FOOTNOTES:

Fig. 39.—Lower jaw and palate ofSebastolobus alascanus. (After Starks.)PA. Palatine.MSPT. Mesopterygoid.PT. Pterygoid.MPT. Metapterygoid.D. Dentary.AR. Articular.AN. Angular.Q. Quadrate.SY. Symplectic.HM. Hyomandibular.POP. Preopercle.IOP. Interopercle.SOP. Subopercle.OP. Opercle.

Fig. 39.—Lower jaw and palate ofSebastolobus alascanus. (After Starks.)

Degeneration.—By degeneration or degradation in biology is meant merely a reduction to a lower degree of complexity or specialization in structure. If in the process of development of the individual some particular organ loses its complexity it is said to be degenerate. If in the geological history of a type the same change takes place the same term is used. Degeneration in this sense is, like specialization, a phase of adaptation. It does not imply disease, feebleness, or mutilation, or any tendency toward extinction. It is also necessary to distinguish clearly phases of primitive simplicity from the apparent simplicity resulting from degeneration.

The Skeleton in Primitive Fishes.—To learn the names of bones we can deal most satisfactorily with the higher fishes, those inwhich the bony framework has attained completion. But to understand the origin and relation of parts we must begin with the lowest types, tracing the different stages in the development of each part of the system.

Fig. 40.—Maxillary and premaxillary ofSebastolobus alascanus. M, maxillary; PM, premaxillary.

In the lancelets (Leptocardii), the vertebral column consists simply of a gelatinous notochord extending from one end of the fish to the other, and pointed at both ends, no skull being developed. The notochord never shows traces of segmentation, although cartilaginous rods above it are thought to forecast apophyses. In these forms there is no trace of jaws, limbs, or ribs.

Fig. 41.—Part of skeleton ofSelene vomer(Linnæus).

In the embryo of the bony fish a similar notochord precedes the segmentation and ossification of the vertebral column. In most of the extinct types of fishes a notochord more or lessmodified persisted through life, the vertebræ being strung upon it spool fashion in various stages of development. In the Cyclostomi (lampreys and hagfishes) the limbs and lower jaw are still wanting, but a distinct skull is developed. The notochord is still present, but its anterior pointed end is wedged into the base of a cranial capsule, partly membranous, partly cartilaginous. There is no trace of segmentation in the notochord itself in these or any other fishes, but neutral arches are foreshadowed in a series of cartilages on each side of the spinal chord. The top of the head is protected by broad plates. There are ring-like cartilages supporting the mouth and other cartilages in connection with the tongue and gill structures.

Fig. 42.—Hyostylic skull ofChiloscyllium indicum, a Scyliorhinoid Shark. (After Parker and Haswell.)

Fig. 43.—Skull ofHeptranchias indicus(Gmelin), a notidanoid shark. (After Parker and Haswell.)

Fig. 44.—Basal bones of pectoral fin of Monkfish,Squatina. (After Zittel.)

The Skeleton of Sharks.—In the Elasmobranchs (sharks, rays, chimæras) the tissues surrounding the notochord are segmented and in most forms distinct vertebræ are developed. Each of these has a conical cavity before and behind, with a central canal through which the notochord is continued. The form and degree of ossification of these vertebræ differ materially in the different groups. The skull in all these fishes is cartilaginous, forming a continuous undivided box containing the brain and lodging the organs of sense. To the skull in the shark is attached a suspensorium of one or two pieces supporting the mandible and the hyoid structures. In the chimæra the mandible is articulated directly with the skull, the hyomandibular and quadrate elements being fused with the cranium. The skull in such case is said to beautostylic, that is, with self-attached mandible. In the shark it is said to behyostylic, the hyomandibular intervening. The upper jaw in the shark consists not of maxillary and premaxillary but of palatine elements, and the two halves of the lower jaw are representatives of Meckel's cartilage, which is the cartilaginous centre of the dentary bone in the bony fishes. These jaw-bones in the higher fishes are in the nature of membrane bones, and in the sharks and their relatives all such bones are undeveloped. The hyoid structures are in the shark relatively simple, as are also the gill-arches, which vary in number. The vertical fins are supported by interneural and interhæmal cartilages, to which the soft fin-rays are attached without articulation.

