The Vertebrata of Lamarck and Cuvier included beasts, birds, reptiles, and fishes, and until recently the group was considered as one of the most sharply limited in the animal kingdom. The progress of anatomical studies very soon rendered it clear that all Vertebrata did not possess bony vertebrae; for, besides the commoner sharks and skates, with their purely cartilaginous skeletons, naturalists became acquainted with the structure of fishes, such as the sturgeons and the lampreys, which possess no vertebrae at all, but merely a continuous elastic rod (the notochord) in the place of the jointed spinal column. The muscles and their skeletal septa were seen in these fishes to be arranged in a series of segments attached to the sides of this continuous rod; and hence the structural character of bony vertebrae, as distinguishing the Vertebrata, gave place to the character of segmental arrangement of the muscles of the body-wall, such muscles being supported by a skeletal axis which might be itself unsegmented (notochord), or replaced by segmental cartilaginous or bony vertebrae. The studies of embryologists furnished a sound foundation for this conception by demonstrating that in the embryos of Vertebrata with true vertebrae these structures are preceded by an unsegmented continuous notochord. The inquiry into the structural characteristics of Vertebrata led further to the recognition of several additional points of structure, the combination of which was present only in the group which had been recognized by Lamarck on superficial grounds. It was found that all Vertebrata possess laterally-placed passages leading from the pharynx to the exterior, serving in the aquatic forms as the exits for water taken in by the mouth, and provided with vascular branchial processes, whilst in the embryos of the higher air-breathing classes they appear only as temporary structures. It was further established that the great mass of nervous tissue lying dorsally above the spinal column, and known as the cerebro-spinal nerve-centre or brain and spinal cord, is in all cases a tube, and originates as part of the dorsal surface of the embryo, which becomes depressed in the form of a long groove and finally closed in by the adhesion of its opposite edges, thus forming a tube or canal. The three structures,—notochord, gill-slits, and tubular dorsal nerve-cord,—were more than twenty years ago recognized as characterizing, together with the metameric segmentation of the musculature of the body-wall, all Vertebrata at some one or other period of their existence.

The establishment by Darwin of the doctrine of organic evolution in 1859 led naturalists consciously to make the attempt to determine the genetic affinities and the probable ancestry of the various groups of animals, and enabled them to recognize in the classifications by “type”, and other such conceptions of earlier systematists, the unconscious striving after genealogical representation of the relationships of organic beings. The question naturally arose in regard to the Vertebrata, as in regard to other great divisions of the animal kingdom, What were the characters of the earliest forms, the ancestors of those now living? Then came the further questions as to whether any surviving Vertebrata closely resemble the ancestral form, and whether any animals are still in existence which retain the general characters of those primeval forms which were the common ancestors at once of Vertebrates and of other large and equally well-marked phyla or branches of the animal kingdom, such as the Molluscs, the , &c. This fascinating subject of inquiry received its most important impulse from the embryological investigations of the Russian naturalist Kowalewsky, and has been for nearly a quarter of a century the fertile source of speculation and its indispensable accompaniments, new observation and research. Kowalewsky published in 1866 an account of the embryology of the lowest and simplest of then recognized Vertebrates, the lancelet (Amphioxus lanceolatus), in which he attempted to trace, cell for cell from the fertilized egg-cell, the origin of the characteristic Vertebrate organs of this animal (2 ). This work alone would not have acquired historic importance, although it is the starting-point of what may be called strict cellular embryology, as compared with the less severely histological works of previous students. But it was accompanied by an account (3 ) of the development of Ascidia mamillata, one of the so-called Tunicate Molluscs, in which it was demonstrated by Kowalewsky, not only that this supposed Mollusc possesses when first hatched from its egg-envelope a notochord, pharyngeal gill-slits, and a tubular dorsal nerve-cord and brain, but that these three characteristically Vertebrate features of organization originate from the same cell-layers of the embryo, and in essentially the same way as in Amphioxus, whilst the cell-layers themselves originate from the egg-cell in the two animals by precisely similar movements of cell division and invagination (see figs. 1 and 2). Kowalewsky’s discoveries established once for all that the Ascidian tadpole is identical in three very special and distinct features of structure with the frog’s tadpole. No classification which pretended to set forth the genetic affinities of animals could henceforth separate the Ascidian from the Vertebrata, and with it the Ascidian brought the whole series of Tunicata.

