It is impossible to determine even approximately, in hundreds or even thousands of years, the real and absolute duration of the phylogenetic period. But for some time now we have, through the research of geologists, been in a position to assign the relative length of the various sections of the organic history of the earth. The immediate data for determining this relative length of the geological periods are found in the thickness of the sedimentary strata—the strata that have been formed at the bottom of the sea or in fresh water from the mud or slime deposited there. These successive layers of limestone, sandstone, slate, marl, etc., which make up the greater part of the rocks, and are often several thousand feet thick, give us a standard for computing the relative length of the various periods.

To make the point quite clear, I must say a word about the evolution of the earth in general, and point out briefly the chief features of the story. In the first place, we encounter the principle that on our planet organic life began to exist at a definite period. That statement is no longer disputed by any competent geologist or biologist. The organic history of the earth could not commence until it was possible for water to settle on our planet in fluid condition. Every organism, without exception, needs fluid water as a condition of existence, and contains a considerable quantity of it. Our own body, when fully formed, contains sixty to seventy per cent of water in its tissues, and only thirty to forty per cent of solid matter. There is even more water in the body of the child, and still more in the embryo. In the earlier stages of development the human foetus contains more than ninety per cent of water, and not ten per cent of solids. In the lower marine animals, especially certain medusae, the body consists to the extent of more than ninety-nine per cent of sea-water, and has not one per cent of solid matter. No organism can exist or discharge its functions without water. No water, no life!

But fluid water, on which the existence of life primarily depends, could not exist on our planet until the temperature of the surface of the incandescent sphere had sunk to a certain point. Up to that time it remained in the form of steam. But as soon as the first fluid water could be condensed from the envelope of steam, it began its geological action, and has continued down to the present day to modify the solid crust of the earth. The final outcome of this incessant action of the water—wearing down and dissolving the rocks in the form of rain, hail, snow, and ice, as running stream or boiling surge—is the formation of mud. As Huxley says in his admirable Lectures on the Causes of Phenomena in Organic Nature, the chief document as to the past history of our earth is mud; the question of the history of past ages resolves itself into a question about the formation of mud.

As I have said, it is possible to form an approximate idea of the relative age of the various strata by comparing them at different parts of the earth's surface. Geologists have long been agreed that there is a definite historical succession of the different strata. The various superimposed layers correspond to successive periods in the organic history of the earth, in which they were deposited in the form of mud at the bottom of the sea. The mud was gradually converted into stone. This was lifted out of the water owing to variations in the earth's surface, and formed the mountains. As a rule, four or five great divisions are distinguished in the organic history of the earth, corresponding to the larger and smaller groups of the sedimentary strata. The larger periods are then sub-divided into a series of smaller ones, which usually number from twelve to fifteen. The comparative thickness of the groups of strata enables us to make an approximate calculation of the relative length of these various periods of time. We cannot say, it is true, "In a century a stratum of a certain thickness (about two feet) is formed on the average; therefore, a layer 1000 feet thick must be 500,000 years old." Different strata of the same thickness may need very different periods for their formation. But from the thickness or size of the stratum we can draw some conclusion as to the RELATIVE length of the period.

The first and oldest of the four or five chief divisions of the organic history of the earth is called the primordial, archaic, or archeozoic period. If we compute the total average thickness of the sedimentary strata at about 130,000 feet, this first period comprises 70,000 feet, or the greater part of the whole. For this and other reasons we may at once conclude that the corresponding primordial or archeolithic period must have been in itself much longer than the whole of the remaining periods together, from its close to the present day. It was probably much longer than the figures I have quoted (7 : 6) indicate—possibly 9 : 6. Of late years the thickness of the archaic rocks has been put at 90,000 feet.

SYNOPSIS OF THE PALEONTOLOGICAL FORMATIONS, OR THE FOSSILIFEROUS STRATA OF THE CRUST.

COLUMN 1 : Groups (V. down to I.).

COLUMN 2 : Systems (XIV. down to I.).

COLUMN 3 : Formations (38 down to 1).

COLUMN 4 : Synonyms of Formations.

V. Anthropolithic group, or anthropozoic (quaternary) group of strata : XIV. Recent (alluvium) : 38. Present : Upper alluvial.

V. Anthropolithic group, or anthropozoic (quaternary) group of strata : XIV. Recent (alluvium) : 37. Recent : Lower alluvial.

V. Anthropolithic group, or anthropozoic (quaternary) group of strata : XIII. Pleistocene (diluvium) : 36. Post-glacial : Upper diluvial.

V. Anthropolithic group, or anthropozoic (quaternary) group of strata : XIII. Pleistocene (diluvium) : 35. Glacial : Lower diluvial.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : XII. Pliocene (neo-tertiary) : 34. Arverne : Upper pliocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : XII. Pliocene (neo-tertiary) : 33. Subapennine : Lower pliocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : XI. Miocene (middle tertiary) : 32. Falun : Upper miocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : XI. Miocene (middle tertiary) : 31. Limbourg : Lower miocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xb. Oligocene (old tertiary) : 30. Aquitaine : Upper oligocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xb. Oligocene (old tertiary) : 29. Ligurium : Lower oligocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xa. Eocene (primitive tertiary) : 28. Gypsum : Upper eocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xa. Eocene (primitive tertiary) : 27. Coarse chalk : Middle eocene.

IV. Cenolithic group, or cenozoic (tertiary) group of strata : Xa. Eocene (primitive tertiary) : 26. London clay : Lower eocene.

III. Mesolithic group, or mesozoic (secondary) group of strata : IX. Chalk (cretaceous) : 25. White chalk. : Upper cretaceous.

III. Mesolithic group, or mesozoic (secondary) group of strata : IX. Chalk (cretaceous) : 24. Green Sand : Middle cretaceous.

III. Mesolithic group, or mesozoic (secondary) group of strata : IX. Chalk (cretaceous) : 23. Neocomian : Lower cretaceous.

III. Mesolithic group, or mesozoic (secondary) group of strata : IX. Chalk (cretaceous) : 22. Wealden : Weald-formation.

III. Mesolithic group, or mesozoic (secondary) group of strata : VIII. Jurassic : 21. Portland : Upper oolithic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VIII. Jurassic : 20. Oxford : Middle oolithic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VIII. Jurassic : 19. Bath : Lower oolithic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VIII. Jurassic : 18. Lias : Liassic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VII. Triassic : 17. Keuper : Upper triassic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VII. Triassic : 16. Muschelkalk : Middle triassic.

III. Mesolithic group, or mesozoic (secondary) group of strata : VII. Triassic : 15. Bunter : Lower triassic.

II. Paleolithic group, or paleozoic (primary) group of strata : VIb. Permian : 14. Zechstein : Upper permian.

II. Paleolithic group, or paleozoic (primary) group of strata : VIb. Permian : 13. Neurot sand : Lower permian.

II. Paleolithic group, or paleozoic (primary) group of strata : VIa. Carboniferous (coal-measures) : 12. Carboniferous sandstone : Upper carboniferous.

II. Paleolithic group, or paleozoic (primary) group of strata : VIa. Carboniferous (coal-measures) : 11. Carboniferous limestone : Lower carboniferous.

II. Paleolithic group, or paleozoic (primary) group of strata : V. Devonian : 10. Pilton : Upper devonian.

II. Paleolithic group, or paleozoic (primary) group of strata : V. Devonian : 9. Ilfracombe : Middle devonian.

II. Paleolithic group, or paleozoic (primary) group of strata : V. Devonian : 8. Linton : Lower devonian.

II. Paleolithic group, or paleozoic (primary) group of strata : IV. Silurian : 7. Ludlow : Upper silurian.

II. Paleolithic group, or paleozoic (primary) group of strata : IV. Silurian : 6. Wenlock : Middle silurian.

II. Paleolithic group, or paleozoic (primary) group of strata : IV. Silurian : 5. Llandeilo : Lower silurian.

I. Archeolithic group, or archeozoic (primordial) group of strata : III. Cambrian : 4. Potsdam : Upper cambrian.

I. Archeolithic group, or archeozoic (primordial) group of strata : III. Cambrian : 3. Longmynd : Lower cambrian.

I. Archeolithic group, or archeozoic (primordial) group of strata : II. Huronian : 2. Labrador : Upper laurentian.

I. Archeolithic group, or archeozoic (primordial) group of strata : I. Laurentian : 1. Ottawa : Lower laurentian.

The primordial period falls into three subordinate sections—the Laurentian, Huronian, and Cambrian, corresponding to the three chief groups of rocks that comprise the archaic formation. The immense period during which these rocks were forming in the primitive ocean probably comprises more than 50,000,000 years. At the commencement of it the oldest and simplest organisms were formed by spontaneous generation—the Monera, with which the history of life on our planet opened. From these were first developed unicellular organisms of the simplest character, the Protophyta and Protozoa (paulotomea, amoebae, rhizopods, infusoria, and other Protists). During this period the whole of the invertebrate ancestors of the human race were evolved from the unicellular organisms. We can deduce this from the fact that we already find remains of fossilised fishes (Selachii and Ganoids) towards the close of the following Silurian period. These are much more advanced and much younger than the lowest vertebrate, the Amphioxus, and the numerous skull-less vertebrates, related to the Amphioxus, that must have lived at that time. The whole of the invertebrate ancestors of the human race must have preceded these.

