Chapter 10

Dipneusts is the first structure of the allantois. We find in these a urinary bladder that proceeds from the lower wall of the hind end of the gut, and serves as receptacle for the renal secretions. This organ has been transmitted to the Amphibia, as we can see in the frog.

The formation of the amnion and the allantois and the complete disappearance of the gills are the chief characteristics that distinguish the Amniotes from the lower Vertebrates we have hitherto considered. To these we may add several subordinate features that are transmitted to all the Amniotes, and are found in these only. One striking embryonic character of the Amniotes is the great curve of the head and neck in the embryo. We also find an advance in the structure of several of the internal organs of the Amniotes which raises them above the highest of the anamnia. In particular, a partition is formed in the simple ventricle of the heart, dividing into right and left chambers. In connection with the complete metamorphosis of the gill-arches we find a further development of the auscultory organs. Also, there is a great advance in the structure of the brain, skeleton, muscular system, and other parts. Finally, one of the most important changes is the reconstruction of the kidneys. In all the earlier Vertebrates we have found the primitive kidneys as excretory organs, and these appear at an early stage in the embryos of all the higher Vertebrates up to man. But in the Amniotes these primitive kidneys cease to act at an early stage of embryonic life, and their function is taken up by the permanent or secondary kidneys, which develop from the terminal section of the prorenal ducts.

(FIGURE 2.264. The lizard (Hatteria punctata = Sphenodon punctatus) of New Zealand. The sole surviving proreptile. (From Brehm.))

Taking all these peculiarities of the Amniotes together, it is impossible to doubt that all the animals of this group--all reptiles, birds, and mammals--have a common origin, and form a single blood-related stem. Our own race belongs to this stem. Man is, in every feature of his organisation and embryonic development, a true Amniote, and has descended from the Protamniote with all the other Amniotes. Though they appeared at the end (possibly even in the middle) of the Paleozoic age, the Amniotes only reached their full development during the Mesozoic age. The birds and mammals made their first appearance during this period. Even the reptiles show their greatest growth at this time, so that it is called "the reptile age." The extinct Protamniote, the ancestor of the whole group, belongs in its whole organisation to the reptile class.

The genealogical tree of the amniote group is clearly indicated in its chief lines by their paleontology, comparative anatomy, and ontogeny. The group succeeding the Protamniote divided into two branches. The branch that will claim our whole interest is the class of the Mammals. The other branch, which developed in a totally different direction, and only comes in contact with the Mammals at its root, is the combined group of the reptiles and birds; these two classes may, with Huxley, be conveniently grouped together as the Sauropsida. Their common stem-form is an extinct lizard-like reptile of the order of the Rhyncocephalia. From this have been developed in various directions the serpents, crocodiles, tortoises, etc.--in a word, all the members of the reptile class. But the remarkable class of the birds has also been evolved directly from a branch of the reptile group, as is now established beyond question. The embryos of the reptiles and birds are identical until a very late stage, and have an astonishing resemblance even later. Their whole structure agrees so much that no anatomist now questions the descent of the birds from the reptiles. On the other hand, the mammal line has descended from the group of the Sauromammalia, a different branch of the Proreptilia. It is connected at its deepest roots with the reptile line, but it then diverges completely from it and follows a distinctive development. Man is the highest outcome of this class, the "crown of creation." The hypothesis that the three higher Vertebrate classes represent a single Amniote-stem, and that the common root of this stem is to be found in the amphibian class, is now generally admitted.

(FIGURE 2.265. Homoeosaurus pulchellus, a Jurassic proreptile from Kehlheim. (From Zittel.))

The instructive group of the Permian Tocosauria, the common root from which the divergent stems of the Sauropsids and mammals have issued, merits our particular attention as the stem-group of all the Amniotes. Fortunately a living representative of this extinct ancestral group has been preserved to our day; this is the remarkable lizard of New Zealand, Hatteria punctata (Figure 2.264). Externally it differs little from the ordinary lizard; but in many important points of internal structure, especially in the primitive construction of the vertebral column, the skull, and the limbs, it occupies a much lower position, and approaches its amphibian ancestors, the Stegocephala. Hence Hatteria is the phylogenetically oldest of all living reptiles, an isolated survivor from the Permian period, closely resembling the common ancestor of the Amniotes. It must differ so little from this extinct form, our hypothetical Protamniote, that we put it next to the Proreptilia. The remarkable Permian Palaehatteria, that Credner discovered in the Plauen terrain at Dresden in 1888, belongs to the same group (Figure 2.266). The Jurassic genus Homoeosaurus (Figure 2.265), of which well-preserved skeletons are found in the Solenhofen schists, is perhaps still more closely related to them.

