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69

ARBOREAL MAN

ARBOREAL MAN

BY

F. WOOD JONES, M.B., D.Sc.

PROFESSOR OF ANATOMY IN THE UNIVERSITY OF LONDON (LONDON SCHOOL OF

MEDICINE FOR WOMEN)

i

LONDON
EDWARD ARNOLD

igi6

[All rii^hts reserved]

D. H. HILL LIBRARY
N. C. STATE UNIVERSITY

To
MY WIFE

PREFACE

The subject, with which the following pages deal, formed
the material for certain Arris and Gale Lectures, de-
livered in the Theatre of the Royal College of Surgeons
of England during the years 1915 and 1916. Chapter XXI.
consists of a condensed, and only partial, account of the
subject treated in one such Lecture on " The Influence
of the Arboreal Habit in the Evolution of the Reproduc-
tive System," delivered on March 22nd, 1915; and the
remaining chapters formed the basis of three Lectures
on " The Influence of the Arboreal Habit in the Ev^olu-
tion of Man," delivered on February 28th, March 1st,
and March 3rd, 1916.

Among their literary defects are those which are in-
separable from their origin, since they are at best but
elaborated notes of separated headings under which the
Lectures were originally planned. The gift, which was
so peculiarly conspicuous a possession of Huxley, of
endowing the written page with the interest felt by the
lecturer in the preparation of his subject, is a rare one.
For the most part, the written notes of lectures are wont
to present themselves as mere disconnected assertions,
woven around a series of apparently disjointed central
ideas. It is this inherent difficulty of reducing the under-
lying thought, and the spoken word to a consecutive
written statement, that is appealed to as an excuse for
the partially woven condition in which the material is
presented to the reader. And this excuse is urged the
more insistently since an alternative one will readily

vii

viii PREFACE

present itself. The want of proper literary sequence
might indicate equally well that the subject had been
but insufficiently thought over, that the sequence of
ideas had been ill considered, and the conclusions hastily
arrived at. That in this case the matter is not so can
best be urged by stating that the substance of these
Lectures had been collected in the form of written notes
some seven years ago. Moreover, many of the details
and the ideas included in these pages I have been accus-
tomed to incorporate in the ordinary routine teaching
of Medical Students at Manchester University, at St.
Thomas's Hospital, and at the London School of Medicine
for Women .

I have endeavoured to acknowledge my indebtedness
to the work of others wherever such a debt has been
incurred. Some debts, however, cannot be considered as
discharged by the mere acknowledgment of the source
of a quoted passage; to Professor Arthur Keith, and
to Professor G. Elliot Smith, I owe far more than is
implied in the few references made directly to their written
works.

The figures which are reproduced here are selected from
those drawn to illustrate the Lectures; they were pre-
pared with especial regard for their appearance when
magnified by the epidiascope rather than when reduced
by the processes of reproduction.

F. W. J.

London,

October, 1916.

CONTENTS

CHAPTER

I. THE PROBLEM OF MAN'S ORIGIN -

II. THE EMANCIPATION OF THE FORE-LIMBS -

III. THE DEVELOPMENT OF THE POWER OF GRASP

IV. THE SKELETON OF THE FORE-LIMB

V. THE CLAVICLE - - - . _

VI. THE MUSCLES OF THE FORE -LIMB -

VII. THE FORE-LIMB: SUMMARY _ - _

VIII. THE FATE OF THE HIND-LIMBS - - -

IX. THE SKELETON OF THE HIND -LIMB

X. THE MUSCLES OF THE HIND-LIMB - - -

XI. OTHER ARBOREAL ADAPTATIONS OF THE HIND-LIMB

XII. THUMBS AND BIG TOES - - - -

XIII. THE HUMAN FOOT- . _ . _

XIV. THE RECESSION OF THE SNOUT REGION

XV. THE RECESSION OF THE JAWS AND REDUCTION OF
THE TOOTH SERIES - - - -

XVI. THE FACE AND THE CRANIUM _ - -

XVII. THE SPINOUS PROCESSES OF THE VERTEBRAL COLUMN

XVIII. THE POISE OF THE HEAD AND THE CURVES OF THE
SPINE ------

XIX. THE PELVIS AND THE VISCERA - - -

XX. THE RESPIRATORY SYSTEM _ _ -

XXI. THE REPRODUCTIVE SYSTEM _ - -

XXII. THE DEVELOPMENT OF THE BRAIN

XXIII. THE STORY OF TACTILE IMPRESSIONS

XXIV. MOTOR IMPRESSIONS _ - - -
XXV. IMPRESSIONS OF SIGHT AND HEARING

XXVI. HIGHER DEVELOPMENTS OF CEREBRAL FUNCTIONS -

ix

PACK

1

7^

19 ^

23

28 t^

32

43

48

53

58

62

67 ^

73 -^

84-^

90

96

101

114 v/

123

131

138

149

157

162

174

180

X CONTENTS

CHAPTKR PAGB

XXVII. HIGHER DEVELOPMENTS OF CEREBRAL FUNCTIONS:

POSSIBLE ANATOMICAL BASIS - - - 190

XXVIII. THE BRAIN AND THE BODY - - - 196

XXIX. THE HUMAN BABY - - _ . 201

XXX. ARBOREAL ACTIVITIES OF MODERN MAN - - 207

XXXI. THE FAILURES OF ARBOREAL LIFE - - 212

XXXII. THE UPRIGHT POSTURE - - - - 221

BIBLIOGRAPHY - - - . . £25

INDEX -----. 227

1

ARBOREAL MA^N'

CHAPTER I

THE PKOBLEM OF MAN'S OKIGIN

It is a strangely difficult thing, for one of our generation,
to picture the acuteness of the upheaval brought about
in 1859 by the publication of the " Origin of Species."
It is hard to realize that there should have been so much
novelty in the ideas expressed in the book that thought
should have been overwhelmed by the new teaching;
that " evolution " should have become a creed; and that
" special creation " should have become an obsession.
So many suggestions had gone forth before, so much of
the path had been paved for evolution, that it seems
strange how the basal idea that species were not specially
created, and definitely fixed types of life, should have
suddenly, as a flame, lit up the fires of the most bitter
controversy carried on in modern times.

It is the more wonderful when we think that, at any
rate as far as the scientific world was concerned, Darwdn
was by no means standing as the pioneer of evolution;
but was only the thoughtful student who w^as putting
forward some easily understood explanation of the
manner in which evolution had been effected.

And yet of the upheaval of thought that occurred we,
separated by more than fifty years from the advent of
that work, can feel the bitter reality when turning the
pages of any contemporary periodical in the columns of
which some of the many battles were waged. Even when

1

D. H. HILL LIBRARY
North Carolina Stafre College

2 ARBOREAL MAN

the opening period of hasty and unreasoning partisanship
was passed, and after the first sl^irmishes had been fought
and won for the principle of evolution, there still remained
the biggest battle of all to be contested. Fifty years ago
even an ardent evolutionist would feel no dijBficulty in
keeping as a mental reservation the belief that, though
no doubt the lesser beasts had been subject to the laws
of gradual change, Man was aloof from all this and was a
divine, a special, and a perfected creation. This mental
reservation is, not unnaturally, still prevalent to-day;
and I think that in 1916 one w^ould give but an ill picture
of the popular progress of the ideas first made definite
by the work of Darwin, if one assumed that, in the dying
of controversy, there had of necessity been a really wide
acceptance of the picture of a simple evolutionary origin
of Man. How comj)letely Man can be separated, by a
series of mental processes, from all the laws known to
govern the modifications and progress of lower animals,
even hy a man of the highest scientific attainments, may
be realized by the reading of such a work as the final
effort of Thomas Dwight, the late distinguished Pro-
fessor of Anatomy of Harvard. What Dwight, pos-
sessed of a vast store of knowledge of the structure and
variations of Man and the lower animals, could do, a
great host of others can do in the comfortable absence
of any such precise knowledge which might influence the
attitude they elected to adopt.

Still, despite the mental reservations of the thinking
few, and the unthinking many, the questions must be
asked and answered: What are the factors of habit or
environment, and what are the steps of " adaptation,"
" variation," or " sporting " which have led to the evolu-
tion of Man as a zoological type ?

We start, therefore, with the assumption that we accept
the principle of evolution as a fact, and that we extend
this principle to embrace Man. " Adaj^tation," "varia-
tion," and " sporting " have been named in that order of

THE PROBLEM OF MAN'S ORIGIN 3

set purpose, and for a very special reason which must be
briefly defined.

Change comes about somehow in animal tj^pes, that
must be admitted, else there could be no groundwork
for the play of evolution. Change might conceivably
come about by " adaptation," and by that is meant the
reaction of the animal to its life surroundings. John
Hunter (1728-1793) had a clear conception of this in-
fluence, and his life work might be summed up by saying
that he saw, with the eye of a genius, the dependence of
structure on function. With the alteration of function
— not uncommonly as a result of change of environment
or habit — structure, in the individual, shows harmonious
change.

As an inheritable, and so as an evolutionary, factor,
this adaptation of structure to function is especially
associated with the name of Jean Baptiste de Monet,
Chevalier de Lamarck (1744-1829), who, quite Hunterian
in his conceptions, appreciated to the full the influences
of " use and disuse " upon organs and systems.

Changes, again, might be brought about, not by special,
definite, and purposive " adaptations," but by slight
" variations," and by " variations " we here mean those
trivial congenital differences, always displayed among
individuals, which are the progeny of parents possessing
varied individualities. Variation, aided by natural selec-
tion, constitutes that particular method of effecting change
in living things especially associated with the name of
Charles Darwin (1809-1882). By " sporting " or "muta-
tion " is understood, not a purposive adaptation, nor a
mere gradually accumulating minor congenital variation,
but the more or less sudden appearance of a " freak," if
one may so express it, among the offspring of an individual.
Evolution, by sporting or by mutation, is a more modern
conception, associated in the main with the name of De
Vries, and a^host of contemporary workers.

These are ideas that are, or have been, current in

4 ARBOREAL MAN

/^accounting for change in the living world. Change comes
L about in some way— that is obvious; by what channel or
fy channels it comes about concerns the present inquiry
// but little. How it is transmitted once it came into being,
\ how: it is accumulated, perfected, and handed on are
questions which, despite an enormous amount of work,
and despite an accumulated literature of dogmatic, and
sometimes unjustified assertion, are at present unknown.
Without touching upon these problems it is proposed to
examine the probable path by which the Primates and
Man have originated, reviewing the influences that have
probably reacted upon them, but leaving aside the ques-
tions as to how changes have come into being, and how
I ysuch changes have been inherited/ We will therefore
/ define our position by saying that change has been
/] effected somehow, and somehow it has been handed on ;
If and that any attempt to chronicle the progress of these
* changes need not be branded as Lamarckian or impossible,
as Darwinian or improbable, as mutationist or orthodox,
unless definite assertions are made as to the exact mode
or means by which these alterations have come into being,
^r have become handed on and stereotyped.

Man has often been discussed as an evolutionary pro-
duct ; the literature of the last fifty years teems with works
upon that special aspect of anthropology which deals with
Man as the highest of the Primates. There is nothing to be
added to the brilliant generalizations of Huxley, nothing
to be altered in the careful analysis of primate and human
characters carried out by Keith. One reason only has
appealed to the writer as an excuse for the publication of
these lectures, and that is the fact that the paleonto-
logical history of Man is rapidly enlarging. Any new
find of a so-called " missing link " may bring us by chance
nearer to deciding in what type human divergence first
manifested itself. Disputes concerning the zoological
rank of such finds will, in all probability, be carried on
with extreme vigour for many years to come. That is

THE PROBLEM OF MAN'S ORIGIN 6

inevitable, and it can only result, in the end, in a gain
to the scientific knowledge of the origin of Man. Mean-
while it is advisable to take stock of what is probable
concerning the phylogenetic story of Man, in order to see
if there is any stage in his evolution at which he, or his
remains, might be labelled as human. Not so long ago
there would have been no hesitation in asserting what^
type was, and what was not, human. Man began as Man,
and that was the beginning and the end of it. We have
definitely passed that stage. To-day we have a bewilder-
ing complexity of genera and species of missing links;
but we still have a more or less definite conception of
what we would term a human being. It has been claimed
that the possession of the ability to speak constitutes an
essential feature of the dawning human being, and it has
even been imagined that, from a study of the fragmentary
physical remains of missing links, the presence or absence
of this faculty could be determined. Physical remains
cannot provide the material from which certain know-
ledge upon this point may be gleaned. There is no more
reason for saying that some such missing link could not
speak because some divergence from the modern human
type is found in the construction of the jaw, than there
is for asserting that a monkey cannot play the piano
because the anatomy of his hand differs in some details
from that of a human pianist. No ape has become an
orator, and no monkey a distinguished performer upon
the piano ; but we must not seek the reason for this in
the departures from the standard human form seen in the
structure of their jaws and hands. Speech, and piano-
playing, are the outcomes of a series of elaborations of
cerebral processes which are present in existing Man, but
not in existing monkeys. We have no certain physical
clue in the fragmentary remains of missing links con-
cerning the presence or absence of these elaborations of
cerebral processes.

There is a very prevalent idea that the a-ssumption of

6 AKBOREAL MAN

the upright posture in terrestrial progression gave to Man
those special attributes which we would term human.
There can be no possible doubt that the faculty for striding
about uj)right upon the surface of the earth marked a very
real phase in evolution. But when we come to examine the
possible influences w^hich preceded this dej)arture, we can
only regard it as a natural and culminating phase of a long
series of changes which had taken place in an altogether
different environment. Were the whole series of missing
links to be paraded before us in the form of their frag-
mentary remains which are yet to be discovered, he would
be a bold man who would point to any individual member
as the one in which tjae features of terrestrial uprightness
jargued humanitjj^Arboreal uprightness preceded ter-
restrial uprightness; and it is the x^urpose of these studies
to show, in some measure, the extent to which Man is
indebted to, and was perfected in, arboreal life.

Man comes of an arboreal stock. Two questions arise.
When in the phylogeny of the Mammals did this stock
become arboreal, and when did it give rise to a creature
which we could possibly term human ? The first ques-
tion is capable of an approximate solution; the second
is unanswerable, but we may say with regard to it that,
if the term " humanoid " may be permitted, such a stock
may have had a very early representation among the
mammalian fauna.

CHAPTER II
THE EMANCIPATION OF THE FORE-LDIBS

ft

We may not here turn aside to inquire into the origin of
limbs, nor pause to consider the questions which of neces-
sity arise out of the fact that, while all Vertebrates are
limited to four limbs, the Invertebrates know no such
limitation.

We will start with the facts as we know them : that
Vertebrates possess four outgrowths from the body seg-
ments, arranged as a symmetrical pectoral, and a sym-
metrical pelvic, pair of limbs ; and that these limbs a^Dpear
probably in their elemental form as the fins of fishes.
We may assume that the most primitive type of Verte-
brate limb is an appendage, which is adapted for the
purpose of ordered and regulated progression. Limbs
may merely propel the aquatic vertebrate body in a
definite direction as oars propel a boat; and yet, even
when we may regard them as a new acquisition in the
Vertebrate phylum, they already subserve other func-
tions. Some fishes propel themselves through the water
by movements of their fins — they " swim " ; but the
source of real propulsion in a great many is the lateral
movements of the tail, the fins serving far more as balanc-
ing and regulating organs than as a means of propulsion
through the water. This possibility of the limbs develop-
ing a balancing function is one that becomes greatly
elaborated in the story of the limbs of higher animals.

We are more immediately concerned here with the
limbs of those Vertebrates higher than the fishes; and
the type of limb from which we will start our comparisons

7

8 ARBOREAL MAN

is that seen in living forms among the tailed Amphibians,
and in some of the less specialized Reptiles. This we
may define as a limb of three segments: arm, forearm,
and hand; thigh, leg, and foot (see Fig. 1). The first
segment consists of muscles massed round one central
bone— humerus or femur; the second segment of two
parallel bones— ulna and radius, or fibula and tibia, and

/r^

Fig. 1.— Diagrammatic Drawing to show the Condition

OF Primitive Limbs.

their regulating muscles ; and the third of a series of small
bones — carpals or tarsals, with the muscles and bones of
the separate digits. This limb is possessed of a high
degree of mobility. It can move in all directions on the
trunk at the shoulder and hip joints; the second segment
is enabled to move on the first by bending or straighten-
ing of the elbow or knee; the two parallel bones of the
second segment may move upon each other, so that the
third segment may be moved with the second, and be
turned (at any rate to some extent) palm or sole up
— supinated, or knuckles up — pronated (see Figs. 2, 3).
Finally the third segment is free to move on the second
in a variety of ways at the wrist and ankle. Such is the
limb which is the heritage of all land-living Vertebrates,
and such a limb is beyond doubt the heritage of the an-
cestral Mammal. The functions of this primitive limb
are simple in the extreme; it enables the animal to walk
about under water, and it serves to drag the animal about

E^UNCIPATION OF THE FORE-LIMBS 9

on land. By its mobility it produces movement, hut if
does not support the weight of the animaVs body. It is a
propelling, but not a supporting, limb (see Fig. 4). A

Fig. 2. — Human Forearm with the Hand turned Palmar
Surface Upwards — Supinated.

Fig. 3. — Human Forearm with the Hand turned Palmar
Surface Downwards — Pronated.

Fig. 4. — Diagrammatic Outline of a Primitive Type op
Vertebrate with Propelling, but not Supporting,
Limbs. i

very near approximation to our ideal primitive limb is
seen in the ordinary water newt. We may readily appre-
ciate, in watching such an animal, the perfection with

h) ARBOREAL MAN

which its limbs enable it to walk at the bottom of its
tank, to clamber over obstacles, or climb aquatic plants.
But we note that when on dry land its activities are con-
siderably hampered, since while its limbs propel it for-
wards they no longer carry its weight, and the body is
dragged along the ground. " On thy belly shalt thou
go " applies to the pioneers of the land-living Vertebrates ;
for their limbs are not yet adapted to supporting their
bodies and carrying them sheer of the ground.

Mobility is the keynote of this primitive limb. With
the permanent exchange of an aquatic for a terrestrial
habitat the limbs took on a new function, for in addition
to acting as mere propellers, they now serve to lift the
body during the act of propulsion. With this change a
new demand is made in the structure of the limb, for
^stability must be added to mobility. There is a gradual
evolution of this new function. The limbs at first support
the body only during the act of propulsion; when the
movement is over, the body sinks to rest upon the ground.
In the next phase the support of the body by the limbs
becomes permanent ; the demand for stability in the limbs
is increased. There is an antagonism in this evolution
between the advantage of elaborating the ancestral, and
useful, mobility of the limb, and the need for the newly
developed, and essential, quality of stability. It is in
this antagonism of developmental needs that the great
interest of the study lies.

In such a question as this the records of paleontology
are likely to furnish much material assistance, and it is
from the paleontologist that the most definite ^^ronounce-
ments may be expected. The remains of animals furnish
some clear guides as to the possibility of their limbs being
supporting as well as propelling organs, and the geological
period at which animals possessing such limbs first
appeared seems to be generally agreed on. We find in
this feature, as we shall repeatedly find again in relation
to other things, that the search for these animals must be

EMANCIPATION OF THE FORE LIMBS 11

pushed very far back in the geological record, and when
it is so pushed back it leads to a curious group of animals
known as the Therapsida, which, presenting a blend of
primitive reptilian and primitive mammalian characters,
flourished in the Triassic. It was, according to Broom,
among the South African members of the Therapsida
especially that the limbs became supporting organs, and
he has said very definitely that " when the Therapsidan
took to walking with its feet underneath and its body
off the ground it first became possible for it to become a
warm-blooded animal." The change that we have been
picturing was, therefore, one which took place very far
back in the geological past; and, according to Broom,
the supporting limb and the mammalian possibilities
made their appearance together, the one being dependent
upon the other. The characters of the supporting limb
as opposed to the purely propelling, but not suj)porting,
limb are so definite that there should be but little hesi-
tation on the part of an anatomist in assigning the proper
functions to the limbs of any extinct form. But it cannot
be said that the geologist, when assuming the r61e of an
articulator of the skeletons of extinct monsters, has always
shown a nice appreciation of these characters. A visit
to the geological galleries of any museum wall reveal
instances of animals, the limbs of w^hich are articulated
for a function that they had no power to perform.

Looking broadly at the Mammals, we may say that the
preservation and elaboration of the inherited mobility of
the fore-limb is an essential for the culmination of evolu-
tion. We may also say that this preservation of mobility
must start very early, before ancestral mobility had
become lost in the development of stability; and that the
most successful Mammals must, by some means or other,
have preserved and stereotyped this mobility almost at
the outset of their mammalian career. Again, we may
say that two distinct lines have been followed. Some
mammals have perfected the new, and mammalian, de-

12 ARBOREAL MAN

mand for stability; and others have retained a primitive
mobility in, at least, the fore-limb.

It is the latter which have been successful and have
become dominant. The problem we are attempting to
solve is : Why have some Mammals retained this primitive
feature of mobility of the fore-limb, and why have these
same Mammals become more successful in the struggle
of evolution ?

We are here face to face with a fundamental problem,
and it is now necessary that we should, as it were, take
sides. Man possesses a mobile fore-limb which takes no
part in the support or the progression of his body. He
is the culmination of a line of ancestors which, on alto-
gether different grounds, is distinct enough in general
outlines. The question is: Does the stock from which
Man arose retain a primitive mobile fore-limb, or has he
evolved his present posture and the present freedom of
his fore-limb from a previously four-footed or quadru-
pedal ancestor ? It may be said with truth that every
teaching of modern orthodox anatomy and anthropology
would lead us to believe that Man had evolved from a
quadrupedal pronograde mammalian stage. With that
it is impossible to disagree so long as it is made perfectly
clear that the stock from which Man is derived was
differentiated so early in the mammalian story, that the
primitive mobility of the fore-limb had never been sacri-
ficed to the needs of stability. Tliere are two ways of
regarding this problem. We may assume that the primi-
tive mammal passed into a regular pronograde four-
footed stage with four supporting limbs, and from that
stage Man evolved into an animal characterized by an
orthograde or upright posture. Or we may imagine that
the stock from which Man wa derived had never been
typically pronograde with four supporting limbs; that in
this stock mobility had never become sacrificed to sta-
bility in all four limbs. It is in the former view, the
assumption of the upright posture from a pronograde

EMANCIPATION OF THE FORE-LIMBS 13

stage, that much of the interest of the modern study of
human morphology is centred. It is the latter view,
that the human stock has never been typically prono-
grade and four-footed, that is here put forward as the
truth.

In attempting to maintain this view, definite answers
must be given to three questions. The first: What was
the factor that saved the particular mammalian stock
which culminated in Man from becoming four-footed
pronogrades?" We will answer by saying at once, " The
arboreal habit." The second: "When did this factor
come into play in the philogenetic history of the Mam-
mals?" We will dismiss this by asserting that it was at the /

very outset, at the very dawn of mammalian life. The ^

anatomical basis for this assertion will be given in detail '
later on. The third: " How did this factor enable that i
particular stock to acquire supremacy?" will be answered, \
so far as is possible, by the study of the influence of the
arboreal habit upon the animal body.

We will deal first with the influence of the arboreal
habit upon the structure and function of the limbs. We
are assuming that the primitive Mammal, new born from
the Therapsidian ancestor, possesses limbs such as we
have deflned, with but little stability, but with a high
degree of mobility, and this mobility includes the power
of rotating the second and third segments around the
central axis of the limb in the actions of pronation and
supination. The effects of mammalian habit upon these
limbs will probably be best appreciated by following the
story as it is unfolded in animals that directed their
newly acquired mammalian possibilities into the natural
channel of supremacy in walking and running over the
surface of the earth.

All four limbs of such an animal will become equally
developed as organs of support and of progression (see
Fig. 5). Mobility at shoulder, elbow and wrist, hip, knee
and ankle will be essential, but stability becomes a prime

14 ARBOREAL MAN

necessity, and the rotation of the parallel bones of the
second segment is a hindrance to perfect stability. Little
by little this power of rotation becomes lost; the muscles
which produce the movements of pronation and supina-
tion disappear, or change their action, the joints between
the two parallel bones become less perfect; finally the

Fig. 5. — Diagrammatic Drawing to show the Condition of
Limbs which have become thoroughly Stable, and
Function both as Propelling and Supporting Organs.

two bones fuse together, and one member of the pair
practically ceases to exist. Again, the digits, except by
virtue of the nails or claws which they bear, cease to be
of great individual importance, and some of them soon
become reduced to the condition of mere rudiments.
The final stage of this process is exemplified in the horse,
where one functional digit alone remains, upon the nail
(hoof) of which the animal is supported. These four
limbs are now stable props which, capable of very definite
and specialized movements, support the animal and

EMANCIPATION OF THE FORE-LIMBS 15

enable it to walk, and run with the very greatest perfec-
tion. As a general statement, we have said that the
evolution that produces limbs of this type also demands
that all four members shall function alike, fore-limbs and
hind-limbs being both supporting and ambulatory organs.
This statement needs some qualification, since there are
certain exceptions to the rule that all four limbs are
functionally of equal importance in the Mammals that
have taken to a pronograde terrestrial life ; and it is these
exceptions that are of interest. In different types of
quadrupedal Mammals there may be well-marked differ-
ences in the actual method of movement of the limbs in
ordinary leisured progression.

A right fore-limb and a left hind-limb may be raised
simultaneously from the ground and swung forwards;
this is the mode of the greater number of quadrupeds.
Again, a right fore-limb and a right hind-limb may be
raised and advanced simultaneously ; this is the ordinary
mode of progression of the giraffe. Or, again, the sequence
of bringing the limbs into play may vary with the pace
at which the animal travels; and then the animal changes
its gait and its stride as the pace varies. An altogether
different method may manifest itself with this change of
gait in response to the demands of pace, and both fore-
limbs and both hind-limbs may be raised and advanced
alternately. This mode is habitual in the ordinary quiet
gait of some animals ; it is the usual way in which a rabbit
moves about when feeding undisturbed. In this method
of movement the fore-limbs and the hind-limbs may play
an equal part, or the hind-limbs may take an increasing
share in the work, both of supporting the bod}^, and of
urging it forward. In this way a more and more j)erfect
hop is developed; when this method of progression has
reached its most advanced stage the fore-limb is freed
very thoroughly from its duties of suj^port. Hojoping is
a specialized development of the pronograde gait; and
it has led to some very interesting develop»ments which

16 ARBOREAL IVIAN

have a bearing upon the present study. To these hop-
ping animals we will return.

We will now come back to our primitive Mammal with
its four mobile limbs, and picture it taking, not to a ter-
restrial, but to an arboreal life. I imagine that the first
stages of this advance could be pictured as being built
upon the ability the animal already possessed of sur-
mounting such obstacles as chanced to lie in its terres-
trial path. The ability which such a primitive Mammal
would have for climbing might perhaps be gauged by
having regard for that skill in clambering which is mani-
fested in the tailed Amphibians, a skill which we must
remember develops within the limits of their own Phylum
(in the Tree Frogs) into perhaps the most perfected tree-
climbing displayed in the Vertebrate series. It may
seem a long way to go back when attempting to unravel
the influences of tree-climbing among the Primates, to
appeal to the clambering activities of the w^ater-newt.
And yet the anatomical condition of the limbs of Man
demands a shifting backward of the inquiry to some such
stage as this. I believe that the truest picture of the
evolution of Primate climbing starts with such a scene
as we are depicting now. The method of this amphibian
or reptilian clambering must be appreciated, for, as we
shall see, climbing may be conducted in several different
ways ; and the particular method practised by any animal
may serve to date the evolutionary stage at which the
habit was adopted. An Amphibian, or unspecialized
Reptile, ascends an obstacle by clambering up; its feet
are applied to the surface of the obstacle up which it
clambers. It makes no attempt to obtain a grip b}^ nails
or claws, but it trusts merely to the apposition of its feet
to the surface to which it clings, and when this fails the
animal falls.

Two points must be especially noticed. As its progress
continues, it repeatedly reaches ahead with one or other
of its fore-limbs for a new hold, and whilst doing this its

E:MANCIP2VTI0N of the fore -limbs 17

body weight is temporarily thrown upon its hind-limbs.
And, again, in reaching out its fore-limb, the freedom of
rotation possessed by the second segment of the limb
allows the animal to apply the palmar surface of its
" hand " against any new hold which may present itself
at almost any angle.

From such a humble beginning great developments are
possible; and here we may observe that, without the
apprenticeship served in this lowly clambering, short cuts
to tree-climbing have never attained the same ultimate
perfection. As arboreal life becomes more complete, the
search for a new foothold will become a far more exact-
ing business than it is in the mere clambering we have
pictured. The more exacting this search becomes, the
more will there tend to be developed that most impor-
tant factor — the specialization of the functions of the fore
and hind limbs. While the animal reaches about with its
fore-limb, the hind-limb becomes the supporting organ.
With the evolution of this process there comes about a
final liberation of the fore-limb from any such servile
function as supporting the weight of the body : it becomes
a free organ full of possibilities, and already capable of
many things. This process I am terming the emancipa-
tion of the fore-limb, and its importance as an evolutionary
factor appears to me to be enormous.

It will be noted that in the little picture we have drawn )
of the process, we have, as it were, rescued the fore-limb;
rescued it while stil] possessed of all its inherited power .
of mobility, saved it from becoming an organ of mere
stability, and handed it over to an enterprising mammalian
stock to adapt to its needs. ^

This picture may seem fanciful, and j^et in reality it
is not so. I have thought it worth while to draw it thus,
since, without such a picture, there are many things very
difficult to understand. I will instance tAvo such cases.
We have hurried almost breathlessly over the process
we have pictured, in a mental anxiety to arrive at emanci-

2

\

(

IS ARBOREAL MAN

pation of the fore-limb before the limb had lost any of
its possibilities of mobility' . This we have done because
of the knowledge that once the limb has become a sup-
porting organ, and given up its birthright of mobility for
the acquired stability, no subsequent degree of liberation,
due to altered habits, will achieve the same great possi-
bilities in evolution. Animals have liberated fore-limbs
already made stable, or partially stable, and they have
not attained the great results which we shall follow in the
stock we have been picturing. It is thus with the jumping
animals we have mentioned previously. The liberation
of the fore-limb may be ver^^ complete, but it i^ a fore-
limb of restricted possibilities that has been liberated.

The arboreal habit alone is not the talisman; other
mammalian stocks have taken to an arboreal habit ; l)ut
they have taken to it after varied periods of quadrupedal
life. Thev have taken to it too late to derive the full
benefits from it, for they took to it with the fore-limbs
already dej^rived of some of their inherited mobility.
Such animals never become jDerfect tree-climbers. They
may acquire an extraordinary skill in running about the
branches of trees as many Rodents do, or they may even
climb in the proper sense of the word, but in this climbing
the grip is not obtained by the application of the palmar
surface of the hand, but by the hook-like action of claws
and nails; this method is practised by many of the Car-
nivora. The maximum of possibilities is not attainable
in any of these cases. It is not enough to have a thor-
oughly emancipated fore-limb, it is not enough to be
thoroughly arboreal. It was a combination of seemingly
humble and unimportant circumstances, acting at the
very dawn of mammalian life, which permitted the eman-
cipation of an unmodified fore-limb in a certain stock,
and so laid the direct path for the evolution of the highest
Mammals, and Man.

CHAPTER III

THE DEVELOPMENT OF THE POSYEK OF GRASP

We have noted that the primitive animal we have been
picturing could place the palm of its hand against any
new hold with which it came into contact, and that the
power of rotation possessed by its forearm enabled this
contact to take place at a variety of angles. Its palms,
for instance, may both be turned inwards so that a branch
or other object can be held between their two opposed
surfaces. This is a power which remains in the possession>^
of many animals even after they have lost much of thef
primitive mobility of the fore-limb in quadrupedal life/
As a general rule, the hopping animals and the semi-
arboreal animals retain sufficient mobility to do this.
Some of them can hold their food pressed between the
two palms, and so are enabled to sit up and eat food held
between their fore-paws. Others, which cannot attain
to this, yet preserve sufficient mobility of the fore-limb
to enable them to use it for a variety of minor purposes.
The more thoroughly quadrupedal the animal is, the less^
is it able to turn its fore-limb to these minor uses.)
Familiarly, we may note that the typically quadrupedal
dog will use its hind-feet for scratching, even the fore-end
of its body; while the cat will scratch and wash its face
with its fore-foot. But we are dealing, in possibilities,
with something far bigger and more important than such
things as these. We must not forget that in rescuing
the fore-limb in its primitive mobile stage, before quadru-
pedal life had in any way impaired its power of rotation,
we saved not only a primitive second segment, but a

19

20 ARBOREAL MAN

primitive third segment as well. We maj' now say that
we have rescued the third segment as a hand, and so
preserved it from ever becoming a mere paw or a hoof.
This is most important— perhaps as important a thing
as ever happened in any evolutionary story— for the
permanent preservation of a primitive hand, affixed to
a primitive rotating forearm, made possible a great
[ number of the most far-reaching developments.

By a primitive hand w^e mean a very definite thing,
and one essential in the make-up of this hand is the
possession of five separate, and fairly equally developed,
digits. We have made use of the water-newi: to picture
some stages of fore-limb development, but Ave may not
press comparisons with this type into minute details.
The hand of existing Amphibians does not fulfil all the
demands of our definition, for only four digits are present
in living tailed members, and four well-developed digits,
with a rudiment of a fifth, in living tailless forms. But
there are several extinct forms of generalized Amphibia
and Reptilia which had what we may truly term a primi-
tive hand, and among the living and unspecialized
Reptilia it is still to be met with. It is a very remark-
/able fact that in the numerical development of the
I individual bones which compose the separate fingers, the
/ Chelonians (Tortoises and Turtles) are the match of
I^Man and his nearest mammalian neighbours. There is
/evidently something extraordinarily primitive about the
I hand that has been preserved and passed on to Man; but
like the primitive rotating forearm, this primitive, simple
and unspecialized five-fingered hand is full of possibilities.
These possibilities are given their chance of development,
and are made the most of under the circumstances we
are picturing — circumstances which include the emanci-
pation of the fore-limb as one of the effects of the dawn
of arboreal life. This primitive hand possesses muscles
Avhich can move it upon the ulna and radius at the wTist-
joint, and muscles which can bend the fingers in towards

DEVELOPMENT OF THE POWER OF GRASP 21

the palm (flexors), and others which can straighten them
out again (extensors). It is these finger muscles which
now become so important. We have noticed that some
of the less jDerfect tree-climbers run with great skill about
the branches, and that others climb rather than run,
but they obtain a grip by the specialized use of nails or
claws. It is a characteristic of the pioneer tree-climbers
we are picturing that they begin to grasp by the flexion
of their fingers, and obtain their grip, not by claws or
foot-pads or nails, but by an actual approximation of
the hand and the fingers to the objects up which they
desire to climb.

The power to grasp with the hand and fingers seems
such a very simple accomplishment that it is difficult to
realize how such an apparently trivial beginning can have
produced the tremendous changes that follow in its
train. In essence its beginning depends upon the pre-
servation of a primitive second segment of the fore-limb,
for this has permitted the animal in its endeavours at
climbing to place the palmar surface of its hand and
fingers flat against the next hold for which it reaches out.
The mobility of the second segment already allows of
an adjustment of the hand to the object encountered.
Next, the hand by virtue of its flexor muscles makes the
adjustment more complete. In this way we may imagine
the flngers are closed over smaller branches, and the
animal begins to grasp. Although the picture is entirely
fanciful, we may iniagine that the higher the animal
climbs (the more perfectly arboreal it becomes), the
smaller the branches encountered, and so the more per-
fect the adjustment of the finger grasp. This picture,
although it may be dismissed as thoroughly outside the
precise demands of science, is nevertheless a useful one,
since in dealing with the modifications of such a primitive
fore-limb it is perfectly true to say that the more thor-
oughly an animal becomes an arboreal creature, the more
perfect becomes its hand grasp. The animal now reaches

22 ARBOREAL MAN

out with its fore-limb, throws its body weight temporarily
upon its hind-limbs, and then with its hand catches hold
of something ahead, and so helps to raise its body. This
is true tree-climbing. It is a critical stage in evolution.
The power of the hand grasp has made possible the fore-
runners of the Primates, has perfected the evolution of
the Primates, and paved the way for the development
of Man.

At first, one would suppose this newly acc^uired powTr
to be used solely for grasping the branches in arboreal
progression, for catching hold of objects ahead, and for
hanging on whilst a new foothold is secured. But with
its develox3ing perfection we may imagine the grasp used
for other purposes, and some of these purposes we ^^ill
enumerate here, but will discuss in detail later.

The animal, from grasping branches, may readily turn
to grasping leaves and fruit — it may learn to grasp its
food in its hand. As a sequel it may learn to convey the
food so grasped to its mouth with its hand, and so become
a liand-feeder.

It may take to grasping other objects which come in
its w^ay. These objects ma^^ be useful for food or they
may not; but the animal will learn to form an estimate
of the object grasped. As a sequel it may learn to feel,
and to test novel objects with its hand. Again, the
mother may learn to grasp her offspring in the precarious
circumstances of an arboreal infancy ; and she may adopt
the habit of carrying and nursing her baby. All these
things are of vast importance, and will be discussed
according to the headings under which they appropriately
fall.

CHAPTER IV

THE SKELETON OF THE F0RE-LDII3

What exactly we are to regard as the most simple con-
dition of the actual skeletal structures of the fore-limb
of the primitive land-living VertelDrate is, of course, open
to some doubt; that we shall not be very far wrong in
assuming it to be generally similar to that which is present
in the most generalized Amphibians and Reptiles is
certain. At the time of mammalian divergence from the
Therapsid ancestor, we may assume the limb to be of
this primitive Reptilian t^q^e, with the added tendency
to a general lengthening, to which Broom has attached
so much importance. Tn such a primitive limb there is
a proximal humerus free to move upon the pectoral
girdle at the shoulder-joint. The next segment consists
of a pre-axial radius and a post-axial ulna, both bones
articulating with the humerus at the elbow-joint, and at
that joint both are flexed and extended on the humerus.
Each bone is free of the other, so that movements of
rotation can take 2^1ace between them. Both bones of
the second segment articulate with the first row of the
carpal bones, so that although the hand is flexed and
extended on the forearm, it is rotated with the forearm.

Discussions as to w^hat is the primitive condition of the
carpus, and how this j^i'iii^itive condition has been de-
parted from in different types, open up the possibilities
for widely divergent views. We will here ado2:)t the
oldest and simplest teaching — that of Gegenbaur, which
is backed by the greatest weight of evidence.

Three bones enter into the formation of the first row

23

24 ARBOREAL MAN

of the carpus — a pre-axial bone (radiale), an intermediate
bone (intermedium), and a post-axial bone (ulnare).
The second row is composed of five small and fairly
miiform bones (Carpalia I.-V.), one being situated at the
base of each metacarpal. Between the two rows, and
situated in the middle of the carpus, is a central element
(centrale). (We may, in modern morphology, be forced
to depart slightly from the classical scheme of Gegenbaur,
in admitting the possible presence of more than one
centrale, but this possibility does not detract from the
simplicity of the main plan.)

