The Riddle of the Universe at the close of the nineteenth century

Since then the situation has, happily, been profoundly modified; while both schools, in their different paths, have pressed onward towards the same high goal, they have recognized their common aspiration, and they draw nearer to a knowledge of the truth in mutual covenant. At the end of the nineteenth century we have returned to that monistic attitude which our greatest realistic poet, Goethe, had recognized from its very commencement to be alone correct and fruitful.[8]

All the different philosophical tendencies may, from the point of view of modern science, be ranged in two antagonistic groups; they represent either a dualistic or a monistic interpretation of the cosmos. The former is usually bound up with teleological and idealistic dogmas, the latter with mechanical and realistic theories. Dualism, in the widest sense, breaks up the universe into two entirely distinct substances—the material world and an immaterial God, who is represented to be its creator, sustainer, and ruler. Monism, on the contrary (likewise taken in its widest sense), recognizes one sole substance in the universe, which is at once “God and nature”; body and spirit (or matter and energy) it holds to be inseparable. The extramundane God of dualism leads necessarily to theism; and the intra-mundane God of the monist leads to pantheism.

The different ideas of monism and materialism, and likewise the essentially distinct tendencies of theoretical and practical materialism, are still very frequently confused. As this and other similar cases of confusion of ideas are very prejudicial, and give rise to innumerable errors, we shall make the following brief observations, in order to prevent misunderstanding:

I. Pure monism is identical neither with the theoretical materialism that denies the existence of spirit, and dissolves the world into a heap of dead atoms, nor with the theoretical spiritualism (lately entitled “energetic” spiritualism by Ostwald) which rejects the notion of matter, and considers the world to be a specially arranged group of “energies” or immaterial natural forces.

II. On the contrary, we hold, with Goethe, that “matter cannot exist and be operative without spirit, nor spirit without matter.” We adhere firmly to the pure, unequivocal monism of Spinoza: Matter, or infinitely extended substance, and spirit (or energy), or sensitive and thinking substance, are the two fundamental attributes or principal properties of the all-embracing divine essence of the world, the universal substance. (Cf. chap. xii.)

CHAPTER II
OUR BODILY FRAME

Fundamental Importance of Anatomy—Human Anatomy—Hippocrates, Aristotle, Galen, Vesalius—Comparative Anatomy—Georges Cuvier—Johannes Müller—Karl Gegenbaur—Histology—The Cellular Theory—Schleiden and Schwann—Kölliker—Virchow—Man a Vertebrate, a Tetrapod, a Mammal, a Placental, a Primate—Prosimiæ and Simiæ—The Catarrhinæ—Papiomorphic and Anthropomorphic Apes—Essential Likeness of Man and the Ape in Corporal Structure

All biological research, all investigation into the forms and vital activities of organisms, must first deal with the visible body, in which the morphological and physiological phenomena are observed. This fundamental rule holds good for man just as much as for all other living things. Moreover, the inquiry must not confine itself to mere observation of the outer form; it must penetrate to the interior, and study both the general plan and the minute details of the structure. The science which pursues this fundamental investigation in the broadest sense is anatomy.

The first stimulus to an inquiry into the human frame arose, naturally, in medicine. As it was usually practised by the priests in the older civilizations, we may assume that these highest representatives of the education of the time had already acquired a certain amount of anatomical knowledge two thousand years before Christ, or even earlier. We do not, however, find more exact observations, founded on the dissection of mammals, and applied, by analogy, to the human frame, until we come to the Greek scientists of the sixth and fifth centuries before Christ—Empedocles (of Agrigentum) and Democritus (of Abdera), and especially the most famous physician of classic antiquity, Hippocrates (of Cos). It was from these and other sources that the great Aristotle, the renowned “father of natural history,” equally comprehensive as investigator and philosopher, derived his first knowledge. After him only one anatomist of any consequence is found in antiquity, the Greek physician Claudius Galenus (of Pergamus), who developed a wealthy practice in Rome in the second century after Christ, under the Emperor Marcus Aurelius. All these ancient anatomists acquired their knowledge, as a rule, not by the dissection of the human body itself—which was then sternly forbidden—but by a study of the bodies of the animals which most closely resembled man, especially the apes; they were all, indeed, comparative anatomists.

The triumph of Christianity and its mystic theories meant retrogression to anatomy, as it did to all the other sciences. The popes were resolved above all things to detain humanity in ignorance; they rightly deemed a knowledge of the human organism to be a dangerous source of enlightenment as to our true nature. During the long period of thirteen centuries the writings of Galen were almost the only source of human anatomy, just as the works of Aristotle were for the whole of natural history. It was not until the sixteenth century, when the spiritual tyranny of the papacy was broken by the Reformation, and the geocentric theory, so intimately connected with papal doctrine, was destroyed by the new cosmic system of Copernicus, that the knowledge of the human frame entered upon a new period of progress. The great anatomists, Vesalius (of Brussels), and Eustachius and Fallopius (of Modena), advanced the knowledge of our bodily structure so much by their own thorough investigations that little remained for their numerous followers to do, with regard to the more obvious phenomena, except the substantiation of details. Andreas Vesalius, as courageous as he was talented and indefatigable, was the pioneer of the movement; he completed in his twenty-eighth year (1543) that great and systematic work De humani corporis fabrica; he gave to the whole of human anatomy a new and independent scope and a more solid foundation. On that account he was, at a later date, at Madrid—where he was physician to Charles V. and Philip II.—condemned to death by the Inquisition as a magician. He only escaped by undertaking a pilgrimage to Jerusalem; in returning he suffered shipwreck on the Isle of Zante, and died there in misery and destitution.

The great merit of the nineteenth century, as far as our knowledge of the human frame is concerned, lies in the founding of two new lines of research of immense importance—comparative anatomy and histology, or microscopic anatomy. The former was intimately associated with human anatomy from the very beginning; indeed, it had to supply the place of the latter so long because the dissection of human corpses was a crime visited with capital punishment—that was the case even in the fifteenth century! But the many anatomists of the next three centuries devoted themselves mainly to a more accurate study of the human organism. The elaborate science which we now call comparative anatomy was born in the year 1803, when the great French zoologist Georges Cuvier (a native of Mömpelgard, in Alsace) published his profound Leçons sur l’anatomie comparée, and endeavored to formulate, for the first time, definite laws as to the organism of man and the beasts. While his predecessors—among whom was Goethe in 1790—had mainly contented themselves with comparing the skeleton of man with those of other animals, Cuvier’s broader vision took in the whole of the animal organization. He distinguished therein four great and mutually independent types: Vertebrata, Articulata, Mollusca, and Radiata. This advance was of extreme consequence for our “question of all questions,” since it clearly brought out the fact that man belonged to the vertebral type, and differed fundamentally from all the other types. It is true that the keen-sighted Linné had already, in his Systema Natuae, made a great step in advance by assigning man a definite place in the class of mammals; he had even drawn up the three groups of half-apes, apes, and men (Lemur, simia, and homo) in the order of primates. But his keen, systematic mind was not furnished with that profound empirical foundation, supplied by comparative anatomy, which Cuvier was the first to attain. Further developments were added by the great comparative anatomists of our own century—Friedrich Meckel (Halle), Johannes Müller (Berlin), Richard Owen, T. Huxley, and Karl Gegenbaur (Jena, subsequently Heidelberg). The last-named, in applying the evolutionary theory, which Darwin had just established, to comparative anatomy, raised his science to the front rank of biological studies. The numerous comparative anatomical works of Gegenbaur are, like his well-known Manual of Human Anatomy, equally distinguished by a thorough empirical acquaintance with their immense multitudes of facts, and by a comprehensive control of his material, and its philosophic appreciation in the evolutionary sense. His recent Comparative Anatomy of the Vertebrata establishes the solid foundation on which our conviction of the vertebral character of man in every aspect is chiefly based.

