THE ORIGIN OF VERTEBRATES, GASKELL 1908

THE

ORIGIN OF VERTEBRATES

WALTER HOLBROOK GASKELL
AND McGILL UNIV.) F.R.S. FELLOW OF TRINITY
HALL AND UNIVERSITY LECTURER IN PHYSIOLOGY, CAMBRIDGE HONORARY FELLOW
CORRESPONDING MEMBER
OF THE ROYAL MEDICAL AND CHIRURGICAL SOCIETY
OF THE IMPERIAL MILITARY ACADEMY OP MEDICINE, ST. PETERSBURG, ETC.
M.A., M.D. (CANTAB.), LL.D. (EDIN.

;

;

;

;

LONGMANS. GREEN, AND
PATERNOSTER ROW, LONDON
NEW YORK, BOMBAY, AND CALCUTTA
39

1908
All

rights reserved

CO.

CONTENTS
Intboduction

......
CHAPTER

PAGE
1

I

The Evidknce op the Central Nervous System

—

Theories of the origin of vertebrates Importance of the central nervous system
Evolution of tissues Evidence of Palaeontology Reasons for choosing
Ammocoetes rather than Amphioxus for the investigation of this problem
Importance of larval forms Comparison of the vertebrate and arthropod
central nervous systems Antagonism between oephalization and alimentation Life-history of lamprey, not a, degenerate animal Brain of AmmoSummary
ccBtes compared with brain of arthropod

—

—

—

—

—

—

—

—

CHAPTER

.

.8

.

II

The Evidence, of the Organs of Vision

— Simple and compound retinas—Upright and inverted
—Median eyes—Median or pineal eyes of Ammocoetes and their
optic ganglia— Comparison with other median eyes — Lateral eyes of vertethe
brates compared with lateral eyes of crustaceans — Peculiarities
the optic diverticula — Evolution
lateral eye of the lamprey— Meaning
vertebrate eyes — Summary

Difierent kinds of eye
retinas

of

...

.68

of

of

CHAPTER

III

The Evidence of the Skeleton
The bony and

—

cartilaginous skeleton considered, not the notoehord Nature of
the earliest cartilaginous skeleton The mesosomatic skeleton of Ammoits topographical arrangement, its structure, its origin in mucocoetes
cartUage The prosomatic skeleton of Ammocoetes
the trabeculae and
parachordals, their structure, their origin in white fibrous tissue The

—

;

—

mesosomatic skeleton

;

Limulus compared with that

—
—

Ammocoetes
similarity of position, of structure, of origin in muco-cartilage The
prosomatic skeleton of Limulus the entosternite, or plastron, compared
with the trabeoulse of Ammocoetes similarity of position, of structure, of
of

of

;

....
;

origin in fibrous tissue

— Summary

....

119


CONTENTS

VI

CHAPTER IV
The Evidence op the Respiratoey Apparatus
l-AGK

—

Branohiaa considered as internal brancliial appendages Innervation of branchial
segments Cranial region older than spinal Three-root system of cranial
nerves: dorsal, lateral, ventral— Explanation of van Wijhe's segments-

—

—

—

Lateral mixed root

The branchial
is appendage-nerve of invertebrate

chamber of Ammoccfites- The branchial unit, not a pouch but an appendage
The origin of the branchial musculature The branchial circulation The
branchial heart of the vertebrate Not homologous with the systemic heart
of the arthropod
Its formation from two longitudinal venous sinuses

—

—

Summary

.

—

—

—

148

.

CHAPTER V
The Evidence op the Thyroid Gland
The value

—
—

—
—

appendage-uuit in non-branchial segments The double nature
hyoid segment— Its branchial part— Its thyroid part—The double
nature of the opercular appendage Its branchial part Its genital part
Unique character of the thyroid gland of Ammocoetes Its structure
Its openings The nature of the thyroid segment The uterus of the
scorpion Its glands Comparison with the thyroid gland of Ammocoetes
Cephalic generative glands of Limulus Interpretation of glandular tissue
filling up the brain-case of Ammocoetes
Function of thyroid gland
Eelation of thyroidgland to sexual functions Summary
of the

of the

—

—

—

—

—

—

....