Unfortunately, the numerous fossil remains of Permian and Triassic Tocosauria that we have found in the last two decades are, for the most part, very imperfectly preserved. Very often we can make only precarious inferences from these skeletal fragments as to the anatomic characters of the soft parts that went with the bony skeleton of the extinct Tocosauria. Hence it has not yet been possible to arrange these important fossils with any confidence in the ancestral series that descend from the Protamniotes to the Sauropsids on the one side and the Mammals on the other. Opinions are particularly divided as to the place in classification and the phylogenetic significance of the remarkable Theromorpha. Cope gives this name to a very interesting and extensive group of extinct terrestrial reptiles, of which we have only fossil remains from the Permian and Triassic strata. Forty years ago some of these Therosauria (fresh-water animals) were described by Owen as Anomodontia. But during the last twenty years the distinguished American paleontologists, Cope and Osborn, have greatly increased our knowledge of them, and have claimed that the stem-forms of the Mammals must be sought in this order. As a matter of fact, the Theromorpha are nearer to the Mammals in the chief points of structure than any other reptiles. This is especially true of the Thereodontia, to which the Pureosauria and Pelycosauria belong (Figure 2.267). The whole structure of their pelvis and hind-feet has attained the same form as in the Monotremes, the lowest Mammals. The formation of the scapula and the quadrate bone shows an approach to the Mammals such as we find in no other group of reptiles. The teeth also are already divided into incisors, canines, and molars. Nevertheless, it is very doubtful whether the Theromorpha really are in the ancestral line of the Sauromammals, or lead direct from the Tocosauria to the earliest Mammals. Other experts on this group believe that it is an independent legion of the reptiles, connected, perhaps, at its lowest root, with the Sauromammals, but developed quite independently of the Mammals--though parallel to them in many ways.

One of the most important of the zoological facts that we rely on in our investigation of the genealogy of the human race is the position of man in the Mammal class. However different the views of zoologists may have been as to this position in detail, and as to his relations to the apes, no scientist has ever doubted that man is a true mammal in his whole organisation and development. Linne drew attention to this fact in the first edition of his famous Systema Naturae (1735). As will be seen in any museum of anatomy or any manual of comparative anatomy; the human frame has all the characteristics that are common to the Mammals and distinguish them conspicuously from all other animals.

(FIGURE 2.266. Skull of a Permian lizard (Palaehatteria longicaudata). (From Credner.) n nasal bone, pf frontal bone, l lachrymal bone, po postorbital bone, sq covering bone, i cheek-bone, vo vomer, im inter-maxillary.)

If we examine this undoubted fact from the point of view of phylogeny, in the light of the theory of descent, it follows at once that man is of a common stem with all the other Mammals, and comes from the same root as they. But the various features in which the Mammals agree and by which they are distinguished are of such a character as to make a polyphyletic hypothesis quite inadmissible. It is impossible to entertain the idea that all the living and extinct Mammals come from a number of separate roots. If we accept the general theory of evolution, we are bound to admit the monophyletic hypothesis of the descent of all the Mammals (including man) from a single mammalian stem-form. We may call this long-extinct root-form and its earliest descendants (a few genera of one family) "primitive mammals" or "stem-mammals" (Promammalia). As we have already seen, this root-form developed from the primitive Proreptile stem in a totally different direction from the birds, and soon separated from the main stem of the reptiles. The differences between the Mammals and the reptiles and birds are so important and characteristic that we can assume with complete confidence this division of the vertebrate stem at the commencement of the development of the Amniotes. The reptiles and birds, which we group together as the Sauropsids, generally agree in the characteristic structure of the skull and brain, and this is notably different from that of the Mammals. In most of the reptiles and birds the skull is connected with the first cervical vertebra (the atlas) by a single, and in the Mammals (and Amphibia) by a double, condyle at the back of the head. In the former the lower jaw is composed of several pieces, and connected with the skull so that it can move by a special maxillary bone (the quadratum); in the Mammals the lower jaw consists of one pair of bony pieces, which articulate directly with the temporal bone. Further, in the Sauropsids the skin is clothed with scales or feathers; in the Mammals with hair. The red blood-cells of the former have a nucleus; those of the latter have not. In fine, two quite characteristic features of the Mammals, which distinguish them not only from the birds and reptiles, but from all other animals, are the possession of a complete diaphragm and of mammary glands that produce the milk for the nutrition of the young. It is only in the Mammals that the diaphragm forms a transverse partition of the body-cavity, completely separating the pectoral from the abdominal cavity. It is only in the mammals that the mother suckles its young, and this rightly gives the name to the whole class (mamma = breast).