As to the number of rays in the distal segment of the
limb, we know that among the lowest Vertebrates which
lead aquatic lives they may be extremely numerous; in
the most primitive of the higher land-living classes this
is changed, and the possession of fiv^e terminal elements
has become the rule. This chancre is undoubtediv
associated with the development of extra-neural ribs and
the formation of intra-costal limb plexuses, the number
of epiblastic segments entering into the limb plexuses
being now restricted to five, represented by the five
roots entering into the formation of the brachial plexus.
These five terminal digits are composed of a series of
separate jointed elements, metacarpals and phalanges, of
which there are primitively (or at any rate in very primi-
tive types, if not in basic form) one metacarpal and three
phalanges, or four separate elements, in each digit
(see Fig. 6).

Taking the Mammalia as a whole, and selecting from
any and every type the most unaltered feature of every
segment of the limb, regardless of the condition of the
other segments, we may estimate the amount of minimal
departure from this archaetype consistent with that stage
of evolution lepresented by the Mammalia. The skeletal
elements of the first and second segments may persist
quite unaltered in a large number of Mammals.

The bones of the first row of the carpus and the os

THE SKELETON OF THE FORE-LIMB

25

centrale may remain in the ideal condition. The bones
of the second row invariably show a fusion between
carpale IV. and carpale V. into the unciform bone. The
primitive five digits may persist, but an invariable
reduction takes place in the elements composing the
pollex, which consists of only three segments instead of
.the ideal four.

The immediate interest lies in the fact that these
conditions of minimal departure from the primitive type
are combined in the individual only within the limits
of the Primates and those other animals which we believe
to^be immediately related to them (see Fig. 7). All
other Mammals, though retaining primitive features of

Fig. 6. — The Skeleton of
THE Hand of a Water
Tortoise {Chehjdea ser-
pentina). (After Gegen-

BAUR.)

Fig. 7. — The Carpus as it
EXISTS IN Some Pri-
mates.

The OS centrale is a separate
element.

the fore-limb here and there, show in some other respects
wider departures from the ideal.

Among the Primates we have skeletal fore-limb elements
so little altered from the ideal type that the humerus

26 ARBOREAL MAN

and ulna and radius remain in tlieir primitive condition.
The first row of the carpal bones articulates Avith both
ulna and radius, and consists of all the ideal elements;
radiale {scaphoid), intermedium (semilunar), and ulnare
(cuneiform) are all separate and normal. The os centrale
is present. Carpale I. (trapezium), carpale II. (trapezoid),
and carpale III. (os magnum) are separate, and not
greatly changed; carpale IV. and carpale V. are fused
into the unciform. The five digits are all present in a
very primitive condition, the pollex alone lacking one
of its elements. In addition to these slight modifications
an ulnar sesamoid (pisiform) is present.

Now all these departures from the ideal which are
manifested in the Primate fore-limb are modifications
which have their parallels in very generalized Reptilia.
Fusion of carj)ale IV. and V. takes place even in the
very generalized chelonian carpus; in the same forms
also is seen the identical loss of an element in the pollex,
and the development of an ulnar sesamoid. Two addi-
tional modifications are present in Man and the giant
AnthrojDoids (see Fig. 8). The centrale is lost as a
separate element by fusion with the scaphoid (radiale)
in Man, the Gorilla and the Chimpanzee; this, again, is
a feature of some of the Lemurs and of so primitive a
carpus as that seen in the Chelonia.

Persistence of the os centrale in the human carpus is
not an exceptionally rare anomaly, and many cases have
been recorded by Gruber. Rosenberg has also shown
its normal presence in the human embryo. Anomalies
of the Gorilla and Chimpanzee are naturally less well
known than are those of Man, but even in these animals
the OS centrale is known to occur exceptionally. In the
orang-utan and in the Gibbon it is normal and well
developed.

In Man, Gorilla, Chimpanzee and Orang-utan the ulna
does not directly articulate with the carpus, but is ex-
cluded from contact with the cuneiform (ulnare) by the

THE SKELETON OF THE FORE-LIMB 27

intervention of the triangular fibro -cartilage. This con-
dition is not found in any of the more primitive forms and
must be regarded as a new and possibly progressive
feature.

It is a fact which cannot be ignored, that in the details
of its skeletal elements the fore-limb of the highest of

Fig. 8. — The Elements of the Carpus as present in Man,
IN the Gorilla, Chimpanzee and Some Lemurs.

The OS centrale is not normally present as a separate element.

the Mammals finds its likeness among living Vertebrates
in such a primitive creature as the Tortoise. Witliout
indulging to the full the speculations which such a fact
may prompt, we are justified in saying that the Primates
have retained a fore -limb skeleton that is sins;ularlv like
that with which we have every reason to believe the
ancestral Mammal was endowed.

CHAPTER V

THE CLAVICLE

One other skeletal element of the fore-limb needs brief
mention, and this is the collar-bone, or clavicle, which is
so well developed in Man. Although the homologues of
the clavicle are perhaps more debated than those of any
other bone in the body, it is not proposed to enter into
any discussion regarding the respective merits of those
theories which find the caudal homologue of the clavicle
now in one, and now in another, element of the pelvic
girdle. Here the clavicle will be regarded, as the work
of Fawcett appears clearly to indicate, as an element
peculiar to the fore-limb girdle. We will assume that
the bone of the clavicle is an intramuscular ossification,
making its appearance in the embryo first (at any rate
in the Mammals), by two ossific centres, the one (lateral)
in the deltoid-trapezius muscle sheet, and the other
(median) in the pectoralis-sterno-cleido-mastoid sheet.
These two ossific intramuscular intersections become
continuous, and, in its complete development, the bony
bar thus formed articulates at its median end with the
sternum and at its lateral end with the shoulder girdle
proper. We will further assume that this bony bar is
a purely functional development, that it is laid down as
a firm strut which keeps the shoulder girdle poised at
the sides of the body, and which makes an acting point for
the separated muscles that are derived from the sheets
in which it is laid down (see Fig. 9). As such a strut,
the clavicle is a very ancient possession of the Vertebrates.
It occurs in the Dipnoi and certain other primitive fish.

28

THE CLAVICLE

29

It may in some fishes become complicated by other
struts not primitively parts of the shoulder girdle, or it
may strut the shoulder girdle, not to the sternum, but
to the skull, or to some outlier of it. A clavicular strut
derived from dermal bone is present also in the majority

Fig. 9. — The Human Shoulder Girdle, to show the Strut

Action of the Clavicle.

of the Amphibians and Keptiles. Within the limits of
these groups great range of variation in development is
seen; the dermal struts may attain great complexity, or
they may be altogether absent, and, on the whole, a
functional rather than a svstematic cause underlies the
degree and condition of their presence. The dermal
strut occurs again in the lowest Mammals, and liere in
perfect reptilian complexity of structure, the condition
present in the Prototheria (Duck-billed Platypus, etc.)

30

ARBOREAL MAN

being likened by all comparative anatomists to that in
the typical pre-mammalian Vertebrates. In the rest of
the mammalian orders the appearance of the clavicle,

if we take the systematic posi-
tion of the animal as our guide,
can only be described as hap-
hazard. Among the Metatheria
(Marsupials), only Perameles (the
Bandicoot) fails to possess a
clavicle. Among Eutheria (higher
Mammals), a complete clavicle
is present in all Insect ivora,
except the aberrent aquatic
Potamogale ; it is present in some
Edentata, in all Cheiroptera, and
all Primates. It is entirely
absent in Cetacea and Sirenia
and Ungulata (see Fig. 10); in
most Carnivora it appears onl}^
as a rudiment, though in some
members of this order it attains
a fair degree of development;
in the Rodent ia it is sometimes
well developed and sometimes
entirely absent . The only under-
lying principle which seems to
explain the rather random de-
velopment of this bone in differ-
ent IMammals appears to be
found in the functional demands
made upon the movements J of
the fore-limb. So long as^ no
more demand is made than the
simple backward and forward movement at the shoulder-
joint, such as is seen in the walking and trotting of pure
quadrupeds, this strut is either not develo^Ded, or attains
no greater perfection than that of a mere isolated inter-

FiG. 10. — The Shoulder
Girdle of an Ungu-
late, TO SHOW THE

Absence of the Clav-
icle.

The fore-limb has no strut
to keej) it poised at the
side of the thorax.

THE CLAVICLE 31

muscular ossification. Even the simple use of the fore-
limb as a paddle is carried out in the absence of any strut,
and the clavicle fails to be developed in aquatic paddlers.
It is the wider range of movements of the shoulder-joint,
such as culminates in the free action of circumduction,
that preserves in full functional development this primi-
tive vertebrate heritage in the mammalian shoulder
girdle.

Within the groups Carnivora and Rodentia, it is easy
to see that freedom of fore-limb and clavicular develop-
ment go hand in hand. It is safe to assert that a Mammal
possessing a fore-limb which, from any cause whatever,
has become in an}^ considerable degree emancipated
from the function of mere quadrupedal progression will
also possess a clavicle of fairly complete development,
no matter what the systematic zoological position of the
Mammal may be.

Emancipation of the fore-limb has preserved the
clavicle, inherited in the general vertebrate make-up of
the first mammalian type, and has insured its survival
in a simple yet very perfect form in Man.

The arboreal habit, as the great factor in preserving
and increasing the original mobility of the fore-limb,
has also been the great factor in preserving in the human
shoulder girdle a well-developed collar-bone.

CHAPTER VI

THE MUSCLES OF THE FOKE-LIMB

Turning from the skeletal features of the fore-limb to
the arrangement of its muscles, we again meet a like
condition of extremely primitive characters in the typical
Primate and in Man. Although most authorities are
agreed as to the general primitive condition of the human
arm and hand — and it is difficult for an osteologist to find
anything other than a very primitive arrangement in the
general plan of the bones — yet the myologist has as a
rule looked upon human musculature from rather a
different standpoint. This is not the place to make
detailed criticisms of the methods employed in com-
parative myology; but one might say that if the human
fore-limb muscles are compared with those of a typical
quadrupedal Mammal, then certain great changes will
be found. It is easy to assume that since Man is a
" high " form, and the horse, for instance, is a " low "
one, then the fore-limb muscles of Man have advanced
considerably in evolution. This type of reasoning
permeates the study of comparative myology, and its
fallacy needs no exposing since it is self-evident. If
there is any truth in the i)resent thesis, that the human
stock has never been quadrupedal, never possessed four
equally supporting limbs, then it is likely that the arrange-
ment of muscles found in the human limb will have its
near counterpart in some very primitive ''Vertebrate.
It is likely that the muscles and the bones will follow
exactly the same story. I think it is jDerfectly obvious
that they do. Far from seeing any signs that the deriva-

32

THE MUSCLES OF THE FORE-LLMB 33

tion of the muscular plan of the human arm is from that
seen in any lower quadrupedal form, it seems quite
obvious that the truly quadru^Dedal pronograde type is
derived from a primitive arrangement such as is retained
in Man.

We have an excellent account of a very generalized
vertebrate type of musculature in the description given
by Humphry of Cryptobranchus japonicus, and to the
stage of evolution of the limb musculature as seen in
this Amphibian we shall have to make frequent reference.
The group of muscles which especially interests us here
is that which produces the rotatory movements of the
second segment of the limb, and in order to limit the
purely anatomical details we will follow the history of
those rotators which produce the movement of pronation
— i.e., turn the back of the hand upwards. This group
is of primary importance, since it produces that mobility
of the second segment which, as we have seen, is incom-
patible with true quadrupedal stability. The pronators
compose a primitive group of muscles which shows the
effects of alteration of function in a very definite manner;
the condition of the members of the group is easily
determined in different animal types, and their disposi-
tion may therefore be taken as a handy index of the
degree to which mobility has been sacrificed to stability
in the forearm. In Cryptobranchus this muscle group is
well developed; but, as is the case with all the muscle
groups of this very generalized animal, it is not so defi-
nitely subdivided into its component elements as it is
in higher forms (see Fig. 11).

The pronators of this Am^Dhibian consist of a super-
ficial part, which arises from the ulnar cond^de of the
humerus in common with the M. flexor carpi radialis,
but, dissociating itself from the common mass, is inserted
into the lateral margin of the radius. Beneath this
portion is a deeper set of fibres, which at its insertion to
the radius becomes continuous with another sheet which

3

34

ARBOBEAL MAN

arises from the ulna. This sheet, which passes from the
ulna to the radius, is oblique in direction above, but
more transverse nearer to the \mst, where it becomes
continuous with another set of fibres, which, arising from
the ulna, passes to the radial side of the ^^Tist and hand.

— P.RT.(1)

P.R.T.(2)

P.M.

-P.R.T.d)
P.R.T.(2)

P.Q.

Fig. 11.

Fig. 12.

Fig. 11. — Diagrammatic Figure of the Pronator Group of

Muscles in CryptohrancJius.

P.E.T. (1), Mammalian superficial head of pionatoiv radii teres.
P.R.T. (2), Deep Mammalian head of same muscle. P.Q.,
Pronator quadratus. P.M., Prooator manus.

Fig. 12. — Diagram of the Pronator Muscles in Man.

P.R.T. (1), Superficial, humeral, head of pronator radii teres.
P.R.T. (2), Deep, ulnar, head of same muscle. P.Q., Pro-
nator quadratus. S.L., Supinator longus. S.B., Supinator
lirevis.

There is therefore in this generalized animal a more
or less continuous pronator sheet, the proximal part of
which is superficial, and passes from humerus to radius,
where it blends with an intermediate part. This second
part is deeper, and passes from ulna to radius, and, in its
turn, blends with a distal part which passes from the
ulna to the radial side of the third segment of the limb.
From this unspecialized condition, advance takes place.

THE MUSCLES OF THE FORE-LIMB 35

as is the general rule, by the segmentation of muscular
sheets into separate muscles; and a comparison may be
made directly between this simple type and the more
specialized condition seen in some other animals. The
superficial portion of the proximal mass becomes the
humeral head of the M. pronator radii teres. The deep
portion of this mass, which is blended with the inter-
mediate sheet, becomes the M. pronator intermedins,
or ulnar head of the M. pronator radii teres. The distal
mass, or M. pronator manus, looses its insertion to the
third segment in many forms, and, united with the lower
portion of the intermediate sheet, it constitutes the
M. pronator quadratus.

With the differentiation of the muscular sheets of
Cryptohranchus there is therefore afforded a myological
basis for (1) a bicipital M. pronator radii teres, and (2) a
definite M. pronator quadratus, which may, or may not,
possess extensions do\\TLwards to the carpus.

This is the condition found in many unspecialized
Reptiles and Amphibians, as well as in the most primitive
Mammals, and it may be regarded as the simplest type
of resolution of the primitive muscle sheets.

But if this condition of the pronator group may be
regarded as primitive, it is obviously one that is very
readily departed from at the dictates of functional
demands for the stability of the limb, and one that has
a very limited representation among the existing
mammalian types.

To gain an insight into the possible modifications of
this muscle group it will be best to take each muscle
separately; and the most simple comparison may be
made in the apparently illogical order of taking tlie
human condition first.

The M. pronator radii teres is a typical bicipital muscle
in Man. The superficial, and larger, portion arises from
the medial aspect of the lower end of the humerus; the
deeper, and smaller, portion is derived by a tendon from

36

ARBOREAL MAN

the coronoid process of the ulna (see Figs. 13 and 14).
Between these two heads of origin the median nerve

S.B."

P.R.T,

P.Q,

P.R.T.d)

--P.R.T.(2)

Fig. 13. — Pronator Muscles of Man.

P.R.T. (1), Site of origin of superficial head, and P.R.T. (2), of
the deep head of the pronator radii teres, the insertion of
which is at P.R.T. P.Q., Pronator quadratus. S.B., Supi-
nator brevis.

passes into the forearm, thereby rendering the two
portions quite distinct. The deeper (ulnar) part is

THE MUSCLES OF THE FORE-LIMB

37

variable in the degree of its development in Man; at
times it constitutes but a small portion of the whole
muscle, and occasionally it is entirely absent. In all

-P.R.T.

Fig. 14. — The Insertion of the Bicipital Pronator Radii

Teres of Man (P.R.T.).

S.B., Supinator brevis.

the Anthropoids this variability of the ulnar part is
displayed, with an increasing tendency to vary in tlie
direction of partial or complete absence.

In the Orang-utan the condition is practically identical
with that seen in Man. In the Chimpanzee, Hepburn

38

ARBOREAL MAN

described the ulnar head as normally present ; but Keith
has shown that it is absent in as many as 10 per cent, of
all examples dissected. Absence of the ulnar head is
more usual in the Gorilla; Hepburn regarded complete
absence as normal, but Keith has determined that it was
present, in some degree, in about 40 per cent, of all
individuals.

In none of the Monkeys is an ulnar head present under
normal conditions, and the Lemurs are alike in this
respect (see Fig. 15).

The ulnar head is absent in all other orders of Eutherian
Mammals, with the exception of at any rate some of

— P.R.T.

— P.Q.

Fig. 16.

Fig. 15. — Diagram of the Pronator Muscles in Typical

Primates.

Lettering as before.

Fig. 16.

-Diagram of the Pronator Muscles in a Tree Shrew
( Tupaia ferruginea).

Lettering as in other diagrams.

the Insectivora. In the Tree Shrews (e.g., Tupaia
ferruginea) there is (despite Kloster's assertion to the
contrary) a portion of the muscle deep to the median
nerve, and this portion arises from the upper extremity
of the ulna and from the internal collateral ligament

THE MUSCLES OF THE FORE-LDIB

39

of the elbow-joint (Figs. 16 and 17). In an Oriental
Pygmy Shrew (Crocidura hottigi), again, the mnscle is in

Fig. 17. — The Pronator Quadratus and Insertion of
Pronator Radii Teres in Tupaia ferruginea.

its bicipital form (Fig. 18). In all the Metatherian
Mammals the only origin of the M. pronator radii teres
is from the humerus, the ulnar head being absent. In

Fig. 18. — The Pronator Radii Teres and Pronator
Quadratus in a Crocidurine Shrew.

the lowly Monotremes the human deep head has been
asserted to be present, but it cannot be regarded as a
normal component of the muscle.* Among the Reptilia

* At the time of delivering the lecture I relied upon the latest
paper published upon the subject — that by Gordon Taylor and
Victor Bonney {Jour. Anat and Phys., vol. 40, p. 34) — which
definitely asserted the presence of the ulnar head in Ornithorliyn-
chus and in Echidna. Since that time 1 have dissected the fore-
limbs of two examples of Ornithorhynchus and one Echidna
(kindly placed at my disposal by Dr. W. C. McKenzie), and
I find no trace of an ulnar head in any of these specimens, tlie
condition agreeing with that described by Rud. Kloster (" Anato-
mische Hefte," 1901, p. 671).

40 ARBOREAL MAN

the same condition is found; an ulnar head constitutes
a bulky portion of the muscle in the Chelonia {e.g., Testudo)
and in the Lacertilia {e.g., Varanus). Among the general-
ized Amphibia, Cryptohranclms shows the same thing in
the ill-differentiated form we have noted x^reviously.

The story of the M. pronator quadratus is equally
striking. In Man an interosseous membrane unites the
two bones of the forearm, and although in this membrane
there is, as is usual in such membranes, a crossed arrange-
ment of its fibres, nevertheless the great bulk of the
strands run from the radius down to the ulna. On a plane
altogether anterior to this membrane the muscle bundles
of the M. pronator quadratus run on the whole in an
op230sed direction, from ulna down to radius. It is
important to notice that behind the M. pronator quad-
ratus the interosseous membrane is quite uninterrupted;
the muscle does not replace the membrane, but lies in
front of it.

Tliis condition is typical of all tlie Primates. It is
found in some of the more mobile-limbed members of
other Eutherian orders. It is present in Tujmia and in
Cfocidura, in typical^ human guise; and it is present
again in generalized Reptiles and in Amphibia. Its con-
dition in CryptohrancJius has previously been mentioned.

A true M. pronator quadratus is not present in those
thoroughly quadrupedal animals in which the rotation of
the two bones of the second segment is lost. The true
M. pronator quadratus is not to be confused \vith the
M. radio-ulnaris, which is a purely interosseous muscle,
homologue of the M. tibio-fibularis of the leg, and is
situated on a plane posterior to the fibres of the distinct
M. pronator quadratus.

The facts of the occurrence of these muscles in the
Vertebrate series must be admitted to be very curious,
since the typically human condition is such a rare mam-
malian feature, and yet is one so closely matched among
the generalized Amphibia and Reptilia. The facts may

THE MUSCLES OF THE FORE-LIMB 41

be interpreted in two ways — the human condition may
be a new and gradual development from the stage seen
in the typical lower Eutherian Mammals, or it may be
a retention of an extremely primitive and generalized
vertebrate type of musculature. Most authorities upon
comparative anatomy appear to regard the ulnar head
of the M. pronator radii teres as a new development in
the Anthropoids — a development which becomes most
fully established and most fully perfected in Man. This
development is, by many, regarded as a reversion. This
point of view was taken by Gordon Taylor and Victor
Bonne}^, and they have concluded regarding this muscle :
" It may be objected that it appears rather strange that
in Man, a Mammal most highly specialized, and the most
highly evolved, the apparently older stage in evolution
of the muscle persists. But we must remember that in
him movement between the bones of the forearm has
reappeared in an extreme degree." Were the human
condition of this muscle to be an isolated phenomenon,
perhaps such an attitude might be justified; but when
the primitive type of every bone and joint of the human
fore-limb is taken into account, we must hesitate before
we name these things as " reappearances " in Man.
That the human ulnar head of the M. pronator radii teres
is a retention of a primitive type appears to me to be a
more reasonable view w^hen all the facts of the anatomy
of the fore-limb are taken into consideration. I therefore
regard this muscle of Man as being more akin to the
ancestral type than anything seen in the rest of the
living members of the Primates.

As for the human M. pronator quadratus, it is usually
regarded as being only a partial survival of a primitively
extensive interosseous muscle, which is best developed
in quadrupedal forms. We have previously pointed out
that the interosseous muscle (M. radio-ulnaris) is on a
plane which is deep to that occupied by the true M.
pronator quadratus, and the nerve-supply points also to

42 ARBOREAL MAN

their entire morjDhological separation. I do not think
the facts justify us in regarding the human M. pronator
quadratus as a degenerated portion of any muscle present
in quadrupedal Mammals, but I imagine that this muscle,
which produces rotation of the fore-limb bones, is absent
in them, and is replaced by a muscle which braces the
immobile bones firmly together. The human M. pronator
quadratus finds its parallel in the same forms as does the
deep head of the M. pronator radii teres, and I imagine
that their story is the same, and that their retention is
due to the same primitive nature of the forearm in these
types.

It is probable that when all power of rotation of the
forearm bones is lost, the ulnar head of the M. pronator
radii teres, being useless under the circumstances, shifts
its origin to the humerus, and joining into the superficial
mass, acts with it as a flexor of the elbow-joint.

Regarded in this way, and solely from this point of \
view, the forearm of Man is more primitive than that of
any living Primate except the Orang-utan; but it finds
its match in the generalized Insect ivora, in the Proto-
theria (in part), and in the unspecialized Reptiles and
Amphibians, and this is a story very like that told by the
bones themselves.

CHAPTER VII

THE FOEE-LIMB: SUMMARY

It would be a difficult matter to find the author who,
writing of the human forearm and the human hand, has
not seen in them the very highest and most perfect
development of the fore-limb found anywhere in the
animal kingdom. It has long been customary to lavish
praise upon this culmination of human perfections, or
climax of evolutionary advances, as writers of different
periods have judged it. The divine plan was most
surelv to be seen in the human hand, that most wonderful
of specially designed members. " The Construction of
the Hand of Man " was especially chosen by the trustees
of the Earl of Bridgwater as a subject in the expounding
of w^hich an apt writer could find outlet for almost in-
exhaustible eulogies, and for countless examples of
perfection of design. It is, perhaps, to be doubted if
Sir Charles Bell, in his completed Bridgwater Treatise,
took full advantage of the wealth of material at his
disposal, or of the iuvsatiable popular appetite for authori-
tative statements upon the human perfections. Bell was
so thorough an anatomist that it was impossible for him
to restrain his admiration for the lion's paw and the
horse's hoof — even the anatomical conditions of the
despised sloth find in him an admirer; but although they
are extremely elegant, his observations upon the liuman
hand are not perhaps coloured with an enthusiasm so
real as that which the noble patron himself entertained.
" Were we to limit our inquiry to the bones of the arm
and hand of Man, no doubt we should soon discover their

43

44 ARBOREAL MAN

provisions for easy, varied, and powerful action, and
conclude that nothing could be more perfectly suited to
their purposes. But we must extend our views to com-
prehend a great deal more — a greater design." This,
and many other similar passages, shows Bell's attitude in
the work he did for his Bridgwater Treatise, and it is to
be regretted that many lesser writers, who were un-
trammelled by the confines imposed by so narrowing a
circumstance, did not follow Bell in this width of outlook.
Those modern authors who have seen so much in the
so-called " attainment of the erect position " (Munro)
have been especially lavish in their praise of the human
hand as a mere anatomical structure. Dr. Munro in his
Presidential Address at the British Association in 1893
permitted himself the expression that the human hand
is "the most complete and perfe.C-t_naechanical_£Li:gaJtL-
Nature has yet produced." Such a statement on the
part of an anatomist can onl}^ be attributed to enthusiasm,
and to a failure to differentiate between the very primitive
anatomical condition of the hand and the perfection of
this simple mechanism when linked to a human brain.
Even John Goodsir was more moderate, for he claimed
no more than that " the human hand is the only perfect
or complete hand."

The hand with its multitude of uses, its better suiting
to human jourposes than such a thing as a hoof or a paw,
its apparent complexity and perfection of movement,
w^as a thing so easily turned to as affording evidence of
design — ^and by design was meant a special and divine
planning. In 1833 almost any anatomist in the United
Kingdom could have done the Bridgwater Treatise more
to its purpose than did Sir Charles Bell. As things were,
and with the height of apparent incongruity, the ])Ook
he wrote in 1833 makes a very suitable introduction to the

\ work of Darwin twenty-six years later.

^ After 1859 the forearm and hand, in common with
everv other feature of the human bodv, came to be

THE FORE-LIMB: SUMMARY 45

regarded, not as a wonderful and specially designed
structure, but as the perfected products of accumulated
ages of evolution — the last thing in animal development
and specialization. It is no overstatement of the case
to say that Man was regarded by many as the last thing
made, the culmination of evolution, and for some op-
ponents of the new teaching and for some of its sup-
porters he was the most modern animal. The orthodox
chronology was accepted, the " highest " form was the
last form made, but instead of being the latest creation,
he was the latest evolution. Huxley soon exposed the
folly of this nocion when it was definitely brought forward
by an opponent. But though the statement of the idea
as expressed by Mr. Gladstone may have been very crude,
and its demolition easy by such powers of argument as
were Huxley's, still, in more subtle guise the same idea
becomes presented under many forms even to-day, and
this not by any means necessarily from opponents of
evolution ; in such forms its refutation is not always easy.
In even the most rigid and strictly scientific investigations
in comparative anatomy this tendency is at times mani- '
fested. The human type of joint, or nerve, or muscle, or y
what not is so often assumed to be the last perfected—
the culminating type. There is a vague idea, which
insinuates itself in many ways, that the human type of
structure must be derived from, and have passed through,
stages seen in a series of " lower " animals. A foolish
argument may be permitted in dealing with a folly.
Were a horse capable of writing works on comparative
anatomy, he would probably, and with far more justice,
regard his race as being the last effort in evolutionary
chronology, and he would, and again with far more
justice, derive his highly specialized limbs from those of \
some such primitive form as Man.

A Bridgwater Treatise upon " The Construction of the \\
Hoof of the Horse," followed by a " Descent of the '
Horse " by a member of the same species, would be a

46 ARBOREAL MAN

most healthy tonic for the human comparative anatomist
as well as for the human philosopher ; in these two hypo-
thetical works there is no doubt that the human fore-
limb would suSer badly. Far from being regarded as
the acme of evolutionary processes, it would be judged
as an extraordinary survival of a very primitive feature
far into the mammalian series, and more would be written
upon its striking similarity to the corresponding member

I in the salamander and the tortoise than of its adaptation

^to the multitude of human functions. This is a silly
argument, and no comparative anatomist not resident
in the kingdom of the Houyhnhnms would enter into
discussion with a quadruped that wrote a thesis showing
that the human fore-limb was very like that of a water-
newt. I have, however, brought the subject forward in
this way of set purpose, for as unbiassed judges of our-
selves we are to say definitely one way or the other :
Is the arrangement of bones and muscles we have seen
in the human arm a gradually elaborated evolutionary
perfection, or is it merely the retention of a condition : o
primitive that it is matched only among its immediate
kin, and by types situated in the vertebrate stock right
at the point of mammalian divergence ? In anatomical

' terms we may say : Have we lost a primitive arrangement
of bones and muscles, and then regained them, in evolu-
tion, upon exactly the same lines, or have we simply

2 retained them comparatively unaltered from the dawn
of mammalian specialization ? We must not overlook
in this the gravity of the second alternative, for it carries
with it the assumption that the human stock began to
be differentiated in that dawn period when the ^Mammals
themselves were evolved from some possible Theromorph
ancestor. With all the evidence that is available I cannot
see how it is possible to avoid this second conclusion.
In bones, and in muscles, the human fore-limb is far
more like that of a tortoise than it is like that of a horse
or a dog. This is no fanciful way of stating the case,

THE FORE-LIMB: STOfflARY 47

nor is it going one whit farther than the ordinarily gross
facts of demonstrable anatomy warrant. Could we
imagine an isolated human arm to be the only relic
extant of the human race, and were this arm to be dis-
sected by some superanatomist, he would find the arrange-
ment of its skeletal and muscular elements matched
very nearly in the Giant Apes and Old-World jMonkeys,
in some of the lowest Lemurs, and some primitive In-
sectivora, as well as in the more unspecialized Reptiles
and some Amphibians; but he would search in vain for
its like among the remaining mammalian groups.

CHAPTER VIII

THE FATE OF THE HIXD-LIMBS

We have hurriedly reviewed the process by which a
primitive Mammal with four undifferentiated and mobile
limbs achieved the emancipation of its fore-limb by its
climbing activities. It is now necessary to make an
attempt to follow the changes which take j)lace in the
hind-limb under the same circumstances. This phase is
rather more complex in the hind-limb, and though the
changes produced are not, perhaps, so great, their sequence
has been more liable to interruption. The most primitive
type of hind-limb we may imagine is an exact counter-
part of the picture we have drawn of the fore-limb. It
corresponds segment for segment, and joint for joint, with
those described in the fore-limb. It has all the same
possibilities; its fate depends in great measure upon the
emancipation of the fore-limb. We have pictured the
animal in its initial stages of tree-climbing as reaching
out, with its fore-limb, to obtain new holds. It is during
this oft-repeated interval that the fate of the hind-limb
is determined, for, during this interval, it becomes the
supporting limb upon which the body weight is thrown.
Here is therefore the dawn of the differentiation in
function of fore and hind limb ; the fore-limb is reaching
ahead for a new hold, the hind-limb is temporarily
supporting the body during the act. It must be noted,
hoAvever, that this supporting is of a very definite kind,
and is not by any means of the same nature as that which
is brought about in those animals which, being purely
terrestrial, have become typically quadruj)edal. In

48

THE FATE OF THE HIND-LIMBS 40

arboreal life, the hind-limb never becomes a mere stable
prop ; it becomes the principal support of the body weight,
but it is a support which is bearing a body undergoing
endless changes of poise. Moreover, it is discharging this
function among the branches of a tree; the foot is not
resting on the ground, it is placed in apposition with a
branch. The sole of the foot becomes applied to the
branch of a tree, in the same manner as does the palm
of the hand. The mobility of the second segment of the
lower limb becomes limited and restricted to definite
lines, but it does not become lost; the simple condition
of the foot becomes retained even if not so completely
as in the fore-limb. The power of grasp of the foot is
developed, though not to the degree of perfection which
is seen in the hand. We may imagine the evolutionary
story to have been carried out somewhat on these lines.
The animal pauses in its attempts to climb, it reaches
for a new hold with its hands, and so trusts to its legs
for its support. Later, the power to grasp becomes
more perfectly developed in the hand, and when it has
secured a new hold it can grasp and suspend the body
weight while the foot reaches farther ahead for a new
foothold; a degree of mobility of the second segment of
the leg is thus retained, and a degree of development of
grasp with the foot is thus developed. From the attain-
ment of this stage, two divergent developments are
possible. The hind-leg may develop a degree of mobility
and of grasp equal, or almost equal, to that of the hand,
a condition which fits the animal for the time-honoured
distinction as quadrumanous or four-handed. The foot
may become, equally with the hand, a grasping and
suspending organ; or the hand and forearm may be
specialized as the mobile grasping-suspending organ, and
the leg and foot as the supporting — still somewhat mobile
and somewhat grasping — organ. It is the grasping-
supporting and not the grasping-suspending leg that has,

from this common point of divergence, led to better things.

•1

f.O ARBOREAL MAN

The one is characteristic of the higher, and the other of
the lower, living members of the Primates. The most
typically arboreal of the Lemurs know but little dis-
tinction of hand and foot ; both are equally grasping-
suspending organs, and as a consequence it matters little
to the animal if it hangs or climbs head upwards or head
downwards. Nycticehus tardigradus positively seems to
prefer an inverted position, and I have noticed that,
when perfect freedom of action is permitted, the animal
nearly always suspends itself by its feet and hangs head
downwards whilst it eats. When going to rest in the
daytime, it will climb to the top of its cage, and then,
turning round, go to sleep upside down like a bat. In
resuming its activity towards evening, it releases the grasp
of its hands, and carries out a careful examination of
everything within its reach before it relaxes the grasp of its
feet. Nycticehus will also grasp food and other objects
with its foot, but shows nevertheless a decided preference
for using its hand for this purpose. This specialization
of the foot as a grasping organ has been carried still
further in the New- World monkeys, and it has conferred
upon that group the title Pedimana, or foot-handed, in
the classification of some former zoologists. In the
American monkeys, the development of the prehensile
tail and specialization of the grasping foot at the expense
of the grasping hand has played a very important part,
and to this question we will return later. The higher
Primates of the Old World, on the other hand, have
differentiated the functions of the hind and fore lim])s
very thoroughly. They suspend themselves only by the
fore-limbs, and use their hind-limbs solely for passive,,
but still grasping, support ; they do not hang or climb
head downw^ards. There is a homely, but not therefore
necessarily unimportant, difference manifested in the
arboreal activity of these two extremes in Primate life.
A Lemur climbs up among the branches head fii'st ;
Nycticehus ascends with extraordinary deliberation,

THE FATE OF THE HIND-LIMBS 51

climbing hand over hand and testing every new hold
that it obtains before finally trusting its weight to it.
When it has reached the limit of its ascent, it commonly
turns round, and, hanging by its feet, eats its meal or
performs its toilet head downwards. It is from such a
position that it descends, and its descent is carried out
in exactly the same manner as its ascent, but naturally
in a reversed position — it crawls and climbs down head
forwards. All this is very easily watched in Nycticehus,
because its actions are so deliberately leisured and orderl}^
but the same head forward descent is typical of all the
Lemurs I have had the opportunity of watching.

The New- World monkeys do the same thing, but in
them the use of the prehensile tails of some species rather
complicates the process of climbing down head foremost.
Now, it is an observation easily made w^herever a higher
Old-World monkey is to be seen, that although it climbs
up a tree, it walks down again hind end foremost. Most
monkeys come down a tree just as a man does, bearing
the weight of the body by the suspending hand grasp
and by the supporting foothold. As a man descends
a ladder, so a higher monkey descends a tree. We may
sum up this process by saying that the lower Primates
climp up trees and climb down again, but the higher
Primates climb up and then walk down.

Now the difference shown in these two simple cases is
in reality a very great one. The arboreal habit conferred
its benefits by emancipating the fore-limb from the duties
of support and progression, and, by differentiating its
functions from that of the hind-limb, it saved the animal
from becoming quadrupedal. In differentiating the
functions of the two sets of limbs, the animal gains a
great deal. Some animals, one might almost say, have
gone too far in adapting themselves to the arboreal habit.
An animal, saved by the arboreal habit from becoming
quadrupedal, does not gain the maximum of the benefits
derivable from its new mode of life, if it is saved from

52 ARBOREAL MAN

this fate only to become quadrumanous. Four feet do
not lead far in the struggle for mammalian supremacy,
four hands do not lead a great deal farther. It was the
differentiation into two hands and two feet that provided
the great strength of the stock from which Man arose.

The active specialization of the fore-limb did much,
but it could not do all, without the accompanying passive
specialization of the hind-limb. Mere ability in climbing,
which usurped the power of any real ability to walk,
was but a poor accomplishment, for to complete the whole
story of evolution the animal which climbed up the tree
had still to walk down — and the Old-World apes still
show in caricature how this was done.

v

CHAPTER IX

THE SKELETON OF THE HIXD-LLMB

We have seen that the human arm and hand exhibit a
strikingly primitive anatomical picture, and that, on
the whole, the resemblance of these parts of Man to the
same parts of the rest of the Primates is very great. We
have arrived at this conclusion despite the rather common
assumption that in the hand of Man there is evinced the
very highest human specialization and refinement.
Compared with the fore-limb, the hind-limb is apt to be
ranked as a rather primitive and unspecialized thing
in Man. Thjs, assumption, again, is contrary to all the'
facts, /for if^we regard the hind-limb as presenting a
more primitive condition in certain Primates (and this
is a well- justified point of view), /we must admit a very
definite alteration from the primitive arrangement in
the leg and foot of Man. ^The human hind-limb has
specialized considerably fimn the condition seen in the

)preal Monkeys, and the arboreal hind-limb is, as we
shall see, far nearer to the primitive Vertebrate type.
Between the anatomical condition of the hind-limb of
the Anthropoids and that seen in Man there is apparently
a somewhat sudden break in the story of the evolution
of the leg. But the changes ^^1lic•h ultimately become
so characteristic of Man are already at work in the
GrTBBons, the Orang-utan, the Chimpanzee, and especially
in the Gorilla. They are already apparent in some of
the Old- World monkeys.

There will be no need to discuss at all fully the anatomi-
cal details of the primitive hind-limb, since the likeness
to the fore-limb, which we have already touched on, is

53

54 ARBOREAL MAN

very great. There is a ventral meeting of the elements of
the pelvic girdle at the pubic symphysis. This meeting
of the ventral parts of the pelvic girdle subserves the
functional role played by the clavicle in the anterior
extremity, and this bone is not represented morpho-
logically in the skeleton of the posterior extremity. The
hip-joint obviously corresponds to the shoulder. The femur
with its muscles recapitulates the features we have noted in
the humerus, the knee-joint corresponds to the elbow.

