We shall pa.s.s over the period of Evolution in which Matter was evolving into finer and still finer forms, until at last it reached a degree of vibration capable of supporting that which we call "life." Of course there is "life" in all matter--even in the atom, as we have shown in previous lessons. But when we speak of "life," as we now do, we mean what are generally called "living forms." The Yogi Teachings inform us that the lowest forms of what we call "life" were evolved from forms of high crystal life, which indeed they very much resemble. We have spoken of this resemblance, in the previous lessons of this series. And, so we shall begin at the point where "living forms" began.

Speaking now of our own planet, the Earth, we find matter emerging from the molten state in which it manifested for ages. Gradually cooling and stratifying, the Earth contained none of those forms that we call living forms. The temperature of the Earth in that period is estimated at about 15,000 times hotter than boiling water, which would, of course, render impossible the existence of any of the present known forms of life. But the Yogi Teachings inform us that even in the molten ma.s.s there were elementary forms that were to become the ancestral forms of the later living forms. These elementary forms were composed of a vaporous, peculiar form of matter, of minute size,--little more than the atoms, in fact, and yet, just a little more advanced. From these elementary forms, there gradually evolved, as the Earth cooled and solidified, other forms, and so on until at last the first "living form" manifested.

As the globe cooled at the poles, there was gradually created a tropical climate, in which the temperature was sufficiently cool to support certain rudimentary forms of life. In the rocks in the far northern lat.i.tudes, there are found abundant traces of fossils, which goes to prove the correctness of the Yogi Teachings of the origin of life at the north pole, from which the living forms gradually spread south toward the equator, as the Earth's surface cooled.

The elementary evolving life forms were of a very simple structure, and were but a degree above the crystals. They were composed of identically the same substance as the crystals, _the only difference being that they displayed a greater degree of mind_. For that matter, even the highest physical form known to us today is composed of simple chemical materials. And these chemical materials are obtained, either directly or indirectly, from the air, water, or earth. The princ.i.p.al materials composing the physical bodies of plants, animals, and man, are oxygen, carbon, hydrogen, nitrogen, with a still smaller proportion of sulphur and phosphorus, and traces of a few other elements. The material part of all living things is alike--the difference lies in the degree of Mind controlling the matter in which it is embodied.

Of these physical materials, carbon is the most important to the living forms. It seems to possess properties capable of drawing to it the other elements, and forcing them into service. From carbon proceeds what is called "protoplasm," the material of which the cells of animal and vegetable life is composed. From protoplasm the almost infinite varieties of living forms have been built up by the process of Evolution, working gradually and by easy stages. Every living form is made up, or composed, of a mult.i.tude of single cells, and their combinations. And every form originates in a single cell which rapidly multiplies and reproduces itself until the form of the amoeba; the plant; the animal; the man, is completed. All living forms are but a single cell multiplied. And every cell is composed of protoplasm.

Therefore we must look for the beginning of life in the grade of matter called protoplasm. In this both modern Science and the Yogi Teachings agree fully.

In investigating protoplasm we are made to realize the wonderful qualities of its princ.i.p.al const.i.tuent--Carbon. Carbon is the wonder worker of the elements. Manifesting in various forms, as the diamond, graphite, coal, protoplasm--is it not ent.i.tled to respect? The Yogi Teachings inform vis that in Carbon we have that form of matter which was evolved as the physical basis of life. If any of you doubt that inorganic matter may be transformed into living forms, let us refer you to the plant life, in which you may see the plants building up cells every day from the inorganic, chemical or mineral substances, in the earth, air, and water. Nature performs every day the miracle of transforming chemicals and minerals into living plant cells. And when animal or man eats these plant cells, so produced, they become transformed into animal cells of which the body is built up. What it took Nature ages to do in the beginning, is now performed in a few hours, or minutes.

The Yogi Teachings, again on all-fours with modern Science, inform us that living forms had their beginning in water. In the slimy bed of the polar seas the simple cell-forms appeared, having their origin in the transitional stages before mentioned. The first living forms were a lowly form of plant life, consisting of a single cell. From these forms were evolved forms composed of groups of cells, and so proceeded the work of evolution, from the lower form to the higher, ever in an upward path.

