We must not underestimate the many difficulties to be encountered, for the field before us is a vast territory of complex human life and of manifold human relations. Without prolonged exercise in scientific methods, it is impossible to view our own kind impersonally, as we do the creatures of lower nature. Furthermore it seems to many that an a.n.a.lysis of human life and biological history, even if it is possible, must alter or degrade mankind in some degree; this is no more true than that a knowledge of the principles of engineering according to which the Brooklyn Bridge has been constructed renders that structure any different or unsafe for travel. Man remains man, whether we are in utter ignorance of his mode of origin, or whether we know all about his ancestry and about the factors that have made him human. It is because our species appears to occupy a superior and isolated position above the rest of nature that the mind seems reluctant to follow the guidance of science when it conducts its investigations into the history of seemingly privileged human nature. And it is feared also, that if evolution is proven for man as well as for all other kinds of animals, our cherished ideas and our outlook upon many departments of human life must be profoundly affected. This may be so, but science endeavors only to find out the truth; it cannot alter truth, nor does it seek to do so. We might well wish that the world were different in many respects and that we were free from the control of many natural laws besides that of evolution, but if the real is what it is, then our duty is plain before us; as we think more widely and deeply on the basis of ripened experience, it becomes ever clearer that a knowledge of human history gives the only sure guidance for human life.
To the zoologist it seems strange that so many are opposed to a scientific inquiry into the facts of human evolution, and to the conclusions established by such an inquiry,--though, to be sure, this opposition is directly proportional to ignorance or misunderstanding of the nature and purpose of scientific investigation and of human evolution. The naturalist comes to view our species as a kind of animal, and as a single one of the hundreds of thousands of known forms of life; thus the question of human origin is but a small part of organic evolution, which is itself only an episode in the great sweep of cosmic evolution, endless in past time and in the future. Were we some other order of beings, and not men, human evolution would appear to us in its proper scientific proportions, namely, as a minute fraction of the whole progress of the world.
While the foregoing statements are true, it is nevertheless right that a close study should be made of the particular case of mankind. No doubt much of the naturalist"s interest in nature at large is due to his conviction that the laws revealed by the organisms of a lower sphere must hold true for man, and may explain many things that cannot be so clearly discerned when only the highest type is the subject of investigation. It is only too evident that little more than a general outline can be given of the wide subject or group of subjects included under the head of human evolution. We must divide the subject logically into parts, so that each one may be taken up without being complicated by questions relating to topics of another category, although the findings in any one department must surely be of importance for comparison with the results established in another section; for if evolution is universally true, the main conclusion in any case must a.s.sist the investigation of another, just as comparative anatomy and embryology supplement and corroborate each other in the larger survey of organic evolution. As before, the ill.u.s.trations of each department of the subject must be selected from the stock of everyday observation and information that we already possess, for we gain much when we realize that evolution includes all the happenings of everyday life and thought, as well as the occurrences of the remote past.
For the present, then, the questions relating to the higher aspects of human life must be put aside, only that they may be taken up at the last.
Social evolution likewise finds its place in a later section, after the phenomena of mind and mental evolution receive due attention and description. At the present juncture, the human species presents itself as a subject for organic a.n.a.lysis and cla.s.sification, merely as a physical organism. Just as the study of locomotives must begin with the detailed structure of machines in the workshop before they can be profitably understood as working mechanisms, so the physical evolution of mankind must first be made intelligible before it is possible to prosecute successfully the studies dealing with the psychology, social relations, and higher conceptions that seem at first to be the exclusive properties of our species.
The problems of physical evolution of man and of men fall into two groups.
Those of the first deal with the origin of the human species as a unit, and its comparative relation to lower organisms, while those of the second part are concerned with the further evolution of human races that have come to be different in certain details of structure since the human type as such arose. In the first part, all men will be a.s.sumed to be alike and the members of a h.o.m.ogeneous species whose fundamental attributes are to be compared with those of other animals; only afterwards will attention be directed to the differences, previously ignored, that divide human beings into well-marked varieties. It must be evident even at this point that the mode of evolution demonstrated by the first investigation will be likely to bear some close relation to the methods by which human races have evolved to their present diverse anatomical situations.
