FINAL REMARKS
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.
[Illustration: FIG. 1.--ARRANGEMENT OF ORGANS IN THE VERTEBRATE (A) AND
ARTHROPOD (B).
_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}[Illustration: FIG. 2.--LARVAL BALANOGLOSSUS (from the Royal Natural
History).]
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.
[Illustration: FIG. 3.--VERTEBRATE CENTRAL NERVOUS SYSTEM COMPARED WITH THE
CENTRAL NERVOUS SYSTEM AND ALIMENTARY CANAL OF THE ARTHROPOD.
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}[Illustration: FIG. 4.--PLAN OF GEOLOGICAL STRATA. (From LANKESTER.)]
{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.
[Illustration: FIG. 5 (from H. WOODWARD).--1. _Limulus polyphemus_ (dorsal
aspect). 2. _Limulus,_ young, in trilobite stage. 3. _Prestwichia
rotundata._ 4. _Prestwichia Birtwelli._ 5. _Hemiaspis limuloides._ 6.
_Pseudoniscus aculeatus._]
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.
[Illustration: FIG. 6.--A TRILOBITE (_Dalmanites_) (after PICTET). Dorsal
view.]
[Illustration: FIG. 7.--_Eurypterus remipes_ (after NIESKOWSKI). Dorsal
view.]
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.
[Illustration: FIG. 8.--A, _Pterygotus Osiliensis_ (from SCHMIDT). B,
_Stylonurus Logani_ (from WOODWARD). C, _Slimonia acuminata_ (from
WOODWARD).]
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.
[Illustration: FIG. 9.--_Apus_ (from the Royal Natural History). Dorsal
view.]
[Illustration: FIG. 10.--_Branchipus stagnalis._ (From CLAUS.)]
{29}We must endeavour to dismiss from our imagination such forms as the
salmon and shark as representatives of the fish-tribe, and the lobster and
spider of the arthropods, and try to picture the kind of animals living in
the seas in the early Devonian and Upper Silurian times, and then we find,
to our surprise, that instead of the contrast between fishes and arthropods
being so striking as to make any comparison between the two seem an
absurdity, the difficulty in the last century, and even now, is to decide
in many cases whether a fossil is an arthropod or a fish.
I have shown what kind of animal the palæostracan was like. What
information is there of the nature of the earliest vertebrate?
The most ancient fishes hitherto discovered have been classified by
Lankester and Smith Woodward into the three orders, Heterostraci,
Osteostraci, and Antiarcha. Of these the Heterostraci contain the genera
Pteraspis and Cyathaspis, and are the very earliest vertebrates yet
discovered, being found in the Lower Silurian. The Osteostraci are divided
into the Cephalaspidæ, Tremataspidæ, etc., and are found in the Upper
Silurian and Devonian beds. The Antiarcha, comprising Pterichthys and
Bothriolepis, belong to the Devonian and are not found in Silurian
deposits. This, then, is the order of their appearance--Pteraspis,
Cephalaspis, and Pterichthys.
In none of these families is there any resemblance to an ordinary fish. In
no case is there any sign of vertebræ or of jaws. They, like the lampreys,
were all agnathostomatous. Strange indeed is their appearance, and it is no
wonder that there should have been a difficulty in deciding whether they
were fish or arthropod. Their great characteristic is their buckler-plated
cephalic shield, especially conspicuous on the dorsal side of the head.
Figs. 11, 14, 15, 16, give the dorsal shields of Pteraspis, Auchenaspis,
Pterichthys, and Bothriolepis.
In 1904, Drevermann discovered a mass of _Pteraspis Dunensis_ embedded in a
single stone, showing the same kind of head-shield as _P. rostrata_, but
the rostrum was longer and the spine at the extremity of the head-shield
much longer and more conspicuous. The whole shape of the animal as seen in
this photograph recalls the shape of a Hemiaspid rather than of a fish. It
is, then, natural enough for the earlier observers to have looked upon such
a fossil as related to an arthropod rather than a fish.
{30}[Illustration: FIG. 11.--_Pteraspis dunensis_ (from DREVERMANN). Dorsal
view of body and spine on the right side. Head-end, showing long rostrum on
the left side.]
[Illustration: FIG. 12.--_Bunodes lunula._ (From SCHMIDT.)]
[Illustration: FIG. 13.--_Auchenaspis (Thyestes) verrucosus_, natural size.
(From WOODWARD.)]
{31}In Figs. 12 and 13 I have placed side by side two Silurian fossils
which are found in the same geological horizon. They are both life size and
possess a general similarity of appearance, yet the one is a Cephalaspidian
fish known by the name of _Auchenaspis_ or _Thyestes verrucosa_, the other
a Palæostracan called _Bunodes lunula_.
[Illustration: FIG. 14.--DORSAL HEAD-SHIELD OF _Thyestes (Auchenaspis)
verrucosus_. (From ROHON.)
_Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum or plate
over brain; _Occ._, occipital region.]
[Illustration: FIG. 15.--_Pterichthys._]
In a later chapter I propose to discuss the peculiarities and the nature of
the head-shields of these earliest fishes, in connection with the question
of the affinities of the animals which bore them. At this point of my
argument I want simply to draw attention to the undoubted fact of the
striking similarity in appearance between the {32}earliest fishes and
members of the Palæostraca, the dominant race of arthropods which swarmed
in the sea at the time: a similarity which could never have been suspected
by any amount of investigation among living forms, but is immediately
revealed when the ages themselves are questioned.
[Illustration: FIG. 16.--_Bothriolepis._ (After PATTEN.)
_An._, position of anus.]
I have not reproduced any of the attempted restorations of these old forms,
as usually given in the text-books, because all such restorations possess a
large element of fancy, due to the personal bias of the observer. I have
put in Rohon's idea of the general shape of Tremataspis (Fig. 17) in order
to draw attention to the lamprey-like appearance of the fish according to
his researches (_cf._ Fig. 18).
[Illustration: FIG. 17.--RESTORATION OF _Tremataspis_. (After ROHON,
slightly modified.)]
[Illustration: FIG. 18.--_Ammocoetes._]
The argument, then, from geology, like that from comparative anatomy and
from the consideration of the importance of the central nervous system in
the upward development of the animal race, not only points directly to the
arthropod group as the ancestor of the {33}vertebrate, but also to a
distinct ancient type of arthropod, the Palæostracan, the only living
example of which is the King-Crab or Limulus; while the nearest approach to
the trilobite group among living arthropods are Branchipus and Apus. It
follows, therefore, that for the following up of this clue, Limulus
especially must be taken into consideration, while Branchipus and Apus are
always to be kept in mind.
AMMOCOETES RATHER THAN AMPHIOXUS IS THE BEST SUBJECT FOR INVESTIGATION.
