Chapter XV: Final Remarks (1)

Problems requiring investigation--

Giant nerve-cells and giant nerve-fibres; their comparison in fishes
and arthropods; blood- and lymph-corpuscles; nature of the skin;
origin of system of unstriped muscles; origin of the sympathetic
nervous system; biological test of relationship.

Criticisms of Balanoglossus theory--Theory of parallel development--
Importance of the theory advocated in this book for all problems of
Evolution 488

BIBLIOGRAPHY AND INDEX OF AUTHORS 501

GENERAL INDEX 517

"_GO ON AND PROSPER; THERE IS NOTHING SO
USEFUL IN SCIENCE AS ONE OF THOSE EARTHQUAKE
HYPOTHESES, WHICH OBLIGE ONE TO FACE
THE POSSIBILITY THAT THE SOLIDEST-LOOKING
STRUCTURES MAY COLLAPSE._"

LETTER FROM PROF. HUXLEY TO
THE AUTHOR. JUNE 2, 1889.

{1}THE

ORIGIN OF VERTEBRATES

_INTRODUCTION_

In former days it was possible for a man like Johannes Müller to be a leader both in physiology and in comparative anatomy. Nowadays all scientific knowledge has increased so largely that specialization is inevitable, and every investigator is confined more and more not only to one department of science, but as a rule to one small portion of that department. In the case of such cognate sciences as physiology and comparative anatomy this limiting of the scope of view is especially deleterious, for zoology without physiology is dead, and physiology in many of its departments without comparative anatomy can advance but little. Then, again, the too exclusive study of one subject always tends to force the mind into a special groove--into a line of thought so deeply tinged with the prevalent teaching of the subject, that any suggestions which arise contrary to such teaching are apt to be dismissed at once as heretical and not worthy of further thought; whereas the same suggestion arising in the mind of one outside this particular line of thought may give rise to new and valuable scientific discoveries.

Nothing but good can, in my opinion, result from the incursion of the non-specialist into the realm of the specialist, provided that the former is in earnest. Over and over again the chemist has given valuable help to the physicist, and the physicist to the chemist, so closely allied are the two subjects; so also is it with physiology and anatomy, the two subjects are so interdependent that a worker in the one may give valuable aid towards the solution of some large problem which is the special territory of the other.

It has been a matter of surprise to many how it came about that {2}I, a worker in the physiological laboratory at Cambridge ever since Foster introduced experimental physiology into English-speaking nations, should have devoted so much time to the promulgation of a theory of the origin of vertebrates--a subject remote from physiology, and one of the larger questions appertaining to comparative anatomy. By what process of thought was I led to take up the consideration of a subject apparently so remote from all my previous work, and so foreign to the atmosphere of a physiological laboratory?

It may perhaps be instructive to my readers to see how one investigation leads to another, until at last, _nolens volens_, the worker finds himself in front of a possible solution to a problem far removed from his original investigation, which by the very magnitude and importance of it forces him to devote his whole energy and time to seeing whether his theory is good.

In the years 1880-1884 I was engaged in the investigation of the action of the heart, and the nature of the nerves which regulate that action. In the course of that investigation I was struck by the ease with which it was possible to distinguish between the fibres of the vagus and accelerator nerves on their way to the heart, owing to the medullation of the former and the non-medullation of the latter. This led me to an investigation of the accelerator fibres, to find out how far they are non-medullated, and so to the discovery that the _rami communicantes_ connecting together the central nervous system and the sympathetic are in reality single, not double, as had hitherto been thought; for the grey _ramus communicans_ is in reality a peripheral nerve which supplies the blood-vessels of the spinal cord and its membranes, and is of the same nature as the grey accelerators to the heart.

This led to the conclusion that there is no give and take between two independent nervous systems, the cerebro-spinal and the sympathetic, as had been taught formerly, but only one nervous system, the cerebro-spinal, which sends special medullated nerve-fibres, characterized by their smallness, to the cells of the sympathetic system, from which fibres pass to the periphery, usually non-medullated. These fine medullated nerves form the system of white _rami communicantes_, and have since been called by Langley the preganglionic nerves. Further investigation showed that such white rami are not universally distributed, but are confined to the thoracico-lumbar region, where their distribution is easily seen in {3}the ventral roots, for the cells of the sympathetic system are entirely efferent in nature, not afferent; therefore, the fibres entering into them from the central nervous system leave the spinal cord by ventral, not dorsal roots.

Following out this clue, I then found that in addition to this thoracico-lumbar outflow of efferent ganglionated visceral nerves, there are similar outflows in the cranial and sacral regions, belonging in the former case especially to the vagus system of nerves, and in the latter to the system of nerves which pass from the sacral region of the cord to the ganglion-cells of the hypogastric plexus, and from them supply the bladder, rectum, etc. To this system of nerves, formerly called the _nervi erigentes_, I gave the name pelvic splanchnics, in order to show their uniformity with the abdominal splanchnics. These investigations led to the conclusion that the organic system of nerves, characterized by the possession of efferent nerve-cells situated peripherally, arises from the central nervous system by three distinct outflows--cranial, thoracico-lumbar, and sacral, respectively. To this system Langley has lately given the name 'autonomic.' These three outflows are separated by two gaps just where the plexuses for the anterior and posterior extremities come in.