The primordial age is followed by a much shorter division, the paleozoic or Primary age. It is divided into four long periods, the Silurian, Devonian, Carboniferous, and Permian. The Silurian strata are particularly interesting because they contain the first fossil traces of vertebrates—teeth and scales of Selachii (Palaeodus) in the lower, and Ganoids (Pteraspis) in the upper Silurian. During the Devonian period the "old red sandstone" was formed; during the Carboniferous period were deposited the vast coal-measures that yield us our chief combustive material; in the Permian (or the Dyas), in fine, the new red sandstone, the Zechstein (magnesian limestone), and the Kupferschiefer (marl-slate) were formed. The collective depth of these strata is put at 40,000 to 45,000 feet. In any case, the paleozoic age, taken as a whole, was much shorter than the preceding and much longer than the subsequent periods. The strata that were deposited during this primary epoch contain a large number of fossils; besides the invertebrate species there are a good many vertebrates, and the fishes preponderate. There were so many fishes, especially primitive fishes (of the shark type) and plated fishes, during the Devonian, and also during the Carboniferous and Permian periods, that we may describe the whole paleozoic period as "the age of fishes." Among the paleozoic plated fishes or Ganoids the Crossopterygii and the Ctenodipterina (dipneusts) are of great importance.

During this period some of the fishes began to adapt themselves to living on land, and so gave rise to the class of the amphibia. We find in the Carboniferous period fossilised remains of five-toed amphibia, the oldest terrestrial, air-breathing vertebrates. These amphibia increase in variety in the Permian epoch. Towards the close of it we find the first Amniotes, the ancestors of the three higher classes of Vertebrates. These are lizard-like animals; the first to be discovered was the Proterosaurus, from the marl at Eisenach. The rise of the earliest Amniotes, among which must have been the common ancestor of the reptiles, birds, and mammals, is put back towards the close of the paleozoic age by the discovery of these reptile remains. The ancestors of our race during this period were at first represented by true fishes, then by dipneusts and amphibia, and finally by the earliest Amniotes, or the Protamniotes.

The third chief section of the organic history of the earth is the Mesozoic or Secondary period. This again is subdivided into three divisions Triassic, Jurassic, and Cretaceous. The thickness of the strata that were deposited in this period, from the beginning of the Triassic to the end of the Cretaceous period, is altogether about 15,000 feet, or not half as much as the paleozoic deposits. During this period there was a very brisk and manifold development in all branches of the animal kingdom. There were especially a number of new and interesting forms evolved in the vertebrate stem. Bony fishes (Teleostei) make their first appearance. Reptiles are found in extraordinary variety and number; the extinct giant-serpents (dinosauria), the sea-serpents (halisauria), and the flying lizards (pterosauria) are the most remarkable and best known of these. On account of this predominance of the reptile-class, the period is called "the age of reptiles." But the bird-class was also evolved during this period; they certainly originated from some division of the lizard-like reptiles. This is proved by the embryological identity of the birds and reptiles and their comparative anatomy, and, among other features, from the circumstance that in this period there were birds with teeth in their jaws and with tails like lizards (Archeopteryx, Odontornis).

Finally, the most advanced and (for us) the most important class of the vertebrates, the mammals, made their appearance during the mesozoic period. The earliest fossil remains of them were found in the latest Triassic strata—lower jaws of small ungulates and marsupials. More numerous remains are found a little later in the Jurassic, and some in the Cretaceous. All the mammal remains that we have from this section belong to the lower promammals and marsupials; among these were most certainly the ancestors of the human race. On the other hand, we have not found a single indisputable fossil of any higher mammal (a placental) in the whole of this period. This division of the mammals, which includes man, was not developed until later, towards the close of this or in the following period.

The fourth section of the organic history of the earth, the Tertiary or Cenozoic age, was much shorter than the preceding. The strata that were deposited during this period have a collective thickness of only about 3,000 feet. It is subdivided into four sections—the Eocene, Oligocene, Miocene, and Pliocene. During these periods there was a very varied development of higher plant and animal forms; the fauna and flora of our planet approached nearer and nearer to the character that they bear to-day. In particular, the most advanced class, the mammals, began to preponderate. Hence the Tertiary period may be called "the age of mammals." The highest section of this class, the placentals, now made their appearance; to this group the human race belongs. The first appearance of man, or, to be more precise, the development of man from some closely-related group of apes, probably falls in either the miocene or the pliocene period, the middle or the last section of the Tertiary period. Others believe that man properly so-called—man endowed with speech—was not evolved from the non-speaking ape-man (Pithecanthropus) until the following, the anthropozoic, age.

In this fifth and last section of the organic history of the earth we have the full development and dispersion of the various races of men, and so it is called the Anthropozoic as well as the Quaternary period. In the imperfect condition of paleontological and ethnographical science we cannot as yet give a confident answer to the question whether the evolution of the human race from some extinct ape or lemur took place at the beginning of this or towards the middle or the end of the Tertiary period. However, this much is certain: the development of civilisation falls in the anthropozoic age, and this is merely an insignificant fraction of the vast period of the whole history of life. When we remember this, it seems ridiculous to restrict the word "history" to the civilised period. If we divide into a hundred equal parts the whole period of the history of life, from the spontaneous generation of the first Monera to the present day, and if we then represent the relative duration of the five chief sections or ages, as calculated from the average thickness of the strata they contain, as percentages of this, we get something like the following relation:—

I. Archeolithic or archeozoic (primordial) age : 53 : 6.

II. Paleolithic or paleozoic (primary) age : 32 : 1.

III. Mesolithic or mesozoic (secondary) age : 11 : 5.

IV. Cenolithic or cenozoic (tertiary) age : 2 : 3

V. Anthropolithic or anthropozoic (quaternary) age : 0 : 5.

Total : 100 : 0.

In any case, the "historical period" is an insignificant quantity compared with the vast length of the preceding ages, in which there was no question of human existence on our planet. Even the important Cenozoic or Tertiary period, in which the first placentals or higher mammals appear, probably amounts to little over two per cent of the whole organic age.

Before we approach our proper task, and, with the aid of our ontogenetic acquirements and the biogenetic law, follow step by step the paleontological development of our animal ancestors, let us glance for a moment at another, and apparently quite remote, branch of science, a general consideration of which will help us in the solving of a difficult problem. I mean the science of comparative philology. Since Darwin gave new life to biology by his theory of selection, and raised the question of evolution on all sides, it has often been pointed out that there is a remarkable analogy between the development of languages and the evolution of species. The comparison is perfectly just and very instructive. We could hardly find a better analogy when we are dealing with some of the difficult and obscure features of the evolution of species. In both cases we find the action of the same natural laws.

All philologists of any competence in their science now agree that all human languages have been gradually evolved from very rudimentary beginnings. The idea that speech is a gift of the gods—an idea held by distinguished authorities only fifty years ago—is now generally abandoned, and only supported by theologians and others who admit no natural development whatever. Speech has been developed simultaneously with its organs, the larynx and tongue, and with the functions of the brain. Hence it will be quite natural to find in the evolution and classification of languages the same features as in the evolution and classification of organic species. The various groups of languages that are distinguished in philology as primitive, fundamental, parent, and daughter languages, dialects, etc., correspond entirely in their development to the different categories which we classify in zoology and botany as stems, classes, orders, families, genera, species, and varieties. The relation of these groups, partly co-ordinate and partly subordinate, in the general scheme is just the same in both cases; and the evolution follows the same lines in both.

When, with the assistance of this tree, we follow the formation of the various languages that have been developed from the common root of the ancient Indo-Germanic tongue, we get a very clear idea of their phylogeny. We shall see at the same time how analogous this is to the development of the various groups of vertebrates that have arisen from the common stem-form of the primitive vertebrate. The ancient Indo-Germanic root-language divided first into two principal stems—the Slavo-Germanic and the Aryo-Romanic. The Slavo-Germanic stem then branches into the ancient Germanic and the ancient Slavo-Lettic tongues; the Aryo-Romanic into the ancient Aryan and the ancient Greco-Roman. If we still follow the genealogical tree of these four Indo-Germanic tongues, we find that the ancient Germanic divides into three branches—the Scandinavian, the Gothic, and the German. From the ancient German came the High German and Low German; to the latter belong the Frisian, Saxon, and modern Low-German dialects. The ancient Slavo-Lettic divided first into a Baltic and a Slav language. The Baltic gave rise to the Lett, Lithuanian, and old-Prussian varieties; the Slav to the Russian and South-Slav in the south-east, and to the Polish and Czech in the west.

We find an equally prolific branching of its two chief stems when we turn to the other division of the Indo-Germanic languages. The Greco-Roman divided into the Thracian (Albano-Greek) and the Italo-Celtic. From the latter came the divergent branches of the Italic (Roman and Latin) in the south, and the Celtic in the north: from the latter have been developed all the British (ancient British, ancient Scotch, and Irish) and Gallic varieties. The ancient Aryan gave rise to the numerous Iranian and Indian languages.

This "comparative anatomy" and evolution of languages admirably illustrates the phylogeny of species. It is clear that in structure and development the primitive languages, mother and daughter languages, and varieties, correspond exactly to the classes, orders, genera, and species of the animal world. In both cases the "natural" system is phylogenetic. As we have been convinced from comparative anatomy and ontogeny, and from paleontology, that all past and living vertebrates descend from a common ancestor, so the comparative study of dead and living Indo-Germanic tongues proves beyond question that they are all modifications of one primitive language. This view of their origin is now accepted by all the chief philologists who have worked in this branch and are unprejudiced.

