CHAPTER XVIII.

ARRANGEMENT OF THE PAPILLARY RIDGES.

The subjects of the preceding, present, and the succeeding chapter are closely allied, from the fact that they all deal with structural changes in the mammalian skin, and that most of these are exhibited for us on our own palms and soles. They certainly comply with the canons of Henri Poincaré as to simplicity, regularity and chance of recurring.

In the last chapter, papillary ridges as organs of touch were briefly referred to, but their mode of development into complicated patterns do not concern the questions here at issue. The general manner in which they are arranged on the hands and feet of man and the Primates below him is very much a matter for such Lamarckian methods of inquiry as I have chosen. In this examination of the ridges I will proceed from man backwards among the Primates and lower still. I described these ridges, in a book previously referred to in the following words, and find no need to alter them here. “The ridges and adjoining furrows which cover the palmar and plantar surfaces of all Primates and a few lower forms in smaller degree, may be compared to the ridges of a ploughed field over which some object, as a light roller, has been passed, the effect of this being to produce a series of ridges with flattened tops. This can be well seen with a lens when the ridges are examined in profile, and is their normal condition in man and many lower animals, in nearly all the palmar, plantar and digital regions.”[64] The reservation in the last sentence is not material here.

[64] _Sense of Touch in Mammals and Birds._

The Hand of Man.

Beginning with the tips of man’s fingers and excluding the wonderful patterns which Galton did so much to elucidate and bring into order, we find the ridges are placed, to a remarkable extent, parallel with the skin-flexures which will be treated in the next chapter. I term the thumb and fingers D 1, 2, 3, 4, 5 for the sake of accuracy (Fig. 59). Over the last joints (distal) of all the digits the ridges suddenly diverge from their directions in the patterns of the pulps, and become arranged transversely to the axis of the digits. This arrangement is observed on the remaining segments of the digits except, very significantly, on the outer or radial side of D 2 and the inner or ulnar side of D 5 where they slope more or less towards the palm. Their lines thus cross slightly those of the skin-flexures in these small areas. On the radial side of D 1 this slope appears in a minor degree, but here it coincides with those of the flexures. On the palm are similar arrangements of the ridges near the radial and ulnar borders, and especially on the two great eminences, thenar and hypothenar, also at the bases of digits 2, 3, 4 and 5. Over the rest of the palm they are arranged in a longitudinal or oblique direction. These brief descriptions are enough to show the close correspondence of the arrangement of the ridges with the flexion of the numerous joints of the hand. An observer can demonstrate this by holding up the open hand in a good light and flexing the fingers slightly, which brings nearly all the ridges adjacent to the joints into directions parallel with one another, the greater lengths of D 3 and 4, and their closer functional connection with one another, producing thus a transverse arrangement, and in D 1, 2 and 5 a more oblique one. In the palm this correspondence of ridges with flexion lines of joints is not found so much except in the central part of this surface. But the oblique and longitudinal ridges of the palm where it becomes concave in the action of folding the hand over a globular object are well shown there also to correspond with such action.

This general grouping of ridges is seen, _mutatis mutandis_, to belong to all the palms and soles of lower Primates, and the illustrations given will speak for themselves, so that little need be said on each.

Reasons for Arrangement Observed.

When one discusses the forces in action on man’s hand which are claimed to have thus arranged the ridges, in regard to the question of use and habit, little more need be added as to those of other Primates, and it is because we know more about ourselves than them, and our own palms and soles are available for inspection, that I have taken man as the example.

The main question is the old and now familiar one: “Are these ridges arranged as we see them _by_ use and habit, or adapted _for_ use?” Dr. Hepburn and the orthodox Selectionist would say that, of course, their mode of arrangement is an adaptation governed by selection for preventing slipping in the action of grasping an object by the hand, and in the foot for preventing slipping in walking. This does not take into account the question as to how the original slight shifting of the ridges in the earliest man and in lower forms could have had selective or survival value, for example, the insignificant sparse groups of ridges on the palm, sole and _tips_ of the digits in a hedgehog or squirrel. As things are now they _do_ subserve these purposes. But I think this matter of prevention of slipping has been much exaggerated, though I may be told that this is a matter of opinion and not a valid argument against the hypothesis.

