EARLY THEORIES ON THE ORIGIN OF VERTEBRATES
Haeckel and Gegenbaur set the fashion for phylogenetic speculation, and
up to the middle 'eighties, when the voice of the sceptics began to make
itself heard, the chief concern of the younger morphologists was the
construction of genealogical trees. The period from about 1865 to 1885
might well be called the second speculative or transcendental period of
morphology, differing only from the first period of transcendentalism by
the greater bulk of its positive achievement. It must be remembered that
the later workers (at least towards the end of this period) had immense
advantages over their predecessors in the matter of equipment and
technique; they possessed well-fitted laboratories in the university
towns and by the sea; they had at their command perfected microscopes
and microtomes; while the whole new technique of microscopical anatomy
with its endless variety of stains and reagents made it possible for the
tyro to confirm in a day what von Baer and Mueller had taken weeks of
painful endeavour to discover.[386] But the democratisation of morphology
which followed upon the facilitation of its means of research left an
evil heritage of detailed and unintelligent work to counterbalance the
very great and real advances which technical improvements alone rendered
possible.
This period of rapid development, which set in soon after the coming of
evolution and multiplied the concrete facts of morphology an
hundredfold, may for our present purpose be conveniently divided into
two somewhat overlapping periods, of which the second may be said to
begin with the enunciation by Haeckel of his Gastraea theory. Within the
first period fall the evolutionary speculations associated with the
names of Kowalevsky, Dohrn, Semper, and others; the characteristic of
the second period is the preponderating influence exercised upon
phylogenetic speculations by the germ-layer doctrine in its two main
evolutionary developments, the Gastraea and Coelom theories.
In the first period we might again distinguish two main tendencies,
according as speculations were based mainly upon anatomical or mainly
upon embryological considerations, and it so happens that these two
tendencies are very well illustrated by the various theories as to the
origin of Vertebrates which began to appear towards the 'seventies. We
shall accordingly, in this chapter, consider very briefly the history of
the earlier views on the phylogeny of the vertebrate stock.
In the early days, before the other claimants to the dignity of
ancestral form to the Vertebrates--_Balanoglossus_, Nemertines and the
rest--had put in an appearance, there were two main views on the
subject, one upheld by Haeckel, Kowalevsky and others, to the effect
that the proximate ancestor of Vertebrates was a form somewhat
resembling the ascidian tadpole, the other supported principally by
Dohrn and Semper that Vertebrates and Arthropods traced their descent to
a common segmented annelid or pro-annelid ancestor. The former view is
historically prior, and arose directly out of the brilliant
embryological investigations of A. Kowalevsky, who proved himself to be
a worthy successor of the great comparative embryologist Rathke. His
work was indeed a true continuation of Rathke's. It was not directly
inspired by evolution, though it supplied much useful confirmation of
the theory--you may read Kowalevsky's earlier memoirs and not realise
that they were written several years after the publication of the
_Origin of Species_.
His first paper of evolutionary importance was a note in Russian on the
development of Amphioxus, published in 1865. This subject was followed
up in two papers which appeared in 1867[387] and 1877.[388] In his
papers on Amphioxus Kowalevsky made out the main features in the
development of this primitive form, and showed that the chief organs
were formed in essentially the same way as in Vertebrates; he described
the formation of the archenteron by invagination, the appearance of the
medullary folds, which coalesced to form the neural canal, the formation
of the notochord and of the gill-slits. At first he made the mistake of
supposing that the body-cavity arose from the segmentation-cavity, but
in his later paper he rightly surmised that it was formed from the
cavities of the "primitive vertebrae," or mesodermal segments. The origin
of the notochord from the endoderm was also not made out by Kowalevsky
in his paper of 1867.
Although many important details remained to be discovered by later
investigators,[389] Kowalevsky's work at once made the development of
Amphioxus the key to vertebrate embryology, the typical ontogeny with
which all others could be compared.
Meanwhile, in 1866 and 1871, Kowalevsky had communicated memoirs of even
greater interest,[390] in which he showed that the simple Ascidians
developed in an extraordinarily similar way to Amphioxus and hence to
Vertebrates in general. His proof that Ascidians also develop on the
vertebrate type aroused great interest at the time, and was naturally
acclaimed by the evolutionists as a striking piece of evidence in favour
of their doctrine. The systematic position of the Ascidians was at that
time quite uncertain; they were grouped, as a rule, with the Mollusca,
and certainly no one suspected that their well-known tailed larvae, first
seen by Savigny, showed any but the most superficial analogy with the
tadpoles of Amphibia. Kowalevsky's papers put a different complexion on
the matter. In the first of them he showed how the nervous system of the
simple Ascidian developed from ectodermal folds just as it did in
Amphioxus and Vertebrates, how gill-slits were formed in the walls of
the pharynx, and how there existed in the ascidian larva a structure
which in position and mode of development was the strict homologue of
the vertebrate notochord. In his second paper he entered into much more
detail, and published some excellent figures, often reproduced since
(see Fig. 13), but the proof of the affinity between Vertebrates and
Ascidians was in all essentials complete in his paper of 1866.
