ZOOLOGY.
One cannot glance at the opening pages of any work on Zoology without being struck with the difficulty that naturalists experience in classifying the objects of their study. This difficulty arises from the fact of there being many animals whose organization presents characters which combine the peculiarities of different families or orders. The Flying Lemur (Galeopithecus volans) of the East Indies was for a long time considered to be a bat; modern anatomists place it among the monkeys, and yet, according to some authorities, the grounds for its determination in the one case are as good as in the other. If the Galeopithecus, the monkeys and the bats originally appeared as we find them now, why should there be such a difficulty in determining their place in the Animal Kingdom? If, however, there has been an order of Galeopitheci, of which the present species are the only surviving representatives, and we regard these extinct forms as the common ancestors of the bats and the monkeys, we have an explanation of the peculiarities which are shared by these three orders:
| Bats. | Galeopithecus. | Monkeys. | |||||||||||||
| Ancestor. | |||||||||||||||
If the theory of the gradual transformation of animals be true, it is quite natural that we should find transitional forms, such as the Flying Lemur. There are many similar examples: the Aye-Aye (Cheiromys) has teeth like a rat, while in other respects it is a monkey. The Duck-bill Platypus of Australia (Ornithorhynchus) combines the organization of lizard in its breast-bone, crocodile in its ribs, and bird in the skull and digestive apparatus, and yet is a four-footed animal.
KINGDOM INTERMEDIATE BETWEEN ANIMALS AND PLANTS.
In modern times, through the assistance of the microscope, many minute beings have been discovered which have been successively classified as plants or animals, according to the botanical or zoological tendencies of their describers. As these microscopic beings present the life of both plant and animal at different stages of their existence, it is quite impossible to say to which kingdom they belong. Many naturalists are, therefore, agreed to consider them as a something apart, an intermediate original kingdom, out of which the plant and the animal worlds have been evolved. If life has been gradually developed, and there has been a progress from beings of low organization to higher, it is natural that such a kingdom should exist, partaking in its nature of animal and plant characters. The origin of life is to be sought, therefore, in this main root, of which animals and plants are the rising diverging branches. The beings of this animal-plant kingdom[1] which still exist are only the descendants of a larger kingdom long since extinct, or perhaps some of the most simple are still formed through spontaneous generation.
MONERA.
The simplest forms of life known are the Monera (Figs. 1, 2, 3), which may be defined as living jelly,—formless, structureless, in every sense of the word. Their movements are restricted to a gliding or crawling, a drawing in or putting out of their jelly-like body; their reproduction is a simple splitting of their body into two halves, each half becoming a new Monas. Such a living slime is seen in Protogenes. The first sign of structure we meet with in this kind of being is where a wall has been exuded inclosing the jelly-like body, as in Protomonas, or as in Amoeba (Figs. 4, 5, 6, 7), where the slime has aggregated in the middle, forming a nucleus. These two different conditions, a nucleated slime and a walled slime, are combined in Arcella, these last being undistinguishable from the young of the simplest water-plants (Algae) and the undeveloped forms of certain jelly-fish (Siphonophorae). In the springtime the ponds are often covered with a green matter, which, when examined with a microscope, is found to consist of Euglena (Fig. 8), minute flask-shaped bodies with little tails; when these bodies are covered with hairs (cilia) they are known as Peridnium. They, like the Arcella, cannot be distinguished from the young of the simplest plants and animalcula (Infusoria). What conclusions can be drawn from the existence of Monera, Amoebae, Euglena? How have they originated? Either they have come from pre-existing forms, or have arisen through spontaneous generation. Where have the pre-existing forms come from, is immediately suggested by the first answer, which only waives the question, and is therefore no answer at all. It will not suffice to say that they were created; as well might an astronomer explain the motion of the moon around the earth by saying it was created so to move. What is meant by spontaneous generation? Let us illustrate by the example of the formation of a crystal out of a simple solution. A nucleus first appears, then increment after increment is added, according to laws, until the crystal is formed. The case of the origin of a Monad is a parallel one. We have a solution; in this solution appears the Monad. There is no more necessity for the pre-existence of a Monad in the solution than that there should have been a pre-existing parent crystal. In both solutions exist the elements of which the Monad and Crystal are formed; the laws according to which they are formed are as susceptible of study in the one case as in the other. Theoretically, therefore, there is no objection to the idea of Spontaneous Generation, the laws of which must be investigated as any other mechanical problem has been; the problem being a question of the redistribution of matter. In fact, according to the experiments of Pouchet, Pennetier, Bastian, Wyman, and others. Vibrios and Bacteria do appear in solutions where there was not previously a trace of these minute beings.
Want of space prevents us from discussing this question in detail. We can only say that at present the evidence seems to us in favor of the view that Spontaneous Generation takes place at the present day under favorable conditions. We turn now to the consideration of living Monera and Amœbae. Whatever their present or remote origin may have been, an Amoeba is a Monas with a nucleus. The Amoebae probably came originally from Monera, if they are not now so produced, and in some cases colonize themselves, forming the Sponge,—this view being suggested by the young of the Sponges, which cannot be distinguished from Amoebae,—or the Amoebae gaining tails and hairs, like the Euglena, gave rise to the Animalcula or Infusoria. But as certain Amoebae and Euglena cannot be distinguished from the spores or young of the simplest plants, the origin of the vegetal world must be sought also in these minute beings. If, now, Spontaneous Generation does not take place, the Monera and Amoebae of the present day, and the other orders of the Intermediate Kingdom also, are the posterity of the long dead original forms, the ancestors of the three kingdoms,—the animal, the vegetal, and the intermediate world. (See Tree I., page 24.)
SPONGES.
The sponge of every-day use (Figs. 11, 12) is composed of a horny, fibrous material, produced by a colony of Amoebae, and if a section of the fresh-water Sponge (Spongilla) be made, a glance will explain by what means the currents of water are produced, which can be seen under the microscope. The outer layer is composed of a number of Amoebae, with little openings, through which the water enters the cavity between the outer and inner layers, this inner layer being also composed of Amoebae, in the deep substance of which are chambers lined with fine hairs (cilia), which, working in the same direction, force the water through them into a common outlet; in this manner strong currents of water pass in and out of the sponge, making a little whirlpool, into which minute particles of matter are dragged.
Sponges are found attached to all kinds of rocks, and often to shells, both in the sea and on the beach, and are of two kinds, soft and hard, of which the soft are probably the ancestors of the hard. The Halisarca, or Slimy Sponge, is found attached to the leathery sea-weed, and is composed of slimy Amoeba-like bodies, in which the canal system just described is only imperfectly developed. From this kind were derived the Gummy Sponges, so called from their gumlike consistency; their canal system foreshadow-s the homologous structure in the Jelly-fish. Sponges like the
Halisarca were probably the ancestors of the first kind of 
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Amoeba-like bodies. Some of the descendants of the Horny Sponges were metamorphosed into the flinty kind, to which our fresh-water sponge belongs, and the Venus's flowerbasket (Euplectella), whose framework rivals in delicacy the most beautiful lace. From the flinty kind were derived most likely the Pitcher Sponges, in which the framework takes the form of a goblet or pitcher. These are often beautifully preserved in a fossil state (Ventriculites, Guettardia); they are nearly allied in their structure to Corals and Anemones. Still closer to the Corals are the calcareous sponges in their affinities. The natural history of Sponges, to those to whom it is known best, is full of evidences in favor of the theory of Evolution.
TREE II.
Actinozoa. Hydrozoa. Anemones. Corals. Jelly-Fish, Calcareous. Pitcher. Gummy^ Flinty. Horny. Slimy. Hard Sponges. Soft Sponges. Original Sponge. Amoeba. Monas.
ZOOPHYTA OR COELENTERATA.
The word Zoophyte means Plant-Animal,—the group taking its name from the striking resemblances that many of its representatives bear to plants; as, for example, in the stationary character, as seen in the coral-builders, in the flowery appearance of the Anemones, in the budding or treelike growth of the Tubularian and Sertularian families.
The word Coelenterata means hollow intestine. The essential character of the animals belonging to this group consists in their radial form and the absence of separate organs for carrying on the functions of digestion, nutrition, circulation, and respiration,—these functions being combined in one system, known as the "gastro-vascular," the digestive cavity being at the same time the general cavity of the body. The Zoophyta divide pretty naturally into two classes. In the first there is no distinct stomach, as seen in the Hydroid jelly-fish and the fresh-water Hydra; in the second, the walls of the body have risen up, prolonging parts of the general cavity of the body into arms (tentacles), thereby cutting off parts of the cavity; the remaining portion not so affected contains a rudimentary stomach. This structure is seen in the Anemones, or Sea-flowers, etc.
ACTINOZOA.
The common Anemones (Fig. 14), Sea-flowers, or Actiniae, are found attached to the under surface of stones and rocks, and principally hanging from those large rocks that are not easily if ever moved by storms. When closed they might be mistaken for some dead matter adhering to the rocks, but when expanded they resemble a beautiful kind of flower. They are variously colored, and present, when found in profusion, the appearance of a flower-bed. The Anemone is a bag attached by its base, or foot, to the rock on which it lives. It has the power of slowly changing its position by a gliding movement, but does not often do this. The free end, with the mouth in the centre, is surrounded by a wreath of arms (tentacles), which contain multitudes of minute vesicles, with coiled threads, which are often projected, and serve to paralyze or entangle living prey. These tentacles are hollow and pierced with small holes. Each tentacle leads into, or is the prolongation of, the general cavity of the body upwards; so when the Anemone shuts itself up, the water which filled it is ejected through the tentacles, making as many little fountains as the Anemone has arms. The stomach, consisting simply of two fold (Figs. 15, a, a), open below, and suspended from the mouth, is held in position by folds, called mesenteries, running to the walls of the body. (Fig. 16.) Hanging to these folds are seen the eggs, which, dropping into the general cavity of the body, come out of the mouth. The general cavity of the body is at once the common system by which the food is digested, and carried to different parts of the body, and the effete matters got rid of There are no intestines, no arteries, veins, gills, or lungs; no separation into organs; little of that division of labor which is so striking in the economy of the higher animals. The curious stones called " brain stones" are constructed by animals allied to the Anemone in structure, as are also the beautiful rocks of madrepore. The coral reefs as large as islands, indeed, often the rocks on which the island verdure has grown, are the results of the unseen, untiring work through countless ages of these minute and beautiful beings. There is nothing which gives one a more profound appreciation of the lapse of ages, of immense periods of time, than the formation of the parts of the earth constructed by such agencies.
