CHAPTER XV
EARLY THEORIES ON THE ORIGIN OF VERTEBRATES
Haeckel and Gegenbaur set the fashion for phylogenetic speculation, and up to the middle 'eighties, when the voice of the sceptics began to make itself heard, the chief concern of the younger morphologists was the construction of genealogical trees. The period from about 1865 to 1885 might well be called the second speculative or transcendental period of morphology, differing only from the first period of transcendentalism by the greater bulk of its positive achievement. It must be remembered that the later workers (at least towards the end of this period) had immense advantages over their predecessors in the matter of equipment and technique; they possessed well-fitted laboratories in the university towns and by the sea; they had at their command perfected microscopes and microtomes; while the whole new technique of microscopical anatomy with its endless variety of stains and reagents made it possible for the tyro to confirm in a day what von Baer and Müller had taken weeks of painful endeavour to discover.[386] But the democratisation of morphology which followed upon the facilitation of its means of research left an evil heritage of detailed and unintelligent work to counterbalance the very great and real advances which technical improvements alone rendered possible.
This period of rapid development, which set in soon after the coming of evolution and multiplied the concrete facts of morphology an hundredfold, may for our present purpose be conveniently divided into two somewhat overlapping periods, of which the second may be said to begin with the enunciation by Haeckel of his Gastræa theory. Within the first period fall the evolutionary speculations associated with the names of Kowalevsky, Dohrn, Semper, and others; the characteristic of the second period is the preponderating influence exercised upon phylogenetic speculations by the germ-layer doctrine in its two main evolutionary developments, the Gastræa and Cœlom theories.
In the first period we might again distinguish two main tendencies, according as speculations were based mainly upon anatomical or mainly upon embryological considerations, and it so happens that these two tendencies are very well illustrated by the various theories as to the origin of Vertebrates which began to appear towards the 'seventies. We shall accordingly, in this chapter, consider very briefly the history of the earlier views on the phylogeny of the vertebrate stock.
In the early days, before the other claimants to the dignity of ancestral form to the Vertebrates—Balanoglossus, Nemertines and the rest—had put in an appearance, there were two main views on the subject, one upheld by Haeckel, Kowalevsky and others, to the effect that the proximate ancestor of Vertebrates was a form somewhat resembling the ascidian tadpole, the other supported principally by Dohrn and Semper that Vertebrates and Arthropods traced their descent to a common segmented annelid or pro-annelid ancestor. The former view is historically prior, and arose directly out of the brilliant embryological investigations of A. Kowalevsky, who proved himself to be a worthy successor of the great comparative embryologist Rathke. His work was indeed a true continuation of Rathke's. It was not directly inspired by evolution, though it supplied much useful confirmation of the theory—you may read Kowalevsky's earlier memoirs and not realise that they were written several years after the publication of the Origin of Species.
His first paper of evolutionary importance was a note in Russian on the development of Amphioxus, published in 1865. This subject was followed up in two papers which appeared in 1867[387] and 1877.[388] In his papers on Amphioxus Kowalevsky made out the main features in the development of this primitive form, and showed that the chief organs were formed in essentially the same way as in Vertebrates; he described the formation of the archenteron by invagination, the appearance of the medullary folds, which coalesced to form the neural canal, the formation of the notochord and of the gill-slits. At first he made the mistake of supposing that the body-cavity arose from the segmentation-cavity, but in his later paper he rightly surmised that it was formed from the cavities of the "primitive vertebræ," or mesodermal segments. The origin of the notochord from the endoderm was also not made out by Kowalevsky in his paper of 1867.
Although many important details remained to be discovered by later investigators,[389] Kowalevsky's work at once made the development of Amphioxus the key to vertebrate embryology, the typical ontogeny with which all others could be compared.
Meanwhile, in 1866 and 1871, Kowalevsky had communicated memoirs of even greater interest,[390] in which he showed that the simple Ascidians developed in an extraordinarily similar way to Amphioxus and hence to Vertebrates in general. His proof that Ascidians also develop on the vertebrate type aroused great interest at the time, and was naturally acclaimed by the evolutionists as a striking piece of evidence in favour of their doctrine. The systematic position of the Ascidians was at that time quite uncertain; they were grouped, as a rule, with the Mollusca, and certainly no one suspected that their well-known tailed larvæ, first seen by Savigny, showed any but the most superficial analogy with the tadpoles of Amphibia. Kowalevsky's papers put a different complexion on the matter. In the first of them he showed how the nervous system of the simple Ascidian developed from ectodermal folds just as it did in Amphioxus and Vertebrates, how gill-slits were formed in the walls of the pharynx, and how there existed in the ascidian larva a structure which in position and mode of development was the strict homologue of the vertebrate notochord. In his second paper he entered into much more detail, and published some excellent figures, often reproduced since (see [Fig. 13]), but the proof of the affinity between Vertebrates and Ascidians was in all essentials complete in his paper of 1866.