Fig. 45.—Pectoral fin ofHeterodontus philippi. (From nature.)

Fig. 46.—Pectoral fin ofHeptranchias indicus(Gmelin). (After Dean.)

The shoulder-girdle is made of a single cartilage, touchingthe back-bone at a distance behind the head. To this cartilage three smaller ones are attached, forming the base of the pectoral fin. These are calledmesopterygium,propterygium, andmetapterygium, the first named being in the middle and more distinctly basal. These three segments are subject to much variation. Sometimes one of them is wanting; sometimes two are grown together. Behind these the fin-rays are attached. In most of the skates the shoulder-girdle is more closely connected with the anterior vertebræ, which are more or less fused together.

Fig. 47.—Shoulder-girdle of a Flounder,Paralichthys californicus(Ayres).

The pelvis, remote from the head, is formed, in the shark, of a single or paired cartilage with smaller elements at the base of the fin-rays. In the males a cartilaginous generative organ, known as the clasper, is attached to the pelvis and the ventral fins. In the Elasmobranchs the tail vertebræ are progressively smaller backward. If a caudal fin is present, the last vertebræ are directed upward (heterocercal) and the greater part of the fin is below the axis. In other forms (sting-rays) the tail degenerates into a whip-like organ (leptocercal), often without fins. In certain primitive sharks (Ichthyotomi), as well as in the Dipnoi and Crossopterygii, the tail isdiphycercal, the vertebræ growing progressively smaller backward and not bent upward toward the tip.

In the chimæras (Holocephali) the notochord persists and is surrounded by a series of calcified rings. The palate with the suspensorium is coalesced with the skull, and the teeth are grown together into bony plates.

Fig. 48.—Shoulder-girdle of a Toadfish,Batrachoides pacifici(Günther).

Fig. 49.—Shoulder-girdle of a Garfish,Tylosurus fodiator(Jordan and Gilbert).

The Archipterygium.—The Dipnoans, Crossopterygians, and Ganoids represent various phases of transition from the ancient cartilaginous types to the modern bony fishes.

In the Ichthyotomous sharks, Dipnoans, and Crossopterygians the segments of the pectoral limb are arranged axially, or one beyond another. This type of fin has been calledarchipterygiumby Gegenbaur, on the theory that it represents the condition shown on the first appearance of the pectoral fin. This theory is now seriously questioned, but it will be convenient to retain the name for the pectoral fin with segmented axis fringed on one or both sides by soft rays.

Fig. 50.—Shoulder-girdle of a Hake,Merluccius productus(Ayres).

The archipterygium of the Dipnoan genusNeoceratodusis thus described by Dr. Günther ("Guide to the Study of Fishes," p. 73): "The pectoral limb is covered with small scales along the middle from the root to the extremity, and is surrounded by a rayed fringe similar to the rays of the vertical fins. A muscle split into numerous fascicles extends all the length of the fin, which is flexible in every part and in every direction. The cartilaginous framework supporting it is joined to the scapular arch by a broad basal cartilage, generally single, sometimesshowing traces of a triple division. Along the middle of the fin runs a jointed axis gradually becoming smaller and thinner towards the extremity. Each joint bears on each side a three-, two-, or one-jointed branch."

In the genusLepidosiren, also a Dipnoan, the pectoral limb has the same axial structure, but is without fin-rays, although in the breeding season the posterior limb or ventral fin in the male is covered with a brush of fine filaments. This structure, according to Prof. J. G. Kerr,[3]is probably without definite function, but belongs to the "category of modifications so often associated with the breeding season (cf. the newts' crest) commonly called ornamental, but which are perhaps more plausibly looked upon as expressions of the intense vital activity of the organisms correlated with its period of reproductive activity." Professor Kerr, however, thinks it not unlikely that this brush of filaments with its rich blood-supply may serve in the function of respiration, a suggestion first made by Professor Lankester.

FOOTNOTES:[3]Philos. Trans., Lond., 1900.

[3]Philos. Trans., Lond., 1900.