The tibia and fibula, free to move on the femur at the
knee-joint, and free to move upon each other, are the
homologues of the radius and ulna (see Fig. 19). The
ideal tarsus consists of nine bones : tibiale, intermedium
and fibulare constituting the first row, tarsalia I.-V. the
second, and an os centra] e is included between the two
rows, this condition obviously reproducing that we have
already noted in the carpus. Five metacarpals and five
digits with their appropriate muscles complete the archi-
tecture of the foot. The digits are composed (as in the
hand) of three separate phalanges, and all the digits are
well developed, with the middle one as the longest member
of the series.

These are the features which we may presume to be
present in the primitive hind-limb, and they will become
modified, now in this direction and now in that, as func-
tion demands it.

There is a strong presumption that the hind-limb will
depart more early than the fore-limb from this primitive
condition, and this presumption is strongly borne out by
the facts. From what we have seen of the effects of even
minimal demands for a supporting function in the case
of the fore-limb, we shall not expect to find a thoroughly
primitive hind-limb at all far up in the land-living verte-
brate stock. The hind-limb was called on, in the land-
living Vertebrates, at the very dawn of mammalian
specialization, as a support for the body weight. Sta-
bility became substituted at the outset of the story for

THE SKELETON OF THE HIND-LIMB 55

mobility. Environmental conditions could not com])ine
to free the hind-limb of its duty of .su])])orting tlie l:)ody
weight and yet preserve it in full functional activity;
the arboreal habit did this for the fore-limb, but there
was no life circumstance that could do the same thing
for the hind-limb. An aerial life might, at fir^jt sight,

Fig. 19. — Diagrammatic Comparison of the Skeletal
Elements of (A) Hind and (B) Fore Limb.

seem to fulfil the necessary conditions, and flight might
seem to afford an escape from the supporting servitude
of the hind-limb. Flying Mammals have achieved many
interesting modifications in hind -limb structure, but they
have not successfully emancipated a hind-limb to give it
other and more highly educational functions. They have
avoided making it a mere prop only to convert it into a
suspending hook. The hind-limb of the Bats is worthy

56 ARBOREAL MAN

of attention for its very special adaptations; but it is not
a member destined to carry its owner far in the race for
mammalian supremacy.

Only a purely aquatic life could produce an animal in
which the hind-limb took no part whatever in the support
of the body weight, and in the thoroughly aquatic forms
(Sirenia and Cetacea) the hind -limb, deprived of this
function, becomes a mere rudiment. Consequently,
even in the most primitive of the prototherian Mammals,
Ave find that the ideal condition is somewhat widely de-
parted from. In the Monotremes the fibula is large, and
from its proximal end a process rises above the point of
articulation with the tibia, and both tibia and fibula are
separated in their whole length: both bones of the second
segment are preserved, and some degree of mobility
between them still survives the demand for stabilitv.

ft/

The mobility of tibia and fibula upon each other is, how-
ever, best retained in the arboreal Metatheria, where,
especially in Phascolarctus and some of the Phalangers,
the power of rotation rivals that displayed between the
radius and ulna. In the arboreal Sloths, again, the fibula
is a large and well-formed bone which articulates with
the tibia at its two extremities. In the Tree Shrews (Tu-
paiadce) the fibula is well developed and entirely sep-
arated from the tibia, whereas in many terrestrial Insec-
4 f. tivora it has become reduced and fused to its neighbour.
In the Primate stock good development, complete
V^ V separation, and even slight mobility of the fibula upon

the tibia, are maintained as a part of the arboreal adap-
tation. In all other mammalian orders the fibula tends
r^ to undergo the reduction we have noted in the ulna of
the fore-limbs of quadrupedal animals; all movement
between it and the tibia is lost earlv, and the fibula be-
comes a rudiment finally blended into the structure of
the dominant tibia.

No Mammal retains the ideal primitive tarsus, but the
same may be said of existing Reptiles and even of their

X

THE SKELETON OF THE HIND-LIMB 57

fossil representatives. The tibiale and intermedium are
fused to form the astragalus, while the fibulare remains
large and distinct as the calcaneum, thus reducing the
first row to two bones. The centrale remains as the
large scaphoid; tarsalia L, IL, and III. persist as distinct
elements, the inner, middle and external cuneiform bones ;
while tarsalia IV. and V. fuse into the cuboid. This con-
stitutes the minimal mammalian reduction (and indeed
the minimal reduction of existing Reptiles), and as such
it is typical of the Primates (see Fig. 20). It is also seen
in the Insect ivora and in some other orders, but it is

Fig. 20. — Skeleton of the Human Foot, to snow the Fate
OF THE Primitive Elements of the Tarsus.

carried farther by other fusions and reductions in most
highly specialized quadrupedal Mammals.

The five primitive digits remain in their elemental
development and relative proportions in the Primates,
as well as in the Insectivora and some other orders, and,
like the thumb, the big to^. has undergone a reduction by
the loss of one element. In most of the members of the
Primate stock the primitive formula of the digits is
retained, and the third or middle toe outstrips its neigh-
bours as does the middle finger.

It may therefore be claimed for the typical Primate
hind-limb that almost as far as original simplicity is
retained in the Mammals it is here present in all the
skeletal elements. In mobility of the second segment
only is the Primate simplicity surpassed by the Mono-
tremes and by the arboreal Metatheria.

CHAPTER X

THE MUSCLES OF THE HIND-LIMB

In the hind-limb we may make a brief review of those
muscles which are the homologues of the rotators, the
history of which we have followed in the fore-limb. The
power of rotation of the second segment of the hind-limb
is, as we have seen, very readily lost when any supporting
function is demanded of the limb. This demand for
support is made at the outset of terrestrial life, and, as
a consequence, the rotator muscles of tibia and fibula
undergo a change very early in the vertebrate series.

In Cryptobranclius japonicus there is a muscle which
arises from the upper and outer aspect of the fibula, and
is inserted to the inner border of the tibia at a lower level.

This muscle rotates the tibia around the fibula; it
corresponds to the ulnar portion of the M. pronator radii
teres of the arm, and it is named M. pronator tibiae.

Superficial to this is a longer muscJe, which, arising
from the outer condyle of the femur, passes to the inner
side of the foot. This muscle is the M. pronator pedis,
and though the comparison cannot be maintained for all
its connections, it contains the element homologous with
the humeral portion of the M. pronator radii teres. In
the hind -limb there is perhaps no true homologue of the
whole of the M. pronator quadratus of human anatomy,
but the M. accessorius is, in all probability, derived from
an element equivalent to its lowest carpal fibres. The
interosseous M. tibio-fibularis is present upon a deeper
plane.

In Varanus, as an example of a reptilian form whose

58

THE MUSCLES OF THE HIND-LIMB 59

limbs have taken on some part of the bodily support, the
condition is as yet unchanged, so far as the fibular origin
of the M. pronator tibiae is concerned, and this muscle is
found as (but perhaps only as part of) the M. po])liteus
of the higher Vertebrates. The superficial portion pos-
sesses practically no power of rotation, and the move-
ment between the two bones is becoming somewhat more
limited. Among the Mammals, the Monotremes and
some of the less specialized Marsupials still possess the
M. popliteus, which is the exact homologue of the deep,
or ulnar, part of the M. pronator radii teres, for it arises
from the upper end of the fibula, and is inserted into the
tibia, upon which bone it produces some degree of rota-
tion. Among some of the Insectivora, the M. popliteus is
in a half-way stage, for it arises from the upper end of
the fibula and from the capsule of the knee-joint. This
is the case in some of the Shrews, and apparently also in
Chrysochloris, the Golden Moles (Dobson). In the common
Hedgehog {Erinacens), the muscle has migrated still
farther towards the femur, but it still arises from the
capsule of the joint as well as from the femoral condyle.
In some of the Lemurs, the M. popliteus still retains
connection with the fibula through the intervention of a
tendon and a sesamoid bone, just as it does in some
Lizards; but in all other Primates the main origin is
entirely from the femur, with occasional slight excursions
to the ligaments of the joint. A small, deeper portion, the
M. peroneo-tibialis, however, retains connection ^\dth the
fibula, but this muscular slip is derived, in all probability,
from the deeper interosseous muscle of the lower Verte-
brates. In Man the popliteus muscle arises entirely
from the femur, and even the peroneo-tibialis is only
present in one out of some seven subjects, according to
Gruber (see Fig. 21). The story of this muscle group
appears to be fairly clear from the functional point of
view. So long as the old mobility was retained, tlie
rotator muscle passed from bone to bone across the second

60

ARBOREAL MAN

segment ; but with the gradual loss of this mobility, and
its substitution by stability, the muscle shifted its origin
as its function changed, and, ascending via the capsule
of the knee-joint, it joined the external condyle of the

Fig. 21. — The Back of the Human Knee-Joint, to show the

PoPLiTEus Muscle.

femur. It then exchanged its rotating function for that
of flexion of the knee-joint. This is a mere repetition of
the story of the ulnar head of the M. pronator radii teres
in those quadrupedal animals in which the fore-limb has
suffered changes identical with those in the hind-limb

THE MUSCLES OF THE HIND-LIMB Gl

The superficial portion of the M. pronator radii teres is
represented in the leg by a muscle which has long since
lost all power of rotation, and in Man is almost certainly
merged with the lateral head of the calf muscle named M.
gastrocnemius.

It is clear, therefore, that these muscle groups of the
fore and hind limbs have undergone very dissimihir
changes in Man. We have seen how strangely primitive
is the retention of the condition of the arm ; but it would
seem that, in the leg, the primitive condition was departed
from, and that some degree of support was demanded
from the leg at an early stage in human evolution. With
a simjDle arrangement of anatomical parts, a slight shifting
of muscular origins has turned a perfectly mobile second
segment into a supporting segment, constructed upon
very simple lines. That these changes are those pro-
duced by the demands of support from the hind-limb in
tree -climbing seems obvious, since they are present in all
arboreal Primates, and as such we maj^ imagine the}- have
been long established in the ancestry of Man.

CHAPTER XI

OTHEE AEBOREAL ADAPTATIONS OF THE

HIND-LIMB

Besides the features which we have already noted in the
arboreal hind-limb there are others of equal, or even
greater, importance in the story of the evolution of Man
as an arboreal animal. These other changes can only be
referred to in outline, since the details of anatomical
arrangements connected A\dth them are legion. In pic-
turing the early stages of the development of climbing,
it was noted how the animal, supporting its body weight
temporarily on its hind-limbs, reached out ahead for a
new hold for its hands. This was the interval which
marked the dawn of specialization of the functions of
fore and hind limbs, and in which stability was demanded
in some measure from the hind-limb. But in this interval
another, and a very important, thing is happening, for
as the animal reaches ahead, its body axis is altered, and
the support of the hind-limbs is called upon in a very
special manner. In this interval of climbing up, the
body axis approaches the vertical, and the animal be-
comes in this way a temporary orthograde. There are
degrees in this development of an orthograde habit, even
if it be only a temporary phase, as in the primitive arboreal
enterprise we are picturing. An animal may carry its
body axis upright as a temporary expedient or as a life
habit, while still retaining its thigh at right angles to its
trunk; or it may hold its trunk erect upon an extended
thigh.

There are many animals which can maintain the trunk

62

ARBOREAL ADAPTATIONS OF HIND-LIMB iy^

in a temporary position of uprightness upon a flexerl
thigh. A dog sitting up to beg, a squirrel eating its nut,
or a bear awaiting its bun, are examples of this degree of
uprightness. There are many animals which adopt this
posture as a life habit, the Kangaroo {Macropus) a.nd the
Jerboa {Dipus) are good examples of Mammals which
are habitually orthograde as far as their trunk axis is
concerned, but in which the thigh is normally flexed so
as to be nearly at right angles with the trunk. But the
posture which is temporarily assumed by the primitive
tree-climber is very different from this, for in the interval
which we are picturing its body axis is tending to become
carried upright upon a thigh which is more or less ex-
tended as the trunk is raised towards the grasping hands.
It is tree-climbing which makes this posture a possi])ility,
and even its temporary adoption marks a great step in
evolution, since, with the increasing perfection of the
arboreal activities, the assumption of this posture is an
oft-recurring one.

With the repetition of this action anatomical changes
are brought about in the limb, for many adaptations must
take place when the femur is brought into line with the
vertebral axis instead of being at right angles to it. These
adaptations will show their first manifestations even when
the demand for the posture is only occasional. Briefly,
the femur becomes capable of a more complete rotation
at the hip-joint, so that its extension may be carried
through a right angle, and it may take up a position
parallel to the axis of the vertebral column.

The capsule of the joint becomes modified to permit of
this extension, and the muscles become accommodated
to the new poise. In the completely adapted arboreal
animal this posture tends to become more or less liabitual.
In some of the Lemurs it is almost as well established as
in the Anthropoids themselves, and under these con-
ditions the anatomical adaptations become more perfect.
The fibres of the capsule of the hip-joint take on a per-

€4

ARBOKEAL MAN

manent twist, such as is seen to perfection in the capsule
of the human hip-joint (see Fig. 22), and the muscles
(e.g., the M. rectus femoris) dispose themselves to the
best mechanical advantage for performing the move-
ments of the joint.

Pig. 22. — The Human Hip-Joint from Behind, to show
Twisting of the Fibres of the Capsule of the Joint.

When the leg has become rotated backwards, and the
muscles and joints have adapted themselves to this
change, there still has to be an elaboration of the support-
ing mechanism in this new position. These forces are
all in action during arboreal life, but they gain an added
importance in the habitual orthograde posture of Man.

ARBOREAL ADAPTATIONS OF HIND-LIMB 05

iW

If

The trunk is first suspended upright from the arms upon
the extended legs (as in the existing Gibbons) (see Fig.
23), next its weight is partially borne
upon the extended legs (as in the
existing Giant Apes), afterwards it is
entirely borne and balanced upon the
fully extended legs in all the ordinary
activities of the animal (as in Man).

The anatomical adaptations which
accompany these changes, as they are
seen in existing Primates, are practically
continuous and harmonious ; but this is
not equivalent to saying that the evo-
lution of the process is seen in progress
among existing types.

In Man, the fascial insertion and the
great increase in size of the M. gluteus
maximus, the extended fascial inser-
tions of other leg muscles, the modifi-
cations of the calf muscles (M. gastro-
cnemius and M. soleus), and, above all,
the development of the M. peroneus ter-
tius, are all instances of the speciali-
zations of muscles for the balancing of
an upright body upon an extended leg.

These are later changes produced by
terrestrial bipedal orthograde habits, in
which the suspending assistance of the
hands is entirely dispensed with, but
all these things had their beginnings
in purely arboreal life.

Only one other change we need

notice here, and that is the finishing

touch of the e version of the foot in Man.

When the arboreal hind-limb has been ^''^"\ ;' drawing?

. ^ - . . made from a plu»-

periected as a supportmg organ in an tograph bv Piot.

extended position, it is still a purely Arthur Keith.

5

Fig. 23. — Diagram
OF A Gibbon (//>/-
lobates lar) SUS-
pended by the
Grasp of its
Hands.

66 ARBOREAL MAN

arboreal grasping-supporting limb. As such, its third
segment is still fitted for application to the branches
upon the grasp of which its powers of support depend.
In conformity with this, its sole is inturned, so that it
may be applied to the rounded sides of the branches along
which the animal walks. The higher Apes and primitive
Man climb up branches with the big toe separated from
the other toes, so that the outer side of the foot tends to
be applied to one side of the branch, whilst the big toe
grasps the other side. In this method of progression the
foot is inverted, the soles look inwards, from opposite
sides of the branch, towards each other. When the
branches are exchanged for the level surface of the earth,
this inversion of the foot is a useful adaptation no longer,
and in terrestrial bipedal progression a new mechanism
is initiated for the eversion of the foot. Into these
changes — which are peculiarly human — it is impossible
to enter, since they are all finishing touches added after
the arboreal habit was abandoned. The eversion of the
foot of Man is a post -arboreal develoj^ment, so also is the
perfected mechanism for balancing the trunk upon the
extended leg; but the extension of the leg upon the trunk,
and the anatomical adaptations it involved, are pm'e out-
comes of the ordinary evolution of the arboreal habit.

CHAPTER XII

THUMBS AXD BIG TOES

So far, we have, in considering the question of the develop-
ment of the grasp,- dealt only with the j)Ower of (1) adap-
ting the palmar surface of the hand and foot to the branch,
and (2) flexing the fingers over it, to make the adaptation
more perfect. For the perfected grasp another factor
comes in, since the hold is made more secure by folding
digits over both sides of the object to be grasped. This
simple requirement has led to the most divergent develop-
ments, when the climbing Vertebrates are looked at as
a whole. Among the existing Reptiles, the Chame-
leons show the most extreme development of arboreal
grasp, and in them the fourth and fifth digits are turned
directly backwards, away from the third, second and
first, which retain their primitive forward direction.

In the perching Birds, some variety exists in the
arrangement of the clasping digits; and the so-called
Zygodactyle foot of the Scansores achieves the same
effect as is attained by the Chameleon.

In the Mammals, and especiall}?- among the Primate

stock, the arboreal life has led to the specialization of one

digit upon hand and foot, which opposes the remaining

four digits. These opposing digits are the thumb (pol-

lex) and the big toe (hallux). By an opposing digit we

mean one that can be turned round so that its palmar

aspect is opposed to the palmar aspect of the remaining

digits, and therefore can be placed, for example, ujx^n

the opposite side of a branch.

The subjects of thumbs and big toes has provided an

67

68 ARBOREAL MAN

arena in which anatomists, philosophers, and even divines
have met and done battle. Man has a well-developed
thumb, which is opposable to the remaining fingers. He
has a big toe, which is w^ell developed, but which is not
opposable to his other toes. The human thumb has
received excessive praise from philosophers, the big toe
has also come in for its share, but upon the question of the
homology of the hallux and pollex there is the widest
difference of opinion. Arguments upon the question have
been carried to extremes. " I have heard a distinguished
naturalist say to a class that he would stake anything,
short of his eternal salvation, that the thumb corresponds
to the little toe, and the little finger to the great toe, and
that he should think his life well spent in establishing the
doctrine" (Dwight).

We need not be led aside into any such controversies,
for it does not matter to us if the thumb finds its strict
serial homologue in the little toe or in the big toe; it is
quite certain that in the latter it finds its exact functional
equivalent. By way of homology we will be quite satis-
fied with the simple fact that, in the mammalian position
of the limbs, it is the digit ivhich is situated nearest the
middle line of the body that is specialized as the opposing
digit. This power to oppose one digit to the remaining
members of the series is no part of the heritage of the
primitive third segment of the limb ; it is a new develop-
ment called forth by the increasing perfection and the
increasing needs of the power to grasp. The mere ana-
tomical arrangement by which opposition may be pro-
duced among the digits is no necessary part of the char-
acteristics of the Primates, nor is it alone within their
phylum that it is displayed. Some of the very thoroughly
arboreal Marsupials have perfected this arrangement, and
most of the Phalangers possess an opposable big toe.
Even developments such as are seen in the Chameleon,
among the Reptiles, and in the Parrots, among Birds, are
hinted at in the hand of the Koala (Phascolarctus), in

THUMBS AND BIG TOES GO

which the two inner digits tend to be separated from, and
opposed to, the outer three. Certain arboreal Rodents
have developed very i3erfectly opposable thumbs and big
toes upon lines exactly similar to the Primates, and this
feature is seen very beautifully in an arboreal mouse
(Mus margarettce) discovered by Charles Hose in Borneo.

Within the Primate phylum some very curious irregu-
larities are apparent in the distribution of the power of
opposing thumbs and big toes among the scattered living
types. It seems strange that no New-World monkey
possesses a perfectly opposable thumb, although all
possess an opposable big toe. Among the New-World
Primates the thumb is not perfectly opposable, and
is always permanently in line with the rest of the digits;
it tends to be small and unimportant, and may be entirely
undeveloped.

At first sight it might seem that this arrangement was
correlated with the development of a prehensile tail, but
all the American Primates are not prehensile-tailed.
Nevertheless, it is not beyond possibility that some func-
tional factor may have been the common cause for both
developments; a common factor may have led to the loss,
or non-development, of the opposable thumb and to the
perfection of the prehensile tail, but these two features
need not occur in combination. This I regard as the most
probable explanation, and in the actual method of climb-
ing characteristic of different groups the origin of the
different developments probabty lies.

The little Marmosets (Hapalidce), which have claws
instead of nails upon all the digits save the big toe, possess
a well-developed, but not opposable, thumb. In the
Cehidce the condition varies. The Howling Monkeys
(Mycetes) possess well-developed thum1)s, prehensile tails,
and the usual opposable big toes. The Sakis (Pithecia)
have well-developed thumbs, but the tail is not prehensile ;
the Night Monkeys {Nyctipithecus) show the same
features. In the prehensile-tailed Spider Monkeys (Atiles)

70

ARBOREAL MAN

the thumb is rudimentary or absent, while the Woolly
Monkeys {Lagothrix) possess a thumb. The Capuchins of
the typical genus Cebus possess a well-developed thumb
and a tail, which, though partially prehensile, is not so
specialized as that of Ateles or Lagothrix.

Fig. 24.

Fig. 25.

Pig. 24. — Plantar Surface op the Eight Foot of Tarsius
Spectrum, showing the Peculiar Development of the
Big Toe.

Pig 25. — Palmar Surface of the Eight Hand of Tarsius
Spectrum.

In all the Old- World Primates the pollex, when present,
is opposable. In the Lemurs the big toe is opposable
and is often extremely specialized, the hallux standing
apart from the foot in some species, such as Tarsius
spectrum, in a manner reminiscent of the split foot of
some of the arboreal members of lower orders (Fig. 24).
The thumb is also well developed and opposable in most
higher or typical Lemurs (see Fig. 25). In the Asiatic

THUMBS AND BIG TOES

monkeys of the genus Semnopithecus, which inchides the
sacred Langurs, the thumb is small but still opposable,
and in the allied African genus Colubus it is reduced to a
mere tubercle, or is altogether absent . The well-developed
big toe is present in all (see Fig. 26). In the Anthropoids,
as a rule, the thumb and the big toe are well developed
and opposable.

We may therefore say that among the scattered and
very diverse living members of the Primates the develop-
ment of the thumb and big toe shows
both from the point of view of mere size,
and as opposable digits, some striking
irregularities. But it is not to be
doubted that the underlying principle
is clear enough, that the arboreal habit
develops the specialized and opposable
thumb and big toe, and that peculiar
habits of climbing account for the actual
condition present in the hand and foot
of any individual species. A freakish
development of tree-climbing, or an
overdoing of the pure ability to climb,
may lead to secondary specializations
away from the simple condition. The
stock from which Man has sprung
shows in this, as in so many other
features, a tempered adaptation to the Fig. 26^ — The
arboreal habit without the development
of any secondary specializations.

We may imagine that, from some
early stage in which both thumb and
big toe were equally specialized for pre-
hension, the human stock cultivated especially the hand
as the grasping organ, and so retained and perfected the
opposable thumb. Some other members of the Primate
stock depended more upon the grasp of the foot, and so
have retained and specialized an opposable big toe, at

Left Foot of
Macacus y ernes -
trinus AS seev
FROM ITS Plan-
tar Aspect.

(

72 ARBOREAL MAN

times even to the extent of suppressing the development
of the thumb. Some have also called in the tail to assist
in the foot grasp, and further deprived the hand and
thumb of their function of suspending the body in
climbing.

We see in this feature a correlated adaptation to the
climbing habit to which I have previously drawn atten-
tion; those Primates which show a tendency to depend
on foot grasp descend a tree head foremost, and those
which depend upon hand grasp walk down feet first.
The human stock walked down, and converted the oppo-
sable big toe into a remarkably useful supporting big toe.
The human thumb is an arboreal grasping organ, per-
fected by ancestors which depended upon their hand
grasp in their arboreal activities. The human big toe
is nothing more than a modified arboreal grasping organ,
the primitive characters of which were stamped upon it
by its specialization as a grasping organ, which supported
the weight of the body in climbing.

We must not overlook the fact that although the
grasping power of the big toe is largely lost in modern
Europeans, it must still be reckoned as a distinctly human
possession. Kohlbrugge has remarked that one would
hardly dare to suggest that the presence of a prehensile
big toe was a sign of human inferiority, were the discus-
sion to take place at an anthropological congress held in
Tokio. In the very primitive negrito races the power of
foot grasp is well retained. For the purposes of petty
theft the Sakai largely relies on the grasp of his toes (Skeat
and Blagden); and very many other instances could be
furnished from races far more highly placed in the human
scale.

CHAPTER XIII

THE HUMAN FOOT

The human hand, a strangely, ahnost shockingly, primi-
tive survival, has received enormous praise mistakenly
lavished by the philosopher and the anatomist; l)ut the
human foot, a wonderfully modified and distinctly human
member, has had but scant appreciation. This assertion
is made in the face of the fact that the human foot has
provided the subject-matter for monographs in several
languages.

The foot is apt to be regarded as a poor relation of the
hand, as a thing which, once being far more useful, has
degenerated, within the narrow confines of a boot, into a
rather distorted and somewhat useless member. Al-
though in modern Man the boot has had its definite in-
fluence (as in limiting the possibilities of the power of
grasp), such generalizations concerning the human foot
are very far from true. If Man should wish to point
with pride to any organ the structure of which definitely
severs him from all other existing Primates, it is to th©v
foot that he should point. If " missing links " are to ^
be tracked with complete success, the foot, far more than ' )
the skull, or the teeth, or the shins, will mark them as
Monkey or as Man. The weakness of Achilles laj^ in his ^
heel; the weakness of the arboreal Primate masquerading
as Man lies in the structure of its foot.

It is in the grades of evolution of the foot that the
stages of the missing link will be most plainly presented
to the future paleontologist, when time and chance shall
have discovered the feet of such forms as Pithecanthropus
and Eoanthro-pus.

73

74 ARBOREAL MAN

There was a period in zoological literature when dis-
cussion was waged earnestly, and without satisfaction,
as to what should be called a hand and what a foot.
The hand and the foot are alike in some animals; they
are quadrupedal or quadrumanous, but some exhibit
differences in the structure of the third segment of the
fore and hind limbs. The point in dispute was the exact
stage at which differentiation in a quadrumanous animal
produced a hand and a foot. In this academic discussion
Etienne Geoffi'oy St. Hillaire, Huxley, Owen, and many
others took their part. We may say, however, that the
problem as it was presented did not offer any very special
difficulty beyond the determination of the function of the
member. This was the solution which Huxley recognized
at once, and to which he always adhered. The Anthro-
poids have hands and feet, and their hands and feet are
differentiated by their function. We will accept such
a solution, and assume that we understand perfectly well
what we mean when we speak of a Gorilla's foot. That
is a simple way out of the dilemma, but we must recognize
it does not do away witli the difficulty whicli presents
itself the moment we attempt to differentiate between
a hand and a foot from the point of view of structure.
A monkey's foot is a definite thing; it has a definite
function which distinguishes it from the hand (see Fig. 27).
The same applies with even more force to the case of an
Anthropoid, but the hand and foot in these animals are,
anatomically, remarkably similar structures in many
ways. The specialization of the foot as a supporting
organ is carried to very definite lengths in the Anthro-
poids; in the Gorilla is seen the best foot developed
among the Giant Apes. We will therefore take this foot
as an anatomical illustration of the stage of development
to which foot differentiation is carried in existing Primates.
The foot of a Gorilla differs from the hand in the fact
that all the digits are placed nearer to the extremity of
the thii'd segment of the hind limb; there is a greater

THE HUIMAN FOOT

75

length of foot behind the base of the great toe than there
is of hand behind the base of the thumb (see Fig. 28).

This posterior elongation of the foot or development
of a heel is present also in many monkeys. The big toe
of the Gorilla is larger and better developed than the

Fig. 27.

Fig. 28.

Fig. 27. — The Left Foot of Cercopitheeus palatinus seen

FROM ITS Plantar Aspect.

Fig. 28. — Plantar Surface of the Left Foot of a Young

Gorilla.
With details of the cutaneous markings, after Duckworth, from
a specimen in the Cambridge Collection.

thumb ; the remaining toes are not so well developed as
the corresponding fingers ; nevertheless, they retain exactly
the same relative proportions. We may speak of a
digital formula for hand and foot, such a formula being
an expression of the relative degree of projection of the

76 ARBOREAL MAN

digits. In the Gorilla, the digital formula for the foot
is exactly the same as that for the hand, and both may-
be expressed as: 3>4>2>5>1. Such a formula is an
exceedingly primitive one, and it is present in the primi-
tive manus of such chelonian reptiles as the water tor-
toises. The strangely primitive human hand has an
identical digital formula, the third being the finger that
reaches farthest forwards, the fourth the next, the second
the next, followed by the fifth, and the thumb is farthest
back of all. There is an almost equally common varia-
tion in the human hand in which the second digit may
be as long as, or longer than, the fourth, and this is
doubtless due to the functional importance of the index-
finger. I am not sure that it should not be considered
as the typical human condition. In such cases the
formula stands thus: 3>2>4>5>1, or 3>2=4>5>1.

Man retains a very primitive digital formula for his
hand. His nearest Primate kinsfolk retain it for both
hands and feet.

It is when we attempt to apply this formula to the
human foot that we see how great is the alteration that
has taken place between the existing Anthropoid with
the best primate foot and Man himself. The digital
formula for the human foot is as a rule: 1>2>3>4>5
(see Figs. 29 and 30). Such a statement holds good for
the feet of the great majority of present-da}^ British
people. It is commonly assumed by artists, and even
by surgeons, that the elongated big toe which projects
in advance of the other four toes is not a natural human
characteristic, but is a result of boot pressure. A long
big toe is regarded rather as a deformity than as a natural
human possession in which to take justifiable pride.

Professor Flower long ago turned his attention to this
point, and he examined the feet of hundreds of the bare-
footed children of Perthshire, and among them all he
found no case in which the big toe did not project beyond
the second toe. We must look upon a big toe which

THE HUMAN FOOT

77

dominates the whole series as a typically human and a
perfectly natural feature. Nevertheless, it is common
enough to see feet in which the second toe is longer than
the big toe. People who have feet with such a digital
formula are apt to be somewhat proud of the fact, for

Fig. 29. — Type of Foot in which the Big Toe is considerably
Longer than any of the other Toes, which diminish
IN Regular Sequence from First to Fifth.

Traced from the outline of the foot.

such a foot is supposed to conform to the " Greek ideal,''
but that this type of foot ever was the ideal of Greek
artists is disputed by some authorities upon the subject,
and certainly we may assume that it is less typically
human, and more ape-like, than the type of foot of the
average hospital patient who possesses a long big toe

78

ARBOREAL MAN

(see Fig. 31). So far we have as the typical digital
formula for the human foot 1>2>>3>4>5, with a not
uncommon variant 2>'1>>3>4>5. There is yet another
type, which seems much less common, in which 2=3>1
>4>5. In the Museum of the Royal College of Surgeons
is the skeleton of a Bushman, in which it is possible that

Fig. 30. Fig. 31.

Fig. 30. — Outline of a Child's Foot.

Fig. 31. — Outline of a Foot in which the Second Toe is
Longer than the Big Toe. The So-called Greek Ideal.

the third digit was longest of all — a distinctlj- anthropoid
condition. The change from the so-called Greek ideal
to the foot with the dominant big toe is almost certainly
no outcome of boot -wearing, nor is any one link in the
whole sequence of the atrophy of the fifth, fourth, third,
and second digits. All are natural processes of evolution,
and all have probably taken place in a series of missing

THE HUMAN FOOT 79

Zoologically speaking, we may say that the very useful
and specialized foot adapted for terrestrial progression
is a foot of few digits. It may, in fact, be a foot composed
of a solitary digit. The evolutionary stages by which
the horse has come to stand solely upon its third digit
are well known. Similar processes produced the two-
digited foot of the deer and of the ostrich. There can
be no doubt that Man is trusting, not to his third digit,
but to his first, and all the others are undergoing a
process of comparative atrophy. This is in reality a
most interesting problem. There is an admitted tendency
to specialize one digit in a thoroughly adapted terrestrial
foot. Man applied an arboreal foot to terrestrial pro-
gression, and in this arboreal foot the best-developed
member Avas the old grasping digit— the first or big toe.
It seems that upon taking to a terrestrial life he has.
started the elaboration of this already specialized toe, and
is tending towards the development of a foot which is quite
unique — a foot in which the first digit is the dominant,
and in the end, perhaps, the sole surviving, member.

It needs no special demonstration to make plain the
fact that the little toe is somewhat of a rudiment in most
Europeans. Usually it is but a poor thing; its nail is.
ill developed, and at times no nail is present. It is
particularly liable to that circulatory disturbance which
manifests itself in chilblains, and not uncommonlv it
seems in a poor state of nutrition. Most people possess
but little power of movement in it, and its skeleton
shows that its atrophic condition has affected the bones
and joints, for the last two phalanges are very commonly
fused together, making it short of a joint as compared
with the rest of the toes. Very commonly its axis is not
straight, and the toe is humped up and also somewhat
bent laterally.

It is easy to assume that all this is merely the result of
wearing boots, but it is perfectly certain that this common
explanation is not the correct one.

80 ARBOREAL MAN

In many races, the members of which are quite innocent
of wearing boots at any period of their lives, the little
toe is just as atroj)hic as it is in the average London
hospital patient, and in some unbooted native races it
is even more degenerated than is common in the booted
Londoner. Among the Malays, the absence of a nail
upon the remarkably stumpy fifth toe is not at all un-
common. The barefooted races in Xubia are no better
off in this matter, and even in the very primitive Sakai
the little toe has suffered.

Vaughan Stevens has noted that the little toe of the
Sakai is not straight, but is " bent like ours," and is
small in proportion; but the Jakuns, according to the
same authority, have little toes which are straight (Skeat
and Blagden).

I imagine that just as the big toe is becoming the
dominant toe, the little toe is becoming a rudiment, and
I presume that, in their turn, the fourtli. third, and
second toes are undergoing a human evolutionary atrophy.
There is a most interesting anatomical feature which is
explained by this trend of human foot development.

In the hand a system of short muscles, which serves
to part the fingers and close them together (M. interosseii),
is ranged symmetrically upon cither side of the third
or middle digit. This digit therefore constitutes the
middle line of the hand from which, and to which, the other
fingers can be moved laterally.

In the Monkeys, with the digital formula of the foot
similar to that of the hand, a like grouping of muscles is
seen about the third toe, which in movements, as well as
in length and axis, constitutes the middle line digit of
the foot.

The same condition is seen in the Chimpanzee and
Orang-utan. In Man, however, the muscle symmetry is
ranged about the second digit, and to and from this second
digit the other toes are moved laterally. The middle line
of the human foot has changed from the third to the

THE HUMAN FOOT 81

second toe. In the Gorilla, a most interesting phase is
seen, for while in most specimens the middle line of the
foot passes through the third toe, " it must be admitted
that many Gorillas possess the human arrangement,
these muscles being grouped about an axis formed by
the second digit " (Duckworth).

When digits atrophy and disappear in phylogeny, it
seems to be the rule that their reduction starts from
their distal extremities ; the terminal phalanx diminishes
first, and the metacarpal is only affected at a much later
period in evolution. Metacarpal bones of absent digits
persist, as the well-known splint bones of the horse. It
is to be expected that the same order should be followed
in the diminishing toes of the outer side of the human
foot. The terminal phalanx of the fifth toe is commonly
a rudiment, and often it is fused to the next phalanx,
of which bone it then constitutes a mere distal tubercle.
Compared with the fingers, there has been a great reduc-
tion in the terminal phalanges of all the toes except the
big toe, and the two basal phalanges are diminished in a
somewhat lesser degree. In the majority of cases, how-
ever, the metacarpal bones have not suffered any very
marked atrophy, and the metacarpal formula is often
that which should exist for the primitive digits. ^luch
individual variation is seen in the skeletons of different
feet, but the metacarpal bone of the big toe is, at times,
shorter than that of the second, which in its turn is shorter
than the third; the fourth, again, is shorter than the
third, and the fifth shorter than the fourth.

One very curious evidence of this skeletal condition is
seen in very many feet, even when the big toe is far in
advance of all the rest. Although a line joining the tips
of the toes slopes without interruption from the first to
the fifth, a line joining the bases of these toes does not
follow the same course. From the first cleft it rises to
the second, and from the fourth cleft it rises to the third,
and there is thus produced a sharp angle in the line which

82 ARBOREAL MAN

usually falls opposite the middle of the base of the third
digit (see Figs. 29, 30 and 31). This line has exactly the
same contour in the hand, and is the typical one seen
in the Monkey's foot. It is the outline of the primitive
foot preserved in its primitive condition by arboreal life.
The contour of the anterior extremity of the human
foot has therefore not undergone much change from its
primitive arboreal condition, the alteration being almost
soirly confined to the phalanges of the free digits.

It is the outer toes which are undergoing atrophy,
and this atrophy has not to any great extent affected
the metacarpal bones, nor altered the outline of the foot
itself.

Human specializations seem to be producing a tendency
to depend upon, and develop especially as supporting
organs, the bones of the inner margin of the foot. The
big toe and its supporting bones are becoming the principal
axis of the foot.

The imperfect efforts at walking upon the feet which
the higher Primates can make have not attained to thi*^
human development. The human baby walks upon the
outer side of its feet when it first learns to walk, and the
bones upon this side of the foot are the first to become
ossified. But a typically human and later change is the
eversion of the foot, which brings its inner margin into
the line transmitting the weight of the body to the
ground. A whole series of finishing touches in human
development is brought into play in this process, but
since they are essentially not arboreal effects, they cannot
be dealt with here.

However, without going into the details of the eversion
of the foot, the general facts are clear enough. Man has.
inherited a primitive and arboreal foot ; purely human
modifications are obviously at work producing a very
typical human type of structure which, adapted in the
first place for support in an arboreal habitat, is now being
fitted for terrestrial progression. The human foot is a

THE HUMAN FOOT 83

definite human evolution, and some may take comfort in
remembering that it is evidence of a high grade of human
evohition to possess a long big toe accompanied by a
steadily diminishing series of toes towards the outer side
of the foot, and that it is not necessary to label as
" sensible " the person, or the fashion, which seeks to
confine this human foot into a boot constructed for the
digital formula of an arboreal Primate.

CHAPTER XIV

THE RECESSION OF THE SNOUT REGION

We have seen that one of the things made possible by
the emancipation of the fore-limb and the development
of the power of grasp is the ability of the animal to seize
its food with its hand and convey it to its mouth. These
are two separate actions, and although their influence is
exerted in the same direction, they are so distinct as to
need somewhat separate treatment. The perfected com-
bination of these actions doubtless came slowly into the
arboreal stock, but the power to seize food with the hands
is^present in some quite lowly forms, and the Tree Shrews
X (Tupaiadce) already possess the power of raising food to
their mouths. In all the Lemurs these actions are
perfected, and passing over the degrees to which they are
developed in different species, we will study the com])leted
process as seen in a typical Primate, and note what
correlated changes may be bound up in its full develop-
ment.

