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THE EVIDENCE FOR EVOLUTION 1

UNICELLULAR AND MULTICELLULAR ANIMALS 23

THE WORMS AND SOME OF THEIR POSTERITY 58

THE EARLY VERTEBRATES AND THE FISHES 79

THE CONQUEST OF THE LAND 104

THE MAMMALS AND MAN 123

EVOLUTION

THE EVIDENCE FOR EVOLUTION

The idea of Evolution is an old one. It is older than the Darwinian hypothesis; it is older than Lamarck, who published his particular theory in 1809, the year that Darwin was born; it is older than Buffon or Kant. In a fairly definite form it is as old as Aristotle. The Evolution idea has thus itself evolved, and is the product of many centuries of thought. Yet it was only the last generation that began to give the idea serious consideration, and it is perhaps only the present that has granted it any general measure of acceptance; and it was Darwin who wrought this change, who raised the conception of Evolution from the status of a vague speculative idea to that of a well-grounded theory, which appeals to the majority of educated minds as satisfactory and reasonable.

We do not here propose to sketch the development of the idea, either before or after Darwin; but only, in the first place, to state the grounds on which the belief in Evolution is based, and, in the second, to trace roughly the lines along which animal Evolution has proceeded. In the first few pages of this book, then, we shall endeavour to bring forward some of the evidence on which the modern Evolution theory rests.

FIG. 2.--Table showing the chronological succession of the stratified rocks, the subdivision of geological time, the approximate position of the earliest fossils of each of the main types of vertebrates, and the period of domination of each group.

As our first witness, we may call the rocks which constitute the outer portion of the earth, and ask them to tell us what they remember of the history of life upon the planet. We cannot hope for the whole truth from them, for their memory is imperfect; and yet they can tell us a great number of important facts.

From the time when the world was sufficiently cooled for water to condense on its surface, a continual process of unbuilding and rebuilding of rocks has gone on. Wind and water, heat and cold have laid their hands to the work, making sand and dust and gravel out of solid stone, and these products of their labours have been carried off to other places, laid down, and cemented together into new rocks. We do not know the exact age of any particular rock that has been made in this way, nor how long the process has been going on. At a rough guess it may be three or four hundreds of millions of years. The chronological succession of the different rock formations is, however, known, and their relative ages may be judged with considerable accuracy. Here and there, as time went on, the body of a plant or an animal was deposited in the sand or mud or chalk, and has remained in the resulting rocks, in the form of a fossil, through all the ages. If, then, we study the occurrence of fossils in this succession of deposits, we ought to get some indications as to the inhabitants of the globe at various stages of its history. And if we do so, we meet unmistakable evidence that the lower and simpler types, both of animals and of plants, were in existence before the higher. Fig. 2 shows the facts with regard to the vertebrates, the great upper class of the animal kingdom. The first appearance of vertebrate fossils is in the Upper Silurian rocks, that is to say, somewhere after the middle of geological time. The fossils represent the lowest group of fishes. In the next great formation, the Devonian, fossils of two higher groups of fishes are to be found. The first land vertebrates, the amphibians, are doubtfully represented in the upper or newer layers of the same formation, and definitely so in the next, the Carboniferous. Towards the end of the Carboniferous or early in the Permian epoch, the first reptiles appear, and in the following period, or after about three-fourths of geological time had passed, the earliest fossils of mammals occur. The significance of this sequence will become plainer when the differences and likenesses of these various groups are explained. Each of these great groups in turn formed the dominant animal population of the globe, and each in turn was superseded, although not entirely, by the next. The mammal group itself appears to be on the wane, overcome in the struggle for dominance by its own latest and most remarkable member, man himself.

THE VARIATION OF PIGEONS UNDER DOMESTICATION.

Centre--Rock Doves.

The broad facts in the history of living things upon the earth are, then, in accordance with the theory of Evolution. The chain of types is indeed a broken one, the gaps being many, and some of them wide. But this is readily to be understood from the comparative scarcity of fossils, and the imperfection of the geological record.

