Volume II Part 6 (1/2)

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_.