(FIGURE 2.267. Skull of a Triassic theromorphum (Galesaurus planiceps), from the Karoo formation in South Africa. (From Owen.) a from the right, b from below, c from above, d tricuspid tooth. N nostrils, NA nasal bone, Mx upper jaw, Prf prefrontal, Fr frontal bone, A eye-pits, S temple-pits. Pa Parietal eye, Bo joint at back of head, Pt pterygoid-bone, Md lower jaw.)

From these pregnant facts of comparative anatomy and ontogeny it follows absolutely that the whole of the Mammals belong to a single natural stem, which branched off at an early date from the reptile-root. It follows further with the same absolute certainty that the human race is also a branch of this stem. Man shares all the characteristics I have described with all the Mammals, and differs in them from all other animals. Finally, from these facts we deduce with the same confidence those advances in the vertebrate organisation by which one branch of the Sauromammals was converted into the stem-form of the Mammals. Of these advances the chief were: (1) The characteristic modification of the skull and the brain; (2) the development of a hairy coat; (3) the complete formation of the diaphragm; and (4) the construction of the mammary glands and adaptation to suckling. Other important changes of structure proceeded step by step with these.

The epoch at which these important advances were made, and the foundation of the Mammal class was laid, may be put with great probability in the first section of the Mesozoic or secondary age--the Triassic period. The oldest fossil remains of mammals that we know were found in strata that belong to the earliest Triassic period--the upper Kueper. One of the earliest forms is the genus Dromatherium, from the North American Triassic (Figure 2.268). Their teeth still strikingly recall those of the Pelycosauria. Hence we may assume that this small and probably insectivorous mammal belonged to the stem-group of the Promammals. We do not find any positive trace of the third and most advanced division of the Mammals--the Placentals. These (including man) are much younger, and we do not find indisputable fossil remains of them until the Cenozoic age, or the Tertiary period. This paleontological fact is very important, because it fully harmonises with the evolutionary succession of the Mammal orders that is deduced from their comparative anatomy and ontogeny.

The latter science teaches us that the whole Mammal class divides into three main groups or sub-classes, which correspond to three successive phylogenetic stages. These three stages, which also represent three important stages in our human genealogy, were first distinguished in 1816 by the eminent French zoologist, Blainville, and received the names of Ornithodelphia, Didelphia, and Monodelphia, according to the construction of the female organs (delphys = uterus or womb). Huxley afterwards gave them the names of Prototheria, Metatheria, and Epitheria. But the three sub-classes differ so widely from each other, not only in the construction of the sexual organs, but in many other respects also, that we may confidently draw up the following important phylogenetic thesis: The Monodelphia or Placentals descend from the Didelphia or Marsupials; and the latter, in turn, are descended from the Monotremes or Ornithodelphia.