Originof the Fins of Fishes.—One of the most interesting problems in vertebrate morphology, and one of the most important from its wide-reaching relations, is that of the derivation of the fins of fishes. This resolves itself at once into two problems, the origin of the median fins, which appear in the lancelets, at the very bottom of the fish-like series, and the origin of the paired fins or limbs, which are much more complex, and which first appear with the primitive sharks.

In this study the problem is to ascertain not what theoretically should happen, but what, as a matter of fact, has happened in the early history of the fish-like groups. That these structures, with the others in the fish body, have sprung from simple origins, growing more complex with the demands of varied conditions, and then at times again simple, through degeneration, there can be no doubt. It is also certain that each structure must have had some element of usefulness in all its stages. In such studies we have, as Hæckel has expressed it, "three ancestral documents, paleontology, morphology, and ontogeny"—the actual history as shown by fossil remains, the sidelight derived from comparison of structures, and the evidence of the hereditary influences shown in the development of the individual. As to the first of these ancestral documents, the evidence of paleontology is conclusive where it is complete. But in very few cases are we sure of any series of details. The records of geology are like a book with half its leaves torn out, the other half confused, displaced, and blotted. Still each record actually existing represents genuine history, and in paleontology we must in time find our final court of appeal in all matters of biological origins.

The evidence of comparative anatomy is most completely secured, but it is often indecisive as to relative age and primitiveness of origin among structures. As to ontogeny, it is, of course, true that through heredity "the life-history of the individual is an epitome of the life-history of the race." "Ontogeny repeats phylogeny," and phylogeny, or line of descent of organisms and structures, is what we are seeking. But here the repetition is never perfect, never nearly so perfect in fact as Hæckel and his followers expected to find it. The demands of natural selection may lead to the lengthening, shortening, or distortion of phases of growth, just as they may modify adult conditions. The interpolation of non-ancestral stages is recognized in several groups. The conditions of the individual development may, therefore, furnish evidence in favor of certain theories of origins, but they cannot alone furnish the absolute proof.

In the process of development the median or vertical fins are doubtless older than the paired fins or limbs, whatever be the origin of the latter. They arise in a dermal keel which is developed in a web fitting and accentuating the undulatory motion of the body. In the embryo of the fish the continuous vertical fin from the head along the back and around the tail precedes any trace of the paired fins.

In this elementary fin-fold slender supports, the rudiments of fin-rays, tend to appear at intervals. These are called by Ryder ray-hairs or actinotrichia. They are the prototype of fin-rays in the embryo fish, and doubtless similarly preceded the latter in geological time. In the development of fishes the caudal fin becomes more and more the seat of propulsion. The fin-rays are strengthened, their basal supports are more and more specialized, and the fin-fold ultimately divides into distinct fins, the longest rays developed where most needed.

That the vertical fins, dorsal, anal, and caudal, have their origin in a median fold of the skin admits of no question. In the lowest forms which bear fins these structures are dermal folds, being supported by very feeble rays. Doubtless at first the vertical fins formed a continuous fold, extending around the tail, this fold ultimately broken, by atrophy of parts not needed, into distinct dorsal, anal, and caudal fins. In the lower fishes, as in the earlier sharks, there is an approach to this condition of primitive continuity, and in the embryosof almost all fishes the same condition occurs. Dr. John A. Ryder points out the fact that there are certain unexplained exceptions to this rule. The sea-horse, pipefish, and other highly modified forms do not show this unbroken fold, and it is wanting in the embryo of the top-minnow,Gambusia affinis. Nevertheless the existence of a continuous vertical fold in the embryo is the rule, almost universal. The codfish with three dorsals, the Spanish mackerel with dorsal and anal finlets, the herring with one dorsal, the stickleback with a highly modified one, all show this character, and we may well regard it as a certain trait of the primitive fish. This fold springs from the ectoblast or external series of cells in the embryo. The fin-rays and bony supports of the fins spring from the mesoblast or middle series of cells, being thrust upward from the skeleton as supports for the fin-fold.

