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 Arboreal man. 
 
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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) 
 
 NEW YORK 
 
 LONGMANS, GREEN AND CO. 
 
 LONDON : EDWARD ARNOLD 
 
 1916 
 
To 
 MY WIFE 
 
 ho, she ©aL dJmeJ °vt W)N.[ 
 
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 Evolu- 
 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 PAGE 
 
 I. THE PROBLEM OF MAN'S ORIGIN - - - 1 
 II. THE EMANCIPATION OF THE FORE-LIMBS - -7 
 
 III. THE DEVELOPMENT OF THE POWER OF GRASP - 19 
 
 IV. THE SKELETON OF THE FORE-LIMB - - 23 
 V. THE CLAVICLE - - - - - 28 
 
 VI. THE MUSCLES OF THE FORE-LIMB - - - 32 
 
 VII. THE FORE-LIMB: SUMMARY - - - 43 
 
 VIII. THE FATE OF THE HIND-LIMBS - - - 48 
 
 IX. THE SKELETON OF THE HIND-LIMB - - 53 
 
 X. THE MUSCLES OF THE HIND-LIMB - - - 58 
 
 XI. OTHER ARBOREAL ADAPTATIONS OF THE HIND-LIMB 62 
 
 XII. THUMBS AND BIG TOES - - - 67 
 
 XIII. THE HUMAN FOOT- - - - - 73 
 
 XIV. THE RECESSION OF THE SNOUT REGION - - 84 
 XV. THE RECESSION OF THE JAWS AND REDUCTION OF 
 
 THE TOOTH SERD3S - - - - 90 
 
 XVI. THE FACE AND THE CRANIUM - - - 96 
 
 XVII. THE SPINOUS PROCESSES OF THE VERTEBRAL COLUMN 101 
 XVIII. THE POISE OF THE HEAD AND THE CURVES OF THE 
 
 SPINE - - - - - -114 
 
 XIX. THE PELVIS AND THE VISCERA - - - 123 
 
 XX. THE RESPIRATORY SYSTEM - 131 
 
 XXI. THE REPRODUCTIVE SYSTEM - 138 
 
 XXII. THE DEVELOPMENT OF THE BRAIN - - 149 
 
 XXIII. THE STORY OF TACTILE IMPRESSIONS - - 157 
 
 XXIV. MOTOR IMPRESSIONS - - - - 162 
 XXV. IMPRESSIONS OF SIGHT AND HEARING - - 174 
 
 XXVI. HIGHER DEVELOPMENTS OF CEREBRAL FUNCTIONS- 180 
 
 ix 
 
x CONTENTS 
 
 CHAPTER PAGE 
 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 ----- 225 
 
 INDEX ------ 227 
 
ARBOREAL MAN 
 
 CHAPTER I 
 
 THE PROBLEM OP MAN'S ORIGIN 
 
 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, Darwin 
 was by no means standing as the pioneer of evolution; 
 but was only the thoughtful student who was 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 
 
2 ARBOREAL MAN 
 
 the opening period of hasty and unreasoning partisanship 
 was passed, and after the first skirmishes 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 difficulty 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 would 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 completely 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 by 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. "Adaptation," "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 types, 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 
 about in some way — that is obvious ; by what channel or 
 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 
 such 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; 
 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, 
 or 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 
 
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Missing Page 
 
CHAPTER II 
 
 THE EMANCIPATION OF THE FORE-LIMBS 
 
 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 appear 
 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 
 
 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 
 
EMANCIPATION OF THE FORE-LIMBS 9 
 
 on land. By its mobility it produces movement, lut it 
 does not support the weight of the animal's body. It is a 
 propelling, but not a supporting, limb (see Fig. 4). A 
 
 Fig. 2. — Human Fokearm 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 of 
 Vertebrate with Propelling, but not Supporting, 
 Limbs. 
 
 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 
 
10 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 pronounce- 
 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 
 
Missing Page 
 
Missing Page 
 
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 
 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 defined, 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 body, and of 
 urging it forward. In this way a more and more perfect 
 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 support. Hopping is 
 a specialized development of the pronograde gait; and 
 it has led to some very interesting developments which 
 
16 ARBOREAL MAN 
 
 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 water-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 by 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 
 
EMANCIPATION 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 still 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 yet 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 two such cases. 
 We have hurried almost breathlessly over the process 
 we have pictured, in a mental anxiety to arrive at emanci- 
 
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 very complete, but it is 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; but 
 they have taken to it after varied periods of quadrupedal 
 life. They have taken to it too late to derive the full 
 benefits from it, for they took to it with the fore-limbs 
 already deprived of some of their inherited mobility. 
 Such animals never become perfect 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 POWER 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 the 
 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 may 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 we mean a very definite thing, 
 and one essential in the make-up of this hand is the 
 possession of five separate, and fairly ecpially developed, 
 digits. We have made use of the water-newt to picture 
 some stages of fore-limb development, but we 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 
 individual bones which compose the separate fingers, the 
 Chelonians (Tortoises and Turtles) are the match of 
 Man and his nearest mammalian neighbours. There is 
 evidently something extraordinarily primitive about the 
 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 
 which can move it upon the ulna and radius at the wrist- 
 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 perfect 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 fingers are closed over smaller branches, and the 
 animal begins to grasp. Although the picture is entirely 
 fanciful, we may imagine 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 acquired power 
 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 developing perfection we may imagine the grasp used 
 for other purposes, and some of these purposes we will 
 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 hand-feeder. 
 
 It may take to grasping other objects which come in 
 its way. These objects may 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 PORE-LIMB 
 
 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 Vertebrate 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 type, with the added tendency 
 to a general lengthening, to which Broom has attached 
 so much importance. In 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 place 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 what is the primitive condition of the 
 carpus, and how this primitive condition has been de- 
 parted from in different types, open up the possibilities 
 for widely divergent views. We will here adopt 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 (radial?), an intermediate 
 bone (intermedium), and a post-axial bone (ulnars). 
 The second row is composed of five small and fairly 
 uniform 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 five terminal elements 
 has become the rule. This change is undoubtedly 
 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 archsetype consistent with that stage 
 of evolution represented 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 (ClteJydea ser- 
 pentina). (After Gegex- 
 baur. ) 
 
 Pig. 7. — The Carpus as it 
 exists ix 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 their primitive condition. 
 The first row of the carpal bones articulates with 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), ca.rpale 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 carpale 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 
 Anthropoids (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. Without 
 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 singularly 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 Gikdle, to show tiie]Strut 
 Action of the Clavicle. 
 
 of the Amphibians and Reptiles. 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 systematic cause underlies the 
 degree and condition of their presence. The dermal 
 strut occurs again in the lowest Mammals, and here 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 Insectivora, 
 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 only 
 as a rudiment, though in some 
 members of this order it attains 
 a fair degree of development; 
 in the Rodentia 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 Mammals appears to be 
 found in the functional demands 
 made upon the movements 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 developed, or attains 
 no greater perfection than that of a mere isolated inter- 
 
 Fig. 10. — The Shoulder 
 Girdle or an Ungu- 
 late, to SHOW THE 
 Absence or the Clav- 
 icle. 
 
 The fore-limb has no strut 
 to keep it poised at the 
 side of the thorax. 
 
THE CLAVICLE 3L 
 
 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 any 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 FORE-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 present thesis, that the human 
 stock has never been quadrupedal, never possessed four 
 ecmally 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 perfectly obvious 
 that they do. Far from seeing any signs that the deriva- 
 
 32 
 
THE MUSCLES OF THE FORE-LIMB 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 quadrupedal 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 Amphibian consist of a super- 
 ficial part, which arises from the ulnar condyle 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 
 
 AKBOREAL 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 wrist, where it becomes 
 continuous with another set of fibres, which, arising from 
 the ulna, passes to the radial side of the wrist and hand. 
 
 P.R.T.(1) 
 
 s.\-.-—M 
 
 *--- -P.R.T.d) 
 
 P.R.T.(2) 
 
 S.B.-f-frj 
 
 JJL-- P.R.T.C2) 
 
 P.Q. 
 
 
 
 P.M. 
 
 
 ■—P.Q. 
 
 Fig. 11. 
 
 Fig. 12. 
 
 Pig. 11. — Diagrammatic Figure of the Pronator Group or 
 Muscles in GryptobrancJius. 
 
 P.R.T. (1), Mammalian superficial head of pronator^radii teres. 
 P.R.T. (2), Deep Mammalian head of same muscle. P.Q., 
 Pronator quadratus. P.M., Pronator 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 
 brevis. 
 
