Creation by Evolution/Embryology and Evolution
EMBRYOLOGY AND EVOLUTION
By Edwin Grant Conklin
Professor of Biology, Princeton University
In early times and among primitive peoples all phenomena were regarded as supernatural. The rising and setting of the sun, the sweet influence of the Pleiades, the coming and going of the winds, storms, lightning, thunder—all the phenomena of life, birth, and death—were supposed to be directly controlled by gods or spirits. In the course of centuries many such events were seen to be natural—that is, lawful or orderly—and were more or less understood, so that gradually the supernatural withdrew to the misty mountain tops of origins. During the last two or three centuries enlightened people everywhere have come to realize that ordinary phenomena occur in accordance with natural laws. But in the matter of beginnings and origins the opinion is still widely held that they do not happen in accordance with nature, but only in response to supernatural action.
The Nature of Development
Even such a constantly recurring phenomenon as the origin of the individual human being was by many regarded as a supernatural phenomenon until a little more than a hundred years ago; and even today many intelligent people believe that the mind and soul of every person is supernaturally created, though few, if any, would go so far as to maintain that the body is supernaturally created, as some of the "preformationists" did in the eighteenth century. This old doctrine of preformation, or "evolutio," as it was called, maintained that the fully-formed but minute organism was encased within the egg or sperm, and the microscopists of the day, with poor instruments but good imagination, thought they could see the "homunculus," or little man, neatly packed away within the human germ cell. Within this homunculus in turn it was held that there must be another generation of germ cells, each containing its homunculus, and so on ad infinitum. Thus arose the doctrine of infinite encasement, or "box in box," and as the schoolmen of the Middle Ages discussed how many angels might stand on the point of a needle, so their successors of the eighteenth century discussed how many fully formed but infinitely minute generations might have been contained in the ovary of Mother Eve. These speculations reached their culmination in the works of Charles Bonnet (1748-1773), the distinguished natural philosopher of Geneva, in which he denied all new formation, all development or generation, and held that in the original creation of the progenitor of each species God created at one stroke all the individuals that would ever come from that progenitor. Thus every individual in the world was supernaturally created.
The actual study of the development of eggs forever put an end to such speculations. Caspar Frederick Wolff (1759) demonstrated that fully formed but minute organisms are not contained in germ cells; that development is not a mere unfolding of that which is already infolded, but that it consists, from inception to maturity, in an increase of complexity; and though he over-emphasized the simplicity of the germ, no one now questions that individual development everywhere consists of progress from a relatively simple to a relatively complex form. Development is not the unfolding of an infolded organism; it is the formation of new structures and functions by combinations and transformations of the relatively simple structures and functions of the germ cells. If anything in the world is natural, this process of development from an egg is natural; but nothing in the world is more wonderful. And yet this process of development is according to nature — a natural development and not a supernatural creation. Religious faith has been able to survive the knowledge of the fact that every human being in the world has come into existence by a process of development. Why should it be supposed that the recognition of an equally natural development of groups of individuals or species would be destructive of religion?
Ontogeny, or the origin of individuals, and phylogeny, or the origin of races, are two aspects of one and the same thing, namely, organic development. There is a remarkable parallelism between the two, and in particular the factors or causes of development are essentially the same in both. Just as the earth rotates on its axis and revolves in its orbit and the whole solar system moves through space in accordance with the law of gravity, so organisms undergo development as embryos, as species, and as larger groups according to the law of organic evolution.
Although the word evolution is now used to specify the development of species and of larger groups of organisms, it was once applied also to the development of individuals, and it could still be so applied, for evolution means only the transformation of an earlier into a later stage according to natural laws. In short, evolution is trans-formation rather than new-formation, natural development rather than supernatural creation, whether we are considering the origin of individuals or of species or larger groups. The Course of Development
Let us consider in brief outline some of the main facts of individual development. In practically all animals and plants development begins with the fertilization of an egg. William Harvey (1651), the discoverer of the circulation of the blood, expressed this fact in his famous dictum, “Omne vivum ex ovo.” The egg is a cell with the structures and functions that are characteristic of cells in general; that is, it contains protoplasm, "the physical basis of life," and this protoplasm is differentiated into a nucleus and a cellbody, and each of these contains other smaller units, many of which differ one from another. But these units are not adult parts in miniature, as the “preformationists” supposed; they are merely the elements out of which adult parts are built; they are like the letters of the alphabet out of which are built words and books and literatures.