The foregoing cla.s.sification of the problems concerned with the nature and origin of the human species renders it possible to restrict the immediate inquiry to a definite and precise question. It is this: does the evidence relating to the physical characteristics of our species prove that man is the product of a supernatural act of creation, or does it show that man"s place in nature has been reached by a gradual process of natural evolution? In order to obtain an equally precise and definite answer to this question, referring to the particular case of most concern to us, it is obvious that the method to be employed is the one which has given us an understanding of organic evolution as an all-inclusive natural process.
The data must be verified, related, and cla.s.sified, so that their meaning may be concisely stated in the form of scientific principles. What are the facts of human structure, comparatively treated? How does the human body develop? Does palaeontology throw any light on the antiquity of man? Do the rules of nature"s order control the lives of men? Our course is now clear; we shall take up serially the anatomy, embryology, and fossil history of the human species, in order to see that there is ample proof of the actual occurrence of evolution, and then, as before, we may look about for the causes which have produced this result by natural methods.
While it is necessary to treat the subject directly, namely, by examining the actual evidences relating to the particular case in question, it is worthwhile before doing so to point out that, as the whole includes a part, human evolution has already been proved beyond question. This conclusion must be accepted, unless reasons can be given for excluding mankind from the rest of the living world as an absolutely unique type, supreme and isolated because of some peculiar endowments not shared with the rest of animate nature. If these reasons are lacking, and the unity of organic nature be recognized, human evolution cannot be denied unless some interpretation more reasonable and logical than evolution can be given for the whole ma.s.s of facts exemplified and discussed in the foregoing chapters. We may accordingly approach the main questions by asking if there are any reasons for regarding the human species as a unique and isolated type of organism.
At the outset, we must recognize that in so far as the human body is material, its movements and ma.s.s relations are controlled by physical principles, like all other ma.s.ses of matter. It is well, indeed, that this is so, for if gravitation and the laws of inertia were not consistent and reliable principles holding true at all times and not intermittently, it would be difficult to order our lives with confidence. In the next place, the general principles of biology hold true for the structure and physiology of the human species as they do for all other living things. A human body is composed of eight systems of organs, whose functions are identical with the eight vital tasks of every other animal. All these organs are made up of cells as ultimate vital units, and the materials of which human cells are composed belong to the cla.s.s of substances called protoplasm. Human protoplasm, like all other living materials, must replenish itself, and respire and oxidize in obedience to biological laws that have been found to be uniform everywhere. Thus the human organism is no more unique in fundamental organic respects than it is apart from the world of physical processes and laws.
How does the matter stand when the general structural plan of a human being is examined? Is it entirely different from everything else? It is a fact of common knowledge that the human body is supported by a bony axis, the vertebral column, to which the skull is articulated and to which also the skeletal framework of the limbs is attached. These characteristics place man inevitably among the so-called vertebrata; he is certainly not an invertebrate, nor is the basic structure of his body such that a third group, outside the invertebrata and vertebrata, can be made to include only the single type--man.
Pa.s.sing now to the cla.s.ses that make up the group of vertebrates, we meet first the lampreys or cyclostomes without jaws, and the others with jaws, such as the fishes, amphibia, reptiles, birds, and mammals, each cla.s.s distinguished by certain definite characters in addition to the vertebral column. The fishes have gills and scales; amphibia of to-day are scaleless, and they are provided with gills when they are young and lungs as adults; reptiles have scales and lungs; birds are warm-blooded and feathered; while mammals are warm-blooded and haired. Is the human species a unique kind of vertebrate, or does it find a place in one of these cla.s.ses? The occurrence of hair, of a four-chambered heart which propels warm blood, of mammary glands, and of other systematic characters marks this species as a kind of mammal and not as a vertebrate in a section by itself.
