The alternation of the cold climates of the glacial stages with the warm temperate climates of the interglacial stages formed great oscillations of temperature (Figs. 13, 14). The fossil plant life indicates that during the periods of the First, Second, and Third Interglacial Stages the climate of western Europe was cooler than it had been during the preceding Pliocene Epoch and somewhat warmer than it is at the present time in the same localities. During the First, Second, and Third Glacial Stages there was certainly a marked lowering of temperature in the regions bordering the great glacial fields. This is indicated by the arrival in the northern glacial border regions of animals and plants adapted to arctic and subarctic climates.

It has been generally believed that the whole of western Europe was extremely cold during these glacial stages, and that the heat-loving animals, the southern elephants, rhinoceroses, and hippopotami, were driven to the south, to return only with the renewed warmth of the next interglacial stage.

There is, however, no proof of the departure of these supposedly less hardy mammals nor of the spread over Europe of the more hardy arctic and steppe types until the advent of the Fourth Glacial Stage. Then, for the first time, all western Europe north of the Pyrenees experienced a general fall of temperature, and conditions of climate prevailed such as are now found in the arctic tundra regions of the north and in the high steppes of central Asia, which are swept by dry and cold winter winds. Fluctuations of temperature, of moisture, and of aridity in Pleistocene time, are evidenced not only by the rise and fall of the snow-line and the advance and retreat of the ice-caps but also by the appearance of plant and animal life in the periods of the 'loess' deposition, indicating the following cycles of climatic change as witnessed from beginning to end of the Third Interglacial Stage:

That great fields of ice and advancing glaciers alone do not constitute proof of very low temperatures is shown at the present time in southeastern Alaska, where very heavy snowfall or precipitation causes the accumulation of vast glaciers, although the mean annual temperature is only 10° Fahr. (5.56° C.) lower than that of southern Germany. Neumayr(69) estimated that during the Ice Age there was a general lowering of temperature in Europe of not more than 6° C. (10.8° Fahr.), and held that even during the glacial advances a comparatively mild climate prevailed in Great Britain. Martins(70) estimated that a lowering of the temperature to the extent of 4° C. (7.2° Fahr.) would bring the glaciers of Chamonix down to the level of the plain of Geneva. Penck estimates that, all the atmospheric conditions remaining the same as at present, a fall of temperature to the extent of 4° to 5° C. would be sufficient to bring back the Glacial Epoch in Europe. These moderate estimates entirely agree with our theory that animals of African and Asiatic habit flourished in western Europe to the very close of the Third Interglacial Stage, and that then for the first time the warm fauna, or faune chaude, gradually disappeared.

Similarly the hypothesis of extremely warm or subtropical conditions prevailing in interglacial times as far north as Britain, which originated with the discovery of the northerly distribution of the hippopotami and rhinoceroses, animals which we now associate with the torrid climate of Africa, is not supported by the study either of the plant life of interglacial stages or by the history of the animals themselves. It is quite probable that both the hippopotami and the rhinoceroses of the 'warm fauna' were protected by hairy covering, although not by the thick undercoating of wool which protected the woolly rhinoceros and woolly mammoth, animals favoring the borders of glaciers and flourishing during the last very cold glacial and Postglacial periods.

The combined evidence from all these great events in western Europe leads us to conclusions somewhat different from those reached by Penck as to the chronology of human culture. In the chart (Fig. 14) on the opposite page, prepared by Dr. C. A. Reeds in collaboration with the author, a new correlation of geologic, climatic, human, industrial, and faunal events is presented. The great waves of glacial advance and retreat (oblique shading) are based upon Penck's estimates of the rise and fall of the snow-line (vertical dotted lines) in the Swiss Alps. (Compare Fig. 13.) The length of these waves corresponds with the relative duration of the glacial and interglacial stages as estimated by the varying amounts of erosion and deposition of materials. The entire Palæolithic or Old Stone Age is thus seen to occupy not more than 125,000 years, or only the last quarter of the Glacial Epoch, which is estimated as extending over a period of 525,000 years. The present opinion of the leading archæologists of France and Germany, which is shared by the author, is that the Pre-Chellean industry is not older than the Third Interglacial Stage. As the Piltdown man was found in deposits containing Pre-Chellean implements, he probably lived in the last quarter of the Glacial Epoch, and not in early Pleistocene times as estimated by some British geologists. This causes us to regard the Piltdown remains as more recent than the jaw of Heidelberg, which all authorities agree is probably of Second Interglacial Age. According to our estimates the Heidelberg man is nearly twice as ancient as the Piltdown man, while Pithecanthropus (Trinil Race) is four times as ancient. Yet the Piltdown man must still be regarded as of very great antiquity, for he is four times as ancient as the final type of Neanderthal man belonging to the Mousterian industrial stage. The various archæologic and palæontologic evidences for this general correlation theory of the Glacial Epoch are fully discussed in the succeeding chapters of this volume.

