The History of Creation Vol. II

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CHAPTER XV.
PERIODS OF CREATION AND RECORDS OF CREATION.
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Reform of Systems by the Theory of Descent.—The Natural System as a Pedigree.—Palæontological Records of the Pedigree.—Petrifactions as Records of Creation.—Deposits of the Neptunic Strata and the Enclosure of Organic Remains.—Division of the Organic History of the Earth into Five Main Periods: Period of the Tangle Forests, Fern Forests, Pine Forests, Foliaceous Forests, and of Cultivation.—The Series of Neptunic Strata.—Immeasurable Duration of the Periods which have elapsed during their Formation.—Deposits of Strata only during the Sinking, not during the Elevation of the Ground.—Other Gaps in the Records of Creation.—Metamorphic Condition of the most Ancient Neptunic Strata.—Small Extent of Palæontological Experience.—Small proportion of Organisms and of Parts of Organisms Capable of Petrifying.—Rarity of many Petrified Species.—Want of Fossilised Intermediate Forms.—Records of the Creation in Ontogeny and in Comparative Anatomy	1
CHAPTER XVI.
PEDIGREE AND HISTORY OF THE KINGDOM OF THE PROTISTA.
Special Mode of Carrying out the Theory of Descent in the Natural System of Organisms.—Construction of Pedigrees.—Descent of all Many-celled from Single-celled Organisms.—Descent of Cells from Monera.—Meaning of Organic Tribes, or Phyla.—Number of the Tribes in the Animal and Vegetable Kingdoms.—The Monophyletic Hypothesis of Descent, or the Hypothesis of one Common Progenitor, and the Polyphyletic Hypothesis of Descent, or the Hypothesis of many Progenitors.—The Kingdom of Protista, or Primæval Beings.—Eight Classes of the Protista Kingdom: Monera, Amœbæ, or Protoplastæ; Whip-swimmers, or Flagellata; Ciliated-balls,Cili Catallacta; Labyrinth-streamers, or Labyrinth-streamers, streamers, or Labyrinthuleæ; Flint-cells, or Diatomeæ; Mucous-moulds, or Myxomycetes; Root-footers (Rhizopoda).—Remarks on the General Natural History of the Protista: Their Vital Phenomena, Chemical Composition, and Formation (Individuality and Fundamental Form).—Phylogeny of the Prostista Kingdomy	36
CHAPTER XVII.
PEDIGREE AND HISTORY OF THE VEGETABLE KINGDOM.
The Natural System of the Vegetable Kingdom.—Division of the Vegetable Kingdom into Six Branches and Eighteen Classes.—The Flowerless Plants (Cryptogamia).—Sub-kingdom of the Thallus Plants.—The Tangles, or Algæ (Primary Algæ), Green Algæ, Brown Algæ, Red Algæ).—The Thread-plants, or Inophytes (Lichens and Fungi).—Sub-kingdom of the Prothallus Plants.—The Mosses, or Muscinæ (Water-mosses, Liverworts, Leaf-mosses, Bog-mosses).—The Ferns, or Filicinæ (Leaf-ferns, Bamboo-ferns, Water-ferns, Scale-ferns).—Sub kingdom of Flowering Plants (Phanerogamia).—The Gymnosperms, or Plants with Naked Seeds (Palm-ferns = Cycadeæ; Pines = Coniferæ).—The Angiosperms, or Plants with Enclosed Seeds.—Monocotylæ.—Dicotylæ.—Cup-blossoms (Apetalæ).—Star-blossoms (Diapetalæ).—Bell-blossoms (Gamopetalæ)	77
CHAPTER XVIII.
PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.
I. Animal-Plants and Worms.
The Natural System of the Animal Kingdom.—Linnæus’ and Lamarck’s Systems.—The Four Types of Bär and Cuvier.—Their Increase to Seven Types.—Genealogical Importance of the Seven Types as Independent Tribes of the Animal Kingdom.—Derivation of Zoophytes and Worms from Primæval Animals.—Monophyletic and Polyphyletic Hypothesis of the Descent of the Animal Kingdom.—Common Origin of the Four Higher Animal Tribes out of the Worm Tribe.—Division of the Seven Animal Tribes into Sixteen Main Classes, and Thirty-eight Classes.—Primæval Animals (Monera, Amœbæ), Synamœbæ), Gregarines, Infusoria, Planæades, and Gastræades (Planula and Gastrula).—Tribe of Zoophytes.—Spongiæ (Mucous Sponges, Fibrous Sponges, Calcareous Sponges).—Sea Nettles, or Acalephæ Corals, Hood-jellies, Comb-jellies).—Tribe of Worms	117
CHAPTER XIX.
PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.
II. Mollusca, Star-Fishes, and Articulated Animals.
Tribe of Molluscs.—Four Classes of Molluscs: Lamp-shells (Spirobranchia); Mussels (Lamellibranchia); Snails (Cochlides); Cuttle-fish (Cephalopoda).—Tribe of Star-fishes, or Echinoderma.—Their Derivation from Ringed Worms (Mailed Worms, or Phracthelminthes).—The Alternation of Generation in the Echinoderma.—Four Classes of Star-fish: Sea-stars (Asteridea); Sea-lilies (Crinoidea); Sea-urchins (Echinidea); Sea-cucumbers (Holothuridea).—Tribe of Articulated Animals, or Arthropoda.—Four Classes of Articulated Animals: Branchiata, or Crustacea, breathing through gills; Jointed Crabs; Mailed Crabs; Articulata Tracheata, breathing through Air Tubes.—Spiders (Long Spiders, Round Spiders).—Myriopods.—Insects.—Chewing and Sucking Insects.—Pedigree and History of the Eight Orders of Insects	154
CHAPTER XX.
PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.
III. Vertebrate Animals.
The Records of the Creation of Vertebrate Animals (Comparative Anatomy, Embryology, and Palæontology).—The Natural System of Vertebrate Animals.—The Four Classes of Vertebrate Animals, according to Linnæus and Lamarck.—Their Increase to Nine Classes.—Main Class of the Tube-hearted, or Skull-less Animals (the Lancelet).—Blood Relationship between the Skull-less Fish and the Tunicates.—Agreement in the Embryological Development of Amphioxus and Ascidiæ.—Origin of the Vertebrate Tribe out of the Worm Tribe.—Main Class of Single-nostriled, or Round-mouthed Animals (Hag and Lampreys).—Main Class of Anamnionate Animals, devoid of Amnion.—Fishes (Primæval Fish, Cartilaginous Fish, Osseous Fish).—Mud-fish, or Dipneusta.—Sea Dragons, or Halisauria.—Frogs and Salamanders, or Amphibia (Mailed Amphibia, Naked Amphibia).—Main Class of Amnionate Animals, or Amniota.—Reptiles (Primary Reptiles, Lizards, Serpents, Crocodiles, Tortoises, Flying Reptiles, Dragons, Beaked Reptiles).—Birds (Feather-tailed, Fan-tailed, Bush-tailed)	192
CHAPTER XXI.
PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.
IV. Mammals.
The System of Mammals according to Linnæus and Blainville.—Three Sub-classes of Mammals (Ornithodelphia, Didelphia, Monodelphia).—Ornithodelphia, or Monotrema.—Beaked Animals (Ornithostoma).—Didelphia, or Marsupials.—Herbivorous and Carnivorous Marsupials.—Monodelphia, or Placentalia (Placental Animals).—Meaning of the Placenta.—Tuft Placentalia.—Girdle Placentalia.—Disc Placentalia.—Non-deciduates, or Indeciduata.—Hoofed Animals.—Single and Double-hoofed Animals.—Whales.—Toothless Animals.—Deciduates, or Animals with Decidua.—Semi-apes.—Gnawing Animals.—Pseudo-hoofed Animals.—Insectivora.—Beasts of Prey.—Bats.—Apes	231
CHAPTER XXII.
ORIGIN AND PEDIGREE OF MAN.
The Application of the Theory of Descent to Man.—Its Immense Importance and Logical Necessity.—Man’s Position in the Natural System of Animals, among Disco-placental Animals.—Incorrect Separation of the Bimana and Quadrumana.—Correct Separation of Semi-apes from Apes.—Man’s Position in the Order of Apes.—Narrow-nosed Apes (of the Old World) and Flat-nosed Apes (of America).—Difference of the two Groups.—Origin of Man from Narrow-nosed Apes.—Human Apes, or Anthropoides.—African Human-apes (Gorilla and Chimpanzee).—Asiatic Human-apes (Orang and Gibbon).—Comparison between the different Human Apes and the different Races of Men.—Survey of the Series of the Progenitors of Man.—Invertebrate Progenitors (Prochordata) and Vertebrate Progenitors	263
CHAPTER XXIII.
MIGRATION AND DISTRIBUTION OF MANKIND. HUMAN SPECIES AND HUMAN RACES.
Age of the Human Race.—Causes of its Origin.—The Origin of Human Language.—Monophyletic or Single, Polyphyletic or Multiple Origin of the Human Race.—Derivation of Man from many Pairs.—Classification of the Human Races.—System of Twelve Species of Men.—Woolly-Haired Men, or Ulotrichis.—Bushy-Haired (Papuans, Hottentots.)—Fleecy-haired (Caffres, Negroes).—Straight-haired Men, or Lissotrichi.—Stiff-haired (Australians, Malays, Mongols, Arctic, and American Tribes).—Curly-haired (Dravidas, Nubians, Midlanders).—Number of Population.—Primæval Home of Man (South Asia, or Lemuria).—Nature of Primæval Men.—Number of Primæval Languages (Monoglottists and Polyglottists).—Divergence and Migration of the Human Race.—Geographical Distribution of the Human Species	296
CHAPTER XXIV.
OBJECTIONS AGAINST, AND PROOFS OF THE TRUTH OF, THE THEORY OF DESCENT.
Objections to the Doctrine of Filiation.—Objections of Faith and Reason.—Immeasurable Length of the Geological Periods.—Transition Forms between Kindred Species.—Dependence of Stability of Form on Inheritance, and of the Variability of Form on Adaptation.—Origin of very Complicated Arrangement of Organisation.—Gradual Development of Instincts and Mental Activities.—Origin of a priori Knowledge from Knowledge a posteriori.—The Knowledge requisite for the Correct Understanding of the Doctrine of Filiation.—Necessary Interaction between Empiricism and Philosophy.—Proofs of the Theory of Descent.—Inner Causal-Connection between all the Biological Series of Phenomena.—The Direct Proof of the Theory of Selection.—Relation of the Theory of Descent to Anthropology.—Proofs of the Animal Origin of Man.—The Pithecoid Theory as an Inseparable Part of the Theory of Descent.—Induction and Deduction.—Gradual Development of the Human Mind.—Body and Mind.—Human Soul and Animal Soul.—A Glance at the Future	334
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List of Works referred to in the Text	371
Appendix (Explanation of the Plates)	379
Index	402

XV.	—Hypothetical Sketch of the Monophyletic Origin of Man			Frontispiece
IV.	—Hand of Nine different Mammals			To face page 34
V.	—Single-Stemmed,or Monophyletic, Pedigree of the Vegetable Kingdom			”      112
VI.	—Historical Growth of the Six Great Stems of Animals			”      122
VII.	—Animal Plants, or Zoophytes			”      140
VIII.	—Star Fishes—First Generation			Between pp. 170, 171
IX.	—Star Fishes—Second Generation
X.	—Naplius-Youth-Form of Six Crab Fish			”      174, 175
XI.	—Adult-Form of the same Six Crab Fish
XII.	—Ascidia and Amphioxus			”      201, 202
XIII.	—Ascidia and Amphioxus
XIV.	—Single, or Monophyletic, Pedigree of Back-boned Animals			To face page 222

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8.—Protamœba Primitiva	52
9.—Bathybius Hæckelii	53
10.—Amœba Sphærococcus	54
11.—Euglena Striata	57
12.—Magosphæra Planula	58
13.—Labyrinthula Macrocystis	59
14.—Navicula Hippocampus	60
15.—Physarum Albipes	61
16.—Cyrtidosphæra Echinoides	66
17.—Caulerpa Denticulata	87
18.—Euastrum Rota	88
19.—Fucus Vesiculosus (egg of)	90

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THE HISTORY OF CREATION.

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The revolutionary influence which the Theory of Descent must exercise upon all sciences, will in all probability affect no branch of science, excepting Anthropology, so much as the descriptive portion of natural history, that which is known as systematic Zoology and Botany. Most naturalists who have hitherto occupied themselves with arranging the different systems of animals and plants, have collected, named, and arranged the different species of these natural bodies with much the same interest as antiquarians and ethnographers collect the weapons and utensils of different nations. Many have not even risen above the degree of intelligence with which people usually collect, label, and arrange crests, stamps, and similar curiosities. In the same manner as some collectors find their pleasure in the similarity of forms, the beauty or rarity of the crests or stamps, and admire in them the inventive art of man, so many naturalists take a delight in the manifold forms of animals and plants, and marvel at the rich imagination of the Creator, at His unwearied creative activity, and at His curious fancy for forming, by the side of so many beautiful and useful organisms, also a number of ugly and useless ones.

This childlike treatment of systematic Zoology and Botany is completely annihilated by the Theory of Descent. In the place of the superficial and playful interest with which most naturalists have hitherto regarded organic structures, we now have the much higher interest of the intelligent understanding which detects in the related forms of organisms their true blood relationships. The Natural System of animals and plants, which was formerly valued either only as a registry of names, to facilitate the survey of the different forms, or as a table of contents for the short expression of their degrees of similarity, receives from the Theory of Descent the incomparably higher value of a true pedigree of organisms. This pedigree is to disclose to us the genealogical connection of the smaller and larger groups. It has to show us in what way the different classes, orders, families, genera, and species of the animal and vegetable kingdoms correspond with the different branches, twigs, and groups of twigs of the pedigree. Every wider and higher category or stage of the system (for example a class, or an order) comprises a number of larger and stronger branches of the pedigree; every narrower and lower category (for example a genus, or a species) only a smaller and thinner group of twigs. It is only when we thus view the natural system as a pedigree that we perceive its true value. (Gen. Morph. ii. Plate XVII. p. 397.)

Since we hold fast this genealogical conception of the Organic System, to which alone undoubtedly the future of classificatory Zoology and Botany belongs, we should now turn our attention to one of the most essential, but also one of the most difficult, tasks of the “non-miraculous history of creation,” namely, to the actual construction of the Organic Pedigree. Let us see how far we are already able to point out all the different organic forms as the divergent descendants of a single or of some few common original forms. But how can we construct the actual pedigree of the animal and vegetable group of forms from our knowledge of them, at present so scanty and fragmentary? The answer to this question lies in what we have already remarked of the parallelism of the three series of development—in the important causal relation which connects the palæontological development of all organic tribes with the embryological development of individuals, and with the systematic development of groups.

In order to accomplish our task we shall first have to direct our attention to palæontology, or the science of petrifactions. For if the Theory of Descent is really true, if the petrified remains of formerly living animals and plants really proceed from the extinct primæval ancestors and progenitors of the present organisms, then, without anything else, the knowledge and comparison of petrifactions ought to disclose to us the pedigree of organisms. However simple and clear this may seem in theory, the task becomes extremely hard and complicated when it is actually taken in hand. Its practical solution would be very difficult even if the petrifactions were to any extent completely preserved. But this is by no means the case. The obvious records of creation which lie buried in petrifactions are imperfect beyond all measure. Hence it is necessary critically to examine these records, and to determine the value which petrifactions possess for the history of the development of organic tribes. As I have previously discussed the general importance of petrifactions as the records of creation, when we were considering Cuvier’s merits in the science of fossils, we may now at once examine the conditions and circumstances under which the remains of organic bodies became petrified and preserved in a more or less recognizable form.

As a rule we find petrifactions or fossils enclosed only in those stones which have been deposited in layers as mud by water, and which are on that account called neptunic, stratified, or sedimentary rocks. The deposition of such strata could of course only commence after the condensation of watery vapour into liquid water had taken place in the course of the earth’s history. After that period, which we considered in our last chapter, not only did life begin on the earth, but also an uninterrupted and exceedingly important transformation of the rigid inorganic crust of the earth. The water began that extremely important mechanical action by which the surface of the earth is perpetually, though slowly, transformed. I may surely presume that it is generally known what an extremely important influence, in this respect, is even yet exercised by water at every moment. As it falls down as rain, trickling through the upper strata of the earth’s crust, and flowing down from heights into hollows, it chemically dissolves different mineral parts of the ground, and mechanically washes away the loose particles. In flowing down from mountains water carries their debris into the plains, or deposits it as mud in stagnant lakes. Thus it continually works at lowering mountains and filling up valleys. In like manner the breakers of the sea work uninterruptedly at the destruction of the coasts and at filling up the bottom of the sea with the debris they wash down. The action of water alone, if it were not counteracted by other circumstances, would in time level the whole earth. There can be no doubt that the mountain masses—which are annually carried down as mud into the sea, and deposited on its floor—are so great that in the course of a longer or shorter period, say a few millions of years, the surface of the earth would be completely levelled and become enclosed by a continuous sheet of water. That this does not happen is owing to the perpetual volcanic action of the fiery-fluid centre of the earth. The surging of the melted nucleus against the firm crust necessitates continual alternations of elevation and depression on the different parts of the earth’s surface. These elevations and depressions for the most part take place very slowly; but, as they continue for thousands of years, by the combined effect of small, interrupted movements, they produce results no less grand than does the counteracting and levelling action of water.

Since the elevations and depressions of the different parts of the earth alternate with one another in the course of millions of years, first this and then that part of the earth’s surface is above or below the level of the sea. I have already given examples of this in the preceding chapter (vol. i. p. 361). Hence, in all probability, there is no part of the outer crust of the earth which has not been repeatedly above and also below the level of the sea. This repeated change explains the variety and the different composition of the numerous neptunic strata of rocks, which in most places have been deposited one above another in considerable thickness. In the different periods of the earth’s history during which these deposits took place there lived various and different populations of animals and plants. When their dead bodies sank to the bottom of the waters, the forms of the bodies impressed themselves upon the soft mud, and imperishable parts, such as hard bones, teeth, shells, etc., became enclosed in it uninjured. These were preserved in the mud, which condensed them into neptunic rock, and as petrifactions they now serve to characterise the respective strata. By a careful comparison of the different strata lying one above another, and the petrifactions preserved in them, it has become possible to decide the relative age of the strata and groups of strata, and to establish, by direct observation, the principal eras of phylogeny, that is to say, the stages in history of the development of animal and vegetable tribes.

The different strata of neptunic rocks deposited one above another, which are composed in very various ways of limestone, clay, and sand, geologists have grouped together into an ideal System or Series, which corresponds with the whole course of the organic history of the earth, or with that portion of the earth’s history during which organic life existed. Just as so-called “universal history” falls into larger and smaller periods, which are characterized by the conditions of development of the most important nations at the respective epochs, and are separated from one another by great events, so we also divide the infinitely longer organic history of the earth into a series of greater and less periods. Each of these periods is distinguished by a characteristic flora and fauna, and by the specially strong development of certain vegetable or animal groups, and each is separated from the preceding and succeeding period by a striking change in the character of its animal and vegetable inhabitants.

In relation to the following survey of the historical course of development which the large animal and vegetable tribes have passed through, it will be desirable to say a few words first as to the systematic classification of the neptunic groups of strata, and the larger and smaller periods corresponding to them. As will be seen directly, we are able to divide the whole of the sedimentary rocks lying one above another into five main groups or periods, each period into several subordinate groups of strata or systems, and each system of strata again into still smaller groups or formations; finally, each formation can again be divided into stages or sub-formations, and each of these again into still smaller layers or beds. Each of the five great rock-groups was deposited during a great division of the earth’s history, during a long era or epoch; each system during a shorter period; each formation during a still shorter period. In thus reducing the periods of the organic history of the earth, and the neptunic strata containing petrifactions deposited during those periods into a connected system, we proceed exactly like the historian who divides the history of nations into the three main divisions of Antiquity, the Middle Ages, and Modern Times, and each of those sections again into subordinate periods and epochs. But the historian by this sharp systematic division, and by fixing the boundary of the periods by particular dates, only seeks to facilitate his survey, and in no way means to deny the uninterrupted connection of events and the development of nations. Exactly the same qualification applies to our systematic division, specification, or classification of the organic history of the earth. Here, too, a continuous thread runs through the series of events unbroken. We must therefore distinctly protest against the idea that by sharply bounding the larger and smaller groups of strata, and the the periods corresponding with them, we in any way wish to adopt Cuvier’s doctrine of terrestrial revolutions, and of repeated new creations of organic populations. That this erroneous doctrine has long since been completely refuted by Lyell, I have already mentioned. (Compare vol. i. p. 127.)

The five great main divisions of the organic history of the earth, or the palæontological history of development, we call the primordial, primary, secondary, tertiary, and quaternary epochs. Each is distinctly characterized by the predominating development of certain animal and vegetable groups in it, and we might accordingly symbolically designate the five epochs, on the one hand by the names of the groups of the vegetable kingdom, and on the other hand by those of the different classes of vertebrate animals. In this case the first, or primordial epoch, would be the era of the Tangles (Algæ) and skull-less Vertebrates; the second, or primary epoch, that of the Ferns and Fishes; the third, or secondary epoch, that of Pine Forests and Reptiles; the fourth, or tertiary epoch, that of Foliaceous Forests and of Mammals; finally, the fifth, or quaternary epoch, the era of Man, and his Civilization. The divisions or periods which we distinguish in each of the five long eras (p. 14) are determined by the different systems of strata into which each of the five great rock-groups is divided (p. 15). We shall now take a cursory glance at the series of these systems, and at the same time at the populations of the five great epochs.

The first and longest division of the organic history of the earth is formed by the primordial epoch, or the era of the Tangle Forests. It comprises the immense period from the first spontaneous generation, from the origin of the first terrestrial organism, to the end of the Silurian system of deposits. During this immeasurable space of time, which in all probability was much longer than all the other four epochs taken together, the three most extensive of all the neptunic systems of strata were deposited, namely, the Laurentian, upon that the Cambrian, and upon that the Silurian system. The approximate thickness or size of these three systems together amounts to 70,000 feet. Of these about 30,000 belong to the Laurentian, 18,000 to the Cambrian, and 22,000 to the Silurian system. The average thickness of all the four other rock groups, the primary, secondary, tertiary, and quaternary, taken together, may amount at most to 60,000 feet; and from this fact alone, apart from many other reasons, it is evident that the duration of the primordial period was probably much longer than the duration of all the subsequent periods down to the present day. Many thousands of millions of years were required to deposit such masses of strata. Unfortunately, by far the largest portion of the primordial group of strata is in the metamorphic state (which we shall directly explain), and consequently the petrifactions contained in them—the most ancient and most important of all—have, to a great extent, been destroyed and become unrecognisable. Only in one portion of the Cambrian strata have petrifactions been preserved in a recognizable condition and in large quantities. The most ancient of all distinctly preserved petrifactions has been found in the lowest Laurentian strata (in the Ottawa formation), which I shall afterwards have to speak of as the “Canadian Life’s-dawn” (Eozoon canadense).

Although only by far the smaller portion of the primordial or archilithic petrifactions are preserved to us in a recognizable condition, still they possess the value of inestimable documents of the most ancient and obscure times of the organic history of the earth. What seems to be shown by them, in the first place, is that during the whole of this immense period there existed only inhabitants of the waters. As yet, at any rate, among all archilithic petrifactions, not a single one has been found which can with certainty be regarded as an organism which has lived on land. All the vegetable remains we possess of the primordial period belong to the lowest of all groups of plants, to the class of Tangles or Algæ, living in water. In the warm primæval sea, these constituted the forests of the period, of the richness of which in forms and density we may form an approximate idea from their present descendants, the tangle forests of the Atlantic Sargasso sea. The colossal tangle forests of the archilithic period supplied the place of the forest vegetation of the mainland, which was then utterly wanting. All the animals, also, whose remains have been found in archilithic strata, like the plants, lived in water. Only crustacea are met with among the animals with articulated feet, as yet no spiders and no insects. Of vertebrate animals, only a very few remains of fishes are known as having been found in the most recent of all primordial strata, in the upper Silurian. But the headless vertebrate animals, which we call skull-less, or Acrania, and out of which fishes must have been developed, we suppose to have lived in great numbers during the primordial epoch. Hence we may call it after the Acrania as well as after the Tangles.

The primary epoch, or the era of Fern Forests, the second main division of the organic history of the earth, which is also called the palæolithic or palæozoic period, lasted from the end of the Silurian formation of strata to the end of the Permian formation. This epoch was also of very long duration, and again falls into three shorter periods, during which three great systems of strata were deposited, namely, first, the Devonian system, or the old red sandstone; upon that, the Carboniferous, or coal system; and upon this, the Permian system. The average thickness of these three systems taken together may amount to about 42,000 feet, from which we may infer the immense length of time requisite for their formation.

The Devonian and Permian formations are especially rich in remains of fishes, of primæval fish as well as enamelled fish (Ganoids), but the bony fish (Teleostei) are absent from the strata of the primary epoch. In coal are found the most ancient remains of animals living on land, both of articulated animals (spiders and insects) as well as of vertebrate animals (amphibious animals, like newts and frogs). In the Permian system there occur, in addition to the amphibious animals, the more highly-developed reptiles, and, indeed, forms nearly related to our lizards (Proterosaurus, etc.). But, nevertheless, we may call the primary epoch that of Fishes, because these few amphibious animals and reptiles are insignificant in comparison with the immense mass of palæozoic fishes. Just as Fishes predominate over the other vertebrate animals, so Ferns, or Filices, predominate among the plants of this epoch, and, in fact, real ferns and tree ferns (leafed ferns, or Phylopteridæ), as well as bamboo ferns (Calamophytæ) and scaled ferns (Lepidophytæ). These ferns, which grew on land, formed the chief part of the dense palæolithic island forests, the fossil remains of which are preserved to us in the enormously large strata of coal of the Carboniferous system, and in the smaller strata of coal of the Devonian and Permian systems. We are thus justified in calling the primary epoch either the era of Ferns or that of Fishes.

The third great division of the palæontological history of development is formed by the secondary epoch, or the era of Pine Forests, which is also called the mesolithic or mesozoic epoch. It extends from the end of the Permian system to the end of the Chalk formation, and is again divided into three great periods. The stratified systems deposited during this period are, first and lowest, the Triassic system, in the middle the Jura system, and at the top the Cretaceous system. The average thickness of these three systems taken together is much less than that of the primary group, and amounts as a whole only to about 15,000 feet. The secondary epoch can accordingly in all probability not have been half so long as the primary epoch.

Just as Fishes prevailed in the primary epoch, Reptiles predominated in the secondary epoch over all other vertebrate animals. It is true that during this period the first birds and mammals originated; at that time, also, there existed important amphibious animals, especially the gigantic Labyrinthodonts, in the sea the wonderful sea-dragons, or Halisaurii, swam about, and the first fish with bones were associated with the many primæval fishes (Sharks) and enamelled fish (Ganoids) of the earlier times; but the very variously developed kinds of reptiles formed the predominating and characteristic class of vertebrate animals of the secondary epoch. Besides those reptiles which were very nearly related to the present living lizards, crocodiles, and turtles, there were, during the mesolithic period, swarms of grotesquely shaped dragons. The remarkable flying lizards, or Pterosaurii, and the colossal land-dragons, or Dinosaurii, of the secondary epoch, are peculiar, as they occur neither in the preceding nor in the succeeding epochs. The secondary epoch may be called the era of Reptiles; but on the other hand, it may also be called the era of Pine Forests, or more accurately, of the Gymnosperms, that is, the epoch of plants having naked seeds. For this group of plants, especially as represented by the two important classes—the pines, or Coniferæ, and the palm-ferns, or Cycadeæ—during the secondary epoch constituted a predominant part of the forests. But towards the end of the epoch (in the Chalk period) the plants of the pine tribe gave place to the leaf-bearing forests which then developed for the first time.

SURVEY
Of the Palæontological Periods, or of the Greater Divisions of the Organic History of the Earth.
I. First Epoch: Archilithic Era.    Primordial Epoch.
(Era of Skull-less Animals and Forests of Tangles.)
1.	Older Primordial Period	or	Laurentian Period.
2.	Middle Primordial Period	”	Cambrian Period.
3.	Later Primordial Period	”	Silurian Period.
II. Second Epoch: Palæolithic Era.    Primary Epoch.
(Era of Fish and Fern Forests.)
4.	Older Primary Period	or	Devonian Period.
5.	Mid Primary Period	”	Coal Period.
6.	Later Primary Period	”	Permian Period.
III. Third Epoch: Mesolithic Era.    Secondary Epoch.
(Era of Reptiles and Pine Forests.)
7.	Older Secondary Period	or	Trias Period.
8.	Middle Secondary Period	”	Jura Period.
9.	Later Secondary Period	”	Chalk Period.
IV. Fourth Epoch: Cænolithic Era.    Tertiary Epoch.
(Era of Mammals and Leaf Forests.)
10.	Older Tertiary Period	or	Eocene Period.
11.	Newer Tertiary Period	”	Miocene Period.
12.	Recent Tertiary Period	”	Pliocene Period.
V. Fifth Epoch: Anthropolithic Era.    Quaternary Epoch.
(Era of Man and Cultivated Forests.)
13.	Older Quaternary Period	or	Ice or Glacial Period.
14.	Newer Quaternary Period	”	Post Glacial Period.
15.	Recent Quaternary Period	”	Period of Culture.
(The Period of Culture is the Historical Period, or the Period of Tradition.)

---

Rock-Groups.		Systems.		Formations.		Synonyms of Formations.
V. Quaternary Group, or Anthropolithic (Anthropozoic) groups of strata.		XIV. Recent (Alluvium)		36.	Present	Upper alluvial
				35.	Recent	Lower alluvial
		XIII. Pleistocene (Diluvium)		34.	Post glacial	Upper diluvial
				33.	Glacial	Lower diluvial
IV. Tertiary Group, or (Cænozoic) groups of strata.		XII. Pliocene (Late tertiary)		32.	Arvernian	Upper pliocene
				31.	Sub-Appenine	Lower pliocene
		XI. Miocene (Late tertiary)		30.	Falunian	Upper miocene
				29.	Limburgian	Lower miocene
		X. Eocene Old tertiary)		28.	Gypsum	Upper eocene
				27.	Nummulitic	Mid eocene
				26.	London clay	Lower eocene
III. Secondary Group, or Mesolithic groups of strata		IX. Cretaceous		25.	White chalk	Upper cretaceous
				24.	Green sand	Mid cretaceous
				23.	Neocomian	Lower cretaceous
				22.	Wealden	The Kentish Weald
		VIII. Jura		21.	Portlandian	Upper oolite
				20.	Oxfordian	Mid oolite
				19.	Bath	Lower oolite
				18.	Lias	Lias formation
		VII. Trias		17.	Keuper	Upper trias
				16.	Muschel-kalk	Mid trias
				15.	Bunter sand	Lower trias
II. Primary Group, or Palæolithic (Palæozoic) groups of strata		VI. Permian		14.	Zechstein	Upper Permian
				13.		Lower Permian
		V. Carbonic (coal)		12.	Carboniferous sandstone	Upper carbonic
				11.	Carboniferous limestone	Lower carbonic
		IV. Devonian (Old red sandstone)		10.	Pilton	Upper Devonian
				9.	Ilfracombe	Mid Devonian
				8.	Linton	Lower Devonian
I. Primordial Group, or Archilithic (Archizoic) groups of strata		III. Silurian		7.	Ludlow	Upper Silurian
				6.	Llandovery	Mid Silurian
				5.	Llandeilo	Lower Silurian
		II. Cambrian		4.	Potsdam	Upper Cambrian
				3.	Longmynd	Lower Cambrian
		I. Laurentian		2.	Labrador	Upper Laurentian
				1.	Ottawa	Lower Laurentian

The fourth main division of the organic history of the earth, the tertiary epoch, or era of Leafed Forests, is much shorter and less peculiar than the three first epochs. This epoch, which is also called the cænolithic or cænozoic epoch, extended from the end of the cretaceous system to the end of the pliocene system. The strata deposited during it amount only to a thickness of about 3,000 feet, and consequently are much inferior to the three first great groups. The three systems also into which the tertiary period is subdivided are very difficult to distinguish from one another. The oldest of them is called eocene, or old tertiary; the newer miocene, or mid tertiary; and the last is the pliocene, or later tertiary system.

The whole population of the tertiary epoch approaches much nearer, on the whole as well as in detail, to that of the present time than is the case in the preceding epochs. From this time the class of Mammals greatly predominates over all other vertebrate animals. In like manner, in the vegetable kingdom, the group—so rich in forms—of the Angiosperms, or plants with covered seeds, predominates, and its leafy forests constitute the characteristic feature of the tertiary epoch. The group of the Angiosperms consists of the two classes of single-seed-lobed plants, or Monocotyledons, and the double-seed-lobed plants, or Dicotyledons. The Angiosperms of both classes had, it is true, made their appearance in the Cretaceous period, and mammals had already occurred in the Jurassic period, and even in the Triassic period; but both groups, the mammals and the plants with enclosed seeds, did not attain their peculiar development and supremacy until the tertiary epoch, so that it may justly be called after them.

