In tracing the genealogy of any American family it is often difficult or impossible to say whether a certain branch is descended from John Oldworthy or his cousin or second cousin. In the latter cases to find the common ancestor we must go back to the grandfather or great-grandfather. The same difficulty, but greatly enhanced, meets us when we try to make a genealogical tree of the animal kingdom. Thus it seems altogether probable that all higher forms are descended from an ancestor of the same general structure and grade of organization as the turbellaria, although probably free swimming, and hence with somewhat different form and development, especially of the muscular system. It seems to me altogether probable that all, except possibly Mollusca, are descended from a common ancestor closely resembling the schematic worm last described. Some would, however, maintain that they diverged rather earlier than even the turbellaria; others after the schematic worm, if such ever existed. As far as our argument is concerned it makes little difference which of these views we adopt.

From our turbellaria, or possibly from some even more primitive ancestor, many lines diverged. And this was to be expected. The coelenterata, as we saw in hydra, had developed rude digestive and reproductive systems. The higher groups of this kingdom had developed all, or nearly all, the tissues used in building the bodies of higher animals muscular, reproductive, connectile, glandular, nervous, etc. But these are mostly very diffuse. The muscular fibrils of a jelly-fish are mostly isolated or parallel in bands, rarely in compact well-defined bundles. The tissues have generally not yet been moulded into compact masses of definite form. There are as yet very few structures to which we can give the name of organs. To form organs and group them in a body of compact definite form was the work pre-eminently of worms. The material for the building was ready, but the architecture of the bilateral animal was not even sketched. And different worms were their own architects, untrammelled by convention or heredity, hence they built very different, sometimes almost fantastic, structures.

We must remember, too, the great age of this group. They are present in highly modified forms in the very oldest palaeozoic strata, and probably therefore came into existence as the first traces of continental areas were beginning to rise above the primeval ocean. They are literally “older than the hills.” They were exposed to a host of rapidly changing conditions, very different in different areas. This prepares us for the fact that the worms represent a stage in animal life corresponding fairly well to the Tower of Babel in biblical history. The animal kingdom seems almost to explode into a host of fragments. Our genealogical tree fairly bristles with branches, but the branches do not seem to form any regular whorls or spirals. Few of them have developed into more than feeble growths. They now contain generally but few species. Many of them are largely or entirely parasitic, and in connection with this mode of life have undergone modifications and degeneration which make it exceedingly difficult to decipher their descent or relationships.

Four of these branches have reached great prominence in numbers and importance. One or two others were formerly equally numerous and have since become almost extinct; so the brachiopoda, which have been almost entirely replaced by mollusks. The same may very possibly be true of others. For of the amount of extinction of larger groups we have generally but an exceedingly faint conception. Indeed in this respect the worms have been well compared to the relics which fill the shelves of one of our grandmother’s china-closets.

The four great branches are the echinoderms, mollusks, articulates, and vertebrates. The echinoderms, including starfishes, sea-urchins, and others straggled early from the great army. We know as yet almost nothing of their history; when deciphered it will be as strange as any romance. The vertebrates are of course the most important line, as including the ancestors of man. But we must take a little glance at mollusks, including our clams, snails, and cuttle-fishes; and at the articulates, including annelids and culminating in insects. The molluscan and articulate lines, though divergent, are of great importance to us as throwing a certain amount of light on vertebrate development; and still more as showing how a certain line of development may seem, and at first really be, advantageous, and still lead to degeneration, or at best to but partial success.

When we compare the forms which represent fairly well the direction of development of these three lines, a snail or a clam with an insect and a fish, we find clearly, I think, that the fundamental anatomical difference lies in the skeleton; and that this resulted from, and almost irrevocably fixed, certain habits of life.

