FLOATING CREATURES OF THE SEA
The Nature of Floating Life—Memoir on Medusæ Accepted by the Royal Society—Old and New Ideas of the Animal Kingdom—What Huxley Discovered in Medusæ—His Comparison of them with Vertebrate Embryos.
As the Rattlesnake sailed through the tropical seas Huxley came in contact with the very peculiar and interesting inhabitants of the surface of the sea, known now to naturalists as pelagic life or "plankton." Although a poet has spoken of the "unvintageable sea," all parts of the ocean surface teem with life. Sometimes, as in high latitudes, the cold is so great that only the simplest microscopic forms are able to maintain existence. In the tropics, animals and plants are abundant, and sometimes by their numbers colour great areas of water; or, as in the drift of the Gulf Stream, make a tangle of animal and plant life through which a boat travels only with difficulty. The basis of the food supply of this vast and hungry floating life is, as on land, vegetable life; for plants are the only creatures capable of building up food from the gases of the air and the simple chemical salts found dissolved in water. Occasionally, in shallow or warm seas, marine floating plants, large and visible like the sea-weeds of the coast, form the floating masses known as Sargasso seas; more often the plants are minute, microscopic specks visible only when a drop of water is placed under the microscope, but occurring in incredible numbers, and, like the green vegetation of the earth, forming the ultimate food-supply of all the living things around them. Innumerable animals, great and small, live on the plants or upon their fellows, and, however far he may be from land, the naturalist has always abundant material got by his daily use of the tow-net. This drifting population floats at the mercy of the waves. Most of the animals are delicate, transparent creatures, their transparency helping to protect them from the attacks of hungry fellows. Nerves, muscles, skin, and the organs generally are clear, pale, and hardly visible. Such structures as the liver, the reproductive organs, and the stomach, which cannot easily become transparent, are grouped together into small knots, coloured brown like little masses of sea-weed. Other floating creatures are vividly coloured, but the hues are bright blues and greens closely similar to the sparkling tints of sea-water in sunlight. The different members of this marine flotsam frequently rise and fall periodically: some of them sinking by day to escape the light, others rising only by day; others, again, appearing on the surface in spring, keeping deep down in winter. Perhaps the majority of them are phosphorescent, sometimes shining by their own light, sometimes borrowing a glory from innumerable phosphorescent bacteria with which they are infested. Nearly every class of the animal kingdom contributes members to this strange population. The young forms of many fish, as for instance of conger, flying gurnards, and some flatfish, are pelagic and have colourless blood, and pale, transparent, gelatinous or cartilaginous skeletons. The tadpole-like stages of the sea-squirts, which in adult life are to be found attached to rocks like weeds, drift about in the surface waters until their time comes for settling down in life. Many other Ascidians pass their whole life as pelagic creatures. A few molluscs, many kinds of worms, echinoderms, and their allies, crab and lobster-like creatures in innumerable different stages of development, are to be found there, while unnumbered polyps and jelly-fish are always present. It would be difficult to imagine a better training for the naturalist than to spend years, as Huxley did, working at this varied assortment of living creatures. Huxley declared that the difficulties of examining such flimsy creatures had been exaggerated.
"At least, with a good light and a good microscope, with the ship tolerably steady, I never failed in procuring all the information I required. The great matter is to obtain a good successive supply of specimens, as the more delicate oceanic species are usually unfit for examination within a few hours after they are taken."
Day after day, as the Rattlesnake crept from island to island, Huxley examined the animals brought up by his tow-net. He made endless dissections, and gradually accumulated a large portfolio of drawings. Much of the time he passed at Sydney was spent in libraries and museums, comparing his own observations with the recorded observations of earlier workers, and receiving from the combination of his own work and the work of others new ideas for his future investigations. It was all entirely a labour of love; it lay outside the professional duties by which he made his living, and for a long time it seemed as if he was not even to gain reputation by the discoveries he knew himself to be making. He writes in his autobiography:
"During the four years of our absence, I sent home communication after communication to the 'Linnæan' Society, with the same result as that obtained by Noah when he sent the raven out of his ark. Tired at last of hearing nothing about them, I determined to do or die, and in 1849 I drew up a more elaborate paper and forwarded it to the Royal Society. This was my dove, if I had only known it; but owing to the movements of the ship I heard nothing of that either until my return to England in the latter end of the year 1850, when I found that it was printed and published, and that a huge packet of separate copies awaited me. When I hear some of my young friends complain of want of sympathy and encouragement, I am inclined to think that my naval life was not the least valuable part of my education."