Fig. 13.—Development of the Ascidian Larva. (After Kowalevsky.)
Kowalevsky's results were accepted by Haeckel, Gegenbaur, Darwin,[391] and many others as conclusive evidence of the origin of Vertebrates from a form resembling the ascidian tadpole; they were extended and amplified by Kupffer[392] in 1870, later by van Beneden and Julin[393] and numerous other workers; they were adversely criticised by Metschnikoff[394] and von Baer,[395] as well as by H. de Lacaze-Duthiers and A. Giard.[396] Lacaze-Duthiers and von Baer both held fast to the old view that Ascidians were directly comparable with Lamellibranch molluscs; they denied the homology of the ascidian nervous system with that of Vertebrates, von Baer being at great pains to show that the ascidian nerve-centre was really ventral in position. He pointed out also that the "notochord" was confined to the tail of the ascidian larva. Giard's attitude was by no means so uncompromising, and the criticisms he passed on the Kowalevsky theory are both subtle and instructive. He admits that there exists a real homology between, for instance, the notochord of Vertebrates and that of Ascidians. "But," he adds, "it is too often forgotten that homology does not necessarily mean an immediate common origin or close relationship. There exist, doubtless, homologies of great atavistic importance—I consider as such, for example, the formation of the cavity of Rusconi [the archenteron] in Ascidians and lower Vertebrates. But there are also adaptive and purely analogical homologies, such as the interdigital palmation of aquatic birds, amphibians and mammals. These are not purely analogous organs, for they can be superposed one on another, which is not the case with simply analogous structures (the bat's wing, for example, cannot be superposed on the bird's wing); they are homologous formations, resulting from the adaptation of the same fundamental organs to identical functions. Such is, in my opinion, the nature of the homology existing between the tail of the ascidian tadpole and that of Amphioxus or of young amphibians. The ascidian larva, having no cilia and being necessarily motile, requires for the insertion of its muscles or contractile organs ... a central flexible axis, a true chorda dorsalis analogous to that of Vertebrates" (pp. 278-9). This point of view is strengthened by the fact that in Molgula, studied by Lacaze-Duthiers, the embryo is practically stationary, and forms no notochord, nor ever develops sense-organs in the cerebral vesicle.
Giard's general conclusion is that "the true homology with Vertebrates ceases after the formation of the cavity of Rusconi and the medullary groove: the homologies established by Kowalevsky for the notochord and the relations of the digestive tube and nervous systems are not atavistic, but adaptive, homologies" (p. 282). There is accordingly no close genetic relationship between Ascidians and Vertebrates.
Giard's criticisms did not avail to check the vogue of the new theory, which soon became an accepted article of faith in most morphological circles.[397] The fall of the Ascidians from their larval high estate provided the text for many a Darwinian sermon.
Some years after the genetic relationship of Ascidians and Vertebrates had been established, a rival theory of the origin of Vertebrates made its appearance—a theory which was practically a rehabilitation in a somewhat altered form of the old Geoffroyan conception that Vertebrates are Arthropods walking on their backs. This was the so-called Annelid theory of Dohrn and Semper. Both Dohrn and Semper started out from the fact that Annelids and Vertebrates are alike segmented animals, and it was an essential part of their theory that this resemblance was due to descent from a common segmented ancestor. Both laid great stress on the fact that the main organs in Vertebrates are arranged in the same way as in an Annelid lying on its back, the nervous system being uppermost, the alimentary system coming next, and below this the vascular.
Dohrn's earlier views are contained in the fascinating little book published in 1875, which bears the title Der Ursprung der Wirbelthiere und das Princip des Functionswechsel (Leipzig). He followed this up by a long series of studies on vertebrate anatomy and embryology,[398] in which he modified his views in certain details. We shall confine our attention to the first sketch of his theory.