In certain particular instances, however, very complete series of fossil forms have been discovered, connecting, by small gradations, modern animals with greatly different extinct types. One of the most complete of such series has been discovered for the horse. The changes that have occurred in the evolution of this animal have been mainly in three directions--increase in size, reduction in the number of toes from the original five to the final one, and deepening of the crowns of the teeth, so as to render them capable of longer wear. From the Eohippus of early tertiary times, an animal of about the size of a fox terrier, with five toes behind, and four with the vestige of the fifth in front, there is a complete connecting series reaching up to the modern horse, with its single remaining toe and the vestiges of two others. A few of the main links in this chain are illustrated in Fig. 3. It is impossible to regard such a series without having the idea of Evolution strongly suggested to the mind.

In the second place, there is evidence for Evolution in the fact that marked changes can and do occur in the characters of living races of organisms. There is ample evidence, for example, that all our modern breeds of pigeons are descended from the wild rock-dove. How markedly some of these differ from their wild ancestor, and among themselves, may be seen from Fig. 4. The size of some is twice as great as that of others. The bill in some is greatly increased in length, is almost ludicrously reduced in others. Colour, feathering, build, even the instincts and the voice, vary enormously as between different varieties. In short, there is hardly any obvious character that has not, in one or other of the breeds, undergone great modification. As Darwin remarked, any naturalist coming upon such a group of forms in nature would have no hesitation in placing them in different species or genera, or even perhaps in different families. Even granting that the conditions of domestication are peculiar, we must admit that if such large changes can occur in a few centuries, it is possible that man has evolved from the lowest of living organisms during a period some hundreds of thousands of times as long.

But marked changes of type occur not only under conditions of domestication; nor is it necessary to infer the occurrence of any such changes without actual direct evidence. The formation of new types occurs in nature, and has taken place under the very eyes of scientific observers. Perhaps the most striking case that can be quoted is that of Lamarck's Evening Primrose, which, under the observation of Prof. De Vries in Amsterdam, produced some half-dozen of 'sports' which seem well entitled to rank as new species. Fig. 5 shows the parent plant and two of the new types that were produced by it. One is a dwarf in habit, the other is characterised by the greatly increased breadth of its foliage. Others showed different peculiarities. One might quote many other instances of violent changes of type--of the appearance of six-fingered children, whose peculiarity was afterwards inherited; of web-footed pigeons, and of new varieties of fruits, flowers, and vegetables. The causes of such 'sports' or mutations are unknown, but their moderately frequent occurrence is abundantly demonstrated. Such facts show, at all events, that the old conception of species as permanently fixed, unchanging types, can no longer reasonably be held.

Not all of the abnormalities which thus suddenly appear, we know not how or wherefore, are new. Many recall characters in lower or older groups, and may reasonably be interpreted as 'reversions.' Thus the horse's leg shown in Fig. 6 bears a well-developed side toe, in place of the small vestige that is normally present. Horses with this peculiarity have occurred with some frequency, probably before, and certainly since, the most famous of their kind, which Julius Caesar rode. It seems reasonable to regard this peculiarity as a return to the old ancestral condition illustrated before, in which the side toes were well developed. The same applies to the instance of a persistent tail and persistent gill slits in man , and to many other instances that might be quoted. One must indeed deal carefully with such cases, for it is always difficult to say what changes are new departures, and what are returns to ancestral types. There is danger of arguing in a circle--of supposing the ancestry from the abnormality, and of terming the latter a reversion because it suggests the supposed ancestry. Nevertheless, when variations occur, suggesting characters which are believed, on other grounds, to be ancestral, they must tend to strengthen the other evidence as to the evolution of the type in question.

Another and a very strong evidence of Evolution is to be found in what are termed vestigeal structures, two of which are illustrated in Figs. 9 and 10. They are, for the most part, obviously useless, and their occurrence has never been satisfactorily explained except by supposing them to be remnants of organs that were functional in the past history of their possessors' race. The appendix of man, for instance, is not only useless, but is frequently a source of danger. But its presence is readily explained by supposing that it represents the blind-gut, which is large and functional in many of the lower animals. Again, how should we account for the presence of small functionless wing-bones in the cassowary, unless by supposing that its ancestors were accustomed to fly like ordinary birds? How should we explain the bones which represent the hind limbs of the whale, unless by regarding the whale as descended from an animal which had functional hind limbs, or the representatives of eyes in animals that live in the dark, unless by supposing that these are descended from ancestors which saw? It has been well said that the bodies of many animals are veritable antiquarian museums, filled with relics of their own ancestors.