—

185

CHAPTER VI
The Evidence of the Olfactory Apparatus

—

Pishes divided into AmphirhiuEe and Monorhinae Nasal tube of the lamprey
Its termination at the inf undibulum The olfactory organs of the scorpion

—

group

—The

oamerostome

—

—

—

Its formation as a tube
Its derivation from a
termination at the true mouth Comparison with the

olfactory tube of Ammocoetes Origin of the nasal tube of Ammocoetes from
the tube of the hypophysis Direct comparison of the hypophysial tube with
the olfactory tube of the scorpion group Summary

pair of antennae

—Its

—

—

—

—

CHAPTER

218

VII

The Peosomatic Segments of Limulus and

—

its

Allies

Comparison of the trigeminal with the prosomatic region The prosomatic
appendages of the Gigantostraca Their number and nature Endognaths
and ectognath The metastoma The coxal glands Prosomatic region of
Eurypterus compared with that of Ammoocetes Prosomatic segmentation
shown by marks on carapace Evidence of coelomio cavities in Limulus

—
—

—

—

Summary

.

—

—

—

233


CONTENTS

CHAPTER
The Segments belonging

The prosomatic segments

vii

VIII

to the Trigeminal Nerve-Group
PAGE

—

the vertebrate Number of segments belonging to
the trigeminal nerve-group History of cranial segments Eye-muscles and
their nerves Comparison with the dorso-ventral somatic muscles of the
scorpion Explanation qf the oculomotor nerve and its group of muscles
Explanation of the trochlear nerve and its dorsal crossing Explanation
of the abduoens nerve
Number of segments supplied by the trigeminal
nerves Evidence of their motor nuclei Evidence of their sensory ganglia
of

—

—

—

—

—

—
—Summary

—

—

257

CHAPTER IX
The Prosomatic Segments of Ammoccetes

—

The prosomatic region in Ammoccetes The suctorial apparatus of the adult
Petromyzon Its origin in Ammoccetes Its derivation from appendages
The segment of the lower lip or the metastomal segment The tentacular
segments The tubular muscles Their segmental arrangement Their

—

—

—

—

—

—
—Their correspondence with the system of veno-pericardial muscles in Limulus — The old mouth or palseostoma — The pituitary
gland—Its comparison with the coxal gland of Limulus — Summary
peculiar innervation

.

.

286

CHAPTER X
The Relationship op Ammocoetes

—THE

to the most Ancient Fishes
OsTRACODERMATA

—

The nose

of the Osteostraci
Comparison of head-shield of Ammocoetes and of
Cephalaspis Ammoccetes only living representative of these ancient fishes
Formation of cranium Closure of old mouth Eohon's primordial

—

—

cranium

—
—
—Primordial cranium of Phrynus and Galeodes — Summary

.

.

326

CHAPTER XI
The Evidence of the Auditory Apparatus and the Organs

of

the Lateral Line

—

—

Lateral line organs Function of this group of organs Poriferous sense-organs
on the appendages in Limulus Branchial sense-organs Prosomatic senseorgans FlabeHum Its structure and position Sense-organs of mandibles
Auditory organs of insects and arachnids Poriferous chordotonal organs
Balancers of Diptera Resemblance to organs of flabeUum Eacquet-organs

—

—

—
—

—

—

—Pectens of scorpions — Large
sense-organs — Origin of parachordals and
Galeodes

—

—

—

nerve to all these special
auditory capsule Reason why
Vllth nerve passes in and out of capsule Evidence of Ammocoetes
Intrusion of glandular mass round brain into auditory capsule Intrusion
of generative and hepatic mass round brain into base of flabeUum
of

Summary

size- of

—

—

—

355


CONTENTS

viii

CHAPTER

XII

The Region of the Spinal Coed
PAGE

—Absence of lateral root—Meristic
variation— Segmentation of coelom— Segmental excretory organs —Development of nephric organs pronephrio, mesonepbric, metanepbrio — Excretory
organs of Ampbioxus— Solenocytes — Excretory organs of Brancbipus and
Peripatus, appendicular and somatic — Comparison
coelom of Peripatus
and of vertebrate — Pronepbric organs compared to eoxal glands — Origin
vertebrate body-cavity (metacoele) — Segmental duct — Summary of formation
of excretory organs — Origin of somatic trunk-musculature—Atrial cavity

of Ampbioxus — Pleural folds — Ventral growtb
pleural folds and somatic
musculature — Pleural folds of Cepbalaspidse and
Trilobita—Meaning of
tbe ductless glands — Alteration in structure
excretory organs wbicb bave
lympbatic
tbeir duct in vertebrates and in invertebrates —Formation
glands — Segmental coxal glands
artbropods and of vertebrates — Origin of
adrenals, pituitary body, tbymus,
tbyroid, and otber ductless glands
—Summary

