Part 4

The arrangement of these bones, peculiar and often complicated, but everywhere essentially the same in all the Tetrapoda, is a striking evidence that man is a descendant from the oldest pentadactyle Amphibia of the Carboniferous period. In man the pentadactyle type has been better preserved by constant heredity than in many other Mammalia, notably the Ungulata.

The oldest Carboniferous Amphibia, the armour-clad Stegocephali, and especially the remarkable Branchiosauri discovered by Credner, are now regarded by all competent zoologists as the indubitable common ancestral group of all Tetrapoda, comprising both Amphibia and Amniota. But whence this most remote group of Tetrapoda? That difficult question is answered by the marvellous progress of modern palæontology, and the answer is in complete harmony with the older results arrived at by comparative anatomy and ontogeny. Thirty-four years ago Carl Gegenbaur, the great living master of comparative anatomy, had demonstrated in a series of works how the skeletal parts of the various classes of Vertebrata, especially the skull and the limbs, still represent a continuous scale of phyletic gradations. Apart from the Cyclostomes, there are the fishes, and among them the Elasmobranchi (sharks and rays), which have best preserved the original structure in all its essential parts of organization. Closely connected with the Elasmobranchi are the Crossopterygii, and with these the Dipneusta or Dipnoi. Among the latter the highest importance attaches to the ancient Australian Ceratodus. Its organization and development is now, at last, becoming well known. This transitional group of Dipnoi, 'fishes with lungs' but without pentadactyle limbs, is the morphological bridge which joins the Ganoids and the oldest Amphibia. With this chain of successive groups of Vertebrata, constructed anatomically, the palæontological facts agree most satisfactorily. Selachians and Ganoids existed in the Silurian times, Dipnoi in the Devonian, Amphibia in the Carboniferous, Reptilia in the Permian, Mammalia in the Trias. These are historical facts of first rank. They connote in the most convincing manner that remarkable ascending scale in the series of vertebrates for our knowledge of which we are indebted to the works of Cuvier and Blainville, Meckel, Johannes Mueller and Gegenbaur, Owen and Huxley. The historical succession of the classes and orders of the Vertebrata in the course of untold millions of years is definitely fixed by the concordance of those leading works, and this invaluable acquisition is much more important for the foundation of our human pedigree than would be a complete series of all possible skeletons of Primates.

See note, p. 97.

Greater and more frequent difficulties arise if we penetrate further into the most remote part of the human phylogeny, and attempt to derive the vertebrate stem from an older stem of invertebrate ancestors. None of those had a skeleton which could be petrified; and the same remark applies to the lowest classes of Vertebrata--to the Cyclostomes and the Acrania. Palæontology, therefore, can tell us nothing about them; and we are limited to the other two great documents of phylogeny--the results of comparative anatomy and ontogeny. The value of their evidence is, however, so great that every competent zoologist can perceive the most important features of the most remote portion of our phylogeny.

Here the first place belongs to the invaluable results which modern comparative ontogeny has gained by the aid of the biogenetic law or the theory of recapitulation. The foundation-stones of vertebrate embryology had been laid by the works of Von Baer, Bischoff, Remak, and Koelliker; but the clearest light was thrown upon it by the famous discoveries of Kowalevsky in 1866. He proved the identity of the first developmental stages of Amphioxus and the Ascidians, and thereby confirmed the divination of Goodsir, who had already announced the close affinity of Vertebrates and Tunicates. The acknowledgment of this affinity has proved of increasing importance, and has abolished the erroneous hypothesis that the Vertebrata may have arisen from Annelids or from other Articulata. Meanwhile, from 1860 to 1872, I myself had been studying the development of the Spongiæ, Medusæ, Siphonophora, and other Coelenterata. Their comparison led me to the statements embodied in the 'Gastræatheorie,' the first abstract of which was published in 1872 in my monograph of the Calcispongiæ.

Wilhelm Bischoff of Munich: works on the history of the development of the rabbit, dog, guinea-pig, roe-deer. 1840-1854.

See note, p. 96.

'Ueber die Entwicklung der einfachen Ascidien,' Mém. Acad. St. Petersbourg, vii. ser., tome x. (1866). Other papers in 'Archiv f. Mikroskop. Anatomie,' vii. (1871); xiii. (1877).

These ideas were carried on and expanded during the subsequent ten years by the help of many excellent embryologists--first of all by E. Ray Lankester and Francis Balfour. The most fruitful result of these widely extended researches was the conclusion that the first stages of embryonic development are essentially the same in all the different Metazoa, and that we may derive from these facts certain views on the common descent of all from one ancestral form. The unicellular egg repeats the stage of our Protozoan ancestors; the Blastula is equivalent to an ancestral coenobium of Magosphæra or Volvox; the Gastrula is the hereditary repetition of the Gastræa, the common ancestor of all the Metazoa.