In all animals the process of fertilization is practically the same, consisting essentially in the union of the nuclei of egg and spermatozoon. In almost all multicellular animals the egg at or near the time of fertilization gives off two minute cells, the polar bodies, which are rudimentary eggs and take no part in development. The fertilized egg undergoes repeated divisions or cleavages, forming a mass of cells, usually a hollow sphere, which is called a blastula, and this in turn becomes a gastrula by the formation in it of a gastric cavity. So far all animals, from sponges to men, travel the same road, although in every group there are minor peculiarities; they travel the same road, but they do not all follow in exactly the same tracks. The germ cells, the cleavage, the blastula, and the gastrula show characteristic differences in different phyla or classes, but their resemblances are more significant than their differences, and within the same phylum these resemblances are very striking. For example, in groups as distinct as flat-worms, annelids, and mollusks corresponding cleavage cells give rise to similar organs, and the larvae of these forms are very much alike, though the adult forms are very different.
In all vertebrates, from Amphioxus to man, there are many fundamental resemblances which distinguish this one phylum from all others. The eggs of vertebrates vary greatly in size, ranging from the microscopic eggs of mammals to the enormous ones of birds, and they differ also in the conditions under which they develop. Some are set free as naked cells to begin their lives independent of their parents; others are enclosed in protective membranes and shells and are further guarded and incubated by the mother or father; and still others undergo their development within the body of the mother. Associated with these varying conditions of development are many differences in the size of eggs, the rate of development, the methods of nutrition and other features. But the striking fact remains that in all vertebrates, from the lowest to the highest, one finds fundamental agreement in the position and structure of the principal organs of the body, and these organs arise from corresponding parts of the egg or embryo in essentially the same manner in all. Thus the brain and the nervous system come from a plate of superficial cells on the dorsal surface of the gastrula — a plate that rolls up at the sides to form a groove and then a tube. The front end of this tube enlarges to form the brain; the hinder part forms the spinal cord. The backbone, which gives the name to the phylum (vertebrate or chordate) appears as a row of cells, the notochord, above the alimentary canal and below the nerve plate. This row of cells is the basis of the backbone; it becomes surrounded by cartilage, is then changed to bone and is segmented into the vertebrae of the vertebral column. In all vertebrates the right and the left side of the pharynx — the cavity behind the mouth and nose — are pierced by a series of gill slits, and between these slits run arterial arches, which in fishes and water-breathing amphibians bring blood to the gills to be aerated; but in all air-breathing vertebrates true gills are lacking, though the arterial arches are still present in the embryo, and they undergo characteristic modifications in different classes of vertebrates. This very brief and general statement of the course of embryonic development applies to all vertebrates, man included.
In all animals development begins with the fertilized egg, which contains none of the structures of the developed animal, though every egg does contain specific kinds of protoplasm, which differ not only in different phyla and species but even in different individuals of the same species. These different kinds of protoplasm in the germ cells constitute the material basis of inherited differences in mature organisms, and the more unlike animals are in their adult state the more unlike is this germinal protoplasm of the egg cells from which they develop. In all organisms the egg contains hereditary units by whose combinations and transformations the characters of the cleavage, blastula, gastrula, embryo, and adult are formed. Development consists in such combinations and transformations, and just as the chemical combination of hydrogen and oxygen to form water gives rise to a substance with new qualities which were not present in the elements that entered into the combination, so new structures and functions develop out of germ cells — structures and functions that were not present as such in the cells. This is "creative synthesis," or "creative evolution," and it is a phenomenon that is found in all nature.
The development of functions goes hand in hand with the development of structures; indeed, function and structure are merely different aspects of life. All the general functions of living things are present in germ cells. In its growth every egg or sperm cell takes in nourishment, which it transforms into its own protoplasm; it divides at intervals and thus reproduces; it is sensitive, or capable of responding to stimuli. These are the fundamental functions of all living things, and every germ cell has them, but as development advances each of these functions becomes more specialized and more perfect. Nutrition, reproduction, sensation, which are all present in the egg cell, become, in the course of development, localized in cells specialized for each of these functions. But just as in the development of structures new parts, which were not present in the germ, appear by a process of “creative synthesis,” so new functions appear in the course of development, which are not merely sorted out of the general functions that were present at the beginning but are created by the combination and interaction of parts and functions already present. In this way the highest and most marvellous functions develop out of egg cells — even the special senses, instincts, and higher psychical faculties of animals and man. All are products of development or evolution — that is, they have come as a result of new combinations and transformations of the functions present in germ cells. Every step in this process is natural; yet that a complex animal, even a man, with all his godlike faculties, can develop out of a germ cell is surely the climax of all wonders!