It is not, however, Limulus that must be investigated in the first
instance, but the vertebrate itself; for it can never be insisted on too
often that in the vertebrate itself its past history will be found, but
that Limulus cannot reveal the future of its race. What vertebrate must be
chosen for investigation? Reasons have been given why our attention should
be fixed upon the king-crab rather than on the lobster on the invertebrate
side; what is the most likely animal on the vertebrate side?
From the evidence already given it is manifest that the earliest mammal
belonged to the lowest group of mammals; that the birds on their first
appearance presented reptilian characteristics, that the earliest reptiles
belonged to a low type of reptile, that the amphibians at their first
appearance were nearer in type to the fishes than were the later forms. As
each of these groups advances in number and power, specialization takes
place in it, and the latest developed members become further and further
removed in type from the earliest. So also it must have been with the
origin of fishes: here too, in the quest for information as to the
structure and nature of the first-formed fishes, we must look to the lowest
rather than to the highest living members of the group.
The lowest fish-like animal at present living is Amphioxus, and on this
ground it is argued that the original vertebrate must have approached in
organization to that of Amphioxus; it is upon the comparison between the
structure of Amphioxus and that of Balanoglossus, that the theory of the
origin of vertebrates from forms like the latter animal is based. For my
own part, I think that in the first instance, at all events, Amphioxus
should be put on one side, although of course its structure must always be
kept in mind, for the following reasons:--
{34}Amphioxus, like the tunicates, does not possess the characteristics of
other vertebrates. In all vertebrates above these forms the great
characteristic is a well-defined brain-region from which arise nerves to
organs of special sense, the eyes and nose. In Amphioxus no eyes exist, for
the pigmented spot at the anterior extremity of the brain-region is no eye
but only a mass of pigment, and the so-called olfactory pit is a very
rudimentary and inferior organ of smell. In connection with the nearly
complete absence of these two most important sense-organs, the most
important part of the central nervous system, the region corresponding to
the cerebral hemispheres, is also nearly completely absent.
Now, the history of the evolution of the central nervous system in the
animal race points directly to its formation as a concentrated mass of
nervous material at the anterior extremity of the body, in consequence of
the formation of special olfactory and visual organs at that extremity. As
already stated, the concentration of nervous material around the mouth as
an oral ring was its beginning. In connection with this there arose special
sense-organs for the guidance of the animal to its food which took the form
of olfactory and optic organs. With the shifting from the radial to the
elongated form these sense-organs remained at the anterior or mouth-end of
the animal, and owing to their immense importance in the struggle for
existence, that part of the central nervous system with which they were
connected developed more than any other part, became the leader to which
the rest of the nervous system was subservient, and from that time onwards
the development of the brain-region was inevitably associated with the
upward progress of animal life.
To those who believe in Evolution and the Darwinian theory of the survival
of the fittest, it is simply inconceivable that a soft-bodied animal living
in the mud, blind, with a rudimentary brain and rudimentary olfactory
organs, such as is postulated when we think of Balanoglossus and Amphioxus,
should hold its own and come victorious out of the struggle for existence
at a time when the sea was peopled with powerful predaceous scorpion- and
crab-like armour-plated animals possessing a well-developed brain, good
eyes and olfactory organs, and powerful means of locomotion. Wherever in
the scale of animal development Amphioxus may ultimately be placed, it
cannot be looked upon as the type of the earliest formed fishes such as
appeared in Silurian times.
{35}The next lowest group of living fishes is the Marsipobranchii which
include the lampreys and hag-fishes. To these naturally we must turn for a
clue as to the organization of the earliest fish, for here we find all the
characteristics of the vertebrates represented: a well-formed brain-region,
well-developed eyes and nose, cranial nerves directly comparable with those
of other vertebrates, and even the commencement of vertebræ.
Among these forms the lamprey is by far the best for investigation, not
only because it is easily obtainable in large quantities, but especially
because it passes a large portion of its existence in a larval condition,
from which it emerges into the adult state by a wonderful process of
transformation, comparable in extent with the transformation of the larval
caterpillar into the adult imago. So long does the lamprey live in this
free larval condition, and so different is it in the adult stage, that the
older anatomists considered that the two states were really different
species, and gave the name of _Ammocoetes branchialis_ to the larval stage,
while the adult form was called _Petromyzon planeri_, or _Petromyzon
fluviatilis_.
This long-continued free-living existence in the larval or Ammocoetes stage
makes the lamprey, more than any other type of lowly organized fish,
invaluable for the present investigation, for throughout the animal kingdom
it is recognized that the larval form approaches nearer to the ancestral
type than the adult form, whether the latter is progressive or degenerate.
Not only are the tissues formed during the stages which are passed through
in a free-living larval form, serviceable tissues comparable to those of
adult life, but also these stages proceed at so much slower a rate than do
those in the embryo _in utero_ or in the egg, as to make the larval form
much more suitable than the embryo for the investigation of ancestral
problems. It is true enough that the free life of the larva may bring about
special adaptations which are not of an ancestral character, as may also
occur during the life of the adult; but the evidence is very strong that
although some of the peculiarities of the larva may be due to such
coenogenetic factors, yet on the whole many of them are due to ancestral
characters, which disappear when transformation takes place, and are not
found in the adult.
Thus if it be supposed that the amphibian arose from the fish, the tadpole
presents more resemblance to the fish than the frog. If {36}it be supposed
that the arthropod arose from the segmented worm, the caterpillar bears out
the suggestion better than the adult imago. If it be supposed that the
tunicate arose from a stock allied to the vertebrate, it is because of the
peculiarities of the larva that such a supposition is entertained. So, too,
if it be supposed that the fish arose from a member of the arthropod group,
the larval form of the fish is most likely to give decisive information on
the point.
For all these reasons the lowest form of fish to be investigated, in the
hopes of finding out the nature of the earliest formed fish, is not
Amphioxus, but Ammocoetes, the larval form of the lamprey--a form which, as
I hope to satisfy my reader after perusal of subsequent pages, more nearly
resembles the ancient Cephalaspidian fishes than any other living
vertebrate.
COMPARISON OF CENTRAL NERVOUS SYSTEMS OF VERTEBRATE AND ARTHROPOD WITHOUT
REVERSAL OF SURFACES.
So far different lines of investigation all point to the origin of the
vertebrate from arthropods, the group of arthropods in question being now
extinct, the nearest living representative being Limulus; also to the fact
that of the two theories of the origin of vertebrates, that one which is
based on the resemblance between the central nervous systems of the
Vertebrata and the Appendiculata (Arthropoda and Annelida) is more in
accordance with this evidence than the other, which is based mainly on the
supposed possession of a notochord among certain animals.
How is it, then, that this theory has been discredited and lost ground?
Simply, I imagine, because it was thought to necessitate the turning over
of the animal. Let us, then, again look at the nervous system of the
vertebrate, and see whether there is any such necessity.