[Illustration: FIG. 13.--Development of the Ascidian Larva. (After
Kowalevsky.)]
Kowalevsky's results were accepted by Haeckel, Gegenbaur, Darwin,[391]
and many others as conclusive evidence of the origin of Vertebrates
from a form resembling the ascidian tadpole; they were extended and
amplified by Kupffer[392] in 1870, later by van Beneden and Julin[393]
and numerous other workers; they were adversely criticised by
Metschnikoff[394] and von Baer,[395] as well as by H. de
Lacaze-Duthiers and A. Giard.[396] Lacaze-Duthiers and von Baer both
held fast to the old view that Ascidians were directly comparable with
Lamellibranch molluscs; they denied the homology of the ascidian
nervous system with that of Vertebrates, von Baer being at great pains
to show that the ascidian nerve-centre was really ventral in position.
He pointed out also that the "notochord" was confined to the tail of
the ascidian larva. Giard's attitude was by no means so
uncompromising, and the criticisms he passed on the Kowalevsky theory
are both subtle and instructive. He admits that there exists a real
homology between, for instance, the notochord of Vertebrates and that
of Ascidians. "But," he adds, "it is too often forgotten that homology
does not necessarily mean an immediate common origin or close
relationship. There exist, doubtless, homologies of great atavistic
importance--I consider as such, for example, the formation of the
cavity of Rusconi [the archenteron] in Ascidians and lower
Vertebrates. But there are also adaptive and purely analogical
homologies, such as the interdigital palmation of aquatic birds,
amphibians and mammals. These are not purely analogous organs, for
they can be superposed one on another, which is not the case with
simply analogous structures (the bat's wing, for example, cannot be
superposed on the bird's wing); they are homologous formations,
resulting from the adaptation of the same fundamental organs to
identical functions. Such is, in my opinion, the nature of the
homology existing between the tail of the ascidian tadpole and that of
Amphioxus or of young amphibians. The ascidian larva, having no cilia
and being necessarily motile, requires for the insertion of its
muscles or contractile organs ... a central flexible axis, a true
chorda dorsalis analogous to that of Vertebrates" (pp. 278-9). This
point of view is strengthened by the fact that in _Molgula_, studied
by Lacaze-Duthiers, the embryo is practically stationary, and forms no
notochord, nor ever develops sense-organs in the cerebral vesicle.
Giard's general conclusion is that "the true homology with Vertebrates
ceases after the formation of the cavity of Rusconi and the medullary
groove: the homologies established by Kowalevsky for the notochord and
the relations of the digestive tube and nervous systems are not
atavistic, but adaptive, homologies" (p. 282). There is accordingly no
close genetic relationship between Ascidians and Vertebrates.
Giard's criticisms did not avail to check the vogue of the new theory,
which soon became an accepted article of faith in most morphological
circles.[397] The fall of the Ascidians from their larval high estate
provided the text for many a Darwinian sermon.
Some years after the genetic relationship of Ascidians and Vertebrates
had been established, a rival theory of the origin of Vertebrates made
its appearance--a theory which was practically a rehabilitation in a
somewhat altered form of the old Geoffroyan conception that Vertebrates
are Arthropods walking on their backs. This was the so-called Annelid
theory of Dohrn and Semper. Both Dohrn and Semper started out from the
fact that Annelids and Vertebrates are alike segmented animals, and it
was an essential part of their theory that this resemblance was due to
descent from a common segmented ancestor. Both laid great stress on the
fact that the main organs in Vertebrates are arranged in the same way as
in an Annelid lying on its back, the nervous system being uppermost, the
alimentary system coming next, and below this the vascular.
Dohrn's earlier views are contained in the fascinating little book
published in 1875, which bears the title _Der Ursprung der Wirbelthiere
und das Princip des Functionswechsel_ (Leipzig). He followed this up by
a long series of studies on vertebrate anatomy and embryology,[398] in
which he modified his views in certain details. We shall confine our
attention to the first sketch of his theory.
