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diff --git a/old/1043.txt b/old/1043.txt new file mode 100644 index 0000000..f4022aa --- /dev/null +++ b/old/1043.txt @@ -0,0 +1,10414 @@ +The Project Gutenberg EBook of The Story of Evolution, by Joseph McCabe + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: The Story of Evolution + +Author: Joseph McCabe + +Posting Date: August 6, 2008 [EBook #1043] +Release Date: September, 1997 + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK THE STORY OF EVOLUTION *** + + + + +Produced by Dianne Bean + + + + + +THE STORY OF EVOLUTION + +By Joseph McCabe + +1912 + + + + +PREFACE + +An ingenious student of science once entertained his generation with a +theory of how one might behold again all the stirring chapters that make +up the story of the earth. The living scene of our time is lit by the +light of the sun, and for every few rays that enter the human eye, and +convey the image of it to the human mind, great floods of the reflected +light pour out, swiftly and indefinitely, into space. Imagine, then, +a man moving out into space more rapidly than light, his face turned +toward the earth. Flashing through the void at, let us say, a million +miles a second, he would (if we can overlook the dispersion of the +rays of light) overtake in succession the light that fell on the French +Revolution, the Reformation, the Norman Conquest, and the faces of the +ancient empires. He would read, in reverse order, the living history of +man and whatever lay before the coming of man. + +Few thought, as they smiled over this fairy tale of science, that +some such panoramic survey of the story of the earth, and even of the +heavens, might one day be made in a leisure hour by ordinary mortals; +that in the soil on which they trod were surer records of the past than +in its doubtful literary remains, and in the deeper rocks were records +that dimly lit a vast abyss of time of which they never dreamed. It +is the supreme achievement of modern science to have discovered and +deciphered these records. The picture of the past which they afford is, +on the whole, an outline sketch. Here and there the details, the colour, +the light and shade, may be added; but the greater part of the canvas is +left to the more skilful hand of a future generation, and even the +broad lines are at times uncertain. Yet each age would know how far its +scientific men have advanced in constructing that picture of the growth +of the heavens and the earth, and the aim of the present volume is to +give, in clear and plain language, as full an account of the story as +the present condition of our knowledge and the limits of the volume will +allow. The author has been for many years interested in the evolution +of things, or the way in which suns and atoms, fishes and flowers, hills +and elephants, even man and his institutions, came to be what they +are. Lecturing and writing on one or other phase of the subject have, +moreover, taught him a language which the inexpert seem to understand, +although he is not content merely to give a superficial description of +the past inhabitants of the earth. + +The particular features which, it is hoped, may give the book a +distinctive place in the large literature of evolution are, first, that +it includes the many evolutionary discoveries of the last few years, +gathers its material from the score of sciences which confine themselves +to separate aspects of the universe, and blends all these facts and +discoveries in a more or less continuous chronicle of the life of the +heavens and the earth. Then the author has endeavoured to show, not +merely how, but why, scene succeeds scene in the chronicle of the earth, +and life slowly climbs from level to level. He has taken nature in the +past as we find it to-day: an interconnected whole, in which the changes +of land and sea, of heat and cold, of swamp and hill, are faithfully +reflected in the forms of its living population. And, finally, he has +written for those who are not students of science, or whose knowledge +may be confined to one branch of science, and used a plain speech which +assumes no previous knowledge on the reader's part. + +For the rest, it will be found that no strained effort is made to trace +pedigrees of animals and plants when the material is scanty; that, if on +account of some especial interest disputable or conjectural speculations +are admitted, they are frankly described as such; and that the more +important differences of opinion which actually divide astronomers, +geologists, biologists, and anthropologists are carefully taken into +account and briefly explained. A few English and American works are +recommended for the convenience of those who would study particular +chapters more closely, but it has seemed useless, in such a work, +to give a bibliography of the hundreds of English, American, French, +German, and Italian works which have been consulted. + + +CONTENTS + + I. THE DISCOVERY OF THE UNIVERSE + II. THE FOUNDATIONS OF THE UNIVERSE + III. THE BIRTH AND DEATH OF WORLDS + IV. THE PREPARATION OF THE EARTH + V. THE BEGINNING OF LIFE + VI. THE INFANCY OF THE EARTH + VII. THE PASSAGE TO THE LAND + VIII. THE COAL-FOREST + IX. THE ANIMALS OF THE COAL-FOREST + X. THE PERMIAN REVOLUTION + XI. THE MIDDLE AGES OF THE EARTH + XII. THE AGE OF REPTILES + XIII. THE BIRD AND THE MAMMAL + XIV. IN THE DAYS OF THE CHALK + XV. THE TERTIARY ERA + XVI. THE FLOWER AND THE INSECT + XVII. THE ORIGIN OF OUR MAMMALS + XVIII. THE EVOLUTION OF MAN + XIX. MAN AND THE GREAT ICE-AGE + XX. THE DAWN OF CIVILISATION + XXI. EVOLUTION IN HISTORY INDEX + + + + + +THE STORY OF EVOLUTION + + + +CHAPTER I. THE DISCOVERY OF THE UNIVERSE + +The beginning of the victorious career of modern science was very +largely due to the making of two stimulating discoveries at the close +of the Middle Ages. One was the discovery of the earth: the other the +discovery of the universe. Men were confined, like molluscs in their +shells, by a belief that they occupied the centre of a comparatively +small disk--some ventured to say a globe--which was poised in a +mysterious way in the middle of a small system of heavenly bodies. The +general feeling was that these heavenly bodies were lamps hung on a not +too remote ceiling for the purpose of lighting their ways. Then certain +enterprising sailors--Vasco da Gama, Maghalaes, Columbus--brought home +the news that the known world was only one side of an enormous globe, +and that there were vast lands and great peoples thousands of miles +across the ocean. The minds of men in Europe had hardly strained +their shells sufficiently to embrace this larger earth when the second +discovery was reported. The roof of the world, with its useful little +system of heavenly bodies, began to crack and disclose a profound +and mysterious universe surrounding them on every side. One cannot +understand the solidity of the modern doctrine of the formation of the +heavens and the earth until one appreciates this revolution. + +Before the law of gravitation had been discovered it was almost +impossible to regard the universe as other than a small and compact +system. We shall see that a few daring minds pierced the veil, and +peered out wonderingly into the real universe beyond, but for the +great mass of men it was quite impossible. To them the modern idea of +a universe consisting of hundreds of millions of bodies, each weighing +billions of tons, strewn over billions of miles of space, would have +seemed the dream of a child or a savage. Material bodies were "heavy," +and would "fall down" if they were not supported. The universe, they +said, was a sensible scientific structure; things were supported in +their respective places. A great dome, of some unknown but compact +material, spanned the earth, and sustained the heavenly bodies. It might +rest on the distant mountains, or be borne on the shoulders of an Atlas; +or the whole cosmic scheme might be laid on the back of a gigantic +elephant, and--if you pressed--the elephant might stand on the hard +shell of a tortoise. But you were not encouraged to press. + +The idea of the vault had come from Babylon, the first home of science. +No furnaces thickened that clear atmosphere, and the heavy-robed priests +at the summit of each of the seven-staged temples were astronomers. +Night by night for thousands of years they watched the stars and +planets tracing their undeviating paths across the sky. To explain their +movements the priest-astronomers invented the solid firmament. Beyond +the known land, encircling it, was the sea, and beyond the sea was a +range of high mountains, forming another girdle round the earth. On +these mountains the dome of the heavens rested, much as the dome of +St. Paul's rests on its lofty masonry. The sun travelled across its +under-surface by day, and went back to the east during the night through +a tunnel in the lower portion of the vault. To the common folk the +priests explained that this framework of the world was the body of an +ancient and disreputable goddess. The god of light had slit her in two, +"as you do a dried fish," they said, and made the plain of the earth +with one half and the blue arch of the heavens with the other. + +So Chaldaea lived out its 5000 years without discovering the universe. +Egypt adopted the idea from more scientific Babylon. Amongst the +fragments of its civilisation we find representations of the firmament +as a goddess, arching over the earth on her hands and feet, condemned to +that eternal posture by some victorious god. The idea spread amongst the +smaller nations which were lit by the civilisation of Babylon and Egypt. +Some blended it with coarse old legends; some, like the Persians and +Hebrews, refined it. The Persians made fire a purer and lighter spirit, +so that the stars would need no support. But everywhere the blue vault +hemmed in the world and the ideas of men. It was so close, some said, +that the birds could reach it. At last the genius of Greece brooded over +the whole chaos of cosmical speculations. + +The native tradition of Greece was a little more helpful than the +Babylonian teaching. First was chaos; then the heavier matter sank to +the bottom, forming the disk of the earth, with the ocean poured round +it, and the less coarse matter floated as an atmosphere above it, +and the still finer matter formed an "aether" above the atmosphere. +A remarkably good guess, in its very broad outline; but the solid +firmament still arched the earth, and the stars were little undying +fires in the vault. The earth itself was small and flat. It stretched +(on the modern map) from about Gibraltar to the Caspian, and from +Central Germany--where the entrance to the lower world was located--to +the Atlas mountains. But all the varied and conflicting culture of the +older empires was now passing into Greece, lighting up in succession the +civilisations of Asia Minor, the Greek islands, and then Athens and its +sister states. Men began to think. + +The first genius to have a glimpse of the truth seems to have been +the grave and mystical Pythagorus (born about 582 B.C.). He taught his +little school that the earth was a globe, not a disk, and that it turned +on its axis in twenty-four hours. The earth and the other planets +were revolving round the central fire of the system; but the sun was a +reflection of this central fire, not the fire itself. Even Pythagoras, +moreover, made the heavens a solid sphere revolving, with its stars, +round the central fire; and the truth he discovered was mingled with so +much mysticism, and confined to so small and retired a school, that it +was quickly lost again. In the next generation Anaxagoras taught that +the sun was a vast globe of white-hot iron, and that the stars were +material bodies made white-hot by friction with the ether. A generation +later the famous Democritus came nearer than any to the truth. The +universe was composed of an infinite number of indestructible particles, +called "atoms," which had gradually settled from a state of chaotic +confusion to their present orderly arrangement in large masses. The sun +was a body of enormous size, and the points of light in the Milky Way +were similar suns at a tremendous distance from the earth. Our universe, +moreover, was only one of an infinite number of universes, and an +eternal cycle of destruction and re-formation was running through these +myriads of worlds. + +By sheer speculation Greece was well on the way of discovery. Then the +mists of philosophy fell between the mind of Greece and nature, and +the notions of Democritus were rejected with disdain; and then, very +speedily, the decay of the brilliant nation put an end to its feverish +search for truth. Greek culture passed to Alexandria, where it met the +remains of the culture of Egypt, Babylonia, and Persia, and one more +remarkable effort was made to penetrate the outlying universe before the +night of the Middle Ages fell on the old world. + +Astronomy was ardently studied at Alexandria, and was fortunately +combined with an assiduous study of mathematics. Aristarchus (about +320-250 B.C.) calculated that the sun was 84,000,000 miles away; a vast +expansion of the solar system and, for the time, a remarkable approach +to the real figure (92,000,000) Eratosthenes (276-196 B.C.) made an +extremely good calculation of the size of the earth, though he held it +to be the centre of a small universe. He concluded that it was a globe +measuring 27,000 (instead of 23,700) miles in circumference. Posidonius +(135-51 B.C.) came even nearer with a calculation that the circumference +was between 25,000 and 19,000 miles; and he made a fairly correct +estimate of the diameter, and therefore distance, of the sun. Hipparchus +(190-120 B.C.) made an extremely good calculation of the distance of the +moon. + +By the brilliant work of the Alexandrian astronomers the old world +seemed to be approaching the discovery of the universe. Men were +beginning to think in millions, to gaze boldly into deep abysses of +space, to talk of vast fiery globes that made the earth insignificant +But the splendid energy gradually failed, and the long line was closed +by Ptolemaeus, who once more put the earth in the centre of the system, +and so imposed what is called the Ptolemaic system on Europe. The keen +school-life of Alexandria still ran on, and there might have been a +return to the saner early doctrines, but at last Alexandrian culture was +extinguished in the blood of the aged Hypatia, and the night fell. +Rome had had no genius for science; though Lucretius gave an immortal +expression to the views of Democritus and Epicurus, and such writers +as Cicero and Pliny did great service to a later age in preserving +fragments of the older discoveries. The curtains were once more drawn +about the earth. The glimpses which adventurous Greeks had obtained of +the great outlying universe were forgotten for a thousand years. The +earth became again the little platform in the centre of a little world, +on which men and women played their little parts, preening themselves on +their superiority to their pagan ancestors. + +I do not propose to tell the familiar story of the revival at any +length. As far as the present subject is concerned, it was literally +a Renascence, or re-birth, of Greek ideas. Constantinople having been +taken by the Turks (1453), hundreds of Greek scholars, with their old +literature, sought refuge in Europe, and the vigorous brain of the young +nations brooded over the ancient speculations, just as the vigorous +young brain of Greece had done two thousand years before. Copernicus +(1473-1543) acknowledges that he found the secret of the movements +of the heavenly bodies in the speculations of the old Greek thinkers. +Galilei (1564-1642) enlarged the Copernican system with the aid of the +telescope; and the telescope was an outcome of the new study of optics +which had been inspired in Roger Bacon and other medieval scholars by +the optical works, directly founded on the Greek, of the Spanish Moors. +Giordano Bruno still further enlarged the system; he pictured the +universe boldly as an infinite ocean of liquid ether, in which the +stars, with retinues of inhabited planets, floated majestically. Bruno +was burned at the stake (1600); but the curtains that had so long been +drawn about the earth were now torn aside for ever, and men looked +inquiringly into the unfathomable depths beyond. Descartes (1596-1650) +revived the old Greek idea of a gradual evolution of the heavens and the +earth from a primitive chaos of particles, taught that the stars stood +out at unimaginable distances in the ocean of ether, and imagined the +ether as stirring in gigantic whirlpools, which bore cosmic bodies in +their orbits as the eddy in the river causes the cork to revolve. + +These stimulating conjectures made a deep impression on the new age. +A series of great astronomers had meantime been patiently and +scientifically laying the foundations of our knowledge. Kepler +(1571-1630) formulated the laws of the movement of the planets; Newton +(1642-1727) crowned the earlier work with his discovery of the real +agency that sustains cosmic bodies in their relative positions. The +primitive notion of a material frame and the confining dome of the +ancients were abandoned. We know now that a framework of the most +massive steel would be too frail to hold together even the moon and the +earth. It would be rent by the strain. The action of gravitation is the +all-sustaining power. Once introduce that idea, and the great ocean of +ether might stretch illimitably on every side, and the vastest bodies +might be scattered over it and traverse it in stupendous paths. Thus it +came about that, as the little optic tube of Galilei slowly developed +into the giant telescope of Herschel, and then into the powerful +refracting telescopes of the United States of our time; as the new +science of photography provided observers with a new eye--a sensitive +plate that will register messages, which the human eye cannot detect, +from far-off regions; and as a new instrument, the spectroscope, endowed +astronomers with a power of perceiving fresh aspects of the inhabitants +of space, the horizon rolled backward, and the mind contemplated a +universe of colossal extent and power. + +Let us try to conceive this universe before we study its evolution. I +do not adopt any of the numerous devices that have been invented for the +purpose of impressing on the imagination the large figures we must +use. One may doubt if any of them are effective, and they are at least +familiar. Our solar system--the family of sun and planets which had been +sheltered under a mighty dome resting on the hill-tops--has turned out +to occupy a span of space some 16,000,000,000 miles in diameter. That is +a very small area in the new universe. Draw a circle, 100 billion miles +in diameter, round the sun, and you will find that it contains only +three stars besides the sun. In other words, a sphere of space measuring +300 billion miles in circumference--we will not venture upon the number +of cubic miles--contains only four stars (the sun, alpha Centauri, +21,185 Lalande, and 61 Cygni). However, this part of space seems to be +below the average in point of population, and we must adopt a different +way of estimating the magnitude of the universe from the number of its +stellar citizens. + +Beyond the vast sphere of comparatively empty space immediately +surrounding our sun lies the stellar universe into which our great +telescopes are steadily penetrating. Recent astronomers give various +calculations, ranging from 200,000,000 to 2,000,000,000, of the number +of stars that have yet come within our faintest knowledge. Let us accept +the modest provisional estimate of 500,000,000. Now, if we had reason to +think that these stars were of much the same size and brilliance as our +sun, we should be able roughly to calculate their distance from their +faintness. We cannot do this, as they differ considerably in size and +intrinsic brilliance. Sirius is more than twice the size of our sun and +gives out twenty times as much light. Canopus emits 20,000 times as much +light as the sun, but we cannot say, in this case, how much larger it is +than the sun. Arcturus, however, belongs to the same class of stars as +our sun, and astronomers conclude that it must be thousands of times +larger than the sun. A few stars are known to be smaller than the sun. +Some are, intrinsically, far more brilliant; some far less brilliant. + +Another method has been adopted, though this also must be regarded +with great reserve. The distance of the nearer stars can be positively +measured, and this has been done in a large number of cases. The +proportion of such cases to the whole is still very small, but, as far +as the results go, we find that stars of the first magnitude are, on the +average, nearly 200 billion miles away; stars of the second magnitude +nearly 300 billion; and stars of the third magnitude 450 billion. If +this fifty per cent increase of distance for each lower magnitude of +stars were certain and constant, the stars of the eighth magnitude would +be 3000 billion miles away, and stars of the sixteenth magnitude would +be 100,000 billion miles away; and there are still two fainter classes +of stars which are registered on long-exposure photographs. The mere +vastness of these figures is immaterial to the astronomer, but he warns +us that the method is uncertain. We may be content to conclude that the +starry universe over which our great telescopes keep watch stretches for +thousands, and probably tens of thousands, of billions of miles. There +are myriads of stars so remote that, though each is a vast incandescent +globe at a temperature of many thousand degrees, and though their +light is concentrated on the mirrors or in the lenses of our largest +telescopes and directed upon the photographic plate at the rate of more +than 800 billion waves a second, they take several hours to register the +faintest point of light on the plate. + +When we reflect that the universe has grown with the growth of our +telescopes and the application of photography we wonder whether we may +as yet see only a fraction of the real universe, as small in comparison +with the whole as the Babylonian system was in comparison with ours. We +must be content to wonder. Some affirm that the universe is infinite; +others that it is limited. We have no firm ground in science for either +assertion. Those who claim that the system is limited point out that, as +the stars decrease in brightness, they increase so enormously in number +that the greater faintness is more than compensated, and therefore, if +there were an infinite series of magnitudes, the midnight sky would be a +blaze of light. But this theoretical reasoning does not allow for dense +regions of space that may obstruct the light, or vast regions of vacancy +between vast systems of stars. Even apart from the evidence that dark +nebulae or other special light-absorbing regions do exist, the question +is under discussion in science at the present moment whether light is +not absorbed in the passage through ordinary space. There is reason to +think that it is. Let us leave precarious speculations about finiteness +and infinity to philosophers, and take the universe as we know it. + +Picture, then, on the more moderate estimate, these 500,000,000 suns +scattered over tens of thousands of billions of miles. Whether they form +one stupendous system, and what its structure may be, is too obscure a +subject to be discussed here. Imagine yourself standing at a point +from which you can survey the whole system and see into the depths and +details of it. At one point is a single star (like our sun), billions +of miles from its nearest neighbour, wearing out its solitary life in a +portentous discharge of energy. Commonly the stars are in pairs, turning +round a common centre in periods that may occupy hundreds of days or +hundreds of years. Here and there they are gathered into clusters, +sometimes to the number of thousands in a cluster, travelling together +over the desert of space, or trailing in lines like luminous caravans. +All are rushing headlong at inconceivable speeds. Few are known to be so +sluggish as to run, like our sun, at only 8000 miles an hour. One of +the "fixed" stars of the ancients, the mighty Arcturus, darts along at +a rate of more than 250 miles a second. As they rush, their surfaces +glowing at a temperature anywhere between 1000 and 20,000 degrees C., +they shake the environing space with electric waves from every tiny +particle of their body at a rate of from 400 billion to 800 billion +waves a second. And somewhere round the fringe of one of the smaller +suns there is a little globe, more than a million times smaller than the +solitary star it attends, lost in the blaze of its light, on which human +beings find a home during a short and late chapter of its history. + +Look at it again from another aspect. Every colour of the rainbow is +found in the stars. Emerald, azure, ruby, gold, lilac, topaz, fawn--they +shine with wonderful and mysterious beauty. But, whether these more +delicate shades be really in the stars or no, three colours are +certainly found in them. The stars sink from bluish white to yellow, and +on to deep red. The immortal fires of the Greeks are dying. Piercing the +depths with a dull red glow, here and there, are the dying suns; and if +you look closely you will see, flitting like ghosts across the light +of their luminous neighbours, the gaunt frames of dead worlds. Here and +there are vast stretches of loose cosmic dust that seems to be gathering +into embryonic stars; here and there are stars in infancy or in +strenuous youth. You detect all the chief phases of the making of a +world in the forms and fires of these colossal aggregations of matter. +Like the chance crowd on which you may look down in the square of a +great city, they range from the infant to the worn and sinking aged. +There is this difference, however, that the embryos of worlds sprawl, +gigantic and luminous, across the expanse; that the dark and mighty +bodies of the dead rush, like the rest, at twenty or fifty miles a +second; and that at intervals some appalling blaze, that dims even the +fearful furnaces of the living, seems to announce the resurrection of +the dead. And there is this further difference, that, strewn about the +intermediate space between the gigantic spheres, is a mass of cosmic +dust--minute grains, or large blocks, or shoals consisting of myriads of +pieces, or immeasurable clouds of fine gas--that seems to be the rubbish +left over after the making of worlds, or the material gathering for the +making of other worlds. + +This is the universe that the nineteenth century discovered and the +twentieth century is interpreting. Before we come to tell the fortunes +of our little earth we have to see how matter is gathered into these +stupendous globes of fire, how they come sometimes to have smaller +bodies circling round them on which living things may appear, how they +supply the heat and light and electricity that the living things need, +and how the story of life on a planet is but a fragment of a larger +story. We have to study the birth and death of worlds, perhaps the most +impressive of all the studies that modern science offers us. Indeed, if +we would read the whole story of evolution, there is an earlier chapter +even than this; the latest chapter to be opened by science, the first to +be read. We have to ask where the matter, which we are going to gather +into worlds, itself came from; to understand more clearly what is the +relation to it of the forces or energies--gravitation, electricity, +etc.--with which we glibly mould it into worlds, or fashion it into +living things; and, above all, to find out its relation to this +mysterious ocean of ether in which it is found. + +Less than half a century ago the making of worlds was, in popular +expositions of science, a comparatively easy business. Take an +indefinite number of atoms of various gases and metals, scatter them +in a fine cloud over some thousands of millions of miles of space, let +gravitation slowly compress the cloud into a globe, its temperature +rising through the compression, let it throw off a ring of matter, which +in turn gravitation will compress into a globe, and you have your earth +circulating round the sun. It is not quite so simple; in any case, +serious men of science wanted to know how these convenient and assorted +atoms happened to be there at all, and what was the real meaning of this +equally convenient gravitation. There was a greater truth than he knew +in the saying of an early physicist, that the atom had the look of a +"manufactured article." It was increasingly felt, as the nineteenth +century wore on, that the atoms had themselves been evolved out of some +simpler material, and that ether might turn out to be the primordial +chaos. There were even those who felt that ether would prove to be the +one source of all matter and energy. And just before the century closed +a light began to shine in those deeper abysses of the submaterial world, +and the foundations of the universe began to appear. + + + +CHAPTER II. THE FOUNDATIONS OF THE UNIVERSE + +To the mind of the vast majority of earlier observers the phrase +"foundations of the universe" would have suggested something enormously +massive and solid. From what we have already seen we are prepared, on +the contrary, to pass from the inconceivably large to the inconceivably +small. Our sun is, as far as our present knowledge goes, one of modest +dimensions. Arcturus and Canopus must be thousands of times larger than +it. Yet our sun is 320,000 times heavier than the earth, and the earth +weighs some 6,000,000,000,000,000,000,000 tons. But it is only in +resolving these stupendous masses into their tiniest elements that we +can reach the ultimate realities, or foundations, of the whole. + +Modern science rediscovered the atoms of Democritus, analysed the +universe into innumerable swarms of these tiny particles, and then +showed how the infinite variety of things could be built up by their +combinations. For this it was necessary to suppose that the atoms were +not all alike, but belonged to a large number of different classes. From +twenty-six letters of the alphabet we could make millions of different +words. From forty or fifty different "elements" the chemist could +construct the most varied objects in nature, from the frame of a man to +a landscape. But improved methods of research led to the discovery +of new elements, and at last the chemist found that he had seventy or +eighty of these "ultimate realities," each having its own very definite +and very different characters. As it is the experience of science to +find unity underlying variety, this was profoundly unsatisfactory, and +the search began for the great unity which underlay the atoms of matter. +The difficulty of the search may be illustrated by a few figures. Very +delicate methods were invented for calculating the size of the atoms. +Laymen are apt to smile--it is a very foolish smile--at these figures, +but it is enough to say that the independent and even more delicate +methods suggested by recent progress in physics have quite confirmed +them. + +Take a cubic millimetre of hydrogen. As a millimetre is less than 1/25th +of an inch, the reader must imagine a tiny bubble of gas that would fit +comfortably inside the letter "o" as it is printed here. The various +refined methods of the modern physicist show that there are 40,000 +billion molecules (each consisting of two atoms of the gas) in this tiny +bubble. It is a little universe, repeating on an infinitesimal scale the +numbers and energies of the stellar universe. These molecules are not +packed together, moreover, but are separated from each other by spaces +which are enormous in proportion to the size of the atoms. Through these +empty spaces the atoms dash at an average speed of more than a thousand +miles an hour, each passing something like 6,000,000,000 of its +neighbours in the course of every second. Yet this particle of gas is +a thinly populated world in comparison with a particle of metal. Take +a cubic centimetre of copper. In that very small square of solid matter +(each side of the cube measuring a little more than a third of an inch) +there are about a quadrillion atoms. It is these minute and elusive +particles that modern physics sets out to master. + +At first it was noticed that the atom of hydrogen was the smallest or +lightest of all, and the other atoms seemed to be multiples of it. +A Russian chemist, Mendeleeff, drew up a table of the elements in +illustration of this, grouping them in families, which seemed to point +to hydrogen as the common parent, or ultimate constituent, of each. When +newly discovered elements fell fairly into place in this scheme the idea +was somewhat confidently advanced that the evolution of the elements was +discovered. Thus an atom of carbon seemed to be a group of 12 atoms of +hydrogen, an atom of oxygen 16, an atom of sulphur 32, an atom of copper +64, an atom of silver 108, an atom of gold 197, and so on. But more +correct measurements showed that these figures were not quite exact, and +the fraction of inexactness killed the theory. + +Long before the end of the nineteenth century students were looking +wistfully to the ether for some explanation of the mystery. It was the +veiled statue of Isis in the scientific world, and it resolutely kept +its veil in spite of all progress. The "upper and limpid air" of the +Greeks, the cosmic ocean of Giordano Bruno, was now an established +reality. It was the vehicle that bore the terrific streams of energy +from star to planet across the immense reaches of space. As the atoms of +matter lay in it, one thought of the crystal forming in its mother-lye, +or the star forming in the nebula, and wondered whether the atom was not +in some such way condensed out of the ether. By the last decade of the +century the theory was confidently advanced--notably by Lorentz and +Larmor--though it was still without a positive basis. How the basis was +found, in the last decade of the nineteenth century, may be told very +briefly. + +Sir William Crookes had in 1874 applied himself to the task of creating +something more nearly like a vacuum than the old air-pumps afforded. +When he had found the means of reducing the quantity of gas in a tube +until it was a million times thinner than the atmosphere, he made the +experiment of sending an electric discharge through it, and found a very +curious result. From the cathode (the negative electric point) certain +rays proceeded which caused a green fluorescence on the glass of the +tube. Since the discharge did not consist of the atoms of the gas, he +concluded that it was a new and mysterious substance, which he called +"radiant matter." But no progress was made in the interpretation of +this strange material. The Crookes tube became one of the toys of +science--and the lamp of other investigators. + +In 1895 Rontgen drew closer attention to the Crookes tube by discovering +the rays which he called X-rays, but which now bear his name. They +differ from ordinary light-waves in their length, their irregularity, +and especially their power to pass through opaque bodies. A number of +distinguished physicists now took up the study of the effect of sending +an electric discharge through a vacuum, and the particles of "radiant +matter" were soon identified. Sir J. J. Thomson, especially, was +brilliantly successful in his interpretation. He proved that they were +tiny corpuscles, more than a thousand times smaller than the atom of +hydrogen, charged with negative electricity, and travelling at the +rate of thousands of miles a second. They were the "electrons" in which +modern physics sees the long-sought constituents of the atom. + +No sooner had interest been thoroughly aroused than it was announced +that a fresh discovery had opened a new shaft into the underworld. Sir +J. J. Thomson, pursuing his research, found in 1896 that compounds of +uranium sent out rays that could penetrate black paper and affect the +photographic plate; though in this case the French physicist, Becquerel, +made the discovery simultaneously' and was the first to publish it. An +army of investigators turned into the new field, and sought to penetrate +the deep abyss that had almost suddenly disclosed itself. The quickening +of astronomy by Galilei, or of zoology by Darwin, was slight in +comparison with the stirring of our physical world by these increasing +discoveries. And in 1898 M. and Mme. Curie made the further discovery +which, in the popular mind, obliterated all the earlier achievements. +They succeeded in isolating the new element, radium, which exhibits the +actual process of an atom parting with its minute constituents. + +The story of radium is so recent that a few lines will suffice to recall +as much as is needed for the purpose of this chapter. In their study of +the emanations from uranium compounds the Curies were led to isolate +the various elements of the compounds until they discovered that the +discharge was predominantly due to one specific element, radium. Radium +is itself probably a product of the disintegration of uranium, the +heaviest of known metals, with an atomic weight some 240 times greater +than that of hydrogen. But this massive atom of uranium has a life that +is computed in thousands of millions of years. It is in radium and its +offspring that we see most clearly the constitution of matter. + +A gramme (less than 15 1/2 grains) of radium contains--we will economise +our space--4x10 (superscript)21 atoms. This tiny mass is, by its +discharge, parting with its substance at the rate of one atom per second +for every 10,000,000,000 atoms; in other words, the "indestructible" +atom has, in this case, a term of life not exceeding 2500 years. In the +discharge from the radium three elements have been distinguished. The +first consists of atoms of the gas helium, which are hurled off at +between 10,000 and 20,000 miles a second. The third element (in the +order of classification) consists of waves analogous to the Rontgen +rays. But the second element is a stream of electrons, which are +expelled from the atom at the appalling speed of about 100,000 miles +a second. Professor Le Bon has calculated that it would take 340,000 +barrels of powder to discharge a bullet at that speed. But we shall see +more presently of the enormous energy displayed within the little system +of the atom. We may add that after its first transformation the radium +passes, much more quickly, through a further series of changes. The +frontiers of the atomic systems were breaking down. + +The next step was for students (notably Soddy and Rutherford) to find +that radio-activity, or spontaneous discharge out of the atomic systems, +was not confined to radium. Not only are other rare metals conspicuously +active, but it is found that such familiar surfaces as damp cellars, +rain, snow, etc., emit a lesser discharge. The value of the new +material thus provided for the student of physics may be shown by +one illustration. Sir J. J. Thomson observes that before these recent +discoveries the investigator could not detect a gas unless about a +billion molecules of it were present, and it must be remembered that the +spectroscope had already gone far beyond ordinary chemical analysis in +detecting the presence of substances in minute quantities. Since these +discoveries we can recognise a single molecule, bearing an electric +charge. + +With these extraordinary powers the physicist is able to penetrate +a world that lies immeasurably below the range of the most powerful +microscope, and introduce us to systems more bewildering than those of +the astronomer. We pass from a portentous Brobdingnagia to a still more +portentous Lilliputia. It has been ascertained that the mass of the +electron is the 1/1700th part of that of an atom of hydrogen, of which, +as we saw, billions of molecules have ample space to execute their +terrific movements within the limits of the letter "o." It has been +further shown that these electrons are identical, from whatever source +they are obtained. The physicist therefore concludes--warning us that +on this further point he is drawing a theoretical conclusion--that the +atoms of ordinary matter are made up of electrons. If that is the case, +the hydrogen atom, the lightest of all, must be a complex system of some +1700 electrons, and as we ascend the scale of atomic weight the clusters +grow larger and larger, until we come to the atoms of the heavier metals +with more than 250,000 electrons in each atom. + +But this is not the most surprising part of the discovery. Tiny as the +dimensions of the atom are, they afford a vast space for the movement of +these energetic little bodies. The speed of the stars in their courses +is slow compared with the flight of the electrons. Since they fly out of +the system, in the conditions we have described, at a speed of between +90,000 and 100,000 miles a second, they must be revolving with terrific +rapidity within it. Indeed, the most extraordinary discovery of all is +that of the energy imprisoned within these tiny systems, which men have +for ages regarded as "dead" matter. Sir J. J. Thomson calculates that, +allowing only one electron to each atom in a gramme of hydrogen, the +tiny globule of gas will contain as much energy as would be obtained by +burning thirty-five tons of coal. If, he says, an appreciable fraction +of the energy that is contained in ordinary matter were to be set free, +the earth would explode and return to its primitive nebulous condition. +Mr. Fournier d'Albe tells us that the force with which electrons repel +each other is a quadrillion times greater than the force of gravitation +that brings atoms together; and that if two grammes of pure electrons +could be placed one centimetre apart they would repel each other with a +force equal to 320 quadrillion tons. The inexpert imagination reels, +but it must be remembered that the speed of the electron is a measured +quantity, and it is within the resources of science to estimate the +force necessary to project it at that speed. [*] + + * See Sir J. J. Thomson, "The Corpuscular Theory of Matter" + (1907) and--for a more elementary presentment--"Light + Visible and Invisible" (1911); and Mr. Fournier d'Albe, "The + Electron Theory" (2nd. ed., 1907). + + +Such are the discoveries of the last fifteen years and a few of the +mathematical deductions from them. We are not yet in a position to say +positively that the atoms are composed of electrons, but it is clear +that the experts are properly modest in claiming only that this is +highly probable. The atom seems to be a little universe in which, in +combination with positive electricity (the nature of which is still +extremely obscure), from 1700 to 300,000 electrons revolve at a speed +that reaches as high as 100,000 miles a second. Instead of being +crowded together, however, in their minute system, each of them has, in +proportion to its size, as ample a space to move in as a single speck of +dust would have in a moderate-sized room (Thomson). This theory not only +meets all the facts that have been discovered in an industrious decade +of research, not only offers a splendid prospect of introducing unity +into the eighty-one different elements of the chemist, but it opens out +a still larger prospect of bringing a common measure into the diverse +forces of the universe. + +Light is already generally recognised as a rapid series of +electro-magnetic waves or pulses in ether. Magnetism becomes +intelligible as a condition of a body in which the electrons revolve +round the atom in nearly the same plane. The difference between positive +and negative electricity is at least partly illuminated. An atom will +repel an atom when its equilibrium is disturbed by the approach of an +additional electron; the physicist even follows the movement of the +added electron, and describes it revolving 2200 billion times a second +round the atom, to escape being absorbed in it. The difference between +good and bad conductors of electricity becomes intelligible. The atoms +of metals are so close together that the roaming electrons pass freely +from one atom to another, in copper, it is calculated, the electron +combines with an atom and is liberated again a hundred million times a +second. Even chemical action enters the sphere of explanation. + +However these hypotheses may fare, the electron is a fact, and the atom +is very probably a more or less stable cluster of electrons. But when +we go further, and attempt to trace the evolution of the electron out +of ether, we enter a region of pure theory. Some of the experts conceive +the electron as a minute whirlpool or vortex in the ocean of ether; +some hold that it is a centre of strain in ether; some regard ether as +a densely packed mass of infinitely small grains, and think that the +positive and negative corpuscles, as they seem to us, are tiny areas +in which the granules are unequally distributed. Each theory has its +difficulties. We do not know the origin of the electron, because we do +not know the nature of ether. To some it is an elastic solid, quivering +in waves at every movement of the particles; to others it is a +continuous fluid, every cubic millimetre of which possesses "an energy +equivalent to the output of a million-horse-power station for 40.000,000 +years" (Lodge); to others it is a close-packed granular mass with a +pressure of 10,000 tons per square centimetre. We must wait. It is +little over ten years since the vaults were opened and physicists began +to peer into the sub-material world. The lower, perhaps lowest, depth is +reserved for another generation. + +But it may be said that the research of the last ten years has given +us a glimpse of the foundations of the universe. Every theory of the +electron assumes it to be some sort of nodule or disturbed area in the +ether. It is sometimes described as "a particle of negative electricity" +and associated with "a particle of positive electricity" in building up +the atom. The phrase is misleading for those who regard electricity as a +force or energy, and it gives rise to speculation as to whether "matter" +has not been resolved into "force." Force or energy is not conceived +by physicists as a substantial reality, like matter, but an abstract +expression of certain relations of matter or electrons. + +In any case, the ether, whether solid or fluid or granular, remains the +fundamental reality. The universe does not float IN an ocean of ether: +it IS an ocean of ether. But countless myriads of minute disturbances +are found in this ocean, and set it quivering with the various pulses +which we classify as forces or energies. These points of disturbance +cluster together in systems (atoms) of from 1000 to 250,000 members, +and the atoms are pressed together until they come in the end to form +massive worlds. It remains only to reduce gravitation itself, which +brings the atoms together, to a strain or stress in ether, and we have +a superb unity. That has not yet been done, but every theory of +gravitation assumes that it is a stress in the ether corresponding to +the formation of the minute disturbances which we call electrons. + +But, it may be urged, he who speaks of foundations speaks of a beginning +of a structure; he who speaks of evolution must have a starting-point. +Was there a time when the ether was a smooth, continuous fluid, without +electrons or atoms, and did they gradually appear in it, like crystals +in the mother-lye? In science we know nothing of a beginning. The +question of the eternity or non-eternity of matter (or ether) is as +futile as the question about its infinity or finiteness. We shall see in +the next chapter that science can trace the processes of nature back +for hundreds, if not thousands, of millions of years, and has ground to +think that the universe then presented much the same aspect as it does +now, and will in thousands of millions of years to come. But if these +periods were quadrillions, instead of millions, of years, they would +still have no relation to the idea of eternity. All that we can say is +that we find nothing in nature that points to a beginning or an end. [*] + + * A theory has been advanced by some physicists that there + is evidence of a beginning. WITHIN OUR EXPERIENCE energy is + being converted into heat more abundantly than heat is being + converted into other energy. This would hold out a prospect + of a paralysed universe, and that stage would have been + reached long ago if the system had not had a definite + beginning. But what knowledge have we of conversions of + energy in remote regions of space, in the depths of stars or + nebulae, or in the sub-material world of which we have just + caught a glimpse? Roundly, none. The speculation is + worthless. + + +One point only need be mentioned in conclusion. Do we anywhere perceive +the evolution of the material elements out of electrons, just as we +perceive the devolution, or disintegration, of atoms into electrons? +There is good ground for thinking that we do. The subject will be +discussed more fully in the next chapter. In brief, the spectroscope, +which examines the light of distant stars and discovers what chemical +elements emitted it, finds matter, in the hottest stars, in an unusual +condition, and seems to show the elements successively emerging from +their fierce alchemy. Sir J. Norman Lockyer has for many years conducted +a special investigation of the subject at the Solar Physics Observatory, +and he declares that we can trace the evolution of the elements out of +the fiery chaos of the young star. The lightest gases emerge first, the +metals later, and in a special form. But here we pass once more from +Lilliputia to Brobdingnagia, and must first explain the making of the +star itself. + + + +CHAPTER III. THE BIRTH AND DEATH OF WORLDS + +The greater part of this volume will be occupied with the things that +have happened on one small globe in the universe during a certain number +of millions of years. It cannot be denied that this has a somewhat +narrow and parochial aspect. The earth is, you remember, a million times +smaller than the sun, and the sun itself is a very modest citizen of +the stellar universe. Our procedure is justified, however, both on the +ground of personal interest, and because our knowledge of the earth's +story is so much more ample and confident. Yet we must preface the story +of the earth with at least a general outline of the larger story of the +universe. No sensible man is humbled or dismayed by the vastness of +the universe. When the human mind reflects on its wonderful scientific +mastery of this illimitable ocean of being, it has no sentiment of being +dwarfed or degraded. It looks out with cold curiosity over the mighty +scattering of worlds, and asks how they, including our own world, came +into being. + +We now approach this subject with a clearer perception of the work we +have to do. The universe is a vast expanse of ether, and somehow or +other this ether gives rise to atoms of matter. We may imagine it as a +spacious chamber filled with cosmic dust; recollecting that the chamber +has no walls, and that the dust arises in the ether itself. The problem +we now approach is, in a word: How are these enormous stretches of +cosmic dust, which we call matter, swept together and compressed +into suns and planets? The most famous answer to this question is the +"nebular hypothesis." Let us see, briefly, how it came into modern +science. + +We saw that some of the ancient Greek speculators imagined their +infinite number of atoms as scattered originally, like dust, throughout +space and gradually coming together, as dust does, to form worlds. The +way in which they brought their atoms together was wrong, but the genius +of Democritus had provided the germ of another sound theory to the +students of a more enlightened age. Descartes (1596-1650) recalled the +idea, and set out a theory of the evolution of stars and planets from a +diffused chaos of particles. He even ventured to say that the earth was +at one time a small white-hot sun, and that a solid crust had gradually +formed round its molten core. Descartes had taken refuge in Sweden from +his persecutors, and it is therefore not surprising that that strange +genius Swedenborg shortly afterwards developed the same idea. In the +middle of the eighteenth century the great French naturalist, Buffon, +followed and improved upon Descartes and Swedenborg. From Buffon's work +it was learned by the German philosopher Kant, who published (1755) +a fresh theory of the concentration of scattered particles into fiery +worlds. Then Laplace (1749-1827) took up the speculation, and gave +it the form in which it practically ruled astronomy throughout the +nineteenth century. That is the genealogy of the famous nebular +hypothesis. It did not spring full-formed from the brain of either Kant +or Laplace, like Athene from the brain of Zeus. + +Laplace had one great advantage over the early speculators. Not only was +he an able astronomer and mathematician, but by his time it was known +that nebulae, or vast clouds of dispersed matter, actually existed in +the heavens. Here was a solid basis for the speculation. Sir William +Herschel, the most assiduous explorer of the heavens, was a contemporary +of Laplace. Laplace therefore took the nebula as his starting-point. + +A quarter of an ounce of solid matter (say, tobacco) will fill a vast +space when it is turned into smoke, and if it were not for the pressure +of the atmosphere it would expand still more. Laplace imagined the +billions of tons of matter which constitute our solar system similarly +dispersed, converted into a fine gas, immeasurably thinner than +the atmosphere. This nebula would be gradually drawn in again by +gravitation, just as the dust falls to the floor of a room. The +collisions of its particles as they fell toward the centre would raise +its temperature and give it a rotating movement. A time would come when +the centrifugal force at the outer ring of the rotating disk would equal +the centripetal (or inward) pull of gravity, and this ring would be +detached, still spinning round the central body. The material of the +ring would slowly gather, by gravitation, round some denser area in it; +the ring would become a sphere; we should have the first, and outermost, +planet circling round the sun. Other rings would successively be +detached, and form the rest of the planets; and the sun is the shrunken +and condensed body of the nebula. + +So simple and beautiful a theory of the solar system could not fail +to captivate astronomers, but it is generally rejected to-day, in the +precise form which Laplace gave it. What the difficulties are which +it has encountered, and the modifications it must suffer, we shall see +later; as well as the new theories which have largely displaced it. It +will be better first to survey the universe from the evolutionary point +of view. But I may observe, in passing, that the sceptical remarks one +hears at times about scientific theories contradicting and superseding +each other are frivolous. One great idea pervades all the theories of +the evolution of worlds, and that idea is firmly established. The +stars and their planets are enormous aggregations of cosmic dust, swept +together and compressed by the action of gravitation. The precise nature +of this cosmic dust--whether it was gas, meteorites and gas, or other +particles--is open to question. + +As we saw in the first chapter, the universe has the word evolution +written, literally, in letters of fire across it. The stars are of all +ages, from sturdy youth to decrepit age, and even to the darkness of +death. We saw that this can be detected on the superficial test of +colour. The colours of the stars are, it is true, an unsafe ground to +build upon. The astronomer still puzzles over the gorgeous colours +he finds at times, especially in double stars: the topaz and azure +companions in beta Cygni, the emerald and red of alpha Herculis, the +yellow and rose of eta Cassiopeiae, and so on. It is at the present time +under discussion in astronomy how far these colours are objective at +all, or whether, if they are real, they may not be due to causes other +than temperature. Yet the significance of the three predominating +colours--blue-white, yellow, and red--has been sustained by the +spectroscope. It is the series of colours through which a white-hot bar +of iron passes as it cools. And the spectroscope gives us good ground to +conclude that the stars are cooling. + +When a glowing gas (not under great pressure) is examined by the +spectroscope, it yields a few vertical lines or bars of light on a +dark background; when a glowing liquid or solid is examined, it gives a +continuous rainbow-like stretch of colour. Some of the nebulae give the +former type of spectrum, and are thus known to be masses of luminous +gas; many of the nebulae and the stars have the latter type of +spectrum. But the stretch of light in the spectrum of a star is crossed, +vertically, by a number of dark lines, and experiment in the laboratory +has taught us how to interpret these. They mean that there is some +light-absorbing vapour between the source of light and the instrument. +In the case of the stars they indicate the presence of an atmosphere +of relatively cool vapours, and an increase in the density of that +atmosphere--which is shown by a multiplication and broadening of +the dark lines on the spectrum--means an increase of age, a loss of +vitality, and ultimately death. So we get the descending scale of +spectra. The dark lines are thinnest and least numerous in the blue +stars, more numerous in the yellow, heavy and thick in the red. As the +body of the star sinks in temperature dense masses of cool vapour gather +about it. Its light, as we perceive it, turns yellow, then red. The next +step, which the spectroscope cannot follow, will be the formation of +a scum on the cooling surface, ending, after ages of struggle, in the +imprisonment of the molten interior under a solid, dark crust. Let us +see how our sun illustrates this theory. + +It is in the yellow, or what we may call the autumnal, stage. Miss +Clerke and a few others have questioned this, but the evidence is too +strong to-day. The vast globe, 867,000 miles in diameter, seems to be a +mass of much the same material as the earth--about forty elements have +been identified in it--but at a terrific temperature. The light-giving +surface is found, on the most recent calculations, to have a temperature +of about 6700 degrees C. This surface is an ocean of liquid or vaporised +metals, several thousand miles in depth; some think that the brilliant +light comes chiefly from clouds of incandescent carbon. Overlying it is +a deep layer of the vapours of the molten metals, with a temperature +of about 5500 degrees C.; and to this comparatively cool and +light-absorbing layer we owe the black lines of the solar spectrum. +Above it is an ocean of red-hot hydrogen, and outside this again is +an atmosphere stretching for some hundreds of thousands of miles into +space. + +The significant feature, from our point of view, is the "sun-spot"; +though the spot may be an area of millions of square miles. These areas +are, of course, dark only by comparison with the intense light of the +rest of the disk. The darkest part of them is 5000 times brighter than +the full moon. It will be seen further, on examining a photograph of +the sun, that a network or veining of this dark material overspreads the +entire surface at all times. There is still some difference of opinion +as to the nature of these areas, but the evidence of the spectroscope +has convinced most astronomers that they are masses of cooler vapour +lying upon, and sinking into, the ocean of liquid fire. Round their +edges, as if responding to the pressure of the more condensed mass, +gigantic spurts and mountains of the white-hot matter of the sun rush +upwards at a rate of fifty or a hundred miles a second, Sometimes they +reach a height of a hundred, and even two hundred, thousand miles, +driving the red-hot hydrogen before them in prodigious and fantastic +flames. Between the black veins over the disk, also, there rise domes +and columns of the liquid fire, some hundreds of miles in diameter, +spreading and sinking at from five to twenty miles a second. The surface +of the sun--how much more the interior!--is an appalling cauldron of +incandescent matter from pole to pole. Every yard of the surface is +a hundred times as intense as the open furnace of a Titanic. From the +depths and from the surface of this fiery ocean, as, on a small scale, +from the surface of the tropical sea, the vapours rise high into the +extensive atmosphere, discharge some of their heat into space, and sink +back, cooler and heavier, upon the disk. + +This is a star in its yellow age, as are Capella and Arcturus and other +stars. The red stars carry the story further, as we should expect. The +heavier lines in their spectrum indicate more absorption of light, and +tell us that the vapours are thickening about the globe; while compounds +like titanium oxide make their appearance, announcing a fall of +temperature. Below these, again, is a group of dark red or "carbon" +stars, in which the process is carried further. Thick, broad, dark lines +in the red end of the spectrum announce the appearance of compounds +of carbon, and a still lower fall of temperature. The veil is growing +thicker; the life is ebbing from the great frame. Then the star sinks +below the range of visibility, and one would think that we can follow +the dying world no farther. Fortunately, in the case of Algol and some +thirty or forty other stars, an extinct sun betrays its existence by +flitting across the light of a luminous sun, and recent research has +made it probable that the universe is strewn with dead worlds. Some of +them may be still in the condition which we seem to find in Jupiter, +hiding sullen fires under a dense shell of cloud; some may already be +covered with a crust, like the earth. There are even stars in which +one is tempted to see an intermediate stage: stars which blaze out +periodically from dimness, as if the Cyclops were spending his last +energy in spasms that burst the forming roof of his prison. But these +variable stars are still obscure, and we do not need their aid. The +downward course of a star is fairly plain. + +When we turn to the earlier chapters in the life of a star, the story +is less clear. It is at least generally agreed that the blue-white stars +exhibit an earlier and hotter stage. They show comparatively little +absorption, and there is an immense preponderance of the lighter gases, +hydrogen and helium. They (Sirius, Vega, etc.) are, in fact, known as +"hydrogen stars," and their temperature is generally computed at between +20,000 and 30,000 degrees C. A few stars, such as Procyon and Canopus, +seem to indicate a stage between them and the yellow or solar type. +But we may avoid finer shades of opinion and disputed classes, and +be content with these clear stages. We begin with stars in which only +hydrogen and helium, the lightest Of elements, can be traced; and the +hydrogen is in an unfamiliar form, implying terrific temperature. In +the next stage we find the lines of oxygen, nitrogen, magnesium, and +silicon. Metals such as iron and copper come later, at first in a +primitive and unusual form. Lastly we get the compounds of titanium +and carbon, and the densely shaded spectra which tell of the thickly +gathering vapours. The intense cold of space is slowly prevailing in the +great struggle. + +What came before the star? It is now beyond reasonable doubt that the +nebula--taking the word, for the moment, in the general sense of a +loose, chaotic mass of material--was the first stage. Professor Keeler +calculated that there are at least 120,000 nebulae within range of +our telescopes, and the number is likely to be increased. A German +astronomer recently counted 1528 on one photographic plate. Many of +them, moreover, are so vast that they must contain the material for +making a great number of worlds. Examine a good photograph of the nebula +in Orion. Recollect that each one of the points of light that are +dotted over the expanse is a star of a million miles or more in diameter +(taking our sun as below the average), and that the great cloud that +sprawls across space is at least 10,000 billion miles away; how much +more no man knows. It is futile to attempt to calculate the extent of +that vast stretch of luminous gas. We can safely say that it is at least +a million times as large as the whole area of our solar system; but it +may run to trillions or quadrillions of miles. + +Nearly a hundred other nebulae are known, by the spectroscope, to be +clouds of luminous gas. It does not follow that they are white-hot, and +that the nebula is correctly called a "fire-mist." Electrical and other +agencies may make gases luminous, and many astronomers think that the +nebulae are intensely cold. However, the majority of the nebulae that +have been examined are not gaseous, and have a very different structure +from the loose and diffused clouds of gas. They show two (possibly more, +but generally two) great spiral arms starting from the central part and +winding out into space. As they are flat or disk-shaped, we see this +structure plainly when they turn full face toward the earth, as does the +magnificent nebula in Canes Venatici. In it, and many others, we clearly +trace a condensed central mass, with two great arms, each apparently +having smaller centres of condensation, sprawling outward like the +broken spring of a watch. The same structure can be traced in the mighty +nebula in Andromeda, which is visible to the naked eye, and it is said +that more than half the nebulae in the heavens are spiral. Knowing that +they are masses of solid or liquid fire, we are tempted to see in them +gigantic Catherine-wheels, the fireworks of the gods. What is their +relation to the stars? + +In the first place, their mere existence has provided a solid basis for +the nebular hypothesis, and their spiral form irresistibly suggests +that they are whirling round on their central axis and concentrating. +Further, we find in some of the gaseous nebulae (Orion) comparatively +void spaces occupied by stars, which seem to have absorbed the nebulous +matter in their formation. On the other hand, we find (in the Pleiades) +wisps and streamers of nebulous matter clinging about great clusters of +stars, suggesting that they are material left over when these clustered +worlds crystallised out of some vast nebula; and enormous stretches of +nebulous material covering regions (as in Perseus) where the stars are +as thick as grains of silver. More important still, we find a type of +cosmic body which seems intermediate between the star and the nebula. +It is a more or less imperfectly condensed star, surrounded by nebular +masses. But one of the most instructive links of all is that at times a +nebula is formed from a star, and a recent case of this character may be +briefly described. + +In February, 1901, a new star appeared in the constellation Perseus. +Knowing what a star is, the reader will have some dim conception of the +portentous blaze that lit up that remote region of space (at least 600 +billion miles away) when he learns that the light of this star increased +4000-fold in twenty-eight hours. It reached a brilliance 8000 times +greater than that of the sun. Telescopes and spectroscopes were turned +on it from all parts of the earth, and the spectroscope showed that +masses of glowing hydrogen were rushing out from it at a rate of nearly +a thousand miles a second. Its light gradually flickered and +fell, however, and the star sank back into insignificance. But the +photographic plate now revealed a new and most instructive feature. +Before the end of the year there was a nebula, of enormous extent, +spreading out on both sides from the centre of the eruption. It was +suggested at the time that the bursting of a star may merely have lit up +a previously dark nebula, but the spectroscope does not support this. A +dim star had dissolved, wholly or partially, into a nebula, as a result +of some mighty cataclysm. What the nature of the catastrophe was we will +inquire presently. + +These are a few of the actual connections that we find between stars and +nebulae. Probably, however, the consideration that weighs most with +the astronomer is that the condensation of such a loose, far-stretched +expanse of matter affords an admirable explanation of the enormous heat +of the stars. Until recently there was no other conceivable source that +would supply the sun's tremendous outpour of energy for tens of millions +of years except the compression of its substance. It is true that the +discovery of radio-activity has disclosed a new source of energy within +the atoms themselves, and there are scientific men, like Professor +Arrhenius, who attach great importance to this source. But, although it +may prolong the limited term of life which physicists formerly allotted +to the sun and other stars, it is still felt that the condensation of a +nebula offers the best explanation of the origin of a sun, and we have +ample evidence for the connection. We must, therefore, see what the +nebula is, and how it develops. + +"Nebula" is merely the Latin word for cloud. Whatever the nature of +these diffused stretches of matter may be, then, the name applies fitly +to them, and any theory of the development of a star from them is still +a "nebular hypothesis." But the three theories which divide astronomers +to-day differ as to the nature of the nebula. The older theory, pointing +to the gaseous nebulae as the first stage, holds that the nebula is a +cloud of extremely attenuated gas. The meteoritic hypothesis (Sir N. +Lockyer, Sir G. Darwin, etc.), observing that space seems to swarm +with meteors and that the greater part of the nebulae are not gaseous, +believes that the starting-point is a colossal swarm of meteors, +surrounded by the gases evolved and lit up by their collisions. The +planetesimal hypothesis, advanced in recent years by Professor Moulton +and Professor Chamberlin, contends that the nebula is a vast cloud of +liquid or solid (but not gaseous) particles. This theory is based mainly +on the dynamical difficulties of the other two, which we will notice +presently. + +The truth often lies between conflicting theories, or they may apply to +different cases. It is not improbable that this will be our experience +in regard to the nature of the initial nebula. The gaseous nebulae, +and the formation of such nebulae from disrupted stars, are facts +that cannot be ignored. The nebulae with a continuous spectrum, and +therefore--in part, at least--in a liquid or solid condition, may very +well be regarded as a more advanced stage of condensation of the same; +their spiral shape and conspicuous nuclei are consistent with this. +Moreover, a condensing swarm of meteors would, owing to the heat +evolved, tend to pass into a gaseous condition. On the tether hand, a +huge expanse of gas stretched over billions of miles of space would be +a net for the wandering particles, meteors, and comets that roam +through space. If it be true, as is calculated, that our 24,000 miles +of atmosphere capture a hundred million meteors a day, what would the +millions or billions of times larger net of a nebula catch, even if the +gas is so much thinner? In other words, it is not wise to draw too fine +a line between a gaseous nebula and one consisting of solid particles +with gas. + +The more important question is: How do astronomers conceive the +condensation of this mixed mass of cosmic dust? It is easy to reply that +gravitation, or the pressure of the surrounding ether, slowly drives the +particles centre-ward, and compresses the dust into globes, as the boy +squeezes the flocculent snow into balls; and it is not difficult for the +mathematician to show that this condensation would account for the +shape and temperature of the stars. But we must go a little beyond this +superficial statement, and see, to some extent, how the deeper students +work out the process. [*] + + * See, especially, Dr. P. Lowell, "The Evolution of Worlds" + (1909). Professor S. Arrhenius, "Worlds in the Making" + (1908), Sir N. Lockyer, "The Meteorite Hypothesis" (1890), + Sir R. Ball, "The Earth's Beginning" (1909), Professor + Moulton, "The Astrophysical Journal (October, 1905), and + Chamberlin and Salisbury, "Geology," Vol. II. (1903). + + +Taking a broad view of the whole field, one may say that the two chief +difficulties are as follows: First, how to get the whole chaotic mass +whirling round in one common direction; secondly, how to account for the +fact that in our solar system the outermost planets and satellites do +not rotate in the same direction as the rest. There is a widespread idea +that these difficulties have proved fatal to the old nebular hypothesis, +and there are distinguished astronomers who think so. But Sir R. Ball +(see note), Professor Lowell (see note), Professor Pickering (Annals of +Harvard College Observatory, 53, III), and other high authorities deny +this, and work out the newly discovered movements on the lines of the +old theory. They hold that all the bodies in the solar system once +turned in the same direction as Uranus and Neptune, and the tidal +influence of the sun has changed the rotation of most of them. The +planets farthest from the sun would naturally not be so much affected +by it. The same principle would explain the retrograde movement of the +outer satellites of Saturn and Jupiter. Sir R. Ball further works out +the principles on which the particles of the condensing nebula would +tend to form a disk rotating on its central axis. The ring-theory of +Laplace is practically abandoned. The spiral nebula is evidently the +standard type, and the condensing nebula must conform to it. In this +we are greatly helped by the current theory of the origin of spiral +nebulae. + +We saw previously that new stars sometimes appear in the sky, and the +recent closer scrutiny of the heavens shows this occurrence to be fairly +frequent. It is still held by a few astronomers that such a cataclysm +means that two stars collided. Even a partial or "grazing" collision +between two masses, each weighing billions of tons, travelling (on the +average) forty or fifty miles a second--a movement that would increase +enormously as they approach each other--would certainly liquefy or +vaporise