Chapter XXIV: Book I: The Making of Our World (2)

Here are three wavering systems of change all going on independently of each other; the precession of the equinoxes, the change in the obliquity of the equator to the orbit, and the changes in the eccentricity of the orbit. Each system tends by itself to produce periods of equability and periods of greater climatic contrast. And all these systems of change interplay with each other. When it happens that at the same time the orbit is most nearly circular, the equator is at its least inclination from the plane of the earth’s orbit, and the spring and autumn are at perihelion and aphelion, then all these causes will be conspiring to make climate warm and uniform; there will be least difference of summer and winter. When, on the other hand, the orbit is in its most eccentric stage of deformation, when also the equator is most tilted up and when further the summer and winter are at aphelion and perihelion, then climates will be at their extremest and winter at its bitterest. There will be great accumulations of ice and snow in winter; the heat of the brief hot summer will be partly reflected back into space by the white snow, and it will be unequal to the task of melting all the winter’s ice before the earth spins away once more towards its chilly aphelion. The earth will accumulate cold so long as this conspiracy of extreme conditions continues.

It does not change in regular periods. It fluctuates through vast ages. As the world’s climate changes, life must change too or perish.]

So our earth’s climate changes and wavers perpetually as these three systems of influence come together with a common tendency towards warmth or severity, or as they contradict and cancel each other.

We can trace in the Record of the Rocks an irregular series of changes due to the interplay of these influences; there have been great ages when the separate rhythms of these three systems kept them out of agreement and the atmosphere was temperate, ages of world-wide warmth, and other ages when they seemed to concentrate bitterly to their utmost extremity, to freeze out and inflict the utmost stresses and hardship upon life.

And in accordance we find from the record in the rocks that there have been long periods of expansion and multiplication when life flowed and abounded and varied, and harsh ages when there was a great weeding out and disappearance of species, genera, and classes, and the learning of stern lessons by all that survived. Such a propitious conjunction it must have been that gave the age of luxuriant low-grade growth of the coal-measures; such an adverse series of circumstances that chilled the closing æons of the Palæozoic time.

It is probable that the warm spells have been long relatively to the cold ages. Our world to-day seems to be emerging with fluctuations from a prolonged phase of adversity and extreme conditions. Half a million years ahead it may be a winterless world with trees and vegetation even in the polar circles. At present we have no certainty in such a forecast, but later on, as knowledge increases, it may be possible to reckon with more precision, so that our race will make its plans thousands of years ahead to meet the coming changes.

§ 2

Another entirely different cause of changes in the general climate of the earth may be due to variations in the heat of the sun. We do not yet understand what causes the heat of the sun or what sustains that undying fire. It is possible that in the past there have been periods of greater and lesser intensity. About that we know nothing; human experience has been too short; and so far we have been able to find no evidence on this matter in the geological record. On the whole, scientific men are inclined to believe that the sun has blazed with a general steadfastness throughout geological time. It may have been cooling slowly, but, speaking upon the scale of things astronomical, it has certainly not cooled very much.

§ 3

A third great group of causes influencing climate are to be found in the forces within the world itself. Throughout the long history of the earth there has been a continuous wearing down of the hills and mountains by frost and rain and a carrying out of their material to become sedimentary rocks under the seas. There has been a continuous process of wearing down the land and filling up the seas, by which the seas, as they became shallower, must have spread more and more over the land. The reverse process, a process of crumpling and upheaval, has also been in progress, but less regularly. The forces of upheaval have been spasmodic; the forces of wearing down continuous. For long ages there has been comparatively little volcanic upheaval, and then have come periods in which vast mountain chains have been thrust up and the whole outline of land and sea changed. Such a time was the opening stage of the Cainozoic period, in which the Alps, the Himalayas, and the Andes were all thrust up from the sea-level to far beyond their present elevations, and the main outlines of the existing geography of the world were drawn.