We may picture to ourselves the primitive ancestor of mollusks as a worm having the short and broad form of the turbellaria, but much thicker or deeper vertically. A fuller description can be found in the “Encyclopædia Britannica,” Art., Mollusca. It was hemi-ovoid in form. It had apparently the perivisceral cavity and nephridia of the schematic worm, and a circulatory system. In this latter respect it stood higher than any form which we have yet studied. Its nervous system also was rather more advanced. It had apparently already taken to a creeping mode of life and the muscles of its ventral surface were strongly developed, while its exposed and far less muscular dorsal surface was protected by a cap-like shell covering the most important internal organs. But the integument of the whole dorsal surface was, as is not uncommon in invertebrates, hardening by the deposition of carbonate of lime in the integument. And this in time increased to such an extent as to replace the primitive, probably horny, shell.

Into the anatomy of this animal or of its descendants we have no time to enter, for here we must be very brief. We have already noticed that the most important viscera were lodged safely under the shell. And as these increased in size or were crowded upward by the muscles of the creeping disk, their portion of the body grew upward in the form of a “visceral hump.” Apparently the animal could not increase much in length and retain the advantage of the protection of the shell; and the shell was the dominating structure. It had entered upon a defensive campaign. Motion, slow at the outset, became more difficult, and the protection of the shell therefore all the more necessary. The shell increased in size and weight and motion became almost impossible. The snail represents the average result of the experiment. It can crawl, but that is about all; it is neither swift nor energetic. Even the earthworm can outcrawl it. It has feelers and eyes, and is thus better provided with sense-organs than almost any worm. It has a supra-oesophageal ganglion of fair size.

The clams and oysters show even more clearly what we might call the logical results of molluscan structure. They increased the shell until it formed two heavy “valves” hanging down on each side of the body and completely enclosing it. They became almost sessile, living generally buried in the mud and gaining their food, consisting mostly of minute particles of organic matter, by means of currents created by cilia covering the large curtain-like gills. Their muscular system disappeared except in the ploughshare-shaped “foot” used mostly for burrowing, and in the muscles for closing the shell. That portion of the body which corresponds to the head of the snail practically aborted with nearly all the sense-organs. The nervous system degenerated and became reduced to a rudiment. They had given up locomotion, had withdrawn, so to speak, from the world; all the sense they needed was just enough to distinguish the particles of food as they swept past the mouth in the current of water. They have an abundance of food, and “wax fat.” The clam is so completely protected by his shell and the mud that he has little to fear from enemies. They have increased and multiplied and filled the mud. “Requiescat in pace.”

But zooelogy has its tragedies as well as human history. Let us turn to the development of a third molluscan line terminating in the cuttle-fishes. The ancestors of these cephalopods, although still possessed of a shell and a high visceral hump, regained the swimming life. First, apparently, by means of fins, and then by a simple but very effective use of a current of water, they acquired an often rapid locomotion. The highest forms gave up the purely defensive campaign, developed a powerful beak, led a life like that of the old Norse pirates, and were for a time the rulers and terrors of the sea. With their more rapid locomotion the supra-oesophageal ganglion reached a higher degree of development, and it was served by sense-organs of great efficiency. They reduced the external shell, and succeeded, in the highest forms, of almost ridding themselves of this burden and encumbrance. Traces of it remain in the squids, but transformed into an internal quill-like, supporting, not defensive, skeleton. They have retraced the downward steps of their ancestors as far as they could. And the high development of their supra-oesophageal ganglion and sense-organs, and their powerful jaws and arms, or tentacles, show to what good purpose they have struggled. But the struggle was in vain, as far as the supremacy of the animal kingdom was concerned. Their ancestors had taken a course which rendered it impossible for their descendants to reach the goal. Their progress became ever slower. They were entirely and hopelessly beaten by the vertebrates. They struggled hard, but too late.

The history of mollusks is full of interest. They show clearly how intimately nervous development is connected with the use of the locomotive organs. The snail crept, and slightly increased its nervous system and sense-organs. The clam almost lost them in connection with its stationary life. The cephalopods were exceedingly active, developed, therefore, keen sense-organs and a very large and complicated supra-oesophagal ganglion, which we might almost call a brain.

