m m m O; ! O m THE EVOLUTION OF MAN Series of Lectures "Delivered under the ^Auspices of the Tale Chapter of Sigma Xi during the ^Academic Tear 1921-1922 JAVA MAN Pithecanthropus erect us. PILTDOWN MAN Eoanthropus Jawsoni. NEANDERTAL MAN CRO-MAGNON MAN Homo neandertalensis. Homo sapiens. Heads modelled by Professor J. H. McGregor, on his restorations of skulls of ancient types of man. These types are not stages in a single evolutionary line, hut probably represent four divergent lines. Original models in the American Museum of Natural History, New York. The Evolution of Man Richard Swann Lull Harry Burr Ferris Gfeorge Howard barker James Rowland Angell Albert Galloway Keller Sdwin Qrant Conklin Edited by Cfeorge Alfred 23 'ait sell NEW HAVEN YALE UNIVERSITY PRESS London : Humphrey Milford : Oxford University 'Press MDCCCCXXIII COPYRIGHT, 1922, BY YALE UNIVERSITY PRESS Published, October, 1922 Second printing, December, 1922 Third printing, February, 1923 CONTENTS PAGE Preface GEORGE ALFRED BAITSELL ix Chapter I. The Antiquity of Man . RICHARD SWANN LULL 1 Chapter II. The Natural History of Man HARRY BURR FERRIS 39 Chapter III. The Evolution of the Nervous System of Man GEORGE HOWARD PARKER 80 Chapter IV. The Evolution of Intelligence JAMES ROWLAND ANGELL 103 Chapter V. Societal Evolution . ALBERT GALLOWAY KELLER 126 Chapter VI. The Trend of Evolution EDWIN GRANT CONKLIN 152 Bibliography 185 Index 191 4- fe 5 ILLUSTRATIONS Models, by Professor J. H. McGregor, of the heads of prehistoric types of man Frontispiece Fig. 1. Skulls from La Chapelle-aux-Saints, representing Neandertal man, and Grotte des Enfants, representing Cro-Magnon man facing page 1 Fig. 2. Map to illustrate the broader lines of dispersal of the principal races of man. After Matthew .... 6 Fig. 3. Pithecanthropus erectus. Skull and face .... 15 Fig. 4. Homo (Paleanthropus) heidelbergensis. Mandible compared with that of a modern European. After Schoetensack 17 Fig. 5. Eoanthropus dawsoni (Piltdown man). Skull and face 21 Fig. 6. Skeleton of Neandertal man compared with that of Cro-Magnon man 25 Fig. 7. Homo (Paleanthropus} neandertalensis. Skull and face 27 Fig. 8. Homo rhodesiensis. Skull and face 29 Fig. 9. Homo sapiens (Cro-Magnon man). Skull and face . 33 Fig. 10. Provisional phylogeny of man and the anthropoids . 36 Fig. 11. Fertilization of the ovum 41 Fig. 12. Four diagrams of early human embryos . facing page 42 Fig. 13. Human embryo of 1.54 mm. (von Spec). X 23. Dorsal view facing page 44 Fig. 14. Human embryo of 4.2 mm., in lateral view (His). X15 facing page 44 Fig. 15. Developing brain from a human embryo of 7 mm. facing page 46 Fig. 16. The pallium and cerebellum in the brains of various vertebrates facing page 48 viii ILLUSTRATIONS Fig. 17. The individual and evolutionary development of the nerve cell 49 Fig. 18. Section through the spinal cord showing the early de- velopment of a spinal nerve .... facing page 50 Fig. 19. Organs developing from the rudimentary alimentary canal 52 Fig. 20. The development of the human heart 54 Fig. 21. The development of the face of a human embryo (His) facing page 56 Fig. 22. The relation of the fetal to the maternal blood in the human placenta facing page 58 Fig. 23. Comparison of the embryos of various mammals at corresponding stages of development . facing page 62 Fig. 24. Changes in proportion during growth 66 Fig. 25. Growth curves for increase in weight and stature in both sexes 68 Fig. 26. The caecum and vermiform appendix in various mam- mals 74 Fig. 27. Supernumerary nipples of man 77 PREFACE The contents of this book have as their basis a series of lec- tures bearing the same title which were given at Yale Univer- sity, during the academic year 1921-1922, under the auspices of the Society of the Sigma Xi. As President of the Yale Chapter for that year it became my duty to arrange the program for the Society, and after considerable thought I came to the con- clusion that the previous successful series of Sigma Xi lectures, which were given in 1916-1917, and later published by the Yale University Press, on "The Evolution of the Earth and Its Inhabitants," could be continued with interest and profit by another series in which specific consideration was given to the question of the evolution of man. Accordingly a series of lectures was arranged as follows: THE EVOLUTION OF MAN Lecture I. The Antiquity of Man, December 2, 1921, Professor Richard Swann Lull. Lecture II. The Natural History of Man, January 20, 1922, Professor Harry Burr Ferris. Lecture III. The Evolution of the Nervous System of Man, February 10, 1922, Professor George Howard Parker. Lecture IV. The Evolution of Intelligence, April 11, 1922, President James Rowland Angell. Lecture V. Societal Evolution, March 10, 1922, Professor Albert Galloway Keller. Lecture VI. The Trend of Evolution, March 22, 1922, Professor Edwin Grant Conklin. The first lecture, by Professor Lull, sets forth the paleonto- logical evidence for the evolution of man. In the second, Professor Ferris gives in detail, largely from the anatomi- cal and embryological standpoints, some of the important x PREFACE evidence for evolution which is to be found in the development and structure of present-day man. The third and fourth lec- tures, by Professor Parker and President Angell respectively, constitute a unit in which the evolution of the highly special- ized and preeminent nervous system of man together with the development of intelligence are given consideration. Pro- fessor Keller, in the fifth chapter, presents the question of evo- lution in the various institutions of human society, and, finally, Professor Conklin sets forth his views with regard to the trend, or future, of evolution. The large attendance at each of the lectures is sufficient evidence of the extreme interest in the question of the evolution of man, and it is hoped that the pub- lication of this book will stimulate an even greater interest in this very important subject. It is believed that the main scientific facts which bear upon the question are here presented from a modern viewpoint in an interesting as well as authori- tative manner. In conclusion, as President of the Society and editor of this volume, I desire to express my deep gratitude to the authors for their willingness to take the time and energy to prepare the lectures and to arrange their manuscripts for publication; to the officers, committees, and members of the Yale Chapter of Sigma Xi who by their enthusiastic cooperation made it pos- sible to carry through the year's program successfully; to my colleagues in the Department of Zoology and elsewhere in the University, who have shown their active interest in the project in many ways, and, finally, to the Yale University Press for the splendid attitude they have shown in all matters connected with the publication of this volume. GEORGE A. BAITSELL, President, Yale Chapter, Sigma Xi. 1921-1922. Osborn Zoological Laboratory, Yale University, July, 1922. FIG. IA. SKULL OF NEANDERTAL MAN From La Chapelle-aux-Saints. Restored by McGregor. R, portion restored. FIG. IB. SKULL OF CRO-MAGNON MAN From Grotte des Enfants. After Verneau. Reproduced by courtesy of Doctors J. H. McGregor and \V. K. Gregory, of the American Museum of Natural History. CHAPTER I THE ANTIQUITY OF MAN RICHARD SWANN LULL PROFESSOR OF VERTEBRATE PALEONTOLOGY, YALE UNIVERSITY THE Mosaic account of the creation, which has been and is yet of wide acceptance, would give us a very recent date for man's advent on this planet. The strictest interpretation of this account is that of Doctor John Lightfoot, a profound Biblical scholar, vice-chancellor of Cambridge University in 1654, who is often quoted because of the exactness of his findings. As a result of careful searching of the Scripture, Doctor Lightfoot was led to declare that "Heaven and earth, centre and circumference were made in the same instance of time, and clouds full of water, and man was created by the Trinity on the 26th of October 4004 B. c. at 9 o'clock in the morning." One questions, however, not the Scriptural account but the exactness of its interpretation. The researches of oriental scholars are bringing more and more into evidence the his- torical truth of the Old Testament narratives, and are estab- lishing from other lines of evidence the historical character of Abraham, Isaac, Jacob, and the other Hebrew patriarchs, but they are also tracing back into a more and more remote period the history of the Near Eastern peoples, as the result of the extensive excavations, with their treasure trove, which are being carried forward in these venerable abiding places of mankind. 