Microscopic anatomy has been developed, in the course of the present century, in a very different fashion from comparative anatomy. At the beginning of the century (1802) a French physician, Bichat, made an attempt to dissect the organs of the human body into their finer constituents by the aid of the microscope, and to show the connection of these various tissues (hista, or tela). This first attempt led to little result, because the scientist was ignorant of the one common element of all the different tissues. This was first discovered (1838) in the shape of the cell, in the plant world, by Matthias Schleiden, and immediately afterwards proved to be the same in the animal world by Theodor Schwann, the pupil and assistant of Johannes Müller at Berlin. Two other distinguished pupils of this great master, who are still living, Albert Kölliker and Rudolph Virchow, took up the cellular theory, and the theory of tissues which is founded on it, in the sixties, and applied them to the human organism in all its details, both in health and disease; they proved that, in man and all other animals, every tissue is made up of the same microscopic particles, the cells, and these “elementary organisms” are the real, self-active citizens which, in combinations of millions, constitute the “cellular state,” our body. All these cells spring from one simple cell, the cytula, or impregnated ovum, by continuous subdivision. The general structure and combination of the tissues are the same in man as in the other vertebrates. Among these the mammals, the youngest and most highly developed class take precedence, in virtue of certain special features which were acquired late. Such are, for instance, the microscopic texture of the hair, of the glands of the skin, and of the breasts, and the corpuscles of the blood, which are quite peculiar to mammals, and different from those of the other vertebrates; man, even in these finest histological relations, is a true mammal.

The microscopic researches of Albert Kölliker and Franz Leydig (at Würzburg) not only enlarged our knowledge of the finer structure of man and the beasts in every direction, but they were especially important in the light of their connection with the evolution of the cell and the tissue; they confirmed the great theory of Carl Theodor Siebold (1845) that the lowest animals, the Infusoria and the Rhizopods, are unicellular organisms.

Our whole frame, both in its general plan and its detailed structure, presents the characteristic type of the vertebrates. This most important and most highly developed group in the animal world was first recognized in its natural unity in 1801 by the great Lamarck; he embraced under that title the four higher animal groups of Linné—mammals, birds, amphibia, and fishes. To these he opposed the two lower classes, insects and worms, as invertebrates. Cuvier (1812) established the unity of the vertebrate type on a firmer basis by his comparative anatomy. It is quite true that all the vertebrates, from the fish up to man, agree in every essential feature; they all have a firm internal skeleton, a framework of cartilage and bone, consisting principally of a vertebral column and a skull; the advanced construction of the latter presents many variations, but, on the whole, all may be reduced to the same fundamental type. Further, in all vertebrates the “organ of the mind,” the central nervous system, in the shape of a spinal cord and a brain, lies at the back of this axial skeleton. Moreover, what we said of its bony environment, the skull, is also true of the brain—the instrument of consciousness and all the higher functions of the mind; its construction and size present very many variations in detail, but its general characteristic structure remains always the same.

We meet the same phenomenon when we compare the rest of our organs with those of the other vertebrates; everywhere, in virtue of heredity, the original plan and the relative distribution of the organs remain the same, although, through adaptation to different environments, the size and the structure of particular sections offer considerable variation. Thus we find that in all cases the blood circulates in two main blood-vessels, of which one—the aorta—passes over the intestine, and the other—the principal vein—passes underneath, and that by the broadening out of the latter in a very definite spot a heart has arisen; this “ventral heart” is just as characteristic of all vertebrates as the “dorsal heart” is of the articulata and mollusca. Equally characteristic of all vertebrates is the early division of the intestinal tube into a “head-gut” (or gill-gut), which serves in respiration, and a “body-gut” (or liver-gut), which co-operates with the liver in digestion; so are, likewise, the ramification of the muscular system, the peculiar structure of the urinary and sexual organs, and so forth. In all these anatomical relations man is a true vertebrate.

Aristotle gave the name of four-footed, or tetrapoda, to all the higher warm-blooded animals which are distinguished by the possession of two pairs of legs. The category was enlarged subsequently, and its title changed into the Latin “quadrupeda,” when Cuvier proved that even “two-legged” birds and men are really “four-footed”; he showed that the internal skeleton of the four legs in all the higher land-vertebrates, from the amphibia up to man, was originally constructed after the same pattern out of a definite number of members. The “arm” of man and the “wing” of bats and birds have the same typical skeleton as the foreleg of the animals which are conspicuously “four-footed.”

The anatomical unity of the fully developed skeleton in the four limbs of all tetrapods is very important. In order to appreciate it fully one has only to compare carefully the skeleton of a salamander or a frog with that of a monkey or a man. One perceives at once that the humeral zone in front and the pelvic zone behind are made up of the same principal parts as in the rest of the quadrupeds. We find in all cases that the first section of the leg proper consists of one strong marrow-bone (the humerus, in the forearm; the femur, behind); the second part, on the contrary, originally always consists of two bones (the ulna and radius, in front; the fibula and tibia, behind). When we further compare the developed structure of the foot proper we are surprised to find that the small bones of which it is made up are also similarly arranged and distributed in every case: in the front limb the three groups of bones of the forefoot (or “hand”) correspond in all classes of the tetrapoda: (1) the carpus, (2) the metacarpus, (3) the five fingers (digiti anteriores); in the rear limb, similarly, we have always the same three osseous groups of the hind foot: (1) the tarsus, (2) the metatarsus, and (3) the five toes (digiti posteriores). It was a very difficult task to reduce all these little bones to one primitive type, and to establish the equivalence (or homology) of the separate parts in all cases; they present extreme variations of form and construction in detail, sometimes being partly fused together and losing their individuality. This great task was first successfully achieved by the most eminent comparative anatomist of our day, Karl Gegenbaur. He pointed out, in his Researches into the Comparative Anatomy of the Vertebrata (1864), how this characteristic “five-toed leg” of the land tetrapods originally (not before the Carboniferous period) arose out of the radiating fin (the breast-fin, or the belly-fin) of the ancient fishes. He had also, in his famous Researches into the Skull of the Vertebrata (1872), deduced the younger skull of the tetrapods from the oldest cranial form among the fishes, that of the shark.