The fifth and last main division of the organic history of the earth is the quaternary epoch, or era of Civilization, which in comparison with the length of the four other epochs almost vanishes into nothing, though with a comical conceit we usually call its record the “history of the world.” As the period is characterized by the development of Man and his Culture, which has influenced the organic world more powerfully and with greater transforming effect than have all previous conditions, it may also be called the era of Man, the anthropolithic or anthropozoic period. It might also be called the era of Cultivated Forests, or Gardens, because even at the lowest stage of human civilization man’s influence is already perceptible in the utilization of forests and their products, and therefore also in the physiognomy of the landscape. The commencement of this era, which extends down to the present time, is geologically bounded by the end of the pliocene stratification.

The neptunic strata which have been deposited during the comparatively short quaternary epoch are very different in different parts of the earth, but they are mostly of very slight thickness. They are reduced to two “systems,” the older of which is designated the diluvial, or pleistocene, and the later the alluvial, or recent. The diluvial system is again divided into two “formations,” the older glacial and the more recent post glacial formations. For during the older diluvial period there occurred that extremely remarkable decrease of the temperature of the earth which led to an extensive glaciation of the temperate zones. The great importance which this “ice” or “glacial period” has exercised on the geographical and topographical distribution of organisms has already been explained in the preceding chapter (vol. i. p. 365). But the post glacial period, or the more recent diluvial period, during which the temperature again increased and the ice retreated towards the poles, was also highly important in regard to the present state of chorological relations.

The biological characteristic of the quaternary epoch lies essentially in the development and dispersion of the human organism and his culture. Man has acted with a greater transforming, destructive, and modifying influence upon the animal and vegetable population of the earth than any other organism. For this reason, and not because we assign to man a privileged exceptional position in nature in other matters, we may with full justice designate the development of man and his civilization as the beginning of a special and last main division of the organic history of the earth. It is probable indeed that the corporeal development of primæval man out of man-like apes took place as far back as the earlier pliocene period, perhaps even in the miocene tertiary period. But the actual development of human speech, which we look upon as the most powerful agency in the development of the peculiar characteristics of man and his dominion over other organisms, probably belongs to that period which on geological grounds is distinguished from the preceding pliocene period as the pleistocene or diluvial. In fact the time which has elapsed from the development of human speech down to the present day, though it may comprise many thousands and perhaps hundreds of thousands of years, almost vanishes into nothing as compared with the immeasurable length of the periods which have passed from the beginning of organic life on the earth down to the origin of the human race.

The tabular view given on page 15 shows the succession of the palæontological rock-groups, systems, and formations, that is, the larger and smaller neptunic groups of strata, which contain petrifactions, from the uppermost, or Alluvial, down to the lowest, or Laurentian, deposits. The table on page 14 presents the historical division of the corresponding eras of the larger and smaller palæontological periods, and in a reversed succession, from the most ancient Laurentian up to the most recent Quaternary period.

Many attempts have been made to make an approximate calculation of the number of thousands of years constituting these periods. The thickness of the strata has been compared, which, according to experience, is deposited during a century, and which amounts only to some few lines or inches, with the whole thickness of the stratified masses of rock, the succession of which we have just surveyed. This thickness, on the whole, may on an average amount to about 130,000 feet; of these 70,000 belong to the primordial, or archilithic; 42,000 to the primary, or palæolithic; 15,000 to the secondary, or mesolithic; and finally only 3,000 to the tertiary, or cænolithic group. The very small and scarcely appreciable thickness of the quaternary, or anthropolithic deposit cannot here come into consideration at all. On an average, it may at most be computed as from 500 to 700 feet. But it is self evident that all these measurements have only an average and approximate value, and are meant to give only a rough survey of the relative proportion of the systems of strata and of the spaces of time corresponding with them.

Now, if we divide the whole period of the organic history of the earth—that is, from the beginning of life on the earth down to the present day—into a hundred equal parts, and if then, corresponding to the thickness of the systems of strata, we calculate the relative duration of the time of the five main divisions or periods according to percentages, we obtain the following result:—

I.	Archilithic, or primordial period	53.6
II.	Palæolithic, or primary period	32.1
III.	Mesolithic, or secondary period	11.5
IV.	Cænolithic, or tertiary period	2.3
V.	Anthropolithic, or quaternary period	0.5
——
Total  . . .  100.0

According to this, the length of the archilithic period, during which no land-living animals or plants as yet existed, amounts to more than one half, more than 53 per cent.; on the other hand the length of the anthropolithic era, during which man has existed, amounts to scarcely one-half per cent. of the whole length of the organic history of the earth. It is, however, quite impossible to calculate the length of these periods, even approximately, by years.

The thickness of the strata of mud at present deposited during a century, and which has been used as a basis for this calculation, is of course quite different in different parts of the earth under the different conditions in which these deposits take place. It is very slight at the bottom of the deep sea, in the beds of broad rivers with a short course, and in inland seas which receive very scanty supplies of water. It is comparatively great on the sea-shores exposed to strong breakers, at the estuaries of large rivers with long courses, and in inland seas with copious supplies of water. At the mouth of the Mississippi, which carries with it a considerable amount of mud, in the course of 100,000 years about 600 feet would be deposited. At the bottom of the open sea, far away from the coasts, during this long period only some few feet of mud would be deposited. Even on the sea-shores where a comparatively large quantity of mud is deposited the thickness of the strata formed during the course of a century may after all amount to no more than a few inches or lines when condensed into solid stone. In any case, however, all calculations based upon these comparisons are very unsafe, and we cannot even approximately conceive the enormous length of the periods which were requisite for the formation of the systems of neptunic strata. Here we can apply only relative, not absolute, measurements of time.

Moreover, we should entirely err were we to consider the size of these systems of strata alone as the measure of the actual space of time which has elapsed during the earth’s history. For the elevations and depressions of the earth’s crust have perpetually alternated with one another, and the mineralogical and palæontological difference—which is perceived between each two succeeding systems of strata, and between each two of their formations at any particular spot—corresponds in all probability with a considerable intermediate space of many thousands of years, during which that particular part of the earth’s crust was raised above the water. It was only after the lapse of this intermediate period, when a new depression again laid the part in question under water, that there occurred a new deposit of earth. As, in the mean time, the inorganic and organic conditions on this part had undergone a considerable transformation, the newly-formed layer of mud was necessarily composed of different earthy constituents and enclosed different petrifactions.

IV. Tertiary Group of Strata, 3,000 feet.	Eocene, Miocene, Pliocene.
III. Mesolithic Group of Strata. Deposits of the Secondary Epoch, about 15,000 feet.	IX. Chalk System. .................................. VIII. Jura System. .................................. VII. Trias System.
II. Palæolithic Group of Strata. Deposits of the Primary Epoch, about 42,000 feet.	VI. Permian System. .................................. V. Coal System. .................................. IV. Devonian System.
I. Archilithic Group of Strata. Deposits of the Primordial Epoch, about 70,000 feet.	III. Silurian System, about 22,000 feet. .................................. II. Cambrian System, about18,000 feet. .................................. I. Laurentian System about 30,000 feet.

The striking differences which so frequently occur between the petrifactions of two strata, lying one above another, are to be explained in a simple and easy manner by the supposition that the same part of the earth’s surface has been exposed to repeated depressions and elevations. Such alternating elevations and depressions take place even now extensively, and are ascribed to the heaving of the fiery fluid nucleus against the rigid crust. Thus, for example, the coast of Sweden and a portion of the west coast of South America are constantly though slowly rising, while the coast of Holland and a portion of the east coast of South America are gradually sinking. The rising as well as the sinking takes place very slowly, and in the course of a century sometimes only amounts to some few lines, sometimes to a few inches, or at most a few feet. But if this action continues uninterruptedly throughout hundreds of thousands of years it is capable of forming the highest mountains.

It is evident that elevations and depressions, such as now can be measured in these places, have uninterruptedly alternated one with another in different places during the whole course of the organic history of the earth. This may be inferred with certainty from the geographical distribution of organisms. (Compare vol. i. p. 350.) But to form a judgment of our palæontological records of creation it is extremely important to show that permanent strata can only be deposited during a slow sinking of the ground under water, but not during its continued rising. When the ground slowly sinks more and more below the level of the sea, the deposited layers of mud get into continually deeper and quieter water, where they can become condensed into stone undisturbed. But when, on the other hand, the ground slowly rises, the newly-deposited layers of mud, which enclose the remains of plants and animals, again immediately come within the reach of the play of the waves, and are soon worn away by the force of the breakers, together with the organic remains which they on close. For this simple but very important reason, therefore, abundant layers, in which organic remains are preserved, can only be deposited during a continuous sinking of the ground. When any two different formations or strata, lying one above the other, correspond with two different periods of depression, we must assume a long period of rising between them, of which period we know nothing, because no fossil remains of the then living animals and plants could be preserved. It is evident, however, that those periods of elevation, which have passed without leaving any trace behind them, deserve a no less careful consideration than the greater or less alternating periods of depression, of whose organic population we can form an approximate idea from the strata containing petrifactions. Probably the former were not of shorter duration than the latter.

From this alone it is apparent how imperfect our records must necessarily be, and all the more so since it can be theoretically proved that the variety of animal and vegetable life must have increased greatly during those very periods of elevation. For as new tracts of land are raised above the water, new islands are formed. Every new island, however, is a new centre of creation, because the animals and plants accidentally cast ashore there, find in the new territory, in the struggle for life, abundant opportunity of developing themselves peculiarly, and of forming new species. The formation of new species has evidently taken place pre-eminently during these intermediate periods, of which, unfortunately, no petrifactions could be preserved, whereas, on the contrary, during the slow sinking of the ground there was more chance of numerous species dying out, and of a retrogression into fewer specific forms. The intermediate forms between the old and the newly-forming species must also have lived during the periods of elevation, and consequently could likewise leave no fossil remains.

In addition to the great and deplorable gaps in the palæontological records of creation—which are caused by the periods of elevation—there are, unfortunately, many other circumstances which immensely diminish their value. I must mention here especially the metamorphic state of the most ancient formations, of those strata which contain the remains of the most ancient flora and fauna, the original forms of all subsequent organisms, and which, therefore, would be of especial interest. It is just these rocks—and, indeed, the greater part of the primordial, or archilithic strata, almost the whole of the Laurentian, and a large part of the Cambrian systems—which no longer contain any recognizable remains, and for the simple reason that these strata have been subsequently changed or metamorphosed by the influence of the fiery fluid interior of the earth. These deepest neptunic strata of the crust have been completely changed from their original condition by the heat of the glowing nucleus of the earth, and have assumed a crystalline state. In this process, however, the form of the organic remains enclosed in them has been entirely destroyed. It has been preserved only here and there by a happy chance, as in the case of the most ancient petrifactions known, the Eozoon canadense, from the lowest Laurentian strata. However, from the layers of crystalline charcoal (graphite) and crystalline limestone (marble), which are found deposited in the metamorphic rocks, we may with certainty conclude that petrified animal and vegetable remains existed in them in earlier times.

Our record of creation is also extremely imperfect from the circumstance that only a small portion of the earth’s surface has been accurately investigated by geologists, namely, England, Germany, and France. But we know very little of the other parts of Europe, of Russia, Spain, Italy, and Turkey. In the whole of Europe, only some few parts of the earth’s crust have been laid open, by far the largest portion of it is unknown to us. The same applies to North America and to the East Indies. There some few tracts have been investigated; but of the larger portion of Asia, the most extensive of all continents, we know almost nothing; of Africa nothing, excepting the Cape of Good Hope and the shores of the Mediterranean; of Australia almost nothing; and of South America but very little. It is clear, therefore, that only quite a small portion, perhaps scarcely the thousandth part of the whole surface of the earth, has been palæontologically investigated. We may therefore reasonably hope, when more extensive geological investigations are made, which are greatly assisted by the constructions of railroads and mines, to find a great number of other important petrifactions. A hint that this will be the case is given by the remarkable petrifactions found in those parts of Africa and Asia which have been minutely investigated,—the Cape districts and the Himalaya mountains. A series of entirely new and very peculiar animal forms have become known to us from the rocks of these localities. But we must bear in mind that the vast bottom of the existing oceans is at the present time quite inaccessible to palæontological investigations, and that the greater part of the petrifactions which have lain there from primæval times will either never be known to us, or at best only after the course of many thousands of years, when the present bottom of the ocean shall have become accessible by gradual elevation. If we call to mind the fact that three-fifths of the whole surface of the earth consists of water, and only two-fifths of land, it becomes plain that on this account the palæontological record must always present an immense gap.

But, in addition to these, there exists another series of difficulties in the way of palæontology which arises from the nature of the organisms themselves. In the first place, as a rule only the hard and solid parts of organisms can fall to the bottom of the sea or of fresh waters, and be there enclosed in the mud and petrified. Hence it is only the bones and teeth of vertebrate animals, the calcareous shells of molluscs, the chitinous skeletons of articulated animals, the calcareous skeletons of star-fishes and corals, and the woody and solid parts of plants, that are capable of being petrified. But soft and delicate parts, which constitute by far the greater portion of the bodies of most organisms, are very rarely deposited in the mud under circumstances favourable to their becoming petrified, or distinctly impressing their external form upon the hardening mud. Now, it must be borne in mind that large classes of organisms, as for example the Medusæ, the naked molluscs without shells, a large portion of the articulated animals, almost all worms, and even the lowest vertebrate animals, possess no firm and hard parts capable of being petrified. In like manner the most important parts of plants, such as the flowers, are for the most part so soft and tender that they cannot be preserved in a recognizable form. We therefore cannot expect to find any petrified remains of these important organisms. Moreover, all organisms at an early stage of life are so soft and tender that they are quite incapable of being petrified. Consequently all the petrifactions found in the neptunic stratifications of the earth’s crust comprise altogether but a very few forms, and of these for the most part only isolated fragments.

We must next bear in mind that the dead bodies of the inhabitants of the sea are much more likely to be preserved and petrified in the deposits of mud than those of the inhabitants of fresh water and of the land. Organisms living on land can, as a rule, become petrified only when their corpses fall accidentally into the water and are buried at the bottom in the hardening layers of mud. But this event depends upon very many conditions. We cannot therefore be astonished that by far the majority of petrifactions belong to organisms which have lived in the sea, and that of the inhabitants of the land proportionately only very few are preserved in a fossil state. How many contingencies come into play here we may infer from the single fact that of many fossil mammals, in fact of all the mammals of the secondary, or mesozoic epoch, nothing is known except the lower jawbone. This bone is in the first place comparatively solid, and in the second place very easily separates itself from the dead body, which floats on the water. Whilst the body is driven away and dissolved by the water, the lower jawbone falls down to the bottom of the water and is there enclosed in the mud. This explains the remarkable fact that in a stratum of limestone of the Jurassic system near Oxford, in the slates of Stonesfield, as yet only the lower jawbones of numerous pouched animals (Marsupials) have been found. They are the most ancient mammals known, and of the whole of the rest of their bodies not a single bone exists. The opponents of the theory of development, according to their usual logic, would from this fact be obliged to draw the conclusion that the lower jawbone was the only bone in the body of those animals.

Footprints are very instructive when we attempt to estimate the many accidents which so arbitrarily influence our knowledge of fossils; they are found in great numbers in different extensive layers of sandstone; for example, in the red sandstone of Connecticut, in North America. These footprints were evidently made by vertebrate animals, probably by reptiles, of whose bodies not the slightest trace has been preserved.1 The impressions which their feet have left on the mud alone betray the former existence of these otherwise unknown animals.

The accidents which, besides these, determine the limits of our palæontological knowledge, may be inferred from the fact that we know of only one or two specimens of very many important petrifactions. It is not ten years since we became acquainted with the imperfect impression of a bird in the Jurassic or Oolitic system, the knowledge of which has been of the very greatest importance for the phylogeny of the whole class of birds. All birds previously known presented a very uniformly organized group, and showed no striking transitional forms to other vertebrate classes, not even to the nearly related reptiles. But that fossil bird from the Jura possessed not an ordinary bird’s tail, but a lizard’s tail, and thus confirmed what had been conjectured upon other grounds, namely, the derivation of birds from lizards. This single fossil has thus essentially extended not only our knowledge of the age of the class of birds, but also of their blood relationship to reptiles. In like manner our knowledge of other animal groups has been often essentially modified by the accidental discovery of a single fossil. The palæontological records must necessarily be exceedingly imperfect, because we know of so very few examples, or only mere fragments of very many important fossils.

Another and very sensible gap in these records is caused by the circumstance that the intermediate forms which connect the different species have, as a rule, not been preserved, and for the simple reason that (according to the principle of divergence of character) they were less favoured in the struggle for life than the most divergent varieties, which had developed out of one and the same original form. The intermediate links have, on the whole, always died out rapidly, and have but rarely been preserved as fossils. On the other hand, the most divergent forms were able to maintain themselves in life for a longer period as independent species, to propagate more numerously, and consequently to be more readily petrified. But this does not exclude the fact that in some cases the connecting intermediate forms of the species have been preserved so perfectly petrified, that even now they cause the greatest perplexity and occasion endless disputes among systematic palæontologists about the arbitrary limits of species.

An excellent example of this is furnished by the celebrated and very variable fresh-water snail from the Stuben Valley, near Steinheim, in Würtemburg, which has been described sometimes as Paludina, sometimes as Valvata, and sometimes as Planorbis multiformis. The snow-white shells of these small snails constitute more than half of the mass of the tertiary limestone hills, and in this one locality show such an astonishing variety of forms, that the most divergent extremes might be referred to at least twenty entirely different species. But all these extreme forms are united by such innumerable intermediate forms, and they lie so regularly above and beside one another, that Hilgendorf was able, in the clearest manner, to unravel the pedigree of the whole group of forms. In like manner, among very many other fossil species (for example, many ammonites, terebratulæ, sea urchins, lily encrinites, etc.) there are such masses of connecting intermediate forms, that they reduce the “dealers in fossil species” to despair.

When we weigh all the circumstances here mentioned, the number of which might easily be increased, it does not appear astonishing that the natural accounts or records of creation formed by petrifactions are extremely defective and incomplete. But nevertheless, the petrifactions actually discovered are of the greatest value. Their significance is of no less importance to the natural history of creation than the celebrated inscription on the Rosetta stone, and the decree of Canopus, are to the history of nations—to archæology and philology. Just as it has become possible by means of these two most ancient inscriptions to reconstruct the history of ancient Egypt, and to decipher all hieroglyphic writings, so in many cases a few bones of an animal, or imperfect impressions of a lower animal or vegetable form, are sufficient for us to gain the most important starting-points in the history of the whole group, and in the search after their pedigree. A couple of small back teeth, which have been found in the Keuper formation of the Trias, have of themselves alone furnished a sure proof that mammals existed even in the Triassic period.

Of the incompleteness of the geological accounts of creation, Darwin, agreeing with Lyell, the greatest of all recent geologists, says:—

“I look at the geological record as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of the slowly-changing language, more or less different in the successive chapters, may represent the forms of life which are entombed in our consecutive formations, and which falsely appear to us to have been abruptly introduced. On this view, the difficulties above discussed are greatly diminished, or even disappear.”—Origin of Species, 6th Edition, p. 289.

If we bear in mind the exceeding incompleteness of palæontological records, we shall not be surprised that we are still dependent upon so many uncertain hypotheses when actually endeavouring to sketch the pedigree of the different organic groups. However, we fortunately possess, besides fossils, other records of the history of the origin of organisms, which in many cases are of no less value, nay, in several cases are of much greater value, than fossils. By far the most important of these other records of creation is, without doubt, ontogeny, that is, the history of the development of the organic individual (embryology and metamorphology). It briefly repeats in great and marked features the series of forms which the ancestors of the respective individuals have passed through from the beginning of their tribe. We have designated the palæontological history of the development of the ancestors of a living form as the history of a tribe, or phylogeny, and we may therefore thus enunciate this exceedingly important biogenetic fundamental principle: “Ontogeny is a short and quick repetition, or recapitulation, of Phylogeny, determined by the laws of Inheritance and Adaptation.” As every animal and every plant from the beginning of its individual existence passes through a series of different forms, it indicates in rapid succession and in general outlines the long and slowly changing series of states of form which its progenitors have passed through from the most ancient times. (Gen. Morph. ii. 6, 110, 300.)

It is true that the sketch which the ontogeny of organisms gives us of their phylogeny is in most cases more or less obscured, and all the more so the more Adaptation, in the course of time, has predominated over Inheritance, and the more powerfully the law of abbreviated inheritance, and the law of correlative adaptation, have exerted their influence. However, this does not lessen the great value which the actual and faithfully preserved features of that sketch possess. Ontogeny is of the most inestimable value for the knowledge of the earliest palæontological conditions of development, just because no petrified remains of the most ancient conditions of the development of tribes and classes have been preserved. These, indeed, could not have been preserved on account of the soft and tender nature of their bodies. No petrifactions could inform us of the fundamental and important fact which ontogeny reveals to us, that the most ancient common ancestors of all the different animal and vegetable species were quite simple cells like the egg-cell. No petrifaction could prove to us the immensely important fact, established by ontogeny, that the simple increase, the formation of cell-aggregates and the differentiation of those cells, produced the infinitely manifold forms of multicellular organisms. Thus ontogeny helps us over many and large gaps in palæontology.

Hand of Nine different Mammals.

Pl. IV.

To the invaluable records of creation furnished by palæontology and ontogeny are added the no less important evidences for the blood relationship of organisms furnished by comparative anatomy. When organisms, externally very different, nearly agree in their internal structure, one may with certainty conclude that the agreement has its foundation in Inheritance, the dissimilarity its foundation in Adaptation. Compare, for example, the hands and fore paws of the nine different animals which are represented on Plate IV., in which the bony skeleton in the interior of the hand and of the five fingers is visible. Everywhere we find, though the external forms are most different, the same bones, and among them the same number, position, and connection. It will perhaps appear very natural that the hand of man (Fig. 1) differs very little from that of the gorilla (Fig. 2) and of the orang-outang (Fig. 3), his nearest relations. But it will be more surprising if the fore feet of the dog also (Fig. 4), as well as the breast-fin (the hand) of the seal (Fig. 5), and of the dolphin (Fig. 6), show essentially the same structure. And it will appear still more wonderful that even the wing of the bat (Fig. 7), the shovel-feet of the mole (Fig. 8), and the fore feet of the duck-bill (Ornithorhynchus) (Fig. 9), the most imperfect of all mammals, is composed of entirely the same bones, only their size and form being variously changed. Their number, the manner of their arrangement and connection has remained the same. (Compare also the explanation of Plate IV., in the Appendix.) It is quite inconceivable that any other cause, except the common inheritance of the part in question from common ancestors, could have occasioned this wonderful homology or similarity in the essential inner structure with such different external forms. Now, if we go down further in the system below the mammals, and find that even the wings of birds, the fore feet of reptiles and amphibious animals, are composed of essentially the same bones as the arms of man and the fore legs of the other mammals, we can, from this circumstance alone, with perfect certainty, infer the common origin of all these vertebrate animals. Here, as in all other cases, the degree of the internal agreement in the form discloses to us the degree of blood relationship.

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By a careful comparison of the individual and the palæontological development, as also by the comparative anatomy of organisms, by the comparative examination of their fully developed structural characteristics, we arrive at the knowledge of the degrees of their different structural relationships. By this, however, we at the same time obtain an insight into their true blood relationship, which, according to the Theory of Descent, is the real reason of the structural relationship. Hence by collecting, comparing, and employing the empirical results of embryology, palæontology, and anatomy for supplementing each other, we arrive at an approximate knowledge of “the Natural System,” which, according to our views, is the pedigree of organisms. It is true that our human knowledge, in all things fragmentary, is especially so in this case, on account of the extreme incompleteness and defectiveness of the records of creation. However, we must not allow this to discourage us, or to deter us from undertaking this highest problem of biology. Let us rather see how far it may even now be possible, in spite of the imperfect state of our embryological, palæontological, and anatomical knowledge, to establish a probable scheme of the genealogical relationships of organisms.

Darwin in his book gives us no answer to these special questions of the Theory of Descent; at the conclusion he only expresses his conjecture “that animals have descended from at most only four or five progenitors, and plants from an equal or less number.” But as these few aboriginal forms still show traces of relationship, and as the animal and vegetable kingdoms are connected by intermediate transitional forms, he arrives afterwards at the opinion “that probably all the organic beings which have ever lived on the earth have descended from some one primordial form, into which life was first breathed by the Creator.” Like Darwin, all other adherents of the Theory of Descent have only treated it in a general way, and not made the attempt to carry it out specially, and to treat the “Natural System” actually as the pedigree of organisms. If, therefore, we venture upon this difficult undertaking, we must take up independent ground.

Four years ago I set up a number of hypothetical genealogies for the larger groups of organisms in the systematic introduction to my General History of Development (Gen. Morph. vol. ii.), and thereby, in fact, made the first attempt actually to construct the pedigrees of organisms in the manner required by the theory of development. I was quite conscious of the extreme difficulty of the task, and as I undertook it in spite of all discouraging obstacles, I claim no more than the merit of having made the first attempt and given a stimulus for other and better attempts. Probably most zoologists and botanists were but little satisfied with this beginning, and least so in reference to the special domain in which each one is specially at work. However, it is certainly in this case much easier to blame than to produce something better, and what best proves the immense difficulty of this infinitely complicated task is the fact that no naturalist has as yet supplied the place of my pedigrees by better ones. But, like all other scientific hypotheses which serve to explain facts, my genealogical hypotheses may claim to be taken into consideration until they are replaced by better ones.

I hope that this replacement will very soon take place; and I wish for nothing more than that my first attempt may induce very many naturalists to establish more accurate pedigrees for the individual groups, at least in the special domain of the animal and vegetable kingdom which happens to be well known to one or other of them. By numerous attempts of this kind our genealogical knowledge, in the course of time, will slowly advance and approach more towards perfection, although it can with certainty be foreseen that we shall never arrive at a complete pedigree. We lack, and shall ever lack, the indispensable palæontological foundations. The most ancient records will ever remain sealed to us, for reasons which have been previously mentioned. The most ancient organisms which arose by spontaneous generation—the original parents of all subsequent organisms—must necessarily be supposed to have been Monera—simple, soft, albuminous lumps, without structure, without any definite forms, and entirely without any hard and formed parts. They and their next offspring were consequently not in any way capable of being preserved in a petrified condition. But we also lack, for reasons discussed in detail in the preceding chapter, by far the greater portion of the innumerable palæontological documents, which are really requisite for a safe reconstruction of the history of animal tribes, or phylogeny, and for the true knowledge of the pedigree of organisms. If we, therefore, in spite of this, venture to undertake their hypothetical construction, we must chiefly depend for guidance on the two other series of records which most essentially supplement the palæontological archives. These are ontogeny and comparative anatomy.

If thoughtfully and carefully we consult these most valuable records, we at once perceive what is exceedingly significant, namely, that by far the greater number of organisms, especially all higher animals and plants, are composed of a great number of cells, and that they originate out of an egg, and that this egg, in animals as well as in plants, is a single, perfectly simple cell—a little lump of albuminous constitution, in which another albuminous corpuscle, the cell-kernel, is enclosed. This cell containing its kernel grows and becomes enlarged. By division it forms an accumulation of cells, and out of these, by division of labour (as has previously been described), there arise the numberless different forms which are presented to us in the fully developed animal and vegetable species. This immensely important process—which we may follow step by step, with our own eyes, any day in the embryological development of any animal or vegetable individual, and which as a rule is by no means considered with the reverence it deserves—informs us more surely and completely than all petrifactions could do as to the original palæontological development of all many-celled organisms, that is, of all higher animals and plants. For as ontogeny, or the embryological development of every single individual, is essentially only a recapitulation of phylogeny, or the palæontological development of its chain of ancestors, we may at once, with full assurance, draw the simple and important conclusion, that all many-celled animals and plants were originally derived from single-celled organisms. The primæval ancestors of man, as well as of all other animals, and of all plants composed of many cells, were simple cells living isolated. This invaluable secret of the organic pedigree is revealed to us with infallible certainty by the egg of animals, and by the true egg-cell of plants. When the opponents of the Theory of Descent assert it to be miraculous and inconceivable that an exceedingly complicated many-celled organism could, in the course of time, have proceeded from a simple single-celled organism, we at once reply that we may see this incredible miracle at any moment, and follow it with our own eyes. For the embryology of animals and plants visibly presents to our eyes in the shortest space of time the same process as that which has taken place in the origin of the whole tribe during the course of enormous periods of time.

Upon the ground of embryological records, therefore, we can with full assurance maintain that all many-celled, as well as single-celled, organisms are originally descended from simple cells; connected with this, of course, is the conclusion that the most ancient root of the animal and vegetable kingdom was common to both. For the different primæval “original cells” out of which the few different main groups or tribes have developed, only acquired their differences after a time, and were descended from a common “primæval cell.” But where did those few “original cells,” or the one primæval cell, come from? For the answer to this fundamental genealogical question we must return to the theory of plastids and the hypothesis of spontaneous generation which we have already discussed (vol. i. p. 327).

As was then shown, we cannot imagine cells to have arisen by spontaneous generation, but only Monera, those primæval creatures of the simplest kind conceivable, like the still living Protamœbæ), Protomyxæ, etc. (vol. i. p. 1186, Fig. 1). Only such corpuscules of mucus without component parts—whose whole albuminous body is as homogeneous in itself as an inorganic crystal, but which nevertheless fulfills the two organic fundamental functions of nutrition and propagation—could have directly arisen out of inorganic matter by autogeny at the beginning (we may suppose) of the Laurentian period. While some Monera remained at the original simple stage of formation, others gradually developed into cells by the inner kernel of the albuminous mass becoming separated from the external cell-substance. In others, by differentiation of the outermost layer of the cell-substance, an external covering (membrane, or skin) was formed round simple cytods (without kernel), as well as round naked cells (containing a kernel). By these two processes of separation in the simple primæval mucus of the Moneron body, by the formation of a kernel in the interior and a covering on the outer surface of the mass of plasma, there arose out of the original most simple cytods, or Monera, those four different species of plastids, or individuals, of the first order, from which, by differentiation and combination, all other organisms could afterwards develop themselves. (Compare vol. i. p. 347.)

The question now forces itself upon us, Are all organic cytods and cells, and consequently also those “original cells” which we previously considered to be the primary parents of the few great main groups of the animal and vegetable kingdoms, descended from a single original form of Moneron, or were there several different organic primary forms, each traceable to a peculiar independent species of Moneron which originated by spontaneous generation? In other words, Is the whole organic world of a common origin, or does it owe its origin to several acts of spontaneous generation? This fundamental question of genealogy seems at first sight to be of exceeding importance. But on a more accurate examination, we shall soon see that this is not the case, and that it is in reality a matter of very subordinate importance.

Let us now pass on to examine and clearly limit our conception of an organic tribe. By tribe, or phylum, we understand all those organisms of whose blood relationship and descent from a common primary form there can be no doubt, or whose relationship, at least, is most probable from anatomical reasons, as well as from reasons founded on historical development. Our tribes, or phyla, according to this idea, essentially coincide with those few “great classes,” or “main classes,” of which Darwin also thinks that each contains only organisms related by blood, and of which, both in the animal and in the vegetable kingdoms, he only assumes either four or five. In the animal kingdom these tribes would essentially coincide with those four, five, or six main divisions which zoologists, since Bär and Cuvier, have distinguished as “main forms, general plans, branches, or sub-kingdoms” of the animal kingdom. (Compare vol. i. p. 53.) Bär and Cuvier distinguished only four of them, namely:—1. The vertebrate animals (Vertebrata); 2. The articulated animals (Articulata); 3. The molluscous animals (Mollusca); and 4. The radiated animals (Radiata). At present six are generally distinguished, since the tribe of the articulated animals is divided into two tribes, those possessing articulated feet (Arthropoda), and the worms (Vermes); and in like manner the tribe of radiated animals is subdivided into the two tribes of the star animals (Echinodermata) and the animal-plants (Zoophyta). Within each of these six tribes, all the included animals, in spite of great variety in external form and inner structure, nevertheless possess such numerous and important characteristics in common, that there can be no doubt of their blood relationship. The same applies also to the six great main classes which modern botany distinguishes in the vegetable kingdom, namely:—1. Flowering plants (Phanerogamia); 2. Ferns (Filicinæ); 3. Mosses (Muscinæ); 4. Lichens (Lichenes); 5. Fungi (Fungi); and 6. Water-weeds (Algæ). The last three groups, again, show such close relations to one another, that by the name of “Thallus plants” they may be contrasted with the three first main classes, and consequently the number of phyla, or main groups, of the vegetable kingdom may be reduced to the number of four. Mosses and ferns may likewise be comprised as “Prothallus plants” (Prothallophyta), and thereby the number of plant tribes reduced to three—Flowering plants, Prothallus plants, and Thallus plants.

