Science and the Criminal

by C. Ainsworth Mitchell

According to Dr. Faulds, the Chinese from time immemorial have caused their convicted criminals to make impressions of their finger-tips as a record, but he gives no details of their system of classifying the prints, if such exists.

The curious markings upon which are based these systems of identification are not confined to the human race, but are also shown by monkeys and to a less pronounced extent by other animals.

The pattern upon the surface of the skin upon the palms of the hand and soles of the feet is formed by the arrangement of what is known as the papillary ridges. It is readily recorded by carefully coating the finger-tips with a fine layer of printing or ordinary ink and pressing them upon paper so as to leave an imprint of the markings upon the finger.

The uses of these ridges is to assist the delicacy of touch, and also to excrete perspiration through the minute pores with which they are covered.

The effect of rough work upon the ridges is to increase their height, and eventually they may become covered up by the horny accretions known as callosities. On the other hand, the ridges upon the palms of people who do very little manual labour are much less apparent, and when the skin is thin are very low. Hence, in the hands of bedridden invalids there is only a slight development of the ridges.

Several circumstances may lead to a temporary obliteration of the ridges, such as, for instance, the constant puncturing of the skin by the head of a needle in sewing, and the imprint of the forefinger of a tailor will therefore often present a very characteristic mottled appearance.

More permanent alterations are produced by cuts or by wounds that have healed and left a white scar. An instance of this is seen in D in the plate ([p. 66]), which represents a print of the left-hand thumb of the present writer. Running across the ridges, and breaking their continuity is a line which marks the place where twenty years ago the slip of a knife nearly severed a piece from the thumb. The effect of this cut has been to add a fresh feature of identity to those furnished by the original ridges, without interfering with the identification of the latter.

In the case of jagged cuts or of scars formed in the healing of an ulcer the ridges may be so distorted as to be practically indistinguishable in that place, or they may even be entirely obliterated. Old age has also an obliterating effect upon the ridges, so much so that the finger-prints of an old man frequently exhibit transverse white markings, indicating signs of the surface disintegration of the skin.

A most important point in the application of finger-prints to the identification of the individual is the persistence of the main details throughout life, since otherwise much of the value of the method would be lost. The observations made by Sir William Herschel in India showed that after the lapse of twenty years there was so little change in the finger-prints of a large number of persons that they could still readily be identified in this way.

Sir Francis Galton has also proved the persistence of the general peculiarities in the prints for periods of over thirty years. He points out that an exact correspondence as to the minutiæ is not always to be expected, since what appears to be a ridge in one print may be really the result of imperfect printing of an enclosure. Apart from possible imperfections in the method, there is also a possibility of variation due to the effect of age rendering the ridges less continuous.