CHAPTER XIX

GEOLOGICAL HISTORY OF VERTEBRATE ANIMALS (INCLUDING MAN)

Vertebrates comprise the highest subkingdom of all animals with man himself at the very top. They are characterized by the possession of a vertebral column, which, in all but the very simple or primitive forms, is an ossified backbone. Their main subdivisions are given in the classification table near the beginning of the preceding chapter. The oldest known Vertebrates, found in fossil form in middle Ordovician strata, were represented by curious and bizarre creatures called ostracoderms, or more popularly "armor fishes." They were not true fishes because they were really somewhat lower in the scale of organization than fishes. Some were distinctly fishlike in appearance, and others notably resembled certain of the Arthropods, so that some students consider them to have formed the connecting link between the highest Invertebrates (Arthropods) and low order fishes of the Vertebrates. The vertebral column always consisted of cartilage or gristle and, in most forms, it extended through tail fin. None had true side fins like fishes, but many were provided with a pair of jointed flappers or paddles. The jawlike portions of the heads moved over each other sidewise as, for example, in beetles and not up and down in true Vertebrate fashion. Two eyes were always very close together. One of the most striking features was the protection of the head and fore part of the body by an armor of bony plates, while the rest of the body had scales. They seldom grew to be more than six or seven inches long. Beginning in the Devonian, they remained rare during the Silurian, and then in the Devonian period they reached their climax of development only to become extinct at its close. Many species were abundantly represented in many parts of the world. By some the Ostracoderms are thought to have been a primitive (sharklike) fish development in the wrong direction, and hence they became extinct.

Fishes, represented only by very primitive sharks, are known to have existed as early as the Silurian period, but the remains are scant. During the Devonian period, however, they showed a marvelous development into many species and countless myriads of individuals. The Devonian is, therefore, commonly called the "Age of Fishes." These very ancient (Devonian) primitive (fish) types of Vertebrate animal life are of profound significance in organic evolution because they were the direct progenitors of the great groups of still higher Vertebrates which since later Paleozoic time gradually increased in diversity and complexity of structure through amphibians, reptiles, birds, and mammals finally to man himself.

In marked contrast to the most typical and highly organized fishes so abundant to-day, all Devonian fishes were of simple types with cartilaginous skeletons and vertebrated tails. Many of them were also generalized types, that is, associated with their clearly defined fish characters were others connecting them with certain higher Vertebrates, as, for example, amphibians and reptiles. Thus all their tail fins were vertebrated as in reptiles; their labyrinthine, internal tooth structure was to be an amphibian feature when those creatures evolved; many had protective armor or bony scales like most early amphibians and many modern reptiles; and many had paired fins which were something like jointed legs. Most abundant and highly organized of the Devonian fishes were "ganoids," characterized by a covering of small plates or bony scales set together but not overlapping like in typical modern fishes. Their intricate tooth structure and limblike fins strongly suggest the amphibians of later Paleozoic time. The skeleton of cartilage gradually became somewhat ossified during succeeding geologic periods. From their great profusion and diversity in the Devonian period the ganoids have steadily fallen away until they now have very few descendants like the gar pike.

Another important group of remarkable fishes, now totally extinct, but common in Devonian and somewhat later time, had heavy, bony armor plates over the fore part of the body. Those which grew to be fifteen to twenty-five feet long were probably the rulers of the middle Paleozoic seas. Another remarkable Devonian fish was able to breathe in both water and air because, like their few modern descendants, they had both gills and lungs. Because of their leglike fins and lung sac, it is commonly believed that they were progenitors of the later Paleozoic amphibians. The simplest of all fishes, the sharks, began in the Silurian, underwent no important change through the millions of years since, and are now of course well represented. During early Cenozoic time the sharks seem to have reached culmination in size--sixty to eighty feet long, with teeth five or six inches long.

Among modern fishes the most abundant by far, and the most highly organized, are the true bony fishes, called the "teleosts," which made their first appearance in the middle of the Mesozoic era. Those earliest forms clearly show their descent from the ganoids. Apparently they have not yet passed their prime.