Suppose the chambers of an Anemone to contract into canals, that this modified Anemone loosens itself, turns upside down, and swims off, stomach hanging downward from an umbrella or bell-shaped body; we would then have a jelly-fish.
HYDROZOA.
Every one who has visited the sea-shore must have had his attention called to the jelly-fishes (Fig. 20), as they floated along by means of the pulsations of their disc-bearing bodies, the animal looking somewhat like an umbrella, and he remembers well the sensations suffered while bathing when his skin came in contact with the long streamers floating about, which are so characteristic of these organisms. The stinging is due to a poison which is contained in vesicles situated in the skin, often millions in number. In the beautiful Blue Physalia, known to sailors as the Portuguese man-of-war, this poison is so powerful that it has been known to cause death. The jelly-fishes and Anemones alike possess these poison-cells; but the Sponges, although having the rudimentary canal system, are devoid of the stinging structures. The most simple example of the Hydrozoa is our common Green Hydra (Fig. 18), so called from the fact that when cut in pieces each piece becomes a new individual. It looks to the naked eye like a piece of green silk thread. When magnified, it is seen to be a simple tube, the digestive cavity and general cavity of the tube being the same; its mouth is surrounded by a circle of arms or tentacles, by means of which it seizes its prey, paralyzing or destroying it by the poison just spoken of The importance of the Hydra, as part of the evidence for the evolution of life, maybe seen in the development of the so-called Hydroid jelly-fishes, such as the Campanularia and Sertularia (Fig. 19), which look like little trees covered with flowers. The branches of these tree-like beings are little tubes, in which the flowers (Hydroid polyps) live; the tubes are all connected, so the colony has a common digestive cavity. Such beings as the Campanula produce, through budding, beautiful little jelly-fishes, which swim freely about. They in their turn produce eggs, from which spring the stationary colonies of Hydrae. There is an alternate generation, Campanulariae producing jelly-fish, jelly-fish producing Campanulariae. We see the Hydra living as an independent organism—the Hydra of our fresh-water ponds—and in a transitory stage, as the Campanula. Sometimes a colony of these Plydroids form a freely swimming organism, as the Portuguese man-of-war. The Ctenophorae, or comb-bearing jelly-fishes, pure as crystal and transparent as glass, are characterized by their organs of motion, which are eight delicate combs, by the graceful movement of which the Beroes and Cydippe glide through the sea. They are intermediate in some respects between the Anemones and common jelly-fish (Aurelia). By glancing at Tree II. we see the probable origin of the Anemones, Corals, and Jelly-fish in the Sponges, the Anemones and Corals coming from the hard sponges, the fossil forms of which, like Ventriculites and Guettardia (Figs. 13, 17), closely resemble in the arrangement of their chambers those of the Anemone and Coral. By comparing the transverse section of a sponge (Fig. 22) with that of the jelly-fish (Fig. 21), we see that the canals of the sponge are the same as those of the jelly-fish, though more simple in their arrangement. Though objections have been, and will be, raised to this view of the origin of the Actinozoa and Hydrozoa, that they have so descended from stationary beings the Anemones and Hydroid polyps are the living proofs. That these stationary ancestors were sponges, or beings allied to them, is rendered very probable by the harmonious evidences of the structure, development, and fossil remains of the entire group.
We will now leave the Coelenterata, considered as a distinct division of the animal kingdom on account of its simple and characteristic structure, and turn to the other descendants of Amoebae, as seen in Tree I.
GREGARINAE
In the alimentary canal of the earth-worm, of cockroaches, etc., are often found sac-like bodies, called Gregarinae. (Fig, 10.) These simple creatures are nearly destitute of organs, having simply in one part of their body a small nucleus and nucleolus, and a delicate muscular fibre. Nourishing itself by imbibing the juices of the animal in which it lives, slowly narrowing or lengthening its body in different directions,—this motion being probably caused by the delicate muscular fibre just mentioned,—the Gregarina passes its existence. At times, however, this motion ceasing, it takes the shape of a sac. (Fig. 10, b.) The nucleus and nucleolus disappear. The substance of the body breaks up into what have been called pseudo-navicellae, from their resemblance to the Navicula. The contents of the navicellae (Fig. 10, d) are changed under favorable circumstances into Amoeba-like bodies (Fig. 10, e, f, g, h), which, in their turn, become Gregarinae. By looking at Tree I. we see Amoebae, or their haired descendants most likely, divided into four groups:—1st, the Sponges, whose supposed progeny we have treated of as Coelenterata; 2d, Gregarinae, whose Amoeba-like development clearly indicates their ancestry, which we now leave; 3d, Infusoria, whose young show in a marked degree their affinity to the Amoebae and to the Worms; 4th, the Noctilucae, the animals (allied to the Infusoria) causing the phosphorescence of the sea by the immense numbers of them found together in tropical climates.
INFUSORIA.
The animalcula of ditches and ponds are made up, in a great measure, of the microscopical beings called Infusoria from their being found in infusions. One of the most common of these forms is known as Paramoecium; and its structure will serve to illustrate this group. The Paramoecium (Fig. 24) is often compared to a slipper-shaped body of semi-fluid consistency (central substance), inclosed in a rind (cortical layer); the rind running insensibly into the semi-fluid substance. This rind is coated on its outside with a delicate layer (cuticula), bearing on certain parts hairs. (Fig. 24, h.) If the animal remain quiet, we can see a depression in the middle of the body, which leads into the so-called mouth; this opens into a kind of gullet. (Fig. 24, g.), This is all the digestive system the Paramoecium possesses. In certain parts of the body one can observe spaces opening and shutting (Fig. 24, c), and through these spaces certain canals are said to be visible, filled most likely with water. It is said these canals or vessels communicate with the exterior by means of holes in the layers forming the walls of the body. If such a system of vessels have really been found in the Infusoria (and many competent observers are confident that they do exist), they furnish an important proof of the derivation of the Worms from the Infusoria, as this rudimentary water-vessel system is much developed in the Worms. (See Aspidogaster.) The hairs on the outer layer of the Paramoecium serve as organs of movement, and, in making currents of water, drag small particles of food, etc. into the body of the animal. These hairs (Fig. 24, h) are called cilia, and their movement ciliary action. The Infusoria have been divided according to the presence or absence of these hairs into Ciliata and Acinetae (sucking); but the transition between the two seems to be furnished by the bell-shaped Vorticellae, which are said to produce Acinetae, while the Acinetae produce Vorticellae. If this be correct, the Acinetae are only a transition stage of Vorticellae, and all Infusoria are Ciliata or haired.
The Infusoria are not to be distinguished in their early stages from Amoebae. Kolpoda-Hke forms, supposed at one time to belong to the Infusoria, have since been shown to be the young of Turbellarian Worms. The Infusoria seem then to be essentially a transition group; so much so that some naturalists have held that the group is not a distinct division of the animal kingdom, but simply a collection of the young of higher animals. It seems proper to mention now the necessity of learning the condition of the young, or embryonic stage of animals, whose origin we are seeking. Supposing the animal kingdom is really represented by a tree, of which the main branches, twigs, and leaflets are the orders, families, and species into which animals are divided, common features of structure in these groups must not be sought at the ends of the branches which are far apart, but at the point where the branches diverge. To make my meaning clear, take the case of young babies, which look very much alike, but owing to certain hereditary influences, and the effects of a different mode of bringing up, can be readily distinguished later in life. The origin of Worms is not to be sought in comparing a highly-organized member of the group with one of the Infusoria, but by placing side by side a simple worm like the Planaria and one of the Animalcula. The proofs of the Worms coming from Infusoria are furnished by the resemblance of the young of the Soft Worms to existing Infusoria, and the peculiarities of structure common to both.
WORMS.
By looking at Tree III. we see the root of the Worms divides into two branches,—the Soft Worms (Scolecida) and Sac-worms,—the Soft Worms giving rise to the Articulated
Worms, in which are seen the beginnings of the Echinodermata and Articulata, while the Sac-worms are the 
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four divisions of the animal kingdom, an interest is excited in them not exceeded by any other group. One must expect then to meet with great difficulties in making a satisfactory arrangement of these organisms, to meet with structures that foreshadow much more complete types, to find many similar forms as well as individuals so different as to make it nearly impossible to say whether they belong to the Worms at all. The classification as shown in Tree III. is principally that of Prof. Haeckel. It is true that this has met with objections, but it must be remembered that no tree could be constructed which would satisfy every anatomist. And this appears to be nearly in harmony with the views of the most eminent naturalists living. Bearing in mind that the nature of the subject will always make a classification with sharply-defined limits impossible, and as Haeckel says that this attempt, like every other of its kind, is provisional, to be corrected or confirmed by the specialists of the future, we will examine it a little more closely, being obliged, however, through the limits of this essay, to give only a general account. The Scolecida, or Soft Worms, supposed to have descended from Infusoria, divide into two groups,—the Flat Worms (Platyelminthes) and the Round Worms (Nematelminthes). The Round Worms include the horse-hair worms (Gordiaceae), so called from the superstition prevalent among country-people that the horse-hairs are changed into these worms. We find among them the Trichina spiralis, so famous in late years as the cause of the Trichiniasis, a disease resembling typhoid fever and acute rheumatism. The Trichina (Fig. 25) lives in its immature condition in a sac. These sac-like bodies are found in the pig, the meat of which, if not sufficiently cooked when eaten, will cause this very fatal disease, as the sac being destroyed by the juices of the intestine, the young Trichina is set free, and deposits eggs, the embryos from which bore through the viscera of the unfortunate one. The Trichina belongs to the family of thread-worms or Nematoda. Associated with the horse-hair and threadworms is the Echinorhynchus, or Bristly-snout (Fig. 27), so called from the proboscis or snout (Fig 27, a), armed with recurved hooks, being the most striking feature in its organization. The three families of Round Worms live within other animals,—the Trichina in the pig and man, the Echinorhynchus in the flounder, the Gordius, or horse-hair worm, in insects. They are sufficiently alike in their general organization to be the descendant of a common ancestor; their modifications are due to their different modes of life. Turning now to the Flat Worms, we see, according to Tree III., that the Turbellaria are probably the oldest of this group. They are arranged in two families, Dendrocoela and Rhabdocoela, according as the intestine is branched, as in the Planaria (Fig. 28), or straight, as in the Prorhynchus (Fig. 29) or Vortex. The Planaria are found principally in fresh water, but also in the sea, adhering to stones or stems of plants. In the haired covering of their bodies, in their internal organization and embryo forms, the Turbellaria are closely allied to the Infusoria, from which, most likely, they have been derived. From the Turbellaria have retrograded the Trematoda, of which the liver fluke (Distoma) (F'ig. 30) is a common example. By retrograding, I mean that the Flukes have lost organs, from want of use, which they would have retained had they not lived as parasites in the viscera of other animals. The Flukes are found in almost every kind of animal. Imagine a long chain of Flukes living as one individual! and we have the Tape-worm, or Taenia (Fig. 31), as a representative of the Cestoda. Either the Tape-worm has split into Flukes, or the Flukes have colonized and made the Tape-worm, or possibly they are both aborted Turbellarians.