Very important facts in the anatomy and the history of development, both in the animal and vegetable kingdoms, support the supposition that even these few main classes or tribes are connected at their roots, that is, that the lowest and most ancient primary forms of all three are related by blood to one another. Nay, by a further examination we are obliged to go still a step further, and to agree with Darwin’s supposition, that even the two pedigrees of the animal and vegetable kingdom are connected at their lowest roots, and that the lowest and most ancient animals and plants are derived from a single common primary creature. According to our view, this common primæval organism can have been nothing but a Moneron which took its origin by spontaneous generation.

In the mean time we shall at all events be acting cautiously if we avoid this last step, and assume true blood relationship only within each tribe, or phylum, where it has been undeniably and surely established by facts in comparative anatomy, ontogeny, and phylogeny. But we may here point to the fact that two different fundamental forms of genealogical hypothesis are possible, and that all the different investigations of the Theory of Descent in relation to the origin of organic groups of forms will, in future, tend more and more in one or the other of these directions. The unitary, or monophyletic, hypothesis of descent will endeavour to trace the first origin of all individual groups of organisms, as well as their totality, to a single common species of Moneron which originated by spontaneous generation (vol. i. p. 343). The multiple, or polyphyletic, hypothesis of descent, on the other hand, will assume that several different species of Monera have arisen by spontaneous generation, and that these gave rise to several different main classes (tribes, or phyla) (vol. i. p. 348). The apparently great contrast between these two hypotheses is in reality of very little importance. For both the monophyletic and the polyphyletic hypothesis of descent must necessarily go back to the Monera as the most ancient root of the one or of the many organic tribes. But as the whole body of a Moneron consists only of a simple, formless mass, without component particles, made up of a single albuminous combination of carbon, it follows that the differences of the different Monera can only be of a chemical nature, and can only consist in a different atomic composition of that mucous albuminous combination. But these subtle and complicated differences of mixture of the infinitely manifold combinations of albumen are not appreciable by the rude and imperfect means of human observation and are, consequently, at present of no further interest to the task we have in hand.

The question of the monophyletic or polyphyletic origin will constantly recur within each individual tribe, where the origin of a smaller or of a larger group is discussed. In the vegetable kingdom, for example, some botanists will be inclined to derive all flowering plants from a single form of fern, while others will prefer the idea that several different groups of Phanerogama have sprung from several different groups of ferns. In like manner, in the animal kingdom, some zoologists will be more in favour of the supposition that all placental animals are derived from a single pouched animal; others will be more in favour of the opposite supposition, that several different groups of placental animals have proceeded from several different pouched animals. In regard to the human race itself, some will prefer to derive it from a single form of ape, while others will be more inclined to the idea that several different races of men have arisen, independently of one another, out of several different species of ape. Without here expressing our opinion in favour of either the one or the other conception, we must, nevertheless, remark that in general the monophyletic hypothesis of descent deserves to be preferred to the polyphyletic hypothesis of descent. In accordance with the chorological proposition of a single “centre of creation” or of a single primæval home for most species (which has already been discussed), we may be permitted to assume that the original form of every larger or smaller natural group only originated once in the course of time, and only in one part of the earth. We may safely assume this simple original root, that is, the monophyletic origin, in the case of all the more highly developed groups of the animal and vegetable kingdoms. (Compare vol. i. p. 353.) But it is very possible that the more complete Theory of Descent of the future will involve the polyphyletic origin of very many of the low and imperfect groups of the two organic kingdoms.

For these reasons I consider it best, in the mean time, to adopt the monophyletic hypothesis of descent both for the animal and for the vegetable kingdom. Accordingly, the above-mentioned six tribes, or phyla, of the animal kingdom must be connected at their lowest root, and likewise the three or six main classes, or phyla, of the vegetable kingdom must be traced to a common and most ancient original form. How the connection of these tribes is to be conceived I shall explain in the succeeding chapters. But before proceeding to this, we must occupy ourselves with a very remarkable group of organisms, which cannot without artificial constraint be assigned either to the pedigree of the vegetable or to that of the animal kingdom. These interesting and important organisms are the primary creatures, or Protista.

All organisms which we comprise under the name of Protista show in their external form, in their inner structure, and in all their vital phenomena, such a remarkable mixture of animal and vegetable properties, that they cannot with perfect justice be assigned either to the animal or to the vegetable kingdom; and for more than twenty years an endless and fruitless dispute has been carried on as to whether they are to be assigned to this or that kingdom. Most of Protista are so small that they can scarcely, if at all, be perceived with the naked eye. Hence the majority of them have only become known during the last fifty years, since by the help of the improved and general use of the microscope these minute organisms have been more frequently observed and more accurately examined. However, no sooner were they better known than endless disputes arose about their real nature and their position in the natural system of organisms. Many of these doubtful primary creatures botanists defined as animals, and zoologists as plants; neither of the two would own them. Others, again, were declared by botanists to be plants, and by zoologists to be animals; each claimed them. These contradictions are not altogether caused by our imperfect knowledge of the Protista, but in reality by their true nature. Indeed, most Protista present such a confused mixture of several animal and vegetable characteristics, that each investigator may arbitrarily assign them either to the animal or vegetable kingdom. Accordingly as he defines these two kingdoms, and as he looks upon this or that characteristic as determining the animal or vegetable nature, he will assign the individual classes of Protista in one case to the animal and in another to the vegetable kingdom. But this systematic difficulty has become an inextricable knot by the fact that all more recent investigations on the lowest organisms have completely effaced, or at least destroyed, the sharp boundary between the animal and vegetable kingdom which had hitherto existed, and to such a degree that its restoration is possible only by means of a completely artificial definition of the two kingdoms. But this definition could not be made so as to apply to many of the Protista.

For this and other reasons it is, in the mean time, best to exclude the doubtful beings from the animal as well as from the vegetable kingdom, and to comprise them in a third organic kingdom standing midway between the two others. This intermediate kingdom I have established as the Kingdom of the Primary Creatures (Protista), when discussing general anatomy in the first volume of my General Morphology, pp. 191-238. In my Monograph of the Monera,(15) I have recently treated of this kingdom, having somewhat changed its limits, and given it a more accurate definition. Of independent classes of the kingdom Protista, we may at present distinguish the following:—

1. The still living Monera; 2. The Amœboidea, or Protoplasts; 3. The Whip-swimmers, or Flagellata; 4. The Flimmer-balls, or Catallacta; 5. The Tram-weavers, or Labyrinthuleæ; 6. The Flint-cells, or Diatomeæ; 7. The Slime-moulds, or Myxomycetes; 8. The Ray-streamers, or Rhizopoda.

The most important groups at present distinguishable in these eight classes of Protista are named in the systematic table on p. 51. Probably the number of these Protista will be considerably increased in future days by the progressive investigations of the ontogeny of the simplest forms of life, which have only lately been carried on with any great zeal. With most of the classes named we have become intimately acquainted only during the last ten years. The exceedingly interesting Monera and Labyrinthuleæ, as also the Catallacta, were indeed discovered only a few years ago. It is probable also that very numerous groups of Protista have died out in earlier periods, without having left any fossil remains, owing to the very soft nature of their bodies. We might add to the Protista from the still living lowest groups of organisms—the Fungi; and in so doing should make a very large addition to its domain. Provisionally we shall leave them among plants, though many naturalists have separated them altogether from the vegetable kingdom.

The pedigree of the kingdom Protista is still enveloped in the greatest obscurity. The peculiar combination of animal and vegetable properties, the indifferent and uncertain character of their relations of forms and vital phenomena, together with a number of several very peculiar features which separate most of the subordinate classes sharply from the others, at present baffle every attempt distinctly to make out their blood relationships with one another, or with the lowest animals on the one hand, and with the lowest plants on the other hand. It is not improbable that the classes specified, and many other unknown classes of Protista, represent quite independent organic tribes, or phyla, each of which has independently developed from one, perhaps from various, Monera which have arisen by spontaneous generation. If we do not agree to this polyphyletic hypothesis of descent, and prefer the monophyletic hypothesis of the blood relationship of all organisms, we shall have to look upon the different classes of Protista as the lower small offshoots of the root, springing from the same simple Monera root, out of which arose the two mighty and many-branched pedigrees of the animal kingdom on the one hand, and of the vegetable kingdom on the other. (Compare pp. 74, 75.) Before I enter into this difficult question more accurately, it will be appropriate to premise something further as to the contents of the classes of Protista given on the next page, and their general natural history.

SYSTEMATIC SURVEY
Of the Larger and Smaller Groups of the Kingdom Protista
Classes of the Protista Kingdom.		Systematic Name of the Classes		Orders of Families of the Classes.		A name of a Genus as an example.
1. Moners			Monera		1. Gymnomonera	Protogenes
					2. Lepomonera	Protomyxa
2. Protoplasts			Amœboida		1. Gymnamœbæ	Amœba
					2. Leptamœbæ	Arcella
					3. Gregarinæ	Monocystis
3. Whip-swimmers			Flagellata		1. Nudiflagellata	Euglena
					2. Cilioflagellata	Peridinium
4. Flimmer-balls			Catallacta		1. Catallacta	Magosphæra
5. Tram-weavers			Labyrinthuleæ		1. Labyrinthuleæ	Labyrinthula
6. Flint-cells			Diatomea		1. Striata	Navicula
					2. Vittata	Tabellaria
					3. Areolata	Coscinodiscus
7. Slime-moulds			Myxomycetes		1. Physareæ	Æthalium
					2. Stemoniteæ	Stemonitis
					3. Trichiaceæ	Arcyria
					4. Lycogaleæ	Reticularia
8. Ray-streamers      or Rhizopods.      (Root-feet)			I. Acyttaria		1. Monothalamia	Gromia
					2. Polythalamia	Nummulina
			II. Heliozoa		1. Heliozoa	Actinosphærium
			III. Radiolaria		1. Monocyttaria	Cyrtidosphæra
					2. Polycyttaria	Collosphæra

 

It will perhaps seem strange that I should here again begin with the remarkable Monera as the first class of the Protista kingdom, as I of course look upon them as the most ancient primary forms of all organisms without exception. Still, what are we otherwise to do with the still living Monera? We know nothing of their palæontological origin, we know nothing of any of their relations to lower animals or plants, and we know nothing of their possible capability of developing into higher organisms. The simple and homogeneous little lump of slime or mucus which constitutes their entire body (Fig. 8) is the most ancient and original form of animal as well as of vegetable plastids. Hence it would evidently be just as arbitrary and unreasonable to assign them to the animal as it would be to assign them to the vegetable kingdom. In any case we shall for the present be acting more cautiously and critically if we comprise the still living Monera—whose number and distribution is probably very great—as a special and independent class, contrasting them with the other classes of the kingdom Protista, as well as with the animal kingdom. Morphologically considered, the Monera—on account of the perfect homogeneity of the albuminous substance of their bodies, on account of their utter want of heterogeneous particles—are more closely connected with anorgana than with organisms, and evidently form the transition between the inorganic and organic world of bodies, as is necessitated by the hypothesis of spontaneous generation. I have described and given illustrations of the forms and vital phenomena of the still living Monera (Protamœba, Protogenes, Protomyxa, etc.) in my Monograph of the Monera,(15) and have briefly mentioned the most important facts in the eighth chapter (vol. i. pp. 183-187). Therefore, only by way of a specimen, I here repeat the drawing of the fresh-water Protamœba (Fig. 8). The history of the life of an orange-red Protomyxa adrantiaca, which I observed at Lanzerote, one of the Canary Islands, is given in Plate I. (see its explanation in the Appendix). Besides this, I here add a drawing of the form of Bathybius, that remarkable Moneron discovered by Huxley, which lives in the greatest depths of the sea in the shape of naked lumps of protoplasm and reticular mucus (vol. i. p. 344).

Fig. 9.—Bathybius Hæckelii, the “creature of primæval slime,” from the greatest depths of the sea. The figure, which is greatly magnified, only shows that form of the Bathybius which consists of a naked network of protoplasm, without the discoliths and cyatholiths which are found in other forms of the same Moneron, and which perhaps may be considered as the products of its secretion.

The Amœbæ of the present day, and the organisms most closely connected with them, Arcellidæ and Gregarinæ, which we here unite as a second class of Protista under the name of Amœboidea (Protoplasta), present no fewer genealogical difficulties than the Monera. These primary creatures are at present usually placed in the animal kingdom without its in reality being understood why. For simple naked cells—that is, shell-less plastids with a kernel—occur as well among real plants as real animals. The generative cells, for example, in many Algæ (spores and eggs) exist for a longer or shorter time in water in the form of naked cells with a kernel, which cannot be distinguished at all from the naked eggs of many animals (for example, those of the Siphonophorous Medusæ). (Compare the figure of a naked egg of a bladder-wrack in Chapter xvii. p. 90.) In reality every naked simple cell, whether it proceeds from an animal or vegetable body, cannot be distinguished from an independent Amœba. For an Amœba is nothing but a simple primary cell, a naked little lump of cell-matter, or plasma, containing a kernel. The contractility of this plasma, which the free Amœba shows in stretching out and drawing in its changing processes, is a general vital property of the organic plasma of all animal as well as of all vegetable plastids. When a freely moving Amœba, which perpetually changes its form, passes into a state of rest, it draws itself together into the form of a globule, and surrounds itself with a secreted membrane. It can then be as little distinguished from an animal egg as from a simple globular vegetable cell (Fig. 10 A).

Naked cells, with kernels, like those represented in Fig. 10 B, which are continuously changing, stretching out and drawing in formless, finger-like processes, and which are on this account called amœboid, are found frequently and widely dispersed in fresh water and in the sea; nay, are even found creeping on land. They take their food in the same way as was previously described in the case of the Protamœba (vol. i. p. 186). Their propagation by division can sometimes be observed. (Fig. 10 C, D.) I have described the processes in an earlier chapter (vol. i. p. 187). Many of these formless Amœbæ have lately been recognized as the early stages of development of other Protista (especially the Myxomycetæ), or as the freed cells of lower animals and plants. The colourless blood-cells of animals, for example, those of human blood, cannot be distinguished from Amœbæ. They, like the latter, can receive solid corpuscles into their interior, as I was the first to show by feeding them with finely divided colouring matters (Gen. Morph. i. 271). However, other Amœbæ (like the one given in Fig. 10) seem to be independent “good species,” since they propagate themselves unchanged throughout many generations. Besides the real, or naked, Amœbæ (Gymnamœbæ), we also find widely diffused in fresh water case-bearing Amœbæ (Lepamœbæ), whose naked plasma body is partially protected by a more or less solid shell (Arcella), sometimes even by a case (Difflugia) composed of small stones. Lastly, we frequently find in the body of many lower animals parasitic Amœbæ (Gregarinæ), which, adapting themselves to a parasitic life, have surrounded their plasma-body with a delicate closed membrane.

The simple naked Amœbæ are, next to the Monera, the most important of all organisms to the whole science of biology, and especially to general genealogy. For it is evident that the Amœbæ originally arose out of simple Monera (Protamœbæ), by the important process of segregation taking place in their homogeneous viscid body—the differentiation of an inner kernel from the surrounding plasma. By this means the great progress from a simple cytod (without kernel) into a real cell (with kernel) was accomplished (compare Fig. 8 A and Fig. 10 B). As some of these cells at an early stage encased themselves by secreting a hardened membrane, they formed the first vegetable cells, while others, remaining naked, developed into the first aggregates of animal cells. The presence or absence of an encircling hard membrane forms the most important, although by no means the entire, difference of form between animal and vegetable cells. As vegetable cells even at an early stage enclose themselves within their hard, thick, and solid cellular shell, like that of the Amœbæ in a state of rest (Fig. 10 A), they remain more independent and less accessible to the influences of the outer world than are the soft animal cells, which are in most cases naked, or merely covered by a thin pliable membrane. But in consequence of this the vegetable cells cannot combine, as do the animal cells, for the construction of higher and composite fibrous tracts, for example, the nervous and muscular tissues. It is probable that, in the case of the most ancient single-celled organisms, there must have developed at an early stage the very important difference in the animal and vegetable mode of receiving food. The most ancient single-celled animals, being naked cells, could admit solid particles into the interior of their soft bodies, as do the Amœbæ (Fig. 10 B) and the colourless blood-cells; whereas the most ancient single-celled plants encased by their membranes were no longer able to do this, and could admit through it only fluid nutrition (by means of diffusion).

Fig. 11.—A single Whip-swimmer (Euglena striata), greatly magnified. Above a thread-like lashing whip is visible; in the centre the round cellular kernel, with its kernel speck.

The Whip-swimmers (Flagellata), which we consider as a third class of the kingdom Protista, are of no less doubtful nature than the Amœbæ. They often show as close and important relations to the vegetable as to the animal kingdom. Some Flagellata at an early stage, when freely moving about, cannot be distinguished from real plants, especially from the spores of many Algæ; whereas others are directly allied to real animals, namely, to the fringed Infusoria (Ciliata). The Flagellata are simple cells which live in fresh or salt water, either singly or united in colonies. The characteristic part of their body is a very movable simple or compound whip-like appendage (whip, or flagellum) by means of which they actively swim about in the water. This class is divided into two orders. Among the fringed whip-swimmers (Cilioflagellata) there exists, in addition to the long whip, a short fringe of vibrating hairs, which is wanting in the unfringed whip-swimmers (Nudoflagellata). To the former belong the flint-shelled yellow Peridinia, which are largely active in causing the phosphorescence of the sea; to the latter belong the green Euglenæ, immense masses of which frequently make our ponds in spring quite green.

Fig. 12.—The Norwegian Flimmer-ball (Magosphæra planula) swimming by means of its vibratile fringes, as seen from the surface.

A very remarkable new form of Protista, which I have named Flimmer-ball (Magosphæra), I discovered only three years ago (in September, 1869), on the Norwegian coast (Fig. 12), and have more accurately described in my Biological Studies(15) (p. 137, Plate V.). Off the island of Gis-oe, near Bergen, I found swimming about, on the surface of the sea, extremely neat little balls composed of a number (between thirty and forty) of fringed pear-shaped cells, the pointed ends of which were united in the centre like radii. After a time the ball dissolved. The individual cells swarmed about independently in the water like fringed Infusoria, or Ciliata. These afterwards sank to the bottom, drew their fringes into their bodies, and gradually changed into the form of creeping Amœbæ (like Fig 10 B). These last afterwards encased themselves (as in Fig. 10 A), and then divided by repeated halvings into a large number of cells (exactly as in the case of the cleavage of the egg, Fig. 6, vol. i. p. 299). The cells became covered with vibratile hairs, broke through the case enclosing them, and now again swam about in the shape of a fringed ball (Fig. 12). This wonderful organism, which sometimes appears like a simple Amœba, sometimes as a single fringed cell, sometimes as a many-celled fringed ball, can evidently be classed with none of the other Protista, and must be considered as the representative of a new independent group. As this group stands midway between several Protista, and links them together, it may bear the name of Mediator, or Catallacta.

Fig. 13.—Labyrinthula macrocystis (much enlarged). Below is a large group of accumulated cells, one of which, on the left, is separating itself; above are two single cells which are gliding along the threads of the retiform labyrinth which form their “tramways.”

The Protista of the fifth class, the Tram-weavers, or Labyrinthuleæ, are of a no less puzzling nature; they were lately discovered by Cienkowski on piles in sea water (Fig. 13). They are spindle-shaped cells, mostly of a yellow-ochre colour, which are sometimes united into a dense mass, sometimes move about in a very peculiar way. They form, in a manner not yet explained, a retiform frame of entangled threads (compared to a labyrinth), and on the dense filamentous “tramways” of this frame they glide about. From the shape of the cells of the Labyrinthuleæ we might consider them as the simplest plants, from their motion as the simplest animals, but in reality they are neither animals nor plants.

Fig. 14.—Navicula hippocampus (greatly magnified). In the middle of the cell the cell-kernel (nucleus) is visible, together with its kernel speck (nucleolus).

The Flint-cells (Diatomeæ), a sixth class of Protista, are perhaps the most closely related to the Labyrinthuleæ. These primary creatures—which at present are generally considered as plants, although some celebrated naturalists still look upon them as animals—inhabit the sea and fresh waters in immense masses, and offer an endless variety of the most elegant forms. They are mostly small microscopic cells, which either live singly (Fig. 14), or united in great numbers, and occur either attached to objects, or glide and creep about in a peculiar manner. Their soft cell-substance, which is of a characteristic brownish yellow colour, is always enclosed by a solid and hard flinty shell, possessing the neatest and most varied forms. This flinty covering is open to the exterior only by one or two slits, through which the enclosed soft plasma-body communicates with the outer world. The flinty cases are found petrified in masses, and many rocks—for example, the Tripoli slate polish, the Swedish mountain meal, etc.,—are in a great measure composed of them.

Fig. 15.—A stalked fruit-body (spore-bladder, filled with spores) of one of the Myxomycetes (Physarum albipes) not much enlarged.

A seventh class of Protista is formed by the remarkable Slime-moulds (Myxomycetes). They were formerly universally considered as plants, as real Fungi, until ten years ago the botanist De Bary, by discovering their ontogeny, proved them to be quite distinct from Fungi, and rather to be akin to the lower animals. The mature body is a roundish bladder, often several inches in size, filled with fine spore-dust and soft flakes (Fig. 15), as in the case of the well-known puff-balls (Gastromycetes). However, the characteristic cellular threads, or hyphæ, of a real fungus do not arise from the germinal corpuscles, or spores, of the Myxomycetes, but merely naked masses of plasma, or cells, which at first swim about in the form of Flagellata (Fig. 11), afterwards creep about like the Amœbæ (Fig. 10 B), and finally combine with others of the same kind to form large masses of “slime,” or “plasmodia.” Out of these, again, there arises, by-and-by, the bladder-shaped fruit-body. Many of my readers probably know one of these plasmodia, the Æthalium septicum, which in summer forms a beautiful yellow mass of soft mucus, often several feet in breadth, known by the name of “tan flowers,” and penetrates tan-heaps and tan-beds. At an early stage these slimy, freely-creeping Myxomycetes, which live for the most part in damp forests, upon decaying vegetable substances, bark of trees, etc., are with equal justice or injustice declared by zoologists to be animals, while in the mature, bladder-shaped condition of fructification they are by botanists defined as plants.

The nature of the Ray-streamers (Rhizopoda), the eighth class of the kingdom Protista, is equally obscure. These remarkable organisms have peopled the sea from the most ancient times of the organic history of the earth, in an immense variety of forms, sometimes creeping at the bottom of the sea, sometimes swimming on the surface. Only very few live in fresh water (Gromia, Actinosphærium). Most of them possess solid calcareous or flinty shells of an extremely beautiful construction, which can be perfectly preserved in a fossil state. They have frequently accumulated in such huge numbers as to form mountain masses, although the single individuals are very small, and often scarcely visible, or completely invisible to the naked eye. A very few attain the diameter of a few lines, or even as much as a couple of inches. The name which the class bears is given because thousands of exceedingly fine threads of protoplasm radiate from the entire surface of their naked slimy body; these rays are quasi-feet, or pseudopodia, which branch off like roots (whence the term Rhizopoda, signifying root-footed), unite like nets, and are observed continually to change form, as in the case of the simpler plasmic feet of the Amœboidea, or Protoplasts. These ever-changing little pseudo-feet serve both for locomotion and for taking food.

The class of the Rhizopoda is divided into three different legions, viz. the chamber-shells, or Acyttaria, the sun-animalcules, or Heliozoa, and the basket-shells, or Radiolaria. The Chamber-shells (Acyttaria) constitute the first and lowest of these three legions; for the whole of their soft body consists merely of simple mucous or slimy cell-matter, or protoplasm, which has not differentiated into cells. However, in spite of this most primitive nature of body, most of the Acyttaria secrete a solid shell composed of calcareous earth, which presents a great variety of exquisite forms. In the more ancient and more simple Acyttaria this shell is a simple chamber, bell-shaped, tubular, or like the shell of a snail, from the mouth of which a bundle of plasmic threads issues. In contrast to these single-chambered forms (Monothalamia), the many-chambered forms (Polythalamia)—to which the great majority of the Acyttaria belong—possess a house, which is composed in an artistic manner of numerous chambers. These chambers sometimes lie in a row one behind the other, sometimes in concentric circles or spirals, in the form of a ring round a central point, and then frequently one above another in many tiers, like the boxes of an amphitheatre. This formation, for example, is found in the nummulites, whose calcareous shells, of the size of a lentil, have accumulated to the number of millions, and form whole mountains on the shores of the Mediterranean. The stones of which some of the Egyptian pyramids are built consist of such nummulitic limestone. In most cases the chambers of the shells of the Polythalamia are wound round one another in a spiral line. The chambers are connected with one another by passages and doors, like rooms of a large palace, and are generally open towards the outside by numerous little windows, out of which the plasmic body can stream or strain forth its little pseudo-feet, or rays of slime, which are always changing form. But in spite of the exceedingly complicated and elegant structure of this calcareous labyrinth, in spite of the endless variety in the structure and the decoration of its numerous chambers, and in spite of the regularity and elegance of their execution, the whole of this artistic palace is found to be the secreted product of a perfectly formless, slimy mass, devoid of any component parts! Verily, if the whole of the recent anatomy of animal and vegetable textures did not support our theory of plastids, if all its important results did not unanimously corroborate the fact that the whole miracle of vital phenomena and vital forms is traceable to the active agency of the formless albuminous combinations of protoplasm, the Polythalamia alone would secure the triumph of that theory. For we may here at any moment, by means of the microscope, point out the wonderful fact, first established by Dujardin and Max Schulze, that the formless mucus of the soft plasma-body, this true “matter of life,” is able to secrete the neatest, most regular, and most complicated structures. This secretive skill is simply a result of inherited adaptation, and by it we learn to understand how this same “primæval slime”—this same protoplasm—can produce in the bodies of animals and plants the most different and most complicated cellular forms.

It is, moreover, a matter of special interest that the most ancient organism, the remains of which are found in a petrified condition, belongs to the Polythalamia. This organism is the “Canadian Life’s-dawn” (Eozoon canadense), which has already been mentioned, and which was found a few years ago in the Ottawa formation (in the deepest strata of the Laurentian system), on the Ottawa river in Canada. If we expected to find organic remains at all in these most ancient deposits of the primordial period, we should certainly look for such of the most simple Protista as are covered with a solid shell, and in the organization of which the difference between animal and plant is as yet not indicated.

We know of but few species of the Sun-animalcules (Heliozoa), the second class of the Rhizopoda. One species is very frequently found in our fresh waters. It was observed even in the last century by a clergyman in Dantzig, Eichhorn by name, and it has been called after him, Actinosphærium Eichhornii. To the naked eye it appears as a gelatinous grey globule of mucus, about the size of a pin’s head. Looking at it through the microscope, we see hundreds or thousands of fine mucous threads radiating from the central plasma body, and perceive that the inner layer of its cell-substance is different from the outer layer, which forms a bladder-like membrane. In consequence of its structure, this, the little sun-animalcule, although wanting a shell, really rises above the structureless Acyttaria, and forms the transition from these to the Radiolaria. The genus Cystophrys is of a nature akin to it.

The Basket-shells (Radiolaria) form the third and last class of the Rhizopoda. Their lower forms are closely allied to the Heliozoa and Acyttaria, whereas their higher forms rise far above them. They are essentially distinguished from both by the fact that the central part of their body is composed of many cells, and surrounded by a solid membrane. This closed “central capsule,” generally of a globular shape, is covered by a mucous layer of plasma, out of which there radiate on all sides thousands of exceedingly fine threads, the branching and confluent so-called pseudopodia. Between these are scattered numerous yellow cells of unknown function, containing grains of starch. Most Radiolaria are characterized by a highly developed skeleton, which consists of flint, and displays a wonderful richness of the neatest and most curious forms. Sometimes this flinty skeleton forms a simple trellice-work ball (Fig. 16 s), sometimes a marvellous system of several concentric trelliced balls, encased in one another, and connected by radial staves. In most cases delicate spikes, which are frequently branched like a tree, radiate from the surface of the balls. In other cases the whole skeleton consists of only one flinty star, and is then generally composed of twenty staves, distributed according to definite mathematical laws, and united in a common central point. The skeletons of other Radiolaria again form symmetrical many-chambered structures, as in the case of the Polythalamia. Perhaps no other group of organisms develop in the formation of their skeletons such an amount of various fundamental forms, such geometrical regularity, and such elegant architecture. Most of the forms as yet discovered, I have given in the atlas accompanying my Monograph of the Radiolaria.(23) Here I shall only give as an example the picture of one of the simplest forms, the Cyrtidosphæra echinoides of Nice. The skeleton in this case consists only of a simple trelliced ball (s), with short radial spikes (a), which loosely surround the central capsule (c). Out of the mucous covering, enclosing the latter, radiate a great number of delicate little pseudopodia (p), which are partly drawn back underneath the shell, and fused into a lumpy mass of mucus. Between these are scattered a number of yellow cells (l).

Most Acyttaria live only at the bottom of the sea, on stones and seaweeds, or creep about in sand and mud by means of their pseudopodia, but most Radiolaria swim on the surface of the sea by means of long pseudopodia extending in all directions. They live together there in immense numbers, but are mostly so small that they have been almost completely overlooked, and have only become accurately known during the last fourteen years. Certain Radiolaria living in communities (Polycyttaria) form gelatinous lumps of some lines in diameter. On the other hand, most of those living isolated (Monocyttaria) are invisible to the naked eye; but still their petrified shells are found accumulated in such masses that in many places they form entire mountains; for example, the Nicobar Islands in the Indian Archipelago, and the Island of Barbadoes in the Antilles.

As most readers are probably but little acquainted with the eight classes of the Protista just mentioned, I shall now add some further general observations on their natural history. The great majority of all Protista live in the sea, some swimming freely on the surface, some creeping at the bottom, and others attached to stones, shells, plants, etc. Many species of Protista also live in fresh water, but only a very small number on dry land (for example, Myxomycetes and some Protoplasta). Most of them can be seen only through the microscope, except when millions of individuals are found accumulated. Only a few of them attain a diameter of some lines, or as much as an inch. What they lack in size of body they make up for by producing astonishing numbers of individuals, and they very considerably influence in this way the economy of nature. The imperishable remains of dead Protista, for instance, the flinty shells of the Diatomeæ and Radiolaria and the calcareous shells of the Acyttaria, often form large rock masses.

In regard to their vital phenomena, especially those of nutrition and propagation, some Protista are more allied to plants, others more to animals. Both in their mode of taking food and in the chemical changes of their living substance, they sometimes more resemble the lower animals, at others the lower plants. Free locomotion is possessed by many Protista, while others are without it; but this does not constitute a characteristic distinction, as we know of undoubted animals which entirely lack free locomotion, and of genuine plants which possess it. All Protista have a soul—that is to say, are “animate”—as well as all animals and all plants. The soul’s activity in the Protista manifests itself in their irritability, that is, in the movements and other changes which take place in consequence of mechanical, electrical, and chemical irritation of their contractile protoplasm. Consciousness and the capability of will and thought are probably wanting in all Protista. However, the same qualities are in the same degree also wanting in many of the lower animals, whereas many of the higher animals in these respects are scarcely inferior to the lower races of human beings. In the Protista, as in all other organisms, the activities of the soul are traceable to molecular motions in the protoplasm.

The most important physiological characteristic of the kingdom Protista lies in the exclusively non-sexual propagation of all the organisms belonging to it. The higher animals and plants multiply almost exclusively in a sexual manner. The lower animals and plants multiply also, in many cases, in a non-sexual manner, by division, the formation of buds, the formation of germs, etc. But sexual propagation almost always exists by the side of it, and often regularly alternates with it in succeeding generations (Metagenesis, vol. i. p. 206). All Protista, on the other hand, propagate themselves exclusively in a non-sexual manner, and in fact, the distinction of the two sexes among them has not been effected—there are neither male nor female Protista.

The Protista in regard to their vital phenomena stand midway between animals and plants, that is to say, between their lowest forms; and the same must be said in regard to the chemical composition of their bodies. One of the most important distinctions between the chemical composition of animal and vegetable bodies consists in the characteristic formation of the skeleton. The skeleton, or the solid scaffolding of the body in most genuine plants, consists of a substance called cellulose, devoid of nitrogen, but secreted by the nitrogenous cell-substance, or protoplasm. In most genuine animals, on the other hand, the skeleton generally consists either of nitrogenous combinations (chitin, etc.) or of calcareous earth. In this respect some Protista are more like plants, others more like animals. In many of them the skeleton is principally or entirely formed of calcareous earth, which is met with both in animal and vegetable bodies. But the active vital substance in all cases is the mucous protoplasm.

In regard to the form of the Protista, it is to be remarked that the individuality of their body almost always remains at an extremely low stage of development. Very many Protista remain for life simple plastids or individuals of the first order. Others, indeed, form colonies or republics of plastids by the union of several individuals. But even these higher individuals of the second order, formed by the combination of simple plastids, for the most part remain at a very low stage of development. The members of such communities among the Protista remain very similar one to another, and never, or only in a slight degree, commence a division of labour, and are consequently as little able to render their community fit for higher functions as are, for example, the savages of Australia. The community of the plastids remains in most cases very loose, and each single plastid retains in a great measure its own individual independence.