2 THE EVOLUTION OF MAN Among the most interesting of these finds has been the bringing to light of several tablets, one of which is preserved in the Yale Babylonian Collection, and which constitute the oldest human documents thus far discovered. These several tablets are of black stone, of no very great size, but bear en- graven on their surfaces characters which give to us a message out of the past, the time of which antedates that of Christ by some 5,500 to 6,000 years; in other words, a thousand or more years before Doctor Lightfoot's date. Nor is this all, for the inscriptions are no longer in the so-called picture writing or ideographs, but in a form of writing undoubtedly derived from this. They have progressed so far along an evolutionary path- way that the original pictures cannot in some instances be even guessed at. This, it would seem, implies a centuries-long de- velopmental period before the beginning of inscriptive writ- ings, and the inference is also justifiable that the protoscript could not have been invented but by peoples of considerable intellectual powers who had long since emerged from savagery and were vastly further yet removed from their ultimate beginnings. The third line of evidence is cultural, based not on inscrip- tions or documents of any sort, but upon the implements and weapons of vanished peoples, with their varying degrees of refinement. Historic times, as is well known, are often spoken of as the Age of Iron, and perhaps the Age of Bronze, while the prehistoric is called the Age of Stone. But the Stone Age again has its subdivisions into, first, the New Stone Age or Neolithic period, in which the distinctive characteristic of the implements is that some of them at least were rubbed smooth or polished after the preliminary fashioning was completed. Back of this period lies the Paleolithic, varying immensely in the degree of perfection of use and workmanship, so that archeologists are agreed upon a number of cultures (see table, infra} , based upon distinctions some of which are evident THE ANTIQUITY OF MAN 3 to the most casual observer, while others are discernible largely to the expert only. Back of the Paleolithic lies the Eolithic, or Dawn Stone implement period, the definition of which originally implied that the stone implements of that time were not artifacts in the sense of purposeful manufacture, but that they were merely pieces of stone of convenient size which showed the effect of use. Later authorities admit of the presence of certain retouching on the margins of implements they still call eoliths, although just where they would draw the line between eoliths on the one hand and paleoliths of crudest workmanship on the other is not clear. Yet other authority would disclaim any human association with these eoliths, either of use or manufacture, and invoke the physical forces of nature to account for their seeming. But our purpose is not to tell again the arguments, but merely to establish, if possible, on the basis of such evidence, a further criterion for the test- ing of man's antiquity. It becomes at once evident, however, that all races of mankind are by no means in the same degree of cultural advancement at a given time, and that for ages of human history the relatively static and the highly advanced peoples must have dwelt contemporaneously in divers portions of the earth, just as, for instance, the native Tasmanians, of whom the last survivor died in 1877, were in a state of culture which some have called Eolithic and others a rather early stage of the Paleolithic, perhaps Mousterian (Osborn) . It becomes necessary therefore to confine ourselves to some definite region which has been the home of mankind from remote ages, in order to establish a chronological series. There is no doubt in my mind that in the course of time certain portions of Asia will provide us with a chronology of great interest and amaz- ing antiquity, but thus far our knowledge of European cultures is at once the most detailed and the most accurately dated, and the one with the fewest omissions in the series. There are readily applied checks on the European chronology, for it has 4 THE EVOLUTION OF MAN been synchronized very accurately with the successive advances and retreats of the ice in the Pleistocene glacial epoch, as well as with the recorded changes in the faunas of these several stages. Hence the European cultural chronology has become the standard for the world, and as it is further possible to synchronize the periods of glaciation in the Old World and in the New, a comparative chronology for the latter may some day be established. The appended table is based upon the highest authority, the American Osborn and the Belgian Rutot. Osborn feels convinced that Pliocene man is established by the Foxhall flints (see page 36), which he says are not Eolithic but Paleo- CHRONOLOGICAL TABLE CHRONOLOGY INDUSTRIES RACES OF MAN Postglacial time Magdalenian ) Upper Aurignacian ) Paleolithic Cro-Magnon Grimaldi ?Rhodesian Quaternary {Pleistocene) 4th Glacial time Mousterian ) Lower Acheulian ) Paleolithic Neandertal 3d Interglacial time 3d Glacial time Chellean ? Piltdown 2d Interglacial time 2d Glacial time Heidelberg 1st Interglacial time 1st Glacial time Cromerian fr O -r*"^ K> es • £ "2 *" V-H' IS *- ~ C O *5 es *" O | S" o u M o: .23 c ~ p ^^ ^ u E = •^" UJ 60 -2 60 u GO *-frH O z Ui crt X C r*i HUMA O CD B? c £ >, 03 O ed |S - _ O O C3 P . O ^U o f^> O O PH « *-H ^5 W D K O 4J O U •" ••*•* ca c O es 3 o (U ^o u ,S c c -2 s s O £* fa 53 E es THE NATURAL HISTORY OF MAN 45 man in the serial arrangement of the vertebras, the ribs, and the spinal nerves. In the lateral mesoderm a cavity develops which is the beginning of the body cavity, or coelom, and later contains the heart, lungs, and viscera of the abdomen. This cavity splits the mesoderm into two layers. The outer layer joins with the ectoderm to form the body wall, and the inner layer with the endoderm to form the wall of the ali- mentary canal which in time becomes entirely enclosed by the mesoderm and ectoderm of the body wall. The human embryo at this stage has acquired the characteristics of a typi- cal vertebrate. LATER PRENATAL DEVELOPMENT The Nervous System. The hollow neural tube early ex- pands at its anterior end into three sacs (Fig. 15), the primary cerebral vesicles, or as they are often called the fore-, mid-, and hind-brain, and later the first and third sacs partially sub- divide making altogether five enlargements. From the walls of these five sacs and from the remainder of the neural tube are developed all parts of the brain and spinal cord and nearly all parts of the cranial and spinal nerves which run to the peripheral regions of the head and body. The developing brain increases in length rapidly and bends in three places, the cervical flexure remaining permanently. A small sac grows out on each side of the hollow anterior brain vesicle and ulti- mately forms the retina of the eye (Fig. 15), which is thus a part of the brain projected to the surface of the head so the light can reach it. By thickenings, thinnings, and outgrowths, in different parts of the walls of the cerebral sacs, and by the development of myriads of nerve cells and fibers growing in many directions, the various parts of the brain are ultimately formed. Another sac grows out from each side of the fore- brain in front of the developing retina which (Fig. 15), in man especially, possesses the power of extensive growth. This sac expands in all directions but especially backward, spreading 46 THE EVOLUTION OF MAN over the rest of the brain tube, and ultimately forming the cere- brum, which is so large in man in comparison with the lower animals. All of this sac, except the lower part, as well as the parts of the adult brain formed from it, is known as the mantle, or pallium (Fig. 16), the olfactory portion of which is the archipallium since in evolution it is the oldest part of the brain, while the rest, being a more recent addition, is called the neo- pallium. The latter is most extensively developed in man and forms the major part of the cerebrum. The early condition of the neopallium in man represents about the extent of the pallium in the adult fish (Fig. 16). As it grows further backward it represents first the extent of the pallium in the next higher class of vertebrates, the amphib- ians, and later the extent in reptiles. Finally, as it begins to cover the cerebellum, we have the extent of the pallium as found in lower mammals and when it covers the cerebellum completely we have the human pallium. Thus we see that the pallium in its development in man passes through successively the various stages represented in the adult forms of the dif- ferent vertebrates, starting with the fish and terminating with the mammalian