The reproduction of the Tape-worm, long involved in obscurity, is now known to be as follows. There exists in the Pig at certain times a sac-like worm called the Cysticercus; this never progresses; but should the part of the pig containing this worm be eaten by man, this sac will be transformed into the Taenia, or Tape-worm. The Leech and the Peripatus are so nearly allied to the Trematoda that they may be regarded as offshoots of that stem. Turning back now to the Turbellarian Worms with a straight intestine (Rhabdocoela), while noticing that the family represented by the Nemertes is given off here (see Tree III.), we see, in following the stem upwards, its importance, in that it furnishes the origin of the Articulated, or Segmented Worms, with their progeny, the Echinodermata and Articulata. Before leaving the Soft Worms, attention must be called to the system of vessels which is found in most, if not all, of this group. It is well developed in the Aspidogaster Conchiola (Fig. 32), a Trematode worm found in the heart-sac of the fresh-water mussel. The worm is shaped somewhat like a vase. Coursing through its body is seen a system of vessels, beginning as large tubes, which, getting smaller, are finally lost as twigs. This system of vessels is supposed to be the same as that observed in an undeveloped condition in the Paramoecium among the Infusoria, and is found also in the Rotatoria, one of the divisions of the Articulated Worms.
The Articulated Worms include the three groups of the
Gephyrea, Annelida, and Rotatoria, They are called articulated or segmented, from the fact of their bodies being
composed of segments or pieces joined together. This
arrangement is carried to the furthest extent in the Annelida, the Nereids (Fig. 34) numbering as many as hundreds
in their segments. This segmentation is only just perceptible in the Sipunculus (Fig. 33), one of the Gephyrea,
The bodies of the Rotatoria (Fig. 35) are inclosed in a transparent case or hardened skin, which is slightly segmented, and through which the jointed intestine may be
seen. The Sipunculus (Fig. 33) resembles somewhat our
earth-worm, and is found at low-water mark buried in the
sand. When the tide comes in, the Sipunculus, rising to
the surface, exhibits a circle of tentacles surrounding its
mouth: this can be drawn in by the animal and quite concealed. They resemble slightly the Sea-cucumbers, animals included in the Echinodermata. Within a few years
a remarkable group of Gephyrea have been found well
preserved in a fossil condition. They have been called
Mailed Worms, or Phractelminthes, and are considered by
Haeckel as furnishing the link between the Worms and
the Star-fishes. We will refer to them again. The Annelida, or second division of Articulated Worms, are among
the most beautiful of living creatures, of every size and
color, sometimes seen as pretty little white or red worms
swimming about in our fresh-water streams and ponds, or
living, as sedentary organisms, in tubes constructed out of
the sand and other materials found near the sea-shore, or
swimming along by a kind of undulatory movement. The
Nereids (Fig. 34) are composed in some examples of many
hundred joints or segments; each segment is furnished with
a little paddle attached to the side. The blood, rushing
into the little tufts of hair, which are seen on the upper
surface of each segment, gives the animal a brilliant appearance,—the little hairs refracting the light make so many
rainbows. The whole effect of the Nereid gliding through
the sea is so beautiful that it has called forth the admiration of the poets. The Annelida increase their length by
adding segments to those already formed. In this respect
they resemble the Centipedes, etc., which belong to the
Myriapoda, of which we will speak again. The Rotatoria
(Fig. 35), or third division of the Articulated Worms, are
microscopical. They live in fresh or salt water; they are composed of a head and a body; sometimes the head and
body coalesce. The head is furnished with fine hairs arranged in different manners, and when these cilia are in
action they look like wheels. The other end of the body
terminates in a jointed foot. Both the wheel-organs and
foot can be drawn within the case in which the body of the
Rotifer is inclosed. This case resembles that of the Crabs.
The Rotatoria possess the water-vascular system of the
Worms, as described in Aspidogaster. The group is intermediate in its structure between the Soft Worms, the
Annelida, and the Crabs,—the Rotatoria having been considered to belong to each of these groups by different
naturalists. They represent very naturally that point of
the tree where the Soft Worms end and the Crabs begin.
Before leaving the Articulated Worms, the position of the
Artisca must be noticed. They have been called Tardigrada, from the slowness of their movement; they are
usually considered as nearly related to the Spiders; others
have looked upon them as Annelids, while some have considered them as the links between the Soft Worms and
Rotatoria. They are placed, therefore, near these groups,
without assigning to them a definite position. From the
difificulty experienced in their classification, the Rotatoria
and Artisca afford a striking proof of the truth of an evolution of these worms in some such manner. Having
called attention briefly to the Soft and Articulated Worms,
we pass to the last division, the Sac-worms, which includes
the Bryozoa and Tunicata.
The Bryozoa resemble living moss, and are found both in fresh and salt water. When observed under the microscope, this moss is seen to be composed of minute tubes, in which the Paludicella, a Bryozoon (Fig. 37), lives. Though this creature is small, it is more complex than many of the animals we have called attention to. The Paludicella has a mouth, gullet, stomach, and intestine, which are entirely shut off from the general cavity of the body; a great advance in structure as compared to that of the Anemone and Jelly-fish. The Bryozoa are usually classed with the Clams and Oysters (Mollusca); but, from their development from worm-like embryos, they are more justly considered as a group of worms. This view of the position of the Bryozoa is confirmed by the recent discoveries of the worm-like development of the Brachiopoda, the first class of the Mollusca. The Bryozoa are probably the root of the Mollusca, and the connecting link between them and the Worms. The Tunicata, the other division of the Sac-worms, are so called from the animals representing them being inclosed in a bag or tunic. They are a very important group, as showing probably that point where the stem of the Fishes originated. The young Ascidian (Fig. 38, a), one of the Tunicata, resembles a tadpole, and in this condition has quite as much of a backbone (Fig. 38, C) as the Amphioxus (Fig. 39, 40, C), the simplest vertebrate known. The Ascidian, when mature, is like a double-necked vase. The arrangement of the nervous system in the Tunicata differs from that of the Bryozoa, and serves as a distinguishing mark. The curious worm Sagitta (Fig. 36), the only representative of the Chaetognathi, has certain affinities with the thread-worm as well as with the simplest of the vertebrata; it is therefore placed between the two.
The Coelenterata are characterized by the want of specialized structures; that division of labor, so conspicuous in the higher animals, begins to be seen in the Worms, the digestive system in them being more or less developed, together with a rudimentary heart, respiratory and excretory apparatus, and the elements of a nervous system. The Tree of Worms is essentially an intermediate one,—its roots intimately connected with the simplest forms of life, Gregarinae, Infusoria, etc.,—its branches expanding into the rest of the animal kingdom. By looking at Tree IV. may be seen the stem of the Gephyrea, giving origin to the Star-fishes, the simplest of the Echinodermata. In the Annelida will be found the roots of the Tracheate, or tube-breathing Articulata, while the Rotatoria lead equally naturally to the Crabs. The Articulated Worms furnish us with the roots of the Echinodermata and Articulata, while the Sac-worms contain the foreshadowing of the Vertebrata and Mollusca.
ECHINODERMATA.