But the point to which I desire particularly to draw the reader's attention in this comparison of the Indo-Germanic languages with the branches of the vertebrate stem is, that one must never confuse direct descendants with collateral branches, nor extinct forms with living. This confusion is very common, and our opponents often make use of the erroneous ideas it gives rise to for the purpose of attacking evolution generally. When, for instance, we say that man descends from the ape, this from the lemur, and the lemur from the marsupial, many people imagine that we are speaking of the living species of these orders of mammals that they find stuffed in our museums. Our opponents then foist this idea on us, and say, with more astuteness than intelligence, that it is quite impossible; or they ask us, by way of physiological experiment, to turn a kangaroo into a lemur, a lemur into a gorilla, and a gorilla into a man! The demand is childish, and the idea it rests on erroneous. All these living forms have diverged more or less from the ancestral form; none of them could engender the same posterity that the stem-form really produced thousands of years ago.

It is certain that man has descended from some extinct mammal; and we should just as certainly class this in the order of apes if we had it before us. It is equally certain that this primitive ape descended in turn from an unknown lemur, and this from an extinct marsupial. But it is just as clear that all these extinct ancestral forms can only be claimed as belonging to the living order of mammals in virtue of their essential internal structure and their resemblance in the decisive anatomic characteristics of each ORDER. In external appearance, in the characteristics of the GENUS or SPECIES, they would differ more or less, perhaps very considerably, from all living representatives of those orders. It is a universal and natural procedure in phylogenetic development that the stem-forms themselves, with their specific peculiarities, have been extinct for some time. The forms that approach nearest to them among the living species are more or less—perhaps very substantially—different from them. Hence in our phylogenetic inquiry and in the comparative study of the living, divergent descendants, there can only be a question of determining the greater or less remoteness of the latter from the ancestral form. Not a single one of the older stem-forms has continued unchanged down to our time.

We find just the same thing in comparing the various dead and living languages that have developed from a common primitive tongue. If we examine our genealogical tree of the Indo-Germanic languages in this light, we see at once that all the older or parent tongues, of which we regard the living varieties of the stem as divergent daughter or grand-daughter languages, have been extinct for some time. The Aryo-Romanic and the Slavo-Germanic tongues have completely disappeared; so also the Aryan, the Greco-Roman, the Slavo-Lettic, and the ancient Germanic. Even their daughters and grand-daughters have been lost; all the living Indo-Germanic languages are only related in the sense that they are divergent descendants of common stem-forms. Some forms have diverged more, and some less, from the original stem-form.

This easily demonstrable fact illustrates very well the analogous case of the origin of the vertebrate species. Phylogenetic comparative philology here yields a strong support to phylogenetic comparative zoology. But the one can adduce more direct evidence than the other, as the paleontological material of philology—the old monuments of the extinct tongue—have been preserved much better than the paleontological material of zoology, the fossilised bones and imprints of vertebrates.

We may, however, trace man's genealogical tree not only as far as the lower mammals, but much further—to the amphibia, to the shark-like primitive fishes, and, in fine, to the skull-less vertebrates that closely resembled the Amphioxus. But this must not be understood in the sense that the existing Amphioxus, or the sharks or amphibia of to-day, can give us any idea of the external appearance of these remote stem-forms. Still less must it be thought that the Amphioxus or any actual shark, or any living species of amphibia, is a real ancestral form of the higher vertebrates and man. The statement can only rationally mean that the living forms I have referred to are COLLATERAL LINES that are much more closely related to the extinct stem-forms, and have retained the resemblance much better, than any other animals we know. They are still so like them in regard to their distinctive internal structure that we should put them in the same class with the extinct forms if we had these before us. But no direct descendants of these earlier forms have remained unchanged. Hence we must entirely abandon the idea of finding direct ancestors of the human race in their characteristic EXTERNAL FORM among the living species of animals. The essential and distinctive features that still connect living forms more or less closely with the extinct common stem-forms lie in the internal structure, not the external appearance. The latter has been much modified by adaptation. The former has been more or less preserved by heredity.

Comparative anatomy and ontogeny prove beyond question that man is a true vertebrate, and, therefore, man's special genealogical tree must be connected with that of the other Vertebrates, which spring from a common root with him. But we have also many important grounds in comparative anatomy and ontogeny for assuming a common origin for all the Vertebrates. If the general theory of evolution is correct, all the Vertebrates, including man, come from a single common ancestor, a long-extinct "Primitive Vertebrate." Hence the genealogical tree of the Vertebrates is at the same time that of the human race.

Our task, therefore, of constructing man's genealogy becomes the larger aim of discovering the genealogy of the entire vertebrate stem. As we now know from the comparative anatomy and ontogeny of the Amphioxus and the Ascidia, this is in turn connected with the genealogical tree of the Invertebrates (directly with that of the Vermalia), but has no direct connection with the independent stems of the Articulates, Molluscs, and Echinoderms. If we do thus follow our ancestral tree through various stages down to the lowest worms, we come inevitably to the Gastraea, that most instructive form that gives the clearest possible picture of an animal with two germinal layers. The Gastraea itself has originated from the simple multicellular vesicle, the Blastaea, and this in turn must have been evolved from the lowest circle of unicellular animals, to which we give the name of Protozoa. We have already considered the most important primitive type of these, the unicellular Amoeba, which is extremely instructive when compared with the human ovum. With this we reach the lowest of the solid data to which we are to apply our biogenetic law, and by which we may deduce the extinct ancestor from the embryonic form. The amoeboid nature of the young ovum and the unicellular condition in which (as stem-cell or cytula) every human being begins its existence justify us in affirming that the earliest ancestors of the human race were simple amoeboid coils.

But the further question now arises: "Whence came these first amoebae with which the history of life began at the commencement of the Laurentian epoch?" There is only one answer to this. The earliest unicellular organisms can only have been evolved from the simplest organisms we know, the Monera. These are the simplest living things that we can conceive. Their whole body is nothing but a particle of plasm, a granule of living albuminous matter, discharging of itself all the essential vital functions that form the material basis of life. Thus we come to the last, or, if you prefer, the first, question in connection with evolution—the question of the origin of the Monera. This is the real question of the origin of life, or of spontaneous generation.

We have neither space nor occasion to go further in this Chapter into the question of spontaneous generation. For this I must refer the reader to the fifteenth chapter of the History of Creation, and especially to the second book of the General Morphology, or to the essay on "The Monera and Spontaneous Generation" in my Studies of the Monera and other Protists.* (* The English reader will find a luminous and up-to-date chapter on the subject in Haeckel's recently written and translated Wonders of Life.—Translator.) I have given there fully my own view of this important question. The famous botanist Nageli afterwards (1884) developed the same ideas. I will only say a few words here about this obscure question of the origin of life, in so far as our main subject, organic evolution in general, is affected by it. Spontaneous generation, in the definite and restricted sense in which I maintain it, and claim that it is a necessary hypothesis in explaining the origin of life, refers solely to the evolution of the Monera from inorganic carbon-compounds. When living things made their first appearance on our planet, the very complex nitrogenous compound of carbon that we call plasson, which is the earliest material embodiment of vital action, must have been formed in a purely chemical way from inorganic carbon-compounds. The first Monera were formed in the sea by spontaneous generation, as crystals are formed in the mother-water. Our demand for a knowledge of causes compels us to assume this. If we believe that the whole inorganic history of the earth has proceeded on mechanical principles without any intervention of a Creator, and that the history of life also has been determined by the same mechanical laws; if we see that there is no need to admit creative action to explain the origin of the various groups of organisms; it is utterly irrational to assume such creative action in dealing with the first appearance of organic life on the earth.

This much-disputed question of "spontaneous generation" seems so obscure, because people have associated with the term a mass of very different, and often very absurd, ideas, and have attempted to solve the difficulty by the crudest experiments. The real doctrine of the spontaneous generation of life cannot possibly be refuted by experiments. Every experiment that has a negative result only proves that no organism has been formed out of inorganic matter in the conditions—highly artificial conditions—we have established. On the other hand, it would be exceedingly difficult to prove the theory by way of experiment; and even if Monera were still formed daily by spontaneous generation (which is quite possible), it would be very difficult, if not impossible, to find a solid proof of it. Those who will not admit the spontaneous generation of the first living things in our sense must have recourse to a supernatural miracle; and this is, as a matter of fact, the desperate resource to which our "exact" scientists are driven, to the complete abdication of reason.

A famous English physicist, Lord Kelvin (then Sir W. Thomson), attempted to dispense with the hypothesis of spontaneous generation by assuming that the organic inhabitants of the earth were developed from germs that came from the inhabitants of other planets, and that chanced to fall on our planet on fragments of their original home, or meteorites. This hypothesis found many supporters, among others the distinguished German physicist, Helmholtz. However, it was refuted in 1872 by the able physicist, Friedrich Zollner, of Leipzig, in his work, On the Nature of Comets. He showed clearly how unscientific this hypothesis is; firstly in point of logic, and secondly in point of scientific content. At the same time he pointed out that our hypothesis of spontaneous generation is "a necessary condition for understanding nature according to the law of causality."

I repeat that we must call in the aid of the hypothesis only as regards the Monera, the structureless "organisms without organs." Every complex organism must have been evolved from some lower organism. We must not assume the spontaneous generation of even the simplest cell, for this itself consists of at least two parts—the internal, firm nuclear substance, and the external, softer cellular substance or the protoplasm of the cell-body. These two parts must have been formed by differentiation from the indifferent plasson of a moneron, or a cytode. For this reason the natural history of the Monera is of great interest; here alone can we find the means to overcome the chief difficulties of the problem of spontaneous generation. The actual living Monera are specimens of such organless or structureless organisms, as they must have boon formed by spontaneous generation at the commencement of the history of life.

CHAPTER 2.19. OUR PROTIST ANCESTORS.