type (Fig. 16). Similarly the structural unit of the nervous system, the nerve cell, or neurone (Fig. 17), passes in its development in man from the very simple neurone of the fish through the increasingly complex forms of the vari- ous vertebrates to its greatest complexity in man. The sensory nerves (Fig. 18), which connect the skin and the various peripheral sense organs to the central nervous sys- tem, develop by a separation of serial groups of cells from the dorsal portion of the neural folds. From each of the cells in these groups two processes grow out. One grows into the central nervous system and the other pushes its way among the cells of the embryo until it reaches the sensory structures at the periphery. The motor nerves (Fig. 18) which control the muscles, on the contrary, grow out from cells situated in Diencephalon Pallium Mesenccphalon Cephalic flexure B Thai am us Pallium Optic cup Pontine flexure Mydencephalon — Meteti- ccphalon Corpus striatum Optic recess Hypothalamus Mesencephulon Isthmus Cerebellum Medulla oblongata FIG. 15. DEVELOPING HUMAN BRAIN From a 7 mm. embryo, showing the vesicles. A, lateral view; B, median sagittal section. The fore-brain consists of the telen- cephalon (region labelled pallium) and diencephalon ; the mid-brain consists of the mesencephalon, and the hind-brain of the metencephalon (cerebellum) and mye- lencephalon (medulla oblongata). The portion of the fore-brain which becomes the retina of the eye is indicated in A as the optic cup. From Prentiss and Arey, Embryology. By permission of W. B. Saunders & Co. THE NATURAL HISTORY OF MAN 47 the anterior part of the spinal cord and thread their way among the embryonic cells till they reach the proper muscle, where they break up into fine fibrils which become embedded in its fibers. Nerve fibers, therefore, are merely the very elon- gated processes of nerve cells. How these fibers are directed to their proper terminations among the myriads of cells, whether by chemical attraction or other force, and so seldom go astray, has never been determined. Before the nerves function they become surrounded by a translucent covering, called the myelin sheath, which is as- sumed at different periods during development, in the various parts of the spinal cord and brain. In general the myelin sheaths are assumed first by the peripheral sensory and motor nerves, thereby completing the reflex mechanism which in man occurs at about the sixth month of prenatal life. Following this the different groups of cells in the spinal cord are asso- ciated by the development of the myelin sheaths on the con- necting nerves, thus arranging for association of the reflex actions. Still later the nerves connecting the spinal cord with the cerebellum myelinate, thus completing the mechanism for coordination of movements. Then the afferent nerves, which connect the spinal cord with the brain, assume their myelin sheaths, thus preparing the pathway for sensory impressions. Not until after birth, however, are the efferent tracts connect- ing the cerebrum and spinal cord myelinated, thus placing the cerebrum in control of the parts below and completing the motor pathway. This late completion of the motor tracts in man explains the great helplessness of the human infant at birth, a condition which is in striking contrast to that in many of the lower animals. It is believed that the assumption of the myelin sheaths in the various association tracts of the cerebrum continues during the period of growth and perhaps until forty years of age. As no brain cells are ever formed after birth, the increasing myelination of the nerve fibers is the chief struc- 48 THE EVOLUTION OF MAN tural change in the brain that can be correlated with the educa- tional process. The Eye. The eye develops from three sources. The retina, as noted above, is a direct outgrowth from the fore- brain on the side of the head. The lens is an ingrowth from the ectoderm, and the sclerotic and choroid membranes, which form protective coverings enclosing the retina, are mostly dif- ferentiated from the mesoderm. Up to the sixth month of prenatal life a membrane stretches across the pupil of the eye which sometimes fails to disappear and thus causes blindness. Also the eyelids are grown together until about the same period, but in man, unlike the condition in certain other animals, such as kittens, a separation of the lids occurs before birth. The Ear. The internal ear forms as a saccular ingrowth from the ectoderm on the side of the head. This invaginated sac becomes greatly modified but eventually forms the semicir- cular canals, which preside over the function of equilibrium, and the highly developed cochlea which is concerned with hear- ing. The internal ear is regarded as a specialization of a part of the lateral line sensory organs of the lower vertebrates, which in the fish is concerned with balancing and movement only. The added cochlea, which is present in terrestrial animals, forms the organ of hearing. Alimentary Canal. The alimentary canal, as stated above, is early folded off from the yolk sac as a tube lying under the notochord of the embryo. Both ends of the tube are closed for a time, but long before birth an anterior and posterior connec- tion with the external surface of the embryo has been made. At the anterior end of the body on each side of the neck, four crevices (Fig. 14) appear in the early embryo which, in the lower vertebrates such as the fish, open directly into the pharyngeal region of the alimentary canal and form the gill clefts. In man, however, these crevices never go on to the 6 SELACHIAN (Torp«do) A.TELE05T (TwutJ £1 BIRO F- MAMMAL FIG. 16. THE PALLIUM AND CEREBELLUM IN THE BRAINS OF VARIOUS VERTEBRATES The cerebellum is in black. After Edinger. From Bailey and Miller, Embryology. By permission of William Wood & Co. THE NATURAL HISTORY OF MAN 49 formation of gill clefts but soon disappear. Their presence, however, is indicative of a fish stage in his development. The lungs, which are the permanent respiratory organs (Fig. 19) of man, are developed from the upper end of the alimentary canal by the formation of a single hollow sac. This later flan Frog Liz arc/ Rat Evolution of the cell of nerre ce// (mamma/) FIG. 17. THE NERVE CELL Its individual and evolutionary development. After Cajal. Redrawn from Donaldson, The Growth of the Brain. bifurcates to form the rudiments of the right and left lungs and these by repeated branching develop the highly ramified, tubu- lar structure of the adult lungs. The early simple, sac-like lung of the human embryo is similar in structure to the per- manent, saccular lung of the adult amphibians. From the neck region of the alimentary tube, three other organs develop as branching outgrowths, all of which finally form organs of internal secretion, namely the thyroid, the 50 THE EVOLUTION OF MAN parathyroid, and the thymus glands (Fig. 19). Another im- portant endocrine organ, the pituitary body, or hypophysis, has a double origin in that a portion of it develops as an upgrowth from the ectoderm which lines the extreme anterior end of the alimentary canal, or buccal cavity, and another por- tion, which fuses with the former, develops as a downgrowth from the brain. These glands, through their internal secre- tions, or hormones, influence development in various ways. All of them ultimately lose their connection with the ali- mentary canal. The thyroid gland moving down the ventral side of the neck stops just above the thorax. The thymus, which in the calf is commonly called the sweetbread, descends into the thorax until it lies just above the heart. Early in pre- natal life the glandular structure of the thymus disappears and a lymphoid tissue, such as we find in the tonsils, takes its place. This organ continues to grow until the child is about two years of age and then gradually diminishes in size, disappearing at about twenty years of age. This same remarkable history of the thymus is found in other mammals as well as in man. Little is known definitely with regard to the function of this organ. Lower down the alimentary canal, just below the spindle- shaped enlargement which represents the developing stomach, two other organs grow out and, by repeated branching, form the liver and pancreas (Fig. 19). These always remain in con- nection with the alimentary canal by ducts which carry their secretions. The pancreas develops as two outgrowths which fuse. Usually only one of the ducts persists. Occasionally it is found in man that both persist and the pancreas pours its secretion into the intestine by two ducts instead of one. The explanation of such cases undoubtedly lies in the double origin. The Vascular System. The heart