As we have said, the single cell is the physical centre, or parent, of every living form. It contains what is known as the _nucleus_, or kernel, which seems to be more highly organized than the rest of the material of the cell--it may be considered as the "brain" of the cell, if you wish to use your imagination a little. The single cell reproduces itself by growth and division, or separation. Each cell manifests the functions of life, whether it be a single-celled creature, or a cell which with billions of others, goes to make up a higher form. It feels, feeds, grows, and reproduces itself. In the single-celled creature, the one cell performs all of the functions, of course. But as the forms become more complex, the many cells composing a form perform certain functions which are allotted to it, the division of labor resulting in a higher manifestation. This is true not only in the case of animal forms, but also in the case of plant forms. The cells in the bone, muscle, nerve-tissue and blood of the animal differ according to their offices; and the same is true in the cells in the sap, stem, root, leaf, seed and flower of the plant.

As we have said, the cells multiply by division, after a period of growth. The cell grows by material taken into its substance, as food.

When sufficient food has been partaken, and enough new material acc.u.mulated to cause the cell to attain a certain size, then it divides, or separates into two cells, the division being equal, and the point of cleavage being at the kernel or nucleus. As the two parts separate, the protoplasm _of_ each groups itself around its nucleus, and two living forms exist where there was but one a moment before. And then each of the two cells proceed to grow rapidly, and then separate, and so on to the end, each cell multiplying into millions, as time pa.s.ses.

Ascending in the scale, we next find the living forms composed of cell-groups. These cell-groups are formed by single cells dividing, and then subdividing, but instead of pa.s.sing on their way they group themselves in cl.u.s.ters, or ma.s.ses. There are millions of forms of these cell-group creatures, among which we find the sponges, polyps, etc.

In the early forms of life it is difficult to distinguish between the animal and the plant forms, in fact the early forms partake of the qualities of both. But as we advance in the scale a little there is seen a decided "branching out," and one large branch is formed of the evolving plant forms, and the other of the evolving animal forms. The plant-branch begins with the sea-weeds, and pa.s.ses on to the fungi, lichens, mosses, ferns, pines and palm-ferns, gra.s.ses, etc., then to the trees, shrubs and herbs. The animal-branch begins with the _monera_, or single-cell forms, which are little more than a drop of sticky, glue-like protoplasm. Then it pa.s.ses on to the _amoebae_, which begins to show a slight difference in its parts. Then on the _foraminifera_, which secretes a sh.e.l.l of lime from the water. Then on a step higher to the _polycystina_, which secretes a sh.e.l.l, or skeleton of flint-like material from the water. Then come the sponges. Then the coral-animals, anemones and jelly-fish. Then come the sea-lilies, star-fish, etc. Then the various families of worms. Then the crabs, spiders, centipedes, insects. Then come the mollusca, which include the oysters, clams and other sh.e.l.l-fish; snails, cuttle-fish, sea-squirts, etc. All of the above families of animal-forms are what are known as "invertebrates," that is, without a backbone.

Then we come to the "vertebrates," or animals having a backbone. First we see the fish family with its thousands of forms. Then come the amphibia, which include the toads, frogs, etc. Then come the reptiles, which include the serpents, lizards, crocodiles, turtles, etc. Then come the great family of birds, with its wonderful variety of forms, sizes, and characteristics. Then come the mammals, the name of which comes from the Latin word meaning "the breast," the characteristic of which group comes from the fact that they nourish their young by milk, or similar fluid, secreted by the mother. The mammals are the highest form of the vertebrates.

First among the mammals we find the aplacentals, or those which bring forth immature young, which are grouped into two divisions, _i.e_., (1) the _monotremes_, or one-vented animals, in which group belong the duck-bills, spiny ant-eaters, etc.; and (2) the _marsupials_, or pouched animals, in which group belong the kangaroo, opossum, etc.

The next highest form among the mammals are known as the _placentals_, or those which bring forth mature young. In this cla.s.s are found the ant-eaters, sloth, manatee, the whale and porpoise, the horse, cow, sheep, and other hoofed animals; the elephant, seal, the dog, wolf, lion, tiger, and all flesh eating animals; the hares, rats, mice, and ail other gnawing animals; the bats, moles, and other insect-feeders; then come the great family of apes, from the small monkeys up to the orang-outang, chimpanzee, and other forms nearly approaching man. And then comes the highest, Man, from the Kaffir, Bush-man, Cave-man, and Digger Indian, up through the many stages until the highest forms of our own race are reached.