Under the guidance of the biogenetic law, and on the basis of the evidence we have obtained, we now turn to the interesting task of determining the series of man's animal ancestors. Phylogeny us a whole is an inductive science. From the totality of the biological processes in the life of plants, animals, and man we have gathered a confident inductive idea that the whole organic population of our planet has been moulded on a harmonious law of evolution. All the interesting phenomena that we meet in ontogeny and paleontology, comparative anatomy and dysteleology, the distribution and habits of organisms—all the important general laws that we abstract from the phenomena of these sciences, and combine in harmonious unity—are the broad bases of our great biological induction.

But when we come to the application of this law, and seek to determine with its aid the origin of the various species of organisms, we are compelled to frame hypotheses that have essentially a DEDUCTIVE character, and are inferences from the general law to particular cases. But these special deductions are just as much justified and necessitated by the rigorous laws of logic as the inductive conclusions on which the whole theory of evolution is built. The doctrine of the animal ancestry of the human race is a special deduction of this kind, and follows with logical necessity from the general inductive law of evolution.

I must point out at once, however, that the certainty of these evolutionary hypotheses, which rest on clear special deductions, is not always equally strong. Some of these inferences are now beyond question; in the case of others it depends on the knowledge and the competence of the inquirer what degree of certainty he attributes to them. In any case, we must distinguish between the ABSOLUTE certainty of the general (inductive) theory of descent and the RELATIVE certainty of special (deductive) evolutionary hypotheses. We can never determine the whole ancestral series of an organism with the same confidence with which we hold the general theory of evolution as the sole scientific explanation of organic modifications. The special indication of stem-forms in detail will always be more or less incomplete and hypothetical. This is quite natural. The evidence on which we build is imperfect, and always will be imperfect; just as in comparative philology.

The first of our documents, paleontology, is exceedingly incomplete. We know that all the fossils yet discovered are only an insignificant fraction of the plants and animals that have lived on our planet. For every single species that has been preserved for us in the rocks there are probably hundreds, perhaps thousands, of extinct species that have left no trace behind them. This extreme and very unfortunate incompleteness of the paleontological evidence, which cannot be pointed out too often, is easily explained. It is absolutely inevitable in the circumstances of the fossilisation of organisms. It is also due in part to the incompleteness of our knowledge in this branch. It must be borne in mind that the great majority of the stratified rocks that compose the crust of the earth have not yet been opened. We have only a few specimens of the innumerable fossils that are buried in the vast mountain ranges of Asia and Africa. Only a part of Europe and North America has been investigated carefully. The whole of the fossils known to us certainly do not amount to a hundredth part of the remains that are really buried in the crust of the earth. We may, therefore, look forward to a rich harvest in the future as regards this science. However, our paleontological evidence will (for reasons that I have fully explained in the sixteenth chapter of the History of Creation) always be defective.

The second chief source of evidence, ontogeny, is not less incomplete. It is the most important source of all for special phylogeny; but it has great defects, and often fails us. We must, above all, clearly distinguish between palingenetic and cenogenetic phenomena. We must never forget that the laws of curtailed and disturbed heredity often make the original course of development almost unrecognisable. The recapitulation of phylogeny by ontogeny is only fairly complete in a few cases, and is never wholly complete. As a rule, it is precisely the earliest and most important embryonic stages that suffer most from alteration and condensation. The earlier embryonic forms have had to adapt themselves to new circumstances, and so have been modified. The struggle for existence has had just as profound an influence on the freely moving and still immature young forms as on the adult forms. Hence in the embryology of the higher animals, especially, palingenesis is much restricted by cenogenesis; it is to-day, as a rule, only a faded and much altered picture of the original evolution of the animal's ancestors. We can only draw conclusions from the embryonic forms to the stem-history with the greatest caution and discrimination. Moreover, the embryonic development itself has only been fully studied in a few species.

Finally, the third and most valuable source of evidence, comparative anatomy, is also, unfortunately, very imperfect; for the simple reason that the whole of the living species of animals are a mere fraction of the vast population that has dwelt on our planet since the beginning of life. We may confidently put the total number of these at more than a million species. The number of animals whose organisation has been studied up to the present in comparative anatomy is proportionately very small. Here, again, future research will yield incalculable treasures. But, for the present, in view of this patent incompleteness of our chief sources of evidence, we must naturally be careful not to lay too much stress in human phylogeny on the particular animals we have studied, or regard all the various stages of development with equal confidence as stem-forms.

In my first efforts to construct the series of man's ancestors I drew up a list of, at first ten, afterwards twenty to thirty, forms that may be regarded more or less certainly as animal ancestors of the human race, or as stages that in a sense mark off the chief sections in the long story of evolution from the unicellular organism to man. Of these twenty to thirty stages, ten to twelve belong to the older group of the Invertebrates and eighteen to twenty to the younger division of the Vertebrates.

In approaching, now, the difficult task of establishing the evolutionary succession of these thirty ancestors of humanity since the beginning of life, and in venturing to lift the veil that covers the earliest secrets of the earth's history, we must undoubtedly look for the first living things among the wonderful organisms that we call the Monera; they are the simplest organisms known to us—in fact, the simplest we can conceive. Their whole body consists merely of a simple particle or globule of structureless plasm or plasson. The discoveries of the last four decades have led us to believe with increasing certainty that wherever a natural body exhibits the vital processes of nutrition, reproduction, voluntary movement, and sensation, we have the action of a nitrogenous carbon-compound of the chemical group of the albuminoids; this plasm (or protoplasm) is the material basis of all vital functions. Whether we regarded the function, in the monistic sense, as the direct action of the material substratum, or whether we take matter and force to be distinct things in the dualistic sense, it is certain that we have not as yet found any living organism in which the exercise of the vital functions is not inseparably bound up with plasm.

The soft slimy plasson of the body of the moneron is generally called "protoplasm," and identified with the cellular matter of the ordinary plant and animal cells. But we must, to be accurate, distinguish between the plasson of the cytodes and the protoplasm of the cells. This distinction is of the utmost importance for the purposes of evolution. As I have often said, we must recognise two different stages of development in these "elementary organisms," or plastids ("builders"), that represent the ultimate units of organic individuality. The earlier and lower stage are the unnucleated cytodes, the body of which consists of only one kind of albuminous matter—the homogeneous plasson or "formative matter." The later and higher stage are the nucleated cells, in which we find a differentiation of the original plasson into two different formative substances—the caryoplasm of the nucleus and the cytoplasm of the body of the cell (cf. Chapter 1.6.)

(FIGURE 2.226. Chroococcus minor (Nageli), magnified 1500 times. A phytomoneron, the globular plastids of which secrete a gelatinous structureless membrane. The unnucleated globule of plasm (bluish-green in colour) increases by simple cleavage (a to d).

The Monera are permanent cytodes. Their whole body consists of soft, structureless plasson. However carefully we examine it with our finest chemical reagents and most powerful microscopes, we can find no definite parts or no anatomic structure in it. Hence, the Monera are literally organisms without organs; in fact, from the philosophic point of view they are not organisms at all, since they have no organs. They can only be called organisms in the sense that they are capable of the vital functions of nutrition, reproduction, sensation, and movement. If we were to try to imagine the simplest possible organism, we should frame something like the moneron.

The Monera that we find to-day in various forms fall into two groups according to the nature of their nutrition—the Phytomonera and the Zoomonera; from the physiological point of view, the former are the simplest specimens of the plant (phyton) kingdom, and the latter of the animal (zoon) world. The Phytomonera, especially in their simplest form, the Chromacea (Phycochromacea or Cyanophycea), are the most primitive and the oldest of living organisms. The typical genus Chroococcus (Figure 2.226) is represented by several fresh-water species, and often forms a very delicate bluish-green deposit on stones and wood in ponds and ditches. It consists of round, light green particles, from 1/7000 to 1/2500 of an inch in diameter.

(FIGURE 2.227. Aphanocapsa primordialis (Nageli), magnified 1000 times. A phytomoneron, the round plastids of which (bluish-green in colour) secrete a shapeless gelatinous mass; in this the unnucleated cytodes increase continually by simple cleavage.)

The whole life of these homogeneous globules of plasm consists of simple growth and reproduction by cleavage. When the tiny particle has reached a certain size by the continuous assimilation of inorganic matter, it divides into two equal halves, by a constriction in the middle. The two daughter-monera that are thus formed immediately begin a similar vital process. It is the same with the brown Procytella primordialis (formerly called the Protococcus marinus); it forms large masses of floating matter in the arctic seas. The tiny plasma-globules of this species are of a greenish-brown colour, and have a diameter of 1/10,000 to 1/5000 of an inch. There is no membrane discoverable in the simplest Chroococcacea, but we find one in other members of the same family; in Aphanocapsa (Figure 2.227) the enveloping membranes of the social plastids combine; in Gloecapsa they are retained through several generations, so that the little plasma-globules are enfolded in many layers of membrane.

Next to the Chromacea come the Bacteria, which have been evolved from them by the remarkable change in nutrition which gives us the simple explanation of the differentiation of plant and animal in the protist kingdom. The Chromacea build up their plasm directly from inorganic matter; the Bacteria feed on organic matter. Hence, if we logically divide the protist kingdom into plasma-forming Protophyta and plasma-consuming Protozoa, we must class the Bacteria with the latter; it is quite illogical to describe them—as is still often done—as Schizomycetes, and class them with the true fungi. The Bacteria, like the Chromacea, have no nucleus. As is well-known, they play an important part in modern biology as the causes of fermentation and putrefaction, and of tuberculosis, typhus, cholera, and other infectious diseases, and as parasites, etc. But we cannot linger now to deal with these very interesting features; the Bacteria have no relation to man's genealogical tree.