The articulate series consists of two groups of animals. The higher group includes the crabs, spiders, thousand-legs, and finally the insects, and forms the kingdom of arthropoda. The lower members are still usually reckoned as worms, and are included under the annelids. Of these our common earthworm is a good example, and near them belong the leeches. But the marine annelids, of which nereis, or a clam-worm, is a good example, are more typical. They are often quite large, a foot or even more in length. They are composed of many, often several hundred, rings or segments. Between these the body-wall is thin, so that the segments move easily upon each other, and thus the animal can creep or writhe.

These segments are very much alike except the first two and the last. If we examine one from the middle of the body we shall find its structure very much like that of our schematic worm. Outside we find a very thin, horny cuticle, secreted by the layer of cells just beneath it, the hypodermis. Beneath the skin we find a thin layer of transverse muscles, and then four heavy bands of longitudinal muscles. These latter have been grouped in the four quadrants, a much more effective arrangement than the cylindrical layer of the schematic worm. Furthermore, the animal has on each segment a pair of fin-like projections, stiffened with bristles, the parapodia. These are moved by special muscles and form effective organs of creeping.

Within the muscles is the perivisceral cavity, and in its central axis the intestine, segmented like the body-wall. The reproductive organs are formed from patches of the lining of the perivisceral cavity, and the reproductive elements, when fully developed, fall into the perivisceral fluid and are carried out by nephridia, just such as we found in the schematic worm. Beside the perivisceral cavity and its fluid there is a special circulatory system. This consists mainly of one long tube above the intestine and a second below, with often several smaller parallel tubes. Transverse vessels run from these to all parts of the body. The dorsal tube pulsates and thus acts as a heart. The surface of the body no longer suffices to gather oxygen, hence we find special feathery gills on the parapodia. But these gills are merely expanded portions of the body wall, arranged so as to offer the greatest possible amount of surface where the capillaries of the blood system can be almost immediately in contact with the surrounding water.

The nervous system consists of a large supra-oesophageal ganglion in the first segment; then of a chain of ganglia, one to each segment, on the ventral side of the body. With one ganglion in each segment there is far more controlling, perceptive, ganglionic material than in lower worms. Furthermore the supra-oesophageal ganglion is relieved of a large part of the direct control of the muscles of each segment, and is becoming more a centre of control and perception for the body as a whole. It is more like our brain, commander-in-chief, the other ganglia constituting its staff. The sense-organs have improved greatly. There are tentacles and otolith vesicles as very delicate organs of feeling, or possibly of hearing also.

But the annelids were probably the first animals to develop an eye capable of forming an image of external objects. The importance of this organ in the pursuit of food or the escape from enemies can scarcely be over-estimated. The lining of the mouth and pharynx can be protruded as a proboscis, and drawn back by powerful muscles, and is armed with two or more horny claws. Eyes and claws gave them a great advantage over their not quite blind but really visionless and comparatively defenceless neighbors, and they must have wrought terrible extinction of lower and older forms. But while we cannot over-estimate the importance of these eyes, we can easily exaggerate their perfectness. They were of short range, fitted for seeing objects only a few inches distant, and the image was very imperfect in detail. But the plan or fundamental scheme of these eyes is correct and capable of indefinitely greater development than the organs of touch or smell, perhaps greater even than the otolith vesicle.

And the reflex influence of the eye on the brain was the greatest advantage of all. Hitherto with feeble muscles and sense-organs it has hardly paid the animal to devote more material to building a larger brain. It was better to build more muscle. But now with stronger muscles at its command, and better sense-organs to report to it, every grain of added brain material is beginning to be worth ten devoted to muscle. The muscular system will still continue to develop, but the brain has begun an almost endless march of progress. The eye becomes of continually increasing advantage and importance because it has a capable brain to use it; and brain is a more and more profitable investment, because it is served by an ever-improving eye.