If the Vertebrate is conceived to have evolved from a primitive Annelid which took to creeping or swimming ventral surface uppermost, a difficulty at once arises with regard to the relative positions of the "brain" and the mouth. In Vertebrates the brain, like the rest of the nervous system, is dorsal to the mouth and the alimentary canal; in an inverted Annelid, however, the brain is ventral to the mouth and is connected with the dorsal nerve cord by commissures passing round the œsophagus. It would seem, therefore, that the primitive Vertebrate must have acquired either a new brain or a new mouth. Dohrn took the latter view. He supposed that the original mouth of the primitive ancestor lay between the crura cerebelli in the fossa rhomboidea, and that in Vertebrates this mouth has been replaced functionally by a new ventrally placed mouth, formed by the medial coalescence of a pair of gill-slits.[399] Probably the two mouths at one period co-existed, and the older one was ousted by the growing functional importance of the newer mouth.
The gill-slits were considered by Dohrn to be derived from the segmental organs of Annelids, which were present originally in every segment of the primitive ancestor. The gills were at first external, like the gills of many Chætopods at the present day. For their support cartilaginous gill-arches naturally arose in the body-wall, and the superficial musculature became attached to these bars. "There existed in all the segments of the Annelid-ancestors of Vertebrates gills with cartilaginous skeleton and gill-arches in the body wall. Each gill had its veins and arteries, each had its branch of the ventral nerve-cord, and between each successive pair of gills a segmental organ opened to the exterior" (p. 14, 1875). The paired fins and limbs of the Vertebrate arose by the functional transformation of two pairs of these gills. The anterior gills became the definitive internal gills of the Vertebrate, for they gradually shifted into the mouths of the anterior segmental organs, which had already acquired an opening into the pharynx and had been transformed into true gill-slits. The posterior gills degenerated and disappeared, but their arches remained as ribs. Gill-arches and ribs were accordingly homologous structures and formed a parietal skeleton. The vertebrate anus, like the mouth, was probably secondary and formed from a pair of gill-slits, the post-anal gut of vertebrate embryos hinting that the original anus was terminal as in Annelids. The unpaired fins of fish were originally paired and possibly arose from the coalescence of rows of parapodia. Dohrn assumed also that the primitive Annelid ancestor must have possessed a notochord to give support in swimming.
If Vertebrates arose from primitive Annelid ancestors, how account for Amphioxus and the Ascidians, which seem to be the most primitive living Vertebrates and yet show no particular annelidan affinities? Dohrn tries to answer this awkward question by showing that these forms are not primitive but degenerate. He points out first that Cyclostomes are degenerate fish, half specialised and half degraded in adaptation to a parasitic mode of life. He thinks that if an Ammocoetes were to become sexually mature and degenerate still further, forms would result which would resemble Amphioxus, and ultimately, if the process of degeneration went far enough, larval Ascidians. Amphioxus therefore might well be considered an extremely simplified and degenerate Cyclostome, and the ascidian larva the last term of this degeneration-series. Both Amphioxus and the Ascidians would accordingly be descended from fish, instead of fish being evolved from them.
Dohrn conceived that the transformation of the Annelid into the Vertebrate took place mainly by reason of an important transforming principle, which he calls the principle of function-change. Each organ, Dohrn thinks, has besides its principal function a number of subsidiary functions which only await an opportunity to become active. "The transformation of an organ takes place by reason of the succession of the functions which one and the same organ possesses. Each function is a resultant of several components, of which one is the principal or primary function, while the others are the subsidiary or secondary functions. The weakening of the principal function and the strengthening of a subsidiary function alters the total function; the subsidiary function gradually becomes the chief function, the total function becomes quite different, and the consequence of the whole process is the transformation of the organ" (p. 60). Examples of function-change are not difficult to find. Thus the stomach in most Vertebrates performs both a chemical and a mechanical function, but in some forms a part of it specialises in the mechanical side of the work and becomes a gizzard, while the remaining part confines its energies to the secretion of the gastric juice. So, too, it is through function-change that certain of the ambulatory appendages of Arthropods have become transformed into jaws—their function as graspers of food has gradually prevailed over their main function as walking limbs. In the evolution of Vertebrates from Annelids the principle came into action in many connections—in the formation of a new mouth from gill-slits, in the transformation of gills into fins and limbs, of segmental organs into gill-slits, and so on. Dohrn tells us that the principle of function-change was suggested to him by Mivart's Genesis of Species (1870), and he points out how it enables a partial reply to be made to the dangerous objection raised against the theory of natural selection that the first beginnings of new organs are necessarily useless in the struggle for existence.