It is especially remarkable that the original number of the toes (five) on each of the four feet, which first appeared in the old amphibia of the Carboniferous period, has, in virtue of a strict heredity, been preserved even to the present day in man. Also, naturally and harmoniously, the typical construction of the joints, ligaments, muscles, and nerves of the two pairs of legs has, in the main, remained the same as in the rest of the “four-footed.” In all these important relations man is a true tetrapod.

The mammals are the youngest and most advanced class of the vertebrates. It is true they are derived from the older class of amphibia, like birds and reptiles: yet they are distinguished from all the other tetrapods by a number of very striking anatomical features. Externally, there is the clothing of the skin with hair, and the possession of two kinds of skin glands—the sweat glands and the sebaceous glands. A local development of these glands on the abdominal skin gave rise (probably during the Triassic period) to the organ which is especially characteristic of the class, and from which it derives its name—the mammarium. This important instrument of lactation is made up of milk glands (mammae) and the “mammar-pouches” (folds of the abdominal skin); in its development the teats appear, through which the young mammal sucks its mother’s milk. In internal structure the most remarkable feature is the possession of a complete diaphragm, a muscular wall which, in all mammals—and only in mammals—separates the thoracic from the abdominal cavity; in all other vertebrates there is no such separation. The skull of mammals is distinguished by a number of remarkable formations, especially in the maxillary apparatus (the upper and lower jaws, and the temporal bones). Moreover, the brain, the olfactory organ, the heart, the lungs, the internal and external sexual organs, the kidneys, and other parts of the body present special peculiarities, both in general and detailed structure, in the mammals; all these, taken collectively, point unequivocally to an early derivation of the mammals from the older groups of the reptiles and amphibia, which must have taken place, at the latest, in the Triassic period—at least twelve million years ago! In all these important characteristics man is a true mammal.

The numerous orders (12-33) which modern systematic zoology distinguishes in the class of mammals had been arranged in 1816 (by Blainville) in three natural groups, which still hold good as sub-classes: (1) the monotrema, (2) the marsupialia, and (3) the placentalia. These three sub-classes not only differ in the important respect of bodily structure and development, but they correspond, also, to three different historical stages in the formation of the class, as we shall see later on. The monotremes of the Triassic period were followed by the marsupials of the Jurassic, and these by the placentals of the Cretaceous. Man belongs to this, the youngest, sub-class; for he presents in his organization all the features which distinguish the placentals from the marsupials and the still older monotremes. First of all, there is the peculiar organ which gives a name to the placentals—the placenta. It serves the purpose of nourishing the young mammal embryo for a long time during its enclosure in the mother’s womb; it consists of blood-bearing tufts which grow out of the chorion surrounding the embryo, and penetrate corresponding cavities in the mucous membrane of the maternal uterus; the delicate skin between the two structures is so attenuated in this spot that the nutriment in the mother’s blood can pass directly into the blood of the child. This excellent contrivance for nourishing the embryo, which makes its first appearance at a somewhat late date, gives the fœtus the opportunity of a longer maintenance and a higher development in the protecting womb; it is wanting in the implacentalia, the two older sub-classes of the marsupials and the monotremes. There are, likewise, other anatomical features, particularly the higher development of the brain and the absence of the marsupial bone, which raise the placentals above all their implacental ancestors. In all these important particulars man is a true placental.

The very varied sub-class of the placentals has been recently subdivided into a great number of orders; they are usually put at from ten to sixteen, but when we include the important extinct forms which have been recently discovered the number runs up to from twenty to twenty-six. In order to facilitate the study of these numerous orders, and to obtain a deeper insight into their kindred construction, it is very useful to form them into great natural groups, which I have called “legions.” In my latest attempt[9] to arrange the advanced system of placentals in phylogenetic order I have substituted eight of these legions for the twenty-six orders, and shown that these may be reduced to four main groups. These, in turn, are traceable to one common ancestral group of all the placentals, their fossil ancestors, the prochoriata of the Cretaceous period. These are directly connected with the marsupial ancestors of the Jurassic period. We will only specify here, as the most important living representatives of these four main groups, the rodentia, the ungulata, the carnivora, and the primates. To the legion of the primates belong the prosimiæ (half-apes), the simiæ (real apes), and man. All the members of these three orders agree in many important features, and are at the same time distinguished by these features from the other twenty-three orders of placentals. They are especially conspicuous for the length of their bones, which were originally adapted to their arboreal manner of life. Their hands and feet are five-fingered, and the long fingers are excellently suited for grasping and embracing the branches of trees; they are provided, either partially or completely, with nails, but have no claws. The dentition is complete, containing all four classes—incisors, canine, premolars, and molars. Primates are also distinguished from all the other placentals by important features in the special construction of the skull and the brain; and these are the more striking in proportion to their development and the lateness of their appearance in the history of the earth. In all these important anatomical features our human organism agrees with that of all the other primates: man is a true primate.

An impartial and thorough comparison of the bodily structure of the primates forces us to distinguish two orders in this most advanced legion of the mammalia—half-apes (prosimiae or hemipitheci) and apes (simiae or pitheci). The former seem in every respect to be the lower and older, the latter to be the higher and younger order. The womb of the half-ape is still double, or two-horned, as it is in all the other mammals. In the true ape, on the contrary, the right and left wombs have completely amalgamated; they blend into a pear-shaped womb, which the human mother possesses besides the ape. In the skull of the apes, just as in that of man, the orbits of the eyes are completely separated from the temporal cavities by an osseous partition; in the prosimiae this is either entirely wanting or very imperfect. Finally, the cerebrum of the prosimia is either quite smooth or very slightly furrowed, and proportionately small; that of the true ape is much larger, and the gray bed especially, the organ of higher psychic activity, is much more developed; the characteristic convolutions and furrows appear on its surface exactly in proportion as the ape approaches to man. In these and other important respects, particularly in the construction of the face and the hands, man presents all the anatomical marks of a true ape.

The extensive order of apes was divided by Geoffroi, in 1812, into two sub-orders, which are still universally accepted in systematic zoology—New World and Old World monkeys, according to the hemisphere they respectively inhabit. The American “New World” monkeys are called Platyrrhinae (flat-nosed); their nose is flat, and the nostrils divergent, with a broad partition. The “Old World” monkeys, on the contrary, are called collectively Catarrhinae (narrow-nosed); their nostrils point downward, like man’s, and the dividing cartilage is narrow. A further difference between the two groups is that the tympanum is superficial in the platyrrhinae, but lies deeper, inside the petrous bone, in the catarrhinae; in the latter a long and narrow bony passage has been formed, while in the former it is still short and wide, or even altogether wanting. Finally, we have a much more important and decisive difference between the two groups in the circumstance that all the Old World monkeys have the same teeth as man—i. e., twenty deciduous and thirty-two permanent teeth (two incisors, one canine, two premolars, and three molars in each half of the jaw). The New World monkeys, on the other hand, have an additional premolar in each half-jaw, or thirty-six teeth altogether. The fact that these anatomical differences of the two simian groups are universal and conspicuous, and that they harmonize with their geographical distribution in the two hemispheres, fully authorizes a sharp systematic division of the two, as well as the phylogenetic conclusion that for a very long period (for more than a million years) the two sub-orders have been developing quite independently of each other in the western and eastern hemispheres. That is a most important point in view of the genealogy of our race; for man bears all the marks of a true catarrhina; he has descended from some extinct member of this sub-order in the Old World.

