Читать книгу Some Salient Points in the Science of the Earth - Sir John William Dawson - Страница 7

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WORLD-MAKING.

G

Geological reading, especially when of a strictly uniformitarian character and in warm weather, sometimes becomes monotonous; and I confess to a feeling of drowsiness creeping over me when preparing material for a presidential address to the American Association for the Advancement of Science in August, 1883. In these circumstances I became aware of the presence of an unearthly visitor, who announced himself as of celestial birth, and intimated to me that being himself free from those restrictions of space and time which are so embarrassing to earthly students, he was prepared for the moment to share these advantages with me, and to introduce me to certain outlying parts of the universe, where I might learn something of its origin and early history. He took my hand, and instantly we were in the voids of space. Turning after a moment, he pointed to a small star and said, "That is the star you call the sun; here, you see, it is only about the third magnitude, and in a few seconds it will disappear." These few seconds, indeed, reduced the whole visible firmament to a mere nebulous haze like the Milky Way, and we seemed to be in blank space. But pausing for a moment I became aware that around us were multitudes of dark bodies, so black that they were, so to speak, negatively visible, even in the almost total darkness around. Some seemed large and massive, some a mere drift of minute particles, formless and without distinct limits. Some were swiftly moving, others stationary, or merely revolving on their own axes. It was a "horror of great darkness," and I trembled with fear. "This," said my guide, "is what the old Hebrew seer called tohu ve bohu, 'formless and void,' the 'Tiamat' or abyss of the old Chaldeans, the 'chaos and old night' of the Greeks. Your mundane physicists have not seen it, but they speculate regarding it, and occupy themselves with questions as to whether it can be lightened and vivified by mere attractive force, or by collision of dark bodies impinging on each other with vast momentum. Their speculations are vain, and lead to nothing, because they have no data wherefrom to calculate the infinite and eternal Power who determined either the attraction or the motion, or who willed which portion of this chaos was to become cosmos, and which was to remain for ever dead and dark. Let us turn, however, to a more hopeful prospect." We sped away to another scene. Here were vast luminous bodies, such as we call nebulæ. Some were globular, others disc-like, others annular or like spiral wisps, and some were composed of several concentric shells or rings. All were in rapid rotation, and presented a glorious and brilliant spectacle. "This," said my guide, "is matter of the same kind with that we have just been considering; but it has been set in active motion. The fiat 'Let there be light!' has been issued to it. Nor is its motion in vain. Each of these nebulous masses is the material of a system of worlds, and they will produce systems of different forms in accordance with the various shapes and motions which you observe. Such bodies are well known to earthly astronomers. One of them, the great nebula of Andromeda, has been photographed, and is a vast system of luminous rings of vapour placed nearly edgewise to the earth, and hundreds of times greater than the whole solar system. But now let us annihilate time, and consider these gigantic bodies as they will be in the course of many millions of years." Instantaneously these vast nebulæ had concentrated themselves into systems of suns and planets, but with this difference from ours, that the suns were very large and surrounded with a wide luminous haze, and each of the planets was self-luminous, like a little sun. In some the planets were dancing up and down in spiral lines. In others they were moving in one plane. In still others, in every variety of direction. Some had vast numbers of little planets and satellites. Others had a few of larger size. There were even some of these systems that had a pair of central suns of contrasting colours. The whole scene was so magnificent and beautiful that I thought I could never weary of gazing on it. "Here," said he, "we have the most beautiful condition of systems of worlds, when considered from a merely physical point of view: the perfection of solar and planetary luminousness, but which is destined to pass away in the interest of things more important, if less showy. This is the condition of the great star Sirius, which the old priest astronomers of the Nile Valley made so much of in their science and religion, and which they called Sothis. It is now known by your star-gazers to be vastly larger than your sun, and fifty times more brilliant.[1] Let us select one of these systems somewhat similar to the solar system, and suppose that the luminous atmospheres of its nearer planets are beginning to wane in brilliancy. Here is one of them, through whose halo of light we can see the body of the planet. What do you now perceive?" The planet referred to was somewhat larger in appearance than our earth, and, approaching near to it, I could see that it had a cloud-bearing firmament, and that it seemed to have continents and oceans, though disposed in more regular forms than on our own planet, and with a smaller proportion of land. Looking at it more closely, I searched in vain for any sign of animal life, but I saw a vast profusion of what might be plants, but not like those of this world.[2] These were trees of monstrous stature, and their leaves, which were of great size and shaped like fronds of seaweeds, were not usually green, but variegated with red, crimson and orange. The surface of the land looked like beds of gigantic specimens of Colias and similar variegated-leaved plants, the whole presenting a most gorgeous yet grotesque spectacle. "This," said my guide, "is the primitive vegetation which clothes each of the planets in its youthful state. The earth was once so clothed, in the time when vegetable life alone existed, and there were no animals to prey upon it, and when the earth was, like the world you now look upon, a paradise of plants; for all things in nature are at first in their best estate. This vegetation is known to you on the earth only by the Carbon and Graphite buried in your oldest rocks. It still lingers on your neighbour Mars,[3] which has, however, almost passed beyond this stage, and we are looking forward before long to see a still more gigantic though paler development of it in altogether novel shapes on the great continents that are being formed on the surface of Jupiter. But look again." And time being again annihilated, I saw the same world, now destitute of any luminous envelope, with a few dark clouds in its atmosphere, and presenting just the same appearance which I would suppose our earth to present to an astronomer viewing it with a powerful telescope from the moon. "Here we are at home again," said my guide; "good-bye." I found myself nodding over my table, and that my pen had just dropped from my hand, making a large blot on my paper. My dream, however, gave me a hint as to a subject, and I determined to devote my address to a consideration of questions which geology has not solved, or has only imperfectly and hypothetically discussed.