Origin of the Paired Fins.—The question of the origin of the paired fins is much more difficult and is still far from settled, although many, perhaps the majority of recent writers favor the theory that these fins are parts of a once continuous lateral fold of skin, corresponding to the vertical fold which forms the dorsal, anal, and caudal. In this view the lateral fold, at first continuous, became soon atrophied in the middle, while at either end it is highly specialized, at first into an organ of direction, then into fan-shaped and later paddle-shaped organs of locomotion. According to another view, the paired fins originated from gill structures, originally both close behind the head, the ventral fin migrating backward with the progress of evolution of the species.

Evidence of Paleontology.—If we had representations of all the early forms of fishes arranged in proper sequence, we could decide once for all, by evidence of paleontology, which form of fin appears first and what is the order of appearance. As to this, it is plain that we do not know the most primitive form of fin. Sharks of unknown character must have existed long before the earliest remains accessible to us. Hence the evidence of paleontology seems conflicting and uncertain. On the whole it lends most support to the fin-fold theory. In the later Devonian, a shark,Cladoselache fyleri, is found in which the paired fins are lappet-shaped, so formed and placed as to suggesttheir origin from a continuous fold of skin. In this species the dorsal fins show much the same form. Other early sharks, constituting the order ofAcanthodei, have fins somewhat similar, but each preceded by a stiff spine, which may be formed from coalescent rays.

Fig. 51.—Cladoselache fyleri(Newberry), restored. Upper Devonian of Ohio. (After Dean.)

Fig. 52.—Fold-like pectoral and ventral fins ofCladoselache fyleri. (After Dean.)

Long after these appears another type of sharks represented byPleuracanthusandCladodus, in which the pectoral fin is a jointed organ fringed with rays arranged serially in one or two rows. This form of fin has no resemblance to a fold of skin, but accords better with Gegenbaur's theory that the pectoral limb was at first a modified gill-arch. In the Coal Measures are found also teeth of sharks (Orodontidæ) which bear a strong resemblance to still existing forms of the family ofHeterodontidæ, which originates in the Permian. The existingHeterodontidæhave the usual specialized form of shark-fin, with three of the basal segments especially enlarged and placed sideby side, the type seen in modern sharks. Whatever the primitive form of shark-fin, it may well be doubted whether any one of these three (Cladoselache,Pleuracanthus, orHeterodontus) actually represents it. The beginning is therefore unknown, though there is some evidence thatCladoselacheis actually more nearly primitive than any of the others. As we shall see, the evidence of comparative anatomy may be consistent with either of the two chief theories, while that of ontogeny or embryology is apparently inconclusive, and that of paleontology is apparently most easily reconciled with the theory of the fin-fold.

Fig. 53.—Pectoral fin of shark,Chiloscyllium. (After Parker and Haswell.)

Development of the Paired Fins in the Embryo.—According to Dr. John A. Ryder ("Embryography of Osseous Fishes," 1882) "the paired fins in Teleostei arise locally, as short longitudinal folds, with perhaps a few exceptions. The pectorals ofLepisosteusoriginate in the same way. Of the paired fins, the pectoral or anterior pair seems to be the first to be developed, the ventral or pelvic pair often not making its appearance until after the absorption of the yolk-sac has been completed, in other cases before that event, as inSalmoand inGambusia. The pectoral fin undergoes less alteration of position during its evolution than the posterior pair."

In the codfish (Gadus callarias) the pectoral fin-fold "appears as a slight longitudinal elevation of the skin on either side of the body of the embryo a little way behind the auditory vesicles, and shortly after the tail of the embryo begins to bud out. At the very first it appears to be merely a dermal fold, and in some forms a layer of cells extends out underneath it from the sides of the body, but does not ascend into it. Itbegins to develop as a very low fold, hardly noticeable, and, as growth proceeds, its base does not expand antero-posteriorly, but tends rather to become narrowed, so that it has a pedunculated form. With the progress of this process the margin of the fin-fold also becomes thinner at its distal border, and at the basal part mesodermal cells make their appearance more noticeably within the inner contour-line. The free border of the fin-fold grows out laterally and longitudinally, expanding the portion outside of the inner contour-line of the fin into a fan-shape. This distal thinner portion is at first without any evidence of rays; further than that there is a manifest tendency to a radial disposition of the histological elements of the fin."