Difference between cranial and spinal regions

;

of

of

of

of

of

of

lost

of

tonsils,

CHAPTER

385

XIII

The Notochoed and Alimentaey Canal

—

Eelationship between uotocbord and gut Position of unsegmented tube of
notocbord Origin of notocbord from a median groove Its function as an
accessory digestive tube Eormation of notocbordal tissue in invertebrates
from closed portions of tbe digestive tube Digestive power of tbe skin of
Ammoooetes Formation of new gut in Ammocoetes at transformation
Innervation of tbe vertebrate gut Tbe tbree outflows of efferent nerves
belonging to tbe organic system Tbe original close contiguity of tbe

—

—

—

respiratory

cbamber

to tbe

—

—

—
—
cloaca —Tbe elongation

of tbe

gut

—Conclusion

433

CHAPTER XIV
The
Tbe law

of recapitulation

Peinciples of Embeyolouy

—

Vindication of tbis law by tbe tbeory advanced in
germ-layer tbeory— -Its present position— A pbysiologioal
not a morpbological conception New fundamental law required Composition of adult body Neuro-epitbelial syncytium and free-living cells
Meaning of tbe blastula— Derivation of tbe Metazoa from tbe ProtozoaImportance of tbe central nervous system for Ontogeny as weU as for
Pbylogeny— Derivation of free-living cells from germ-ceUs— Meaning of
tbis

book— Tbe

—

coelom- Formation

— Summary

of

—

—

neural canal— Gastrula of Ampbioxus and. of Lucifer

455


CONTENTS

ix

CHAPTER XV
Pinal Remarks
PAOB

Problems requiring investigation
Giant nerve-cells and giant nerve-fibres; their comparison in fishes and
origin of
arthropods blood- and lymph-corpuscles nature of the skin
system of unstriped muscles origin of the sympathetic nervous system
;

;

;

;

biological test of relationship.

—Theory of parallel development —Importance

Criticisms of Balanoglossus theory
of

the theory advocated in this book for

all

problems of Evolution

.

.

488

Bibliography and Index op Authors

501

GenbraIi Index

517


"

Go ON AND PROSPER ; THERE

IS

NOTHING SO

USEFUL IN SCIENCE AS ONE OF THOSE EARTH-

QUAKE HYPOTHESES, WHICH OBLIGE ONE TO FACE
THE POSSIBILITY THAT THE SOLIDEST-LOOKING
STRUCTURES

MAY

COLLAPSE:'

Letter from Prof. Huxley to
THE Author. June z, 1889.


THE

ORIGIN OF VERTEBRATES
INTRODUCTION
In former days
to be a

it

was possible

leader both in

Nowadays aU

for a

man

like

Johannes Miiller

physiology and in comparative anatomy.

knowledge has increased so largely that
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
specialization

scientific

is

a special groove

inevitable,

—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

mind of one outside this particular line
to new and valuable scientific discoveries.

arising in the

may

give rise

Nothing but good can, in

my

of thought

opinion, result from the incursion

of the non-specialist into the .realm of the specialist, provided that

Over and over again the chemist has
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
the former

is

in earnest.

given valuable help to the physicist,

;

L

B


THE ORIGIN OF VERTEBRATES

2
I,

a worker in the physiological laboratory at Cambridge ever since

experimental physiology into English-speaking
nations, should have devoted so much time to the promulgation of
Foster introduced

a theory of the origin of vertebrates

—a

subject remote from phy-

and one of the larger questions appertaining
anatomy. By what process of thought was I led
siology,

to

comparative

to take

consideration of a subject apparently so remote from all

my

up the

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

the vagus and accelerator nerves on their

way

between the
to the heart,

fibres of

owing

to

the meduUation of the former and the non-medullation of the latter.