their substance; but the astronomer, accustomed to see cosmic +bodies escape each other by increasing their speed, is generally +disinclined to believe in collisions. Some have made the new star plunge +into the heart of a dense and dark nebula; some have imagined a shock of +two gigantic swarms of meteors; some have regarded the outflame as the +effect of a prodigious explosion. In one or other new star each or any +of these things may have occurred, but the most plausible and accepted +theory for the new star of 1901 and some others is that two stars had +approached each other too closely in their wandering. Suppose that, +in millions of years to come, when our sun is extinct and a firm crust +surrounds the great molten ball, some other sun approaches within a few +million miles of it. The two would rush past each other at a terrific +speed, but the gravitational effect of the approaching star would tear +open the solid shell of the sun, and, in a mighty flame, its molten and +gaseous entrails would be flung out into space. It has long been one +of the arguments against a molten interior of the earth that the sun's +gravitational influence would raise it in gigantic tides and rend the +solid shell of rock. It is even suspected now that our small earth +is not without a tidal influence on the sun. The comparatively near +approach of two suns would lead to a terrific cataclysm. + +If we accept this theory, the origin of the spiral nebula becomes +intelligible. As the sun from which it is formed is already rotating +on its axis, we get a rotation of the nebula from the first. The mass +poured out from the body of the sun would, even if it were only a small +fraction of its mass, suffice to make a planetary system; all our sun's +planets and their satellites taken together amount to only 1/100th of +the mass of the solar system. We may assume, further, that the outpoured +matter would be a mixed cloud of gases and solid and liquid particles; +and that it would stream out, possibly in successive waves, from more +than one part of the disrupted sun, tending to form great spiral trails +round the parent mass. Some astronomers even suggest that, as there are +tidal waves raised by the moon at opposite points of the earth, similar +tidal outbursts would occur at opposite points on the disk of the +disrupted star, and thus give rise to the characteristic arms starting +from opposite sides of the spiral nebula. This is not at all clear, +as the two tidal waves of the earth are due to the fact that it has a +liquid ocean rolling on, not under, a solid bed. + +In any case, we have here a good suggestion of the origin of the spiral +nebula and of its further development. As soon as the outbursts are +over, and the scattered particles have reached the farthest limit to +which they are hurled, the concentrating action of gravitation will +slowly assert itself. If we conceive this gravitational influence as the +pressure of the surrounding ether we get a wider understanding of the +process. Much of the dispersed matter may have been shot far enough into +space to escape the gravitational pull of the parent mass, and will be +added to the sum of scattered cosmic dust, meteors, and close shoals +of meteors (comets) wandering in space. Much of the rest will fall +back upon the central body But in the great spiral arms themselves the +distribution of the matter will be irregular, and the denser areas will +slowly gather in the surrounding material. In the end we would thus get +secondary spheres circling round a large primary. + +This is the way in which astronomers now generally conceive the +destruction and re-formation of worlds. On one point the new +planetesimal theory differs from the other theories. It supposes that, +since the particles of the whirling nebula are all travelling in the +same general direction, they overtake each other with less violent +impact than the other theories suppose, and therefore the condensation +of the material into planets would not give rise to the terrific heat +which is generally assumed. We will consider this in the next chapter, +when we deal with the formation of the planets. As far as the central +body, the sun, is concerned, there can be no hesitation. The 500,000,000 +incandescent suns in the heavens are eloquent proof of the appalling +heat that is engendered by the collisions of the concentrating +particles. + +In general outline we now follow the story of a star with some +confidence. An internal explosion, a fatal rush into some dense nebula +or swarm of meteors, a collision with another star, or an approach +within a few million miles of another star, scatters, in part or whole, +the solid or liquid globe in a cloud of cosmic dust. When the violent +outrush is over, the dust is gathered together once more into a star. At +first cold and attenuated, its temperature rises as the particles come +together, and we have, after a time, an incandescent nucleus shining +through a thin veil of gas--a nebulous star. The temperature rises still +further, and we have the blue-hot star, in which the elements seem to +be dissociated, and slowly re-forming as the temperature falls. After, +perhaps, hundreds of millions of years it reaches the "yellow" stage, +and, if it has planets with the conditions of life, there may be a +temporary opportunity for living things to enjoy its tempered energy. +But the cooler vapours are gathering round it, and at length its +luminous body is wholly imprisoned. It continues its terrific course +through space, until some day, perhaps, it again encounters the mighty +cataclysm which will make it begin afresh the long and stormy chapters +of its living history. + +Such is the suggestion of the modern astronomer, and, although we seem +to find every phase of the theory embodied in the varied contents of +the heavens, we must not forget that it is only a suggestion. The +spectroscope and telescopic photography, which are far more important +than the visual telescope, are comparatively recent, and the field to be +explored is enormous. The mist is lifting from the cosmic landscape, but +there is still enough to blur our vision. Very puzzling questions remain +unanswered. What is the origin of the great gaseous nebulae? What is +the origin of the triple or quadruple star? What is the meaning of stars +whose light ebbs and flows in periods of from a few to several hundred +days? We may even point to the fact that some, at least, of the spiral +nebulae are far too vast to be the outcome of the impact or approach of +two stars. + +We may be content to think that we have found out some truths, by no +means the whole truth, about the evolution of worlds. Throughout this +immeasurable ocean of ether the particles of matter are driven together +and form bodies. These bodies swarm throughout space, like fish in the +sea; travelling singly (the "shooting star"), or in great close shoals +(the nucleus of a comet), or lying scattered in vast clouds. But the +inexorable pressure urges them still, until billions of tons of +material are gathered together. Then, either from the sheer heat of the +compression, or from the formation of large and unstable atomic systems +(radium, etc.), or both, the great mass becomes a cauldron of fire, +mantled in its own vapours, and the story of a star is run. It dies out +in one part of space to begin afresh in another. We see nothing in +the nature of a beginning or an end for the totality of worlds, the +universe. The life of all living things on the earth, from the formation +of the primitive microbes to the last struggles of the superman, is a +small episode of that stupendous drama, a fraction of a single scene. +But our ampler knowledge of it, and our personal interest in it, magnify +that episode, and we turn from the cosmic picture to study the formation +of the earth and the rise of its living population. + + + +CHAPTER IV. THE PREPARATION OF THE EARTH + +The story of the evolution of our solar system is, it will now be seen, +a local instance of the great cosmic process we have studied in the last +chapter. We may take one of the small spiral nebulae that abound in the +heavens as an illustration of the first stage. If a still earlier stage +is demanded, we may suppose that some previous sun collided with, or +approached too closely, another mighty body, and belched out a large +part of its contents in mighty volcanic outpours. Mathematical reasoning +can show that this erupted material would gather into a spiral nebula; +but, as mathematical calculations cannot be given here, and are less +safe than astronomical facts, we will be content to see the early shape +of our solar system in a relatively small spiral nebula, its outermost +arm stretching far beyond the present orbit of Neptune, and its great +nucleus being our present sun in more diffused form. + +We need not now attempt to follow the shrinking of the central part of +the nebula until it becomes a rounded fiery sun. That has been done in +tracing the evolution of a star. Here we have to learn how the planets +were formed from the spiral arms of the nebula. The principle of their +formation is already clear. The same force of gravitation, or the same +pressure of the surrounding ether, which compresses the central mass +into a fiery globe, will act upon the loose material of the arms and +compress it into smaller globes. But there is an interesting and acute +difference of opinion amongst modern experts as to whether these smaller +globes, the early planets, would become white-hot bodies. + +The general opinion, especially among astronomers, is that the +compression of the nebulous material of the arms into globes would +generate enormous heat, as in the case of the sun. On that view the +various planets would begin their careers as small suns, and would pass +through those stages of cooling and shrinking which we have traced in +the story of the stars. A glance at the photograph of one of the spiral +nebulae strongly confirms this. Great luminous knots, or nuclei, are +seen at intervals in the arms. Smaller suns seem to be forming in them, +each gathering into its body the neighbouring material of the arm, +and rising in temperature as the mass is compressed into a globe. The +spectroscope shows that these knots are condensing masses of white-hot +liquid or solid matter. It therefore seems plain that each planet will +first become a liquid globe of fire, coursing round the central sun, and +will gradually, as its heat is dissipated and the supply begins to fail, +form a solid crust. + +This familiar view is challenged by the new "planetesimal hypothesis," +which has been adopted by many distinguished geologists (Chamberlin, +Gregory, Coleman, etc.). In their view the particles in the arms of +the nebula are all moving in the same direction round the sun. They +therefore quietly overtake the nucleus to which they are attracted, +instead of violently colliding with each other, and much less heat +is generated at the surface. In that case the planets would not pass +through a white-hot, or even red-hot, stage at all. They are formed by +a slow ingathering of the scattered particles, which are called +"planetesimals" round the larger or denser masses of stuff which were +discharged by the exploding sun. Possibly these masses were prevented +from falling back into the sun by the attraction of the colliding body, +or the body which caused the eruption. They would revolve round the +parent body, and the shoals of smaller particles would gather about them +by gravitation. If there were any large region in the arm of the nebula +which had no single massive nucleus, the cosmic dust would gather about +a number of smaller centres. Thus might be explained the hundreds of +planetoids, or minor planets, which we find between Mars and Jupiter. If +these smaller bodies came within the sphere of influence of one of +the larger planets, yet were travelling quickly enough to resist its +attraction, they would be compelled to revolve round it, and we could +thus explain the ten satellites of Saturn and the eight of Jupiter. Our +moon, we shall see, had a different origin. + +We shall find this new hypothesis crossing the familiar lines at +many points in the next few chapters. We will consider those further +consequences as they arise, but may say at once that, while the new +theory has greatly helped us in tracing the formation of the planetary +system, astronomers are strongly opposed to its claim that the planets +did not pass through an incandescent stage. The actual features of our +spiral nebulae seem clearly to exhibit that stage. The shape of the +planets--globular bodies, flattened at the poles--strongly suggests that +they were once liquid. The condition in which we find Saturn and Jupiter +very forcibly confirms this suggestion; the latest study of those +planets supports the current opinion that they are still red-hot, and +even seems to detect the glow of their surfaces in their mantles of +cloud. These points will be considered more fully presently. For +the moment it is enough to note that, as far as the early stages of +planetary development are concerned, the generally accepted theory +rests on a mass of positive evidence, while the new hypothesis is +purely theoretical. We therefore follow the prevailing view with some +confidence. + +Those of the spiral nebulae which face the earth squarely afford an +excellent suggestion of the way in which planets are probably formed. In +some of these nebulae the arms consist of almost continuous streams +of faintly luminous matter; in others the matter is gathering about +distinct centres; in others again the nebulous matter is, for the most +part, collected in large glowing spheres. They seem to be successive +stages, and to reveal to us the origin of our planets. The position +of each planet in our solar system would be determined by the chance +position of the denser stuff shot out by the erupting sun. I have seen +Vesuvius hurl up into the sky, amongst its blasts of gas and steam, +white-hot masses of rock weighing fifty tons. In the far fiercer +outburst of the erupting sun there would be at least thinner and denser +masses, and they must have been hurled so far into space that their +speed in travelling round the central body, perhaps seconded by the +attraction of the second star, overcame the gravitational pull back to +the centre. Recollect the force which, in the new star in Perseus, drove +masses of hydrogen for millions of miles at a speed of a thousand miles +a second. + +These denser nuclei or masses would, when the eruption was over, begin +to attract to themselves all the lighter nebulous material within their +sphere of gravitational influence. Naturally, there would at first be a +vast confusion of small and large centres of condensation in the arms +of the nebula, moving in various directions, but a kind of natural +selection--and, in this case, survival of the biggest--would ensue. The +conflicting movements would be adjusted by collisions and gravitation, +the smaller bodies would be absorbed in the larger or enslaved as +their satellites, and the last state would be a family of smaller suns +circling at vast distances round the parent body. The planets, moreover, +would be caused to rotate on their axes, besides revolving round the +sun, as the particles at their inner edge (nearer the sun) would move +at a different speed from those at the outer edge. In the course of time +the smaller bodies, having less heat to lose and less (or no) atmosphere +to check the loss, would cool down, and become dark solid spheres, lit +only by the central fire. + +While the first stage of this theory of development is seen in the +spiral nebula, the later stages seem to be well exemplified in the +actual condition of our planets. Following, chiefly, the latest research +of Professor Lowell and his colleagues, which marks a considerable +advance on our previous knowledge, we shall find it useful to glance at +the sister-planets before we approach the particular story of our earth. + +Mercury, the innermost and smallest of the planets, measuring only some +3400 miles in diameter, is, not unexpectedly, an airless wilderness. +Small bodies are unable to retain the gases at their surface, on account +of their feebler gravitation. We find, moreover, that Mercury always +presents the same face to the sun, as it turns on its axis in the same +period (eighty-eight days) in which it makes a revolution round the sun. +While, therefore, one half of the globe is buried in eternal darkness, +the other half is eternally exposed to the direct and blistering rays +of the sun, which is only 86,000,000 miles away. To Professor Lowell it +presents the appearance of a bleached and sun-cracked desert, or "the +bones of a dead world." Its temperature must be at least 300 degrees C. +above that of the earth. Its features are what we should expect on the +nebular hypothesis. The slowness of its rotation is accounted for by +the heavy tidal influence of the sun. In the same way our moon has been +influenced by the earth, and our earth by the sun, in their movement of +rotation. + +Venus, as might be expected in the case of so large a globe (nearly +as large as the earth), has an atmosphere, but it seems, like Mercury, +always to present the same face to the sun. Its comparative nearness +to the sun (67,000,000 miles) probably explains this advanced effect of +tidal action. The consequences that the observers deduce from the fact +are interesting. The sun-baked half of Venus seems to be devoid of water +or vapour, and it is thought that all its water is gathered into a rigid +ice-field on the dark side of the globe, from which fierce hurricanes +must blow incessantly. It is a Sahara, or a desert far hotter than the +Sahara, on one side; an arctic region on the other. It does not seem +to be a world fitted for the support of any kind of life that we can +imagine. + +When we turn to the consideration of Mars, we enter a world of unending +controversy. With little more than half the diameter of the earth, Mars +ought to be in a far more advanced stage of either life or decay, but +its condition has not yet been established. Some hold that it has a +considerable atmosphere; others that it is too small a globe to +have retained a layer of gas. Professor Poynting believes that its +temperature is below the freezing-point of water all over the globe; +many others, if not the majority of observers, hold that the white cap +we see at its poles is a mass of ice and snow, or at least a thick coat +of hoar-frost, and that it melts at the edges as the springtime of Mars +comes round. In regard to its famous canals we are no nearer agreement. +Some maintain that the markings are not really an objective feature; +some hold that they are due to volcanic activity, and that similar +markings are found on the moon; some believe that they are due to +clouds; while Professor Lowell and others stoutly adhere to the familiar +view that they are artificial canals, or the strips of vegetation along +such canals. The question of the actual habitation of Mars is still +open. We can say only that there is strong evidence of its possession of +the conditions of life in some degree, and that living things, even on +the earth, display a remarkable power of adaptation to widely differing +conditions. + +Passing over the 700 planetoids, which circulate between Mars and +Jupiter, and for which we may account either by the absence of one +large nucleus in that part of the nebulous stream or by the disturbing +influence of Jupiter, we come to the largest planet of the system. Here +we find a surprising confirmation of the theory of planetary development +which we are following. Three hundred times heavier than the earth +(or more than a trillion tons in weight), yet a thousand times less +in volume than the sun, Jupiter ought, if our theory is correct, to be +still red-hot. All the evidence conspires to suggest that it is. It has +long been recognised that the shining disk of the planet is not a solid, +but a cloud, surface. This impenetrable mass of cloud or vapour is drawn +out in streams or belts from side to side, as the giant globe turns on +its axis once in every ten hours. We cannot say if, or to what extent, +these clouds consist of water-vapour. We can conclude only that this +mantle of Jupiter is "a seething cauldron of vapours" (Lowell), and +that, if the body beneath is solid, it must be very hot. A large red +area, at one time 30,000 miles long, has more or less persisted on the +surface for several decades, and it is generally interpreted, either as +a red-hot surface, or as a vast volcanic vent, reflecting its glow upon +the clouds. Indeed, the keen American observers, with their powerful +telescopes, have detected a cherry-red glow on the edges of the +cloud-belts across the disk; and more recent observation with the +spectroscope seems to prove that Jupiter emits light from its surface +analogous to that of the red stars. The conspicuous flattening of its +poles is another feature that science would expect in a rapidly rotating +liquid globe. In a word, Jupiter seems to be in the last stage of +stellar development. Such, at some remote time, was our earth; such one +day will be the sun. + +The neighbouring planet Saturn supports the conclusion. Here again we +have a gigantic globe, 28,000 miles in diameter, turning on its axis +in the short space of ten hours; and here again we find the conspicuous +flattening of the poles, the trailing belts of massed vapour across +the disk, the red glow lighting the edges of the belts, and the +spectroscopic evidence of an emission of light. Once more it is +difficult to doubt that a highly heated body is wrapped in that thick +mantle of vapour. With its ten moons and its marvellous ring-system--an +enormous collection of fragments, which the influence of the planet +or of its nearer satellites seems to have prevented from +concentrating--Saturn has always been a beautiful object to observe; it +is not less interesting in those features which we faintly detect in its +disk. + +The next planet, Uranus, 32,000 miles in diameter, seems to be another +cloud-wrapt, greatly heated globe, if not, as some think, a sheer mass +of vapours without a liquid core. Neptune is too dim and distant for +profitable examination. It may be added, however, that the dense masses +of gas which are found to surround the outer planets seem to confirm the +nebular theory, which assumes that they were developed in the outer and +lighter part of the material hurled from the sun. + +From this encouraging survey of the sister-planets we return with more +confidence to the story of the earth. I will not attempt to follow +an imaginative scheme in regard to its early development. Take four +photographs--one of a spiral nebula without knots in its arms, one of +a nebula like that in Canes Venatici, one of the sun, and one of +Jupiter--and you have an excellent illustration of the chief stages in +its formation. In the first picture a section of the luminous arm of the +nebula stretches thinly across millions of miles of space. In the next +stage this material is largely collected in a luminous and hazy sphere, +as we find in the nebula in Canes Venatici. The sun serves to illustrate +a further stage in the condensation of this sphere. Jupiter represents +a later chapter, in which the cooler vapours are wrapped close about the +red-hot body of the planet. That seems to have been the early story of +the earth. Some 6,000,000,000 billion tons of the nebulous matter were +attracted to a common centre. As the particles pressed centreward, the +temperature rose, and for a time the generation of heat was greater than +its dissipation. Whether the earth ever shone as a small white star we +cannot say. We must not hastily conclude that such a relatively small +mass would behave like the far greater mass of a star, but we may, +without attempting to determine its temperature, assume that it runs an +analogous course. + +One of the many features which I have indicated as pointing to a former +fluidity of the earth may be explained here. We shall see in the course +of this work that the mountain chains and other great irregularities of +the earth's surface appear at a late stage in its development. Even +as we find them to-day, they are seen to be merely slight ridges and +furrows on the face of the globe, when we reflect on its enormous +diameter, but there is good reason to think that in the beginning the +earth was much nearer to a perfectly globular form. This points to +a liquid or gaseous condition at one time, and the flattening of the +sphere at the poles confirms the impression. We should hardly expect +so perfect a rotundity in a body formed by the cool accretion of solid +fragments and particles. It is just what we should expect in a fluid +body, and the later irregularities of the surface are accounted for by +the constant crumpling and wearing of its solid crust. Many would find +a confirmation of this in the phenomena of volcanoes, geysers, and +earthquakes, and the increase of the temperature as we descend the +crust. But the interior condition of the earth, and the nature of these +phenomena, are much disputed at present, and it is better not to rely on +any theory of them. It is suggested that radium may be responsible for +this subterraneous heat. + +The next stage in the formation of the earth is necessarily one that we +can reach only by conjecture. Over the globe of molten fire the vapours +and gases would be suspended like a heavy canopy, as we find in Jupiter +and Saturn to-day. When the period of maximum heat production was +passed, however, the radiation into space would cause a lowering of +the temperature, and a scum would form on the molten surface. As may be +observed on the surface of any cooling vessel of fluid, the scum would +stretch and crack; the skin would, so to say, prove too small for the +body. The molten ocean below would surge through the crust, and bury it +under floods of lava. Some hold that the slabs would sink in the ocean +of metal, and thus the earth would first solidify in its deeper layers. +There would, in any case, be an age-long struggle between the molten +mass and the confining crust, until at length--to employ the old Roman +conception of the activity of Etna--the giant was imprisoned below the +heavy roof of rock. + +Here again we seem to find evidence of the general correctness of the +theory. The objection has been raised that the geologist does not find +any rocks which he can identify as portions of the primitive crust +of the earth. It seems to me that it would be too much to expect the +survival at the surface of any part of the first scum that cooled on +that fiery ocean. It is more natural to suppose that millions of years +of volcanic activity on a prodigious scale would characterise this +early stage, and the "primitive crust" would be buried in fragments, or +dissolved again, under deep seas of lava. Now, this is precisely what we +find, The oldest rocks known to the geologist--the Archaean rocks--are +overwhelmingly volcanic, especially in their lower part. Their +thickness, as we know them, is estimated at 50,000 feet; a thickness +which must represent many millions of years. But we do not know how much +thicker than this they may be. They underlie the oldest rocks that have +ever been exposed to the gaze of the geologist. They include sedimentary +deposits, showing the action of water, and even probable traces of +organic remains, but they are, especially in their deeper and older +sections, predominantly volcanic. They evince what we may call a +volcanic age in the early story of the planet. + +But before we pursue this part of the story further we must interpolate +a remarkable event in the record--the birth of the moon. It is now +generally believed, on a theory elaborated by Sir G. Darwin, that when +the formation of the crust had reached a certain depth--something over +thirty miles, it is calculated--it parted with a mass of matter, which +became the moon. The size of our moon, in comparison with the earth, +is so exceptional among the satellites which attend the planets of our +solar system that it is assigned an exceptional origin. It is calculated +that at that time the earth turned on its axis in the space of four or +five hours, instead of twenty-four. We have already seen that the tidal +influence of the sun has the effect of moderating the rotation of the +planets. Now, this very rapid rotation of a liquid mass, with a thin +crust, would (together with the instability occasioned by its cooling) +cause it to bulge at the equator. The bulge would increase until the +earth became a pear-shaped body. The small end of the pear would draw +further and further away from the rest--as a drop of water does on the +mouth of a tap--and at last the whole mass (some 5,000,000,000 cubic +miles of matter) was broken off, and began to pursue an independent +orbit round the earth. + +There are astronomers who think that other cosmic bodies, besides our +moon, may have been formed in this way. Possibly it is true of some of +the double stars, but we will not return to that question. The further +story of the moon, as it is known to astronomers, may be given in a few +words. The rotational movement of the earth is becoming gradually slower +on account of tidal influence; our day, in fact, becomes an hour longer +every few million years. It can be shown that this had the effect of +increasing the speed, and therefore enlarging the orbit, of the moon, +as it revolved round the earth. As a result, the moon drew further and +further away from the earth until it reached its present position, about +240,000 miles away. At the same time the tidal influence of the earth +was lessening the rotational movement of the moon. This went on until +it turned on its axis in the same period in which it revolves round +the earth, and on this account it always presents the same face to the +earth. + +Through what chapters of life the moon may have passed in the meantime +it is impossible to say. Its relatively small mass may have been unable +to keep the lighter gases at its surface, or its air and water may, as +some think, have been absorbed. It is to-day practically an airless and +waterless desert, alternating between the heat of its long day and the +intense cold of its long night. Careful observers, such as Professor +Pickering, think that it may still have a shallow layer of heavy gases +at its surface, and that this may permit the growth of some stunted +vegetation during the day. Certain changes of colour, which are observed +on its surface, have been interpreted in that sense. We can hardly +conceive any other kind of life on it. In the dark even the gases will +freeze on its surface, as there is no atmosphere to retain the heat. +Indeed, some students of the moon (Fauth, etc.) believe that it is an +unchanging desert of ice, bombarded by the projectiles of space. + +An ingenious speculation as to the effect on the earth of this +dislodgment of 5,000,000,000 cubic miles of its substance is worth +noting. It supposes that the bed of the Pacific Ocean represents the +enormous gap torn in its side by the delivery of the moon. At each side +of this chasm the two continents, the Old World and the New, would +be left floating on their molten ocean; and some have even seen a +confirmation of this in the lines of crustal weakness which we trace, by +volcanoes and earthquakes, on either side of the Pacific. Others, again, +connect the shape of our great masses of land, which generally run to +a southern point, with this early catastrophe. But these interesting +speculations have a very slender basis, and we will return to the story +of the development of the earth. + +The last phase in preparation for the appearance of life would be the +formation of the ocean. On the lines of the generally received nebular +hypothesis this can easily be imagined, in broad outline. The gases +would form the outer shell of the forming planet, since the heavier +particles would travel inward. In this mixed mass of gas the oxygen and +hydrogen would combine, at a fitting temperature, and form water. +For ages the molten crust would hold this water suspended aloft as a +surrounding shell of cloud, but when the surface cooled to about 380 +degrees C. (Sollas), the liquid would begin to pour on it. A period of +conflict would ensue, the still heated crust and the frequent volcanic +outpours sending the water back in hissing steam to the clouds. At +length, and now more rapidly, the temperature of the crust would sink +still lower, and a heated ocean would settle upon it, filling the +hollows of its irregular surface, and washing the bases of its +outstanding ridges. From that time begins the age-long battle of the +land and the water which, we shall see, has had a profound influence on +the development of life. + +In deference to the opinion of a number of geologists we must glance +once more at the alternative view of the planetesimal school. In their +opinion the molecules of water were partly attracted to the surface out +of the disrupted matter, and partly collected within the porous outer +layers of the globe. As the latter quantity grew, it would ooze upwards, +fill the smaller depressions in the crust, and at length, with the +addition of the attracted water, spread over the irregular surface. +There is an even more important difference of opinion in regard to the +formation of the atmosphere, but we may defer this until the question +of climate interests us. We have now made our globe, and will pass on +to that early chapter of its story in which living things make their +appearance. + +To some it will seem that we ought not to pass from the question of +origin without a word on the subject of the age of the earth. All that +one can do, however, is to give a number of very divergent estimates. +Physicists have tried to calculate the age of the sun from the rate +of its dissipation of heat, and have assigned, at the most, a hundred +million years to our solar system; but the recent discovery of a source +of heat in the disintegration of such metals as radium has made their +calculations useless. Geologists have endeavoured, from observation of +the action of geological agencies to-day, to estimate how long it will +have taken them to form the stratified crust of the earth; but even the +best estimates vary between twenty-five and a hundred million years, and +we have reason to think that the intensity of these geological agencies +may have varied in different ages. Chemists have calculated how long it +would take the ocean, which was originally fresh water, to take up from +the rocks and rivers the salt which it contains to-day; Professor Joly +has on this ground assigned a hundred million years since the waters +first descended upon the crust. We must be content to know that the +best recent estimates, based on positive data, vary between fifty and a +hundred million years for the story which we are now about to narrate. +The earlier or astronomical period remains quite incalculable. Sir G. +Darwin thinks that it was probably at least a thousand million years +since the moon was separated from the earth. Whatever the period of time +may be since some cosmic cataclysm scattered the material of our solar +system in the form of a nebula, it is only a fraction of that larger and +illimitable time which the evolution of the stars dimly suggests to the +scientific imagination. + + + +THE GEOLOGICAL SERIES + +[The scale of years adopted--50,000,000 for the stratified rocks--is +merely an intermediate between conflicting estimates.] + + ERA. PERIOD. RELATIVE LENGTH. + + Quaternary Holocene 500,000 years + Pleistocene + + + Tertiary Pliocene 5,500,000 years + or Miocene + Cenozoic Oligocene + Eocene + + + Secondary Cretaceous 7,200,000 years + or Jurassic 3,600,000 " + Mesozoic Triassic 2,500,000 " + + + Primary Permian 2,800,000 years + or Carboniferous 6,200,000 " + Palaeozoic Devonian 8,000,000 " + Silurian 5,400,000 " + Ordovician 5,400,000 " + Cambrian 8,000,000 " + + Archaean Keweenawan Unknown (probably + Animikie at least + Huronian 50,000,000 years) + Keewatin + Laurentian + + + +CHAPTER V. THE BEGINNING OF LIFE + +There is, perhaps, no other chapter in the chronicle of the earth that +we approach with so lively an interest as the chapter which should +record the first appearance of life. Unfortunately, as far as the +authentic memorials of the past go, no other chapter is so impenetrably +obscure as this. The reason is simple. It is a familiar saying that life +has written its own record, the long-drawn record of its dynasties and +its deaths, in the rocks. But there were millions of years during which +life had not yet learned to write its record, and further millions of +years the record of which has been irremediably destroyed. The first +volume of the geological chronicle of the earth is the mass of the +Archaean (or "primitive") rocks. What the actual magnitude of that +volume, and the span of time it covers, may be, no geologist can say. +The Archaean rocks still solidly underlie the lowest depth he has ever +reached. It is computed, however, that these rocks, as far as they are +known to us, have a total depth of nearly ten miles, and seem therefore +to represent at least half the story of the earth from the time when it +rounded into a globe, or cooled sufficiently to endure the presence of +oceans. + +Yet all that we read of the earth's story during those many millions of +years could be told in a page or two. That section of geology is still +in its infancy, it is true. A day may come when science will decipher a +long and instructive narrative in the masses of quartz and gneiss, and +the layers of various kinds, which it calls the Archaean rocks. But we +may say with confidence that it will not discover in them more than +a few stray syllables of the earlier part, and none whatever of the +earliest part, of the epic of living nature. A few fossilised remains of +somewhat advanced organisms, such as shell-fish and worms, are found +in the higher and later rocks of the series, and more of the same +comparatively high types will probably appear. In the earlier strata, +representing an earlier stage of life, we find only thick seams of black +shale, limestone, and ironstone, in which we seem to see the ashes of +primitive organisms, cremated in the appalling fires of the volcanic +age, or crushed out of recognition by the superimposed masses. Even if +some wizardry of science were ever to restore the forms that have been +reduced to ashes in this Archaean crematorium, it would be found that +they are more or less advanced forms, far above the original level of +life. No trace will ever be found in the rocks of the first few million +years in the calendar of life. + +The word impossible or unknowable is not lightly uttered in science +to-day, but there is a very plain reason for admitting it here. The +earliest living things were at least as primitive of nature as the +lowest animals and plants we know to-day, and these, up to a fair level +of organisation, are so soft of texture that, when they die, they leave +no remains which may one day be turned into fossils. Some of them, +indeed, form tiny shells of flint or lime, or, like the corals, make for +themselves a solid bed; but this is a relatively late and higher stage +of development. Many thousands of species of animals and plants lie +below that level. We are therefore forced to conclude, from the aspect +of living nature to-day, that for ages the early organisms had no hard +and preservable parts. In thus declaring the impotence of geology, +however, we are at the same time introducing another science, biology, +which can throw appreciable light on the evolution of life. Let us first +see what geology tells us about the infancy of the earth. + +The distribution of the early rocks suggests that there was +comparatively little dry land showing above the surface of the Archaean +ocean. Our knowledge of these rocks is not at all complete, and we must +remember that some of this primitive land may be now under the sea or +buried in unsuspected regions. It is significant, however, that, up to +the present, exploration seems to show that in those remote ages only +about one-fifth of our actual land-surface stood above the level of the +waters. Apart from a patch of some 20,000 square miles of what is now +Australia, and smaller patches in Tasmania, New Zealand, and India, +nearly the whole of this land was in the far North. A considerable area +of eastern Canada had emerged, with lesser islands standing out to the +west and south of North America. Another large area lay round the basin +of the Baltic; and as Greenland, the Hebrides, and the extreme tip of +Scotland, belong to the same age, it is believed that a continent, of +which they are fragments, united America and Europe across the North +Atlantic. Of the rest of what is now Europe there were merely large +islands--one on the border of England and Wales, others in France, +Spain, and Southern Germany. Asia was represented by a large area in +China and Siberia, and an island or islands on the site of India. Very +little of Africa or South America existed. + +It will be seen at a glance that the physical story of the earth +from that time is a record of the emergence from the waters of larger +continents and the formation of lofty chains of mountains. Now this +world-old battle of land and sea has been waged with varying fortune +from age to age, and it has been one of the most important factors +in the development of life. We are just beginning to realise what a +wonderful light it throws on the upward advance of animals and plants. +No one in the scientific world to-day questions that, however imperfect +the record may be, there has been a continuous development of life from +the lowest level to the highest. But why there was advance at all, why +the primitive microbe climbs the scale of being, during millions +of years, until it reaches the stature of humanity, seems to many a +profound mystery. The solution of this mystery begins to break upon us +when we contemplate, in the geological record, the prolonged series of +changes in the face of the earth itself, and try to realise how these +changes must have impelled living things to fresh and higher adaptations +to their changing surroundings. + +Imagine some early continent with its population of animals and plants. +Each bay, estuary, river, and lake, each forest and marsh and solid +plain, has its distinctive inhabitants. Imagine this continent slowly +sinking into the sea, until the advancing arms of the salt water meet +across it, mingling their diverse populations in a common world, making +the fresh-water lake brackish or salt, turning the dry land into swamp, +and flooding the forest. Or suppose, on the other hand, that the land +rises, the marsh is drained, the genial climate succeeded by an icy +cold, the luscious vegetation destroyed, the whole animal population +compelled to change its habits and its food. But this is no imaginary +picture. It is the actual story of the earth during millions of years, +and it is chiefly in the light of these vast and exacting changes in the +environment that we are going to survey the panorama of the advance of +terrestrial life. + +For the moment it will be enough to state two leading principles. The +first is that there is no such thing as a "law of evolution" in +the sense in which many people understand that phrase. It is now +sufficiently well known that, when science speaks of a law, it does not +mean that there is some rule that things MUST act in such and such a +way. The law is a mere general expression of the fact that they DO act +in that way. But many imagine that there is some principle within +the living organism which impels it onward to a higher level of +organisation. That is entirely an error. There is no "law of progress." +If an animal is fitted to secure its livelihood and breed posterity +in certain surroundings, it may remain unchanged indefinitely if these +surroundings do not materially change. So the duckmole of Australia and +the tuatara of New Zealand have retained primitive features for millions +of years; so the aboriginal Australian and the Fuegian have remained +stagnant, in their isolation, for a hundred thousand years or more; so +the Chinaman, in his geographical isolation, has remained unchanged +for two thousand years. There is no more a "conservative instinct" +in Chinese than there is a "progressive instinct" in Europeans. The +difference is one of history and geography, as we shall see. + +To make this important principle still clearer, let us imagine some +primitive philosopher observing the advance of the tide over a level +beach. He must discover two things: why the water comes onward at all, +and why it advances along those particular channels. We shall see later +how men of science explain or interpret the mechanism in a living thing +which enables it to advance, when it does advance. For the present it +is enough to say that new-born animals and plants are always tending to +differ somewhat from their parents, and we now know, by experiment, that +when some exceptional influence is brought to bear on the parent, the +young may differ considerably from her. But, if the parents were already +in harmony with their environment, these variations on the part of the +young are of no consequence. Let the environment alter, however, and +some of these variations may chance to make the young better fitted than +the parent was. The young which happen to have the useful variation will +have an advantage over their brothers or sisters, and be more likely to +survive and breed the next generation. If the change in the environment +(in the food or climate, for instance) is prolonged and increased for +hundreds of thousands of years, we shall expect to find a corresponding +change in the animals and plants. + +We shall find such changes occurring throughout the story of the earth. +At one important point in the story we shall find so grave a revolution +in the face of nature that twenty-nine out of every thirty species of +animals and plants on the earth are annihilated. Less destructive and +extreme changes have been taking place during nearly the whole of the +period we have to cover, entailing a more gradual alteration of the +structure of animals and plants; but we shall repeatedly find them +culminating in very great changes of climate, or of the distribution of +land and water, which have subjected the living population of the earth +to the most searching tests and promoted every variation toward a more +effective organisation. [*] + + * This is a very simple expression of "Darwinism," and will + be enlarged later. The reader should ignore the occasional + statement of non-scientific writers that Darwinism is "dead" + or superseded. The questions which are actually in dispute + relate to the causes of the variation of the young from + their parents, the magnitude of these variations' and the + transmission of changes acquired by an animal during its own + life. We shall see this more fully at a later stage. The + importance of the environment as I have described it, is + admitted by all schools. + + +And the second guiding principle I wish to lay down in advance is that +these great changes in the face of the earth, which explain the progress +of organisms, may very largely be reduced to one simple agency--the +battle of the land and the sea. When you gaze at some line of cliffs +that is being eaten away by the waves, or reflect on the material +carried out to sea by the flooded river, you are--paradoxical as it may +seem--beholding a material process that has had a profound influence on +the development of life. The Archaean continent that we described was +being reduced constantly by the wash of rain, the scouring of rivers, +and the fretting of the waves on the coast. It is generally thought that +these wearing agencies were more violent in early times, but that is +disputed, and we will not build on it. In any case, in the course of +time millions of tons of matter were scraped off the Archaean continent +and laid on the floor of the sea by its rivers. This meant a very +serious alteration of pressure or weight on the surface of the globe, +and was bound to entail a reaction or restoration of the balance. + +The rise of the land and formation of mountains used to be ascribed +mainly to the cooling and shrinking of the globe of the earth. The skin +(crust), it was thought, would become too large for the globe as it +shrank, and would wrinkle outwards, or pucker up into mountain-chains. +The position of our greater mountain-chains sprawling across half the +earth (the Pyrenees to the Himalaya, and the Rocky Mountains to the +Andes), seems to confirm this, but the question of the interior of the +earth is obscure and disputed, and geologists generally conceive the +rise of land and formation of mountains in a different way. They are due +probably to the alteration of pressure on the crust in combination with +the instability of the interior. The floors of the seas would sink still +lower under their colossal burdens, and this would cause some draining +of the land-surface. At the same time the heavy pressure below the seas +and the lessening of pressure over the land would provoke a reaction. +Enormous masses of rock would be forced toward and underneath the +land-surface, bending, crumpling, and upheaving it as if its crust were +but a leather coat. As a result, masses of land would slowly rise above +the plain, to be shaped into hills and valleys by the hand of later +time, and fresh surfaces would be dragged out of the deep, enlarging the +fringes of the primitive continents, to be warped and crumpled in their +turn at the next era of pressure. + +In point of geological fact, the story of the earth has been one +prolonged series of changes in the level of land and water, and in their +respective limits. These changes have usually been very gradual, but +they have always entailed changes (in climate, etc. ) of the greatest +significance in the evolution of life. What was the swampy soil of +England in the Carboniferous period is now sometimes thousands of feet +beneath us; and what was the floor of a deep ocean over much of Europe +and Asia at another time is now to be found on the slopes of lofty Alps, +or 20,000 feet above the sea-level in Thibet. Our story of terrestrial +life will be, to a great extent, the story of how animals and plants +changed their structure in the long series of changes which this endless +battle of land and sea brought over the face of the earth. + +As we have no recognisable remains of the animals and plants of the +earliest age, we will not linger over the Archaean rocks. Starting from +deep and obscure masses of volcanic matter, the geologist, as he +travels up the series of Archaean rocks, can trace only a dim and +most unsatisfactory picture of those remote times. Between outpours of +volcanic floods he finds, after a time, traces that an ocean and rivers +are wearing away the land. He finds seams of carbon among the rocks of +the second division of the Archaean (the Keewatin), and deduces from +this that a dense sea-weed population already covered the floor of +the ocean. In the next division (the Huronian) he finds the traces of +extensive ice-action strangely lying between masses of volcanic rock, +and sees that thousands of square miles of eastern North America were +then covered with an ice-sheet. Then fresh floods of molten matter are +poured out from the depths below; then the sea floods the land for a +time; and at last it makes its final emergence as the first definitive +part of the North American continent, to enlarge, by successive fringes, +to the continent of to-day. [*] + + * I am quoting Professor Coleman's summary of Archaean + research in North America (Address to the Geological Section + of the British Association, 1909). Europe, as a continent, + has had more "ups and downs" than America in the course of + geological time. + + +This meagre picture of the battle of land and sea, with interludes of +great volcanic activity and even of an ice age, represents nearly all +we know of the first half of the world's story from geology. It is +especially disappointing in regard to the living population. The very +few fossils we find in the upper Archaean rocks are so similar to those +we shall discuss in the next chapter that we may disregard them, and the +seams of carbon-shales, iron-ore, and limestone, suggest only, at the +most, that life was already abundant. We must turn elsewhere for some +information on the origin and early development of life. + +The question of the origin of life I will dismiss with a brief account +of the various speculations of recent students of science. Broadly +speaking, their views fall into three classes. Some think that the germs +of life may have come to the earth from some other body in the universe; +some think that life was evolved out of non-living matter in the early +ages of the earth, under exceptional conditions which we do not at +present know, or can only dimly conjecture; and some think that life +is being evolved from non-life in nature to-day, and always has been so +evolving. The majority of scientific men merely assume that the earliest +living things were no exception to the general process of evolution, but +think that we have too little positive knowledge to speculate profitably +on the manner of their origin. + +The first view, that the germs of life may have come to this planet on a +meteoric visitor from some other world, as a storm-driven bird may take +its parasites to some distant island, is not without adherents to-day. +It was put forward long ago by Lord Kelvin and others; it has been +revived by the distinguished Swede, Professor Svante Arrhenius. The +scientific objection to it is that the more intense (ultra-violet) rays +of the sun would frill such germs as they pass through space. But a +broader objection, and one that may dispense us from dwelling on it, is +that we gain nothing by throwing our problems upon another planet. We +have no ground for supposing that the earth is less capable of evolving +life than other planets. + +The second view is that, when the earth had passed through its white-hot +stage, great masses of very complex chemicals, produced by the great +heat, were found on its surface. There is one complex chemical substance +in particular, called cyanogen, which is either an important constituent +of living matter, or closely akin to it. Now we need intense heat to +produce this substance in the laboratory. May we not suppose that masses +of it were produced during the incandescence of the earth, and that, +when the waters descended, they passed through a series of changes which +culminated in living plasm? Such is the "cyanogen hypothesis" of +the origin of life, advocated by able physiologists such as Pfluger, +Verworn, and others. It has the merit of suggesting a reason why life +may not be evolving from non-life in nature to-day, although it may have +so evolved in the Archaean period. + +Other students suggest other combinations of carbon-compounds and water +in the early days. Some suggest that electric action was probably far +more intense in those ages; others think that quantities of radium +may have been left at the surface. But the most important of these +speculations on the origin of life in early times, and one that has the +merit of not assuming any essentially different conditions then than we +find now, is contained in a recent pronouncement of one of the greatest +organic chemists in Europe, Professor Armstrong. He says that such +great progress has been made in his science--the science of the chemical +processes in living things--that "their cryptic character seems to have +disappeared almost suddenly." On the strength of this new knowledge of +living matter, he ventures to say that "a series of lucky accidents" +could account for the first formation of living things out of non-living +matter in Archaean times. Indeed, he goes further. He names certain +inorganic substances, and says that the blowing of these into pools by +the wind on the primitive planet would set afoot chemical combinations +which would issue in the production of living matter. [*] + + * See his address in Nature, vol. 76, p. 651. For other + speculations see Verworn's "General Physiology," Butler + Burke's "Origin of Life" (1906), and Dr. Bastian's "Origin + of Life" (1911). + + +It is evident that the popular notion that scientific men have declared +that life cannot be evolved from non-life is very far astray. This +blunder is usually due to a misunderstanding of the dogmatic statement +which one often reads in scientific works that "every living thing comes +from a living thing." This principle has no reference to remote ages, +when the conditions may have been different. It means that to-day, +within our experience, the living thing is always born of a living +parent. However, even this is questioned by some scientific men of +eminence, and we come to the third view. + +Professor Nageli, a distinguished botanist, and Professor Haeckel, +maintain that our experience, as well as the range of our microscopes, +is too limited to justify the current axiom. They believe that life may +be evolving constantly from inorganic matter. Professor J. A. Thomson +also warns us that our experience is very limited, and, for all we know, +protoplasm may be forming naturally in our own time. Mr. Butler Burke +has, under the action of radium, caused the birth of certain minute +specks which strangely imitate the behaviour of bacteria. Dr. Bastian +has maintained for years that he has produced living things from +non-living matter. In his latest experiments, described in the book +quoted, purely inorganic matter is used, and it is previously subjected, +in hermetically sealed tubes, to a heat greater than what has been found +necessary to kill any germs whatever. + +Evidently the problem of the origin of life is not hopeless, but our +knowledge of the nature of living matter is still so imperfect that we +may leave detailed speculation on its origin to a future generation. +Organic chemistry is making such strides that the day may not be far +distant when living matter will be made by the chemist, and the secret +of its origin revealed. For the present we must be content to choose the +more plausible of the best-informed speculations on the subject. + +But while the origin of life is obscure, the early stages of its +evolution come fairly within the range of our knowledge. To the inexpert +it must seem strange that, whereas we must rely on pure speculation +in attempting to trace the origin of life, we can speak with more +confidence of those early developments of plants and animals which are +equally buried in the mists of the Archaean period. Have we not said +that nothing remains of the procession of organisms during half the +earth's story but a shapeless seam of carbon or limestone? + +A simple illustration will serve to justify the procedure we are +about to adopt. Suppose that the whole of our literary and pictorial +references to earlier stages in the development of the bicycle, the +locomotive, or the loom, were destroyed. We should still be able to +retrace the phases of their evolution, because we should discover +specimens belonging to those early phases lingering in our museums, in +backward regions, and elsewhere. They might yet be useful in certain +environments into which the higher machines have not penetrated. In the +same way, if all the remains of prehistoric man and early civilisation +were lost, we could still fairly retrace the steps of the human race, by +gathering the lower tribes and races, and arranging them in the order +of their advancement. They are so many surviving illustrations of the +stages through which mankind as a whole has passed. + +Just in the same way we may marshal the countless species of animals and +plants to-day in such order that they will, in a general way, exhibit +to us the age-long procession of life. From the very start of living +evolution certain forms dropped out of the onward march, and have +remained, to our great instruction, what their ancestors were millions +of years ago. People create a difficulty for themselves by imagining +that, if evolution is true, all animals must evolve. A glance at our own +fellows will show the error of this. Of one family of human beings, as +a French writer has said, one only becomes a Napoleon; the others remain +Lucien, Jerome, or Joseph. Of one family of animals or trees, some +advance in one or other direction; some remain at the original level. +There is no "law of progress." The accidents of the world and hereditary +endowment impel some onward, and do not impel others. Hence at nearly +every great stage in the upward procession through the ages some +regiment of plants or animals has dropped out, and it represents to-day +the stage of life at which it ceased to progress. In other words, when +we survey the line of the hundreds of thousands of species which we +find in nature to-day, we can trace, amid their countless variations and +branches, the line of organic evolution in the past; just as we could, +from actual instances, study the evolution of a British house, from +the prehistoric remains in Devonshire to a mansion in Park Lane or a +provincial castle. + +Another method of retracing the lost early chapters in the development +of life is furnished by embryology. The value of this method is not +recognised by all embryologists, but there are now few authorities who +question the substantial correctness of it, and we shall, as we proceed, +see some remarkable applications of it. In brief, it is generally +admitted that an animal or plant is apt to reproduce, during its +embryonic development, some of the stages of its ancestry in past time. +This does not mean that a higher animal, whose ancestors were at one +time worms, at another time fishes, and at a later time reptiles, will +successively take the form of a little worm, a little fish, and a little +reptile. The embryonic life itself has been subject to evolution, and +this reproduction of ancestral forms has been proportionately disturbed. +Still, we shall find that animals will tend, in their embryonic +development, to reproduce various structural features which can only be +understood as reminiscences of ancestral organs. In the lower animals +the reproduction is much less disturbed than in the higher, but even in +the case of man this law is most strikingly verified. We shall find +it useful sometimes at least in confirming our conclusions as to the +ancestry of a particular group. + +We have, therefore, two important clues to the missing chapters in the +story of evolution. Just as the scheme of the evolution of worlds is +written broadly across the face of the heavens to-day, so the scheme of +the evolution of life is written on the face of living nature; and it +is written again, in blurred and broken characters, in the embryonic +development of each individual. With these aids we set out to restore +the lost beginning of the epic of organic evolution. + + + +CHAPTER VI. THE INFANCY OF THE EARTH + +The long Archaean period, into which half the story of the earth is so +unsatisfactorily packed, came to a close with a considerable uplift of +the land. We have seen that the earth at times reaches critical stages +owing to the transfer of millions of tons of matter from the land to the +depths of the ocean, and the need to readjust the pressure on the crust. +Apparently this stage is reached at the end of the Archaean, and a great +rise of the land--probably protracted during hundreds of thousands of +years--takes place. The shore-bottoms round the primitive continent are +raised above the water, their rocks crumpling like plates of lead +under the overpowering pressure. The sea retires with its inhabitants, +mingling their various provinces, transforming their settled homes. A +larger continent spans the northern ocean of the earth. + +In the shore-waters of this early continent are myriads of living +things, representing all the great families of the animal world below +the level of the fish and the insect. The mud and sand in which their +frames are entombed, as they die, will one day be the "Cambrian" rocks +of the geologist, and reveal to him their forms and suggest their +habits. No great volcanic age will reduce them to streaks of shapeless +carbon. The earth now buries its dead, and from their petrified remains +we conjure up a picture of the swarming life of the Cambrian ocean. + +A strange, sluggish population burrows in the mud, crawls over the sand, +adheres to the rocks, and swims among the thickets of sea-weed. The +strangest and most formidable, though still too puny a thing to survive +in a more strenuous age, is the familiar Trilobite of the geological +museum; a flattish animal with broad, round head, like a shovel, its +back covered with a three-lobed shell, and a number of fine legs or +swimmers below. It burrows in the loose bottom, or lies in it with +its large compound eyes peeping out in search of prey. It is the chief +representative of the hard-cased group (Crustacea) which will later +replace it with the lobster, the shrimp, the crab, and the water-flea. +Its remains form from a third to a fourth of all the buried Cambrian +skeletons. With it, swimming in the water, are smaller members of the +same family, which come nearer to our familiar small Crustacea. + +Shell-fish are the next most conspicuous inhabitants. Molluscs are +already well represented, but the more numerous are the more elementary +Brachiopods ("lampshells"), which come next to the Trilobites in number +and variety. Worms (or Annelids) wind in and out of the mud, leaving +their tracks and tubes for later ages. Strange ball or cup-shaped little +animals, with a hard frame, mounted on stony stalks and waving irregular +arms to draw in the food-bearing water, are the earliest representatives +of the Echinoderms. Some of these Cystids will presently blossom into +the wonderful sea-lily population of the next age, some are already +quitting their stalks, to become the free-moving star-fish, of which +a primitive specimen has been found in the later Cambrian. Large +jelly-fishes (of which casts are preserved) swim in the water; +coral-animals lay their rocky foundations, but do not as yet form reefs; +coarse sponges rise from the floor; and myriads of tiny Radiolaria and +Thalamophores, with shells of flint and lime, float at the surface or at +various depths. + +This slight sketch of the Cambrian population shows us that living +things had already reached a high level of development. Their story +evidently goes back, for millions of years, deep into those mists of the +Archaean age which we were unable to penetrate. We turn therefore to +the zoologist to learn what he can tell us of the origin and +family-relations of these Cambrian animals, and will afterwards see how +they are climbing to higher levels under the eye of the geologist. + +At the basis of the living world of to-day is a vast population of +minute, generally microscopic, animals and plants, which are popularly +known as "microbes." Each consists, in scientific language, of one cell. +It is now well known that the bodies of the larger animals and plants +are made up of millions of these units of living matter, or cells--the +atoms of the organic world--and I need not enlarge on it. But even a +single cell lends itself to infinite variety of shape, and we have to +penetrate to the very lowest level of this luxuriant world of one-celled +organisms to obtain some idea of the most primitive living things. +Properly speaking, there were no "first living things." It cannot be +doubted by any student of nature that the microbe developed so gradually +that it is as impossible to fix a precise term for the beginning of life +as it is to say when the night ends and the day begins. In the course of +time little one-celled living units appeared in the waters of the earth, +whether in the shallow shore waters or on the surface of the deep is a +matter of conjecture. + +We are justified in concluding that they were at least as rudimentary +in structure and life as the lowest inhabitants of nature to-day. The +distinction of being the lowest known living organisms should, I think, +be awarded to certain one-celled vegetal organisms which are very common +in nature. Minute simple specks of living matter, sometimes less than +the five-thousandth of an inch in diameter, these lowly Algae are so +numerous that it is they, in their millions, which cover moist surfaces +with the familiar greenish or bluish coat. They have no visible +organisation, though, naturally, they must have some kind of structure +below the range of the microscope. Their life consists in the absorption +of food-particles, at any point of their surface, and in dividing into +two living microbes, instead of dying, when their bulk increases. A very +lowly branch of the Bacteria (Nitrobacteria) sometimes dispute their +claim to the lowest position in the hierarchy of living nature, but +there is reason to suspect that these Bacteria may have degenerated from +a higher level. + +Here we have a convenient starting-point for the story of life, and +may now trace the general lines of upward development. The first great +principle to be recognised is the early division of these primitive +organisms into two great classes, the moving and the stationary. The +clue to this important divergence is found in diet. With exceptions +on both sides, we find that the non-moving microbes generally feed on +inorganic matter, which they convert into plasm; the moving microbes +generally feed on ready-made plasm--on the living non-movers, on each +other, or on particles of dead organic matter. Now, inorganic food is +generally diffused in the waters, so that the vegetal feeders have no +incentive to develop mobility. On the other hand, the power to move +in search of their food, which is not equally diffused, becomes a most +important advantage to the feeders on other organisms. They therefore +develop various means of locomotion. Some flow or roll slowly along +like tiny drops of oil on an inclined surface; others develop minute +outgrowths of their substance, like fine hairs, which beat the water as +oars do. Some of them have one strong oar, like the gondolier (but in +front of the boat); others have two or more oars; while some have their +little flanks bristling with fine lashes, like the flanks of a Roman +galley. + +If we imagine this simple principle at work for ages among the primitive +microbes, we understand the first great division of the living world, +into plants and animals. There must have been a long series of earlier +stages below the plant and animal. In fact, some writers insist that the +first organisms were animal in nature, feeding on the more elementary +stages of living matter. At last one type develops chlorophyll (the +green matter in leaves), and is able to build up plasm out of inorganic +matter; another type develops mobility, and becomes a parasite on the +plant world. There is no rigid distinction of the two worlds. Many +microscopic plants move about just as animals do, and many animals live +on fixed stalks; while many plants feed on organic matter. There is so +little "difference of nature" between the plant and the animal that the +experts differ in classifying some of these minute creatures. In fact, +we shall often find plants and animals crossing the line of division. We +shall find animals rooting themselves to the floor, like plants, though +they will generally develop arms or streamers for bringing the food to +them; and we shall find plants becoming insect-catchers. All this +merely shows that the difference is a natural tendency, which special +circumstances may overrule. It remains true that the great division +of the organic world is due to a simple principle of development; +difference of diet leads to difference of mobility. + +But this simple principle will have further consequences of a most +important character. It will lead to the development of mind in one half +of living nature and leave it undeveloped in the other. Mind, as we know +it in the lower levels of life, is not confined to the animal at +all. Many even of the higher plants are very delicately sensitive +to stimulation, and at the lowest level many plants behave just like +animals. In other words, this sensitiveness to stimuli, which is +the first form of mind, is distributed according to mobility. To the +motionless organism it is no advantage; to the pursuing and pursued +organism it is an immense advantage, and is one of the chief qualities +for natural selection to foster. + +For the moment, however, we must glance at the operation of this and +other natural principles in the evolution of the one-celled animals +and plants, which we take to represent the primitive population of +the earth. As there are tens of thousands of different species even of +"microbes," it is clear that we must deal with them in a very summary +way. The evolution of the plant I reserve for a later chapter, and I +must be content to suggest the development of one-celled animals on +very broad lines. When some of the primitive cells began to feed on each +other, and develop mobility, it is probable that at least two distinct +types were evolved, corresponding to the two lowest animal organisms in +nature to-day. One of these is a very minute and very common (in vases +of decaying flowers, for instance) speck of plasm, which moves about by +lashing the water with a single oar (flagellum), or hair-like extension +of its substance. This type, however, which is known as the Flagellate, +may be derived from the next, which we will take as the primitive and +fundamental animal type. It is best seen in the common and familiar +Amoeba, a minute sac of liquid or viscid plasm, often not more than a +hundredth of an inch in diameter. As its "skin" is merely a finer kind +of the viscous plasm, not an impenetrable membrane, it takes in food at +any part of its surface, makes little "stomachs," or temporary cavities, +round the food at any part of its interior, ejects the useless matter at +any point, and thrusts out any part of its body as temporary "arms" or +"feet." + +Now it is plain that in an age of increasing microbic cannibalism the +toughening of the skin would be one of the first advantages to secure +survival, and this is, in point of fact, almost the second leading +principle in early development. Naturally, as the skin becomes firmer, +the animal can no longer, like the Amoeba, take food at, or make limbs +of, any