It did not seem necessary to repeat the calculation for couplets of digits of different names, situated on opposite hands, as those that were calculated on closely the same data for similar couplets situated on the same hands, suffice for both. It is evident from the irregularity in the run of the figures that the units in the several entries cannot be more than vaguely approximate. They have, however, been retained, as being possibly better than nothing at all.
TABLE VIII.
_Approximate Measures of Relationship between the various Digits, on a Centesimal Scale._
(0 = no relationship; 100 = the utmost feasible likeness.)
+------------------------------------------------------------+Couplets.Loops.Whorls.Means.---------------------------------------------------------_Digits of the same name._Right and left thumbs576461" " fore-fingers375948" " middle fingers345243" " ring fingers617065---------------------------------------------------------Means476154---------------------------------------------------------_Digits of different names onthe same or on opposite hands._Thumb and fore-finger192924" middle finger193427" ring-finger334439Fore and middle finger526860" ring finger133423Middle and ring finger317452---------------------------------------------------------Means284737+------------------------------------------------------------+
The arches were sufficiently numerous in the fore-fingers (17 per cent) to fully justify the application of this method of calculation. The result was 43, which agrees fairly with 48, the mean of the loops and the whorls. In the middle finger the frequency of the arches was only half the above amount and barely suffices for calculation. It gave the result of 38, which also agrees fairly with 43, the mean of the loops and the whorls for that finger.
Some definite results may be gathered from this table notwithstanding the irregularity with which the figures run. Its upper and lower halves clearly belong to different statistical groups, the entries in the former being almost uniformly larger than those in the latter, in the proportion of 54 to 37, say 3 to 2, which roughly represents in numerical terms the nearer relationship between digits of the same name, as compared to that between digits of different names. It seems also that of the 6 couplets of digits bearing different names, the relationship is closest between the middle finger and the two adjacent ones (60 and 52, as against 24, 27, 39 and 23). It is further seen in every pair of entries that whorls are related together more closely than loops. I note this, but cannot explain it. So far as my statistical inquiries into heredity have hitherto gone, all peculiarities were found to follow the same law of transmission, none being more surely inherited than others. If there were a tendency in any one out of many alternative characters to be more heritable than the rest, that character would become universally prevalent, in the absence of restraining influences. But it does not follow that there are no peculiar restraining influences here, nor that what is true for heredity, should be true, in all its details, as regards the relationships between the different digits.
CHAPTER IX
METHODS OF INDEXING
In this chapter the system of cla.s.sification by Arches, Loops, and Whorls described in Chapter V. will be used for indexing two, three, six or ten digits, as the case may be.
An index to each set of finger marks made by the same person, is needful in almost every kind of inquiry, whether it be for descriptive purposes, for investigations into race and heredity, or into questions of symmetry and correlation. It is essential to possess an index to the finger marks of known criminals before the method of finger prints can be utilised as an organised means of detection.
The ideal index might be conceived to consist of a considerable number of compartments, or their equivalents, each bearing a different index-heading, into which the sets of finger prints of different persons may be severally sorted, so that all similar sets shall lie in the same compartment.
The principle of the proposed method of index-headings is, that they should depend upon a few conspicuous differences of pattern in many fingers, and not upon many minute differences in a few fingers. It is carried into effect by distinguishing the A. L. W. cla.s.s of pattern on each digit in succession, by a letter,--_a_ for Arch, _l_ for Loop, _w_ for Whorl; or else, as an alternative method, to subdivide _l_ by using _i_ for a loop with an Inner slope, and _o_ for one with an Outer slope, as the case may be. In this way, the cla.s.s of pattern in each set of ten digits is described by a sequence of ten letters, the various combinations of which are alphabetically arranged and form the different index-headings. Let us now discuss the best method of carrying out this principle, by collating the results of alternative methods of applying it.
We have to consider the utility of the _i_ and _o_ as compared to the simple _l_, and the gain through taking all ten digits into account, instead of only some of them.
It will be instructive to print here an actual index to the finger prints of 100 different persons, who were not in any way selected, but taken as they came, and to use it as the basis of a considerable portion of the following remarks, to be checked where necessary, by results derived from an index to 500 cases, in which these hundred are included.
This index is compiled on the principle shortly to be explained, ent.i.tled the "_i_ and _o_ fore-finger" method.