As previously mentioned, the comparison of the two central nervous systems
showed such close resemblances as to force those anatomists who supported
this theory to the conclusion that the infundibular tube was in the
position of the original oesophagus; they therefore looked for the remains
of a mouth opening in the dorsal roof of the brain, but did not attempt to
explain the extraordinary fact that the infundibular tube is only a ventral
offshoot from the tube of the central nervous system. Yet this latter tube
{37}is one, if not the most striking, of the peculiarities which
distinguish the vertebrate; a tubular central nervous system such as that
of the vertebrate is totally unlike any other nervous system, and the very
fact that the two nervous systems of the vertebrate and arthropod are so
similar in their nervous arrangements, makes it still more extraordinary
that the nervous system should be grouped round a tube in the one case and
not in the other.
Now, in the arthropod the oesophagus leads directly into the stomach, which
is situated in the head-region, and from this a straight intestine passes
directly along the length of the body to the anus, where it terminates. The
relations of mouth, oesophagus, alimentary canal, and nervous system in
these animals are represented in the diagram (Fig. 3).
Any tube, therefore, such as that of the infundibulum, which would
represent the oesophagus of such an animal, must have opened into the mouth
on the ventral side, and into the stomach on the dorsal side, and the
lining epithelium of such an oesophagus must have been continuous with that
of the stomach, and so of the whole intestinal tract.
Supposing, then, the animal is not turned over, but that the dorsal side
still remains dorsal and ventral ventral, then the original mouth-opening
of the oesophagus must be looked for on the ventral surface of the
vertebrate brain in the region of the pituitary body or hypophysis, and on
the dorsal side the tube representing the oesophagus must be continuous
with a large cephalically dilated tube, which ought to pass into a small
canal, to run along the length of the body and terminate in the anus.
This is exactly what is found in the vertebrate, for the infundibular tube
passes into the third ventricle of the brain, which forms, with the other
ventricles of the brain, the large dilated cephalic portion of the
so-called nerve tube, and at the junction of the medulla oblongata and
spinal cord, this dilated anterior part passes into the small, straight,
central canal of the spinal cord, which in the embryo terminates in the
anus by way of the neurenteric canal. If the animal is regarded as not
having been turned over, then the conclusion that the infundibulum was the
original oesophagus leads immediately to the further conclusion that the
ventricles of the vertebrate brain represent the original cephalic stomach,
and the central canal of the spinal cord the straight intestine of the
arthropod ancestor.
{38}For the first time a logical, straightforward explanation is thus given
of the peculiarities of the tube of the central nervous system, with its
extraordinary termination in the anus in the embryo, its smallness in the
spinal cord, its largeness in the brain region, and its offshoot to the
ventral side of the brain as the infundibular channel. It is so clear that,
if the infundibular tube be looked on as the old oesophagus, then its
lining epithelium is the lining of that oesophagus; and the fact that this
lining epithelium is continuous with that of the third ventricle, and so
with the lining of the whole nerve-tube, must be taken into account and not
entirely ignored as has hitherto been the case. If, then, we look at the
central nervous system of the vertebrate in the light of the central
nervous system of the arthropod without turning the animal over, we are led
immediately to the conclusion that what has hitherto been called the
vertebrate nervous system is in reality composed of two parts, viz. a
nervous part comparable in all respects with that of the arthropod
ancestor, which has grown over and included into itself, to a greater or
less extent, a tubular part comparable in all respects with the alimentary
canal of the aforesaid ancestor. If this conclusion is correct, it is
entirely wrong to speak of the vertebrate central nervous system as being
tubular, for the tube does not belong to the nervous system, but was
originally a simple epithelial tube, such as characterizes the oesophagus,
cephalic stomach, and straight intestine of the arthropod.
Here, then, is the crux of the position--either the so-called nervous tube
of the vertebrate is composed of two separate factors, consisting of a true
non-tubular nervous system and a non-nervous epithelial tube, these two
elements having become closely connected together; or it is composed of one
factor, an epithelial tube which constitutes the nervous system, its
elements being all nervous elements.
If this latter hypothesis be accepted, then it is necessary to explain why
parts of that tube, such as the roof of the fourth ventricle, the choroid
plexuses of the various ventricles, which are parts of the original roof
inserted into the ventricles, are not composed of nervous material, but
form simple single-layered epithelial sheets, which by no possibility can
be included among functional nervous structures. The upholders of this
hypothesis can only explain the nature of these thin epithelial parts of
the nervous tube in one of two ways; either the tube was originally formed
of nervous {39}material throughout, and for some reason parts of it have
lost their nervous function and thinned down; or else these thin epithelial
parts are on their way to become nervous material, are still in an
embryonic condition, and are of the nature of epiblast-epithelium, from
which the central nervous system originally arose.
The first explanation is said to be supported by embryology, for at first
the nerve-tube is formed in a uniform manner, and then later, parts of the
roof appear to thin out and so form the thin epithelial parts. If this were
the right explanation, then it ought to be found that in the lowest
vertebrates there is greater evidence of a uniformly nervous tube than in
the higher members of the group: while conversely, if, on the contrary, as
we descend the vertebrate phylum, it is found that more and more of the
tube presents the appearance of a single layer of epithelium, and the
nervous material is limited more and more to certain parts of that tube,
then the evidence is strong that the tubular character of the central
nervous system is not due to an original nervous tube, but to a non-nervous
epithelial tube with which the original nervous system has become closely
connected.
The comparison of the brain region of the different groups of vertebrates
(Fig. 19) is most instructive, for it demonstrates in the most conclusive
manner how the roof of the nervous tube in that region loses more and more
its nervous character, and takes on the appearance of a simple epithelial
tube, as we descend lower and lower; until at last, in the brain of
Ammocoetes, as represented in the figures, the whole of the brain-roof,
from the region of the pineal eye to the commencement of the spinal cord,
is composed of fold upon fold of a thin epithelial membrane forming an
epithelial bag, which is constricted in only one place, where the fourth
cranial nerve crosses over it.
Further, the brain of Ammocoetes (Fig. 20) shows clearly not only that it
is composed of two parts, an epithelial tube and a nervous system, but also
that the nerve-masses are arranged in the same relative position with
respect to this tube as are the nerve-masses in the invertebrate with
respect to the cephalic stomach and oesophagus. This evidence is so
striking, so conclusive, that it is impossible to resist the conclusion
that the tube did not originate as part of the central nervous system, but
was originally independent of the central nervous system, and has been
invaded by it.
{40}[Illustration: FIG. 19.--COMPARISON OF VERTEBRATE BRAINS.
_CB._, cerebellum; _PT._, pituitary body; _PN._, pineal body; _C. STR._,
corpus striatum; _G.H.R._, right ganglion habenulæ. _I._, olfactory; _II._,
optic nerves.]
{41}[Illustration: FIG. 20.--BRAIN OF AMMOCOETES.
A, dorsal view; B, lateral view; C, ventral view. _C.E.R._, cerebral
hemispheres; _G.H.R._, right ganglion habenulæ; _PN._, right pineal eye;
_CH_2_, _CH_3_, choroid plexuses; _I.-XII._ cranial nerves; _C.P._, _Conus
post-commissuralis_.]