 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 intermedius, 
 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 
 Cryptdbranchus 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 downwards 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 the 
 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 
 
 P.R.T, 
 
 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 the 
 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. 
 
 — P.R.T.d) 
 -P.R.T.(2) 
 
 P.Q- 
 
 Fig. 15. 
 
 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-LIMB 39 
 
 of the elbow-joint (Figs. 16 and 17). In an Oriental 
 Pyg m Y Shrew (Crocidura bottigi), again, the muscle is in 
 
 Fig. 17. — The Pronator Quadratus and Insertion of 
 Pronator Kadii 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. Anal, and Phys., vol. 40, p. 34) — which 
 definitely asserted the presence of the ulnar head in Ornithorhyn- 
 chus and in Echidna. Since that time I 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, the 
 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, Cryptobranchus shows the same thing in 
 the ill-differentiated form we have noted previously. 
 
 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 
 opposed 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. 
 
 This condition is typical of all the Primates. It is 
 found in some of the more mobile-limbed members of 
 other Eutherian orders. It is present in Tupaia and in 
 Crocidura, in typically human guise; and it is present 
 again in generalized Reptiles and in Amphibia. Its con- 
 dition in Cryptobranchus 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 with 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 
 Bonney, 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 when 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 morphological 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 Insectivora, 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 FORE-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 
 surely 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 which 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 insatiable 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 human 
 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 perfect mechanical organ 
 Nature has yet produced." Such a statement on the 
 part of an anatomist can only 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 purposes than such a thing as a hoof or a paw, 
 its apparent complexity and perfection of movement, 
 was 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 book 
 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 
 every other feature of the human body, 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 notion 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 
 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 suffer 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 
 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 
 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: SUMMARY 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 Monkeys, 
 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 HIND-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 place 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 neAv hold, the hind-limb is temporarily 
 supporting the body during the act. It must be noted, 
 however, 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 quadrupedal. In 
 
 48 
 
THE FATE OF THE HIND-LIMBS 49 
 
 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. 
 
 4 
 
50 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. Nycticebus 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. Nycticebus 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 limbs 
 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 downwards. 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 first; 
 Nycticebus 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 Nycticebus, 
 because its actions are so deliberately leisured and orderly, 
 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 wherever 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. 
 
CHAPTER IX 
 
 THE SKELETON OF THE HIND-LIMB 
 
 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. This 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 from the condition seen in the 
 arboreal 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 which ultimately become 
 so characteristic of Man are already at work in the 
 Gibbons, 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-jointobviouslycorrespondstothe shoulder. Thefemur 
 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 centrale 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 OP THE HIND-LIMB 55 
 
 mobility. Environmental conditions could not combine 
 to free the hind-limb of its duty of supporting the body 
 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 first sight, 
 
 Fig. 19. — Diagrammatic Comparison of the Skeletal 
 Elements op (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, 
 we 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 stability. 
 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- 
 paiadte) the fibula is well developed and entirely sep- 
 arated from the tibia, whereas in many terrestrial Insec- 
 tivora it has become reduced and fused to its neighbour. 
 In the Primate stock good development, complete 
 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 
 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 early, 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 
 
THE SKELETON OF THE HIND-LIMB 57 
 
 fossil representatives. The tibiale and intermedium are 
 fused to form the astragalus, while the nbulare remains 
 large and distinct as the calcaneum, thus reducing the 
 first row to two bones. The centrale remains as the 
 large scaphoid; tarsalia I., II., 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 exist ing Reptiles), and as such 
 it is typical of the Primates (see Fig. 20). It is also seen 
 in the Insectivora and in some other orders, but it is 
 
 Fig. 20. — Skeleton of the Human Foot, to show the Fate 
 of the Primitive Elements of the Taksus. 
 
 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 OP 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 Cryptobranchus japonicits 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 tibia?. 
 
 Superficial to this is a longer muscle, which, arising 
 from the outer condjde 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 tibia? is concerned, and this muscle is 
 found as (but perhaps only as part of) the M. poplitcus 
 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 (Erinaceus), 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 with 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, the 
 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 01 
 
 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 dissimilar 
 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 simple 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 may imagine they have 
 been long established in the ancestry of Man. 
 
CHAPTER XI 
 
 OTHER ARBOREAL 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 with 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 
 bod}^ 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 63 
 
 in a temporary position of uprightness upon a flexed 
 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) and 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 possibility, 
 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 habitual. 
 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- 
 
64 
 
 ARBOREAL 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. 
 
 Fig. 22. — The Human Hip-Joint fkom 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 65 
 
 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 kg 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 eversion of the foot in Man. 
 When the arboreal hind-limb has been 
 perfected as a supporting organ in an 
 extended position, it is still a purely 
 
 ll 
 
 Pig. 23. — Diagram 
 of a Gibbon (Hy- 
 lobates lay) sus- 
 pended BT THE 
 
 Grasp op its 
 Hands. 
 
 From a drawing 
 made from a pho- 
 tograph by Prof. 
 Arthur Keith. 
 5 
 
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 development, 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 pure out- 
 comes of the ordinary evolution of the arboreal habit. 
 
CHAPTER XII 
 
 THUMBS AND BIG TOES 
 
 So far, we have, in considering the question of the develop- 
 ment of the grasp, dealt only with the power 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 especially 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, upon 
 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 well 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 " (D wi ght ) . 
 
 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 69 
 
 which the two inner digits tend to be separated from, and 
 opposed to, the outer three. Certain arboreal Rodents 
 have developed very perfectly 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 probably 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 
 Cebidm the condition varies. The Howling Monkeys 
 (Mycetes) possess well-developed thumbs, 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 (Ateles) 
 
•70 
 
 ARBOREAL MAN 
 
 the thumb is rudimentary or absent, while the Woolly 
 Monkeys (Lagothrix) possess a thumb. The Capuchins pf 
 the typical genus Cebus possess a well-developed thumb 
 and a tail, which, though partially prehensile, is not so 
 specialized as that of Aides or Lagothrix. 
 
 Fig. 24. 
 
 Fig. 25. 
 
 Fig. 24. — Plantar Surface of the Eight Foot of Tarsius 
 Spectrum, showing the Peculiar Development of the 
 Big Toe. 
 
 Fig 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 
 
 71 
 
 monkeys of the genus Semnopithecus, which includes 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 opiDOsable 
 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 
 
 Maeacua N ernes- 
 trinus as seen 
 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, almost shockingly, primi- 
 tive survival, has received enormous praise mistakenly 
 lavished by the philosopher and the anatomist; but 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 the 
 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 lay 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 Eoawthropus. 
 
 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 Geoffroy 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 with the difficulty which 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 third segment of the hind limb; there is a greater 
 
THE HUMAN 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 op Cercopithecus 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 ABBOREAL 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-day 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 
 
 ( 
 
 y\ 
 
 p\V 
 
 \ 
 
 P\\ \\ 
 
 
 p\v vck 
 
 
 
 \ 
 
 
 
 
 i 
 
 Fig. 29. — Type of Foot in which, the Big Toe is considerably 
 Longer than any of the other Toes, which diminish 
 in Keghxar 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 
 
 AEBOREAL 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 op 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 distinctly 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 
 links. 
 
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 was 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 uncommonly 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 atrophic 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 Nubia 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 fourth, 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 either 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. Much 
 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 
 solely 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- 
 it self. 
 
 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 this 
 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 
 evolution 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 
 (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 ars 
 developed in different species, we will study the completed 
 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 
 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 
 harmoniously. 
 
 A typical Primate obtains its food with its hand instead 
 of adopting the common mammalian method of taking 
 it with 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 
 the 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 
 between 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 87 
 
 kill it with its teeth. I have seen Nycticebus tear grass- 
 hoppers to pieces with its hands, but birds it always 
 killed by biting. The monkeys were adepts at catching 
 birds, and although chained they had no difficulty 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. Generally 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 mobile 
 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 pre- 
 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 suppose 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 and 
 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 yet 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 will 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 RECESSION OF THE JAWS AND REDUCTION 
 OF THE TOOTH SERIES 
 
 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 problem, 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 acquired 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 RECESSION 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 familiar 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 replaced 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 when 
 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 surely, 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 writings 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 Egypt 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 
 they 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 93 
 
 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 thrown 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 the dawn 
 of the Christian era in Nubia. Among the hundreds of 
 cases examined, not one case of caries of the milk denti- 
 tion was 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 MAN 
 
 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 being 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 grinding 
 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 flat, 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 very 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 
 
 o 
 
 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 
 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 evolution; and yet the recession 
 of the modern jaw is not to be denied. 
 