See note, p. 115--Theory of cells.

Man agrees in all these respects with the other vertebrates, and must have descended with them from the same common root.

Particularly obscure is that part of our phylogeny which extends from the Gastræa to Amphioxus. The morphological importance of this last small creature had been perceived by Johannes Mueller, who in 1842 gave the first accurate description of it. It would not, of course, be correct to proclaim the modern Amphioxus the common ancestor of all the vertebrates; but he must be regarded as closely related to them, and as the only survivor of the whole class of Acrania. If the Amphioxidæ had through some unfortunate accident become extinct, we should not have been able to gain anything like a positive glimpse at our most remote vertebrate ancestor. On the one hand, Amphioxus is closely connected with the early larva of the Cyclostomes, which are the oldest Craniota, and the pre-Silurian ancestors of the fishes. On the other hand, the ontogeny of Amphioxus is in harmony with that of the Ascidians, and if this agreement is not merely coincidental, but due to relationship, we are justified in reconstructing for both Ascidians and Amphioxus one common ancestral group of chordate animals, the hypothetical Prochordonia. The modern Copelata give us a remote idea of their structure. The curious Balanoglossus, the only living form of Enteropneusta, seems to connect these Prochordonia with the Nemertina and other Vermalia, which we unite in one large class--Frontonia.

No doubt these pre-Cambrian Vermalia, and the common root of all Metazoa, the Gastræades, were connected during the Laurentian period by a long chain of intermediate forms, and probably among these were some older forms of Rotatoria and Turbellaria; but at present it is not possible to fill this wide gap with hypotheses that are satisfactory, and we have to admit that here indeed are many missing links in the older history of the Invertebrata. Still, every zoologist who is convinced of the truth of transformism, and is accustomed to phylogenetic speculations, knows very well that their results are most unequal, often incomplete.

III.

Let us now recapitulate the ancestral chain of man, as it is set forth in the accompanying diagram (p. 55), which represents our present knowledge of our descent. For simplicity's sake the many side-issues or branches which lead to groups not in the main line of our descent have been left out, or have been indicated merely. Many of the stages are of course hypothetical, arrived at by the study of comparative anatomy and ontogeny; but an example for each of them has been taken from those living or fossil creatures which seem to be their nearest representatives.

1. The most remote ancestors of all living organisms were living beings of the simplest imaginable kind, organisms without organs, like the still existing Monera. Each consisted of a simple granule of protoplasm, a structureless mass of albuminous matter or plasson, like the recent Chromaceæ and Bacteriæ. The morphological value of these beings is not yet that of a cell, but that of a cytode, or cell without a nucleus. Cytoplasm and nucleus were still undifferentiated.

I assume that the first Monera owe their existence to spontaneous creation out of so-called anorganic combinations, consisting of carbon, hydrogen, oxygen, and nitrogen. An explanation of this hypothesis I have given in my 'Generelle Morphologie.'

The Monera probably arose early in the Laurentian period. The oldest are the Phytomonera, with vegetable metabolism. They possessed the power (characteristic of plants) of forming albumin by synthesis from carbon, water, and ammonia. From some of these plasma-forming Monera arose the plasmophagous Zoomonera with animal metabolism, living directly upon the produce of their plasmodomous or plasma-forming sisters. This is the first instance of the great principle of division of labour.

2. The second stage is that of the simple and single cell, a bit of protoplasm with a nucleus. Such unicellular organisms are still very common. The Amoebæ are their simplest representatives. The morphological value of such beings is the same as that of the egg of any animal. The naked egg cells of the sponges creep about in an amoeboid fashion, scarcely distinguishable from Amoeba. The same remark applies to the egg-cell of man himself in its early stages before it is enclosed in a membrane. The first unicellular organisms arose from Monera through differentiation of the inner nucleus from the outer protoplasm.

3. Repeated division of the unicellular organism produces the Synamoebium, or community of Amoebæ, provided the divisional products, or new generations of the original cell, do not scatter, but remain together. The existence of such a Coenobium, a number of equal and only loosely-connected cells, as a separate stage in the ancestral history of animals, is made highly probable by the fact that the eggs of all animals undergo after fertilization such a process of repeated self-division, or 'cleavage,' until the single egg cell is transformed into a heap of cells closely packed together, not unlike a mulberry (morula)--hence morula stage in ontogeny.