If the Vertebrate is conceived to have evolved from a primitive Annelid
which took to creeping or swimming ventral surface uppermost, a
difficulty at once arises with regard to the relative positions of the
"brain" and the mouth. In Vertebrates the brain, like the rest of the
nervous system, is dorsal to the mouth and the alimentary canal; in an
inverted Annelid, however, the brain is ventral to the mouth and is
connected with the dorsal nerve cord by commissures passing round the
oesophagus. It would seem, therefore, that the primitive Vertebrate must
have acquired either a new brain or a new mouth. Dohrn took the latter
view. He supposed that the original mouth of the primitive ancestor lay
between the _crura cerebelli_ in the _fossa rhomboidea_, and that in
Vertebrates this mouth has been replaced functionally by a new ventrally
placed mouth, formed by the medial coalescence of a pair of
gill-slits.[399] Probably the two mouths at one period co-existed, and the
older one was ousted by the growing functional importance of the newer
mouth.
The gill-slits were considered by Dohrn to be derived from the segmental
organs of Annelids, which were present originally in every segment of
the primitive ancestor. The gills were at first external, like the gills
of many Chaetopods at the present day. For their support cartilaginous
gill-arches naturally arose in the body-wall, and the superficial
musculature became attached to these bars. "There existed in all the
segments of the Annelid-ancestors of Vertebrates gills with
cartilaginous skeleton and gill-arches in the body wall. Each gill had
its veins and arteries, each had its branch of the ventral nerve-cord,
and between each successive pair of gills a segmental organ opened to
the exterior" (p. 14, 1875). The paired fins and limbs of the Vertebrate
arose by the functional transformation of two pairs of these gills. The
anterior gills became the definitive internal gills of the Vertebrate,
for they gradually shifted into the mouths of the anterior segmental
organs, which had already acquired an opening into the pharynx and had
been transformed into true gill-slits. The posterior gills degenerated
and disappeared, but their arches remained as ribs. Gill-arches and ribs
were accordingly homologous structures and formed a _parietal_ skeleton.
The vertebrate anus, like the mouth, was probably secondary and formed
from a pair of gill-slits, the post-anal gut of vertebrate embryos
hinting that the original anus was terminal as in Annelids. The unpaired
fins of fish were originally paired and possibly arose from the
coalescence of rows of parapodia. Dohrn assumed also that the primitive
Annelid ancestor must have possessed a notochord to give support in
swimming.
If Vertebrates arose from primitive Annelid ancestors, how account for
Amphioxus and the Ascidians, which seem to be the most primitive living
Vertebrates and yet show no particular annelidan affinities? Dohrn tries
to answer this awkward question by showing that these forms are not
primitive but degenerate. He points out first that Cyclostomes are
degenerate fish, half specialised and half degraded in adaptation to a
parasitic mode of life. He thinks that if an _Ammocoetes_ were to become
sexually mature and degenerate still further, forms would result which
would resemble Amphioxus, and ultimately, if the process of degeneration
went far enough, larval Ascidians. Amphioxus therefore might well be
considered an extremely simplified and degenerate Cyclostome, and the
ascidian larva the last term of this degeneration-series. Both Amphioxus
and the Ascidians would accordingly be descended from fish, instead of
fish being evolved from them.
Dohrn conceived that the transformation of the Annelid into the
Vertebrate took place mainly by reason of an important transforming
principle, which he calls the principle of function-change. Each organ,
Dohrn thinks, has besides its principal function a number of subsidiary
functions which only await an opportunity to become active. "The
transformation of an organ takes place by reason of the succession of
the functions which one and the same organ possesses. Each function is a
resultant of several components, of which one is the principal or
primary function, while the others are the subsidiary or secondary
functions. The weakening of the principal function and the strengthening
of a subsidiary function alters the total function; the subsidiary
function gradually becomes the chief function, the total function
becomes quite different, and the consequence of the whole process is the
transformation of the organ" (p. 60). Examples of function-change are
not difficult to find. Thus the stomach in most Vertebrates performs
both a chemical and a mechanical function, but in some forms a part of
it specialises in the mechanical side of the work and becomes a gizzard,
while the remaining part confines its energies to the secretion of the
gastric juice. So, too, it is through function-change that certain of
the ambulatory appendages of Arthropods have become transformed into
jaws--their function as graspers of food has gradually prevailed over
their main function as walking limbs. In the evolution of Vertebrates
from Annelids the principle came into action in many connections--in the
formation of a new mouth from gill-slits, in the transformation of gills
into fins and limbs, of segmental organs into gill-slits, and so on.
Dohrn tells us that the principle of function-change was suggested to
him by Mivart's _Genesis of Species_ (1870), and he points out how it
enables a partial reply to be made to the dangerous objection raised
against the theory of natural selection that the first beginnings of new
organs are necessarily useless in the struggle for existence.
