The Evolution of Man, V.2, Ernst Haeckel [free ebook reader for pc txt] 📗

The ontogenetic loss of the gills and the tail in the frog and toad can only be explained on the assumption that they are descended from long-tailed Amphibia of the salamander type. This is also clear from the comparative anatomy of the two groups. This remarkable metamorphosis is, however, also interesting because it throws a certain light on the phylogeny of the tail-less apes and man. Their ancestors also had long tails and gills like the gilled Amphibia, as the tail and the gill-arches of the human embryo clearly show.

For comparative anatomical and ontogenetic reasons, we must not seek these amphibian ancestors of ours--as one would be inclined to do, perhaps--among the tail-less Batrachia, but among the tailed lower Amphibia.

The vertebrate form that comes next to the Amphibia in the series of our ancestors is a lizard-like animal, the earlier existence of which can be confidently deduced from the facts of comparative anatomy and ontogeny. The living Hatteria of New Zealand (Figure 2.264) and the extinct Rhyncocephala of the Permian period (Figure 2.265) are closely related to this important stem-form; we may call them the Protamniotes, or Primitive Amniotes. All the Vertebrates above the Amphibia--or the three classes of reptiles, birds, and mammals--differ so much in their whole organisation from all the lower Vertebrates we have yet considered, and have so great a resemblance to each other, that we put them all together in a single group with the title of Amniotes. In these three classes alone we find the remarkable embryonic membrane, already mentioned, which we called the amnion; a cenogenetic adaptation that we may regard as a result of the sinking of the growing embryo into the yelk-sac.

All the Amniotes known to us--all reptiles, birds, and mammals (including man)--agree in so many important points of internal structure and development that their descent from a common ancestor can be affirmed with tolerable certainty. If the evidence of comparative anatomy and ontogeny is ever entirely beyond suspicion, it is certainly the case here. All the peculiarities that accompany and follow the formation of the amnion, and that we have learned in our consideration of human embryology; all the peculiarities in the development of the organs which we will presently follow in detail; finally, all the principal special features of the internal structure of the full-grown Amniotes--prove so clearly the common origin of all the Amniotes from single extinct stem-form that it is difficult to entertain the idea of their evolution from several independent stems. This unknown common stem-form is our primitive Amniote (Protamnion). In outward appearance it was probably something between the salamander and the lizard.

It is very probable that some part of the Permian period was the age of the origin of the Protamniotes. This follows from the fact that the Amphibia are not fully developed until the Carboniferous period, and that the first fossil reptiles (Palaehatteria, Homoeosaurus, Proterosaurus) are found towards the close of the Permian period. Among the important changes of the vertebrate organisation that marked the rise of the first Amniotes from salamandrine Amphibia during this period the following three are especially noteworthy: the entire disappearance of the water-breathing gills and the conversion of the gill-arches into other organs, the formation of the allantois or primitive urinary sac, and the development of the amnion.

One of the most salient characteristics of the Amniotes is the complete loss of the gills. All Amniotes, even if living in water (such as sea-serpents and whales), breathe air through lungs, never water through gills. All the Amphibia (with very rare exceptions) retain their gills for some time when young, and have for a time (if not permanently) branchial respiration; but after these there is no question of branchial respiration. The Protamniote itself must have entirely abandoned water-breathing. Nevertheless, the gill-arches are preserved by heredity, and develop into totally different (in part rudimentary) organs--various parts of the bone of the tongue, the frame of the jaws, the organ of hearing, etc. But we do not find in the embryos of the Amniotes any trace of gill-leaves, or of real respiratory organs on the gill-arches.

With this complete abandonment of the gills is probably connected the formation of another organ, to which we have already referred in embryology--namely, the allantois or primitive urinary sac (cf.

Chapter 1.

15). It is very probable that the urinary bladder of the 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.

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