Back Aa

Romance of Modern Geology

<h2 class="nobreak" id="CHAPTER_X">CHAPTER X</h2> <h3>THE EARTH AS THE ABODE OF LIFE</h3></div> In the last chapter we spoke of the formation of the atmosphere of the earth and of the growth of the oceans. We must now consider the formation of these more closely, and we must distinguish between the great vaporous clouds which rolled about the earth in its molten state and the settled atmosphere which formed about it when it grew cooler. After the earth had begun to solidify it was at first covered with a collection of porous fragments of rock covering the earth like a shell and containing the elements of water. Such materials in general appearance would be not unlike the pumice stone which is expelled from volcanoes to-day. Those who have never had the fortune to see volcanic eruptions for themselves usually imagine that the volcano throws out nothing but fire and smoke. As a matter of fact it throws out vast quantities of vapour, of which, according to Sir Archibald Geikie, 999 parts in 1000 are steam. At the great eruption of Mount Pelée the cloud of steam continually arising from the volcano for months in succession was several cubic -109- miles in measurement. Consequently it will be seen that the porous volcanic rocks with which the young earth was covered contained all the materials for water-manufacture within themselves. As the water began to form, squeezed out of the porous rocks as we can squeeze it out of a sponge (or as we might steam it out if we put the moist sponge in an oven), it gathered itself into reservoirs underground. As it increased in bulk it rose nearer to the surface; because, of course, owing to the heat of the inner portions of the earth it could never succeed in sinking below a certain depth. Doubtless it first appeared at the bottom of the pits which had been sunk by volcanoes or volcanic action. There must have been innumerable depressions in the earth's surface as widespread and deeper than those which we can perceive on the rugged surface of the Moon. We may gain an idea on a very minute scale of what the first pits of water were like from the examples (formed, however, at a much later period and probably in a different way) of the crater lakes that are left to-day. Some curious examples of "crater lakes" are to be found in the Eifel district of Germany, an ancient volcanic region which lies in the triangle formed by the junction of the Rhine and the Moselle at Coblenz. One of the pleasing peculiarities of this district is that, owing to the volcanic nature of the soil, the neighbourhood is seldom dusty, even in August or September, after the dry continental summer. It is well worth visiting for its castles as well as for its crater lakes and other volcanic relics, and it is the scene of R. L. Stevenson's romance Prince Otto. The chief crater -110- lakes are between Daun and Manderscheid. There are, of course, many other and larger crater lakes in existence, but we select these because they are so easily accessible. The flow of the lakes into one another followed. Innumerable lakelets developed into rivers or chains of lakes on the surface of the young planet, continually becoming larger bodies of water, till they developed into the vast irregular oceans of to-day. This evolution is of great importance from a geological point of view, because it leads the way to the origin of the ocean basins and the great platforms of land which we call continents. It is easy to see that because of the weight of water in the depressions the earth under the waters tended to become more and more depressed, so that the water areas tended to grow larger and deeper. The wash of earths from the land tended to build its borders out into the water basins, but the deepening and spreading of the water basins is believed to have been the most marked feature of the earth's early growth. All this time the earth was growing in diameter and circumference.[9] When this growth ceased other causes and effects came into play, and the proportions of sea and land became better balanced. <div class="footnote"> [9] For reasons which are a little too complex to be considered here. We can only indicate the general line of reasoning by saying that the central heat as it moved outwards from the rocks nearer the surface expanded them. </div> There is nothing in our human knowledge to tell us with certainty when or how life first appeared on the earth. We have already spoken of animal and vegetable remains that for ages are preserved in the rocks. But clearly no such remains could ever be found in the -111- volcanic or molten rocks of the earliest stages of the earth's life. Think for a moment of what the simplest forms of life are. A great deal has been heard of microbes and bacteria during recent years, and we may therefore assume that every one has some knowledge of the structure of these simplest living things. They may be compared to tiny bladders of jelly—so small that the microscope is necessary in order to see them, and sometimes so much smaller than this that the best microscopes cannot distinguish them. Such forms of life are called "unicellular organisms," because they consist of a single cell, which contains the jelly-like substance called protoplasm, and a smaller body, smaller even than these tiny cells themselves, which is called the nucleus. These are the simplest forms of life. But all the higher forms of life, and we may say, roughly speaking, any form of life that the unaided eye can see, is made up not of one cell but of many cells. A human being, for example, is made up of uncounted millions of cells; and millions of cells go to the formation of a worm, a fish, a gnat, or indeed to the formation of the simplest well-known animal that the ordinary person could name. Similarly millions of cells go to the formation of a leaf or a twig. These higher forms of life are called "multicellular organisms," because they have many cells, and most often many different kinds of cells. For instance, in the body of a man there are different kinds of cells to form the skin, or the lining of the mouth, or the substance of the eyes, or the red or white corpuscles of the blood, or the grey matter of the -112- brain, or the roots of the hair—to name only a few. Thus we see how complicated the structure of animals has become since life first made its appearance on the earth. The cells joined themselves to form tissues; and the tissues joined themselves to form organs; and these things had to happen before anything like a complete animal of the higher type, or even a complete vegetable, made its appearance. Suppose by some great cataclysm, not so great as that which we have imagined in an earlier chapter, but still world-wide in its effects, the whole world should once again be swept by a great outbreak of lava and molten rocks, which of all the living things would leave traces of its existence? Perhaps a few of the animals with great bones, or the great trees, might leave an impress of themselves in the depths of the overwhelming rocks, just as we can stamp the impress of the skin of our finger-tips on hot sealing-wax; but it is fairly evident that all the soft-tissued animals and vegetables would disappear entirely and leave not a trace behind—certainly no trace that anybody could recognise many millions of years afterwards. It is still more certain that the simplest forms of life, "the unicellular organisms," would leave no trace at all. We know that side by side with the complicated organisms that we can see the