We may note in passing that a somewhat similar idea was later applied by Kleinenberg to the explanation of some of the ancestral features of development. He pointed out in his classical memoir on the embryology of the Annelid Lopadorhynchus[400] that many embryonic organs seem to be formed for the sole purpose of providing the necessary stimulus for the development of the definitive organs. Thus the notochord is the necessary forerunner of the vertebral column, cartilage the precursor of bone. "From this point of view," he writes, "many rudimentary organs appear in a different light. Their obstinate reappearance throughout long phylogenetic series would be hard to understand were they really no more than reminiscences of bygone and forgotten stages. Their significance in the processes of individual development may in truth be far greater than is generally recognised. When in the course of the phylogeny they have played their part as intermediary organs (Vermittelungsorgane) they assume the same function in the ontogeny. Through the stimulus or by the aid of these organs, now become rudimentary, the permanent parts of the embryo appear and are guided in their development; when these have attained a certain degree of independence, the intermediary organ, having played its part, may be placed upon the retired list."[401]
Dohrn was well aware of the functional, or as he calls it, the physiological, orientation of his principle, and he rightly regarded this as one of its chief merits. He held that morphology became too abstract and one-sided if it disregarded physiology completely; he saw clearly that the evolution of function was quite as important a problem as the evolution of form, and that neither could be solved in isolation from the other. "The concept of function-change is purely physiological;" he writes, "it contains the elements out of which perhaps a history of the evolution of function may gradually arise, and for this very reason it will be of great utility in morphology, for the evolutionary history of structure is only the concrete projection of the content and course of the evolution of function, and cannot be comprehended apart from it" (p. 70).[402]
It is very instructive in this connection to note that Dohrn was not, like so many of his contemporaries, a dogmatic materialist, but upheld the commonsense view that vital phenomena must, in the first instance at least, be accepted as they are. "It is for the time being irrelevant," he writes, "to squabble over the question as to whether life is a result of physico-chemical processes or an original property (Urqualität) of all being.... Let us take it as given" (p. 75).
Semper's speculations on the genetic affinity of Articulates and Vertebrates are contained in two papers[403] which appeared about the same time as Dohrn's. He openly acknowledges that his work is essentially a continuation of Geoffroy's transcendental speculations, and gives in his second paper a good historical account of the views of his great predecessor. It is a significant fact that evolutionary morphologists very generally held that Geoffroy was right in maintaining against Cuvier[404] the unity of plan of the whole animal kingdom, for they saw in this a strong argument for the monophyletic descent of all animals from one common ancestral form.
In his first paper Semper does little more than break ground; he insists on the fact that both Annelids and Vertebrates are segmented animals, and he points out how close is the analogy between the nephridia or "segmental organs" of the former and the excretory (mesonephric) tubules of the latter, upon which he published in the same volume an extensive memoir. At this time he considered Balanoglossus—by reason of its gill-slits (its notochord he did not know)—to be the nearest living representative of the ancestral form of Vertebrates and Annelida.
His second paper is a more exhaustive piece of work and deals with every aspect of the problem, both from an anatomical and from an embryological standpoint. It is consciously and admittedly an attempt to apply Geoffroy's principle of the unity of plan and composition to the three great metameric groups, the Annelida, Arthropoda, and Vertebrata. Semper follows Geoffroy's lead very closely in maintaining that it is not the position of the organs relative to the ground that must be taken into account in establishing their homologies, but solely their spatial relations one to another. He holds that dorsum and venter are terms of purely physiological import, and he proposes to substitute for them the terms neural and cardial (better, hæmal) surfaces, either of which may be either dorsal or ventral in position.
Having established this primary principle, Semper has little difficulty in showing that the main organs of the body lie to one another in the same relative positions in Annelida, Arthropoda, and Vertebrata; and this, together with the metameric segmentation common to them all, constitutes his first great argument in favour of their genetic relationship. But he has still to show that Annelids possess at least the rudiments of certain organs which seem to be peculiar to Vertebrates, as the gill-slits, the notochord, and a nervous system developed from the ectoderm of the "dorsal" surface. He takes particular cognisance also of the old distinction drawn by von Baer, that Vertebrates show a "double-symmetrical" mode of development (evolutio bigemina), the dorsal muscle-plates forming a tube above the notochord, the ventral plates a tube below the notochord, whereas Articulates do not possess this axis, and form only one tube, namely, that round the "vegetative" organs (evolutio gemina). Semper is at pains to prove that evolutio bigemina is characteristic also of Annelidan development.