We may now turn to consider the remarkable Protamoeba, or unnucleated Amoeba. I have, in the first volume, pointed out the great importance of the ordinary Amoeba in connection with several weighty questions of general biology. The tiny Protamoebae, which are found both in fresh and salt water, have the same unshapely form and irregular movements of their simple naked body as the real Amoebae; but they differ from them very materially in having no nucleus in their cell-body. The short, blunt, finger-like processes that are thrust out at the surface of the creeping Protamoeba serve for getting food as well as for locomotion. They multiply by simple cleavage (Figure 2.228).

(FIGURE 2.228. A moneron (Protamoeba) in the act of reproduction. A The whole moneron, moving like an ordinary amoeba by thrusting out changeable processes. B It divides into two halves by a constriction in the middle. C The two halves separate, and each becomes an independent individual. (Highly magnified.))

The next stage to the simple cytode-forms of the Monera in the genealogy of mankind (and all other animals) is the simple cell, or the most rudimentary form of the cell which we find living independently to-day as the Amoeba. The earliest process of inorganic differentiation in the structureless body of the Monera led to its division into two different substances—the caryoplasm and the cytoplasm. The caryoplasm is the inner and firmer part of the cell, the substance of the nucleus. The cytoplasm is the outer and softer part, the substance of the body of the cell. By this important differentiation of the plasson into nucleus and cell-body, the organised cell was evolved from the structureless cytode, the nucleated from the unnucleated plastid. That the first cells to appear on the earth were formed from the Monera by such a differentiation seems to us the only possible view in the present condition of science. We have a direct instance of this earliest process of differentiation to-day in the ontogeny of many of the lower Protists (such as the Gregarinae).

The unicellular form that we have in the ovum has already been described as the reproduction of a corresponding unicellular stem-form, and to this we have ascribed the organisation of an Amoeba (cf. Chapter 1.6). The irregular-shaped Amoeba, which we find living independently to-day in our fresh and salt water, is the least definite and the most primitive of all the unicellular Protozoa (Figure 1.16). As the unripe ova (the protova that we find in the ovaries of animals) cannot be distinguished from the common Amoebae, we must regard the Amoeba as the primitive form that is reproduced in the embryonic stage of the amoeboid ovum to-day, in accordance with the biogenetic law. I have already pointed out, in proof of the striking resemblance of the two cells, that the ova of many of the sponges were formerly regarded as parasitic Amoebae (Figure 1.18). Large unicellular organisms like the Amoebae were found creeping about inside the body of the sponge, and were thought to be parasites. It was afterwards discovered that they were really the ova of the sponge from which the embryos were developed. As a matter of fact, these sponge-ova are so much like many of the Amoebae in size, shape, the character of their nucleus, and movement of the pseudopodia, that it is impossible to distinguish them without knowing their subsequent development.

Our phylogenetic interpretation of the ovum, and the reduction of it to some ancient amoeboid ancestral form, supply the answer to the old problem: "Which was first, the egg or the chick?" We can now give a very plain answer to this riddle, with which our opponents have often tried to drive us into a corner. The egg came a long time before the chick. We do not mean, of course, that the egg existed from the first as a bird's egg, but as an indifferent amoeboid cell of the simplest character. The egg lived for thousands of years as an independent unicellular organism, the Amoeba. The egg, in the modern physiological sense of the word, did not make its appearance until the descendants of the unicellular Protozoon had developed into multicellular animals, and these had undergone sexual differentiation. Even then the egg was first a gastraea-egg, then a platode-egg, then a vermalia-egg, and chordonia-egg; later still acrania-egg, then fish-egg, amphibia-egg, reptile-egg, and finally bird's egg. The bird's egg we have experience of daily is a highly complicated historical product, the result of countless hereditary processes that have taken place in the course of millions of years.

The earliest ancestors of our race were simple Protophyta, and from these our protozoic ancestors were developed afterwards. From the morphological point of view both the vegetal and the animal Protists were simple organisms, individualities of the first order, or plastids. All our later ancestors are complex organisms, or individualities of a higher order—social aggregations of a plurality of cells. The earliest of these, the Moraeada, which represent the third stage in our genealogy, are very simple associations of homogeneous, indifferent cells—undifferentiated colonies of social Amoebae or Infusoria. To understand the nature and origin of these protozoa-colonies we need only follow step by step the first embryonic products of the stem-cell. In all the Metazoa the first embryonic process is the repeated cleavage of the stem-cell, or first segmentation-cell (Figure 2.229). We have already fully considered this process, and found that all the different forms of it may be reduced to one type, the original equal or primordial segmentation (cf. Chapter 1.8). In the genealogical tree of the Vertebrates this palingenetic form of segmentation has been preserved in the Amphioxus alone, all the other Vertebrates having cenogenetically modified forms of cleavage. In any case, the latter were developed from the former, and so the segmentation of the ovum in the Amphioxus has a great interest for us (cf. Figure 1.38). The outcome of this repeated cleavage is the formation of a round cluster of cells, composed of homogeneous, indifferent cells of the simplest character (Figure 2.230). This is called the morula (= mulberry-embryo) on account of its resemblance to a mulberry or blackberry.

(FIGURE 2.229. Original or primordial ovum-cleavage. The stem-cell or cytula, formed by fecundation of the ovum, divides by repeated regular cleavage first into two (A), then four (B), then eight (C), and finally a large number of segmentation-cells (D).

FIGURE 2.230. Morula, or mulberry-shaped embryo.)

It is clear that this morula reproduces for us to-day the simple structure of the multicellular animal that succeeded the unicellular amoeboid form in the early Laurentian period. In accordance with the biogenetic law, the morula recalls the ancestral form of the Moraea, or simple colony of Protozoa. The first cell-communities to be formed, which laid the early foundation of the higher multicellular body, must have consisted of homogeneous and simple amoeboid cells. The oldest Amoebae lived isolated lives, and even the amoeboid cells that were formed by the segmentation of these unicellular organisms must have continued to live independently for a long time. But gradually small communities of Amoebae arose by the side of these eremitical Protozoa, the sister-cells produced by cleavage remaining joined together. The advantages in the struggle for life which these communities had over the isolated cells favoured their formation and their further development. We find plenty of these cell-colonies or communities to-day in both fresh and salt water. They belong to various groups both of the Protophyta and Protozoa.

To have some idea of those ancestors of our race that succeeded phylogenetically to the Moraeada, we have only to follow the further embryonic development of the morula. We then see that the social cells of the round cluster secrete a sort of jelly or a watery fluid inside their globular body, and they themselves rise to the surface of it (Figure 1.29 F, G). In this way the solid mulberry-embryo becomes a hollow sphere, the wall of which is composed of a single layer of cells. We call this layer the blastoderm, and the sphere itself the blastula, or embryonic vesicle.

This interesting blastula is very important. The conversion of the morula into a hollow ball proceeds on the same lines originally in the most diverse stems—as, for instance, in many of the zoophytes and worms, the ascidia, many of the echinoderms and molluscs, and in the amphioxus. Moreover, in the animals in which we do not find a real palingenetic blastula the defect is clearly due to cenogenetic causes, such as the formation of food-yelk and other embryonic adaptations. We may, therefore, conclude that the ontogenetic blastula is the reproduction of a very early phylogenetic ancestral form, and that all the Metazoa are descended from a common stem-form, which was in the main constructed like the blastula. In many of the lower animals the blastula is not developed within the foetal membranes, but in the open water. In those cases each blastodermic cell begins at an early stage to thrust out one or more mobile hair-like processes; the body swims about by the vibratory movement of these lashes or whips (Figure 1.29 F).

We still find, both in the sea and in fresh water, various kinds of primitive multicellular organisms that substantially resemble the blastula in structure, and may be regarded in a sense as permanent blastula-forms—hollow vesicles or gelatinous balls, with a wall composed of a single layer of ciliated homogeneous cells. There are "blastaeads" of this kind even among the Protophyta—the familiar Volvocina, formerly classed with the infusoria. The common Volvox globator is found in the ponds in the spring—a small, green, gelatinous globule, swimming about by means of the stroke of its lashes, which rise in pairs from the cells on its surface. In the similar Halosphaera viridis also, which we find in the marine plancton (floating matter), a number of green cells form a simple layer at the surface of the gelatinous ball; but in this case there are no cilia.

Some of the infusoria of the flagellata-class (Signura, Magosphaera, etc.) are similar in structure to these vegetal clusters, but differ in their animal nutrition; they form the special group of the Catallacta. In September, 1869, I studied the development of one of these graceful animals on the island of Gis-Oe, off the coast of Norway (Magosphaera planula), Figures 2.231 and 2.232). The fully-formed body is a gelatinous ball, with its wall composed of thirty-two to sixty-four ciliated cells; it swims about freely in the sea. After reaching maturity the community is dissolved. Each cell then lives independently for some time, grows, and changes into a creeping amoeba. This afterwards contracts, and clothes itself with a structureless membrane. The cell then looks just like an ordinary animal ovum. When it has been in this condition for some time the cell divides into two, four, eight, sixteen, thirty-two, and sixty-four cells. These arrange themselves in a round vesicle, thrust out vibratory lashes, burst the capsule, and swim about in the same magosphaera-form with which we started. This completes the life-circle of the remarkable and instructive animal.

If we compare these permanent blastulae with the free-swimming ciliated larvae or blastulae, with similar construction, of many of the lower animals, we can confidently deduce from them that there was a very early and long-extinct common stem-form of substantially the same structure as the blastula. We may call it the Blastaea. Its body consisted, when fully formed, of a simple hollow ball, filled with fluid or structureless jelly, with a wall composed of a single stratum of ciliated cells. There were probably many genera and species of these blastaeads in the Laurentian period, forming a special class of marine protists.

