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.