Now, a time of high mountains and deep seas would mean a larger dry land surface for the world, and a more restricted sea surface, and a time of low lands would mean a time of wider and shallower seas. High mountains precipitate moisture from the atmosphere and hold it out of circulation as snow and glaciers, while smaller oceans mean a lesser area for surface evaporation. Other things being equal, lowland stages of the world’s history would be ages of more general atmospheric moisture than periods of relatively greater height of the mountains and greater depth of the seas. But even small increases in the amount of moisture in the air have a powerful influence upon the transmission of radiant heat through that air. The sun’s heat will pass much more freely through dry air than through moist air, and so a greater amount of heat would reach the land surfaces of the globe under the conditions of extremes of elevation and depth, than during the periods of relative lowness and shallowness. Dry phases in the history of the earth mean, therefore, hot days. But they also mean cold nights, because for the same reason that the heat comes abundantly to the earth, it will be abundantly radiated away. Moist phases mean, on the other hand, cooler days and warmer nights. The same principle applies to the seasons, and so a phase of great elevations and depressions of the surface would also be another contributory factor on the side of extreme climatic conditions.

And a stage of greater elevation and depression would intensify its extreme conditions by the gradual accumulation of ice caps upon the polar regions and upon the more elevated mountain masses. This accumulation would be at the expense of the sea, whose surface would thus be further shrunken in comparison with the land.

Here, then, is another set of varying influences that will play in with and help or check the influence of the astronomical variations stated in § 1 and § 2. There are other more localized forces at work into which we cannot go in any detail here, but which will be familiar to the student of the elements of physical geography; the influence of great ocean currents in carrying warmth from equatorial to more temperate latitudes; the interference of mountain chains with the moisture borne by prevalent winds and the like. As in the slow processes of nature these currents are deflected or the mountain chains worn down or displaced by fresh upheavals, the climate over great areas will be changed and all the conditions of life changed with it. Under the incessant slow variations of these astronomical, telluric, and geographical influences life has no rest. As its conditions change it must change or perish.

§ 4

And while we are enumerating the forces that change climate and the conditions of terrestrial life, we may perhaps look ahead a little and add a fourth set of influences, at first unimportant in the history of the world so far as the land surface is concerned, but becoming more important after the age of Reptiles, to which we shall proceed in our next chapter. These are the effects produced upon climate by life itself. Particularly great is the influence of vegetation, and especially that of forests. Every tree is continually transpiring water vapour into the air; the amount of water evaporated in summer by a lake surface is far less than the amount evaporated by the same area of beech forest. As in the later Mesozoic and the Cainozoic Age, great forests spread over the world, their action in keeping the air moist and mitigating and stabilizing climate by keeping the summer cool and the winter mild must have become more and more important. Moreover, forests accumulate and protect soil and so prepare the possibility of agricultural life.

Water-weeds again may accumulate to choke and deflect rivers, flood and convert great areas into marshes, and so lead to the destruction of forests or the replacement of grass-lands by boggy wildernesses.

Finally, with the appearance of human communities, came what is perhaps the most powerful of all living influences upon climate. By fire and plough and axe man alters his world. By destroying forests and by irrigation man has already affected the climate of great regions of the world’s surface. The destruction of forests makes the seasons more extreme; this has happened, for instance, in the northeastern states of the United States of America. Moreover, the soil is no longer protected from the scour of rain, and is washed away, leaving only barren rock beneath. This has happened in Spain and Dalmatia and, some thousands of years earlier, in South Arabia. By irrigation, on the other hand, man restores the desert to life and mitigates climate. This process is going on in Northwest India and Australia. In the future, by making such operations worldwide and systematic, man may be able to control climate to an extent at which as yet we can only guess.

VI

THE AGE OF REPTILES

§ 1. _The Age of Lowland Life._ § 2. _Flying Dragons._ § 3. _The
First Birds._ § 4. _An Age of Hardship and Death._ § 5. _The First
Appearance of Fur and Feathers._

§ 1

We know that for hundreds of thousands of years the wetness and warmth, the shallow lagoon conditions that made possible the vast accumulations of vegetable matter which, compressed and mummified,[14] are now coal, prevailed over most of the world. There were some cold intervals, it is true; but they did not last long enough to destroy the growths. Then that long age of luxuriant low-grade vegetation drew to its end, and for a time life on the earth seems to have undergone a period of world-wide bleakness.

When the story resumes again, we find life entering upon a fresh phase of richness and expansion. Vegetation has made great advances in the art of living out of water. While the Palæozoic plants of the coal-measures probably grew with swamp water flowing over their roots, the Mesozoic flora from its very outset included palm-like cycads and low-ground conifers that were distinctly land plants growing on soil above the water level. The lower levels of the Mesozoic land were no doubt covered by great fern brakes and shrubby bush and a kind of jungle growth of trees. But there existed as yet no grass, no small flowering plants, no turf nor greensward. Probably the Mesozoic was not an age of very brightly coloured vegetation. It must have had a flora green in the wet season and brown and purple in the dry. There were no gay flowers, no bright autumn tints before the fall of the leaf, because there was as yet no fall of the leaf. And beyond the lower levels the world was still barren, still unclothed, still exposed without any mitigation to the wear and tear of the wind and rain.