A second structural characteristic, which next to their low stage of individuality especially distinguishes the Protista, is the low stage of development of their stereometrical fundamental forms. As I have shown in my theory of fundamental forms (in the fourth book of the General Morphology), a definite geometrical fundamental form can be pointed out in most organisms, both in the general form of the body and in the form of the individual parts. This ideal fundamental form, or type, which is determined by the number, position, combination, and differentiation of the component parts, stands in just the same relation to the real organic form as the ideal geometrical fundamental form of crystals does to their imperfect real form. In most bodies and parts of the bodies of animals and plants this fundamental form is a pyramid. It is a regular pyramid in the so-called “regular radiate” forms, and an irregular pyramid in the more highly differentiated, so-called “bilaterally symmetrical” forms. (Compare the plates in the first volume of my General Morphology, pp. 556-558.) Among the Protista this pyramidal type, which prevails in the animal and vegetable kingdom, is on the whole rare, and instead of it we have either quite irregular (amorphous) or more simple, regular geometrical types; especially frequent are the sphere, the cylinder, the ellipsoid, the spheroid, the double cone, the cone, the regular polygon (tetrahedron, hexahedron, octahedron, dodecahedron, icosahedron), etc. All the fundamental forms of the pro-morphological system, which are of a low rank in that system, prevail in the Protista. However, in many Protista there occur also the higher, regular, and bilateral types, fundamental forms which predominate in the animal and vegetable kingdoms. In this respect some of the Protista are frequently more closely allied to animals (as the Acyttaria), others more so to plants (as the Radiolaria).

With regard to the palæontological development of the kingdom Protista, we may form various, but necessarily very unsafe, genealogical hypotheses. Perhaps the individual classes of the kingdom are independent tribes, or phyla, which have developed independently of one another and independently of the animal and the vegetable kingdoms. Even if we adopt the monophyletic hypothesis of descent, and maintain a common origin from a single form of Moneron for all organisms, without exception, which ever have lived and still live upon the earth, even in this case the connection of the neutral Protista on the one hand with the vegetable kingdom, and on the other hand with the animal kingdom, must be considered as very vague. We must regard them (compare p. 74) as lower offshoots which have developed directly out of the root of the great double-branched organic pedigree, or perhaps out of the lowest tribe of Protista, which may be supposed to have shot up midway between the two diverging high and vigorous trunks of the animal and vegetable kingdoms. The individual classes of the Protista, whether they are more closely connected at their roots in groups, or only form a loose bunch of root offsets, must in this case be regarded as having nothing to do either with the diverging groups of organisms belonging to the animal kingdom on the right, or to the vegetable kingdom on the left. They must be supposed to have retained the original simple character of the common primæval living thing more than have genuine animals and genuine plants.

But if we adopt the polyphyletic hypothesis of descent, we have to imagine a number of organic tribes, or phyla, which all shoot up by spontaneous generation out of the same ground, by the side of and independent of one another. (Compare p. 75.) In that case numbers of different Monera must have arisen by spontaneous generation whose differences would depend only upon slight, to us imperceptible, differences in their chemical composition, and consequently upon differences in their capability of development. A small number of Monera would then have given origin to the animal kingdom, and, again, a small number would have produced the vegetable kingdom. Between these two groups, however, there would have developed, independently of them, a large number of independent tribes, which have remained at a lower stage of organization, and which have neither developed into genuine plants nor into genuine animals.

A safe means of deciding between the monophyletic and polyphyletic hypotheses is as yet quite impossible, considering the imperfect state of our phylogenetic knowledge. The different groups of Protista, and those lowest forms of the animal kingdom and of the vegetable kingdom which are scarcely distinguishable from the Protista, show such a close connection with one another and such a confused mixture of characteristics, that at present any systematic division and arrangement of the groups of forms seem more or less artificial and forced. Hence the attempt here offered must be regarded as entirely provisional. But the more deeply we penetrate into the genealogical secrets of this obscure domain of inquiry, the more probable appears the idea that the vegetable kingdom and the animal kingdom are each of independent origin, and that midway between these two great pedigrees a number of other independent small groups of organisms have arisen by repeated acts of spontaneous generation, which on account of their indifferent neutral character, and in consequence of their mixture of animal and vegetable properties, may lay claim to the designation of independent Protista.

II. Vegetable Kingdom Plantæ		III. Animal Kingdom Animalia
Flowering Plants Phanerogamia				Vertebrate Animals Vertebrata
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Ferns Filicinæ │		Star-fishes Echinoderma	│ │ │	│ │ │	Molluscous Animals Mollusca
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Mosses Muscinæ	Lichens Lichenes
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Algæ	Fungi Fungi			│ │	│ │
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Primæval Plants Protophyta		Neutral Primæval Creatures Protista		Primæval Animals Protozoa
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Archigonic Monera

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II. Vegetable Kingdom Vegetabilia

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III. Animal Kingdom Animalia

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N.B.—The lines marked with a ♱ indicate extinct tribes of Protista, which have arisen independently by repeated acts of Spontaneous Generation.

Thus, if we assume one entirely independent trunk for the vegetable kingdom, and a second for the animal kingdom, we may set up a number of independent stems of Protista, each of which has developed, quite independently of other stems and trunks, from a special archigonic form of Monera. In order to make this relation more clear, we may imagine the whole world of organisms as an immense meadow which is partially withered, and upon which two many-branched and mighty trees are standing, likewise partially withered. The two great trees represent the animal and vegetable kingdoms, their fresh and still green branches the living animals and plants; the dead branches with withered leaves represent the extinct groups. The withered grass of the meadow corresponds to the numerous extinct tribes, and the few stalks, still green, to the still living phyla of the kingdom Protista. But the common soil of the meadow, from which all have sprung up, is primæval by protoplasm.

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Every attempt that we make to gain a knowledge of the pedigree of any small or large group of organisms related by blood must, in the first instance, start with the evidence afforded by the existing “natural system” of this group. For although the natural system of animals and plants will never become finally settled, but will always represent a merely approximate knowledge of true blood relationship, still it will always possess great importance as a hypothetical pedigree. It is true, by a “natural system” most zoologists and botanists only endeavour to express in a concise way the subjective conceptions which each has formed of the objective “form-relationships” of organisms. These form-relationships, however, as the reader has seen, are in reality the necessary result of true blood relationship. Consequently, every morphologist in promoting our knowledge of the natural system, at the same time promotes our knowledge of the pedigree, whether he wishes it or not. The more the natural system deserves its name, and the more firmly it is established upon the concordance of results obtained from the study of comparative anatomy, ontogeny, and palæontology, the more surely may we consider it as the approximate expression of the true pedigree of the organic world.

In entering upon the task contemplated in this chapter, the genealogy of the vegetable kingdom, we shall have, according to this principle, first to glance at the natural system of the vegetable kingdom as it is at present (with more or less important modifications) adopted by most botanists. According to the system generally in vogue, the whole series of vegetable forms is divided into two main groups. These main divisions, or sub-kingdoms, are the same as were distinguished more than a century ago by Charles Linnæus, the founder of systematic natural history, and which he called Cryptogamia, or secretly-blossoming plants, and Phanerogamia, or openly-flowering plants. The latter, Linnæus, in his artificial system of plants, divided, according to the different number, formation, and combination of the anthers, and also according to the distribution of the sexual organs, into twenty-three different classes, and then added the Cryptogamia to these as the twenty-fourth and last class.

The Cryptogamia, the secretly-blossoming or flowerless plants, which were formerly but little observed, have in consequence of the careful investigations of recent times been proved to present such a great variety of forms, and such a marked difference in their coarser and finer structure, that we must distinguish no less than fourteen different classes of them; whereas the number of classes of flowering plants, or Phanerogamia, may be limited to four. However, these eighteen classes of the vegetable kingdom can again be naturally grouped in such a manner that we are able to distinguish in all six main divisions or branches of the vegetable kingdom. Two of these six branches belong to the flowering, and four to the flowerless plants. The table on page 82 shows how the eighteen classes are distributed among the six branches, and how these again fall under the sub-kingdoms of the vegetable kingdom.

The one sub-kingdom of the Cryptogamia may now be naturally divided into two divisions, or sub-kingdoms, differing very essentially in their internal structure and in their external form, namely, the Thallus plants and the Prothallus plants. The group of Thallus plants comprises the two large branches of Tangles, or Algæ, which live in water, and the Thread-plants, or Inophytes (Lichens and Fungi), which grow on land, upon stones, bark of trees, upon decaying bodies, etc. The group of Prothallus plants, on the other hand, comprises the two branches of Mosses and Ferns, containing a great variety of forms.

All Thallus plants, or Thallophytes, can be directly recognized from the fact that the two morphological fundamental organs of all other plants, stem and leaves, cannot be distinguished in their structure. The complete body of all Algæ and of all Thread-plants is a mass composed of simple cells, which is called a lobe, or thallus. This thallus is as yet not differentiated into axial-organs (stem and root) and leaf-organs. On this account, as well as through many other peculiarities, the Thallophytes contrast strongly with all remaining plants—those comprised under the two sub-kingdoms of Prothallus plants and Flowering plants—and for this reason the two latter sub-kingdoms are frequently classed together under the name of Stemmed plants, or Cormophytes. The following table will explain the relation of these three sub-kingdoms to one another according to the two different views:—

I. Flowerless Plants. (Cryptogamia)		A. Thallus Plants (Thallophyta)		I. Thallus Plants (Thallophyta)
		B. Prothallus Plants (Prothallophyta)		II. Stemmed Plants (Cormophyta)
II. Flowering Plants (Phanerogamia)		C. Flowering Plants (Phanerogamia)

The stemmed plants, or Cormophytes, in the organization of which the difference of axial-organs (stem and root) and leaf-organs is already developed, form at present, and have, indeed, for a very long period formed, the principal portion of the vegetable world. However, this was not always the case. In fact, stemmed plants, not only of the flowering group, but even of the prothallus group, did not exist at all during that immeasurably long space of time which forms the beginning of the first great division of the organic history of the earth, under the name of the archilithic, or primordial period. The reader will recollect that during this period the Laurentian, Cambrian, and Silurian systems of strata were deposited, the thickness of which, taken as a whole, amounts to about 70,000 feet. Now, as the thickness of all the more recent superincumbent strata, from the Devonian to the deposits of the present time, taken together, amounts to only about 60,000 feet, we were enabled from this fact alone to draw the conclusion—which is probable also for other reasons—that the archilithic, or primordial, period was of longer duration than the whole succeeding period down to the present time. During the whole of this immeasurable space of time, which probably comprises many millions of centuries, vegetable life on our earth seems to have been represented exclusively by the sub-kingdom of Thallus plants, and, moreover, only by the class of marine Thallus plants, that is to say, the Algæ. At least all the petrified remains which are positively known to be of the primordial period belong exclusively to this class. As all the animal remains of this immense period also belong exclusively to animals that lived in water, we come to the conclusion that at that time organisms adapted to a life on land did not exist at all.

SYSTEMATIC VIEW
Of the Six Branches and Eighteen Classes of the Vegetable Kingdom
Primary Groups or Sub-Kingdoms of the Vegetable Kingdom.		Branches or Clades of the Vegetable Kingdom.		Classes of the Vegetable Kingdom.	Systematic Name of the Classes.
A. Thallus Plants Thallophyta		I. Tangles Algæ		1. Primæval algæ	1. Archephyceæ     (Protophyta)
				2. Green algæ	2. Chlorophyceæ     (Chloroalgæ)
				3. Brown algæ	3. Phæophyceæ     (Fucoideæ)
				4. Red algæ	4. Rhodophyceæ     (Florideæ)
		II. Thread-plants Inophyta		5. Lichens	5. Lichenes
				6. Fungi	6. Fungi
B. Prothallus Plants Prothallophyta		III. Mosses Muscinæ		7. Tangle-mosses	7. Charobrya     (Characeæ)
				8. Liverworts	8. Thallobrya     (Hepaticæ)
				9. Frondose-mosses	9. Phyllobrya     (Frondosæ)
				10. Turf-mosses	10. Sphagnobrya     (Sphagnaceæ)
		IV. Ferns Felicinæ		11. Shaft-ferns	11. Calamariæ     (Calamophyta)
				12. Frondose-ferns	12. Filices     (Pterideæ)
				13. Aquatic ferns	13. Rhizocarpeæ     (Hydropterides)
				14. Scale-ferns	14. Selagineæ     (Lepidophyta)
C. Flowering Plants Phanerogamia		V. Plants with Naked Seeds Gymnosperma		15. Palm-ferns	15. Cycadeæ
				16. Pines	16. Coniferæ
		VI. Plants with Enclosed Seeds Angiosperma		17. Plants with one         seed lobe	17. Monocotylæ
				18. Plants with two         seed lobes	18. Dicotylæ

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PEDIGREE OF VEGETABLE KINGDOM.
			Gamopetalæ (Flowers with corolla)
			│
			Dialypetalæ (Star-shaped flowers)
			│
			Monochlamydeæ (Flowers with calyx)	Monocotyledonæ (One seed-lobed plants)
			│	│
			Dicotyledonæ (Two seed-lobed plants)	│ │ │
			│	│
Cycadeæ (Palm-ferns)	Coniferæ (Pines)	Gnetaceæ	Angiospermæ (Plants with enclosed seeds)
│	│	│	│
Gymnospermæ (Plants with naked seeds)
Selagineæ (Scaled-ferns)	Rhizocarpeæ (Water-ferns)	│ │ │	Phanerogamæ Flowering plants	Pterideæ (Frondose-ferns)	Calamariæ (Shaft-ferns)
│	│	│	│	│	│
	Filicinæ (Ferns)
Frondosæ (Leaf-mosses)		Sphagnaceæ (Turf-mosses)	│ │ │
│		│	│
				Characeæ (Tangle-mosses)
Hepaticæ (Liverworts)				│ │
│				│
Muscinæ (Mosses)
	Florideæ (Red Algæ)	Fucoideæ (Brown Algæ)	Chlorophyceæ (Green Algæ)	Lichenes (Lichens)
	│	│	│	│
				│
	Algæ (Tangles)			Fungi Inophyta (Thread-plants)
	│			│
		Protophyta (Primæval Plants)
		│
		Vegetable Monera

For these reasons the first and most imperfect of the great provinces or branches of the vegetable kingdom, the division of the Algæ, or Tangles, must be of special interest to us. But, in addition, there is the interest which this group offers when viewed by itself. In spite of the exceedingly simple composition of their constituent cells, which are but little differentiated, the Algæ show an extraordinary variety of different forms. To them belong the simplest and most imperfect of all forms, as well as very highly developed and peculiar forms. The different groups of Algæ are distinguished as much by size of body as by the perfection and variety of their outer form. At the lowest stage we find such species as the minute Protococcus, several hundred thousands of which occupy a space no larger than a pin’s head. At the highest stage we marvel at the gigantic Macrocysts, which attain a length of from 300 to 400 feet, the longest of all forms in the vegetable kingdom. It is possible that a large portion of the coal has been formed out of Algæ. If not for these reasons, yet the Algæ must excite our special attention from the fact that they form the beginning of vegetable life, and contain the original forms of all other groups of plants, supposing that our monophyletic hypothesis of a common origin for all groups of plants is correct. (Compare p. 83.)

Most people living inland can form but a very imperfect idea of this exceedingly interesting branch of the vegetable kingdom, because they know only its proportionately small and simple representatives living in fresh water. The slimy green aquatic filaments and flakes of our pools and ditches and springs, the light green slimy coverings of all kinds of wood which have for any length of time been in contact with water, the yellowish green, frothy, and oozy growths of our village ponds, the green filaments resembling tufts of hair which occur everywhere in fresh water, stagnant and flowing, are for the most part composed of different species of Algæ. Only those who have visited the sea-shore, and wondered at the immense masses of cast-up seaweed, and who, from the rocky coast of the Mediterranean, have seen through the clear blue waters the beautifully-formed and highly-coloured vegetation of Algæ at the bottom, know how to estimate the importance of the class of Algæ. And yet, even these marine Algæ-forests of European shores, so rich in forms, give only a faint idea of the colossal forests of Sargasso in the Atlantic ocean, those immense banks of Algæ, covering a space of about 40,000 square miles—the same which made Columbus, on his voyage of discovery, believe that a continent was near. Similar but far more extensive forests of Algæ grew in the primæval ocean, probably in dense masses, and what countless generations of these archilithic Algæ have died out one after another is attested, among other facts, by the vast thickness of Silurian alum schists in Sweden, the peculiar composition of which proceeds from those masses of submarine Algæ. According to the recently expressed opinion of Frederick Mohr, a geologist of Bonn, even the greater part of our coal seams have arisen out of the accumulated dead bodies of the Algæ forests of the ocean.

Within the branch of the Algæ we distinguish four different classes, each of which is again divided into several orders and families. These again contain a large number of different genera and species. We designate these four classes as Primæval Algæ, or Archephyceæ, Green Algæ, or Chlorophyceæ, Brown Algæ, or Phæophyceæ, and Red Algæ, or Rhodophyceæ.

The first class of Algæ, the Primæval Algæ (Archephyceæ), might also be called primæval plants, because they contain the simplest and most imperfect of all plants, and, among them, those most ancient of all vegetable organisms out of which all other plants have originated. To them therefore belong those most ancient of all vegetable Monera which arose by spontaneous generation in the beginning of the Laurentian period. Further, we have to reckon among them all those vegetable forms of the simplest organization which first developed out of the Monera in the Laurentian period, and which possessed the form of a single plastid. At first the entire body of one of these small primary plants consisted only of a most simple cytod (a plastid without kernel), and afterwards attained the higher form of a simple cell, by the separation of a kernel in the plasma. (Compare above, vol. i. p. 345.) Even at the present day there exist various most simple forms of Algæ which have deviated but little from the original primary plants. Among them are the Algæ of the families Codiolaceæ, Protococcaceæ, Desmidiaceæ, Palmellaceæ, Hydrodictyeæ, and several others. The remarkable group of Phycochromaceæ (Chroococcaceæ and Oscillarineæ) might also be comprised among them, unless we prefer to consider them as an independent tribe of the kingdom Protista.

The monoplastic Protophyta—that is, those primary Algæ formed by a single plastid—are of the greatest interest, because the vegetable organism in this case completes its whole course of life as a perfectly simple “individual of the first order,” either as a cytod without kernel, or as a cell containing a kernel.

Among the primary plants consisting of a single cytod are the exceedingly remarkable Siphoneæ, which are of considerable size, and strangely “mimic” the forms of higher plants. Many of the Siphoneæ attain a size of several feet, and resemble an elegant moss (Bryopsis), or in some cases a perfect flowering plant with stalks, roots, and leaves (Caulerpa) (Fig. 17). Yet the whole of this large body, externally so variously differentiated, consists internally of an entirely simple sack, possessing the negative characters of a simple cytod.

These curious Siphoneæ, Vaucheriæ, and Caulerpæ show us to how great a degree of elaboration a single cytod, although a most simple individual of the first order, can develop by continuous adaptation to the relations of the outer world. Even the single-celled primary plants—which are distinguished from the monocytods by possessing a kernel—develop into a great variety of exquisite forms by adaptation; this is the case especially with the beautiful Desmidiaceæ, of which a species of Euastrum is represented in Fig. 18 as a specimen.

Fig. 18.—Euastrum rota, a single-celled Desmid, much enlarged. The whole of the star-shaped body of this primæval plant has the formal value of a simple cell. In its centre lies the kernel, and within this the kernel corpuscle, or speck.

It is very probable that similar primæval plants, the soft body of which, however, was not capable of being preserved in a fossil state, at one time peopled the Laurentian primæval sea in great masses and varieties, and in a great abundance of forms, without, however, going beyond the stage of individuality of a simple plastid.

The group of Green Tangles (Chlorophyceæ), or Green Algæ (Cloroalgæ), are the second class, and the most closely allied to the primæval group. Like the majority of the Archephyceæ, all the Chlorophyceæ are coloured green, and by the same colouring matter—the substance called leaf-green, or chlorophyll—which colours the leaves of all the higher plants.

To this class belong, besides a great number of low marine Algæ, most of the Algæ of fresh water, the common water hair-weeds, or Confervæ, the green slime-balls, or Glœosphæræ, the bright green water-lettuce, or Ulva, which resembles a very thin and long lettuce leaf, and also numerous small microscopic algæ, dense masses of which form a light green shiny covering to all sorts of objects lying in water—wood, stones, etc.

These forms, however, rise above the simple primary Algæ in the composition and differentiation of their body. As the green Algæ, like the primæval Algæ, mostly possess a very soft body, they are but rarely capable of being petrified. However, it can scarcely be doubted that this class of Algæ—which was the first to develop out of the preceding one—most extensively and variously peopled the fresh and salt waters of the earth in early times.

In the third class, that of the Brown Tangles (Phæophyceæ), or Black Algæ (Fucoideæ), the branch of the Algæ attains its highest stage of development, at least in regard to size and body. The characteristic colour of the Fucoid is more or less dark brown, sometimes tending more to an olive green or yellowish green, sometimes more to a brownish red or black colour.

Among these are the largest of all Algæ, which are at the same time the longest of all plants, namely, the colossal giant Algæ, amongst which the Macrocystis pyrifera, on the coast of California, attains a length of 400 feet. Also, among our indigenous Algæ, the largest forms belong to this group. Especially I may mention here the stately sugar-tangle (Laminaria), whose slimy, olive green thallus-body, resembling gigantic leaves of from 10 to 15 feet in length, and from a half to one foot in breadth, are thrown up in great masses on the coasts of the North and Baltic seas.

To this class belongs also the bladder-wrack (Fucus vesiculosus) common in our seas, whose fork-shaped, deeply-cut leaves are kept floating on the water by numerous air bladders (as is the case, too, with many other brown Algæ). The freely floating Sargasso Alga (Sargasso bacciferum), which forms the meadows or forests of the Sargasso Sea, also belongs to this class.

Although each individual of these large alga-trees is composed of many millions of cells, yet at the beginning of its existence it consists, like all higher plants, of a single cell—a simple egg. This egg—for example, in the case of our common bladder-wrack—is a naked, uncovered cell, and as such is so like the naked egg-cells of lower marine animals—for example, those of the Medusæ—that they might easily be mistaken one for another (Fig. 19).

Fig. 19.—The egg of the common bladder-wrack (Fucus vesiculosus), a simple naked cell, much enlarged. In the centre of the naked globule of protoplasm the bright kernel is visible.

It was probably the Fucoideæ, or Brown Algæ, which during the primordial period, to a great extent constituted the characteristic alga-forests of that immense space of time. Their petrified remains, especially those of the Silurian period, which have been preserved, can, it is true, give us but a faint idea of them, because the material of these Algæ, like that of most others, is ill-suited for preservation in a fossil state. As has already been remarked, a large portion of coal is perhaps composed of them.

Less important is the fourth class of Algæ, that of the Rose-coloured Algæ (Rhodophyceæ), or Red Sea-weeds (Florideæ). This class, it is true, presents a great number of different forms; but most of them are of much smaller size than the Brown Algæ. Although they are inferior to the latter in perfection and differentiation, they far surpass them in some other respects. To them belong the most beautiful and elegant of all Algæ, which on account of the fine plumose division of their leaf-like bodies, and also on account of their pure and delicate red colour, are among the most charming of plants. The characteristic red colour sometimes appears as a deep purple, sometimes as a glowing scarlet, sometimes as a delicate rose tint, and may verge into violet and bluish purple, or on the other hand into brown and green tints of marvellous splendour. Whoever has visited one of our sea-coast watering places, must have admired the lovely forms of the Florideæ, which are frequently dried on white paper and offered for sale.

Most of the Red Algæ are so delicate, that they are quite incapable of being petrified; this is the case with the splendid Ptilotes, Plocamia, Delesseria, etc. However, there are individual forms, like the Chondria and Sphærococca, which possess a harder thallus, often almost as hard as cartilage, and of these fossil remains have been preserved—principally in the Silurian, Devonian, and Carboniferous strata, and later in the oolites. It is probable that this class also had an important share in the composition of the archilithic Algæ flora.

If we now again take into consideration the flora of the primordial period, which was exclusively formed by the group of Algæ, we can see that it is not improbable that its four subordinate classes had a share in the composition of those submarine forests of the primæval oceans, similar to that which the four types of vegetation—trees with trunks, flowering shrubs, grass, and tender leaf-ferns and mosses—at present take in the composition of our recent land forests.

We may suppose that the submarine tree forests of the primordial period were formed by the huge Brown Algæ, or Fucoideæ. The many-coloured flowers at the foot of these gigantic trees were represented by the gay Red Algæ, or Florideæ. The green grass between was formed by the hair-like bunches of Green Algæ, or Chloroalgæ. Finally, the tender foliage of ferns and mosses, which at present cover the ground of our forests, fill the crevices left by other plants, and even settle on the trunks of the trees, at that time probably had representatives in the moss and fern-like Siphoneæ, in the Caulerpa and Bryopsis, from among the class of the primary Algæ, Protophyta, or Archephyceæ.

With regard to the relationships of the different classes of Algæ to one another and to other plants, it is exceedingly probable that the Primary Algæ, or Archephyceæ, as already remarked, form the common root of the pedigree, not merely for the different classes of Algæ, but for the whole vegetable kingdom. On this account they may with justice be designated as primæval plants, or Protophyta.

Out of the naked vegetable Monera, in the beginning of the Laurentian period, enclosed cytods were probably the first to arise (vol. i. p. 345), by the naked, structureless, albuminous substance of the Monera becoming condensed in the form of a pellicle on the surface, or by secreting a membrane. At a later period, out of these enclosed cytods genuine vegetable cells probably arose, as a kernel or nucleus separated itself in the interior from the surrounding cell-substance or plasma.

The three classes of Green Algæ, Brown Algæ, and Red Algæ, are perhaps three distinct classes, which have arisen independently of one another out of the common radical group of Primæval Algæ, and then developed themselves further (each according to its kind), and have variously branched off into orders and families. The Brown and Red Algæ possess no close blood relationship to the other classes of the vegetable kingdom. These latter have most probably arisen out of the Primæval Algæ, either directly or by the intermediate step of the Green Algæ.

It is probable that Mosses (out of which, at a later time, Ferns developed) proceeded from a group of Green Algæ, and that Fungi and Lichens proceeded from a group of Primæval Algæ. The Phanerogamia developed at a much later period out of Ferns.

As a second class of the Vegetable Kingdom we have above mentioned the Thread-plants (Inophyta). We understood by this term the two closely related classes of Lichens and Fungi. It is possible that these Thallus plants have not arisen out of the Primæval Algæ, but out of one or more Monera, which, independently of the latter, arose by spontaneous generation. It appears conceivable that many of the lowest Fungi, as for example, many ferment-causing fungi (forms of Micrococcus, etc.), owe their origin to a number of different archigonic Monera (that is, Monera originating by spontaneous generation).

In any case the Thread-plants cannot be considered as the progenitors of any of the higher vegetable classes. Lichens, as well as fungi, are distinct from the higher plants in the composition of their soft bodies, consisting as it does of a dense felt-work of very long, variously interwoven, and peculiar threads or chains of cells—the so-called hyphæ, on which account we distinguish them as a province under the name Thread-plants. From their peculiar nature they could not leave any important fossil remains, and consequently we can form only a very vague guess at their palæontological development.

The first class of Thread-plants, the Fungi, exhibit a very close relationship to the lowest Algæ; the Algo-fungi, or Phycomycetes (the Saprolegniæ and Peronosporæ) in reality only differ from the bladder-wracks and Siphoneæ (the Vaucheria and Caulerpa) mentioned previously by the want of leaf-green, or chlorophyll. But, on the other hand, all genuine Fungi have so many peculiarities, and deviate so much from other plants, especially in their mode of taking food, that they might be considered as an entirely distinct province of the vegetable kingdom.

Other plants live mostly upon inorganic food, upon simple combinations which they render more complicated. They produce protoplasm by the combination of water, carbonic acid, and ammonia. They take in carbonic acid and give out oxygen. But the Fungi, like animals, live upon organic food, consisting of complicated combinations of carbon, which they receive from other organisms and assimilate. They inhale oxygen and give out carbonic acid like animals. They also never form leaf-green, or chlorophyll, which is so characteristic of most other plants. In like manner they never produce starch. Hence many eminent botanists have repeatedly proposed to remove the Fungi completely out of the vegetable kingdom, and to regard them as a special and third kingdom, between that of animals and plants. By this means our kingdom of Protista would be considerably increased. The Fungi in this case would, in the first place, be allied to the so-called “slime moulds,” or Myxomycetes (which, however, never form any hyphæ). But as many Fungi propagate in a sexual manner, and as most botanists, according to the prevalent opinion, look upon Fungi as genuine plants, we shall here leave them in the vegetable kingdom, and connect them with lichens, to which they are at all events most nearly related.

The phyletic origin of Fungi will probably long remain obscure. The close relationship already hinted at between the Phycomycetes and Siphoneæ (especially between the Saprolegniæ and Vaucheriæ) suggests to us that they are derived from the latter. Fungi would then have to be considered as Algæ, which by adaptation to a parasitical life have become very peculiarly transformed. Many facts, however, support the supposition that the lowest fungi have originated independently from archigonic Monera.

The second class of Inophyta, the Lichens (Lichenes), are very remarkable in relation to phylogeny; for the surprising discoveries of late years have taught us that every Lichen is really composed of two distinct plants—of a low form of Alga (Nostochaceæ, Chroococcaceæ), and of a parasitic form of Fungus (Ascomycetes), which lives as a parasite upon the former, and upon the nutritive substances prepared by it. The green cells, containing chlorophyll (gonidia), which are found in every lichen, belong to the Alga. But the colourless threads (hyphæ) which, densely interwoven, form the principal mass of the body of Lichens, belong to the parasitic Fungus. But in all cases the two forms of plants—Fungus and Alga—which are always considered as members of two quite distinct provinces of the vegetable kingdom, are so firmly united, and so thoroughly interwoven, that nearly every one looks upon a Lichen as a single organism.

Most Lichens form small, more or less formless or irregularly indented, crust-like coverings to stones, bark of trees, etc. Their colour varies through all possible tints, from the purest white to yellow, red, green, brown, and the deepest black.

Many lichens are important in the economy of nature from the fact that they can settle in the driest and most barren localities, especially on naked rocks upon which no other plant can live. The hard black lava, which covers many square miles of ground in volcanic regions, and which for centuries frequently presents the most determined opposition to the life of every kind of vegetation, is always first occupied by Lichens. It is the white or grey Lichens (Stereocaulon) which, in the most desolate and barren fields of lava, always begin to prepare the naked rocky ground for cultivation, and conquer it for subsequent higher vegetation. Their decaying bodies form the first mould in which mosses, ferns, and flowering plants can afterwards take firm root. Hardy Lichens are also less affected by the severity of climate than any other plants. Hence the naked rocks, even in the highest mountains—for the most part covered by eternal snow, on which no plant could thrive—are encrusted by the dry bodies of Lichens.

Leaving now the Fungi, Lichens, and Algæ, which are comprised under the name of Thallus plants, we enter upon the second sub-kingdom of the vegetable kingdom, that of the Prothallus plants (Prothallophyta), which by some botanists are called phyllogonic Cryptogamia (in contradistinction to the Thallus plants, or thallogonic Cryptogamia). This sub-kingdom comprises the two provinces of Mosses and Ferns.

Here we meet with (except in a few of the lowest forms) the separation of the vegetable body into two different fundamental organs, axial-organs (stem and root) and leaves (or lateral organs). In this the Prothallus plants resemble the Flowering plants, and hence the two groups have recently often been classed together as stemmed plants, or Cormophytes.

But, on the other hand, Mosses and Ferns resemble the Thallus plants, in the absence of the development of flowers and seeds, and even Linnæus classed them with these, as Cryptogamia, in contradistinction to the plants forming seeds; that is, flowering plants (Anthophyta or Phanerogamia).

Under the name of “Prothallus plants” we combine the closely-related Mosses and Ferns, because both exhibit a peculiar and characteristic “alternation of generation” in the course of their individual development. For every species exhibits two different generations, of which the one is usually called the Prothallium, or Fore-growth, the other is spoken of as the Cormus, or actual Stem of the moss or fern.

The first and original generation, the Fore-growth, or Prothallus, also called Protonema, still remains in that lower stage of elaboration manifested throughout life by all Thallus plants; that is to say, stem and leaf-organs have as yet not differentiated, and the entire cell-mass of the Fore-growth corresponds to a simple thallus. The second and more perfect generation of mosses and ferns—the Stem, or Cormus—develops a much more highly elaborate body, which has differentiated into stalk and leaf (as in the case of flowering plants), except in the lowest mosses, where this generation also remains in the lower stage of the thallus.