Fig. 14.—Transverse Section (Inverted) of the Worm Nais. (After Semper.)
| a.c. Alimentary canal. | sp.g. Spinal ganglion. | d.p. Neural muscle-plate. |
| n.c. Nerve cord. | n. Notechord. | v.p. Haemal muscle-plate. |
He gets his facts from an elaborate study of the process of budding in the Naidæ, making the somewhat risky assumption that regeneration takes essentially the same course as embryonic development.
He succeeds in showing—to his own satisfaction at least—that in the formation of new segments in Nais and Chætogaster a strand of cells appears between the alimentary canal and the nerve-cord, and that from this axial strand the hæmal muscle-plates grow out dorsally round the alimentary canal and the neural muscle-plates ventrally round the nerve-cord (see [Fig. 14]).
This strand of cells, he concludes, must clearly be the notochord, and the type of development is obviously the double-symmetrical met with in Vertebrates.
The nervous system Semper found to develop in the buds of Nais and Chætogaster by an ectodermal thickening, just as in some Vertebrates. The cerebral ganglion was formed by the ends of the nerve-cord growing up round the œsophagus and fusing with the paired "sense-plates" which develop from the ectoderm of the head. The cerebral ganglion is accordingly only secondarily hæmal in position, and there is no need therefore to seek in Vertebrates for the homologue of the œsophageal commissures of Annelids, as, for instance, Schneider did.
Since the mouth opens on the neural surface in Annelids and on the hæmal surface in Vertebrates, Semper considers that they cannot be equivalent structures, and he finds the homologue of the Vertebrate mouth in a little pit on the hæmal surface of the head in the leech Clepsine (also in the true mouth of Turbellaria and the proboscis-opening in Nemertines). The primitive Annelid mouth, however, does not appear in the embryogeny of Vertebrates, for the great development of the brain crowds it out of existence.
The homologues of the gill-slits Semper finds in two little canals in the head of Chætogaster, which open from the pharynx to the exterior. In Sabellids he describes an elaborate system of gill-canals, with a supporting cartilaginous framework which forms a real Kiemenkorb or gill-basket, comparable with that of Amphioxus.
Gill-slits, notochord, relation of nervous system, mesonephric tubules, are thus common to Annelids and Vertebrates—what further proof could one desire of the close relationship of these groups? Yet Semper enters into refinements of comparison, seeing, for instance, in the lateral portions of the ventral ganglia ([Fig. 14], sp. g.) the homologues of the spinal ganglia of Vertebrates, and comparing the lateral line of sense organs in Annelids with the lateral line in Anamnia.
He will not admit that Amphioxus and the Ascidians show a closer resemblance to Vertebrates than his beloved Annelids. Amphioxus, he thinks, is not a Vertebrate, and Ascidians, though sharing with Annelids the possession of a notochord, gill-slits, and a "dorsal" nervous system, yet are further removed from Vertebrates than the latter by reason of their lacking that essential characteristic of Vertebrates, metameric segmentation.
Not content with establishing the unity of plan of Annelids, Arthropods, and Vertebrates, Semper tries to link on the Annelids, as the most primitive group of the three, to the unsegmented worms, and particularly to the Turbellaria. His speculations on this matter may be summed up somewhat as follows:—The common ancestor of all segmented animals is a segmented worm-like form, not quite like any existing type, resembling the Turbellaria in having two nerve strands on the dorsal side and no œsophageal ring, potentially able to develop either the Vertebrate or the Annelid mouth, and so to give origin both to the Articulate and to the Vertebrate series. The common ancestor alike of unsegmented worms and of all segmented types is probably the trochosphere larva, which in the Vertebrates is represented by the simple Keimblase or blastula.