The Recapitulation Theory
At the beginning of the nineteenth century the belief was general that higher animals pass through stages in their development that correspond to the adult condition of lower animals. In 1828 von Baer, “the father of comparative embryology,” recognized that this view was not wholly correct, and he modified it as follows: “An embryo never resembles an adult animal and is only to be compared with the embryos of other animals. The more different two animal forms are in their end stages the farther back in their development must one go in order to find agreement between them.” This has often been called “von Baer's Law.” Louis Agassiz, in his famous Essay on Classification (1858), pointed out the fact that there is a parallelism between embryology, palaeontology, and classification in that the stages that an animal passes through in its development from the egg resemble certain animal forms that have appeared in the past history of the earth and also certain lower forms now living.
The full significance of this parallelism was not appreciated until the revival of the doctrine of evolution under Darwin. In the fourteenth chapter of the Origin of Species Darwin discusses this parallelism and the significance of the homologies of embryos, and he closes his discussion of embryology with these carefully guarded words: “Embryology rises greatly in interest when we look at the embryo as a picture, more or less obscured, of the progenitor, either in its adult or larval state, of all the members of the same great class.”
It was Ernst Haeckel, in his Generelle Morphologie (1866) and in many later books, who announced that “Ontogeny is a short recapitulation of Phylogeny” — that is, the successive embryonic stages in the development of an animal correspond to the successive adult stages of the phylum to which it belongs. This is Haeckel's Fundamental Law of Biogeny (“Biogenetisches Grundgesetzt”), which is more frequently called the theory of embryonic recapitulation.
Inasmuch as many phenomena of development are mere adaptations to the conditions of embryonic or larval life and could never have been present in adult animals, Haeckel separated such characters, which he called “cænogenetic,” from the truly ancestral ones, which he called “palingenetic.” Unfortunately there was no certain method of always distinuishing these two types of embryonic characters, but in spite of this difficulty embryology was supposed to afford a short and easy method of determining the ancestral history of every group. Since every animal in its development from the egg to the adult condition was believed to climb its own ancestral tree, one can imagine the feverish zeal with which the study of embryology was pursued. Here was a method which promised to reveal more important secrets of the past than would the unearthing of all the buried monuments of antiquity — in fact nothing less than a complete genealogical tree of all the diversified forms of life which inhabit the earth. It promised to reveal not only the animal ancestry of man and the line of his descent but also the method of origin of his mental, social, and ethical faculties.
Unfortunately there was no certain criterion by which the palingenetic or ancestral features of development could be distinguished from the cænogenetic or recently acquired ones, and what one embryologist regarded as ancestral another might consider a recent addition. Furthermore, when there were no living or fossil animals resembling certain embryological forms the fancy was given free rein to invent hypothetical ancestors corresponding to such forms.
As a result of such speculations multitudes of phylogenetic trees sprang up in the thin soil of embryological fact and developed a capacity of branching and producing hypothetical ancestors which was in inverse proportion to their hold on solid ground. For a time embryology was studied chiefly to learn the course of past evolution, but owing to the highly speculative character of such studies and to the differences of opinion as to what were original (palingenetic) and what were acquired (cænogenetic) characters, there gradually arose a widespread skepticism concerning the value of embryology for this purpose. Gegenbaur, in 1889, voiced the growing opinion among zoölogists in these words: “If we are compelled to admit that cænogenetic characters are intermingled with palingenetic, then we cannot regard ontogeny as a pure source of evidence regarding phyletic relationships. Ontogeny accordingly becomes a field in which an active imagination may have full scope for its dangerous play, but in which positive results are by no means everywhere to be attained. To attain such results the palingenetic and the cænogenetic phenomena must be sifted apart, an operation that requires more than one critical granum salis.” Since the time this was written there have been many other less moderate utterances to the same effect, some even declaring that there is no evidence that ontogeny ever recapitulates phylogeny and that Haeckel's “biogenetic law” has no foundation in fact.
But after all, these criticisms of certain details of the recapitulation theory have not destroyed the general and fundamental truth of that theory — namely, that many features of individual development repeat ancestral features. There are many remarkable and undoubted instances in which ontogeny repeats phylogeny and in which the relationships of organisms can be determined only by their embryological history. The most severe critics of Haeckel's “biogenetic law” do not deny this; their criticisms apply to details rather than to foundation principles.
