There are, as has been mentioned, two kinds of minute glands in the skin: one, to secrete that complex excretion, the sweat; the other, to provide a certain greasiness to hair. The latter are found chiefly in other parts than the palms, and serve to secure that slight oiliness of the surface of our bodies which is very well seen in taking one’s bath. However thoroughly that thin film of surface greasiness is removed with the use of soap and vigorous scrubbing, in a moment or two water is seen to act on the cutaneous surface as it would on a slightly greasy platter or a duck’s back. The importance of this point will become apparent when we come to deal with some practical applications of dactylography in searching for invisible greasy finger-marks, which may be made visible.
Looking carefully at the visible texture of the fingers and palms, we see, then, that the cutaneous ridges lie, for the most part, closely and evenly, like furrows in a well-ploughed field. But just as in some fields the ploughman has perforce had to swerve and veer round some fast embedded boulder or old tree stump, varying his intended pattern, so, too, in our fingers curious divergent lineations are found to occur, and we cannot very well tell the reason why. Coloured patches may be designed like so many pretty wall-paper designs, to enclose these patterns in books on finger-prints, but I, for one, cannot see that they throw any light on their genuine nature and origin. We find, under purely mechanical conditions, similar patterns produced in the ripples of a sub-aerial sand-drift and on a tidal shore. While writing this chapter, I saw to-day similar deltas, junctions, forks, and the like, on a lake whose frozen surface was thinly sprinkled with fine dry snow. The lines were mostly parallel, but where certain gusts or eddies had occurred they had been broken up into patterns not unlike those of finger-tips.
Ripple Marks in Sand (After Lyell).
In human skin, and in the anthropoid apes, those scroll-like patterns present almost infinite varieties of detail, and they often resemble a condensed railway plan, showing junctions, blind sidings, loops, triangles, and curves. There is one important distinction to be observed. The lineations of skin ridges are not always quite uniform in breadth, but broaden out sometimes or dwindle away. Again, they are dotted with sweat-pores and do not always, when printed from, show those pores in the same degree of patency or openness. Hence a little variation is inevitable when the same finger is several times impressed under varying conditions. It is not to be forgotten that, to a limited extent, this is true of a rigid box-wood engraving or steel plate, or lithographic stone, which give somewhat divergent results with varying degrees of pressure in printing, moisture of atmosphere or paper, and other conditions.
In this country the feet do not afford a favourable field of study to the dactylographer. So far as identification is concerned, little use could be made of them practically. In the East, however, it is different, and many years’ residence there gave me opportunities to observe that the toes, unrestrained by the use of stiff leather boots, are mobile and powerful, grasping as fingers do. The carpenter in Japan, for example, uses his toes to grip and steady the board he is sawing or hewing, while many of my readers must be familiar with the extraordinary agility of Japanese acrobats in the use of their feet and toes. In those cases the ridges are often varied in grouping, and well defined in development. A European baby generally begins life with similar simian-like powers. But so far as my own observations go, the patterns in the hands usually show a somewhat higher degree of evolution, a more complex and intricate network of lines, than those exhibited by the feet of the same person. Hence, apart from the greater convenience of inspecting them, the finger-prints have greater value for the purpose of identification. Cases, however, of crime, might readily occur even in this country, where the imprints of naked feet might yield important and irrefutable evidence of one’s presence at a scene of evil-doing.
But there are other important points of scientific interest besides their evidential value for identification. An important problem in evolutionary development, on which a considerable amount of literature begins to accumulate, is the serial relation of the limbs. Professor Bowditch, the distinguished biologist, of Harvard University, U.S., wrote me, of date November 18th, 1880, thus:—
“Dear Sir,—I have just read in Nature of October 28th, your article on the skin-furrows of the hand. The subject interested me because it so happened that fourteen years ago, at the suggestion of the late Professor Jeffries Wyman, I made some prints of the finger and toe tips with the hope of throwing some light on the question of the antero-posterior symmetry of the body. Since reading your article I have made some new impressions from the same individual, and it is interesting to notice the unchanged character of the cutaneous furrows.”
Some additional particulars are added in the letter, and a fine finger imprint was enclosed.
It is well to remember that the comparison of the ridges to those of a ploughed field does not always, and in every way, hold good. As I have elsewhere said:[C]