VERTEBRATE ANATOMY
The Theory of the Vertebrate Skull--Goethe, Oken, Cuvier, and
Owen--Huxley Defends Goethe--His Own Contributions to the
Theory--The Classification of Birds--Huxley Treats them as
"Extinct Animals"--Geographical Distribution--Sclater's
Regions--Huxley's Suggestions.
We have seen that some of the most important of the contributions made
by Huxley to zoölogical knowledge were in the field of the lower
animals, especially of those marine forms for the study of which he
had so great opportunities on the _Rattlesnake_. A great bulk of his
zoölogical work, however, related to the group of back-boned animals.
These, by their natural affinities and anatomical structure, are more
closely related to man, and, as Huxley began his scientific work as a
medical student, the groundwork of all his knowledge was study of the
anatomy and physiology of man. Moreover, throughout the greater part
of his working life, he had more to do with the extinct forms of life.
The vertebrate animals, from the great facility for preservation which
their hard skeleton presents, as well as from the extremely important
anatomical characters of the skeleton, bulk more largely in the study
of palæontology than does any other group. In each of the great
groups of vertebrate animals, in fishes, amphibia, reptiles, birds,
and mammals, Huxley did important work. Much of this is embodied in
his treatise on _Vertebrate Anatomy_, but to some particular parts of
it special attention may now be directed, as much because these serve
as excellent examples of his method of work as because of their
intrinsic importance.
The skull is the most striking feature in the skeleton of vertebrate
animals, and to the theory and structure of the vertebrate skull
Huxley paid special attention, and his views and summary of the views
of others form the basis of our modern knowledge. This work was put
before the public in the course of a series of lectures on Comparative
Anatomy given in 1863, while Huxley was Hunterian Professor at the
Royal College of Surgeons, and the beginnings of it were contained in
a Croonian lecture to the Royal Society in 1858.
The theory of the skull which held the field was known as the
vertebral theory. The great bulk of the nervous system of vertebrate
animals consists of a mass of tissue lying along the dorsal line of
the body and enclosed in a cartilaginous or bony sheath. The nerve
tissue is the brain and spinal cord; the sheath is the skull in front
and the vertebral column along the greater part of the length of the
animal. The brain may be taken simply as an anterior portion of the
nerve mass, corresponding in a general way to an expansion of the
spinal cord in the region of the anterior limbs and an expansion in
the region of the hind limbs, the latter indeed having recently been
shown in some extinct creatures to surpass the brain in size. In a
similar simple fashion the skull may be taken as an expanded anterior
part of the vertebral column, serving as an expanded box for the
brain, just as in the regions of the pectoral and pelvic expansions
of the cord there are similar expansions of the surrounding bony case.
We know now, from greater knowledge of its embryological development,
that the brain contains structures quite peculiar to itself, and
differs from the spinal cord in kind as well as in size; but, at the
same time, when the vertebral theory of the skull was inaugurated,
embryological knowledge and the importance of its relation to
anatomical structure were less considered. What Huxley did was to show
that the skull, in its mode of origin and real nature, was not merely
an expanded portion of the vertebral column, but that it differed from
it in kind.
The hypothesis of the vertebral structure of the skull was due both to
Goethe, the great German poet, and Oken, a most able but somewhat
mystic German anatomist. An attempt had been made by a well-known
English anatomist to cast on Goethe the stigma of having tried to rob
Oken of the credit for this theory. Huxley set that matter finally at
rest, disproving and repelling with indignation the unworthy
suggestion. Oken gave out his theory in 1807, and described how it had
been first suggested to his mind by the accident of picking up a dried
and battered sheep's skull, in which the apparent vertebral structure
was very obvious, as, indeed, anyone may see at a glance. It was in
1820, long after the theory had been made current, that the poet first
publicly narrated that in a similar way he had long before come to the
same conclusion; but Huxley was able to show that, although announcing
it later, Goethe had in reality anticipated the anatomist. A passage
occurs in a letter to a friend, of a date in 1790, which admits of no
doubt. "By the oddest happy chance, my servant picked up a bit of an
animal's skull in the Jews' cemetery at Venice, and, by way of a
joke, held it out to me as if he were offering me a Jew's skull. I
have made a great step in the formation of animals." It is an
interesting trait in Huxley's character, to find him zealous in
defence of the reputation of a great man, even although that man had
been dead more than half a century; but it may be added that his just
zeal was at least stimulated by the fact that the maligner of Goethe
was Owen, the conduct of whom, with regard to Darwin and Huxley,
Huxley had had just reason for resenting.
