182 Life and Letters of Francis Galton
markings, whether they be on the finger, on other parts of the palmar surface of the hand, or on the sole of the foot. At the same time they are out of all proportion more numerous than any other measurable feature ; about thirty-five of them are situated on the bulb of each of the ten digits, in addition to more than 100 on the ball of the thumb, which is not one-fifth of the superficies of the rest of the palmar surface. The total number of points suitable for comparison on the two hands must therefore be not less than one thousand and nearer to two ; an estimate which I verified by a rough count on my own hand ; similarly in respect of the feet. The dimensions of the limbs and body alter in the course of growth and decay ; the colour, quantity and quality of the hair, the tint and quality of the skin, the number and set of the teeth, the expression of the features, the gestures, the handwriting, even the eye colour, change after many years. There seems no persistence in the visible parts of the body, except in these minute
and hitherto too much disregarded ridges." (pp. 97-8.)
Chapter VII (pp. 100-113) is entitled Evidential Value. Its object is to give an approximate numerical idea of the value of finger-prints as a measure of Personal Identification. Galton's method is a somewhat elaborate one. If we take a square of one ridge interval, and place this on our finger-print, we can almost certainly draw on its surface correctly the ridge or ridges which lie behind it. When we take an opaque square of side 6-ridge intervals, and fasten this blank square to the finger-print and then reconstruct the system of ridges which lies behind it we are rather more often wrong than right in our reconstructed ridges. Galton thinks that a square of 5-ridge intervals would probably allow reconstruction as often right as wrong. He made two series of experiments of this character, with the enlargements double and sixfold. Then he made a twentyfold enlargement, and placed upon it a chequerboard arrangement of 6-ridge interval squares ; he reconstructed the whole finger-print, each square from the four adjacent ones, which bordered the unseen square. There were in this case seven rightly and sixteen wrongly constructed. He now makes a rather drastic assumption
"that any one of these reconstructions represents lineations that might have occurred in Nature, in association with the conditions outside the square, just as well as the lineations of the actual finger-print (p. 107).... It therefore seems right to look upon the squares as independent variables, in the sense that when the surrounding conditions are alone taken into account, the ridges may either run in the observed way or in a different way, the chance of these two contrasted events
being taken (for safety's sake) as approximately equal." (p. 108.)
There being about 24 6-ridge interval squares in any finger-print, Galton makes 1/2' to be the chance of the actual system of ridges appearing. He now proceeds to give a rough approximation to two other chances, which he considers to be involved : the first concerns guessing correctly the general course of the ridges adjacent to each square, and the second of guessing rightly the number of ridges that enter and issue from the square. He takes these in round numbers to be 1/2' and 1/28, so that the whole chance of the observed system is 1/2'. Now the total number of persons in the world has been reckoned at about 16,000,000,000 and the chance of a particular observed arrangement is of the order 1/64,000,000,000, or the odds are very roughly 39 to 1 against the particular arrangement occurring on a single definite digit of any existing human being*.
* Galton in his own copy has a pencil note "repeat calculations" and corrects the total population of the world which in his text he has made ten times too great. I have corrected the figures in the last paragraph of his p. 110 accordingly.