The admission of Tunicata as a group of Vertebrata was proposed by the present writer as long ago as 1877 (4 ); but it required the intermediate proposition by Balfour of a group Chordata, to comprise the two divisions Tunicata and Vertebrata, in order to render the final admission of Tunicata to their proper association with the Vertebrata of Cuvier palatable to systematists. As an objection to the simple inclusion of Tunicata in the great phylum Vertebrata it has been urged that Tunicata do not possess vertebrae,—a proposition which is equally true of Amphioxus and of some Fishes. Shifting the objection, some writers have maintained that the vertebration of the Vertebrata may be understood as having reference to the segmentation of the muscles of the body-wall, which is exhibited by all Cuvier’s Vertebrata without exception, inclusive of Amphioxus, though not by Tunicata. To this it may be replied that the Ascidian tadpole, and more clearly the free-swimming Tunicate Appendicularia (see fig. 9), do exhibit a segmentation of the muscles of the hinder part of the body-wall similar to and identical with that of Amphioxus, whilst no such strict application of a name in its original descriptive sense is desirable in systematic nomenclature. All Gastropoda (it has been pointed out) are not gastropodous; all Arthropoda are not arthropodous; and many Echinoderma are not echinodermous. It is, in the present writer’s opinion, better to retain an historic and familiar name for the great branch of the animal pedigree to which it has become necessary to admit forms whose affinities therewith were at one time unsuspected rather than to sacrifice historical significance to a futile striving after etymological accuracy.

The admission of Tunicata to association with Cuvier’s Vertebrata has been followed by a further innovation. The remarkable marine worm Balanoglossus— originally described by Della Chiaje at the end of the 18th century— was shown in 1866 by Kowalewsky (5) to possess a series of pharyngeal gill-slits similar to those of Tunicata and Amphioxus. Later researches by Bateson (ό) have demonstrated that Balanoglossus develops in embryonic life a short notochord, whilst its nerve-cord is, in part at least, tubular, and similar in position and relations to the median epidermal tract by the infolding of which the nerve-tube of Tunicata and the other Vertebrata is formed. Hence it seems impossible to exclude Balanoglossus from a place in the phylum Vertebrata. The possession of pharyngeal gill-slits alone might not justify the association; but, when this is combined with the presence of the notochord, though rudimentary and with a special condition and position of the main nerve-centre, it becomes impossible to ignore Balanoglossus in our conception of the Vertebrate phylum. The recent discoveries of Harmer (7 ) with regard to Cephalodiscus will hereafter render it necessary to associate that form, and in all probability Rhabdopleura also, with Balanoglossus in the Vertebrate phylum. The further consequences of such an association in regard especially to the affinities of Polyzoa and of Gephyraea open up a large field of speculation and of consequent embryological and anatomical research.