This peculiar arrangement of the white _rami communicantes_ set me thinking, for the gaps corresponded to an increase of somatic musculature to form the muscles of the fore and hind limbs, so that if, as seemed probable, the white _rami communicantes_ arise segmentally from the spinal cord, then a marked distinction must exist in structure between the spinal cord in the thoracic region, where the visceral efferent nerves are large in amount and the body musculature scanty, and in the cervical or lumbar swellings, where the somatic musculature abounds, and the white _rami communicantes_ scarcely exist.

I therefore directed my attention in the next place to the structure of the central nervous system in the endeavour to associate the topographical arrangement of cell-groups in this system with the outflow of the different kinds of nerve-fibres to the peripheral organs.

This investigation forcibly impressed upon my mind the uniformity in the arrangement of the central nervous system as far as the centres of origin of all the segmental nerves are concerned, {4}both cranial and spinal, and also the original segmental character of this part of the nervous system.

I could not, therefore, help being struck by the force of the comparison between the central nervous systems of Vertebrata and Appendiculata as put forward again and again by the past generation of comparative anatomists, and wondered why it had been discredited. There in the infundibulum was the old oesophagus, there in the cranial segmental nerves the infraoesophageal ganglia, there in the cerebral hemispheres and optic and olfactory nerves the supraoesophageal ganglia, there in the spinal cord the ventral chain of ganglia. But if the infundibulum was the old oesophagus, what then? The old oesophagus was continuous with and led into the cephalic stomach. What about the infundibulum? It was continuous with and led into the ventricles of the brain, and the whole thing became clear. The ventricles of the brain were the old cephalic stomach, and the canal of the spinal cord the long straight intestine which led originally to the anus, and still in the vertebrate embryo opens out into the anus. Not having been educated in a morphological laboratory and taught that the one organ which is homologous throughout the animal kingdom is the gut, and that therefore the gut of the invertebrate ancestor must continue on as the gut of the vertebrate, the conception that the central nervous system has grown round and enclosed the original ancestral gut, and that the vertebrate has formed a new gut did not seem to me so impossible as to prevent my taking it as a working hypothesis, and seeing to what it would lead.

This theory that the so-called central nervous system of the vertebrate is in reality composed of two separate parts, of which the one, the segmented part, corresponds to the central nervous system of the highest invertebrates, while the other, the unsegmented tube, was originally the alimentary canal of that same invertebrate, came into my mind in the year 1887. The following year, on June 23, 1888, I read a paper on the subject before the Anatomical Society at Cambridge, which was published in the _Journal of Anatomy and Physiology_, vol. 23, and more fully in the _Journal of Physiology_, vol. 10. Since that time I have been engaged in testing the theory in every possible way, and have published the results of my investigations in a series of papers in different journals, a list of which I append at the end of this introductory chapter.

{5}It is now twenty years since the theory first came into my mind, and the work of those twenty years has convinced me more and more of its truth, and yet during the whole time it has been ignored by the morphological world as a whole rather than criticized. Whatever may have been the causes for such absence of criticism, it is clear that the serial character of its publication is a hindrance to criticism of the theory as a whole, and I hope, therefore, that the publication of the whole of the twenty years' work in book-form will induce those who differ from my conclusions to come forward and show me where I am wrong, and why my theory is untenable. Any one who has been thinking over any one problem for so long a time becomes obsessed with the infallibility of his own views, and is not capable of criticizing his own work as thoroughly as others would do. I have been told that it is impossible for one man to consider so vast a subject with that thoroughness which is necessary, before any theory can be accepted as the true solution of the problem. I acknowledge the vastness of the task, and feel keenly enough my own shortcomings. For all that, I do feel that it can only be of advantage to scientific progress and a help to the solution of this great problem, to bring together in one book all the facts which I have been able to collect, which appeal to me as having an important bearing on this solution.

In this work I have been helped throughout by Miss R. Alcock. It is not too much to say that without the assistance she has given me, many an important link in the chain of evidence would have been missing. With extraordinary patience she has followed, section by section, the smallest nerves to their destination, and has largely helped to free the transformation process in the lamprey from the mystery which has hitherto enveloped it. She has drawn for me very many of the illustrations scattered through the pages in this book, and I feel that her aid has been so valuable and so continuous, lasting as it does over the whole period of the work, that her name ought fittingly to be associated with mine, if perchance the theory of the Origin of Vertebrates, advocated in the pages of this book, gains acceptance.

I am also indebted to Mr. J. Stanley Gardiner and to Dr. A. Sheridan Lea for valuable assistance in preparing this book for the press. I desire to express my grateful thanks to the former for valuable criticism of the scientific evidence which I have brought {6}forward in this book, and to the latter for his great kindness in undertaking the laborious task of collecting the proofs.

LIST OF PREVIOUS PUBLICATIONS BY THE AUTHOR, CONCERNING THE ORIGIN OF VERTEBRATES.

1888. "Spinal and Cranial Nerves." _Proceedings of the Anatomical
Society_, June, 1888. _Journal of Anatomy and Physiology_,
vol. xxiii.

1889. "On the Relation between the Structure, Function, Distribution,
and Origin of the Cranial Nerves; together with a Theory of the
Origin of the Nervous System of Vertebrata." _Journal of
Physiology_, vol. x., p. 153.

1889. "On the Origin of the Central Nervous System of Vertebrates."
_Brain_, vol. xii., p. 1.

1890. "On the Origin of Vertebrates from a Crustacean-like Ancestor."
_Quarterly Journal of Microscopical Science_, vol. xxxi., p. 379.

1895. "The Origin of Vertebrates." _Proceedings of the Cambridge
Philosophical Society_, vol. ix., p. 19.

