Initiative in Evolution

Skin of Palm and Sole.

One can examine in more detail the remarkable form of skin which is found to cover the palmar and plantar surfaces in many mammals. It is highly specialised and appears in many degrees of efficiency for the purposes, or uses, of walking and climbing, grasping and discrimina­tion of objects. With two or three insignificant exceptions these are the only regions even of man’s body where hairs do not grow in the normal state, and in most other mammals hair is absent from the component parts or pads, which correspond to our palms and soles. In the absence of hairs and sebaceous glands and the presence of as many as 320 sweat-glands to the square centimetre, and especially the papillary ridges, the mammalian hand and foot present a fruitful field for study. They have been studied by none more earnestly and thoroughly than Dr. H. Wilder Harris and Mrs. Wilder Harris (née Inez Whipple). This small area of skin as an organ for grasping and discrimina­tion has been studied by persons from different, but not conflicting points of view. Time would fail me even to mention these, but I would recall here one aspect of the matter, that is the name given to it by these eminent authorities, Friction Skin. I think I do them no injustice, nay even honour, when I claim them as allies for us “Old Contemptibles” in the struggle, Lamarck v. Darwin in respect of these characters of the “mammalian chiridium.” This is a term employed by them for the hand and foot of all mammals, and is very convenient for descriptive purposes. From this point of view this organ has been produced from more generalised ancestral structures by reason of friction and pressure, and not for the purpose of resisting them, at least in their initial stages—again, adapted by and not adapted for meeting those forces. There are other views of the matter held by Pan-Selectionists, notably that of Dr. Hepburn, in regard to the papillary ridges. He would, as I gather, treat them as primarily induced, by selection, for the better grasping of objects cylindrical or more or less globular. I have referred elsewhere63 at some length to this in a book describing the examina­tion of the hands and feet of eighty-six species of mammals. The varieties of epidermis were divided into the smooth, corrugated, scaly, nodular, hairy, rod-like and ridge-covered forms, also four mixed varieties, such as corrugated with coarse transverse ridges on the digits, corrugated with papillary ridges, nodular with papillary ridges, and hairy with coarse transverse ridges and smooth pads. Of these the species with smooth epidermis and hair are few and unimportant, and the largest group examined was that of the Primates, thirty in all, in which papillary ridges were always present. It is highly probable that the causes of these modifications of the epidermis in diverse groups of animals could be traced to the habits and modes of life of each, but I make no attempt here to do this. It is also matter for inquiry, upon which no agreement has apparently been reached, how it came to pass that man has virtually lost his hairy coat, and in regard to the palms and soles of animals, what may be the reason that so few have any hair on them, and why man has no sebaceous glands, but has very numerous sweat-glands in these regions.

This is all of great interest, and possibly some day the Mendelians will solve for us the mysteries thereof. But here I need only ask how it would have been possible for hairs to grow, or, if growing, not to be promptly worn away on a surface used by animals from monotreme to man for walking-pads, and by most of them also for grasping and discrimina­tion between objects as well. We are so familiar with the thickening of the skin on the hands of manual workers and on the feet of those who walk much, to say nothing of what we call a “corn,” from pressure of tight boots, that we are in danger of forgetting that the protecting skin over the hands and feet of animals was of necessity adjusted in a crude way to the measure and kind of walking in past ages and in all levels of life, and that it is maintained in that adjusted condition by the use, or disuse, of each life. Another familiar example is that of knee-pads, as in the gnu and other ungulates. Some such process it is legitimate to assume whether it be reckoned backwards to monotremes or later levels of life-forms. We see then before our eyes how this living tissue becomes adapted in varied ways by response to the stimuli of friction and pressure, and the modifications thus slowly effected must, one would suppose, be transmitted to offspring ultimately from the original groups with which the process began, when by frequent repeti­tion small changes of structure have arisen at last. I acknowledge the limited force of the answer, that this picture involves the continuance in each succeeding genera­tion of the stimuli which initiated the changes, but the fact remains that ex hypothesi the changes are there, written in tablets of animal tissue, and that the making-up of an organism in course of many ages is not and cannot be conceived as being governed alone by the “tyranny,” even in the good Greek sense of that word, of rigid unit-characters.