differentiates from a por- tion of the mesoderm lying underneath the pharynx in the head region of the embryo. Two straight tubes are first formed (Fig. 20). These quickly fuse for part of their length form- - . - •••-"•^ 'A' '• . .. •" ••/-• FIG. 18. SPINAL NERVK Transverse section through the spinal cord of a 76-hour chick embryo showing early development of a spinal nerve. A, motor root; B, sensory root; C, bifurca- tion of fibers of sensory root; a, b, c, d, neu- rones in various stages of differentiation. After Cajal. From Bailey and Miller, Embryology. By permission of William Wood & Co. THE NATURAL HISTORY OF MAN 51 ing a single tube which is bifurcated at each end. At this stage of development, the human heart resembles that found in the adult of the lowest vertebrates. Later the single tube of the developing heart partially subdivides into two cavities, the auricle and the ventricle, and it now resembles the heart of the adult of the next higher vertebrates, the fish. The auricle is then subdivided into two cavities, and the human heart of three cavities resembles the fully developed heart of the next higher vertebrate, the amphibian. Later the ventricle is sub- divided and the human heart contains four cavities, which is characteristic of the adult heart of the highest vertebrates. Thus it is evident that the human heart in its development passes through stages representing the different adult stages of the various ascending vertebrate classes. This again is an illustration of the so-called Law of Recapitulation, which holds in essence that man in his individual development repeats the evolutionary history of the race, or that "ontogeny repeats phylogeny." The blood vessels, consisting of the arteries, capillaries, and veins, are tubes which differentiate from the mesoderm cells in all parts of the body and become connected with the heart. The blood cells, which are present in countless numbers in the blood, likewise are derived from mesoderm cells, and we find that the red blood cells of the human embryo when first formed are large and nucleated. In this stage they resemble those of the fishes and amphibians; later their structure is similar to those of the reptiles. Finally, before birth, they become in man, as in all mammals, non-nucleated and biconcave. The complete natural history of the human red blood cor- puscles has never been learned. It is known that they are con- stantly formed in the adult from the cells of the red marrow in the ends of the long bones and that they are nucleated at first but lose their nuclei before they enter the circulation. Although it is believed that the corpuscles are constantly being 52 THE EVOLUTION OF MAN destroyed, little is known as to where or how the destructive process occurs. Sometimes they seem to be destroyed much faster than they are formed and this results in a great reduc- tion of the number of the red blood cells of the body from Middle lobe of thyroid gland. Thymus gland. Lateral lobe of thyroid gland. Trachea. Lung. Right lobe of liver. Vitelline duct. Pharyngeal pouches. Stomach. Pancreas. Left lobe of liver. Small intestine. Large intestine. FIG. 19. ORGANS DEVELOPING FROM THE RUDIMENTARY ALIMENTARY CANAL After Bonnet. From Heisler, Embryology. By permission of W. B. Saunders & Co. 5,000,000 to perhaps 1,000,000 per cubic millimeter, together with the appearance also of a certain number of the embry- onic type of nucleated red cells in the blood stream. Evidently the immature cells are drawn into the blood stream in the endeavor to make good the excessive loss. This is the condi- tion which exists in that quite fatal disease, pernicious anaemia. THE NATURAL HISTORY OF MAN 53 Reproductive System. The gonads, in which the germinal cells form, are developed in the body cavity just at the lower end of the kidneys, probably from mesodermal cells, and early begin to move downward in the abdomen. In the female they go as far as the pelvic cavity where they remain. In the male they pass through a canal in the lower part of the abdominal wall, called the inguinal canal, to the exterior of the body. Similar migrations of the gonads occur in most mammals. The reasons for these migrations are not known, but the inguinal canal remains as a weak spot in the abdominal wall and under special stress may permit loops of the intestines to pass through, thus forming a hernia. This is the penalty man is still paying for the lack of structural adjustment to the upright position. The external genital organs develop in a similar way in both sexes and no structural distinction is to be observed until near the beginning of the third month of fetal life. At this time the indifferent sex condition begins to develop into one sex or the other by the greater growth of some parts and the partial suppression of others, depending upon the sex to be formed. In rare cases the indifferent condition of the sex organs may persist permanently in either sex, a condition known as spurious hermaphroditism, which has particular in- terest to criminologists and sociologists. The urinary system develops in close relationship to the genital system and, before the permanent adult condition is reached, passes through a remarkable series of changes, many of which are typical of the permanent adult condition in the lower vertebrates. The Muscles. As noted above, the mesoderm, which lies along each side of the developing spinal cord, becomes trans- versely divided into a linear series of segments (Fig. 14). This process begins when the embryo is about two weeks old and continues until a total of thirty-eight segments are formed which extend the entire length of the embryo. From these 54 THE EVOLUTION OF MAN segments are developed all the muscles of the body, the axial part of the skeleton, and the membranes surrounding the brain and spinal cord. Although the muscles are at first FIG. 20. THE HUMAN HEART Diagram showing its development and changes of form and external appearance at different stages. Modified from His's models. Figs. IIIB and IVB are side views; the others are front views, a, primitive ventral aorta; b, bulbus cordis; c, ventricle; c', left ventricle; d, auricle; d', left auricle; e, sinus venosus; f, atrio-ventricular canal ; g, position of orifice of atrio- ventricular canal; h, vitelline vein. Redrawn from Cunningham, Anatomy. THE NATURAL HISTORY OF MAN 55 separate masses, by a process of fusion of the segments and then a later splitting, either lengthwise or tangentially, the long, flat muscles in the front and back of the body are formed. Also many muscles shift from their early positions to some quite distant place. This is the case, for example, with the muscular diaphragm, which forms in the neck and is supplied by a nerve from this region but which, during development, moves posteriorly dragging its nerve with it, until in the final position it separates the thorax from the abdomen. All of this differentiation and shifting of muscles has been com- pleted some time before the child is born and therefore is not produced by any functional necessities after birth, but is due to heredity. The Limbs. Until the beginning of the fourth week of de- velopment, man is a limbless vertebrate (Fig. 23). The rudi- ments of the limbs then grow out as buds from the side of the body and, as they elongate, five projections appear at the end of each of the buds which represent the developing fingers and toes. This peripheral cleavage into five parts perhaps is representative of the five main segments of the body from which the limb projects. Inasmuch as the five-fingered, or pentadactyl, limb is the common vertebrate type it is believed that polydactylism, or an extra number of fingers or toes, is not a reversion to an ancestral form but is due to an unknown cause. The Skeleton. The skeleton is preceded in very early pre- natal life by two kinds of material, cartilage and membrane. While most of the bones of the skeleton are ossified from car- tilage, the bones of the cranium are largely developed from membrane. The cranial bones begin ossification at their centers near the end of the second month of prenatal life, the growth of the cranium being due chiefly to additions at the edges of the bones. In the long bones, like the femur and humerus, which are mapped out in cartilage, the centers of 56 THE EVOLUTION OF MAN ossification appear about the eighth week of prenatal life in the center of the shaft, and, at birth, while the shaft is bone, both extremities are still cartilaginous. The Skin. The outside portion of the skin, or epidermis, is derived from the ectoderm, while the deeper portion, or dermis, is developed from the mesoderm. The human skin at the end of the second prenatal month is