{42}The second explanation is hardly worth serious consideration, for it
supposes that the nervous system, for no possible reason, was laid down in
its most important parts--the brain-region--as an epithelial tube with
latent potential nervous functions; that even up to the highest vertebrate
yet evolved these nervous functions are still in abeyance over the whole of
the choroid plexuses and the roof of the fourth ventricle. Further, it
supposes that this prophetic epithelial tube originally developed into true
nervous material only in certain parts, and that these parts, curiously
enough, formed a nervous system absolutely comparable to that of the
arthropod, while the dormant prophetic epithelial part was formed so as
just to mimic, in relation to the nervous part, the alimentary canal of
that same arthropod.
The mere facts of the case are sufficient to show the glaring absurdity of
such an explanation. This is not the way Nature works; it is not consistent
with natural selection to suppose that in a low form nervous material can
be laid down as non-nervous epithelial material in order to provide in some
future ages for the great increase in the nervous system.
Every method of investigation points to the same conclusion, whether the
method is embryological, anatomical, or pathological.
First, take the embryological evidence. On the ground that the individual
development reproduces to a certain extent the phylogenetic development,
the peculiarities of the formation of the central nervous system in the
vertebrate embryo ought to receive an appropriate explanation in any theory
of phylogenetic development. Hitherto such explanation has been totally
lacking; any suggestion of the manner in which a tubular nervous system may
have been formed takes no account whatever of the differences between
different parts of the tube; its dilated cephalic end with its infundibular
projection ventrally, its small straight spinal part, and its termination
in the anus. My theory, on the other hand, is in perfect harmony with the
embryological history, and explains it point by point.
From the very first origin of the central nervous system there is evidence
of two structures--the one nervous, and the other an epithelial
surface-layer which ultimately forms a tube; this was first described by
Scott in Petromyzon, and later by Assheton in the frog. In the latter case
the external epithelial layer is pigmented, while the underlying nervous
layer contains no pigment; a marked {43}and conspicuous demarcation exists,
therefore, between the two layers from the very beginning, and it is easy
to trace the subsequent fate of the two layers owing to this difference of
pigmentation. The pigmented cells form the lining cells of the central
canal, and becoming elongated, stretch out between the cells of the nervous
layer; while the latter, on their side, invade and press between the
pigmented cells. In this case, owing to the pigmentation of the epithelial
layer, embryology points out in the clearest possible manner how the
central nervous system of the vertebrate is composed of two structures--an
epithelial non-nervous tube, on the outside of which the central nervous
system was originally grouped; how, as development proceeds, the elements
of these two structures invade each other, until at last they become so
involved together as to give rise to the conception that we are dealing
with one single nerve tube. It is impossible for embryology to give a
clearer clue to the past history than it does in this case, for it actually
shows, step by step, how the amalgamation between the central nervous
system and the old alimentary canal took place.
Further, consider the shape of the tube when it is first formed, how
extraordinary and significant that is. It consists of a simple dilated
anterior end leading into a straight tube, the lumen of which is much
larger than that of the ultimate spinal canal, and terminates by way of the
neurenteric canal in the anus.
Why should the tube take this peculiar shape at its first formation? No
explanation is given or suggested in any text-book of embryology, and yet
it is so natural, so simple: it is simply the shape of the invertebrate
alimentary canal with its cephalic stomach and straight intestine ending in
the anus. Again embryology indicates most unmistakably the past history of
the race. How are the nervous elements grouped round this tube when it is
first formed? Here embryology shows that a striking difference exists
between the part of the tube which forms the spinal cord and the dilated
cephalic part. Fig. 21, A (2), represents the relation between the nervous
masses and the epithelial tube in the first instance. At this stage the
nervous material in the spinal cord lies laterally and ventrally to this
tube, and at a very early stage the white anterior commissure is formed,
joining together these two lateral masses; as yet there is no sign of any
posterior fissure, the tube with its open lumen extends right to the dorsal
surface.
{44}The interpretation of this stage is that in the invertebrate ancestor
the nerve-masses were situated laterally and ventrally to the epithelial
tube, and were connected together by commissures on the ventral side of the
tube (Fig. 21, A (1)); in other words, the chain of ventral ganglia and
their transverse commissures lying just ventrally to the intestine, which
are so characteristic of the arthropod nervous system, is represented at
this stage.
[Illustration: FIG. 21.--A, METHOD OF FORMATION OF THE VERTEBRATE SPINAL
CORD FROM THE VENTRAL CHAIN OF GANGLIA AND THE INTESTINE OF AN ARTHROPOD,
REPRESENTED IN 1; B, METHOD OF FORMATION OF THE VERTEBRATE MEDULLA
OBLONGATA FROM THE INFRA-OESOPHAGEAL GANGLIA AND THE CEPHALIC STOMACH OF AN
ARTHROPOD.]
Subsequently, by the growth dorsalwards of nervous material to form the
posterior columns, the original epithelial tube is compressed dorsally and
laterally to such an extent that those parts lose all signs of lumen, the
one becoming the posterior fissure and the others the _substantia
gelatinosa Rolandi_ on each side. The original tube is thus reduced to a
small canal formed by its ventral portion only (Fig. 21, A (3)). In this
way the spinal cord is formed, and the walls of the original epithelial
tube are finally visible only as the lining of the central canal (Fig. 21,
A (4)).
When we pass to the brain-region, to the anterior dilated portion of the
tube, embryology tells a different story. Here, as in the spinal cord, the
nervous masses are grouped at first laterally and ventrally to the
epithelial tube, as is seen in Fig. 21, B (2), but owing to the large size
of its lumen here, the nervous material is not able to enclose it
completely, as in the case of the spinal cord; {45}consequently there is no
posterior fissure formed; but, on the contrary, the dorsal roof, not
enclosed by the nerve-masses, remains epithelial, and so forms the
membranous roof of the fourth ventricle and of the other ventricles of the
brain (Fig. 21, B (3)). In the higher animals, owing to the development of
the cerebrum and cerebellum, this membranous roof becomes pushed into the
larger brain cavity, and thus forms the choroid plexuses of the third and
lateral ventricles. In the lower vertebrates, as in Ammocoetes and the
Dipnoi, it still remains as a dorsal epithelial roof and forms a most
striking characteristic of such brains.
In this part of the nervous system, then, the nervous material is all
grouped in its original position on the ventral side of the tube; and yet
it is the same nervous material as that of the spinal cord, all the
elements are there, giving origin here to the segmental cranial nerves just
as lower down they give rise to the segmental spinal nerves, connecting
together the separate segments each with the other and all with the higher
brain-centres--the supra-infundibular centres--just as they do in the
spinal region.