Thus we must regard as the twenty-first stage in our genealogical tree the earliest and lowest chief group of the Mammals--the sub-class of the Monotremes ("cloaca-animals," Ornithodelphia, or Prototheria, Figures 2.269 and 2.270). They take their name from the cloaca which they share with all the lower Vertebrates. This cloaca is the common outlet for the passage of the excrements, the urine, and the sexual products. The urinary ducts and sexual canals open into the hindmost part of the gut, while in all the other Mammals they are separated from the rectum and anus. The latter have a special uro-genital outlet (porus urogenitalis). The bladder also opens into the cloaca in the Monotremes, and, indeed, apart from the two urinary ducts; in all the other Mammals the latter open directly into the bladder. It was proved by Haacke and Caldwell in 1884 that the Monotremes lay large eggs like the reptiles, while all the other Mammals are viviparous. In 1894 Richard Semon further proved that these large eggs, rich in food-yelk, have a partial segmentation and discoid gastrulation, as I had hypothetically assumed in 1879; here again they resemble their reptilian ancestors. The construction of the mammary gland is also peculiar in the Monotremes. In them the glands have no teats for the young animal to suck, but there is a special part of the breast pierced with holes like a sieve, from which the milk issues, and the young Monotreme must lick it off. Further, the brain of the Monotremes is very little advanced. It is feebler than that of any of the other Mammals. The fore-brain or cerebrum, in particular, is so small that it does not cover the cerebellum. In the skeleton (Figure 2.270) the formation of the scapula among other parts is curious; it is quite different from that of the other Mammals, and rather agrees with that of the reptiles and Amphibia. Like these, the Monotremes have a strongly developed caracoideum. From these and other less prominent characteristics it follows absolutely that the Monotremes occupy the lowest place among the Mammals, and represent a transitional group between the Tocosauria and the rest of the Mammals. All these remarkable reptilian characters must have been possessed by the stem-form of the whole mammal class, the Promammal of the Triassic period, and have been inherited from the Proreptiles.

(FIGURE 2.268. Lower jaw of a Primitive Mammal or Promammal (Dromatherium silvestre) from the North American Triassic. i incisors, c canine, p premolars, m molars. (From Doderlein.))

During the Triassic and Jurassic periods the sub-class of the Monotremes was represented by a number of different stem-mammals. Numerous fossil remains of them have lately been discovered in the Mesozoic strata of Europe, Africa, and America. To-day there are only two surviving specimens of the group, which we place together in the family of the duck-bills, Ornithostoma. They are confined to Australia and the neighbouring island of Van Diemen's Land (or Tasmania); they become scarcer every year, and will soon, like their blood-relatives, be counted among the extinct animals. One form lives in the rivers, and builds subterraneous dwellings on the banks; this is the Ornithorhyncus paradoxus, with webbed feet, a thick soft fur, and broad flat jaws, which look very much like the bill of a duck (Figures 2.269 and 2.270). The other form, the land duck-bill, or spiny ant-eater (Echidna hystrix), is very much like the anteaters in its habits and the peculiar construction of its thin snout and very long tongue; it is covered with needles, and can roll itself up like a hedgehog. A cognate form (Parechidna Bruyni) has lately been found in New Guinea.

These modern Ornithostoma are the scattered survivors of the vast Mesozoic group of Monotremes; hence they have the same interest in connection with the stem history of the Mammals as the living stem-reptiles (Hatteria) for that of the reptiles, and the isolated Acrania (Amphioxus) for the phylogeny of the Vertebrate stem.

The Australian duck-bills are distinguished externally by a toothless bird-like beak or snout. This absence of real bony teeth is a late result of adaptation, as in the toothless Placentals (Edentata, armadillos and ant-eaters). The extinct Monotremes, to which the Promammalia belonged, must have had developed teeth, inherited from the reptiles. Lately small rudiments of real molars have been discovered in the young of the Ornithorhyncus, which has horny plates in the jaws instead of real teeth.

(FIGURE 2.269. The Ornithorhyncus or Duck-mole. (Ornithorhyncus paradoxus).

FIGURE 2.270. Skeleton of the Ornithorhyncus.)

The living Ornithostoma and the stem-forms of the Marsupials (or Didelphia) must be regarded as two widely diverging lines from the Promammals. This second sub-class of the Mammals is very interesting as a perfect intermediate stage between the other two. While the Marsupials retain a great part of the characteristics of the Monotremes, they have also acquired some of the chief features of the Placentals. Some features are also peculiar to the Marsupials, such as the construction of the male and female sexual organs and the form of the lower jaw. The Marsupials are distinguished by a peculiar hook-like bony process that bends from the corner of the lower jaw and points inwards. As most of the Placentals have not this process, we can, with some probability, recognise the Marsupial from this feature alone. Most of the mammal remains that we have from the Jurassic and Cretaceous deposits are merely lower jaws, and most of the jaws found in the Jurassic deposits at Stonesfield and Purbeck have the peculiar hook-like process that characterises the lower jaw of the Marsupial. On the strength of this paleontological fact, we may suppose that they belonged to Marsupials. Placentals do not seem to have existed at the middle of the Mesozoic age--not until towards its close (in the Cretaceous period). At all events, we have no fossil remains of indubitable Placentals from that period.