This first successful paper was a memoir On the Anatomy and the Affinities of the Family of Medusæ, and was sent at Captain Stanley's suggestion to that officer's father, the Bishop of Norwich, who communicated it to the Royal Society. It is a curious circumstance that Huxley, who afterwards met with so virulent opposition from bishops, owed his first public success to one of them. Professor Sir Michael Foster writes of this period in Huxley's life:
"The career of many a successful man has shewn that obstacles often prove the mother of endeavour, and never was this lesson clearer than in the case of Huxley. Working amidst a host of difficulties, in want of room, in want of light, seeking to unravel the intricacies of minute structure with a microscope lashed to secure steadiness, cramped within a tiny cabin, jostled by the tumult of a crowded ship's life, with the scantiest supply of books of reference, with no one at hand of whom he could take counsel on the problems opening up before him, he gathered for himself during these four years a large mass of accurate, important, and in most cases novel, observations and illustrated them with skilful, pertinent drawings. Even his intellectual solitude had its good effects: it drove him to ponder over the new facts which came before him, and all his observations were made alive with scientific thought."
Afterwards, in England, he received the Royal Medal of the Royal Society for this memoir on Medusæ, sharing this supreme distinction of scientific England with men so illustrious as Joule, the discoverer of the relation between force and heat, Stokes, the great investigator of optical physics, and Humboldt, the traveller, all of whom received medals in the same year. In making the presentation to Huxley, the Earl of Rosse, then President of the Royal Society, declared:
"In those papers you have for the first time fully developed their structure (that of the Medusæ), and laid the foundation of a rational theory for their classification. In your second paper, on the anatomy of Salpa and Pyrosoma, the phenomena have received the most ingenious and elaborate elucidations, and have given rise to a process of reasoning, the results of which can scarcely yet be anticipated, but must bear in a very important degree upon some of the most abstruse points of what may be called transcendental physiology."
Many reasons make it difficult for us to realise, now, the singular novelty and importance of Huxley's memoir on the Medusæ. The first is a reason which often prevents great discoveries in almost every subject from receiving in after years their due respect. The years that have passed since 1850 have seen not only the most amazing progress in our knowledge of comparative anatomy, but almost a revolution in the methods of studying it. Huxley's work has been incorporated in the very body of science. A large number of later investigators have advanced upon the lines he laid down; and just as the superstructures of a great building conceal the foundations, so later anatomical work, although it has only amplified and extended Huxley's discoveries, has made them seem less striking to the modern reader. The present writer, for instance, learned all that he knows of anatomy in the last ten years, and until he turned to it for the purpose of this volume he had never referred to Huxley's original paper. When he did so, he found from beginning to end nothing that was new to him, nothing that was strange: all the ideas in the memoir had passed into the currency of knowledge and he had been taught them as fundamental facts. It was only when he turned to the text-books of anatomy and natural history current in Huxley's time that he was able to realise how the conclusions of the young ship-surgeon struck the Fellows and President of the Royal Society as luminous and revolutionary ideas.