We may note in passing that a somewhat similar idea was later applied by
Kleinenberg to the explanation of some of the ancestral features of
development. He pointed out in his classical memoir on the embryology of
the Annelid _Lopadorhynchus_[400] that many embryonic organs seem to be
formed for the sole purpose of providing the necessary stimulus for the
development of the definitive organs. Thus the notochord is the
necessary forerunner of the vertebral column, cartilage the precursor of
bone. "From this point of view," he writes, "many rudimentary organs
appear in a different light. Their obstinate reappearance throughout
long phylogenetic series would be hard to understand were they really no
more than reminiscences of bygone and forgotten stages. Their
significance in the processes of individual development may in truth be
far greater than is generally recognised. When in the course of the
phylogeny they have played their part as intermediary organs
(_Vermittelungsorgane_) they assume the same function in the ontogeny.
Through the stimulus or by the aid of these organs, now become
rudimentary, the permanent parts of the embryo appear and are guided in
their development; when these have attained a certain degree of
independence, the intermediary organ, having played its part, may be
placed upon the retired list."[401]
Dohrn was well aware of the functional, or as he calls it, the
physiological, orientation of his principle, and he rightly regarded
this as one of its chief merits. He held that morphology became too
abstract and one-sided if it disregarded physiology completely; he saw
clearly that the evolution of function was quite as important a problem
as the evolution of form, and that neither could be solved in isolation
from the other. "The concept of function-change is purely
physiological;" he writes, "it contains the elements out of which
perhaps a history of the evolution of function may gradually arise, and
for this very reason it will be of great utility in morphology, for the
evolutionary history of structure is only the concrete projection of the
content and course of the evolution of function, and cannot be
comprehended apart from it" (p. 70).[402]
It is very instructive in this connection to note that Dohrn was not,
like so many of his contemporaries, a dogmatic materialist, but upheld
the commonsense view that vital phenomena must, in the first instance at
least, be accepted as they are. "It is for the time being irrelevant,"
he writes, "to squabble over the question as to whether life is a result
of physico-chemical processes or an original property (_Urqualitaet_) of
all being.... Let us take it as given" (p. 75).
Semper's speculations on the genetic affinity of Articulates and
Vertebrates are contained in two papers[403] which appeared about the same
time as Dohrn's. He openly acknowledges that his work is essentially a
continuation of Geoffroy's transcendental speculations, and gives in his
second paper a good historical account of the views of his great
predecessor. It is a significant fact that evolutionary morphologists
very generally held that Geoffroy was right in maintaining against
Cuvier[404] the unity of plan of the whole animal kingdom, for they saw in
this a strong argument for the monophyletic descent of all animals from
one common ancestral form.
In his first paper Semper does little more than break ground; he insists
on the fact that both Annelids and Vertebrates are segmented animals,
and he points out how close is the analogy between the nephridia or
"segmental organs" of the former and the excretory (mesonephric) tubules
of the latter, upon which he published in the same volume an extensive
memoir. At this time he considered _Balanoglossus_--by reason of its
gill-slits (its notochord he did not know)--to be the nearest living
representative of the ancestral form of Vertebrates and Annelida.
His second paper is a more exhaustive piece of work and deals with every
aspect of the problem, both from an anatomical and from an embryological
standpoint. It is consciously and admittedly an attempt to apply
Geoffroy's principle of the unity of plan and composition to the three
great metameric groups, the Annelida, Arthropoda, and Vertebrata. Semper
follows Geoffroy's lead very closely in maintaining that it is not the
position of the organs relative to the ground that must be taken into
account in establishing their homologies, but solely their spatial
relations one to another. He holds that dorsum and venter are terms of
purely physiological import, and he proposes to substitute for them the
terms neural and cardial (better, haemal) surfaces, either of which may
be either dorsal or ventral in position.
Having established this primary principle, Semper has little difficulty
in showing that the main organs of the body lie to one another in the
same relative positions in Annelida, Arthropoda, and Vertebrata; and
this, together with the metameric segmentation common to them all,
constitutes his first great argument in favour of their genetic
relationship. But he has still to show that Annelids possess at least
the rudiments of certain organs which seem to be peculiar to
Vertebrates, as the gill-slits, the notochord, and a nervous system
developed from the ectoderm of the "dorsal" surface. He takes particular
cognisance also of the old distinction drawn by von Baer, that
Vertebrates show a "double-symmetrical" mode of development (_evolutio
bigemina_), the dorsal muscle-plates forming a tube above the notochord,
the ventral plates a tube below the notochord, whereas Articulates do
not possess this axis, and form only one tube, namely, that round the
"vegetative" organs (_evolutio gemina_). Semper is at pains to prove
that _evolutio bigemina_ is characteristic also of Annelidan
development.