The numerous types of catarrhinae which still survive in Asia and Africa have been formed into two sections for some time—the tailed, doglike apes (the cynopitheci) and the tailless, manlike apes (the anthropomorpha). The latter are much nearer to man than the former, not only in the absence of a tail and in the general build of the body (especially of the head), but also on account of certain features which are unimportant in themselves but very significant in their constancy. The sacrum of the anthropoid ape, like that of man, is made up of the fusion of five vertebræ; that of the cynopithecus consists of three (more rarely four) sacral vertebræ. The premolar teeth of the cynopitheci are greater in length than breadth; those of the anthropomorpha are broader than they are long; and the first molar has four protuberances in the former, five in the latter. Furthermore, the outer incisor of the lower jaw is broader than the inner one in the manlike apes and man; in the doglike ape it is the smaller. Finally, there is a special significance in the fact, established by Selenka in 1890, that the anthropoid apes share with man the peculiar structure of the discoid placenta, the decidua reflexa, and the pedicle of the allantois. In fact, even a superficial comparison of the bodily structure of the anthropomorpha which still survive makes it clear that both the Asiatic (the orang-outang and the gibbous ape) and the African (the gorilla and chimpanzee) representatives of this group are nearer to man in build than any of the cynopitheci. Under the latter group we include the dog-faced papiomorpha, the baboon, and the long-tailed monkey, at a very low stage. The anatomical difference between these low papiomorpha and the most highly developed anthropoid apes is greater in every respect, whatever organ we take for comparison, than the difference between the latter and man. This instructive fact was established with great penetration by the anatomist Robert Hartmann, in his work on The Anthropoid Apes;[10] he proposed to divide the order of Simiae in a new way—namely, into the two great groups of primaria (man and the anthropoid ape) and the simiae proper, or pitheci (the rest of the catarrhinæ and all the platyrrhinæ). In any case, we have a clear proof of the close affinity of man and the anthropoid ape.

Thus comparative anatomy proves to the satisfaction of every unprejudiced and critical student the significant fact that the body of man and that of the anthropoid ape are not only peculiarly similar, but they are practically one and the same in every important respect. The same two hundred bones, in the same order and structure, make up our inner skeleton; the same three hundred muscles effect our movements; the same hair clothes our skin; the same groups of ganglionic cells build up the marvellous structure of our brain; the same four chambered heart is the central pulsometer in our circulation; the same thirty-two teeth are set in the same order in our jaws; the same salivary, hepatic, and gastric glands compass our digestive process; the same reproductive organs insure the maintenance of our race.

It is true that we find, on close examination, certain minor differences in point of size and shape in most of the organs of man and the ape; but we discover the same, or similar, differences between the higher and lower races of men, when we make a careful comparison—even, in fact, in a minute comparison of the various individuals of our own race. We find no two persons who have exactly the same size and form of nose, ears, eyes, and so forth. One has only to compare attentively these special features in many different persons in any large company to convince one’s self of the astonishing diversity of their construction and the infinite variability of specific forms. Not infrequently even two sisters are so much unlike as to make their origin from the same parents almost incredible. Yet all these individual variations do not weaken the significance of the fundamental similarity of structure; they are traceable to certain minute differences in the growth of the individual features.

CHAPTER III
OUR LIFE

Development of Physiology in Antiquity and the Middle Ages: Galen—Experiment and Vivisection—Discovery of the Circulation of the Blood by Harvey—Vitalism: Haller—Teleological and Vitalistic Conception of Life—Mechanical and Monistic View of the Physiological Processes—Comparative Physiology in the Nineteenth Century: Johannes Müller—Cellular Physiology: Max Verworn—Cellular Pathology: Virchow—Mammal Physiology—Similarity of all Vital Activity in Man and the Ape

It is only in the nineteenth century that our knowledge of human life has attained the dignity of a genuine, independent science; during the course of the century it has developed into one of the highest, most interesting, and most important branches of knowledge. This “science of the vital functions,” physiology, had, it is true, been regarded at a much earlier date as a desirable, if not a necessary, condition of success in medical treatment, and had been constantly associated with anatomy, the science of the structure of the body. But it was only much later, and much more slowly, than the latter that it could be thoroughly studied, as it had to contend with much more serious difficulties.

The idea of life, as the opposite of death, naturally became the subject of speculation at a very early age. In the living man, just as in other living animals, there were certain peculiar changes, especially movements, which were wanting in lifeless nature: spontaneous locomotion, the beat of the heart, the drawing of the breath, speech, and so forth. But the discrimination of such “organic movements” from similar phenomena in inorganic bodies was by no means easy, and was frequently impossible; the flowing stream, the flickering flame, the rushing wind, the falling rock, seemed to man to exhibit the same movements. It was quite natural that primitive man should attribute an independent life to these “dead” bodies. He knew no more of the real sources of movement in the one case than in the other.

We find the earliest scientific observations on the nature of man’s vital functions (as well as on his structure) in the Greek natural philosophers and physicians of the sixth and fifth centuries before Christ. The best collection of the physiological facts which were known at that time is to be found in the Natural History of Aristotle; a great number of his assertions were probably taken from Democritus and Hippocrates. The school of the latter had already made attempts to explain the mystery; it postulated as the ultimate source of life in man and the beasts a volatile “spirit of life” (Pneuma); and Erasistratus (280 B.C.) already drew a distinction between the lower and the higher “spirit of life,” the pneuma zoticon in the heart and the pneuma psychicon in the brain.

The credit of gathering these scattered truths into unity, and of making the first attempt at a systematic physiology, belongs to the great Greek physician Galen; we have already recognized in him the first great anatomist of antiquity (cf. p. 23). In his researches into the organs of the body he never lost sight of the question of their vital activity, their functions; and even in this direction he proceeded by the same comparative method, taking for his principal study the animals which approach nearest to man. Whatever he learned from these he applied directly to man. He recognized the value of physiological experiment; in his vivisection of apes, dogs, and swine he made a number of interesting experiments. Vivisection has been made the object of a violent attack in recent years, not only by the ignorant and narrow-minded, but by theological enemies of knowledge and by perfervid sentimentalists; it is, however, one of the indispensable methods of research into the nature of life, and has given us invaluable information on the most important questions. This was recognized by Galen seventeen hundred years ago.

