The Story of Evolution

by Joseph McCabe

* 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.