[Illustration: FIG. 14.--Transverse Section (Inverted) of the Worm
_Nais_. (After Semper.)]
He gets his facts from an elaborate study of the process of budding in
the _Naidae_, making the somewhat risky assumption that regeneration
takes essentially the same course as embryonic development.
He succeeds in showing--to his own satisfaction at least--that in the
formation of new segments in _Nais_ and _Chaetogaster_ a strand of cells
appears between the alimentary canal and the nerve-cord, and that from
this axial strand the haemal muscle-plates grow out dorsally round the
alimentary canal and the neural muscle-plates ventrally round the
nerve-cord (see Fig. 14).
This strand of cells, he concludes, must clearly be the notochord, and
the type of development is obviously the double-symmetrical met with in
Vertebrates.
The nervous system Semper found to develop in the buds of _Nais_ and
_Chaetogaster_ by an ectodermal thickening, just as in some Vertebrates.
The cerebral ganglion was formed by the ends of the nerve-cord growing
up round the oesophagus and fusing with the paired "sense-plates" which
develop from the ectoderm of the head. The cerebral ganglion is
accordingly only secondarily haemal in position, and there is no need
therefore to seek in Vertebrates for the homologue of the oesophageal
commissures of Annelids, as, for instance, Schneider did.
Since the mouth opens on the neural surface in Annelids and on the haemal
surface in Vertebrates, Semper considers that they cannot be equivalent
structures, and he finds the homologue of the Vertebrate mouth in a
little pit on the haemal surface of the head in the leech _Clepsine_ (also
in the true mouth of Turbellaria and the proboscis-opening in
Nemertines). The primitive Annelid mouth, however, does not appear in
the embryogeny of Vertebrates, for the great development of the brain
crowds it out of existence.
The homologues of the gill-slits Semper finds in two little canals in
the head of _Chaetogaster_, which open from the pharynx to the exterior.
In Sabellids he describes an elaborate system of gill-canals, with a
supporting cartilaginous framework which forms a real _Kiemenkorb_ or
gill-basket, comparable with that of Amphioxus.
Gill-slits, notochord, relation of nervous system, mesonephric tubules,
are thus common to Annelids and Vertebrates--what further proof could
one desire of the close relationship of these groups? Yet Semper enters
into refinements of comparison, seeing, for instance, in the lateral
portions of the ventral ganglia (Fig. 14, _sp. g._) the homologues of
the spinal ganglia of Vertebrates, and comparing the lateral line of
sense organs in Annelids with the lateral line in Anamnia.
He will not admit that Amphioxus and the Ascidians show a closer
resemblance to Vertebrates than his beloved Annelids. Amphioxus, he
thinks, is not a Vertebrate, and Ascidians, though sharing with Annelids
the possession of a notochord, gill-slits, and a "dorsal" nervous
system, yet are further removed from Vertebrates than the latter by
reason of their lacking that essential characteristic of Vertebrates,
metameric segmentation.
Not content with establishing the unity of plan of Annelids, Arthropods,
and Vertebrates, Semper tries to link on the Annelids, as the most
primitive group of the three, to the unsegmented worms, and particularly
to the Turbellaria. His speculations on this matter may be summed up
somewhat as follows:--The common ancestor of all segmented animals is a
segmented worm-like form, not quite like any existing type, resembling
the Turbellaria in having two nerve strands on the dorsal side and no
oesophageal ring, potentially able to develop either the Vertebrate or
the Annelid mouth, and so to give origin both to the Articulate and to
the Vertebrate series. The common ancestor alike of unsegmented worms
and of all segmented types is probably the trochosphere larva, which in
the Vertebrates is represented by the simple _Keimblase_ or blastula.
The Annelid theory of Dohrn and Semper was perhaps not so widely
accepted as the rival Ascidian theory, but it counted not a few
adherents and gave a certain stimulus to comparative morphology. F. M.
Balfour, who pointed out about the same time as Semper the analogy
between the nephridia of Annelids and the mesonephric tubules of
Vertebrates,[405] while not accepting the actual theories of Dohrn and
Semper, took up a distinctly favourable attitude to the general idea
that Annelids and Vertebrates were descended from a common segmented
ancestor. Discussing this question in his classical work on the
development of Elasmobranch fishes,[406] Balfour came to the conclusion
"that we must look for the ancestors of the Chordata, not in allies of
the present Chaetopoda, but in a stock of segmented forms descended from
the same unsegmented types as the Chaetopoda, but in which two lateral
nerve-cords, like those of Nemertines, coalesced dorsally instead of
ventrally to form a median nervous cord. This group of forms, if my
suggestion as to their existence is well founded, appears now to have
perished."[407]
He held that while there was much to be said for the interchange of
dorsal and ventral surfaces postulated by Dohrn and Semper, the
difficulties involved in the supposition were too great; he preferred,
therefore, to assume that the present Vertebrate mouth was primitive,
and not a secondary formation.