Galen reduces all the different functions of the body to three groups, which correspond to the three forms of the pneuma, or vital spirit. The pneuma psychicon—the soul—which resides in the brain and nerves, is the cause of thought, sensation, and will (voluntary movement); the pneuma zoticon—the heart—is responsible for the beat of the heart, the pulse, and the temperature; the pneuma physicon, seated in the liver, is the source of the so-called vegetative functions, digestion and assimilation, growth and reproduction. He especially emphasized the renewal of the blood in the lungs, and expressed a hope that we should some day succeed in isolating the permanent element in the atmosphere—the pneuma, as he calls it—which is taken into the blood in respiration. More than fifteen centuries elapsed before this pneuma—oxygen—was discovered by Lavoisier.

In human physiology, as well as in anatomy, the great system of Galen was for thirteen centuries the Codex aureus, the inviolable source of all knowledge. The influence of Christianity, so fatal to scientific culture, raised the same insuperable obstacles in this as in every other branch of secular knowledge. Not a single scientist appeared from the third to the sixteenth century who dared to make independent research into man’s vital activity, and transcend the limits of the Galenic system. It was not until the sixteenth century that experiments were made in that direction by a number of distinguished physicians and anatomists (Paracelsus, Servetus, Vesalius, and others). In 1628 Harvey published his great discovery of the circulation of the blood, and showed that the heart is a pump, which drives the red stream unceasingly through the connected system of arteries and veins by a rhythmic, unconscious contraction of its muscles. Not less important were Harvey’s researches into the procreation of animals, as a result of which he formulated the well-known law: “Every living thing comes from an egg” (omne vivum ex ovo).

The powerful impetus which Harvey gave to physiological observation and experiment led to a great number of discoveries in the sixteenth and seventeenth centuries. These were co-ordinated for the first time by the learned Albrecht Haller about the middle of the last century; in his great work, Elementa Physiologiae, he established the inherent importance of the science, independently of its relation to practical medicine. In postulating, however, a special “sensitive force or sensibility” for neural action, and a special “irritability” for muscular movement, Haller gave strong support to the erroneous idea of a specific “vital force” (vis vitalis).

For more than a century afterwards, from the middle of the eighteenth until the middle of the nineteenth century, medicine and (especially) physiology were dominated by the old idea that a certain number of the vital processes may be traced to physical and chemical causes, but that others are the outcome of a special vital force which is independent of physical agencies. However much scientists differed in their conceptions of its nature and its relation to the “soul,” they were all agreed as to its independence of, and essential distinction from, the chemico-physical forces of ordinary “matter”; it was a self-contained force (archaeus), unknown in inorganic nature, which compelled ordinary forces into its service. Not only the distinctly psychical activity, the sensibility of the nerves and the irritability of the muscles, but even the phenomena of sense activity, of reproduction, and of development seemed so wonderful and so mysterious in their sources that it was impossible to attribute them to simple physical and chemical processes. As the free activity of the vital force was purposive and conscious, it led, in philosophy, to a complete teleology; especially did this seem indisputable when even the “critical” philosopher Kant had acknowledged, in his famous critique of the teleological position, that, though the mind’s authority to give a mechanical interpretation of all phenomena is theoretically unlimited, yet its actual capacity for such interpretation does not extend to the phenomena of organic life; here we are compelled to have recourse to a purposive—therefore supernatural—principle. This divergence of the vital phenomena from the mechanical processes of life became, naturally, more conspicuous as science advanced in the chemical and physical explanation of the latter. The circulation of the blood and a number of other phenomena could be traced to mechanical agencies; respiration and digestion were attributable to chemical processes like those we find in inorganic nature. On the other hand, it seemed impossible to do this with the wonderful performances of the nerves and muscles, and with the characteristic life of the mind; the co-ordination of all the different forces in the life of the individual seemed also beyond such a mechanical interpretation. Hence there arose a complete physiological dualism—an essential distinction was drawn between inorganic and organic nature, between mechanical and vital processes, between material force and life force, between the body and the soul. At the beginning of the nineteenth century this vitalism was firmly established in France by Louis Dumas, and in Germany by Reil. Alexander Humboldt had already published a poetical presentation of it in 1795, in his narrative of the Legend of Rhodes; it is repeated, with critical notes, in his Views of Nature.

In the first half of the seventeenth century the famous philosopher Descartes, starting from Harvey’s discovery of the circulation of the blood, put forward the idea that the body of man, like that of other animals, is merely an intricate machine, and that its movements take place under the same mechanical laws as the movements of an automaton of human construction. It is true that Descartes, at the same time, claimed for man the exclusive possession of a perfectly independent, immaterial soul, and held that its subjective experience, thought, was the only thing in the world of which we have direct and certain cognizance (“Cogito, ergo sum”). Yet this dualism did not prevent him from doing much to advance our knowledge of the mechanical life processes in detail. Borelli followed (1660) with a reduction of the movements of the animal body to purely physical laws, and Sylvius endeavored, about the same time, to give a purely chemical explanation of the phenomena of digestion and respiration; the former founded the iatromechanical, the latter the iatrochemical, school of medicine. However, these rational tendencies towards a natural, mechanical explanation of the phenomena of life did not attain to a universal acceptance and application; in the course of the eighteenth century they fell entirely away before the advance of teleological vitalism. The final disproof of the latter and a return to mechanism only became possible with the happy growth of the new science of comparative physiology in the forties of the present century.

Our knowledge of the vital functions, like our knowledge of the structure of the human body, was originally obtained, for the most part, not by direct observation of the human organism itself, but by a study of the more closely related animals among the vertebrates, especially the mammals. In this sense the very earliest beginning of human anatomy and physiology was “comparative.” But the distinct science of “comparative physiology,” which embraces the whole sphere of life phenomena, from the lowest animal up to man, is a triumph of the nineteenth century. Its famous creator was Johannes Müller, of Berlin (born, the son of a shoemaker, at Coblentz, in 1801). For fully twenty-five years—from 1833 to 1858—this most versatile and most comprehensive biologist of our age evinced an activity at the Berlin University, as professor and investigator, which is only comparable with the associated work of Haller and Cuvier. Nearly every one of the great biologists who have taught and worked in Germany for the last sixty years was, directly or indirectly, a pupil of Johannes Müller. Starting from the anatomy and physiology of man, he soon gathered all the chief groups of the higher and lower animals within his sphere of comparison. As, moreover, he compared the structure of extinct animals with the living, and the healthy organism with the diseased, endeavoring to bring together all the phenomena of life in a truly philosophic fashion, he attained a biological knowledge far in advance of his predecessors.