This division of the animal kingdom includes the Starfishes, Feather-stars, Sea-urchins, and Sea-cucumbers. (Figs. 41, 43, 44, 45.) Every one who has visited the seashore is familiar with the appearance of the star-fish. (Fig. 41.) From the mouth, which lies in the centre of the body, fork out five arms, which run insensibly into each other, the mouth lying in the middle of the space formed by the union of the diverging or radiating arms. The number of arms in some star-fishes is as many even as forty, but the most common number in all the Echinodermata is five. Each arm in the star-fish is composed of movable segments. There exists also a water-vascular system, which terminates externally in suckers, serving as organs of locomotion. There is a rudimentary blood-vessel system, beginning as a tube in the body of the star-fish, and which courses outwards. On the under surface of the arm is found a fine nervous thread, coming from a ring surrounding the mouth, and, finally, at the free end of each arm eyes are found. The arm of a star-fish is, in fact, a worm; not simply resembling one, but structurally the same, the segmentation, the water-vascular system, the nervous cord in each arm of the star-fish being exactly the same as that of an articulated worm. The star-fish has probably been produced through the union of five worms, the worms having united at their posterior ends, since the eyes are seen at the free ends of the star-fish. This interpretation of the structure of the star-fish is not without a parallel among the worms. The Botryllus, one of the sac-worms, is really composed of many little Ascidians living as one individual. There is nothing more extraordinary in five worms living together as a star-fish, than in many little Ascidians living together as a Botryllus. This view of the origin and the structure of the star-fish, first proposed by Haeckel, is in perfect harmony, according to the same author, with the facts of its development. The egg of the star-fish is transformed into a larva, provided with an intestine from the inner part of the body of the larva. Around its mouth appear five distinct layers, which, uniting at their posterior ends, form the body and arms of the mature animal. The same kind of reproduction is seen in the Sipunculi, which are supposed to be indirectly the ancestors of the star-fish, and also in the Nemertian worms, from which, or their allies, the Sipunculi and other articulated worms have descended. Within a few years there have been found a very well-preserved group of fossil worms,—the Phractelminthes, or mailed worms. These are considered by Haeckel to be intermediate between the Sipunculus and the star-fish, they being scarcely distinguishable from the arms of the latter. Through the union of worms, like the Phractelminthes, have the star-fishes been produced. The origin of the Asteridae, or star-fishes, from the worms, is in perfect harmony with the structure, development, and petrified remains of the group. The most striking facts of their economy are explainable on such a theory, but are perfectly meaningless on any other. The star-fishes are probably the ancestors of the remaining Echinodermata. Passing over the Ophiuridae, which differ but little from the star-fishes, we come to the Feather-stars, or Comatula (Fig. 43), which, when young, live in a stationary condition, rooted by a stem; in this immature state it was supposed for a long time to be a distinct animal, known as the Pentacrinus. (Fig. 42.) After a time, however, the stem disappears, and the little creature floats off as the very pretty Comatula. The Comatula is very interesting, as its early Pentacrinus stage is the only living representative of the Crinoids, known commonly as lily stones, and as St. Cuthbert's beads, when segments of the stem alone are preserved. The Crinoids are now extinct but are preserved in great profusion in the fossil state. As the star-fishes in one stage of their existence are more or less fixed, and as the Crinoids have died out, save the only living example, the young of the Comatula, it is possible that the Crinoids are the earliest of the Echinodermata. Haeckel, however, considers the Crinoids as a very ancient offshoot of the star-fishes, adapted to the fixed state of living. Perhaps the Crinoids and star-fishes are the diverging stems of an intermediate group, partaking in its nature of the peculiarities of both these classes. In either case the star-fishes are the progenitors of the sea-urchins, and they of the sea-cucumbers. Imagine the five arms of the star-fish bending down until their free ends, uniting in the middle, form a ball-shaped figure; suppose the empty spaces between the arms to be filled up, and a sphere will be formed. Such are the relations of the star-fish to the sea-urchin, or Echinus. Many intermediate fossil forms have been found connecting these extremes. The sea-urchins, or sea-eggs, are covered with innumerable spines or bristles; hence their name of Echini. (Fig. 44.) These spines are movable, being loosely articulated to little knobs covering the body. When one watches a living Echinus, there may be seen protruding between the spines sucker-like appendages, which serve as a means of progression. If the spines be removed, the body of the Echinus is seen to be a hollow sphere, composed of arms (ambulacral plates) and intermediate arms (interambulacral plates). The arms are pierced with holes, hence their name of ambulacra; through these holes or ambulacra are protruded the sucker-like bodies just mentioned. There are ten ambulacral plates, arranged in pairs, and between these ten interambulacral plates, also in pairs; so, starting from right to left, we have two ambulacral plates united, then two interambulacral plates united, and so on around the shell. The plates are composed of still smaller pieces, these minute plates being formed through the secreting power of the skin, which dips down between the different plates. The shell of an Echinus, with its innumerable pieces, plates, spines, and suckers, is therefore quite a complex organism. If we turn to the interior of the animal, we find the intestine loosely attached, but possessing in its jaws a most complicated apparatus, the so-called Lantern of Aristotle. This lantern-shaped apparatus is composed of five triangular pieces, united at their bases. Crowning the apex of each triangle is seen a tooth, the sides of the triangle being furnished with fine saw-like teeth. The five triangles are kept firm by clamps, and movable through delicate muscles, the whole forming a most efficient, though delicate, arrangement. The nervous system is a simple ring surrounding the mouth, with radiating threads; the organs of reproduction are arranged in a direction corresponding to that of the arms. The structure of the Echinus is essentially radiate. Suppose, however, that an Echinus be drawn out until its length exceeds its breadth, and the mouth be encircled by a wreath of tentacles, we would have then a sea-cucumber, or Holothuria. (Fig. 45.) The Echinodermata agree in the structure of their water-vascular locomotor system, in the peculiar lining or hardening of the skin which incloses their bodies, and in many other respects. They may be regarded, therefore, from their structure and manner of development, as a distinct division of the animal kingdom. The origin of the Echinodermata from the Soft Worms, with which they are most closely allied, is in harmony with the views of the most eminent naturalists of the present day.
ARTICULATA.
We turn now to a consideration of the Articulata, so called from their bodies being composed of distinct pieces jointed or articulated together. They include the Centipedes, known as the Myriapoda, from their numerous feet, the Spiders, or Arachnida, with eight feet, and the Insects, which have only six feet. We will pass over for the present the Crustacea, or last order of Articulata, and confine ourselves for the moment to the Centipedes, Insects, and Spiders (Figs. 46, 48, 49), which agree in breathing by means of fine tubes opening externally. These tubes pass from the walls of the body, getting smaller and smaller, until they are lost in a net-work. In the inside of these tubes, arranged in the form of a spiral, is found a delicate wire, which serves to keep the tubes expanded. This respiratory system is known as the Tracheate: hence the Centipedes, Insects, and Spiders are joined in one division, the Tracheata. A Myriapod, or Centipede (Fig. 46), is composed of numerous segments resembling a Nereid; in fact, it has been observed that the Myriapoda are to the land what the Annelida are to the water. In the Insect (Fig. 48) we can distinguish only three segments, known as head, thorax, and abdomen. There are seen usually in Insects two pairs of wings, less often one pair, and in some cases none are apparent. In the Spider (Fig. 49) the head and breast are soldered into one piece, known as cephalo-thorax. So in the Arachnida we find only two segments. While the Myriapoda, Insecta, and Arachnida breathe by tracheae, the Crustacea, including the Crabs, Lobsters, etc., breathe by gills. They live in the water, and are of every size, shape, and color. The question now arises, Do the Tracheata come from the Crustacea, or are they modified Annelida? Plausible arguments have been advanced for the first of these views; but the second, or that of the Tracheata coming from the Annelida, will be the one here adopted. On seeing for the first time the minute worm resembling an Annelid (Fig. 47), moving through the water, it would surprise the observer to learn that it was the larva, or undeveloped young, of an Insect. The numerous segments of which the immature Insect and Spider are composed gradually coalesce, until finally the perfect Insect exhibits only three pieces, the Spider two. In the young of the Ephemera, or the one-day fly, and of the Libellula, small respiratory tufts are found externally, exactly as in the Annelida, which were alluded to in speaking of the brilliant colors of the Nereis. The existence in the larva of these external respiratory tufts, the manner of the development of the young of Insects and Spiders, furnishes the clue to their origin.
The development, however, of the Myriapoda is just the reverse of that of the Insects, the Centipedes, etc. increasing instead of diminishing the number of their segments. Originally the body is composed of a congeries of cells, segment after segment being added, exactly in the same manner as in the case of the Annelida, with which in structure they closely agree, being adapted, however, to live on land. The Annelida seem then to be the ancestors of the Myriapoda, Insects, and Spiders, the Myriapoda retaining much of the Annelid structure through life, whereas the Insect is an Annelid or Myriapod only when in a larval or undeveloped condition. That the development of the different kinds of insects has been gradual. Geology seems to show, the evidences for which will be brought forward in the chapter on that subject. By looking at Tree IV. the Hymenoptera will be seen very high up; this family includes the Bees, Ants, etc., whose economy has always been the subject of admiration on the part of naturalists. The Articulata are the most complex in structure of the Invertebrata, or animals without a backbone. The nervous system is highly developed, compound eyes are present, the digestive system has various parts, excretory glands and ducts have been discovered, respiration is carried on by the beautiful system of tracheae, and of their powers of jumping, flying, stinging, biting, and making noises every one is aware. The remaining division of the Articulata, including the Crabs, etc. (Fig. 51), though differing greatly in shape, size, etc., are all alike in their early stages. The Nauplius (Fig. 50), or primitive stage of every Crustacean, seems to be more nearly allied to the Rotatoria than any other group of animals. Some of the microscopic forms of the Crustacea, as Cypris, Daphnia, Cyclops, furnish the transitions from the Rotatoria to the Crustacea; indeed, the Rotatoria have been considered as a group of the Crustacea by many naturalists.
MOLLUSCA.
The most striking difference in the Mollusca, as compared with the Articulata, is seen in the entire want of that segmentation which is so apparent in the Insects or Centipedes. The body of an Oyster, a molluscous animal, is a soft mass, and, though possessed of organs, never exhibits the slightest trace of joints, as seen in the higher worms, insects, etc. The nervous system is composed of a few scattered nervous masses or ganglia, there being no distinct chain of ganglia running through the body from head to tail. Indeed, some of the Mollusca have no head, being known as the Acephala. For this reason the Acephala include the Brachiopoda and Conchifera. The Brachiopoda,
Lamp Shells, or Arm-foot Mollusca, are better called Spirobranchiae, as their branchiae, or gills, are arranged in the 
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by very few genera, Lingula, Terebratula, etc. These spiral-shaped gills were supposed to be used as arms or feet: hence their common name of Arm-feet or Brachiopoda. The Conchifera are better named Lamellibranchiata, from the gills in this class being arranged in the shape of plates or lamellae. This class includes the oysters, clams, mussels. The remaining classes of the Mollusca, the Gasteropoda and Cephalopoda, differ from the Acephala not only in having heads, but in many other respects. The Gasteropoda, or Belly-feet, are so called from these creatures moving on that part of their body. This class is represented by the Whelks (Fig. 52), most of the shells on the sea-shore, and the Snails. The common garden-snail is remarkable on account of the great number of teeth which arise from the tongue: as many as twenty-five thousand are said, according to competent authority, to have been discovered. The Cephalopoda (Fig. 54) are distinguished by having long arms or feet radiating from the head; hence the name of this class, which includes the Cuttle-fish and the Pearly Nautilus of the Indian Ocean. Gasteropoda like the Dentalium are so rudimentary as regards the development of the head, that they may be looked upon as offering the transition from the Gasteropoda to the Lamellibranchiata. The Spirobranchiata, or Brachiopoda, in their development and structure are so closely allied to the Bryozoa that perhaps they ought to be considered rather as part of the Worms than as belonging to the Mollusca. The curious affinities of the Brachiopoda, Bryozoa, and Worms are .striking proofs for the view that the Mollusca have come from the Bryozoa, or animals allied to them, and they from the Worms. That the Brachiopoda were the first Mollusca that appeared on the earth is. at once suggested from the enormous number of fossil forms that are found in the oldest rocks. They are so numerous that the name "Age of Mollusca" has been given to the time during which these creatures lived, there being found in great numbers also representatives of the other classes of Mollusca, though not in so great a profusion as Brachiopods. The Brachiopoda of the present seas are restricted to a few genera, the class having nearly died out. This is true, comparatively speaking, of the rest of the Mollusca, though not in so great degree. This fact of the abundance of the Mollusca in the oldest rocks, and of the Brachiopoda in particular, harmonizes with the facts of their structure and development in showing that the group must have branched off from the main trunk (worms) very early in time, and that of the Mollusca the Brachiopoda are the oldest. In some Gasteropoda the feet appear modified as wings, as in Hyalsea and Cleodora, constituting the Pteropoda, and offering the transition to the Cephalopoda. This view is confirmed by the embryos of the Gasteropoda, Pteropoda, and Cephalopoda (Fig. 53) being so very similar. The Gasteropoda breathe both by gills and lungs; examples of the gill-breathing kind are seen in the Whelks (Buccinum) (Fig. 52) often picked up on the sea-shore while the garden-snail will represent the lung-breathing kind. The beautiful Carinaria, with its delicate propeller, is a highly specialized gill-breather. The Cephalopoda are the most highly organized of the Mollusca. The Cuttle-fishes (Fig. 54), with their long arms, are familiar to all who have read the "Toilers of the Sea." In them we find the nervous system well developed, eyes are present, the viscera are large, while blood circulates through arteries and veins. The Cuttle-fish is able to conceal itself through emitting a very brownish-black fluid, which is contained in the so-called ink-bag. They breathe by two gills, but in one genus, the Pearly Nautilus, four gills are present. The Nautilus is the only living representative of myriads of fossil forms, the Ammonites, and is probably the ancestor of the two-gilled Cephalopods. The evidences of ANIMAL KINGDOM. ' Protozoa. ' Monads. Amoebse. - Gregarinse. Infusoria. Sponges. Coelenterata. '-Fishes. Vermes, Worms. Star-Fish es. [ Sea- Cucumbers, Crabs. Insects. [ Spiders. ' Lamp Shells. Oysters. Snails. Cuttle-Fishes. ' Fishes. Batrachia. - Reptiles. Birds. ^ Mammals. Articulata. Centipedes.