From the Monera to Man is a long path, containing many stages, but it is a path including all the intermediate forms. The Yogi Teachings hold to the theory of evolution, as maintained by modern Science, but it goes still further, for it holds not only that the physical forms are subject to the evolutionary process, but that also the "souls" embodied in these forms are subject to the evolutionary process. In other words the Yogi Teachings hold that there is a twin-process of evolution under way, the main object of which is to develop "souls," but which also finds it necessary to evolve higher and higher forms of physical bodies for these constantly advancing souls to occupy.

Let us take a hasty glance at the ascending forms of animal life, as they rise in the evolutionary scale. By so doing we can witness the growth of the soul, within them, as manifested by the higher and higher physical forms which are used as channels of expression by the souls within. Let us first study soul-evolution from the outer viewpoint, before we proceed to examine it from the inner. By so doing we will have a fuller idea of the process than if we ignored the outer and proceed at once to the inner. Despise not the outer form, for it has always been, and is now, the Temple of the Soul, which the latter is remodelling and rebuilding in order to accommodate its constantly increasing needs and demands.

Let us begin with the _Protozoa_, or one-celled forms--the lowest form of animal life. The lowest form of this lowest cla.s.s is that remarkable creature that we have mentioned in previous lessons--the _Moneron_.

This creature lives in water, the natural element in which organic life is believed to have had its beginning. It is a very tiny, shapeless, colorless, slimy, sticky ma.s.s--something like a tiny drop of glue--alike all over and in its ma.s.s, and without organs or parts of any kind. Some have claimed that below the field of the microscope there may be something like elementary organs in the Moneron, but so far as the human eye may discover there is no evidence of anything of the kind. It has no organs or parts with which to perform particular functions, as is the case with the higher forms of life. These functions, as you know, may be cla.s.sed into three groups, _i.e._, nutrition, reproduction, and relation--that is, the function of feeding, the function of reproducing its kind, and the function of receiving and responding to the impressions of the outside world. All of these three cla.s.ses of functions the Moneron performs--but _with any part of its body, or with all of it_.

Every part, or the whole, of the Moneron absorbs food and oxygen--it is all mouth and lungs. Every part, or the whole, digests the food--it is all stomach. Every part, or the whole, performs the reproductive function--it is all reproductive organism. Every part of it senses the impressions from outside, and responds to it--it is all organs of sense, and organs of motion. It envelops its prey as a drop of glue surrounds a particle of sand, and then absorbs the substance of the prey into its own substance. It moves by prolonging any part of itself outward in a sort of tail-like appendage, which it uses as a "foot," or "finger" with which to propel itself; draw itself to, or push itself away from an object. This prolongation is called a _pseudopod_, or "false-foot." When it gets through using the "false-foot" for the particular purpose, it simply draws back into itself that portion which had been protruded for the purpose.

It performs the functions of digestion, a.s.similation, elimination, etc., perfectly, just as the higher forms of life--but it has no organs for the functions, and performs them severally, and collectively with any, or all parts of its body. What the higher animals perform with intricate organs and parts--heart, stomach, lungs, liver, kidneys, etc., etc.--this tiny creature performs _without organs_, and with its entire body, or any part thereof. The function of reproduction is startlingly simple in the case of the Moneron. It simply divides itself in two parts, and that is all there is to it. There is no male or female s.e.x in its case--it combines both within itself. The reproductive process is even far more simple than the "budding" of plants. You may turn one of these wonderful creatures inside out, and still it goes on the even tenor of its way, in no manner disturbed or affected. It is simply a "living drop of glue," which eats, digests, receives impressions and responds thereto, and reproduces itself. This tiny glue-drop performs virtually the same life functions as do the higher complex forms of living things. Which is the greater "miracle"--the Moneron or Man?