We must first turn aside to note that there are other
animals than the Primates and their kindred which can
grasp food in their hands and convey it to their mouths.
There are even animals showing no trace of being arboreal
which can do this action with great address. It is the
same with all systems and organs, and it is a story to
which we shall repeatedly have to turn, for, as we have
seen, the fore-limb may receive some degree of emancipa-
tion by other activities than tree-climbing. Jumping f
and hopping animals, and animals which sit upright, can
use their hands for many skilled movements. Jerboas

84

THE RECESSION OF THE SNOUT REGION 85

(Dipodince), and many other hopping Rodents, eat food
held between their fore-paws. Beavers can hold a small
object clasped by the palm and claws of one hand, and
Marmots (Arctomys) even go farther than this, for when
sitting erect they can pick up their food from the ground.
Truly arboreal animals of other stocks again possess this
power, and squirrels, phalangers, and opossums are good
examples of limited hand -feeding arboreal types. So
far as the process goes in any of these animals, the changes
which we are picturing in the Primate stock take place
harmoniouslv.

A typical Primate obtains its food with its hand instead
of adopting the common mammalian method of taking
it wdth its mouth: one function of the mouth, that of
food-getting, is therefore relegated to the hand in the
Primates.

It may be said on broad lines that throughout the
whole of the animal kingdom the mouth parts show a
development depending upon the nature of the animal's
food and the method of taking it. If it is the hand
which becomes the grasping organ, the mouth and the
anatomical structures connected with it need no longer
be developed in any special way to carry on this function.
The food-grasping power of the Primate hand renders
unnecessary the development of grasping lips and a long
series of grasping teeth. Again, the fact that the food
once grasped by the hand is conveyed by the hand to the
mouth renders the mouth and its associated parts merely
an organ for dealing with food already grasped and carried
to it. A mouth merely adapted for the reception of food
already grasped and brought to it is a structure very
different from a mouth adapted for the purposes of reach-
ing out for food, seizing the food so reached, and subse-
quently dealing with it.

When the mouth is the food-obtaining organ, there is a
necessity for its situation being advanced from the face,
and especially that part of the face in which the eyes are

86 ARBOREAL MAN

situated. A long snout with a mouth opening far in
advance of the eyes is a necessity in any animal which
uses its mouth alone, in all the processes of obtaining
food. The grazing herbivores must carry their food-
getting mouth far in advance of their eyes. The long
face of the horse may serve as a familiar example. The
animals which catch insects must have a similar structure,
and the " snouty " insectivorous Shrews are typical of
such animals. The more the fore-limbs serve to obtain
or to hold the food, the less is this snout developed, and
I am terming the change which hand -feeding produces
iJie recession of the snout region. In herbivorous animals
the transition is very easily seen; the long-faced horse
may be contrasted (solely from the point of view of this
function) with the short-faced squirrel which holds food
betw^een its fore-paws. In carnivorous animals and mixed
feeders another factor comes in, for the mouth may be
used, not only for grasping, but for killing the food, or
the fore-limb may take over this function in part.

The long-faced dog grasps and kills its food with its
mouth, the shorter-faced cat holds its food with its fore-
paws and kills either with its paws or with its mouth;
but the tiger, in which the snout region has shortened
very considerably, kills as a rule with its fore-limb, and
holds the kill with its paws. I have noticed, in this
respect, some interesting phases among the Primate stock.
I had at the same time, living as nearly as possible in
their natural state, some Lemurs {Nycticebus tardigradus)
and some Monkeys. Both of these animals, although
mixed feeders, are in a state of nature very fond of animal
food, the Lemur delighting in insects, especially grass-
hoppers, and young birds, the monkey always ready to
kill and eat anything, from a cockroach to a chicken.

The Lemur would catch a grasshopper with its
hand (or its foot), and would catch a bird put into its
cage in exactly the same way, but after a preliminary
squeezing would almost invariably put it to its mouth to

THE RECESSION OF THE SNOUT REGION S7

kill it with its teeth. I have seen Nycticebus tear grasn-
hoj)pers to pieces with its hands, but Ijirds it always
killed by biting. The monkeys were adepts at catchinpr
birds, and although chained they had no diffioulty in
seizing the confiding Java sparrows that were attracted
by their food. The bird was caught in one hand, and
was then killed by being pulled and twisted between the
two hands. Generall}^ the monkey wrung the bird's neck
so thoroughly that it succeeded in pulling it altogether
apart; it killed with its hands, and then conveyed the
kill to its mouth with its hands.

In the Primates, owing to the preponderant use of the
fore-limb, there is no need for a mouth which reaches out
for food, or for a mouth which seizes food or kills it when
seized, all these functions being discharged by the moljile
and grasping fore-limb.

Now teeth are developed for different purposes. They
are developed for cutting herbage, for seizing animal
food, and for killing prey as well as for biting it up j^re-
paratory to swallowing. Some teeth subserve the func-
tion of obtaining a variety of food in a variety of ways,
and some subserve the function of preparing this food for
the processes of digestion. With the adaptation of the
hand for obtaining food, the need for the specialization
of teeth for this purpose will no longer be felt so strongly,
and it is natural to suj^pose that the tooth series will
become abbreviated, only those teeth which are necessary
for dealing with food brought to them by the hand
remaining fully functional. We may assume, as a ground-
plan of the mammalian tooth series, an upper and a lower
set, composed of three incisors, one canine, four pre-
molars, and three molars upon each side of the jaw.
Such a tooth series comprises forty-four teeth, and it
is seen, for example, in the elongated jaws of the
omnivorous pig.

If we trace this tooth series through the Primates rnd
their probable next of kin, we find the full forty-four in

88

ARBOREAL MAN

the terrestrial Insectivora. In the arboreal Tree Shrews
(Tupaiadce) it has become reduced to thirty-eight by the
loss of one upper incisor and an upper and lower pre-
molar upon each side. In the Lemurs only thirty-six
teeth remain, for a corresponding lower incisor has been
lost. In the Old- World Monkeys, the Anthropoid Apes,
and Man, one more premolar is lost in each jaw upon
either side, and the dentition is reduced to a set of thirty-
two teeth. In Man there are signs of a further reduction.
The reduction of the tooth series and the shortening of
the jaw in the arboreal stock go on step by step together,
and for the same reason, and vet to a certain extent the
two developments are independent of each other. The
tooth series may diminish and may even disappear, yet
if food is still reached for and is seized by the mouth the
snout region will remain elongated. A toothless animal
may still be a long-jawed animal if long jaws are needed
to take food which when taken requires no teeth for its
killing or for its mastication. With hand-feeding the
recession of the snout may outstrip the reduction of the
tooth series, and this process is evidenced in the stock
we are considering. Starting with a full mammalian
series of forty-four teeth, the snout region may yet be so
long that gaps exist between the teeth, and different
groups of teeth may be widely separated from each other.
Reduction in the tooth series in this stock does not in-
crease the gaps, but the gaps diminish faster than the
tooth series is reduced. The ultimate result of this
process is that Man. with a reduced number of teeth,
has the most crowded dentition. Man is the only living
Primate that has its teeth arranged in a continuous series,
and it is one of his distinctions that there are no gaps
between them. The process of the shortening of the
snout, outstripping the process of reduction of the dental
series, gives rise to one of the great problems of modern
dentistry — the proper treatment of the many evils arising
from overcrowded jaws. To this subject we \^'ill turn

THE RECESSION OF THE SNOUT REGION 89

again, but we can here sum up the last phases of the
process by saying that if primitive and natural Man has
no gaps between his adult teeth, his children always have
gaps between the smaller teeth of their first set; but the
children of modern and civilized Man are losing even these
gaps with the shortening of the jaws.

CHAPTER XV

THE EECESSION OF THE JAWS AND REDUCTION

OF THE TOOTH SEEIES

With the business of hand-feeding, Man has gone a great
deal farther than any other member of the Primates,
and that comparatively modern development — civilized
Man — has gone still farther. The highest Primates select
their food with their hands, they even do more than this,
for, to a certain extent, they prepare it for eating with
their hands. But this preparation, though an enormous
stride, does not go to very great lengths beyond peeling
a banana or husking a thin-shelled nut with the fingers;
for anything much more exacting the teeth are re-
quisitioned. We have seen the amount of work that the
hands have already saved the teeth in the evolution of
an arboreal stock, and there is obviously a tendency in
the highest apes for the hands to assume further duties.
Man has applied his brain and his mobile hands more fully
to this jDroblem, and he has saved his teeth to the utmost
limits, but has made a sorry bargain. The general
bearing of these factors did not escape the notice of
Darwin, but, strangely enough, he confined his argument
practically to the fact that the hands of the human
ancestors, armed with primitive weapons, tended to take
the place of the fighting canine teeth. " As they gradu-
ally acc[uired the habit of using stones, clubs, and other
weapons, for fighting with their enemies, they would have
used their jaws and teeth less and less. In this case, the
jaws, together with the teeth, would have become reduced
in size, as we may feel sure from innumerable analogous

90

THE KECESSION OF THE JAWS 91

cases." In many ways, therefore, human hands have
replaced the functions of human teeth. There is no need
to trace the stages in a story famihar to all. Man has
ground, husked, prepared, cleaned, and finally cooked
his food. He has freed it from hard parts, and made it
" tender " in every conceivable way. His canine teeth
he has re]3laced by the use of his hands; his flint or his
knife has usurped the function of his incisors; and his
molars he has relegated to the kitchen premises as a
pestle and mortar in some form or other. Even wlien
he had done all this he had not run the whole gamut of
robbing his teeth and jaws of their legitimate occupation,
for there is still the knife and fork of the Europeanized
to perform outside the mouth those duties formerly
performed within it.

Every organ which loses its function must undergo a
change, and unless this change leads to the assumption of
new functions, the ultimate result will be an atrophy of
that organ. The human teeth, deprived in great measure
of their normal functions, acquire no new ones — even
speaking and whistling through the teeth may not save
them from their ultimate destiny — and it is not to be
denied that, slowly, of course, but still surelj^ they are
undergoing atrophy. Among existing races of mankind
the fact is patent, the observation is a commonplace of
anthropology. " The possession of an ample palate and
large well-formed teeth by the black races is a matter
of common knowledge (as is the fact that in the crania of
the prehistoric inhabitants of Europe the size and quality
of the teeth were superior to those at present obtaining
in the same geographical area). It is therefore impossible
to overlook the inference that reduction in the size of
the teeth is at least attendant (if not dependent) upon
the acquisition of higher grades of civilization and directly
upon diet and the ^preparation of food."

This, from the wiitings of Dr. Duckworth, may be
taken as an orthodox statement of the general position

92 ARBOREAL MAN

as summed up in modern anthropology. The more
primitive races have larger and better formed teeth,
rooted in more roomy palates, than members of more
civilized races can boast of. There is but little need to
dilate upon so well known a circumstance, but some few
facts may be cited.

The third molars, wisdom teeth, being the last to be
erupted in the already diminished jaws, show the maxi-
mum effects of the atrophic influences of disuse. In
modern civilized Man these teeth are erupted late, fre-
quently in a condition of defective development, and
usually in such a manner as to restrict, if not entirely to
obviate, their functional utility. In civilized Man they
are always smaller than the first or second molars, and
as a rule all their biting cusps are not fully developed.
They may not all be erupted; sometimes they are present
in one jaw, and not in the other, and often when present
in both jaws they do not meet and bite together.
Frequently they altogether fail to be erupted.

In primitive races they are rarely absent, they are cut
earlier, and are but little if any smaller than the other
molars, and they bite and grind together in a perfectly
even manner.

In the skulls of the ancient inhabitants of Eg\'pt and
Nubia this perfection of the third molars is very striking.
Among the modern Egyptians — even those of them lead-
ing a town life in Cairo — the wisdom teeth are cut at
times well before the eighteenth year — full six years before
the}^ make their imperfect appearance in most Europeans.
In the more primitive living races the third molars are
usually very fine teeth, erupted early, and fully capable
of all the typical molar-grinding functions. In the skulls
of the earliest human remains in which the dental series
is preserved, the molar teeth are large, and the molar
series diminishes little, if at all, from the first to the
third molar; indeed, in some cases the primitive con-
dition of a reversed state of affairs is seen, and the

THE RECESSION OF THE JAWS u:i

third molar is larger than the second, which in turn
is larger than the first. In modern Man the first molar
is markedly larger than the second, which is again
markedly larger than the third. In the higher Apes the
third molar is the largest tooth of the molar series, and
it erupts before the canine. There is one other circum-
stance connected with the molar teeth that is worthy of
note. It is not extremely rare for a fourth molar tooth
to be developed in the roomy jaws of the skulls of ancient
races, and it is not at all uncommon for some diminished
remnant of this tooth to be present in modern primitive
Man. The presence of this fourth molar is all the more
remarkable, since its normal presence has to be sought
in a stock so apparently remote as the metatherian
(Marsupial) Mammals.

Apart altogether from the anatomical development of
individual teeth, some light is throw^n upon this question
by studying the quality of the teeth when developed.
We know that the teeth readily decay, and that caries in
modern civilized man affects the permanent set of the
adult and the temporary set of the child. Caries of the
milk teeth is so common as to be the rule in modern city
children, and few children shed their milk teeth without
decay having played some havoc among a set of teeth
the normal functional life of which is naturally brief.
But although the milk teeth were not intended to remain
in functional activity for more than ten years, they were
not meant to decay before or at that time. It is a striking
fact that, in the work of the Archaeological Survey of
Nubia (1907-08), no case of caries of the milk teeth
was found in the skull of any child living before tlu^dawn
of the Christian era in Nubia. Among the hundreds of
cases examined, not one case of caries of the milk denti-
tion w^as discovered in the children of the early Egyptians.
The children of the more primitive living races rarely
show any traces of decay in the teeth of their infantile
set. In modern civilized children living a city life, not

94

ARBOREAL MAX

only is caries of the milk teeth almost the rule, but dental
surgeons commonly " stop " milk teeth nowadays, so
early is the onset of their decay. The decay of the
permanent teeth needs no description, its almost universal
occurrence in civilized modern man ])eing familiar to all.
But remarkably good sets of permanent teeth are found
in skulls of historical date, and in the remains of ancient
races extensive caries of the permanent teeth becomes
increasingly rare. One definite factor has certainly
played a large part in this deterioration of the teeth of
modern Man, and this factor is the loss of the reaction to
wear and tear — the loss of the power of repair.

When a horse grinds its molars together it wears them
down, and the enamel with which the surfaces of the cusps
are covered is worn off, and the main substance of the
tooth (dentine) is exposed below. Year after year the
cusps are worn flatter, and an ever-changing pattern is
produced, the pattern being made by areas of exposed
dentine surrounded by margins of enamel. The dentine
which is exposed in this way reacts to the grind; Kg
influences, and becomes hardened on its surface, and
changed to a condition known as secondary dentine, which
provides almost as hard a surface as that of the original
enamel. By this process the animal will gradually wear
its teeth Hat. but these worn and flattened teeth are
perfectly sound teeth. The teeth of ancient races and
of modern primitive races show well this dentine reaction.
Human teeth may be ground down by wear and tear, and
react to the grinding influence. But in modern civilized
Man the reaction is verv much diminished, and in the
majority of cases, when the enamel is worn through, the
fate of the tooth is sealed, since the dentine, instead of
reacting, becomes the site of decay.

We have seen that in the whole Primate (and the whole
arboreal) stock there has been a recession of the jaw.
In one way this feature has been carried to very definite
human lengths by very definite human methods. When

THE RECESSION OF THE JAWS 95

arrived at its highest Anthropoid stage, the still relatively
large tooth-bearing (alveolar) margin of the jaws is well
in advance of the rest of the jaws. The tooth-bearing
surfaces and the teeth project. The upper jaw is prog-
nathous, and the lower jaw has a very receding chin.
With the abbreviation of the alveolar margin in Man, the
prognathism disappears and the chin makes its appear-
ance. The gradations in this change are very beautifully
seen in the remains of ancient Man and also in the jaws
of existing primitive Man (see Fig. 32). The mandible

A B

Fig. 32. — One Half of the Lower Jaw of (A) Homo and (B)
A Monkey, to show the Prominence of the Tooth-
bearing Margin in B and of the Chin in A.

of the " Piltdown individual " is notorious, with its
advanced alveolar margin and receding chin. The
prognathous savage is proverbial. Man developed a chin
by the shrinkage of the alveolar margin of his jaw, con-
sequent upon the diminishing demands made upon the j
teeth, the functions of which were so largely usurped by
the hands. Modern and civilized Man seems to be in
some danger of losing even his chin as the whole mandible
becomes reduced. First, the recession of the tooth-
bearing margin makes the lower margin conspicuous,
and a chin is developed ; and now the lower margin seems
to be disposed to follow in the train of the upper. The
dawn of a chinless aristocracy is no pleasing picture in ■
the later stages of human evokition ; and yet the recession
of the modern jaw is not to be denied.

CHAPTER XVI

THE FACE AND THE CRANIUM

We have followed some of the phases by which hand-
feeding and alteration of life-habit have led to the re-
cession of the snout region, and we have seen the influence
which this recession exerts upon the dental scries; but
its influences are felt in man}^ other ways, and some other
of the accompanying phenomena must receive passing
attention. The snout recedes as a part of a general
evolution proceeding in arboreal life, and one other
feature (to be dealt with later) which accompanies it is the
steadily increasing growth of the brain. The skull as
a whole may be said to consist of two parts — a part which
is a containing bony case for the brain and the sense
organs, and a part which is the skeleton of the mechanism
for obtaining and masticating food (see Fig. 33).

In the lowest arboreal animals, the second or facial
part is preponderately large in proportion to the first or
cranial part, but the relations of the two ])arts are soon
altered in arboreal evolution. The growing brain de-
mands a large brain case; the diminishing jaws require
less bony basis. In this way the configuration of the
skull is profoundly altered, for not only do the jaws
shrink back, but the brain case protrudes. Apart
altogether from the mere evidence of the relative sizes
of the bony parts in a skeleton, there is seen in this
evolution a gradual change in the arteries and veins
associated with these two constituents of the head region.
The internal^carotid artery enters the cranium to supply
the brain, the external carotid artery runs to the face

96

THE FACE AND THE CRANIUM 97

and supplies the whole facial area with blood; the internal
jugular vein returns the blood from the brain, and the
external jugular vein drains the area supplied by the
external carotid artery. These two sets of vessels are
of varying importance in different animals. Primitively,
the external carotids and the external jugulars are ])y
far the largest and most important vessels in the head
region, and the internal carotids and internal jugulars
are, relatively, inconsiderable channels. But in arboreal
life, with hand-feeding and increasing brain growth, their

Fig. 33. — Outline of the Skull of a Dog, to show the
Relative Portions devoted to the Skeleton of the
Face and to the Skeleton of the Brain Cavity.

relative importance becomes altered, and finally in Man
and in the higher Primates the internal carotids and in-
ternal jugulars far outweigh in size and importance the
arteries and veins of the facial area. The gross changes
which take place in the contour of the head have already
been touched on, and they need no further emphasis,
since they are conspicuous. A piimitive arboreal Insec-
tivore, such as Tupaia, has a relatively small head and a
long snouty face (see Fig. 37); many Lemurs have snout ^
regions almost as long, but others, such as Tarsius spec-
trum, have faces which are already distinctly flat (see
Fig. 70). In Monkeys and Anthropoids, although the
jaws protrude considerably in the adults of some species,
the rounded skull case and flattened forehead dominate

7

98

ARBOREAL MAN

the snout region as features of the head and face (see
Fig. 84). It is noteworthy that when for any reason
elongation of the nose becomes characteristic of any
species of monkey, this elongation is produced quite
apart from any reversion to a condition of j)rolonged
snout region. In the Proboscis Monkey (Nasalis larva-
lus) the nose reaches a remarkable degree of prominence,
but there is no involvement of the maxilla or mandible
m this new departure, and the same tendency is shown,

Fig. 34. — Outline of a Human Skull, to show the Relative
Portions devoted to the Skeleton of the Face and to
the Skeleton of the Brain Cavity.

but to a lesser degree, in the curious Snub-Nosed Monkey
[Rh inopitJi ecus roxella nee).

These general alterations of the configuration of the
head and face lead to several changes, especially in the
position of the sense organs, which are probably of the
greatest importance. When an animal has a fully elon-
gated snout region, it may be said to possess a long face
with an eye situated upon each side of it; but when the
snout region has undergone complete recession, it may
be said to have a flat face with two eyes situated upon
the front of it. The mere fact of the recession of the
snout produces this change, for the two eyes are turned

THE FACE AND THE CRANIU.M U9

to the front as the elongated muzzle shrinks between
them.

As the eyes begin to take up a forward position, a bar
of bone forms behind them and intervenes between them
and the sj^ace at the side of the skull in which the muscles
of the jaw lie. In the Tree Shrews, the orbit has already
commenced its separation from the temporal fossa ; in
Tarsius spectru7n, the separation is complete, as we might
imagine from the shortness of its face, but in all the other
Lemurs the orbit and the temporal fossa communicate
freely. In all the Monkeys and Anthropoid Apes, as
in Man, the orbital cavity is an entirely separated com-
partment surrounded by bony walls and containing the
eveball and its associated muscles, nerves, and vessels.
The bringing of the eyes to the front of the face and their
lodgement in separated bony orbital cavities has, in all
probability, far-reaching effects; but it must be looked
upon only as a part of the general process of change in
head formation brought about by hand-feeding in arboreal
life. Another factor not to be disregarded is the change
to which w^e will allude more fully later — the alteration
of the head j)oise. Some consideration of this change is
inseparable from a study of the recession of the snout
region. When the face becomes so short that the whole
skull is balanced upon its condyles, a complete change
takes place in the axis of the principal movements of
the head upon the trunk, and a greatly increased range
is given to these movements. The arboreal Primates
may nod their heads backwards and forw^ards, as in the
human method of saying " Yes." This movement takes
place betw^een the condyles of the skull and the first
cervical vertebra, and it is the primitive movement of
raising and lowering the head common to all IMammals.
But they also have an enormously increased power of
turning the head from side to side, in the human method
of signifying " No." This movement takes place between
the first and second, as well as, to a lesser extent, bct^^•een

100 ARBOREAL MAN

the other cervical vertebrae ; but a wide range of movement
is permitted in the neighbourhood of the skull before the
other joints of the neck are involved.

This ability to turn the head quickly in any direction
has had its influence upon the principal sense organs.
Both eyes may be directed immediately, and at the same
time, towards an object which attracts attention. In
Tarsius, which possesses a wonderfully mobile head poise,
there seems almost a tendency for the head movements
to replace the movements of the enormous eyes, but in
all other Primates, the head mobility merely supplements
and aids the mobility of the eyeballs.

The head may be tilted into any conceivable position,
so as to be placed at the greatest advantage to catch a
sound proceeding from any direction. The sense organs
may be brought into greater harmony and their teachings
may be correlated by this mobility of the head, and,
indeed, it is this mobility which has replaced that seen
in lower animals in the pinna itself. An arboreal animal
Avhich has arrived at this stage does not " cock " or
" prick " its ears when it hears a sound, but turns its
mobile head so that it can catch the sound to greatest
advantage, and at the same time bring the cause of the
sound under the observation of its eyes.

Man and the Anthropoids have lost all trace of the
useful movements of the external ear upon the scalp,
but he and the arboreal Primates have compensated for
this in the increased power to move the head — a power
permitted by the altered configuration of the skull con-
sequent upon the recession of the snout region.

CHAPTER XVII

THE SPINOUS PROCESSES OF THE VERTEBRAL

COLUMN

In all works which deal with Comparative Anatomy, or
with Anthropology, much attention is devoted to the
human distinctions of poise of body. Various archi-
tectural features of the human body are modelled upon
a plan somewhat different from that seen in most other
animals, and these alterations of structural details are,
for the most part, associated with a typical human poise
of body. All these points have been eagerly seized upon
as definite and measurable human features, and without
turning aside now for any discussion of theories con-
cerning them, it is our business to see if any of these
features were impressed upon the body of Man as a result
of his philogenetic youth spent among the branches.
Most of the problems concern, in some measure, the
vertebral column as the central axis around which the
rest of the body is disposed. There are, for example,
the questions of the poise of the head upon the neck,
the presence of the sinuous curves of the vertebral column
—cervical, dorsal, lumbar, and sacral curvatures; the
actual method by which vertebra articulates with
vertebra; the varying size, shape, and number of the
elements which compose different regions of the cohimn;
and finally the manner in which the column articulates
with the pelvic girdle.

We will start our examination of the backbone in a
somewhat irregular way by considering, not the general
curves and articulations of the whole column, but the

101

102 ARBOREAL MAN

characters of those processes which project, one from the
dorsal aspect of each vertebra, and which are named
spinous processes or neural spines. These spinous ]iro-
cesses stand up in line all do\Mi the middle of the back,
and to them are attached portions of the great muscle
(M. erector spinge) which acts upon the vertel)ral colunni.
In actual disposition these spinous processes differ greatly
in different animals; and the most conspicuous differences
are to be noted in the direction in which they slope.
Some, or all of them, may stand up quite straight, or
they may lean towards the head end, or the tail end. of
the animal. In the Reptiles, the arrangement of the
spines is comparatively simple, for in most existing types
all the processes stand directly u])wards. or they are
directed slightly backwards at their free tips. Tiure is
a very primitive ]ilan. soon in many Reptiles, lioth living
and extinct, as well as in some existing and many extinct
Mammals, in which the vertebra to which the pelvic
girdle is united forms a definite landmark by })ossessing
an u])right s])inous process. ^Vll the vertebra? in front
of this one may have their spines directed slightly back-
wards towards it, or with some variation displayed in the
forward spines, uprightness is again found in the spine
of this pelvic or sacral vertebra.

In many extinct reptilian forms the only truly u])right
spine is at the pelvis. In the Gavial some ten anterior
spines point backwards, about nine are upright, and five
are directed slightly forwards. In the modern Nilotic
Crocodile some seventeen anterior spines are directed
backwards, the last rib-bearing vertebra having an erect
spine, and being followed by some five elements in which
the spines are directed slightly forwards; and after that
comes the pelvic region with an upright spine again.

The upright spine of the pelvic vertebra of the Reptiles
entitles this vertebra especially to the distinctive name
of anticlinal vertebra. This may be the only spine in the
whole vertebral column which does not slope, but we

THE SPINOUS PROCESSES

103

have already seen that in the Crocodile (and in some other
existing Reptiles) another upright spine is beginning to
be evident at the hind end of the rib series, and this is
the one to which the term anticlinal vertebra is usual ly
applied in mammalian anatomy.

Within the limits of the Mammalia, the condition of
the trend of the spinous processes varies enormously.
Among the Prototheria, Echidna and Proechidna show a
series of cervical, dorsal, and lumbar spines which point
uniformly backwards towards the sacral region (see
Fig. 35). This is apparently the primitive mammalian

*4i*A4i'

Fig. 35. — Diagram of the Vertebral Column of an Animal
IN WHICH All the Spinous Processes are Retroverted.

as well as the primitive reptilian condition, and, as such,
is seen in an extraordinarily varied collection of extinct
species embracing such forms as Toxodon, Arsinotherium,
Mylodon, etc. In Ornithordhynchus, there is a change,
for though all the cervical and dorsal spines slope acutely
backwards, those of the three lumbar vertebras slope
forwards, there being an anticlinal element at the hind
end of the rib-bearing series. In the Metatheria and
Eutheria, the very widest divergence in spinal inclination
is seen, and it seems most probable that some functional
demand determines the variations met with. Owen
paid attention to this point, and recognized clearly the

104 ARBOREAL MAN

underlying cause of the variations. Some fift}' j^ears ago
Paul Topinard dealt partially with this problem, which
had previously engaged the attention of Paul Broca.
But these two authors considered little more than the
end of the story, for they took most note of those changes
which have taken place in so short a chapter as that
comprised in the study of the higher Primates and Man.
It is, however, necessarv to embrace far more than this
in the study of what Topinard termed " anteversion and
retroversion " of the spinous processes. If we take the
skeleton of such a well-known animal as the dog it is at
once apparent that the spines of the cervical and most
of the dorsal vertebrae are " retroverted," that the
penultimate dorsal vertebra is " anticlinal," and the two
last dorsal and all the lumbar vertebrae have spines that
are " anteverted," another upright spinous process ap-
pearing on the sacrum. The anticlinal vertebra which
is situated, in the dog, near the end of the dorsal, or rib-
bearing, series has also been termed the region of the
" centre of motion " ; and it is easy to realize, in watching
a greyhound looping along, that this is a perfectly well
justified term. The anticlinal vertebra indicates that
the animal possessing it has the power of bending its
vertebral column as a spring is bent, and that the apex
of the bend is situated at this particular point. The
presence of such a vertebra in the backbone, whether
of a recent animal or a fossil, shows clearly that the
animal could flex and extend its vertebral column about
this central point, and that its spine could be bent, and
could be straightened out again as a spring in the ordinary
activities of the animal (see Fig. 36). Tliese things are
clear enough when we look at the skeleton of a dog, or a
hare, and weave into the bones the picture of the animal
laying itself out, and doubling itself up, as it goes at full
speed. But there is, as we have seen, another condition
— that seen in some primitive Reptiles and Mammals —
in which the anticlinal vertebra is situated, not in the

THE SPINOUS PROCESSES

ior>

middle of the back, but at the point where the legs and
pelvis hinge upon the spine, at the junction of tail and
body. In these animals the mechanism of spinal move-
ments is obviously of a different nature, and a whole
series of correlated anatomical details makes it clear that
no spring-like bending of the backbone takes place at or

tf^i

Fig. 36. — Dragram of the Vertebral Column of an Animal

IN WHICH THE SPINOUS PROCESSES ARE AnTEVERTED AND

Retroverted to a Definite Centre of Movement.

near its mid-point. But the pelvic anticlinal vertebra
is a true centre of movement in animals built upon this
type. Towards the fixed pelvic girdle the head and neck
and the whole of the trunk may be pulled, and raised, as
the arm of a crane; towards the same fixed point the
usually elongated tail may be similarly pulled up. This
is a simple and primitive anatomical plan, and it is
adapted to simple and primitive types of movement.
The range of body movements possible with this plan of
arrangement of spinous processes and their associated
muscles would appear to comprise such actions as crawl-
ing, waddling, shuffling, that type of running best termed
ambling, and simple aquatic paddling.

In all these actions there is no regional bending of the
vertebral column, no centre of movement save that
situated at the point above the hips. This is the mechan-
ism, and these are the movements of most of the existing

106 ARBOREAL MAN

limbed Reptiles; the same type of movement, Ave may
fairly hazard, was characteristic of the extinct gigantic
forms of which the bony evidences of the mechanism are
so clearly preserved.

Among the plastic Mammals, the evidence of function
is very easily seen. Animals which hop, jump, spring,
leap, or gallop show the presence of well-marked paite^
version and retroversion of the spinous processes. Nearly
all Rodents and Insect ivores possess this feature. Of
the Carnivora, the cats which spring, and the dogs which
leap and gallojD, have a strongly divided series of spinous
processes, while the shuffling bears show a vertebral
column of which all the spines anterior to the sacrum
are directed backwards. Leaping and galloping LTngu-
lates, which can use both fore-limbs and both hind-limbs
alternately in their full stride, provide the classical
example of the anticlinal spinous process at the pen-
ultimate rib-bearing vertebra. Some very striking ex-
ceptions are worth noticing among the Ungulates. We
have already called attention to the curious gait of the
Giraffe, which in quiet progression advances both limbs
of the same side at the same time. It is interesting to
find that in this animal all the cervical, dorsal, and lumbar
spinous processes slope backwards — there is no centre
of movement until the region of the hips is reached. I
should imagine that, even when hard pressed, a Giraffe
cannot break into a gallop, and that it possesses little or
no power of jumping, but I know of no authoritative
observations upon these points. The spinous processes
of Okapia are arranged upon the same simple plan, and
I presume that it possesses the same peculiar gait, and
the same probable limitations of activity as the Giraffe.
It is not surprising that the lumbering Elephant, with
its peculiarly rigid backbone, should have no dorsal
centre of movement, and no anticlinal vertebra, and the
same feature is shared by such simple paddlers as the
Sirenia.

THE SPINOUS PROCESSES io7

It would take us too far aside from our present purpose
to discuss the question, but we may note the observation
that, judging by such skeletal remains as have been
preserved, the Mammals, at an early period of their
history, were represented by an extraordinary number of
forms the gait of which we may presume to have ])een
but little better than a simple reptilian shuffle. The
active galloping and springing animals are their changed
and modern representatives.

In the case of arboreal animals the problem is ap-
parently complicated at the outset by the fact that,
while some perfected tree-climbers show a highly special-
ized series of anteverted and retro verted spines, separated
by a well-marked anticlinal vertebra, others, none the
less well fitted for a thoroughly arboreal life, have a
series of uniformly directed spines, all being retro verted
(even if only slightly so) towards the pelvis.

It is easy to furnish a satisfactory explanation for these
differences by appealing to the varied, and perfectly
distinctive, methods of tree-climbing adopted by different
arboreal animals, but it is by no means easy to determine
what may be the relation of these two forms to each other.
Either type (the divided, or the uniform spinous series)
might be primitive in the tree-climbers, and the one
might subsequently be derived from the other by altera-
tion of function as displayed in climbing methods.
Again, both types may have been definite legacies in
arboreal animals derived from differently constructed
primitive stocks; both may have been inherited types
with which the animals took to an arboreal life, and on
which they have moulded their arboreal activities. Or,
both of these alternative factors may be in action, when
we regard the whole wide range of arboreal Mammals.
It seems not unlikely that this last supposition is true.

Taking a group of animals so perfectly arboreal as the
Edentate Sloths of South America (Brady podidcv), we
see, combined with a very peculiar fashion of arboreal

108 ARBOREAL MAN

activity, a vertebral column possessing a uniformly
sloping series of spinous processes. The question that
presents itself is, Does this spinal arrangement represent
the inherited handicap of these arboreal animals, an
ancestral birthright which has determined and limited
their peculiar climbing habits ; or have their individualities
as tree-clingers modified a spinal column which may at
one time have possessed the doubly sloping series of
spines indicative of greater activity ? The present-day
arboreal Sloths possess a backbone of the lumbering-
terrestrial walkers. Are they derived from a lumbering-
walking stock of which some of the smaller, lighter
members have taken to the trees and become lumbering
tree-clingers, since that was the limit of their arboreal
possibilities ? The evidence of paleontology certainly
points towards the last conclusion as being nearer the
truth.

The extinct relations of the Sloths are well known.
On the strength of the evidence afforded b}' Megatherium,
Mylodon, and other well-studied gigantic fossil Edentates,
it seems justifiable to regard the modern Sloths as diminu-
tive descendants of lumbering animals, and to look upon
their restricted arboreal activities as the necessary result
of their ancestry. We may assume the correctness of
Owen's conclusion that Mylodon robustus reared itself
against the trees in tripod fashion, and pulling down
the branches, browsed upon their leaves.

From such a beginning we would picture some smaller
members of the same stock going farther than this, and
clinging to the branches in their search for food; and in
this manner we would picture the Sloths becoming
arboreal.

Even when they had reached the tree-tops, and had
definitely made their homes among them, they were still
limited by the handicap which their ancestral terrestrial
shuffling gait had imposed upon them; and though no
animals are more thoroughly arboreal than the existing

THE SPINOUS PROCESSES 109

Sloths, it cannot be said that their arboreal activities,
although distinctive enough, tend to lead far in tlie
struggle of evolution.

If this be the true history of the arboreal Brady podicbje,
it would seem to be one not easily applied to the origin
of the arboreal Primate stock. No lumbering gait, or
mere clinging to branches, seems to have led them to the
tree-tops; and, indeed, as we have seen in a previous
section, an early acquired mammalian activity appears
the most probable factor in bringing about the enter])i-ise.
Taken as a whole, the Primates show a distinct I \' retro-
verted and anteverted series of dorso-lumbar spinous
processes, the two sets being separated by an anticlinal
vertebra marking a centre of movement, which is very
obvious in the arboreal activities of most monkevs.

t.-

The same conditions are present in most members of
the Insectivora. Among the Menotyplilidoe, the Oriental
Tree Shrews (Tupaiadce), which are deserving of especial
notice, have twelve ribs, the spines of the vertebra?
anterior to the tenth dorsal slope backwards, the tenth
or eleventh is upright, and the nine posterior spines sk)j)e
forw^ards. These are active arboreal creatures jumping
from branch to branch, and having a very definite centre
of movement at the hind end of the thoracic region
(see Fig. 37).

There are many reasons for supposing that, in some
such form as a primitive tree-haunting Insect ivore, a
picture of an earlier stage of the Primate phyhnn is to
be seen most perfectly among living Mammals. Some
exceedingly primitive form, of which a very much elabo-
rated modern evolution may be seen in the existing
Tupaiadce, probably pioneered the Primate stock in the
conquest of the branches; this pioneer form was, in all
probability, a small active animal, perhaps with tli<'
commencing possession of a centre of movement situated
at the hinder end of the thoracic vertebra\

Great interest centres round the Lemurs in the study

110

AKBOREAL MAN

Fig. 37. — A Typical Tree Shrew. Adult Female of Tupaia

ferruginea.

From a spirit specimen.

THE SPINOUS PROCESSES m

of the disposition of the spinous processes. In this
feature the majority of them follow the Tree Shrews,
and those that are characterized by special activity
present a remarkably double-sloped series of spines.
Some Lemurs might almost be called arboreal jumpers,
and among them the Bornean Tarsius and the African
Galagos are most prominent; in these animals the forward
slope of the lumbar spines is particularly acute. But
with some other Lemurs the most puzzling feature of the
problem is introduced, for Nycticebus, as a type of the
Asiatic Slow Lemurs, presents a series of spines as uni-
formly retroverted as that seen in the Sloths themselves.
Some confusion between the Sloths and Slow Lemurs
has, in bygone days, been a stumbling-block in systematic
zoology. Is this similiarity of the backbone another
feature which might cause the animals to be confounded
and is it one that might point to any real philogenetic
affinity in the stocks of the Slow Lemurs and the Sloths ?
In this feature we have seen some reason for believing
that Sloths were derived from a lumbering terrestrial
stock, and it may fairly be asked if the same reasoning
should not apply to the case of Nycticebus. "We have
pictured the stock of the Lemurs as arising most probal)]y
from a small active animal; are we to regard the Slow
Lemurs as having a different origin ?

Probably the correct answer is that the Slow Lemurs
show so many points of affinity with the rest of the
Lemurs that they can only be regarded as altered members
of the same stock. " Although they vary considerably in
structure from the more typical Lemurs, there can l)e
no doubt that the Slow Lemurs possess a true Lemurine
structure in many important particulars, so tlial they
must have had a common origin with the true Lenuirs "
(Sclater). We are bound, therefore, by our ]>rcsrnt
limitations of knowledge to regard this as a case of con-
vergence. We have previously noted the peculiarities
of the arboreal habits of Nycticebus. It is a trce-clinger

112 ARBOREAL MAN

more nearly than a true tree-climber, and moreover it
shows a definite tendency to carry out its arboreal
activities in an inverted position. It is therefore to be
presumed that the adoption of this habit has led to a
modification of its erector spinse muscle and its spinous
processes, and that by its slothful habits Xycticebus has,
in this respect, arrived at the state to which the Edentate
Sloths were bound by their inherited disabilities.