The Annelid theory of Dohrn and Semper was perhaps not so widely accepted as the rival Ascidian theory, but it counted not a few adherents and gave a certain stimulus to comparative morphology. F. M. Balfour, who pointed out about the same time as Semper the analogy between the nephridia of Annelids and the mesonephric tubules of Vertebrates,[405] while not accepting the actual theories of Dohrn and Semper, took up a distinctly favourable attitude to the general idea that Annelids and Vertebrates were descended from a common segmented ancestor. Discussing this question in his classical work on the development of Elasmobranch fishes,[406] Balfour came to the conclusion "that we must look for the ancestors of the Chordata, not in allies of the present Chætopoda, but in a stock of segmented forms descended from the same unsegmented types as the Chætopoda, but in which two lateral nerve-cords, like those of Nemertines, coalesced dorsally instead of ventrally to form a median nervous cord. This group of forms, if my suggestion as to their existence is well founded, appears now to have perished."[407]
He held that while there was much to be said for the interchange of dorsal and ventral surfaces postulated by Dohrn and Semper, the difficulties involved in the supposition were too great; he preferred, therefore, to assume that the present Vertebrate mouth was primitive, and not a secondary formation.
His views as to the phylogeny of the Chordata and the genetic relation of the various classes to one another are exhibited in the following schema,[408] names of hypothetical groups being printed in capitals, names of degenerate groups in italics:—
The hypothetical ancestral forms (Protochordata) possessed a notochord, a ventral suctorial mouth and numerous gill-slits, and were presumably descended from the common ancestor of Annelids and Vertebrates. Amphioxus and the Ascidians found their place in this schema as degenerate offshoots of the ancestral Protochordates, while the Cyclostomes were in the same way the degenerate modern representatives of the ancestral Protovertebrates.
Balfour's suggestion, that the nervous system in Annelids and Vertebrates might have arisen by the dorsal or ventral coalescence of the lateral nerve cords found in their common ancestor, bore fruit in the speculations of Hubrecht,[409] on the relation of Nemertines to Vertebrates.
The Annelid theory was firmly supported by Eisig, who in his elaborate monograph on the Capitellidæ[410] maintained against Fürbringer the genetic identity of the Annelidan nephridia with the kidney tubules of Vertebrates. The independent discovery by E. Meyer[411] and J. T. Cunningham,[412] of an internal segmental duct in Lanice, into which several nephridia opened, seemed to strengthen this view.
Following Ehlers,[413] Eisig found the homologue of the notochord in the accessory intestine of the Capitellidæ and Eunicidæ, which he supposed might easily be transformed, according to the principle of function-change, from a respiratory to a supporting organ. He finally disposed of the alternative notion that the notochord was represented in Annelids by the "giant-fibres" or neurochordal strands which lie close above the nerve-cord, a view held by Kowalevsky,[414] and for a time by Semper. These strands were shown by Eisig, and by Spengel, to be the neurilemmar sheaths of thick nerve fibres which had in many cases degenerated. The view that the content of the neurochordal tubes was nervous in nature was first promulgated by Leydig in 1864.
Much difference of opinion reigned as to the true homologies of the brain and mouth of Annelids and Vertebrates. Beard[415] and others got over the difficulty of the hæmal position of the cerebral ganglion in Annelids by supposing that it degenerated and disappeared altogether in the Annelidan ancestor of Vertebrates, and that accordingly it had no homologue in the Vertebrate nervous system. Beard put forward also the ingenious theory that the hypophysis represents the old Annelidan mouth.
Van Beneden and Julin[416] assumed that in the ancestors of Vertebrates the œsophagus shifted forward between the still unconnected lobes of the brain to open on the hæmal surface.
The fundamental assumption of the Annelid theory, that dorsal and ventral surfaces are morphologically interchangeable, seemed rather bold to many zoologists, and Gegenbaur[417] voiced a common opinion when he rejected as unscientific the comparison of the ventral nerve cord of Articulates with the dorsal nervous system of Vertebrates.
