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The doctrine of natural selection took form in the mind of Darwin mainly on account of three potent influences; these were, first, the geological doctrine of uniformitarianism proposed by Lyell, second, his own observations of wild life in many lands and his analysis of the breeder's results with domesticated animals, and third, the writings of Malthus dealing with overpopulation. As Darwin had read the works of Buffon, Lamarck, and Erasmus Darwin, his grandfather, who had written a famous treatise under the title of "Zoonomia," he was familiar with the evidences known in his student days tending to prove that organic evolution was a real natural process. Lyell's doctrine of uniform geological history made an early and deep impression upon his mind, and it led him to ask himself whether the efficient causes of past evolution might not be revealed by an analysis of the present workings of nature. As naturalist of the "Beagle" during its four years' cruise around the world, Darwin saw many new lands and observed varied circumstances under which the organisms of the tropics and other regions lived their lives. The fierce struggle for existence waged by the denizens of the jungle recalled to him the views of Malthus regarding overpopulation and its results. These and other influences led him to begin the remarkable series of note-books, from which it is interesting indeed to learn how the doctrine of natural selection began to assume a definite and permanent form in his mind, as year followed year, and evidence was added to evidence. And it is a valuable lesson to the student of science that for twenty-five years Darwin devoted all his time to the acquisition of facts before he gave his doctrine to the world in the famous "Origin of Species."
Darwin was particularly impressed by the way mankind has dealt with the various species of domesticated animals, and he was the first naturalist to point out the correspondence between the breeder's method of "artificial selection," and the world-wide process of natural selection. As every one knows, the breeder of race horses finds that colts vary much in their speed; discarding the slower animals, he uses only the swifter for breeding purposes, and so he perfects one type of horse. With other objects in view, the heavy draught horse, the spirited hackney, and the agile polo pony have been severally bred by exactly the same method. Among cattle many kinds occur, again the products of an artificial or human selection; hornless breeds have been originated, as well as others with wide-spreading or sharply curved horns; the Holstein has been bred for an abundant supply of milk as an object, while Jerseys and Alderneys excel in the rich quality of their milk. Various kinds of domesticated sheep and rabbits and cats also owe their existence to the employment of the selfsame method, unconsciously copied by man from nature; for men have found variations arising naturally among their domesticated animals, and they have simply substituted their practical purposes or their fancy for nature's criterion of adaptive fitness, preserving those that they wish to perfect and eliminating those unfitted to their requirements or ideas.
In the case of many of these and other examples, wild forms still occur which seem to be like the ancestral stock from which the domesticated forms have been produced. All the varied forms of dogs—from mastiff to toy-terrier, and from greyhound to dachshund and bulldog—find their prototypes in wild carnivora like the wolf and jackal. In Asia and Malaysia the jungle fowl still lives, while its domesticated descendants have altered under human direction to become the diverse strains of the barnyard, and even the peculiar Japanese product with tail feathers sometimes as long as twenty feet. That far-reaching changes can be brought about in a relatively short time is proved by the history of the game cock, which has nearly doubled in height since 1850, while at the same time its slender legs, long spurs, and other qualities have been perfected for the cruel sport for which it has been bred. Again, the wild rock pigeon seems to be the ancestral form from which the fantail and pouter and carrier-pigeon with their diverse characters have taken their origin.
It is true that some biologists have urged certain technical objections to the employment of domesticated animals and their history as analogies to the processes and results in wild nature. To my mind, however, artificial selection is truly a part of the whole process of natural selection. Man is but one element of the environment of tame forms, and his fancy or need is therefore one of the varied series of external criteria that must be met if survival is to be the result; failing this, elimination follows as surely as under the conditions of an area uninhabited or uninfluenced by mankind. Congenital variation is real, selection is real and the heredity of the more fit modification is equally real. Surely Darwin was right in contending that the facts of this class amplify the conception of natural selection developed on the basis of an analysis of wild life.
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Knowing the elements of the selective process, it is possible to analyze and to understand many significant phenomena of nature, and to gain a clearer conception of the results of the struggle for existence, especially when the human factor is involved. Let us see how much is revealed when the foregoing results are employed in a further study of some of nature's vital situations.