We shall now consider the next higher group of Vertebrates, the amphibians, which breathe by gills when young and later develop lungs. Many live both on land and in water like the frogs. Unlike fishes they have legs with toes and not fins. Beginning probably in the Devonian as a branch of the fishes, amphibians showed a marvelous development during later Paleozoic and very early Mesozoic times when they reached their climax, after which they fell off remarkably, being now relatively unimportant like the frogs and salamanders. They are of special significance because they were the first of all the back-boned animals (Vertebrates) to inhabit the land which they dominated only until the great rise of reptiles of Mesozoic time. The reptiles in fact evolved from the amphibians in the late Paleozoic when many transition forms occurred. (Plate 15.) During those ancient days the numerous and very diversified amphibians were like giant salamanders, commonly five to eight feet long, with one Triassic form fifteen to twenty feet long, and with heavily armored skulls two to four feet long.

Turning now to the reptiles we find that they are much more distinctly land animals than the preceding types of Vertebrates. Reptilian life of the earth began in late Paleozoic time as an evolutionary branch of the amphibians. The earliest forms were in many ways much like the amphibians, but gradually they diversified and progressed so that before the close of the Mesozoic era, which has long been called the "Age of Reptiles," they were the rulers of the world. "They covered the land with gigantic herbivorous and carnivorous forms; they swarmed in the sea, and, as literal dragons, they dominated the air." (Scott.) Mesozoic reptiles are of special interest and significance not only in themselves, but also because from one of their branches the birds were evolved, and from another the mammals. "In advancing from the amphibian to the reptile the evolution of the Vertebrates was far from finished. The cold-blooded, clumsy and sluggish, small-brained and unintelligent reptile is as far inferior to the higher mammals, whose day was still to come, as it is superior to the amphibian and the fish." (Norton.)

Since the reptiles of the Mesozoic era constitute one of the few most remarkable and diversified classes of animals which ever inhabited the earth, we shall attempt to give the reader a fair idea of the most typical groups which have been totally extinct since the close of the Mesozoic era some millions of years ago. Of the swimming reptiles which lived in the seas many types are known and only a few will be described. Among these one important type was the ichthyosaur, a fishlike form which not uncommonly grew to be twenty to even forty feet long (Plate 18). The large head, sometimes four or five feet long, contained as many as 200 big sharp teeth and enormous eyes up to a foot in diameter. The body was heavy set, and the neck very short. There were four short, stout swimming paddles, and the tail was vertebrated. Some specimens of ichthyosaurs have been so perfectly preserved in Mesozoic strata that even the unborn young are plainly seen in the bodies! In some cases it is actually possible to tell what was the last meal of a particular ichthyosaur those millions of years ago; in one specimen, for example, remains of 200 creatures of the "cuttle-fish" tribe having been found in the exact position of the stomach.

The mosasaurs of the late Mesozoic were the only real sea serpents of the geologic ages. They were something like the ichthyosaurs, but with smaller heads and much longer, more slender, serpentlike bodies. Some grew to be thirty or forty feet long.

Plesiosaurs were perhaps the strangest of all the Mesozoic marine reptiles. They grew to be forty to fifty feet long, with stout body, very long, slender neck, small head, short tail, and four long, powerful swimming paddles which were distinctly leglike. These and the mosasaurs were both flesh eaters, as shown by the sharp teeth.

The most remarkable walking reptiles of all time were the dinosaurs or "terrible lizards." We shall describe enough types of these unique creatures to give the reader a fair idea of their appearance and habits. Most astonishing of all were the sauropods including the largest animals which ever trod the earth. They grew to be as much as sixty to ninety feet or more in length. Remarkably well preserved skeletons have been found, one from Utah, eighty-seven feet long, being mounted in the Carnegie Museum of Pittsburgh. The largest of these brutes stood fifteen to twenty feet high and they must have weighed thirty to fifty tons. The very long, serpentlike neck and tail, and very small head were grotesque features. Considering the structure of the dinosaurs, the kind of strata in which they are embedded, and the associated fossil remains, it seems clear that they mostly lived in and near fresh water and on near-by lowlands. The character of their teeth shows that they fed entirely on soft plants which they must have habitually bolted because their teeth were not well adapted to grinding food. It is difficult to believe that a single huge beast could have consumed less than a few hundred pounds of vegetable matter per day, and, on account of the very small size of the head, he must have spent most of his time eating. Also the comparatively very small size of the brain, and its simplicity of structure, render it certain that they were extremely stupid creatures. "To make up for this they had an enormous enlargement of the spinal cord in the sacral region (i.e., over the hind legs). This sacral brain--if we may so call it--was ten to twenty times bigger than the cranial brain. It was necessary in order to work the powerful hind legs and tail." (Le Conte.)

