In discussing the development of plants and animals during the Tertiary Era we have already perceived the shadow of the approaching Ice-Age. We found that in the course of the Tertiary the types which were more sensitive to cold gradually receded southward, and before its close Europe, Asia, and North America presented a distinctly temperate aspect. This is but the penumbra of the eclipse. When we pass the limits of the Tertiary Era, and enter the Quaternary, the refrigeration steadily proceeds, and, from temperate, the aspect of much of Europe and North America becomes arctic. From six to eight million square miles of the northern hemisphere are buried under fields of snow and ice, and even in the southern regions smaller glacial sheets spread from the foot of the higher ranges of mountains.

It is unnecessary to-day to explain at any length the evidences by which geologists trace this enormous glaciation of the northern hemisphere. There are a few works still in circulation in which popular writers, relying on the obstinacy of a few older geologists, speak lightly of the "nightmare" of the Ice-Age. But the age has gone by in which it could seriously be suggested that the boulders strewn along the east of Scotland—fragments of rock whose home we must seek in Scandinavia—were brought by the vikings as ballast for their ships. Even the more serious controversy, whether the scratches and the boulders which we find on the face of Northern Europe and America were due to floating or land ice, is virtually settled. Several decades of research have detected the unmistakable signs of glacial action over this vast area of the northern hemisphere. Most of Europe north of the Thames and the Danube, nearly all Canada and a very large part of the United States, and a somewhat less expanse of Northern Asia, bear to this day the deep scars of the thick, moving ice-sheets. Exposed rock-surfaces are ground and scratched, beds of pebbles are twisted and contorted hollows are scooped out, and moraines—the rubbish-heaps of the glaciers—are found on every side. There is now not the least doubt that, where the great Deinosaurs had floundered in semi-tropical swamps, where the figs and magnolias had later flourished, where the most industrious and prosperous hives of men are found to-day, there was, in the Pleistocene period, a country to which no parallel can be found outside the polar circles to-day.

The great revolution begins with the gathering of snows on the mountains. The Alps and Pyrenees had now, we saw, reached their full stature, and the gathering snows on their summits began to glide down toward the plains in rivers of ice. The Apennines (and even the mountains of Corsica), the Balkans, Carpathians, Caucasus, and Ural Mountains, shone in similar mantles of ice and snow. The mountains of Wales, the north of England, Scotland, and Scandinavia had even heavier burdens, and, as the period advanced, their sluggish streams of ice poured slowly over the plains. The trees struggled against the increasing cold in the narrowing tracts of green; the animals died, migrated to the south, or put on arctic coats. At length the ice-sheets of Scandinavia met the spreading sheets from Scotland and Wales, and crept over Russia and Germany, and an almost continuous mantle, from which only a few large areas of arctic vegetation peeped out, was thrown over the greater part of Europe. Ten thousand feet thick where it left the hills of Norway and Sweden, several thousand feet thick even in Scotland, the ice-sheet that resulted from the fusion of the glaciers gradually thinned as it went south, and ended in an irregular fringe across Central Europe. The continent at that time stretched westward beyond the Hebrides and some two hundred miles beyond Ireland. The ice-front followed this curve, casting icebergs into the Atlantic, then probably advanced up what is now the Bristol Channel, and ran across England and Europe, in a broken line, from Bristol to Poland. South of this line there were smaller ice-fields round the higher mountains, north of it almost the whole country presented the appearance that we find in Greenland to-day.

In North America the glaciation was even more extensive. About four million square miles of the present temperate zone were buried under ice and snow. From Greenland, Labrador, and the higher Canadian mountains the glaciers poured south, until, in the east, the mass of ice penetrated as far as the valley of the Mississippi. The great lakes of North America are permanent memorials of its Ice-Age, and over more than half the country we trace the imprint and the relics of the sheet. South America, Australia, Tasmania, and New Zealand had their glaciated areas. North Asia was largely glaciated, but the range of the ice-sheet is not yet determined in that continent.