This led

me

an investigation of the accelerator fibres, to find out
non-meduUated, 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

how

to

far they are

hitherto

the grey ramus communicans is in
which supplies the blood-vessels of the
membranes, and is of the same nature as the

been thought;

for

reality a peripheral nerve

spinal cord and

its

grey accelerators

to the heart.

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

This led to

system, the cerebro-spinal, which sends special meduUated nerve-

by their smallness, to the cells of the sympathetic
from which fibres pass to the periphery, usually nonThese fine meduUated nerves form the system of
medullated.
white rami communicantes, and have since been called by Langley
fibres, characterized

system,

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


INTRODUCTION

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, belong-

ing 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

nerves, formerly called the nervi erigentes, I gave the

pelvic

their uniformity with the

abdominal
These investigations led to the conclusion that the

splanchnics, in order to

splanchnics.

show

system of

name

organic system

of-

nerves, characterized

by the possession

of efferent

nerve- cells situated peripherally, arises from the central nervous

—

system by three distinct ontflows cranial, thoracico-lumbar, and
To this system Langley has lately given the
sacral, respectively.

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 rarni communicantes
scarcely exist.
I

therefore

directed

my

attention in

the

next place to the

structure of the central nervous system in the endeavour to asso-

arrangement of cell-groups in this system
with the outflow of the different kinds of nerve-fibres to the
ciate the topographical

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,


THE ORIGIN OF VERTEBRATES

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 gene-

why it had been
There in the infundibulum was the old oesophagus,

ration of comparative anatomists, and wondered
discredited.

there in the cranial segmental nerves the infracesophageal ganglia,
there in the cerebral hemispheres and optic and olfactory nerves the

supraoesophageal ganglia, there in the spinal cord the ventral chain

But if the infundibulum was the old oesophagus, what
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
of ganglia.

then

?

the vertebrate, the conception that the central nervous system has

grown round and enclosed the
as to prevent

what

it

would

my

original ancestral gut,

and that the

me

so impossible

new gut

vertebrate has formed a

taking

it

as a

did not seem to

working hypothesis, and seeing

to

lead.

This theory that the so-called central nervous system of the
vertebrate

is

in reality

segmented

composed

of

two separate

parts, of

which

corresponds to the central nervous
system of the highest invertebrates, while the other, the unsegthe one, the

part,

was originally the alimentary canal of that same
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
mented

tube,

invertebrate,

Physiology, vol. 10.

Since that time I have been engaged in testing
and have published the results of

the theory in every possible way,

my

investigations in a series of papers in different journals, a list of
I append at the end of this introductory chapter.

which

INTRODUCTION

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

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
Any one
where I am wrong, and why my theory is untenable.
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
of the theory as a whole,

bearing on this solution.

In
It

is

this

work

not too

I

much

have been helped throughout by Miss E. Alcock.
to say that

without the assistance she has given

many an important link in the chain
been missing. With extraordinary patience
me,

of evidence

would have

she has followed, section

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

by

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,
it does over the whole period of the work, that her name

lasting as

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
I desire to express my grateful thanks to the former for
press.
valuable criticism of the scientific evidence which I have brought


THE ORIGIN OF VERTEBRATES

6

forward in this book, and to the latter for his great kindness in

undertaking the laborious task of correcting the proofs.

LIST OF PREVIOUS PUBLICATIONS BY THE AUTHOR, CON-

CERNING THE ORIGIN OF VERTEBRATES.
1888.

1889.

" Spinal

and Cranial Nerves."

Proceedings of the Anatomical Society,

June, 1888. Journal of Anatomy and Physiology, vol. xxiii.
" 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

1890.

"

On

1895.

"

The Origin

the Origin of the Central Nervous System of
Brain, vol. xii.. p. 1.

Vertebrates."

the Origin of Vertebrates from a Crustacean-like Ancestor.''
Quarterly Journal of Microscopical Science, vol. xxxi., p. 379.
of Vertehrates.''

sophical Society, vol.

Proceedings of the Cambridge Philo-

ix., p. 19.

to Section I. at the meeting of the British
Association for the Advancement of Science in Liverpool. Report

1896.

Presidential Address

1899.

"

of the British Association, 1896, p. 942.

