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. [*]