translucent and has many points in common with that of fishes and amphibians. In the third month a delicate, superficial, horny stratum ap- pears, a stage which has been held to represent the evolution from an aquatic to a terrestrial form of life. Hairs are outgrowths from the epidermis and are developed in groups and lines. Their arrangement can best be explained on the supposition that originally the skin was covered by scales and that the hair grew out in groups at their tessellated junctions as is found in certain of the edentates. The skin of man, in comparison with that of the other primates, is com- paratively hairless, which is probably a recently acquired character. At the seventh month of prenatal life the chim- panzee and gorilla have well-developed hair on the scalp, eye- brows, and lips, while the rest of the body is covered with fine hair. This is also the condition of the human fetus at a corre- sponding period. The hair slopes in man are also very similar to those of the apes. Some hairs, such as those present in the eyebrows, perhaps originally had a sensory function and, in general, hair appears to be a modification of certain glandu- lar and sensory structures found in the skin of the amphibians. Evidence has accumulated which shows that the development of the hair is regulated, at least to some degree, by an internal secretion of the thyroid and sex glands. Face and Nose. The face is developed from a series of paired processes surrounding the primitive mouth (Fig. 21). These grow in from the sides toward the mid-ventral line where they normally fuse by the third prenatal month, by a FIG. 21. DEVELOPMENT OF THE FACE OF A HUMAN EMBRYO (His.) From Prentiss and Arey, Embryology. By permission of W. B. Saunders & Co. THE NATURAL HISTORY OF MAN 57 method somewhat akin to the healing of wounds, to form the nose and lips. A failure of proper fusion of these processes results in harelip and various other facial deformities. In the development of the human nasal cavity a recapitula- tion of the conditions present in the various classes of verte- brates is to be noted. At the end of the third week of fetal life the rudiments of the olfactory organ appear as two thick- ened plates of ectoderm which are in contact with the under surface of the fore-brain. A week later these plates become depressed and are found at the bottom of the olfactory pits. This condition is similar to that found in fishes. Two weeks later the depression has deepened and a nasal cavity is formed which is somewhat similar in structure to that found in the air-breathing or lung fishes. By the seventh week this primi- tive nasal cavity has rapidly enlarged and a communication has been made with the mouth as in amphibians. The mam- malian condition is reached by the third month. At this time the roof of the mouth is completed by the growth to the center and fusion of two lateral shelves. A failure of these shelves to unite produces cleft palate. The teeth are formed by local outgrowths of the ectoderm and mesoderm lining the mouth, the enamel being formed from the ectoderm, and the dentine and pulp from the meso- derm. The human molars with their several cusps have evolved from the simple conical teeth of fishes and reptiles either by a process of partial fusion or by an outgrowth of cusps from the conical tooth. The Prenatal Nourishment of the Embryo. During pre- natal life the fetus is enclosed in a fluid-filled sac, the wall of which is composed of two membranes, the inner called the amnion and the outer the chorion, which serve for protection and nutrition (Fig. 12). The development of an embryo in nt-ero is dependent upon its ability to secure nourishment and to eliminate certain wastes. In order to do this it must get 58 THE EVOLUTION OF MAN into contact with the maternal blood supply in the walls of the uterus. It appears that, in this early stage, the outer layer of embryonic cells is able to secrete a ferment, or enzyme, which actually eats away, or digests, a portion of the uterine wall with which it is in contact. This process furnishes the embryo with a temporary supply of food which can be ab- sorbed by the cells and also enables it to imbed itself com- pletely in the uterine wall where it is surrounded with extra- vascular blood, the result of the erosion of maternal blood vessels. In a short time a certain region of the outer layer of cells together with the underlying chorion becomes modified to form a highly specialized structure, the placenta, through which an interchange of materials between the mother and embryo can take place. The fetal placental tissues become actually fused with the uterine wall (Fig. 22), and the under- lying tissue of the latter is gradually eroded to such an extent that the ringer-like processes, or villi, of the fetal tissues of the placenta, which project into the maternal tissue, are sur- rounded by intervillous spaces of considerable size which are filled with the maternal blood (Fig. 22). Although the pla- cental villi are richly supplied with fetal blood vessels there is never any direct connection between the maternal and fetal blood vessels in the placenta, and no nerves pass from the mother to the child. The interchange of materials, oxygen and food from the maternal to the fetal circulation, and carbon dioxide and liquid metabolic wastes from the fetal to the maternal, all takes place by osmosis and specific selection. The structure of the placenta in the different mammals shows considerable variation. That found in the anthropoid apes is very similar to the human type. The maternal and fetal blood streams are separated by the embryonic tissue of the placental villi (Fig. 22). This separa- tion can be proved by the microscopic examination of the con- tents of the fetal vessels in the placental villi and the maternal FIG. 22. THE RELATION OF THE FETAL TO THE MATERNAL BLOOD IN THE PLACENTA Arrows indicate the supply and exhaust of the maternal blood in the large intervillous spaces. a, sinus of uterine vein; b, muscle tissue of uterine wall; c, uterine vein; d, uterine artery; e, sinus of uterine vein; f, decidua basalis; g, uterine artery; h, intervillous space filled with maternal blood; i, syncytium; j, villus; k, umbilical vein (fetal) ; 1, umbilical artery (fetal) ; m, uterine artery. From Prentiss and Arey, Embryology. By permission of W. B. Saunders & Co. THE NATURAL HISTORY OF MAN 59 blood in the placental sinuses in the early stages of develop- ment. Many nucleated red corpuscles which are found only in the blood of the fetus will be seen in the vessels of the placental villi but there are none in the placental sinuses. Further, the vessels in the placental villi can be injected from the fetal but not from the maternal blood vessels. That gases can be transmitted from the maternal to the fetal circulation is shown by the fact that the blood in the um- bilical vein, which carries blood from the placenta to the fetus, is redder and contains more oxygen than the darker blood of the umbilical artery, which carries blood from the child to the placenta. Also ether or chloroform administered to the mother can be demonstrated in the blood of the child. Substances in solution also can pass from the maternal to the fetal blood. This is indicated by the fact that the child grows. Also it has been experimentally demonstrated in the case of many drugs that when they are administered to the mother they will be found in the blood of the child. Among these drugs are bromide of potassium, arsenic, strychnine, quinine, and morphine. The work of recent observers seems to show that fats and proteins are not passed unchanged through the walls of the placental villi from maternal to fetal blood, but are broken down into simpler compounds and later recombined, thus mak- ing the process of absorption in the placenta somewhat analo- gous to that occurring in the intestine. The placental tissue apparently has not only the power to change some substances but also the power of specific absorption. Proteolytic, lipo- lytic, and glycolytic ferments have been demonstrated in the placental villi, also a ferment which changes certain of the amino acids into ammonia. Observers now generally agree that, while the placenta usually acts as an efficient filter against bacteria, occasionally bacteria may be transmitted from mother to child as has been 60 THE EVOLUTION OF MAN known to occur in some cases of typhoid fever and, in rare instances, of tuberculosis. On the contrary, it has been demon- strated that the toxins and antitoxins of diphtheria, tetanus, and typhoid fever readily pass from the maternal to the fetal blood stream in the placenta. Furthermore, biologists agree that there is no foundation for the