The case of the Slow Lemurs is all the more interesting
since the absence of a dorso-himbar centre of movement,
and the presence of a practically uniform series of spines,
are seen in another group of arboreal Primates. Xj/ctice-
bus does not leap from branch to branch, it takes no spring
from its hind-limbs, it does not jump to its next arboreal
station, but reaches out for it and gras])s it, and upon
the ground it crawls and shuffles along. Now, much the
same conditions are seen in the Anthropoids. The Giant
Apes, though active enough and expert climbers, do not
spring from their hind-limbs, or leap about the branches
as the smaller monkeys do. Wallace has described the
Orang-utan in its natural state as " moving slowly along,
hanging from the ])ranches by its arms," and as " moving
along a large liml) of a tree in a semi-erect posture."
Both these modes of progression are typical of the An-
thropoids; the first, according to Wallace, is unusual in
Simla satyrus, but it is the one which is characteristic
of the smaller agile Anthropoids known as the Gibbons
(Hylobates).

The Chimpanzee adopts the same methods of climbing,
methods which involve a semi-erect foot balance com-
bined with a dependence upon a powerful hand grasp.
More rapid translation from branch to branch, or from
tree to tree, is not performed by a spring from the resting
feet, but by a swing from the grasping hands. A group
of monkeys passing from tree to tree in the jungle will
jump those gaps where branches fail to meet, but a party
of Gibbons will swing themselves across the gap, releasing

THE SPINOUS PROCESSES 113

one hand-grasp only to gain another. It is in this fashion
that human performers on the high trapeze pass from
one swinging bar to another.

It is this factor, a purely arboreal one, that has led to
the typical condition of the anthropoid vertebral column,
and determined the disposition of its muscles and the
arrangement of its bony prominences. The springing
point in the middle of the backbone is absent, and the
column acts as a whole; its spines are in uniform series,
and its accompanying muscles support it as a pillar,
rather than bend it as a spring.

There is nothing in this that is peculiar to Man, nothing
that has any relation to the attainment of any distinctive
human attribute; it w^as among the branches, as an
outcome of the arboreal life, that the uniformly sloping
series of spinous processes, seen in the human vertebral
column, was attained.

Among the Anthropoids themselves some minor varia-
tions are seen in the disposition of the sj)inous processes.
In the Gibbons the series is quite uniform, but in the
large Anthropoids the spines in the cervical region are
peculiarly elongated. More than this, in the Gorilla the
spine of the third cervical vertebra is strongly ant everted,
and at times the fourth and fifth share in a slight forward
slope. Again, at the hind end of the series some varia-
bility is seen, for some of the posterior spines are not
distinctly retroverted. In Man this variability in the
posterior spines is also present, for the lumbar spinous
processes do not always slope in quite the same manner.
In some primitive human races there is even a tendency
to anteversion in the jDOsterior spines, which shows itself
at times quite distinctly in the first sacral vertebra.

8

CHAPTER XVIII

THE POISE OF THE HEAD AND THE CURVES OF

THE SPINE

The differences seen in the disposition of the cervical
spinous processes in the Anthropoids and in Man are due
tO; and involve, yet another factor which may be termed
the poise of the head upon the vertebral column.

This subject has been so fully investigated by com-
parative anatomists that little need be said here con-
cerning it.

The skull is hinged to the foremost vertebra by its two
condyles, and between these occipital condyles and the
articulating surfaces of the first vertebra the human
movement of nodding the head to and fro takes place.
The position of the two condyles relativel}' to other
anatomical features of the skull varies enormous] v in
animals adopting different life postures. The condyles
may be situated right at the hinder end of the skull,
they may be just beneath the hind end, or they may be
situated some distance forward along its base. In
pronograde quadrupedal animals, such as the dog, the
head is jointed to the vertebral column by condyles
situated at the extreme hind end of the skull; the nose
is directed forwards in line with the vertebral column,
and the skull is braced in position by a strong ligament —
the ligamentum nuchse — and by muscles passing from the
vertebrae to the back of the cranium (see Fig. 38). In
animals which are not purely pronograde quadrupeds, an
alteration takes place; the poise of the head becomes
changed and the site of the condyles shifts upon the skull.
In arboreal animals this change becomes very evident,

114

THE POISE OF THE HEAD

115

for with the body held even partially, and only occasion-
ally upright, the eyes and face still need to be directed
fonvards, and an angle is introduced between the long

Fig. 38. — Base of the Skull of a Dog.

This and the following three figures are drawn with the diopto-
grapli from skulls which are orientated strictly in the s;une
plane. The relative positions of the condyles are thcrctore
directly comparable in the series of drawings.

axis of the skull and face, and the long axis of the vertebral
column. This angle can be produced, as occasion de-
mands, in the articulations of an ordinary quadruped.

116

ARBOREAL MAN

In the normal method of progression a dog carries its
head nearly in line with its vertebral column, but when
it sits up to beg it bends its head and neck so that its
eyes and face are still directed forwards, and the head
becomes almost at right angles to the axis of its vertebral
column. This position becomes habitual in the arboreal

Fig. 39. — Base of the Skull of a Baboon {Cynocephalus).

Primates, for in them the trunk has so frequently to be
more or less upright, and in proportion to the permanency
of this position there comes about a shifting of the site
of the condyles.

Posture alone determines this change, for the more
quadrupedal Baboons (Cynocephalus) do not share so
fully in this feature, which is so characteristic of their
truly arboreal allies (see Fig. 39). In most monkeys the
occipital condyles are situated well forward upon the

THE POISE OF THE HEAD 117

base of the skull, and in the Anthropoids they are still
further forward. In Man the head is practically balanced
upon the first cervical vertebra (see Figs. 40 and 41).

The general factor which underlies this forward migra-
tion of the condyles is involved early in arboreal life, and
it is one that proceeds far in animals that are still purely

Fig. 40. — Base of the Skull of a Monkey [CercopUhccm).

arboreal. The final changes which have taken place in
Man might be ranked among the finishing touches of
human development, for they consist, not so much in
any further forward migration of the site of the condyles,
as in the culmination of that other process which we
have termed the recession of the snout region. In the
giant Anthropoids the head itself has already attained

118

ARBOKEAL MAN

all the essentials of the human poise, but the preponder-
ately large face and jaws, in the adult Gorilla especially,
demand for their proper balance a large muscular leverage
applied to the back of the head. It is this muscle mass
which gives these animals their apparently short bull-

FiG. 41. — Base of the Human Skull.

necks, and which, acting from the spinous processes of
the cervical vertebrae, elongates these processes and pulls
them towards the skull.

The poise of the skull, and the forward migration of the
occipital condyles and foramen magnum, are arboreal
features. Finishing touches have been put upon the

THE POISE OF THE HEAD 1 1 9

human condition by the final phases of the recession of
the snout region.

With the question of the curves of tlie verte})ral cohiimi
we need deal but briefly, for the subject is one whicli
finds ample discussion in every work upon anthroijoloiiy.
In pronograde quadrupeds the backbone rises as one
long, low^-pitched arch from the point where it is Mip-
ported by the fore-limb, to a maximum in the dorso-
lumbar region, and then falls again to the point whore it
is supported by the hind-limbs. The weight of the tiiink
is carried from an arch which is supported upon pillars
(limbs) at its two extremities. It is this arch whicli in
some animals has a springing point at its centre. In
front of the anterior supporting pillar the spine bends
up again for the carriage of the head. This tmie the
arch is reversed, for while the curve of the back is convex
upon its dorsal side, the curve of the neck is convex
vent rally. Again, behind the posterior supporting pillar
the spine is also bent upwards ; here, at the sacro-vertel)ral
angle, the bending dorsalwards is more acute; but from
this point, the curve is slightly downwards once more,
the posterior or sacral arch being like the dorsal arch in
miniature, but generally still more flattened (see Figs. 35
and 36).

In arboreal animals these curves are also well marked,
and the changes which they undergo are quite definite.
Arboreal uprightness, in the sense of the assumption of
a sitting posture, has a well-marked influence upon the
primitive curves. Some monlveys, as they sit up, spend
the greater part of their time with the trunk supported
vertical upon their ischial prominences, and in these
animals the dorso-lumbar curve tends to l)e, not one
long pitch as in the quadrupeds, but an arch subdivided
into an anterior sharper curve over the thoracic part of
the body, and a more gradual curve over the abdominal
part. The dorso-lumbar curve tends to be concentrated
as a dorsal curve, while the lumbar region is scarcely

120 ARBOREAL MAX

arched at all. This is, of course, merely an adaptation
to posture, and as such it is seen in other, and non
arboreal, animals which tend to carry the trunk axis
vertical, no matter what may be the relation of the hind-
limb to the trunk. A flattened lumbar region is present
in the Kangaroos {3Iacropus), and it is the same in the
Jerboas (Dipus), which hop with the trunk axis nearly
vertical. Arboreal life brought about a lumbar flattening
early, since trunk uprightness is an easily attained out-
come of the climbing habit, but it also — in the Anthropoid
Apes — carried it a stage further than this.

A lumbar flattening suffices for an animal which holds
its trunk upright upon the basis of its flexed lower limbs,
and it suffices for animals which sit and hop upright.
It will not suffice, however, when the trunk uprightness
has to be combined with extended lower limbs. If an
animal is to maintain its trunk and its lower limbs in one
continuous axis, something more than a lumbar flattening
is required, and a reversed lumbar curve is introduced.
In most monkeys the reversed lumbar curve is already
present in some slight degree. In Cercojiithecus jycilaiimis
it is perfectly definite, and the same may be said for all
thoroughly arboreal monkeys, when the vertebral column
is examined in its natural state; but the curvature dis-
appears altogether in the skeleton (see Fig. 42). The
reason for this is that the curvature is caused bv the
shaping of the soft intervertebral discs, rather than by
any change in the bones themselves, and when these
discs are lost in the preparation of the skeleton, the
presence of the curves is ignored in the subsequent
mounting of the specimen. If our knowledge be derived
from the actual animals, rather than from museum
skeletons, we cannot denv that a lumbar curve convex
forwards is already present in the monkeys.

The straightening of the lower limb upon the trunk is
an extremely important factor in primate evolution, and
we will follow Professor Keith in regarding the habit of

V

THE POISE OF THE HEAD

121

hand suspension, seen in the Gibbons, as the agent which
made it a definite possession of the Anthropoids. As the
Gibbon travels about among the branches, its trunk and

Fig. 42.— Section of the Hinder Part of the Body of a
Monkey {Cercopithecus palatinus), to show the Incipient
Forward Curve in the Lumbar Kegion of the Spine.

hind-limbs are dependent while it swings with its long
arms from branch to branch. The body and leg axis
is straightened almost as much as it is in upright walking
Man. The Gibbons show a curvature in the lumbar

122

ARBOREAL MAN

region, the convexity of which is directed forwards and
which is better marked than the same curve in any
monkey. In the Gibbons the bones themselves have
begun to share in the change, and the curvature is evident
in the dried skeleton. This lumbar curve is present with

Fig. 43. — The Normal Curves of a Human Vertebral Column
as seen in a section through an upright body.

an ever-increasing perfection through the Giant Apes to
the lower, and finally to the higher, races of mankind
(see Fig. 43).

No doubt it is a feature which is called into being by
the erectness of the trunk upon the lower limbs, but it
must not be regarded as a feature stamped upon the
human frame by terrestrial bipedal orthograde habits; it
was begot among the branches, it led to greater possi-
bilities, and only its finishing touches were put on by
upright walking upon the surface of the earth

CHAPTER XIX

THE PELVIS AND THE VISCERA

The arboreal alteration of body poise makes itself felt
in other skeletal and visceral features than those related
solely to the backbone and the skull; for, at the other
end of the vertebral column, the pelvis undergoes marked
changes in arboreal life. The primitive pelvis, such as
the earliest Mammals inherited, is a very definite structure
articulated in a very definite manner, and in all essentials
it is of the same type as that seen in the generalized
Reptiles both living and extinct.

Such a primitive pelvis consists of two lateral halves,
each half being composed of three elements: one a dorsal
element, articulating with the vertebral column, and the
other two, which are ventral elements, articulating \\ith
each other in the middle line of the ventral surface of the
body. The dorsal element (ilium) articulates with the
vertebra] column at the sacrum, over a sacro-iliac joint
surface which involves both the rib element (pleura po-
physis) and the transverse process element (diapophysis),
which enter into the formation of the sacrum. The two
ventral elements articulate at an elongated symphysis,
which involves both bones (pubis and ischium), and is
therefore an ischio-pubic symphysis. These types of
sacro-iliac and ischio-pubic joints are characteristic ot
quadrupedal animals that have four equally developed
supporting limbs, and they are obviously dependent upon
the mechanical demands for supporting the body upon
the limbs in pronograde animals. With a change of
body poise, an alteration in pelvic architecture, to meet

123

124

ARBOREAL MAN

the new conditions, is evidenced in a wide series of
vertebrate forms, and, with an exchange of pronograde
quadrupedal progression for arboreal uprightness, the
pelvis becomes greatly modified. In a simple mechanical
way we may regard the sacrum of the pronograde as
slung between the two ilia, slung from two separate
points of suspension, the one on its costal portion {phiira-
pophysis), and the other on its transverse process element

Fig. 44. — Purely Diagrammatic Representation of the
Pelvis of a thoroughly Quadrupedal Mammal.

Note the way in which the sacrum is articulated with the ilia at
the sacro-iliac joint, and the meeting of the pubes and
ischia at the ischio-pubic symphysis.

(diajyophysis). The visceral weight is supported upon an
elongated ventral symphysis, which constitutes only one
element in the supporting developments of the structures
in the mid-ventral line of the body. Such a pelvis tends
to be narrow from side to side, but elongated in its dorsi-
ventral axis. With the assumption (even to a partial
extent) of arboreal uprightness of the body axis, the
body weight tends to be disposed round the vertebral
column as round a vertical pillar, rather than to be slung
from it as from a horizontal pole; and now the sacrum
tends to be wedged between the two iliac bones, as the
keystone of an arch disposed in a cranio-caudal rather

THE PELVIS AND THE VISCERA 125

than in a dorsi-ventral direction. This change produces
an effect u]3on the sacro-iliac joint that we may sum uj)
by saying that an elongated dorsi-ventral contact area
becomes unnecessary, and is gradually replaced by an
elongated cranio-caudal contact area.

Although there are some exceptions and irregularities
in the application of this principle to the pelves of existing
Mammals, still the exceptions are capable of explanation
and do not detract from the general rule that with a
change from quadrupedal to bipedal progression more
sacral vertebrae enter into the formation of the joint, but
less of the dorsi-ventral area of each vertebra is engaged
by the ilia.

A common type of sacro-iliac joint, in purely prono
grade quadrupedal Mammals, is that in which but one
sacral element is articulated with the ilia; and this one
element is sunk deep between the two ilia, so that the
joint surface involves the whole of its dorsi-ventral area,
engaging both pleurapophysis and diapophysis elements
which are represented in this area. In most of the
Lemurs one whole sacral element, and from a quarter to
a half of the next caudal element, are engaged in the
sacro-iliac joint. In many New- World Monkeys the
condition is the same, and in both cases these elements
are deeply sunk between the ilia, so that the diapophyses
are articular. In the Baboons one whole element, and
three-quarters of another, are engaged. In most Old-
World Monkeys nearly the whole of two sacral elements
enter into the articulation, and the same condition is
present in the pelvis of the upright Indris among the
Lemurs. In the Anthropoids from two and a half to
three sacral elements are involved, but the condition is
subject to a considerable degree of variation in different
individuals. In the Gibbons (Hylobates Jar) nearly the
whole of three elements is articular, and the articulation
involves both pleurapophysis and diapophysis of the
sacral vertebra. In the Orang-utan the articulation

126 ARBOREAL MAN

varies; it may involve no more than the best part of two
sacral vertebrae, or nearly the whole of three may enter
into the joint. But the diapophyses of only two of the
sacral vertebrge are, as a rule, involved. In both the
Gorilla and the Chimpanzee the joint surface occupies
from two and a half to three sacral vertebrae, but involves
the diapophysis of only two elements. In Man the
greatest variation is sexual, and the female articulation,
as a rule, comprises only two or two and a half sacral
vertebra?, while that of the male embraces from two and
a half to three. In an}- case, an articulation evolving
a diapophyseal element is a somewhat uncommon human
anomaly.

It would therefore seem that the sacrum is being
received between the ilia in a greater proportion of its
length, but is freeing itself from articulation in some
portion of its depth by (if it may be expressed thus)
liberating its dorsal surface from joint contact with the
ilia.

Hand in hand with this alteration of the sacro-iliac
joint a change is proceeding in the ventral symphysis.
This change may best be summed up by saying that the
symphysial area becomes shortened progressively in the
transition from pronograde poise to arboreal orthograde
habit. The svm])hvsis, which is first of all an ischio-
pubic union in the mid-line, becomes opened from its
caudal aspect, and more and more of the ischia are freed
from the ventral union. By gradual stages the whole
length of the ischia becomes liberated from the symphysis,
and the two bones are splayed aside from each other.
Next, the pubes share in the process, and from the condi-
tion of a true pubic symphj^sis, in \\liicli the whole of
the ventral ends of the pubic bones are involved, there is
developed the pubic symphysis typical of Man, in which
only about a half of the ventral ends of the pubes enter
into the articulation.

That this is an outcome of the modified method of

THE PELVIS AND THE VISCERA 127

supporting the body weight is not to be doubted, despite
some apparent contradictions, and it is to be remarked
that m animals in which the body weight is not borne at
all upon the hind-limbs (as in the Bats, etc.) no portion
of the pelvic girdle meets in the mid-ventral line, and
consequently no symphysis is developed.

Fig. 45. — Purely Diagrammatic Eepresentation of the
Pelvis of an Orthograde Mammal.

Note the way in which the sacrum articulates with the ilia at the
sacro-iliac joint, and the meeting of only a small area of the
pubes at the pubic symphysis.

The pelvis has now, in its altered relation to the hiiid-
limb, an entirely different mechanical design. There i^
a main w^eight-supj)orting arch behind, composed of the
ilia, with the sacrum as the keystone of this arch. A
subsidiary weight-supporting arch is developed in front,
and this is represented by the subpubic arch. The old
ventro-dorsal weight-sujDporting arch is superseded, and
now the cavity of the pelvis need no longer be moulded

128

ARBOREAL MAN

in an elongated form from back to front ; it rather becomes
rounded, or even broadened from side to side. These
changes in the bony architecture of the pelvic girdle lead
naturally to changes in the disposition of the viscera
most intimately related to the pelvis. In particular the
splaying open of the hinder end of the symphysis produces

A B.

Fig. 46. — Diagrammatic Representations of (A) Primitive
Mammalian and (B) Human Type of Ventral Sym-
physis.

marked alterations in the visceral outlets, since the
perineum shares in the changes. The form of the ex-
ternal genitalia in both sexes becomes modified by this
opening movement of the symphysis; and the external
reproductive orifices become situated beneath the j^ubic
arch, instead of occuj^ying the hind end of an elongated
pelvic tunnel. The internal organs also become pro-
foundly modified, and those channels which connect the
hollow viscera with the surface of the body become
abbreviated with the outfolding of the hind end of the
symphysis. Other visceral changes come about hand in
hand with this pelvic adaptation, and this for the reason
that both are the results of the altered poise in arboreal
activities. As the body axis becomes increasingly
upright, the disposition of the viscera within the body
cavities undergoes a purely mechanical alteration. It
may be said that the method of packing the organs in
the cavities is changed, simply for convenience, as the

THE PELVIS AND THE VISCERA 120

axis of the cavities becomes modified. Since most of
the organs are suspended from the walls of the cavities
it is this method of suspension which naturally becomes

Fig. 47. — Median Section of a Young Female Pig, to show
THE Relation of the Ventral Symphysis (V.S.) to the
Viscera.

Fig. 48. — Median Section of a Full-Term Human Female

FCETUS, TO SHOW THE RELATION OF THE VENTUAL SYM-
PHYSIS (V.S.) TO THE Viscera.

most markedly affected. Briefly, we may nay that wlwn
the trunk axis becomes more upright, all the viscera tend
to sink towards the hind end of the body, and they tend
to be suspended from the head end of the cavities rather
than from the dorsal aspect. There is no need to describe

130 ARBOREAL MAN

these changes in any detail, since almost every individual
manifestation of them that is especially well marked in
Man has been seized upon as an example of the alterations
due to his orthograde bipedal habits, and, as a conse-
quence, the subject has received sufficient attention in
the literature of physical anthropology. The heart
comes to be suspended from the cephalic end of the
thorax, rather than from its dorsal side, and so it rests
upon the upper surface of the diaphragm, rather than
upon the anterior wall of the chest. Changes in the
disposition of the lungs follow harmoniously this read-
justment of the position of the heart.

In the abdominal cavity the same general effects are
seen. The viscera sink backwards. The liver is sus-
pended more strongly from the lower surface of the
diaphragm, and the intestines obtain attachments which
sling them from the upper part of the cavity, as well as
from its dorsal wall. All these things are adjustments
to trunk uprightness — that trunk uprightness which is
early brought about in arboreal life — and, as such, they
make their appearance long before the stage at which
Man walked upright upon his two feet; for it must never
be forgotten that the trunk of an animal which climbs
up a tree, or sits upon a branch, is just as upright as is
that of a Man standing erect. As far as concerns the
abdominal and thoracic viscera, a man is as upright when
he sits as when he stands, and an arboreal animal which
sits and climbs among the branches is in a like case.

The upright poise of Man has been lauded as one of
his greatest distinctions. This praise of human upright-
ness has, without doubt, been carried to absurd extremes,
so also has the tendency to ascribe to this same uprightness
a multitude of human weaknesses and disabilities. This
visceral uprightness is no new thing, the readjustment
has been gradual, and some measure of it has been very
long established. It is easy to overdo the praise of the
poise. It is equally easy to overdo the condemnation of
it as a cause of many human ills.

CHAPTER XX

THE RESPIRATORY SYSTEM

We will not deal directly with that portion of the re-
spiratory system which is concerned with the production
of voice, since the factors underlying the changes produced
in these structures are not those that bring about the
modifications in the disposition of the organs purely
concerned in breathing.

Here we will only consider the effects produced uf)on
the chest and lungs, and the method by which air is
taken into the lungs. The alterations in the general
shape of the chest, which have differentiated the human
form from that seen in purely quadrupedal animals, are
very well known, and are discussed in all works dealing
with human morphology.

These alterations are, as a rule, ascribed to the upright
posture — and rightly; but, again, we must remember
that the upright posture need not mean the erect walking
posture, but may imply nothing more than mere arboreal
uprightness. In brief, we may say that a tj'pical quad-
rupedal animal, such as a dog, is narrow-chested. Its
breastbone marks the keel of a chest, deep from breast-
bone to backbone, but narrow from side to side. An
upright animal, on the other hand, tends to have a broad
chest, the breastbone no longer protrudes as a keel, and
the chest is shallow from breastbone to backbone, and
broad from side to side. Man shows the extreme of this
flattening of the chest; but the rounding of ihv quad-
rupedal chest is well seen in arboreal types quite low
down in the mammalian scale. Indeed, although we

131

132 ARBOREAL MAN

speak of a phylogenetic flattening of the chest, we must
also be prepared to admit that the narrow quadrupedal
type of chest is itself a modification from the presumably
flatter chest that was the possession of the first mammalian
forms.

The change from the narrow pointed chest to the broad
flat chest is, for the most part, effected by the falling
backwards, towards the backbone, of the whole chest,
as the animal becomes more adapted to maintaining its
body axis upright. The ribs of a quadrupedal animal
tend to dispose themselves as oval hoops, hung in their
long axis from the backbone; the ribs of an animal with
an upright axis tend to dispose themselves as rounded
hoops, surrounding the backbone as hooj)S surround a
pole.

In this way the backbone tends to become, not the
ridge from which the hoops are hung, but a central prop
within the circle of the hoops; to do this the backbone
has to sink into the back of the series of hoops, pushing
them in before it as it goes. This simple mechanical
alteration effects a double change. In the quadrupedal
animal the breastbone projects as a keel; the backbone
projects as a ridge. In the animal with the upright axis
the breastbone is merely part of the evenly rounded front
of the chest, the backbone merely a part of the evenly
rounded back. In Man the process culminates in a chest
which is flat in front, and a back which is flat behind.
This is a simple mechanical process ; in no sense can it be
said to be due to the assumption of the upright jDosture,
if by that posture the erect walking posture is meant.
It did not come suddenly into the possession of the human
stock when that stock took to walking with the soles of
the feet planted flat upon the level surface of the earth;
it was already in being when, in life among the branches,
the animal squatted or hung with its body erect. This
is a change of bodily conformation, and, as such, needs
treatment elsewhere; it does not so directlv concern the

THE RESPIRATORY SYSTEM ua

function of respiration. This function has, however,
been modified very distinctly by the habit of trec-cliinbing,
and especially by its most important development, tlie
emancipation of the fore-limb. The story of the changes
in the method of respiration is a singularly complicated
one, since it is so inextricably interwoven with the changes
produced in other systems that its main thread is apt to
be lost in the complications which occur in every cliai)ter.
The primitive air-breathing Vertebrates draw air into
their lungs by creating a suction wdthin the spaces inside
their bodies, and this they do by drawing their ribs
upwards and outwards towards their fore-limbs. They
" heave " their chests forwards, as one would pull out
one end of a concertina, and so suck air into the lungs
within their body cavity.

Inspiration in these animals (tailed Amphibians and
unspecialized Reptiles) is produced by muscles that pull
the ribs towards the fore-limbs; expiration by a reversal
of the process, and by muscles which squeeze the internal
cavity of the body and so drive the air out again.

In the most primitive of the Mammals a great change
has come in, for the internal cavity of the body is sub-
divided into a headward chest cavity, and a taihvard
abdominal cavity, and the lungs are separated from the
abdominal viscera by a muscular partition — the dia-
phragm. When the muscular diaphragm acts, it com-
presses the abdominal cavity — this is its primitive
function — but it can also create a suction in the cliest
by pulling its floor tailwards, or, to continue the simile,
by pulling out the other end of the concertina. The
diaphragm therefore becomes capable of assisting in
drawing air into the lungs in inspiration.

There are, therefore, in the Mammals two possible
mechanisms of inspiration; first, the original air-breathing
vertebrate method of elevating the ribs to the shoulder
girdle, and second, the new method of lowering the
floor of the chest cavity. The one was named l)y Sir

134 ARBOREAL MAN

Charles Bell the *' external " and the other the " in-
ternal " respn-atory system. It is possible for these two
systems to be combined, and to act in consort. It is
easy to realize that, with the action of the external
respiratory system, a more effective inspiration will be
produced if the internal respiratory system acts, even if
it comes into place only in order to resist passive move-
ment. In other words, the diaphragm must at least
resist being sucked up into the chest during inspiration.
This, for the most part, covers the range of activity of
the diaphragm in the inspiration of most animals.

For the most advantageous functioning of the external
respiratory system, it is necessary that the shoulder
girdle and the fore-limb should be sufficiently fixed, at
the moment of inspiration, to form a firm acting point
for the muscles which pass from them to the ribs.

This is the condition present in the truly quadrupedal
animals. In these animals the muscles which, arising
from the fore-limbs and shoulder girdle, pass to the ribs
pull the movable ribs towards the fixed limbs when they
contract (see Fig. 49). A contracting muscle, however,
is like a stretched elastic band; it pulls upon both of its
attached ends, and will move that attachment which is
least firmly fixed. In a quadrupedal animal, standing
with its fore-limb firmly planted on the ground, the ribs
are pulled to the relatively fixed fore-limb, but if the fore-
limb be free and movable, it will be pulled towards the
relatively fixed ribs. This is what actually happens in
animals which have developed mobility of the fore-limb,
at the expense of its stability. It reaches its climax in
those animals which have completely emancipated the
fore-limb, for in these animals the muscles of the external
respiratory system have become muscles which produce
added movements of the mobile fore-limb. The mobility
and range of movements of the fore-limb are increased,
but the efficiency of the primitive respiratory mechanism
is impaired in projDortion. It is now that the internal

THE RESPIRATORY SYSTEM 135

respiratory system becomes of increasing imporlunce,
and the animal of necessity begins to depend more and
more upon its diaphragm as an inspiratory muscle
(see Fig. 50).

We may say, therefore, that, as a general rule. i)urc
quadrupedal animals use their external rcspirator\ sy.«;tcm

Fig. 49. — Diagram of a Quadrupedal Animal, to show the
Muscles passing from the Kelatively Fixed Fore-Limb
TO the Kelatively Movable Ribs.

Only one muscle (serratus magnus) is represented. It produces
elevation of the ribs and the inspiration of air.

most, but animals with emancipated fore-limbs depend
more and more upon their internal respiratory system.
So far is this the case, that in Man the original external
respiratory muscles are almost universally regarded as
no more than mere " extraordinary or accessory muscles
of respiration."

It is perhaps worth turning aside to note how Man.
when he needs added respiratory mechanism, attempts
to take up a quadrupedal position, or at least change
the mobility of his fore-limb back again to stability in
order to bring his pnmitive external respiratory musclen
into play, A runner who is " blown " will grasj) his

136

ARBOREAL MAN

Fig. 50. — Diagram of a Human Skeleton', to show the
Muscles passing from the Relatively Fixed Ribs to
the relatntely movable fore-llmb.

Only one muscle (serratus magniis) is represented. It produces

a round arm-blow.

THE RESPIRATORY SYSTE.AI 137

knees with his hands, and so fix his fore-limbs; a patient \
with embarrassed respiration will grasp the back of a '
chair, or adopt any convenient hold which may make
him functionally a quadruped for the time being.
Arboreal life has done this for the Primate stock — it has
given them fiat chests and flat backs, has brought about
a greater degree of dependence upon the diaphragm as
a mechanism of inspiration, and at the same time, has
added to the mobile fore-limb an increased source of
mobility in the muscles of the external respiratory
system.

i

CHAPTER XXI

THE REPRODUCTIVE SYSTEM

The influences of tree-climbing upon the reproductive
habits, and consequently upon the reproductive system,
are very great. They may be considered under two
headings : as due to the arboreal life alone, or as they are
outcomes of the emancipation of the fore-limb, which is
itself a consequence of the arboreal life. The great factoi'
involved under the first heading is the necessity for the
reduction of the family in arboreal life. Arboreal animals
tend to have small families, and some of the influences
which have brought about this result are perfectly obvious.
In the first place, large litters are, as a rule, produced
among animals living such a life as affords rest and
protection for the female during pregnancy. Pregnancy
with a large litter and active arboreal life are almost
incompatible. No matter what the underlying regulating
factor may be, it is quite definitely in action, and although
nest-building may offer a temporary expedient in a race
of arboreal animals, reduction in the number of offspring
produced at a birth will be the ultimate result. Again,
apart altogether from the disabilities of pregnancy, there
are the difficulties of dealing with large families when
born to an arboreal mother. Helpless offspring in large
numbers may be managed and cared for in some safe
terrestrial nursery, but up a tree even were large numbers
of such offspring produced, it is doubtful if many would
survive. We know that the Tree Shrews build a nest,
and so do some of the Lemurs (Cheirogaleus), as well as
arboreal animals of other stocks, such as Rodents

138

THE REPRODUCTIVE SYSTEM 139

(Squirrels and Dormice, etc.), Marsupials (Phascologale),
and in the nest the offspring are nursed during their most
helpless stage. But nest-building is only a temporary
expedient in mammalian evolution, and reduction of the
number of young produced at a birth is the ultimate
outcome in a truly arboreal life. This is the result that
has been arrived at in the Primate stock. The terrestrial
Insectivora produce large families, Centetes even having
a litter of twenty; Tiipaia in its arboreal nest still begets
three or four offspring at a birth. The family of the
Marmosets very constantly numbers three. Among the
Lemurs two young may be born at a time, and among
all the Monkeys one offspring is the general rule, though
two are not infrequently born.

Multiple pregnancies are, of course, primitive; and
reduction of the family is acquired under the conditions
of arboreal life. This reduction of the family produces
its changes in the reproductive system. In the first
place, the prenatal nidus designed to accommodate, say,
twenty embryos may well be expected to show a structure
anatomically different from one that is designed to accom-
modate a single embryo. In an animal in which the
pregnancy is habitually multiple, the genital tract is
divisible into three distinct parts (see Fig. 51). Leading
from each ovary, from which the egg cells are shed, there
are, on each side of the body, (1) thin tortuous tubes
(the Falloppian tubes), which are merely ducts down which
the egg cells pass into (2) the uterine cornua, which form
the bilateral nidus in which the fertilized egg cells develop
into the embryos; these two uterine cornua meet in
(3) a small common median chamber, the body of the
uterus, which opens into the vagina, and so to the ex-
terior. In animals which have multiple offs})ring the
embryos are developed in the uterine cornua. In Cen-
tetes, which we have already instanced, ten embryos
might be expected in one cornu, and ten in the other.
As the family becomes reduced so do the uterine cornua

140

ARBOREAL MAN

diminish, and the dwindling of the uterine cornua is
harmonious with the diminishing family. Finally, when,
in arboreal life, the begetting of a single offspring is the
long-established habit, the cornua disappear altogether,
and the single offspring is lodged in the single median

Fig. 51. — The Type of Uterus associated with Multiple
Pregnancies. Small Uterine Chamber, Large Uterine
Cornua, Small Falloppian Tubes.

The diagram is taken from the uterus of the dog.

chamber — the uterine bod3\ In the higher Primates,
therefore, and in other typical arboreal animals such as
the Sloths, the uterine cornua have practically disap-
peared, and the genital tract consists of (1) the Falloppian
tubes, or egg ducts, leading into (3) the uterine body,
which in turn opens to the vagina, and so to the exterior

THE REPRODUCTIVE SYSTE:^!

141

(see Fig. 52). A single median uterus for the accom-
modation of a single offspring is the outcome of the
reduced family, typical of all perfected arboreal animals,
and so typical of the Primates and Man.

The reduction of the family, and its final result in the
production of only a single offspring at a birth, has had
its effect also upon the development of the mammary
glands. Mammary glands, as is well known, are serial

Fig. 52. — The Type of Uterus associated with Single
Pregnancy. Large Uterine Chamber, No Uterine
CoRNUA, and Large Falloppian Tubes.

The diagram is taken from the human uterus.

structures occurring upon a definite line — the mammary,
or milk, line — which stretches from the axilla, along the
sides of the chest and abdomen, over the inguinal region
to the base of the tail (see Fig. 53). At intervals along
this line mammary glands are developed in all the
Eutherian mammals. The number of functional milk-
secreting glands that are developed varies. In C en fetes,
whose large family Ave have already noted, twenty-two
fully functioning glands — eleven on each side of the body
— are developed. In the domestic sow the large mam-
mary series is obvious and well known.

It is a general rule throughout the mannnaiian series

142

ARBOREAL MAN

that the development of the mammary glands is in har-
mony with the number of offspring produced at a birth,

Fig. 53. — The Mammary Line upon a Hypothetical Mammal,

TO SHOW THE SiTES AT WHICH MAMMARY GlANDS AND NiPPLES
MAY BE DEVELOPED.

and so suckled simultaneously. Animals with large
families possess multiple mammary glands for the suckling

THE REPRODUCTIVE SYSTEM 143

of the numerous offspring. With the reduction of the
family, reduction of the mammary series takes place,
and animals which produce few offspring at a birth possess
few functional mammary glands. The manner of reduc-
tion of the mammary series in response to the lessened
demands of a diminishing family is in no way hapliazard,
and, in a general way, it may be said that with a reduced
family, those mammary glands are retained which are
most convenient for the nursing of the offspring. If, in
phylogeny, the family is reduced from ten to two, then
instead of ten functional mammary glands persisting as
the normal of the species, the number will probably be
reduced to correspond to the diminished number of the
offspring, and the glands selected for survival will be the
pair at which it is most convenient for the mother to
suckle the young. This convenience is ruled by the
bodily habit and adaptations both of mother and offspring.
It is here that the emancipation of the fore-limb enters
as an important factor, for the infant is now enabled to
grasp the mother, and the mother to grasp the infant.
The young of the Lemur grasps tight to the mother's
ventral fur, and in this way are carried about by her as
she climbs among the branches. The method in which
the young hangs on to the mother is curious, for while
the mother is engaged in active climbing movements, the
infant clings with its head towards the mother's tail.
The legs encircle the mother's waist, and the hands grasp
the hair of the mother's flanks, the infant's head being
pressed against the inguinal region of the mother. The
position taken up by the young is doubtless to permit of
free arm movements on the part of the mother. Whilst
in this position, the infant grasps in its mouth one member
of a pair of inguinal nipples which are present in all
Lemurs (and some Bats) (see Fig. 54). These inguinal
nipples appear to be developed — or rather to persist—
for a very definite reason, and they do not in the majority
of cases convey any nourishment to the young, but merely

144 ARBOREAL MAN

serve to provide an extra hold for it during its mother's
excursions about the branches. For this reason I have
named them in a previous paper the anclioring nipples.

f

•i:- ■■•■ .•■..'^ \'5;:-/

■■■:<

mm

■\-::-^ifi^£f^.i^: ;.:;-::n;v^v.v,:.;::. ■■■■■ ■ -A

.-•/A-

^ - ■ (

•^?^l@^i:^;-"--:-;P;

v.'' ■--•''

V'_ — -■_ *

Fig. 54. — The Mammary Glands of a Lemur.

The pectoral pair are functional organs, the inguinal pair serving
only as anchoring nipples for the young.

Anchoring nipples are present in the Rhinolophid Bats,
and have been named in these animals, by Rollinat and
Trouessart, who have especially studied them, " faux
tetons du pubis." In Bats the single offspring clings
to these nipples during the mother's flight, just as the
young Lemur clings whilst its mother climbs. These

THE REPRODUCTIVE SYSTEM 145

anchoring nii^xDles are unassociated with milk secretory
glands, but among the Marsupial animals the unusual
circumstances of pouch life have led to the develop-
ment of a peculiar type of nipple, which is both an
anchoring and a milk-conveying nipple. Now, when the
mother Lemur (or Bat) is at rest, the young one reverses

Fig. 55. — The Pectoral Mammary Gland of a Typical Bat

{Brachi/plijjlla cavernarum).

its position, and clinging with its legs round its mother's
waist and grasping the fur of her chest with its hands,
takes into its mouth one of a pair of pectoral nipples,
and from this it suckles (see Figs. 55 and 50 ). The
pectoral nipples are associated with pectoral mamma ry
glands, and are the source of supply of infantile nutrition.
The aberrant Galeopithecus volans probably combines the
functions of milk secretion and of anchoring in its pec- 1 oral
mammary glands (see Fig. 57). Now in the Primates
higher than the Lemurs the inguinal anchoring nipples are

10

146

ARBOREAL MAN

Fig. 56. — The Pectoral MAMiiAKY Gland of Tarsi us

spectrum.

There is anotlier pair of nipples on the lower part of the abdominal

wall.

Fig. 57. — The Pectoral Mam:siary Gland of Galeojvthecus
volans, WHICH is probably both a Milk-Secreting ^and
AN Anchoring Organ.

THE REPRODUCTIVE SYSTEM

U'

Fig. 58. — Semnopithecus pileatus (the Capped Langur)

NURSING ITS Offspring.

