Outlines of the Earth"s History.
by Nathaniel Southgate Shaler.
PREFACE.
The object of this book is to provide the beginner in the study of the earth"s history with a general account of those actions which can be readily understood and which will afford him clear understandings as to the nature of the processes which have made this and other celestial spheres. It has been the writer"s purpose to select those series of facts which serve to show the continuous operations of energy, so that the reader might be helped to a truer conception of the nature of this sphere than he can obtain from ordinary text-books.
In the usual method of presenting the elements of the earth"s history the facts are set forth in a manner which leads the student to conceive that history as in a way completed. The natural prepossession to the effect that the visible universe represents something done, rather than something endlessly doing, is thus re-enforced, with the result that one may fail to gain the largest and most educative impression which physical science can afford him in the sense of the swift and unending procession of events.
It is well known to all who are acquainted with the history of geology that the static conception of the earth--the idea that its existing condition is the finished product of forces no longer in action--led to prejudices which have long r.e.t.a.r.ded, and indeed still r.e.t.a.r.d, the progress of that science. This fact indicates that at the outset of a student"s work in this field he should be guarded against such misconceptions. The only way to attain the end is by bringing to the understanding of the beginner a clear idea of successions of events which are caused by the forces operating in and on this sphere. Of all the chapters of this great story, that which relates to the history of the work done by the heat of the sun is the most interesting and awakening. Therefore an effort has been made to present the great successive steps by which the solar energy acts in the processes of the air and the waters.
The interest of the beginner in geology is sure to be aroused when he comes to see how very far the history of the earth has influenced the fate of men. Therefore the aim has been, where possible, to show the ways in which geological processes and results are related to ourselves; how, in a word, this earth has been the well-appointed nursery of our kind.
All those who are engaged in teaching elementary science learn the need of limiting the story they have to tell to those truths which can be easily understood by beginners. It is sometimes best, as in stating such difficult matters as those concerning the tides, to give explanations which are far from complete, and which, as to their mode of presentation, would be open to criticism were it not for the fact that any more elaborate statements would most likely be incomprehensible to the novice, thus defeating the teacher"s aim.
It will be observed that no account is here given of the geological ages or of the successions of organic life. Chapters on these subjects were prepared, but were omitted for the reason that they made the story too long, and also because they carried the reader into a field of much greater difficulty than that which is found in the physical history of the earth.
N.S.S.
_March, 1898._
CHAPTER I.
AN INTRODUCTION TO THE STUDY OF NATURE.
The object of this book is to give the student who is about to enter on the study of natural science some general idea as to the conditions of the natural realm. As this field of inquiry is vast, it will be possible only to give the merest outline of its subject-matter, noting those features alone which are of surpa.s.sing interest, which are demanded for a large understanding of man"s place in this world, or which pertain to his duties in life.
In entering on any field of inquiry, it is most desirable that the student should obtain some idea as to the ways in which men have been led to the knowledge which they possess concerning the world about them. Therefore it will be well briefly to sketch the steps by which natural science has come to be what it is. By so doing we shall perceive how much we owe to the students of other generations; and by noting the difficulties which they encountered, and how they avoided them, we shall more easily find our own way to knowledge.
The primitive savages, who were the ancestors of all men, however civilized they may be, were students of Nature. The remnants of these lowly people who were left in different parts of the world show us that man was not long in existence before he began to devise some explanation concerning the course of events in the outer world.
Seeing the sun rise and set, the changes of the moon, the alternation of the seasons, the incessant movement of the streams and sea, and the other more or less orderly successions of events, our primitive forefathers were driven to invent some explanation of them. This, independently, and in many different times and places, they did in a simple and natural way by supposing that the world was controlled by a host of intelligent beings, each of which had some part in ordering material things. Sometimes these invisible powers were believed to be the spirits of great chieftains, who were active when on earth, and who after death continued to exercise their power in the larger realms of Nature. Again, and perhaps more commonly, these movements of Nature were supposed to be due to the action of great though invisible beasts, much like those which the savage found about him. Thus among our North American Indians the winds are explained by the supposition that the air is fanned by the wings of a great unseen bird, whose duty it is to set the atmosphere into motion. That no one has ever seen the bird doing the work, or that the task is too great for any conceivable bird, is to the simple, uncultivated man no objection to this view. It is long, indeed, before education brings men to the point where they can criticise their first explanations of Nature.