more or less general belief that maternal impressions are responsible for malformations of a child. It can be understood, however, that a general dis- turbance of the fundamental metabolic processes in the mother may result in similar disturbances in the fetus and thus may interfere with the normal 'developmental processes. Birth. It has been seen that the human embryo is a para- site which engrafts itself on the maternal tissues and by the remarkable and highly specialized organ, the placenta, is supplied with nourishment and relieved of its wastes. This connection is normally retained for a period of approximately nine months, at which time the 'host,' for reasons largely un- known, refuses longer to support the 'parasite.' Rhythmic contractions begin in the walls of the uterus. These increase in strength and frequency and finally result in the birth of the child. In separating the placenta and tearing the large ma- ternal vessels of the uterus, without causing an excessive hemorrhage, nature performs a wonderful surgical operation. The moment the child breathes and the lungs expand, the course of the circulation is changed. This change is due to the expansion of the lungs and the consequent large flow of blood to them. As the result of this, the opening between the two auricles (foramen ovale) closes, the course of the circula- tion in the heart is also changed and the blood, which was formerly aerated in the placenta, is now aerated in the lungs. Sometimes this interauricular opening fails to close in the proper manner and this permits a mixture of venous with arterial blood which causes a dark color in the skin and, there- fore, what is commonly known as a 'blue baby.' THE NATURAL HISTORY OF MAN 61 In the prenatal condition the child needs to produce very little heat because so little is radiated and the mother supplies all of the necessary food and oxygen and removes all the wastes. After birth the radiation of heat is greater and the child is obliged to produce much more in order to maintain its body temperature. The alimentary canal must take in and digest its own food; through its lungs the child must obtain oxygen and free itself of carbon dioxide, and its excretory organs must remove its own wastes. It is evident, from the above, that there is a great contrast between the prenatal and postnatal environment, and the fundamental changes in circulation, nutrition, and excretion which occur at the time of birth make this period a very critical one for the child. At birth the child weighs about seven pounds, and is twenty inches in height. Its upper extremities are relatively long, and the lower extremities short. The legs are partly flexed, the great toe abducted and the soles of the feet turned in. The latter is believed to be indicative of a persistence of the climb- ing position which existed in arboreal man. Additional evi- dence along this line is to be found also in the remarkable power of the hand grip of the child during the first month of its life. At this time the infant is able to hold its weight sus- pended by its hands, a power which is later lost. When the body proportions of the newly born child are compared with those of the adult it is found that the child is four times its head height while the adult is eight times; that the upper and lower extremities of the infant are equal in length while in the adult the lower extremities are longer than the upper. The long arms, short legs, flat nose, inverted feet, and lack of cervical and lumbar curves in the spinal column of a newly born infant constitute a remarkable series of structural re- semblances to the ape which later disappear as the adult condi- tion is reached. The child at birth can feel, see, taste, and suffer pain but it is deaf for about twenty-four hours. Its 62 THE EVOLUTION OF MAN most striking characteristics, in contrast to the lower animals, are its helplessness and educability. Its helplessness has un- doubtedly been an important factor in the development of the family, while the long period of growth before maturity is reached has exerted a great influence in the development of man's psychic powers, which distinguish him so markedly from all other animals. Statistics of various animals show that the proportion of male and female births is approximately equal; the ratio in man being about 106 males to 100 females. Multiple births are the usual rule in most of the lower mammals but not in the primates. Occasionally, however, it occurs and in man even sextuplets have been recorded. There have been some 500 theories proposed as to the cause of sex determination and as to methods for controlling it, but the accumulated evidence of the last few years clearly shows that sex is determined at the time of fertilization, just as are all somatic characters, and that it cannot later be changed by any means. Inasmuch as the ossification of the bones of a female embryo precedes slightly that of the male embryo, a radiograph may perhaps aid in ascertaining the sex of the child before birth. If we summarize the facts regarding man's early develop- ment, which are given above, it is evident that he starts his development as a unicellular animal, becomes a metazoan of the invertebrate type, develops the structure of a vertebrate, and ultimately becomes a mammal. Many of the structures of his body pass through temporary stages which are typical of the adult condition in various classes of the vertebrates and, in general, his organs are developed in a manner similar to that in other animals. Man's development, however, parallels that of the anthropoid apes for a longer time than that of other mammals and at birth the child has ape-like characters that later disappear. These facts of human development associate man closely with the anthropoid apes and give evi- PIG RABBIT MONKEY MAK A FIG. 23. EMBRYOS OF THE PIG, RABBIT, MONKEY, AND MAN Comparison at corresponding stages of development. The embryos of each animal are arranged in the vertical columns according to age, beginning with the youngest stage at the top. Stage A of the human embryo is full}* labelled, and the corresponding structures in the other embryos can be noted. a, head region; b, eye; c, ear; d, gill slits; e, heart; f, fore limb; g, primitive muscle segments: h, hind limb; i, tail region. Slightly modified from K. (nienther, after Keibel. THE NATURAL HISTORY OF MAN 63 dence that the latter reproduce, in many respects, a compara- tively recent phase in the history of human evolution. POSTNATAL DEVELOPMENT In the final analysis, growth in any living organism is the result of the division of a cell into two daughter cells each of which, when first formed, is half the size of the original cell. The daughter cells very soon, by the process of intussuscep- tion which undoubtedly involves complex physico-chemical processes, grow to the size of the original cell and then the process can be repeated. In prenatal life, the embryonic cells, in general, divide comparatively rapidly and, as a rule, are not as large as in postnatal development. The increase in size after birth is believed to be due primarily to a general increase in the size of the cells of the body rather than to the formation, by cell division, of additional cells. Development is therefore accomplished by cell division, cell enlargement, and cell differ- entiation. The ultimate size of an animal depends both upon the rate and the duration of growth. Minot has shown that man becomes larger than the rabbit, not because of a more rapid growth, but because he grows for a longer period of time. The rabbit, on the other hand, becomes larger than the guinea pig because of a more rapid rate of growth. Large animals, with few exceptions, continue to grow during a longer period and live longer than do small animals. Statistical studies show that the rate of growth is most rapid in early prenatal life and steadily diminishes until birth. During the prenatal period of the human embryo the weight increases more than 5,000,- ooo times, while after birth the increase is only 20.6 times. As the rate of growth is a constantly decreasing one, its cessation seems to be the final term of a diminishing series. During the most rapid period of growth the cells have large nuclei but, with advancing age, the cytoplasm relatively 64 THE EVOLUTION OF MAN increases and this change in the relative size of nucleus and cytoplasm constitutes the chief structural difference between the cells of the young and of the old. A decreasing rate of growth is also accompanied by a diminution in the water content of the body, as is shown by the fact that in the first month of prenatal life the percentage of water in the body is 97.5 per cent, at birth it has