The next argument for Evolution to which we would refer is based on the similar structure and origin of organs or members that have entirely different uses. In Fig. 11 are figured the bones of the fore limbs of four different mammals, a whale, a bat, a dog, and man. The first is used for swimming, the second for flight, the third for locomotion on land, and the fourth as a grasping and holding organ. If these organs had been specially designed, each for its specific purpose, we should expect to find fundamental differences in structure. Actually the general arrangement of bones is the same in each case. A fact like this points strongly to a common origin of the four types mentioned, and to a general primitive arrangement of the bones of the limb. This primary type, it seems natural to suppose, has been modified for various special purposes in many different directions, the general features remaining recognisable. Many other cases of homology, or similarity of structure and origin, in organs whose function is dissimilar, might be quoted. Thus the poison gland of the poison snakes is not an organ which has been specially developed, but is a modified portion of one of the salivary glands. The hoof of the horse and the finger nail of man can evidently be satisfactorily explained as modifications of a general type of terminal claw, and the scales of the scaly ant-eater and the quills of the porcupine are only modified hairs. The significance of facts like these, when carefully considered, is very great.

The study of the geographical distribution of animals has brought forth a great mass of facts which, considered by themselves, seem chaotic and meaningless, but which, in the light of Evolution, are full of significance. Observe, for example, the distribution of the Marsupials or pouch-bearing animals, shown on the accompanying map . Australia is full of them, while they are relatively meagrely represented in a few other parts of the world. At the same time the greater and higher group of mammals was represented in Australia, at the time of its discovery, only by the bushman and his dog and a few species of mice. It is not as if the Australian environment were specially well adapted for marsupials, or specially ill-adapted for higher mammals; for the sheep has proved itself splendidly adapted for the conditions, and the rabbit most inconveniently so. Why, then, this curious state of affairs? It is an undoubted fact that the marsupials are both lower in their position in the animal kingdom, and older, than the main group to which all our European mammals belong. Now it is believed that Australia was once connected by land with the Asiatic Continent, and that it was finally separated from it before the higher mammals were in existence. The great step of further progress occurred elsewhere than in Australia, and the mammals of the latter continent were left in their obsolete condition, preserved through lack of competition of that higher type which elsewhere became dominant.

Madagascar offers a similar case. It abounds with forest vegetation and seems to offer a highly suitable environment for the monkey tribe. Yet there are no apes on the island. Their place is occupied by the Lemur tribe, which, there is every reason to believe, is the older group of the two, and that from which the apes have sprung. It is supposed, then, that Madagascar was separated from Africa before the ape had evolved. The lemurs thenceforward were free from the competition of their more highly developed relatives, and have branched out into a great variety of types, while still remaining on a relatively low plane of intelligence and specialisation. The distribution of the Lemurs is shown in Fig. 13.

Finally, we may mention the evidence that has been gathered from the study of embryology and development. It has been stated, in a metaphor which is perhaps more clever than it is exact, that every animal climbs up its own ancestral tree; and while it would be absurd to say, for instance, that a mammalian embryo resembles successively a fish, an amphibian, and a reptile, still many of the broad facts in the evolution of a race seem to be repeated, in a more or less blurred and indistinct fashion, in the development of the individual. Thus, for example, gill-slits and a tail are possessed in common by the embryos of all higher animals, only afterwards to disappear in those types in which the adult animal is without these structures. The heart of the mammal or bird is at first simple, then two chambered like that of a fish, then three chambered like an amphibian's, and finally four chambered. Some of the main phases in the development of the rabbit and of man are shown in Figs. 14 and 15 respectively.