His views as to the phylogeny of the Chordata and the genetic relation
of the various classes to one another are exhibited in the following
schema,[408] names of hypothetical groups being printed in capitals, names
of degenerate groups in italics:--
Mammalia. Sauropsida.
| |
|____________________________|
|
Proto-Amniota. Amphibia.
| |
|_____________________|
|
Proto-Pentadactyloidei.
|
Teleostei. |
| |
Ganoidei. |____________Dipnoi
| |
|__________________|
|
Proto-Ganoidei.
|
|____________Holocephali.
|
|____________Elasmobranchii.
|
Proto-Gnathostomata.
|
____________________|
| |
_Cyclostomata_. |
|
|
Proto-Vertebrata.
|
|
|
|
____________________|______________________
| |
_Cephalochorda_. Protochordata. _Urochorda_.
The hypothetical ancestral forms (Protochordata) possessed a notochord,
a ventral suctorial mouth and numerous gill-slits, and were presumably
descended from the common ancestor of Annelids and Vertebrates.
Amphioxus and the Ascidians found their place in this schema as
degenerate offshoots of the ancestral Protochordates, while the
Cyclostomes were in the same way the degenerate modern representatives
of the ancestral Protovertebrates.
Balfour's suggestion, that the nervous system in Annelids and
Vertebrates might have arisen by the dorsal or ventral coalescence of
the lateral nerve cords found in their common ancestor, bore fruit in
the speculations of Hubrecht,[409] on the relation of Nemertines to
Vertebrates.
The Annelid theory was firmly supported by Eisig, who in his elaborate
monograph on the _Capitellidae_[410] maintained against Fuerbringer the
genetic identity of the Annelidan nephridia with the kidney tubules of
Vertebrates. The independent discovery by E. Meyer[411] and J. T.
Cunningham,[412] of an internal segmental duct in _Lanice_, into which
several nephridia opened, seemed to strengthen this view.
Following Ehlers,[413] Eisig found the homologue of the notochord in the
accessory intestine of the _Capitellidae_ and _Eunicidae_, which he
supposed might easily be transformed, according to the principle of
function-change, from a respiratory to a supporting organ. He finally
disposed of the alternative notion that the notochord was represented in
Annelids by the "giant-fibres" or neurochordal strands which lie close
above the nerve-cord, a view held by Kowalevsky,[414] and for a time by
Semper. These strands were shown by Eisig, and by Spengel, to be the
neurilemmar sheaths of thick nerve fibres which had in many cases
degenerated. The view that the content of the neurochordal tubes was
nervous in nature was first promulgated by Leydig in 1864.
Much difference of opinion reigned as to the true homologies of the
brain and mouth of Annelids and Vertebrates. Beard[415] and others got
over the difficulty of the haemal position of the cerebral ganglion in
Annelids by supposing that it degenerated and disappeared altogether in
the Annelidan ancestor of Vertebrates, and that accordingly it had no
homologue in the Vertebrate nervous system. Beard put forward also the
ingenious theory that the hypophysis represents the old Annelidan mouth.
Van Beneden and Julin[416] assumed that in the ancestors of Vertebrates
the oesophagus shifted forward between the still unconnected lobes of
the brain to open on the haemal surface.
The fundamental assumption of the Annelid theory, that dorsal and
ventral surfaces are morphologically interchangeable, seemed rather bold
to many zoologists, and Gegenbaur[417] voiced a common opinion when he
rejected as unscientific the comparison of the ventral nerve cord of
Articulates with the dorsal nervous system of Vertebrates.
The _Balanoglossus_ theory of Vertebrate descent also belongs, at least
in its first form, to the earlier group of evolutionary speculations.