When one speaks of conifers in the Mesozoic the reader must not think of the pines and firs that clothe the high mountain slopes of our time. He must think of low-growing evergreens. The mountains were still as bare and lifeless as ever. The only colour effects among the mountains were the colour effects of naked rock, such colours as make the landscape of Colorado so marvellous to-day.

Amidst this spreading vegetation of the lower plains the reptiles were increasing mightily in multitude and variety. They were now in many cases absolutely land animals. There are numerous anatomical points of distinction between a reptile and an amphibian; they held good between such reptiles and amphibians as prevailed in the carboniferous time of the Upper Palæozoic; but the fundamental difference between reptiles and amphibia which matters in this history is that the amphibian must go back to the water to lay its eggs, and that in the early stages of its life it must live in and under water. The reptile, on the other hand, has cut out all the tadpole stages from its life cycle, or, to be more exact, its tadpole stages are got through before the young leave the egg case. The reptile has come out of the water altogether. Some had gone back to it again, just as the hippopotamus and the otter among mammals have gone back, but that is a further extension of the story to which we cannot give much attention in this _Outline_.

In the Palæozoic period, as we have said, life had not spread beyond the swampy river valleys and the borders of sea lagoons and the like; but in the Mesozoic, life was growing ever more accustomed to the thinner medium of the air, was sweeping boldly up over the plains and towards the hillsides. It is well for the student of human history and the human future to note that. If a disembodied intelligence with no knowledge of the future had come to earth and studied life during the early Palæozoic age, he might very reasonably have concluded that life was absolutely confined to the water, and that it could never spread over the land. It found a way. In the Later Palæozoic Period that visitant might have been equally sure that life could not go beyond the edge of a swamp. The Mesozoic Period would still have found him setting bounds to life far more limited than the bounds that are set to-day. And so to-day, though we mark how life and man are still limited to five miles of air and a depth of perhaps a mile or so of sea, we must not conclude from that present limitation that life, through man, may not presently spread out and up and down to a range of living as yet inconceivable.

Late Mesozoic

Reptiles]

The earliest known reptiles were beasts with great bellies and not very powerful legs, very like their kindred amphibia, wallowing as the crocodile wallows to this day; but in the Mesozoic they soon began to stand up and go stoutly on all fours, and several great sections of them began to balance themselves on tail and hind legs, rather as the kangaroos do now, in order to release the fore limbs for grasping food. The bones of one notable division of reptiles which retained a quadrupedal habit, a division of which many remains have been found in South African and Russian Early Mesozoic deposits, display a number of characters which approach those of the mammalian skeleton, and because of this resemblance to the mammals (beasts) this division is called the _Theriomorpha_ (beastlike). Another division was the crocodile branch, and another developed towards the tortoises and turtles. The _Plesiosaurs_ and _Ichthyosaurs_ were two groups which have left no living representatives; they were huge reptiles returning to a whale-like life in the sea. _Pliosaurus_, one of the largest plesiosaurs, measured thirty feet from snout to tail tip--of which half was neck. The _Mosasaurs_ were a third group of great porpoise-like marine lizards. But the largest and most diversified group of these Mesozoic reptiles was the group we have spoken of as kangaroo-like, the _Dinosaurs_, many of which attained enormous proportions. In bigness these greater _Dinosaurs_ have never been exceeded, although the sea can still show in the whales creatures as great. Some of these, and the largest among them, were herbivorous animals; they browsed on the rushy vegetation and among the ferns and bushes, or they stood up and grasped trees with their fore legs while they devoured the foliage. Among the browsers, for example, were the _Diplodocus carnegii_, which measured eighty=four feet in length, and the _Atlantosaurus_. The _Gigantosaurus_, disinterred by a German expedition in 1912 from rocks in East Africa, was still more colossal. It measured well over a hundred feet! These greater monsters had legs, and they are usually figured as standing up on them; but it is very doubtful if they could have supported their weight in this way, out of water. Buoyed up by water or mud, they may have got along. Another noteworthy type we have figured is the _Triceratops_. There were also a number of great flesh-eaters who preyed upon these herbivores. Of these, _Tyrannosaurus_ seems almost the last word in “frightfulness” among living things. Some species of this genus measured forty feet from snout to tail. Apparently it carried this vast body kangaroo fashion on its tail and hind legs. Probably it reared itself up. Some authorities even suppose that it leapt through the air. If so, it possessed muscles of a quite miraculous quality. A leaping elephant would be a far less astounding idea. Much more probably it waded half submerged in pursuit of the herbivorous river saurians.