This summary statement will convey some idea of the extraordinary phase through which the earth passed in the early part of the present geological era. But it must be added that a singular circumstance prolonged the glacial regime in the northern hemisphere. Modern geologists speak rather of a series of successive ice-sheets than of one definite Ice-Age. Some, indeed, speak of a series of Ice-Ages, but we need not discuss the verbal question. It is now beyond question that the ice-sheet advanced and retreated several times during the Glacial Epoch. The American and some English geologists distinguished six ice-sheets, with five intermediate periods of more temperate climate. The German and many English and French geologists distinguish four sheets and three interglacial epochs. The exact number does not concern us, but the repeated spread of the ice is a point of some importance. The various sheets differed considerably in extent. The wide range of the ice which I have described represents the greatest extension of the glaciation, and probably corresponds to the second or third of the six advances in Dr. Geikie's (and the American) classification.

Before we consider the biological effect of this great of refrigeration of the globe, we must endeavour to understand the occurrence itself. Here we enter a world of controversy, but a few suggestions at least may be gathered from the large literature of the subject, which dispel much of the mystery of the Great Ice-Age.

It was at one time customary to look out beyond the earth itself for the ultimate causes of this glaciation. Imagine the sheet of ice, which now spreads widely round the North Pole, shifted to another position on the surface of the planet, and you have a simple explanation of the occurrence. In other words, if we suppose that the axis of the earth does not consistently point in one direction—that the great ball does not always present the same average angle in relation to the sun—the poles will not always be where they are at present, and the Pleistocene Ice-Age may represent a time when the north pole was in the latitude of North Europe and North America. This opinion had to be abandoned. We have no trace whatever of such a constant shifting of the polar regions as it supposes, and, especially, we have no trace that the warm zone correspondingly shifted in the Pleistocene.

A much more elaborate theory was advanced by Dr. Croll, and is still entertained by many. The path of the earth round the sun is not circular, but elliptical, and there are times when the gravitational pull of the other planets increases the eccentricity of the orbit. It was assumed that there are periods of great length, separated from each other by still longer periods, when this eccentricity of the orbit is greatly exaggerated. The effect would be to prolong the winter and shorten the summer of each hemisphere in turn. The total amount of heat received would not alter, but there would be a long winter with less heat per hour, and a short summer with more heat. The short summer would not suffice to melt the enormous winter accumulations of ice and snow, and an ice-age would result. To this theory, again, it is objected that we do not find the regular succession of ice-ages in the story of the earth which the theory demands, and that there is no evidence of an alternation of the ice between the northern and southern hemispheres.

More recent writers have appealed to the sun itself, and supposed that some prolonged veiling of its photosphere greatly reduced the amount of heat emitted by it. More recently still it has been suggested that an accumulation of cosmic or meteoric dust in our atmosphere, or between us and the sun, had, for a prolonged period, the effect of a colossal "fire-screen." Neither of these suppositions would explain the localisation of the ice. In any case we need not have recourse to purely speculative accidents in the world beyond until it is clear that there were no changes in the earth itself which afford some explanation.

This is by no means clear. Some writers appeal to changes in the ocean currents. It is certain that a change in the course of the cold and warm currents of the ocean to-day might cause very extensive changes of climate, but there seems to be some confusion of ideas in suggesting that this might have had an equal, or even greater, influence in former times. Our ocean currents differ so much in temperature because the earth is now divided into very pronounced zones of climate. These zones did not exist before the Pliocene period, and it is not at all clear that any redistribution of currents in earlier times could have had such remarkable consequences. The same difficulty applies to wind-currents.

On the other hand, we have already, in discussing the Permian glaciation, discovered two agencies which are very effective in lowering the temperature of the earth. One is the rise of the land; the other is the thinning of the atmosphere. These are closely related agencies, and we found them acting in conjunction to bring about the Permian Ice-Age. Do we find them at work in the Pleistocene?

It is not disputed that there was a very considerable upheaval of the land, especially in Europe and North America, at the end of the Tertiary Era. Every mountain chain advanced, and our Alps, Pyrenees, Himalaya, etc., attained, for the first time, their present, or an even greater elevation. The most critical geologists admit that Europe, as a whole, rose 4000 feet above its earlier level. Such an elevation would be bound to involve a great lowering of the temperature. The geniality of the Oligocene period was due, like that of the earlier warm periods, to the low-lying land and very extensive water-surface. These conditions were revolutionised before the end of the Tertiary. Great mountains towered into the snow-line, and vast areas were elevated which had formerly been sea or swamp.