The gill-slits of _Balanoglossus_ were discovered by Kowalevsky as early
as 1866.[418] _Tornaria_ was discovered by J. Mueller in 1850, but by him
considered an Asterid larva; its true nature as the larva of
_Balanoglossus_ was made out by Metschnikoff in 1870, who also remarked
upon its extraordinary likeness to the larvae of Echinoderms.[419] That it
had some relationship with Vertebrates was recognised by Semper,
Gegenbaur and others, but the full working-out of its Vertebrate
affinities is due to Bateson.[420]
Bateson broke completely with the Dohrn-Semper view that the metamerism
of Articulates and Vertebrates must be put down to inheritance from a
common ancestor. He held that metamerism was merely a special
manifestation of the general property of repetition, common to all
living things (_cf._ Owen's "vegetative force"), and that accordingly
"however far back a segmented ancestor of a segmented descendant may
possibly be found, yet ultimately the form has still to be sought for in
which these repetitions had their origin" (p. 549). The meaning of the
phenomenon was obscure, but he was convinced that the explanation was
not to be found in ancestry. "This much alone is clear," he wrote, "that
the meaning of cases of complex repetition will not be found in the
search for an ancestral form, which, itself presenting this same
character, may be twisted into a representation of its supposed
descendant. Such forms there may be, but in finding them the real
problem is not even resolved a single stage; for from whence was their
repetition derived? The answer to this question can only come in a
fuller understanding of the laws of growth and of variation, which are
as yet merely terms" (pp. 548-9). It was in following up this line of
thought that Bateson produced his monumental _Materials for the Study of
Variation_ (1894).
He found a strong positive argument for his theory that Vertebrates are
descended from unsegmented forms in the fact that the notochord arises
as an unsegmented structure. With the notochord he homologised the
supporting rod in the proboscis of _Balanoglossus_, which like the
notochord arises from the dorsal wall of the archenteron, and has a
vacuolated structure. The gill-slits of _Balanoglossus_, with their
close resemblance in detail to those of Amphioxus, Bateson also used as
an argument in favour of the phylogenetic relationship of the
Enteropneusta and Vertebrata, together with the formation from the
ectoderm of a dorsal nerve tube.
Bateson's views attracted considerable attention, and were thought by
many to lighten appreciably the obscurity in which the origin of
Vertebrates was wrapped. Thus Lankester wrote in his article on
Vertebrates[421] in the _Encyclopedia Britannica_:--"It seems that in
_Balanoglossus_ we at last find a form which, though no doubt
specialised for its burrowing sand-life, and possibly to some extent
degenerate, yet has not to any large extent fallen from an ancestral
eminence. The ciliated epidermis, the long worm-like form, and the
complete absence of segmentation of the body-muscles lead us to forms
like the Nemertines. The great proboscis of _Balanoglossus_ may well be
compared to the invaginable organ similarly placed in the Nemertines.
The collar is the first commencement of a structure destined to assume
great importance in _Cephalochorda_ and _Craniata_, and perhaps
protective of a single gill-slit in _Balanoglossus_ before the number of
those apertures had been extended. Borrowing, as we may, the nephridia
from the Nemertines, and the lateral in addition to the dorsal nerve, we
find that _Balanoglossus_ gives the most hopeful hypothetical solution
of the pedigree of Vertebrates."
Much doubt was cast upon the Chordate affinities of the Enteropneusta by
Spengel in his monograph of the group,[422] but when the development of
the coelom came to be more thoroughly worked out in _Balanoglossus_ and
Amphioxus, the striking resemblance in this respect between the two
forms gave additional support to the Batesonian view.[423]
[386] The stages in the development of microscopical
technique are well summarised by R. Burckhardt,
_Geschichte der Zoologie_, p. 121, Leipzig 1907.
[387] "Entwickelungsgeschichte des Amphioxus lanceolatus,"
_Mem. Acad. Sci. St Petersbourg_ (Petrograd) (vii.),
xi., No. 4, 1867, 17 pp., 3 pls.
[388] "Weitere Studien ue. die Entwickelungsgeschichte des
Amphioxus lanceolatus," _Arch. fuer mikr. Anat._, xiii.,
pp. 181-204, 1877.
[389] Particularly by Hatschek (1881) and Boveri (1892).
[390] "Entwickelungsgeschichte der einfachen Ascidien,"
_Mem. Acad. Sci. St Petersbourg_ (Petrograd), (vii.),
x., No. 15, 1866, 19 pp., 3 pls. "Weitere Studien ue. die
Entwicklung der einfachen Ascidien," _Arch. f. mikr.
Anat._, vii., pp. 101-130, 1871.
[391] _Descent of Man_, i., p. 205, 1871.
[392] _Arch. f. mikr. Anat._, vi., 1870, and viii., 1872.
[393] _Archives de Biologie_, 1884, 1885, and 1887.
[394] _Bull. Acad. Sci. St Petersbourg_ (Petrograd) xiii.,
1869, and _Zeits. f. wiss. Zool._, xxii., 1872.
[395] _Mem. Acad. Sci. St Petersbourg_(Petrograd)(7),
xix., 1873.