As men in their advance come to see how much n.o.bler are their own natures than those of the lower animals, they gradually put aside the explanation of events by the actions of beasts, and account for the order of the world by the supposition that each and every important detail is controlled by some immortal creature essentially like a man, though much more powerful than those of their own kind. This stage of understanding is perhaps best shown by the mythology of the Greeks, where there was a great G.o.d over all, very powerful but not omnipotent; and beneath him, in endless successions of command, subordinate powers, each with a less range of duties and capacities than those of higher estate, until at the bottom of the system there were minor deities and demiG.o.ds charged with the management of the trees, the flowers, and the springs--creatures differing little from man, except that they were immortal, and generally invisible, though they, like all the other deities, might at their will display themselves to the human beings over whom they watched, and whose path in life they guided.
Among only one people do we find that the process of advance led beyond this early and simple method of accounting for the processes of Nature, bringing men to an understanding such as we now possess. This great task was accomplished by the Greeks alone. About twenty-five hundred years ago the philosophers of Greece began to perceive that the early notion as to the guidance of the world by creatures essentially like men could not be accepted, and must be replaced by some other view which would more effectively account for the facts.
This end they attained by steps which can not well be related here, but which led them to suppose separate powers behind each of the natural series--powers having no relation to the qualities of mankind, but ever acting to a definite end. Thus Plato, who represents most clearly this advance in the interpretation of facts, imagined that each particular kind of plant or animal had its shape inevitably determined by something which he termed an idea, a shape-giving power which existed before the object was created, and which would remain after it had been destroyed, ever ready again to bring matter to the particular form. From this stage of understanding it was but a short step to the modern view of natural law. This last important advance was made by the great philosopher Aristotle, who, though he died about twenty-two hundred years ago, deserves to be accounted the first and in many ways the greatest of the ancient men of science who were informed with the modern spirit.
With Aristotle, as with all his intellectual successors, the operations of Nature were conceived as to be accounted for by the action of forces which we commonly designate as natural laws, of which perhaps the most familiar and universal is that of gravitation, which impels all bodies to move toward each other with a degree of intensity which is measured by their weight and the distance by which they are separated.
For many centuries students used the term law in somewhat the same way as the more philosophical believers in polytheism spoke of their G.o.ds, or as Plato of the ideas which he conceived to control Nature. We see by this instance how hard it is to get rid of old ways of thinking.
Even when the new have been adopted we very often find that something of the ancient and discarded notions cling in our phrases. The more advanced of our modern philosophers are clear in their mind that all we know as to the order of Nature is that, given certain conditions, certain consequences inevitably follow.
Although the limitations which modern men of science perceive to be put upon their labours may seem at first sight calculated to confine our understanding within a narrow field of things which can be seen, or in some way distinctly proved to exist, the effect of this limitation has been to make science what it is--a realm of things known as distinct from things which may be imagined. All the difference between ancient science and modern consists in the fact that in modern science inquirers demand a businesslike method in the interpretation of Nature. Among the Greeks the philosopher who taught explanations of any feature in the material world which interested him was content if he could imagine some way which would account for the facts. It is the modern custom now to term the supposition of an explanation a _working hypothesis_, and only to give it the name of theory after a very careful search has shown that all the facts which can be gathered are in accordance with the view. Thus when Newton made his great suggestion concerning the law of gravitation, which was to the effect that all bodies attracted each other in proportion to their ma.s.ses, and inversely as the square of their distance from each other, he did not rest content, as the old Greeks would have done, with the probable truth of the explanation, but carefully explored the movements of the planets and satellites of the solar system to see if the facts accorded with the hypothesis. Even the perfect correspondence which he found did not entirely content inquirers, and in this century very important experiments have been made which have served to show that a ball suspended in front of a precipice will be attracted toward the steep, and that even a ma.s.s of lead some tons in weight will attract toward itself a small body suspended in the manner of a pendulum.