Why should there be this striking difference between the formation of the
infra-infundibular region of the brain and that of the spinal cord? Do the
advocates of the origin of vertebrates from Balanoglossus give the
slightest reason for it? They claim that their view also provides a tubular
nervous system for the vertebrate, but give not the slightest sign or
indication as to why the nervous material should be grouped entirely on the
ventral side of an epithelial tube in the infra-infundibular region and yet
surround it in the spinal cord region. And the explanation is so natural,
so simple: embryology does its very best to tell us the past history of the
race, if only we look at it the right way.
The infra-infundibular nervous mass is naturally confined to the ventral
side of the epithelial tube, because it represents the infra-oesophageal
ganglia, situated as they are on the ventral side of the cephalic stomach,
and, owing to the size of the stomach, they could not enclose it by dorsal
growth, as they do in the case of the formation of the spinal cord (Fig.
21, B (1)). Still these nervous masses have grown dorsalwards, have
commenced to involve the walls of the cephalic stomach even in the lowest
vertebrate, as is seen in Ammocoetes, in which animal a ventral portion of
the epithelial bag has been evidently compressed and its lumen finally
obliterated {46}by the growth of the nerve-masses on each side of it.
Throughout the whole vertebrate kingdom this obliterated portion still
leaves its mark as the _raphé_ or seam, which is so characteristic of the
infra-infundibular portion of the brain.
[Illustration: FIG. 22.--HORIZONTAL SECTION THROUGH THE BRAIN OF
AMMOCOETES.
_Cr._, membranous cranium; _I_, olfactory nerves; _l.v._, lateral
ventricles; _gl._, glandular tissue which fills up the cranial cavity.]
Here, again, it is seen how simple is the explanation of a peculiarity
which has always puzzled anatomists--why should there be this seam in the
infra-infundibular portion of the brain and not in the supra-infundibular
or in the spinal cord? The corresponding compression in the upper
brain-region forms the lateral ventricles, as is seen in the accompanying
figure of the brain of Ammocoetes (Fig. 22).
[Illustration: FIG. 23.--SECTION THROUGH RHOMBOIDAL SINUS OF BIRD.]
In yet another instance it is seen how markedly the nervous masses are
arranged in the same position with respect to the central tube as are the
nerve ganglia with respect to the intestinal tube in the case of the
invertebrate. Thus in birds a portion of the spinal cord in the
lumbo-sacral region presents a very different appearance from the rest of
the cord; it is known as the rhomboidal sinus, and a section of the cord of
an adult pigeon across this region is given in Fig. 23. As is seen, the
nervous portions are entirely confined to two masses connected together by
the white anterior commissures which are situated laterally and ventrally
to a median gelatinous mass; the small central canal is visible and {47}the
whole dorsal area of the cord is taken up by a peculiar non-nervous
wedge-shaped mass of tissue. At its first formation this portion of the
cord is formed exactly in the same manner as the rest of the cord; instead,
however, of the nervous material invading the dorsal part of the tube to
form the posterior fissure, it has been from some cause unable to do so,
the walls of the original non-nervous tube have become thickened dorsally,
been transformed into this peculiar tissue, and so caused the peculiar
appearance of the cord here. The nervous parts have not suffered in their
development; the mechanism for walking in the bird is as well developed as
in any other animal; their position only is different, for they still
retain the original ventro-lateral position, but the non-nervous tube, the
remains of the old intestine, has undergone a peculiar gelatinous
degeneration just where it has remained free from invasion by the nervous
tissue.
Throughout the whole of that part of the nervous system which gives origin
to the cranial and spinal segmental nerves, the evidence is absolutely
uniform that the nervous material was originally arranged bilaterally and
ventrally on each side of the central tube, exactly in the same way as the
nerve-masses of the infra-oesophageal and ventral chain of ganglia are
arranged with respect to the cephalic stomach and straight intestine of the
arthropod. But, in addition, we find in the vertebrate nervous masses, the
cerebral hemispheres, the corpora quadrigemina and the cerebellum situated
on the dorsal side of the central tube in the brain-region; this nervous
material is, however, of a different character to that which gives origin
to the spinal and cranial segmental nerves. How is the presence of these
dorsal masses to be explained on the supposition that the dilated anterior
part of the nerve-tube was originally the cephalic stomach of the arthropod
ancestor? The cerebral hemispheres are simple enough, for they represent
the supra-oesophageal ganglia, which of necessity, as they increased in
size, would grow round the anterior end of the cephalic stomach and become
more and more dorsal in position.
The difficulty lies rather in the position of the cerebellum and corpora
quadrigemina, and the solution is as simple as it is conclusive.
Let us again turn to embryology and see what help it gives. In all
vertebrates the dilated anterior portion of the nerve-tube does not, {48}as
it grows, increase in size uniformly, but a constriction appears on its
dorsal surface at one particular place, so as to divide it into an anterior
and posterior vesicle; then the latter becomes divided into two portions by
a second constriction. In this way three cerebral vesicles are formed;
these three primary cerebral vesicles indicate the region of the
fore-brain, mid-brain, and hind-brain respectively. Subsequently the first
cerebral vesicle becomes divided into two to form the prosencephalon and
thalamencephalon, while the third cerebral vesicle is also divided into two
to form the region of the cerebellum and medulla oblongata.
These constrictions are in the position of commissural bands of nervous
matter; of these the limiting nervous strands between the thalamencephalon
and mesencephalon and between the mesencephalon and the hind-brain are of
primary importance. The first of these commissural bands is in the position
of the posterior commissure connecting the two optic thalami. In close
connection with this are found, on the mid-dorsal region, the two pineal
eyes with their optic ganglia, the so-called _ganglia habenulæ_. From these
ganglia a peculiar tract of fibre, known as Meynert's bundle, passes on
each side to the ventral infra-infundibular portion of the brain. In other
words, the first constriction of the dilated tube is due to the presence
and growth of nervous material in connection with the median pineal eyes.
Here in precisely the same spot, as will be fully explained in the next
chapter, there existed in the arthropod ancestor a pair of median eyes
situated dorsally to the cephalic stomach, the pre-existence of which
explains the reason for the first constriction.
The second primary constriction separating the mid-brain from the
hind-brain is still more interesting, for it is coincident with the
position of the trochlear or fourth cranial nerve. In all vertebrates
without exception this nerve takes an extraordinary course; all other
nerves, whether cranial or spinal, pass ventralwards to reach their
destination. This nerve passes dorsalwards, crosses its fellow mid-dorsally
in the valve of Vieussens, where the roof of the brain is thin, and then
passes out to supply the superior oblique muscle of the eye of the opposite
side. The two nerves form an arch constricting the dilated tube at this
place. In the lowest vertebrate (Ammocoetes) the constriction formed by
this nerve-pair is evident not only in the embryonic condition as in other
vertebrates, but during the whole larval stage. As Fig. 20, A and B, shows,
the whole of the dorsal {49}region of the brain up to the region of the
pineal eye and _ganglion habenulæ_ is one large membranous bag, except for
the single constriction where the fourth nerve on each side crosses over.