part of it. There must be permanent pores in the membrane to +receive food or let out rays of the living substance to act as oars +or arms. Thus we get an immense variety amongst these Protozoa, as the +one-celled animals are called. Some (the Flagellates) have one or two +stout oars; some (the Ciliates) have numbers of fine hairs (or cilia). +Some have a definite mouth-funnel, but no stomach, and cilia drawing +the water into it. Some (Vorticella, etc.), shrinking from the open +battlefield, return to the plant-principle, live on stalks, and have +wreaths of cilia round the open mouth drawing the water to them. Some +(the Heliozoa) remain almost motionless, shooting out sticky rays of +their matter on every side to catch the food. Some form tubes to live +in; some (Coleps) develop horny plates for armour; and others develop +projectiles to pierce their prey (stinging threads). + +This miniature world is full of evolutionary interest, but it is too +vast for detailed study here. We will take one group, which we know +to have been already developed in the Cambrian, and let a study of its +development stand for all. In every lecture or book on "the beauties of +the microscope" we find, and are generally greatly puzzled by, minute +shells of remarkable grace and beauty that are formed by some of these +very elementary animals They are the Radiolaria (with flinty shells, as +a rule) and the Thalamophora (with chalk frames). Evolution furnishes a +simple key to their remarkable structure. + +As we saw, one of the early requirements to be fostered by natural +selection in the Archaean struggle for life was a "thick skin," and +the thick skin had to be porous to let the animal shoot out its viscid +substance in rays and earn its living. This stage above the Amoeba +is beautifully illustrated in the sun-animalcules (Heliozoa). Now the +lowest types of Radiolaria are of this character. They have no shell +or framework at all. The next stage is for the little animal to develop +fine irregular threads of flint in its skin, a much better security +against the animal-eater. These animalcules, it must be recollected, +are bits of almost pure plasm, and, as they live in crowds, dividing +and subdividing, but never dying, make excellent mouthfuls for a small +feeder. Those with the more flint in their skins were the more apt to +survive and "breed." The threads of flint increase until they form a +sort of thorn-thicket round a little social group, or a complete lattice +round an individual body. Next, spikes or spines jut out from the +lattice, partly for additional protection, partly to keep the little +body afloat at the surface of the sea. In this way we get a bewildering +variety and increasing complexity of forms, ascending in four divergent +lines from the naked ancestral type to the extreme grace and intricacy +of the Calocyclas monumentum or the Lychnaspis miranda. These, however, +are rare specimens in the 4000 species of Radiolaria. I have hundreds of +them, on microscopic slides, which have no beauty and little regularity +of form. We see a gradual evolution, on utilitarian principles, as +we run over the thousands of forms; and, when we recollect the +inconceivable numbers in which these little animals have lived and +struggled for life--passively--during tens of millions of years, we are +not surprised at the elaborate protective frames of the higher types. + +The Thalamophores, the sister-group of one-celled animals which largely +compose our chalk and much of our limestone, are developed on the same +principle. The earlier forms seem to have lived in a part of the ocean +where silica was scarce, and they absorbed and built their protective +frames of lime. In the simpler types the frame is not unlike a +wide-necked bottle, turned upside-down. In later forms it takes the +shape of a spirally coiled series of chambers, sometimes amounting to +several thousand. These wonderful little houses are not difficult to +understand. The original tiny animal covers itself with a coat of lime. +It feeds, grows, and bulges out of its chamber. The new part of its +flesh must have a fresh coat, and the process goes on until scores, or +hundreds, or even thousands, of these tiny chambers make up the spiral +shell of the morsel of living matter. + +With this brief indication of the mechanical principles which have +directed the evolution of two of the most remarkable groups of the +one-celled animals we must be content, or the dimensions of this volume +will not enable us even to reach the higher and more interesting types. +We must advance at once to the larger animals, whose bodies are composed +of myriads of cells. + +The social tendency which pervades the animal world, and the evident use +of that tendency, prepare us to understand that the primitive +microbes would naturally come in time to live in clusters. Union means +effectiveness in many ways, even when it does not mean strength. We +have still many loose associations of one-celled animals in nature, +illustrating the approach to a community life. Numbers of the Protozoa +are social; they live either in a common jelly-like matrix, or on a +common stalk. In fact, we have a singularly instructive illustration of +the process in the evolution of the sponges. + +It is well known that the horny texture to which we commonly give +the name of sponge is the former tenement and shelter of a colony of +one-celled animals, which are the real Sponges. In other groups the +structure is of lime; in others, again, of flinty material. Now, the +Sponges, as we have them to-day, are so varied, and start from so low +a level, that no other group of animals "illustrates so strikingly the +theory of evolution," as Professor Minchin says. We begin with colonies +in which the individuals are (as in Proterospongia) irregularly +distributed in their jelly-like common bed, each animal lashing the +water, as stalked Flagellates do, and bringing the food to it. Such a +colony would be admirable food for an early carnivore, and we soon find +the protective principle making it less pleasant for the devourer. The +first stage may be--at least there are such Sponges even now--that the +common bed is strewn or sown with the cast shells of Radiolaria. However +that may be, the Sponges soon begin to absorb the silica or lime of +the sea-water, and deposit it in needles or fragments in their bed. The +deposit goes on until at last an elaborate framework of thorny, or limy, +or flinty material is constructed by the one-celled citizens. In the +higher types a system of pores or canals lets the food-bearing water +pass through, as the animals draw it in with their lashes; in the +highest types the animals come still closer together, lining the walls +of little chambers in the interior. + +Here we have a very clear evolutionary transition from the solitary +microbe to a higher level, but, unfortunately, it does not take us far. +The Sponges are a side-issue, or cul de sac, from the Protozoic world, +and do not lead on to the higher. Each one-celled unit remains an +animal; it is a colony of unicellulars, not a many-celled body. We +may admire it as an instructive approach toward the formation of +a many-celled body, but we must look elsewhere for the true upward +advance. + +The next stage is best illustrated in certain spherical colonies of +cells like the tiny green Volvox (now generally regarded as vegetal) +of our ponds, or Magosphoera. Here the constituent cells merge +their individuality in the common action. We have the first definite +many-celled body. It is the type to which a moving close colony of +one-celled microbes would soon come. The round surface is well adapted +for rolling or spinning along in the water, and, as each little cell +earns its own living, it must be at the surface, in contact with the +water. Thus a hollow, or fluid-filled, little sphere, like the Volvox, +is the natural connecting-link between the microbe and the many-celled +body, and may be taken to represent the first important stage in its +development. + +The next important stage is also very clearly exhibited in nature, and +is more or less clearly reproduced in the embryonic development of all +animals. We may imagine that the age of microbes was succeeded by an age +of these many-celled larger bodies, and the struggle for life entered +upon a new phase. The great principle we have already recognised came +into play once more. Large numbers of the many-celled bodies shrank from +the field of battle, and adopted the method of the plant. They rooted +themselves to the floor of the ocean, and developed long arms or lashes +for creating a whirlpool movement in the water, and thus bringing the +food into their open mouths. Forfeiting mobility, they have, like the +plant, forfeited the greater possibilities of progress, and they remain +flowering to-day on the floors of our waters, recalling the next phase +in the evolution of early life. Such are the hydra, the polyp, the +coral, and the sea-anemone. It is not singular that earlier observers +could not detect that they were animals, and they were long known in +science as "animal-plants" (Zoophytes). + +When we look to the common structure of these animals, to find the +ancestral type, we must ignore the nerve and muscle-cells which they +have developed in some degree. Fundamentally, their body consists of a +pouch, with an open mouth, the sides of the pouch consisting of a double +layer of cells. In this we have a clue to the next stage of animal +development. Take a soft india-rubber ball to represent the first +many-celled animal. Press in one half of the ball close upon the other, +narrow the mouth, and you have something like the body-structure of the +coral and hydra. As this is the course of embryonic development, and as +it is so well retained in the lowest groups of the many-celled animals, +we take it to be the next stage. The reason for it will become clear on +reflection. Division of labour naturally takes place in a colony, and in +that way certain cells in the primitive body were confined to the work +of digestion. It would be an obvious advantage for these to retire into +the interior, leaving the whole external surface free for the adjustment +of the animal's relations to the outer world. + +Again we must refrain from following in detail the development of +this new world of life which branches off in the Archaean ocean. The +evolution of the Corals alone would be a lengthy and interesting +story. But a word must be said about the jelly-fish, partly because the +inexpert will be puzzled at the inclusion of so active an animal, and +partly because its story admirably illustrates the principle we are +studying. The Medusa really descends from one of the plant-like animals +of the early Archaean period, but it has abandoned the ancestral stalk, +turned upside down, and developed muscular swimming organs. Its past is +betrayed in its embryonic development. As a rule the germ develops into +a stalked polyp, out of which the free-swimming Medusa is formed. This +return to active and free life must have occurred early, as we find +casts of large Medusae in the Cambrian beds. In complete harmony with +the principle we laid down, the jelly-fish has gained in nerve and +sensitiveness in proportion to its return to an active career. + +But this principle is best illustrated in the other branch of the early +many-celled animals, which continued to move about in search of food. +Here, as will be expected, we have the main stem of the animal world, +and, although the successive stages of development are obscure, certain +broad lines that it followed are clear and interesting. + +It is evident that in a swarming population of such animals the most +valuable qualities will be speed and perception. The sluggish Coral +needs only sensitiveness enough, and mobility enough, to shrink behind +its protecting scales at the approach of danger. In the open water the +most speedy and most sensitive will be apt to escape destruction, +and have the larger share in breeding the next generation. Imagine a +selection on this principle going on for millions of years, and the +general result can be conjectured. A very interesting analogy is found +in the evolution of the boat. From the clumsy hollowed tree of Neolithic +man natural selection, or the need of increasing speed, has developed +the elongated, evenly balanced modern boat, with its distinct stem and +stern. So in the Archaean ocean the struggle to overtake food, or escape +feeders, evolved an elongated two-sided body, with head and tail, and +with the oars (cilia) of the one-celled ancestor spread thickly along +its flanks. In other words, a body akin to that of the lower water-worms +would be the natural result; and this is, in point of fact, the next +stage we find in the hierarchy of living nature. + +Probably myriads of different types of this worm-like organisation were +developed, but such animals leave no trace in the rocks, and we can +only follow the development by broad analogies. The lowest flat-worms +of to-day may represent some of these early types, and as we ascend +the scale of what is loosely called "worm" organisation, we get some +instructive suggestions of the way in which the various organs develop. +Division of labour continues among the colony of cells which make up +the body, and we get distinct nerve-cells, muscle-cells, and digestive +cells. The nerve-cells are most useful at the head of an organism which +moves through the water, just as the look-out peers from the head of the +ship, and there they develop most thickly. By a fresh division of labour +some of these cells become especially sensitive to light, some to the +chemical qualities of matter, some to movements of the water; we have +the beginning of the eyes, the nose, and the ears, as simple little +depressions in the skin of the head, lined with these sensitive cells. A +muscular gullet arises to protect the digestive tube; a simple drainage +channel for waste matter forms under the skin; other channels permit +the passage of the fluid food, become (in the higher worms) muscular +blood-vessels, and begin to contract--somewhat erratically at first--and +drive the blood through the system. + +Here, perhaps, are millions of years of development compressed into +a paragraph. But the purpose of this work is chiefly to describe the +material record of the advance of life in the earth's strata, and show +how it is related to great geological changes. We must therefore abstain +from endeavouring to trace the genealogy of the innumerable types of +animals which were, until recently, collected in zoology under the +heading "Worms." It is more pertinent to inquire how the higher classes +of animals, which we found in the Cambrian seas, can have arisen from +this primitive worm-like population. + +The struggle for life in the Archaean ocean would become keener and more +exacting with the appearance of each new and more effective type. That +is a familiar principle in our industrial world to-day, and we shall +find it illustrated throughout our story. We therefore find the various +processes of evolution, which we have already seen, now actively at +work among the swarming Archaean population, and producing several +very distinct types. In some of these struggling organisms speed is +developed, together with offensive and defensive weapons, and a line +slowly ascends toward the fish, which we will consider later. In others +defensive armour is chiefly developed, and we get the lines of the +heavy sluggish shell-fish, the Molluscs and Brachiopods, and, by a +later compromise between speed and armour, the more active tough-coated +Arthropods. In others the plant-principle reappears; the worm-like +creature retires from the free-moving life, attaches itself to a +fixed base, and becomes the Bryozoan or the Echinoderm. To trace the +development of these types in any detail is impossible. The early +remains are not preserved. But some clues are found in nature or in +embryonic development, and, when the types do begin to be preserved in +the rocks, we find the process of evolution plainly at work in them. We +will therefore say a few words about the general evolution of each type, +and then return to the geological record in the Cambrian rocks. + +The starfish, the most familiar representative of the Echinoderms, +seems very far removed from the kind of worm-like ancestor we have been +imagining, but, fortunately, the very interesting story of the starfish +is easily learned from the geological chronicle. Reflect on the +flower-like expansion of its arms, and then imagine it mounted on a +stalk, mouth side upward, with those arms--more tapering than they +now are--waving round the mouth. That, apparently, was the past of the +starfish and its cousins. We shall see that the earliest Echinoderms we +know are cup-shaped structures on stalks, with a stiff, limy frame and +(as in all sessile animals) a number of waving arms round the mouth. +In the next geological age the stalk will become a long and flexible +arrangement of muscles and plates of chalk, the cup will be more +perfectly compacted of chalky plates, and the five arms will taper and +branch until they have an almost feathery appearance; and the animal +will be considered a "sea-lily" by the early geologist. + +The evidence suggests that both the free-moving and the stalked +Echinoderms descend from a common stalked Archaean ancestor. Some +primitive animal abandoned the worm-like habit, and attached itself, +like a polyp, to the floor. Like all such sessile animals, it developed +a wreath of arms round the open mouth. The "sea-cucumber" (Holothurian) +seems to be a type that left the stalk, retaining the little wreath of +arms, before the body was heavily protected and deformed. In the others +a strong limy skeleton was developed, and the nerves and other organs +were modified in adaptation to the bud-like or flower-like structure. +Another branch of the family then abandoned the stalk, and, spreading +its arms flat, and gradually developing in them numbers of little "feet" +(water-tubes), became the starfish. In the living Comatula we find a +star passing through the stalked stage in its early development, when it +looks like a tiny sea-lily. The sea-urchin has evolved from the star by +folding the arms into a ball. [*] + + * See the section on Echinoderms, by Professor MacBride, in + the "Cambridge Natural History," I. + + +The Bryozoa (sea-mats, etc.) are another and lower branch of the +primitive active organisms which have adopted a sessile life. In the +shell-fish, on the other hand, the principle of armour-plating has its +greatest development. It is assuredly a long and obscure way that +leads from the ancestral type of animal we have been describing to +the headless and shapeless mussel or oyster. Such a degeneration is, +however, precisely what we should expect to find in the circumstances. +Indeed, the larva, of many of the headless Molluscs have a mouth and +eyes, and there is a very common type of larva--the trochosphere--in the +Mollusc world which approaches the earlier form of some of the +higher worms. The Molluscs, as we shall see, provide some admirable +illustrations of the process of evolution. In some of the later +fossilised specimens (Planorbis, Paludina, etc.) we can trace the animal +as it gradually passes from one species to another. The freshening of +the Caspian Sea, which was an outlying part of the Mediterranean quite +late in the geological record, seems to have evolved several new genera +of Molluscs. + +Although, therefore, the remains are not preserved of those primitive +Molluscs in which we might see the protecting shell gradually +thickening, and deforming the worm-like body, we are not without +indications of the process. Two unequal branches of the early wormlike +organisms shrank into strong protective shells. The lower branch became +the Brachiopods; the more advanced branch the Molluscs. In the Mollusc +world, in turn, there are several early types developed. In the +Pelecypods (or Lamellibranchs--the mussel, oyster, etc.) the animal +retires wholly within its fortress, and degenerates. The Gastropods +(snails, etc.) compromise, and retain a certain amount of freedom, so +that they degenerate less. The highest group, the Cephalopods, "keep +their heads," in the literal sense, and we shall find them advancing +from form to form until, in the octopus of a later age, they discard the +ancestral shell, and become the aristocrats of the Mollusc kingdom. + +The last and most important line that led upward from the chaos of +Archaean worms is that of the Arthropods. Its early characteristic was +the acquisition of a chitinous coat over the body. Embryonic indications +show that this was at first a continuous shield, but a type arose in +which the coat broke into sections covering each segment of the body, +giving greater freedom of movement. The shield, in fact, became a fine +coat of mail. The Trilobite is an early and imperfect experiment of the +class, and the larva of the modern king-crab bears witness that it has +not perished without leaving descendants. How later Crustacea increase +the toughness of the coat by deposits of lime, and lead on to the +crab and lobster, and how one early branch invades the land, develops +air-breathing apparatus, and culminates in the spiders and insects, will +be considered later. We shall see that there is most remarkable evidence +connecting the highest of the Arthropods, the insect, with a remote +Annelid ancestor. + +We are thus not entirely without clues to the origin of the more +advanced animals we find when the fuller geological record begins. +Further embryological study, and possibly the discovery of surviving +primitive forms, of which Central Africa may yet yield a number, may +enlarge our knowledge, but it is likely to remain very imperfect. +The fossil records of the long ages during which the Mollusc, the +Crustacean, and the Echinoderm slowly assumed their characteristic forms +are hopelessly lost. But we are now prepared to return to the record +which survives, and we shall find the remaining story of the earth a +very ample and interesting chronicle of evolution. + + + +CHAPTER VII. THE PASSAGE TO THE LAND + +Slender as our knowledge is of the earlier evolution of the Invertebrate +animals, we return to our Cambrian population with greater interest. +The uncouth Trilobite and its livelier cousins, the sluggish, skulking +Brachiopod and Mollusc, the squirming Annelids, and the plant-like +Cystids, Corals, and Sponges are the outcome of millions of years of +struggle. Just as men, when their culture and their warfare advanced, +clothed themselves with armour, and the most completely mailed +survived the battle, so, generation after generation, the thicker and +harder-skinned animals survived in the Archaean battlefield, and the +Cambrian age opened upon the various fashions of armour that we there +described. But, although half the story of life is over, organisation +is still imperfect and sluggish. We have now to see how it advances to +higher levels, and how the drama is transferred from the ocean to a new +and more stimulating environment. + +The Cambrian age begins with a vigorous move on the part of the land. +The seas roll back from the shores of the "lost Atlantis," and vast +regions are laid bare to the sun and the rains. In the bays and hollows +of the distant shores the animal survivors of the great upheaval adapt +themselves to their fresh homes and continue the struggle. But the +rivers and the waves are at work once more upon the land, and, as the +Cambrian age proceeds, the fringes of the continents are sheared, and +the shore-life steadily advances upon the low-lying land. By the end of +the Cambrian age a very large proportion of the land is covered with +a shallow sea, in which the debris of its surface is deposited. The +levelling continues through the next (Ordovician) period. Before its +close nearly the whole of the United States and the greater part of +Canada are under water, and the new land that had appeared on the site +of Europe is also for the most part submerged. The present British Isles +are almost reduced to a strip of north-eastern Ireland, the northern +extremity of Scotland, and large islands in the south-west and centre of +England. + +We have already seen that these victories of the sea are just as +stimulating, in a different way, to animals as the victories of the +land. American geologists are tracing, in a very instructive way, the +effect on that early population of the encroachment of the sea. In each +arm of the sea is a distinctive fauna. Life is still very parochial; the +great cosmopolitans, the fishes, have not yet arrived. As the land is +revelled, the arms of the sea approach each other, and at last mingle +their waters and their populations, with stimulating effect. Provincial +characters are modified, and cosmopolitan characters increase in the +great central sea of America. The vast shallow waters provide a greatly +enlarged theatre for the life of the time, and it flourishes enormously. +Then, at the end of the Ordovician, the land begins to rise once more. +Whether it was due to a fresh shrinking of the crust, or to the simple +process we have described, or both, we need not attempt to determine; +but both in Europe and America there is a great emergence of land. +The shore-tracts and the shallow water are narrowed, the struggle is +intensified in them, and we pass into the Silurian age with a greatly +reduced number but more advanced variety of animals. In the Silurian +age the sea advances once more, and the shore-waters expand. There is +another great "expansive evolution" of life. But the Silurian age closes +with a fresh and very extensive emergence of the land, and this time +it will have the most important consequences. For two new things have +meantime appeared on the earth. The fish has evolved in the waters, and +the plant, at least, has found a footing on the land. + +These geological changes which we have summarised and which have been +too little noticed until recently in evolutionary studies, occupied +7,000,000 years, on the lowest estimate, and probably twice that period. +The impatient critic of evolutionary hypotheses is apt to forget the +length of these early periods. We shall see that in the last two or +three million years of the earth's story most extraordinary progress +has been made in plant and animal development, and can be very fairly +traced. How much advance should we allow for these seven or fourteen +million years of swarming life and changing environments? + +We cannot nearly cover the whole ground of paleontology for the period, +and must be content to notice some of the more interesting advances, and +then deal more fully with the evolution of the fish, the forerunner of +the great land animals. + +The Trilobite was the most arresting figure in the Cambrian sea, and its +fortunes deserve a paragraph. It reaches its climax in the Ordovician +sea, and then begins to decline, as more powerful animals come upon the +scene. At first (apparently) an eyeless organism, it gradually develops +compound eyes, and in some species the experts have calculated that +there were 15,000 facets to each eye. As time goes on, also, the eye +stands out from the head on a kind of stalk, giving a wider range of +vision. Some of the more sluggish species seem to have been able to +roll themselves up, like hedgehogs, in their shells, when an enemy +approached. But another branch of the same group (Crustacea) has +meantime advanced, and it gradually supersedes the dwindling Trilobites. +Toward the close of the Silurian great scorpion-like Crustaceans +(Pterygotus, Eurypterus, etc.) make their appearance. Their development +is obscure, but it must be remembered that the rocks only give the +record of shore-life, and only a part of that is as yet opened by +geology. Some experts think that they were developed in inland waters. +Reaching sometimes a length of five or six feet, with two large compound +eyes and some smaller eye-spots (ocelli), they must have been the giants +of the Silurian ocean until the great sharks and other fishes appeared. + +The quaint stalked Echinoderm which also we noticed in the Cambrian +shallows has now evolved into a more handsome creature, the sea-lily. +The cup-shaped body is now composed of a large number of limy plates, +clothed with flesh; the arms are long, tapering, symmetrical, and richly +fringed; the stalk advances higher and higher, until the flower-like +animal sometimes waves its feathery arms from the top of a flexible +pedestal composed of millions of tiny chalk disks. Small forests of +these sea-lilies adorn the floor of the Silurian ocean, and their broken +and dead frames form whole beds of limestone. The primitive Cystids +dwindle and die out in the presence of such powerful competitors. Of +250 species only a dozen linger in the Silurian strata, though a new and +more advanced type--the Blastoid--holds the field for a time. It is the +age of the Crinoids or sea-lilies. The starfish, which has abandoned the +stalk, does not seem to prosper as yet, and the brittle-star appears. +Their age will come later. No sea-urchins or sea-cucumbers (which would +hardly be preserved) are found as yet. It is precisely the order of +appearance which our theory of their evolution demands. + +The Brachiopods have passed into entirely new and more advanced species +in the many advances and retreats of the shores, but the Molluscs show +more interesting progress. The commanding group from the start is that +of the Molluscs which have "kept their head," the Cephalopods, and +their large shells show a most instructive evolution. The first great +representative of the tribe is a straight-shelled Cephalopod, which +becomes "the tyrant and scavenger of the Silurian ocean" (Chamberlin). +Its tapering, conical shell sometimes runs to a length of fifteen +feet, and a diameter of one foot. It would of itself be an important +evolutionary factor in the primitive seas, and might explain more than +one advance in protective armour or retreat into heavy shells. As the +period advances the shell begins to curve, and at last it forms a +close spiral coil. This would be so great an advantage that we are not +surprised to find the coiled type (Goniatites) gain upon and gradually +replace the straight-shelled types (Orthoceratites). The Silurian +ocean swarms with these great shelled Cephalopods, of which the little +Nautilus is now the only survivor. + +We will not enlarge on the Sponges and Corals, which are slowly +advancing toward the higher modern types. Two new and very powerful +organisms have appeared, and merit the closest attention. One is the +fish, the remote ancestor of the birds and mammals that will one day +rule the earth. The other may be the ancestor of the fish itself, or it +may be one of the many abortive outcomes and unsuccessful experiments of +the stirring life of the time. And while these new types are themselves +a result of the great and stimulating changes which we have reviewed +and the incessant struggle for food and safety, they in turn enormously +quicken the pace of development. The Dreadnought appears in the +primitive seas; the effect on the fleets of the world of the evolution +of our latest type of battleship gives us a faint idea of the effect, on +all the moving population, of the coming of these monsters of the deep. +The age had not lacked incentives to progress; it now obtains a more +terrible and far-reaching stimulus. + +To understand the situation let us see how the battle of land and sea +had proceeded. The Devonian Period had opened with a fresh emergence of +the land, especially in Europe, and great inland seas or lakes were left +in the hollows. The tincture of iron which gives a red colour to our +characteristic Devonian rocks, the Old Red Sandstone, shows us that +the sand was deposited in inland waters. The fish had already been +developed, and the Devonian rocks show it swarming, in great numbers and +variety, in the enclosed seas and round the fringe of the continents. + +The first generation was a group of strange creatures, half fish and +half Crustacean, which are known as the Ostracoderms. They had large +armour-plated heads, which recall the Trilobite, and suggest that they +too burrowed in the mud of the sea or (as many think) of the inland +lakes, making havoc among the shell-fish, worms, and small Crustacea. +The hind-part of their bodies was remarkably fish-like in structure. But +they had no backbone--though we cannot say whether they may not have +had a rod of cartilage along the back--and no articulated jaws like the +fish. Some regard them as a connecting link between the Crustacea +and the fishes, but the general feeling is that they were an abortive +development in the direction of the fish. The sharks and other large +fishes, which have appeared in the Silurian, easily displace these +clumsy and poor-mouthed competitors One almost thinks of the aeroplane +superseding the navigable balloon. + +Of the fishes the Arthrodirans dominated the inland seas (apparently), +while the sharks commanded the ocean. One of the Arthrodirans, the +Dinichthys ("terrible fish"), is the most formidable fish known to +science. It measured twenty feet from snout to tail. Its monstrous head, +three feet in width, was heavily armoured, and, instead of teeth, its +great jaws, two feet in length, were sharpened, and closed over the +victim like a gigantic pair of clippers. The strongly plated heads of +these fishes were commonly a foot or two feet in width. Life in the +waters became more exacting than ever. But the Arthrodirans were +unwieldy and sluggish, and had to give way before more progressive +types. The toothed shark gradually became the lord of the waters. + +The early shark ate, amongst other things, quantities of Molluscs and +Brachiopods. Possibly he began with Crustacea; in any case the practice +of crunching shellfish led to a stronger and stronger development of the +hard plate which lined his mouth. The prickles of the plate grew +larger and harder, until--as may be seen to-day in the mouth of a young +shark--the cavity was lined with teeth. In the bulk of the Devonian +sharks these developed into what are significantly called "pavement +teeth." They were solid plates of enamel, an inch or an inch and a half +in width, with which the monster ground its enormous meals of Molluscs, +Crustacea, sea-weed, etc. A new and stimulating element had come into +the life of the invertebrate world. Other sharks snapped larger victims, +and developed the teeth on the edges of their jaws, to the sacrifice +of the others, until we find these teeth in the course of time solid +triangular masses of enamel, four or five inches long, with saw-like +edges. Imagine these terrible mouths--the shears of the Arthrodiran, +and the grindstones and terrible crescents of the giant sharks--moving +speedily amongst the crowded inhabitants of the waters, and it is easy +to see what a stimulus to the attainment of speed and of protective +devices was given to the whole world of the time. + +What was the origin of the fish? Here we are in much the same position +as we were in regard to the origin of the higher Invertebrates. Once +the fish plainly appears upon the scene it is found to be undergoing a +process of evolution like all other animals. The vast majority of our +fishes have bony frames (or are Teleosts); the fishes of the Devonian +age nearly all have frames of cartilage, and we know from embryonic +development that cartilage is the first stage in the formation of bone. +In the teeth and tails, also, we find a gradual evolution toward the +higher types. But the earlier record is, for reasons I have already +given, obscure; and as my purpose is rather to discover the agencies +of evolution than to strain slender evidence in drawing up pedigrees, I +need only make brief reference to the state of the problem. + +Until comparatively recent times the animal world fell into two clearly +distinct halves, the Vertebrates and the Invertebrates. There were +several anatomical differences between the two provinces, but the most +conspicuous and most puzzling was the backbone. Nowhere in living nature +or in the rocks was any intermediate type known between the backboned +and the non-backboned animal. In the course of the nineteenth century, +however, several animals of an intermediate type were found. The +sea-squirt has in its early youth the line of cartilage through the +body which, in embryonic development, represents the first stage of the +backbone; the lancelet and the Appendicularia have a rod of cartilage +throughout life; the "acorn-headed worm" shows traces of it. These are +regarded as surviving specimens of various groups of animals which, in +early times, fell between the Invertebrate and Vertebrate worlds, and +illustrate the transition. + +With their aid a genealogical tree was constructed for the fish. It was +assumed that some Cambrian or Silurian Annelid obtained this stiffening +rod of cartilage. The next advantage--we have seen it in many cases--was +to combine flexibility with support. The rod was divided into connected +sections (vertebrae), and hardened into bone. Besides stiffening the +body, it provided a valuable shelter for the spinal cord, and its upper +part expanded into a box to enclose the brain. The fins were formed of +folds of skin which were thrown off at the sides and on the back, as +the animal wriggled through the water. They were of use in swimming, and +sections of them were stiffened with rods of cartilage, and became the +pairs of fins. Gill slits (as in some of the highest worms) appeared in +the throat, the mouth was improved by the formation of jaws, and--the +worm culminated in the shark. + +Some experts think, however, that the fish developed directly from a +Crustacean, and hold that the Ostracoderms are the connecting link. A +close discussion of the anatomical details would be out of place here, +[*] and the question remains open for the present. Directly or +indirectly, the fish is a descendant of some Archaean Annelid. It is +most probable that the shark was the first true fish-type. There are +unrecognisable fragments of fishes in the Ordovician and Silurian rocks, +but the first complete skeletons (Lanarkia, etc.) are of small shark- +like creatures, and the low organisation of the group to which the shark +belongs, the Elasmobranchs, makes it probable that they are the most +primitive. Other remains (Palaeospondylus) show that the fish-like +lampreys had already developed. + + * See, especially, Dr. Gaskell's "Origin of Vertebrates" + (1908). + + +Two groups were developed from the primitive fish, which have great +interest for us. Our next step, in fact, is to trace the passage of the +fish from the water to the land, one of the most momentous chapters in +the story of life. To that incident or accident of primitive life we +owe our own existence and the whole development of the higher types of +animals. The advance of natural history in modern times has made this +passage to the land easy to understand. Not only does every frog reenact +it in the course of its development, but we know many fishes that +can live out of water. There is an Indian perch--called the "climbing +perch," but it has only once been seen by a European to climb a +tree--which crosses the fields in search of another pool, when its own +pool is evaporating. An Indian marine fish (Periophthalmus) remains +hunting on the shore when the tide goes out. More important still, +several fishes have lungs as well as gills. The Ceratodus of certain +Queensland rivers has one lung; though, I was told by the experts in +Queensland, it is not a "mud-fish," and never lives in dry mud. However, +the Protopterus of Africa and the Lepidosiren of South America have two +lungs, as well as gills, and can live either in water or, in the dry +season, on land. + +When the skeletons of fishes of the Ceratodus type were discovered in +the Devonian rocks, it was felt that we had found the fish-ancestor of +the land Vertebrates, but a closer anatomical examination has made this +doubtful. The Devonian lung-fish has characters which do not seem to +lead on to the Amphibia. The same general cause probably led many groups +to leave the water, or adapt themselves to living on land as well as in +water, and the abundant Dipoi or Dipneusts ("double-breathers") of the +Devonian lakes are one of the chief of these groups, which have +luckily left descendants to our time. The ancestors of the Amphibia +are generally sought amongst the Crossopterygii, a very large group of +fishes in Devonian times, with very few representatives to-day. + +It is more profitable to investigate the process itself than to make a +precarious search for the actual fish, and, fortunately, this inquiry +is more hopeful. The remains that we find make it probable that the fish +left the water about the beginning of the Devonian or the end of the +Silurian. Now this period coincides with two circumstances which throw a +complete light on the step; one is the great rise of the land, catching +myriads of fishes in enclosed inland seas, and the other is the +appearance of formidable carnivores in the waters. As the seas +evaporated [*] and the great carnage proceeded, the land, which was +already covered with plants and inhabited by insects, offered a safe +retreat for such as could adopt it. Emigration to the land had been +going on for ages, as we shall see. Curious as it must seem to the +inexpert, the fishes, or some of them, were better prepared than most +other animals to leave the water. The chief requirement was a lung, or +interior bag, by which the air could be brought into close contact with +the absorbing blood vessels. Such a bag, broadly speaking, most of the +fishes possess in their floating-bladder: a bag of gas, by compressing +or expanding which they alter their specific gravity in the water. In +some fishes it is double; in some it is supplied with blood-vessels; in +some it is connected by a tube with the gullet, and therefore with the +atmosphere. + + * It is now usually thought that the inland seas were the + theatre of the passage to land. I must point out, however, + that the wide distribution of our Dipneusts, in Australia, + tropical Africa, and South America, suggests that they were + marine though they now live in fresh water. But we shall see + that a continent united the three regions at one time, and + it may afford some explanation. + + +Thus we get very clear suggestions of the transition from water to land. +We must, of course, conceive it as a slow and gradual adaptation. +At first there may have been a rough contrivance for deriving oxygen +directly and partially from the atmosphere, as the water of the lake +became impure. So important an advantage would be fostered, and, as +the inland sea became smaller, or its population larger or fiercer, the +fishes with a sufficiently developed air-breathing apparatus passed to +the land, where, as yet, they would find no serious enemy. The fact is +beyond dispute; the theory of how it occurred is plausible enough; the +consequences were momentous. Great changes were preparing on the land, +and in a comparatively short time we shall find its new inhabitant +subjected to a fierce test of circumstances that will carry it to an +enormously higher level than life had yet reached. + +I have said that the fact of this transition to the land is beyond +dispute. The evidence is very varied, but need not all be enlarged upon +here. The widespread Dipneust fishes of the Devonian rocks bear +strong witness to it, and the appearance of the Amphibian immediately +afterwards makes it certain. The development of the frog is a +reminiscence of it, on the lines of the embryonic law which we saw +earlier. An animal, in its individual development, more or less +reproduces the past phases of its ancestry. So the free-swimming +jelly-fish begins life as a fixed polyp; a kind of star-fish (Comatula) +opens its career as a stalked sea-lily; the gorgeous dragon-fly is at +first an uncouth aquatic animal, and the ethereal butterfly a worm-like +creature. But the most singular and instructive of all these embryonic +reminiscences of the past is found in the fact that all the higher +land-animals of to-day clearly reproduce a fish-stage in their embryonic +development. + +In the third and fourth weeks of development the human embryo shows four +(closed) slits under the head, with corresponding arches. The bird, the +dog, the horse--all the higher land animals, in a word, pass through the +same phase. The suggestion has been made that these structures do not +recall the gill-slits and gill-arches of the fish, but are folds due +to the packing of the embryo in the womb. In point of fact, they appear +just at the time when the human embryo is only a fifth of an inch +long, and there is no such compression. But all doubt as to their +interpretation is dispelled when we remove the skin and examine the +heart and blood-vessels. The heart is up in the throat, as in the fish, +and has only two chambers, as in the fish (not four, as in the bird +and mammal); and the arteries rise in five pairs of arches over the +swellings in the throat, as they do in the lower fish, but do not in +the bird and mammal. The arrangement is purely temporary--lasting only +a couple of weeks in the human embryo--and purposeless. Half these +arteries will disappear again. They quite plainly exist to supply fine +blood-vessels for breathing at the gill-clefts, and are never used, for +the embryo does not breathe, except through the mother. They are a most +instructive reminder of the Devonian fish which quitted its element and +became the ancestor of all the birds and mammals of a later age. + +Several other features of man's embryonic development--the budding of +the hind limbs high up, instead of at the base of, the vertebral column, +the development of the ears, the nose, the jaws, etc.--have the same +lesson, but the one detailed illustration will suffice. The millions of +years of stimulating change and struggle which we have summarised have +resulted in the production of a fish which walks on four limbs (as the +South American mud-fish does to-day), and breathes the atmosphere. + +We have been quite unable to follow the vast changes which have meantime +taken place in its organisation. The eyes, which were mere pits in +the skin, lined with pigment cells, in the early worm, now have a +crystalline lens to concentrate the light and define objects on the +nerve. The ears, which were at first similar sensitive pits in the skin, +on which lay a little stone whose movements gave the animal some sense +of direction, are now closed vesicles in the skull, and begin to be +sensitive to waves of sound. The nose, which was at first two blind, +sensitive pits in the skin of the head, now consists of two nostrils +opening into the mouth, with an olfactory nerve spreading richly +over the passages. The brain, which was a mere clump of nerve-cells +connecting the rough sense-impressions, is now a large and intricate +structure, and already exhibits a little of that important region (the +cerebrum) in which the varied images of the outside world are combined. +The heart, which was formerly was a mere swelling of a part of one of +the blood-vessels, now has two chambers. + +We cannot pursue these detailed improvements of the mechanism, as we +might, through the ascending types of animals. Enough if we see more or +less clearly how the changes in the face of the earth and the rise of +its successive dynasties of carnivores have stimulated living things to +higher and higher levels in the primitive ocean. We pass to the clearer +and far more important story of life on land, pursuing the fish through +its continuous adaptations to new conditions until, throwing out +side-branches as it progresses, it reaches the height of bird and mammal +life. + + + +CHAPTER VIII. THE COAL-FOREST + +With the beginning of life on land we open a new and more important +volume of the story of life, and we may take the opportunity to make +clearer certain principles or processes of development which we may seem +hitherto to have taken for granted. The evolutionary work is too often +a mere superficial description of the strange and advancing classes of +plants and animals which cross the stage of geology. Why they change and +advance is not explained. I have endeavoured to supply this explanation +by putting the successive populations of the earth in their respective +environments, and showing the continuous and stimulating effect on them +of changes in those environments. We have thus learned to decipher +some lines of the decalogue of living nature. "Thou shalt have a thick +armour," "Thou shalt be speedy," "Thou shalt shelter from the more +powerful," are some of the laws of primeval life. The appearance of each +higher and more destructive type enforces them with more severity; and +in their observance animals branch outward and upward into myriads of +temporary or permanent forms. + +But there is no consciousness of law and no idea of evading danger. +There is not even some mysterious instinct "telling" the animal, as +it used to be said, to do certain things. It is, in fact, not strictly +accurate to say that a certain change in the environment stimulates +animals to advance. Generally speaking, it does not act on the advancing +at all, but on the non-advancing, which it exterminates. The procedure +is simple, tangible, and unconscious. Two invading arms of the sea meet +and pour together their different waters and populations. The habits, +the foods, and the enemies of many types of animals are changed; the +less fit for the new environment die first, the more fit survive longest +and breed most of the new generation. It is so with men when they +migrate to a more exacting environment, whether a dangerous trade or +a foreign clime. Again, take the case of the introduction of a giant +Cephalopod or fish amongst a population of Molluscs and Crustacea. The +toughest, the speediest, the most alert, the most retiring, or the least +conspicuous, will be the most apt to survive and breed. In hundreds or +thousands of generations there will be an enormous improvement in the +armour, the speed, the sensitiveness, the hiding practices, and the +protective colours, of the animals which are devoured. The "natural +selection of the fittest" really means the "natural destruction of the +less fit." + +The only point assumed in this is that the young of an animal or plant +tend to differ from each other and from their parents. Darwin was +content to take this as a fact of common observation, as it obviously +is, but later science has thrown some light on the causes of these +variations. In the first place, the germs in the parent's body may +themselves be subject to struggle and natural selection, and not share +equally in the food-supply. Then, in the case of the higher animals (or +the majority of animals), there is a clear source of variation in +the fact that the mature germ is formed of certain elements from two +different parents, four grandparents, and so on. In the case of the +lower animals the germs and larvae float independently in the water, +and are exposed to many influences. Modern embryologists have found, +by experiment, that an alteration of the temperature or the chemical +considerable effect on eggs and larvae. Some recent experiments have +shown that such changes may even affect the eggs in the mother's +ovary. These discoveries are very important and suggestive, because the +geological changes which we are studying are especially apt to bring +about changes of temperature and changes in the freshness or saltiness +of water. + +Evolution is, therefore, not a "mere description" of the procession of +living things; it is to a great extent an explanation of the procession. +When, however, we come to apply these general principles to certain +aspects of the advance in organisation we find fundamental differences +of opinion among biologists, which must be noted. As Sir E. Ray +Lankester recently said, it is not at all true that Darwinism is +questioned in zoology to-day. It is true only that Darwin was not +omniscient or infallible, and some of his opinions are disputed. + +Let me introduce the subject with a particular instance of evolution, +the flat-fish. This animal has been fitted to survive the terrible +struggle in the seas by acquiring such a form that it can lie almost +unseen upon the floor of the ocean. The eye on the under side of the +body would thus be useless, but a glance at a sole or plaice in a +fishmonger's shop will show that this eye has worked upward to the top +of the head. Was the eye shifted by the effort and straining of the +fish, inherited and increased slightly in each generation? Is the +explanation rather that those fishes in each generation survived and +bred which happened from birth to have a slight variation in that +direction, though they did not inherit the effect of the parent's effort +to strain the eye? Or ought we to regard this change of structure as +brought about by a few abrupt and considerable variations on the part +of the young? There you have the three great schools which divide modern +evolutionists: Lamarckism, Weismannism, and Mendelism (or Mutationism). +All are Darwinians. No one doubts that the flat-fish was evolved from an +ordinary fish--the flat-fish is an ordinary fish in its youth--or that +natural selection (enemies) killed off the old and transitional types +and overlooked (and so favoured) the new. It will be seen that the +language used in this volume is not the particular language of any +one of these schools. This is partly because I wish to leave seriously +controverted questions open, and partly from a feeling of compromise, +which I may explain. [*] + + * Of recent years another compromise has been proposed + between the Lamarckians and Weismannists. It would say that + the efforts of the parent and their effect on the position + of the eye--in our case--are not inherited, but might be of + use in sheltering an embryonic variation in the direction of + a displaced eye. + + +First, the plain issue between the Mendelians and the other two +schools--whether the passage from species to species is brought about +by a series of small variations during a long period or by a few large +variations (or "mutations") in a short period--is open to an obvious +compromise. It is quite possible that both views are correct, in +different cases, and quite impossible to find the proportion of each +class of cases. We shall see later that in certain instances where the +conditions of preservation were good we can sometimes trace a perfectly +gradual advance from species to species. Several shellfish have been +traced in this way, and a sea-urchin in the chalk has been followed, +quite gradually, from one end of a genus to the other. It is significant +that the advance of research is multiplying these cases. There is no +reason why we may not assume most of the changes of species we have +yet seen to have occurred in this way. In fact, in some of the lower +branches of the animal world (Radiolaria, Sponges, etc.) there is often +no sharp division of species at all, but a gradual series of living +varieties. + +On the other hand we know many instances of very considerable sudden +changes. The cases quoted by Mendelists generally belong to the plant +world, but instances are not unknown in the animal world. A shrimp +(Artemia) was made to undergo considerable modification, by altering the +proportion of salt in the water in which it was kept. Butterflies have +been made to produce young quite different from their normal young by +subjecting them to abnormal temperature, electric currents, and so on; +and, as I said, the most remarkable effects have been produced on eggs +and embryos by altering the chemical and physical conditions. Rats--I +was informed by the engineer in charge of the refrigerating room on +an Australian liner--very quickly became adapted to the freezing +temperature by developing long hair. All that we have seen of the past +changes in the environment of animals makes it probable that these +larger variations often occur. I would conclude, therefore, that +evolution has proceeded continuously (though by no means universally) +through the ages, but there were at times periods of more acute change +with correspondingly larger changes in the animal and plant worlds. + +In regard to the issue between the Lamarckians and Weismannists--whether +changes acquired by the parent are inherited by the young--recent +experiments again suggest something of a compromise. Weismann says that +the body of the parent is but the case containing the germ-plasm, so +that all modifications of the living parent body perish with it, and do +not affect the germ, which builds the next generation. Certainly, when +we reflect that the 70,000 ova in the human mother's ovary seem to have +been all formed in the first year of her life, it is difficult to see +how modifications of her muscles or nerves can affect them. Thus we +cannot hope to learn anything, either way, by cutting off the tails of +cows, and experiments of that kind. But it is acknowledged that certain +diseases in the blood, which nourishes the germs, may affect them, and +recent experimenters have found that they can reach and affect the germs +in the body by other agencies, and so produce inherited modifications +in the parent. [*] If this claim is sustained and enlarged, it may be +concluded that the greater changes of environment which we find in the +geological chronicle may have had a considerable influence of this kind. + + * See a paper read by Professor Bourne to the Zoological + Section of the British Association, 1910. It must be + understood that when I speak of Weismannism I do not refer + to this whole theory of heredity, which, he acknowledges, + has few supporters. The Lamarckian view is represented in + Britain by Sir W. Turner and Professor Darwin. In other + countries it has a larger proportion of distinguished + supporters. On the whole subject see Professor J. A. + Thomson's "Heredity" (1909), Dewar and Finn's "Making of + Species" (1909--a Mendelian work), and, for essays by the + leaders of each school, "Darwinism and Modern Science" + (1909). + + +The general issue, however, must remain open. The Lamarckian and +Weismannist theories are rival interpretations of past events, and we +shall not find it necessary to press either. When the fish comes to +live on land, for instance, it develops a bony limb out of its fin. The +Lamarckian says that the throwing of the weight of the body on the main +stem of the fin strengthens it, as practice strengthens the boxer's arm, +and the effect is inherited and increased in each generation, until +at last the useless paddle of the fin dies away and the main stem +has become a stout, bony column. Weismann says that the individual +modification, by use in walking, is not inherited, but those young are +favoured which have at birth a variation in the strength of the stem of +the fin. As each of these interpretations is, and must remain, purely +theoretical, we will be content to tell the facts in such cases. But +these brief remarks will enable the reader to understand in what precise +sense the facts we record are open to controversy. + +Let us return to the chronicle of the earth. We had reached the Devonian +age, when large continents, with great inland seas, existed in North +America, north-west Europe, and north Asia, probably connected by a +continent across the North Atlantic and the Arctic region. South America +and South Africa were emerging, and a continent was preparing to stretch +from Brazil, through South Africa and the Antarctic, to Australia and +India. The expanse of land was, with many oscillations, gaining on the +water, and there was much emigration to it from the over-populated seas. +When the fish went on land in the Devonian, it must have found a diet +(insects, etc.) there, and the insects must have been preceded by a +plant population. We have first, therefore, to consider the evolution of +the plant, and see how it increases in form and number until it covers +the earth with the luxuriant forests of the Carboniferous period. + +The plant world, we saw, starts, like the animal world, with a great +kingdom of one-celled microscopic representatives, and the same +principles of development, to a great extent, shape it into a large +variety of forms. Armour-plating has a widespread influence among them. +The graceful Diatom is a morsel of plasm enclosed in a flinty box, often +with a very pretty arrangement of the pores and markings. The Desmid has +a coat of cellulose, and a less graceful coat of cellulose encloses the +Peridinean. Many of these minute plants develop locomotion and a degree +of sensitiveness (Diatoms, Peridinea, Euglena, etc.). Some (Bacteria) +adopt animal diet, and rise in power of movement and sensitiveness until +it is impossible to make any satisfactory distinction between them +and animals. Then the social principle enters. First we have loose +associations of one-celled plants in a common bed, then closer clusters +or many-celled bodies. In some cases (Volvox) the cluster, or the +compound plant, is round and moves briskly in the water, closely +resembling an animal. In most cases, the cells are connected in chains, +and we begin to see the vague outline of the larger plant. + +When we had reached this stage in the development of animal life, we +found great difficulty in imagining how the chief lines of the +higher Invertebrates took their rise from the Archaean chaos of early +many-celled forms. We have an even greater difficulty here, as plant +remains are not preserved at all until the Devonian period. We can only +conclude, from the later facts, that these primitive many-celled plants +branched out in several different directions. One section (at a quite +unknown date) adopted an organic diet, and became the Fungi; and a later +co-operation, or life-partnership, between a Fungus and a one-celled +Alga led to the Lichens. Others remained at the Alga-level, and grew in +great thickets along the sea bottoms, no doubt rivalling or surpassing +the giant sea-weeds, sometimes 400 feet long, off the American coast +to-day. Other lines which start from the level of the primitive +many-celled Algae develop into the Mosses (Bryophyta), Ferns +(Pteridophyta), Horsetails (Equisetalia), and Club-mosses +(Lycopodiales). The mosses, the lowest group, are not preserved in the +rocks; from the other three classes will come the great forests of the +Carboniferous period. + +The early record of plant-life is so poor that it is useless to +speculate when the plant first left the water. We have somewhat obscure +and disputed traces of ferns in the Ordovician, and, as they and the +Horsetails and Club-mosses are well developed in the Devonian, we may +assume that some of the sea-weeds had become adapted to life on land, +and evolved into the early forms of the ferns, at least in the Cambrian +period. From that time they begin to weave a mantle of sombre green over +the exposed land, and to play a most important part in the economy of +nature. + +We saw that at the beginning of the Devonian there was a considerable +rise of the land both in America and Europe, but especially in Europe. +A distant spectator at that time would have observed the rise of a chain +of mountains in Scotland and a general emergence of land north-western +Europe. A continent stretched from Ireland to Scandinavia and North +Russia, while most of the rest of Europe, except large areas of Russia, +France, Germany, and Turkey, was under the sea. Where we now find our +Alps and Pyrenees towering up to the snow-line there were then level +stretches of ocean. Even the north-western continent was scooped +into great inland seas or lagoons, which stretched from Ireland to +Scandinavia, and, as we saw, fostered the development of the fishes. + +As the Devonian period progressed the sea gained on the land, and must +have restricted the growth of vegetation, but as the lake deposits now +preserve the remains of the plants which grow down to their shores, or +are washed into them, we are enabled to restore the complexion of the +landscape. Ferns, generally of a primitive and generalised character, +abound, and include the ferns such as we find in warm countries to-day. +Horsetails and Club-mosses already grow into forest-trees. There are +even seed-bearing ferns, which give promise of the higher plants to +come, but as yet nothing approaching our flower and fruit-bearing trees +has appeared. There is as yet no certain indication of the presence of +Conifers. It is a sombre and monotonous vegetation, unlike any to be +found in any climate to-day. + +We will look more closely into its nature presently. First let us see +how these primitive types of plants come to form the immense forests +which are recorded in our coal-beds. Dr. Russel Wallace has lately +represented these forests, which have, we shall see, had a most +important influence on the development of life, as somewhat mysterious +in their origin. If, however, we again consult the geologist as to the +changes which were taking place in the distribution of land and water, +we find a quite natural explanation. Indeed, there are now distinguished +geologists (e.g. Professor Chamberlin) who doubt if the Coal-forests +were so exceptionally luxuriant as is generally believed. They think +that the vegetation may not have been more dense than in some other +ages, but that there may have been exceptionally good conditions for +preserving the dead trees. We shall see that there were; but, on the +whole, it seems probable that during some hundreds of thousands of +years remarkably dense forests covered enormous stretches of the earth's +surface, from the Arctic to the Antarctic. + +The Devonian period had opened with a rise of the land, but the sea eat +steadily into it once more, and, with some inconsiderable oscillations +of the land, regained its territory. The latter part of the Devonian +and earlier part of the Carboniferous were remarkable for their great +expanses of shallow water and low-lying land. Except the recent chain +of hills in Scotland we know of no mountains. Professor Chamberlin +calculates that 20,000,000, or 30,000,000 square miles of the present +continental surface of Europe and America were covered with a +shallow sea. In the deeper and clearer of these waters the earliest +Carboniferous rocks, of limestone, were deposited. The "millstone grit," +which succeeds the "limestone," indicates shallower water, which is +being rapidly filled up with the debris of the land. In a word, all the +indications suggest the early and middle Carboniferous as an age of vast +swamps, of enormous stretches of land just above or below the sea-level, +and changing repeatedly from one to the other. Further, the climate +was at the time--we will consider the general question of climate +later--moist and warm all over the earth, on account of the great +proportion of sea-surface and the absence of high land (not to speak of +more disputable causes). + +These were ideal conditions for the primitive vegetation, and it spread +over the swamps with great vigour. To say that the Coal-forests +were masses of Ferns, Horsetails, and Club-mosses is a lifeless and +misleading expression. The Club-mosses, or Lycopodiales, were massive +trees, rising sometimes to a height of 120 feet, and probably averaging +about fifty feet in height and one or two feet in diameter. The largest +and most abundant of them, the Sigillaria, sent up a scarred and fluted +trunk to a height of seventy or a hundred feet, without a branch, +and was crowned with a bunch of its long, tapering leaves. The +Lepidodendron, its fellow monarch of the forest, branched at the summit, +and terminated in clusters of its stiff, needle-like leaves, six' or +seven inches long, like enormous exaggerations of the little cones at +the ends of our Club-mosses to-day. The Horsetails, which linger +in their dwarfed descendants by our streams to-day, and at their +exceptional best (in a part of South America) form slender stems +about thirty feet high, were then forest-trees, four to six feet in +circumference and sometimes ninety feet in height. These Calamites +probably rose in dense thickets from the borders of the lakes, their +stumpy leaves spreading in whorls at every joint in their hollow +stems. Another extinct tree, the Cordaites, rivalled the Horsetails and +Club-mosses in height, and its showers of long and extraordinary leaves, +six feet long and six inches in width, pointed to the higher plant +world that was to come. Between these gaunt towering trunks the graceful +tree-ferns spread their canopies at heights of twenty, forty, and even +sixty feet from the ground, and at the base was a dense undergrowth +of ferns and fern-like seed-plants. Mosses may have carpeted the moist +ground, but nothing in the nature of grass or flowers had yet appeared. + +Imagine this dense assemblage of dull, flowerless trees pervaded by a +hot, dank atmosphere, with no change of seasons, with no movement +but the flying of large and primitive insects among the trees and the +stirring of the ferns below by some passing giant salamander, with no +song of bird and no single streak of white or red or blue drawn across +the changeless sombre green, and you have some idea of the character of +the forests that are compressed into our seams of coal. Imagine these +forests spread from Spitzbergen to Australia and even, according to the +south polar expeditions, to the Antarctic, and from the United States to +Europe, to Siberia, and to China, and prolonged during some hundreds +of thousands of years, and you begin to realise that the Carboniferous +period prepared the land for the coming dynasties of animals. Let some +vast and terrible devastation fall upon this luxuriant world, entombing +the great multitude of its imperfect forms and selecting the higher +types for freer life, and the earth will pass into a new age. + +But before we describe the animal inhabitants of these forests, the +part that the forests play in the story of life, and the great cataclysm +which selects the higher types from the myriads of forms which the +warm womb of the earth has poured out, we must at least glance at the +evolutionary position of the Carboniferous plants themselves. Do they +point downward to lower forms, and upward to higher forms, as the theory +of evolution requires? A close inquiry into this would lead us deep into +the problems of the modern botanist, but we may borrow from him a few +of the results of the great labour he has expended on the subject within +the last decade. + +Just as the animal world is primarily divided into Invertebrates and +Vertebrates, the plant world is primarily divided into a lower kingdom +of spore-bearing plants (the Cryptogams) and an upper kingdom of +seed-bearing plants (the Phanerogams). Again, just as the first half of +the earth's story is the age of Invertebrate animals, so it is the age +of Cryptogamous plants. So far evolution was always justified in the +plant record. But there is a third parallel, of much greater interest. +We saw that at one time the evolutionist was puzzled by the clean +division of animals into Invertebrate and Vertebrate, and the sudden +appearance of the backbone in the chronicle: he was just as much puzzled +by the sharp division of our plants into Cryptogams and Phanerogams, and +the sudden appearance of the latter on the earth during the Coal-forest +period. And the issue has been a fresh and recent triumph for evolution. + +Plants are so well preserved in the coal that many years of microscopic +study of the remains, and patient putting-together of the crushed and +scattered fragments, have shown the Carboniferous plants in quite a new +light. Instead of the Coal-forest being a vast assemblage of Cryptogams, +upon which the higher type of the Phanerogam is going suddenly to +descend from the clouds, it is, to a very great extent, a world of +plants that are struggling upward, along many paths, to the higher +level. The characters of the Cryptogam and Phanerogam are so mixed up +in it that, although the special lines of development are difficult to +trace, it is one massive testimony to the evolution of the higher +from the lower. The reproductive bodies of the great Lepidodendra are +sometimes more like seeds than spores, while both the wood and the +leaves of the Sigillaria have features which properly belong to the +Phanerogam. In another group (called the Sphenophyllales) the characters +of these giant Club-mosses are blended with the characters of the giant +Horsetails, and there is ground to think that the three groups have +descended from an earlier common ancestor. + +Further, it is now believed that a large part of what were believed to +be Conifers, suddenly entering from the unknown, are not Conifers at +all, but Cordaites. The Cordaites is a very remarkable combination of +features that are otherwise scattered among the Cryptogams, Cycads, and +Conifers. On the other hand, a very large part of what the geologist had +hitherto called "Ferns" have turned out to be seed-bearing plants, half +Cycad and half Fern. Numbers of specimens of this interesting group--the +Cycadofilices (cycad-ferns) or Pteridosperms (seed-ferns)--have been +beautifully restored by our botanists. [*] They have afforded a new and +very plausible ancestor for the higher trees which come on the scene +toward the close of the Coal-forests, while their fern-like characters +dispose botanists to think that they and the Ferns may be traced to a +common ancestor. This earlier stage is lost in those primitive ages from +which not a single leaf has survived in the rocks. We can only say +that it is probable that the Mosses, Ferns, Lycopods, etc., arose +independently from the primitive level. But the higher and more +important development is now much clearer. The Coal-forest is not simply +a kingdom of Cryptogams. It is a world of aspiring and mingled types. +Let it be subjected to some searching test, some tremendous spell of +adversity, and we shall understand the emergence of the higher types out +of the luxuriant profusion and confusion of forms. + + * See, especially, D. H. Scott, "Studies of Fossil Botany" + (2nd ed., 1908), and "The Evolution of Plants" (1910--small + popular manual). + + + +CHAPTER IX. THE ANIMALS OF THE COAL-FOREST + +We have next to see that when this period of searching adversity +comes--as it will in the next chapter--the animal world also offers a +luxuriant variety of forms from which the higher types may be selected. +This, it need hardly be said, is just what we find in the geological +record. The fruitful, steaming, rich-laden earth now offered tens of +millions of square miles of pasture to vegetal feeders; the waters, on +the other hand, teemed with gigantic sharks, huge Cephalopods, large +scorpion-like and lobster-like animals, and shoals of armour-plated, +hard-toothed fishes. Successive swarms of vegetarians--Worms, Molluscs, +etc.