The Cuvierian Vertebrata, Amphioxus, Tunicata, and Balanoglossus being thus indisputably connected by a remarkable combination of structural points, which admit of no explanation consistent with the principles of evolutional morphology except that of the genetic relationship of the forms thus enumerated, we are at once confronted by those questions as to the ancestral history of Vertebrata which have been already mentioned above as stimulated by Kowalewsky’s discoveries. Undoubtedly Amphioxus is lower and simpler in structure than any Fish, Tunicata as low as or lower than Amphioxus, and Balanoglossus, in some respects, more archaic than either Amphioxus or the Ascidian tadpole. The first tendency arising from the discovery of the affinities of these simpler forms with the Cuvierian Vertebrata was to see in them the representatives of the ancestors of all Vertebrata. Amphioxus has been pointed to by authorities in morphology as the living presentation of our common Vertebrate ancestor; a similar position corresponding to an earlier stage of development has been admitted by no less an authority than Darwin for the Ascidian. It appears, nevertheless, that all such simple solutions of the problem of Vertebrate ancestry are without warrant. They arise from a very common tendency of the mind, against which the naturalist has to guard himself,—a tendency which finds expression in the very widespread notion that the existing anthropoid apes, and more especially the gorilla, must be looked upon as the ancestors of mankind, if once the doctrine of the descent of man from ape-like forefathers is admitted. A little reflexion suffices to show that any given living form, such as the gorilla, cannot possibly be the ancestral form from which man was derived, since ex hypothesi that ancestral form underwent modification and development, and in so doing ceased to exist. The same considerations apply to the question of the ancestry of Vertebrata. Probably no existing low form of Vertebrate closely represents the ancestral form by the modification of which higher forms have been developed. We have no justification for assuming that such low forms do more than present to us a collateral branch of the family, and that collateral branch must, in all probability, have experienced its own special series of modifications of structure. Not only this, but we have no sufficient ground for assuming that, even in respect of the simplicity of their structure, any given animal forms at present existing exhibit a mere survival of a corresponding degree of simplicity in their remote ancestors. Such an assumption was almost universally made, until a more correct view was pressed on the attention of naturalists by Dr Anton Dohrn, the founder of the zoological station of Naples (8 ). So far from its being the case that simplicity of organization necessarily implies the continuous hereditary transmission of a low stage of structural development from remote ancestors, there are numerous instances in which it is certain that the existing simplicity of structure is due to a process of degeneration, and that an existing form of simple structure is thus descended from ancestors of far higher complexity of organization than itself. Such are various parasitic worms and Crustacea. The evidence in favour of the occurrence of progressive simplification of structure or degeneration, in place of progressive elaboration, depends (1) upon the comparison of the adult structure of the degenerate organism with that of its nearest allies, by which it is often rendered clear that the ensemble of the organization of the simpler organism cannot be explained on the hypothesis that it represents an ancestral or archaic condition common to it and its more elaborate congeners, and (2) on the direct evidence of individual development or life-history. The latter evidence is conclusive, when we find, as in the case of Cirrhipede Crustaceans and of Ascidian Tunicates, that the embryo on its way to the adult condition passes through-stages of development presenting a higher degree of organization than that ultimately reached, so that, as in the Cirrhipede larva and the Ascidian tadpole, the young form resembles allied organisms of a higher stage of development, and subsequently degenerates from the point of progressive elaboration to which it had attained, and becomes greatly simplified in the final stage of its growth. Conclusive as such evidence is, there is no law of development which necessitates its preservation. If it be an advantage to the organism, the full force of heredity has play, and what are called the “recapitulative phases” of ancestral development are passed through by the individual in the course of development from the egg. But with remorseless thoroughness all such hereditary tendencies may be removed when such removal is an advantage to the organism, and the development from the egg may proceed directly to the adult degenerate form. Such is the case with many Tunicata, the young of which never exhibit notochord and tadpole form; indeed, were it not for the preservation of a few exceptional cases, like that of the Ascidian section of the group, we should have no direct evidence of the degeneration of Tunicata from tadpole-like ancestry.

The general result of the considerations which have been urged with regard to degeneration is this, that it is prima facie as legitimate an hypothesis, that any existing animal has developed by progressive simplification from more elaborate ancestors, as it is that such an animal has developed by a continuous and unbroken progress in elaboration from simpler ancestors; and we are specially called upon to apply the hypothesis of degeneration where the animal under consideration is likely from its mode of life to have undergone that process. Such modes of life, tending to degeneration, are parasitism, sessile or adherent habit, burrowing in the sea-bottom, and diffuse feeding. The animal which pursues living prey, and contends with other organisms for the dominion of the regions of earth and water that are flooded with light and richly supplied with oxygen gas, is the animal which represents the outcome of a longer or shorter period of progressive elaboration. It is worth while noting in parenthesis that in all cases the “whirligig of time” has probably brought its revenges, and that the ancestry of a form evolved through a long period of progressive elaboration was at an antecedent period subject to simplification and degeneration, whilst in the past records of the present exemplars of the latter process there must certainly have been long stretches of continuous elaboration.