The members of the cla.s.s mammalia differ much among themselves; and now that we recognize clearly that man is a mammalian vertebrate, the next question is whether an order exists to which our type must be a.s.signed, or whether we have at last reached a point where it is justifiable to establish an isolated division to contain the human species alone. We are familiar with many representatives of different mammalian orders and with the kind of structural characteristics that serve as convenient distinctions in denoting their relationships. Horses and cattle, sheep, and goats and pigs resemble one another in many respects besides their hoofs, and they form one natural order; the well-developed gnawing teeth of rats and rabbits and squirrels place these forms together in the order rodentia; the structures adapting their possessors for a flesh-eating and predatory life unite the tribes of the lion, wolf, bear, and seal, in the order carnivora. Among these and other orders of mammalia is one to which the lemurs, monkeys, and apes are a.s.signed, because all these forms agree in certain structural respects that place them apart from the other mammalia, in the same way, for example, that the races of white men may be recognized as a group distinct from the black and red races. But comparative studies, prosecuted not only by those who have been forced to adopt the evolutionary interpretation, but also by believers in special creation like Linnaeus and Cuvier and other more modern opponents of evolution, have shown that the peculiar qualities of this order are shared by the human species. Indeed, the name of primates was given to this section by Linnaeus himself, because the human body found a place in the array which begins at the lower extreme with the lemurs and the monkeys and ends with man at the other end. Again it is found that no separate order of mammals exists to include only the genus _h.o.m.o_.
To one unacquainted with the facts of vertebrate comparative anatomy, the distinguishing characteristics of the primates seem to be trivial in nature. It is surprising to find how insignificant are the details to which appeal must be made in order to draw a line between our own division of mammalia and the others. It is well to review them as they are given in the standard text-books of comparative anatomy. Primates are eutheria, or true mammalia possessing a placental attachment of the young within the parent. The first digits, namely, the "great toe" and the "thumb," are freely movable and opposable to the others, so that the limbs are prehensile and clasping structures; usually but not always the animals of this order are tree-dwellers in correlation with the grasping powers of the feet and hands. The permanent teeth succeed a shorter series of so-called "milk teeth," and they are diverse in structure, being incisors, canines, or "eye teeth," premolars, and molars; the particular numbers of each kind are almost invariable throughout the order and markedly different from those of other orders. The number of digits is always five, and with few exceptions they bear nails instead of claws. The clavicles, or "collar bones," are well developed in correlation with the prehensile nature of the fore limbs; a bony ring surrounds the orbit or eye socket.
Finally there are two mammary glands by which the young are suckled. It is because any other details of difference between man and other forms are far less marked than the agreements in these respects, that the human species must be regarded as a primate mammalian vertebrate.
The comparative study of the human organism as a structural type has now been narrowed down to a review of the various members of the order of primates. It is the duty of science to arrange these organisms according to the minor differences beneath the agreements in major qualities, and to show how they are related in an order of evolution. It will appear, when this is done, that the supreme place is given to the human species on account of four and only four characteristics; these are (1) an entirely erect posture, (2) greater brain development, (3) the power of articulate speech, and (4) the power of reason. As we are treating the human body as a subject for comparative structural study, the third and fourth characters do not concern us here; but it is well to point out that they depend entirely upon the second, and that they are the functional concomitants of the improved type of brain belonging to the highest type.
Two characters remain, and in both cases it is significant that differences in degree only are to be found by even the closest a.n.a.lysis.
The human brain is the same kind of brain that lower primates possess; its structure is unique in no general respect. And as regards the first-mentioned character, comparative anatomy shows, in the first place, that this also is something differing only in degree, and in the second place, that it is due directly to the development of the brain. For these reasons a survey of the various members of the order of primates must deal largely with the progressive elaboration of the brain and the entailed effects of this enlargement.
The order of primates is subdivided as follows :--
Sub-order 1. _PROSIMII_. Lemurs.
Sub-order 2. _ANTHROPOIDEA_.
Family 1. _Hapalidae_. The marmosets.
Family 2. _Cebidae_. The American or tailed monkeys.
Family 3. _Cercopithecidae_. The baboons.
Family 4. _Simiidae_. The true apes.
Family 5. _Hominidae_. The human species. Primates
Each one of these subdivisions is interesting in its own way, either because its members depart from the typical condition of the whole order in some respects, or because of some character that foreshadows and leads to a more developed element of the animals placed in the higher sections.