--followed the plant on to the land; and swarms of carnivores +followed the vegetarians, and assumed strange, new forms in adaptation +to land-life. The migration had probably proceeded throughout the +Devonian period, especially from the calmer shores of the inland seas. +By the middle of the Coal-forest period there was a very large and +varied animal population on the land. Like the plants, moreover, +these animals were of an intermediate and advancing nature. No bird or +butterfly yet flits from tree to tree; no mammal rears its young in the +shelter of the ferns. But among the swarming population are many types +that show a beginning of higher organisation, and there is a rich and +varied material provided for the coming selection. + +The monarch of the Carboniferous forest is the Amphibian. In that age +of spreading swamps and "dim, watery woodlands," the stupid and sluggish +Amphibian finds his golden age, and, except perhaps the scorpion, +there is no other land animal competent to dispute his rule. Even the +scorpion, moreover, would not find the Carboniferous Amphibian very +vulnerable. We must not think of the smooth-skinned frogs and toads and +innocent newts which to-day represent the fallen race of the Amphibia. +They were then heavily armoured, powerfully armed, and sometimes as +large as alligators or young crocodiles. It is a characteristic of +advancing life that a new type of organism has its period of triumph, +grows to enormous proportions, and spreads into many different types, +until the next higher stage of life is reached, and it is dethroned by +the new-comers. + +The first indication--apart from certain disputed impressions in the +Devonian--of the land-vertebrate is the footprint of an Amphibian on an +early Carboniferous mud-flat. Hardened by the sun, and then covered with +a fresh deposit when it sank beneath the waters, it remains to-day +to witness the arrival of the five-toed quadruped who was to rule the +earth. As the period proceeds, remains are found in great abundance, +and we see that there must have been a vast and varied population of the +Amphibia on the shores of the Carboniferous lagoons and swamps. There +were at least twenty genera of them living in what is now the island of +Britain, and was then part of the British-Scandinavian continent. Some +of them were short and stumpy creatures, a few inches long, with weak +limbs and short tails, and broad, crescent-shaped heads, their +bodies clothed in the fine scaly armour of their fish-ancestor (the +Branchiosaurs). Some (the Aistopods) were long, snake-like creatures, +with shrunken limbs and bodies drawn out until, in some cases, the +backbone had 150 vertebrae. They seem to have taken to the thickets, in +the growing competition, as the serpents did later, and lost the use of +their limbs, which would be merely an encumbrance in winding among +the roots and branches. Some (the Microsaurs) were agile little +salamander-like organisms, with strong, bony frames and relatively long +and useful legs; they look as if they may even have climbed the trees in +pursuit of snails and insects. A fourth and more formidable sub-order, +the Labyrinthodonts--which take their name from the labyrinthine folds +of the enamel in their strong teeth--were commonly several feet in +length. Some of them attained a length of seven or eight feet, and had +plates of bone over their heads and bellies, while the jaws in their +enormous heads were loaded with their strong, labyrinthine teeth. Life +on land was becoming as eventful and stimulating as life in the waters. + +The general characteristic of these early Amphibia is that they very +clearly retain the marks of their fish ancestry. All of them have tails; +all of them have either scales or (like many of the fishes) plates of +bone protecting the body. In some of the younger specimens the gills can +still be clearly traced, but no doubt they were mainly lung-animals. We +have seen how the fish obtained its lungs, and need add only that this +change in the method of obtaining oxygen for the blood involved certain +further changes of a very important nature. Following the fossil +record, we do not observe the changes which are taking place in the +soft internal organs, but we must not lose sight of them. The heart, for +instance, which began as a simple muscular expansion or distension of +one of the blood-vessels of some primitive worm, then doubled and +became a two-chambered pump in the fish, now develops a partition in +the auricle (upper chamber), so that the aerated blood is to some extent +separated from the venous blood. This approach toward the warm-blooded +type begins in the "mud-fish," and is connected with the development +of the lungs. Corresponding changes take place in the arteries, and we +shall find that this change in structure is of very great importance in +the evolution of the higher types of land-life. The heart of the higher +land-animals, we may add, passes through these stages in its embryonic +development. + +Externally the chief change in the Amphibian is the appearance of +definite legs. The broad paddle of the fin is now useless, and its main +stem is converted into a jointed, bony limb, with a five-toed foot, +spreading into a paddle, at the end. But the legs are still feeble, +sprawling supports, letting the heavy body down almost to the ground. +The Amphibian is an imperfect, but necessary, stage in evolution. It is +an improvement on the Dipneust fish, which now begins to dwindle very +considerably in the geological record, but it is itself doomed to give +way speedily before one of its more advanced descendants, the Reptile. +Probably the giant salamander of modern Japan affords the best +suggestion of the large and primitive salamanders of the Coal-forest, +while the Caecilia--snake-like Amphibia with scaly skins, which live +underground in South America--may not impossibly be degenerate survivors +of the curious Aistopods. + +Our modern tailless Amphibia, frogs and toads, appear much later in the +story of the earth, but they are not without interest here on account of +the remarkable capacity which they show to adapt themselves to different +surroundings. There are frogs, like the tree-frog of Martinique, and +others in regions where water is scarce, which never pass through the +tadpole stage; or, to be quite accurate, they lose the gills and tail in +the egg, as higher land-animals do. On the other hand, there is a modern +Amphibian, the axolotl of Mexico, which retains the gills throughout +life, and never lives on land. Dr. Gadow has shown that the lake in +which it lives is so rich in food that it has little inducement to leave +it for the land. Transferred to a different environment, it may pass to +the land, and lose its gills. These adaptations help us to understand +the rich variety of Amphibian forms that appeared in the changing +conditions of the Carboniferous world. + +When we think of the diet of the Amphibia we are reminded of the other +prominent representatives of land life at the time. Snails, spiders, and +myriapods crept over the ground or along the stalks of the trees, and a +vast population of insects filled the air. We find a few stray wings in +the Silurian, and a large number of wings and fragments in the Devonian, +but it is in the Coal-forest that we find the first great expansion of +insect life, with a considerable development of myriapods, spiders, and +scorpions. Food was enormously abundant, and the insect at least had no +rival in the air, for neither bird nor flying reptile had yet appeared. +Hence we find the same generous growth as amongst the Amphibia. +Large primitive "may-flies" had wings four or five inches long; great +locust-like creatures had fat bodies sometimes twenty inches in length, +and soared on wings of remarkable breadth, or crawled on their six long, +sprawling legs. More than a thousand species of insects, and nearly +a hundred species of spiders and fifty of myriapods, are found in the +remains of the Coal-forests. + +From the evolutionary point of view these new classes are as obscure in +their origin, yet as manifestly undergoing evolution when they do +fully appear, as the earlier classes we have considered. All are of a +primitive and generalised character; that is to say, characters +which are to-day distributed among widely different groups were then +concentrated and mingled in one common ancestor, out of which the later +groups will develop. All belong to the lowest orders of their class. No +Hymenopters (ants, bees, and wasps) or Coleopters (beetles) are found +in the Coal-forest; and it will be many millions of years before the +graceful butterfly enlivens the landscapes of the earth. The early +insects nearly all belong to the lower orders of the Orthopters +(cockroaches, crickets, locusts, etc.) and Neuropters (dragon-flies, +may-flies, etc.). A few traces of Hemipters (now mainly represented by +the degenerate bugs) are found, but nine-tenths of the Carboniferous +insects belong to the lowest orders of their class, the Orthopters and +Neuropters. In fact, they are such primitive and generalised insects, +and so frequently mingle the characteristics of the two orders, that +one of the highest authorities, Scudder, groups them in a special +and extinct order, the Palmodictyoptera; though this view is not now +generally adopted. We shall find the higher orders of insects making +their appearance in succession as the story proceeds. + +Thus far, then, the insects of the Coal-forest are in entire harmony +with the principle of evolution, but when we try to trace their origin +and earlier relations our task is beset with difficulties. It goes +without saying that such delicate frames as those of the earlier insects +had very little chance of being preserved in the rocks until the special +conditions of the forest-age set in. We are, therefore, quite prepared +to hear that the geologist cannot give us the slenderest information. +He finds the wing of what he calls "the primitive bug" (Protocimex), +an Hemipterous insect, in the later Ordovician, and the wing of a +"primitive cockroach" (Palaeoblattina) in the Silurian. From these we +can merely conclude that insects were already numerous and varied. But +we have already, in similar difficulties, received assistance from the +science of zoology, and we now obtain from that science a most important +clue to the evolution of the insect. + +In South America, South Africa, and Australasia, which were at one +time connected by a great southern continent, we find a little +caterpillar-like creature which the zoologist regards with profound +interest. It is so curious that he has been obliged to create a special +class for it alone--a distinction which will be appreciated when I +mention that the neighbouring class of the insects contains more than a +quarter of a million living species. This valuable little animal, with +its tiny head, round, elongated body, and many pairs of caterpillar-like +legs, was until a few decades ago regarded as an Annelid (like the +earth-worm). It has, in point of fact, the peculiar kidney-structures +(nephridia) and other features of the Annelid, but a closer study +discovered in it a character that separated it far from any worm-group. +It was found to breathe the air by means of tracheae (little tubes +running inward from the surface of the body), as the myriapods, spiders, +and insects do. It was, in other words, "a kind of half-way animal +between the Arthropods and the Annelids" ("Cambridge Natural History," +iv, p. 5), a surviving kink in the lost chain of the ancestry of the +insect. Through millions of years it has preserved a primitive frame +that really belongs to the Cambrian, if not an earlier, age. It is one +of the most instructive "living fossils" in the museum of nature. + +Peripatus, as the little animal is called, points very clearly to an +Annelid ancestor of all the Tracheates (the myriapods, spiders, and +insects), or all the animals that breathe by means of trachere. To +understand its significance we must glance once more at an early chapter +in the story of life. We saw that a vast and varied wormlike population +must have filled the Archaean ocean, and that all the higher lines of +animal development start from one or other point in this broad kingdom. +The Annelids, in which the body consists of a long series of connected +rings or segments, as in the earth-worm, are one of the highest groups +of these worm-like creatures, and some branch of them developed a pair +of feet (as in the caterpillar) on each segment of the body and a +tough, chitinous coat. Thus arose the early Arthropods, on tough-coated, +jointed, articulated animals. Some of these remained in the water, +breathing by means of gills, and became the Crustacea. Some, +however, migrated to the land and developed what we may almost call +"lungs"--little tubes entering the body at the skin and branching +internally, to bring the air into contact with the blood, the tracheae. + +In Peripatus we have a strange survivor of these primitive +Annelid-Tracheates of many million years ago. The simple nature of its +breathing apparatus suggests that the trachere were developed out of +glands in the skin; just as the fish, when it came on land, probably +developed lungs from its swimming bladders. The primitive Tracheates, +delivered from the increasing carnivores of the waters, grew into +a large and varied family, as all such new types do in favourable +surroundings. From them in the course of time were evolved the three +great classes of the Myriapods (millipedes and centipedes), the +Arachnids (scorpions, spiders, and mites), and the Insects. I will +not enter into the much-disputed and Obscure question of their nearer +relationship. Some derive the Insects from the Myriapods, some the +Myriapods from the Insects, and some think they evolved independently; +while the rise of the spiders and scorpions is even more obscure. + +But how can we see any trace of an Annelid ancestor in the vastly +different frames of these animals which are said to descend from it? It +is not so difficult as it seems to be at first sight. In the Myriapod +we still have the elongated body and successive pairs of legs. In +the Arachnid the legs are reduced in number and lengthened, while the +various segments of the body are fused in two distinct body-halves, the +thorax and the abdomen. In the Insect we have a similar concentration +of the primitive long body. The abdomen is composed of a large number +(usually nine or ten) of segments which have lost their legs and fused +together. In the thorax three segments are still distinctly traceable, +with three pairs of legs--now long jointed limbs--as in the caterpillar +ancestor; in the Carboniferous insect these three joints in the thorax +are particularly clear. In the head four or five segments are fused +together. Their limbs have been modified into the jaws or other +mouth-appendages, and their separate nerve-centres have combined to form +the large ring of nerve-matter round the gullet which represents the +brain of the insect. + +How, then, do we account for the wings of the insect? Here we can +offer nothing more than speculation, but the speculation is not without +interest. It may be laid down in principle that the flying animal begins +as a leaping animal. The "flying fish" may serve to suggest an early +stage in the development of wings; it is a leaping fish, its extended +fins merely buoying it, like the surfaces of an aeroplane, and so +prolonging its leap away from its pursuer. But the great difficulty is +to imagine any part of the smooth-coated primitive insect, apart from +the limbs (and the wings of the insect are not developed from legs, +like those of the bird), which might have even an initial usefulness in +buoying the body as it leaped. It has been suggested, therefore, that +the primitive insect returned to the water, as the whale and seal did +in the struggle for life of a later period. The fact that the mayfly and +dragon-fly spend their youth in the water is thought to confirm this. +Returning to the water, the primitive insects would develop gills, like +the Crustacea. After a time the stress of life in the water drove them +back to the land, and the gills became useless. But the folds or +scales of the tough coat, which had covered the gills, would remain as +projecting planes, and are thought to have been the rudiment from +which a long period of selection evolved the huge wings of the early +dragon-flies and mayflies. It is generally believed that the wingless +order of insects (Aptera) have not lost, but had never developed, wings, +and that the insects with only one or two pairs all descend from an +ancestor with three pairs. + +The early date of their origin, the delicacy of their structure, and +the peculiar form which their larval development has generally assumed, +combine to obscure the evolution of the insect, and we must be content +for the present with these general indications. The vast unexplored +regions of Africa, South America, and Central Australia, may yet yield +further clues, and the riddle of insect-metamorphosis may some day +betray the secrets which it must hold. For the moment the Carboniferous +insects interest us as a rich material for the operation of a coming +natural selection. On them, as on all other Carboniferous life, a great +trial is about to fall. A very small proportion of them will survive +that trial, and they trill be the better organised to maintain +themselves and rear their young in the new earth. + +The remaining land-life of the Coal-forest is confined to worm-like +organisms whose remains are not preserved, and land-snails which do +not call for further discussion. We may, in conclusion, glance at the +progress of life in the waters. Apart from the appearance of the +great fishes and Crustacea, the Carboniferous period was one of great +stimulation to aquatic life. Constant changes were taking place in the +level and the distribution of land and water. The aspect of our coal +seams to-day, alternating between thick layers of sand and mud, shows +a remarkable oscillation of the land. Many recent authorities have +questioned whether the trees grew on the sites where we find them +to-day, and were not rather washed down into the lagoons and shallow +waters from higher ground. In that case we could not too readily imagine +the forest-clad region sinking below the waves, being buried under the +deposits of the rivers, and then emerging, thousands of years later, to +receive once more the thick mantle of sombre vegetation. Probably +there was less rising and falling of the crust than earlier geologists +imagined. But, as one of the most recent and most critical authorities, +Professor Chamberlin, observes, the comparative purity of the coal, the +fairly uniform thickness of the seams, the bed of clay representing +soil at their base, the frequency with which the stumps are still found +growing upright (as in the remarkable exposed Coal-forest surface in +Glasgow, at the present ground-level), [*] the perfectly preserved fronds +and the general mixture of flora, make it highly probable that the +coal-seam generally marks the actual site of a Coal-forest, and there +were considerable vicissitudes in the distribution of land and water. +Great areas of land repeatedly passed beneath the waters, instead of a +re-elevation of the land, however, we may suppose that the shallow water +was gradually filled with silt and debris from the land, and a fresh +forest grew over it. + + * The civic authorities of Glasgow have wisely exposed and + protected this instructive piece of Coal-forest in one of + their parks. I noticed, however that in the admirable + printed information they supply to the public, they describe + the trees as "at least several hundred thousand years old." + There is no authority in the world who would grant less than + ten million years since the Coal-forest period. + + +These changes are reflected in the progress of marine life, though their +influence is probably less than that of the great carnivorous monsters +which now fill the waters. The heavy Arthrodirans languish and +disappear. The "pavement-toothed" sharks, which at first represent +three-fourths of the Elasmobranchs, dwindle in turn, and in the +formidable spines which develop on them we may see evidence of the great +struggle with the sharp-toothed sharks which are displacing them. The +Ostracoderms die out in the presence of these competitors. The smaller +fishes (generally Crossopterygii) seem to live mainly in the inland and +shore waters, and advance steadily toward the modern types, but none of +our modern bony fishes have yet appeared. + +More evident still is the effect of the new conditions upon the +Crustacea. The Trilobite, once the master of the seas, slowly yields +to the stronger competitors, and the latter part of the Carboniferous +period sees the last genus of Trilobites finally extinguished. The +Eurypterids (large scorpion-like Crustacea, several feet long) suffer +equally, and are represented by a few lingering species. The stress +favours the development of new and more highly organised Crustacea. One +is the Limulus or "king-crab," which seems to be a descendant, or near +relative, of the Trilobite, and has survived until modern times. Others +announce the coming of the long-tailed Crustacea, of the lobster and +shrimp type. They had primitive representatives in the earlier periods, +but seem to have been overshadowed by the Trilobites and Eurypterids. As +these in turn are crushed, the more highly organised Malacostraca take +the lead, and primitive specimens of the shrimp and lobster make their +appearance. + +The Echinoderms are still mainly represented by the sea-lilies. The +rocks which are composed of their remains show that vast areas of the +sea-floor must have been covered with groves of sea-lilies, bending on +their long, flexible stalks and waving their great flower-like arms in +the water to attract food. With them there is now a new experiment in +the stalked Echinoderm, the Blastoid, an armless type; but it seems to +have been a failure. Sea-urchins are now found in the deposits, +and, although their remains are not common, we may conclude that the +star-fishes were scattered over the floor of the sea. + +For the rest we need only observe that progress and rich diversity of +forms characterise the other groups of animals. The Corals now form +great reefs, and the finer Corals are gaining upon the coarser. The +Foraminifers (the chalk-shelled, one-celled animals) begin to form thick +rocks with their dead skeletons; the Radiolaria (the flinty-shelled +microbes) are so abundant that more than twenty genera of them have been +distinguished in Cornwall and Devonshire. The Brachiopods and Molluscs +still abound, but the Molluscs begin to outnumber the lower type of +shell-fish. In the Cephalopods we find an increasing complication of the +structure of the great spiral-shelled types. + +Such is the life of the Carboniferous period. The world rejoices in a +tropical luxuriance. Semi-tropical vegetation is found in Spitzbergen +and the Antarctic, as well as in North Europe, Asia, and America, and in +Australasia; corals and sea-lilies flourish at any part of the earth's +surface. Warm, dank, low-lying lands, bathed by warm oceans and steeped +in their vapours, are the picture suggested--as we shall see more +closely--to the minds of all geologists. In those happy conditions the +primitive life of the earth erupts into an abundance and variety that +are fitly illustrated in the well-preserved vegetation of the forest. +And when the earth has at length flooded its surface with this seething +tide of life; when the air is filled with a thousand species of insects, +and the forest-floor feels the heavy tread of the giant salamander and +the light feet of spiders, scorpions, centipedes, and snails, and the +lagoons and shores teem with animals, the Golden Age begins to close, +and all the semi-tropical luxuriance is banished. A great doom is +pronounced on the swarming life of the Coal-forest period, and from +every hundred species of its animals and plants only two or three will +survive the searching test. + + + +CHAPTER X. THE PERMIAN REVOLUTION + +In an earlier chapter it was stated that the story of life is a story +of gradual and continuous advance, with occasional periods of more rapid +progress. Hitherto it has been, in these pages, a slow and even advance +from one geological age to another, one level of organisation to +another. This, it is true, must not be taken too literally. Many +a period of rapid change is probably contained, and blurred out of +recognition, in that long chronicle of geological events. When a region +sinks slowly below the waves, no matter how insensible the subsidence +may be, there will often come a time of sudden and vast inundations, as +the higher ridges of the coast just dip below the water-level and the +lower interior is flooded. When two invading arms of the sea meet at +last in the interior of the sinking continent, or when a land-barrier +that has for millions of years separated two seas and their populations +is obliterated, we have a similar occurrence of sudden and far-reaching +change. The whole story of the earth is punctuated with small +cataclysms. But we now come to a change so penetrating, so widespread, +and so calamitous that, in spite of its slowness, we may venture to call +it a revolution. + +Indeed, we may say of the remaining story of the earth that it is +characterised by three such revolutions, separated by millions of years, +which are very largely responsible for the appearance of higher types of +life. The facts are very well illustrated by an analogy drawn from the +recent and familiar history of Europe. + +The socio-political conditions of Europe in the eighteenth century, +which were still tainted with feudalism, were changed into the +socio-political conditions of the modern world, partly by a slow and +continuous evolution, but much more by three revolutionary movements. +First there was the great upheaval at the end of the eighteenth century, +the tremors of which were felt in the life of every country in Europe. +Then, although, as Freeman says, no part of Europe ever returned +entirely to its former condition, there was a profound and almost +universal reaction. In the 'thirties and 'forties, differing in +different countries, a second revolutionary disturbance shook Europe. +The reaction after this upheaval was far less severe, and the conditions +were permanently changed to a great extent, but a third revolutionary +movement followed in the next generation, and from that time the +evolution of socio-political conditions has proceeded more evenly. + +The story of life on the earth since the Coal-forest period is +similarly quickened by three revolutions. The first, at the close of +the Carboniferous period, is the subject of this chapter. It is the most +drastic and devastating of the three, but its effect, at least on the +animal world, will be materially checked by a profound and protracted +reaction. At the end of the Chalk period, some millions of years later, +there will be a second revolution, and it will have a far more enduring +and conspicuous result, though it seem less drastic at the time. Yet +there will be something of a reaction after a time, and at length +a third revolution will inaugurate the age of man. If it is clearly +understood that instead of a century we are contemplating a period of at +least ten million years, and instead of a decade of revolution we have +a change spread over a hundred thousand years or more, this analogy will +serve to convey a most important truth. + +The revolutionary agency that broke into the comparatively even +chronicle of life near the close of the Carboniferous period, dethroned +its older types of organisms, and ushered new types to the lordship of +the earth, was cold. The reader will begin to understand why I dwelt +on the aspect of the Coal-forest and its surrounding waters. There +was, then, a warm, moist earth from pole to pole, not even temporarily +chilled and stiffened by a few months of winter, and life spread +luxuriantly in the perpetual semi-tropical summer. Then a spell of cold +so severe and protracted grips the earth that glaciers glitter on the +flanks of Indian and Australian hills, and fields of ice spread over +what are now semitropical regions. In some degree the cold penetrates +the whole earth. The rich forests shrink slowly into thin tracts of +scrubby, poverty-stricken vegetation. The loss of food and the bleak and +exacting conditions of the new earth annihilate thousands of species +of the older organisms, and the more progressive types are moulded into +fitness for the new environment. It is a colossal application of natural +selection, and amongst its results are some of great moment. + +In various recent works one reads that earlier geologists, led astray by +the nebular theory of the earth's origin, probably erred very materially +in regard to the climate of primordial times, and that climate has +varied less than used to be supposed. It must not be thought that, in +speaking of a "Permian revolution," I am ignoring or defying this view +of many distinguished geologists. I am taking careful account of it. +There is no dispute, however, about the fact that the Permian age +witnessed an immense carnage of Carboniferous organisms, and a very +considerable modification of those organisms which survived the +catastrophe, and that the great agency in this annihilation and +transformation was cold. To prevent misunderstanding, nevertheless, it +will be useful to explain the controversy about the climate of the earth +in past ages which divides modern geologists. + +The root of the difference of opinion and the character of the +conflicting parties have already been indicated. It is a protest of the +"Planetesimalists" against the older, and still general, view of the +origin of the earth. As we saw, that view implies that, as the heavier +elements penetrated centreward in the condensing nebula, the gases were +left as a surrounding shell of atmosphere. It was a mixed mass of gases, +chiefly oxygen, hydrogen, nitrogen, and carbon-dioxide (popularly known +as "carbonic acid gas"). When the water-vapour settled as ocean on the +crust, the atmosphere remained a very dense mixture of oxygen, nitrogen, +and carbon-dioxide--to neglect the minor gases. This heavy proportion of +carbon-dioxide would cause the atmosphere to act as a glass-house over +the surface of the earth, as it does still to some extent. Experiment +has shown that an atmosphere containing much vapour and carbon-dioxide +lets the heat-rays pass through when they are accompanied by strong +light, but checks them when they are separated from the light. In other +words, the primitive atmosphere would allow the heat of the sun to +penetrate it, and then, as the ground absorbed the light, would retain +a large proportion of the heat. Hence the semi-tropical nature of the +primitive earth, the moisture, the dense clouds and constant rains that +are usually ascribed to it. This condition lasted until the rocks and +the forests of the Carboniferous age absorbed enormous quantities of +carbon-dioxide, cleared the atmosphere, and prepared an age of chill and +dryness such as we find in the Permian. + +But the planetesimal hypothesis has no room for this enormous percentage +of carbon-dioxide in the primitive atmosphere. Hinc illoe lachrymoe: in +plain English, hence the acute quarrel about primitive climate, and +the close scanning of the geological chronicle for indications that the +earth was not moist and warm until the end of the Carboniferous period. +Once more I do not wish to enfeeble the general soundness of this +account of the evolution of life by relying on any controverted theory, +and we shall find it possible to avoid taking sides. + +I have not referred to the climate of the earth in earlier ages, except +to mention that there are traces of a local "ice-age" about the middle +of the Archaean and the beginning of the Cambrian. As these are many +millions of years removed from each other and from the Carboniferous, +it is possible that they represent earlier periods more or less +corresponding to the Permian. But the early chronicle is so compressed +and so imperfectly studied as yet that it is premature to discuss the +point. It is, moreover, unnecessary because we know of no life on land +in those remote periods, and it is only in connection with life on land +that we are interested in changes of climate here. In other words, as +far as the present study is concerned, we need only regard the climate +of the Devonian and Carboniferous periods. As to this there is no +dispute; nor, in fact, about the climate from the Cambrian to the +Permian. + +As the new school is most brilliantly represented by Professor +Chamberlin, [*] it will be enough to quote him. He says of the Cambrian +that, apart from the glacial indications in its early part, "the +testimony of the fossils, wherever gathered, implies nearly uniform +climatic conditions... throughout all the earth wherever records of the +Cambrian period are preserved" (ii, 273). Of the Ordovician he says: +"All that is known of the life of this era would seem to indicate that +the climate was much more uniform than now throughout the areas where +the strata of the period are known" (ii, 342). In the Silurian we have +"much to suggest uniformity of climate"--in fact, we have just the same +evidence for it--and in the Devonian, when land-plants abound and afford +better evidence, we find the same climatic equality of living things +in the most different latitudes. Finally, "most of the data at hand +indicate that the climate of the Lower Carboniferous was essentially +uniform, and on the whole both genial and moist" (ii, 518). The "data," +we may recall, are in this case enormously abundant, and indicate the +climate of the earth from the Arctic regions to the Antarctic. Another +recent and critical geologist, Professor Walther ("Geschichte der +Erde und des Lebens," 1908), admits that the coal-vegetation shows +a uniformly warm climate from Spitzbergen to Africa. Mr. Drew ("The +Romance of Modern Geology," 1909) says that "nearly all over the globe +the climate was the same--hot, close, moist, muggy" (p. 219). + + * An apology is due here in some measure. The work which I + quote as of Professor Chamberlin ("Geology," 1903) is really + by two authors, Professors Chamberlin and Salisbury. I + merely quote Professor Chamberlin for shortness, and because + the particular ideas I refer to are expounded by him in + separate papers. The work is the finest manual in modern + geological literature. I have used it much, in conjunction + with the latest editions of Geikie, Le Conte, and Lupparent, + and such recent manuals as Walther, De Launay, Suess, etc., + and the geological magazines. + + +The exception which Professor Chamberlin has in mind when he says "most +of the data" is that we find deposits of salt and gypsum in the Silurian +and Lower Carboniferous, and these seem to point to the evaporation +of lakes in a dry climate. He admits that these indicate, at the most, +local areas or periods of dryness in an overwhelmingly moist and warm +earth. It is thus not disputed that the climate of the earth was, during +a period of at least fifteen million years (from the Cambrian to the +Carboniferous), singularly uniform, genial, and moist. During that vast +period there is no evidence whatever that the earth was divided into +climatic zones, or that the year was divided into seasons. To such an +earth was the prolific life of the Coal-forest adapted. + +It is, further, not questioned that the temperature of the earth fell in +the latter part of the Carboniferous age, and that the cold reached +its climax in the Permian. As we turn over the pages of the geological +chronicle, an extraordinary change comes over the vegetation of the +earth. The great Lepidodendra gradually disappear before the close of +the Permian period; the Sigillariae dwindle into a meagre and expiring +race; the giant Horsetails (Calamites) shrink, and betray the adverse +conditions in their thin, impoverished leaves. New, stunted, hardy +trees make their appearance: the Walchia, a tree something like the low +Araucarian conifers in the texture of its wood, and the Voltzia, the +reputed ancestor of the cypresses. Their narrow, stunted leaves suggest +to the imagination the struggle of a handful of pines on a bleak +hill-side. The rich fern-population is laid waste. The seed-ferns +die out, and a new and hardy type of fern, with compact leaves, the +Glossopteris, spreads victoriously over the globe; from Australia it +travels northward to Russia, which it reaches in the early Permian, and +westward, across the southern continent, to South America. A profoundly +destructive influence has fallen on the earth, and converted its rich +green forests, in which the mighty Club-mosses had reared their crowns +above a sea of waving ferns, into severe and poverty-stricken deserts. + +No botanist hesitates to say that it is the coming of a cold, dry +climate that has thus changed the face of the earth. The geologist finds +more direct evidence. In the Werribee Gorge in Victoria I have seen the +marks which Australian geologists have discovered of the ice-age which +put an end to their Coal-forests. From Tasmania to Queensland they +find traces of the rivers and fields of ice which mark the close of the +Carboniferous and beginning of the Permian on the southern continent. +In South Africa similar indications are found from the Cape to the +Transvaal. Stranger still, the geologists of India have discovered +extensive areas of glaciation, belonging to this period, running down +into the actual tropics. And the strangest feature of all is that the +glaciers of India and Australia flowed, not from the temperate zones +toward the tropics, but in the opposite direction. Two great zones of +ice-covered land lay north and south of the equator. The total area was +probably greater than the enormous area covered with ice in Europe and +America during the familiar ice-age of the latest geological period. + +Thus the central idea of this chapter, the destructive inroad of a +colder climate upon the genial Carboniferous world, is an accepted fact. +Critical geologists may suggest that the temperature of the Coal-forest +has been exaggerated, and the temperature of the Permian put too low. +We are not concerned with the dispute. Whatever the exact change of +temperature was, in degrees of the thermometer, it was admittedly +sufficient to transform the face of the earth, and bring a mantle of ice +over millions of square miles of our tropical and subtropical regions. +It remains for us to inquire into the causes of this transformation. + +It at once occurs to us that these facts seem to confirm the prevalent +idea, that the Coal-forests stripped the air of its carbon-dioxide until +the earth shivered in an atmosphere thinner than that of to-day. +On reflection, however, it will be seen that, if this were all that +happened, we might indeed expect to find enormous ice-fields extending +from the poles--which we do not find--but not glaciation in the tropics. +Others may think of astronomical theories, and imagine a shrinking or +clouding of the sun, or a change in the direction of the earth's axis. +But these astronomical theories are now little favoured, either +by astronomers or geologists. Professor Lowell bluntly calls them +"astrocomic" theories. Geologists think them superfluous. There is +another set of facts to be considered in connection with the Permian +cold. + +As we have seen several times, there are periods when, either owing to +the shrinking of the earth or the overloading of the sea-bottoms, or a +combination of the two, the land regains its lost territory and emerges +from the ocean. Mountain chains rise; new continental surfaces are +exposed to the sun and rain. One of the greatest of these upheavals of +the land occurs in the latter half of the Carboniferous and the Permian. +In the middle of the Carboniferous, when Europe is predominantly a flat, +low-lying land, largely submerged, a chain of mountains begins to rise +across its central part. From Brittany to the east of Saxony the great +ridge runs, and by the end of the Carboniferous it becomes a chain of +lofty mountains (of which fragments remain in the Vosges, Black Forest, +and Hartz mountains), dragging Central Europe high above the water, and +throwing the sea back upon Russia to the north and the Mediterranean +region to the south. Then the chain of the Ural Mountains begins to +rise on the Russian frontier. By the beginning of the Permian Europe was +higher above the water than it had ever yet been; there was only a sea +in Russia and a southern sea with narrow arms trailing to the northwest. +The continent of North America also had meantime emerged. The rise of +the Appalachia and Ouachita mountains completes the emergence of the +eastern continent, and throws the sea to the west. The Asiatic continent +also is greatly enlarged, and in the southern hemisphere there is a +further rise, culminating in the Permian, of the continent ("Gondwana +Land") which united South America, South Africa, the Antarctic land, +Australia and New Zealand, with an arm to India. + +In a word, we have here a physical revolution in the face of the earth. +The changes were generally gradual, though they seem in some places to +have been rapid and abrupt (Chamberlin); but in summary they amounted +to a vast revolution in the environment of animals and plants. The +low-lying, swampy, half-submerged continents reared themselves upward +from the sea-level, shook the marshes and lagoons from their face, and +drained the vast areas that had fostered the growth of the Coal-forests. +It is calculated (Chamberlin) that the shallow seas which had covered +twenty or thirty million square miles of our continental surfaces in the +early Carboniferous were reduced to about five million square miles in +the Permian. Geologists believe, in fact, that the area of exposed land +was probably greater than it is now. + +This lifting and draining of so much land would of itself have a +profound influence on life-conditions, and then we must take account of +its indirect influence. The moisture of the earlier period was probably +due in the main to the large proportion of sea-surface and the absence +of high land to condense it. In both respects there is profound +alteration, and the atmosphere must have become very much drier. As this +vapour had been one of the atmosphere's chief elements for retaining +heat at the surface of the earth, the change will involve a great +lowering of temperature. The slanting of the raised land would aid this, +as, in speeding the rivers, it would promote the circulation of water. +Another effect would be to increase the circulation of the atmosphere. +The higher and colder lands would create currents of air that had not +been formed before. Lastly, the ocean currents would be profoundly +modified; but the effect of this is obscure, and may be disregarded for +the moment. + +Here, therefore, we have a massive series of causes and effects, all +connected with the great emergence of the land, which throw a broad +light on the change in the face of the earth. We must add the lessening +of the carbon dioxide in the atmosphere. Quite apart from theories of +the early atmosphere, this process must have had a great influence, +and it is included by Professor Chamberlin among the causes of the +world-wide change. The rocks and forests of the Carboniferous period are +calculated to have absorbed two hundred times as much carbon as there is +in the whole of our atmosphere to-day. Where the carbon came from we may +leave open. The Planetesimalists look for its origin mainly in volcanic +eruptions, but, though there was much volcanic activity in the later +Carboniferous and the Permian, there is little trace of it before the +Coal-forests (after the Cambrian). However that may be, there was a +considerable lessening of the carbon-dioxide of the atmosphere, and +this in turn had most important effects. First, the removal of so much +carbon-dioxide and vapour would be a very effective reason for a general +fall in the temperature of the earth. The heat received from the sun +could now radiate more freely into space. Secondly, it has been shown by +experiment that a richness in carbon-dioxide favours Cryptogamous plants +(though it is injurious to higher plants), and a reduction of it would +therefore be hurtful to the Cryptogams of the Coal-forest. One may +almost put it that, in their greed, they exhausted their store. Thirdly, +it meant a great purification of the atmosphere, and thus a most +important preparation of the earth for higher land animals and plants. + +The reader will begin to think that we have sufficiently "explained" +the Permian revolution. Far from it. Some of its problems are as yet +insoluble. We have given no explanation at all why the ice-sheets, which +we would in a general way be prepared to expect, appear in India and +Australia, instead of farther north and south. Professor Chamberlin, +in a profound study of the period (appendix to vol. ii, "Geology"), +suggests that the new land from New Zealand to Antarctica may have +diverted the currents (sea and air) up the Indian Ocean, and caused a +low atmospheric pressure, much precipitation of moisture, and perpetual +canopies of clouds to shield the ice from the sun. Since the outer polar +regions themselves had been semi-tropical up to that time, it is very +difficult to see how this will account for a freezing temperature in +such latitudes as Australia and India. There does not seem to have been +any ice at the Poles up to that time, or for ages afterwards, so that +currents from the polar regions would be very different from what +they are today. If, on the other hand, we may suppose that the rise of +"Gondwana Land" (from Brazil to India) was attended by the formation +of high mountains in those latitudes, we have the basis, at least, of +a more plausible explanation. Professor Chamberlin rejects this +supposition on the ground that the traces of ice-action are at or near +the sea-level, since we find with them beds containing marine fossils. +But this only shows, at the most, that the terminations of the glaciers +reached the sea. We know nothing of the height of the land from which +they started. + +For our main purpose, however, it is fortunately not necessary to clear +up these mysteries. It is enough for us that the Carboniferous land +rises high above the surface of the ocean over the earth generally. +The shallow seas are drained off its surface; its swamps and lagoons +generally disappear; its waters run in falling rivers to the ocean. The +dense, moist, warm atmosphere that had so long enveloped it is changed +into a thinner mantle of gas, through which, night by night, the +sun-soaked ground can discharge its heat into space. Cold winds blow +over it from the new mountains; probably vast regions of it are swept by +icy blasts from the glaciated lands. As these conditions advance in the +Permian period, the forests wither and shrink. Of the extraordinarily +mixed vegetation which we found in the Coal-forests some few types are +fitted to meet the severe conditions. The seed-bearing trees, the thin, +needle-leafed trees, the trees with stronger texture of the wood, are +slowly singled out by the deepening cold. The golden age of Cryptogams +is over. The age of the Cycad and the Conifers is opening. Survivors +of the old order linger in the warmer valleys, as one may see to-day +tree-ferns lingering in nooks of southern regions while an Antarctic +wind is whistling on the hills above them; but over the broad earth +the luscious pasturage of the Coal-forest has changed into what is +comparatively a cold desert. We must not, of course, imagine too abrupt +a change. The earth had been by no means all swamp in the Carboniferous +age. The new types were even then developing in the cooler and drier +localities. But their hour has come, and there is great devastation +among the lower plant population of the earth. + +It follows at once that there would be, on land, an equal devastation +and a similar selection in the animal world. The vegetarians suffered an +appalling reduction of their food; the carnivores would dwindle in +the same proportion. Both types, again, would suffer from the enormous +changes in their physical surroundings. Vast stretches of marsh, with +teeming populations, were drained, and turned into firm, arid plains or +bleak hill-sides. The area of the Amphibia, for instance, was no less +reduced than their food. The cold, in turn, would exercise a most +formidable selection. Before the Permian period there was not on the +whole earth an animal with a warm-blooded (four-chambered) heart or a +warm coat of fur or feathers; nor was there a single animal that gave +any further care to the eggs it discharged, and left to the natural +warmth of the earth to develop. The extermination of species in the egg +alone must have been enormous. + +It is impossible to convey any just impression of the carnage which this +Permian revolution wrought among the population of the earth. We can but +estimate how many species of animals and plants were exterminated, and +the reader must dimly imagine the myriads of living things that are +comprised in each species. An earlier American geologist, Professor Le +Conte, said that not a single Carboniferous species crossed the line +of the Permian revolution. This has proved to be an exaggeration, but +Professor Chamberlin seems to fall into an exaggeration on the other +side when he says that 300 out of 10,000 species survived. There are +only about 300 species of animals and plants known in the whole of the +Permian rocks (Geikie), and most of these are new. For instance, of the +enormous plant-population of the Coal-forests, comprising many thousands +of species, only fifty species survived unchanged in the Permian. We +may say that, as far as our knowledge goes, of every thirty species +of animals and plants in the Carboniferous period, twenty-eight were +blotted out of the calendar of life for ever; one survived by undergoing +such modifications that it became a new species, and one was found +fit to endure the new conditions for a time. We must leave it to the +imagination to appreciate the total devastation of individuals entailed +in this appalling application of what we call natural selection. + +But what higher types of life issued from the womb of nature after so +long and painful a travail? The annihilation of the unfit is the seamy +side, though the most real side, of natural selection. We ignore it, or +extenuate it, and turn rather to consider the advances in organisation +by which the survivors were enabled to outlive the great chill and +impoverishment. + +Unfortunately, if the Permian period is an age of death, it is not an +age of burials. The fossil population of its cemeteries is very scanty. +Not only is the living population enormously reduced, but the areas that +were accustomed to entomb and preserve organisms--the lake and shore +deposits--are also greatly reduced. The frames of animals and plants now +rot on the dry ground on which they live. Even in the seas, where life +must have been much reduced by the general disturbance of conditions, +the record is poor. Molluscs and Brachiopods and small fishes fill the +list, but are of little instructiveness for us, except that they show a +general advance of species. Among the Cephalopods, it is true, we find +a notable arrival. On the one hand, a single small straight-shelled +Cephalopod lingers for a time with the ancestral form; on the other +hand, a new and formidable competitor appears among the coiled-shell +Cephalopods. It is the first appearance of the famous Ammonite, but +we may defer the description of it until we come to the great age of +Ammonites. + +Of the insects and their fortunes in the great famine we have no direct +knowledge; no insect remains have yet been found in Permian rocks. We +shall, however, find them much advanced in the next period, and must +conclude that the selection acted very effectively among their thousand +Carboniferous species. + +The most interesting outcome of the new conditions is the rise and +spread of the reptiles. No other sign of the times indicates so clearly +the dawn of a new era as the appearance of these primitive, clumsy +reptiles, which now begin to oust the Amphibia. The long reign of +aquatic life is over; the ensign of progress passes to the land animals. +The half-terrestrial, half-aquatic Amphibian deserts the water entirely +(in one or more of its branches), and a new and fateful dynasty is +founded. Although many of the reptiles will return to the water, when +the land sinks once more, the type of the terrestrial quadruped is now +fully evolved, and from its early reptilian form will emerge the lords +of the air and the lords of the land, the birds and the mammals. + +To the uninformed it may seem that no very great advance is made when +the reptile is evolved from the Amphibian. In reality the change implies +a profound modification of the frame and life of the vertebrate. Partly, +we may suppose, on account of the purification of the air, partly on +account of the decrease in water surface, the gills are now entirely +discarded. The young reptile loses them during its embryonic life--as +man and all the mammals and birds do to-day--and issues from the egg a +purely lung-breathing creature. A richer blood now courses through the +arteries, nourishing the brain and nerves as well as the muscles. The +superfluous tissue of the gill-structures is used in the improvement of +the ear and mouth-parts; a process that had begun in the Amphibian. The +body is raised up higher from the ground, on firmer limbs; the ribs and +the shoulder and pelvic bones--the saddles by which the weight of the +body is adjusted between the limbs and the backbone--are strengthened +and improved. Finally, two important organs for the protection and +nurture of the embryo (the amnion and the allantois) make their +appearance for the first time in the reptile. In grade of organisation +the reptile is really nearer to the bird than it is to the salamander. + +Yet these Permian reptiles are so generalised in character and so +primitive in structure that they point back unmistakably to an Amphibian +ancestry. The actual line of descent is obscure. When the reptiles first +appear in the rocks, they are already divided into widely different +groups, and must have been evolved some time before. Probably they +started from some group or groups of the Amphibia in the later +Carboniferous, when, as we saw, the land began to rise considerably. +We have not yet recovered, and may never recover, the region where the +early forms lived, and therefore cannot trace the development in detail. +The fossil archives, we cannot repeat too often, are not a continuous, +but a fragmentary, record of the story of life. The task of the +evolutionist may be compared to the work of tracing the footsteps of a +straying animal across the country. Here and there its traces will be +amply registered on patches of softer ground, but for the most part they +will be entirely lost on the firmer ground. So it is with the fossil +record of life. Only in certain special conditions are the passing forms +buried and preserved. In this case we can say only that the Permian +reptiles fall into two great groups, and that one of these shows +affinities to the small salamander-like Amphibia of the Coal-forest (the +Microsaurs), while the other has affinities to the Labyrinthodonts. + +A closer examination of these early reptiles may be postponed until we +come to speak of the "age of reptiles." We shall see that it is probable +that an even higher type of animal, the mammal, was born in the throes +of the Permian revolution. But enough has been said in vindication of +the phrase which stands at the head of this chapter; and to show how +the great Primary age of terrestrial life came to a close. With its new +inhabitants the earth enters upon a fresh phase, and thousands of its +earlier animals and plants are sealed in their primordial tombs, to +await the day when man will break the seals and put flesh once more on +the petrified bones. + + + +CHAPTER XI. THE MIDDLE AGES OF THE EARTH + +The story of the earth from the beginning of the Cambrian period to the +present day was long ago divided by geologists into four great eras. +The periods we have already covered--the Cambrian, Ordovician, Silurian, +Devonian, Carboniferous, and Permian--form the Primary or Palaeozoic +Era, to which the earlier Archaean rocks were prefixed as a barren +and less interesting introduction. The stretch of time on which we now +enter, at the close of the Permian, is the Secondary or Mesozoic Era. +It will be closed by a fresh upheaval of the earth and disturbance of +life-conditions in the Chalk period, and followed by a Tertiary Era, in +which the earth will approach its modern aspect. At its close there will +be another series of upheavals, culminating in a great Ice-age, and the +remaining stretch of the earth's story, in which we live, will form the +Quaternary Era. + +In point of duration these four eras differ enormously from each other. +If the first be conceived as comprising sixteen million years--a very +moderate estimate--the second will be found to cover less than eight +million years, the third less than three million years, and the fourth, +the Age of Man, much less than one million years; while the Archaean +Age was probably as long as all these put together. But the division +is rather based on certain gaps, or "unconformities," in the geological +record; and, although the breaches are now partially filled, we saw that +they correspond to certain profound and revolutionary disturbances in +the face of the earth. We retain them, therefore, as convenient and +logical divisions of the biological as well as the geological chronicle, +and, instead of passing from one geological period to another, we may, +for the rest of the story, take these three eras as wholes, and devote +a few chapters to the chief advances made by living things in each era. +The Mesozoic Era will be a protracted reaction between two revolutions: +a period of low-lying land, great sea-invasions, and genial climate, +between two upheavals of the earth. The Tertiary Era will represent a +less sharply defined depression, with genial climate and luxuriant life, +between two such upheavals. + +The Mesozaic ("middle life") Era may very fitly be described as the +Middle Ages of life on the earth. It by no means occupies a central +position in the chronicle of life from the point of view of time or +antiquity, just as the Middle Ages of Europe are by no means the centre +of the chronicle of mankind, but its types of animals and plants are +singularly transitional between the extinct ancient and the actual +modern types. Life has been lifted to a higher level by the Permian +revolution. Then, for some millions of years, the sterner process of +selection relaxes, the warm bosom of the earth swarms again with a +teeming and varied population, and a rich material is provided for the +next great application of drastic selective agencies. To a poet it might +seem that nature indulges each succeeding and imperfect type of living +thing with a golden age before it is dismissed to make place for the +higher. + +The Mesozoic opens in the middle of the great revolution described in +the last chapter. Its first section, the Triassic period, is at first a +mere continuation of the Permian. A few hundred species of animals and +hardy plants are scattered over a relatively bleak and inhospitable +globe. Then the land begins to sink once more. The seas spread in great +arms over the revelled continents, the plant world rejoices in the +increasing warmth and moisture, and the animals increase in number and +variety. We pass into the Jurassic period under conditions of great +geniality. Warm seas are found as far north and south as our present +polar regions, and the low-lying fertile lands are covered again with +rich, if less gigantic, forests, in which hordes of stupendous animals +find ample nourishment. The mammal and the bird are already on the +stage, but their warm coats and warm blood offer no advantage in that +perennial summer, and they await in obscurity the end of the golden age +of the reptiles. At the end of the Jurassic the land begins to rise once +more. The warm, shallow seas drain off into the deep oceans, and the +moist, swampy lands are dried. The emergence continues throughout the +Cretaceous (Chalk) period. Chains of vast mountains rise slowly into the +air in many parts of the earth, and a new and comparatively rapid +change in the vegetation--comparable to that at the close of the +Carboniferous--announces the second great revolution. The Mesozoic +closes with the dismissal of the great reptiles and the plants on which +they fed, and the earth is prepared for its new monarchs, the flowering +plants, the birds, and the mammals. + +How far this repeated levelling of the land after its repeated upheavals +is due to a real sinking of the crust we cannot as yet determine. The +geologist of our time is disposed to restrict these mysterious rises +and falls of the crust as much as possible. A much more obvious and +intelligible agency has to be considered. The vast upheaval of nearly +all parts of the land during the Permian period would naturally lead to +a far more vigorous scouring of its surface by the rains and rivers. The +higher the land, the more effectively it would be worn down. The cooler +summits would condense the moisture, and the rains would sweep more +energetically down the slopes of the elevated continents. There would +thus be a natural process of levelling as long as the land stood out +high above the water-line, but it seems probable that there was also +a real sinking of the crust. Such subsidences have been known within +historic times. + +By the end of the Triassic--a period of at least two million years--the +sea had reconquered a vast proportion of the territory wrested from it +in the Permian revolution. Most of Europe, west of a line drawn from the +tip of Norway to the Black Sea, was under water--generally open sea +in the south and centre, and inland seas or lagoons in the west. The +invasion of the sea continued, and reached its climax, in the Jurassic +period. The greater part of Europe was converted into an archipelago. +A small continent stood out in the Baltic region. Large areas remained +above the sea-level in Austria, Germany, and France. Ireland, Wales, +and much of Scotland were intact, and it is probable that a land bridge +still connected the west of Europe with the east of America. Europe +generally was a large cluster of islands and ridges, of various sizes, +in a semi-tropical sea. Southern Asia was similarly revelled, and it +is probable that the seas stretched, with little interruption, from the +west of Europe to the Pacific. The southern continent had deep wedges +of the sea driven into it. India, New Zealand, and Australia were +successively detached from it, and by the end of the Mesozoic it was +much as we find it to-day. The Arctic continent (north of Europe) was +flooded, and there was a great interior sea in the western part of the +North American continent. + +This summary account of the levelling process which went on during the +Triassic and Jurassic will prepare us to expect a return of warm climate +and luxurious life, and this the record abundantly evinces. The enormous +expansion of the sea--a great authority, Neumayr, believes that it was +the greatest extension of the sea that is known in geology--and lowering +of the land would of itself tend to produce this condition, and it +may be that the very considerable volcanic activity, of which we find +evidence in the Permian and Triassic, had discharged great volumes of +carbon-dioxide into the atmosphere. + +Whatever the causes were, the earth has returned to paradisiacal +conditions. The vast ice-fields have gone, the scanty and scrubby +vegetation is replaced by luscious forests of cycads, conifers, and +ferns, and warmth-loving animals penetrate to what are now the Arctic +and Antarctic regions. Greenland and Spitzbergen are fragments of a +continent that then bore a luxuriant growth of ferns and cycads, and +housed large reptiles that could not now live thousands of miles +south of it. England, and a large part of Europe, was a tranquil blue +coral-ocean, the fringes of its islands girt with reefs such as we +find now only three thousand miles further south, with vast shoals +of Ammonites, sometimes of gigantic size, preying upon its living +population or evading its monstrous sharks; while the sunlit lands were +covered with graceful, palmlike cycads and early yews and pines and +cypresses, and quaint forms of reptiles throve on the warm earth or in +the ample swamps, or rushed on outstretched wings through the purer air. + +It was an evergreen world, a world, apparently, of perpetual summer. +No trace is found until the next period of an alternation of summer and +winter--no trees that shed their leaves annually, or show annual rings +of growth in the wood--and there is little trace of zones of climate as +yet. It is true that the sensitive Ammonites differ in the northern and +the southern latitudes, but, as Professor Chamberlin says, it is not +clear that the difference points to a diversity of climate. We may +conclude that the absence of corals higher than the north of England +implies a more temperate climate further north, but what Sir A. Geikie +calls (with slight exaggeration) "the almost tropical aspect" of +Greenland warns us to be cautious. The climate of the mid-Jurassic was +very much warmer and more uniform than the climate of the earth to-day. +It was an age of great vital expansion. And into this luxuriant world we +shall presently find a fresh period of elevation, disturbance, and cold +breaking with momentous evolutionary results. Meantime, we may take a +closer look at these interesting inhabitants of the Middle Ages of the +earth, before they pass away or are driven, in shrunken regiments, into +the shelter of the narrowing tropics. + +The principal change in the aspect of the earth, as the cold, arid +plains and slopes of the Triassic slowly yield the moist and warm +ow-lying lands of the Jurassic, to consists in the character of the +vegetation. It is wholly intermediate in its forms between that of the +primitive forests and that of the modern world. The great Cryptogams of +the Carboniferous world--the giant Club-mosses and their kindred--have +been slain by the long period of cold and drought. Smaller Horsetails +(sometimes of a great size, but generally of the modern type) and +Club-mosses remain, but are not a conspicuous feature in the landscape. +On the other hand, there is as yet--apart from the Conifers--no trace of +the familiar trees and flowers and grasses of the later world. The vast +majority of the plants are of the cycad type. These--now confined to +tropical and subtropical regions--with the surviving ferns, the new +Conifers, and certain trees of the ginkgo type, form the characteristic +Mesozoic vegetation. + +A few words in the language of the modern botanist will show how this +vegetation harmonises with the story of evolution. Plants are broadly +divided into the lower kingdom of the Cryptogams (spore-bearing) and the +upper kingdom of the Phanerogams (seed-bearing). As we saw, the Primary +Era was predominantly the age of Cryptogams; the later periods witness +the rise and supremacy of the Phanerogams. But these in turn are broadly +divided into a less advanced group, the Gymnosperms, and a more advanced +group, the Angiosperms or flowering plants. And, just as the Primary Era +is the age of Cryptogams, the Secondary is the age of Gymnosperms, and +the Tertiary (and present) is the age of Angiosperms. Of about 180,000 +species of plants in nature to-day more than 100,000 are Angiosperms; +yet up to the end of the Jurassic not a single true Angiosperm is found +in the geological record. + +This is a broad manifestation of evolution, but it is not quite an +accurate statement, and its inexactness still more strongly confirms the +theory of evolution. Though the Primary Era was predominantly the age of +Cryptogams, we saw that a very large number of seed-bearing plants, with +very mixed characters, appeared before its close. It thus prepares the +way for the cycads and conifers and ginkgoes of the Mesozoic, which +we may conceive as evolved from one or other branch of the mixed +Carboniferous vegetation. We next find that the Mesozoic is by no means +purely an age of Gymnosperms. I do not mean merely that the Angiosperms +appear in force before its close, and were probably evolved much +earlier. The fact is that the Gymnosperms of the Mesozoic are often of +a curiously mixed character, and well illustrate the transition to the +Angiosperms, though they may not be their actual ancestors. This will be +clearer if we glance in succession at the various types of plant which +adorned and enriched the Jurassic world. + +The European or American landscape--indeed, the aspect of the earth +generally, for there are no pronounced zones of climate--is still +utterly different from any that we know to-day. No grass carpets the +plains; none of the flowers or trees with which we are familiar, except +conifers, are found in any region. Ferns grow in great abundance, +and have now reached many of the forms with which we are acquainted. +Thickets of bracken spread over the plains; clumps of Royal ferns and +Hartstongues spring up in moister parts. The trees are conifers, cycads, +and trees akin to the ginkgo, or Maidenhair Tree, of modern Japan. +Cypresses, yews, firs, and araucarias (the Monkey Puzzle group) grow +everywhere, though the species are more primitive than those of today. +The broad, fan-like leaves and plum-like fruit of the ginkgoales, of +which the temple-gardens of Japan have religiously preserved a solitary +descendant, are found in the most distant regions. But the most frequent +and characteristic tree of the Jurassic landscape is the cycad. + +The cycads--the botanist would say Cycadophyta or Cycadales, to mark +them off from the cycads of modern times--formed a third of the whole +Jurassic vegetation, while to-day they number only about a hundred +species in 180,000, and are confined to warm latitudes. All over +the earth, from the Arctic to the Antarctic, their palm-like foliage +showered from the top of their generally short stems in the Jurassic. +But the most interesting point about them is that a very large branch of +them (the Bennettiteae) went far beyond the modern Gymnosperm in their +flowers and fruit, and approached the Angiosperms. Their fructifications +"rivalled the largest flowers of the present day in structure and +modelling" (Scott), and possibly already gave spots of sober colour to +the monotonous primitive landscape. On the other hand, they approached +the ferns so much more closely than modern cycads do that it is often +impossible to say whether Jurassic remains must be classed as ferns or +cycads. + +We have here, therefore, a most interesting evolutionary group. The +botanist finds even more difficulty than the zoologist in drawing up the +pedigrees of his plants, but the general features of the larger groups +which he finds in succession in the chronicle of the earth point very +decisively to evolution. The seed-bearing ferns of the Coal-forest point +upward to the later stage, and downward to a common origin with the +ordinary spore-bearing ferns. Some of them are "altogether of a +cycadean type" (Scott) in respect of the seed. On the other hand, the +Bennettiteae of the Jurassic have the mixed characters of ferns, cycads, +and flowering plants, and thus, in their turn, point downward to a lower +ancestry and upward to the next great stage in plant-development. It +is not suggested that the seed-ferns we know evolved into the cycads we +know, and these in turn into our flowering plants. It is enough for the +student of evolution to see in them so many stages in the evolution of +plants up to the Angiosperm level. The gaps between the various groups +are less rigid than scientific men used to think. + +Taller than the cycads, firmer in the structure of the wood, and +destined to survive in thousands of species when the cycads would be +reduced to a hundred, were the pines and yews and other conifers of the +Jurassic landscape. We saw them first appearing, in the stunted Walchias +and Voltzias, during the severe conditions of the Permian period. Like +the birds and mammals they await the coming of a fresh period of cold +to give them a decided superiority over the cycads. Botanists look +for their ancestors in some form related to the Cordaites of the +Coal-forest. The ginkgo trees seem to be even more closely related to +the Cordaites, and evolved from an early and generalised branch of that +group. The Cordaites, we may recall, more or less united in one tree +the characters of the conifer (in their wood) and the cycad (in their +fruit). + +So much for the evolutionary aspect of the Jurassic vegetation in +itself. Slender as the connecting links are, it points clearly enough +to a selection of higher types during the Permian revolution from +the varied mass of the Carboniferous flora, and it offers in turn a +singularly varied and rich group from which a fresh selection may choose +yet higher types. We turn now to consider the animal population which, +directly or indirectly, fed upon it, and grew with its growth. To the +reptiles, the birds, and the mammals, we must devote special chapters. +Here we may briefly survey the less conspicuous animals of the Mesozoic +Epoch. + +The insects would be one of the chief classes to benefit by the renewed +luxuriance of the vegetation. The Hymenopters (butterflies) have not yet +appeared. They will, naturally, come with the flowers in the next +great phase of organic life. But all the other orders of insects are +represented, and many of our modern