On

A

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

series of papers on " The Origin of Vertebrates, deduced from the
study of Ammocoetes," in the Journal of Anatomy and Physiology, as

f oUows

1898.

Part
.,

:

I.

"

II. "

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

Facial Nerve," vol. xxxiii.,
1900.

..

V.

"

The Origin

Segment

:

the

Meaning

Segmentation the Meaning
and Eye-muscle Nerves," vol. xxxiv.,

of the Pro-otic

of the Trigeminal

of the

p. 638.
:

p. 465.

1900.

.,

VI.

"

The Old Mouth and

the OKactory

Organ

:

the

Meaning

of the First Nerve," vol. xxxiv., p. 514.

1900.

1900.

„

„

VII.

VIII.

"

"

The Evidence
as given

of Prosomatic Appendages in Ammocoetes,
by the Course and Distribution of the Trigeminal

Nerve,"

vol. xxxiv., p. 537.

The

Palfeontological

Evidence

:

Ammocoetes a Cepha-

laspid," vol. xxxiv., p. 562.

1901.

„

IX. " The Origin of the Optic Apparatus
Optic Nerves,'"

vol. xxxv., p. 224.

:

the

Meaning

of the


INTRODUCTION
1902.

Part

1903.

„

X.

"

XI.

"

7

The Origin of the Auditory Organ the Meaning of the
Vlllth Cranial Nerve," vol. xxxvi., p. 164
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.

„

1906.

,,

XII.
XIII.

"

"

The Principles of Embryology," vol. xxxix., p. 371.
The Origin of the Notochord and Alimentary Canal,"
vol. xl., p. 306.


CHAPTER

I

THE EVIDEXCE OF THE CENTRAL NERVOUS SYSTEM

—

Theories of tte origin of vertelirates. Importance of the central nervous
Evidence of Pateontology.— Reasons for
system. Evolution of tissues.
choosing Ammoccetes rather than Amphioxus. Importance of larval forms.
Comparison of the vertebrate and arthropod central nervous systems.

—

—

—

—

Antagonism between

lamprey

:

brain of arthropod.

At

the present time

and

cephalization

not a degenerate animal.

— Summary.
it is

— Brain

alimentation.

— Life-history

of

Ammoccetes compared with

of

no longer a debatable question whether or
Since the time of Darwin the accu-

no Evolution has taken

place.

mulation of facts in

support has been so overwhelming that all

zoologists look

upon

its

and desire now to find
Here two
investigation, which can be and are

this question as settled,

out the manner in which such evolution has taken place.

problems

offer

themselves for

treated separately
of heredity

—the

one dealing with the question of those laws

and variation which have brought about

in the past

are still causing in the present the evolution of living beings,

i.e.

and
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
first,

the finding

any one group

is

of a twofold character

:

out of the steps by which the higher forms in

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,

groups of animals exist which have so

many

it is

clear that large

coiiimon characteristics

as to necessitate their being placed in one larger

group or kingdom


THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM

9

thus zoologists are able to speak definitely of the Vertebrata, Arthropoda, Annelida, Echinodermata, Porifera, Ccelenterata, MoUusca,

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

etc.

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

we trace the progress from the lowest fishes
man, and already the discoveries of palaeontology 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

place in the Vertebrata as
to

matter of doubt
solution of
of the

;

which

members

the connected problems are matters of detail, the
is

of

certain sooner or later.

any

The same may be

said

of the other great natural groups, such as

the Arthropoda, the Annelida, the Echinodermata, etc.
It is different, however,

two of the main

when an attempt

is

made

to connect

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
divisions themselves.

It is true

in other cases.

In the very case

of the origin of the

Appendiculata we are con-

— the origin of
— the solution of which not

fronted with one of the large problems of evolution

segmented from non-segmented animals

is

yet known.

Theories of the Oeigin of Vertebrates.

The other

large problem, perhaps the most important of

question of the relationship of the great

kingdom

all, is

the

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


THE ORIGIN OF VERTEBRATES

lO

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.
jSTow, if

we take a broad and comprehensive view

of the inver-

tebrate kingdom, without arguing out each separate case,

we

find that

D

Fig.

Abeangbmbnt op Obgans

1.