It is this incessant revision of the facts, in order to see if they accord with the a.s.sumed rule or law, which has given modern science the sound footing that it lacked in earlier days, and which has permitted our learning to go on step by step in a safe way up the heights to which it has climbed. All explanations of Nature begin with the work of the imagination. In common phrase, they all are guesses which have at first but little value, and only attain importance in proportion as they are verified by long-continued criticism, which has for its object to see whether the facts accord with the theory. It is in this effort to secure proof that modern science has gathered the enormous store of well-ascertained facts which const.i.tutes its true wealth, and which distinguishes it from the earlier imaginative and to a great extent unproved views.
In the original state of learning, natural science was confounded with political and social tradition, with the precepts of duty which const.i.tute the law of the people, as well as with their religion, the whole being in the possession of the priests or wise men. So long as natural action was supposed to be in the immediate control of numerous G.o.ds and demiG.o.ds, so long, in a word, as the explanation of Nature was what we term polytheistic, this a.s.sociation of science with other forms of learning was not only natural but inevitable. Gradually, however, as the conception of natural law replaced the earlier idea as to the intervention of a spirit, science departed from other forms of lore and came to possess a field to itself. At first it was one body of learning. The naturalists of Aristotle"s time, and from his day down to near our own, generally concerned themselves with the whole field of Nature. For a time it was possible for any one able and laborious man to know all which had been ascertained concerning astronomy, chemistry, geology, as well as the facts relating to living beings. The more, however, as observation acc.u.mulated, and the store of facts increased, it became difficult for any one man to know the whole. Hence it has come about that in our own time natural learning is divided into many distinct provinces, each of which demands a lifetime of labour from those who would know what has already been done in the field, and what it is now important to do in the way of new inquiries.
The large divisions which naturalists have usually made of their tasks rest in the main on the natural part.i.tions which we may readily observe in the phenomenal world. First of all comes astronomy, including the phenomena exhibited in the heavens, beyond the limits of the earth"s atmosphere. Second, geology, which takes account of all those actions which in process of time have been developed in our own sphere. Third, physics, which is concerned with the laws of energy, or those conditions which affect the motion of bodies, and the changes which are impressed upon them by the different natural forces. Fourth, chemistry, which seeks to interpret the principles which determine the combination of atoms and the molecules which are built of them under the influence of the chemical affinities. Fifth, biology, or the laws of life, a study which pertains to the forms and structures of animals and plants, and their wonderful successions in the history of the world. Sixth, mathematics, or the science of s.p.a.ce and number, that deals with the principles which underlie the order of Nature as expressed at once in the human understanding and in the material universe. By its use men were made able to calculate, as in arithmetic, the problems which concern their ordinary business, as well as to compute the movements of the celestial bodies, and a host of actions which take place on the earth that would be inexplicable except by the aid of this science. Last of all among the primary sciences we may name that of psychology, which takes account of mental operations among man and his lower kindred, the animals.
In addition to the seven sciences above mentioned, which rest in a great measure on the natural divisions of phenomena, there are many, indeed, indefinitely numerous, subdivisions which have been made to suit the convenience of students. Thus astronomy is often separated into physical and mathematical divisions, which take account either of the physical phenomena exhibited by the heavenly bodies or of their motions. In geology there are half a dozen divisions relating to particular branches of that subject. In the realm of organic life, in chemistry, and in physics there are many parts of these sciences which have received particular names.