It is an interesting fact that in the plant kingdom also the simple hollow sphere is found to be an elementary form of the multicellular organism. At the surface and below the surface (down to a depth of 2000 yards) of the sea there are green globules swimming about, with a wall composed of a single layer of chlorophyll-bearing cells. The botanist Schmitz gave them the name of Halosphaera viridis in 1879.

The next stage to the Blastaea, and the sixth in our genealogical tree, is the Gastraea that is developed from it. As we have already seen, this ancestral form is particularly important. That it once existed is proved with certainty by the gastrula, which we find temporarily in the ontogenesis of all the Metazoa (Figure 1.29 J, K). As we saw, the original, palingenetic form of the gastrula is a round or oval uni-axial body, the simple cavity of which (the primitive gut) has an aperture at one pole of its axis (the primitive mouth). The wall of the gut consists of two strata of cells, and these are the primary germinal layers, the animal skin-layer (ectoderm) and vegetal gut-layer (entoderm).

The actual ontogenetic development of the gastrula from the blastula furnishes sound evidence as to the phylogenetic origin of the Gastraea from the Blastaea. A pit-shaped depression appears at one side of the spherical blastula (Figure 1.29 H). In the end this invagination goes so far that the outer or invaginated part of the blastoderm lies close on the inner or non-invaginated part (Figure 1.29 J). In explaining the phylogenetic origin of the gastraea in the light of this ontogenetic process, we may assume that the one-layered cell-community of the blastaea began to take in food more largely at one particular part of its surface. Natural selection would gradually lead to the formation of a depression or pit at this alimentary spot on the surface of the ball. The depression would grow deeper and deeper. In time the vegetal function of taking in and digesting food would be confined to the cells that lined this hole; the other cells would see to the animal functions of locomotion, sensation, and protection. This was the first division of labour among the originally homogeneous cells of the blastaea.

(FIGURE 2.231. The Norwegian Magosphaera planula, swimming about by means of the lashes or cilia at its surface.

FIGURE 2.232. Section of Magosphaera planula, showing how the pear-shaped cells in the centre of the gelatinous ball are connected by a fibrous process. Each cell has a contractile vacuole as well as a nucleus.)

The effect, then, of this earliest histological differentiation was to produce two different kinds of cells—nutritive cells in the depression and locomotive cells on the surface outside. But this involved the severance of the two primary germinal layers—a most important process. When we remember that even man's body, with all its various parts, and the body of all the other higher animals, are built up originally out of these two simple layers, we cannot lay too much stress on the phylogenetic significance of this gastrulation. In the simple primitive gut or gastric cavity of the gastrula and its rudimentary mouth we have the first real organ of the animal frame in the morphological sense; all the other organs were developed afterwards from these. In reality, the whole body of the gastrula is merely a "primitive gut." I have shown already (Chapters 1.8 and 1.9) that the two-layered embryos of all the Metazoa can be reduced to this typical gastrula. This important fact justifies us in concluding, in accordance with the biogenetic law, that their ancestors also were phylogenetically developed from a similar stem-form. This ancient stem-form is the gastraea.

The gastraea probably lived in the sea during the Laurentian period, swimming about in the water by means of its ciliary coat much as free ciliated gastrulae do to-day. Probably it differed from the existing gastrula only in one essential point, though extinct millions of years ago. We have reason, from comparative anatomy and ontogeny, to believe that it multiplied by sexual generation, not merely asexually (by cleavage, gemmation, and spores), as was no doubt the case with the earlier ancestors. Some of the cells of the primary germ-layers probably became ova and others fertilising sperm. We base these hypotheses on the fact that we do to-day find the simplest form of sexual reproduction in some of the living gastraeads and other lower animals, especially the sponges.

The fact that there are still in existence various kinds of gastraeads, or lower Metazoa with an organisation little higher than that of the hypothetical gastraea, is a strong point in favour of our theory. There are not very many species of these living gastraeads; but their morphological and phylogenetic interest is so great, and their intermediate position between the Protozoa and Metazoa so instructive, that I proposed long ago (1876) to make a special class of them. I distinguished three orders in this class—the Gastremaria, Physemaria, and Cyemaria (or Dicyemida). But we might also regard these three orders as so many independent classes in a primitive gastraead stem.

The Gastremaria and Cyemaria, the chief of these living gastraeads, are small Metazoa that live parasitically inside other Metazoa, and are, as a rule, 1/50 to 1/25 of an inch long, often much less (Figure 2.233, 1 to 15). Their soft body, devoid of skeleton, consists of two simple strata of cells, the primary germinal layers; the outer of these is thickly clothed with long hair-like lashes, by which the parasites swim about in the various cavities of their host. The inner germinal layer furnishes the sexual products. The pure type of the original gastrula (or archigastrula, Figure 1.29 I) is seen in the Pemmatodiscus gastrulaceus, which Monticelli discovered in the umbrella of a large medusa (Pilema pulmo) in 1895; the convex surface of this gelatinous umbrella was covered with numbers of clear vesicles, of 1/25 to 1/8 inch in diameter, in the fluid contents of which the little parasites were swimming. The cup-shaped body of the Pemmatodiscus (Figure 2.233, 1) is sometimes rather flat, and shaped like a hat or cone, at other times almost curved into a semi-circle. The simple hollow of the cup, the primitive gut (g), has a narrow opening (o). The skin layer (e) consists of long slender cylindrical cells, which bear long vibratory hairs; it is separated by a thin structureless, gelatinous plate (f) from the visceral or gut layer (i), the prismatic cells of which are much smaller and have no cilia. Pemmatodiscus propagates asexually, by simple longitudinal cleavage; on this account it has recently been regarded as the representative of a special order of gastraeads (Mesogastria).

Probably a near relative of the Pemmatodiscus is the Kunstleria Gruveli (Figure 2.233, 2). It lives in the body-cavity of Vermalia (Sipunculida), and differs from the former in having no lashes either on the large ectodermic cells (e) or the small entodermic (i); the germinal layers are separated by a thick, cup-shaped, gelatinous mass, which has been called the "clear vesicle" (f). The primitive mouth is surrounded by a dark ring that bears very strong and long vibratory lashes, and effects the swimming movements.

Pemmatodiscus and Kunstleria may be included in the family of the Gastremaria. To these gastraeads with open gut are closely related the Orthonectida (Rhopalura, Figure 2.233, 3 to 5). They live parasitically in the body-cavity of echinoderms (Ophiura) and vermalia; they are distinguished by the fact that their primitive gut-cavity is not empty, but filled with entodermic cells, from which the sexual cells are developed. These gastraeads are of both sexes, the male (Figure 1.3) being smaller and of a somewhat different shape from the oval female (Figure 1.4).

The somewhat similar Dicyemida (Figure 1.6) are distinguished from the preceding by the fact that their primitive gut-cavity is occupied by a single large entodermic cell instead of a crowded group of sexual cells. This cell does not yield sexual products, but afterwards divides into a number of cells (spores), each of which, without being impregnated, grows into a small embryo. The Dicyemida live parasitically in the body-cavity, especially the renal cavities, of the cuttle-fishes. They fall in several genera, some of which are characterised by the possession of special polar cells; the body is sometimes roundish, oval, or club-shaped, at other times long and cylindrical. The genus Conocyema (Figures 1.7 to 1.15) differs from the ordinary Dicyema in having four polar pimples in the form of a cross, which may be incipient tentacles.

The classification of the Cyemaria is much disputed; sometimes they are held to be parasitic infusoria (like the Opalina), sometimes platodes or vermalia, related to the suctorial worms or rotifers, but having degenerated through parasitism. I adhere to the phylogenetically important theory that I advanced in 1876, that we have here real gastraeads, primitive survivors of the common stem-group of all the Metazoa. In the struggle for life they have found shelter in the body-cavity of other animals.

(FIGURE 2.233. Modern gastraeads. Figure 1. Pemmatodiscus gastrulaceus (Monticelli), in longitudinal section. Figure 2. Kunstleria gruveli (Delage), in longitudinal section. (From Kunstler and Gruvel.) Figures 3 to 5. Rhopalura Giardi (Julin): Figure 3 male, Figure 4 female, Figure 5 planula. Figure 6. Dicyema macrocephala (Van Beneden). Figures 7 to 15. Conocyema polymorpha (Van Beneden): Figure 7 the mature gastraead, Figures 8 to 15 its gastrulation. d primitive gut, o primitive mouth, e ectoderm, i entoderm, f gelatinous plate between e and i (supporting plate, blastocoel).)

The small Coelenteria attached to the floor of the sea that I have called the Physemaria (Haliphysema and Gastrophysema) probably form a third order (or class) of the living gastraeads. The genus Haliphysema (Figures 2.234 and 2.235) is externally very similar to a large rhizopod (described by the same name in 1862) of the family of the Rhabdamminida, which was at first taken for a sponge. In order to avoid confusion with these, I afterwards gave them the name of Prophysema. The whole mature body of the Prophysema is a simple cylindrical or oval tube, with a two-layered wall. The hollow of the tube is the gastric cavity, and the upper opening of it the mouth (Figure 2.235 m). The two strata of cells that form the wall of the tube are the primary germinal layers. These rudimentary zoophytes differ from the swimming gastraeads chiefly in being attached at one end (the end opposite to the mouth) to the floor of the sea.

In Prophysema the primitive gut is a simple oval cavity, but in the closely related Gastrophysema it is divided into two chambers by a transverse constriction; the hind and smaller chamber above furnishes the sexual products, the anterior one being for digestion.