It is certainly no mere accident that practically all animals being their individual existence as fertilized eggs; that before or during fertilization all eggs produce two polar bodies, or rudimentary eggs, which undergo no further development; that in all animals fertilization occurs in essentially the same way; that all eggs undergo a series of divisions or cleavages which lead to the formation of a hollow sphere, the blastula, and that the blastula gets a digestive cavity and becomes a two-layered embryo, the gastrula, comparable to a simple sponge or hydroid. It is certainly no accident that the cleavage of the egg in types as distinct as flatworms, annelids, and moUusks is almost cell for cell the same, and that, where differences exist, rudimentary cells have been found that correspond to well-developed cells in other forms. It is no accident that the eggs of all chordates, for example, have the same type of organization, that all develop a notochord and nervous system and sense organs and gill slits and excretory organs and hundreds of other structures in essentially the same way. These fundamental resemblances, or homologies, as they are technically called, call for some explanation, and the only natural explanation that has ever been proposed is evolution.
Some of these homologies between different embryos and between these and adult forms of lower animals are worthy of more detailed mention. The earliest stages of ontogeny are in many respects like the lowest living organisms. The egg carries us back to the protozoan; cleavage recalls the protozoan colony; the blastula suggests volvox-like forms; the radial gastrula suggests the sponge or hydroid forms; and the bilateral gastrula suggests polyclad-like forms.
From the gastrula stage onward different phyla usually follow different paths, but all the members of each phylum show many fundamental resemblances in embryonic and larval stages, though they may differ notably in adult structure. For example, crustaceans that differ widely in adult form have larvae that are strikingly alike. Zoölogists had long classified barnacles as mollusks, until a study of their larvae showed conclusively that they were crustaceans. Many parasitic crustaceans are mere sacs of eggs or spermatozoa in the adult stage, but they have typical crustacean larvae. Ascidians, or sea-squirts, were classified as mollusks until Kowalevsky showed that their larvae are little tadpoles with notochord, dorsal nerve-tube, and gill-slits like any typical vertebrate. These and many other examples show that natural affinities or ancestral relationships are often shown in embryos and larvae long after they have been obscured or lost in adult stages.
All vertebrates, from fishes to mammals, pass through a fish-like stage in their development in which they have (1) gill slits, (2) five or six pairs of aortic arches, (3) a simple tubular heart with one auricle and one ventricle, (4) a notochord (the basis of the backbone in higher vertebrates), (5) a primitive type of kidney (the pronephros), etc. These organs persist throughout life in the lowest fishes, but they undergo many changes in higher forms.
Entirely similar conditions are found in the development of the brain and central nervous system; the eye and ear; the limbs and muscular system; the digestive, respiratory and reproductive systems. In fact, nearly all the important organs and systems of higher vertebrates pass through stages in their development which in lower vertebrates remain permanently.
It is true that some of these embryonic reminiscences of lower forms have been modified and greatly abbreviated, but they are nevertheless all there and are recognizable. Among higher forms there are many adaptations to peculiar conditions of embryonic or larval life which have no counterpart in lower forms; such are the embryonic membranes of higher vertebrates, modifications due to the presence of much yolk, and extraordinary modifications of parasitic larvae. Such modifications indicate that evolutionary changes may occur in embryonic as well as in adult stages — in fact, that they may affect any part of the life history. But this does not disprove the thesis that in general “ontogeny recapitulates phylogeny.” The fundamental resemblances or homologies between embryos, larvae, and adults which have been cited above are just as genuine homologies as those between adult structures, and the only natural explanation that has ever been found for such homologies is inheritance from common ancestors.
The origin of the individual (ontogeny) is not only a key to the conditions and causes of the origin of the race (phylogeny) but is also an actual evolution — that is, an origin of new forms by transformation from old ones. In the long series of stages which an egg passes through in its transformation into a mature animal one sees not only an actual evolution — the evolution of an individual — but also, more or less obscurely, a repetition of the stages of the long-past evolution of the ancestors of the species.