Another dinosaur, in some respects like the sauropod, was the stegosaur which grew to be twenty to thirty feet long, and heavier than the elephant. Unlike the sauropod, it had a short neck and was armored with a double row of great plates over its back, and sharp spines (one to three feet long) toward the end of the tail. The excessive stupidity of the creature is proved by the fact that its very simple brain weighed less than three ounces! Stegosaurs were plant eaters as indicated by the tooth structure, and, though they looked ferocious, they were probably not fighters, certainly at least nothing like the carnivorous types of dinosaurs we shall soon describe.

The ferocious dinosaurs of Mesozoic time were carnivorous, or flesh eaters, as shown by their numerous sharp teeth in relatively large heads. The largest known type is the tyrannosaur, an almost perfect skeleton of which, 40 feet long and 16 feet high, is mounted in the American Museum of Natural History in New York (Plate 17). So far as known, this was the largest carnivorous animal which ever walked on the earth. It is evident from the structure that it walked on its hind legs, the front ones having been much shorter and used something like arms. There were also various other smaller forms of two-legged flesh-eating dinosaurs, many of the wonderfully preserved tracks in the Triassic sandstones of the Connecticut River Valley having been made by such creatures when they walked around over soft, sandy mud flats at least eight or ten million years ago. The sandy mud with its tracks became somewhat hardened and then deeply buried under much more sediment which, through the ages, has been eroded off, thus exposing to view certain of the layers covered with tracks. Some bones of dinosaurs have also been found in the Connecticut Valley.

Another remarkable type of two-legged dinosaur was much like the flesh eaters just described, but they were plant eaters. The largest of these grew to be 30 feet long and 15 to 20 feet high, comparable, therefore, to the tyrannosaur in size. A wonderful collection of almost perfect skeletons may be seen in the museum in Brussels, Belgium. In mining coal 1,000 feet below the surface in Belgium, twenty-two complete skeletons and several partial skeletons were found in an ancient river deposit of Cretaceous Age. A marvelously preserved specimen of one of these two-legged plant eaters found in Wyoming, has been called a "dinosaur mummy" because the skin and much of the flesh of the creature had shriveled down upon its bones. The minutest details of the texture of its skin are almost perfectly preserved.

Another type of dinosaur, so different from the others, should be briefly described. This was triceratops, or the "three-horned face" beast, so named because of the three powerful horns which projected forward from the top of the very large, flattened skull. It grew to be twenty to twenty-five feet long. Skulls six to eight feet long have been unearthed. Just where the brain might have developed, the skull dished downward, and so one authority considers triceratops to have had the largest head and smallest brain of all the great reptiles.

It is well known that dinosaurs of many types lived during the great "Age of Reptiles," though by no means all types ranged through the whole era. No dinosaurs are definitely known to have crossed the line into the Cenozoic era. One of the most astonishing facts in the history of animal life is the extinction of the mighty dinosaurs, but no very satisfactory explanation has yet been offered. Probably their great size was a contributing factor, for it is well known "that while very large animals spend nearly all their time in eating, small animals spend a small proportion of theirs, and most of it in other activities. Now, as long as food is abundant, the larger animals of a race have the better chances, but if a scarcity of food ensues, the larger animals may all be suddenly swept out of existence." (Matthew.) Whatever may have been the real reason for dinosaur extinction we can at least be sure "that with the extensive changes in the elevation of land areas (Rocky Mountain Revolution) which mark the close of the Mesozoic, came the withdrawing of the great inland Cretaceous seas along the low-lying shores of which the dinosaurs had their home, and with the consequent restriction of old haunts, came the blotting out of a heroic race. Their career was not a brief one, for the duration of their recorded evolution was twice that of the subsequent mammalian (Cenozoic) age. They do not represent a futile attempt on the part of nature to people the world with creatures of insignificant moment, but are comparable in majestic rise, slow culmination, and dramatic fall to the greatest nations of antiquity." (Schuchert.)

Among the most extraordinary animals not only of the Mesozoic, but also of all time, were the flying reptiles or literal dragons of the air. Some were very small, while others were the largest creatures which ever flew, with a spread of wing of twenty to twenty-five feet--twice that of any modern bird. Unlike birds they had no feathers, but the two wings consisted of large membranes (batlike) supported by one enormously elongated finger of each front limb. The other fingers were armed with sharp claws. The early Mesozoic flying reptiles had sharp teeth, while the later ones were mostly entirely toothless, but all were carnivorous. Their short bodies were supplied with tails of varying lengths, one long-tailed species having a rudder at the end. Their heads were fairly large, but of light build. The creature called "pteranodon" was not only the largest of the flying reptiles, but also probably the most highly specialized creature which ever lived, everything possible apparently having been sacrificed to facilitate flight (Plate 18). The hollow bones were so wonderfully light and strong that it has been estimated that the living animal, with twenty-five foot spread of wing, and head four feet long, could not have weighed more than twenty-five pounds! The rear portions of the body and hind limbs were very weak.