The annelid had hit upon a most advantageous line of development, which led ultimately to the insect. The study of the insect will show us clearly the advantages and defects of the annelid plan. First of all, the insect, like the mollusk, has an external skeleton. But the skeleton of the mollusk was purely protective, a hindrance to locomotion. That of the insect is still somewhat protective, but is mainly, almost purely, locomotive. It is never allowed to become so heavy as to interfere with locomotion. In the second place, the insect has three body regions, having each its own special functions or work. And one of these is a head. The annelid had two anterior segments differing from those of the rest of the body; these may, perhaps, be considered as the foreshadowings of a structure not yet realized; they can only by courtesy be called a head. Thirdly, the insect has legs. The annelid had fin-like parapodia, approaching the legs of insects about as closely as the fins of a fish approach the legs of a mammal. The reproductive and digestive systems, while somewhat improved, are not very markedly higher than those of annelids. The excretory system has more work to perform and reaches a rather higher development.

But in these organs there is no great or striking change; the time for marked and rapid development of the digestive and reproductive systems has gone by. Material can be more profitably invested in brain or muscle. Air is carried to all parts of the body by a special system of air-sacks and tubes. This is a very advantageous structure for small animals with an external skeleton. In very large animals, or where the skeleton is internal, it would hardly be practicable; the risk of compression of the tubes at some point, and of thus cutting off the air-supply of some portion of the body, would be altogether too great.

The circulatory system is very poor. It consists practically only of a heart, which drives the blood in an irregular circulation between the other organs of the body much as with a syringe you might keep up a system of currents in a bowl of water. But the rapidity of the flow of the blood in our bodies is mainly to furnish a supply of oxygen to the organs. A tea-spoonful of blood can carry a fair amount of dissolved solid nutriment like sugar, it can carry at each round but a very little gas like oxygen. Hence the blood must make its rounds rapidly, carrying but a little oxygen at each circuit. But in the insect the blood conveys only the dissolved solid nutriment, the food; hence a comparatively irregular circulation answers all purposes.

The skeleton is a thickening of the horny cuticle of the annelid on the surface of each segment. The horny cylinder surrounding each segment is composed of several pieces, and on the abdomen these are united by flexible, infolded membranes. This allows the increase in the size of the segment corresponding to the varying size of the digestive and reproductive systems. In this part of the body the skeletal ring of each segment is joined to that of the segments before and behind it in the same manner. But in other parts of the body we shall find the skeletal pieces of each segment and the rings of successive segments fused in one plate of mail. The legs are the parapodia of annelids carried to a vastly higher development. They are slender and jointed, and yet often very powerful. A large portion of the muscular system of the body is attached to these appendages.

But the insect has also jaws. The annelid had teeth or claws attached to the proboscis. But true jaws are something quite different. They always develop by modifying some other organ. In the insect they are modified legs. This is shown first by their embryonic development. But the king- or horseshoe-crab has still no true jaws, but uses the upper joints of its legs for chewing. There are primitively three pairs of jaws of various forms for the different kinds of food of different species or higher groups. But some of them may disappear and the others be greatly modified into awls for piercing, or a tube for sucking honey. Into the wonderful transformations of these modified legs we cannot enter.

The muscles are no longer arranged to form a sack as in annelids. Transverse muscles, running parallel to the unyielding plates of chitin or horn could accomplish nothing. They have largely disappeared. The work of locomotion has been transferred from the trunk to the legs.

The abdomen of the insect is as clearly composed of distinct segments as the body of the annelid. Of these there are perhaps typically eleven. The thorax is composed of three segments, distinct in the lowest forms, fused in the highest. This fusion of segments in the thorax of the highest forms furnishes a very firm framework for the attachment of wings and muscles. These wings are a new development, and how they arose is still a question. But they give the insect the capability of exceedingly rapid locomotion.