1896. Presidential Address to Section I. at the meeting of the British
Association for the Advancement of Science in Liverpool. _Report
of the British Association_, 1896, p. 942.

1899. "On the Meaning of the Cranial Nerves." Presidential Address to
the Neurological Society for the year 1899. _Brain_, vol. xxii.,
p. 329.

A series of papers on "The Origin of Vertebrates, deduced from the study of Ammocoetes," in the _Journal of Anatomy and Physiology_, as follows:--

1898. Part I. "The Origin of the Brain," vol. xxxii., p. 513.

" II. "The Origin of the Vertebrate Cranio-facial Skeleton,"
vol. xxxii., p. 553.

" III. "The Origin of the Branchial Segmentation," vol.
xxxiii., p. 154.

1899. " IV. "The Thyroid, or Opercular Segment: the Meaning of the
Facial Nerve," vol. xxxiii., p. 638.

1900. " V. "The Origin of the Pro-otic Segmentation: the Meaning
of the Trigeminal and Eye-muscle Nerves," vol.
xxxiv., p. 465.

1900. " VI. "The Old Mouth and the Olfactory Organ: the Meaning
of the First Nerve," vol. xxxiv., p. 514.

1900. " VII. "The Evidence of Prosomatic Appendages in Ammocoetes,
as given by the Course and Distribution of the
Trigeminal Nerve," vol. xxxiv., p. 537.

1900. " VIII. "The Palæontological Evidence: Ammocoetes a
Cephalaspid," vol. xxxiv., p. 562.

1901. " IX. "The Origin of the Optic Apparatus: the Meaning of the
Optic Nerves," vol. xxxv., p. 224.

1902. " X. "The Origin of the Auditory Organ: the Meaning of the
VIIIth Cranial Nerve," vol. xxxvi., p. 164.

1903. " XI. "The Origin of the Vertebrate Body-cavity and Excretory
Organs: the Meaning of the Somites of the Trunk and
of the Ductless Glands," vol. xxxvii., p. 168.

1905. " XII. "The Principles of Embryology," vol. xxxix., p. 371.

1906. " XIII. "The Origin of the Notochord and Alimentary Canal,"
vol. xl., p. 305.

{8}CHAPTER I

_THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM_

Theories of the origin of vertebrates.--Importance of the central nervous
system.--Evolution of tissues.--Evidence of Palæontology.--Reasons for
choosing Ammocoetes rather than Amphioxus.--Importance of larval
forms.--Comparison of the vertebrate and arthropod central nervous
systems.--Antagonism between cephalization and
alimentation.--Life-history of lamprey: not a degenerate animal.--Brain
of Ammocoetes compared with brain of arthropod.--Summary.

At the present time it is no longer a debatable question whether or no Evolution has taken place. Since the time of Darwin the accumulation of facts in its support has been so overwhelming that all zoologists look upon this question as settled, and desire now to find out the manner in which such evolution has taken place. Here two problems offer themselves for investigation, which can be and are treated separately--the one dealing with the question of those laws of heredity and variation which have brought about in the past and are still causing in the present the evolution of living beings, _i.e._ the causes of evolution; the other concerned with the relationship of animals, or groups of animals, rather than with the causes which have brought about such relationship, _i.e._ the sequence of evolution.

It is the latter problem with which this book deals, and, indeed, not with the whole question at all, but only with that part of it which concerns the origin of vertebrates.

This problem of the sequence of evolution is of a twofold character: first, the finding out of the steps by which the higher forms in any one group of animals have been evolved from the lower; and secondly, the evolution of the group itself from a lower group.

In any classification of the animal kingdom, it is clear that large groups of animals exist which have so many common characteristics as to necessitate their being placed in one larger group or kingdom; {9}thus zoologists are able to speak definitely of the Vertebrata, Arthropoda, Annelida, Echinodermata, Porifera, Coelenterata, Mollusca, etc. In each of these groups affinities can be traced between the members, so that it is possible to speak of the progress from lower to higher members of the group, and it is conceivable, given time to work out the details, that the natural relationships between the members of the whole group will ultimately be discovered.

Thus no one can doubt that a sequence of the kind has taken place in the Vertebrata as we trace the progress from the lowest fishes to man, and already the discoveries of palæontology and anatomy give us a distinct clue to the sequence from fish to amphibian, from amphibian to reptile, from reptile to mammal on the one hand, and to bird on the other. That the different members of the vertebrate group are related to each other in orderly sequence is no longer a matter of doubt; the connected problems are matters of detail, the solution of which is certain sooner or later. The same may be said of the members of any of the other great natural groups, such as the Arthropoda, the Annelida, the Echinodermata, etc.

It is different, however, when an attempt is made to connect two of the main divisions themselves. It is true enough that there is every reason to believe that the arthropod group has been evolved from the segmented annelid, and so the whole of the segmented invertebrates may be looked on as forming one big division, the Appendiculata, all the members of which will some day be arranged in orderly sequence, but the same feeling of certainty does not exist in other cases.

In the very case of the origin of the Appendiculata we are confronted with one of the large problems of evolution--the origin of segmented from non-segmented animals--the solution of which is not yet known.

THEORIES OF THE ORIGIN OF VERTEBRATES.

The other large problem, perhaps the most important of all, is the question of the relationship of the great kingdom of the Vertebrata: from what invertebrate group did the vertebrate arise?