This rise of the land involved a great decrease in the proportion of moisture in the atmosphere. The sea surface was enormously lessened, and the mountains would now condense the moisture into snow or cloud to a vastly greater extent than had ever been known before There would also be a more active circulation of the atmosphere, the moist warm winds rushing upward towards the colder elevations and parting with their vapour. As the proportion of moisture in the atmosphere lessened the surface-heat would escape more freely into space, the general temperature would fall, and the evaporation—or production of moisture would be checked, while the condensation would continue. The prolonging of such conditions during a geological period can be understood to have caused the accumulation of fields of snow and ice in the higher regions. It seems further probable that these conditions would lead to a very considerable formation of fog and cloud, and under this protecting canopy the glaciers would creep further down toward the plains.

We have then to consider the possibility of a reduction of the quantity of carbon-dioxide in the atmosphere The inexpert reader probably has a very exaggerated idea of the fall in temperature that would be required to give Europe an Ice-Age. If our average temperature fell about 5-8 degrees C. below the average temperature of our time it would suffice; and it is further calculated that if the quantity of carbon-dioxide in our atmosphere were reduced by half, we should have this required fall in temperature. So great a reduction would not be necessary in view of the other refrigerating agencies. Now it is quite certain that the proportion of carbon-dioxide was greatly reduced in the Pleistocene. The forests of the Tertiary Era would steadily reduce it, but the extensive upheaval of the land at its close would be even more important. The newly exposed surfaces would absorb great quantities of carbon. The ocean, also, as it became colder, would absorb larger and larger quantities of carbon-dioxide. Thus the Pleistocene atmosphere, gradually relieved of its vapours and carbon-dioxide, would no longer retain the heat at the surface. We may add that the growth of reflective surfaces—ice, snow, cloud, etc.—would further lessen the amount of heat received from the sun.

Here, then, we have a series of closely related causes and effects which would go far toward explaining, if they do not wholly suffice to explain, the general fall of the earth's temperature. The basic cause is the upheaval of the land—a fact which is beyond controversy, the other agencies are very plain and recognisable consequences of the upheaval. There are, however, many geologists who do not think this explanation adequate.

It is pointed out, in the first place, that the glaciation seems to have come long after the elevation. The difficulty does not seem to be insurmountable. The reduction of the atmospheric vapour would be a gradual process, beginning with the later part of the elevation and culminating long afterwards. The reduction of the carbon-dioxide would be even more gradual. It is impossible to say how long it would take these processes to reach a very effective stage, but it is equally impossible to show that the interval between the upheaval and the glaciation is greater than the theory demands.

It is also said that we cannot on these principles understand the repeated advance and retreat of the ice-sheet.

This objection, again, seems to fail. It is an established fact that the land sank very considerably during the Ice-Age, and has risen again since the ice disappeared. We find that the crust in places sank so low that an arctic ocean bathed the slopes of some of the Welsh mountains; and American geologists say that their land has risen in places from 2000 to 3000 feet (Chamberlin) since the burden of ice was lifted from it. Here we have the possibility of an explanation of the advances and retreats of the glaciers. The refrigerating agencies would proceed until an enormous burden of ice was laid on the land of the northern hemisphere. The land apparently sank under the burden, the ice and snow melted at the lower level and there was a temperate interglacial period. But the land, relieved of its burden, rose once more, the exposed surface absorbed further quantities of carbon, and a fresh period of refrigeration opened. This oscillation might continue until the two sets of opposing forces were adjusted, and the crust reached a condition of comparative stability.

Finally, and this is the more serious difficulty, it is said that we cannot in this way explain the localisation of the glacial sheets. Why should Europe and North America in particular suffer so markedly from a general thinning of the atmosphere? The simplest answer is to suggest that they especially shared the rise of the land. Geology is not in a position either to prove or disprove this, and it remains only a speculative interpretation of the fact We know at least that there was a great uprise of land in Europe and North America in the Pliocene and Pleistocene and may leave the precise determination of the point to a later age. At the same time other local causes are not excluded. There may have been a large extension of the area of atmospheric depression which we have in the region of Greenland to-day.