The next point of interest is found in the change of position of the pectoral fin by a rotation on its base. This is associated with changes in the development of the fish itself. The ventral fin is also, in most fishes, a short horizontal fold and just above the preanal part of the median vertical fold which becomes anal, caudal, and dorsal. But in the top-minnow (Gambusia), of the order Haplomi, the ventral first appears as "a little papilla and not as a fold, where the body-walls join the hinder upper portion of the yolk-sac, a very little way in front of the vent." "These two modes of origin," observes Dr. Ryder, "are therefore in striking contrast and well calculated to impress us with the protean character of the means at the disposal of Nature to achieve one and the same end."

Current Theories as to Origin of Paired Fins.—There are three chief theories as to the morphology and origin of the paired fins. The earliest is that of Dr. Karl Gegenbaur, supported by various workers among his students and colleagues. In his view the pectoral and ventral fins are derived from modifications of primitive gill-arches. According to this theory, the skeletal arrangements of the vertebrate limb are derived from modifications of one primitive form, a structure made up of successive joints, with a series of fin-rays on one or both sides of it. To this structure Gegenbaur gives the name of archipterygium. It is found in the shark,Pleuracanthus, inCladodus, and in all the Dipnoan and Crossopterygian fishes, its primitive form being still retained in the Australian genus of Dipnoans,Neoceratodus. This biserial archipterygium with its limb-girdle isderived from a series of gill-rays attached to a branchial arch. The backward position of the ventral fin is due to a succession of migrations in the individual and in the species.

As to this theory, Mr. J. Graham Kerr observes:

Fig. 54.—Skull and shoulder-girdle ofNeoceratodus forsteri(Günther), showing the archipterygium.

"The Gegenbaur theory of the morphology of vertebrate limbs thus consists of two very distinct portions. The first, that the archipterygium is the ground-form from which all other forms of presently existing fin skeletons are derived, concerns us only indirectly, as we are dealing here only with theoriginof the limbs, i.e., their origin from other structures that were not limbs.

"It is the second part of the view that we have to do with, that deriving the archipterygium, the skeleton of the primitive paired fin, from a series of gill-rays and involving the idea that the limb itself is derived from the septum between two gill-clefts.

"This view is based on the skeletal structures within the fin. It rests upon (1) the assumption that the archipterygium is the primitive type of fin, and (2) the fact that amongst the Selachians is found a tendency for one branchial ray to become larger than the others, and, when this has happened, for the base of attachment of neighboring rays to show a tendency to migrate from the branchial arch on to the base of the larger or, as we may call it, primary ray; a condition coming about which, were the process to continue rather farther than it is known to do in actual fact, would obviously result in a structure practically identical with the archipterygium. Gegenbaur suggests that the archipterygium actually has arisen in this way in phylogeny."

Fig. 55.—Acanthoessus wardi(Egerton). Carboniferous. FamilyAcanthoessidæ. (After Woodward.)

Fig. 56.—Shoulder-girdle ofAcanthoessus. (After Dean.)

Fig. 57.—Pectoral fin ofPleuracanthus. (After Dean.)

The fin-fold theory of Balfour, adopted by Dohrn, Weidersheim, Thacher, Mivart, Ryder, Dean, Boulenger, and others, and now generally accepted by most morphologists as plausible, is this: that "The paired limbs are persisting and exaggerated portions of a fin-fold once continuous, which stretched along each side of the body and to which they bear an exactly similar phylogenetic relation as do the separate dorsal and anal fins to the once continuous median fin-fold."

"This view, in its modern form, was based by Balfour on his observation that in the embryos of certain Elasmobranchsthe rudiments of the pectoral and pelvic fins are at a very early period connected together by a longitudinal ridge of thickened epiblast—of which indeed they are but exaggerations. In Balfour's own words referring to these observations: 'If the account just given of the development of the limb is an accurate record of what really takes place, it is not possible to deny that some light is thrown by it upon the first origin of the vertebrate limbs. The facts can only bear one interpretation, viz., that the limbs are the remnants of continuous lateral fins.'

Fig. 58.—Shoulder-girdle ofPolypterus bichir. Specimen from the White Nile.