The great difficulty which presents itself in attempting a solution of this question is not so much, as used to be thought, the difficulty of deriving a group of animals possessing an internal bony and {10}cartilaginous skeleton from a group possessing an external skeleton of a calcareous or chitinous nature, but rather the difficulty caused by the fundamental difference of arrangement of the important internal organs, especially the relative positions of the central nervous system and the digestive tube.

_Al_, gut; _H_, heart; _C.N.S._, central nervous system; V, ventral side; D, dorsal side.]

Now, if we take a broad and comprehensive view of the invertebrate kingdom, without arguing out each separate case, we find that it bears strongly the stamp of a general plan of evolution derived from a coelenterate animal, whose central nervous system formed a ring surrounding the mouth. Then when the radial symmetry was given up, and an elongated, bilateral, segmented form evolved, the central nervous system also became elongated and segmented, but, owing to its derivation from an oral ring, it still surrounded the mouth-tube, or oesophagus, and thus in its highest forms is divided into supra-oesophageal and infra-oesophageal nervous masses. These latter {11}nervous masses are of necessity ventral to the digestive tube, because the mouth of the coelenterate is on the ventral side. The striking characteristic, then, of the invertebrate kingdom is the situation of a large portion of the central nervous system ventrally to the alimentary canal and the piercing of the nervous system by a tube--the oesophagus--leading from the mouth to the alimentary canal. The equally striking characteristic of the vertebrate is the dorsal position of the central nervous system and the ventral position of the alimentary canal combined with the absence of any piercing of the central nervous system by the oesophagus.

So fundamentally different is the arrangement of the important organs in the two groups that it might well give rise to a feeling of despair of ever hoping to solve the problem of the Origin of Vertebrates; and, to my mind, this is the prevalent feeling among morphologists at the present time. Two attempts at solution have been made. The one is associated with the name of Geoffrey St. Hilaire, and is based on the supposition that the vertebrate has arisen from the invertebrate by turning over on its back, swimming in this position, and so gradually converting an originally dorsal surface into a ventral one, and _vice versâ_; at the same time, a new mouth is supposed to have been formed on the new ventral side, which opened directly into the alimentary canal, while the old mouth, which had now become dorsal, was obliterated.

The other attempt at solution is of much more recent date, and is especially associated with the name of Bateson. It supposes that bilaterally symmetrical, elongated, segmented animals were formed from the very first in two distinct ways. In the one case the digestive tube pierced the central nervous system, and was situated dorsally to its main mass. In the other case the segmented central nervous system was situated from the first dorsally to the alimentary canal, and was not pierced by it. In the first case the highest result of evolution led to the Arthropoda; in the second case to the Vertebrata.

Neither of these views is based on evidence so strong as to cause universal acceptance. The great difficulty in the way of accepting the second alternative is the complete absence of any evidence, either among animals living on the earth at the present day or among those known to have existed in the past, of any such chain of intermediate animal forms as must, on this hypothesis, have existed in order to link together the lower forms of life with the vertebrates.

{12}

It has been supposed that the Tunicata and the Enteropneusta (_Balanoglossus_) (Fig. 2) are members of this missing chain, and that in Amphioxus the vertebrate approaches in organization to these low invertebrate forms. The tunicates, indeed, are looked upon as degenerate members of an early vertebrate stock, which may give help in picturing the nature of the vertebrate ancestor but are not themselves in the direct line of descent. Balanoglossus is supposed to have arisen from the Echinodermata, or at all events to have affinities with them, so that to fill up the enormous gap between the Echinodermata and the Vertebrata on this theory there is absolutely nothing living on the earth except Balanoglossus, Rhabdopleura, and Cephalodiscus. The characteristics of the vertebrate upon which this second theory is based are the notochord, the respiratory character of the anterior part of the alimentary canal, and the tubular nature of the central nervous system; it is claimed that in Balanoglossus the beginnings of a notochord and a tubular central nervous system are to be found, while the respiratory portion of the gut is closely comparable to that of Amphioxus.

The strength of the first theory is essentially based on the comparison of the vertebrate central nervous system with that of the segmented invertebrate, annelid or arthropod. In the latter the central nervous system is composed of--

1. The supra-oesophageal ganglia, which give origin to the nerves of the eyes and antennules, _i.e._ to the optic and olfactory nerves, for the first pair of antennæ are olfactory in function. These are connected with the infra-oesophageal ganglia by the oesophageal commissures which encircle the oesophagus.

2. The infra-oesophageal ganglia and the two chains of ventral ganglia, which are segmentally-arranged sets of ganglia. Of these, {13}each pair gives rise to the nerves of its own segment, and these nerves are not nerves of special sense as are the supra-oesophageal nerves, but motor and sensory to the segment; nerves by the agency of which food is taken in and masticated, respiration is effected, and the animal moves from place to place.

In the vertebrate the central nervous system consists of--

1. The brain proper, from which arise only the olfactory and optic nerves.

A. Vertebrate central nervous system. _S. Inf. Br._, supra-infundibular brain; _I. Inf. Br._, infra-infundibular brain and cranial segmental nerves; _C.Q._, corpora quadrigemina; _Cb._, cerebellum; _C.C._, crura cerebri; _C.S._, corpus striatum; _Pn._, pineal gland.

B. Invertebrate central nervous system. _S. Oes. G._, supra-oesophageal ganglia; _I. Oes. G._, infra-oesophageal ganglia; _Oes. Com._, oesophageal commissures.]

2. The region of the mid-brain, medulla oblongata, and spinal cord; from these arises a series of nerves segmentally arranged, which, as in the invertebrate, gives origin to the nerves governing mastication, respiration, and locomotion.