CHAPTER XVI 
 
 THE PACE 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 series ; but 
 its influences are felt in many 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 parts 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 by 
 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 Cavitt. 
 
 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 primitive arboreal Insec- 
 tivore, such as Tupaia, has a relatively small head and a 
 long snouty face (see Eig. 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. 34). 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 prolonged 
 snout region. In the Proboscis Monkey (Nasalis larva- 
 ins) the nose reaches a remarkable degree of prominence, 
 but there is no involvement of the maxilla or mandible 
 in 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 Cavitt. 
 
 but to a lesser degree, in the curious Snub-Nosed Monkey 
 {Rhino pith ecus roxellcmce). 
 
 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 CRANIUM 99 
 
 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 space 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 
 Torsius spectrum, 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 
 eyeball 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 we will allude more fully later — the alteration 
 of the head poise. 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 forwards, as in the 
 human method of saying " Yes." This movement takes 
 place between 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 Mammals. 
 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, between 
 
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 
 which 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 column; 
 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 pro- 
 cesses stand up in line all down the middle of the back, 
 and to them are attached portions of the great muscle 
 (M. erector spinas) which acts upon the vertebral column. 
 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 upwards, or they are 
 directed slightly backwards at their free tips. There is 
 a very primitive plan, seen in many Reptiles, both 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 possessing 
 an upright spinous process. All the vertebras 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 upright 
 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 J03 
 
 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 usually 
 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 
 
 ~4**ttt 
 
 Mi** 
 
 Fig. 35. — Diagram op 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 Omithordhynchus, there is a change, 
 for though all the cervical and dorsal spines slope acutely 
 backwards, those of the three lumbar vertebrae 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 ARBOEEAL MAN 
 
 underlying cause of the variations. Some fifty years 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, necessary 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 vertebra? 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. 3G). These 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 
 
 105 
 
 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 
 
 ►>W>>* 
 
 Fig. 36. — Dragrah of the Vertebral Column of an Animal 
 in which the spinous processes are anteverted and 
 
 EeTROVERTED 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 7 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, we may 
 fair]y 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 ante- 
 version and retroversion of the spinous processes. Nearly 
 all Rodents and Insectivores possess this feature. Of 
 the Carnivora, the cats which spring, and the dogs which 
 leap and gallop, 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 L'ngu- 
 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 107 
 
 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 been 
 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 retroverted 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 retroverted 
 (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 podidce), 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 hare taken to the trees and become lumbering 
 tree-clingers, since that was the limit of their arboreal 
 possibilities 1 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 by 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 the 
 struggle of evolution. 
 
 If this be the true history of the arboreal Brady podidce, 
 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 enterprise. 
 Taken as a whole, the Primates show a distinctly 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 monkeys. 
 
 The same conditions are present in most members of 
 the Insectivora. Among the Menotyphlidoe, the Oriental 
 Tree Shrews (Tupaiadce), which are deserving of especial 
 notice, have twelve ribs, the spines of the vertebrae 
 anterior to the tenth dorsal slope backwards, the tenth 
 or eleventh is upright, and the nine posterior spines slope 
 forwards. 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 Insectivore, a 
 picture of an earlier stage of the Primate phylum 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 the 
 commencing possession of a centre of movement situated 
 at the hinder end of the thoracic vertebrae. 
 
 Great interest centres round the Lemurs in the study 
 
110 
 
 ARBOREAL MAN 
 
 Fig. 37. — A Typical Tree Shrew. Adult Female of Tupaia 
 
 ferruginea. 
 
 From a spirit specimen. 
 
THE SPINOUS PROCESSES 111 
 
 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 probably 
 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 be 
 no doubt that the Slow Lemurs possess a true Lemurine 
 structure in many important particulars, so that they 
 must have had a common origin with the true Lemurs " 
 (Sclater). We are bound, therefore, by our present 
 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 tree-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 spinas muscle and its spinous 
 processes, and that by its slothful habits Nycticebus 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-lumbar centre of movement, 
 and the presence of a practically uniform series of spines, 
 are seen in another group of arboreal Primates. Nyctice- 
 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 grasps 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 branches by its arms," and as " moving 
 along a large limb 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 
 Si?nia satyrus, but it is the one which is characteristic 
 of the smaller agile Anthropoids known as the Gibbons 
 (Hylobatcs). 
 
 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 hi 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 was 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 spinous 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 anteverted, 
 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 maimer. 
 In some primitive human races there is even a tendency 
 to anteversion in the posterior spines, which shows itself 
 at times quite distinctly in the first sacral vertebra. 
 
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 relatively to other 
 anatomical features of the skull varies enormously 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 Math the vertebral column, 
 and the skull is braced in position by a strong ligament — 
 the ligamentum nuchse — and by muscles passing from the 
 vertebras 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 
 forwards, 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- 
 graph from skulls which are orientated strictly in the same 
 plane. The relative positions of the condyles are therefore 
 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 (CynocepMlus). 
 
 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 (Cercopithecus). 
 
 arboreal. The final changes which have taken place in 
 MaD 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 
 
 ARBOREAL 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 119 
 
 human condition by the final phases of the recession of 
 the snout region. 
 
 With the question of the curves of the vertebral column 
 we need deal but briefly, for the subject is one which 
 finds ample discussion in every work upon anthropology. 
 In pronograde quadrupeds the backbone rises as one 
 long, low-pitched arch from the point where it is sup- 
 ported by the fore-limb, to a maximum in the dorso- 
 lumbar region, and then falls again to the point where it 
 is supported by the hind-limbs. The weight of the trunk 
 is carried from an arch which is supported upon pillars 
 (limbs) at its two extremities. It is this arch which 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 time the 
 arch is reversed, for while the curve of the back is convex 
 upon its dorsal side, the curve of the neck is convex 
 ventrally. Again, behind the posterior supporting pillar 
 the spine is also bent upwards; here, at the sacro-vertebral 
 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 Eigs. 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 monkeys, 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 be, 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 MAN 
 
 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 (Macropus), 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 Cercopithecus palatinus 
 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 by 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 deny 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 
 
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 palatimis), to show the Incipient 
 Forward Curve in the Lumbar Eegion 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 with 
 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 (pleurapo- 
 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 of 
 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 maj' 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 (pleura- 
 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. 
 
 (diapophysis). 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 upon the sacro-iliac joint that we may sum up 
 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 lar) 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 vertebrse, or nearly the whole of three may enter 
 into the joint. But the diapophyses of only two of the 
 sacral vertebrae are, as a rule, involved. In both the 
 Gorilla and the Chimpanzee the joint surface occupies 
 from two and a half to three sacral vertebras, 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 
 vertebrse, while that of the male embraces from two and 
 a half to three. In any 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 symphysis, 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 symphysis, in which 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 in 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 Kepresentation of the 
 Pelvis of ax 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 hind- 
 limb, an entirely different mechanical design. There is 
 a main weight-supporting 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-supporting 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. 
 
 Pig. 46. — Diageammatic 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 externa] 
 reproductive orifices become situated beneath the pubic 
 arch, instead of occupying 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 
 
 129 
 
 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 Ventral Sym- 
 physis (V.S.) to the Viscera. 
 
 most markedly affected. Briefly, we may say that when 
 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 
 
 9 
 
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 upon 
 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 typical 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 the 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 hoops 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 posture, 
 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 directly concern the 
 
THE RESPIRATORY SYSTEM 133 
 
 function of respiration. This function has, however, 
 been modified very distinctly by the habit of tree-climbing, 
 and especially by its most important development, the 
 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 sj-stems that its main thread is apt to 
 be lost in the complications which occur in every chapter. 
 The primitive air-breathing Vertebrates draw air into 
 their lungs by creating a suction within 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 tailward 
 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 chest 
 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 by Sir 
 
134 ARBOREAL MAN 
 
 Charles Bell the " external " and the other the " in- 
 ternal " respiratory 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 proportion. It is now that the internal 
 
THE RESPIRATORY SYSTEM 
 
 135 
 
 respiratory system becomes of increasing importance, 
 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, pure 
 quadrupedal animals use their external respiratory system 
 
 Fig. 49. — Diagram of a Quadrupedal Animal, to show the 
 Muscles passing from the Eelatively Fixed Fore-Limb 
 to the Relatively Movable Kibs. 
 