With the exception of these latter forms the first generation of Mosses and Ferns (the thallus-shaped Fore-growth) always produces a second generation with stem and leaves; the latter in its turn produces the thallus of the first generation, and so on. Thus, in this case, as in the ordinary cases of alternation of generation in animals, the first generation is like the third, fifth, etc., the second like the fourth, sixth, etc. (Compare vol. i. p. 206).

Of the two main classes of Prothallus plants, the Mosses in general are at a much lower stage of development than the Ferns, and their lowest forms (especially in an anatomical respect) form the transition from the Thallus plants through the Algæ to Ferns. The genealogical connection of Mosses and Ferns which is indicated by this fact can, however, be inferred only from the case of the most imperfect forms of the two classes; for the more perfect and higher groups of mosses and ferns do not stand in any close relation to one another, and develop in completely opposite directions. In any case Mosses have arisen directly out of Thallus plants, and probably out of Green Algæ.

Ferns, on the other hand, are probably derived from extinct unknown Mosses, which were very nearly related to the lowest liverworts of the present day. In the history of creation, Ferns are of greater importance than Mosses.

The branch of Mosses (Muscinæ, also called Musci, or Bryophyta) contains the lower and more imperfect plants of the group of Prothallophytes, which as yet do not possess vessels. Their bodies are mostly so tender and perishable that they are very ill-suited for being preserved in a recognizable state as fossils. Hence the fossil remains of all classes of Mosses are rare and insignificant. It is probable that Mosses developed in very early times out of the Thallus plants, or, to be more precise, out of the Green Algæ. It is probable that in the primordial period there existed aquatic forms of transition from the latter to Mosses, and in the primary period to those living on land. The Mosses of the present day—out of the gradually differentiating development of which comparative anatomy may draw some inferences as to their genealogy—are divided into two different classes, namely: (1) Liverworts; (2) Leafy Mosses.

The first and oldest class of Mosses, which is directly allied to the Green Algæ, or Confervæ, is formed by the Liverworts (Hepaticæ, or Thallobrya). The mosses belonging to them are, for the most part, small and insignificant in form, and are little known. Their lowest forms still possess, in both generations, a simple thallus like the Thallus plants; as for example, the Ricciæ and Marchantiaceæ. But the more highly developed liverworts, the Jungermanniaceæ and those akin to them, gradually commence to differentiate stem and leaf, and their most highly-developed forms are closely allied to leaf-mosses. By this transitional series the liverworts show their direct derivation from the Thallophytes, and more especially from the Green Algæ.

The Mosses, which are generally the only ones known to the uninitiated—and which, in fact, form the principal portion of the whole branch—belong to the second class, or Leafy Mosses (Musci frondosi, called Musci in a narrow sense, also Phyllobrya). Among them are most of those pretty little plants which, united in dense groups, form the bright glossy carpet of moss in our woods, or which, in company with liverworts and lichens, cover the bark of trees. As reservoirs, carefully storing up moisture, they are of the greatest importance in the economy of nature. Wherever man mercilessly cuts down and destroys forests, there, as a consequence, disappear the leafy mosses which covered the bark of the trees, or, protected by their shade, clothed the ground, and filled the spaces between the larger plants. Together with the leafy mosses disappear the useful reservoirs which stored up rain and dew for times of drought. Thus arises a disastrous dryness of the ground, which prevents the growth of any rich vegetation. In the greater part of Southern Europe—in Greece, Italy, Sicily, and Spain—mosses have been destroyed by the inconsiderate extirpation of forests, and the ground has thereby been robbed of its most useful stores of moisture; once flourishing and rich tracts of land have been changed into dry and barren wastes. Unfortunately in Germany, also, this rude barbarism is beginning to prevail more and more. It is probable that the small frondose mosses have played this exceedingly important part in nature for a very long time, possibly from the beginning of the primary period. But as their tender bodies are as little suited as those of all other mosses for being preserved in a fossil state, palæontology can give us no information about this.

We learn from the science of petrifactions much more than we do in the case of Mosses of the importance which the second branch of Prothallus plants—that is, Ferns—have had in the history of the vegetable world. Ferns, or more strictly speaking, the “plants of the fern tribe” (Filicineæ, or Pterideæ, also called Pteridophyta, or Vascular Cryptogams), formed during an extremely long period, namely, during the whole primary or palæolithic period, the principal portion of the vegetable world, so that we may without hesitation call it the era of Fern Forests. From the beginning of the Devonian period, in which organisms living on land appeared for the first time, namely, during the deposits of the Devonian, Carboniferous, and Permian strata, plants like Ferns predominated so much over all others, that we are justified in giving this name to that period. In the stratifications just mentioned, but above all, in the immense layers of coal of the Carboniferous or coal period, we find such numerous and occasionally well preserved remains of Ferns, that we can form a tolerable vivid picture of the very peculiar land flora of the palæolithic period. In the year 1855 the total number of the then known palæolithic species of plants amounted to about a thousand, and among these there were no less than 872 Ferns. Among the remaining 128 species were 77 Gymnosperms (pines and palm-ferns), 40 Thallus plants (mostly Algæ), and about 20 not accurately definable Cormophyta (stem-plants).

As already remarked, Ferns probably developed out of the lower liverworts in the beginning of the primary period. In their organization Ferns rise considerably above Mosses, and in their more highly developed forms even approach the flowering plants. In Mosses, as in Thallus plants, the entire body is composed of almost equi-formal cells, little if at all differentiated; but in the tissues of Ferns we find those peculiarly differentiated strings of cells which are called the vessels of plants, and which are universally met with in flowering plants. Hence Ferns are sometimes united as “vascular Cryptogams” with Phanerogams, and the group so formed is contrasted as that of the “vascular plants” with “cellular plants,”—that is, with “cellular cryptogams” (Mosses and Thallus plants). This very important process in the organization of plants—the formation of vessels—first occurred, therefore, in the Devonian period, consequently in the beginning of the second and smaller half of the organic history of the earth.

The branch of Ferns, or Filicinæ, is divided into five distinct classes: (1) Frondose Ferns, or Pteridæ; (2) Reed Ferns, or Calamariæ; (3) Aquatic Ferns, or Rhizocarpeæ; (4) Snakes Tongues, or Ophioglossæ; and (5) Scale Ferns, or Lepidophyta. By far the most important of these five classes, and also the richest in forms, were first the Frondose Ferns, and then the Scale-ferns, which formed the principal portion of the palæolithic forests. The Reed Ferns, on the other hand, had at that time already somewhat diminished in number; and of the Aquatic Ferns, we do not even know with certainty whether they then existed. It is difficult for us to form any idea of the very peculiar character of those gloomy palæolithic fern forests, in which the whole of the gay abundance of flowers of our present flora was entirely wanting, and which were not enlivened by any birds. Of the flowering plants there then existed only the two lowest classes, the pines and palm ferns, with naked seeds, whose simple and insignificant blossoms scarcely deserve the name of flowers.

The phylogeny of Ferns, and of the Gymnosperms which have developed out of them, has been made especially clear by the excellent investigations which Edward Strasburger published in 1872, on “The Coniferæ and Gnetaceæ,” as also “On Azolla.” This thoughtful naturalist and Charles Martins, of Montpellier, are among the few botanists who have thoroughly understood the fundamental value of the Theory of Descent, and the mechanical-causal connection between ontogeny and phylogeny. The majority of botanists do not even yet know the important difference between homology and analogy, between the morphological and physiological comparison of parts—which has long since been recognized in zoology—but Strasburger has employed this distinction and the principle of evolution in his “Comparative Anatomy of the Gymnosperms,” in order to sketch the outlines of the blood relationship of this important group of plants.

The class among Ferns which has developed most directly out of the Liverworts is the class of real Ferns, in the narrow sense of the word, the Frondose Ferns (Filices, or Phyllopterides, also called Pteridæ). In the present flora of the temperate zones this class forms only a subordinate part, for it is in most cases represented only by low forms without trunks. But in the torrid zones, especially in the moist, steaming forests of tropical regions, this class presents us with the lofty palm-like fern trees. These beautiful tree-ferns of the present day, which form the chief ornament of our hot-houses, can however give us but a faint idea of the stately and splendid frondose ferns of the primary period, whose mighty trunks, densely crowded together, then formed entire forests. These trunks, accumulated in super-incumbent masses, are found in the coal seams of the Carboniferous period, and between them, in an excellent state of preservation, are found the impressions of the elegant fan-shaped leaves, crowning the top of the trunk in an umbrella-like bush. The varied outlines and the feather-like forms of these fronds, the elegant shape of the branching veins or bunches of vessels in their tender foliage, can still be as distinctly recognized in the impressions of the palæolithic fronds as in the fronds of ferns of the present day. In many cases even the clusters of fruit, which are distributed on the lower surface of the fronds, are distinctly preserved. After the Carboniferous period, the predominance of frondose ferns diminished, and towards the end of the secondary period they played almost as subordinate a part as they do at the present time.

The Calamariæ, Ophioglossæ, and Rhizocarpeæ seem to have developed as three diverging branches out of the Frondose Ferns, or Pteridæ. The Calamariæ, or Calamophyta, have remained at the lowest level among these three classes. The Calamariæ comprise three different orders, of which only one now exists, namely, the Horse-tails (Equisetaceæ). The two other orders, the Giant Reeds (Calamiteæ), and the Star-leaf Reeds (Asterophylliteæ), are long since extinct. All Calamariæ are characterized by a hollow and jointed stalk, stem, or trunk, upon which the branches and leaves (in cases where they exist) are set so as to encircle the jointed stem in whorls. The hollow joints of the stalk are separated from one another by partition walls. In Horse-tails and Calamiteæ the surface is traversed by longitudinal ribs running parallel, as in the case of a fluted column, and the outer skin contains so much silicious earth in the living forms, that it is used for cleansing and polishing. In the Asterophylliteæ, the star-shaped whorls of leaves were more strongly developed than in the two other orders. There exist, at present, of the Calamariæ only the insignificant Horse-tails (Equisetum), which grow in marshes and on moors; but during the whole of the primary and secondary periods they were represented by great trees of the genus Equisetites. There existed, at the same time, the closely related order of the Giant Reeds (Calamites), whose strong trunks grew to a height of about fifty feet. The order of the Asterophyllites, on the other hand, contained smaller and prettier plants, of a very peculiar form, and belongs exclusively to the primary period.

Among all Ferns, the history of the third class, that of the Root, or Aquatic Ferns (Rhizocarpeæ, or Hydropteridæ), is least known to us. In their structure these ferns, which live in fresh water, are on the one hand allied to the frond ferns, and on the other to the scaly ferns, but they are more closely related to the latter. Among them are the but little known moss ferns (Salvinia), clover ferns (Marsilea), and pill ferns (Pilularia) of our fresh waters; further, the large Azolla which floats in tropical ponds. Most of the aquatic ferns are of a delicate nature, and hence ill-suited for being petrified. This is probably the reason of their fossil remains being so scarce, and of the oldest of those known to us having been found in the Jura system. It is probable, however, that the class is much older, and that it was already developed during the palæolithic period out of other ferns by adaptation to an aquatic life.

The fourth class of ferns is formed by the Tongue Ferns (Ophioglossæ, or Glossopterides). These ferns, to which belongs the Botrychium, as well as the Ophioglossum (adder’s-tongue) of our native genera, were formerly considered as forming but a small subdivision of the frondose ferns. But they deserve to form a special class, because they represent important transitional forms from the Pterideæ and Lepidophytes towards higher plants, and must be regarded as among the direct progenitors of the flowering plants.

The fifth and last class is formed by the Scale Ferns (Lepidophytes, or Selagines). In the same way as the Ophioglossæ arose out of the frondose forms, the scale ferns arose out of the Ophioglossæ. They were more highly developed than all other ferns, and form the transition to flowering plants, which must have developed out of them. Next to the frondose ferns they took the largest part in the composition of the palæolithic fern forests. This class also contains, as does the class of reed ferns, three nearly related but still very different orders, of which only one now exists, the two others having become extinct towards the end of the Carboniferous period. The scaled ferns still existing belong to the order of the club-mosses (Lycopodiaceæ). They are mostly small, pretty moss-like plants, whose tender, many-branched stalk creeps in curves on the ground like a snake, and is densely encompassed and covered by small scaly leaves. The pretty creeping Lycopodium of our woods, which mountain tourists twine round their hats, is known to all, as also the still more delicate Selaginella, which under the name of creeping moss is used to adorn the soil of our hot-houses in the form of a thick carpet. The largest club-mosses of the present day are found in the Sunda Islands, where their stalks rise to the height of twenty-five feet, and attain half a foot in thickness. But in the primary and secondary periods even larger trees of this kind were widely distributed, the most ancient of which probably were the progenitors of the pines (Lycopodites). The most important dimensions were, however, attained by the class of scale trees (Lepidodendreæ), and by the seal trees (Sigillarieæ). These two orders, with a few species, appear in the Devonian period, but do not attain their immense and astonishing development until the Carboniferous period, and become extinct towards the end of it, or in the Permian period directly following upon it. The scale trees, or Lepidodendreæ, were probably more closely related to club-mosses than to Sigillarieæ. They grew into splendid, straight, unbranching trunks which divided at the top into numerous forked branches. They bore a large crown of scaly leaves, and like the trunk were marked in elegant spiral lines by the scars left at the base of the leaf stalks which had fallen off. We know of scale-marked trees from forty to sixty feet in length, and from twelve to fifteen feet in diameter at the root. Some trunks are said to be even more than a hundred feet in length. In the coal are found still larger accumulations of the no less highly developed but more slender trunks of the remarkable seal trees, Sigillarieæ, which in many places form the principal part of coal seams. Their roots were formerly described as quite a distinct vegetable form (under the name of Stigmaria). The Sigillarieæ are in many respects very like the scale-trees, but differ from them and from ferns in general in many ways. They were possibly closely related to the extinct Devonian Lycopterideæ, combining characteristic peculiarities of the club-mosses and the frondose ferns, which Strasburger considers as the hypothetical primary form of flowering plants.

In leaving the dense forests of the primary period, which were principally composed of frond ferns (Lepidodendreæ and Sigillarieæ), we pass onwards to the no less characteristic pine forests of the secondary period. Thus we leave the domain of the Cryptogamia, the plants forming neither flowers nor seeds, and enter the second main division of the vegetable kingdom, namely, the sub-kingdom of the Phanerogamia, flowering plants forming seeds. This division, so rich in forms, containing the principal portion of the present vegetable world, and especially the majority of plants living on land, is certainly of a much more recent date than the division of Cryptogamia. For it can have developed out of the latter only in the course of the palæolithic period. We can with full assurance maintain that, during the whole archilithic period, hence during the first and longer half of the organic history of the earth, no flowering plants as yet existed, and that they first developed during the primary period out of Cryptogamia of the fern kind. The anatomical and embryological relation of Phanerogamia to the latter is so close, that from it we can with certainty infer their genealogical connection, that is, their true blood relationship. Flowering plants cannot have directly arisen out of thallus plants, nor out of mosses; but only out of ferns, or Filicines. Most probably the scaled ferns, or Lepidophyta, and more especially amongst these the Lycopodiaceæ, forms closely related to the Selaginella of the present day, have been the direct progenitors of the Phanerogamia.

On account of its anatomical structure and its embryological development, the sub-kingdom of the Phanerogamia has for a long time been divided into two large branches, into the Gymnosperms, or plants with naked seeds, and the Angiosperms, or plants with enclosed seeds. The latter are in every respect more perfect and more highly organized than the former, and developed out of them only at a late date during the secondary period. The Gymnosperms, both anatomically and embryologically, form the transition group from Ferns to Angiosperms.

The lower, more imperfect, and the older of the two main classes of flowering plants, that of the Archispermeæ, or Gymnosperms (with naked seeds), attained its most varied development and widest distribution during the mesolithic or secondary epoch. It was no less characteristic of this period, than was the fern group of the preceding primary, and the Angiosperms of the succeeding tertiary, epoch. Hence we might call the secondary epoch that of Gymnosperms, or after its most important representatives, the era of Pine Forests. The Gymnosperms are divided into three classes: the Coniferæ, Cycadeæ, and Gnetaceæ. We find fossil remains of the pines, or Conifers, and of the Cycads, even in coal, and must infer from this that the transition from scaled ferns to Gymnosperms took place during the Coal, or possibly even in the Devonian period. However, the Gymnosperms play but a very subordinate part during the whole of the primary epoch, and do not predominate over Ferns until the beginning of the secondary epoch.

Of the two classes of Gymnosperms just mentioned, that of the Palm Ferns (Zamiæ, or Cycadeæ) stands at the lowest stage, and is directly allied to ferns, as the name implies, so that some botanists have actually included them in the fern group. In their external form they resemble palms, as well as tree ferns (or tree-like frond ferns), and are adorned by a crown of feathery leaves, which is placed either on a thick, short trunk, or on a slender, simple trunk like a pillar. At the present day this class, once so rich in forms, is but scantily represented by a few forms living in the torrid zones, namely, by the coniferous ferns (Zamia), the thick-trunked bread-tree (Encephalartos), and the slender-trunked Caffir bread-tree (Cycas). They may frequently be seen in hot-houses, and are generally mistaken for palms. A much greater variety of forms than occurs among the still existing palm ferns (Cycadeæ) is presented by the extinct and fossil Cycads, which occurred in great numbers more towards the middle of the secondary period, during the Jura, and which at that time principally determined the character of the forests.

The class of Pines, or coniferous trees (Coniferæ), has preserved down to our day a greater variety of forms than have the palm ferns. Even at the present time the trees belonging to it—cypresses, juniper trees, and trees of life (Thuja), the box and ginko trees (Salisburya), the araucaria and cedars, but above all the genus Pinus, which is so rich in forms, with its numerous and important species, spruces, pines, firs, larches, etc.—still play a very important part in the most different parts of the earth, and almost of themselves constitute extensive forests. Yet this development of pines seems but weak in comparison with the predominance which the class had attained over other plants during the early secondary period, that of the Trias. At that time mighty coniferous trees—with but proportionately few genera and species, but standing together in immense masses of individuals—formed the principal part of the mesolithic forests. This fact justifies us in calling the secondary period the “era of the pine forests,” although the remains of Cycadeæ predominate over those of coniferous trees in the Jura period.2

From the pine forests of the mesolithic, or secondary period, we pass on into the leafy forests of the cænolithic, or tertiary period, and we arrive thus at the consideration of the sixth and last class of the vegetable kingdom, that of the Metaspermæ, Angiospermæ, or plants with enclosed seeds. The first certain and undoubted fossils of plants with enclosed seeds are found in the strata of the chalk system, and indeed we here find, side by side, remains of the two classes into which the main class of Angiosperms is generally divided, namely, the one seed-lobed plants, or monocotylæ, and the two seed-lobed plants, or dicotylæ. However, the whole group probably originated at an earlier period during the Trias. For we know of a number of doubtful and not accurately definable fossil remains of plants from the Oolitic and Trias (sic) periods, which some botanists consider to be Monocotylæ, whilst others consider them as Gymnosperms. In regard to the two classes of plants with enclosed seeds, the Monocotylæ and Dicotylæ, it is exceedingly probable that the Dicotyledons developed out of the Gnetaceæ, but that the Monocotyledons developed later out of a branch of the dicotyledons.

The class of one seed-lobed plants (Monocotylæ, or Monocotyledons, also called Endogenæ) comprises those flowering plants whose seeds possess but one germ leaf or seed lobe (cotyledon). Each whorl of its flower contains in most cases three leaves, and it is very probable that the mother plants of all Monocotyledons possessed a regular triple blossom. The leaves are mostly simple, and traversed by simple, straight bunches of vessels or “nerves.” To this class belong the extensive families of the rushes, grasses, lilies, irids, and orchids, further a number of indigenous aquatic plants, the water-onions, sea grasses, etc., and finally the splendid and highly developed families of the Aroideæ and Pandaneæ, the bananas and palms. On the whole, the class of Monocotyledons—in spite of the great variety of forms which it developed, both in the tertiary and the present period—is much more simply organized than the class of the Dicotyledons, and its history of development also offers much less of interest. As their fossil remains are for the most part difficult to recognize, it still remains at present an open question in which of the three great secondary periods—the Trias, Jura, or chalk period—the Monocotyledons originated. At all events they existed in the chalk period as surely as did the Dicotyledons.

Haeckel-History of Creation

Pl. V.

The second class of plants with enclosed seeds, the two seed-lobed (Dicotylæ, or Dicotyledons, also called Exogenæ) presents much greater historical and anatomical interest in the development of its subordinate groups. The flowering plants of this class generally possess, as their name indicates, two seed lobes or germ leaves (cotyledons). The number of leaves composing its blossom is generally not three, as in most Monocotyledons, but four, five, or a multiple of those numbers. Their leaves, moreover, are generally more highly differentiated and more composite than those of the Monocotyledons; they are traversed by crooked, branching bunches of vessels or “veins.” To this class belong most of the leafed trees, and as they predominate in the tertiary period as well as, at present, over the Gymnosperms and Ferns, we may call the cænolithic period that of leafed forests.

Although the majority of Dicotyledons belong to the most highly developed and most perfect plants, still the lowest division of them is directly allied to the Gymnosperms, and particularly to the Gnetaceæ. In the lower Dicotyledons, as in the case of the Monocotyledons, calyx and corolla are as yet not differentiated. Hence they are called Apetalous (Monochlamydeæ, or Apetalæ). This sub-class must therefore doubtless be looked upon as the original group of the Angiosperms, and existed probably even during the Trias and Jura periods. Among them are most of the leafed trees bearing catkins—birches and alders, willows and poplars, beeches and oaks; further, the plants of the nettle kind—nettles, hemp, and hops, figs, mulberries, and elms; finally, plants like the spurges, laurels, and amaranth.

It was not until the chalk period that the second and more perfect class of the Dicotyledons appeared, namely, the group with corollas (Dichlamydeæ, or Corollifloræ). These arose out of the Apetalæ from the simple cover of the blossoms of the latter becoming differentiated into calyx and corolla. The sub-class of the Corollifloræ is again divided into two large main divisions or legions, each of which contains a large number of different orders, families, genera, and species. The first legion bears the name of star-flowers, or Diapetalæ, the second that of the bell-flowers, or Gamopetalæ.

The lower and less perfect of the two legions of the Corollifloræ are the star-flowers (also called Diapetalæ or Dialypetalæ). To them belong the extensive families of the Umbelliferæ, or umbrella-worts (wild carrot, etc.), the Cruciferæ, or cruciform blossoms (cabbage, etc.); further, the Ranunculaceæ (buttercups) and Crassulaceæ, the Mallows and Geraniums, and, besides many others, the large group of Roses (which comprise, besides roses, most of our fruit trees), and the Pea-blossoms (containing, among others, beans, clover, genista, acacia, and mimosa). In all these Diapetalæ the blossom-leaves remain separate, and never grow together, as is the case in the Gamopetalæ. These latter developed first in the tertiary period out of the Diapetalæ, whereas the Diapetalæ appeared in the chalk period together with the Apetalæ.

The highest and most perfect group of the vegetable kingdom is formed by the second division of the Corollifloræ, namely, the legion of bell-flowers (Gamopetalæ, also called Monopetalæ or Sympetalæ). In this group the blossom-leaves, which in other plants generally remain separate, grow regularly together into a more or less bell-like, funnel-shaped, or tubular flower. To them belong, among others, the Bell-flowers and Convolvulus, Primroses and Heaths, Gentian and Honeysuckle, further the family of the Olives (olive trees, privet, elder, and ash), and finally, besides many other families, the extensive division of the Lip-blossoms (Labiatæ) and the Composites. In these last the differentiation and perfection of the Phanerogamic blossoms attain their highest stage of development, and we must therefore place them at the head of the vegetable kingdom, as the most perfect of all plants. In accordance with this, the legion of the Gamopetalæ appear in the organic history of the earth later than all the main groups of the vegetable kingdom—in fact, not until the cænolithic or tertiary epoch. In the earliest tertiary period the legion is still very rare, but it gradually increases in the mid-tertiary, and attains its full development only in the latest tertiary and the quaternary period.

Now if, having reached our own time, we look back upon the whole history of the development of the vegetable kingdom, we cannot but perceive in it a grand confirmation of the Theory of Descent. The two great principles of organic development which have been pointed out as the necessary results of natural selection in the Struggle for Life, namely, the laws of differentiation and perfecting, manifest themselves everywhere in the development of the larger and smaller groups of the natural system of plants. In each larger or smaller period of the organic history of the earth, the vegetable kingdom increases both in variety and perfection, as a glance at Plate IV. will clearly show. During the whole of the long primordial period there existed only the lowest and most imperfect group, that of the Algæ. To these are added, in the primary period, the higher and more perfect Cryptogamia, especially the main-class of Ferns. During the coal period the Phanerogamia begin to develop out of the latter; at first, however, they are represented only by the lower main-class, that of Gymnosperms. It was not until the secondary period that the higher main-class, that of Angiosperms, arose out of them. Of these also there existed at first only the lower groups without distinct corollas, the Monocotyledons and the Apetalæ. It was not until the chalk period that the higher Corollifloræ developed out of the latter. But even this most highly developed group is represented, in the chalk period, only by the lower stage of Star-flowers, or Diapetalæ, and only at quite a late date, in the tertiary period, did the more highly developed Bell-blossoms, Gamopetalæ, arise out of them, which at the same time are the most perfect of all flowering plants. Thus, in each succeeding later division of the organic history of the earth the vegetable kingdom gradually rose to a higher degree of perfection and variety.

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I. Animal-Plants and Worms.

The natural system of organisms which we must employ in the animal as well as in the vegetable kingdom, as a guide in our genealogical investigations, is in both cases of but recent origin, and essentially determined by the progress of comparative anatomy and ontogeny (the history of individual development) during the present century. Almost all the attempts at classification made in the last century followed the path of the artificial system, which was first established in a consistent manner by Charles Linnæus. The artificial system differs essentially from the natural one, in the fact that it does not make the whole organization and the internal structure (depending upon the blood relationship) the basis of classification, but only employs individual, and for the most part external, characteristics, which readily strike the eye. Thus Linnæus distinguished his twenty-four classes of the vegetable kingdom principally by the number, formation, and combination of the stamens. In like manner he distinguished six classes in the animal kingdom principally by the nature of the heart and blood. These six classes were: (1) Mammals; (2) Birds; (3) Amphibious Animals; (4) Fishes; (5) Insects; and (6) Worms.

But these six animal classes of Linnæus are by no means of equal value, and it was an important advance when, at the end of the last century, Lamarck comprised the first four classes as vertebrate animals (Vertebrata), and put them in contrast with the remaining animals (the insects and worms of Linnæus), of which he made a second main division—the invertebrate animals (Invertebrata). In reality Lamarck thus agreed with Aristotle, the father of Natural History, who had distinguished these two main groups, and called the former blood-bearing animals, the latter bloodless animals.

The next important progress towards a natural system of the animal kingdom was made some decades later by two most illustrious zoologists, Carl Ernst Bär and George Cuvier. As has already been remarked, they established, almost simultaneously and independently of one another, the proposition that it was necessary to distinguish several completely distinct main groups in the animal kingdom, each of which possessed an entirely peculiar type or structure (compare above, vol. i. p. 53). In each of these main divisions there is a tree-shaped and branching gradation from most simple and imperfect forms to those which are exceedingly composite and highly developed. The degree of development within each type is quite independent of the peculiar plan of structure, which forms the basis of the type and gives it a special characteristic. The “type” is determined by the peculiar relations in position of the most important parts of the body, and the manner in which the organs are connected. The degree of development, however, is dependent upon the greater or less division of labour among organs, and on the differentiation of the plastids and organs. This extremely important and fruitful idea was established by Bär, who relied more distinctly and thoroughly upon the history of individual development than did Cuvier. Cuvier based his argument upon the results of comparative anatomy. But neither of them recognized the true cause of the remarkable relationships pointed out by them, which is first revealed to us by the Theory of Descent. It shows us that the common type or plan of structure is determined by inheritance, and the degree of development or differentiation by adaptation. (Gen. Morph. ii. 10).

Both Bär and Cuvier distinguished four different types in the animal kingdom, and divided it accordingly into four great main divisions (branches or circles). The first of these is formed by the vertebrate animals (Vertebrata), and comprises Linnæus’ first four classes—mammals, birds, amphibious animals, and fishes. The second type is formed by the articulated animals (Articulata), containing Linnæus’ insects, consequently the six-legged insects, and also the myriopods, spiders, and crustacea, but besides these, a large number of the worms, especially the ringed worms. The third main division comprises the molluscous animals (Mollusca)—slugs, snails, mussels, and some kindred groups. Finally, the fourth and last circle of the animal kingdom comprises the various radiated animals (Radiata), which at first sight differ from the three preceding types by their radiated, flower-like form of body. For while the bodies of molluscs, articulated animals, and vertebrated animals consist of two symmetrical lateral halves—of two counterparts or antimera, of which the one is the mirror of the other—the bodies of the so-called radiated animals are composed of more than two, generally of four, five, or six counterparts grouped round a common central axis, as in the case of a flower. However striking this difference may seem at first, it is, in reality, a very subordinate one, and the radial form has by no means the same importance in all “radiated animals.”

The establishment of these natural main groups or types of the animal kingdom by Bär and Cuvier was the greatest advance in the classification of animals since the time of Linnæus. The three groups of vertebrated animals, articulated animals, and molluscs are so much in accordance with nature that they are retained, even at the present day, little altered in extent. But a more accurate knowledge soon showed the utterly unnatural character of the group of the radiated animals. Leuckart, in 1848, first pointed out that two perfectly distinct types were confounded under the name, namely, the Star-fishes (Echinoderma)—the sea-stars, lily encrinites, sea-urchins, and sea-cucumbers; and, on the other hand, the Animal-plants, or Zoophytes (Cœlenterata or Zoophyta)—the sponges, corals, hood-jellies, and comb-jellies. At the same time, Siebold united the Infusoria with the Rhizopoda, under the name of Protozoa (lowest animals), into a special main division of the animal kingdom. By this the number of animal types was increased to six. It was finally increased to seven by the fact that modern zoologists separated the main division of the articulated animals into two groups: (a) those possessing articulated feet (Arthropoda), corresponding to Linnæus’ Insects, namely, the Flies (with six legs), Myriopods, Spiders, and Crustacea; and (b) the footless Worms (Vermes), or those possessing non-articulated feet. These latter comprise only the real or genuine Worms (ring-worms, round worms, planarian worms, etc.), and therefore in no way correspond with the Worms of Linnæus, who had included the molluscs, the radiates, and many other lower animals under this name.

Thus, according to the views of modern zoologists, which are given in all recent manuals and treatises on zoology, the animal kingdom is composed of seven completely distinct main divisions or types, each of which is distinguished by a characteristic plan of structure peculiar to it, and perfectly distinct from every one of the others. In the natural system of the animal kingdom—which I shall now proceed to explain as its probable pedigree—I shall on the whole agree with this usual division, but not without some modifications, which I consider very important in connection with genealogy, and which are rendered absolutely necessary in consequence of our view as to the history of the development of animals.

We evidently obtain the greatest amount of information concerning the pedigree of the animal kingdom (as well as concerning that of the vegetable kingdom) from comparative anatomy and ontogeny. Besides these, palæontology also throws much valuable light upon the historical succession of many of the groups. From numerous facts in comparative anatomy, we may, in the first place, infer the common origin of all those animals which belong to one of the seven “types.” For in spite of all the variety in the external form developed within each of these types, the essential relative position of the parts of the body which determines the type, is so constant, and agrees so completely in all the members of every type, that on account of their relations of form alone we are obliged to unite them, in the natural system, into a single main group. But we must certainly conclude, moreover, that this conjunction also has its expression in the pedigree of the animal kingdom. For the true cause of the intimate agreement in structure can only be the actual blood relationship. Hence we may, without further discussion, lay down the important proposition that all animals belonging to one and the same circle or type must be descended from one and the same original primary form. In other words, the idea of the circle or type, as it is employed in zoology since Bär and Cuvier’s time to designate the few principal main groups or “sub-kingdoms” of the animal kingdoms, coincides with the idea of “tribe” or “phylum,” as employed by the Theory of Descent.

If, then, we can trace all the varieties of animal forms to these seven fundamental forms, the following question next presents itself to us as a second phylogenetic problem—Where do these seven animal tribes come from? Are they seven original primary forms of an entirely independent origin, or are they also distantly related by blood to one another?

Haeckel-History of Creation

Pl. VI.

At first we might be inclined to answer this question in a polyphyletic sense, by saying that we must assume, for each of the seven great animal tribes, at least one independent primary form completely distinct from the others. On further considering this difficult problem, we arrive in the end at the notion of a monophyletic origin of the animal kingdom, viz., that these seven primary forms are connected at their lowest roots, and that they are derived from a single, common primæval form. In the animal as well as in the vegetable kingdom, when closely and accurately considered, the monophyletic hypothesis of descent is found to be more satisfactory than the polyphyletic hypothesis.