Further, the vertebrate central nervous system possesses the peculiarity, found nowhere else, of being tubular, and the tube is of a striking character. In the spinal region it is a small, simple canal of uniform calibre, which at the front end dilates to form the ventricles of the region of the brain. From that part of this dilated {14}portion, known as the third ventricle, a narrow tube passes to the ventral surface of the brain. This tube is called the _infundibulum_, and, extraordinary to relate, lies just anteriorly to the exits of the third cranial or oculomotor nerves; in other words, it marks the termination of the series of spinal and cranial segmental nerves. Further, on each side of this infundibular tube are lying the two thick masses of the _crura cerebri_, the strands of fibres which connect the higher brain-region proper with the lower region of the medulla oblongata and spinal cord. Not only, then, are the nerve-masses in the two systems exactly comparable, but in the very place where the oesophageal tube is found in the invertebrate, the infundibular tube exists in the vertebrate, so that if the words infundibular and oesophageal are taken to be interchangable, then in every respect the two central nervous systems are comparable. The brain proper of the vertebrate, with its olfactory and optic nerves, becomes the direct descendant of the supra-oesophageal ganglia; the crura cerebri become the oesophageal commissures, and the cranial and spinal segmental nerves are respectively the nerves belonging to the infra-oesophageal and ventral chain of ganglia.

This overwhelmingly strong evidence has always pointed directly to the origin of the vertebrate from some form among the segmented group of invertebrates, annelid or arthropod, in which the original oesophagus had become converted into the infundibulum, and a new mouth formed. So far, the position of this school of anatomists was extremely sound, for it is impossible to dispute the facts on which it is based. Still, however, the fact remained that the gut of the vertebrate lies ventrally to the nervous system, while that of the invertebrate lies dorsally; consequently, since the infundibulum was in the position of the invertebrate oesophagus, it must originally have entered into the gut, and since the vertebrate gut was lying ventrally to it, it could only have opened into that gut in the invertebrate stage by the shifting of dorsal and ventral surfaces. From this argument it followed that the remains of the original mouth into which the infundibulum, _i.e._ oesophagus, opened were to be sought for on the dorsal side of the vertebrate brain. Here in all vertebrates there are two spots where the roof of the brain is very thin, the one in the region of the pineal body, and the other constituting the roof of the fourth ventricle. Both of these places have had their advocates as the position of the old mouth, the former being upheld by Owen, the latter by Dohrn.

{15}The discovery that the pineal body was originally an eye, or, rather, a pair of eyes, has perhaps more than anything else proved the impossibility of accepting this reversal of surfaces as an explanation of the genesis of the vertebrate from the annelid group. For whereas a pair of eyes close to the mid-dorsal line is not only likely enough, but is actually found to exist among large numbers of arthropods, both living and extinct, a pair of eyes situated close to the mid-ventral line near the mouth is not only unheard of in nature, but so improbable as to render impossible the theory which necessitates such a position.

Yet this very discovery gives the strongest possible additional support to the close identity in the plan of the central nervous system of vertebrate and appendiculate.

A truly paradoxical situation! The very discovery which may almost be said to prove the truth of the hypothesis, is the very one which has done most to discredit it, because in the minds of its authors the only possible solution of the transition from the one group to the other was by means of the reversal of surfaces.

Still, as already said, even if the theory advanced to explain the facts be discredited, the facts remain the same; and still to this day an explanation is required as to why such extraordinary resemblances should exist between the two nervous systems, unless there is a genetic connection between the two groups of animals. An explanation may still be found, and must be diligently sought for, which shall take into account the strong evidence of this relationship between the two groups, and yet not necessitate any reversal of surfaces. It is the object of this book to consider the possibility of such an explanation.

What are the lines of investigation most likely to meet with success? Is it possible to lay down any laws of evolution? It is instructive to consider the nature of the investigations which have led to the two theories just mentioned, for the fundamental starting-point is remarkably different in the two cases. The one theory is based upon the study of the vertebrate itself, and especially of its central nervous system, and its supporters and upholders have been and are essentially anatomists, whose chief study is that of vertebrate and human anatomy. The other theory is based upon the study of the invertebrate, and consists especially of an attempt to find in the invertebrate some structure resembling a notochord, such {16}organ being considered by them as the great characteristic of the vertebrate; indeed, so much is this the case, that a large number of zoologists speak now of Chordata rather than of Vertebrata, and in order to emphasize their position follow Bateson, and speak of the Tunicata as Uro-chordata, of Amphioxus as Cephalo-chordata, of the Enteropneusta as Hemi-chordata, and even of Actinotrocha (to use Masterman's term), as Diplo-chordata.

The upholders of this theory lay no stress on the nature of the central nervous system in vertebrates, they are essentially zoologists who have made a special study of the invertebrate rather than of the vertebrate.

Of these two methods of investigating the problem, it must be conceded that the former is more likely to give reliable results. By putting the vertebrate to the question in every possible way, by studying its anatomy and physiology, both gross and minute, by inquiring into its past history, we can reasonably hope to get a clue to its origin, but by no amount of investigation can we tell with any certainty what will be its future fate; we can only guess and prophesy in an uncertain and hesitating manner. So it must be with any theory of the origin of vertebrates, based on the study of one or other invertebrate group. Such theory must partake rather of the nature of prophecy than of deduction, and can only be placed on a firm basis when it so happens that the investigation of the vertebrate points irresistibly to its origin from the same group; in fact, "never prophesy unless you know."