Foot of Man.

The point may be best understood by considering the foot of man, of which Fig. 60 shows a good example. The value of the roughened surface of the foot with its papillary ridges can hardly have been great, even in the days when man’s foot was naked, at any rate so little that for him to acquire by a selectional process such a remarkable _change_ of arrangement as we see when we look at the foot of man and of any other Primate involves on our part a tremendous stretch of imagination as to its _modus operandi_. These low, soft ridges of man’s foot could do little to prevent him from slipping on such surfaces as grass, sand, rock, wet or dry, and from the time when he began to protect his feet with coverings this small value would be further reduced. _Underneath his developing plantar arch it would not exist at all, and yet here especially he has changed their direction._ As to the papillary ridges, man’s foot has sadly embarked on the pathway of degeneration much as his little toe has done. Not only has he here a much simpler arrangement than any ape or monkey, but the individual ridges are blurred and flattened on much of the plantar surface. This comes of his pride in acquiring his human distinction, or title of nobility, of a plantar arch and his coincident increase of pedestrian locomotion. On the triple bases of support, heel, ball of great and little toe, the ridges are still strongly marked and coarse; transverse on the heel, whorl-like on the ball of the great toe, and oblique or nearly transverse on that of the little toe. On the rest of the surface they are vulgarly transverse. And I may add that the toe-prints of man are simplicity itself compared with his finger-prints. It would seem that this example of arrangement of ridges on man’s foot is strongly in favour of the hypothesis that they are so disposed by flexion of the foot in walking, and not by some need for prevention of slipping under the guidance of selection.

Lower Animals.

At the other end of the scale the scanty ridges of a hedgehog’s or squirrel’s foot would be negligible in preventing slipping, however useful they would be, as I hold, as early organs of touch. Between these extremes the _slow loris_ affords a valuable example to study, with the help of Fig. 61. The foot, as more concerned with prevention of slipping than the hand, is chosen for observation, but with little exception the hand agrees closely with it. On the tips of four digits, D 1, 3, 4 and 5, omitting D 2 for the moment, the ridges are arranged nearly in a longitudinal direction, and would on that account have little or no effect in preventing slipping of the foot. If this be disputed one can but reply that if the need of preventing slipping in this tiny area were to call forth selective value this is not the arrangement of the ridges that best serves the purpose. It may be remarked here that the pulps of _lemurs_, the _marmoset_ and _squirrel monkey_ all show this indifferent mode of grouping of ridges. The aborted D 2 of the _loris_, with its hooked nail overhanging the circular pattern of ridges, is obviously quite unadapted for any non-slipping effect of its skin, as a glance at the figure shows. On the remaining segments of the digits the ridges in the main slope from each side of each digit in the distal direction and fail here also to obtain the best, or transverse direction for preventing slipping in locomotion. The corresponding surface of D 1 is not different from its pulp as to direction of ridges, and it is here to be noted and admitted that when this muscular great toe is tightly applied to a branch, which from its shape it must cross at a right angle, the non-slipping effect of the longitudinal ridges would be very effective. One must then notice that over the middle of the sole of this foot the ridges have again changed their direction and lie in a transverse direction. Between this and the basis of the digits are three fleshy pads and an intervening area of longitudinal ridges.