become 74.7 per cent, and at maturity 58.5 per cent. While man and the higher vertebrates have a definite period of growth some fishes and amphibians continue to grow throughout life. The forces which stimulate or inhibit growth are largely unknown. Evidence is accumulating to show that certain of the internal secretions, notably those given off by the thyroid and pituitary glands, play a large part in the con- trol of growth. It is believed that too much secretion of the latter may increase the growth of local areas of the body such as the hands and face. This condition is to be seen in the rare disease known as acromegaly. On the other hand, a dimin- ished secretion of the thyroid results in a failure of growth of the entire body as well as a lack of mental development, a condition known as cretinism. Various factors such as un- favorable climatic conditions, poor food, lack of proper pro- teins and vitamines, and severe illness retard growth. However, these factors do not offer any explanation as to why most animals stop growing aft'er reaching the size of the species. A mouse never grows to the size of an elephant, nor does an elephant remain as small as a mouse. It is apparent that there is a fundamental hereditary factor involved which in some way, possibly through the internal secretions, controls growth. That this hereditary influence has not always re- mained constant is shown in the size variation of the same species in different geologic ages. This is notably shown in the evolution of the horse. The question of growth is closely associated with the power of regeneration, that is, the ability THE NATURAL HISTORY OF MAN 65 of an animal to replace lost parts. The power of regeneration in an animal, in general, is found to be in inverse ratio to the degree of specialization, or differentiation, which it exhibits. For example, among the lower vertebrates, the amphibia are able to regenerate entire limbs, whereas in the more highly specialized animals such as man this power of regeneration is present only in certain tissues, notably the connective and epithelial, by the agency of which wounds are healed. Cancer is generally regarded as a localized lawless and unrestrained growth of epithelium, the cells having become parasites and attacked the host. The only cure thus far discovered is an early destruction or removal of the abnormal parasitic cells. The causation of cancer apparently lies in the disturbed bal- ance of the forces stimulating and restraining growth in the affected cells and is probably essentially a faulty cellular chemistry. The child sits up by the sixth month, creeps by the tenth month, and walks by the fifteenth month, thus passing from a quadrupedal gait to the erect position in a few months, an accomplishment which in evolution may have occupied ages. All parts of the child do not grow at the same rate and, as a result, the body proportions continually change during growth (Fig. 24). Even the shaping of the features is due to the different rates of growth of the various parts of the face. Comparing the condition at birth with that of maturity it is found that while the head doubles in height, the body increases three times, the upper limbs four times, and the lower limbs five times. These changes in the body proportions are well shown if, with the adult proportions, we compare the pro- portions of the child when expanded to the height of the adult. Such a comparison will show, as has been previously noted, that the head of the adult is relatively smaller, the arms shorter, and the legs longer (Fig. 24) . Likewise it is known that various organs grow at different 66 THE EVOLUTION OF MAN rates with the result that they bear a different ratio to the total body weight at different ages. Here is a field that needs much further study inasmuch as this changing relationship in size may have an important bearing on the incidence of disease at different ages. The studies which have been made on the 2 mo. (fetal) 5 mo. Newborn 2 yrs. 6 yrs. 12 yrs. 25 yrs. FIG. 24. CHANGES IN PROPORTION DURING PRENATAL AND POSTNATAL GROWTH After Stratz. From Morris's Human Anatomy. By permission of P. Blakiston's Son & Co. relative growth of the different organs show that the same proportionate weight of the skeleton, fat, and skin, which is about 39 per cent of the total weight of the new-born, persists in the adult; that the relative weight of the organs of circula- tion, respiration, and alimentation is about twice as large in the new-born as in the adult; that the weight of the muscles of the new-born is relatively one half as large as in the adult, and, that the central nervous system of the new-born is relatively eight times as large as in the adult. It is possible that the very rapid growth of the brain in early childhood accounts in part, at least, for the nervous instability of that period. Considerable data have accumulated with regard to the growth of bone. Membrane bones, such as certain bones of the THE NATURAL HISTORY OF MAN 67 cranium, grow by additions along their edges which form the sutures. Beginning at about forty years of age, when the brain stops growing, the cranial sutures disappear. In the long limb bones in early postnatal life, one or more centers of ossifica- tion appear in the terminal cartilages and form caps of bone. The growth in length occurs in the cartilage under the bony caps. This was shown experimentally many years ago by John Hunter, who inserted shot in the shaft and in the end cap of a femur in a young animal, and later found that the dis- tance between them had increased. Usually by the twenty-first year these caps have joined to the shaft thus making any further growth in length impossible. The growth in the cir- cumference of a bone takes place by additions under the outer fibrous membrane, known as the periosteum. This has been shown experimentally by feeding growing animals with madder, which gives a yellowish stain to the new bone. The child, like the young of all mammals, is born without teeth. The first set, twenty in number, begins to erupt about the sixth month after birth and is completed when the child is nearly two and one half years of age. During the next few years the permanent teeth, thirty-two in number, are forming beneath the temporary teeth and also behind them, and, by the absorption of their roots, replace them. The first permanent tooth to erupt is the first molar which appears behind all the temporary teeth at the sixth year. By thirteen years of age all of the permanent teeth have erupted except the third molars, or wisdom teeth, which appear about the seventeenth year. The latter may be small in size, be abnormally placed, appear late, or even fail entirely to appear. A study of the growth curves for height and weight (Fig. 25), which have been plotted by averaging the results obtained from weighing and measuring many children at various ages, but mostly over five years of age, shows a general agreement of results. There is a loss of weight for the first few days 68 THE EVOLUTION OF MAN after birth, the postnatal retardation. During the first year, growth in both weight and height is rapid — the boy increasing AGE o 5 10 is YEARS SO 25 70 60 50 3O .-VX Males Females I4O too 80 60 FIG. 25. GROWTH CURVES FOR INCREASE IN WEIGHT AND STATURE FOR BOTH SEXES From Donaldson, after Roberts. THE NATURAL HISTORY OF MAN 69 more rapidly than the girl — then the rate of growth goes on more slowly up to seven years of age. This is followed by a more rapid increase in growth rate up to about seventeen years in the boy and sixteen years in the girl, with a prepubertal acceleration present in both sexes. The growth rate then de- creases to twenty-five years of age, at which time growth prac- tically ceases. During the early years the boy is slightly taller and heavier than the girl but, owing to an earlier prepubertal acceleration in the girl, the two sexes are equal in height at twelve years of age and for the next three years the girl is the taller. Simi- larly, at thirteen years of age, the two sexes are of equal weight and for the next three years, the girl is the heavier. After fifteen years of age the boy surpasses the girl in height and after sixteen years in weight. The average height for the people in the United States is about five feet and eight inches for men, and five feet and four inches for women. On the other hand some of the dwarf races of central Africa scarcely attain four and one half feet in height. The average weight of the male is 150 pounds and of the female 125 pounds. Puberty. At puberty certain marked physical and psychic characters, known as the secondary sexual characters, develop which quite clearly distinguish the sexes. These