The lemurs are small animals very much like squirrels in their general form and in their tree-climbing habits. They live now almost exclusively on the island of Madagascar, but palaeontology shows that they were more widely spread at an earlier time. Their teeth are exactly like our own, except that there is one more premolar on each side of each jaw. The "fingers" and "toes" bear nails like ours, again with an exception in the case of the second digits of the hind limbs, which bear claws. The details of structure that set these animals apart from all the rest of the primates are too small to deserve comment in the present connection.
Pa.s.sing to the true anthropoids, or man-like primates and man himself, the first forms encountered are the little marmosets, which are like the lemurs in some ways, but in other respects they resemble the familiar tailed monkeys. They are peculiar in having three premolars and two molars on either side of both upper and lower jaws, and also in the fact that the "thumb" is not opposable to the other fingers, while all the digits except the "great toes" bear claws instead of manlike nails. The proportion of brain-case and face does not differ much from that in the lemurs and even lower forms like cats, for the brain has not increased greatly in total ma.s.s, though the cerebrum is more convoluted than in the lower forms.
The true monkeys, or Cebidae, are more interesting, and at the same time they are much more familiar to every one, as they are the commonest anthropoids of the menagerie and circus. Their wonderful agility and sureness in climbing about is partly due to the perfect grasping power of the lower limb. To all intents and purposes the foot is a hand; the first toe is shorter than the others, and its free motion is unrestricted as in the thumb of the hand. These animals usually possess a long tail which they can use as a prehensile organ, curling it about the branch of a tree with hand-like ease and grasp. When they run on all fours, they plant the palms and soles flat upon the ground. The feature of primary importance in a comparative sense is the advanced structure of the skull. These anthropoids are much more intelligent than the lower forms, which is a correlate of their larger and more convoluted brains. The increase in the total bulk of the brain has wrought considerable change, not only in the head, but also in the relation of head to the trunk. The cranium, or brain-case of bone, is relatively larger than the "face," and it bulges upward so as to lie no longer behind the latter as it does in the lower mammalia. In consequence of this cranial enlargement, the face and eyes are swung downward, as it were, so that the line of vision is not straight ahead, but depressed below the horizontal. In order to look to the front and to the immediate foreground to which it is progressing or to where its food or enemies may be, the monkey must bend back its head; if it is still, it finds greater ease in the upright sitting posture which it a.s.sumes readily and naturally.
The next division, called the Cercopithecidae, includes the baboons of the Old World. These animals also run upon all fours, and their feet are handlike as before, but the tail is much reduced. The general appearance of the head is doglike, and the brain-case arches little more than it does in the monkeys, but the face projects forward as a long muzzle, with terminal nostrils close together. In some respects the baboons stand somewhat away from the line leading from the lower to higher anthropoids; in other characters they approach the latter, for in the teeth especially they are identical with the apes and with the human species.
The Simiidae, or true apes, possess an overwhelming importance, far beyond that of the baboons and monkeys. There are only four princ.i.p.al kinds now existing, namely, the gibbon, orang-outang, chimpanzee, and the gorilla, of which the first is much less familiar than the others. The known species of gibbons occur in Indo-China and the Malay Peninsula. The typical animal stands about three feet high; its overarching braincase, enlarged in conformity with the much greater brain development, has pushed the eyes and face still further around underneath, so that if the animal walks upon all fours the eyes look almost straight into the ground.
Therefore it must bend back its head at an extremely uncomfortable angle if it is to remain upon all four feet, but it prefers to raise itself up into the human sitting posture, or, when it walks, it stands erect upon its hind limbs. Hence we who are accustomed to think of ourselves as the only erect animals must revise our opinion, for we find in the gibbon an organism that is nearly, if not quite, as advanced in this respect as we are. One peculiar difference may be pointed out,--the walking gibbon stretches out its great long arms to the sides in order to preserve its balance. The animal seems awkward to us, perhaps, but it is possible that the human method of balancing the body by vigorously swinging the arms might seem quite as awkward to a gibbon as its grotesque posture does to us.