 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 primitive external respiratory muscles 
 into play. A runner who is " blown " will grasp his 
 
136 
 
 ARBOREAL MAN 
 
 Fig. 50. — Diagram of a Human Skeleton", to show the 
 Muscles passing from the Relatively Fixed PiIbs to 
 the Relatively Movable Fore-Limb. 
 
 Only one muscle (serratus rnagnus) is iepresented. It produces 
 a round arm-blow. 
 
THE RESPIRATORY SYSTEM 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 flat 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. 
 
CHAPTER XXI 
 
 THE EEPEODUCTIVE 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 factor 
 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 ; Tupaia 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 offspring 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 body. 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 SYSTEM 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 Faxloppian 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 Centetes, 
 whose large family we 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 mammalian series 
 
142 
 
 ARBOEEAL 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 haphazard, 
 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. 5±). 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 anchoring nipples. 
 
 ^. 
 
 v. ■.'.'-.•■ .<-.« ■■;:..■</ 
 
 w ■ 
 
 -,,JM; 
 
 ■s^ : 
 
 Fig. 54. — The Mammary Glands of a Lemuk. 
 
 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 nipples 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 
 (Brachyphylla 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 56). The 
 pectoral nipples are associated with pectoral mammary 
 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 pectoral 
 mammary glands (see Fig. 57). Now in the Primates 
 higher than the Lemurs the inguinal anchoring nipples are 
 
 10 
 
14(5 
 
 ARBOREAL MAN 
 
 Pig.'. 56. — The Pectoral Mammary Gland of Tarsius 
 
 spectrum. 
 
 There is another pair of nipples on the lower part of the abdominal 
 
 wall. 
 
 Fig. 57. — The Pectoral Mammary Gland of Galeopithecus 
 volans, which is probably both aJMilk-Secreting and 
 an Anchoring Organ. 
 
THE REPRODUCTIVE SYSTEM 
 
 147 
 
 Fig. 58. — Semnopithecm pileatus (the Capped Langur} 
 
 nursing its Offspring. 
 
 From a photograph by D. Seth Smith, Esq., taken in the Gardens 
 
 of the Zoological Society of London. 
 
148 ARBOREAL MAN 
 
 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 reproductive system, and these effects will be con- 
 sidered elsewhere. 
 
CHAPTER XXII 
 
 THE DEVELOPMENT OF THE BRAIX 
 
 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 evidently 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 = archepalliuin. 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 which 
 impressions are sorted, associated, and stored, so that 
 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 slowly 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 repeatedly 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 
 
 lished 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 op a Brain in a Further Stage of Evo- 
 lution THAN THAT REPRESENTED IN FlG. 59. 
 
 The 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. 61). 
 
 Fig. 61. — Diagram of a Primitive Mammalian Type of Brain 
 illustrating the b.ise of the neopallium (coarsely 
 Dotted Area). 
 
 In the Mammal, not only do smell and taste impressions 
 gain pallial representation, but impressions of touch, of 
 sight, and of hearing 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 BRAIN 15S 
 
 these things, and these additions constitute what has 
 been named by Elliot Smith the Neopallium. Following 
 the same guide, we may therefore give comparatively 
 simple answers to our two original questions: (1) The 
 earliest Mammal inherited an archepallium 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 1 For some aspects of this question 
 we can again turn to the paper by Professor Elliot Smith 
 to which we have already 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 hemisphere, 
 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 in the out- 
 side world was judged primarily and predominantly by 
 its smell: the sense of touch, vision, and hearing being 
 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 sense 
 
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 
 disappear 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 ShreAv (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 
 has 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 Lemuridse 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 roies 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 subject of 
 memories and experiences, still shows in Man a subtle 
 
156 ARBOREAL MAN 
 
 power as a memory sense. Dudley Kidd has noted this 
 feature in investigating the psychology of Kafir children. 
 " When 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<m 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. Moreover, 
 
 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 efficiency, by the hand. 
 
THE STORY OF TACTILE IMPRESSIONS 15 & 
 
 The physicah;changes are great and obvious, but 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 striking 
 lines of progress. The picture of the lowly animal which 
 noses its way 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 
 in 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 with 
 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 will 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. Much 
 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 power 
 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 the most- 
 
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 emancipated, 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 the 
 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 the 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 power to pick up the object and to examine it from 
 all points of view was added. 
 
THE STORY OF TACTILE IMPRESSIONS 161 
 
 There are many other ramifications, and many other 
 possibilities, of this educational gain in the possession 
 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 tine 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 emancipated 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 impulses 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 strangely primitive anatomical 
 structure, becomes applied to all the finer and more 
 skilful movements which the life necessities of the animal 
 can demand of it. Every increase in cerebral develop- 
 ment will make new demands ujaon 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 
 bygone age, were termed the " curious " it will appeal 
 as an interesting theme that this hand, anatomically one 
 of the most primitive parts of Man's body, one to be so 
 nearly matched among the " hands " of the lowest 
 
 162 
 
MOTOR IMPRESSIONS 163 
 
 limbed Vertebrates, has responded to all the exacting 
 calls made upon its functions by the myriad promptings 
 of the complex human brain. We will, 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 evolution 
 from the dawn stage we have pictured. We are con- 
 cerned only with the problem of the possible manner in 
 which these things have affected brain development. 
 In the present state of knowledge this problem is a highly 
 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 prin- 
 ciple involved is a calling of the nervous seat of origin 
 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 well- 
 defined anatomical position in the central nervous 
 system, then, in its immediate neighbourhood, and 
 attracted to it, will be the motor centre which governs 
 the movements of the parts most intimately related to 
 this particular sensibility. It may be that this particular 
 sensibility is intimately related to different movable parts 
 in different animals, and then in each will be found a 
 different motor centre appropriately attracted into the 
 closest anatomical relation to the sensory centre. This 
 general principle has been shown by Ariens Kappers to 
 hold good in the case of the ganglionic centres of the 
 cranial nerves, and to account for their otherwise inex- 
 plicable positions in the brain stem. This principle of 
 neurobiotaxis is, I believe, capable of extension from 
 the ganglionic masses in the brain stem, where Ariens 
 
164 ARBOREAL 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 snoiit 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- 
 ment in which the hand is added to the snout as a tactile 
 sensory organ, and in which the co-ordinated fine move- 
 
MOTOR IMPRESSIONS 165 
 
 nients of the hand are increasing in perfection. These 
 things we are picturing as demanding pallial representa- 
 tion, and it is likely that the hand-tactile area will be 
 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. 6?. 
 
 Fig. 64. — Cerebral Hemisphere of Jlacroscelides (the Jump- 
 ing Shrew), to show the Cortical Areas as determined 
 bt Professor Elliot Smiih. (From Duckworth.) 
 
 M, Motor. S. Sensory. V, visual. A. auditory areas. The 
 white areas are olfactory. 
 
 Fig. 65. — Cerebral Hemisphere of Tapaia (the Tree Shrew), 
 to show the Coriical Areas as determined bt Pro- 
 fessor Elliot Smith. (From Duckworth.) 
 
 Xot€ the development of a prefrontal area in front of the motor 
 
 cortex (51). 
 
 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 movement 
 areas; snout tactile and snout movement areas; hand 
 tactile and hand movement areas, as localized portions 
 of cortex spreading from the old archepallial olfactory 
 area over the unfolding neopallium. Meanwhile we will 
 
166 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 
 
 Pig. 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 sensation*- 
 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 body, 
 
MOTOR IMPRESSIONS 167 
 
 and an animal may now treat its own body as a novel 
 object and learn all about it. In the second place (as a 
 result of the altered poise of the head, etc.) the eyes may 
 also examine almost all of the body, and the animal then 
 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 observations 
 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 wide 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 emancipated 
 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 working 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 cortical 
 area in which this representation is localized is, as we 
 should expect, an extension of the tactile nose and hand 
 area in the developing neopallium. Some difficulty has 
 always been felt in defining the qualities represented in 
 
1G8 
 
 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 " kinesthetic 
 area." Some of the difficulties would, 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 
 
 Pig. 67. — Cerebral 'Hemisphere of a Lemue, to show the 
 Cortical Areas as determined by Brodmann. (From 
 Duckworth.) 
 
 Note the general enlargement of the " association " areas from 
 the stage seen in Tarsius. 
 
 these words were clearly understood. In this area are 
 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 impression of pictured parts 
 are added, possibly by the agency implied by neurobio- 
 taxis, the. centres concerned with the pictured movements 
 of these parts. 
 
