The History of Creation Volume II Part 6

The lower and less perfect of the two legions of the Corolliflorae are the star-flowers (also called Diapetalae or Dialypetalae). To them belong the extensive families of the Umbelliferae, or umbrella-worts (wild carrot, etc.), the Cruciferae, or cruciform blossoms (cabbage, etc.); further, the Ranunculaceae (b.u.t.tercups) and Cra.s.sulaceae, 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 Diapetalae the blossom-leaves remain separate, and never grow together, as is the case in the Gamopetalae. These latter developed first in the tertiary period out of the Diapetalae, whereas the Diapetalae appeared in the chalk period together with the Apetalae.

The highest and most perfect group of the vegetable kingdom is formed by the second division of the Corolliflorae, namely, the legion of bell-flowers (Gamopetalae, also called Monopetalae or Sympetalae). 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 (l.a.b.i.atae) 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 Gamopetalae appear in the organic history of the earth later than all the main groups of the vegetable kingdom-in fact, not until the caenolithic 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 Algae. To these are added, in the primary period, the higher and more perfect Cryptogamia, especially the main-cla.s.s 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-cla.s.s, that of Gymnosperms.

It was not until the secondary period that the higher main-cla.s.s, 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 Apetalae. It was not until the chalk period that the higher Corolliflorae 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 Diapetalae, and only at quite a late date, in the tertiary period, did the more highly developed Bell-blossoms, Gamopetalae, 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.

CHAPTER XVIII.

PEDIGREE AND HISTORY OF THE ANIMAL KINGDOM.

I. ANIMAL-PLANTS AND WORMS.

The Natural System of the Animal Kingdom.-Linnaeus and Lamarck's Systems.-The Four Types of Bar 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 Primaeval 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 Cla.s.ses, and Thirty-eight Cla.s.ses.-Primaeval Animals (Monera, Ambae, Synambae), Gregarines, Infusoria, Planaeades, and Gastraeades (Planula and Gastrula).-Tribe of Zoophytes.-Spongiae (Mucous Sponges, Fibrous Sponges, Calcareous Sponges).-Sea Nettles, or Acalephae (Corals, Hood-jellies, Comb-jellies).-Tribe of 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 cla.s.sification made in the last century followed the path of the artificial system, which was first established in a consistent manner by Charles Linnaeus. 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 relations.h.i.+p) the basis of cla.s.sification, but only employs individual, and for the most part external, characteristics, which readily strike the eye. Thus Linnaeus distinguished his twenty-four cla.s.ses of the vegetable kingdom princ.i.p.ally by the number, formation, and combination of the stamens. In like manner he distinguished six cla.s.ses in the animal kingdom princ.i.p.ally by the nature of the heart and blood. These six cla.s.ses were: (1) Mammals; (2) Birds; (3) Amphibious Animals; (4) Fishes; (5) Insects; and (6) Worms.

But these six animal cla.s.ses of Linnaeus 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 cla.s.ses as vertebrate animals (Vertebrata), and put them in contrast with the remaining animals (the insects and worms of Linnaeus), 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 ill.u.s.trious zoologists, Carl Ernst Bar 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 Bar, 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 relations.h.i.+ps 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 Bar 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 Linnaeus' first four cla.s.ses-mammals, birds, amphibious animals, and fishes. The second type is formed by the articulated animals (Articulata), containing Linnaeus' 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 Bar and Cuvier was the greatest advance in the cla.s.sification of animals since the time of Linnaeus. 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-cuc.u.mbers; and, on the other hand, the _Animal-plants_, or _Zoophytes_ (Clenterata 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 Linnaeus' 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 Linnaeus, 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, palaeontology 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 relations.h.i.+p. 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 Bar and Cuvier's time to designate the few princ.i.p.al 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?

[Ill.u.s.tration: _PL. VI._

Historical Growth of the six great stems of Animals. _See the Explanation._]

At first we might be inclined to answer this question in a _polyphyletic_ sense, by saying that we must a.s.sume, 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 primaeval 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 a.s.sumption 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 a.s.sumed 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 primaeval animal-which may still be seen in a living state in the _Ambae_ of the present day-as the _second_ step in the series of phylogenetic forms of the animal kingdom. Like the still living simple Ambae, and like the naked egg-cells of many lower animals (for example, of Sponges and Medusae, etc.), which cannot be distinguished from them, the remote phyletic primary Ambae also were perfectly simple naked-cells, which moved about in the Laurentian primaeval ocean, creeping by means of the ever-changing processes of their body-substance, and nouris.h.i.+ng and propagating themselves in the same way as the Ambae of the present day. (Compare vol. i. p. 188, and vol. ii. p. 54.) The existence of this Amba-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, equi-formal, and equivalent cells. Even at the present day, in the ontogenetic development of every animal egg-cell, there first arises a globular heap of equi-formal naked cells, by the repeated self-division of the primary cell. (Compare vol. i. p. 190 and the Frontispiece, Fig. 3.) We called this acc.u.mulation 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 circ.u.mstance 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 Amba-like primaeval cells, one similar to the other. We shall call this most ancient community of Ambae-this most simple acc.u.mulation of animal cells-which is recapitulated in individual development by the Morula-the _Synamba_.