The most valuable fruit of these comprehensive studies of Johannes Müller was his Manual of Human Physiology. This classical work contains much more than the title indicates; it is the sketch of a comprehensive “comparative biology.” It is still unsurpassed in respect of its contents and range of investigation. In particular, we find the methods of observation and experiment applied in it as masterfully as the philosophic processes of induction and deduction. Müller was originally a vitalist, like all the physiologists of his time. Nevertheless, the current idea of a vital force took a novel form in his speculations, and gradually transformed itself into the very opposite. For he attempted to explain the phenomena of life mechanically in every department of physiology. His “transfigured” vital force was not above the physical and chemical laws of the rest of nature but entirely bound up with them. It was, in a word, nothing more than life itself—that is, the sum of all the movements which we perceive in the living organism. He sought especially to give them the same mechanical interpretation in the life of the senses and of the mind as in the working of the muscles; the same in the phenomena of circulation, respiration, and digestion as in generation and development. Müller’s success was chiefly due to the fact that he always began with the simplest life phenomena of the lowest animals, and followed them step by step in their gradual development up to the very highest, to man. In this his method of critical comparison proved its value both from the physiological and from the anatomical point of view. Johannes Müller is, moreover, the only great scientist who has equally cultivated these two branches of research, and combined them with equal brilliancy. Immediately after his death his vast scientific kingdom fell into four distinct provinces, which are now nearly always represented by four or more chairs—human and comparative anatomy, pathological anatomy, physiology, and the history of evolution. This sudden division of Müller’s immense realm of learning in 1858 has been compared to the dissolution of the empire which Alexander the Great had consolidated and ruled.

Among the many pupils of Johannes Müller who, either during his lifetime or after his death, labored hard for the advancement of the various branches of biology, one of the most fortunate—if not the most important—was Theodor Schwann. When the able botanist Schleiden, in 1838, indicated the cell as the common elementary organ of all plants, and proved that all the different tissues of the plant are merely combinations of cells, Johannes Müller recognized at once the extraordinary possibilities of this important discovery. He himself sought to point out the same composition in various tissues of the animal body—for instance, in the spinal cord of vertebrates—and thus led his pupil, Schwann, to extend the discovery to all the animal tissues. This difficult task was accomplished by Schwann in his Microscopic Researches into the Accordance in the Structure and Growth of Plants and Animals (1839). Thus was the foundation laid of the “cellular theory,” the profound importance of which, both in physiology and anatomy, has become clearer and more widely recognized in each subsequent year. Moreover, it was shown by two other pupils of Johannes Müller that the activity of all organisms is, in the ultimate analysis, the activity of the components of their tissues, the microscopic cells—these were the able physiologist Ernst Brücke, of Vienna, and the distinguished histologist Albert Kölliker, of Würzburg. Brücke correctly denominated the cells the “elementary organisms,” and showed that, in the body of man and of all other animals, they are the only actual, independent factors of the life process. Kölliker earned special distinction, not only in the construction of the whole science of histology, but particularly by showing that the animal ovum and its products are simple cells.

Still, however widely the immense importance of the cellular theory for all biological research was acknowledged, the “cellular physiology” which is based on it only began an independent development very recently. In this Max Verworn (of Jena) earned a twofold distinction. In his Psycho-physiological Studies of the Protistae (1889) he showed, as a result of an ingenious series of experimental researches, that the “theory of a cell-soul” which I put forward in 1866[11] is completely established by an accurate study of the unicellular protozoa, and that “the psychic phenomena of the protistæ form the bridge which unites the chemical processes of inorganic nature with the mental life of the highest animals.” Verworn has further developed these views, and based them on the modern theory of evolution, in his General Physiology. This distinguished work returns to the comprehensive point of view of Johannes Müller, in opposition to the one-sided and narrow methods of those modern physiologists who think to discover the nature of the vital phenomena by the exclusive aid of chemical and physical experiments. Verworn showed that it is only by Müller’s comparative method and by a profound study of the physiology of the cell that we can reach the higher stand-point which will give us a comprehensive survey of the wonderful realm of the phenomena of life. Only thus do we become convinced that the vital processes in man are subject to the same physical and chemical laws as those of all other animals.

The fundamental importance of the cellular theory for all branches of biology was made clear in the second half of the nineteenth century, not only by the rapid progress of morphology and physiology, but also by the entire reform of that biological science which has always been deemed most important on account of its relation to practical medicine—pathology, or the science of disease. Many even of the older physicians were convinced that human diseases were natural phenomena, like all other manifestations of life, and should be studied scientifically, like other vital functions. Particular schools of medicine—the Iatrophysical and the Iatrochemical—had already, in the seventeenth century, attempted to trace the sources of disease to certain physical and chemical changes. However, the imperfect condition of science at that period precluded any lasting results of these efforts. Many of the older theories, which sought the nature of disease in supernatural and mystical causes, were almost universally accepted down to the middle of the nineteenth century.

It was then that Rudolf Virchow, another pupil of Müller, conceived the happy idea of transferring the cellular theory from the healthy to the diseased organism; he sought in the more minute metamorphoses of the diseased cells and the tissues they composed the true source of those larger changes which, in the form of disease, threaten the living organism with peril and death. Especially during the seven years of his professorship at Würzburg (1849-56) Virchow pursued his great task with such brilliant results that his Cellular Pathology (published in 1858) turned, at one stroke, the whole of pathology and the dependent science of practical medicine into new and eminently fruitful paths. This reform of medicine is significant for our present purpose in that it led us to a monistic and purely scientific conception of disease. In sickness, no less than in health, man is subject to the same eternal “iron laws” of physics and chemistry as all the rest of the organic world.

Among the numerous classes of animals which modern zoology distinguishes the mammals occupy a pre-eminent position, not only on morphological grounds, but also for physiological reasons. As man belongs to the class of mammals (see p. 27) by every portion of his frame, we must expect him to share his characteristic functions with the rest of the mammals. Such we find to be the case. The circulation of the blood and respiration are accomplished in man under precisely the same laws and in the same manner as in all the other mammals—and in these alone; they are determined by the peculiar structure of their heart and lungs. In mammals only is all the arterial blood conducted from the left ventricle of the heart to the body by one, the left, branch of the aorta, while in birds it passes along the right branch, and in reptiles along both branches. The blood of mammals is distinguished from that of any other vertebrate by the circumstance that its red cells have lost their nucleus (by reversion). The respiratory movements are effected largely by the diaphragm in this class of animals alone, because only in them does it form a complete partition between the pectoral and abdominal cavities. Special importance, however, in this highest class of animals, attaches to the production of milk in the breasts (mammae), and to the peculiar method of the rearing of the young, which entails the supplying of the offspring with the mother’s milk. As this nutritive process reacts most powerfully on the other vital functions, and the maternal affection of mammals must have arisen from this intimate form of rearing, the name of the class justly reminds us of its great importance. In millions of pictures, most of them produced by painters of the highest rank, the “madonna with the child” is revered as the purest and noblest type of maternal love—the instinct which is found in its extreme form in the exaggerated tenderness of the mother-ape.