The theory, then, which had dropped stillborn from Goethe, but which
Oken developed, was simply that the skull consisted of a series of
expanded vertebræ. Each vertebra consists of a basal piece or centrum,
the anterior and posterior faces of which are closely applied to the
face of an adjoining vertebra, and of a bony arch or ring which
encloses and protects the nervous cord. Oken supposed that there were
four such vertebræ in the skull, the centra being firmly fused and the
arches expanded to form the dome of the skull. Quite correctly, he
divided the skull into four regions, corresponding to what he called
an ear vertebra, at the back, through which the auditory nerves
passed; a jaw vertebra, in the sphenoidal region, through which the
nerves to the jaws passed; an eye vertebra in front, pierced by the
optic nerves, and again in front a nose vertebra, the existence of
which he doubted at first. Quite rightly, he discriminated between the
ordinary bones of the skull and the special structures surrounding the
inner ear which he declared to be additions derived from another
source. So far it cannot be doubted that the vertebral theory made a
distinct advance in our knowledge of the skull. It was to a certain
extent, however, thrown into disrepute by various fantastic theories
with which Oken surrounded it. Later on, Cuvier removed from it these
wilder excrescences, and amplified the basis of observation upon which
the underlying theory of the unity of type of the skull throughout the
vertebrates was based. Cuvier, however, came to reject the theory,
except so far as it applied to the posterior or occipital segment of
the skull. Later on, Owen resuscitated the theory, first throwing
doubt on the merit of Goethe, and then suggesting that Oken, instead
of relying on the observed facts, had deduced the whole theory from
his own imagination. Owen, although he made no new contribution to
fact or theory in this matter, practically claimed the whole credit of
it as a scientific hypothesis.
When Huxley took up the subject, the position was that the vertebral
theory was in full possession of the field, under the auspices of
Owen. Huxley began afresh from observed facts. The first object of his
investigation was to settle once for all the question as to whether
the skulls of all vertebrates were essentially modifications of the
same type. He took in succession the skulls of man, sheep, bird,
turtle, and carp, and showed that in all these there were to be
distinguished the same four basi-cranial regions: the basi-occipital,
basi-sphenoid, pre-sphenoid, and ethmoid. These were essentially
identical with the centra of the four vertebræ of Oken. Similarly, he
showed the composition of the lateral and dorsal walls, proving the
essential identity of the structures involved and of their relations
to the nerve exits in the great types he had chosen. In the series of
lectures delivered before the College of Surgeons, he extended his
observations to a much larger series of vertebrates, and substantially
laid down the main lines of our knowledge of the skull. In two
important respects his statements were not merely a codification of
existing knowledge, but an important extension of it. He distinguished
the different modes in which the jaws may be suspended to the skull,
and established for these different kinds of suspensoria the names
which have ever since been employed. He proved clearly what had been
suggested by Oken, that the region of the ear is a lateral addition to
the skull, and he distinguished in it three bones, his names for which
have since become the common property of anatomists. Finally, he made
it plain beyond any possible doubt that the skulls of all vertebrates
were built upon a common plan.
Having established the facts, he proceeded to enquire into the theory.
There was now a new method for investigating such problems, the method
of embryology, which, practically, had not been available to Oken, and
of which neither Cuvier nor Owen had made proper use. By putting
together the investigations of a number of embryologists, by adding to
these himself, and, lastly, by interpreting the facts which his
investigations into comparative anatomy had brought to light, he
shewed that the vertebral theory could not be maintained. He shewed,
by these methods, that, though both skull and vertebral column are
segmented, the one and the other, after an early stage, are fashioned
on lines so different as to exclude the possibility of regarding the
details of each as mere modifications of a common type. "The spinal
column and the skull start from the same primitive condition, whence
they immediately begin to diverge." "It may be true to say that there
is a primitive identity of structure between the spinal or vertebral
column and the skull; but it is no more true that the adult skull is
a modified vertebral column than it would be to affirm that the
vertebral column is a modified skull." Taking the embryological facts,
he shewed that the skull arose out of elements quite different from
those of the vertebral column. The notochord alone is common to both.