It should not be thought that the above-described groups of reptiles were the only ones which existed during Mesozoic time. There were also certain groups still living, like turtles, lizards, and crocodiles, but they were doubtless mostly completely under the dominance of certain of the now long-extinct types above described. The oldest-known fossil snakes are from very late Mesozoic rocks, where they are small and comparatively rare. More than likely they evolved from lizards by deterioration of the legs. Poisonous snakes were not evolved until early in the next (Cenozoic) era.

We shall now turn our attention to next to the highest class of Vertebrate animals--the birds. They and the mammals are the only warm-blooded animals. What is their ancestry? From what original stock did they branch off? The oldest-known bird lived during the Jurassic period, and it was so decidedly reptilian in character as to render it practically certain that birds are specialized descendants of certain Mesozoic reptiles, though not, as might be supposed, of the flying reptiles. The few known specimens of the Jurassic birds were found in the famous lithographic limestone quarries of Bavaria. At least two of the specimens are in a marvelous state of preservation, with practically the whole skeleton intact and almost perfect impressions of the feathers on the rock. That the creature was really a bird is proved not only by its feathers, but also its beak, brain, limb bones, and feet. Among the reptilian characters are its long, vertebrated tail, teeth set in sockets, and long claws on the wings. This reptilian bird was about the size of a small crow.

By late Cretaceous time the birds made notable evolutionary progress and they became diversified, more than thirty species being known from Cretaceous rocks. These were distinctly more modern in structure and appearance than the Jurassic bird. The only important reptilian characteristic still retained was the possession of teeth. The tail had become much shortened and the brain was still relatively smaller than in modern birds. One type, about nine inches high, was a powerful flier, as shown by the strong keel and wing bones. Another important Cretaceous type was almost wholly a water dweller, with powerfully developed legs used in swimming. Its teeth were set in grooves instead of in sockets, thus indicating degeneration of tooth structure. This type was notable for its size--five to six feet in length.

During the early part of the Cenozoic era birds became still more advanced and numerous, with many modern groups represented. Some of the more primitive types were, however, still left over during the Tertiary, as, for example, a toothed bird, in which the teeth were merely dentations of the bill, thus being the most degenerate of all types of tooth structure.

Mammals comprise the highest class of all animals. They are, of course, all warm blooded and characterized by suckling their young. So far as known, mammal life began in the early Mesozoic era as a branch of primitive reptiles, but they made little progress throughout the era when they occupied a very subordinate position in the animal world. They were few in number, small, and primitive in structure. There is no evidence for the Mesozoic existence of any of the higher forms of mammals, that is, those which give birth to well-formed young which are prenatally attached to the mother by the so-called placentum. "During the eons of the Mesozoic, from late Triassic time until its close, the mammals (including the remote progenitors of humanity) were in existence, but held in such effective check (by reptiles) that their evolutionary progress was practically insignificant. This curb is strikingly illustrated by the wonderful series of tiny jaws and teeth of these diminutive creatures found in the Comanchian (early Cretaceous) of Wyoming, in actual association with the single tooth of a carnivorous dinosaur, many times the bulk of the largest mammalian jaw. The removal of this check resulted (in the Tertiary period) in the speedy evolution of the archaic mammals." (Schuchert.)

The phenomenal development of mammals during the Tertiary period forms one of the most wonderful chapters in the whole evolution of organisms. Even very early in the Tertiary, many important higher (placental) types of mammals had evolved, and the simpler, more primitive Mesozoic forms became very subordinate. By the close of the Tertiary the higher types of mammals had become marvelously differentiated into most of the present-day groups or types. A very significant feature of the evolution was the steady increase in relative size of brain. The vast numbers of fossil skeletons and bones of mammals found in Tertiary strata is scarcely believable. In our brief discussion we can do no more than describe a few representative examples of the Cenozoic evolution of mammals.