§ 2

One special development of the dinosaurian type of reptile was a light, hopping, climbing group of creatures which developed a bat-like web between the fifth finger and the side of the body, which was used in gliding from tree to tree after the fashion of the flying squirrels. These bat-lizards were the _Pterodactyls_. They are often described as _flying_ reptiles, and pictures are drawn of Mesozoic scenery in which they are seen soaring and swooping about. But their breastbone has no keel such as the breastbone of a bird has for the attachment of muscles strong enough for long-sustained flying. They must have flitted about like bats. They must have had a grotesque resemblance to heraldic dragons, and they played the part of bat-like birds in the Mesozoic jungles. But bird-like though they were, they were not birds nor the ancestors of birds. The structure of their wings was altogether different from that of birds. The structure of their wings was that of a hand with one long finger and a web; the wing of a bird is like an arm with feathers projecting from its hind edge. And these Pterodactyls had no feathers.

§ 3

Far less prevalent at this time were certain other truly birdlike creatures, of which the earlier sorts also hopped and clambered and the later sorts skimmed and flew. These were at first--by all the standards of classification--Reptiles. They developed into true birds as they developed wings and as their reptilian scales became long and complicated, fronds rather than scales, and so at last, by much spreading and splitting, feathers. Feathers are the distinctive covering of birds, and they give a power of resisting heat and cold far greater than that of any other integumentary covering except perhaps the thickest fur. At a very early stage this novel covering of feathers, this new heatproof contrivance that life had chanced upon, enabled many species of birds to invade a province for which the pterodactyl was ill equipped. They took to sea fishing--if indeed they did not begin with it--and spread to the north and south polewards beyond the temperature limits set to the true reptiles. The earliest birds seem to have been carnivorous divers and water birds. To this day some of the most primitive bird forms are found among the sea birds of the Arctic and Antarctic seas, and it is among these sea birds that zoologists still find lingering traces of teeth, which have otherwise vanished completely from the beak of the bird.

The earliest known bird (the _Archæopteryx_) had no beak; it had a row of teeth in a jaw like a reptile’s. It had three claws at the forward corner of its wing. Its tail too was peculiar. All modern birds have their tail feathers set in a short compact bony rump; the _Archæopteryx_ had a long bony tail with a row of feathers along each side.

§ 4

(the earliest known bird)]

This great period of Mesozoic life, this second volume of the book of life, is indeed an amazing story of reptilian life proliferating and developing. But the most striking thing of all the story remains to be told. Right up to the latest Mesozoic Rocks we find all these reptilian orders we have enumerated still flourishing unchallenged. There is no hint of an enemy or competitor to them in the relics we find of their world. Then the record is broken. We do not know how long a time the break represents; many pages may be missing here, pages that may represent some great cataclysmal climatic change. When next we find abundant traces of the land plants and the land animals of the earth, this great multitude of reptile species had gone. For the most part they have left no descendants. They have been “wiped out.” The pterodactyls have gone absolutely; of the plesiosaurs and ichthyosaurs none is alive; the mosasaurs have gone; of the lizards a few remain, the monitor of the Dutch East Indies is the largest; all the multitude and diversity of the dinosaurs have vanished. Only the crocodiles and the turtles and tortoises carry on in any quantity into Cainozoic times. The place of all these types in the picture that the Cainozoic fossils presently unfold to us is taken by other animals not closely related to the Mesozoic reptiles and certainly not descended from any of their ruling types. A new kind of life is in possession of the world.

This apparently abrupt ending up of the reptiles is, beyond all question, the most striking revolution in the whole history of the earth before the coming of mankind. It is probably connected with the close of a vast period of equable warm conditions and the onset of a new austerer age, in which the winters were bitterer and the summers brief but hot. The Mesozoic life, animal and vegetable alike, was adapted to warm conditions and capable of little resistance to cold. The new life, on the other hand, was before all things capable of resisting great changes of temperature.