It is comparative ontogeny (embryology) which first and foremost leads to the assumption of the monophyletic origin of the whole animal kingdom (the Protista excepted of course). The zoologist who has thoughtfully compared the history of the individual development of various animals, and has understood the importance of the biogenetic principle (p. 33), cannot but be convinced that a common root must be assumed for the seven different animal tribes, and that all animals, including man, are derived from a single, common primary form. The result of the consideration of the facts of embryology, or ontogeny, is the following genealogical or phylogenetic hypothesis, which I have put forward and explained in detail in my “Philosophy of Calcareous Sponges” (Monograph of the Calcareous Sponges, vol. i. pp. 464, 465, etc.,—“the Theory of the Layers of the Embryo, and the Pedigree of Animals.”)

The first stage of organic life in the Animal kingdom (as in the Vegetable and Protista kingdoms) was formed by perfectly simple Monera, originating by spontaneous generation. The former existence of this simplest animal form is, even at present, attested by the fact that the egg-cell of many animals loses its kernel directly after becoming fructified, and thus relapses to the lower stage of development of a cytod without a kernel, like a Moneron. This remarkable occurrence I have interpreted, according to the law of latent inheritance (vol. i. p. 205), as a phylogenetic relapse of the cellular form into the original form of a cytod. The Monerula, as we may call this egg-cytod without a kernel, repeats then, according to the biogenetic principle (vol. ii. p. 33), the most ancient of all animal forms, the common primary form of the animal kingdom, namely, the Moneron.

The second ontogenetic process consists in a new kernel being formed in the Monerula, or egg-cytod, which thus returns again to the value of a true egg-cell. According to this, we must look upon the simple animal cell, containing a kernel, or the single-celled primæval animal—which may still be seen in a living state in the Amœbæ of the present day—as the second step in the series of phylogenetic forms of the animal kingdom. Like the still living simple Amœbæ, and like the naked egg-cells of many lower animals (for example, of Sponges and Medusæ, etc.), which cannot be distinguished from them, the remote phyletic primary Amœbæ also were perfectly simple naked-cells, which moved about in the Laurentian primæval ocean, creeping by means of the ever-changing processes of their body-substance, and nourishing and propagating themselves in the same way as the Amœbæ of the present day. (Compare vol. i. p. 188, and vol. ii. p. 54.) The existence of this Amœba-like, single-celled primary form of the whole animal kingdom is unmistakably indicated by the exceedingly important fact that the egg of all animals, from those of sponges and worms up to those of the ant and man, is a simple cell.

Thirdly, from the “single-cell” state arose the simplest multicellular state, namely, a heap or a small community of simple, equiformal, and equivalent cells. Even at the present day, in the ontogenetic development of every animal egg-cell, there first arises a globular heap of equiformal naked cells, by the repeated self-division of the primary cell. (Compare vol. i. p. 190 and the Frontispiece, Fig. 3.) We called this accumulation of cells the mulberry state (Morula), because it resembles a mulberry or blackberry. This Morula-body occurs in the same simple form in all the different tribes of animals, and on account of this most important circumstance we may infer—according to the biogenetic principle—that the most ancient, many-celled, primary form of the animal kingdom resembled a Morula like this, and was in fact a simple heap of Amœba-like primæval cells, one similar to the other. We shall call this most ancient community of Amœbæ—this most simple accumulation of animal cells—which is recapitulated in individual development by the Morula—the Synamœba.

Out of the Synamœbæ, in the early Laurentian period, there afterwards developed a fourth primary form of the animal kingdom, which we shall call the ciliated germ (Planæa). This arose out of the Synamœba by the outer cells on the surface of the cellular community beginning to extend vibrating fringes called cilia, and becoming “ciliated cells,” and thus differentiating from the inner and unchanged cells. The Synamœbæ consisted of completely equi-formed and naked cells, and crept about slowly, at the bottom of the Laurentian primæval ocean, by means of movements like those of an Amœba. The Planæa, on the other hand, consisted of two kinds of different cells—inner ones like the Amœbæ, and external “ciliated cells.” By the vibrating movements of the cilia the entire multicellular body acquired a more rapid and stronger motion, and passed over from the creeping to the swimming mode of locomotion. In exactly the same manner the Morula, in the ontogenesis of lower animals, still changes into a ciliated form of larva, which has been known, since the year 1847, under the name of Planula. This Planula is sometimes a globular, sometimes an oval body, which swims about in the water by means of a vibrating movement; the fringed (ciliated) and smaller cells of the surface differ from the larger inner cells, which are unfringed. (Fig. 4 of the Frontispiece.)

Out of this Planula, or fringed larva, there then develops, in animals of all tribes, an exceedingly important and interesting animal form, which, in my Monograph of the Calcareous Sponges, I have named Gastrula (that is, larva with a stomach or intestine). (Frontispiece, Fig. 5, 6). This Gastrula externally resembles the Planula, but differs essentially from it in the fact that it encloses a cavity which opens to the outside by a mouth. The cavity is the “primary intestine,” or “primary stomach,” the progaster, the first beginning of the alimentary canal; its opening is the “primary mouth” (prostoma). The wall of the progaster consists of two layers of cells: an outer layer of smaller ciliated cells (outer skin, or ectoderm), and of an inner layer of larger non-ciliated cells (inner skin, or entoderm). This exceedingly important larval form, the “Gastrula,” makes its appearance in the ontogenesis of all tribes of animals—in Sponges, Medusæ, Corals, Worms, Sea-squirts, Radiated animals, Molluscs, and even in the lowest Vertebrata (Amphioxus: compare p. 200, Plate XII., Fig. B 4; see also in the same place the Ascidian, Fig. A 4).

Definition of the forms of the five first stages of the development of the animal body.	Ontogenesis.	Phylogenesis.
	The five first stages The five first stages of the individual development.	The five first stages of the phyletic or historical development.
First Stage of Development.	1. Monerula.	1. Moneron.
A simple cytod (a plastid without a kernel.)	Animal egg without a kernel (when the egg-kernel has disappeared, after being fructified).	Most ancient animal Monera, originating by spontaneous generation.
│ │	│ │	│ │
Second Stage of Development.	2. Ovulum.	2. Amœbæ.
A simple cell (a plastid containing a kernel.)	Animal egg with kernel (a simple egg-cell).	Animal Amœbæ.
│ │	│ │	│ │
Third Stage of Development.	3. Morula. (Mulberry form.)	3. Synamœba.
A community (an aggregation of identical simple cells).	Globular heap of homogeneous “cleavage spheres.”	An aggregation of Amœbæ.
│ │	│ │	│ │
Fourth Stage of Development.	4. Planula. (Ciliated larva.)	4. Planæa.
A solid or bladder-shaped, globular, or oval body, composed of two kinds of different cells: externally ciliated, internally non-ciliated cells.	Many-celled larva without mouth, composed of different cells.	Many-celled primæval animal without mouth, composed of two kinds of different cells.
│ │	│ │	│ │
Fifth Stage of Development.	5. Gastrula. (Larva with mouth.)	5. Gastræa.
A globular or oval body with simple intestinal cavity and mouth-opening. Body wall composed of two layers; an externally ciliated ectoderm (dermal layer), an internally non-ciliated entoderm (gastral layer).	Many-celled with intestines and mouth; intestinal wall with two layers.	Many-celled primæval animal with intestine and mouth; intestinal wall with two layers. (Primary form of zoophytes and worms.)

From the ontogenetic occurrence of the Gastrula in the most different animal classes, from Zoophytes up to Vertebrata, we may, according to the biogenetic principle, safely draw the conclusion that during the Laurentian period there existed a common primary form of the six higher anima tribes, which in all essential points was formed like the Gastrula, and which we shall call the Gastræa. This Gastræa possessed a perfectly simple globular or oval body, which enclosed a simple cavity of like form, namely, the progaster; at one of the poles of the longitudinal axis the primary intestine opened by a mouth which served for the reception of nutrition. The body wall (which was also the intestinal wall) consisted of two layers of cells, the unfringed entoderm, or intestinal layer, and the fringed ectoderm, or skin-layer; by the motion of the cilia or fringes of the latter the Gastræa swam about freely in the Laurentian ocean. Even in those higher animals, in the ontogenesis of which the original Gastrula form has disappeared, according to the laws of abbreviated inheritance (vol. i. p. 212), the composition of the Gastræa body has been transmitted to the phase of development which directly arises out of the Morula. This phase is an oval or round disc consisting of two cell-layers or membranes: the outer cell-layer, the animal or dermal layer (ectoblast), corresponds to the ectoderm of the Gastræa; out of it develops the external, loose skin (epidermis), with its glands and appendages, as well as the central nervous system. The inner cell-layer, the vegetative or intestinal layer (hypoblast), is originally the entoderm of the Gastræa; out of it develops the inner membrane (epithelium) of the intestinal canal and its glands. (Compare my Monograph of the Calcareous Sponges, vol. i. p. 466, etc.)

By ontogeny we have already gained five primordial stages of development of the animal kingdom: (1) the Moneron; (2) the Amœba; (3) the Synamœba; (4) the Planæa; and (5) the Gastræa. The former existence of these five oldest primary forms, which succeeded one another, and which must have lived in the Laurentian period, follows as a consequence of the biogenetic principle; that is to say, from the parallelism and the mechanico-causal connection of ontogenesis and phylogenesis. (Compare vol. i. p. 309.) In our genealogical system of the animal kingdom we may class all these animal forms, long since extinct, and, which on account of the soft nature of their bodies could leave no fossil remains, among the tribe of Primæval animals (Protozoa), which also comprises the still living Infusoria and Gregarinæ.

The phyletic development of the six higher animal tribes, which are all derived from the Gastræa, deviated at this point in two directions. In other words, the Gastræads (as we may call the group of forms characterized by the Gastræa-type of structure), divided into two divergent lines or branches; the one branch of Gastræads gave up free locomotion, adhered to the bottom of the sea, and thus, by adopting an adhesive mode of life, gave rise to the Protascus, the common primary form of the Animal-plants (Zoophyta). The other branch of the Gastræads retained free locomotion, did not become adherent and later on developed into the Prothelmis, the common primary form of Worms (Vermes). (Compare p. 133.)

This latter tribe (as limited by modern zoology) is of the greatest interest in the study of genealogy. For among Worms, as we shall see later, there are, besides very numerous peculiar families, and besides many independent classes, also very remarkable forms, which may be considered as forms of direct transition to the four higher animal tribes. Both comparative anatomy and the ontogeny of these worms enable us to recognize in them the nearest blood relations of those extinct animal forms which were the original primary forms of the four higher animal tribes. Hence these latter, the Molluscs, Star-fishes, Articulated animals, and Vertebrate animals, do not stand in any close blood relationship to one another, but have originated independently in four different places out of the tribe of Worms.

In this way comparative anatomy and phylogeny lead us to the monophyletic pedigree of the animal kingdom, the outlines of which are given on p. 133. According to it the seven phyla, or tribes, of the animal kingdom are of different value in regard to genealogy. The original primary group of the whole animal kingdom is formed by the Primæval animals (Protozoa), including the Infusoria and Gastræads. Out of these latter arose the two tribes of Animal-plants (Zoophyta) and Worms as diverging branches. Out of four different groups of the Worm tribe, the four higher tribes of the animal kingdom were developed—the Star-fishes (Echinoderma) and Insects (Arthropoda) on the one hand, and the Molluscs (Mollusca) and Vertebrated animals (Vertebrata) on the other.

Having thus sketched out the monophyletic pedigree of the animal kingdom in its most important features, we must now turn to a closer examination of the historical course of development which the seven tribes of the animal kingdom, and the classes distinguished in them, have passed through (p. 132). There is a much larger number of classes in the animal than in the vegetable kingdom, owing to the simple reason that the animal body, in consequence of its more varied and perfect vital activity, could differentiate and develop in very many more different directions than could the vegetable body. Thus, while we were able to divide the whole vegetable kingdom into six main classes and nineteen classes, we have to distinguish, at least, sixteen main classes and thirty-eight classes in the animal kingdom These are distributed among the seven different tribes of the animal kingdom in the way shown in the Systematic Survey on pages 132 and 133.

The group of Primæval animals (Protozoa) within the compass which we here assign to this tribe, comprises the most ancient and the simplest primary forms of the animal kingdom; for example, the five oldest phyletic stages of development previously mentioned, and besides these the Infusoria and Gregarinæ, as well as all those imperfect animal forms, for which, on account of their simple and indifferent organization, no place can be found in any of the other six animal tribes. Most zoologists, in addition to these, Include among the Protozoa a larger or smaller portion of those lowest organisms, which we mentioned in our neutral kingdom of Protista (in Chapter XVI.). But these Protista, especially the large division of the Rhizopoda, which are so rich in forms, cannot be considered as real animals for reasons previously given. Hence, if we here leave them out of the question, we may accept two main classes or provinces of real Protozoa, namely, Egg animals (Ovularia) and Germ animals (Blastularia). To the former belong the three classes of Archezoa, Gregarinæ, and Infusoria, to the latter the two classes of Planæads and Gastræads.

SYSTEMATIC SURVEY
Of the 16 Main Classes and 38 Classes of the Animal Kingdom.
Tribes or Phyla of the Animal Kingdom.		Main Classes, Branches or Clades of the Animal Kingdom.		Classes of the Animal Kingdom.		Systematic Name of the Classes.
A. Primæval Animals Protozoa		I. Egg-animals Ovularia		1.	Archaic animals	1.	Archezoa
				2.	Gregarines	2.	Gregarinæ
				3.	Infusoria	3.	Infusoria
		II. Mulberry animals Ovularia		4.	Planæads	4.	Planæadas
				5.	Gastræads	5.	Gastræadas
B. Animal Plants Zoophyta		III. Sponges Spongiæ		6.	Sponges	6.	Porifera
		IV. Sea-nettles Acalephæ		7.	Corals	7.	Coralla
				8.	Hood-jellies	8.	Hydromedusæ
				9.	Comb-jellies	9.	Ctenophora
C. Worms Vermes		V. Bloodless worms Acœlomi		10.	Planary worms	10.	Platyhelminthes
		V. Bloodless worms Acœlomi		11.	Round worms	11.	Nemathelminthes
				12.	Moss-polyps	13.	Bryozoa
				13.	Sac-worms	13.	Tunicata
				14.	Proboscideans	14.	Rhynchocœla
				15.	Star-worms	15.	Gephyrea
				16.	Wheel animalcules	16.	Rotatoria
				17.	Ring-worms	17.	Annelida
D. Molluscs Mollusca		VII. Headless shellfish Acephala		18.	Lamp-shells	18.	Spirobranchia
				19.	Mussels	19.	Lamellibranchia
		VIII. Head-bearing Eucephala		20.	Snails	20.	Cochlides
				21.	Cuttles	21.	Cephalopoda
E. Star-fishes Echinoderma		IX. Ringed-arms Colobrachia		22.	Sea-stars	22.	Asterida
				23.	Lily-stars	23.	Crinoida
		X. Armless Lipobrachia		24.	Sea-urchins	24.	Echinida
				25.	Sea-cucumbers	25.	Holothuriæ
F. Articulated Animals Arthropoda		XI. Gill-breathers Carides		26.	Crab-fish	26.	Crustacea
		X. Armless Lipobrachia		27.	Spiders	27.	Arachnida
				28.	Centipedes	28.	Myriopoda
				29.	Flies	29.	Insecta
G. Vertebrate Animals Vertebrata		XIII. Skull-less Acrania		30.	Lancelets	30.	Leptocardia
		XIV. Single-nostriled Lipobrachia		31.	Lampreys	31.	Cyclostoma
		XV. Amnion-less Anamnia		32.	Fishes	32.	Pisces
				33.	Mud-fish	33.	Dipneusta
				34.	Sea dragons	34.	Halisauria
				35.	Amphibians	35.	Amphibia
		XVI. Amnion-bearing Amniota		36.	Reptiles	36.	Reptilia
				37.	Birds	37.	Aves
				38.	Mammals	38.	Mammalia

---

					Vertebrata (Vertebrated animals) Craniota
			Arthropoda (Articulated Animals) Tracheata		│ │ │ │			Mollusca (Molluscs) Eucephala
Echinoderma (Star-fishes)			│ │		│ │			│ │
Lipobrachia			Crustacea		Acrania			│
│		Annelida	│		│	Tunicata		Acephala
Colobrachia │		│ │	│ │		│ │	│ │	Bryozoa │	│ │
│	Gephyrea	│	│	Rotatoria
│	│	│	│	│	│		│
Vermes (Worms)					│ │
				│
				Cœlomati (Worms with a body-cavity)
				│	Platyhelminthes
				│	│
	Zoophyta (Animal Plants)			Acœlomi (Worms without body-cavity)
	Spongiæ	Acalephæ		│
	│	│		│
				│
	Protascus			Prothelmis
	│			│
			│
			│ │ │	Protozoa (Primæval animals)
			Gastræa
			│	Infusoria
			Planæa	│	Gregarinæ
			│	│	│
			Synamœbæ
			│	│
			│
			Amœbæ
			│
			Monera

The first province of the Protozoa consists of the Egg animals (Ovularia); we include among them all single-celled animals, all animals whose body, in the fully developed state, possesses the form-value of a simple plastid (of a cytod or a cell), also those simple animal forms whose body consists of an aggregation of several cells perfectly similar one to another.

The Archaic animals (Archezoa) form the first class in the series of Egg animals. It contains only the most simple and most ancient primary forms of the animal kingdom, whose former existence we have proved by means of the fundamental law of biogenesis; they are, (1) Animal Monera; (2) Animal Amœbæ; (3) Animal Synamœbæ. We may, if we choose, include among them a portion of the still living Monera and Amœbæ, but another portion (according to the discussion in Chapter XVI.) must on account of their neutral nature be considered as Protista, and a third portion, on account of their vegetable nature, must be considered as plants.

A second class of the egg animals consists of the Gregarines (Gregarinæ), which live as parasites in the intestines and body-cavities of many animals. Some of these Gregarines are perfectly simple cells like the Amœbæ; some form chains of two or three identical cells, one lying behind the other. They differ from the naked Amœbæ by possessing a thick, simple membrane, which surrounds their cell-body; they can be considered as animal Amœbæ which have adopted a parasitical mode of life, and in consequence have surrounded themselves with a secreted covering.

As a third class of egg animals, we adopt the real Infusoria (Infusoria), embracing those forms to which modern zoology almost universally limits this class of animals. The principal portion of them consists of the small ciliated Infusoria (Ciliata), which inhabit all the fresh and salt waters of the earth in great numbers, and which swim about by means of a delicate garb of vibratile fringes. A second and smaller division consists of the adherent sucking Infusoria (Acinetæ), which take their food by means of fine sucking-tubes. Although during the last thirty years numerous and very careful investigations have been made on these small animalcules,—which are mostly invisible to the naked eye,—still we are even now not very sure about their development and form-value. We do not even yet know whether the Infusoria are single or many-celled; but as no investigator has as yet proved their body to be a combination of cells, we are, in the mean time, justified in considering them as single-celled, like the Gregarines and the Amœbæ.

The second main class of primæval animals consists of the Germ animals (Blastularia). This name we give to those extinct Protozoa which correspond to the two ontogenetic embryonic forms of the six higher animal tribes, namely, the Planula and the Gastrula. The body of these Blastularia, in a perfectly developed state, was composed of many cells, and these cells moreover differentiated—in two ways at least—into an external (animal or dermal) and an internal (vegetative or gastral) mass. Whether there still exist representatives of this group is uncertain. Their former existence is undoubtedly proved by the two exceedingly important ontogenetic animal forms which we have already described as Planula and Gastrula, and which still occur as a transient stage of development in the ontogeny of the most different tribes of animals. Corresponding to these, we may, according to the biogenetic principle, assume the former existence of two distinct classes of Blastularia, namely, the Planæada and Gastræada. The type of the Planæada is the Planæa—long since extinct—but whose historical portrait is still presented to us at the present day in the widely distributed ciliated larva (Planula). (Frontispiece, Fig. 4.) The type of the Gastræada is the Gastræa, of whose original nature the mouth-and-stomach larva (Gastrula), which recurs in the most different animal tribes, still gives a faithful representation. (Frontispiece Fig. 5, 6.) Out of the Gastræa, as we have previously mentioned, there were at one time developed two different primary forms, the Protascus and Prothelmis; the former must be looked upon as the primary form of the Zoophytes, the latter as the primary form of Worms. (Compare the enunciation of this hypothesis in my Monograph of the Calcareous Sponges, vol i. p. 464.)

The Animal-plants (Zoophyta, or Cœlenterata) which constitute the second tribe of the animal kingdom, rise considerably above the primitive animals in the characters of their whole organisation, while they remain far below most of the higher animals. For in the latter (with the exception only of the lowest forms) the four distinct functions of nutrition—namely, digestion, circulation of the blood, respiration, and excretion—are universally accomplished by four perfectly different systems of organs; by the intestines, the vascular system, the organs of respiration, and the urinary apparatus. In Zoophytes, however, these functions and their organs are not yet separate, and are all performed by a single system of alimentary canals, by the so-called gastro-vascular system, or the cœlenteric apparatus of the intestinal cavity. The mouth, which is also the anus, leads into a stomach, into which the other cavities of the body also open. In Zoophytes the body-cavity, or “cœloma,” possessed by the four higher tribes of animals is still completely wanting, likewise the vascular system and blood, as also the organs of respiration, etc.

All Zoophytes live in water; most of them in the sea, only a very few in fresh water, such as fresh-water sponges (Spongilla) and some primæval polyps (Hydra, Cordylophora). A specimen of the pretty flower-like forms which are met with in great variety among Zoophytes is given on Plate VII. (Compare its explanation in the Appendix.)

The tribe of animal-plants, or Zoophytes, is divided into two distinct provinces, the Sponges, or Spongiæ, and the Sea-nettles, or Acalephæ (p. 144). The latter are much richer in forms and more highly organized than the former. In all Sponges the entire body, as well as the individual organs, are differentiated and perfected to a much less extent than in Sea-nettles. All Sponges lack the characteristic nettle-organs which all Sea-nettles possess.

The common primary form of all Zoophytes must be looked for in the Protascus, an animal form long since extinct, but whose existence is proved according to the biogenetic principle by the Ascula. This Ascula is an ontogenetical development form which, in Sponges as well as in Sea-nettles, proceeds from the Gastrula. (Compare the Ascula of the calcareous sponge on the Frontispiece, Fig. 7, 8.) For after the Gastrula of zoophytes has for a time swum about in the water it sinks to the bottom, and there adheres by that pole of its axis which is opposite to the opening of the mouth. The external cells of the ectoderm draw in their vibrating, ciliary hairs, whereas, on the contrary, the inner cells of the entoderm begin to form them. Thus the Ascula, as we call this changed form of larva, is a simple sack, its cavity (the cavity of the stomach or intestine) opening by a mouth externally, at the upper pole of the longitudinal axis (opposite the basal point of fixture). The entire body is here in a certain sense a mere stomach or intestinal canal, as in the case of the Gastrula. The wall of the sack, which is both body wall and intestinal wall, consists of two layers or coats of cells, a fringed entoderm, or gastral layer (corresponding with the inner or vegetative germ-layer of the higher animals), and an unfringed exoderm or dermal layer (corresponding with the external or animal germ-layer of the higher animals). The original Protascus, a true likeness of which is still furnished by the Ascula, probably formed egg-cells and sperm-cells out of its gastral layer.

The Protascads—as we will call the most ancient group of vegetable animals, represented by the Protascus-type—divided into two lines or branches, the Spongiæ and the Sea-nettles, or Acalephæ. I have shown in my Monograph of the Calcareous Sponges (vol. i. p. 485) how closely these two main classes of Zoophytes are related, and how they must both be derived, as two diverging forms, from the Protascus-form. The primary form of Spongiæ, which I have there called Archispongia, arose out of the Protascus by the formation of pores through its body-wall; the primary form of Sea-nettles, which I there called Archydra, developed out of the Protascus by the formation of nettle-organs, as also by the formation of feelers or tentacles.

The main-class or branch of the Sponges, Spongiæ, or Porifera, lives in the sea, with the single exception of the green fresh-water Sponge (Spongilla). These animals were long considered as plants, later as Protista; in most Manuals they are still classed among the primæval animals, or Protozoa. But since I have demonstrated their development out of the Gastrula, and the construction of their bodies of two cellular germ-layers (as in all higher animals), their close relationship to Sea-nettles, and especially to the Hydrapolyps, seems finally to be established. The Olynthus especially, which I consider as the common primary form of calcareous sponges, has thrown a complete and unmistakable light upon this point.

The numerous forms comprised in the class of Spongiæ have as yet been but little examined; they may be divided into three legions and eight orders. The first legion consists of the soft, gelatinous Mucous Sponges (Myxospongiæ), which are characterized by the absence of any hard skeleton. Among them are, on the one hand, the long-since-extinct primary forms of the whole class, the type of which I consider to be the Archispongia; on the other hand there are the still living, gelatinous sponges, of which the Halisarca is best known. We can obtain a notion of the Archispongia, the most ancient primæval sponge, if we imagine the Olynthus (see Frontispiece), to be deprived of its radiating calcareous spiculæ.

The second legion of Spongiæ contains the Fibrous Sponges (Fibrospongiæ), the soft body of which is supported by a firm, fibrous skeleton. This fibrous skeleton often consists merely of so-called “horny fibres,” formed of a very elastic, not readily destructible, organic substance. This is the case for instance in our common bathing Sponge (Euspongia officinalis), the purified skeleton of which we use every morning when washing. Blended with the horny, fibrous skeleton of many of these Sponges, there are numerous flinty spicula; this is the case for example with the fresh-water Sponge (Spongilla). In others the whole skeleton consists of only calcareous or silicious spicula which are frequently interwoven into an extremely beautiful lattice-work, as in the celebrated Venus’ Flower Basket (Euplectella). Three orders of fibrous sponges may be distinguished according to the different formation of the spicula, namely, Chalynthina, Geodina, and Hexactinella. The natural history of the fibrous sponges is of especial interest to the Theory of Descent, as was first shown by Oscar Schmidt, the greatest authority on this group of animals. In no other group, perhaps, can the unlimited pliability of the specific form, and its relation to Adaptation and Inheritance, be so clearly followed step by step; perhaps in no other group is the species so difficult to limit and define.

Pl. VII.

E. Haeckel del.

Lagesse sc.

This proposition, which applies to the great legion of the Fibrous Sponges, applies in a still higher degree to the smaller but exceedingly interesting legion of the calcareous sponges (Calcispongiæ), on which in 1872, after five years’ careful examination, I published a comprehensive Monograph. The sixty plates of figures accompanying this Monograph explain the extreme pliability of these small sponges “good species” of which, in fact, cannot be spoken of in the usual systematic sense. We find among them only varying series of forms, which do not even completely transmit their specific form to their nearest descendants, but by adaptation to subordinate, external conditions of existence, perpetually change. It frequently occurs here, that there arise out of one and the same stock different form-species, which according to the usual system would belong to several quite distinct genera; this is the case, for instance, with the remarkable Ascometra (Frontispiece, Fig. 10.) The entire external bodily form is much more pliable and protean in Calcareous Sponges than in the silicious sponges, which are characterized by possessing silicious spicula, forming a beautiful skeleton. Through the study of the comparative anatomy and ontogeny of calcareous sponges, we can recognise, with the greatest certainty, the common primary form of the whole group, namely, the sack-shaped Olynthus, whose development is represented in the Frontispiece (compare its explanation in the Appendix). Out of the Olynthus (Fig. 9 on the Frontispiece), the order of the Ascones was the first to develop, out of which, at a later period, the two other orders of Calcareous Sponges, the Leucones and Sycones, arose as diverging branches. Within these orders, the descent of the individual forms can again be followed step by step. Thus the Calcareous Sponges in every respect confirm the proposition which I have elsewhere maintained: that “the natural history of sponges forms a connected and striking argument in favour of Darwin.”

The second main class or branch in the tribe of Zoophytes is formed by the Sea-nettles (Acalephæ, or Cnidæ). This interesting group of animals, so rich in forms, is composed of three different classes, namely, the Hood-jellies (Hydromedusæ), the Comb-jellies (Ctenophora), and the Corals (Coralla). The hypothetical, extinct Archydra must be looked upon as the common primary form of the whole group; it has left two near relations in the still living fresh-water polyps (Hydra and Cordylophora). The Archydra was very closely related to the simplest forms of Spongiæ (Archispongia and Olynthus), and probably differed from them only by possessing nettle organs, and by the absence of cutaneous pores. Out of the Archydra there first developed the different Hydroid polyps, some of which became the primary forms of Corals, others the primary forms of Hydromedusæ. The Ctenophora developed later out of a branch of the latter.

The Sea-nettles differ from the Spongiæ (with which they agree in the characteristic formation of the system of the alimentary canal) principally by the constant possession of nettle organs. These are small bladders filled with poison, large numbers—generally millions—of which are dispersed over the skin of the sea nettles, and which burst and empty their contents when touched. Small animals are killed by this; in larger animals this nettle poison causes a slight inflammation of the skin, just as does the poison of our common nettles. Any one who has often bathed in the sea, will probably have at times come in contact with large Hood-jellies (Jelly-fish), and become acquainted with the unpleasant burning feeling which their nettle organs can produce. The poison in the splendid blue Jelly-fish, Physalia, or Portuguese Man-of-war, acts so powerfully that it may lead to the death of a human being.

The class of Corals (Coralla) lives exclusively in the sea, and is more especially represented in the warm seas by an abundance of beautiful and highly-coloured forms like flowers. Hence they are also called Flower-animals (Anthozoa). Most of them are attached to the bottom of the sea, and contain an internal calcareous skeleton. Many of them by continued growth produce such immense stocks that their calcareous skeletons have formed the foundation of whole islands, as is the case with the celebrated coral reefs and atolls of the South Seas, the remarkable forms of which were first explained by Darwin.(13) In corals the counterparts, or antimera—that is, the corresponding divisions of the body which radiate from and surround the central main axis of the body—exist sometimes to the number of four, sometimes to the number of six or eight. According to this we distinguish three legions, the Fourfold (Tetracoralla), Sixfold (Hexacoralla), and Eightfold corals (Octocoralla). The fourfold corals form the common primary group of the class, out of which the sixfold and eightfold have developed as two diverging branches.

SYSTEMATIC VIEW
Of the 4 Classes and 30 Orders of the Animal Plants, or Zoophytes.
Class of the Zoophytes.		Legions of the Zoophytes.		Orders of the Zoophytes.		A Genus Name as example.
I. Sponges Spongiæ or Porifera		I. Myxospongiæ Mucous Sponges		1.	Archispongina	Archispongia
				2.	Halisarcina	Halisarca
		II. Fibrospongiæ Fibrous Sponges		3.	Chalynthina	Spongilla
				4.	Geodina	Ancorina
				5.	Hexactinella	Euplectella
		III. Calcispongiæ Calcareous Sponges		6.	Ascones	Olynthus
				7.	Leucones	Dyssycus
				8.	Sycones	Sycurus
II. Corals Coralla or Anthozoa		IV. Tetracoralla Fourfold Corals		9.	Rugosa	Cyathophyllum
				10.	Paranemeta	Cereanthus
		V. Hexacoralla Sixfold Corals		11.	Cauliculata	Antipathes
				12.	Madreporaria	Astræa
				13.	Halirhoda	Actinia
		VI. Octocoralla Eightfold Corals		14.	Alcyonida	Lobularia
				15.	Gorgonida	Isis
				16.	Pennatulida	Veretillum
III. Jelly-polyps Hydromedusæ or Hood-jellies Medusa		VII. Archydræ Primæval Polyps		17.	Hydraria	Hydra
		VIII. Leptomedusæ Soft Jelly-fish		18.	Vesiculata	Sertularia
				19.	Ocellata	Tubularia
				20.	Siphonophora	Physophora
		IX. Trachymedusæ Hard Jelly-fish		21.	Marsiporchida	Trachynema
				22.	Phyllorchida	Geryonia
				23.	Elasmorchida	Charybdæ
		X. Calycozoa Stalked Jellies		24.	Podactinaria	Lucernaria
		XI. Discomedusæ Disc-jellies		25.	Semæostomeæ	Aurelia
				26.	Rhizostomeæ	Crambessa
IV. Comb-jellies Ctenophora		XII. Eurystoma Wide-mouthed		27.	Beroida	Beroe
		XIII. Stenostoma Narrow-mouthed		28.	Saccata	Cydippe
				29.	Lobata	Eucharis
				30.	Tæniata	Cestum

---

		Ctenophora					Hydromedusæ
		Tæniata	Lobata				Rhizostomeæ
		│ │	│ │				│ │
		Saccata Stenostoma					Semæostomeæ Discomedusæ
		│		Trachymedusæ			│	Siphonophora
		│		│			│	│
		Eurystoma		│ │	Lucernaria Calycozoa		│ │	│ │
		│ │		│ │	│ │		│ │	│ │
		│		│	│		│	│
		Leptomedusæ
				Coralla		│
				Hexacoralla	Octocoralla	│
				│	│	│
						│
				Tetracoralla		│
Spongiæ					│	│
Fibrospongiæ			Calcispongiæ		│	│
Chalynthina			Leucones	Sycones	│	│
Hexactinella	│	Geodina	Dyssycus	Sycurus	│	Hydroida
│	│	│	│	│	│	│	Cordylophora
│	│	│	│	│	│	│	│	Hydra
					│	│	│	│
│			Ascones		│	│	│	│
│ │	Myxospongia Halisarcina		│ │		│ │	Hydroida
Chalynthus	│		Olynthus		│	│
│	│		│		│	│
Archispongiæ					Hydroida
	│			│
	│			Archydra
	│			│
Protascus
│
Gastræa

The second class of Sea-nettles is formed by the Hood-jellies (Medusæ) or Polyp-jellies (Hydromedusæ). While most corals form stocks like plants, and are attached to the bottom of the sea, the Hood-jellies generally swim about freely in the form of gelatinous bells. There are, however, numbers of them, especially the lower forms, which adhere to the bottom of the sea, and resemble pretty little trees. The lowest and simplest members of this class are the little fresh-water polyps (Hydra and Cordylophora). We may look upon them as but little changed descendants of those Primæval polyps (Archydræ), from which, during the primordial period, the whole division of the Sea-nettles originated. Scarcely distinguishable from the Hydra are the adherent Hydroid polyps (Campanularia, Tubularia), which produce freely swimming medusæ by budding, and out of the eggs of these there again arise adherent polyps. These freely swimming Hood-jellies are mostly of the form of a mushroom, or of an umbrella, from the rim of which many long and delicate tentacles hang. They are among the most beautiful and most interesting inhabitants of the sea. The remarkable history of their lives, and especially the complicated alternation of generation of polyps and medusæ, are among the strongest proofs of the truth of the theory of descent. For just as Medusæ still daily arise out of the Hydroids, did the freely swimming medusa-form originally proceed, phylogenetically, out of the adherent polyp-form. Equally important for the theory of descent is the remarkable division of labour of the individuals, which among some of them is developed to an astonishingly high degree, more especially in the splendid Siphonophora.(37) (Plate VII. Fig. 13).