 Without an extended discussion of the anatomical 
 details of the central nervous system, we may fall back 
 
MOTOR IMPRESSIONS 169 
 
 upon the axiom — agreed to by physiologists, patho- 
 logists, and anatomists — that " movements, not muscles, 
 are represented in the cerebral cortex." I think we might 
 extend this axiom by elaiming that only " pictured 
 movements " are represented in thecortex. 
 
 This axiom in reality teaches a great deal, for the brain 
 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 knowledge of the form and movements of 
 the different parts of its body. 
 
 We may gain considerable knowledge of the functions 
 of this pictured movement area by the consideration of 
 the results of experiments, which have been carried out 
 by different investigators, upon a series of animals of 
 varying zoological position. The movements of the 
 different parts of the body are not carried out in 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 decapitated fowl is not a 
 paralyzed fowl, 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 pigeon, 
 yet it is capable of flight when 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 Mammals 
 by removal of the pictured movement area of the cortex. 
 
 3. " On the day of removal of the cerebral centres for the 
 limbs, a rabbit will jump vigorously " (Kinnier Wilson). 
 
 4. If this area is removed in a puppy it does not become 
 parah-zed, and one week afterwards, when 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 
 capacity 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 paralyzed 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 type of 
 paralysis, there is one strange clinical fact which may be 
 expressed in the usual axiomatic manner, by saying that 
 " a muscle which can perform two movements may be 
 paralyzed for one movement, and not 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, provided 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 
 
 merits not being represented in the pictured movement 
 area. So much for some of the facts ; what is the probable 
 interpretation of them ? 
 
 It is obvious that in birds, and vertebrates lower than 
 birds, there is a co-ordinating mechanism in the spinal 
 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 working 
 of this reflex mechanism. In the rabbit and dog this 
 reflex mechanism of the cord is considerably 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 of 
 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 in the cortical 
 pictured movement area. 
 
 In Man all the pictured 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 of 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 kinaesthetic region of the human brain 
 (see Fig. 71, p. 192). It is perhaps not beyond possibility 
 that the full lodgment of all pictured movements is not 
 yet permanently effected in all human brains, and that 
 the process is still in progress . There is certain anatomical 
 support for such a supposition, but its exact nature does 
 
172 ARBOREAL MAX 
 
 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 kinesthetic 
 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 anywhere 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 initiating 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 which 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. 
 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-testing, 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 neuro- 
 biotaxis is involved) of the motor centres from a basal 
 ganglionic mass into the kinesthetic, 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 
 
 IMPRESSIONS OF SIGHT AND HEARING 
 
 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 pallial 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 very 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 
 may be termed the increased mobility 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 with the greatest rapidity, towards any 
 object which attracts its attention by any sensory 
 channel. 
 
 174 
 
IMPRESSIONS OF SIGHT AND HEARING 175 
 
 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 eves 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. 65. — Cieebbax HzinsPHXKE of a Moxket ( Matacus), to 
 show ihz cortical areas as dxteritixxd by bttodmaxx. 
 (From Duckwobth.) 
 
 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 many 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 veiy limited. 
 Even the dog is given to r unning with its head turned 
 somewhat sideways, a position which, affecting the car- 
 riage of the whole of its bodv, 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 
 Tig. 70). Although in Man the two eyes see slightly 
 
 Pig. 69. — Human Cerebrum, to show the Main Cortical 
 
 Areas. 
 
 Note the genera] 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 
 
DEPRESSIONS OF SIGHT AND HEARING 177 
 
 Fig. 70. — Ax Adult Female Tarsius 
 
 ectrum. 
 
 From a spirit specimen collected by Dr. Charles Hose in the 
 Baram District of Sarawak, Borneo. 
 
 12 
 
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 ease 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 kinesthetic 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 Reil), while the rest of the 
 hollow hemispheres enlarge around it. As the cerebral 
 hemispheres expand, the main 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, grows downwards and forwards as it revolves 
 below the island of Reil, pushed on from above by the 
 enlarging visual field. Not only 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 179 
 
 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 then 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. 
 
CHAPTER XXVI 
 
 HIGHER DEVELOPMENTS OF CEREBRAL 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. Dwight, 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. Dwight 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 the problem 
 becomes, in great measure, centred in the ascertainable 
 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 this 
 way we may regard the intelligence of a Mammal 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 imper- 
 fect, intelligence as a summation of cerebral possibilities 
 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 faculty 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 proposition 
 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 of 
 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 
 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. The roaming 
 Ungulates, ready to flee upon the least apprehension of 
 danger, have no natural nursery for their young, and in 
 all of them the family is reduced. The pelagic C'etacea 
 are in the same condition, and so also are the Sirenia. 
 
 Large families can only be indulged in by animals that 
 can have a safe retreat in which to rear their numerous 
 young, or by animals sufficiently equipped with weapons 
 to guard them. 
 
 Of those animals which, having no nursery to hand, 
 have a reduced litter, there are two distinct classes. 
 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 beasts, 
 but upon the swiftness of theh open escape. These 
 cannot be successful if the females are handicapped by 
 the disabilities of pregnancy with large litters, or by the 
 nursing of helpless offspring. In them the number of 
 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 the 
 arboreal animals. There is no natural nursery among 
 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. Xo doubt nest-building was 
 resorted to as a temporary expedient in the arboreal 
 stock; and among all the arboreal and semi-arboreal 
 animals derived from many orders, nest -building, in 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 Monkeys are 
 born immature and comparatively helpless, 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 phrase " 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 offsjjring 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 FUNCTIONS 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. On 
 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, though 
 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 helpless 
 baby clinging to the mother's fur. With Monkeys, 
 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, Monkeys show 
 the greatest concern in the well-being of then offspring. 
 That they will defend them from attack is nothing, for 
 such a display of maternal instinct is the common property 
 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 ARBOREAL MAN 
 
 in which the Gibbons attend to their young, 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 (Hylobates 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 years, and is not fully adult 
 until it is fifteen (Forbes). 
 
 This prolongation of infancy, 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 (Hylobates 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 general biological law; 
 but it also follows that if infantile helplessness is pro- 
 longed, his parental duties are liable to a similar exten- 
 sion. Here is evidently the beginning of that association 
 of mother, father, and child which, lasting beyond a 
 brief period comprised in courtship, the suckling of help- 
 less young, and the guarding of mother and offspring, 
 lays the foundation of the family. 
 
 When infancy is brief, the family bond is similarly of 
 short duration; and, the period of suckling being ended, 
 there comes a time of expansion of infantile enterprises, 
 a time marked by some internecine strife and much 
 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 recurrence 
 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 these very 
 details of Primate economy, enough has been recorded 
 to warrant some general statements. The Anthropoid 
 Apes are met with almost invariably as family parties, 
 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 Chim- 
 panzee either lives in separate families, or in small groups 
 
188 ARBOREAL MAN 
 
 of families " (Hartman). " 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) ; S&mnoptiheciis 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 maintained 
 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 isolated in 
 pairs, or limited to family parties. Very little of the 
 intimate details of then lives has been studied, but 
 Tarsius spectrum definitely " lives in pahs " (Hose), and 
 so does Xycticebus tardigradus. 
 
 If higher ideals of conduct are admitted to be mere 
 extensions of a natural cerebral evolution of which so 
 many other developments are certainly known, it wiU be 
 under such conditions as those we have been picturing 
 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 then rudiments will be 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 
 
 HIGHER DEVELOPMENTS OF CEREBRAL FUNCTIONS: 
 POSSIBLE ANATOMICAL BASIS 
 
 When we come to make any attempt to attach a precise 
 location to the neopallial representation of such higher 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 carefully 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 basis 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 191 
 
 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 highest point, 
 in which channels of education were multiplied, and in 
 which cortical areas were elaborated and associated in a 
 bewildering complexity, culminating in the highest 
 possible receptive, sorting, associating, and storing 
 mechanism evidenced by a prodigy of intelligence or 
 intellect, but in which higher ideals of conduct were 
 absent . 
 
 If. th r -n, we can imagine what constitutes the anatomi- 
 cal basis of intelligence, what picture hare we of the 
 physical seat of " higher ideals of conduct "1 There is 
 a well-known cortical area, which is situated at the 
 anterior end of the neopallium, that has yielded up 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 ma}- 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 area? 
 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 their 
 borders. This prefrontal region marches upon the 
 borders of only one such area — the so-called '"motor" 
 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 co- 
 
192 
 
 ARBOREAL MAN 
 
 ordinated ideas," etc., it seems strange that its only 
 associated areas should be the " motor " centre (see Figs. 
 64-69, 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- 
 
 plg. 71.- diagram of the left cerebral hemisphere of a 
 
 Human Brain, to show the Order of Representation 
 in the " Pictured Movement " Abe a. 
 