The orang-outang comes next in this series. It inhabits the islands of Borneo and Sumatra, where we find two distinct species. It is a reddish colored animal standing about four feet four inches high, with rather long hair. It is bulky, slow and deliberate in action, and when it walks in a semi-erect position it rests its knuckles upon the ground, swinging its long arms as crutch-like supports. Like the gibbon, it does not walk upon all four feet in the way that the monkeys and baboons do, and we find in the still further development of the brain and the higher arch of the cranium the reasons for its semi-erectness. It cannot remain with its hands and feet upon the ground and bend back its head so as to direct its vision forward.
The chimpanzee of intertropical Africa brings us to a still less monkey-like and more manlike stage. This creature attains the height of five feet, which is more than that of some of the lower races of man. It possesses large ears and heavy overarching brows; its thumb and great toe are more like those of man, though its foot is still practically a hand.
Its lower limb curves like those of the other apes, and its soles are turned toward one another; in brief, it is naturally bow-legged, a character that adapts it for a tree-climbing life. This animal also is nearly, though not quite, erect. It shows a most marked advance in the matter of the brain, for the cerebrum is richly folded or convoluted, and with this higher degree of physical complexity is correlated its superior intelligence; it is well known that chimpanzees can be taught to wear clothing and to use a cup and spoon and bowl like a human child. Indeed, in mental respects, the chimpanzee surpa.s.ses all of the other mammalia, with the sole exception of man. An eminent psychologist has stated that it is about the equal, in mental ability, of a nine months" old human infant.
The last form among the apes, the gorilla, is one that brings us to a realization of our own human physical degeneracy. The animal lives in West Equatorial Africa, and it is a veritable giant in bulk, though its height may not exceed five feet six inches. The heavy ridges over the eyes, the upturned nostrils and triangular nose, place it near to the orang-outang, but it is superior to that form in its relatively greater brain-box, and in the fact that its heavy lower jaws do not protrude so greatly. It, too, is semi-erect, so that the line of the vertebral axis makes an angle with the plane of the ground of about seventy degrees. Its anterior limbs, or arms, are again very long and bulky; and like the chimpanzee, it rests its knuckles upon the ground in walking.
It is a short step further to the human organism, whose brain has become larger and more complex, with a corresponding advance in the functional powers of reason and the like that owe their existence to the improved structural basis. After what has been said earlier regarding the relation between the erect att.i.tude in walking and the increased size of the cranial part of the skull as compared with the face, it will not be difficult to see how inevitably the former is the result of the latter.
Should we get upon the ground upon our hands and knees in the position of a tailed monkey, the eyes look straight into the ground, for the bulging cranium has pushed out over the jaws and face so that they lie _under_ the brain-case instead of in front. A person in this position can bend back the head so as to look ahead, but the strain is too great for comfort.
Rising to the knees, and lifting the hands from the ground, a feeling of ease at once succeeds that of tension. In the course of evolution accomplished primarily by the increase of the higher portions of the brain, the erect position has been a.s.sumed gradually and naturally, and to maintain it has necessitated many other changes in skeleton and muscles; for example, the pelvis has broadened to support the intestines, which bear downwards instead of upon the abdominal walls; a double curve has arisen in the axis of the vertebral column, giving an easier balance to the upper part of the body and the head. Countless structures of the human frame testify to an originally four-footed position and to a rotation of the longer axis through an angle of ninety degrees, as evolution has produced the human type.
The conclusion that the human brain has made mankind is thus established as one of fundamental importance. Proceeding further, we learn that this organ proves to be essentially the same as the brain of lower primates; it does not gain its greater size and efficiency by the origination of wholly new and unique parts, but solely by the further elaboration of the ones present in lower forms. In a word, it is only a difference in _degree_ and not in essential _kind_ that separates man from the apes and other primates. Human nature is animal nature, and human structure is animal structure, for nowhere can final and absolute differences be found. This does not mean that no differences appear, for it would be absurd to contend that man and the apes are identical in every respect; but it does mean that the resemblances are fundamental and comprehensive, and any details of dissimilarity are in the degree of complexity only. The supreme place in nature attained by man is therefore due to progressive evolution in the nervous system. The other systems have degenerated to a greater or less degree, but such regressive changes are more than compensated for by the superior control exerted by the improved brain. In purely physical and mechanical respects, the human body is a degenerate as compared with a gorilla; the arm of the latter is more powerful than the lower limb of the former, while the gorilla"s chest is more than twice as broad as the human, and more than four times as capacious. It is not through superior physique, but by superior ability to direct the activities of his body, that man excels in the struggle for existence with the lower animals.