It must not be supposed that these sciences have the independence of each other which their separate names would imply. In fact, the student of each, however, far he may succeed in separating his field from that of the other naturalists, as we may fitly term all students of Nature, is compelled from time to time to call in the aid of his brethren who cultivate other branches of learning. The modern astronomer needs to know much of chemistry, or else he can not understand many of his observations on the sun. The geologists have to share their work with the student of animal and vegetable life, with the physicists; they must, moreover, know something of the celestial spheres in order to interpret the history of the earth. In fact, day by day, with the advance of learning, we come more clearly to perceive that all the processes of Nature are in a way related to each other, and that in proportion as we understand any part of the great mechanism, we are forced in a manner to comprehend the whole. In other words, we are coming to understand that these divisions of the field of science depend upon the limitations of our knowledge, and not upon the order of Nature itself. For the purposes of education it is important that every one should know something of the great truths which each science has disclosed. No mortal man can compa.s.s the whole realm of this knowledge, but every one can gain some idea of the larger truths which may help him to understand the beauty and grandeur of the sphere in which he dwells, which will enable him the better to meet the ordinary duties of life, that in almost all cases are related to the facts of the world about us. It has been of late the custom to term this body of general knowledge which takes account of the more evident facts and important series of terrestrial actions physiography, or, as the term implies, a description of Nature, with the understanding that the knowledge chosen for the account is that which most intimately concerns the student who seeks information that is at once general and important. Therefore, in this book the effort is made first to give an account as to the ways and means which have led to our understanding of scientific problems, the methods by which each person may make himself an inquirer, and the outline of the knowledge that has been gathered since men first began to observe and criticise the revelations the universe may afford them.
CHAPTER II.
WAYS AND MEANS OF STUDYING NATURE.
It is desirable that the student of Nature keep well in mind the means whereby he is able to perceive what goes on in the world about him. He should understand something as to the nature of his senses, and the extent to which these capacities enable him to discern the operations of Nature. Man, in common with his lower kindred, is, by the mechanism of the body, provided with five somewhat different ways by which he may learn something of the things about him. The simplest of these capacities is that of touch, a faculty that is common to the general surface of the body, and which informs us when the surface is affected by contact with some external object. It also enables us to discern differences of temperature. Next is the sense of taste, which is limited to the mouth and the parts about it. This sense is in a way related to that of touch, for the reason that it depends on the contact of our body with material things. Third is the sense of smell, so closely related to that of taste that it is difficult to draw the line between the two. Yet through the apparatus of the nose we can perceive the microscopically small parts of matter borne to us through the air, which could not be appreciated by the nerves of the mouth.
Fourth in order of scope comes the hearing, which gives us an account of those waves of matter that we understand as sound. This power is much more far ranging than those before noted; in some cases, as in that of the volcanic explosions from the island of Krakatoa, in the eruption of 1883, the convulsions were audible at the distance of more than a thousand miles away. The greater cannon of modern days may be heard at the distance of more than a hundred miles, so that while the sense of touch, taste, and smell demand contact with the bodies which we appreciate, hearing gives us information concerning objects at a considerable distance. Last and highest of the senses, vastly the most important in all that relates to our understanding of Nature, is sight, or the capacity which enables us to appreciate the movement of those very small waves of ether which const.i.tute light. The eminent peculiarity of sight is that it may give us information concerning things which are inconceivably far away; it enables us to discern the light of suns probably millions of times as remote from us as is the centre of our own solar system.
Although much of the pleasure which the world affords us comes through the other senses, the basis of almost all our accurate knowledge is reported by sight. It is true that what we have observed with our eyes may be set forth in words, and thus find its way to the understanding through the ears; also that in many instances the sense of touch conveys information which extends our perceptions in many important ways; but science rests practically on sight, and on the insight that comes from the training of the mind which the eyes make possible.