As the apes approach nearest to man of all the mammals in point of structure, we shall expect to hear the same of their vital functions; and that we find to be the case. Everybody knows how closely the habits, the movements, the sense activity, the mental life, and the parental customs of apes resemble those of man. Scientific physiology proves the same significant resemblance in other less familiar processes, particularly in the working of the heart, the division of the breasts, and the sexual life. In the latter connection it is especially noteworthy that the mature females of many kinds of apes suffer a periodical discharge of blood from the womb, which corresponds to the menstruation of the human female. The secretion of the milk in the glands and the suctorial process also take place in the female ape in precisely the same fashion as in women.

Finally, it is of especial interest that the speech of apes seems on physiological comparison to be a stage in the formation of articulate human speech. Among living apes there is an Indian species which is musical; the hylobates syndactylus sings a full octave in perfectly pure, harmonious half-tones. No impartial philologist can hesitate any longer to admit that our elaborate rational language has been slowly and gradually developed out of the imperfect speech of our Pliocene simian ancestors.

CHAPTER IV
OUR EMBRYONIC DEVELOPMENT

The Older Embryology—The Theory of Preformation—The Theory of Scatulation: Haller and Leibnitz—The Theory of Epigenesis: C. F. Wolff—The Theory of Germinal Layers: Carl Ernst Baer—Discovery of the Human Ovum: Remak, Kölliker—The Egg-Cell and the Sperm-Cell—The Theory of the Gastræa—Protozoa and Metazoa—The Ova and the Spermatozoa: Oscar Hertwig—Conception—Embryonic Development in Man—Uniformity of the Vertebrate Embryo—The Germinal Membranes in Man—The Amnion, the Serolemma, and the Allantois—The Formation of the Placenta and the “After-Birth”—The Decidua and the Funiculus Umbilicalis—The Discoid Placenta of Man and the Ape

Comparative ontogeny, or the science of the development of the individual animal, is a child of the nineteenth century in even a truer sense than comparative anatomy and physiology. How is the child formed in the mother’s womb? How do animals evolve from ova? How does the plant come forth from the seed? These pregnant questions have occupied the thoughtful mind for thousands of years. Yet it is only seventy years since the embryologist Baer pointed out the correct means and methods for penetrating into the mysteries of embryonic life; it is only forty years since Darwin, by his reform of the theory of descent, gave us the key which should open the long-closed door, and lead to a knowledge of embryonic agencies. As I have endeavored to give a complete, popular presentation of this very interesting but difficult study in the first section of my Anthropogeny, I will confine myself here to a brief survey and discussion of the most important phenomena. Let us first cast a historical glance at the older ontogeny, and the theory of preformation which is connected with it.

The classical works of Aristotle, the many-sided “father of science,” are the oldest known scientific sources of embryology, as we found them to be for comparative anatomy. Not only in his great natural history, but also in a special small work, Five Books on the Generation and Development of Animals, the great philosopher gives us a host of interesting facts, adding many observations on their significance; it was not until our own days that many of them were fully appreciated, and, indeed, we may say, discovered afresh. Naturally, many fables and errors are mixed up with them; it was all that was known at that time of the hidden growth of the human germ. Yet during the long space of the next two thousand years the slumbering science made no further progress. It was not until the commencement of the seventeenth century that there was a renewal of activity. In 1600 the Italian anatomist Fabricius ab Aquapendente published at Padua the first pictures and descriptions of the embryos of man and some of the higher animals; in 1687 the famous Marcello Malpighi, of Bologna, a distinguished pioneer alike in zoology and botany, published the first consistent exposition of the growth of the chick in the hatched egg.

All these older scientists were possessed with the idea that the complete body, with all its parts, was already contained in the ovum of animals, only it was so minute and transparent that it could not be detected; that, therefore, the whole development was nothing more than a growth, or an “unfolding,” of the parts that were already “infolded” (involutae). This erroneous notion, almost universally accepted until the beginning of the present century, is called the “preformation theory”; sometimes it is called the “evolution theory” (in the literal sense of “unfolding”); but the latter title is accepted by modern scientists for the very different theory of “transformation.”

Closely connected with the preformation theory, and as a logical consequence of it, there arose in the last century a further theory which keenly interested all thoughtful biologists—the curious “theory of scatulation.” As it was thought that the outline of the entire organism, with all its parts, was present in the egg, the ovary of the embryo had to be supposed to contain the ova of the following generation; these, again, the ova of the next, and so on in infinitum! On that basis the distinguished physiologist Haller calculated that God had created together, 6000 years ago—on the sixth day of his creatorial labors—the germs of 200,000,000,000 men, and ingeniously packed them all in the ovary of our venerable mother Eve. Even the gifted philosopher Leibnitz fully accepted this conclusion, and embodied it in his monadist theory; and as, on his theory, soul and body are in eternal, inseparable companionship, the consequence had to be accepted for the soul; “the souls of men have existed in organized bodies in their ancestors from Adam downward—that is, from the very beginning of things.”

In the month of November, 1759, a young doctor of twenty-six years, Caspar Friedrich Wolff (son of a Berlin tailor), published his dissertation for the degree at Halle, under the title, Theoria Generationis. Supported by a series of most laborious and painstaking observations, he proved the entire falsity of the dominant theories of preformation and scatulation. In the hatched egg there is at first no trace of the coming chick and its organs; instead of it we find on top of the yolk a small, circular, white disk. This thin “germinal disk” becomes gradually round, and then breaks up into four folds, lying upon each other, which are the rudiments of the four chief systems of organs—the nervous system above, the muscular system underneath, the vascular system (with the heart), and, finally, the alimentary canal. Thus, as Wolff justly remarked, the embryonic development does not consist in an unfolding of the preformed organs, but in a series of new constructions; it is a true epigenesis. One part arises after another, and all make their appearance in a simple form, which is very different from the later structure. This only appears after a series of most remarkable formations. Although this great discovery—one of the most important of the eighteenth century—could be directly proved by a verification of the facts Wolff had observed, and although the “theory of generation” which was founded on it was in reality not a theory at all, but a simple fact, it met with no sympathy whatever for half a century. It was particularly retarded by the high authority of Haller, who fought it strenuously with the dogmatic assertion that “there is no such thing as development: no part of the animal body is formed before another; all were created together.” Wolff, who had to go to St. Petersburg, was long in his grave before the forgotten facts he had observed were discovered afresh by Oken at Jena, in 1806.