The explanation of this peculiarity is given in Chapter VII., and follows
simply from the facts of the arrangement of that musculature in the
scorpion-group which gave rise to the eye-muscles of the vertebrate.
In Ammocoetes both cerebellum and posterior corpora quadrigemina can hardly
be said to exist, but upon transformation a growth of nervous material
takes place in this region, and it is seen that this commencing cerebellum
and the corpora quadrigemina arise from tissue that is present in
Ammocoetes along the course of the fourth nerve.
Here, then, again Embryology does its best to tell us how the vertebrate
arose. The formation of the two primary constrictions in the dilated
anterior vesicle whereby the brain is divided into fore-brain, mid-brain,
and hind-brain is simply the representation ontogenetically of the two
nerve-tracts which crossed over the cephalic stomach in the prevertebrate
stage, in consequence of the mid-dorsal position of the pineal eyes and of
the insertion of the original superior oblique muscles.
The subsequent constriction by which the prosencephalon is separated from
the thalamencephalon is in the position of the anterior commissure, that
commissure which connects the two supra-infundibular nerve-masses, and is
one of the first-formed commissures in every vertebrate. This naturally is
simply the commissure between the two supra-oesophageal ganglia; anterior
to it, in the middle line, equally naturally, the anterior end of the old
stomach wall still exists as the _lamina terminalis_.
The other division in the hind-brain region, which separates the region of
the cerebellum from the medulla oblongata, is due to the growth of the
cerebellum, and indicates its posterior limit. In such an animal as the
lamprey, where the cerebellum is only commencing, this constriction does
not occur in the embryo.
From such simple beginnings as are seen in Ammocoetes, the higher forms of
brain have been evolved, to culminate in that of man, in which the massive
cerebrum and cerebellum conceals all sign of the dorsal membranous roof,
those parts of the simple epithelial tube which still remain being tucked
away into the cavities to form the various choroid plexuses.
{50}In the whole evolution from the brain of Ammocoetes to that of man, the
same process is plainly visible, viz. growth and extension of nervous
material over the epithelial tube; extension dorsally and posteriorly of
the supra-infundibular nervous masses (as seen in Fig. 19), combined with a
dorsal growth of parts of the infra-infundibular nervous masses to form the
cerebellum and posterior corpora quadrigemina.
Especially instructive is the formation of the cerebellum. It consists at
first of a small mass of nervous tissue accompanying the fourth nerve, then
by the growth of that mass surrounding and constricting a fold of the
membranous roof, the _worm_ of the cerebellum is formed, as in the
dog-fish. This very constriction causes the membrane to be thrown into a
lateral fold on each side, as seen in Fig. 24, and in the dog-fish the
nervous material on each side, known as the fimbriæ, is already commencing
to grow from the ventral mass of the medulla oblongata to surround these
lateral membranous folds. These _fimbriæ_ develop more and more in higher
forms, and thus form the cerebellar hemispheres.
Not only does comparative anatomy confirm the teachings of embryology, but
also pathology gives its quota in the same direction.
[Illustration: FIG. 24.--CEREBELLUM OF DOG-FISH.
_v_, worm of cerebellum; _IV._, membranous roof of fourth ventricle
continuous with the membranous folds on each side. Through these the
fimbriæ (_fb._) can be dimly seen.]
One of the striking facts about malformations and disease of the central
nervous system is the frequency of cystic formations; _spina bifida_ is a
well-known instance. These cysts are merely epithelial non-nervous cysts
formed from the epithelium of the central canal, difficult to understand if
the whole nerve tube is one and entirely nervous, either actually or
potentially, but natural and easy if we are really dealing with a simple
epithelial tube on the outside of which the nervous material was originally
grouped. The cystic formation belongs naturally enough to this tube, not to
the nervous system.
Again, where animals such as lizards have grown a new tail, owing to the
breaking off of the original one, it is found that the central canal
extends into this new tail for some distance, but not {51}the nervous
material surrounding it; all the nerves supplying the new tail arise from
the uninjured spinal cord above, the central canal with its lining layer of
epithelial cells alone grows into the new-formed appendage.
To all intents and purposes the same thing is seen in the termination of
the spinal cord in a bird-embryo; more and more, as the end of the tail is
approached, does the nervous matter of the spinal cord grow less and less,
until at last a naked central canal with its lining epithelium is alone
left to represent the so-called nerve-tube.
All these different methods of investigation lead irresistibly to the one
conclusion that the tubular nature of the central nervous system has been
caused by the central nervous system enclosing to a greater or less extent
a pre-existing, non-nervous, epithelial tube.
This must always be borne strictly in mind. The problem, therefore, which
presents itself is the comparison of these two factors separately, in order
to find out the relationship of the vertebrate to the invertebrate. The
nervous system without the tube must be compared to other nervous systems,
and the tube must be considered apart from the nervous system.
THE PRINCIPLE OF CONCENTRATION AND CEPHALIZATION.
The central nervous system of the vertebrate resembles that of all the
Appendiculata in the fact that it is composed of segments joined together
which give origin to segmental nerves. There is, however, a great
difference between the two systems: the division into separate segments is
not obvious to the eye in the vertebrate nervous system, while in the
invertebrate we can see that it is composed of a series of separate pairs
of ganglia joined together longitudinally by nervous strands known as
connectives and transversely by the nerve-commissures. Such a simple
segmented system is found in the segmented worms, and in the lower
arthropods, such as Branchipus, no great advance has been made on that of
the annelid. In the higher forms, however, a greater and greater tendency
to fusion of separate ganglia exists, especially in the head-region, so
that the infra-oesophageal ganglia, which, in the lower forms are as
separate as those of the ventral chain, in the higher forms are fused
together to form a single nervous mass.
{52}This is the great characteristic of the advancement of the central
nervous system among the Invertebrata, its concentration in the region of
the head. It may be called the principle of cephalization, and is
characteristic not only of higher organization in a group, but also of the
adult as distinguished from the larval form. Thus in the imago greater
concentration is found than in the caterpillar.
The segmented annelid type of nervous system consists of a
supra-oesophageal ganglion, composed of the fused ganglia belonging to the
pre-oral segments, and an infra-oesophageal chain of separate ganglia. With
the concentration and modification around the mouth of the most anterior
locomotor appendages to form organs for prehension and mastication of food,
a corresponding concentration and fusion of the ganglia belonging to these
segments takes place, so that finally, in the higher annelids, and in most
of the great arthropod group, a fusion of a number of the most anterior
ganglia has taken place to form the infra-oesophageal ganglion-mass.
The infra-oesophageal ganglia which are the first to fuse are those which
supply the most anterior portion of the animal with nerves, and include
always those anterior appendages which are modified for mastication
purposes. To this part the name _prosoma_ has been given; in many cases it
forms a well-defined, distinct portion of the animal.