A slight step upward from the Moneron brings us to the _Amoeba_. The name of this new creature is derived from the Greek word meaning "change," and has been bestowed because the creature is constantly changing its shape. This continual change of shape is caused by a continuous prolongation and drawing-in of its pseudopods, or "false-feet," which also gives the creature the appearance of a "many-fingered" organism. This creature shows the first step toward "parts," for it has something like a membrane or "skin" at its surface, and a "nucleus" at its centre, and also an expanding and contracting cavity within its substance, which it uses for holding, digesting and distributing its food, and also for storing and distributing its oxygen--an elementary combination of stomach and lungs! So you see that the amoeba has taken a step upward from the moneron, and is beginning to appreciate the convenience of parts and organs. It is interesting to note, in this connection, that while the ordinary cells of the higher animal body resemble the _monera_ in many ways, still the white corpuscles in the blood of man and the animals bear a startling resemblance to the _amoebae_ so far as regards size, general structure, and movements, and are in fact known to Science as "amoeboids." The white corpuscles change their shape, take in food in an intelligent manner, and live a comparatively independent life, their movements showing independent "thought" and "will."

Some of the amoebae (the diatoms, for instance) secrete solid matter from the water, and build therefrom sh.e.l.ls or houses, which serve to protect them from their enemies. These sh.e.l.ls are full of tiny holes, through which the pseudopods are extended in their search for food, and for purposes of movement. Some of these sh.e.l.ls are composed of secreted lime, and others of a flinty substance, the "selection" of these substances from the ether mineral particles in the water, evidencing a degree cf "thought," and mind, even in these lowly creatures. The skeletons of these tiny creatures form vast deposits of chalk and similar substances.

Next higher in the scale are the _Infusoria_. These creatures differ from the amoebae inasmuch as instead of pseudopods, they have developed tiny vibrating filaments, or thread-like appendages, which are used for drawing in their prey and for moving about. These filaments are permanent, and are not temporary like the pseudopods of the monera or amoebae--they are the _first signs of permanent hands and feet_. These creatures have also discovered the possibilities of organs and parts, to a still greater degree than have their cousins the amoebae, and have evolved something like a mouth-opening (very rudimentary) and also a short gullet through which they pa.s.s their food and oxygen--_they have developed the first signs of a throat, wind-pipe and food-pa.s.sage_.

Next come the family of Sponges, the soft skeletons of which form the useful article of everyday use. There are many forms who weave a home of far more delicacy and beauty than their more familiar and homely brothers. The sponge creature itself is a slimy, soft creature, which fills in the s.p.a.ces in its spongy skeleton. It is fastened to one spot, and gathers in its food from the water around it (and oxygen as well), by means of numerous whip-like filaments called _cilia_, which flash through the water driving in the food and oxygen to the inner positions of its body. The water thus drawn in, as well as the refuse from the food, is then driven out in the same manner. It is interesting to note that in the organisms of the higher animals, including man, there are numerous _cilia_ performing offices in connection with nutrition, etc.

When Nature perfects an instrument, it is very apt to retain it, even in the higher forms, although in the latter its importance may be dwarfed by higher ones.

The next step in the ascending scale of life-forms is occupied by the _polyps_, which are found in water, fastened to floating matter. The polyps fasten themselves to this floating matter, with their mouths downward, from the latter dangling certain tentacles, or thin, long arms. These tentacles contain small thread-like coils in contact with a poisonous fluid, and enclosed in a cell. When the tentacles come in contact with the prey of the creature, or with anything that is sensed as a possible enemy, they contract around the object and the little cells burst and the tiny thread-like coils are released and twist themselves like a loop around the object, poisoning it with the secreted fluid. Some of the polyps secrete flint-like tubes, which they inhabit, and from the ends of which they emerge like flowers. From these parent polyps emerge cl.u.s.ters of young, resembling buds. These bud-like young afterwards become what are known as jelly-fishes, etc., which in turn reproduce themselves--but here is a wonder--the jelly-fish lay eggs, which when hatched produce stationary polyps like their grandparent, and not moving creatures like their parents. The jelly-fishes have a comparatively complex organism. They have an intricate system of ca.n.a.l-like pa.s.sages with which to convey their food and oxygen to the various parts. They also have something like muscles, which contract and enable the creature to "swim." They also possess a "nervous system," and, most wonderful of all, they have _rudimentary eyes and ears_. Their tentacles, like those of the parent-polyp, secrete the poisonous fluid which is discharged into prey or enemy.

Akin to the polyps are the sea-anemones, with their beautiful colors, and still more complex structure and organism, the tentacles of which resemble the petals of a flower. Varying slightly from these are the coral-creatures, which form in colonies and the skeletons of which form the coral trees and branches, and other forms, with which we are familiar.