In the assumed process the correcting force of the Lamarckian drill-sergeant is always at hand, as it superintends the construc­tion of tissues and parts, and I doubt if even Professor Thomson will here interpose the difficulty of “correla­tion with useful characters,” for the only important functions which are invoked as the invariable antecedent of these structures are the elementary habits of walking, climbing or grasping objects in certain different ways, and without these habits or functions there would be neither lemur, monkey nor man to interest the mind of a biologist from Mars. As I am desirous of condensing such replies as I can make to certain opinions of opponents and objections, I will remind the reader that Professor Bateson in the Jubilee Volume of 1909, pp. 100, 101, uses a metaphor to illustrate his view that among the facts of nature we meet certain definite structures and patterns in which we ought not, if desiring rightly to interpret them, to expect to find purposefulness. He says: “Such things are, as often as not, I suspect rather of the nature of tool-marks, mere incidents of manufacture, benefiting their possessor not more than the wire-marks in a sheet of paper, or the ribbing on the bottom of an oriental plate renders these objects more attractive in our eyes.” Metaphors are both indispensable and delightful, they are the very salt of scientific and other sober writings, but they have a rather “slim” way of betraying their employers. They express at times the truth too well, and at others when vague and inaccurate lead the reader right astray. Thinking of this metaphor of tool marks I was in a modern church the other day and saw just before me a stone pillar the pediment of which was marked with oblique parallel marks of a mason’s tool. Here then there were marks left by a human hand at some date or other and by means of some tool or other. I know one may not reason by an analogy from inorganic to organic phenomena in which the push and force of life is in full blast, and that inheritance in the former is ruled out; but, taking the metaphor seriously, you have to account for the appearance of the ribbing of paper and the mason’s marks on the stone. To call them “by-products” or “tool marks” or obiter facta, or by any suggestive name, does not advance the reply to the question, “Whence came this great multitude?” If I were unwary enough to be here trying to attack Selection and to respond to the invita­tion of the more learned arachnida to walk into his parlour with a scheme of organic evolution for him to demolish at his leisure, I should have to enter upon the question of adapta­tion, specific difference and perhaps other great disputed doctrines. But, knowing my own limits, and desiring to keep to the self-imposed limits of the title of this book, I again plead that I am here contending, as all through it, for the origin of initial modifications by use and habit, and for nothing else. No one who reads of the immense amount of research and learning that are being carried on by the students of Mendelism and Mutationism can fail to admire them. But, as I have remarked before, these are systems of thought which in the main deal with characters by distribu­tion or “unpacking,” as it is called. Such a process of course leads to new characters by amphimixis, and no one of whom I know denies it. Such work is concerned with fresh views of the origin of species, but with lamentable cowardice, or humility, I leave all that great sphere to those who are incomparably more fit for it, and just seek to mind my own business.

In subsequent chapters on modifications and their origin I shall not need to repeat these observations.

Some Chosen Examples of Palms and Soles.

The facts then of a few selected examples of the palms and soles of mammals are shortly these.

A heavy, burrowing animal, the earth wolf of the Cape, has a very smooth, hard epidermis covering its foot-pads and is thus a generalised structure which I have found in no other animal.

The common mole which uses its broad strong fore-feet like a pair of spades, and depends chiefly for discrimina­tion of its habitat on the delicate sensory nerve-endings of its snout, has a hard nodular skin which is much less developed on the hind feet than the fore feet, the latter being less active tools. It has no papillary ridges, in accordance with this fact, and is a very efficient miner that never practises ca’ canny, as we know to our cost when we go out in the morning and find great heaps of soft earth thrown up in the line of its advance from its base or fortress. Such a mode of life lends itself remarkably to the kind of skin on its feet, and this is now at any rate adapted to its environment.

The capybara is a large, heavily-built rodent, and has rather a smooth epidermis not specially thick, with long and efficient papillæ of the corium shown in microscopical sections. Being largely aquatic in its habitat, and given to frequenting marshy ground and to enjoying as much sleep as it can manage, it depends a good deal for discrimina­tion of objects on its sensitive corium, and its epidermis is not much specialised for, or by friction and pressure in walking. It does not acquire by reason of stimuli and response any unnecessary tools.

With this may be classed the echidna or Australian ant-eater which has sparse hairs set on a hard and slightly corrugated epidermis, and, being mainly a nocturnal animal and living a secluded life, it does not walk much or far in its stealthy pursuit of worms and insects, and the stimuli of friction or pressure encountered by it are few.

A similar condition is found on the feet of many small carnivores.

Animals with scales on their feet, which are held to constitute the earliest stage of the Primate modifica­tion of papillary ridges are such as the potoroo, wallaby, kangaroo and giant ant-eater. Such scales register a long, long series of stimuli of friction and pressure in these and their ancestors, in a level of life before any delicate discrimina­tion of surfaces came into operation.