When we turn to the question of chronology we have the same acute difference of opinion as we have found in regard to all questions of geological time. It used to be urged, on astronomical grounds, that the Ice-Age began about 240,000 years ago, and ended about 60,000 years ago, but the astronomical theory is, as I said, generally abandoned. Geologists, on the other hand, find it difficult to give even approximate figures. Reviewing the various methods of calculation, Professor Chamberlin concludes that the time of the first spread of the ice-sheet is quite unknown, the second and greatest extension of the glaciation may have been between 300,000 and a million years ago, and the last ice-extension from 20,000 to 60,000 years ago; but he himself attaches "very little value" to the figures. The chief ice-age was some hundreds of thousands of years ago, that is all we can say with any confidence.

In dismissing the question of climate, however, we should note that a very serious problem remains unsolved. As far as present evidence goes we seem to be free to hold that the ice-ages which have at long intervals invaded the chronicle of the earth were due to rises of the land. Upheaval is the one constant and clearly recognisable feature associated with, or preceding, ice-ages. We saw this in the case of the Cambrian, Permian, Eocene, and Pleistocene periods of cold, and may add that there are traces of a rise of mountains before the glaciation of which we find traces in the middle of the Archaean Era. There are problems still to be solved in connection with each of these very important ages, but in the rise of the land and consequent thinning of the atmosphere we seem to have a general clue to their occurrence. Apart from these special periods of cold, however, we have seen that there has been, in recent geological times, a progressive cooling of the earth, which we have not explained. Winter seems now to be a permanent feature of the earth's life, and polar caps are another recent, and apparently permanent, acquisition. I find no plausible reason assigned for this.

The suggestion that the disk of the sun is appreciably smaller since Tertiary days is absurd; and the idea that the earth has only recently ceased to allow its internal heat to leak through the crust is hardly more plausible. The cause remains to be discovered.

We turn now to consider the effect of the great Ice-Age, and the relation of man to it. The Permian revolution, to which the Pleistocene Ice-Age comes nearest in importance, wrought such devastation that the overwhelming majority of living things perished. Do we find a similar destruction of life, and selection of higher types, after the Pleistocene perturbation? In particular, had it any appreciable effect upon the human species?

A full description of the effect of the great Ice-Age would occupy a volume. The modern landscape in Europe and North America was very largely carved and modelled by the ice-sheet and the floods that ensued upon its melting. Hills were rounded, valleys carved, lakes formed, gravels and soils distributed, as we find them to-day. In its vegetal aspect, also, as we saw, the modern landscape was determined by the Pleistocene revolution. A great scythe slowly passed over the land. When the ice and snow had ended, and the trees and flowers, crowded in the southern area, slowly spread once more over the virgin soil, it was only the temperate species that could pass the zone guarded by the Alps and the Pyrenees. On the Alps themselves the Pleistocene population still lingers, their successful adaptation to the cold now preventing them from descending to the plains.

The animal world in turn was winnowed by the Pleistocene episode. The hippopotamus, crocodile, turtle, flamingo, and other warm-loving animals were banished to the warm zone. The mammoth and the rhinoceros met the cold by developing woolly coats, but the disappearance of the ice, which had tempted them to this departure, seems to have ended their fitness. Other animals which became adapted to the cold—arctic bears, foxes, seals, etc.—have retreated north with the ice, as the sheet melted. For hundreds of thousands of years Europe and North America, with their alternating glacial and interglacial periods, witnessed extraordinary changes and minglings of their animal population. At one time the reindeer, the mammoth, and the glutton penetrate down to the Mediterranean, in the next phase the elephant and hippopotamus again advance nearly to Central Europe. It is impossible here to attempt to unravel these successive changes and migrations. Great numbers of species were destroyed, and at length, when the climatic condition of the earth reached a state of comparative stability, the surviving animals settled in the geographical regions in which we find them to-day.