genera are fully evolved. The giant +insects of the Coal-forest, with their mixed patriarchal features, have +given place to more definite types. Swarms of dragon-flies, may-flies, +termites (with wings), crickets, and cockroaches, may be gathered from +the preserved remains. The beetles (Coleopters) have come on the scene +in the Triassic, and prospered exceedingly. In some strata three-fourths +of the insects are beetles, and as we find that many of them +are wood-eaters, we are not surprised. Flies (Dipters) and ants +(Hymenopters) also are found, and, although it is useless to expect to +find the intermediate forms of such frail creatures, the record is of +some evolutionary interest. The ants are all winged. Apparently there +is as yet none of the remarkable division of labour which we find in the +ants to-day, and we may trust that some later period of change may throw +light on its origin. + +Just as the growth of the forests--for the Mesozoic vegetation has +formed immense coal-beds in many parts of the world, even in Yorkshire +and Scotland--explains this great development of the insects, they would +in their turn supply a rich diet to the smaller land animals and flying +animals of the time. We shall see this presently. Let us first glance at +the advances among the inhabitants of the seas. + +The most important and stimulating event in the seas is the arrival of +the Ammonite. One branch of the early shell-fish, it will be remembered, +retained the head of its naked ancestor, and lived at the open mouth of +its shell, thus giving birth to the Cephalopods. The first form was +a long, straight, tapering shell, sometimes several feet long. In the +course of time new forms with curved shells appeared, and began to +displace the straight-shelled. Then Cephalopods with close-coiled +shells, like the nautilus, came, and--such a shell being an obvious +advantage--displaced the curved shells. In the Permian, we saw, a new +and more advanced type of the coiled-shell animal, the Ammonite, made +its appearance, and in the Triassic and Jurassic it becomes the ogre or +tyrant of the invertebrate world. Sometimes an inch or less in diameter, +it often attained a width of three feet or more across the shell, at the +aperture of which would be a monstrous and voracious mouth. + +The Ammonites are not merely interesting as extinct monsters of the +earth's Middle Ages, and stimulating terrors of the deep to the animals +on which they fed. They have an especial interest for the evolutionist. +The successive chambers which the animal adds, as it grows, to the +habitation of its youth, leave the earlier chambers intact. By removing +them in succession in the adult form we find an illustration of the +evolution of the elaborate shell of the Jurassic Ammonite. It is an +admirable testimony to the validity of the embryonic law we have often +quoted--that the young animal is apt to reproduce the past stages of +its ancestry--that the order of the building of the shell in the late +Ammonite corresponds to the order we trace in its development in +the geological chronicle. About a thousand species of Ammonites were +developed in the Mesozoic, and none survived the Mesozoic. Like the +Trilobites of the Primary Era, like the contemporary great reptiles +on land, the Ammonites were an abortive growth, enjoying their hour of +supremacy until sterner conditions bade them depart. The pretty +nautilus is the only survivor to-day of the vast Mesozoic population of +coiled-shell Cephalopods. + +A rival to the Ammonite appeared in the Triassic seas, a formidable +forerunner of the cuttle-fish type of Cephalopod. The animal now boldly +discards the protecting and confining shell, or spreads over the outside +of it, and becomes a "shell-fish" with the shell inside. The octopus of +our own time has advanced still further, and become the most powerful of +the invertebrates. The Belemnite, as the Mesozoic cuttle-fish is called, +attained so large a size that the internal bone, or pen (the part +generally preserved), is sometimes two feet in length. The ink-bags of +the Belemnite also are sometimes preserved, and we see how it could balk +a pursuer by darkening the waters. It was a compensating advantage for +the loss of the shell. + +In all the other classes of aquatic animals we find corresponding +advances. In the remaining Molluscs the higher or more effective types +are displacing the older. It is interesting to note that the oyster is +fully developed, and has a very large kindred, in the Mesozoic seas. +Among the Brachiopods the higher sloping-shoulder type displaces the +square-shoulder shells. In the Crustacea the Trilobites and Eurypterids +have entirely disappeared; prawns and lobsters abound, and the earliest +crab makes its appearance in the English Jurassic rocks. This sudden +arrival of a short-tailed Crustacean surprises us less when we learn +that the crab has a long tail in its embryonic form, but the actual +line of its descent is not clear. Among the Echinoderms we find that the +Cystids and Blastoids have gone, and the sea-lilies reach their climax +in beauty and organisation, to dwindle and almost disappear in the last +part of the Mesozoic. One Jurassic sea-lily was found to have 600,000 +distinct ossicles in its petrified frame. The free-moving Echinoderms +are now in the ascendant, the sea-urchins being especially abundant. +The Corals are, as we saw, extremely abundant, and a higher type (the +Hexacoralla) is superseding the earlier and lower (Tetracoralla). + +Finally, we find a continuous and conspicuous advance among the fishes. +At the close of the Triassic and during the Jurassic they seem to +undergo profound and comparatively rapid changes. The reason will, +perhaps, be apparent in the next chapter, when we describe the gigantic +reptiles which feed on them in the lakes and shore-waters. A greater +terror than the shark had appeared in their environment. The Ganoids and +Dipneusts dwindle, and give birth to their few modern representatives. +The sharks with crushing teeth diminish in number, and the sharp-toothed +modern shark attains the supremacy in its class, and evolves into forms +far more terrible than any that we know to-day. Skates and rays of a +more or less modern type, and ancestral gar-pikes and sturgeons, +enter the arena. But the most interesting new departure is the first +appearance, in the Jurassic, of bony-framed fishes (Teleosts). Their +superiority in organisation soon makes itself felt, and they enter upon +the rapid evolution which will, by the next period, give them the first +place in the fish world. + +Over the whole Mesozoic world, therefore, we find advance and the +promise of greater advance. The Permian stress has selected the fittest +types to survive from the older order; the Jurassic luxuriance is +permitting a fresh and varied expansion of life, in preparation for the +next great annihilation of the less fit and selection of the more fit. +Life pauses before another leap. The Mesozoic earth--to apply to it the +phrase which a geologist has given to its opening phase--welcomes the +coming and speeds the parting guest. In the depths of the ocean a new +movement is preparing, but we have yet to study the highest forms of +Mesozoic life before we come to the Cretaceous disturbances. + + + +CHAPTER XII. THE AGE OF REPTILES + +From one point of view the advance of life on the earth seems to proceed +not with the even flow of a river, but in the successive waves of an +oncoming tide. It is true that we have detected a continuous advance +behind all these rising and receding waves, yet their occurrence is a +fact of some interest, and not a little speculation has been expended on +it. When the great procession of life first emerges out of the darkness +of Archaean times, it deploys into a spreading world of strange +Crustaceans, and we have the Age of Trilobites. Later there is the +Age of Fishes, then of Cryptogams and Amphibia, and then of Cycads and +Reptiles, and there will afterwards be an Age of Birds and Mammals, and +finally an Age of Man. But there is no ground for mystic speculation on +this circumstance of a group of organisms fording the earth for a few +million years, and then perishing or dwindling into insignificance. We +shall see that a very plain and substantial process put an end to the +Age of the Cycads, Ammonites, and Reptiles, and we have seen how the +earlier dynasties ended. + +The phrase, however, the Age of Reptiles, is a fitting and true +description of the greater part of the Mesozoic Era, which lies, like +a fertile valley, between the Permian and the Chalk upheavals. From the +bleak heights of the Permian period, or--more probably--from its more +sheltered regions, in which they have lingered with the ferns and +cycads, the reptiles spread out over the earth, as the summer of the +Triassic period advances. In the full warmth and luxuriance of the +Jurassic they become the most singular and powerful army that ever trod +the earth. They include small lizard-like creatures and monsters more +than a hundred feet in length. They swim like whales in the shallow +seas; they shrink into the shell of the giant turtle; they rear +themselves on towering hind limbs, like colossal kangaroos; they even +rise into the air, and fill it with the dragons of the fairy tale. They +spread over the whole earth from Australia to the Arctic circle. Then +the earth seems to grow impatient of their dominance, and they shrink +towards the south, and struggle in a diminished territory. The colossal +monsters and the formidable dragons go the way of all primitive life, +and a ragged regiment of crocodiles, turtles, and serpents in the +tropics, with a swarm of smaller creatures in the fringes of the warm +zone, is all that remains, by the Tertiary Era, of the world-conquering +army of the Mesozoic reptiles. + +They had appeared, as we said, in the Permian period. Probably they +had been developed during the later Carboniferous, since we find +them already branched into three orders, with many sub-orders, in the +Permian. The stimulating and selecting disturbances which culminated in +the Permian revolution had begun in the Carboniferous. Their origin is +not clear, as the intermediate forms between them and the amphibia are +not found. This is not surprising, if we may suppose that some of the +amphibia had, in the growing struggle, pushed inland, or that, as the +land rose and the waters were drained in certain regions, they had +gradually adopted a purely terrestrial life, as some of the frogs have +since done. In the absence of water their frames would not be preserved +and fossilised. We can, therefore, understand the gap in the record +between the amphibia and the reptiles. From their structure we gather +that they sprang from at least two different branches of the amphibia. +Their remains fall into two great groups, which are known as the Diapsid +and the Synapsid reptiles. The former seem to be more closely related to +the Microsauria, or small salamander-like amphibia of the Coal-forest; +the latter are nearer to the Labyrinthodonts. It is not suggested that +these were their actual ancestors, but that they came from the same +early amphibian root. + +We find both these groups, in patriarchal forms, in Europe, North +America, and South Africa during the Permian period. They are usually +moderate in size, but in places they seem to have found good conditions +and prospered. A few years ago a Permian bed in Russia yielded a most +interesting series of remains of Synapsid reptiles. Some of them were +large vegetarian animals, more than twelve feet in length; others were +carnivores with very powerful heads and teeth as formidable as those +of the tiger. Another branch of the same order lived on the southern +continent, Gondwana Land, and has left numerous remains in South Africa. +We shall see that they are connected by many authorities with the origin +of the mammals. [*] The other branch, the Diapsids, are represented +to-day by the curiously primitive lizard of New Zealand, the tuatara +(Sphenodon, or Hatteria), of which I have seen specimens, nearly two +feet in length, that one did not care to approach too closely. The +Diapsids are chiefly interesting, however, as the reputed ancestors of +the colossal reptiles of the Jurassic age and the birds. + + * These Synapsid reptiles are more commonly known as + Pareiasauria or Theromorpha. + + +The purified air of the Permian world favoured the reptiles' being +lung-breathers, but the cold would check their expansion for a time. +The reptile, it is important to remember' usually leaves its eggs to +be hatched by the natural warmth of the ground. But as the cold of the +Permian yielded to a genial climate and rich vegetation in the course of +the Triassic, the reptiles entered upon their memorable development. The +amphibia were now definitely ousted from their position of dominance. +The increase of the waters had at first favoured them, and we find more +than twenty genera, and some very large individuals, of the amphibia +in the Triassic. One of them, the Mastodonsaurus, had a head three feet +long and two feet wide. But the spread of the reptiles checked them, and +they shrank rapidly into the poor and defenceless tribe which we find +them in nature to-day. + +To follow the prolific expansion of the reptiles in the semi-tropical +conditions of the Jurassic age is a task that even the highest +authorities approach with great diffidence. Science is not yet wholly +agreed in the classification of the vast numbers of remains which the +Mesozoic rocks have yielded, and the affinities of the various groups +are very uncertain. We cannot be content, however, merely to throw on +the screen, as it were, a few of the more quaint and monstrous types out +of the teeming Mesozoic population, and describe their proportions and +peculiarities. They fall into natural and intelligible groups or orders, +and their features are closely related to the differing regions of +the Jurassic world. While, therefore, we must abstain from drawing +up settled genealogical trees, we may, as we review in succession the +monsters of the land, the waters, and the air, glance at the most recent +and substantial conjectures of scientific men as to their origin and +connections. + +The Deinosaurs (or "terrible reptiles"), the monarchs of the land and +the swamps, are the central and outstanding family of the Mesozoic +reptiles. As the name implies, this group includes most of the colossal +animals, such as the Diplodocus, which the illustrated magazine has made +familiar to most people. Fortunately the assiduous research of American +geologists and their great skill and patience in restoring the dead +forms enable us to form a very fair picture of this family of medieval +giants and its remarkable ramifications. [*] + + * See, besides the usual authorities, a valuable paper by + Dr. R. S. Lull, "Dinosaurian Distribution" (1910). + + +The Diapsid reptiles of the Permian had evolved a group with horny, +parrot-like beaks, the Rhyncocephalia (or "beak-headed" reptiles), of +which the tuatara of New Zealand is a lingering representative. New +Zealand seems to have been cut off from the southern continent at the +close of the Permian or beginning of the Triassic, and so preserved +for us that very interesting relic of Permian life. From some primitive +level of this group, it is generally believed, the great Deinosaurs +arose. Two different orders seem to have arisen independently, or +diverged rapidly from each other, in different parts of the world. One +group seems to have evolved on the "lost Atlantis," the land between +Western Europe and America, whence they spread westward to America, +eastward over Europe, and southward to the continent which still united +Africa and Australia. We find their remains in all these regions. +Another stock is believed to have arisen in America. + +Both these groups seem to have been more or less biped, rearing +themselves on large and powerful hind limbs, and (in some cases, at +least) probably using their small front limbs to hold or grasp their +food. The first group was carnivorous, the second herbivorous; and, as +the reptiles of the first group had four or five toes on each foot, +they are known as the Theropods (or "beast-footed" ), while those of +the second order, which had three toes, are called the Ornithopods (or +"bird-footed"). Each of them then gave birth to an order of quadrupeds. +In the spreading waters and rich swamps of the later Triassic some of +the Theropods were attracted to return to an amphibious life, and became +the vast, sprawling, ponderous Sauropods, the giants in a world of +giants. On the other hand, a branch of the vegetarian Ornithopods +developed heavy armour, for defence against the carnivores, and +became, under the burden of its weight, the quadrupedal and monstrous +Stegosauria and Ceratopsia. Taking this instructive general view of the +spread of the Deinosaurs as the best interpretation of the material we +have, we may now glance at each of the orders in succession. + +The Theropods varied considerably in size and agility. The Compsognathus +was a small, active, rabbit-like creature, standing about two feet high +on its hind limbs, while the Megalosaurs stretched to a length of +thirty feet, and had huge jaws armed with rows of formidable teeth. The +Ceratosaur, a seventeen-foot-long reptile, had hollow bones, and we find +this combination of lightness and strength in several members of the +group. In many respects the group points more or less significantly +toward the birds. The brain is relatively large, the neck long, and +the fore limbs might be used for grasping, but had apparently ceased to +serve as legs. Many of the Theropods were evidently leaping reptiles, +like colossal kangaroos, twenty or more feet in length when they were +erect. It is the general belief that the bird began its career as a +leaping reptile, and the feathers, or expanded scales, on the front +limbs helped at first to increase the leap. Some recent authorities +hold, however, that the ancestor of the bird was an arboreal reptile. + +To the order of the Sauropods belong most of the monsters whose +discovery has attracted general attention in recent years. Feeding +on vegetal matter in the luscious swamps, and having their vast bulk +lightened by their aquatic life, they soon attained the most formidable +proportions. The admirer of the enormous skeleton of Diplodocus (which +ran to eighty feet) in the British Museum must wonder how even such +massive limbs could sustain the mountain of flesh that must have +covered those bones. It probably did not walk so firmly as the skeleton +suggests, but sprawled in the swamps or swam like a hippopotamus. But +the Diplodocus is neither the largest nor heaviest of its family. The +Brontosaur, though only sixty feet long, probably weighed twenty tons. +We have its footprints in the rocks to-day, each impression measuring +about a square yard. Generally, it is the huge thigh-bones of these +monsters that have survived, and give us an idea of their size. The +largest living elephant has a femur scarcely four feet long, but the +femur of the Atlantosaur measures more than seventy inches, and the +femur of the Brachiosaur more than eighty. Many of these Deinosaurs must +have measured more than a hundred feet from the tip of the snout to the +end of the tail, and stood about thirty feet high from the ground. The +European Sauropods did not, apparently, reach the size of their American +cousins--so early did the inferiority of Europe begin--but our Ceteosaur +seems to have been about fifty feet long and ten feet in height. +Its thigh-bone was sixty-four inches long and twenty-seven inches in +circumference at the shaft. And in this order of reptiles, it must be +remembered, the bones are solid. + +To complete the picture of the Sauropods, we must add that the whole +class is characterised by the extraordinary smallness of the brain. +The twenty-ton Brontosaur had a brain no larger than that of a new-born +human infant. Quite commonly the brain of one of these enormous animals +is no larger than a man's fist. It is true that, as far as the muscular +and sexual labour was concerned, the brain was supplemented by a great +enlargement of the spinal cord in the sacral region (at the top of the +thighs). This inferior "brain" was from ten to twenty times as large as +the brain in the skull. It would, however, be fully occupied with the +movement of the monstrous limbs and tail, and the sex-life, and does +not add in the least to the "mental" power of the Sauropods. They were +stupid, sluggish, unwieldy creatures, swollen parasites upon a luxuriant +vegetation, and we shall easily understand their disappearance at the +end of the Mesozoic Era, when the age of brawn will yield to an age of +brain. + +The next order of the Deinosaurs is that of the biped vegetarians, the +Ornithopods, which gradually became heavily armoured and quadrupedal. +The familiar Iguanodon is the chief representative of this order in +Europe. Walking on its three-toed hind limbs, its head would be +fourteen or fifteen feet from the ground. The front part of its jaws was +toothless and covered with horn. It had, in fact, a kind of beak, and it +also approached the primitive bird in the structure of its pelvis and in +having five toes on its small front limbs. Some of the Ornithopods, such +as the Laosaur, were small (three or four feet in height) and +active, but many of the American specimens attained a great size. The +Camptosaur, which was closely related to the Iguanodon in structure, was +thirty feet from the snout to the end of the tail, and the head +probably stood eighteen feet from the ground. One of the last great +representatives of the group in America, the Trachodon, about thirty +feet in length, had a most extraordinary head. It was about three and +a half feet in length, and had no less than 2000 teeth lining the mouth +cavity. It is conjectured that it fed on vegetation containing a large +proportion of silica. + +In the course of the Jurassic, as we saw, a branch of these biped, +bird-footed vegetarians developed heavy armour, and returned to the +quadrupedal habit. We find them both in Europe and America, and must +suppose that the highway across the North Atlantic still existed. + +The Stegosaur is one of the most singular and most familiar +representatives of the group in the Jurassic. It ran to a length of +thirty feet, and had a row of bony plates, from two to three feet in +height, standing up vertically along the ridge of its back, while its +tail was armed with formidable spikes. The Scleidosaur, an earlier +and smaller (twelve-foot) specimen, also had spines and bony plates to +protect it. The Polacanthus and Ankylosaur developed a most effective +armour-plating over the rear. As we regard their powerful armour, we +seem to see the fierce-toothed Theropods springing from the rear upon +the poor-mouthed vegetarians. The carnivores selected the vegetarians, +and fitted them to survive. Before the end of the Mesozoic, in fact, the +Ornithopods became aggressive as well as armoured. The Triceratops had +not only an enormous skull with a great ridged collar round the neck, +but a sharp beak, a stout horn on the nose, and two large and sharp +horns on the top of the head. We will see something later of the +development of horns. The skulls of members of the Ceratops family +sometimes measured eight feet from the snout to the ridge of the collar. +They were, however, sluggish and stupid monsters, with smaller brains +even than the Sauropods. + +Such, in broad outline, was the singular and powerful family of the +Mesozoic Deinosaurs. Further geological research in all parts of the +world will, no doubt, increase our knowledge of them, until we can fully +understand them as a great family throwing out special branches to meet +the different conditions of the crowded Jurassic age. Even now they +afford a most interesting page in the story of evolution, and their +total disappearance from the face of the earth in the next geological +period will not be unintelligible. We turn from them to the remaining +orders of the Jurassic reptiles. + +In the popular mind, perhaps, the Ichthyosaur and Plesiosaur are the +typical representatives of that extinct race. The two animals, however, +belong to very different branches of the reptile world, and are by no +means the most formidable of the Mesozoic reptiles. Many orders of the +land reptiles sent a branch into the waters in an age which, we saw, was +predominantly one of water-surface. The Ichthyosauria ("fish-reptiles") +and Thalattosauria ("sea-reptiles") invaded the waters at their first +expansion in the later Triassic. The latter groups soon became extinct, +but the former continued for some millions of years, and became +remarkably adapted to marine life, like the whale at a later period. + +The Ichthyosaur of the Jurassic is a remarkably fish-like animal. Its +long tapering frame--sometimes forty feet in length, but generally less +than half that length--ends in a dip of the vertebral column and an +expansion of the flesh into a strong tail-fin. The terminal bones of the +limbs depart more and more from the quadruped type, until at last they +are merely rows of circular bony plates embedded in the broad paddle +into which the limb has been converted. The head is drawn out, sometimes +to a length of five feet, and the long narrow jaws are set with two +formidable rows of teeth; one specimen has about two hundred teeth. In +some genera the teeth degenerate in the course of time, but this +merely indicates a change of diet. One fossilised Ichthyosaur of the +weaker-toothed variety has been found with the remains of two hundred +Belemnites in its stomach. It is a flash of light on the fierce struggle +and carnage which some recent writers have vainly striven to attenuate. +The eyes, again, which may in the larger animals be fifteen inches in +diameter, are protected by a circle of radiating bony plates. In fine, +the discovery of young developed skeletons inside the adult frames has +taught us that the Ichthgosaur had become viviparous, like the mammal. +Cutting its last connection with the land, on which it originated it +ceased to lay eggs, and developed the young within its body. + +The Ichthyosaur came of the reptile group which we have called the +Diapsids. The Plesiosaur seems to belong to the Synapsid branch. In the +earlier Mesozoic we find partially aquatic representatives of the line, +like the Nothosaur, and in the later Plesiosaur the adaptation to a +marine life is complete. The skin has lost its scales, and the front +limbs are developed into powerful paddles, sometimes six feet in length. +The neck is drawn out until, in some specimens, it is found to consist +of seventy-six vertebrae: the longest neck in the animal world. It is +now doubted, however, if the neck was very flexible, and, as the jaws +were imperfectly joined, the common picture of the Plesiosaur darting +its snake-like neck in all directions to seize its prey is probably +wrong. It seems to have lived on small food, and been itself a rich diet +to the larger carnivores. We find it in all the seas of the Mesozoic +world, varying in length from six to forty feet, but it is one of the +sluggish and unwieldy forms that are destined to perish in the coming +crisis. + +The last, and perhaps the most interesting, of the doomed monsters +of the Mesozoic was the Pterosaur, or "flying reptile." It is not +surprising that in the fierce struggle which is reflected in the arms +and armour of the great reptiles, a branch of the family escaped into +the upper region. We have seen that there were leaping reptiles with +hollow bones, and although the intermediate forms are missing, there +is little doubt that the Pterosaur developed from one or more of these +leaping Deinosaurs. As it is at first small, when it appears in the +early Jurassic--it is disputed in the late Triassic--it probably came +from a small and agile Deinosaur, hunted by the carnivores, which relied +on its leaping powers for escape. A flapperlike broadening of the fore +limbs would help to lengthen the leap, and we must suppose that this +membrane increased until the animal could sail through the air, like the +flying-fish, and eventually sustain its weight in the air. The wing +is, of course, not a feathery frame, as in the bird, but a special skin +spreading between the fore limb and the side of the body. In the bat +this skin is supported by four elongated fingers of the hand, but in +the Pterosaur the fifth (or fourth) finger alone--which is enormously +elongated and strengthened--forms its outer frame. It is as if, in +flying experiments, a man were to have a web of silk stretching from his +arm and an extension of his little finger to the side of his body. + +From the small early specimens in the early Jurassic the flying reptiles +grow larger and larger until the time of their extinction in the +stresses of the Chalk upheaval. Small Pterosaurs continue throughout the +period, but from these bat-like creatures we rise until we come to such +dragons as the American Pteranodon, with a stretch of twenty-two feet +between its extended wings and jaws about four feet long. There were +long-tailed Pterosaurs (Ramphorhyncus), sometimes with a rudder-like +expansion of the end of the tail, and short-tailed Pterosaurs +(Pterodactyl), with compact bodies and keeled breasts, like the bird. In +the earlier part of the period they all have the heavy jaws and numerous +teeth of the reptile, with four or five well-developed fingers on the +front limbs. In the course of time they lose the teeth--an advantage +in the distribution of the weight of the body while flying--and develop +horny beaks. In the gradual shaping of the breast-bone and head, also, +they illustrate the evolution of the bird-form. + +But the birds were meantime developing from a quite different stock, +and would replace the Pterosaurs at the first change in the environment. +There is ground for thinking that these flying reptiles were +warm-blooded like the birds. Their hollow bones seem to point to the +effective breathing of a warm-blooded animal, and the great vitality +they would need in flying points toward the same conclusion. Their +brain, too, approached that of the bird, and was much superior to that +of the other reptiles. But they had no warm coats to retain their heat, +no clavicle to give strength to the wing machinery, and, especially in +the later period, they became very weak in the hind limbs (and therefore +weak or slow in starting their flight). The coming selection will +therefore dismiss them from the scene, with the Deinosaurs and +Ammonites, and retain the better organised bird as the lord of the air. + +There remain one or two groups of the Mesozoic reptiles which are still +represented in nature. The turtle-group (Chelonia) makes its appearance +in the Triassic and thrives in the Jurassic. Its members are extinct and +primitive forms of the thick-shelled reptiles, but true turtles, both +of marine and fresh water, abound before the close of the Mesozoic. +The sea-turtles attain an enormous size. Archelon, one of the primitive +types, measured about twelve feet across the shell. Another was thirteen +feet long and fifteen feet from one outstretched flipper to the other. +In the Chalk period they form more than a third of the reptile remains +in some regions. They are extremely interesting in that they show, to +some extent, the evolution of their characteristic shell. In some of the +larger specimens the ribs have not yet entirely coalesced. + +The Crocodilians also appear in the later Triassic, abound in the +Jurassic, and give way before the later types, the true Crocodiles, in +the Cretaceous. They were marine animals with naked skin, a head and +neck something like that of the Ichthyosaur, and paddles like those of +the Plesiosaur. Their back limbs, however, were not much changed after +their adaptation to life in the sea, and it is concluded that they +visited the land to lay their eggs. The Teleosaur was a formidable +narrow-spouted reptile, somewhat resembling the crocodiles of the Ganges +in the external form of the jaws. The modern crocodiles, which replaced +this ancient race of sea-crocodiles, have a great advantage over them +in the fact that their nostrils open into the mouth in its lower depths. +They can therefore close their teeth on their prey under water and +breathe through the nose. + +Snakes are not found until the close of the Mesozoic, and do not figure +in its characteristic reptile population. We will consider them later. +But there was a large group of reptiles in the later Mesozoic seas which +more or less correspond to the legendary idea of a sea-serpent. These +Dolichosaurs ("long reptiles") appear at the beginning of the Chalk +period, and develop into a group, the Mososaurians, which must have +added considerably to the terrors of the shore-waters. Their slender +scale-covered bodies were commonly twenty to thirty feet in length. The +supreme representative of the order, the Mososaur, of which about forty +species are known, was sometimes seventy-five feet long. It had two +pairs of paddles--so that the name of sea-serpent is very imperfectly +applicable--and four rows of formidable teeth on the roof of its mouth. +Like the Deinosaurs and Pterosaurs, the order was doomed to be entirely +extinguished after a brief supremacy in its environment. + +From this short and summary catalogue the reader will be able to form +some conception of the living inhabitants of the Mesozoic world. It is +assuredly the Age of Reptiles. Worms, snails, and spiders were, we may +assume, abundant enough, and a great variety of insects flitted from +tree to tree or sheltered in the fern brakes. But the characteristic +life, in water and on land, was the vast and diversified family of +the reptiles. In the western and the eastern continent, and along the +narrowing bridge that still united them, in the northern hemisphere and +the southern, and along every ridge of land that connected them, these +sluggish but formidable monsters filled the stage. Every conceivable +device in the way of arms and armour, brute strength and means of +escape, seemed to be adopted in their development, as if they were the +final and indestructible outcome of the life-principle. And within a +single geological period the overwhelming majority of them, especially +the larger and more formidable of them, were ruthlessly slain, leaving +not a single descendant on the earth. Let us see what types of animals +were thus preferred to them in the next great application of selective +processes. + + + +CHAPTER XIII. THE BIRD AND THE MAMMAL + +In one of his finest stories, Sur La Pierre Blanche, Anatole France +has imagined a group of Roman patricians discussing the future of their +Empire. The Christians, who are about to rise to power on their ruin, +they dismiss with amiable indifference as one of the little passing +eccentricities of the religious life of their time. They have not the +dimmest prevision, even as the dream of a possibility, that in a century +or two the Empire of Rome will lie in the dust, and the cross will tower +above all its cities from York to Jerusalem. If we might for a moment +endow the animals of the Mesozoic world with AEsopian wisdom, we could +imagine some such discussion taking place between a group of Deinosaur +patricians. They would reflect with pride on the unshakable empire of +the reptiles, and perhaps glance with disdain at two types of animals +which hid in the recesses or fled to the hills of the Jurassic world. +And before another era of the earth's story opened, the reptile +race would be dethroned, and these hunted and despised and feeble +eccentricities of Mesozoic life would become the masters of the globe. + +These two types of organisms were the bird and the mammal. Both existed +in the Jurassic, and the mammals at least had many representatives +in the Triassic. In other words, they existed, with all their higher +organisation, during several million years without attaining power. The +mammals remained, during at least 3,000,000 years, a small and obscure +caste, immensely overshadowed by the small-brained reptiles. The birds, +while making more progress, apparently, than the mammals, were far +outnumbered by the flying reptiles until the last part of the Mesozoic. +Then there was another momentous turn of the wheel of fate, and they +emerged from their obscurity to assume the lordship of the globe. + +In earlier years, when some serious hesitation was felt by many to +accept the new doctrine of evolution, a grave difficulty was found in +the circumstance that new types--not merely new species and new genera, +but new orders and even sub-classes--appeared in the geological record +quite suddenly. Was it not a singular coincidence that in ALL cases the +intermediate organisms between one type and another should have wholly +escaped preservation? The difficulty was generally due to an imperfect +acquaintance with the conditions of the problem. The fossil population +of a period is only that fraction of its living population which +happened to be buried in a certain kind of deposit under water of a +certain depth. We shall read later of insects being preserved in resin +(amber), and we have animals (and even bacteria) preserved in trees from +the Coal-forests. Generally speaking, however, the earth has buried only +a very minute fraction of its land-population. Moreover, only a fraction +of the earth's cemeteries have yet been opened. When we further reflect +that the new type of organism, when it first appears, is a small and +local group, we see what the chances are of our finding specimens of +it in a few scattered pages of a very fragmentary record of the earth's +life. We shall see that we have discovered only about ten skeletons +or fragments of skeletons of the men who lived on the earth before the +Neolithic period; a stretch of some hundreds of thousands of years, +recorded in the upper strata of the earth. + +Whatever serious difficulty there ever was in this scantiness of +intermediate types is amply met by the fact that every fresh decade of +search in the geological tombs brings some to light. We have seen many +instances of this--the seed-bearing ferns and flower-bearing cycads, for +example, found in the last decade--and will see others. But one of the +most remarkable cases of the kind now claims our attention. The bird was +probably evolved in the late Triassic or early Jurassic. It appears in +abundance, divided into several genera, in the Chalk period. Luckily, +two bird-skeletons have been found in the intermediate period, the +Jurassic, and they are of the intermediate type, between the reptile +and the bird, which the theory of evolution would suggest. But for +the fortunate accident of these two birds being embedded in an ancient +Bavarian mud-layer, which happened to be opened, for commercial +purposes, in the second half of the nineteenth century, critics of +evolution--if there still were any in the world of science--might be +repeating to-day that the transition from the reptile to the bird was +unthinkable in theory and unproven in fact. + +The features of the Archaeopteryx ("primitive bird") have been described +so often, and such excellent pictorial restorations of its appearance +may now be seen, that we may deal with it briefly. We have in it a most +instructive combination of the characters of the bird and the reptile. +The feathers alone, the imprint of which is excellently preserved in +the fine limestone, would indicate its bird nature, but other anatomical +distinctions are clearly seen in it. "There is," says Dr. Woodward, "a +typical bird's 'merrythought' between the wings, and the hind leg +is exactly that of a perching bird." In other words, it has the +shoulder-girdle and four-toed foot, as well as the feathers, of a bird. +On the other hand, it has a long tail (instead of a terminal tuft of +feathers as in the bird) consisting of twenty-one vertebrae, with +the feathers springing in pairs from either side; it has biconcave +vertebrae, like the fishes, amphibia, and reptiles; it has teeth in its +jaws; and it has three complete fingers, free and clawed, on its front +limbs. + +As in the living Peripatus, therefore, we have here a very valuable +connecting link between two very different types of organisms. It is +clear that one of the smaller reptiles--the Archaeopteryx is between a +pigeon and a crow in size--of the Triassic period was the ancestor of +the birds. Its most conspicuous distinction was that it developed a +coat of feathers. A more important difference between the bird and the +reptile is that the heart of the bird is completely divided into four +chambers, but, as we saw, this probably occurred also in the other +flying reptiles. It may be said to be almost a condition of the greater +energy of a flying animal. When the heart has four complete chambers, +the carbonised blood from the tissues of the body can be conveyed direct +to the lungs for purification, and the aerated blood taken direct to the +tissues, without any mingling of the two. In the mud-fish and amphibian, +we saw, the heart has two chambers (auricles) above, but one (ventricle) +below, in which the pure and impure blood mingle. In the reptiles a +partition begins to form in the lower chamber. In the turtle it is +so nearly complete that the venous and the arterial blood are fairly +separated; in the crocodile it is quite complete, though the arteries +are imperfectly arranged. Thus the four-chambered heart of the bird and +mammal is not a sudden and inexplicable development. Its advantage is +enormous in a cold climate. The purer supply of blood increases the +combustion in the tissues, and the animal maintains its temperature and +vitality when the surrounding air falls in temperature. It ceases to be +"cold-blooded." + +But the bird secures a further advantage, and here it outstrips the +flying reptile. The naked skin of the Pterosaur would allow the heat to +escape so freely when the atmosphere cooled that a great strain would be +laid on its vitality. A man lessens the demand on his vitality in cold +regions by wearing clothing. The bird somehow obtained clothing, in +the shape of a coat of feathers, and had more vitality to spare for +life-purposes in a falling temperature. The reptile is strictly limited +to one region, the bird can pass from region to region as food becomes +scarce. + +The question of the origin of the feathers can be discussed only from +the speculative point of view, as they are fully developed in the +Archaeopteryx, and there is no approach toward them in any other living +or fossil organism. But a long discussion of the problem has convinced +scientific men that the feathers are evolved from the scales of the +reptile ancestor. The analogy between the shedding of the coat in a +snake and the moulting of a bird is not uninstructive. In both cases the +outer skin or epidermis is shedding an old growth, to be replaced by +a new one. The covering or horny part of the scale and the feather are +alike growths from the epidermis, and the initial stages of the growth +have certain analogies. But beyond this general conviction that the +feather is a development of the scale, we cannot proceed with any +confidence. Nor need we linger in attempting to trace the gradual +modification of the skeleton, owing to the material change in habits. +The horny beak and the reduction of the toes are features we have +already encountered in the reptile, and the modification of the pelvis, +breast-bone, and clavicle are a natural outcome of flight. + +In the Chalk period we find a large number of bird remains, of about +thirty different species, and in some respects they resume the story of +the evolution of the bird. They are widely removed from our modern types +of birds, and still have teeth in the jaws. They are of two leading +types, of which the Ichthyornis and Hesperornis are the standard +specimens. The Ichthyornis was a small, tern-like bird with the power of +flight strongly developed, as we may gather from the frame of its wings +and the keel-shaped structure of its breast-bone. Its legs and feet were +small and slender, and its long, slender jaws had about twenty teeth on +each side at the bottom. No modern bird has teeth; though the fact that +in some modern species we find the teeth appearing in a rudimentary +form is another illustration of the law that animals tend to reproduce +ancestral features in their development. A more reptilian character in +the Ichthyornis group is the fact that, unlike any modern bird, but like +their reptile ancestors, they had biconcave vertebrae. The brain was +relatively poor. We are still dealing with a type intermediate in +some respects between the reptile and the modern bird. The gannets, +cormorants, and pelicans are believed to descend from some branch of +this group. + +The other group of Cretaceous birds, of the Hesperornis type, show an +actual degeneration of the power of flight through adaptation to an +environment in which it was not needed, as happened, later, in the kiwi +of New Zealand, and is happening in the case of the barn-yard fowl. +These birds had become divers. Their wings had shrunk into an abortive +bone, while their powerful legs had been peculiarly fitted for diving. +They stood out at right angles to the body, and seem to have developed +paddles. The whole frame suggests that the bird could neither walk nor +fly, but was an excellent diver and swimmer. Not infrequently as large +as an ostrich (five to six feet high), with teeth set in grooves in +its jaws, and the jaws themselves joined as in the snake, with a great +capacity of bolting its prey, the Hesperornis would become an important +element in the life of the fishes. The wing-fingers have gone, and the +tail is much shortened, but the grooved teeth and loosely jointed jaws +still point back to a reptilian ancestry. + +These are the only remains of bird-life that we find in the Mesozoic +rocks. Admirably as they illustrate the evolution of the bird from the +reptile, they seem to represent a relatively poor development and spread +of one of the most advanced organisms of the time. It must be understood +that, as we shall see, the latter part of the Chalk period does not +belong to the depression, the age of genial climate, which I call the +Middle Ages of the earth, but to the revolutionary period which closes +it. We may say that the bird, for all its advances in organisation, +remains obscure and unprosperous as long as the Age of Reptiles +lasts. It awaits the next massive uplift of the land and lowering of +temperature. + +In an earlier chapter I hinted that the bird and the mammal may have +been the supreme outcomes of the series of disturbances which closed +the Primary Epoch and devastated its primitive population. As far as +the bird is concerned, this may be doubted on the ground that it first +appears in the upper or later Jurassic, and is even then still largely +reptilian in character. We must remember, however, that the elevation +of the land and the cold climate lasted until the second part of the +Triassic, and it is generally agreed that the bird may have been evolved +in the Triassic. Its slow progress after that date is not difficult to +understand. The advantage of a four-chambered heart and warm coat would +be greatly reduced when the climate became warmer. The stimulus to +advance would relax. The change from a coat of scales to a coat of +feathers obviously means adaptation to a low temperature, and there is +nothing to prevent us from locating it in the Triassic, and indeed no +later known period of cold in which to place it. + +It is much clearer that the mammals were a product of the Permian +revolution. They not only abound throughout the Jurassic, in which they +are distributed in more than thirty genera, but they may be traced into +the Triassic itself. Both in North America and Europe we find the +teeth and fragments of the jaws of small animals which are generally +recognised as mammals. We cannot, of course, from a few bones deduce +that there already, in the Triassic, existed an animal with a fully +developed coat of fur and an apparatus, however crude, in the breast for +suckling the young. But these bones so closely resemble the bones of the +lowest mammals of to-day that this seems highly probable. In the latter +part of the long period of cold it seems that some reptile exchanged its +scales for tufts of hair, developed a four-chambered heart, and began +the practice of nourishing the young from its own blood which would give +the mammals so great an ascendancy in a colder world. + +Nor can we complain of any lack of evidence connecting the mammal with a +reptile ancestor. The earliest remains we find are of such a nature that +the highest authorities are still at variance as to whether they should +be classed as reptilian or mammalian. A skull and a fore limb from +the Triassic of South Africa (Tritylodon and Theriodesmus) are in +this predicament. It will be remembered that we divided the primitive +reptiles of the Permian period into two great groups, the Diapsids and +Synapsids (or Theromorphs). The former group have spread into the +great reptiles of the Jurassic; the latter have remained in comparative +obscurity. One branch of these Theromorph reptiles approach the mammals +so closely in the formation of the teeth that they have received the +name "of the Theriodonts", or "beast-toothed" reptiles. Their teeth are, +like those of the mammals, divided into incisors, canines (sometimes +several inches long), and molars; and the molars have in some cases +developed cusps or tubercles. As the earlier remains of mammals which +we find are generally teeth and jaws, the resemblance of the two groups +leads to some confusion in classifying them, but from our point of view +it is not unwelcome. It narrows the supposed gulf between the reptile +and the mammal, and suggests very forcibly the particular branch of the +reptiles to which we may look for the ancestry of the mammals. We cannot +say that these Theriodont reptiles were the ancestors of the mammals. +But we may conclude with some confidence that they bring us near to the +point of origin, and probably had at least a common ancestor with the +mammals. + +The distribution of the Theriodonts suggests a further idea of interest +in regard to the origin of the mammals. It would be improper to press +this view in the present state of our knowledge, yet it offers a +plausible theory of the origin of the mammals. The Theriodonts seem to +have been generally confined to the southern continent, Gondwana Land +(Brazil to Australia), of which an area survives in South Africa. It is +there also that we find the early disputed remains of mammals. Now we +saw that, during the Permian, Gondwana Land was heavily coated with ice, +and it seems natural to suppose that the severe cold which the glacial +fields would give to the whole southern continent was the great agency +in the evolution of the highest type of the animal world. From this +southern land the new-born mammals spread northward and eastward with +great rapidity. Fitted as they were to withstand the rigorous conditions +which held the reptiles and amphibia in check, they seemed destined to +attain at once the domination of the earth. Then, as we saw, the +land was revelled once more until its surface broke into a fresh +semi-tropical luxuriance, and the Deinosaurs advanced to their triumph. +The mammals shrank into a meagre and insignificant population, a +scattered tribe of small insect-eating animals, awaiting a fresh +refrigeration of the globe. + +The remains of these interesting early mammals, restricted, as they +generally are, to jaws and teeth and a few other bones that cannot +in themselves be too confidently distinguished from those of certain +reptiles, may seem insufficient to enable us to form a picture of their +living forms. In this, however, we receive a singular and fortunate +assistance. Some of them are found living in nature to-day, and their +distinctly reptilian features would, even if no fossil remains were in +existence, convince us of the evolution of the mammals. + +The southern continent on which we suppose the mammals to have +originated had its eastern termination in Australia. New Zealand seems +to have been detached early in the Mesozoic, and was never reached by +the mammals. Tasmania was still part of the Australian continent. To +this extreme east of the southern continent the early mammals spread, +and then, during either the Jurassic or the Cretaceous, the sea +completed its inroad, and severed Australia permanently from the rest of +the earth. The obvious result of this was to shelter the primitive life +of Australia from invasion by higher types, especially from the great +carnivorous mammals which would presently develop. Australia became, in +other words, a "protected area," in which primitive types of life were +preserved from destruction, and were at the same time sheltered from +those stimulating agencies which compelled the rest of the world to +advance. "Advance Australia" is the fitting motto of the present human +inhabitants of that promising country; but the standard of progress has +been set up in a land which had remained during millions of years the +Chinese Empire of the living world. Australia is a fragment of the +Middle Ages of the earth, a province fenced round by nature at least +three million years ago and preserving, amongst its many invaluable +types of life, representatives of that primitive mammal population which +we are seeking to understand. + +It is now well known that the Duckbill or Platypus (Ornithorhyncus) +and the Spiny Anteater (Echidna) of Australia and Tasmania--with one +representative of the latter in New Guinea, which seems to have been +still connected--are semi-reptilian survivors of the first animals to +suckle their young. Like the reptiles they lay tough-coated eggs +and have a single outlet for the excreta, and they have a reptilian +arrangement of the bones of the shoulder-girdle; like the mammals, they +have a coat of hair and a four-chambered heart, and they suckle the +young. Even in their mammalian features they are, as the careful +research of Australian zoologists has shown, of a transitional type. +They are warm-blooded, but their temperature is much lower than that +of other mammals, and varies appreciably with the temperature of their +surroundings. [*] Their apparatus for suckling the young is primitive. +There are no teats, and the milk is forced by the mother through simple +channels upon the breast, from which it is licked by the young. The +Anteater develops her eggs in a pouch. They illustrate a very early +stage in the development of a mammal from a reptile; and one is almost +tempted to see in their timorous burrowing habits a reminiscence of the +impotence of the early mammals after their premature appearance in the +Triassic. + + * See Lucas and Le Soulf's Animals of Australia, 1909. + + +The next level of mammal life, the highest level that it attains in +Australia (apart from recent invasions), is the Marsupial. The pouched +animals (kangaroo, wallaby, etc.) are the princes of pre-human life in +Australia, and represent the highest point that life had reached when +that continent was cut off from the rest of the world. A few words on +the real significance of the pouch, from which they derive their name, +will suffice to explain their position in the story of evolution. + +Among the reptiles the task of the mother ends, as a rule, with the +laying of the egg. One or two modern reptiles hatch the eggs, or show +some concern for them, but the characteristic of the reptile is to +discharge its eggs upon the warm earth and trouble no further about its +young. It is a reminiscence of the warm primitive earth. The bird and +mammal, born of the cooling of the earth, exhibit the beginning of +that link between mother and offspring which will prove so important an +element in the higher and later life of the globe. The bird assists the +development of the eggs with the heat of her own body, and feeds the +young. The mammal develops the young within the body, and then feeds +them at the breast. + +But there is a gradual advance in this process. The Duckbill lays its +eggs just like the reptile, but provides a warm nest for them at the +bottom of its burrow. The Anteater develops a temporary pouch in its +body, when it lays an egg, and hatches the egg in it. The Marsupial +retains the egg in its womb until the young is advanced in development, +then transfers the young to the pouch, and forces milk into its mouth +from its breasts. The real reason for this is that the Marsupial falls +far short of the higher mammals in the structure of the womb, and cannot +fully develop its young therein. It has no placenta, or arrangement by +which the blood-vessels of the mother are brought into connection with +the blood-vessels of the foetus, in order to supply it with food until +it is fully developed. The Marsupial, in fact, only rises above the +reptile in hatching the egg within its own body, and then suckling the +young at the breast. + +These primitive mammals help us to reconstruct the mammal life of +the Mesozoic Epoch. The bones that we have are variously described +in geological manuals as the remains of Monotremes, Marsupials, and +Insectivores. Many of them, if not most, were no doubt insect-eating +animals, but there is no ground for supposing that what are technically +known as Insectivores (moles and shrews) existed in the Mesozoic. On +the other hand, the lower jaw of the Marsupial is characterised by a +peculiar hooklike process, and this is commonly found in Mesozoic jaws. +This circumstance, and the witness of Australia, permit us, perhaps, +to regard the Jurassic mammals as predominantly marsupial. It is more +difficult to identify Monotreme remains, but the fact that Monotremes +have survived to this day in Australia, and the resemblance of some +of the Mesozoic teeth to those found for a time in the young Duckbill +justify us in assuming that a part of the Mesozoic mammals correspond to +the modern Monotremes. Not single specimen of any higher, or placental, +mammal has yet been found in the whole Mesozoic Era. + +We must, however, beware of simply transferring to the Mesozoic world +the kinds of Monotremes and Marsupials which we know in nature to-day. +In some of the excellent "restorations" of Mesozoic life which are found +in recent illustrated literature the early mammal is represented with an +external appearance like that of the Duckbill. This is an error, as the +Duckbill has been greatly modified in its extremities and mouth-parts +by its aquatic and burrowing habits. As we have no complete skeletons +of these early mammals we must abstain from picturing their external +appearance. It is enough that the living Monotreme and Marsupial so +finely illustrate the transition from a reptilian to a mammalian form. +There may have been types more primitive than the Duckbill, and others +between the Duckbill and the Marsupial. It seems clear, at least, +that two main branches, the Monotremes and Marsupials, arose from the +primitive mammalian root. Whether either of these became in turn the +parent of the higher mammals we will inquire later. We must first +consider the fresh series of terrestrial disturbances which, like some +gigantic sieve, weeded out the grosser types of organisms, and cleared +the earth for a rapid and remarkable expansion of these primitive birds +and mammals. + +We have attended only to a few prominent characters in tracing the line +of evolution, but it will be understood that an advance in many organs +of the body is implied in these changes. In the lower mammals the +diaphragm, or complete partition between the organs of the breast and +those of the abdomen, is developed. It is not a sudden and mysterious +growth, and its development in the embryo to-day corresponds to the +suggestion of its development which the zoologist gathers from the +animal series. The ear also is now fully developed. How far the fish +has a sense of hearing is not yet fully determined, but the amphibian +certainly has an organ for the perception of waves of sound. Parts of +the discarded gill-arches are gradually transformed into the three bones +of the mammal's internal ear; just as other parts are converted into +mouth cartilages, and as--it is believed--one of the gill clefts is +converted into the Eustachian tube. In the Monotreme and Marsupial the +ear-hole begins to be covered with a shell of cartilage; we have the +beginning of the external ear. The jaws, which are first developed +in the fish, now articulate more perfectly with the skull. Fat-glands +appear in the skin, and it is probably from a group of these that the +milk-glands are developed. The origin of the hairs is somewhat obscure. +They are not thought to be, like the bird's feathers, modifications of +the reptile's scales, but to have been evolved from other structures in +the skin, possibly under the protection of the scales. + +My purpose is, however, rather to indicate the general causes of +the onward advance of life than to study organs in detail--a vast +subject--or construct pedigrees. We therefore pass on to consider the +next great stride that is taken by the advancing life of the earth. +Millions of years of genial climate and rich vegetation have filled +the earth with a prolific and enormously varied population. Over this +population the hand of natural selection is outstretched, as it were, +and we are about to witness another gigantic removal of older types of +life and promotion of those which contain the germs of further advance. +As we have already explained, natural selection is by no means inactive +during these intervening periods of warmth. We have seen the ammonites +and reptiles, and even the birds and mammals, evolve into hundreds +of species during the Jurassic period. The constant evolution of more +effective types of carnivores and their spread into new regions, the +continuous changes in the distribution of land and water, the struggle +for food in a growing population, and a dozen other causes, are ever at +work. But the great and comprehensive changes in the face of the earth +which close the eras of the geologist seem to give a deeper and quicker +stimulus to its population and result in periods of especially rapid +evolution. Such a change now closes the Mesozoic Era, and inaugurates +the age of flowering plants, of birds, and of mammals. + + + +CHAPTER XIV. IN THE DAYS OF THE CHALK + +In accordance with the view of the later story of the earth which was +expressed on an earlier page, we now come to the second of the three +great revolutions which have quickened the pulse of life on the earth. +Many men of science resent the use of the word revolution, and it is +not without some danger. It was once thought that the earth was really +shaken at times by vast and sudden cataclysms, which destroyed its +entire living population, so that new kingdoms of plants and animals +had to be created. But we have interpreted the word revolution in a very +different sense. The series of changes and disturbances to which we give +the name extended over a period of hundreds of thousands of years, +and they were themselves, in some sense, the creators of new types +of organisms. Yet they are periods that stand out peculiarly in +the comparatively even chronicle of the earth. The Permian period +transformed the face of the earth; it lifted the low-lying land into +a massive relief, drew mantles of ice over millions of miles of its +surface, set volcanoes belching out fire and fumes in many parts, +stripped it of its great forests, and slew the overwhelming majority +of its animals. On the scale of geological time it may be called a +revolution. + +It must be confessed that the series of disturbances which close the +Secondary and inaugurate the Tertiary Era cannot so conveniently be +summed up in a single formula. They begin long before the end of the +Mesozoic, and they continue far into the Tertiary, with intervals of +ease and tranquillity. There seems to have been no culminating point in +the series when the uplifted earth shivered in a mantle of ice and +snow. Yet I propose to retain for this period--beginning early in the +Cretaceous (Chalk) period and extending into the Tertiary--the name of +the Cretaceous Revolution. I drew a fanciful parallel between the three +revolutions which have quickened the earth since the sluggish days of +the Coal-forest and the three revolutionary movements which have changed +the life of modern Europe. It will be remembered that, whereas the first +of these European revolutions was a sharp and massive upheaval, +the second consisted in a more scattered and irregular series of +disturbances, spread over the fourth and fifth decades of the nineteenth +century; but they amounted, in effect, to a revolution. + +So it is with the Cretaceous Revolution. In effect it corresponds very +closely to the Permian Revolution. On the physical side it includes a +very considerable rise of the land over the greater part of the globe, +and the formation of lofty chains of mountains; on the botanical side +it means the reduction of the rich Mesozoic flora to a relatively +insignificant population, and the appearance and triumphant spread of +the flowering plants, on the zoological side it witnesses the complete +extinction of the Ammonites, Deinosaurs, and Pterosaurs, an immense +reduction of the reptile world generally, and a victorious expansion of +the higher insects, birds, and mammals; on the climatic side it provides +the first definite evidence of cold zones of the earth and cold seasons +of the year, and seems to represent a long, if irregular, period of +comparative cold. Except, to some extent, the last of these points, +there is no difference of opinion, and therefore, from the evolutionary +point of view, the Cretaceous period merits the title of a revolution. +All these things were done before the Tertiary period opened. + +Let us first consider the fundamental and physical aspect of this +revolution, the upheaval of the land. It began about the close of the +Jurassic period. Western and Central Europe emerged considerably from +the warm Jurassic sea, which lay on it and had converted it into an +archipelago. In North-western America also there was an emergence of +large areas of land, and the Sierra and Cascade ranges of mountains were +formed about the same time. For reasons which will appear later we must +note carefully this rise of land at the very beginning of the Cretaceous +period. + +However, the sea recovered its lost territory, or compensation for it, +and the middle of the Cretaceous period witnessed a very considerable +extension of the waters over America, Europe, and southern Asia. The +thick familiar beds of chalk, which stretch irregularly from Ireland to +the Crimea, and from the south of Sweden to the south of France, plainly +tell of an overlying sea. As is well known, the chalk consists mainly +of the shells or outer frames of minute one-celled creatures +(Thalamophores) which float in the ocean, and form a deep ooze at its +bottom with their discarded skeletons. What depth this ocean must have +been is disputed, and hardly concerns us. It is clear that it must have +taken an enormous period for microscopic shells to form the thick masses +of chalk which cover so much of southern and eastern England. On the +lowest estimates the Cretaceous period, which includes the deposit of +other strata besides chalk, lasted about three million years. And as +people like to have some idea of the time since these things happened, +I may add that, on the lowest estimate (which most geologists would at +least double), it is about three million years since the last stretches +of the chalk-ocean disappeared from the surface of Europe. + +But while our chalk cliffs conjure up a vision of England lying deep--at +least twenty or thirty fathoms deep--below a warm ocean, in which +gigantic Ammonites and Belemnites and sharks ply their deadly trade, +they also remind us of the last phase of the remarkable life of the +earth's Middle Ages. In the latter part of the Cretaceous the land +rises. The chalk ocean of Europe is gradually reduced to a series of +inland seas, separated by masses and ridges of land, and finally to a +series of lakes of brackish water. The masses of the Pyrenees and Alps +begin to rise; though it will not be until a much later date that they +reach anything like their present elevation. In America the change is +even greater. A vast ridge rises along the whole western front of the +continent, lifting and draining it, from Alaska to Cape Horn. It is +the beginning of the Rocky Mountains and the Andes. Even during the +Cretaceous period there had been rich forests of Mesozoic vegetation +covering about a hundred thousand square miles in the Rocky Mountains +region. Europe and America now begin to show their modern contours. + +It is important to notice that this great uprise of the land and the +series of disturbances it entails differ from those which we summed +up in the phrase Permian Revolution. The differences may help us to +understand some of the changes in the living population. The chief +difference is that the disturbances are more local, and not nearly +simultaneous. There is a considerable emergence of land at the end of +the Jurassic, then a fresh expansion of the sea, then a great rise of +mountains at the end of the Cretaceous, and so on. We shall find our +great mountain-masses (the Pyrenees, Alps, Himalaya, etc.) rising at +intervals throughout the whole of the Tertiary Era. However, it suffices +for the moment to observe that in the latter part of the Mesozoic and +early part of the Tertiary there were considerable upheavals of the land +in various regions, and that the Mesozoic Era closed with a very much +larger proportion of dry land, and a much higher relief of the +land, than there had been during the Jurassic period. The series of +disturbances was, says Professor Chamberlin, "greater than any that had +occurred since the close of the Palaeozoic." + +From the previous effect of the Permian upheaval, and from the fact that +the living population is now similarly annihilated or reduced, we should +at once expect to find a fresh change in the climate of the earth. Here, +however, our procedure is not so easy. In the Permian age we had +solid proof in the shape of vast glaciated regions. It is claimed by +continental geologists that certain early Tertiary beds in Bavaria +actually prove a similar, but smaller, glaciation in Europe, but this is +disputed. Other beds may yet be found, but we saw that there was not +a general upheaval, as there had been in the Permian, and it is quite +possible that there were few or no ice-fields. We do not, in fact, know +the causes of the Permian icefields. We are thrown upon the plant +and animal remains, and seem to be in some danger of inferring a cold +climate from the organic remains, and then explaining the new types of +organisms by the cold climate. This, of course, we shall not do. The +difficulty is made greater by the extreme disinclination of many recent +geologists, and some recent botanists who have too easily followed the +geologists, to admit a plain climatic interpretation of the facts. Let +us first see what the facts are. + +In the latter part of the Jurassic we find three different zones of +Ammonites: one in the latitude of the Mediterranean, one in the latitude +of Central Europe, and one further north. Most geologists conclude that +these differences indicate zones of climate (not hitherto indicated), +but it cannot be proved, and we may leave the matter open. At the same +time the warm-loving corals disappear from Europe, with occasional +advances. It is said that they are driven out by the disturbance of the +waters, and, although this would hardly explain why they did not spread +again in the tranquil chalk-ocean, we may again leave the point open. + +In the early part of the Cretaceous, however, the Angiosperms (flowering +plants) suddenly break into the chronicle of the earth, and spread with +great rapidity. They appear abruptly in the east of the North American +continent, in the region of Virginia and Maryland. They are small in +stature and primitive in structure. Some are of generalised forms that +are now unknown; some have leaves approaching those of the oak, willow, +elm, maple, and walnut; some may be definitely described as fig, +sassafras, aralia, myrica, etc. Eastern America, it may be recalled, is +much higher than western until the close of the Cretaceous period. The +Angiosperms do not spread much westward; they appear next in Greenland, +and, before the middle of the Cretaceous, in Portugal. They have +travelled over the North Atlantic continent, or what remains of it. The +process seems very rapid as we write it, but it must be remembered that +the first half of the Cretaceous period means a million or a million and +a half years. + +The cycads, and even the conifers, shrink before the higher type of +tree. The landscape, in Europe and America, begins to wear a modern +aspect. Long before the end of the Cretaceous most of the modern genera +of Angiosperm trees have developed. To the fig and sassafras are now +added the birch, beech, oak, poplar, walnut, willow, ivy, mulberry, +holly, laurel, myrtle, maple, oleander, magnolia, plane, bread-fruit, +and sweet-gum. Most of the American trees of to-day are known. The +sequoias (the giant Californian trees) still represent the conifers in +great abundance, with the eucalyptus and other plants that are now found +only much further south. The ginkgoes struggle on for a time. The cycads +dwindle enormously. Of 700 specimens in one early Cretaceous deposit +only 96 are Angiosperms; of 460 species in a later deposit about 400 are +Angiosperms. They oust the cycads in Europe and America, as the cycads +and conifers had ousted the Cryptogams. The change in the face of the +earth would be remarkable. Instead of the groves of palm-like cycads, +with their large and flower-like fructifications, above which the pines +and firs and cypresses reared their sombre forms, there were now forests +of delicate-leaved maples, beeches, and oaks, bearing nutritious fruit +for the coming race of animals. Grasses also and palms begin in the +Cretaceous; though the grasses would at first be coarse and isolated +tufts. Even flowers, of the lily family (apparently), are still detected +in the crushed and petrified remains. + +We will give some consideration later to the evolution of the +Angiosperms. For the moment it