"A similar view to that of Balfour was enunciated almost synchronously by Thacher and a little later by Mivart—in each case based on anatomical investigation of Selachians—mainly relating to the remarkable similarity of the skeletal arrangements in the paired and unpaired fins."

A third theory is suggested by Mr. J. Graham Kerr (Cambridge Philos. Trans., 1899), who has recently given a summary of the theories on this subject. Mr. Kerr agrees with Gegenbaur as to the primitive nature of the archipterygium, but believes that it is derived, not from the gill-septum, but from an external gill. Such a gill is well developed in the young of all the living sharks, Dipnoans and Crossopterygians, and in the latter types of fishes it has a form analogous to that of the archipterygium, although without bony or cartilaginous axis.

We may now take up the evidence in regard to each of the different theories, using in part the language of Kerr, the paragraphs in quotation-marks being taken from his paper. We may first consider Balfour's theory of the lateral fold.

Balfour's Theory of the Lateral Fold.—"The evidence in regard to this view may be classed under three heads, as ontogenetic, comparative anatomical, and paleontological. The ultimate fact on which it was founded was Balfour's discovery that in certain Elasmobranch embryos, but especially inTorpedo(Narcobatus), the fin rudiments were, at an early stage, connected by a ridge of epiblast. I am not able to make out what were the other forms in which Balfour found this ridge, but subsequent research, in particular by Mollier, a supporter of the lateral-fold view, is to the effect that it does not occur in such ordinary sharks asPristiurusandMustelus, while it is to be gathered from Balfour himself that it does not occur inScyllium(Scyliorhinus).

"It appears to me that the knowledge we have now that the longitudinal ridge is confined to the rays and absent in the less highly specialized sharks greatly diminishes its security as a basis on which to rest a theory. In the rays, in correlation with their peculiar mode of life, the paired fins have undergone (in secondary development) enormous extension along the sides of the body, and their continuity in the embryo may well be a mere foreshadowing of this.

Fig. 59.—Arm of a frog.

"An apparently powerful support from the side of embryology came in Dohrn and Rabl's discoveries that inPristiurusall the interpterygial myotomes produce muscle-buds. This, however, was explained away by the Gegenbaur school as being merely evidence of the backward migration of the hind limb—successive myotomes being taken up and left behind again as the limb moved farther back. As either explanation seems an adequate one, I do not think we can lay stress upon this body of facts as supporting either one view or the other. Thefacts of the development of the skeleton cannot be said to support the fold view; according to it we should expect to find a series of metameric supporting rays produced which later on become fused at their bases. Instead of this we find alongitudinalbar of cartilage developing quite continuously, the rays forming as projections from its outer side.

"The most important evidence for the fold view from the side of comparative anatomy is afforded by (1) the fact that the limb derives its nerve supply from a large number of spinal nerves, and (2) the extraordinary resemblance met with between the skeletal arrangements of paired and unpaired fins. The believers in the branchial arch hypothesis have disposed of the first of these in the same way as they did the occurrence of interpterygial myotomes, by looking on the nerves received from regions of the spinal cord anterior to the attachment of the limb as forming a kind of trail marking the backward migration of the limb.

"The similarity in the skeleton is indeed most striking, though its weight as evidence has been recently greatly diminished by the knowledge that the apparently metameric segmentation of the skeletal and muscular tissues of the paired fins is quite secondary and does not at all agree with the metamery of the trunk. What resemblance there is may well be of a homoplastic character when we take into account the similarity in function of the median and unpaired fins, especially in such forms asRaja, where the anatomical resemblances are especially striking. There is a surprising dearth of paleontological evidence in favor of this view."

The objection to the first view is its precarious foundation. Such lateral folds are found only in certain rays, in which they may be developed as a secondary modification in connection with the peculiar form of these fishes. Professor Kerr observes that this theory must be looked upon and judged: "Just as any other view at the present time regarding the nature of the vertebrate limb, rather as a speculation, brilliant and suggestive though it be, than as a logically constructed theory of the now known facts. It is, I think, on this account allowable to apply to it a test of a character which is admittedly very apt to mislead, that of 'common sense.'