The only question into which we may enter with any fullness is that of the relation of human development to this grave perturbation of the condition of the globe. The problem is sometimes wrongly conceived. The chief point to be determined is not whether man did or did not precede the Ice-Age. As it is the general belief that he was evolved in the Tertiary, it is clear that he existed in some part of the earth before the Ice-Age. Whether he had already penetrated as far north as Britain and Belgium is an interesting point, but not one of great importance. We may, therefore, refrain from discussing at any length those disputed crude stone implements (Eoliths) which, in the opinion of many, prove his presence in northern regions before the close of the Tertiary. We may also now disregard the remains of the Java Ape-Man. There are authorities, such as Deniker, who hold that even the latest research shows these remains to be Pliocene, but it is disputed. The Java race may be a surviving remnant of an earlier phase of human evolution.

The most interesting subject for inquiry is the fortune of our human and prehuman forerunners during the Pliocene and Pleistocene periods. It may seem that if we set aside the disputable evidence of the Eoliths and the Java remains we can say nothing whatever on this subject. In reality a fact of very great interest can be established. It can be shown that the progress made during this enormous lapse of time—at least a million years—was remarkably slow. Instead of supposing that some extraordinary evolution took place in that conveniently obscure past, to which we can find no parallel within known times, it is precisely the reverse. The advance that has taken place within the historical period is far greater, comparatively to the span of time, than that which took place in the past.

To make this interesting fact clearer we must attempt to measure the progress made in the Pliocene and Pleistocene. We may assume that the precursor of man had arrived at the anthropoid-ape level by the middle of the Miocene period. He is not at all likely to have been behind the anthropoid apes, and we saw that they were well developed in the mid-Tertiary. Now we have a good knowledge of man as he was in the later stage of the Ice-Age—at least a million years later—and may thus institute a useful comparison and form some idea of the advance made.

In the later stages of the Pleistocene a race of men lived in Europe of whom we have a number of skulls and skeletons, besides vast numbers of stone implements. It is usually known as the Neanderthal race, as the first skeleton was found, in 1856, at Neanderthal, near Dusseldorf. Further skeletons were found at Spy, in Belgium, and Krapina, in Croatia. A skull formerly found at Gibraltar is now assigned to the same race. In the last five years a jaw of the same (or an earlier) age has been found at Mauer, near Heidelberg, and several skeletons have been found in France (La Vezere and Chapelle-aux-Saints). From these, and a few earlier fragments, we have a confident knowledge of the features of this early human race.

The highest appreciation of the Neanderthal man—a somewhat flattering appreciation, as we shall see—is that he had reached the level of the Australian black of to-day. The massive frontal ridges over his eyes, the very low, retreating forehead, the throwing of the mass of the brain toward the back of the head, the outthrust of the teeth and jaws, and the complete absence (in some cases) or very slight development of the chin, combine to give the head what the leading authorities call a "bestial" or "simian" aspect. The frame is heavy, powerful, and of moderate height (usually from two to four inches over five feet). The thigh-bones are much more curved than in modern man. We cannot enter here into finer anatomical details, but all the features are consistent and indicate a stage in the evolution from ape-man to savage man.

One point only calls for closer inquiry. Until a year or two ago it was customary to state that in cranial capacity also—that is to say, in the volume of brain-matter that the skull might contain—the Neanderthal race was intermediate between the Ape-Man and modern man. We saw above that the cranial capacity of the highest ape is about 600 cubic centimetres, and that of the Ape-Man (variously given as 850 and 950) is about 900. It was then added that the capacity of the Neanderthal race was about 1200, and that of civilised man (on the average) 1600. This seemed to be an effective and convincing indication of evolution, but recent writers have seriously criticised it. Sir Edwin Ray Lankester, Professor Sollas, and Dr. Keith have claimed in recent publications that the brain of Neanderthal man was as large as, if not larger than, that of modern man. [*] Professor Sollas even observes that "the brain increases in volume as we go backward." This is, apparently, so serious a reversal of the familiar statement in regard to the evolution of man that we must consider it carefully.

