Every prisoner for a month or longer nowadays has his prints taken a little before he is discharged. These prints, if they are not already in the records of Scotland Yard, are added to them, and a number gives the key to the man's record in the Habitual Criminals Registry.
In this manner there has accumulated since 1901, when the system was first put in force, a collection of more than two hundred thousand prints. It is all a matter of system, of scientific and literal exactness, and there is no margin of error. A mistake in identification by finger-prints is literally impossible.
As everyone knows, the ridges at the tips of the fingers maintain their formation from birth to death, and even after. Nothing can change them. It is a possibility, though I believe it has never been known to happen, that there are two people in the world who have the markings on one finger-tip exactly alike. But even that incredible chance is guarded against, by taking the markings of the whole ten fingers. It will be realised how great a miracle it would be for two persons to have exactly the same lines, broken in exactly the same way, in exactly the same order on their two hands. That fact is the root principle of the finger-print work.
It is necessary to point out that the existence of the department is not so much for the purpose of detecting crime as of detecting criminals. In the administration of justice a judge takes the past career of a prisoner into consideration when passing sentence. The main work of the department is to furnish the clue to a past career by scrutinising the finger-prints of persons on remand to discover whether they are habitual criminals or not.
A thousand aliases will not help a man, no change of appearance, no protestations of mistake, if his prints correspond with those in the files. But it is all so simply done. There is nothing spectacular, nothing imposing about the process. Practically all that is needed is a piece of tin, some printer's ink, and a sheet of paper. Within a few minutes afterwards his record can be known.
Compare this with the old Bertillon system of anthropometric measurements. Bertillon's system depends on the fact that after a person reaches maturity certain portions of the body are always the same in measurement. The theory is sound, but the difficulties in the way of applying it are immense.
In his book Sir Edward Henry has pointed out the defects of the system. The instruments are costly, measurers have to be specially trained, and even so may make a mistake—an error of two twenty-fifths of an inch will prevent identification—the search among the records may take an hour or more, and, moreover, through carelessness or inattention, the whole data may be wrong. For six years—from 1895 to 1901—this system was in force at Scotland Yard. The maximum number of identifications in any one year was 500. In 1913, by the aid of finger-prints, 10,607 persons were identified.
Roughly, it is all a matter of classification into "arches," "loops," "whorls," and "composites." It is intricate to describe, but simple to carry out. To the uninitiated it inevitably suggests the old problem "think of a number, double it—."
What happens is this: Every print for primary classification purposes is considered as a loop or a whorl. The fingers are taken in pairs and put down something like this:
L. L. W. L. L.