The skull is built up of longitudinal cartilaginous pieces, now known
as the "parachordals" and "trabeculæ," of sense capsules enclosing the
nose and ear, and of various roofing bones. In the historical
development of the skull three grades become apparent; a primitive
stage, as seen in Amphioxus, where there is nothing but a fibrous
investment of the nervous structures; a cartilaginous grade, as seen
in the skate or shark, where the skull is formed of cartilage, very
imperfectly hardened by earthy deposits; a bony stage, seen in most of
the higher animals. He shewed that in actual development of the higher
animals these historical grades are repeated, the skull being at first
a mere membranous or fibrous investment of the developing nervous
masses, then becoming cartilaginous, and, lastly, bony. He made some
important prophetic remarks as to the probable importance that future
embryological work would give to the distinction between cartilage and
membrane bones--a prophecy that has been more than fully realised by
the investigations of Hertwig and of others. Our present knowledge of
the skull differs from Huxley's conception practically only in a
fuller knowledge of details. We know now that throughout the series
there is a primitive set of structures common to all animals higher in
the scale than Amphioxus, and forming the base and lateral walls of
the skull. This is termed the Chondrocraninm, because it is laid down
in cartilage; it is composed of the separate elements which Huxley
indicated, and, in different animals, as Huxley suggested, the exact
limits of the ossification of the primitive cartilages differ in
extent, but occur in homologous situations. This primitive skull is
roofed over by a series of membrane bones which have no connection in
origin with the other portions of the skull, and which have no
representative in the vertebral column, but which are the direct
descendants of the bony scales clothing the external skin in
cartilaginous fishes. In one respect only was Huxley erroneous. Partly
by inadvertence, and partly because the minute details of vertebrate
embryology became really familiar to zoölogists only after the
elaborate work of Balfour of Cambridge, Huxley, in his account of the
formation of the first beginnings of the skeleton in the embryo, made
confusion between the walls of the primitive groove, which, in
reality, give rise to the nervous structures, and those embryonic
tissues which form the skeletal system.
The next great piece of work which we may take as typical of Huxley's
contributions to vertebrate anatomy, is his classical study on the
classification of birds. The great group of birds contains a larger
number of species than is known in any other group of vertebrates,
and, in this vast assemblage of forms there is strikingly little
anatomical difference. The ostrich and the humming-bird might perhaps
be taken as types of the extremest differences to be found, and yet,
although these differ in size, plumage, adaptations, habits, mode of
life, and almost everything that can separate living things, the two
conform so closely to the common type of bird structure that knowledge
of the anatomy of one would be a sufficient guide, down to minute
details, for dissection of the other. None the less, there are
hundreds of thousands of species of birds between these two types. It
is not surprising that to reduce this vast assemblage of similar
creatures to an ordered system of classification has proved one of the
most difficult tasks attempted by zoölogists. Before Huxley, it had
been attempted by a number of distinguished zoölogists; but, for the
most part, these had relied too much on merely external characters and
on superficial modifications in obvious relation to habits. When
Huxley, in the course of a set of lectures on Comparative Anatomy, was
about to approach the subject of birds he was asked by a zoölogist how
he proposed to treat them. "I intend," he replied, "to treat them as
extinct animals." By that he meant that it was his purpose to make a
prolonged study of their skeletal structures the basis of his
grouping, following the lines which Cuvier, Owen, and he himself had
pursued so successfully in the case of the fossil remains of
vertebrates. The result was that this first systematic study of even
one set of the anatomical characters of the group completely reformed
the method by which all subsequent workers have tried to grapple with
the problem; ornithology was raised from a process akin to
stamp-collecting to a reasoned scientific study. The immediate
practical results were equally important. He was able to shew that
among the innumerable known forms there were three grades of
structure. The lowest had already been recognised and named by
Haeckel; it consisted of the Saururæ, or reptile-like, birds, and
contained a single fossil form, Archæopteryx, distinguished from all
living birds by the presence of a hand-like wing in which the
metacarpal bones were well developed and freely movable, and by the
possession of a long lizard-like tail actually exceeding in length
the remainder of the spinal column. The next group of Ratites,
although it contained only the Ostrich, Rhea, Emu, Cassowary, and
Apteryx, he shewed to be equivalent in anatomical coherence to the
third great group of Carinates, which includes the vast majority of
living birds. In his arrangement of the latter group, he laid most
stress on the characters of the bony structures which form the palate,
and by this simple means was able to lay down clearly at least the
main lines of a natural classification of the group.