The young flat-fish is like an ordinary member of the fish tribe, with an eye on either side of its head, and its body built on the ordinary symmetrical lines. It is only later, when it begins habitually to be upon one side on the sea bottom, that the eye from the under side wanders round to the opposite aspect beside its fellow, and the upper side becomes pigmented, while the lower remains white.

In similar fashion a primitive form of kidney is, as it were, sketched in, in the development of the higher animals, only to be erased at a later stage and replaced by a better form. The human child has a complete body covering of hair, which disappears soon after birth. In these and many more instances, one cannot avoid the impression that the organism has not been specially designed for what it finally comes to be. It cannot forget, and must needs repeat, or so it seems, some considerable part of the history of its race.

Manifestly, then, all this evidence, gleaned from many different sources, points to a common origin of living things, and to the gradual evolution of the higher from the lower types. It may also be said that there is no scientific evidence against such a view.

UNICELLULAR AND MULTICELLULAR ANIMALS

We must now turn to the main project of this book, which is to attempt to trace out the lines along which animal Evolution has proceeded, with special reference to that particular line which leads up to man. Indeed, we shall have to stick somewhat closely to this one main highway, and can but barely pause to glance along the numerous branch roads, interesting though the travelling there might be.

It is perhaps necessary to say, at the outset, that the history of the Evolution of man cannot be written as a plain, matter-of-fact tale. Many portions of this history are tolerably well understood, but there are other periods, in some of which notable steps of progress were made, of which no record has ever been discovered. We must therefore expect occasionally to be reduced to speculation, and here and there to meet with controversy and with opposing theories.

To begin at the beginning of our tale, we may ask ourselves what are the lowest, simplest, living things that are known. The question does not admit of any very definite answer. For as we look around among a number of the most simple forms, we find ourselves handicapped in our attempt to judge between them, by a lack of knowledge of their nature. We come upon organisms so small that they appear, even under the most powerful microscope, only as the tiniest specks; whose size is to be measured in hundredths of thousandths of an inch. We even find good evidence that living things exist which we are unable, in any manner whatsoever, to see. Among the smallest known forms, and also among some of the larger, we find organisms that we can only describe as practically structureless, that appear as specks of almost homogeneous protoplasm; but it seems reasonable to suppose that this appearance is due rather to our imperfect observation than to an actual absence of differentiation.

It is certain, however, that the lowest of the great groups is that of the one-celled organisms. As all the higher types are built up of large numbers of cells, essentially similar to those which constitute the unicellular forms, it is important that we should know something of the nature of this organic unit. A typical cell is illustrated in Fig. 16. It consists of a mass of protoplasm, with a distinctly differentiated portion called the nucleus. The function of the nucleus is that of directing and controlling the activities of the cell; if it is removed, the remaining portion of the cell soon dies; while, on the other hand, a small portion of the cell, if it contains the nucleus, may frequently live, and build up new protoplasm to replace what was lost. Cells are formed only from previously existing cells, by a process of division, which is usually simply one of halving. This process is begun in the nucleus; it undergoes a complex rearrangement of its parts, the object of which appears to be to insure an absolute equality in the halves, and finally divides in two. The bulk of the protoplasm then separates into two portions, a portion remaining round each of the nuclei. The process of cell division is illustrated in Fig. 17.

Now it is a somewhat remarkable fact that we do not know whether or not all the humbler forms of life possess a nucleus. It was formerly believed that a considerable number of one-celled organisms were devoid of the body in question, but in most of such it has been shown that nuclear matter is present, though it may be distributed, in small portions, throughout the cell. If organisms do exist which consist of a cell without a nucleus, we must regard them as the simplest of living things. In any case, the formation of a nucleus, a process by which a kind of central government was formed, was probably one of the great early steps of Evolution.

The life-history of an ordinary one-celled organism may be briefly summed up. It absorbs nourishment and energy, adds to its substance until it reaches a certain fairly definite size, and then divides in two, the halves separating, and going each its own way. In the world of one-celled organisms there is no 'death from natural causes.' The individual is potentially immortal, except in so far as we may regard the individual life as ceasing when division takes place. Death occurs only, as we say, accidentally--for example, from starvation or from the attacks of enemies. A number of simple unicellular organisms are shown in Figs. 18, 19, and 20.