4. The morula of most animals further changes into a Blastula, a hollow ball filled with fluid, the wall being formed by a single layer of cells, the blastoderm or germinal layer. This modification is brought about by the action of the cells--they conveying nourishing fluid into the interior of the whole cell colony and thereby being themselves forced towards the surface. The Blastula of most Invertebrata, and even that of Amphioxus, is possessed of fine ciliæ, or hair-like processes, the vibrating motion of which causes the whole organism to rotate and advance in the water. Living representatives of such Blastæads, namely, globular gelatinous colonies of cells enclosing a cavity, are Volvox and Magosphæra.

5. The Blastula of most animals assumes a new larval form called Gastrula, in which the essential characteristics are that a portion of the blastoderm by invagination converts the Blastula into a cup with double walls, enclosing a new cavity, the primitive gut. This invagination or bulging-in obliterates the original inner cavity of the Blastula. The outer layer of the Gastrula is the ectoderm, the inner the endoderm; both pass into each other at the blastoporus, or opening of the gut cavity. The Gastrula is a stage in the embryonic development of the various great groups of animals, and some such primitive form as ancestral to all Metazoa is thus indicated. This hypothetical Gastræa is still very essentially represented by the lower Coelenterates--e.g., Olynthus, Hydra.

6. The sixth stage--that of the Platodes, or flat-worms--is very hypothetical. They are bilateral gastræads, with a flattened oblong body, furnished with ciliæ, with a primitive nervous system, simple sensory and reproductive organs, but still without appendages, body cavity, vent, and blood-vessels. The nearest living representatives of such creatures are the acoelous Turbellarians--e.g., Convoluta, a free-swimming, ciliated creature.

7. The next higher stage is represented by such low animals as the Gastrotricha--e.g., Chætonotus among the Rotatoria, which differ from the rhabdocoelous Turbellarians chiefly by the formation of a vent and the beginnings of a coelom, or cavity, between gut and body wall. The addition of a primitive vascular system and a pair of nephridia, or excretory organs, is first met with in the Nemertines.

8. These, together with the Enteropneusta (Balanoglossus), are comprised under the name of Frontonia, or Rhynchelminthes, and form the highest group of the Vermalia.

The Enteropneusta especially fix our attention, because they alone, although essentially 'worms,' exhibit certain characteristics which make it possible to bridge over the gulf which still separates the Invertebrata from the vertebrate phylum. The anterior portion of the gut is transformed into a breathing apparatus--hence Gegenbaur's term of Enteropneusta, or Gut-breathers. Moreover, Balanoglossus and Cephalodiscus possess another modification of the gut--namely, a peculiar diverticulum, which, in the present state of our knowledge, may be looked upon as the forerunner of the chorda dorsalis.

9. Stage of Prochordonia, as indicated by the larval form, called Chordula, which is common to the Tunicata and all the Vertebrata. These two groups possess three most important features: (a) A chorda dorsalis, a stiff rod lying in the long axis of the body, dorsally from the gut and below the central nervous system. This latter, for the first time in the animal kingdom, appears in the shape of a spinal cord. (b) The use of the anterior portion of the gut for respiratory purposes. (c) The larval development of the Tunicata is essentially the same as that of the Vertebrata in its early stages. Only the free-swimming Copelata or Appendicularia among the Tunicates retain most of these features. The others, which become sessile--namely, the Ascidiæ, or sea-squirts--degenerate and specialize away from the main line.

10. Stage of the Acrania, represented by Amphioxus. The early development of this little marine creature agrees closely with that of the Tunicates; but one important feature is added to its organization--namely, metamerism, segmentally arranged mesoderm. Amphioxus still possesses neither skull nor vertebræ, neither ribs nor jaws, and no limbs. But it is a member of the Vertebrata if we define these as follows: Bilateral symmetrical animals with segmentally arranged mesoderm, with a chorda dorsalis between the tubular nervous system and the gut, and with respiratory organs which arise from the anterior portion of the gut. We do not assume that Amphioxus stands in the direct ancestral line; it is probably much specialized, partly degenerated, and represents a side-branch; but it is, nevertheless, the only creature, hitherto known, which satisfactorily connects the Vertebrata with their invertebrate ancestors. Many other efforts have been made to solve the mystery of the origin of the Vertebrata--all less satisfactory than the present suggestion, or even absolutely futile. This remark applies especially to the attempts to derive them from either Articulata or Echinoderms. The other great and highly developed phylum, the Mollusca, is quite out of the question. We have to go back to a level at which all these principal phyla meet, and there we find the Vermalia, the lower of which alone permit connection in an upward direction with the higher phyla.