Huxley's work upon birds, like his work in many other branches of
anatomy, has been so overlaid by the investigations of subsequent
zoölogists that it is easy to overlook its importance. His employment
of the skeleton as the basis of classification was succeeded by the
work of others who made a similar use of the muscular anatomy, of the
intestinal canal, of the windpipe, of the tendons of the feet, and
many other structures which display anatomical modifications in
different birds. The modern student finds that all these new sets of
facts are much greater in bulk than the work of Huxley, and it is easy
for him to remain in ignorance that they were all suggested and
inspired by the method which Huxley employed. He finds that further
research has supplanted some of Huxley's conclusions, and it is easy
for him to remain in ignorance that the conclusions themselves
suggested the investigations which have modified them. Huxley's
anatomical work was essentially living and stimulating, and too often
it has become lost to sight simply because of the vast superstructures
of new facts to which it gave rise.
Closely associated with vertebrate anatomy is the subject of
geographical distribution. In 1857 the study of this important
department of zoölogy was placed on a scientific basis, practically
for the first time, by a memoir on the geographical distribution of
birds published in the _Journal_ of the Linnæan Society of London. It
was known in a general way that different kinds of creatures were
found in different parts of the world, but little attempt had been
made to map out the world into regions characterised by their animal
and vegetable inhabitants, as the political divisions of the world are
characterised by their different governments and policies. Mr.
Sclater, who two years later became secretary of the Zoölogical
Society of London, in his memoir introduced the subject in the
following words:
"It is a well-known and universally acknowledged fact that we can
choose two portions of the globe of which the respective fauna
and flora shall be so different that we should not be far wrong
in supposing them to have been the result of distinct creations.
Assuming, then, that there are, or may be, more areas of creation
than one, the question naturally arises how many of them are
there, and what are their respective extents and boundaries; or,
in other words, what are the most natural primary ontological
divisions of the earth's surface?"
Mr. Sclater's answer was that there are six great regions;
Neotropical, Nearctic, Palæarctic, Ethiopian, Indian, and Australian,
and his answer, with minor alterations and the addition of a great
wealth of detail, has been accepted by zoölogy.
Two years later, however, Darwin gave a new meaning and a new
importance to Sclater's work, by the new interpretation he caused to
be placed on the words "centres of creation." Sclater's facts and
areas remained the same; Darwin rejected the idea of separate
creations in the older sense of the words, and laid stress on the
impossibility of accounting for the resemblances within a region and
for the differences between regions by climatic differences and so
forth. He raised the questions of modes of dispersal and of barriers
to dispersal, of similarities due to common descent, and of the
modifying results produced by isolation. He gave, in fact, a theory of
the "creations" which Mr. Sclater had shewn to be a probable
assumption. It was in the nature of things that Huxley should make a
contribution to a set of problems so novel and of so much importance
to zoölogy. In 1868, in the course of a memoir on the anatomy of the
gallinaceous birds and their allies, he made a useful attempt, nearly
the first of its kind, to correlate anatomical facts with geographical
distribution. Having shewn the diverging lines of anatomical structure
that existed in the group of creatures he had been considering, he
went on to shew that there was a definite relation between the
varieties of structure and the different positions on the surface of
the globe occupied at the present time by the creatures in question.
He made, in fact, the geographical position a necessary part of the
whole idea of a species or of a group, and so introduced a conception
which has become a permanent part of zoölogical science.