The reader will have observed that we have referred to the group under consideration in general terms, and without endeavouring to classify its members as plants or animals. And indeed it is impossible to carry this great distinction down to the lowest group of the organic world. This stands below the first great forking of the tree of life; its members remain in what has been described as a condition of 'chronic indecision,' neither clearly vegetable nor definitely animal. But very soon, in the march of progress, the forking of the roads was reached, and whosoever was bent on journeying farther had perforce to make the choice. We must here briefly consider what this choice was, and wherein the fundamental distinction between a plant and an animal consists; for, strange as the statement may seem, the basis of this distinction is by no means generally appreciated.

The typical plant lives by absorbing carbon dioxide gas, water, and mineral salts from the surrounding media. These substances, by means of energy which it gathers from the rays of the sun, the plant builds up into organic substances, to be used in the maintenance of life, and for growth and reproduction. This process of chemical construction occurs only in the green, exposed parts of the plant, and indeed can occur only in the presence of chlorophyll, the green colouring matter of the leaves.

The animal, on the other hand, lives by appropriating, either directly or indirectly, what the plant has produced. All flesh is indeed grass, in a different sense from that originally intended by the statement. It is this essential difference which lies at the root of all the plain and obvious distinctions between animals and plants. The plant has neither the necessity to go forth in search of its food materials, which nature brings to it, nor has it to spare of its painfully collected energy for the labour of locomotion. Hence it remains stationary. The animal must of necessity go to seek its more elaborate fare, therefore it moves. Moreover, to be successful in its search, the animal obviously requires a nervous system to direct and control its movements, which system, except in the simplest and crudest forms, is absent from the plant. In the main, then, the plant builds up and saves, the animal breaks down and spends. The plant is the producer, the animal the consumer.

Turning now to those of the lower organisms that are somewhat more definitely animal in nature, we may describe the common Amoeba. Microscopic in size, this creature consists of a speck of semi-liquid protoplasm, which is irregular and ever-changing in shape. It is continually pushing out finger-like projections from various parts of its surface, feeling, in a dim, vague way, for its food. It moves, if but slowly, by withdrawing its substance in one direction and pouring it forth in another. It indulges in such fare as bacteria or particles of dead organic matter and feeds by the simple method of surrounding the food particle with its protoplasm, and gradually digesting and absorbing whatever it contains of nutriment. Undigested portions are simply left behind as the creature moves on. The waste products are drained into a simple cavity in the protoplasm called the contractile vacuole, which empties itself periodically to the outside. The Amoeba reproduces by the ordinary process of simple fission, illustrated, with the creature in its ordinary condition, in Figs. 21 and 22.

There are many groups of one-celled animals other than those typified in the Amoeba and the Paramoecium, but they do not appear to have any significance so far as the descent of the higher animals is concerned, and they therefore do not immediately concern us.

We have already mentioned that water is the life medium of the slipper animalcule. It was destined to remain the natural element, both of animals and of plants, throughout many subsequent stages of progress. The reason of this is not far to seek. Active protoplasm consists to the extent of about three-fourths of water, and a plentiful supply of this is one of the essentials for the continuance of active life. Therefore, before the conquest of the dry land could be accomplished, devices had to be evolved both for maintaining and for conserving the water supply--roots in the plant; in the animal, some method of locomotion by land or air, so that water could be frequently reached; protection against evaporation, in the form of a skin, in both; and numerous other special devices. Add to this the fact that locomotion on land presents much greater difficulties than that in water, and it will hardly occasion surprise that vast ages were yet to be required before the Evolution process could produce a land animal.

A striking analogy may be drawn between animal Evolution, from this point onwards, and social Evolution. In the latter case we begin with men, brought by a slow process of Evolution to a high state of individual perfection, living in a state of savage individualism. Each thinks and acts for himself, provides his own food, raiment, and dwelling; constitutes his own standing army and police. From this condition of affairs there has gradually been developed the modern social arrangement, by which each individual helps to carry out some distinctly special part of work for the community--be it wheat-growing, cloth-weaving, bricklaying, or the arresting of burglars--and trusts to the community for his requirements in all other directions. These requirements themselves have so multiplied during the course of social Evolution that innumerable forms of activity have sprung up between those occupations which provide the original necessities of life. The essence of the whole process has been co-operation and the division of labour.