The three pairs of jaws, modified legs, in the rear half of the head show that this portion is composed of three segments. For only one pair of legs is ever developed on a single segment. Embryology has shown that the portion of the head in front of the mouth is also composed of three segments. Possibly between the prae- and post-oral portions still another segment should be included, making a total of seven in the head. The head has thus been formed by drawing forward segments from the trunk, and fusing them successively with the first or primitive head segment. This is difficult to conceive of in the fully developed insect, where the boundary between head and thorax is very sharp. But the ancestors of insects looked more like thousand-legs or centipedes, and here head and thorax are much less distinct. But in the annelid the mouth is on the second segment; here it is on the fourth. It has evidently travelled backward. That the mouth of an animal can migrate seems at first impossible, but if we had time to examine the embryology of annelids and insects, it would no longer appear inconceivable or improbable. And its backward migration brought it among the legs which were grasping and chewing the food. And in vertebrates the mouth has changed its position, though not in exactly the same way. Our present mouth is probably not at all the mouth of the primitive ancestor of vertebrates. Thus in the insect three segments have fused around the mouth, and three, possibly four, in front of it. This makes a head worthy of the name. The ganglia of the three post-oral segments, which bear the jaws, have fused in one compound ganglion innervating the mouth and jaws. Those of the three prae-oral segments have fused to form a brain. Eyes are well developed, giving images sometimes accurate in detail, sometimes very rude. Ears are not uncommon. The sense of smell is often keen.

Perhaps the greatest advance of the insect is its adaptation to land life. This gives it a larger supply of oxygen than any aquatic animal could ever obtain. This itself stimulates every function, and all the work of the body goes on more energetically. Then the heat produced is conducted off far less rapidly than in aquatic forms. Water is a good conductor of heat, and nearly all aquatic animals are cold-blooded. The few which are warm-blooded are protected by a thick layer of non-conducting fat. In all land animals, even when cold-blooded, the work of the different systems is aided by the longer retention of the heat in the body.

Let us recapitulate. The schematic worm had a body composed of two concentric tubes. The outer was composed of the muscles of the body covered by the protective integument. The inner tube was the alimentary canal with its special muscles. Between these two was the perivisceral cavity, filled with nutritive fluid, lymph, and furnishing a safe lodging-place for the more delicate viscera. It represented fairly the trunk of higher animals.

The annelid added segmentation, and thus greater freedom of motion by the parapodia. But the segments were still practically alike. In the insect division of labor took place, that is, each group of segments was allotted its own special work; and these groups of segments were modified in structure to best suit the performance of this part of the work of the body. The abdomen was least modified and its eleven segments were devoted to digestion, reproduction, and excretion the old vegetative functions. Three segments were united in the thorax; all their energy was turned to locomotion, and the insect became thus an exceedingly active, swift animal. The third body-region, the head, includes six segments, of which three surrounded the mouth and furnished the jaws, while two more were crowded or drawn forward in order that their ganglia might be added to the old supraoesophageal ganglion and form a brain. It is interesting to note that a form, peripatus, still exists which stands almost midway between annelids and insects and has only four segments in the head. The formation of the head was thus a gradual process, one segment being added after another.

In the turbellaria the dominant functions were digestion and reproduction, and their organs composed almost the whole body. Here only eleven segments at most are devoted to these functions, and nine in head and thorax to locomotion and brain. Head and thorax have increased steadily in importance, while the abdomen has decreased as steadily in number of segments. And the brain is increasing thus rapidly because there are now muscles and sense-organs of sufficient power to make such a brain of value. And this brain perceives not only objects and qualities, but invisible relations between these, and this is an advance amounting to a revolution. It remembers, and uses its recollections. It is capable of learning a little by experience and observation. The A, B, C of thinking was probably learned long before the insect’s time, and the bee shows a fair amount of intelligence.

