Science and the Criminal

by C. Ainsworth Mitchell

In one of the examples given by him finger-prints were taken of a child of two and a half in 1877, and again thirteen years later. Between two of the prints there were forty-two points of resemblance and only one point of difference. This was a small forked ridge which appeared in the print of the baby, but had been filled up in the print from the boy. This instance is mentioned as unique, for in every other case examined by Sir Francis Galton, comparing prints of the boy with the man, and the man with the old man there was perfect correspondence between the selected points. He therefore concludes that “we are justified in inferring that between birth and death there is absolutely no change in, say, 699 out of 700 of the numerous characteristics in the markings of the same person such as can be impressed by them whenever it is desirable to do so.”

An interesting series of photographs was recently exhibited by the Chief Commissioner of the Police. These included the portraits of three men who so closely resembled one another that they would readily have been mistaken for one another in photographs. Their finger-prints, however, were quite distinct.

So persistent are these distinctive markings that they last as long as the skin itself, and may be clearly seen upon the fingers of Egyptian mummies.

However much the general dimensions of the pattern of the prints may be changed by the advance of age or the effect of disease, the number of the pattern will still remain. To use the apt illustration of Sir Francis Galton, the changes to be expected are comparable with those seen in a piece of lace. The material may be stretched in one or the other direction or shrunken to half its former dimensions, but the individual loops and knots may be identified with those in the original fabric.

As is the case with all the other measurements of the human body alterations will occur in the size of the markings; for the pattern as a whole increases with the growth of the finger, but this growth does not affect the arrangement of the loops and ridges that make up the markings upon the skin.

In no other way than a study of the finger-prints is it possible to find over a thousand points of comparison upon which to establish the identity of an individual.

In estimating the value of finger-prints as evidence of identity, Sir Francis Galton found that out of 1,000 thumb-prints the collection could be classified into 100 groups each containing prints with a more or less close resemblance to one another. He further found that on the average it was impossible to put great reliance upon the general resemblance between two given prints as a proof that they were produced by the same finger, though obvious difference was a proof that they were produced by different fingers.

But on studying the minutiæ of the patterns, and calculating the chances that the print of a single finger should agree in all particulars with the print of another finger, he concluded that it was as one is to about sixty-four millions; so that the chance of two persons giving similar prints from a single finger would be less than one in four. If the comparisons were extended to two fingers the improbability of agreement in all details would be squared, “reaching a figure altogether beyond the range of imagination.”

The general conclusion drawn from these numerical results was that even after making all allowance for ambiguities and for possible alterations caused by accident or disease, a complete, or nearly complete, agreement between two prints of one finger and infinitely more so between two or more fingers, afforded evidence, which did not stand in need of corroboration, that the prints were derived from the fingers of one and the same person.

In finger-prints, therefore, we have the only means of proving the identity of an individual beyond all question.