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.

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 D1, 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 D2 and the inner or ulnar side of D5 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 D1 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 D3 and 4, and their closer functional connec­tion with one another, producing thus a transverse arrangement, and in D1, 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, D1, 3, 4 and 5, omitting D2 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 D2 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 D1 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 D1, 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 D3, 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.

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.