The early inquirers had no resources except those their bodies afforded; but man is a tool-making creature, and in very early days he began to invent instruments which helped him in inquiry. The earliest deliberate study was of the stars. Science began with astronomy, and the first instruments which men contrived for the purpose of investigation were astronomical. In the beginning of this search the stars were studied in order to measure the length of the year, and also for the reason that they were supposed in some way to control the fate of men. So far as we know, the first pieces of apparatus for this purpose were invented in Egypt, perhaps about four thousand years before the Christian era. These instruments were of a simple nature, for the magnifying gla.s.s was not yet contrived, and so the telescope was impossible. They consisted of arrangements of straight edges and divided circles, so that the observers, by sighting along the instruments, could in a rough way determine the changes in distance between certain stars, or the height of the sun above the horizon at the various seasons of the year. It is likely that each of the great pyramids of Egypt was at first used as an observatory, where the priests, who had some knowledge of astronomy, found a station for the apparatus by which they made the observations that served as a basis for casting the horoscope of the king.
In the progress of science and of the mechanical invention attending its growth, a great number of inventions have been contrived which vastly increase our vision and add inconceivably to the precision it may attain. In fact, something like as much skill and labour has been given to the development of those inventions which add to our learning as to those which serve an immediate economic end. By far the greatest of these scientific inventions are those which depend upon the lens.
By combining shaped bits of gla.s.s so as to control the direction in which the light waves move through them, naturalists have been able to create the telescope, which in effect may bring distant objects some thousand times nearer to view than they are to the naked eye; and the microscope, which so enlarges minute objects as to make them visible, as they were not before. The result has been enormously to increase our power of vision when applied to distant or to small objects. In fact, for purposes of learning, it is safe to say that those tools have altogether changed man"s relation to the visible universe. The naked eye can see at best in the part of the heavens visible from any one point not more than thirty thousand stars. With the telescope somewhere near a hundred million are brought within the limits of vision. Without the help of the microscope an object a thousandth of an inch in diameter appears as a mere point, the existence of which we can determine only under favourable circ.u.mstances. With that instrument the object may reveal an extended and complicated structure which it may require a vast labour for the observer fully to explore.
Next in importance to the aid of vision above noted come the scientific tools which are used in weighing and measuring. These balances and gauges have attained such precision that intervals so small as to be quite invisible, and weights as slight as a ten-thousandth of a grain, can be accurately measured. From these instruments have come all those precise examinations on which the accuracy of modern science intimately depends. All these instruments of precision are the inventions of modern days. The simplest telescopes were made only about two hundred and fifty years ago, and the earlier compound microscopes at a yet later date. Accurate balances and other forms of gauges of s.p.a.ce, as well as good means of dividing time, such as our accurate astronomical clocks and chronometers, are only about a century old. The instruments have made science accurate, and have immensely extended its powers in nearly all the fields of inquiry.
Although the most striking modern discoveries are in the field which was opened to us by the lens in its manifold applications, it is in the chemist"s laboratory that we find that branch of science, long cultivated, but rapidly advanced only within the last two centuries, which has done the most for the needs of man. The ancients guessed that the substances which make up the visible world were more complicated in their organization than they appear to our vision. They even suggested the great truth that matter of all kinds is made up of inconceivably small indivisible bits which they and we term atoms. It is likely that in the cla.s.sic days of Greece men began to make simple experiments of a chemical nature. A century or two after the time of Mohammed, the Arabians of his faith, a people who had acquired Greek science from the libraries which their conquests gave them, conducted extensive experiments, and named a good many familiar chemical products, such as alcohol, which still bears its Arabic name.