changes, as experimental work on the lower animals has shown, are brought about by the internal secretions of the sex glands and can be prevented by their early removal. Also some of the sexual characters in the human female may be modified to somewhat resemble the male after the climacteric when the internal secretions of the sex glands diminish or cease. In the male the growth in weight and height is usually greater, the brain and face grow larger, the vocal cords elongate rapidly, which causes the pitch of the voice to drop an octave, the beard develops, the muscles become larger, giving a more angular appearance to the surface of the body in marked contrast to 70 THE EVOLUTION OF MAN the curves of the female, the waist is broader, the pelvis smaller, the shoulders less sloping, and the hands and feet larger. SENESCENCE The normal life cycle has three phases, development, ma- turity, and decline. Each of the first two phases lasts about thirty years and the third should be at least of equal length. But this may be greatly exceeded as shown, for example, by the longest life yet recorded in modern times; that of Thomas Parr of England who was one hundred fifty-two years and nine months old when he died. Old age in itself is a physiological condition which results from a state of lessened vitality and activity of the various organs. The essential factor in grow- ing old is a progressive degeneration of the tissues, as a result of changes in the cells which compose them, thereby bringing about a gradually diminishing functional capacity of the various organs until, finally, the activity of some vital organ or organs ceases and physiological death results. Certain visible external structural changes in the body are increasingly in evidence as old age comes on, all of which are due funda- mentally to degenerative changes in the individual cells of the various tissues. The hair turns gray, the skin loses its elas- ticity and becomes wrinkled. Extensive changes occur in the jaws which are due to a loss of the teeth, with a consequent absorption of that part of the jaw which held them, resulting in a return to the infantile type of jaw. The walls of the arteries harden (arteriosclerosis) and the blood pressure in- creases. There is a progressive loss of strength and elasticity in the muscles. Some of the cartilages may calcify, the bones become more fragile, the cranial sutures disappear, and there is a decrease in the stature. The near point of vision recedes, due to a loss in elasticity of the mechanism of accommodation. Changes also take place in the nervous system. These are THE NATURAL HISTORY OF MAN 71 associated with a shrinkage of the brain, due essentially to the atrophy of nerve cells in the cerebral cortex, and bring about a loss in muscular coordination and in memory, and a lack of acuteness in the senses. Normally, physical decadence occurs earlier than mental. It is very difficult to distinguish between normal senile de- generative changes and those due to disease. For example, it seems impossible at present to tell whether the hardening of the arteries is to be considered normal or abnormal in old age. Also we know very little of the chemical alterations in the body which are associated with advancing age, except that a relative increase in the salt content has been found. In fact, the whole subject of senescence needs renewed investigation to determine what constitutes normal senile changes. The striking experiments of Steinach and others have shown, in the case of both the lower animals and man, that at least a temporary rejuvenescence can be produced in the old by en- grafting the gonads or, in the male, by ligating the vas deferens. There are three principal theories with regard to the bio- logical significance of old age and natural death. The first theory is that old age is a pathological condition due to lack of internal adjustment to external environment. Metchnikoff was an advocate of this and held that there is a constant con- flict between certain cells of the body, some trying to destroy the others which are essential to life. This they may accom- plish if the cells acquire a lessened resistance as a result of the absorption of toxins released by certain bacteria in the intestines. Hence another species of bacteria, Bacillus bul- garicus, by interfering with the growth of the toxin-producing bacteria, becomes, according to Metchnikoff, a "fountain of youth." According to the second theory, which was advocated by Weismann, the higher organisms have acquired senility and natural death through natural selection as a character which 72 THE EVOLUTION OF MAN is of advantage to the existence of the species. The third theory, which was advocated by Maupas and Minot, holds that, at fertilization, a stimulus is supplied to the developing organism which is gradually dissipated, during the later growth and differentiation, until finally none is left and the organism dies of old age. As the phenomenon of old age is a common one to all metazoan animals it would seem that the third theory is more in accord with the facts. Biologists have estimated that an animal should live from five to seven times the period of growth. This means that man should live to be from one hundred to one hundred and forty years old. The facts in regard to different animals, how- ever, do not seem to support this view. Rubner has estimated that man requires about four times as many food calories for the period from the cessation of growth till the end of life as other mammals and he interprets this as meaning that the hu- man cells have a much greater total capacity for obtaining energy from foodstuffs than those of other mammals. The cells seem to be able to make only a limited number of chemical transformations, after which physiological death ensues. Rub- ner believes that the cells in man can make a greater number of such transformations than those of most other mammals. VESTIGIAL STRUCTURES The normal life cycle of man having now been considered, attention should be given to another valuable line of evidence as to man's origin, namely, certain structural features known as the vestigial organs which, although practically functionless in man at the present time, are believed to be the remains of well-developed, functional organs in the past. A few typical examples may be noted. At the inner corner of the eye is a fold of the conjunctiva, the mucous membrane covering the front of the eyeball, called THE NATURAL HISTORY OF MAN 73 the plica semilunaris, which is relatively larger during prenatal life. It is believed to be a reduced third eyelid, such as is regularly found in amphibia and birds, by which the eye is closed. Man possesses muscles which are arranged to move the ear either as a whole or in part, but he has entirely lost the con- trol of them or only exceptionally retains partial control. Probably when man assumed the erect posture with eyes look- ing forward the head became more movable and rendered unnecessary the mobility of the ear with the resulting loss of function in its muscles. At one period during prenatal de- velopment the human ear is pointed and resembles the ear of certain monkeys. This point persists in many people as a projection on the rolled or unrolled rim (helix) of the ear. It is known as Darwin's tubercle and is the vestigial remains of man's ancestral ear point. Near the junction of the large and small intestine in man (Fig. 26) there is a narrow, blind process, about three and a half inches in length, known as the vermiform appendix. The appendix in man is a vestigial structure and represents the func- tionless, shriveled, terminal remains of the caecum, the blind beginning of the large intestine. In an herbivorous animal, the caecum is a large, nutritive organ of great importance. In carnivorous animals, the caecum is reduced. The reduction of the terminal portion of the caecum to form an appendix occurs only in man (Fig. 26), the anthropoid apes, and some rodents. The frequent pathological condition of the appendix in man has given rise to the aphorism "that vestigial structures are particularly prone to disease." There are various structures in the body which, although not vestigial, give evidence of a retrogression. Such a condi- tion is to be noted in certain of the sense organs. The olfac- tory organ, for example, which is of the greatest importance to the lower mammals and other classes of the vertebrates, 74 THE EVOLUTION OF MAN has become very much reduced in man and is represented only by a small part of the nasal mucous membrane. In corre- spondence with this the olfactory portion of the brain is also very much reduced. Other structures showing signs of retrogression are the c aecum