TABLE IX.--INDEX TO 100 SETS OF FINGER PRINTS.
+------------------------------------+A B C DOrderRight. Left. Rt. Lt.of-----------------------------Entry.F.M.R. F.M.R. T.L. T.L.-----------------------------------1_a a a a a a a a l a_2_ " " a l a l_3_ " " " " _4_ " " w l l l_5_a a l a a l a l a l_6_ " " l l l l_7_ " " " " _8_ " a a w l l l l_9_ " a l l l l l l_10_ " " l w w l_11_ " o l l l l l l_12_a a w a a l l l l l_13_ " a l l l l l l_14_a l a a a a l a l a_15_ " " l a l w_16_ " o l l w l l l_17_a l l a a l l l a l_18_ " " l l l l_19_ " " " " _20_ " " " " _21_ " " " " _22_ " " " " _23_ " a l w l l l l_24_ " i l l l l l l_25_ " " " " _26_a l l i l l w l l l_27_ " o a l w l l l_28_ " o l l w l l l_29_ " w w w w l l l_30_a l w i l w l l l l_31_ " o a l l l l l_32_ " o l l l w l l_33_ " " w l w l_34_ " o l w a l a l_35_i l l a l l w l l l_36_ " " w l w l_37_ " i l l l l l l_38_ " " " " _39_ " " " " _40_ " " " " _41_i l l i l l w l l l_42_ " i w w w l w l_43_i l w i l l l l w l_44_ " " w w w l_45_ " i l w w w w l_46_ " i w l l l l l_47_ " w l w w l w l_48_ " w w l l l l l_49_i w w a l l w l w l_50_ " w w w w l w l_51_ " " " " _52_o a w o l l l l l l_53_o l l o l l l l l l_54_ " " " " _55_ " " " " _56_ " " w l w l_57_ " i l l l l l l_58_ " " " " _59_ " " " " _60_ " o l l l l l l_61_ " " " " _62_ " " " " _63_ " " " " _64_ " " " " _65_ " " " " _66_ " w a l l l w l_67_ " w w w l l w l_68_o l w a l l l l l l_69_ " " w l w l_70_ " i l l w l w l_71_ " o l l l l l l_72_ " " " " _73_ " o l w l l l l_74_ " " " " _75_w l l i l l l l w l_76_ " " " " _77_w l l w l l l l l l_78_ " " " " _79_ " " w l w l_80_ " w l w l l l l_81_w l w o l w l l l l_82_ " " l l a l_83_ " " w l l l_84_ " w w w w l w l_85_ " " w w l l_86_ " " w w l w_87_ " " w w w w_88_ " " " " _89_w w l i l l l l l l_90_ " w l l w l l l_91_w w w o l w w l l l_92_ " w l w w l w l_93_ " " " " _94_ " w w l l l l w_95_ " w w w i l l l_96_ " " w l l l_97_ " " w l w l_98_ " " w w w l_99_ " " " " _100_ " " w w w w_+------------------------------------+
The sequence in which the digits have been registered is not from the thumb outwards to the little finger, but, on account of various good reasons that will be appreciated as we proceed, in the following order.
The ten digits are registered in four groups, which are distinguished in the Index by the letters A, B, C, D:--
A. _First._ The fore, middle, and ring-fingers of the _right_ hand taken in that order.
B. _Second._ The fore, middle, and ring-fingers of the _left_ hand taken in that order.
C. _Third._ The thumb and little finger of the _right_ hand.
D. _Fourth._ The thumb and little finger of the _left_ hand.
Consequently an index-heading will be of the form--
First Second Third Fourth group. group. group. group.
_a a l_ _a a w_ _l l_ _l l_
These index-headings are catalogued in alphabetical order. The method used in the Index is that which takes note of no slopes, except those of loops in the fore-finger of either hand. Consequently the index-heading for my own digits, printed on the t.i.tle-page, is _wlw oll wl wl_. Those of the eight sets in Plate VI. are as follows:--
_i l w i l l w w w l_ _o l w o l w w l l l_ _o l w o l w w l l l_ _o l w o l l l l l l_ _i l w i l w w l w l_ _i l w i w l l l l l_ _i l l w w l l l l l_ _o l l a a l l l a l_ _o a a a a a l a l a_
For convenience of description and reference, the successive entries in the specimen index have been numbered from 1 to 100, but that is no part of the system: those figures would be replaced in a real index by names and addresses.