"If there is any soundness in zoological speculation at all, I think it must be admitted that the more primitive vertebrates were creatures possessing a notochordal axial skeleton near the dorsal side, with the main nervous axis above it, the main viscera below it, and the great mass of muscle lying in myotomes along its sides. Now such a creature is well adapted to movements of the character of lateral flexure, and not at all for movements in the sagittal plane—which would be not only difficult to achieve, but would tend to alternately compress and extend its spinal cord and its viscera. Such a creature would swim through the water as does a Cyclostome, or aLepidosiren, or any other elongated vertebrate without special swimming organs. Swimming like this, specialization for more and more rapid movement would mean flattening of the tail region and is extension into an at first not separately mobile median tail-fold. It is extremely difficult to my mind to suppose that a new purelyswimmingarrangement should have arisen involving up-and-down movement, and which, at its first beginnings, while useless as a swimming organ itself, must greatly detract from the efficiency of that which already existed."

Objections to Gegenbaur's Theory.—We now return to the Gegenbaur view—that the limb is a modified gill-septum.

"Resting on Gegenbaur's discovery already mentioned, that the gill-rays in certain cases assume an arrangement showing great similarity to that of the skeletal elements of the archipterygium, it has, so far as I am aware, up to the present time received no direct support whatever of a nature comparable with that found for the rival view in the fact that, in certain forms at all events, the limbs actually do arise in the individual in the way that the theory holds they did in phylogeny. No one has produced either a form in which a gill-septum becomes the limb during ontogeny, or the fossil remains of any form which shows an intermediate condition.

"The portion of Gegenbaur's view which asserts that the biserial archipterygial fin is of an extremely primitive character is supported by a large body of anatomical facts, and is rendered further probable by the great frequency with which fins apparently of this character occur amongst the oldest known fishes. On the lateral-fold view we should have toregard these as independently evolved, which would imply that fins of this type are of a very perfect character, and in that case we may be indeed surprised at their so complete disappearance in the more highly developed forms, which followed later on."

Fig. 60.—Pleuracanthus decheni(Goldfuss). (After Dean.)

As to Gegenbaur's theory it is urged that no form is known in which a gill-septum develops into a limb during the growth of the individual. The main thesis, according to Professor Kerr, "that the archipterygium was derived from gill-rays, is supported only by evidence of an indirect character. Gegenbaur in his very first suggestion of his theory pointed out, as a great difficulty in the way of its acceptance, the position of the limbs, especially of the pelvic limbs, in a position far removed from that of the branchial arches. This difficulty has been entirely removed by the brilliant work of Gegenbaur's followers, who have shown from the facts of comparative anatomy and embryology that the limbs, and the hind limbs especially, actually have undergone, and in ontogeny do undergo, an extensive backward migration. In some cases Braus has been able to find traces of this migration as far forward as a point just behind the branchial arches. Now, when we consider the numbers, the enthusiasm, and the ability of Gegenbaur's disciples, we cannot help being struck by the fact that theonlyevidence in favor of this derivation of the limbs has been that which tends to show that a migration of the limbs backwards has taken place from a region somewhere near the last branchial arch, and that they have failed utterly to discover any intermediate steps between gill-rays and archipterygial fin. And if for a moment we apply the test of common sense we cannot but be impressed by the improbability of the evolution of a gill-septum, which in all the lower forms of fishes is fixedfirmly in the body-wall, and beneath its surface, into an organ of locomotion.

Fig. 61.—Embryos ofHeterodontus japonicusMaclay and Macleay, a Cestraciont shark, showing the backward migration of the gill-arches and the forward movement of the pectoral fin.a,b,c, representing different stages of growth. (After Dean.)

"May I express the hope that what I have said is sufficient to show in what a state of uncertainty our views are regarding the morphological nature of the paired fins, and upon what an exceedingly slender basis rest both of the two views which at present hold the field?"