Moreover, the human body is a veritable museum of rare and interesting relics of antiquity. This characterization is justified by those vestigial and rudimentary structures that represent organs of value to human relatives among the lower animals, though they play a less active part at the present time in human economy. There is scarcely a single system that does not exhibit many or fewer of these rudimentary structures, but only a few need be specified. As compared with those of the apes, the human wisdom teeth are degenerate; in the gorilla they are cut at the same time as the other molars; and in the lower human races they come through the gums in early youth, while in the more advanced Caucasic races they are cut only in later life or not at all. The reduced vermiform appendix of man, a source of much ill health, is another structure that is a counterpart of a relatively larger and useful part of the digestive tract in the lower primates and other animals. Furthermore, the human tail is a reality, not a fiction. Now and then an individual is born with a tail that may reach a length in later life of eight or ten inches; such structures are, of course, abnormal. But in every normal human being there is a series of little bones at the lower end of the vertebral column, const.i.tuting the coccyx, and this is just where the abbreviated tail of the ape and the still longer prehensile tail of the monkey arises from the body. Unless the coccyx is a tail, what can it be? And if it does not represent a reduced counterpart of the tails of other mammals, what does it represent?
Many of the vestigial structures of man appear more clearly in infancy and in embryonic development. The human embryo possesses a complete coat of hair, called the lanugo, which usually disappears before birth. This hair cannot be regarded as any less significant than the coat of hair which the infant whale possesses; it means a completely haired ancestor. The elements of this coat are arranged precisely as they are in the apes; upon the arm, for example, they point from shoulder to elbow and from wrist to elbow. Unless the anterior limb of the hairy human ancestor was held in the position of the climbing ape"s, this arrangement would be disadvantageous, for the hair as a rain-shedding thatch would be effective only upon the upper arm, while the hairs upon the forearm would catch the rain. In a word, this vestigial coat indicates in the clearest possible manner that the ancestor of the human species was not only hairy, but also arboreal in its mode of life.
Every human infant is bow-legged at birth, and the natural position of its curved limbs is like that of the gorilla"s, for the soles of the feet are turned toward one another. Again, the so-called great toe is at first shorter than the others, and for a time it retains the power of free movement that indicates a handlike character of the lower limb in the ancestor. Many savage human races, however, whose feet remain unshod, make use of the primitive grasping power of the foot which the higher races lose completely. An Australian and Polynesian can pick up small objects with the foot very much as we may with the hand.
Among the wonderful reminiscent characters displayed by the human infant is the firm clasping power of the hand, which it possesses for a time after birth and which enables it to hang suspended for several minutes from a stick placed in its grasp. The muscles which enable the infant to do this gradually dwindle, so that the two-year-old child can hang suspended for only a few seconds. This grasping muscle is a heritage from the ape, where there is an obvious necessity for the newborn individual to have a firm hold upon the hairy coat of its tree-climbing mother. When the newborn child hangs in this way, it bends its curved lower limbs so that the soles of the feet are turned toward one another, thus increasing its resemblance to the ape.
Let us realize that these curious relics found in so many places in the framework of man are not unique, and that they are reduced counterparts of larger and more valuable structures in the ape. Unless evolution is true, they have absolutely no sensible reasons for existence. Science prefers the evolutionary explanation of their occurrence because this explanation is more in harmony with the facts known about other organisms, and it is more reasonable than any other.
When we dealt with the general doctrine of natural transformation, it appeared that the evidence of embryology was in many respects more cogent and conclusive than that derived from the comparative study of animal structures. In the case of man, as before, no one could demand any surer or more convincing proof that an organic mechanism with one structure can change into an organic mechanism with a different structure, than the obvious facts of development. The embryo, which is not an infant or an adult, becomes an infant which must work its way onward by the gradual acc.u.mulation of slight changes here and there and everywhere in its anatomy, until it becomes mature. Each and every one of us has actually undergone the process of organic change in becoming what we are, and we cannot deny the reality of such a process without challenging the evidence of our senses.