is chiefly important to notice a feature +of them to which the botanist pays less attention. In his technical view +the Angiosperm is distinguished by the structure of its reproductive +apparatus, its flowers, and some recent botanists wonder whether the +key to this expansion of the flowering plants may not be found in a +development of the insect world and of its relation to vegetation. In +point of fact, we have no geological indication of any great development +of the insects until the Tertiary Era, when we shall find them deploying +into a vast army and producing their highest types. In any case, such +a view leaves wholly unexplained the feature of the Angiosperms which +chiefly concerns us. This is that most of them shed the whole of their +leaves periodically, as the winter approaches. No such trees had yet +been known on the earth. All trees hitherto had been evergreen, and we +need a specific and adequate explanation why the earth is now covered, +in the northern region, with forests of trees which show naked boughs +and branches during a part of the year. + +The majority of palaeontologists conclude at once, and quite +confidently, from this rise and spread of the deciduous trees, that a +winter season has at length set in on the earth, and that this new type +of vegetation appears in response to an appreciable lowering of the +climate. The facts, however, are somewhat complex, and we must proceed +with caution. It would seem that any general lowering of the temperature +of the earth ought to betray itself first in Greenland, but the flora +of Greenland remains far "warmer," so to say, than the flora of Central +Europe is to-day. Even toward the close of the Cretaceous its plants +are much the same as those of America or of Central Europe. Its fossil +remains of that time include forty species of ferns, as well as cycads, +ginkgoes, figs, bamboos, and magnolias. Sir A. Geikie ventures to say +that it must then have enjoyed a climate like that of the Cape or of +Australia to-day. Professor Chamberlin finds its flora like that of +"warm temperate" regions, and says that plants which then flourished in +latitude 72 degrees are not now found above latitude 30 degrees. + +There are, however, various reasons to believe that it is unsafe to draw +deductions from the climate of Greenland. There is, it is true, some +exaggeration in the statement that its climate was equivalent to that +of Central Europe. The palms which flourished in Central Europe did not +reach Greenland, and there are differences in the northern Molluscs +and Echinoderms which--like the absence of corals above the north of +England--point to a diversity of temperature. But we have no right to +expect that there would be the same difference in temperature between +Greenland and Central Europe as we find to-day. If the warm current +which is now diverted to Europe across the Atlantic--the Gulf +Stream--had then continued up the coast of America, and flowed along +the coast of the land that united America and Europe, the climatic +conditions would be very different from what they are. There is a more +substantial reason. We saw that during the Mesozoic the Arctic continent +was very largely submerged, and, while Europe and America rise again at +the end of the Cretaceous, we find no rise of the land further north. A +difference of elevation would, in such a world, make a great difference +in temperature and moisture. + +Let us examine the animal record, however, before we come to any +conclusion. The chronicle of the later Cretaceous is a story of +devastation. The reduction of the cyeads is insignificant beside the +reduction or annihilation of the great animals of the Mesozoic world. +The skeletons of the Deinosaurs become fewer and fewer as we ascend the +upper Cretaceous strata. In the uppermost layer (Laramie) we find +traces of a last curious expansion--the group of horned reptiles, of the +Triceratops type, which we described as the last of the great +reptiles. The Ichthyosaurs and Plesiosaurs vanish from the waters. The +"sea-serpents" (Mososaurs) pass away without a survivor. The flying +dragons, large and small, become entirely extinct. Only crocodiles, +lizards, turtle, and snakes cross the threshold of the Tertiary Era. In +one single region of America (Puerco beds) some of the great reptiles +seem to be making a last stand against the advancing enemy in the dawn +of the Tertiary Era, but the exact date of the beds is disputed, and +in any case their fight is soon over. Something has slain the most +formidable race that the earth had yet known, in spite of its marvellous +adaptation to different environments in its innumerable branches. + +We turn to the seas, and find an equal carnage among some of its most +advanced inhabitants. The great cuttlefish-like Belemnites and the whole +race of the Ammonites, large and small, are banished from the earth. The +fall of the Ammonites is particularly interesting, and has inspired +much more or less fantastic speculation. The shells begin to assume such +strange forms that observers speak occasionally of the "convulsions" or +"death-contortions" of the expiring race. Some of the coiled shells take +on a spiral form, like that of a snail's shell. Some uncoil the shell, +and seem to be returning toward the primitive type. A rich eccentricity +of frills and ornamentation is found more or less throughout the whole +race. But every device--if we may so regard these changes--is useless, +and the devastating agency of the Cretaceous, whatever it was, removes +the Ammonites and Belemnites from the scene. The Mollusc world, like the +world of plants and of reptiles, approaches its modern aspect. + +In the fish world, too, there is an effective selection in the course of +the Cretaceous. All the fishes of modern times, except the large +family of the sharks, rays, skates, and dog-fishes (Elasmobranchs), the +sturgeon and chimaera, the mud-fishes, and a very few other types, are +Teleosts, or bony-framed fishes--the others having cartilaginous frames. +None of the Teleosts had appeared until the end of the Jurassic. They +now, like the flowering plants on land, not only herald the new age, +but rapidly oust the other fishes, except the unconquerable shark. They +gradually approach the familiar types of Teleosts, so that we may say +that before the end of the Cretaceous the waters swarmed with primitive +and patriarchal cod, salmon, herring, perch, pike, bream, eels, and +other fishes. Some of them grew to an enormous size. The Portheus, +an American pike, seems to have been about eight feet long; and the +activity of an eight-foot pike may be left to the angler's imagination. +All, however, are, as evolution demands, of a generalised and unfamiliar +type: the material out of which our fishes will be evolved. + +Of the insects we have very little trace in the Cretaceous. We shall +find them developing with great richness in the following period, but, +imperfect as the record is, we may venture to say that they were checked +in the Cretaceous. There were good conditions for preserving them, but +few are preserved. And of the other groups of invertebrates we need only +say that they show a steady advance toward modern types. The sea-lily +fills the rocks no longer; the sea-urchin is very abundant. The Molluscs +gain on the more lowly organised Brachiopods. + +To complete the picture we must add that higher types probably arose +in the later Cretaceous which do not appear in the records. This is +particularly true of the birds and mammals. We find them spreading +so early in the Tertiary that we must put back the beginning of the +expansion to the Cretaceous. As yet, however, the only mammal remains +we find are such jaws and teeth of primitive mammals as we have already +described. The birds we described (after the Archaeopteryx) also belong +to the Cretaceous, and they form another of the doomed races. Probably +the modern birds were already developing among the new vegetation on the +higher ground. + +These are the facts of Cretaceous life, as far as the record has yielded +them, and it remains for us to understand them. Clearly there has been +a great selective process analogous to, if not equal to, the winnowing +process at the end of the Palaeozoic. As there has been a similar, if +less considerable, upheaval of the land, we are at once tempted to think +that the great selective agency was a lowering of the temperature. When +we further find that the most important change in the animal world is +the destruction of the cold-blooded reptiles, which have no concern for +the young, and the luxuriant spread of the warm-blooded animals, which +do care for their young, the idea is greatly confirmed. When we add +that the powerful Molluscs which are slain, while the humbler Molluscs +survive, are those which--to judge from the nautilus and octopus--love +warm seas, the impression is further confirmed. And when we finally +reflect that the most distinctive phenomenon of the period is the rapid +spread of deciduous trees, it would seem that there is only one possible +interpretation of the Cretaceous Revolution. + +This interpretation--that cold was the selecting agency--is a familiar +idea in geological literature, but, as I said, there are recent writers +who profess reserve in regard to it, and it is proper to glance at, or +at least look for, the alternatives. + +Before doing so let us be quite clear that here we have nothing to +do with theories of the origin of the earth. The Permian cold--which, +however, is universally admitted--is more or less entangled in that +controversy; the Cretaceous cold has no connection with it. Whatever +excess of carbon-dioxide there may have been in the early atmosphere +was cleared by the Coal-forests. We must set aside all these theories in +explaining the present facts. + +It is also useful to note that the fact that there have been great +changes in the climate of the earth in past time is beyond dispute. +There is no denying the fact that the climate of the earth was warm from +the Arctic to the Antarctic in the Devonian and Carboniferous periods: +that it fell considerably in the Permian: that it again became at least +"warm temperate" (Chamberlin) from the Arctic to the Antarctic in the +Jurassic, and again in the Eocene: that some millions of square miles +of Europe and North America were covered with ice and snow in the +Pleistocene, so that the reindeer wandered where palms had previously +flourished and the vine flourishes to-day; and that the pronounced zones +of climate which we find today have no counterpart in any earlier +age. In view of these great and admitted fluctuations of the earth's +temperature one does not see any reason for hesitating to admit a fall +of temperature in the Cretaceous, if the facts point to it. + +On the other hand, the alternative suggestions are not very convincing. +We have noticed one of these suggestions in connection with the origin +of the Angiosperms. It hints that this may be related to developments +of the insect world. Most probably the development of the characteristic +flowers of the Angiosperms is connected with an increasing relation +to insects, but what we want to understand especially is the deciduous +character of their leaves. Many of the Angiosperms are evergreen, so +that it cannot be said that the one change entailed the other. In fact, +a careful study of the leaves preserved in the rocks seems to show +the deciduous Angiosperms gaining on the evergreens at the end of the +Cretaceous. The most natural, it not the only, interpretation of this is +that the temperature is falling. Deciduous trees shed their leaves so as +to check their transpiration when a season comes on in which they cannot +absorb the normal amount of moisture. This may occur either at the +on-coming of a hot, dry season or of a cold season (in which the roots +absorb less). Everything suggests that the deciduous tree evolved to +meet an increase of cold, not of heat. + +Another suggestion is that animals and plants were not "climatically +differentiated" until the Cretaceous period; that is to say, that they +were adapted to all climates before that time, and then began to be +sensitive to differences of climate, and live in different latitudes. +But how and why they should suddenly become differentiated in this way +is so mysterious that one prefers to think that, as the animal remains +also suggest, there were no appreciable zones of climate until the +Cretaceous. The magnolia, for instance, flourished in Greenland in the +early Tertiary, and has to live very far south of it to-day. It is much +simpler to assume that Greenland changed--as a vast amount of evidence +indicates--than that the magnolia changed. + +Finally, to explain the disappearance of the Mesozoic reptiles without a +fall in temperature, it is suggested that they were exterminated by +the advancing mammals. It is assumed that the spreading world of the +Angiospermous plants somewhere met the spread of the advancing mammals, +and opened out a rich new granary to them. This led to so powerful +a development of the mammals that they succeeded in overthrowing the +reptiles. + +There are several serious difficulties in the way of this theory. The +first and most decisive is that the great reptiles have practically +disappeared before the mammals come on the scene. Only in one series of +beds (Puerco) in America, representing an early period of the Tertiary +Era, do we find any association of their remains; and even there it +is not clear that they were contemporary. Over the earth generally the +geological record shows the great reptiles dying from some invisible +scourge long before any mammal capable of doing them any harm appears; +even if we suppose that the mammal mainly attacked the eggs and the +young. We may very well believe that more powerful mammals than the +primitive Mesozoic specimens were already developed in some part of the +earth--say, Africa--and that the rise of the land gave them a bridge +across the Mediterranean to Europe. Probably this happened; but the +important point is that the reptiles were already almost extinct. The +difficulty is even greater when we reflect that it is precisely the +most powerful reptiles (Deinosaurs) and least accessible reptiles +(Pterosaurs, Ichthyosaurs, etc.) which disappear, while the smaller land +and water reptiles survive and retreat southward--where the mammals are +just as numerous. That assuredly is not the effect of an invasion of +carnivores, even if we could overlook the absence of such carnivores +from the record until after the extinction of the reptiles in most +places. + +I have entered somewhat fully into this point, partly because of +its great interest, but partly lest it be thought that I am merely +reproducing a tradition of geological literature without giving due +attention to the criticisms of recent writers. The plain and common +interpretation of the Cretaceous revolution--that a fall in temperature +was its chief devastating agency--is the only one that brings harmony +into all the facts. The one comprehensive enemy of that vast reptile +population was cold. It was fatal to the adult because he had a +three-chambered heart and no warm coat; it was fatal to the Mesozoic +vegetation on which, directly or indirectly, he fed; it was fatal to his +eggs and young because the mother did not brood over the one or care +for the other. It was fatal to the Pterosaurs, even if they were +warm-blooded, because they had no warm coats and did not (presumably) +hatch their eggs; and it was equally fatal to the viviparous +Ichthyosaurs. It is the one common fate that could slay all classes. +When we find that the surviving reptiles retreat southward, only +lingering in Europe during the renewed warmth of the Eocene and Miocene +periods, this interpretation is sufficiently confirmed. And when +we recollect that these things coincide with the extinction of the +Ammonites and Belemnites, and the driving of their descendants further +south, as well as the rise and triumph of deciduous trees, it is +difficult to see any ground for hesitating. + +But we need not, and must not, imagine a period of cold as severe, +prolonged, and general as that of the Permian period. The warmth of the +Jurassic period is generally attributed to the low relief of the land, +and the very large proportion of water-surface. The effect of this would +be to increase the moisture in the atmosphere. Whether this was assisted +by any abnormal proportion of carbon-dioxide, as in the Carboniferous, +we cannot confidently say. Professor Chamberlin observes that, since +the absorbing rock-surface was greatly reduced in the Jurassic, the +carbon-dioxide would tend to accumulate in its atmosphere, and help to +explain the high temperature. But the great spread of vegetation and the +rise of land in the later Jurassic and the Cretaceous would reduce this +density of the atmosphere, and help to lower the temperature. + +It is clear that the cold would at first be local. In fact, it must be +carefully realised that, when we speak of the Jurassic period as a time +of uniform warmth, we mean uniform at the same altitude. Everybody knows +the effect of rising from the warm, moist sea-level to the top of even +a small inland elevation. There would be such cooler regions throughout +the Jurassic, and we saw that there were considerable upheavals of +land towards its close. To these elevated lands we may look for the +development of the Angiosperms, the birds, and the mammals. When +the more massive rise of land came at the end of the Cretaceous, the +temperature would fall over larger areas, and connecting ridges would +be established between one area and another. The Mesozoic plants and +animals would succumb to this advancing cold. What precise degree of +cold was necessary to kill the reptiles and Cephalopods, yet allow +certain of the more delicate flowering plants to live, is yet to be +determined. The vast majority of the new plants, with their winter +sleep, would thrive in the cooler air, and, occupying the ground of +the retreating cycads and ginkgoes would prepare a rich harvest for the +coming birds and mammals. + + + +CHAPTER XV. THE TERTIARY ERA + +We have already traversed nearly nine-tenths of the story of terrestrial +life, without counting the long and obscure Archaean period, and still +find ourselves in a strange and unfamiliar earth. With the close of the +Chalk period, however, we take a long stride in the direction of the +modern world. The Tertiary Era will, in the main, prove a fresh period +of genial warmth and fertile low-lying regions. During its course our +deciduous trees and grasses will mingle with the palms and pines over +the land, our flowers will begin to brighten the landscape, and the +forms of our familiar birds and mammals, even the form of man, will be +discernible in the crowds of animals. At its close another mighty period +of selection will clear the stage for its modern actors. + +A curious reflection is prompted in connection with this division of +the earth's story into periods of relative prosperity and quiescence, +separated by periods of disturbance. There was--on the most modest +estimate--a stretch of some fifteen million years between the Cambrian +and the Permian upheavals. On the same chronological scale the interval +between the Permian and Cretaceous revolutions was only about seven +million years, and the Tertiary Era will comprise only about three +million years. One wonders if the Fourth (Quaternary) Era in which we +live will be similarly shortened. Further, whereas the earth returned +after each of the earlier upheavals to what seems to have been its +primitive condition of equable and warm climate, it has now entirely +departed from that condition, and exhibits very different zones of +climate and a succession of seasons in the year. One wonders what the +climate of the earth will become long before the expiration of those ten +million years which are usually assigned as the minimum period during +which the globe will remain habitable. + +It is premature to glance at the future, when we are still some millions +of years from the present, but it will be useful to look more closely +at the facts which inspire this reflection. From what we have seen, +and shall further see, it is clear that, in spite of all the recent +controversy about climate among our geologists, there has undeniably +been a progressive refrigeration of the globe. Every geologist, indeed, +admits "oscillations of climate," as Professor Chamberlin puts it. +But amidst all these oscillations we trace a steady lowering of +the temperature. Unless we put a strained and somewhat arbitrary +interpretation on the facts of the geological record, earlier ages knew +nothing of our division of the year into pronounced seasons and of +the globe into very different climatic zones. It might plausibly be +suggested that we are still living in the last days of the Ice-Age, +and that the earth may be slowly returning to a warmer condition. +Shackleton, it might be observed, found that there has been a +considerable shrinkage of the south polar ice within the period of +exploration. But we shall find that a difference of climate, as compared +with earlier ages, was already evident in the middle of the Tertiary +Era, and it is far more noticeable to-day. + +We do not know the causes of this climatic evolution--the point will be +considered more closely in connection with the last Ice-Age--but we see +that it throws a flood of light on the evolution of organisms. It is +one of the chief incarnations of natural selection. Changes in the +distribution of land and water and in the nature of the land-surface, +the coming of powerful carnivores, and other agencies which we have +seen, have had their share in the onward impulsion of life, but the most +drastic agency seems to have been the supervention of cold. The higher +types of both animals and plants appear plainly in response to a +lowering of temperature. This is the chief advantage of studying the +story of evolution in strict connection with the geological record. We +shall find that the record will continue to throw light on our path to +the end, but, as we are now about to approach the most important era +of evolution, and as we have now seen so much of the concrete story of +evolution, it will be interesting to examine briefly some other ways of +conceiving that story. + +We need not return to the consideration of the leading schools of +evolution, as described in a former chapter. Nothing that we have seen +will enable us to choose between the Lamarckian and the Weismannist +hypothesis; and I doubt if anything we are yet to see will prove +more decisive. The dispute is somewhat academic, and not vital to a +conception of evolution. We shall, for instance, presently follow the +evolution of the horse, and see four of its toes shrink and disappear, +while the fifth toe is enormously strengthened. In the facts themselves +there is nothing whatever to decide whether this evolution took place +on the lines suggested by Weismann, or on the lines suggested by Lamarck +and accepted by Darwin. It will be enough for us merely to establish +the fact that the one-toed horse is an evolved descendant of a primitive +five-toed mammal, through the adaptation of its foot to running on firm +ground, its teeth and neck to feeding on grasses, and so on. + +On the other hand, the facts we have already seen seem to justify the +attitude of compromise I adopted in regard to the Mutationist theory. It +would be an advantage in many ways if we could believe that new species +arose by sudden and large variations (mutations) of the young from the +parental type. In the case of many organs and habits it is extremely +difficult to see how a gradual development, by a slow accentuation of +small variations, is possible. When we further find that experimenters +on living species can bring about such mutations, and when we reflect +that there must have been acute disturbances in the surroundings of +animals and plants sometimes, we are disposed to think that many a +new species may have arisen in this way. On the other hand, while +the palaeontological record can never prove that a species arose by +mutations, it does sometimes show that species arise by very gradual +modification. The Chalk period, which we have just traversed, affords +a very clear instance. One of our chief investigators of the English +Chalk, Dr. Rowe, paid particular attention to the sea-urchins it +contains, as they serve well to identify different levels of chalk. He +discovered, not merely that they vary from level to level, but that +in at least one genus (Micraster) he could trace the organism very +gradually passing from one species to another, without any leap or +abruptness. It is certainly significant that we find such cases as this +precisely where the conditions of preservation are exceptionally good. +We must conclude that species arise, probably, both by mutations and +small variations, and that it is impossible to say which class of +species has been the more numerous. + +There remain one or two conceptions of evolution which we have not +hitherto noticed, as it was advisable to see the facts first. One of +these is the view--chiefly represented in this country by Professor +Henslow--that natural selection has had no part in the creation of +species; that the only two factors are the environment and the organism +which responds to its changes. This is true enough in the sense that, as +we saw, natural selection is not an action of nature on the "fit," but +on the unfit or less fit. But this does not in the least lessen the +importance of natural selection. If there were not in nature this body +of destructive agencies, to which we apply the name natural selection, +there would be little--we cannot say no--evolution. But the rising +carnivores, the falls of temperature, etc., that we have studied, have +had so real, if indirect, an influence on the development of life that +we need not dwell on this. + +Another school, or several schools, while admitting the action of +natural selection, maintain that earlier evolutionists have made nature +much too red in tooth and claw. Dr. Russel Wallace from one motive, and +Prince Krapotkin from another, have insisted that the triumphs of +war have been exaggerated, and the triumphs of peace, or of social +co-operation, far too little appreciated. It will be found that such +writers usually base their theory on life as we find it in nature +to-day, where the social principle is highly developed in many groups +of animals. This is most misleading, since social co-operation among +animals, as an instrument of progress, is (geologically speaking) quite +a recent phenomenon. Nearly every group of animals in which it is found +belongs, to put it moderately, to the last tenth of the story of life, +and in some of the chief instances the animals have only gradually +developed social life. [*] The first nine-tenths of the chronicle of +evolution contain no indication of social life, except--curiously +enough--in such groups as the Sponges, Corals, and Bryozoa, which are +amongst the least progressive in nature. We have seen plainly that +during the overwhelmingly greater part of the story of life the +predominant agencies of evolution were struggle against adverse +conditions and devouring carnivores; and we shall find them the +predominant agencies throughout the Tertiary Era. + + * Thus the social nature of man is sometimes quoted as one + of the chief causes of his development. It is true that it + has much to do with his later development, but we shall see + that the statement that man was from the start a social + being is not at all warranted by the facts. On the other + hand, it may be pointed out that the ants and termites had + appeared in the Mesozoic. We shall see some evidence that + the remarkable division of labour which now characterises + their life did not begin until a much later period, so that + we have no evidence of social life in the early stages. + + +Yet we must protest against the exaggerated estimate of the conscious +pain which so many read into these millions of years of struggle. +Probably there was no consciousness at all during the greater part +of the time. The wriggling of the worm on which you have accidentally +trodden is no proof whatever that you have caused conscious pain. The +nervous system of an animal has been so evolved as to respond with great +disturbance of its tissue to any dangerous or injurious assault. It is +the selection of a certain means of self-preservation. But at what level +of life the animal becomes conscious of this disturbance, and "feels +pain," it is very difficult to determine. The subject is too vast to be +opened here. In a special investigation of it. [*] I concluded that there is +no proof of the presence of any degree of consciousness in the +invertebrate world even in the higher insects; that there is probably +only a dull, blurred, imperfect consciousness below the level of the +higher mammals and birds; and that even the consciousness of an ape is +something very different from what educated Europeans, on the ground of +their own experience, call consciousness. It is too often forgotten that +pain is in proportion to consciousness. We must beware of such fallacies +as transferring our experience of pain to a Mesozoic reptile, with an +ounce or two of cerebrum to twenty tons of muscle and bone. + + * "The Evolution of Mind" (Black), 1911. + + +One other view of evolution, which we find in some recent and reputable +works (such as Professor Geddes and Thomson's "Evolution," 1911), calls +for consideration. In the ordinary Darwinian view the variations of the +young from their parents are indefinite, and spread in all directions. +They may continue to occur for ages without any of them proving an +advantage to their possessors. Then the environment may change, and +a certain variation may prove an advantage, and be continuously and +increasingly selected. Thus these indefinite variations may be so +controlled by the environment during millions of years that the fish at +last becomes an elephant or a man. The alternative view, urged by a few +writers, is that the variations were "definitely directed." The phrase +seems merely to complicate the story of evolution with a fresh and +superfluous mystery. The nature and precise action of this "definite +direction" within the organism are quite unintelligible, and the facts +seem explainable just as well--or not less imperfectly--without as with +this mystic agency. Radiolaria, Sponges, Corals, Sharks, Mudfishes, +Duckbills, etc., do not change (except within the limits of their +family) during millions of years, because they keep to an environment +to which they are fitted. On the other hand, certain fishes, reptiles, +etc., remain in a changing environment, and they must change with it. +The process has its obscurities, but we make them darker, it seems to +me, with these semi-metaphysical phrases. + +It has seemed advisable to take this further glance at the general +principles and current theories of evolution before we extend our own +procedure into the Tertiary Era. The highest types of animals and plants +are now about to appear on the stage of the earth; the theatre itself is +about to take on a modern complexion. The Middle Ages are over; the new +age is breaking upon the planet. We will, as before, first survey the +Tertiary Era as a whole, with the momentous changes it introduces, and +then examine, in separate chapters, the more important phases of its +life. + +It opens, like the preceding and the following era, with "the area +of land large and its relief pronounced." This is the outcome of the +Cretaceous revolution. Southern Europe and Southern Asia have risen, and +shaken the last masses of the Chalk ocean from their faces; the whole +western fringe of America has similarly emerged from the sea that had +flooded it. In many parts, as in England (at that time a part of the +Continent), there is so great a gap between the latest Cretaceous +and the earliest Tertiary strata that these newly elevated lands must +evidently have stood out of the waters for a prolonged period. On their +cooler plains the tragedy of the extinction of the great reptiles comes +to an end. The cyeads and ginkgoes have shrunk into thin survivors of +the luxuriant Mesozoic groves. The oak and beech and other deciduous +trees spread slowly over the successive lands, amid the glare and +thunder of the numerous volcanoes which the disturbance of the crust has +brought into play. New forms of birds fly from tree to tree, or linger +by the waters; and strange patriarchal types of mammals begin to move +among the bones of the stricken reptiles. + +But the seas and the rains and rivers are acting with renewed vigour +on the elevated lands, and the Eocene period closes in a fresh age of +levelling. Let us put the work of a million years or so in a sentence. +The southern sea, which has been confined almost to the limits of our +Mediterranean by the Cretaceous upheaval, gradually enlarges once more. +It floods the north-west of Africa almost as far as the equator; it +covers most of Italy, Turkey, Austria, and Southern Russia; it spreads +over Asia Minor, Persia, and Southern Asia, until it joins the Pacific; +and it sends a long arm across the Franco-British region, and up the +great valley which is now the German Ocean. + +From earlier chapters we now expect to find a warmer climate, and the +record gives abundant proof of it. To this period belongs the "London +Clay," in whose thick and--to the unskilled eye--insignificant bed the +geologist reads the remarkable story of what London was two or three +million years ago. It tells us that a sea, some 500 or 600 feet deep, +then lay over that part of England, and fragments of the life of the +period are preserved in its deposit. The sea lay at the mouth of +a sub-tropical river on whose banks grew palms, figs, ginkgoes, +eucalyptuses, almonds, and magnolias, with the more familiar oaks and +pines and laurels. Sword-fishes and monstrous sharks lived in the sea. +Large turtles and crocodiles and enormous "sea-serpents" lingered +in this last spell of warmth that Central Europe would experience. +A primitive whale appeared in the seas, and strange large +tapir-like mammals--remote ancestors of our horses and more familiar +beasts--wandered heavily on the land. Gigantic primitive birds, +sometimes ten feet high, waded by the shore. Deposits of the period at +Bournemouth and in the Isle of Wight tell the same story of a land +that bore figs, vines, palms, araucarias, and aralias, and waters that +sheltered turtles and crocodiles. The Parisian region presented the same +features. + +In fact, one of the most characteristic traces of the southern sea which +then stretched from England to Africa in the south and India in the +east indicates a warm climate. It will be remembered that the Cretaceous +ocean over Southern Europe had swarmed with the animalcules whose dead +skeletons largely compose our chalk-beds. In the new southern ocean +another branch of these Thalamophores, the Nummulites, spreads with such +portentous abundance that its shells--sometimes alone, generally with +other material--make beds of solid limestone several thousand feet in +thickness. The pyramids are built of this nummulitic limestone. The +one-celled animal in its shell is, however, no longer a microscopic +grain. It sometimes forms wonderful shells, an inch or more in diameter, +in which as many as a thousand chambers succeed each other, in spiral +order, from the centre. The beds containing it are found from the +Pyrenees to Japan. + +That this vast warm ocean, stretching southward over a large part +of what is now the Sahara, should give a semitropical aspect even to +Central Europe and Asia is not surprising. But this genial climate was +still very general over the earth. Evergreens which now need the warmth +of Italy or the Riviera then flourished in Lapland and Spitzbergen. +The flora of Greenland--a flora that includes magnolias, figs, and +bamboos--shows us that its temperature in the Eocene period must have +been about 30 degrees higher than it is to-day. [*] The temperature of the +cool Tyrol of modern Europe is calculated to have then been between 74 +and 81 degrees F. Palms, cactuses, aloes, gum-trees, cinnamon trees, +etc., flourished in the latitude of Northern France. The forests that +covered parts of Switzerland which are now buried in snow during a great +part of the year were like the forests one finds in parts of India and +Australia to-day. The climate of North America, and of the land which +still connected it with Europe, was correspondingly genial. + + * The great authority on Arctic geology, Heer, who makes + this calculation, puts this flora in the Miocene. It is now + usually considered that these warmer plants belong to the + earlier part of the Tertiary era. + + +This indulgent period (the Oligocene, or later part of the Eocene), +scattering a rich and nutritious vegetation with great profusion over +the land, led to a notable expansion of animal life. Insects, birds, and +mammals spread into vast and varied groups in every land. Had any of the +great Mesozoic reptiles survived, the warmer age might have enabled them +to dispute the sovereignty of the advancing mammals. But nothing more +formidable than the turtle, the snake, and the crocodile (confined +to the waters) had crossed the threshold of the Tertiary Era, and the +mammals and birds had the full advantage of the new golden age. The +fruits of the new trees, the grasses which now covered the plains, and +the insects which multiplied with the flowers afforded a magnificent +diet. The herbivorous mammals became a populous world, branching into +numerous different types according to their different environments. +The horse, the elephant, the camel, the pig, the deer, the rhinoceros +gradually emerge out of the chaos of evolving forms. Behind them, +hastening the course of their evolution, improving their speed, arms, +and armour, is the inevitable carnivore. He, too, in the abundance of +food, grows into a vast population, and branches out toward familiar +types. We will devote a chapter presently to this remarkable phase of +the story of evolution. + +But the golden age closes, as all golden ages had done before it, and +for the same reason. The land begins to rise, and cast the warm shallow +seas from its face. The expansion of life has been more rapid and +remarkable than it had ever been before, in corresponding periods of +abundant food and easy conditions; the contraction comes more quickly +than it had ever done before. Mountain masses begin to rise in nearly +all parts of the world. The advance is slow and not continuous, but as +time goes on the Atlas, Alps, Pyrenees, Apennines, Caucasus, Himalaya, +Rocky Mountains, and Andes rise higher and higher. When the geologist +looks to-day for the floor of the Eocene ocean, which he recognises +by the shells of the Nummulites, he finds it 10,000 feet above the +sea-level in the Alps, 16,000 feet above the sea-level in the Himalaya, +and 20,000 feet above the sea-level in Thibet. One need not ask why the +regions of London and Paris fostered palms and magnolias and turtles in +Tertiary times, and shudder in their dreary winter to-day. + +The Tertiary Era is divided by geologists into four periods: the +Eocene, Oligocene, Miocene, and Pliocene. "Cene" is our barbaric way of +expressing the Greek word for "new," and the classification is meant +to mark the increase of new (or modern and actual) types of life in +the course of the Tertiary Era. Many geologists, however, distrust +the classification, and are disposed to divide the Tertiary into two +periods. From our point of view, at least, it is advisable to do this. +The first and longer half of the Tertiary is the period in which the +temperature rises until Central Europe enjoys the climate of South +Africa; the second half is the period in which the land gradually +rises, and the temperature falls, until glaciers and sheets of ice cover +regions where the palm and fig had flourished. + +The rise of the land had begun in the first half of the Tertiary, but +had been suspended. The Pyrenees and Apennines had begun to rise at +the end of the Eocene, straining the crust until it spluttered with +volcanoes, casting the nummulitic sea off large areas of Southern +Europe. The Nummulites become smaller and less abundant. There is also +some upheaval in North America, and a bridge of land begins to +connect the north and south, and permit an effective mingling of their +populations. But the advance is, as I said, suspended, and the Oligocene +period maintains the golden age. With the Miocene period the land +resumes its rise. A chill is felt along the American coast, showing a +fall in the temperature of the Atlantic. In Europe there is a similar +chill, and a more obvious reason for it. There is an ascending movement +of the whole series of mountains from Morocco and the Pyrenees, through +the Alps, the Caucasus, and the Carpathians, to India and China. Large +lakes still lie over Western Europe, but nearly the whole of it emerges +from the ocean. The Mediterranean still sends an arm up France, and with +another arm encircles the Alpine mass; but the upheaval continues, and +the great nummulitic sea is reduced to a series of extensive lakes, cut +off both from the Atlantic and Pacific. The climate of Southern Europe +is probably still as genial as that of the Canaries to-day. Palms still +linger in the landscape in reduced numbers. + +The last part of the Tertiary, the Pliocene, opens with a slight return +of the sea. The upheaval is once more suspended, and the waters +are eating into the land. There is some foundering of land at the +south-western tip of Europe; the "Straits of Gibraltar" begin to connect +the Mediterranean with the Atlantic, and the Balearic Islands, Corsica, +and Sardinia remain as the mountain summits of a submerged land. Then +the upheaval is resumed, in nearly every part of the earth. + +Nearly every great mountain chain that the geologist has studied +shared in this remarkable movement at the end of the Tertiary Era. The +Pyrenees, Alps, Himalaya, etc., made their last ascent, and attained +their present elevation. And as the land rose, the aspect of Europe +and America slowly altered. The palms, figs, bamboos, and magnolias +disappeared; the turtles, crocodiles, flamingoes, and hippopotamuses +retreated toward the equator. The snow began to gather thick on the +rising heights; then the glaciers began to glitter on their flanks. As +the cold increased, the rivers of ice which flowed down the hills +of Switzerland, Spain, Scotland, or Scandinavia advanced farther and +farther over the plains. The regions of green vegetation shrank before +the oncoming ice, the animals retreated south, or developed Arctic +features. Europe and America were ushering in the great Ice-Age, which +was to bury five or six million square miles of their territory under a +thick mantle of ice. + +Such is the general outline of the story of the Tertiary Era. We +approach the study of its types of life and their remarkable development +more intelligently when we have first given careful attention to this +extraordinary series of physical changes. Short as the Era is, compared +with its predecessors, it is even more eventful and stimulating than +they, and closes with what Professor Chamberlin calls "the greatest +deformative movements in post-Cambrian history." In the main it has, +from the evolutionary point of view, the same significant character +as the two preceding eras. Its middle portion is an age of expansion, +indulgence, exuberance, in which myriads of varied forms are thrown upon +the scene, its later part is an age of contraction, of annihilation, +of drastic test, in which the more effectively organised will be chosen +from the myriads of types. Once more nature has engendered a vast brood, +and is about to select some of her offspring to people the modern world. +Among the types selected will be Man. + + + +CHAPTER XVI. THE FLOWER AND THE INSECT + +AS we approach the last part of the geological record we must neglect +the lower types of life, which have hitherto occupied so much of +our attention, so that we may inquire more fully into the origin and +fortunes of the higher forms which now fill the stage. It may be noted, +in general terms, that they shared the opulence of the mid-Tertiary +period, produced some gigantic specimens of their respective families, +and evolved into the genera, and often the species, which we find living +to-day. A few illustrations will suffice to give some idea of the later +development of the lower invertebrates and vertebrates. + +Monstrous oysters bear witness to the prosperity of that ancient and +interesting family of the Molluscs. In some species the shells were +commonly ten inches long; the double shell of one of these Tertiary +bivalves has been found which measured thirteen inches in length, eight +in width, and six in thickness. In the higher branch of the Mollusc +world the naked Cephalopods (cuttle-fish, etc.) predominate over the +nautiloids--the shrunken survivors of the great coiled-shell race. Among +the sharks, the modern Squalodonts entirely displace the older types, +and grow to an enormous size. Some of the teeth we find in Tertiary +deposits are more than six inches long and six inches broad at the base. +This is three times the size of the teeth of the largest living shark, +and it is therefore believed that the extinct possessor of these +formidable teeth (Carcharodon megalodon) must have been much more than +fifty, and was possibly a hundred, feet in length. He flourished in +the waters of both Europe and America during the halcyon days of the +Tertiary Era. Among the bony fishes, all our modern and familiar types +appear. + +The amphibia and reptiles also pass into their modern types, after a +period of generous expansion. Primitive frogs and toads make their first +appearance in the Tertiary, and the remains are found in European beds +of four-foot-long salamanders. More than fifty species of Tertiary +turtles are known, and many of them were of enormous size. One carapace +that has been found in a Tertiary bed measures twelve feet in length, +eight feet in width, and seven feet in height to the top of the back. +The living turtle must have been nearly twenty feet long. Marine +reptiles, of a snake-like structure, ran to fifteen feet in length. +Crocodiles and alligators swarmed in the rivers of Europe until the +chilly Pliocene bade them depart to Africa. + +In a word, it was the seven years of plenty for the whole living world, +and the expansive development gave birth to the modern types, which were +to be selected from the crowd in the subsequent seven years of famine. +We must be content to follow the evolution of the higher types of +organisms. I will therefore first describe the advance of the Tertiary +vegetation, the luxuriance of which was the first condition of the great +expansion of animal life; then we will glance at the grand army of +the insects which followed the development of the flowers, and at the +accompanying expansion and ramification of the birds. The long and +interesting story of the mammals must be told in a separate chapter, +and a further chapter must be devoted to the appearance of the human +species. + +We saw that the Angiosperms, or flowering plants, appeared at the +beginning of the Cretaceous period, and were richly developed before the +Tertiary Era opened. We saw also that their precise origin is unknown. +They suddenly invade a part of North America where there were conditions +for preserving some traces of them, but we have as yet no remains +of their early forms or clue to their place of development. We may +conjecture that their ancestors had been living in some elevated inland +region during the warmth of the Jurassic period. + +As it is now known that many of the cycad-like Mesozoic plants bore +flowers--as the modern botanist scarcely hesitates to call them--the +gap between the Gymnosperms and Angiosperms is very much lessened. There +are, however, structural differences which forbid us to regard any of +these flowering cycads, which we have yet found, as the ancestors of the +Angiosperms. The most reasonable view seems to be that a small and local +branch of these primitive flowering plants was evolved, like the rest, +in the stress of the Permian-Triassic cold; that, instead of descending +to the warm moist levels with the rest at the end of the Triassic, and +developing the definite characters of the cycad, it remained on the +higher and cooler land; and that the rise of land at the end of the +Jurassic period stimulated the development of its Angiosperm features, +enlarged the area in which it was especially fitted to thrive, and so +permitted it to spread and suddenly break into the geological record as +a fully developed Angiosperm. + +As the cycads shrank in the Cretaceous period, the Angiosperms deployed +with great rapidity, and, spreading at various levels and in different +kinds of soils and climates, branched into hundreds of different types. +We saw that the oak, beech, elm, maple, palm, grass, etc., were well +developed before the end of the Cretaceous period. The botanist divides +the Angiosperms into two leading groups, the Monocotyledons (palms, +grasses, lilies, orchises, irises, etc.) and Dicotyledons (the vast +majority), and it is now generally believed that the former were +developed from an early and primitive branch of the latter. But it is +impossible to retrace the lines of development of the innumerable types +of Angiosperms. The geologist has mainly to rely on a few stray leaves +that were swept into the lakes and preserved in the mud, and the +evidence they afford is far too slender for the construction of +genealogical trees. The student of living plants can go a little +further in discovering relationships, and, when we find him tracing such +apparently remote plants as the apple and the strawberry to a common +ancestor with the rose, we foresee interesting possibilities on the +botanical side. But the evolution of the Angiosperms is a recent and +immature study, and we will be content with a few reflections on the +struggle of the various types of trees in the changing conditions of +the Tertiary, the development of the grasses, and the evolution of +the flower. In other words, we will be content to ask how the modern +landscape obtained its general vegetal features. + +Broadly speaking, the vegetation of the first part of the Tertiary Era +was a mixture of sub-tropical and temperate forms, a confused mass of +Ferns, Conifers, Ginkgoales, Monocotyledons, and Dicotyledons. Here is +a casual list of plants that then grew in the latitude of London and +Paris: the palm, magnolia, myrtle, Banksia, vine, fig, aralea, sequoia, +eucalyptus, cinnamon tree, cactus, agave, tulip tree, apple, plum, +bamboo, almond, plane, maple, willow, oak, evergreen oak, laurel, beech, +cedar, etc. The landscape must have been extraordinarily varied and +beautiful and rich. To one botanist it suggests Malaysia, to another +India, to another Australia. + +It is really the last gathering of the plants, before the great +dispersion. Then the cold creeps slowly down from the Arctic regions, +and begins to reduce the variety. We can clearly trace its gradual +advance. In the Carboniferous and Jurassic the vegetation of the Arctic +regions had been the same as that of England; in the Eocene palms can +flourish in England, but not further north; in the Pliocene the palms +and bamboos and semi-tropical species are driven out of Europe; in the +Pleistocene the ice-sheet advances to the valleys of the Thames and the +Danube (and proportionately in the United States), every warmth-loving +species is annihilated, and our grasses, oaks, beeches, elms, apples, +plums, etc., linger on the green southern fringe of the Continent, and +in a few uncovered regions, ready to spread north once more as the ice +creeps back towards the Alps or the Arctic circle. Thus, in few words, +did Europe and North America come to have the vegetation we find in them +to-day. + +The next broad characteristic of our landscape is the spreading carpet +of grass. The interest of the evolution of the grasses will be seen +later, when we shall find the evolution of the horse, for instance, +following very closely upon it. So striking, indeed, is the connection +between the advance of the grasses and the advance of the mammals that +Dr. Russel Wallace has recently claimed ("The World of Life," 1910) +that there is a clear purposive arrangement in the whole chain of +developments which leads to the appearance of the grasses. He says that +"the very puzzling facts" of the immense reptilian development in +the Mesozoic can only be understood on the supposition that they were +evolved "to keep down the coarser vegetation, to supply animal food for +the larger Carnivora, and thus give time for higher forms to obtain a +secure foothold and a sufficient amount of varied form and structure" +(p. 284). + +Every insistence on the close connection of the different strands in the +web of life is welcome, but Dr. Wallace does not seem to have learned +the facts accurately. There is nothing "puzzling" about the Mesozoic +reptilian development; the depression of the land, the moist warmth, +and the luscious vegetation of the later Triassic and the Jurassic +amply explain it. Again, the only carnivores to whom they seem to have +supplied food were reptiles of their own race. Nor can the feeding of +the herbivorous reptiles be connected with the rise of the Angiosperms. +We do not find the flowering plants developing anywhere in those vast +regions where the great reptiles abounded; they invade them from some +single unknown region, and mingle with the pines and ginkgoes, while the +cyeads alone are destroyed. + +The grasses, in particular, do not appear until the Cretaceous, and do +not show much development until the mid-Tertiary; and their development +seems to be chiefly connected with physical conditions. The meandering +rivers and broad lakes of the mid-Tertiary would have their fringes +of grass and sedge, and, as the lakes dried up in the vicissitudes of +climate, large areas of grass would be left on their sites. To these +primitive prairies the mammal (not reptile) herbivores would be +attracted, with important results. The consequences to the animals +we will consider presently. The effect on the grasses may be well +understood on the lines so usefully indicated in Dr. Wallace's book. The +incessant cropping, age after age, would check the growth of the larger +and coarser grasses give opportunity to the smaller and finer, and lead +in time to the development of the grassy plains of the modern world. +Thus one more familiar feature was added to the landscape in the +Tertiary Era. + +As this fresh green carpet spread over the formerly naked plains, +it began to be enriched with our coloured flowers. There were large +flowers, we saw, on some of the Mesozoic cycads, but their sober yellows +and greens--to judge from their descendants--would do little to brighten +the landscape. It is in the course of the Tertiary Era that the mantle +of green begins to be embroidered with the brilliant hues of our +flowers. + +Grant Allen put forward in 1882 ("The Colours of Flowers") an +interesting theory of the appearance of the colours of flowers, and it +is regarded as probable. He observed that most of the simplest flowers +are yellow; the more advanced flowers of simple families, and the +simpler flowers of slightly advanced families, are generally white or +pink; the most advanced flowers of all families, and almost all the +flowers of the more advanced families, are red, purple, or blue; and the +most advanced flowers of the most advanced families are always +either blue or variegated. Professor Henslow adds a number of equally +significant facts with the same tendency, so that we have strong reason +to conceive the floral world as passing through successive phases of +colour in the Tertiary Era. At first it would be a world of yellows +and greens, like that of the Mesozoic vegetation, but brighter. In time +splashes of red and white would lie on the face of the landscape; and +later would come the purples, the rich blues, and the variegated colours +of the more advanced flowers. + +Why the colours came at all is a question closely connected with the +general story of the evolution of the flower, at which we must glance. +The essential characteristic of the flower, in the botanist's judgment, +is the central green organ which you find--say, in a lily--standing out +in the middle of the floral structure, with a number of yellow-coated +rods round it. The yellow rods bear the male germinal elements (pollen); +the central pistil encloses the ovules, or female elements. "Angiosperm" +means "covered-seed plant," and its characteristic is this protection +of the ovules within a special chamber, to which the pollen alone may +penetrate. Round these essential organs are the coloured petals of the +corolla (the chief part of the flower to the unscientific mind) and the +sepals, often also coloured, of the calyx. + +There is no doubt that all these parts arose from modifications of the +leaves or stems of the primitive plant; though whether the bright +leaves of the corolla are directly derived from ordinary leaves, or are +enlarged and flattened stamens, has been disputed. And to the question +why these bright petals, whose colour and variety of form lend such +charm to the world of flowers, have been developed at all, most +botanists will give a prompt and very interesting reply. As both male +and female elements are usually in one flower, it may fertilise itself, +the pollen falling directly on the pistil. But fertilisation is more +sure and effective if the pollen comes from a different individual--if +there is "cross fertilisation." This may be accomplished by the simple +agency of the wind blowing the pollen broadcast, but it is done much +better by insects, which brush against the stamens, and carry grains of +the pollen to the next flower they visit. + +We have here a very fertile line of development among the primitive +flowers. The insects begin to visit them, for their pollen or juices, +and cross-fertilise them. If this is an advantage, attractiveness to +insects will become so important a feature that natural selection will +develop it more and more. In plain English, what is meant is that those +flowers which are more attractive to insects will be the most surely +fertilised and breed most, and the prolonged application of this +principle during hundreds of thousands of years will issue in the +immense variety of our flowers. They will be enriched with little stores +of honey and nectar; not so mysterious an advantage, when we reflect on +the concentration of the juices in the neighbourhood of the seed. Then +they must "advertise" their stores, and the strong perfumes and bright +colours begin to develop, and ensure posterity to their possessors. The +shape of the corolla will be altered in hundreds of ways, to accommodate +and attract the useful visitor and shut out the mere robber. These +utilities, together with the various modifying agencies of different +environments, are generally believed to have led to the bewildering +variety and great beauty of our floral world. + +It is proper to add that this view has been sharply challenged by a +number of recent writers. It is questioned if colours and scents do +attract insects; though several recent series of experiments seem to +show that bees are certainly attracted by colours. It is questioned if +cross-fertilisation has really the importance ascribed to it since the +days of Darwin. Some of these writers believe that the colours and the +peculiar shape which the petals take in some flowers (orchises, for +instance) have been evolved to deter browsing animals from eating them. +The theory is thus only a different application of natural selection; +Professor Henslow, on the other hand, stands alone in denying the +selection, and believing that the insects directly developed the scents, +honeys, colours, and shapes by mechanical irritation. The great majority +of botanists adhere to the older view, and see in the wonderful Tertiary +expansion of the flowers a manifold adaptation to the insect friends and +insect foes which then became very abundant and varied. + +Resisting the temptation to glance at the marvellous adaptations +which we find to-day in our plant world--the insect-eating plants, the +climbers, the parasites, the sensitive plants, the water-storing plants +in dry regions, and so on--we must turn to the consideration of the +insects themselves. We have already studied the evolution of the insect +in general, and seen its earlier forms. The Tertiary Era not only +witnessed a great deployment of the insects, but was singularly rich in +means of preserving them. The "fly in amber" has ceased to be a puzzle +even to the inexpert. Amber is the resin that exuded from pine-like +trees, especially in the Baltic region, in the Eocene and Oligocene +periods. Insects stuck in the resin, and were buried under fresh layers +of it, and we find them embalmed in it as we pick up the resin on the +shores of the Baltic to-day. The Tertiary lakes were also important +cemeteries of insects. A great bed at Florissart, in Colorado, is +described by one of the American experts who examined it as "a Tertiary +Pompeii." It has yielded specimens of about a thousand species of +Tertiary insects. Near the large ancient lake, of which it marks the +site, was a volcano, and the fine ash yielded from the cone seems to +have buried myriads of insects in the water. At Oeningen a similar +lake-deposit has, although only a few feet thick, yielded 900 species of +insects. + +Yet these rich and numerous finds throw little light on the evolution of +the insect, except in the general sense that they show species and even +genera quite different from those of to-day. No new families of insects +have appeared since the Eocene, and the ancient types had by that time +disappeared. Since the Eocene, however, the species have been almost +entirely changed, so that the insect record, from its commencement +in the Primary Era, has the stamp of evolution on every page of it. +Unfortunately, insects, especially the higher and later insects, +are such frail structures that they are only preserved in very rare +conditions. The most important event of the insect-world in the Tertiary +is the arrival of the butterflies, which then appear for the first time. +We may assume that they spread with great rapidity and abundance in +the rich floral world of the mid-Jurassic. More than 13,000 species of +Lepidoptera are known to-day, and there are probably twice that number +yet to be classified by the entomologist. But so far the Tertiary +deposits have yielded only the fragmentary remains of about twenty +individual butterflies. + +The evolutionary study of the insects is, therefore, not so much +concerned with the various modifications of the three pairs of jaws, +inherited from the primitive Tracheate, and the wings, which have +given us our vast variety of species. It is directed rather to the more +interesting questions of what are called the "instincts" of the insects, +the remarkable metamorphosis by which the young of the higher orders +attain the adult form, and the extraordinary colouring and marking of +bees, wasps, and butterflies. Even these questions, however, are so +large that only a few words can be said here on the tendencies of recent +research. + +In regard to the psychic powers of insects it may be said, in the +first place, that it is seriously disputed among the modern authorities +whether even the highest insects (the ant, bee, and wasp) have any +degree whatever of the intelligence which an earlier generation +generously bestowed on them. Wasmann and Bethe, two of the leading +authorities on ants, take the negative view; Forel claims that they show +occasional traces of intelligence. It is at all events clear that the +enormous majority of, if not all, their activities--and especially +those activities of the ant and the bee which chiefly impress the +imagination--are not intelligent, but instinctive actions. And the +second point to be noted is that the word "instinct," in the old sense +of some innate power or faculty directing the life of an animal, has +been struck out of the modern scientific dictionary. The ant or bee +inherits a certain mechanism of nerves and muscles which will, in +certain circumstances, act in the way we call "instinctive." The problem +is to find how this mechanism and its remarkable actions were slowly +evolved. + +In view of the innumerable and infinitely varied forms of "instinct" +in the insect world we must restrict ourselves to a single +illustration--say, the social life of the ants and the bees. We are not +without indications of the gradual development of this social life. In +the case of the ant we find that the Tertiary specimens--and about a +hundred species are found in Switzerland alone, whereas there are only +fifty species in the whole of Europe to-day--all have wings and are, +apparently, of the two sexes, not neutral. This seems to indicate +that even in the mid-Tertiary some millions of years after the first +appearance of the ant, the social life which we admire in the ants today +had not yet been developed. The Tertiary bees, on the other hand, are +said to show some traces of the division of labour (and modification +of structure) which make the bees so interesting; but in this case the +living bees, rising from a solitary life through increasing stages of +social co-operation, give us some idea of the gradual development of +this remarkable citizenship. + +It seems to me that the great selective agency which has brought about +these, and many other remarkable activities of the insects (such as the +storing of food with their eggs by wasps), was probably the occurrence +of periods of cold, and especially the beginning of a winter season in +the Cretaceous or Tertiary age. In the periods of luxuriant life (the +Carboniferous, the Jurassic, or the Oligocene), when insects swarmed and +varied in every direction, some would vary in the direction of a more +effective placing of the eggs; and the supervening period of cold and +scarcity would favour them. When a regular winter season set in, this +tendency would be enormously increased. It is a parallel case to the +evolution of the birds and mammals from the reptiles. Those that varied +most in the direction of care for the egg and the young would have the +largest share in the next generation. When we further reflect that since +the Tertiary the insect world has passed through the drastic disturbance +of the climate in the great Ice-Age, we seem to have an illuminating +clue to one of the most remarkable features of higher insect life. + +The origin of the colour marks' and patterns on so many of the higher +insects, with which we may join the origin of the stick-insects, +leaf-insects, etc., is a subject of lively controversy in science +to-day. The protective value of the appearance of insects which +look almost exactly like dried twigs or decaying leaves, and of an +arrangement of the colours of the wings of butterflies which makes them +almost invisible when at rest, is so obvious that natural selection was +confidently invoked to explain them. In other cases certain colours +or marks seemed to have a value as "warning colours," advertising the +nauseousness of their possessors to the bird, which had learned to +recognise them; in other cases these colours and marks seemed to be +borrowed by palatable species, whose unconscious "mimicry" led to their +survival; in other cases, again, the patterns and spots were regarded as +"recognition marks," by which the male could find his mate. + +Science is just now passing through a phase of acute criticism--as +the reader will have realised by this time--and many of the positions +confidently adopted in the earlier constructive stage are challenged. +This applies to the protective colours, warning colours, mimicry, etc., +of insects. Probably some of the affirmations of the older generation of +evolutionists were too rigid and extensive; and probably the denials of +the new generation are equally exaggerated. When all sound criticism has +been met, there remains a vast amount of protective colouring, shaping, +and marking in the insect world of which natural selection gives us the +one plausible explanation. But the doctrine of natural selection does +not mean that every feature of an animal shall have a certain utility. +It will destroy animals with injurious variations and favour animals +with useful variations; but there may be a large amount of variation, +especially in colour, to which it is quite indifferent. In this way much +colour-marking may develop, either from ordinary embryonic variations +or (as experiment on butterflies shows) from the direct influence of +surroundings which has no vital significance. In this way, too, small +variations of no selective value may gradually increase until they +chance to have a value to the animal. [*] + + * For a strong statement of the new critical position see + Dewar and Finn's "Making of Species," 1909, ch. vi. + + +The origin of the metamorphosis, or pupa-stage, of the higher +insects, with all its wonderful protective devices, is so obscure and +controverted that we must pass over it. Some authorities think that +the sleep-stage has been evolved for the protection of the helpless +transforming insect; some believe that it occurs because movement would +be injurious to the insect in that stage; some say that the muscular +system is actually dissolved in its connections; and some recent experts +suggest that it is a reminiscence of the fact that the ancestors of the +metamorphosing insects were addicted to internal parasitism in their +youth. It is one of the problems of the future. At present we have no +fossil pupa-remains (though we have one caterpillar) to guide us. We +must leave these fascinating but difficult problems of insect life, and +glance at the evolution of the birds. + +To the student of nature whose interest is confined to one branch +of science the record of life is a mysterious Succession of waves. A +comprehensive view of nature, living and non-living, past and present, +discovers scores of illuminating connections, and even sees at times +the inevitable sequence of events. Thus if the rise of the Angiospermous +vegetation on the ruins of the Mesozoic world is understood in the light +of geological and climatic changes, and the consequent deploying of +the insects, especially the suctorial insects, is a natural result, the +simultaneous triumph of the birds is not unintelligible. The grains +and fruits of the Angiosperms and the vast swarms of insects provided +immense stores of food; the annihilation of the Pterosaurs left a whole +stratum of the earth free for their occupation. + +We saw that a primitive bird, with very striking reptilian features, was +found in the Jurassic rocks, suggesting very clearly the evolution of +the bird from the reptile in the cold of the Permian or Triassic period. +In the Cretaceous we found the birds distributed in a number of genera, +but of two leading types. The Ichthyornis type was a tern-like flying +bird, with socketed teeth and biconcave vertebrae like the reptile, but +otherwise fully evolved into a bird. Its line is believed to survive in +the gannets, cormorants, pelicans, and frigate-birds of to-day. The less +numerous Hesperornis group were large and powerful divers. Then there +is a blank in the record, representing the Cretaceous upheaval, and it +unfortunately conceals the first great ramification of the bird world. +When the light falls again on the Eocene period we find great numbers +of our familiar types quite developed. Primitive types of gulls, herons, +pelicans, quails, ibises, flamingoes, albatrosses, buzzards, hornbills, +falcons, eagles, owls, plovers, and woodcocks are found in the Eocene +beds; the Oligocene beds add parrots, trogons, cranes, marabouts, +secretary-birds, grouse, swallows, and woodpeckers. We cannot suppose +that every type has been preserved, but we see that our bird-world was +virtually created in the early part of the Tertiary Era. + +With these more or less familiar types were large ostrich-like survivors +of the older order. In the bed of the sea which covered the site +of London in the Eocene are found the remains of a toothed bird +(Odontopteryx), though the teeth are merely sharp outgrowths of the +edge of the bill. Another bird of the same period and region (Gastornis) +stood about ten feet high, and must have looked something like a wading +ostrich. Other large waders, even more ostrich-like in structure, lived +in North America; and in Patagonia the remains have been found of a +massive bird, about eight feet high, with a head larger than that of +any living animal except the elephant, rhinoceros, and hippopotamus +(Chamberlin). + +The absence of early Eocene remains prevents us from tracing the lines +of our vast and varied bird-kingdom to their Mesozoic beginnings. +And when we appeal to the zoologist to supply the missing links of +relationship, by a comparison of the structures of living birds, we +receive only uncertain and very general suggestions. [*] He tells us that +the ostrich-group (especially the emus and cassowaries) are one of the +most primitive stocks of the bird world, and that the ancient Dinornis +group and the recently extinct moas seem to be offshoots of that stock. +The remaining many thousand species of Carinate birds (or flying birds +with a keel [carina]-shaped breast-bone for the attachment of the flying +muscles) are then gathered into two great branches, which are "traceable +to a common stock" (Pycraft), and branch in their turn along the later +lines of development. One of these lines--the pelicans, cormorants, +etc.