A preliminary way of obtaining an idea of the differentiating power of an index is to count the number of the different headings that are required to cla.s.sify a specified number of cases. A table is appended which shows the numbers of the headings in the three alternative methods (1) of noting slopes of all kinds in all digits, (2) of noting slopes of Loops only and in the fore-fingers only, and (3) of disregarding the slopes altogether.
Also in each of these three cases taking account of--
(_a_) All the ten digits;
(_b_) the fore, middle, and ring-fingers of both hands;
(_c_) those same three fingers, but of the right hand only;
(_d_) the fore and middle fingers of the right hand.
TABLE X.
_No. of different index-heads in 100 sets of Finger Prints._
+---------------------------------------------------------------+Account taken ofNo. of------------------------------digitsDigits noted.All_i_ and _o_Noregarded.slopes.in fore-slope.fingers.-----------------------------------------------------------10All the 10 digits827671Fore, middle,6and ring-fingers655043of both hands3Of right hand only2516142Fore and middle of1287right hand only+---------------------------------------------------------------+
The column headed "all slopes" refers to the method first used with success, and described in my Memoir, already alluded to (_Proc. Roy.
Soc._, 1891), accompanied by a specimen index, from which the present one was derived. There the direction of the slope of every pattern that has one, is taken into account, and in order to give as much scope as possible to the method, the term Arch (I then called it a Primary) was construed somewhat over-liberally (see p. 114). It was made to include the forked-arch Fig. 12 (~2~), and even the nascent-loop (~9~), so long as not more than a single recurved ridge lay within the outline of the pattern; therefore many of the so-called arches had slopes. It is not necessary to trouble the reader with the numerical nomenclature that was then used, the method itself being now obsolete. Full particulars of it are, however, given in the Memoir.
A somewhat large experience in sorting finger prints in various ways and repeatedly, made it only too evident that the mental strain and risk of error caused by taking all slopes into account was considerable. The judgment became fatigued and the eye puzzled by having to a.s.sign opposite meanings to the same actual direction of a slope in the right and left hands respectively. There was also a frequent doubt as to the existence of a slope in large whorls of the spiral- and circlet-in-loop patterns (Fig.
13, ~21~, ~22~) when the impressions had not been rolled. A third objection is the rarity of the inner slopes in any other digit than the fore-finger. It acted like a soporific to the judgment not only of myself but of others, so that when an inner slope did occur it was apt to be overlooked. The first idea was to discard slopes altogether, notwithstanding the accompanying loss of index power, but this would be an unnecessarily trenchant measure. The slope of a loop, though it be on the fore-finger alone, decidedly merits recognition, for it differentiates such loops into two not very unequal cla.s.ses. Again, there is little chance of mistake in noting it, the impression of the thumb on the one side and those of the remaining fingers on the other, affording easy guidance to the eye and judgment. These considerations determined the method I now use exclusively, by which Table IX. was compiled, and to which the second column of Table X., headed "_i_ and _o_ in fore-fingers,"
refers.
The heading of the third column, "no slope," explains itself, no account having been there taken of any slopes whatever, so _i_ and _o_ disappear, having become merged under _l_.
The table gives a very favourable impression of the differentiating power of all these methods of indexing. By the "_i_ and _o_ fore-finger" method, it requires as many as 76 different index-headings to include the finger prints of 100 different persons, 195 of 300 persons, and 285 of 500.
The number of entries under each index-heading varies greatly; reference to the index of 100 sets showing no less than six entries (Nos. 60-65) under one of them, and four entries (Nos. 18-21 and 37-40) under each of two others. Thus, although a large portion of the 100 sets are solitary entries under their several headings, and can be found by a single reference, the remainder are grouped together like the commoner surnames in a directory. They are troublesome to distinguish, and cannot be subdivided at all except by supplementary characteristics, such as the number of ridges in some specified part of the pattern, or the character of the cores.
In other respects the difference of merit between the three methods is somewhat greater, as is succinctly indicated by the next table.
TABLE XI.--_In 100 Sets._
+-----------------------------------------------------+No. of different index-headings.Number of Entries--------------------------------under the same head.All_i_ and _o_Noslopes.fore-fingersslope.only.--------------------------------------------------17163582108931314...225.........61......13......1--------------------------------------------------Total837671+-----------------------------------------------------+