The great diversity of modern placental animals may be suggested by a few examples, as the tiger, dog, horse, camel, elephant, squirrel, hedgehog, whale, monkey, and man. Forms like these, traced back through their ancestors to the very early part of the Tertiary period, gradually become less and less distinct until they cannot be at all distinguished as separate groups, but rather there are ancestral generalized forms which show combinations of features of the later groups. Those early Tertiary generalized placental mammals had four feet of primitive character, with five toes on each foot; the whole foot, which from toe to heel touched the ground, was not adapted to swift running; the teeth were simple (primitive) in type and of full original number (forty-four); the toes were supplied with nails which were about intermediate between real claws and hoofs in structure; and the brain was relatively much smaller and simpler in structure than in most modern mammals.

The history of the horse family furnishes an excellent illustration of certain evolutionary changes among mammals. Skeletons of many species, ranging from the early Tertiary to the present, have been found in remarkable state of preservation representing every important change in the history of the horse family. A study of the chart will make clear some of the most striking changes which have taken place. The oldest member of the horse family represented on the chart was about the size of a small fox, with four toes and a degenerated fifth toe (splint) on the front foot, and three toes and splint on the hind foot. Since the chart was made a still more primitive form, even more closely resembling the original five-toed ancestor, has been found. Gradually the middle toe enlarged, while the others disappeared except the two splints or very degenerate toes still left in the modern horse. Increase in size of the animal and brain capacity accompanied these changes. Also the teeth underwent notable change, and two originally separate bones (radius and ulna) of the foreleg became consolidated into a single stronger bone.

The even-toed hoofed mammals of to-day, like the deer, pig, and camel, are also the product of evolution much like that of the horse, except that two of the original five toes have been equally developed, while the others have either greatly degenerated, as in the pig, or disappeared entirely, as in the camel.

The elephants, or trunk-bearing animals, illustrate a very different kind of evolution. They seem to have reached their climax of development in the late Tertiary when they grew to be as much as 14 feet high, and were more abundant and widespread over the earth than at any other time. The modern elephant, like the horse, has been traced back through many intermediate forms to its primitive early Tertiary ancestry. Some of the most important evolutionary changes took place in the head portion. The trunk is a highly developed form of snout, the earliest form of which was much like that of the modern tapir. The tusks are highly specialized and elongated teeth. During the earlier history the chin was very long and supported short tusks, so that there were then four tusks.

Carnivorous mammals, like tigers and wolves, and gnawers, like rats and squirrels, may also be traced back to generalized early Tertiary types.

Another kind of evolution is well illustrated by certain mammals which, even in early Tertiary time, so thoroughly adapted themselves to a water environment as to become whales, porpoises, etc.

The primates include the highest group of all Vertebrates, and therefore of all animals. Monkeys, apes, and man belong to the primates. There is no evidence whatever for the appearance of even the simplest and most primitive forms before the opening of the Cenozoic era, but even very early in Tertiary time, lemurs and primitive types of monkeys existed. Later in the Tertiary true monkeys and apes were common, and by the close of the period some apes were highly enough developed to strongly resemble certain of the oldest and most primitive types of man. We have, however, no positive knowledge of the existence of man in even the latest Tertiary. In the light of much evidence in regard to the antiquity of man, it seems improbable that true human fossils will ever be found in rocks older than the Quaternary, though if we are willing to descend (far enough in the human scale toward apes) it is not unlikely that man-apes may be discovered in very late Tertiary rocks. The difficulty comes in the classification. Where are we to draw the line between the higher apes and the lowest forms of man? But this very difficulty is one of the strongest arguments in favor of the organic evolution of man because practically all intermediate forms between true man and certain other high-grade primates are known from the strata. The following tabular summary of the geological history of man is based upon the work of most of the ablest students of the subject.

==========================+===============================+============== 3. Homo sapiens |Historic (bronze and iron) age.|Modern (e.g., modern man) | | |Neolithic ("recent stone") age |Postglacial | (carefully shaped and |but | polished stone implements) |prehistoric --------------------------+-------------------------------+-------------- 2. Homo primigenius |Upper Paleolithic ("ancient |Late Glacial (e.g., Neanderthal | stone") age (rough bone | man) | and stone implements, cave | | frescoes, bone carvings, | | etc.) | |Lower Paleolithic ("ancient |Middle Glacial | stone") age (rude stone | | implements of so-called | | "river man") | --------------------------+-------------------------------+-------------- 1. Early ancestral forms |Possibly some very crude |Early Glacial (e.g., Pithecanthropus | stone implements |and possibly erectus) | |late Tertiary --------------------------+-------------------------------+--------------

Of the early ancestral forms, that is, those which were rather distinctly man-apes, two will be very briefly referred to. One of these, known as _Pithecanthropus erectus_, was a remarkable creature whose partial skeleton, consisting of the upper part of a skull, lower jaw, several teeth, and a thigh bone, was found in early Quaternary deposits in Java in 1891. It was certainly a man-ape or possibly ape-man of low order, about 5-1/2 feet high. The skull has a low crown, very receding forehead, and prominent brow ridges, but the brain capacity is 850 cubic centimeters, as compared to 500 cubic centimeters in ordinary higher apes, and nearly 1,500 cubic centimeters in the average modern man. The very recently extinct very low-type aborigines of Tasmania had a skull capacity of 1,199 cubic centimeters.