The first question that arises in the attempt to analyse so complex a grouping on a strange member like the foot of a _loris_ is this--what is the primary function subserved by the ridges and their mode of arrangement, and what may be their secondary uses? In the book referred to I have maintained throughout, in opposition to Mrs. Wilder Harris and others such as Dr. Hepburn, that the sense of touch is the primary, and prevention of slipping the secondary adaptation secured by the ridges. If this be true (and I know it is _sub judice_) there is a very clear reason why the ridges should be longitudinal on the tips of the digits on account of the better discrimination of small objects secured by this arrangement, though it does not well assist the _loris_ to avoid slipping. On D 1, as mentioned, the non-slipping effect is secured by its ridges, and this digit is necessarily less employed for discrimination than support. On the other hand the sloping arrangement on the rest of the segments of D 3, 4, 5 is decidedly less effective in preventing slipping than a transverse arrangement would have been. I think I am justified in saying that too much has been made of this secondary effect of the ridges in the prevention of slipping. I know that the string wound round the handle of a cricket bat is very effective for its purpose, but one can also understand that a casual strand wound here and there on the handle as the ridges are on a _hedgehog’s_ and _squirrel’s_ hand and foot would be of little use for the purpose.

On the other hand if the view may be entertained that on the palm and sole of _hedgehog_, _squirrel_, _loris_ and man, we have written in rows of papillary ridges and their modes of arrangement a register of long-continued flexion of hand and foot in flexion and correlated actions, we find the facts of these and numerous other Primates agree in a remarkable manner with the hypothesis; whereas the exclusive non-slipping rival has many awkward facts to explain, or disregard.

Further as one has always to bear in mind the Mendelian analysis it should be observed that the extreme variability, within certain limits, of the arrangements of papillary ridges throughout the Primates renders the hypothesis of unit-characters segregated, according to Mendelian laws, wholly inapplicable to the _manner of their arrangement_ even though perhaps not so to the _existence_ of papillary ridges.

It may be bluntly asserted that the ridges are arranged as we find them because, hands and feet being used as they are, the ridges “can do no other,” and that there’s an end of it, and that we cannot derive any help as to the origin of specific difference from such a trifle, the next item on the agenda should be called for. As a piece of dialectics that would be effective, but if taken literally it only goes to prove my simple contention.

It will be enough to mention the hand alone of the remaining series with a note as to each animal.

Fig. 64 gives the hand of a _chimpanzee_ with ridges on the pulps resembling those of all the _apes_, _monkeys_ and _lemurs_, arched groups on the digits and longitudinal ones on the centre of the palm, both of these last two being exactly what would be found arising from the actions of climbing branches and discriminating globular objects in the palm.

Fig. 65 is that of a _gorilla_ and its general features resemble closely those of the _chimpanzee_ and of Fig. 66 which is that of an _orang_.

Fig. 67 of a _Hainan gibbon_ is very different on the palm from the other three apes for its ridges are nearly all longitudinal or slightly oblique, precisely as one would find this part if the _palm_ were used very little for grasping boughs and much for discriminating globular objects procured for its repasts. The wonderful long digits of the gibbon form its main organ for supporting itself on branches and swinging its body rapidly from branch to branch, and the arched or nearly transverse ridges on the digits are placed just as the endless use of them for this purpose would be likely to follow from it. This example is a very clear one for showing, if it exist, the effect of use and habit on the disposition of the ridges.

Fig. 68 shows the arrangement of papillary ridges in a _lemur_ and 69 that of a brown _sapajou_.

Fig. 70 of the _Chacma baboon_, playfully called by the Boers Adonis, is a very active and wary animal which lives on the rough rocky slopes of the Cape. It is very much of a pedestrian and the response of its mode of life and use of its forefoot is shown in five great pads of muscle and efficient whorls of ridges for touch, those on the digits being very nearly all transverse in accordance with simple flexion of these joints. This again is what one would expect if my hypothesis be sound. The purely non-slipping mechanism supposed by the rival view is not here well supported by the facts.

Neither the arrangements of ridges (Fig. 61), in _loris_, nor the _hedgehog_ (Fig. 62), nor the _squirrel_ (Fig. 63), need further reference, but they are all, I think, very consistent with the prolonged effects of use and habit.