Man no less than other mammals develops from a fertilized egg, which passes through cleavage, blastula, and gastrula stages. The human embryo has gill slits and aortic arches, which undergo exactly the same transformations that take place in other mammals. Man's heart is at first like that of a fish, consisting of one auricle and one ventricle. His backbone begins as a notochord, is next a segmented cartilaginous rod, then each segment or vertebra consists of five separate bones, and finally each fuses into a single bone. He has in the course of his development three different pairs of kidneys, first a pronephros (or fore-kidney), like that of the lower fishes, then a mesonephros (or mid-kidney), like that of the frogs, and finally a metanephros (or hind-kidney) like that of reptiles, birds, and mammals, which alone survives in the adult. His brain, eye, ear, in fact, all his organs, pass through stages in development that are characteristic of lower vertebrates. Even in those adult features that are distinctively human, such as the peculiar form of the hand and the foot, the number of bones in the ankle and wrist, the number of pairs of ribs, the absence of a tail and the relative hairlessness of the skin — in all these respects the human foetus resembles anthropoid apes more than adult man.
Why are not these and a hundred other structures made directly? Why this roundabout process of making a man? There is no answer but evolution.
Embryology bears indubitable testimony to the truth of evolution; ancestral history is repeated in individual history, but the record is like an ancient palimpsest that has been erased and written over again and again. Traces of the old record are still there; some are obscure, some are almost entirely obliterated, but wherever they are decipherable they, tell the old, old story of the common origin of animal and man — their similarities and their kinship.
The Causes of Development and Evolution
Ontogeny recapitulates certain stages and features of phylogeny, but the whole course of evolution through past geologic ages can be followed only by a study of fossils. The record is necessarily incomplete, for we rarely find all the stages of evolution represented in fossils. Nevertheless palaeontology is a certain guide to the general succession of living things during past ages. But evolution is a present process, going on to-day, and to see it at work it is not necessary to explore “the dark backward and abysm of time.” To be sure, it goes slowly; species are not made in a day any more than Rome was, but they are being made here and now, and many biologists are studying the steps, conditions, and causes of evolutionary changes in animals and plants. All such study of contemporary evolution is necessarily a study of successive generations of individuals, and all analytical or experimental study of the causes of evolution resolves itself into a study of the factors involved in the genesis of individuals; there is no other possible method of approaching the problem. The study of the factors involved in the genesis of individuals under various conditions of inheritance and environment reveals all that can certainly be known regarding the methods and causes of the evolution of races and species.
The causes of the development of an individual or of the evolution of a species are twofold, internal and external. The internal causes are represented by the organization of the germ cell, the external by surrounding conditions; the internal causes may be called heredity, the external causes environment.
An egg cell, like every other kind of cell, functions in response to stimuli. When a muscle cell is stimulated it contracts, when a gland cell is stimulated it secretes, when an egg cell is stimulated it develops. The stimulus comes, in the first instance at least, from the environment; an egg will start to develop only when stimulated by a spermatozoon or by certain salts or chemicals, or by changes in temperature, and it will continue to develop only so long as environmental stimuli of water, oxygen, food, temperature, etc., remain favorable to its development.
The character of development depends primarily upon the nature (that is, the hereditary organization) of the egg concerned, and secondarily upon the environmental stimuli. The former determines all the possibilities of development and its main course; the latter determines which of these possibilities are realized and modifies more or less the course of development.
Entirely similar causes are at work in the evolution of races or species. With true insight Charles Darwin wrote, many years ago: “Although every variation is either directly or indirectly caused by some change in the surrounding conditions, we must never forget that the nature of the organization which is acted on essentially governs the result.” Whether these variations are first wrought in mature organisms and then transferred in some unknown way to the germ cells, as Lamarckians assert, or whether they first appear in the germ cells, as Weismann and his followers maintain, is a secondary, although important, consideration, into which we will not enter here. In conclusion it may be confidently asserted that the causes or factors of the evolution of species and of the development of an individual are fundamentally the same.
Development, Evolution, and Religion
What bearings do these scientific evidences as to the origin of individuals and species have on religious faith? It might satisfy our pride to believe that every human being sprang into existence fully formed and armed, like Minerva, from the brain of Jove, but however pleasing such a belief might be it could not be held by sane and enlightened people. We know well that every human being, even the greatest that ever trod the earth, was once a baby, an embryo, a germ cell, and this knowledge has not destroyed our belief in the dignity of man nor in the existence of God.
It pleases many persons to believe that the first man sprang into existence fully formed and perfectly endowed, coming directly from the hand of God by an act of supernatural creation. But such a belief cannot be held by enlightened persons who have really studied and appreciated the evidences of man's evolution. Such persons know well that every human being bears in his body the marks of his animal origin, and that the human embryo shows that man's ancestors were once water-breathers and later hairy quadrupeds before they became men; yet this knowledge need not destroy belief in the dignity of man nor in the existence of God.