Al, gut

;

H, heart

;

in

the Vebtebbate

C.N. 8., central nervous system

;

(A)

and Abthbopod

V, ventral side

;

D, dorsal

(B)-

side.

bears strongly the stamp of a general plan of evolution derived
from a coelenterate animal, whose central nervous system formed a
Then when the radial symmetry was
ring surrounding the mouth.
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-cesophageal nervous masses. These latter
it


THE EVIDENCE OF

TftS

CENTRAL NERVOUS SYSTEM

II

nervous masses are of necessity ventral to the digestive tube, because
the

mouth

of the ccelenterate

is

on the ventral

characteristic, then, of the invertebrate

kingdom

side.
is

The

striking

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

phagus

—leading

from the mouth

to

—the

the alimentary canal.

equally striking characteristic of the vertebrate

is

ceso-

The

the dorsal position

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 Verteof the central nervous system

brates

;

and,

to

my

mind, this

is

prevalent

the

Two

morphologists at the present time.

feeling

among

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 versa

;

at the

same time, a new

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

mouth

especially associated with the

name

bilaterally symmetrical, elongated,

of Bateson.

It supposes that

segmented animals were formed

in two distinct ways.

In the one case the digestive tube pierced the central nervous system, and was situated dorsally
In the other case the segmented central nervous
to its main mass.
system was situated from the first dorsally to the alimentary canal,
from the very

first

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
The great difficulty in the way of accepting
universal acceptance.
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.

THE ORIGIN OF VERTEBRATES

12
It has

been supposed that the Tunicata and the Enteropneusta

{Balanoglossus) (Fig. 2) are

members

and that

of this missing chain,

in

Amphioxus the

tebrate

ver-

approaches

organization

in

these

to

low invertebrate forms.

The

indeed,

tnnicates,

upon as degenerate members of an
are looked

early vertebrate

which

may

stock,

give help in

picturing the nature of

the vertebrate ancestor

but are not themselves

Pig.

2.

Larval Balanoglossus (from the Eoyal

in

Natural History).

is

arisen from the Echinodermata, or at

with them, so that to

fill

the

descent.

all

line

of

Balanoglossus

supposed

to

have

events to have aflSnities

up the enormous

Echinodermata and the Vertebrata on

direct

gap

between the

this theory there is absolutely

nothing living on the earth except Balanoglossus, Ehabdopleura,

The characteristics of the vertebrate upon
and Cephalodiscus.
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

In the latter the
segmented invertebrate, annelid or arthropod.
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 antenute are olfactory in function.

These are

connected with the infra-cesophageal ganglia by the oesophageal

commissures which encircle the oesophagus.
2. The infra-cesophageal ganglia and the two chains of ventral
ganglia, which are segmentally-arranged sets of ganglia.
Of these.


THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM
each pair gives

rise

bo

the nerves of

its

own segment, and

1

these

nerves are not nerves of special sense as are the supra-cesophageal

motor and sensory to the segment nerves by the agency
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, but

;

of

nerves.

DORSAL
Spinal canal

Spinol Copd

in/undtUum

*

Segmental

Neureateric canaX

'SiTvea

VENTRAL

DORSAL

VENTRAL

/OEiopKaja.
(jbs

Fig.

Com

3.

Vbbtebbate Oentbal Nervous System compabbd with the Obntbal
Nbbvous System and Alimentary Canal op the Abthbopod.

A. Vertebrate central nervous system.
/.

Inf. Br., infra-infundibular brain

quadrigemina

;

S. Inf. Br., supra-infundibular brain;

and cranial segmental nerves; C.Q., corpora

Cb., cerebellum; C.C., crura cerebri;

C.S., corpus striatum;

Pw.,

pineal gland.

B. Invertebrate central nervous system.
I.

S. CEs. G., supra-oesophageal ganglia

CEs. G., infra-oBsophageal ganglia; CEs. Com,, oesophageal commissures.

2.

cord;

The region
from

of the mid-brain, medulla oblongata,

and spinal

segmentally arranged,

these arises a series of nerves

which, as in the invertebrate, gives origin to the nerves governing
mastication, respiration,

and locomotion.

system possesses the
nowhere else, of being tubular, and the tube is
In the spinal region it is a small, simple
of a striking character.
canal of uniform calibre, which at the front end dilates to form the
Further, the vertebrate central nervous

peculiarity, found

ventricles of the region of the brain.