As a consequence of the many-sided struggle for existence, the interrelations of a series of species will approach a condition of equilibrium in an area where the natural circumstances remain relatively undisturbed for a long time. For example, among the field-mice of one generation, just as many individuals will survive as will be able to find food and to escape hereditary foes such as cats and snakes and owls. The number of owls, in their turn, will be determined by the number of available mice and other food organisms, as well as by the severity of the adverse circumstances that cause elimination of the less fit among the fledglings brought into the world. The vital chain of connections is sometimes astonishingly long and intricate. One remarkable illustration is given by Fiske, as an elaboration of an example cited by Darwin. He points out that the fine quality of the traditional roast beef of England is directly determined by the number of elderly spinsters in that country. The chain of circumstances is as follows: the quality of the clover fields, furnishing the best food for cattle, depends largely upon the visits to the clover-blossoms by wild bees, that accomplish the fertilization of the flowers by carrying pollen upon their bodies from one plant to another. Field-mice devour the young in the nests of these bees, so if there are few field-mice there will be many bees, and consequently better grazing for the cattle. The number of field-mice will vary according to the abundance of cats, and so the number of these domestic animals will exert an influence upon the whole foregoing chain of forms. But, as Fiske points out, cats are the favorite companions of elderly spinsters; therefore, if there are many of the latter, there will be more cats, fewer field-mice, more bees, richer clover fields, and finer cattle! Each link is real and the whole chain is a characteristic example of the countless ways that the natural destinies of living things are interrelated and intertwined.
The reality of such organic interrelationships is revealed with wonderful clearness in the numerous instances where some disturbing factor has altered one or another element of the balanced system. The invasion of the new world by Europeans has directly led to the partial or complete extinction of the tribes of Indians to whom the land formerly belonged; they have disappeared almost entirely from our state of New York, together with the bear and wolf and many other species of animals that formerly existed here. Wild horses and bison have also vanished before the advances of civilization and the alteration of their homes. Sometimes the extermination of one pest has resulted in an increase in the number of another through human interference with nature's equilibrium. In some of our Western states, a bounty was offered for the scalps of wolves, so as to lessen the number of these predatory foes of sheep. But when the wolves were diminished in number, their wild food-animals, the prairie dogs, found their lot much bettered, and they have multiplied so rapidly that in some places they have become even more destructive than the wolves.
One of the most remarkable illustrations is that of the rabbits introduced into Australia. This island continent was cut off from the surrounding lands long before the higher mammals evolved in far distant regions, so that the balance of nature was worked out without reference to animals like the rabbit. When the first of these were introduced they found a territory without natural enemies where everything was favorable. They promptly multiplied so rapidly that within a few years their descendants were numerous enough to eat up practically every green thing they could reach. Two decades ago, the single province of Queensland was forced to expend $85,000,000 in a vain effort to put down the rabbit plague. The remarkable statement has been made that in some places nature has taken a hand in causing a new type of rabbit to evolve. Finding the situation desperate, some of the animals have begun to develop into tree-climbing creatures. The animals exist in such numbers that the available food upon the ground is insufficient for all, and so some elimination results. But the young rabbits with longer claws, varying in this way on account of congenital factors, have an advantage over their fellows because they can climb some of the trees and so obtain food inaccessible to the others. If the facts are correctly reported, and if the process of selection on the basis of longer claws and the climbing habit is continued, the original type of animal is splitting up into a form that will remain the same and live upon the ground, and another that will be to all intents and purposes a counterpart of our familiar squirrel. All the evidence goes to show that squirrels have evolved from terrestrial rodents; if the data relating to Australian rabbits are correct, nature is again producing a squirrel-like animal by evolution in a region where the former natural situation has been interfered with by man.
The laws of biological inheritance have received close and deep study by numerous investigators of Darwinian and post-Darwinian times, because from the first it was clearly recognized that a complete description of nature's method of accomplishing evolution must show how species maintain the same general characteristics from generation to generation, and also how new qualities may be fixed in heredity as species transform in the course of time. Before our modern era in biology, the fact of inheritance was accepted as self-sufficient; now much is known that supplements and extends the incomplete account given by natural selection of the way evolution takes place.