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make it most probable that the Tree of the Mollusca is such as represented.
With the Mollusca we leave the Invertebrata, or animals without a backbone, and turn to the Vertebrata.
VERTEBRATA.
This division includes—1st, the Fishes; 2d, the Batrachia (frogs, etc.); 3d, the Reptiles (snakes, etc.); 4th, the Birds; 5th, the Mammals (animals suckling their young). They all possess a backbone, rudimentary in some fishes. This backbone is composed of separate bony pieces known as vertebrae; hence the name of Vertebrata given to the five classes just mentioned. Running through this backbone, spine, or vertebral column, as it is differently called, is seen the marrow or spinal cord,—a nervous cord which expands into the brain, which is inclosed by the skull-bones. Such a structure is never seen in a star-fish, insect, or mollusk. The Vertebrata never possess more than two pairs of limbs. The muscles moving these limbs are attached to bones, which, together with the skull and backbone, form the skeleton. The skeleton is the most characteristic feature of the Vertebrata, and nothing like it is met with in the Invertebrata, called also Evertebrata, that is, without vertebrae. There are apparent exceptions, such as the wing of an insect, among the Invertebrates, which is used like the wing of a bird; but the wing of the insect is only an expansion of the skin, whereas the wing of a bird is always supported by bone. The wing of the insect and that of the bird are said to be analogous, because they are used for the same purpose; they are not homologous, because they have not the same structure. The jaws of a Vertebrate are always parts of the head, never, as in many of the Crabs, modifications of the anterior limbs. There are found at different stages of existence in the Vertebrata, behind the mouth, thickenings; they are known as the visceral arches. (Fig. 178, c.) The spaces between these thickenings finally disappear, so that the interior of the mouth communicates with the exterior. Such a condition is retained in fishes, where we see the water entering the mouth and passing out through the gills. The visceral arches are never seen among the Invertebrates. Such structures as those just mentioned were often quoted as separating the Vertebrates entirely from the Invertebrates; and these differences, as well as others, were so great that they were considered as offering an insuperable objection to the view that the Vertebrata had been developed from the Invertebrata. Recently it has been shown, however, that the Ascidia, one of the Tunicate sac-worms, develops in the same manner as the Amphioxus, the simplest fish known. The young Ascidian (Fig. 38, a) resembles a tadpole, and swims freely about by means of its tail. In this state it has as much of a backbone as the Amphioxus. After it matures it becomes stationary (Fig. 38), remaining attached to objects by means of a rootlike foot. The gulf between the Vertebrates and Invertebrates is now bridged over by this discovery of the identical development of the Amphioxus and Ascidia. The Amphioxus is the only living representative of a group probably long since extinct. This group, allied to the sac-worms in its structure, has in one direction retrograded, the Ascidians, in another progressed, the Amphioxus.
The structure of the skull offers one of the most striking proofs for the common origin of the Vertebrata. If we compare in this respect a fish, turtle, bird, mouse, elephant, and man, we shall find that, notwithstanding the great difference in appearance of these animals, their skulls are fundamentally composed of the same bones arranged in the same manner.
Remembering the different uses of the arm of man and monkey, the wing of bat and bird, the pectoral fins of whale and Ichthyosaurus, the fore limb of horse and frog, one would not believe that they are all identical structures. Nevertheless, Comparative Osteology has shown that the fore limb in every Vertebrate is composed of the same bones, joined in the same way (Figs. 82 to 86), giving attachments to the same kind of muscles, though serving very different purposes, as in the cases just mentioned. There seems to be but one explanation for the existence of these similar parts with dissimilar uses, namely, that the Vertebrata have descended from one common ancestor, and that their posterity, subjected to different conditions of existence, have had their originally similar structures more or less modified.
Embryology has shown that the early conditions of all Vertebrata are alike, so much so that it is impossible to distinguish the young turtle, chicken, dog, and man (Figs. 178 to 181) from one another at certain stages of their existence; and that in proportion as the animals are alike when mature, the longer will their young resemble each other, whereas in those animals which are most unlike when adult, it will be found that their young early indicate difference; and that what is transitory, in the higher animals is retained permanently in the lower,—the higher animals representing at some time the lower. These facts can only be explained by the theory that the Vertebrata are the descendants of a common ancestor. Geology has shown that the earth has experienced great changes through past time,—the sea washing away the land, the land filling up the sea, together with other causes, changing entirely the conditions of existence. Some of the animals living at that day, not capable of resisting such changes, perished, in many cases leaving their skeletons well preserved, as imperishable proof of their having lived. Such are known as fossils, and the study of these ancient remains constitutes Paleontology. This science has shown that forms so different as the Horse and Rhinoceros are linked together by the fossil forms of Coryphodon, Paleotherium, etc.; that the Pig and Hippopotamus represent another group, connected by Anoplotherium and Dichotrum, etc.; that the Fishes and Batrachia form one great division, the Reptiles and Birds another,—forms linking indissolubly together these divisions of the Vertebrates having been discovered in different parts of the world. The structure, the development, and fossil remains all harmonize in proving without a doubt that they are only the modified posterity of a class now extinct, which the Amphioxus nearly represents. That the Amphioxus came from Worms in their structure allied to Ascidians, is highly probable. But, given the Amphioxus, that the genealogy of the Vertebrata is represented by a tree, like that provisionally offered in Tree V., seems to us to follow, without doubt, from the facts of Anatomy, Geology, and Embryology.
FISHES.
The Amphioxus, or Lancelet, is a little animal about two inches long, found generally buried in the sand on the coasts of different seas. It does not possess head, brain, eyes, or limbs, and yet there exists a backbone in a rudimentary condition (notochord), and marrow. Its gills are not like those of fishes, but its branchial apparatus is that of an Ascidian (Fig. 40), confirming the view of the origin of the Amphioxus from the Ascidian Worms, suggested by their identical development. What is the Amphioxus? It seems to be an intermediate animal, a link connecting the Ascidian with the Fishes. The part of the body containing the mouth is usually regarded as the head, and is membranous in structure, which condition is found in fishes at certain periods of their existence. We may designate the class of fishes of which the Amphioxus is the representative, then, as Membranous Fishes. The first step in complexity of structure is presented by the simplest of the gristly (cartilaginous) fishes, Lamprey. They possess a gristly skull, with brain, etc., but there is no lower jaw attached to the skull, their mouth being of the sucking kind (Cyclostomi). There are no traces of limbs as yet; but the sucking fishes have a distinct heart, differing from the Amphioxus, wherein we find only slight dilatations of the blood-vessels. The Myxine, or Hag-fish, and the Petromyzon, or Lamprey, are representatives of this order. In the Chimaera we find a lower jaw, but its suspensorium is still immovable. It furnishes the transition from the Lamprey kind to the Sharks. (Fig. 55.) The Sharks and Ray.s (Devil-fish) are still gristly in structure, but their jaws are very freely movable, and furnished with numerous teeth, which are very characteristic in the different kinds. These teeth are found fossil in great numbers in the early rocks, and prove that the gristly fishes were among the first Vertebrates that appeared in the seas. The Sharks possess two pairs of fins, and their intestine is furnished with valves arranged in a spiral or transversely. We come next to a class of fishes known as Ganoids, that is, shining. In some of these, as in the Sturgeon (Fig. 56), we have the backbone still gristly, while in others, as in the Gar-pike, it is bony. The outer part of the body is covered either with shiny plates (Placoganoids), as in the Coccosteus. Sturgeon, or with shiny scales (Lepidoganoids), as in the Gar-pike. It is by means of these shiny plates and scales, as well as the whole fish, found in great profusion, well preserved in the early rocks, that we know that the Ganoids are very old fish, and that they existed in great numbers in the early ages of the earth; whereas at the present day the Ganoids are represented only by half a dozen kinds, the Sturgeon, Gar-pike, Polypterus, etc. The Ganoids agree with the Sharks in the structure of their heart and optic nerves, and Polypterus has the spiral intestinal valve of the Sharks, but their skulls have true bones, and they possess a gill-cover (opercular appendage). In this respect they agree with the Teliosts, or bony fish, of the present day. If we compare the tail of one of our common fish, a Cod, or Shad, or Perch (Fig. 57), with the tail of a Shark (Fig. 55) or a Sturgeon, we see that in the Perch the end of the tail divides into two equal parts, whereas in the Sturgeon (Fig. 56) the tail divides unequally. The unequally-ending or heterocercal tail is characteristic of these Ganoid fishes, and the equally-ending or homocercal tail is equally characteristic of our common fishes; but the tail of the embryo of one of our common fishes ending unequally is as heterocercal as the tail of the Sturgeon. The embryo fish is composed also of gristle, as regards its backbone and skull. Hence the transitory stage or embryo condition of our common fish represents the permanent stage of the Sharks and Sturgeons,—a striking proof of the truth of the view that the Bony Fish, or Teliosts, are the posterity of the Ganoids and Sharks. Fossil Ganoids, like the Ccelacanthes, Holoptychii, Coccolepis, and