Mammals of Five Distinct Geographic Regions

(Compare Color Map, Pl. II, and Fig. 15)

As we have already observed, during the whole history of mammalian life in various parts of the world never did there prevail conditions so unusual and so complex as those which surrounded the men of the Old Stone Age in Europe. The successive races of Palæolithic men in Europe were all flesh eaters, depending upon the chase. The mammals, first pursued only for food, utensils, and clothing, finally became subjects of artistic appreciation and endeavor which resulted in a remarkable æsthetic development.

From the beginning to the end of Palæolithic times the various races of man witnessed the assemblage in Europe of animals indigenous to every continent on the globe except South America and Australia and adapted to every climatic life-zone, from the warm and dry plains of southern Asia and northern Africa to the temperate forests and meadows of Eurasia; from the heights of the Alps, Himalayas, Pyrenees, and Altai Mountains to the high, arid, dry steppes of central Asia with their alternating heat of summer and cold of winter; from the tundras or barren grounds of Scandinavia, northern Europe, and Siberia to the mild forests and plains of southern Europe.(71) Members of all these highly varied groups of animals had been evolving in various parts of the northern hemisphere from the Eocene Epoch onward. In Pliocene times they had become thoroughly adapted to their various habitats. Throughout early Pleistocene times, with the increasing cold extending southward from the arctic circle, such mammals as the elephant, rhinoceros, musk-ox, and reindeer had become thoroughly adapted to the climate of the extreme north. There is every reason to believe that when these tundra quadrupeds first arrived in Europe, during early mid-glacial stages, they had already acquired the heavy coat of hair and undercoating of wool, such as now characterizes the musk-ox, one of the living representatives of this northern fauna.

MIGRATIONS AND EXTINCTIONS OF MAMMALIAN LIFE DURING THE
FOUR GLACIAL, THREE INTERGLACIAL, AND POSTGLACIAL STAGES

The five great sources of mammalian migration into western Europe in Pleistocene times were accordingly as follows:

In the warm plains, forests, and rivers of southern Asia and northern Africa there developed the elephants, rhinoceroses, hippopotami, lions, hyænas, and jackals, which, taken together, may be known as the African-Asiatic fauna. It contains altogether fourteen species of mammals. The great geographic area from the far east to the far west over which ranged similar or identical species of these pachyderms and carnivores is indicated by the oblique lines in the geographic chart (Fig. 15).

The north temperate belt of Asia and Europe, with its hardy forests and genial meadows, was the home of the even more highly varied Eurasiatic Forest and Meadow fauna. This includes twenty-six or more species. Of these the red deer, or stag, was most characteristic of the forests and the bison and wild cattle[M] of the meadows. Even at the very beginning of Pleistocene times there appear the stag, the wild boar, and the roe-deer with their natural pursuers, the wolf and the brown bear. From the northern woods came the moose and the wolverene. Most of these mammals were so similar to existing forms that the older naturalists placed them in existing species, but the tendency now is to separate them or place them in distinct subspecies. Mingled with these forest and meadow mammals were a few others which have since become extinct, such as the giant deer (Megaceros), the giant beaver (Trogontherium), and the primitive forest and meadow horses. From this region also there developed the cave-bear (Ursus spelæus). Certainly it is astonishing to find the remains of these mammals mingled with those from southern Asia and Africa, as is frequently the case. In early glacial times the bison and wild cattle mingled freely with the hippopotami and rhinoceroses, but in late glacial and Postglacial times they occurred as companions of the mammoth and the woolly rhinoceros. In prehistoric times they survived with the mammals brought from the Orient by the Neolithic agriculturists.

During a great glaciation, but especially during the severe climate of late Pleistocene times, the Alpine mammals were driven down from the heights into the plains and among the lower mountains and foot-hills. Thus the ibex, chamois, and argali sheep from the Altai Mountains are represented both in drawing and in sculpture by the men of the Reindeer Period.

Still more remarkable is the arrival in Europe of the Steppe Fauna of Russia and of western Siberia, mammals which now survive in the vast Kirghiz steppes, east of the Caspian Sea and the Ural Mountains, where the climate is one of hot, dry summers and prolonged cold winters, with sweeping dust and snow storms. These animals are very hardy, alert, and swift of foot, such as the jerboa, the saiga antelope, the wild asses, and the wild horses, including the Przewalski type, which still survives in the desert of Gobi. From this region also came the Elasmothere (E. sibiricum), with its single giant horn above the eyes. Very distinctive of the fauna frequenting the caverns are the small rodents, including the dwarf pikas, the steppe hamsters, and the lemmings. These animals were attracted into Europe during the 'steppe' and 'loess' periods of cold, dry climate.