It is not possible in the present brief outline to describe all the results of recent investigations, but some of them are too important to be passed over. Perhaps the most interesting one is that the laws of heredity seem to be the same for man and other kinds of living creatures, as proved by Galton and Pearson and many others who have dealt with such characters as human stature, human eye color, and an extensive series of the peculiarities of lower animals and even of plants.
The researches dealing with the physical basis of inheritance and its location in the organism have yielded the most striking and brilliant results. Darwin himself realized that the doctrine of natural selection was incomplete, as it accepted at its face value the inheritance of congenital racial qualities without attempting to describe the way an egg or any other germ bears them, and he endeavored to round out his doctrine of selection by adding the theory of pangenesis. According to this, every cell of every tissue and organ of the body produces minute particles called gemmules, which partake of the characters of the cells that produce them. The gemmules were supposed to be transported throughout the entire body, and to congregate in the germ-cells, which in a sense would be minute editions of the body which bears them, and would then be capable of producing the same kind of a body. If true, this view would lead to the acceptance of Lamarck's or even Buffon's doctrine, for changes induced in any organ by other than congenital factors could be impressed upon the germ-cell, and would then be transported together with the original specific characters to future generations. Darwin was indeed a good Lamarckian.
But the researches of post-Darwinians, and especially those of the students of cellular phenomena, have demonstrated that such a view has no real basis in fact. Many naturalists, like Naegeli and Wiesner, were convinced that there was a specific substance concerned with hereditary qualities as in a larger way protoplasm is the physical basis of life. It remained for Weismann to identify this theoretical substance with a specific part of the cell, namely, the deeply staining substance, or chromatin, contained in the nucleus of every cell. Bringing together the accumulating observations of the numerous cytologists of his time, and utilizing them for the development of his somewhat speculative theories, Weismann published in 1882 a volume called "The Germ Plasm," which is an immortal foundation for all later work on inheritance. The essential principles of the germ-plasm theory are somewhat as follows. The chromatin of the nucleus contains the determinants of hereditary qualities. In reproduction, the male sex-cell, which is scarcely more than a minute mass of chromatin provided with a thin coat of protoplasm and a motile organ, fuses with the egg, and the nuclei of the two cells unite to form a double body, which contains equal contributions of chromatin from the two parental organisms. This gives the physical basis for paternal inheritance as well as for maternal inheritance, and it shows why they may be of the same or equivalent degree. When, now, the egg divides, at the first and later cleavages, the chromatin masses or chromosomes contained in the double nucleus are split lengthwise and the twin portions separate to go into the nuclei of the daughter-cells. As the same process seems to hold for all the later divisions of the cleavage-cells whose products are destined to be the various tissue elements of the adult body, it follows that all tissue-cells would contain chromatin determinants derived equally from the male and female parents. As of course only the germ-cells of an adult organism pass on to form later generations, and as their content of chromatin is derived not from the sister organs of the body, but from the original fertilized egg, there is a direct stream of the germ plasm which flows continuously from the germ-cell to germ-cell through succeeding generations. It would seem, therefore, that the various organic systems are, so to speak, sister products in embryonic origin. The reproductive organs are not produced by the other parts of the body, but their cells are the direct descendants of the common starting-point namely, the egg. As the cells of the reproductive organs are the only ones that pass over and into the next and later generations, it will be evident, in the first place, that the germ plasm of their nuclei is the only essential substance that connects parent and offspring. This stream of germ plasm passes on in direct continuity through successive generations—from egg to the complete adult, including its own germ-cells, through these to the next adult, with its germ-cells, and so on and on as long as the species exists. It does not flow circuitously from egg to adult and then to new germ-cells, but it is direct and continuous, and apparently it cannot pick up any of the body-changes of an acquired nature. Now we see why individual acquisitions are not transmitted. The hereditary stream of germ plasm is already constituted before an animal uses its parts in adult life; we cannot see how alterations in the structure of mature body parts through use and adjustment to the environment can be introduced into it to become new qualities of the species.
It must be clear, I am sure, that this theory supplements natural selection, for it describes the physical basis of inheritance, it demonstrates the efficiency of congenital or germ-plasmal factors of variation in contrast with the Lamarckian factors, and finally in the way that in the view of Weismann it accounts for the origin of variations as the result of the commingling of two differing parental streams of germ plasm.