[1] In evidence of these and other statements I may refer to Huggins' recent address as President of the British Association, and to the "Story of the Heavens," etc., by Sir Robert Ball.

[2] We shall see farther on that there is reason to believe that the primitive land vegetation was more different from that of the Devonian and Carboniferous than it is from that of the present day.

[3] Mars is probably a stage behind the earth in its development, and the ruddy hue of its continents would seem to b: due to some organic covering.

Such unsolved or partially solved questions must necessarily exist in a science which covers the whole history of the earth in time. At the beginning it allies itself with astronomy and physics and celestial chemistry. At the end it runs into human history, and is mixed up with archæology and anthropology. Throughout its whole course it has to deal with questions of meteorology, geography and biology. In short, there is no department of physical or biological science, with which this many-sided study is not allied, or at least on which the geologist may not presume to trespass. When, therefore, it is proposed to discuss in the present chapter some of the unsolved problems and disputed questions of this universal science, the reader need not be surprised if it should be somewhat discursive.

Perhaps we may begin at the utmost limits of the subject by remarking that in matters of natural and physical science we are met at the outset with the scarcely solved question as to our own place in the nature which we study, and the bearing of this on the difficulties we encounter. The organism of man is decidedly a part of nature. We place ourselves, in this aspect, in the sub-kingdom vertebrata and class mammalia, and recognise the fact that man is the terminal link in a chain of being, extending throughout geological time. But the organism is not all that belongs to man, and when we regard him as a scientific inquirer, we raise a new question. If the human mind is a part of nature, then it is subject to natural law, and nature includes mind as well as matter. Indeed, without being absolute idealists we may hold that mind is more potent than matter, and nearer to the real essence of things. Our science is in any case necessarily dualistic, being the product of the reaction of mind on nature, and must be largely subjective and anthropomorphic. Hence, no doubt, arises much of the controversy of science, and much of the unsolved difficulty. We recognise this when we divide science into that which is experimental, or depends on apparatus, and that which is observational and classificatory—distinctions these which relate not so much to the objects of science as to our methods of pursuing them. This view also opens up to us the thought that the domain of science is practically boundless, for who can set limits to the action of mind on the universe, or of the universe on mind. It follows that science, as it exists at any one time, must be limited on all sides by unsolved mysteries; and it will not serve any good purpose to meet these with clever guesses. If we so treat the enigmas of the sphinx nature, we shall surely be devoured. Nor, on the other hand, must we collapse into absolute despair, and resign ourselves to the confession of inevitable ignorance. It becomes us rather boldly to confront the unsolved questions of nature, and to wrestle with their difficulties till we master such as we can, and cheerfully leave those we cannot overcome to be grappled with by our successors.

Fortunately, as a geologist, I do not need to invite attention to those transcendental questions which relate to the ultimate constitution of matter, the nature of the ethereal medium filling space, the absolute difference or identity of chemical elements, the cause of gravitation, the conservation and dissipation of energy, the nature of life, or the primary origin of bioplasmic matter. I may take the much more humble rôle of an inquirer into the unsolved or partially solved problems which meet us in considering that short and imperfect record which geology studies in the rocky layers of the earth's crust, and which leads no farther back than to the time when a solid rind had already formed on the earth, and was already covered with an ocean. This record of geology covers but a small part of the history of the earth and of the system to which it belongs, nor does it enter at all into the more recondite problems involved; still it forms, I believe, some necessary preparation at least to the comprehension of these. If we are to go farther back, we must accept the guidance of physicists rather than of geologists, and I must say that in this physical cosmology both geologists and general readers are likely to find themselves perplexed with the vagaries in which the most sober mathematicians may indulge. We are told that the original condition of the solar system was that of a vaporous and nebulous cloud intensely heated and whirling rapidly round, that it probably came into this condition by the impact of two dark solid bodies striking each other so violently, that they became intensely heated and resolved into the smallest possible fragments. Lord Kelvin attributes this impact to their being attracted together by gravitative force. Croll[4] argues that in addition to gravitation these bodies must have had a proper motion of great velocity, which Lord Kelvin thinks "enormously" improbable, as it would require the solid bodies to be shot against each other with a marvellously true aim, and this not in the case of the sun only, but of all the stars. It is rather more improbable than it would be to affirm that in the artillery practice of two opposing armies, cannon balls have thousands of times struck and shattered each other midway between the hostile batteries. The question, we are told, is one of great moment to geologists, since on the one hypothesis the duration of our system has amounted to only about twenty millions of years; on the other, it may have lasted ten times that number.[5] In any case it seems a strange way of making systems of worlds, that they should result from the chance collision of multitudes of solid bodies rushing hither and thither in space, and it is almost equally strange to imagine an intelligent Creator banging these bodies about like billiard balls in order to make worlds. Still, in that case we might imagine them not to be altogether aimless. The question only becomes more complicated when with Grove and Lockyer we try to reach back to an antecedent condition, when there are neither solid masses nor nebulæ, but only an inconceivably tenuous and universally diffused medium made up of an embryonic matter, which has not yet even resolved itself into chemical elements. How this could establish any motion within itself tending to aggregation in masses, is quite inconceivable. To plodding geologists laboriously collecting facts and framing conclusions therefrom, such flights of the mathematical mind seem like the wildest fantasies of dreams. We are glad to turn from them to examine those oldest rocks, which are to us the foundation stones of the earth's crust.