Out of the Synambae, in the early Laurentian period, there afterwards developed a fourth primary form of the animal kingdom, which we shall call the ciliated germ (Planaea). This arose out of the Synamba 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 Synambae consisted of completely equi-formed and naked cells, and crept about slowly, at the bottom of the Laurentian primaeval ocean, by means of movements like those of an Amba. The Planaea, on the other hand, consisted of two kinds of different cells-inner ones like the Ambae, and external "ciliated cells." By the vibrating movements of the cilia the entire multicellular body acquired a more rapid and stronger motion, and pa.s.sed 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 ca.n.a.l; 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, Medusae, 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_ | +Ontogenesis.+ | +Phylogenesis.+ of the five first stages | The five first stages | The five first stages of the development of | of the individual | of the phyletic or the animal body. | development. | historical development.

-------------------------------+------------------------------+------------------------ | | _First Stage of Development._ | 1. | 1.

| +Monerula.+ | +Moneron.+ A simple cytod (a | | plastid without a kernel.) | Animal egg without a | Most ancient animal | | kernel (when the egg-kernel | Monera, originating by | | has disappeared, | spontaneous generation.

| | after being fructified). | | | | | | | | | | | | _Second Stage of Development._ | 2. | 2.

| +Ovulum.+ | +Ambae.+ A simple cell (a | | plastid containing a | Animal egg with kernel | Animal Ambae.

kernel.) | (a simple egg-cell). | | | | | | | | | | | | | | | | | _Third Stage of Development._ | 3. | 3.

| +Morula.+ | +Synamba.+ A community (an | (_Mulberry form._) | aggregation of identical | | An aggregation of simple cells). | Globular heap of h.o.m.ogeneous | Ambae.

| | "cleavage spheres." | | | | | | | | | | | | _Fourth Stage of Development._ | 4. | 4.

| +Planula.+ | +Planaea.+ A solid or bladder-shaped, | (_Ciliated larva_.) | globular, or oval | | Many-celled primaeval body, _composed of two | Many-celled larva | animal without kinds of different cells_: | without mouth, composed | mouth, composed of externally ciliated, | of different cells. | two kinds of different internally non-ciliated | | | cells.

cells. | | | | | | | | | | | | | | _Fifth Stage of Development._ | 5. | 5.

| +Gastrula.+ | +Gastraea.+ A globular or oval | (_Larva with mouth._) | _body with simple intestinal | Many-celled with intestines | Many-celled primaeval cavity and mouth-opening. | and mouth; intestinal | animal with intestine Body wall composed | wall with two | and mouth; intestinal of two layers_; an | layers. | wall with two externally ciliated ectoderm | | layers. (Primary form (dermal layer), an | | of zoophytes and internally non-ciliated | | worms.) entoderm (gastral layer). | |

From the ontogenetic occurrence of the Gastrula in the most different animal cla.s.ses, 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 animal tribes, which in all essential points was formed like the Gastrula, and which we shall call the Gastraea. This Gastraea 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 Gastraea 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 Gastraea 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 Gastraea; 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 Gastraea; out of it develops the inner membrane (epithelium) of the intestinal ca.n.a.l 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 Amba; (3) the Synamba; (4) the Planaea; and (5) the Gastraea. 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 cla.s.s 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 Primaeval animals (Protozoa), which also comprises the still living Infusoria and Gregarinae.

The phyletic development of the six higher animal tribes, which are all derived from the Gastraea, deviated at this point in two directions. In other words, the _Gastraeads_ (as we may call the group of forms characterized by the Gastraea-type of structure), divided into two divergent lines or branches; the one branch of Gastraeads 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 Gastraeads 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 cla.s.ses, 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 relations.h.i.+p 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 Primaeval animals (Protozoa), including the Infusoria and Gastraeads. 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 cla.s.ses distinguished in them, have pa.s.sed through (p. 132). There is a much larger number of cla.s.ses 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 cla.s.ses and nineteen cla.s.ses, we have to distinguish, at least, sixteen main cla.s.ses and thirty-eight cla.s.ses 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 _Primaeval animals_ (Protozoa) within the compa.s.s which we here a.s.sign 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 Gregarinae, 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 cla.s.ses or provinces of real Protozoa, namely, _Egg animals_ (Ovularia) and _Germ animals_ (Blastularia). To the former belong the three cla.s.ses of Archezoa, Gregarinae, and Infusoria, to the latter the two cla.s.ses of Planaeads and Gastraeads.

SYSTEMATIC SURVEY

_Of the 16 Main Cla.s.ses and 38 Cla.s.ses of the Animal Kingdom._

------------------+-----------------------+----------------------+----------------- _Tribes or Phyla_ | _Main Cla.s.ses_, | _Cla.s.ses_ |_Systematic Name_ _of the_ |_Branches or Clades_ | _of the_ | _of the_ _Animal Kingdom._ | _of the_ | _Animal Kingdom._ | _Cla.s.ses._ | _Animal Kingdom._ | | ------------------+-----------------------+----------------------+------------------

A. { =Primaeval= { I. Egg-animals { 1. Archaic animals 1. Archezoa =Animals= { _Ovularia_ { 2. Gregarines 2. Gregarinae { { 3. Infusoria 3. Infusoria +Protozoa+ { {II. Mulberry animals { 4. Planaeads 4. Planaeadas { _Blastularia_ { 5. Gastraeads 5. Gastraeadas

B. { =Animal= { III. Sponges { 6. Sponges 6. Porifera =Plants= { _Spongiae_ { { +Zoophyta+ { IV. Sea-nettles { 7. Corals 7. Coralla { _Acalephae_ { 8. Hood-jellies 8. Hydromedusae { { 9. Comb-jellies 9. Ctenophora