At first, for many reasons, Weismann's theories did not meet with general acceptance, but during recent years there has been a marked return to many of his positions, mainly as the result of further cytological discoveries, and of the formulation of Mendel's Law and of De Vries's mutation theory. The first-named law was propounded by Gregor Mendel on the basis of extensive experiments upon plants conducted during many years, 1860 and later, in the obscurity of his monastery garden at Altbrünn, in Austria. It was rescued from oblivion by De Vries, who found it buried in a mass of literature and brought it to light when he published his renowned Mutation Theory in 1901. Mendelian phenomena of inheritance, confirmed and extended by numerous workers with plants and animals, prove that in many cases portions of the streams of germ plasm that combine to form the hereditary content of organisms may retain their individuality during embryonic and later development, and that they may emerge in their original purity when the germ-cells destined to form a later generation undergo the preparatory processes of maturation. They demonstrate also the apparent chance nature of the phenomena of inheritance. To my mind the most striking and significant result in this field is the demonstration that a particular chromosome or chromatin mass determines a particular character of an adult organism, which is quite a different matter from the reference of all the hereditary characters to the chromatin as a whole. Wilson and others have brought forward convincing proof that the complex character of sex in insects actually resides in or is determined by particular and definite masses of this wonderful physical basis of inheritance.
Mendel's principles also account in the most remarkable way for many previously obscure phenomena, like reversion, or a case where a child resembles its grandparent more than it does either of its parents; such phenomena are due, so to speak, to the rise to the surface of a hidden stream of germ plasm that had flowed for one or many generations beneath its accompanying currents. I believe that the law is replacing more and more the laws of Galton and Pearson, formulated as statistical summaries of certain phenomena of human inheritance taken en masse. According to Galton's celebrated law of ancestral inheritance, the qualities of any organism are determined to the extent of a certain fraction by its two parents taken together as a "mid-parent," that a smaller definite fraction is contributed by the grandparents taken together as a mid-grandparent, and so on to earlier generations. But Mendel's Law has far greater definiteness, it explains more accurately the cases of alternative inheritance, and it may be shown to hold for blended and mosaic inheritance as well.
De Vries's new "mutation theory" is clearly not an alternative but a complementary theory to natural selection, the Weismannian and Mendelian theories. Like these last, it emphasizes the importance of the congenital hereditary qualities contained in the germ plasm, though unlike the Darwinian doctrine it shows that sometimes new forms may arise by sudden leaps and not necessarily by the slow and gradual accumulation of slight modifications or fluctuations. The mutants like any other variants must present themselves before the jury of environmental circumstances, which passes judgment upon their condition of adaptation, and they, too, must abide by the verdict that means life or death.
From what has been said of these post-Darwinian discoveries, the Lamarckian doctrine, which teaches that acquired non-congenital characters are transmitted, seems to be ruled out. I would not lead you to believe that the matter is settled. I would say only that the non-transmission of racial mutilations, negative breeding experiments upon mutilated rats and mice, the results of further study of supposedly transmitted immunity to poisons—that all these have led zoölogists to render the verdict of "not proved." The future may bring to light positive evidence, and cases like Brown-Séquard's guinea-pigs, and results like those of MacDougal with plants, and of Tower with beetles, may lead us to alter the opinion stated. But as it stands now most investigators hold that there are strong general grounds for disbelief in the principle, and also that it lacks experimental proof.
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The explanation of natural evolution given by Darwinism and the principles of Weismann, Mendel, and De Vries, still fails to solve the mystery completely, and appeal has been made to other agencies, even to teleology and to "unknown" and "unknowable" causes as well as to circumstantial factors. A combination of Lamarckian and Darwinian factors has been proposed by Osborn, Baldwin, and Lloyd Morgan, in the theory of organic selection. The theory of orthogenesis propounded by Naegeli and Eimer, now gaining much ground, holds that evolution takes place in direct lines of progressive modification, and is not the result of apparent chance. Of these and similar theories, all we can say is that if they are true, they are not so well substantiated as the ones we have reviewed at greater length.
The task of experimental zoölogy is to work more extensively and deeply upon inheritance and variation, combining the methods and results of cellular biology, biometrics, and experimental breeding. We may safely predict that great advances will be made during the next few years in analyzing the method of evolution; and that a few decades hence men will look back to the present time as a period of transition like the era of reawakened interest and renewed investigation that followed the appearance of the "Origin of Species." For the present, we can justly say "that evolution, so far as it is understood, is a real and natural process."