Succeeding this prosoma or masticatory region, there occurs in all
gill-bearing arthropods a respiratory region, in many cases more or less
distinctly defined, which has received the name of _mesosoma._ The rest of
the body is called the _metasoma_.
In accordance with this nomenclature the central nervous system of many of
the Arthropoda may be divided as follows:--
1. Pre-oral, or supra-oesophageal ganglia.
2. Infra-oral, or infra-oesophageal ganglia and ventral chain, which
consist of three groups: prosomatic, mesosomatic, and metasomatic ganglia.
The infra-oesophageal ganglion-mass, then, in most of the Arthropoda may be
spoken of as formed by the fusion of the prosomatic or mouth-ganglia, the
mesosomatic and metasomatic remaining separate and distinct. The number of
ganglia which have fused may be observed by examination of the embryo, in
which it is easy to see indications of the individual ganglia or
_neuromeres_, although all such indication has disappeared in the adult;
thus the {53}infra-oesophageal ganglia of the cray-fish have been shown to
be constituted of six prosomatic ganglia.
In Fig. 25 I give figures of the central nervous system (with the exception
of the abdominal or metasomatic ganglia) of Branchipus, Astacus, Limulus,
Scorpio, Androctonus, Thelyphonus, and Ammocoetes. In all the figures the
supra-oesophageal ganglia are lined horizontally, and their nerves shown,
viz. optic (lateral eyes (II) and median eyes (II[prime])), olfactory (I)
(first antennæ, camerostome, nose); then come the prosomatic ganglia
(dotted), with their nerves (A) supplying the mouth parts, and the second
antennæ or cheliceræ; then the mesosomatic (lined horizontally), with their
nerves (B) supplying respiratory appendages. These figures show that the
concentrated brain mass around the oesophagus of an arthropod which has
arrived at the stage of Astacus, is represented by the supra-oesophageal
ganglia and the fused prosomatic ganglia.
The next stage in the evolution of the brain is seen in the gradual
inclusion of the mesosomatic ganglia, one after the other, into the
infra-oesophageal mass of the already fused prosomatic ganglia. With this
fusion is associated the loss of locomotion in these mesosomatic
appendages, and their entire subservience to the function of respiration.
Dana urges that cephalization is a consequence of functional alteration in
the appendages, from organs of locomotion to those of mastication and
respiration. Whether this be true or not, it is certainly a fact that in
Limulus, the ganglion supplying the first mesosomatic appendage has fused
with the prosomatic, infra-oesophageal mass. It is also a fact that the
prosomatic appendages are the organs of mastication, their basal parts
being arranged round the mouth so as to act as foot-jaws, while the
mesosomatic appendages, though still free to move, have been reduced to
such an extent as to consist mainly of their basal parts, which are all
respiratory in function, except in the case of the first pair, where they
carry the terminal ducts of the genital organs. In the next stage, that, of
the scorpion, in which the mesosomatic appendages have lost all power of
free locomotion, and have become internal branchiæ, another mesosomatic
ganglion has fused with the brain mass, while in Androctonus two of the
branchial mesosomatic ganglia have fused; and finally, in Thelyphonus and
Phrynus, all the mesosomatic ganglia have coalesced with the fused
prosomatic ganglia, while the metasomatic ganglia have themselves fused
together in the caudal region to form what is known as the caudal brain.
{54}[Illustration: FIG. 25.--COMPARISON OF INVERTEBRATE BRAINS FROM
BRANCHIPUS TO AMMOCOETES.]
{55}The brain in these animals may be spoken of as composed of three
parts--(1) the fused supra-oesophageal ganglia, (2) the fused prosomatic
ganglia, and (3) the fused mesosomatic ganglia. Such a brain is strictly
homologous with the vertebrate brain, which also is built up of three
parts--(1) the part in front of the notochord, the prechordal or
supra-infundibular brain, which consists of the cerebral hemispheres,
together with the basal and optic ganglia and corresponds, therefore, to
the supra-oesophageal mass, with its olfactory and optic divisions lying in
front of the oesophagus; (2 and 3) the epichordal brain, composed of (2) a
trigeminal and (3) a vagus division, of which the first corresponds
strictly to the fused prosomatic ganglia, and the second to the fused
mesosomatic ganglia. Further, just as in the embryo of an arthropod it is
possible, with more or less accuracy, to see the number of neuromeres or
original ganglia which have fused to form the supra- and infra-oesophageal
portions of its brain, so also in the embryo of a vertebrate we are able at
an early stage to gain an indication, more or less accurate, of the number
of neuromeres which have built up the vertebrate brain. The further
consideration of these neuromeres, and the evidence they afford as to the
number of the prosomatic and mesosomatic ganglia which have formed the
epichordal part of the vertebrate brain, must be left to the chapter on the
segmentation of the cranial nerves.
The further continuation of this process of concentration of separate
segments, together with the fusion of the nervous system with the tube of
the alimentary canal, leads in the simplest manner to the formation of the
spinal cord of the vertebrate from the metasomatic ganglia of the ventral
chain of the arthropod.
THE ANTAGONISM BETWEEN CEPHALIZATION AND ALIMENTATION.
This concentration of the nervous system in the head-region, together with
an actual increase in the bulk of the cephalic nervous masses, constitutes
the great principle upon which the law of upward progress or evolution in
the animal kingdom is based, and it illustrates in a striking manner the
blind way in which natural selection works; for, as already explained, the
central nervous system arose as a ring round the mouth, in consequence of
which, with the progressive {56}evolution of the animal kingdom, the
oesophagus necessarily pierced the central nervous system at the cephalic
end. At the same time, the very fact that the evolution was progressive
necessitated the concentration and increase of the nervous masses in this
very same oesophageal region.
Progress on these lines must result in a crisis, owing to the inevitable
squeezing out of the food-channel by the increasing nerve-mass; and,
indeed, the fact that such a crisis had in all probability arisen at the
time when vertebrates first appeared is apparent when we examine the
conditions at the present time.
Those invertebrates whose central nervous system is most concentrated at
the cephalic end belong to the arachnid group, among which are included the
various living scorpion-like animals, such as Thelyphonus, Androctonus,
etc.
As already mentioned, the giants of the Palæostracan age were Pterygotus,
Slimonia, etc., all animals of the scorpion-type--in fact, sea-scorpions.
Now, all these animals, spiders and scorpions, without exception, are
blood-suckers, and in all of them the concentrated cephalic mass of nervous
material surrounds an oesophagus the calibre of which is so small that
nothing but a fluid pabulum can be taken into the alimentary canal; and
even for that purpose a special suctorial apparatus has in some species
been formed on the gastric side of the oesophagus for the purpose of
drawing blood through this exceedingly narrow tube.