Pa.s.sing on to the next highest family of life-forms, we see the spiny-bodied sea-creatures, such as the sea-urchin, star-fish, etc., which possess a thick, hard skin, covered by spines or p.r.i.c.kly projections. These creatures abound in numerous species. The star-fish has rays projecting from a common centre, which gives it its name, while the sea-urchin resembles a ball. The sea-lilies, with their stems and flowers (so-called) belong to this family, as do also the sea-cuc.u.mbers, whose name is obtained from their shape and general appearance, but which are animals possessing a comparatively complex organism, one of the features of which is a stomach which may be discarded at will and replaced by a new one. These creatures have a well defined nervous system, and have eyes, and some of them even rudimentary eyelids.

Ascending the scale of life-forms, we next observe the great family of the _Annulosa_, or jointed creatures, which comprises the various families of the worm, the crab, the spider, the ant, etc. In this great family are grouped nearly four-fifths of the known life-forms. Their bodies are well formed and they have nervous systems running along the body and consisting of two thin threads, knotted at different points into ganglia or ma.s.ses of nerve cells similar to those possessed by the higher animals. They possess eyes and other sense organs, in some cases highly developed. They possess organs, corresponding to the heart, and have a well-developed digestive apparatus. Note this advance in the nutritive organism: the _moneron_ takes its food at any point of its body; the _amoeba_ takes its food by means of its "false-feet," and drives it through its body by a rhythmic movement of its substance; the _polyp_ distributes its food to its various parts by means of the water which it absorbs with the food; the _sea-urchin and star-fish_ distribute their food by ca.n.a.ls in their bodies which open directly into the water; in the higher forms of the _annulosa_, the food is distributed by a fluid resembling blood, which carries the nourishment to every part and organ, and which carries away the waste matter, the blood being propelled through the body by a rudimentary heart. The oxygen is distributed by each of these forms in a corresponding way, the higher forms having rudimentary lungs and respiratory organs. Step by step the life-forms are perfected, and the organs necessary to perform certain definite functions are evolved from rudimentary to perfected forms.

The families of worms are the humblest members of the great family of the Annulosa. Next come the creatures called Rotifers, which are very minute. Then come the Crustacea, so called from their crustlike sh.e.l.l.

This group includes the crabs, lobsters, etc., and closely resembles the insects. In fact, some of the best authorities believe that the insects and the crustacea spring from the same parent form, and some of the Yogi authorities hold to this belief, while others do not attempt to pa.s.s upon it, deeming it immaterial, inasmuch as all life-forms have a common origin. The western scientists pay great attention to outward details, while the Oriental mind is apt to pa.s.s over these details as of slight importance, preferring to seek the cause back of the outward form. On one point both the Yogi teachers and the scientists absolutely agree, and that is that the family of insect life had its origin in some aquatic creature. Both hold that the wings of the insect have been evolved from organs primarily used for breathing purposes by the ancestor when it took short aerial flights, the need for means of flight afterwards acting to develop these rudimentary organs into perfected wings. There need be no more wonder expressed at this change than in the case of the transformation of the insect from grub to chrysalis, and then to insect. In fact this process is a reproduction of the stages through which the life-form pa.s.sed during the long ages between sea-creature and land-insect.

We need not take up much of your time in speaking of the wonderful complex organism of some of the insect family, which are next on the scale above the crustacea. The wonders of spider-life--the almost human life of the ants--the spirit of the beehive--and all the rest of the wonders of insect life are familiar to all of our readers. A study of some good book on the life of the higher forms of the insect family will prove of value to anyone, for it will open his or her eyes to the wonderful manifestation of life and mind among these creatures.

Remember the remark of Darwin, that the brain of the ant, although not much larger than a pin point, "is one of the most marvelous atoms of matter in the world, perhaps more so than the brain of man."