[396] Giard, _Arch. zool. exper. gen._, i., 1872, and
Lacaze-Duthiers, _ibid._, iii., 1874.
[397] For the later history of the Amphioxus-Ascidian
theory the reader may be referred to A. Willey's
well-known work, _Amphioxus and the Ancestry of the
Vertebrates_, New York and London, 1894, and to Delage
et Herouard, _Traite de Zoologie concrete_, Tome viii.,
Paris, 1898.
[398] "Studien zur Urgeschichte des Wirbelthierkoerpers,"
_Mittheil. Zool. Stat. Neapel_, 1882-1907.
[399] Leydig (_Vom Baue des thierischen Koerpers_,
Tuebingen, 1864), who, in a measure, forestalled Dohrn
and Semper by comparing Vertebrates with reversed
Arthropods, specially insects, supposed the old mouth to
pass between the _crura cerebri_.
[400] _Zeits. f. wiss. Zool._, xliv., 1886.
[401] Quoted by E. B. Wilson, _Wood's Holl Biological
Lectures for 1894_, p. 121.
[402] _Cf._ Metschnikoff, _Quart. Journ. Microsc. Sci._,
xxiv., pp. 89-111, 1884.
[403] "Die Stammesverwandschaft der Wirbelthiere und
Wirbellosen," _Arb. zool.-zoot. Instit. Wuerzburg_, ii.,
pp. 25-76, 1875; "Die Verwandschaftsbeziehungen der
gegliederten Thiere," _Ibid._, iii., pp. 115-404,
1876-7.
[404] Abuse of Cuvier also dates from the early days of
evolution, see Radl, ii., pp. 12-17.
[405] "On the origin and history of the urino-genital
organs of Vertebrates," _Journ. Anat. Phys._, x., 1876.
The conclusions of Balfour and Semper were adversely
criticised by M. Fuerbringer (_Morph. Jahrb._, iv.,
1878), and were negatived by later research.
[406] _A Monograph on the Development of Elasmobranch
Fishes_, London, 1878.
[407] _A Treatise on Comparative Embryology_, vol. ii., p.
311, London, 1881.
[408] _Loc. cit._, vol. ii., p. 327.
[409] "On the Ancestral Form of the Chordata," _Q.J.M.S._,
xxiii., 1883. "The Relation of the Nemertea to the
Vertebrata," _ibid._, xxvii., 1887. Hubrecht gives the
credit for the first indication of the relationship of
Nemertines and Vertebrates to Harting (_Leerboek van de
Grondbeginselen der Dierkunde_, 1874).
[410] "Monographie der Capitelliden des Golfes von
Neapel," _Fauna u. Flora des Golfes von Neapel_, Monog.
xvi., Berlin, 1887.
[411] _Mitt. Zool. Stat. Neapel_, vii., 1887.
[412] _Nature_, xxxvi., p. 162, 1887.
[413] "Nebendarm und Chorda dorsalis," _Nachr. Ges. Wiss.
Goettingen_, p. 390, 1885.
[414] "Embryologische Studien an Wuermern u. Arthropoden,"
_Mem. Acad. Sci. St Petersbourg_ (Petrograd), (7), xvi.,
1870. And in _Arch. f. mikr. Anat._, vii., p. 122, 1871.
[415] "The Old Mouth and the New," _Anat. Anz._, iii.,
1888. _Nature_, xxxix., 1889.
[416] "Recherches sur la Morphologie des Tuniciers,"
_Arch. de Biol._, vi., 1887.
[417] "Die Stellung u. Bedeutung der Morphologie," _Morph.
Jahrb._, i., pp. 1-19, 1876.
[418] "Anatomie des Balanoglossus," _Mem. Acad. Sci. St
Petersbourg_ (Petrograd), (7), x., 1866.
[419] _Zeit. f. wiss. Zool._, xx., 1870. For a recent view
of the relation of the Enteropneusta to the Echinoderma,
see J. F. Gemmill, _Phil. Trans._ B., ccv., pp. 213-94,
1914.
[420] In a series of papers published in 1884-6, the
speculative results being discussed in his memoir on
"The Ancestry of the Chordata," _Q.J.M.S._ (n.s.), xxvi.,
pp. 535-71, 1886.
[421] Reprinted in _Zoological Articles_, London, 1891.
[422] "Die Enteropneusten des Golfes von Neapel," _Fauna
und Flora des Golfes von Neapel_, Monog. xviii., Berlin,
1893.
[423] See Macbride, "A Review of Prof. Spengel's Monograph
on Balanoglossus," _Q.J.M.S._, xxxvi., 1894, and "The
Early Development of Amphioxus," _Q.J.M.S._, xl., 1898.