The nodular form of skin is present in the Canadian tree porcupine, where rough nodules cluster closely on the surface of both feet, and it is a significant fact that it shares with the American opossum the peculiarity of nodules on the ventral surface of the powerful prehensile tail. This adapta­tion tends to efficiency in its arboreal life, and may well have been produced by infinitely small degrees of response in structure in the course of a long evolution.

The rabbit alone have I found with rod-like projections of the epidermic cells, among which are set in dense order the soft, long, delicate hairs and which thus conduce to its wonderful power of treading on sharp objects without injury. We thus see the inner meaning of dear old Brer Rabbit’s jeer of triumph to Brer Fox, “Born and bred in a brier bush.” This adapta­tion might be an unit-character segregated from the ancestral stock of the Leporidæ, or it might not, but at any rate the rabbit leads a life in which its walking or running is no more prominent or frequent than is a good “run” on the part of a hunter which pursues the hare with his beagles, and one may say at least this—that its mode of life has not produced a hard rough nodular surface on its feet by stimuli of pressure and friction and response.

One may observe that there’s a divinity doth shape our ends, rough hew them as we may, even if some objection be taken to the present view of rough-hewing of parts of our organism on the ground of its piecemeal character, rather than dealing with the organism as a whole. To which it may be replied that the Mendelians give high support to the piecemeal study of the profound subject of genetics, and further that the business here is to look separately and simply at a few selected attributes of parts of an organism, and see how they began to grow big enough to avoid passing through the meshes of the sieve.

The foregoing examples of animals in which papillary ridges are absent have been given not in their zoological order, nor as representative of a great many groups, but as taken from the eighty-six species I examined myself. The following belong to the same series, but all present papillary ridges in an ascending scale towards perfec­tion in man.

Examples of Ridge-covered Palms and Soles.

The common hedgehog though a burrowing animal like the mole is not always underground as his distant relative is. He is not always mining and though of ancient lineage he is a “slacker” compared with the mole, hibernating for months, and spending also much time in his nest and prowling slowly about above ground for insects. He has thus acquired his somewhat indifferent epidermis that one finds, but with the addition of sparse papillary ridges. It is the species among this list with the fewest of these tactile structures, for there are but three or four separate ridges on six of the ten digits, and radiating groups on only three of all the palmar and plantar pads. So quâ touch it is ill-equipped, though it has adapted a higher form of tool than the rabbit.

The common squirrel, that sits much and walks mainly on branches of trees just as much as it needs to do, has an epidermis little differentiated, and one which is corrugated with scanty papillary ridges on the palmar and plantar pads, and none on the digits.

The squirrel-like phalanger which flies always more or less downwards by a kind of parachute-arrangement has most of its palmar and plantar skin covered with papillary ridges encroaching upon its corrugated areas, and a response to more delicate tactile experience has been thus produced by its intermittent performance of ordinary progression.

Azara’s opossum presents about as large a part of the surface covered with nodules as with papillary ridges, the latter highly-developed for an animal so low, zoologically-speaking, but one in which delicate discrimina­tion is much practised.

The kinkajou, another arboreal animal which walks about on trees more than it uses its feet for prehension, trusting much to its prehensile tail, shows its corrugated epidermis and papillary ridges developed in about equal proportions.

These five mammals thus show that the stimuli of pressure and friction and the response to them are being complicated by the addition of the more delicate tactile organs known as papillary ridges, and these, perhaps, in a secondary way are becoming useful in preventing friction. But I must not omit to point out that, quâ preven­tion of slipping, the few sparse papillary ridges of the hedgehog, squirrel, kinkajou and flying-phalanger, especially those on the extreme tips of the digits, could have no effect in this preven­tion and no survival value. It is otherwise when they are developed in large areas as in the succeeding groups.

Primates.

All the thirty species of Primates possessed papillary ridges to such an extent that only small areas of the palmar and plantar skin of the lemurs showed any other than these remarkable characters. It is so much a property of the Primate hand and foot to possess these that it might be almost made a matter of ordinal rank belonging to the Primates, were it not that a few stray lower mammals also possess it.

The black-headed lemur is the lowest Primate examined and it is characterised by highly developed patterns of ridges on the palm and sole, and these are interspersed with nodules on the regions less exposed to pressure. The complexity of the patterns of another, the ring-tailed lemur, is greater still. Now these nodules are distinguished from the rough undifferentiated nodules of lower forms, such as the Canadian tree-porcupine, and from the scales in others. When examined with a lens the separate nodules show small groups of papillary ridges two, three or four on each nodule, arranged in a direction parallel to those of neighbouring nodules. They are in fact papillary ridges in embryo, and shortly above this lemur-stage in the ascent of animal life they are merged into papillary ridges in patterns. All this is well told at length by Dr. and Mrs. H. Wilder Harris. I refer to it here because the disappearance of the rough, plain, nodular or corrugated epidermis in mammals is coincident with increasing activity and intelligence in forms who employ or acquire a more delicate sense of touch in their hands and feet. The cruder response of structure to stimuli of friction and pressure, evident in the lower forms, is abandoned in the higher, as tactile delicacy in prehension comes more into play. Here, for example, may be a subtle case of the co-operation of the mould and sieve in action.