It is a curious fact that many persons who are seriously disturbed by scientific teachings as to the evolution or gradual development of the human race accept with equanimity the universal observations as to the development of the human individual. The animal ancestry of the race should be no more disturbing to philosophical and religious beliefs than the germinal origin of the individual, yet the latter is a fact of universal observation, which cannot be relegated to the domain of theory and which cannot be successfully denied. If we admit the fact of the development of the entire individual from the egg, surely it matters little to our religious beliefs to admit the development or evolution of the race from some animal ancestor; for who will maintain that a germ cell is more complex, more perfect, or more intelligent than man's nearest relative in the animal world?
If the idea of the evolution of a species is atheistic, as some persons assert, so is the idea of the development of an individual, for individual development involves the same principles as race evolution. If one concedes the fact of individual development according to natural laws and without supernatural suspension of those laws, one might as well concede the fact of evolution without supernatural creation, so far at least as its effect on theology is concerned. It is surprising that the so-called “Fundamentalists” have not denied the development of the individual as they deny the development of the species; if they are consistent they will demand that we return to the teachings of the “preformationists” of the eighteenth century—to the idea of endless encasement of one generation within an earlier one and hence to the special and supernatural creation of every child of Adam in the creation of Adam himself. When that comes to pass, there will probably be a demand that the teaching of embryology shall be abolished in all schools and colleges.
How much truer and better is the view that God made the first man as he has made the last and that divine power and wisdom are shown just as fully in the development of the last human child as in the origin of the first! The actual facts of development are no less wonderful than any conceivable acts of creation—indeed they are vastly more wonderful than any that were ever conceived in prescientific times. Just as astronomy and geology and physics and chemistry have given us grander views of the universe than were ever dreamed of before, so biology, and especially the study of development and evolution, have given us grander views of the living world—its unity, its antiquity, its mystery—than were ever before held or suspected.
REFERENCES
I. Older Classical Works
- Agassiz, Louis. Methods of Study in Natural History, Boston, 1863.
- Baer, Karl Ernst von. Ueber Entwicklungsgeschichte der Thiere. Königsberg, 1828.
- Darwin, Charles. The Origin of Species. First edition, 1859; sixth edition, 1877. Appleton, New York. The Descent of Man. First edition, 1871; second edition, 1874. Appleton, New York.
- Haeckel, Ernst. Natural History of Creation (English translation, 1870), 2 vols. Appleton, 1906. The Evolution of Man (English translation, 1879). Putnam, 1910.
- Müller, Fritz. Facts and Arguments for Darwin (English translation). London, 1869.
- Wheeler, William M. Casper Friedrich Wolff and the “Theoria Generationis.” Woods Hole Lectures, 1899.
- Whitman, Charles O. Evolution and Epigenesis. Woods Hole Lectures, 1895. Bonnet’s Theory of Evolution. Woods Hole Lectures, 1895. Palingenesis and the Germ Doctrine of Bonnet. Woods Hole Lectures, 1895.
II. More Recent Works
- Conklin, Edwin G. The Mechanism of Evolution. Scientific Monthly, Dec. 1919—May 1920. Heredity and Environment in the Development of Men. Princeton Univ. Press, 5th ed., 1925.
- Hertwig, Oscar. The Biological Problem of To-day, etc. English translation, London, 1896.
- Hurst, C. H. Biological Theories III. The Recapitulation Theory. Natural Science, vol. II, 1893.
- Morgan, Thomas H. Evolution and Adaptation. Macmillan, 1903.
- Romanes, J. G. Darwin and After Darwin. Open Court. Chicago.
- Scott, William B. The Theory of Evolution. Macmillan, 1917.
- Wiedersheim, R. The Structure of Man, an Index to His Past History. English translation. Macmillan, 1895.
Among the greatest and most astonishing discoveries in the modern scientific world is the fact that the story told by the gradual development of the embryo gives a summary of the rise and development of its race. A story covering millions of years, if told in a few months or days, must necessarily be very abbreviated, condensed and modified, but the general lines of the two stories agree.
Another astonishing fact shown by the development of the embryo is that it follows the same line of ascent that is shown by the story told by the fossil rocks, thus doubly confirming the fact that the course of nature is from the simple to the complex, as from amoeba to man; that things in nature have come about by gradual change and development, instead of finished and perfect in the beginning.—Editor.