From a photograph by D. Seth Smith, Esq., taken in the Gardens

of the Zoological Society of Loudon.

U8 ARBOREAL MAX

not developed (although as anomalies they may occur in
so high a Primate as Man), and their disappearance
becomes completed as the perfection of the emancipation
of the fore-limb culminates in the power of the definitive
hand grasp.

The young of Monkeys are held by their mothers, and
they are nursed by their mothers, as Owen has described
it, " in very human fashion " — the mother holds the off-
spring whilst it suckles at the pectoral mammary gland
(see Fig. 58). Lemurs do not hold and carry their off-
spring, but the offspring clings tight to the fur of the
mother, and Charles Hose has observed that when Tarsius
is called upon to pick up and carry her baby, she does
it with her teeth, as cats commonly do. But all the
Monkeys carry their babies, and hold them in their arms,
nursing them " in very human fashion." When this
stage is arrived at, the need for inguinal anchoring nipples
is past, and the more convenient pectoral mammae become
the permanent Primate milk-secreting glands. The
importance of the Primate ability to carry and nurse a
baby cannot be over-estimated; many of its effects are
produced very far beyond the limits of mere adaptations
of the rejDroductive system, and these effects will be con-
sidered elsewhere.

CHAPTER XXII

THE DEVELOPMENT OF THE BRAIN

It seems at first sight impossible to derive any advances
in brain development from the mere habit of tree-climb-
ing, and yet it is precisely these important and dominating
advances which can most surely be linked up with the
changing fortunes of the arboreal stock. Since any
story of brain evolution is of necessity extremely com-
plex, and since the different chapters in this story are
interwoven in a very complicated manner, it is necessary
to be quite certain of a tolerable degree of agreement
about two things; the first, What sort of brain was that
inherited by the earliest mammal ? And the second.
In what way will environmental possibilities of education
affect such a brain ? Fortunately, within rather wide
limits, we may expect agreement upon these two points,
and as the working basis of this review I shall take
unreservedly the researches of Professor Elliot Smith.
In that writer's paper on the " Origin of Mammals " the
following statement occurs: "In spite of the certainty
that the mammalian brain passed through a reptilian
stage in its phylogeny, the brain of no living reptile
fulfils the conditions required in the actual ancestor of
the Protomammalia." Here we have evidentlv the same
story, some of the pages of which we have turned pre-
viously. Somewhere, the Protomammal and the Primitive
Reptile meet, somewhere in the geological past these
two stocks branch off from a common ancestor. There
is every reason to imagine that among the Cynodontia
of the Triassic there was this blend of primitive Reptile

149

150

ARBOREAL MAN

and primitive Mammal which constitutes the ideal
ancestor — the ancestor which possessed the ideal Proto-
mammalian brain.

Although the brains of all existing Reptiles are too
highly specialized, in one direction or another, in harmony
with the specialized lives of their possessors, still it is
to them, and to the much more lowly Dipnoi, that we
must turn to obtain any concrete picture of the brain

Fig. 59. — Diagrammatic Outline of a Primitive Type of

Vertebrate Brain.

C.H., Cerebral hemisphere, practically all of which is devoted
to the sense of smell = arcliepallium. A small area, repre-
sented by coarser dots, indicates a portion of the cortex
connected with non-olfactory impressions.

of the earliest Mammal. The anatomical features need
not be discussed in detail. Upon broad lines, such a
brain consists of a collection of ganglionic masses, each
mass definitely allotted to a particular sense or a particular
function. To such a brain, impressions from special
sense organs come by definite channels each to its definite
anatomical station within the brain, and these central
ganglionic stations are in free communication with each
other. In addition to all this there is, as an outgrowth
from each side of the brain, a small cerebral hemisphere,
forerunner of the great cerebral cortex of the higher
types (see Fig. 59). It is in these cerebral hemispheres
that all the possibilities of evolution lie. It is the func-
tion of the cortex that it provides an organ in which are
blended the impressions that come by the several channels

THE DEVELOPMENT OF THE BRAIN 151

to the appropriate ganglionic masses — an organ in wliich
impressions Ure sorted, associated, and stored, so tliat
in the process such a complicated state as consciousness
is evoked, and judgment and memory are made possible.
Upon the completed cortex, complex pictures are woven
and subsequently interwoven with others, and stored in
that endless array of memories which constitutes ex-
perience, and forms the basis of judgments. But these
things came sloAvly in evolution. The reflections from
different centres and different channels found their way
to the cortex gradually, and in definite order. First to
obtain cerebral re-representation was the sense of smell,
and this, of course, for the reason that smell impressions
play such a predominant part in the life of a lowly animal.
Placed right at the extreme fore-end of the primitive
animal body, the great olfactory sense organs and the
olfactory parts of the brain may be regarded as giving
the animal its first impression of anything with which it
came in contact. As the animal moved through life it
tested and learned of life by this the most forward sense
channel, and it was this which first demanded something
more in brain development — it demanded a place in which
to store up the impressions it was repeatedlj^ gathering.
The most primitive cerebral cortex is an olfactory cortex,
and, following the nomenclature of Elliot Smith, we will
term it the Archepallium (see Figs. 59 and 60). This, then,
was the birthright of the Protomammal, a definite cerebral
pallium, a small and limited storehouse, but a storehouse
full of possibilities; for it was one in which impressions
of one kind were already laid by ready for use at any
time, to which others might conceivably find their way,
and in which all might possibly be blended and retained
in that intellectual medley comprised in memory and
experience. Even in existing Reptiles some slight
advance upon a pure olfactory cortex is seen, for " tactile
paths have made their way into the hitherto almost
exclusively olfactory cerebral hemispheres, and estab-

152

ARBOREAL MAN

lislied some definite representation for the sense of touch
in this dominant part of the brain " (Elliot Smith).

The slight advance made in the brains of existing
Reptiles shows the initial stage of the attainment of the

Fig. 60. — Diagram of a Brain in a Further Stage of Evo-
lution THAN THAT REPRESENTED IN FiG. 59.

Tlie coarsely dotted, non-olfactory cortex or neopallium occupies
a larger portion of the cerebral hemisphere.

enormous possibilities which the possession of a cerebral
pallium offered; but it was the uprising mammalian
stock which took full advantage of all the possibilities
(see Fig. Gl).

Fig. 61. — Diagram of a Primitive Mammalian Type of Brain
illustrating the rise of the neopallium (coarselt
Dotted Area).

In the Mammal, not onl}' do smell and taste impressions
gain pallial representation, but impressions of touch, of
sight, and of hearhig streaming into the brain also demand
their reflexion places in the receptive cerebral cortex.
The originally small pallium becomes augmented by new
areas for the reception, the blending, and the storage of

THE DEVELOPMENT OF THE BRAIX 153

these things, and these additions constitute what has
been named by Elliot Smith the NeopaUiinn. Following
the same guide, we may therefore give comparatively
simple answers to our two original questions: (1) The
earliest Mammal inherited an archej)alliuin capable of
great achievements; (2) the environmental possibilities
of education will affect such a brain by increasing elabora-
tions of the neopallium, which the early Mammals started
to develop.

How will arboreal life in particular influence this cere-
bral development ? For some aspects of this question
we can again turn to the paper by Professor Elliot Smith
to which we have alrea^dy made reference, and the best
introduction may be made in the form of a quotation:

" In the forerunners of the Mammalia the cerebral
hemisphere was predominantly olfactory in function;
and even when the true Mammal emerged, and all the
other senses received due representation in the neopallium,
the animal's behaviour was still influenced to a much
greater extent by smell impressions than by those of the
other senses. This was due not only to the fact that
the sense of smell had already installed its instruments
in, and taken possession of, the cerebral hemis[)liere,
long before the advent in this dominant part of the brain
of any adequate representation of the other senses, but
also, and chiefly, because to a small land-grubbing animal
the guidance of smell impressions, whether in search for
food or as a means of recognition of friends or enemies,
was much more serviceable than all the other senses.
Thus the small creature's mental life was lived essentially
in an atmosphere of odours, and every object iu tlie out-
side world was judged primarily and predominantly by
its smell: the sense of touch, vision, and hearing IxMug
merely auxiliary to the compelling influence of smell.

" Once such a creature left the solid earth and took to
an aquatic or an arboreal life all this was changed, for
away from the ground the guidance of the olfactory sciiiie

154 ARBOREAL MAN

lost much of its usefulness; and in the case of aquatic
Mammals, the whole smell apparatus atrophied, and in
some cases vanished. We need not stop to consider the
aquatic Mammal, because a life in the water calls for such
marked specialization of structure that such creatures
disaj)pear from the race for mammalian supremacy. But
the case is very different with arboreal Mammals. Life
amidst the branches limits the usefulness of the olfactory
organs."

So much seems evident; the only difficulty is for us,
with our manifold channels of information, to realize how
thoroughly the lowly terrestrial Vertebrates live their
whole lives dominated by dependence upon the sense
of smell. Friends, and food, are found by their scent,
foes are avoided by the same sense, and the whole sexual
life of the animal is lived in a like atmosphere. This is
very largely the case even with the lowest Eutherian
Mammals, and perhaps as familiar an example of the
scent-dominated Mammal as can be chosen is the common
English Shrew (Sorex araneus). In one feature this
inquiry may be removed from the realms of the psychical
into the domain of gross anatomy, and that altogether
apart from a study of the structure of the brain. Scent
glands of various kinds are most important anatomical
features of these small and primitive Insectivora, and in
them they reach a bewildering degree of complexity of
development; but scent glands diminish steadily in those
stocks which are truly arboreal. No trail of scent is
laid among the branches of a tree, and for an animal that
bas become truly arboreal these glands are comparatively
useless structures. In the tree-haunting Insectivora
they have diminished, the anal glands being their last
survivals. " In Chiromys and some other Lemuridae the
anal glands are reduced to two shallow cutaneous pits
at the sides and upper part of the vent: in the higher
■Quadrumana this trace disappears " (Owen).

In the olfactory parts of the brain, and in the sensory

THE DEVELOPMENT OF THE BRAIN 155

olfactory apparatus itself, the atrophy in the arboreal
stock is extremely well marked, and smell impressions
play but little part in the more important rCles in the
lives of the Primates. In Man the sense of smell, and

Fig. 62. — Diagram of a Typical" Mammalian Form of Brain
IN which the Neopallium has expanded at the Expense
OF the Archepallium (Finely Dotted Area), and
OCCUPIES the Greater Part of the Lateral Portion of
the Cerebral Hemisphere.

Fig. 63.— The Finished Mammalian Brain.

Diagram showing the general expansion of the cerebral hemi-
sphere (neopallium), upon which fissures are beginning to
appear. The archepallium is represented only by an in-
considerable (finely dotted) margin of cortex.

what may be termed the smell-life, are very minor
factors in the whole physiological economy (see Figs. 62

and 63).

Nevertheless, this early sense which first gained a
pallial representation, and became the first siil)jcct of
memories and experiences, still shows in Man a subtle

156

ARBOREAL MAX

power as a memory sense. Dudley Kidd has noted this
feature in investigating the psychology of Kafir children.
" AVhen Kafirs are questioned as to their earliest remem-
bered impressions they usually state that these were
connected with the senses of taste and smell. The next
things they remember are connected with the sense of
colour: th'^n impressions of sound and of form seem to
follow last of all." In still more primitive races the
importance of smell impressions is probably' greater; and
there are few of us who have not some complex memory
picture associated with an early impression of smell.

CHAPTER XXIII

THE STORY OF TACTILE IMPRESSIONS

In picturing the activities of a primitive Mammal we
have seen how large a share the sense of smell takes in
regulating the life of the animal, and in guiding it about
its habitat. A primitive Mammal may be said to " nose **
its way about the world, and it " noses " its path through
life in more senses than one Just as its nose leads the
way, and gives the first impression of novel objects by
permitting the animal to become acquainted with their
scent, so it gives the second impression of them by
imparting to the animal a knowledge of their '' feel."
Such an animal is guided to an object by olfactory
stimuli from the nose; afterwards, it tests the object with
its snout. This is a form of activity well seen among the
Shrews; tactile impressions of everything with which
they come into contact being conveyed by the elongated
snout. Touch tests for novel objects are carried on by
the extreme anterior end of the animal body in all lower
forms of life, and long before a " nose " is developed
the animal is guided through life by touching those
objects with which the fore-end of its moving body comes
into contact. When a definite nose is present, an animal
may be said to learn tactile experience of its surroundings
by bumping its nose into them. In the lower Mammals
this function is very obvious, and the anatomical adapta-
tions to subserve it are numerous. The snout region has
set apart for its special innervation that great ganglionated
cranial nerve known as the trigeminal, the branches of
which convey sensory impulses from the whole of the
skin area which surrounds the muzzle, ^lorcovcr,

157

158 ARBOREAL MAN

special tactile sense organs are lodged in the skin of this
region, and special tactile sensory hairs — the whiskers,
etc. — are connected with them. When a Shrew, nosing
its way about its habitat, comes across a novel object
it learns much of it by its smell; but by the multiple
stimuli imparted to the tactile sense organs, through the
whiskers of its elongated and mobile muzzle, it consider-
ably reinforces this knowledge, by adding an idea of
size, form, etc., of the object, the smell of which has been
tested. Snout-tactile, or fifth cranial nerve, impressions,
therefore, soon find their way to the pallium, and the
long muzzle becomes typical and emblematic of this
association in all primitive Mammals, Prototherian,
Metatherian, or Eutherian.

In dealing with the story of the fore-limb, we have
seen what is the fate of this elongated snout in the evolu-
tion of the arboreal animals. With the emancipation of
the fore-limb, and the development of the power of hand-
grasp, there is seen harmonious recession of the snout
region, the grasping hand taking on the functions of the
grasping jaws.

But there is something more important and far-
reaching than this in the process, for the grasping hand
becomes also the testing and touching hand. Not only
does the hand come to take over the crude grasping
functions of the teeth and jaws, but in gradual stages it
slowly but surely usurps the delicate tactile duties of
the muzzle. The recession of the snout is therefore a
vastly important thing, for not only are the characters
of the jaws and teeth and the general build of the face
profoundly altered, but the principal tactile organ of the
animal body is transferred, as a whole, from one part to
another. The liberated hand takes on the duties of the
snout, and the exchange is effected very completely and
harmoniously, so that all those functions formerly dis-
charged by the snout are now carried on, and with far
greater efficienc}-, by the hand.

THE STORY OF TACTILE IMPRESSIONS loi>

The physical changes are great and obvious. l)ut as
possibilities of progress in evolution they are trivial,
compared with the new avenues opened up for cerebral
development.

The enormous difference which the translation of the
receptive mechanism for touch impressions makes in
animal economy is difficult to appreciate. Change of
conduct, however, makes apparent the more striknig
lines of progress. The picture of the lowly animal which
noses its w^ay through life smelling with its nose, and
examining with its snout all novel objects with which it
comes in contact, is familiar to everyone, and is one that
contrasts strongly with the behaviour of an animal that
has become arboreal. Although it is a very long step
to take, much may be learned by going straight to a
Lemur and watching its treatment of novel objects.
Here, handling obviously takes the place of nosing,
although the scent test is by no means omitted, especially
m all cases where the suitability of the object as an
article of food is concerned. If Nycticebus is given some
fruit which is new to it, it will examine the fruit \\ith
its fingers, pick it up with its hands if it be small, and
then, as a rule, it will hold it to its nose. It will also
smell its hands, and if these tests produce no result,
some animals wdll proceed to rub the fruit, or hammer
it on the ground, in order to obtain the scent from a
bruised or scraped surface. All this is done before any
attempt is made to eat any unfamiliar object. .Aluch
the same behaviour is shown when the animal tests an
object which is merely a novelty, and is not regarded as
a possible article of food. The superiority of hand-
tactile information is at once seen by watching such an
animal, and the possibilities of education of this new-
touch organ are easily realized. Even before the jiower
of grasp is developed, we may imagine the dawn stages
of educational advances initiated by hand touch. In
the first place, the mere physical separation of thu nu-^i

160 ARBOREAL MAN

important tactile organ from the seat of the nasal scent im-
pressions is important, for other things than those smelled
out or bumped into come constantly under examination.
The evolution is evidently harmonious in its details.
The more the fore-limb becomes emancij^ated, the less
is the hand called upon for menial duties which in other
stocks necessitate the development of skin thickenings,
pads, callosities, or hoofs. It is the freed hand which is
permitted to become the sensitive hand, and it is the
freed and sensitive hand which now, so to speak, goes in
advance of the animal and feels its way as it climbs
through life. The animal no longer smells out an object,
subsequently to feel it with its nose; but it feels with its
hand some object which comes within its reach in the
ordinary course of its arboreal activities, and it may or
may not subsequently add to its knowledge of the object
by smelling it. Tactile impressions gained through tlie
hand are therefore perpetually streaming into the brain
of an arboreal animal, and new avenues of learning about
its surroundings are being opened up as additions to the
old olfactory and snout-tactile routes. With tlie develop-
ment of the power of grasp, new and great possibilities
come in. Much may be learned of an object that can be
felt by the hand; much more of an object that may be
grasped, lifted and examined in the hands. When an
object can be grasped and lifted it can be examined from
every point of view, and the eyes must play a large part
in this examination. Its whole outline, the texture of
its surface, its hardness or softness, its size, temperature,
and weight, can all be ascertained. It is difficult for
us, with our perfected cerebration, to appreciate the
difference which the power to grasp an object makes to an
animal attempting to learn the nature of objects with
which it comes in contact, but we may be sure that the
difference was very great, and was made greater when
the j^ower to pick up the object and to examine it from
all points of view was added.

THE STORY OF TACTILE IMPRESSIONS IGl

There are many other ramifications, and many otlier
possibilities, of this educational gain in the poRsessi(ju
of a sensitive tactile hand; there are the enormous advan-
tages of the opportunities of correlating and checking
impressions gained by other channels ; the encouragement
of the development of fine movements of the hands;
the ultimate possibility of using in the hand an object
judged to be useful (as a weapon or implement) by the
hand; and a host of others. These we will not discuss
here, but will leave for treatment, where then importance
demands it, as separate entities.

11

CHAPTER XXIV
MOTOR IMPRESSIONS

The very fact that the sense of touch becomes lodged,
to so large an extent, in the emancij)ated hand of the
arboreal animal becomes a guarantee that this hand will
be called upon to discharge its tactile function in a variety
of ways. All sorts of uses, previously quite foreign to it,
will be demanded of it in virtue of its possibilities as a
tactile organ. The combination of the increasing tactile
perceptions, and the freedom of movement, creates a
condition which ultimately leads to the most important
developments.

The sensory stimuli streaming from the hand towards
the central nervous system must become associated in the
most intimate way with the motor imj^ulses streaming
to the mobile fingers. There is, in the end result, no
gross alteration of the mechanism of the hand, but there
is an enormous alteration in the nervous control over
the hand, and the purposive skill with which it can be
used. The hand, as a strangeh^ primitive anatomical
structure, becomes applied to all the finer and more
skilful movements which the life necessities of the animal
can demand of it. Ever}' increase in cerebral develop-
ment will make new demands upon it, and these demands
are met by an increase of range of controlled, co-ordinated,
fine movements. To those who, in the literature of a
bj'gone age, were termed the " curious " it will ajopeal
as an interesting theme that this hand, anatomicall}- one
of the most j^i^iniitive parts of Man's body, one to be so
nearly matched among the " hands " of the lo\vcst

1G2

MOTOR IMPRESSIONS io3

limbed Vertebrates, has responded to ail llie exacting
calls made upon its functions by the myriad promptings
of the complex human brain. We ^^•ill, however, not
pursue this theme.

It is the necessity for the close association of the func-
tions of sensation and mobility, which are subserved by
the emancipated hand, that is of interest in rvolntif.ii
from the dawn stage we have pictured. ^^^• are con-
cerned only with the problem of the possible manner in
which these things have affected brain deveIoi)ment.
In the present state of knowledge this problem is a lii^hly
complex one, but there can be no doubt that, on broad
lines, fairly simple underlying processes act harmoniously
in the evolution of the brain. There has been enunciated,
by Dr. Ariens Kappers, of Amsterdam, a theory of one
such underlying principle to which he has given the name
of " neurobiotaxis.'' Put into simple language, the])rin-
ciple involved is a calling of the nervous seat of orij^nn
of the outgoing motor impulses towards the site to which
the associated incoming sensory impulses stream. Sup-
pose, for instance, the primitive nervous centre which is
associated with any definite sensibility to have a ^^ell-
defined anatomical position in the central nervous
system, then, in its immediate neighl)ourliood. i\\u\
attracted to it, will be the motor centre which governr-.
the movements of the parts most intimately related to
this particular sensibility. It may be that this i^articular
sensibility is intimately related to different mova])le parts
in different animals, and then in each will be found n
different motor centre appropriately attracted into tlie
closest anatomical relation to the sensory centre. This
general principle has been shown b}^ Ariens Kaj)pers to
hold good in the case of the ganglionic centres of the
cranial nerves, and to account for their otherwise ine.x-
I)licable positions in the brain stem. This principle of
neurobiotaxis is, I believe, capable of extension fn m
the ganglionic masses in the brain stem where Ariens

164 ARBOKEAL MAN

Kappers demonstrated its reality, to the pallial areas in
the cerebral cortex, where these ganglionic masses gain
re-representation.

There is an order in cortical representation of functions
which is probably brought about by the same agency as
that which determines the order of the basal ganglionic
masses. We have seen that the first function to gain a
representation in the cortical pallium is the sense of
smell, and we have pictured snout- tactile impressions as
following in its train. It is therefore likely that the
site of representation of these snout tactile impressions will
be in that part of the pallium which is in the immediate
proximity of the olfactory area. Likely, too, that the
associated mouth sense of taste will be in its near neigh-
bourhood. Further, since in the brain stem the motor
centres are attracted to sensory ones, it is likely that a
pallial area associated with snout movements will also
be developed in the neighbourhood of these sensory
areas. We should now have a brain the cortex of which
consisted of an archepallial olfactory area, and grouped
in its immediate neighbourhood in the developing neo-
pallium areas devoted to the storing, sorting, and associa-
tion of impressions of taste, snout sensations, and snout
movements. So far, our outline of brain building has
been upon purely hypothetical grounds, but we can pass
from this stage to reality at any moment by examining
such functional charts as have been made of simple mam-
malian brains (see Figs. 64, 65, 66, 67 and 68). In the chart
of such an animal the rather large olfactory area, or arche-
pallium, has as its immediate neighbours in the neo-
pallium a taste area, and an associated area related to
tongue movements; and a tactile area in which sensations
from the snout are stored, with, as a forward extension
of this, an area which, when stimulated, evokes snout ;
movements. We have now^ imagined a further develop- j
ment in which the hand is added to the snout as a tactile
sensory organ, and in which the co-ordinated fine move-

r
I 1

MOTOR IMPRESSIONS ig5

merits of the hand are increasing in perfection. Those
things we are picturing as demanding ])allial roj)resenta.
tion, and it is likely that the hand-tactile area N\iil bo
added as a new neopallial area beyond that devoted to
snout touch, and that its corresponding motor area will
be attracted to it as a distal addition to the snout move-
ment area. This, again, is a condition which passes from

Fig. 64. Fig. 65.

Fig. 64. — Cerebral Hemisphere of Macroscelides (the Jump-
ing Shrew), to show the Cortical Areas as determined
BY Professor Elliot Smith. (From Duckworth.)

M, Motor. S, Sensory. V, visual. A, auditory areas. Tiie

white areas are olfactory.

Fig. 65. — Cerebral Hemisphere of Tiipaia (the Tree Siikew).

TO SHOW THE CORTICAL ArEAS AS DETERMINED BY PRO-
FESSOR Elliot Smith. (From Duckworth.)

Note the development of a prefrontal area in front of tin; motor

cortex (M).

the hypothetical to the actual, for the sensory and motor
association areas of the hand are laid down on the un-
folding neopallium as outliers to those we have already
seen to be realities.

For the present we will leave brain architecture at
this point where neopallial representation is comprised,
in our limited survey, to taste and tongue niovenient
areas; snout tactile and snout movement areas; hand
tactile and hand movement areas, as local i'/ed portions
of cortex spreading from the old archepallial olfactory
area over the unfolding neopallium. IMeanwhile A\e will

1()()

ARBOREAL MAN

return to our arboreal animal to study the ever-increasing
possibilities of its education.

The greatest difference between the process of gathering
tactile impressions by the snout region, and receiving
them by the hand, is that in the latter case the examina-
tion of a novel object is carried out to a far greater extent
under the observation of the eyes. It is true that when

Fig. 66. — Cerebral Hemisphere of Tarsius spectrum, to show
THE Cortical Areas as determined by Professor Elliot
Smith. (From Duckworth.)

Note the development of intervening " association " areas be-
tween the visual (V), sensory (S), and auditory (A) fields, as
well as the enlargement of the prefrontal area.

the snout region is the tactile organ, the object tested is
brought into greater proximity with the eyes; but it is
exposed to a far more limited and restricted view than when
it is examined by the hand. Tactile impressions from the
examining hand become correlated with visual impres-
sions as simultaneous observations. Visual sensatioiij-
will gain an added possibility as avenues of education,
and their pallial representation will, in all probability, be
augmented.

Again, two other factors must be added as affording
paths by which education may advance. These two
factors at first sight seem obscure, and possibly trivial,
and yet it is not at all improbable that they have had
marked effects upon cortical development. In the first
place the emancipated hand may feel, examine, and test
practically every part of the external surface of the bod}^

MOTOR IMPRESSIONS InT

and an animal may now treat its own Ijudy as a novt-l
object and learn all about it. In the second ])lace (as a
result of the altered poise of the lioad. etc.) tlie eyes niav
also examine almost all of the body, and tlie animal thm
has a picture of its own external anatomy — a picture
more perfect for some parts than for others. These two
factors are correlated by the simultaneous observat ictus
of hand and eye, just as are the impressions gained by
the examination of any object such as a nut or a grass-
hopper. The meaning of this may perhaps be made more
clear by taking examples. Some animals must of neces-
sity possess an extremely limited knowledge of their
own bodies. A tapir, for example, can see but little of
its body, and can examine with its tactile nose only a very
limited portion of it. An elephant would know next to
nothing of its general form were it not enabled to gather
touch impressions of those parts of its body accessible
to its trunk. A horse can reach and touch a limited
area with its nose, and can gather impressions in this
way, and supplement these impressions by those gathered
by its eyes. A dog can touch with its nose a ^^ ide area
of its body, and can bring a great deal of it under the
observation of its eyes. A very great advance is seen
in any arboreal animal which possesses an emanci]iated
fore-limb and a mobile head; there is little that a monkey
does not know about its own external anatomy.

An arboreal animal gains a precise knowledge of its
own body; it can realize its form, and it has, to a certain
extent, a w^orking idea of the alterations in its form which
are the outcomes of the movements of its several parts.

I imagine that it is mostly in this way that the whole of
the body gains cortical representation in the neopallium
in ordered sequence, from nose to perineum. The eortieal
area in which this representation is localized is. as wo
should expect, an extension of the tactile nose and hand
area in the developing neopallium. Some dilliculty has
always been felt in defining the qualities represented in

168

ARBOREAL MAN

this area. It could be conceived that there might be
separated sensory and motor areas adjoining each other,
or the two might be combined in one complex sensori-
motor area, which, in the hope of providing a rather
wider connotation, has been named the " kincesthetic
area." Some of the difficulties w^ould, I believe, be
removed by naming it the ''pictured movement area,"
and for " pictured " we might substitute the words
" realized " or " known," provided the connotation of

Fig. 67. — Cerebral Hemisphere of a Lemur, to show the
Cortical Areas as determined by Brodmaxn. (From
Duckworth.)

Note the general enlargement of the " association " areas from

the stage seen in Tarsius.

these words were clearlv understood. In this area are

ft/

represented the impressions of those parts of the body
of which the animal has concrete knowledge. The hand,
the forearm, the elbow-joint, the arm, the shoulder;
the trunk, the thigh, hip, knee, leg, ankle, foot, and
perineum, all have their specially allotted areas in Man.
And to these centres of the im2:)ression of pictured parts
are added, possibly by the agency implied by neurobio-
taxis, the centres concerned with the pictured movements
of these parts.

\Yithout an extended discussion of the anatomical
details of the central nervous system, we may fall back

MOTOR IMPRESSIONS \m

upon the axiom— agreed to by physiologists. ])atlio-
logists, and anatomists — that " movements, not muscles,
are represented in the cerebral cortex.'' 1 think we might
extend this axiom by claiming that only *' pictured
movements " are represented in the cortex.

This axiom in reality teaches a great deal, for the braiu
knows nothing of muscles, since the animal is itself
ignorant; but for movements it has a vast storehouse,
the contents of which are in direct ratio to the animal's
own pictured know^ledge of the form and movements nf
the different parts of its body.

We may gain considerable knowledge of the functions
of this pictured movement area by the consideration uf
the results of experiments, which have been carried out
by different investigators, upon a series of animals oi
varying zoological position. The movements of the
different parts of the body are not carried out hi all
animals by the same nervous centres. We will tabulate
the experimental findings in order.

1. If a bird, or a Vertebrate lower than a bird, be
deprived of its brain altogether, it can continue the
movements of its limbs; a deca])itated fowl is not a
paralyzed fowd, for it will continue to run and flap its
wings for some time despite the entire loss of its brain.
" It is possible to remove the entire cerebrum of a ])ige(in,
yet it is capable of flight w^hen thrown into the air an
hour later " (Kinnier Wilson).

2. With a Mammal there is no activity, at all compar-
able to this manifestation, in the absence of a brain, but
very varying effects are produced in different .Mannnals
by removal of the pictured movement area of the cortix.

3. '* On the day of removal of the cerebral centres for t ho
limbs, a rabbit will jump vigorously " (Kinnier Wilson).

4. If this area is removed in a puppy it does not beeonie
paralyzed, and one week afterwards, ^^-hen it has recovered
from the operation, it can carry out all the movements
proper to a normal dog.

170 ARBOREAL MAN

5. With a monkey the effects of removal of the pictured
movement area are much more grave, for the animal is
very considerably damaged by the operation. It is not
completely, but it is partially paralyzed, and the paralysis
affects the hand movements far more than the leg move-
ments. There is still, however, a very well-marked
ca]3acity for recovery.

6. In Man the effects of injury to, or disease of, this
area, or of the fibres coming from the area, are very well
known, and a Man whose pictured movement area is
entirely destroyed is completely paralj^zecl on one side
of his body. Moreover this paralysis is permanent.
This unilateral paralysis, or hemiplegia, is of a very
special type (upper neurone type), for the muscles them-
selves are perfectly capable of acting, are perfectly well
nourished, and in a good state of tone; but their move-
ments cannot be initiated for any pictured movement.
Now, in connection with this upper neurone t^^pe of
paralysis, there is one strange clinical fact which may be
expressed in the usual axiomatic manner, by saying that
" a muscle which can 2^^^'form two movements may he
paralyzed for one movement, and 7iot for the other '^ One
pictured movement centre may be damaged, while a
neighbouring one may be spared, and the muscle is only
deprived of its power to take part in one of its previously
possible movements. But more interesting still are those
cases in which a muscle is entirely paralyzed for all
pictured movements by the destruction of these areas,
for then the muscle may still act, j^rovided it plays some
part in any movement which is not represented in the
pictured movement area.

For example, a muscle (M. trapezius) which acts upon
the shoulder and arm, and also upon the ribs, may be
quite unable to perform its pictured movements upon
the shoulder and the arm, after such a lesion (hemi-
plegia), but is quite competent to act when the patient
labours in respiration, coughs, or sneezes, these move-

MOTOR IMPRESSIONS 171

meiits not being represented in the pictured nioveinciit
area. So much for some of the facts ; what is the prohahlo
interpretation of them ?

It is obvious that in birds, and vertebrates h)\ver llian
birds, there is a co-ordinating mechanism in the s])inal
cord — a reflex mechanism — which is capable of carrying
on the movements of the body in the entire absence of
the brain; that the brain is not necessary for the workini;
of this reflex mechanism. In the rabbit and d()<; this
reflex mechanism of the cord is considera])lv lessened,
and the centre for the initiation of movement is in the
brain; but it is obviously not (in its essential part) in the
cortical pictured movement area. As a matter of fact,
experimental evidence has proved it to be in a ganglionic
mass connected with the cortical neopallium which is
named the corpus striatum (for details see the work .)f
Kinnier Wilson and others).

In a Monkey, although some movement is, without
doubt, still initiated from the corpus striatum, much
(and quite a definite part) is now lodged iu the cortical
pictured movement area.

In Man all the j)ictured movements are initiated from
this area, but movements of which the individual has no
definite pictured cognizance — such as the movements of
the heart and viscera, and the movements of respiration
— are still lodged in the ganglionic masses of the brain
stem.

It would seem probable that the representation ut the
pictured movements are arranged in the neopallial cortex
in a perfectly definite order, and that the sequence of their
establishment is evidenced by the well-known distribution
of the areas in the kinsesthetic region of the human l.rain
(see Fig. 71, p. 192). It is perhaps not beyond ])ossil)ility
that the full lodgment of all pictured movements is not
yet permanently effected in all human brains, and iliat
the process is still in progress. There is certain anatomical
support for such a supposition, but its exact nature dors

172 AKbUKi^AL. xAlAA

not concern us here, and we may rest content with the
usual clinical conclusion that the lodgment has been so
complete that damage of the whole of this kina?sthetic
area causes a total inability to perform all " voluntary '*
— or more accurately — " pictured " movements. It
should be noted that, although in popular usage it is
commonly assumed that such oft -repeated things as the
movements of the legs and feet in walking become
" reflex," such an expression is totally inaccurate. No
repetitive pictured movement ever becomes reflex in the
sense that it is initiated anv where than in the cortex of
the neopallium. Its site of initiation is lodged in the
neopallium, and it cannot be substituted by any " lower "
centre. Were walking, for instance, ever to be performed
as a true reflex, the power to walk would still be present
in cases of hemiplegia. It is this complete translation
of the mitiating centre to the cortex which demands an
education of the human motor functions. A human
baby has to learn to walk, it has to learn all purposive
pictured movements. The newborn young of a lower
Mammal does not have to pass through the probationary
period necessary to an animal, in wliich such movement
is represented only in the cortex. There is a gradual
scale in this feature displayed in the mammalian series,
and to this we shall have to return in a subsequent
chapter, since it is concerned with the problem of infancy, j,
From the point of view of cortical representation of %
motor impressions the arboreal habit has therefore '♦
probably effected a great deal. It has permitted of .
hand-testmg, and it has enabled this testing to comprise
a correlated study by the hands and the eyes. It has
given scope for a wide range of fine hand movements, and
it has demanded a high degree of co-ordination of these
movements. It has also called forth a very special
co-ordination of movements in the balancing, necessary
in an arboreal life. And it has permitted the animal to
know, and to picture, all the outward features of its

MOTOR IMPRESSIONS 173

bodily activities. All this has demanded cortical repre-
sentation in the developing neopallium, and has effected
a translation (in which probably the principle of ncuro-
biotaxis is involved) of the motor centres from a basal
ganglionic mass into the kingesthetic, or pictured, move-
ment area. So far as I know, no human being, be he
anatomist, physiologist, or clinician, has yet conceived so
concrete a picture of the visceral movements involved
in respiration, circulation, and the processes of alimenta-
tion, carried out in his own body, as to insure these
movements a representation in his own cerebral cortex.
And it is well that it is not so, for in that case the physi-
cian's attendance would be in more frequent demand,
and hemiplegia would be inevitably fatal.

CHAPTER XXV
IMPEESSIOXS OF SIGHT AND HEARIXG

The two other senses, sight and hearing, which gain an
early neopallial representation in the Mammals become
of enhanced value to the arboreal animal. Their in-
creasing importance in this stock is, in the first place,
largely the outcome of the diminishing dominance of the
sense of smell. When an animal ceases to find its way

«

about the world guided almost entirely by olfactory
impressions it begins to rely more and more upon other
senses for its guidance. It is not that the sense of smell
or its pal Hal representation becomes lost, but it ceases
to be the main channel through which the animal gains
knowledge of its surroundings. Sight, especially, becomes
the principal guiding sense of the arboreal animal.

Both visual and auditory neopallial areas are well
developed in terrestrial Mammals. There is nothing
whatever distinctive of arboreal life in the mere cortical
representation of these senses, but the arboreal life has
a ver}' definite influence upon the development of these
areas.

Two essentially physical factors come prominently

into play in the elaboration of the neopallial areas devoted

to sight and hearing in arboreal animals. The first is

one to which reference has been already made, and which

ma}^ be termed the increased mobilit}^ of the poise of

the head. There are obvious educational possibilities for

the animal which can turn head and eyes and ears all

together, and Avith the greatest rapidity, towards an}*

object which attracts its attention h^^ any sensory

channel.

174

IMPRESSIONS OF SIGHT AXD HEARIXG 17r>

Then, again, there is that process which we have termed
the recession of the snout region, which effects so many
changes, and among them brings the two eyen to the
front of the face. This purely physical change produces
great and new possibilities of vision. In an animal in
which the snout region is prolonged, the eyes are lateral,
and the correlated vision of the two eyes is necessarily
imperfect,, each eye possessing a more or less independent

Fig. 68. — ^Cerebral Hemisphere of a Monkey {JIacacus), to
SHOW THE Cortical Areas as determined by Brodmaxx.
(From Duckworth.)

A further advance is seen from the stage represented by the
lemur, especially in the development of the prefrontal area.

field of vision; and the blending of the two visual fields
into stereoscopic effects can be only very partially effected.
There are, probably, not complete conditions of isolation
of the two visual fields in Mammals, although among the
Reptiles their complete separation is common enough,
but the separation in maii}^ mammalian forms must
approach completeness.

The power to look directly forwards with both eyes at
once is present in all arboreal Mammals, but in many
terrestrial quadrupedal pronogrades it is very limited.
Even the dog is given to running with its head tui-ncd
somewhat sideways, a position which, affecting the car-
riage of the Avhole of its body, is alternated at intervals

176

ARBOREAL MAN

from side to side as the animal runs. When, with the
shortening of the face, the eyes are permanently brought
to the front, they both control one visual field. It is
noteworthy that in Tarsius spectrum the mobility of the
head seems almost to replace the mobility of the enormous
eyes, a change akin to that by which the mobility of the
head has already replaced the mobility of the ears (see
Pig. 70). Although in Man the two eyes see slightly

Fig. 69. — Human Cerebrum, to show the Main Cortical

Areas.

Note the general change in position of the areas with cortical
growth and also their wide separation by " association " areas.

different pictures when either is used alone, these pictures
overlap so greatly that with binocular vision they blend
into a common field in which stereoscopic effects are
produced. It is difficult to estimate the changes that this
has produced in the educational possibilities of the sense
of sight, but it is easy to realize that they have been great,
and I imagine it is to be appreciated crudely by studying
the different mental pictures produced by a simple photo-
graph and stereoscopic views examined through the
appropriate lenses.