The third class of Sea-nettles—the peculiar division of Comb-jellies (Ctenophora), probably developed out of a branch of the Hood-jellies. The Ctenophora, which are also called Ribbed-jellies, possess a body of the form of a cucumber, which, like the body of most Hood-jellies, is as clear and transparent as crystal or cut glass. Comb or Ribbed-jellies are characterized by their peculiar organs of motion, namely, by eight rows of paddling, ciliated leaflets, which run in the form of eight ribs from one end of the longitudinal axis (from the mouth) to the opposite end. Those with narrow mouths (Stenostoma) probably developed later out of those with wide mouths (Eurystoma). (Compare Plate VII. Fig. 16.)

The third tribe of the animal kingdom, the phylum of Worms or worm-like animals (Vermes, or Helminthes), contains a number of diverging branches. Some of these numerous branches have developed into well-marked and perfectly independent classes of Worms, but others changed long since into the original, radical forms of the four higher tribes of animals. Each of these four higher tribes (and likewise the tribe of Zoophytes) we may picture to ourselves in the form of a lofty tree, whose branches represent the different classes, orders, families, etc. The phylum of Worms, on the other hand, we have to conceive as a low bush or shrub, out of whose root a mass of independent branches shoot up in different directions. From this densely branched shrub, most of the branches of which are dead, there rise four high stems with many branches. These are the four lofty trees just mentioned as representing the higher phyla—the Echinoderma, Articulata, Mollusca, and Vertebrata. These four stems are directly connected with one another at the root only, to wit, by the common primary group of the Worm tribe.

The extraordinary difficulties which the systematic arrangement of Worms presents, for this reason merely, are still more increased by the fact that we do not possess any fossil remains of them. Most of the Worms had and still have such soft bodies that they could not leave any characteristic traces in the neptunic strata of the earth. Hence in this case again we are entirely confined to the records of creation furnished by ontogeny and comparative anatomy. In making then the exceedingly difficult attempt to throw a few hypothetical rays of light upon the obscurity of the pedigree of Worms, I must therefore expressly remark that this sketch, like all similar attempts possesses only a provisional value.

The numerous classes distinguished in the tribe of Worms, and which almost every zoologist groups and defines according to his own personal views, are, in the first place, divided into two essentially different groups or branches, which in my Monograph of the Calcareous Sponges I have termed Acœlomi and Cœlomati. For all the lower Worms which are comprised in the class of Flat-worms (Platyhelminthes), (the Gliding-worms, Sucker-worms, Tape-worms), differ very strikingly from other Worms, in the fact that they possess neither blood nor body-cavity (no cœlome); they are, therefore, called Acœlomi. The true cavity, or cœlome, is completely absent in them as in all the Zoophytes; in this important respect the two groups are directly allied. But all other Worms (like the four higher tribes of animals) possess a genuine body-cavity and a vascular system connected with it, which is filled with blood; hence we class them together as Cœlomati.

The main division of Bloodless Worms (Acœlomi) contains, according to our phylogenetic views, besides the still living Flat-worms, the unknown and extinct primary forms of the whole tribe of Worms, which we shall call the Primæval Worms (Archelminthes). The type of these Primæval Worms, the ancient Prothelmis, may be directly derived from the Gastræa (p. 133). Even at present the Gastrula-form—the faithful historical portrait of the Gastræa—recurs in the ontogenesis of the most different kinds of worms as a transient larva-form. The ciliated Gliding-worms (Turbellaria), the primary group of the present Planary or Flat-worms (Platyhelminthes), are the nearest akin to the Primæval Worms. The parasitical Sucker-worms (Trematoda) arose out of the Gliding-worms, which live freely in water, by adaptation to a parasitical mode of life; and out of them later on—by an increasing parasitism—arose the Tape-worms (Cestoda).

Out of a branch of the Acœlomi arose the second main division of the Worm tribe, the Worms with blood and body-cavity (Cœlomati): of these there are seven different classes.

The Pedigree on p. 151 shows how the obscure phylogeny of the seven classes of Cœlomati may be supposed to stand. We shall, however, mention these classes here quite briefly, as their relationships and derivation are, at present, still very complicated and obscure. More numerous and more accurate investigations of the ontogeny of the different Cœlomati will at some future time throw light upon their phylogenesis.

SYSTEMATIC VIEW
Of the 8 Classes and 22 Orders of the Worm Tribe.
(Compare Gen. Morph. II. Plate V. pp. 75-77.)
Classes of the Worm Tribe.		Orders of the Worm Tribe.		Systematic Name of the Orders of Worms.		Name of a Genus as example.
1. Flat Worms Platyhelminthes		1.	Primæval worms	1.	Archelminthes	Prothelmis
		2.	Gliding-worms	2.	Turbellaria	Planaria
		3.	Sucker-worms	3.	Trematoda	Distoma
		4.	Tape-worms	4.	Cestoda	Tænia
2.Round Worms Nemathelminthes		5.	Arrow-worms	5.	Chætognatha	Sagitta
		6.	Thread-worms	6.	Nematoda	Trichina
		7.	Hook-headed-worms	7.	Acanthocephala	Echinorhynchus
3. Moss Polyps Bryozoa		8.	Horse-shoe-lipped	8.	Lophopoda	Alcyonella
		9.	Circle-lipped	9.	Stelmopoda	Retepora
4. Sea-sacs Tunicata		10.	Sea-squirts	10.	Ascidia	Phallusia
		11.	Sea-barrels	11.	Thaliacea	Salpa
5. Proboscideans Rhynchocœla		12.	Tongue-worms	12.	Enteropneusta	Balanoglossus
		13.	Cord-worms	13.	Nemertina	Borlasia
6. Star-Worms Gephyrea		14.	Star-worms without bristles	14.	Sipunculida	Sipunculus
		15.	Star-worms with bristles	15.	Echiurida	Echiurus
7. Wheel Animalcule Rotifera		16.	Wheel-worms	16.	Rotatoria	Hydatina
8. Ring Worms Annelida		17.	Bear-worm	17.	Arctisca	Macrobiotus
		18.	Worms with claws	18.	Onychophora	Peripatus
		19.	Leeches	19.	Hirudinea	Hirudo
		20.	Land-worms	20.	Drilomorpha	Lumbricus
		21.	Mailed worms	21.	Phracthelminthes	Crossopodia
		22.	Bristle-footed worms	22.	Chætopoda	Aphrodite

---

				Chætopoda
				│		Drilomorpha
				│		│
					│
				Phracthelminthes	│
				│	│
			Echiurida	│	│	Hirudinea
		Sipunculida	│	│	│	│
		│	│	│	│	│
				│	│	│	Onychophora
		Gephyrea		│	│	│	│	Arctisca
		│		│	│	│	│	│
		│		│	│	│
	Chætognatha	│				│	│
Nematoda	│	│		│
│	│	│		│		│
		│
│ │	Acantho-cephala	│ │	Stelmopoda	Annelida
│	│	│	│		│			Enteropneusta
		│	│		│			│
Nemathelminthes		│ │	Lophopoda Bryozoa		│ │		Ascidia	│ │
│		│	│		│	Thalicea	│	│	Nemertina
│		│	│		│	│	│	│	│
│		│	│	Rotifera	│	│	│
│		│	│	│	│	│	│	Rhynchocœla
│		│	│					│
│		│	│	│		Tunicata		│
│		│	│	│		│		│
Cœlomati (_worms with body-cavity_)
					│
			Cestoda		│
			│		│
			│		│
			Trematoda		│
			│		│
			│		│
			Turbellaria		│
			Platyhelminthes		│
			│		│
			│		│
			Acœlomi (_worms without body-cavity_)
			│
			│
			Archelminthes
			Prothelmis
			│
			│
			Gastræa

The Round Worms (Nemathelminthes) which we mention as the first class of the Cœlomati, and which are characterized by their cylindrical form, consist principally of parasitical Worms which live in the interior of other animals. Of human parasites, the celebrated Trichinæ, the Maw-worms, Whip-worms, etc., for example, belong to them. The Star-worms (Gephyrea) which live exclusively in the sea are allied to round worms, and the comprehensive class of Ring-worms (Annelida) are allied to the former. To the Ring-worms, whose long body is composed of a number of segments, all alike in structure, belong the Leeches (Hirudinea), Earth-worms (Lumbricina), and all the marine bristle-footed Worms (Chætopoda). Nearly akin to them are the Snout-worms (Rhynchocœla), and the small microscopic Wheel-worms (Rotifera). The unknown, extinct, primary forms of the tribe of Sea-stars (Echinoderma), and of the tribe of the articulated animals (Arthropoda), were nearest akin to the Ring-worms. On the other hand, we must probably look for the primary forms of the great tribe of Molluscs in extinct Worms, which were very closely related to the Moss-polyps (Bryozoa) of the present day; and for the primary forms of the Vertebrata in the unknown Cœlomati, whose nearest kin of the present day are the Sea-sacs, especially the Ascidia.

The class of Sea-sacs (Tunicata) is one of the most remarkable among Worms. They all live in the ocean, where some of the Ascidiæ adhere to the bottom, while others (the sea-barrels, or Thaliacea) swim about freely. In all of them the non-jointed body has the form of a simple barrel-shaped sack, which is surrounded by a thick cartilaginous mantle. This mantle consists of the same non-nitrogenous combination of carbon, which, under the name of cellulose, plays an important part in the Vegetable Kingdom, and forms the largest portion of vegetable cellular membranes, and consequently also the greater part of wood. The barrel-shaped body generally possesses no external appendages. No one would recognise in them a trace of relationship to the highly differentiated vertebrate animals. And yet this can no longer be doubted, since Kowalewsky’s investigations, which in the year 1867 suddenly threw an exceedingly surprising and unmistakable light upon them. From these investigations it has become clear that the individual development of the adherent simple Ascidian Phallusia agrees in most points with that of the lowest vertebrate animal, namely, the Lancelet (Amphioxus lanceolatus). The early stages of the Ascidia possess the beginnings of the spinal marrow and the spinal column (chorda dorsalis) lying beneath it, which are the two most essential and most characteristic organs of the vertebrate animal. Accordingly, of all invertebrate animals known to us, the Tunicates are without doubt the nearest blood relations of the Vertebrates, and must be considered as the nearest relations of those Worms out of which the vertebrate tribe has developed. (Compare Plates XII. and XIII.)

While thus different branches of the Cœlomatous group of the Worms furnish us with several genealogical links leading to the four higher tribes of animals, and give us important phylogenetic indications of their origin, the lower group of Acœlomi, on the other hand, show close relationships to the Zoophytes, and to the Primæval animals. The great phylogenetic interest of the Worm tribe rests upon this peculiar intermediate position.

---

II. Mollusca, Star-fishes, and Articulated Animals.

The great natural main groups of the animal kingdom, which we have distinguished as TRIBES, or PHYLA (“types” according to Bär and Cuvier), are not all of equal systematic importance for our phylogeny or history of the pedigree of the living world. They can neither be classed in a single series of stages, one above another, nor be considered as entirely independent stems, nor as equal branches of a single family-tree. It seems rather (as we saw in the last chapter) that the tribe of Protozoa, the so-called primæval animals, is the common radical group of the whole animal kingdom. Out of the Gastræada—which we class among the Protozoa—the Zoophytes and the Worms have developed, as two diverging branches. We must now in turn look upon the varied and much-branching tribe of Worms as the common primary group, out of which (from perfectly distinct branches) arose the remaining tribes, the four higher phyla of the animal kingdom. (Compare the Pedigree, p. 133.)

Let us now take a genealogical look at these four higher tribes of animals, and try whether we cannot make out the most important outlines of their pedigree. Even should this attempt prove defective and imperfect, we shall at all events have made a beginning, and paved the road for subsequent and more satisfactory attempts.

It does not matter in what succession we take up the examination of the four higher tribes. For these four phyla have no close relationship whatever among one another, but have grown out from entirely distinct branches of the group of Worms (p. 133). We may consider the tribe of Molluscs as the most imperfect and the lowest in point of morphological development. We nowhere meet among them with the characteristic articulation or segmented formation of the body, which distinguishes even the Ring-worms, and which in the other three higher tribes—the Echinoderma, Articulata, and Vertebrata—is most essentially connected with the high development of their forms, their differentiation, and perfection. The body in all Molluscs—in mussels, snails, etc.—is a simple non-jointed sack, in the cavity of which lie the intestines. The nervous system consists not of a cord but of several distinct (generally three) pairs of knots loosely connected with one another. For these and many other anatomical reasons, I consider the tribe of Molluscs (in spite of the high physiological development of its most perfect forms) to be morphologically the lowest among the four higher tribes of animals.

Whilst, for reasons already given, we exclude the Moss-polyps, and Tunicates—which have hitherto been generally classed with the tribe of Molluscs—we retain as genuine Molluscs the following four classes: Lamp-shells, Mussels, Snails, and Cuttles. The two lower classes of Molluscs, the Lamp-shells and Mussels, possess neither head nor teeth, and they can therefore be comprised under one main class, or branch, as headless animals (Acephala), or toothless animals (Anodontoda). This branch is also frequently called that of the clam-shells (Conchifera, or Bivalvia), because all its members possess a two-valved calcareous shell. In contrast to these the two higher classes of Molluscs, the snails and cuttles, may be represented as a second branch with the name of Head-bearers (Cephalophora), or Tooth-bearers (Odontophora), because both head and teeth are developed in them.

The soft, sack-shaped body in most Molluscs is protected by a calcareous shell or house, which in the Acephala (lamp-shells and mussels) consists of two valves, but in the Cephalophora (snails and cuttles) is generally a spiral tube (the so-called snail’s house). Although these hard skeletons are found in large quantities in a petrified state in all the neptunic strata, yet they tell us but little of the historical development of the tribe, which must have taken place for the most part in the primordial period. Even in the Silurian strata we find fossil remains of all the four classes of Molluscs, one beside the other, and this, conjointly with much other evidence, distinctly proves that the tribe of Molluscs had then obtained a strong development, when the higher tribes, especially the Articulates and Vertebrates, had scarcely got beyond the beginning of their historical development. In subsequent periods, especially in the primary and secondary periods, these higher tribes increased in importance more and more at the expense of Molluscs and Worms, which were no match for them in the struggle for life, and accordingly decreased in number. The still living Molluscs and Worms must be considered as only a proportionately small remnant of the vast molluscan fauna, which greatly predominated in the primordial and primary periods over the other tribes. (Compare Plate VI. and explanation in the Appendix.)

No tribe of animals shows more distinctly than do the Molluscs, how very different the value of fossils is in geology and in phylogeny. In geology the different species of the fossil shells of Molluscs are of the greatest importance because they serve as excellent marks whereby to characterize the different groups of strata, and to fix their relative ages. As far as relates to the genealogy of Molluscs, however, they are of very little value, because, on the one hand, the shells are parts of quite subordinate morphological importance, and because the actual development of the tribe belongs to the earlier primordial period, from which no distinct fossils have been preserved. If therefore we wish to construct the pedigree of Molluscs, we are mainly dependent upon the records of ontogeny and comparative anatomy from which we obtain something like the following result. (Gen. Morph. ii. Plate VI. pp. 102-116.)

The lowest stage of the four classes of genuine Molluscs known to us, is occupied by the Lamp-shells or Spiral-gills (Spirobranchia), frequently but inappropriately called Arm-footers (Brachiopoda), which have become attached to the bottom of the sea. There now exist but few forms of this class; for instance, some species of Lingula, Terebratula, and others akin to them, which are but feeble remnants of the great variety of forms which represented the Lamp-shells in earlier periods of the earth’s history. In the Silurian period they constituted the principal portion of the whole Mollusc tribe. From the agreement which, in many respects, their early stage of development presents with the Moss animals, it has been concluded that they have developed out of Worms, which were nearly related to this class. Of the two sub-classes of Lamp-shells, the Hinge-less (Ecardines) must be looked upon as the lower and more imperfect, the Hinged (Testicardines) as the higher and more fully developed group.

The anatomical difference between the Lamp-shells and the three other classes of Molluscs is so considerable that the latter may be distinguished from the former by the name of Otocardia. All the Otocardia have a heart with chamber (ventricle) and ante-chamber (auricle), whereas Lamp-shells do not possess the ante-chamber. Moreover, the central nervous system is developed only in the former (and not in the latter) in the shape of a complete pharyngeal ring. Hence the four classes of Molluscs may be grouped in the following manner:—

I. Molluscs without head. Acephala.		1. Lamp-shells (Spirobranchia)		I. Haplocardia (with simple heart)
		2. Mussels (Lamellibranchia)		II. Otocardia (with chamber and ante-chamber to the heart)
I. Molluscs with head. Cephalophora.		3. Snails (Cochlides).
		4. cuttles cephalopoda.

The result of these structural dispositions for the history of the pedigree of Molluscs, which is confirmed by palæontology, is that Lamp-shells stand much nearer to the primæval root of the whole tribe of Molluscs than do the Otocardia. Probably Mussels and Snails developed as two diverging branches out of Molluscs, which were nearly akin to the Lamp-shells.

Mussels, or Plate-gills (Lamellibranchia), possess a bivalved shell like the Lamp-shells. In the latter, one of the two valves covers the back, the other the belly of the animal; whereas in Mussels the two valves lie symmetrically on the right and left side of the body. Most Mussels live in the sea, only a few in fresh water. The class is divided into two sub-classes, Asiphonia and Siphonida, of which the latter were developed at a later period out of the former. Among the Asiphonia are Oysters, mother-of-pearl Shells, and fresh water Mussels; among the Siphonida, which are characterized by a respiratory tube, are the Venus-shells, Razor-shells, and Burrowing Clams. The higher Molluscs seem to have developed at a later period out of those without head and teeth; they are distinguished from the latter by the distinct formation of the head, and more especially by a peculiar kind of tooth apparatus. Their tongue presents a curious plate, armed with a great number of teeth. In our common Vineyard Snail (Helix pomatia) the number of teeth amount to 21,000, and in the large Garden Slug (Limax maximus) to 26,800.

SYSTEMATIC SURVEY
Of the 4 Classes, 8 Sub-classes, and 21 Orders of Molluscs.
Classes of Molluscs.		Sub-classes of Molluscs.		Orders of Molluscs.		Systematic Name of the Orders.
I. Molluscs without head or teeth: Acephala or Anodontoda.
I. Lamp-shells Spirobranchia or Brachiopoda		I. Ecardines Hinge-less		1.	Stalked	1.	Lingulida
				2.	Flattened	2.	Craniada
		II. Testicardines Hinge-less		3.	Fleshy armed	3.	Sarcobrachia
				4.	Calcareous-armed	4.	Sclerobrachia
II. Mussels or Plate-gills Lamellibranchia or Phyllobranchia		III. Asiphonia Mussels without respiratory tubes		5.	One-muscled	5.	Monomya
				6.	Uneven-muscled	6.	Heteromya
				7.	Even-muscled	7.	Isomya
		IV. Siphonida Mussels with respiratory tubes		8.	Round-mantled	8.	Integripallia
				9.	Ray-mantled	9.	Sinupalliata
				10.	Tube-mussels	10.	Inclusa
II. Molluscs with head and teeth: Cephalophora or Odontophora.
III. Snails Cochlides or Gasteropoda		V. Stump-headed Perocephala		11.	Tube-snails	11.	Scaphopoda
				12.	Butterfly-snails	12.	Pteropoda
		VI. Large-headed Delocephala		13.	With hind gills	13.	With hind gills
				14.	With fore gills	14.	Prosobranchia
				15.	Swimming-snails	15.	Heteropoda
				16.	Beetle-snails	16.	Chitonoida
				17.	Snails with lungs	17.	Pulmonata
IV. Cuttles or Poulps Cephalopoda		VII. Chamber-Poulps with four gills Tetraranchia		18.	Pearl boats	18.	Nautilida
				19.	Ammon's horns	19.	Ammonitida
		VIII. Ink-Poulps with two gills Dibranchia		20.	Ten-armed	20.	Decabrachiones
				21.	Eight-armed	21.	Octobrachiones

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					Dibranchia
		Heteropoda			│
		│	Prosobranchia		│
Pulmonata │ │	Lipobranchia │ │	│ │ │	│ │ │		Tetrabranchia Cephalopoda (Cuttles or Poulps)
│	Gymnobranchia	│	│		│
│	│	│	│		│
│ │	Pleurobranchia Opisthobranchia	│		Chitonides	│ │
│	│	│		│	│
					│
		Delocephala			│
		│			│
		Inclusa		│	Pteropoda
		│		│	│
		│
		Sinupalliata		│
		│	Scaphopoda	│
		│	│	│
	Sclerobrachia	│ Integripalliata Siphoniata │	Perocephala Cochlides (Snails)
	│	│	│
	Sarcobrachia Testicardines	Asiphonia Lamellibranchia (Mussels)	│ │ │
	│	│	│
	│
	Ecardines Spirobranchia (Lamp-shells)	│ Otocardia (Molluscs with chamber and ante-chamber to the heart)
	│	│
Promollusca (Primæval Molluscs) Molluscs with simple heart
│
(Worms)
│
Gastræa

We distinguish two sub-classes among the Snails (Cochlides, or Gasteropoda), namely, the Stump-headed and the Large-headed Snails. The Stump-headed Snails (Perocephala) are very closely allied to Mussels (through the Tooth-shells), and also to the Cuttle-fish (through the Butterfly-snails). The more highly developed Snails, with large heads (Delocephala), can be divided into Snails with gills (Branchiata) and Snails with lungs (Pulmonata). Among the latter are the Land-snails, the only Molluscs which have left the water and become habituated to a life on land. The great majority of Snails live in the sea, only a few live in fresh water. Some River-snails in the tropics (the Ampullaria) are amphibious, living sometimes on land, sometimes in water, and at one time they breathe through gills, at another through lungs. They have both kinds of respiratory organs, like the Mud-fish and Gilled Newts among the Vertebrata.

The fourth and last class, and at the same time the most highly developed class of Molluscs, is that of the Cuttles, or Poulps, also called Cephalopoda (foot attached to the head). They all live in the sea, and are distinguished from Snails by eight, ten, or more long arms, which surround the mouth in a circle. The Cuttles existing in our recent oceans—the Sepia, Calamary, Argonaut, and Pearly Nautilus—are, like the few Spiral-gill Lamp-shells of the present time, but a poor remnant of the host which represents this class in the oceans of the primordial, primary, and secondary periods. The numerous fossil “Ammon’s horns” (Ammonites), “pearl boats” (Nautilus), and “thunderbolts” (Belemnites) are evidences of the long since extinct splendour of the tribe. The Poulps, or Cuttles, have probably developed out of a low branch of the snail class, out of the Butterfly-snails (Pteropoda) or kindred forms.

The different sub-classes and orders, distinguished in the four classes of Molluscs, whose systematic succession is given on the Table (p. 160), furnish various proofs of the validity of the law of progress by their historical development and by the systematic development corresponding to it. As however these subordinate groups of Molluscs are in themselves of no further special interest, I must refer to the sketch of their pedigree on p. 161, and to the detailed pedigree of Molluscs which I have given in my General Morphology, and I shall now at once turn to the consideration of the tribe of Star-fishes.

The Star-fishes (Echinoderma, or Estrellæ) among which are the four classes of Sea-stars, Sea-lilies, Sea-urchins, and Sea-cucumbers are one of the most interesting divisions of the animal kingdom, and yet we know less about them than about any. They all live in the sea. Every one who has been at the sea shore must have seen at least two of their forms, the Sea-stars and the Sea-urchins. The tribe of Star-fishes must be considered as a completely independent tribe of the animal kingdom on account of its very peculiar organization, and must be carefully distinguished from the Animal-plants—Zoophytes, or Cœlenterata, with which it is still frequently but erroneously classed under the name Radiata (as for example, by Agassiz, who even to this day defends this error of Cuvier’s, together with many others).

All Echinoderma are characterized, and at the same time distinguished from all other animals, by a very remarkable apparatus for locomotion, which consists of a complicated system of canals or tubes, filled with sea water from without. The sea water in these aqueducts is moved partly by the strokes of the cilia, or vibratile hairs lining their walls, and partly by the contractions of the muscular walls of the tubes themselves, which resemble india-rubber bags. The water is pressed from the tubes into a number of little hollow feet, which thereby become widely distended, and are then employed for walking and suction. The Sea-stars are moreover characterized by a peculiar calcareous formation in the skin, which in most cases forms a firm, well-closed coat of mail, composed of a number of plates. In almost all Echinoderma the body consists of five radii (counterparts, or antimera) standing round the main axis of the body, where they meet. It is only in some species of Sea-stars that the number of these radii amount to more than five—to 6-9, 10-12, or even to 20-40; and in this case the number of radii is generally not constant, but varies in different individuals of one species.

The historical development and the pedigree of the Echinoderma are completely revealed to us by their numerous and, in most cases, excellently preserved fossil remains, by their very remarkable individual developmental history, and by their interesting comparative anatomy; this is the case with no other tribe of animals, even the Vertebrata themselves are not to be excepted. By a critical use of those three archives, and by a careful comparison of the results derived from their study, we obtain the following genealogy of the Star-fishes, which I have already published in my General Morphology (vol. ii. Plate IV. pp. 62-77.)

The most ancient and original group of the Star-fishes, the primary form of the whole phylum, consists of the class of the true Sea-stars (Asterida). This is established by numerous and important arguments in anatomy and the history of development, but above all by the irregular and varying number of the radii, or antimera, which in all other Echinoderma is limited, without exception, to five. Every Star-fish consists of a central, small, body-disc, all round the circumference of which are attached five or several long articulated arms. Each arm of the Star-fish essentially corresponds in its organisation with an articulated worm of the class of Ring-worms, or Annelida (p. 149). I therefore consider the Star-fish as a genuine stock or cormus of five or more articulated worms, which have arisen by the star-wise growth of a number of buds out of a central mother-worm. The connected members, thus grouped like the rays of a star, have inherited from the mother-worm the common opening of the mouth, and the common digestive cavity (stomach) lying in the central body-disc. The end by which they have grown together, and which fuses in the common central disc, probably corresponds to the posterior end of the original independent worms.

In exactly the same way several individuals of certain kinds of worms are united so as to form a star-like cormus. This is the case in the Botryllidæ, compound Ascidians, belonging to the class of the Tunicata. Here also the posterior ends of the individual worms have grown together, and have formed a common outlet for discharges, a central cloaca; whereas at the anterior end each worm still possesses its own mouth. In Star-fishes the original mouths have probably become closed in the course of the historical development of the cormus, or colony, whereas the cloaca has developed into a common mouth for the whole cormus.

 

SYSTEMATIC SURVEY
Of the 4 Classes, 9 Sub-classes, and 20 Orders of Star-fishes.
(Compare Gen. Morph. II. Plate IV. pp. 62-67.)
Classes of Star-fishes.		Sub-classes of Star-fishes.		Orders of Star-fishes.			Systematic Name of the Orders.
I. Sea Stars Asterida		I. Sea Stars with radiated stomach Actinogastra		1.	Primary Stars	1.	Tecastra
				2.	Articulated Stars	2.	Colastra
				3.	Brisinga Stars	3.	Brisingastra
		II. Sea Stars with disc-shaped stomach Discogastra		4.	Serpent Stars	4.	Ophiastra
				5.	Tree Stars	5.	Phytastra
				6.	Lily Stars	6.	Crinastra
II. Sea Lilies Crinoida		III.Lilies with arms Brachiata		7.	Plated Lilies with arms	7.	Phatnocrinida
				8.	Articulated Lilies with arms	8.	Colocrinida
		IV. Lilies with buds Blastoidea		9.	Regularly budding Lilies	9.	Pentremitida
				10.	Lilies budding on two sides	10.	Eleutherocrina
		V. Bladder Lilies Cystidea		11.	Bladder Lilies without stalks	11.	Agelacrinida
				12.	Bladder Lilies with stalks	12.	Sphæronitida
III. Sea Urchins Echinida		VI. Older Sea Urchins (with more than 20 rows of plates) Palechinida		13.	Palechinida with more than 10 rows of ambulacral plates	13.	Melonitida
				14.	Palechinida with 10 rows of ambulacral plates	14.	Eocidaria
		VII. More recent Sea Urchins (with 20 rows of plates) Autechinida		15.	Autechinida with band-like ambulacra	15.	Desmosticha
				16.	Autechnidia with branching tentacles	16.	Petalosticha
IV. Sea Cucumbers Holothuriæ		VIII. Sea Cucumbers with aquatic feet Eupodia		17.	Eupodia with scutiform tentacles	17.	Aspidochirota
				18.	Eupodia with branching tentacles	18.	Dendrochirota
		IX. Sea Cucumbers without aquatic feet Apodia		19.	Apodia with water-lungs	19.	Liodermatida
				20.	Apodia without water-lungs	20.	Synaptida

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					Clypeastridæ
				Spatangidæ	│
			Dysasteridæ	│	│
		Aspidochirota	│ │
	Synaptida	│ │	│ │	Cassidulidæ Petalosticha
	│	│	│	│
	Liodermatida Apodia	│ │				Echinonidæ
	│	│	│			│
	│	│	Galeritidæ			│
	│	│		│	Echinometridæ	│
	│	│		│	│	│
	Dendrochirota Eupodia Holothuriæ		Salenidæ	Latistellæ
			│	│
	│
	│ │ │		Angustistellæ Desmosticha Autechinida
	│		│		Colocrinæ
Phytastra	│		│	Sphæronitidæ	│
│	│		Eocidaridæ	│	│	Eleutherocrina
│	│		│	│	│	│
Ophiastra Discogastra	│ │		│ │	Agelacrinæ Cystidea	│ │	│ │
│	│		│	│	│	│
│ │ │	│ │ │		Melonitida Palechinida Echinida	│ │ │	Phatnocrinæ Brahiata	│ │ │
Brisingastra	│ │		│ │	│ │	│ │	Pentremitida Blastoidea
│	│		│	│	│	│
Colastra
│ │ │	│ │ │			Brachiata Crinoida Crinastra
Tocastra Actinogastra Asterida
│
Phracthelminthes
│
Cœlomati
│
Gastræa

Hence the Star-fishes would be compound stocks of worms which, by the radial formation of buds, have developed out of true articulated worms, or Annelids. This hypothesis is most strongly supported by the comparative anatomy, and by the ontogeny of some Star-fishes (Colastra), and of segmented worms. The many-jointed Ring-worms (Annelida) in their inner structure are closely allied to the individual arms or radii of the Star-fishes, that is to the original single worms, which each arm represents. Each of the five worms of the Star-fish is a chain composed of a great number of equi-formal members, or metamera, lying one behind the other, like every segmented Worm, and every Arthropod. As in the latter a central nervous cord, the ventral nerve cord runs along the central line of the ventral wall of each segment. On each metameron there is a pair of non-jointed feet, and besides these, in most cases, one or more hard thorns or bristles similar to those of many Ring-worms. A detached arm of a Star-fish can lead an independent life, and can then, by the radially-directed growth of buds at one end, again become a complete star.