In the first half of the century, a conception of the animal kingdom prevailed which was entirely different from our modern ideas. We know now that all animals are bound together by the bond of a common descent, and we seek in anatomy a clue to the degrees of relationship existing among the different animals we know. We regard the animal kingdom as a thicket of branches all springing from a common root. Some of these spring straight up from the common root unconnected with their fellows. Others branch repeatedly, and all the branches of the same stem have features in common. What we see in the living world is only the surface of the thicket, the tops of the twigs; and it is by examination of the structure of this surface that we reconstruct in imagination the whole system of branches, and know that certain twigs, from their likeness, meet each other a little way down; that others are connected only very deep down, and that others, again, spring free almost from the beginning. The fossils of beds of rock of different geological ages give us incomplete views of the surface of the thicket of life, as it was in earlier times. These views we have of the past aspects of the animal kingdom are always much more incomplete than our knowledge of the existing aspect; partly because many animals, from the softness of their bodies, have left either no fossil remains at all, or only very imperfect casts of the external surfaces of their bodies; and partly because the turning of any animal into a fossil, and its subsequent discovery by a geologist, are occasional accidents; but, although the evidence is much less perfect than we could wish, there is enough of it to convince anatomists that existing animals are all in definite blood-relationship to each other, and to make them, in the investigation of any new animal, study its anatomy with the definite view of finding out its place in the family tree of the living world.
When Huxley made his first discoveries, entirely different ideas prevailed. The animal kingdom was supposed to offer a series of types, of moulds, into which the Creator at the beginning of the world had cast the substance of life. These types were independent of each other, and had been so since the beginning of things. Anatomists were concerned chiefly with systematic work, with detecting and recording the slight differences that existed among the numbers of animals grouped around each type. No attempt was made to see connection between type and type, for where these had been separately created there was nothing to connect them except possibly some idea in the mind of the Creator. This apparently barren attitude to nature was stronger in men's minds because it had inspired the colossal achievements of Cuvier, a genius who, under whatever misconceptions he had worked, would have added greatly to knowledge. As we have seen in the first chapter, Huxley, through Wharton Jones, and through his own reading, had been brought under the more modern German thought of Johannes Mueller and Von Baer. He had learned to study the problems of living nature in the spirit of a physicist making investigations into dead nature. In the anatomy of animals, as in the structure of rocks and crystals, there were to be sought out "laws of growth" and shaping and moulding influences which accounted for the form of the structures. To use the technical term, he was a morphologist: one who studied the architecture of animals not merely in a spirit of admiring wonder, but with the definite idea of finding out the guiding principles which had determined these shapes.
Not only was the prevailing method of investigation faulty, but actual knowledge of a large part of the animal kingdom was extremely limited. In the minds of most zoölogists the animal kingdom was divided into two great groups: the vertebrates and invertebrates. The vertebrate, or back-boned, animals were well known; comparatively speaking they are all built upon the type of man; and human anatomists, who indeed made up the greater number of all anatomists, using their exact knowledge of the human body, had studied many other vertebrates with minute care, and, from man to fishes, had arranged living vertebrates very much in the modern order. But the invertebrates were a vague and ill-assorted heap of animals. It was not recognised that among them there were many series of different grades of ascending complexity, and there was no well-known form to serve as a standard of comparison for all the others in the fashion that the body of man served as a standard of comparison for all vertebrates. Here and there, a few salient types such as insects and snails had been picked out, but knowledge of them helped but little with a great many of the invertebrates. The great Linnæus had divided the animal kingdom into four groups of vertebrates: mammals, birds, reptiles, and fishes, but for the invertebrates he had done no more than to pick out the insects as one group and to call everything else "Vermes" or worms. The insects included all creatures possessed of an external skeleton or hard skin divided into jointed segments, and included forms so different as insects, spiders, crabs, and lobsters. But Vermes included all the members of the animal kingdom that were neither vertebrates nor insects. Cuvier advanced a little. He got rid of the comprehensive title Vermes—the label of the rubbish-heap of zoölogists. He divided animals into four great subkingdoms: Vertebrates, Mollusca, Articulata, Radiata. These names, however, only covered very superficial resemblances among the animals designated by them. The word Mollusca only meant that the creatures grouped together had soft bodies, unsupported by internal or external articulated skeletons; and this character, or, rather, absence of character, was applied alike to many totally dissimilar creatures. The term Articulata included not only Linnæus's insects but a number of soft-skinned, apparently jointed, worm-like animals such as the leech and earthworm. Lastly, the name Radiata meant no more than that the organs of the creatures so designated were more or less disposed around a centre, as the sepals and petals of a flower are grouped around the central pistil; and it included animals so different as the starfish and sea-anemones and Medusæ. The names used in the classification were not only loosely applied but were based on the most superficial observation, and took no account of the intimate structures of the tissues and organs of the animals. With slight modifications, due to individual taste or special knowledge of small groups, later writers had followed Linnæus and Cuvier.