V
THE PHYSICAL EVOLUTION OF THE HUMAN SPECIES AND OF HUMAN RACES
The teachings of science that relate to the origin and history of the human species constitute for us the most important part of the whole doctrine of organic evolution and now, having completely outlined this doctrine as a general one, we are brought to the point where we must deal frankly and squarely with the insistent questions arising on all sides as to the way that mankind is involved in the vast mechanism of nature's order. These questions have been ignored heretofore, in order that the natural history of animals in general might be discussed without any interference on the part of purely human interest and concern. It now becomes our privilege, and our duty as well, to employ and apply the principles we have learned in order to understand more completely the origin of the human body as an organic type, the history of human races, the development of human faculty and of social institutions, and the evolution finally of even the highest elements of human life. These are scientific problems, and if we are to solve them we must employ the now familiar methods of science which only yield sure results.
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 analysis 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 zoölogist 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 assist 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 illustrations 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 analysis and classification, 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 assumed to be alike and the members of a homogeneous 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.
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The foregoing classification 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 classified, 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 palæontology 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 mass 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 mass relations are controlled by physical principles, like all other masses 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 class 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.
Passing now to the classes 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 class 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 classes? 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 class 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 assigned, 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 assigned, 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 Linnæus 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 Linnæus 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 Homo.
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.
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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 analysis. 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. Hapalidæ. The marmosets.
Family 2. Cebidæ. The American or tailed monkeys.
Family 3. Cercopithecidæ. The baboons.
Family 4. Simiidæ. The true apes.
Family 5. Hominidæ. 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 palæontology 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.
Passing 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 mass, though the cerebrum is more convoluted than in the lower forms.
The true monkeys, or Cebidæ, 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 assumes readily and naturally.
The next division, called the Cercopithecidæ, 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 Simiidæ, or true apes, possess an overwhelming importance, far beyond that of the baboons and monkeys. There are only four principal 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 surpasses 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 attitude 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 assumed 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.
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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, constituting 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.
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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 accumulation 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 passes 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 compassed 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.
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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.
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The foregoing facts illustrate the conclusive evidence brought forward by science that human evolution in physical respects is true. Even if we wished to do so, we cannot do away with the facts of structure and development and fossil history, nor is there any other explanation more reasonable than evolution for these facts. If now we should inquire into the causes of this process, we would find again that the present study of man and men reveals their subjection to the laws of nature which accomplish evolution elsewhere in the organic world.
The fact of human variation requires no elucidation; it is as real for men as for insects and trees. Indeed, some of the most significant facts of variation have been first made out in the case of the human species. The struggle for existence can be seen in everyday life. We cannot doubt its reality when scores perish annually because of their failure to withstand the extreme degrees of temperature during midwinter and midsummer; when starvation causes so many deaths, and when the incessant combat with bacterial enemies alone brings the list of casualties on the human side in our own country to more than two hundred and fifty thousand a year. As in nature at large, the more unfit are eliminated as a result of this struggle, while the more adapted succeed. In the long run, that particular applicant for a clerkship or any other work who may be the more fitted is the one who gets it. While the severity of competition may be somewhat mitigated as the result of social organization, and while our altruistic charitable institutions enable many to prolong a more or less efficient existence, the struggle for existence cannot be entirely done away with. Heredity also is a real human process, and it follows the same course as in animals at large; as in the case of variation, some of the fundamental laws of its operation have been first worked out in the case of human phenomena, and have been found subsequently to be of general application.
Reverting to the specific question as to the earliest divergence of man from the apes, we can readily see how the superior development of the ape-man's brain gave him a great advantage over his nearest competitors, and how truly human ingenuity enabled the earliest men to employ weapons and crude instruments instead of brute force. Thus the gap between men and apes widened more and more, as reasoning power increased through successive generations. This is another aspect of the statement that the supreme position of man has been gained, not by superior organization in physical respects outside of the nervous system, but by the superior control of human organization by the higher organs of this system.
The unity of nature and of its processes is established more and more surely as the naturalist classifies the facts of structure, development, fossil history, and evolutionary method. Our own species is not unique; it takes its high place among other organic forms whose lives are controlled in every way by the uniform consistent laws of the world.
