the scarring of various body parts, or the forced application of tattoos. Advances in civilization and criminal rights laws put an end to these pragmatic, yet unquestionably barbaric, means of identification by the latter part of the 1800's, but no practical methods were devised to take their place. As a result, fewer and fewer people were seen walking the streets with scarlet A's carved on their foreheads, and more and more people were being brought to trial or admitted to prison as "first offenders."
The absence of a truly effective means of personal identification put the legal authorities of the day at a formidable disadvantage. The only way to identify a criminal as a repeat offender was to have someone present at his trial who could recognize him from a prior offense. This was hardly a reliable method, and the well-intentioned practice of having officers stare "hard and fixedly" at offenders in an attempt to memorize their images did little toward increasing its efficiency.
The advent of photography eased the strain on the officers' memories somewhat, but the inability to categorize and file photographs -- not to mention the effects of age, hair dye and shaving equipment -- eliminated photography as a total solution to the identification conundrum.
One popular method of criminal identification in England during the late 1800's was the Tattoo Index. Since it was noted that many in the criminal profession were "addicted to tattoos," it was deemed advisable to begin detailing their locations and appearance. These records were classified and placed in a central index for future retrieval.
Identifications were made through this index but, quite predictably, criminals soon developed the habit of periodically altering their tattoos.
The net result was that, toward the end of the nineteenth century, it was still impossible to determine accurately the identity of anyone who wished to remain unknown. This gave career criminals an advantage over law enforcement, an advantage that then appeared they would never lose.