The most important proofs, however, of the truth of my hypothesis are furnished by the ontogeny or the individual development of the Echinoderma. The most remarkable facts of this ontogeny were first discovered in the year 1848 by the great zoologist, Johannes Müller of Berlin. Some of its most important stages are represented on Plates VIII. and IX. (Compare their explanation in the Appendix.) Fig. A on Plate IX. shows us a common Sea-star (Uraster), Fig. B, a Sea-lily (Comatula), Fig. C, a Sea-urchin (Echinus), and Fig. D, a Sea-cucumber (Synapta). In spite of the extraordinary difference of form manifested by these four representatives of the different classes of Star-fishes, yet the beginning of their development is identical in all cases. Out of the egg an animal-form develops which is utterly different from the fully developed Star-fish, but very like the ciliated larvæ of certain segmented Worms (Star-worms and Ring-worms). This peculiar animal-form is generally called the “larva,” but more correctly the “nurse” of these Star-fish. It is very small and transparent, swims about by means of a fringe of cilia, and is always composed of two equal symmetrical halves or sides. The fully grown Echinoderm, however—which is frequently more than a hundred times larger, and quite opaque—creeps at the bottom of the sea, and is always composed of at least five co-ordinate pieces, or antimera, in the form of radii. Plate VIII. shows the development of the “nurses” of the four Echinoderms represented on Plate IX.

The fully developed Echinoderm arises by a very remarkable process of budding in the interior of the “nurse,” of which it retains little more than the stomach. The nurse, erroneously called the “larva,” of the Echinoderm, must accordingly be regarded as a solitary worm, which by internal budding produces a second generation, in the form of a stock of star-shaped and connected worms. The whole of this process is a genuine alternation of generations, or metagenesis, not a “metamorphosis,” as is generally though erroneously stated. A similar alternation of generations also occurs in many other worms, especially in some star worms (Sipunculidæ), and cord worms (Nemertinæ). Now if, bearing in mind the fundamental law of biogeny, we refer the ontogeny of Echinoderma to their phylogeny, then the whole historical development of the Star-fishes suddenly becomes clear and intelligible to us, whereas without this hypothesis it remains an insoluble mystery. (Compare Gen. Morph. ii pp. 95-99.)

Besides the reasons mentioned, there are many other facts (principally from the comparative anatomy of Echinoderma) which most distinctly prove the correctness of my hypothesis. I established this hypothesis in 1866, without having any idea that fossil articulated worms still existed, apparently answering to the hypothetical primary forms. Such have in the mean time, however, really been discovered. In a treatise “On the Equivalent of the North American Taconic Schist in Germany,”3 Geinitz and Liebe, in 1867, have described a number of articulated Silurian worms, which completely confirm my suppositions. Numbers of these very remarkable worms are found in an excellent state of preservation in the slates of Würzbach, in the upper districts of Reusz. They are of the same structure as the articulated arm of a Star-fish, and evidently possessed a hard coat of mail, a much denser, more solid cutaneous skeleton than other worms in general. The number of body-segments, or metamera, is very considerable, so that the worms, although no more than a quarter or half an inch in breadth, attained a length of from two to three feet. The excellently preserved impressions, especially those of the Phyllodocites thuringiacus and Crossopodia Henrici, are so like the arms of many Star-fish (Colastra) that their true blood relationship seems very probable. This primæval group of worms, which are most probably the ancestors of Star-fish, I call Mailed worms (Phracthelminthes, p. 150.)

The three other classes of Echinoderma evidently arose at a later period out of the class of Sea-stars which have most faithfully retained the original form of the stellate colony of worms. The Sea-lilies, or Crinoida, differ least from them, but having given up the free, slow motion possessed by other Sea-stars, they have become adherent to rocks, etc., and form for themselves a long stalk. Some Encrinites, however (for example, the Comatulæ, Fig. B, on Plates VIII. and IX.), afterwards detach themselves from their stalk. The original worm individuals in the Crinoida are indeed no longer preserved in the same independent condition as in the case of the common star-fish; but they nevertheless always possess articulated arms extending from a common central disc. Hence we may unite the Sea-lilies and Sea-stars into a main-class, or branch, characterized as possessing articulated arms (Colobrachia).

In the other two classes of Echinoderma, the Sea-urchins and Sea-cucumbers, the articulated arms are no longer present as independent parts, but, by the increased centralization of the stock, have completely fused so as to form a common, inflated, central disc, which now looks like a simple box or capsule without arms. The original stock of five individuals has apparently degenerated to the form-value of a simple individual, a single person. Hence we may represent these two classes as a branch characterized as being without arms (Lipobrachia), equivalent to those which possess articulated arms. The first of these two classes, that of Sea-urchins (Echinida) takes its name from the numerous and frequently very large thorns which cover the hard shell, which is itself artistically built up of calcareous plates. (Fig. C, Plates VIII. and IX.) The fundamental form of the shell itself is a pentagonal pyramid. The Sea-urchins probably developed directly out of the group of Sea-stars. The different classes and orders of marine lilies and stars which are given in the following table, illustrate the laws of progress and differentiation in a striking manner. In each succeeding period of the earth’s history we see the individual classes continually increasing in variety and perfection. (Gen. Morph. ii. Plate IV.)

The history of three of these classes of Star-fish is very minutely recorded by numerous and excellently preserved fossils, but on the other hand, we know almost nothing of the historical development of the fourth class, that of the Sea-cucumbers (Holothuriæ). These curious sausage-shaped Star-fish manifest externally a deceptive similarity to worms. (Fig. D, Plates VIII. and IX.) The skeletal structures in their skin are very imperfect, and hence no distinct remains of their elongated, cylindrical, worm-like body could be preserved in a fossil state. However, from the comparative anatomy of the Holothuriæ, we can infer that they have arisen, by the softening of the cutaneous skeleton, from members of the class of Sea-urchins.

From the Star-fish we turn to the fifth and most highly developed tribe of the invertebrate animals, namely, the phylum of Articulata, or those with jointed feet (Arthropoda). As has already been remarked, this tribe corresponds to Linnæus’ class of Insects. It contains four classes: (1) the genuine six-legged Insects, or Flies; (2) the eight-legged Spiders; (3) the Centipedes, with numerous pairs of legs; and (4) the Crabs, or Crustacea, whose legs vary in number. The last class breathe water through gills, and may therefore be contrasted as the main-class of gill-breathing Arthropoda, or Gilled Insects (Carides), with the three first classes. The latter breathe air by means of peculiar wind-pipes, or tracheæ, and may therefore appropriately be united to form the main-class of the trachea-breathing Arthropoda, or Tracheate Insects (Tracheata).

In all animals with articulated feet, as the name indicates, the legs are distinctly articulated, and by this, as well as by the strong differentiation of the separate parts of the body, or metamera, they are sharply distinguished from Ringed worms, with which Bär and Cuvier classed them. They are, however, in every respect so like the Ringed worms that they can scarcely be considered altogether distinct from them. They, like the Ringed worms, possess a very characteristic form of the central nervous system, the so-called ventral marrow, which commences in a gullet-ring encircling the mouth. From other facts also, it is evident that the Arthropoda developed at a late period out of articulated worms. Probably either the Wheel Animalcules or the Ringed worms are their nearest blood relations in the Worm tribe. (Gen. Morph. ii. Plate V. pp. 85-102.)

Now, although the derivation of the Arthropoda from ringed Worms may be considered as certain, still it cannot with equal assurance be maintained that the whole tribe of the former has arisen out of one branch of the latter. For several reasons seem to support the supposition that the Gilled Arthropods have developed out of a branch of articulated worms, different from that which gave rise to the Tracheate Arthropods. But on the whole it remains more probable that both main-classes have arisen out of one and the same group of Worms. In this case the Tracheate Insects—Spiders, Flies, and Centipedes—must have branched off at a later period from the gill-breathing Insects, or Crustacea.

The pedigree of the Arthropoda can on the whole be clearly made out from the palæontology, comparative anatomy, and ontogeny of its four classes, although here, as everywhere else, many details remain very obscure. Not until the history of the individual development of all the different groups has become more accurately known than it is at present, can this obscurity be removed. The history of the class of Gilled Insects, or Crabs (Carides), is at present that best known to us; they are also called encrusted animals (Crustacea), on account of the hard crust or covering of their body. The ontogeny of these animals is extremely interesting and, like that of Vertebrate animals, distinctly reveals the essential outlines of the history of their tribe, that is, their phylogeny. Fritz Müller, in his work, “Für Darwin,”(16) which has already been referred to, has explained this remarkable series of facts in a very able manner.

Nauplius. Youth-form of six Crab-fish.

Pl. X.

Adult form of the same six Crab-fish.

Pl. XI.

 

The common primary form of all Crabs, which in most cases is even now the first to develop out of the egg, is originally one and the same, the so-called Nauplius. This remarkable primæval crab represents a very simple form of articulated animal, the body of which in general has the form of a roundish, oval, or pear-shaped disc, and has on its ventral side only three pairs of legs. The first of these is uncloven, the two subsequent pairs are forked. In front, above the mouth, lies a simple, single eye. Although the different orders of the Crustacean class differ very widely from one another in the structure of their body and its appendages, yet the early Nauplius form always remains essentially the same. In order to be convinced of this, let the reader look attentively at Plates X. and XI., a more detailed explanation of which is given in the Appendix. On Plate XI. we see the fully developed representatives of six different orders of Crabs, a Leaf-footed Crab (Limnetis, Fig. A c); a Stalked Crab (Lepas, Fig. D c); a Root Crab, (Sacculina, Fig. E c); a Boatman Crab (Cyclops, Fig. B c); a Fish Louse (Lernæocera, Fig. C c); and, lastly, a highly developed Shrimp (Peneus, Fig. F c). These six crabs vary very much, as we see, in the entire form of body, in the number and formation of the legs, etc. When, however, we look at the earliest stages, or “nauplius,” of these six different classes, after they have crept out of the egg—those marked with corresponding letters on Plate X. (Fig. A n-F n)—we shall be surprised to find how much they agree. The different forms of Nauplius of these six orders differ no more from one another than would six different “good species” of one genus. Consequently, we may with assurance infer a common derivation of all those orders from a common Primæval Crab, which was essentially like the Nauplius of the present day.

 

SYSTEMATIC SURVEY
Of the 7 Legions and 20 Orders of Crabs, or Crustacea.
Legions of the Crustaceæ.		Orders of the Crustaceæ.		Systematic name of the Orders.		Name of a Genus as an example.
I. Entomostraca, Lower Crustacea, or Segmented Crabs (not passing through the actual Zoëa form in youth).
I. Branchiopoda Gill-footed Crabs		1.	Primæval Crabs	1.	Archicarida	Nauplius
		2.	Leaf-foot Crabs	2.	Phyllopoda	Limnetis
		3.	Trilobites	3.	Trilobita	Paradoxides
		4.	Water Fleas	4.	Cladocera	Daphnia
		5.	Bivalve Crabs	5.	Ostracoda	Cypris
II. Pectostraca Fixed Crabs		6.	Barnacle Crabs	6.	Cirripedia	Lepas
		7.	Root Crabs	7.	Rhizocephala	Sacculina
III. Copepoda Oar-footed Crabs		8.	Boatmen Crabs	8.	Eucopepoda	Cyclops
		9.	Fish Lice	9.	Siphonostoma	Lernæocera
IV. Pantopoda No-body Crabs		10.	No-body Crabs	10.	Pycnogonida	Nymphon
V. Pœcilopoda Oar-footed Crabs		11.	Spear-tails	11.	Xiphosura	Limulus
		12.	Giant Crabs	12.	Gigantostraca	Eurypterus
II. Malacostraca, Higher Crustacea, or Mailed Crabs (passing through the Zoëa form in youth).
V. Pœcilopoda Oar-footed Crabs		13.	Zoëa Crabs	13.	Zoëpoda	Zoëa
		14.	Split-legged Crabs	14.	Schizopoda	Mysis
		15.	Mouth-footed Crabs	15.	Stomatopoda	Squilla
		16.	Ten-footed Crabs	16.	Decapoda	Peneus
VI. Edriophthalma Mailed Crabs with sessile eyes		17.	Cuma Crabs	17.	Cumacea	Cuma
		18.	Flea Crabs	18.	Amphipoda	Gammarus
		19.	Wizard Crabs	19.	Læmodipoda	Caprella
		20.	Crabs	20.	Isopoda	Oniscus

---

 

	Brachyura						Isopoda
	│					Læmodipoda	│
	Anomura					│	│
	│					Amphipoda	│
	│					│	│
	Macrura	Stomatopoda
	Decapoda	│				Cumacea
	│	│				Edriophthalma
	│	│				│
						│
	│					│
				chizopoda Podophthalma
				│
				Zoëpoda Malacostraca
	Gigantostraca			│			Rhizocephala
	│			│			│
	│			│	Siphonostoma		│
	│ │	Xiphosuræ		Zoëa	│ │		Cirripediæ Pectostraca
	│	│		│	│		│
	│	│		│	│		│
				│	│		│
	Pœcilopoda			│	│		│
	│		Nebaliæ	│	│		│
	│ │		│ │	│ │	Eucopepoda Copepoda		│ │
	Belinurœ		│ │	│ │	│ │		│ │	Pycnogonida Pantopoda
	│		│	│	│		│	│
	Trilobita		│	│	│		│	│
	│		Phyllopoda		│	Ostracoda	│	│
Cladoceræ	│		│		│	│	│	│
│	│		│		│	│	│	│
					│			│
Branchiopoda					│	│		│
│					│	│		│
	Nauplius
	│
	Archicaridæ
	(Articulated Worms)

The pedigree on p. 177 will show how we may at present approximately conceive the derivation of the twenty orders of Crustacea enumerated on p. 176, from the common primary form of the Nauplius. Out of the Nauplius form—which originally existed as an independent genus—the five legions of lower Crabs developed as diverging branches in different directions, which in the systematic survey of the class are united as Segmented Crabs (Entomostraca). The higher division of Mailed Crabs (Malacostraca) have likewise originated out of the common Nauplius form. The Nebalia is still a direct form of transition from the Phyllopods to the Schizopods, that is, to the primary form of the stalk-eyed and sessile-eyed Mailed Crabs. The Nauplius at this stage gives rise to another larva form, the so-called Zoëa, which is of great importance. The order of Schizopoda, those with cloven feet (Mysis, etc.), probably originated from this curious Zoëa; they are at present still directly allied, through the Nebalia to the Phyllopoda, those with foliaceous feet. But of all living crabs the Phyllopods are the most closely allied to the original primary form of the Nauplius. Out of the Schizopoda the stalk-eyed and sessile-eyed Mailed Crabs, or Malacostraca, developed as two diverging branches in different directions: the former through shrimps (Peneus, etc.), the latter through the Cumacea (Cuma, etc.), which are still living and closely allied to the Schizopoda. Among those with stalked eyes is the river crab (cray-fish), the lobster, and the others with long tails, or the Macrura, out of which, in the chalk period, the short-tailed crabs, or Brachyura, developed by the degeneration of the tail. Those with sessile eyes divide into the two branches of Flea-crabs (Amphipoda) and Louse-crabs (Isopoda); among the latter are our common Rock-slaters and Wood-lice.

The second main-class of Articulated animals, that of the Tracheata, or air-breathing Tracheate Insects4 (Spiders, Centipedes, and Flies) did not develop until the beginning of the palæolithic era, after the close of the archilithic period, because all these animals (in contrast with the aquatic crabs) are originally inhabitants of land. It is evident that the Tracheata can have developed only after the lapse of the Silurian period when terrestrial life first began. But as fossil remains of spiders and insects have been found, even in the carboniferous beds, we can pretty accurately determine the time of their origin. The development of the first Tracheate Insects out of gill-bearing Zoëa-crabs, must have taken place between the end of the Silurian and the beginning of the coal period, that is, in the Devonian period.

Gegenbaur, in his excellent “Outlines of Comparative Anatomy,”(21) has lately endeavoured to explain the origin of the Tracheata by an ingenious hypothesis. The system of tracheæ, or air pipes, and the modifications of organization dependent upon it, distinguish Flies, Centipedes, and Spiders so much from other animals, that the conception of its first origin presents no inconsiderable difficulties to phylogeny. According to Gegenbaur, of all living Tracheate Insects, the Primæval Flies, or Archiptera, are most closely allied to the common primary form of the Tracheata. These insects—among which we may especially mention the delicate Day flies (Ephemera), and the agile dragon-flies (Libellula)—in their earliest youth, as larvæ, frequently possess external tracheate gills which lie in two rows on the back of the body, and are shaped like a leaf or paint-brush. Similar leaf or paint-brush shaped organs are met with as real water-breathing organs or gills, in many crabs and ringed worms, and, moreover, in the latter as real dorsal appendages or limbs. The “tracheate gills,” found in the larvæ of many primæval winged insects, must in all probability be explained as “dorsal limbs,” and as having developed out of the corresponding appendages of the Annelida, or possibly as having really arisen out of similar parts in Crustacea long since extinct. The present tracheal respiration of the Tracheata developed at a later period out of respiration through the “tracheate gills.” The tracheate gills themselves, however, have in some cases disappeared, and in others become transformed into the wings of the Flies. They have disappeared entirely in the classes of Spiders and Centipedes, and these groups must accordingly be conceived of as degenerated or peculiarly developed lateral branches of the Fly class, which at an early period branched off from the common primary form of Flies; Spiders probably did so at an earlier period than Centipedes. Whether that common primary form of all Tracheata, which in my General Morphology I have named Protracheata, did develop directly out of genuine Ringed worms, or at first out of Crustacea of the Zoëa form (Zoëpoda, p. 212) will probably be settled at some future time by a more accurate knowledge and comparison of the ontogeny of the Tracheata, Crustacea, and Annelida. However, the root of the Tracheata, as well as that of the Crustacea, must in any case be looked for in the group of Ringed worms.

The genuine Spiders (Arachnida) are distinguished from Flies by the absence of wings, and by four pairs of legs; but, as is distinctly seen in the Scorpion-spiders and Tarantulæ, they, like Flies, possess in reality only three pairs of genuine legs. The apparent “fourth pair of legs” in spiders (the foremost) are in reality a pair of feelers. Among the still existing Spiders, there is a small group which is probably very closely allied to the common primary form of the whole class; this is the order of Scorpion-spiders, or Solifugæ, (Solpuga, Galeodes), of which several large species live in Africa and Asia, and are dreaded on account of their poisonous bite. Their body consists—as we suppose to have been the case in the common ancestor of the Tracheata—of a head possessing several pairs of feelers like legs, of a thorax, to the three rings of which are attached three pairs of legs, and of a hinder, body, or abdomen, consisting of many distinct rings. In the articulation of their body, the Solifugæ are therefore in reality more closely related to flies than to other spiders. Out of the Devonian Primæval Spiders, which were nearly related to the Solifugæ of the present day, the Long Spiders, the Tailor Spiders, and the Round Spiders probably developed as three diverging branches.

The Long Spiders (Arthrogastres), in which the earlier articulation of body has been better preserved than in Round Spiders, appear to be the older and more original forms. The most important members of this sub-class are the scorpions, which are connected with the Solifugæ through the Tarantella (or Phrynidæ). The small book scorpions, which inhabit our libraries and herbariums, appear as a degenerate lateral branch from the true scorpions. Mid-way between the Scorpions and Round Spiders are the long-legged Tailor-spiders (Opiliones) which have possibly arisen out of a special branch of the Solifugæ. The Pycnogonida, or No-body Crabs, and the Arctisca, or Bear Worms—still generally included among Long Spiders—must be completely excluded from the class of Spiders; the former belong to the Crustacea, the latter to Ringed worms.

SYSTEMATIC SURVEY
Of the 3 Classes and 17 Orders of the Tracheata.
Classes of Tracheata.		Sub-classes of Tracheata.		Orders of Tracheata.		Two Names of Genera as examples.
I. Spiders Arachnida		I. Long spiders Arthrogastres		1.	Scorpion spiders Solifugæ		Solpuga Galeodes
				2.	Tarantella Phrynida		Phrynus Thelyphonus
				3.	Scorpions Scorpioda		Scorpio Buthus
				4.	Book scorpions Pseudoscorpioda		Obisium Chelifer
				5.	Tailor spiders Opilionida		Phalangium Opilio
		II. Round spiders Sphærogastres		6.	Spinning spiders Araneæ		Epeira Mygale
				7.	Mites Acarida		Sarcoptes Demodex
II. Centipedes Scolopendria or Myriapoda		III. Simple-footed Chilopoda		8.	Simple-footed Chilopoda		Scolopendra Geophilus
		IV.Double-footed Diplopoda		9.	Double-footed Diplopoda		Julus Polydesmus
III. Flies Hexapoda		V. Chewing Masticantia		10.	Primitive flies Archiptera		Ephemera Libellula
				11.	Gauze-wings Neuroptera		Hemerobius Phryganea
				12.	Straight-wings Orthoptera		Locusta Forficula
				13.	Beetles Coleoptera		Cicindela Melolontha
				14.	Bee-wings Hymenoptera		Apis Formica
		VI. Sucking Sugentia		15.	Bugs Hemiptera		Aphis Cimex
				16.	Two-wings Diptera		Culex Musca
				17.	Butterflies Lepidoptera		Bombyx Papilio

---

 

			Butterflies Lepidoptera	Isopoda
	Bees Hymenoptera		│ │	Two-wings Diptera
	│		│	│
Beetles Coleoptera	│ │		│ │	│ │	Bugs Hemiptera
│	│		│	│	│
│				│	│
Straight-wings Orthoptera	Gauze wings Neuroptera
│	│			│
				Primæval Flies Archiptera
			Scorpions Archiptera	│ │	Double-footed Diplopoda
	Tailor Spiders Opiliones		│ │	│ │	│ │
	│ │	Book Scorpions Pseudoscorpioda	│ │	│ │	│ │
Mites Acarida	│ │	│ │	│ │	│ │	│ │
│	│			│	│
│ │	│ │	Tarantella Phrynida		│ │	│ │
Weaving Spiders Araneæ │ │	│ │ │ │	│ │ │ │		│ │ │ │	Simple-footed Chilopoda Centipedes Myriapoda
				│	│
Scorpion Spiders Solifugæ Spiders. Arachnida				│ │ │ │	│ │ │ │
│ │				Flies. Insecta Hexapoda	│ │
│				│	│
	Primary Air-breathing Arthropods Protracheata
	│
	Articulated Worms Coelminthes

 

Fossil remains of Long Spiders are found in the Coal. The second sub-class of the Arachnida, the Round Spiders (Sphærogastres), first appear in the fossil state in the Jura, that is, at a very much later period. They have developed out of a branch of the Solifuga, by the rings of the body becoming more and more united with one another. In the true Spinning Spiders (Araneæ), which we admire on account of their delicate skill in weaving, the union of the joints of the trunk, or metamera, goes so far, that the trunk now consists of only two pieces, of a head-breast (cephalo-thorax) with jaws, feelers, and four pairs of legs, and of a hinder body without appendages, where the spinning warts are placed. In Mites (Acarida), which have probably arisen by degeneration (especially by parasitism) out of a lateral branch of Spinning Spiders, even these two trunk pieces have become united and now form an unsegmented mass.

The class of Scolopendria, Myriapoda, or Centipedes, the smallest and poorest in forms of the four classes of Arthropoda, is characterized by a very elongated body, like that of a segmented Ringed worm, and often possesses more than a hundred pairs of legs. But these animals also originally developed out of a six-legged form of Tracheata, as is distinctly proved by the individual development of the millipede in the egg. Their embryos have at first only three pairs of legs, like genuine insects, and only at a later period do the posterior pairs of legs bud, one by one, from the growing rings of the hinder body. Of the two orders of Centipedes (which in our country live under barks of trees, in moss, etc.) the round, double-footed ones (Diplopoda) probably did not develop until a later period out of the older flat, single-footed ones (Chilopoda), by successive pairs of rings of the body uniting together. Fossil remains of the Chilopoda are first met with in the Jura period.

The third and last class of the Arthropoda breathing through tracheæ, is that of the Flies, or Insects, in the narrow sense of the word (Insecta, or Hexapoda), the largest of all classes of animals, and next to that of Mammalia, also the most important. Although Flies develop a greater variety of genera and species than all other animals taken together, yet these are all in reality only superficial variations of a single type, which is entirely and constantly preserved in its essential characteristics. In all Flies the three divisions of the trunk—head, breast (thorax), and hinder body—are quite distinct. The hinder body, or abdomen, as in the case of spiders, has no articulated appendages. The central division, the breast or thorax, has on its ventral side three pairs of legs, on its back two pairs of wings. It is true that, in very many Flies, one or both pairs of wings have become reduced in size or have even entirely disappeared; but the comparative anatomy of Flies distinctly shows that this deficiency has arisen only gradually by the degeneration of the wings, and that all the Flies existing at present are derived from a common, primary Fly, which possessed three pairs of legs and two pairs of wings. (Compare p. 256.) These wings, which so strikingly distinguish Flies from all other Arthropoda, probably arose, as has been already shown, out of the tracheate gills which may still be observed in the larvæ of the ephemeral flies (Ephemera) which live in water.

The head of Flies universally possesses, besides the eyes, a pair of articulated feelers, or antennæ, and also three jaws upon each side of the mouth. These three pairs of jaws, although they have arisen in all Flies from the same original basis, by different kinds of adaptation, have become changed to very varied and remarkable forms in the various orders, and are therefore employed for distinguishing and characterizing the main divisions of the class. In the first place, we may distinguish two main divisions, namely, Flies with chewing mandibles (Masticantia) and Flies with sucking mouths (Sugentia). On a closer examination each of these two divisions may again be divided into two sub-groups. Among chewing Flies, or Masticantia, we may distinguish the biting and the licking ones. Biting flies (Mordentia) comprise the most ancient and primæval winged Flies, the gauzy-winged (Neuroptera), straight-winged (Orthoptera), and beetles (Coleoptera). Licking flies (Lambentia) are represented by the one order of skin-winged (Hymenoptera) Flies. We distinguish two groups of Sucking Flies, or Sugentia, namely, those which prick and those which sip. There are two orders of pricking Flies (Pungentia), those with half wings (Hemiptera) and gnats and blow-flies (Diptera); butterflies are the only sipping Flies (Sorbentia), Lepidoptera.

Biting Flies, and indeed the order of Primæval Flies (Archiptera, or Pseudoneuroptera) are nearest akin to the still living Flies, and include the most ancient of all Flies, the primary forms of the whole class (hence also those of all Tracheata). Among them are, first of all, the Ephemeral Flies (Ephemera) whose larvæ which live in water, in all probability still show us in their tracheæ-gills the organs out of which the wings of Flies were originally developed. This order further contains the well known dragon-flies, or Libellula, the wine-glass sugar mites (Lepisma), the hopping Flies with bladder-like feet (Physopoda), and the dreaded Termites, fossil remains of which are found even in coal. The order of Gauze-winged Flies (Neuroptera), probably developed directly out of the primæval Flies, which differ from them only by their perfect series of transformations. Among them are the gauze-flies (Planipennia), caddis-flies (Phryganida), and fan-flies (Strepsiptera). Fossil Flies, which form the transition from the primæval Flies (Libellula) to the gauze-winged (Sialidæ), are found even in coal (Dictyophylebia).

The order of Straight-winged Flies (Orthoptera) developed at an early period out of another branch of the primæval Flies by differentiation of the two pairs of wings. This division is composed of one group with a great variety of forms—cockroaches, grasshoppers, crickets, etc. (Ulonata)—and of a smaller group consisting only of the well-known earwigs (Labidura), which are characterised by nippers at the hinder end of their bodies. Fossil remains of cockroaches, as well as of crickets and grasshoppers, have been found in coal.

Fossil remains of the fourth order of Biting Flies, beetles (Coleoptera) likewise occur in coal. This extremely comprehensive order—the favourite one of amateurs and collectors—shows more clearly than any other what infinite variety of forms can be developed externally by adaptation to different conditions of life, without the internal structure and the original form of the body being in any way essentially changed. Beetles have probably developed out of a branch of the straight-winged Flies, from which they differ only in their transformations (larva, pupa, etc.).

The one order of Licking Flies, namely, the interesting group of the Bees, or Skin-winged Flies (Hymenoptera), is closely allied to the four orders of biting Flies. Among them are those Flies which have risen to such an astonishing degree of mental development, of intellectual perfection, and strength of character, by their extensive division of labour, formation of communities and states, and surpass in this not merely most invertebrate animals, but even most animals in general. This may be said especially of all ants and bees, also of wasps, leaf-wasps, wood-wasps, gall-wasps, etc. They are first met with in a fossil state in the oolites, but they do not appear in greater numbers until the tertiary period. Probably these insects developed either out of a branch of the primæval Flies or the gauze-winged Flies.

Of the two orders of Pricking Flies (Hemiptera and Diptera), that containing the Half-winged Flies (Hemiptera), also called Beaked Flies (Rhynchota), is the older of the two. It includes three sub-orders, viz., the leaf-lice (Homoptera), the bugs (Heteroptera), and lice (Pediculina). Fossil remains of the first two classes are found in the oolites; but an ancient Fly (Eugereon) is found in the Permian system, and seems to indicate the derivation of the Hemiptera from the Neuroptera. Probably the most ancient of the three sub-orders of the Hemiptera are the Homoptera, among which, besides the actual leaf-lice, are the shield-lice, leaf-fleas, and leaf-crickets, or Cicadæ. Lice have probably developed out of two different branches of Homoptera, by continued degeneration (especially by the loss of wings); bugs, on the other hand, by the perfecting and differentiation of the two pairs of wings.

The second order of pricking flies, namely, the Two-winged Flies (Diptera), are also found in a fossil state in the oolites, together with Half-winged Flies; but they probably developed out of the Hemiptera by the degeneration of the hind wings. In Diptera the fore wings alone have remained perfect. The principal portion of this order consists of the elongated gnats (Nemocera) and of the compact blow-flies and house-flies (Brachycera), the former of which are probably the older of the two. However, remains of both are found in the oolitic period. The two small groups of lice-flies (Pupipara) forming chrysales, and the hopping-fleas (Aphaniptera), probably developed out of the Diptera by degeneration resulting from parasitism.

The eighth and last order of Flies, and at the same time the only one with mouth-parts adapted to sipping liquids, consists of moths and butterflies (Lepidoptera). This order appears, in several morphological respects, to be the most perfect class of Flies, and accordingly was the last to develop. For we only know of fossil remains of this order from the tertiary period, whereas the three preceding orders extend back to the oolites, and the four biting orders even to the coal period. The close relationship between some moths (Tineæ) and (Noctuæ), and some caddis-flies (Phryganida) renders it probable that butterflies have developed from this group, that is, out of the order of Gauze-winged Flies, or Neuroptera.

The whole history of Flies, and, moreover, the history of the whole tribe of Arthropoda, essentially confirms the great laws of differentiation and perfecting which, according to Darwin’s theory of selection, must be considered as the necessary results of Natural Selection. The whole tribe, so rich in forms, begins in the Archilithic period with the class of Crabs breathing by gills, and with the lowest Primæval Crabs, or Archicaridæ. The form of these Primæval Crabs, which were developed out of segmented worms, is still approximately preserved by the remarkable Nauplius, in the common larval stage of so many Crabs. Out of the Nauplius, at a later period, the curious Zoëa was developed, which is the common larval form of all the higher or mailed crabs (Malacostraca), and, at the same time, possibly of that Arthopod which at first breathed through tracheæ, and became the common ancestor of all Tracheata. This Devonian ancestor, which must have originated between the end of the Silurian and the beginning of the Coal period, was probably most closely related to the still living Primæval Flies, or Archiptera. Out of these there developed, as the main tribe of the Tracheata, the class of Flies, from the lowest stage of which the spiders and centipedes separated as two diverging branches. Throughout a long period there existed only the four biting orders of Flies—the Primæval flies, Gauze-wings, Straight-wings, and the Beetles, the first of which is probably the common primary form of the three others. It was only at a much later period that the Licking, Pricking, and Sipping flies developed out of the Biting ones, which retained the original form of the three pairs of jaws most distinctly. The following table will show once more how these orders succeeded one another in the history of the earth.

 

CLASSIFICATION OF FLIES.
A. Flies with Chewing Mouths Sugentia		I. Biting Flies Mordentia		1.	Primæval winged Archiptera		M.I. A.A.
				2.	Gauze-winged Neuroptera		M.C. A.A.
				3.	Straight-winged Orthoptera		M.I. A.D.
				4.	Beetles Coleoptera		M.C. A.D.
		II. Licking Flies Lambentia		5.	Skin-winged Hymenoptera		M.C. A.A.
B. Flies with Sucking Mouths Sugentia		III. Stinging Flies Pungentia		6.	Half-winged Hemiptera		M.I. A.A.
				7.	Tway-flies Diptera		M.C. A.D.
		IV. Sipping Flies Sorbentia		8.	Butterflies Lepidoptera		M.C. A.A.

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