 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 193 
 
 possible to imagine that an area which is an association 
 or extension area of this field, in which an animal'* 
 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. Probably the 
 first occupation of this new area is effected by memories 
 of pietiu'ed movements, and the sorting of experiences 
 gained by this source. From calling up pictures of past 
 associations of pictured movements, there is perhaps 
 a step towards constructing conceptions of future move- 
 ments evoked by pictured hypothetical 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 terms of 
 action. We may say that in the gaming of this prefrontal 
 neopalhal 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 kina?sthetic area performs 
 all its actions in the absence of any pictured consciousness 
 of the action. An a nim al with a kinesthetic 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. It 
 knows what it is doing, and it remembers what it has done. 
 An animal with an elaborated prefrontal area has. in 
 addition, the faculty for building up pictures of possible 
 future actions. It knows what it is doi)ig, it remembers 
 what it has done, and it can estimate what it might do. 
 We mav 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 conduct. 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 by 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 whom the problems of 
 conscience and conduct bulk so large as to be a definitely 
 
HIGHER FUNCTIONS: ANATOMICAL BASIS 195 
 
 one-sided development. Again, there are others 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 tits 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 whom damage of an associa- 
 tion area prevents his intellectual mind from finding the 
 least meaning in the spoken words of his fellow-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 be 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 XXVIII 
 
 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. We 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 worthy 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 may 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 primarily arboreal. 
 
 106 
 
THE BRAIN AND THE BODY 197 
 
 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 upon 
 them, and yet it is obvious that the advantage that they 
 have taken of it has been slight. There are rnetatherian 
 convergent mimics of Carnivora, Rodent ia, Insect ivora, 
 and of most other Eutherian orders, but there is no 
 rnetatherian convergent mimic of the eutherian Primates. 
 It would not be unnatural, therefore, to assume that the 
 full advantage coidd not be grasped by the rnetatherian 
 animals, since the ground-plan of their brain would not 
 permit it. Climbing metatherians with perfectly mobile 
 fore-limbs and grasping members were at one time classed, 
 upon the strength of this feature, amongst the Cheiropeds, 
 a group which included only them {Didelphidce, 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 
 lingers, 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 seen 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 rnetatherian from the eutherian brain, and 
 if this distinction will in any way account for the very 
 slight evolutionary advances made by thoroughly arboreal 
 
198 ARBOREAL MAN 
 
 metatherians. Anatomically, this question receives an 
 almost perfect answer. Without entering into a bewilder- 
 ing array of interesting anatomical details which, deter- 
 mined by Owen, were somewhat obscured by later 
 writers, only to be denned 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 perfection. 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 instrument 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 coincidently. 
 
 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 placed 
 within its reach. The earliest eutherian Mammal 
 
 Fig. 72 — Diagram taken from the Drawing by Professor 
 
 G. Elliot Smith of the Brain of Omithorhynchus. 
 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 showD 
 
 in Fig. 72. Note the size of the corpus callosum. 
 
 possessed the cerebral condition which made it possible 
 for it to take full advantage of the physical advance. 
 
200 AKBOREAL 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 which 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 
 physical adaptations are by no means to be ignored. 
 A Master may perform marvels upon the violin, but his 
 expression will be seriously hampered if there is nothing 
 better to hand than an empty cigar-bos strung with a 
 few strings. Man 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 str ikin g illustration of the converse of this expecta- 
 tion may perhaps make the argument more clear. Among 
 the birds, the whole family of the Terns (Sternidce) is 
 characterized by a typically terrestrial habit of incubation, 
 for their eggs are laid upon 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 f amity (Anous stolidus), this nest may 
 rise, as a collection of sea-wrack, to the dignity 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. 
 Xo 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 will 
 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. No 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 White Tern shows so 
 well its terrestrial inheritance, whilst its parents are 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 when 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. 76), the 
 
 Fig. 74. — Human Embryo 
 105 mm. in Total 
 Length. 
 
 Fig. 75. — Human Embkto 
 195 mm. in total 
 Length. 
 
 disproportionate growth of the human leg being largely 
 a post-natal development. At one stage of human 
 embryonic development the arm is longer than 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 human 
 lengthening of the leg proceeds more rapidly; when 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 
 
 Fig. 76. — Newborn 
 Babt. 
 
 Fig. 77.' — Child Eighteen 
 Months Old. 
 
 Fig. 78.— Child Six 
 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 worthy 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 power of toe- 
 grasp displayed by infants and young children 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 baby's feet 
 are turned inwards so completely that they can be 
 pressed flat against each other, this, indeed, being a 
 common position of rest in an infant, as in an arboreal 
 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 which 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 baby entwine its fingers in his hair, 
 or has permitted a slightly older one to grapple with his 
 watch and chain, will doubt the very real power 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 
 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 two minutes, 
 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 AEBOEEAL ACTIVITIES OF MODE EX MAN 
 
 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 into 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 learned to 
 climb. Schoolboys are taught to climb a rope upon 
 lines altogether different from those employed by then* 
 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 European small 
 boy climbs a tree in true monkey fashion till he comes 
 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 irons, which 
 are furnished with spikes at the points where the legs 
 are most adopted to hugging the branch. 
 
 This method of climbing is. however, a mere adaptation 
 to the handicap imposed by long civilization and the habit 
 of wearing boots. It is a confession that the plastic foot- 
 grasp is lost. Unbooted races do not " swarm " or 
 " shin " up trees, but many of them have learned some 
 
 207 
 
208 ARBOREAL MAN 
 
 mechanical way of assisting their waning powers of foot- 
 grasp. One widespread method is the adoption of a 
 hoop or girdle which 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 purpose. 
 
 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 readify 
 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 MODERN MAN 209 
 
 the spathe, he depends entirely upon the natural grasp 
 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 or climbs 
 up exactly as a monkey would under similar circum- 
 stances. 
 
 Races more primitive than the Malays can climb the 
 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 the Primates. 
 " 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 " (Skeat and 
 Blagden). Sea-going Malays adopt the same method 
 when climbing masts or ropes aboard ship, 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 overlooked, 
 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 ability to climb 
 manifested upon exactly the same lines as the climbing 
 function of the arboreal Primates, and differing only in 
 
 14 
 
210 
 
 ARBOREAL MAN 
 
 Fig. 79. — A Sakai Tree-climbing. 
 
 From a photograph by Ccrruti reproduced in "The Pagan Races 
 of the Malay Peninsula," by W. W. Skeat and C. 0. Blagden. 
 
ARBOREAL ACTIVITIES OF MODERN MAN 211 
 
 that it is somewhat less perfectly performed. Modern 
 European man has no doubt lost his foot-grasp so thor- 
 oughly that when he takes to climbing he learns a new 
 and human method, but his less trammelled brother 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 FAILURES OF ARBOREAL LIFE 
 
 There would seem to be a general law applicable 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 varying 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 plasticity. 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, 
 " Hardly 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 definite 
 and special type of food, has proved the downfall of many 
 a promising animal type. The Primate and human stock 
 has not been led astray in this direction; for it has pre- 
 served throughout that well-balanced habit of dietary, 
 only to be termed omnivorous. To talk in the fashion 
 of human successes in life, an animal may use or abuse 
 its life surroundings. We may say that when it uses 
 them rightly it undergoes the successful minimal adaptive 
 specialization, but when it abuses them it runs riot in 
 specializations — specializations which ultimately make it 
 the slave of its environment. 
 
 An animal which chances to come into possession of 
 a habitat of which one feature is the presence of water — 
 be it rivers, lakes, or oceans — in which food is to be ob- 
 tained will open up a wider field for its activities, gain a 
 new series of educational possibilities, and perhaps place 
 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 further 
 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 become re- 
 duced to paddles ; smell, hearing, and even sight, become 
 restricted senses, and an animal wholly dependent on a 
 thoroughly aquatic life is one debarred from real mam- 
 malian progress. The Sirenia, Cetacea (toothed and 
 toothless) and even the Pinnipeda among the Mammals, 
 are examples of types which, having become slaves of 
 an aquatic habit, and leading singularly restricted, 
 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, finished 
 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 (Tcdpidce), 
 the Golden Moles (Chrysochloridce), the Rodent Mole Rats 
 (Spalacidce), and the marsupial Notoryctes, are examples 
 of highly specialized 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. 
 