From

that part of this dilated


THE ORIGIN OF VERTEBRATES

14
portion,

knowu

as the third ventricle, a

ventral surface of the brain.

This tube

narrow tube passes to the
is

called the infuTidihulwm,

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 criora 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

the words infundibular and

if

oesophageal are taken to be interchangable, then in every respect

The brain proper

the two central nervous systems are comparable.
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 infraoesophageal and ventral chain of ganglia.

This overwhelmingly strong evidence has always pointed directly
to the origin of the vertebrate

froa 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,

extremely sound, for
it

is

based.

Still,

the position of this school of anatomists was

it is

impossible to dispute the facts on which

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

spots where the roof of the brain

is

all

vertebrates there are two

very thin, the one in the region of

the pineal body, and the other constituting the roof of the fourth venBoth of these places have had their advocates as the position of

tricle.

the old mouth, the former being upheld by Owen, the latter by

Dohm.


THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM

1

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.

whereas a pair of eyes close to the mid-dorsal line
enough, but

is

actually found io

exist

among

is

For

not only likely

large

numbers

of

arthropods, both living and extinct, a pair of eyes situated close

mouth

to the mid-ventral line near the

is

not only unheard of in

nature, but so improbable as to render impossible the theory which

necessitates such a position.

Yet
support

this very discovery gives the strongest possible additional
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,

which has done most

to discredit

it,

is

the very one

because in the minds of

its

authors the only possible solution of the transition from the one

group

to the other

Still, as

was by means

already said, even

if

of the reversal of surfaces.

the theory advanced to explain the

and still to this day
aa 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.
Kr>. 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
It is the object of this book to consider the possibility
of surfaces.
of such an explanation.
What are the lines of investigation most likely to meet with
Is it possible to lay down any laws of evolution ?
It
success ?
is instructive to consider the nature of the investigations which
have led to the two theories just mentioned, for the fundamental
The one
starting-point is remarkably different in the two cases.
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
The other theory is based upon the
vertebrate and human anatomy.

study of the invertebrate, and consists especially of an attempt to
find in the invertebrate some structure resembling a notochord, such
facts be discredited, the facts

remain the same

;


THE ORIGIN OF VERTEBRATES

1

organ being considered by them as the great characteristic of the
indeed, so

vertebrate;
of zoologists

speak

now

much

is

this the case, that a large

number

and

of Chordata rather than of Vertebrata,

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
is more likely to give reliable results.
the
vertebrate
to
the question in every possible way, by
putting
By
anatomy
and

physiology,
both gross and minute, by
studying its
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
conceded that the former

;

;

:

find out the origin of vertebrates, inquire, in the first place, of the

vertebrate

itself.

Impoetance of the Central Nebvous 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

upon which evolution depends, it has gradually dawned upon the scientific mind that
the fittest may be produced in two diametrically opposite ways
the law of the

'

'

'

survival of the fittest'

is

the factor


THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM
by progress upwards to
lower type of animal. The

1

by degeneration

either

a superior form, or

a

principle of degeneration as a factor

which
nowadays uuiversally admitted.

in the formation of groups of animals,

to

are thereby enabled

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
to survive,

is

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

Such forms are

arose.

of

considerable interest, but

cannot help, except negatively, to decide what factor

especially

is

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,
Not only

especially of the region of the brain.

manifest in distinguishing
to the latter as well.

man from

By

the

is

this point

the lower animals, but

amount

it

most

applies

of convolution of the brain,

the amount of grey matter in the cerebral hemispheres, the enlarge-

ment and increasing complexity

of the higher parts of the central

nervous system, the anthropoid apes are differentiated from the lower

mammals from the lower. In the recent work
and of Edinger, most conclusive proof is given that

forms, and the higher
of Elliot Smith,

the

upward progress

in the vertebrate

phylum

is

correlated with the

increase of brain-power, and the latter writer shows

remarkable

how

steady and

the increase in substance and in complexity of the

is

we

brain-region as

pass from the fishes, through the amphibians and

and mammals.
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 brainreptiles, to the birds

The study

of the forms

;

power

to the lords of creation in after-times.

What

applies to the vertebrate

tebrate groups.

Here

also

phylum

applies also to the inver-

an upward progress

is

recognized as

c

we