When the full import of this history is realized, and when we look further into the nature of these preliminary conditions through which the human organism pa.s.ses in development, we are forcibly impressed by other facts than the one to which I have directed your attention, for not only do we find natural transformation, as in the other mammals, but the embryonic stages are marvelously similar to the earlier conditions in other mammals.
Not very long before birth the human embryo is strikingly similar to the embryo of the ape; still earlier, it presents an appearance very like that of the embryos of other mammals lower in the scale, like the cat and the rabbit,--forms which comparative anatomy independently holds to be more remote relatives of the human species. Indeed, as we trace back the still earlier history, more and more characters are found which are the common properties of wider and wider arrays of organisms, for at one time the embryo exhibits gill-slits in the sides of its throat which in all essential respects are just like those of the embryos of birds and reptiles and amphibia, as well as of other embryo mammals and these gill-slits are furthermore like those of the fishes which use them throughout life. All the other organic systems exhibit everywhere the common characteristics in which the embryos of the so-called higher animals agree with one another and with the adult forms among lower creatures; the human embryo possesses a fishlike heart and brain and primitive backbone, fishlike muscles and alimentary tract. Can we reasonably regard these resemblances as indications of anything else but a community of ancestry of the forms that exhibit them?
Yet a still more wonderful fact is revealed by the study of the very earliest stages of individual development. The human embryo begins its very existence as a single cell,--nothing more and nothing less; in general structure the human egg, like the eggs of all other many-celled organisms, is just one of the unitary building blocks of the entire organic world. And yet the egg may ultimately become the adult man. Does this mean that man and all the other higher forms have evolved from protozoa in the course of long ages? Science asks if it can mean anything else. When the comparative anatomist bids us look upon the wide and varied series of adult animals lower than man as his relatives, because they display similar structural plans beneath their minor differences, it may be difficult at first to obey him. But in the brief time necessary for the human egg to develop into an adult, the entire range is compa.s.sed from the single cell to the highest adult we know. There are no breaks in the series of embryonic stages like those between the diverse adult animals of the comparative array. I do not think we could ask nature for more complete proof that human beings have evolved from one-cell ancestors as simple as modern protozoa beyond the obvious facts of human transformation during development. They at least are real and not the logical deductions of reason; yet their very reality and familiarity render us blind to the deeper meaning revealed to us only when science places the facts in intelligible order.
And now, in the third place, we may look to nature for fossil evidence regarding the ancestry of our species. Much is known about the remains of many kinds of men who lived in prehistoric times, but we need consider here only one form which lived long before the glacial period in the so-called Tertiary times. In 1894 a scientist named Dubois discovered in Java some of the remains of an animal which was partly ape and partly man.
So well did these remains exhibit the characters of Haeckel"s hypothetical ape-man, _Pithecanthropus_, that the name fitted the creature like a glove. Specifically, the cranium presents an arch which is intermediate between that of the average ape and of the lowest human beings. It possessed protruding brows like those of the gorilla. The estimated brain capacity was about one thousand cubic centimeters, four hundred more than that of any known ape, and much less than the average of the lower human races. Even without other characters, these would indicate that the animal was actually a "missing link" in the scientific sense,--that is, a form which is near the common progenitors of the modern species of apes and of man. We would not expect to find a missing link that was actually intermediate in all respects between modern apes and modern men, any more than we should look for actual connecting bands of tissue between any two leaves upon a tree. A missing link, in the true sense, is like a bud of earlier years which stood near the point from which two twigs of the present day now diverge. So _Pithecanthropus_ is a part of the chain leading to man, not far from the place where the human line sprang from a lower primate ancestor.
Of the fossil remains of true prehistoric men, little need be said. We cannot know whether the races now living in the regions where these remains are found are really the descendants of the older types, and so a direct comparison cannot be made. It is true that the brain capacities of the man of Spy, of the Neanderthal, and of the English caverns are lower than those of modern civilized races, but the differences are not so striking and not so clearly indicative of the apelike ancestor of man as in the case of the previous comparison of _Pithecanthropus_ with apes and men.