Amia of the present day, were probably the ancestors of fishes in which the air-bladder has a duct, as seen in the Carp, Herring, Salmon, while they were probably the progenitors of those fishes in which the duct is absent or rudimentary, as in the Perch, Cod, Sole. We turn now to a consideration of the remaining order of fishes, known as Dipnoi, and represented by the Lepidosiren (Fig. 59) of South America and the African rivers. During the rainy season in Africa, large tracts of land are overflowed by the rising of the rivers. With the retreating waters are carried most of the fish; but the Lepidosiren remains, and, burrowing in the mud (hence its name of mud-fish), constructs a hole, leaving only a small opening for the passage of air. Exuding a sort of slime as a covering for its body, and remaining in this torpid condition, it breathes by means of lungs until the return of the water, when it rises to the surface and breathes by its gills. Hence the Lepidosiren is both Fish and Amphibian. As regards its respiration, it is truly an Amphibian. It differs from the ordinary fish in the structure of its heart, which is composed of three chambers in the Lepidosiren and Amphibian (Siren, Frog, etc.), whereas in the Fishes the heart is composed of only two. The Lepidosiren and Polypterus both have the spiral valve in the intestine, so characteristic of the Sharks. The air-bladder in Polypterus, and the lungs of Lepidosiren, are the same in their structure as regards the arteries of these parts and the relations of their air-ducts. The form of the brain is the same in Lepidosiren and Polypterus. The skull of Lepidosiren is intermediate between the gristly and the bony fishes. The backbone is gristly; in this respect it agrees more with the Fishes than with the Amphibia. In the structure of the liver apparatus and the limbs it agrees with the Amphibia. What is the Lepidosiren? Is it a Fish, or is it an Amphibian? The Lepidosiren is the intermediate form linking the Fishes and Amphibia together, and is more closely allied to the Ganoid Polypterus than any living fish. Among the fossil Ganoids the Coccosteus would represent the Lepidosiren should its skeleton be fossilized. The Ganoid fishes, although intermediate between the Sharks and common bony fishes or Teliosts, have many affinities with the Amphibia: thus, the Amia and Lepidosteus, among the Ganoids, have the air-bladder filled with air-vesicles and resembling strongly the lung of the Amphibia. So the back-bone of the Lepidosteus in the ball-and-socket joint of the pieces forming its spine differs from all Fishes, and agrees with many of the Amphibia. The structure of the Ganoids,
Lepidosiren, and Amphibia seems to warrant the conclusion 
one end and the true Reptiles at the other.[2] Among the most perfectly preserved fossils are the Ichthyosauri (Fig. 58) and Plesiosauri. They seem, on the whole, to be more allied to Fishes in the structure of their paddles, backbone, etc., and Amphibia in other respects, than to true Reptiles, and must have diverged early from the main fish stem. Their position is somewhere near the Lepidosiren, Archegosaurus, and Labyrinthodon stems.
BATRACHIA.
The Batrachia, or Amphibia, as they are often called, include the Frogs, Salamanders, Siredons, Tritons, Ccecilia, and the fossil Archegosaurus and extinct Labyrinthodons, They breathe by gills, at least at some period of their existence, and in this respect agree Mnth Fishes. Some of the Batrachia, as the Siren, Proteus (Fig. 61), and Menobranchus, retain their gills throughout life, and for this reason are called Perennibranchiata; whereas others, as the Frog (Fig. 64), lose them after passing through their tadpole stage. (Figs. 62, 63.) The Batrachia present two types for consideration: in the one we find the body covered over with bony plates or scales, as in the extinct Archegosaurus (Fig. 60), Labyrinthodons, and Coecilia; in the other, the body is naked, as in the Siren, Salamander, and Frog. A considerable advance in structure is seen on comparing the Batrachia with Fishes; but the Lepidosiren links together the Ganoid Fishes with the Frog division of the Batrachia, while the Archegosaurus leads up from the Ganoids through the Labyrinthodon to the Coecilia.
The Archegosaurus (Fig. 60), when first discovered, was supposed to be a fish; but more careful study has shown equal affinities with the Batrachia. The Labyrinthodon is another extinct form, with a very large skull, sometimes three feet in length and two in breadth. The bones of the skull in Archegosaurus and Labyrinthodon recall strongly the skull of the Gar-pike and Sturgeon. The persistence of a gristly backbone in Archegosaurus is the same as in the Sturgeon. The Lepidosiren and Archegosaurus agree in the structure of their backbone, and the retention of the branchial (gill) arches, and in the manner in which their skulls are joined to the backbone (absence of occipital condyles). The teeth are of the same kind (labyrinthic) in the Gar-pike, Archegosaurus, and Labyrinthodon. The large throat-plates in Archegosaurus are like those of Megalichthys (fish) and the Gar-pike; whereas, in the structure of the jaws (Fig. 60, B), certain bones called hyoid, and in the shoulder-girdle and extremities (Fig. 60, C, D), we see striking proofs of the relation of Archegosaurus to Batrachia like Proteus, whose jaws and extremities are (Fig. 60, b, c, d) very like those of Archegosaurus. The Archegosaurus form is the link between the Fishes and Batrachia, on the one hand leading through the Labyrinthodon to the Ccecilia, on the other to the Frogs through Siredon forms. The Archegosaurus came either directly from the Ganoids, or indirectly through the Lepidosiren. Supposing the latter view to be the true one, then the Ganoids divided into two branches, one being transformed into the common fish, the other giving rise to Lepidosiren-like forms, these leading insensibly to the Archegosaurus, the earliest of the Amphibia, the long type represented by Labyrinthodon and Coecilia forming one stem, the Siredon and Frogs, naked Amphibia, the other. The naked Batrachia are among the most striking proofs of the truth of the Derivation theory, as the links are all living. The Siredons and Proteus (Fig. 65, 61) strongly resemble the
Lepidosiren and Archegosaurus; they have tails and external 
the tails are retained, but the external gills are lost; finally, the Frog has neither gills nor tail, but the tadpole or the immature frog has both, so that in one stage of its existence (Fig. 62) the Frog is a Siredon, later it is more like a Salamander, finally it (Fig. 63) resembles neither.
REPTILES.
Leaving the Fishes and Batrachia, and turning to the Reptiles, we see that the Fishes and Batrachia breathe by means of gills (the Batrachia at some stage of their existence), whereas the Reptiles always breathe by lungs, as a bird or four-footed creature. The Vertebrates have been divided by some naturalists, for this reason, into the two divisions of the gill-breathing. Fish, Batrachia; and the lung-breathing, Reptiles, Birds, Mammals. The reptiles, birds, and mammals agree with each other in possessing, during embryo life, an amnion and an allantois (see Embryology); the fishes and batrachia never, at any stage of their existence, possess either. The amnion is a transparent sac filled with a fluid (liquor amnii) in which the young bird or reptile floats. The allantois is a vesicle starting from the under part of the body of the bird or reptile, and filling up the interior of the egg. The allantois is filled with blood-vessels; and as the porosity of the egg-shell permits the passing out of the pernicious carbonic acid, and the passing in of the life-sustaining oxygen, it is by means of the allantois that respiration is effected. The visceral arches in the young bird and reptile (Figs. 179, 178 c) are converted through growth into part of the jaws and part of the organ of hearing; the visceral arches in the fishes are modified into gills. We see, therefore, that a great progress has been made on comparing the structure of the gill- and lung-breathing division of Vertebrates.
The Reptiles of the present day include, 1st, the Lacertilia (Monitors, Chameleons, Wall-lizards); 2d, the Ophidia (Snakes); 3d, the Crocodilia (Crocodile, Alligator); 4th, the Chelonia (Turtles); and numerous extinct forms. As the reptiles that live at the present day are but a small portion left of those that have once lived, and as these extinct forms are not always entirely preserved, and from the nature of petrifaction very little of their soft parts can be known except from analogy, naturally the ancestors of the reptilian class have not been positively determined. Premising that the tree of the Reptiles, like all other such trees, is only a provisional one, the following line of descent is offered with diffidence. As long ago as 1710 the Proterosaurus—which, when translated, means "first lizard"—was described by Spener, a physician of Berlin. Since that time other reptiles, allied to Proterosaurus, have been discovered, as Belodon, Paleosaurus, etc., which have been classed together as Thecodonts. The skeleton of Proterosaurus resembles most closely, among living reptiles, that of the Varanus, the large African lizard; but among the Thecodonts have been found also scales of a crocodilian nature, so that the Thecodont group seems to be the forerunner in the Proterosaurus of the lizards and crocodiles, while the Paleosaurus and Belodon are the first of a series leading to the Dinosauria. The Snakes are probably an offshoot of the Lizard, to which they are closely allied; the Sepidae (Fig. 68), among the Lacertilia, leading to the Anguidae (Fig. 67) among the Snakes. The Anemodonts, of which the Pterodactyle is a remarkable representative, lead to the Turtles through forms like Rhynchosaurus. The Dinosauria were represented by huge reptiles like Iguanodon and Hadrosaurus, of which some were more than thirty feet long. They are very interesting on account of their affinities to birds. The different orders of Reptiles seem to have branched off from a common stock represented by Thecodont forms, which are allied to the Salamanders among the Batrachia. Until some better theory of the origin of Reptiles is offered, this one will be provisionally accepted.
TREE VI.
Oscines. (Singers.) i Clamatores. (Crow, etc.) Raptores. (Birds of Prey.) Scansores. (Climbers.) Doves. Dodo. Pigeons. Leaving egg blind. Grallatores. (Waders. ) Rasores. Natatores. (Scratchers.) (Swimmers.) Peneiopidse. Ostrich. Rhea. Emeu. Saurophalli. (Birds leaving egg seeing.) ( Reptile-like Birds.) Archeopteryx. (Most ancient Bird.) I (Bird-like Reptiles.) Compsognathus. Apteryx. . I Dinornis. I Cassowary. Dinosaurian Reptiles.