The Balanoglossus theory of Vertebrate descent also belongs, at least in its first form, to the earlier group of evolutionary speculations. The gill-slits of Balanoglossus were discovered by Kowalevsky as early as 1866.[418] Tornaria was discovered by J. Müller in 1850, but by him considered an Asterid larva; its true nature as the larva of Balanoglossus was made out by Metschnikoff in 1870, who also remarked upon its extraordinary likeness to the larvæ of Echinoderms.[419] That it had some relationship with Vertebrates was recognised by Semper, Gegenbaur and others, but the full working-out of its Vertebrate affinities is due to Bateson.[420]
Bateson broke completely with the Dohrn-Semper view that the metamerism of Articulates and Vertebrates must be put down to inheritance from a common ancestor. He held that metamerism was merely a special manifestation of the general property of repetition, common to all living things (cf. Owen's "vegetative force"), and that accordingly "however far back a segmented ancestor of a segmented descendant may possibly be found, yet ultimately the form has still to be sought for in which these repetitions had their origin" (p. 549). The meaning of the phenomenon was obscure, but he was convinced that the explanation was not to be found in ancestry. "This much alone is clear," he wrote, "that the meaning of cases of complex repetition will not be found in the search for an ancestral form, which, itself presenting this same character, may be twisted into a representation of its supposed descendant. Such forms there may be, but in finding them the real problem is not even resolved a single stage; for from whence was their repetition derived? The answer to this question can only come in a fuller understanding of the laws of growth and of variation, which are as yet merely terms" (pp. 548-9). It was in following up this line of thought that Bateson produced his monumental Materials for the Study of Variation (1894).
He found a strong positive argument for his theory that Vertebrates are descended from unsegmented forms in the fact that the notochord arises as an unsegmented structure. With the notochord he homologised the supporting rod in the proboscis of Balanoglossus, which like the notochord arises from the dorsal wall of the archenteron, and has a vacuolated structure. The gill-slits of Balanoglossus, with their close resemblance in detail to those of Amphioxus, Bateson also used as an argument in favour of the phylogenetic relationship of the Enteropneusta and Vertebrata, together with the formation from the ectoderm of a dorsal nerve tube.
Bateson's views attracted considerable attention, and were thought by many to lighten appreciably the obscurity in which the origin of Vertebrates was wrapped. Thus Lankester wrote in his article on Vertebrates[421] in the Encyclopedia Britannica:—"It seems that in Balanoglossus we at last find a form which, though no doubt specialised for its burrowing sand-life, and possibly to some extent degenerate, yet has not to any large extent fallen from an ancestral eminence. The ciliated epidermis, the long worm-like form, and the complete absence of segmentation of the body-muscles lead us to forms like the Nemertines. The great proboscis of Balanoglossus may well be compared to the invaginable organ similarly placed in the Nemertines. The collar is the first commencement of a structure destined to assume great importance in Cephalochorda and Craniata, and perhaps protective of a single gill-slit in Balanoglossus before the number of those apertures had been extended. Borrowing, as we may, the nephridia from the Nemertines, and the lateral in addition to the dorsal nerve, we find that Balanoglossus gives the most hopeful hypothetical solution of the pedigree of Vertebrates."
Much doubt was cast upon the Chordate affinities of the Enteropneusta by Spengel in his monograph of the group,[422] but when the development of the cœlom came to be more thoroughly worked out in Balanoglossus and Amphioxus, the striking resemblance in this respect between the two forms gave additional support to the Batesonian view.[423]
[386] The stages in the development of microscopical technique are well summarised by R. Burckhardt, Geschichte der Zoologie, p. 121, Leipzig 1907.
[387] "Entwickelungsgeschichte des Amphioxus lanceolatus," Mém. Acad. Sci. St Pétersbourg (Petrograd) (vii.), xi., No. 4, 1867, 17 pp., 3 pls.
[388] "Weitere Studien ü. die Entwickelungsgeschichte des Amphioxus lanceolatus," Arch. für mikr. Anat., xiii., pp. 181-204, 1877.
[389] Particularly by Hatschek (1881) and Boveri (1892).
[390] "Entwickelungsgeschichte der einfachen Ascidien," Mém. Acad. Sci. St Pétersbourg (Petrograd), (vii.), x., No. 15, 1866, 19 pp., 3 pls. "Weitere Studien ü. die Entwicklung der einfachen Ascidien," Arch. f. mikr. Anat., vii., pp. 101-130, 1871.
[391] Descent of Man, i., p. 205, 1871.
[392] Arch. f. mikr. Anat., vi., 1870, and viii., 1872.
[393] Archives de Biologie, 1884, 1885, and 1887.
[384] Bull. Acad. Sci. St Pétersbourg (Petrograd) xiii., 1869, and Zeits. f. wiss. Zool., xxii., 1872.
[395] Mém. Acad. Sci. St Pétersbourg(Petrograd)(7), xix., 1873.