The simplest sponges (Olynthus, Figure 2.238) have the same organisation as the Physemaria. The only material difference between them is that in the sponge the thin two-layered body-wall is pierced by numbers of pores. When these are closed they resemble the Physemaria. Possibly the gastraeads that we call Physemaria are only olynthi with the pores closed. The Ammoconida, or the simple tubular sand-sponges of the deep-sea (Ammolynthus, etc.), do not differ from the gastraeads in any important point when the pores are closed. In my Monograph on the Sponges (with sixty plates) I endeavoured to prove analytically that all the species of this class can be traced phylogenetically to a common stem-form (Calcolynthus).

(FIGURES 2.234 AND 2.235. Prophysema primordiale, a living gastraead.

FIGURE 2.234. The whole of the spindle-shaped animal (attached below to the floor of the sea).

FIGURE 2.235. The same in longitudinal section. The primitive gut (d) opens above at the primitive mouth (m). Between the ciliated cells (g) are the amoeboid ova (e). The skin-layer (h) is encrusted with grains of sand below and sponge-spicules above.

FIGURES 2.236 TO 2.237. Ascula of gastrophysema, attached to the floor of the sea. Figure 2.236 external view, 2.237 longitudinal section. g primitive gut, o primitive mouth, i visceral layer, e cutaneous layer. (Diagram.)

FIGURE 2.238. Olynthus, a very rudimentary sponge. A piece cut away in front.)

The lowest form of the Cnidaria is also not far removed from the gastraeads. In the interesting common fresh-water polyp (Hydra) the whole body is simply an oval tube with a double wall; only in this case the mouth has a crown of tentacles. Before these develop the hydra resembles an ascula (Figures 2.236 and 2.237). Afterwards there are slight histological differentiations in its ectoderm, though the entoderm remains a single stratum of cells. We find the first differentiation of epithelial and stinging cells, or of muscular and neural cells, in the thick ectoderm of the hydra.

In all these rudimentary living coelenteria the sexual cells of both kinds—ova and sperm cells—are formed by the same individual; it is possible that the oldest gastraeads were hermaphroditic. It is clear from comparative anatomy that hermaphrodism—the combination of both kinds of sexual cells in one individual—is the earliest form of sexual differentiation; the separation of the sexes (gonochorism) was a much later phenomenon. The sexual cells originally proceeded from the edge of the primitive mouth of the gastraead.

CHAPTER 2.20. OUR WORM-LIKE ANCESTORS.

The gastraea theory has now convinced us that all the Metazoa or multicellular animals can be traced to a common stem-form, the Gastraea. In accordance with the biogenetic law, we find solid proof of this in the fact that the two-layered embryos of all the Metazoa can be reduced to a primitive common type, the gastrula. Just as the countless species of the Metazoa do actually develop in the individual from the simple embryonic form of the gastrula, so they have all descended in past time from the common stem-form of the Gastraea. In this fact, and the fact we have already established that the Gastraea has been evolved from the hollow vesicle of the one-layered Blastaea, and this again from the original unicellular stem-form, we have obtained a solid basis for our study of evolution. The clear path from the stem-cell to the gastrula represents the first section of our human stem-history (Chapters 1.8, 1.9, and 2.19).

The second section, that leads from the Gastraea to the Prochordonia, is much more difficult and obscure. By the Prochordonia we mean the ancient and long-extinct animals which the important embryonic form of the chordula proves to have once existed (cf. Figures 1.83 to 1.86). The nearest of living animals to this embryonic structure are the lowest Tunicates, the Copelata (Appendicaria) and the larvae of the Ascidia. As both the Tunicates and the Vertebrates develop from the same chordula, we may infer that there was a corresponding common ancestor of both stems. We may call this the Chordaea, and the corresponding stem-group the Prochordonia or Prochordata.

From this important stem-group of the unarticulated Prochordonia (or "primitive chorda-animals") the stems of the Tunicates and Vertebrates have been divergently evolved. We shall see presently how this conclusion is justified in the present condition of morphological science.

We have first to answer the difficult and much-discussed question of the development of the Chordaea from the Gastraea; in other words, "How and by what transformations were the characteristic animals, resembling the embryonic chordula, which we regard as the common stem-forms of all the Chordonia, both Tunicates and Vertebrates, evolved from the simplest two-layered Metazoa?"

The descent of the Vertebrates from the Articulates has been maintained by a number of zoologists during the last thirty years with more zeal than discernment; and, as a vast amount has been written on the subject, we must deal with it to some extent. All three classes of Articulates in succession have been awarded the honour of being considered the "real ancestors" of the Vertebrates: first, the Annelids (earth-worms, leeches, and the like), then the Crustacea (crabs, etc.), and, finally, the Tracheata (spiders, insects, etc.). The most popular of these hypotheses was the annelid theory, which derived the Vertebrates from the Worms. It was almost simultaneously (1875) formulated by Carl Semper, of Wurtzburg, and Anton Dohrn, of Naples. The latter advanced this theory originally in favour of the failing degeneration theory, with which I dealt in my work, Aims and Methods of Modern Embryology.

This interesting degeneration theory—much discussed at that time, but almost forgotten now—was formed in 1875 with the aim of harmonising the results of evolution and ever-advancing Darwinism with religious belief. The spirited struggle that Darwin had occasioned by the reformation of the theory of descent in 1859, and that lasted for a decade with varying fortunes in every branch of biology, was drawing to a close in 1870-1872, and soon ended in the complete victory of transformism. To most of the disputants the chief point was not the general question of evolution, but the particular one of "man's place in nature"—"the question of questions," as Huxley rightly called it. It was soon evident to every clear-headed thinker that this question could only be answered in the sense of our anthropogeny, by admitting that man had descended from a long series of Vertebrates by gradual modification and improvement.

In this way the real affinity of man and the Vertebrates came to be admitted on all hands. Comparative anatomy and ontogeny spoke too clearly for their testimony to be ignored any longer. But in order still to save man's unique position, and especially the dogma of personal immortality, a number of natural philosophers and theologians discovered an admirable way of escape in the "theory of degeneration." Granting the affinity, they turned the whole evolutionary theory upside down, and boldly contended that "man is not the most highly developed animal, but the animals are degenerate men." It is true that man is closely related to the ape, and belongs to the vertebrate stem; but the chain of his ancestry goes upward instead of downward. In the beginning "God created man in his own image," as the prototype of the perfect vertebrate; but, in consequence of original sin, the human race sank so low that the apes branched off from it, and afterwards the lower Vertebrates. When this theory of degeneration was consistently developed, its supporters were bound to hold that the entire animal kingdom was descended from the debased children of men.

This theory was most strenuously defended by the Catholic priest and natural philosopher, Michelis, in his Haeckelogony: An Academic Protest against Haeckel's Anthropogeny (1875). In still more "academic" and somewhat mystic form the theory was advanced by a natural philosopher of the older Jena school—the mathematician and physicist, Carl Snell. But it received its chief support on the zoological side from Anton Dohrn, who maintained the anthropocentric ideas of Snell with particular ability. The Amphioxus, which modern science now almost unanimously regards as the real Primitive Vertebrate, the ancient model of the original vertebrate structure, is, according to Dohrn, a late, degenerate descendant of the stem, the "prodigal son" of the vertebrate family. It has descended from the Cyclostoma by a profound degeneration, and these in turn from the fishes; even the Ascidia and the whole of the Tunicates are merely degenerate fishes! Following out this curious theory, Dohrn came to contest the general belief that the Coelenterata and Worms are "lower animals"; he even declared that the unicellular Protozoa were degenerate Coelenterata. In his opinion "degeneration is the great principle that explains the existence of all the lower forms."

If this Michelis-Dohrn theory were true, and all animals were really degenerate descendants of an originally perfect humanity, man would assuredly be the true centre and goal of all terrestrial life; his anthropocentric position and his immortality would be saved. Unfortunately, this trustful theory is in such flagrant contradiction to all the known facts of paleontology and embryology that it is no longer worth serious scientific consideration.

But the case is no better for the much-discussed descent of the Vertebrates from the Annelids, which Dohrn afterwards maintained with great zeal. Of late years this hypothesis, which raised so much dust and controversy, has been entirely abandoned by most competent zoologists, even those who once supported it. Its chief supporter, Dohrn, admitted in 1890 that it is "dead and buried," and made a blushing retraction at the end of his Studies of the Early History of the Vertebrate.

Now that the annelid-hypothesis is "dead and buried," and other attempts to derive the Vertebrates from Medusae, Echinoderms, or Molluscs, have been equally unsuccessful, there is only one hypothesis left to answer the question of the origin of the Vertebrates—the hypothesis that I advanced thirty-six years ago and called the "chordonia-hypothesis." In view of its sound establishment and its profound significance, it may very well claim to be a THEORY, and so should be described as the chordonia or chordaea theory.

I first advanced this theory in a series of university lectures in 1867, from which the History of Creation was composed. In the first edition of this work (1868) I endeavoured to prove, on the strength of Kowalevsky's epoch-making discoveries, that "of all the animals known to us the Tunicates are undoubtedly the nearest blood-relatives of the Vertebrates; they are the most closely related to the Vermalia, from which the Vertebrates have been evolved. Naturally, I do not mean that the Vertebrates have descended from the Tunicates, but that the two groups have sprung from a common root. It is clear that the real Vertebrates (primarily the Acrania) were evolved in very early times from a group of Worms, from which the degenerate Tunicates also descended in another and retrogressive direction." This common extinct stem-group are the Prochordonia; we still have a silhouette of them in the chordula-embryo of the Vertebrates and Tunicates; and they still exist independently, in very modified form, in the class of the Copelata (Appendicaria, Figure 2.225).