Whatever it was that led to the extinction of the Mesozoic reptiles, it was probably some very far-reaching change indeed, for the life of the seas did at the same time undergo a similar catastrophic alteration. The crescendo and ending of the Reptiles on land was paralleled by the crescendo and ending of the Ammonites, a division of creatures like squids with coiled shells which swarmed in those ancient seas. All through the rocky record of this Mesozoic period there is a vast multitude and variety of these coiled shells; there are hundreds of species, and towards the end of the Mesozoic period they increased in diversity and produced exaggerated types. When the record resumes, these too have gone. So far as the reptiles are concerned, people may perhaps be inclined to argue that they were exterminated because the Mammals that replaced them competed with them, and were more fitted to survive; but nothing of the sort can be true of the Ammonites, because to this day their place has not been taken. Simply they are gone. Unknown conditions made it possible for them to live in the Mesozoic seas, and then some unknown change made life impossible for them. No genus of Ammonite survives to-day of all that vast variety, but there still exists one isolated genus very closely related to the Ammonites, the Pearly Nautilus. It is found, it is to be noted, in the warm waters of the Indian and Pacific oceans.[15]

And as for the Mammals competing with and ousting the less fit reptiles, a struggle of which people talk at times, there is not a scrap of evidence of any such direct competition. To judge by the Record of the Rocks as we know it to-day, there is much more reason for believing that first the reptiles in some inexplicable way perished, and then that later on, after a very hard time for all life upon the earth, the mammals, as conditions became more genial again, developed and spread to fill the vacant world.

§ 5

Were there mammals in the Mesozoic period?

This is a question not yet to be answered precisely. Patiently and steadily the geologists gather fresh evidence and reason out completer conclusions. At any time some new deposit may reveal fossils that will illuminate this question. Certainly either mammals, or the ancestors of the mammals, must have lived throughout the Mesozoic period. In the very opening chapter of the Mesozoic volume of the Record there were those Theriomorphous Reptiles to which we have already alluded, and in the later Mesozoic a number of small jaw-bones are found, entirely mammalian in character. But there is not a scrap, not a bone, to suggest that there lived any Mesozoic Mammal which could look a dinosaur in the face. The Mesozoic mammals or mammal-like reptiles--for we do not know clearly which they were--seem to have been all obscure little beasts of the size of mice and rats, more like a down-trodden order of reptiles than a distinct class; probably they still laid eggs and were developing only slowly their distinctive covering of hair. They lived away from big waters, and perhaps in the desolate uplands, as marmots do now; probably they lived there beyond the pursuit of the carnivorous dinosaurs. Some perhaps went on all fours, some chiefly went on their hind legs and clambered with their fore limbs. They became fossils only so occasionally that chance has not yet revealed a single complete skeleton in the whole vast record of the Mesozoic rocks by which to check these guesses.

(_Reptilian wingless water-bird_)]

These little Theriomorphs, these ancestral mammals, developed hair. Hairs, like feathers, are long and elaborately specialized scales. Hair is perhaps the clue to the salvation of the early mammals. Leading lives upon the margin of existence, away from the marshes and the warmth, they developed an outer covering only second in its warmth-holding (or heat-resisting) powers to the down and feathers of the Arctic sea-birds. And so they held out through the age of hardship between the Mesozoic and Cainozoic ages, to which most of the true reptiles succumbed.

All the main characteristics of this flora and sea and land fauna that came to an end with the end of the Mesozoic age were such as were adapted to an equable climate and to shallow and swampy regions. But in the case of their Cainozoic successors, both hair and feathers gave _a power of resistance to variable temperatures_ such as no reptile possessed, and with it they gave a range far greater than any animal had hitherto attained.

The range of life of the Lower Palæozoic Period was confined to warm water.

The range of life of the Upper Palæozoic Period was confined to warm water or to warm swamps and wet ground.

The range of life of the Mesozoic Period as we know it was confined to water and fairly low-lying valley regions under equable conditions.

Meanwhile in each of these periods there were types involuntarily extending the range of life beyond the limits prevailing in that period; and when ages of extreme conditions prevailed, it was these marginal types which survived to inherit the depopulated world.