The line of development which the insect followed was comparatively easy and its course probably rapid. Certain crustacea, aquatic arthropoda, are among the oldest fossils, and it is possible that insects lived on the land before the first fish swam in the sea. They had fine structure and powers; and yet during the later geologic periods they have scarcely advanced a step, and are now apparently at a standstill. They ran splendidly for a time, and then fell out of the race. What hindered and stopped them?

One vital defect in their whole plan of organization is evident. The external skeleton is admirably suited to animals of small size, but only to these. In larger animals living on land it would have to be made so heavy as to be unwieldy and no longer economical. Their mode of breathing also is fitted only for animals of small size having an external skeleton. Whatever may be our explanation the fact remains that insects are always small. This is in itself a disadvantage. Very small animals cannot keep up a constant high temperature unless the surrounding air is warm, for their radiating surface is too large in comparison with their heat-producing mass. At the first approach of even cool weather they become chilled and sluggish, and must hibernate or die. They are conformed to but a limited range of environment in temperature.

But small size is, as a rule, accompanied by an even greater disadvantage. It seems to be almost always correlated with short life. Why this is so, or how, we do not know. There are exceptions; a crow lives as long as a man; or would, if allowed to. But, as a rule, the length of an animal’s days is roughly proportional to the size of its body. And the insect is, as a rule, very short-lived. It lives for a few days or weeks, or even months, but rarely outlasts the year. It has time to learn but little by experience. The same experience must be passed, the same emergency arise and be met, over and over again during the lifetime of the same individual if the animal is to learn thereby. And intelligence is based upon experience. Hence insects can and do possess but a low grade of intelligence. But instinct is in many cases habit fixed by heredity and improved by selection. The rapid recurrence of successive generations was exceedingly favorable to the development of instincts, but very unfavorable to intelligence. Insects are instinctive, the highest vertebrates intelligent. The future can never belong to a tiny animal governed by instincts. Mollusks and insects have both failed to reach the goal; another plan of structure than theirs must be sought if the animal kingdom is to have a future.

The future belonged to the vertebrate. To begin with less characteristic organs the digestive system is much like that of the annelid or schematic worm, but with greatly increased glandular and absorptive surfaces. The present mouth of nearly all vertebrates is probably not primitive. It is almost certainly one of the gill-slits of some old ancestor of fish, such as now are used to discharge the water which is used for respiration. The jaws are modified branchial arches or the cartilaginous or bony rods which in our present fish support the fringe of gills. These have formed a pair of exceedingly effective and powerful jaws. The reproductive system holds still to the old type and shows little if any improvement. The excretory organs, kidneys, are composed primitively of nephridial tubes like those of the schematic worm or annelid, but immensely increased in number, modified, and improved in certain very important particulars. The muscles in simplest forms are composed of heavy longitudinal bands, especially developed toward the dorsal surface of the body to the right and left of the axial skeleton. Locomotion was produced by lashing the tail right and left, as still in fish. There is improvement in all these organs, except perhaps the reproductive, but nothing very new or striking. The great improvement from this time on was not to be sought in the vegetative organs, or even directly to any great extent in muscles.

The new and characteristic organ was not the vertebral column, or series of vertebrae, or backbone, from which the kingdom has derived its name. This was a later production. The primitive skeleton was the notochord, still appearing in the embryos of all vertebrates and persisting throughout life in fish. This is an elastic rod of cartilage, lying just beneath the spinal marrow or nerve-cord, which runs backward from the brain. The nerve-centres are therefore here all dorsal, and the notochord or skeleton lies between these and the digestive or alimentary canal. The skeleton of the clam or snail is purely protective and a hindrance to locomotion. That of the insect is almost purely locomotive, but external, that of the vertebrate purely locomotive and internal. It does not lie outside even of the nervous system, although this system especially required, and was worthy of, protection. It does not protect even the brain; the skull of vertebrates is an after-thought. It is almost the deepest seated of all organs. But lying in the central axis of the body it furnishes the very best possible attachment for muscles. Around this primitive notochord was a layer of connectile tissue which later gave rise to the vertebrae forming our backbone.