So far the arboreal animal has come to have a greater
dependence upon the impressions of sight and of hearing,
for the reason that its former guiding sense of smell is

IMPRESSIONS OF SIGHT AND HEARING 177

Fig. 70. — An Adult Female Tarsius spectrum.

From a spirit specimen collected by Dr. Charles llosc iit the

Baram District of Sarawak, Borueo,

\-2

178 ARBOREAL MAN

now of only minor importance. It has increased the
possibilities of both these senses by the greater mobility
of its head, and, in the case of sight, by the recession of
the snout region and the bringing of the eyes to the
front. It is easy to see that these things can only in-
crease the educational adaptability of these two senses,
and add to the possibilities of the association of their
impressions with (1) each other, and (2), in the case of
sight, with the sense of touch.

This is exactly the condition which the neopallial
development of arboreal animals would lead us to expect
from pure anatomical and experimental evidences. When
the auditory area is first laid down in the neopallium, it
is situated at the hinder and lower portion of the cerebral
hemisphere; the visual area lies immediately above it,
and the kinassthetic area lies in front of both. When
the cerebral hemispheres expand with the increased
demand for neopallial representation, the expansion does
not take place equally. One portion of the cortex is
in lateral apposition with the ganglionic mass of the
corpus striatum, and this portion does not share in the
expansion which affects those parts of the cerebral
vesicle which have no such rigid anchoring mass in rela-
tion with them. This portion consequently remains more
or less fixed (as the island of Rcil), while the rest of the
hollow hemispheres enlarge around it. As the cerebi'al
hemispheres expand, the mam direction of their growth
is backwards, and in this way it comes about that the
enlarging hollow hemispheres revolver ound this fixed point
as they grow (see Figs. 64-69).

The auditory portion of the neopallium, which was
situated at the posterior inferior extremity of the primitive
brain, sfrows downwards and forwards as it revolves
below the island of Reil, pushed on from above by the
enlarging visual field. Not onl}^ this, but, as it expands
round the island of Reil, it becomes separated from the
visual field by an intervening cortical area. The visual

IMPRESSIONS OF SIGHT AND HEARING 17<J

field in its turn is pushed backwards by the expansion of
the neopallium, and so comes to occupy the posterior limit
of the cerebral hemisphere; and it, in its turn, becomes
separated from the sensory area by an intervening area
of cortex. An altogether new field, the prefrontal or
silent area, is developed at the anterior extremity of the
hemisphere, and it, by its growth and expansion, is largely
instrumental in bringing about the rotation around the
island of Reil, which has been described. In the brain
of a Lemur, and more markedly in the brain of a Monkey,
the original neopallial areas allotted to the several senses
have (1) migrated from before backwards round the
island of Reil as a centre, (2) have tended by their growth
to submerge this fixed field of cortex, and (3) have become
separated from each other by ever-widening fields of
intervening cortex (see Figs. 67 and 68). It is these
intervening areas, known as " association areas," which
are of interest. In them there is every reason to believe
that the impressions represented in the bounding areas
are blended and co-ordinated. It is these association
areas between the auditory and the visual, and between
the visual and the sensory fields which, enlarging in
the arboreal Primates, become the distinctive feature of
the human :brain (see Fig. 69). And this anatomical
condition is the result we should expect from studying
the educational possibilities of the arboreal life.

i*.

CHAPTER XXVI

HIGHEK DEVELOPMENTS OF CEREBEAL FUNCTIONS

It has often been assumed that the anatomist is incapable
of making any real contribution towards the knowledge
of the origin of Man, since he treats, and rather prides
himself on treating, his material as though, to use Huxley's
well-known expression, it were sent from some other
planet, preserved, it may be, in a cask of rum. He takes
cognizance only of muscles, bones, and other organs,
but, it is urged there is something far more subtle than
a mere assemblage of anatomical structures to be con-
sidered in the evolution of Man. D wight, as an anatomist,
has put forward this view with most cogency, but still,
even when conducted with his skilful handling, and
backed by his special knowledge, the argument cannot
be considered as a reasonable one. D wight has said that
to regard an animal merely as an anatomical entity to
which to assign a zoological position, '' is a very narrow-
minded and one-sided view to take of any organism, and,
above all, of so high an organism as Man, whose intel-
ligence (be its origin what you will) places him in an
order of his own. The problem is of a higher sphere
than that of morphology."

That the problem of the evolution of intelligence is
beyond the reach of investigations undertaken in pure
morphology is a proposition very difficult to combat
when we are forced to take as objects for study " a bee
or an ant or a wasp," such as Dwight postulates. But
the difficulty is one imposed rather by the limitations
of knowledge in the particular field selected, than by any

180

HIGHER FUNCTIONS 181

inherent inadequacy of a morphological method of study.
In dealing with the intelligence of Mammals tho pro})l('m
becomes, in great measure, centred in the ascertainahle
channels of cerebral education afforded by the several
senses, and the study of those educable fields of cerebral
cortex with which these senses are associated. In tliis
way we may regard the intelligence of a ^lammal as a
thing not wholly separated from its anatomical structure,
and therefore not wholly outside the province of, nor
entirely unexplained by, a purely morphological study.
Within the limits of knowledge, admittedly very imj)er-
fect, intelligence as a summation of cerebral possi])ilities
may be said to be a thing which falls within the province
of the anatomist; a thing concerning the evolution of
which he can glean some definite ideas by the methods
of comparative anatomy. It becomes merely a question
of academic argument to deal with the next general
proposition as enunciated by Dwight: " Reason, involving
as it does general ideas, can by no possibility have been
evolved." If we regard this formation of " general ideas "
merely as the product of a specially perfected type of
cerebral mechanism, a mechanism which we may see in
every stage of increasing perfection in existing forms,
then we are bound to admit that the facult}' of reason
itself is merely an extension of evolutionary development
of the neopallium; and, indeed, there is no adequate
ground for doubting this. Despite the pitfalls that may
occur over the use of mere words, " reason" is a product
of evolution just as much as is, say, the tactile association
area of the cerebral cortex. Dwight's third ]irnposition
that, "It is very evident that no process of survival
of the fittest could have led to higher ideals of conduct."
is only likely to catch us tripping by the introduction <>f
the phrase "process of survival of the fittest "' in place
of the previously used word " evolution." The employ-
ment of this phrase is evidently not due to chance, since
I it gives occasion for a picture, so easily drawn., of savage

1S2 ARBOREAL MAN

nature, fierce struggles, and the consequent elimination
of the weak, but possibly moral, individual, and all the
other non-social tendencies, that the law of self-preserva-
tion in an active, possibly bloody, life-contest connotes.
With this proposition we will deal further, leaving alto-
gether untouched the fourth extension of Dwight's line
of reasoning — " that the evolution of the soul is untenable
as a scientific proposition."

For the present purposes we will take as our stand-
point the thesis, that the rise of the neopallium is the
tangible anatomical evidence of the perfection of cerebral
processes, and that in the ordinary sequence of evolution
the neopallial dominance and complexity culminate in
the production of those faculties ordinarily connoted in
the term " intelligence." An elaboration of intelligence,
which we conceive to be simply attained in the ordinary
workings of evolution, demands the rights of recognition
as a more or less distinct faculty called " reason." It
remains for us to ascertain if there is any indication in
this evolution that an extension of the process along its
normal lines could possibly lead to the formation of any
basis for what is termed " higher ideals of conduct."
It is necessary, first of all, to rid the problem of any sug-
gestion that these things came about by a process of
*' survival of the fittest," in the sense that this survival
means dominance in physical contest, in the elimination
of the unfit in the sense of the physical weakling. There
may be a much more peaceful evolution — but an evolution
none the less — and I regard the arboreal life as a school
in which some of the lessons of conduct were learned.

We have seen that arboreal life tends towards the reduc-
tion of the number of young produced at a birth, and that,
in the Primate stock, it is the rule that but a single off-
spring is begot at each pregnancy. This, as I have
pointed out in a previous chapter and elsewhere, is a
mere adaptation to life circumstances — an application
of the general rule that when no natural nursery is to

HIGHER FUNCTIONS 183

hand there will be no large families. Tlir i naming
Ungulates, ready to flee upon the least ai)i)ic|iiiisi<)ii of
danger, have no natural nursery iur tlieir young, and in
all of them the family is reduced. The ])elagic (Vtacea
are in the same condition, and so also arc the Sirenia.

Large families can only be indulged in by animals tliat
can have a safe retreat in Avhich to rear their nunu-rous
young, or by animals sufficiently equipped witli weapons
to guard them.

Of those animals which, having no nursery to liand,
have a reduced litter, there are two distinrt cla.-ses.
The first class, for which we may turn to the horse (as
a representative of the Ungulates) for an example, is
made up of animals whose roaming life is composed of
a series of escapes from danger; animals that depend
for theh^ safety, not upon their retreat into burrows,
holes, or any other fastness open to some smaller bea>ts,
but upon the swiftness of their open escape. These
camiot be successful if the females are handicap])ed by
the disabilities of pregnancy with large litters, or by the
nursing of helpless offspring. In them the number A
offspring is reduced, and the usually solitary infant is
born singularly mature, so that it may share as soon as
possible in the life-saving activities of its species.

The solitary young of such animals is born " grown
up," it can flee at its mother's side within a few hours of
its birth. Its period of dependence upon its mother is
relatively short, and there is but little infancy, or child-
hood, for such a baby. In the second class come tlie
arboreal animals. There is no natural nursery anion^;
the tree-tops, and the disabilities of pregnancy with a
large litter are felt as keenly in active tree-climbers as
in any class of animals. No doubt nest-buildinu wa--
resorted to as a temporary expedient in the ari)oreal
stock; and among all the arboreal and semi-ar])oreal
animals derived from many orders, nest-building, m some
members, is still the rule. But nest-building only over-

184 ARBOREAL MAN

came a temporary disability, and in the end, reduction of
the family solved the problem.

The baby of the perfectly adapted arboreal animals
of the Primate stock is solitary; but it is a baby very
different from that we have pictured in the previous
group. The arboreal baby is born immature, and it is
singularly dependent upon its mother in the precarious
circumstances of life among the branches. There would
seem to be no alternative in such a life; the baby must
either be born a perfected tree-climber, or it must be
a more or less immature creature dependent upon others
for its safe conduct about the branches. As a matter
of fact, the offspring of the Lemurs and ^lonkeys are
born immature and comparatively h'jlpless, save for the
power of grasp which is well developed in their hands.
Naturally they cannot immediately follow their mother
upon her arboreal excursions; and among the Lemurs it
is the rule for the young to grasp the mother, and among
the Monkeys for the mother to assist by grasping the
young. The Simian mother has to carry the baby with
her wherever she goes; this, at the outset, is a new factor
in the relation of mother and offspring. We may surmise
that in this new relation there is given a wider scope for
the working of that very primitive display of instinct
summed up in the commonly used j^hrase " maternal
care." Maternal care is, of course, perfectly well mani-
fested in animals situated very differently from those
we are studying; it is, in some of its manifestations, a
widespread and primitive animal instinct. But the
phrase " maternal care " when applied to a mother that,
in time of danger, defends a dozen helpless offspring
connotes something rather different from its extension
to a mother that carries a solitary offspring which clings
to her throughout a somewhat prolonged infancy.

It is to be regretted that observations upon the intimate
details of the lives of the Primates in their natural state
are not made more frequently by those having the

HIGHER rUNCTIOXS 185

opportunity to do so. Among the Lemurs, Charles Hose
has noted how Tarsius carries its baby in the way common
among cats, by picking it up with the teeth. It evidently
does not nurse its offspring.

The young of Nycticebus tardigradus clings tight to the
mother, and the mother makes but little effort to handle
its young. It will bite savagely if an attempt is made
to remove the baby from its fur, but, as a rule, it resents
any other interference in exactly the same manner. ( )n
one occasion a female Nycticebus escaped from its cage
at night, and left its baby, which was still suckling, to
its fate. The baby, which was reared on the bottle,
used its voice freely each evening, but the mother, thouj^h
living in some trees quite close to its cage, never returned
to it. The voice of the mother was heard on rare occa-
sions, but five years passed before her actual home was
discovered; even then she was still within a few paces
of the spot in which she started her freedom, and in the
meanwhile the young one had died.

I do not know of any recorded observations which show
that in the Lemurs the maternal instinct is very much
developed beyond its display in carrying the heli)]ess
baby clinging to the mother's fur. With :\lonkeys,
however, the care for the young is very real, and several
observations have been recorded upon this point. Both
in their natural state, and in captivity, :\Ionkeys show
the greatest concern in the well-being of their offspring.
That they will defend them from attack is nothing, for
such a display of maternal instinct is the common ])roperty
of most living creatures, but Monkeys go further than
this in the development of those numerous tendernesses
for their young which in all accounts are, and can only
be, likened to human parallels.

With the Anthropoids, so far as opportunities for study
in their natural state have permitted, there is every
evidence that maternal and paternal care is carried still
further. Many observers have noted the human manner

186 AKBOREAL MAN

in which the Gibbons attend to their j^onng, and the
mothers have been seen to take their babies to the water
and carefully wash and dry them (Bock); even the
Gorilla has been seen to correct its offspring by boxing
its ears when it misbehaved (Koppenfels). Not only is
the display of maternal care much more marked in all
these higher arboreal Primates, but it is exercised for a
very much longer period than in any other animals.
Arboreal Primate babies have a very long babyhood and
a long infancy. The baby Gibbon (Hylohates lar) clings to
its mother for about seven months (Blanford), and it
is not fully mature until it is fourteen or fifteen years old
(Hartman). The young Orang-utan is dependent upon
its mother for about two 3'ears, and is not fully adult
until it is fifteen (Forbes).

This prolongation of infanc}^ and the period of youthful
dependence, has probably a rather widely reaching
influence. It calls for a much more prolonged exercise
of parental care and control, and causes these attributes
to be more or less permanent characteristics, rather than
periodically recurring manifestations of an instinct.
Again, the prolongation of infancy may be said to be
the especial factor which created the family as a social
unit. In almost all the higher Vertebrates it is the habit
of the male parent to remain with the mother during the
helpless early stages of the offspring, and in many in-
stances (in several orders) he even plays his part in caring
for the young during their most dependent period. In
the Primates, the share that the male takes in the duties
of parenthood has often been noted. The males have
repeatedly been seen to carry the young on their arboreal
journeys, and it has even been asserted that the male
of the Siamang Gibbon (Hylohates syndactylus) always
carries the baby if it be a male, the female parent only
carrying a female offspring (Diard).

In whatever degree parental duties to the helpless
offspring are discharged by the male arboreal Primate,

HIGHER FUNCTIONS 187

it is evident he is only fulfilling a genenil Ijiijjogicul law;
but it also follows that if infantile lK'l])lessness is jjio-
longed, his parental duties are liable to a similar exten-
sion. Here is evidently the beginning of tiiat asM)c-iati<.ii
of mother, father, and child which, lasting Ih-voihI a
brief j)eriod comprised in courtship, the suckling of lii-lp-
less young, and the guarding of mother and olTs]jring,
lays the foundation of the family.

When infancy is brief, the family bond is simijarlv <.f
short duration; and, the period of suckling being ended,
there comes a time of expansion of infantile enterj)rises,
a time marked by some internecine strife and nnich
parental intolerance. It becomes a necessity for the
mother to repel the young when mammary activity is
ended; it devolves upon the father to chastise any possible
rivals: and in most large littered animals the family tie
loosens and dissolves as soon as the young are fully
capable of fending for themselves. As the period of
dependence of the solitary offspring becomes more pro-
tracted, the advent of the dissolution of the family is
naturally delayed — it may be delayed until the recin-rence
of the next natural parental sexual season. This i
imagine to be a very important factor. If the bond of
the helpless offspring keeps the male in attendance until
the next sexual period of the female, there is likely to be
a recurrence of the whole process, and a step towards the
permanence of their union.

Although, as Professor Hickson has observed, there
is a striking poverty of observations upon llioc viiy
details of Primate economy, enough has been recorded
to warrant some general statements. The Anthropoid
Apes are met with almost invariably as family ])ariies,
or as solitary wandering individuals, and it is believed
that pairing lasts for life. " The gorilla lives in a society
consisting of male and female and their young of varying
ages " (Koppenfels, quoted by Hartman). " The C'liini-
panzee either lives in separate families, or in small groups

188 ARBOREAL MAN

of families " (Hart man). " Each male lives with his
own single female " (Forbes).

The Orang-utan — at any rate the male — seems to be
rather more solitary, for he is generally encountered alone
(Wallace), but " the female is generally accompanied by
one of her progeny, sometimes by two, the one always
an infant, and the other a more or less grown but im-
mature individual of a previous birth " (Forbes). In the
Gibbons is seen that amalgamation of families into
groups which so frequently forms the basis of Monkey
communities. There is room for very many more ac-
curate observations upon the formation of these social
communities, which, especially in the genus Semnopithecus,
embrace a large number of individuals banded into an
apparently fairly- well -defined group.

The Proboscis Monkey {Nasalis larvatus) lives in small
communities embracing up to thirteen individuals
(Hornaday) ; Seyniiojntkecus femoralis in groups of from
ten to thirty (Hose) ; with S. cephaloterus parties of from
twenty to thirty (Tennant); and with S. Barbii from
thirty to fifty (Anderson) are usual.

Most of the genus Cercopithecus live in communal
groups which may contain from thirty to fifty individuals
of such species as C. campbelli (Forbes). The Macaques
also are group monkeys, M. nemestrinus sometimes
forming considerable communities. The typical African
Baboons live in extremely large packs, some companies
being said to comprise as many as two thousand indi-
viduals (Slack), but these animals, being for the most
part non-arboreal, do not so directly interest us.

The aberrant Black Baboon of the Celebes (Cyno-
pithecus niger) is, however, an arboreal animal, and it is
" usually seen in pairs, but sometimes a family of seven
or eight may be found together feeding in a tree. Such
families invariably consist of a pair of adults and a number
of young ones " (Hickson). According to the natives
these baboons pair for life. ^

HIGHER FUNCTIONS 189

Most of the New World Monkeys live in small com-
munities, nevertheless the family unit is long maintjiincd
in some forms {Lagothrix, etc.), and in some is said to be
permanent {Pithecia).

Amongst the Lemurs conditions vary greatly. Some
live in small groups, but the majority remain isolattni in
pairs, or limited to family parties. Very little of the
intimate details of their lives has been studied, but
Tarsius spectrum definitely " lives in pairs " (Hose), and
so does Nycticebus tardigradus.

If higher ideals of conduct are admitted tu Ijc mere
extensions of a natural cerebral evolution of wliich so
many other developments are certainly known, it w ill be
under such conditions as those we have been })icturing
that they will be called into being. If higher ideals of
conduct are to be acquired as an evolutionary process,
it is in the family circle that their rudiments will l)e laid
down, and it is in the family circle and in the society
composed of families that these rudiments will be per-
fected.

CHAPTER XXVII

HIGHEE DEVELOPMENTS OF CEREBRAL FUNCTIONS:
POSSIBLE ANATOMICAL BASIS

When we come to make any attempt to attach a precise
location to the neo^^allial representationof such higlier cere-
bral developments as " intelligence " and " higher ideals of
conduct," we are at once met with an overwhelming
difficulty, a difficulty almost as great as that which con-
fronted the mediaeval anatomists who sought a structural
habitat for the " soul." The difficulties are so great, and
imagination must play so large a part in attempting to
overcome them, that very considerable latitude must be
permitted in their treatment. It is, however, possible,
and permissible, to make guesses, provided the guesses
are carefulh* deprived of any pretence to be a part of, or
take equal rank with, knowledge derived from ascertain-
able facts. It is for this reason that a discussion of an
anatomical Iiasis of " intelligence " and " higher ideals of
conduct " is isolated from the study of those other things
which an anatomist can, and must, investigate with
scalpel and forceps. In the first place, I conceive that
" intelligence," " reason," " intellect," the '' mind," or
any other word which has a like connotation, denotes a
thing somewhat different from " higher ideals of conduct."
It is not the most intellectual person who necessarily
has the highest ideals of conduct. I have regarded
intelligence as an expression for the summation of the
cerebral possibilities of an animal. The channels by
which education can come to the cortex, the development
of the cortical areas and their correlation and association,

190

HIGHER FUNCTIONS: ANATOMICAL BASIS loi

compose the physical basis of an animal's intelligence.
It is even possible to conceive a creature in which neo-
pallial development had reached its very lii^hest point,
in which channels of education were multiplied, and in
which cortical areas were elaborated and associated in a
bewildering complexity, culminating in the liighest
possible receptive, sorting, associating, and storincr
mechanism evidenced by a prodigy of intelligence or
intellect, but in which higher ideals of conduct were
absent.

If, then, we can imagine what constitutes the anatonii-
cal basis of intelligence, what picture hav^e we of the
physical seat of "higher ideals of conduct"? There is
a well-known cortical area, which is situated at tlio
anterior end of the neopallium, that has yielded uj) no
secrets to the experimental investigator. It is called at
times the " silent area," since stimulation of it produces
no result in the ordinary methods of experiment ; from
its anatomical position, it is also named the " frontal '*
or " prefrontal " area. This portion of the brain is
already beginning to differentiate in the Tree Shrews; it
increases through the whole Primate stock, and is de-
veloped to its greatest extent in Man. We may regard
the neopallial cortex as a mantle in which are situated
receptive centres of different impressions, and we may
regard the elaboration of the neopallium as a growth of
"association areas" interposed between the areas
allotted to these different impressions.

In " association areas " are blended impressions from
different receptive centres, and in them are formed,
sorted, and stored memories and experiences derived from
the several senses, the centres for which march with tluir
borders. This prefrontal region marches upon the
borders of only one such area — the so-called "nu)tor"
area. If this prefrontal area be— as it is generally
assumed to be — the seat of "memory, judgment, and
imagination " or of " higher mental faculties, of cu-

192

ARBOREAL IVIAN

ordinated ideas," etc., it seems strange that its only
associated areas should be the " motor " centre (see Figs.
€4-69, and 71). We have seen that there appears to be an
underlying functional order in the massing of the neopallial
areas, and it is therefore disconcerting to find this purely
ethical centre developed as an extension of, and associated
only with, the motor area. But we have already postu-

Perina3um

Pig. 71. — Diagram of the Left Cerebral Hemisphere of a
Human Brain, to show the Order of Representation
IN the " Pictured Movement " Area.

lated that this so-called motor area is a field devoted to
a very special motor function which we have attempted
to express as " pictured movements." We are assuming
that it is an area in which are lodged impressions of the
movements of which the animal has present cognizance,
a function which may be crudely expressed by saying
that it comprises the movements which an animal can
see and feel itself doing. It is, therefore, not so im-
probable that this new anterior silent area, which has
connection with no other neopallial areas, is simply an
extension from this specialized pictured movement area,
and probably an association area of it. It is not im-

HIGHER FUNCTIONS: ANATOMICAL BASIS rJ3

possible to imagine that an area which is an association
or extension area of this field, in which an aniniart:
actual pictured movements are stored and sorted, might
be connected with a further elaboration — in the form of
an idealization — of pictured movements. From a con-
ception of a concrete movement performed under actual
circumstances, it may be that passage is made to the
idealization of a possible movement performed under
hypothetical circumstances, and that this latter process
takes place in the silent prefrontal area. Proba])ly the
first occupation of this new area is effected by memories
of pictured movements, and the sorting of experiences
gained by this source. From calling up pictures of past
associations of pictured movements, there is perha])s
a step towards constructing conceptions of future move-
ments evoked by pictured h3'j)othetical circumstances.
The picturing of action in hypothetical circumstances
seems to me to be almost synonymous with such a concept
as conduct. Conduct can only be pictured in term^ of
action. We may say that in the gaining of this prefrontal
neopallial area the animal passes from a state in which
it has a conception of its present and actual movements
to a state in which it has memories of past movements
and pictured concepts of possible future movements.
The animal without a neopallial kinaesthetic area performs
all its actions in the absence of any pictured consciousness
of the action. An animal with a kinsesthetic area per-
forms actions of which it has a definite mental pictured
conception. It knows what it is doing.

An animal with a developing prefrontal association
area has, in addition, memories of its past actions. //
knows what it is doing, and it remembers what it has dune.
An animal with an elaborated prefrontal area lias, in
addition, the faculty for building up pictures of possible
future actions. It knows what it is doing, it remembers
what it has done, and it can estimate what if might do.
We may translate this into the phraseology usual in the

13

194 ARBOREAL MAN

description of human mentality. That it knows what
it is doing presupposes the existence of consciousness.
That it remembers what it has done argues the dawning
of a conscience. That it can estimate what it might
do implies the laying of the foundation stone for building
ideals of cojidnct. Here is at least the basis for the
formation of that grade of moral social behaviour that
results from the lessons taught by experience. If ideals
of conduct be the answers to an ever-insistent series of
problems comprised in the question, " What shall I
do ?" then the area in which ideals of conduct are lodged
is, very probably, the prefrontal silent area. It must
be pointed out that in thus approaching the question of
the function of this area we are proceeding by more or
less logical steps; we are not merely localizing vague
functions, of which we can obtain no physical signs, in
an area from which no response can be elicited b}' experi-
ment. We are not forced by the extremities which
urged Descartes to assign the habitation of the soul to
the pineal body, but we are attempting to determine
the functions of this association area, just as we should
determine the function of any such area, by ascertaining
the probable characters of its neighbours.

But this finding brings us face to face with the diffi-
culty, that in imagining the intellect to be represented
anatomically in the summation of all the neopallial areas,
and ideals of conduct to be lodged in the prefrontal areas,
we are supposing a rather definite separation of these
two factors in cortical representation. I believe that
this is, as a matter of fact, no difficulty at all, but is in
many ways a clue to understanding some normal and
abnormal conditions displayed in human mentality. It
should be possible to have a very definite separation of
these qualities displayed by their very unequal develop-
ment in different individuals. There are certainly
persons in whom no very special qualities of the intel-
lectual mind are present, but to A\hom the problems of
conscience and conduct bulk so large as to be a definitely

HIGHER FUNCTIONS: ANATOMICAL BASIS 105

one-sided development. Again, there are otliers whose
intellectual mind is particularly well developed, but whose
conceptions of conduct and of conscience are distinctly
below the average. Disease may apparently affect these
two qualities separately, and I imagine that advances in
knowledge are likely to be made only by attacking the
problem along these lines.

The views of Charles Mercier have been vividly ex-
pressed to the medical profession, but apparently they
have been but little comprehended. " Alienists still
deny that insanity is disorder of conduct, though they
witness such disorder in every case of insanity that comes
before them; they still declare that disorder of mind is
insanity, in the face of many mental disorders in which
not a trace of insanity can be found " (Mercier). Most
physicians are familiar with the patient whose abnormal
conduct demands his confinement within the walls of an
asylum, but whose intellect would be envied by many
whose conduct fits them to live without those walls.
Equally familiar is the patient whose intellectual estima-
tion of the abnormalities of conduct displayed by his
fellow-inmates is perfectly sound, but whose own conduct
is possibly even more abnormal than that which he
criticizes adversely in others. On the other hand, the
conduct of an individual in w^hom damage of an associa-
tion area prevents his intellectual mind from finding the
least meaning in the spoken words of his fenoA\-men may
be perfect.

Should reason and intelligence be the outcome of the
perfection of cortical representations of the several senses
and the development of ample association areas, and
should the formation of higher ideals of conduct ])e a
concomitant phenomenon dependent upon the develop-
ment of a prefrontal association area, then the rise of
these things may be followed (by the ordinary methods
of the anatomist and physiologist) in the elaborating
cerebral hemispheres of the arboreal stock, which cul-
minates in Man.

CHAPTER XX\aiI
THE BRAIN AND THE BODY

We have seen that arboreal life may be regarded as
offering opportunities for educational possibilities un-
known in terrestrial life. W'c have also seen that it
probably brings about certain bodily modifications. We
are now confronted by a problem: Did the cerebral
advance create the physical adaptations, or did the
physical adaptations make possible a cerebral advance ?
It would seem, at first sight, that upon such a problem
the argument might be as long sustained, and as futile,
as that expended upon the question of the priority of the
hen or the egg. And yet the question is a very interesting
one, and one well \\orthy of attention. It is certainly
not to be dismissed by a series of confident and epigram-
matic assertions. It is possible that at least a partial
solution can be given.

Using a form of words wellnigh meaningless, but never-
theless well understood, we ma}^ say that Nature has
made several experiments in brain-building. Vertebrate
brains are not built all upon one plan; even within the
limits of the [Mammals, brain architecture varies con-
siderably in basal design in the Prototheria (Monotremes),
Metatheria (Marsupials), and Eutheria (higher Mammals).
There is no living prototherian animal which has adopted
the arboreal habit, and the few existing members of the
Monotremes lead lives of particularly restricted possi-
bilities. But many of the Metatheria lead lives as truly
arboreal as that of any animal, and, indeed, the Marsupial
stock is regarded by some as being 2^1'iniarily arboreal.

196

THE BRAIN AND THE BODY 107

These arboreal Metatherians have had all the educational
advantages of a thoroughly arboreal life; nothing that
we have pictured has failed to exert its influences ujxm
them, and yet it is obvious that the advantage that they
have taken of it has been slight. There are metatherian
convergent mimics of Carnivora, Rodentia, Insect ivora,
and of most other Eutherian orders, but there is no
metatherian convergent mimic of the eutherian Primates.
It would not be unnatural, therefore, to assume tliat the
full advantage could not be grasped by the metatherian
animals, since the ground-plan of their brain would not
permit it. Climbing metatherians Avith perfectly mobile
fore-limbs and grasping members were at one time classed,
upon the strength of this feature, amongst the Cheirojx'ds,
a grouj) which included only them {DidelphidoB, etc.)
and the Primates; but they were sorry companions for
the Monkeys and the Lemurs in all other respects. Life
habit has made them physical mimics, in some degree,
of the Eutherian Primates ; it has not made them mimics
in any cerebral feature. Rotating forearms, grasping
fingers, opposable thumbs — all these features are found
in perfect combination in the arboreal metatherians, and
yet far short of a human, no anthropoid, no simian, and
no lemurine evolution is seen in the Metatheria. Obvi-
ously, it is not the bodily adaptations alone that have
sufficed to create the possibilities of Primate brain develop-
ment. We have followed the changes in physical ad-
vances and S3en how these have affected Primate evolution,
each physical adaptation leading to new possibilities of
cerebral advance. All these physical changes could be
followed equally well in the Metatheria, but we should
fail to note a corresponding advance in cerebral perfec-
tion. It is, therefore, natural to ask if there is any gross
condition of brain architecture which will serve to dis-
tinguish the metatherian from the eutherian brain, and
if this distinction will in any way account for the very
slight evolutionary advances made by thoroughly arl)oreal

198 ARBOREAL :\L\N

metatherians. Anatomically, this question receives an
almost perfect answer. Without entering into a bcAvilder-
ing array of interesting anatomical details which, deter-
mined by Owen, were somewhat obscured by later
writers, only to be defined with more striking emphasis
by Elliot Smith and other recent workers, we may assume
that, on the whole, it was the development of the corpus
callosum and all its associated structures that gave the
eutherian brain its psychical as well as its anatomical
distinction (see Figs. 72 and 73). A true corpus callosum
— the great cross-connecting bond of the two neopallial
areas — is the outstanding feature of the eutherian brain,
and is the index of its neopallial j^erfection. Without
neopallial possibilities, educational advantages and physi-
cal perfections come in vain to the animal.

The evolution of the free and mobile fore-limb in
arboreal life may be likened to the production of a musical
instrument — an insti-ument upon which it is impossible
for the animal to produce a full range of harmony, or to
appreciate the psychical connotations of this harmony,
unless adequate cerebration is developed coincident ly.

Once again in the evolutionary story we are forced
back to consider a combination of seemingly trivial, and
apparently chance, associations; in this case the dawning
possibilities of neopallial developments combined with
the physical adaptations due directly to environmental
influences.

Some authorities have ascribed great, and possibly
undue, influence to the changes in brain architecture,
while some have concentrated upon the purely bodily
adaptations. The solution of the problem lies probably
in the consideration of the mean of these two influences.
Physical perfections of adaptation are useless, unless
advantage can be taken of them by a specialized type of
brain; but specialization of the cerebral architecture
cannot proceed in the absence of, yet cannot create,
physical specializations in evolution. The earliest

THE BRAIN AND THE BODY

199

Mammal possibly had the physical advance ])laced
within its reach. The earliest eiitherian Mannnal

Fig. 72. — Diagram taken from the Drawing by Professor

G. Elliot Smith of the Brain of Oniithorhj/nchus.
The brain is in medial section and the commissures are cut across.

There is no corpus callosum.

Fig. 73. — Diagram of the Human Brain in Medial Section.

The commissures are marked in a similar manner to those sliown
in Fig. 72. Note the size of the corpus callosum.

possessed the cerebral condition which made it ]inssihle
for it to take full advantage of the physical advance.

200 ARBOREAL MAN

Neopallial perfections did not, for instance, create the
hand, but cerebral advances made possible the full
utilization of this very primitive yet very plastic member.

Large-brained Man has invented schemes of classifica-
tion which embrace all living things, and he has agreed
that the brain perfection wliich he possesses is to be
adjudged, in his schemes, as the qualification for the
highest rank. We may therefore say that, from a human
point of view, evolution consists of increasing perfection
of the brain, and that an animal's place in the scale of
Nature may be determined, in the last resort, by an
appeal to its cerebral development. In this sense, the
brain has led the way in evolution, and physical adapta-
tions may be regarded as following in its train. Yet the
])hysical adaptations are by no means to be ignored.
A Master maj^ perform marvels upon the violin, but his
expression will be seriously ham])crcd if there is nothing
better to hand than an empty cigar-box strung with a
few strings. ]\Ian may execute a bewildering array of
highly skilled movements with his thumb and five fingers,
but it is difficult to see how the human brain could have
coped with a fore-limb in which stability had predomi-
nated in the culmination of a second segment devoid of
the power of rotation and furnished with a terminal hoof.

CHAPTER XXIX

THE HUMAN BABY

It is to the young of animals that we look, as a rule, to
find evidences of the lingering of ancestral habits. Evi-
dences of an ancestral arboreal habit might possibly
linger under some guise or other in the young of an animal
which, descended from an arboreal stock, has ceased to
make its home among the branches.

A striking illustration of the conv^erse of this expecta-
tion may perhaps make the argument more clear. Among
the birds, the whole family of the Terns {Steniidce) is
characterized by a typically terrestrial habit of incubation,
for their eggs are laid uj^on the bare ground. It is true
that some species make a slight attempt at nest-building,
and some meagre wisps are brought together to line a
shallow depression in the beach shingle. In the case of
one member of the family {Anous stolidus), this nest may
rise, as a collection of sea-wrack, to the dignit}' of being
a little mound; but the general rule is that the egg is
laid bare upon the ground. The Tropic Island "White
Tern (Gygis Candida) has, however, taken to an arboreal
life, and it lays its solitary egg upon the branch of a tree.
No nest whatever is constructed, and no attempt is made
to insure the safety of the egg beyond selecting a spot
upon a branch where some irregularity of the bark \\\\\
prevent it freely rolling away. Although in this business
of finding a suitable place^ in balancing a naked egg
upon a bare branch, and in the whole process of sitting
upon this delicately poised egg, the adults show a very
complete adaptation to their new surroundings, the
offspring is hatched as an obviously terrestrial creature.

201

202 ARBOREAL MAN

It is not so completely helpless as is the typical inhabitant
of an arboreal nest, nor is it hatched perfected for arboreal
life, but it exhibits just that ability for early terrestrial
enterprise that the typical terrestrially hatched members
of its family possess.

It is this retention of the old terrestrial adaptation of
the young that causes a defect in this otherwise singularly
successful assumption of arboreal habit, for the dangerous
degree of ability in terrestrial enterprise, which the young
still possesses, leads at times to its early destruction by
falling from the branch. Xo doubt there are good
reasons — probably in the shape of land-crabs and rats —
for the adoption of this strange nesting habit by Gygis
Candida, but, even were there no typical Terns in which
the ancestral customs could be studied, an examination
of the young would at once reveal the fact that the
parental arboreal life was a comparatively recent assump-
tion by the species. As the baby "Wliite Tern shows so
well its terrestrial inheritance, whilst its parents ^re so
perfectly adapted to an arboreal life, it is not unlikely
that the human baby will show its arboreal inheritance
better than its terrestrially modified parent.

We will first turn to so obvious a point as the relative
lengths of the arm and leg. In typical arboreal Primates
the arm is longer than the leg, and in some forms, such
as the Orang-utan, the disproportion is very well marked.
This disproportion may be expressed by means of an
*' intermembral index," which, without further discussion,
we may accept as an arithmetical expression of the
relation of fore and hind limb lengths, which is high
when the arm is relatively long, and low Avhen it is rela-
tively short. In the Orang-utan this index is about 140,
in the Gorilla about 118, and in the Chimpanzee only 104.
In adult Man the alteration has been so great that,
though the index is as high as 83-6 for the Bambute
Pygmies (Shrubshall), it averages no more than 67 in
most Europeans (Duckworth).

THE HUMAN BABY 203

But it is to be noted that the anthropoid proportions
are retained in the human foetus until a relatively late
stage (see Figs. 74 and 75), and that even in the human
baby the proportion of arm length to leg length ap-
proaches the index of the Chimpanzee (see Fig. 70), the

Fig. 74.^ — Human Embryo
105 MM. IN Total
Length.

Fig. 75. — Human Embryo
195 mm. in Total
Length.

I

disproportionate growth of the human leg being largelj-
a post-natal development. At one stage of human
embryonic development the arm is longer tluin the leg
— a typically arboreal Primate feature; later the two
members are equal, and then the leg outstrips the arm
in relative growth. When the baby is born this Iniman
lengthening of the leg proceeds more rapidly; ^\heu the
child begins to walk the disproportion becomes more
marked (see Fig. 77), and the influence of this factor is
marked until about the period of the fifteenth year of

204

ARBOREAL MAN

life (see Fig. 78). This later human growth of the leg
may be expressed more crudely, but perhaps more
strikingly, in another way. When a baby is born, its
umbilicus is below the middle point of its entire body
length, measured from the soles of its feet to the crown

Pig. 76. — Xewborn Fig. 77. — Child Eighteen Fig. 78. — Child Six
Baby. Months Old. Years Old.

of its head. But as the post-natal growth proceeds, the
umbilicus moves relatively upwards, and by the end of
the eighteenth month it is the central point of the body
length. By the fifteenth year it is well above this point,
which is now situated in the region of the pubic symphysis.
It is worth}^ of note that in this feature the male has
advanced more than the female, since the preponderant
growth of the legs has exerted a more marked influence
in displacing the body centre in the adult man than in
the adult woman.

THE HUMAN BABY

205

In the relative proportion of arm and leg the human
baby is, therefore, far more like an Anthropoid— far more
like an arboreal animal — than are its parents.