With regard to the number and limits of the zoölogical regions into
which the world may be divided, Huxley raised a number of problems
which have not yet reached a full solution. Mr. Sclater had divided
the world into six great regions: the Nearctic, including the
continent of North America, with an overlap into what is called South
America by geographers; the Palæarctic, comprising Europe and the
greater part of Asia; the Oriental, containing certain southern
portions of Asia, such as India south of the Himalayas and many of the
adjacent islands; the Ethiopian, including Africa, except north of
the Sahara, and Madagascar; the Australian, containing Australia and
New Zealand and some of the more southeastern of the islands of Malay;
the Neotropical, including South America. Huxley first called
attention to certain noteworthy resemblances between the Neotropical
and the Australian regions of Sclater, and held that a primary
division of the world was into _Arctogæa_, comprising the great land
masses of the Northern Hemisphere with a part of their extension
across the equator, and _Notogæa_, which contained Australia but not
New Zealand and South America. Although this acute suggestion has not
been generally accepted as a modification of Mr. Sclater's scheme, it
called attention in a striking fashion to some very remarkable
features in the distribution of animals. Subsequent writers have
considerably extended Huxley's conception of the similarities to be
found among the more southern land areas. They have pointed out that
the most striking idea of the distribution of land and water on the
surface of the globe is to be got by considering the globe alternately
from one pole and from the other. In the south, a clump of ice-bound
land, well within the Antarctic Circle, surrounds the pole. All else
is a wide domain of ocean broken only where tapering and isolated
tongues of land, South America, the Cape, Australia, lean down from
the great land masses of the north. On the other hand, all the great
land masses expand in the Northern Hemisphere, and shoulder one
another round the North Pole. America is separated from Asia only by
the shallowest and narrowest of straits; an elevation of a few fathoms
would unite Greenland with Europe. Science points definitely to some
part of the great northern land area as the centre of life for at
least the larger terrestrial forms of life. We know that these arose
successively, primitive birds like the ostriches being older than
higher forms like the parrots and singing birds; the pouched
marsupials preceding the antelopes and the lion; the lemurs coming
before the man-like apes. Each wave of life spread over the whole area
producing after its kind; then, pressing round the northern land area,
it met a thousand different conditions of environment, different
foods, enemies, and climates, and broke up into different genera and
species. But there was never a wave of life that was not followed by
another wave. In the struggle for existence between the newer and the
older forms, the older forms were gradually driven southwards towards
the diverging fringes of the land masses. The vanquished left behind
them on the field of battle only their bones, to become fossils.
Sometimes succeeding waves swept along to the extreme limits of the
land, and many early types were utterly destroyed. But others found
sanctuary in the ends of the South, and such survivors of older and
earlier types of life cause a similarity between the southern lands
that Huxley called Notogæa, although the extent of his region must be
increased.
Recently, however, there has been a recurrence to Huxley's suggested
union of South America and Australia, based on new evidence of a
direct kind, quite different from that which had just been given.
Various groups of naturalists have stated that there are similarities
between the invertebrate inhabitants of Australia and of South America
of a kind which makes the existence of a direct land connection in the
Southern Hemisphere extremely probable. Moreover, Ameghino has
recently described some marsupial fossils from South America which, he
states, belong to the Australian group of Dasyuridæ, and Oldfield
Thomas has described a new mammal from South America which is unlike
the opossums of America and like the diprotodonts of Australia. So
that, while the general opinion has been against Huxley's division,
Notogæa, in the strict meaning which he gave to it, there has recently
been an opinion growing in its favour.
Huxley also made minor alterations in Mr. Sclater's scheme by forming
an additional circumpolar region for the Northern Hemisphere, and by
elevating New Zealand into a separate region, distinct from Australia.
On these points there is a balance of opinion against his views.
Before leaving the subject of Huxley's contributions to vertebrate
anatomy, the actual details of which would occupy far too much space,
it is necessary to mention the great importance to zoölogy of the new
terms and new ideas he introduced into classification. His mind was,
above all things, orderly and comprehensive, and while, in innumerable
minute points, from the structure of the palate of birds to the
structure of the roots of human hair (actually the subject of Huxley's
first published contribution to scientific knowledge), he added to the
number of known facts, he did even more important work in
co-ordinating and grouping together the known body of facts. To him
are due not only the names, but the idea, that the mammalian animals
fall into three grades of ascending complexity of organisation: the
reptile-like Prototheria, which lay large eggs, and which have many
other reptilian characters; the Metatheria, or marsupial animals; the
Eutheria, or higher animals, which include all the common animals from
the mole or rabbit up to man. In a similar fashion, he grouped the
vertebrates into three divisions, and named them: Ichthyopsida, which
include the fish and Amphibia, creatures in which the aquatic habit
dominates the life history and the anatomical structure; Sauropsida,
including birds and reptiles, on the close connection between which he
threw so much light; Mammalia.