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The physical evolution of human races is the next major division of the large subject before us. Heretofore the obvious differences displayed by various races have been disregarded and the species has been treated as a unit, in order that its evolution from pre-human ancestors might be made clear. Knowing now how the facts of structure show that the supreme position of our kind has been attained mainly as the result of the progressive elaboration of the higher portions of the brain, and not because new and unique structures have been developed, we are prepared to turn our attention to the diverse characteristics of human races; and during this inquiry anatomical matters will still be the only ones to be reviewed. The intellectual and social characters of numerous races belong to the category of physiological or functional phenomena, which are to receive due consideration at a later time. It is the meaning of the facts of racial diversity for which we are now to look.
For many reasons this subject is more difficult to describe in a concise outline than those taken up before. It is true that every one is familiar with different types of human beings, such as the Negro and Japanese and Chinese, while furthermore the obvious differences between such races as the Norwegian and Italian are sufficiently marked to strike the attention of any one who looks about at his fellow-passengers in a crowded street car. But few indeed have a comprehensive knowledge of the wider range of racial variation in which these familiar examples find their place. Anthropology, or the science of mankind, is a large and well-organized department of knowledge, dealing with the entire array of structural and physiological characters of all men. One of its subdivisions, anthropometry, is almost an independent discipline with methods of its own; it describes the characteristics of human races as these are determined by statistical methods of a somewhat technical nature. There is still another science, ethnology, which deals more particularly with institutions, customs, beliefs, and languages rather than with physical matters, although it is clear that ethnology and anthropology cannot be sharply separated, and that each must employ the results of the other for its own particular purposes.
Because men have always been interested in the study of themselves, the subject of racial evolution is literally enormous, and the attempt to give anything like a complete description of what is known would obviously be futile. But it is possible to obtain a clear conception of certain of the fundamental principles that fall into line with the other parts of the doctrine of organic evolution with which we have now become acquainted. The main questions, therefore, may be stated in simple terms. The first deals with the evidences as to the reality of evolution during the historical and prehistoric development of the various types of man from earlier common ancestors; the second asks whether the lines of racial evolution are further continuations of the line leading from ape-like ancestors to the human species as a type. In order to give the proper perspective, it will be well to state at the present juncture, first, that the various kinds of men do not vary from each other in a chance manner so as to show all possible types and varieties, but that they fall into natural groups or families distinguished by certain common characteristics, just as do all other kinds of species of animals; in the second place, it appears that some of the differences between the races denoted higher on structural accounts and the lowest forms of man are of the same nature as those observed in the review of the various species of primates from the lemurs to man.
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It is best to look at the whole question in a very simple and common-sense way before undertaking an extended examination of the details of human diversity. The most casual survey of the peoples that we know best because of our own individual nearness to them enables us to realize that the races now upon the earth have not existed forever and ever, or even for the age of 6000 years as contended by Archbishop Ussher. They have all come into existence as such, and they differ from their known antecedents; so that at the very outset common-sense leads us to accept evolution as true, if we admit that human races have changed during the course of recent centuries. We know, for example, that the so-called Mexicans of to-day are a people produced by a fusion of Spanish conquerors and Indian aborigines the Mexican is neither Spaniard nor Indian, though he may resemble both in certain respects; he is a product of natural evolution, accomplished in this case by an amalgamation of two contrasted types. When we speak of the American people, we must realize that it too has come into existence as such, and even, indeed, that it is in the actual process of evolution at the present time. The various foreign elements that have been added during the last few decades by the hundreds of thousands are becoming merged with the people who preceded them, just as the Dutch and the French and the English coalesced during the days of early settlement to form the young American nation. Perhaps most of us call ourselves Anglo-Saxon, but we are in reality somewhat different even in physical respects from the Englishmen of Queen Elizabeth's time, who alone deserved the name Anglo-Saxon. This very term indicates an evolution of a type that differs from both the Angles and the early Saxons of King Alfred's age. These are simple examples which illustrate many features of the universal history of human races wherever they are to be found. Even in the comparatively peaceful times of our modern era the history of any race is a veritable turmoil of constant changes; conquerors impress their characters upon the vanquished, while the victors often adopt some of the features of the conquered. Colonies split off from the mother nation to follow out their destinies under other conditions. Nowhere does the naturalist find evidence of long-established permanence, or an unentwined course of an uninterrupted and unmodified line of racial descent.