Some Undesigned Experiments in Ridges.

This section of the subject has afforded a good supply of indirect evidence, but so far no direct proof that papillary ridges can be created and disposed in their lines by pressure, friction and response. The clearest case is one I brought forward at the Zoological Society of London in 1905, and which was published in its proceedings of April 18th. It was an instance of the hand of a _chimpanzee_ with papillary ridges produced in an aberrant or abnormal situation by walking, and was given as follows:--

“In the course of an examination of the papillary ridges in some specimens of anthropoid apes and monkeys certain groups of ridges were found on the _extensor_ surface of the terminal phalanges of the hand, apparently identical with those of the palmar and plantar surfaces. Three specimens of chimpanzee living in the Society’s menagerie were examined, of the ages: one year eight months, two-and-a-half years and six years. In the oldest of these, called “Mickie,” the ridges were definite and well-developed, on the second, third and fourth digits on both hands; in the youngest specimen, “Jack,” they were absent; and in “Jimmie,” two-and-a-half years old, they were small and ill-defined, as if in process of development.

Direction of Ridges.

_Mickie._ Ridges longitudinal and reaching to the matrix of the nail on the second, third and fourth digits.

_Jimmie._ Showed ridges as follows:--

R. hand 1st D none. L. hand 1st D none. 2nd " oblique. 2nd " oblique. 3rd " transverse at base of D. 3rd " " 4th " " " " 4th " " 5th " nearly longitudinal. 5th " none.

_In these three specimens ridges were absent from the corresponding surfaces of the foot._

“The well-defined longitudinal direction of the ridges in “Mickie” is worth notice. It must be remembered in this connection that a chimpanzee walks with the extensor surfaces of the phalanges touching the ground and the digits turned inwards, so that their long axis are at right angles to the line of progression of the animal, and accordingly the ridges of this part also occupy the same relative position. There is no correlation in this instance between the act of prehension and the direction of the ridges, though it agrees closely with the general rule which obtains in so many regions, that the ridges lie at right angles to the line of incidence of the predominating pressure on the part.”

In this example of ridges developed on an abnormal situation we see what is, perhaps, an undesigned experiment as to the production of ridges by a more frequent habit of walking in captivity than would be found to occur in the wild state, for, as Lydekker says in the _Royal Natural History_, Vol. I, p. 27, “When the chimpanzee goes on all-fours, he generally supports himself on the backs of his closed fingers rather than on the palm of the hand (see Fig. 6 of the illustration on p. 15) and he goes _sometimes_ on the soles of his feet and _sometimes_ on his closed toes.”

I have underlined purposely this word “sometimes,” for in the instance I have described, not only the presence of the ridges and their direction on the backs of the fingers but their absence on the backs of the toes is significant, and I suggest that the _chimpanzees_ examined have not sufficiently often exposed the backs of their toes to pressure and friction for the production of ridges, whereas those on the backs of the fingers have done so. Another point worth notice is that in the oldest of the three _chimpanzees_, “Mickie,” æt six years, the greatest number of ridges is present; in “Jimmie,” æt two-and-a-half years, they were “small and ill-defined as if in process of development,” and in “Jack,” æt twenty months they were absent. This would agree at any rate with the hypothesis that the element of time and frequent repetition of stimuli enter into the causation of aberrant ridges.

A similar condition, with aberrant papillary ridges, has been found on the digits of the hand of the _orang_.

On the heel of adult man ridges are found surrounding it, of the average depth of one inch from the plantar surface, and in one particular case of a woman aged forty-nine, the depth of this area on each foot measured was one and a half inches from the plantar surface.

The extensor surface, or back, of the little toe shows ridges when it is distorted by ill-fitting boots.

In man ridges frequently appear on the radial side of the back or extensor surface of the index finger to nearly the middle line of the finger, and this is often more on the right than the left hand.