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 Coelenterata, the name
implying that the creatures are simply hollow stomachs, and it is
contrasted in the strongest way with the group Coelomata, 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
Coelenterata, 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 coelome, 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
Coelenterata 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 Coelenterata 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.