From this lemur-level the degree of development in the Primate palm and sole rises and falls, but always advances through the lemuroidea, monkeys and anthropoid apes to man. No attempt at the tracing of the lineage is made here, and from the present limited point of view little remains to be said about different Primates. Only two of those examined will be briefly referred to, the slow loris and man.

The slow loris shares with many monkeys and apes a very soft moist skin of the palm and sole, and in this and other refinements of this region it is much beyond many more intelligent, active and higher Primates. I have never had social intercourse with a loris, but I have shaken the friendly little hand of a chimpanzee with a combina­tion of pleasure, mild shock and perhaps memories of my own palms in the more nervous moments of early life. It is a strange, cool, soft and damp surface, but the sensation conveyed by the skin of a loris lately dead show that in life it is a wonderfully sensitive and tender structure. The whole of the palm and sole is covered with well-developed patterns of papillary ridges especially on the palmar and plantar pads. No trace of old-fashioned nodules, scales or corruga­tion is to be found. The structures due to stimuli of friction and pressure in its ancestors have disappeared for ever from this specialised and small group, and we may fairly hold, in accordance with the law of conserva­tion of energy, that the past is somehow enwrapped in the present in the strange hands and feet of the loris. The adaptations of the hand and foot of the loris are most obviously now of value to it in its wary and dangerous life in the branches of trees, but are equally unfitted for that higher life which, in his case, consists in going lower down, on the ground. The extraordinary deliberate life of the loris has been often described. As he moves from place to place on a branch, fixing one limb before he moves another, much as we do in going up a ladder, he is subjected much to the stimuli of pressure, but hardly at all to those of friction. He sets us a good example of leaving nothing to chance. Thus his soft sensitive skin suits well his mode of progression, but he would find the harder, rougher skin of an African baboon very inferior for the purpose. Here, indeed, I have ventured on the edge of Tom Tiddler’s ground, and the Pan-Selectionist or Mendelian will make a grab at me so that I escape with just the loss of a portion of clothing. After escaping I have only to observe to him as to the adaptations of a loris’s hand and foot that in human life, of which we know a little, one can in a measure forecast what a man will be like if we are told on reliable authority what he and his ancestors have not done in the way of muscular or cerebral output, without informa­tion as to what he has done. This is too obvious, but also too complex to prove here by numerous illustrations and it may be left as a mere sugges­tion as to the past life of the loris and his ancestors for many generations. He has not walked in the ordinary method of terrestrial mammals, he has always moved very slowly about the branches of trees, he sleeps most of the day in a hollow of a tree, curled up like a ball, and his home is in moist, tropical regions. No habits and conditions of life could be better calculated to soften and moisten the skin over his palms and soles or expose it less to stimuli of friction, while even those of pressure in his tenacious grasp of boughs are decidedly intermittent. Unless one may assume the appearance in the distant past of some unit-character of soft, moist skin in this and other Primates, it seems difficult to refuse the Lamarckian claim of long, long absence of effectual stimuli of friction and equally long presence of enervating “negative” conditions. Proof of such a view is, of course, wanting.

Palm and Sole of Man.

The palm of man’s hand is a miracle of adaptations for touch and grasping, but has lost most of the coarse structure formed in response to stimuli of pressure and friction which we saw were common in lower mammals. This indeed he shares with most simian forms. The skin of our hands is now very much what we make it and responds very soon to fresh positive or passive conditions. The horny, cracked epidermis on palm and digit of the old sailor may be contrasted with the soft and flexible and pale surface of his twin-brother, the bank clerk, who is of studious habits and has neither the vice of gardening nor golf. If one compares the hand of the ordinary maid with that of her mistress the difference is striking. But if one compares the hand of that mistress with that of her spinster sister who has lain for twenty years in bed or on a couch, the difference is equally significant. Indeed the sofa-and-bed-ridden invalid, of whom I knew a few once, but who have gone out of fashion, gives the observer some useful thoughts as to the why and wherefore of the strange skin of the hands of the slow loris previously referred to. And if he be disposed also to the pleasant pursuit of moralizing at the expense of others he will feel led to reflect over harshly on the invalid and compare her outlook on life with that of the loris. Even in this concrete case of the hand of an invalid there may be evidence of positive as well as negative response, if one examines the right forefinger so much used in sewing, where the skin becomes hard and thick.