[396] Giard, Arch. zool. expér. gén., i., 1872, and Lacaze-Duthiers, ibid., iii., 1874.
[397] For the later history of the Amphioxus-Ascidian theory the reader may be referred to A. Willey's well-known work, Amphioxus and the Ancestry of the Vertebrates, New York and London, 1894, and to Delage et Hérouard, Traité de Zoologie concrète, Tome viii., Paris, 1898.
[398] "Studien zur Urgeschichte des Wirbelthierkörpers," Mittheil. Zool. Stat. Neapel, 1882-1907.
[399] Leydig (Vom Baue des thierischen Körpers, Tübingen, 1864), who, in a measure, forestalled Dohrn and Semper by comparing Vertebrates with reversed Arthropods, specially insects, supposed the old mouth to pass between the crura cerebri.
[400] Zeits. f. wiss. Zool., xliv., 1886.
[401] Quoted by E. B. Wilson, Wood's Holl Biological Lectures for 1894, p. 121.
[402] Cf. Metschnikoff, Quart. Journ. Microsc. Sci., xxiv., pp. 89-111, 1884.
[403] "Die Stammesverwandschaft der Wirbelthiere und Wirbellosen," Arb. zool.-zoot. Instit. Würzburg, ii., pp. 25-76, 1875; "Die Verwandschaftsbeziehungen der gegliederten Thiere," Ibid., iii., pp. 115-404, 1876-7.
[404] Abuse of Cuvier also dates from the early days of evolution, see Rádl, ii., pp. 12-17.
[405] "On the origin and history of the urino-genital organs of Vertebrates," Journ. Anat. Phys., x., 1876. The conclusions of Balfour and Semper were adversely criticised by M. Fürbringer (Morph. Jahrb., iv., 1878), and were negatived by later research.
[406] A Monograph on the Development of Elasmobranch Fishes, London, 1878.
[407] A Treatise on Comparative Embryology, vol. ii., p. 311, London, 1881.
[408] Loc. cit., vol. ii., p. 327.
[409] "On the Ancestral Form of the Chordata," Q.J.M.S., xxiii., 1883. "The Relation of the Nemertea to the Vertebrata," ibid., xxvii., 1887. Hubrecht gives the credit for the first indication of the relationship of Nemertines and Vertebrates to Harting (Leerboek van de Grondbeginselen der Dierkunde, 1874).
[410] "Monographie der Capitelliden des Golfes von Neapel," Fauna u. Flora des Golfes von Neapel, Monog. xvi., Berlin, 1887.
[411] Mitt. Zool. Stat. Neapel, vii., 1887.
[412] Nature, xxxvi., p. 162, 1887.
[413] "Nebendarm und Chorda dorsalis," Nachr. Ges. Wiss. Göttingen, p. 390, 1885.
[414] "Embryologische Studien an Würmern u. Arthropoden," Mém. Acad. Sci. St Pétersbourg (Petrograd), (7), xvi., 1870. And in Arch. f. mikr. Anat., vii., p. 122, 1871.
[415] "The Old Mouth and the New," Anat. Anz., iii., 1888. Nature, xxxix., 1889.
[416] "Recherches sur la Morphologie des Tuniciers," Arch. de Biol., vi., 1887.
[417] "Die Stellung u. Bedeutung der Morphologie," Morph. Jahrb., i., pp. 1-19, 1876.
[418] "Anatomie des Balanoglossus," Mém. Acad. Sci. St Pétersbourg (Petrograd), (7), x., 1866.
[419] Zeit. f. wiss. Zool., xx., 1870. For a recent view of the relation of the Enteropneusta to the Echinoderma, see J. F. Gemmill, Phil. Trans. B., ccv., pp. 213-94, 1914.
[420] In a series of papers published in 1884-6, the speculative results being discussed in his memoir on "The Ancestry of the Chordata," Q.J.M.S. (n.s.), xxvi., pp. 535-71, 1886.
[421] Reprinted in Zoological Articles, London, 1891.
[422] "Die Enteropneusten des Golfes von Neapel," Fauna und Flora des Golfes von Neapel, Monog. xviii., Berlin, 1893.
[423] See Macbride, "A Review of Prof. Spengel's Monograph on Balanoglossus," Q.J.M.S., xxxvi., 1894, and "The Early Development of Amphioxus," Q.J.M.S., xl., 1898.