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.
[Illustration: Fig. 59.--W. K. Right hand drawing of papillary ridges,
made from impressions.]
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.
[Illustration: Fig. 60 S. K. Right foot drawing of papillary ridges
made from impression.]
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.
[Illustration: Fig. 61.--Slow loris--right foot.]
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.
[Illustration: Fig. 62. Hedgehog--right foot.]
[Illustration: Fig. 62A. Hedgehog--right hand.]
[Illustration: Fig. 63. Common squirrel--left foot.]
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.
[Illustration: Fig. 64.--Chimpanzee--right hand.]
[Illustration: Fig. 66.--Orang--right hand.]
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_.
[Illustration: Fig. 65.--Gorilla--left hand.]
[Illustration: Fig. 67.--Gibbon--left hand.]
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.
[Illustration: Fig. 68.--Left foot of ring-tailed lemur.]
[Illustration: Fig. 69.--Brown sapajou, right hand.]
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.”
[Illustration: Fig. 70.--Left hand of chacma baboon.]
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.