--seems to be a continuation of the Ichthyornis type of the +Cretaceous, with the Odontopteryx as an Eocene offshoot; the divers, +penguins, grebes, and petrels represent another ancient stock, which +may be related to the Hesperornis group of the Cretaceous. Dr. Chalmers +Mitchell thinks that the "screamers" of South America are the nearest +representatives of the common ancestor of the keel-breasted birds. But +even to give the broader divisions of the 19,000 species of living birds +would be of little interest to the general reader. + + * The best treatment of the subject will be found in W. P. + Pycraft's History of Birds, 1910. + + +The special problems of bird-evolution are as numerous and unsettled +as those of the insects. There is the same dispute as to "protective +colours" and "recognition marks", the same uncertainty as to the origin +of such instinctive practices as migration and nesting. The general +feeling is that the annual migration had its origin in the overcrowding +of the regions in which birds could live all the year round. They +therefore pushed northward in the spring and remained north until the +winter impoverishment drove them south again. On this view each group +would be returning to its ancestral home, led by the older birds, in the +great migration flights. The curious paths they follow are believed by +some authorities to mark the original lines of their spread, preserved +from generation to generation through the annual lead of the older +birds. If we recollect the Ice-Age which drove the vast majority of the +birds south at the end of the Tertiary, and imagine them later following +the northward retreat of the ice, from their narrowed and overcrowded +southern territory, we may not be far from the secret of the annual +migration. + +A more important controversy is conducted in regard to the gorgeous +plumage and other decorations and weapons of the male birds. Darwin, as +is known, advanced a theory of "sexual selection" to explain these. +The male peacock, to take a concrete instance, would have developed its +beautiful tail because, through tens of thousands of generations, the +female selected the more finely tailed male among the various suitors. +Dr. Wallace and other authorities always disputed this aesthetic +sentiment and choice on the part of the female. The general opinion +today is that Darwin's theory could not be sustained in the range and +precise sense he gave to it. Some kind of display by the male in the +breeding season would be an advantage, but to suppose that the females +of any species of birds or mammals had the definite and uniform taste +necessary for the creation of male characters by sexual selection is +more than difficult. They seem to be connected in origin rather with the +higher vitality of the male, but the lines on which they were selected +are not yet understood. + +This general sketch of the enrichment of the earth with flowering +plants, insects, and birds in the Tertiary Era is all that the limits of +the present work permit us to give. It is an age of exuberant life +and abundant food; the teeming populations overflow their primitive +boundaries, and, in adapting themselves to every form of diet, every +phase of environment, and every device of capture or escape, the +spreading organisms are moulded into tens of thousands of species. We +shall see this more clearly in the evolution of the mammals. What we +chiefly learn from the present chapter is the vital interconnection of +the various parts of nature. Geological changes favour the spread of +a certain type of vegetation. Insects are attracted to its nutritious +seed-organs, and an age of this form of parasitism leads to a signal +modification of the jaws of the insects themselves and to the lavish +variety and brilliance of the flowers. Birds are attracted to the +nutritious matter enclosing the seeds, and, as it is an advantage to the +plant that its seeds be scattered beyond the already populated area, by +passing through the alimentary canal of the bird, and being discharged +with its excrements, a fresh line of evolution leads to the appearance +of the large and coloured fruits. The birds, again, turn upon the +swarming insects, and the steady selection they exercise leads to +the zigzag flight and the protective colour of the butterfly, the +concealment of the grub and the pupa, the marking of the caterpillar, +and so on. We can understand the living nature of to-day as the outcome +of that teeming, striving, changing world of the Tertiary Era, just as +it in turn was the natural outcome of the ages that had gone before. + + + +CHAPTER XVII. THE ORIGIN OF OUR MAMMALS + +In our study of the evolution of the plant, the insect, and the bird we +were seriously thwarted by the circumstance that their frames, +somewhat frail in themselves, were rarely likely to be entombed in good +conditions for preservation. Earlier critics of evolution used, when +they were imperfectly acquainted with the conditions of fossilisation, +to insinuate that this fragmentary nature of the geological record was a +very convenient refuge for the evolutionist who was pressed for positive +evidence. The complaint is no longer found in any serious work. Where +we find excellent conditions for preservation, and animals suitable +for preservation living in the midst of them, the record is quite +satisfactory. We saw how the chalk has yielded remains of sea-urchins +in the actual and gradual process of evolution. Tertiary beds which +represent the muddy bottoms of tranquil lakes are sometimes equally +instructive in their fossils, especially of shell-fish. The Paludina of +a certain Slavonian lake-deposit is a classical example. It changes +so greatly in the successive levels of the deposit that, if the +intermediate forms were not preserved, we should divide it into several +different species. The Planorbis is another well-known example. In this +case we have a species evolving along several distinct lines into forms +which differ remarkably from each other. + +The Tertiary mammals, living generally on the land and only coming by +accident into deposits suitable for preservation, cannot be expected to +reveal anything like this sensible advance from form to form. They were, +however, so numerous in the mid-Tertiary, and their bones are so well +calculated to survive when they do fall into suitable conditions, that +we can follow their development much more easily than that of the birds. +We find a number of strange patriarchal beasts entering the scene in the +early Eocene, and spreading into a great variety of forms in the genial +conditions of the Oligocene and Miocene. As some of these forms advance, +we begin to descry in them the features, remote and shadowy at first, of +the horse, the deer, the elephant, the whale, the tiger, and our other +familiar mammals. In some instances we can trace the evolution with a +wonderful fullness, considering the remoteness of the period and +the conditions of preservation. Then, one by one, the abortive, the +inelastic, the ill-fitted types are destroyed by changing conditions or +powerful carnivores, and the field is left to the mammals which filled +it when man in turn began his destructive career. + +The first point of interest is the origin of these Tertiary mammals. +Their distinctive advantage over the mammals of the Mesozoic Era was-the +possession by the mother of a placenta (the "after-birth" of the higher +mammals), or structure in the womb by which the blood-vessels of the +mother are brought into such association with those of the foetus that +her blood passes into its arteries, and it is fully developed within the +warm shelter of her womb. The mammals of the Mesozoic had been small and +primitive animals, rarely larger than a rat, and never rising above the +marsupial stage in organisation. They not only continued to exist, and +give rise to their modern representatives (the opossum, etc.) during +the Tertiary Era, but they shared the general prosperity. In Australia, +where they were protected from the higher carnivorous mammals, they +gave rise to huge elephant-like wombats (Diprotodon), with skulls two +or three feet in length. Over the earth generally, however, they were +superseded by the placental mammals, which suddenly break into the +geological record in the early Tertiary, and spread with great vigour +and rapidity over the four continents. + +Were they a progressive offshoot from the Mesozoic Marsupials, or +Monotremes, or do they represent a separate stock from the primitive +half-reptile and half-mammal family? The point is disputed; nor does the +scantiness of the record permit us to tell the place of their origin. +The placental structure would be so great an advantage in a cold and +unfavourable environment that some writers look to the northern land, +connecting Europe and America, for their development. We saw, however, +that this northern region was singularly warm until long after the +spread of the mammals. Other experts, impressed by the parallel +development of the mammals and the flowering plants, look to the +elevated parts of eastern North America. + +Such evidence as there is seems rather to suggest that South Africa +was the cradle of the placental mammals. We shall find that many of +our mammals originated in Africa; there, too, is found to-day the most +primitive representative of the Tertiary mammals, the hyrax; and there +we find in especial abundance the remains of the mammal-like reptiles +(Theromorphs) which are regarded as their progenitors. Further search +in the unexplored geological treasures and dense forests of Africa is +needed. We may provisionally conceive the placental mammals as a group +of the South African early mammals which developed a fortunate variation +in womb-structure during the severe conditions of the early Mesozoic. In +this new structure they would have no preponderant advantage as long as +the genial Jurassic age favoured the great reptiles, and they may have +remained as small and insignificant as the Marsupials. But with the +fresh upheaval and climatic disturbance at the end of the Jurassic, and +during the Cretaceous, they spread northward, and replaced the dying +reptiles, as the Angiosperms replaced the dying cycads. When they met +the spread of the Angiosperm vegetation they would receive another great +stimulus to development. + +They appear in Europe and North America in the earliest Cretaceous. The +rise of the land had connected many hitherto isolated regions, and +they seem to have poured over every bridge into all parts of the four +continents. The obscurity of their origin is richly compensated by their +intense evolutionary interest from the moment they enter the geological +record. We have seen this in the case of every important group of plants +and animals, and can easily understand it. The ancestral group was +small and local; the descendants are widely spread. While, therefore, +we discover remains of the later phases of development in our casual +cuttings and quarries, the ancestral tomb may remain for ages in some +unexplored province of the geological world. If this region is, as we +suspect, in Africa, our failure to discover it as yet is all the more +intelligible. + +But these mammals of the early Tertiary are still of such a patriarchal +or ancestral character that the student of evolution can dispense with +their earlier phase. They combine in their primitive frames, in an +elementary way, the features which we now find distributed in widely +removed groups of their descendants. Most of them fall into two large +orders: the Condylarthra, the ancestral herbivores from which we shall +find our horses, oxen, deer, elephants, and hogs gradually issuing, and +the Creodonta, the patriarchal carnivores, which will give birth to our +lions and tigers, wolves and foxes, and their various cousins. As yet +even the two general types of herbivore and carnivore are so imperfectly +separated that it is not always possible to distinguish between them. +Nearly all of them have the five-toed foot of the reptile ancestor; and +the flat nails on their toes are the common material out of which the +hoof of the ungulate and the claw of the carnivore will be presently +fashioned. Nearly all have forty-four simply constructed teeth, from +which will be evolved the grinders and tusks of the elephant or the +canines of the tiger. They answer in every respect to the theory that +some primitive local group was the common source of all our great +mammals. With them are ancestral forms of Edentates (sloths, etc.) and +Insectivores (moles, etc.), side-branches developing according to their +special habits; and before the end of the Eocene we find primitive +Rodents (squirrels, etc.) and Cheiroptera (bats). + +From the description of the Tertiary world which we have seen in the +last chapter we understand the rapid evolution of the herbivorous +Condylarthra. The rich vegetation which spreads over the northern +continents, to which they have penetrated, gives them an enormous +vitality and fecundity, and they break into groups, as they increase +in number, adapted to the different conditions of forest, marsh, or +grass-covered plain. Some of them, swelling lazily on the abundant food, +and secure for a time in their strength, become the Deinosaurs of their +age, mere feeding and breeding machines. They are massive, sluggish, +small-brained animals, their strong stumpy limbs terminating in broad +five-toed feet. Coryphodon, sometimes as large as an ox, is a typical +representative. It is a type fitted only for prosperous days, and these +Amblypoda, as they are called, will disappear as soon as the great +carnivores are developed. + +Another doomed race, or abortive experiment of early mammal life, were +the remarkable Deinocerata ("terrible-horned" mammals). They sometimes +measured thirteen feet in length, but had little use for brain in the +conditions in which they were developed. The brain of the Deinoceras was +only one-eighth the size of the brain of a rhinoceros of the same +bulk; and the rhinoceros is a poor-brained representative of the modern +mammals. To meet the growing perils of their race they seem to have +developed three pairs of horns on their long, flat skulls, as we find +on them three pairs of protuberances. A late specimen of the group, +Tinoceras, had a head four feet in length, armed with these six horns, +and its canine teeth were developed into tusks sometimes seven or +eight inches in length. They suggest a race of powerful but clumsy and +grotesque monsters, making a last stand, and developing such means of +protection as their inelastic nature permitted. But the horns seem to +have proved a futile protection against the advancing carnivores, and +the race was extinguished. The horns may, of course, have been mainly +developed by, or for, the mutual butting of the males. + +The extinction of these races will remind many readers of a theory on +which it is advisable to say a word. It will be remembered that the +last of the Deinosaurs and the Ammonites also exhibited some remarkable +developments in their last days. These facts have suggested to some +writers the idea that expiring races pass through a death-agony, and +seem to die a natural death of old age like individuals. The Trilobites +are quoted as another instance; and some ingenious writers add the +supposed eccentricities of the Roman Empire in its senile decay and a +number of other equally unsubstantial illustrations. + +There is not the least ground for this fantastic speculation. The +destruction of these "doomed races" is as clearly traceable to external +causes as is the destruction of the Roman Empire; nor, in fact, did the +Roman Empire develop any such eccentricities as are imagined in this +superficial theory. What seem to our eye the "eccentricities" and +"convulsions" of the Ceratopsia and Deinocerata are much more likely +to be defensive developments against a growing peril, but they were +as futile against the new carnivores as were the assegais of the Zulus +against the European. On the other hand, the eccentricities of many +of the later Trilobites--the LATEST Trilobites, it may be noted, +were chaste and sober specimens of their race, like the last Roman +patricians--and of the Ammonites may very well have been caused by +physical and chemical changes in the sea-water. We know from experiment +that such changes have a disturbing influence, especially on the +development of eggs and larvae; and we know from the geological record +that such changes occurred in the periods when the Trilobites and +Ammonites perished. In fine, the vast majority of extinct races passed +through no "convulsions" whatever. We may conclude that races do not +die; they are killed. + +The extinction of these races of the early Condylarthra, and the +survival of those races whose descendants share the earth with us +to-day, are quite intelligible. The hand of natural selection lay heavy +on the Tertiary herbivores. Apart from overpopulation, forcing groups +to adapt themselves to different regions and diets, and apart from the +geological disturbances and climatic changes which occurred in nearly +every period, the shadow of the advancing carnivores was upon them. +Primitive but formidable tigers, wolves, and hyenas were multiplying, +and a great selective struggle set in. Some groups shrank from the +battle by burrowing underground like the rabbit; some, like the squirrel +or the ape, took refuge in the trees; some, like the whale and seal, +returned to the water; some shrank into armour, like the armadillo, or +behind fences of spines, like the hedgehog; some, like the bat, escaped +into the air. Social life also was probably developed at this time, and +the great herds had their sentinels and leaders. But the most useful +qualities of the large vegetarians, which lived on grass and leaf, were +acuteness of perception to see the danger, and speed of limb to escape +it. In other words, increase of brain and sense-power and increase of +speed were the primary requisites. The clumsy early Condylarthra failed +to meet the tests, and perished; the other branches of the race were +more plastic, and, under the pressure of a formidable enemy, were +gradually moulded into the horse, the deer, the ox, the antelope, and +the elephant. + +We can follow the evolution of our mammals of this branch most easily +by studying the modification of the feet and limbs. In a running +attitude--the experiment may be tried--the weight of the body is shifted +from the flat sole of the foot, and thrown upon the toes, especially the +central toes. This indicates the line of development of the Ungulates +(hoofed animals) in the struggle of the Tertiary Era. In the early +Eocene we find the Condylarthra (such as Phenacodus) with flat five-toed +feet, and such a mixed combination of characters that they "might serve +very well for the ancestors of all the later Ungulata" (Woodward). +We then presently find this generalised Ungulate branching into three +types, one of which seems to be a patriarchal tapir, the second +is regarded as a very remote ancestor of the horse, and the third +foreshadows the rhinoceros. The feet have now only three or four toes; +one or two of the side-toes have disappeared. This evolution, however, +follows two distinct lines. In one group of these primitive Ungulates +the main axis of the limb, or the stress of the weight, passes through +the middle toe. This group becomes the Perissodactyla ("odd-toed" +Ungulates) of the zoologist, throwing out side-branches in the tapir +and the rhinoceros, and culminating in the one-toed horse. In the other +line, the Artiodactyla (the "even-toed" or cloven-hoofed Ungulates), the +main axis or stress passes between the third and fourth toes, and the +group branches into our deer, oxen, sheep, pigs, camels, giraffes, and +hippopotamuses. The elephant has developed along a separate and very +distinctive line, as we shall see, and the hyrax is a primitive survivor +of the ancestral group. + +Thus the evolutionist is able to trace a very natural order in the +immense variety of our Ungulates. He can follow them in theory as they +slowly evolve from their primitive Eocene ancestor according to their +various habits and environments; he has a very rich collection of fossil +remains illustrating the stages of their development; and in the hyrax +(or "coney") he has one more of those living fossils, or primitive +survivors, which still fairly preserve the ancestral form. The hyrax has +four toes on the front foot and three on the hind foot, and the feet are +flat. Its front teeth resemble those of a rodent, and its molars +those of the rhinoceros. In many respects it is a most primitive and +generalised little animal, preserving the ancestral form more or less +faithfully since Tertiary days in the shelter of the African Continent. + +The rest of the Ungulates continued to develop through the Tertiary, and +fortunately we are enabled to follow the development of two of the most +interesting of them, the horse and the elephant, in considerable detail. +As I said above, the primitive Ungulate soon branches into three types +which dimly foreshadow the tapir, the horse, and the rhinoceros, the +three forms of the Perissodactyl. The second of these types is the +Hyracotherium. It has no distinct equine features, and is known only +from the skull, but the authorities regard it as the progenitor (or +representative of the progenitors) of the horse-types. In size it must +have been something like the rabbit or the hyrax. Still early in the +Eocene, however, we find the remains of a small animal (Eohippus), about +the size of a fox, which is described as "undoubtedly horse-like." It +had only three toes on its hind feet, and four on its front feet; though +it had also a splint-bone, representing the shrunken and discarded fifth +toe, on its fore feet. Another form of the same period (Protorohippus) +shows the central of the three toes on the hind foot much enlarged, and +the lateral toes shrinking. The teeth, and the bones and joints of the +limbs, are also developing in the direction of the horse. + +In the succeeding geological period, the Oligocene, we find several +horse-types in which the adaptation of the limbs to running on the firm +grassy plains and of the teeth to eating the grass continues. Mesohippus +has lost the fourth toe of the fore foot, which is now reduced to a +splintbone, and the lateral toes of its hind foot are shrinking. In the +Miocene period there is a great development of the horse-like mammals. +We have the remains of more than forty species, some continuing the main +line of development on the firm and growing prairies of the Miocene, +some branching into the softer meadows or the forests, and giving rise +to types which will not outlive the Tertiary. They have three toes on +each foot, and have generally lost even the rudimentary trace of the +fourth toe. In most of them, moreover, the lateral toes--except in the +marsh-dwelling species, with spreading feet--scarcely touch the ground, +while the central toe is developing a strong hoof. The leg-bones are +longer, and have a new type of joint; the muscles are concentrated near +the body. The front teeth are now chopping incisors, and the grinding +teeth approach those of the modern horse in the distribution of the +enamel, dentine, and cement. They are now about the size of a donkey, +and must have had a distinctly horsy appearance, with their long necks +and heads and tapering limbs. One of them, Merychippus, was probably in +the direct line of the evolution of the horse. From Hipparion some +of the authorities believe that the zebras may have been developed. +Miohippus, Protohippus, and Hypohippus, varying in size from that of a +sheep to that of a donkey, are other branches of this spreading family. + +In the Pliocene period the evolution of the main stem culminates in +the appearance of the horse, and the collateral branches are destroyed. +Pliohippus is a further intermediate form. It has only one toe on each +foot, with two large splint bones, but its hoof is less round than that +of the horse, and it differs in the shape of the skull and the length +of the teeth. The true horse (Equus) at length appears, in Europe and +America, before the close of the Tertiary period. As is well known, it +still has the rudimentary traces of its second and fourth toes in the +shape of splint bones, and these bones are not only more definitely +toe-shaped in the foal before birth, but are occasionally developed and +give us a three-toed horse. + +From these successive remains we can confidently picture the evolution, +during two or three million years, of one of our most familiar mammals. +It must not, of course, be supposed that these fossil remains all +represent "ancestors of the horse." In some cases they may very well +do so; in others, as we saw, they represent sidebranches of the family +which have become extinct. But even such successive forms as the +Eohippus, Mesohippus, Miohippus, and Pliohippus must not be arranged +in a direct line as the pedigree of the horse. The family became most +extensive in the Miocene, and we must regard the casual fossil specimens +we have discovered as illustrations of the various phases in the +development of the horse from the primitive Ungulate. When we recollect +what we saw in an earlier chapter about the evolution of grassy plains +and the successive rises of the land during the Tertiary period, and +when we reflect on the simultaneous advance of the carnivores, we can +without difficulty realise this evolution of our familiar companion from +a hyrax-like little animal of two million years ago. + +We have not in many cases so rich a collection of intermediate forms as +in the case of the horse, but our fossil mammals are numerous enough +to suggest a similar development of all the mammals of to-day. The +primitive family which gave birth to the horse also gave us, as we saw, +the tapir and the rhinoceros. We find ancestral tapirs in Europe and +America during the Tertiary period, but the later cold has driven them +to the warm swamps of Brazil and Malaysia. The rhinoceros has had a long +and interesting history. From the primitive Hyrochinus of the Eocene, in +which it is dimly foreshadowed, we pass to a large and varied family +in the later periods. In the Oligocene it spreads into three great +branches, adapted, respectively, to life on the elevated lands, the +lowlands, and the water. The upland type (Hyracodon) was a light-limbed +running animal, well illustrating the close relation to the horse. The +aquatic representative (Metamynodon) was a stumpy and bulky animal. +The intermediate lowland type was probably the ancestor of the modern +animal. All three forms were yet hornless. In the Miocene the lowland +type (Leptaceratherium, Aceratherium, etc.) develops vigorously, while +the other branches die. The European types now have two horns, and in +one of the American species (Diceratherium) we see a commencement of +the horny growths from the skull. We shall see later that the rhinoceros +continued in Europe even during the severe conditions of the glacial +period, in a branch that developed a woolly coat. + +There were also in the early Tertiary several sidebranches of the +horse-tapir-rhinoceros family. The Palaeotheres were more or less +between the horse and the tapir in structure; the Anoplotheres between +the tapir and the ruminant. A third doomed branch, the Titanotheres, +flourished vigorously for a time, and begot some strange and monstrous +forms (Brontops, Titanops, etc.). In the larger specimens the body was +about fourteen feet long, and stood ten feet from the ground. The long, +low skull had a pair of horns over the snout. They perished like the +equally powerful but equally sluggish and stupid Deinocerata. The +Tertiary was an age of brain rather than of brawn. As compared with +their early Tertiary representatives' some of our modern mammals have +increased seven or eight-fold in brain-capacity. + +While the horses and tapirs and rhinoceroses were being gradually +evolved from the primitive types, the Artiodactyl branch of the +Ungulates--the pigs, deer, oxen, etc.--were also developing. We must +dismiss them briefly. We saw that the primitive herbivores divided early +in the Eocene into the "odd-toed" and "even-toed" varieties; the name +refers, it will be remembered, not to the number of toes, but to the +axis of stress. The Artiodactyl group must have quickly branched in +turn, as we find very primitive hogs and camels before the end of the +Eocene. The first hog-like creature (Homacodon) was much smaller than +the hog of to-day, and had strong canine teeth, but in the Oligocene +the family gave rise to a large and numerous race, the Elotheres. These +"giant-pigs," as they have been called, with two toes on each foot, +flourished vigorously for a time in Europe and America, but were +extinguished in the Miocene, when the true pigs made their appearance. +Another doomed race of the time is represented by the Hyopotamus, +an animal between the pig and the hippopotamus; and the Oreodontids, +between the hog and the deer, were another unsuccessful branch of the +early race. The hippopotamus itself was widespread in Europe, and +a familiar form in the rivers of Britain, in the latter part of the +Tertiary. + +The camel seems to be traceable to a group of primitive North American +Ungulates (Paebrotherium, etc.) in the later Eocene period. The +Paebrotherium, a small animal about two feet long, is followed by +Pliauchenia, which points toward the llamas and vicunas, and Procamelus, +which clearly foreshadows the true camel. In the Pliocene the one branch +went southward, to develop into the llamas and vicunas, and the other +branch crossed to Asia, to develop into the camels. Since that time they +have had no descendants in North America. + +The primitive giraffe appears suddenly in the later Tertiary deposits of +Europe and Asia. The evidence points to an invasion from Africa, and, +as the region of development is unknown and unexplored, the evolution of +the giraffe remains a matter of speculation. Chevrotains flourished in +Europe and North America in the Oligocene, and are still very primitive +in structure, combining features of the hog and the ruminants. Primitive +deer and oxen begin in the Miocene, and seem to have an earlier +representative in certain American animals (Protoceras), of which the +male has a pair of blunt outgrowths between the ears. The first true +deer are hornless (like the primitive muskdeer of Asia to-day), but by +the middle of the Miocene the males have small two-pronged antlers, and +as the period proceeds three or four more prongs are added. It is some +confirmation of the evolutionary embryonic law that we find the antlers +developing in this way in the individual stag to-day. A very +curious race of ruminants in the later Tertiary was a large antelope +(Sivatherium) with four horns. It had not only the dimensions, but +apparently some of the characters, of an elephant. + +The elephant itself, the last type of the Ungulates, has a clearer line +of developments. A chance discovery of fossils in the Fayum district in +Egypt led Dr. C. W. Andrews to make a special exploration, and on the +remains which he found he has constructed a remarkable story of the +evolution of the elephant. [*] It is clear that the elephant was developed +in Africa, and a sufficiently complete series of remains has been found +to give a good idea of the origin of its most distinctive features. +In the Eocene period there lived in the Egyptian region an animal, +something like the tapir in size and appearance, which had its second +incisors developed into small tusks and--to judge from the nasal opening +in the skull--a somewhat prolonged snout. This animal (Moeritherium) +only differed from the ordinary primitive Ungulate in these incipient +elephantine features. In the later Eocene a larger and more advanced +animal, the Palaeomastodon, makes its appearance. Its tusks are larger +(five or six inches long), its molars more elephantine, the air-cells +at the back of the head more developed. It would look like a small +elephant, except that it had a long snout, instead of a flexible trunk, +and a projecting lower jaw on which the snout rested. + + *See this short account, "Guide to the Elephants in the + British Museum," 1908. + + +Up to the beginning of the Miocene, Africa was, as we saw, cut off from +Europe and Asia by the sea which stretched from Spain to India. Then the +land rose, and the elephant passed by the new tracts into the north. Its +next representative, Tetrabelodon, is found in Asia and Europe, as +well as North Africa. The frame is as large as that of a medium-sized +elephant, and the increase of the air-cells at the back of the skull +shows that an increased weight has to be sustained by the muscles of the +neck. The nostrils are shifted further back. The tusks are from twenty +to thirty inches long, and round, and only differ from those of the +elephant in curving slightly downward, The chin projects as far as the +tusks. The neck is shorter and thicker, and, as the animal increases in +height, we can understand that the long snout--possibly prehensile at +its lower end--is necessary for the animal to reach the ground. But +the snout still lies on the projecting lower jaw, and is not a trunk. +Passing over the many collateral branches, which diverge in various +directions, we next kind that the chin is shortening (in Tetrabelodon +longirostris), and, through a long series of discovered intermediate +forms, we trace the evolution of the elephant from the mastodon. The +long supporting skin disappears, and the enormous snout becomes a +flexible trunk. Southern Asia seems to have been the province of this +final transformation, and we have remains of some of these primitive +elephants with tusks nine and a half feet long. A later species, which +wandered over Central and Southern Europe before the close of the +Tertiary, stood fifteen feet high at the shoulder, while the mammoth, +which superseded it in the days of early man, had at times tusks more +than ten feet in length. + +It is interesting to reflect that this light on the evolution of one of +our most specialised mammals is due to the chance opening of the soil +in an obscure African region. It suggests to us that as geological +exploration is extended, many similar discoveries may be made. The +slenderness of the geological record is a defect that the future may +considerably modify. + +From this summary review of the evolution of the Ungulates we must now +pass to an even briefer account of the evolution of the Carnivores. The +evidence is less abundant, but the characters of the Carnivores consist +so obviously of adaptations to their habits and diet that we have little +difficulty in imagining their evolution. Their early Eocene ancestors, +the Creodonts, gave rise in the Eocene to forms which we may regard as +the forerunners of the cat-family and dog-family, to which most of our +familiar Carnivores belong. Patriofelis, the "patriarchal cat," about +five or six feet in length (without the tail), curiously combines +the features of the cat and the seal-family. Cyonodon has a wolf-like +appearance, and Amphicyon rather suggests the fox. Primitive weasels, +civets, and hyaenas appear also in the Eocene. The various branches of +the Carnivore family are already roughly represented, but it is an age +of close relationships and generalised characters. + +In the Miocene we find the various groups diverging still further from +each other and from the extinct stocks. Definite wolves and foxes abound +in America, and the bear, civet, and hyaena are represented in Europe, +together with vague otter-like forms. The dog-family seems to have +developed chiefly in North America. As in the case of the Ungulates, +we find many strange side-branches which flourished for a time, but are +unknown to-day. Machoerodus, usually known as "the sabre-toothed tiger," +though not a tiger, was one of the most formidable of these transitory +races. Its upper canine teeth (the "sabres") were several inches in +length, and it had enormously distensible jaws to make them effective. +The great development of such animals, with large numbers of hyaenas, +civets, wolves, bears, and other Carnivores, in the middle and later +Tertiary was probably the most effective agency in the evolution of +the horse and deer and the extinction of the more sluggish races. The +aquatic branch of the Carnivores (seals, walruses, etc.) is little +represented in the Tertiary record. We saw, however, that the most +primitive representatives of the elephant-stock had also some characters +of the seal, and it is thought that the two had a common origin. + +The Moeritherium was a marsh-animal, and may very well have been cousin +to the branch of the family which pushed on to the seas, and developed +its fore limbs into paddles. + +The Rodents are represented in primitive form early in the Eocene +period. The teeth are just beginning to show the characteristic +modification for gnawing. A large branch of the family, the Tillodonts, +attained some importance a little later. They are described as combining +the head and claws of a bear with the teeth of a rodent and the general +characters of an ungulate. In the Oligocene we find primitive squirrels, +beavers, rabbits, and mice. The Insectivores also developed some of the +present types at an early date, and have since proved so unprogressive +that some regard them as the stock from which all the placental mammals +have arisen. + +The Cetacea (whales, porpoises, etc.) are already represented in the +Eocene by a primitive whale-like animal (Zeuglodon) of unknown origin. +Some specimens of it are seventy feet in length. It has large teeth, +sometimes six inches long, and is clearly a terrestrial mammal that +has returned to the waters. Some forms even of the modern whale develop +rudimentary teeth, and in all forms the bony structure of the fore limbs +and degenerate relic of a pelvis and back limbs plainly tell of the +terrestrial origin. Dolphins appear in the Miocene. + +Finally, the Edentates (sloths, anteaters, and armadilloes) are +represented in a very primitive form in the early Eocene. They are then +barely distinguishable from the Condylarthra and Creodonta, and seem +only recently to have issued from a common ancestor with those groups. +In the course of the Tertiary we find them--especially in South America, +which was cut off from the North and its invading Carnivores during +the Eocene and Miocene--developed into large sloths, armadilloes, and +anteaters. The reconnection with North America in the Pliocene allowed +the northern animals to descend, but gigantic sloths (Megatherium) and +armadilloes (Glyptodon) flourished long afterwards in South America. +The Megatherium attained a length of eighteen feet in one specimen +discovered, and the Glyptodon often had a dorsal shield (like that of +the armadillo) from six to eight feet long, and, in addition, a stoutly +armoured tail several feet long. + +The richness and rapidity of the mammalian development in the Tertiary, +of which this condensed survey will convey some impression, make it +impossible to do more here than glance over the vast field and indicate +the better-known connections. It will be seen that evolution not only +introduces a lucid order and arrangement into our thousands of species +of living and fossil mammals, but throws an admirable light on the +higher animal world of our time. The various orders into which the +zoologist puts our mammals are seen to be the branches of a living tree, +approaching more and more closely to each other in early Tertiary times, +in spite of the imperfectness of the geological record. We at last trace +these diverging lines to a few very primitive, generalised, patriarchal +groups, which in turn approach each other very closely in structure, +and plainly suggest a common Cretaceous ancestor. Whether that common +ancestor was an Edentate, an Insectivore, or Creodont, or something more +primitive than them all, is disputed. But the divergence of nearly all +the lines of our mammal world from those patriarchal types is admirably +clear. In the mutual struggle of carnivore and herbivore, in adaptation +to a hundred different environments (the water, the land, and the air, +the tree, the open plain, the underground, the marsh, etc.) and forms +of diet, we find the descendants of these patriarchal animals gradually +developing their distinctive characters. Then we find the destructive +agencies of living and inorganic nature blotting out type after type, +and the living things that spread over the land in the later Tertiary +are found to be broadly identical with the living things of to-day. The +last great selection, the northern Ice-Age, will give the last touches +of modernisation. + + + +CHAPTER XVIII. THE EVOLUTION OF MAN + +We have reserved for a closer inquiry that order of the placental +mammals to which we ourselves belong, and on which zoologists have +bestowed the very proper and distinguishing name of the Primates. Since +the days of Darwin there has been some tendency to resent the term +"lower animals," which man applies to his poorer relations. But, though +there is no such thing as an absolute standard by which we may judge the +"higher" or "lower" status of animals or plants, the extraordinary power +which man has by his brain development attained over both animate and +inanimate nature fully justifies the phrase. The Primate order is, +therefore, of supreme interest as the family that gave birth to man, and +it is important to discover the agencies which impelled some primitive +member of it to enter upon the path which led to this summit of organic +nature. + +The order includes the femurs, a large and primitive family with +ape-like features--the Germans call them "half-apes"--the monkeys, +the man-like apes, and man. This classification according to structure +corresponds with the successive appearance of the various families in +the geological record. The femurs appear in the Eocene; the monkeys, and +afterwards the apes, in the Miocene, the first semi-human forms in the +Pleistocene, though they must have been developed before this. It is +hardly necessary to say that science does not regard man as a descendant +of the known anthropoid apes, or these as descended from the monkeys. +They are successive types or phases of development, diverging early from +each other. Just as the succeeding horse-types of the record are not +necessarily related to each other in a direct line, yet illustrate the +evolution of a type which culminates in the horse, so the spreading and +branching members of the Primate group illustrate the evolution of a +type of organism which culminates in man. The particular relationship of +the various families, living and dead, will need careful study. + +That there is a general blood-relationship, and that man is much +more closely related to the anthropoid apes than to any of the lower +Primates, is no longer a matter of controversy. In Rudolph Virchow there +died, a few years ago, the last authoritative man of science to express +any doubt about it. There are, however, non-scientific writers who, by +repeating the ambiguous phrase that it is "only a theory," convey the +impression to inexpert readers that it is still more or less an open +question. We will therefore indicate a few of the lines of evidence +which have overcome the last hesitations of scientific men, and closed +the discussion as to the fact. + +The very close analogy of structure between man and the ape at once +suggests that they had a common ancestor. There are cases in which two +widely removed animals may develop a similar organ independently, but +there is assuredly no possibility of their being alike in all organs, +unless by common inheritance. Yet the essential identity of structure in +man and the ape is only confirmed by every advance of science, and would +of itself prove the common parentage. Such minor differences as there +are between man and the higher ape--in the development of the cerebrum, +the number of the teeth or ribs, the distribution of the hair, and so +on--are quite explicable when we reflect that the two groups must have +diverged from each other more than a million years ago. + +Examining the structure of man more closely, we find this strong +suggestion of relationship greatly confirmed. It is now well known that +the human body contains a number of vestigial "organs"--organs of no +actual use, and only intelligible as vestiges of organs that were once +useful. Whatever view we take of the origin of man, each organ in +his frame must have a meaning; and, as these organs are vestigial and +useless even in the lowest tribes of men, who represent primitive man, +they must be vestiges of organs that were of use in a remote pre-human +ancestor. The one fact that the ape has the same vestigial organs as man +would, on a scientific standard of evidence, prove the common descent of +the two. But these interesting organs themselves point back far earlier +than a mixed ape-human ancestor in many cases. + +The shell of cartilage which covers the entrance to the ear--the gristly +appendage which is popularly called the ear--is one of the clearest and +most easily recognised of these organs. The "ear" of a horse or a cat is +an upright mobile shell for catching the waves of sound. The human ear +has the appearance of being the shrunken relic of such an organ, and, +when we remove the skin, and find seven generally useless muscles +attached to it, obviously intended to pull the shell in all directions +(as in the horse), there can be no doubt that the external ear is a +discarded organ, a useless legacy from an earlier ancestor. In cases +where it has been cut off it was found that the sense of hearing was +scarcely, if at all, affected. Now we know that it is similarly useless +in all tribes of men, and must therefore come from a pre-human ancestor. +It is also vestigial in the higher apes, and it is only when we descend +to the lower monkeys and femurs that we see it approaching its primitive +useful form. One may almost say that it is a reminiscence of the far-off +period when, probably in the early Tertiary, the ancestors of the +Primates took to the trees. The animals living on the plain needed +acute senses to detect the approach of their prey or their enemies; +the tree-dweller found less demand on his sense of hearing, the +"speaking-trumpet" was discarded, and the development of the internal +ear proceeded on the higher line of the perception of musical sounds. + +We might take a very large number of parts of the actual human body, and +discover that they are similar historical or archaeological monuments +surviving in a modern system, but we have space only for a few of the +more conspicuous. + +The hair on the body is a vestigial organ, of actual use to no race of +men, an evident relic of the thick warm coat of an earlier ancestor. It +in turn recalls the dwellers in the primeval forest. In most cases--not +all, because the wearing of clothes for ages has modified this +feature--it will be found that the hairs on the arm tend upward from the +wrist to the elbow, and downward from the shoulder to the elbow. This +very peculiar feature becomes intelligible when we find that some of the +apes also have it, and that it has a certain use in their case. They put +their hands over their heads as they sit in the trees during ram, and in +that position the sloping hair acts somewhat like the thatched roof of a +cottage. + +Again, it will be found that in the natural position of standing we are +not perfectly flat-footed, but tend to press much more on the outer than +on the inner edge of the foot. This tendency, surviving after ages +of living on the level ground, is a lingering effect of the far-off +arboreal days. + +A more curious reminiscence is seen in the fact that the very young +infant, flabby and powerless as it is in most of its muscles, is so +strong in the muscles of the hand and arm that it can hang on to a stick +by its hands, and sustain the whole weight of its body, for several +minutes. Finally, our vestigial tail--for we have a tail comparable to +that of the higher apes--must be mentioned. In embryonic development +the tail is much longer than the legs, and some children are born with +a real tail, which they move as the puppy does, according to their +emotional condition. Other features of the body point back to an even +earlier stage. The vermiform appendage--in which some recent medical +writers have vainly endeavoured to find a utility--is the shrunken +remainder of a large and normal intestine of a remote ancestor. This +interpretation of it would stand even if it were found to have a certain +use in the human body. Vestigial organs are sometimes pressed into a +secondary use when their original function has been lost. The danger of +this appendage in the human body to-day is due to the fact that it is +a blind alley leading off the alimentary canal, and has a very narrow +opening. In the ape the opening is larger, and, significantly enough, it +is still larger in the human foetus. When we examine some of the lower +mammals we discover the meaning of it. It is in them an additional +storage chamber in the alimentary system. It is believed that a change +to a more digestible diet has made this additional chamber superfluous +in the Primates, and the system is slowly suppressing it. + +Other reminiscences of this earlier phase are found in the many +vestigial muscles which are found in the body to-day. The head of the +quadruped hangs forward, and is held by powerful muscles and ligaments +in the neck. We still have the shrunken remainder of this arrangement. +Other vestigial muscles are found in the forehead, the scalp, the +nose--many people can twitch the nostrils and the scalp--and under the +skin in many parts of the body. These are enfeebled remnants of the +muscular coat by which the quadruped twitches its skin, and drives +insects away. A less obvious feature is found by the anatomist in +certain blood-vessels of the trunk. As the blood flows vertically in a +biped and horizontally in a quadruped, the arrangement of the valves in +the blood-vessels should be different in the two cases; but it is the +same in us as in the quadruped. Another trace of the quadruped ancestor +is found in the baby. It walks "on all fours" so long, not merely from +weakness of the limbs, but because it has the spine of a quadruped. + +A much more interesting fact, but one less easy to interpret, is that +the human male has, like the male ape, organs for suckling the young. +That there are real milk-glands, usually vestigial, underneath the teats +in the breast of the boy or the man is proved by the many known cases in +which men have suckled the young. Several friends of the present writer +have seen this done in India and Ceylon by male "wet-nurses." As there +is no tribe of men or species of ape in which the male suckles the young +normally, we seem to be thrown back once more upon an earlier ancestor. +The difficulty is that we know of no mammal of which both parents +suckle the young, and some authorities think that the breasts have been +transferred to the male by a kind of embryonic muddle. That is difficult +to believe, as no other feature has ever been similarly transferred to +the opposite sex. In any case the male breasts are vestigial organs. +Another peculiarity of the mammary system is that sometimes three, four, +or five pairs of breasts appear in a woman (and several have been known +even in a man). This is, apparently, an occasional reminiscence of an +early mammal ancestor which had large litters of young and several pairs +of breasts. + +But there are features of the human body which recall an ancestor even +earlier than the quadruped. The most conspicuous of these is the little +fleshy pad at the inner corner of each eye. It is a common feature in +mammals, and is always useless. When, however, we look lower down in the +animal scale we find that fishes and reptiles (and birds) have a third +eyelid, which is drawn across the eye from this corner. There is little +room to doubt that the little fleshy vestige in the mammal's eye is the +shrunken remainder of the lateral eyelid of a remote fish-ancestor. + +A similar reminiscence is found in the pineal body, a small and useless +object, about the size and shape of a hazel-nut, in the centre of the +brain. When we examine the reptile we find a third eye in the top of +the head. The skin has closed over it, but the skull is still, in +many cases, perforated as it is for the eyes in front. I have seen it +standing out like a ball on the head of a dead crocodile, and in the +living tuatara--the very primitive New Zealand lizard--it still has a +retina and optic nerve. As the only animal in nature to-day with an eye +in this position (the Pyrosome, a little marine animal of the sea-squirt +family) is not in the line of reptile and mammal ancestry, it is +difficult to locate the third eye definitely. But when we find the skin +closing over it in the amphibian and reptile, then the bone, and then +see it gradually atrophying and being buried under the growing brain, we +must refer it to some early fish-ancestor. This ancestor, we may recall, +is also reflected for a time in the gill-slits and arches, with their +corresponding fish-like heart and blood-vessels, during man's embryonic +development, as we saw in a former chapter. + +These are only a few of the more conspicuous instances of vestigial +structures in man. Metchnikoff describes about a hundred of them. Even +if there were no remains of primitive man pointing in the direction of +a common ancestry with the ape, no lower types of men in existence with +the same tendency, no apes found in nature to-day with a structure so +strikingly similar to that of man, and no fossil records telling of the +divergence of forms from primitive groups in past time, we should be +forced to postulate the evolution of man in order to explain his actual +features. The vestigial structures must be interpreted as we interpret +the buttons on the back of a man's coat. They are useless reminiscences +of an age in which they were useful. When their witness to the past +is supported by so many converging lines of evidence it becomes +irresistible. I will add only one further testimony which has been +brought into court in recent years. + +The blood consists of cells, or minute disk-shaped corpuscles, floating +in a watery fluid, or serum. It was found a few years ago, in the course +of certain experiments in mixing the blood of animals, that the serum of +one animal's blood sometimes destroyed the cells of the other animal's +blood, and at other times did not. When the experiments were multiplied, +it was found that the amount of destructive action exercised by one +specimen of blood upon another depended on the nearness or remoteness of +relationship between the animals. If the two are closely related, there +is no disturbance when their blood is mixed; when they are not closely +related, the serum of one destroys the cells of the other, and the +intensity of the action is in proportion to their remoteness from each +other. Another and more elaborate form of the experiment was devised, +and the law was confirmed. On both tests it was found by experiment that +the blood of man and of the anthropoid ape behaved in such a way as to +prove that they were closely related. The blood of the monkey showed a +less close relationship--a little more remote in the New World than in +the Old World monkeys; and the blood of the femur showed a faint and +distant relationship. + +The FACT of the evolution of man and the apes from a common ancestor is, +therefore, outside the range of controversy in science; we are concerned +only to retrace the stages of that evolution, and the agencies which +controlled it. Here, unfortunately, the geological record gives us +little aid. Tree-dwelling animals are amongst the least likely to +be buried in deposits which may preserve their bones for ages. The +distribution of femur and ape remains shows that the order of the +Primates has been widespread and numerous since the middle of the +Tertiary Era, yet singularly few remains of the various families have +been preserved. + +Hence the origin of the Primates is obscure. They are first foreshadowed +in certain femur-like forms of the Eocene period, which are said in some +cases (Adapis) to combine the characters of pachyderms and femurs, and +in others (Anaptomorphus) to unite the features of Insectivores and +femurs. Perhaps the more common opinion is that they were evolved from +a branch of the Insectivores, but the evidence is too slender to justify +an opinion. It was an age when the primitive placental mammals were just +beginning to diverge from each other, and had still many features in +common. For the present all we can say is that in the earliest spread +of the patriarchal mammal race one branch adopted arboreal life, and +evolved in the direction of the femurs and the apes. The generally +arboreal character of the Primates justifies this conclusion. + +In the Miocene period we find a great expansion of the monkeys. These in +turn enter the scene quite suddenly, and the authorities are reduced to +uncertain and contradictory conjectures as to their origin. Some think +that they develop not from the femurs, but along an independent line +from the Insectivores, or other ancestors of the Primates. We will not +linger over these early monkeys, nor engage upon the hopeless task of +tracing their gradual ramification into the numerous families of the +present age. It is clear only that they soon divided into two main +streams, one of which spread into the monkeys of America and the other +into the monkeys of the Old World. There are important anatomical +differences between the two. The monkeys remained in Central and +Southern Europe until near the end of the Tertiary. Gradually we +perceive that the advancing cold is driving them further south, and +the monkeys of Gibraltar to-day are the diminished remnant of the great +family that had previously wandered as far as Britain and France. + +A third wave, also spreading in the Miocene, equally obscure in its +connection with the preceding, introduces the man-like apes to the +geologist. Primitive gibbons (Pliopithecus and Pliobylobates), +primitive chimpanzees (Palaeopithecus), and other early anthropoid +apes (Oreopithecus, Dryopithecus, etc.), lived in the trees of Southern +Europe in the second part of the Tertiary Era. They are clearly +disconnected individuals of a large and flourishing family, but from the +half-dozen specimens we have yet discovered no conclusion can be drawn, +except that the family is already branching into the types of anthropoid +apes which are familiar to us. + +Of man himself we have no certain and indisputable trace in the Tertiary +Era. Some remains found in Java of an ape-man (Pithecanthropus), +which we will study later, are now generally believed, after a special +investigation on the spot, to belong to the Pleistocene period. Yet no +authority on the subject doubts that the human species was evolved in +the Tertiary Era, and very many, if not most, of the authorities believe +that we have definite proof of his presence. The early story of mankind +is gathered, not so much from the few fragments of human remains we +have, but from the stone implements which were shaped by his primitive +intelligence and remain, almost imperishable, in the soil over which he +wandered. The more primitive man was, the more ambiguous would be +the traces of his shaping of these stone implements, and the earliest +specimens are bound to be a matter of controversy. It is claimed by many +distinguished authorities that flints slightly touched by the hand of +man, or at least used as implements by man, are found in abundance +in England, France, and Germany, and belong to the Pliocene period. +Continental authorities even refer some of them to the Miocene and the +last part of the Oligocene. + +The question whether an implement-using animal, which nearly all would +agree to regard as in some degree human, wandered over what is now +the South of England (Kent, Essex, Dorsetshire, etc.) as many hundred +thousand years ago as this claim would imply, is certainly one of great +interest. But there would be little use in discussing here the question +of the "Eoliths," as these disputed implements are called. A very keen +controversy is still being conducted in regard to them, and some of the +highest authorities in England, France, and Germany deny that they show +any trace of human workmanship or usage. Although they have the support +of such high authorities as Sir J. Prestwich, Sir E. Ray Lankester, Lord +Avebury, Dr. Keane, Dr. Blackmore, Professor Schwartz, etc., they are +one of those controverted testimonies on which it would be ill-advised +to rely in such a work as this. + +We must say, then, that we have no undisputed traces of man in the +Tertiary Era. The Tertiary implements which have been at various times +claimed in France, Italy, and Portugal are equally disputed; the remains +which were some years ago claimed as Tertiary in the United States are +generally disallowed; and the recent claims from South America are under +discussion. Yet it is the general feeling of anthropologists that man +was evolved in the Tertiary Era. On the one hand, the anthropoid apes +were highly developed by the Miocene period, and it would be almost +incredible that the future human stock should linger hundreds of +thousands of years behind them. On the other hand, when we find the +first traces of man in the Pleistocene, this development has already +proceeded so far that its earlier phase evidently goes back into the +Tertiary. Let us pass beyond the Tertiary Era for a moment, and examine +the earliest and most primitive remains we have of human or semi-human +beings. + +The first appearance of man in the chronicle of terrestrial life is a +matter of great importance and interest. Even the least scientific of +readers stands, so to say, on tiptoe to catch a first glimpse of +the earliest known representative of our race, and half a century of +discussion of evolution has engendered a very wide interest in the early +history of man. [*] + + * A personal experience may not be without interest in this + connection. Among the many inquiries directed to me in + regard to evolution I received, in one month, a letter from + a negro in British Guiana and an extremely sensible query + from an inmate of an English asylum for the insane! The + problem that beset the latter of the two was whether the + Lemuranda preceded the Lemurogona in Eocene times. He had + found a contradiction in the statements of two scientific + writers. + + +Fortunately, although these patriarchal bones are very scanty--two +teeth, a thigh-bone, and the skull-cap--we are now in a position to form +some idea of the nature of their living owner. They have been subjected +to so searching a scrutiny and discussion since they were found in +Java in 1891 and 1892 that there is now a general agreement as to their +nature. At first some of the experts thought that they were the remains +of an abnormally low man, and others that they belonged to an abnormally +high ape. The majority held from the start that they belonged to a +member of a race almost midway between the highest family of apes and +the lowest known tribe of men, and therefore fully merited the name +of "Ape-Man" (Pithecanthropus). This is now the general view of +anthropologists. + +The Ape-Man of Java was in every respect entitled to that name. The +teeth suggest a lower part of the face in which the teeth and lips +projected more than in the most ape-like types of Central Africa. +The skull-cap has very heavy ridges over the eyes and a low receding +forehead, far less human than in any previously known prehistoric skull. +The thigh-bone is very much heavier than any known human femur of the +same length, and so appreciably curved that the owner was evidently in +a condition of transition from the semi-quadrupedal crouch of the ape +to the erect attitude of man. The Ape-Man, in other words, was a heavy, +squat, powerful, bestial-looking animal; of small stature, but above the +pygmy standard; erect in posture, but with clear traces of the proneness +of his ancestor; far removed from the highest ape in brainpower, but +almost equally far removed from the lowest savage that is known to +us. We shall see later that there is some recent criticism, by weighty +authorities, of the earlier statements in regard to the brain of +primitive man. This does not apply to the Ape-Man of Java. The average +cranial capacity (the amount of brain-matter the skull may contain) of +the chimpanzees, the highest apes, is about 600 cubic centimetres. +The average cranial capacity of the lowest races of men, of moderate +stature, is about 1200. And the cranial capacity of Ape-Man was about +900 + +It is immaterial whether or no these bones belong to the same +individual. If they do not, we have remains of two or three individuals +of the same intermediate species. Nor does it matter whether or no this +early race is a direct ancestor of the later races of men, or an extinct +offshoot from the advancing human stock. It is, in either case, an +illustration of the intermediate phase between the ape and man The more +important tasks are to trace the relationship of this early human stock +to the apes, and to discover the causes of its superior evolution. + +The first question has a predominantly technical interest, and the +authorities are not agreed in replying to it. We saw that, on the +blood-test, man showed a very close relationship to the anthropoid apes, +a less close affinity to the Old World monkeys, a more remote affinity +to the American monkeys, and a very faint and distant affinity to the +femurs. A comparison of their structures suggests the same conclusion. +It is, therefore, generally believed that the anthropoid apes and man +had a common ancestor in the early Miocene or Oligocene, that this group +was closely related to the ancestral group of the Old World monkeys, and +that all originally sprang from a primitive and generalised femur-group. +In other words, a branch of the earliest femur-like forms diverges, +before the specific femur-characters are fixed, in the direction of the +monkey; in this still vague and patriarchal group a branch diverges, +before the monkey-features are fixed, in the direction of the +anthropoids; and this group in turn spreads into a number of types, some +of which are the extinct apes of the Miocene, four become the gorilla, +chimpanzee, orang, and gibbon of to-day, and one is the group that will +become man. To put it still more precisely, if we found a whole series +of remains of man's ancestors during the Tertiary, we should probably +class them, broadly, as femur-remains in the Eocene, monkey-remains in +the Oligocene, and ape-remains in the Miocene. In that sense only man +"descends from a monkey." + +The far more important question is: How did this one particular group of +anthropoid animals of the Miocene come to surpass all its cousins, and +all the rest of the mammals, in brain-development? Let us first rid the +question of its supposed elements of mystery and make of it a simple +problem. Some imagine that a sudden and mysterious rise in intelligence +lifted the progenitor of man above its fellows. The facts very quickly +dispel this illusion. We may at least assume that the ancestor of man +was on a level with the anthropoid ape in the Miocene period, and we +know from their skulls that the apes were as advanced then as they are +now. But from the early Miocene to the Pleistocene is a stretch of +about a million years on the very lowest estimate. In other words, +man occupied about a million years in travelling from the level of the +chimpanzee to a level below that of the crudest savage ever discovered. +If we set aside the Java man, as a possible survivor of an earlier +phase, we should still have to say that, much more than a million years +after his departure from the chimpanzee level, man had merely advanced +far enough to chip stone implements; because we find no other trace +whatever of intelligence than this until near the close of the +Palaeolithic period. If there is any mystery, it is in the slowness of +man's development. + +Let us further recollect that it is a common occurrence in the calendar +of life for a particular organ to be especially developed in one +member of a particular group more than in the others. The trunk of the +elephant, the neck of the giraffe, the limbs of the horse or deer, the +canines of the satire-toothed tiger, the wings of the bat, the colouring +of the tiger, the horns of the deer, are so many examples in the mammal +world alone. The brain is a useful organ like any other, and it is easy +to conceive that the circumstances of one group may select it just as +the environment of another group may lead to the selection of speed, +weapons, or colouring. In fact, as we saw, there was so great and +general an evolution of brain in the Tertiary Era that our modern +mammals quite commonly have many times the brain of their Tertiary +ancestors. Can we suggest any reasons why brain should be especially +developed in the apes, and more particularly still in the ancestors of +man? + +The Primate group generally is a race of tree-climbers. The appearance +of fruit on early Tertiary trees and the multiplication of carnivores +explain this. The Primate is, except in a few robust cases, a +particularly defenceless animal. When its earliest ancestors came in +contact with fruit and nut-bearing trees, they developed climbing power +and other means of defence and offense were sacrificed. Keenness of +scent and range of hearing would now be of less moment, but sight would +be stimulated, especially when soft-footed climbing carnivores came on +the scene. There is, however, a much deeper significance in the adoption +of climbing, and we must borrow a page from the modern physiology of the +brain to understand it. + +The stress laid in the modern education of young children on the use of +the hands is not merely due to a feeling that they should handle objects +as well as read about them. It is partly due to the belief of many +distinguished physiologists that the training of the hands has a direct +stimulating effect on the thought-centres in the brain. The centre in +the cerebrum which controls the use of the hands is on the fringe of +the region which seems to be concerned in mental operations. For reasons +which will appear presently, we may add that the centres for controlling +the muscles of the face and head are in the same region. Any finer +training or the use of the hands will develop the centre for the fore +limbs, and, on the principles, may react on the more important region of +the cortex. Hence in turning the fore foot into a hand, for climbing +and grasping purposes, the primitive Primate entered upon the path +of brain-development. Even the earliest Primates show large brains in +comparison with the small brains of their contemporaries. + +It is a familiar fact in the animal world that when a certain group +enters upon a particular path of evolution, some members of the group +advance only a little way along it, some go farther, and some outstrip +all the others. The development of social life among the bees will +illustrate this. Hence we need not be puzzled by the fact that the +lemurs have remained at one mental level, the monkeys at another, and +the apes at a third. It is the common experience of life; and it is +especially clear among the various races of men. A group becomes fitted +to its environment, and, as long as its surroundings do not change, it +does not advance. A related group, in a different environment, receives +a particular stimulation, and advances. If, moreover, a group remains +unstimulated for ages, it may become so rigid in its type that it loses +the capacity to advance. It is generally believed that the lowest +races of men, and even some of the higher races like the Australian +aboriginals, are in this condition. We may expect this "unteachability" +in a far more stubborn degree in the anthropoid apes, which have been +adapted to an unchanging environment for a million years. + +All that we need further suppose is--and it is one of the commonest +episodes in terrestrial life--that one branch of the Miocene +anthropoids, which were spread over a large part of the earth, received +some stimulus to change which its cousins did not experience. It is +sometimes suggested that social life was the great advantage which led +to the superior development of mind in man. But such evidence as there +is would lead us to suppose that primitive man was solitary, not social. +The anthropoid apes are not social, but live in families, and are very +unprogressive. On the other hand, the earliest remains of prehistoric +man give no indication of social life. Fire-places, workshops, caves, +etc., enter the story in a later phase. Some authorities on prehistoric +man hold very strongly that during the greater part of the Old Stone +Age (two-thirds, at least, of the human period) man wandered only in the +company of his mate and children. [*] + + * The point will be more fully discussed later. This account + of prehistoric life is well seen in Mortillet's + Prehistorique (1900). The lowest races also have no tribal + life, and Professor Westermarck is of opinion that early man + was not social. + + +We seem to have the most plausible explanation of the divergence of man +from his anthropoid cousins in the fact that he left the trees of his +and their ancestors. This theory has the advantage of being a fact--for +the Ape-Man race of Java has already left the trees--and providing a +strong ground for brain-advance. A dozen reasons might be imagined for +his quitting the trees--migration, for instance, to a region in +which food was more abundant, and carnivores less formidable, on the +ground-level--but we will be content with the fact that he did. Such a +change would lead to a more consistent adoption of the upright attitude, +which is partly found in the anthropoid apes, especially the gibbons. +The fore limb would be no longer a support of the body; the hand would +be used more for grasping; and the hand-centre in the brain would be +proportionately stimulated. The adoption of the erect attitude would +further lead to a special development of the muscles of the head and +face, the centre for which is in the same important region in the +cortex. There would also be a direct stimulation of the brain, as, +having neither weapons nor speed, the animal would rely all the more on +sight and mind. If we further suppose that this primitive being extended +the range of his hunting, from insects and small or dead birds to small +land-animals, the stimulation would be all the greater. In a word, the +very fact of a change from the trees to the ground suggests a line of +brain-development which may plausibly be conceived, in the course of a +million years, to evolve an Ape-Man out of a man-like ape. And we are +not introducing any imaginary factor in this view of human origins. + +The problem of the evolution of man