Closely allied to the crustacea is the sub-family of the _mollusca_, which includes the oyster, clams, and similar creatures; also the snails, cuttle-fish, slugs, nautilus, sea-squirts, etc., etc. Some are protected by a hard sh.e.l.l, while others have a gristly outer skin, serving as an armor, while others still are naked. Those having sh.e.l.ls secrete the material for their construction from the water. Some of them are fixed to rocks, etc., while others roam at will. Strange as it may appear at first sight, some of the higher forms of the mollusca show signs of a rudimentary vertebra, and science has hazarded the opinion that the sea-squirts and similar creatures were descended from some ancestor from whom also descended the vertebrate animals, of which man is the highest form known today on this planet. We shall mention this connection in our next lesson, where we will take up the story of "The Ascent of Man" from the lowly vertebrate forms.

And now, in closing this lesson, we must remind the reader that we are _not_ teaching Evolution as it is conceived by modern science. We are viewing it from the opposite viewpoint of the Yogi Teaching. Modern Science teaches that Mind is a by-product of the evolving material forms--while the Yogi Teachings hold that there was Mind _involved_ in the lowest form, and that that Mind constantly pressing forward for unfoldment _compelled_ the gradual evolution, or unfoldment of the slowly advancing degrees of organization and function. Science teaches that "function precedes organization," that is, that a form performs certain functions, imperfectly and crudely, before it evolves the organs suitable for the functioning. For instance the lower forms digested food before they evolved stomachs--the latter coming to meet the need. But the Yogi Teachings go further and claim that "desire precedes function," that is, that the lowly life form "desires" to have digestive apparatus, in order to proceed in the evolutionary scale, before it begins the functioning that brings about the more complex organism. There is ever the "urge" of the Mind which craves unfoldment, and which the creature feels as a dim desire, which grows stronger and stronger as time goes on. Some yield more readily to the urge, and such become the parents of possible higher forms. "Many are called, but few are chosen," and so matters move along slowly from generation to generation, a few forms serving to carry on the evolutionary urge to their descendants. But is always the Evolutionary Urge of the imprisoned Mind striving to cast aside its sheaths and to have more perfect machinery with which, and through which, to manifest and express itself? This is the difference between the "Evolution" of Modern Science and the "Unfoldment" of the Yogi Teachings. The one is all material, with mind as a mere by-product, while the other is all Mind, with matter as a tool and instrument of expression and manifestation.

As we have said in this lesson--and as we shall point out to you in detail in future lessons--accompanying this evolution of bodies there is an evolution of "souls" producing the former. This evolution of souls is a basic principle of the Yogi Teachings, but it is first necessary that you acquaint yourselves with the evolution of bodies and forms, before you may fully grasp the higher teachings.

Our next lesson will be ent.i.tled "The Ascent of Man," in which the rise of man--that is, his body--from the lowly forms of the vertebrates is shown. In the same lesson we shall begin our consideration of the "evolution of souls." We trust that the students are carefully studying the details of each lesson, for every lesson has its part in the grand whole of the Teachings.

THE EIGHTH LESSON

THE ASCENT OF MAN.

In our last lesson we led you by successive steps from the beginnings of Life in living forms up to the creatures closely resembling the family of vertebrates--the highest family of living forms on this planet. In this present lesson we take up the story of the "Ascent of Man" from the lowly vertebrate forms.

The large sub-family of forms called "The Vertebrates" are distinguished from the Invertebrates by reason of the former possessing an _internal_ bony skeleton, the most important feature of which is the vertebra or spinal column. The vertebrates, be it remembered, possess practically the same organs as the lower forms of life, but differ from them most materially by the possession of the _internal_ skeleton, the lower forms having an _external_ or outside _skeleton_, which latter is merely a hardening of the skin.

The flexibility of the vertebra creates a wonderful strength of structure, combined with an ease of movement peculiar to the vertebrates, and which renders them the natural forms of life capable of rapid development and evolution. By means of this strength, and ease, these forms are enabled to move rapidly in pursuit of their prey, and away from their pursuers, and also to resist outside pressure or attack. They are protected in a way similar to the invertebrates having sh.e.l.ls, and yet have the additional advantage of easy movement.

Differing in shape and appearance as do the numerous members of the sub-family of vertebrates, still their structure is easily seen to spring from a single form--all are modifications of some common pattern, the differences arising from the necessities of the life of the animal, as manifested through the desire and necessities of the species.