These chemical studies were continued in Europe by the alchemists, a name also of Arabic origin, a set of inquirers who were to a great extent drawn away from scientific studies by vain though unending efforts to change the baser metals into gold and silver, as well as to find a compound which would make men immortal in the body. By the invention of the accurate balance, and by patient weighing of the matters which they submitted to experiment, by the invention of hypotheses or guesses at truth, which were carefully tested by experiment, the majestic science of modern chemistry has come forth from the confused and mystical studies of the alchemists. We have learned to know that there are seventy or more primitive or apparently unchangeable elements which make up the ma.s.s of this world, and probably const.i.tute all the celestial spheres, and that these elements in the form of their separate atoms may group themselves in almost inconceivably varied combinations. In the inanimate realm these a.s.sociations, composed of the atoms of the different substances, forming what are termed molecules, are generally composed of but few units. Thus carbonic-acid gas, as it is commonly called, is made up of an aggregation of molecules, each composed of one atom of carbon and two of oxygen; water, of two atoms of hydrogen and one of oxygen; ordinary iron oxide, of two atoms of iron and three of oxygen. In the realm of organic life, however, these combinations become vastly more complicated, and with each of them the properties of the substance thus produced differ from all others. A distinguished chemist has estimated that in one group of chemical compounds, that of carbon, it would be possible to make such an array of substances that it would require a library of many thousand ordinary volumes to contain their names alone.
It is characteristic of chemical science that it takes account of actions which are almost entirely invisible. No contrivances have been or are likely to be invented which will show the observer what takes place when the atoms of any substance depart from their previous combination and enter on new arrangements. We only know that under certain conditions the old atomic a.s.sociations break up, and new ones are formed. But though the processes are hidden, the results are manifest in the changes which are brought about upon the ma.s.ses of material which are subjected to the altering conditions. Gradually the chemists of our day are learning to build up in their laboratories more and more complicated compounds; already they have succeeded in producing many of the materials which of old could only be obtained by extracting them from plants. Thus a number of the perfumes of flowers, and many of the dye-stuffs which a century ago were extracted from vegetables, and were then supposed to be only obtainable in that way, are now readily manufactured. In time it seems likely that important articles of food, for which we now depend upon the seeds of plants, may be directly built up from the mineral kingdom. Thus the result of chemical inquiry has been not only to show us much of the vast realm of actions which go on in the earth, but to give us control of many of these movements so that we may turn them to the needs of man.
Animals and plants were at an early day very naturally the subjects of inquiry. The ancients perceived that there were differences of kind among these creatures, and even in Aristotle"s time the sciences of zoology and botany had attained the point where there were considerable treatises on those subjects. It was not, however, until a little more than a century ago that men began accurately to describe and cla.s.sify these species of the organic world. Since the time of Linnaeus the growth of our knowledge has gone forward with amazing swiftness. Within a century we have come to know perhaps a hundred times as much concerning these creatures as was learned in all the earlier ages. This knowledge is divisible into two main branches: in one the inquirers have taken account of the different species, genera, families, orders, and cla.s.ses of living forms with such effect that they have shown the existence at the present time of many hundred thousand distinct species, the vast a.s.semblage being arranged in a cla.s.sification which shows something as to the relationship which the forms bear to each other, and furthermore that the kinds now living have not been long in existence, but that at each stage in the history of the earth another a.s.semblage of species peopled the waters and the lands.
At first naturalists concerned themselves only with the external forms of living creatures; but they soon came to perceive that the way in which these organisms worked, their physiology, in a word, afforded matters for extended inquiry. These researches have developed the science of physiology, or the laws of bodily action, on many accounts the most modern and extensive of our new acquisitions of natural learning. Through these studies we have come to know something of the laws or principles by which life is handed on from generation to generation, and by which the gradations of structure have been advanced from the simple creatures which appear like bits of animated jelly to the body and mind of man.
The greatest contribution which modern naturalists have made to knowledge concerns the origin of organic species. The students of a century ago believed that all these different kinds had been suddenly created either through natural law or by the immediate will of G.o.d. We now know that from the beginning of organic life in the remote past to the present day one kind of animal or plant has been in a natural and essentially gradual way converted into the species which was to be its successor, so that all the vast and complicated a.s.semblage of kinds which now exists has been derived by a process of change from the forms which in earlier ages dwelt upon this planet. The exact manner in which these alterations were produced is not yet determined, but in large part it has evidently been brought about by the method indicated by Mr. Darwin, through the survival of the fittest individuals in the struggle for existence.