is often approached in a frame of +mind not far removed from that of the educated, but inexpert, European +who stands before the lowly figure of the chimpanzee, and wonders by +what miracle the gulf between it and himself was bridged. That is to lay +a superfluous strain on the imagination. The proper term of comparison +is the lowest type of human being known to us, since the higher types of +living men have confessedly evolved from the lower. But even the lowest +type of existing or recent savage is not the lowest level of humanity. +Whether or no the Tasmanian or the Yahgan is a primitive remnant of the +Old Stone Age, we have a far lower depth in the Java race. What we have +first to do is to explain the advance to that level, in the course of +many hundreds of thousands of years: a period fully a hundred times as +long as the whole history of civilisation. Time itself is no factor +in evolution, but in this case it is a significant condition. It means +that, on this view of the evolution of man, we are merely assuming that +an advance in brain-development took place between the Miocene and the +Pleistocene, not similar to, but immeasurably less than, the advance +which we know to have been made in the last fifty thousand years. In +point of fact, the most mysterious feature of the evolution of man was +its slowness. We shall see that, to meet the facts, we must suppose man +to have made little or no progress during most of this vast period, and +then to have received some new stimulation to develop. What it was we +have now to inquire. + + + +CHAPTER XIX. MAN AND THE GREAT ICE-AGE + +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. + + *See especially an address by Professor Sollas in the + Quarterly Journal of the Geological Society, Vol. LXVI. + (1910). + + +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. + + *See his article in Anthropologie, Vol. XX. (1909), p. 257. + As Professor Sollas mainly relies on Boule, it is important + to see that there is a very great difference between the + two. + + +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. + + + +CHAPTER XX. THE DAWN OF CIVILISATION + +The story of man before the discovery of metal and the attainment of +civilisation is notoriously divided into a Palaeolithic (Old Stone) Age, +and a Neolithic (New Stone) Age. Each of these ages is now subdivided +into stages, which we will review in succession. But it is important +to conceive the whole story of man in more correct proportion than this +familiar division suggests. The historical or civilised period is now +computed at about ten thousand years. The Neolithic Age, which preceded +civilisation, is usually believed to be about four or five times as +long, though estimates of its duration vary from about twenty to a +hundred thousand years. The Palaeolithic Age in turn is regarded as at +least three or four times as long as the Neolithic; estimates of time +vary from a hundred to five hundred thousand years. And before this +there is the vast stretch of time in which the ape slowly became a +primitive human. + +This long, early period is, as we saw, still wrapped in mist and +controversy. A few bones tell of a race living, in semi-human shape, in +the region of the Indian Ocean; a few crude stones are held by many to +indicate that a more advanced, but very lowly race, wandered over the +south of Europe and north of Africa before the Ice-Age set in. The +starting-point or cradle of the race is not known. The old idea of +seeking the patriarchal home on the plains to the north of India is +abandoned, and there is some tendency to locate it in the land which has +partly survived in the islands of the Indian Ocean. The finding of early +remains in Java is not enough to justify that conclusion, but it obtains +a certain probability when we notice the geographical distribution of +the Primates. The femurs and the apes are found to-day in Africa and +Asia alone; the monkeys have spread eastward to America and westward to +Europe and Africa; the human race has spread north-eastward into Asia +and America, northwestward into Europe, westward into Africa, and +southward to Australia and the islands. This distribution suggests +a centre in the Indian Ocean, where there was much more land in the +Tertiary Era than there is now. We await further exploration in that +region and Africa. + +There is nothing improbable in the supposition that man wandered into +Europe in the Tertiary, and has left in the Eoliths the memorials of his +lowly condition. The anthropoid apes certainly reached France. However +that may be, the Ice-Age would restrict all the Primates to the south. +It will be seen, on a glance at the map, that a line of ice-clad +mountains would set a stern barrier to man's advance in the early +Pleistocene, from the Pyrenees to the Himalaya, if not to the Pacific. +He therefore spread westward and southward. One branch wandered into +Australia, and was afterwards pressed by more advanced invaders (the +present blacks of Australia) into Tasmania, which seems to have been +still connected by land. Another branch, or branches, spread into +Africa, to be driven southward, or into the central forests, by later +and better equipped invaders. They survive, little changed (except by +recent contact with Europeans), in the Bushmen and in large populations +of Central Africa which are below the level of tribal organisation. +Others remained in the islands, and we seem to have remnants of them +in the Kalangs, Veddahs, etc. But these islands have been repeatedly +overrun by higher races, and the primitive life has been modified. + +Comparing the most isolated of these relics of early humanity, we obtain +many suggestions about the life of that remote age. The aboriginal +Tasmanians, who died out about forty years ago, were of great +evolutionary interest. It is sometimes said that man is distinguished +from all other animals by the possession of abstract ideas, but the very +imperfect speech of the Tasmanians expressed no abstract ideas. Their +mind seems to have been in an intermediate stage of development. They +never made fire, and, like the other surviving fragments of early +humanity, they had no tribal organisation, and no ideas of religion or +morality. + +The first effect of the Ice-Age on this primitive humanity would be to +lead to a beginning of the development of racial characters. The pigment +under the skin of the negro is a protection against the actinic rays +of the tropical sun; the white man, with his fair hair and eyes, is a +bleached product of the northern regions; and the yellow or brown skin +seems to be the outcome of living in dry regions with great extremes +of temperature. As the northern hemisphere divided into climatic zones +these physical characters were bound to develop. The men who went +southward developed, especially when fully exposed to the sun on open +plains, the layer of black pigment which marks the negroid type. There +is good reason, as we shall see to think that man did not yet wear +clothing, though he had a fairly conspicuous, if dwindling, coat +of hair. On the other hand the men who lingered further north, in +South-western Asia and North Africa, would lose what pigment they +had, and develop the lighter characters of the northerner. It has been +noticed that even a year in the arctic circle has a tendency to make +the eyes of explorers light blue. We may look for the genesis of +the vigorous, light-complexioned races along the fringe of the great +ice-sheet. It must be remembered that when the limit of the ice-sheet +was in Central Germany and Belgium, the climate even of North Africa +would be very much more temperate than it is to-day. + +As the ice-sheet melted, the men who were adapted to living in the +temperate zone to the south of it penetrated into Europe, and the long +story of the Old Stone Age opened. It must not, of course, be supposed +that this stage of human culture only began with the invasion of Europe. +Men would bring their rough art of fashioning implements with them, but +the southern regions are too little explored to inform us of the earlier +stage. But as man enters Europe he begins to drop his flints on a soil +that we have constant occasion to probe--although the floor on which +he trod is now sometimes forty or fifty feet below the surface--and we +obtain a surer glimpse of the fortunes of our race. + +Most European geologists count four distinct extensions of the +ice-sheet, with three interglacial periods. It is now generally believed +that man came north in the third interglacial period; though some high +authorities think that he came in the second. As far as England is +concerned, it has been determined, under the auspices of the British +Association, that our oldest implements (apart from the Eoliths) are +later than the great ice-sheet, but there is some evidence that they +precede the last extension of the ice. + +Two stages are distinguished in this first part of the Palaeolithic +Age--the Acheulean and Chellean--but it will suffice for our purpose to +take the two together as the earlier and longer section of the Old Stone +Age. It was a time of temperate, if not genial, climate. The elephant +(an extinct type), the rhinoceros, the hippopotamus, the hyaena, and +many other forms of animal life that have since retired southward, were +neighbours of the first human inhabitant of Europe. Unfortunately, we +have only one bone of this primitive race, the jaw found at Mauer in +1907, but its massive size and chinless contour suggest a being midway +between the Java man and the Neanderthal race. His culture confirms the +supposition. There is at this stage no clear trace of fire, clothing, +arrows, hefted weapons, spears, or social life. As the implements are +generally found on old river-banks or the open soil, not in caves, we +seem to see a squat and powerful race wandering, homeless and unclad, +by the streams and broad, marshy rivers of the time. The Thames and the +Seine had not yet scooped out the valleys on the slopes of which London +and Paris are built. + +This period seems, from the vast number of stone implements referred to +it, to have lasted a considerable time. There is a risk in venturing to +give figures, but it may be said that few authorities would estimate it +at less than a hundred thousand years. Man still advanced with very +slow and uncertain steps, his whole progress in that vast period being +measured by the invention of one or two new forms of stone implements +and a little more skill in chipping them. At its close a great +chill comes over Europe--the last ice-sheet is, it seems, spreading +southward--and we enter the Mousterian period and encounter the +Neanderthal race which we described in the preceding chapter. + +It must be borne in mind that the whole culture of primitive times is +crushed into a few feet of earth. The anthropologist is therefore quite +unable to show us the real succession of human stages, and has to be +content with a division of the whole long and gradual evolution into a +few well-marked phases. These phases, however, shade into each other, +and are merely convenient measurements of a continuous story. The +Chellean man has slowly advanced to a high level. There is no sudden +incoming of a higher culture or higher type of man. The most impressive +relics of the Mousterian period, which represent its later epoch, are +merely finely chipped implements. There is no art as yet, no pottery, +and no agriculture; and there is no clear trace of the use of fire or +clothing, though we should be disposed to put these inventions in the +chilly and damp Mousterian period. There is therefore no ground for +resenting the description, "the primeval savage," which has been applied +to early man. The human race is already old, yet, as we saw, it is +hardly up to the level of the Australian black. The skeleton found at +Chapelle-aux-Saints is regarded as the highest known type of the race, +yet the greatest authority on it, M. Boule, says emphatically: "In no +actual race do we find the characters of inferiority--that is to say, +the ape-like features--which we find in the Chapelle-aux-Saints head." +The largeness of the head is in proportion to the robust frame, but +in its specifically human part--the front--it is very low and bestial; +while the heavy ridges over the large eyes, the large flat stumpy nose, +the thick bulge of the lips and teeth, and the almost chinless jaw, show +that the traces of his ancestry cling close to man after some hundreds +of thousands of years of development. + +The cold increases as we pass to the last part of the Old Stone Age, the +Solutrean and Magdalenian periods; and nothing is clearer than that the +pace of development increases at the same time. Short as the period is, +in comparison with the preceding, it witnesses a far greater advance +than had been made in all the rest of the Old Stone Age. Beyond a doubt +men now live in caves, in large social groups, make clothing from the +skins of animals, have the use of fire, and greatly improve the quality +of their stone axes, scrapers, knives, and lance-heads. There is at +last some promise of the civilisation that is coming. In the soil of +the caverns in which man lived, especially in Southern France and the +Pyrenean region, we find the debris of a much larger and fuller life. +Even the fine bone needles with which primitive man sewed his skin +garments, probably with sinews for thread, survive in scores. In other +places we find the ashes of the fires round which he squatted, often +associated with the bones of the wild horses, deer, etc., on which he +lived. + +But the most remarkable indication of progress in the "cave-man" is his +artistic skill. Exaggerated conclusions are sometimes drawn from the +statuettes, carvings, and drawings which we find among the remains of +Magdalenian life. Most of them are crude, and have the limitations of a +rustic or a child artist. There is no perspective, no grouping. Animals +are jumbled together, and often left unfinished because the available +space was not measured. There are, however, some drawings--cut on bone +or horn or stone with a flint implement--which evince great skill in +line-drawing and, in a few cases, in composition. Some of the caves +also are more or less frescoed; the outlines of animals, sometimes of +life-size and in great numbers, are cut in the wall, and often filled in +with pigment. This skill does not imply any greater general intelligence +than the rest of the culture exhibits. It implies persistent and +traditional concentration upon the new artistic life. The men who drew +the "reindeer of Thayngen" and carved the remarkable statuettes of women +in ivory or stone, were ignorant of the simplest rudiments of pottery or +agriculture, which many savage tribes possess. + +Some writers compare them with the Eskimo of to-day, and even suggest +that the Eskimo are the survivors of the race, retreating northward with +the last ice-sheet, and possibly egged onward by a superior race from +the south. It is, perhaps, not a very extravagant claim that some +hundreds of thousands of years of development--we are now only a few +tens of thousands of years from the dawn of civilisation--had lifted +man to the level of the Eskimo, yet one must hesitate to admit +the comparison. Lord Avebury reproduces an Eskimo drawing, or +picture-message, in his "Prehistoric Times," to which it would be +difficult to find a parallel in Magdalenian remains. I do not mean +that the art is superior, but the complex life represented on the +picture-message, and the intelligence with which it is represented, are +beyond anything that we know of Palaeolithic man. I may add that nearly +all the drawings and statues of men and women which the Palaeolithic +artist has left us are marked by the intense sexual exaggeration--the +"obscenity," in modern phraseology--which we are apt to find in coarse +savages. + +Three races are traced in this period. One, identified by skeletons +found at Mentone and by certain statuettes, was negroid in character. +Probably there was an occasional immigration from Africa. Another race +(Cro-Magnon) was very tall, and seems to represent an invasion from some +other part of the earth toward the close of the Old Stone Age. The third +race, which is compared to the Eskimo, and had a stature of about five +feet, seem to be the real continuers of the Palaeolithic man of Europe. +Curiously enough, we have less authentic remains of this race than +of its predecessor, and can only say that, as we should expect, the +ape-like features--the low forehead, the heavy frontal ridges, the +bulging teeth, etc.--are moderating. The needles we have found--round, +polished, and pierced splinters of bone, sometimes nearly as fine as a +bodkin--show indisputably that man then had clothing, but it is curious +that the artist nearly always draws him nude. There is also generally a +series of marks round the contour of the body to indicate that he had a +conspicuous coat of hair. Unfortunately, the faces of the men are merely +a few unsatisfactory gashes in the bone or horn, and do not picture +this interesting race to us. The various statuettes of women generally +suggest a type akin to the wife of the Bushman. + +We have, in fine, a race of hunters, with fine stone knives and +javelins. Toward the close of the period we find a single representation +of an arrow, which was probably just coming into use, but it is not +generally known in the Old Stone Age. One of the drawings seems to +represent a kind of bridle on a horse, but we need more evidence than +this to convince us that the horse was already tamed, nor is there any +reason to suppose that the dog or reindeer had been tamed, or that the +ground was tilled even in the most rudimentary way. Artistic skill, the +use of clothing and fire, and a finer feeling in the shaping of weapons +and implements, are the highest certain indications of the progress made +by the end of the Old Stone Age. + +But there was probably an advance made which we do not find recorded, +or only equivocally recorded, in the memorials of the age. Speech was +probably the greatest invention of Magdalenian man. It has been pointed +out that the spine in the lower jaw, to which the tongue-muscle is +attached, is so poorly developed in Palaeolithic man that we may +infer from it the absence of articulate speech. The deduction has been +criticised, but a comparison of the Palaeolithic jaw with that of the +ape on one hand and modern man on the other gives weight to it. Whatever +may have been earlier man's power of expression, the closer social life +of the Magdalenian period would lead to a great development of it. Some +writers go so far as to suggest that certain obscure marks painted +on pebbles or drawn on the cavern-walls by men at the close of the +Palaeolithic Age may represent a beginning of written language, or +numbers, or conventional signs. The interpretation of these is obscure +and doubtful. It is not until ages afterwards that we find the first +clear traces of written language, and then they take the form of +pictographs (like the Egyptian hieroglyphics or the earliest Chinese +characters). + +We cannot doubt, however, that articulate speech would be rapidly +evolved in the social life of the later Magdalenian period, and the +importance of this acquisition can hardly be exaggerated. Imagine even a +modern community without the device of articulate language. A very large +proportion of the community, who are now maintained at a certain level +by the thought of others, communicated to them by speech, would sink +below the civilised standard, and the transmission and improvement of +ideas would be paralysed. It would not be paradoxical to regard the +social life and developing speech of Magdalenian man as the chief cause +of the rapid advance toward civilisation which will follow in the next +period. + +And it is not without interest to notice that a fall in the temperature +of the earth is the immediate cause of this social life. The building +of homes of any kind seems to be unknown to Magdalenian man. The artist +would have left us some sketchy representation of it if there had been +anything in the nature of a tent in his surroundings. The rock-shelter +and the cave are the homes which men seek from the advancing cold. As +these are relatively few in number, fixed in locality, and often of +large dimensions, the individualism of the earlier times is replaced by +collective life. Sociologists still dispute whether the clan arose +by the cohesion of families or the family arose within the clan. Such +evidence as is afforded by prehistoric remains is entirely in favour +of the opinion of Professor Westermarck, that the family preceded the +larger group. Families of common descent would now cling together and +occupy a common cavern, and, when the men gathered at night with the +women for the roasting and eating of the horse or deer they had hunted, +and the work of the artist and the woman was considered, the uncouth +muttering and gesticulating was slowly forged into the great instrument +of articulate speech. The first condition of more rapid progress was +instinctively gained. + +Our story of life has so often turned on this periodical lowering of the +climate of the earth that it is interesting to find this last and most +important advance so closely associated with it that we are forced +once more to regard it as the effective cause. The same may be said of +another fundamental advance of the men of the later Palaeolithic age, +the discovery of the art of making fire. It coincides with the oncoming +of the cold, either in the Mousterian or the Magdalenian. It was more +probably a chance discovery than an invention. Savages so commonly make +fire by friction--rubbing sticks, drills, etc.--that one is naturally +tempted to regard this as the primitive method. I doubt if this was the +case. When, in Neolithic times, men commonly bury the dead, and put some +of their personal property in the grave with them, the fire-kindling +apparatus we find is a flint and a piece of iron pyrites. Palaeolithic +man made his implements of any kind of hard and heavy stone, and it +is probable that he occasionally selected iron ore for the purpose. +An attempt to chip it with flint would cause sparks that might fall on +inflammable material, and set it alight. Little intelligence would be +needed to turn this discovery to account. + +Apart from these conjectures as to particular features in the life +of prehistoric man, it will be seen that we have now a broad and firm +conception of its evolution. From the ape-level man very slowly mounts +to the stage of human savagery. During long ages he seems to have made +almost no progress. There is nothing intrinsically progressive in his +nature. Let a group of men be isolated at any stage of human evolution, +and placed in an unchanging environment, and they will remain stationary +for an indefinite period. When Europeans began to traverse the globe in +the last few centuries, they picked up here and there little groups of +men who had, in their isolation, remained just where their fathers had +been when they quitted the main road of advance in the earlier stages +of the Old Stone Age. The evolution of man is guided by the same laws +as the evolution of any other species. Thus we can understand the long +period of stagnation, or of incalculably slow advance. Thus, too, we can +understand why, at length, the pace of man toward his unconscious goal +is quickened. He is an inhabitant of the northern hemisphere, and +the northern hemisphere is shaken by the last of the great geological +revolutions. From its first stress emerges the primeval savage of the +early part of the Old Stone Age, still bearing the deep imprint of his +origin, surpassing his fellow-animals only in the use of crude stone +implements. Then the stress of conditions relaxes--the great ice-sheet +disappears--and again during a vast period he makes very little +progress. The stress returns. The genial country is stripped and +impoverished, and the reindeer and mammoth spread to the south of +Europe. But once more the adversity has its use, and man, stimulated +in his hunt for food, invigorated by the cold, driven into social life, +advances to the culmination of the Old Stone Age. + +We are still very far from civilisation, but the few tens of thousands +of years that separate Magdalenian man from it will be traversed with +relative speed--though, we should always remember, with a speed far +less than the pace at which man is advancing to-day. A new principle now +enters into play: a specifically human law of evolution is formulated. +It has no element of mysticism, and is merely an expression of the fact +that the previous general agencies of development have created in man +an intelligence of a higher grade than that of any other animal. In his +larger and more plastic brain the impressions received from the outer +world are blended in ideas, and in his articulate speech he has a unique +means of entering the idea-world of his fellows. The new principle +of evolution, which arises from this superiority, is that man's chief +stimulus to advance will now come from his cultural rather than his +physical environment. Physical surroundings will continue to affect +him. One race will outstrip another because of its advantage in soil, +climate, or geographical position. But the chief key to the remaining +and more important progress of mankind, which we are about to review, is +the stimulating contact of the differing cultures of different races. + +This will be seen best in the history of civilisation, but the principle +may be recognised in the New Stone Age which leads from primeval +savagery to civilisation, or, to be more accurate and just, to the +beginning of the historical period. It used to be thought that there was +a mysterious blank or gulf between the Old and the New Stone Age. +The Palaeolithic culture seemed to come to an abrupt close, and the +Neolithic culture was sharply distinguished from it. It was suspected +that some great catastrophe had destroyed the Palaeolithic race in +Europe, and a new race entered as the adverse conditions were removed. +This was especially held to be the case in England. The old Palaeolithic +race had never reached Ireland, which seems to have been cut oft from +the Continent during the Ice-Age, and most of the authorities still +believe--in spite of some recent claims--that it never reached Scotland. +England itself was well populated, and the remains found in the caves +of Derbyshire show that even the artist--or his art--had reached that +district. This Palaeolithic race seemed to come to a mysterious end, +and Europe was then invaded by the higher Neolithic race. England was +probably detached from the Continent about the end of the Magdalenian +period. It was thought that some great devastation--the last ice-sheet, +a submersion of the land, or a plague--then set in, and men were unable +to retreat south. + +It is now claimed by many authorities that there are traces of a +Middle Stone (Mesolithic) period even in England, and nearly all the +authorities admit that such a transitional stage can be identified +in the Pyrenean region. This region had been the great centre of the +Magdalenian culture. Its large frescoed caverns exhibit the culmination +of the Old Stone life, and afford many connecting links with the new. +It is, however, a clearly established and outstanding fact that the +characteristic art of Magdalenian man comes to an abrupt and complete +close, and it does not seem possible to explain this without supposing +that the old race was destroyed or displaced. If we could accept +the view that it was the Eskimo-like race of the Palaeolithic that +cultivated this art, and that they retreated north with the reindeer +and the ice, and survive in our Eskimo, we should have a plausible +explanation. In point of fact, we find no trace whatever of this slow +migration from the south of Europe to the north. The more probable +supposition is that a new race, with more finished stone implements, +entered Europe, imposed its culture upon the older race, and gradually +exterminated or replaced it. We may leave it open whether a part of the +old race retreated to the north, and became the Eskimo. + +Whence came the new race and its culture? It will be seen on reflection +that we have so far been studying the evolution of man in Europe only, +because there alone are his remains known with any fullness. But the +important region which stretches from Morocco to Persia must have been +an equally, if not more, important theatre of development. While Europe +was shivering in the last stage of the Ice-Age, and the mammoth and +reindeer browsed in the snows down to the south of France, this +region would enjoy an excellent climate and a productive soil. We may +confidently assume that there was a large and stirring population of +human beings on it during the Magdalenian cold. We may, with many of the +authorities, look to this temperate and fertile region for the slight +advance made by early Neolithic man beyond his predecessor. As the cold +relaxed, and the southern fringe of dreary steppe w as converted once +more into genial country, the race would push north. There is evidence +that there were still land bridges across the Mediterranean. From Spain +and the south of France this early Neolithic race rapidly spread over +Europe. + +It must not be supposed that the New Stone Age at first goes much beyond +the Old in culture. Works on prehistoric man are apt to give as features +of "Neolithic man" all that we know him to have done or discovered +during the whole of the New Stone Age. We read that he not only gave a +finer finish to, and sometimes polished, his stone weapons, but built +houses, put imposing monuments over his dead, and had agriculture, tame +cattle, pottery, and weaving. This is misleading, as the more advanced +of these accomplishments appear only late in the New Stone Age. The +only difference we find at first is that the stone axes, etc., are more +finely chipped or flaked, and are frequently polished by rubbing on +stone moulds. There is no sudden leap in culture or intelligence in the +story of man. + +It would be supremely interesting to trace the evolution of human +industries and ideas during the few tens of thousands of years of the +New Stone Age. During that time moral and religious ideas are largely +developed, political or social forms are elaborated, and the arts of +civilised man have their first rude inauguration. The foundations of +civilisation are laid. Unfortunately, precisely because the period is +relatively so short and the advance so rapid, its remains are crushed +and mingled in a thin seam of the geological chronicle, and we cannot +restore the gradual course of its development with any confidence. +Estimates of its duration vary from 20,000 to 70,000 years; though +Sir W. Turner has recently concluded, from an examination of marks +on Scottish monuments, that Neolithic man probably came on foot from +Scandinavia to Scotland, and most geologists would admit that it must be +at least a hundred thousand years since one could cross from Norway +to Scotland on foot. As usual, we must leave open the question of +chronology, and be content with a modest provisional estimate of 40,000 +or 50,000 years. + +We dimly perceive the gradual advance of human culture in this important +period. During the Old Stone Age man had made more progress than he had +made in the preceding million years; during the New Stone Age--at least +one-fourth as long as the Old--he made even greater progress; and, we +may add, in the historical period, which is one-fourth the length of the +Neolithic Age, he will make greater progress still. The pace of advance +naturally increases as intelligence grows, but that is not the whole +explanation. The spread of the race, the gathering of its members into +tribes, and the increasing enterprise of men in hunting and migration, +lead to incessant contacts of different cultures and a progressive +stimulation. + +At first Neolithic man is content with finer weapons. His stone axe is +so finely shaped and polished that it sometimes looks like forged or +moulded metal. He also drills a clean hole through it--possibly by means +of a stick working in wet sand--and gives it a long wooden handle. He +digs in the earth for finer flints, and in some of his ancient shafts +(Grimes, Graves and Cissbury) we find picks of reindeer horn +and hollowed blocks of chalk in which he probably burned fat for +illumination underground. But in the later part of the Neolithic--to +which much of this finer work also may belong--we find him building +huts, rearing large stone monuments, having tame dogs and pigs and oxen, +growing corn and barley, and weaving primitive fabrics. He lives in +large and strong villages, round which we must imagine his primitive +cornfields growing and his cattle grazing, and in which there must have +been some political organisation under chiefs. + +When we wish to trace the beginning of these inventions we have the same +difficulty that we experienced in tracing the first stages of new animal +types. The beginning takes place in some restricted region, and our +casual scratching of the crust of the earth or the soil may not touch +it for ages, if it has survived at all. But for our literature and +illustrations a future generation would be equally puzzled to know how +we got the idea of the aeroplane or the electric light. In some cases we +can make a good guess at the origin of Neolithic man's institutions. Let +us take pottery. Palaeolithic man cooked his joint of horse or reindeer, +and, no doubt, scorched it. Suppose that some Palaeolithic Soyer had +conceived the idea of protecting the joint, and preserving its juices, +by daubing it with a coat of clay. He would accidentally make a clay +vessel. This is Mr. Clodd's ingenious theory of the origin of pottery. +The development of agriculture is not very puzzling. The seed of corn +would easily be discovered to have a food-value, and the discovery of +the growth of the plant from the seed would not require a very high +intelligence. Some ants, we may recall, have their fungus-beds. It would +be added by many that the ant gives us another parallel in its keeping +of droves of aphides, which it "milks." But it is now doubted if the ant +deliberately cultivates the aphides with this aim. Early weaving might +arise from the plaiting of grasses. If wild flax were used, it might be +noticed that part of it remained strong when the rest decayed, and so +the threads might be selected and woven. + +The building of houses, after living for ages in stone caverns, would +not be a very profound invention. The early houses were--as may be +gathered from the many remains in Devonshire and Cornwall--mere rings of +heaped stones, over which, most probably, was put a roof of branches or +reeds, plastered with mud. They belong to the last part of the New Stone +Age. In other places, chiefly Switzerland, Neolithic man lived in wooden +huts built on piles in the shallow shores of lakes. It is an evidence +that life on land is becoming as stimulating as we find it in the age of +Deinosaurs or early mammals. These pile-villages of Switzerland lasted +until the historical period, and the numerous remains in the mud of the +lake show the gradual passage into the age of metal. + +Before the metal age opened, however, there seem to have been fresh +invasions of Europe and changes of its culture. The movements of the +various early races of men are very obscure, and it would be useless +to give here even an outline of the controversy. Anthropologists have +generally taken the relative length and width of the skull as a standard +feature of a race, and distinguished long-headed (dolichocephalic), +short-headed (brachycephalic), and middle-headed (mesaticephalic) races. +Even on this test the most divergent conclusions were reached in regard +to early races, and now the test itself is seriously disputed. Some +authorities believe that there is no unchanging type of skull in a +particular race, but that, for instance, a long-headed race may become +short-headed by going to live in an elevated region. + +It may be said, in a few words, that it is generally believed that two +races invaded Europe and displaced the first Neolithic race. The race +which chiefly settled in the Swiss region is generally believed to have +come from Asia, and advanced across Europe by way of the valley of the +Danube. The native home of the wheat and barley and millet, which, as +we know, the lake-dwellers cultivated, is said to be Asia. On the other +hand, the Neolithic men who have left stone monuments on our soil are +said to be a different race, coming, by way of North Africa, from Asia, +and advancing along the west of Europe to Scandinavia. A map of the +earth, on which the distribution of these stone monuments--all probably +connected with the burial of the dead--is indicated, suggests such a +line of advance from India, with a slighter branch eastward. But the +whole question of these invasions is disputed, and there are many who +regard the various branches of the population of Europe as sections of +one race which spread upward from the shores of the Mediterranean. + +It is clear at least that there were great movements of population, much +mingling of types and commercial interchange of products, so that we +have the constant conditions of advance. A last invasion seems to have +taken place some two or three thousand years before the Christian era, +when the Aryans overspread Europe. After all the controversy about the +Aryans it seems clear that a powerful race, representing the ancestors +of most of the actual peoples of Europe and speaking the dialects which +have been modified into the related languages of the Greeks, Romans, +Germans, Celts, Lithuanians, etc., imposed its speech on nearly the +whole of the continent. Only in the Basques and Picts do we seem to find +some remnants of the earlier non-Aryan tongues. But whether these Aryans +really came from Asia, as it used to be thought, or developed in the +east of Europe, is uncertain. We seem justified in thinking that a very +robust race had been growing in numbers and power during the Neolithic +Age, somewhere in the region of South-east Europe and Southwest Asia, +and that a few thousand years before the Christian Era one branch of +it descended upon India, another upon the Persian region, and another +overspread Europe. We will return to the point later. Instead of being +the bearers of a higher civilisation, these primitive Aryans seem to +have been lower in culture than the peoples on whom they fell. + +The Neolithic Age had meantime passed into the Age of Metal. Copper was +probably the first metal to be used. It is easily worked, and is found +in nature. But the few copper implements we possess do not suggest a +"Copper Age" of any length or extent. It was soon found, apparently, +that an admixture of tin hardened the copper, and the Bronze Age +followed. The use of bronze was known in Egypt about 4800 B.C. (Flinders +Petrie), but little used until about 2000 B.C. By that time (or a few +centuries later) it had spread as far as Scandinavia and Britain. The +region of invention is not known, but we have large numbers of beautiful +specimens of bronze work--including brooches and hair-pins--in all parts +of Europe. Finally, about the thirteenth century B.C., we find the first +traces of the use of iron. The first great centre for the making of iron +weapons seems to have been Hallstatt, in the Austrian Alps, whence it +spread slowly over Europe, reaching Scandinavia and Britain between +500 and 300 B.C. But the story of man had long before this entered the +historical period, to which we now turn. + + + +CHAPTER XXI. EVOLUTION IN HISTORY + +In the preceding chapters I have endeavoured to show how, without +invoking any "definitely directed variations," which we seem to have +little chance of understanding, we may obtain a broad conception of the +way in which the earth and its living inhabitants came to be what they +are. No one is more conscious than the writer that this account is +extremely imperfect. The limits of the volume have permitted me to use +only a part of the material which modern science affords, but if the +whole of our discoveries were described the sketch would still remain +very imperfect. The evolutionary conception of the world is itself +undergoing evolution in the mind of man. Age by age the bits of fresh +discovery are fitted into the great mosaic. Large areas are still +left for the scientific artist of the future to fill. Yet even in +its imperfect state the evolutionary picture of the world is most +illuminating. The questions that have been on the lips of thoughtful men +since they first looked out with adult eyes on the panorama of nature +are partly answered. Whence and Why are no longer sheer riddles of the +sphinx. + +It remains to be seen if evolutionary principles will throw at least an +equal light on the progress of humanity in the historical period. +Here again the questions, Whence and Why, have been asked in vain for +countless ages. If man is a progressive animal, why has the progress +been confined to some of the race? If humanity shared at first a common +patrimony, why have the savages remained savages, and the barbarians +barbaric? Why has progress been incarnated so exceptionally in the white +section of the race, the Europeans? We approach these questions more +confidently after surveying the story of terrestrial life in the light +of evolutionary principles. Since the days of the primeval microbe it +has happened that a few were chosen and many were left behind. There was +no progressive element in the advancing few that was not shared by the +stagnant many. The difference lay in the environment. Let us see if this +principle applies to the history of civilisation. + +In the last chapter I observed that, with the rise of human +intelligence, the cultural environment becomes more important than the +physical. Since human progress is a progress in ideas and the emotions +which accompany them, this may seem to be a truism. In point of fact it +is assailed by more than one recent historical writer. The scepticism +is partly due to a misunderstanding. No one but a fanatical adherent of +extreme theories of heredity will deny that the physical surroundings of +a race continue to be of great importance. The progress of a particular +people may often be traced in part to its physical environment; +especially to changes of environment, by migration, for instance. +Further, it is not for a moment suggested that a race never evolves its +own culture, but has always to receive it from another. If we said that, +we should be ultimately driven to recognise culture, like the early +Chinese, as a gift of the gods. What is meant is that the chief key to +the progress of certain peoples, the arrest of progress in others, +and the entire absence of progress in others, is the study of their +relations with, or isolation from, other peoples. They make progress +chiefly according to the amount of stimulation they get by contact with +a diverse culture. + +Let us see if this furnishes a broad explanation of the position of the +various peoples of the world. The Ethnologist tells us that the +lowest peoples of the earth are the Yahgans of Tierra del Fuego, the +Hottentots, a number of little-understood peoples in Central Africa, +the wild Veddahs of Ceylon, the (extinct) Tasmanians, the Aetas in the +interior of the Philippines, and certain fragments of peoples on islands +of the Indian Ocean. There is not the least trace of a common element +in the environment of these peoples to explain why they have remained +at the level of primitive humanity. Many of them lived in the most +promising and resourceful surroundings. What is common to them all is +their isolation from the paths of later humanity. They represent the +first wave of human distribution, pressed to the tips of continents or +on islands by later waves, and isolated. The position of the Veddahs +is, to some extent, an exception; and it is interesting to find that the +latest German students of that curious people think that they have been +classed too low by earlier investigators. + +We cannot run over all the peoples of the earth in this way, but will +briefly glance at the lower races of the various continents. A branch +of the second phase of developing humanity, the negroid stock, spread +eastward over the Asiatic islands and Australia, and westward into +Africa. The extreme wing of this army, the Australian blacks, too +clearly illustrates the principle to need further reference. It has +retained for ages the culture of the middle Palaeolithic. The negritos +who penetrated to the Philippines are another extreme instance of +isolation. The Melanesians of the islands of the Indian and Pacific +Ocean are less low, because those islands have been slowly crossed by +a much higher race, the Polynesians. The Maoris of New Zealand, the +Tongans, Hawaians, etc., are people of our own (Caucasic) stock, +probably diverging to the south-east while our branch of the stock +pressed westward. This not only explains the higher condition of the +Maoris, etc., but also shows why they have not advanced like their +European cousins. Their environment is one of the finest in the world, +but--it lies far away from the highways of culture. + +In much the same way can we interpret the swarming peoples of Africa. +The more primitive peoples which arrived first, and were driven south or +into the central forests by the later and better equipped invaders from +the central zone, have remained the more primitive. The more northern +peoples, on the fringe of, or liable to invasion from, the central +zone, have made more advance, and have occasionally set up rudimentary +civilisations. But the movements from the north to the south in early +historical times are too obscure to enable us to trace the action of +the principle more clearly. The peoples of the Mediterranean fringe +of Africa, living in the central zone of stimulation, have proved very +progressive. Under the Romans North Africa was at least as civilised as +Britain, and an equally wise and humane European policy might lead to +their revival to-day. + +When we turn to Asia we encounter a mass of little-understood peoples +and a few civilisations with obscure histories, but we have a fairly +clear application of the principle. The northern, more isolated +peoples, are the more primitive; the north-eastern, whose isolation +is accentuated by a severe environment, are most primitive of all. The +Eskimo, whether they are the survivors of the Magdalenian race or a +regiment thrown off the Asiatic army as it entered America, remain at +the primitive level. The American peoples in turn accord with this view. +Those which penetrate furthest south remain stagnant or deteriorate; +those which remain in the far north remain below the level of +civilisation, because the land-bridge to Asia breaks down; but those +which settle in Central America evolve a civilisation. A large zone, +from Mexico to Peru, was overspread by this civilisation, and it was +advancing steadily when European invaders destroyed it, and reduced the +civilised Peruvians to the Quichas of to-day. + +There remain the civilisations of Asia, and here we have a new and +interesting aspect of the question. How did these civilisations develop +in Asia, and how is it that they have remained stagnant for ages, while +Europe advanced? The origin of the Asiatic civilisations is obscure. +The common idea of their vast antiquity has no serious ground. The +civilisation of Japan cannot be traced back beyond about the eighth +century B.C. Even then the population was probably a mixed flotsam from +neighbouring lands--Ainus, Koreans, Chinese, and Malays. What was the +character of the primitive civilisation resulting from the mixture +of these different cultures we do not know. But the chief elements +of Japanese civilisation came later from China. Japan had no written +language of any kind until it received one from China about the sixth +century of the Christian Era. + +The civilisation of China itself goes back at least to about 2300 +B.C., but we cannot carry it further back with any confidence. The +authorities, endeavouring to pick their steps carefully among old +Chinese legends, are now generally agreed that the primitive Chinese +were a nomadic tribe which slowly wandered across Asia from about the +shores of the Caspian Sea. In other words, they started from a region +close to the cradle of western civilisation. Some students, in +fact, make them akin to the Akkadians, who founded civilisation in +Mesopotamia. At all events, they seem to have conveyed a higher culture +to the isolated inhabitants of Western Asia, and a long era of progress +followed their settlement in a new environment. For more than two +thousand years, however, they have been enclosed in their walls and +mountains and seas, while the nations of the remote west clashed +unceasingly against each other. We need no other explanation of their +stagnation. To speak of the "unprogressiveness" of the Chinese is pure +mysticism. The next generation will see. + +The civilisation of India is also far later than the civilisation of the +west, and seems to be more clearly due to borrowing from the west. The +primitive peoples who live on the hills about India, or in the jungles, +are fragments, apparently, of the Stone Age inhabitants of India, or +their descendants. Their culture may have degenerated under the adverse +conditions of dislodgement from their home, but we may fairly conclude +that it was never high. On these primitive inhabitants of the plains +of India there fell, somewhere about or before 1000 B.C., the Asiatic +branch of the Aryan race. + +A very recent discovery (1908) has strongly confirmed and illumined this +view of the origin of Indian civilisation. Explorers in the ruins of the +ancient capital of the Hittite Empire (in North Syria and Cappadocia) +found certain treaties which had been concluded, about 1300 B.C., +between the Hittites and the king of the Aryans. The names of the +deities which are mentioned in the treaties seem to show that the +Persian and Indian branches of the Aryan race were not yet separated, +but formed a united kingdom on the banks of the Euphrates. They seem to +have come from Bactria (and possibly beyond), and introduced the horse +(hitherto unknown to the Babylonians) about 1800 B.C. It is surmised by +the experts that the Indian and Persian branches separated soon +after 1300 B.C., possibly on account of religious quarrels, and the +Sanscrit-speaking branch, with its Vedic hymns and its Hinduism, +wandered eastward and northward until it discovered and took possession +of the Indian peninsula. The long isolation of India, since the +cessation of its commerce with Rome until modern times, explains the +later stagnation of its civilisation. + +Thus the supposed "non-progressiveness" of the east, after once +establishing civilisation, turns out to be a question of geography and +history. We have now to see if the same intelligible principles will +throw light on the "progressiveness" of the western branch of the Aryan +race, and on the course of western civilisation generally. [*] + + * In speaking of Europeans as Aryans I am, of course, + allowing for an absorption of the conquered non-Aryans. A + European nation is no more Aryan, in strict truth, than the + English are Anglo-Saxon. + + +The first two centres of civilisation are found in the valley of the +Nile and the valley of the Tigris and Euphrates; the civilisations of +Egypt and Babylon, the oldest in the world. There is, however, a good +deal of evidence by which we may bring these civilisations nearer to +each other in their earliest stages, so that we must not confidently +speak of two quite independent civilisations. The civilisation which +developed on the Euphrates is found first at Susa, on the hills +overlooking the plains of Mesopotamia, about 6000 B.C. A people akin +to the Turkish or Chinese lives among the hills, and makes the vague +advance from higher Neolithic culture to primitive civilisation. About +the same time the historical or dynastic civilisation begins in Egypt, +and some high authorities, such as Mr. Flinders Petrie, believe that the +evidence suggests that the founders of this dynastic civilisation came +from "the mountainous region between Egypt and the Red Sea." From the +northern part of the same region, we saw, the ancestors of the Chinese +set out across Asia. + +We have here a very suggestive set of facts in connection with early +civilisation. The Syro-Arabian region seems to have been a thickly +populated centre of advancing tribes, which would be in striking accord +with the view of progress that I am following. But we need not press the +disputed and obscure theory of the origin of the historic Egyptians. The +remains are said to show that the lower valley of the Nile, which must +have been but recently formed by the river's annual deposit of mud, was +a theatre of contending tribes from about 8000 to 6000 B.C. The fertile +lands that had thus been provided attracted tribes from east, west, and +south, and there is a great confusion of primitive cultures on its soil. + +It is not certain that the race which eventually conquered and founded +the historical dynasties came from the mountainous lands to the east. It +is enough for us to know that the whole region fermented with jostling +peoples. Why it did so the previous chapters will explain. It is +the temperate zone into which men had been pressed by the northern +ice-sheet, and from Egypt to the Indian Ocean it remained a fertile +breeding-ground of nations. + +These early civilisations are merely the highest point of Neolithic +culture. The Egyptian remains show a very gradual development of +pottery, ornamentation, etc., into which copper articles are introduced +in time. The dawn of civilisation is as gradual as the dawn of the +day. The whole gamut of culture--Eolithic, Palaeolithic, Neolithic, and +civilised--is struck in the successive layers of Egyptian remains. +But to give even a summary of its historical development is neither +necessary nor possible here. The maintenance of its progress is as +intelligible as its initial advance. Unlike China, it lay in the main +region of human development, and we find that even before 6000 B.C. it +developed a system of shipping and commerce which kept it in touch with +other peoples over the entire region, and helped to promote development +both in them and itself. + +Equally intelligible is the development of civilisation in Mesopotamia. +The long and fertile valley which lies between the mountainous region +and the southern desert is, like the valley of the Nile, a quite recent +formation. The rivers have gradually formed it with their deposit in the +course of the last ten thousand years. As this rich soil became covered +with vegetation, it attracted the mountaineers from the north. As I +said, the earliest centre of the civilisation which was to culminate in +Babylon and Nineveh is traced at Susa, on the hills to the north, about +6000 B.C. The Akkadians (highlanders) or Sumerians, the Turanian people +who established this civilisation, descended upon the rivers, and, about +5000 B.C., set up the early cities of Mesopotamia. As in the case of +Egypt, again, more tribes were attracted to the fertile region, and +by about 4000 B.C. we find that Semitic tribes from the north have +superseded the Sumerians, and taken over their civilisation. + +In these ancient civilisations, developing in touch with each other, and +surrounded by great numbers of peoples at the high Neolithic level from +which they had themselves started, culture advanced rapidly. Not +only science, art, literature, commerce, law, and social forms were +developed, but moral idealism reached a height that compares well +even with that of modern times. The recovery in our time of the actual +remains of Egypt and Babylon has corrected much of the libellous legend, +which found its way into Greek and European literature, concerning +those ancient civilisations. But, as culture advances, human development +becomes so complex that we must refrain from attempting to pursue, even +in summary, its many outgrowths. The evolution of morality, of art, of +religion, of polity, and of literature would each require a whole volume +for satisfactory treatment. All that we can do here is to show how the +modern world and its progressive culture are related to these ancient +empires. + +The aphorism that "all light comes from the east" may at times be +pressed too literally. To suggest that western peoples have done no more +than receive and develop the culture of the older east would be at once +unscientific and unhistorical. By the close of the Neolithic age a great +number of peoples had reached the threshold of civilisation, and it +would be extremely improbable that in only two parts of the world the +conditions would be found of further progress. That the culture of +these older empires has enriched Europe and had a great share in its +civilisation, is one of the most obvious of historical truths. But we +must not seek to confine the action of later peoples to a mere borrowing +of arts or institutions. + +Yet some recent historical writers, in their eagerness to set up +indigenous civilisations apart from those of Egypt and Mesopotamia, pass +to the opposite extreme. We are prepared to find civilisation developing +wherever the situation of a people exposes it to sufficient stimulation, +and we do find advance made among many peoples apart from contact with +the great southern empires. It is uncertain whether the use of bronze +is due first to the southern nations or to some European people, but the +invention of iron weapons is most probably due to European initiative. +Again, it is now not believed that the alphabets of Europe are derived +from the hieroglyphics of Egypt, though it is an open question whether +they were not derived, through Phoenicia, from certain signs which we +find on ancient Egyptian pottery. + +If we take first a broad view of the later course of civilisation we see +at a glance the general relation of east and west. Some difficulty would +arise, if we pressed, as to the exact stage in which a nation may be +said to become "civilised," but we may follow the general usage of +archaeologists and historians. They tell us, then, that civilisation +first appears in Egypt about 8000 B.C. (settled civilisation about +6000 B.C.), and in the Mesopotamian region about 6000 B.C. We next find +Neolithic culture passing into what may be called civilisation in Crete +and the neighbouring islands some time between 4000 and 3000 B.C., or +two thousand years after the development of Egyptian commerce in that +region. We cannot say whether this civilisation in the AEgean sea +preceded others which we afterwards find on the Asiatic mainland. +The beginning of the Hittite Empire in Asia Minor, and of Phoenician +culture, is as yet unknown. But we can say that there was as yet no +civilisation in Europe. It is not until after 1600 that civilisation +is established in Greece (Mycenae and Tiryns) as an offshoot of AEgean +culture. Later still it appears among the Etruscans of Italy--to which, +as we know, both Egyptian and AEgean vessels sailed. In other words, the +course of civilisation is very plainly from east to west. + +But we must be careful not to imagine that this represents a mere +transplantation of southern culture on a rude northern stock. The whole +region to the east of the Mediterranean was just as fitted to develop a +civilisation as the valley of the Nile. It swarmed with peoples having +the latest Neolithic culture, and, as they advanced, and developed +navigation, the territory of many of them became the high road of more +advanced peoples. A glance at the map will show that the easiest line of +expansion for a growing people was westward. The ocean lay to the right +of the Babylonians, and the country north and south was not inviting. +The calmer Mediterranean with its fertile shores was the appointed field +of expansion. The land route from Egypt lay, not to the dreary west in +Africa, but along the eastern shore of the Mediterranean, through Syria +and Asia Minor. The land route from Babylon lay across northern +Syria and Asia Minor. The sea route had Crete for its first and most +conspicuous station. Hence the gradual appearance of civilisation in +Phoenicia, Cappadocia, Lydia, and the Greek islands is a normal and +natural outcome of the geographical conditions. + +But we must dismiss the later Asiatic civilisations, whose remains are +fast coming to light, very briefly. Phoenicia probably had less part +in the general advance than was formerly supposed. Now that we have +discovered a powerful civilisation in the Greek islands themselves, we +see that it would keep Tyre and Sidon in check until it fell into decay +about 1000 B.C. After that date, for a few centuries, Phoenicia had a +great influence on the development of Europe. The Hittites, on the other +hand, are as yet imperfectly known. Their main region was Cappadocia, +where, at least as far back as 1500 B.C., they developed so +characteristic a civilisation, that its documents or inscriptions are +almost undecipherable. They at one time overran the whole of Asia Minor. +Other peoples such as the Elamites, represent similar offshoots of the +fermenting culture of the region. The Hebrews were probably a small and +unimportant group, settled close round Jerusalem, until a few centuries +before the Christian Era. They then assimilated the culture of the +more powerful nations which crossed and recrossed their territory. The +Persians were, as we saw, a branch of the Aryan family which slowly +advanced between 1500 and 700 B.C., and then inherited the empire of +dying Babylon. + +The most interesting, and one of the most recently discovered, of these +older civilisations, was the AEgean. Its chief centre was Crete, but it +spread over many of the neighbouring islands. Its art and its script are +so distinctive that we must recognise it as a native development, not +a transplantation of Egyptian culture. Its ruins show it gradually +emerging from the Neolithic stage about 4000 B.C., when Egyptian +commerce was well developed in its seas. Somewhere about 2500 B.C. +the whole of the islands seem to have been brought under the Cretan +monarchy, and the concentration of wealth and power led to a remarkable +artistic development, on native lines. We find in Crete the remains of +splendid palaces, with advanced sanitary systems and a great luxuriance +of ornamentation. It was this civilisation which founded the centre +at Mycenae, on the Greek mainland, about the middle of the second +millennium B.C. + +But our inquiry into the origin of European civilisation does not +demand any extensive description of the AEgean culture and its Mycenaean +offshoot. It was utterly destroyed between 1500 and 1000 B.C., and +this was probably done by the Aryan ancestors of the later Greeks or +Hellenes. About the time when one branch of the Aryans was descending +upon India and another preparing to rival decaying Babylonia, the third +branch overran Europe. It seems to have been a branch of these that +swept down the Greek peninsula, and crossed the sea to sack and destroy +the centres of AEgean culture. Another branch poured down the Italian +peninsula; another settled in the region of the Baltic, and would prove +the source of the Germanic nations; another, the Celtic, advanced to the +west of Europe. The mingling of this semi-barbaric population with +the earlier inhabitants provided the material of the nations of modern +Europe. Our last page in the story of the earth must be a short account +of its civilisation. + +The first branch to become civilised, and to carry culture to a greater +height than the older nations had ever done, was the Hellenes. There is +no need for us to speculate on the "genius" of the Hellenes, or even +to enlarge on the natural advantages of the lower part of the peninsula +which they occupied. A glance at the map will explain why European +civilisation began in Greece. The Hellenes had penetrated the region +in which there was constant contact with all the varied cultures of the +older world. Although they destroyed the AEgean culture, they could not +live amidst its ruins without receiving some influence. Then the traders +of Phoenicia, triumphing in the fall of their AEgean rivals, brought the +great pacific cultural influence of commerce to bear on them. After +some hundreds of years of internal trouble, barbaric quarrels, and fresh +arrivals from the north, Greece began to wear an aspect of civilisation. +Many of the Greeks passed to Asia Minor, as they increased, and, freed +from the despotism of tradition, in living contact with the luxury and +culture of Persia, which had advanced as far as Europe, they evolved the +fine civilisation of the Greek colonies, and reacted on the motherland. +Finally, there came the heroic struggle against the Persian invaders, +and from the ashes of their early civilisation arose the marble city +which will never die in the memory of Europe. + +The Romans had meantime been advancing. We may neglect the older Italian +culture, as it had far less to do with the making of Italy and Europe +than the influence of the east. By about 500 B.C. Rome was a small +kingdom with a primitive civilisation, busy in subduing the neighbouring +tribes who threatened its security, and unconsciously gathering the +seeds of culture which some of them contained. By about 300 B.C. the +vigour of the Romans had united all the tribes of Italy in a powerful +republic, and wealth began to accumulate at Rome. Not far to the east +was the glittering civilisation of Greece; to the south was Carthage, a +busy centre of commerce, navigation, and art; and from the Mediterranean +came processions of ships bringing stimulating fragments and stories +of the hoary culture of the east. Within another two hundred years Rome +annihilated Carthage, paralysed and overran Greece, and sent its legions +over the Asiatic provinces of the older empires. By the beginning of +the Christian Era all that remained of the culture of the old world was +gathered in Rome. All the philosophies of Greece, all the religions +of Persia and Judea and Egypt, all the luxuries and vices of the east, +found a home in it. Every stream of culture that had started from the +later and higher Neolithic age had ended in Rome. + +And in the meantime Rome had begun to disseminate its heritage over +Europe. Its legions poured over Spain and Gaul and Germany and Britain. +Its administrators and judges and teachers followed the eagles, and set +up schools and law-courts and theatres and baths and temples. It flung +broad roads to the north of Britain and the banks of the Rhine and +Danube. Under the shelter of the "Roman Peace" the peoples of Europe +could spare men from the plough and the sword for the cultivation of art +and letters. The civilisations of Britain, France, Germany, Spain, North +Africa, and Italy were ushered into the calendar of mankind, and were +ready to bear the burden when the mighty city on the Tiber let the +sceptre fall from its enfeebled hands. + +Rome fell. The more accurate historians of our time correct the old +legend of death from senile decay or from the effect of dissipation. +Races of men, like races of animals, do not die; they are killed. The +physical deterioration of the citizens of Rome was a small matter in its +fall. Fiscal and imperial blunders loosed the frame of its empire. The +resources were still there, but there was none to organise and unify +them. The imperial system--or chaos--ruined Rome. And just when the +demoralisation was greatest, and the Teutonic tribes at the frontiers +were most numerous and powerful, an accident shook the system. A fierce +and numerous people from Asia, the Huns, wandered into Europe, threw +themselves on the Teutonic tribes, and precipitated these tribes upon +the Empire. A Diocletian might still have saved the Empire, but there +was none to guide it. The northern barbarians trod its civilisation +underfoot, and Europe passed into the Dark Ages. + +One more application of the evolutionary principle, and we close the +story. The "barbarians"--the Goths and Vandals and their Germanic +cousins--were barbaric only in comparison with the art and letters +of Rome. They had law, polity, and ideals. European civilisation owes +elements to them, as well as to Rome. To say simply that the barbarians +destroyed the institutions of Rome is no adequate explanation of the +Dark Ages. Let us see rather how the Dark Ages were enlightened. + +It is now fully recognised that the reawakening of Europe in the twelfth +and thirteenth centuries was very largely due to a fresh culture-contact +with the older civilisations. The Arabs had, on becoming civilised, +learned from the Nestorians, who had been driven out of the Greek world +for their heresies, the ancient culture of Greece. They enshrined it +in a brilliant civilisation which it inspired them to establish. By the +ninth century this civilisation was exhibited in Spain by its Moorish +conquerors, and, as its splendour increased, it attracted the attention +of Europe. Some Christian scholars visited Spain, as time went on, but +the Jews were the great intermediaries in disseminating its culture +in Europe. There is now no question about the fact that the rebirth of +positive learning, especially of science, in Europe was very largely due +to the literature of the Moors, and their luxury and splendour gave an +impulse to European art. Europe entered upon the remarkable intellectual +period known as Scholasticism. Besides this stimulus, it must be +remembered, the scholars of Europe had at least a certain number of old +Latin writers whose works had survived the general wreck of culture. + +In the fifteenth century the awakening of Europe was completed. The +Turks took Constantinople, and drove large numbers of Greek scholars to +Italy. Out of this catastrophe issued the great Renaissance, or rebirth, +of art, science, and letters in Italy, and then in France, Germany, +and England. In the new intellectual ferment there appeared the great +artists, great thinkers and inventors, and great navigators who led the +race to fresh heights. The invention of printing alone would almost have +changed the face of Europe. But it was accompanied by a hundred +other inventions and discoveries, by great liberating and stimulating +movements like the Reformation, by the growth of free and wealthy +cities, and by the extension of peace over larger areas, and the +concentration of wealth and encouragement of art which the growth and +settlement of the chief European powers involved. Europe entered upon +the phase of evolution which we call modern times. + +***** + +The future of humanity cannot be seen even darkly, as in a glass. No +forecast that aspires beyond the immediate future is worth considering +seriously. If it be a forecast of material progress, it is rendered +worthless by the obvious consideration that if we knew what the future +will do, we would do it ourselves. If it is a forecast of intellectual +and social evolution, it is inevitably coloured by the intellectual +or social convictions of the prophet. I therefore abstain wholly from +carrying the story of evolution beyond realities. But I would add two +general considerations which may enable a reflective reader to answer +certain questions that will arise in his mind at the close of this +survey of the story of evolution. + +Are we evolving to-day? Is man the last word of evolution? These are +amongst the commonest questions put to me. Whether man is or is not the +last word of evolution is merely a verbal quibble. Now that language is +invented, and things have names, one may say that the name "man" will +cling to the highest and most progressive animal on earth, no matter how +much he may rise above the man of to-day. But if the question is +whether he WILL rise far above the civilisation of to-day, we can, in +my opinion, give a confident answer. There is no law of evolution, but +there is a fact of evolution. Ten million years ago the highest animal +on the earth was a reptile, or, at the most, a low, rat-like marsupial. +The authorities tell us that, unless some cosmic accident intervene, the +earth will remain habitable by man for at least ten million years. It +is safe to conclude that the man of that remote age will be lifted above +the man of to-day as much as we transcend the reptile in intelligence +and emotion. It is most probable that this is a quite inadequate +expression of the future advance. We are not only evolving, but evolving +more rapidly than living thing ever did before. The pace increases +every century. A calm and critical review of our development inspires a +conviction that a few centuries will bring about the realisation of the +highest dream that ever haunted the mind of the prophet. What splendours +lie beyond that, the most soaring imagination cannot have the dimmest +perception. + +And the last word must meet an anxiety that arises out of this very +confidence. Darwin was right. It is--not exclusively, but mainly--the +struggle for life that has begotten higher types. Must every step of +future progress be won by fresh and sustained struggle? At least we may +say that the notion that progress in the future depends, as in the past, +upon the pitting of flesh against flesh, and tooth against tooth, is +a deplorable illusion. Such physical struggle is indeed necessary to +evolve and maintain a type fit for the struggle. But a new thing has +come into the story of the earth--wisdom and fine emotion. The processes +which begot animal types in the past may be superseded; perhaps must be +superseded. The battle of the future lies between wit and wit, art and +art, generosity and generosity; and a great struggle and rivalry may +proceed that will carry the distinctive powers of man to undreamed-of +heights, yet be wholly innocent of the passion-lit, blood-stained +conflict that has hitherto been the instrument of progress. + + + + + +End of the Project Gutenberg EBook of The Story of Evolution, by Joseph McCabe + +*** END OF THIS PROJECT GUTENBERG EBOOK THE STORY OF EVOLUTION *** + +***** This file should be named 1043.txt or 1043.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/1/0/4/1043/ + +Produced by Dianne Bean + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. 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