The advance of the great Scandinavian glaciers from the north crowded to the south the Tundra or Barren Ground fauna of the arctic circle. The herald of this fauna during the First Glacial Stage was the musk-ox, which appears in Sussex, and then came the reindeer of the existing Scandinavian type. These animals are followed by the woolly mammoth (E. primigenius) and the woolly rhinoceros (D. antiquitatis) with their panoply of hair and wool which had long been developing in the north. Finally in the Fourth Glacial Stage arrived the lemming of the river Obi, also the more northern banded lemming, the arctic fox, the wolverene, and the ermine, as well as the arctic hare. These tundra mammals for a short period mingled in places with survivors of the African-Asiatic fauna, such as Merck's rhinoceros and the straight-tusked elephant (E. antiquus). In general, they swept southward as far as the Pyrenees over country which had long been enjoyed by the African-Asiatic mammals, while the hippopotami and the southern elephants retreated still farther south and became extinct.

The only survivors of the great African-Asiatic fauna in Fourth Glacial and Postglacial times were the hyænas (H. crocuta spelæa) and the lions (Felis leo spelæa). The lion frequently appears in the drawings of the cavemen.

The various species belonging to these five great faunæ apparently succeed each other, and wherever their remains are mingled with the palæoliths, as along the rivers Somme, Marne, and Thames, or in the hearths of the shelters and caverns, they become of extreme interest both in their bearing on the chronology of man and on the development of human culture, art, and industry. They also tell the story of the sequence of climatic conditions both in the regions bordering the glaciers and in the more temperate regions remote from the ice-caps. Thus they guide the anthropologist over the difficult gaps where the geologic record is limited or undecipherable. The general succession of these great faunæ is illustrated in Fig. 14 and also in the above table.

CHAPTER I

None of these fossil anthropoids either of Europe or of Asia can be regarded as ancestral to man, although both Neopithecus and Dryopithecus have been placed in or near the line of human ancestry by such high authorities as Branco and Gaudry. When Dryopithecus was first discovered by Lartet, Gaudry(1) considered it to be by far the most manlike of all the apes, even attributing to it sufficient intelligence for the working of flints, but fuller knowledge of this animal has shown that some of the living anthropoids are more manlike than Dryopithecus. This animal is closely related to the ancestral stock of the chimpanzee, gorilla, and orang. The jaw, it is true, resembles that of the Piltdown man (Eoanthropus), but the grinding-teeth are much more primitive and there is little reason to think that it is ancestral to any human type.[N]

Among these fossil anthropoids, as well as among the four living forms, we discover no evidence of direct relationship to man but very strong evidence of descent from the same ancestral stock. These proofs of common ancestry, which have already been observed in the existing races of man, become far more conspicuous in the ancient Palæolithic races; in fact, we cannot interpret the anatomy of the men of the Old Stone Age without a survey of the principal characters of the existing anthropoid apes, the gibbon, the orang, the chimpanzee, and the gorilla.

Fig. 17. The orang has a high rounded skull and long face.
Photograph from the New York Zoological Park.

The gibbon is the most primitive of living apes in its skull and dentition, but the most specialized in the length of its arms and its other extreme adaptations to arboreal life. As in the other anthropoids, the face is abbreviated, the narial region is narrow, i. e., catarrhine, and the brain-case is widened, but the top of the skull is smooth, and the forehead lacks the prominent ridges above the orbits; thus the profile of the skull of the gibbon (Fig. 16) is more human than that of the other anthropoid apes. When on the ground the gibbon walks erect and is thus afforded the free use of its arms and independent movements of its fingers. In the brain there is a striking development of the centres of sight, touch, and hearing. It is these characteristics of the modern gibbon which preserve with relatively slight changes the type of the original ancestor of man, as noted by Elliot Smith.(2)

The limbs of the orang are less elongated and less extremely specialized for arboreal life than those of the gibbon but more so than those of the chimpanzee and the gorilla. The skull is rounded and of great vertical height, with broad, bony ridges above the orbits and a great median crest on top of the skull in old males. The lower jaw of the orang is stout and deep, and, although used as a fighting weapon, the canine tusks are much less prominent than in either the gibbon, chimpanzee, or gorilla. Of all anthropoids this jaw most nearly resembles that of the Piltdown man.