As to the backward migration of the ventral fins, Dr. Bashford Dean has recently brought forward evidence from the embryo of a very ancient type of shark (Heterodontus japonicus) that this does not actually occur in that species. On the otherhand, we have a forward migration of the pectoral fin, which gradually takes its place in advance of the hindmost gill-arches. The accompanying cut is from Dean's paper, "Biometric Evidence in the Problem of the Paired Limbs of the Vertebrates" (American Naturalist for November, 1902). Dean concludes that inHeterodontus"there is no evidence that there has ever been a migration of the fins in the Gegenbaurian sense." "The gill region, at least in its outer part, shows no affinity during proportional growth with the neighboring region of the pectoral fin. In fact from an early stage onward, they are evidently growing in opposite directions."

Kerr's Theory of Modified External Gills.—"It is because I feel that in the present state of our knowledge neither of the two views I have mentioned has a claim to any higher rank than that of extremely suggestive speculations that I venture to say a few words for the third view, which is avowedly a mere speculation.

"Before proceeding with it I should say that I assume the serial homology of fore and hind limbs to be beyond dispute. The great and deep-seated resemblances between them are such as to my mind seem not to be adequately explicable except on this assumption.

"In the Urodela (salamanders) the external gills are well-known structures—serially arranged projections from the body-wall near the upper ends of certain of the branchial arches. When one considers the ontogenetic development of these organs, from knob-like outgrowth from the outer face of the branchial arch, covered with ectoderm and possessing a mesoblastic core, and which frequently if not always appear before the branchial clefts are open, one cannot but conclude that they are morphologically projections of the outer skin and that they have nothing whatever to do with the gill-pouches of the gut-wall. Amongst the Urodela one such gill projects from each of the first three branchial arches. InLepidosirenthere is one on each of the branchial arches I-IV. InPolypterusandCalamoichthys(Erpetoichthys) there is one on the hyoid arch. Finally, in many Urodelan larvæ we have present at the same time as the external gills a pair of curious structures called balancers. At an early stage of my work onLepidosiren,while looking over other vertebrate embryos and larvæ for purposes of comparison, my attention was arrested by these structures, and further examinations, by section or otherwise, convinced me that there were serial homologues of the external gills, situated on the mandibular arch. On then looking up the literature, I found that I was by no means first in this view. Rusconi had long ago noticed the resemblance, and in more recent times both Orr and Maurer had been led to the same conclusion as I had been. Three different observers having been independently led to exactly the same conclusions, we may, I think, fairly enough regard the view I have mentioned of the morphological nature of the balancers as probably a correct one.

"Here, then, we have a series of homologous structures projecting from each of the series of visceral arches. They crop up on the Crossopterygii, the Dipnoi, and the Urodela, i.e., in three of the most archaic of the groups of Gnathostomata. But we may put it in another way. The groups in which they do not occur are those whose young possess a very large yolk-sac (or which are admittedly derived from such forms). Now wherever we have a large yolk-sac we have developed on its surface a rich network of blood-vessels for purposes of nutrition. But such a networkmust necessarilyact as an extraordinarily efficient organ of respiration, and did we not know the facts we might venture to prophesy that in forms possessing it any other small skin-organ of respiration would tend to disappear.

"No doubt these external gills are absent also in a few of the admittedly primitive forms such as, e.g., (Neo-)Ceratodus. But I would ask that in this connection one should bear in mind one of the marked characteristics of external gills—their great regenerative power. This involves their being extremely liable to injury and consequently a source of danger to their possessor. Their absence, therefore, in certain cases may well have been due to natural selection. On the other hand, thepresencein so many lowly forms of these organs, the general close similarity in structure that runs through them in different forms, and the exact correspondence in their position and relations to the body can, it seems to me,onlybe adequately explained by looking on them as being homologous structuresinherited from a common ancestor and consequently of great antiquity in the vertebrate stem."

As to the third theory, Professor Kerr suggests tentatively that the external gill may be the structure modified to form the paired limbs. Of the homology of fore and hind limbs and consequently of their like origin there can be no doubt.

The general gill-structures have, according to Kerr, "the primary function of respiration. They are also, however, provided with an elaborate muscular apparatus comprising elevators, depressors, and adductors, and larvæ possessing them may be seen every now and then to give them a sharp backward twitch. They are thuspotentiallymotor organs. In such a Urodele asAmblystomatheir homologues on the mandibular arch are used as supporting structures against a solid substratum exactly as are the limbs of the youngLepidosiren.

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