Applying these considerations to the construction of the genealogical tree of Vertebrata, we find that the task is by no means simplified. We cannot with the earliest evolutionists adopt a scale or ladder-like series, placing the simplest form on the lowest step ; nor can we be satisfied with a tree-like arrangement, in which the forms at the ends of the branches are always more elaborate than those nearer the trunk. Our genealogical tree will more strictly conform to that of a parvenu human family, if we take worldly prosperity in the latter case as corresponding to elaboration of structure in the former. The strict family genealogist will include in the successive ramifications of the tree the five sons of the founder of the family, one of whom remained an agricultural labourer, whilst two became brewers and two emigrated. The cousins in the next generation will be set forth in place, the sons of one brewer becoming paupers, whilst those of the other advance to the position of Government employes, and one to the peerage. Thus in successive branchings of the family history there may be alternate progress and degeneration. And so it must be in the genealogical trees constructed by the naturalist: the fact that a branch is later in origin will not imply that it is higher in elaboration than those below it, and accordingly we must not expect to draw our tree so as to be able to trace all simpler forms to lower off-sets of the tree.

The structural features of those animals which must be admitted to the Vertebrate phylum in consequence of possessing notochord, pharyngeal gill-slits, and dorsal nerveplate, tubular or unrolled, are such as enable us very readily to group them in four great divisions, which appear to be equally distinct from one another. As to what may be the genetic relations to one another of these four groups we will inquire subsequently; for the present we term these groups “branches.” They are as follows :— [table]

The Vertebrata thus limited may be defined as Ccelomate Enterozoa (Metazoa) with well-developed coelom. In all, with the exception of the more degenerate members of Urochorda, an elastic skeletal rod—the notochord—is developed dorsally by an outgrowth of cells forming the wall of the primitive archenteron; the notochord may or may not persist in adult life. Pharyngeal gill-slits, which may or may not persist in adult life, are developed in all Vertebrates. In all, except in certain Urochorda, the chief nerve-centre has the form of a dorsal, median, elongate tract, derived from the epiblast, which becomes sunk below the surface and invaginated so as to form a tube. In all there is a tendency to metameric repetition of parts, which may find its expression in a strongly-marked segmentation of the musculature of the body-wall and its skeleton, or may be recognizable only in a limited degree, as exhibited by the successive gill-slits or successive gonads.

We shall now examine the distinctive features of each of these large groups, and form an estimate of their relations to one another, and of their probable ancestry, this being the task to which we must limit ourselves in the brief space here afforded.

The Craniata.

The Craniata are Vertebrata in which the tubular cerebro-spinal nerve-mass is swollen anteriorly to form a brain, consisting primarily of three successive vesicles, in connexion with the anterior of which the special nerves of the olfactory organs and of the eyes originate. The notochord, whilst extending posteriorly to the extremity of the body, does not reach quite so far forward anteriorly as the termination of the nerve-tube. A cartilaginous cranium or brain-case develops round the anterior extremity of the nerve-cord, and rises up laterally so as to enclose and protect the brain (hence Craniata). Cartilage is developed in other parts of the body as a skeletal substance, though it may be subsequently replaced in the cranium, as elsewhere, by bone. The longitudinal muscles of the body-wall are divided by transverse fibrous septa into a series of segments, varying in the adult from ten to one hundred or more in number. Cartilaginous neural arches, corresponding in number and position to the fibrous septa, and resting on the notochord, are developed so as to protect the nerve-cord. Cartilaginous bars also pass outwards, with a direction at first horizontal and then ventral, from the sides of the notochord into the intermuscular fibrous septa. Very generally, but not always, a tubular cartilaginous sheath forms round the notochord; this sheath with rare exceptions becomes segmented to form a series of vertebral bodies, which lie in the planes of the fibrous intersegmental septa, and, increasing in thickness by encroaching upon the substance of the notochord, finally obliterate it almost entirely.

The pharyngeal slits follow closely upon the mouth, and in existing Craniata never number more than eight pairs (see fig. 3). They are separated from one another and their apertures strengthened by a series of cartilaginous hoops, the first of which, that between the mouth and the first gill-slit, forms the primitive upper and lower jaw in all but the small and degenerate group Cyclostoma. The gill-slits when functional are generally protected by an opercular fold of the body-wall, which overhangs them and corresponds to the epipleural fold of Amphioxus, the collar of Balanoglossus, and doubtfully to the wall of the atrial chamber of Urochorda. The extension of this fold along the sides of the middle third of the body (between the pharyngeal region and the anus) acquired in ancestral Craniata the function of a continuous right and left lateral fin (see fig. 4). At the...