In the chimpanzee we observe the very prominent bony ridges above the eyes, like those in the Trinil and Neanderthal races of men. The prognathous or protruding tooth rows and receding chin suggest those in the Heidelberg, Piltdown, and Neanderthal races. When the chimpanzee is walking (Fig. 18) the arms reach down below the level of the knees, whereas in the higher races of man they reach only half-way down the thighs. Thus, the fore limb, although much shorter than that of the gibbon, is relatively longer than that of any human race, recent or ancient. We observe also in the walking chimpanzee (Fig. 18) that the upper part of the leg, the thigh-bone, or femur, is relatively long, while the lower part, the shin-bone, or tibia, is relatively short. Indeed, both in the arm and in the leg the upper bones are relatively long and the lower bones are relatively short. These proportions, which are inheritances of arboreal life, are in very marked contrast to those observed in the arms and legs of the Neanderthal race of men, in which the limbs are of the terrestrial or walking type.

ANCESTRAL TREE OF THE ANTHROPOID APES AND OF MAN

We observe also in the chimpanzee a contrast between the grasping power of the big toe, which is a kind of thumb, and the lack of that power in the hand, in which the thumb is nearly useless; in all apes this function is characteristic of the foot, in man of the hand alone. The opposable thumb, with its power of bringing the thumb against each of the fingers, is the one character which is lacking in every one of the anthropoid apes and which was early developed among the ancestors of man.

The skull of the chimpanzee is longer than that of the orang, the most prominent feature in the top view being the extreme protuberance of the orbits, which are surrounded by a supraorbital and circumorbital bony ridge, which is also strongly developed in the Neanderthal skull as well as in the Pithecanthropus or Trinil skull but, so far as we know, is entirely lacking in that of Piltdown. As in the orang and the gorilla, a crest develops along the middle of the top of the skull for the insertion of the powerful muscles of the jaws, a crest which is wholly wanting in the gibbon and probably wanting in all the true ancestors of man.

The gorilla illustrates in the extreme the specializations which are begun in the chimpanzee, and which are attributable to a life partly arboreal, partly terrestrial, with the skull and jaws used as powerful fighting organs. The head is lengthened by the forward growth of the muzzle into an extreme prognathism. The limbs and body of the gorilla show a departure from the primitive, slender-limbed, arboreal type of apes and are partly adapted to a bipedal, ground-dwelling habit.

As regards psychic evolution,(3) Elliot Smith observes that the arboreal mode of life of the early ancestors of man developed quick, alert, and agile movements which stimulated the progressive development of the posterior and lateral portions of the brain. The sense of smell had been well developed in a previous terrestrial life, but once these creatures left the earth and took to the trees, guidance by the olfactory sense was less essential, for life amidst the branches of the trees is most favorable to the high development of the senses of vision, touch, and hearing. Moreover, it demands an agility and quickness of movement that necessitate efficient motor centres in the brain to co-ordinate and control such actions as tree life calls for. The specialization of sight awakens curiosity to examine objects with greater minuteness and guides the hands to more precise and skilled movements.

The anatomy of man is full of remote reminders of this original arboreal existence, which also explains the very large and early development of the posterior portions of the brain, in which the various senses of sight, touch, and hearing are located.

The first advance from arboreal to terrestrial life is marked by the power of walking more or less erect on the hind limbs and thus releasing the arms; this power is developed to a greater or less degree in all the anthropoid apes; with practice they become expert walkers. The additional freedom which the erect attitude gives to the arms and to the movements of the hands and the separate movements of the fingers is especially noticeable in the gibbon. The cultivation of the powers of the hand reacts upon the further growth and specialization of the brain; thus the brain and the erect attitude react upon each other. In the gibbon there is a marked increase in the size of those portions of the brain which supply the centres of touch, vision, and hearing.

Discussion as to how the ancestors of man were fashioned has chiefly dealt with the rival claims of four lines of structural evolution: first, the assumption of the erect attitude; second, the development of the opposable thumb; third, the growth of the brain; and fourth, the acquisition of the power of speech. The argument for the erect attitude suggested by Lamarck, and ably put by Munro(4) in 1893, indicates that the cultivation of skill with the hands and fingers lies at the root of man's mental supremacy. Elliot Smith's argument that the steady growth and specialization of the brain itself has been the chief factor in leading the ancestors of man step by step upward indicates that such an advance as the erect attitude was brought about because the brain had made possible the skilled movements of the hands.