The foot of man has a good deal of negative evidence in favour of my conten­tion as well as positive. As to the latter, in the thickening of the skin over the heel and ball of the great toe in those who walk much we find changes precisely similar to those on the hand. The negative or degenerative changes visible on man’s foot consist chiefly in the remarkable simplicity of pattern of the papillary ridges as well as their flattening and blurring, through wasting of those which occupy mainly the arch of the foot. These will be shown in the next chapter in a drawing. When this portion of skin is compared with that of the foot of any monkey or anthropoid ape it is clear that in this respect the skin of man’s foot has undergone even more degenera­tion than his hand has shown of higher development. This degenera­tion has coincided with two facts, first that man’s terrestrial locomo­tion has advanced far beyond that of any other Primate, and second, that he alone has a plantar arch. This subject belongs to a later chapter and is referred to here because the possession of an arch to his foot has caused man to escape, on the under surface of it, a vast propor­tion of the stimuli of pressure and friction involved in his mode of walking, and the extreme simplicity of his plantar papillary ridges, and relatively thin, soft skin under the plantar arches affords a fairly conclusive example of change of structure from disuse per se.

I have thus only selected and used two striking types of the Primates, the loris and man, not wishing to burden this part of the subject unduly with intervening and less characteristic forms of life. It may be legitimate here to say in defence of this long chapter that it illustrates what I desire to keep before me all through, the fact that use, habit, environment and selection go ever hand in hand. In all matters of science one has to descend to particulars, so it seemed necessary to select a few scattered phenomena in the best known groups of higher animals and endeavour to understand how certain “characters” or better “modifications” began to grow big enough to avoid passing through the meshes of the sieve.

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.

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 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. 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. 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.

CHAPTER XIX.
FLEXURES OF THE PALM AND SOLE.

Those flexures of the palmar and plantar skin which are called by Galton chiromantic creases, and said by him to be no more significant to others than palmists than the creases of old clothes, have received a remarkable amount of pseudo-scientific attention since earliest times in Chinese and Greek history. The former even added podoscopy to their chiromancy. The line of life, the line of the head, the line of the heart, the line of fortune and that of the liver, figure freely in fortune-telling of modern drawing-rooms by women who ought to be in Holloway gaol, but are not. The gipsies, their predecessors and equally honest teachers, did not employ such high-sounding words, but I believe that by observing closely the bearing, looks, dress and manner of their dupes, while pretending to study their palms, both classes of practitioners, like phrenologists, are able to tell a good deal of what their customers are, and being shrewd persons they are able to guess pretty well what they will be and will do.

I agree with Galton that these creases of hand and foot are no more significant than those of an old coat-sleeve, a pair of trousers, or boots; but they are not less significant of certain muscular habits of the wearers of those articles.65

The flexures in question are in line with the subjects of the two preceding chapters, and require little more descrip­tion in detail than is afforded by the accompanying illustra­tion of mammalian hands and feet.

Description of Flexures.

There are two classes which may be conveniently called here Primary and Secondary, the latter being too variable and accidental for further notice. The former lie in three main directions and are longitudinal, oblique or transverse. They represent in graphic characters the nature and degree of the functions exercised by muscles moving the joints which underlie them, and are often called “flexion-lines.” They are “folds so disposed that the thick skin shall be capable of bending in grasping while it at the same time requires to be tightly bound down to the skeleton of the hands and feet, so as to prevent slipping of the skin which would necessarily lead to insecurity of prehension, just as the quilting and buttoning down of the covers of furniture by upholsterers keeps them from slipping. For this purpose the skin is tied by fibres of white fibrillar tissue to the deep layer of the dermis along the lateral and lower edges of the palmar fascia and to the sheaths of the flexor tendons. The folds, therefore, which are disposed for the purpose of making the grasp secure, vary with the relative lengths of the metacarpal bones, with the mutual relations of the sheaths of the tendons and the edge of the palmar and plantar fascia. . . . The sulci are emphasised because the subcutaneous fat, which is copious in order to pad the skin for the purpose of holding, being restricted to the interval between the lines along which the skin is tied down, makes these intervals project, and these are the monteculi.”66

This account of them from a leading anatomist shows that not for nothing have these creases been evolved. They are inherited, have an important function and are worthy of study in their humble way: they may be even dignified with the name “character.”

They are often double over the joints of the fingers and toes, but, from the functional point of view and for simplicity, may be reckoned as single.

Chief Types.