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 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 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, 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 œsophagus, there in the cranial segmental nerves the infraœsophageal ganglia, there in the cerebral hemispheres and optic and olfactory nerves the supraœsophageal ganglia, there in the spinal cord the ventral chain of ganglia. But if the infundibulum was the old œsophagus, what then? The old œsophagus 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.

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 forward in this book, and to the latter for his great kindness in undertaking the laborious task of collecting the proofs.

A series of papers on "The Origin of Vertebrates, deduced from the study of Ammocœtes," in the Journal of Anatomy and Physiology, as follows:—