The existing Marsupials, of which the plant-eating kangaroo and the carnivorous opossum (Figure 2.272) are the best known, differ a good deal in structure, shape, and size, and correspond in many respects to the various orders of Placentals. Most of them live in Australia, and a small part of the Australian and East Malayan islands. There is now not a single living Marsupial on the mainland of Europe, Asia, or Africa. It was very different during the Mesozoic and even during the Cenozoic age. The sedimentary deposits of these periods contain a great number and variety of marsupial remains, sometimes of a colossal size, in various parts of the earth, and even in Europe. We may infer from this that the existing Marsupials are the remnant of an extensive earlier group that was distributed all over the earth. It had to give way in the struggle for life to the more powerful Placentals during the Tertiary period. The survivors of the group were able to keep alive in Australia and South America because the one was completely separated from the other parts of the earth during the whole of the Tertiary period, and the other during the greater part of it.

(FIGURE 2.271. Lower jaw of a Promammal (Dryolestes priscus), from the Jurassic of the Felsen strata. (From Marsh.))

From the comparative anatomy and ontogeny of the existing Marsupials we may draw very interesting conclusions as to their intermediate position between the earlier Monotremes and the later Placentals. The defective development of the brain (especially the cerebrum), the possession of marsupial bones, and the simple construction of the allantois (without any placenta as yet) were inherited by the Marsupials, with many other features, from the Monotremes, and preserved. On the other hand, they have lost the independent bone (caracoideum) at the shoulder-blade. But we have a more important advance in the disappearance of the cloaca; the rectum and anus are separated by a partition from the uro-genital opening (sinus urogenitalis). Moreover, all the Marsupials have teats on the mammary glands, at which the new-born animal sucks. The teats pass into the cavity of a pouch or pocket on the ventral side of the mother, and this is supported by a couple of marsupial bones. The young are born in a very imperfect condition, and carried by the mother for some time longer in her pouch, until they are fully developed (Figure 2.272). In the giant kangaroo, which is as tall as a man, the embryo only develops for a month in the uterus, is then born in a very imperfect state, and finishes its growth in the mother's pouch (marsupium); it remains in this about nine months, and at first hangs continually on to the teat of the mammary gland.

(FIGURE 2.272. The crab-eating Opossum (Philander cancrivorus). The female has three young in the pouch. (From Brehm.)

From these and other characteristics (especially the peculiar construction of the internal and external sexual organs in male and female) it is clear that we must conceive the whole sub-class of the Marsupials as one stem group, which has been developed from the Promammalia. From one branch of these Marsupials (possibly from more than one) the stem-forms of the higher Mammals, the Placentals, were afterwards evolved. Of the existing forms of the Marsupials, which have undergone various modifications through adaptation to different environments, the family of the opossums (Didelphida or Pedimana) seems to be the oldest and nearest to the common stem-form of the whole class. To this family belong the crab-eating opossum of Brazil (Figure 2.272) and the opossum of Virginia, on the embryology of which Selenka has given us a valuable work (cf. Figures 1.63 to 1.67 and 1.131 to 1.135). These Didelphida climb trees like the apes, grasping the branches with their hand-shaped hind feet. We may conclude from this that the stem-forms of the Primates, which we must regard as the earliest Lemurs, were evolved directly from the opossum. We must not forget, however, that the conversion of the five-toed foot into a prehensile hand is polyphyletic. By the same adaptation to climbing trees the habit of grasping their branches with the feet has in many different cases brought about that opposition of the thumb or great toe to the other toes which makes the hand prehensile. We see this in the climbing lizards (chameleon), the birds, and the tree-dwelling mammals of various orders.

Some zoologists have lately advanced the opposite opinion, that the Marsupials represent a completely independent sub-class of the Mammals, with no direct relation to the Placentals, and developing independently of them from the Monotremes. But this opinion is untenable if we examine carefully the whole organisation of the three sub-classes, and do not lay the chief stress on incidental features and secondary adaptations (such as the formation of the marsupium). It is then clear that the Marsupials--viviparous Mammals without placenta--are a necessary transition from the oviparous Monotremes to the higher Placentals with chorion-villi. In this sense the Marsupial class certainly contains some of man's ancestors.