The first principle, then, I would lay down is this: In order to find out the origin of vertebrates, inquire, in the first place, of the vertebrate itself.

IMPORTANCE OF THE CENTRAL NERVOUS SYSTEM.

Does the history of evolution pick out any particular organ or group of organs as more necessary than another for upward progress? If so, it is upon that organ or group of organs that special stress must be laid.

Since Darwin wrote the "Origin of Species," and laid down that the law of the 'survival of the fittest' is the factor upon which evolution depends, it has gradually dawned upon the scientific mind that 'the fittest' may be produced in two diametrically opposite ways: {17}either by progress upwards to a superior form, or by degeneration to a lower type of animal. The principle of degeneration as a factor in the formation of groups of animals, which are thereby enabled to survive, is nowadays universally admitted. The most striking example is to be found in the widely distributed group of Tunicata, which live, in numbers of instances, a sedentary life upon the rocks, have the appearance of very low forms of animal life, propagate by budding, have lost all the characteristics of higher forms, and yet are considered to be derived from an original vertebrate stock. Such degenerate forms remain degenerate, and are never known to regenerate and again to reach the higher stage of evolution from which they arose. Such forms are of considerable interest, but cannot help, except negatively, to decide what factor is especially important for upward progress.

At the head of the animal race at the present day stands man, and in mankind itself some races are recognized as higher than others. Such recognition is given essentially on account of their greater brain-power, and without doubt the great characteristic which puts man at the head is the development of his central nervous system, especially of the region of the brain. Not only is this point most manifest in distinguishing man from the lower animals, but it applies to the latter as well. By the amount of convolution of the brain, the amount of grey matter in the cerebral hemispheres, the enlargement and increasing complexity of the higher parts of the central nervous system, the anthropoid apes are differentiated from the lower forms, and the higher mammals from the lower. In the recent work of Elliot Smith, and of Edinger, most conclusive proof is given that the upward progress in the vertebrate phylum is correlated with the increase of brain-power, and the latter writer shows how steady and remarkable is the increase in substance and in complexity of the brain-region as we pass from the fishes, through the amphibians and reptiles, to the birds and mammals.

The study of the forms which lived on the earth in past ages confirms and emphasizes this conclusion, for it is most striking to see how small is the cranium among the gigantic Dinosaurs; how in the great reptilian age the denizens of the earth were far inferior in brain-power to the lords of creation in after-times.

What applies to the vertebrate phylum applies also to the invertebrate groups. Here also an upward progress is recognized as we {18}pass from the sponges to the arthropods--a progress which is manifested, first by the concentration of nervous material to form a central nervous system, and then by the increase in substance and complexity of that nervous system to form a higher and a higher type, until the culmination is reached in the nervous system of the scorpions and spiders. No upward progress is possible with degeneration of the central nervous system, and in all those cases where a group owes its existence to degeneration, the central nervous system takes part in the degeneration.

This law of the paramount importance of the growth of the central nervous system for all upward progress in the evolution of animals receives confirmation from the study of the development of individuals, especially in those cases where a large portion of the life of the animal is spent in a larval condition, and then, by a process of transformation, the larva changes into the adult form. Such cases are well known among Arthropoda, the familiar instance being the change from the larval caterpillar to the adult imago. Among Vertebrata, the change from the tadpole to the frog, from the larval form of the lamprey (_Ammocoetes_) to the adult form (_Petromyzon_), are well-known instances. In all such cases the larva shows signs of having attained a certain stage in evolution, and then a remarkable transformation takes place, with the result that an adult animal emerges, whose organization reaches a higher stage of evolution than that of the larva.

This transformation process is characterized by a very great destruction of the larval tissues and a subsequent formation of new adult tissues. Most extensive is the destruction in the caterpillar and in the larval lamprey. But one organ never shares in this process of histolysis, and that is the central nervous system; amidst the ruins of the larva it remains, leading and directing the process of re-formation. In the Arthropoda, the larval alimentary canal may be entirely destroyed and eaten up by phagocytes, but the central nervous system not only remains intact but increases in size, and by the concentration and cephalization of its infra-oesophageal ganglia forms in the adult a central nervous system of a higher type than that of the larva.

So, too, in the transformation of the lamprey, there is not the slightest trace of any destruction in the central nervous system, but simply a development and increase in nervous material, which {19}results in the formation of a brain region more like that of the higher vertebrates than exists in Ammocoetes.

In these cases the development is upward--the adult form is of a higher type than that of the larva. It is, however, possible for the reverse to occur, so that the individual development leads to degeneration, not to a higher type. Instances are seen in the Tunicata, and in various parasitic arthropod forms, such as Lernæa, etc. In these cases, the transformation from the larval to the adult form leads to degradation, and in this degradation the central nervous system is always involved.

It is perhaps a truism to state that upward progress is necessarily accompanied by increased development of the central nervous system; but it is necessary to lay special stress upon the importance of the central nervous system in all problems of evolution, because there is, in my opinion, a tendency at the present time to ignore this factor to too great an extent.

The law of progress is this--The race is not to the swift, nor to the strong, but to the wise.

This law carries with it the necessary corollary that the immediate ancestor of the vertebrate must have had a central nervous system nearly approaching that of the lowest undegenerated vertebrate. Among all the animals living on the earth at the present time, the highest invertebrate group, the Arthropoda, possesses a central nervous system most closely resembling that of the vertebrate.

The law, then, of the paramount importance of a steady development of the central nervous system for the upward progress of the animal kingdom, points directly to the arthropod as the most probable ancestor of the vertebrate.