The true conception of prehuman evolution, which occurred during Miocene and Pliocene times, is rather that of the coincident development of these four distinctively human powers. It appears from the limb proportions in the Neanderthal race that the partly erect attitude and walking gait were assumed much earlier in geologic time than we formerly imagined. The intimate relation between the use of the opposable thumb and the development of the higher mental faculties of man is sustained to-day by the discovery that one of the best methods of developing the mind of the child is to insist upon the constant use of the hands, for the action and reaction between hand and brain is found to develop the mind. A similar action and reaction between foot and brain developed the erect gait which released the hand from its locomotive and limb-grasping function, and by the resultant perfecting of the motion of thumbs and fingers turned the hand into an organ ready for the increasing specialization demanded by the manufacture of flint implements.

This is the stage reached, we believe, in late Pliocene times in which the human ancestor emerges from the age of mammals and enters the age of man, the period when the prehistory of man properly begins. The attitude is erect, the hand has a well-developed opposable thumb, the centres of the brain relating to the higher senses and to the control of all the motions of the limbs, hands, and fingers are well developed. The power of speech may still be rudimentary. The anterior centres of the brain for the storing of experience and the development of ideas are certainly very rudimentary.

Change of Environment in Europe

Considering that the origin and development of any creature are best furthered by a struggle for existence sufficiently severe to demand the full and frequent exercise of its powers of mind and body, it is interesting to trace the sequence of natural events which prepared western Europe for the entrance of the earliest branches of the human race. The forests and plants portray even more vividly than the animals the changing conditions of the environment and temperature which marked the approach and various vicissitudes of the great Ice Age.

The forests of central France in Pliocene times, as well as those of the valley of the Arno in northern Italy, were very similar to the forests of the middle United States at the present time, comprising such trees as the sassafras, the locust, the honey-locust, the sumach, the bald cypress, and the tulip. Thus the regions which harbored the rich forest and meadow fauna of northern Italy in Upper Pliocene times abounded in trees familiar to-day in North and South Carolina, including even such distinctively American forms as the sweet gum (Liquidambar styraciflua), the sour gum (Nyssa sylvatica), and the bay, beside those above mentioned. To the south, along the Mediterranean, there also flourished trees incident to a more tropical climate, the bamboo, the sabal palm, and the dwarf fan-palm; most interesting is the presence of the sabal, which now flourishes in the subtropical rain forests of central Florida. The sequoia also was abundant. Toward the close of the Pliocene the first indications of the coming Glacial Epoch were a lowering of the temperature, and, in the higher mountainous areas perhaps, a beginning of the glacial stages.

The ancestors of the modern forests of Europe predominated in central France: the oak, the beech, the poplar, the willow, and the larch. It is these forests, which survived the vicissitudes of glacial times, that gave descent to the forests of Postglacial Europe, while all the purely American types disappeared from Europe and are now found only in the temperate regions of the United States.(5)

We have seen that anthropoid apes have not been discovered either in the Middle or Upper Pliocene of Europe; the gibbon-ape line disappears with the Pliohylobates of the Upper Pliocene. These animals are, however, rarely found in fossil form, owing to their retreat to the trees in times of flood and danger, so that we need not necessarily assume that the anthropoids had actually become extinct in France. The primates which are found in the Upper Pliocene belong to the lower types of the Old World monkeys, related to the living langur of India and to the macaque and baboon. The evidence, as far as it goes, indicates that the ancestors of man were at this time evolving in Asia and not in Europe. This evidence, nevertheless, would be completely offset if it could be proven that the eoliths, or primitive flints, found in various parts of Europe from Oligocene to Pleistocene times are really artifacts of human or prehuman origin.

The mammals of Europe in Pliocene times were derived by very remote migrations from North America and, more directly, from southern Asia. The Oriental element is very strong, including types of rhinoceroses now peculiar to Sumatra and southern Asia, numerous mastodons very similar to the south Asiatic types of the times, gazelles and antelopes, including types related to the existing elands, and primitive types of horses and of tapirs. Among the carnivores in Europe similar to south Asiatic species were the hyænas, the dog bears (Hyænarctos), the civets, and the pandas (Ailurus); there were also the sabre-tooth tigers and numerous other felines. In the trees were found the south Asiatic and north African monkeys; and in the forests the axis deer, now restricted to Asia. But the most distinctive African-Asiatic animal of this period was found in the rivers; namely, the hippopotamus, which arrived in Italy in the early Pliocene and ranged south by way of the Sicilian land bridge into northern Africa and east along the southern shores of the Black Sea to the Siwalik hills of India. Thus, many of the ancestors of what we have termed the African-Asiatic mammal group of Pleistocene times had already found their way into Europe early in Pliocene times. In middle and late Pliocene times there arrived three very important types of mammals which played a great rôle in the early Pleistocene. These are: