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diff --git a/old/1043-h.zip b/old/1043-h.zip Binary files differnew file mode 100644 index 0000000..6bbdf33 --- /dev/null +++ b/old/1043-h.zip diff --git a/old/1043-h/1043-h.htm b/old/1043-h/1043-h.htm new file mode 100644 index 0000000..756e879 --- /dev/null +++ b/old/1043-h/1043-h.htm @@ -0,0 +1,11003 @@ +<?xml version="1.0" encoding="us-ascii"?> + +<!DOCTYPE html + PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" + "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd" > + +<html xmlns="http://www.w3.org/1999/xhtml" lang="en"> + <head> + <title> + The Story of Evolution, by Joseph McCabe + </title> + <style type="text/css" xml:space="preserve"> + + body { margin:5%; background:#faebd0; text-align:justify} + P { text-indent: 1em; margin-top: .25em; margin-bottom: .25em; } + H1,H2,H3,H4,H5,H6 { text-align: center; margin-left: 15%; margin-right: 15%; } + hr { width: 50%; text-align: center;} + .foot { margin-left: 20%; margin-right: 20%; text-align: justify; text-indent: -3em; font-size: 90%; } + blockquote {font-size: 97%; font-style: italic; margin-left: 10%; margin-right: 10%;} + .mynote {background-color: #DDE; color: #000; padding: .5em; margin-left: 10%; margin-right: 10%; font-family: sans-serif; font-size: 95%;} + .toc { margin-left: 10%; margin-bottom: .75em;} + .toc2 { margin-left: 20%;} + div.fig { display:block; margin:0 auto; text-align:center; } + div.middle { margin-left: 20%; margin-right: 20%; text-align: justify; } + .figleft {float: left; margin-left: 0%; margin-right: 1%;} + .figright {float: right; margin-right: 0%; margin-left: 1%;} + .pagenum {display:inline; font-size: 70%; font-style:normal; + margin: 0; padding: 0; position: absolute; right: 1%; + text-align: right;} + pre { font-style: italic; font-size: 90%; margin-left: 10%;} + +</style> + </head> + <body> +<pre xml:space="preserve"> + +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 + +Release Date: August 6, 2008 [EBook #1043] +Last Updated: February 6, 2013 + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK THE STORY OF EVOLUTION *** + + + + +Produced by Dianne Bean, and David Widger + + + + + +</pre> + <p> + <br /><br /> + </p> + <h1> + THE STORY OF EVOLUTION + </h1> + <p> + <br /> + </p> + <h2> + By Joseph McCabe + </h2> + <p> + <br /> + </p> + <h3> + 1912 + </h3> + <p> + <br /> <br /> + </p> + <hr /> + <p> + <br /> <br /> <a name="link2H_PREF" id="link2H_PREF"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + PREFACE + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <br /> <br /> + </p> + <hr /> + <p> + <br /> <br /> + </p> + <blockquote> + <p class="toc"> + <big><b>CONTENTS</b></big> + </p> + <p> + <br /> + </p> + <p class="toc"> + <a href="#link2H_PREF"> PREFACE </a> + </p> + <p> + <br /> + </p> + <p class="toc"> + <a href="#link2H_4_0002"> <big><b>THE STORY OF EVOLUTION</b></big> </a> + </p> + <p> + <br /> <a href="#link2HCH0001"> CHAPTER I. </a> THE DISCOVERY + OF THE UNIVERSE <br /><br /> <a href="#link2HCH0002"> CHAPTER II. </a> THE + FOUNDATIONS OF THE UNIVERSE <br /><br /> <a href="#link2HCH0003"> CHAPTER + III. </a> THE BIRTH AND DEATH OF WORLDS <br /><br /> <a + href="#link2HCH0004"> CHAPTER IV. </a> THE PREPARATION OF THE + EARTH <br /><br /> <a href="#link2HCH0005"> CHAPTER V. </a> THE + BEGINNING OF LIFE <br /><br /> <a href="#link2HCH0006"> CHAPTER VI. </a> THE + INFANCY OF THE EARTH <br /><br /> <a href="#link2HCH0007"> CHAPTER VII. + </a> THE PASSAGE TO THE LAND <br /><br /> <a + href="#link2HCH0008"> CHAPTER VIII. </a> THE COAL-FOREST + <br /><br /> <a href="#link2HCH0009"> CHAPTER IX. </a> THE + ANIMALS OF THE COAL-FOREST <br /><br /> <a href="#link2HCH0010"> CHAPTER + X. </a> THE PERMIAN REVOLUTION <br /><br /> <a + href="#link2HCH0011"> CHAPTER XI. </a> THE MIDDLE AGES OF THE + EARTH <br /><br /> <a href="#link2HCH0012"> CHAPTER XII. </a> THE + AGE OF REPTILES <br /><br /> <a href="#link2HCH0013"> CHAPTER XIII. </a> THE + BIRD AND THE MAMMAL <br /><br /> <a href="#link2HCH0014"> CHAPTER XIV. + </a> IN THE DAYS OF THE CHALK <br /><br /> <a + href="#link2HCH0015"> CHAPTER XV. </a> THE TERTIARY ERA <br /><br /> + <a href="#link2HCH0016"> CHAPTER XVI. </a> THE FLOWER AND THE + INSECT <br /><br /> <a href="#link2HCH0017"> CHAPTER XVII. </a> THE + ORIGIN OF OUR MAMMALS <br /><br /> <a href="#link2HCH0018"> CHAPTER XVIII. + </a> THE EVOLUTION OF MAN <br /><br /> <a + href="#link2HCH0019"> CHAPTER XIX. </a> MAN AND THE GREAT + ICE-AGE <br /><br /> <a href="#link2HCH0020"> CHAPTER XX. </a> THE + DAWN OF CIVILISATION <br /><br /> <a href="#link2HCH0021"> CHAPTER XXI. + </a> EVOLUTION IN HISTORY <br /><br /> + </p> + </blockquote> + <p> + <br /> <br /> + </p> + <hr /> + <p> + <br /> <br /> <a name="link2H_4_0002" id="link2H_4_0002"> + <!-- H2 anchor --> </a> + </p> + <h1> + THE STORY OF EVOLUTION + </h1> + <p> + <a name="link2HCH0001" id="link2HCH0001"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER I. THE DISCOVERY OF THE UNIVERSE + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0002" id="link2HCH0002"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER II. THE FOUNDATIONS OF THE UNIVERSE + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * 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). +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> + <p> + <a name="link2HCH0003" id="link2HCH0003"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER III. THE BIRTH AND DEATH OF WORLDS + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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? + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + "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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * 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). +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0004" id="link2HCH0004"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER IV. THE PREPARATION OF THE EARTH + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + THE GEOLOGICAL SERIES + </p> + <p> + [The scale of years adopted—50,000,000 for the stratified rocks—is + merely an intermediate between conflicting estimates.] + </p> +<pre xml:space="preserve"> + ERA. PERIOD. RELATIVE LENGTH. + + Quaternary Holocene 500,000 years + Pleistocene +</pre> +<pre xml:space="preserve"> + Tertiary Pliocene 5,500,000 years + or Miocene + Cenozoic Oligocene + Eocene +</pre> +<pre xml:space="preserve"> + Secondary Cretaceous 7,200,000 years + or Jurassic 3,600,000 " + Mesozoic Triassic 2,500,000 " +</pre> +<pre xml:space="preserve"> + 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 +</pre> + <p> + <a name="link2HCH0005" id="link2HCH0005"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER V. THE BEGINNING OF LIFE + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + [*] + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * 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). +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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? + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0006" id="link2HCH0006"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER VI. THE INFANCY OF THE EARTH + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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." + </p> + <p> + 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). + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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). + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * See the section on Echinoderms, by Professor MacBride, in + the "Cambridge Natural History," I. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0007" id="link2HCH0007"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER VII. THE PASSAGE TO THE LAND + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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? + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + * See, especially, Dr. Gaskell's "Origin of Vertebrates" + (1908). +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0008" id="link2HCH0008"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER VIII. THE COAL-FOREST + </h2> + <p> + 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. + </p> + <p> + 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." + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + * 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). +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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). + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + * See, especially, D. H. Scott, "Studies of Fossil Botany" + (2nd ed., 1908), and "The Evolution of Plants" (1910—small + popular manual). +</pre> + <p> + <a name="link2HCH0009" id="link2HCH0009"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER IX. THE ANIMALS OF THE COAL-FOREST + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0010" id="link2HCH0010"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER X. THE PERMIAN REVOLUTION + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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). + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0011" id="link2HCH0011"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER XI. THE MIDDLE AGES OF THE EARTH + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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). + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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). + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0012" id="link2HCH0012"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER XII. THE AGE OF REPTILES + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + * These Synapsid reptiles are more commonly known as + Pareiasauria or Theromorpha. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * See, besides the usual authorities, a valuable paper by + Dr. R. S. Lull, "Dinosaurian Distribution" (1910). +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0013" id="link2HCH0013"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER XIII. THE BIRD AND THE MAMMAL + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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." + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + * See Lucas and Le Soulf's Animals of Australia, 1909. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0014" id="link2HCH0014"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER XIV. IN THE DAYS OF THE CHALK + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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." + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0015" id="link2HCH0015"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER XV. THE TERTIARY ERA + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> +<pre xml:space="preserve"> + * "The Evolution of Mind" (Black), 1911. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0016" id="link2HCH0016"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER XVI. THE FLOWER AND THE INSECT + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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). + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * For a strong statement of the new critical position see + Dewar and Finn's "Making of Species," 1909, ch. vi. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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). + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + * The best treatment of the subject will be found in W. P. + Pycraft's History of Birds, 1910. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0017" id="link2HCH0017"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER XVII. THE ORIGIN OF OUR MAMMALS + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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). + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + *See this short account, "Guide to the Elephants in the + British Museum," 1908. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0018" id="link2HCH0018"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER XVIII. THE EVOLUTION OF MAN + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> + <p> + 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 + </p> + <p> + 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. + </p> + <p> + 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." + </p> + <p> + 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. + </p> + <p> + 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? + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0019" id="link2HCH0019"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER XIX. MAN AND THE GREAT ICE-AGE + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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? + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + It is also said that we cannot on these principles understand the repeated + advance and retreat of the ice-sheet. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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? + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + *See especially an address by Professor Sollas in the + Quarterly Journal of the Geological Society, Vol. LXVI. + (1910). +</pre> + <p> + 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. + </p> + <p> + 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. + </p> +<pre xml:space="preserve"> + *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. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0020" id="link2HCH0020"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER XX. THE DAWN OF CIVILISATION + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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). + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <a name="link2HCH0021" id="link2HCH0021"> + <!-- H2 anchor --> </a> + </p> + <div style="height: 4em;"> + <br /><br /><br /><br /> + </div> + <h2> + CHAPTER XXI. EVOLUTION IN HISTORY + </h2> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. [*] + </p> +<pre xml:space="preserve"> + * 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. +</pre> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <hr /> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + 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. + </p> + <p> + <br /><br /> + </p> +<pre xml:space="preserve"> + + + + + +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-h.htm or 1043-h.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, and David Widger + +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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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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FOR PUBLIC DOMAIN ETEXTS*Ver.04.29.93*END* + + + + + +This etext was scanned with OmniPage Pro OCR software donated by +Caere by Dianne Bean, Chino Valley, AZ. from a 1921 edition. + + + + + +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 bc 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 hunter!, 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 Etext of The Story of Evolution, by McCabe + diff --git a/old/old/tsoev10.zip b/old/old/tsoev10.zip Binary files differnew file mode 100644 index 0000000..30904a4 --- /dev/null +++ b/old/old/tsoev10.zip |