Largeness of brain in an individual is no indication of intelligence, and smallness of brain no proof of low mentality. Some of the greatest thinkers, such as Aristotle and Leibnitz, had abnormally small heads. Further, the size of the brain is of no significance whatever except in strict relation to the size and weight of the body. Woman has five or six ounces less brain-matter than man, but in proportion to her average size and the weight of the vital tissue of her body (excluding fat) she has as respectable a brain as man. When, however, these allowances have been made, it has usually been considered that the average brain of a race is in proportion to its average intelligence. This is not strictly true. The rabbit has a larger proportion of brain to body than the elephant or horse, and the canary a larger proportion than the chimpanzee. Professor Sollas says that the average cranial capacity of the Eskimo is 1546 cubic centimetres, or nearly that assigned to the average Parisian.

Clearly the question is very complex, and some of these recent authorities conclude that the cranial capacity, or volume of the brain, has no relation to intelligence, and therefore the size of the Neanderthal skull neither confirms nor disturbs the theory of evolution. The wise man will suspend his judgment until the whole question has been fully reconsidered. But I would point out that some of the recent criticisms are exaggerated. The Gibraltar skull is estimated by Professor Sollas himself to have a capacity of about 1260; and his conclusion that it is an abnormal or feminine skull rests on no positive grounds. The Chapelle-aux-Saints skull ALONE is proved to have the high capacity of 1620; and it is as yet not much more than a supposition that the earlier skulls had been wrongly measured. But, further, the great French authority, M. Boule, who measured the capacity of the Chapelle-aux Saints skull, observes [*] that "the anomaly disappears" on careful study. He assures us that a modern skull of the same dimensions would have a capacity of 1800-1900 cubic centimetres, and warns us that we must take into account the robustness of the body of primitive man. He concludes that the real volume of the Neanderthal brain (in this highest known specimen) is "slight in comparison with the volume of the brain lodged in the large heads of to-day," and that the "bestial or ape-like characters" of the race are not neutralised by this gross measurement.

We must therefore hesitate to accept the statement that primitive man had as large a brain, if not a larger brain, than a modern race. The basis is slender, and the proportion of brain to body-tissue has not been taken into account. On the other hand, the remains of this early race are, Professor Sollas says, "obviously more brutal than existing men in all the other ascertainable characters by which they differ from them." Nor are we confined to precarious measurements of skulls. We have the remains of the culture of this early race, and in them we have a surer trace of its mental development.

Here again we must proceed with caution, and set aside confused and exaggerated statements. Some refer us to the artistic work of primitive man. We will consider his drawings and carvings presently, but they belong to a later race, not the Neanderthal race. Some lay stress on the fact, apparently indicated in one or two cases out of a dozen, that primitive man buried his dead. Professor Sollas says that it indicates that even Neanderthal man had reached "a comparatively high stage in the evolution of religious ideas "; but the Australians bury their dead, and the highest authorities are not agreed whether they have any idea whatever of a supreme being or of morality. We must also disallow appeals to the use of fire, the taming of animals, pottery, or clothing. None of these things are clearly found in conjunction with the Neanderthal race.

The only certain relic of Neanderthal culture is the implement which the primitive savage fashioned, by chipping or pressure, of flint or other hard stone. The fineness of some of these implements is no indication of great intelligence. The Neanderthal man inherited a stone culture which was already of great antiquity. At least one, if not two or three, prolonged phases of the Old Stone Age were already over when he appeared. On the most modest estimate men had by that time been chipping flints for several hundred thousand years, and it is no argument of general intelligence that some skill in the one industry of the age had been developed. The true measure of Neanderthal man's capacity is that, a million years or so after passing the anthropoid-age level, he chipped his stones more finely and gave them a better edge and contour. There is no evidence that he as yet hefted them. It is flattering to him to compare him with the Australian aboriginal. The native art, the shields and spears and boomerangs, and the elaborate tribal and matrimonial arrangements of the Australian black are not known to have had any counterpart in his life.

It would therefore seem that the precursors of man made singularly little, if any, progress during the vast span of time between the Miocene and the Ice-Age, and that then something occurred which quickened the face of human evolution. From the Neanderthal level man will advance to the height of modern civilisation in about one-tenth the time that it took him to advance from the level of the higher ape to that of the lowest savage. Something has broken into the long lethargy of his primitive career, and set him upon a progressive path. Let us see if a careful review of the stages of his culture confirms the natural supposition that this "something" was the fall in the earth's temperature, and how it may have affected him.