The nervous system on the dorsal surface of the notochord consists of the brain in the head and the spinal marrow running down the back. The brain of all except the very lowest vertebrates consists of four portions: 1. The cerebrum, or cerebral lobes, or simply “forebrain,” the seat of consciousness, thought, and will, and from which no nerves proceed. Whether the primitive vertebrate had any cerebrum is still uncertai. The mid-brain, which sends nerves to the eyes, and in this respect reminds us of the brain of insects. Its anterior portion appears from embryology to be very primitiv. The small brain, or cerebellum, which in all higher forms is the centre for co-ordination of the motions of the bod. The medulla, which controls especially the internal organs. The spinal marrow, or that portion of the nervous system which lies outside of the head, is at the same time a great nerve-trunk and a centre for reflex action of the muscles of the body. But the development of these distinct portions and the division of labor between them must have been a long and gradual process.

We have every reason to believe that here, as in insects, the head has been formed by annexation of segments from the rump and the fusion of their nervous matter with that of the brain. But here, instead of only three segments, from nine to fourteen have been fused in the head to furnish the material for the brain. Notochord and backbone may be the most striking and apparent characteristic of vertebrates, but their predominant characteristic is brain. On this system they lavished material, giving it from three to four times as much as any lower or earlier group had done. They very early set apart the cerebral lobes to be the commander-in-chief and centre of control for all other nerve-centres. To this all report, and from it all directly or indirectly receive orders. It can say to every other organ in the body, “Starve that I may live.” It is the seat of thought and will. The other portions of the brain report to it what they have gathered of vision or sound; it explains the vision or song or parable. It is relieved as far as possible from all lower and routine work that it may think and remember and govern. The vertebrate built for mind, not neglecting the body.

Every trait of vertebrates is a promise of a great future. Its internal skeleton gives it the possibility of large size. This gave it in time the victory in the struggle with its competitors, as to whether it should eat or be eaten. It is vigorous and powerful, for all its organs are at the best. It gives the possibility of later, on land, becoming warm-blooded, i.e., of maintaining a constant high temperature. It is thus resistant to climate and hardship. In time its descendants will face the arctic winter as well as the heat of the tropics.

But it has started on the road which leads to mind. The greater size is correlated with longer life. The lessons of experience come to it over and over again, and it can and must learn them. It is the intelligent, remembering, thinking type. The insect had begun to peer into the world of invisible and intangible relations, the vertebrate will some day see them. This much is prophecied in his very structure. He must be heir to an indefinite future.

You have probably noticed that the vertebrate differs greatly from all his predecessors. The gulf between him and them is indeed wide and deep. His origin and ancestry are yet far from certain. But an attempt to decipher his past history, though it may lead to no sure conclusions, will yet be of use to us. Practically all aquatic vertebrates lead a swimming life, neither sessile nor creeping. The embryonic development of our appendages leads to the same conclusion. We must never forget that the embryonic development of the individual recapitulates briefly the history of the development of the race. Now the legs and arms, or fore- and hind-legs, of higher vertebrates and the corresponding paired fins of fish develop in the embryo as portions of a long ridge extending from front to rear of the side of the body.

This justifies the inference that the primitive vertebrate ancestor had a pair of long fins running along the sides of the body, but bending slightly downward toward the rear so as to meet one another and continue as a single caudal fin behind the anal opening. Such fins, like the feathers of an arrow, could be useful only to keep the animal “on an even keel” as it was forced through the water by the lateral sweeps of the tail. They would have been useless for creeping.