That perhaps is the most general statement we can make about the story of the geological record; it is a story of widening range. Classes, genera, and species of animals appear and disappear, but the range widens. It widens always. Life has never had so great a range as it has to-day. Life to-day, in the form of man, goes higher in the air than it has ever done before; man’s geographical range is from pole to pole, he goes under the water in submarines, he sounds the cold, lifeless darkness of the deepest seas, he burrows into virgin levels of the rocks, and in thought and knowledge he pierces to the centre of the earth and reaches out to the uttermost star. Yet in all the relics of the Mesozoic time we find no certain memorials of his ancestry. His ancestors, like the ancestors of all the kindred mammals, must have been creatures so rare, so obscure, and so remote that they have left scarcely a trace amidst the abundant vestiges of the monsters that wallowed rejoicing in the steamy air and lush vegetation of the Mesozoic lagoons, or crawled or hopped or fluttered over the great river plains of that time.[16]

VII

THE AGE OF MAMMALS

§ 1. _A New Age of Light._ § 2. _Tradition Comes into the World._ §
3. _An Age of Brain Growth._ § 4. _The World Grows Hard Again._ §
5. _Chronology of the Ice Age._

§ 1

The third great division of the geological record, the Cainozoic, opens with a world already physically very like the world we live in to-day. Probably the day was at first still perceptibly shorter, but the scenery had become very modern in its character. Climate was, of course, undergoing, age by age, its incessant and irregular variations; lands that are temperate to-day have passed, since the Cainozoic age began, through phases of great warmth, intense cold, and extreme dryness; but the landscape, if it altered, altered to nothing that cannot still be paralleled to-day in some part of the world or other. In the place of the cycads, sequoias, and strange conifers of the Mesozoic, the plant names that now appear in the lists of fossils include birch, beech, holly, tulip trees, ivy, sweet gum, bread-fruit trees. Flowers had developed concurrently with bees and butterflies. Palms were now very important. Such plants had already been in evidence in the later levels of the (American Cretaceous) Mesozoic, but now they dominated the scene altogether. Grass was becoming a great fact in the world. Certain grasses, too, had appeared in the later Mesozoic, but only with the Cainozoic period came grass plains and turf spreading wide over a world that was once barren stone.

The period opened with a long phase of considerable warmth; then the world cooled. And in the opening of this third part of the record, this Cainozoic period, a gigantic crumpling of the earth’s crust and an upheaval of mountain ranges was in progress. The Alps, the Andes, the Himalayas, are all Cainozoic mountain ranges; the background of an early Cainozoic scene, to be typical, should display an active volcano or so. It must have been an age of great earthquakes.

Geologists make certain main divisions of the Cainozoic period, and it will be convenient to name them here and to indicate their climate. First comes the _Eocene_ (dawn of recent life), an age of exceptional warmth in the world’s history, subdivided into an older and newer Eocene; then the _Oligocene_ (but little of recent life), in which the climate was still equable. The _Miocene_ (with living species still in a minority) was the great age of mountain building, and the general temperature was falling. In the _Pliocene_ (more living than extinct species), climate was very much at its present phase; but with the _Pleistocene_ (a great majority of living species) there set in a long period of extreme conditions--it was the Great Ice Age. Glaciers spread from the poles towards the equator, until England to the Thames was covered in ice. Thereafter to our own time came a period of partial recovery.

§ 2

In the forests and following the grass over the Eocene plains there appeared for the first time a variety and abundance of mammals. Before we proceed to any description of these mammals, it may be well to note in general terms what a mammal is.

From the appearance of the vertebrated animals in the Lower Palæozoic Age, when the fish first swarmed out into the sea, there has been a steady progressive development of vertebrated creatures. A fish is a vertebrated animal that breathes by gills and can live only in water. An amphibian may be described as a fish that has added to its gill-breathing the power of breathing air with its swimming-bladder in adult life, and that has also developed limbs with five toes to them in place of the fins of a fish. A tadpole is for a time a fish; it becomes a land creature as it develops. A reptile is a further stage in this detachment from water; it is an amphibian that is no longer amphibious; it passes through its tadpole stage--its fish stage, that is--in an egg. From the beginning it must breathe in air; it can never breathe under water as a tadpole can do. Now, a modern mammal is really a sort of reptile that has developed a peculiarly effective protective covering, hair; and that also retains its eggs in the body until they hatch so that it brings forth living young (viviparous), and even after birth it cares for them and feeds them by its mammæ for a longer or shorter period. Some reptiles, some vipers for example, are viviparous, but none stand by their young as the real mammals do. Both the birds and the mammals, which escaped whatever destructive forces made an end to the Mesozoic reptiles, and which survived to dominate the Cainozoic world, have these two things in common: first, a far more effective protection against changes of temperature than any other variation of the reptile type ever produced; and, secondly, a peculiar care for their eggs, the bird by incubation and the mammal by retention, and a disposition to look after the young for a certain period after hatching or birth. There is by comparison the greatest carelessness about offspring in the reptile.