After Wolff’s “epigenesis theory” had been established by Oken and Neckel (whose important work on the development of the alimentary canal was translated from Latin into German), a number of young German scientists devoted themselves eagerly to more accurate embryological research. The most important and successful of these was Carl Ernst Baer. His principal work appeared in 1828, with the title, History of the Development of Animals: Observations and Reflections. Not only the phenomena of the formation of the germ are clearly illustrated and fully described in it, but it adds a number of very pregnant speculations. In particular, the form of the embryo of man and the mammals is correctly presented, and the vastly different development of the lower invertebrate animals is also considered. The two leaflike layers which appear in the round germ disk of the higher vertebrates first divide, according to Baer, into two further layers, and these four germinal layers are transformed into four tubes, which represent the fundamental organs—the skin layer, the muscular layer, the vascular layer, and the mucous layer. Then, by very complicated evolutionary processes, the later organs arise, in substantially the same manner, in man and all the other vertebrates. The three chief groups of invertebrates, which in their turn differ widely from each other, have a very different development.

One of the most important of Baer’s many discoveries was the finding of the human ovum. Up to that time the little vesicles which are found in great numbers in the human ovary and in that of all other mammals had been taken for the ova. Baer was the first to prove, in 1827, that the real ova are enclosed in these vesicles—the “Graafian follicles”—and much smaller, being tiny spheres 1-120th inch in diameter, visible to the naked eye as minute specks under favorable conditions. He discovered likewise that from this tiny ovum of the mammal there develops first a characteristic germ globule, a hollow sphere with liquid contents, the wall of which forms the slender germinal membrane, or blastoderm.

Ten years after Baer had given a firm foundation to embryological science by his theory of germ layers a new task confronted it on the establishment of the cellular theory in 1838. What is the relation of the ovum and the layers which arise from it to the tissues and cells which compose the fully developed organism? The correct answer to this difficult question was given about the middle of this century by two distinguished pupils of Johannes Müller—Robert Remak, of Berlin, and Albert Kölliker, of Würzburg. They showed that the ovum is at first one simple cell, and that the many germinal globules, or granules, which arise from it by repeated segmentation, are also simple cells. From this mulberry-like group of cells are constructed first the germinal layers, and subsequently by differentiation, or division of labor, all the different organs. Kölliker has the further merit of showing that the seminal fluid of male animals is also a mass of microscopic cells. The active pin-shaped “seed-animalcules,” or spermatozoa, in it are merely ciliated cells, as I first proved in the case of the seed-filaments of the sponge in 1866. Thus it was proved that both the materials of generation, the male sperm and the female ova, fell in with the cellular theory. That was a discovery of which the great philosophic significance was not appreciated until a much later date, on a close study of the phenomena of conception in 1875.

All the older studies in embryonic development concern man and the higher vertebrates, especially the embryonic bird, since hens’ eggs are the largest and most convenient objects for investigation, and are plentiful enough to facilitate experiment; we can hatch them in the incubator, as well as by the natural function of the hen, and so observe from hour to hour, during the space of three weeks, the whole series of formations, from the simple germ cell to the complete organism. Even Baer had only been able to gather from such observations the fact that the different classes of vertebrates agreed in the characteristic form of the germ layers and the growth of particular organs. In the innumerable classes of invertebrates, on the other hand—that is, in the great majority of animals—the embryonic development seemed to run quite a different course, and most of them seemed to be altogether without true germinal layers. It was not until about the middle of the century that such layers were found in some of the invertebrates. Huxley, for instance, found them in the medusæ in 1849, and Kölliker in the cephalopods in 1844. Particularly important was the discovery of Kowalewsky (1886) that the lowest vertebrate—the lancelot, or amphioxus—is developed in just the same manner (and a very original fashion it is) as an invertebrate, apparently quite remote, tunicate, the sea-squirt, or ascidian. Even in some of the worms, the radiata and the articulata, a similar formation of the germinal layers was pointed out by the same observer. I myself was then (since 1886) occupied with the embryology of the sponges, corals, medusæ, and siphonophoræ, and, as I found the same formation of two primary germ layers everywhere in these lowest classes of multicellular animals, I came to the conclusion that this important embryonic feature is common to the entire animal world. The circumstance that in the sponges and the cnidaria (polyps, medusæ, etc.) the body consists for a long time, sometimes throughout life, merely of two simple layers of cells, seemed to me especially significant. Huxley had already (1849) compared these, in the case of the medusæ, with the two primary germinal layers of the vertebrates. On the ground of these observations and comparisons I then, in 1872, in my Philosophy of the Calcispongiae, published the “theory of the gastræa,” of which the following are the essential points:

I. The whole animal world falls into two essentially different groups, the unicellular primitive animals (Protozoa) and the multicellular animals with complex tissues (Metazoa). The entire organism of the protozoon (the rhizopods of the infusoria) remains throughout life a single simple cell (or occasionally a loose colony of cells without the formation of tissue, a coenobium). The organism of the metazoon, on the contrary, is only unicellular at the commencement, and is subsequently built up of a number of cells which form tissues.

II. Hence the method of reproduction and development is very different in each of these great categories of animals. The protozoa usually multiply by non-sexual means, by fission, gemmation, or spores; they have no real ova and no sperm. The metazoa, on the contrary, are divided into male and female sexes, and generally propagate sexually, by means of true ova, which are fertilized by the male sperm.

III. Hence, further, true germinal layers, and the tissues which are formed from them, are found only in the metazoa; they are entirely wanting in the protozoa.

IV. In all the metazoa only two primary layers appear at first, and these have always the same essential significance; from the outer layer the external skin and the nervous system are developed; from the inner layer are formed the alimentary canal and all the other organs.

V. I called the germ, which always arises first from the impregnated ovum, and which consists of these two primary layers, the “gut-larva,” or the gastrula; its cup-shaped body with the two layers encloses originally a simple digestive cavity, the primitive gut (the progaster or archenteron), and its simple opening is the primitive mouth (the prostoma or blastoporus). These are the earliest organs of the multicellular body, and the two cell layers of its enclosing wall, simple epithelia, are its earliest tissues; all the other organs and tissues are a later and secondary growth from these.

VI. From this similarity, or homology, of the gastrula in all classes of compound animals I drew the conclusion, in virtue of the biogenetic law (p. 81), that all the metazoa come originally from one simple ancestral form, the gastraea, and that this ancient (Laurentian), long-extinct form had the structure and composition of the actual gastrula, in which it is preserved by heredity.

VII. This phylogenetic conclusion, based on the comparison of ontogenetic facts, is confirmed by the circumstance that there are several of these gastræades still in existence (gastraemaria, cyemaria, physemaria, etc.), and also some ancient forms of other animal groups whose organization is very little higher (the olynthus of the sponges, the hydra, or common fresh-water polyp, of the cnidaria, the convoluta and other cryptocæla, or worms of the simplest type, of the platodes).

VIII. In the further development of the various tissue-forming animals from the gastrula we have to distinguish two principal groups. The earlier and lower types (the coelenteria or acoelomia) have no body cavity, no vent, and no blood; such is the case with the gastræades, sponges, cnidaria, and platodes. The later and higher types (the caelomaria or bilateria), on the other hand, have a true body cavity, and generally blood and a vent; to these we must refer the worms and the higher types of animals which were evolved from these later on, the echinodermata, mollusca, articulata, tunicata, and vertebrata.