 PhascoJurctus, among the Metatheria, is a mere arboreal 
 dinger with 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 LIFE 215 
 
 and first finger are opposed to the other three digits. 
 The eutherian Sloths {Brady pod id a?) show to perfection 
 the fatal effects of mere arboreal clinging. These animals 
 spend their lives for the most part among the branches 
 of trees, to which they cling hooked up in an inverted 
 position by a reduced and highly specialized 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 upon 
 specific senility. With the phylogenetic history, and the 
 affinities of the Bradypodidce 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 Xycticebus tardigradus. This 
 Lemur may easily be appealed to as an example of a 
 tendency to arboreal clinging which may possibly be 
 exaggerated, and so lead astray from the line of true 
 Primate development. Xycticebus may also be pointed 
 to as showing possible tendencies to two other outcomes 
 of the arboreal habit which prove pitfalls of special- 
 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 Xew World Monkeys. It is a strange 
 feature of the South American arboreal animals that they 
 have assisted and perfected the foot-grasp by the develop- 
 ment of a prehensile tail. It might seem that the accpiire- 
 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. Yet it has 
 proved to be a specialization which turned aside 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 arboreal specialization which 
 culminated in the power of flight is difficult to determine 
 with 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 they were derived. 
 The curious Flying Lemur Galeopithecus volans (see 
 Fig. 80) has been regarded by 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 by many 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 the 
 rodent Flying Squirrels (Pteromys, Anomalurus, etc.) 
 which have made some progress towards flight. But it 
 
 Pi V<3 
 
 «W 
 
 / / 
 
 [;fc 
 
 *>* 
 
 Fig. 80. — Young Female Oaleopithecus. 
 From a specimen collected by Charles Hose. 
 
 is to be doubted if the truly flying Mammals, such as the 
 Bats, started their career on these lines. 
 
 According to Willey, " the facts seem to show clearly 
 that it is not merely the habit of taking flying leaps, like 
 
218 
 
 ARBOREAL MAN 
 
 Fig. 81. — The Naked Bat {Cheiromeles torquatus) which 
 shows, particularly in the structure of its hlnd- 
 Limbs, Adaptations to a Tree-climbing Habit. 
 
 From a spirit specimen collected by Dr. Charles Hose, Sarawak, 
 
 Borneo. 
 
THE FAILURES OF ARBOREAL LIFE 219 
 
 monkeys, for example, that has led to the formation of 
 organs of flight." Certainly there is nothing in the 
 anatomy of Cheiromehs or of Galeopithecus to indicate 
 any inheritance of a power of arboreal leaping. Assuming 
 that the Bats are monophylic and that Cheiromehs might 
 show an evolutionary phase representative of the fore- 
 runners of all the members of the order (an assumption 
 I believe to be by no means justified), one might be 
 inclined to imagine that the specialization of foot-grasping 
 and the consequent adaptation of an inverted position, 
 such as we have noted in Nycticebus, was an early phase 
 of the evolution of true mammalian flight. It is of 
 interest to remark here that more than one existing Lemur 
 shows a definite development of a lateral skin fold such 
 as constitutes, when fully developed, a flying membrane 
 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 flight any 
 further, nor turn aside to inquire if all the flying Mammals 
 grouped as the Cheiroptera, or Bats, have sprung at the 
 same tirm, 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 that 
 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 habitat or 
 environment; geographical barriers, which limit the 
 activity and spread of the stock from which it sprang, 
 offer no unsurmountable boundaries to its enterprises. 
 Indeed, the geographical distribution of the Cheiroptera 
 demonstrates the reality of this advantage. 
 
 The power of flight, whilst offering an abundant change 
 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 
 which 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 which we have seen are so essential in the final 
 education of the neopallium which makes for real evolu- 
 tionary progress. No matter from what 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 very 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 foregoing 
 chapters that, in the main, I have attempted to derive 
 most of the peculiar features of Man, and of his kindred, 
 from adaptations and advantages gained during an 
 arboreal apprenticeship. To this source of derivation of 
 these adaptations^! can see no real alternative; but it 
 must be pointed out that most of the physical details 
 to which I have called attention are generally explained 
 as being outcomes of the " attainment of the erect 
 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 slowjand painful acquisition 
 of a radically new posture " at which Dwight 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 with 
 Dwight when he says that " as an explanation it has been 
 terribly overworked. "J^ Walking upright upon the surface 
 of the earth has produced its changes in the human body, 
 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 much 
 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 MAN 
 
 would agree to name as a human being. We have now a 
 complexity of species, and even genera, of " human " 
 remains, and yet, as is indeed inevitable, we have no 
 criterion by which all will agree to judge such remains 
 as belonging to an evolutionary stage universally 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 Man became definitely human. 
 
 Munro has stated his conviction clearly, and, for him, 
 Homo sapiens came into being with the " attainment of 
 the erect position," and the consequent possession of its 
 accompanying benefits. But the statement of his case 
 needs examination: "With the attainment of the erect 
 position and the consequent specialization of his limbs 
 into hands and feet, Man entered on a new phase of 
 existence. With the advantage of manipulative organs 
 and progressive brain he became Homo sapiens." 
 
 If it be these things which determine Homo sapiens as 
 a species, then Homo sapiens need not be limited to Man 
 the upright, for all these things are effects 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 upright posture all 
 those changes and benefits accumulated 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 22a 
 
 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 extra ordinary 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 supposing 
 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 dawn 
 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 Mammal 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 Mammal must 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 as 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, and Now 
 Edition of vol. i., 1915. 
 
 Dwight, Thomas. 
 
 Thoughts of a Catholic Anatomist. London, 1911. 
 Elliot Smith, G. 
 
 The Origin of Mammals. Discussion, Section D. British Associa- 
 tion, 1911. 
 
 Presidential Address. Section H. British Association, 1912. 
 
 Arris and Gale Lectures on "The Evolution of the Brain." Lancet, 
 1910, p. 153. 
 
 Ellis, 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, 1896. 
 
 Gadow, Hans. 
 
 Observations in Comparative Myology. Journ. Anat. and Phys., 
 vol. xvi., p. 493. 
 
 Goodsir, John. 
 
 Anatomical Memoirs. Edited by William Turner. 1868. 
 
 Hartman, Robert. 
 
 Anthropoid Apes. Second edition. London, 1889. 
 
 Hickson, Sydney J. 
 
 A Naturalist in North Celabes. London, 1889. 
 
 225 15 
 
226 ARBOREAL MAN 
 
 Humphry, Sir G. M. 
 
 Observations in Myology. London, 1872. 
 
 The Human Foot and the Human Hand. London, 1861. 
 
 See also Journ. Anat. and Phys., vol. Hi., 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. 
 
 Heith, Arthur. 
 
 Introduction to the Study of Anthropoid Apes. London, 1897. 
 On the Chimpanzees and their Relationship to the Gorilla. 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 
 Hefte, 1901, p. 671. 
 
 Lawrence, Sir William. 
 
 Lectures on the Comparative Anatomy, 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. and Phys., 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 
 Phys., vol. xxxii., p. 428, etc. 
 
 Sclater.W. L. andP.L. 
 
 The Geography of Mammals. London, 1899. 
 
 Skeat, W. W., and Blagden, C. 0. 
 
 Pagan Races of the Malay Peninsula. London, 1906. 
 
 Taylor, Gordon, and Bonney, Victor. 
 
 Homology and Morphology of the Poplitcus Muscle. Journ. 
 Anat. and Phys., vol. xl., p. 34. 
 
 Topinard, Paul. 
 
 Anthropology. English edition, 1890. 
 
 Wallace, A. R. 
 
 The Malay Archipelago. 
 
 Willey, Arthur. 
 
 Convergence in Evolution. London, 1911. 
 