It was with a view of the animal kingdom not much clearer than this that Huxley began his work on the Medusæ of the tropic seas. He began to study them no doubt simply because they were among the most abundant of the animals that could be obtained from the ship. He made endless dissections and drawings, and, above all, studied their minute anatomy with the microscope. They were all placed among Cuvier's Radiata, but, as Huxley said in the first line of his memoir:
"Perhaps no class of animals has been investigated with so little satisfactory and comprehensive result, and this not for the want of patience and ability on the part of the observers, but rather because they have contented themselves with stating matters of detail concerning particular genera and species, instead of giving broad and general views of the whole class, considered as organised upon a given type, and inquiring into its relations with other families."
He found that fully developed Medusæ consisted each of a disc with tentacles and vesicular bodies at the margins, a stomach, and canals proceeding from it, and generative organs. He traced this simple common structure through the complications and modifications in which it appeared in the different groups of Medusæ, in all this work bringing out the prevailing features of the anatomy in contrast to the individual peculiarities. He shewed that microscopically all the complicated systems of canals and organs were composed of two "foundation-membranes," two thin webs of cells, one of which formed the outermost layer of the body, while the inner formed the lining of the stomach and canals in the thinner parts of the body, such as the edges of the umbrella-like disc, and towards the ends of the tentacles. These thin webs formed practically all the body. In the thicker parts there was interposed between them an almost structureless layer of jelly, placed like padding between the lining and the cloth of a coat. He shewed that blood-vessels and blood were absent, in which he has been confirmed by all other observers. He declared more doubtfully against the existence of a special nervous system, and it was not until long after, when the methods of microscopic investigation were much more perfect, that the delicate nerve-cells and nerve-fibres, which we now know to exist, were discovered.
Having thus shewn the peculiar organisation of the group he turned to seek out its allies among other families. The Medusæ consisted essentially of two membranes inclosing a variously shaped cavity inasmuch as all its organs were so composed. The generative organs were external, being variously developed processes of the two membranes. The peculiar organs called thread-cells—poisoned darts by the discharge of which prey could be paralysed—were universally present. What other families presented these peculiarities?
There are to be found abundantly in sea-water, and less frequently in fresh water, innumerable forms of animal life called Zoöphytes or animal plants because they occur as encrusting masses like lichens, or branched forests like moss, on the surface of stones and shells. A common habit gave this set of creatures their common name; but, although they were grouped together, there was no greater affinity among them than there is racial affinity among people who clothe themselves for an evening party in the same conventional dress. Huxley examined a large number of these, and picked out from them two great families of polyps, the Hydroid and Sertularian polyps, which each consist of colonies of creatures very much like the little fresh-water hydra. He shewed that the tubular body of these and the ring of tentacles surrounding the mouth were composed of the same two foundation-membranes of which all the organs of Medusæ are composed. He found in them the poisoned arrows or thread-cells of the Medusæ, and the same external position of the reproductive organs. And, lastly, he separated from all other creatures, and associated with his new group, some of the strangest and most beautiful animals of the tropic seas, known to science as the Physophoridæ and the Diphyidæ. The best-known of these is the "Portuguese man-of-war," the body of which consists of a large pear-shaped vesicle which floats on the water like a bladder. From the lower part of this depend into the water large and small nutritive branches, each ending in a mouth surrounded by a circle of waving tentacles armed with batteries of thread-cells, while another set of hanging protrusions bear the grape-like reproductive organs. On the upper surface of the bladder is fixed a purple sail of the most brilliant colour, by which the floating creature is blown through the water. When the weather is rough, the bladder empties, and the creature sinks down into the quiet water below the waves, to rise again when the storm is over. This, and its equally wonderful allies, Huxley showed to be a complicated colony of hydra-like creatures, each part being composed of two membranes, and therefore essentially similar to Medusæ. Thus, by a great piece of constructive work, an assemblage of animals was gathered into a new group and shewn to be organised upon one simple and uniform plan, and, even in the most complex and aberrant forms, reducible to the same type. The group, and Huxley's conception of its