The chordaea-theory received the most valuable and competent support from Carl Gegenbaur. This able comparative morphologist defended it in 1870, in the second edition of his Elements of Comparative Anatomy; at the same time he drew attention to the important relations of the Tunicates to a curious worm, Balanoglossus: he rightly regards this as the representative of a special class of worms, which he called "gut-breathers" (Enteropneusta). Gegenbaur referred on many other occasions to the close blood-relationship of the Tunicates and Vertebrates, and luminously explained the reasons that justify us in framing the hypothesis of the descent of the two stems from a common ancestor, an unsegmented worm-like animal with an axial chorda between the dorsal nerve-tube and the ventral gut-tube.

The theory afterwards received a good deal of support from the research made by a number of distinguished zoologists and anatomists, especially C. Kupffer, B. Hatschek, F. Balfour, E. Van Beneden, and Julin. Since Hatschek's Studies of the Development of the Amphioxus gave us full information as to the embryology of this lowest vertebrate, it has become so important for our purpose that we must consider it a document of the first rank for answering the question we are dealing with.

The ontogenetic facts that we gather from this sole survivor of the Acrania are the more valuable for phylogenetic purposes, as paleontology, unfortunately, throws no light whatever on the origin of the Vertebrates. Their invertebrate ancestors were soft organisms without skeleton, and thus incapable of fossilisation, as is still the case with the lowest vertebrates—the Acrania and Cyclostoma. The same applies to the greater part of the Vermalia or worm-like animals, the various classes and orders of which differ so much in structure. The isolated groups of this rich stem are living branches of a huge tree, the greater part of which has long been dead, and we have no fossil evidence as to its earlier form. Nevertheless, some of the surviving groups are very instructive, and give us clear indications of the way in which the Chordonia were developed from the Vermalia, and these from the Coelenteria.

While we seek the most important of these palingenetic forms among the groups of Coelenteria and Vermalia, it is understood that not a single one of them must be regarded as an unchanged, or even little changed, copy of the extinct stem-form. One group has retained one feature, another a different feature, of the original organisation, and other organs have been further developed and characteristically modified. Hence here, more than in any other part of our genealogical tree, we have to keep before our mind the FULL PICTURE of development, and separate the unessential secondary phenomena from the essential and primary. It will be useful first to point out the chief advances in organisation by which the simple Gastraea gradually became the more developed Chordaea.

We find our first solid datum in the gastrula of the Amphioxus (Figure 1.38). Its bilateral and tri-axial type indicates that the Gastraeads—the common ancestors of all the Metazoa—divided at an early stage into two divergent groups. The uni-axial Gastraea became sessile, and gave rise to two stems, the Sponges and the Cnidaria (the latter all reducible to simple polyps like the hydra). But the tri-axial Gastraea assumed a certain pose or direction of the body on account of its swimming or creeping movement, and in order to sustain this it was a great advantage to share the burden equally between the two halves of the body (right and left). Thus arose the typical bilateral form, which has three axes. The same bilateral type is found in all our artificial means of locomotion—carts, ships, etc.; it is by far the best for the movement of the body in a certain direction and steady position. Hence natural selection early developed this bilateral type in a section of the Gastraeads, and thus produced the stem-forms of all the bilateral animals.

The Gastraea bilateralis, of which we may conceive the bilateral gastrula of the amphioxus to be a palingenetic reproduction, represented the two-sided organism of the earliest Metazoa in its simplest form. The vegetal entoderm that lined their simple gut-cavity served for nutrition; the ciliated ectoderm that formed the external skin attended to locomotion and sensation; finally, the two primitive mesodermic cells, that lay to the right and left at the ventral border of the primitive mouth, were sexual cells, and effected reproduction. In order to understand the further development of the gastraea, we must pay particular attention to: (1) the careful study of the embryonic stages of the amphioxus that lie between the gastrula and the chordula; (2) the morphological study of the simplest Platodes (Platodaria and Turbellaria) and several groups of unarticulated Vermalia (Gastrotricha, Nemertina, Enteropneusta).

We have to consider the Platodes first, because they are on the border between the two principal groups of the Metazoa, the Coelenteria and the Coelomaria. With the former they share the lack of body-cavity, anus, and vascular system; with the latter they have in common the bilateral type, the possession of a pair of nephridia or renal canals, and the formation of a vertical brain or cerebral ganglion. It is now usual to distinguish four classes of Platodes: the two free-living classes of the primitive worms (Platodaria) and the coiled-worms (Turbellaria), and the two parasitic classes of the suctorial worms (Trematoda) and the tape-worms (Cestoda). We have only to consider the first two of these classes; the other two are parasites, and have descended from the former by adaptation to parasitic habits and consequent degeneration.

(FIGURE 2.239. Aphanostomum Langii (Haeckel), a primitive worm of the platodaria class, of the order of Cryptocoela or Acoela. This new species of the genus Aphanostomum, named after Professor Arnold Lang of Zurich, was found in September, 1899, at Ajaccio in Corsica (creeping between fucoidea). It is one-twelfth of an inch long, one-twenty-fifth of an inch broad, and violet in colour. a mouth, g auditory vesicle, e ectoderm, i entoderm, o ovaries, a spermaries, f female aperture, m male aperture.)

The primitive worms (Platodaria) are very small flat worms of simple construction, but of great morphological and phylogenetic interest. They have been hitherto, as a rule, regarded as a special order of the Turbellaria, and associated with the Rhabdocoela; but they differ considerably from these and all the other Platodes (flat worms) in the absence of renal canals and a special central nervous system; the structure of their tissue is also simpler than in the other Platodes. Most of the Platodes of this group (Aphanostomum, Amphichoerus, Convoluta, Schizoprora, etc.) are very soft and delicate animals, swimming about in the sea by means of a ciliary coat, and very small (1/10 to 1/20 inch long). Their oval body, without appendages, is sometimes spindle-shaped or cylindrical, sometimes flat and leaf-shaped. Their skin is merely a layer of ciliated ectodermic cells. Under this is a soft medullary substance, which consists of entodermic cells with vacuoles. The food passes through the mouth directly into this digestive medullary substance, in which we do not generally see any permanent gut-cavity (it may have entirely collapsed); hence these primitive Platodes have been called Acoela (without gut-cavity or coelom), or, more correctly, Cryptocoela, or Pseudocoela. The sexual organs of these hermaphroditic Platodaria are very simple—two pairs of strings of cells, the inner of which (the ovaries, Figure 2.239 o) produce ova, and the outer (the spermaria, s) sperm-cells. These gonads are not yet independent sexual glands, but sexually differentiated cell-groups in the medullary substance, or, in other words, parts of the gut-wall. Their products, the sex-cells, are conveyed out behind by two pairs of short canals; the male opening (m) lies just behind the female (f). Most of the Platodaria have not the muscular pharynx, which is very advanced in the Turbellaria and Trematoda. On the other hand, they have, as a rule, before or behind the mouth, a bulbous sense-organ (auditory vesicle or organ of equilibrium, g), and many of them have also a couple of simple optic spots. The cell-pit of the ectoderm that lies underneath is rather thick, and represents the first rudiment of a neural ganglion (vertical brain or acroganglion).

The Turbellaria, with which the similar Platodaria were formerly classed, differ materially from them in the more advanced structure of their organs, and especially in having a central nervous system (vertical brain) and excretory renal canals (nephridia); both originate from the ectoderm. But between the two germinal layers a mesoderm is developed, a soft mass of connective tissue, in which the organs are embedded. The Turbellaria are still represented by a number of different forms, in both fresh and sea-water. The oldest of these are the very rudimentary and tiny forms that are known as Rhabdocoela on account of the simple construction of their gut; they are, as a rule, less than a quarter of an inch long and of a simple oval or lancet shape (Figure 2.240). The surface is covered with ciliated epithelium, a stratum of ectodermic cells. The digestive gut is still the simple primitive gut of the gastraea (d), with a single aperture that is both mouth and anus (m). There is, however, an invagination of the ectoderm at the mouth, which has given rise to a muscular pharynx (sd). It is noteworthy that the mouth of the Turbellaria (like the primitive mouth of the Gastraea) may, in this class, change its position considerably in the middle line of the ventral surface; sometimes it lies behind (Opisthostomum), sometimes in the middle (Mesostomum), sometimes in front (Prosostomum). This displacement of the mouth from front to rear is very interesting, because it corresponds to a phylogenetic displacement of the mouth. This probably occurred in the Platode ancestors of most (or all?) of the Coelomaria; in these the permanent mouth (metastoma) lies at the fore end (oral pole), whereas the primitive mouth (prostoma) lay at the hind end of the bilateral body.

In most of the Turbellaria there is a narrow cavity, containing a number of secondary organs, between the two primary germinal layers, the outer or animal layer of which forms the epidermis and the inner vegetal layer the visceral epithelium. The earliest of these organs are the sexual organs; they are very variously constructed in the Platode-class; in the simplest case there are merely two pairs of gonads or sexual glands—a pair of testicles (Figure 2.241 h) and a pair of ovaries (e). They open externally, sometimes by a common aperture (Monogonopora), sometimes by separate ones, the female behind the male (Digonopora, Figure 2.241). The sexual glands develop originally from the two promesoblasts or primitive mesodermic cells (Figure 1.83 p). As these earliest mesodermic structures extended, and became spacious sexual pouches in the later descendants of the Platodes, probably the two coelom-pouches were formed from them, the first trace of the real body-cavity of the higher Metazoa (Enterocoela).