Part 6 (1/2)

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 palaeontology 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 ma.s.s 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 ma.s.s, 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 surpa.s.s the brain in size. In a similar simple fas.h.i.+on 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 antic.i.p.ated the anatomist. A pa.s.sage 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 vertebrae. 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 vertebrae 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 pa.s.sed; a jaw vertebra, in the sphenoidal region, through which the nerves to the jaws pa.s.sed; 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 vertebrae of Oken. Similarly, he showed the composition of the lateral and dorsal walls, proving the essential ident.i.ty 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 fas.h.i.+oned 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 ident.i.ty 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 ”trabeculae,” 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 ma.s.ses, 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 h.o.m.ologous 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 zoologists 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 cla.s.sical study on the cla.s.sification 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 a.s.semblage 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 a.s.semblage of similar creatures to an ordered system of cla.s.sification has proved one of the most difficult tasks attempted by zoologists. Before Huxley, it had been attempted by a number of distinguished zoologists; 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 zoologist 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 Saururae, or reptile-like, birds, and contained a single fossil form, Archaeopteryx, 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 Rat.i.tes, although it contained only the Ostrich, Rhea, Emu, Ca.s.sowary, 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 cla.s.sification 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 zoologists that it is easy to overlook its importance. His employment of the skeleton as the basis of cla.s.sification was succeeded by the work of others who made a similar use of the muscular anatomy, of the intestinal ca.n.a.l, 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 a.s.sociated with vertebrate anatomy is the subject of geographical distribution. In 1857 the study of this important department of zoology 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 Linnaean 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 Zoological 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.

a.s.suming, 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, Palaearctic, Ethiopian, Indian, and Australian, and his answer, with minor alterations and the addition of a great wealth of detail, has been accepted by zoology.

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 a.s.sumption. 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 zoology. 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 zoological science.

With regard to the number and limits of the zoological 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 Palaearctic, 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 _Arctogaea_, comprising the great land ma.s.ses of the Northern Hemisphere with a part of their extension across the equator, and _Notogaea_, 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 fas.h.i.+on 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 ma.s.ses of the north. On the other hand, all the great land ma.s.ses 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 ma.s.ses. 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 Notogaea, 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 Dasyuridae, 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, Notogaea, 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 circ.u.mpolar 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 s.p.a.ce, it is necessary to mention the great importance to zoology of the new terms and new ideas he introduced into cla.s.sification. 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 fas.h.i.+on, 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.

CHAPTER IX

MAN AND THE APES