simplest organisms exist now; and must have existed at the beginning of life. Yet when we examine the records of living things in the rocks which were formed in the youth of the world, and go back right to the earliest of these forms of life that have ever been discovered, we find that such specimens are all of the rather higher (if -113- not of the highest forms of life). From that we infer that life must have existed many ages before the period of such remains, though, as we should have expected, all examples of the earlier forms have disappeared. Where did this life come from? Lord Kelvin once rashly committed himself to the notion that life might have been brought to the earth on one of those flying pieces of rock, which we have already spoken of, which are named meteorites, or on a fragment of some other planetary body that had been cast out into space. The speculation is not so wildly improbable as it has sometimes been considered to be, because recent researches have shown that it is not impossible for life to survive at the very low temperatures which a meteorite would experience on its way through space, and also that the time which a small body would occupy in travelling, let us say from Mars to the earth, would not be too great for the prolonged existence of some germ of life on the meteorite. On the other hand, there is nothing in known meteorites to suggest that they came from worlds where conditions exist suitable for life as we know it; and, moreover, even if we shut our eyes to these improbabilities, we are no nearer to a solution of the problem of where and how life began. To say that it was brought to earth on a meteorite is merely to throw back the problem another stage, for we have still to ask how life began on the meteorite and on the planet from which it came. The indirect evidence regarding the probable beginning of the era of life on the earth is also extremely difficult to examine, and we can only say that the best geological -114- authority leans to the idea that the conditions when life would have been possible on the earth were finished long before the earth had finished growing, and that these conditions may have existed when the earth was about the size of Mars. Consequently the first beginnings of life may have existed at depths hundreds of miles below the earth's surface of to-day. The life was then, however, only of the very simplest kind; it was probably vegetable life. Probably also the first life appeared in the ocean, though it is not altogether unlikely that it may have begun, and have gone on developing, in fresh water—in those great pits which were first formed by volcanoes and which afterwards became lakes and then seas. For our purpose, however, it will be sufficient to say that life began in the great bodies of water which were accumulating on the globe, and which owing to the washing down by rain and rivers and stream and wave action of land materials were becoming "saltier," or more highly charged with mineral salts of various kinds. The early forms of life were of the nature of jelly-fish, or simple organisms which were permeated by the fluid in which they dwelt. The sea was then warmer than it is now, and there are reasons for believing that it was something above 100° F., perhaps higher—perhaps rather hotter than we should now care to bathe in. It was also at the beginning an ocean which was much less salt and had much less lime in it than now. Its water was a good deal "softer." It was, however, becoming much more hard, more like the Dead Sea, which, as everybody knows, is a body of water so -115- charged with mineral salts and accumulations that a bather cannot sink in it, and will emerge from his bath encrusted with salty deposits. As the early ocean became more and more saturated (with lime and carbonates, etc.), the more vigorous of the living forms in the water began to resist the change in various ways. They tried to meet it, or to alter themselves so as not to be incommoded by it. This is a very familiar occurrence in natural life and evolution. Perhaps the commonest example of it that we can select is the formation of corns on the human feet and hands. A corn, properly considered, is the defence raised by the skin against unusual or discommoding pressure or friction. When a boot is too tight, or when a plough handle or a cricket bat or a golf club is continually clasped tightly, a "callosity" or horn-like defence is formed. In some cases a blister is antecedent to the corn; and we may regard this not only as showing the need for the hardening of the skin, but as being a stage preliminary to it. There are many other instances. Hair is formed as a protection to the body, and owing to nature's economies is not usually formed when clothing takes its place or heat renders it unnecessary. If the heat is too great, or the light beating on the unprotected skin is too strong, then another form of protection takes the place of hair. Why is it that races living near the Equator wear "the burnished livery of the burning sun," and show black or brown pigmentation of the skin? It is because this pigmentation arrests the penetration of -116- the rays of the sun more effectively than a garment. Lately Dr. Sambon has pointed out that the white linen clothes which Europeans generally wear in the tropics, though they look cool, are not sufficient screens against the rays of light and heat, and has suggested that white men's clothes, to be properly protective against the sun, ought to be woven of threads of two colours, so that the garments should be white outside and black inside. Apply these principles to animals in sea-water, who were distressingly affected by conditions to which they had never been subjected. What would happen? The weaker would probably be killed by the change in the conditions, just as some fresh-water fishes and animalculæ would be killed now if plunged into sea-water. The stronger would, however, become acclimatised, and would in the course of successive generations struggle to adapt their bodies to the new conditions. Thus the living organisms in the earth's early sea contrived to cut themselves off from being bathed with the novel carbonaceous water. They cut themselves off from it in the course of generations by closing themselves off from it with skin or membrane. Many of them stirred up their cells to secrete lime and exude it so as to form for themselves a more or less impervious covering or shell. Finally, as they grew to like the mineral water less, they continually made fresh experiments to avoid it, and the more enterprising and adventurous got out of the ocean altogether, and migrated to the air or the land, perhaps by way of the shore sands and -117- muds. This period, when the ocean seems to have passed its best stage for supporting all forms of life, appears to have been that which is geologically known as the Cambrian. After this period there was a wealth of ferns, of animals able to leave hard traces of themselves in the fossil records. Before this period there was no physiological need for either skin or shell. But once the skin and shell had been developed, primarily as a defence against the sea-water, their great advantages for purposes of the struggle for life among all forms of animals soon made themselves felt, and so they were retained. <hr class="chap x-ebookmaker-drop" /> -118- <div style="break-after:column;"></div>

1 of 2

2 of 2

Update Required To play the media you will need to either update your browser to a recent version or update your Flash plugin.