BIRDS.
Although the class of Birds presents great variety as regards differences in shape, color, manner of living, etc., they can be represented by the types of swimming (Fig. 74) (duck), wading (Fig. 75) (snipe), scratching (Fig. 76) (chicken), singing (Fig. 78) (lark), flying (humming-bird), climbing (woodpecker), birds of prey (Fig. 77) (eagle), and running birds (Fig. 73) (ostrich). This classification is only useful as a means of superficially arranging the eight thousand species of birds known to naturalists. An examination of the skeleton, however, offers us only three orders, the first of which, possessing a long bony tail, is represented by a single genus, the extinct Archeopteryx; the second, in which the breast-bone is furnished with a ridge or keel, whence the name Carinata, includes ordinary birds; the third order has no keel on its breast-bone, and is represented by the Ostrich, Emeu, etc. Although birds differ considerably from reptiles, yet they agree with one another in very many important characters, such as peculiarities in the structure and articulation of the skull and backbone, and in the structure and articulation of the lower jaw with the skull. The brain, heart, and great cavities of the body are alike in these orders. These common characters, as well as others, seem to warrant the union of birds and reptiles in a natural group, of which the birds are the most advanced in specialized structure. This view is confirmed by the embryology and fossil remains of birds and reptiles. The Ostrich family, among existing birds, perhaps approaches nearest the reptiles. Nevertheless, the gap was very great between these classes. By the discovery of the existence in past time of the Dinosaurian reptiles and Archeopteryx, this gap has been bridged over. The Archeopteryx offers the only known example of a bird with a bony tail, that is, among adult birds, as birds, when in an embryonic condition, have quite as much of a tail as a turtle. The bones of the hand (metacarpal), in the Archeopteryx, are not soldered together (anchylosed) as in birds, but remain distinct. In these marked and important peculiarities the Archeopteryx agrees with reptiles and differs from birds. The Archeopteryx, in fact, is
a reptile-like bird. In the Compsognathus, a fossil reptile, 
we find the bones of the foot (first series of tarsal bones) soldered together and with the tibia as in birds, whereas in most reptiles these bones remain distinct. The Compsognathus in this, as well as in other respects, is a very bird-like reptile. The Compsognathus is considered by some anatomists to belong to the order of Dinosaurian reptiles. The Dinosauria agree in many respects with the Ostrich family, perhaps being more nearly allied to them than to any other order of birds. They used their hind limbs only as a means of progression; in this respect they resembled birds more than reptiles; their feet were terminated with claws (Fig, 70), and the curious arrangement by which the bones of the leg (tibia and fibula) are united to those of the foot (astragalus) in birds seems to have been exactly the same in these huge reptiles. The bones of the leg of the embryo bird (Fig. 72) are like those of the adult Dinosaurian and Reptile. (Figs. 70, 69.) There is good evidence for supposing that the muscles moving the foot had the same disposition in some of the Dinosauria as exhibited in the chicken. According to a high authority on this subject, " if the whole hind quarters from the ilium (haunch bone) to the toes of a half-hatched chicken could be suddenly enlarged, ossified, and fossilized as they are, they would furnish us with the last step of the transition between birds and reptiles, for there would be nothing in their characters to prevent us from referring them to the Dinosauria." And according to the same high authority (Prof. Huxley), if certain bones of the Hypsilopodon had been found alone, they would have been certainly described as belonging to a bird. The idea of these huge Dinosaurs having so much in common with birds is not a mere theory, but a truth, whatever inferences may be drawn from it, as the bones of some of them (the Megalosaurus, etc.), at least in reference to the posterior extremities, are absolutely the same as those of a bird. The Compsognathus, in the great number of neck-bones (cervical vertebrae), in the light character of the bones of the head, together with the structure of the foot, is probably the most bird-like of reptiles, and is to be considered together with the Megalosaurus, Hypsilopodon, and other Dinosauria, as the representative of the ancestors of birds. It is not possible to say exactly which Dinosaur was the progenitor of birds, but as the Compsognathus is the most bird-like of reptiles, we take it as our example. The idea of birds coming from reptiles is suggested by the following facts: 1st, existing birds have much in common with existing reptiles; 2d, birds and reptiles, at an early period of their existence, cannot be distinguished from one another, as in the case of the embryo chicken and turtle; 3d, the most ancient bird (Archeopteryx) is very reptilian, while certain extinct reptiles (Dinosauria) are very bird-like. These statements are facts accepted by naturalists, whether they are evolutionists or not; they seem to us to harmonize in warranting the conclusion that birds are modified reptiles. Among existing birds the Ostrich family is particularly interesting, as they seem to be the representatives of a class once much larger. The Ostrich is found in Africa, the Rhea in South America, the Emeu in New Holland, and the Cassowary in the East Indies. They are known as the running birds, their wings being 'quite rudimentary. With the Ostrich family is generally placed the Apteryx of New Zealand. It is a little bird, the miniature of the gigantic Dinornis, which stood nearly twice as high as the Ostrich. The Dinornis has died out very recently, if it be indeed extinct. This very wide distribution of the Ostrich family suggests that the order was once much larger, extending all over the earth, and that the present representatives are the sole survivors. This seems more natural than to suppose that the Ostrich, Rhea, and Cassowary appeared independently in parts of the earth so remote. If this view be correct, then the running birds are older than the ordinary birds; this harmonizes with the fact that the running birds are of existing birds the most nearly allied to the Dinosauria, the supposed ancestors of birds. The birds called Penelope (Cranes) are generally classed with the scratching birds, but they seem to be more nearly allied to the Rhea, Emeu, and Cassowary, and are joined with them, and called by Haeckel Saurophalli. The tree of descent would be then Dinosaurian reptiles, represented by Compsognathus, etc. Changed into Archeopteryx-like forms, the most ancient of birds, the modified posterity of the Archeopteryx would be represented by the Penelope, Rhea, Emeu, Cassowary, having three toes. The African Ostrich, having only two toes, is probably more modern than the three-toed South American kind, or. Rhea. The Cassowary, through the Dinornis, leads up to the Apteryx, while the Emeu and its posterity seem to have remained unchanged. The Penelopidse are probably the ancestors of the scratching birds, to which are nearly allied the recently extinct Dodo of the Mauritius, and the doves and pigeons. The ducks seem to form the transition between the Saurophalli and the swimming birds, though it must be remembered that the Penguin in its separated metatarsals (bones of the foot) would indicate an ancient bird. The swimming birds gave rise probably to the wading birds. All these birds, excepting the doves, leave the egg in a condition fitted to nourish themselves; whereas the doves, pigeons, etc., and their descendants, leave the egg blind, and are nourished by their parents. The pigeons and doves, descending through the Pteroclidae from the scratching birds, probably divided into two branches, the Clamatores (crows, etc.) and the Climbers (woodpecker, etc.); the Clamatores were gradually improved into our singing birds, and the climbing birds into birds of prey.
MAMMALIA.
ORDER, Ornithodelphia, Monotremata. Ornithorhynchus. Didelphia. Marsupialia. Opossum. ' Carnivora. l^og. Cetacea. Whale. Prosimise. Lemur. ' Simise. Man, Monkey. Rodentia. Beaver, Rat. Hyracoidea. Hyrax. ° ^ P Proboscidea. Elephant. Cheiroptera. Bat. Insectivora. Hedgehog. Edentata. Sloth. Ungulata, Horse, Pig. Sirenia. Sea-Cow, Dugong.
MAMMALIA.
The class Mammalia is so called from the females suckling their young, and includes the domestic animals as well as many other less well-known forms. The mammals differ from the reptiles and birds in many important characters, as in the manner in which the skull and backbone are joined together (two condyles instead of one), in the simple structure of the lower jaw, it being composed of only one piece on each side in the Mammalia, whereas in the reptiles and birds it is made up of several. The skin of the mammals is covered more or less with hairs, never with feathers, as in birds. They bring their young into the world living, and nourish them for a longer or shorter time with milk. These peculiarities, as well as others, separate the birds and reptiles from the mammals. The Mammalia can be divided into three sub-classes, each of which
offers well-marked peculiarities, which serve to distinguish 
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so called from the terminal arrangement of the abdominal viscera being the same as that of Birds and Reptiles. The Ornithorhynchus and Echidna are the only representatives of the Ornithodelphia, and are limited to Australia and Tasmania. They seem to be the survivors of a class once much larger, and now extinct. The Didelphia include the Kangaroos (Fig. 80), Wombats, Opossums, etc. With the exception of the Opossums, they are also confined to Australia and the adjacent isles. The most striking feature in this sub-class is the pouch in which the young are protected in their helpless condition. The third sub-class of Mammalia, the Monodelphia, contains as many as twelve orders, of which the following will serve as examples: Dog, Whale, Lemur (a sort of Monkey, one of the Prosimiae), Ape, Man, Beaver, Hyrax, Elephant, Bat, Hedgehog, Sloth, Horse, Pig, and Sea-cow. The names of the orders of which these animals are examples may be seen in the accompanying diagram. It is to be understood that there are many other examples of each order, which want of space prevents us from inserting. Of all Mammalia, the Ornithorhynchus (Fig. 79) and Echidna approach nearest the Birds and Reptiles, not only in the characteristic arrangement of the abdominal viscera, but also in the skeleton. The collar-bone (clavicle), the breast -bone (sternum), and the coracoid process of the shoulder-blade (scapula) form together a fork-shaped bone similar to that of Birds and Lizards. This fork-shaped bone is not present in the other Mammalia. The ribs in the Ornithorhynchus offer the same arrangement as seen in the Crocodile, while the skull is very bird-like in the articulations of its bones, and in the arrangement of the organs of hearing (semi-circular canals) and the nerve of smell.
While, therefore, there can be little doubt that the class represented by the Ornithorhynchus is the posterity of the Sauropsida (birds and reptiles), yet the imperfect knowledge of the development of the Ornithorhynchus, and the total absence, so far, of fossil remains, make it impossible to designate which particular order of Sauropsida ought to be considered as the progenitor of the Ornithorhynchus, and through it of the rest of the Mammalia. In seeking the origin of the pouch-bearing mammals, we meet with the same difficulties, though not in the same degree. The Marsupialia are intermediate, in many respects, between the Monotremata (Ornithorhynchus) and the ordinary Mammalia. In the present state of our knowledge, it seems more advisable to regard simply some of the Monotremata as the ancestors of the Marsupialia than to attempt to designate which particular one was that ancestor, or exactly the manner of their development. That the Marsupialia came after the Monotremata (Ornithorhynchus, etc.) seems most probable, from the fact of their young, in their transitory condition, offering the arrangement of the viscera so characteristic of the Ornithorhynchus, while in their adult condition they agree with the ordinary Mammalia. The pouch-bearing Mammalia offer examples of meat and vegetal feeders, as well as of the leaping, burrowing, and climbing kinds.