EVOLUTION OF TISSUES.

In the whole scheme of evolution we can recognize, not only an upward progress in the organization of the animal as a whole, but also a distinct advance in the structure of the tissues composing an individual, which accompanies that upward progress. Thus it is possible to speak of an evolution of the supporting tissues from the simplest form of connective tissue up to cartilage and thence to bone; of the contractile tissues, from the simplest contractile protoplasm {20}to unstriped muscle, and thence to the highest forms of striated muscle; of the nervous connecting strands, from undifferentiated to fine strands, then to thicker, more separated ones, resembling non-medullated fibres, and finally to well-differentiated separate fibres, each enclosed in a medullated sheath.

In the connective tissue group, bone is confined to the vertebrates, cartilage is found among invertebrates, and the closest resemblance to vertebrate embryonic or parenchymatous cartilage is found in the cartilage of Limulus. Also, as Gegenbaur has pointed out, Limulus, more than any other invertebrate, possesses a fibrous connective tissue resembling that of vertebrates.

In the muscular group, Biedermann, who has made a special study of the physiology of striated muscle, says that among invertebrates the striated muscle of the arthropod group resembles most closely that of the vertebrate.

In the nervous group the resemblance between the nerve-fibres of Limulus and Ammocoetes, both of which are devoid of any marked medullary sheath, is very apparent, and Retzius points out that the only evidence of medullation, so characteristic of the vertebrates, is found in a species of prawn (_Palæmon_). In all these cases the nearest resemblance to the vertebrate tissues is to be found in the arthropod.

THE EVIDENCE OF PALÆONTOLOGY.

Perhaps the most important of all the clues likely to help in the solution of the origin of vertebrates is that afforded by Geology, for although the geological record is admittedly so imperfect that we can never hope by its means alone to link together the animals at present in existence, yet it does undoubtedly point to a sequence in the evolution of animal forms, and gives valuable information as to the nature of such sequence. In different groups of animals there are times when the group can be spoken of as having attained its most flourishing period. During these geological epochs the distribution of the group was universal, the numbers were very great, the number of species was at the maximum, and some of them had attained a maximal size. Such races were at that time dominant, and the struggle for existence was essentially among members of the same group. At the present time the dominant race is man, and the {21}struggle for existence is essentially between the members of that race, and not between them and any inferior race.

The effect of such conditions is, as Darwin has pointed out, to cause great variation in that group; in consequence of that variation and that dominance the evolution of the next higher group is brought about from some member of the dominant group. Thus the present age is the outcome of the Tertiary period, a time when giant mammals roamed the earth and left as their successors the mammals of the present day; a time of dominance of quadruped mammals; a time of which the period of maximum development is long past, and we now see how the dominance of the biped mammal, man, is accompanied by the rapid diminution and approaching extermination of the larger mammals. No question can possibly arise as to the immediate ancestor of the biped mammal; he undoubtedly arose from one of the dominant quadrupedal mammals.

Passing along to the next evidence of the rocks, we find an age of reptiles in the Mesozoic period. Here, again, the number and variety is most striking; here, again, the size is enormous in comparison with that of the present-day members of the group. This was the dominant race at the time when the birds and mammals first appeared on the earth, and anatomists recognize in these extinct reptilian forms two types; the one bird-like, the other more mammalian in character. From some members of the former group birds are supposed to have been evolved, and mammals from members of the other group. There is no question of their origin directly from lower fish-like forms; the time of their appearance on the earth, their structure, all point irresistibly to the same conclusion as we have arrived at from the consideration of the origin of the biped from the quadruped mammal, viz. that birds and mammals arose, in consequence of the struggle for existence, from some members of the reptilian race which at that time was the dominant one on earth.

Passing down the geological record, we find that when the reptiles first appear in the Carboniferous age there is abundant evidence of the existence of numbers of amphibian forms. At this time the giant Labyrinthodonts flourished. Here among the swamps and marshes of the coal-period the prevalent vertebrate was amphibian in structure. Their variety and number were very great, and at that period they attained their greatest size. Here, again, from the geological record we draw the same conclusion as before, that the reptiles arose from the race which was then predominant on the earth--the Amphibia.

{22}

{23}Again, another point of great interest is seen here, and that is that these Labyrinthodonts, as Huxley has pointed out, possess characters which bring them more closely than the amphibians of the present day into connection with the fishes; and further, the fish-like characters they possessed are those of the Ganoids, the Marsipobranchs, the Dipnoans, and the Elasmobranchs, rather than of the Teleosteans.

Now, it is a striking fact that the ancient fishes at the time when the amphibians appeared had not reached the teleostean stage. The ganoids and elasmobranchs swarmed in the waters of the Devonian and Carboniferous times. Dipnoans and marsipobranchs were there, too, in all probability, but teleosteans do not appear until the Mesozoic period. The very kinds of fish, then, which swarmed in the seas at that time, and were the predominant race before the Carboniferous epoch, are those to which the amphibians at their first appearance show the closest affinity. Here, again, the same law appears; from the predominant race at the time, the next higher race arose, and arose by a most striking modification, which was the consequence of altering the medium in which it lived. By coming out of the water and living on the land, or, rather, being able to live partly on land and partly in the water, by the acquisition of air-breathing respiratory organs or lungs in addition to, and instead of, water-breathing organs or gills, the amphibian not only arose from the fish, but made an entirely new departure in the sequence of progressive forms.

This was a most momentous step in the history of evolution--one fraught with mighty consequences and full of most important suggestions.