Hair was evidently the earliest distinction of the mammals from the rest of the reptiles. It is doubtful if the particular Theriodont reptiles who were developing hair in the early Mesozoic were viviparous. Two mammals survive to this day which not only do not suckle their young,[17] but which lay eggs, the _Ornithorhynchus_ and the _Echidna_, and in the Eocene there were a number of allied forms. They are the survivors of what was probably a much larger number and variety of small egg-laying hairy creatures, hairy reptiles, hoppers, climbers, and runners, which included the Mesozoic ancestors of all existing mammals up to and including man.

Now we may put the essential facts about mammalian reproduction in another way. _The mammal is a family animal._ And the family habit involved the possibility of a new sort of continuity of experience in the world. Compare the completely closed-in life of an individual lizard with the life of even a quite lowly mammal of almost any kind. The former has no mental continuity with anything beyond itself; it is a little self-contained globe of experience that serves its purpose and ends; but the latter “picks up” from its mother, and “hands on” to its offspring. All the mammals, except for the two genera we have named, had already before the lower Eocene age arrived at this stage of pre-adult dependence and imitation. They were all more or less imitative in youth and capable of a certain modicum of education; they all, as a part of their development, received a certain amount of care and example and even direction from their mother. This is as true of the hyæna and rhinoceros as it is of the dog or man; the difference of educability is enormous, but the fact of protection and educability in the young stage is undeniable. So far as the vertebrated animals go, these new mammals, with their viviparous, young-protecting disposition, and these new birds, with their incubating, young-protecting disposition, introduce at the opening of the Cainozoic period a fresh thing into the expanding story of life, namely, social association, the addition to hard and inflexible instinct of _tradition_, and the nervous organization necessary to receive tradition.

All the innovations that come into the history of life begin very humbly. The supply of blood-vessels in the swimming-bladder of the mudfish in the lower Palæozoic torrent-river, that enabled it to pull through a season of drought, would have seemed at that time to that bodiless visitant to our planet we have already imagined, a very unimportant side fact in that ancient world of great sharks and plated fishes, sea-scorpions, and coral reefs and seaweed; but it opened the narrow way by which the land vertebrates arose to predominance. The mudfish would have seemed then a poor refugee from the too crowded and aggressive life of the sea. But once lungs were launched into the world, every line of descent that had lungs went on improving them. So, too, in the upper Palæozoic, the fact that some of the Amphibia were losing their “amphibiousness” by a retardation of hatching of their eggs, would have appeared a mere response to the distressful dangers that threatened the young tadpole. Yet that prepared the conquest of the dry land for the triumphant multitude of the Mesozoic reptiles. It opened a new direction towards a free and vigorous land-life along which all the reptilian animals moved. And this viviparous, young-tending training that the ancestral mammalia underwent during that age of inferiority and hardship for them, set going in the world a new continuity of perception, of which even man to-day only begins to appreciate the significance.

§ 3

A number of types of mammal already appear in the Eocene. Some are differentiating in one direction, and some in another, some are perfecting themselves as herbivorous quadrupeds, some leap and climb among the trees, some turn back to the water to swim, but all types are unconsciously exploiting and developing the brain which is the instrument of this new power of acquisition and educability. In the Eocene rocks are found small early predecessors of the horse (Eohippus), tiny camels, pigs, early tapirs, early hedgehogs, monkeys and lemurs, opossums and carnivores. Now, all these were more or less ancestral to living forms, and all have brains relatively much smaller than their living representatives. There is, for instance, an early rhinoceros, _Titanotherium_, with a brain not one tenth the size of that of the existing rhinoceros. The latter is by no means a perfect type of the attentive and submissive student, but even so it is ten times more observant and teachable than its predecessor. This sort of thing is true of all the orders and families that survive until to-day. All the Cainozoic mammals were doing this one thing in common under the urgency of a common necessity; they were all growing brain. It was a parallel advance. In the same order or family to-day, the brain is usually from six to ten times what it was in the Eocene ancestor.