In other features the same tendency is shown, and we
will only note in passing the far greater ])ower of toe-
grasp displayed by infants and young childien than is
ever seen in European adults. This point is merely
noted, and no stress is laid on it, since the habit of wearing
boots is so readily appealed to as the factor which has
deprived the adult European foot of its grasping powers.
One other detail with regard to the foot of the human
baby should be mentioned, and that is the inturning of
the soles, which, characteristic of the arboreal Primates,
is so well marked in infants. The soles of a babj-'s feet
are turned inwards so completely that they can ])e
pressed fiat against each other, this, indeed, being a
common position of rest in an infant, as in an arl)oreal
Anthropoid.

When children learn to walk, it is upon the outer side
of their feet that they trust their weight, exactly as the
Anthropoids are wont to do. The bones upon the outer
side of the feet are first ossified, and it is the outer margin
of the foot which first bears the body weight ; the eversion
of the foot is a later and a human characteristic. It is
this inherited arboreal foot-poise which leads children to
make holes in the outer sides of the soles of their boots
before the inner margin are subjected to any great degree
of wearing.

Only one other arboreal characteristic of the human
baby will be noted here, and that is one whieh has so
often been discussed, as to be well within the bounds of
homely and domestic knowledge. No one who has let
even a very young bab}^ entwine its fingers in his luur.
or has permitted a slightly older one to gra])pk' with Ins
watch and chain, will doubt the very real i)ower of an
infant's hand-grasp. This extraordinary power of hand-
grasp, although a very homely thing, is one of the most

206 ARBOREAL MAN

astonishing features of a newborn baby. It is generally
known that a baby within an hour of its birth can support
its body weight by hanging with its hands for at least
ten seconds. One observer (Dr. Louis Robinson) has
recorded the fact that twelve infants under one hour old
supported themselves thus for thirty seconds, and that
three or four could hold on for almost a minute. When
li the child is between a fortnight and a month old, it can
support its body weight by its hands for a longer period,
some even being capable of hanging on for ^wQ_iIiii^^^tes,
but after a month the baby generally refuses to be tried
by any such test, and relaxes its grasp when any strain
is exerted upon its arms.

The suspension of the body weight for even a minute
by a baby a fortnight old may not seem to be a very
astonishing feat, and yet it is quite as much as most
adults can do. The suspension for two minutes thirty-
five seconds which Dr. Louis Robinson records for a baby
of three weeks is a truly remarkable performance, since
it is longer than that possible for the average healthy
schoolboy, and far longer than that attainable by most
adults.

This curious strength of the grasp and of the arms is
an obvious arboreal adaptation of the human baby. It
is the survival of the grip which enabled it to cling to
its mother, and to the branches of its arboreal home, and
as such it wanes in the human body after the first few
months of its life, and becomes still less when the power
of walking upright is fully acquired in infancy.

CHAPTER XXX

THE ARBOREAL ACTIVITIES OF M(Jl)i:iiN MAX

If tree-climbing has done so much for the human stock,
and if the arboreal habit is, so to speak, so near to the
basis of humanity, it is natural to inquire in+o the evidences
of the retention of this ancestral habit in existing man.
What abilities to lead an arboreal life are manifested in
existing man ?

In such an inquiry we are liable to be led astray by
many things, but none more likely to distort our outlook
than the fact that modern civilized man has learn f<l to
climb. Schoolboys are taught to climb a roj)e upon
lines altogether different from those employed by their
Primate ancestors. A white man " shins " up a pole
in a fashion foreign to the arboreal Primates: he clasps
it with his knees, and with his locked legs and feet, and
by approximating this hold to his hand-grasp, he clumsily
and slowly progresses upwards. The Euro])ean small
boy climbs a tree in true monkey fashion till he ('(^nes
to a branch which is nearly perpendicular, and then his
only resort is to " swarm " up it. The European man has
perfected his knee and leg grasp by a mechanical con-
trivance known to schoolboys as climbing inuis. which
are furnished with spikes at the points where the hi^s
are most adopted to hugging the ])ran('h.

This method of climbing is, however, a mere ada])tation
to the handicap imposed by long civilization and the habit
of wearing boots. It is a confession that Ww plastic foot-
grasp is lost. Unbooted races do not
" shin "

or

swarm
up trees, but many of them have learned Mune

207

208 ARBOREAL i\IAN

mechanical way of assisting their waning powers of foot-
grasp. One widespread method is the adoption of a
hoop or girdle Avhich encircles the tree and the man's
waist, and so allows him to lean back from the trunk
while his feet are firmly planted against it. This is a
natural mechanical contrivance which enables the climber
to use his hands for other purposes than for mere hanging
on. His foot-grasp is not good enough to trust to, and
an extra support is gained by the waist girdle, which
allows a free use to be made of the hands for gathering
fruit, incising the bark, or any other j)urpose.

Some races do not use the waist girdle, and they rely
still more upon the foot-grasp, but supplement it by
running a thong between the two big toes. This method
is often made use of by Malays in climbing the almost
vertical stems of coconut trees. The two feet are
pressed firmly against the trunk, and the thong (about
one foot long) stretching between the big toes readil}^
adapts itself to the annular irregularities of the bark.
The security afforded by this hold is very great.

But, again, other and more primitive people use no
mechanical contrivance at all ; they depend entirely
upon a foot-grasp just as monkeys do. In some parts
of the world coconuts are gathered from the trees before
they are ripe enough to fall, and then very commonly,
and as a matter of convenience for repeated climbing,
the upright stems are notched, producing the so-called
" monkey ladder." These notches will not enable an
ordinary European to climb the tree in native fashion,
but for the native they provide an ascent but little more
difficult than the mounting of a stairway. The natives
walk up these trees with great facility by taking advantage
of the slight irregularities afforded by the notches.

But in other places coconuts are not gathered — they
are permitted to fall when ripe, and then no monkey
ladder is made upon the trees. In these places when a
native climbs a tree to obtain a drinking nut, or to tap

ARBOREAL ACTIVITIES OF :y[Or)EKX MAX 2(.9

the spathe, he depends entirely upon the natin-al grasjj
which his hands and feet afford him. He does not shin
or swarm up, but approximating the palms of his hands
and the soles of his feet to the trunk, he walks (.r i limbs
up exactly as a monlvcy would under simihir eiicum-
stances.

Races more j)rimitive than the Malays can climl) tlie
perpendicular trunks of jungle trees with the greatest
ease. The Sakai "can climb about like monkeys"
(Skeat and Blagden) (see Fig. 79.) The Semangs,
although they are not ignorant of mechanical aids, are
skilful climbers in the typical manner of tlic rrimates.
*' I myself once saw two of the Kedah Semang run
several yards up trees by putting the flat of their feet
against the trunk and their arms round it " f Skeat and
Blagden). Sea-going Malays adopt the same method
when climbing masts or ropes aboard shi]), and in all
these feats the grasp of the big toe is a very essential
feature.

Tree-living habits also must not be forgotten in any
review of arboreal man. Arboreal houses, or even mere
arboreal leaf shelters, are well-known ethnological details
of the domestic economy of some primitive races. Nor
must the origin of these arboreal homes be overl(»oked.
since their purpose is that, while the human occupants
may freely climb to and fro, they are inaccessible to the
more dangerous jungle beasts. The Semangs regard a
shelter high up in the branches as the safest place for
human babies, and they usually gain access to these
houses by a slanting bamboo made purposely shiny and
difficult for predatory animals to climb.

It does not matter to us how ethnologists might be
disposed to regard these cases — they might label them
as primitive or as degenerated — but for us they certainly
show that in Man, as he is, there is an ahility to clinih
manifested upon exactly the same lines as the ilimbing
function of the arboreal Primates, and difTeriuL' only in

14

210

ARBOPxEAL MAX

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_•*•■• •' * ",

.**•■'.'"?: •'

^^■■li>":V-i

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11

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Fig. 79. — A Sakai Tree-climbing.

From a photograpli by CeiTuti reproduced in "The Pagan Races
of the Malay Peninsula," by W. W. Skeat and C. 0. Blagden.

ARBOREAL ACTIVITIES OF MODERN MxVX 211

that it is somewhat less perfectly performed. Mcjdorn
European man has no doubt lost his foot-grasp so thor-
oughly that when he takes to climbing ho learns a new
and human method, but his less trammelled ])r{)ther still
conducts the business upon its primitive lines, and does
it far better. Nor is the original method beyond the reach
of the European, for upon the stage acrobats and animal
impersonators periodically appear, possessing every
feature of the typical arboreal activities of the Primates.

CHAPTER XXXI

THE FAILUKES OF ARBOEEAL LIFE

There would seem to be a general law apjDlicable to
animal adaptations — a law which we might term the
law of successful minimal adaptive specialization. A
plastic stock, given unlimited scope of development in
varied environment, tends to differentiate. Different
races will specialize towards the needs of their environ-
ment. Different environments offer var^^ng possibilities
of education, expansion, and advance, but the full educa-
tional possibilities are not necessarily grasped solely, or
to the full, by the animal which becomes most completely
specialized. This is a fact made clear by a whole sequence
of geological types which have seized upon their environ-
mental opportunities, and have become specialized in
an extraordinary degree to fit their environment, only
to arrive at specific senility, and be supplanted by less
specialized and more plastic types. A complete, early,
and all-absorbing specialization is almost synonymous
with specific senility. An animal which specializes to
the limits, in response to its environment, becomes a
slave to its environment, and loses its greatest evolution-
ary asset of plasticit3\ This, in the end, spells the doom
of progress. It does not matter greatly in what venture
the all-absorbing specialization is cultivated. It may be
in response to environment; it may be in protective
mechanisms, it may be in diet. As Willey has said,
" Hardl}^ anything proclaims a finished organization, the
culmination of a phyletic career, so plainly as an exclusive
diet."

A specialization for blood-sucking, a specialization for

212

THE FAILURES OF ARBOREAL LIFE 213

eating ants, or an adaptation for any other very (ie-tinitf
and special type of food, has proved the downfall of many
a promising animal type. The Primate and iiuman stock
has not been led astray in this direction; for it has pre-
served throughout that well-balanced habit (»f dietary,
only to be termed omnivorous. To talk in the fashicm
of human successes in life, an animal may use or abuse
its life surroundings. We may say that wlu-u it uses
them rightly it undergoes the successful viinitnal adaptive
specialization, but when it abuses them it runs riot in
specializations — specializations which ultimatelv make it
the slave of its environment.

An animal which chances to come into possession of
a habitat of which one feature is the j^resence of water —
be it rivers, lakes, or oceans — in Avhich food is to'})e ob-
tained will open up a wider field for its activities, gain a
new series of educational possibilities, and j)erha])s ])Iace
itself beyond the competition of a rival by acquiring, in
some degree, an aquatic habit. To be at home both
upon the land and in the water offers a wider field under
normal circumstances, and a useful, possibly life-saving
alternative under abnormal circumstances, that is an
obvious asset to the animal. But to go much furtlur
than this in the cultivation of an aquatic habit is to court
disaster, since a purely aquatic life is one singularly
barren of educational possibilities. Limbs l)ecome re-
duced to paddles; smell, hearing, and even sight, become
restricted senses, and an animal wholly de])endent on a
thoroughly aquatic life is one debarred from real mam-
malian progress. The Sirenia. Cetacea (toothed and
toothless) and even the Pinnij^eda among the M annua Is,
are examples of types which, having ])eeome slaves of
an aquatic habit, and leading singularly restriettxl.
though highly specialized lives, have fallen behind in the
march of progress. It is notorious how long in geological
history these animals have been, as it were, tinisiicd
organisms.

214 ARBOREAL MAN

The same story could be told of every condition of
distinctive environment. Some animals have acquired
a highly useful power of burrowing in the earth for the
purpose of making safe retreats for themselves and their
young, or for obtaining food below the surface of the
earth; some animals have become highly specialized
slaves to this habit. The Insectivorous Moles (Talpidce),
the Golden Moles {Chrysochloridce), the Rodent Mole Rats
(Spalacidce), and the marsupial Notoryctes, are examples
of highly s^Decialized failures in this direction.

It is not likely that a habitat so attractive and so
universally present as the tree-tops would fail to be
abused by some members of the stocks which have taken
possession of it. It is the distinction of the human stock
— a distinction to which we have had frequent occasion
to allude — that it never became the slave of its arboreal
environment, for it became adapted to tree life in a
strictly tempered manner, and it specialized to the
successful minimum degree.

It will be best to note the particular specializations
which arboreal animals are likely to develop to such an
extent as to imperil their future evolutionary progress.
First are those special adaptations for clinging tight to
branches, securing for the animal a high degree of arboreal
safety at the initial expense of some of its activity. The
more the clinging adaptations are developed the more
hampered become the real climbing powers, and the less
the chance of producing a truly emancipated fore-limb.
All four climbing limbs become clinging limbs, the grasp-
ing hands and feet become alike mere claw-like adapta-
tions of the members to the branches, and even claws
and nails may turn into hooks.

Phascolarctus, among the Metatheria, is a mere arboreal
dinger w^ith activities greatly reduced, and its educational
possibilities almost gone. It possesses an opposable big
toe, but its hand has undergone a change reminiscent of
ventures seen in avian and reptilian orders, for the thumb

THE FAILURES OF ARBOREAL LTFL 215

and first finger are opposed to the other three digits.
The eutherian Sloths (BradypocUda) show to perfection
the fatal effects of mere arboreal clinging. Tlu-se aniinals
spend their lives for the most part among the l)ranches
of trees, to which they cling hooked uj) in an inverted
position by a reduced and highly special i/cd series of
digits. The educational possibilities that the arboreal
habit offers to a Sloth are extremely limited: even the
range of its diet becomes restricted, and an animal that
has become an arboreal dinger is an animal entering ii))oii
specific senility. With the phylogenetic history, and the
affinities of the Bradypodidoe we are not here concerned,
but perhaps they are not beyond the suspicion of having
certain Primate linkages, and it would be easy to point
the moral of the tendency to such a sloth-like condition
already manifested in Nycticebus tardifjradus. This
Lemur may easily be appealed to as an exam]»l«' nf a
tendency to arboreal clinging which may possibly be
exaggerated, and so lead astray from the line of true
Primate development. Nycticebus may also be pointed
to as showing possible tendencies to two other outcomes
of the arboreal habit which prove pitfalls of s])ecial-
ization in arboreal animals. We have noticed the
tendency shown by this animal to trust to the suspending
grasp of its feet rather than to that of its hands; it fre-
quently turns upside down, and hangs head downwards.
This is apparently a somewhat similar manifestation of
the trust to foot-grasp which has become so highly
elaborated in some New World ^Monkeys. It is a strange
feature of the South American arboreal animals that they
have assisted and perfected the foot-grasp by the di-vi-Iop-
ment of a prehensile tail. It might seem that the acquire-
ment of this new grasping organ, with all its beautiful
motor and sensory adaptations, would be a distinct
advance in the evolution of the Primates. \et it has
proved to be a specialization which turned n<uh« its
possessors from real progress.

216 ARBOREAL MAN

The South American monkeys are sometimes named
"four-handed," but some of them might, with equal
justice, be termed five-handed, so perfect is the specializa-
tion of the tail for all grasping and tactile functions.
Yet in this multiplicity of hands there is no evolutionary
gain. The true hands lose some of their perfections in
this sharing of their duties by other members, and the
animal becomes so much a perfected arboreal acrobat,
that advances in any other direction are wellnigh
impossible.

With the other specialized results of arboreal habit
it is less easy to deal. That flying Mammals have
originated from arboreal Mammals is certain. The Indo-
Malayan and Australasian faunas teem with the repre-
sentatives of several orders which, having become
thoroughly arboreal, have gained some powers of aerial
flight. The particular ai^boreal specialization which
culminated in the power of flight is difficult to determine
w^th certainty, since comparative anatomy helps but
little, and paleontology not at all. There are, for instance,
no geological evidences of the types which linked the
Bats (already fully perfected in Eocene times) with any
other mammalian order from which thev were derived.
The curious Flying Lemur Gahopithecus volans (see
Fig. 80) has been regarded ]iy many anatomists as an
existing remnant of such a link, and Cheiromeles tor-
quatus, a Bornean Bat, possesses many of the characters
of a true tree-climber (see Fig. 81). But these creatures
hardly tell us how the habit of flight was acquired as an
arboreal specialization. It is natural to assume that an
arboreal animal which has learned to leap from branch
to branch in the astonishing manner evinced bv manv of
the Lemurs should progress in its special line by launching
itself into the air and increasing the lengths in its leaps
by gliding, or planing, on an outstretched membrane
derived from some part of its anatomy. There are many
leaping aerial gliders: we may instance the marsupial

THE FAILURES OF ARBOREAL LIFE 217

Flying Phalanges (Petaurus, Acrobates, etc) and tlic-
rodent Flying Squirrels {Pteromys, Anomalurus, oU- )
which have made some progress towards flight 15ut it

Fig. 80. — Young Female Galeopithecm.
From a specimen collected by Charles Hose.

is to be doubted if the truly flying ^Mammals, sucli as the
Bats, started their career on these lines.

According to Willey, " the facts seem to show rloarly
that it is not merely the habit of taking flying leaps, h'ko

218

ARBOREAL MAN

Fig. 81. — The Naked Bat {Cheiromeles torquaius) which

SHOWS, PARTICULARLY IN THE STRUCTURE OF ITS HlXD-

LiMBS, Adaptations to a Tree-climbing Habit.

From a spirit specimen collected by Dr. Charles Hose, Sarawak,

Borneo.

THE FAILURES OF ARBOREAL LIl'i: l>1!.

monkeys, for example, that has led to the formation of
organs of flight." Certainly there is nothing in tho
anatomy of Cheiromeles or of Galeointhecns \^^ indicate?
any inheritance of a power of arboreal leaping. Assuming
that the Bats are monophylic and that Cheiromeles might
show an evolutionary phase representative of tlu* fore-
runners of all the members of the order (an assumption
T believe to be by no means justified), one miglit be
inclined to imagine that the specialization of foot -grasping
and the consequent adaptation of an inverted ])osition,
such as we have noted in Nycticebus, was an early phase
of the evolution of true mammalian fliglit. it is of
interest to remark here that more than one existing Lennir
show^s a definite development of a lateral skin fokl sueh
as constitutes, when fully developed, a flying mem))rane
or patagium. Beddard has called attention to such a
rudiment in Propithecus, and more recently Anthony
and Bortnowsky have described a pleuropatagium in
Microcebus (cheirogaleus) minor under the name of " un
appareil aerien de type particulier."

We will not probe the origin of mammalian fliglit any
further, nor turn aside to inquire if all the flying Mammals
grouped as the Cheiroptera, or Bats, have sprung at the
same tim^., and in the same manner, from the arboreal
mammalian stem; we will be content to see to what ends
this new acquisition led. At first sight, it would seem t hat
the ability to fly would be an enormous asset to a Mammal
already passed through the apprenticeship of arboreal
life. A flying animal knows no limits of hal)itat or
environment; geographical barriers, whieii limit tho
activity and spread of the stock from whieh il sprang,
offer no unsurmountable boundaries to its enterjirisej?.
Indeed, the geographical distribution of the Cheiroptera
demonstrates the reality of this advantage.

The power of flight, whilst offering an abundant ehange
of habitat, affords also an almost unlimited range of
dietary; it facilitates escape from enemies, and provides

220 ARBOREAL MAN

a ready means of avoiding local overcrowding, rivalry,
or temporary local adversity. All these things are assets
— enormous assets — in the preservation and multiplication
of the type; and the specific richness, the enormous
numbers of individuals, and wide- world distribution of
the Bats, are evidence of this. But it must be remembered
that, despite the undoubted successes of the flying
Mammals in these limited directions, there has been an
evolutionary stasis in the group extending over a very
long geological period. They have obviously gained
their freedom, and their specific plasticity at the expense
of some very vital evolutionary asset. The thing which
they have lost in taking to an aerial life is the very thing
w^hich they won in their arboreal life, the factor which
made their aerial enterprises possible — the emancipation
of the fore-limb. Their fore-limbs have become purely
specialized as " wings " ; they are no longer useful for
grasping, for touch, for examination and for all the other
functions Avhich we have seen are so essential in the final
education of the neopallium \\hich makes for real evolu-
tionary progress. No matter from \\ liat sources, and by
what routes, the whole of the flying Mammals comprised
within the limits of the order Cheiroptera were derived,
we may regard them all as animals which, having sacrificed
the very valuable freedom of the fore-limb to the powers
of flight, had flourished exceedingly as a consequence of
their enterprise, but had progressed but little in real
evolution, since the ver}- factor which enabled them to
take their momentous step had been altogether absorbed
in taking the step.

CHAPTER XXXII

THE UPRIGHT POSTURE

It will be gathered from a perusal of the iovc^om^
chapters that, in the main, I have attempted to derive
most of the peculiar features of Man, and of liis kindred,
from adaptations and advantages gained during an
arboreal apprenticeship. To this source of derivation of
these adaptations|I can see no real alternative: ])nt it
must be pointed out that most of the physical di-tails
to which I have called attention are generally explaiiuxl
as being outcomes of the " attainment of the erwt
posture." The problem of making Man has, indeed,
commonly been regarded as " the turning of an ordinary
quadruped a quarter of a circle into the vertical plane "
(Robert Munro). There is here evinced that unnatural
and thoroughly mechanical picture of " the far-reaching
effects on the organism of this slov.'[and painful acquisition
of a radically new posture " at which D wight and some
few others, have scoffed, but which underlies so tena-
ciously much modern anthropological teaching. The
erect position of Man is obvious, but I heartily agree w ith
D wight when he says that " as an explanation it has been
terribly overworked. "^Walking upright upon the surface
of the earth has produced its changes in the human hody.
of this there is no doubt; but we must be careful to dis-
tinguish between these ''finishing touches" and those
other changes which are so much older and so nuich
more important — the adaptations to arboreal life.

We may not say at what point in the evolutionary
story the rising member of this stock became what all

221

222 ARBOREAL MAX

would agree to name as a human being. "We have now a
complexity of species, and even genera, of " human "
remains, and vet. as is indeed inevitable, we have no
criterion by which all will agree to judge such remains
as belonging to an evolutionary stage universall}^ recog-
nized as being " human." Some, it is true, have boldly
taken this question in hand — or, rather, have made
assertions as to when the change took place, and when
the ancestor of ^lan became definitely human.

Munro has stated his conviction clearl}-, and, for him,
Homo sapiens came into being with the '' attainment of
the erect position," and the consccpient possession of its
accompanying benefits. But the statement of his case
needs examination: " With the attainment of the erect
position and the consccpient specialization of his liml)s
into hands and feet, Man entered on a new phase of
existence. With the advantage of manipulative organs
and progressive brain lie became Homo sapiens.''^

If it be these things \\ liich determine Homo sajnens as
a species, then Homo sapiens need not be limited to Man
the upright, for all these things are efi"ects of an arboreal
life, and we know not to what lengths they had carried
evolution while the animal was still arboreal. Even if
we are to limit our ideas of *' Man " to an animal which
walks upright upon its two feet, we must not fall into the
very usual error of ascribing to this ujiright posture all
those changes and benefits accumuhited among the
branches. Different anatomists have assigned varying
importance to the upright posture, and its accompanying
blessings. Among the earlier of them it was customary
to see something very distinctive — typically human, if
not partially divine — in this posture. '* In the external
conformation of man we immediately remark his upright
stature; that majestic attitude which announces his
superiority over all the other inhabitants of the globe."
This is the statement of William Lawrence, a man who
in 1820 was regarded by the authorities of St. Bartho-

THE UPRIGHT POSTURE 223

lemew's as a very dangerous and unorthodox thinker
and teacher concerning the zoological status of Man.
It is certainly not so enthusiastically eulogistic as are the
statements of almost all who went before, and many
who came after him. In 1862 John Goodsir chose as
the subject for his summer session lectures, " The Dignity
of the Human Body." It is easy to picture the circum-
stances under which these lectures were given to the
students of Edinburgh University; it is easy to understand
the enthusiasm which Goodsir put into their composition ;
but it is extremely difficult to realize how the fascination
of such a subject could lead so competent an anatomist
to pen some of the extraordinary nonsense contained in
these lectures. It would be easy to furnish a long list
of quotations from the works of modern anthropologists
to show the enormous importance commonly assigned to
this matter of standing and walking upright. It would
be equally easy to show that, in most cases, the changes
which they are picturing as being produced by it are in
reality due to the much older climbing activities of the
animal. It is far more difficult to find any written word
of dissent from such views.

Nevertheless Dwight made his position clear when he
wrote: "The upright position is certainly one of the
great human characteristics, but I am not carried away
by the enthusiasm with which some authors dilate on it."
Elliot Smith alludes to " the common fallacy of suj^posing
that the erect attitude is Man's distinctive prerogative,
and of regarding the assumption of that position and
mode of progression as the determining factor in the
evolution of Man." Klaatsch has asserted, with more
directness, that " Man and his ancestors were never
quadrupeds as the dog, or the elephant, or the horse."

With this plain statement it is quite impossible to dis-
agree, when one studies the condition of the bones and
muscles of the human fore-limb. Right from that tiawn
period in which the Therapsida of the Triassic gave birth

224 ARBOREAL MAN

to the ancestors of the Mammals, the fore-limb of the
mammalian stock from which Man sprang has been spared
from the servile function of merely supporting the body
weight in quadrupedal progression. " Man and his
ancestors were never quadrupeds;" there has never been
*' a slow and painful acquisition of a radically new
position." Until Man walked upon the earth in " that
majestic attitude which announces his superiority over
all the other inhabitants of the globe," he and his fore-
bears climbed and walked about the branches of the trees.

No " ordinary quadruped " was turned through " a
quarter of a circle into the vertical plane." But some
extremely primitive ^lammal climbed a tree, lived and
evolved among its branches, and after long ages walked
to earth again as that Primate destined to be the dominant
member of the animal kingdom. That the upright habit
is of the very first importance as an evolutionary factor
and as a human possession must be freely admitted.
But that this upright habit is the distinct prerogative
of Man is a proposition not to be entertained for a moment.

That there is an alternative to the all too common
idea that a four-footed pronograde ]\Iammal nuist have
become upright in process of the making of mankind is,
I think, obvious. And that this alternative is the gradual
readjustment incidental to an arboreal life, I conceive
to be certain. The human child sits up before it stands;
the human stock sat up before it stood.

BIBLIOGRAPHY

Bell, Sir Charles.

The Hand, its Mechanism and Vital Endowments aj evincing
Design. Bridgwater Treatise No. IV. London, 1833.

Blagden, C. 0.

See Skeat.

Bolton, Joseph Shaw.

The Functions of the Frontal Lobes. Brain, 1903, p. 215.

Bonney, Victor.
See Taylor.

Darwin, Charles.

The Descent of Man. First edition. London, 1871.

Duckworth, W. L. H.

Morphology and Anthropology. Cambridge, 1904, cavA New
Edition of vol. i., 1915.

Dwight, Thomas.

Thoughts of a Catholic Anatomist. London, 1911.

Elliot Smith, G.

The Origin of Mammals. Discussion, Section D. Brltisli Associa-
tion, 1911.

Presidential Address. Section H. British Association, IDl'i.

Arris and Gale Lectures on "The Ev^olution of the Brain." Lancet,
1910, p. 153.

EUis, Thomas S.

The Human Foot, its Form and Structure, Functions and Clothing.
London, 1889.

Fortes, Henry 0.

A Handbook of the Primates. Two vols. London, ISDli.

Gadow, Hans.

Observations in Comparative Myology. Jonrn. Anat. end Ihys.,
vol. xvi., p. 493.

Goodsir, John.

Anatomical Memoirs. Edited by William Turner. 18GS.

Haitman, Robert.

Anthropoid Apes. Second CLlition. London, 1S*^9.

Hickson, Sydney J.

A Naturalist in North Celabes. London, 1889.

225 I'i

226 ARBOREAL MAN

Humphry, Sir G. M.

Observations in Myology. London, 1872.

The Human Foot and the Human Hand. London, 186L

See also Journ. Anat. andPlnjs., vol. iii., p. 320, and vol. vi., p. 1.

Huntington, G. S.

Numerous Studies in Myology. See especially American Journal
of Anatomy, vol. ii., No. 2, p. 157.

Huxley, Thomas Henry.

Evidence as to Man s Place in Nature. Third thousand. London,
1864.

Keith, Arthur.

Introduction to the Study of Anthropoid Apes. London, 1897.
On the Chimpanzees and their Relationship to the C4orilla. Proc.

Zool. Soc, 1899, p. 296.
Hunterian Lectures on " Certain Phases in the Evolution of Man."

Brit. Med. Journ., 1912, pp. 734 and 788.

Kidd, Dudley.

Savage Childhood, a Study of Kafir Children. London, 1906.

Kloster, Rudolph.

On the M. Pronator Radii Teres of the Mammals. Anatomische
He ftp, 1901, p. 671.

Lawrence, Sir "William.

Lectures on the Comparative Anatomj', Physiology, Zoology, and
the Natural History of Man. Ninth edition. London, 1844.

Macalister, Alex.

On the Arrangement of the Pronator Muscles in the Limbs of
Vertebrate Animals. Journ. Anat. andPhys., vol. iii., p. 335.

Munro, Robert.

Presidential Address. Anthropological Section, British Associa-
tion, 1893.

Parsons, F. G.

Numerous Studies in Myology. See especially Journ. Anat. and
Phf/s.. vol. xxxii., p. 428, etc.

Sclater, W. L. and P. L.

The Geography of Mammals. London, 1899.

Skeat, W. W., and Blagden, C. 0.

Pagan Races of the ^Nlalay Peninsula. London, 1906.

Taylor, Gordon, and Bonney, Victor.

Homology and Morphology of the Popliteus Muscle. Journ.
Anat. andPhys., vol. xl., p. 34.

Topinard, Paul.

Anthroi)ology. English edition, 1890.

Wallace, A. R.

The Malay Archipelago.

Willey, Arthm-.

Convergence in Evolution. London, 1911.

INDEX

Accessory muscles of respiration,

135
Adaptation to environment, 3
Aerial life, 216
American monkeys, 69, 215
Amphibia, 16, 20
Anchoring nipples, 144
Anous stolidus, nesting of, 201
Anthony, Professor Raoul, 219
Anticlinal vertebra, 103
Aquatic animals, 56, 213
Arciomys, 85
Archepallium, 151
Ariens Kappers, Dr., 163
Association areas, 191
Association of senses, 178
Auditory impressions, 174

B

Baboon, skull of, 116

pelvis of, 125
Balancing muscles, 65
Bambute Pvgmies, limbs of, 202
Bats, hind-limb of, 55

pelvis of, 127

mammary glands of, 143

as failures, 217
Beddard on lemurs, 219
Bell, Sir Charles, on the hand, 43

on respiration, 134
Big toe, grasping power of, 72
Binocular vision, 176
Birds, feet of, 67
Boots, wearing out, 205
Brady podidce, 215
Brain, evolution of, 194

importance in evolution, 200
Bridgwater treatise, 43
Broom on limbs, 11
Burrowing animals, 214

Canine teeth, 90
Carotid arteries, 96
Carpus, 23
Cat washing face, 19
Centetes, litter of, 139

mammary glands of, 141

Centre of motion, 104
Ceiropithecu.i, verU'hr.il cohinin of,

120
Cerebral hemisphere^, 150
Chameleon, feet of, in
Che ircHjalt a s, 138
Cheiromdes, 216
Cheiropods, 197
Chelonians, 2(», 40
Chest, shape of, 131
Chimpanzee, 37, 112, 126, 187, 202
Chin a human feature, 95
Chiromys, 154
Chrysochloris, 59
Clavicle, 28
Climbing, dawn f)f, 16
Coconuts, method of gathering,

208
Conduct, evolution of. ISl

conception of, 193
Conscience, 194
Convergent mimics, 197
Corpus callosum, 198

striatum, 171
Cortex, development of, 15(»

method of building of, 1'''4
Croi'idura, 39

Crocodile, vertebral column of, 102
Cryptohranchus, 33, 58
Curves of spine, 119
Cynocephahis, 1 16
Cynodontia, 149

D

Darwin, 1, 3, 90

Decapitation of bird, 169

Dental caries, 93

Dentine, 94

Descartes, 194

Descending trees, methods of, 50

De Vries, 3

Dia})hragni, 133

Diet, s]HHiali7.ation in, 212

Digital formula, 75

Dipnoi, brain of, 1.50

Dog, 19, 167, 175

Duckworth, Dr. W. L- "•• '^l- •"

202 ^

Dwifht, Professor Thomft'', 2. 1^0,

221

227

228

ARBOREAL MAN

E

Edentates, 108,215

Egyptians, teeth of, 92

Elephant, 106, 167

Elliot l^mith, Professor G., 149, 152,

223
Emancipation of fore-limb, 7, 17
Erinaceus, 59
Eversion of foot, 205
External respirator^' system, 134
Extinct animals, skeletons of, 11
Eyes, position of, 175

Ealloppian tubes, 139
Eamily, formation of, 188
Flower, Professor, 76
Flying mammals, do, 216
Foetus, limbs of, 203
Food, ])reparation of, 91
Foot, eversion of, 63
Fore-limb, bones of, 23

muscles of, 32
Frontal cortex, 191

G

Gait of animals, 15

GiUago, 111

(ialeopithenis, 145, 21<>

(Javial. vertebral column of, 102

( iegcnbaur on caqms, 23

Gibbons, cbmbing of, 112

vertebral cf)lumn of, 121
pelvis of, 125
young of, 186
Giraffe, gait of, 15

vertebral column of, 106
Golden nioU's, 59
Goodsir, John, 223
Gorilla, 38

foot of, 74
pelvis of, 126
young of, 186
limbs of, 202
Grasp, development of, 19, 22
Greek ideal foot, 77
Grip of baby. 205
Growth of cortex, 178
Gruber on anomalies, 23, 59
Cyf/jf Candida, young of, 201

H

Hand, primitive condition of, 20
Hand-feeding, 84
Head, poise of, 114
Hearing, impressions of, 174
Hedgehog, 59

Hemijilegia, 170
Hepburn, Professor, 37
Hickson, Professor 8., 187
Hind-limb, changes in, 53
Hip-joint, 63
Homo sapiens, 222
Hopping animals, 15, 19, 120
Horse, 14, 45, 167
Hose, Charles, 148
Humphry, J. M., 33
Hunter, John, 3
Huxley, T. H., 4, 45, 180

Ideals of conduct, 190
Infancy, duration of, 186
Inguinal nipples, 143
Insanity, 195

Intellect, cortical site of, 194
Intelligence, evolution of, 180
Intermembral index, 202
Internal respiratory system, 134
Interossei muscles, 80
Ischio-pubic symphysis, 123

Jerboa, 84
Joint, hi]), 63

sacro-iliac, 125
Jugular veins, 96

Kafir children, 156

Keith. Professor Arthur, 4. 38,

12(.
Kidd, Dudley, 156
Killing prey, method of, 86
Kinesthetic area, 168
Kinnier Wil-on, 169
Klaatsch on upright posture, 223
Klostcr, Rud.,38'

I^marck, 3

I^wrcnce. Sir William, 222

Lemurs, 125, 138,219

Limbs, origin of, 7

Little toe, 79

Lower neurone lesion, 170

M

Malays, climbing methods of, 20S
Mammary line, 141
, Marmosets, 69
Marsupials, n?st -building, 139

arboreal, 196
Maternal care, 184
MeKenzie, Lr. W. C, 39

INDEX

229

Memory, development of, 151
Mercier, Charles, 195
Jletatheria, arboreal, 19G
Middle line digit, 80
Milk teeth, decay of, 93
Missing links, feet of, 73
Molar teeth, 92
Monkey communities, 188
Monotremes, 39
Motor area, 107

impressions, 162
Munro, Dr., 44, 221
Mus margarettcB, 69
Mutation, 3
Myology, comparative, 32

N

Nasalis larvatus, 98
Neopallium, 153
Nest-building, 138
Neurobiotaxis, 163
Novel objects, testing of, 159
Nursery, importance of, 183
Nursing habits, 143

of monkeys, 148
Nycticehus tardigradus, 50, 86, 111,
159, 185, 215

O

Okapia, vertebral column, 106
Opposable digits, 67
Orang-utan, 37

climbing of, 112

pelvis of, 125

young of, 186

limbs of, 202
Orbit, development of, 99
Os centrale, 26
Overcrowded jaws, 88
Owen, Sir R., on scent glands, 154

Pairing, duration of, 186
Parallel developments, 197
Paralysis, types of, 170
Pelvis, primitive type of, 123

different forms of, 125
Phalanges, 24
Phascolarctus, 68, 214
Pictured movements, 168, 192
Pigeon, cerebrum of, 169
Pinna, movements of, 100
Prefrontal area, 191
Prehensile tails, 69, 215
Proboscis monkey, 98
Prognathism, 95
Pronator muscles, 33

Protothtria, 56, .j<», UKi
Pubic symphysis, 123
Pupj)y, experiment on, 16'J
Pygmies, linibn of, 202

Q

Quad rum ana, 49

R

Kabbit, exj)eriment on, 169

Radio-ulnaris muscle, 41

Reason, evolutinn of, l8l

Reflex action, 172

Reil, Island of, 178

Reptiles, vertebral column of, I02

brain of, 151
Respiration, methods of, 133
Rhinolophid bats, nipples of, 144
BJmiopithecus, 98
Robinson, Dr. Louis, 205
Rodents, nests of, 139
Rotation of tibia and fibula, 59

of hip-joint, 63
Rosenberg, 26

8

Sacro-iliac joint, 123
Sakai, toes of, 72, SO

methods of climbing, 209
Scent glands, 154
Scmangs, 209
Shrew, pygmy, 39

senses of, 154
Sight, impressions of, 174
Silent area, 191
Sloths, 56, 215
Slow lemurs, 14
Smell, importance of, 153
Snub-nosed monkey, 9S
Social habits of monkeys, 188
Specialization, results of, 212
Speech, acquirement of, 5
Stability of limbs, 10
Sternidce, nesting habits of, 201
Successful rdaptation, 212
Suspension-power of baby, 20.)
Symphysis pubis, 12«»

Tapir, 167

Tarsias spectrum, 54, 57

foot of, 70

face of, 97

vertebral column of, 11 1

nursii\c of, 14S

eyes of, I0(i, 176
Taylor, Gordon, 39, 4J
Teeth, functions of, 87

230

ARBOREAL MAN

Terns, nesting habits of, 201

Terrestrial life, limbs adapted to, 13

Testing by hand, 158

Therapsida, 11

Thigh, extension of, 63

Topinard, Paul, 104

Tortoise, carpus of, 27

Touch, sense of, 157

Trapezius muscle, 170

Trigeminal nerve, 157

Troups of monkeys, 188

Twpaia, 38, 56, 84, 88

face of, 97

vertebral column of, 109

nest-building, 139

U

Umbilicus, site of, 204
Ungulates, offspring of, 183

Upper neurone lesion, 170

Upright posture, 0

Uterus, different types of, 139

Varanus, 58

Variations, 3

Vertebral column, curves of, 119

Viscera, disposition of, 129

Visceral outlets, 128

Visual impressions, 174

W

Walking, nervous mechanism of, 172
Water newt, 9
Wearing of teeth, 94
White teni, nesting of, 201
Willey on diet, 212
Wisdom teeth, 92

BILLING AND SONS, LTD., PRINTERS, GUILDFORD, ENGLAND

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