structure, are now absolutely accepted by anatomists, and have made one of the corner-stones of our modern idea of the arrangement of the animal kingdom. With the exception of sponges, concerning the exact relations of which there is still dispute, and of a few sets of parasitic and possibly degenerate creatures, all animals, the bodies of which are multicellular, from the simple fresh-water hydra up to man, are divided into two great groups. The structure of the simpler of these groups is exactly what Huxley found to be of importance in the Medusæ. The body wall, from which all the organs protrude, consists merely of a web of cells arranged in two sheets or membranes, and the single cavity consists of a central stomach, surrounded by these membranes, the cavity remaining simple or giving rise to a number of branching canals. The members of this great division of the animal kingdom are the creatures which Huxley selected and placed together, with the addition of the sea-anemones and the medusa-like Ctenophora, which, indeed, he mentioned in his memoir as being related to the others, but reserved fuller consideration for a future occasion. This group is now called the Cœlenterata, the name implying that the creatures are simply hollow stomachs, and it is contrasted in the strongest way with the group Cœlomata, in which are placed all the higher animals, from the simplest worm up to man; animals in which, in addition to the two foundation-membranes of the Cœlenterata, there is a third foundation-membrane, and in which, in addition to the simple stomach cavity with its offshoots, there is a true body-cavity or cœlome, and usually a set of spaces and channels containing a blood-fluid. The older method of naming groups of animals after some obvious superficial character lingered on for some years in text-books and treatises, but in this memoir the young ship-surgeon had replaced it by the modern scientific method of grouping animals together only because of real identity of structure.
There is yet left to be noticed perhaps the most wonderful of all the ideas in this first memoir by Huxley. In the course of describing the two foundation membranes of the Medusæ he remarks:
"It is curious to remark, that throughout, the outer and inner membranes appear to bear the same physiological relation to one another as do the serous and mucous layers of the germ: the outer becoming developed into the muscular system, and giving rise to the organs of offence and defence: the inner on the other hand appearing to be more closely subservient to the purposes of nutrition and generation."
In the whole range of science it would be difficult to select an utterance more prophetic of future knowledge than these few words. Huxley had been reading the investigations of Von Baer into the early development of back-boned animals. He had learned from them the great generalisation, that the younger stages of these animals resemble one another more closely than the adult stages, and that in an early stage in the development of all these animals the beginning of the embryo consists of two layers of cells, in fact of two foundation-membranes, one forming specially the wall of the future digestive canal, the other forming the most external portion of the future animal. In these days nothing could have seemed a remoter or more unlikely comparison than one instituted between Medusæ and the embryonic stages of back-boned animals. But Huxley made it, not allowing the evidence brought before his reason to be swamped by preconceived ideas. At the time he did no more than to make the comparison. It was much later that the full importance of it became known, when more extended work on the embryology of vertebrates and of the different groups of the invertebrates had made it plain that the two foundation-membranes of Huxley occur in all animals from the Medusæ up to man. In the group of Cœlenterata the organisation remains throughout life as nothing more than a folding in and folding out of these membranes. The early stages of all the higher animals similarly consist of complications of the two membranes; but later on there is added to them a third membrane. Thus the group that Huxley gathered together comprises those animals that as adults remain in a condition of development which is passed through in the embryonic life of all higher animals. The immense importance of this conclusion becomes plain, and the conclusion itself seems obvious, when seen in the light of the doctrine of descent. The group of Cœlenterata represents a surviving, older condition in the evolution of animals. Huxley himself, when on the Rattlesnake, regarded evolution only as a vague metaphysical dream, and he made the comparison which has been described without any afterthought of what it implied. In this we have the earliest authentic instance of the peculiar integrity of mind which was so characteristic of him in his dealings with philosophy and tradition. He never allowed any weight of authority or any apparent disturbance of existing ideas to alter the conclusions to which his reason led him. This intellectual courage made him fitted to be the leader in the battle for evolution and against traditional thought, and we shall find again and again in consideration of his work that it was the keynote of his life.