In the story of animal Evolution we have reached a point where a highly perfected individual cell has been produced, which carries out for itself, and for itself alone, all the activities of life. From now onwards, co-operation and specialisation are the watchwords of progress. There is a clubbing together, first of a few cells, then of hundreds, and finally of millions upon millions, to form the bodies corporate which we recognise as individual higher animals. Division and distribution, subdivision and further distribution of the life activities proceed at the same time, until we reach the condition prevailing in the higher animals, where the degree of specialisation almost passes conception. In such there is, to begin with, a vast frontier army of skin cells, occupied in securing peace, as far as possible, for the industries that go on within. There are directors and controllers of these industries--the brain cells--with a myriad of workers under their guidance, and a great and complex telegraph system between. The workers themselves are of all descriptions--common labourers like the cells of the muscles; transport workers like those of the circulatory system; skilled factory hands like those of the glands; even scavengers in the shape of the sweat gland and kidney cells. Nay, there is even a numerous police force, of white blood corpuscles, which patrol everywhere, arresting intruders and disposing of them by the summary method of swallowing them whole.

Our information regarding the early history of this co-operative movement is fragmentary and incomplete, for only an odd species or so seems to survive of the group which we regard as the earliest of multicellular animals. In certain forms which are still essentially unicellular, such as the Spondylomorum shown in Fig. 26, there is a tendency to form smaller or larger cell colonies. When the individual cell divides, the two daughter cells do not separate, but remain somewhat loosely attached to each other, and the process of division without separation continues until a considerable group is produced. From this colony occasional individuals break away and proceed to form new colonies. From such a type it is a comparatively easy step to the Magosphaera described by Haeckel and illustrated in Fig. 27. This consists of a simple ball of ciliated cells, which reproduces by the occasional breaking away of an individual member, which divides and redivides until a new sphere is produced. Unfortunately this animal has only once been discovered, and many hold that it has not been sufficiently investigated. No other of the same type is known.

If we turn to the plant kingdom, however, we find a comparatively common organism of somewhat similar form. This is the Volvox, a plant which consists of some thousands of cells, and reaches a size of about a pin-head. It has the form of a hollow sphere, the wall of which is one cell thick, and the cavity of which contains only water. The cells bear whip-like cilia on their outer surface, by whose means the organism is able to move, swimming by a rotary motion round a definite axis. The individual cells are separated by layers of a gelatinous substance, through which, however, pass connecting strands of protoplasm. The cells, of course, contain the green colouring matter common to plants in general. Distributed among the ordinary cells occur a few that are distinguished by their larger size, and by the fact that they lack cilia. These are the special reproductive members of the colony. When the Volvox reaches maturity, these cells begin to divide, and form new growths which take the form of hollow sacs, which project into the cavity of the parent sphere. Later they separate from the wall of the parent, and begin to move about, in the internal cavity, by means of the cilia which they have developed. Finally the parent breaks up and dies, and the progeny are set free to commence life for themselves.

The next great groups of animals are, on the one hand, that of the sponges, and, on the other, that which includes the sea-anemones, jelly-fishes, corals, etc. At first sight their structure seems vastly different to that of the Volvox, from some form similar to which they have probably been derived. The evidence obtained from the study of their individual development, however, strongly suggests a process by which we suppose that they evolved from Volvox-like ancestors. We shall therefore briefly describe the earlier stages of the development of a coral. The sexually produced individual starts life as a single cell, the fertilised egg. This divides and redivides until a hollow ball of cells is produced, which cells, like those of the Volvox, bear cilia. Although simply spherical in shape, the creature moves by rotating round a definite axis, like a planet. Moreover, nutriment is absorbed not by any or every part of the surface, but only by a small area round the lower pole. Now as development proceeds, the cells at this pole divide more rapidly than the rest, with the natural result that the ball begins to get out of shape. The distended portion, however, develops to the inside, so that one part of the sphere is, as it were, pushed into the other. When this process has been completed, the original internal cavity is almost entirely eliminated, and a form is produced which resembles a double-walled flask or vase. Such a form may be taken as the fundamental architectural type of the groups that we are now to consider. The meaning of this further step of Evolution is again specialisation. The inner layer of cells takes on the functions of digestion and absorption of food, there having been evolved, in fact, the simplest possible form of mouth and stomach. Such other functions as those of locomotion, protection, and support are exercised by the outer layer. This process is illustrated in Fig. 31.