It is the task of the student of human evolution to unravel the tangled threads of human histories. The task is relatively simple when it is concerned with recent times where the aid of written history may be summoned but when the events of remote and prehistoric ages are to be placed in order, the difficulties seem well-nigh insuperable. All is not known, nor can it ever be known; but wherever facts can be established, science can deal with them. By a study of the present races of mankind, much of their earlier history can be worked out, for their genetic relations may be determined by employing the principle that likeness means consanguinity. Let us suppose an alien visitor to reach our planet from somewhere else; if he were endowed with only ordinary human common-sense, he would very soon ascertain the common origin of the English-speaking people in Canada, the United States, Australia and New Zealand, South Africa, and many other places. Even if he could not understand a word of the English language, he would be justified in regarding them all as the descendants of common ancestors because they agree in so many physical qualities. The anthropologist works according to the same common-sense principle, obtaining results that find no explanation other than evolution when the varying characters that are used to determine social relationship are properly classified and related. It is to these characters that we must now give some attention.
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The average stature of adults varies in different races from four feet one inch in certain blacks to nearly six feet and seven inches, as among the Patagonians. These are the extreme values for normal averages, although dwarfs only fifteen inches high have been known, while "giants" sometimes occur with a height of nine feet and five inches. Such individuals are of course rare and abnormal, and are not to be taken into account in establishing the average stature of a race for use in comparison with that of another group.
The color of the skin is another criterion of racial relationship, though it is more variable in races of common descent than we are wont to assume. We are familiar with the fair and florid skin of the northern European, the fair and pale skin in middle and southern Europe, the coppery red of the American Indian, the brown of the Malay, of the Polynesian and of the Moor, the yellowish cast of the Chinese and Japanese, and the deeper velvety black of the Zulu; but it has been found that many of the close relatives of the black are lighter in skin color than some of our Caucasian relatives, so that this character cannot be taken by itself as a single criterion of racial affinity.
Perhaps the most conservative and most reliable character that serves for the broad classification of the human races is the shape of the individual hairs of the head. We are familiar with the straight lank hair of the Mongolian peoples and of the various tribes of American Indians, in whom the hair possesses these peculiarities because each element grows as a nearly perfect cylinder from the cells of the skin at the bottom of a tiny pit or hair-follicle. The familiar wavy hair of white men owes its character to the fact that the individual elements are formed by the skin, not as pencil-like rods, but as flattened cylinders. They are oval or elliptical in cross-section, and when they emerge from the skin they grow into a long spiral. If, now, the hair is formed as a very much flattened rod about one-half as wide in one diameter as in the other, it curls into a very tight close spiral and gives the frizzly or woolly head-covering of the Papuan and of the Negro.
In the next place, the shape of the cranium is a character of much value. This is determined as the proportion between the transverse diameter of the skull above the ears to the long diameter, namely, the line that runs from the middle of the brow to the most posterior point of the skull. In the so-called "long-headed" or dolichocephalic races, the proportion is seventy-five to one hundred, while in those forms that have more rounded or brachycephalic heads, like the Polynesian and the black pygmy, the relation is eighty-three to one hundred. The cranial capacity again varies considerably, from nine hundred cubic centimeters to twenty-two hundred cubic centimeters. Many striking variations are also found in the projection of the jaws. A line drawn from the lower end of the nose to the chin makes a certain angle with the line drawn from the chin to the posterior end of the lower jaw; if the jaw projects very greatly, this angle will be much less than when they do not. In most of the Caucasian peoples, the lines meet at an angle of eighty-nine degrees, or very nearly a right angle, but in some of the lower races the figure may be only fifty-one degrees. Additional characters of the teeth and of the palate are also taken into account, and have proved their utility. Finally, the nose exhibits a wide range of variation from the small delicate feature of the Chinaman to the large, well-arched nose of the Indian. It may be hollowed out at the bridge instead of arched; again, it may be nearly an equilateral triangle in outline, as in the Veddahs, and the nostrils may open somewhat forward instead of downward. As many as fifteen distinct varieties of the human nose have been catalogued by Bertillon.