Science shows the direct relationship between the Vertebrates, and the Invertebrates by means of several connecting-links, the most noticeable of which is the Lancelot, a creature resembling the fish-form, and yet also closely resembling the lower (invertebrate) forms of life. This creature has no head, and but one eye. It is semi-transparent, and possesses _cilia_ for forcing in the water containing its food. It has something like gills, and a gullet like the lower forms. It has no heart, the blood being circulated by means of contracting vessels or parts. Strictly speaking, it has no back-bone, or vertebra, but still Science has been compelled to cla.s.s it among the vertebrates because is has a gristly cartilage where the back-bone is found in the higher forms. This gristle may be called an "elementary spine." It has a nervous system consisting of a single cord which spreads into a broadened end near the creature's mouth, and which may therefore be regarded as "something like a brain." This creature is really a developed form of Invertebrate, shaped like a Vertebrate, and showing signs of a rudimentary spine and nervous system of the latter. It is a "connecting-link."

The lowest forms of the true Vertebrates are the great families of Fishes. These Fish families include fishes of high and low degree, some of the higher forms being as different from the lowest as they (the highest) are different from the Reptile family. It is not necessary to go into detail regarding the nature of the fish families, for every student is more or less familiar with them.

Some peculiar forms of fish show a shading into the Reptile family, in fact they seem to belong nearly as much to the latter as to their own general family. Some species of fish known as the _Dipnoi_ or "double-breathers," have a remarkable dual system of breathing. That is, they have gills for breathing while in the water, and also have a primitive or elementary "lung" in the shape of an air-bladder, or "sound," which they use for breathing on land. The Mud-fish of South America, and also other forms in Australia and other places, have a modification of fins which are practically "limbs," which they actually use for traveling on land from pond to pond. Some of these fish have been known to travel enormous distances in search of new pools of water, or new streams, having been driven from their original homes by droughts, or perhaps by instincts similar to the migrating instinct of birds. Eels are _fish_ (although many commonly forget this fact) and many of their species are able to leave the water and travel on land from pond to pond, their breathing being performed by a peculiar modification of the gills. The climbing perch of India are able to live out of water, and have modified gills for breathing purposes, and modified fins for climbing and walking. So you see that without leaving the fish family proper, we have examples of land living creatures which are akin to "connecting links."

But there are real "connecting-links"' between the Fish and the Reptiles. Pa.s.sing over the many queer forms which serve as links between the two families, we have but to consider our common frog's history for a striking example. The Tadpole has gills, has no limbs, uses its tail like a fish's fin, eats plants, etc. Pa.s.sing through several interesting stages the Tadpole reaches a stage in which it is a frog with a tail--then it sheds its tail and is a full fledged Frog, with four legs; web-feet; no tail; and feeding on animals. The Frog is amphibious, that is, able to live on land or in water--and yet it is compelled to come to the surface of the water for air to supply its lungs. Some of the amphibious animals possess both lungs and gills, even when matured; but the higher vertebrates living in the water breathe through lungs which are evolved from the air-bladder of fishes, which in turn have been evolved from the primitive gullet of the lower forms. There are fishes known which are warm-blooded. Students will kindly remember that the Whale is not a fish, but an aquatic animal--a mammal, in fact, bringing forth its young alive, and suckling it from its b.r.e.a.s.t.s.

So we readily see that it is but a step, and a short step at that, between the land-traveling and climbing fishes and the lower forms of Reptiles. The Frog shows us the process of evolution between the two families, its life history reproducing the gradual evolution which may have required ages to perfect in the case of the species. You will remember that the embryo stages of all creatures reproduce the various stages of evolution through which the species has pa.s.sed--this is true in Man as well as in the Frog.

We need not tarry long in considering the Reptile family of living forms. In its varieties of serpents, lizards, crocodiles, turtles, etc., we have studied and observed its forms. We see the limbless snakes; the lizards with active limbs; the huge, clumsy, slow crocodiles and alligators--the armor-bearing turtles and tortoises--all belonging to the one great family of Reptiles, and nearly all of them being degenerate descendants of the mighty Reptile forms of the geological Age of Reptiles, in which flourished the mighty forms of the giant reptiles--the monsters of land and water. Amidst the dense vegetation of that pre-historic age, surrounded by the most favorable conditions, these mighty creatures flourished and lived, their fossilized skeleton forms evidencing to us how far their descendants have fallen, owing to less favorable conditions, and the development of other life-forms more in harmony with their changed environment.