But there is another piece of evidence that he was a free swimming form. All vertebrates breathe by gills or lungs, and these are modified portions of the digestive system, of the walls of the oesophagus, from which even the lung is an embryonic outgrowth. Now practically all invertebrates breathe through modified portions of the integument or outer surface of the body, and their gills are merely expansions of this. In the annelid they are projections of the parapodia, in the mollusk expansions of the skin, where the foot or creeping sole joins the body. Why did the vertebrate take a new and strange, and, at first sight, disadvantageous mode of breathing? There must have been some good reason for this. The most natural explanation would seem to be that he had no projections on his outer surface which could develop into gills, and farther, that he could not afford to have any. Now projections on the lower portion of the sides of the body would be an advantage in creeping, but a hindrance in any such mode of swimming as we have described, or indeed in any mode of writhing through the water.

Furthermore, if he lived, not a creeping life on the bottom, but swimming in the water above, he would have to live almost entirely on microscopic animals and embryos; and these would be most easily captured by a current of water brought in at the mouth. The whole branchial apparatus in its simplest forms would seem to be an apparatus for sifting out the microscopic particles of food and only later a purely respiratory apparatus. Moreover, we have seen that the parapodia of annelids naturally point to the development of an external skeleton, for their muscles are already a part of the external body-wall and attached to the already existing horny cuticle. The logical goal of their development was the insect.

Now I do not wish to conceal from you that many good zooelogists believe that the vertebrate is descended from annelids; but for this and other reasons such a descent appears to me very improbable. It would seem far more natural to derive the vertebrate from some free swimming form like the schematic worm, whose largest nerve-cord lay on the dorsal surface because its branches ran to heavy muscles much used in swimming. Later the other nerve-cords degenerated, for such a degeneration of nerve-cords is not at all impossible or improbable. “No thoroughfare” is often written across paths previously followed by blood or nervous impulses, when other paths have been found more economical or effective.

But where did the notochord come from? I do not know. It always forms in the embryo out of the entoderm or layer which becomes the lining of the intestine. Now this is a very peculiar origin for cartilage, and the notochord is a very strange cartilage even if we have not made a mistake in calling it cartilage at all. My best guess would be that it is simply a thickened portion of the upper median surface of the intestine to keep the “balls” of digesting nutriment or other hard particles in the intestine from “grinding” against the nerve-cord as they are crowded along in the process of digestion. Once started its elasticity would be a great aid in swimming.

Professor Brooks has called attention to the fact that the higher a group stands in development, the longer its ancestors have maintained a swimming life. Thus we have noticed that the sponges were the first to settle; then a little later the mass of the coelenterates followed their example. But the etenophora, the nearest relatives of bilateral animals, have remained free swimming. Then the flat worms and mollusks took to a creeping mode of life, while the annelids and vertebrates still swam. Then the annelids settled to the bottom and crept, and all their descendants remained creeping forms. The vertebrates alone remained swimming, and probably neither they nor their descendants ever crept until they emerged on the land, or as amphibia were preparing for land life. If this be true, it is a fact worthy of our most careful consideration. The swimming life would appear to be neither as easy nor as economical as the creeping. It is certainly hard to believe that food would not have been obtained with less effort and in greater abundance at the bottom than in the water above. The swimming life gave rise to higher and stronger forms; but did its maintenance give immediate advantage in the struggle for existence? This is an exceedingly interesting and important question, and demands most careful consideration. But we shall be better prepared to answer it in a future lecture.

The period of development of mollusks, articulates, and vertebrates, is really one. They developed to a certain extent contemporaneously. The development of vertebrates was slow, and they were the last to appear on the stage of geological history.

You must all have noticed that development, during this period, takes on a much more hopeful form than during that described in the last chapter. Then digestion and reproduction were dominant. Now muscle is of the greatest importance. If this fails of development, as in mollusks, the group is doomed to degeneration or at best stagnation. But we have seen the dawn of a still higher function. In insects and vertebrates the brain is becoming of importance, and absorbing more and more material. This is the promise of something vastly higher and better. Better sense-organs are appearing, fitted to aid in a wider perception of more distant objects. The vertebrate has discovered the right path; though a long journey still lies before it. The night is far spent, the day is at hand.