INDEX 
 
 A 
 
 Accessory muscles of respiration, 
 
 135 
 Adaptation to environment, 3 
 Aerial life, 216 
 American monkeys, 69, 215 
 Amphibia, 10, 20 
 Anchoring nipples, 144 
 Anous stolidus, nesting of. 201 
 Anthony, Professor Kaoul, 219 
 Anticlinal vertebra, 103 
 Aquatic animals, 50, 213 
 Arctomys, S5 
 Arehepallium, 151 
 Ariens Kappers, Dr., 103 
 Association areas, 101 
 Association of senses, 178 
 Auditory impressions, 174 
 
 B 
 
 Baboon, skull of, 110 
 
 pelvis of, 125 
 Balancing muscles, 05 
 Bambute Pvgmics, limbs of, 202 
 Bats, hind-limb of, 55 
 
 pelvis of, 127 
 
 mammary glands of, 143 
 
 as failures, 217 
 Beddard on lemurs, 210 
 Bell, Sir Charles, on the hand, 43 
 
 on respiration, 134 
 Big toe, grasping power of, 72 
 Binocular vision. 170 
 Birds, feet of, 07 
 Boots, wearing out, 205 
 Bmdypodidcr, 215 
 Brain, evolution of, 194 
 
 importance in evolution, 200 
 Bridgwater treatise, 43 
 Broom on limbs, 11 
 Burrowing animals, 214 
 
 C 
 Canine teeth, 90 
 Carotid arteries, 90 
 Carpus, 23 
 Cat washing face, 19 
 Centetes, litter of, 139 
 
 mammary glands of, 141 
 
 I Centre of motion, 104 
 ' Cercopilhecus, vertebral column of, 
 120 
 
 Cerebral hemispheres, 150 
 
 Chameleon, feet of, 67 
 
 Cheirogaleas, 138 
 | Cheiromdes, 216 
 ! Cheiropods, 197 
 
 Chelonians, 20, 40 
 
 Chest, shape of, 131 
 
 Chimpanzee, 37, 112, 120, 187, 202 
 
 Chin a human feature, 95 
 
 Chiromys, 154 
 
 ChrysoclUoris, 59 
 
 Clavicle, 2S 
 
 Climbing, dawn of, 16 
 
 Coconuts, method of gathering, 
 208 
 
 Conduct, evolution of, 181 
 conception of, 193 
 
 Conscience, 1 94 
 
 Convergent mimics, 197 
 
 Corpus callosum, 198 
 striatum, 171 
 
 Cortex, development of, 150 
 method of building of, 104 
 
 Crocidura, 39 
 
 Crocodile, vertebral column of, 102 
 
 Cri/plobmnchus, 33, 58 
 
 Curves of spine, 119 
 
 Cyuocephalus, 110 
 
 Cynodontia, 149 
 
 D 
 Darwin, 1.3,90 
 Decapitation of bird, 109 
 Dental caries, 93 
 Dentine, 94 
 Descartes, 194 
 
 Descending trees, methods of, 50 
 De Vries, 3 
 Diaphragm. 133 
 Diet, specialization in, 212 
 Digital formula, 75 
 Dipnoi, brain of. 150 
 Dog, 19, 107, 175 
 Duckworth, Dr. W, L. H., 81, 91, 
 
 202 
 Dwight, Professor Thomas, 2, 180, 
 
 221 
 
 227 
 
228 
 
 ARBOREAL MAN 
 
 E 
 
 Edentates, 108, 215 
 
 Egyptians, teeth of, 92 
 
 Elephant, 106, 167 
 
 Elliot Smith, Professor G., 149, 152, 
 
 223 
 Emancipation of fore-limb, 7, 17 
 Erinaceus, 59 
 Eversion of foot, 205 
 External respiratory system, 134 
 Extinct animals, skeletons of, 11 
 Eyes, position of, 175 
 
 Falloppian tubes, 139 
 Family, formation of, 188 
 Flower, Professor, 76 
 Flying mammals, 55, 216 
 Foetus, limbs of, 203 
 Food, preparation of, 91 
 Foot, eversion of, 63 
 Fore-limb, bones of, 23 
 
 muscles of, 32 
 Frontal cortex, 191 
 
 G 
 
 Gait of animals, 15 
 
 Galago, 111 
 
 Galeopilhecus, 145, 216 
 
 Gavial, vertebral column of, 102 
 
 Gegenbaur on carpus, 23 
 
 Gibbons, climbing of, 112 
 
 vertebral column of, 121 
 
 pelvis of, 125 
 
 young of, 186 
 Giraffe, gait of, 15 
 
 vertebral column of, 106 
 Golden moles, 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 
 Gygis Candida, young of, 201 
 
 H 
 
 Hand, primitive condition of, 20 
 Hand-feeding, 84 
 Head, poise of, 114 
 Hearing, impressions of, 174 
 Hedgehog, 59 
 
 Hemiplegia, 170 
 Hepburn, Professor, 37 
 Hickson, Professor S., 187 
 Hind-limb, changes in, 53 
 Hip-joint, 63 
 Hcnno 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 
 
 I 
 
 Ideals of conduct, 190 
 Infancy, duration of, 186 
 Inguinal nipples, 143 
 Insanity, 195 
 
 Intellect, cortical site of, 194 
 Intelligence, evolution of, 180 
 lntermembral index, 202 
 Internal respiratory system, 134 
 Interossei muscles, 80 
 Ischio-pubic symphysis, 123 
 
 Jerboa, 84 
 Joint, hip, 63 
 
 sacro-iliac, 125 
 Jugular veins, 96 
 
 K 
 
 Kafir children, 156 
 
 Keith, Professor Arthur, 4, 38, 
 
 120 
 Kidd, Dudley, 156 
 Killing prey, method of, 86 
 Kinesthetic area, 168 
 Kinnier Wilson, 169 
 Klaatsch on upright posture, 223 
 Kloster, Rud., 38 
 
 Lamarck, 3 
 
 Lawrence, Sir William, 222 
 
 Lemurs, 125, 138, 219 
 
 Limbs, origin of, 7 
 
 Little toe, 79 
 
 Lower neurone lesion, 170 
 
 M 
 
 Malays, climbing methods of, 208 
 Mammary line, 141 
 Marmosets, 69 
 Marsupials, n?st-building, 139 
 
 arboreal, 196 
 Maternal care, ] 84 
 McKenzie, Lr. W. C, 39 
 
INDEX 
 
 229 
 
 Memory, development of, 151 
 Mercier, Charles, 195 
 Metatheria, arboreal, 196 
 Middle line digit, 80 
 Milk teeth, decay of, 93 
 Missing links, feet of, 73 
 Molar teeth, 92 
 Monkey communities, 188 
 Monotremes, 39 
 Motor area, 167 
 
 impressions, 162 
 Munro, Dr., 44, 221 
 Mus margarettm, 69 
 Mutation, 3 
 Myology, comparative, 32 
 
 N 
 
 Nasalis larvatus, 98 
 Neopallium, 153 
 Nest-building, 138 
 Neuro bio taxis, 163 
 Novel objects, testing of, 159 
 Nursery, importance of, 183 
 Nursing habits, 143 
 
 of monkeys, 148 
 Nycticebus tardwjradus, 50, 86, 111, 
 159, 185, 215 
 
 
 
 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 
 
 Prototheria, 56, 59, 103 
 Pubic symphysis, 123 
 Puppy, experiment on, 169 
 Pygmies, limbs of, 202 
 
 Quadrumana, 49 
 
 R 
 
 Rabbit, experiment on, 169 
 
 Radio-ulnaris muscle, 41 
 
 Reason, evolution of, 181 
 
 Reflex action, 172 
 
 Reil, Island of, 178 
 
 Reptiles, vertebral column of, 102 
 
 brain of, 151 
 Respiration, methods of, 133 
 Rhmolophid bats, nipples of, 144 
 Rhinopithecus, 98 
 Robinson, Dr. Louis, 205 
 Rodents, nests of, 139 
 Rotation of tibia and fibula, 59 
 
 of hip-joint, 63 
 Rosenberg, 26 
 
 S 
 
 Sacro-iliac joint, 123 
 Sakai, toes of, 72, 80 
 
 methods of climbing, 209 
 Scent glands, 154 
 Semangs, 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, 98 
 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, 205 
 Symphysis pubis, 126 
 
 Tapir, 167 
 
 Tarsius spectrum, 54, 57 
 
 foot of, 70 
 
 face of, 97 
 
 vertebral column of, 111 
 
 nursing of, 148 
 
 eyes of, 100, 170 
 Taylor, Gordon, 39, 41 
 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, 6 
 
 Uterus, different types of, 139 
 
 Varanus, 58 
 
 Variations, 3 
 
 Vertebral column, curves of, 119 
 
 Viscera, disposition of, 129 
 
 Visceral outlets, 128 
 
 Visualimpressions, 174 
 
 W 
 
 Walking, nervous mechanism of, 172 
 Water newt, 9 
 Wearing of teeth, 94 
 White tern, nesting of, 201 
 Willey on diet, 212 
 Wisdom teeth, 92 
 
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