Until men came to have a clear conception as to the spherical form of the earth, it was impossible for them to begin any intelligent inquiries concerning its structure or history. The Greeks knew the earth to be a sphere, but this knowledge was lost among the early Christian people, and it was not until about four hundred years ago that men again came to see that they dwelt upon a globe. On the basis of this understanding the science of geology, which had in a way been founded by the Greeks, was revived. As this science depends upon the knowledge which we have gained of astronomy, physics, chemistry, and biology, all of which branches of learning have to be used in explaining the history of the earth, the advance which has been made has been relatively slow. Geology as a whole is the least perfectly organized of all the divisions of learning. A special difficulty peculiar to this science has also served to hinder its development.
All the other branches of learning deal mainly, if not altogether, with the conditions of Nature as they now exist. In this alone is it necessary at every step to take account of actions which have been performed in the remote past.
It is an easy matter for the students of to-day to imagine that the earth has long endured; but to our forefathers, who were educated in the view that it had been brought from nothingness into existence about seven thousand years ago, it was most difficult and for a time impossible to believe in its real antiquity. Endeavouring, as they naturally did, to account for all the wonderful revolutions, the history of which is written in the pages of the great stone book, the early geologists supposed this planet to have been the seat of frequent and violent changes, each of which revolutionized its shape and destroyed its living tenants. It was only very gradually that they became convinced that a hundred million years or more have elapsed since the dawn of life on the earth, and that in this vast period the march of events has been steadfast, the changes taking place at about the same rate in which they are now going on. As yet this conception as to the history of our sphere has not become the general property of the people, but the fact of it is recognised by all those who have attentively studied the matter. It is now as well ascertained as any of the other truths which science has disclosed to us.
It is instructive to note the historic outlines of scientific development. The most conspicuous truth which this history discloses is that all science has had its origin and almost all its development among the peoples belonging to the Aryan race. This body of folk appears to have taken on its race characteristics, acquired its original language, its modes of action, and the foundations of its religion in that part of northern Europe which is about the Baltic Sea. Thence the body of this people appear to have wandered toward central Asia, where after ages of pastoral life in the high table lands and mountains of their country it sent forth branches to India, Asia Minor and Greece, to Persia, and to western Europe. It seems ever to have been a characteristic of these Aryan peoples that they had an extreme love for Nature; moreover, they clearly perceived the need of accounting for the things that happened in the world about them. In general they inclined to what is called the pantheistic explanation of the universe. They believed a supreme G.o.d in many different forms to be embodied in all the things they saw. Even their own minds and bodies they conceived as manifestations of this supreme power. Among the Aryans who came to dwell in Europe and along the eastern Mediterranean this method of explaining Nature was in time changed to one in which humanlike G.o.ds were supposed to control the visible and invisible worlds. In that marvellous centre of culture which was developed among the Greeks this conception of humanlike deities was in time replaced by that of natural law, and in their best days the Greeks were men of science essentially like those of to-day, except that they had not learned by experience how important it was to criticise their theories by patiently comparing them with the facts which they sought to explain. The last of the important Greek men of science, Strabo, who was alive when Christ was born, has left us writings which in quality are essentially like many of the able works of to-day. But for the interruption in the development of Greek learning, natural science would probably have been fifteen hundred years ahead of its present stage. This interruption came in two ways.
In one, through the conquest of Greece and the destruction of its intellectual life by the Romans, a people who were singularly incapable of appreciating natural science, and who had no other interest in it except now and then a vacant and unprofitable curiosity as to the processes of the natural world. A second destructive influence came through the fact that Christianity, in its energetic protest against the sins of the pagan civilization, absolutely neglected and in a way despised all forms of science.