From this time onwards the struggle for existence by which upward progress ensued took place on the land, not in the sea, and, as has been pointed out, led to the evolution of reptiles from amphibians, birds and quadrupedal mammals from reptiles, and man from quadrupeds. In the sea the fishes were left to multiply and struggle among themselves, their only opponents being the giant cephalopods, which themselves had been evolved from a continual succession of the Mollusca. For this reason the struggle for existence between the fishes and the higher race evolved from them did not {24}take place until some members of that higher race took again to the water, and so competed with the fish-tribe in their own element.

Another most important conclusion to be derived from the uprising of the Amphibia is that at that time there was no race of animals living on the land which had a chance against them. No race of land-living animals had been evolved whose organization enabled them to compete with and overcome these intruders from the sea in the struggle for existence. For this reason that the whole land was their own, and no serious competition could arise from their congeners, the fish, they took possession of it, and increased mightily in size; losing more and more the habit of going into the water, becoming more and more truly terrestrial animals. Henceforth, then, in trying to find out the sequence of evolution, we must leave the land and examine the nature of the animals living in the sea; the air-breathing animals which lived on the land in the Upper Silurian and Devonian times cannot have reached a stage of organization comparable with that of the fishes, seeing how easily the amphibians became dominant.

We arrive, then, at the conclusion that the ancestors of the fishes must have lived in the sea, and applying still the same principles that have held good up to this time, the ancestors of the fishes must have arisen from some member of the race predominant at the time when they first appeared, and also the earliest fishes must have much more closely resembled the ancestral form than those found in later times or at the present day.

What, then, is the record of the rocks at the time of the first appearance of fish-like forms? What kind of fishes were they, and what was the predominant race at the time?

We have now reached the Upper Silurian and Lower Devonian times, and most instructive and suggestive is the revelation of the rocks. Here, when the first vertebrates appeared, the sea was peopled with corals, brachiopods, early forms of cephalopods, and other invertebrates; but, above all, with the great tribe of trilobites (Fig. 6) and their successors. From the trilobites arose, as evidenced by their larval form, the king-crab group, called the Xiphosura (Fig. 5). Closely connected with them, and forming intermediate stages between trilobites and king-crabs, numerous forms have been discovered, known as Belinurus, Prestwichia, Hemiaspis, Bunodes, etc. (Fig. 5 and Fig. 12). From them also arose the most striking group {25}of animals which existed at this period--the giant sea-scorpions, or Gigantostraca. This group was closely associated with the king-crabs, and the two groups together are classified under the title Merostomata.

The appearance of these sea-scorpions is given in Figs. 7 and 8, representing Stylonurus, Slimonia, Pterygotus, Eurypterus. They must have been in those days the tyrants of the deep, for specimens of Pterygotus have been found over six feet in length.

At this time, then, by every criterion hitherto used, by the multitude of species, by the size of individual species, which at this period reached the maximum, by their subsequent decay and final extinction, we must conclude that these forms were in their zenith, that the predominant race at this time was to be found in this group of arthropods. Just previously, the sea swarmed with trilobites, and right into the period when the Gigantostraca flourished, the trilobites {26}are still found of countless forms, of great difference in size. The whole period may be spoken of as the great trilobite age, just as the Tertiary times form the mammalian age, the Mesozoic times the reptilian age, etc. From the trilobites the Gigantostraca and Xiphosura arose, as evidenced by the embryology of Limulus, and, therefore, in the term trilobite age would be included the whole of those peculiar forms which are classified by the names Trilobita, Gigantostraca, Xiphosura, etc. Of all these the only member alive at the present time is Limulus, or the King-Crab.

As, however, the term 'trilobite' does not include the members of the king-crab or sea-scorpion groups, it is advisable to use some other term to represent the whole group. They cannot be called crustaceans or arachnids, for in all probability they gave origin to both; the nearest approach to the Trilobite stage of development at the present time is to be found perhaps in Branchipus (Fig. 10) and Apus (Fig. 9), just as the nearest approach to the Eurypterid {27}form is Limulus. Crustaceans such as crabs and lobsters are of much later origin, and do not occur in any quantity until the late Mesozoic period. The earliest found, a kind of prawn, occurs in the Carboniferous age.

Korschelt and Heider have accordingly suggested the name _Palæostraca_ for this whole group, and _Protostraca_ for the still earlier {28}arthropod-like animals which gave origin to the trilobites themselves. This name I shall adopt, and speak, therefore, of the _Palæostraca_ as the dominant race at the time when vertebrates first appeared.

If, then, there is no break in the law of evolution here, the race which was predominant at the time when the vertebrate first appeared must have been that from which the first fishes arose, and these fishes must have resembled, not the crustacean proper, or the arachnid proper, but a member of the palæostracan group. Moreover, just as the Labyrinthodonts show special affinities to the fishes which were then living, so we should expect that the forms of the earliest fish would resemble the arthropodan type dominant at the time more closely than the fish of a later era.

At first sight it seems too great an absurdity even to imagine the possibility of any genetic connection between a fish and an arthropod, for to the mind's eye there arises immediately the picture of a salmon or a shark and a lobster or a spider. So different in appearance are the two groups of animals, so different their methods of locomotion, that it is apparently only an inmate of a lunatic asylum who could possibly suggest such a connection. Much more likely is it that a fish-like form should have been developed out of a smooth, wriggling, worm-like animal, and it is therefore to the annelids that the upholders of the theory of the reversal of surfaces look for the ancestor of the vertebrate.