But there is no known type of animal which, in its adult form, shows quite the simple structure that we have described. Perhaps the nearest approach is to be found in the lower sponges, in which two modifications of the original plan have already been introduced. In the first place, the creature is sedentary, being fixed, in an inverted position, to some solid basis. It has, so to speak, ceased to be a hunter, and is become a fisher. Secondly, its wall is pierced in many places, so as to permit of a freer circulation, through the digestive cavity, of the water which contains the food material. The water passes in through these numerous perforations, and out through the main central opening or 'mouth.' The sponges do not appear to represent a stage in the main line of Evolution, but lead us almost immediately into a cul-de-sac. We therefore cannot pause to describe fully the many peculiar and interesting developments which occurred in the group. An ordinary 'sponge,' by the way, bears the same relation to the creature which produces it as does a 'coral' to the coral animal. It represents, that is to say, the skeletons of a large colony of individuals. The structure of a sponge is shown in Fig. 32.

If now we make a brief general survey of the group to which the Hydra belongs, we find in it two somewhat strikingly different types. On the one hand are sedentary forms that resemble, in a general way, the Hydra; that consist of a tube-shaped body, with the mouth, surrounded by a ring of tentacles, at the upper end. The sea-anemones and corals are examples of this type, in which, however, the structure shows various complexities as compared with that of the Hydra, which complexities we cannot here pause to describe. On the other hand is the well-known Medusa form, of which the common jelly-fish is a typical example. This creature, as is well known, is mushroom shaped, with tentacles round the edge. The mouth is in the middle of the lower aspect, at the end of a short 'stalk.' This type is very different in general appearance from the Hydra or sea-anemone, yet the one form may be somewhat easily derived from the other; we have only to imagine that a Hydra is turned upside down, that it is squashed, vertically, until the internal cavity is greatly reduced, and the circumference, especially in the region of the tentacles, greatly increased, and we should have something resembling a Medusa. That the two types are actually closely related is shown by the fact that there is in the life-history of one group of Coelenterates a regular alternation between the one and the other.

If the above general conception of the structure of the Medusa be borne in mind, its details will be easily understood. The internal cavity, instead of being simple, has become complicated, through the obliteration of certain parts of it, where the upper and lower walls come in contact. What is left is a comparatively small cavity immediately above the mouth, a number of symmetrically arranged canals radiating out from this, and a ring canal connecting the ends of these with each other. Another special characteristic is that there is a great mass of gelatinous substance between the outer and inner cell layers. The reproductive cells, as in the sea-anemones, are produced by the inner cell layer, and escape by the mouth.

Something remains to be said regarding the specialisation of tissues in this group. We have already mentioned the stinging cells, and the beginnings of muscular tissue, in Hydra. The former are a constant feature of the Coelenterates, while the latter reaches a very considerable development in the higher forms, as may be judged from the surprising rapidity with which the Medusa can swim, or from the strength with which the sea-anemone can retract its tentacles and draw itself together. Important, further, is the nerve tissue. This consists of cells whose business it is to receive and transmit stimuli. They have long fibrous projections connecting them with each other, so that there is a network of communication throughout the whole animal. In the Medusa, where co-ordinated movements of various portions is necessary, there is a concentration of nerve cells into a double ring near the edge. Here also there are special organs, probably of sight and of the sense of balance; but as these cannot be regarded as the forerunners of the analogous organs in higher animals, we need not pause to describe them. The anatomy of the Coelenterates will be better understood if the reader will study the diagrams in Figs. 35 and 36, while some idea of the beauty and variety met with in the group may be obtained from Fig. 37.

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