Grass was now spreading over the world, and with this extension arose some huge graminivorous brutes of which no representative survives to-day. Such were the Uintatheres and the Titanotheres. And in pursuit of such beasts came great swarms of primitive dogs, some as big as bears, and the first cats, one in particular (_Smilodon_), a small fierce-looking creature with big knife-like canines, the first sabre-toothed tiger, which was to develop into greater things. American deposits in the Miocene display a great variety of camels, giraffe camels with long necks, gazelle camels, llamas, and true camels. North America, throughout most of the Cainozoic period, appears to have been in open and easy continuation with Asia, and when at last the glaciers of the Great Ice Age, and then the Bering Strait, came to separate the two great continental regions, the last camels were left in the old world and the llamas in the new.

In the Eocene the first ancestors of the elephants appear in northern Africa as snouted creatures; the elephant’s trunk dawned on the world in the Miocene.

One group of creatures is of peculiar interest in a history that is mainly to be the story of mankind. We find fossils in the Eocene of monkeys and lemurs, but of one particular creature we have as yet not a single bone. It was half ape, half monkey; it clambered about the trees and ran, and probably ran well, on its hind legs upon the ground. It was small-brained by our present standards, but it had clever hands with which it handled fruits and beat nuts upon the rocks and perhaps caught up sticks and stones to smite its fellows. It was our ancestor.

§ 4

Through millions of simian generations the spinning world circled about the sun; slowly its orbit, which may have been nearly circular during the equable days of the early Eocene, was drawn by the attraction of the circling outer planets into a more elliptical form. Its axis of rotation, which had always heeled over to the plane of its orbit, as the mast of a yacht under sail heels over to the level of the water, heeled over by imperceptible degrees a little more and a little more. And each year its summer point shifted a little further from perihelion round its path. These were small changes to happen to a one-inch ball, circling at a distance of 330 yards from a flaming sun nine feet across, in the course of a few million years. They were changes an immortal astronomer in Neptune, watching the earth from age to age, would have found almost imperceptible. But from the point of view of the surviving mammalian life of the Miocene, they mattered profoundly. Age by age the winters grew on the whole colder and harder and a few hours longer relatively to the summers in a thousand years; age by age the summers grew briefer. On an average the winter snow lay a little later in the spring in each century, and the glaciers in the northern mountains gained an inch this year, receded half an inch next, came on again a few inches....

The Record of the Rocks tells of the increasing chill. The Pliocene was a temperate time, and many of the warmth-loving plants and animals had gone. Then, rather less deliberately, some feet or some inches every year, the ice came on.

An arctic fauna, musk ox, woolly mammoth, woolly rhinoceros, lemming, ushers in the Pleistocene. Over North America, and Europe and Asia alike, the ice advanced. For thousands of years it advanced, and then for thousands of years it receded, to advance again. Europe down to the Baltic shores, Britain down to the Thames, North America down to New England, and more centrally as far south as Ohio, lay for ages under the glaciers. Enormous volumes of water were withdrawn from the ocean and locked up in those stupendous ice caps so as to cause a world-wide change in the relative levels of land and sea. Vast areas were exposed that are now again sea bottom.

The world to-day is still coming slowly out of the last of four great waves of cold. It is not growing warmer steadily. There have been fluctuations. Remains of bog oaks, for example, which grew two or three thousand years ago, are found in Scotland at latitudes in which not even a stunted oak will grow at the present time. And it is amidst this crescendo and diminuendo of frost and snow that we first recognize forms that are like the forms of men. The Age of Mammals culminated in ice and hardship and man.

§ 5

Guesses about the duration of the great age of cold are still vague, but in the Time diagram on page 60 we follow H. F. Osborn in accepting as our guides the estimates of Albrecht Penck[18] and C. A. Reeds.[19]

The reader should compare this diagram carefully with our first time diagram, Chapter II, § 2, p. 14. That diagram, if it were on the same scale as this one, would be between 41 and 410 feet long. The position of the Eoanthropus is very uncertain: it may be as early as the Pliocene]