In Fig. 25 this increasing antagonism between brain-power and alimentation,
as we pass from such a form as Branchipus to the scorpion, is illustrated,
and in Fig. 26 the relative sizes of the oesophagus and the brain-mass
surrounding it is shown. The section shows that the food channel is
surrounded by the white and grey matter of the brain as completely as the
central canal of the spinal cord of the vertebrate is surrounded by the
white and grey nervous material.
[Illustration: FIG. 26.--TRANSVERSE SECTION THROUGH THE BRAIN OF A YOUNG
THELYPHONUS.
_A_, supra-oesophageal ganglia; _B_, infra-oesophageal ganglia; _Al_,
oesophagus.]
{57}Truly, at the time when vertebrates first appeared, the direction and
progress of variation in the Arthropoda was leading, owing to the manner in
which the brain was pierced by the oesophagus, to a terrible
dilemma--either the capacity for taking in food without sufficient
intelligence to capture it, or intelligence sufficient to capture food and
no power to consume it.
Something had to be done--some way had to be found out of this difficulty.
The atrophy of the brain meant degeneration and the reduction to a lower
stage of organization, as is seen in the Tunicata. The further development
of the brain necessitated the establishment of a new method of alimentation
and the closure of the old oesophagus, its vestiges still remaining as the
infundibular canal of the vertebrate, meant the enormous upward stride of
the formation of the vertebrate.
At first sight it might appear too great an assumption even to imagine the
possibility of the formation of a new gut in an animal so highly organized
as an arthropod, but a little consideration will, I think, show that such
is not the case.
In the higher animals we are accustomed to speak of certain organs as vital
and necessary for the further existence of the animal; these are
essentially the central nervous system, the respiratory system, the
circulatory system, and the digestive system. Of these four vital systems
the first cannot be touched without the chance of degeneration; but that is
not the case with the second. The passage from the fish to the amphibian,
from the water-breathing to the air-breathing animal, has actually taken
place, and was effected by the modification of the swim-bladder to form new
respiratory organs--the lung; the old respiratory organs--the
gills--becoming functionless, but still persisting in the embryo as
vestiges. The necessity arose in consequence of the passage of the animal
from water to land, and with this necessity nature found a means of
overcoming the difficulty; air-breathing vertebrates arose, and from the
very fact of their being able to extend over the land-surfaces, increased
in numbers and developed in complexity in the manner already sketched out.
For a respiratory system all that is required is an arrangement {58}by
means of which blood should be brought to the surface, so as to interchange
its gases with those of the external medium; and it is significant to find
that of all vertebrates the Amphibia alone are capable of an effective
respiration by means of the skin.
As to the circulatory system, it is exceedingly easily modified. An animal
such as Amphioxus has no heart; in some the heart is systemic, in others
branchial; in some there are more than one heart; in others there are
contractile veins in addition to a heart. There is no difficulty here in
altering and modifying the system according to the needs of the individual.
For a digestive system all that is required is an arrangement for the
digestion and absorption of food, a mechanism which can arise easily if
some of the cells of the skin possess digestive power. Now Miss Alcock has
shown that some of the surface-cells of crustaceans secrete a fluid which
possesses digestive powers, and she has also shown that certain of the
cells in the skin of Ammocoetes possess digestive power.
The difficulty, then, of forming a new digestive system in the passage from
the arthropod to the vertebrate is very much the same as the difficulty in
forming a new respiratory system in the passage from the water-breathing
fish to the air-breathing amphibian--a change which does not strike us as
inconceivable, because we know it has taken place.
The whole argument so far leads to the conclusion that vertebrates arose
from ancient forms of arthropods by the formation of a new alimentary
canal, and the enclosure of the old canal by the growing central nervous
system. If this conclusion is true, then it follows that we possess a
well-defined starting-point from which to compare the separate organs of
the arthropod with those of the vertebrate, and if, in consequence of such
working hypothesis, each organ of the arthropod is found in the vertebrate
in a corresponding position and of similar structure, then the truth of the
starting-point is proved as fully as can possibly be expected by deductive
methods. It is, in fact, this method of comparative anatomy which has
proved the descent of man from the ape, the frog from the fish, etc.
Let us, then, compare all the organs of such a low vertebrate as Ammocoetes
with those of an arthropod of the ancient type.
{59}LIFE HISTORY OF THE LAMPREY--NOT A DEGENERATE ANIMAL.
The striking peculiarity of the lamprey is its life-history. It lives in
fresh water, spending a large portion of its life in the mud during the
period of its larval existence: then comes a somewhat sudden
transformation-stage, characterized, as in the lepidopterous larva, by a
process of histolysis, by which many of the larval tissues are destroyed
and new ones formed, with the result that the larval lamprey, or
Ammocoetes, is transformed into the adult lamprey, or Petromyzon. This
transformation takes place in August, at all events in the neighbourhood of
Cambridge, and later in the year the transformed lamprey migrates to the
sea, grows in size and maturity, and returns to the river the following
spring up to its spawning beds, where it spawns and forthwith dies. How
long it lives in the Ammocoetes stage is unknown; I myself have kept some
without transformation for four years, and probably they live in the rivers
longer than that before they change from their larval state. It is
absolutely certain that very much the longest part of the animal's life is
spent in the larval stage, and that with the maturity of the sexual organs
and the production of the fertilized ova the life of the individual ends.
Now, the striking point of this transformation is that it produces an
animal more nearly comparable with higher vertebrates than is the larval
form; in other words, the transformation from larva to adult is in the
direction of upward progress, not of degeneration. It is, therefore,
inaccurate to speak of the adult lamprey as degenerate from a higher race
of fishes represented by its larval form--Ammocoetes. Its transformation
does not resemble that of the tunicates, but rather that of the frog, so
that, just as in the case of the tadpole, the peculiarities of its larval
form may be expected to afford valuable indications of its immediate
ancestry. The very peculiarities to which attention must especially be paid
are those discarded at transformation, and, as will be seen, these are
essentially characteristic of the invertebrate and are not found in the
higher vertebrates. In fact, the transformation of the lamprey from the
Ammocoetes to the Petromyzon stage may be described as the casting off of
many of its ancestral invertebrate characters and the putting on of the
characteristics of the vertebrate type. It is this double individuality of
the lamprey, together with its long-continued existence in the larval form,
which makes Ammocoetes more {60}valuable than any other living vertebrate
for the study of the stock from which vertebrates sprang.
Many authorities hold the view that the lamprey, like Amphioxus, must be
looked upon as degenerate, and therefore as no more suitable for the
investigation of the problem of vertebrate ancestry than is Amphioxus
itself. This charge of degeneracy is based on the statement that the
lamprey is a parasite, and that the eyes in Ammocoetes are under the skin.
The whole supposition of the degeneracy of the Cyclostomata arose because
of the prevailing belief of the time that the earliest fishes were
elasmobranchs, and therefore gnathostomatous. From such gnathostomatous
fishes the cyclostomes were supposed to have descended, having lost their
jaws and become suctorial in habit in consequence of their parasitism.