So striking is the parallel between the different kinds of pouch-bearing mammals and the different orders of the ordinary Mammalia that many naturalists seem disposed to consider the different orders of the ordinary mammals as having come directly or indirectly from the corresponding kinds of pouch-bearers; that is, extinct Marsupials, like the grazing and browsing Kangaroo, were the ancestors of the orders represented at the present time by the Pig and Horse. Pouch-bearers like the Opossums, using their big toe as a thumb, gave rise to Monkeys, improperly called four-handed; while the meat-eaters (Dog) are the posterity of extinct Marsupials allied to the Thylacinus, This view has much in its favor, as the transition from the pouch-bearers to the ordinary mammals is very gradual, as, for example, the smaller Opossums lead up to the Insectivora (Hedgehog), the Wombat to the Beaver, etc. The fact of Australia, with few exceptions, containing only the reptile-bird-like and pouch-bearing mammals at the present day seems to confirm the view of the ordinary mammals coming from the pouch-bearers in some such way; for while we find in other parts of the world fossil remains of Marsupials, with the exception of the Opossums, the living pouch-bearers are found only in Australia and the adjacent islands, the fossil remains being Marsupials, but of a much larger size. Australia seems to offer us a living picture of what Europe once has been. Just as in Europe and other places, among the ordinary mammals, we have various kinds preying on each other, so we find the same thing among the existing pouch-bearers in Australia; and this relation existed also in past time. The Diprotodon (a gigantic Kangaroo) was warred upon by the Thylacoleo, a meat-eater of the size of a lion. Supposing this view of the origin of the ordinary Mammalia to be correct, the number of branches descending from the pouch-bearers will depend on the view taken by naturalists of the affinities of the different orders. Although there are twelve orders in the ordinary Mammalia, some of them seem directly or indirectly to be more nearly related than others. Thus, the order of odd-toed (Rhinoceros, Tapir, Horse) and that of even-toed (Pig, Hippopotamus, Sheep, Deer) are joined in one group, the Ungulata, or hoofed animals: they would represent one stem. The Half-Apes, the gnawing animals, with the Hyrax and Elephant, the insect-eaters, the bats, and true Apes, with man, would make a second stem. The meat-eaters (Lion, Fig. 81, Dog, Seal, etc.) may represent a third stem; while the Edentata, or animals without incisor (front) teeth, like the Sloth and Ant-eaters, which have no teeth at all, seem to make a fourth stem.
Without attempting a detailed account of these orders, we will try to call attention to the most important peculiarities connected with their organization and possible origin. The group of odd-toed (Perissodactyla) is so called from its representatives having an uneven number of toes, the Rhinoceros three, the Tapir three (at least in hind foot), the Horse one. These animals, however different in appearance externally, agree further in the structure of the skull and teeth, the number of pieces in the backbone (not less than twenty-two dorso-lumbar vertebrae), the simple stomach, and the peculiar character of the intestine (caecum). These animals are linked together by fossil forms, the whole series forming a very natural group, the odd-toed, of which the Paleotherium (Fig. 151) is the oldest. The Artiodactyle or even-toed group—the Hippopotamus, etc., having four toes, the Cow, Sheep, Deer, etc., two—agree in the structure of the skull and teeth, and in the number of dorso-lumbar vertebrae (nineteen), while some of them in the complex digestive system form the sub-group of the Ruminants, in which there exist three or four stomachs, one of which serves to hold the food until it is chewed a second time, while in the Camel and Llama the second stomach is modified to hold water. The living even-toed animals, linked together by extinct forms, make the second natural order of the Ungulata. The oldest even-toed is the Anoplotherium (Fig. 152). In the age preceding that in which the Anoplotherium and Paleotherium appeared there lived the Lophiodon, Coryphodon, Pliolophus, etc., animals which, in their dentition, seem to have combined the peculiarities of both the even- and the odd-toed orders. They are considered to be the common ancestors of the Ungulata, and the posterity of the Diprotodon and Nototherium, animals allied to the browsing Kangaroo. The line of descent would be: Marsupials like the Diprotodon gave rise to the Lophiodon-like animals; they divided into the Paleotherium and Anoplotherium, the roots of the odd- and even-toed orders. The Rhinoceros, of living even-toed, is the most ancient, the Horse the most modern, the Tapir being intermediate. The links binding the Horse and its ancestor, the Paleotherium, are furnished by the Hipparion and Anchitherium; these extinct animals, in , the structure of their teeth and feet, offering us a picture of what we see now in the Horse only in an embryonic condition: that is, the Horse, at one stage of its existence, is an Hipparion, while still earlier it is an Anchitherium. While the Paleotherium, descending from the Lophiodon, originated the odd-toed order, the Anoplotherium, coming from the same stock, divides into the Xiphodon and Anthracotherium branches. The Xiphodon, together with the Dichodon and Dichobune, were the earliest of Ruminants, of which there are the branches of the hollow-horned. Cow, Sheep, Goat, Antelope; the solid-horned. Deer, Giraffe; while the Camel and Llama, resembling each other in many respects, are represented by a separate stem. The Anthracotherium, the other branch coming from the Anoplotherium, divides into the stems of the Pig and Hippopotamus; nearly allied to the latter are the Sea-cow and Dugong, large herbivorous animals, found in bays and at the mouths of large rivers.
Leaving now the stem of the hoofed animals, or Ungulata, and turning to that of the Monkeys, etc., we find that many of the Prosimiae or Half-Apes are found in Madagascar, from which island the order spreads to the East Indies and Africa. These Half-Apes were regarded for a long time as Monkeys, but they differ from the true Monkeys in the number and structure of their teeth, as well as being characterized by the claw on the second toe. The different kinds of Half-Apes indicate and are the transitions to the beginnings of other orders. Thus, the Galeopithecus, flying Lemur, is a perfect link between the Half-Apes and Bats, the Cheiromys foreshadows the order of gnawers, resembling in appearance, as well as in the structure of the teeth, the gnawers (Rat, Squirrel) more than the Half-Monkeys. The short-footed Lemurs (Makis and Loris) are more like the true Monkeys, while the long-footed Tarsius is allied to the Insect-eaters (Cladobates, Hedgehog). A century ago the half-apes, gnawers, bats, and apes, with man, were joined together by Linnaeus, and called Primates; and modern research seems but to have confirmed his generalization. Strange as it may at first appear, the Elephant is more nearly allied to the gnawers (Rodentia) in its skeleton, as well as in many other respects, than to any other order of the Mammalia. One can hardly conceive of a mouse and an elephant having anything in common; but it must be remembered that size has nothing to do with community of structure, and that there are Rodents, like the Capybara, as large as a dog. The position of the little Hyrax in the animal kingdom has been a constant subject for discussion since the days of Cuvier. According to some authorities, it stands near the Elephant and Rodentia, while others place it near the Tapir, among the odd-toes. I follow Haeckel in placing it near the Elephant. The true apes have descended from the half-apes, but as, zoologically, man and the true apes are not to be separated, we reserve for a separate chapter the consideration of the Simise, the highest order of the Mammalia. The half-apes are probably the posterity of extinct Marsupials allied to the opossums. The Carnivora or meat-eaters include the lion (Fig. 81), dog, cat, bear, walrus, seal, as well as other animals. They have so many characters in common, and differ so much from all other orders, that we regard them as a distinct stem, descending from Marsupials like the Thylacinus or dog-headed opossum. The transition from the seals to the whales (Cetacea) is made through the extinct Zeuglodon, which combines perfectly the peculiarities of both these orders. It must be mentioned, however, that Haeckel considers the whales, etc. as being more nearly allied to the Sea-cow, etc. With the exception of the Ant-eater of South Africa and the Pangolin common to Asia and Africa, the Edentata, so called from many of them having no front or incisor teeth, and some no teeth at all, are confined to South America, represented there by the Sloths, Armadillos, and Ant-eaters. The Sloths differ from all other Mammalia in having more than seven bony pieces in the neck, there being nine cervical vertebrae in the three-toed Sloth, and in the great number of ribs (twenty-three) in the two-toed Sloth, as well as in the bird-reptile arrangement of the viscera, agreeing in this peculiarity with the Ornithorhynchus; in many other respects the Edentata show a low grade of organization. From the wide geographical distribution, the gradual extinction, and the reptile-bird-like organization of the Edentata, we consider them as the survivors of an order which must have diverged very early from the main stem of the Mammalia. Gigantic fossils belonging to this order, like the Megatherium, Megalonyx, Mylodon, have been found in remote parts of the earth, showing the extent and size of the order in past time. The Megatherium (twenty-two feet in length) combines the head of the Sloth with the backbone and extremities of the Ant-eater. In the present state of Paleontology and Embryology, it is impossible to indicate the progenitors of these extinct Edentates.
With the Edentata we leave the Mammalia, and, for the present, the structure of the animal kingdom. We have endeavored to show—following principally Haeckel—that there is a main trunk of life, beginning in the Monads, ending in Man; here and there large branches are given off, terminating in twigs and leaflets. Allusions have been made to extinct animals, often forming an essential part of these branches. The relation, in time, that these extinct animals bear to each other, and to those now living, will be treated of in the chapter on Geology, while the Development or Embryology of the groups will be more detailed in the chapter on that subject. Though Geology and Embryology confirm the view of the gradual production of a tree of life, it seems to us that the structure of animals, without any other evidence, suggests such a conclusion, though we were never able to show the cause of it. Few astronomers after the time of Kepler doubted that the orbits of planets were ellipses: it remained for Newton to show that the attraction of gravitation was the cause of the ellipse; Lamarck and others have been to Biology what Kepler was to Astronomy; if future biologists confirm Darwin's views as to the cause of the evolution of life, as Laplace, Lagrange, D'Alembert, and Euler placed the Newtonian theory on a more secure foundation, then Darwin will be, as he has been already called, the Newton of Natural History.