In 1907 the lower jaw of an anthropoid or manlike ape set with rather human teeth was found associated with very crude stone implements seventy-five feet below the surface in river-deposited sand in Germany. It is of either early or middle Glacial time and quite certainly represents a lower order creature than the oldest Paleolithic man as described below.

Many bones and implements of Paleolithic man (see above table) have been found mainly in river gravels and caves. The relative ages of Paleolithic human bones and implements are best determined by the associated fossil animals. Thus the most ancient truly human fossils are found directly associated with bones of very old types of elephants, rhinoceroses, and hippopotamuses which are definitely known to have lived during middle or early middle Glacial (Quaternary) time corresponding to early Paleolithic time. A very conservative estimate would make the age of such very old human remains at least 150,000 to 250,000 years because the Ice Age was at least 500,000 years long. In a later human stage there are many associations with extinct animals like an older type of mammoth, cave bear, cave hyena, and others of later Glacial time estimated at 50,000 to 150,000 years ago. Last of all was the latest Paleolithic stage corresponding to the close of the Ice Age, the human remains of which are found associated with reindeer and the latest mammoths which roamed in great numbers across Europe. This was probably not more than 30,000 to 50,000 years ago.

Paleolithic man is so called because he fashioned stone weapons and implements. The structure of skull and skeleton shows him to have been a low-type savage, something over five feet high on the average, with a forward stooping carriage. The average Paleolithic brain was not greatly inferior in size to that of modern civilized man, but it was not so highly organized and occupied a thick skull with much lower forehead and heavy brow ridges. The bushmen of Australia and the recently extinct Tasmanians are the nearest modern resemblances. Many fine specimens of Paleolithic man have been found, especially in cave deposits. That he was an expert hunter is proved by the great accumulation of bones of now extinct animals found in and about his haunts or camps, bones representing at least 100,000 horses having been found around a single camp site!

Only two among the many known Paleolithic man localities will be briefly described. In the Perigord district of southwestern France a number of caves contain human relics ranging in age from early to late Paleolithic. Of special interest among these relics are fishhooks made of bone, and crude sketches of animals such as the mammoth and reindeer now extinct in that region. The Aurignac cave, also in France, was no doubt a family or tribal burial place. Seventeen Paleolithic human skeletons, associated with bones of extinct animals and crude art works, were found in the cave. Near the entrance there were ashes and charcoal mixed with burned and split bones of extinct animals. Certain of the caves occupied by late Paleolithic man have their walls decorated with sketches and even colored pictures. These are, therefore, the oldest known art galleries. An excellent example is the cave at Altamira in northern Spain. "As we glance at the pictures one of the first things to impress us is the excellence of the drawing, the proportions and postures being unusually good.... The next observation may be that, in spite of this perfection of technique, there is no perspective composition--that is, no attempt to combine or group the figures.... It is also clear that the work of many different artists is represented, covering a considerable period of time. The walls show traces of many other paintings that were erased to make way for new work." (Wissler.)

The Neolithic, or "recent stone" age was a gradual development from the late Paleolithic, and man was then more highly developed and more similar in structure to modern man. His stone implements were more perfectly made, and often more or less polished and ground at the edges. "The remains of Neolithic man are found, much as are those of the North American Indians, upon or near the surface, in burial mounds, in shell heaps (the refuse heaps of their settlements), in peat bogs, caves, recent flood-plain deposits, and in beds of lakes near shore where they sometimes built their dwellings upon piles.... Neolithic man in Europe had learned to make pottery, to spin and weave linen, to hew timber, and build boats, and to grow wheat and barley. The dog, horse, ox, sheep, goat, and hog had been domesticated." (Norton.)

"Man is linked to the past through the system of life, of which he is the last, the completing creation. But, unlike other species of that closing system of the past, he, through his spiritual nature, is more intimately connected with the opening future." (J. D. Dana.)